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Electrochemical Methods for the Preparation of Gold-Coated TiO2 Nanoparticles with Variable Coverages Yu-Chuan Liu*,† and Lain-Chuen Juang‡ Department of Chemical Engineering and Department of Environmental Engineering, Vanung University, 1, Van Nung Road, Chung-Li City, Taiwan, Republic of China Received March 23, 2004. In Final Form: May 24, 2004 We report here the first electrochemical methods to prepare elemental Au(0)-coated TiO2 nanoparticles with controllable coverages. First, Au substrates were cycled in a deoxygenated aqueous solution containing 0.1 N HCl and 1 mM TiO2 nanoparticles from -0.28 to +1.22 V versus Ag/AgCl at 500 mV/s with different numbers of scans. The durations at the cathodic and anodic vertexes were 10 and 5 s, respectively. After this process, positively charged Au-coated TiO2 nanoparticles were formed in the solutions. Then a cathodic overpotential of 0.6 V from the open circuit potential of ca. 0.82 V versus Ag/AgCl was applied under sonication to synthesize elemental Au(0)-coated TiO2 nanoparticles. The coverage of Au shells in the elemental Au-coated TiO2 nanoparticles is varied from 10% to 95% by increasing the number of scans from 10 to 50 in preparing the positively charged Au-coated TiO2 nanoparticles. The extremely high coverage of 95% in this study is notable, as compared with other methods to prepare Au-coated TiO2 nanoparticles.
Introduction Nanostructured materials have been the focus of scientific research1,2 due to their unusual optical,3 chemical,4 photoelectrochemical,5 and electronic6 properties. Therein, nanoscale titania is one of the most investigated oxide materials recently owing to its important applications in environmental cleanup,7 photocatalysts,8 solar cells,9 and gas sensors.10 Recently, gold dispersed on some oxide supports has become interesting because of its unusual catalytic properties.11-13 On the other hand, the photocatalytic efficiency of gold-capped oxide supports can be greatly improved due to the charge transfer between the oxide and the gold.14,15 Rodriguez et al.14 reported the adsorption and reaction of SO2 on TiO2/Au. The deposition of Au nanoparticles on TiO2 produces a system with an * To whom correspondence should be addressed. Tel: 886-34515811 ext 540. Fax: 886-2-86638557. E-mail: liuyc@ msa.vnu.edu.tw. † Department of Chemical Engineering. ‡ Department of Environmental Engineering. (1) Guo, L. J.; Cheng, X.; Chou, C. F. Nano Lett. 2004, 4, 69. (2) Xu, Q.; Gates, B. D.; Whitesides, G. M. J. Am. Chem. Soc. 2004, 126, 1332. (3) Krolikowska, A.; Kudelski, A.; Michota, A.; Bukowska, J. Surf. Sci. 2003, 532-535, 227. (4) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (5) Chandrasekharan, N.; Kamat, P. V. J. Phys. Chem. B 2000, 104, 10851. (6) Peto, G.; Molnar, G. L.; Paszti, Z.; Geszti, O.; Beck, A.; Guczi, L. Mater. Sci. Eng., C 2002, 19, 95. (7) Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist, A. J. Phys. Chem. B 2002, 106, 10674. (8) Tada, H.; Suzuki, F.; Ito, S.; Akita, T.; Tanaka, K.; Kawahara, T.; Kobayashi, H. J. Phys. Chem. B 2002, 106, 8714. (9) Gratzel, M. Nature 2001, 414, 338. (10) Rothschild, A.; Levakov, A.; Shapira, Y.; Ashkenasy, N.; Komem, Y. Surf. Sci. 2003, 532-535, 456. (11) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B 2002, 106, 7634. (12) Guo, Y. G.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2003, 107, 5441. (13) Manzoli, M.; Chiorino, A.; Boccuzzi, F. Surf. Sci. 2003, 532535, 377. (14) Rodriguez, J. A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2002, 124, 5242. (15) Zwijnenburg, A.; Goossens, A.; Sloof, W. G.; Craje, M. W. J.; Kraan, A. M.; Jongh, L. J.; Makkee, M.; Moulijn, J. A. J. Phys. Chem. B 2002, 106, 9853.
