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Composite materials based on nanoporous SiO2 matrices and squarylium dye
Journal of Non-Crystalline Solids 389 (2014) 11–16
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Composite materials based on nanoporous SiO2 matrices and squarylium dye O.N. Bezkrovnaya a,⁎, I.M. Pritula a, A.G. Plaksyi a, V.F. Tkachenko a, О.М. Vovk a, Yu.L. Slominskii b, A.D. Kachkovskiy b, Yu.A. Gurkalenko c, S.N. Krivonogov a, A.V. Lopin a a b c
Institute for Single Crystals, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001 Kharkiv, Ukraine Institute for Organic Chemistry, NAS of Ukraine, Murmanskaya 5, Kiev-94, 253660, Ukraine Institute for Scintillation Material, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001, Kharkiv, Ukraine
a r t i c l e
i n f o
Article history: Received 4 November 2013 Received in revised form 15 January 2014 Available online xxxx Keywords: Sol–gel SiO2 matrices; Nanopores; Transmission electron microscopy; Atomic force microscopy; Squarylium dyes; Luminescence spectra
a b s t r a c t Sol–gel SiO2 matrices with different contents of HF acid were synthesized. The matrices were annealed at 600 °C and impregnated with squarylium dye SqD1 in methylmethacrylate (MMA) solvent. Xerogel nanoporous structure of SiO2 matrices was found to contain 5–12 nm and 60–600 nm pores. The size of large pores increases with the concentration of HF acid. For the synthesis of SiO2 matrices the ratio n(HF/TEOS) = 0.25–0.38 is the most optimal. At other HF concentrations the formed matrices are prone to cracking at drying, annealing and impregnation with the dye solution in MMA. The location of the maxima of the absorption and luminescence bands of SqD1 dye in the composite SiO2:SqD1:ММА is found to be influenced by the surface state of SiO2 xerogel depending on the concentration of HF acid at the synthesis of the matrices. © 2014 Published by Elsevier B.V.
1. Introduction Recent decade has seen increased interest in optical-quality nanoporous structures based on SiO2 synthesized by the sol–gel method (bulk monoliths [1–5], glasses [2,6–8], fibres [7] and films [9]) meant for the creation of new optoelectronic devices. SiO2 matrices possess high transparency in the visible and near-IR spectral regions, and are characterized by high mechanical strength and bulk laser damage threshold, as well as high absorption ability and chemical stability. Such matrices may be used as a base for active solid media with minimal light scattering, good thermal conductivity and low temperature coefficient of the change of the refractive index [1]. In a number of papers there is reported the obtaining of nanoporous silicate matrices with isolated 5–15 nm PbS nanocrystals distributed in the matrix bulk. Such matrices are applied for the creation of narrow-band-gap semiconductors [10] and SiO2 matrices with LiIO3 nanocrystals for second-order nonlinear optics [11]. SiO2 matrices do not possess nonlinear optical (NLO) properties, but due to low optical losses they serve as ideal matrices for nonlinear materials. Considered in [12] is the use of the sol–gel method for the synthesis of three types of nanocomposites with NLO properties: semiconductor-glass, metal cluster-glass, and organics-glass nanocomposites. Another topical problem is the creation of laser media on the base of nanoporous SiO2
⁎ Corresponding author. Tel.: +380 573410157. E-mail address: [email protected] (O.N. Bezkrovnaya). 0022-3093/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.052
matrices with incorporated organic dyes which allow to obtain generation of wide-band radiation while developing tuneable solid lasers [1,2,13,14]. Advance of laser technologies has aroused considerable interest in the creation of optical limiting materials used for protecting solid-state sensors and eyes from intense laser beams [15–17]. Thereat, particular attention is being paid to organic dyes (squarylium [15], polymethine [15,17], etc.) with conjugate π-bonds which have a delocalized electron. Reported in [16] are the results of studying the optical limiting behaviour of acid blue ethanol solutions under the influence of a low-power CW He-Ne laser. In [15] the non-linear properties of polymethine and squarylium dyes are investigated in the elastopolymeric material polyurethane acrylate and in ethanol solutions. Creation of optical limiting devices for protecting sensitive optical elements involves the use of active components with high reverse saturable absorption (RSA) [15]. The mechanism of RSA is based on induced absorption from excited singlet or triplet states of the medium. This is one of low-threshold and effective physical mechanisms leading to nonlinear attenuation of radiation intensity. The necessary condition for the existence of RSA mechanism in dye molecules is the excess of the crosssection of absorption from the excited state over that of linear absorption from the ground state at the pumping wavelength [15,16,18,19]. High values of RSA in the visible spectral region are characteristic of organic dyes including polymethine and squarylium which have highly polarizable π-electron systems [15,17,18]. Symmetric squarylium dyes are essentially linear molecules with identical donor groups at both ends bound with the central acceptor group C4O2, and hence they
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have no permanent dipole moments and exhibit high nonlinearities. In particular, polymer matrices based on polymethylmethacrylate doped with squarylium dyes possess highly nonlinear optical response [19]. In solid matrix of polyurethane [15] photostability of squarylium dye is higher in comparison with that of ethanol solutions, therefore it seems interesting to introduce such a dye in solid SiO2 matrix for creating optical limiting materials. As shown in a number of papers [15,20], the media with low viscosity may show non-radiative decay caused by internal rotation of the C–C bonds between the central C4O2 group and electron-donor end fragments in the excited state. For squarylium dyes non-radiative excitedstate decay may be inhibited by hydrogen bonds between the donor and acceptor parts of the dye molecule [15]. The presence of a hydroxyl group in the ortho-position of aromatic donor groups in squarylium dye molecule such as SqD1 (Fig. 1), leads to the formation of hydrogen bonds between these hydroxyl groups and the polar oxygen atoms of the central ring C4O2 [15,20]. The structure of porous SiO2 matrices meant for