Unlocking photocatalytic NO removal potential in an S‐type UiO‐66‐NH2/ZnS(en)0.5 heterostructure

The contamination of nitric oxide presents a significant environmental challenge, necessitating the development of efficient photocatalysts for remediation. Conventional heterojunctions encounter obstacles such as large contact barriers, sluggish charge transport, and compromised redox capacity. Here, we introduce an innovative S‐type heterostructure photocatalyst, UiO‐66‐NH2/ZnS(en)0.5, designed specifically to overcome these challenges. The synthesis, employing a unique microwave solvothermal method, strategically aligns the lowest unoccupied molecular orbital of UiO‐66‐NH2 with the highest occupied molecular orbital of ZnS(en)0.5, fostering the formation of a stepped heterojunction. The resulting intimate interface contact generates a built‐in electric field, facilitating charge separation and migration, as evidenced by time‐resolved photoluminescence spectroscopy and photoelectrochemical tests. The abundant active sites in the porous UiO‐66‐NH2 counterpart provide adsorption and activation sites for nitrogen monoxide (NO) oxidation. Performance evaluation reveals exceptional photocatalytic NO removal, achieving 70% efficiency and 99% selectivity toward nitrates under simulated solar illumination. Evidence from X‐ray photoelectron spectroscopy and trapping experiments supports the effectiveness of the S‐type heterostructure, showcasing refined reactive oxygen species, particularly superoxide. Thus, this study introduces a new perspective on advanced NO oxidation and unlocks the potential of S‐scheme heterojunctions to refine reactive oxygen species for NO remediation.

