Water-mediated Recycling of Gold, Palladium, and Platinum Using Semimetallic TiS2 and TaS2 Nanosheets

Water-mediated Recycling of Gold, Palladium, and Platinum Using Semimetallic TiS2 and TaS2 Nanosheets | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Water-mediated Recycling of Gold, Palladium, and Platinum Using Semimetallic TiS 2 and TaS 2 Nanosheets yang Su, Jianhong Wei, Miaofei Huang, Huanjing Liang, Fei Li, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5882578/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Nov, 2025 Read the published version in National Science Review → Version 1 posted You are reading this latest preprint version Abstract The intensive and irreplaceable consumption of precious metals (PMs) including Au, Pd, and Pt in the electronic and catalysis industries, coupled with their scarcity in the earth’s crust, demand innovative recycling solutions for PM sustainability 1-8 . However, efforts to recycle PMs from leachates of their waste are frustrated by an unsatisfactory extraction capacity at low concentrations and remain predominantly focused on gold, leaving other PMs unexplored 9-13 . We report the ultrahigh reductive recycling of PM ions and their simultaneous aqueous-phase deposition on semimetallic transition metal dichalcogenides (TMD) of TiS 2 and TaS 2 nanosheets . Notably, TiS 2 shows unprecedented high extraction capacities of ~8 g/g, 2.3 g/g, and 1.15 g/g for Au, Pd, and Pt ions, respectively, and the adsorbed PM ions directly transformed into nanoparticles deposited on the nanosheets. Mechanistic studies reveal that water-mediated electron donation from the sulfur site of the semimetallic TiS 2 and TaS 2 nanosheets is responsible for the ultrahigh extraction capacity, with a single TiS 2 molecule donating more than 13 electrons to gold ions. This electron transfer is mediated by the formation of sulfur-oxygen species during water dissociation. We further demonstrate the selective and complete recovery of Au, Pd, and Pt from real-world waste streams including electronic waste, spent catalysts, and automotive catalytic converters. Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials Physical sciences/Nanoscience and technology/Nanoscale materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Precious metals (PMs) including gold (Au), platinum (Pt), palladium (Pd), and others are the cornerstones of many industrial processes and devices, ranging from catalysis and renewable energy devices to modern electronic devices 1-6 . For instance, Au is essential in electronics, while Pt and Pd are crucial for industrial catalysis 7-12 . However, the high annual consumptions of ~4600 tons of Au, ~220 tons of Pt, and ~300 tons of Pd 13-15 , their low concentrations in the earth's crust (~4, 13, and 5 mg/ton) 16 , and their low recycling rates <20% present sustainability challenges 14, 15 . Recycling PMs from wastes offers a sustainable alternative to ore mining as it reduces energy consumption, resource extraction, and the environmental impact caused by landfilling of such wastes 9, 17 . Hydrometallurgy is frequently used for PM recycling, and involves leaching to dissolve the PMs and co-existing elements, followed by selective adsorption and reduction of the PM ions 13 . Selective adsorption is a critical step and relies on the efficiency of the adsorbents 12, 18 . Traditional adsorbents have limitations on unsatisfactory selectivity and insufficient extraction capacity ( Q e ) 19-22 . Recent advances show that by incorporating redox sites in porous materials 12, 23, 24 , the resulting adsorbents have a Q e of 300-4000 mg/g (at an Au concentration of < 200 ppm) with simultaneous reduction of the ionic gold to the element 11, 17, 25-28 . While gold adsorbents have been extensively studied, adsorbents for other PMs, for example, Pt, is largely unexplored despite their high economic value and critical application in the catalysis industry, for example, existing adsorbents for Pt rely on either weak intermolecular forces 29, 30 or use gold-specific adsorbents 10, 17, 31, 32 , and suffer from an unsatisfactory adsorption performance for Pt ions because of its lower reduction potential than Au. Recently emerging 2D materials, with their large surface area and important surface chemistry, present new opportunities for efficient PM recycling 24, 27, 30 . Our previous study has shown that reduced graphene oxide (rGO) is highly efficient for Au recycling, but is ineffective for [PtCl 6 ] 2- due to its inability to reductively adsorb Pt ion 24 . Though other 2D materials are reported to reductively adsorb Pt ion, they also exhibit low Q e even at a high concentration due to their weak electron donation capability 28, 3 1- 33 , and suffer from either a complicated fabrication process and/or a high cost, undermining their economic viability for practical recycling PMs. Materials that enable efficient electron donation to PM ions should be promising adsorbents for PM recycling. We then become interested in 2D transition metal dichalcogenides (TMDs) , in particular, semimetallic TMDs, such as TiS 2 and TaS 2 , they have narrow bandgaps (~0.2 eV and ~0.1 eV), and high Fermi levels of ~-4.13 eV and ~-4.49 eV 34-38 , respectively, well above the equivalent energy level for the reduction potentials of [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- ions , may offer more efficient electron donation and superior PM adsorption than other TMDs. Here, we report ultrahigh and selective PM extraction by semimetallic TiS 2 and TaS 2 nanosheets, with TiS 2 having Q e values of 8073 mg/g, 2359 mg/g, and 1154 mg/g for [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- , respectively, making them ideal candidates for nearly complete recycling of PMs from their electronic waste (e-waste), spent fuel cell catalysts, and automotive catalytic converters (ACCs). In addition, our work highlights the efficient extraction of PM ions accompanied by the simultaneous formation of PM/TMD heterostructures, providing a synthetic pathway for advanced catalysis and contact engineering of TMD electronic devices, and opening an avenue to address PM sustainability. Adsorption performance of semimetallic TiS 2 and TaS 2 nanosheets The 1T-TiS 2 and 2H-TaS 2 nanosheets were exfoliated by sonication of their thick flakes in a LiOH aqueous solution 39 , and Raman analysis showed that the exfoliated nanosheets had no phase change during the process (Fig. S1 and supplementary section 1) 40 . Transmission electron microscope (TEM) and atomic force microscope (AFM) analysis showed that both types of nanosheets had a lateral size of ~ 2 μm and a thickness of 0.9 - 2.5 nm (Fig. S2 and Fig. S3), suggesting a mono- and few-layer structure and a large specific surface area 41 . X-ray diffraction (XRD) and high-resolution TEM (HR-TEM) showed high crystallinity (Fig. S2 and S4). The Fermi energy levels of TiS 2 and TaS 2 allowed its electron transfer to the [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- ions (Fig. 1a), promising their strong adsorption for PM ions. An initial adsorption test showed that both TiS 2 and TaS 2 nanosheets had an immediate color change when mixed with [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- solutions (initial concentration C 0 = 100 ppm), respectively, suggesting their strong interaction with and rapid adsorption of the PM ions (Fig. 1b). Taking 1T-TiS 2 as an example, we measured the Q e values of [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- ( C 0 ~100 ppm) were 8073, 2359, and 1154 mg/g, respectively (Fig. 1c). Given that TiS 2 was the lightest and also possibly the cheapest TMD (~14 RMB/g) 42, 43 , the measured ultrahigh Q e values indicated its strong economic viability for PM recycling (Fig. 1c inset and supplementary section 2). We next quantified the adsorption behavior of the TMD for PM ions. The Q e values for PM ions with C 0 varying from 0.1 to 100 ppm (Fig. 2a) were studied. Notably, TiS 2 had a Q e of 6620 mg/g to 0.1 ppm [AuCl 4 ] - , then became saturated at ~8000 mg/g when the C 0 increased to 1 ppm. Note that such a Q e at a low concentration of [AuCl 4 ] - was significantly higher than previously reported gold adsorbent as summarized in Fig. 2c, promising its application for gold recycling at minute concentrations. For [PdCl 4 ] 2- and [PtCl 6 ] 2- , the Q e started to saturate when C 0 was 10 ppm or higher, specifically, the Q e for [PdCl 4 ] 2- was 1892, 2217, and 2359 mg/g and that for [PtCl 6 ] 2- was ~323, 813 and 1154 mg/g when C 0 = 1, 10 and 100 ppm respectively. The adsorption kinetics (Fig. 2b) showed that TiS 2 extracts >99% of the Au and Pd within 10 minutes ( C 0 = 10 ppm), significantly faster than the value for rGO we reported previously 24 . In contrast, the adsorption of [PtCl 6 ] 2- was slow, with only 3.2% of [PtCl 6 ] 2- extracted in 10 min, and 68% and >95% removal efficiencies were achieved for 15 and 24 h adsorption. We also investigated the influence of pH on Q e and removal efficiency. According to the Pourbaix diagram which describes ion stability at different pH values, the pH ranges for stable [AuCl 4 ] - and [PtCl 6 ] 2- were ~1-7, and that for [PdCl 4 ] 2- was pH<5 32, 44 . We therefore studied the Q e value in these pH ranges (Fig. S5). For [AuCl 4 ] - ( C 0 = 10 ppm), TiS 2 had the highest Q e (~8076 mg/g) at pH=3-5, and decreasing or increasing the pH results in a lower Q e , which probably was due to a trade-off between the surface charge of TiS 2 nanosheets and stability of [AuCl 4 ] - at different pH values (Supplementary Section 2). For [PdCl 4 ] 2- , the Q e increases with pH, while for [PtCl 6 ] 2- , it remained relatively stable in the tested pH range (Supplementary Section 2) 10, 24 . Furthermore, TiS 2 showed a ~100% removal efficiency for the three types of PM ions (Fig. 2b inset). Not only do we found an ultrahigh Q e for TiS 2 , we also found that the TaS 2 nanosheets were highly efficient for the extraction of the three PM ions, their Q e values for [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- were 4100, 1100 and 452 mg/g (Fig. 2d and Table S1), slightly lower than that of TiS 2 . Nevertheless, as summarized in Fig. 2 c-d and Table S1, both TMDs had significantly higher Q e values than the previously reported PM adsorbents, for example, MXene, graphene, MOFs, COFs, and pyridine-modified disulfide 24, 25, 27, 28, 45 , especially in the sub-ppm to tens of ppm range relevant to the practical recycling PM-containing waste streams. Adsorption mechanism To understand the efficient adsorption of PM by semimetallic TMD, we used TEM (Figs. 3a-c, Fig. S6), XRD (Fig. 3d), and X-ray photoelectron spectroscopy (XPS, Fig. 3e) to study TiS 2 @PM precipitates after adsorption. The TEM observation revealed the deposition of a high density of nanoparticles on the TiS 2 nanosheets (particle size 3-18 nm). For [AuCl 4 ] - and [PdCl 4 ] 2- , nanoparticles had lattice spacings consistent with the (111) plane of metallic Au and Pd 8, 25, 46 , whereas, the nanoparticles deposited after adsorption of [PtCl 6 ] 2- had a lattice spacing corresponding to the (110) plane of PtS (Fig. 3c and Fig. S7) 47 . The XRD patterns showed prominent peaks corresponding to elemental Au and Pd, and to PtS 47 , consistent with the TEM observations (Fig. 3d). The deconvoluted XPS peaks of PM deposited on TiS 2 quantitatively showed that >99%, ~92%, and ~ 82% of [AuCl 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- were reduced to Au(0), Pd(0), and Pt(II), respectively, with the rest of the adsorbed PM being the Pd(II) and Pt(IV) (Fig. 3e and Fig. S8) 47-49 , Collectively, these results suggested the dominant adsorption mechanism of PM by TiS 2 was reductive adsorption, and supported that the high Fermi level of semimetallic TMDs enables their efficient electron donation to PM ions 4, 50 . We also noted that the reductive adsorption of [PtCl 6 ] 2- yields PtS, which could be understood by the fact that the reduction of [PtCl 6 ] 2- to Pt 0 was a two-step process with Pt 2+ being an intermediate product, and its subsequent bonding to a sulfur site of TiS 2 inhibited its further reduction to Pt 0 because of the good chemical stability of PtS 51 . Nevertheless, because PtS was a primary ore for mining Pt, this suggested the PtS could easily be purified using established methods 51 . Further insight regarding the reductive adsorption of PM ions by TiS 2 was gained by measuring its Q e values of a series of PM ions with decreasing reduction potentials. As plotted in Fig. 3f, TiS 2 has a Q e greater than 1100 mg/g for ions with reduction potentials >0.59 V (versus a standard hydrogen electrode (SHE)), including [AuCl 4 ] - , [AuBr 4 ] - , AgNO 3 , [AuI 4 ] - , [PdCl 4 ] 2- , and [PtCl 6 ] 2- , but a small Q e (<200 mg/g) for ions with reduction potentials <0.59 V, for example, [RhCl 6 ] 3- and [Au(S 2 O 3 ) 2 ] 3- , confirming the redox reaction-driven reductive adsorption of PMs (Fig. S9). It was worth noting that the high Q e observed for a wide range of gold ions expanded the potential use of TiS 2 beyond the commonly seen [AuCl 4 ]⁻ ion. For example, [AuI 4 ] - , a product of the standard wet etching of a gold coating in the electronics industry, could be effectively recycled from wastewater using TiS 2 , as evidenced by a high Q e of 3458 mg/g. The redox-dominated adsorption also provided a way to selectively adsorb PM ions based on their reduction potentials. To validate this, we measured the PM removal efficiency of TiS 2 nanosheets in a simulated solution containing 15 interfering ions (~100 ppm each of Na + , K + , Mg 2+ , Ca 2+ , Cd 2+ , Mn 2+ , Co 2+ , Cu 2+ , Zn 2+ , Ni 2+ , Al 3+ , Fe 3+ , Ce 3+ , La 3+ , and Cr 3+ ) and 4 types of