Metal selenide photocatalysts for visible-light-driven Z-scheme pure water splitting

Center for Energy & Environmental Scienc Japan. E-mail: domen@chemsys.t.u-tokyo.ac Department of Chemical System Engineerin Tokyo, Tokyo 113-8656, Japan National Synchrotron Radiation Laborato Chemistry for Energy Materials, University 230029, China Institute of Engineering Innovation, The Un Japan Technological Research Association Process (ARPChem), Tokyo 113-8656, Japan Mitsui Chemicals, Inc., Chiba 299-0265, Ja † Electronic supplementary information images, PESA result, Mott–Schottky plots DOI: 10.1039/c9ta00768g ‡ Current affiliation: School of Physical S University, Shanghai 201210, China. § Current affiliation: Department of C Lenseld Road, Cambridge, CB2 1EW, UK { Current affiliation: School of Chemist University, Hefei 230601, China. Cite this: J. Mater. Chem. A, 2019, 7, 7415


Introduction
][8][9][10][11][12] Metal selenide semiconductors have narrower bandgaps than corresponding metal (oxy)suldes due to shallower valence bands formed by Se 4p orbitals than those formed by O 2p and S 3p orbitals.][15] However, their application in photocatalytic pure water splitting has not been achieved so far, which is mainly because the valence band maxima (VBM) of most selenides are more negative than the water oxidation potential and photogenerated holes preferentially oxidize the selenides themselves (photocorrosion) instead of water.For these reasons, selenide semiconductors have been used only for the hydrogen evolution reaction in the presence of sacricial reagents. 16][19][20] Recently, our group has developed a particulate photocatalyst sheet system for Z-scheme pure water splitting, in which the HEP and OEP are bridged by a conductive layer that transfers electrons from the OEP to the HEP. 21Notably, these three components are integrated into one composite having low interfacial resistance, enabling efficient charge transfer and recombination among them.Accordingly, it is considered to apply the photocorrosive metal selenides in the sheet system to enable pure water splitting.
Herein, we show that particulate photocatalyst sheets for visible-light-responsive Z-scheme pure water splitting can be constructed using metal selenides loaded with reduction cocatalysts as HEPs along with CoO x /BiVO 4 and Au as the OEP and electron mediator, respectively.The metal selenides employed here are solid solutions of zinc selenide (ZnSe) and copper gallium selenide (CGSe), which are denoted as ZnSe:CGSe.3][24] More importantly, by tuning the molar ratio of n-type ZnSe and p-type CGSe, the semiconductor character (p/ n type) could also be nely controlled.Therefore, ZnSe:CGSe with different compositions were studied in this work, with the aim of establishing a probable structure-performance relationship.Key factors affecting the Z-scheme process and overall water splitting activity were identied, and guidelines for developing selenide-based photocatalytic systems were developed and provided herein.

Preparation of photocatalysts
ZnSe:CGSe was prepared by a solid-state reaction based on our previous report. 23Typically, Cu 2 Se (99.9%, High Purity Chemicals), Ga 2 Se 3 (99.9%,High Purity Chemicals) and ZnSe (99.99%, High Purity Chemicals) were mixed at Zn/(Zn + Cu) and Ga/Cu molar ratios of 0-1 and 1.0-3.5, respectively, in a glovebox under a nitrogen atmosphere for 30 min.The mixture (1 g in total) was then sealed in a quartz ampule aer drying in a vacuum for 2 h.The sealed ampule was then heated at 1173 K for 10 h.Aer the sample was naturally cooled to room temperature, the product was ground into a powder.
BiVO 4 was synthesized via a solid-liquid reaction. 25Typically, 2.40 g of Bi(NO) 3 $5H 2 O (99.99%, Kanto Chemical Co., Inc.) and 0.45 g of V 2 O 5 (99.6%,Aldrich) were stirred in 0.75 mol L À1 nitric acid aqueous solution (100 mL) for 48 h.The molar ratio of bismuth to vanadium was unity in this suspension.Subsequently, BiVO 4 powder was obtained by ltration and washing with distilled water several times before drying.A traditional impregnation and calcination method was used to load a CoO x cocatalyst on the surface of BiVO 4 .Co(NO 3 ) 2 (99.5%,Wako Pure Chemical Industries, Ltd.) was used as a cobalt precursor.Aer impregnating and drying treatments, 0.5 g of the resultant powder was calcined in air at 673 K for 2 h to obtain a CoO x /BiVO 4 photocatalyst.The content of CoO x (calculated based on the cobalt element) was 0.5 wt% unless otherwise noted.

