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Jan. 2008, Volume 2, No.1 (Serial No.2) Journal of Environmental Science and Engineering, ISSN1934-8932, USA Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles YU Chung-chin1, LIU Yu-chuan2, CHIU Wen-hui2, WANG Cheng-cai1 (1. Department of Environmental Engineering, Nano Materials Applications R & D Center, Vanung University, Taiwan; 2. Department of Chemical and Materials Engineering, Nano Materials Applications R & D Center, Vanung University, Taiwan) Abstract: The synthesis of gold nanoparticles caped with visible light-responsible TiO2 nanoparticles was prepared by using electrochemical Oxidation-Reduction Cycles (ORC) in 0.1 M HCl aqueous solution containing 60 mM visible light-responsible TiO2 nanoparticles. Firstly, an Au substrate was cycled in a deoxygenated aqueous solution containing 0.1 M HCl and 60 mM anatase TiO2 nanoparticles from -0.28 to +1.22 V vs Ag/AgCl at 500 mV/s with 25 scans. The durations at the cathodic and anodic vertexes are 10 and 5 s, respectively. After this process, Au-and TiO2-containing complexes were left in the solution. Then a Pt electrode immediately replaced the Au working electrode, and a cathodic overpotential of 0.6 V from the Open Circuit Potential (OCP) was applied under sonification to synthesize Au nanoparticles. Encouragingly, the prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles are more active for the decomposition of formaldehyde than pure visible light-responsible TiO2 nanoparticles are in the same condition. After 5 days testing, the formaldehyde was decomposed ca. 35% in containing Au nanoparticles caped with visible light-responsible TiO2 nanoparticles, but the formaldehyde was decomposed only ca. 25% in containing pure visible light-responsible TiO2 nanoparticles. Key words: formaldehyde; TiO2 nanoparticles; Oxidation-Reduction Cycles (ORC) 1. Introduction The nanoparticles exhibit their unique electronic structure and extremely large surface areas, so there widely applications have been researched, especially in and catalysts modification [1]. There are several developed methods for nanoparticles fabrication including chemical reduction [2], sonochemical * Acknowledgements: This work was supported by the fund of the Vanung University. YU Chung-chin (1963- ), male, assistant professor; main research field: environmental material. E-mail: [email protected]. 36 reduction[3], annealing from high-temperature solutions[4], metal evaporation [5], and laser ablation [6] etc.. It is useful to develop effective methods for sizeand shape-controlled synthesis of metal nanoparticles due to these properties can significantly effect their corresponding characterization [7,8]. Kobayashi et al [9] reported a sol-gel processing of silica-coated gold nanoparticles for preparation of gels with different particle sizes and shapes. WANG et al. [10] reported polyelectrolyte multi-layers for preparing silver nanoparticles composites with controlled metal concentrations and particle sizes by adjusting the pH value of solutions. Generally, the advantages of electrochemical methods over the chemical ones are the high purity of the particles and the control of particle size by adjusting applied potentials or current densities [7,11]. In the nanoscale field, titania is one of the most investigated oxide materials recently owning to its important applications in environmental cleanup [12], photocatalysts [13], solar cells [14] and polarimetric interference sensor [15]. With the development of visible light-responsible TiO2 nanoparticles [16,17], it is now possible to combine the effects of TiO2 and metal nanoparticles on photoreactions to improve the environmental cleanup effects. In the studies of surface-enhanced Raman scattering (SERS), the electrochemical oxidation-reduction cycles (ORC) procedure [18,19] is a better way to produce SERS-active metal substrates because a controllable and reproduced surface roughness can be easily generated [20,21]. In this work, we use an electrochemical ORC roughening procedure Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles to obtain Au-containing complexes in a 0.1 M HCl (US pharmacopoeia grade for following applications) aqueous solution from an Au substrate. Then Au caped with visible light-responsible TiO2 nanoparticles are immediately synthesized in the same solution with visible light-responsible TiO2 nanoparticles by using a sonoelectrochemical reduction method. The prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles are examined for the catalytic decomposition of formaldehyde in reactor. Formaldehyde will cause damage to lung of people in indoor air. The improved environmental cleanup effects and the reduced formaldehyde were investigated. 