PM (~10 ppm each of Au 3+ , Pt 4+ , Pd 2+ , and Rh 3+ ). The nanosheets showed a removal efficiency of >97% for Au 3+ , Pt 4+ , and Pd 2+ , while a low removal efficiency of <8% for the remaining ions including Rh 3+ (Fig. 3g), promising a good PM selectivity for real-world PM recycling involving complex ion mixtures. In addition, the work function of TiS 2 was -3.89 eV as measured by ultraviolet photoelectron spectroscopy (Fig. S10), consistent with the reported value 36 , whereas, the reduction potentials (E Red ( vs SHE), Fig. S11) of [PdCl 4 ] 2- and [RhCl 6 ] 3- were +0.59 V and +0.43 V respectively, which translated into E Red values of -5.03 and -4.87 eV (E Red = - E Red ( vs SHE) - 4.44), respectively. Note that TiS 2 had a high Q e for [PdCl 4 ] 2- but was incapable of adsorbing [RhCl 6 ] 3- , which indicated that the energy barrier required for efficient adsorption by TiS 2 should be ~1-1.14 eV. This was in good agreement with previous results 4, 28 , and was explained by the energy needed for ion diffusion, ion desolvation, and, in our case, electron donation and crystal nucleation and growth. We next investigated why TiS 2 had such a high Q e , using gold as an example. We first compared the Q e values achieved by using thick unexfoliated TiS 2 flakes and the exfoliated ones. For a C 0 = 100 ppm, unexfoliated TiS 2 had a Q e for gold of 476.8 mg/g (Fig. S12), nearly ten times lower than that of the exfoliated nanosheets. Similar results were also found for exfoliated and unexfoliated TaS 2 (Fig. S12). Because our HR-TEM and Raman analysis showed the TiS 2 nanosheets had a highly crystalline structure, such a large difference in Q e could not be caused by defects, but was due to an increased specific surface area which provided abundant adsorption sites for achieving a high Q e 24, 25 . Furthermore, a Q e value of 8073 mg/g for [AuCl 4 ] - and the complete reductive adsorption of gold ions by TiS 2 suggested that each TiS 2 molecule donates >13 electrons during the adsorption process (Supplementary Section 3), we therefore studied the active site that donates such a large number of electrons by analyzing the structure change of TiS 2 nanosheets after adsorption. XPS analysis (Fig. S13) showed that the binding energies of Ti and S increased compared to pristine TiS 2 . The XPS spectra of Ti2p after adsorption were deconvoluted into two peaks (~ 464.4 and 458.7 eV), which do not fit with the pristine Ti-S bond, but align with the typical Ti-O bond (Fig. S13) 52 . For sulfur, we measured the dissolved salt after adsorption and found multiple binding energies at 168.6 eV and 169.8 eV were assigned to the sulfate species (Fig. 4a) 53, 54 . This indicated each TiS 2 molecule could, in principle, donate no more than 16 electrons to the adsorbed [AuCl 4 ] - , in quantitative agreement with the observed high Q e . Note that the XPS analysis also showed a similar transformation of the sulfur in TaS 2 after [AuCl 4 ] - adsorption (Fig. S14), with the difference that a valency change of tantalum (Ta 4+ →Ta 5+ ) was also observed 55, 56 . This suggested that one TaS 2 molecule can donate no more than 17 electrons to [AuCl 4 ] - , however, the observed slightly lower Q e for the adsorption of [AuCl 4 ] - by TaS 2 must be attributed to the fact that its higher weight fraction than TiS 2 reduces the weight-based PM extraction capacity. Collectively, these results provided unambiguous evidence that the sulfur of the TMD was the dominant site for extremely high electron donation which was responsible for the ultrahigh Q e values. The formation of sulfate species after adsorption suggested that oxygen was needed for electron donation, but was absent in the PM salt used, also the good crystal structure of the TMD nanosheets (Fig. S2) did not explain the presence of such a large amount of oxygen after the reductive adsorption. It was therefore reasonable to suggest that the oxygen comes from the dissociation of the water solvent used in the adsorption. To testify this, we first considered that dissociation of water would lead to a release of proton in the solvent, indeed, the pH of the gold solution decreased from 3.96 to 3.77 after gold extraction (Fig. S15). We then replaced the water with the non-protonic solvent acetone and found that the Q e of TiS 2 nanosheets with an acetone solution of 100 ppm [AuCl 4 ] - was only ~1180 mg/g (Fig. S16 and Fig. 4b), ~7 times lower than the Q e value measured in water. In addition, adding 30 vol% water to acetone resulted in an increased Q e of 5217 mg/g. Because all the experiments were performed in the same ambient environment, the change in Q e produced by a change of solvent indicated that it was oxygen from the solvent rather than from the environment that dictates the PM adsorption. To understand such an interesting role of water in the extraction process, we performed theoretical calculations using spin-polarized density functional theory (DFT) and TiS 2 crystal cells to evaluate the adsorption energy ( E ad ) of the [AuCl 4 ] - ion. Fig. 4c showed the energy diagram of the dechlorination reduction process of [AuCl 4 ] - by TiS 2 in an aqueous solution (Supplementary Section 3). Firstly, the calculation indicated the favorable adsorption of [AuCl 4 ] - ion onto TiS 2 (forming the “TiS 2 *AuCl 3 ” species) with an E ad of -0.88 eV. This adsorption was accompanied by a dichlorination step, which made the gold atom directly bond to a sulfur atom of TiS 2 , forming an electron donation site from sulfur to gold. Subsequently, a water molecule was adsorbed on the TiS 2 *AuCl 3 intermediate, with the oxygen atom of water substituting one Cl - , followed by the dissociation of H 2 O, generating a TiS 2 *AuCl 2 OH intermediate in a thermodynamically favorable process with an E ad of -0.76 eV, and forming a HCl byproduct, which explained the observed pH decrease after the gold extraction. Next, another water molecule approached the TiS 2 *AuCl 2 OH intermediate, initiating a second dechlorination reaction that resulted in the formation of the TiS 2 *AuClOH intermediate, which involved the dissociation of the second water molecule. The overall process led to the reduction of Au from Au (III) to Au (II) by electron donation from the sulfur site and the formation of an S-O bond, which may eventually lead to the formation of sulfate species, in good agreement with the XPS results. The mechanism revealed by DFT showed that the S atom in TiS 2 was the electron donor, while the entire process relied on the dissociation of the water molecule which forms the S-O bond and releases HCl. Taking these results together, we propose that water-mediated electron donation from the sulfur site of semimetallic TiS 2 and TaS 2 nanosheets is primarily responsible for the observed ultrahigh adsorption and deposition of PMs. The sulfur atoms of TiS 2 /TaS 2 are mostly converted to positive hexavalent sulfur (Fig. 4a), such sulfur oxidation is enabled by the disassociation of water which provides oxygen to form sulfate species. In addition, the large surface area of the nanosheets allows efficient and rapid adsorption. Thermodynamically, the electronic structures of TiS 2 and TaS 2 nanosheets (a narrow bandgap of 0.1-0.2 eV, and a Fermi level that is at least ~1 eV higher than the reduction potential of PM ions), allow the electron donation from the nanosheets to the PM ions (Fig. 3e). PM recovery from e-waste and catalyst scrap The observed ultrahigh Q e for PM ions and good selectivity indicated that TiS 2 can be used to recycle PMs from their corresponding waste. To demonstrate this, we recycled Au, Pd, and Pt from their corresponding wastes including e-waste, a Pd/C catalyst for chemical manufacturing, a Pt/C catalyst for fuel cells, and scrap ACCs. For the e-waste (Fig. 5a), TiS 2 recovered >99% gold from the leachate of computer central processing unit (CPU) boards, and showed negligible adsorption of the co-existing ions, including Cu 2+ and Ni 2+ ions (Fig. 5a). For the leachate of the Pd/C catalyst, TiS 2 recovered 99% Pd (Supplementary Section 4). For the Pt/C catalyst from scrap fuel cells (Fig. 5b), Co or Ni was frequently used as a co-catalyst, the TiS 2 can directly recover 95.5% Pt from the leachate of the scrap catalyst while absorbing only 6.9% and 5.4% of Co 2+ and Ni 2+ respectively. For the spent ACCs, it had a complex composition containing Pd, Pt, and Rh catalysts typically supported on ceramic substrates (e.g., MgO 2 , Al 2 O 3 , SiO 2 ). In our case, after acid digestion of the ACCs, the amounts of Pd 2+ , Pt 4+ , and Rh 3+ were 36.3, 18.5, and 18.1 ppm, respectively, while those of the co-existing Al 3+ , La 3+ , Ce 3+ , and Mg 2+ ions were respectively ~3475, 1222, 501, and 88.5 ppm (Fig. 5c). Different from the three types of PM-containing wastes mentioned previously, Pd and Pt co-exist in the ACCs, and require selective adsorption to separate them. Because TiS 2 had much faster adsorption kinetics of Pd than Pt (Fig. 2b), we believed that these two PMs could be separated based on their distinct adsorption kinetics. As shown in Fig. 5d, a two-step extraction process for their recovery and separation was designed. We added TiS 2 nanosheets to the leachate to adsorb Pd 2+ for 10 min, and then removed them, resulting in the recovery of 99% of Pd but only 8% of Pt. We then added a second batch of TiS 2 to extract the Pt for 36 h, and 97% Pt was recovered with no detectable Pd observed in TiS 2 . This efficient recovery of the Pd and Pt allowed further extraction of the Rh. We used iron powder to initiate a replacement reaction which recovered 98% of the Rh, leaving the rest of the co-existing ions remaining in the leachate (Fig. 5e). These steps allowed the selective recovery of ~90% to ~100% Pd, Pt, and Rh. Following the previous report 57 , the TiS 2 with adsorbed PMs (including recycled from scrap CPUs, Pd/C catalysts, Pt/C catalysts, and ACCs) was dissolved in aqua regia and chemically reduced to separate the PM from the insoluble Ti-containing precipitates (Fig. S17). Energy dispersive spectroscopy (EDS) results showed that the purities of the recycled Au, Pt, and Pd from their corresponding scrap CPUs, Pt/C catalysts, and Pd/C catalysts all were > 97 wt% (Fig. S17). For the recycled Pd and Pt from ACCs, the Pd purity of the product after the first purification step was ~88 wt%, with ~11 wt% of Pt, and the Pt purity of the product after the second-step was~98 wt%, respectively (Fig. S17). Finally, the insoluble Ti-containing precipitates were shown by EDS to contain mostly Ti (61 wt%) and O (33 wt%) (Fig. S17), Such a high abundance of Ti indicated that it could be used for the synthesis of other Ti-based compounds, for example, TiS 2 , suggesting a closed-loop regeneration process. Conclusion We have discovered that semimetallic TiS 2 and TaS 2 nanosheets are highly efficient for the adsorption of PM ions including Au, Pd, and Pt. Benefiting from a near zero bandgap and an appropriate Fermi energy level, these nanosheets donate ~13 electrons/molecule to the PM ions primarily from their sulfur sites, resulting in an ultrahigh Q e and an excellent adsorption selectivity. We have also demonstrated the use of TiS 2 nanosheets for the recovery of PMs from their wastes, including e-waste, scrap catalysts, and automotive catalytic convertors, promising their use for PM recycling. Our study has revealed a complicated electron donation behavior from the adsorbent to the ions, which is a result of the interplay between adsorbate, adsorbent, and solvent, which provides insight on designing novel adsorbents. Furthermore, the observed aqueous-phase PM deposition on the TiS 2 and TaS 2 nanosheets provides a new strategy and insight for the surface functionalization of TMD nanosheets which are interesting for the interfacial engineering of TMD-based electronic devices and catalysts. Given the irreplaceable role of PMs in modern industry, our finding opens a way to use 2D materials to address global PM sustainability. Methods Extraction capacity of TiS 2 and TaS 2 nanosheets for PM ions A KAuCl 4 aqueous solution was mixed with a TiS 2 or TaS 2 suspension to form mixtures with initial gold concentrations of 0.1, 1, 10, 50, and 100 ppm. The pH values of the solutions were adjusted by 0.1 M HCl or NaOH solutions. The weight ratio between the Au ions and the TiS 2 /TaS 2 was 10:1, and these mixtures were stirred for 24 hours at 25°C to determine the extraction capacity of the nanosheets. After extraction, the mixtures were filtered through a syringe filter with a 0.22 µm pore size polyethersulfone (PES) membrane, and then the ion concentrations were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Finally, the extraction capacity ( Q e , mg/g) was calculated using the following equation, Q e = ( C 0 - C e ) ·V/m, where C 0 (mg/L) and C e (mg/L) were the initial concentration and equilibrium concentrations, respectively. V (L) and m (g) were respectively the volume of the solution and the mass of the TMD nanosheets. To determine the Q e of TiS 2 for [AuBr 4 ] - , [AuI 4 ] - (obtained by following previous report to mix [AuCl 4 ] - with KI solution 58 ), [Au(S 2 O 3 ) 2 ] 3- , [RhCl 6 ] 3- , and Ag + (from AgNO 3 ) ions, we used the same procedure as that for [AuCl 4 ] - , with an initial ion concentration of 100 ppm. During the extraction, the pH was~3-4, except for [Au(S 2 O 3 ) 2 ] 3- , which was conducted at a pH of ~10, due to its known instability in acidic solutions 24 . A similar procedure was used to obtain the extraction capacity for Pd and Pt from their corresponding ions ([PdCl 4 ] 2- and [PtCl 6 ] 2- ), and the weight ratio of the Pd and Pt ions to TiS 2 or TaS 2 was set at 3:1 and 2:1 respectively. Selective extraction for precious metals from a multi-component solution To determine the extraction selectivity of TiS 2 , TiS 2 nanosheets (5 mg) were added to the 100 mL solution containing Au 3+ (~10 ppm), Pt 4+ (~10 ppm), Pd 2+ (~10 ppm), and Rh 3+ (~10 ppm), as well as 15 interfering metals (~100 ppm for each ion) including Na + , K + , Mg 2+ , Ca 2+ , Cd 2+ , Mn 2+ , Co 2+ , Cu 2+ , Zn 2+ , Ni 2+ , Al 3+ , Fe 3+ , Ce 3+ , La 3+ , and Cr 3+ . After stirring for 24 hours at 25 ℃ (pH=~2), the mixture was filtered and the ion concentrations in the filtrate were analyzed using ICP-OES and used to calculate the removal efficiency (Supplementary section 2). Characterization Powder XRD patterns were obtained using a Rigaku MiniFlex600 X-ray diffractometer with Cu Kα radiation. High-resolution TEM images of TiS 2 , TaS 2 nanosheets, and TiS 2 @[AuCl 4 ] - , TiS 2 @[PdCl 4 ] 2- , and TiS 2 @[PtCl 6 ] 2- were obtained using a double spherical aberration-corrected microscope (Spectra 300) or FEI Tecnai G2 F30 transmission electron microscope. The thickness of TMD nanosheets was measured by AFM (Bruker Dimension Icon). Raman spectroscopy measurements of TMD nanosheets were performed under ambient conditions with a Thermo Fisher DXR2xi Raman imaging microscope using a 532 nm laser. XPS spectra were obtained using a Thermo Fisher ESCALAB Xi+ equipped with Al Kα radiation. The work functions of TMD nanosheets were measured by UPS (Thermo Fisher Nexsa). The concentration of metal ions was analyzed using ICP-OES (SPECTRO ARCOS Ⅱ MV) and UV-visible spectrophotometer (JASCO V760). Declarations Contributions Y.S. and H.