Photodegradation of methyl orange by photocatalyst of CNTs/P-TiO2 under UV and visible-light irradiation

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Abstract

A novel nanoscale photocatalyst CNTs/P-TiO2 was successfully prepared by hydrothermal method. The morphology and the physicochemical properties of the prepared samples were investigated using TEM, XPS, XRD, BET, FTIR, TG-DSC and UV–vis DRS spectroscopy. The photocatalytic activity was evaluated by degradation of methyl orange (MO) dye. The results demonstrated that CNTs/P-TiO2 nanoparticles could effectively photodegrade MO not only under UV irradiation but also under visible-light (VL) irradiation. The MO degradation performance on CNTs/P-TiO2 was superior to that of the commercial P25. The optimal mass ratio of CNTs to P-TiO2 in the nanocomposite catalyst was 5:100. The synergetic effect was discussed in terms of different roles played by phosphorus doping and introducing CNTs into the composite catalysts.

Introduction

Since the discovery of photocatalytic splitting of water on TiO2 electrodes by Fujishima and Honda in 1972 [1], heterogeneous photocatalysis by semiconductors has attracted much interest due to its applications in environmental purification and solar energy conversion [2]. It especially provides an economical and ecological method for the remediation of contaminated water and air [3], [4]. Among various semiconductor materials widely used in photocatalysis, TiO2 has proved to be the most suitable one because of its many desirable properties such as high activity, chemical stability, robustness against photocorrosion, low toxicity, no secondary pollution, low cost and water insolubility under most conditions [5].

However, many problems remain unresolved in TiO2 photocatalytic system for practical applications, such as low photon utilization efficiency and narrow spectrum responsive range (λ < 388 nm). Several methods have been applied to improve the photocatalytic efficiency of TiO2. Doping with non-metals, such as N, P, B, C, S, F, Cl and Br, has been widely used for the modification of TiO2 to improve its photocatalytic activity or to extend its light absorption into the visible region [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Among them, the phosphorous-doped TiO2 has recently attracted increasing interest due to its enhanced photocatalytic efficiency [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Compared with pure TiO2, the as-prepared P-doped TiO2 shows a narrower band gap. In addition, it has an absorption tail in the visible-light region. Consequently, it is more effective in the photocatalytic degradation of organic contaminants under visible-light irradiation. Shi et al. [19] found that P-doped TiO2 prepared by a sol–gel method with NaH2PO4 as precursor demonstrated a higher photocatalytic activity under visible-light irradiation than pure TiO2. Ozaki et al. [22] prepared TiO2 modified with various elements and investigated the physical and photocatalytic properties of the samples. They found that phosphorus was the most effective dopant for photocatalytic decomposition of acetaldehyde under visible-light irradiation. Zhu and co-workers [25] synthesized the P-doped TiO2 with high crystallinity and large surface area by hydrothermal method. The methylene blue degradation performance on the P-doped TiO2 was significantly enhanced and superior to that of the commercial P25.

Another approach for enhancing the photocatalytic efficiency of TiO2 involves adding a co-adsorbent such as activated carbon (AC) [26], [27], [28], graphite [29], [30], and carbon nanotubes (CNTs) [31], [32]. It has been reported that the presence of carbon materials in TiO2 photocatalysts can also induce some beneficial effects on their photocatalytic activities [11], [33]. Faria and Wang [34] reviewed the application of various carbon materials in photocatalysis during the last decade. Recently, CNTs have been regarded as more attractive catalyst supports than activated carbons because of their combination of electronic, adsorption, mechanical and thermal properties [35]. CNTs can conduct electrons, and have strong adsorption and specific semiconducting characteristics. CNTs/TiO2 composite catalysts have been successfully prepared by various processes [36], [37], [38], [39], [40]. The studies on CNTs/TiO2 reveal a considerable synergy effect with metal oxides and carbon phases. Furthermore, researchers have shown that CNTs can increase the adsorption and photocatalytic activity of TiO2 in the presence of UV and VL irradiation. Therefore, CNTs can be used as a promising material for environmental cleaning, and can be adopted to improve the photocatalytic efficiency of TiO2.

Our work aims to synthesize a composite photocatalyst with higher photocatalytic activity and wider spectrum responsive range than pure TiO2. In this study, the nanocatalysts of CNTs/P-TiO2 (labeled as CPT) with varying mass ratios of CNTs to P-TiO2 were synthesized by hydrothermal method. The influence of phosphorus and CNTs on the structural behavior of the TiO2 samples was studied by XPS, XRD, BET, TEM, UV–vis DRS. The photocatalytic degradation of methyl orange (MO) dye under UV and VL irradiation was investigated over nanosized CNTs/P-TiO2 photocatalysts.

Section snippets

Materials

Multi-walled carbon nanotubes (MWCNTs), 10–20 nm in diameter and 5–15 μm in length, were purchased from Shenzhen Nanotech Port Co., Ltd., China. Titanium (IV) n-butoxide (Ti(OBu)4, 98%) was chosen as a Ti precursor, which was chemical pure grade. P25 TiO2 (ca. 80% anatase, 20% rutile) was obtained from the Degussa AG Company in Germany. MO was analytical grade reagent from Tianjin Tianxin Fine Chemical Development Center in China and used without further purification. Distilled water was

XPS analysis

The chemical forms of surface elements in the CNTs/P-TiO2 sample were investigated by XPS analysis. The XPS survey spectrum of the CPT005 sample is shown in Fig. 1(a). The characteristic energy spectrum of four kinds of atoms, titanium, oxygen, carbon and phosphorus can be observed. The photoelectron peak for Ti 2p appeares clearly at a binding energy, Eb, of 459 eV, with O 1s at Eb = 531 eV, C 1s at Eb = 285 eV and P 2p at Eb = 134 eV. The C 1s peak corresponds to the CNTs, while the Ti 2p and O 1s

Conclusions

A new kind of composite photocatalysts by coupling CNTs with phosphorus-doped TiO2 was successfully prepared by hydrothermal method. It is found that the novel CNTs/P-TiO2 photocatalyst has smaller crystalline size, larger surface area and stronger absorption in the visible range than pure TiO2. A synergetic effect on the photocatalytic degradation of MO is observed for the CNTs/P-TiO2 composite catalyst, which exhibits higher photocatalytic activity than P25 and pure TiO2 under both UV and VL

Acknowledgements

We are greatly indebted to the State Ministry of Science and Technology (2008BAE64B05, 2006BAJ04A12-4), the State Key Laboratory of Subtropical Building Science (2008ZA09, 2009ZB05) of China, and the Department of Science and Technology of Guangdong Province (2007A032500005) for their financial support. We also thank Professor Elwood Powell for linguistic correction.

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