Materials Science Forum Vols. 587-588 (2008) pp 849-853 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland
Nanocrystalline CNT-TiO2 Composites Produced by an Acid Catalyzed Sol-gel Method Cláudia G. Silva1,a, Wendong Wang2,b and Joaquim L. Faria1,c 1
Laboratório de Catálise e Materiais (LCM), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal 2
Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China a
[email protected],
[email protected],
[email protected]
Keywords: Nanostructured materials, Titanium dioxide, Carbon nanotubes, Photochemistry
Abstract. Carbon nanotube-TiO2 composites and pure TiO2 are prepared by a modified sol-gel method. The nanoscaled materials obtained are extensively characterized by spectroscopic, microscopic and calorimetric techniques. Their photoactivity is tested in the degradation of phenol under visible illumination. A correlation between the CNT content and the changes in the UV–vis absorption properties was found. The effect induced by CNT on the composite catalysts is explained in terms of an interphase interaction between CNT and TiO2 in the composite catalysts. Introduction Carbon nanotubes (CNT) are nanometer scaled structures which can be used on their own, or incorporated in various matrices as key components to produce advanced composites with applications in sensors, electronic and optical devices, catalysts and other devices for energy production and storage. All of them are of environmental relevance [1]. In a recent review, attention has been drawn to the fact that CNT can compete with activated carbon as catalyst supports due to the combination of their electronic, adsorption, mechanical and thermal properties [2]. On the other hand, titanium dioxide has been widely used as photocatalytic material for solving environmental problems, especially for removing chemical pollutants from waste waters [3-5]. The widespread use of TiO2 in photocatalytic applications is due to its strong oxidizing power, high chemical stability and relative inexpensiveness. TiO2 exists in different crystalline forms such as anatase, rutile and brookite. Anatase and brookite are thermodynamically metastable and can be irreversibly transformed to the more stable rutile following a temperature treatment. TiO2 can be prepared by both liquid and gas phase processes. Amongst the solution routes, the sol-gel method is one of the most used techniques to synthesize thin films, powders and membranes. The sol-gel technique has many advantages over other production methods, including ease of processing, and control over the composition, purity and homogeneity of the materials obtained. The sol-gel route is well established as an excellent method to prepare the TiO2-based materials. Likewise, nanoscale composite materials containing carbon nanotubes and titania can be prepared by using a modified sol-gel method. In the present study, we describe the preparation of TiO2 following an acid-catalyzed sol-gel method using a titanium alkoxide precursor. The xerogel calcination temperature was controlled in order to optimize surface and morphological properties of the catalysts. Composite CNT-TiO2 materials were also prepared through an acid-catalyzed sol-gel method, with the aim of being used as photocatalysts in water treatment processes. We take advantage of the unique properties of CNT to optimize the surface and catalytic properties of the resulting composite materials.
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Experimental TiO2 was prepared by an acid-catalyzed sol-gel method using titanium isopropoxide as precursor as described elsewhere [6]. The gel obtained was aged in air for one week. The xerogel was then ground into a fine powder and dried at room temperature. The powder was calcined at several temperatures (673 to 973K) in a nitrogen flow for 2 hours. CNT-TiO2 composite catalysts were prepared similarly, by introducing a certain amount of carbon nanotubes (Shenzhen NTP, China) in the Ti(OC3H7)4 ethanol solution. Catalysts are named as X-CNT-TiO2, where X (5, 10, and 20) corresponds to the weight ratio of CNT to a 100 weight basis of pure TiO2. The xerogel obtained was then calcined at 673K. Pure TiO2 and composites were comprehensively characterized by thermogravimetric analysis, differential scanning calorimetry (DSC), nitrogen adsorption-desorption isotherms, powder X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and diffuse reflectance UV-vis absorption spectroscopy. Catalysts were tested in the photocatalytic degradation of phenol under visible radiation (λexc=366, 405 and 436 nm). Reaction intermediates were isolated by HPLC analysis. Degradation efficiency was determined by total organic carbon (TOC) measurements performed at given reaction times. Results and Discussion Nanocrystalline TiO2 was obtained by hydrolysis of titanium isopropoxide (≡Ti–OC3H7) in acidic media (Eq. 1) followed by condensation of the polymeric chains (Eqs. 2 and 3). Acid catalyzed hydrolysis provides control over the rate of this step avoiding TiO2 precipitation. HNO3 ≡ Ti − OC 3 H 7 → ≡ Ti − OH
(1)
≡ Ti − OH + C3 H 7 O − Ti ≡ → ≡ Ti − O − Ti ≡ +C3 H 7 OH
(2)
≡ Ti − OH + HO − Ti ≡ → ≡ Ti − O − Ti ≡ + H 2 O
(3)
The material was then thermally treated under nitrogen flow. Lower calcinations temperatures resulted in materials with smaller crystallite sizes and higher surface area (Fig. 1).
Fig. 1 Evolution of anatase crystallite dimensions (dA) and surface area (S) of TiO2 with calcination temperature.
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XRD analysis (not shown) revealed that only the anatase crystalline phase is observed for calcination temperatures below 723K. Further increase in the calcination temperature induces phase transformation from anatase to more thermodynamically stable rutile phase. Differential scanning calorimetric profiles revealed that crystal phase transition starts to occur between 723 and 773K. At 973K only the rutile phase is observed. With increasing calcination temperature, titania crystallites tend to agglomerate leading to the formation of bigger particles. At 973K the observed particle dimensions were typically of a magnitude of 100-200 nanometers, as shown by AFM analysis (Fig. 2).
