Defect-engineered Zr-MOFs with enhanced O₂ adsorption and activation for photocatalytic H₂O₂ synthesis

Abstract

Defect engineering has recently received much attention as an effective approach for improving photocatalytic performances. Herein, UiO-66-NH₂ with missing ligand defects (DUiO-66-NH₂) wrapped by ZnIn₂S₄ was investigated for photocatalytic H₂O₂ production under visible light. The defects in DUiO-66-NH₂ enable efficient charge separation and O₂ capture, which are the key factors in accelerating H₂O₂ production. Specifically, the O₂ adsorption capacity over ZnIn₂S₄/DUiO-66-NH₂ was improved by a factor of 3.1 as compared to its counterpart without defects. As a result, in ambient air and pure water without any sacrificial agents, the H₂O₂ yield of ZnIn₂S₄/DUiO-66-NH₂ was 340 μmol L⁻¹, which was significantly higher than that of ZnIn₂S₄/UiO-66-NH₂ (150 μmol L⁻¹). The main H₂O₂ formation pathway over ZnIn₂S₄/DUiO-66-NH₂ is an indirect oxygen reduction reaction with ·O₂⁻ as the intermediate, and the defects in UiO-66-NH₂ could act as active sites to adsorb and activate O₂ to produce ·O₂⁻. However, the H₂O₂ generation over ZnIn₂S₄/UiO-66-NH₂ undergoes both indirect and direct oxygen reduction reactions. This work could provide new insights and inspire further research into defect engineering of MOFs and photocatalytic H₂O₂ synthesis.

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1. Introduction

H₂O₂ is a green, versatile and high-energy oxidant, and has great application values in the fields of bleaching, pollutant removal and chemical synthesis.^1,2 Moreover, H₂O₂ also has the ability to store hydrogen.³ Under such a background, the global demand for H₂O₂ will reach a staggering level in the next decades.⁴ In industry, the anthraquinone process is used to produce H₂O₂, which is complicated and would produce a large amount of inorganic salt waste.^5,6 It is therefore of great significance to realize the green production of H₂O₂. Photocatalytic H₂O₂ synthesis through the O₂ reduction reaction or the water oxidation reaction has been verified as a promising strategy,^7,8 in which the O₂ reduction reaction is readily conducted and has been broadly explored. Since O₂ is a major reactant, its capture and activation will inevitably affect the formation of H₂O₂, especially when the reactions take place in ambient air. Meanwhile, few studies have considered the effect of O₂ adsorption and activation,⁹ and increasing O₂ adsorption and activation would kinetically and thermodynamically accelerate H₂O₂ production.

Metal–organic frameworks (MOFs) are multifunctional materials constructed by coordination bonds between inorganic nodes (metal ions/metal clusters) and organic ligands,^10,11 which have a broad range of applications.^12–14 Particularly, due to their semiconducting behavior and tunable optical properties, MOFs are in the spotlight in photocatalysis.^15–17 Nonetheless, photocatalytic performances of pristine MOFs are usually poor owing to the rapid recombination of photo-induced charge carriers.^18–20 To address the above problem, defect engineering on MOFs has recently emerged as a viable countermeasure by modulating the electronic energy band structure.²¹ Investigations of defective MOFs mainly focus on Zr-MOFs (e.g. UiO-66 and UiO-66-NH₂) because of their excellent structural stability.^22,23 Jiang et al. found that moderate defects in UiO-66-NH₂ enabled an energy decrease of the unoccupied d orbitals of Zr atoms, leading to improved charge separation and transfer.²⁴ For the photocatalytic production of H₂O₂, the Yamashita group found that the defects could also restrain the decomposition of H₂O₂ (ref. 25) and the nonradiative relaxation of ligands.⁷ Additionally, defects in MOFs may result in a high porosity and large surface area, as well as coordination-unsaturated metal sites that can act as catalytically active centers.^26,27 Based on these, it is an essential task to elucidate the correlation between MOF defects and O₂ adsorption/activation in the reaction of H₂O₂ generation from O₂ reduction.

