Article pubs.acs.org/JPCC
Photoelectrochemical Hydrogen Evolution from Water Using Copper Gallium Selenide Electrodes Prepared by a Particle Transfer Method Hiromu Kumagai, Tsutomu Minegishi, Yosuke Moriya, Jun Kubota, and Kazunari Domen* Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: Photocathodes prepared using p-type semiconductor photocatalyst powders of copper gallium selenides (CGSe) were investigated for visible-light-driven photoelectrochemical water splitting. The CGSe powders were prepared by solid-state reaction of selenide precursors with various Ga/Cu ratios. The CGSe photoelectrodes prepared by the particle transfer method showed cathodic photocurrent in an alkaline electrolyte. Pt modification was conducted for all the photoelectrodes by photoassisted electrodeposition. CGSe particles with a Ga/Cu ratio of 2, consisting of the CuGa3Se5 phase and an intermediate phase between CuGaSe2 and CuGa3Se5, yielded the largest cathodic photocurrent. By surface modification with a CdS semiconductor layer, the photocurrent density and onset potential clearly increased, indicating enhancement of charge separation caused by the formed p-n junction with appropriate band alignment at solid−liquid interfaces. A multilayer structure on the particles was confirmed to be beneficial for enhancing the photocurrent, as in the case of thin-film photoelectrodes. A Pt/CdS/CGSe electrode (Ga/Cu = 2) was demonstrated to work as a photocathode contributing stoichiometric hydrogen evolution from water for 16 h under visible light irradiation.
1. INTRODUCTION Photoelectrochemical (PEC) water splitting using sunlight is a beneficial method to efficiently produce hydrogen as a clean, renewable, and sustainable fuel. Since photoelectrolysis using a TiO2 electrode was reported by Fujishima and Honda for the first time,1 a number of attempts have been made to achieve highly efficient hydrogen production from PEC water splitting.2 In thermal equilibrium, an electric field in a semiconductor electrode, which can be represented as band bending in the band diagram, is induced at the solid−liquid interface. Photogenerated electrons and holes are separated by this electric field. Because p-type and n-type semiconductor electrodes have opposite electric field polarities, reduction and oxidation reactions of water occur at the interfaces of the ptype and n-type semiconductor electrodes against an electrolyte, respectively. P-type semiconductor electrodes can act as photocathodes. Si materials,3−8 phosphides,9−12 and chalcogenides13−15 have been intensively investigated for their use as photocathodes for water splitting because of the ability to control their semiconducting properties to be p-type and their appropriate absorption edges for sunlight utilization. Among these, chalcogenides with a chalcopyrite structure have attracted significant attention because of their high optical absorption coefficients of more than 105 cm−1, p-type conductivity, and usability in the polycrystalline state.16−20 Copper gallium selenides (CGSe) are promising candidates for photocathodes for sunlight-driven water splitting because of their stability, long absorption edges, and ability to tune band structure by controlling composition.18−20 The most typical © XXXX American Chemical Society
phase of CGSe is CuGaSe2, which has a chalcopyrite structure with a band gap of 1.7 eV.21 The first application of this material to PEC water splitting was reported by Marsen et al.16 Recently, Moriya et al. reported that CuGaSe2 modified with CdS and Pt showed stable hydrogen evolution for over 10 days in an alkaline solution with an applied bias photon-to-current efficiency (ABPE) of 0.83% at 0.2 V versus reversible hydrogen electrode (RHE).20 Although CuGaSe2 possesses outstanding capabilities in this regard, a CuGaSe2 photocathode requires a large external bias voltage because of the shallow potential of the valence band edge. Kim et al. showed the possibility of tuning the band structure of CGSe through increasing the compositional ratio of Ga/Cu.18,19 Cu-deficient CGSe structures, namely CuGa3Se5 and CuGa5Se8, have a deeper potential at the valence band edge and a larger band gap than CuGaSe2. In most cases of application of CGSe to PEC water splitting, thin films prepared by the vacuum coevaporation method have been employed as photoelectrodes. Using this method, polycrystalline thin films with high quality can be prepared. However, high-vacuum systems are required for this preparation and the materials are synthesized under nonequilibrium conditions. Therefore, this method has difficulties for largescale manufacturing and for the preparation of substances in equilibrium. In the present study, photocathodes prepared Special Issue: Michael Grätzel Festschrift Received: October 6, 2013 Revised: January 16, 2014
