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Scientific paper
Photocatalytic Deposition of Gold on Nanocrystalline TiO2 Films
Ivanka Ruvarac-Bugar~i},1 Ivana Jankovi},1 Zorica Konstantinovi},2 Zoran Šaponji}1 and Jovan Nedeljkovi}1*
1Vin~a Institute of Nuclear Sciences, P. O. Box 522, 11001 Belgrade, Serbia
2Departament de Física Fonamental and Institut de Nanociència i Nanotecnologia, Universitat de Barcelona, Martíi Franquès 1, 08028 Barcelona, Spain
* Corresponding author: E-mail: jovned@vin.bg.ac.yu
Received: 09-07-2007
Abstract
Nanocrystalline TiO2 films were prepared on glass supports by the dip coating technique using colloidal solutions consisting of 45 Å TiO2 particles as a precursor in order to probe the feasibility of exploiting solid state photocatalytic reactions for efficient deposition of gold. The reduction of Au3+ ions was maintained by photoirradiation of unmodified Ti-O2 films with metal ions being attached directly to the surface of TiO2. Absorption spectrum as well as atomic force microscopy measurements revealed presence of gold grains in the nanosize regime. Conductivity of deposited gold was measured in the temperature range from 45 to 295 K.
Keywords: TiO2 films, gold, photocatalytic deposition, conductivity
1. Introduction
Photocatalytic reactions on TiO2 surfaces are very important in environmental cleanup and remediation, such as oxidation of organic materials1–3 and reduction of heavy metal ions4–8 in industrial waste streams. We have recently shown that nanocrystalline TiO2 material differs from the bulk material, having larger surface area and unique surface chemistry due to distortions in surface Ti sites from the octahedral geometry to penta-coordinate square pyramid with one double Ti–O bond.9–11 Photoirradiation of TiO2 nanoparticles, with photon energy larger than the band gap (3.2 eV), creates electron – hole pairs. Following irradiation, the TiO2 na-noparticles act as either electron or hole donors to reduce or oxidize materials in the surrounding media. However, the photo-induced charge separation in bare TiO2 nanoparticles has a very short lifetime due to charge recombination. Therefore, it is important to prevent electron – hole recombination before a designated redox reaction occurs.
Recently, the performance improvement of TiO2 nanocrystals for photocatalytic reduction of heavy metal
ions was achieved by using multifunctional ligands, which bind both particle surface and heavy metal ions.6,12 In these studies it was shown that surface modification of 45 Å TiO2 particles with bidentate and tridentate ligands that have mercapto, carboxyl and amino groups in different positions results in increased adsorption rate of metal ions compared to bare particles, enlarged initial charge separation distances, and overall enhanced photoreduction chemistry. In this work we report efficient reduction of Au3+ ions to its metallic form on the surface of dry unmodified nanocrystalline TiO2 film. Atomic force microscopy (AFM) and absorption spec-troscopy were used to confirm the presence of gold grains in the nanosize regime. Conductivity of photoca-talytically deposited gold was measured in the temperature range from 45 to 295 K.
2. Experimental
2. 1. Synthesis of 45 Å TiO2 Colloids
The TiO2 colloids with mean particle diameter of 45 Å were prepared by controlled hydrolysis of titanium (IV)
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chloride, as described elsewhere.13 The concentration of TiO2 (0.1–0.6 M) was determined from the concentration of the peroxide complex obtained after dissolving the colloid in 1.44 M H2SO4.14
2. 2. Preparation of Nanocrystalline TiO2 Films
Nanocrystalline TiO2 films were prepared on glass supports by the dip coating technique using titania colloidal solutions consisting of 45 Å TiO2 particles with anata-se crystalline structure, as precursor. After dipping the glass slides in 0.12 M TiO2 colloidal solution, the samples were dried at elevated temperature (150 °C) for 30 min. The nanocrystalline TiO2 films having different thickness (d = 240–560 nm) were prepared repeating the above-mentioned procedure different number of times (typically, the number of depositions varied from 3 to 7).