The excessive release of exhaust gases, including nitrogen oxides (NO x ), from fossil fuel combustion in vehicles, industries, and power plants poses severe health and environmental risks. [1,2]NO x , comprised of nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ), is a notorious air pollutant associated with issues such as cardiovascular diseases and environmental degradation. [3,4]Additionally, NO x serves as an indirect greenhouse gas, contributing to ground-level ozone formation through atmospheric photochemical reactions. [5]To address these concerns, various strategies have been developed to mitigate NO x emissions, with selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) emerging as prominent methodologies. [6]SCR involves injecting a reducing agent (e.g., NH 3 ) over a catalyst, but its drawback lies in significant energy consumption, especially at elevated temperatures. [7]On the other hand, SNCR is simpler but less efficient and susceptible to by-products.An emerging alternative, photocatalysis, utilizes light-activated catalysts for NO x oxidation, providing environmental benefits and the potential for integration into existing systems. [4]y harnessing the power of photons to drive catalytic reactions, photocatalysis represents a cutting-edge avenue for addressing the challenges posed by NO x emissions, providing a sustainable and efficient solution.
10][11] This process encompasses the generation of long-lived electron-hole pairs and reactive oxygen species (ROS), along with the establishment of efficient adsorption mechanisms for reactant molecules such as NO and O 2 . [12]Notably, metal-organic framework (MOF) materials, known for their rich and well-ordered porous structures, demonstrate a robust capacity to capture and enrich these reactant molecules. [13][16][17] The NH 2 functional groups play a pivotal role in narrowing the bandgap and inducing a positive shift in the valence band (VB).However, this modification results in an energy reduction in the VB edge, leading to a diminished oxidation capability of holes for deep oxidation of NO molecules.Furthermore, the sluggish electron transfer within UiO-66-NH 2 , from the excited linker to the unoccupied d-orbitals of the node, contributes to a sluggish electron-hole separation rate.20][21][22] Jiang and co-workers incorporated Pt nanoparticles into or onto UiO-66-NH 2 , aiming to facilitate electron transport and suppress electron-hole recombination. [23]In conventional type II heterojunctions, photogenerated electrons accumulate in the conduction band (CB) with weak reduction potential, while photogenerated holes gather in the VB with weak oxidation potential, potentially hindering the redox ability of photogenerated carriers. [24,25]Wang et al. established a Z-scheme charge transfer configuration with heterostructured Bi 5 O 7 I/UiO-66-NH 2 photocatalysts for ciprofloxacin degradation. [26]Although the Z-type heterostructure showed improved charge-separation efficiency, it was limited to the solution phase and associated with side reactions.In contrast, S-scheme heterojunctions, with their higher redox potential, offer an advantage over Z-scheme configurations for ROS in gas-solid phase reactions. [27]For example, He et al. assembled an Sscheme UiO-66-NH 2 /CdIn 2 S 4 heterojunction by anchoring UiO-66-NH 2 nanoparticles on the porous CdIn 2 S 4 microspheres, in which more negative CB of CdIn 2 S and more positive VB of UiO-66-NH 2 can be activated with enhanced redox capacities. [28]The S-scheme heterostructure configuration offers a novel insight into electron-hole transport in heterostructured photocatalysts.Hence, constructing an Stype heterojunction by integrating UiO-66-NH 2 with suitable semiconductors holds great promise for a breakthrough in promoting ROS generation and photocatalytic NO oxidation. [29,30]erein, ZnS(en) 0.5 (en = ethylenediamine) was selected as the semiconductor counterpart due to its rapid charge migration and well-matched band structure with UiO-66-NH 2 .Through the alignment of the lowest unoccupied molecular orbital (LUMO) of the UiO-66-NH 2 with the highest occupied molecular orbital (HOMO) of ZnS(en) 0.5 , an S-type UiO-66-NH 2 /ZnS (en) 0.5 heterostructure was constructed via a microwave superhot hydrothermal method, wherein the UiO-66-NH 2 nanoparticles grows on the flower-like ZnS(en) 0.5 microspheres.This configuration allows spatial separation of photogenerated electron-hole pairs and reserves the strong redox ability.In addition, porous UiO-66-NH 2 frameworks provide abundant reactive sites for the adsorption and activation of reactant molecules (NO and O 2 ).The fine-tuned band structure facilitates the generation of highly active electrons, driving the production of robust superoxide radicals (•O 2 -) for enhanced photocatalytic NO oxidation.Contributed by these merits, the optimized UiO-66-NH 2 /ZnS(en) 0.5 stepped heterojunction composite achieves a 70% NO removal rate under simulated solar light irradiation, with a selectivity close to 99% toward nitrates, surpassing the performance of individual UiO-66-NH 2 and ZnS(en) 0.5 .
Hence, this study reveals the potential of S-type heterostructure photocatalysts for activating molecular oxygen, presenting an effective strategy for NO removal.
In Figure 1B, Fourier-transform infrared spectroscopy (FT-IR) spectra of UIO, ZS, and ZSU4 are presented.UIO F I G U R E 1 (A) X-ray diffraction patterns and (B) Fourier-transform infrared spectroscopy spectra of UIO, ZS, and ZSU4 samples.Scanning electron microscopy images of (C-E) ZS samples at different magnifications and (F-H) ZSU4 samples at different magnifications.and ZSU4 samples exhibit similar peak shapes in the range of 1480-1752 cm −1 , corresponding to the skeletal vibrations of the phenyl ring of the H 2 ATA ligand.Similarly, peaks in the range of 1346-1467 cm −1 are attributed to the stretching vibration of the corresponding carboxyl group in both UIO and ZSU4 samples.The two prominent peaks at 668 and 772 cm −1 in the UIO component are associated with the transverse and longitudinal modes of the Zr-O bond in the zirconium oxide cluster.The broad peak observed at 3300-3500 cm −1 originates from adsorbed water molecules on the sample surface.In the FT-IR spectrum of the ZS sample, the characteristic peaks at 3117 and 3246 cm −1 correspond to the stretching vibration of N-H, and the peak at 1585 cm −1 can be attributed to the shear vibration of -NH 2 . [33]The peaks at 1357, 2869, and 2939 cm −1 are indicative of -CH 2 , and the peaks at 1033 and 1079 cm −1 are detected for the C-N bond, both originating from the ethylenediamine, as reported in previous studies. [34]The prominent peak at around 630 cm −1 is due to the Zn-S coordination bond.The introduction of ZS into the composite system causes a slight shift of the longitudinal Zr-O peak to the negative direction, from 668 to 646 cm −1 , due to the influence of the Zn-S bond.Moreover, peaks at 2868, 2937, 3114, and 3244 cm −1 are ascribed to the characteristic -CH 2 and N-H stretching vibrations of ZnS (en) 0.5 .Owing to their high peak intensity, these features are not completely concealed by the broad peak at 3300-3500 cm −1 .These observations further affirm the successful fabrication of ZnS(en) 0.5 /UiO-66-NH 2 composites.
The morphological features of UIO, ZS, and ZSU4 were identified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses.The ZS microparticles have a flower-like structure with a diameter of around 0.5-1.0μm, which are composed of numerous nanosheets (Figure 1C).At higher magnification (Figure 1D,E), a clear stacking pattern is observed in the ZS nanosheets, forming a structure with a thick center and thin edges, indicative of a wrinkled stacking configuration.ZSU4 exhibits notable distinctions in microstructure compared to individual ZS samples.SEM images of ZSU4 (Figure 1F-H) show that UiO-66-NH 2 nanoparticles uniformly cover the ZS surface, resulting in a rough granular surface.TEM and highresolution TEM (HRTEM) images provide additional insights into the structural features of the ZSU4 composite.TEM image of the ZS reveals a thin-layered structure similar to the SEM image (Figure 2A).As shown in Figure 2B, clear lattice fringes of ZS with an interplanar spacing of 0.17 nm and 0.29 nm correspond to the (103) and ( 101) facets, consistent with the XRD results discussed above.The diffraction rings in the selected area electron diffraction pattern in Figure 2C can also be associated with the (101), (110), and (103) facets of ZS.The TEM image of ZSU4 reveals numerous UiO-66-NH 2 nanoparticles densely growing on the ZnS (en) 0.5 nanosheets, with some nanoparticles clearly marked and circled in red (Figure 2D-E).Adjacent to the circled UiO-66-NH 2 nanoparticles, clear lattice fringes of ZnS(en) 0.5 corresponding to the (103) facet can be observed, indicating their contact interface.The HAADF-TEM image in Figure 2F designates the testing area for energy-dispersive X-ray mapping (Figure 2G-L) of the ZSU4.The uniform distribution of N, Zr, and O elements from UiO-66-NH 2 in the area of Zn and S elements from ZnS(en) 0.5 indicates the homogeneous growth of UiO-66-NH 2 nanoparticles on ZnS(en) 0.5 nanosheets.
The specific surface area and pore structure characteristics of the catalysts were investigated using Brunauer-Emmett-Teller (BET) and pore diameter distribution analyses.As depicted in Supporting Information S1: Figure S1A, the N 2 adsorption-desorption isotherms of the UIO, ZS, and ZSU4 samples exhibit distinct characteristics.The UIO sample displays a typical type I isotherm, indicating its nature as a microporous material with a specific surface area of 839.0 m 2 /g.In contrast, the N 2 adsorption-desorption isotherm of the ZS sample exhibits a type III isotherm, suggesting that most of the pores and gas adsorption sites arise from the interstices of the laminar stacking structure, predominantly composed of mesopores and macropores, with virtually no micropores.This results in a significantly smaller specific surface area of 109.3 m 2 /g compared to UIO.For the ZSU4 sample, the N 2 adsorption-desorption isotherm approaches the Y-axis at low pressure, indicating a strong adsorption affinity for N 2 and similar characteristics to UIO.At high pressure, a hysteresis loop of a type V adsorption-desorption isotherm appears, suggesting a high proportion of mesoporous structures in the ZS composite.The pore size distribution further confirms these observations (Supporting Information S1: Figure S1B).Micropores, with an average diameter of around 1.7 nm, are predominant in both the UIO and ZSU4.In contrast, the pore structure of ZS is primarily composed of mesopores with diameters ranging from 10 to 20 nm, as well as larger pores exceeding 50 nm.The introduction of ZS results in a decrease in the relative content of micropores in the ZSU4, accompanied by an increase in the distribution of mesopores.The specific surface area of the ZSU4 catalyst experiences an approximately fourfold increase compared to that of ZS, reaching 415.8 m 2 /g.The expanded specific surface area and hierarchical pore structure significantly enhance the adsorption capability of UiO-66-NH 2 /ZnS(en) 0.5 composites for reactant molecules, providing substantial advantages in the gas-solid phase photocatalytic oxidation of NO pollutants.

