Modified Sol-Gel Synthesis of Carbon Nanotubes Supported Titania Composites with Enhanced Visible Light Induced Photocatalytic Activity

Abstract

Multiwalled carbon nanotube (MWCNT) enhanced MWCNT/TiO2 nanocomposites were synthesized by surface coating of carbon nanotube with mixed phase of anatase and rutile TiO2 through a modified sol-gel approach using tetrabutyl titanate as raw material. The morphological structures and physicochemical properties of the nanocomposites were characterized by FT-IR, XRD, DTA-TG, TEM, and UV-Vis spectra. The results show that TiO2 nanoparticles with size of around 15 nm are closely attached on the sidewall of MWCNT. The nanocomposites possess good absorption properties not only in the ultraviolet but also in the visible light region. Under irradiation of ultraviolet lamp, the prepared composites have the highest photodegradation efficiency of 83% within 4 hours towards the degradation of Methyl Orange (MO) aqueous solution. The results indicate that the carbon nanotubes supported TiO2 nanocomposites exhibit high photocatalytic activity and stability, showing great potentials in the treatment of wastewater.

Quanjie Wang 1 and Yanqing Wang 1,2 and Baorong Duan 1 and Mengmeng Zhang 1

Academic Editor:Xujie Lü

1, Department of Light Industry of Technology and Engineering, College of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China
2, Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agro-Ecology, Chinese Academy of Sciences, Changchun 130102, China

Received 28 July 2015; Revised 8 November 2015; Accepted 12 November 2015

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Treatment of the industrial wastewater, especially the organic pollutants which are difficult to be biochemically degraded, is currently viewed as an active research area [1, 2]. Titanium dioxide (TiO₂ ) is an environmental-friendly photocatalyst material due to its high catalytic activity, thermal stability, strong oxidizing power, and nontoxic, low cost, and other unique advantages [3-5]. However, its photoefficiency is not high enough and the speed of ultraviolet photoresponse is not satisfactory [6, 7]. On the other hand, there is a measurable reduction in the photocatalytic activity for the recycled use of the photocatalysts. The disadvantage of TiO₂ semiconductor is that it only absorbs a small portion of solar spectrum in the ultraviolet region, which limits its applications. Therefore, the development of modified TiO₂ with enhanced visible light induced properties is needed to increase the photocatalytic activity for the organic pollutants [8-10].

Carbon nanotube (CNT) is a new category of carbon structure, which was founded in 1991 by Iijima [11]. The ideal carbon nanotubes own seamless, hollow tube structure rolled by graphite surfaces slice layer composed of hexagon carbon atom. According to the number of graphite surface layers, they can be divided into single walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT). CNTs are considered to be ideal catalyst carriers due to their huge specific surface area, remarkable chemical stability, unique electronic structure, nanoscale hollow tube property, and good absorbability [12-15].

It was demonstrated in the paper that CNTs are regarded as the carrier of TiO₂ nanoparticle with the aim of improving the photocatalytic activity effectively and make TiO₂ easy to recycle. Herein, MWCNT/TiO₂ nanocomposites were synthesized by surface coating of carbon nanotubes with anatase and rutile types TiO₂ through a modified sol-gel approach using tetrabutyl titanate as the raw material. The obtained photocatalysts exhibit higher performance for Methyl Orange (MO) than pure TiO₂ .

2. Results and Discussion

The morphology and microstructure of the acid modified MWCNT and MWCNT/TiO₂ nanocomposites were observed by Scanning Electron Microscope (SEM) and transmission electron microscope (TEM) investigations, respectively. As shown in Figure 1(a), the acid modified MWCNTs were dispersed uniformly with no or less aggregation. The weaved interconnected networks of carbon nanotubes contribute largely to the homogenous supporting of photocatalyst nanoparticles. After being supported by TiO₂ photocatalysts, as shown in Figure 1(b), the interval space of these weaved MWCNT networks was filled with nanosized TiO₂ .

Figure 1: Typical SEM morphology of (a) modified MWCNT and (b) MWCNT/TiO₂ nanocomposites.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

In the TEM image with low magnification (Figure 2(a)), MWCNTs coated with TiO₂ nanoparticles are identified with minor agglomeration of MWCNT. From the TEM image with higher magnification (Figure 2(b)), it can be seen that the TiO₂ nanoparticles with an average size of 15 nm are closely attached on the walls of MWCNT, which agrees with the XRD results below. Furthermore, the long and tube-like aperture structure of carbon nanotubes with large surface area contributes to the adsorption of the organic molecules. On the other hand, it is observed that the surfaces of MWCNT were not fully covered by TiO₂ nanoparticles, since the number of active sites on MWCNT generated by the acid treatment is not sufficient for the full covering [16].

