Integrating Zeolite-Type Chalcogenide with Titanium DioxideNanowires for Enhanced Photoelectrochemical ActivityChengyu Mao,†,‡,⊥ Yanxiang Wang,‡ Wei Jiao,‡,§ Xitong Chen,‡ Qipu Lin,‡ Mingli Deng,§ Yun Ling,§

Yaming Zhou,§ Xianhui Bu,*,∥ and Pingyun Feng*,†,‡

†Materials Science and Engineering Program, University of California, Riverside, California 92521, United States‡Department of Chemistry, University of California, Riverside, California 92521, United States§Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai200433, China∥Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach,California 90840, United States

*S Supporting Information

ABSTRACT: Developing photoanodes with efficient visible-light harvesting and excellent charge separation still remains akey challenge in photoelectrochemical water splitting. Herezeolite-type chalcogenide CPM-121 is integrated with TiO2nanowires to form a heterostructured photoanode, in whichcrystalline CPM-121 particles serve as a visible light absorberand TiO2 nanowires serve as an electron conductor. Owing tothe small band gap of chalcogenides, the hybrid electrodedemonstrates obvious absorption in visible-light range.Electrochemical impedance spectroscopy (EIS) shows thatelectron transport in the hybrid electrode has beensignificantly facilitated due to the heterojunction formation.A >3-fold increase in photocurrent is observed on the hybrid electrode under visible-light illumination when it is used as aphotoanode in a neutral electrolyte without sacrificial agents. This study opens up a new avenue to explore the potentialapplications of crystalline porous chalcogenide materials for solar-energy conversion in photoelectrochemistry.

1. INTRODUCTION

Heterometallic chalcogenides are among the most advancedsolid-state materials and have been widely investigated in recentyears.1,2 By combining synthetic and structural concepts frommetal chalcogenide chemistry and nanoporous solids, a numberof highly stable porous chalcogenides with structural andfunctional diversity have been developed.3 Compared to thewell-developed crystalline porous zeolites, which are at thecenter of many industrial revolutions like petroleum refiningand gas separation,4 chalcogenides possess favorable electronicband structures in semiconducting5,6 instead of insulatingrange, and their atomic and electronic structures can beconveniently tuned through either direct synthesis orpostsynthetic modifications by varying cluster composition,type, and size, intercluster connectivity, or doping clusters withheteroatoms.7 Moreover, thanks to the size-tunable buildingblocks in chalcogenides such as supertetrahedral clusters,8 apore size much larger than that in zeolites can be achieved,which makes porous chalcogenides ideal as host frameworks forguest molecules.9 Such integration between unique electronicproperties10 and porosity makes chalcogenides promisingcandidates for many important applications.11,12

The quest for efficient solar-energy conversion into chemicalenergy has stimulated worldwide interest in artificial photosyn-thesis research13,14 since the first discovery of TiO2 photo-catalyst four decades ago.15 Despite their strong opticalabsorption, advantageous band edge positions, high stabilitytoward photocorrosion, low toxicity, and abundance, theefficiency of metal oxide photocatalysts is limited by the largeband gap and severe charge recombination.16 In comparison,heterometallic chalcogenides are known to consist of environ-mentally benign elements and exhibit tunable gaps in thevisible-light range. As such, they are well-suited for solar-energyharvest.17−19 As photovoltaic materials, high solar-energyconversion efficiency has been achieved in solar cell devicesby employing various quaternary chalcogenides such asCu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe), andCuInxGa1−xSe4 (CIGS).20 However, for photocatalytic reac-tions, hole-induced oxidative photocorrosion represents asignificant hindrance for the practical application of metalchalcogenide photocatalysts.21 Moreover, low surface area of

Received: July 12, 2017Revised: October 2, 2017Published: November 15, 2017

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© 2017 American Chemical Society 13634 DOI: 10.1021/acs.langmuir.7b02403Langmuir 2017, 33, 13634−13639

Cite This: Langmuir 2017, 33, 13634-13639

many previously reported chalcogenides also limits the availablesurface-active sites.Integrating merits of different types of materials by forming

hybrid structures could provide a feasible pathway tocircumvent some limitations of single-component photo-catalysts.22−24 For instance, chalcogenides can be anchoredonto a wide band gap photocatalyst to form a heterojunction.25

