Photocarrier Recombination Dynamics in BiVO₄ for Visible Light-Driven Water Oxidation

Abstract

Photocarrier recombination dynamics of BiVO4 powders synthesized at different temperatures were studied by temperature-dependent steady-state and time-resolved photoluminescence (PL). Structural analysis indicates that BiVO4 materials synthesized at low temperatures contain mixed-phase crystals including monoclinic and tetragonal scheelite phase, showing poor photocatalytic performance. Relatively higher synthesis temperatures improve the photocatalyst efficiency by promoting the formation of single-phase monoclinic BiVO4 with larger grains. Excitation-power dependence along with temperature dependence of the PL of BiVO4 suggests that the donor-to-acceptor transitions are the dominant radiative recombination mechanism. Furthermore, hole effective lifetimes observed in PL decays were found in the order of nanoseconds, which is far behind the ideal radiative lifetime of ∼6 μs, calculated theoretically using van Roosbroeck-Shockley relation. This suggests that the photocarrier recombination in BiVO4 occurs predominately nonradiatively via multiphonon emission, plausibly through deep-level defects. In addition, the coexistence of tetragonal and monoclinic phases might indirectly induce additional trap states, leading to an increase of the nonradiative recombination rate and subsequently poor photocatalytic efficiency in samples synthesized at lower temperatures. Thus, the nonradiative recombination which is associated with a short photocarrier lifetime and small holes diffusion length is the most limiting process for BiVO4 performance.

Mechanism of Incorporation of Zirconium into BiVO4 Visible-Light Photocatalyst

“…To characterize the effect of the Zr additive on photoexcited charge recombination in BVO, the steady-state and time-dependent PL were measured for undoped and doped samples. No bandto-band emission (expected ∼2.4 eV) is observed, but a broad pronounced emission peak centered at ∼1.85 eV is detected (Figure 3a), consistent with our previous report 26 and other works. 27−30 Based on our previous study, the observed PL emission is due to donor−acceptor pair (DAP) transitions from a deep donor at 0.58 eV below the conduction band (CB) into a shallow acceptor at 0.1 eV above the valence band (VB).…”

“…Fitted TRPL parameters are summarized in Table S2. All observed lifetimes are far behind the ideal radiative lifetime, τ r , of 6 μs estimated using the van Roosbroeck–Shockley relation for BVO . The results suggest that photoexcited charges predominately recombine nonradiatively (τ PL ≈ τ nr ), likely via the Shockley Read Hall (SRH) recombination.…”

“…All observed lifetimes are far behind the ideal radiative lifetime, τ r , of 6 μs estimated using the van Roosbroeck−Shockley relation for BVO. 26 The results suggest that photoexcited charges predominately recombine nonradiatively (τ PL ≈ τ nr ), likely via the Shockley Read Hall (SRH) recombination. During SRH recombination, photoexcited charge carriers recombine through deep trap states localized inside the band gap.…”

Manipulating Luminescence and Photocatalytic Activities of BiVO4 by Eu3+ Ions Incorporation

“…3,4 More and more strategies have been conducted to improve photocatalytic efficiencies. 5,6 Rare-earth (RE) ion doping is effective in optimizing photocatalysis via a narrowing band gap, extending absorption, modifying band position, and promoting charge carrier separation. 7−9 RE elements with an incompletely occupied 4f orbital can increase the optical absorption and separate the photogenerated charges.…”

“…Photoenergy conversion is one of the most popular topics in the development of functional materials, which usually has two kinds of well-known behaviors, photoluminescence and photocatalysis. Photoluminescence investigation has a long history in lighting and displays; , however, photocatalysis is still not developed in practical applications because of the fast electron–hole recombination in a semiconductor photocatalyst. , More and more strategies have been conducted to improve photocatalytic efficiencies. , Rare-earth (RE) ion doping is effective in optimizing photocatalysis via a narrowing band gap, extending absorption, modifying band position, and promoting charge carrier separation. − RE elements with an incompletely occupied 4f orbital can increase the optical absorption and separate the photogenerated charges. , Among them, the Eu 3+ ion is a good candidate widely used in photocatalysts because of its highly active optical activities. For example, Eu 3+ -doped TiO 2 photocatalysts with enhanced photocatalysis have been widely reported. − …”