Journal of Sol-Gel Science and Technology 17, 247–254 (2000) c  2000 Kluwer Academic Publishers. Manufactured in The Netherlands. Sol-Gel Synthesis of Humidity-Sensitive P 2 O 5 -SiO 2 Amorphous Films M. D’APUZZO Department of Electrical Engineering, University of Naples “Federico II” Via Claudio, 21, 80125 Napoli, Italy A. ARONNE, S. ESPOSITO AND P. PERNICE Department of Materials and Production Engineering, University of Naples “Federico II” Piazzale Tecchio, 80125 Napoli, Italy Received April 21, 1999; Accepted October 28, 1999 Abstract. The sol-gel synthesis of silicophosphate gels using phosphoryl chloride and tetraethoxysilane as molecular precursors is reported and discussed. Gel-derived glasses and films having the molar compositions 10P 2 O 5 · 90SiO 2 and 30P 2 O 5 · 70SiO 2 have been obtained. The structure of the dried gels as well as the structural modifications that occur during the transformations in gel-derived glasses are analyzed by Fourier transform infrared spectroscopy (FTIR). It has been found that the evidence of the P O Si linkages begins to appear only on the FTIR spectra of the bulk gels heat treated up to 400 ◦ C while they are well resolved on the FTIR spectra of the bulk gel samples heated up to 1000 ◦ C indicating that at this temperature the transformation in the corresponding gel-derived glasses occurs. The humidity sensitive properties of the gel-films have been evaluated by electrochemical impedance spectroscopy (EIS). The phosphorous content as well as the temperature of the heat treatments strongly affect the sensitivity to RH of the gel-derived films. Keywords: sol-gel, ceramic thin film, FTIR, humidity sensor 1. Introduction Conventional ceramic humidity sensors are mainly based on porous sintered oxide [1]. Recent progress in ceramic film deposition technologies provides new possibilities for the improvements of ceramic active elements in the field of chemical sensors, necessary for the fabrication of integrated sensors [2]. The sol- gel synthesis is a low cost technology that allows the preparation of high purity ceramics in their final film shape, by dipping or spin-coating, with a homogeneous distributions of components on the atomic scale [3, 4]. Some studies on humidity sensitive ionic-type ceramic oxides in thin film form, prepared by sol-gel method, were recently reported in the literature [5–8]. In these papers different systems were investigated, LiCl doped SiO 2 [5], P 2 O 5 -TiO 2 [6], TiO 2 [7] and alkali doped ti- tania films [8], even if the sensing mechanisms have not been thoroughly explained. Traversa et al. [8] sug- gested that the conductivity of alkali ions was the main contribute to the sensing mechanism for alkali doped ti- tania films. On the contrary, Makita et al. [6] suggested that a protonic sensing mechanism occurs in the P 2 O 5 - TiO 2 thin films [6]. These authors obtained chemically durable glass films with P 2 O 5 content up to 70 mol % (nominal composition) and they found that only the high P 2 O 5 content film was sensitive to the change of the relative humidity ranging from 50 to 90% [6]. This result was interpreted as due to coexistence of molecu- lar water with protons in the glass structure for the high P 2 O 5 content glasses [6]. The chemistry of the sol-gel synthesis of phos- phosilicate glasses has been extensively studied [9– 13]. Livage et al. [9] have shown that phosphate es- ters, PO(OR) 3 , cannot be easily used as molecular precursors of the P 2 O 5 in the sol-gel synthesis of