dye incorporation is to allow penetration of the solvent into their bulk and to remain undisturbed at saturation with the dye solvent. The pore size in the matrices can be controlled by technological conditions of the obtaining of the material taking into account the size of the objects to be incorporated. It should be noted that the micro-environment of the dye molecules incorporated into SiO2 matrices differs from that in the solution. This is due to the interaction of the dye with the pore surface. The character of the distribution of the pores in the bulk of nanoporous materials based on SiO2 was investigated by the methods of scanning electron microscopy [21,22], transmission electron microscopy (TEM) [10,22,23] and atomic force microscopy (AFM) [24–26]. In the present paper we report the data on the synthesis of SiO2 matrices with different porosity, as well as on the conditions of the obtaining of composite SiO2:SqD1:ММА materials and on their spectral properties. It is shown that the use of HF acid with different concentrations in the process of sol–gel synthesis of SiO2 matrices leads to the formation of not only nanometric (up to 12 nm [2,9]), but also much larger pores (200 nm and more). The size of the latter is defined by the concentration of HF acid. 2. Experimental 2.1. Materials and synthesis of SiO2 matrices The silica gel was synthesized using tetraethoxysilane (TEOS; Aldrich), additionally purified ethyl alcohol, HF (40%, Aldrich), twice distilled water. In the capacity of active molecules there was applied squarylium dye SqD1 (Fig. 1) produced at the Institute of Organic Chemistry (Kiev, Ukraine) by the method described in [27]. Methylmethacrylate (MMA; Aldrich) was used for impregnation of SiO2 matrices. SiO2 matrices were obtained using the sol–gel method by TEOS hydrolysis with the addition of nitric acid as a reaction catalyst [2]. Ethanol and TEOS were being mixed during 30 minutes. Then there were added twice distilled water, a few drops of nitric acid, and different quantities of hydrofluoric acid (HF), thereat the molar ratio n(HF/TEOS) was varied from 0.08 to 0.76. The resulting mixture was being stirred during 2 h. The synthesized sol was poured into plastic cuvettes, the latter were hermetically sealed and stored till the gel was formed. Then the cuvettes were
Fig. 1. SqD1 dye structural formula.
opened and the samples were being dried during 3–4 weeks at room temperature and at 60 °С during the next 7–10 days. The solid xerogels were shaped as parallelepipeds with the dimensions 0.5 × 0.5 × 1.5 cm. To raise their mechanical strength, the samples were annealed in air at temperatures up to 600 °С, the rate of temperature rise and drop was 80 °С/hr. The density and porosity of the samples of SiO2 matrices were determined by the method of hydrostatic weighing and from their geometric size and weight. The annealed samples were being impregnated with the solutions of squarylium dye SqD1 (Fig. 1) in methylmethacrylate during 3 days and then polymerized at 45–50 °С during 7 days. 2.2. Characterization The absorption spectra of the samples were recorded by a spectrophotometer Lambda 35 UV/Vis Spectrophotometers (Perkin-Elmer, USA) in 200–1100 nm region (the wavelength reproducibility was ±0.05 nm, the photometric accuracy (using NIST 930D filter) and photometric reproducibility being ± 0.001 Å and b 0.001 Å, respectively. The measurement of the luminescence spectra was realized on a fluorimeter FluoroMax-4 (Horiba Jobin Yuon, USA), thereat the accuracy and repeatability were 0.5 nm and 0.1 nm, respectively, the integration time varied from 0.001 to 160 sec. The Fourier transforms infrared spectra of the crystals and of the powders were recorded at room temperature in 400–4000 cm−1 region using Spectrum One PerkinElmer with a resolution of 1 cm−1 by the KBr pellet technique. For preparation of the pellets there were used equal weighed samples each containing the matrix substance (0.0005 g) and 0.3 g of KBr. The size of SiO2 nanoparticles forming the xerogel structure was checked on a transmission electron microscope (EM-125) with an accelerating voltage of 100 kV. The samples were prepared according to the standard procedure by placing the replicas from the inner cleavage of the matrices on copper meshes coated with thin carbon film with subsequent drying. The surface microrelief was investigated by means of an atomic force microscope Solver P47H PRO (Russia). AFM data are qualitative for all the dimensions in a grey scale: dark and light tones represent the low and high features, respectively. The linear attenuation coefficient of X-ray passed through the sample (μ, cm− 1) was determined using a DRON–ЗМ diffractometer in СоKα1 – radiation (λ = 1.54051 Å [28,29]). The measurements of μ were realized on specially prepared plane-parallel samples with a thickness of 2 mm cut out from SiO2 matrices along the directions a (the horizontal direction) and z (the vertical one) of the sample. The value of the coefficient μ was calculated from the relation I = I0 · e−μd; μ = −(lnI/I0)/d, where I is the intensity of the incident X-ray beam; I0, the intensity of the X-ray beam transmitted through the sample, d, the thickness of the investigated sample. The error at determination of μ includes those of determination of I, I0 and d. The intensities I and I0 were measured with an error of 0.5%, for d the error was 1%. The error of determination of μ did not exceed 5%. 3. Results and discussion 3.1. Influence of HF acid concentration on the matrices porosity Fig. 2 presents the photographs of annealed pure SiO2 matrices. It is found that at the increase of the molar ratio n(HF/TEOS) up to 0.08, 0.380 and 0.760 the density of SiO2 matrices diminishes to 1.31, 0.85 and 0.62 g/сm3, respectively, and their open porosity increases (32.3, 54.6 and 74.7%). At the rise of the concentration of HF during the synthesis of the matrices the transmittance of the samples reduces (Fig. 3a). In particular, at n(HF/TEOS) = 0.08, 0.38 and 0.76 the transmittance of the samples at 600 nm is 71%, 51% and 25%, respectively (Fig. 3a). For the matrices synthesized at the same concentration of nitric acid, but without HF acid, the considered value amounts to 80% at 600 nm (its open porosity is 21%). At the same time, the transmittance
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Fig. 2. Appearance of the samples of SiO2 matrices with different open porosity: 32.3% (а) and 74.7% (b). The matrices were annealed at temperatures up to 600°С.