-M.C. conceived the idea and supervised the project. J.H.W. prepared the sample, performed the adsorption experiments, and analyzed the results with the help of H.J.L., F.L., K.Q.Z., F.L.C., and Y.B.G. M.F.H. performed theoretical simulation under the supervision of K.Y. All authors analyzed the data, J.H.W, Y.S., H.-M.C., M.F.H., and K.Y. wrote the manuscript with input from all authors. Acknowledgments This research was supported by the National Key Research and Development Program of China (2022YFA1205300), the National Natural Science Foundation of China (52472298), Guangdong Innovative and Entrepreneurial Research Team Program (2023ZT10L039), Peacock Team Project (KQTD20210811090112002), Natural Science Foundation of Guangdong Province, China (2024A1515012424), Shenzhen Science and Technology Program (JCYJ20240813112114020) and Shenzhen Geim Graphene Center. This work made use of the TEM facilities at the Institute of Materials Research, Tsinghua Shenzhen International Graduate School. References Wang, Z. et al. Take responsibility for electronic-waste disposal. Nature 536 , 23-25 (2016). Olivetti, E.A. & Cullen, J.M. Toward a sustainable materials system. Science 360 , 1396-1398 (2018). Chen, Y. et al. Precious metal recovery. Joule 5 , 3097-3115 (2021). Sun, Y. et al. Interface-mediated noble metal deposition on transition metal dichalcogenide nanostructures. Nature Chemistry 12 , 284-293 (2020). Raabe, D. The Materials Science behind Sustainable Metals and Alloys. Chemical Reviews 123 , 2436-2608 (2023). Cheng, K. et al. Maximizing noble metal utilization in solid catalysts by control of nanoparticle location. Science 377 , 204-208 (2022). Chen, Y. et al. Selective recovery of precious metals through photocatalysis. Nature Sustainability 4 , 618-626 (2021). Zhang, Q. et al. Metal/MXene composites via in situ reduction. Nature Synthesis https://doi.org/10.1038/s44160-024-00660-z (2024). Yang, K. et al. Graphene/chitosan nanoreactors for ultrafast and precise recovery and catalytic conversion of gold from electronic waste. Proceedings of the National Academy of Sciences 121 , e2414449121 (2024). Shi, J. et al. Porous Polypyrrolidines for Highly Efficient Recovery of Precious Metals through Reductive Adsorption Mechanism. Advanced Materials 36 , 2405731 (2024). Fu, K. et al. Utilizing cost-effective pyrocarbon for highly efficient gold retrieval from e-waste leachate. Nature Communications 15 , 6137 (2024). Li, X. et al. Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste. Nature Communications 14 , 263 (2023). Niu, B. et al. Physicochemical reactions in e-waste recycling Nature Reviews Chemistry 8 , 569-586 (2024). Fritz, B. et al. Environmental impact of high-value gold scrap recycling. The International Journal of Life Cycle Assessment 25 , 1930-1941 (2020). Zhang, S. et al. Supply and demand of platinum group metals and strategies for sustainable management. Renewable and Sustainable Energy Reviews 204 , 114821 (2024). Kanický, V. et al. Comparison of some analytical performance characteristics in inductively coupled plasma spectrometry of platinum group metals and gold. Talanta 48 , 859-866 (1999). Shin, S.S. et al. Efficient recovery and recycling/upcycling of precious metals using hydrazide-functionalized star-shaped polymers. Nature Communications 15 , 3889 (2024). Cao, J. et al. Tailoring the Asymmetric Structure of NH 2 -UiO-66 Metal-Organic Frameworks for Light-promoted Selective and Efficient Gold Extraction and Separation. Angewandte Chemie International Edition 62 , e202302202 (2023). Kim, S. et al. Silanol-rich ordered mesoporous silica modified thiol group for enhanced recovery performance of Au(III) in acidic leachate solution. Chemical Engineering Journal 351 , 1027-1037 (2018). Xia, J., Mahandra, H. & Ghahreman, A. Efficient Gold Recovery from Cyanide Solution Using Magnetic Activated Carbon. ACS Applied Materials & Interfaces 13 , 47642-47649 (2021). Xu, B. et al. Kinetic studies of gold leaching from a gold concentrate calcine by thiosulfate with cobalt-ammonia catalysis and gold recovery by resin adsorption from its pregnant solution. Separation and Purification Technology 213 , 368-377 (2019). Fan, R. et al. Selective adsorption and recovery of Au(III) from three kinds of acidic systems by persimmon residual based bio-sorbent: A method for gold recycling from e-wastes. Bioresource Technology 163 , 167-171 (2014). Wu, H. et al. High-efficiency gold recovery by additive-induced supramolecular polymerization of β-cyclodextrin. Nature Communications 14 , 1284 (2023). Li, F. et al. Highly efficient and selective extraction of gold by reduced graphene oxide. Nature Communications 13 , 4472 (2022). Xue, T. et al. A customized MOF-polymer composite for rapid gold extraction from water matrices. Science Advances 9 , eadg4923 (2023). Lin, S. et al. Effective adsorption of Pd(II), Pt(IV) and Au(II) by Zr(IV)-based metal–organic frameworks from strongly acidic solutions. Journal of Materials Chemistry A 5 , 13557-13564 (2017). Jung, Y. et al. Nitrogen-Doped Titanium Carbide (Ti 3 C 2 T x ) MXene Nanosheet Stack For Long-Term Stability and Efficacy in Au and Ag Recovery. Small 19 , 2305247 (2023). Wang, R. et al. Interfacial Coordination Bonding-Assisted Redox Mechanism-Driven Highly Selective Precious Metal Recovery on Covalent-Functionalized Ultrathin 1T-MoS 2 . ACS Applied Materials & Interfaces 15 , 9331-9340 (2023). Chen, Z. et al. A Supramolecular Strategy toward an Efficient and Selective Capture of Platinum(II) Complexes. Journal of the American Chemical Society 141 , 11204-11211 (2019). Liu, Y. et al. Super-Stable, Highly Efficient, and Recyclable Fibrous Metal–Organic Framework Membranes for Precious Metal Recovery from Strong Acidic Solutions. Small 15 , 1805242 (2019). Hong, Y. et al. Precious metal recovery from electronic waste by a porous porphyrin polymer. Proceedings of the National Academy of Sciences 117 , 16174-16180 (2020). Wang, R. et al. Interfacial Coordinational Bond Triggered Photoreduction Membrane for Continuous Light-Driven Precious Metals Recovery. Nano Letters 23 , 2219-2227 (2023). Feng, B. et al. Highly efficient and selective recovery of Au(III) from a complex system by molybdenum disulfide nanoflakes. Chemical Engineering Journal 350 , 692-702 (2018). Fang, C. et al. Bulk and surface electronic structure of 1T-TiS 2 and 1T-TiSe 2 . Physical Review B 56 , 4455-4463 (1997). Li, Z. et al. Beyond Conventional Charge Density Wave for Strongly Enhanced 2D Superconductivity in 1H-TaS 2 Superlattices. Advanced Materials 36 , 2312341 (2024). Huckaba, A.J. et al. Low-Cost TiS 2 as Hole-Transport Material for Perovskite Solar Cells. Small Methods 1 , 1700250 (2017). Stolovas, D. et al. Electrical Properties of LaS-TaS 2 Misfit Layered Compound Nanotubes. Israel Journal of Chemistry 62 , e202100072 (2022). Lorenz, T. et al. Charge Transfer Variability in Misfit Layer Compounds: Comparison of SnS-SnS 2 and LaS-TaS 2 . Israel Journal of Chemistry 57 , 553-559 (2017). Deng, F. et al. Regulating the Electrical and Mechanical Properties of TaS 2 Films via van der Waals and Electrostatic Interaction for High Performance Electromagnetic Interference Shielding. Nano-Micro Letters 15 , 106 (2023). Wang, S. et al. Organic covalent modification to improve thermoelectric properties of TaS 2 . Nature Communications 13 , 4401 (2022). Wu, J. et al. Acid-Assisted Exfoliation toward Metallic Sub-nanopore TaS 2 Monolayer with High Volumetric Capacitance. Journal of the American Chemical Society 140 , 493-498 (2018). Tang, J. et al. MXene derived TiS 2 nanosheets for high-rate and long-life sodium-ion capacitors. Energy Storage Materials 26 , 550-559 (2020). Kumar, A. et al. Photo-Rechargeable Li-Ion Batteries using TiS 2 Cathode. Small 19 , 2303319 (2023). Peng, Y. et al. Laser Assisted Solution Synthesis of High Performance Graphene Supported Electrocatalysts. Advanced Functional Materials 30 , 2001756 (2020). Qiu, J. et al. Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold. Angewandte Chemie International Edition 62 , e202300459 (2023). Li, F. et al. A portable and washable solar steam evaporator based on graphene and recycled gold for efficient point-of-use water purification. Science China Materials 67 , 3700-3709 (2024). Huang, M. et al. Switching the Oxygen Reduction Route on Pt Using S through d–p Hybridization. ACS Catalysis 14 , 2095-2106 (2024). Liu, M. et al. Modulating Skeletons of Covalent Organic Framework for High-Efficiency Gold Recovery. Angewandte Chemie International Edition 63 , e202317015 (2024). Ren, M. et al. Synergy between palladium single atoms and small nanoparticles co-anchored on carbon atom self-doped graphitic carbon nitride boosting photocatalytic H 2 generation. Applied Catalysis B: Environment and Energy 345 , 123680 (2024). Kim, J. et al. Enhanced Electrocatalytic Properties of Transition-Metal Dichalcogenides Sheets by Spontaneous Gold Nanoparticle Decoration. Journal of Physical Chemistry Letters 4 , 1227-1232 (2013). Huang, J. et al. Surface-State Assisted Carrier Recombination and Optical Nonlinearities in Bulk to 2D Nonlayered PtS. ACS Nano 13 , 13390-13402 (2019). Guo, M. et al. In Situ Built Nanoconfined TiO 2 Particles in Robust-Flexible MXene@rGO Conductive Framework Enabling High-Performance Hybrid Magnesium–Sulfur Batteries. Advanced Energy Materials 13 , 2300417 (2023). Chen, Q. et al. Highly reversible oxygen redox in layered compounds enabled by surface polyanions. Nature Communications 11 , 3411 (2020). Liao, H. et al. Unveiling Role of Sulfate Ion in Nickel-Iron (oxy)Hydroxide with Enhanced Oxygen-Evolving Performance. Advanced Functional Materials 31 , 2102772 (2021). Wang, W. et al. Tantalum pentoxide (Ta 2 O 5 and Ta 2 O 5-x )-based memristor for photonic in-memory computing application. Nano Energy 106 , 108072 (2023). Shi, J. et al. Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst. Nature Communications 8 , 958 (2017). Shang, H. et al. Scalable and selective gold recovery from end-of-life electronics. Nature Chemical Engineering 1 , 170-179 (2024). Chen, D. et al. In-situ growth of gold nanoparticles on electrospun flexible multilayered PVDF nanofibers for SERS sensing of molecules and bacteria. Nano Research 14 , 4885-4893 (2021). