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J. Phys. D: Appl. Phys. 33 (2000) 3152­3155. Printed in the UK

PII: S0022-3727(00)13723-4

Surface segregation of transition metals in sol­gel silica films
V B Prokopenko, V S Gurin§, A A Alexeenko , V S Kulikauskas and D L Kovalenko
Advanced Materials Laboratory, Gomel State University, 104 Sovetskaya Street, 246699 Gomel, Belarus § Physico-Chemical Research Institute, Belarusian State University, 220080 Minsk, Belarus Laboratory of Technical Ceramics and Silicates, Gomel State Technical University, 48 October av, 246746 Gomel, Belarus ¶ Moscow State University, Moscow, Russia E-mail: vitpr@usa.net Received 3 May 2000, in final form 21 July 2000
Abstract. Silica sol­gel films (SGFs) doped with various transition metals derived by spin-coating on glass substrates and silicon wafers have been studied. Precursor solutions were prepared by mixing tetraethylorthosilicate, ethanol, water and nitrates of the transition metals (Cr, Mn, Fe, Co, Ni and Cu) using HCl as a catalyst. Remarkable segregation of metals at the surface was observed for films doped with Cu and Co. For Mn- and Ni-doped films this effect was much less, and it vanished for the films doped with Cr or Fe. The influence of boron and phosphorus upon the chemical state and the distribution of metal atoms was also analysed. The structure, optical properties and composition of the SGFs were studied and some pathways to control the dopant segregation effect were realized.

1. Introduction

Transition metal oxides supported on various substrates or incorporated in matrices exhibit a number of unique properties that are of interest from both fundamental and technological standpoints. One of the basic requirements in optoelectronics and magneto-optics for films and glasses is uniformity both around the whole working area and with depth. However, in the case of multicomponent systems a priori this uniformity is not provided due to chemical interaction of components and transport processes, essentially at elevated temperatures. The sol­gel technique is one of the prospective preparation methods for glass-like silica­metal oxide systems. The flexibility to control matrix properties, the type and concentration of dopants and the usability of different substrates opens up a lot of developments in the field of silica sol­gel science and technology [1, 2]. However, the structural features, distribution of dopants and their chemistry as influenced both by the matrix and the environment have not been investigated in detail. In the course of the sol­gel process a liquid sol and solid products from it (gels, xerogels, films) are assumed to be initially uniform, and secondary processes in the solid phase contribute to the distribution of metal atoms in the matrix. This redistribution can be rather complicated because a variety of factors occurs in every multicomponent system [3­5].
Present address: Uppsala University, Department of Earth Science, Villavagen 16, SE-752 36 Uppsala, Sweden.

The phenomenon of surface segregation is well known in solids [6], but in the case of metal­oxide and oxide­oxide systems understanding is far from complete due to the multiplicity of reaction products in such multicomponent systems, which depend on many factors. There are a number of strategies to control and overcome the segregation effect for metals in solid matrices: (i) binding of metal ions with the silica matrix, for example due to remnant hydroxyl groups in silica [7, 8]; (ii) covalent tethering of the metal with additional ligands [9]; and (iii) an adjustable post-treatment of the films in different atmospheres [10, 11]. All these approaches involve new chemistry in metal­matrix systems which affects the chemical state of the metal and its ability to be transferred. Evidently, a purely thermodynamic explanation of the segregation on the basis of an energetic benefit under some conversion like MOx M + x O is not valid here in contrast to metallic alloys because the contribution of the environment (matrix, atmosphere) should be considered as a component of the system. For instance, recently, Van den Oetelaar et al [12] have established a reversible chemical bonding of copper with silica in Cu/SiO2 catalysts depending on the atmosphere at heat treatment (reductive, oxidative, inert) resulting in the migration of copper through the interface. We also may not exclude an interaction of silica with metals and oxides, which leads to different silicate compounds and complicates any full simulation. Our work covers a series of films with transition metal oxides (Cr, Mn, Fe, Co, Ni and Cu) incorporated within the silica sol­gel films (SGFs) and includes additional modification by other glass-forming elements

0022-3727/00/243152+04$30.00 © 2000 IOP Publishing Ltd


Transition metals in sol­gel silica films

(B, P). We have analysed the films with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS) and optical transmission in order to elucidate the segregation of components under the steps of post-fabrication and modification. The effects of the dopant type and concentration upon the final chemical state and the optical properties was studied and the pathways to control the dopant segregation effect by B and P were shown.
2. Experimental details

SGFs were prepared from precursor sols by the spin-coating on quartz substrates and silicon wafers. A precursor sol was obtained by mixing tetraethylorthosilicate (TEOS), ethanol, water and HCl in the molar ratio 1:7:10:0.1. After a week of storage in a closed vessel the metal doping was carried out by addition of a certain amount of the corresponding nitrates (Cr, Mn, Fe, Co, Ni and Cu, 10­50 wt% in sols) followed by the subsequent SGF formation. Another set of film samples was fabricated from such a sol additionally doped with boric or phosphoric acids. After spin-coating all the samples were heated up to 500 C in air and kept at this temperature for 15 min. According to infraredspectroscopy data after heating all the organic components from the solvents and TEOS are completely removed from the SGFs and amorphous silicon dioxide is formed. The thickness of the films was about 600 nm. High adhesion to the glass substrate was attained together with surface uniformity and good optical quality under fine-tuning of the above described preparation procedure.
3. Results and discussion

Figure 1. Transmission spectra of silica SGFs deposited on a quartz substrate for different concentrations of metal nitrates incorporated into the silica sol.

