Enhancing Majority Carrier Transport in WO₃Water Oxidation Photoanode via Electrochemical Doping

Abstract

Here we report an electrochemical reduction-induced photocurrent enhancement up to an order of magnitude for monoclinic tungsten trioxide (WO 3 ) particulate photoanodes. Electrochemical impedance and photoelectrochemical measurements suggest that the improved performance is attributed to the increase in majority carrier concentration (from 1.5 × 10 18 to 2.5 × 10 21 cm −3 ). This results in higher conductivity and improved electron transport. Minority carrier extraction can be increased by reducing the WO 3 particle size from 200 nm to 50 and 30 nm. The larger solid-liquid interface area promotes the water oxidation rate. By balancing electron/hole extraction with photon absorption in an optimized 30 nm WO 3 particulate electrode, a record water oxidation photocurrent of 3.8 mA/cm 2 , under +1.36 V (vs. RHE) applied bias in 0.1 M Na 2 SO 4 solution at pH = 3.5 is achieved with 50 mW/cm 2 unfiltered Xe illumination. Photoelectrochemical water splitting is an efficient way of converting solar energy to chemical fuel by decomposing water into hydrogen and oxygen.1-9 Among inorganic materials that catalyze the photoelectrolysis of water, WO 3 with a bandgap of 2.6 eV is attractive due to its visible absorption and its photochemical stability in acid aqueous solution up to pH 5. [10][11][12][13] Since it was first reported as a potential photoanode for photoelectrochemical cells (PEC) 27-32 and enhancing WO 3 stability in neutral solution using surface coating. 27 Recently, Yat Li's group reported that a hydrogen treatment at 350• C of hydrothermally synthesized WO 3 films enhanced photoactivity and photostability of the WO 3 electrode, due to the incorporation of W 5+ donors and oxygen vacancies. 24 The W 5+ states are believed to be more resistant to photocorrosion by peroxo-intermediates formed during water oxidation and they improve electron transport in the WO 3 films. Herein, we demonstrate that a similar performance increase can be achieved by in-situ electrochemical reduction of WO 3 particle films at +0.17 V vs. RHE (−0.04 V vs. NHE) in aqueous electrolyte. The reduction increases the water oxidation photocurrent of WO 3 by over an order of magnitude, up to 3.8 mA/cm 2 for an optimized device. This is comparable to the highest observed water oxidation photocurrents for WO 3 films reported in the literature (>2.5 mA cm −2 , AM 1.5, E App = 1.0 V vs. NHE). 18,23,[33][34][35][36] Furthermore, this photocurrent enhancement is achieved without the utilization of sophisticated film geometry design 18 or water oxidation electrocatalysts. [27][28][29][30][31] The relative photocurrent enhancement is most pronounced for films composed of bulk WO 3 particles (enhancement factor of 14 fold), followed by films of 50 nm particles (10 fold) and 30 nm particles (2 fold). The reduction treatment also improves the stability of WO 3 against photocorrosion; after reduction, 50% of the photocurrent persists after 1 hr operation, compared to 10% for an untreated film. The enhancement is preserved in air, when films are at roo...

Defects in Porous Networks of WO3 Particle Aggregates

“…Our observations are qualitatively in line with a recent report by Osterloh and co‐workers . By using electrochemical impedance spectroscopy, these authors showed that the photocurrent enhancement by up to an order of magnitude upon moderate electrochemical doping of monoclinic WO 3 electrodes was accompanied by an up to three orders of magnitude increase in majority carrier concentration and by an increase in the conductivity of the porous film.…”

“…Based on our results and in line with a previous report, we attribute the photocurrent increase observed for WO 3 (450) electrodes after moderate doping to the accumulation of electrons in the semiconductor particles (Figure ). Adsorption and insertion of protons from the electrolyte compensate the accumulated negative charge, thus forming (e − /H + ) centers in the oxide.…”

“…Adsorption and insertion of protons from the electrolyte compensate the accumulated negative charge, thus forming (e − /H + ) centers in the oxide. Importantly, such an electrochemical doping may take place not only upon the application of an external potential, as reported previously, but constitutes a real in situ doping process that takes place even at open circuit potential upon band gap excitation (Figure ). This is of high relevance for photocatalysis, in which the active material is operated under open‐circuit conditions.…”

“…The positive effect of such a charge transfer reductive doping on the photocatalytic and photovoltaic performance of mesoporous TiO 2 films was attributed to accelerated charge transport together with reduced recombination, resulting in improved electron collection efficiencies . Recently, it was shown that a similar effect can be observed in WO 3 electrodes . The interrelation of in situ processes connected to charge accumulation, on the one hand, and the exploitation of photogenerated charge carriers, on the other hand, has recently been highlighted .…”

The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalysts

“…The increase in surface states is also known to pin the Fermi level of the photocatalyst, and results in an anodic shift in the onset potential. 29 This offsets the Figure 9 Energy band alignment diagrams for Co3O4/WO3, Conduction and valence band edge positions of Co3O4 and WO3 material are obtained from literature. 21,22 cathodic shift normally observed with the introduction of a cocatalyst, and may account for why only a marginal shift in onset potential has been observed ( Figure 6).…”