Letter www.acsami.org
Exposure of WO3 Photoanodes to Ultraviolet Light Enhances Photoelectrochemical Water Oxidation Tengfei Li, Jingfu He, Bruno Peña, and Curtis P. Berlinguette* Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T1Z1, Canada S Supporting Information *
ABSTRACT: Exposure of WO3 photoanodes to sustained irradiation by ultraviolet (UV) light induces a morphology change that enhances the photoelectrochemical (PEC) activity towards the oxygen evolution reaction (OER). A 30% enhancement in photocurrent density at 1.23 V vs RHE was measured despite a nominal change in onset potential. A structural and electrochemical analysis of the films before and after exposure to UV irradiation indicates that a higher film porosity and correspondingly higher specific surface area is responsible for the enhancement in PEC activity. The effect of prolonged UV irradiation on the WO3 films is fundamentally different to that which was previously observed for BiVO4 films. KEYWORDS: tungsten oxide, photoelectrocatalysis, solar energy, ultraviolet light, water splitting
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increase in PEC activity towards the OER reaction under AM 1.5 sunlight. The increase in PEC enhancement in the case of WO3 is due to a larger accessible surface area created by the UV irradiation step. This result starkly contrasts the effect UV light has on BiVO4, which results in a lower film porosity (a suppression of surface defects was instead responsible for the enhanced PEC activity).15 The WO3 photoanodes were synthesized by a modified spincoating method previously reported by Guo and co-workers.16 Tungsten hexachloride (0.397 g, 1.00 mmol) was dissolved in 4.00 mL of ethanol and sonicated for 15 min prior to spincoating on a conducting layer of fluorine-doped tin oxide (FTO) at 1000 rpm for 30 s (see Supporting Information). Each of the six coats were annealed at 500 °C for 10 min prior to an 8-h annealing step at 500 °C after the last coat was applied. The as-prepared WO3 samples were exposed to a UV lamp in air for 4 h (λmax ≈ 254 nm and 185 nm; flux ≈ 10 mW/ cm2). The WO3 samples before (denoted “as-prepared”) and after (denoted “irradiated”) the films were subjected to UV irradiation to study changes in morphology and physical properties caused by UV radiation. The thickness of the WO3 films were around 1 mm based on cross-sectional SEM images (Figure S1). Scanning electron microscopy (SEM) images of as-prepared WO3 (Figure 1a) reveal a morphology feature with densely packed grains of WO3, which is in accordance with previous descriptions of
hotoelectrochemical (PEC) water splitting offers a direct means to store solar energy as hydrogen fuels.1−4 The efficiency of this process is linked to several parameters, including how effectively light is absorbed and how the kinetically challenging oxygen evolution reaction (OER) is managed.3−5 There is therefore a large effort to increase the performance of PEC cell assemblies through the discovery and modification of photoanodes, with and without auxiliary electrocatalyst coatings, that are competent at both absorbing incident visible light and mediating the OER at a fast rate with minimal energy losses.6−11 Composite photoanodes (i.e. a photoanode coated with an electrocatalyst layer) are widely used to separate the light-absorbing and catalyst functions, but are susceptible to many issuessuch as the electrocatalyst layer absorbing incident lightthat uncoated photoelectrodes do not suffer from.12,13 Driving up the efficiency of photoelectrodes still requires a better understanding of the physical and chemical processes that occur at the solid/liquid interface. Smith and co-workers recently reported that the PEC performance of BiVO 4 photoanodes can be increased by exposure to AM 1.5 illumination in solution.14 We have previously demonstrated that exposure of BiVO4 to sustained UV photolysis in air can produce a marked cathodic shift in onset potential and doubling of photocurrent at 1.23 V vs RHE due to the elimination of deleterious surface defects.15 This UV treatment is a simple, stable, and efficient approach for enhancing BiVO4 photoanode performance, raising the question of how UV irradiation affects other semiconductor photoanode materials. We show herein that exposure of WO3 to sustained UV irradiation in air prior to PEC testing also yields a substantial © XXXX American Chemical Society
Received: July 13, 2016 Accepted: August 29, 2016
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DOI: 10.1021/acsami.6b08152 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. (a, b) SEM images, (c) powder XRD diffractograms, and (d) UV−vis absorptance data recorded on WO3 films coated on FTO glass before (as-prepared) and after (irradiated) exposure to 4 h of UV irradiation. Reflections attributed to FTO are indicated by black diamonds in (c).