Herein, UiO-66-NH₂ with missing ligand defects (DUiO-66-NH₂) was prepared and then decorated by two-dimensional ZnIn₂S₄ for photocatalytic H₂O₂ production in pure water and ambient air under visible light. The introduction of ZnIn₂S₄ boosts the visible-light response and constructs a heterojunction with MOFs for optimizing charge separation. In comparison with ZnIn₂S₄/UiO-66-NH₂, ZnIn₂S₄/DUiO-66-NH₂ exhibited a much higher H₂O₂ yield. Based on a series of experiments and characterization, the effects of defects in DUiO-66-NH₂ on the textural properties and H₂O₂ synthesis reactions were clarified. It is noted that the defects could accelerate the charge separation and O₂ adsorption and activation. And interestingly, the H₂O₂ formation pathway changed from two modes (direct and indirect O₂ reduction reactions) on ZnIn₂S₄/UiO-66-NH₂ to one mode (indirect O₂ reduction reaction) on ZnIn₂S₄/DUiO-66-NH₂. This study could offer a novel insight into defective MOFs in the green synthesis of H₂O₂.

2. Experimental

2.1. Materials

Thioacetamide (TAA, ≥99%), N,N-dimethylformamide (DMF), formic acid and ethanol were purchased from Sinopharm. Zirconium chloride (ZrCl₄, 98%) and p-benzoquinone were bought from Aladdin. Zinc chloride (ZnCl₂, ≥98%) and anhydrous acetone (≥98%) were obtained from Nanjing Chemical Reagent. Indium chloride tetrahydrate (InCl₃·4H₂O, >99%) was obtained from Macklin. 2-Aminoterephthalic acid (ATA, ≥98%) was purchased from Sigma-Aldrich.

2.2. Preparation of UiO-66-NH₂ and DUiO-66-NH₂

UiO-66-NH₂ was synthesized by a solvothermal method at 120 °C for 24 h in a 100 mL autoclave with 40 mL of DMF, 190 mg of ZrCl₄ and 146 mg of ATA. The resultant solid was obtained by centrifugation, rinsed with DMF, anhydrous acetone and ethanol, and dried at 80 °C overnight. DUiO-66-NH₂ was prepared with the same processes, but extra formic acid (3 mL) was added to the precursor solution. In addition, the final defective samples were gained after heat treatment at 270 °C for 2 h to remove the coordinated formic acid to form MOFs with missing ligand defects. The preparation illustrations of UiO-66-NH₂ and DUiO-66-NH₂ are displayed in Fig. 1a.

Fig. 1 Preparation illustrations of UiO-66-NH₂ and DUiO-66-NH₂ (a), XRD patterns of UiO-66-NH₂, DUiO-66-NH₂, ZnIn₂S₄, Z/U and Z/DU (b), TG plots (c) and ¹H NMR spectra (d) of UiO-66-NH₂ and DUiO-66-NH₂, EPR spectra of Z/U and Z/DU after 10 min illumination (e), N₂ adsorption–desorption isotherms (f) and pore size distribution curves (g) of UiO-66-NH₂, DUiO-66-NH₂, ZnIn₂S₄, Z/U and Z/DU.

2.3. Preparation of ZnIn₂S₄/UiO-66-NH₂ and ZnIn₂S₄/DUiO-66-NH₂

In general, 282 mg of Zr-MOFs was dispersed in 100 mL of deionized water and sonicated for 30 min, after which 587 mg of InCl₃·4H₂O, 137 mg of ZnCl₂ and 451 mg of TAA were added and stirred for another 30 min, and the resultant suspension was sealed and stirred for 2 h at 80 °C in an oil bath. Then, the solid was obtained after centrifugation and washed three times with water and ethanol, and finally dried at 80 °C. Notably, ZnIn₂S₄ growth on DUiO-66-NH₂ and UiO-66-NH₂ were named Z/DU and Z/U, respectively.

