to make a great contribution to both environmental treatment (emission cleaning and water purification) and renewable energy. Over the past few decades, the number of applications based on photocatalysis increased sharply; while a wide range of materials systems have been developed [73]. Photocatalytic H2 production from water is one of the most promising ways to realize a hydrogen economy for three reasons. (1) This technology is based on photon (or solar) energy, which is a clean, permanent source of energy, and mainly water, which is a renewable resource; (2) it is an environmental safe technology without undesirable by-products and pollutants; and (3) the photochemical conversion of solar energy into a storable form of energy, i.e. hydrogen allows to deal with the intermittent character and seasonal variation of the solar influx. Nano science and nanotechnology have boosted the modification of existing photocatalysts and the discovery and development of new candidate materials [74]. The rapidly increasing number of scientific publications constitutes clear bibliographical evidence for the significance of this hot topic. The valence holes are good oxidants and the conduction electrons are reductants. In most cases, the organic compounds are degraded by oxidation reaction involving photogenerated holes. Since 1972, scientists discovered the photocatalytic splitting of water on the illuminated single crystal TiO2 electrodes [75], the photocatalysis as a research area is of significant interest and photocatalytic processes have been extensively studied.
The adoption of TiO2 photoanode as well as a Pt counter electrode soaked in an aqueous electrolytic solution made it possible for the water to be incised, which was caused by UV light. This was conducted by Honda and Fujishima in 1972. It has been issued that the charge clipping and photocatalytic performance can be improved by combination of semiconductor with a wide band gap like SnO2 with TiO2 [76, 77]. There is an effective of promoting the photocatalytic performance of visible light, namely TiO2 matrix is inserted with the combination of metal ions. The principle of this approach is through blocked charge carrier recombination [78]. The insertion of the metal ion promotes the shape of Ti3+ ions, thus improving the photocatalytic performance. Transforming the TiO2 through incorporation of two or more than two dopants is issued, which makes great combination influence. On the contrary, the undoped TiO2 or the TiO2 with one ion incorporated is less effective [79, 80]. Surface spots, oxygen vacancies and polar planes contribute the difference in photocatalytic performance of ZnO. With solvothermal technique, Xu et al. synthesized carious forms of ZnO and adopted them as photocatalyst to degrade the phenol [81].
These researchers proposed that nanoflowers and NPs indicated boosted photodegradation outcomes in comparison to nanoflowers, nanorods, nanotubes, as well as hour-glass-like ZnO spheres. To photodegrade phenol, Liu et al. used TiO2 nanostructures with various forms such as microspheres, NPs and nanorods though hydrothermal approach [82]. With nanorods to be photocatalyst, this group of researchers gained marvelous photodegradation outcomes. ZnO has come into the researchers’ focus since 1935, but its excellent features are discovered through modern methods and improved equipment [83]. Liang et al. [84] found that the generated graphene–TiO2 nanocrystal combination featured advanced photocatalytic performance in contrast to other TiO2 materials like P25, bare TiO2 and mixture of P25 and GO handled by hydrothermal procedure, in the splitting process of rhodamine B with UV irradiation, boosting a three-fold photocatalytic influence on P25. Metal oxide appearances indicate that it is good at decomposing organic molecules with great oxidizing ability for and superhydrophilicity [85, 86] and such traits could be adopted to generate wettability patterns, which have been adopted in many areas like in printed-circuit boards and offset printing, and can be used for fluid microchips in the future [87, 88].
1.5 Metal Oxide Nanomaterials for Sensor Applications
Research into imidazole chemistry has been quite comprehensive, single-site functionalizations and substitution as well as various annulation strategies has been well documented [89]. Recently, the interest in this heterocyclic system has been widened as it is a precursor to a class of compounds, called room temperature ionic liquids. Microwave-assisted synthesis of the substituted imidazoles on a solid support under solvent-free conditions in a three-component reaction [90] and efficient synthesis of imidazoles from aldehydes, 1,2-diketones and ammonium acetate in acetic acid have been reported [91]. Noble metal Au, Ag, or Pt coated ZnO is important for photoelectron transfer (PET) in the bulk and interface of ZnO semiconductors. Under illumination of UV light, the exciton absorption bands of ZnO are strongly bleached due to the accumulation of conduction band electrons. Thus, the efficiency of both the photocatalysis and photoelectric energy conversion can be greatly enhanced by depositing noble metals on the surface of ZnO [92–94]. The principle terms involved in a photoactive semiconductor are conduction band (CB), valence bands (VB), band gap, trap sites and Fermi level. The bands are the allowed energy states that an electron can occupy in a material. The highest energy band occupied by an electron is called the valence band while the next available lowest empty energy level, next to valence band is called the conduction band. The metal oxide samples were deoxygenated by bubbling with pure nitrogen gas. An ethanolic solution of the imidazole derivative of required concentration was mixed with nanoparticles dispersed in ethanol at different loading and the absorbance and emission spectra were recorded. The nanocrystals were dispersed under sonication in ethanol using ethylene glycol followed by dilution with ethanol. Surface modification of Fe2O3 has been performed as follows [95]. 2 g of Fe2O3 nanoparticles has been kept in a vacuum chamber at 110 °C for 75 min and then dispersed in acetone by stirring for 1 h at ambient temperature and finally has been sonicated for 20 min. Then, 1 g APTS (50 wt%) has been gradually added to the dispersed solution and stirred for further 24 h at ambient temperature.
Fluorescence enhancing arises due to formation of APTS–ZnO complex. The energy transfer competence is related not only to the distance between the acceptor ZnO and donor APTS (r0) but also to critical energy transfer distance (R0). The critical energy transfer distance (R0),
1.6 Metal Oxide Nanocomposites and its Thermal Property Analysis
Metal nanoparticles, due to their special properties and also small dimensions, find important applications in optical, magnetic, thermal, sensoric devices, catalysis, etc. Metals are dominated by the collective oscillation of conduction electrons resulting from the interaction with electromagnetic radiation. In addition to these, many production processes include heat transfer in various forms; it might be the cooling of a machine tool, pasteurization