of the matrices synthesized at n(HF/TEOS) = 0.38 (at 600 nm) which have been immersed into ethylene glycol during 24 h till saturation of the pores with the solvent, increases from 51% to 80% (at 600 nm)
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(Fig. 3b). Addition of HF acid in the process of synthesis of the matrices increases their open porosity. Low light transmittance is evidently caused by the scattering in the matrices synthesized at n(HF/TEOS) = 0.38–0.76, due to the presence of a great number of large pores which size (400–700 nm and more) is comparable with the incident light wavelength. At HF concentrations lesser than n(HF/TEOS) = 0.25 the obtained matrices have low porosity and high density, that results in their destruction at annealing or impregnation with methylmethacrylate and impedes penetration of active molecules into the matrix bulk. At HF concentration higher than n(HF/TEOS) = 0.38 the matrices have large pores leading to the formation of cracks in the samples in the process of drying and annealing. As is known, the quality of silicate sol–gel matrices essentially depends on the composition of the initial reaction mixtures, the synthesis and drying conditions. The use of the acid as a catalyst results in the formation of a three-dimensional network consisting of SiO2 nanoparticles. The density of this network, its compression degree and the pore size can be regulated by changing the acidity of the medium in the course of sol–gel synthesis [30–33]. According to [30], due to the rise of the catalyst basicity the diameter of the silica gel pores grows from 0.5 nm to 4 nm. HF which is a weak acid is used for reducing the influence of the acidity of the medium on the structure of the formed threedimensional network while synthesizing SiO2 sol. As a result, the porosity of the material changes [2,9,34]. The density of bulk SiO2 matrices studied in [2] was 0.76 and 0.56 g/cm3, their porosity was 16.47% and 77.44% at n(HF/TEOS) = 0.15 and 0.30, respectively. According to literature data, the characteristic pore size for SiO2 films is 2–10 nm [9]; 1.4–18.4 nm at the increase of the ratio n(HF/TEOS) from 0 to 0.12 in silica membranes [34]; 9.04 nm and 30.02 nm at n(HF/TEOS) = 0.15 and 0.30, respectively, in bulk SiO2 matrices [2]. 3.2. Study of porosity in SiO2 matrices by transmission electron microscopy, atomic force microscopy and X-ray methods
Fig. 3. Transmittance spectra of pure SiO2 matrices synthesized at n(HF/TEOS): 0.08 (2), 0.38 (3), 0.76 (4) and without of HF (1) (a) and spectrum of SiO2 matrix synthesized at n(HF/TEOS) = 0.38 and saturated with ethylene glycol (b). The matrices were annealed at temperatures up to 600оС.
TEM study of the synthesized SiO2 matrices shows that their structure is formed by chains of 6–12 nm nanoparticles, as well as by small (5–12 nm) (Fig. 4а-с) and large (60–400 nm and more) pores (Fig. 4d). The pore size increases with the concentration of HF. The small pores may be rounded (at HF concentrations of 0.38–0.76 mole per 1 mole of TEOS) or elongated (when these concentrations diminish to 0.20–0.08 mole per 1 mole of TEOS). The analysis of the obtained data shows that with the rise of the concentration of HF at the synthesis of the matrices the size of the formed SiO2 particles exceeds that in the samples with lower HF concentration (Fig. 4). For instance, at n(HF/TEOS) equal to 0.08 and 0.24 mole per 1 mole of TEOS the pore size is 60–140 nm and 100–200 nm, respectively. HF acid possesses weak alkalinity and regulates the rates of TEOS hydrolysis and condensation. As a result, the SiO2 particles formed in the xerogel network are larger in comparison with the ones obtained while using a strong acid as a catalyst [9]. The concentration of HF also influences the formation of branched structure consisting of SiO2 nanoparticles in the samples. Due to high concentrations of the catalyst HF (which shortens the period of gelation), TEOS does not undergo the complete stage of hydrolysis and condensation, and such a branched structure is not formed. It is known that with the rise of the HF/TEOS molar ratio the surface area of the samples decreases, while the pore volume and the pore radii increase [2,34]. The surface area (m2/g) of bulk SiO2 matrices is 827 m2/g, 679 m2/g and 216 m2/g at the ratio n(HF/TEOS) equal to 0.09, 0.15 and 0.30, respectively [2]. As shown in [9], SiOF thin film contains natural Si-O tetrahedron, destroyed F-O tetrahedron, and bridge oxide, which lead to relaxation of the network structure leaving more space and more pores in the films. The method of АFM allowed to reveal the presence of pores with a size of 100–300 nm at n(HF/TEOS) = 0.08 and 300–600 nm and larger at n(HF/TEOS) = 0.38 (Fig. 5). The pore sizes are confirmed by the
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Fig. 4. TEM image of the surface of SiO2 matrices synthesized at different molar ratio n(HF/TEOS): 0.08 (а), 0.38 (b), 0.76 (с) and 0.24 (d).
profiles of AFM images. Thus, SiO2 matrices are porous xerogel structures which consist of small (5–12 nm) and large (60–600 nm) pores. The size of the pores increases with the content of HF.
To estimate the character of pore distribution in the matrices we measured linear attenuation coefficient μ of monochromatic СоKα1 of incident X-ray. This coefficient characterizes relative reduction of the
Fig. 5. AFM image of the surface of SiO2 matrices synthesized at different molar ratio n(HF/TEOS): 0.08 (a) and 0.38 (c), and the corresponding surface profiles: n(HF/TEOS): 0.08 (b) and 0.38 (d).