The nanocrystalline TiO2 films were immersed in a solution of 0.013 M AuCl3 in 0.007 M HCl (pH ? 2.5–3) and stored in a dark for a week. As-prepared samples were rinsed with distilled water and dried in a stream of argon.
2. 3. Apparatus
A photochemical apparatus UV 17 F Helios Ital-quartz that consists of the low pressure Hg lamp, ATE 215 M, placed into quartz shielding was employed for steady-state illumination. UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 35 spectrophotometer. The atomic force microscopy (AFM) studies were performed by using Digital Instruments Dimension 3100 in the tapping mode. The ac electrical measurements were carried out by a four probe technique using a lock-in amplifier and an ac constant current source in a frequency range of 10–6000 Hz. The dc resistance was measured at very low frequencies (57 Hz) in a closed circuit helium cryostat, varying the sample temperature between 45 K and room temperature. Average composition of the sample prepared for transport measurements was determined by microprobe analyses.
this approximation, which is applicable for weakly absorbing films, extrema occur at wavelengths specified by
m = 1, 2,…
(1)
i. e. the locations of the extrema are uniquely determined by nd. If m’ is the order of an extremum from an arbitrary reference extremum (of order m0), then m = m’ + m0 and
i  I
vb¥-
'Und J
(2)
Hence, for a constant value of nd, a plot of 1/?ext vs. m’ will yield a straight line with a slope 1/4nd and an intercept m0/4nd.
Figure 1: Transmission spectra of nanocrystalline TiO2 films as a function of number of depositions: (a) 3, (b) 4, (c) 5, (d) 6 and (e) 7
3. Results and Discussion
The thickness of the nanocrystalline TiO2 films on the glass slides as a function of number of depositions was followed from the transmission spectra measured in the spectral range of 350–1000 nm (Figure 1). The method proposed by Babu et al.15 was used to calculate thickness of TiO2 films from the position of interference fringes in transmission spectra. Briefly, locations of the extrema in the transmission curve are uniquely determined in a first approximation by specifying the product nd, where n is refractive index and d is the thickness of the thin film. In
Knowing the value of the refractive index of anata-se TiO2 particles (n = 2.524), the thickness of nanocry-stalline film was estimated to increase for about 80 nm per each deposition. It should be pointed out that the dip coating technique led to the formation of nanocrystalline TiO2 films having a fairly uniform thickness across the film.
The illumination of unmodified TiO2 films where gold ions (Au3+) are directly linked to the TiO2 surface, leads to the formation of metallic form of gold in nanosi-ze regime16,17 as confirmed with the appearance of surface plasmon resonance band at 527 nm in absorption spectrum. The absorption spectrum of photocatalytically de-
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Acta Chim. Slov. 2008, 55, 268–272
Figure 2: Absorption spectra of photocatalytically deposited metallic gold on the surface of nanocrystalline TiO2 film.
posited metallic Au on unmodified nanocrystalline TiO2 film obtained upon illumination is presented in Figure 2.
The atomic force microscopy image (Figure 3) shows that a homogeneous gold thin film consisting of well resolved grains is successfully assembled.
Recently, it has been shown that the addition of multifunctional ligands (cysteine, alanine, arginine) leads to the surface modification of TiO2 colloid particles enhancing their photocatalytic reduction properties. The increase of metal ions adsorption on the surface of modified TiO2 particles is the consequence of the replacement of TiO2 surface OH groups upon coordination of surface Ti atoms with carboxylic groups.6, 12 In order to extend the investigations of photocatalytic ability of dry nano-crystalline surface modified TiO2 films with alanine or arginine, we followed the three electron-transfer reduction process of Au3+ ions. Contrary to copper or silver12
ions which are almost instantaneously reduced to corresponding metals upon illumination of dried modified samples, in the case of gold ions the formation of its metallic form was not noticed.
