Facet-selective electrostatic assembling of 2D MXene onto anisotropic single-crystal metal oxides for enhanced photocatalysis

Facet-selective electrostatic assembling of 2D MXene onto anisotropic single-crystal metal oxides for enhanced photocatalysis | 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 Article Facet-selective electrostatic assembling of 2D MXene onto anisotropic single-crystal metal oxides for enhanced photocatalysis Shun Kashiwaya, Stephen Myakala, Sho Nekita, Yuta Tsuji, Yuran Niu, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5717389/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This work demonstrates an electrostatic assembly strategy for the facet-selective deposition of two-dimensional (2D) transition metal carbides (MXenes) onto anisotropic single-crystal semiconducting metal oxides. By precisely controlling the solution pH, we modulated the surface charge of the MXenes and the distinct crystallographic facets of the metal oxides, enabling selective deposition driven by electrostatic attraction. Specifically, negatively charged Mo 4/3 C MXenes were selectively deposited on the electron-rich (101) surface of TiO 2 exposed with {101} and {001} facets at pH 3, the (100) surface of Cu 2 O, exposed with {100} and {111} facets at pH 11, and the (010) surface of BiVO 4 , exposed with {010} and {110} facets at pH 1.5. The high degree of facet selectivity was confirmed through a combination of advanced techniques, including electron microscopy, electron spectroscopy, and synchrotron-based spectromicroscopy. This selective interfacial engineering promotes spatially separated charge carrier migration towards distinct facets of the oxides, while Schottky barriers form at the MXenes/oxides interfaces, further enhancing charge separation. The MXenes act as efficient reduction co-catalysts, facilitating the rapid consumption of electrons trapped at the Schottky barriers, thereby enhancing photocatalytic hydrogen evolution. Physical sciences/Materials science/Nanoscale materials/Structural properties Physical sciences/Materials science/Nanoscale materials/Synthesis and processing Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Solar energy, as a sustainable and renewable free resource, is crucial for tackling environmental and energy challenges. Photocatalysis on semiconductors presents a promising solution for its capacity of splitting water into hydrogen with minimal carbon dioxide emissions when used in fuel cells, reducing CO 2 into value-added chemicals and fuels, and purifying air and water by degrading pollutants 1 , 2 . However, current photocatalysts are not yet efficient enough for economic conditions. Photocatalytic redox reactions involve complex physical and chemical processes, with one major challenge being the recombination of photogenerated charge carriers, which hinders photocatalytic efficiency 3 . Therefore, ensuring efficient separation of charge carriers under light irradiation is a critical step in improving the photocatalysis process. Recent advances have shown that photogenerated electrons and holes migrate towards different crystal facets of various single-crystalline particulate semiconductors, such as BiVO 4 4,5 , Cu 2 O 6 , and TiO 2 7 . Compared to their polycrystalline counterparts, anisotropic single crystals terminated with well-defined crystallographic surfaces offer distinct advantages that arise from different electronic structures of their heterogeneous facets. Such configuration facilitates improved charge transfer and separation, thereby enhancing photocatalytic efficiency 8 . The heterogeneity also provides facet-dependent active sites, influencing chemical and physical processes such as adsorption and catalysis 9 , 10 . The aqueous interface, crucial for optimizing catalytic performance and ensuring system durability, behaves differently contingent on the crystal facet type exposed on the surface of TiO 2 11,12 and perovskites 13 . Moreover, the overpotential for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on co-catalysts varies depending on the TiO 2 facets on which noble metal co-catalysts are selectively deposited 14 . Inspired by these potential benefits, novel synthetic methods have been attempted to control the facet ratios of anisotropic single-crystal photocatalysts—facet-engineering 15 . A conventional approach to enhance charge separation involves creating heterostructures by coupling photocatalysts with metallic co-catalysts to form a Schottky barrier or with semiconducting metal oxides to build a p-n junction or a direct Z-scheme heterojunction at their interface 16 . The internal electric field built in the space charge region gives rise to a vectorial separation of charge carriers, preventing their recombination. Our strategy to further reinforce the charge separation and photocatalytic performance involves combining facet-engineered anisotropic single-crystal nanoparticles with facet-selectively deposited co-catalysts. Co-catalysts not only govern energy junctions at the interface with semiconductor supports, but also catalyze redox reactions, facilitating the efficient consumption of charge carriers by lowering the activation energy for photocatalytic reactions and the overpotential for photoelectrochemical reactions. Noble metals, such as Pt and Rh, are highly active reduction co-catalysts for HER, although rare and expensive 17 . To date, the highest photocatalytic HER performance has been achieved using Rh-based co-catalysts and electronic heterogeneity of SrTiO 3 supports 18 – 21 . We propose that MXenes (Mn + 1 Xn + 1T z ), a class of 2D inorganic compounds 22 , could be used instead of noble metals without compromising catalytic activity. MXenes consist of abundant elements in the form of transition metals (M), carbon/nitrogen (X), and surface terminations like -O, -F and -OH (T) 23 . Among MXenes, Mo 2 C is predicted to exhibit high HER activity, comparable to Pt 24 , 25 . Recently, Mo 4/3 C MXenes with ordered divacancies have recently shown superior electrochemical HER activity compared to Mo 2 C MXenes 26 , 27 . Photo-deposition, a well-established technique, allows for precise and selective deposition of co-catalysts onto specific facets of single crystals 4 . However, it requires converting precursors into targeted co-catalysts, typically in the form of spherical polycrystals, through photocatalytic redox reactions; thus, this method is not readily applicable to pre-synthesized co-catalysts to be selectively deposited onto specific facets of support crystals, nor does it allow for the deposition of co-catalysts with well-defined structures. Although assembling pre-synthesized co-catalysts onto supports using electrostatic forces has been attempted, it lacked facet selectivity. 28 Herein, our concept of facet-selective electrostatic assembling (FSEA) builds on the fact that different crystal facets exhibit various electrostatic potentials on their surfaces, which vary depending on the pH of the solution. The pH at which a surface is uncharged is known as the point of zero charge (PZC) or isoelectric point (IEP). Most metal oxides have PZC in the range of 2 and 11 29 , while MXenes have extremely low PZC, around 1 30 . Recent research suggests that pH values have substantial effects on the facet-selectivity of photo-deposition on various single-crystal metal oxides, indicating that the surface electrostatic potential may vary depending on the nature and structure of the facets 31 , 32 . Experimental evidence has confirmed different surface charges for diverse facets of SrTiO 3 33 and BiVO 4 34 . Typically, the PZC of one facet (A) of single-crystal semiconducting particles is slightly higher than that of another facet (B) due to a difference in their surface configurations. At a pH between the PZC values of the two facets, surface A is positively charged, while surface B is negatively charged, leading to the selective affinity of the highly negatively charged MXene toward the positively charged surface A. In this work, we present the method of FSEA to selectively couple 2D MXene co-catalysts with specific facets of various single-crystal metal oxides: TiO 2 , Cu 2 O, and BiVO 4 . As model systems, we investigate the combination of 2D Mo 4/3 C MXene, an alternative to noble metals, with decahedron TiO 2 co-exposed with {101} and {001} facets (d-TiO 2 ). The PZC values of Mo 4/3 C and the oxides are determined using zeta potential measurements. The effect of pH on the assembly process is systematically studied using scanning electron microscopy (SEM). Facet-selective assembling with optimal selectivity is observed at pH 2–3 for the combination of Mo 4/3 C MXenes and d-TiO 2 . This facet selectivity is further corroborated by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (STEM-EDX and STEM-EELS), as well as synchrotron-based photoemission electron microscopy with X-ray absorption spectroscopy (XAS-PEEM). The electronic structures of the resulting Mo 4/3 C/d-TiO 2 composite and their charge carrier dynamics are described by electronic properties derived from photoelectron spectroscopy. By leveraging the synergistic effects of electronic band alignment and the catalytic function of MXenes, an optimized composite is obtained, exhibiting efficient photocatalytic hydrogen evolution. Consequently, we propose a general strategy for applying FSEA to 2D co-catalysts and multi-faceted single-crystal semiconductor particles with various PZC combinations. Results and discussion Facet-selective electrostatic assembling (FSEA) Figure 1 a shows a schematic of the FSEA process with selective assembling of Mo 4/3 C onto the (101) surface of d-TiO 2 by adjusting the acidity of the aqueous solution. Zeta potential curves for Mo 4/3 C and d-TiO 2 as a function of pH show a similar trend: as pH increases, the surface charge becomes more negative (Fig. 1 b). The PZC values of Mo 4/3 C and d-TiO 2 are determined to be 0.82 and 6.55, respectively, as derived from the intersection of their zeta potential curves with the horizontal line at 0 mV. The PZC of Mo 4/3 C is lower than previously reported PZC values for other MXenes: 3.3 for Ti 3 