extraordinary ability to adsorb and dissociate SO2. A further study of a comparison of the behavior of SO2 on MgO(100)/Au and TiO2(110)/Au shows how important can be the effects of the oxide support for the activation of Au nanoparticles.16 Dawson and Kamat17 reported the photoinduced fusion and photocatalysis of gold-capped TiO2 nanoparticles. With TiO2/Au nanoparticles capped with a low concentration of the noble metal, more than 40% enhancement in the thiocyanate oxidation efficiency was found in their study. The major goal to prepare nanocomposites with coreshell structures is to improve the catalytic or sensing properties or to turn the material luminescent. In the Au-capped TiO2 system, many methods have been developed to achieve this requirement. Therein, chemically reducing HAuCl4 on the surface of TiO2 nanoparticles was most popularly used in the literature.5,17,18 In this method, the key point is that the pH of the solution should be lower than the isoelectric point of the amphoteric TiO2 (IEPTiO2 ) 6).11,17 Then the AuCl4- complex is readily adsorbed on the surface of TiO2. Rodriguez et al.14 used vapor deposition of gold onto the TiO2 surface at 300 K. Zanella et al.11 proposed a way to prepare TiO2/Au catalysts by calcination treatment, which leads to the decomposition of the Au(III) complexes adsorbed on the TiO2 nanoparticles into gold metal particles. In the study of the deposition of Au nanocrystals on TiO2 crystallites reported by Schaaff and Blom,19 the adsorption situations of Au nanocrystals at boundaries between adjacent TiO2 crystallites in toluene solutions were observed based on the self-assembled monolayer approach. Photodeposition for depositing noble metals on TiO2 nanoparticles was also shown in the literature.17,18 Certainly, the catalytic activity of the Au-capped TiO2 nanoparticles is sensitive to the ratio between Au and TiO2, which is dependent on the Au complex concentration, the pH, and the temperature of the solution and the calcination temperature in prepara(16) Rodriguez, J. A.; Perez, M.; Jirsak, T.; Evans, J.; Hrbek, J.; Gonzalez, L. Chem. Phys. Lett. 2003, 378, 526. (17) Dawson, A.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 960. (18) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Langmuir 2003, 19, 469. (19) Schaaff, T. G.; Blom, D. A. Nano Lett. 2004, 4, 507.
10.1021/la049234c CCC: $27.50 © 2004 American Chemical Society Published on Web 06/30/2004
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tion. Meanwhile, the formation of positively charged Au cations also can lead to activation of the Au, as shown in a recent article.20 In our previous studies of surface-enhanced Raman scattering (SERS) spectroscopies of polypyrrole (PPy),21,22 we reported evidence of the chemical effect on SERS of PPy electrodeposited on gold roughened by electrochemical oxidation-reduction cycles (ORC) and the relationship between crystalline orientations of gold and SERS of PPy deposited on it. Meanwhile, during roughening of Au substrates by the ORC treatment, stable Au-containing nanocomplexes were found existing in a 0.1 N HCl aqueous solution without any other additive. These novel nanocomplexes existing in aqueous solutions with very pale yellow color are usually ignored. We successfully applied the electrochemical activity of the Au-containing nanocomplexes in preparing Au/PPy core-shell nanocomposites.22 In this work, we first use the electrochemical ORC roughening process to prepare the precursor of positively charged gold-coated TiO2 nanoparticles in an aqueous solution containing 0.1 N HCl and 1 mM TiO2 nanoparticles. Then elemental Au(0)-coated TiO2 nanoparticles with controllable coverages are immediately synthesized in the same solution without addition of any stabilizer by a sonoelectrochemical reduction method. Experimental Section Chemical Reagents. HCl was used as received without further purification. The reagents (p.a. grade) were purchased from Acros Organics. Anatase TiO2 nanoparticles were purchased from Desunnano Co., Ltd., Taiwan. All of the solutions were prepared using deionized 18 MΩ cm water. Preparation of Au-Coated TiO2 Nanoparticles. All the electrochemical experiments were performed in a three-compartment cell at room temperature, 24 °C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). A sheet of polycrystalline gold foil with bare surface area of 4 cm2, a 2 × 4 cm2 platinum sheet, and KCl-saturated silver-silver chloride (Ag/AgCl) were employed as the working, counter, and reference electrodes, respectively. Before