Leveraging crystal defect modulation of Zirconium nano-MOF for enhanced water vapour confinement and photocatalytic behaviours

Leveraging crystal defect modulation of Zirconium nano-MOF for enhanced water vapour confinement and photocatalytic behaviours | 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 Research Article Leveraging crystal defect modulation of Zirconium nano-MOF for enhanced water vapour confinement and photocatalytic behaviours Liang Ying Ee, Sean Yi Rong Chia, Regina Pei Woon Tan, Qian Ma This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6727927/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 Metal-organic frameworks (MOFs) have emerged as promising materials for various applications, including photocatalysis and moisture harvesting. This present study undertakes an analytical exploration of the impact of acid modulation on the structural and functional attributes of amine-functionalized UiO66 (UiO66NH 2 ), focusing predominantly on improving its photocatalytic proficiency and efficiency in water vapor confinement. A crucial highlight of the study was the ideal crystallization of UiO66NH 2 nanoparticles achieved with the use of 150 equivalents of acetic acid modulator. Empirical findings have shown more than a significant escalation in water vapor adsorption capabilities presented by these nanoparticles. The minimal hysteresis observed in the process underscores the potential these particles hold in being utilized fruitfully for moisture harvesting applications. In addition to these, the prepared MOF nanoparticles showcase exceptional performance in the degradation of Rhodamine B dye, primarily attributable to the optimal visible light bandgap and extraordinary stabilizing capacity of superoxide radicals. The results derived from this study accentuate the integral role acid modulation can play in specifically tailoring the properties of MOFs to suit dedicated applications. It provides a meaningful perspective and understanding that can guide the future design of advanced MOF-based materials. Figures Figure 1 Figure 2 Figure 3 Figure 4 1. INTRODUCTION The tuneable and large surface area, porosity and abundance of functional sites ensue from rational fabrication design and precise control of reaction variables favour metal-organic frameworks (MOFs) to be effective adsorbents for sensing and environmental remediation applications. 1–4 Besides, many researchers have uncovered the excellent homogeneity and water stability of MOFs in composites and extended their applications in separation membranes and photocatalytic decontamination. 5–10 Zirconium-based (Zr-) MOFs represent one of the popularly studied subclasses that are formed by six Zr nodes each in tetravalent oxidation state and coordinated with oxo-, hydroxo- and carboxylate ligands. 11 The most common Zr-MOFs have been designated UiO-66 to UiO-68 (UiO refers to University of Oslo), differing in the dicarboxylate linkers. 12, 13 Despite the wide environmental application of Zr-MOFs by exploiting a range of synthetic modifications, 14 their preparation process is still difficult to reproduce and replicate due to the lack of knowledge about structure-activity relationships. Moreover, the mechanisms are different in every environmental application, while most literature perfunctory reports their large surface area and porosity for adsorption applications. Advanced photocatalytic water treatment processes aim to overcome roadblocks including the need for high-energy light irradiation at ultraviolet wavelengths and low specificity in degrading targeted contaminants. 15 However, the yield of these unstable radicals is highly reliant on the photocatalytic material, its energy band gap, and the possibility of electron-hole recombination. 15, 16 Today, tailored MOF with high surface area and porosity has been considered to be a good alternative to metal oxides (e.g., TiO 2 ) for enhanced photocatalytic activity and leading to innocuous degraded products. Despite potentially being unstable in water due to the weaker bond between the metal nodes and organic linkers, 17 the MOF photocatalytic property and porosity can be easily designed and controlled in-situ synthesis or post-synthesis. Numerous procedures have been reported for the preparation of photocatalytic doped MOFs for the visible-light degradation of rhodamine B, 18 methylene blue, and tetracyclines. 19, 20 Tailoring the optical absorption properties of MOFs has been evidenced to be an important consideration for photocatalytic applications, which typically employ doping or increments of π-conjugation along organic linkers. 21 Nonetheless, when looking beyond linker manipulation, there is a research gap on the effect of crystal growth and morphology on the optical absorption properties. Structural modification and defect modulation is an important engineering approach to modifying the performance behaviours, and physicochemical properties of MOFs. 22–24 Acid modulators mainly act as the capping, deprotonating, or reactive agent to introduce defect sites arising from missing linkers or metal clusters. These defects are known to alter pore size, binding energy, wettability, electronic band gap, and catalytic activity. 25, 26 In the preparation of highly porous UiO-66(Zr), monocarboxylic acids such as formic acid have been shown to coordinatively reduce 4 linkers in a missing cluster defect, which shows a larger influence on increasing the specific surface area than a missing linker defect. 27, 28 The modulated UiO-66(Zr) crystal has reported an enormously large surface area of about 1,800 m 2 g − 1 , which resulted from the low pK a value of the acid modulator that readily dissociates and hinders deprotonation of ligands. Nonetheless, the presence of water content in precursor materials can also lead to crystal defects as it becomes difficult for the ligands to displace the coordinated hydroxyl ions and water molecules. 29, 30 Therefore, high controllability of water and moisture content during synthesis is essential to ensure reproducibility. Various applications of the modulation approach in the design of suitable MOF crystals have been demonstrated, ranging from fuel storage to the catalytic degradation of environmental pollutants. 31–33 Despite the growing studies on crystal defect engineering, there is still an ongoing controversy about the modulated crystallization mechanism while ensuring the electroneutrality condition through ligand substitution. The high photocatalytic activity of MOF from modulated synthesis has been demonstrated in many literature articles reporting on the photoreduction of CO 2 , the photodegradation of environmental contaminants, and other energy conversion applications. 34–36 Du et al. have discussed the feasibility of using an acetic acid-modulated UiO66NH 2 (Zr/Hf) membrane to reduce carcinogenic hexavalent chromium into less toxic trivalent chromium under sunlight irradiation, achieving a photoreduction efficiency of 98% in 120 minutes. 37 Although it has great potential as a facile reusable solution to remove pollutants under photocatalytic effect, the question of how the modulation strategy can improve the rate of photoreduction with high reproducibility is the area to which this research now turns. Convincing results have been reported on the precise modulation of the binding energy of the first adsorbed water molecule and the water uptake behaviour of MOF crystals. 38–41 Particularly, different acid-modulated Zr-MOFs are more efficient and stable as water adsorbent materials with high reversibility in the adsorption-desorption cycle, making them suitable for applications in aqueous or high humidity environments. 42–44 However, further investigation is needed to reveal how the modulator amount influences the water uptake behaviour with careful reticular design. Therefore, herein the present study, we investigate the effect of the acid modulator on the structure and morphology of UiO66NH 2 (Zr) MOF nanocrystals. Besides the study on water adsorption-desorption behaviour, the influence of the amount of acid modulator on the MOFs’ photocatalytic activity in the degradation of environmental pollutants is further examined. This will better shed a new perspective on the modulation approach to the design and construction of MOF nanomaterials for advanced photocatalytic and water sorption applications. 2. EXPERIMENTAL PROCEDURE 2.1 Chemicals Zirconium (IV) chloride (ZrCl 4 , ≥ 99.5% trace metals basis), 2-aminoterephthalic acid (ATA, 99%), glacial acetic acid (ReagentPlus®, ≥ 99%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), barium sulfate (99.99% trace metals basis), rhodamine B (HPLC, ≥ 95%), sodium deuteroxide (40 wt. % in D 2 O, 99.5 atom % D), and SnakeSkin™ dialysis tubing (10 K MWCO, 35 mm) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The ultrapure water (UPW) used was produced by Milli-Q® water purification system (EMD Millipore Co., Billerica, MA, USA). 2.2 Synthesis of UiO66NH 2 Zr-MOF ZrCl 4 (1.03 mmol, 0.24 g), ATA (1.05 mmol, 0.19 g) and 1.8 mL glacial acetic acid were added into a 250-mL beaker containing 60 mL of DMF. The mixture was stirred for 30 min before being transferred into a Teflon-lining stainless steel hydrothermal flask for autoclave at 120 o C for 24 h. The synthesized material was centrifuged at 8,000 rpm for 8 min and washed three times with fresh DMF. Finally, the washed UiO66NH 2 was dialyzed with UPW using 10 K MWCO SnakeSkin™ dialysis tubing over three days before lyophilization. UiO66NH 2 synthesized with different crystallization times (24, 48 and 72 h) during the autoclave process were synthesized to determine the optimal reaction time. Large pore size and pore volume were achieved with 72 h crystallization time based on Figure S1 .1a . The modulator effect on synthesized UiO66NH 2 crystal morphologies and properties was investigated. The synthesis procedure was repeated with different molar equivalents of the glacial acetic acid modulator (i.e., 0, 50, 100, 150 and 200 eq). The prepared UiO66NH 2 samples are then labelled according to the molar equivalence of modulator (e.g., UiO66NH 2 _50eq refers to UiO66NH 2 prepared with 50 eq of modulator). 