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformationWatermediatedRecyclingofGoldPalladiumandPlatinumUsingSemimetallicTiS2andTaS2Nanosheets.pdf supplementary information Cite Share Download PDF Status: Published Journal Publication published 19 Nov, 2025 Read the published version in National Science Review → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5882578","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":412341270,"identity":"2a833a01-755e-476e-b5b1-eb1b8a7e7024","order_by":0,"name":"yang Su","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABHUlEQVRIiWNgGAWjYBADOQhlAEZAwEZYizEQMzaQpCWxAayFgQgt8jMS2CR+7qhNn99/gP3hj4I7dtslkh8wfCg7zMA/uwGrFoMbCWySvWeO5244cICxmcfgWfLOGWkGjDPOHWaQuHMAuxYJoC28bcdyNzA2MDYzGBxOBhpiwMzbdhgkhdNhkn/bjqXLNzMwNv4Aa0n/wPwXjxYGoMOkedtqEhiOAb3PY3DYzuBGjgEzIx4tBmceNlvLth0w3HCGsXE2UEuCwZk3BQd7zqXzSNzA4bD25IM337bVycv3Hz7w8cefw/YGx9M3PvhRZi3HPwOHwxgYWyQYGA4zwKIFFEEMB4CYB4d6EGD+wMBQB+fZ41E5CkbBKBgFIxQAAI+nYrD7CtSKAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-7042-0524","institution":"Tsinghua University","correspondingAuthor":true,"prefix":"","firstName":"yang","middleName":"","lastName":"Su","suffix":""},{"id":412341271,"identity":"8b04a6d9-7142-41bc-a9f2-745d67d0e418","order_by":1,"name":"Jianhong Wei","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Jianhong","middleName":"","lastName":"Wei","suffix":""},{"id":412341272,"identity":"7f163237-6856-4bf5-bfb9-a6f15efc62d0","order_by":2,"name":"Miaofei Huang","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Miaofei","middleName":"","lastName":"Huang","suffix":""},{"id":412341273,"identity":"f8aa10c8-bf89-4012-a0c3-19a7f7a5ac3d","order_by":3,"name":"Huanjing Liang","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Huanjing","middleName":"","lastName":"Liang","suffix":""},{"id":412341274,"identity":"c9fb57e1-450a-4f65-a0cd-89d6d37694a0","order_by":4,"name":"Fei Li","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Fei","middleName":"","lastName":"Li","suffix":""},{"id":412341275,"identity":"2f2cae79-2abc-41d5-96e1-af419b0775a2","order_by":5,"name":"Kaiqiang Zheng","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Kaiqiang","middleName":"","lastName":"Zheng","suffix":""},{"id":412341276,"identity":"6a762cfd-8715-448f-ac64-63953e511cf2","order_by":6,"name":"Fangluo Chen","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Fangluo","middleName":"","lastName":"Chen","suffix":""},{"id":412341277,"identity":"1e54e51c-de66-4e5d-a492-0542920ef38c","order_by":7,"name":"Yibo Gao","email":"","orcid":"","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Yibo","middleName":"","lastName":"Gao","suffix":""},{"id":412341278,"identity":"5db08fa8-77b7-41c9-9709-965807578c01","order_by":8,"name":"Kuang Yu","email":"","orcid":"https://orcid.org/0000-0001-9142-5263","institution":"Institute of Materials Research, Tsinghua Shenzhen International Graduate School","correspondingAuthor":false,"prefix":"","firstName":"Kuang","middleName":"","lastName":"Yu","suffix":""},{"id":412341279,"identity":"847b64ff-1fff-44fc-a1c5-689743b35c5d","order_by":9,"name":"Hui-Ming Cheng","email":"","orcid":"https://orcid.org/0000-0002-5387-4241","institution":"Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Hui-Ming","middleName":"","lastName":"Cheng","suffix":""}],"badges":[],"createdAt":"2025-01-22 16:41:25","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5882578/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5882578/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1093/nsr/nwaf522","type":"published","date":"2025-11-20T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":76766713,"identity":"40a234bf-6251-49d4-bd7c-c031d21748e5","added_by":"auto","created_at":"2025-02-20 13:37:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":747747,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHighly efficient extraction of PM ions\u003c/strong\u003e. (a) Band alignment diagram for semiconducting SnS\u003csub\u003e2\u003c/sub\u003e, MoS\u003csub\u003e2\u003c/sub\u003e, semimetallic TiS\u003csub\u003e2\u003c/sub\u003e, and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets and [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e/Au\u003csup\u003e0\u003c/sup\u003e, [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e/[PtCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, [PtCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e/Pt\u003csup\u003e0\u003c/sup\u003e, and [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e/Pd\u003csup\u003e0\u003c/sup\u003e. Vertical bars span the bandgaps, the blue lines were the reduction potentials (for ions) and Fermi levels (for the TMD). (b) Schematic of the extraction process using semimetallic TMD nanosheets. After mixing with PM ions (100 ppm), the TMD suspension immediately changes its color from dark blue to clear. (c) Extraction capacity of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets for PM ions. Inset was an economic analysis of the estimated PM recovery using TiS\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/3c75debe6464ff8b8f084487.png"},{"id":76766711,"identity":"249d9c62-96ba-4f52-bbf3-8066274ead80","added_by":"auto","created_at":"2025-02-20 13:37:33","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":583629,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAdsorption performance of TiS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and TaS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e nanosheets.\u003c/strong\u003e (a) Extraction capacities of TiS\u003csub\u003e2\u003c/sub\u003e for Au, Pt, and Pd at the \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e of 0.1-100 ppm. (b) Removal efficiency of TiS\u003csub\u003e2\u003c/sub\u003e for Au, Pt, and Pd at \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e = 10 ppm. Inset was the removal efficiency of TiS\u003csub\u003e2\u003c/sub\u003e for Au, Pt, and Pd in solutions with multiple pH values. Comparison of (c) Au, (d) Pt, and Pd extraction performance by semimetallic TMD nanosheets with other adsorbents. The data in the shaded area were taken from previously reported adsorbents listed in the figure and Table S1.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/d298209bbd648a4b8fcdb2f7.png"},{"id":76766714,"identity":"deb7e9b2-30e5-4096-8da0-2d7330f79db4","added_by":"auto","created_at":"2025-02-20 13:37:33","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1980298,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure analysis of adsorbed PM ions and TiS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e nanosheets, and the adsorption selectivity. \u003c/strong\u003eTEM images of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets after adsorption of (a) [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, (b) [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and (c) [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e. (d) XRD patterns of TiS\u003csub\u003e2\u003c/sub\u003e after adsorption of PM ions (color-coded), the corresponding samples were denoted as TiS\u003csub\u003e2\u003c/sub\u003e@[AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, TiS\u003csub\u003e2\u003c/sub\u003e@[PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and TiS\u003csub\u003e2\u003c/sub\u003e@[PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e\u003csub\u003e.\u003c/sub\u003e (e) XPS spectra of Au4f, Pd3d, and Pt4f of TiS\u003csub\u003e2\u003c/sub\u003e@[AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e- \u003c/sup\u003e(left panel), TiS\u003csub\u003e2\u003c/sub\u003e@[PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2- \u003c/sup\u003e(middle panel), and TiS\u003csub\u003e2\u003c/sub\u003e@[PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2- \u003c/sup\u003e(right panel). (f) Extraction capacity of TiS\u003csub\u003e2\u003c/sub\u003e for PM ions with various reduction potentials. (g) Removal efficiencies of TiS\u003csub\u003e2\u003c/sub\u003e for 15 types of interfering ions and\u003csub\u003e \u003c/sub\u003e4 types of PM ions (color-coded). Inset: redox potentials of the used metal ions.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/41de9e6bc670608ef7906265.png"},{"id":76766710,"identity":"730df119-54e7-451e-a7aa-774084788358","added_by":"auto","created_at":"2025-02-20 13:37:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":529109,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eUnderstanding the role of H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO in the PM adsorption by TiS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e (a) Deconvoluted XPS S2p spectra of dissolved compounds collected after PM adsorption. The red line represents TiS\u003csub\u003e2\u003c/sub\u003e@[AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e. Inset was the XPS S2p spectra of TiS\u003csub\u003e2\u003c/sub\u003e@[PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2- \u003c/sup\u003e(blue line) and TiS\u003csub\u003e2\u003c/sub\u003e@[PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2- \u003c/sup\u003e(black line). (b) Extraction capacity as a function of time for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e solutions using water, acetone, and 30 vol%-water-70 vol%-acetone mixture as the solvent (color coded). Inset summarized \u003cem\u003eQ\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e \u003c/em\u003evalues for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e measured using water, acetone, and 30 vol%-water-70 vol%-acetone mixture as the solvent. (c) Calculated energy diagram of the early dechlorination reduction of TiS\u003csub\u003e2\u003c/sub\u003e and Au species in an aqueous solution.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/dc7b354251acd0d2fedfbb7e.png"},{"id":76767656,"identity":"86573a13-1336-4cc0-821d-46ef1167ec2c","added_by":"auto","created_at":"2025-02-20 13:45:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":423597,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePM Recovery from their corresponding wastes. \u003c/strong\u003eConcentration changes of metal ions in (a) the CPU board leaching solution, (b) the Pt/C catalyst leachate, and (c) the ACCs leachate before and after recovery by the TiS\u003csub\u003e2 \u003c/sub\u003enanosheets. (d) Schematic of the two-step PM extraction process from the ACCs. (e) Corresponding removal efficiency of Ps and co-existing metal ions by TiS\u003csub\u003e2 \u003c/sub\u003enanosheets.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/cf32090008d4597aa6659d6f.png"},{"id":96760972,"identity":"092e140b-e6e7-43a3-82d3-a9fe15f41d41","added_by":"auto","created_at":"2025-11-25 19:09:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6034942,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/2c27ecfa-1b3f-4c38-b37e-5372f28ad958.pdf"},{"id":76766716,"identity":"927cb173-e2b7-4bed-94ea-7e442db0ca7c","added_by":"auto","created_at":"2025-02-20 13:37:34","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2484596,"visible":true,"origin":"","legend":"supplementary information","description":"","filename":"SupplementaryInformationWatermediatedRecyclingofGoldPalladiumandPlatinumUsingSemimetallicTiS2andTaS2Nanosheets.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5882578/v1/8a3605e2b1c88dea16f41bd8.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eWater-mediated Recycling of Gold, Palladium, and Platinum Using Semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e Nanosheets\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePrecious metals (PMs) including gold (Au), platinum (Pt), palladium (Pd), and others are the cornerstones of many industrial processes and devices, ranging from catalysis and renewable energy devices to modern electronic devices\u003csup\u003e1-6\u003c/sup\u003e. For instance, Au is essential in electronics, while Pt and Pd are crucial for industrial catalysis\u003csup\u003e7-12\u003c/sup\u003e. However, the high annual consumptions of ~4600 tons of Au, ~220 tons of Pt, and ~300 tons of Pd\u003csup\u003e13-15\u003c/sup\u003e, their low concentrations in the earth\u0026apos;s crust (~4, 13, and 5 mg/ton)\u003csup\u003e16\u003c/sup\u003e, and their low recycling rates \u0026lt;20% present sustainability challenges\u003csup\u003e14, 15\u003c/sup\u003e. Recycling PMs from wastes offers a sustainable alternative to ore mining as it reduces energy consumption, resource extraction, and the environmental impact caused by landfilling of such wastes\u003csup\u003e9, 17\u003c/sup\u003e. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHydrometallurgy is frequently used for PM recycling, and involves leaching to dissolve the PMs and co-existing elements, followed by selective adsorption and reduction of the PM ions\u003csup\u003e13\u003c/sup\u003e. Selective adsorption is a critical step and relies on the efficiency of the adsorbents\u003csup\u003e12, 18\u003c/sup\u003e. Traditional adsorbents have limitations on unsatisfactory selectivity and insufficient extraction capacity (\u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e)\u003csup\u003e19-22\u003c/sup\u003e. Recent advances show that by incorporating redox sites in porous materials\u003csup\u003e12, 23, 24\u003c/sup\u003e, the resulting adsorbents have a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of 300-4000 mg/g (at an Au concentration of \u0026lt; 200 ppm) with simultaneous reduction of the ionic gold to the element\u003csup\u003e11, 17, 25-28\u003c/sup\u003e. While gold adsorbents have been extensively studied, adsorbents for other PMs, for example, Pt, is largely unexplored despite their high economic value and critical application in the catalysis industry, for example, existing adsorbents for Pt rely on either weak intermolecular forces\u003csup\u003e29, 30\u003c/sup\u003e or use gold-specific adsorbents\u003csup\u003e10, 17, 31, 32\u003c/sup\u003e, and suffer from an unsatisfactory adsorption performance for Pt ions because of its lower reduction potential than Au.\u003c/p\u003e\n\u003cp\u003eRecently emerging 2D materials, with their large surface area and important surface chemistry, present new opportunities for efficient PM recycling\u003csup\u003e24, 27, 30\u003c/sup\u003e. Our previous study has shown that reduced graphene oxide (rGO) is highly efficient for Au recycling, but is ineffective for [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e due to its inability to reductively adsorb Pt ion\u003csup\u003e24\u003c/sup\u003e. Though other 2D materials are reported to reductively adsorb Pt ion, they also exhibit low \u003cem\u003eQ\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e even at a high concentration due to their weak electron donation capability\u003csup\u003e28, 3\u003c/sup\u003e\u003csup\u003e1-\u003c/sup\u003e\u003csup\u003e33\u003c/sup\u003e, and suffer from either a complicated fabrication process and/or a high cost, undermining their economic viability for practical recycling PMs.\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eMaterials\u003csup\u003e\u0026nbsp;\u003c/sup\u003ethat enable efficient electron donation to PM ions should be promising adsorbents for PM recycling. We then become interested in 2D transition metal dichalcogenides (TMDs)\u003cstrong\u003e, in particular, semimetallic TMDs, such as TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e, they have narrow bandgaps (~0.2 eV and ~0.1 eV), and high\u0026nbsp;\u003c/strong\u003eFermi levels of ~-4.13 eV and ~-4.49 eV\u003csup\u003e34-38\u003c/sup\u003e, respectively, well above the equivalent energy level for the reduction potentials of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003eions\u003cstrong\u003e, may offer more efficient electron donation and superior PM adsorption than other TMDs.