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Photocatalytically Deposited Silver Nanoparticles on Mesoporous TiO2 Films Elias Stathatos and Panagiotis Lianos* Engineering Science Department, University of Patras, 26500 Patras, Greece Polykarpos Falaras Institute of Physical Chemistry, NCSR ‘’Demokritos’’, 15310 Athens, Greece A. Siokou ICEHT/FORTH, P.O. Box 1414, University Campus, 26500 Rion, Patras, Greece Received December 31, 1998. In Final Form: June 22, 1999
1. Introduction Use of titania in heterogeneous photocatalysis for the total oxidation of organic and inorganic water pollutants is an object of extensive research in the recent years. Many studies have been devoted to the improvement of titania photoactivity by depositing noble metals.1-4 In particular, deposition of silver on TiO2 colloidal particles has received an extended interest that goes beyond photocatalytic degradation. Thus metallic silver can be photocatalytically deposited on TiO2, and this process can be used in silver recovery, e.g., from waste photographic effluents.5 The photoexcitation of electrons is enhanced if metal particles are incorporated in a TiO2 electrode. Thus, an increase in the anodic photocurrent in the visible region was observed for both the Au and Ag particle dispersed electrodes, which was thought to result from the surface plasmon resonance of the metal particles.6 Thermally or photocatalytically made cermet materials of silver nanoparticles associated with titania and silica present interesting electrical and optical properties. Electron tunneling between metal islands can allow electrical conductivity, at the same time offering optical transparency, thanks to the low size of the metal particles.7,8 The technique of coloring the surface of glasses by applying a coat of silver salts, followed by thermal treatment, has been empirically known for centuries.9 We now know that color is obtained by surface plasmon frequencies associated with silver nanoparticles.1,7 Having all these interesting applications of silvermetal oxide combinations in mind, we have in the present work photocatalytically deposited silver nanoparticles on titania nanoparticle films and have studied them by atomic * To whom correspondence may be addressed: Fax: 30-61997803. E-mail:
[email protected]. (1) Herrmann, J.-M.; Tahiri, H.; Ait-Ichou, Y.; Lassaletta, G.; Gonzalez-Elipe, A. R.; Fernandez, A. Appl. Catal., B: Environmental 1997, 13, 219. (2) Karakitsou, K. E.; Verykios, X. E. J. Catal. 1992, 134, 629. (3) Ohtani, B.; Iwai, K.; Nishimoto, S.-i.; Sato, S. J. Phys. Chem. 1997, 101, 3349. (4) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (5) Sahyun, M. R. V.; Serpone, N. Langmuir 1997, 13, 5082. (6) Zhao, G.; Kozyka, H.; Yoko, T. Thin Solid Films 1996, 277, 147. (7) Lassaletta, G.; Gonzalez-Elipe, A. R.; Justo, A.; Fernandez, A.; Ager, F. J.; Respaldiza, M. A.; Soars, J. C.; Da Silva, M. F. J. Mater. Sci. 1996, 31, 2325. (8) Roy, B.; Jain, B.; Roy, S.; Chakravorty, D. J.Non-Cryst. Solids 1997, 222, 102. (9) Vilegas, M. A.; Fernandez-Navarro, J. M.; Paje, S. E.; Llopis, J. Phys. Chem. Glasses 1996, 37, 248.