2.4. Characterization

The crystal structures of the prepared samples were studied by X-ray diffraction (XRD, Rigaku Ultima IV). N₂ adsorption–desorption isotherms were acquired using a Micromeritics TriStar II apparatus. Their microstructures were observed by scanning electron microscopy (SEM, Hitachi Regulus 8100) and transmission electron microscopy (TEM, JEOL JEM-2100). Energy dispersive X-ray spectrometry (EDS, Hitachi Regulus 8100) was used for elemental analyses of samples. UV-vis diffuse reflectance spectra (DRS) were documented using a Shimadzu UV-2600 photometer. Fourier transform infrared spectroscopy (FTIR) was carried out on a Thermo Electron Nicolet-360 instrument. X-ray photoelectron spectroscopy (XPS) was conducted using an AXIS UltraDLD machine. Photocurrent and electrochemical impedance spectra (EIS), linear sweep voltammetry (LSV) curves and Mott–Schottky plots were tested on a CHI-760E electrochemical workstation. Steady-state photoluminescence (PL) and time-resolved photoluminescence decay (TRPD) spectra were measured using a FluoroMax-4. Electron paramagnetic resonance (EPR) was conducted on a Bruker EMXPLUS spectrometer. Thermogravimetric analysis (TGA) was carried out on a STA2500 thermal analyzer. Nuclear magnetic resonance (NMR) spectra were monitored on an AVANCE III HD equipment.

2.5. Photocatalytic experiments

Typically, 20 mg of photocatalyst was scattered in 40 mL of deionized water. After 30 min stirring, the photocatalytic reaction was triggered through exposure under a 300 W xenon lamp (400–780 nm). At specific time points, a certain amount of the suspension was pipetted and filtered. The production of H₂O₂ was detected at 350 nm based on iodometry (the corresponding standard curve is displayed in Fig. S1†). The apparent quantum yield (AQY) was studied using the following equation at different irradiation wavelengths:

3. Results and discussion

3.1. Material characterization

XRD patterns indicate the high crystallinity of both UiO-66-NH₂ and DUiO-66-NH₂ (Fig. 1b),²⁸ suggesting that the proper missing ligand defects do not destroy the crystal structure. After ZnIn₂S₄ growth, the characteristic peaks of ZnIn₂S₄ emerge on the patterns of Z/U and Z/DU, indicating the successful decoration of ZnIn₂S₄ to Zr-MOFs. The FTIR spectra of UiO-66-NH₂, DUiO-66-NH₂, Z/U and Z/DU are similar overall (Fig. S2†), but the bands at 1257 (C–N), 1501 (C=C) and 1624 (N–H) cm⁻¹ for DUiO-66-NH₂ and Z/DU²⁹ are weaker than those of UiO-66-NH₂ and Z/U, respectively, implying the partial loss of ATA ligands. To further determine the defects, TGA, ¹H NMR and EPR spectra were employed to explore the defects in DUiO-66-NH₂. TGA plots of Zr-MOFs show three weight loss segments: the loss of water of crystallization in 25–100 °C, the removal of solvent DMF from 100 to 320 °C and the combustion and fracture of NH₂-BDC ligands after 320 °C (Fig. 1c).³⁰ It is worth noting that the final phase of weight loss marks the defective extent of ligands. From the fracture of ligands to the complete transformation into ZrO₂, the weight loss ratio of DUiO-66-NH₂ is obviously smaller than that of UiO-66-NH₂, suggesting the existence of ligand defects in DUiO-66-NH₂.³¹ For ¹H NMR measurements, a D₂O solution of NaOH was used to digest UiO-66-NH₂ and DUiO-66-NH₂ (Fig. 1d).³² The chemical shifts at 6.99, 7.06 and 7.49 ppm show the benzene ring structures of ATA,³³ and the peak at 2.06 ppm is ascribed to the formation of Zr–OH. Notably, DUiO-66-NH₂ shows an extra but small H peak from HCOO⁻ at the chemical shift of 1.71 ppm,³⁴ which is triggered by the residual HCOO– in the structure of DUiO-66-NH₂, indicating that ATA was partially substituted by HCOOH to coordinate with Zr–O clusters. In addition, the EPR spectrum of Z/DU shows more apparent peaks at g = 2.011 and 2.004 than Z/U after 10 min illumination (Fig. 1e). The two peaks are allocated to Zr(III) produced from ligand to metal charge transfer in UiO-66-NH₂, which implies a higher charge transfer efficiency due to the existence of ligand defects.³⁵ It is known that the ligand-missing defects have an impact on the specific surface area and porosity of MOFs. As illustrated in Fig. 1f, the specific surface areas of DUiO-66-NH₂ and UiO-66-NH₂ are 814 and 467 m² g⁻¹, respectively. After integration with ZnIn₂S₄, the specific surface areas still remain as 624 and 271 m² g⁻¹ for Z/DU and Z/U, respectively. Accordingly, the pore volumes of UiO-66-NH₂, DUiO-66-NH₂, Z/U and Z/DU are 0.204, 0.379, 0.127 and 0.302 cm³ g⁻¹, respectively (Fig. 1g). The boosted specific surface areas and pore volumes of DUiO-66-NH₂ and Z/DU are attributed to more pore space formed in the presence of missing ligand defects.³⁵ By now, the missing ligand defects in DUiO-66-NH₂ and Z/DU are confirmed indubitably.

