Nano titanium dioxide coated on multi-wall carbon nanotubes for photocatalytic application

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0047 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 176-181 This paper is available online at NANO TITANIUM DIOXIDE COATED ONMULTI-WALL CARBON NANOTUBES FOR PHOTOCATALYTIC APPLICATION Nguyen Cao Khang Center for Nano Science and Technology, Hanoi National University of Education Abstract. Titanium dioxide (TiO2) coated on multi-walled carbon nanotubes (CNTs) was synthesized with CNTs and TiO2 using the grinded method. The XRS spectra include that of the obtained single-phase anatase sample. The UV-Vis diffuse reflectance spectra show that the composite material can absorb at a higher wavelength and the absorption covers the whole visible region. The application of the catalysts to photocatalytic degradation of methylene blue (MB) was tested under visible light irradiation. From the photocatalytic result, we noted that the TiO2/CNTs composite catalysts exhibit higher MB degradation activity than neat TiO2. Keywords: TiO2, CNTs, composite, photocatalytic. 1. Introduction TiO2 has been considered to be a promising material for use in dynamic random access memories, dye-sensitized solar cells, photocatalysts for environmental remediation and water splitting, coating materials to obtain superhydrophilic surfaces, and optical devices [1–3]. However, because of the wide band gap of titanium dioxide, its practical application is limited due to low photon utilization efficiency and the spectrum need of an ultraviolet excitation source which accounts for only a small fraction of solar light. Therefore, it is important and challenging to develop a new TiO2 photocatalytic system with enhanced activities under both UV and visible light irradiation compared to that of TiO2, improving the utilization efficiency of solar energy [4-6]. Recently, the authors noted that the photocatalytic activity of some semiconductors can be improved by making a composite with CNTs. It has been reported that the CNTs not only provide a large surface area for catalyst support, but also stabilize the charge separation by trapping electrons transferred from the semiconductor, thereby hindering charge recombination [7-9]. Their outstanding charge transfer abilities can favor the excited electron in the conduction band of the nanocrystal semiconductor to migrate into the CNTs, thereby decreasing recombination of the electron-hole pairs [10] and increasing photocatalytic activity under visible light. Received September 18, 2016. Accepted October 14, 2016. Contact Nguyen Cao Khang, e-mail address: khangdhsp@gmail.com 176 Nano titanium dioxide coated on multi-wall carbon nanotubes for photocatalytic application In this study, we report on the synthesis of a TiO2/CNTs composite and the components of its photocatalytic effect under visible light using a simple grinded method. The photocatalytic activities of samples were assessed using photodegradation of MB. We believe that knowledge of electron transference between CNTs and TiO2 is important for an understanding of why the visible light photocatalytic activity of TiO2/CNTs composite is higher than that of neat TiO2. 2. Content 2.1. Experiment The synthesis of TiO2 nanocrystals is accomplished with a drop wise addition of Ti[OCH(CH3)2]4 dissolved in isopropyl alcohol to doubly distilled water. By adjusting the pH of the solution, TiO2 nanocrystals can be synthesized. The white TiO2 precursor was then mixed with CNTs in ratios of 2/1, 4/1 and 6/1 using the grinded method. The mixture was ground for 6 h in an agate mortar and dried for 4 h in vacuum at 100 ◦C. The photocatalytic activity of the prepared samples was evaluated by measuring the decomposition of MB under visible-light irradiation. The light source used was a 150 W high-pressure Xenon lamp with a cut-off filter of 400 nm. For a typical photodecomposition experiment, 25 mg photocatalyst was mixed with a 50 ml MB solution. Before turning on the light, the suspension containing the MB and photocatalyst was magnetically stirred in the dark with continuous stirring until there was no change in the absorbance of the solution, this done to make sure that physical adsorption does not play a role in reducing the MB concentration. The structures of the TiO2 samples were determined using an X-ray diffractometer D5005 (Siemen) with CuKα radiation. Their morphology and sizes were investigated using a Field Emission Scanning Electron Microscope (FE-SEM). Optical absorption spectra were measured using a V-670 spectrophotometer. 