Preparation and characterization of Au-dispersed Ti02 thin films by a liquid-phase deposition method Shigehito Deki,* Yoshifumi Aoi, Hiroshi Yanagimoto, Katsuyuki Ishii, Kensuke Akamatsu, Minoru Mizuhata and Akihiko Kajinami Department of Chemical Science & Engineering, Faculty of Engineering, Kobe University, Rokkodai-cho, Nada-ku, Kobe 657, Japan Au-dispersed Ti02 (anatase) thin films have been prepared by a novel method, liquid-phase deposition (LPD) The deposited film\ were characterized by XRD, XPS, TEM and UV-VIS absorption spectroscopy The results showed that the titanium oxide thin film containing Au"' ions was formed from a mixed solution of ammonium hexafluorotitanate, boric acid and tetrachloroauric acid Heat treatment above 200 "C of the deposited film under flowing air produced dispersed Au metal particles, accompanied by the crystallization of titanium oxide as a matrix The mean particle size of the dispersed Au particles was ca 15 nm The optical absorption band due to the surface plasmon resonance of the dispersed Au particles shifted toward longer wavelengths with increasing heat- treatment temperature Microcrystals of metals and semiconductor-doped glasses have been studied in detail due to their large optical non-linearity These glasses are expected to be used in optical logic devices in an optical information context Generally, these metal- dispersed glass films or thin layers are prepared by r f sputter-ing,' ion-implantation2 and the sol-gel method For dry processes such as r f sputtering and ion-implantation, special apparatus and high energies are required for the preparation of films In addition, they are not suitable for the preparation of films on substrates with large surface areas, because they need vacuum or low pressure in operation For the sol-gel method, it is difficult to prepare thin films on substrates with complex surface morphologies Recently, we have developed a very simple wet process for the preparation of TiO2 thin films, the liquid-phase deposition (LPD) method In this process, transparent anatase thin films form directly on the substrate immersed in a mixed solution of ammonium hexafluorotitanate [( NH,),TiF,] and boric acid ( H3B03) In solution, the following ligand-exchange (hydroly- sis) equilibrium reaction of (NH,),TiF, is presumed TiFG2 +nH20+TiF6-n(OH)n2-+rzHF (1) This equilibrium reaction is shifted to the right-hand side by the addition of boric acid which reacts readily with F-and forms stable complex ion as follows H3B03+4HF BF,-+H30++2H20 (2) The addition of H3BO3 leads to the consumption of non-coordinated F-ions and accelerates the ligand-exchange (hydrolysis) reaction ( 1) Consequently, titanium oxide thin films form on the substrate immersed in the solution6 This process can be applied readily to the preparation of thin films on substrates which have complex morphologies and large surface areas without special equipment, because the LPD process is performed in an aqueous solution system Multi-component oxide thin films can be formed by the addition of the objective metal ion to the treatment solution In this study, we tried to prepare an Au-dispersed T10, thin film using the LPD method by adding tetrachloroauric acid (HAuC1,) solu-tion to the solution of (NH,),TiF, and H3B03 The films deposited were characterized by XRD, XPS, TEM and optical absorption spectroscopy Experimental As parent solutions, (NH,),TiF, (Kishida Chemical Co Ltd ) and H3B03 (Nacalai Tesque Inc) were dissolved in distilled water at concentrations of 0 5 mol dmP3, and HAuC1, (Wako Pure Chemical Industries Ltd ) was dissolved in distilled water at a concentration of 24mmol dm-3 These solutions were mixed at various compositions and used as the treatment solution for deposition The films were formed at a concen- tration of 0 1 mol dm-3 for (NH,),TiF, and of 0 2 mol dm-3 for H3BO3 This solution composition is that corresponding to the concentration at which the transparent anatase thin film was formed6 The concentration of HAuC1, in the treat- ment solution was