CNT z , 2.4 for Ti 3 C 2 T z , and 0.97 for V 2 CT z 13 , indicating that Mo 4/3 C forms a more stable colloidal solution and remains negatively charged over a broader pH range compared to other MXenes. Optimal facet-selective assembly of Mo 4/3 C and d-TiO 2 was achieved through electrostatic self-assembling in aqueous solution at pH 3. Zeta potential curves for {101} and {001} facets are modelled based on both experimental data 31 and theoretical predictions 35 . The resulting curve for the (001) surface is shifted to lower pH compared to the (101) surface, meaning the PZC of the (001) surface is lower than that of the (101) surface. At pH 3, the (101) surface of d-TiO 2 is expected to exhibit a stronger positive charge compared to the (001) facet. This difference in surface potential leads to a preferential attraction of the highly negatively charged Mo 4/3 C MXenes towards the (101) surface. Figure 1 c-e presents SEM and STEM-EDX images of pristine d-TiO 2 with a truncated bipyramidal structure terminated by well-defined {101} and {001} facets and d-Mo 4/3 C/d-TiO 2 self-assembled at pH 3, demonstrating that TiO 2 (101) is selectively covered by 2D Mo 4/3 C flakes, while the (001) surface remains pristine. The selective assembly of Mo 4/3 C on the (101) surface of d-TiO 2 was also observed at pH 2; however, in line with the proposed mechanism, no facet selectivity was observed at other pH levels (Supplementary section S1). While complete selectivity might not be achieved due to factors such as MXene size variability and the potential for weaker interactions with the (001) facet, this preferential affinity promotes the formation of the Mo 4/3 C/d-TiO 2 composite with a higher concentration of MXenes on the (101) facet. Notably, a sufficiently high surface potential is crucial for driving the deposition process. For example, at pH 6.6, near the PZC of d-TiO 2 (Supplementary section S1), the electrostatic attraction is diminished, resulting in negligible facet selectivity despite the presence of opposite charges on the (101) and (001) facets and the MXenes. Further investigation into the quantitative relationship between surface potential and facet coverage, considering the influence of MXene size distribution, is warranted to elucidate the complex interplay of factors governing this selective assembly process. STEM-EELS spectra acquired from the region around the d-TiO 2 (101) surface of the Mo 4/3 C/d-TiO 2 assembly (highlighted by a green rectangle in Fig. 2 a) display electron energy loss near edge structures (ELNES) of M-edge doublets typical of Mo compounds 36 . These spectra closely resemble those of pristine MXene and are consistent with our DFT-simulated ELNES spectra, confirming the facet-selectivity of the assembly from a side-view of d-TiO 2 crystals (Fig. 2 ) (Supplementary section S2). To corroborate the facet selectivity of FSEA, we conducted synchrotron-based XAS-PEEM measurements on pristine Mo 4/3 C and optimized Mo 4/3 C/d-TiO 2 composites self-assembled at pH 3 (Fig. 3 ). Notably, while pristine Mo 4/3 C sheets are typically on the micrometer scale, the size of the MXene sheets can be tuned to match that of the sub-µm TiO 2 facets 37 . X-ray absorption near-edge structure (XANES) spectra at Mo-M 2,3 edges for pristine Mo 4/3 C are consistent with its ELNES spectra observed by STEM-EELS. XAS-PEEM images of a single Mo 4/3 C/d-TiO 2 assembly were acquired from a 2D projection looking down the [001] axis of d-TiO 2 , as indicated by the square shape in the top view of truncated bipyramidal d-TiO 2 . XANES spectra were selectively acquired from the (001) and (101) surface region of the single Mo 4/3 C/d-TiO 2 assembly, with only the (101) surface exhibiting Mo-M 2,3 edges characteristic of Mo 4/3 C MXenes. This validates the selective deposition of Mo 4/3 C onto the TiO 2 (101) facet of the Mo 4/3 C/d-TiO 2 assembly. No distinctive difference was observed between the (001) and (101) surfaces of pristine d-TiO 2 for Ti-L 2,3 edges (Supplementary section S3), indicating that the electronic heterogeneity of the two facets is attributed to a difference in their work function. The facet-selective configuration of Mo 4/3 C/d-TiO 2 composites was confirmed from various angle projections using SEM, STEM-EDX/EELS, and synchrotron-based XAS-PEEM. Electronic band alignment Figure 4 shows the electronic band structures of the Mo 4/3 C/d-TiO 2 composites prepared at pH 3, using electronic potentials derived from ultraviolet photoelectron spectroscopy (Supplementary section S4). The work function values of Mo 4/3 C and the single-crystal anatase-TiO 2 (101) surface, after surface cleaning and exposure to air, are 4.65 and 3.65–3.90 eV, respectively. The difference in Fermi level with respect to a vacuum level leads to a space charge layer of Schottky barrier between Mo 4/3 C and the (101) surface of d-TiO 2 as a result of electronic equilibrium formation upon contact, according to the electron affinity rule 38 . Photogenerated electrons and holes migrate to the (101) and (001) surfaces at the facet interface due to the difference in their Fermi levels 8 , 39 . Despite the upward band bending in d-TiO 2 at the Mo 4/3 C/d-TiO 2 interface, some photo-excited electrons can readily transfer to the Mo 4/3 C MXenes due to the favorable energy difference between the conduction band of d-TiO 2 and the Fermi level of the MXenes. When the incident photon energy exceeds the semiconductor band gap, photoexcited electrons are promoted to high-energy states within the conduction band above the conduction band minimum 40 . In our case, the photon energy is approximately 3.4 eV, which is well above the 3.2 eV band gap of TiO 2 . These electrons, generated on a femtosecond timescale, can transfer to the metallic side of the Schottky interface prior relaxing to the conduction band minimum. This phenomenon, in which electrons transfer from semiconductors to metals despite upward band bending, has been observed in semiconductor/noble metal systems, where the higher work function of noble metals compared to MXenes leads to even more pronounced upward band bending 41 . In the present Mo 4/3 C/d-TiO 2 system, the transferred electrons are subsequently trapped by the Mo 4/3 C MXene due to the Schottky junction formed at the interface, which arises from the larger work function of the MXene compared to the TiO 2 (101) surface. This electronic structure promotes effective charge separation, while the MXene functions as a superior reduction co-catalyst, reducing the overpotential and activation energy of reduction redox reactions and efficiently consuming the accumulated electrons. Photocatalysis By leveraging the synergistic effect of charge separation and co-catalyst function, the Mo 4/3 C/d-TiO 2 composite self-assembled at pH 3 demonstrates a 147-fold increase in photocatalytic hydrogen evolution under UV irradiation compared to pristine d-TiO 2 (Fig. 4 ). A similar enhanced level of activity was observed for the Mo 4/3 C/d-TiO 2 composite prepared at pH 4.5, which falls within the optimal pH range for FSEA (Fig. 1 b). Remarkably, the pristine d-TiO 2 exhibited enhanced photocatalytic activity across various reaction systems, surpassing commercially available TiO 2 benchmarks, including FP-6 (Showa Denko Ceramics) and the industry-standard P25 (Evonik), renowned for its high photocatalytic performance 42 , 43 . In contrast, the Mo 4/3 C/d-TiO 2 assembly mixed at pH 1.5 exhibited no co-catalytic effect of Mo 4/3 C despite the much higher H + concentration. These photocatalytic results confirm that the co-catalytic effect of Mo 4/3 C can only be harvested when facet-selective assembly with d-TiO 2 occurs under an optimized surface charge configuration. Strategy of FSEA To demonstrate the wider applicability of FSEA, we use two different metal oxide photocatalysts: BiVO 4 with a low PZC and Cu 2 O with a high PZC, compared to TiO 2 . Figure 5 shows the selective deposition of Mo 4/3 C MXenes onto the (010) surface of single-crystal octahedral BiVO 4 , co-exposed with {010} and {110} facets at pH 1.5, and onto the (100) surface of tetradecahedral Cu 2 O, co-exposed with {100} and {111} facets at pH 11, via FSEA (Supplementary section S5). These results indicate that the {110} facets are electrostatically more positively charged than the {010} facets of BiVO 4 , and the {100} facets are more positively charged than the {111} facets of Cu 2 O, consistent with previous experimental reports 6 , 34 , thus enabling selective assembly with the negatively charged MXenes. We propose a general strategy of FSEA for 2D co-catalysts and multi-faceted semiconductor particles with various combinations of PZCs. In this model, consider semiconductor facets of type A and B, where the PZC of A is larger than that of B, making facet A more positively charged. Two cases can be considered (Fig. 6 ): Case 1 The PZC of the 2D co-catalysts is lower than that of the single-crystal semiconductor (PZC 2D < PZC semi ). In this case, negatively charged 2D co-catalysts are attracted to the A facets. Case 2 The PZC of the 2D co-catalyst is higher than that of the single-crystal semiconductor (PZC semi < PZC 2D ). In this case, positively charged 2D co-catalysts are attracted to the B facets. The absolute difference between the PZC values of the semiconductors and 2D co-catalysts should be larger than at least 1 on the pH scale, as the PZC difference between facets can be as large as 2–3 units 33 , 34 . FSEA would function at pH levels between PZC 2D and PZC semi in both cases. However, the practical optimal range may be narrower, as demonstrated with TiO 2 , Cu 2 O, and BiVO 4 . This is because 2D co-catalysts and facets A or B should have opposite electrostatic charges high enough to attract each other, while the electrostatic potentials of A and B facets should differ enough to selectively attract the 2D co-catalysts. FSEA is broadly applicable to multi-faceted polyhedral semiconductors, including metal oxides, sulfides and perovskites, for efficient photocatalysis and solar cells 13 , 44 – 46 . Additionally, a variety of 2D MXenes and other candidates, such as elemental 