2.3 Characterization techniques of synthesized UiO66NH 2 Zr-MOF The UiO66NH 2 samples were dried in the DZ-1BC vacuum oven at 115 o C overnight before characterization. Attenuated Fourier-Transform Infrared (ATR-FTIR) spectroscopic analysis was performed with IR Prestige-21 spectrophotometer (Shimadzu Co., Kyoto, Japan) between 4,000 cm − 1 and 500 cm − 1 wavenumbers for 40 scans and 4 cm − 1 resolution. Powder XRD (PXRD) analysis was performed using D8 Advance X-ray diffractometer (Bruker Daltonics Inc., Billerica, MA, USA) with Cu-Kα radiation (λ = 0.15418 nm) at a voltage of 30 kV and current of 10 mA. PXRD data were collected in a 2θ range from 3 o to 50 o at an increment of 0.03 o s − 1 . The interplanar spacing and crystallite size are calculated using Equations 2.1–2.2 . \(\:{d}_{hkl}\:=\frac{{\lambda\:}}{2\text{sin}\theta\:}\) , derived from Bragg’s Eq. (2 .1) d hkl is interplanar spacing (Å), \(\:{\lambda\:}\) is Cu K α X-ray wavelength (1.5406 Å), θ is Bragg’s angle ( o ) \(\:D\:=\frac{\text{K}{\lambda\:}}{\beta\:\text{cos}\theta\:}\) , Scherrer Eq. (2 .2) D is crystallite size (nm), K is Scherrer constant (0.94 for spherical crystallites), \(\:{\lambda\:}\) is Cu K α X-ray wavelength (1.5406 Å), \(\:\beta\:\) is line broadening at FWHM (radians), θ is Bragg’s angle ( o ) Small-angle X-ray scattering (SAXS) was performed on MOF samples with Xeuss 2.0 system (Xenocs, France) using Cu Kα radiation (λ = 0.15418 nm) and point collimation at sample-to-detector distances of 0.1652 m and 1.195 m. 1 H NMR was recorded on 600 MHz Varian NMR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) between − 2 ppm and 14 ppm for 32 scans. Before NMR analysis, MOF samples (15.0 mg) were vortexed and incubated in 1.2 mL of 1 M NaOD/D 2 O overnight. Malvern Zetasizer Nano (Malvern Panalytical Ltd., Malvern, WR, UK) was used to measure the hydrodynamic size distribution and surface zeta potential of the MOF samples at 25 o C. Before analysis, the MOF samples were dispersed and sonicated in UPW at 0.1 wt% and filtered through a 0.45-µm hydrophilic PTFE syringe filter. Brunauer-Emmett-Teller (BET) analysis to determine the total surface area and pore size probed by nitrogen gas adsorbate were performed using ASAP-2020 (Micromeritics Instrument Co., Norcross, GA, USA). Prior to BET analysis, MOF samples were degassed for 12 h at 120 o C and residual pressure of at least 8 x 10 − 3 mm Hg. The thermal stability of the samples was characterized by thermal gravimetric analysis (TGA) using Discovery TGA 5500 (TA Instruments, New Castle, DE, USA) from 25 o C to 600 o C at a heating rate of 10 o C min − 1 under nitrogen gas flow. TGA was also performed to quantify the defect level of the MOF samples wherein the samples were heated from room temperature to 105 o C and isothermally for 2 min before heating to 850 o C at a heating rate of 5 o C min − 1 under an airflow of 50 mL min − 1 . The structural defects of the samples were calculated using Equations 2.3 – 2.6 . TGA coupled with Pfizer mass spectrometer (TGA-MS, Mettler Toledo International Inc., Columbus, OH, USA) was performed between room temperature and 600 o C in a nitrogen flow at a heating rate of 10 o C min − 1 . The mass spectrometry analysis was set up to detect OH + , H 2 O + , [CH 3 CO] + , CO 2 + and NO 2 + and their integrals compared semi-quantitatively. $$\:\raisebox{1ex}{$Ligands$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.=\frac{Exp\:dehydr.\:mass\%\:-\:Final\:mass\%}{Ideal\:dehydr.\:mass\%}$$ 2.3 \(\:\raisebox{1ex}{$Ligands$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.\) is the experimentally determined average number of ligands per formula unit. $$\:\raisebox{1ex}{$Defects$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.=6\:-\raisebox{1ex}{$Ligands$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.$$ 2.4 \(\:\raisebox{1ex}{$Defects$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.\) is the experimentally determined average number of ligand defects per formula unit. $$\:\%\:Defects=\left(\frac{\raisebox{1ex}{$Defects$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.}{6}\right)\times\:100\%$$ 2.5 $$\:Theoretical\:dehydr.\:mass\:\%\:with\:defects\:=\frac{a\times\:{MW}_{L}+(6+(6-a\left)\right)\times\:15.999+(6\times\:91.224)}{(6\times\:123.223)}\times\:100\%$$ 2.6 Theoretical dehydration mass % with defects is the theoretical mass % of desolvated and dehydrated MOF calculated using number of calculated ligand defects and balancing charge of missing ligands with oxide. $$\:a\:=\raisebox{1ex}{$Ligands$}\!\left/\:\!\raisebox{-1ex}{$fu$}\right.\:,\:{MW}_{L}\:=molar\:mass\:of\:ligand\:(i.e.\:179.15\:g\:{mol}^{-1})$$ JEOL JSM-6701F Field Emission Scanning Electron Microscope (FESEM) coupled with JEOL JED-2300F Energy Dispersive X-ray Spectrometer (EDX) (JEOL Co., Akishima, Japan) was used to image and conduct elemental analysis on the MOF samples. Samples were sputter-coated with platinum at 20 mA for 30 s before imaging. For transmission electron microscopy (TEM) imaging with JEOL JEM-3011 TEM (JEOL Co., Akishima, Japan), 0.1% w/v MOF samples were first dispersed and sonicated in ethanol before introducing onto carbon-coated copper TEM grid (300-mesh) and dried overnight. X-ray photoelectron spectroscopy (XPS) of powdered samples on copper tape was performed with Kratos Axis Ultra DLD (Shimadzu Co., Kyoto, Japan) using Al Kα irradiation operated at a residual vacuum of below 2.0 x 10 − 9 mbar. All characterizations were repeated at least thrice to ensure reproducibility. 2.4 Water confinement analysis of synthesized UiO66NH 2 Zr-MOF All MOF samples were vacuum-dried overnight at 120 o C prior to water sorption analysis using the Aquadyne DVS water sorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The samples were first equilibrated at 66 o C and 0.5% relative humidity before their initial mass was measured. Thereafter, their masses were measured at every 10% increment in relative humidity up to 90% before being reduced to 10% while the temperature was maintained at 25.7 o C. The mass difference was calculated as the amount of water adsorbed or desorbed from the MOF samples. A combined TGA and differential scanning calorimetry (DSC) method was performed to evaluate the heat enthalpy of water desorption. All MOF samples were vacuum dried overnight at 120 o C before being stored in a humidity and temperature-controlled enclosure at 60% relative humidity and 25 o C for 24 h. Approximately 10 mg of the moisture-saturated samples were immediately transferred to both TGA and DSC under atmospheric conditions for analysis. The conditions in TGA and DSC were kept the same wherein the temperature was ramped from 40 o C to 80 o C at a heating rate of 0.5 o C min − 1 in nitrogen flow of 30 mL min − 1 . The temperature was then cooled to 40 o C before heating back to 80 o C at a heating rate of 1 o C min − 1 in nitrogen flow. An Aluminium DSC sample pan with a pinhole in the lid was used to facilitate the removal of moisture vapour during desorption. The heat enthalpy of water desorption by the MOF samples was then calculated using Equations 2.7 –2.9. All experiments were repeated at least three times to ensure reproducibility. $$\:Water\:content\:\left(g\:{g}^{-1}\:MOF\right)\:=\frac{Mass\:loss\:from\:dehydration\:\left(g\right)}{Amoung\:of\:MOF\:in\:TGA\:\left(g\right)}$$ 2.7 $$\:{\varDelta\:H}_{water\:desorption}\:\left(kJ\:{g}^{-1}\right)\:=\frac{{\varDelta\:H}_{enthalpy\:dehydr.}\:\left(kJ\:{g}^{-1}\right)}{Water\:content\:\left(g\:{g}^{-1}\right)}$$ 2.8 \(\:{\varDelta\:H}_{water\:desorption}\:\left(kJ\:{mol}^{-1}\right)\:=\) \(\:{\varDelta\:H}_{water\:desorption}\:\left(kJ\:{g}^{-1}\right)\:\times\:\:18.02\:g\:{mol}^{-1}\) (2.9) 2.5 Photocatalytic degradation of RhB dye with synthesized UiO66NH 2 Zr-MOF Solid-state UV-Vis diffuse reflectance (DRS) spectroscopy was performed using UV-2600i (Shimadzu Co., Kyoto, Japan) to determine the optical band gap of the MOF samples by Tauc plot derived from the DRS spectrum. MOF sample (20 mg) was added to barium sulfate (400 mg) before grinding the solid mixture with a mortar and pestle for DRS measurement. Oxygen vacancy in the defect MOF samples was examined with steady-state electron paramagnetic resonance (EPR) spectroscopy upon UV irradiation using the JEOL FA200 EPR spectrometer (JEOL Co., Akishima, Japan). The EPR was carried out in the X band (9.5 GHz) for 20 scans ranging from 0 to 600 mT on each MOF sample at room temperature. The photocatalytic activity of MOF samples was evaluated based on their degradation efficiency towards the rhodamine B (RhB) dye. The MOF sample (15 mg) was loaded into a 100-mL beaker containing 50 mL of RhB aqueous solution (10 mg L − 1 ). The suspension was magnetically stirred in the dark for 30 min to reach adsorption-desorption equilibrium before being irradiated under a 300 W xenon lamp at a fixed distance of 120 mm. 1 mL of the suspension was withdrawn at regular intervals of 10 min, including the initial suspension before irradiation, and filtered using a 0.22-µm hydrophilic PTFE syringe filter. The photocatalytic experiment was stopped after 80 min and the absorbance of the filtered solution was measured using a Cary 60 UV-Vis spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA at 554 nm, which corresponds to λ max of the RhB dye. The UV photocatalytic degradation of the RhB dye by the MOF samples is expressed as C/C 0 , where C is the absorbance of the withdrawn solution after irradiation at a given point and C 0 is the absorbance of the RhB dye at the initial concentration. All experiments were repeated at least three times to ensure reproducibility. 3. RESULTS AND DISCUSSION 3.1 Modulator effect on morphological characteristics of synthesized UiO66NH 2 Figure S1 .1a-e present a series of characterization outcomes for the synthesized UiO66NH 2 . As observed in the N 2 adsorption-desorption isotherm at 77 K, the unmodulated UiO66NH 2 _0eq exhibits a type-II isotherm, along with a total BET surface area of 380.7 m 2 g − 1 (with the micropore area equalling 103.1 m 2 g − 1 ) and a type-C hysteresis loop pattern, suggesting the presence of slit-shaped pores with an average diameter of 1.9 nm. The modulation in the synthesis process results in an increased micropore surface area of 308.9 m 2 g − 1 for UiO66NH 2 _150eq, and an average pore diameter of 2.8 nm. An assessment of the zeta potential of the synthesized UiO66NH 2 revealed an upward trend, increasing from − 17.4 mV in UiO66NH 2 _0eq to 14.1 mV in UiO66NH2_200eq. These findings are consistent with the work of Morris et al. (2017) who also noted an acid-promoted synthesis of UiO66. 46 This further implies that the modulation process introduced more surface defects and improved colloidal stability. Some of the 2-aminoterephthalic acid organic linkers were also substituted with the acetic acid modulator, a change detected in the ATR-FTIR spectra (the C = O vibration of acetic acid being identifiable at 1,740 cm − 1 ). Furthermore, 1 H qNMR was employed to quantify the quantity of organic linkers replaced, with UiO66NH 2 _200eq reaching a modulator:linker ratio of 0.34. Figures S1 .2 - S1.3 and Table S1 .2 show both TEM and SEM micrographs of the synthesized UiO66NH 2 . Modulation resulted in the formation of larger nanoparticle sizes from 13.7 nm in UiO66NH 2 _0eq to 39.0 nm in UiO66NH 2 _150eq due to the intergrown and aggregation into ortho-octahedral crystals. Table 1 PXRD analysis results of UiO66NH 2 samples. Sample Crystallite Size (nm) Crystallinity (%) 2θ 111 / o d-spacing 111 (nm) 2θ 200 / o d-spacing 200 (nm) UiO66NH 2 _0eq 91.4 80.2 7.42 1.19 8.60 1.03 UiO66NH 2 _50eq 50.2 88.5 7.22 1.22 8.29 1.07 UiO66NH 2 _100eq 37.5 90.1 7.16 1.23 8.29 1.07 UiO66NH 2 _150eq 40.9 95.7 7.19 1.23 8.32 1.06 UiO66NH 2 _200eq 44.1 95.6 7.19 1.23 8.29 1.07 The pore structure and morphological information of as-synthesized UiO66NH 2 under acid modulation (0, 50, 100, 150, and 200 molar equivalence of glacial acetic acid) was characterized by both SAXS and PXRD and the results are presented in Fig. 1 and Table 1 . The recorded SAXS 2D-scattering patterns of the UiO66NH 2 nanoparticles reveal that the intensities of the lattice fringes increased with modulation (Fig. 1 a-e), corresponding to increased crystallinity. This is in good agreement with the calculated crystallinity index from PXRD analysis, observing a significant increase from 80.2% in UiO66NH 2 _0eq to 95.7% in UiO66NH 2 _200eq. One of the key findings is that two distinct peaks in the corresponding wide-angle X-ray scattering (WAXS) region at q = 4.51 nm − 1 and 4.86 nm − 1 are observed from the 1D Guinier-Porod intensity profile in Fig. 1 f. The calculated peak positions and their corresponding d-spacings of 1.39 nm and 1.29 nm are consistently close to and assigned as the PXRD (111) and (200) indices, respectively (Fig. 1 g). Additionally, the WAXS profile shows that the intensities of the two featured peaks increase with acid modulation because of a predicted increase in specific surface area and pore volume. 