WO3/Zn0.5Cd0.5S S 型異質(zhì)結(jié)光催化產(chǎn)氫耦合有機(jī)物轉(zhuǎn)化機(jī)理研究

摘要：開發(fā)新型納米材料實(shí)現(xiàn)光催化產(chǎn)氫耦合有機(jī)物轉(zhuǎn)化、提高太陽能到化學(xué)能的轉(zhuǎn)換效率，在解決能源和環(huán)境危機(jī)方面具有巨大潛力。三元金屬硫化物具有可調(diào)控的帶隙和優(yōu)異的可見光響應(yīng)，在光催化分解水產(chǎn)氫方面引起了廣泛關(guān)注。其中，Zn0.5Cd0.5S是一種帶隙較窄、導(dǎo)帶位置較高、耐光腐蝕的還原型光催化劑；然而，單一Zn0.5Cd0.5S中光生電子和空穴的復(fù)合率較高，只有少部分光生載流子參與光催化反應(yīng)，導(dǎo)致量子效率較低而無法達(dá)到實(shí)際需求。WO3是一種典型的氧化型光催化劑，具有較低的價帶位置和較強(qiáng)的氧化能力，是與Zn0.5Cd0.5S耦合構(gòu)建S型異質(zhì)結(jié)的理想半導(dǎo)體?；诖?， 本文通過靜電紡絲和水熱方法將Zn0.5Cd0.5S納米片垂直生長在WO3納米纖維上， 制備了具有核殼結(jié)構(gòu)的WO3/Zn0.5Cd0.5S異質(zhì)結(jié)。功函數(shù)的差異驅(qū)動Zn0.5Cd0.5S的電子轉(zhuǎn)移到WO3上，在界面處形成內(nèi)建電場并使能帶彎曲。通過原位光照X射線光電子能譜、電子順磁共振和時間分辨熒光光譜分析，發(fā)現(xiàn)在內(nèi)建電場、彎曲能帶和庫侖吸引力的作用下，WO3導(dǎo)帶上的光生電子遷移到Zn0.5Cd0.5S價帶上并與其光生空穴復(fù)合，表明WO3和Zn0.5Cd0.5S之間形成了S型異質(zhì)結(jié)，實(shí)現(xiàn)了具有強(qiáng)氧化還原能力的載流子的高效分離。得益于獨(dú)特的S型光催化機(jī)制以及反應(yīng)物在催化劑表面的有效吸附與活化，沒有貴金屬助催化劑的情況下，WO3/Zn0.5Cd0.5S異質(zhì)結(jié)在產(chǎn)氫（715 μmol?g?1?h?1）和乳酸轉(zhuǎn)化為丙酮酸方面表現(xiàn)出增強(qiáng)的光催化活性，實(shí)現(xiàn)了光生電子和空穴的高效利用。原位漫反射傅里葉變換紅外光譜和密度泛函理論計(jì)算揭示了光催化產(chǎn)氫和有機(jī)物轉(zhuǎn)化的反應(yīng)機(jī)理。本工作為設(shè)計(jì)和研究新型S型異質(zhì)結(jié)光催化劑、實(shí)現(xiàn)高效產(chǎn)氫耦合有機(jī)物轉(zhuǎn)化提供了新的見解。

關(guān)鍵詞：三氧化鎢；S型異質(zhì)結(jié)；產(chǎn)氫；有機(jī)物轉(zhuǎn)化；化學(xué)吸附與活化

中圖分類號：O643

Insights into Photocatalytic Mechanism of H2 Production Integrated with Organic Transformation over WO3/Zn0.5Cd0.5S S-Scheme Heterojunction

