Selective Removal of Photocatalytically Active Anatase TiO₂ Phase from Mixed‐Phase TiO₂‐ZnO Nanocomposites: Impact on Physicochemical Properties and Photocatalytic Activity

Abstract

Heterogeneous photocatalysis employing semiconductor nanoparticles, especially TiO 2 nanoparticles, has attained considerable research interest in recent years. [1] TiO 2 is known to exist in three polymorphic forms, viz. anatase, rutile, and brookite. [2] Often, anatase TiO 2 is considered to be the most photocatalytically active due to high adsorption affinity for organic molecules even at low concentration [3] and low recombination of charge carriers, owing to inherently present oxygen vacancies. [4] However, the inability to utilize visible light limits the practical application of TiO 2 for photocatalysis. [5] Therefore, in recent years, synthesis and application of photocatalytic heterostructures based on TiO 2 and other semiconductors/noble metals have emerged as a strategy to facilitate visible light activity. [6-9] Nanocomposites made of two of the most versatile semiconductor oxides, viz. TiO 2 and ZnO have drawn considerable attention for photocatalytic processes. [10] TiO 2-ZnO nanocomposites have shown good potential for degradation of a variety of organic pollutants, [11,12] and activity under visible irradiation was feasible due to favorable alignment of energy states of both TiO 2 and ZnO. [12-14] Heterostructures comprising of anatase TiO 2 and other metal oxides, [15] and anatase with other TiO 2 phases, such as rutile, [10] are reported to be active under visible irradiation. Their superior photocatalytic activity was attributed to the presence of anatase or a favorable anatase/rutile ratio. The superior photocatalytic activity of anatase TiO 2 is well established. [16] According to literature reports, anatase exhibits an indirect bandgap in addition to its direct bandgap, which enhances charge carrier lifetime. [17] Anatase TiO 2 , being a quasi-stable polymorph, is susceptible to formation of defects and, hence, midgap defect states. [18] On the other hand, pure rutile phase of TiO 2 is reported to be photocatalytically inferior to anatase phase. [3] Contrary to such theories, recent studies have suggested that phase transformations were controlled by the addition of ZnO, such that, at optimal Ti:Zn molar ratios, a unique mix of various TiO 2 , ZnO, and zinc titanate phases coexisted which helped in tailoring the bandgap. This facilitated visible light activity even when anatase was present in negligible amounts or even absent. [19] Interestingly,

Ag-enriched TiO2 nanocoating apposite for self-sanitizing/ self-sterilizing/ self-disinfecting of glass surfaces

“…The Raman mode emerging around 790–797 cm −1 band attributed to B 2g mode as the first overtone of the B g mode is caused by the symmetric stretching of the O–Ti–O bond in the TiO 2 lattice [26]. The two specific bands arising about 564–566 and 1095-1098 cm −1 corresponded to Si–O stretching vibration bond of the glass substrate [ 41 ].…”

Entangled ZnO on Ultrathin Hollow Fibers for UV-Aided Pollutant Decomposition

“…Compared to the conventional substrates (such as polyimide membranes, − silicon wafers, glass, or ceramics), natural fibers can significantly reduce the secondary contamination from used heterogeneous catalysts, especially, in the liquid-phase processes. For example, during water pollution removal by reducing the contamination, the metal oxide (like ZnO) based heterogeneous catalysts used for the photodecomposition technique could lead to nanocrystal exfoliation in water causing heavy metal/solid pollution. − …”

Earth‐Abundant CaCO3‐Based Photocatalyst for Enhanced ROS Production, Toxic By‐Product Suppression, and Efficient NO Removal

“…Artificial photosynthesis has received interdisciplinary attention in view of its great potential in harnessing solar energy to address environmental pollution and energy shortage. [1][2][3][4][5] Recently, semiconductor-based photocatalysis as a green and economical technology is widely reported, which could achieve the conversion of solar energy into chemical energy for pollutant removal. [6][7][8][9][10] In general, photoinduced reactive oxygen species (ROS) that are generated from the reactions with photogenerated carriers are considered as the active species participating in the degradation of pollutant.…”