New Insights into the SnO₂ Sensing Mechanism Based on the Properties of Shape Controlled Tin Oxide Nanoparticles

We report on the sensing behavior of SnO₂ shape controlled nanocrystals in order to evaluate the role of their exposed crystal surfaces in the sensing mechanism. Octahedral (OCT), elongated dodecahedral (DOD), and nanobar shaped (NBA) nanocrystals were synthesized by previously reported procedures and their performances were evaluated in the sensing toward CO. Singly ionized oxygen vacancies (V_O^•) were detected by electron spin resonance (ESR), and their abundance and reactivity were associated to the exposed crystal faces and, in turn, to the sensing responses of the nanocrystals. Results indicated that the electrical properties and the formation/reactivity of the V_O^• centers are interconnected and are relatable to the nanoparticle specific surfaces. Two different temperature-dependent sensing mechanisms were proposed, depending on the prevalence of the surface structure or of the specific surface area on the sensing ability of shape controlled SnO₂ nanoparticles

Crystal Surfaces and Fate of Photogenerated Defects in Shape-Controlled Anatase Nanocrystals: Drawing Useful Relations to Improve the H₂ Yield in Methanol Photosteam Reforming

We comprehensively explored the photocatalytic properties, in H₂ production by methanol photosteam reforming, of anatase nanocrystals with nearly rectangular (<i>RC</i>), rhombic (<i>R</i>), and nanobar (<i>NB</i>) shapes having exposed {001}, {101}, and {010} surfaces. The aim was to relate the reactivity both to the type of crystal facets and to the photogenerated defects. The electron spin resonance (ESR) spectra reveal that the amount of Ti³⁺ (electron traps) is parallel to the H₂ evolution rate and becomes a maximum for the <i>RC</i> nanocrystals, which display the highest area of {001} surfaces and the lowest {101} area but also involve a significant area of {010} facets. This points out that the H₂ production cannot be related only to the envisaged reducing {101} facets, but that the {010} facets also play a key role. We suggest that the contiguous {001}, {101}, and {010} facets form a highly effective “surface heterojunction” within a <i>RC</i> nanoparticle which drives the electrons photogenerated on {001} facets not just toward the {101} but also to the {010} facets, while the holes are driven toward the {001} facets. This transfer improves the charge separation, thus boosting the photoefficiency of <i>RC</i> nanocrystals compared to that of <i>NB</i> and <i>R</i> nanocrystals. The ESR spectra performed after ultraviolet excitation in the presence of MeOH show the partial annihilation of the Ti³⁺ features, mainly for highly reactive <i>RC</i> nanocrystals. Because H₂ production involves an electron transfer to the proton, a relevant role in H⁺ photoreduction of the Ti³⁺ centers present on the exposed {010} and {101} surfaces is suggested. These findings underline the importance of determining the relationship between the photogenerated defects and the exposed crystal surfaces to optimize the photocatalytic properties of anatase nanocrystals

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