2. Experimental section 2.1 Chemical reagents The electrolytes reagents (p. a. grade) purchased from Acros organics were used as received without further purification. Visible light-responsible TiO2 nanoparticles with particle sizes from 20 to 30 nm were purchased from QF-NANO TECH. Co., Ltd, Taiwan. All of the solutions were prepared using deionized 18.2 M cm water provided from a MilliQ system. 2.2 Preparation of containing Au nanoparticles caped with visible light-responsible TiO2 nanoparticles All of the electrochemical experiments were performed in a three-compartment cell at room temperature, 24℃, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). A sheet of silver foil with a bare surface area of 0.238 cm2, a 2 × 2 cm2 platinum sheet, and a silver-silver chloride (Ag/AgCl) electrode were employed as the working, counter, and reference electrodes, respectively. Before the Oxidation-Reduction Cycles (ORC) treatment, the silver electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 m of alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated 0.1 M HCl aqueous solution containing 60 mM TiO2 nanoparticles from -0.28 to +1.22 V vs Ag/AgCl at 500 mV/s for three scans without any duration at the cathodic and anodic vertexes (called modified roughen Au substrate prepared with this procedure). Finally, the potential was hold at the cathodic vertex before the roughened Au electrode was took from the solution and rinsed throughout with deionized water. For comparison, roughened Au substrates without the additive of TiO2 were also prepared by using the same roughening condition (called unmodified roughen Au substrate prepared with this procedure). The ultrasonic irradiation was performed by using an ultrasonic generator (model XL2000, Microson) and operated at 20 kHz with a barium titanate oscillator of 3.2 mm diameter to deliver a power of 100 W. 2.3 Characteristics of gold nanoparticles and decomposition of formaldehyde A single drop of the sample-containing solution was placed on a 300 mesh Cu/carbon film transmission electron microscopy (TEM) sample grid and was allowed to be dried in a vacuum oven. Then the sample was examined by using a Philips Tecnai G2 F20 transmission electron microscopy (TEM). Continuous concentrations of prepared 2 ppmv formaldehyde were added in the reactor (30 ㎝×30 ㎝ ×20 ㎝ ) and kept temperature at 22℃. The gas detector tube (KITAGAWA, NO710A) of formaldehyde analyzed concentration of formaldehyde gas after 5 days from irradiating visible light (>400 nm) to visible light-responsible TiO2 nanoparticles for photoreaction. The HCHO will react with (NH2OH)3, H3PO4 in the tube and then produce H3PO4 and HCH=NOH in which the color change yellowish orange for reddish orange. The measuring range of the detector tube is 0.05-2.00 ppm and the detecting limit is 0.005 ppm. 3. Results and discussion 37 Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles Because the controllable and reproduced surface roughness can be easily generated, the electrochemical Oxidation-Reduction Cycles (ORC) procedure is a better way to produce SERS-active metal substrates. After the ORC procedure of roughening the Au substrate, it would leave some unreduced species, possibly positively charged Au clusters, on the Au surface, which was shown in previous studies [18,22]. Fig. 1 shows the Cyclic Voltammograms (CV) for the dissolution and redeposition of Au substrates in 0.1 M HCl. 0.04 Current/mA 0.02 maximum band is observed. As shown in spectrum b of Fig. 2, the absorbance maximum of Au-containing complexes after the ORC treatment and before the sonoelectrochemical reduction appears approximately at 310 nm, which is markedly different from that of zero-valent Au located at ca. 520 nm [23,24]. Correspondingly, the appearance of the characteristic absorbance maximum at ca. 550 nm, as shown in spectrum c of Fig. 1, after the sonoelectrochemical reduction at an overpotential of 0.6 V reveals that the elemental Au (0) caped with visible light-responsible TiO2 nanoparticles can be readily synthesized by the electrochemical reduction at room temperature under ultrasonication in the solution containing Au complexes, which is prepared in this study. 