Abstract： Developing novel nanostructures to enhance the efficiency of solar-tochemicalconversion through integrated photocatalytic hydrogen （H2） evolution andorganic transformation holds great promise in addressing pressing energy andenvironmental crises. Ternary metal sulfides have garnered considerable attention inphotocatalytic H2 production due to their tunable bandgap and excellent visible lightresponse. Among them， Zn0.5Cd0.5S stands out as a reduction photocatalyst with anarrow bandgap， a high conduction band level， and excellent resistance tophotocorrosion. However， unitary Zn0.5Cd0.5S suffers from a high recombination rate ofphotogenerated electron/hole pairs， resulting in only a small fraction of charge carriersbeing involved in the photoreactions， leading to a low quantum efficiency that falls shortof practical demand. WO3， a typical oxidation photocatalyst with a lower valence bandposition and strong oxidization ability， is an ideal candidate for constructing an S-scheme heterojunction with Zn0.5Cd0.5S.Herein， a core-shell structured WO3/Zn0.5Cd0.5S heterojunction with Zn0.5Cd0.5S nanosheets vertically growing out of WO3nanofibers is fabricated through electrospinning and hydrothermal methods. The distinct disparity in work functions leadsto the transfer of electrons from Zn0.5Cd0.5S to WO3 upon contact， creating an interfacial electric field （IEF） andsimultaneously bending the energy bands at the interface. As a consequence of IEF， bent energy bands， and coulombattraction， the photogenerated electrons in the conduction band of WO3 migrate to the valence band of Zn0.5Cd0.5S andrecombine with its photoinduced holes， signifying the formation of an S-scheme heterojunction between WO3 andZn0.5Cd0.5S and enabling efficient separation of powerful charge carriers， as evidenced by in situ irradiated X-rayphotoelectron spectroscopy， electron paramagnetic resonance， and time-resolved fluorescence spectroscopy analyses.Benefiting from the unique S-scheme photocatalytic mechanism， along with the effective chemisorption and activation ofreactants on the catalyst， the optimized WO3/Zn0.5Cd0.5S heterostructures exhibit exceptional photocatalytic performancein H2 production （715 μmol?g?1?h?1） and the transformation from lactic acid to pyruvic acid without the need for any noblemetal cocatalyst， achieving the full utilization of photoinduced electrons and holes. In situ diffuse reflectance infraredFourier transform spectroscopy， as well as density functional theory simulations， reveal the photoreaction mechanism ofH2 production and organic transformation. This work offers valuable insights into the design and investigation of themechanism behind novel S-scheme heterojunction photocatalysts， enabling high-performance H2 production andsimultaneous organic transformation.

Key Words： Tungsten oxide; S-scheme heterojunction; Hydrogen production; Organic transformation;Chemisorption and activation

1 Introduction

The overuse of fossil fuels in recent decades has led to theongoing energy and environmental crises， posing a significantthreat to the sustainable development of human society 1–5. Cleanhydrogen （H2） energy has emerged as a compelling alternativedue to its regenerative nature， minimal pollution， and highenergy density 6?11. Solar-driven photocatalytic water splittingoffers a host of advantages， including harnessing inexhaustiblesolar energy， ensuring sustainability， cost-effectiveness， andachieving reasonable solar-to-hydrogen efficiency. It stands outas one of the most promising and economically viableapproaches to reduce fossil fuel consumption and associatedgreenhouse gas emissions 12?15. Currently， significant researchefforts are dedicated to the photocatalytic half-reaction for H2production from water. This involves introducing holescavengers， such as methanol and triethanolamine， into thesystem to extend the lifespan of photogenerated electrons，thereby enhancing H2 production performance 16–20. However，the presence of molecular scavengers hinders the effectiveutilization of energetic holes， resulting in inefficient energydissipation. Additionally， using sacrificial agents for H2production is environmentally unfriendly and increases the costof photocatalytic processes. Thus， making full use ofphotogenerated electrons and holes upon light illumination is anurgent challenge that needs to be addressed 21–26.