force microscopy (AFM), UV-visible spectroscopy, and X-ray photoelectron spectroscopy (XPS). The novelty of the present work is related with the simple and straightforward preparation conditions for both mesoporous TiO2 films and silver nanoparicle layer, which provide an optically interesting material. Materials and Methods All chemicals used in the present work were from Aldrich and were used as received. Millipore water was used in all experiments. TiO2 mesoporous films were made by the sol-gel method according to the following procedure. We first prepared a reverse micellar solution of Triton X-100 in cyclohexane.10 Surfactant concentration was 0.2 M and water concentration was 0.4 M. At temperatures below 25 °C, this mixture is turbid; however, it clears out upon addition of titanium isopropoxide.10 It is, nevertheless, preferable to work with clear solutions, i.e., above 25 °C, to get well-defined final products. Titanium isopropoxide was added to the reverse micellar solution under vigorous stirring. Films were subsequently made by dipping a glass microscope slide into the mixture. Prior to dipping, slides were cleaned by leaving them overnight in sulfochromic solution and were dried in a stream of N2. Films were, finally, calcinated by slowly heating to 450 °C in air. To deposit Ag+ on these films, we dipped freshly prepared TiO2 films in AgNO3 aqueous solutions. The concentration of silver salt varied from 0.01 to 0.1 M. No waiting time is necessary to adsorb silver ions on TiO2. The film can be dipped for only a few seconds in the AgNO3 solution and then it can be withdrawn. We have, however, found that more concentrated solutions yield larger adsorbent quantities on TiO2. After dipping, films were copiously rinsed with pure water and dried with N2. These films that contain Ag ions are completely transparent and they do not show any visible sign of the adsorbed ions. That the ions are there is revealed upon further treatment of the films. Finally, the films were irradiated in air, about 3-4 min, with UV radiation. They then obtained a dark color, revealing the presence of silver particles. Irradiation was done with a 400 W Osram lamp (Ultratech-400). UV-vis absorption spectra were recorded with a Cary 1E spectrophotometer. AFM images were obtained with a Nanoscope III, Digital Instruments, in the tapping mode. X-ray photoelectron spectra were taken in an ultrahigh vacuum (UHV) system equipped with a SPECS LH-10 hemispherical electron energy analyzer and a twin anode X-ray gun. For these measurements, the unmonochromatized Mg KR line at 1253.6 eV and an analyzer pass energy of 97 eV were used.
3. Results and Discussion A TiO2 film made by the above procedure is optically transparent. Figure 1 shows its absorption spectrum, which contains the near-UV band-gap absorption of TiO2 semiconductor particles. The absorption onset is hypsochromically displaced with respect to crystalline TiO2, due to size limitations in the nanoparticles.11 The oscillation of the curve shape above 400 nm is due to interference of light. The thickness of these films ranges between 150 and 200 nm. The adsorption of silver ions from the AgNO3 solution did not change the absorption spectrum of TiO2 films. However, after UV irradiation, a strong wide absorption band, peaking around 442 nm, dominated the absorption spectrum, as seen in Figure 1. Increase of AgNO3 solution concentration did, of course, increase the absorbance of the film. The above spectrum is charac(10) Stathatos, E.; Lianos, P.; DelMonte, F.; Levy, D.; Tsiourvas, D. Langmuir 1997, 13, 4295. (11) Brus, L. J. Phys. Chem. 1986, 90, 2555.
10.1021/la981783t CCC: $19.00 © 2000 American Chemical Society Published on Web 01/22/2000
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Figure 3. The Ag 3d region of the XP spectrum for theTiO2 film after Ag+ adsorption and subsequent UV irradiation. Adsorption of Ag+ was done from a 0.01 M aqueous AgNO3 solution. Figure 1. Absorption spectra of a plain TiO2 film (1) and of the same film after Ag+ adsorption and subsequent UV irradiation (2). Adsorption of Ag+ was done from a 0.01 M aqueous AgNO3 solution.