47 It was further found that the major XRD diffraction peaks of all UiO66NH 2 samples were well consistent with the simulated literature pattern, 48 and the calculated crystallite size was reduced by approximately 55% with 150 equivalents of acetic acid modulation. Explicitly, the presence of broad XRD peaks in the 2θ range of 5.2–9 o is an indication of missing-linker defects in the MOF nanocrystals, 49 which were observed with UiO66NH 2 _0eq and UiO66NH 2 _200eq samples. Moreover, the quantification of the missing-linker defects was achieved through TGA analysis ( Figure S1 .4 and Table S1 .3 ). The outcome of this defect analysis through TGA reveals that UiO66NH 2 _0eq and UiO66NH 2 _200eq displayed missing-linker defects of 16.8% and 25.2%, respectively, with ideally six ligands per formula unit of the MOF nanocrystal. These results correspond closely with the findings derived from SAXS/WAXS profiling. Contrarily, UiO66NH2_150eq displays the minimum defect (11.2%) and the maximum degree of crystallinity. When considering the ratio between the modulator and the linker, which was calculated using qNMR, along with the evaluation of missing-linker defects derived from TGA, it becomes anticipate that unmodulated UiO66NH 2 has the highest content of 2-aminoterephthalate. In comparison, UiO66NH 2 _200eq appears to possess the lowest content of this organic linker. These anticipated results could potentially be validated by referring to the analysis results produced from a combination of TGA-MS. A summary of these results has also been displayed in Figure S1 .5 and Table S1 .4 . 3.2 Modulator effect on water confinement behaviour of synthesized UiO66NH 2 Water vapour sorption behaviour is undeniably vital to study before selecting materials for water purification processes. Apart from that, their stability in water is a critical property to be considered along with the water loading capacity. The shape of the water vapour isotherm can also provide important information on the hydrophilicity and water transport mechanism of the material. 50 Fig. 2 a-b illustrate the well-defined water adsorption-desorption isotherms carried out at 25.7 o C for all water-stable UiO66NH 2 samples. In particular, the unmodulated UiO66NH 2 _0eq shows a different water sorption behaviour compared to the modulated crystals wherein it exhibits a convex type III reversible adsorption isotherm, 51 which means that there was weak adsorbent-adsorbate interaction. On the other hand, the acid-modulated UiO66NH 2 has strong adsorbent-adsorbate interactions that resulted in type II isotherms through monolayer and multilayer pore filling mechanisms. 52 UiO66NH 2 _150eq and UiO66NH 2 _200eq specifically show rapid uptake of water content reaching saturation. Little hysteresis is observed for the modulated samples, while a large hysteresis between 0.5–0.8 P/P 0 is seen during water vapour desorption for the unmodulated crystal, which is associated with capillary condensation and hydrophobicity. In light of the reported results from water vapour sorption, it is conceivable that the introduction of acid modulation can lead to more hydrophilic UiO66NH 2 due to the replacement of aminoterephthalate with acetate linker. Very good agreement is observed when referenced to the heat of adsorption determined by the combined TGA/DSC method (Fig. 2 e). Both UiO66NH 2 _150eq and UiO66NH 2 _200eq with a high degree of modulation found that their exothermic heats of adsorption are 23.9 kJ mol − 1 and 26.2 kJ mol − 1 , respectively, twice that of the unmodulated UiO66NH 2 _0eq, suggesting that acid modulation in the synthesis of MOF thermodynamically favour water uptake. However, the heat of adsorption calculated in this work concurs with some studies but is lower than the value previously reported by others possibly due to the unconventional analysis method adopted here. 53–55 The postulation that water uptake and hydrophilicity improve with increased acid modulation is solely based on the results from water sorption isotherm and heat of adsorption, hence should be interpreted with caution. The defect level of the synthesized UiO66NH 2 generally decreases with increasing concentration of acid modulation. Some findings are surprising as UiO66NH 2 _200eq exhibits the highest defect at 25.2% while UiO66NH 2 _150eq with 11.2% defect is the most ideal crystal synthesized. It is hence worth remarking here that the underlying mechanisms for the high water uptake by both acid-modulated crystals are different. The improvement in water sorption behaviour in UiO66NH 2 _150eq is conclusively consistent with the previous hypothesis that replacing 2-aminoterephthalate with acetate linkers under acid modulation yielded higher hydrophilicity, and water molecules were diffused around the octahedral and tetrahedral pores. On the contrary, the missing linker defects leading to compensation by -OH and/or -OH 2 groups are the presiding reason for the improvement of the water sorption behaviour in UiO66NH 2 _200eq, where most of the water molecules are concentrated at the missing-linker nanoregions. The presence of missing linker defects into the UiO66NH 2 on one hand can increase their pore accessibility, while on the other hand, reduces their thermal stability. Colm et al., cited several factors influencing the thermal stability of MOFs. 56 In particular, the nature and position of functional groups, metal hardness and the presence of coordinated solvent molecules were listed to significantly affect the decomposition temperature. The loss of linkers can inevitably affect the stability of both the ligands and the nodes present within the modulated UiO66NH 2 . This implicates the applicability of the MOF especially as adsorbent substituents. Repeated cycles of absorption and desorption via hydrothermal means can cause degradation in the defected structure of the modulated UiO66NH 2 and heavily impact their water loading capacity if their structure is compromised. It is postulated that the ideally synthesized UiO66NH 2 with minimal defect level of 11.2% have high stability in both its ligands and nodes and could possibly have a higher thermal resistance. A cycling test with consecutive adsorption/desorption cycles is necessary to tease this out. Modulator effect on photocatalytic behaviour of synthesized UiO66NH 2 Figures S2.1 – S2.2 show the wide survey and deconvoluted high-resolution C 1s XPS spectra of the synthesized UiO66NH 2 . Significant differences in the O 1s, N 1s, and Zr 3d XPS spectra between UiO66NH 2 _0eq and UiO66NH 2 _150eq are observed in Fig. 3 . After splitting the peak of O 1s for both UiO66NH 2 _0eq and UiO66NH 2 _150eq, it can be observed that the -OH peak appears to be missing in UiO66NH 2 _150eq. However, this does not necessarily meant that -OH peaks on the carboxyl group on the MOF was removed during the acid modulation process since their low percentage could possibly be masked by the large percentage of Zr-O and C = O peaks. It is also unsurprising to detect the lower percentage of Zr-O within UiO66NH 2 _150eq compared to the unadulterated UiO66NH 2 _0eq. This supports the idea of missing linker detects as a result of modulation by acetic acid After deconvoluting the peak of N 1s, three peaks were obtained (Fig. 3 (aii) ): peaks at 396.0 eV, 398.1 eV and 400.3 eV are attributed to N-C, -NH 2 sp 2 and -NH 3 + within UiO66NH 2 _0eq. Similar bonding energy were ascribed to the split peaks in the N 1s spectrum for UiO66NH 2 _150eq, which could be understood as hydrogen bonding/ electrostatic attraction between -NH 2 sp 2 / -NH 3 + and RhB during the photodegradation. In terms of Zr-Zr and Zr-O bonding, there is insignificant percentage difference which indicates that the acid modulation effect have little to no effect on the internal chemical makeup of the UiO66NH 2 core. However, one of the Zr-Zr split peaks from UiO66NH 2 _150eq was found to have an significant increase from 176.9 eV in UiO66NH 2 _0eq to 177.3 eV. This is surprising as one would expect the modulating effects of acetic acid to weaken the Zr-Zr bond energies. One plausible explanation is due the missing linker defect which cause the Zr atoms to be closer to another. In turn, stronger metal-metal interactions result from close proximity. This unexpected twist support the idea of UiO66NH 2 _150eq being a stable acid modulated MOF with optimal porosity for water uptake. The spectra of the respective UiO66NH 2 are characterized by DRS in Fig. 4 a and their band gap energy are estimated with the empirical Eq. (3.1) , respectively, the figure is displayed in Fig. 4 b. αh ν = k (h ν − E g ) n/2 (3.1) where k is a constant and the value of n is 1 based on direct optical transition. The E g of pristine UiO66NH 2 are estimated to be 2.52 eV. With increasing amount of acetic acid used for the modulation process, an increasing trend of E g deviating away from 2.52 eV was observed. Specifically, a substantial rise in E g was observed from UiO66NH 2 _150eq and UiO66NH 2 _200eq. It is widely accepted that the most suitable bandgap for creating a visible light photocatalyst is roughly between 2 and 2.7 eV. Consequently, UiO66NH 2 _150eq is predicted to provide an optimal reaction to visible light, while ensuring lowered probability of electron-hole recombination, for the photocatalytic breakdown of environmental pollutants. EPR was used as a method to suggest the existence of paramagnetic d 1 Zr 3+ entities at the Zr-oxo accumulation points, and/or single electron repositories in oxygen deficiencies within these accumulations. The demonstrative evidence from Fig. 4 c unveils greatest EPR signals notably in UiO66NH 2 _150eq while EPR signals for UiO66NH 2 _200eq were marginally less conspicuous when measured against the EPR spectrum for UiO66NH 2 _0eq as the reference. The signal in proximity to 350 mT can be correlated to Zr 3+ ions while a separate signal at an approximate 275 mT could conceivably be linked to superoxide anions stabilized on the surface of Zr 4+ centers. It is worth noting that the latter signal may also comprehend amine-centered gaps. Ibrahim et al.'s proposed pathway solidified the understanding of free radical species formation during the degradation of methylene blue, alongside highlighting the crucial role of the amino group in mediating this process. 57 According to Fig. 4 d, the modulated MOF variant, UiO66NH 2 _150eq, demonstrated superior photocatalytic efficiency within the context of rhodamine dye degradation. This ties back to their proposed pathway, wherein a missing linker defect induced by the careful modulation through acetic acid in optimal proportions likely amplified the dyes' accessibility within the MOF cages. This modification also afforded ample space for the generation of crucial free hydroxyl radicals under ultraviolet irradiation. 58, 59 Conversely, UiO66NH 2 _200eq depicted similar tendencies, although photodegradation rates exhibited a marginal decline. This can potentially be ascribed to overmodulation, brought about by an excessive utilization of acetic acid. Such overmodulation resulted in an augmented distance between the amino group and the catalyst active site, ensuing a relative retardation in the photocatalytic degradation process facilitation. 