Ternary metal sulfides have attracted considerable attention inphotocatalytic H2 production due to their tunable bandgap andexcellent visible light response. Among them， ZnxCd1?xS standsout as an n-type reduction photocatalyst with a narrow bandgap，a high conduction band （CB） level， and excellent resistance tophotocorrosion 27–31. By adjusting the ratio of Zn2+ and Cd2+ions， photocatalysts with optimal band structures can besynthesized， with Zn0.5Cd0.5S demonstrating the highestphotocatalytic performance. However， similar to othermonocomponent photocatalysts， unitary Zn0.5Cd0.5S suffersfrom a high recombination rate of photoinduced electron/holepairs 32–34. Only a small fraction of electrons and holes areinvolved in the photoreactions， resulting in a low quantumefficiency that falls short of practical demand 35–38. Therefore， itis desirable， yet challenging， to develop Zn0.5Cd0.5S-basedheterojunction photocatalysts with efficient separation ofphotogenerated electron/hole pairs and high quantumefficiency 39–43.

The construction of S-scheme heterojunctions involvingZn0.5Cd0.5S for integrated photocatalytic H2 production andorganic photosynthesis is acknowledged as a potential solutionto address the aforementioned challenges 44–46. In this process，ineffective photoinduced electrons and holes tend to recombine，while the powerful photoelectrons in the CB of the reductionphotocatalyst and photoholes in the valence band （VB） of the oxidation photocatalyst are preserved to reduce H2O to yield H2and drive the oxidative organic transformation to produce valueaddedchemicals， respectively. Such a strategy enables theefficient separation of charge carriers with strong redox abilitiesand maximizes the utilization of the reactive electrons and holesderived from solar conversion 47–53. Tungsten oxide （WO3）， atypical oxidation photocatalyst with a lower VB position andstrong oxidization ability， is an ideal candidate for constructingan S-scheme heterojunction with Zn0.5Cd0.5S 54. Specifically，one-dimensional （1D） WO3 nanofibers， prepared throughelectrospinning， have garnered significant interest owing to theirimproved specific surface areas， reduced charge transport length，inert aggregation and increased active sites， etc. In this work， wesynthesized electrospun WO3 nanofibers and then grewZn0.5Cd0.5S nanosheets on the nanofiber surface via a simplelow-temperature hydrothermal method to constructWO3/Zn0.5Cd0.5S nanostructures for photocatalytic H2production and organic transformation. Density functionaltheory （DFT） calculations， in situ X-ray photoelectronspectroscopy （XPS）， electron paramagnetic resonance （EPR），and time-resolved fluorescence spectroscopy （TRPL） analysesconfirmed the formation of an S-scheme heterojunction betweenWO3 nanofibers and Zn0.5Cd0.5S nanosheets， endowing thecomposite with exceptional photocatalytic performance for H2Oreduction and lactic acid （LA） oxidation. This work providesvaluable insights into the design of novel S-schemeheterojunction photocatalysts for high-performance H2production and concurrent organic transformation.

2 Experimental details

2.1 Preparation of photocatalysts

WO3 nanofibers were synthesized via the electrospinningmethod. Firstly， tungsten hexachloride （WCl6， 1.0 g） andpolyacrylonitrile （PAN， 0.5 g） were dissolved in 5 mL of N，Ndimethylformamide（DMF） through magnetic stirring for 12 h atroom temperature. The resulting dark-blue solution was thenloaded into an electrospinning setup using a syringe. Theelectrical potential and solution-feeding rate were set at 20 kVand 0.2 mL?h?1， respectively. Subsequently， the obtained WO3nanofiber precursors were calcinated at 300 °C for 1 h in air，followed by further heating at 500 °C for another 5 h to obtainpale yellow WO3 nanofibers.

For the synthesis of WO3/Zn0.5Cd0.5S nanohybrids， zincacetate dihydrate （0.8 mmol）， cadmium acetate dihydrate （0.8mmol） and thiourea （3.2 mmol） were dissolved in 40 mL ofethanol to form a homogeneous solution. Then， 60 mg of WO3nanofibers were added to the solution. After vigorous stirring for10 min， the resulting suspension was transferred to a Teflonlinedstainless-steel autoclave and heated at 120 °C for 12 h.Upon cooling， the yellow WO3/Zn0.5Cd0.5S product wasseparated by centrifugation， thoroughly washed with water andethanol， and then dried in an oven at 60 °C. For comparison， pureZn0.5Cd0.5S was also prepared under the same conditions without the addition of WO3 nanofibers. The synthesizedWO3/Zn0.5Cd0.5S photocatalysts are denoted as WZCx （x = 20，30， 40）， where W and ZC represent WO3 and Zn0.5Cd0.5S，respectively; x indicates the mass percentage of WO3 toZn0.5Cd0.5S.