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intensity of the beam transmitted through a layer of absorber. As seen from the data presented in Fig. 6, the coefficient μ diminishes with the rise of the concentration of HF, due to the increase of the porosity of the formed matrices. Thereat, if the concentration of HF is lower than 0.38 mole per a mole of TEOS, the coefficient μ has different values for the samples of the same matrices cut out from it along the directions а and z (Fig. 6). This testifies that the pores are extended along the direction z. If the matrix density decreases the coefficient μ has the same value for the directions а and z, and diminishes practically by half for n(HF/TEOS) = 0.76 in contrast to n(HF/TEOS) = 0.38 samples. This may be explained by the formation of a porous structure with an open porosity of 74.7% and the same density along the directions а and z. For the samples cut out from different parts in the bulk of the same SiO2 matrix, the linear coefficients of X-radiation attenuation μ have close values. This shows that the matrix is homogeneous. 3.3. Spectral properties of SiO2 matrices with incorporated Squayilium dye SiO2 matrices to be impregnated with the solution of SqD1 dye in methylmethacrylate (SiO2:SqD1:MMA) were synthesized at n(HF/ TEOS) ratios equal to 0.08, 0.38 and 0.76, the dye concentration varied from 7.1 · 10−6 М to 1.5 · 10−4 М. The absorption band maximum of 1 · 10−5 М SqD1 in methylmethacrylate, ethanol and SiO2 sole is 640 nm, 641 nm and 646 nm, respectively (Fig. 7). The observed insignificant bathochromic shift of the absorption maximum of SqD1 in SiO2 with respect to that in the solution is caused by changes in the microenvironment of SqD1 dye and by intensification of the interaction between its molecules and the medium. Typical absorption and luminescence spectra of SiO2:SqD1:ММА composites are presented in Figs. 7 and 8b. It is found that in these composites synthesized at n(HF/TEOS) = 0.08–0.38, the dye absorption and luminescence maxima are 646 nm and 671 nm, respectively. It is to be noted that the increase of the concentration of HF acid up to n(HF/TEOS) = 0.76 at the synthesis of the matrices gives rise to the shift of the absorption maximum of SiO2: SqD1:ММА composites to 634 nm. Thereat, the luminescence maxima of these composites synthesized at n(HF/TEOS) = 0.08, 0.38 and 0.76 are 675 nm, 675 nm and 653 nm, respectively (Fig. 8b). Such shifts in the absorption and luminescence band maxima may be due to changes in the dye micro-environment in the pores. To estimate the state of the xerogel surface we measured the IR spectra of the matrices synthesized at different HF concentrations. As is known, the surface of pores in Er2O3-SiO2 xerogels is characterized by the presence of Si–OH stretching vibration free silanol groups; moreover, the residual porosity and incomplete densification of the samples are defined by the dopant concentration [6]. The structure of
Fig. 6. Dependences of the linear attenuation coefficient μ of X-radiation passing through the investigated part of SiO2 matrices, on the molar ratio of the synthesis precursors n(HF/ TEOS) and the porosity of annealed matrices (Po).
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Fig. 7. Absorption spectra of 1 · 10−5 М SqD1 dye in different environments: methylmethacrylate solution (1), ethanol solution (2), SiO2 sol (3), and in annealed SiO2 matrices (n(HF/TEOS) = 0.24) with ММА (4).
silica consists of assemblies of Si-O-Si rings of various sizes in which each Si-O-Si belongs to a cyclic structure [35]. The bands between 1080 cm−1 and 1200 cm−1 correspond to Si-O(− Si) vibrations. The peaks observed at ~ 1100 cm−1 correspond to stretching vibration of Si-O(− Si), belonging to a more linear and less cross-linked structure [35]. For the synthesized matrices there was observed the shift of the position of peaks towards higher frequencies from 1089 cm−1, 1097 cm− 1 to 1104 cm− 1 with the rise of the HF concentration (Fig. 9). The absorption band around 1100 cm−1 is related to asymmetric stretching of Si-O-Si bonds [36]. The observed shift of the stretching vibration of Si-O-Si to higher frequencies caused by increased
Fig. 8. Normalized absorption (a) and luminescence (b) spectra of SiO2 matrices, synthesized at different molar ratio n(HF/TEOS): 0.76 (1), 0.38 (2), 0.08 (3) and saturated with 1.2 · 10−5 М SqD1 dye in ММА. SiO2:SqD1:MMA composite was excited at λex = 590 nm (1), λex = 640 nm (2) and λex = 640 nm (3).
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concentrations of HF acid will crack at drying, annealing or impregnation with methylmethacrylate. At a porosity of 50% and higher the density of the distribution of the matrices pores along the horizontal and vertical directions is the same. The properties of SqD1 dye in the matrices pores are influenced by its micro-environment. The observed hypochromic shift of the absorption and luminescence band maxima of SqD1 dye in the matrices synthesized at (n(HF/TEOS) = 0.76) with respect to the maxima in the matrices synthesized at n(HF/TEOS) =0.38–0.08 is caused by diminution of the quantity of ОН-groups on the xerogel surface.
Acknowledgments The authors are grateful to Dr. D. Sophronov for measurement of the FTIR-spectra (Institute for Single Crystals, NAS of Ukraine). Fig. 9. FTIR-spectra of pure SiO2 matrices synthesized at n(HF/TEOS): 0.08 (1), 0.38 (2), 0.76 (3). The matrices were annealed at temperatures up to 600оС.