Preparation of photocatalyst sheets
Photocatalyst sheets based on particulate ZnSe:CGSe and CoO x / BiVO 4 embedded into a Au layer were prepared by the particle transfer method and denoted as ZnSe:CGSe/Au/BiVO 4 . 21nSe:CGSe and CoO x /BiVO 4 particles (10 mg each) were dispersed in 0.5 mL of isopropanol by sonication for 5 min.The obtained suspension was drop-cast on a glass substrate (3 Â 3 cm).Aer drying naturally at room temperature, a thin Au layer was deposited by vacuum evaporation (VFR-200 M/ERH, ULVAC KIKO, Inc.).Then, a post-annealing treatment at 473 K for 10 min in air was carried out.The resulting Au layer holding the photocatalyst particles was rmly attached to a second glass substrate using adhesive carbon tape (Nisshin EM Co., Ltd.), and then the initial glass plate was lied off.Excess particles that had piled up on the particle layer were subsequently removed by sonication treatment in distilled water.

Photodeposition of cocatalysts
Nanoparticulate Pt species was photodeposited on the photocatalyst sheet from H 2 PtCl 6 $6H 2 O (Kanto Chemicals) in distilled water (40 mL).A Cr 2 O 3 layer was subsequently photodeposited from an aqueous K 2 CrO 4 (Kanto Chemicals) solution.Here, 0.16 and 0.08 mmol of H 2 PtCl 6 $6H 2 O and K 2 CrO 4 were used, respectively.The photodeposition reactions were carried out under visible light irradiation (l > 420 nm) from a 300 W Xe lamp (CX-04E, INOTEX CO., LTD., Japan) using a closed circulation system for 2 h each.
Photocatalytic pure-water splitting Z-scheme pure-water splitting reactions were carried out in the same closed gas circulation system with top irradiation using a 300 W Xe lamp equipped with a cut-off lter (l > 420 nm).A photocatalyst sheet (3 Â 3 cm) was placed at the bottom of the reactor containing 40 mL of distilled water.Before photoirradiation, the reaction system was evacuated to ensure complete air removal.A ow of cooling water was used to keep the reaction suspension at approximately 285 K. Gas chromatography (Shimadzu GC-8A with a thermal conductivity detector, 5 Å molecular sieve columns and Ar carrier gas) was used to analyze the evolved gases.

Quantum yield measurement
The apparent quantum yield (AQY) of Z-scheme water splitting was measured using the same experimental setup but with a 420 nm band-pass lter.The number of photons received by the photocatalyst sheet was measured using a grating spectroradiometer (EKO Instruments Co., Ltd., LS-100), and the AQY was calculated using the following equation: where n(H 2 ) and n(photons) indicate the number of H 2 molecules generated and the number of incident photons, respectively.It was assumed that all incident photons were absorbed by the photocatalysts.A coefficient of 4 denotes that the twostep photo-excitation processes are involved.

Solar to hydrogen (STH) energy conversion efficiency measurement
The STH value during photocatalytic Z-scheme pure-water splitting was measured using the same experimental apparatus but with illumination from a solar simulator (Asahi Spectra Co., Ltd., HAL-320).The STH value was calculated by the following equation: where R(H 2 ), DG, P, and S are the rate of hydrogen evolution, the Gibbs free energy for the overall water splitting reaction (H 2 O (l) / H 2 (g) + 1/2O 2 (g)), the energy intensity of the solar light irradiation, and the effective irradiation area, respectively.The value of DG was 226 kJ mol À1 at 285 K at a background pressure of 5 kPa, and the energy intensity of the solar light irradiation was 100 mW cm À2 .