| Surface electronic structures
X-ray photoelectron spectroscopy (XPS) analysis of UIO, ZS, and ZSU4 samples (Figure 3) reveals crucial insights into surface bonding states.In the C 1s spectrum of UIO (Figure 3A), peaks at 284.8, 286.3, and 288.7 eV correspond to C-C/C=C, C-O, and C-N in the H 2 ATA ligand, respectively. [35,36]The N 1s spectrum (Figure 3B) of pristine UIO displayed peaks at 399.3 and 400.3 eV, representing N-H and N-C in the amino group and the amino group connected to the benzene ring of the H 2 ATA ligand.Pure ZS exhibits a single peak at 284.8 eV, indicating C-C/C=C from the ethylenediamine component of ZnS(en) 0.5 and carbon contamination during testing.Additionally, characteristic peaks of N-H and N-C at 399.6 and 401.4 eV from ethylenediamine were observed in pure ZS.The C 1s spectrum of ZSU4 maintained peaks representing C-C/ C=C, C-O, and C-N, suggesting that the addition of ZS does not alter the molecular structure of UIO (Figure 3A).Notably, the N 1s spectrum of ZSU4 indicates a significant increase in the proportion of N-C bonds compared to before the combination, indicating the formation of new N-C bonds (Figure 3B).This could be attributed to the coordination of nitrogen in the ethylenediamine component of ZnS(en) 0.5 with the carbon on the benzene ring skeleton of the UiO-66-NH 2 .The increased proportion of C-N bonds in the C 1s spectrum of ZSU4 further supports this bonding mode.In Figure 3C, Zr 3d double peaks (Zr 3d 5/2 and Zr 3d 3/2 ) show a positive shift in the binding energy of ZSU4 by 0.4 eV compared to UIO.This shift signifies electron loss in UIO during the combination with ZS, resulting in reduced electron density in the UIO component and an increased binding energy.In Figure 3D, the binding energy of Zn 2p (2p 1/2 and 2p 3/2 ) decreases by 0.5 eV after compounding compared to pure ZS, indicating electron gain in the ZS component and an increased electron density.Similar changes are observed in the N 1s spectrum, where the main peak position in ZSU4 shifts by +0.2 and −0.2 eV relative to UIO and ZS, respectively.The observed shifts in binding energy correspond to alterations in the electron density of the UIO and ZS components in the composite.These shifts demonstrate a spontaneous migration of electrons occurs from UIO to ZS, establishing an electron depletion layer in UIO and an electron accumulation layer in ZS.Consequently, UIO acquires a positive charge, and ZS becomes negatively charged.This process sets up an internal electric field directing from UIO to ZS, facilitating the transfer of photogenerated electrons from UIO to ZS.This observation underscores the robust formation of a heterojunction.