4. CONCLUSIONS From the investigation, the profound influence of acid modulation on the structural and functionality aspects of amine-functionalized UiO66 (UiO66NH 2 ) has been comprehensively studied, with a principal focus on enhancing its photocatalytic efficacy and its potential for moisture capture. The study reveals the successful crystallization of UiO66NH 2 nanoparticles with lowest degree of defect (11.2%) using 150 equivalents of acetic acid modulator. Empirical evidence from this research indicates a substantial increase in the nanoparticles’ capacity to adsorb water vapor due to its high BET surface area of 797.1 m 2 g − 1 and exothermic heats of adsorption (23.9 kJ mol − 1 ). The minimum hysteresis observed in this study indicates that these particles have remarkable potential for prolific use in applications necessitating moisture harvesting. Additionally, the produced UiO66NH 2 _150eq nanoparticles display exceptional proficiency in the degradation of Rhodamine B dye, achieving more than 50% efficiency within 1 h without the use of commonly reported composite MOF-based photocatalysts. This outstanding performance is predominantly attributable to the ideal visible light bandgap (2.67 eV) along with an excellent capacity for stabilizing superoxide radicals revealed by EPR analysis. The outcomes of this research underline the indispensable function of acid modulation in customizing the attributes of MOFs to match specific application requirements. This study offers useful insights into the specific roles that the MOFs can play and how their properties can be tailored to specific uses. These insights can be instrumental in guiding future research and developments in the design of advanced MOF-based materials. Declarations Supporting Information The Supporting Information is available free of charge at ___________. Supporting Information 1: Characterization results Corresponding Author Liang Ying Ee – Chemical Engineering Program, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, 23955 − 6900, Saudi Arabia. Email: [email protected] Ethics approval and consent to participate Not applicable Consent for publication Not applicable Competing interests The authors declare no competing interests. FUNDING The research was financially supported by a grant (R-143-000-B24-592) co-funded by the Singapore National Additive Manufacturing – Innovation Cluster (NAMIC) and MIPS Innovations Pte. Ltd.. The authors acknowledge the support from PUB, Singapore’s National Water Agency, and the facility support from NUS Chemical, Molecular and Materials Analysis Centre (CMMAC), NUS Environmental Research Institute, and Agency for Science, Technology and Research. AUTHOR INFORMATION Author Contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. L.Y. Ee conceptualized the research, carried out main research and wrote main manuscript. S. Y. R. Chia, R. P. W. Tan, and Q. 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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-6727927","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":462841014,"identity":"5acaa9d8-8186-4a8f-85b8-5b40887260d8","order_by":0,"name":"Liang Ying Ee","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA70lEQVRIiWNgGAWjYLCCBAYGOQkom7GBWC3GYC0HiNYCBIkziNbCz97AJvFwR236zPYzxp8/MNjIbjjA/vADPi2SPQfYJBLPHM+dzZNjJnGAIc14wwEeYwl8WgxuJAC1tB3LnceQYwZ02OFEoBYGvFrs7z8Aa0mX439j/OEAw3+gFvbHP/DaIsEA0lKTIC2RYwB02AGgFgYzvLZInElstkhsO2A4c8azMokzBsnGMw/zmFng08LffvjgzZ9tdfIS55M3f6iosJPtO97++AY+LcCIaAE64zDMnUDMjF89CDADY6GOsLJRMApGwSgYuQAAU8RNI/TVDLEAAAAASUVORK5CYII=","orcid":"","institution":"King Abdullah University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Liang","middleName":"Ying","lastName":"Ee","suffix":""},{"id":462841015,"identity":"904c57fc-2131-4ddb-a9b0-b81b8c97e796","order_by":1,"name":"Sean Yi Rong Chia","email":"","orcid":"","institution":"National University of Singapore","correspondingAuthor":false,"prefix":"","firstName":"Sean","middleName":"Yi Rong","lastName":"Chia","suffix":""},{"id":462841016,"identity":"cdc67b6e-9ea8-4f40-b300-92fd8ea39480","order_by":2,"name":"Regina Pei Woon Tan","email":"","orcid":"","institution":"National University of Singapore","correspondingAuthor":false,"prefix":"","firstName":"Regina","middleName":"Pei Woon","lastName":"Tan","suffix":""},{"id":462841017,"identity":"b02c99f0-a80e-42ab-b953-85d503b11358","order_by":3,"name":"Qian Ma","email":"","orcid":"","institution":"National University of Singapore","correspondingAuthor":false,"prefix":"","firstName":"Qian","middleName":"","lastName":"Ma","suffix":""}],"badges":[],"createdAt":"2025-05-22 21:38:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6727927/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6727927/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83599222,"identity":"26dae8c9-e00e-4d98-884a-9f7e0181a710","added_by":"auto","created_at":"2025-05-29 08:45:27","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":666702,"visible":true,"origin":"","legend":"\u003cp\u003e2D SAXS spectra of (a) UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq, (b) UiO66NH\u003csub\u003e2\u003c/sub\u003e_50eq, (c)\u003cstrong\u003e \u003c/strong\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_100eq, (d) UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq and (e) UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq, (f)\u003cstrong\u003e \u003c/strong\u003e1D SAXS/WAXS Guinier-Porod intensity profile (scattering intensity as function of scattering vector) and (g) PXRD spectra of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples and simulated UiO66NH\u003csub\u003e2\u003c/sub\u003e for quantitative Rietveld refinement to determine crystal structure and morphology.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/fa5cc7137475301a4dae516b.jpg"},{"id":83599493,"identity":"31b441cc-9cf7-494d-94b3-345f0829fc4e","added_by":"auto","created_at":"2025-05-29 08:53:27","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":460485,"visible":true,"origin":"","legend":"\u003cp\u003e(a)\u003cstrong\u003e \u003c/strong\u003eWater adsorption-desorption profiles with (b) highlighted sorption characteristics of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples. Type II sorption typically involves partial adsorption, complete monolayer, to multilayer and finally saturation; (c)\u003cstrong\u003e \u003c/strong\u003eTGA and (d) DSC thermograms of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples after equilibration in 60 % relative humidity; (e) Calculated heat enthalpy of moisture adsorption from TGA-DSC profiles, and (f) BET surface area (grey bar) and pore radius (blue bar) of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples using nitrogen probe adsorbate.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/bb052bf364600981657aac9a.jpg"},{"id":83600106,"identity":"9e132d94-eec0-4445-8c4a-029b95054af7","added_by":"auto","created_at":"2025-05-29 09:01:27","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":300457,"visible":true,"origin":"","legend":"\u003cp\u003eDeconvoluted high-resolution O 1s, N 1s, and Zr 3d XPS spectra of (ai) – (aiii) UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq and (bi) – (biii) UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/ad69b5258a3f7df2bf2452b5.jpg"},{"id":83599227,"identity":"715c91c0-3c47-4599-a64e-3c6f86bcc4c8","added_by":"auto","created_at":"2025-05-29 08:45:27","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":250896,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV-Vis diffuse reflectance spectroscopy (DRS) spectra, (b)\u003cstrong\u003e \u003c/strong\u003eTauc plot (with annotated band gaps), (c) electron paramagnetic resonance (EPR) under irradiation and (d) RhB dye photocatalytic degradation efficiencies of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples over 80 min, determined with calibrated UV-Vis spectrophotometry method. UiO66NH\u003csub\u003e2\u003c/sub\u003e samples were suspended in the RhB dye solution for 30 min to achieve equilibrium in adsorption-desorption before the photocatalytic experiment.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/f38fc5569a90149a5c523123.jpg"},{"id":89486558,"identity":"b3cf6de9-f86d-4da2-98ff-1b5fb1e6c2df","added_by":"auto","created_at":"2025-08-20 12:54:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2858100,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/2631e322-fc8d-4c59-b6c7-164187cf8093.pdf"},{"id":83599232,"identity":"1f9a6559-a403-4a87-a945-173740eeccf0","added_by":"auto","created_at":"2025-05-29 08:45:28","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":16314990,"visible":true,"origin":"","legend":"","description":"","filename":"UiO66NH2Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/59d6f950fb704117502d62d6.docx"},{"id":83599226,"identity":"d784ebba-d6c3-4ae2-90dc-7cd3f2963456","added_by":"auto","created_at":"2025-05-29 08:45:27","extension":"jpg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":68717,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical abstract\u003c/p\u003e","description":"","filename":"Graphicalabstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6727927/v1/c9e68041f6f596d75687e72e.jpg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Leveraging crystal defect modulation of Zirconium nano-MOF for enhanced water vapour confinement and photocatalytic behaviours","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eThe tuneable and large surface area, porosity and abundance of functional sites ensue from rational fabrication design and precise control of reaction variables favour metal-organic frameworks (MOFs) to be effective adsorbents for sensing and environmental remediation applications.\u003csup\u003e1\u0026ndash;4\u003c/sup\u003e Besides, many researchers have uncovered the excellent homogeneity and water stability of MOFs in composites and extended their applications in separation membranes and photocatalytic decontamination.\u003csup\u003e5\u0026ndash;10\u003c/sup\u003e Zirconium-based (Zr-) MOFs represent one of the popularly studied subclasses that are formed by six Zr nodes each in tetravalent oxidation state and coordinated with oxo-, hydroxo- and carboxylate ligands.\u003csup\u003e11\u003c/sup\u003e The most common Zr-MOFs have been designated UiO-66 to UiO-68 (UiO refers to University of Oslo), differing in the dicarboxylate linkers.\u003csup\u003e12, 13\u003c/sup\u003e Despite the wide environmental application of Zr-MOFs by exploiting a range of synthetic modifications,\u003csup\u003e14\u003c/sup\u003e their preparation process is still difficult to reproduce and replicate due to the lack of knowledge about structure-activity relationships. Moreover, the mechanisms are different in every environmental application, while most literature perfunctory reports their large surface area and porosity for adsorption applications.\u003c/p\u003e \u003cp\u003eAdvanced photocatalytic water treatment processes aim to overcome roadblocks including the need for high-energy light irradiation at ultraviolet wavelengths and low specificity in degrading targeted contaminants.\u003csup\u003e15\u003c/sup\u003e However, the yield of these unstable radicals is highly reliant on the photocatalytic material, its energy band gap, and the possibility of electron-hole recombination.\u003csup\u003e15, 16\u003c/sup\u003e Today, tailored MOF with high surface area and porosity has been considered to be a good alternative to metal oxides (e.g., TiO\u003csub\u003e2\u003c/sub\u003e) for enhanced photocatalytic activity and leading to innocuous degraded products. Despite potentially being unstable in water due to the weaker bond between the metal nodes and organic linkers,\u003csup\u003e17\u003c/sup\u003e the MOF photocatalytic property and porosity can be easily designed and controlled in-situ synthesis or post-synthesis. Numerous procedures have been reported for the preparation of photocatalytic doped MOFs for the visible-light degradation of rhodamine B,\u003csup\u003e18\u003c/sup\u003e methylene blue, and tetracyclines.\u003csup\u003e19, 20\u003c/sup\u003e Tailoring the optical absorption properties of MOFs has been evidenced to be an important consideration for photocatalytic applications, which typically employ doping or increments of π-conjugation along organic linkers.\u003csup\u003e21\u003c/sup\u003e Nonetheless, when looking beyond linker manipulation, there is a research gap on the effect of crystal growth and morphology on the optical absorption properties.\u003c/p\u003e \u003cp\u003eStructural modification and defect modulation is an important engineering approach to modifying the performance behaviours, and physicochemical properties of MOFs.\u003csup\u003e22\u0026ndash;24\u003c/sup\u003e Acid modulators mainly act as the capping, deprotonating, or reactive agent to introduce defect sites arising from missing linkers or metal clusters. These defects are known to alter pore size, binding energy, wettability, electronic band gap, and catalytic activity.