Graphene-spindle shaped TiO₂ mesocrystal composites: Facile synthesis and enhanced visible light photocatalytic performance

Highlights

• Graphene-TiO₂ mesocrystal composites were fabricated via a facile approach.

• Graphene sheets were decorated with spindle-like TiO₂ mesocrystals.

• Graphene causes enhanced light absorbance and visible light photocatalytic activity.

• Oxygen-containing radicals are believed to responsible for its improved activity.

Abstract

Graphene (GR)-TiO₂ mesocrystal composites were prepared by a facile template-free process based on the combination of sol–gel and solvothermal methods, and were characterized using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (UV–vis DRS), nitrogen absorption and electron spin resonance (ESR). Visible light photocatalytic performance of GR-TiO₂ composites was evaluated for photocatalytic degradation of organic dye Rhodamine B. It was found that the amount of graphene oxide (GO) added obviously affects morphologies of TiO₂ mesocrystals and photocatalytic activities of as-prepared nanocomposites. Composites prepared in the presence of different amounts of GO all exhibit higher photocatalytic activity than pure TiO₂ mesocrystals and P25, the composite obtained by using 20 mg GO presents the most uniform TiO₂ mesocrystals in the composite and shows the highest photocatalytic efficiency. The mechanism for the generation of TiO₂ mesocrystals in the GR-TiO₂ composite is proposed and possible reasons for the enhancement in visible light photocatalytic efficiency are also discussed.

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.