Nanostructured SnO₂–ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO₂–ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO₂ particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO₂ particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption analyses, transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectroscopy. The SnO₂–ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO₂ nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV–visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO₂–ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO₂, that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO₂–ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO₂–ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO₂ and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO₂–ZnO photocatalyst because of the energy difference between the conduction band edges of SnO₂ and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO₂–ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications

Preparation of RuO₂/TiO₂ Mesoporous Heterostructures and Rationalization of Their Enhanced Photocatalytic Properties by Band Alignment Investigations

Nanoporous RuO₂/TiO₂ heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO₂ nanoparticles in a ruthenium­(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO₂ amount (0.5–5 wt %), the as-prepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO₂ nanocrystallites modified with RuO₂ according to N₂ sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO₂/TiO₂ heterojunction. At first, a model experiment involving stepwise deposition of RuO₂ on the TiO₂ film and an <i>in situ</i> XPS measurement showed a shift of Ti 2p_3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO₂/TiO₂ heterojunction. The band bending for the heterostructure RuO₂/TiO₂ nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO₂/TiO₂ nanocatalyst led to twice higher activities than pure anatase TiO₂ and reference commercial TiO₂ P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO₂/TiO₂ heterojunction. Finally, these mesoporous RuO₂–TiO₂ heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications

New Insights into the Photocatalytic Properties of RuO₂/TiO₂ Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition

Photocatalytic activities of mesoporous RuO₂/TiO₂ heterojunction nanocomposites for organic dye decomposition and H₂ production by methanol photoreforming have been studied as a function of the RuO₂ loading in the 1–10 wt % range. An optimum RuO₂ loading was evidenced for both kinds of reaction, the corresponding nanocomposites showing much higher activities than pure TiO₂ and commercial reference P25. Thus, 1 wt % RuO₂/TiO₂ photocatalyst led to the highest rates for the degradation of cationic (methylene blue) and anionic (methyl orange) dyes under UV light illumination. To get a better understanding of the mechanisms involved, a comprehensive investigation on the photogenerated charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of O^–, Ti³⁺, and O₂^– trapping centers, was performed. Along with the key role of superoxide paramagnetic species in the photodecomposition of organic dyes, ESR measurements revealed a higher amount of trapped holes in the case of the 1 wt % RuO₂/TiO₂ photocatalyst that allowed rationalizing the trends observed. On the other hand, a maximum average hydrogen production rate of 618 μmol h^–1 was reached with 5 wt % RuO₂/TiO₂ photocatalyst to be compared with 29 μmol h^–1 found without RuO₂. Favorable band bending at the RuO₂/TiO₂ interface and the key role of photogenerated holes have been proposed to explain the highest activity of the RuO₂/TiO₂ photocatalysts for hydrogen production. These findings open new avenues for further design of RuO₂/TiO₂ nanostructures with a fine-tuning of the RuO₂ nanoparticle distribution in order to reach optimized vectorial charge distribution and enhanced photocatalytic hydrogen production rates