Lifetime Enhancement of Visible Light Induced Photocharges in Tungsten and Nitrogenin situCodoped TiO₂:WN Thin Films

Abstract

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 TiO2–x ones (0.026 min–1), is a direct consequence of the increased photocharge lifetimes in tandem with visible light photosensitivity.

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“…34,35 After illumination, the TRMC signals rise following ultrafast charge separation process (<200 fs), 36 which is mainly due to mobile electrons at the surface of TiO 2 , since it is usually assumed that holes with a limited mobility remain on the bulk. The time-dependent change of microwave power transmission ∆ / is proportional to the photoconductivity ∆σ t variation and therefore proportional to the density of mobile charge carriers, which can be expressed as: 37,38 ∆ A ∆σ t ∑∆ (2) where ∆ defines the density of photogenerated charge carriers " " at known time " ", and defines their mobility. In the present study, all TRMC measurements are recorded under 360 nm UV illumination, and with a light energy density received by the samples set at 1.4 mJ cm −2 .…”

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“…After illumination, the TRMC signals rise following ultrafast charge separation process (<200 fs), 36 which is mainly due to mobile electrons at the surface of TiO2, since it is usually assumed that holes with a limited mobility remain on the bulk. The time-dependent change of microwave power transmission ∆ ( )/ is proportional to the photoconductivity ∆σ(t) variation and therefore proportional to the density of mobile charge carriers, which can be expressed as: 37,38 ∆ ( ) = A • ∆σ(t) = • ∑∆ ( )…”

“…After illumination, the TRMC signals rise following ultrafast charge separation process (<200 fs), 36 which is mainly due to mobile electrons at the surface of TiO2, since it is usually assumed that holes with a limited mobility remain on the bulk. The time-dependent change of microwave power transmission ∆𝑃(𝑡)/𝑃is proportional to the photoconductivity ∆σ(t) variation and therefore proportional to the density of mobile charge carriers, which can be expressed as: 37,38 ∆𝑃 𝑃 (𝑡) = A • ∆σ(t) = 𝐴 • ∑∆𝑛 𝑖 (𝑡)𝜇 𝑖 (2) where ∆𝑛 𝑖 (𝑡) defines the density of photogenerated charge carriers "𝑖" at known time "𝑡", and 𝜇 𝑖 defines their mobility. In the present study, all TRMC measurements are recorded under 360 nm UV illumination, and with a light energy density received by the samples set at 1.4 mJ cm −2 .…”

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