Role of Tungsten Doping on the Surface States in BiVO₄ Photoanodes for Water Oxidation: Tuning the Electron Trapping Process

Abstract

The nanostructured BiVO 4 photoanodes were prepared by electrospinning and were further characterized by XRD, SEM, and XPS, confirming the bulk and surface modification of the electrodes attained by W addition. The role of surface states (SS) during water oxidation for the asprepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (N SS) with respect to the charge donor density (N d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest N d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V 5+ /V 4+ in semiconductor bulk to form water oxidation intermediates through the electron trapping process. Otherwise, the irreversible surface reductive reaction of VO 2 + to VO 2+ though the electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of the BiVO 4 electrode. It can tune the electron trapping process to obtain a high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.

Tungsten Trioxide Nanostructures for Photoelectrochemical Water Splitting: Material Engineering and Charge Carrier Dynamic Manipulation

“…Generally, data obtained by EIS is expressed graphically in a Nyquist plot or a Bode plot . More importantly, EIS can be conducted at any bias, while IMVS is usually performed under open circuit condition . As a result, researchers can study the charge transport and recombination kinetics over a wide range of operating conditions using EIS technique …”

“…BiVO 4 is a semiconductor with proper energy band structure, which is nearly straddle the water reduction and oxidation potentials . The smaller band gap of BiVO 4 (≈2.4 eV) than that of WO 3 (2.5–2.8 eV) means that BiVO 4 has larger light absorption range, which is conducive to PEC water splitting . Lee et al prepared WO 3 /BiVO 4 composite film photoanode through layer‐by‐layer deposition .…”

New aspects of improving the performance of WO3 thin films for photoelectrochemical water splitting by tuning the ultrathin depletion region

“…This ratio can also be used to reect the kinetics of water oxidation at photoanode surface. 24,42,44,45 As shown in Fig. 7a, the ratios of R trap /R ct for pristine and reduced WO 3 photoanodes generally increased as the applied potential increased, while the minimum values of this ratio were observed at the onset potential (0.5 V RHE ) ( Fig.…”

“…These variables include a chemical capacitance (C ss ) representing the chemical capacitance of the traps, a trapping resistance (R trap ) representing the trapping/detrapping resistance of electrons that go from the conduction band to the surface traps, and a charge-transfer resistance (R ct ) representing the resistance of hole transfer. 42,43 The dependence of these variables (C ss , R trap , and R ct ) on the applied potential measured for our samples is shown in the ESI (Fig. S7, ESI †).…”

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…The stability issues can be classified as the following: (1) degradation of the LbL‐assembled film by electrochemical reactions on photoelectrodes and (2) detachment or delamination of the film. Nevertheless, we believe there will be many opportunities for non‐covalent, LbL approaches, because it has more flexibility in design and fabrication and also can be readily combined with other conventional approaches, such as doping,, defect engineering,, and heterojunction formation, , of an underlying photoelectrode, for the practical application of artificial photosynthesis.…”