The mechanism of formation of platinum nanoparticles via chemical reduction of Na2PtCl4 in aqueous solution was investigated by UV-Visible Spectroscopy and Transmission Electron Microscopy. Sodium borohydride (NaBH4) was used as the reducing agent, and tri-Sodium Citrate was used for stabilising the nanoparticles. It is possible to monitor various stages of the reduction process on an observable time scale when the Pt(II) solution is suitably aged. Under appropriate experimental conditions, the theoretically predicted plasmon resonance absorption peak from the well-dispersed Pt(s) nanoparticles is observed at 215nm for the Pt suspensions in citrate medium. It is found that an increased concentration of citrate stabilizer decreases the reaction rate, although there is only a narrow concentration range of stabilizer which produces a stable suspension with well-separated Pt(s) nanoparticles. This conclusion was also supported by the TEM observation of the nanoparticles, which had a very narrow size distribution (between 2 to 6nm).
The sol-gel spin-coating technique is increasingly being used for the fabrication of thin-films with diverse applications in areas that include advanced optics, microelectronics and sensors. The performance quality of the thin film devices is affected by the thickness and homogenity of the film, which are in turn controlled by parameters such as substrate surface quality, viscoelastic properties of the sol, and the spin coating parameters. Processing high quality thin-films is simpler when the desired thickness can be achieved by a single-step coating of sol-gel film. We report on the fabrication of TiO2 thin films by single-step spin-coating of a well characterised sol on a single crystal Si substrate. By monitoring the viscoelasticity of a titanium alkoxide sol, the thickness of the coated film can be controlled and with proper calibration, the viscoelasticity of the sol can be used as a parameter to fabricate a quality film with desired thickness. The thickness of the spin coated sol-gel films and that of the processed TiO2 films was evaluated using SEM and ellipsometry. The optical interference phenomenon of Newton's colours, which are obtained for the thin transparent films on reflecting surfaces, can be used to estimate the thickness of the TiO2 thin-films. This presentation provides details on how this concept can be applied to estimate the thickness of the TiO2 thin-films in the range of 50 to 200nm. The value determined using this method was comparable to that obtained through ellipsometric and SEM measurements.
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