18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS PREPARATION AND CHARACTERIZATION OF TIO 2 /GRAPHENE HYBRID BY SOL-GEL PROCESS ASSISTED WITH MICROWAVE IRRADIATION Dinh Huong Nguyen, Han Na Kim, and Dai Soo Lee* School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea * Corresponding author(dslee@jbnu.ac.kr) Keywords : TiO 2 /graphene hybrid, microwave surface area for absorption of reactant and active site 1 Introduction [5, 9-11]. Graphene, a two-dimensional sp 2 carbon network, Methods reported to prepare the hybrids of has attracted much attention due to its superior TiO 2 /CNM can be classified into three. The first is electrical, mechanical, and thermal properties. It has simple mixing of nanocrystalline TiO 2 with CNM been studied for various applications as [9], usually resulting in rather heterogenous reinforcements, conductive fillers, and functional dispersion of TiO 2 nanoparticles on the surface of materials due to its high surface area, charge carrier CNM. The second is sol-gel calcinations process. mobility, mechanical flexibility, and optical Amorphous TiO 2 deposited on the surface of CNM were calcinated at high temperature over 400 o C for transparency [1, 2]. It is also used as substrates for nanostructured metal or semiconductor metal oxide several hours in order to form nano crystalline TiO 2 nanoparticles of Pt, Au, Ag, and TiO 2 for catalytic [10]. Both rutile and anatase TiO 2 can be obtained applications [3-5] by this process. The third is hydrothermal process in Titanium dioxide is very popular and widely used in which the hybrid of crystalline TiO 2 /CNM is many applications as photovoltaic, catalyst, battery, prepared in autoclave reactor at high pressure for and hydrogen production because of its nontoxic several hours to crystallize TiO 2 [5]. Usually only nature, low cost with wide band gap, and high nanoparticles of anatase TiO 2 are formed on the photocatalytic activity [6, 7]. Various methods have surface of graphene in the hydrothermal processes. been studied to enhance the photocatalytic and However, those processes employ the conventional photoelectrochemical performances of TiO 2 in metal heating for several hours and high pressure. particle loadings, co-catalysts, dye sensitization, and Microwave is popular in industries as well as in metallic or non metallic doping [8]. Recently, the household applications because of direct and fast hybrid of TiO 2 with carbonous nano materials heating with high efficiency. By microwave (CNM) especially carbon nanotube and graphene irradiation for short time, nanocrystalline TiO 2 is have been investigated to enhance performance of formed homogenously and effectively [6]. Moreover, TiO 2 because of their unique and controllable graphene is also prepared rapidly from graphite structure and electrical properties. The significant oxide (GO) under microwave irradiation [2]. Taking enhancements of photocatalytic and into account of the advantages of microwave photoelectrochemical behavior of the hybrid of CNT irradiations, we studied a new, simple, and effective or graphene with TiO 2 have been reported. The method to prepare TiO 2 /graphene hybrid from GO enhancement may be due to the following and titanium precursor by microwave irradiation. possibility: the band-gab turning, retardation of The effects of TiO 2 content and microwave electron-hole recombination, provision of high rradiation time on the properties of TiO 2 /graphene hybrids are reported in this paper.