0 Absorbance/ a.u. b -0.02 -0.04 -0.4 0 0.4 0.8 1.2 c 1.6 E/ V vs. Ag/ AgCl Fig. 1 In preparing well-dispersed metal nanoparticles in aqueous solutions by electrochemical reduction methods, one necessary requirement is to accelerate the transfer of gold clusters formed from the cathodic vicinity to the bulk solution. The sonification and the mechanical stirring used in this study can satisfy this requirement. Further experiments indicate that only the mechanical stirring would result in serious deposition of Au nanoparticles on the substrate. Similarly, only sonification would cause the diffusion issue for the reactant of Au-containing complexes. Fig. 2(a) shows the UV-vis absorption spectrum of a blank solution of 0.1 M HCl, which was used as electrolytes in the ORC treatment. As expected, no marked absorption 38 a I-E curve for roughening gold substrates 200 400 600 800 Wavelength/nm Fig. 2 UV-vis spectra of various solutions: (a) 0.1 M HCl blank solution; (b) Au complexes-containing solution after roughing the Au substrate; (c) Au nanoparticles-containing solution after a sonoelectrochemical reduction treatment to the Au complexes-containing solution Fig. 3 illustrates the sonoelectrochemically cathodic reduction process at an overpotential of 0.6 V from positively charged Au-containing complexes to elemental Au nanoparticles caped with visible light-responsible TiO2 nanoparticles. The dispersion and the particle size of the gold nanoparticles caped Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles with visible light-responsible TiO2 nanoparticles prepared at a cathodic overpotential of 0.6 V with sonification in an aqueous solution are examined by using the TEM micrograph, as shown in Fig. 2. The nanoparticles with a diameter of ca. 50 nm demonstrate gold nanoparticles (shadow in the figure) caped with visible light-responsible TiO2 nanoparticles. Fig. 3 TEM micrograph of Au nanoparticles, prepared at a cathodic overpotential of 0.6 V from the OCP of 0.78 V vs Ag/AgCl with sonification, showing size and dispersion Unsupported Au nanoparticles exhibit activity for the CO oxidation at room temperature [25]. However, as shown in the literature [26,27], Au nanoparticles supported on some oxidize supports can active for the low-temperature oxidation of CO, too. By further investigating their applications in other fields, it is found that the electrochemically prepared Au nanoparticles also demonstrate a catalytic activity for the decomposition of aldehyde in alcohol solution [28]. It is known formaldehyde in air can cause damage to lung of people. Removal of formaldehyde from air would be contributive to human healthy. In this work, the reactor containing 2 ppmv formaldehyde in air was prepared to evaluate the catalytic activity of the prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles for the decomposition of formaldehyde. The gas detector tube (KITAGAWA, NO710A) of formaldehyde analyzed concentration of formaldehyde gas after 5 days from irradiating visible light (>400 nm) to visible light-responsible TiO2 nanoparticles for photoreaction. The HCHO will react with (NH2OH)3, H3PO4 in the tube and then produce H3PO4 and HCH=NOH in which the color change yellowish orange for reddish orange. The measuring range of the detector tube is 0.05-2.00 ppm and the detecting limit is 0.005 ppm. The content of formaldehyde in the reactor is reduced from original 2 to 1.5 ppmv by adding visible light-responsible TiO2 nanoparticles in reactor after 5 days, so the formaldehyde was decomposed ca. 25%. Varying visible light-responsible TiO2 nanoparticles with the prepared Au nanoparticles, the contents of formaldehyde in air can be further reduced to 2 and 1.3 ppmv, so the formaldehyde was decomposed ca. 35%, respectively. This result indicates that the electrochemically prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles in this work are higher catalysts (about 40%) for the decomposition of formaldehyde than pure visible light-responsible TiO2 nanoparticles. 4. Conclusions In this work, the prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles were prepared based on sonoelectrochemical methods in 0.1 M HCl of US pharmacopoeia grade from gold substrates. The prepared Au nanoparticles caped with visible light-responsible TiO2 nanoparticles are active catalysts for the decomposition of 2 ppmv formaldehyde in reactor. After 5 days testing, the formaldehyde was decomposed by ca. 35% under visible light irritation. In the same condition, the formaldehyde was decomposed by ca. 25% under visible light irritation with pure visible light-responsible TiO2 nanoparticles. This effect can be extended to commercial visible light-responsible TiO2 39 Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles nanoparticles for completely removing formaldehyde in indoor air. References: [1] ZHANG J, Vukmirovic M B, Sasaki K, Nilekar A U, Mavrikakis M, Adzic R R. 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