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Seminar | Center for Molecular Engineering

Deposition and Characterization of Graphenated MWCNT and Visible-Light-Active in situ codoped TiO2:WN

IME Seminar

Abstract: Two selected material deposition and characterization projects will be presented: (1) PECVD and PLD graphenated multiwall carbon nanotubes (g-MWCNTs) for field electron emission applications  and (2) visible-light-active TiO2:WN for photocatalytic applications.

Nitrogen doping has been widely used to photosensitize TiO2 by narrowing its bandgap. This approach is, however, known to lead to the formation of oxygen vacancies (VOs), thereby reducing the per-photon efficiency of the N-doped TiO2. To overcome this limitation, we developed an approach based on acceptor-donor passivation, achieved through the in situ codoping of TiO2 by both W and N dopants, using an RF-MS deposition process. Thus, codoped TiO2:WN films with a wide incorporation range for both W and N dopants (0-5 at.% and 0-10 at.% for W and N, respectively) were investigated.

Compositional and structural analyses revealed that both dopants are mostly of a substitutional nature (WTi″ and NO) in the TiO2 lattice. N incorporation was found to narrow the Eg of TiO2 films from 3.2 to ~2.3 eV. On the other hand, high-frequency dielectric spectroscopy measurements pointed out an optimal doping level of ~2.5 at% of W, which corresponds to a significant reduction (by 2 orders of magnitude) of the 2[TiIII]-[VO”] defect pair contribution to ε′. Our results clearly showed that the codoping approach reduces the VO density in the TiO2:WN films as compared with the TiO2:N monodoped ones.

This reduction in defects was directly correlated to a photocharge lifetime’s 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 (0.08, 0.75, and 11.5 µs, respectively) were found to be about double those of their nitrogen monodoped TiO2:N counterparts (0.03, 0.35, and 6.8 µs), quantitatively confirming the effective passivating outcome of the tungsten-nitrogen codoping approach developed here.

Finally, the codoped TiO2:WN films were integrated into large (6-in-diameter) photoanodes and shown to be effective for the electro-photocatalytic degradation of atrazine. Indeed, the pseudo-first-order constants of the atrazine degradation reaction were found to increase drastically from -0.026 min-1 for TiO2 photoanodes to -0.077 min-1 for the TiO2:WN ones.