Electrons generated under visible-light irradiation in theconduction band (CB) of chalcogenides can be effectivelytransferred into the conduction band of the other photocatalyst,which allows for a more efficient solar-light absorption andphotogenerated electron−hole separation. TiO2 nanowirephotoanode sensitized with CdS quantum dots (QDs) orshells is a classic example of heterostructured photoelectrodefor photoelectrochemical (PEC) water splitting.26,27 Morerecently, reduced graphene oxide28 (RGO) sheets, which canoffer excellent electron mobility and high optical trans-parency,29,30 are widely used as an electron acceptor topromote charge separation. In our own work, by integratingRGO sheets and OCF-40-ZnGaSnS chalcogenide (OCF standsfor organically directed chalcogenide frameworks) T4 super-tetrahedral clusters (T4 means there are 4 metal sites along theedge of the tetrahedron), we were able to fabricate a hybridmaterial31 using a two-step approach. The resulting hybridexhibited enhanced photoelectrochemical performance andmuch improved photostability. Despite these aforementionedadvances, hybridizing heterometallic chalcogenide clusters withwide band gap materials as photoanode for visible-lightphotoelectrochemical reactions without sacrificial agent hasnot been explored so far.32

Here, we first report the fabrication of heterostructuralphotoanode by integrating TiO2 with a zeolite-type chalcoge-nide denoted CPM-121-ZnGeGaS, or simply CPM-12133

(CPM stands for crystalline porous materials), and its use forvisible-light photoelectrochemical reactions in neutral electro-lyte without any sacrificial agents. CPM-121-ZnGeGaS was in

situ formed on the vertically aligned TiO2 substrate grown onthe fluorine-doped tin oxide (FTO) current collector. Theporous crystalline CPM-121 serves as a visible-light absorberand could provide more active sites due to its porosity and highsurface area. Simultaneously, TiO2 nanowires work as anelectron conductor to transfer electrons to the FTO currentcollector. The in situ formed heterojunction between thechalcogenide cluster and TiO2 nanowires could facilitate thecharge-separation process and offer a much-improved chargetransfer compared to clusters deposited by postsyntheticelectrophoresis or spin-coating, reported previously.34−36 Theas-fabricated hybrid photoanode shows a significantly enhancedphotocurrent compared to either TiO2 photoanode or CPM-121 photoanode directly deposited on FTO, demonstrating thesynergetic effect of both components.

2. EXPERIMENTAL SECTIONMaterials Synthesis. To integrate zeolite-type chalcogenide CPM-

121-ZnGeGaS with TiO2, TiO2 nanowires were first grown on asubstrate using a hydrothermal method. A clean FTO substrate withconductive side facing down was immersed into the autoclave filledwith a solution of hydrochloric acid and Ti(IV) isopropoxide. Theautoclave was put in a preheated 200 °C oven for 90 min. Afterward,FTO substrate covered with TiO2 nanowires was recovered from theautoclave, dried overnight first, and annealed in air at 400 °C for 1 h.

CPM-121-ZnGeGaS was then fabricated on FTO-TiO2. A slurrycontaining GeO2, Zn(NO3)2·6H2O, Ga(NO3)3·xH2O, sulfur, and 3mL of N-(2-aminoethyl) morpholine (AEM) was first stirredovernight until no aggregates existed. The as-obtained slurry wasdropped onto the TiO2 side of FTO-TiO2 by using a glass rod. FTO-TiO2 was held vertically to remove extra slurry. The process wasrepeated for 3−5 times until the whole FTO-TiO2 surface was coveredby the slurry uniformly. The FTO-TiO2 was then moved into anempty autoclave. The vessel was then sealed and heated at 190 °C for1 week. After cooling to room temperature, the FTO was recoveredand the yellow crystals were grown on the TiO2 surface.

Characterization. The X-ray diffraction (XRD) patterns weremeasured using a Bruker D8 Advance powder diffractometer with CuKα emission. Scanning electron microscopy (SEM) images and

Figure 1. (A) PXRD patterns of FTO-TiO2 and FTO-TiO2-CPM-121. (B) Adamantane cage in CPM-121. (C, D) SEM image and EDX spectra ofFTO-TiO2-CPM-121.

