<HTML> <HEAD>    <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">    <META NAME="GENERATOR" CONTENT="Mozilla/4.03 [es] (WinNT; I) [Netscape]">    <TITLE>Raman and IR analysis of Cu incorporated to SiO2 matrix prepared by the sol-gel method</TITLE> </HEAD> <BODY TEXT="#000053" BGCOLOR="#FFFFFF" LINK="#257AA3" VLINK="#800000" ALINK="#387272">  <CENTER><IMG SRC="../../imagenes/ciencias.gif" ALT="[ Facultad de Ciencias - Artculos de Investigacin ]" HEIGHT=100 WIDTH=770></CENTER>  <CENTER> <H2> <FONT SIZE=+2>LABORATORIO DE MATERIALES</FONT></H2></CENTER>  <CENTER><B>FACULTAD DE CIENCIAS</B></CENTER>  <CENTER><B>&nbsp;</B></CENTER>  <CENTER><B>UNIVERSIDAD AUTONOMA DE SAN LUIS POTOSI</B></CENTER>   <P><FONT SIZE=-2>&nbsp;</FONT> <CENTER><B><FONT SIZE=+2>Raman and IR analysis of Cu incorporated to SiO<SUB>2</SUB> matrix prepared by the sol-gel method</FONT></B></CENTER>   <P>&nbsp; <CENTER><B>J. R. Mart&iacute;nez</B></CENTER>  <CENTER>Facultad de Ciencias, Universidad Aut&oacute;noma de San Luis Potos&iacute;</CENTER>  <CENTER>78000 San Luis Potos&iacute;, S.L.P., M&eacute;xico</CENTER>  <CENTER><B>F. Ruiz</B></CENTER>  <CENTER>Instituto de F&iacute;sica "Manuel Sandoval Vallarta", Universidad Aut&oacute;noma de San Luis Potos&iacute;</CENTER>  <CENTER>78000 San Luis Potos&iacute;, S.L.P., M&eacute;xico</CENTER>  <CENTER><B>J. Gonz&aacute;lez-Hern&aacute;ndez </B>and<B> S. Jim&eacute;nez-Sandoval</B></CENTER>  <CENTER>Laboratorio de Investigaci&oacute;n en Materiales, Centro de Investigaci&oacute;n y de Estudios Avanzados del IPN,</CENTER>  <CENTER>Facultad de Qu&iacute;mica, Universidad Aut&oacute;noma de Quer&eacute;taro</CENTER>  <CENTER>Cerro de las Campanas s/n, 76010 Quer&eacute;taro, Qro., M&eacute;xico</CENTER>   <P>&nbsp; <HR SIZE=4 WIDTH="100%"> <CENTER> <H3> Abstract</H3></CENTER> Raman spectroscopy has been used to analyze the incorporation of Cu in SiO<SUB>2</SUB> matrix in samples prepared by the sol-gel method. Samples contain Cu at 30 %wt and are studied as a function of annealing temperature. It is found that the heat treatments have a remarkable influence on the structure. With this analysis we infer that the final structure of the sample is like copper orthosilicate (CuSiO<SUB>4</SUB>) and we discuss the mechanism that carry out that structure. Finally we compare this result with IR spectra obtained in this and a previous work. <BR>&nbsp;  <P><B>Introduction</B>  <P>Raman and infrared spectra are among the most direct structural probes of non-crystalline solids [1]. However, there is no way of calculating the structure directly from the observed spectra [2], in part because the number of structural parameter is greater than the number of observables in the spectra. This is particularly true for glasses. Raman spectroscopy is very well suited for studying gels on a molecular level.  <P>In a previous work the infrared spectroscopy was used to analyze the incorporation of Cu and Fe in SiO<SUB>2</SUB> matrix in samples prepared by the sol-gel method [3]. In the present work we analyze the incorporation of Cu in SiO<SUB>2</SUB> matrix prepared by the sol-gel method as a function of annealing temperature using Raman spectroscopy and we compared with previously reported results obtained by IR analysis. <BR>&nbsp;  <P><B>Experimental methods</B>  <P>The starting solutions were prepared by mixing tetraethyl orthosilicate (TEOS), water, ethanol and copper nitrates. The mol ratios of ethanol to TEOS and water to TEOS were 4:1 and 11.67:1, respectively. The concentration of Cu nitrates were calculated to obtain 30 wt% of metal oxide, assuming that it is in its maximum oxidation state. The nitrates used for the addition of copper were Cu(NO<SUB>3</SUB>)<SUB>2<FONT FACE="Symbol">&middot;</FONT> </SUB>2.5H<SUB>2</SUB>O. To catalyzed the hydrolysis/condensation reactions small amounts of nitric acid were added. The metal nitrate was dissolved in the water and separately the TEOS and the ethanol were also mixed, a homogenous solution of all components was obtained mixing them for about 15 minutes using magnetic stirrer. The solution was placed, in the same container, in a constant bath temperature at 35 <FONT FACE="Symbol">&deg;</FONT> C. Soft pieces of the glass were obtained after about 48 hr. Previously the bath temperature was fixed at 50 <FONT FACE="Symbol">&deg;</FONT> C during 2 hr. For subsequent annealing the samples were introduced in the oven at the desired temperature for 15 minutes at atmospheric air conditions. Those pieces were grounded to form a fine powder.  <P>The Raman spectra were obtained at room temperature in a Labram-Dilor micro-Raman system in which the sample was excited with a He-Ne laser and the signal analyzed with a low-noise CCD detector. To check for possible changes in the sample induced by the laser, the laser spot diameter was varied in the 1-10<FONT FACE="Symbol">m</FONT> m range and the power density changed by using neutral density filters. <BR>&nbsp;  <P><B>Results</B>  <P>Fig. 1 shows typical Raman spectra in the range of 200 to 1100 cm<SUP>-1</SUP> for a 70SiO<SUB>2</SUB>:30CuO powdered sample at 100, 200, 400 and 600 <FONT FACE="Symbol">&deg;</FONT> C for 15 minutes, for comparison it is included a typical spectrum from a SiO<SUB>2</SUB> without Cu sample prepared using the same sol-gel method. This sample was annealed at 350 <FONT FACE="Symbol">&deg;</FONT> C for twenty minutes.  <P>The spectra for Cu at 30 wt% show a close similarity with the spectra of silica gel indicating a similarity in the molecular structure.  <P>The peak observed in the sample without Cu at 975 cm<SUP>-1</SUP> reveals the presence of SiO<SUB>4</SUB> tetrahedral with two non-bridging oxygen ions, when Cu is incorporated the broad is diminish and the peak is presented in all temperatures. The two non-bridging oxygen are binding to OH groups. In the case of the fuzzed quartz this peak is absent, which is a evidence that the four oxygen are used as bridge.  <P><IMG SRC="Image33.gif" HEIGHT=547 WIDTH=718> <CENTER><B><I><FONT SIZE=-1>Figure 1</FONT>. Raman spectra for SiO<SUB>2</SUB> and for a 70SiO<SUB>2</SUB>:30CuO powdered sample at 100, 200, 400 and 600<FONT FACE="Symbol">&deg;</FONT> C</I></B></CENTER>   <P>&nbsp; <BR>The intense peak observed at 1065 cm<SUP>-1</SUP> for the sample annealed at 100 <FONT FACE="Symbol">&deg;</FONT> C correspond to nitrates. When the sample is treated to high temperatures this peak tend to disappears and the intensity of the peak at 305 and 355 cm<SUP>-1</SUP> increase, which corresponds to Cu-O bonds. This indicate us that Cu substituted to Si atoms how we report in a previous work using IR analysis [3]. When the annealing temperature is 100 <FONT FACE="Symbol">&deg;</FONT> C the peak at 286 cm<SUP>-1</SUP> correspond to Cu<SUB>2</SUB>O and this peak tend to disappears conform the annealing temperature is increased until the peaks at 305 and 355 cm<SUP>-1</SUP> which correspond to CuO are evident, how we indicate previously.  <P>The intensity of the band at 600 cm<SUP>-1</SUP> increase when the temperature is increased. In some reports this band is associated to defect in equal manner the band at 490 cm<SUP>-1</SUP> (named D<SUB>2</SUB> and D<SUB>1</SUB>, respectively). However Raman measurements have argued that both the bands have to be thought of in terms of non-random structural features and associated to vibrational modes localized on rings of various sizes and/or to elongated Si-O bonds. Galleener [4] has argued that these bands are due to three- and four-membered planar Si-O-Si rings in the SiO<SUB>2</SUB> structure, Krol [5] in accordance with this assignment discuss the temperature-dependent behavior. We can observed that the behavior of the bands obtained by us is in agreement with this interpretation.  <P>The band at 440 cm<SUP>-1</SUP> is relatively weak and his width is very extended. This we can be associated to the absence of the D<SUB>2</SUB> band in correspondence to some interpretation of other authors [6-8]. Thus only four-member rings or greater are formed and the angle Si-O-Si distributions is from 140<FONT FACE="Symbol">&deg;</FONT> to greater values.  <P>A relationship between bands at 600 and 975 cm<SUP>-1</SUP> exist. Agree with previous report this band reveals the presence of SiO<SUB>4</SUB> tetrahedral with two non-bridging oxygen ions. The two non-bridging oxygen are binding to OH groups. The absence of the 600 cm<SUP>-1</SUP> band correspond to the presence of the 975 cm<SUP>-1</SUP> band and in this case the Si-O-Si angles are greater that 140<FONT FACE="Symbol">&deg;</FONT> thus only four-member rings are formed represented for the D<SUB>1</SUB> band located at 490 cm<SUP>-1</SUP>.  <P><IMG SRC="Image34.gif" HEIGHT=552 WIDTH=717> <CENTER><B><I><FONT SIZE=-1>Figure 2</FONT>. Raman spectra for the sample at 100 <FONT FACE="Symbol">&deg;</FONT> C for a long time of gelation</I></B></CENTER>   <P>&nbsp;  <P>The appearance of the D<SUB>2</SUB> band is associated to the dehidroxylation process diminish the Si-O-Si angle to values smaller than 140<FONT FACE="Symbol">&deg;</FONT> making possible the formation of three-members rings. Thus on the surface oxygen atoms binding to silicon atoms increase the oxygen numbers used how oxygen brinding. However in our case the principal factor associated to the appearance of the D<SUB>2</SUB> band is the Cu incorporation to the SiO<SUB>2</SUB> matrix due that in the pure sample show in figure 1 annealed at 350 <FONT FACE="Symbol">&deg;</FONT> C, D<SUB>2</SUB> band is absent, whereas the sample with Cu in a temperature of 200 <FONT FACE="Symbol">&deg;</FONT> C presented the band D<SUB>2</SUB> and the sample at 100 <FONT FACE="Symbol">&deg;</FONT> C for a long time of gelation, figure 2, presented the D<SUB>2</SUB> band too. We can conclude thus the Cu incorporation in the SiO<SUB>2</SUB> matrix favours the three-members rings formations. This situation is associated with a significantly diminish of the intensity and broad band at 975 cm<SUP>-1</SUP>. In samples with Cu we observed three bands situated at 975, 950 and 920 cm<SUP>-1</SUP>. Those bands can be associated to bonding of Si and Cu to [OH]. The first band is the Si-[OH] and the latests can be Cu-[OH]. The band at 950 cm<SUP>-1</SUP> at a temperature of 100 <FONT FACE="Symbol">&deg;</FONT> C show a significantly broad wich diminish when the annealing temperature is increased.  <P>Our interpretation of the formation of three-members rings is in agreement with the IR results report previously [3] in which for an Cu-doped glasses show a considerable increase in the width of the band centered at 1075 cm<SUP>-1</SUP> accompanied by the decrease in the width of the band at 1165 cm<SUP>-1</SUP>. The variation of the width of the band 1075 cm<SUP>-1</SUP> is approximately two times larger than the variation of the other band. This increase in the width can be interpretated as a increase range variety in the Si-O-Si angle distribution around 148<FONT FACE="Symbol">&deg;</FONT> . The width of this band extendly to lower values of frequency of about 900 cm<SUP>-1</SUP> cover the peaks that correspond to the silanol -OH, for this range of frequency the Si-O-Si angles are very lower that 148<FONT FACE="Symbol">&deg;</FONT> thus the Cu favours the presence of Si-O-Si angle of 137<FONT FACE="Symbol">&deg;</FONT> created three-members ring and the D<SUB>2</SUB> band to appear. Thus the Cu is incorporated producing a drastical changes in the Si-O-Si distribution angles, breaking the continuity of the SiO<SUB>4</SUB> network.  <P><IMG SRC="Image35.gif" HEIGHT=552 WIDTH=717> <CENTER><B><I><FONT SIZE=-1>Figure 3</FONT>. Raman spectra for the Sample annealed at 800 <FONT FACE="Symbol">&deg;</FONT> C</I></B></CENTER>   <P>In the pure sample (without Cu) and in sample with Cu incorporation at 100 <FONT FACE="Symbol">&deg;</FONT> C, where organic groups are still presents, the 600 cm<SUP>-1</SUP> bands is absent or weak and the 490 cm<SUP>-1</SUP> band is very strong. This indicate that initially only four-membered rings are formed. This is in accordance with the fact that in organic cyclosiloxanes four-membered rings are most often encountered [9]. It is observed that the 490 and 600 cm<SUP>-1</SUP> band depends of the temperature.  <P>The formation of the sample is strong dependent of the time. The process of formation continues and is relatively equivalent to the temperature treatment as we can observe in the figure 2 for the sample at 100 <FONT FACE="Symbol">&deg;</FONT> C obtained approximately one month after under the same conditions. In this figure we observed that the diminish peak at 1065 cm<SUP>-1</SUP> which correspond to nitrates, it is connected to a increase of the peaks at 305, 355 and 600 cm<SUP>-1</SUP> which correspond to Cu-O bonds and Si-O bond respectively how we had indicate. In the same figure we show the spectrum for the sample without temperature treat. A double peak at 1065 cm<SUP>-1 </SUP>is very intense, peaks at 400, 730 and 765 cm<SUP>-1</SUP> are observed. Those peaks correspond to nitrates. Sample annealed at 800 <FONT FACE="Symbol">&deg;</FONT> C is showed in figure 3. In this temperature whole Cooper is incorporated to SiO<SUB>2</SUB> matrix and the fully structure is similar to cooper orthosilicate (CuSiO<SUB>4</SUB>). In this figure we can observe that the peaks positions correspond to first and second harmonics of the Cu-O (300 cm<SUP>-1</SUP>) and Si-O (600 cm<SUP>-1</SUP>) bonds which indicate us that the majority of the four oxygen are used as bridge forming rings of Si-O bonds coupled to other rings through Cu-O bonds using three oxygen as bridge. The structure is similar to a three-dimensional network. This is in agreement with the IR spectrum show in figure 4 where the sample annealed to 800 <FONT FACE="Symbol">&deg;</FONT> C is compared with the sample without termic treatment. The treatment at higher temperature "pushes" the doping atoms in the substitution (for Si) sites, which cause the changes of the IR spectra [3]. <CENTER><IMG SRC="Image36.gif" HEIGHT=425 WIDTH=638></CENTER>  <CENTER><B><FONT SIZE=-1><I>Figure 4</I>. Infrared spectrum for sample annealed in 800 <FONT FACE="Symbol">&deg;</FONT> C and for the sample without termic treatment</FONT></B></CENTER>   <P><B>Conclusions</B>  <P>The Raman spectra show that the incorporation of Cu in a SiO<SUB>2</SUB> matrix in samples prepared by a sol-gel method is very sensitive to termic treatment. The total incorporation of Cu occurs at higher temperatures above 600 <FONT FACE="Symbol">&deg;</FONT> C. When the temperature is 800 <FONT FACE="Symbol">&deg;</FONT> C the structure of the sample is similar to cooper orthosilicate structure. In this temperature a three-dimensional network appear. This interpretation is complemented with the IR spectra obtained in this and a previous work. The incorporation of Cu is carried out substituting Si atoms and a set of Cu-O bonds are formed. Thus the Cu is incorporated producing a drastical changes in the Si-O-Si distribution angles, breaking the continuity of the SiO<SUB>4</SUB> network. The temperature dependent behavior of the 490 and 600 cm<SUP>-1</SUP> SiO<SUB>2</SUB> bands in the gels is in accordance with the assignment of these bands to planar four- and three-membered Si-O rings.  <P> <HR SIZE=4 WIDTH="100%"> <H3> <B>References</B></H3>  <OL> <LI> A.S. Barker and A.J. Sievers, Rev. Mod. Phys. <B>47</B> (1975) 51-5179</LI>  <LI> T. Furukawa and W.B. White, J. Non-Cryst. Solids <B>38</B> &amp; <B>39</B> (1980) 87</LI>  <LI> J.F. P&eacute;rez-Robles, J. Gonz&aacute;lez-Hern&aacute;ndez, Y.V. Vorobiev, F. Ruiz and J.R. Parga-Torres, To be publisher</LI>  <LI> F.L. Galeener, Solid State Comun., <B>44</B> (1982) 1037</LI>  <LI> D.M. Krol and J.G. van Lierop, J. Non-Cryst. Solids <B>63</B> (1984) 131</LI>  <LI> V. Gottardi, M.Guglielmi, A. Bertoluzza, C. Fagnano and M.A. Morelli, J. Non-Cryst. Solids, <B>63</B> (1984) 71</LI>  <LI> R.J. Hemley, H.K. Mao, P.M. Bell and B.O. Mysen, Phys. Rev. Lett., <B>57</B>, (1986) 747</LI>  <LI> P. McMillan and B. Piriou, Bull. Mineral, <B>106</B>, (1983) 57</LI>  <LI> D.A. Armitage, <I>Inorganic rings and cages</I>, Edward Arnold, London (1972)</LI> </OL>  <HR SIZE=4 WIDTH="100%"><IMG SRC="../../imagenes/backslp.gif" ALT="[ UASLP ]" HEIGHT=30 WIDTH=770> <CENTER><FONT FACE="Arial,Helvetica"><FONT SIZE=-1>| <A HREF="index.html">Indice de Articulos</A> | <A HREF="../index.html">Facultad de Ciencias</A> |</FONT></FONT></CENTER>  <CENTER><FONT FACE="Arial,Helvetica"><FONT SIZE=-1><B>Ultima modificaci&oacute;n:</B> 08/Julio/1998</FONT></FONT></CENTER>  <CENTER><FONT FACE="Arial,Helvetica">&nbsp;</FONT></CENTER>  <CENTER><B><FONT FACE="Arial,Helvetica">Sugerencias y Comentarios al <A HREF="../../comentarios.asp">Webmaster</A></FONT></B></CENTER>  </BODY> </HTML> 