According to our experimental conditions, before illumination TiO2 film was immersed in acidic solution (0.007M HCl) of AuCl3 in order to prevent undesired hydrolysis of the salt. Hence, at pH -2.5-3 hydroxyl groups on the surface of TiO2 nanocrystalline film are pro-tonized while Au3+ is coordinated with Cl– ions in the form of the halogeno complex [AuCl4]–. Electrostatic interaction between protonized surface hydroxyl groups on TiO2 and [AuCl4J– ions could be the explanation for the efficient adsorption and consequent photoreduction of gold ions on the surface of unmodified TiO2 nanocrystalline film according to redox reaction:
AuCl4– + 3e– = Au0 + 4Cl–       E0 = +1.002 V (NHE)
Since the electrochemical potential for the reduction of gold ions is significantly more positive than the position of conduction band of TiO2, photocatalytic reduction is thermodynamically favorable even upon changing E0 (AuCl4–/Au0) due to interaction with TiO2 surface. The halogeno complex form of gold ions might be the explanation for unsuccessful reduction of Au3+ on the surface of TiO2 modified with alanine or arginine. In acidic medium (pH -2.5-3) carboxylate groups of amino acids are negatively charged disabling the approach of halogeno complex form of gold to the surface modified TiO2 nanocrystals.
With the aim to check the conductivity of photoca-talytically deposited layer of gold on the surface of nanocrystalline TiO2 film we measured the resistivity as a function of temperature. In Figure 4 is presented R vs. T
Figure 3: Typical AFM image of gold grains on the surface of nanocrystalline TiO2 film
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Figure 4: Temperature dependence of the resistivity of Au–TiO2 film on glass support. Insets: Modulus of the impedance |Z| (left) and voltage phase drop ? (right) vs. frequency
dependence obtained for Au–TiO2 film which contains 20 vol. % of Au0 (or 55.19 wt. %) determined by microprobe analyses. Very slight changes of the resistivity with temperature indicate that the sample is near the percolation threshold. The modulus of the impedance |Z| decreases with frequency at all temperatures indicating the presence of capacitive conduction (see T = 45 K and T = 295 K in left inset of Fig. 4). This is even more clear from the huge frequency dependence of the voltage phase drop ? (right inset of Fig. 4).18 At frequencies close to dc limit, ? is roughly zero, corresponding to the dc current circulation essentially between small closely packed particles. By increasing frequency, curves display a phase drop as the electron current, due to the capacitive effect between particles, becomes more and more important. As the dc current is only slightly dependent on T and the capacitive current between distantly separated particles is T-indepen-dent, the phase drop appears to be essentially the same for all temperatures (see T = 45 K and T = 295 K in right inset of Fig. 4). The negative value of the temperature coefficient of the resistivity indicates the sample containing nonelectrically connected nanoparticles i. e. samples situated in the dielectric region.19
4. Conclusions
Layer of gold grains photocatalytically deposited on the surface of nanocrystalline TiO2 film was prepared. It was found that the gold ions directly attached on the surface of unmodified TiO2 nanocrystalline film are reduced via multielectron transfer process from TiO2 conduction band. The position of the surface plasmon absorption band reveals presence of gold grains in the nanosize regime. Conductivity measurements show that sample con-
tains nonelectrically connected metallic particles near percolation threshold.
5. Acknowledgments
Financial support for this study was granted by the Ministry of Science and Environmental Protection of the Republic of Serbia (Project 142066). The authors gratefully acknowledge to Prof. Günes and Prof. Sariciftci for helpful discussion and AFM measurements.
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Povzetek
Preverjali smo mo`nost izkori{~anja trdnih fotokatalizatorjev za u~inkovito depozicijo zlata. Nanokristalini~ne tanke sloje TiO2 smo pripravili na steklenih nosilcih s tehniko pomakanja v koloidno raztopino delcev TiO2 velikosti 45 Å, kot prekurzorjev. Redukcijo Au3+ smo dosegli z obsevanjem tankih slojev TiO2 z adsorbiranim zlatom na povr{ini. Meritve absorpcijskih spektrov in mikroskopija na atomsko silo so potrdile prisotnost nanodelcev zlata. Elektri~no prevodnost zlata, nane{enega po opisanem postopku, smo izmerili v obmo~ju 45 do 295 K.
Ruvarac-Bugar~i} et al.:  Photocatalytic Deposition of Gold ...