| Optical and band structure analysis
The ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) in Figure 4A provide insights into the optical properties of UIO, ZS, and ZSU4.ZnS(en) 0.5 , a typical wide bandgap semiconductor, exhibits strong UV absorption, characterized by an absorption edge around 350 nm, consistent with prior studies. [37]On the other hand, UIO demonstrates pronounced absorption in the visible region around 400 nm due to the incorporation of the amino moiety chromophore.ZSU4 presents absorption features resembling ZS at approximately 250 nm and absorption peaks akin to UIO in the 300-420 nm range.Notably, ZSU4 exhibits slightly enhanced absorption in the near-UV and visible regions compared to individual components.Bandgap energy (E g ) of UIO and ZS was calculated using the Kubelka-Munk equation (Equation 1) from the UV-vis DRS spectra after transformation.The calculated E g values are 2.75 eV for UIO and 3.54 eV for ZS (Figure 4B).
To determine the energy band positions of UIO and ZS, Mott-Schottky (M-S) curves were further measured at frequencies of 1000, 2000, and 3000 Hz (Figure 4C,D).The flat-band potentials of UIO and ZS samples, obtained by intercepting the tangent lines with the X-axis, were −1.09V for UIO and −0.66 V for ZS versus Ag/AgCl.The flat-band potentials of UIO and ZS relative to the normal hydrogen electrode (NHE) potential were −0.45 and −0.02 V, respectively, obtained by transforming the Ag/AgCl electrode potential using Equation (2).Typically, the flat-band position of a semiconductor is lower than its CB position by approximately 0.1-0.3V. [38] This implies CB positions of −0.75 V for UIO and −0.17 V for ZS.Combining these data with the bandgap energy, the theoretical energy band structures of UIO and ZS are determined, as illustrated in Figure 4E.
The VB-XPS spectra of UIO and ZS, shown in Supporting Information S1: Figure S2, display intersection points obtained by extending horizontal portions below 0 eV, representing the energy positions of the VB edges.By applying Equation (3), the calculated VB positions were transformed to the NHE potential, being 2.02 V for UIO and 3.34 V for ZS. [39]These values closely align with those derived from flat-band potential and bandgap energy calculations, showing an error of only around 0.02 V.This consistency reinforces the accuracy of the determined energy band structures of UIO and ZS.
In this equation, E VB,NHE represents the standard hydrogen electrode, E VB,XPS signifies the VB potential, and φ denotes the work function.catalysts The photocatalytic NO oxidation performance of the ZS, UIO, and ZSU was evaluated in a gas-solid reactor, monitored by an online NO-NO 2 -NO x analyzer under simulated solar light irradiation.As shown in Figure 5A, ZS and UIO exhibit relatively low NO removal abilities, approximately 38% and 50%, respectively.In contrast, ZSU composites demonstrate superior NO removal rates and efficiency (Supporting Information S1: Figure S3).Among the ZSU composites with varying amounts of ZS and UIO, ZSU4 stands out by eliminating about 70% of NO with approximately 99% selectivity toward nitrate.This suggests that robust interfacial contact facilitates charge transfer and molecular oxygen activation, enabling complete NO oxidation removal without toxic NO 2 .Crucially, ZSU4 exhibits enduring activity and high nitrate selectivity in long-term photocatalytic NO oxidation (Figure 5B).This sustained NO removal performance is likely attributed to the abundance of active sites on the surface of ZSU4, which helps mitigate catalyst deactivation.To identify the dominant ROS involved in NO oxidation, we conducted typical trapping experiments by introducing corresponding scavengers during NO oxidation removal.Specifically, we employed 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as a scavenger for electrons, potassium iodide for holes, coumarin for hydroxyl radicals, and para-benzoquinone (PBQ) for •O 2 -radicals.Based on scavenger-involved performance

