Characterization of Titanium Dioxide thin films produced by RF plasma sputtering technique for photocatalytic applications

In the present work Titanium Dioxide thin films were successfully prepared both on FTO and (1 0 0) Silicon substrates by means of radiofrequency plasma sputtering technique. AFM, XRD, SEM and contact angle measurements were performed in order to characterize the samples. XRD measurements revealed a crystalline structure for all the samples, with a mean globular size in the range of 20 30 nm. All samples showed a reversible hydrophilic behavior due to UV irradiation, that can be correlated with their photocatalytic properties. Introduction Search of clean fuels is a primary objective in research activity devoted to the development of new technologies and innovative materials for energy applications. Among the clean fuels, hydrogen is a promising candidate, because it is energy-efficient, environmentally friendly and abundant in nature. Looking for a new way to produce it, not depending on steam reforming process and fossil fuels, photocatalytic water splitting seems to be a good alternative. This process uses solar energy to decompose water into hydrogen and oxygen in the presence of a photocatalyst. A photocatalyst is a semiconductor which absorbs solar energy and creates an electron hole pair. These charge carriers migrate on the semiconductor surface without recombination and promote redox reactions in the water. The net effect is the hydrogen production with a clean process. TiO2 is a well-known semiconductor intensively investigated in the fields of photocatalysis since 1970s [1], because of its high photo-oxidation, photostability, low cost and non-toxicity [4, 5]. However, because of its wide band gap (≈ 3,2 eV) [4], it shows a weak photoresponse to the visible light region, as well as a high electron-hole pair recombination rate greatly reducing the quantum efficiency. The method most often used to synthetize titanium dioxide thin films is a sol-gel technique [6, 7, 8], but other ways have been demonstrated to be interesting: reactive magnetron sputtering [9], ion beam sputtering [10], chemical vapor deposition [11]. Lately, a different method has been investigated [12, 13, 14]: plasma enhanced chemical vapor deposition technique. The principal feature of this method lies in the effective use of radio frequency induced plasma to enhance and catalyze chemical reactions during the deposition process. On the basis of the discussion above, we report a first experimental approach of characterization of thin films of TiO2 deposited with RF plasma sputtering, in order to identify experimental parameters able to produced thin films with relevant photocatalytic properties. Experimental Setup Two experimental campaigns were carried out. Thin films of TiO2 were prepared by means of RF (13,56 MHz) plasma sputtering, both on (1 0 0) Silicon and FTO substrates. Substrates were carefully placed on the grounded substrate holder under the RF powered electrode. The vacuum before deposition was less than 1 x 10 mbar and the substrate temperature was monitored by using a thermocouple placed in contact with the sample. The sample holder had the possibility to perform deposition both at TRoom and with the substrate heated by a heating system: temperature achieved in this way was in the range 340-360 °C. Argon (99.999%) and Oxygen (99.999%) gases were introduced into the chamber through a mass flow controller with different percentages: Argon plasma in a reactive Oxygen environment promotes sputtering of Titanium target. Different film thicknesses were achieved, depending on Argon and Oxygen quantities and on target-substrate distance. Working pressure was fixed at 10 mbar and the upper electrode was kept at 1500 V. Subsequently samples have been characterized by means of Atomic Force Microscope (AFM), X-Ray Diffraction (XRD), Contact Angle (CA) Measurement and Scanning Electron Microscopy (SEM). AFM measurements were made in air by a Nano-RTM AFM System (Pacific Nanotechnology, Santa Clara, CA, USA) operating in contact mode. Silicon conical tips of 10 nm radius mounted on silicon cantilevers of 125 m length, 42 N/m force constant and 320 kHz resonance frequency were used. Images were processed and analyzed by means of the NanoRule+TM software provided by Pacific Nanotechnology. The structural properties studied by X-ray diffraction measurements were performed with a wide angle Siemens D-500 diffractometer (WAXD) equipped with a Siemens FK 60-1