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The in-situ doping of sputtered titanium dioxide thin-films for their efficient visible light sensitization and use for the electro-photocatalytic degradation of pollutants in water.

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Delegan, Nazar (2018). The in-situ doping of sputtered titanium dioxide thin-films for their efficient visible light sensitization and use for the electro-photocatalytic degradation of pollutants in water. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 274 p.

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The purpose of this work is to develop photoactive TiO2 films for the electro-photocatalytic degradation of water pollutants under sunlight. Radio frequency magnetron sputtering (RF-MS) was used to synthesize in-situ N-doped TiO2:N films in order to shift their photoactivity from the UV to visible light domain. Careful tuning of the nitrogen-to-argon mass flow rates during sputterdeposition allowed the growth of TiO2:N films with a wide range of N-dopant contents ranging from 0 at.% to 13 at.%. These studies indicated that most of the N-doping atoms were in the desired substitutional states (i.e. NO). UV-Vis absorption and spectroscopic ellipsometry measurements were used to quantify the bandgap (Eg) variation of the TiO2:N thin films as a function of their N content. This was used to point out the existence of an optimal nitrogen loading level that reduces the bandgap from 3.2 eV for TiO2 to 2.2 eV for ~3.5 at.% nitrogen doped TiO2:N. This gain in visible-light photosensitivity was assessed for the 1.5AM light driven electro-photocatalytic (EPC) degradation of an emerging pollutant, chlortetracycline (CTC). Thus, TiO2:N photoanodes have been shown to achieve CTC degradation efficiencies of up to 98% (within 2 h treatments under simulated solar light). Moreover, the EPC performance of the TiO2:N was shown to be directly correlated to their optoelectronic properties, sharing a common optimal point between Eg shrinkage and EPC degradation efficiency. However, a more exhaustive analysis revealed that the increase in EPC performance was disproportional with the photosensitization, with many photogenerated charges seemingly lost. Further studies revealed that nitrogen doping was synonymous with the formation of oxygen vacancy (VO) defects that can reduce the per-photon efficiency of the material by acting as charge recombination centers. In order to circumvent this limitation, tungsten doped TiO2:W thin films were developed, as they were theoretically proposed to increase VO defect formation energy. To this end, RF-MS deposition was used to fabricate undoped TiO2, oxygen deficient (TiO2–x), and tungsten doped (TiO2:W) films with varying dopant levels. The compositional analysis of the W-dopant bonding states revealed the presence of substitutional WVI (W00 Ti) and WIV (W× Ti) type dopants with the total concentration in from 0 at.% to 10 at.%. Additionally, XPS studies revealed a significant recovery of oxygen stoichiometry upon small W incorporations. On the other hand, high frequency spectroscopy measurements (HF-DS) confirmed an optimal tungsten doping of ~2.5 at.% associated with the lowest ε' contribution from the 2 TiIII V00 O defect pair (two orders of magnitude reduction of the VO dielectric contribution as compared to TiO2–x). Consequently, this reduction in VO was exploited by integrating the optimally doped TiO2:W films as photoanodes in visiblelight- driven electro-photocatalytic degradation of atrazine (ATZ), another emerging pollutant. The pseudo-first order degradation kinetic constants were shown to increase from 0.027 min−1 for TiO2 and VO-doped TiO2–x to 0.053 min−1 for the optimally doped TiO2:W photoanodes as a direct result of the reduction in VO. Interestingly, it was revealed that optimally doped TiO2:W photoanodes performed on par with the visible-light photosensitized TiO2:N ones. This confirmed that while TiO2:N had higher EPC performance due to an increased amount of usable photons, TiO2:W had better photocharge transport properties, resulting in higher relative per-photon efficiency. In light of these results, and guided by theoretical models, RF-MS was used to synthesize acceptor-donor passivated, in-situ WN-codoped TiO2:WN thin films. Thus, by varying the reactive RF-MS deposition parameters, we were able to tune the in-situ incorporation of both N and W dopants in the TiO2:WN films over a wide concentration range. The objective of the co-doping was twofold: (i) narrow the bandgap of the TiO2:WN films through N-doping to extend their photosensitivity as far as possible in the visible, and (ii) passivate the N-doping induced VO defect centers through appropriate WN-codoping. Systematic analysis by means of XPS and XRD techniques revealed that both W and N dopants were mostly of substitutional nature. Nitrogen doping was found to be the key component in narrowing the optical-band-gap down to 2.3 eV for both TiO2:N & TiO2:WN. Most importantly, XPS analysis hinted that the codoping approach greatly reduced the density of VO in the TiO2:WN films as compared to TiO2:N ones. This reduction in defects translated into improved crystalline structure, and increased dopant solubility. The suppression of VO via the acceptor-donor passivating approach was directly confirmed by HF-DS measurements showing a marked reduction in the density of 2 TiIII V00 O defect pairs with the codoping of tungsten and nitrogen as compared to N-monodoped samples. This defect reduction was shown to increase photocharge characteristic lifetimes 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 μs, 0.75 μs, and 11.5 μs, respectively) were found to be about 2.5 times longer than those of their nitrogen monodoped TiO2:N counterparts (i.e. 0.03 μs, 0.35 μs, and 6.8 μs). This quantitatively confirms the effective passivation of the WN-codoping approach developed here. Finally, the developed TiO2:WN’s practicality was confirmed by integrating them as photoanodes for the visible-light driven EPC decontamination of ATZ. A significant increase in the degradation kinetics, resulting in up to a four-fold increase in the pseudo-first order degradation constant k for the optimally codoped TiO2:WN photoanodes (k = 0.106 min−1), in comparison with the undoped TiO2–x & TiO2 ones (both having a k = 0.026 min−1). Most importantly, the optimal TiO2:WN photoanodes showed a twofold increase in degradation kinetics as compared to TiO2:W (k = 0.057 min−1) & TiO2:N (k = 0.047 min−1) as a direct consequence of both increased photocharge lifetimes and visible light photosensitivity.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: El Khakani, My Ali
Co-directeurs de mémoire/thèse: Drogui, Patrick
Mots-clés libres: RF-magnetron sputtering; in-situ doping; nitrogen and tungsten codoping; TiO2:WN; photoanodes; bandgap narrowing; electronic acceptor-donor passivation; sunlight driven electrophotocatalysis; pollutant degradation
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 avr. 2019 21:14
Dernière modification: 09 avr. 2019 21:14
URI: http://espace.inrs.ca/id/eprint/8017

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