Measurement of PEC performance
ZnSe:CGSe and CoO x /BiVO 4 photoelectrodes were prepared by a particle transfer method, which is similar to the preparation process of the ZnSe:CGSe/Au/BiVO 4 sheet.A Au layer with a thickness of about 2 mm was deposited by vacuum evaporation on a powder layer, which was drop-cast on a glass substrate.The layers holding the particulate particles were then transferred to another glass plate using adhesive carbon tape.Finally, excess particles piled up on the electrode surface were removed by sonication treatment in distilled water so that only a few particles which were in direct contact with the Au layer remained on the surface.A lead wire was connected to the Au layer of the prepared electrode using indium solder, followed by covering the unnecessary part of the electrode with epoxy resin.A three-electrode setup connected with a potentiostat (HSV-110, Hokuto Denko) was used to deposit a Pt cocatalyst and characterize the PEC properties of the prepared photoelectrodes.A Pt wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively.For the photo-electrodeposition of a Pt cocatalyst, a solar simulator (XES-401S2, SAN-EI Electric) producing AM 1.5G simulated sunlight at 100 mW cm À2 was employed as a light source.A constant potential of À0.8 V versus Ag/AgCl was applied to the photoelectrode in an aqueous electrolyte (100 mL) containing 10 mmol L À1 H 2 PtCl 6 , 100 mmol L À1 NaOH and 0.1 mol L À1 Na 2 SO 4 until the saturation of photocurrent.This deposition process lasted for about 1 h.Aer the Pt deposition, the modied photoelectrode was transferred to another cell containing an aqueous solution (pH 6.9) consisting of 0.1 mol L À1 K 2 SO 4 , 0.025 mol L À1 KH 2 PO 4 , and 0.025 mol L À1 Na 2 HPO 4 for photoelectrochemical measurements. 26All of those experiments were performed under purging with an Ar ow.

Materials characterization
The morphologies and elemental compositions of the photocatalyst sheets were assessed by scanning electron microscopy (SEM, S-4700, Hitachi and SU8020, Hitachi) and SEM with energy dispersive X-ray spectroscopy (SEM-EDS, JSM-7001FA, JEOL).X-ray diffraction (XRD) patterns were obtained using a Rigaku MiniFlex 300 powder diffractometer.UV-vis diffuse reectance spectra (DRS) were recorded with a spectrophotometer (V-670, JASCO) equipped with an integrating sphere, with a Spectralon standard as a reference for baseline correction.To determine the work function of the selenides, photoelectron spectroscopy in air (PESA) was conducted using a surface analyzer (AC-3, Riken-Keiki Co., Ltd).A potentiostat (VersaSTAT 3, Princeton Applied Research) was used for the measurement of Mott-Schottky plots.