3.1. Film composition and optical properties Homogeneously coloured coatings were obtained for all the SGFs under study. An amorphous structure was observed using XRD analysis. Only the wide diffuse halo located at 2 = 15­30 , corresponding to the most intensive reflection for amorphous SiO2 , was detected. The SGFs were coloured with varying hues (yellow, brown, orange, olive and blue) and intensities depending on the type and concentration of the dopants (figure 1). A limitation on the maximum amount of metal (corresponding to the salt concentration introduced in sols) to attain homogeneity and a good optical quality of the films was observed. In the case of Mn and Ni the maximum content of the nitrates in the sol was 20 wt%, Cr and Cu, 25 wt%; Co- and Fe-doped SGFs allowed increased levels of metal content up to 70 wt%. Transmission and reflection spectra in the UV/visible range for a series of silica SGF deposited on silica glass substrates exhibited a gradual reduction in transmittance with an increase of the metal concentration without a change of the initial colour for Mn, Fe, Ni and Cu. For Co-doped films the colour changed from light blue to olive with an increase in the amount of Co salt in the sols higher than 15 wt%. Such behaviour can be associated with a change of the cobalt valence state (between the Co (II) and Co (III) species). Moreover, the olive Co-doped SGFs formed on the silica substrate are highly reflective. An explicit redistribution

Figure 2. Transmission spectra of Co-doped silica SGFs derived from sols (30 wt% of cobalt nitrate) with and without boric or phosphoric acids (10 wt%).

of components was also observed for the Cu-doped films. The SGFs fabricated using sols with 25 wt% of Cu(NO3 )2 combined the light green colour of the bulk films and a top surface dark layer (easily mechanically removable by wiping). The optical features of other metal-doped SGFs were similar to the results described in [13­15]. The typical glass forming elements, boron and phosphorus, were used for modification of the silica network to increase the amount of metals in the films with no deterioration of their optical quality. These modifiers are typical additives (in the form of oxides) for multicomponent float glass preparation. An appreciable influence of B and P on the properties of the metal-doped SGFs was found with concentrations of boric or phosphoric acids in the sol above 6 wt%. The presence of boron resulted in a significant improvement in the mechanical and optical properties of the highly-doped films, and a significant decrease in reflectance for silica films doped with Co were observed. At the same time the olive colour of the SGFs changed to light blue when phosphorus was added (figure 2). Doping with phosphorus together with Cr, Mn, Fe, Ni or Cu produced colourless films supporting the suggestion that metal oxides are included in the glass matrix with no precipitation of separate species like metal oxide particles.
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Table 1. Relative atomic ratios of silicon/metal in the surface layers (thickness = 10 nm) and remaining bulk films.

Metal Cr Surface Si/Me ratio Bulk Si/Me ratio 7.0 7.0 Fe 1.1 1.1 Mn 1.7 2.0 Ni 1.7 2.0 Cu 0.3 1.1 Co 0.05 2.5 Co + B 2.1 2.1 Co + P 1.5 1.5

Figure 4. XPS spectra of Co 2p Figure 3. Typical RBS spectra of silica SGFs doped with different

metal oxides and boron or phosphorus: (a) Cr-, Fe-, Mn- and Ni-doped films; and (b) Cu-, Co-, Co + P- and Co + B-doped films.

3/2 in silica SGFs derived from sols (30 wt% of cobalt nitrate) with and without boric or phosphoric acids (10 wt%).