| Density and migration of photogenerated carriers
To investigate charge carrier transport dynamics, the charge carrier concentration was determined using equation (Equation 4), coupled with photoelectrochemical tests.It is evident that N D is proportional to the slope dE/d(1/C 2 ), ( ) where N D represents the photo-generated carrier density, e is the elementary charge value (1.60 × 10 −19 C), ε 0 is the vacuum dielectric constant (8.85 × 10 −14 F cm −1 ), ε is the semiconductor dielectric coefficient (2.05 for UiO-66-NH 2 ), C is the space charge capacitance, and E is the applied potential.
As shown in Figure 6A, the curve for ZSU4 displays a markedly greater slope than the UIO sample curve, and their carrier concentrations are calculated to be 1.71 × 10 22 and 9.03 × 10 21 , respectively.This indicates the staggered heterojunction effectively increases the concentration of photo-generated charges compared to the parent UIO.The separation efficiency of photogenerated charge carriers in the as-prepared samples was assessed through transient photocurrent response tests.In Figure 6B, ZS exhibits the weakest photoelectric response, while UIO displays a higher initial photocurrent intensity that gradually weakens.In stark contrast, ZSU4 demonstrates the most robust and persistent photocurrent signals.These results highlight that ZSU4 exhibits improved charge carrier density and outstanding separation efficiency.photo-generated electron-hole pairs.In Figure 6C, under 350 nm laser excitation, the UIO sample exhibits a strong broad emission peak at 455 nm, while the ZS sample shows a distinctive broad peak signal around 420 nm.Remarkably, the emission peak of the ZSU4 composite material is similar to that of UIO, with only a weak broad peak at 455 nm.Higher PL intensity implies increased recombination of charge carriers, while lower PL intensity signifies enhanced separation of charge carriers.Thus, the observed reduction in PL intensity in the ZSU4 composite indicates an effective suppression of carrier recombination, allowing a greater number of carriers to actively participate in redox reactions.The time-resolved photoluminescence (TRPL) analysis was conducted to investigate the dynamics of separation and transfer of photo-generated carriers (Figure 6D).The fitting of TRPL spectra reveals distinct decay processes and lifetimes of photo-generated carriers.Detailed parameters for UIO, ZS, and ZSU samples are provided in Table 1.Using Equation ( 5), the average PL lifetimes of carriers in the UIO, ZS, and ZSU4 samples were calculated to be 1.21, 1.12, and 1.04 ns, respectively.The shortest average PL lifetime in ZSU4 indicates the fastest carrier separation and transfer after light irradiation, attributed to the delicate construction of the staggered heterojunction.This structural design enhances the efficiency of separating and transferring photo-generated charge carriers