Figure 2. AFM images of a plain TiO2 film (a) and the same film after Ag+ adsorption and subsequent UV irradiation. Adsorption of Ag+ was done from a 0.1 M aqueous AgNO3 solution.
teristic of surface plasmon absorption corresponding to silver particles.1,7 AFM images obtained with plain TiO2 and with silver/ TiO2 films are shown in Figure 2. TiO2 nanoparticles seen in image 2a are monodisperse spherical particles of about 15 nm diameter. Monodispersity is the advantage of the reverse micellar route for the synthesis of TiO2 nanopar-
ticles by hydrolysis of titanium isopropoxide. Hydrolysis is slow, due to the fact that the small quantity of water present in solution is contested between the hydration of surfactant polar heads and the chemical consumption for alkoxide hydrolysis. Restructuring of surfactant molecules around the polar species formed during hydrolysis10 results in growth limitations and uniform particle sizes. In addition to particle diameter, AFM image analysis also gives the values of two other characteristic parameters of the surface of the film: mean square roughness R and fractal dimension f. R is the standard deviation of the surface z-dimension and f is the fractal dimensionality of the surface (2 < f < 3). Both are automatically calculated by the instrument. In particular, the algorithm employed for fractal analysis divides the surface into a series of triangles, starting with a 1 × 1 cell size. In the first iteration, the surface is divided into two triangles. The surface area of each triangle is calculated and recorded. In the second iteration, each cell is further divided in half, resulting in eight triangular cells. In the third iteration 32 triangles are defined and so on, until the image has been divided into a maximum number of triangles, each having a cell size of one pixel. As the number of triangles increases, the total surface area of the analyzed sample increases. The logarithmic plot of the cell size versus the surface area determines the fractal value of the surface. In fact, the fractal dimension is defined as the slope of the line obtained by plotting the log of the cell size versus the log of the cell surface area. Plain TiO2 films had R ) 1.14 and f ) 2.67. After silver adsorption and UV irradiation, the surface of the film gave the image of Figure 2b. Silver nanoparticles are prominent in the foreground of this image, while TiO2 nanoparticles are hardly distinguished in the background. The size of silver particles varied between 35 and 60 nm. Their number density varied in the same direction as the concentration of the original AgNO3 solution and the absorbance of the film. The roughness of the film also increased with the quantity of the deposited silver while the fractal dimension slightly decreased. Thus for the image of Figure 2b, R ) 7.52 and f ) 2.58. The increase of the standard height deviation and the concomitant decrease of the fractal dimension after silver deposition can be attributed to the presence of a more complex surface topography. In fact, on the high surface area nanocrystalline TiO2 film a number of surface features are now “hidden” under the silver particles of
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greater size. As a result, the surface roughness increases but the real specific surface area of the sample is now reduced. The optical absorption spectra revealed the presence of metallic silver on TiO2 films while the AFM image showed their nanoparticle structure. There is, however, a question: Are the silver nanoparticles actually made of pure metallic silver or are some other species also present? To answer this question, we have subjected the films to XPS measurements. The wide-scan XP spectrum shows only oxygen, titanium, silver, and some traces of adventitious carbon on the surface. Figure 3 shows the silver 3d spectrum region. To compensate for sample charging, binding energies were referenced to that of the adventitious carbon 1s peak at 285.0 ((0.2) eV. The O 1s peak then appears at 530.5 ((0.2) eV, whereas the Ti 2p doublet (12) Briggs D., Seah, M. P., Eds. Practical Surface Analysis; John Wiley and Sons: New York, 1983.
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exhibits a splitting of 5.6 eV with Ti 2p3/2 at 459.0 ((0.2) eV. These values correspond to TiO2.12 The Ag 3d5/2 peak (Figure 3) appears at a binding energy of 368.0 ((0.2) eV, its fwhm is 1.9 eV, whereas the splitting of the 3d doublet is 6.0 eV. These values correspond to metallic silver.12 In conclusion, we have presented a simple method of making mesoporous TiO2 films consisting of monodispersed nanoparticles. These films readily adsorb silver ions from aqueous silver salt. Ions yield silver nanoparticles by UV irradiation. The concentration of the nanoparticles on the film is mainly influenced only by the original concentration of silver salt in solution, indicating the large capacity of these films to adsorb silver ions. Acknowledgment. We acknowledge financial aid from the program “K.KAPAΘEO∆ΩPHΣ” of the University of Patras. LA981783T
