Elsevier

Carbon

Volume 50, Issue 7, June 2012, Pages 2472-2481
Carbon

TiO2 nanoparticles loaded on graphene/carbon composite nanofibers by electrospinning for increased photocatalysis

https://doi.org/10.1016/j.carbon.2012.01.069Get rights and content

Abstract

Graphene/carbon composite nanofibers (CCNFs) with attached TiO2 nanoparticles (TiO2–CCNF) were prepared, and their photocatalytic degradation ability under visible light irradiation was assessed. They were characterized using scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible diffuse spectroscopy. The results suggest that the presence of graphene embedded in the composite fibers prevents TiO2 particle agglomeration and aids the uniform dispersion of TiO2 on the fibers. In the photodegradation of methylene blue, a significant increase in the reaction rate was observed with TiO2–CCNF materials under visible light. This increase is due to the high migration efficiency of photoinduced electrons and the inhibition of charge–carrier recombination due to the electronic interaction between TiO2 and graphene. The TiO2–CCNF materials could be used for multiple degradation cycles without a decrease in photocatalytic activity.

Introduction

TiO2 is one of the most promising catalysts because of its superior photocatalytic performance, easy availability, long-term stability, and nontoxicity [1], [2]. Typically, photoexcited electron–hole pairs can be generated by irradiation with light with an energy greater than the band gap energy of TiO2 (Ebg = 3.2 eV for anatase). However, several problems can arise when TiO2 is applied as a photocatalyst: (1) photogenerated electron–hole pairs can recombine quickly, which affects the photocatalytic efficiency [3], [4], [5], [6], [7], and (2) TiO2 can only be excited with ultraviolet (UV) light, which is less than 5% of solar light, because of its wide band gap [7], [8], [9]. These disadvantages of TiO2 result in a low photocatalytic activity in practical application. Therefore, a number of recent studies have focused on the preparation and modification of TiO2 to narrow its band gap and enhance the photocatalytic activity under visible light radiation [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

Among the carbon nanostructures (e.g., C60, carbon nanotubes, and graphenes), graphenes offer new opportunities in photovoltaic conversion and photocatalysis by the hybrid structures with a variety of nanomaterials, due to their excellent charge carrier mobility, a large specific surface area, and good electrical conductivity [31], [32], [33], [34], [35], [36]. For example, TiO2 combined with graphene acts as an electron trap, which promotes electron–hole separation and facilitates interfacial electron transfer [14], [24], [25], [26], [30]. Furthermore, graphene can help control the morphology of TiO2 nanoparticles because it controls nucleation and growth of TiO2 nanoparticles and allows for optimal chemical interactions and bonding between nanoparticles and graphene.

In this work, carbon nanofibers (CNFs) containing micropores were used to support TiO2 because CNFs exhibit a high adsorption capability and adsorption rate due to the shallow and uniform pore structure. Shallow micropores open directly to the surface, which results in a large adsorption capacity and fast adsorption/desorption [37], [38]. In addition, electrospinning has been widely used as a versatile technique to fabricate various hybrid nanofibers [39], [40]. However, reports have seldom been published on the fabrication and evaluation of TiO2 nanoparticles loaded on graphene/carbon composite nanofibers (TiO2–CCNF) produced by electrospinning techniques.

In the present work, TiO2–CCNF catalysts were prepared by electrospinning to determine if they exhibit improved photocatalytic efficiency in the visible spectrum, as illustrated in Fig. 1. By increasing the photocatalytic activity of the TiO2–CCNF materials, the current study aims to (i) extend the light absorption spectrum into the visible region, (ii) reduce electron/hole pair recombination, and (iii) enhance the adsorption capability and high adsorption/desorption rate of organic pollutants, which increases the reaction efficiency because of the high specific surface area of the CNFs.

Section snippets

Materials and methods

The graphenes used in this study were xGNP-C750-grade materials produced by XG Science, USA. The elementary analysis of graphene was characterized as 88.68% carbon, 0.79% hydrogen, 1.11% nitrogen, and 7.65% oxygen using the Mettler method (Metler-Toledo AG, Switzerland). The graphene with functional group can be dispersed in organic solvent. The 3 wt.% grpahene (3 wt.% relative to PAN) sample was immersed in dimethylformamide (DMF) and sonicated in a bath-type sonicator. The PAN (Mw = 150,000,

Results and discussion

Fig. 2 presents SEM images of TiO2–CCNF and TiO2–CNF hybrids prepared with and without graphene. The electrospun PAN based- and PAN/graphene based nanofibers showed a smooth surface with an average diameter of 180 and 270 nm, respectively, before coating of TiO2. It was observed that the nanoparticles were uniformly distributed across the surface of the TiO2–CCNF materials (Fig. 2a) without aggregation, whereas the TiO2 on pure PAN/based fibers (TiO2–CNF) consisted of large, aggregated

Conclusion

Small TiO2 nanoparticles were successfully deposited on electrospun CCNFs by the sol–gel method, and these composites were very active photocatalysts in the photodegradation of MB under visible light irradiation. Furthermore, TiO2–CCNF materials could be recycled without a decrease in the photocatalytic activity. The results suggest that graphene acts as an electron acceptor and a photosensitizer, which causes an increase in the photodegradation rate and reduces electron–hole pair

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0005890) and the Ministry of Education, Science and Technology (MEST) (K20903002024-11E0100-04010).

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