WO3 photoanodes.16,17 Images of irradiated WO3 (Figure 1b) show a higher film porosity. Atomic force microscopy (AFM) imaging also indicated a higher roughness factor for the films subjected to UV irradiation (Figure S2). The diffractograms of WO3 did not show any discernable differences in reflections caused by UV irradiation (Figure 1c), nor did the X-ray photoelectron spectra (XPS) show any differences in the W 4f and O 1s signals before and after irradiation (Figure S3 and Table S1). Moreover, the absorption spectra (Figure 1d) of the as-prepared and irradiated WO3 films were also superimposable, ruling out differences in absorptivity being responsible for the PEC enhancement. The photocurrent densities of as-prepared and photolyzed WO3 photoanodes were measured under AM 1.5 front-side illumination in a three-electrode cell containing a 0.1-M potassium phosphate (KPi) solution buffered to pH 7 (Figure 2). The as-prepared WO3 samples that were not exposed to UV
then kept in air for 3 days prior to PEC measurements (Figure S4). This behavior was reproducible (Table S2). The photocurrents were stable for at least 2 h (Figure S5) and the morphology of WO3 did not change during 2 h of electrolysis (Figure S6). The photocurrent densities increased progressively for up to 4 h of UV irradiation, with no meaningful differences measured with additional UV exposure (Figure S7). The PEC current densities measured in the presence of H2O2, a more efficient hole scavenger than water, also displayed a 30% enhancement from 0.9 to 1.2 mA/cm2 at 1.23 V vs RHE after being subjected to UV irrradiation (Figure 2). Notably, the onset potentials of both PEC water and H2O2 oxidation at 0.65 and 0.48 V vs RHE, respectively, were not shifted after UV photolysis. This observation is consistent with the photovoltages measured at open-circuit potentials and the flat band potentials measured by Mott−Schottky plots that were also unchanged (Figures S8 and S9, respectively). The static onset potential and concurrent increase in photocurrent of the films after UV irradiation imply that the irradiation of WO 3 enhances PEC performance through a different mechanism than previously reported protocols.6,8,15,19 Recombination at the WO3 surface was studied by measuring the photocurrent transient profiles at 1.23 V vs RHE (Figure S10). Measurements recorded before and after irradiation both showed a square profile with an initial spike in current consistent with charge accumulation and recombination at the surface of WO3.6,15 The insensitivity of the transient behavior to UV irradiation is consistent with the unshifted onset potentials shown in Figure 2. We consider these results to rule out the reduction of potential that is consumed by surface charge recombination being responsible for PEC enhancement.6,15,19 The observations that UV irradiation increases the porosity of the films with no discernable change in crystalline phase (Figure 1), enhances the photocurrent density, and does not affect the onset potential (Figure 2) collectively indicate that a larger specific surface area is the primary reason for WO3 photocurrent enhancement. Further support of this hypothesis is provided by the charging currents measured in the dark at different scan rates (Figure 3a, b) from 0.9 to 1.7 V vs RHE, a potential window where only double-layer charging and discharging occur. The charging currents of irradiated films
Figure 2. Photocurrent (AM1.5 front-side illumination) densities of as-prepared (black) and UV irradiated (red) WO3 samples, in KPi buffered solutions (solid) and after (dashed) the addition of 0.1 M H2O2. Data were recorded in a three-electrode cell in KPi buffered solutions (pH 7).
photolysis displayed an onset potential at 0.65 V vs RHE and a photocurrent density of 0.53 mA/cm2 at 1.23 V vs RHE, both of which commensurate with previously reported WO 3 films.16−18 The irradiated WO3 photoanodes, however, displayed a 30% enhancement of photocurrent density from 0.53 to 0.69 mA/cm2 at 1.23 V vs RHE. This enhancement of photocurrent was retained for films that were irradiated and B
DOI: 10.1021/acsami.6b08152 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. Cyclic voltammetry (recorded in the dark) for WO3 (a) before and (b) after UV irradiation at different scan rates (50, 100, 150, 200, 250 mV/s) show significant enhancement of charging current after UV irradiation. (c) Charging current densities of WO3 at different scan rates. (d) Cyclic voltammetry with a scan rate of 10 mV/s in the dark. Data were recorded in a three-electrode cell in KPi buffered solutions (pH 7).
were found to be higher than the as-prepared sample at all scan rates tested. The slope of charging currents plotted against scan rates (Figure 3c), which reflects the double-layer capacitance and assumed to be proportional to the specific surface area of the electrode, increased from 43 μF to 62 μF after UV irradiation. The fact that the irradiated WO3 displays higher charging currents and capacitance than the as-prepared WO3 is consistent with a larger accessible specific surface area. These measurements are further corroborated by the higher dark current densities for irradiated WO3 (Figure 3d). Comparing these results to our previous study of irradiated BiVO4 reveal some unexpected difference in PEC activity towards the OER under AM 1.5 illumination. While both photoanodes displayed an enhancement of photocurrent after being conditioned by UV irradiation, the changes in microstructure and film surface are fundamentally different.15 Consider the following: (i) BiVO4 displayed a substantial cathodic shift (∼230 mV) in onset potential upon UV irradiation, whereas the same treatment of WO3 did not produce any change in onset potential. (ii) The photocurrent transients of BiVO4 show less prominent current spikes after UV irradiation, which is consistent with reduced surface recombination. This observation is also consistent with the lower onset potential owing to the fact that surface recombination consumes additional bias. The photocurrent transients of as-prepared and irradiated WO3, however, both showed similar and square profiles suggestive of minimal differences in surface charge recombination. (iii) The XPS peak of non-lattice oxygen attributed to deleterious surface recombination sites in BiVO4 were diminished after UV irradiation. No significant changes in the corresponding XPS signatures were observed after UV irradiation of WO3. (iv) SEM images and charging current measurements show that UV irradiation yields a less porous surface for BiVO4 and a higher accessible surface area for WO3. The larger specific surface area for WO3 leads to the same degree of
photocurrent enhancement (30%) for water oxidation and H2O2 oxidation as did BiVO4 despite the onset potentials being held at parity. This study demonstrates that exposure of WO3 photoanodes to UV irradiation can lead to improvements of PEC performance by increasing the specific surface area, which is fundamentally different to what occurs for BiVO4 when subjected to the same treatment. These differences provide useful insights into how UV irradiation influences semiconductor materials used in PEC schemes.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08152. Experimental details, X-ray photoelectron spectrum, photocurrent of photolyzed WO3 kept in air for 3 days, open-circuit potentials, Mott−Schottky plots, photocurrent transients, sample-to-sample variation of photocurrent densities (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding from The University of British Columbia 4YF Program, Canada Foundation for Innovation, Canada Research Chairs, and CIFAR is gratefully acknowledged.
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DOI: 10.1021/acsami.6b08152 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