| Band structure and carrier mobility
The determination of work function (Φ) is crucial for unraveling the intricacies of the charge transfer dynamics within the ZSU composite.The work function can be calculated using the contact potential difference (ΔV = Φ − φ), where φ is the work function XPS, being 4.20 eV.The VB spectra in the binding energy range of 0-10 eV reveal a ΔV of 1.80 eV for ZS, resulting in a work function (Φ) of 6.00 eV (Figure 7A).Similarly, work function of ZSU4 is calculated as 5.32 eV (Figure 7B).Previous studies found the work function of UIO to be 4.83 eV. [31]Based on the work functions and the determined band positions of UIO and ZS, the band structures of UIO and ZS before contact, after contact, and under irradiation are shown in Figure 7C-E.Figure 7C illustrates that UIO exhibits higher CB and VB positions, along with a smaller work function compared to ZS. Upon close contact, electrons spontaneously migrate from UIO to ZS, creating an electron depletion layer in UIO and an electron accumulation layer in ZS, resulting in UIO becoming positively charged and ZS negatively charged.This establishes an internal electric field directing from UIO to ZS facilitating the transfer of photogenerated electrons from UIO to ZS, as depicted in Figure 7D, consistent with the XPS results in Figure 3.When UIO and ZS contact, their Fermi energies align to the same level, causing an upward and downward shift in the Fermi levels of UIO and ZS, respectively.Upon light excitation, this band bending and Coulombic attraction between holes and electrons at the interface induces recombination of photogenerated electrons in the LUMO of ZS and holes in the HOMO of UIO at the interface region (Figure 7E). [23]Due to this S-scheme charge transfer, the heterostructure exhibits robust redox ability, facilitating the generation of superoxide and hydroxy free radicals for their involvement in photocatalytic NO oxidation reactions.Therefore, the exceptional photocatalytic performance of the ZSU4 composite can be ascribed to several pivotal factors: the expanded specific surface area, ensuring abundant adsorption sites; the creation of a staggered heterojunction, enabling efficient spatial separation of charge carriers; and, notably, the fine-tuned band structure, fostering the generation of potent ROS.The synergistic effect of these factors substantially enhances the efficiency of the photocatalytic NO oxidation reaction.

| CONCLUSION
In this study, we successfully synthesized an S-type heterojunction photocatalyst, UiO-66-NH 2 /ZnS(en) 0.5 , using a microwave solvothermal method.This innovative structure overcomes the limitations of traditional heterojunctions, including large contact barriers, sluggish charge transport, and compromised redox capacity.The close contact in S-type heterostructure generates a builtin electric field at the UiO-66-NH 2 /ZnS(en) 0.5 interface, facilitating charge separation and migration, as corroborated by TRPL.Notably, the heterostructured photocatalyst exhibits outstanding performance with a 70% NO removal efficiency and 99% selectivity toward nitrates under simulated solar illumination.The superior photocatalytic NO oxidation is attributed to the unique Sscheme, as confirmed by XPS and trapping experiments.The S-scheme, preserving the maximum redox capacity of spatially separated electrons and holes, enables the refinement of ROS to active •O 2 -, which originates from the reduction of adsorbed molecular oxygen by highly active electrons at the LUMO of UiO-66-NH 2 .This study marks a significant advancement in the design of stepped heterostructure photocatalysts, offering valuable insights into their potential for ROS refinement and broader applications in environmental remediation and catalysis.Anhydrous zinc acetate (183.5 mg) was dissolved in a mixture of ethylenediamine (30 mL) and deionized water (10 mL) by stirring for 15 min.Thiourea (228.4 mg) was added to the resulting solution, and the mixture was stirred for an additional 10 min until the translucent crystals dissolved completely.The solution was then transferred to a custom-made 80-mL Teflon reactor and heated in a microwave workstation at a heating rate of 10°C min −1 , holding at 180°C for 1 h.After cooling to room temperature, the precipitate was collected by centrifugation, washed thrice with deionized water and anhydrous ethanol, and dried at 80°C for 12 h.The resulting white powder was designated as ZS.

