Graphene-spindle shaped TiO2 mesocrystal composites: Facile synthesis and enhanced visible light photocatalytic performance
Graphical abstract
Introduction
Mesocrystals are colloidal crystals whose elemental units are nanocrystals (size within the mesoscale) that all grow oriented in the same crystallographic direction [1]. Mesocrystals are a very interesting generalization of colloidal crystals, as they extend the well-recognized class of colloidal crystals formed from monodisperse, spherical tectons to those constructed from nonspherical building units. Further, these units can also show considerable polydispersity in size and shape, which offers significant new possibilities in terms of superstructure formation [2], [3]. Over the past few years, mesocrystals have experienced rapidly increasing attention of researchers [4], [5]. The interest in mesocrystals or in mesocrystal-like porous nanostructures (MLPNs) is mainly because of their behavior similar to single crystals while preserving nanoscale capabilities. It is well-known that nanoscale capabilities are essential in many applications including catalysis, energy storage, drug and gene delivery, however many properties also depend on the nature and structural characteristic of specific materials. Thus the design and facile synthesis of mesocrystals or MLPNs with unique characteristics, such as good and uniform crystallinity, high surface area, narrow size distribution, and pore accessibility are of great importance to potential applications of functional materials.
Anatase TiO2 is one of the most studied semiconductor photocatalysts because of its strong oxidizing power, high chemical inertness, low cost, and long-term stability. However, a large bandgap of anatase TiO2 restricts its use only to the narrow light-response range of ultraviolet (only about 3–5% of total sunlight). Its photocatalytic performance has not yet been satisfactory for practical use due to the low photocatalytic efficiency under visible light irradiation.
Graphene, an sp2-bonded carbon sheet with a thickness of single atom, has recently received high attention from materials scientists because of its unique properties including large surface area, good flexibility, high electrical conductivity, high chemical stability [6]. Its large surface area and tunable surface properties allow it to be a competitive host substrate for the heterogeneous growth of desired active guest materials since surface functional groups such as hydroxyl groups on functional graphene can act as favorable nucleation sites for guest materials [7]. Moreover, the competitive growth of nucleated clusters on the functionalized graphene surface may tailor the size and structure of materials. It is suggested that, when graphene is hybridized with other materials, graphene can slow the recombination of photo- or electro-chemically generated electron–hole pairs, increasing charge transfer rate of electrons and surface-adsorbed amount of chemical molecules through π–π interactions [8], [9]. Thus the integration of nanosized anatase TiO2 with two-dimensional graphene nanosheets offers a great opportunity to design and synthesize TiO2-graphene hybrid materials with improved visible-light photocatalytic activity.
Although many efforts have been made to prepare TiO2 mesocrystals [10], [11], [12], [13], [14], [15], and to incorporate different TiO2 nanostructures [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37] and TiO2-based hybrids [38], [39], [40], [41] into graphene sheets, however, few works have been conducted on the facile one-pot synthesis of graphene-TiO2 composites where graphene sheets are decorated with shaped-controlled anatase TiO2 mesocrystals assembled from well-defined TiO2 nanocrystals. Herein we report on facile template-free solvothermal synthesis of graphene-anatase TiO2 mesocrystals that highly enhance the photocatalytic activity of TiO2 under visible-light irradiation. The graphene-anatase TiO2 composite materials where graphene nanosheets are decorated with anchored, well-dispersed spindle-like anatase TiO2 mesocrystals were prepared by mixing graphene oxide (GO) with acetic acid under ultrasonication, followed by the dropwise addition of tetrabutyl titanate (TBT) into the above suspension. The size and morphologies of as-prepared TiO2 were found to depend on the amount of graphene oxide employed in the process. The application of as-synthesized GR-TiO2 composites as the photocatalyst for the degradation of organic dye RhB under visible-light irradiation has also been investigated. As expected, GR-TiO2 composites show excellent photocatalytic activity in comparison to a reference TiO2 and commercially available P25.
Section snippets
Synthesis of samples
All reagents was purchased from Sinophram Chemical Reagent Co. Ltd. (Shanghai, China) and used as received without further purification. GO was synthesized from natural graphite by a modified Hummers’ method with additional KMnO4 [42].
In a typical synthesis, the as-synthesized graphene oxide (20 mg) was firstly dispersed in glacial acetic acid (70 mL) and sealed using parafilm, followed by ultrasonication with water bath for 3 h, then tetrabutyl titanate (TBT, 2 mL) was dropwise added into the
Characterization of GR-TiO2 mesocrystal composites
Fig. 1(a) displays a typical SEM image of the as-synthesized GR-TiO2 hybrid materials where a large amount of spindle-like TiO2 mesocrystals are covered by graphene sheets, SEM observations of selected uncovered TiO2 mesocrystals at a higher magnification shown in Fig. 1(b) reveal that each spindle-like TiO2 mesocrystal is actually self-assembled by secondary TiO2 nanocrystals. Fig. 1(c) and (d) display typical low-magnification TEM images of as-prepared GR-TiO2 composites, which indicate that
Conclusions
In summary, we have presented a facile solvothermal process for the synthesis of well-defined GR-TiO2 composites by using GO and tetrabutyl titanate as precursors in the presence of acetic acid. The resulting products exhibit a uniform distribution of nanoporous anatase TiO2 mesocrystals on the graphene sheets. Moreover, studies of their photocatalytic performance have indicated that GR-TiO2 composites exhibit much higher activities than pure TiO2 for the degradation of dye molecules and phenol
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (51102116), Natural Science Foundation of Jiangsu (BK2011480 and BK2011534), a Project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (10KJB430001) and Scientific Research Foundation for Advanced Talents, Jiangsu University (10JDG057).
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