Effect of Pt Concentration on the Grain Growth of TiO 2 sol-gel Films

Pt metallic has been supported on TiO2 surface using different methods, Here, Pt doped TiO2 (Pt-TiO2) sol gel thin film were successfully produced by reducing chloroplatinc acid (H2PtCl6). The structures of prepared composites were investigated using X-ray diffraction (XRD). The physical morphologies of the composites were examined using transmission electron microscope (TEM). The grain size of Pt-TiO2 thin film after annealing was also measured by atomic force microscope (AFM) images.


Introduction
Recently, extensive research for titanium dioxide (TiO2) has been carried out in efforts to develop variety of application fields including capacitors for photovoltaic cells [1], sensors [2], antireflection films [3], white pigments [4], optical coatings [5], and high density dynamic random access memory devices [6].TiO2 have three different crystal phases, such as anatase, rutile, and brookite [7].Rutile is the most common natural form of TiO2.Brookite phase is crystallized in the orthorhombic system and exhibits no photocatalytic activity.Anatase TiO2 has excellent photocatalytic activity, physical and chemical stabilities and antimicrobial activity [8][9][10][11].
Earlier studies have revealed that the photocatalytic activity of TiO2 can be improved significantly by doping with noble metals such as Pt, Au, Ag, etc [12,13].It is well known that platinization of TiO2 often shows a high photocatalytic activity.The doping of platinum on TiO2 can form the Schottky diode barrier among the metalsand the electronic potential barrier at the metal-semiconductor heterojunction, and the platinized TiO2 traps the photogenerated electrons efficiently [14].The doping of Pt on TiO2 surface has been widely reported to improve photocatalytic performance for the split of water and the degradation of different harmful compounds [15].
In this paper, we report characteristic properties of TiO2 and Pt composite including UV-visible spectra, transmission electron microscope (TEM) images, x-ray diffraction (XRD) patterns and atomic force microscope (AFM) images.

Experimental details
Titanium isopropoxide was used as a sol-gel precursor.The metal alkoxide was mixed with ethanol.The resulting solution became a milk color, and white precipitation was observed.Hydrochloric acid was slowly added to the solution with vigorous stirring until the solution become transparent.Various amounts of chloroplatinc acid (H2PtCl6) (8wt.% in water) was added to the TiO2 sol-gel solution.Sodium borohydride (NaBH4) was dispersed in ethanol and slowly added into the H2PtCl6/TiO2 solution to reduce [PtCl6]2-to Pt0.The aged sol was spin coated onto the glass and silicon substrateby spin-coated at 2000 rpm and heated at 200 qC for 5 min.. UV-visible spectra were obtained with diode array spectrophotometer (Hewlett-Packard 8452 A).The sol-gel was diluted to ethanol and drop to the copper TEM grid.Using this copper grid sample, TEM images were obtained with CM200 (Phylips) microscope.The TiO2 thin films coated onto silicon wafer were annealed in agas environment tube furnace (EM Tech).The annealing process started from room temperature to 750qC in steps of 5 qC/min and then maintained the temperature for 1 h.The films naturally cooled down to room temperature.The TiO2 thin films doped with Pt were subject to XRD analysis with X'Pert MPD Pro diffractometer (Phylips).The surface of the film also analyzed with NanoScope 3D (Veeco) microscope.The UV-visible spectra show the possible uniform distribution of the Pt-nanoparticles throughout the sol-gel films.To obtain the particle size and distribution, the TEM was employed.Figure 2   Figure 3 (a) shows XRD patterns for Pt-TiO2films prepared by sol-gel method with various amount of Pt.All these samples show typical TiO2diffraction peaks in form of anatase.It shows almost coincide with a pure TiO2 diffraction peaks and shows no diffraction peaks due to the doped Pt.It can be assumed that the amount of Pt doped particle was very low, which resulted in non-appearance of the Pt crystalline peaks [17,18].However, diffraction intensity increased with the increase of Pt concentration.The crystalline size of the Pt-TiO2 samples after annealing were estimated from line broadening using Scherrer equation based on (101) peak of anatase TiO2 [19].where λ is the X-ray wavelength of Copper Kα radiation, θ is Bragg's angle and β is the pure full width of the diffraction line at half of the maximum intensity (Figure 3(b)).The calculated grain sizes are 15 nm (0.01%), 20 nm (0.05%), 35 nm (0.08%) and 40 nm (0.1%), separately.The result refers to that doping Pt also retard the grain growth of TiO2 thin film.
AFM images of the Pt-TiO2 surface were used to monitor the grain size of the sol-gel thin film depends on Pt concentration. 1 μm × 1 μm scans revealed it is clear that the grain size of Pt-TiO2 gradually increased with Pt loading.

Conclusions
Metallic doping is one factor for TiO2anatase phase grain growth.The intensity of the main anatase diffraction peak increased with increasing Pt concentration.The grain sizes estimated from anatase XRD major diffraction peaks were 15, 20, 35 and 40 nm for the sample with Pt concentration of 0.01%, 0.05%, 0.085 and 0.1%, respectively.The measured grain sizes using AFM images also increased depends on Pt concentration.The governing factor to determine the grain size of the TiO2 thin films is Pt doping.
(a)  shows the TEM image for the 0.1 % of Pt-TiO2film with scale bar size of 100.The aggregated area was further focused and obtained the image with scale bar size of 20 nm as shown in Figure2 (b).Pt-nanoparticles do not have big aggregates, distribute relatively uniformly, and distribute particle sizes between 4-8 nm.