the ORC treatment, the gold electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 µm of alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated 0.1 N HCl aqueous solution containing 1 mM rutile TiO2 nanoparticles from -0.28 to +1.22 V versus Ag/AgCl at 500 mV/s with different scans. The durations at the cathodic and anodic vertexes were 10 and 5 s, respectively. Unless mentioned otherwise, 50 scans in the ORC treatment were used in the text. Before the ORC treatment, the solution pH (0.98) was lower than the IEPTiO2. This is responsible for the anion adsorption.11 After the ORC treatment, positively charged Au-coated TiO2 nanoparticles were formed in the aqueous solution at pH 0.95. To remove any Au species not coating on the TiO2 nanoparticles, these positively charged Au-coated TiO2 nanoparticles in solution were centrifuged and washed with deionized water for cycling four times. Then the treated powders were dissolved in 0.1 N HCl solutions. Finally, the gold working electrode used before was replaced by a platinum substrate with the same bare surface area of 4 cm2. A cathodic overpotential of 0.6 V from the open circuit potential (OCP) of ca. 0.82 V versus Ag/AgCl was applied under sonication and slight stirring for 60 min to synthesize elemental Au-coated TiO2 nanoparticles. 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. Characteristics of Au-Coated TiO2 Nanoparticles. In X-ray photoelectron spectroscopy (XPS) experiments, a Physical (20) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (21) Liu, Y. C. Langmuir 2002, 18, 174. (22) Liu, Y. C.; Jang, L. Y. J. Phys. Chem. B 2002, 106, 6748.
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Figure 1. XRD pattern of pure TiO2 nanoparticles. Electronics PHI 1600 spectrometer with monochromatized Mg KR radiation, 15 kV and 250 W, and an energy resolution of 0.1-0.8% ∆E/E was used. To compensate for surface charging effects, all XPS spectra are referred to the C 1s neutral carbon peak at 284.6 eV. Surface chemical compositions were determined from peak-area ratios corrected with the approximate instrument sensitivity factors. Then the coverages of gold on TiO2 were estimated based on their individual surface chemical compositions. The powder samples for the XPS analyses were collected by centrifugation at 16 000 rpm for 10 min to separate them from the solution. Then the separated powders were suspended in water under stirring for washing and centrifuged again. These procedures were repeated four times to remove residuals of electrolytes and Au species not coating on the TiO2 nanoparticles. The X-ray diffraction studies were performed by a MAC Science MXP3 X-ray diffractometer, 40 kV and 30 mA, with a copper target at a scanning rate of 5°/min. Ultraviolet-visible absorption spectroscopic measurements were carried out on a Perkin-Elmer Lambda 25 spectrophotometer in 1 cm quartz curvettes.
Results and Discussion Figure 1 shows the XRD pattern of TiO2 nanoparticles used in this study. The characteristic peak at 25.3° represents the anatase TiO2 nanoparticles.24,25 Figure 2 shows the results of the 50th scan of cyclic voltammograms (CVs) for the dissolution of the Au substrate and the redeposition of the Au-containing nanoparticles in 0.1 N HCl with and without additives of TiO2 nanoparticles. In the ORC treatment, the chloride electrolyte was selected since this facilitates the metal dissolution-deposition process that is known to yield SERS-active roughened surfaces.26 The CV plots are basically similar to those reported before.21,22 Under an electric field, the notable phenomenon is the additive of TiO2 nanoparticles demonstrating a positive effect on increasing the anodic peak, located at 0.5-0.75 V versus Ag/AgCl, which is responsible for the enhancement in the SERS of PPy.22 The enhancing anodic peak may also be ascribed to the increased surface area due to the incorporation of TiO2 nanoparticles with the redeposition of Au species on the roughened Au substrate. Further XPS analyses show that the content of TiO2 nanoparticles in the roughened Au substrate is about 10 mol %. In this study, the main goal is to deposit the Au-containing species, which result from the dissolu(23) Liu, Y. C.; Chuang, T. C. J. Phys. Chem. B 2003, 107, 12383. (24) Weng, C. C.; Wei, K. H. Chem. Mater. 2003, 15, 2936. (25) Tom, R. T.; Nair, A. S.; Singh, N.; Aslam, M.; Nagendra, C. L.; Philip, R.; Vijayamohanan, K.; Pradeep, T. Langmuir 2003, 19, 3439. (26) Chang, R. K.; Laube, B. L. CRC Crit. Rev. Solid State Mater. Sci. 1984, 12, 1.