References strengthening of the network in the matrices [37]. The peak around 960 cm−1 corresponds to the Si-O(H) stretching vibration [35,38]. As seen from Fig. 9, the ratio of the intensities of the absorption bands at ~ 1200–1080 cm−1 increases in comparison with that of the band with a peak at 961 cm−1 with the rise of the HF concentration. There is observed the decrease in the intensity of the absorption corresponding to Si-O(H) stretching vibration as against that of Si-O(− Si) stretching vibration. This is caused by the diminution of the number of-OH groups on the silica surface with simultaneous strengthening of the network of silica. Moreover, some part of OH-groups on the surface of SiO2 xerogel nanoparticles may be replaced by fluorine ions. The temperature of Si-F bond breaking may be as high as 700 °C, therefore during the annealing at temperatures up to 600 °C fluorine ions remain on the xerogel surface. As a result, the micro-environment of the dye molecules in the pores changes, and the interaction between the dye and the surface reduces. It is known that with the rise of the concentration of HF acid at the synthesis of the matrices their porosity increases, and the surface area diminishes due to the formation of large pores [2]. The influence of the xerogel porosity and of the presence of nanometric and larger pores reveals itself in the decreased quantity of the absorbed molecules of SqD1 dye in the matrices at the rise of their porosity. At n(HF/TEOS) = 0.08, 0.38 and 0.76 (and a porosity of 32.3%, 54.6% and 74.7%) the concentration of SqD1 dye molecules adsorbed in the matrices pores is 6.2 · 10−5 mole/dm3, 3.7 · 10−5 mole/cm3 and 2.6 · 10−5 mole/cm3, respectively. The matrices were saturated with SqD1 dye solution in ММА containing 1.2 · 10−5 М of the dye. The content of the dye in the matrix per unit volume is larger in comparison with the one in the solution, since the dye molecules are concentrated in the matrix. An essential diminution of the dye concentration in the matrices with a porosity of 74.7% and 54.6% in comparison with that in the matrices with a porosity of 32.3% is due to predominance of large pores with a size of several hundreds nanometers reducing the xerogel surface area. Thus, for the formation of the composite material (SiO2:SqD1:MMA) the matrices synthesized at the value of n(HF/TEOS) ranging between 0.25 to 0.38. are most preferable. 4. Conclusions It is shown that at the synthesis of SiO2 matrices using HF acid there is created a porous xerogel structure formed by both nanometric (5–12 nm) and considerably larger (60–600 nm) pores. The size of the large pores increases with the concentration of HF acid. To obtain composite materials based on SiO2 matrices annealed at temperatures up to 600 °С and higher, their synthesis is to be realized at the ratio n(HF/ TEOS) ranging between 0.25 and 0.38. The matrices formed at other
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Composite materials based on nanoporous SiO2 matrices and squarylium dye O.N. Bezkrovnaya a,⁎, I.M. Pritula a, A.G. Plaksyi a, V.F. Tkachenko a, О.М. Vovk a, Yu.L. Slominskii b, A.D. Kachkovskiy b, Yu.A. Gurkalenko c, S.N. Krivonogov a, A.V. Lopin a a b c
Institute for Single Crystals, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001 Kharkiv, Ukraine Institute for Organic Chemistry, NAS of Ukraine, Murmanskaya 5, Kiev-94, 253660, Ukraine Institute for Scintillation Material, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001, Kharkiv, Ukraine
a r t i c l e
i n f o
Article history: Received 4 November 2013 Received in revised form 15 January 2014 Available online xxxx Keywords: Sol–gel SiO2 matrices; Nanopores; Transmission electron microscopy; Atomic force microscopy; Squarylium dyes; Luminescence spectra
a b s t r a c t Sol–gel SiO2 matrices with different contents of HF acid were synthesized. The matrices were annealed at 600 °C and impregnated with squarylium dye SqD1 in methylmethacrylate (MMA) solvent. Xerogel nanoporous structure of SiO2 matrices was found to contain 5–12 nm and 60–600 nm pores. The size of large pores increases with the concentration of HF acid. For the synthesis of SiO2 matrices the ratio n(HF/TEOS) = 0.25–0.38 is the most optimal. At other HF concentrations the formed matrices are prone to cracking at drying, annealing and impregnation with the dye solution in MMA. The location of the maxima of the absorption and luminescence bands of SqD1 dye in the composite SiO2:SqD1:ММА is found to be influenced by the surface state of SiO2 xerogel depending on the concentration of HF acid at the synthesis of the matrices. © 2014 Published by Elsevier B.V.
1. Introduction Recent decade has seen increased interest in optical-quality nanoporous structures based on SiO2 synthesized by the sol–gel method (bulk monoliths [1–5], glasses [2,6–8], fibres [7] and films [9]) meant for the creation of new optoelectronic devices. SiO2 matrices possess high transparency in the visible and near-IR spectral regions, and are characterized by high mechanical strength and bulk laser damage threshold, as well as high absorption ability and chemical stability. Such matrices may be used as a base for active solid media with minimal light scattering, good thermal conductivity and low temperature coefficient of the change of the refractive index [1]. In a number of papers there is reported the obtaining of nanoporous silicate matrices with isolated 5–15 nm PbS nanocrystals distributed in the matrix bulk. Such matrices are applied for the creation of narrow-band-gap semiconductors [10] and SiO2 matrices with LiIO3 nanocrystals for second-order nonlinear optics [11]. SiO2 matrices do not possess nonlinear optical (NLO) properties, but due to low optical losses they serve as ideal matrices for nonlinear materials. Considered in [12] is the use of the sol–gel method for the synthesis of three types of nanocomposites with NLO properties: semiconductor-glass, metal cluster-glass, and organics-glass nanocomposites. Another topical problem is the creation of laser media on the base of nanoporous SiO2
