Materials Science Forum Vol. 855

Abstract: The image of CO₂ is turning quickly because there are increasing attempts to consider it as resources for hydrocarbon based fuels rather a green house gas. Owing to the limited amount of non-renewable readily available energy sources, the paradigm of energy supply is changing from conventional energy sources to inexhaustible renewable sources such as sunlight, wind, tidal energy. But the current scientific interest is concentrated in the efficient recycling of CO₂ from a waste combustion product into a solar fuel by photo reduction method that can be used within the current energy infrastructure. Solar energy as direct solar irradiations is excessively available and it is imperious to utilize it for solar fuel products. In other words, solar to chemical conversion by photo-reduction process is an effective route. Moreover, fuels from solar energy are complementary to solar to electrical energy conversion, but there is still intense research is needed before its successful commercialization. Solar fuels produced from CO₂ in comparison with H₂ are analyzed and it is seen that these solar-hydrocarbons fuels involves easy transportation and storage than H₂ fuel. Photoreduction of CO₂ is considered as one of the scientific challenges and has been carried out by different photocatalysts. But the nanostructured photocatalyst owing to their unique optical and electrical property are gaining much attention. Several nanostructured semiconductor photocatalyst such as: metal oxides, heterojunctions, porous materials, layered materials, materials with hierarchical structure, and nanobiocatalysts are acknowledged as good candidate for CO₂ photo reduction. This technology not only provides an alternative way to produce the sustainable fuels, but also convert the waste CO₂ into valuable chemicals, which is important for keeping our environment clean and sustainable. However, there are still several limitations present in the process of CO₂ photoreduction and various strategies have been developed to overcome them. Numerous efforts are required to improve the competence of the photo reduction reaction by developing the novel and efficient photocatalyst with considerable activity, high reaction selectivity. In this chapter, we have summarized several scientific attempts that lead to the design of efficient nanocatalysts for CO₂ photo reduction along with their mechanistic pathways.

Abstract: Heavy industrialization, specifically in the developing countries, has generated several unwanted environmental pollution. A variety of toxic organic compounds is produced in chemical and petroleum industries, which have resulted in collectively hazardous effects on the environment that needs immediate attention for remediation. Degradation of these pollutants has been tried through the various mechanism, out of which photocatalytic degradation seems to be one of the most promising approaches to reduce environmental pollution specifically in waste water treatment. Photocatalytic degradation has potential for the effective decomposition of organic pollutants due to efficiency to convert light energy into chemical energy. Additionally, the photocatalytic oxidation process is an advanced technique as it offers high degradation and effective mineralization at moderate temperature and specific radiation wavelength. Among various known photocatalysts, TiO₂ is regarded as the one of the potential photocatalysts because of its hydrophilic property, high reactivity, reduced toxicity, chemical stability and lower costs. Therefore, the present chapter focuses on the role of TiO₂ as the photocatalyst for the degradation of organic pollutants. The general mechanism of degradation of organic pollutants along with properties of TiO₂ as the photocatalyst, existing mechanism of degradation via TiO₂ was explained. The possible approaches to enhance degradation via TiO₂ nanoparticle along with existing bottlenecks have been also discussed.

Abstract: TiO₂ and Sm³⁺ doped TiO₂ nanocrystalline has been successfully synthesized by a modified sol-gel method. As synthesized samples of TiO₂ and Sm³⁺ doped TiO₂ were calcined at 300, 500, 700 and 800^OC and characterized by various techniques such as XRD, UV/Vis Reflectance spectroscopy, FTIR, SEM-EDS and TEM. The crystallite size of Sm³⁺ doped TiO₂ at all calcination temperature is lower than that of TiO₂ due the doping Sm³⁺ ion and thereby induced more nanobehavior. FTIR spectroscopy confirmed the presence of Ti-O and Ti-O-Ti bond in TiO₂, in Sm³⁺ doped TiO₂ along with Ti-O and Ti-O-Ti, the presence Sm-O and Ti-O-Sm bonds are confirmed. Diffuse reflectance spectra showed that the Sm³⁺ doped TiO₂ have a significant shift to longer wavelengths and an extension of the absorption in the visible region compared to the TiO₂. SEM images confirmed that the particles are agglomerated and the particle size was decreased in the Sm³⁺ doped TiO₂ in comparison with the TiO₂. EDS analysis showed the presence of Sm³⁺ ion present in the lattice of TiO₂ in doped sample. Finally the photocatalytic activity of TiO₂ and Sm³⁺ doped TiO₂ at various calcinations temperatures was investigated by the degradation of methylene blue solution under UV light and visible light. Doping with the samarium ions significantly enhanced the overall photocatalytic activity for MB degradation under both UV and visible light irradiation. The results showed that the Sm³⁺ doped TiO₂ sample calcined at 700 ^OC shows the highest photocatalytic activity under UV light and visible light irradiation.