2.2 H2O photoreduction integrated with organicphotooxidation

The performance of the photoreduction of H2O coupled withthe photooxidation of organics was evaluated using a sealed andN2-filled 150 mL three-necked Pyrex flask. Typically， 20 mg ofphotocatalysts were dispersed in 50 mL of an aqueous solutioncontaining LA at a concentration of 0.1 vol% （volume fraction）under magnetic stirring. To maintain an anaerobic condition， thesystem was purged with N2 for 30 min using a multi-channelatmosphere controller （PLA-MAC1005， Beijing Perfectlight，China）. Subsequently， the system was irradiated for 1 h using a300 W Xenon arc lamp （PLS-SXE300+， Beijing Perfectlight，China）. After the photocatalytic reaction， the generated gas wasanalyzed using a gas chromatograph （GC-14C， Shimadzu，Japan）， and the liquid products were detected using highperformanceliquid chromatography equipped with a UV-visiblelight detector （SPD-20A， Shimadzu， Japan）， a column oven（CTO-20A， Shimadzu， Japan）， a low-pressure gradient unit （LC-20AD， Shimadzu， Japan）， and a degasser （DGU-20A5R，Shimadzu， Japan）. The mobile phase consisted of a 10 mmol?L?1potassium dihydrogen phosphate solution with a flow rate of 1.0mL?min?1.

3 Results and discussion

3.1 Characterization of WO3/Zn0.5Cd0.5S nanostructures

The field emission scanning electron microscopy （FESEM）image of pristine WO3 unveils a uniform nanofibrousmorphology characterized by diameters ranging from 200 to 300nm and a conspicuously rough surface （Fig. 1a）. Subsequent tothe solvothermal process， an abundance of vertically alignedZn0.5Cd0.5S nanosheets emerges from the WO3 nanofibers， asexemplified in Fig. 1b，c. The high-resolution transmissionelectron microscopy （HRTEM） image of the WO3/Zn0.5Cd0.5Snanostructure （inset of Fig. 1c） reveals an interplanar crystalspacing of 0.31 nm， precisely coinciding with the （111） facet ofZn0.5Cd0.5S. No observable lattice spacing of WO3 is detected asit resides within the composite nanofibers. The energy dispersiveX-ray spectroscopy （EDS） elemental mappings （Fig. 1d）demonstrate a homogeneous distribution of W， O， Zn， Cd and Selements， thus validating the successful hybridization of WO3and Zn0.5Cd0.5S.

The X-ray diffraction （XRD） pattern of pristine WO3nanofibers exhibits characteristic peaks corresponding to themonoclinic phase （JCPDS No.20-1324） （Fig. 1e）. The obtainedpure Zn0.5Cd0.5S displays crystal properties associated with thehexagonal wurtzite phase of CdS （JCPDS No.41-1049） 55，56 andZnS （JCPDS No.36-1450） 57， albeit with slight peak shifts. This occurrence arises from the larger atomic radius of Cd comparedto Zn， which induces changes in the lattice constant andinterplanar spacing during the formation of a solid solution （Fig.S1）. For the composite WZCx， diffraction peaks of both WO3and Zn0.5Cd0.5S are detected， further confirming the formation ofWO3/Zn0.5Cd0.5S nanohybrids and agreeing well with the aboveanalyses. The optical absorption capabilities of WO3，Zn0.5Cd0.5S， and WO3/Zn0.5Cd0.5S heterojunctions wereinvestigated via ultraviolet-visible diffuse reflectance spectrum（UV-Vis DRS）. As shown in Fig. 1f， bare WO3 and Zn0.5Cd0.5Sexhibit intrinsic absorption edges at around 510 and 480 nm，indicating bandgaps （Eg） of 2.40 and 2.62 eV， respectively. Theobtained WZC30 demonstrates enhanced absorption of visiblelight due to the strong light absorption properties of WO3， whichis beneficial for efficient light harvesting to improve thephotocatalytic performance.