varied from 0 1 to 10 mmol dm Most of the experiments were performed with the concentration of HAuCI, at 0 29 mmol dmP3 Non-alkali glass (Corning, #7059) was used as the substrate After being degreased and washed ultrasonically, the substrate was immersed in the treatment solution and suspended therein vertically for 20 h The sub- strate was then removed from the solution, washed with distilled water and dried at ambient temperature Heat treat- ment was carried out in an air flow for 1 h at various temperatures from 100 to 600°C The atomic ratios of Au/Ti in the deposited films were determined by inductively coupled plasma atomic emission spectroscopic analysis (ICP-AES, Seiko Instruments Inc , SRS 1500VR) of the solutions produced by dissolving the films with dilute hydrochloric acid XPS analyses of the deposited films were carried out on a Shimadzu ESCA 750 instrument XRD studies of the deposited films were measured on a Rigaku RINT 2100 diffractometer with thin film attachment, using Cu-Ka radiation (40 kV, 40 mA, scanning step 0 Oln) Crystallite sizes of the dispersed gold were calculated using Scherrer's equation, L=O 94/2/8 cos 8, where L is the mean dimension of the crystallites, is the full width in radians subtended by the half maximum intensity width of the diffrac- tion peak, 0 is the diffraction Fngle and n is the wavelength of the Cu-KN radiation (154 A) In order to calculate the crystallite size, the intensities were accumulated by repeating the measurement 10 times Optical absorption spectra were measured with a UVIDEC 660 (Japan Spectroscopic Co Ltd ) Particle sizes of the dispersed Au metal were measured directly on a Hitachi H-7100TE transmission electron microscope Results and Discussion The deposited film was colourless, transparent and showed strong adherence to the substrate The film changed from colourless to purple on heat treatment J Muter Chem, 1996, 6(12), 1879-1882 1879 o 0.2 0.4 0.6 0.6 I [HAuCl,]/mmol dm3 Fig.1 Relationship between the Au/Ti atomic ratio of the deposited film and the concentration of HAuCl, in the treatment solution. Concentration of (NH,),TiF,: 0.1 rnol dmP3 and of H3BO3: 0.2 mol dmP3. Reaction time: 20 h. Fig. 1 shows the relationship between the concentration of HAuCl, in the treatment solution and the Au/Ti atomic ratio of the deposited films. The Au content of the deposited film increased up to cu. 0.16 with an increasing HAuCI, concen- tration in the treatment solution. This indicates that the Au content of the film is controllable over a wide range by controlling the concentration of HAuCl, in the treatment solution. In the XP spectra of the deposited films, the binding energy of Au 4f7,, was 86.3 eV for the as-deposited film, and was 84.3 eV for the film heat-treated at 600°C.The binding energy of Au 4f,,, for the as-deposited film was close to that of Au"' and that for the heat-treated film was assigned to Au metal.* Based on these data, gold exists as Au"' ionic species in the as-deposited film and these species decompose and become Au metal upon the heat treatment. The change of the colour of the film from transparent to purple indicates the formation of Au metal particles in the film caused by the heat treatment. The XRD patterns of the deposited films which were heat- treated at various temperatures for 1 h are shown in Fig. 2. The measurements were made at an X-ray incidence angle of 1".The as-deposited film and the film heat-treated at 100°C were amorphous without any significant diffraction peak. The diffraction peaks assigned to Au metal were observed for the deposited films after heat-treatment above 200 "C. The decomposition of Au"' ionic species with formation of Au I 0 1 10 20 30 40 50 28/degrees Fig. 2 X-Ray diffraction patterns of the deposited films heat-treated at various temperatures for 1 h. a, As-deposited film; b-g, films heat- treated at 100, 200, 300, 400, 500 and 600°C, respectively. 