Preparation of Gold-Coated TiO2 Nanoparticles
Figure 2. I-E curves for roughening Au substrates with a scan rate of 500 mV/s and 50 scans in different electrolytes. (a) In 0.1 N HCl and 1 mM TiO2 nanoparticles. (b) In 0.1 N HCl.
Figure 3. UV-vis absorbance spectra of various colloidal solutions. (a) 1 mM TiO2 nanoparticles in 0.1 N HCl before coating of positively charged Au by roughening Au substrates. (b-f) Positively charged Au-coated TiO2 nanoparticles after roughening Au substrates in 0.1 N HCl and 1 mM TiO2 nanoparticles with 10, 20, 30, 40, and 50 scans, respectively. (g) Positively charged Au complexes after roughening the Au substrate in 0.1 N HCl with 50 scans.
tion of the Au substrate, on the surface of TiO2 nanoparticles in the solution during the ORC treatment. Figure 2 just reveals some interaction between the Au species and the TiO2 nanoparticles. Figure 3 illustrates the change of the UV-vis absorbance maximum bands of TiO2 nanoparticles with increasing the loading of positively charged Au on TiO2 nanoparticles by roughening the Au substrates in 0.1 N HCl and 1 mM TiO2 nanoparticles with increasing scans. Spectra a and g in this figure with absorbance maxima at ca. 325 and 225 nm (and 314 nm) represent pure TiO2 nanoparticles and positively charged Au- and negatively charged Cl-containing nanocomplexes,23 respectively. Clearly, the absorbance band of TiO2 nanoparticles is depressed with increasing the ORC scans; instead, the characteristic absorbance band of positively charged Au species shows markedly. Since only the materials on the surface of nanoparticles are responsible for the surface plasmon resonance, the diminishing characteristic absorbance band of TiO2 nanoparticles indicates that
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Figure 4. UV-vis absorbance spectra of various colloidal solutions. (a) Positively charged Au-coated TiO2 nanoparticles after roughening the Au substrate in 0.1 N HCl and 1 mM TiO2 nanoparticles with 50 scans. (b) Elemental Au-coated TiO2 nanoparticles synthesized by sonoelectrochemical reduction at a cathodic overpotential of 0.6 V from the OCP of 0.82 V vs Ag/AgCl for 60 min in the solution containing positively charged Au-coated TiO2 nanoparticles.
positively charged Au is successfully deposited on the surface of TiO2 nanoparticles with an increase of coverage. With 50 scans in the ORC treatment, TiO2 nanoparticles are almost coated with the positively charged Au. This result can be further confirmed from the XPS analyses. As shown in spectrum a of Figure 4, the absorbance maxima of the positively charged Au species, coating on TiO2 nanoparticles, before the sonoelectrochemical reduction appears approximately at 225 and 314 nm, which are markedly different from those of zerovalent Au nanoparticles located at ca. 520 nm,27,28 and their oxidation states can be confirmed from the XPS analysis. Correspondingly, the appearance of the characteristic absorbance maximum at ca. 530 nm, as shown in spectrum b of Figure 4, after the sonoelectrochemical reduction at a cathodic overpotential of 0.6 V for 60 min, reveals that the elemental Au(0)-coated TiO2 nanoparticles can be readily synthesized by the electrochemical reduction at room temperature under ultrasonication in this study. Meanwhile, the disappearance of the absorption band of TiO2 nanoparticles at ca. 325 nm means that the TiO2 nanoparticles are almost coated by the elemental Au(0) from the precursor prepared with 50 scans in the ORC treatment. Figure 5 shows the XPS survey spectrum of the elemental Au-coated TiO2 nanoparticles synthesized by roughening the Au substrate in 0.1 N HCl and 1 mM TiO2 nanoparticles with the ORC treatment for 50 scans, followed by the sonoelectrochemical reduction. Before the sonoelectrochemical reduction, TiO2 nanoparticles are coated with positively charged Au- and negatively charged Cl-containing complexes. The disappearance of the Cl signal around 200 eV means that the positively charged Au is completely reduced to the elemental one. Because Au still adsorbs on the surface of TiO2 nanoparticles, the signal of Ti is therefore shielded. Figure 6 displays the XPS Ti 2p3/2-1/2 core-level spectra of TiO2 nanoparticles before and after the coating with elemental Au. The signal (27) Ship, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789. (28) Henry, M. C.; Hsueh, C. C.; Timko, B. P.; Freund, M. S. J. Electrochem. Soc. 2001, 148, D155.
