DOI: 10.1007/s00339-007-4232-1 Appl. Phys. A 89, 987–993 (2007) Materials Science & Processing Applied Physics A s. mahalingam m.j. edirisinghe ✉ Characteristics of electrohydrodynamically prepared titanium dioxide films Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received: 8 December 2006/Accepted: 17 July 2007 Published online: 28 August 2007 • © Springer-Verlag 2007 ABSTRACT A titanium dioxide precursor sol flowing through a needle at a flow rate of 10 −10 m 3 s −1 was subjected to an electric field of 4.5 kV to generate droplets in the size range 0.3–6 μ m. The droplets were collected on a silicon substrate to form uniformly thick, dense films. Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy and UV/Vis spectroscopy were used to characterize as-deposited and annealed films. Raman spectra show the annealed films were anatase phase with annealing converting it to the rutile phase. The energy bandgap of the titanium dioxide film an- nealed to 500 ◦ C shows an indirect bandgap energy of 3.50 eV and a direct bandgap energy of 3.95 eV. PACS 81.15.Rs; 81.07.-b; 78.20.-e; 78.30.-j; 78.67.-n; 78.70.ck 1 Introduction Titanium dioxide (TiO 2 ) has been investigated as a transition metal oxide for both industrial applications and scientific research in the last few years [1–3]. Due to their unique dielectric and optical properties, TiO 2 films are used in memory cell capacitors [4], anti-reflection coating, optical interference filtering and optical wave guides [5–7]. TiO 2 is a well known material for having noticeable photocatalytic properties [8]; it has been studied for antifouling, antibacterial coating and antifogging coating applications [9]. More impor- tantly it has found application in dye-sensitised solar cells [10] to increase the photoconversion efficiencies. Due to its high permittivity, and high physical and chemical stability in mod- ern CMOS fabrication, TiO 2 is largely investigated for the replacement of SiO 2 in DRAMS and MEMS [11]. TiO 2 shows three distinct phases namely rutile, anatase and brookite with rutile and anatase being utilised for practi- cal applications. The rutile phase is thermodynamically stable at all temperatures whereas the anatase phase is metastable. There are various techniques applied to produce TiO 2 thin films such as electron beam evaporation [12], pulsed laser de- position (PLD) [13], chemical vapour deposition (CVD) [14], sputtering techniques [15, 16], sol–gel techniques [17, 18], and atomic layer deposition [19]. ✉ Fax: 00442073880180, E-mail m.edirisinghe@ucl.ac.uk In this paper we are reporting on electric field assisted de- position of TiO 2 films. This route uses an electric field to gen- erate a jet, which later breaks down to droplets, hence leading to a spray, which is a good source of material to prepare films on a substrate. Electrohydrodynamic jetting and droplet gen- eration was first explored by Zeleny [20, 21] and increased in technological importance over the last two decades. There are several different modes of electrohydrodynamic atomi- sation, namely micro-dripping, spindle, stable cone-jet, and multi-jet together with their numerous manifestations. Stable cone-jet [22–26] is considered to be more desirable and is fre- quently studied as it generates near mono-dispersed droplets of a few micrometers in size and in this way the droplet size can be controlled by varying the flow rate, applied voltage and the physical properties of the electrosprayed liquid. The electrohydrodynamic route has been exploited for producing powders [27], films [28, 29], scaffolds [30] and bio- ceramics [31]. Compared with other atomisation techniques, it offers easy generation of droplets, avoids coalescence of droplets due to electric charge of the same polarity in the droplets and achieves a narrow size distribution of droplets via the stable cone-jet mode [32]. 2 Experimental details 2.1 Sol preparation and characterisation Titanium (IV) isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 ) supplied by Sigma-Aldrich, was used as a precursor in our ex- periments. The precursor sol was prepared by transferring 2% volume of precursor to an air tight bottle containing 99.7% ethanol (VWR International), and the resulting solution was stirred using a magnetic stirrer for 3h at ambient conditions. The important parameters for achieving the stable cone-jet mode of electrohydrodynamic atomisation are density, sur- face tension, viscosity, electrical conductivity and relative permittivity [22, 33–36] of the solution, and these physical properties were measured. All the equipment used for charac- terisation was calibrated against reference data. Experiments were performed at ambient temperature. Electrical conductivity was measured using a HI-8733 (Hanna Instrument) conductivity probe. Surface tension was measured using a Kruss Tensiometer K9 (Wilhelmy’s plate method). Around 200 ml of the sol was used and five consecu- tive surface tension readings were taken to obtain a value. The