Electron Spin Resonance (Esr) Study of VO 2+ Doped Germanium Dioxide Synthesized Via the Sol-Gel Process
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ABSTRACT VOSO 4 .3H 2 0 was used as doping agent to prepare GeO2 xerogels, via the sol-gel process. Samples were analyzed by Thermal Gravimetric Analysis (TGA), Differential Thermal Analysis, (DTA), Diffuse reflectance (UV-Vis) spectroscopy and electron spin resonance (ESR). Study of thermally treated samples was done. VO2+ were found in V20 5 at 350 °C. VO2+ ions were incorporated to the GeO2 network after thermal treatment at 700 OC. From the results we propose that VO2+ acts as crystal former in these samples.
INTRODUCTION In recent years a growing interest on the study of GeO 2 glass matrices has been noticed (1]. This research field is connected with the development of new materials for optics and fiber optics. The study of glass-former activator and the crystal activator composition is important to elucidate the structure and bonding of vitreous and crystalline systems. Electron spin resonance (ESR) technique have been used in the present study to obtain information about the changes in the GeO 2 network. VOSO 4 .3H 2 0 was employed in this research as VO2+ source to dope GeO2 glasses. Previous studies have been done in the V205-GeO2 system [2,3]. However, in those cases remarkable interest was confined to the synthesis of amorphous vanadium oxide thin films, with GeO 2 employed as a network former. The goal of this study is to explain the structural an ESR study.
changes of VO2+ doped GeO2 systems, mainly through
EXPERIMENTAL GeO2__qass 3
7.9x10- mol of Ge(OEt) 4 (OEt: OCH 2 CH3 ) (Pfaltz & Bauer) were dissolved in EtOH (Baker, 99.9%) to prepare a solution with a molar ratio nGe/nEtOH = 1/6. The solution was stirred for 15 min. An alcoholic solution with the molar composition nH2 0/nEtOH = 1/3 was added by dropping in a one-hour period. The obtained gel was stirred for one hour. The final molar ratio nGe/nH2 0 was 1/2.
GeO2 doped with VO 6.2 Following the aforementioned procedure, four solutions were prepared using VOSO4 .3H 2 0 (Aldrich, 99.99%) as doping agent. Final concentrations of VO2+ were 0.5, 1.0, 1.5 and 2.0 wt. %. The samples were labeled VO(0.5), VO(I.0), VO(I.5) and VO(2.0), respectively. Light blue color samples were obtained. Gels were dried at 100 0C in an oven for 12 h. The samples were calcined at a) 350 0C and b) 700 0C for a 12 h. period in air. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were done in N2 atmosphere from 25 0C to 1000 0C with a Shimadzu DT-30 equipment. Diffuse reflectance results were obtained in a Varian Cary 1 spectrophotometer from 12500 cm- to 50000 cm-, using MgO as reference. ESR study was performed in a Bruker 200-D SRC-A spectrometer. This analysis included the temperatures 77 °K 0 and 298 K. All the results presented here correspond to 77 °K. 243 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
RESULTS TGA and DTA cases, TGA of the xerogels not shown here exhibit a single slight In all weight loss step. The corresponding value for pure GeO 2 is 4.0 wt. % in the temperature range studied
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