Indirect Bandgap and Optical Properties of Monoclinic Bismuth Vanadate

Abstract

Monoclinic scheelite bismuth vanadate (m-BiVO 4 ) is a promising semiconductor photoanode for photoelectrochemical (PEC) water splitting. Despite considerable recent progress in achieving improved photocurrents and photovoltages, there remain open questions about the basic optoelectronic properties of this material. Indeed, there is disagreement about the nature of its fundamental bandgap, with theoretical predictions and some experimental observations pointing to an indirect bandgap and other experimental studies to a direct bandgap. Knowledge of this property is critical for understanding light absorption and photocarrier properties, as well as for establishing rational approaches to improved efficiency. Here, experimental spectroscopic techniques are used to resolve this issue and provide a fundamental portrait of the optical properties of the material. Resonant inelastic X-ray scattering proves conclusively that m-BiVO 4 is an indirect bandgap semiconductor. These measurements are supported by UV−vis absorption spectroscopy and spectroscopic ellipsometry, which confirm this finding and also indicate the presence of a direct transition located at 200 meV above the indirect one. The spectral dependence of the optical constants is determined by development of a photophysical model for the ellipsometric data. Photogenerated carrier dynamics are probed by transient absorption spectroscopy, which reveals a relatively long lifetime compared to other commonly utilized metal oxide photoanodes and is attributed to the indirect nature of the fundamental gap. The combination of strong visible light absorption and relatively long excited state lifetime provides the basis for the high performance that has been achieved from BiVO 4 photoanodes for water splitting.

Determination of the Intrinsic Defect at the Origin of Poor H2 Evolution Performance of the Monoclinic BiVO4 Photocatalyst Using Density Functional Theory

“…42 The band gap type was found to be indirect in line with previous reports. 43,44 Figure S2 (with x = 0.0625) has given a reduced band gap with a value around 2.3 eV (Figure 4c), which is very close to the experimentally reported ones. 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

Role of Hydrogen in Defining the n-Type Character of BiVO4 Photoanodes

“…5 The DOS related to O vac are found deep in the gap, at 1.44 eV above the VBM. By scaling for the discrepancy in the bandgap using the scissor operator, the narrow DOS introduced by O vac are predicted to lie approximately 1.7 eV above the VBM.…”

“…Bismuth vanadate (BiVO 4 ) is one of the most actively investigated oxide semiconductors for PEC energy conversion due to its moderate indirect bandgap of 2.5 eV, 5 favourable conduction band position, 6 and relatively long photocarrier lifetimes. [6][7][8] However, poor majority carrier transport, arising from formation of electron small polarons, has been identified as one of the primary limitations on its PEC performance.…”

Does the low optical band gap of yellow Bi3YO6 guarantee the photocatalytical activity under visible light illumination?

“…4a. There are materials which exhibit two types of transitions [35,36]. However, taking into account the UV-Vis spectrum in Fig.…”