\u0026nbsp;\u003c/strong\u003eHere, we report ultrahigh and selective PM extraction by semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets,\u0026nbsp;with TiS\u003csub\u003e2\u003c/sub\u003e having \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values of 8073 mg/g, 2359 mg/g, and 1154 mg/g for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, respectively, making them ideal candidates for nearly complete recycling of PMs from their electronic waste (e-waste), spent fuel cell catalysts, and automotive catalytic converters (ACCs). In addition, our work highlights the efficient extraction of PM ions accompanied by the simultaneous formation of PM/TMD heterostructures, providing a synthetic pathway for advanced catalysis and contact engineering of TMD electronic devices, and opening an avenue to address PM sustainability.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdsorption performance of semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe 1T-TiS\u003csub\u003e2\u003c/sub\u003e and 2H-TaS\u003csub\u003e2\u003c/sub\u003e nanosheets were exfoliated by sonication of their thick flakes in a LiOH aqueous solution\u003csup\u003e39\u003c/sup\u003e, and Raman analysis showed that the exfoliated nanosheets had no phase change during the process (Fig. S1 and supplementary section 1)\u003csup\u003e40\u003c/sup\u003e. Transmission electron microscope (TEM) and atomic force microscope (AFM) analysis showed that both types of nanosheets had a lateral size of ~ 2 \u0026mu;m and a thickness of 0.9 - 2.5 nm (Fig. S2 and Fig. S3), suggesting a mono- and few-layer structure and a large specific surface area\u003csup\u003e41\u003c/sup\u003e. X-ray diffraction (XRD) and high-resolution TEM (HR-TEM) showed high crystallinity (Fig. S2 and S4).\u003c/p\u003e\n\u003cp\u003eThe Fermi energy levels of TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eallowed its electron transfer to the [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e ions (Fig. 1a), promising their strong adsorption for PM ions. An initial adsorption test showed that both TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets had an immediate color change when mixed with [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e solutions (initial concentration \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e = 100 ppm),\u003csup\u003e\u0026nbsp;\u003c/sup\u003erespectively, suggesting their strong interaction with and rapid adsorption of the PM ions (Fig. 1b). Taking 1T-TiS\u003csub\u003e2\u003c/sub\u003e as an example, we measured the \u003cem\u003eQ\u003csub\u003ee\u0026nbsp;\u003c/sub\u003e\u003c/em\u003evalues of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e (\u003cem\u003eC\u003csub\u003e0\u0026nbsp;\u003c/sub\u003e\u003c/em\u003e~100 ppm) were 8073, 2359, and 1154\u0026nbsp;mg/g, respectively (Fig. 1c). Given that TiS\u003csub\u003e2\u003c/sub\u003e was the lightest and also possibly the cheapest TMD (~14 RMB/g)\u003csup\u003e42, 43\u003c/sup\u003e, the measured ultrahigh\u003cem\u003e\u0026nbsp;Q\u003csub\u003ee\u0026nbsp;\u003c/sub\u003e\u003c/em\u003evalues indicated its strong economic viability for PM recycling (Fig. 1c inset and supplementary section 2). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe next quantified the adsorption behavior of the TMD for PM ions. The \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values for PM ions with \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e varying from 0.1 to 100 ppm (Fig. 2a) were studied. Notably, TiS\u003csub\u003e2\u003c/sub\u003e had a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of 6620 mg/g to 0.1 ppm [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, then became saturated at ~8000 mg/g when the \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e increased to 1 ppm. Note that such a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e at a low concentration of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e was significantly higher than previously reported gold adsorbent as summarized in Fig. 2c, promising its application for gold recycling at minute concentrations. For [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e and\u0026nbsp;[PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, the \u003cem\u003eQ\u003csub\u003ee\u0026nbsp;\u003c/sub\u003e\u003c/em\u003estarted to saturate when \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e was 10 ppm or higher, specifically, the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e was 1892, 2217, and 2359 mg/g and that for [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e was ~323, 813 and 1154 mg/g when \u003cem\u003eC\u003csub\u003e0\u0026nbsp;\u003c/sub\u003e\u003c/em\u003e= 1, 10 and 100 ppm respectively. The adsorption kinetics (Fig. 2b) showed that TiS\u003csub\u003e2\u003c/sub\u003e extracts \u0026gt;99% of the Au and Pd within 10 minutes (\u003cem\u003eC\u003csub\u003e0\u0026nbsp;\u003c/sub\u003e\u003c/em\u003e= 10 ppm), significantly faster than the value for rGO we reported previously\u003csup\u003e24\u003c/sup\u003e. In contrast, the adsorption of [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e was slow, with only 3.2% of [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e extracted in 10 min, and 68% and \u0026gt;95% removal efficiencies were achieved for 15 and 24 h adsorption. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe also investigated the influence of pH on \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u0026nbsp;\u003c/em\u003eand removal efficiency. According to the Pourbaix diagram which describes ion stability at different pH values, the pH ranges for stable [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e were ~1-7, and that for [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003ewas\u003csup\u003e\u0026nbsp;\u003c/sup\u003epH\u0026lt;5\u003csup\u003e32, 44\u003c/sup\u003e. We therefore studied the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e value in these pH ranges (Fig. S5). For [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e (\u003cem\u003eC\u003csub\u003e0\u0026nbsp;\u003c/sub\u003e\u003c/em\u003e= 10 ppm), TiS\u003csub\u003e2\u003c/sub\u003e had the highest \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e (~8076 mg/g) at pH=3-5, and decreasing or increasing the pH results in a lower \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e, which probably was due to a trade-off between the surface charge of TiS\u003csub\u003e2\u0026nbsp;\u003c/sub\u003enanosheets and stability of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e at different pH values (Supplementary Section 2). For [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e increases with pH, while for [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, it remained relatively stable in the tested pH range (Supplementary Section 2)\u003csup\u003e10, 24\u003c/sup\u003e. Furthermore, TiS\u003csub\u003e2\u003c/sub\u003e showed a ~100% removal efficiency for the three types of PM ions (Fig. 2b inset). Not only do we found an ultrahigh \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for TiS\u003csub\u003e2\u003c/sub\u003e, we also found that the TaS\u003csub\u003e2\u003c/sub\u003e nanosheets were highly efficient for the extraction of the three PM ions, their \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e were 4100, 1100 and 452 mg/g (Fig. 2d and Table S1), slightly lower than that of TiS\u003csub\u003e2\u003c/sub\u003e. Nevertheless, as summarized in Fig. 2 c-d and Table S1, both TMDs had significantly higher \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values than the previously reported PM adsorbents, for example, MXene, graphene, MOFs, COFs, and pyridine-modified disulfide\u003csup\u003e24, 25, 27, 28, 45\u003c/sup\u003e, especially in the sub-ppm to tens of ppm range relevant to the practical recycling PM-containing waste streams.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdsorption\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003emechanism\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo understand the efficient\u0026nbsp;adsorption of PM by semimetallic TMD, we used TEM (Figs. 3a-c, Fig. S6), XRD (Fig. 3d), and X-ray photoelectron spectroscopy (XPS, Fig. 3e) to study TiS\u003csub\u003e2\u003c/sub\u003e@PM precipitates after adsorption. The TEM observation revealed the deposition of a high density of nanoparticles on the TiS\u003csub\u003e2\u003c/sub\u003e nanosheets (particle size 3-18 nm). For [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e and [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, nanoparticles had lattice spacings consistent with the (111) plane of metallic Au and Pd\u003csup\u003e8, 25, 46\u003c/sup\u003e, whereas, the nanoparticles deposited after adsorption of [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e had a lattice spacing corresponding to the (110) plane of PtS (Fig. 3c and Fig. S7)\u003csup\u003e47\u003c/sup\u003e. The XRD patterns showed prominent peaks corresponding to elemental Au and Pd, and to PtS\u003csup\u003e47\u003c/sup\u003e, consistent with the TEM observations (Fig. 3d). The deconvoluted XPS peaks of PM deposited on TiS\u003csub\u003e2\u003c/sub\u003e quantitatively showed that \u0026gt;99%, ~92%, and ~ 82% of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e,\u003csup\u003e\u0026nbsp;\u003c/sup\u003eand [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e were reduced to Au(0), Pd(0), and Pt(II), respectively, with the rest of the adsorbed PM being the Pd(II) and Pt(IV) (Fig. 3e and Fig. S8)\u003csup\u003e47-49\u003c/sup\u003e, Collectively, these results suggested the dominant adsorption mechanism of PM by TiS\u003csub\u003e2\u003c/sub\u003e was reductive adsorption, and supported that the high Fermi level of semimetallic TMDs enables their efficient electron donation to PM ions\u003csup\u003e4, 50\u003c/sup\u003e. We also noted that the reductive adsorption of [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003eyields PtS, which could be understood by the fact that the reduction of [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003eto Pt\u003csup\u003e0\u003c/sup\u003e was a two-step process with Pt\u003csup\u003e2+\u003c/sup\u003e being an intermediate product, and its subsequent bonding to a sulfur site of TiS\u003csub\u003e2\u003c/sub\u003e inhibited its further reduction to Pt\u003csup\u003e0\u003c/sup\u003e because of the good chemical stability of PtS\u003csup\u003e51\u003c/sup\u003e. Nevertheless, because PtS was a primary ore for mining Pt, this suggested the PtS could easily be purified using established methods\u003csup\u003e51\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFurther insight regarding the reductive adsorption of PM ions by TiS\u003csub\u003e2\u003c/sub\u003e was gained by measuring its \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values of a series of PM ions with decreasing reduction potentials. As plotted in Fig. 3f, TiS\u003csub\u003e2\u003c/sub\u003e has a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e greater than 1100 mg/g for ions with reduction potentials \u0026gt;0.59 V (versus a standard hydrogen electrode\u003csub\u003e\u0026nbsp;\u003c/sub\u003e(SHE)), including [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [AuBr\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, AgNO\u003csub\u003e3\u003c/sub\u003e, [AuI\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, but a small \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e (\u0026lt;200 mg/g) for ions with reduction potentials \u0026lt;0.59 V, for example, [RhCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e and [Au(S\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e, confirming the redox reaction-driven reductive adsorption of PMs (Fig. S9). It was worth noting that the high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e observed for a wide range of gold ions expanded the potential use of TiS\u003csub\u003e2\u003c/sub\u003e beyond the commonly seen [AuCl\u003csub\u003e4\u003c/sub\u003e]⁻ ion. For example, [AuI\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, a product of the standard wet etching of a gold coating in the electronics industry, could be effectively recycled from wastewater using TiS\u003csub\u003e2\u003c/sub\u003e, as evidenced by a high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of 3458 mg/g.