The construction of defects may influence the crystal growth of UiO-66-NH₂ and the subsequent growth of ZnIn₂S₄. The SEM image of pristine UiO-66-NH₂ shows an irregular structure and a rough surface (Fig. 2a),³⁶ while DUiO-66-NH₂ possesses a regular and uniform octahedral structure with a size of ∼260 nm (Fig. 2b). This is due to the addition of monocarboxylic acid (formic acid) competing with the ligands (ATA, dicarboxylic acid) for coordination and timely cutting of the crystal cell intergrowth.³⁷ ZnIn₂S₄ presents dense flower-shaped microspheres with interlaced layered structures (Fig. S3†). Interestingly, with ZnIn₂S₄in situ growth, UiO-66-NH₂ was thoroughly buried by ZnIn₂S₄ nanosheets (Fig. 2c). However, ZnIn₂S₄ nanosheets can be exfoliated by DUiO-66-NH₂ and coated on the surface of DUiO-66-NH₂ uniformly (Fig. 2d). TEM and high-resolution TEM images (Fig. 2e and f) of Z/DU further distinguish the surface ZnIn₂S₄ lamellas and the interface between ZnIn₂S₄ and DUiO-66-NH₂. Such a structure of Z/DU is more beneficial for the mass and charge transfer compared with that of Z/U. The above results indicate that the defects in DUiO-66-NH₂ have a huge impact on the growth of ZnIn₂S₄ and could effectively inhibit the aggregation of ZnIn₂S₄ nanosheets. EDS mapping images shed light on the even distribution of the elements S, In, Zn, Zr, C, N and O in Z/DU (Fig. S4†) and valence states of these elements were determined by XPS (Fig. S5†), where the valence state of each element is in line with the compositions of Z/DU and Z/U.

Fig. 2 SEM images of UiO-66-NH₂ (a), DUiO-66-NH₂ (b), Z/U (c) and Z/DU (d), TEM images of Z/DU (e and f).

