A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation
Introduction
Hydrogen has been considered as one of the most ideal fuel for the next generation due to its high energy conversion efficiency and zero-carbon emission. Since the discovery of hydrogen production through the photoelectrochemical (PEC) splitting of water on TiO2 electrodes [1], the semiconductor based photocatalytic water splitting has been considered a promising way to generate hydrogen. Several metal-oxide-based semiconductor photocatalyst such as Cu2O [2], WO3 [3], ZnO [4], TiO2 [5], g-C3N4 [6], α-Fe2O3 [7] have subsequently been studied for developing photoelectrochemical cell devices. TiO2 has been considered as the most promising material for photocatalytic water splitting application due to its availability, low cost, nontoxicity and high photochemical stability [8], [9]. However, a wide band gap (∼3.2 eV), high recombination rate of photogenerated charge carriers and rapid backward reaction between O2 and H2 limits its practical applications [10]. TiO2 is active only under UV light radiation (which is 4% of the solar spectrum), whereas the visible light in the solar spectrum is 46%. Therefore, extending the photoresponse of TiO2 to the visible light range and reducing the recombination of photoexcited electron–hole pairs are the two big challenges for efficient hydrogen production using solar energy from water. Various strategies such as, doping TiO2 with non metallic elements [11], co-catalyst doping [12] and coupling TiO2 with other semiconductor [13], has been explored to extend its visible light photoactivity. However, efficiency and stability of photocatalyst are still compromised with these strategies.
In recent years, the plasmonic photoactive system by the combination of noble metal nanoparticles like Au and Ag with semiconductor has proved to be an effective way to improve the visible light photoactivity of wide band gap semiconductors, due to their surface plasmon resonance (SPR) effect [14], [15]. Plasmonic metal nanoparticles have been shown to act as photosensitizers, which absorb the visible light irradiation due to their SPR effect and inject charge carriers directly from excited metal nanoparticles into the adjacent semiconductor [16], [17]. The coupling of TiO2 with Ag nanoparticles is a promising method for water splitting under visible light irradiation.
In addition, multi walled carbon nanotubes (MWCNT) with high electrical and thermal conductivity and large surface area is a promising material in the field of photocatalytic water splitting [18], [19]. Due to its excellent electrical conductivity, MWCNT has been used as a promoter for charge carrier transfer in various photocatalysts [20]. Therefore, the incorporation of MWCNTs can enhance the PEC performance of Ag/TiO2 binary nanocomposite by facilitating the separation of electron-hole pairs. It has been demonstrated that photoactivity of ternary hybrid nanocomposites such as CdS-GR-TiO2 [21], P25/GO/Pt [22], g-C3N4/Pt/ZnO [23] and TiO2−CMK-3/Ag [24] is superior to the binary one. Therefore, efforts in the direction of preparing carbon nanotubes based ternary nanocomposites can lead to better PEC water splitting performance. There are few published reports on the formation of nanocomposite utilizing all the three components Ag, TiO2 and CNT for the antibacterial activity, photocatalytic degradation of thiophene and methylene blue (MB) [25], [26] and for lithium-ion-batteries applications [27]. However, no study has been reported for exploring Ag/TiO2/CNT ternary nanocomposite for PEC water splitting. For scaling up the synthesis, it is always desirable to synthesize the materials at lower temperature and using soft chemical route. Azam et al. [25] synthesized the Ag/TiO2/CNT ternary by sol-gel approach in which 400 °C annealing temperature was used. Koo et al. [28] synthesized the ternary composite by physical mixing of P25 and CNT. Zhang et al. [29] synthesized the ternary composite by heat treatment of gel at 700 °C.
In this paper, we report the synthesis of ternary Ag/TiO2/CNT nanocomposite photocatalyst in two step processes: synthesis of TiO2/CNT nanocomposite by hydrothermal method followed by the decoration of Ag nanoparticles over TiO2/CNT using soft chemical route in which as-synthesized TiO2/CNT nanocomposite was mixed to AgNO3 aqueous solution and stirred at room temperature. The PEC water splitting performance of Ag/TiO2/CNT ternary nanocomposite was systematically investigated. The photoactivity of ternary nanocomposite was found superior to binary nanocomposite. The roles of the MWCNT and Ag nanoparticles in the PEC water splitting process have been discussed in detail.
Section snippets
Synthesis of multi-walled carbon nanotubes
MWCNTs were synthesized using the thermal chemical vapor deposition method. Details of the synthesis of MWCNT have been presented elsewhere [30]. Briefly, ferrocene/xylene of concentration 30 mg/ml was used as precursor solution and a double zone furnace heated to 850 °C was used to get the MWCNT deposition on the wall of quartz tube. The black powder deposited on the walls of the quartz tube was collected and purified to remove any metal catalyst and amorphous carbon. Functionalization of
Characterization of photocatalyst
Fig. 1 shows the X-ray diffraction (XRD) patterns of TiO2, TiO2/CNT and Ag/TiO2/CNT samples. The XRD peaks in the diffraction patterns of TiO2, TiO2/CNT and Ag/TiO2/CNT nanostructures observed at 2θ = 25.3°, 37.7°, 47.9°, 54.0°, 54.9° correspond to (101), (004), (200), (105) and (211) crystal planes of anatase TiO2 (JCPDS No. 21-1272). However, no diffraction peak was observed corresponding to MWCNT in the XRD pattern of TiO2/CNT and Ag/TiO2/CNT samples, which might be due to the shielding of
Conclusions
Ag/TiO2/CNT ternary nanocomposite was prepared by the chemically decoration of Ag nanoparticles over hydrothermally grown TiO2/CNT nanocomposite. The as prepared ternary Ag/TiO2/CNT nanocomposite photoanode showed improved visible light photoresponse and PEC performance. Ag/TiO2/CNT ternary nanocomposite photoanode exhibits a four times higher photocurrent density (0.91 mA/cm2) as compared to the binary TiO2/CNT photoanode (0.23 mA/cm2). With the synergistic effect of Ag nanoparticles and CNT
Acknowledgements
The financial support from DeitY (Government of India) (RP02395) is gratefully acknowledged. One of the authors (DC) is thankful to Indian Institute of Technology Delhi (IIT Delhi) for senior research fellowship.
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