TiO 2 -CeO 2 Nanometric Powders Prepared by Sol-Gel Method

1. TiO2-CeO2 Nanometric Powders Prepared by Sol-Gel Method M.Zaharescu*, V.S.Teodorescu**, A.Barau*, C.Andronescu*, N.Preda**, F.Papa* (*)Institute of Physical Chemistry "Ilie Murgulescu" - Roumanian Academy 202 Splaiul Independentei, 060021 Bucharest, ROUMANIA (**)National Institute of Material Physics, 105 bis Atomistilor Street, 077125 Bucharest-Măgurele, ROUMANIA Aim of the work:TiO2-CeO2 powders were prepared for different applications such as catalysts, electrochromic devices materials, oxygen sensors. The precursors employed were Ti(iOPr)4 as TiO2 source and Ce(NO3)3.6H2O or (CH3CO2)3Ce.xH2O as CeO2 source. Several parameters were varied, like cerium precursor (cerium nitrate or cerium acetate) and the TiO2:CeO2 molar ratios. For the both cerium sources binary powders with TiO2:CeO2 = 4:1 or 1:1 were prepared. The prepared material was dried overnight in the oven at 100 oC and thermally treated for 1 h at 400oC with a heating rate of 1°C/min in order to eliminate the water and organic residues. The prepared sol-gel powders were structural and morphologic characterized by thermal analysis (DTA/TGA), transmission electron microscopy (TEM) and X-ray diffraction (XRD), IR spectroscopy, FT-Raman spectroscopy, XPS spectroscopy and BET characterization. Experimental The FT-Raman spectra measured on the TiO2:CeO2 samples 80:20, thermally treated at 400 oC h are presented in Fig. 4 • Powder preparation: The TiO2-CeO2 powder was prepared by the sol-gel method at a ratios of TiO2:CeO2 of 4:1 and 1:1 using: • precursors: Ti(iOPr)4 and Ce(NO3)3.6H2O or (CH3CO2)3Ce.xH2O • reaction medium: absolute ethylic alcohol • -catalyst:NH4OH (25%) • molar ratios : [ROH]/[Precursors] = 35.6; [H2O]/[Precursors] = 3 and [NH4OH]/[Precursors] =3 for TiO2 and samples from Ce nitrate; 6 from samples from Ce acetate. • reaction conditions: for all samples the pH value was set to 5 and the slurries were stirred 2 h at 80 oC. • The resulted precipitate was filtered, dried in the oven for 48 h and then annealed 1 h at 400°C with a heating rate of 1°C/min in order to eliminate the water and organic residues. • Powder characterization: • thermal analysis and thermogravimetric analysis using a Mettler Toledo TGA/SDTA 851e equipment, • transmission electron microscopy (TEM) using a JEOL 200 CX microscope and a Topcon 00B instrument, • BET specific surface area using the nitrogen adsorption method at liquid nitrogen temperature, • XPS determinations using a AVG ESCA 3 MkII-EUROSCAN instrument, • FT-Raman using a Bruker RFS 100 equipment, • FT-IR using a FT-Bruker Vertex 70 instrument. The binary powder starting with Ce-nitrate have shown an reaction between the TiO2-CeOx due to the absence of the Raman vibration of the monocomponent oxides. The binary powder starting with Ce-acetate is a mixture of the individual oxides. Results and discussion Fig. 4. The FT-Raman spectra for the TiO2-CeO2 (80:20) samples, thermally treated at 400 oC, from Ce-nitrate a) and from Ce-acetate b) The TEM micrographs for the as prepared ant thermally treated at 400 oC monocomponent and bi-component powders are presented in Figs. 5 - 7 a) b) c) d) Fig. 5. TEM micrographs for the monocomponent powders: TiO2 as prepared (a); TiO2 thermally treated at 400 oC (b); CeO2 from Ce(NO3)3.6H2O as prepared (c) and CeO2 from (CH3CO2)3Ce·xH2O thermally treated at 400 oC (d). Fig. 1. DTA/TG curves for the TiO2:CeO2 (80:20) from Ti(O-iC3H7)4 and Ce(NO3)3.6H2O (a) and Ti(O-iC3H7)4 and (CH3CO2)3Ce·xH2O (b) Sample a: - up to 400 oC - water and organic residues were eliminated - 708 oC the reaction between TiO2 and CeOx with Ce2O3 oxidation Sample b: - up to 500 oC – water and organic residues were eliminated - 500 – 650 oC Ce2O3 oxidation - 700 oC reaction between TiO2 and CeO2 a) b) c) d) Fig 6. TEM micrographs for the binary powders in the TiO2-CeO2 system from Ce(NO3)3.6H2O; a) Ti:Ce = 80:20 as prepared, b) Ti:Ce = 50:50 as prepared, c) Ti:Ce = 80:20 thermally treated at 400 oC and d) Ti:Ce = 50:50 thermally treated at 400 oC We present in Fig. 2 the XPS spectra recorded for the sample prepared with the cerium nitrate and in Fig 3 the XPS spectra recorded for the sample prepared from cerium acetate. c) d) a) b) Fig.2 The XPS spectra for the TiO2:CeO2 (80:20) sample, thermally treated at 400 oC, from Ti(O-iC3H7)4 and Ce(NO3)3.6H2O Fig 7. TEM micrographs for the binary powders in the TiO2-CeO2 system from (CH3CO2)3Ce·xH2Oa) Ti:Ce = 80:20 as prepared, b) Ti:Ce = 50:50 as prepared, c) Ti:Ce = 80:20 thermally treated at 400 oC and d) Ti:Ce = 50:50 thermally treated at 400 oC BET surface area determinations had shown that all the samples have relatively high surface areas, the highest values being recorded for the samples prepared from the cerium acetate precursor (between 67-146 m2/g). The results for the BET measurements are presented in Table 1. Table 1. BET surface area for the as-prepared monocomponent and bicomponent powders Fig.3 The XPS spectra for the TiO2:CeO2 (80:20) sample, thermally treated at 400 oC, from Ti(O-iC3H7)4 and (CH3CO2)3Ce · xH2O The recorded binding energy suggested that that regardless of the Ce precursor employed, in the TiO2:CeO2 samples 80:20, thermally treated at 400 oC h, the titanium is present in the Ti4+ valence state while cerium is present in two valence states Ce4+ and Ce3+, the proportion being more or less the same. Conclusions Monocomponent and bi-component powders from different precurssors were successfully prepared by sol-gel method in the TiO2-CeO2 system. All prepared powders are in nanometric range, the dried material is constituted from amorphous aggregates of particles in nanometric range while the 400 oC thermally treated samples are formed from crystallite of 4 – 5 nm. The FT-Raman results suggested the formation of a Ti-O-Ce type bonds in the case of bi-component thermally treated samples prepared from Ce nitrate. The type of Ce precursor plays and important role in the the formation of the binary oxide powders in the TiO2-CeO2 system. Acknowledgement: "This poster and the work it concerns were generated in the context of the MULTIPROTECT project, funded by the European Community as contract N° NMP3-CT-2005-011783 under the 6th Framework Programme for Research and Technological Development“ and it was co-financed by the Roumanian National Project SUPRACOM under contrac no. 05-D11-42/06.10.2005