The CO₂ photoreduction process to produce light hydrocarbons is known to be influenced by the presence of CuO nanoparticles, but the actual role of this material, whether as a catalyst or a reactant, has not yet been revealed. In this work, we investigate the role of CuO nanoparticles produced by a solvothermal method as a catalyst in CO₂-saturated water reaction media under UV light, considering the effects of different electrolytes (Na₂C₂O₄, KBrO₃, and NaOH) and temperatures on nanoparticle phase and activity. The electrolyte strongly influenced product selectivity (NaOH led to evolution of CH₄, Na₂C₂O₄ to CO, and KBrO₃ to O₂) and induced CuO phase change. A long-term analysis of these processes indicated that during the initial steps, CuO acted as a reactant, rather than as a catalyst, and was converted to CuCO₃.Cu(OH)₂, while the as-converted material acted as a catalyst in CO₂ photoreduction, with conversion values comparable to those reported in the literature.

Nanomaterials based on metal oxides are extensively studied for several applications due to their versatility. Improvements in their performances can be obtained due to specific structural modifications. One possible modification is by doping the crystal structure, which can affect the materials structure and properties, especially in nanosized particles. Electronic features are among the properties that can be modified through the doping process, consequently morphological and optical parameters can also be controlled by this process. In this sense, this review presents some modifications to tin dioxide (SnO₂), one the most studied materials, mainly through the doping process and their impact on several properties. The article starts by describing the SnO₂ structural features and the computational models used to explain the role of the doping process on these features. Based on those models, some applications of doped SnO₂, such as photocatalytic degradation of pollutants, CO₂ reduction, and desulfurization of fossil fuels are presented and discussed. Additionally, the review describes many biological applications related to antimicrobial activity for doped SnO₂ and its nanostructures. Although most of the examples presented in this article are based on the doped SnO₂, it also presents examples related to SnO₂ composites with other nanomaterials forming heterojunctions. The metal oxides SnO₂, doped-SnO₂ and their nanostructures are promising materials, with results reported in many fields presented in this review, such as theoretical and computational chemistry, environmental remediation, nanoparticle morphology control, fossil fuels improvement, and biomedical applications. Although widely explored, there are still fields for innovation and advances with tin dioxide nanostructures, for example, in transparent conducting oxides, in forensics as materials for latent fingerprints visualization, and sensors in medicine for detection of exhaled volatile organic compounds. Therefore, this article aims to be a reference regarding correlating the doping processes and the properties presented by the SnO₂ nanostructures.