⁎ Corresponding author. Tel.: +380 573410157. E-mail address: [email protected] (O.N. Bezkrovnaya). 0022-3093/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.052
matrices with incorporated organic dyes which allow to obtain generation of wide-band radiation while developing tuneable solid lasers [1,2,13,14]. Advance of laser technologies has aroused considerable interest in the creation of optical limiting materials used for protecting solid-state sensors and eyes from intense laser beams [15–17]. Thereat, particular attention is being paid to organic dyes (squarylium [15], polymethine [15,17], etc.) with conjugate π-bonds which have a delocalized electron. Reported in [16] are the results of studying the optical limiting behaviour of acid blue ethanol solutions under the influence of a low-power CW He-Ne laser. In [15] the non-linear properties of polymethine and squarylium dyes are investigated in the elastopolymeric material polyurethane acrylate and in ethanol solutions. Creation of optical limiting devices for protecting sensitive optical elements involves the use of active components with high reverse saturable absorption (RSA) [15]. The mechanism of RSA is based on induced absorption from excited singlet or triplet states of the medium. This is one of low-threshold and effective physical mechanisms leading to nonlinear attenuation of radiation intensity. The necessary condition for the existence of RSA mechanism in dye molecules is the excess of the crosssection of absorption from the excited state over that of linear absorption from the ground state at the pumping wavelength [15,16,18,19]. High values of RSA in the visible spectral region are characteristic of organic dyes including polymethine and squarylium which have highly polarizable π-electron systems [15,17,18]. Symmetric squarylium dyes are essentially linear molecules with identical donor groups at both ends bound with the central acceptor group C4O2, and hence they
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have no permanent dipole moments and exhibit high nonlinearities. In particular, polymer matrices based on polymethylmethacrylate doped with squarylium dyes possess highly nonlinear optical response [19]. In solid matrix of polyurethane [15] photostability of squarylium dye is higher in comparison with that of ethanol solutions, therefore it seems interesting to introduce such a dye in solid SiO2 matrix for creating optical limiting materials. As shown in a number of papers [15,20], the media with low viscosity may show non-radiative decay caused by internal rotation of the C–C bonds between the central C4O2 group and electron-donor end fragments in the excited state. For squarylium dyes non-radiative excitedstate decay may be inhibited by hydrogen bonds between the donor and acceptor parts of the dye molecule [15]. The presence of a hydroxyl group in the ortho-position of aromatic donor groups in squarylium dye molecule such as SqD1 (Fig. 1), leads to the formation of hydrogen bonds between these hydroxyl groups and the polar oxygen atoms of the central ring C4O2 [15,20]. The structure of porous SiO2 matrices meant for dye incorporation is to allow penetration of the solvent into their bulk and to remain undisturbed at saturation with the dye solvent. The pore size in the matrices can be controlled by technological conditions of the obtaining of the material taking into account the size of the objects to be incorporated. It should be noted that the micro-environment of the dye molecules incorporated into SiO2 matrices differs from that in the solution. This is due to the interaction of the dye with the pore surface. The character of the distribution of the pores in the bulk of nanoporous materials based on SiO2 was investigated by the methods of scanning electron microscopy [21,22], transmission electron microscopy (TEM) [10,22,23] and atomic force microscopy (AFM) [24–26]. In the present paper we report the data on the synthesis of SiO2 matrices with different porosity, as well as on the conditions of the obtaining of composite SiO2:SqD1:ММА materials and on their spectral properties. It is shown that the use of HF acid with different concentrations in the process of sol–gel synthesis of SiO2 matrices leads to the formation of not only nanometric (up to 12 nm [2,9]), but also much larger pores (200 nm and more). The size of the latter is defined by the concentration of HF acid. 2. Experimental 2.1. Materials and synthesis of SiO2 matrices The silica gel was synthesized using tetraethoxysilane (TEOS; Aldrich), additionally purified ethyl alcohol, HF (40%, Aldrich), twice distilled water. In the capacity of active molecules there was applied squarylium dye SqD1 (Fig. 1) produced at the Institute of Organic Chemistry (Kiev, Ukraine) by the method described in [27]. Methylmethacrylate (MMA; Aldrich) was used for impregnation of SiO2 matrices. SiO2 matrices were obtained using the sol–gel method by TEOS hydrolysis with the addition of nitric acid as a reaction catalyst [2]. Ethanol and TEOS were being mixed during 30 minutes. Then there were added twice distilled water, a few drops of nitric acid, and different quantities of hydrofluoric acid (HF), thereat the molar ratio n(HF/TEOS) was varied from 0.08 to 0.76. The resulting mixture was being stirred during 2 h. The synthesized sol was poured into plastic cuvettes, the latter were hermetically sealed and stored till the gel was formed. Then the cuvettes were
Fig. 1. SqD1 dye structural formula.