2D materials (e.g., graphene and goldene 47 ), 2D transition-metal chalcogenides, and metal/covalent organic frameworks (MOFs/COFs), could also be utilized as low-dimensional co-catalysts. Conclusions We demonstrate facet-selective electrostatic assembling of 2D Mo 4/3 C and decahedron TiO 2 single crystals terminated by well-defined {101} and {001} facets, where Mo 4/3 C is preferentially attached to TiO 2 (101) by adjusting the acidity of the aqueous solution. Mo 4/3 C is found to have an exceptionally low PZC of 0.82, whereas d-TiO 2 has a PZC of 6.65. As the TiO 2 (101) surface is expected to have a slightly higher PZC than the TiO 2 (001) surface, at pH levels below the PZC of d-TiO 2 , Mo 4/3 C is negatively charged while TiO 2 (101) is more positively charged than TiO 2 (001). Consequently, Mo 4/3 C exhibits a preferential affinity toward the TiO 2 (101) surface. The optimal pH level for combining 2D Mo 4/3 C and d-TiO 2 is 3. In this configuration, photogenerated electrons and holes move to the (101) and (001) facets, respectively. Moreover, Mo 4/3 C and likely other MXenes, effectively utilize the electrons accumulated due to the Schottky junction formed at the interface with the (101) surface, further promoting charge separation. By leveraging the synergistic effects of charge separation and co-catalyst functionality, the Mo 4/3 C/d-TiO 2 composite self-assembled via FSEA exhibits enhanced photocatalytic HER activity. This work widens the design space for composite photocatalysts by combining facet-engineered anisotropic semiconductors and 2D co-catalysts, optimizing them for efficient photocatalytic redox reactions. Methods Sample preparation d-TiO 2 nanoparticles were synthesized via a gas-phase reaction process 43 . Single-crystal tetradecahedral-Cu 2 O particles, co-exposed with {100} and {111} facets, were synthesized via a controlled reduction of copper salts in an alkaline aqueous solution. First, 780 mg of CuCl 2 and 780 mg of Cu(CO 2 CH 3 ) 2 were dissolved in 500 ml of deionized (DI) water maintained at 55°C under continuous stirring. A separate solution of 4 g NaOH in 40 ml DI water was prepared. Upon complete dissolution of the copper salts, the temperature of the solution was raised to 75°C, and the NaOH solution was introduced dropwise, inducing a color change to turquoise. Following 5 min. of stirring, 4 g of D-(+)-glucose was added to initiate the reduction for 30 min., after which the resulting Cu 2 O powder was collected via vacuum filtration, washed with DI water and ethanol (50 ml each), and dried at 60°C overnight. Single-crystal octahedral BiVO 4 particles, co-exposed with {010} and {110} facets, were synthesized using a microwave-assisted method. In a 30 ml microwave vial, 600 mg of Bi(NO 3 ) 3 ·5H 2 O was dissolved in 2 ml of DI water with the addition of 50 µl of concentrated HNO 3 . Concurrently, 150 mg of NH 4 VO 3 was dissolved in 2.5 ml of DI water at 90–95°C. The Bi(NO 3 ) 3 solution was then slowly added to the NH 4 VO 3 solution, and the mixture was heated to 190°C for 5 min. in a microwave reactor. The resulting BiVO 4 was thoroughly washed with 50 ml of DI water and dried at 60°C overnight. 2D Mo 4/3 C MXene flakes were produced by etching a 3D atomic laminate of (Mo 2/3 Sc 1/3 ) 2 AlC, followed by delamination, as previously reported 26 . Only small-sized flakes were used for the FSEA. The assembly of Mo 4/3 C/d-TiO 2 was prepared by mixing d-TiO 2 with 5 wt.% Mo 4/3 C in DI water at various pH levels with no light exposure. The pH of the mixture solution was adjusted by using diluted HNO 3 and NaOH. The resulting solution was ultrasonicated for 10 min. For electron microscopy and spectroscopy measurements, samples were prepared by drop-casting the solution onto Si substrates for SEM, UPS, and XAS-PEEM and onto TEM grids with a 10 nm thick amorphous carbon support membrane for STEM-EDX/EELS at room temperature. Commercial MoO 2 and MoO 3 (Sigma-Aldrich) were used as reference materials for STEM-EELS measurements. Zeta potential analysis The surface zeta potentials of the aqueous solutions containing MXene (50 µg/mL) and d-TiO 2 (250 µg/mL) as a function of pH were measured using dynamic light scattering (DLS) with a Zetasizer Nano-ZS90 (Malvern Instruments) in DTS1070 capillary cells (Malvern instruments). The zeta potential was calculated based on electrophoretic mobility, which was determined by the electrophoretic light scattering technique of Zetasizer. For each pH value, three independent measurements of the zeta potential were performed, and the mean values were plotted. The pH of the prepared solutions was adjusted using HNO 3 and KOH, with pH levels monitored by a SevenCompact pH/Ion meter (S220 Mettler Toledo). Prior to the measurements, the electrode was calibrated using three technical buffer solutions at pH 2, 7, and 11. Electron microscopy and spectroscopy SEM imaging of the prepared samples was performed using a LEO 1550. Annular dark-field (ADF)-STEM images, EDX maps, and EELS spectra were acquired using a Thermo Fisher Scientific Titan cube G2 60–300, equipped with a Schottky electron source, a monochromator, a spherical aberration corrector (DCOR, CEOS) for the probe-forming lens system, a Super-X system for EDX, and a GATAN Quantum 965 imaging filter for EELS. The microscope was operated at an accelerating voltage of 300 kV. For EELS, the energy resolution, measured from the full width of half-maximum of the zero-loss peak (ZLP), was 0.3 eV. The pixel sizes for STEM-EELS mapping were on the nanometer scale for pristine Mo 4/3 C and on the subnanometer scale for the Mo 4/3 C/d-TiO 2 (101) interfaces. The typical acquisition time per pixel was 0.1 s. The STEM-EELS spectra presented in Fig. 2 were obtained by accumulating signals from multiple regions for the pristine Mo 4/3 C and the Mo 4/3 C/d-TiO 2 (101) interfaces (Supplementary section S6). Photocatalytic hydrogen evolution The hydrogen evolution experiments were conducted in a slurry-type, water-cooled reactor illuminated from the side by a monochromatic UV LED light source centered at 365 ± 6 nm (SOLIS, Thorlabs), with an incident light intensity of 576 mW (power density of 183 mW/cm 2 ). The experiments were carried out in batch-type mode, with the reaction solution maintained at 15°C under constant stirring at 650 rpm throughout the experiment. A stock solution of the catalyst was prepared by mixing 90 mL of an aqueous HNO 3 solution at pH 1.5, 3, and 4.5 with 10 mL of HPLC-grade methanol, followed by a re-adjustment of the pH to 1.5, 3, and 4.5 using diluted HNO 3 . 80 mL of the obtained solution was mixed with 80 mg of d-TiO 2 and ultrasonicated for 10 minutes to achieve a homogenous suspension. To this, 220 µL of the fresh Mo 4/3 C solution (approx. concentration of 9 mg mL − 1 ), corresponding to approximately 2.5 wt.% MXene in TiO 2 , was added to enable uniform deposition of Mo 4/3 C flakes onto the (101) surface of d-TiO 2 via FSEA. For each HER experiment, 2 mL of the d-TiO 2 /Mo 4/3 C solution mixture was transferred into the reactor and purged with Ar (flow rate of 10 mL min − 1 ) to remove dissolved oxygen. During light irradiation, the reactor was sealed to maintain an airtight environment. The headspace was probed every 30 minutes using a gas-tight syringe and analysed by gas chromatography (Shimadzu GC-2030, equipped with a barrier discharge ionization detector). A 5 point calibration profile was used to accurately quantify the amount of hydrogen evolved, and the results were translated into mole values as presented in the main text. Synchrotron-based XAS-PEEM measurements The XAS-PEEM measurements were conducted using the AC-SPELEEM (Elmitec GmbH) endstation of MAXPEEM beamline at MAX IV Laboratory, Sweden. Detailed descriptions of the analysis setup can be found in reference 48 . During the measurement, a stack of PEEM images was acquired with a photon energy increment of 0.2 eV across the Mo-M 2,3 edges, where 3p electrons are excited to unoccupied valence band states. All PEEM images were captured using secondary photoelectrons with a kinetic energy of 1.9 eV. 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Two-dimensional molybdenum carbide (MXene) as an efficient electrocatalyst for hydrogen evolution. ACS Energy Letters 1, 589–594 (2016). Tao, Q. et al. Two-dimensional Mo 1. 33 C MXene with divacancy ordering prepared from parent 3D laminate with in-plane chemical ordering. Nature Communications 8, 14949 (2017). Lind, H. et al. Hydrogen evolution reaction for vacancy-ordered i-MXenes and the impact of proton absorption into the vacancies. Advanced Sustainable Systems 5, 2000158 (2021). Xie, X., Zhang, N., Tang, Z.-R., Anpo, M. & Xu, Y.-J. Ti 3 C 2 T x MXene as a Janus cocatalyst for concurrent promoted photoactivity and inhibited photocorrosion. Applied Catalysis B: Environmental 237, 43–49 (2018). Kosmulski, M. The pH dependent surface charging and points of zero charge. X. Update. Advances in Colloid and Interface Science, 102973 (2023). Naguib, M., Unocic, R. R., Armstrong, B. L. & Nanda, J. 