We report a facile hydrothermal method for the synthesis of niobium pentoxide (Nb2O5) under two different temperatures, 120 °C (Nb2O5-120) and 150 °C (Nb2O5-150). The obtained materials were characterized by structural, optical and morphological techniques. Also, the photocatalytic properties of the Nb2O5 samples were evaluated in two reactions under ultraviolet (UVC) irradiation: degradation of methylene blue (MB) and indigo carmine (IC) dyes; and esterification of free fatty acids (FFA) present in waste cooking oil (WCO). The Nb2O5 was formed at different temperatures of synthesis, and the increase in temperature did not cause significant changes in the structural and optical characteristics, but resulted in an increased surface area of the synthesized materials. Both synthesized materials showed excellent efficiency for dye photodegradation. The sample Nb2O5-150 presented the best performance for the MB photodegradation, with almost 85% of the removal efficiency. In this case, the adsorption of MB molecules on the surface of the material was high due to the favorable electrostatic interaction and also because of its high surface area. For the IC photodegradation, the adsorption was insignificant, and both samples presented approximately 100% of the removal efficiency. These materials were also promising for the reduction of free fat acids in waste oil by photoesterification. NbO Nb O (Nb2O5 12 C Nb2O5120 (Nb2O5-120 15 Nb2O5150. Nb2O5150 . (Nb2O5-150) techniques Also Nb2O UVC (UVC irradiation (MB (IC dyes FFA (FFA WCO. WCO (WCO) characteristics photodegradation Nb2O5-15 85 case insignificant 100 photoesterification (Nb2O 1 Nb2O512 (Nb2O5-12 Nb2O515 (Nb2O5-150 (WCO Nb2O5-1 8 10 Nb2O51 (Nb2O5-1 (Nb2O5-15 Nb2O5- (Nb2O5-

Titanium (Ti)-layered materials have interesting characteristics for photocatalytic applications, such as the high specific surface area and ease of intercalation of species in the interlamellar region, which are extremely favorable for the surface processes. In this sense, this review addresses the data available in the literature related to the preparation, structure and main photocatalytic applications of Ti-layered materials. The main synthesis methodologies of these materials were addressed: the hydrothermal, which is mainly used to obtain powders (containing TiO2 or titanates) and the anodization process that is commonly used to obtain films composed mainly of TiO2 nanotubes. Prominent photocatalytic applications for semiconductors were also addressed, focusing on the use of pure or associated Ti-layered materials for the photodegradation of emerging pollutants, photoreduction of CO2 or metals, hydrogen production and the inactivation of microorganisms. It was possible to observe that, despite these being extremely promising materials and with outstanding efficiency in some processes, such materials can still be considered little explored and deserve greater attention from researchers in the area.

Heterostructured semiconductors are materials with unique features formed by combination of two or more semiconductors with suitable interface between them. The study of the heterostructures aim the development of physico-chemical and photocatalytic properties higher than their isolated phases, which makes it possible to obtain higher efficiency and lower cost in several technological processes. Recently the heterostructured materials have been extensively used in the development of alternative technologies either for environmental pollution control or for renewable energy production, among other applications. However, the obtaining of a heterostructure with suitable features is not trivial. In this way, this review article presents a broad discussion about methods of obtaining heterostructures formed by semiconductors and their applications in the photochemical processes, mainly in the environmental field. It will be addressed the use of the heterostructures in photocatalytic application for the organic pollutants degradation, heavy metals removal, the inactivation of pathogenic microorganisms, beyond of the volatile organic compounds (VOC’s) photooxidation and the photocatalytic abatement of nitrogen oxide (NOx) in gaseous phase. Finally, a discussion about the carbon dioxide (CO2) photoreduction and hydrogen (H2) production focusing mainly in the chemical energy production will be presented.

Advanced oxidative processes (AOPs) are based on chemical processes that can generate free radicals, such as hydroxyl radicals (.OH) which are strong, non-selective oxidant species that react with the vast majority of organic compounds. Nanostructured semiconductors, especially titanium dioxide (TiO2) in the anatase phase, are well-established photocatalysts for this process, which have proved to be useful in the degradation of dyes, pesticides and other contaminants. Research in different strategies for the synthesis of nanostructured semiconductors, with particular characteristic is currently a topic of interest in many studies. Thus, this paper presents a review about various synthesis strategies of nanostructured photocatalysts.