ARTICLE pubs.acs.org/JPCC
Effect of Thermal Treatment on the Crystallographic, Surface Energetics, and Photoelectrochemical Properties of Reactively Cosputtered Copper Tungstate for Water Splitting Yuancheng Chang,† Artur Braun,†,‡ Alexander Deangelis,† Jess Kaneshiro,† and Nicolas Gaillard*,† † ‡
Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, € berlandstrasse 129, CH - 8600 D€ubendorf, Switzerland U ABSTRACT: In this Article, we report the effect of postdeposition thermal treatment on reactively cosputtered copper tungstate (CuWO4) thin films and its impact on photoelectrochemical performances. This study indicates that CuWO4 films fabricated at 275 °C were amorphous and did not show significant photoresponse in 0.33 M H3PO4 electrolyte when irradiated with air mass 1.5Global simulated illumination. However, a major improvement in photoelectrochemical performance was observed in identical test conditions after a postdeposition treatment performed at 500 °C in pure argon for 8 h. Indeed, a photocurrent density of approximately 400 μA cm 2 at 1.6 V vs saturated calomel electrode was measured on the annealed CuWO4 samples. Subsequent X-ray diffraction analysis revealed a clear transformation of as-deposited amorphous thin films into a triclinic CuWO4 structure after the annealing step. The polycrystalline CuWO4 films exhibited n-type conductivity, an indirect band gap of 2.25 eV, and a flat-band potential of 0.35 V vs saturated calomel electrode.
I. INTRODUCTION Copper tungstate, also known as copper tungsten oxide (CuWO4), has been widely studied for optoelectronic applications such as detectors,1 optical fibers,2 and lasers.3 In its crystalline form (wolframite-type structure), CuWO4 owns n-type conductivity and an indirect optical band gap (EG) of 2.3 eV,4,5 making this material a promising photoanode in photoelectrochemical cells (PEC) for water splitting applications.6 Extensive studies have been carried out on CuWO4 growth using a wide range of synthesis methods including double-decomposition chemical reaction,7 solid-state reaction,8,9 top-seeded solution growth,10 and spray deposition.11 As far as its use in PEC water splitting applications, only a few studies have been reported so far. Arora et al. prepared CuWO4 single crystals (EG = 3.88 eV) using the flux reaction method, followed by thermal annealing in an argon atmosphere at 750 °C for as long as 80 h.12 Subsequent electrochemical measurement showed that their CuWO4 had a donor concentration of 7.79 1017 cm 3 and a flat-band potential of 0.48 V vs saturated calomel electrode (SCE) in KCl/H2SO4 electrolyte (pH 1.5). Recently, Yourey et al. reported the synthesis of copper tungstate by electrochemical deposition.13 After annealing in air at 500 °C for 2 h, electrodeposited CuWO4 films exhibited n-type conductivity, an indirect band gap of 2.25 eV, and a donor density of 2.7 1021 cm 3. Subsequent PEC tests done on these films in a pH 7 potassium phosphate buffer pointed out that electrodeposited CuWO4 was photoactive, with a photocurrent density of 160 μA cm 2 at 0.5 V vs Ag/AgCl. Interestingly, the authors reported that their electrodeposited CuWO4 r 2011 American Chemical Society
films were more stable under both dark and illuminated conditions than polycrystalline WO3. Finally, physical vapor deposition was recently used by Chen et al. as a method to synthesize amorphous copper tungsten oxide thin films.14 The resulting films were mainly p-type and had a modest photoresponse in pH 13 NaOH solution to visible light illumination. In this Article, we report the results of our studies on reactive magnetron co-sputtering for the synthesis of copper tungstate thin films. The effect of postdeposition thermal annealing on the morphology and crystalline structure of CuWO4 thin films is presented. The PEC properties of as-deposited and annealed CuWO4 are compared using electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). A tentative description of CuWO4 surface energetic is also presented.