opened and the samples were being dried during 3–4 weeks at room temperature and at 60 °С during the next 7–10 days. The solid xerogels were shaped as parallelepipeds with the dimensions 0.5 × 0.5 × 1.5 cm. To raise their mechanical strength, the samples were annealed in air at temperatures up to 600 °С, the rate of temperature rise and drop was 80 °С/hr. The density and porosity of the samples of SiO2 matrices were determined by the method of hydrostatic weighing and from their geometric size and weight. The annealed samples were being impregnated with the solutions of squarylium dye SqD1 (Fig. 1) in methylmethacrylate during 3 days and then polymerized at 45–50 °С during 7 days. 2.2. Characterization The absorption spectra of the samples were recorded by a spectrophotometer Lambda 35 UV/Vis Spectrophotometers (Perkin-Elmer, USA) in 200–1100 nm region (the wavelength reproducibility was ±0.05 nm, the photometric accuracy (using NIST 930D filter) and photometric reproducibility being ± 0.001 Å and b 0.001 Å, respectively. The measurement of the luminescence spectra was realized on a fluorimeter FluoroMax-4 (Horiba Jobin Yuon, USA), thereat the accuracy and repeatability were 0.5 nm and 0.1 nm, respectively, the integration time varied from 0.001 to 160 sec. The Fourier transforms infrared spectra of the crystals and of the powders were recorded at room temperature in 400–4000 cm−1 region using Spectrum One PerkinElmer with a resolution of 1 cm−1 by the KBr pellet technique. For preparation of the pellets there were used equal weighed samples each containing the matrix substance (0.0005 g) and 0.3 g of KBr. The size of SiO2 nanoparticles forming the xerogel structure was checked on a transmission electron microscope (EM-125) with an accelerating voltage of 100 kV. The samples were prepared according to the standard procedure by placing the replicas from the inner cleavage of the matrices on copper meshes coated with thin carbon film with subsequent drying. The surface microrelief was investigated by means of an atomic force microscope Solver P47H PRO (Russia). AFM data are qualitative for all the dimensions in a grey scale: dark and light tones represent the low and high features, respectively. The linear attenuation coefficient of X-ray passed through the sample (μ, cm− 1) was determined using a DRON–ЗМ diffractometer in СоKα1 – radiation (λ = 1.54051 Å [28,29]). The measurements of μ were realized on specially prepared plane-parallel samples with a thickness of 2 mm cut out from SiO2 matrices along the directions a (the horizontal direction) and z (the vertical one) of the sample. The value of the coefficient μ was calculated from the relation I = I0 · e−μd; μ = −(lnI/I0)/d, where I is the intensity of the incident X-ray beam; I0, the intensity of the X-ray beam transmitted through the sample, d, the thickness of the investigated sample. The error at determination of μ includes those of determination of I, I0 and d. The intensities I and I0 were measured with an error of 0.5%, for d the error was 1%. The error of determination of μ did not exceed 5%. 3. Results and discussion 3.1. Influence of HF acid concentration on the matrices porosity Fig. 2 presents the photographs of annealed pure SiO2 matrices. It is found that at the increase of the molar ratio n(HF/TEOS) up to 0.08, 0.380 and 0.760 the density of SiO2 matrices diminishes to 1.31, 0.85 and 0.62 g/сm3, respectively, and their open porosity increases (32.3, 54.6 and 74.7%). At the rise of the concentration of HF during the synthesis of the matrices the transmittance of the samples reduces (Fig. 3a). In particular, at n(HF/TEOS) = 0.08, 0.38 and 0.76 the transmittance of the samples at 600 nm is 71%, 51% and 25%, respectively (Fig. 3a). For the matrices synthesized at the same concentration of nitric acid, but without HF acid, the considered value amounts to 80% at 600 nm (its open porosity is 21%). At the same time, the transmittance
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Fig. 2. Appearance of the samples of SiO2 matrices with different open porosity: 32.3% (а) and 74.7% (b). The matrices were annealed at temperatures up to 600°С.
of the matrices synthesized at n(HF/TEOS) = 0.38 (at 600 nm) which have been immersed into ethylene glycol during 24 h till saturation of the pores with the solvent, increases from 51% to 80% (at 600 nm)
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(Fig. 3b). Addition of HF acid in the process of synthesis of the matrices increases their open porosity. Low light transmittance is evidently caused by the scattering in the matrices synthesized at n(HF/TEOS) = 0.38–0.76, due to the presence of a great number of large pores which size (400–700 nm and more) is comparable with the incident light wavelength. At HF concentrations lesser than n(HF/TEOS) = 0.25 the obtained matrices have low porosity and high density, that results in their destruction at annealing or impregnation with methylmethacrylate and impedes penetration of active molecules into the matrix bulk. At HF concentration higher than n(HF/TEOS) = 0.38 the matrices have large pores leading to the formation of cracks in the samples in the process of drying and annealing. As is known, the quality of silicate sol–gel matrices essentially depends on the composition of the initial reaction mixtures, the synthesis and drying conditions. The use of the acid as a catalyst results in the formation of a three-dimensional network consisting of SiO2 nanoparticles. The density of this network, its compression degree and the pore size can be regulated by changing the acidity of the medium in the course of sol–gel synthesis [30–33]. According to [30], due to the rise of the catalyst basicity the diameter of the silica gel pores grows from 0.5 nm to 4 nm. HF which is a weak acid is used for reducing the influence of the acidity of the medium on the structure of the formed threedimensional network while synthesizing SiO2 sol. As a result, the porosity of the material changes [2,9,34]. The density of bulk SiO2 matrices studied in [2] was 0.76 and 0.56 g/cm3, their porosity was 16.47% and 77.44% at n(HF/TEOS) = 0.15 and 0.30, respectively. According to literature data, the characteristic pore size for SiO2 films is 2–10 nm [9]; 1.4–18.4 nm at the increase of the ratio n(HF/TEOS) from 0 to 0.12 in silica membranes [34]; 9.04 nm and 30.02 nm at n(HF/TEOS) = 0.15 and 0.30, respectively, in bulk SiO2 matrices [2]. 3.2. Study of porosity in SiO2 matrices by transmission electron microscopy, atomic force microscopy and X-ray methods
Fig. 3. Transmittance spectra of pure SiO2 matrices synthesized at n(HF/TEOS): 0.08 (2), 0.38 (3), 0.76 (4) and without of HF (1) (a) and spectrum of SiO2 matrix synthesized at n(HF/TEOS) = 0.38 and saturated with ethylene glycol (b). The matrices were annealed at temperatures up to 600оС.