3.2. Photocatalytic performances of H₂O₂ production and mechanism

UiO-66-NH₂ with different HCOOH/ATA molar ratios was prepared for photocatalytic H₂O₂ production first (Fig. S6†) and the results demonstrate that DUiO-66-NH₂ with a HCOOH/ATA molar ratio of 100 : 1 (3 mL of HCOOH addition) has the highest H₂O₂ yield of 57 μmol L⁻¹, but the performance is still far from ideal due to the rapid recombination of photo-excited charge carriers. ZnIn₂S₄ also has poor activity (52 μmol L⁻¹, Fig. 3a), which is because of the limited active sites and fast charge recombination as well. Z/DU and Z/U possess an appreciably enhanced H₂O₂ yield of 340 and 150 μmol L⁻¹, respectively. Additionally, different weight ratios of DUiO-66-NH₂ in Z/DU were tested for photocatalytic H₂O₂ production (Fig. S7†) and Z/DU with a 40% weight ratio of DUiO-66-NH₂ is the best sample. The H₂O₂ production rate of Z/DU is competitive to the related MOF-based photocatalysts (Table S1†). Subsequently, the performance reproducibility of H₂O₂ production over Z/DU was tested. A slight decrease in the H₂O₂ yields over Z/DU was detected after four-time cycling reactions (Fig. S8†), and the crystal structure, morphology, valence states and chemical structure of Z/DU have no obvious changes (Fig. S9–S11†), indicating the favorable recyclability and stability. To investigate the dynamic process of photocatalytic H₂O₂ production over Z/DU and Z/U, the rate constants of H₂O₂ formation (K_f, μmol L⁻¹ min⁻¹) and decomposition (K_d, min⁻¹) were estimated based on [H₂O₂] = (K_f/K_d)(1 − exp(−K_dt)) (Fig. S12†).³⁸ Compared with Z/U, Z/DU possesses a higher K_f but a lower K_d, which is conducive to the H₂O₂ generation. Furthermore, AQYs of Z/DU for photocatalytic H₂O₂ production at 360, 400, 550 and 650 nm wavelengths were tested (Fig. 3b). The results were consistent with the trend of UV-vis DRS, indicating the excellent utilization of light in the process of generating H₂O₂ over Z/DU.

Fig. 3 Photocatalytic H₂O₂ production over ZnIn₂S₄, UiO-66-NH₂, DUiO-66-NH₂, Z/U and Z/DU (a), AQYs of Z/DU for H₂O₂ production and UV-vis DRS of Z/DU (b), photocatalytic H₂O₂ production over Z/DU (c and e) and Z/U (d and f) in various gas atmospheres, with and without p-benzoquinone.

Photocatalytic production of H₂O₂ in different gas atmospheres (O₂ and N₂) is studied to elucidate the formation pathway of H₂O₂. For both Z/DU and Z/U (Fig. 3c and d), in comparison with reactions in air, reactions in O₂ accelerate the H₂O₂ generation but the H₂O₂ generation in N₂ has a pronounced shrinkage, proving that the H₂O₂ formation mainly stems from O₂ reduction rather than water oxidation. Of note, the H₂O₂ yield in O₂ merely has a slight enhancement by 6% compared with that in air over Z/DU, but this enhancement is 57% over Z/U, implying that Z/DU has a quite higher utilization of O₂ in air than that of Z/U. Since the O₂ reduction reaction to produce H₂O₂ is distinguished as direct (O₂ + 2H⁺ + 2e⁻ → H₂O₂) and indirect pathways (O₂ + e⁻ → ·O₂⁻, ·O₂⁻ + e⁻ + 2H⁺ → H₂O₂),^39,40 to determine this, ·O₂⁻ trapping experiments using p-benzoquinone as a scavenger were conducted over Z/DU and Z/U. As displayed in Fig. 3e, after addition of p-benzoquinone, the H₂O₂ yield over Z/DU has a sharp decrease by 86%, which demonstrates that the formation of H₂O₂ over Z/DU mainly undergoes an indirect O₂ reduction reaction with ·O₂⁻ as intermediates. Nevertheless, the H₂O₂ yield only decreases by 57% over Z/U with the existence of p-benzoquinone, suggesting that H₂O₂ production over Z/U experiences both indirect and direct O₂ reduction reactions. The above difference in H₂O₂ formation pathways is probably due to missing ligand defects weakening the proton-coupled capacity of Z/DU, because the abatement of amino groups on ligands may decrease the hydrogen-bond interaction between Z/DU and water. In addition, to determine the average electron transfer number (n) of the O₂ reduction reaction, the rotating disk electrode test was performed. The n values of both Z/DU and Z/U are close to 2, indicating that they have mainly undergone a process of two-electron reduction of O₂ to H₂O₂ (Fig. S13†).

