Tailoring the structural and electronic features of N-DOPED TiO2/SnO2 photocatalysts for pollutant degradaton

TiO2/SnO2 composites hold promise in numerous cutting-edge research fields, such as gas sensors (Chen 2015), fuel cells (An 2013) and photocatalysis (Toloman 2019). Thanks to the near isomorphism of TiO2 rutile (P42/mnm, a=4.5937 Å, c=2.9587 Å) and SnO2 cassiterite (P42/mnm, a=4.7382 Å, c=3.1871 Å), stable heterojunctions between the two semiconductors can be obtained. Furthermore, SnO2 and TiO2 present different work functions (4.4 and 4.2 eV, respectively). The Fermi energy of TiO2 is higher than that of SnO2, which can promote electron transfer from the TiO2 conduction band to the SnO2 one, with the ensuing formation of an interface contact potential (Floriano 2014). These electron transfer phenomena can be exploited to promote the separation of charge carriers in photocatalytic applications, leading to more efficient materials. Extending the lifetime of photo-generated charges is a crucial aspect particularly for N-doped TiO2, as in these materials visible-light activity is generally achieved at the expense of faster charge carriers recombination (Asahi 2014; Rimoldi 2015). We have recently shown that addition of Sn species can promote the photocatalytic activity under solar irradiation of N-doped TiO2 (Rimoldi 2018). In this work, N-doped TiO2/SnO2 materials are investigated as photocatalyst for the light activated degradation of both water and gas phase pollutants. N-doped TiO2/SnO2 and TiO2/SnO2 samples were synthesized in a broad range of Sn:Ti molar ratios (up to 20%) adopting three different methods (mechanical mixing, co-synthesis and seeded growth) followed by calcination at 400°C. Commercial and ad hoc synthesized TiO2 and SnO2 samples were also analyzed as references. The structural, morphological, electronic and surface features of each set of samples were investigated in detail. The local structure, defectivity and charge transfer phenomena were also studied via in situ X-ray absorption spectroscopy experiments. High resolution X-ray diffraction and X-ray absorption curves at the Sn L1-edge and Ti K-edge showed that the adopted synthetic strategies control the microstructure in terms of both phase composition and concentration of defects, which in turn imply significant changes in both the long- and short-range structures. In particular, the coprecipitation route gives rise to notable differences with respect to the other two methods, showing no peaks due to SnO2 cassiterite and promoted growth of TiO2 rutile (Fig. 1b) as well as a different Sn coordination geometry (Fig. 1a). Furthermore, the operando measurements provided evidence of electron transfer mechanisms taking place upon UV light irradiation. The distinctive character of samples from coprecipitation route is also appreciable in terms of light absorption features (Fig. 2), where the copromotion with Sn and N gives rise to synergistic effects. Samples were tested under UV and solar irradiation towards the remediation of an emerging pollutant (tetracycline) in water and of a model VOC (ethanol) in the gas phase. Both the molecule disappearance, the reaction intermediate and final mineralization were monitored, showing promising results in terms of photocatalytic activity of the copromoted samples (Fig. 2). The photocatalytic performances were correlated with the structural and spectroscopic results also on the grounds of the reaction pathways.