ARTICLE pubs.acs.org/JPCC
Effects of Annealing Temperature and SiO2 Matrix on the Structure and Optical Properties of Co-Doped ZnAl2O4/SiO2 Nanoglass Ceramic Composites Xiulan Duan,* Xinqiang Wang, Fapeng Yu, and Duorong Yuan State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People's Republic of China ABSTRACT: Co-doped glass ceramics containing spinel nanocrystals attach much attention as optical materials. The composition and preparation conditions of the material will influence its structure and optical properties. In this study, the samples (100 x)Co0.1Zn0.9Al2O4 xSiO2 (x = 0 95) were prepared by the sol gel method and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and 27Al solid-state NMR spectroscopy were used to study the chemical environments of cations and oxygen in the nanocomposites as a function of annealing temperature and SiO2 concentration. The results show that all the samples are composed of a silica matrix and enclosed ZnAl2O4 nanocrystalline particles. Besides the tetrahedral, octahedral, and second octahedral coordination sites present in Co0.1Zn0.9Al2O4 nanocrystals, the five-coordinate sites of Al3+ ions are observed in the nanocomposites. The inversion parameter (two times the fraction of Al3+ ions in the tetrahedral sites) increases with the decrease of SiO2 concentration and annealing temperature. The absorption spectra indicate that Co2+ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanocomposites. The intensity of the absorption peak corresponding to octahedral Co2+ ions decreases with the increase of annealing temperature and SiO2 concentration.
1. INTRODUCTION Spinel-type oxides with the formula of AB2O4, where A and B are two different metal cations, have a wide range of applications in catalyst, ceramic, magnetic, and host materials because of their good thermal and chemical stability.1 4 The spinel structure is characterized by two types of cation sites: octahedral and tetrahedral. The distribution of A and B cations in these two sites, affected by the combination and nature of the two cations, depends strongly on the annealing temperature, impurity content, and particle size.5 8 The cation distribution can be characterized by the inversion parameter (y), which is defined as the fraction of the divalent cations in the octahedral sites (or two times the fraction of Al3+ ions in the tetrahedral sites). Studies of cation distribution in spinels have attracted much attention because they may allow better understanding of the correlations between structure and properties such as color, magnetic behavior, catalytic activity, optical properties, etc., which are strongly dependent on the occupation of these two sites by metals.9 12 Among the spinel-type oxides, Co2+-doped ZnAl2O4 and MgAl2O4 crystals have been used as saturable absorber passive Q-switches for 1.34 and 1.54 μm lasers or can be considered as possible candidates for tunable solid-state lasers in the visible and near-infrared regions.13 17 Transparent glass ceramics are of great interest as hosts for transition metals, since they could combine the advantage of crystalline hosts with the relative ease of the manufacturing technology of glass. Recently, glass ceramics containing Co2+-doped ZnAl2O4 and MgAl2O4 nanocrystals r 2012 American Chemical Society
have been reported as saturable absorber passive Q-switches.18,19 The microscopic structure of nanoglass ceramic composites will differ from that of pure nanocrystals and glass materials. Therefore, the optical properties of these materials will differ greatly. The properties of the nanoglass ceramic composites depend, with a resonable approximation, on the linear combination of the individual characteristics of the two phases and are largely governed by the interfacial interactions between the two components. Recent reports on nanocomposite system have shown that the matrix has certain influence on the structure and the properties of the system.20,21 The structure feature of Co-doped ZnAl2O4 spinel has been previously studied.22 26 However, the structural properties of Co-doped ZnAl2O4 nanocrystals embedded in glass matrix and its correlation with UV vis optical properties have not been studied so far. In this paper, the effects of SiO2 concentration and annealing temperature on the structure of Co:ZnAl2O4/SiO2 nanoglass ceramic composites obtained by sol gel method were studied by means of X-ray photoelectron spectroscopy (XPS) and 27Al solid state NMR spectroscopy. The UV vis absorption spectra of the materials were studied. Furthermore the relationship between the structure and the optical properties was investigated.
Received: October 13, 2011 Revised: December 5, 2011 Published: January 17, 2012 2313
dx.doi.org/10.1021/jp209837q | J. Phys. Chem. C 2012, 116, 2313–2321
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Photographs of (100 x)Co0.1Zn0.9Al2O4 xSiO2 glass ceramics: (a) x = 95; (b) x = 90; (c) x = 85; (d) x = 80; (e) x = 60; (f) x = 20.
Figure 3. TEM images of 20Co0.1Zn0.9Al2O4 80SiO2 glass ceramics annealed at different temperatures: (a) 900 °C; (b) 1000 °C; (c) 1100 °C. (d) HR-TEM image marked in (b).
Figure 2. XRD patterns of (a) 20Co0.1Zn0.9Al2O4 80SiO2 glass ceramics annealed at different temperatures. (b) (100 x)Co0.1Zn0.9Al2O4 xSiO2 glass ceramics annealed at 1000 °C with different x values.
2. EXPERIMENTAL METHODS 2.1. Preparation. The gels with the theoretical composition of (100 x)Co0.1Zn0.9Al2O4 xSiO2 (x = 0 95) were prepared by the sol gel process. The starting materials were tetraethoxy silane (TEOS), Zn(NO3)2 3 6H2O, Al(NO3)3 3 9H2O and Co(NO3)2 3 6H2O. All the reagents were analytical-grade and were
used without further purification. First, a mixture of TEOS, ethanol and water in a molar ratio 1:3:1 was stirred for 1 h at room temperature. After this hydrolysis, Zn(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and Co(NO3)2 3 6H2O, which had been dissolved in water, were added to give a H2O/TEOS ratio of 15. This solution was stirred for 1 h, poured into glass containers, and allowed to gel at room temperature. After gelation, the samples were aged at 40 °C for three days and then dried at 60 °C for about one month. The dried gels were then calcined in the temperature range of 900 1100 °C for 5 h. 2.2. Characterization. X-ray powder diffraction (XRD) patterns of the samples were carried out on a Japan Rigaku D/MaxrA diffractometer using a Cu-target tube (λ = 0.15418 nm) and a graphite monochromator. Transmission electron microscopy (TEM) images were recorded with a JEOL-2010 transmission electron microscope. For the sample preparation, the samples were grinded into powders and were ultrasonicated in ethanol and then a droplet of the suspension was deposited and dried on the sample support. X-ray photoelectron spectra (XPS) were measured using a Thermofisher ESCALAB 250 X-ray photoelectron spectrometer with monochromatized Al Kα X-ray radiation in ultrahigh vacuum (