\u003csup\u003e25, 26\u003c/sup\u003e In the preparation of highly porous UiO-66(Zr), monocarboxylic acids such as formic acid have been shown to coordinatively reduce 4 linkers in a missing cluster defect, which shows a larger influence on increasing the specific surface area than a missing linker defect.\u003csup\u003e27, 28\u003c/sup\u003e The modulated UiO-66(Zr) crystal has reported an enormously large surface area of about 1,800 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which resulted from the low pK\u003csub\u003ea\u003c/sub\u003e value of the acid modulator that readily dissociates and hinders deprotonation of ligands. Nonetheless, the presence of water content in precursor materials can also lead to crystal defects as it becomes difficult for the ligands to displace the coordinated hydroxyl ions and water molecules.\u003csup\u003e29, 30\u003c/sup\u003e Therefore, high controllability of water and moisture content during synthesis is essential to ensure reproducibility. Various applications of the modulation approach in the design of suitable MOF crystals have been demonstrated, ranging from fuel storage to the catalytic degradation of environmental pollutants.\u003csup\u003e31\u0026ndash;33\u003c/sup\u003e Despite the growing studies on crystal defect engineering, there is still an ongoing controversy about the modulated crystallization mechanism while ensuring the electroneutrality condition through ligand substitution.\u003c/p\u003e \u003cp\u003eThe high photocatalytic activity of MOF from modulated synthesis has been demonstrated in many literature articles reporting on the photoreduction of CO\u003csub\u003e2\u003c/sub\u003e, the photodegradation of environmental contaminants, and other energy conversion applications.\u003csup\u003e34\u0026ndash;36\u003c/sup\u003e Du et al. have discussed the feasibility of using an acetic acid-modulated UiO66NH\u003csub\u003e2\u003c/sub\u003e(Zr/Hf) membrane to reduce carcinogenic hexavalent chromium into less toxic trivalent chromium under sunlight irradiation, achieving a photoreduction efficiency of 98% in 120 minutes.\u003csup\u003e37\u003c/sup\u003e Although it has great potential as a facile reusable solution to remove pollutants under photocatalytic effect, the question of how the modulation strategy can improve the rate of photoreduction with high reproducibility is the area to which this research now turns. Convincing results have been reported on the precise modulation of the binding energy of the first adsorbed water molecule and the water uptake behaviour of MOF crystals.\u003csup\u003e38\u0026ndash;41\u003c/sup\u003e Particularly, different acid-modulated Zr-MOFs are more efficient and stable as water adsorbent materials with high reversibility in the adsorption-desorption cycle, making them suitable for applications in aqueous or high humidity environments.\u003csup\u003e42\u0026ndash;44\u003c/sup\u003e However, further investigation is needed to reveal how the modulator amount influences the water uptake behaviour with careful reticular design.\u003c/p\u003e \u003cp\u003eTherefore, herein the present study, we investigate the effect of the acid modulator on the structure and morphology of UiO66NH\u003csub\u003e2\u003c/sub\u003e(Zr) MOF nanocrystals. Besides the study on water adsorption-desorption behaviour, the influence of the amount of acid modulator on the MOFs\u0026rsquo; photocatalytic activity in the degradation of environmental pollutants is further examined. This will better shed a new perspective on the modulation approach to the design and construction of MOF nanomaterials for advanced photocatalytic and water sorption applications.\u003c/p\u003e"},{"header":"2. EXPERIMENTAL PROCEDURE","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Chemicals\u003c/h2\u003e \u003cp\u003eZirconium (IV) chloride (ZrCl\u003csub\u003e4\u003c/sub\u003e, \u0026ge; 99.5% trace metals basis), 2-aminoterephthalic acid (ATA, 99%), glacial acetic acid (ReagentPlus\u0026reg;, \u0026ge; 99%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), barium sulfate (99.99% trace metals basis), rhodamine B (HPLC, \u0026ge; 95%), sodium deuteroxide (40 wt. % in D\u003csub\u003e2\u003c/sub\u003eO, 99.5 atom % D), and SnakeSkin\u0026trade; dialysis tubing (10 K MWCO, 35 mm) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The ultrapure water (UPW) used was produced by Milli-Q\u0026reg; water purification system (EMD Millipore Co., Billerica, MA, USA).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of UiO66NH\u003csub\u003e2\u003c/sub\u003e Zr-MOF\u003c/h2\u003e \u003cp\u003eZrCl\u003csub\u003e4\u003c/sub\u003e (1.03 mmol, 0.24 g), ATA (1.05 mmol, 0.19 g) and 1.8 mL glacial acetic acid were added into a 250-mL beaker containing 60 mL of DMF. The mixture was stirred for 30 min before being transferred into a Teflon-lining stainless steel hydrothermal flask for autoclave at 120 \u003csup\u003eo\u003c/sup\u003eC for 24 h. The synthesized material was centrifuged at 8,000 rpm for 8 min and washed three times with fresh DMF. Finally, the washed UiO66NH\u003csub\u003e2\u003c/sub\u003e was dialyzed with UPW using 10 K MWCO SnakeSkin\u0026trade; dialysis tubing over three days before lyophilization. UiO66NH\u003csub\u003e2\u003c/sub\u003e synthesized with different crystallization times (24, 48 and 72 h) during the autoclave process were synthesized to determine the optimal reaction time. Large pore size and pore volume were achieved with 72 h crystallization time based on \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.1a\u003c/b\u003e. The modulator effect on synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e crystal morphologies and properties was investigated. The synthesis procedure was repeated with different molar equivalents of the glacial acetic acid modulator (i.e., 0, 50, 100, 150 and 200 eq). The prepared UiO66NH\u003csub\u003e2\u003c/sub\u003e samples are then labelled according to the molar equivalence of modulator (e.g., UiO66NH\u003csub\u003e2\u003c/sub\u003e_50eq refers to UiO66NH\u003csub\u003e2\u003c/sub\u003e prepared with 50 eq of modulator).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization techniques of synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e Zr-MOF\u003c/h2\u003e \u003cp\u003eThe UiO66NH\u003csub\u003e2\u003c/sub\u003e samples were dried in the DZ-1BC vacuum oven at 115 \u003csup\u003eo\u003c/sup\u003eC overnight before characterization. Attenuated Fourier-Transform Infrared (ATR-FTIR) spectroscopic analysis was performed with IR Prestige-21 spectrophotometer (Shimadzu Co., Kyoto, Japan) between 4,000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e wavenumbers for 40 scans and 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e resolution. Powder XRD (PXRD) analysis was performed using D8 Advance X-ray diffractometer (Bruker Daltonics Inc., Billerica, MA, USA) with Cu-Kα radiation (λ\u0026thinsp;=\u0026thinsp;0.15418 nm) at a voltage of 30 kV and current of 10 mA. PXRD data were collected in a 2θ range from 3 \u003csup\u003eo\u003c/sup\u003e to 50 \u003csup\u003eo\u003c/sup\u003e at an increment of 0.03 \u003csup\u003eo\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The interplanar spacing and crystallite size are calculated using \u003cb\u003eEquations 2.1\u0026ndash;2.2\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{d}_{hkl}\\:=\\frac{{\\lambda\\:}}{2\\text{sin}\\theta\\:}\\)\u003c/span\u003e \u003c/span\u003e, derived from Bragg\u0026rsquo;s Eq.\u0026nbsp;(2\u003cb\u003e.1)\u003c/b\u003e\u003c/p\u003e \u003cp\u003ed\u003csub\u003ehkl\u003c/sub\u003e is interplanar spacing (\u0026Aring;), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\lambda\\:}\\)\u003c/span\u003e\u003c/span\u003e is Cu K\u003csub\u003eα\u003c/sub\u003e X-ray wavelength (1.5406 \u0026Aring;), θ is Bragg\u0026rsquo;s angle (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:D\\:=\\frac{\\text{K}{\\lambda\\:}}{\\beta\\:\\text{cos}\\theta\\:}\\)\u003c/span\u003e \u003c/span\u003e, Scherrer Eq.\u0026nbsp;(2\u003cb\u003e.2)\u003c/b\u003e\u003c/p\u003e \u003cp\u003eD is crystallite size (nm), K is Scherrer constant (0.94 for spherical crystallites), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\lambda\\:}\\)\u003c/span\u003e\u003c/span\u003e is Cu K\u003csub\u003eα\u003c/sub\u003e X-ray wavelength (1.5406 \u0026Aring;), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\beta\\:\\)\u003c/span\u003e\u003c/span\u003e is line broadening at FWHM (radians), θ is Bragg\u0026rsquo;s angle (\u003csup\u003eo\u003c/sup\u003e)\u003c/p\u003e \u003cp\u003eSmall-angle X-ray scattering (SAXS) was performed on MOF samples with Xeuss 2.0 system (Xenocs, France) using Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;0.15418 nm) and point collimation at sample-to-detector distances of 0.1652 m and 1.195 m. \u003csup\u003e1\u003c/sup\u003eH NMR was recorded on 600 MHz Varian NMR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) between \u0026minus;\u0026thinsp;2 ppm and 14 ppm for 32 scans. Before NMR analysis, MOF samples (15.0 mg) were vortexed and incubated in 1.2 mL of 1 M NaOD/D\u003csub\u003e2\u003c/sub\u003eO overnight.\u003c/p\u003e \u003cp\u003eMalvern Zetasizer Nano (Malvern Panalytical Ltd., Malvern, WR, UK) was used to measure the hydrodynamic size distribution and surface zeta potential of the MOF samples at 25 \u003csup\u003eo\u003c/sup\u003eC. Before analysis, the MOF samples were dispersed and sonicated in UPW at 0.1 wt% and filtered through a 0.45-\u0026micro;m hydrophilic PTFE syringe filter. Brunauer-Emmett-Teller (BET) analysis to determine the total surface area and pore size probed by nitrogen gas adsorbate were performed using ASAP-2020 (Micromeritics Instrument Co., Norcross, GA, USA). Prior to BET analysis, MOF samples were degassed for 12 h at 120 \u003csup\u003eo\u003c/sup\u003eC and residual pressure of at least 8 x 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mm Hg. The thermal stability of the samples was characterized by thermal gravimetric analysis (TGA) using Discovery TGA 5500 (TA Instruments, New Castle, DE, USA) from 25 \u003csup\u003eo\u003c/sup\u003eC to 600 \u003csup\u003eo\u003c/sup\u003eC at a heating rate of 10 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under nitrogen gas flow. TGA was also performed to quantify the defect level of the MOF samples wherein the samples were heated from room temperature to 105 \u003csup\u003eo\u003c/sup\u003eC and isothermally for 2 min before heating to 850 \u003csup\u003eo\u003c/sup\u003eC at a heating rate of 5 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under an airflow of 50 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The structural defects of the samples were calculated using Equations \u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e2.3\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e2.6\u003c/span\u003e. TGA coupled with Pfizer mass spectrometer (TGA-MS, Mettler Toledo International Inc., Columbus, OH, USA) was performed between room temperature and 600 \u003csup\u003eo\u003c/sup\u003eC in a nitrogen flow at a heating rate of 10 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The mass spectrometry analysis was set up to detect OH\u003csup\u003e+\u003c/sup\u003e, H\u003csub\u003e2\u003c/sub\u003eO\u003csup\u003e+\u003c/sup\u003e, [CH\u003csub\u003e3\u003c/sub\u003eCO]\u003csup\u003e+\u003c/sup\u003e, CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and their integrals compared semi-quantitatively.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\raisebox{1ex}{$Ligands$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.=\\frac{Exp\\:dehydr.\\:mass\\%\\:-\\:Final\\:mass\\%}{Ideal\\:dehydr.\\:mass\\%}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:\\raisebox{1ex}{$Ligands$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.\\)\u003c/span\u003e \u003c/span\u003e is the experimentally determined average number of ligands per formula unit.