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe redox-dominated adsorption also provided a way to selectively adsorb PM ions based on their reduction potentials. To validate this, we measured the PM removal efficiency of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets in a simulated solution containing 15 interfering ions (~100 ppm each of Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, Cd\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Zn\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Al\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, Ce\u003csup\u003e3+\u003c/sup\u003e, La\u003csup\u003e3+\u003c/sup\u003e, and Cr\u003csup\u003e3+\u003c/sup\u003e) and 4 types of PM (~10 ppm each of Au\u003csup\u003e3+\u003c/sup\u003e, Pt\u003csup\u003e4+\u003c/sup\u003e, Pd\u003csup\u003e2+\u003c/sup\u003e, and Rh\u003csup\u003e3+\u003c/sup\u003e). The nanosheets showed a removal efficiency of \u0026gt;97% for Au\u003csup\u003e3+\u003c/sup\u003e, Pt\u003csup\u003e4+\u003c/sup\u003e, and Pd\u003csup\u003e2+\u003c/sup\u003e, while a low removal efficiency of \u0026lt;8% for the remaining ions including Rh\u003csup\u003e3+\u003c/sup\u003e (Fig. 3g), promising a good PM selectivity for real-world PM recycling involving complex ion mixtures. In addition, the work function of TiS\u003csub\u003e2\u003c/sub\u003e was -3.89 eV as measured by ultraviolet photoelectron spectroscopy (Fig. S10), consistent with the reported value\u003csup\u003e36\u003c/sup\u003e, whereas, the reduction potentials (E\u003csub\u003eRed\u0026nbsp;\u003c/sub\u003e(\u003cem\u003evs\u0026nbsp;\u003c/em\u003eSHE), Fig. S11) of [PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e and [RhCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e were +0.59 V and +0.43 V respectively, which translated into E\u003csub\u003eRed\u003c/sub\u003e values of -5.03 and -4.87 eV (E\u003csub\u003eRed\u0026nbsp;\u003c/sub\u003e= - E\u003csub\u003eRed\u0026nbsp;\u003c/sub\u003e(\u003cem\u003evs\u0026nbsp;\u003c/em\u003eSHE) - 4.44), respectively. Note that TiS\u003csub\u003e2\u003c/sub\u003e had a high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for\u0026nbsp;[PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e but was incapable of adsorbing [RhCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e, which indicated that the energy barrier required for efficient adsorption by TiS\u003csub\u003e2\u003c/sub\u003e should be ~1-1.14 eV. This was in good agreement with previous results\u003csup\u003e4, 28\u003c/sup\u003e, and was explained by the energy needed for ion diffusion, ion desolvation, and, in our case, electron donation and crystal nucleation and growth.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe next investigated why TiS\u003csub\u003e2\u003c/sub\u003e had such a high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e, using gold as an example. We first compared the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values achieved by using thick unexfoliated TiS\u003csub\u003e2\u003c/sub\u003e flakes and the exfoliated ones. For a \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e = 100 ppm, unexfoliated TiS\u003csub\u003e2\u003c/sub\u003e had a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for gold of 476.8 mg/g (Fig. S12), nearly ten times lower than that of the exfoliated nanosheets. Similar results were also found for exfoliated and unexfoliated TaS\u003csub\u003e2\u003c/sub\u003e (Fig. S12). Because our HR-TEM and Raman analysis showed the TiS\u003csub\u003e2\u003c/sub\u003e nanosheets had a highly crystalline structure, such a large difference in \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e could not be caused by defects, but was due to an increased specific surface area which provided abundant adsorption sites for achieving a high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e24, 25\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurthermore, a \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e value of 8073 mg/g for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand the complete reductive adsorption of gold ions by TiS\u003csub\u003e2\u003c/sub\u003e suggested that each TiS\u003csub\u003e2\u003c/sub\u003e molecule donates \u0026gt;13 electrons during the adsorption process (Supplementary Section 3), we therefore studied the active site that donates such a large number of electrons by analyzing the structure change of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets after adsorption. XPS analysis (Fig. S13) showed that the binding energies of Ti and S increased compared to pristine TiS\u003csub\u003e2\u003c/sub\u003e. The XPS spectra of Ti2p after adsorption were deconvoluted into two peaks (~ 464.4 and 458.7 eV), which do not fit with the pristine Ti-S bond, but align with the typical Ti-O bond (Fig. S13)\u003csup\u003e52\u003c/sup\u003e. For sulfur, we measured the dissolved salt after adsorption and found multiple binding energies at 168.6 eV and 169.8 eV were assigned to the sulfate species (Fig. 4a)\u003csup\u003e53, 54\u003c/sup\u003e. This indicated each TiS\u003csub\u003e2\u003c/sub\u003e molecule could, in principle, donate no more than 16 electrons to the adsorbed [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, in quantitative agreement with the observed high \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e. Note that the XPS analysis also showed a similar transformation of the sulfur in TaS\u003csub\u003e2\u003c/sub\u003e after [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003eadsorption (Fig. S14), with the difference that a valency change of tantalum (Ta\u003csup\u003e4+\u003c/sup\u003e\u0026rarr;Ta\u003csup\u003e5+\u003c/sup\u003e) was also\u0026nbsp;observed\u003csup\u003e55, 56\u003c/sup\u003e. This suggested that one TaS\u003csub\u003e2\u003c/sub\u003e molecule can donate no more than 17 electrons to [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, however, the observed slightly lower \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for the adsorption of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e by TaS\u003csub\u003e2\u003c/sub\u003e must be attributed to the fact that\u0026nbsp;its higher weight fraction than TiS\u003csub\u003e2\u003c/sub\u003e reduces the weight-based PM extraction capacity. Collectively, these results provided unambiguous evidence that the sulfur of the TMD was the dominant site for extremely high electron donation which was responsible for the ultrahigh\u003cem\u003e\u0026nbsp;Q\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e values.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe formation of sulfate species after adsorption suggested that oxygen was needed for electron donation, but was absent in the PM salt used, also the good crystal structure of the TMD nanosheets (Fig. S2) did not explain the presence of such a large amount of oxygen after the reductive adsorption. It was therefore reasonable to suggest that the oxygen comes from the dissociation of the water solvent used in the adsorption. To testify this, we first considered that dissociation of water would lead to a release of proton in the solvent, indeed, the pH of the gold solution decreased from 3.96 to 3.77 after gold extraction (Fig. S15). We then replaced the water with the non-protonic solvent acetone and found that the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets with an acetone solution of 100 ppm [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e was only ~1180 mg/g (Fig. S16 and Fig. 4b), ~7 times lower than the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e value measured in water. In addition, adding 30 vol% water to acetone resulted in an increased \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of 5217 mg/g. Because all the experiments were performed in the same ambient environment, the change in \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e produced by a change of solvent indicated that it was oxygen from the solvent rather than from the environment that dictates the PM adsorption.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo understand such an interesting role of water in the extraction process, we performed theoretical calculations using spin-polarized density functional theory (DFT) and TiS\u003csub\u003e2\u003c/sub\u003e crystal cells to evaluate the adsorption energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e) of the [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eion. Fig. 4c showed the energy diagram of the dechlorination reduction process of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e by TiS\u003csub\u003e2\u003c/sub\u003e in an aqueous solution (Supplementary Section 3). Firstly, the calculation indicated the favorable adsorption of [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e ion onto TiS\u003csub\u003e2\u003c/sub\u003e (forming the \u0026ldquo;TiS\u003csub\u003e2\u003c/sub\u003e*AuCl\u003csub\u003e3\u003c/sub\u003e\u0026rdquo; species) with an \u003cem\u003eE\u003csub\u003ead\u003c/sub\u003e\u003c/em\u003e of -0.88 eV. This adsorption was accompanied by a dichlorination step, which made the gold atom directly bond to a sulfur atom of TiS\u003csub\u003e2\u003c/sub\u003e, forming an electron donation site from sulfur to gold. Subsequently, a water molecule was adsorbed on the TiS\u003csub\u003e2\u003c/sub\u003e*AuCl\u003csub\u003e3\u003c/sub\u003e intermediate, with the oxygen atom of water substituting one Cl\u003csup\u003e-\u003c/sup\u003e, followed by the dissociation of H\u003csub\u003e2\u003c/sub\u003eO, generating a TiS\u003csub\u003e2\u003c/sub\u003e*AuCl\u003csub\u003e2\u003c/sub\u003eOH intermediate in a thermodynamically favorable process with an \u003cem\u003eE\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e of -0.76 eV, and forming a HCl byproduct, which explained the observed pH decrease after the gold extraction. Next, another water molecule approached the TiS\u003csub\u003e2\u003c/sub\u003e*AuCl\u003csub\u003e2\u003c/sub\u003eOH intermediate, initiating a second dechlorination reaction that resulted in the formation of the TiS\u003csub\u003e2\u003c/sub\u003e*AuClOH intermediate, which involved the dissociation of the second water molecule. The overall process led to the reduction of Au from Au (III) to Au (II) by electron donation from the sulfur site and the formation of an S-O bond, which may eventually lead to the formation of sulfate species, in good agreement with the XPS results. The mechanism revealed by DFT showed that the S atom in TiS\u003csub\u003e2\u003c/sub\u003e was the electron donor, while the entire process relied on the dissociation of the water molecule which forms the S-O bond and releases HCl.\u003c/p\u003e\n\u003cp\u003eTaking these results together, we propose that water-mediated electron donation from the sulfur site of semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets is primarily responsible for the observed ultrahigh adsorption and deposition of PMs. The sulfur atoms of TiS\u003csub\u003e2\u003c/sub\u003e/TaS\u003csub\u003e2\u003c/sub\u003e are mostly converted to positive hexavalent sulfur (Fig. 4a), such sulfur oxidation is enabled by the disassociation of water which provides oxygen to form sulfate species. In addition, the large surface area of the nanosheets allows efficient and rapid adsorption. Thermodynamically, the electronic structures of TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets (a narrow bandgap of 0.1-0.2 eV, and a Fermi level that is at least ~1 eV higher than the reduction potential of PM ions), allow the electron donation from the nanosheets to the PM ions (Fig. 3e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePM recovery\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;from e-waste and catalyst scrap\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe observed ultrahigh \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e for PM ions and good selectivity indicated that TiS\u003csub\u003e2\u003c/sub\u003e can be used to recycle PMs from their corresponding waste. To demonstrate this, we recycled Au, Pd, and Pt from their corresponding wastes including e-waste, a Pd/C catalyst for chemical manufacturing, a Pt/C catalyst for fuel cells, and scrap ACCs. For the e-waste (Fig. 5a), TiS\u003csub\u003e2\u003c/sub\u003e recovered \u0026gt;99% gold from the leachate of computer central processing unit (CPU) boards, and showed negligible adsorption of the co-existing ions, including Cu\u003csup\u003e2+\u003c/sup\u003e and Ni\u003csup\u003e2+\u003c/sup\u003e ions (Fig. 5a). For the leachate of the Pd/C catalyst, TiS\u003csub\u003e2\u003c/sub\u003e recovered 99% Pd (Supplementary Section 4). For the Pt/C catalyst from scrap fuel cells (Fig. 5b), Co or Ni was frequently used as a co-catalyst, the TiS\u003csub\u003e2\u003c/sub\u003e can directly recover 95.5% Pt from the leachate of the scrap catalyst while absorbing only 6.9% and 5.4% of Co\u003csup\u003e2+\u003c/sup\u003e and Ni\u003csup\u003e2+\u003c/sup\u003e respectively.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;For the spent ACCs, it had a complex composition containing Pd, Pt, and Rh catalysts typically supported on ceramic substrates (e.g., MgO\u003csub\u003e2\u003c/sub\u003e, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, SiO\u003csub\u003e2\u003c/sub\u003e). In our case, after acid digestion of the ACCs, the amounts of Pd\u003csup\u003e2+\u003c/sup\u003e, Pt\u003csup\u003e4+\u003c/sup\u003e, and Rh\u003csup\u003e3+\u003c/sup\u003e were 36.3, 18.5, and 18.1 ppm, respectively, while those of the co-existing Al\u003csup\u003e3+\u003c/sup\u003e, La\u003csup\u003e3+\u003c/sup\u003e, Ce\u003csup\u003e3+\u003c/sup\u003e, and Mg\u003csup\u003e2+\u003c/sup\u003e ions were respectively ~3475, 1222, 501, and 88.5 ppm (Fig. 5c). Different from the three types of PM-containing wastes mentioned previously, Pd and Pt co-exist in the ACCs, and require selective adsorption to separate them. Because TiS\u003csub\u003e2\u003c/sub\u003e had much faster adsorption kinetics of Pd than Pt (Fig. 2b), we believed that these two PMs could be separated based on their distinct adsorption kinetics. As shown in Fig. 5d, a two-step extraction process for their recovery and separation was designed. We added TiS\u003csub\u003e2\u003c/sub\u003e nanosheets to the leachate to adsorb Pd\u003csup\u003e2+\u003c/sup\u003e for 10 min, and then removed them, resulting in the recovery of 99% of Pd but only 8% of Pt. We then added a second batch of TiS\u003csub\u003e2\u003c/sub\u003e to extract the Pt for 36 h, and 97% Pt was recovered with no detectable Pd observed in TiS\u003csub\u003e2\u003c/sub\u003e. This efficient recovery of the Pd and Pt allowed further extraction of the Rh. We used iron powder to initiate a replacement reaction which recovered 98% of the Rh, leaving the rest of the co-existing ions remaining in the leachate (Fig. 5e). These steps allowed the selective recovery of ~90% to ~100% Pd, Pt, and Rh.