Abstract: Anatase TiO₂ nanotubes (ATNT) was synthesised by hydrothermal method using anatase TiO₂ nanoparticles (AT) as precursor and calcined at two different temperatures (250 & 450 °C) for 2 h. The AT and synthesized ATNT photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, N₂ adsorption-desorption measurements, UV-vis diffuse reflectance and Fourier Transform Infra-red spectroscopy techniques for their structural, textural and electronic properties. The photocatalytic degradation of Indigo carmine (IC) dye aqueous solution has carried out using ATNT-250 and ATNT-450 photocatalysts under UVLED irradiation. The kinetic analysis has also revealed that the degradation of IC dye solution follows first order kinetic model. The overall study demonstrates the appropriate band gap of the photocatalysts used and the suitable irradiation source which could accelerate the rate of photocatalytic degradation. The band gap of the synthesised ATNT is not much affected due to the change in morphology from nanoparticle to nanotube. The results demonstrated that the irradiation of UV-LED could be utilised for the degradation of organic dyes

Abstract: World is presently facing two major problems: Energy crisis and ever increasing environmental pollution as the fossil fuels used today are polluting the environment and these resources are limited only for a few coming decades. nanosized materials are being used these days to provide alternative energy sources to fossil fuels, which is environmentally clean also. The development of newer photocatalytic nanomaterials will enable us to produce and store solar energy in the form of hydrogen. Hydrogen has been advocated as the fuel of future and it can be produced by photo-splitting of water in presence of photocatalytic materials. nanosized photocatalytic materials have also been utilized in solar cells and photocatalytic reduction of carbon dioxide (a step towards artificial photosynthesis). Although, the use of nanosized photocatalytic materials has long miles to go to compete with present day technology (Photovoltaics and use of fossil fules), but there is lot of hopes from this kind of material in years to come. This chapter deals with use of nanomaterials in conversion of solar energy into electricity, photogeneration of hydrogen, and photocatalytic reduction of carbon dioxide. Presently, majority of photovoltaic power comes from bulk semiconductors, and only a limited use has been made of nanosized semiconductors, but there is likely U-turn in coming decades so that nanosemiconductors will have an edge over bulk semiconductors.

Abstract: This review focuses on the recent developments of high temperature stable anatase TiO₂ photocatalyst. Eventhough TiO₂ exists in different forms anatase, rutile and brookite, anatase phase stabilization is often the key to obtain the highest photocatalytic performance for TiO₂, particularly for the use as an antibacterial and self-cleaning coatings in high temperature processed ceramics. Different methods available for the anatase stabilization in literature are critically reviewed and emphasis is placed on relatively recent developments. Currently available methods of anatase stabilizations are classified in to four categories viz (i) doping with metal ions (ii) doping with non-metal ions (iii) co-doping with metal and non-metal ions and (iv) dopant free stabilization by oxygen richness. Further to this, the application of these high temperature stabilized anatase TiO₂ photocatalyst on various ceramics substrates such as tile, glass and sanitary wares as self-cleaning and antibacterial coatings are also been briefly discussed.

Abstract: Textile and dyes industries are generating huge amount of waste water contains significant amount of dyes which are toxic and hazardous for the environment. Recently, advanced oxidation processes (AOPs) have considerable attention because it offers an attractive method for degradation of organic compounds and color from wastewater. The present work is focused on the heterogeneous photocatalytic degradation of Alizarin Cyanine Green and Acid Orange 7 dye using UV/TiO₂ process. The degradation of dyes is strongly dependent on the various operational parameters like initial concentration of dye, pH and loading of TiO₂. The kinetics of degradation of dye in the solutions was found to pseudo first order kinetics. A statistical technique response surface methodology was employed to study the influence of various operational parameters on degradation efficiency. Three-factor-three-level Face Centered Design was used for design of experiments. For 100 mg/L concentration of Alizarin Cyanine Green and Acid Orange 7 dyes, the optimum parameters were found to be 1.2 gm/L and 1.4 gm/L TiO₂ loading for maximum degradation of 97% and 65% respectively.

Abstract: Tungsten trioxide (WO₃) is an oxygen deficient metal oxide and well known semiconductor with a small band gap of between 2.4 and 2.8 eV. It is also used as a photo-catalyst for degradation of organic pollutants present in aqueous environment. It has stable physico-chemical properties and shows strong absorption of solar spectrum and thus can be used in visible-light driven photocatalysis. WO₃ has a conduction band (E_CB) of +0.4 V versus NHE (normal hydrogen electrode) at pH = 0. Therefore, pure WO₃ has lower light energy conversion efficiency as compared to other widely used photocatalysts such as zinc oxide (ZnO) and titanium oxide (TiO₂). This is because the reduction potential of the electrons in WO₃ is low due to its low conduction band level. O₂ cannot be efficiently trapped in the conduction band electrons to yield superoxide radicals and fast recombination of charge carriers takes place resulting in lesser photocatalytic activity of WO₃. However, holes in the valence band (E_VB = +3.1 V) are energetically favorably situated to oxidize water to hydrogen. To modify the energy band position and reduce the charge carrier recombination, doping or surface modification of WO₃ is necessary. This review article demonstrates the effect of dopants (low band semiconductor catalyst) on the surface modification of WO₃ to enhance the photo catalytic activity which helps in degradation of the organic pollutants present in the wastewater.

Abstract: In the current era where pesticides play a vital role in one’s everyday life, large quantities of various pesticides some of which are highly toxic are being used routinely by industries and consumers. Extensive use of these chemicals provides greater risk to plants, animals and human population which has been reviewed from time to time. Apart from the biological degradation, photochemical removal holds considerable promise for the abatement of these pesticides in wastewaters. This paper reviews the photochemical degradation of pesticides. It is evident from the review that removal depends on several factors such as pH of the solution, catalysts loading, initial concentration of the pesticides, support based catalysts or suspended catalysts, light intensity and so on and so forth. Since the pesticides are ubiquitously present in the wastewaters, photochemical technology seems imperative to alleviate the pollution problems associated with the pesticides. However, commercial application of this technology has to be clearly assessed.