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energy-dispersive X-ray spectroscopy (EDX) were obtained using aNova Nano-SEM450 equipped with Oxford Instruments Aztec energy-dispersive spectroscopy detector. Optical properties were investigatedby a SHIMADZU UV-3150 UV−vis scanning spectrophotometerequipped with an integration sphere.Photoelectrochemical Characterization. Photoelectrochemical

study was performed in a three-electrode configuration with samples asa working electrode, Pt as a counter electrode, and Ag/AgCl as areference electrode. A 150 W xenon lamp coupled with a 420 nmcutoff filter was applied as the visible-light source, and 0.5 M Na2SO4aqueous solution at pH = 6.3 was employed as the electrolyte. ASolartron 1287 electrochemistry interface coupled with a Solartron1260 impedance/gain-phase analyzer was used to collect the data. Thegaseous product on the cathode was collected by electrolyte-displacement method and analyzed by a gas chromatograph(Shimadzu GC-8A) equipped with a thermal conductivity detector(TCD). Stability test was also performed in 0.5 M Na2SO4 solution at1.23 V vs reversible hydrogen electrode (RHE) with a 455 nm cutofffilter.

3. RESULTS AND DISCUSSION

Rutile TiO2 nanowires were grown on FTO by a hydrothermalmethod.37 Both powder X-ray diffraction (PXRD) and SEMcharacterizations (Figure 1A and Figure S1A) confirmed theformation of TiO2 nanowires. Energy-dispersive X-ray spec-troscopy (EDX) collected at the same spot as the SEM imagesreveals that the nanowires are composed of Ti and O (FigureS1B). With its high chemical stability, TiO2 nanowires couldserve as the backbone during the following solvothermalreactions, and crystallized chalcogenide clusters can be grownon its surface. The crystallization of Ge/Sn chalcogenidezeotypes induced by a small amount of divalent metal ions wassystematically investigated in our previous study. Built ofsupertetrahedral T2 clusters, the as-obtained CPM-121 crystalscould be simplified into a noninterpenetrated superdiamond,and its adamantane cage is depicted in Figure 1B. The detailedprocedure for CPM-121 formation on the TiO2 backbone bythe reaction of Zn(NO3)2·6H2O, Ga(NO3)3·xH2O, GeO2, andS with the assistance of N-(2-aminoethyl) morpholine (AEM)template is described in the Experimental Section andSupporting Information. As shown in the SEM image ofFigure S2, TiO2 nanowires kept their original morphology afterits recovery from the autoclave while octahedral-shaped crystalsin a few micrometers were obtained on TiO2 surfaces. From alarger-scale SEM image in Figure 1C, uniformly distributedoctahedral crystals with a narrow size distribution wereobserved on the TiO2 substrate. Diffraction peaks ascribed toCPM-121 were also spotted in the PXRD pattern of the as-obtained sample (Figure 1A) in addition to those signalscoming from TiO2 substrate. The signatures from Zn, Ge, Ga,

and S in EDX spectra (Figure 1D) further verified theformation of CPM-121. Moreover, the elemental mapping(Figure S3) revealed that those signals only came from thelocations where the octahedral crystals were formed. Morespecifically, S and Ge elements were homogeneouslydistributed on the octahedral-shaped crystals while Ti signalswere coming from the nanowires. Given the small amount ofZn and Ga in the chalcogenide cluster, their signals were notstrong in the elemental mapping. Powder samples prepared inthe autoclave were also examined by powder X-ray diffraction.Their crystallinity and purity were further verified by comparingthe X-ray diffraction pattern obtained from the powders andthat simulated from single-crystal data (Figure S4).The optical properties of the heterostructural electrodes as

