The nature of photogenerated charge separation among different crystal facets of BiVO₄studied by density functional theory

Abstract

Charge separation among different crystal facets of a semiconductor has been observed experimentally, but the underlying reasons behind this phenomenon are unknown. In this work, the activation energies of carrier hopping and the mobility of electron/hole transport along seven low-index crystal orientations of bulk BiVO4 have been calculated using a small polaron model. The calculated mobility and our previous experimental results reveal that there is a parallel relationship between the carrier mobility along the crystal axis and the carrier preferred accumulation on the corresponding crystal facets. It is proposed that the mobility of electrons (or holes) along the crystal axis [hkl] might be essentially related to the charge separation among the indices of corresponding facets (hkl); namely, the mobility of electrons (or holes) along the crystal axis [hkl] is the largest among all possible crystal axes, and the photogenerated electrons (or holes) tend to be accumulated on the indices of the corresponding facet (hkl) when the surface factors like surface band bending, surface energetic differences, etc. are not considered.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals

“…2 (a) Electron doping and the existence of inter-band gap trap states and polaron-like states in BiVO4 were the subject of two recent studies using hybrid DFT methods. 7,10 The methods were employed due to issues of conventional DFT in accurately predicting charge localization.…”

“…5,9 To probe the nature and behaviour of excess electrons in 'doped' BiVO4, DFT+U hybrid computational methods have been applied. 7,10 Excess electrons were predicted to localize at V 3dz 2 sites, which resulted in the formation of inter-band gap small-polaron states. The data was consistent with electronic transport measurements indicating that despite an increase in the PEC activity of BiVO4 with doping, the electronic mobility remains low, yet consistent with small-polaron hopping, ∼10 -4 cm 2 V -1 s -1 (upper limit in the small-polaron hopping regime is ~0.1-1 cm 2 V -1 s -1 ).…”

A Theoretical Perspective on Charge Separation and Transfer in Metal Oxide Photocatalysts for Water Splitting

“…To understand this selective photodeposition phenomenon, Liu et al . combined DFT+U method with a small polaron model based on Marcus‐EHAM theory to calculate the carriers mobility and hopping activation energy in seven low‐index facets of BiVO 4 . They tested the effect of different U values of V 3d and O 2p orbitals on the band gap and localizing the electron or hole.…”

Recent Advances in Nanostructured Vanadium Oxides and Composites for Energy Conversion

“…[249] Engineering nanostructures of BiVO 4 have resulted specific exposed crystal facets, including [001], [250] [010], [31,114,248,251,252] 2D BiVO 4 nanoplates with exposed [001] facets were compared with BiVO 4 nanorods. [250] Even though the specific surface area of BiVO 4 nanorods was measured more than three times greater than BiVO 4 nanoplates, the oxygen evolution rate of BiVO 4 nanoplates was measured nine times higher than that of BiVO 4 nanorods.…”