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Figure 5. XPS survey spectrum of Au-coated TiO2 nanoparticles.
Figure 6. XPS Ti 2p3/2-1/2 core-level spectra of different TiO2based nanoparticles. (a) Pure TiO2 nanoparticles. (b) Elemental Au-coated TiO2 nanoparticles.
of the elemental Au-coated TiO2 nanoparticles is diminished. Nevertheless, the positions of the main peaks of Ti are basically identical to those of pure TiO2 nanoparticles. This indicates that the TiO2 nanoparticles are stable during the coating procedure in the electrochemical treatments. Figure 7 displays the XPS Au 4f7/2-5/2 corelevel spectra of Au-coated TiO2 nanoparticles before (spectrum a) and after (spectrum b) treating the positively charged Au-coated TiO2 nanoparticles by the sonoelectrochemical reduction at a cathodic overpotential of 0.6 V. As shown in spectrum b, the doublet peaks located at 84 and 87.7 eV can be assigned to Au(0) according to the XPS handbook and the previous report.28 Undoubtedly, elemental Au-coated TiO2 nanoparticles are successfully synthesized by the electrochemical methods in this study. In a comparison of spectrum a, representing the positively charged Au-coated TiO2 nanoparticles, with spectrum b, representing the elemental nanoparticles Au(0), it is found that there are extra oxidized components of Au shown on the higher binding energy side. The oxidized Au shown in spectrum a can be assigned to monovalent Au(I) and trivalent Au(III) at 85.2 and 86.7 eV, respectively.29 No further deconvolution was made, and the term “positively charged Au” was thus adopted in this study. The coverage
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Figure 7. XPS Au 4f7/2-5/2 core-level spectra of different Aucoated TiO2 nanoparticles. (a) Positively charged Au-coated TiO2 nanoparticles. (b) Elemental Au-coated TiO2 nanoparticles.
Figure 8. Correlation of coverage of Au shells in various elemental Au-coated TiO2 nanoparticles synthesized by sonoelectrochemical reduction at a cathodic overpotential of 0.6 V in the solution containing positively charged Au-coated TiO2 nanoparticles prepared by roughening Au substrates in 0.1 N HCl and 1 mM TiO2 nanoparticles with different numbers of scans.
of gold on TiO2 can also be estimated from the XPS data, as mentioned in the Experimental Section. The result shows an extremely high coverage of 95%, as compared with other methods to prepare Au-coated TiO2 nanoparticles. The electrochemical methods developed in this work can provide an almost complete covering of Au on TiO2 nanoparticles. Variable coverage of elemental Au coated on the surface of TiO2 nanoparticles can be achieved by just changing the number of scans in the ORC treatment for roughening the Au substrate in 0.1 N HCl and 1 mM TiO2 nanoparticles, followed by the sonoelectrochemical reduction. Figure 8 shows this result. Basically, the coverage increases with the increase of the number of scans used in the ORC treatment. We tried to use higher scans, more than 60, but the coverage is still around 95%. Thus 95% coverage is the topmost value in this system. (29) Suzer, S.; Ertas, N.; Kumser, S.; Ataman, O. Y. Appl. Spectrosc. 1997, 51, 1537.
Preparation of Gold-Coated TiO2 Nanoparticles
Conclusion In this study, we develop an easy electrochemical pathway to prepare elemental Au(0)-coated TiO2 nanoparticles. By varying the number of scans in the ORC treatment for roughening the Au substrate in 0.1 N HCl and 1 mM TiO2 nanoparticles, followed by sonoelectrochemical reduction, the coverage of Au on TiO2 nanoparticles is readily controlled from 10% to 95% with increasing the number of scans from 10 to 50. The extremely high coverage of 95% is notable, as compared with other
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methods to prepare Au-coated TiO2 nanoparticles. The study of the potential application of this methodology to other noble metals or bimetal-coated oxide supports is currently underway. Acknowledgment. The authors thank the National Science Council of the Republic of China (NSC-91-2214E-238-001) and Vanung University for their financial support. LA049234C
