Structural and electronic properties of N-doped TiO2/SnO2 photocatalysts for air pollutant remediation

Introduction TiO2/SnO2 heterojunctions bear relevance to numerous research fields, such gas sensors, fuel cells and photocatalysis [1]. Actually, the isomorphism of SnO2 cassiterite and TiO2 rutile (both P42/mnm) makes them suitable for developing stable junctions. Moreover, the relative positions of the two semiconductors’ Fermi energies favour electron transfer from the TiO2 conduction band to the SnO2 one, promoting charge carrier separation and increasing the e–-h+ life-time. In turn, this improves the device efficiency in photo-electrochemical applications with respect to N-doped TiO2, one of the most investigated visible-light active photocatalysts, which suffers from enhanced recombination of photo-generated charges [2]. Results and Discussion N-doped TiO2-SnO2 photocatalysts were prepared in a broad range of Sn:Ti ratios (0-20%) by three different synthetic approaches (mechanical mixing, seeded growth and co-synthesis), followed by calcination at 400°C. Samples were characterized from the structural, morphological and electronic point of view, also via in situ synchrotron radiation-based XAS studies. Structural properties are remarkably modified by the adopted synthetic procedure (Fig.1a). Mechanical mixing leads to mixed phases in almost stoichiometric ratios. The seeded growth gives rise to partial segregation of SnO2, while for co-synthesis, the only appreciable component is rutile TiO2, despite the low calcination temperature. Such a tailoring of the structural features reflects in tuneable optical properties (Fig.1b), which show synergistic effects when both N and Sn diffuse in bulk TiO2. The samples’ photocatalytic activity was tested toward the gas phase degradation of VOCs, under both UV and visible light irradiation, monitoring the pollutant disappearance, the main intermediates’ formation and the mineralization degree. Conclusions By tuning the synthetic procedure, mixed phases or Sn doping within the TiO2 matrix could be obtained, leading to different optical and electronic properties. High-resolution XRPD and XAS analyses shed light on (i) the actual location and chemical nature of Sn species, (ii) the defectivity of the TiO2 lattice, and (iii) the interplay between the dopants. Structural and spectroscopic results were correlated with the photocatalytic activity in the visible region. References [1] G. Kelp et al., Nanoscale, 2016, 8, 7056; H. An et al., Electrochim. Acta, 2013, 92, 176. [2] Asahi et al. Chem Rev, 2014, 114, 9824; Rimoldi et al. J Phys Chem C, 2015, 119, 24410.