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Journal of the American Chemical Society (2024). Gao, F. & Zhao, Q. Facet engineering: a promising pathway toward highly efficient and stable perovskite photovoltaics. The Journal of Physical Chemistry Letters 14, 4409–4418 (2023). Kashiwaya, S. Synthesis of goldene comprising single-atom layer gold. Nature Synthesis 3, 744–751 (2024). Niu, Y. et al. MAXPEEM: a spectromicroscopy beamline at MAX IV laboratory. Journal of Synchrotron Radiation 30, 468–478 (2023). Schindelin, J. et al. Fiji: an open-source platform for biological-image analysis. Nature Methods 9, 676–682 (2012). Tseng, Q. Template Matching and Slice Alignment. https://sites.google.com/site/qingzongtseng/template-matching-ij-plugin. Additional Declarations There is NO Competing Interest. Supplementary Files 20241226SupplementaryMXeneTiO2ver10.0.docx SUPPLEMENTARY INFORMATION Cite Share Download PDF Status: Posted 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-5717389","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":401426765,"identity":"79073694-4510-4462-937a-1311199b1a0b","order_by":0,"name":"Shun Kashiwaya","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYDACCRBxgIHBgBnMtZFhI1VLGg8JWiDcwzwEdcjP7jF7wHDGzt6cncfwM0/NeR4+iQTmDx/waDG4c8bcgOFGcuLOZh5jaZ5jt3nYJBLYJGfg0yKRYybB8IE5weAw7wZpHjaIFmZ8zpOfAdZSbw/Usvk3z79zIC3Mn//g88wNkJYbhxk3HObdJs3bdgCkhUEanw6DG2llEglnjiduOMz/zXJuXzIPG8/DNskevA5L3ibx4Vi1vcH5Y8k33nyzk5NvTz784Qc+a0AgAUozQXzN2EBIAwIwEjR8FIyCUTAKRiQAAB3WRzG55eGkAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-0578-8218","institution":"Linköping University","correspondingAuthor":true,"prefix":"","firstName":"Shun","middleName":"","lastName":"Kashiwaya","suffix":""},{"id":401426766,"identity":"bd480c46-f6c4-4d34-b10f-135c98222895","order_by":1,"name":"Stephen Myakala","email":"","orcid":"","institution":"Technische Universität Wien","correspondingAuthor":false,"prefix":"","firstName":"Stephen","middleName":"","lastName":"Myakala","suffix":""},{"id":401426767,"identity":"b8ac7ec3-595a-4c41-bb67-96d9caae9e3a","order_by":2,"name":"Sho Nekita","email":"","orcid":"","institution":"Kyushu University","correspondingAuthor":false,"prefix":"","firstName":"Sho","middleName":"","lastName":"Nekita","suffix":""},{"id":401426768,"identity":"bde56f70-dc37-4393-a34f-9c05f468e2dd","order_by":3,"name":"Yuta Tsuji","email":"","orcid":"https://orcid.org/0000-0003-4224-4532","institution":"Kyushu University","correspondingAuthor":false,"prefix":"","firstName":"Yuta","middleName":"","lastName":"Tsuji","suffix":""},{"id":401426769,"identity":"d2dba482-756a-4448-a83c-58d3d9529275","order_by":4,"name":"Yuran Niu","email":"","orcid":"","institution":"Lund University","correspondingAuthor":false,"prefix":"","firstName":"Yuran","middleName":"","lastName":"Niu","suffix":""},{"id":401426770,"identity":"b96369da-feed-4af9-8886-79c97f434186","order_by":5,"name":"Liu Xianjie","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"prefix":"","firstName":"Liu","middleName":"","lastName":"Xianjie","suffix":""},{"id":401426771,"identity":"923df659-4316-4081-8d60-5abb12614b41","order_by":6,"name":"Leiqiang Qin","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"prefix":"","firstName":"Leiqiang","middleName":"","lastName":"Qin","suffix":""},{"id":401426772,"identity":"fdca8be6-e892-4dd9-9786-2a21da1a9348","order_by":7,"name":"Alexei Kakharov","email":"","orcid":"","institution":"Lund University","correspondingAuthor":false,"prefix":"","firstName":"Alexei","middleName":"","lastName":"Kakharov","suffix":""},{"id":401426773,"identity":"ca392787-515a-4b82-a689-435cc5ec1b31","order_by":8,"name":"Lars Hultman","email":"","orcid":"https://orcid.org/0000-0002-2837-3656","institution":"Linköping University","correspondingAuthor":false,"prefix":"","firstName":"Lars","middleName":"","lastName":"Hultman","suffix":""},{"id":401426774,"identity":"f43c9a14-38e3-47a6-b105-f512f63b1332","order_by":9,"name":"Eder Dominik","email":"","orcid":"","institution":"Technische Universität Wien","correspondingAuthor":false,"prefix":"","firstName":"Eder","middleName":"","lastName":"Dominik","suffix":""},{"id":401426775,"identity":"305517da-4baf-4ecf-a515-33546260d7cc","order_by":10,"name":"Hikaru Saito","email":"","orcid":"","institution":"Kyushu University","correspondingAuthor":false,"prefix":"","firstName":"Hikaru","middleName":"","lastName":"Saito","suffix":""},{"id":401426776,"identity":"b81ce12f-5b1e-411c-ba41-0912df7b2b62","order_by":11,"name":"Alexey Cherevan","email":"","orcid":"","institution":"Technische Universität Wien","correspondingAuthor":false,"prefix":"","firstName":"Alexey","middleName":"","lastName":"Cherevan","suffix":""},{"id":401426777,"identity":"a978ef5e-4d4b-425c-ab30-fb4a64f19a75","order_by":12,"name":"Johanna Rosen","email":"","orcid":"","institution":"Linköping University","correspondingAuthor":false,"prefix":"","firstName":"Johanna","middleName":"","lastName":"Rosen","suffix":""}],"badges":[],"createdAt":"2024-12-26 17:05:25","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5717389/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5717389/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":73749500,"identity":"451f7ee7-c275-4254-8c19-37ded7495aa3","added_by":"auto","created_at":"2025-01-14 09:30:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1006444,"visible":true,"origin":"","legend":"\u003cp\u003eFacet-selective assembling of MXenes with the (101) surface of decahedron anatase TiO\u003csub\u003e2\u003c/sub\u003e. a) Schematic illustration of the facet-selective assembling. b) Zeta potential of Mo\u003csub\u003e4/3\u003c/sub\u003eC and TiO\u003csub\u003e2\u003c/sub\u003e as a function of pH. c) SEM images of pristine TiO\u003csub\u003e2\u003c/sub\u003e and Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. d) STEM-EDX images of Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. e) High-resolution STEM images of Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. Left and right images correspond to the magnified regions marked in the middle image, highlighting the TiO\u003csub\u003e2\u003c/sub\u003e (101) and (001) surface, respectively.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/5d04f377f15f937384adb64c.png"},{"id":73748368,"identity":"6def4300-f52a-4dee-b8e0-d64707581ad7","added_by":"auto","created_at":"2025-01-14 09:22:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":524087,"visible":true,"origin":"","legend":"\u003cp\u003eSTEM-EELS measurements on pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC and Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. a,b) Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3 (a) and pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene (a) and with selected regions (green) for EELS measurements. c) EELS Mo-M\u003csub\u003e2,3\u003c/sub\u003e spectra for Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene on TiO\u003csub\u003e2, \u003c/sub\u003epristine MXene, and references of MoO\u003csub\u003e2\u003c/sub\u003e and MoO\u003csub\u003e3\u003c/sub\u003e particulates. d) EELS O-K spectra for MXene and the Mo oxides references.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/a322d3914a93210b9e301af2.png"},{"id":73748366,"identity":"fe4d8e81-09e7-4e91-9557-953ee74f7906","added_by":"auto","created_at":"2025-01-14 09:22:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":559161,"visible":true,"origin":"","legend":"\u003cp\u003eSynchrotron-based XAS-PEEM measurements on pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC and Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. a) XAS spectra for pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC, agglomerated Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composites prepared at pH 3, and the (101) and (001) surfaces of a single Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3. The emission for the pristine MXene comes from the green circle spot presented in b). The emission for the agglomerate of assemblies originates from the entire region of c. The emissions of the (101) and (001) surfaces originate from the circle (cyan) and rectangular (magenta) in d, respectively. b) XAS-PEEM image of pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC. The right image corresponds to the square region of the left image. c) XAS-PEEM image and illustration for the agglomerated Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composites prepared at pH 3. d) XAS-PEEM image and illustration for a single Mo\u003csub\u003e4/3\u003c/sub\u003eC/TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 3.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/c7dc98648f232fc8b76c0329.png"},{"id":73748377,"identity":"68aef093-a30b-4dd6-8049-47a373dc003f","added_by":"auto","created_at":"2025-01-14 09:22:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":157658,"visible":true,"origin":"","legend":"\u003cp\u003eThe electronic band structure of the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite (left) and its photocatalytic hydrogen evolution at pH 1.5, 3, and 4.5 (right). The band structure of the composite, where Mo\u003csub\u003e4/3\u003c/sub\u003eC is selectively deposited on the (101) facet of TiO\u003csub\u003e2\u003c/sub\u003e at pH 3, is derived from potentials acquired by photoelectron spectroscopy. Error bars correspond to the fluctuations of the detected amount of H\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/112417ffb5248783face2b8a.png"},{"id":73748383,"identity":"00e7cd96-f808-45cc-af50-041315627ac1","added_by":"auto","created_at":"2025-01-14 09:22:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":629494,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of pristine BiVO\u003csub\u003e4\u003c/sub\u003e (a), pristine Cu\u003csub\u003e2\u003c/sub\u003eO (b), Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene and BiVO\u003csub\u003e4\u003c/sub\u003e self-assembled at pH 1.5, where MXene is selectively attached to the BiVO\u003csub\u003e4\u003c/sub\u003e(110) surface (c), and Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene and Cu\u003csub\u003e2\u003c/sub\u003eO self-assembled at pH 1, where MXene is selectively attached to the Cu\u003csub\u003e2\u003c/sub\u003eO(100) surface (d).\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/7fa164aba64888dd992ed7fa.png"},{"id":73750318,"identity":"4550b013-1f53-4fcd-86fa-458e2f7de850","added_by":"auto","created_at":"2025-01-14 09:38:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":203738,"visible":true,"origin":"","legend":"\u003cp\u003eGeneral strategy of FSEA for 2D co-catalysts and anisotropic single-crystal particulate semiconductors co-exposed with multiple facets for two cases:\u0026nbsp; case 1) a point of zero charge of 2D co-catalysts is lower than a point of zero charge of single-crystal semiconductors (PZC\u003csub\u003e2D\u003c/sub\u003e \u0026lt; PZC\u003csub\u003esemi\u003c/sub\u003e), and case 2) the opposite scenario (PZC\u003csub\u003esemi\u003c/sub\u003e \u0026lt; PZC\u003csub\u003e2D\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/b292867a61a48cf1662e66dd.png"},{"id":76846474,"identity":"50f2cd0f-b0ae-4c54-8c12-9e2eb0936ed0","added_by":"auto","created_at":"2025-02-21 11:03:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4146134,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/78e5deda-9b03-45c3-8b2f-5809ddd6b06f.pdf"},{"id":73748376,"identity":"ae39b929-478c-4d80-b111-937e54635a86","added_by":"auto","created_at":"2025-01-14 09:22:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":16970703,"visible":true,"origin":"","legend":"SUPPLEMENTARY INFORMATION","description":"","filename":"20241226SupplementaryMXeneTiO2ver10.0.docx","url":"https://assets-eu.researchsquare.com/files/rs-5717389/v1/fcf75f4e89acaf5fc0e73bd5.