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFollowing the previous report\u003csup\u003e57\u003c/sup\u003e, the TiS\u003csub\u003e2\u003c/sub\u003e with adsorbed\u003csub\u003e\u0026nbsp;\u003c/sub\u003ePMs\u003csub\u003e\u0026nbsp;\u003c/sub\u003e(including recycled from scrap CPUs, Pd/C catalysts, Pt/C catalysts, and ACCs) was dissolved in aqua regia and chemically reduced to separate the PM from the insoluble Ti-containing precipitates (Fig. S17).\u0026nbsp;Energy dispersive spectroscopy (EDS) results showed that the purities of the recycled Au, Pt, and Pd from their corresponding scrap CPUs, Pt/C catalysts, and Pd/C catalysts all were \u0026gt; 97 wt% (Fig. S17). For the recycled Pd and Pt from ACCs, the Pd purity of the product after the first purification step was ~88 wt%, with ~11 wt% of Pt, and the Pt purity of the product after the second-step was~98 wt%, respectively (Fig. S17). Finally, the insoluble Ti-containing precipitates were shown by EDS to contain mostly Ti (61 wt%) and O (33 wt%) (Fig. S17), Such a high abundance of Ti indicated that it could be used for the synthesis of other Ti-based compounds, for example, TiS\u003csub\u003e2\u003c/sub\u003e, suggesting a closed-loop regeneration process. \u0026nbsp;\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe have discovered that semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets are highly efficient for the adsorption of PM ions including Au, Pd, and Pt. Benefiting from a near zero bandgap and an appropriate Fermi energy level, these nanosheets donate ~13 electrons/molecule to the PM ions primarily from their sulfur sites, resulting in an ultrahigh \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e and an excellent adsorption selectivity. We have also demonstrated the use of TiS\u003csub\u003e2\u003c/sub\u003e nanosheets for the recovery of PMs from their wastes, including e-waste, scrap catalysts, and automotive catalytic convertors, promising their use for PM recycling. Our study has revealed a complicated electron donation behavior from the adsorbent to the ions, which is a result of the interplay between adsorbate, adsorbent, and solvent, which provides insight on designing novel adsorbents. Furthermore, the observed aqueous-phase PM deposition on the TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets provides a new strategy and insight for the surface functionalization of TMD nanosheets which are interesting for the interfacial engineering of TMD-based electronic devices and catalysts. Given the irreplaceable role of PMs in modern industry, our finding opens a way to use 2D materials to address global PM sustainability.\u0026nbsp;\u003c/p\u003e\n"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eExtraction capacity of TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets for PM ions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA KAuCl\u003csub\u003e4\u003c/sub\u003e aqueous solution was mixed with a TiS\u003csub\u003e2\u003c/sub\u003e or TaS\u003csub\u003e2\u003c/sub\u003e suspension to form mixtures with initial gold concentrations of 0.1, 1, 10, 50, and 100 ppm. The pH values of the solutions were adjusted by 0.1 M HCl or NaOH solutions. The weight ratio between the Au ions and the TiS\u003csub\u003e2\u003c/sub\u003e/TaS\u003csub\u003e2\u003c/sub\u003e was 10:1, and these mixtures were stirred for 24 hours at 25\u0026deg;C to determine the extraction capacity of the nanosheets. After extraction, the mixtures were filtered through a syringe filter with a 0.22 \u0026micro;m pore size polyethersulfone (PES) membrane, and then the ion concentrations were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Finally, the extraction capacity (\u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e, mg/g) was calculated using the following equation, \u003cem\u003eQ\u003csub\u003ee\u0026nbsp;\u003c/sub\u003e=\u0026nbsp;\u003c/em\u003e(\u003cem\u003eC\u003csub\u003e0\u0026nbsp;\u003c/sub\u003e-\u0026nbsp;\u003c/em\u003e\u003cem\u003eC\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e)\u003cem\u003e\u0026nbsp;\u0026middot;V/m,\u0026nbsp;\u003c/em\u003ewhere \u003cem\u003eC\u003csub\u003e0\u003c/sub\u003e\u003c/em\u003e (mg/L) and \u003cem\u003eC\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e (mg/L) were the initial concentration and equilibrium concentrations, respectively. \u003cem\u003eV\u003c/em\u003e (L) and \u003cem\u003em\u003c/em\u003e (g) were respectively the volume of the solution and the mass of the TMD nanosheets.\u003c/p\u003e\n\u003cp\u003eTo determine the \u003cem\u003eQ\u003csub\u003ee\u003c/sub\u003e\u003c/em\u003e of TiS\u003csub\u003e2\u003c/sub\u003e for [AuBr\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e,\u003csup\u003e\u0026nbsp;\u003c/sup\u003e[AuI\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u0026nbsp;\u003c/sup\u003e(obtained by following previous report to mix [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e with KI solution\u003csup\u003e58\u003c/sup\u003e),\u003csup\u003e\u0026nbsp;\u003c/sup\u003e[Au(S\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e, [RhCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e,\u003csup\u003e\u0026nbsp;\u003c/sup\u003eand Ag\u003csup\u003e+\u003c/sup\u003e (from AgNO\u003csub\u003e3\u003c/sub\u003e) ions, we used the same procedure as that for [AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, with an initial ion concentration of 100 ppm. During the extraction, the pH was~3-4, except for [Au(S\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e, which was conducted at a pH of ~10, due to its known instability in acidic solutions\u003csup\u003e24\u003c/sup\u003e. A similar procedure was used to obtain the extraction capacity for Pd and Pt from their corresponding ions ([PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003eand [PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e), and the weight ratio of the Pd and Pt ions to TiS\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eor TaS\u003csub\u003e2\u003c/sub\u003e was set at 3:1 and 2:1 respectively.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSelective extraction for precious metals from a multi-component solution\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo determine the extraction selectivity of TiS\u003csub\u003e2\u003c/sub\u003e, TiS\u003csub\u003e2\u003c/sub\u003e nanosheets (5\u0026thinsp;mg) were added to the 100\u0026thinsp;mL solution containing Au\u003csup\u003e3+\u003c/sup\u003e (~10 ppm), Pt\u003csup\u003e4+\u003c/sup\u003e (~10 ppm), Pd\u003csup\u003e2+\u003c/sup\u003e (~10 ppm), and Rh\u003csup\u003e3+\u003c/sup\u003e (~10 ppm), as well as 15 interfering metals (~100 ppm for each ion) including Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, Cd\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, Co\u003csup\u003e2+\u003c/sup\u003e, Cu\u003csup\u003e2+\u003c/sup\u003e, Zn\u003csup\u003e2+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Al\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, Ce\u003csup\u003e3+\u003c/sup\u003e, La\u003csup\u003e3+\u003c/sup\u003e, and Cr\u003csup\u003e3+\u003c/sup\u003e. After stirring for 24\u0026thinsp;hours at 25 ℃ (pH=~2), the mixture was filtered and the ion concentrations in the filtrate were analyzed using ICP-OES and used to calculate the removal efficiency (Supplementary section 2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePowder XRD patterns were obtained using a Rigaku MiniFlex600 X-ray diffractometer with Cu K\u0026alpha; radiation. High-resolution TEM images of TiS\u003csub\u003e2\u003c/sub\u003e, TaS\u003csub\u003e2\u003c/sub\u003e nanosheets, and TiS\u003csub\u003e2\u003c/sub\u003e@[AuCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e-\u003c/sup\u003e, TiS\u003csub\u003e2\u003c/sub\u003e@[PdCl\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e2-\u003c/sup\u003e, and TiS\u003csub\u003e2\u003c/sub\u003e@[PtCl\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e2-\u0026nbsp;\u003c/sup\u003ewere obtained using a double spherical aberration-corrected microscope (Spectra 300) or FEI Tecnai G2 F30 transmission electron microscope. The thickness of TMD nanosheets was measured by AFM (Bruker Dimension Icon). Raman spectroscopy measurements of TMD nanosheets were performed under ambient conditions with a Thermo Fisher DXR2xi Raman imaging microscope using a 532\u0026thinsp;nm laser. XPS spectra were obtained using a Thermo Fisher ESCALAB Xi+ equipped with Al K\u0026alpha; radiation. The work functions of TMD nanosheets were measured by UPS (Thermo Fisher Nexsa). The concentration of metal ions was analyzed using ICP-OES (SPECTRO ARCOS Ⅱ MV) and UV-visible spectrophotometer (JASCO V760).\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eY.S. and H.-M.C. conceived the idea and supervised the project. J.H.W. prepared the sample, performed the adsorption experiments, and analyzed the results with the help of H.J.L., F.L., K.Q.Z., F.L.C., and Y.B.G. M.F.H.\u0026nbsp;performed theoretical simulation under the supervision of K.Y. All authors analyzed the data, J.H.W, Y.S., H.-M.C.,\u0026nbsp;M.F.H., and K.Y. wrote the manuscript with input from all authors.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the National Key Research and Development Program of China (2022YFA1205300), the National Natural Science Foundation of China (52472298), Guangdong Innovative and Entrepreneurial Research Team Program (2023ZT10L039), Peacock Team Project (KQTD20210811090112002), Natural Science Foundation of Guangdong Province, China (2024A1515012424), Shenzhen\u0026nbsp;Science and Technology Program (JCYJ20240813112114020) and Shenzhen Geim Graphene Center. This work made use of the TEM facilities at the Institute of Materials Research, Tsinghua Shenzhen International Graduate School.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWang, Z. et al. Take responsibility for electronic-waste disposal. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e536\u003c/strong\u003e, 23-25 (2016).\u003c/li\u003e\n\u003cli\u003eOlivetti, E.A. \u0026amp; Cullen, J.M. Toward a sustainable materials system. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e360\u003c/strong\u003e, 1396-1398 (2018).\u003c/li\u003e\n\u003cli\u003eChen, Y. et al. Precious metal recovery. \u003cem\u003eJoule\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 3097-3115 (2021).\u003c/li\u003e\n\u003cli\u003eSun, Y. et al. Interface-mediated noble metal deposition on transition metal dichalcogenide nanostructures. \u003cem\u003eNature Chemistry\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 284-293 (2020).\u003c/li\u003e\n\u003cli\u003eRaabe, D. The Materials Science behind Sustainable Metals and Alloys. \u003cem\u003eChemical Reviews\u003c/em\u003e \u003cstrong\u003e123\u003c/strong\u003e, 2436-2608 (2023).\u003c/li\u003e\n\u003cli\u003eCheng, K. et al. Maximizing noble metal utilization in solid catalysts by control of nanoparticle location. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e377\u003c/strong\u003e, 204-208 (2022).\u003c/li\u003e\n\u003cli\u003eChen, Y. et al. Selective recovery of precious metals through photocatalysis. \u003cem\u003eNature Sustainability\u003c/em\u003e \u003cstrong\u003e4\u003c/strong\u003e, 618-626 (2021).\u003c/li\u003e\n\u003cli\u003eZhang, Q. et al. Metal/MXene composites via in situ reduction. \u003cem\u003eNature Synthesis\u003c/em\u003e https://doi.org/10.1038/s44160-024-00660-z (2024).\u003c/li\u003e\n\u003cli\u003eYang, K. et al. Graphene/chitosan nanoreactors for ultrafast and precise recovery and catalytic conversion of gold from electronic waste. \u003cem\u003eProceedings of the National Academy of Sciences\u003c/em\u003e \u003cstrong\u003e121\u003c/strong\u003e, e2414449121 (2024).\u003c/li\u003e\n\u003cli\u003eShi, J. et al. Porous Polypyrrolidines for Highly Efficient Recovery of Precious Metals through Reductive Adsorption Mechanism. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2405731 (2024).\u003c/li\u003e\n\u003cli\u003eFu, K. et al. Utilizing cost-effective pyrocarbon for highly efficient gold retrieval from e-waste leachate. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 6137 (2024).\u003c/li\u003e\n\u003cli\u003eLi, X. et al. Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 263 (2023).\u003c/li\u003e\n\u003cli\u003eNiu, B. et al. Physicochemical reactions in e-waste recycling \u003cem\u003eNature Reviews Chemistry\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 569-586 (2024).\u003c/li\u003e\n\u003cli\u003eFritz, B. et al. Environmental impact of high-value gold scrap recycling. \u003cem\u003eThe International Journal of Life Cycle Assessment\u003c/em\u003e \u003cstrong\u003e25\u003c/strong\u003e, 1930-1941 (2020).\u003c/li\u003e\n\u003cli\u003eZhang, S. et al. Supply and demand of platinum group metals and strategies for sustainable management. \u003cem\u003eRenewable and Sustainable Energy Reviews\u003c/em\u003e \u003cstrong\u003e204\u003c/strong\u003e, 114821 (2024).\u003c/li\u003e\n\u003cli\u003eKanick\u0026yacute;, V. et al. Comparison of some analytical performance characteristics in inductively coupled plasma spectrometry of platinum group metals and gold. \u003cem\u003eTalanta\u003c/em\u003e \u003cstrong\u003e48\u003c/strong\u003e, 859-866 (1999).\u003c/li\u003e\n\u003cli\u003eShin, S.S. et al. Efficient recovery and recycling/upcycling of precious metals using hydrazide-functionalized star-shaped polymers. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 3889 (2024).\u003c/li\u003e\n\u003cli\u003eCao, J. et al. Tailoring the Asymmetric Structure of NH\u003csub\u003e2\u003c/sub\u003e-UiO-66 Metal-Organic Frameworks for Light-promoted Selective and Efficient Gold Extraction and Separation. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202302202 (2023).\u003c/li\u003e\n\u003cli\u003eKim, S. et al. Silanol-rich ordered mesoporous silica modified thiol group for enhanced recovery performance of Au(III) in acidic leachate solution. \u003cem\u003eChemical Engineering Journal\u003c/em\u003e \u003cstrong\u003e351\u003c/strong\u003e, 1027-1037 (2018).\u003c/li\u003e\n\u003cli\u003eXia, J., Mahandra, H. \u0026amp; Ghahreman, A. Efficient Gold Recovery from Cyanide Solution Using Magnetic Activated Carbon. \u003cem\u003eACS Applied Materials \u0026amp; Interfaces\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 47642-47649 (2021).\u003c/li\u003e\n\u003cli\u003eXu, B. et al. Kinetic studies of gold leaching from a gold concentrate calcine by thiosulfate with cobalt-ammonia catalysis and gold recovery by resin adsorption from its pregnant solution. \u003cem\u003eSeparation and Purification Technology\u003c/em\u003e \u003cstrong\u003e213\u003c/strong\u003e, 368-377 (2019).\u003c/li\u003e\n\u003cli\u003eFan, R. et al. Selective adsorption and recovery of Au(III) from three kinds of acidic systems by persimmon residual based bio-sorbent: A method for gold recycling from e-wastes. \u003cem\u003eBioresource Technology\u003c/em\u003e \u003cstrong\u003e163\u003c/strong\u003e, 167-171 (2014).\u003c/li\u003e\n\u003cli\u003eWu, H. et al. High-efficiency gold recovery by additive-induced supramolecular polymerization of \u0026beta;-cyclodextrin. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 1284 (2023).\u003c/li\u003e\n\u003cli\u003eLi, F. et al. Highly efficient and selective extraction of gold by reduced graphene oxide. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 4472 (2022).\u003c/li\u003e\n\u003cli\u003eXue, T. et al. A customized MOF-polymer composite for rapid gold extraction from water matrices. \u003cem\u003eScience Advances\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, eadg4923 (2023).\u003c/li\u003e\n\u003cli\u003eLin, S. et al. Effective adsorption of Pd(II), Pt(IV) and Au(II) by Zr(IV)-based metal\u0026ndash;organic frameworks from strongly acidic solutions. \u003cem\u003eJournal of Materials Chemistry A\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 13557-13564 (2017).\u003c/li\u003e\n\u003cli\u003eJung, Y. et al. Nitrogen-Doped Titanium Carbide (Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e) MXene Nanosheet Stack For Long-Term Stability and Efficacy in Au and Ag Recovery. \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e19\u003c/strong\u003e, 2305247 (2023).\u003c/li\u003e\n\u003cli\u003eWang, R. et al. Interfacial Coordination Bonding-Assisted Redox Mechanism-Driven Highly Selective Precious Metal Recovery on Covalent-Functionalized Ultrathin 1T-MoS\u003csub\u003e2\u003c/sub\u003e. \u003cem\u003eACS Applied Materials \u0026amp; Interfaces\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 9331-9340 (2023).\u003c/li\u003e\n\u003cli\u003eChen, Z. et al. A Supramolecular Strategy toward an Efficient and Selective Capture of Platinum(II) Complexes. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e141\u003c/strong\u003e, 11204-11211 (2019).\u003c/li\u003e\n\u003cli\u003eLiu, Y. et al. Super-Stable, Highly Efficient, and Recyclable Fibrous Metal\u0026ndash;Organic Framework Membranes for Precious Metal Recovery from Strong Acidic Solutions. \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 1805242 (2019).\u003c/li\u003e\n\u003cli\u003eHong, Y. et al. Precious metal recovery from electronic waste by a porous porphyrin polymer. \u003cem\u003eProceedings of the National Academy of Sciences\u003c/em\u003e \u003cstrong\u003e117\u003c/strong\u003e, 16174-16180 (2020).\u003c/li\u003e\n\u003cli\u003eWang, R. et al. Interfacial Coordinational Bond Triggered Photoreduction Membrane for Continuous Light-Driven Precious Metals Recovery. \u003cem\u003eNano Letters\u003c/em\u003e \u003cstrong\u003e23\u003c/strong\u003e, 2219-2227 (2023).\u003c/li\u003e\n\u003cli\u003eFeng, B. et al. Highly efficient and selective recovery of Au(III) from a complex system by molybdenum disulfide nanoflakes. \u003cem\u003eChemical Engineering Journal\u003c/em\u003e \u003cstrong\u003e350\u003c/strong\u003e, 692-702 (2018).\u003c/li\u003e\n\u003cli\u003eFang, C. et al. Bulk and surface electronic structure of 1T-TiS\u003csub\u003e2\u003c/sub\u003e and 1T-TiSe\u003csub\u003e2\u003c/sub\u003e. \u003cem\u003ePhysical Review B\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 4455-4463 (1997).\u003c/li\u003e\n\u003cli\u003eLi, Z. et al. Beyond Conventional Charge Density Wave for Strongly Enhanced 2D Superconductivity in 1H-TaS\u003csub\u003e2\u003c/sub\u003e Superlattices. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2312341 (2024).\u003c/li\u003e\n\u003cli\u003eHuckaba, A.J. et al. Low-Cost TiS\u003csub\u003e2\u003c/sub\u003e as Hole-Transport Material for Perovskite Solar Cells. \u003cem\u003eSmall Methods\u003c/em\u003e \u003cstrong\u003e1\u003c/strong\u003e, 1700250 (2017).\u003c/li\u003e\n\u003cli\u003eStolovas, D. et al. Electrical Properties of LaS-TaS\u003csub\u003e2\u003c/sub\u003e Misfit Layered Compound Nanotubes. \u003cem\u003eIsrael Journal of Chemistry\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202100072 (2022).\u003c/li\u003e\n\u003cli\u003eLorenz, T. et al. Charge Transfer Variability in Misfit Layer Compounds: Comparison of SnS-SnS\u003csub\u003e2\u003c/sub\u003e and LaS-TaS\u003csub\u003e2\u003c/sub\u003e. \u003cem\u003eIsrael Journal of Chemistry\u003c/em\u003e \u003cstrong\u003e57\u003c/strong\u003e, 553-559 (2017).\u003c/li\u003e\n\u003cli\u003eDeng, F. et al. Regulating the Electrical and Mechanical Properties of TaS\u003csub\u003e2\u003c/sub\u003e Films via van der Waals and Electrostatic Interaction for High Performance Electromagnetic Interference Shielding. \u003cem\u003eNano-Micro Letters\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 106 (2023).\u003c/li\u003e\n\u003cli\u003eWang, S. et al. Organic covalent modification to improve thermoelectric properties of TaS\u003csub\u003e2\u003c/sub\u003e. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 4401 (2022).\u003c/li\u003e\n\u003cli\u003eWu, J. et al. Acid-Assisted Exfoliation toward Metallic Sub-nanopore TaS\u003csub\u003e2\u003c/sub\u003e Monolayer with High Volumetric Capacitance. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e140\u003c/strong\u003e, 493-498 (2018).\u003c/li\u003e\n\u003cli\u003eTang, J. et al. MXene derived TiS\u003csub\u003e2\u003c/sub\u003e nanosheets for high-rate and long-life sodium-ion capacitors. \u003cem\u003eEnergy Storage Materials\u003c/em\u003e \u003cstrong\u003e26\u003c/strong\u003e, 550-559 (2020).\u003c/li\u003e\n\u003cli\u003eKumar, A. et al. Photo-Rechargeable Li-Ion Batteries using TiS\u003csub\u003e2\u003c/sub\u003e Cathode. \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e19\u003c/strong\u003e, 2303319 (2023).\u003c/li\u003e\n\u003cli\u003ePeng, Y. et al. Laser Assisted Solution Synthesis of High Performance Graphene Supported Electrocatalysts. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e30\u003c/strong\u003e, 2001756 (2020).\u003c/li\u003e\n\u003cli\u003eQiu, J. et al. Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202300459 (2023).\u003c/li\u003e\n\u003cli\u003eLi, F. et al. A portable and washable solar steam evaporator based on graphene and recycled gold for efficient point-of-use water purification. \u003cem\u003eScience China Materials\u003c/em\u003e \u003cstrong\u003e67\u003c/strong\u003e, 3700-3709 (2024).\u003c/li\u003e\n\u003cli\u003eHuang, M. et al. Switching the Oxygen Reduction Route on Pt Using S through d\u0026ndash;p Hybridization. \u003cem\u003eACS Catalysis\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 2095-2106 (2024).\u003c/li\u003e\n\u003cli\u003eLiu, M. et al. Modulating Skeletons of Covalent Organic Framework for High-Efficiency Gold Recovery. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e63\u003c/strong\u003e, e202317015 (2024).\u003c/li\u003e\n\u003cli\u003eRen, M. et al. Synergy between palladium single atoms and small nanoparticles co-anchored on carbon atom self-doped graphitic carbon nitride boosting photocatalytic H\u003csub\u003e2\u003c/sub\u003e generation. \u003cem\u003eApplied Catalysis B: Environment and Energy\u003c/em\u003e \u003cstrong\u003e345\u003c/strong\u003e, 123680 (2024).\u003c/li\u003e\n\u003cli\u003eKim, J. et al. Enhanced Electrocatalytic Properties of Transition-Metal Dichalcogenides Sheets by Spontaneous Gold Nanoparticle Decoration. \u003cem\u003eJournal of Physical Chemistry Letters\u003c/em\u003e \u003cstrong\u003e4\u003c/strong\u003e, 1227-1232 (2013).\u003c/li\u003e\n\u003cli\u003eHuang, J. et al. Surface-State Assisted Carrier Recombination and Optical Nonlinearities in Bulk to 2D Nonlayered PtS. \u003cem\u003eACS Nano\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 13390-13402 (2019).\u003c/li\u003e\n\u003cli\u003eGuo, M. et al. In Situ Built Nanoconfined TiO\u003csub\u003e2\u003c/sub\u003e Particles in Robust-Flexible MXene@rGO Conductive Framework Enabling High-Performance Hybrid Magnesium\u0026ndash;Sulfur Batteries. \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 2300417 (2023).\u003c/li\u003e\n\u003cli\u003eChen, Q. et al. Highly reversible oxygen redox in layered compounds enabled by surface polyanions. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, 3411 (2020).\u003c/li\u003e\n\u003cli\u003eLiao, H. et al. Unveiling Role of Sulfate Ion in Nickel-Iron (oxy)Hydroxide with Enhanced Oxygen-Evolving Performance. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e31\u003c/strong\u003e, 2102772 (2021).\u003c/li\u003e\n\u003cli\u003eWang, W. et al. Tantalum pentoxide (Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5-x\u003c/sub\u003e)-based memristor for photonic in-memory computing application. \u003cem\u003eNano Energy\u003c/em\u003e \u003cstrong\u003e106\u003c/strong\u003e, 108072 (2023).\u003c/li\u003e\n\u003cli\u003eShi, J. et al. Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 958 (2017).\u003c/li\u003e\n\u003cli\u003eShang, H. et al. Scalable and selective gold recovery from end-of-life electronics. \u003cem\u003eNature Chemical Engineering\u003c/em\u003e \u003cstrong\u003e1\u003c/strong\u003e, 170-179 (2024).\u003c/li\u003e\n\u003cli\u003eChen, D. et al. In-situ growth of gold nanoparticles on electrospun flexible multilayered PVDF nanofibers for SERS sensing of molecules and bacteria. \u003cem\u003eNano Research\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 4885-4893 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5882578/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5882578/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe intensive and irreplaceable consumption of precious metals (PMs) including Au, Pd, and Pt in the electronic and catalysis industries, coupled with their scarcity in the earth’s crust, demand innovative recycling solutions for PM sustainability\u003csup\u003e1-8\u003c/sup\u003e. However, efforts to recycle PMs from leachates of their waste are frustrated by an unsatisfactory extraction capacity at low concentrations and remain predominantly focused on gold, leaving other PMs unexplored\u003csup\u003e9-13\u003c/sup\u003e. We report the ultrahigh reductive recycling of PM ions and their simultaneous aqueous-phase deposition on semimetallic transition metal dichalcogenides (TMD) of TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets\u003csub\u003e. \u003c/sub\u003eNotably, TiS\u003csub\u003e2\u003c/sub\u003e shows unprecedented high extraction capacities of ~8 g/g, 2.3 g/g, and 1.15 g/g for Au, Pd, and Pt ions, respectively,\u0026nbsp;and the adsorbed PM ions directly transformed into nanoparticles deposited on the nanosheets. Mechanistic studies reveal that water-mediated electron donation from the sulfur site of the semimetallic TiS\u003csub\u003e2\u003c/sub\u003e and TaS\u003csub\u003e2\u003c/sub\u003e nanosheets is responsible for the ultrahigh extraction capacity, with a single TiS\u003csub\u003e2 \u003c/sub\u003emolecule donating more than 13 electrons to gold ions. This electron transfer is mediated by the formation of sulfur-oxygen species during water dissociation. We further demonstrate the selective and complete recovery of Au, Pd, and Pt from real-world waste streams including electronic waste, spent catalysts, and automotive catalytic converters.\u003c/p\u003e","manuscriptTitle":"Water-mediated Recycling of Gold, Palladium, and Platinum Using Semimetallic TiS2 and TaS2 Nanosheets","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-20 13:37:28","doi":"10.21203/rs.3.rs-5882578/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"35f4cf17-c5e8-4679-8d42-86194b96e6ea","owner":[],"postedDate":"February 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":44630158,"name":"Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials"},{"id":44630159,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials"}],"tags":[],"updatedAt":"2025-11-25T19:09:41+00:00","versionOfRecord":{"articleIdentity":"rs-5882578","link":"https://doi.org/10.1093/nsr/nwaf522","journal":{"identity":"national-science-review","isVorOnly":true,"title":"National Science Review"},"publishedOn":"2025-11-20 00:00:00","publishedOnDateReadable":"November 20th, 2025"},"versionCreatedAt":"2025-02-20 13:37:28","video":"","vorDoi":"10.1093/nsr/nwaf522","vorDoiUrl":"https://doi.org/10.1093/nsr/nwaf522","workflowStages":[]},"version":"v1","identity":"rs-5882578","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5882578","identity":"rs-5882578","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}