Results and discussion
Structural and optical properties of ZnSe:CGSe solid solutions Fig. 1A and B show the XRD patterns of ZnSe:CGSe samples with a xed Ga/Cu molar ratio of 2.5 and different Zn/(Zn + Cu) values over the entire compositional range (denoted as (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx ).Herein, the samples for which x ¼ 0 and 1 correspond to CuGa 2.5 Se 4.25 with a chalcopyrite structure and ZnSe with a zincblende structure, respectively.The CuGa 2.5 Se 4.25 specimen was composed of the transition phase of CuGaSe 2 and CuGa 3 Se 5 . 23As shown in Fig. 1B, the diffraction peak at approximately 28 gradually shied to lower angles with increases in Zn/(Zn + Cu), indicating the formation of solid solutions of CuGa 2.5 Se 4.25 and ZnSe.In addition, the peak splitting became smaller as the Zn/(Zn + Cu) value was increased due to the higher symmetry of the (ZnSe) x (CuGa 2.5 -Se 4.25 ) 1Àx crystal structure. 27SEM images indicated that the (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx particles had undened shapes and that the particle sizes ranged from the submicrometer to micrometer scale (Fig. S1 †).Increasing the Zn/(Zn + Cu) ratio produced specimens with larger particle sizes.
To investigate the effect of the Zn/(Zn + Cu) ratio on the band structures of the (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx semiconductors, UVvis DRS of those specimens with different Zn/(Zn + Cu) molar ratios were recorded and are given in Fig. 1C.The absorption edge wavelengths of these selenide solid solutions could be tuned over the range of 480-750 nm.With increases in the Zn/ (Zn + Cu) value, the absorption edge initially shied to longer wavelengths and then to shorter wavelengths in a monotonic fashion, and their relationship is depicted in Fig. S2.† The ionization potentials of the materials were determined by PESA to evaluate the relationship between the VBM potential and the Zn/(Zn + Cu) value.Fig. S3 † gives the dependence of the photoelectron yield on the incident light energy.It is known that the cube root of photoemission yield (Y 1/3 ) is linear to the incident photon energy for semiconductors.The threshold energy corresponds to the ionization potential, which can be determined by the extrapolation method.This obtained energy value is equal to the VBM potential against the vacuum level for semiconductors. 28It is shown that the threshold energy once decreased as the molar ratio of Zn/(Zn + Cu) increased from 0 to 1/6 and then increased gradually with further increase in the Zn/(Zn + Cu) molar ratio.
The band alignments of the (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx samples estimated from the band gap energy and VBM potential data are summarized in Fig. 1D.On the basis of thermodynamics, each of these materials would be expected to reduce protons to produce hydrogen and hence is potentially applicable as a HEP.The potentials of the conduction band minimum (CBM) and VBM were dependent on the Zn/(Zn + Cu) molar ratio but not in a monotonic manner.This is exceptional if one considers the change of the constituent elements alone.The CBM and VBM of (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx are mainly composed of the hybridizations of Zn 4s4p + Ga 4s and Cu 3d + Se 4p orbitals, respectively. 29,30Judging from the relative band positions of ZnSe and CuGa 2.5 Se 4.25 , Zn 4s4p forms a conduction band at a higher (more negative) level than Ga 4s, and the introduction of the Cu 3d orbital shis the VBM potential negatively.This means that the CBM and the VBM of (ZnSe) x (-CuGa 2.5 Se 4.25 ) 1Àx would be higher and lower than those of CuGa 2.5 Se 4.25 , respectively.However, the prepared selenides with x # 1/3 did not follow such a tendency, suggesting contribution from other factors.A similar phenomenon was also reported for the solid solutions of ZnS and CuGaS 2 , and it was deduced that the distortion of the crystal structure and/or MS 4 tetrahedra and the repulsion between Cu 3d + S 3p and Zn 3d might contribute to the aforementioned anomalous result. 27he effects of the Ga/Cu ratio on the structural and optical properties of these ZnSe:CGSe selenides were similarly investigated.Fig. 2A and B show the XRD patterns of (ZnSe) 0.5 (CGSe) 0.5 with the Ga/Cu ratios ranging from 1.0 to 3.5.All of the diffraction peaks are assigned to chalcopyrite structures, indicating the formation of solid solutions of ZnSe and CGSe.With increasing the molar ratio of Ga/Cu, the main diffraction peak located at around 28 gradually shied to higher angles due to rearrangement of the structures. 23SEM images showed that these materials were composed of particles having undened morphologies, coarse surfaces and a wide size distribution, ranging from hundreds of nanometers to several micrometers (Fig. S4 †).UV-vis DRS (Fig. 2C) and the concluded change of the band gap energy (Fig. S5 †) indicated that the band gap energy was increased by approximately 0.1 eV upon increasing the Ga/ Cu molar ratio from 1.5 to 3.5.The PESA results demonstrated that the VBM of these selenides was also slightly shied, from 5.2 to 5.3 eV, upon increasing the Ga/Cu ratio (Fig. S6 †).Considering the similarity in these changes, it is thought that the observed increase in the band gap energy can be primarily attributed to the positive shi in the VBM over this composition range.It is known that the repulsion of Cu 3d and Se 4p orbitals can shi the valence band to higher energies in chalcopyrite compounds such as CuGaSe 2 . 31Therefore, it can be understood that the positive shi of the VBM is mainly due to the decreased repulsion degree with increases in the Ga/Cu molar ratio.