XPS was employed to investigate the chemical states andsurface composition of WO3， Zn0.5Cd0.5S， and theWO3/Zn0.5Cd0.5S heterostructure. Fig. 2a illustrates the presenceof W， O， Zn， Cd， and S elements in WZC30. The high-resolutionXPS spectra of W 4f in Fig. 2b display two pairs of doublets，where the peaks at 35.7 and 37.8 eV are assigned to W6+， whilethe others located at 35.2 and 37.3 eV are attributed to W5+ 28，58.Deconvolution of the O 1s XPS spectra （Fig. 2c） uncoversdistinct peaks corresponding to lattice oxygen （530.5 eV），surface-adsorbed oxygen （ ― OH） （532.1 eV）， and vacancyoxygen （533.3 eV） in the samples. Additionally， the detection ofZn 2p， Cd 3d， and S 2p doublets in pure Zn0.5Cd0.5S and WZC30composite （Fig. 2d–f） provides compelling evidence for theexistence of Zn2+， Cd2+， and S2? in both samples. It is worth noting that in the dark， the binding energies （BEs） of W 4f andO 1s in WZC30 shift negatively compared to pure WO3， whilethe peaks of Zn 2p， Cd 3d， and S 2p exhibit positive shifts withrespect to pristine Zn0.5Cd0.5S. Such an interesting phenomenonsuggests the transfer of electrons from Zn0.5Cd0.5S to WO3 uponhybridization.

3.2 Insights into charge transfer and separationmechanism

To investigate the mechanism of charge transfer and separationin the WO3/Zn0.5Cd0.5S heterostructures， the band structure ofWO3 and Zn0.5Cd0.5S was initially examined. The VB-XPSspectra （Fig. S2） reveal that the difference between the VBmaximum and the flat band potential （Efb） of WO3 andZn0.5Cd0.5S is determined to be 2.43 and 1.30 eV， respectively.According to the Mott-Schottky plots as depicted in Fig. S3， theEfb values of WO3 and Zn0.5Cd0.5S are derived to be +0.1 and?0.5 V （vs. RHE）， respectively. By incorporating the bandgapsdisclosed in the inset of Fig. 1f， the band structure of WO3 andZn0.5Cd0.5S is calculated and illustrated in Fig. S4， where WO3functions as the oxidation photocatalyst and Zn0.5Cd0.5S acts asthe reduction photocatalyst.

The work function （Φ） plays a crucial role in exploring thepathway of charge transfer. Analysis of the electrostatic potentialprofiles reveals that the Φ values of WO3 （001） and Zn0.5Cd0.5S（100） are determined to be 6.8 and 5.1 eV， respectively，suggesting a lower Fermi level （EF） of WO3 compared toZn0.5Cd0.5S （Fig. 3a，b）. Furthermore， the results obtained fromultraviolet photoelectron spectroscopy （UPS） estimate the EFvalues of WO3 and Zn0.5Cd0.5S to be ?4.22 and ?1.02 eV，respectively， relative to the vacuum level （Fig. S5）， which alignswith the above DFT simulation. As a consequence of thedifference in work function and Fermi level， electrons will migrate from Zn0.5Cd0.5S to WO3 upon their contact untilequilibrium is reached at the interface. Such charge transferresults in the formation of an internal electric field （IEF） directedfrom Zn0.5Cd0.5S to WO3， and meanwhile bends their energybands at the nanostructure interfaces （Fig. 3c）， corroborating theabove XPS analysis.