II. EXPERIMENTAL METHODS Copper tungstate thin films were deposited by reactive cosputtering method using metallic Cu (99.99%) and W (99.95%) targets (2 in. diameter). The substrates (SnO2:F-coated glass, TEC 15 Hartford Glass) were first cleaned in a soap/DI-water/ acetone/DI-water/methanol/DI-water/isopropanol sequence, then placed in the deposition chamber, about 20 cm above the targets, and heated to ∼275 °C (temperature measured on the SnO2:F-side of the substrate). After reaching a base pressure of Received: August 1, 2011 Revised: November 7, 2011 Published: November 14, 2011 25490
dx.doi.org/10.1021/jp207341v | J. Phys. Chem. C 2011, 115, 25490–25495
The Journal of Physical Chemistry C 2 10 6 Torr or lower, argon (Ar) and oxygen (O2) gas flow was set for an [O2]/([Ar]+[O2]) ratio of 36%. A manual gate valve was used to fix the working pressure at 9.0 mTorr. The copper to tungsten ratio in the films was controlled by adjusting the radio frequency (RF) power applied to each target. Specifically, the RF power applied to the copper target was fixed at 250 W, whereas the one applied to the W target varied from 50 to 200 W. As the co-sputtering deposition method naturally yields composition nonuniformity across samples, particular care was taken to perform analyses in a systematic fashion on samples located at the exact same position on the substrate holder during each deposition run. Film thicknesses (typically 2 μm) were measured by profilometry using a Tencor Alpha Step 200 profilometer. Postdeposition thermal treatment was performed in argon at 500 °C in a ceramic tube furnace (Micropyretics Heater International, Inc.). The surface morphology before and after annealing was characterized with a Hitachi S-4800 scanning electron microscope (SEM) at an acceleration voltage of 20 kV. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS) using an Oxford INCA Energy 250 3 with INCA PentaFETx-3 SiLi detector. To investigate the effect of thermal treatment on the CuWO4 crystal structure, X-ray diffraction (XRD) analysis was performed before and after thermal treatment with a Rigaku Miniflex II X-ray diffractometer using a Cu cathode (Kα, 1.546 Å) powered at 20 kV and 20 mA. The reflectance and transmission of annealed CuWO4 films were measured with a Lambda 2 Perkin-Elmer spectrophotometer from 350 to 1100 nm. All PEC characterizations presented in this Article were carried out according standard protocols published by Chen et al.15 Briefly, electrochemical analyses were performed with a Gamry Reference 600 potentiostat. The test cell consisted of a CuWO4 PEC electrode (typically 1.5 cm2), a platinum foil counter electrode (10 cm2), and an SCE reference electrode. Aqueous 0.33 M H3PO4 electrolytes (pH 1.35) were used for all tests. Photoanodes were irradiated with simulated air mass 1.5Global (AM1.5G) illumination provided by an Oriel class A solar simulator equipped with a xenon bulb and an AM1.5G filter. The irradiance of the simulator was calibrated using an International Light Technologies 900 spectrophotoradiometer. This calibration was done such that the Xe bulb output power within CuWO4 absorption range (up to 560 nm) matches with the AM1.5G radiance in the same spectral range. Potentiostatic electrochemical impedance spectroscopy (EIS) was performed at frequencies ranging from 0.1 Hz to 1 MHz (AC voltage = 10 mV rms) to identify CuWO4 flat-band potential and carrier concentration. Linear sweep voltammetry was measured in three-electrode configurations with a scan rate of 25 mV s 1. Finally, photocurrent spectroscopy was performed under monochromatic light (Spectral Products) to verify the band gap of CuWO4 samples.
III. RESULTS AND DISCUSSION As expected with co-sputtering deposition, the W-to-Cu ratio was found to be greatly dependent on the sputtering RF powers. It was observed that tungsten-poor films were obtained with W target RF power below 200 W. Specifically, atomic [W]/[Cu] ratios of 0.08, 0.30, and 0.74 were obtained for RF powers of 50, 100, and 150 W, respectively. However, with a W target RF power of 200 W, stoichiometric ([W]/[Cu] = 1.05) CuWO4 was obtained. Subsequent thermal annealing (500 °C, in argon for 8 h)
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Figure 1. SEM micrographs acquired on (a) as-deposited and (b) annealed CuWO4 films.
was performed on stoichiometric CuWO4 synthesized with these specific deposition parameters. EDS analysis showed that the composition of the films did not change with annealing. The surface morphology of CuWO4 thin films before and after annealing is presented in Figure 1. The as-deposited CuWO4 films are made of small, globular grains with diameters ranging from 50 to 100 nm. The topography of these as-deposited films is smooth, as all grain surfaces appear to be flat and on the same plane. However, after annealing, these grains coalesced to form large clusters, exhibiting sharper edges. In this case, the surface looks rather rough, made of jagged grains. This change in grain morphology suggests that a structural transformation took place during the annealing treatment. To confirm this hypothesis, X-ray diffractograms were recorded on CuWO4 samples before and after thermal treatment (Figure 2). Before annealing, all observed peaks correspond to F-doped SnO2 (FTO) substrate (Powder Diffraction File, PDF 77-0447), indicating that a reactive sputtering process performed at 275 °C led to amorphous CuWO4 thin films. However, after annealing, XRD analysis reveals that amorphous CuWO4 thin films became crystalline, with a triclinic structure (PDF 21-0307). Copper tungstate thin film crystallization with similar annealing conditions was also reported elsewhere.11,13 Figure 3 presents Tauc plots of as-deposited and annealed CuWO4 films derived from reflection and transmission spectra, assuming indirect optical transition.16 Here, optical band gaps of 2.17 ((0.05) and 2.25 ((0.05) eV were obtained for as-deposited and annealed CuWO4, respectively. These band gap values are typical as are others reported for this material.4,13 When compared to other metal oxides such as tungsten oxide (EG = 2.6 eV17,18), the absorption properties of CuWO4 are more suitable for practical PEC applications, in which a band gap value of approximately 2.0 eV 25491
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The Journal of Physical Chemistry C
Figure 2. X-ray diffraction spectra measured on as-deposited and annealed CuWO4 thin films (scan rate: 0.025 deg s 1).