Figure 2: Typical TEM images of acid modified MWCNT coated with nanosized TiO₂ materials in low magnification (a) and high magnification (b).

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

Figure 3 presents the Fourier transform infrared (FT-IR) spectroscopy spectrum of MWCNT/TiO₂ nanocomposites. The appearance of two strong absorption peaks at 2920 cm^-1 and 2850 cm^-1 is attributed to symmetrical and asymmetric telescopic vibration of methylene group (-CH₂ -) of carbon nanotubes, which indicates that the methylene structure of carbon nanotubes is not destroyed. Two absorption peaks at 1728 cm^-1 and 1160 cm^-1 demonstrate the formation of carboxyl groups and carbonyl groups in the modification stage [17]. Additionally, the band in the low wavenumber around 670 cm^-1 in the spectrum corresponds to the characteristic absorption peak of TiO₂ [18].

Figure 3: FT-IR spectrum of MWCNT/TiO₂ nanocomposites.

[figure omitted; refer to PDF]

In order to describe the crystalline structure of the obtained nanocomposites, X-ray diffraction (XRD) spectra were involved in the characterization. As shown in Figure 4, a characteristic diffraction peak at [figure omitted; refer to PDF] ° is the typical feature of carbon nanotubes. In curve (b), the peaks at [figure omitted; refer to PDF] ° and 27.48° are attributed to anatase and rutile structure of TiO₂ , respectively [19]. The main characteristic diffraction peak of carbon nanotubes at 25.33° is overlapped with characteristic feature of TiO₂ . Furthermore, the obtained TiO₂ particles with around 15 nm were calculated by Scherrer equation using the peak of [figure omitted; refer to PDF] ° [20], which is consistent with TEM images.

Figure 4: XRD patterns of MWCNT (a) and MWCNT/TiO₂ (b) nanocomposites.

[figure omitted; refer to PDF]

In order to evaluate and confirm the chemical compositions of the prepared samples, differential thermal analysis and thermogravimetric (DTA-TG) analysis of carbon nanotubes and MWCNT/TiO₂ nanocomposites is shown in Figure 5. There is an obviously exothermic peak at 650°C, which is attributed to the peak of carbon nanotube. TG curve shows that a weight loss happens at 600°C and it can be supposed that the framework of carbon nanotubes was destroyed and then volatilized. The loading amount of MWCNT in the MWCNT/TiO₂ nanocomposites is about 12 wt.%, which is in agreement with the results of previous reports that the low ratio of carbon-based supporter contributes to the dispersion of catalysts on the supporters [21].

Figure 5: DTA-TG curves of MWCNT (a) and MWCNT/TiO₂ (b) nanocomposites.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

Figure 6 shows the UV-Vis spectra of the pure TiO₂ , MWCNT, and MWCNT/TiO₂ nanocomposites. The absorption spectrum for the composites in 400 nm is much higher than that of pure TiO₂ . The composites also have stable absorption in the region of visible light from 400 to 800 nm with slight increase of absorption intensity between 700 and 800 nm in the wavelength. The composites have good absorption properties not only in the ultraviolet area but also in the visible light region, which is of significance for us to exploit and utilize solar energy resources in the application of environmental remediation [22].

Figure 6: UV-Vis absorption spectra of pure TiO₂ , MWCNT, and MWCNT/TiO₂ nanocomposites.

[figure omitted; refer to PDF]

To explore the photocatalytic activity of MWCNT/TiO₂ nanocomposites, photocatalytic experiments are performed using MO as the model pollutant under irradiation of ultraviolet lamp. Figure 7 shows the relation of the irradiation time and the degradation of MO by MWCNT/TiO₂ , pure TiO₂ , and MWCNT. The prepared composites have higher degradation efficiency than pure TiO₂ , which is comparable with the previous result that metal-doped TiO₂ (P25) has a much higher photocatalytic activity than pure TiO₂ with a degradation ratio of less than 10% under visible light and around 30% even under a 500 W high-pressure Hg lamp [23]. Within 4 h, the highest photodegradation efficiency can reach 83%. However, only about 53% and 34% of MO were degraded by using pure TiO₂ and MWCNT, respectively. The photocatalytic activities of TiO₂ were largely improved through the addition of MWCNT. The reason can be concluded as follows: firstly, MWCNT can absorb dissolved oxygen and organic matter on the outside of its surface due to the large surface area and special aperture structure; secondly, MWCNTs are eminent electronic conductors that can orderly export electrons from the surface of TiO₂ and quickly reduce electronic accumulation on TiO₂ [24, 25].