well as FTO-TiO2 were investigated by UV−vis absorbancespectra.38 As shown in Figure S5A, crystalline CPM-121powder showed strong absorption in the entire range ofwavelengths, making it an ideal light absorber for efficient solar-energy harvest. The band gap was determined to be 1.94 eVfrom its corresponding Tauc plot (Figure S5B). On the otherhand, Figure 2A showed that TiO2 nanowires grown on FTOcurrent collector only demonstrated a strong absorbance below420 nm, consistent with its intrinsic large band gap of 2.97 eV.When the TiO2 photoelectrode was integrated with thechalcogenide crystals, an obvious absorption in the visible-light region was observed. Furthermore, the light absorption inthe UV region was also enhanced. The results confirmed theeffectiveness of the heterostructure for light harvest andsuggested that the hybrid electrode may absorb more photonsto generate electrons and holes and thus enhance thephotoelectrochemical activity. Tauc plot of the heterostruc-tured photoelectrode in Figure 2B revealed two distinct bandgaps at 2.99 and 1.84 eV, corresponding to the band gap ofrutile TiO2 and CPM-121, respectively. An X-ray photoelectronspectroscopy valence band (XPS-VB) spectrum was alsocollected for CPM-121 (Figure S6), and the valence bandmaximum was determined to be at 0.56 eV. Conduction bandminimum of rutile and CPM-121 could then be estimated usingtheir measured band gap, which were at −1.12 and −1.28 eV,respectively.39 As a result, a type-II gap could be formedbetween rutile and CPM-121 to facilitate charge transport. Theslight deviation of the band gap from each individualcomponent may be the result of strong interfacial interactionbetween the two components.The visible-light-driven photoelectrochemical activity of the

hybrid electrode was studied here. The transient photocurrentsof the FTO-TiO2-CPM-121, FTO-TiO2, and CPM-121 crystalsdeposited directly on the FTO by electrophoresis were

Figure 2. (A) UV−vis spectra and (B) Tauc plots of FTO-TiO2-CPM-121 and FTO-TiO2.

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recorded in Figure 3A during repeated on/off illuminationcycles at 1.0 V vs RHE. The RHE potential was converted fromthe saturated Ag/AgCl reference electrode using ERHE =E(Ag/AgCl) + 0.57 V. All samples exhibited prompt andreproducible responses toward light illumination, with photo-currents decreasing to almost zero upon irradiation interruptionand fully recovering upon light switching on. The spikeobserved upon irradiation was due to the rapid effect uponpower excitation, and then it quickly returned to the steadystate. The photocurrent of FTO-TiO2-CPM-121 reached 55μA/cm2 upon visible-light illumination, whereas FTO-TiO2 isonly 16 μA/cm2. The >3-fold enhancement in photocurrentconfirmed the synergistic effect of the heterostructure, whereCPM-121 acts as an efficient visible-light absorber while TiO2substrate contributed to the electron transfer and suppressedcharge recombination. The gaseous product generated on thecounter electrode (Pt) was collected by electrolyte displace-ment and analyzed by gas chromatography, and the results arepresented in Figure S10. Signals from air were used as areference. Only a prominent H2 peak was identified from thegaseous product on the cathode side and confirmed as theproduct of the photoelectrochemical reaction. Action spectra ofphotocurrents of FTO-TiO2-CPM-121 and FTO-TiO2 werealso obtained by measuring the photocurrents under each 20nm wavelength interval from 300 to 600 nm. The spectra inFigure S8 further demonstrated that the hybrid electrode has asuperior activity in both UV and visible-light range due to thesynergistic effect between the small band gap CPM-121 andTiO2 nanowires, which was consistent with their opticalproperties and the photocurrent measurements discussedabove. Although CPM-121 powder demonstrated strong lightadsorption in the entire light range, it only exhibits a limitedphotocurrent of 1.4 μA/cm2 upon visible-light illumination.The poor photoelectrochemical performance should beascribed to the difficult electron transport between the directlydeposited crystals and the current collector, which could lead tosevere photoexcited electron−hole pairs recombination. Notethat the photocurrent achieved by the FTO-TiO2-CPM-121here was even >2 times higher than that by Pt-loaded CPM-120photoanode with sacrificial agent.33 This superior performanceby FTO-TiO2-CPM-121 hybrid electrode may result frommore efficient charge separation owing to the heterojunctionformation, as well as the enhanced light harvesting. This resultwas further confirmed by the linear sweep voltammetry (LSV)in Figure 3B, where FTO-TiO2 only achieved a photocurrent of30.5 μA/cm2 at 1.23 V vs RHE. In contrast, FTO-TiO2-CPM-121 also exhibited a substantially higher photocurrent over theentire potential range. At the same 1.23 V applied voltage,

