Effects of precipitation, liquid formation, and intervalence charge transfer on the properties and photocatalytic performance of cobalt- or vanadium-doped TiO2 thin films

Chen, Wen-Fan; Koshy, Pramod; Ying, Hai; Adabifiroozjaei, Esmaeil; Yao, Yin; Sorrell, Charles Christopher

doi:10.1016/j.ijhydene.2016.08.115

Cited by 45 publications

(45 citation statements)

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“…While the basic assumptions already have been stated, other factors may be relevant, particularly the influence of localized surface equilibria and whether or not the redox couples are in equilibrium [55,56]. These are clearly key issues owing to the Ce 4+ /Ce 3+ and Fe 3+ /Fe 2+ redox couples, the potential for intervalence charge transfer effects [57,58], and the effects of these equilibria on the overall and localized redox potential of the solution. …”

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confidence: 99%

Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles

et al. 2017

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Abstract:The present work describes the effects of water on Fe-doped nanoparticulate CeO 2 , produced by flame spray pyrolysis, which represent a critical environmental issue because CeO 2 is not stable in typical atmospheric conditions. It is hygroscopic and absorbs~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO 2 initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of~69 mol % solid CeO 2 ·2H 2 O and~30 mol % gelled Ce(OH) 4 . Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH) 4 gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce 4+ -H 2 O, Ce 3+ -H 2 O, Fe 3+ -H 2 O, and Fe 2+ -H 2 O systems; and (c) the Pourbaix diagrams for the Ce-H 2 O and Fe-H 2 O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO 2 component of the hydrated phase CeO 2 ·2H 2 O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO 2 upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present.

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Aqueous and Surface Chemistries of Photocatalytic Fe-Doped CeO2 Nanoparticles

et al. 2017

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“…Consequently, the peak shifts indicating valence increases (greater binding energies) and decreases (lower binding energies) provide information on charge transfer effects. In particular, electron transfer and complementary valence shifts between codopants, between dopant and matrix, and between codopants and matrix can be used to support the conclusion of IVCT [11,39] or multivalence [41]. In addition, these data can indicate the presence of direct redox effects, such as…”

Section: Resultsmentioning

confidence: 99%

“…Surface chemical analysis by XPS has emerged as a standard tool in materials characterisation because it quantitatively identifies valence states [11,35,36]. Consequently, the peak shifts indicating valence increases (greater binding energies) and decreases (lower binding energies) provide information on charge transfer effects.…”

Section: Resultsmentioning

confidence: 99%

“…According to Hume-Rothery's rules for substitutional solid solubility [36], it is very likely that this type of solid solubility occurred owing to the similar crystal radii of Ti 4+ , Fe 4+ , Fe 3+ (high spin), Fe 2+ (low spin) for Fe-doping, and Mo 6+ , Mo 5+ , Mo 4+ and Mo 3+ for Mo-doping. However, it also is possible that interstitial solid solubility occurred since each of the two interstices adjacent to the central Ti in the elongated TiO 6 octahedron of anatase has a radius of 0.0782 nm [11]; Fe 4+ , Fe 3+ (low spin), and Mo 6+ can fit in this position. According to the crystal field stabilisation energies, Fe 3+ and Fe 2+ are likely to be high-spin complexes [36].…”

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confidence: 99%

“…One of the most common methods to improve these characteristics in semiconducting oxides is through the use of dopants. One of the most important of these parameters is the optical indirect band gap, which can be lowered by convergence of the conduction and valence band positions as well as the introduction of shallow midgap energy levels, which derive from the introduction of lattice defects [9], the presence of biaxial tension in the a-b plane of TiO 2 imposed by dopants [10], increasing the crystallinity [11], and/or activating intervalence charge transfer (IVCT) [11]. Another important parameter that can be manipulated is extension of the electron-hole pair recombination time, which can be enhanced through the presence of deep midgap energy levels, which again derive from lattice defects [12].…”

Section: Introductionmentioning

confidence: 99%

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Intervalence charge transfer and thermodynamic effects on the photocatalytic performance of Fe/Mo single and codoped TiO2 thin films

et al. 2019

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Fe/Mo single doped and codoped TiO 2 thin films were spin coated on polished fused silica substrates and annealed in air at 450 °C for 2 h. The XPS data for the anatase thin films were distinctive in showing that Fe-doping caused Ti 4+ reduction, Mo-doping caused Ti 4+ oxidation, and codoping did not alter the Ti valence. The XPS data also showed that the precursor valences of Fe 3+ and Mo 5+ were reduced upon annealing. Analysis of the potential roles of thermodynamics and intervalence charge transfer (IVCT) demonstrates that the latter drives the equilibria both in solution and during annealing. Photocatalytic performance testing indicated that Fe-doping was slightly deleterious, Mo doping had a consistent positive effect, and codoping exhibited a clear negative trend as a function of doping concentration. These results are interpreted in terms of the effect of IVCT on the matrix valence, dopant valences, and charge carrier trapping. Fe doping exhibited reduced performance because both matrix Ti (4−x)+ and dopant Fe (3−y)+ acted as electron traps. Mo doping exhibited enhanced performance because the matrix Ti (4+x)+ acted as a hole trap and the dopant Mo (5−x)+ and Mo (4−x)+ acted as electron traps, thereby promoting charge separation. Codoping exhibited a clearly detected negative trend on the performance because, while Ti 4+ played no role, Fe (3−y)+ , Mo (5−x)+ , and Mo (4−x)+ all acted as traps for the majority charge carrier electrons.

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