| Synthesis of UiO-66-NH 2
In a typical synthesis, 204 mg of ZrCl 4 was dissolved in 20 mL of N,N-dimethylformamide (DMF) and stirred for 10 min to obtain solution A. Then, 145 mg of 2-aminoterephthalic acid (H 2 BDC-NH 2 ) was dissolved in 20 mL of DMF and stirred for 10 min to obtain solution B. The obtained precursor solution was mixed, and 5 mL of acetic acid was added and stirred for 10 min until the solution was completely clear.The obtained precursor solution was transferred to an 80 mL polytetrafluoroethylene (PTFE) reaction vessel and moved to a microwave workstation, at a heating rate of 15°C min −1 , holding at 150°C for 30 min, with a stirring speed of 60 r/min.After cooling to room temperature, the suspension precipitate was collected by centrifugation, washed three times with DMF and methanol, and dried at 80°C for 12 h to finally obtain a light-yellow powder named UIO.The preparation steps of UiO-66-NH 2 /ZnS(en) 0.5 composite catalyst are like those of UIO.After obtaining a clear and transparent precursor solution of UiO-66-NH 2 , x mg of ZS sample (x = 80, 100, 120) was added, stirred for 10 min, and sonicated for 20 min to completely disperse the ZS sample in the precursor solution.The mixed suspension was transferred to an 80 mL PTFE reaction vessel and moved to a microwave workstation, at a heating rate of 15°C min −1 , holding at 150°C for 30 min, with a stirring speed of 60 r/min.After cooling to room temperature, the suspension was collected by centrifugation, and the precipitate was washed three times with DMF and methanol, respectively, and dried at 80°C for 12 h to obtain a light-yellow powder, which was named ZSU2, ZSU4, and ZSU6, respectively.

| Characterizations
Using the Bruker D8 Advance XRD, Cu Kα radiation was adopted as the X-ray source, with a testing voltage of 40 kV and a current range of 20-30 mA.The scanning range was from 2θ = 5°to 80°at a scanning rate of 3°/min.PerkinElmer Spectrum 100 FT-IR spectrometer was used with the potassium bromide pellet method and a wavelength range between 500 and 4000 cm −1 .FEI Quanta FEG field-emission scanning electron microscope was utilized for observing the microscopic structure and morphology of the sample surface with nano-resolution.FEI Tecnai G2 F30 TEM was used for observing the fine structural lattice spacing of the sample.Steady-state PL measurements were conducted using a Hitachi F-7100 fluorescence spectrometer, with excitation light having a wavelength of 385/ 350 nm to detect the recombination of photo-generated carriers.To further test the lifetime of photo-generated electrons, a transient fluorescence spectrometer (FLS-1000) from Edinburgh, United Kingdom was used with an excitation wavelength of 380 nm and an emission wavelength of 450 nm to measure time-resolved transient fluorescence spectra.UV-vis spectrophotometer (PerkinElmer Lam750S) was used to set the test wavelength range at 200-800 nm, while the UV-vis-near infrared test was carried out in the range of 800-2000 nm, calibrated with BaSO 4 blank reference samples to correct the baseline.Thermo Fisher K-Alpha+ XPS analyzer was used (Al Kα as the excitation source) to analyze the surface state of the catalyst with the calibration of carbon standard at 284.6 eV.Micromeritics ASAP2020 automatic gas adsorption instrument was used to measure and calculate the specific surface area of catalysts by the BET equation, while the corresponding pore size and pore volume were calculated using the Barret-Joyner-Halenda cylindrical model.The sample pretreatment method involved vacuum drying at 120°C for 12 h.