FTO-TiO2-CPM-121 showed a nearly 2-fold enhancement inphotocurrent density, up to 59.6 μA/cm2. On the other hand,CPM-121 only exhibited an inferior photoelectrochemicalperformance and a much smaller photocurrent of 12.2 μA/cm2 at 1.23 V vs RHE.To gain further insight into the charge-transport behaviors of

all samples, electrochemical impedance spectroscopy (EIS)measurements were performed on the electrodes studied above.A Mott−Schottky plot was generated based on the capacitancederived from the impedance at different applied potentials ofFTO-TiO2-CPM-121 (Figure S9) as well. The positive slope inthe linear part of the Mott−Schottky plot described thebehavior of an n-type semiconductor, which was working as aphotoanode in the photoelectrochemical cell. The semicircle inthe as-obtained Nyquist plots was associated with the charge-transfer process, and a smaller arc corresponds to smallerimpedance and a better charge separation. As shown in Figure4A, FTO-TiO2-CPM-121 photoanode exhibited a much smaller

radius than FTO-TiO2, further proving the advantageouscharge separation upon heterojunction formation in the as-fabricated photoanode. The equivalent circuit shown in FigureS11 was employed to further analyze the Nyquist plot, where Rsstands for a series resistance, Rct stands for charge-transferresistance, CPE stands for a constant phase element forimperfect capacitance, and Ws is Nernst diffusion impedance.While Rs did not change much between FTO-TiO2 and FTO-TiO2-CPM-121(from 46.9Ω to 40.3Ω), Rct decreased signifi-cantly from 11 891Ω to 1 889Ω, proving the beneficial chargetransfer in the hybrid material. Meanwhile, CPM-121 powderdeposited on FTO current collector had a significantly largersemicircle (Figure 4B), suggesting the poor electron transportwithin the electrode and consistent with our observations inphotoelectrochemical tests. The photocurrent of FTO-TiO2-CPM-121 in the presence and the absence of H2O2 under AM

Figure 3. (A) Transient photocurrents and (B) linear sweep voltammetry of FTO-TiO2-CPM-121, FTO-TiO2, and CPM-121 deposited on theFTO electrode.

Figure 4. (A) Nyquist plot of FTO-TiO2-CPM-121, FTO-TiO2, and(B) CPM-121 deposited on the FTO electrode.

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1.5 illumination was measured to estimate the hole-injectionyield. Because the injection yield becomes 100% when H2O2 isin the electrolyte, the charge-injection yield into water can beestimated by dividing two photocurrents. As shown in FigureS12, the hole-injection yield rises steeply for FTO-TiO2-CPM-121 and remains close to 100% at high potentials. Stability testwas also performed on FTO-TiO2-CPM-121, and the result isdisplayed in Figure S13. The as-fabricated photoanodeexhibited a steady activity, with 90% photocurrent remainingafter 30 min of visible-light illumination.

4. CONCLUSIONS

In summary, we have rationally designed a heterostructuredphotoanode consisting of heterometallic zeolite-type chalcoge-nide CPM-121 and TiO2 nanowires for efficient solar-energyconversion. The crystalline porous chalcogenides act as avisible-light absorber to generate electrons and holes uponirradiation, while the TiO2 nanowire serves as the scaffold forchalcogenide formation and facilitates the electron transferduring the photoelectrochemical reactions. The interfaceformed between two components significantly promotes chargeseparation and reduces the electrochemical impedance. As aresult, a profoundly enhanced photoelectrochemical activity inthe neutral electrolyte is realized under visible-light illuminationon the hybrid electrode. Given the richness of compositionaland topological variations in crystalline porous chalcogenidematerials and their tunable optical and electronic properties,this study opens up a new avenue to explore their potentialapplications for solar-energy conversion in photoelectrochem-istry.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.lang-muir.7b02403.

Experimental details, XRD patterns, SEM, EDX, UV−visspectra, Mott−Schottky plot, and GC profile (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: xianhui.bu@csulb.edu.*E-mail: pingyun.feng@ucr.edu.

ORCIDYun Ling: 0000-0002-6956-4504Yaming Zhou: 0000-0003-2720-9536Xianhui Bu: 0000-0002-2994-4051Pingyun Feng: 0000-0003-3684-577XPresent Address⊥C.M.: Energy and Transportation Science Division, Oak RidgeNational Laboratory, One Bethel Valley Road, P.O. Box 2008,Oak Ridge, TN 37831, United States.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by the National ScienceFoundation (DMR-1506661, P.F.). W.J. thanks the financialsupport of Fudan University and the NSFC (51402314, W.J.)

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DOI: 10.1021/acs.langmuir.7b02403Langmuir 2017, 33, 13634−13639

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