Fig. 2 AFM images of TiO2 particles (tapping mode) annealed at 973K. The dimensions of the TiO2 particles obtained at 673K revealed to be of a magnitude of 10 to 20 nm, being constituted by few anatase crystallites. On the other hand, XRD analysis of composite catalysts revealed that only the anatase phase is present with crystallites of about 8.5 nm aggregated together with CNT in particles of 15 to 20 nm of diameter. The surface area of the composite catalysts is lower than estimated theoretically in proportion to the TiO2 and CNT contents, suggesting the development of a strong interface interaction between the two solid phases. The existence of a contact surface was confirmed by scanning electron microscopy analysis. Diffuse reflectance UV-vis spectra shows that the TiO2 characteristic absorption sharp edge at 400 nm changes with the introduction of the carbon phase into the TiO2 matrix, leading to a rise of material absorption in the visible region (Fig.3).
Fig. 3 Diffuse reflectance UV-vis spectra of TiO2 and CNT -TiO2 composite catalysts.
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The observed effect revealed itself to be proportional to the CNT content of the composite catalysts, supporting the creation of an interphase interaction. Photocatalytic degradation of phenol using both TiO2 and CNT-TiO2 catalysts under visible irradiation follows pseudo-first order kinetics. Hydroquinone and catechol were detected as the main intermediates (Fig. 4), which is in line with the expected reactivity of phenol with hydroxyl radical concerning hydrogen substitution [7]. OH OH OH Catechol Products
OH Phenol
OH Hydroquinone
Fig. 4 Simplified mechanism of the photocatalytic degradation of phenol. A synergetic effect was observed with the introduction of CNT into the titania matrix. The performance of the catalysts prepared was evaluated by comparing the apparent first order rate constants (kapp) listed in Table 1. A synergy factor (R), used to quantify the synergetic effect, was defined as the ratio between apparent rate constant using composite catalysts and neat titania, as follows: R=
k app ( X − CNT − TiO2 ) k app (TiO2 )
.
(4)
The introduction of 5 and 10% of CNT into the TiO2 matrix resulted in an enhancement of its performance for the photocatalytic oxidation of phenol of 20%. 20-CNT-TiO2 composite catalyst creates an obvious kinetic synergy effect in phenol degradation with an increase in the rate constant by a factor of 2.1. At the same time, an increase in the total organic carbon removal is observed. The complete disappearance of phenol (99%) using 20-CNT-TiO2 could be achieved after 8 hours of irradiation (Fig. 5) contrasting with the 77% removal obtained with TiO2. Table 1 Phenol conversion (X4h) and TOC removal (XTOC,4h) after 4 hours of visible irradiation, apparent rate constants (kapp) and synergy factor (R) obtained for the different catalysts. Catalyst X4h [%] XTOC,4h [%] R kapp [×10-3 min-1] TiO2 47 40 --2.8 ± 0.1 5-CNT-TiO2 52 38 1.2 3.3 ± 0.1 10-CNT-TiO2 57 37 1.2 3.5 ± 0.1 20-CNT-TiO2 75 51 2.1 5.9 ± 0.2 An obvious correlation between the optical properties of the catalysts and the increase in the synergetic effect can be observed. Therefore, it is reasonable to ascribe this effect to CNT acting as photosensitizer by injecting electrons into the TiO2 conduction band suppressing the recombination of electron/hole pairs and triggering the formation of bulk O2•– and HO• radicals, which are responsible for the degradation of phenol.
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Fig. 5 Phenol conversion during irradiation experiments using TiO2 and 20-CNT-TiO2 catalysts. The results described show that is possible to develop supported titania materials with photocatalytic activity, which can be incorporated in many different structures, from chemical reactors to building materials. Conclusions TiO2 and CNT-TiO2 composite catalysts have been prepared by an acid catalyzed sol-gel method. Material characterization indicates the creation of a strong interaction between CNT and TiO2. A synergetic effect is observed for the CNT-TiO2 composite catalysts on the degradation of phenol under visible light irradiation. This effect may be explained in terms of CNT acting as a photosensitizer. The increase of CNT/TiO2 ratio from 5 to 20% favors the enhancement of the synergetic effect on phenol disappearance, which could be correlated to the UV-vis spectra changes of the solids. The results presented point out the existence of an intimate contact between CNT and TiO2 phases in the composite catalysts, able to develop unique electron transfer properties on the resulting materials. Acknowledgments The authors gratefully acknowledge FCT and FEDER for financial assistance (fellowship SFRH/BD/16966/2004 and projects POCI/EQU/58252/2004 and POCTI/1181/2003). This work was also partly supported by China Postdoctoral Science Foundation. References [1] L. Theodore and R.G. Kunz: Nanotechnology: Environmental Applications and Solutions (John Wiley & Sons, Inc., Hoboken, New Jersey 2005). [2] P. Serp, M. Corrias and P. Kalck: Appl. Catal. A Vol. 253 (2003), p. 337. [3] D.S. Bhatkhande, V.G. Pangarkar and A.A.C.M. Beenackers: J. Chem. Technol. Biotechnol. Vol. 77 (2002), p. 102. [4] C.G. Silva and J.L. Faria: J. Photochem. Photobiol. A Vol. 155 (2003), p. 133. [5] C.G. Silva, W. Wang and J.L. Faria: J. Photochem. Photobiol. A Vol. 181 (2006), p. 314. [6] W. Wang, C.G. Silva and J.L. Faria: Appl. Catal. B Vol. 70 (2007), p. 470. [7] Z. Wang, W. Cai, X. Hong, X. Zhao, F. Xu and C. Cai: Appl. Catal. B Vol. 57 (2005), p. 223.
