Enhanced photocatalytic hydrogen production ...

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Enhanced photocatalytic hydrogen production activity of noble metal free MWCNT-TiO2 nanocomposites N. Ramesh Reddy a, M. Mamatha Kumari a,*, K.K. Cheralathan b, M.V. Shankar a a

Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa 516003, Andhra Pradesh, India b Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India

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abstract

Article history:

Nanocomposite photocatalysts, MWCNT-TiO2 were prepared by hydrothermal method.

Received 5 June 2017

The photocatalysts were characterized by X-ray diffraction, Transmission electron mi-

Received in revised form

croscopy (TEM), Raman spectroscopy, UV-Visible diffuse reflectance spectroscopy and

18 December 2017

photoluminescence (PL) spectroscopy to understand the crystal structure, morphology, and

Accepted 1 January 2018

optical properties. The catalyst synthesis parameters such as calcination temperature and

Available online 1 February 2018

loading of MWCNTs were optimized for better hydrogen (H2) production in 5 vol% glycerol aqueous solution under UV-visible light irradiation. Among the prepared nanocomposites,

Keywords:

0.1 wt% CNT loaded TiO2 calcined at 450 C for 2 h showed the highest H2 production rate of

MWCNT

8.8 mmol g1 h1. This higher H2 production rate obtained can be ascribed to effective

Titanium dioxide

utilization of the photo generated electrons and holes for redox reactions.

H2 production

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Nanocomposite

Introduction In 21st century, eco-friendly, clean energy sources are necessary to meet the global energy demand with reduced fossil fuel dependence and coupled environmental damage. In this connection, generating H2 through photocatalysis process using environmentally benign renewable energy sources is the best choice. Hydrogen is the best alternative fuel over the other fossil fuels as it is clean and eco-friendly as combustion of it produces only water. Titanium dioxide (TiO2) is one of the potential strategic semiconductor for the photocatalytic water splitting due to its

high chemical stability, favourable energy band structure, ready availability, inexpensive and non-toxic nature [1]. However, TiO2 suffers from some serious disadvantages such as (i) ability to absorb only UV light covering very small ( MT0.5 > MT-1 > MT-0.05 > TiO2. The highest H2 production rate (8.8 mmol h1 g1) is observed on MT-0.1 nanocomposite. The rate is nearly nine fold higher than that of the pure TiO2. Ahmmad et al. evaluated H2 production rate of MWNT-TiO2 under UV light and found it to be 1.3 mmol h1 g1 [42]. In the present work, the observed hydrogen production rate of MT0.1 is 8.8 mmol h1 g1, which is nearly six fold higher than that reported by Ahmmad et al. One of the reasons for this enhanced catalytic activity observed in MT-0.1 could be due to effective formation of TieOeC bond at the interface of TiO2 and MWCNTs which would favour the effective charge transfer between TiO2 and MWCNTs. This charge transfer from TiO2 to MWCNTs will allow the flow of electrons in a particular direction, limiting electronehole recombination as proved by us previously [55,56]. The enhanced H2 production observed in MT-0.1 may also be due to defects in TiO2 that enable better charge carrier transfer between surface interfaces [57]. At lower MWCNTs loading, i.e. in MT-0.05, the H2 production rate is closer to TiO2. In MT-0.05, TiO2 may not be dispersed well on the MWCNTs due to smaller amount of MWCNTs present leading to loss of contact between TiO2 and MWCNTs. This loss of contact will reduce the charge transfer and decrease the chance for prevention of electronehole recombination. The decrease in H2 production rate in MT-0.5 and MT-1 above the optimal loading (0.1 wt%, MT-0.1) of


Fig. 7 e H2 production performances of nanocomposites with various MWCNTs loadings.

MWCNTs is mainly ascribed to increase in opaqueness of the suspension, which promotes light scattering effects. Photocatalytic stability of MT-0.1 was tested for 4 h in 5 cycles (5 days) and the results are depicted in Fig. 8. These results reveal that almost identical amount of H2 was generated in all the five cycles. At the reported photocatalytic experimental conditions, oxidation of glycerol will lead to formation of reaction intermediates. Adsorption of these reaction intermediates onto the exterior of photocatalysts could affect the light penetration onto the surface of the photocatalyst and it may also lead to poor surface interaction of glycerol with TiO2 and hence a slight drop in the total amount of H2 generated was observed after 3rd cycle.

Mechanism of photocatalytic activity The mechanism proposed for the photocatalytic activity of MWCNTs/TiO2 nanocomposite under UV-Visible light irradiation is based on electron transfer (Fig. 9) between TiO2 and MWCNTs which is supported by the presented experimental data. Irradiation of UV-Visible light on the photocatalyst will lead to generation of electron-hole pairs in the conduction and

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Fig. 9 e Possible photocatalysis mechanism of MWCNTTiO2 composite.

valance bands of TiO2. The conduction band electrons of TiO2 can be transferred to MWCNTs via the interface between them leaving holes in valance band available for oxidation of glycerol or water. MWCNTs can be viewed only as co-catalyst and not as a small band gap semiconductor [5,31] since only UVvisible light is used for irradiation. In general, CNTs play a significant role in photocatalysis by acting either as a cocatalyst or as a sensitizer or both. Ahmmed et al. suggested that in SWCNT/TiO2 nanocomposite, SWCNT acts as cocatalyst which hinders the recombination of charge carriers and favours efficient electron conductance onto the surface active sites for enhanced H2 production [58]. Changwei et al. reported that in DWCNT-TiO2 composite, DWCNTs work as a sensitizer for visible light absorption as well as good electron conductor [19]. However, in the present nanocomposite system, we believe that MWCNTs acts as electron sinks and allow only uni-directional flow of electrons, thus limiting the electron-hole recombination (Fig. 9).

Conclusion In conclusion, this work resulted in synthesis of MWCNTs/ TiO2 nanocomposite by hydrothermal method for enhanced H2 production from aqueous solution of glycerol. By loading an optimum amount of MWCNTs on TiO2 (0.1 wt%), the photocatalytic H2 production rate could be increased from 1.3 mmol h1 g1 to 8.8 mmol h1 g1. The TiO2 nanoparticles and rods are found to be located on the surface of MWCNTs and the surface interaction between TiO2 and MWCNTs leads to uni-directional electron transfer between them. In this way, carbon nanotubes act as co-catalysts facilitating effective utilization of electron-hole pairs for redox reactions.

Acknowledgements

Fig. 8 e Photocatalytic stability of MT-0.1 catalyst.

Financial support from University Grants Commission (UGC), New Delhi, India (F. No. 43-405/2014), is gratefully acknowledged. We are also grateful to the help rendered by M. Madhuri in revising the manuscript.

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Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.01.011.

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