The role of graphene oxide content on the adsorption-enhanced photocatalysis of titanium dioxide/graphene oxide composites
Introduction
TiO2-based materials are the most commonly used semiconductor oxide photocatalysts due to their low environmental impact. However, there are numerous obstacles impeding the maximization of photocatalytic activity in these materials, including low adsorption ability, detrimental recombination of charge carriers, and light utilization [1], [2]. In the past few decades, doping with metal ions, coupling with a second semiconductor, and anchoring TiO2 particles onto large-surface-area materials, such as mesoporous materials, zeolites or carbon-based materials, have all been elucidated as efficient techniques to improve the photodegradability of semiconductor oxide photocatalysts [3], [4], [5]. Among them, TiO2/carbon nanotube composites have been established as viable potential photocatalysts for use in both water and air purifications [6], [7], [8]. The synergetic effect of carbon nanotubes on photocatalyst enhancement, in which carbon nanotubes act as the electron sink for the hindrance of charge carrier recombination [6] or as the photosensitizer to generate a greater density of electron/hole pairs [7], has been previously demonstrated. Carbon nanotubes also behave as impurities, resulting in the formation of Ti–O–C bonds and, therefore, expanding the light absorption to longer wavelengths [8].
Alternatively, the synthesis of graphene, a single atom-thick planar sheet of sp2-bonded carbon atoms, has recently attracted scientific interest due to its fascinating electronic, mechanical, thermal and optical characteristics [9], [10]. Graphene is considered as the parent of all graphitic forms and can be curled, rolled or stacked to form buckyball fullerenes, carbon nanotubes or graphite [11]. Graphene-based materials have been widely used as transparent conducting electrodes, supercapacitors, optoelectronic devices, composites, and catalysts [12], [13], [14], [15].
Graphene can be synthesized by chemical vapor deposition, micromechanical exfoliation of graphite, epitaxial growth on electrically insulating surfaces, solvothermal synthesis and reduction of graphene oxide by thermal or chemical treatment [15], [16], [17], [18], [19]. The combination of TiO2 and graphene oxide and/or graphene is predicted to generate a synergistic effect that potentially enhances the photodegradation of organic contaminants in both gaseous and aqueous media due to the possible improvements in the adsorbability and efficient charge transfer rate. Few reports related to the synthesis of such graphene-based composites with TiO2 photocatalyst have been published [20], [21], [22], [23]. Williams et al. [24] reported that graphene oxide was successfully reduced to graphene with the assistance of TiO2 nanoparticles upon UV irradiation due to the transfer of accumulated electrons in excited TiO2 to the graphene oxide sheets or to the reduction of certain functional groups. Manga et al. [25] monitored the ultrafast electron transfer between TiO2, graphene oxide and graphene sheets in femtosecond transient absorption spectroscopy, reflecting the higher separation efficiencies of photo-induced electrons and holes. They also determined that the photo-reductions in the transformation of graphene oxide to graphene created a continuous electron conducting network through cross-surface charge percolation and allowed graphene to function as an efficient exciton sink.
Commercial Degussa P25 (Degussa Company, Germany) is a well-known efficient TiO2 photocatalyst; therefore, the preparation of a P25-based composite with intact intrinsic properties is of great importance. Recently, Zhang et al. [22] applied commercial P25 and graphene oxide as TiO2 precursor and carbonaceous source; subsequently, they prepared P25-solvothermally reduced graphene composite by hydrothermal treatment with ca. 1 wt% of carbon content. The photocatalytic activities of pure P25, P25–graphene and P25–carbon nanotube composites were compared through the photodegradation of methylene blue upon UV and visible lights. However, to our knowledge, the simple preparation of P25/graphene oxide composite as the photocatalyst has not been reported yet. It is predicted to be more advantageous to the adsorbability and corresponding photoactivity when preparing a TiO2/graphene oxide composite system due to the presence of abundant functional groups at the edge or on the surface of graphene oxide layers compared to those of graphene sheets. Furthermore, the one-step colloidal blending is an environmentally friendly and simple method that preserves the TiO2 characteristics and combines the outstanding advantages of graphene oxide sheets, including excellent electronic conductivity and large surface area.
In the present study, a one-step colloidal blending method for the preparation of TiO2/graphene oxide composites for application in methylene blue photodegradation is described. The role of graphene oxide content in the superior adsorptivities and photocatalytic activities under both UV and visible irradiations was achieved for the first time.
Section snippets
Preparation of TiO2/graphene oxide composites
The composites were prepared by one-step colloidal blending using commercial TiO2 (P25, Degussa), graphene oxide and deionized water. Graphene oxide was synthesized in the laboratory, and the procedure was described in detail elsewhere [26]. In a typical preparation, an aqueous dispersion of graphene oxide (8.4 mg/mL) was dissolved in 200 mL of deionized water. TiO2 powder (P25, Degussa) was dispersed in deionized water and subsequently added to the graphene oxide solution. The mixture was
Photocatalyst properties
The percentages of graphene oxide in the composites determined using thermogravimetric analysis are shown in Table 1. The evaluated graphene oxide concentrations were quite consistent with the calculated values. The porosities of the xGO/Ti composites as measured by N2 adsorption–desorption isotherms were significantly enhanced in comparison with that of pure TiO2 powder. The porosity enhanced according to graphene oxide content, and the composite containing 5 wt% graphene oxide possessed the
Conclusions
TiO2/graphene oxide composites were synthesized using a simple, environmentally friendly, one-step colloidal blending method. Superior adsorptivity and photocatalytic activity under UV and visible light were obtained on the composite system, in which increasing graphene oxide content resulted in higher degradation efficiencies of methylene blue. Graphene oxide plays the roles of adsorbent, electron acceptor and photosensitizer in order to accelerate photodecomposition.
Acknowledgement
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0008810).
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