| Photocatalytic NO oxidation performance
The gas-phase photocatalytic oxidation of NO was carried out in a continuous flow reactor with a volume of 10.5 L cuboid (35 cm × 20 cm × 15 cm) at the ambient temperature.The upper surface of the reactor was sealed with quartz glass, and a 200 W xenon lamp was used to simulate sunlight for photocatalytic reaction.In each experiment, ultrapure air flow containing 1 ppm NO was allowed to pass through an 80 mg photocatalyst at a rate of 4.0 L/min.When the adsorption-desorption equilibrium is reached on the photocatalyst, the light is turned on to initiate the photocatalytic reaction.The concentration of NO and NO 2 was continuously measured by a chemiluminescence analyzer (Thermo Scientific 42i-TL).The NO removal rate (%) was calculated based on the following equation: NO removal rate (%) = (C 0 − C)/ C 0 × 100%, where C 0 and C refer to the NO concentration determined before and after the reaction.

| Photoelectrochemical properties
A standard three-electrode system was employed using 0.5 M aqueous Na 2 SO 4 solution as the electrolyte.The photocatalysts coated on FTO conductive glass were used as the working electrodes.An Ag/AgCl electrode was used as the reference electrode, with a saturated potassium chloride solution, and a platinum plate was used as the counter electrode.
Photocurrent testing and electrochemical impedance spectroscopy (EIS): A simulated solar light source was created using a xenon lamp located 10 cm from the electrolytic cell, and the power supply was switched once every 30 s.The EIS was measured at an open circuit voltage and recorded in a frequency range of 0.01-1 × 10 6 Hz with an AC amplitude of 10 mV.
Mott-Schottky (M−S) testing: According to the M-S equation (Equation 6), the inverse square of the capacitance is linearly related to the applied voltage.A tangent line is drawn on the longest straight part of the M-S curve.The intercept of this tangent line on the Xaxis represents the flat-band potential (E fb ) relative to the reference electrode Transmission electron microscopy (TEM) images, (B) high-resolution TEM (HRTEM) and (C) SEAD images of ZS. (D) TEM and (E) HRTEM images of ZSU4.(F) HAADF-TEM image and (G-L) energy-dispersive X-ray mapping images of ZSU4.

F
I G U R E 4 (A) Ultraviolet-visible diffuse reflectance spectra and (B) bandgap energies calculated by Kubelka-Munk transformation of UIO, ZS, and ZSU4 samples.Mott-Schottky curves of UIO samples (C) and ZS samples (D).(E) Energy band positions of UIO and ZS.CB, conduction band; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; NHE, normal hydrogen electrode; VB, valence band.tests (Figure 5C,D), the addition of PBQ and DDQ significantly inhibits NO oxidation over ZSU4, indicating that O 2 − is the dominant species responsible for NO oxidation.
Photoluminescence (PL) was employed to probe the separation of photogenerated charge carriers as PL signals typically originate from the recombination of

F
I G U R E 6 (A) Linear fits of Mott-Schottky plots of UIO and ZSU4 samples.(B) Transient photocurrent responses of ZS, UIO, and ZSU4.(C) Photoluminescence spectra of different samples excited at 350 nm.(D) Time-resolved photoluminescence spectra of ZS, UIO and ZSU4.T A B L E 1 Fluorescence lifetime parameters fitted by bi-exponential in TRPL spectra.

F
I G U R E 7 Valence band X-ray photoelectron spectroscopy spectra of (A) ZS and (B) ZSU4.(C-E) Schematic of UiO-66-NH 2 /ZnS(en) 0.5 heterojunction with staggered band configuration: (C) before contact, (D) after contact, (E) upon light irradiation in S-scheme mode.CB, conduction band; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

4. 1 . 3 |
Synthesis of UiO-66-NH 2 /ZnS(en) 0.5 ACKNOWLEDGMENTSThis work was supported by the National Natural Science Foundation of China (22106105, 22201180), the Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-E00015), the Shanghai Scientific and Technological Innovation Project (21DZ1206300), the Central Guidance on Local Science and Technology Development Fund of Shanghai (YDZX20213100003002), the Science and Technology Commission of Shanghai Municipality (20060502200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Shanghai Sailing Program (20YF1432200).