Evaluating the Catalytic Effects of Carbon Materials on the Photocatalytic Reduction and Oxidation Reactions of TiO₂

Abstract

TiO 2 composites with seven different carbon materials (activated carbons, graphite, carbon fibers, singlewalled carbon nanotubes, multi-walled carbon nanotubes, graphene oxides, and reduced graphene oxides) that are virgin or treated with nitric acid are prepared through an evaporation method. The photocatalytic activities of the as-prepared samples are evaluated in terms of H 2 production from aqueous methanol solution (photocatalytic reduction: PCR) and degradation of aqueous pollutants (phenol, methylene blue, and rhodamine B) (photocatalytic oxidation: PCO) under AM 1.5-light irradiation. Despite varying effects depending on the kinds of carbon materials and their surface treatment, composites typically show enhanced PCR activity with maximum 50 times higher H 2 production as compared to bare TiO 2 . Conversely, the carbon-induced synergy effects on PCO activities are insignificant for all three substrates. Colorimetric quantification of hydroxyl radicals supports the absence of carbon effects. However, platinum deposition on the binary composites displays the enhanced effect on both PCR and PCO reactions. These differing effects of carbon materials on PCR and PCO reactions of TiO 2 are discussed in terms of physicochemical properties of carbon materials, coupling states of TiO 2 /carbon composites, interfacial charge transfers. Various surface characterizations of composites (UV-Vis diffuse reflectance, SEM, FTIR, surface area, electrical conductivity, and photoluminescence) are performed to gain insight on their photocatalytic redox behaviors.

Effect of Different Activated Carbon as Carrier on the Photocatalytic Activity of Ag-N-ZnO Photocatalyst for Methyl Orange Degradation under Visible Light Irradiation

“…The absorption edge is therefore extended to visible light, leading to the generation of electron-hole pairs, which enhances the photocatalytic properties of the composite photocatalyst [ 25 ]. The loading of the Ag-N-ZnO on AC results in a narrowing of the band-gap and the raising of the absorbance in the visible region, which is likely due to the formation of Zn-O-C just like carbon doping ZnO [ 26 ].…”

“…The smaller particle could facilitate the rapid diffusion of photogenerated carriers to the semiconductor surface, resulting in decreasing the recombination rate of the carrier. A lower PL intensity means a lower electron-hole recombination rate and a higher separation efficiency, hence carriers have a longer lifetime [ 26 ]. Therefore, the efficient charge separation and inhibited electron-hole recombination could probably enhance the photocatalytic activity of photocatalysts.…”

“…Moreover, when the adsorption of the photocatalyst was eliminated, the degradation efficiency of MO can still reach 97.23% within 120 min under visible light radiation ( Figure 7 b). This is due to three reasons: (1) The particle size of Ag-N-ZnO/CHAC is the smallest, which is in favor of adsorption of O 2 and –OH to generate ·OH and ·O 2 − [ 23 ]; (2) The surface area of Ag-N-ZnO/CHAC is the largest, which may contribute to more MO molecules gathering around Ag-N-ZnO [ 10 ]; and (3) The intensity of the PL of Ag-N-ZnO/CHAC is the weakest, which indicates a decrease in the recombination rate of electrons and holes, thus increasing the life of electrons and holes [ 26 ]. These factors resulted in a highest photocatalytic activity of Ag-N-ZnO/CHAC.…”

SWNTs-catalyzed solar hydrogen production

“…Recently, inexpensive carbon materials including nanotubes, nanofibers, graphites, and graphenes have been found to effectively catalyze hydrogen production from water. [6][7][8][9][10] The electric conductivity of carbon materials was found to be a primary factor in determining the STH efficiency of semiconductor/carbon composites. 10 Among these carbon materials, single-walled carbon nanotubes (SWNTs) were found to be capable of producing hydrogen under visible light even in the absence of semiconductor particles.…”

“…It has been shown that surface oxidation of CNTs by annealing and acid treatment remove disordered or dangling carbons. [6][7][8][9][10] These treatments increased the graphitic property and electrical conductivity, whereas surface area was not altered significantly. Borohydride treatment (for Pt loading) also could affect the surface carbon states in a similar way.…”