O O OH O O O-Ti(OIP) 3 OH OH O-Ti(OIP) 3 O O OH O OH O O-Ti(OIP) 3 OH O-Ti(OIP) 3 OH OH O O OH O O Microwave Adding H 2 O irradiation Adding TTP Crystalline TiO 2 /GO GO in DMF TTP modified GO TiO 2 /Graphene After hydrolysis Scheme 1. Typical process to prepare TiO 2 /Graphene hybrids. (c) (b) 200 nm (a) Figure 1. Morphological features of GO prepared: (a) TEM image; (b) AFM image; (c) Height profile in (b). (XRD), X’pert PRO MRD from Philips, using Cu 2 Experimental details K α radiation and operating at 30kV and 13 mA, FTIR spectrometer (JASCO 4100), high resolution 2.1 Material and preparation transmittance electron microscopy (HR-TEM, JEOL Synthetic graphite particle and titanium JEM-2010), and X-ray photoelectron spectroscopy tetrapropoxide (TTP, reagent grade) from Aldrich (XPS), AXIS-NOVA (Kratos. Inc). Chemical were used without further purification. GO were prepared by modified Hummer method Table 1. Sample codes for TiO 2 /graphene hybrids of [12]. The process to prepare TiO 2 /graphene hybrids different compositions and microwave irradiation is summarized in Scheme 1. Typically, 0.4 g of GO times was dispersed in 400ml of DMF and 1.42g of TTP Sample TTP/ Irradiation TiO 2 A/R were added followed by ultrasonication for 4 hours content a Codes GO time ratio to obtain homogenous solution. Excess amount of ratio (second) (%) deionized water were added dropwise under rigorous TiO 2 0 30’ 0 stirring for 1 hour. Then centrifuge, washing with GT1 1.77 30’ 55.8 0.36 deionized water, and drying were carried out to get GT2 3.55 30’ 70.5 0.28 powders of amorphous TiO 2 /GO. Finally, GT3 7.10 30’ 86.7 0.39 nanocrystalline TiO 2 /Graphene hybrids were GT4 14.2 30’ 87.1 0.43 obtained by irradiating the powder in a conventional GT5 3.55 30’+20’ 75.3 0.29 microwave oven (Samsung Electronics, 750W) for GT6 3.55 30’+60’ 83.7 0.55 different periods. The compositions and irradiation time is shown in Table 1. a: TiO 2 content obtained from TGA data of the hybrids after microwave irradiation. 2.2 Characterization 3 Rresults and discussion GO and the hybrids of TiO 2 /graphene were Figure 1 shows TEM and AFM images of GO characterized employing a X-ray diffractometer prepared by modified Hummer’s methods. After
PAPER TITLE ultrasonication in DMF, thin layers of GO were obtained as shown in Figure 1. In Figure 1(c), the thickness of GO is about 3.1nm (two layers of GO (a) sheet). FTIR spectra of GO is shown in Figure 2(a). 1730 1620 1212 (b) The broad peak from 3000cm -1 to 3600cm -1 is (c) attributed to stretching of hydroxyl groups. The characteristic peaks at 1730 cm -1 , 1620 cm -1 and (d) 1212 cm -1 correspond to stretching of carbonyl (C=O), skeletal vibration of unoxidized graphitic domains and stretching of C-OH respectively [13]. (e) Figure 2(b) shows the IR spectrum of TTP with characteristic peaks of CH 2 and CH 3 (around 4000 3500 3000 2500 2000 1500 1000 500 2850cm -1 -3000cm -1 ). After TTP was added into GO, -1 ) Wave number (cm the broad peak of hydroxyl group in GO disappeared Figure 2. FTIR spectra of GO (a), TTP (b), TTP (Figure 2(c)). It is due to the exchange reaction of modified GO (c), TiO 2 (d), and TiO 2 /Graphene. TTP with OH groups of GO (scheme 1), making GO more stable in DMF solution. The spectrum of pure A R R: Rutile TiO 2 in Figure 2(d) shows characteristic peaks of R A A: Anatase hydroxyl groups at 3270 cm -1 , and water absorbed at R R 1623 cm -1 . No characteristic peaks of GO or TTP A R R R (e) were observed in the spectrum of TiO 2 /Graphene after microwave irradiation for 30 seconds (Figure (d) 2(e)). The results indicate that after hydrolysis and (c) microwave irradiation, TTP was converted into TiO 2 and GO were reduced to graphene [2]. (b) Figure 3 shows TEM images of TiO 2 /GO and (a) TiO 2 /Graphene. Before microwave irradiation, the 20 30 40 50 60 70 80 2 Degree) layer of amorphous TiO 2 covered the surface of GO as shown in Figure 3(a). After microwave irradiation Figure 4. XRD data of GT2 before microwave irradiation (a), GT1 (b), GT2(c), GT3 (d), GT4 (e) (30 seconds), the nanoparticles of TiO 2 in range of 20 nm were homogenously formed on the surface of after microwave irradiations. graphene (Figure 3(b)). The crystal structure of TiO 2 Figure 4 shows XRD patterns of TiO 2 /GO and (green arrows) and few layers structure of graphene (red arrows) were observed in HR-TEM image of TiO 2 /graphene hybrids. Before microwave TiO 2 /Graphene (Figure 3(c)). Those results indicate irradiation, no characteristic diffraction peak of crystalline TiO 2 was observed, indicating amorphous that crystalline TiO 2 nanoparticles were formed by microwave irradiation. nature of TiO 2 ((a) in Figure 4). After microwave 100 nm 5 nm 100 nm (a) (b (c) Figure 3. TEM images of GT2: (a) after hydrolysis; (b) after microwave irradiation; (c) HR- TEM image of GT2. 3
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