0,Au; 0, TiO, (anatase). Concentration of (NH4),TiF,: 0.1 mol dmP3; of H3BO3: 0.2 mol dm-3 and of HAuCl,: 0.29 mmol dm-3. Reaction time: 20 h. 1880 J. Muter. Chern., 1996, 6(12), 1879-1882 microcrystals occurred in the temperature range 100-200 "C.The full-width at half-maximum of the diffraction peaks of Au( 1 11) decreased as the heat-treatment temperature increased, indicating that the Au metal crystals aggregated and grew in crystallite size during the heat treatment. The crystallite sizes calculated using the Scherrer equation7 for the diffraction line from Au( 111) are summarized in Table 1. For the films heat-treated above 400 "C, diffraction peaks assigned to anatase as the matrix phase of the film were also observed. This indicates that the oxide phase as the matrix was transformed from amorphous to crystalline anatase by the heat-treatment simultaneously with the formation and aggregation of the Au metal particles. A TEM photograph of the deposited film after heat treat- ment at 400°C is shown in Fig.3. In order to study them with the TEM, gold-dispersed thin films were removed from the substrates by exposure to hydrogen fluoride (HF) vapour. The Au metal particles are spherical and uniformly dispersed in the film. Particle-size distributions of the dispersed Au metal for the films heat-treated at 400, 500 and 600°C are shown in Fig. 4. 300 Particles were counted to obtain the distributions shown. The size distributions became broad as the heat- treatment temperature increased. The mean particle sizes of dispersed Au metal are summarized in Table 1. The optical absorption spectra of the deposited films after heat-treatment for 1 h are shown in Fig. 5. The absorption spectrum of the as-deposited film showed no absorption bands except for that below 400 nm due to the interband transition of the TiO, matrix.The absorption bands due to the surface plasmon resonance of the Au metal fine particles were observed for the deposited films heat-treated above 200 "C. The plasmon band increased in intensity and shifted toward longer wave- lengths from 555 to 608 nm, with increasing heat-treatment temperature. The change of the absorption peak position of the deposited films heat-treated at various temperatures is Table 1 Crystallite and mean particle sizes of the dispersed Au metal particles in the films heat-treated at various temperatures heat-treatment crystallite mean temperature/"C size/nm particle size/nm 200 8.5 300 11.5 - 400 12.4 14.6 500 14.0 17.4 600 14.0 15.6 Fig. 3 TEM photograph of the deposited film heat-treated at 400 "C for 1 h. Concentration of (NH,),TiF,: 0.1 mol dm-3; of H3B03: 0.2 mol dm-3 and of HAuC1,: 0.29 mmol dmP3.Reaction time: 20 h. t 620I30 0 10 20 30 40 2\ 20 c a,;10-L 0 0 10 20 30 40 30t I 0 10 20 30 40 diameterhm Fig. 4 Size distributions of dispersed Au metal particles in the deposited films heat-treated at 400 (a), 500 (b) and 600°C (c). Concentration of (NH,),TiF,: 0.1 mol drn-,; of H3B03: 0.2 mol dm-3 and of HAuC1,: 0.29 mmol dm-3. Reaction time: 20 h. ----a I 400 500 600 700 800 wavelengthhm Fig. 5 Optical absorption spectra of the deposited films heat-treated at various temperatures for 1 h.a, As-deposited film, b-g, films heat- treated at 100, 200, 300, 400, 500 and 600 "C, respectively. Concentration of (NH,),TiF,: 0.1 mol dm-3; of H3B03:0.2 rnol dm-3 and of HAuCl,: 0.29 mmol drn-,. Reaction time: 20 h. shown in Fig. 6. The peak wavelength shifted slightly to longer wavelengths up to 300 "C, then shifted markedly with increas- ing heat- trea tmen t temperature. For other gold-dispersed glasses, similar red shifts have been rep~rted.'.~'~ The plasmon band is affected by the dispersed metal particle size, the relative permittivity of the surrounding matrix and the aggregation of the metal particles.'