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Facet-selective electrostatic assembling of 2D MXene onto anisotropic single-crystal metal oxides for enhanced photocatalysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSolar energy, as a sustainable and renewable free resource, is crucial for tackling environmental and energy challenges. Photocatalysis on semiconductors presents a promising solution for its capacity of splitting water into hydrogen with minimal carbon dioxide emissions when used in fuel cells, reducing CO\u003csub\u003e2\u003c/sub\u003e into value-added chemicals and fuels, and purifying air and water by degrading pollutants\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. However, current photocatalysts are not yet efficient enough for economic conditions. Photocatalytic redox reactions involve complex physical and chemical processes, with one major challenge being the recombination of photogenerated charge carriers, which hinders photocatalytic efficiency\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Therefore, ensuring efficient separation of charge carriers under light irradiation is a critical step in improving the photocatalysis process.\u003c/p\u003e \u003cp\u003eRecent advances have shown that photogenerated electrons and holes migrate towards different crystal facets of various single-crystalline particulate semiconductors, such as BiVO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e4,5\u003c/sup\u003e, Cu\u003csub\u003e2\u003c/sub\u003eO\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, and TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e7\u003c/sup\u003e. Compared to their polycrystalline counterparts, anisotropic single crystals terminated with well-defined crystallographic surfaces offer distinct advantages that arise from different electronic structures of their heterogeneous facets. Such configuration facilitates improved charge transfer and separation, thereby enhancing photocatalytic efficiency\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. The heterogeneity also provides facet-dependent active sites, influencing chemical and physical processes such as adsorption and catalysis\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The aqueous interface, crucial for optimizing catalytic performance and ensuring system durability, behaves differently contingent on the crystal facet type exposed on the surface of TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e11,12\u003c/sup\u003e and perovskites\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Moreover, the overpotential for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on co-catalysts varies depending on the TiO\u003csub\u003e2\u003c/sub\u003e facets on which noble metal co-catalysts are selectively deposited\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Inspired by these potential benefits, novel synthetic methods have been attempted to control the facet ratios of anisotropic single-crystal photocatalysts\u0026mdash;facet-engineering\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eA conventional approach to enhance charge separation involves creating heterostructures by coupling photocatalysts with metallic co-catalysts to form a Schottky barrier or with semiconducting metal oxides to build a p-n junction or a direct Z-scheme heterojunction at their interface\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The internal electric field built in the space charge region gives rise to a vectorial separation of charge carriers, preventing their recombination. Our strategy to further reinforce the charge separation and photocatalytic performance involves combining facet-engineered anisotropic single-crystal nanoparticles with facet-selectively deposited co-catalysts.\u003c/p\u003e \u003cp\u003eCo-catalysts not only govern energy junctions at the interface with semiconductor supports, but also catalyze redox reactions, facilitating the efficient consumption of charge carriers by lowering the activation energy for photocatalytic reactions and the overpotential for photoelectrochemical reactions. Noble metals, such as Pt and Rh, are highly active reduction co-catalysts for HER, although rare and expensive\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. To date, the highest photocatalytic HER performance has been achieved using Rh-based co-catalysts and electronic heterogeneity of SrTiO\u003csub\u003e3\u003c/sub\u003e supports\u003csup\u003e\u003cspan additionalcitationids=\"CR19 CR20\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. We propose that MXenes (Mn\u0026thinsp;+\u0026thinsp;1 Xn\u0026thinsp;+\u0026thinsp;1T\u003csub\u003e\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003e), a class of 2D inorganic compounds\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, could be used instead of noble metals without compromising catalytic activity. MXenes consist of abundant elements in the form of transition metals (M), carbon/nitrogen (X), and surface terminations like -O, -F and -OH (T)\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Among MXenes, Mo\u003csub\u003e2\u003c/sub\u003eC is predicted to exhibit high HER activity, comparable to Pt\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Recently, Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes with ordered divacancies have recently shown superior electrochemical HER activity compared to Mo\u003csub\u003e2\u003c/sub\u003eC MXenes\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003ePhoto-deposition, a well-established technique, allows for precise and selective deposition of co-catalysts onto specific facets of single crystals\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. However, it requires converting precursors into targeted co-catalysts, typically in the form of spherical polycrystals, through photocatalytic redox reactions; thus, this method is not readily applicable to pre-synthesized co-catalysts to be selectively deposited onto specific facets of support crystals, nor does it allow for the deposition of co-catalysts with well-defined structures. Although assembling pre-synthesized co-catalysts onto supports using electrostatic forces has been attempted, it lacked facet selectivity.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eHerein, our concept of facet-selective electrostatic assembling (FSEA) builds on the fact that different crystal facets exhibit various electrostatic potentials on their surfaces, which vary depending on the pH of the solution. The pH at which a surface is uncharged is known as the point of zero charge (PZC) or isoelectric point (IEP). Most metal oxides have PZC in the range of 2 and 11\u003csup\u003e29\u003c/sup\u003e, while MXenes have extremely low PZC, around 1\u003csup\u003e30\u003c/sup\u003e. Recent research suggests that pH values have substantial effects on the facet-selectivity of photo-deposition on various single-crystal metal oxides, indicating that the surface electrostatic potential may vary depending on the nature and structure of the facets\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Experimental evidence has confirmed different surface charges for diverse facets of SrTiO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e33\u003c/sup\u003e and BiVO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e34\u003c/sup\u003e. Typically, the PZC of one facet (A) of single-crystal semiconducting particles is slightly higher than that of another facet (B) due to a difference in their surface configurations. At a pH between the PZC values of the two facets, surface A is positively charged, while surface B is negatively charged, leading to the selective affinity of the highly negatively charged MXene toward the positively charged surface A.\u003c/p\u003e \u003cp\u003eIn this work, we present the method of FSEA to selectively couple 2D MXene co-catalysts with specific facets of various single-crystal metal oxides: TiO\u003csub\u003e2\u003c/sub\u003e, Cu\u003csub\u003e2\u003c/sub\u003eO, and BiVO\u003csub\u003e4\u003c/sub\u003e. As model systems, we investigate the combination of 2D Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene, an alternative to noble metals, with decahedron TiO\u003csub\u003e2\u003c/sub\u003e co-exposed with {101} and {001} facets (d-TiO\u003csub\u003e2\u003c/sub\u003e). The PZC values of Mo\u003csub\u003e4/3\u003c/sub\u003eC and the oxides are determined using zeta potential measurements. The effect of pH on the assembly process is systematically studied using scanning electron microscopy (SEM). Facet-selective assembling with optimal selectivity is observed at pH 2\u0026ndash;3 for the combination of Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes and d-TiO\u003csub\u003e2\u003c/sub\u003e. This facet selectivity is further corroborated by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (STEM-EDX and STEM-EELS), as well as synchrotron-based photoemission electron microscopy with X-ray absorption spectroscopy (XAS-PEEM). The electronic structures of the resulting Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite and their charge carrier dynamics are described by electronic properties derived from photoelectron spectroscopy. By leveraging the synergistic effects of electronic band alignment and the catalytic function of MXenes, an optimized composite is obtained, exhibiting efficient photocatalytic hydrogen evolution. Consequently, we propose a general strategy for applying FSEA to 2D co-catalysts and multi-faceted single-crystal semiconductor particles with various PZC combinations.