Elsevier

Materials Letters

Volume 165, 15 February 2016, Pages 247-251
Materials Letters

Enhanced photocatalytic activity of nanohybrids TiO2/CNTs materials

https://doi.org/10.1016/j.matlet.2015.12.004Get rights and content

Highlights

  • The nanohybrids TiO2/CNTs materials were synthesized by hydrolysis method.

  • The TiO2 attached on CNTs were dependent on the surface oxidation of CNTs.

  • The percent of dyes decomposed highest at the sample with [TiO2]/[CNT]=5/1.

  • ΔEads are −0.898 eV suggest that the adsorption processes are spontaneous.

Abstract

The nanohybrids TiO2/CNTs materials were synthesized by hydrolysis method. The results show that, compared to pure TiO2 or CNTs nanoparticles, nanohybrids TiO2/CNTs materials exhibit higher catalytic activities in degradation of methylene blue (MB), or methylene orange (MO). The highest degradation percentages, observed in samples with [TiO2]/[CNTs] ratio of 5/1, are around 33% for MO and 38% for MB, respectively. This can be attributed to the significantly enhanced visible light absorption of nanohybrids TiO2/CNTs due to the attachment of the TiO2 nanoparticles on the sidewall of CNTs. The density functional-theory (DFT) calculations indicate that the stability of nanohybrids TiO2/CNTs is due to the favored bonding state between nanocrytal TiO2 and CNTs at the interface. Hopefully, this work can help promote better understanding of the role of semiconductor-CNTs interface in increasing the efficiency of using the solar energy, by enhancing the photocatalytic performance in the visible region.

Introduction

Development of novel hybrid nanostructures can significantly increase the complexity and expand the functionality of nanomaterials, benefiting nanotechnology-based electronics [1], medicine [2], catalysis [3], and sensors [4]. Among the photocatalytic active materials, TiO2 is the most widely used due to its high catalytic activity, chemical and biological stability and low cost [5]. However, the main drawback of TiO2 materials is the large band gap (3.0–3.2 eV) which limits its application to the UV region which constitutes only about 4% of the total solar spectrum. It is therefore important and a great challenge to develop new TiO2 photocatalytic systems that are both UV and visible light active.

The issue was further solved by doping effect which results in the sub-bandgap absorption in the visible region. The experimental works demonstrate that nonmetal or metal dopants extend photoresponse of TiO2 from the UV to the visible light region [6], [7], [8], [9]. Theoretical calculations also confirm that anion doping considerably affects the band gap of TiO2 [10]. However, the high concentration of dopants is difficult to obtain and the synthesized materials are usually unstable to photo-corrosion [11]. Furthermore, dopant-mediated electron hole pair recombination in the lattice can be a significant problem, restricting practical application. Recently, some studies reported that the photocatalytic activity of some semiconductors can be improved by making composites with CNTs [12], [13], [14]. In the nanohybrids TiO2/CNTs, a dense heterojunctions through Ti–O–CNTs structure is a reason enhanced photoactivity [15]. Not only do CNTs provide a large surface area supporting the catalysts, but they also stabilize the charge separation by trapping the electrons transferred from semiconductors and thus hinder electron–pair recombination. Therefore, TiO2/CNTs can have applications in optical or electronic technologies. [16], [17].

In this work, we report the efficiency of enhancing photocatalytic activities of nanohybrids TiO2/CNTs materials. The nanocrystal TiO2 was attached on the surface of CNTs via hydrolysis method. The bonding between TiO2 nanocrystal and CNTs enhances the visible light absorption ability and photocatalytic activity for the degradation of dyes (in comparison with pure TiO2 or CNTs). The experimental results were combined with DFT calculations of the electronic band structures and density of states (DOS) to understand the bonding states between TiO2 and CNTs, proving the stability of the TiO2/CNTs system.

Section snippets

Experiment

CNTs were surface-activated, using a mixture of concentrated H2SO4 and concentrated HNO3 (H2SO4/HNO3=1/3). After stirring vigorously for 12 h at 120 °C, the resulting solution was filtered and washed with deionized water, then dried at 80 °C in air for 24 h. C12H28O4Ti was then added dropwise to the prepared CNTs dispersion in an iced bath with various concentrations ([TiO2]/[CNT]=3/1, 5/1, 10/1, or 20/1). The prepared particles were dried at 80 °C for 24 h and heat treated at 400 °C for 2 h. To

Results and discussions

Fig. 1(a) shows the XRD patterns of the nanohybrids TiO2/CNTs materials. The peaks in 2θ of XRD patterns were indexed as anatase TiO2 phase and hexagonal graphite structure, similar to pristine CNTs. The peak at 26.0° of CNTs is absent in the samples with TiO2/CNTs ratios of 20/1, 10/1, and 5/1. However, the broadening and asymmetry of the 25.5° peak of TiO2 in the samples with the ratios of 3/1 and 1/1 clearly show the effect of CNTs on XRD spectrum of TiO2. This indicate that our samples

Conclusion

The highly adsorption of TiO2 along CNTs was obtained from CNTs of which surface is acid modified. The nanohybrids TiO2/CNTs exhibits the ability to absorb longer wavelength lights, the absorption spectrum even covers the whole range of visible region, due to the bonding between nanocrystal TiO2 and CNTs at interfaces. The photocatalytic activities in nanohybrids TiO2/CNTs were found to be higher than those of pure CNTs or neat anatase nanocrystal TiO2. The highest rates of decomposition of MB

Acknowledgments

The research is financed by Nafosted No.103.02.2014.21

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