Under light irradiation， electrons in both WO3 and Zn0.5Cd0.5Sare initially excited from the VB to the CB. Driven by the IEF，band bending， and coulomb interactions， the photogeneratedelectrons in the WO3 CB preferentially transfer to the Zn0.5Cd0.5SVB and recombine with its photoexcited holes， therebypreserving powerful photoelectrons in the Zn0.5Cd0.5S CB andphotoholes in the WO3 VB to participate in the photoreactions.This distinctive charge transfer and separation mechanism signify the formation of an S-scheme heterojunction betweenWO3 and Zn0.5Cd0.5S， enabling efficient separation ofphotogenerated carriers and imparting the heterojunction withexceptional redox ability （Fig. 3c）. In situ irradiated XPS spectrareveal that the BEs of W 4f and O 1s in WZC30 exhibit positiveshifts， while the peaks of Zn 2p， Cd 3d and S 2p shift towardslower binding energy with reference to those recorded in the dark（Fig. 2b–f）. These intriguing findings demonstrate the transportof photoelectrons from WO3 to Zn0.5Cd0.5S， reaffirming the Sschemephotocatalytic mechanism 59–61.

EPR measurements were conducted using 5，5-dimethyl-1-pyrroline N-oxide （DMPO） as a spin trapping agent to elucidatethe charge transfer pathway within the WO3/Zn0.5Cd0.5Sheterostructure. As depicted in Fig. 3d， the mono-componentWO3 and Zn0.5Cd0.5S exhibit relatively weaker DMPO-·O2? andDMPO-·OH signals， which can be attributed to their bandstructures and the rapid recombination of charge carriers. Incontrast， the WZC30 composite displays stronger DMPO-·O2?and DMPO-·OH signals， indicating the effective separation andpreservation of highly energetic electrons in the Zn0.5Cd0.5S CBand holes in the WO3 VB， providing further compelling evidencefor the S-scheme charge separation mechanism. The deeperinsights into the charge separation efficiency and charge transferdynamics of S-scheme heterojunctions were further investigatedvia TRPL. According to the steady-state photoluminescencespectra of pure WO3 and Zn0.5Cd0.5S （Fig. S6）， TRPL spectrawere recorded at an emission wavelength of 630 nm， where thefluorescence signal is predominantly contributed by Zn0.5Cd0.5S.As depicted in Fig. 3e， the heterostructure WZC30 exhibitslonger lifetimes compared to pristine Zn0.5Cd0.5S， indicating themigration of photogenerated electrons from the WO3 CB to theZn0.5Cd0.5S VB， where they recombine with holes.Consequently， a greater accumulation of photoelectrons occursin the Zn0.5Cd0.5S CB， extending the lifetime and furtheraffirming the S-scheme charge transfer route within theWO3/Zn0.5Cd0.5S heterojunctions. Notably， the introduction oflactic acid （LA） to the system significantly reduces the averagelifetime of WZC30， indicating the efficient photoreduction ofH2O integrated with the photooxidation of LA， thereby leavingfew photocarriers available for the recombination. Furthermore，the photoelectrochemical results （Fig. S7） demonstrate thatWZC30 exhibits a higher photocurrent density and smallerradius of Nyquist plots with respect to pure WO3 andZn0.5Cd0.5S， testifying the enhanced charge separation efficiencyand reduced charge transfer resistance in the WO3/Zn0.5Cd0.5Sheterojunction. These analyses suggest that the formation of anS-scheme heterojunction between WO3 nanofibers andZn0.5Cd0.5S nanosheets promotes efficient charge transfer andenables effective separation of powerful electrons and holes，thereby boosting the photocatalytic performance.