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\raisebox{1ex}{$Defects$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.=6\\:-\\raisebox{1ex}{$Ligands$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:\\raisebox{1ex}{$Defects$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.\\)\u003c/span\u003e \u003c/span\u003e is the experimentally determined average number of ligand defects per formula unit.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\%\\:Defects=\\left(\\frac{\\raisebox{1ex}{$Defects$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.}{6}\\right)\\times\\:100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:Theoretical\\:dehydr.\\:mass\\:\\%\\:with\\:defects\\:=\\frac{a\\times\\:{MW}_{L}+(6+(6-a\\left)\\right)\\times\\:15.999+(6\\times\\:91.224)}{(6\\times\\:123.223)}\\times\\:100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eTheoretical dehydration mass % with defects is the theoretical mass % of desolvated and dehydrated MOF calculated using number of calculated ligand defects and balancing charge of missing ligands with oxide.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:a\\:=\\raisebox{1ex}{$Ligands$}\\!\\left/\\:\\!\\raisebox{-1ex}{$fu$}\\right.\\:,\\:{MW}_{L}\\:=molar\\:mass\\:of\\:ligand\\:(i.e.\\:179.15\\:g\\:{mol}^{-1})$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eJEOL JSM-6701F Field Emission Scanning Electron Microscope (FESEM) coupled with JEOL JED-2300F Energy Dispersive X-ray Spectrometer (EDX) (JEOL Co., Akishima, Japan) was used to image and conduct elemental analysis on the MOF samples. Samples were sputter-coated with platinum at 20 mA for 30 s before imaging. For transmission electron microscopy (TEM) imaging with JEOL JEM-3011 TEM (JEOL Co., Akishima, Japan), 0.1% w/v MOF samples were first dispersed and sonicated in ethanol before introducing onto carbon-coated copper TEM grid (300-mesh) and dried overnight. X-ray photoelectron spectroscopy (XPS) of powdered samples on copper tape was performed with Kratos Axis Ultra DLD (Shimadzu Co., Kyoto, Japan) using Al Kα irradiation operated at a residual vacuum of below 2.0 x 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e mbar. All characterizations were repeated at least thrice to ensure reproducibility.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Water confinement analysis of synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e Zr-MOF\u003c/h2\u003e \u003cp\u003eAll MOF samples were vacuum-dried overnight at 120 \u003csup\u003eo\u003c/sup\u003eC prior to water sorption analysis using the Aquadyne DVS water sorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The samples were first equilibrated at 66 \u003csup\u003eo\u003c/sup\u003eC and 0.5% relative humidity before their initial mass was measured. Thereafter, their masses were measured at every 10% increment in relative humidity up to 90% before being reduced to 10% while the temperature was maintained at 25.7 \u003csup\u003eo\u003c/sup\u003eC. The mass difference was calculated as the amount of water adsorbed or desorbed from the MOF samples.\u003c/p\u003e \u003cp\u003eA combined TGA and differential scanning calorimetry (DSC) method was performed to evaluate the heat enthalpy of water desorption. All MOF samples were vacuum dried overnight at 120 \u003csup\u003eo\u003c/sup\u003eC before being stored in a humidity and temperature-controlled enclosure at 60% relative humidity and 25 \u003csup\u003eo\u003c/sup\u003eC for 24 h. Approximately 10 mg of the moisture-saturated samples were immediately transferred to both TGA and DSC under atmospheric conditions for analysis. The conditions in TGA and DSC were kept the same wherein the temperature was ramped from 40 \u003csup\u003eo\u003c/sup\u003eC to 80 \u003csup\u003eo\u003c/sup\u003eC at a heating rate of 0.5 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in nitrogen flow of 30 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The temperature was then cooled to 40 \u003csup\u003eo\u003c/sup\u003eC before heating back to 80 \u003csup\u003eo\u003c/sup\u003eC at a heating rate of 1 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in nitrogen flow. An Aluminium DSC sample pan with a pinhole in the lid was used to facilitate the removal of moisture vapour during desorption. The heat enthalpy of water desorption by the MOF samples was then calculated using Equations \u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e2.7\u003c/span\u003e\u0026ndash;2.9. All experiments were repeated at least three times to ensure reproducibility.\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:Water\\:content\\:\\left(g\\:{g}^{-1}\\:MOF\\right)\\:=\\frac{Mass\\:loss\\:from\\:dehydration\\:\\left(g\\right)}{Amoung\\:of\\:MOF\\:in\\:TGA\\:\\left(g\\right)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.7\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:{\\varDelta\\:H}_{water\\:desorption}\\:\\left(kJ\\:{g}^{-1}\\right)\\:=\\frac{{\\varDelta\\:H}_{enthalpy\\:dehydr.}\\:\\left(kJ\\:{g}^{-1}\\right)}{Water\\:content\\:\\left(g\\:{g}^{-1}\\right)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2.8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:H}_{water\\:desorption}\\:\\left(kJ\\:{mol}^{-1}\\right)\\:=\\)\u003c/span\u003e \u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:H}_{water\\:desorption}\\:\\left(kJ\\:{g}^{-1}\\right)\\:\\times\\:\\:18.02\\:g\\:{mol}^{-1}\\)\u003c/span\u003e\u003c/span\u003e \u003cb\u003e(2.9)\u003c/b\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Photocatalytic degradation of RhB dye with synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e Zr-MOF\u003c/h2\u003e \u003cp\u003eSolid-state UV-Vis diffuse reflectance (DRS) spectroscopy was performed using UV-2600i (Shimadzu Co., Kyoto, Japan) to determine the optical band gap of the MOF samples by Tauc plot derived from the DRS spectrum. MOF sample (20 mg) was added to barium sulfate (400 mg) before grinding the solid mixture with a mortar and pestle for DRS measurement. Oxygen vacancy in the defect MOF samples was examined with steady-state electron paramagnetic resonance (EPR) spectroscopy upon UV irradiation using the JEOL FA200 EPR spectrometer (JEOL Co., Akishima, Japan). The EPR was carried out in the X band (9.5 GHz) for 20 scans ranging from 0 to 600 mT on each MOF sample at room temperature.\u003c/p\u003e \u003cp\u003eThe photocatalytic activity of MOF samples was evaluated based on their degradation efficiency towards the rhodamine B (RhB) dye. The MOF sample (15 mg) was loaded into a 100-mL beaker containing 50 mL of RhB aqueous solution (10 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The suspension was magnetically stirred in the dark for 30 min to reach adsorption-desorption equilibrium before being irradiated under a 300 W xenon lamp at a fixed distance of 120 mm. 1 mL of the suspension was withdrawn at regular intervals of 10 min, including the initial suspension before irradiation, and filtered using a 0.22-\u0026micro;m hydrophilic PTFE syringe filter. The photocatalytic experiment was stopped after 80 min and the absorbance of the filtered solution was measured using a Cary 60 UV-Vis spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA at 554 nm, which corresponds to λ\u003csub\u003emax\u003c/sub\u003e of the RhB dye. The UV photocatalytic degradation of the RhB dye by the MOF samples is expressed as C/C\u003csub\u003e0\u003c/sub\u003e, where C is the absorbance of the withdrawn solution after irradiation at a given point and C\u003csub\u003e0\u003c/sub\u003e is the absorbance of the RhB dye at the initial concentration. All experiments were repeated at least three times to ensure reproducibility.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Modulator effect on morphological characteristics of synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003e \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.1a-e\u003c/b\u003e present a series of characterization outcomes for the synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e. As observed in the N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm at 77 K, the unmodulated UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq exhibits a type-II isotherm, along with a total BET surface area of 380.7 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (with the micropore area equalling 103.1 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and a type-C hysteresis loop pattern, suggesting the presence of slit-shaped pores with an average diameter of 1.9 nm. The modulation in the synthesis process results in an increased micropore surface area of 308.9 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq, and an average pore diameter of 2.8 nm. An assessment of the zeta potential of the synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e revealed an upward trend, increasing from \u0026minus;\u0026thinsp;17.4 mV in UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq to 14.1 mV in UiO66NH2_200eq.\u0026nbsp;These findings are consistent with the work of Morris et al. (2017) who also noted an acid-promoted synthesis of UiO66.\u003csup\u003e46\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThis further implies that the modulation process introduced more surface defects and improved colloidal stability. Some of the 2-aminoterephthalic acid organic linkers were also substituted with the acetic acid modulator, a change detected in the ATR-FTIR spectra (the C\u0026thinsp;=\u0026thinsp;O vibration of acetic acid being identifiable at 1,740 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Furthermore, \u003csup\u003e1\u003c/sup\u003eH qNMR was employed to quantify the quantity of organic linkers replaced, with UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq reaching a modulator:linker ratio of 0.34. \u003cb\u003eFigures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.2 - S1.3\u003c/b\u003e and \u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.2\u003c/b\u003e show both TEM and SEM micrographs of the synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e. Modulation resulted in the formation of larger nanoparticle sizes from 13.7 nm in UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq to 39.0 nm in UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq due to the intergrown and aggregation into ortho-octahedral crystals.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePXRD analysis results of UiO66NH\u003csub\u003e2\u003c/sub\u003e samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCrystallite Size (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCrystallinity (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2θ\u003csub\u003e111\u003c/sub\u003e/ \u003csup\u003eo\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ed-spacing\u003csub\u003e111\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2θ\u003csub\u003e200\u003c/sub\u003e/ \u003csup\u003eo\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003ed-spacing\u003csub\u003e200\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e91.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_50eq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e88.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_100eq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e90.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e95.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e44.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e95.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe pore structure and morphological information of as-synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e under acid modulation (0, 50, 100, 150, and 200 molar equivalence of glacial acetic acid) was characterized by both SAXS and PXRD and the results are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The recorded SAXS 2D-scattering patterns of the UiO66NH\u003csub\u003e2\u003c/sub\u003e nanoparticles reveal that the intensities of the lattice fringes increased with modulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-e), corresponding to increased crystallinity. This is in good agreement with the calculated crystallinity index from PXRD analysis, observing a significant increase from 80.2% in UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq to 95.7% in UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq.\u0026nbsp;One of the key findings is that two distinct peaks in the corresponding wide-angle X-ray scattering (WAXS) region at q\u0026thinsp;=\u0026thinsp;4.51 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 4.86 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are observed from the 1D Guinier-Porod intensity profile in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef.\u003c/p\u003e \u003cp\u003eThe calculated peak positions and their corresponding d-spacings of 1.39 nm and 1.29 nm are consistently close to and assigned as the PXRD (111) and (200) indices, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg). Additionally, the WAXS profile shows that the intensities of the two featured peaks increase with acid modulation because of a predicted increase in specific surface area and pore volume.\u003csup\u003e47\u003c/sup\u003e It was further found that the major XRD diffraction peaks of all UiO66NH\u003csub\u003e2\u003c/sub\u003e samples were well consistent with the simulated literature pattern,\u003csup\u003e48\u003c/sup\u003e and the calculated crystallite size was reduced by approximately 55% with 150 equivalents of acetic acid modulation. Explicitly, the presence of broad XRD peaks in the 2θ range of 5.2\u0026ndash;9 \u003csup\u003eo\u003c/sup\u003e is an indication of missing-linker defects in the MOF nanocrystals,\u003csup\u003e49\u003c/sup\u003e which were observed with UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq samples.\u003c/p\u003e \u003cp\u003eMoreover, the quantification of the missing-linker defects was achieved through TGA analysis (\u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.4\u003c/b\u003e and \u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.3\u003c/b\u003e). The outcome of this defect analysis through TGA reveals that UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq displayed missing-linker defects of 16.8% and 25.2%, respectively, with ideally six ligands per formula unit of the MOF nanocrystal. These results correspond closely with the findings derived from SAXS/WAXS profiling. Contrarily, UiO66NH2_150eq displays the minimum defect (11.2%) and the maximum degree of crystallinity.\u003c/p\u003e \u003cp\u003eWhen considering the ratio between the modulator and the linker, which was calculated using qNMR, along with the evaluation of missing-linker defects derived from TGA, it becomes anticipate that unmodulated UiO66NH\u003csub\u003e2\u003c/sub\u003e has the highest content of 2-aminoterephthalate. In comparison, UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq appears to possess the lowest content of this organic linker. These anticipated results could potentially be validated by referring to the analysis results produced from a combination of TGA-MS. A summary of these results has also been displayed in \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.5\u003c/b\u003e and \u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.4\u003c/b\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Modulator effect on water confinement behaviour of synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWater vapour sorption behaviour is undeniably vital to study before selecting materials for water purification processes. Apart from that, their stability in water is a critical property to be considered along with the water loading capacity. The shape of the water vapour isotherm can also provide important information on the hydrophilicity and water transport mechanism of the material.\u003csup\u003e50\u003c/sup\u003e Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-b illustrate the well-defined water adsorption-desorption isotherms carried out at 25.7 \u003csup\u003eo\u003c/sup\u003eC for all water-stable UiO66NH\u003csub\u003e2\u003c/sub\u003e samples. In particular, the unmodulated UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq shows a different water sorption behaviour compared to the modulated crystals wherein it exhibits a convex type III reversible adsorption isotherm,\u003csup\u003e51\u003c/sup\u003e which means that there was weak adsorbent-adsorbate interaction.\u003c/p\u003e \u003cp\u003eOn the other hand, the acid-modulated UiO66NH\u003csub\u003e2\u003c/sub\u003e has strong adsorbent-adsorbate interactions that resulted in type II isotherms through monolayer and multilayer pore filling mechanisms.\u003csup\u003e52\u003c/sup\u003e UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq specifically show rapid uptake of water content reaching saturation. Little hysteresis is observed for the modulated samples, while a large hysteresis between 0.5\u0026ndash;0.8 P/P\u003csub\u003e0\u003c/sub\u003e is seen during water vapour desorption for the unmodulated crystal, which is associated with capillary condensation and hydrophobicity.\u003c/p\u003e \u003cp\u003eIn light of the reported results from water vapour sorption, it is conceivable that the introduction of acid modulation can lead to more hydrophilic UiO66NH\u003csub\u003e2\u003c/sub\u003e due to the replacement of aminoterephthalate with acetate linker. Very good agreement is observed when referenced to the heat of adsorption determined by the combined TGA/DSC method (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). Both UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq with a high degree of modulation found that their exothermic heats of adsorption are 23.9 kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 26.2 kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, twice that of the unmodulated UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq, suggesting that acid modulation in the synthesis of MOF thermodynamically favour water uptake. However, the heat of adsorption calculated in this work concurs with some studies but is lower than the value previously reported by others possibly due to the unconventional analysis method adopted here.\u003csup\u003e53\u0026ndash;55\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe postulation that water uptake and hydrophilicity improve with increased acid modulation is solely based on the results from water sorption isotherm and heat of adsorption, hence should be interpreted with caution. The defect level of the synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e generally decreases with increasing concentration of acid modulation. Some findings are surprising as UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq exhibits the highest defect at 25.2% while UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq with 11.2% defect is the most ideal crystal synthesized. It is hence worth remarking here that the underlying mechanisms for the high water uptake by both acid-modulated crystals are different.\u003c/p\u003e \u003cp\u003eThe improvement in water sorption behaviour in UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq is conclusively consistent with the previous hypothesis that replacing 2-aminoterephthalate with acetate linkers under acid modulation yielded higher hydrophilicity, and water molecules were diffused around the octahedral and tetrahedral pores. On the contrary, the missing linker defects leading to compensation by -OH and/or -OH\u003csub\u003e2\u003c/sub\u003e groups are the presiding reason for the improvement of the water sorption behaviour in UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq, where most of the water molecules are concentrated at the missing-linker nanoregions.\u003c/p\u003e \u003cp\u003eThe presence of missing linker defects into the UiO66NH\u003csub\u003e2\u003c/sub\u003e on one hand can increase their pore accessibility, while on the other hand, reduces their thermal stability. Colm et al., cited several factors influencing the thermal stability of MOFs.\u003csup\u003e56\u003c/sup\u003e In particular, the nature and position of functional groups, metal hardness and the presence of coordinated solvent molecules were listed to significantly affect the decomposition temperature. The loss of linkers can inevitably affect the stability of both the ligands and the nodes present within the modulated UiO66NH\u003csub\u003e2\u003c/sub\u003e. This implicates the applicability of the MOF especially as adsorbent substituents. Repeated cycles of absorption and desorption via hydrothermal means can cause degradation in the defected structure of the modulated UiO66NH\u003csub\u003e2\u003c/sub\u003e and heavily impact their water loading capacity if their structure is compromised. It is postulated that the ideally synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e with minimal defect level of 11.2% have high stability in both its ligands and nodes and could possibly have a higher thermal resistance. A cycling test with consecutive adsorption/desorption cycles is necessary to tease this out.\u003c/p\u003e \u003cp\u003e \u003cb\u003eModulator effect on photocatalytic behaviour of synthesized UiO66NH\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFigures S2.1 \u0026ndash; S2.2\u003c/b\u003e show the wide survey and deconvoluted high-resolution C 1s XPS spectra of the synthesized UiO66NH\u003csub\u003e2\u003c/sub\u003e. Significant differences in the O 1s, N 1s, and Zr 3d XPS spectra between UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq are observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. After splitting the peak of O 1s for both UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq, it can be observed that the -OH peak appears to be missing in UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq.\u0026nbsp;However, this does not necessarily meant that -OH peaks on the carboxyl group on the MOF was removed during the acid modulation process since their low percentage could possibly be masked by the large percentage of Zr-O and C\u0026thinsp;=\u0026thinsp;O peaks. It is also unsurprising to detect the lower percentage of Zr-O within UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq compared to the unadulterated UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq.\u0026nbsp;This supports the idea of missing linker detects as a result of modulation by acetic acid\u003c/p\u003e \u003cp\u003eAfter deconvoluting the peak of N 1s, three peaks were obtained (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e(aii)\u003c/b\u003e): peaks at 396.0 eV, 398.1 eV and 400.3 eV are attributed to N-C, -NH\u003csub\u003e2\u003c/sub\u003e sp\u003csup\u003e2\u003c/sup\u003e and -NH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e within UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq.\u0026nbsp;Similar bonding energy were ascribed to the split peaks in the N 1s spectrum for UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq, which could be understood as hydrogen bonding/ electrostatic attraction between -NH\u003csub\u003e2\u003c/sub\u003e sp\u003csup\u003e2\u003c/sup\u003e / -NH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and RhB during the photodegradation.\u003c/p\u003e \u003cp\u003eIn terms of Zr-Zr and Zr-O bonding, there is insignificant percentage difference which indicates that the acid modulation effect have little to no effect on the internal chemical makeup of the UiO66NH\u003csub\u003e2\u003c/sub\u003e core. However, one of the Zr-Zr split peaks from UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq was found to have an significant increase from 176.9 eV in UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq to 177.3 eV. This is surprising as one would expect the modulating effects of acetic acid to weaken the Zr-Zr bond energies. One plausible explanation is due the missing linker defect which cause the Zr atoms to be closer to another. In turn, stronger metal-metal interactions result from close proximity. This unexpected twist support the idea of UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq being a stable acid modulated MOF with optimal porosity for water uptake.