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eFacet-selective electrostatic assembling (FSEA)\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows a schematic of the FSEA process with selective assembling of Mo\u003csub\u003e4/3\u003c/sub\u003eC onto the (101) surface of d-TiO\u003csub\u003e2\u003c/sub\u003e by adjusting the acidity of the aqueous solution. Zeta potential curves for Mo\u003csub\u003e4/3\u003c/sub\u003eC and d-TiO\u003csub\u003e2\u003c/sub\u003e as a function of pH show a similar trend: as pH increases, the surface charge becomes more negative (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The PZC values of Mo\u003csub\u003e4/3\u003c/sub\u003eC and d-TiO\u003csub\u003e2\u003c/sub\u003e are determined to be 0.82 and 6.55, respectively, as derived from the intersection of their zeta potential curves with the horizontal line at 0 mV. The PZC of Mo\u003csub\u003e4/3\u003c/sub\u003eC is lower than previously reported PZC values for other MXenes: 3.3 for Ti\u003csub\u003e3\u003c/sub\u003eCNT\u003csub\u003e\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003e, 2.4 for Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003e\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003e, and 0.97 for V\u003csub\u003e2\u003c/sub\u003eCT\u003csub\u003e\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003e\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, indicating that Mo\u003csub\u003e4/3\u003c/sub\u003eC forms a more stable colloidal solution and remains negatively charged over a broader pH range compared to other MXenes. Optimal facet-selective assembly of Mo\u003csub\u003e4/3\u003c/sub\u003eC and d-TiO\u003csub\u003e2\u003c/sub\u003e was achieved through electrostatic self-assembling in aqueous solution at pH 3. Zeta potential curves for {101} and {001} facets are modelled based on both experimental data\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e and theoretical predictions\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. The resulting curve for the (001) surface is shifted to lower pH compared to the (101) surface, meaning the PZC of the (001) surface is lower than that of the (101) surface. At pH 3, the (101) surface of d-TiO\u003csub\u003e2\u003c/sub\u003e is expected to exhibit a stronger positive charge compared to the (001) facet. This difference in surface potential leads to a preferential attraction of the highly negatively charged Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes towards the (101) surface.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec-e presents SEM and STEM-EDX images of pristine d-TiO\u003csub\u003e2\u003c/sub\u003e with a truncated bipyramidal structure terminated by well-defined {101} and {001} facets and d-Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e self-assembled at pH 3, demonstrating that TiO\u003csub\u003e2\u003c/sub\u003e(101) is selectively covered by 2D Mo\u003csub\u003e4/3\u003c/sub\u003eC flakes, while the (001) surface remains pristine. The selective assembly of Mo\u003csub\u003e4/3\u003c/sub\u003eC on the (101) surface of d-TiO\u003csub\u003e2\u003c/sub\u003e was also observed at pH 2; however, in line with the proposed mechanism, no facet selectivity was observed at other pH levels (Supplementary section S1). While complete selectivity might not be achieved due to factors such as MXene size variability and the potential for weaker interactions with the (001) facet, this preferential affinity promotes the formation of the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite with a higher concentration of MXenes on the (101) facet. Notably, a sufficiently high surface potential is crucial for driving the deposition process. For example, at pH 6.6, near the PZC of d-TiO\u003csub\u003e2\u003c/sub\u003e (Supplementary section S1), the electrostatic attraction is diminished, resulting in negligible facet selectivity despite the presence of opposite charges on the (101) and (001) facets and the MXenes. Further investigation into the quantitative relationship between surface potential and facet coverage, considering the influence of MXene size distribution, is warranted to elucidate the complex interplay of factors governing this selective assembly process.\u003c/p\u003e \u003cp\u003eSTEM-EELS spectra acquired from the region around the d-TiO\u003csub\u003e2\u003c/sub\u003e(101) surface of the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e assembly (highlighted by a green rectangle in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) display electron energy loss near edge structures (ELNES) of M-edge doublets typical of Mo compounds\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. These spectra closely resemble those of pristine MXene and are consistent with our DFT-simulated ELNES spectra, confirming the facet-selectivity of the assembly from a side-view of d-TiO\u003csub\u003e2\u003c/sub\u003e crystals (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) (Supplementary section S2).\u003c/p\u003e \u003cp\u003eTo corroborate the facet selectivity of FSEA, we conducted synchrotron-based XAS-PEEM measurements on pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC and optimized Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composites self-assembled at pH 3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Notably, while pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC sheets are typically on the micrometer scale, the size of the MXene sheets can be tuned to match that of the sub-\u0026micro;m TiO\u003csub\u003e2\u003c/sub\u003e facets\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. X-ray absorption near-edge structure (XANES) spectra at Mo-M\u003csub\u003e2,3\u003c/sub\u003e edges for pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC are consistent with its ELNES spectra observed by STEM-EELS. XAS-PEEM images of a single Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e assembly were acquired from a 2D projection looking down the [001] axis of d-TiO\u003csub\u003e2\u003c/sub\u003e, as indicated by the square shape in the top view of truncated bipyramidal d-TiO\u003csub\u003e2\u003c/sub\u003e. XANES spectra were selectively acquired from the (001) and (101) surface region of the single Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e assembly, with only the (101) surface exhibiting Mo-M\u003csub\u003e2,3\u003c/sub\u003e edges characteristic of Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes. This validates the selective deposition of Mo\u003csub\u003e4/3\u003c/sub\u003eC onto the TiO\u003csub\u003e2\u003c/sub\u003e(101) facet of the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e assembly. No distinctive difference was observed between the (001) and (101) surfaces of pristine d-TiO\u003csub\u003e2\u003c/sub\u003e for Ti-L\u003csub\u003e2,3\u003c/sub\u003e edges (Supplementary section S3), indicating that the electronic heterogeneity of the two facets is attributed to a difference in their work function. The facet-selective configuration of Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composites was confirmed from various angle projections using SEM, STEM-EDX/EELS, and synchrotron-based XAS-PEEM.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eElectronic band alignment\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the electronic band structures of the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composites prepared at pH 3, using electronic potentials derived from ultraviolet photoelectron spectroscopy (Supplementary section S4). The work function values of Mo\u003csub\u003e4/3\u003c/sub\u003eC and the single-crystal anatase-TiO\u003csub\u003e2\u003c/sub\u003e(101) surface, after surface cleaning and exposure to air, are 4.65 and 3.65\u0026ndash;3.90 eV, respectively. The difference in Fermi level with respect to a vacuum level leads to a space charge layer of Schottky barrier between Mo\u003csub\u003e4/3\u003c/sub\u003eC and the (101) surface of d-TiO\u003csub\u003e2\u003c/sub\u003e as a result of electronic equilibrium formation upon contact, according to the electron affinity rule\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Photogenerated electrons and holes migrate to the (101) and (001) surfaces at the facet interface due to the difference in their Fermi levels\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Despite the upward band bending in d-TiO\u003csub\u003e2\u003c/sub\u003e at the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e interface, some photo-excited electrons can readily transfer to the Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes due to the favorable energy difference between the conduction band of d-TiO\u003csub\u003e2\u003c/sub\u003e and the Fermi level of the MXenes. When the incident photon energy exceeds the semiconductor band gap, photoexcited electrons are promoted to high-energy states within the conduction band above the conduction band minimum\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. In our case, the photon energy is approximately 3.4 eV, which is well above the 3.2 eV band gap of TiO\u003csub\u003e2\u003c/sub\u003e. These electrons, generated on a femtosecond timescale, can transfer to the metallic side of the Schottky interface prior relaxing to the conduction band minimum. This phenomenon, in which electrons transfer from semiconductors to metals despite upward band bending, has been observed in semiconductor/noble metal systems, where the higher work function of noble metals compared to MXenes leads to even more pronounced upward band bending\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. In the present Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e system, the transferred electrons are subsequently trapped by the Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene due to the Schottky junction formed at the interface, which arises from the larger work function of the MXene compared to the TiO\u003csub\u003e2\u003c/sub\u003e(101) surface. This electronic structure promotes effective charge separation, while the MXene functions as a superior reduction co-catalyst, reducing the overpotential and activation energy of reduction redox reactions and efficiently consuming the accumulated electrons.