Figure 7: Effect of photocatalytic decomposition of Methyl Orange with irradiation time.

[figure omitted; refer to PDF]

3. Experimental Section

3.1. Materials

MWCNT (diameter 10-20 nm, length 1-2 μ m, purity: 98%, and ashes 0.2 wt.%) was purchased from Nano Port of Shenzhen Inc.; tetrabutyl titanate (CP) and other chemicals were purchased from Sinopharm Chemical Reagent Limited Corporation.

3.2. Purification of MWCNT

1.0 g MWCNT were acidified in a mixed solution of concentrated sulfuric (98%) and nitric acids (65%-68%) with a volume ratio of 3 : 1 under ultrasonication at 70°C for 3 h. Then, the MWCNTs were separated by filtration, followed by the washing with distilled water until pH = 7. The purified MWCNTs were dried at 100°C and ground for the further using.

3.3. Preparation of MWCNT/TiO₂ Composites

In a typical experiment for the preparation of composites, titanium dioxide coated MWCNT was prepared by the following procedure. A precursor solution was prepared by the mixing of 5 mL of ethanol and 1.5 mL of glacial acetic acid as inhibitors. 20 mL of tetrabutyl titanate as a TiO₂ precursor was added dropwise to the solution under stirring. The resulting solution was designated as A. 20 mg of surface modified carbon nanotubes was dispersed in a mixture solution of nitric acid, deionized water, and ethanol. After ultrasonic vibration for 20 min, the obtained suspension was mixed with solution A under constant stirring until the gel formation. The gel was aged for 48 h at room temperature, followed by the drying in air at 105°C for about 8 h and the further grinding to a powder. The powder was then calcined at 450°C in air for 2 h to produce MWCNT/TiO₂ nanocomposites.

3.4. Photocatalytic Degradation Experiment

200 mg nanocomposites were added into 250 mL Methyl Orange (MO) solution with concentration of 20 mg/L. After the pH value of degradation solution was adjusted to about 3, the solution under vigorous stirring was irradiated by 200 W medium mercury lamp with a main wavelength of 365 nm (Beijing Institute of Light Sources). The dispersion was kept in the dark for 60 min for dark adsorption experiments, after which photodegradation was carried out. The dark adsorption was designed to be 60 min because the adsorption results indicate that MO molecules were absorbed to saturation on the surface of catalysts (data not shown). A certain amount of aliquots (3 mL) was taken from solutions every 30 min to determine the concentration of the sample. The concentration of the MO dye was monitored by UV-Vis spectroscopy by recording the absorbance of the characteristic peak of MO at 465 nm. The change of MO concentration was regarded as the evaluation of photocatalytic activity of the composites.

3.5. Characterization and Testing

The FT-IR spectra were recorded on a Shimadzu IR-440 infrared spectrometer. The morphology of the particles was observed by JEM-1230 transmission electron microscope and JSM-5610LV scanning electron microscope. DTA-TG measurement was carried out in static air condition at room temperature. The X-ray diffraction (XRD) patterns were recorded by a PANalytical X'Pert PRO diffractometer operating at 50 kV with Cu kα radiation. The XRD intensity was measured by step scanning in the 2 [figure omitted; refer to PDF] range 10°-70° with a step of 0.0167°. The UV-Vis spectra of the powder solids were measured on TU-1901 UV-Vis spectrophotometry.

4. Conclusions

In conclusion, MWCNT-based MWCNT/TiO₂ nanocomposites have higher photocatalytic activity than single TiO₂ photocatalyst for the degradation of MO aqueous solution under ultraviolet light irradiation, which is attributable to the uniform coating of TiO₂ nanoparticles and widening of absorption wavelength. This study provides an avenue for the preparation of carbon nanotube-based photocatalysts that utilize ultraviolet as an energy source in the application of environmental remediation. Researches on the composites of including both highly dispersed TiO₂ nanoparticles and highly dispersed carbon nanotubes are anticipated.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (no. 2177142) and the Important Deployment Project of Chinese Academy of Sciences (KZZD-EW-TZ-16).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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