3.3 Photocatalytic performance towards H2Oreductionand LA-oxidation

The adsorption and activation of reactants on catalysts play a molecule exhibits strong chemical adsorption on the Zn0.5Cd0.5Ssurface， characterized by an adsorption energy （Eads） of ?1.65eV， along with charge transfer as revealed in the charge densitydifference （Fig. S8a）. The integrated crystal orbital Hamiltonpopulation （ICOHP） value of O-H pairs in free H2O is predictedto be ?7.39 eV （Fig. S8b）. Upon adsorption on the Zn0.5Cd0.5Ssurface， the ICOHP value increases to ?7.01 eV， and new O-Cdspecies are formed with an ICOHP value of ?0.50 eV （Fig. S8c），providing further evidence of the activation of the H2O moleculeon the Zn0.5Cd0.5S surface. Furthermore， when the LA moleculeadsorbs onto the WO3 surface， LA donates 0.15 electrons toWO3 with an Eads of ?1.91 eV （Fig. S9）， indicating a strongchemisorption interaction between LA and WO3. These DFTresults suggest that Zn0.5Cd0.5S and WO3 are the preferred activesites for H2O photoreduction and LA photooxidation，respectively.

The photocatalytic performance of all the samples in H2Oreduction and LA oxidation was evaluated through experimentsin which water was split in a 0.1 vol% LA solution under UVvisiblelight irradiation. Control experiments were carried out toverify that no products were detectable in the absence ofphotocatalysts or light irradiation. Fig. 4a illustrates that theWO3/Zn0.5Cd0.5S S-scheme heterojunctions exhibit significantlyenhanced H2 production performance compared to pure WO3and Zn0.5Cd0.5S， achieving a maximum production rate of 14.3μmol?h?1 （715 μmol?g?1?h?1 at 20 mg of photocatalysts） overWZC30 with an apparent quantum efficiency （AQE） of 8.3% at420 nm. Further analysis of the liquid products unveils a gradualoxidation of LA to pyruvic acid （PA） accompanied by thegeneration of H2 （Fig. 4b）. The conversion of LA and theselectivity of PA over WO3， WZCx， and Zn0.5Cd0.5S after a sixhourirradiation demonstrate that the composite WZC30 exhibitsthe highest LA conversion and PA production， with a PAselectivityof ～82% （Fig. 4c，d）. Such outstanding performancetowards H2O reduction and LA oxidation of theWO3/Zn0.5Cd0.5S nanohybrids aligns with our expectation and ismainly attributed to the efficient activation of reactants on thecatalyst surface， as well as the unique S-scheme chargeseparation mechanism.

Gibbs free energy calculations and in situ diffuse reflectanceinfrared Fourier transform spectroscopy （DRIFTS） wereemployed to investigate the reaction mechanism of H2Ophotoreduction and LA photooxidation. The Gibbs free energyof H* （ΔGH*） was first analyzed to assess the efficiency ofphotocatalytic H2 production. A ΔGH* value approaching zeroindicates a more favorable condition for H* adsorption， therebyenhancing the evolution of H2. Fig. 4e demonstrates that activeH* exhibits a preference for adsorption on S sites， with a ΔGH*value of 0.91 eV， compared to Zn and Cd （instability） sites，suggesting that S serves as the active site for H2Ophotoreduction.

In situ DRIFTS spectra （Fig. 4f） reveal that after introducing H2O and LA into the system in the dark， the detectedcharacteristic peaks assigned to the out-of-plane bendingvibration of ―OH （675 cm?1）， the deformation vibration ofmethylene （1300 cm?1）， the symmetric bending of ―CH3 （1360cm?1）， the stretching vibration of ―COOH （1550 cm?1）， thestretching vibration of the carbonyl group （1730 cm?1）， thestretching vibration of hypomethyl （3000 cm?1）， the stretchingvibration of ―OH （3200 cm?1）， and the H2O signal （3500 cm?1）strongly support the chemisorption of reactants on the WZC30surface. Upon light irradiation， the peaks at 1300， 3000， 3200，and 3500 cm?1 diminish， while a new signal corresponding topyruvate emerges at 1210 cm?1， suggesting continuousconsumption of the adsorbed H2O and LA， leading to thegeneration of H2 and PA. Furthermore， Gibbs free energy change（ΔG） calculations were performed to investigate the elementaryreactions involved in the conversion of LA to PA. As depicted in Figs. 4g and S10， the rate-limiting step for both WO3 as thecatalyst and the reaction without a catalyst is thedehydrogenation of the adsorbed LA. Notably， WO3 has a lowerΔG value compared to the reaction with no catalyst （0.71 vs. 1.73eV）， highlighting the feasibility of LA photooxidation over theWO3 surface.