From the results of photocatalytic H₂O₂ production, it can be seen that there is an increase in the H₂O₂ yield by 2.3 fold over Z/DU compared to Z/U, where defects play a vital role in this process. On the one hand, defects in DUiO-66-NH₂ would expedite charge separation and transportation.⁴¹ To explore this, behaviors of charge separation and transfer over Z/U and Z/DU were characterized by a series of characterization techniques including photocurrent spectroscopy, EIS, LSV, PL and TRPD. On account of the heterostructure formation, Z/U and Z/DU exhibited stronger photocurrent signals than their monomers (Fig. 4a). Additionally, the photocurrent signal over Z/DU is more obvious than that over Z/U. Meanwhile, the resistance on Z/DU is also smaller (Fig. S14†), and the LSV curves imply that the overpotential on Z/DU is lower than that on Z/U (Fig. 4b).^42,43 Furthermore, the weaker photoluminescence intensity of Z/DU suggests a low recombination frequency of photo-induced electrons and holes (Fig. 4c), and the charge lifespan on Z/DU (0.91 ns) is also longer than that on Z/U (0.73 ns, Fig. 4d). The above results undoubtedly confirm that more efficient charge transportation and separation over Z/DU are achieved in comparison with those over Z/U.⁴⁴

Fig. 4 Photocurrent spectra of ZnIn₂S₄, UiO-66-NH₂, DUiO-66-NH₂, Z/U and Z/DU (a), LSV curves (b), PL (c) and TRPL (d) spectra and O₂ adsorption isotherms (e) of Z/U and Z/DU, and EPR spectra of DMPO–·O₂⁻ adducts in methanol over Z/U and Z/DU in the dark and 10 min irradiation (f).

On the other hand, defects in DUiO-66-NH₂ can prompt O₂ adsorption and activation to convert into ·O₂⁻ since the H₂O₂ formation over Z/DU mainly experiences an indirect O₂ reduction reaction. The O₂ adsorption capacities of Z/DU and Z/U were measured using O₂ adsorption isotherms. The maximum O₂ adsorption capacity of Z/DU is 53 cm³ g⁻¹, which is 3.1 fold larger than that of Z/U (Fig. 4e), suggesting that defects could promote the O₂ adsorption. The promoting effect is likely to be aroused by the boosted surface area and coordination-unsaturated metal sites in the presence of missing ligand defects. The generation of ·O₂⁻ was detected by EPR (Fig. 4f). No corresponding signals appeared in the absence of light illumination, and evident characteristic peaks of ·O₂⁻ are present with light illumination, suggesting that ·O₂⁻ comes from light excitation.⁴⁵ In comparison, the ·O₂⁻ signal over Z/DU is stronger than that over Z/U, mainly due to the more efficient charge separation and O₂ capture, indirectly confirming that defects in DUiO-66-NH₂ are conducive to O₂ activation to convert into ·O₂⁻.

To disclose the effects of missing ligand defects on light absorption and band configuration, and finally define the charge transfer routes for photocatalytic H₂O₂ production over Z/DU, UV-vis DRS, XPS valence band (VB) spectra and Mott–Schottky plots were detected. The light absorption edge of DUiO-66-NH₂ has a slight blue shift compared to that of UiO-66-NH₂ (Fig. 5a), which is caused by the reduction of amino groups along with the ligand defects. It is known that amino substituents can act as auxochromic and bathochromic groups in aromatic rings that is the reason for UiO-66-NH₂ with visible-light absorption. After combination with ZnIn₂S₄, both Z/DU and Z/U have good visible-light absorption. Based on the curves of (αhv)²versus hv, bandgaps of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ were determined as 2.80, 2.75 and 2.42 eV, respectively (Fig. 5b). Moreover, the highest occupied molecular orbital (HOMO)/VB potentials of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ were estimated as 2.34, 2.20 and 1.40 eV, respectively, according to their XPS VB/HOMO spectra (Fig. 5c). To determine their lowest unoccupied molecular orbital (LUMO)/conduction band (CB) potentials, Mott–Schottky plots of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ were assayed at 500, 1000 and 1500 Hz (Fig. 5d–f), and all samples were proved to be n-type semiconductors. Since the Ag/AgCl electrode is the reference electrode, the flat-band potentials of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ were −0.363, −0.473 and −0.803 V vs. NHE (E_NHE = E_Ag/AgCl + 0.197 V), respectively. For n-type semiconductors, the LUMO/CB potentials are 0.1 V less than the corresponding flat-band potentials.⁴⁶ Hence, the LUMO potentials of DUiO-66-NH₂ and UiO-66-NH₂ and the CB potential of ZnIn₂S₄ are −0.463, −0.573 and −0.903 V, respectively. In accordance with the equation of E_bandgap = E_VB/HOMO − E_CB/LUMO,⁴⁷ the bandgaps of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ were calculated as 2.803, 2.773 and 2.303 eV, respectively, which are close to the results obtained from the corresponding curves of (αhv)²versus hv.