Photocatalytic performance of ZnSe:CGSe/Au/BiVO 4 sheets
Using these ZnSe:CGSe selenides with varying compositions, photocatalyst sheets were fabricated via a particle transfer method, together with CoO x /BiVO 4 (see the XRD pattern and UVvis DRS in Fig. S7 †) and Au as the OEP and electron mediator, respectively (Fig. S8 †). 21Top-view SEM and the corresponding EDS mapping images of a representative sheet, where the selenide has a Zn/(Zn + Cu) value of 0.5 and a Ga/Cu ratio of 2.5, are presented in Fig. 3A and S9.† These images demonstrate that both (ZnSe) 0.5 (CuGa 2.5 Se 4.25 ) 0.5 and CoO x /BiVO 4 particles cover the Au layer.Cross-sectional SEM images and SEM-EDS mapping images (Fig. 3 and S10 †) conrmed that these two different photocatalyst particles were well-distributed and anchored in the Au layer, with a thickness of approximately 500 nm.The photocatalytic pure water splitting activities of ZnSe:CGSe/Au/BiVO 4 sheets with different values of Zn/(Zn + Cu) and Ga/Cu were evaluated under visible light irradiation.Each sheet was modied with a Pt/Cr 2 O 3 cocatalyst prior to the reaction (see the ESI † for details).As shown in Fig. 4A, the incorporation of ZnSe into the CuGa 2.5 Se 4.25 signicantly affected the photocatalytic activity.The water splitting rate was dependent on the Zn/(Zn + Cu) value in a reversed V-shape, peaking at a ratio of 0.5.When the Zn/(Zn + Cu) value was equal to and greater than 5/6, overall water splitting did not occur.Based on this optimal Zn/(Zn + Cu) ratio, (ZnSe) 0.5 (CGSe) 0.5 /Au/ BiVO 4 sheets with different Ga/Cu ratios were subsequently evaluated.As seen from Fig. 4B, there was a similar relationship between the water splitting rate and the Ga/Cu ratio, with the highest activity at a Ga/Cu ratio of 2.5.Time courses of overall water splitting on these sheets indicated that the activity was maintained over the testing time period of 6 h, and the molar ratio of the produced H 2 and O 2 was close to the theoretical value of 2 (Fig. S11 and S12 †).The stability test showed that the optimal (ZnSe) 0.5 (CuGa 2.5 Se 4.25 ) 0.5 /Au/BiVO 4 sheet (AQY: 0.54% at 420 nm and STH energy conversion efficiency: 0.01%) could split water in a stable manner over a span of 17 h under visible light irradiation, indicating that the selfoxidation of (ZnSe) 0.5 (CuGa 2.5 Se 4.25 ) 0.5 was effectively suppressed (Fig. 4C).For a control sheet sample without the Au layer, only H 2 gas with decreased evolution rate was detected, implying that the Z-scheme process was not operational and the selenide photocatalyst underwent photo-corrosion during the reaction (Fig. S13 †).That is to say, the successful stable overall water splitting using the selenides in the form of the photocatalyst sheet is primarily due to the conductive Au layer, which facilitates electron transfer from the BiVO 4 to the ZnSe:CGSe and allows recombination of the transferred electrons with holes to protect the selenide photocatalysts from being oxidized.This result represents the rst-ever demonstration that selenide-based photocatalytic systems can split pure water under visible light irradiation, further extending the application eld of metal selenides in solar energy conversion.Considering that absorption edges of selenides are generally longer than those of sulde and oxide semiconductors composed of the same metal cations, higher solar-to-hydrogen energy conversion efficiency can be theoretically achieved based on selenide semiconductors. 4ysical properties of ZnSe:CGSe dominating the performance of the photocatalyst sheets Our previous work regarding oxysulde-based sheet systems indicated that overall water splitting activity was strongly correlated with the activity of the HEP as a photocathode rather than as a suspended photocatalyst. 32,33Therefore, the PEC properties of ZnSe:CGSe semiconductors embedded into a Au  contact layer and loaded with a Pt cocatalyst were investigated.The Au contact layer and Pt cocatalyst were selected based on the composition of the photocatalyst sheets previously assessed so that their features could be linked with each other.As shown in Fig. 5, these ZnSe:CGSe photoelectrodes generated either cathodic or anodic photocurrents depending on their compositions due to the different semiconductor types.The Mott-Schottky data showed that the (ZnSe) x (CuGa 2.5 Se 4.25 ) 1Àx changed from p-type to n-type when the x value reached 5/6 (Fig. S14 †).The p/n-type transformation was similarly found for the solid solutions of n-type CuInSe 2 and p-type CuGaSe 2 with varied compositions. 