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dx.doi.org/10.1021/jp409921f | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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which directly contacted the metal layer remained on the surface. A lead wire was connected to the metal layer of the prepared electrode using indium followed by covering the unnecessary part of the electrode with epoxy resin before PEC measurements. 2.4. Surface Modification with CdS. CdS layers were formed on the surface of the CGSe electrodes by the chemical bath deposition (CBD) method. An aqueous solution containing Cd(CH3COO)2 (Kanto Chemical, 98.0%), SC(NH2)2 (Kanto Chemical, 98.0%), and NH3 (Wako Pure Chemical, 28 wt %) with concentrations of 25 mM, 375 mM, and 14 wt %, respectively, was used as a chemical bath. All of the reagents were added to 25 mL of distilled water and heated to 343 K with mechanical stirring. The prepared CGSe electrodes were immersed in this chemical bath for 30 s. After CBD of CdS, the sample was rinsed with distilled water followed by postannealing for 1 h in air at 473 K. 2.5. Surface Modification with Pt. Pt particles were deposited on the prepared electrodes as hydrogen evolution catalysts by photoassisted electro-deposition. The Pt deposition was performed using a 3-electrode PEC system, and the details of the PEC system are given in Photoelectrochemical Measurements. The electrodes were exposed to the light from a 300 W Xe lamp equipped with filters in the solution containing 10 μM H2PtCl6(Kanto, 98.5%) and 0.1 M Na2SO4 at the potential of −0.80 V versus Ag/AgCl until the saturation of photocurrent. Both the deposition of Pt and the hydrogen evolution reaction occurred on the electrode surface competitively with an increase in the photocurrent because of the enhancement of reaction sites of hydrogen evolution. When the photocurrent was saturated, the Pt deposition reaction might become negligible and only H2 evolution seemed to take place on the electrode surface. The Pt deposition lasted for 0.5−2 h. The example in the case of CGSe electrode (Ga/Cu = 2) is shown in Figure S1 of the Supporting Information. 2.6. Photoelectrochemical Measurements. A typical three-electrode setup was used in the PEC measurements. A Pt wire and Ag/AgCl in saturated KCl were used as the counter and reference electrodes, respectively. All of the PEC measurements were performed under an Ar-saturated atmosphere using a 0.1 M Na2SO4 aqueous solution (pH adjusted to 9.5 by NaOH addition) as the electrolyte. The potentials in each measurement were converted into the values against reversible hydrogen electrode (RHE) by the Nernst equation. A 300 W Xe lamp equipped with a 420 nm cutoff filter (HOYA, L42) and a cold mirror (OPTO-LINE, CM-1) was employed as the light source. (The spectrum of the Xe lamp is shown in Figure S2 of the Supporting Information. The integral light intensity of the visible region (400−800 nm) is 680 mW cm−2.) 2.7. Calculations of the Band Alignments of CGSe Electrodes. The band alignments of the CGSe and CdS/ CGSe electrodes at solid−liquid interfaces were calculated by the finite-difference time-domain (FDTD) method using Poisson’s equation. The difference between the Fermi level (EF) and the potential of the valence band maximum (VBM, EVBM) of CGSe and the difference between EF and the potential of the conduction band minimum (CBM, ECBM) of CdS were assumed to be 0.2 eV. The reported flat band potentials of CGSe and CdS of +0.55 V20 and −0.1 V versus RHE24 at pH 9 were used in the calculations. The band gap values of CGSe and CdS were set to 1.7 and 2.4 eV,25 respectively, and the value of the VBM offset between CGSe and CdS was 0.98 eV.26
using CGSe powder materials are investigated. From the viewpoint of large-scale applicability, the use of powder materials as photoelectrodes is preferable because the preparation method is simple and the cost is low. CGSe powder materials with various Ga/Cu ratios were prepared by a solid-state reaction from selenide precursors. We note that CGSe prepared by the solid-state reaction method can show PEC properties different from those prepared using the vacuum coevaporation method. The photoelectrodes are prepared from CGSe powders through the particle transfer (PT) method.22 The effects of the composition on PEC properties of CGSe and fabrication of a multilayer structure on the electrode are examined here.
