Multiple Effects Induced by Mo⁶⁺ Doping in BiVO₄ Photoanodes

Abstract

Mo6+ doping increases the photoelectrochemical performance of BiVO4 photoanodes in water oxidation. Herein, the underlying mechanisms is elucidated through a systematic structural, morphological, and photoelectrochemical investigation on photoelectrodes of pure and Mo6+ doped BiVO4 prepared by a novel multistep spin‐coating deposition approach, leading to multilayer flat films with high optical transparency. Transient absorption spectroscopy in the nano‐ to microsecond time scale reveals a longer lifetime of photogenerated holes in the doped films. Besides confirming that Mo6+ ions improve the electron transport in the material bulk, impedance spectroscopy also reveals the crucial role of the dopant on the surface properties of BiVO4 photoanodes. The presence of intra‐bandgap states, acting as traps of photogenerated charge carriers in pure BiVO4, is detected through the build‐up of the interfacial surface state capacitance. The limited activity of pure BiVO4 in water oxidation is largely improved upon 3 at% Mo6+ incorporation, ensuring a more efficient charge carrier transport with respect to pure BiVO4, together with the beneficial passivation of its trap surface states.

Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

“…In previous studies, we investigated the charge carrier dynamics in the WO 3 /BiVO 4 system through transient absorption spectroscopy (TAS) with detection either in the visible range to observe the hole dynamics in BiVO 4 18 , 26 − 28 or in the mid-infrared to follow the electron dynamics in WO 3 and BiVO 4 . 29 We also identified wavelength-dependent processes by tuning the excitation wavelength across the WO 3 absorption edge (ca.…”

“… 21 , 29 On the other hand, τ 2 , which is ascribed to the recombination of trapped holes with trapped electrons, increases from ∼1 to 6.5 ns with increasing BiVO 4 layer thickness as more holes get trapped in bulk sites. 28 , 36 , 37 …”

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

“…For example, Mo doping in BiVO 4 improves PEC performances by increasing electron mobility, disfavoring recombination, and enhancing charge separation. 60,61 These strategies induce similar chemical and electronic effects in CuWO 4 and translate into efficiency improvements (from 0.30 to 0.39 mA cm −2 for hydrogenation, 44 from 0.35 to 0.62 mA cm −2 for Mo doping, 22 from 0.27 to 0.42 mA cm −2 for Fe doping, 49,50 from 0.38 to 0.57 mA cm −2 for fluorine doping). 52 The moderate photocurrent increase observed with CuWO 4 in the presence of hole scavengers such as H 2 O 2 (Figure 5C) indicates that few holes reach the electrode/electrolyte interface, where they are rapidly consumed at surface catalytic sites.…”

Femtosecond transient absorption spectroscopy investigation into the electron transfer mechanism in photocatalysis

“…36,37 Besides, fs-TAS can reveal the relaxation process of photogenerated carriers from high-energy excited states to the ground states, such as the electron transition from the conduction band (CB) to the valence band (VB) in semiconductors. 38,39 More importantly, the electron transfer kinetics between energy levels in HPs can be obtained by analyzing the fs-TAS results. [40][41][42] In practical fs-TAS measurements, a semiconductor photocatalyst is excited by a high-energy femtosecond monochromatic laser (pump pulse).…”