Fig. 5 (αhv)²versus hv curves of ZnIn₂S₄, UiO-66-NH₂ and DUiO-66-NH₂ (a), UV-vis DRS of ZnIn₂S₄, UiO-66-NH₂, DUiO-66-NH₂, Z/U and Z/DU (b), VB/HOMO XPS spectra of ZnIn₂S₄, UiO-66-NH₂ and DUiO-66-NH₂ (c), Mott–Schottky plots of DUiO-66-NH₂ (d), UiO-66-NH₂ (e) and ZnIn₂S₄ (f) at 500, 1000 and 1500 Hz, band configurations of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ (g).

Now, the band configurations of DUiO-66-NH₂, UiO-66-NH₂ and ZnIn₂S₄ are affirmed and presented in Fig. 5g. All of them are easy to stimulate to produce electrons and holes under visible light due to their narrow bandgaps, and their electrons on the CB are able to reduce O₂ to generate H₂O₂via either indirect or direct pathway. As a summary, H₂O₂ production pathways over Z/DU and Z/U were also exhibited. To infer the charge transfer behavior on Z/DU and Z/U, the presence or absence of ·OH generation is an important index (E(OH⁻/·OH) = 1.99 eV).⁴⁸ However, the EPR characteristic peaks of ·OH over both Z/DU and Z/U are not obvious after illumination (Fig. S15 and S16†), suggesting that little ·OH is produced and holes on the VB of DUiO-66-NH₂ or UiO-66-NH₂ are migrated to the VB of ZnIn₂S₄. Hence, a type II heterostructure would be formed between DUiO-66-NH₂ (UiO-66-NH₂) and ZnIn₂S₄, which facilitates the charge transfer. In addition, the presence of defects in DUiO-66-NH₂ could further promote the charge separation, and, significantly, O₂ adsorption and activation are accelerated as well, which finally improves the H₂O₂ production.

4. Conclusions

In summary, DUiO-66-NH₂ and UiO-66-NH₂ were prepared and modified by ZnIn₂S₄ for photocatalytic H₂O₂ synthesis in pure water and ambient air under visible light. Z/DU exhibited a higher H₂O₂ yield of 350 μmol L⁻¹ than Z/U (150 μmol L⁻¹). Z/DU and Z/U produce H₂O₂ in different O₂ reduction modes. Z/DU primarily experiences an indirect O₂ reduction reaction with ·O₂⁻ as an intermediate product, while Z/U has both indirect and direct O₂ reduction modes. The performance enhancement over Z/DU is allocated to the efficient charge separation and O₂ capture initiated by the missing ligand defects, and finally facilitating the activation of O₂ to transform into ·O₂⁻. This study provides a new perspective on defect engineering of MOFs and we hope that it will shed light on the photocatalytic H₂O₂ synthesis from oxygen reduction reactions.

Conflicts of interest

There are no conflicts of interest.

Acknowledgements

The authors are grateful for the financial support from the Natural Science Foundation of Jiangsu Province Youth Fund (BK20210628) and the Scientific Research Foundation from Nanjing Forestry University. We also thank the Advanced Analysis & Testing Center, Nanjing Forestry University for sample tests.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01396k

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