3.2. The surface segregation revealed by RBS The RBS method was used for measurements of the elemental composition and distribution with depth in SGF on a polished silicon substrate (figure 3). The 1.6 MeV 4 He++ beam normally incident on the samples from a Van der Graaf accelerator (Moscow State University) was detected at a scattering angle of 160 . The analysis of these RBS data was carried out using RUMP software. The calculated Si/Me ratio at the surface and in the bulk of films are summarized in table 1. The depth distribution of the atoms depends strongly on the type of metal and the presence of boron or phosphorus modifiers. The SGF doped with Cr or Fe oxides were homogeneous with depth for all investigated concentrations (up to 50 wt% metal nitrates in sol). According to [16] the SGFs doped with Mn or Ni and Cu or Co can be referred to as weakly and strongly segregating systems, respectively. We noted that the amount of silicon atoms is very small on the surface of the Co doped films (table 1). This was the reason for the high intensity of the optical reflection for this composition. The thickness of this layer was about 10 nm. The addition of B or P resulted in the fabrication of the films with increased depth homogeneity. Such behaviour can be a consequence of the formation of new chemical bonds (complexes) in the amorphous silica network. The most significant modification of the structural characteristics, optical properties and elemental distribution were observed
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for Co-doped SGFs, and in the further description of results we restrict ourselves to the examples of these films. Typical RBS spectra of the SGFs derived with identical concentrations of Co in the sol (20 wt%) and doped with B or P (figure 3(b)) show the evident composition change. The basic reason for the variety of behaviours of transition metals in sol­gel silica matrices lies in the difference of their chemical reactivity to form oxides and the ability of the oxides to coexist in contact with SiO2 . Polyvalent Fe and Cr are assumed to be most active in their ability to form network structures in a glass matrix (taking into account their own ability to form glassy oxides and hydroxides). The metals which indicated an ability to segregate (Cu, Co, Ni and Mn) produce oxides with a lower stability at high temperatures. The reference data on free energy values at 500 C for these oxides [17] show the lowest - G for Cu2 O (55 kJ mol-1 ), medium values for CoO and NiO (about 90 kJ mol-1 ) and the highest values of - G of formation from elements for FeO, Fe3 O4 and Cr2 O3 (>100 kJ mol-1 ). In the case of Mn, its oxide MnO belongs to the highly-stable compounds; however, a presence of Mn (III) and Mn (IV) oxides is expected in the films heated up to 500 C in air. Thus, the thermodynamics of the metal oxides can be another factor influencing the segregation together with the above-mentioned reactivity with respect to silica matrix. The G(T ) diagrams (Ellingham diagrams) are linear for these oxides in the range of T under investigation, and the above relationship between values of G are conserved within this range. So, one can suggest that the segregation


Transition metals in sol­gel silica films

observed proceeds by a sequence of chemical processes with the oxides followed by the transport of more mobile components. For example, CuO and Co3 O4 are decomposed under normal pressure at temperatures above 800 C [18]. Under heat treatment the decomposition of the metal oxides in the surface layer may in part lead to an outward gradient in the depth distribution. An incomplete decomposition of copper and cobalt oxides can take place at 500 C. An appearance of only a thin highly-reflective metallic layer on the surface of the Cu- and Co-doped films confirms the partial decomposition of the corresponding oxides. An additional source of surface segregation is the different reactivity of the metal oxides or pure metals with respect to silica. It should be noted that the heat treatment under temperatures of 400 C in an hydrogen atmosphere resulted in no surface segregation for both Cu and Co. Particles of metallic copper and cobalt are easily produced from the oxides in a reducing atmosphere. As a result, the chemical compositions of the SGFs become different, and the reactivity of the free metals (Cu, Co) is much less with respect to silica. The changes of the local environment of the metals by the addition of glass forming elements, both B and P, provided a significant decrease of the surface segregation effect. These elements are known to increase strongly the ability of metal oxides to be incorporated into the glassy matrix [19]. 3.3. XPS The chemical state of the elements in the SGFs was studied by XPS and is shown here for the example of Co-doped films (figure 4). The XPS measurements were carried out using Mg K (1253.6 eV) radiation with an ES2401 spectrometer. The C 1s line (284.6 eV) from the naturally occurring surface carbon was used for calibration. The value of the binding energy (BE) for the Co 2p3/2 core levels (779.7 eV) appeared to be essentially higher than the BE of the reference metallic Co (778.2 eV), but also a little less than the BE for the stoichiometric cobalt oxides: Co2 O3 , 779.9 eV; Co3 O4 , 780.2 eV; and CoO, 781.5 eV [20­22]. Thus, taking into account the fact that the XPS method analyses a top surface layer, which is anomalous in the above described RBS data, we may suggest the partial oxidation of cobalt in it (the less oxidation degree than Co (II)). Similar states of copper compounds in SGFs were shown by us recently [23]. It should be noted that the data on the surface composition obtained with XPS correlate with the RBS data. The Co-doped SGFs with B and P reveal a much more positive shift of the Co 2p3/2 levels to 781.4 eV and 783.0 eV, respectively, and a noticeable peak broadening (figure 4). Taking into account the optical properties and the location of other XPS peaks (Si, P and O not shown here), the appearance of complexes consisting of three or more atoms can take place. Thus, boron and phosphorus effectively promote the complete oxidation of cobalt and the inclusion of it in a glassy network.
4. Conclusions

used as the dopant in the sols using similar preparation procedures and can be significantly reduced by modifiers containing boron or phosphorus. Strong segregation occurred for the Cu- and Co-doped SGFs heated in air derived from sols with a metal salt concentration above 20 wt%. The Fe- and Cr-doped SGFs did not show segregation. The addition of B or P in the form of acids in the sol up to 10 wt% allowed us to attain homogeneity of the films with depth for both weakly and strongly segregating types of metals. This phenomenon is associated with the peculiarities of the chemical processes in the composite system `transition metal oxide­amorphous silica'.
Acknowledgments

The authors thank Dr K N Kasparov and DrEATyavlovskaya for XPS measurements. This work was supported in part by the Belarus Fundamental Foundation.
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We have established that the surface segregation of silica SGFs depends significantly on the type of transition metal

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