2.2. Results and discussion An FE-SEM image of composite in Figure 1 shows that TiO2 nanoparticles about 8 nm in size are attached to the sidewall of CNTs. Also observed were clusters of TiO2 particles. This conductive network of CNTs facilitates electron transfer between the adsorbed MB molecules and the catalyst substrate. This would be beneficial for photocatalytic reactions because photocatalytic reactions are carried out on the surface of the TiO2/MWCNTs composites catalysts and the CNTs network. Therefore, the TiO2/CNTs composites should show excellent photocatalytic activity. In order to examine the crystallization behaviors of anatase, various samples were prepared using the sol-gel and hydrolytic synthesis methods. Figure 2 shows the XRD spectra of the prepared nanoparticles. The peaks of 2θ at 25.5 ◦C, 37.8 ◦C, 48.1 ◦C, 54.0 ◦C and 62.8 ◦C can be perfectly attributed to the crystal planes of the , , , and of the anatase TiO2. The width of the peak broadens indicating that the particle size had decreased. The average crystalline size of TiO2 powder was estimated from the full width at half maximum (FWHM) of the (101) XRD peak using Scherrer’s equation: d = kλ/βcosθ, where d is the average crystalline size, k is the constant, λ is the X-ray wavelength, β is the full width at half maximum 177 Nguyen Cao Khang of the diffraction line, and θ is the angle of diffraction. The result shows that the size of the TiO2 particles is about 7.6 nm. Figure 1. SEM image of the sample TiO2/CNTs=2/1 Figure 2. XRD patterns of TiO2/CNTs samples Figure 3. UV–Vis spectra of TiO2/CNTs composite To investigate the optical properties of the TiO2/CNTs composite materials, the UV-Vis spectra were recorded and shown in Figure 3. An enhancement of visible absorption of TiO2 on the CNTs is evident. The composite material can absorb from 430 nm to 800 nm and the absorption covers the whole visible range. Photocatalytic efficiency was evaluated from the intensity peak at 665 nm in the absorption spectra of the MB solution. The percent of degradation of the MB solution was calculated using the following equation 178 Nano titanium dioxide coated on multi-wall carbon nanotubes for photocatalytic application D = A−Ao Ao .100% where D is the percent of degradation, and Ao and A are the maximum absorbance at 665 nm in the absorption spectra of the initial and constant MB solution, respectively. The absorbance spectral changes of the MB solution in the presence of TiO2, and the TiO2/CNTs composites are shown in Figure 4. From Figure 4, it can be seen that a photocatalytic process of MB with fast degradation efficiency was observed with the TiO2/CNTs composite (listed in Table 1). The percentage of MB removed by the TiO2/CNTS samples is 4 to 5 times more than that of pure TiO2. The best photocatalytic MB degradation result is observed for the sample TiO2/CNTs = 2/1, which destroys up to 37% of MB molecules within 4 h of irradiation. Compared with the previous research, the TiO2/CNTs samples in this study were high performance for decomposition of MB [11, 12]. It is thought that the decrease of MB concentration in the aqueous solution can occur due to physical phenomena such as adsorption by CNTs and photocatalytic decomposition by TiO2, and that in this case it was mainly due to photocatalytic decomposition. This indicates that the decrease of MB can be concluded due to the combined effects of photocatalytic decomposition by TiO2 and assistance from CNT network. The increase in photocatalytic performance is also due to a large surface area of the nanohybrids TiO2/CNTs. Moreover, the CNTs have a high adsorption capacity for MB molecules, so the concentration of MB in the vicinity of CNTs is higher than those in other places in the reaction system. This is increases the possibility of contact between the TiO2 with MB. Figure 4. Absorbance spectral changes of MB solution in the presence of TiO2, and TiO2/CNTs composite 179 Nguyen Cao Khang Table 1. Percentage of MB decomposed after 4 hours Only TiO2 TiO2/CNTs=2/1 TiO2/CNTs=4/1 TiO2/CNTs=6/1 7% 37% 33% 32% Figure 5. The proposed mechanisms for CNT-mediated enhancement of photocatalysis As a plausible mechanism, it is quite reasonable to describe the combination effect with CNT acting as electron sensitizer and donator in the composite photocatalysts. The schematic diagram for the mechanism was shown in Figure 5. CNT may accept the photo-induced electron (e−) by light irradiation. It is thought that the electrons in CNT transfer into the conduction band in the TiO2 particles. The photogenerated electron is injected into the conduction band of the TiO2, enabling the formation of superoxide radicals by adsorbed molecular oxygen. Once this occurs, the positively charged nanotubes remove an electron from the valence band of the TiO2 leaving a hole. The now positively charged TiO2 can then react with adsorbed water to form hydroxyl radicals. Simultaneously, a positive charged hole (h+) might be formed with electron transfer from the valence bond in TiO2 to CNT. The positive charged hole (h+) may react with the OH− derived from H2O, which can trigger the formation of hydroxyl radicals (HO·). These radical groups are responsible for the decomposition of the organic compounds. 3. Conclusion TiO2/CNTs composite photocatalysts containing CNTs were prepared using a grinded method. The composite material can absorb at higher wavelength and absorption occurs in the whole visible range. The photocatalytic degradation of MB was observed over TiO2/CNTs composite catalysts, which exhibit 5 times higher photocatalytic activity in comparison with neat TiO2. 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