-13 For colloidal gold particles in water, Bloemer et reported that the peak wavelength of the surface plasmon resonance shifted ca.12.5nm toward longer wavelengths with increasing particle size from 5 to 30 nm. In our samples, the peak wavelength shifted by ca. 55 nm towards longer wavelengths with increasing heat-treatment tempera- ture, although the mean particle sizes of the dispersed gold are almost constant around 15 nm for the films heat-treated above 400°C (Table 1). The absorption coefficient, ct, of the plasmon 6oo' 1 s0c 580' 3 Y3 560' a c ~nL2-L-uJ4U0 100 200 300 400 500 600 heat-treatment temperature/"C Fig. 6 Relationship between the peak wavelength of the surface plas- mon band of dispersed Au metal particles in the film and the heat- treatment temperature of the film.Concentration of (NH,),TiF6: 0.1 mol dm-3; of H,BO,: 0.2 rnol dmP3 and of HAuCI,: 0.29 mmol dmP3. Reaction time: 20 h. band of gold-dispersed glass is expressed as follows:2 w cc=p -lfJ2Ern/' (3)nc (4) Where &,(a) =E,' +i~," is the relative permittivity of the gold particles, &d is the relative permittivity of the matrix, p is the volume fraction of gold particles, co is the wavelength, n is the refractive index of the matrix, c is the velocity of light andf, is the local field factor. The maximum absorption is given by The peak wavelength of the plasmon resonance depends on the relative permittivity of the dispersed gold particles, which depends on the particle size,, and that of the matrix. As shown in Fig. 6, the peak wavelength shifted markedly towards longer wavelengths for the films heat-treated above 300°C.From the XRD patterns of the films (Fig. 2), the TiO, as matrix phase was transformed from amorphous to crystalline by heat-treatment above 300°C. Based on these results, we conclude that the shift of the plasmon band toward longer wavelengths by the heat-treatment was caused by the change in relative permittivity of the matrix due to the crystallization of TiO,. Conclusion We have developed a very simple process for the preparation of Au-dispersed TiO, thin films. The TiO, thin film containing Au"' ions was formed by the LPD method from a mixed solution of (NH,),TiF,, H3B03 and HAuC1, at ambient temperature and atmosphere. The LPD method is milder and more cost-effective than the conventional methods such as r.f.sputtering or ion-implantation, because it does not require special equipment. The Au content in the film was controlled easily by controlling the concentration of HAuCl, in the treatment solution. Heat-treatment of the deposited film above 200 "C produced Au metal particles which were ca. 15 nm in diameter. The size distribution of the dispersed Au particles became broad as the heat-treatment temperature increased. Crystallization of TiO, as the matrix occurred simultaneously with formation of Au metal particles during the heat treatment. Surface plasmon resonance bands were observed for the films heat-treated above 200°C. The peak wavelength shifted by ca. 55 nm towards longer wavelengths following the heat treat- ment.The shift of the peak position is caused by the change of the relative permittivity of the TiO, as the matrix crystallized. J. Muter. Chem., 1996, 6(12), 1879-1882 1881 The authors wish to thank Dr Keisuke Oguro and Ms Yasuko Ehara of Hydrogen Energy Section of Osaka National Research Institute, AIST for XPS measurements 4 5 H Shinojima, J Yumoto and S Uesugi, Appl Phys Lett, 1992, 60,298 B L Justus, M E Seaver, J A Ruller and A J Campillo, Appl Phys Lett, 1990,57,1381 6 S Deki, Y Aoi, 0 Hiroi and A Kajinami, Chem Lett, 1996,433 7 B E Warren, in X-Raj Difractron, Dover Publications, New References 8 York, 1969, p 253 J J Pireaux, M Liehr, P A Thiry, J P Delrue and R Caudano, T Kineri, M Mori, K Kadono, T Sakaguchi, M Miya, H Wakabayashi and T Tsuchiya, J Ceram Soc Jpn, 1993, 101, 1340 9 10 Surf Scr ,1984,141,221 U Kreibig and L Ganzel, Surf Scr , 1985, 156,678 U Kreibig, J Phys Coll C, 1977, 2,97 K Fukumi, A Chayahara, K Kadono, T Sakaguchi, Y Horino, M Miya, K Fujii, J Hayakawa and M Satou, J Appl Phys , 1994, 75,3075 J Matsuoka, R Mizutani, S Kaneko, H Nasu, K Kamiya, 11 12 13 R H Doremus, J Chem Phys, 1964,40,2389 M J Bloemer, J W Haus and P R Ashley, J Opt Soc Am B, 1990,7,790 G W Arnold, J Appl Phys, 1975,46,4466 K Kadono, T Sakaguchi and M Miya, J Ceram Soc Jpn , 1993, 101,53 Paper 6/04806D, Received 9th July, 1996 1882 J Muter Chem, 1996, 6(12), 1879-1882