A cyclic H2 production test was conducted to investigate thephotostability of the nanohybrids. As revealed in Fig. S11， theWO3/Zn0.5Cd0.5S heterojunction （WZC30） shows a negligibledecrease in H2 production after four cycles， indicating thesatisfactory stability of the composite. The XRD pattern andFESEM image （Fig. S12） of the spent WZC30 demonstrate nosignificant alterations in phase and morphology when comparedto the fresh one， reaffirming the photostability ofWO3/Zn0.5Cd0.5S heterostructures. Comparing the XPS spectraof WZC30 before and after photoreaction （Fig. S13）， it can beobserved that the BEs of S 2p and O 1s shift negatively andpositively， respectively， which is attributed to the weakened IEFfollowing the photoreaction. The chemical states of all elementsreveal negligible change， underscoring the satisfying stability ofthe WO3/Zn0.5Cd0.5S heterojunction photocatalyst. Generally，the severe photocorrosion of sulfides mainly results from theoxidation of S2? ions by photogenerated holes. However， in theWO3/Zn0.5Cd0.5S heterojunction， the S-scheme charge transfermechanism enables the photogenerated electrons in the WO3 CBto effectively recombine with the holes in the Zn0.5Cd0.5S VB，thereby preventing the oxidation of S2? in Zn0.5Cd0.5S and thusmitigating the issue of photocorrosion to improve thephotostability.

4 Conclusion

In conclusion， unique WO3/Zn0.5Cd0.5S S-schemeheterojunction photocatalysts were synthesized usingelectrospinning and low-temperature hydrothermal methods.DFT calculations and UPS results demonstrated that the Fermilevel of WO3 was lower than that of Zn0.5Cd0.5S， resulting inelectron transfer from Zn0.5Cd0.5S to WO3 upon contact， therebygenerating an IEF and bending the energy bands at the interface.Under light irradiation， the photogenerated electrons migratedfrom the WO3 CB to the Zn0.5Cd0.5S VB driven by the IEF， bentenergy bands and coulomb attraction， as supported by in situXPS and TRPL analysis. Such charge transfer mechanismresulted in the formation of an S-scheme heterojunction betweenWO3 and Zn0.5Cd0.5S， facilitating efficient separation ofphotoinduced charge carriers with high redox capabilities.Thanks to this unique S-scheme photocatalytic mechanism，combined with the effective chemisorption and activation ofreactants on the catalyst surface， the resulting WO3/Zn0.5Cd0.5SS-scheme heterostructures exhibited outstanding photocatalyticperformance in H2O reduction and LA oxidation， without theneed for any noble metal cocatalysts， thus fully utilizingphotogenerated electrons and holes. In situ DRIFTS， togetherwith DFT simulations， provided insights into the photoreaction mechanism involved in the production of H2 and PA. This studypresents a fresh perspective on the design and fabrication ofnovel S-scheme heterojunction photocatalysts， enabling theenhancement of solar-to-chemical conversion efficiency throughthe integration of H2 evolution with value-added chemicalsproduction.

Author Contribution： S.C. and F.X. conceived anddesigned the experiments. S.C. conducted material synthesis andphotocatalytic test， and wrote the draft. S.C.， B.Z.， C.B.， B.C.and F.X. contributed to data analysis. F.X. supervised theproject， performed DFT calculations， and revised themanuscript. All authors discussed the results and commented onthe manuscript.

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國家重點(diǎn)研究與發(fā)展計(jì)劃（2022YFB3803600， 2022YFE0115900）， 國家自然科學(xué)基金（52003213， 22238009， 22261142666， 52073223， 22278324， 51932007）以及湖北省自然科學(xué)基金（2022CFA001）資助