34Interestingly, this is also the x value above which the photocatalyst sheets no longer exhibit water splitting activity.Therefore, it is evident that the p-type character of the ZnSe:CGSe semiconductor is a prerequisite to obtaining an effective Z-scheme overall water splitting process when assembled with the n-type BiVO 4 photocatalyst.This result is similar to the case of Z-scheme systems based on reduced graphene oxide as the electron mediator. 26t has been reported that Z-scheme water splitting systems based on solid-state electron mediators require that the HEP photocathode and OEP photoanode generate photocurrents within the same potential window. 35As shown in Fig. 5, all ptype selenide photocathodes showed onset potentials more positive than 0.32 V vs. RHE, which was the onset potential of the n-type CoO x /BiVO 4 photoanode.In this circumstance, these selenide photocatalysts could be coupled with CoO x /BiVO 4 to obtain a Z-scheme process, where electrons from the CoO x / BiVO 4 would be spontaneously transferred via the Au layer to the p-type selenide photocatalyst without an external bias.It is also important to note that the trends in the onset potential data are similar to those shown by the photocatalytic activities in Fig. 4A and B. This result conrms that a selenide photocathode having a positive onset potential and a high photocurrent can drive an efficient photocatalytic Z-scheme water splitting process.The photocatalyst sheet can be regarded as an assembly of numerous miniature p-n PEC cells. 21It is required that the used conductive layer should allow the HEP and the OEP to work efficiently in the forms of photocathode and photoanode, respectively.Thus, it is generally favorable to use large  and small work function materials for HEP and OEP materials, respectively, to minimize the Schottky-type barrier.In the present case, Au with a work function as high as 5.1 eV is applicable because the corresponding CoO x /BiVO 4 photoanode can generate decent photocurrent.The photocurrent and working potential of such cells are determined by the intersection of the current-potential curves of the photocathode and photoanode.As such, a photocathode with a more positive onset potential and higher photocurrent will generate higher working photocurrent in a PEC cell.This working photocurrent without an external bias is generally considered as another indicator of photocatalytic activity in a Z-scheme water splitting process.The higher the working photocurrent, the easier and more efficient the Z-scheme overall water splitting is.Hence, it can be concluded that the PEC characteristics of these materials are closely related to the photocatalytic performance of the particulate photocatalyst sheet system and thus may provide useful information with regard to improving the activity of the system.

Conclusions
This work demonstrated that particulate metal selenides with narrow bandgaps (absorption edges ranging from 480 to 750 nm) could be applied to stable Z-scheme pure water splitting.In this photocatalytic system, ZnSe:CGSe photocatalysts were used as HEPs, in conjunction with CoO x /BiVO 4 and Au as the OEP and electron mediator, respectively.The p-type semiconductor character of the ZnSe:CGSe selenides and the presence of a potential window in which the selenide photocathode and BiVO 4 photoanode can generate photocurrent are essential to obtaining a Z-scheme process.It was also conrmed that the selenides exhibiting a relatively positive onset potential and high photocurrent as photocathodes could achieve higher performance in Z-scheme pure water splitting.This study opens up a new application eld of photocatalytic pure water splitting for metal selenides and provides insights into the construction of selenide-based Z-scheme overall water splitting systems, which are believed to be extended to other solar energy conversion processes.