As the τ of electron which is decided by the hole trapping time is now a constant, R (or Γ) will be independent of the excitation power, i.e., R (or Γ) = const. Once the power exceeds a critical value (trap filling intensity), the photogenerated hole density is much higher than the find more trap density and the traps will be fully occupied. Under this condition, the trapping

effect can be ignored and photocarriers will follow the bimolecular recombination mechanism [40–42]. The recombination after trap filling results in the decrease of τ with the increase of I, making an intensity-dependent R (or Γ) following an inverse power law, i.e., R (or Γ) ∝ I -k , where the theoretical k = 1/2 [42]. The aforementioned model www.selleckchem.com/products/AG-014699.html agrees with the two-stage power-dependent R (or Γ) result in Figure  2c and i p in Figure  2b. The trap filling intensity is roughly at 5 W m-2, and the fitted k value is 0.62 ± 0.04 for the V2O5 NWs. The change of recombination

behavior can be further verified by the power-dependent τ measurement. Figure  3a illustrates the normalized photocurrent rise curves under selected light intensity. The result shows that the rise time or photoresponse time increases with the decrease of power density. By fitting the photoresponse curves using stretched exponential function i p(t) = i p0 exp[-(t/τ) β ], where i p0 is the steady-state photocurrent and β is the stretching factor smaller than unity; the dependence of τ on power density can be obtained and is depicted in Figure  3b. The result shows that the τ also follows the similar two-stage power dependence as R (or Γ), which further confirms the lifetime-dominant hole trapping PC mechanism in the V2O5 NWs. Figure 3 Normalized photocurrent rise curves and fitted carrier lifetime versus intensity. (a) The normalized photocurrent rise curves under inter-band excitation (λ = 325 nm) with selected intensity and (b) fitted carrier lifetime versus intensity measured at a bias of 0.1 V for the V2O5 NW with d = 800 nm and

l = 2.5 (-)-p-Bromotetramisole Oxalate μm. According to literature reports, the photoconductivity of metal oxide semiconductor NWs, such as ZnO, SnO2, TiO2, and WO3, mostly follow a common oxygen-sensitized (OS) PC mechanism [36, 37, 43–45]. The mechanism is controlled by the interaction of foreign oxygen molecule and semiconductor in the near surface area. According to the OS model, the PC process includes four steps: (1) In the dark and in the atmospheric ambience, as oxygen plays a role of electron trap state in the metal oxide semiconductor surface, through oxygen adsorption, the electron is captured on the surface and creates negatively charged surface states (or oxygen ions) [O2(g) + e - → O2 -(ad)]. The effect induces an enhanced upward bending of the energy band at the surface. (2) Under light illumination, electron–hole pairs are generated [hυ → e - + h +] and (3) subsequently separated by the surface electric field or band bending.