Textured nanoporous Mo:BiVO₄ photoanodes with high charge transport and charge transfer quantum efficiencies for oxygen evolution

Abstract

We have developed a simple spin coating method to make high-quality nanoporous photoelectrodes of monoclinic BiVO 4 and studied the ability of these electrodes to transport photogenerated carriers to oxidize sulfite and water. Samples containing molybdenum and featuring [001] out-of-plane crystallographic texture show a photocurrent and external quantum efficiency (EQE) for sulfite oxidation as high as 3.1 mA cm À2 and 60%, respectively, at 1.23 V versus reversible hydrogen electrode. By using an optical model of the electrode stack to accurately determine the fraction of electrode absorptance due to the BiVO 4 active layer, we estimate that on average 70 AE 5% of all photogenerated carriers escape recombination. A comparison of internal quantum efficiency as a function of film processing, illumination direction, and film thickness shows that electron transport is efficient and hole transport limits the photocurrent (hole diffusion length o40 nm). We find that Mo addition primarily improves electron transport and texturing mostly improves hole transport. Mo enhances electron transport by thinning the surface depletion layer or passivating traps and recombination centers at grain boundaries and interfaces, while improved hole transport in textured films may result from more efficient lateral hole extraction due to the texturing itself or the reduced density of deep gap states observed in photoemission measurements.Photoemission data also reveal that the films have bismuth-rich, vanadium-and oxygen-deficient surface layers, while ion scattering spectroscopy indicates a Bi-V-O surface termination. Without added catalysts, the plain BiVO 4 electrodes oxidized water with an initial photocurrent and peak EQE of 1.7 mA cm À2 and 30%, respectively, which equates to a hole transfer efficiency to water of 464% at 1.23 V. The electrodes quickly photocorrode during water oxidation but show good stability during sulfite oxidation and indefinite stability in the dark. By improving the hole transport efficiency and coating these nanoporous BiVO 4 films with an appropriate protective layer and oxygen evolution catalyst, it should be possible to achieve highly efficient and stable water oxidation at a practical pH.

Enhancing Mo:BiVO4 Solar Water Splitting with Patterned Au Nanospheres by Plasmon‐Induced Energy Transfer

“…Under these optimized conditions, the Au-BVO photoanode achieved a current density of 2.83 mA cm −2 at 1.23 V (versus RHE), which is ≈2.2 times higher than that of the bare BVO photoanode (1.29 mA cm −2 ). [34] The simulated absorptance (A) was calculated with the numerical rigorous coupled wave analysis using the experimental geometry for Au-BVO in Figure 1c. The inset of Figure 2b shows that there is a significant EQE enhancement in the range of ≈500-700 nm.…”

Insight into the PEC and interfacial charge transfer kinetics at the Mo doped BiVO4photoanodes

“…[15][16][17][18] However, the slow surface catalytic activity, short hole diffusion length, fast electron-hole recombination are major challenges with the BiVO 4 . 6,[19][20][21][22] To overcome these challenges, a number of the strategies have been widely investigated such as; nanostructure control, [23][24][25][26] band engineering, 17,[27][28][29][30][31][32][33][34] heteroatom doping, [35][36][37][38][39][40][41] generation of oxygen vacancy 22,[42][43][44] and the oxygen evolution catalyst (OEC) incorporation. 18,[45][46][47][48][49][50][51][52][53] SnO 2 has been used for the heterojunction formation in the BiVO 4 system which suppresses the back electron-hole recombination process.…”

Dual Modification of a BiVO4 Photoanode for Enhanced Photoelectrochemical Performance

“…To date, researchers have conducted a variety of investigations to improve the PEC performance of BiVO 4 , such as doping, morphology tuning, and surface modification; the construction of a heterojunction has been a prevalent strategy . In particular, the type II heterojunction has attracted much attention, of which the VB and conduction band (CB) of the top layer are both higher than that of the bottom layer, and thus, electrons can diffuse to the bottom layer and holes can diffuse to the top layer.…”