\u003c/p\u003e\n\u003ch3\u003ePhotocatalysis\u003c/h3\u003e\n\u003cp\u003eBy leveraging the synergistic effect of charge separation and co-catalyst function, the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite self-assembled at pH 3 demonstrates a 147-fold increase in photocatalytic hydrogen evolution under UV irradiation compared to pristine d-TiO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). A similar enhanced level of activity was observed for the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite prepared at pH 4.5, which falls within the optimal pH range for FSEA (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Remarkably, the pristine d-TiO\u003csub\u003e2\u003c/sub\u003e exhibited enhanced photocatalytic activity across various reaction systems, surpassing commercially available TiO\u003csub\u003e2\u003c/sub\u003e benchmarks, including FP-6 (Showa Denko Ceramics) and the industry-standard P25 (Evonik), renowned for its high photocatalytic performance\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. In contrast, the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e assembly mixed at pH 1.5 exhibited no co-catalytic effect of Mo\u003csub\u003e4/3\u003c/sub\u003eC despite the much higher H\u003csup\u003e+\u003c/sup\u003e concentration. These photocatalytic results confirm that the co-catalytic effect of Mo\u003csub\u003e4/3\u003c/sub\u003eC can only be harvested when facet-selective assembly with d-TiO\u003csub\u003e2\u003c/sub\u003e occurs under an optimized surface charge configuration.\u003c/p\u003e\n\u003ch3\u003eStrategy of FSEA\u003c/h3\u003e\n\u003cp\u003eTo demonstrate the wider applicability of FSEA, we use two different metal oxide photocatalysts: BiVO\u003csub\u003e4\u003c/sub\u003e with a low PZC and Cu\u003csub\u003e2\u003c/sub\u003eO with a high PZC, compared to TiO\u003csub\u003e2\u003c/sub\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the selective deposition of Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes onto the (010) surface of single-crystal octahedral BiVO\u003csub\u003e4\u003c/sub\u003e, co-exposed with {010} and {110} facets at pH 1.5, and onto the (100) surface of tetradecahedral Cu\u003csub\u003e2\u003c/sub\u003eO, co-exposed with {100} and {111} facets at pH 11, via FSEA (Supplementary section S5). These results indicate that the {110} facets are electrostatically more positively charged than the {010} facets of BiVO\u003csub\u003e4\u003c/sub\u003e, and the {100} facets are more positively charged than the {111} facets of Cu\u003csub\u003e2\u003c/sub\u003eO, consistent with previous experimental reports\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, thus enabling selective assembly with the negatively charged MXenes.\u003c/p\u003e \u003cp\u003eWe propose a general strategy of FSEA for 2D co-catalysts and multi-faceted semiconductor particles with various combinations of PZCs. In this model, consider semiconductor facets of type A and B, where the PZC of A is larger than that of B, making facet A more positively charged. Two cases can be considered (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e):\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCase 1\u003c/strong\u003e \u003cp\u003eThe PZC of the 2D co-catalysts is lower than that of the single-crystal semiconductor (PZC\u003csub\u003e2D\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;PZC\u003csub\u003esemi\u003c/sub\u003e). In this case, negatively charged 2D co-catalysts are attracted to the A facets.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCase 2\u003c/strong\u003e \u003cp\u003eThe PZC of the 2D co-catalyst is higher than that of the single-crystal semiconductor (PZC\u003csub\u003esemi\u003c/sub\u003e \u0026lt; PZC\u003csub\u003e2D\u003c/sub\u003e). In this case, positively charged 2D co-catalysts are attracted to the B facets.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe absolute difference between the PZC values of the semiconductors and 2D co-catalysts should be larger than at least 1 on the pH scale, as the PZC difference between facets can be as large as 2\u0026ndash;3 units\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. FSEA would function at pH levels between PZC\u003csub\u003e2D\u003c/sub\u003e and PZC\u003csub\u003esemi\u003c/sub\u003e in both cases. However, the practical optimal range may be narrower, as demonstrated with TiO\u003csub\u003e2\u003c/sub\u003e, Cu\u003csub\u003e2\u003c/sub\u003eO, and BiVO\u003csub\u003e4\u003c/sub\u003e. This is because 2D co-catalysts and facets A or B should have opposite electrostatic charges high enough to attract each other, while the electrostatic potentials of A and B facets should differ enough to selectively attract the 2D co-catalysts. FSEA is broadly applicable to multi-faceted polyhedral semiconductors, including metal oxides, sulfides and perovskites, for efficient photocatalysis and solar cells\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan additionalcitationids=\"CR45\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. Additionally, a variety of 2D MXenes and other candidates, such as elemental 2D materials (e.g., graphene and goldene\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e), 2D transition-metal chalcogenides, and metal/covalent organic frameworks (MOFs/COFs), could also be utilized as low-dimensional co-catalysts.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWe demonstrate facet-selective electrostatic assembling of 2D Mo\u003csub\u003e4/3\u003c/sub\u003eC and decahedron TiO\u003csub\u003e2\u003c/sub\u003e single crystals terminated by well-defined {101} and {001} facets, where Mo\u003csub\u003e4/3\u003c/sub\u003eC is preferentially attached to TiO\u003csub\u003e2\u003c/sub\u003e(101) by adjusting the acidity of the aqueous solution. Mo\u003csub\u003e4/3\u003c/sub\u003eC is found to have an exceptionally low PZC of 0.82, whereas d-TiO\u003csub\u003e2\u003c/sub\u003e has a PZC of 6.65. As the TiO\u003csub\u003e2\u003c/sub\u003e(101) surface is expected to have a slightly higher PZC than the TiO\u003csub\u003e2\u003c/sub\u003e(001) surface, at pH levels below the PZC of d-TiO\u003csub\u003e2\u003c/sub\u003e, Mo\u003csub\u003e4/3\u003c/sub\u003eC is negatively charged while TiO\u003csub\u003e2\u003c/sub\u003e(101) is more positively charged than TiO\u003csub\u003e2\u003c/sub\u003e(001). Consequently, Mo\u003csub\u003e4/3\u003c/sub\u003eC exhibits a preferential affinity toward the TiO\u003csub\u003e2\u003c/sub\u003e(101) surface. The optimal pH level for combining 2D Mo\u003csub\u003e4/3\u003c/sub\u003eC and d-TiO\u003csub\u003e2\u003c/sub\u003e is 3. In this configuration, photogenerated electrons and holes move to the (101) and (001) facets, respectively. Moreover, Mo\u003csub\u003e4/3\u003c/sub\u003eC and likely other MXenes, effectively utilize the electrons accumulated due to the Schottky junction formed at the interface with the (101) surface, further promoting charge separation. By leveraging the synergistic effects of charge separation and co-catalyst functionality, the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e composite self-assembled via FSEA exhibits enhanced photocatalytic HER activity. This work widens the design space for composite photocatalysts by combining facet-engineered anisotropic semiconductors and 2D co-catalysts, optimizing them for efficient photocatalytic redox reactions.\u003c/p\u003e "},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003eSample preparation\u003c/h2\u003e \u003cp\u003ed-TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles were synthesized via a gas-phase reaction process\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Single-crystal tetradecahedral-Cu\u003csub\u003e2\u003c/sub\u003eO particles, co-exposed with {100} and {111} facets, were synthesized via a controlled reduction of copper salts in an alkaline aqueous solution. First, 780 mg of CuCl\u003csub\u003e2\u003c/sub\u003e and 780 mg of Cu(CO\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e were dissolved in 500 ml of deionized (DI) water maintained at 55\u0026deg;C under continuous stirring. A separate solution of 4 g NaOH in 40 ml DI water was prepared. Upon complete dissolution of the copper salts, the temperature of the solution was raised to 75\u0026deg;C, and the NaOH solution was introduced dropwise, inducing a color change to turquoise. Following 5 min. of stirring, 4 g of D-(+)-glucose was added to initiate the reduction for 30 min., after which the resulting Cu\u003csub\u003e2\u003c/sub\u003eO powder was collected via vacuum filtration, washed with DI water and ethanol (50 ml each), and dried at 60\u0026deg;C overnight. Single-crystal octahedral BiVO\u003csub\u003e4\u003c/sub\u003e particles, co-exposed with {010} and {110} facets, were synthesized using a microwave-assisted method. In a 30 ml microwave vial, 600 mg of Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;5H\u003csub\u003e2\u003c/sub\u003eO was dissolved in 2 ml of DI water with the addition of 50 \u0026micro;l of concentrated HNO\u003csub\u003e3\u003c/sub\u003e. Concurrently, 150 mg of NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e was dissolved in 2.5 ml of DI water at 90\u0026ndash;95\u0026deg;C. The Bi(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e solution was then slowly added to the NH\u003csub\u003e4\u003c/sub\u003eVO\u003csub\u003e3\u003c/sub\u003e solution, and the mixture was heated to 190\u0026deg;C for 5 min. in a microwave reactor. The resulting BiVO\u003csub\u003e4\u003c/sub\u003e was thoroughly washed with 50 ml of DI water and dried at 60\u0026deg;C overnight. 2D Mo\u003csub\u003e4/3\u003c/sub\u003eC MXene flakes were produced by etching a 3D atomic laminate of (Mo\u003csub\u003e2/3\u003c/sub\u003eSc\u003csub\u003e1/3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eAlC, followed by delamination, as previously reported\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Only small-sized flakes were used for the FSEA. The assembly of Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e was prepared by mixing d-TiO\u003csub\u003e2\u003c/sub\u003e with 5 wt.