2. EXPERIMENTAL SECTION 2.1. Synthesis of CGSe Powders. Micron-sized particles of CGSe were prepared by a solid-state reaction method. The precursor materials, Cu2Se (High Purity Chemicals, 99.9%) and Ga2Se3 (High Purity Chemicals, 99.9%), were mixed in an inert atmosphere in a nitrogen gas-filled glove box. In the mixing process, the ratio of the precursors was changed by changing the molar ratio of Ga/Cu to 1, 2, 2.5, 3, 4, and 5. These mixtures were sealed in quartz ampules after drying in vacuum for 1 h at 423 K. Calcination of the mixtures in ampules at 1173 K for 10 h resulted in the formation of a polycrystalline chunk of CGSe. Micron-sized particles of CGSe were then obtained by the grinding of this chunk using an alumina mortar and pestle. 2.2. Characterizations of CGSe Powders. Structural and compositional analyses of the prepared CGSe powders and electrodes were performed simultaneously using scanning electron microscopy (SEM; Hitachi, S-4700) and energy dispersive X-ray spectroscopy (EDX; Horiba, EMAX-7000), respectively. The CGSe powders were characterized by X-ray diffraction (XRD; Rigaku, RINT-Ultima III) using the Cu Kα line to confirm these crystalline properties and by using UV− visible diffuse reflectance spectroscopy (UV−vis DRS; V-560, Jasco) to reveal the absorption edges. 2.3. Fabrication of Electrodes from CGSe Powders. Photoelectrodes of these CGSe particles were prepared by a particle transfer (PT) method.22 First, a suspension of 30 mg of CGSe powder in 500 μL of isopropanol was dropped onto the primary glass substrate with an amount of 100 μL cm−2 and then dried in air at room temperature to form a CGSe powder layer which completely covered the substrate to a thickness of several tens of micrometers. Two metal layers of Mo and Ti were deposited on the formed powder layer by the radio frequency (RF) magnetron sputtering method as a contact and a conductor layer in sequence. It should be noted that a thin contact layer of Mo was employed for formation of an ohmic contact23 to achieve smooth transfer of carriers between CGSe and a thick conductor layer of Ti. The preparation procedures for the metal layers were as follows. The sample was loaded into a vacuum chamber with a base pressure 1 (Figure 2B), those found at 28.0− 28.2° can be assigned to defect chalcopyrite phases such as CuGa3Se5 and CuGa5Se8.28 In the case of Ga/Cu ratios of 2 C
dx.doi.org/10.1021/jp409921f | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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presence of the intermediate phase seems to increase the photocurrent of the CGSe photocathode. To increase the photocurrent, surface modification with CdS layers on the CGSe electrodes was investigated. Figure 6 shows
Figure 4. (A) Top view and (B) cross-sectional SEM images of CGSe (Ga/Cu = 2) electrode prepared by the particle transfer method.
anchored by the conductor layer (Figure 4B). The Mo layer, which was formed as a contact layer between the CGSe particles and Ti conductor layer, was not clearly observed in the images because it is thinner than 100 nm. We note that these underlying metal layers are exposed to electrolytes during the PEC measurements; however, the exposure do not affect PEC properties at the potential between 0 and +1.23 V versus RHE because of the electrochemical inactivity of Ti layers and thinness of Mo layers. Figure 5 shows current−potential curves for the CGSe photoelectrodes modified with Pt (Pt/CGSe) with various Ga/ Figure 6. Current−potential curves for CGSe, Pt/CGSe, and Pt/CdS/ CGSe electrodes. 0.1 M Na2SO4(aq) (pH adjusted to 9.5 by NaOH addition) and 300 W Xe lamp equipped with filters were used as the electrolyte and light source, respectively. The potential was swept toward the positive direction at 10 mV s−1.
the current−potential curves of the CGSe (Ga/Cu = 2) electrodes with and without surface modifications with Pt and CdS. The photocurrent values at 0 V versus RHE were −0.75, −2.49, and −9.31 mA cm−2 for CGSe, Pt/CGSe, and CGSe electrodes modified with CdS and Pt (Pt/CdS/CGSe), respectively. On the other hand, the onset potentials of the cathodic photocurrent of CGSe, Pt/CGSe, and Pt/CdS/CGSe were found to be +0.41, +0.47, and +0.81 V versus RHE. Through the surface modifications, both the cathodic photocurrent and the onset potential were significantly increased by 12.4 times and 0.40 V, respectively. We note that stoichiometric water splitting using the Pt/CdS/CGSe in the 3-electrode cell was confirmed (see Figure S4 of the Supporting Information) and Pt/CdS/CGSe showed a photocurrent of −2.15 mA cm−2 at 0 V versus RHE under simulated sunlight (see Figure S5 of the Supporting Information). These results clearly demonstrate the usefulness of the surface modification with hydrogen evolution catalysts and thin semiconductor materials even in the case of powder-based photoelectrodes. As reported by our group previously,20 a p-n junction was formed at the CdS/CGSe interface because CdS is ordinarily an n-type semiconductor material. Because the deposited CdS layer was very thin, it was fully in the depletion layer and showed upward band bending to solid−liquid interfaces without the formation of opposite band bending at a potential lower than the onset potential of the cathodic photocurrent as calculated below. The calculated band alignments at electrolyte−electrode interfaces for CdS/CGSe and bare CGSe electrodes are shown in Figure 7. Assuming carrier concentrations of 1017 and 1016 cm−3 for CGSe and CdS, respectively, the thin CdS layer with a thickness of 20 nm was surprisingly positioned entirely within the depletion layer at the applied potential of
