Abstract
Graphene-based cocatalysts can improve the photocatalytic properties of semiconductors, but sometimes, they also function as barrier-like materials, influencing the photoactivity of composites. However, in a multi-cocatalyst system, less attention is paid to these negative effects of graphene on the performance of other cocatalysts. In this study, by adjusting the loading sequence of graphene and Ag cocatalyst on the surface of TiO2 spheres, the barrier effect of graphene sheets on Ag nanoparticles could be controlled effectively. As a result, these ternary composites with almost no Ag nanoparticles wrapped by graphene possessed improved properties for the photocatalytic reduction of nitro-aromatics as compared to those with some Ag nanoparticles covered by graphene. Furthermore, this phenomenon of barrier effect caused by graphene could be found in the control reaction with metal silver as the main catalyst; this indicated that by avoiding the possible negative influence of graphene on other cocatalysts, the properties of composites with graphene-containing multi cocatalysts could be further improved.
Highlights
By adjusting the loading sequence of graphene and Ag cocatalyst on the surface of TiO2 spheres, the barrier effect of graphene sheets on Ag nanoparticles could be controlled effectively. These ternary composites with almost no Ag nanoparticles wrapped by graphene possessed improved properties for the photocatalytic reduction of nitro-aromatics as compared to those with some Ag nanoparticles covered by graphene
To enable graphene sheets to better wrap the metal nanoparticles on the surface of semiconductors, small Graphene oxide (GO) sheets and TiO2 spheres (400 nm) were chosen as the starting materials to construct ternary composites based on our previous research.[36]
Ag nanoparticles were decorated on the surface of TiO2 spheres by traditional photo-deposition methods in an ethanol solution
Summary
Graphene-based sheets have been widely used as cocatalysts to improve the photocatalytic properties of semiconductors due to their unique physicochemical properties.[1,2,3,4] To expand their application in photocatalysis areas, graphenecontaining binary cocatalysts, in particular those combined with noble metal nanoparticles (such as Au and Ag), have been adopted to modify the semiconductor photocatalysts such as ZnO, TiO2, Bi2WO6, La2Ti2O7, etc.[5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] Owing to good electron collection/transport abilities and light response properties of both graphene sheets and noble nanoparticles, these multicocatalysts exhibit combined or synergistic effects on improving the performance of these semiconductors in many photocatalytic reactions.Generally, there are three strategies to obtain graphene and metal-containing composite photocatalysts: (1) using graphene sheets to wrap the metal-loaded semiconductors,[5,6,7,8,9] (2) using metal nanoparticles to modify graphene–semiconductor composites,[10,11,12,13,14,15,16,17,23,24] and (3) using a one-step synthesis process, for example, a solvothermal method, to prepare these ternary composites.[25,26,27,28,29,30] all these methods can produce graphene/metal-containing photocatalysts, the relative positions of these two kinds of cocatalysts on the surface of semiconductors may be slightly different. A er a certain degree of reduction, the areas of oxygen-containing functional groups decrease in the C1s spectra.[48,49] By comparing the changes in these groups (C–O/C]O) in the TA0.5GO0.5 and TA0.5G0.5 samples, it was found that these added GO sheets could be reduced through photocatalytic reduction (Fig. 3B(a) and (b)), which was consistent with previous studies.[44,45] similar C1s spectra in both TA0.5G0.5 and TG0.5A0.5 indicated that GO sheets in these two kinds of ternary composites possessed approximately the same reduction degree, which could be further supported by Raman results (Fig. S3†).[50,51] In addition, the contents of graphene sheets in TA0.5G0.5 and TG0.5A0.5 composites were similar to each other, which were con rmed by thermogravimetric analysis (Fig. S4†).
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