% Mo\u003csub\u003e4/3\u003c/sub\u003eC in DI water at various pH levels with no light exposure. The pH of the mixture solution was adjusted by using diluted HNO\u003csub\u003e3\u003c/sub\u003e and NaOH. The resulting solution was ultrasonicated for 10 min.\u003c/p\u003e \u003cp\u003eFor electron microscopy and spectroscopy measurements, samples were prepared by drop-casting the solution onto Si substrates for SEM, UPS, and XAS-PEEM and onto TEM grids with a 10 nm thick amorphous carbon support membrane for STEM-EDX/EELS at room temperature. Commercial MoO\u003csub\u003e2\u003c/sub\u003e and MoO\u003csub\u003e3\u003c/sub\u003e (Sigma-Aldrich) were used as reference materials for STEM-EELS measurements.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e\n\u003ch3\u003eZeta potential analysis\u003c/h3\u003e\n\u003cp\u003eThe surface zeta potentials of the aqueous solutions containing MXene (50 \u0026micro;g/mL) and d-TiO\u003csub\u003e2\u003c/sub\u003e (250 \u0026micro;g/mL) as a function of pH were measured using dynamic light scattering (DLS) with a Zetasizer Nano-ZS90 (Malvern Instruments) in DTS1070 capillary cells (Malvern instruments). The zeta potential was calculated based on electrophoretic mobility, which was determined by the electrophoretic light scattering technique of Zetasizer. For each pH value, three independent measurements of the zeta potential were performed, and the mean values were plotted. The pH of the prepared solutions was adjusted using HNO\u003csub\u003e3\u003c/sub\u003e and KOH, with pH levels monitored by a SevenCompact pH/Ion meter (S220 Mettler Toledo). Prior to the measurements, the electrode was calibrated using three technical buffer solutions at pH 2, 7, and 11.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eElectron microscopy and spectroscopy\u003c/h2\u003e \u003cp\u003eSEM imaging of the prepared samples was performed using a LEO 1550. Annular dark-field (ADF)-STEM images, EDX maps, and EELS spectra were acquired using a Thermo Fisher Scientific Titan cube G2 60\u0026ndash;300, equipped with a Schottky electron source, a monochromator, a spherical aberration corrector (DCOR, CEOS) for the probe-forming lens system, a Super-X system for EDX, and a GATAN Quantum 965 imaging filter for EELS. The microscope was operated at an accelerating voltage of 300 kV. For EELS, the energy resolution, measured from the full width of half-maximum of the zero-loss peak (ZLP), was 0.3 eV. The pixel sizes for STEM-EELS mapping were on the nanometer scale for pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC and on the subnanometer scale for the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e(101) interfaces. The typical acquisition time per pixel was 0.1 s. The STEM-EELS spectra presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e were obtained by accumulating signals from multiple regions for the pristine Mo\u003csub\u003e4/3\u003c/sub\u003eC and the Mo\u003csub\u003e4/3\u003c/sub\u003eC/d-TiO\u003csub\u003e2\u003c/sub\u003e(101) interfaces (Supplementary section S6).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic hydrogen evolution\u003c/h2\u003e \u003cp\u003eThe hydrogen evolution experiments were conducted in a slurry-type, water-cooled reactor illuminated from the side by a monochromatic UV LED light source centered at 365\u0026thinsp;\u0026plusmn;\u0026thinsp;6 nm (SOLIS, Thorlabs), with an incident light intensity of 576 mW (power density of 183 mW/cm\u003csup\u003e2\u003c/sup\u003e). The experiments were carried out in batch-type mode, with the reaction solution maintained at 15\u0026deg;C under constant stirring at 650 rpm throughout the experiment. A stock solution of the catalyst was prepared by mixing 90 mL of an aqueous HNO\u003csub\u003e3\u003c/sub\u003e solution at pH 1.5, 3, and 4.5 with 10 mL of HPLC-grade methanol, followed by a re-adjustment of the pH to 1.5, 3, and 4.5 using diluted HNO\u003csub\u003e3\u003c/sub\u003e. 80 mL of the obtained solution was mixed with 80 mg of d-TiO\u003csub\u003e2\u003c/sub\u003e and ultrasonicated for 10 minutes to achieve a homogenous suspension. To this, 220 \u0026micro;L of the fresh Mo\u003csub\u003e4/3\u003c/sub\u003eC solution (approx. concentration of 9 mg mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), corresponding to approximately 2.5 wt.% MXene in TiO\u003csub\u003e2\u003c/sub\u003e, was added to enable uniform deposition of Mo\u003csub\u003e4/3\u003c/sub\u003eC flakes onto the (101) surface of d-TiO\u003csub\u003e2\u003c/sub\u003e via FSEA. For each HER experiment, 2 mL of the d-TiO\u003csub\u003e2\u003c/sub\u003e/Mo\u003csub\u003e4/3\u003c/sub\u003eC solution mixture was transferred into the reactor and purged with Ar (flow rate of 10 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) to remove dissolved oxygen. During light irradiation, the reactor was sealed to maintain an airtight environment. The headspace was probed every 30 minutes using a gas-tight syringe and analysed by gas chromatography (Shimadzu GC-2030, equipped with a barrier discharge ionization detector). A 5 point calibration profile was used to accurately quantify the amount of hydrogen evolved, and the results were translated into mole values as presented in the main text.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eSynchrotron-based XAS-PEEM measurements\u003c/h2\u003e \u003cp\u003eThe XAS-PEEM measurements were conducted using the AC-SPELEEM (Elmitec GmbH) endstation of MAXPEEM beamline at MAX IV Laboratory, Sweden. Detailed descriptions of the analysis setup can be found in reference\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. During the measurement, a stack of PEEM images was acquired with a photon energy increment of 0.2 eV across the Mo-M\u003csub\u003e2,3\u003c/sub\u003e edges, where 3p electrons are excited to unoccupied valence band states. All PEEM images were captured using secondary photoelectrons with a kinetic energy of 1.9 eV. At this energy, photoelectrons from both the top and side facets could pass through the angle-limited aperture of the microscope, enabling simultaneous observation of these facets in the PEEM images. Given that only a subset of the secondary photoelectrons was collected, the resulting XAS data represent a partial electron yield. The acquired PEEM images were drift-corrected using the Template Matching plugin in FiJi\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e,\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. The final XANES spectra were subsequently background-subtracted and normalized.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this work are available within the article, the corresponding Supplementary Information and the public data repository.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFujishima, A., Zhang, X. \u0026amp; Tryk, D. A. TiO\u003csub\u003e2\u003c/sub\u003e photocatalysis and related surface phenomena. Surface Science Reports 63, 515\u0026ndash;582 (2008).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRyan, A. J. \u0026amp; Rothman, R. H. Engineering chemistry to meet COP26 targets. Nature Reviews Chemistry 6, 1\u0026ndash;3 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCorby, S., Rao, R. R., Steier, L. \u0026amp; Durrant, J. R. 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Template Matching and Slice Alignment. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://sites.google.com/site/qingzongtseng/template-matching-ij-plugin.\u003c/span\u003e\u003cspan address=\"https://sites.google.com/site/qingzongtseng/template-matching-ij-plugin.\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"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-5717389/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5717389/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis work demonstrates an electrostatic assembly strategy for the facet-selective deposition of two-dimensional (2D) transition metal carbides (MXenes) onto anisotropic single-crystal semiconducting metal oxides. By precisely controlling the solution pH, we modulated the surface charge of the MXenes and the distinct crystallographic facets of the metal oxides, enabling selective deposition driven by electrostatic attraction. Specifically, negatively charged Mo\u003csub\u003e4/3\u003c/sub\u003eC MXenes were selectively deposited on the electron-rich (101) surface of TiO\u003csub\u003e2\u003c/sub\u003e exposed with {101} and {001} facets at pH 3, the (100) surface of Cu\u003csub\u003e2\u003c/sub\u003eO, exposed with {100} and {111} facets at pH 11, and the (010) surface of BiVO\u003csub\u003e4\u003c/sub\u003e, exposed with {010} and {110} facets at pH 1.5. The high degree of facet selectivity was confirmed through a combination of advanced techniques, including electron microscopy, electron spectroscopy, and synchrotron-based spectromicroscopy. This selective interfacial engineering promotes spatially separated charge carrier migration towards distinct facets of the oxides, while Schottky barriers form at the MXenes/oxides interfaces, further enhancing charge separation. The MXenes act as efficient reduction co-catalysts, facilitating the rapid consumption of electrons trapped at the Schottky barriers, thereby enhancing photocatalytic hydrogen evolution.\u003c/p\u003e","manuscriptTitle":"Facet-selective electrostatic assembling of 2D MXene onto anisotropic single-crystal metal oxides for enhanced photocatalysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-14 09:22:01","doi":"10.21203/rs.3.rs-5717389/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":"ad3815dc-57e9-434a-9474-1e997db3341c","owner":[],"postedDate":"January 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":42777899,"name":"Physical sciences/Materials science/Nanoscale materials/Structural properties"},{"id":42777900,"name":"Physical sciences/Materials science/Nanoscale materials/Synthesis and processing"}],"tags":[],"updatedAt":"2025-02-21T10:55:27+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-14 09:22:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5717389","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5717389","identity":"rs-5717389","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}