Figure 3. Tauc plots of as-deposited (9) and annealed (O) CuWO4 thin films derived from transmission and reflection spectra.
is considered as ideal.19,20 It should be mentioned that the best strategy to reduce the band gap of WO3 is to raise the position of the valence band maximum (VBM),21 located approximately 1.75 V below the oxygen evolution reaction potential.22 In the case of CuWO4, it has been experimentally demonstrated that the presence of the Cu(3d) orbital raises the position of VBM by 0.5 eV when compared to that in pure WO3,13 as predicted also by first principal calculation.23 Thus, as far as its optical properties and band-edges positions are concerned, CuWO4 should be a promising candidate for solar water splitting in photoelectrochemical cells. Subsequent EIS analyses were performed on CuWO4 films. Measurements were done under AM1.5G illumination at potentials between 0.1 and 0.5 V vs SCE for as-deposited samples and between 0.1 and 0.9 V for annealed samples, with frequencies ranging from 0.1 Hz to 1 MHz. Figure 4 presents the Nyquist plots obtained at 0.5 V vs SCE under illumination on asdeposited and annealed CuWO4. A distinct electrochemical behavior between amorphous and polycrystalline CuWO4 was observed here. For as-deposited samples, Nyquist plots obtained at all applied potentials were incomplete semicircles in the frequency range employed in this experiment. It should be mentioned that the charge transfer resistance estimated from the impedance spectra (>10 000 Ω cm2) is rather large. In the case of polycrystalline samples, Nyquist plots obtained at all potentials contain one complete semicircle and one incomplete semicircle at high and low frequencies, respectively. A similar Nyquist plot was
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Figure 4. Nyquist plots of as-deposited (9) and annealed (O) CuWO4 thin films measured in 0.33 M H3PO4 at 0.5 V vs SCE under AM1.5G illumination.
Figure 5. Mott Schottky plots of as-deposited (9) and annealed (O) CuWO4 thin films measured in 0.33 M H3PO4 under AM1.5G illumination.
also reported on single crystalline CuWO4.16 Here, polycrystalline CuWO4 exhibited a lower charge transfer resistance (approximately 2200 Ω cm2) when compared to its amorphous counterpart. However, it should be mentioned that this value is 1 order of magnitude larger than the charge transfer resistance observed for reactively sputtered polycrystalline tungsten oxide24 (approximately 200 Ω cm2). Depletion capacitances of the CuWO4 thin films were then calculated from the EIS data using constant phase element (CPE) equivalent circuits (Rsol Csc// (Rct ΦCPE), where Rsol, Csc, Rct, and ΦCPE represent the resistance of the H3PO4 solution, the capacitance of the space charge region in CuWO4, the charge transfer resistance in CuWO4, and the constant phase element, respectively). Next, Mott Schottky (MS) plots were created to determine conductivity type, flatband potential (Vfb) and charge carrier density (ND) (Figure 5). Amorphous and crystalline CuWO4 were concluded to be p-type (negative MS slope) and n-type (positive MS slope), respectively, which support the findings of previous studies.12 14 The intercepts of the two MS plots with the potential axis indicate flatband potentials of 0.35 and +0.54 V vs SCE for crystalline and amorphous CuWO4, respectively. The flat-band potential measured on annealed samples is within other values reported for monocrystalline12 ( 0.48 V vs SCE in pH 1.5 solution) and polycrystalline11 ( 0.14 V vs SCE in pH 1.0 solution) CuWO4. As compared to the flat-band potential reported on WO324 (+0.15 V vs SCE), this value corresponds to a cathodic shift of 25492
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The Journal of Physical Chemistry C approximately 500 mV. To the best of our knowledge, no flatband value has been reported for amorphous CuWO4 yet. Charge carrier densities were then calculated using the slopes of the MS curves12 (using a dielectric constant of 83 for CuWO425). For the annealed CuWO4 films, the charge carrier density ND was calculated to be 4.68 1019 cm 3, a value in the same order of magnitude as that reported for monocrystalline CuWO416 (5.50 1019 cm 3) but lower than others recently reported for polycrystalline CuWO4 thin films13 (2.7 1021 cm 3). Nevertheless, it should be mentioned that all of these ND values are relatively high when compared to the ones generally admitted for nondegenerated semiconductors (