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe spectra of the respective UiO66NH\u003csub\u003e2\u003c/sub\u003e are characterized by DRS in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and their band gap energy are estimated with the empirical \u003cb\u003eEq.\u0026nbsp;(3.1)\u003c/b\u003e, respectively, the figure is displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb.\u003c/p\u003e \u003cp\u003eαh\u003cem\u003eν\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003ek\u003c/em\u003e(h\u003cem\u003eν\u003c/em\u003e\u0026thinsp;\u0026minus;\u0026thinsp;E\u003csub\u003eg\u003c/sub\u003e)\u003csup\u003en/2\u003c/sup\u003e \u003cb\u003e(3.1)\u003c/b\u003e\u003c/p\u003e \u003cp\u003ewhere k is a constant and the value of n is 1 based on direct optical transition. The E\u003csub\u003eg\u003c/sub\u003e of pristine UiO66NH\u003csub\u003e2\u003c/sub\u003e are estimated to be 2.52 eV. With increasing amount of acetic acid used for the modulation process, an increasing trend of E\u003csub\u003eg\u003c/sub\u003e deviating away from 2.52 eV was observed. Specifically, a substantial rise in E\u003csub\u003eg\u003c/sub\u003e was observed from UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq and UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq.\u0026nbsp;It is widely accepted that the most suitable bandgap for creating a visible light photocatalyst is roughly between 2 and 2.7 eV. Consequently, UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq is predicted to provide an optimal reaction to visible light, while ensuring lowered probability of electron-hole recombination, for the photocatalytic breakdown of environmental pollutants.\u003c/p\u003e \u003cp\u003eEPR was used as a method to suggest the existence of paramagnetic d\u003csup\u003e1\u003c/sup\u003e Zr\u003csup\u003e3+\u003c/sup\u003e entities at the Zr-oxo accumulation points, and/or single electron repositories in oxygen deficiencies within these accumulations. The demonstrative evidence from Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec unveils greatest EPR signals notably in UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq while EPR signals for UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq were marginally less conspicuous when measured against the EPR spectrum for UiO66NH\u003csub\u003e2\u003c/sub\u003e_0eq as the reference. The signal in proximity to 350 mT can be correlated to Zr\u003csup\u003e3+\u003c/sup\u003e ions while a separate signal at an approximate 275 mT could conceivably be linked to superoxide anions stabilized on the surface of Zr\u003csup\u003e4+\u003c/sup\u003e centers. It is worth noting that the latter signal may also comprehend amine-centered gaps.\u003c/p\u003e \u003cp\u003eIbrahim et al.'s proposed pathway solidified the understanding of free radical species formation during the degradation of methylene blue, alongside highlighting the crucial role of the amino group in mediating this process.\u003csup\u003e57\u003c/sup\u003e According to Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, the modulated MOF variant, UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq, demonstrated superior photocatalytic efficiency within the context of rhodamine dye degradation. This ties back to their proposed pathway, wherein a missing linker defect induced by the careful modulation through acetic acid in optimal proportions likely amplified the dyes' accessibility within the MOF cages. This modification also afforded ample space for the generation of crucial free hydroxyl radicals under ultraviolet irradiation.\u003csup\u003e58, 59\u003c/sup\u003e Conversely, UiO66NH\u003csub\u003e2\u003c/sub\u003e_200eq depicted similar tendencies, although photodegradation rates exhibited a marginal decline. This can potentially be ascribed to overmodulation, brought about by an excessive utilization of acetic acid. Such overmodulation resulted in an augmented distance between the amino group and the catalyst active site, ensuing a relative retardation in the photocatalytic degradation process facilitation.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. CONCLUSIONS","content":"\u003cp\u003eFrom the investigation, the profound influence of acid modulation on the structural and functionality aspects of amine-functionalized UiO66 (UiO66NH\u003csub\u003e2\u003c/sub\u003e) has been comprehensively studied, with a principal focus on enhancing its photocatalytic efficacy and its potential for moisture capture. The study reveals the successful crystallization of UiO66NH\u003csub\u003e2\u003c/sub\u003e nanoparticles with lowest degree of defect (11.2%) using 150 equivalents of acetic acid modulator. Empirical evidence from this research indicates a substantial increase in the nanoparticles\u0026rsquo; capacity to adsorb water vapor due to its high BET surface area of 797.1 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and exothermic heats of adsorption (23.9 kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The minimum hysteresis observed in this study indicates that these particles have remarkable potential for prolific use in applications necessitating moisture harvesting.\u003c/p\u003e \u003cp\u003eAdditionally, the produced UiO66NH\u003csub\u003e2\u003c/sub\u003e_150eq nanoparticles display exceptional proficiency in the degradation of Rhodamine B dye, achieving more than 50% efficiency within 1 h without the use of commonly reported composite MOF-based photocatalysts. This outstanding performance is predominantly attributable to the ideal visible light bandgap (2.67 eV) along with an excellent capacity for stabilizing superoxide radicals revealed by EPR analysis. The outcomes of this research underline the indispensable function of acid modulation in customizing the attributes of MOFs to match specific application requirements. This study offers useful insights into the specific roles that the MOFs can play and how their properties can be tailored to specific uses. These insights can be instrumental in guiding future research and developments in the design of advanced MOF-based materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eSupporting Information\u003c/h2\u003e \u003cp\u003eThe Supporting Information is available free of charge at ___________.\u003c/p\u003e \u003cp\u003eSupporting Information 1: Characterization results\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eCorresponding Author\u003c/h2\u003e \u003cp\u003eLiang Ying Ee \u0026ndash; Chemical Engineering Program, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, 23955\u0026thinsp;\u0026minus;\u0026thinsp;6900, Saudi Arabia. Email: [email protected]\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eEthics approval and consent to participate\u003c/h2\u003e \u003cp\u003eNot applicable\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent for publication\u003c/strong\u003e \u003cp\u003eNot applicable\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCompeting interests\u003c/strong\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFUNDING\u003c/h2\u003e \u003cp\u003eThe research was financially supported by a grant (R-143-000-B24-592) co-funded by the Singapore National Additive Manufacturing \u0026ndash; Innovation Cluster (NAMIC) and MIPS Innovations Pte. Ltd.. The authors acknowledge the support from PUB, Singapore\u0026rsquo;s National Water Agency, and the facility support from NUS Chemical, Molecular and Materials Analysis Centre (CMMAC), NUS Environmental Research Institute, and Agency for Science, Technology and Research.\u003c/p\u003e \u003cp\u003eAUTHOR INFORMATION\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. L.Y. Ee conceptualized the research, carried out main research and wrote main manuscript. S. Y. R. Chia, R. P. W. Tan, and Q. Ma carried out research work and contributed to the manuscript drafts.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe research was financially supported by a grant (R-143-000-B24-592) co-funded by the Singapore National Additive Manufacturing \u0026ndash; Innovation Cluster (NAMIC) and MIPS Innovations Pte. Ltd.. The authors acknowledge the support from PUB, Singapore\u0026rsquo;s National Water Agency, and the facility support from NUS Chemical, Molecular and Materials Analysis Centre (CMMAC), NUS Environmental Research Institute, and Agency for Science, Technology and Research.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eCai, G.; Yan, P.; Zhang, L.; Zhou, H. C.; Jiang, H. L. Metal-Organic Framework-Based Hierarchically Porous Materials: Synthesis and Applications. \u003cem\u003eChem Rev \u003c/em\u003e\u003cstrong\u003e2021\u003c/strong\u003e, \u003cem\u003e121\u003c/em\u003e (20), 12278-12326. DOI: 10.1021/acs.chemrev.1c00243.\u003c/li\u003e\n\u003cli\u003eEe, L. Y.; Tan, R. P. W.; Li, S. F. Y. 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Photo-catalytic dye degradation of methyl orange using zirconia\u0026ndash;zeolite nanoparticles. \u003cem\u003eBulletin of Materials Science \u003c/em\u003e\u003cstrong\u003e2019\u003c/strong\u003e, \u003cem\u003e42\u003c/em\u003e, 1-11.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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-6727927/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6727927/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMetal-organic frameworks (MOFs) have emerged as promising materials for various applications, including photocatalysis and moisture harvesting. This present study undertakes an analytical exploration of the impact of acid modulation on the structural and functional attributes of amine-functionalized UiO66 (UiO66NH\u003csub\u003e2\u003c/sub\u003e), focusing predominantly on improving its photocatalytic proficiency and efficiency in water vapor confinement. A crucial highlight of the study was the ideal crystallization of UiO66NH\u003csub\u003e2\u003c/sub\u003e nanoparticles achieved with the use of 150 equivalents of acetic acid modulator. Empirical findings have shown more than a significant escalation in water vapor adsorption capabilities presented by these nanoparticles. The minimal hysteresis observed in the process underscores the potential these particles hold in being utilized fruitfully for moisture harvesting applications. In addition to these, the prepared MOF nanoparticles showcase exceptional performance in the degradation of Rhodamine B dye, primarily attributable to the optimal visible light bandgap and extraordinary stabilizing capacity of superoxide radicals. The results derived from this study accentuate the integral role acid modulation can play in specifically tailoring the properties of MOFs to suit dedicated applications. It provides a meaningful perspective and understanding that can guide the future design of advanced MOF-based materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"Leveraging crystal defect modulation of Zirconium nano-MOF for enhanced water vapour confinement and photocatalytic behaviours","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-29 08:45:22","doi":"10.21203/rs.3.rs-6727927/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":"d1a275b1-7f9e-442c-8741-3d68fa775e97","owner":[],"postedDate":"May 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-08-20T12:54:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-29 08:45:22","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6727927","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6727927","identity":"rs-6727927","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}