Lifetime Enhancement of Visible Light Induced Photocharges in Tungsten and Nitrogen in situ Codoped TiO₂:WN Thin Films

We report on one-step in situ codoped TiO₂ thin films synthesized by cosputtering. The purpose of this acceptor-donor passivated codoping approach is to overcome the optoelectronic limitations that arise for monodoped TiO₂ in photocatalytic applications. To evaluate these added benefits, the TiO₂:WN thin films were characterized by different techniques. X-ray diffraction patterns and X-ray photoelectron spectral analysis revealed that both N and W dopants are mostly present in the desired substitutional locations. Additionally, the codoping approach was found to reduce the internal strain and defect density of the TiO₂:WN films as compared to their monodoped TiO₂:N counterparts. This defect reduction is confirmed via photocharge lifetime variation obtained using visible light flash photolysis time-resolved microwave conductivity measurements (FP-TRMC). Photocharge lifetime analysis indicated the presence of three distinct decay processes: charge trapping, recombination, and surface reactions. These characteristic lifetimes of the codoped TiO₂:WN films (i.e., 0.08, 0.75, and 11.5 μs, respectively) were found to be about double those of their nitrogen monodoped TiO₂:N counterparts (i.e., 0.03, 0.35, and 6.8 μs), quantitatively confirming the effective passivating outcome of the tungsten-nitrogen codoping approach developed here. The practicality of this method was confirmed by integrating the TiO₂:WN films as photoanodes for the electro-photocatalytic, solar light driven degradation of a real pollutant (i.e., atrazine). A significant increase in the degradation kinetics, leading to a 4-fold increase in the pseudo-first-order degradation constant for the optimally doped TiO₂:WN photoanodes (0.106 min^-1) from the undoped TiO_2-x ones (0.026 min^-1), is a direct consequence of the increased photocharge lifetimes in tandem with visible light photosensitivity.