TEM study of the synthesized SiO2 matrices shows that their structure is formed by chains of 6–12 nm nanoparticles, as well as by small (5–12 nm) (Fig. 4а-с) and large (60–400 nm and more) pores (Fig. 4d). The pore size increases with the concentration of HF. The small pores may be rounded (at HF concentrations of 0.38–0.76 mole per 1 mole of TEOS) or elongated (when these concentrations diminish to 0.20–0.08 mole per 1 mole of TEOS). The analysis of the obtained data shows that with the rise of the concentration of HF at the synthesis of the matrices the size of the formed SiO2 particles exceeds that in the samples with lower HF concentration (Fig. 4). For instance, at n(HF/TEOS) equal to 0.08 and 0.24 mole per 1 mole of TEOS the pore size is 60–140 nm and 100–200 nm, respectively. HF acid possesses weak alkalinity and regulates the rates of TEOS hydrolysis and condensation. As a result, the SiO2 particles formed in the xerogel network are larger in comparison with the ones obtained while using a strong acid as a catalyst [9]. The concentration of HF also influences the formation of branched structure consisting of SiO2 nanoparticles in the samples. Due to high concentrations of the catalyst HF (which shortens the period of gelation), TEOS does not undergo the complete stage of hydrolysis and condensation, and such a branched structure is not formed. It is known that with the rise of the HF/TEOS molar ratio the surface area of the samples decreases, while the pore volume and the pore radii increase [2,34]. The surface area (m2/g) of bulk SiO2 matrices is 827 m2/g, 679 m2/g and 216 m2/g at the ratio n(HF/TEOS) equal to 0.09, 0.15 and 0.30, respectively [2]. As shown in [9], SiOF thin film contains natural Si-O tetrahedron, destroyed F-O tetrahedron, and bridge oxide, which lead to relaxation of the network structure leaving more space and more pores in the films. The method of АFM allowed to reveal the presence of pores with a size of 100–300 nm at n(HF/TEOS) = 0.08 and 300–600 nm and larger at n(HF/TEOS) = 0.38 (Fig. 5). The pore sizes are confirmed by the
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Fig. 4. TEM image of the surface of SiO2 matrices synthesized at different molar ratio n(HF/TEOS): 0.08 (а), 0.38 (b), 0.76 (с) and 0.24 (d).
profiles of AFM images. Thus, SiO2 matrices are porous xerogel structures which consist of small (5–12 nm) and large (60–600 nm) pores. The size of the pores increases with the content of HF.
To estimate the character of pore distribution in the matrices we measured linear attenuation coefficient μ of monochromatic СоKα1 of incident X-ray. This coefficient characterizes relative reduction of the
Fig. 5. AFM image of the surface of SiO2 matrices synthesized at different molar ratio n(HF/TEOS): 0.08 (a) and 0.38 (c), and the corresponding surface profiles: n(HF/TEOS): 0.08 (b) and 0.38 (d).
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intensity of the beam transmitted through a layer of absorber. As seen from the data presented in Fig. 6, the coefficient μ diminishes with the rise of the concentration of HF, due to the increase of the porosity of the formed matrices. Thereat, if the concentration of HF is lower than 0.38 mole per a mole of TEOS, the coefficient μ has different values for the samples of the same matrices cut out from it along the directions а and z (Fig. 6). This testifies that the pores are extended along the direction z. If the matrix density decreases the coefficient μ has the same value for the directions а and z, and diminishes practically by half for n(HF/TEOS) = 0.76 in contrast to n(HF/TEOS) = 0.38 samples. This may be explained by the formation of a porous structure with an open porosity of 74.7% and the same density along the directions а and z. For the samples cut out from different parts in the bulk of the same SiO2 matrix, the linear coefficients of X-radiation attenuation μ have close values. This shows that the matrix is homogeneous. 3.3. Spectral properties of SiO2 matrices with incorporated Squayilium dye SiO2 matrices to be impregnated with the solution of SqD1 dye in methylmethacrylate (SiO2:SqD1:MMA) were synthesized at n(HF/ TEOS) ratios equal to 0.08, 0.38 and 0.76, the dye concentration varied from 7.1 · 10−6 М to 1.5 · 10−4 М. The absorption band maximum of 1 · 10−5 М SqD1 in methylmethacrylate, ethanol and SiO2 sole is 640 nm, 641 nm and 646 nm, respectively (Fig. 7). The observed insignificant bathochromic shift of the absorption maximum of SqD1 in SiO2 with respect to that in the solution is caused by changes in the microenvironment of SqD1 dye and by intensification of the interaction between its molecules and the medium. Typical absorption and luminescence spectra of SiO2:SqD1:ММА composites are presented in Figs. 7 and 8b. It is found that in these composites synthesized at n(HF/TEOS) = 0.08–0.38, the dye absorption and luminescence maxima are 646 nm and 671 nm, respectively. It is to be noted that the increase of the concentration of HF acid up to n(HF/TEOS) = 0.76 at the synthesis of the matrices gives rise to the shift of the absorption maximum of SiO2: SqD1:ММА composites to 634 nm. Thereat, the luminescence maxima of these composites synthesized at n(HF/TEOS) = 0.08, 0.38 and 0.76 are 675 nm, 675 nm and 653 nm, respectively (Fig. 8b). Such shifts in the absorption and luminescence band maxima may be due to changes in the dye micro-environment in the pores. To estimate the state of the xerogel surface we measured the IR spectra of the matrices synthesized at different HF concentrations. As is known, the surface of pores in Er2O3-SiO2 xerogels is characterized by the presence of Si–OH stretching vibration free silanol groups; moreover, the residual porosity and incomplete densification of the samples are defined by the dopant concentration [6]. The structure of
Fig. 6. Dependences of the linear attenuation coefficient μ of X-radiation passing through the investigated part of SiO2 matrices, on the molar ratio of the synthesis precursors n(HF/ TEOS) and the porosity of annealed matrices (Po).
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Fig. 7. Absorption spectra of 1 · 10−5 М SqD1 dye in different environments: methylmethacrylate solution (1), ethanol solution (2), SiO2 sol (3), and in annealed SiO2 matrices (n(HF/TEOS) = 0.24) with ММА (4).
silica consists of assemblies of Si-O-Si rings of various sizes in which each Si-O-Si belongs to a cyclic structure [35]. The bands between 1080 cm−1 and 1200 cm−1 correspond to Si-O(− Si) vibrations. The peaks observed at ~ 1100 cm−1 correspond to stretching vibration of Si-O(− Si), belonging to a more linear and less cross-linked structure [35]. For the synthesized matrices there was observed the shift of the position of peaks towards higher frequencies from 1089 cm−1, 1097 cm− 1 to 1104 cm− 1 with the rise of the HF concentration (Fig. 9). The absorption band around 1100 cm−1 is related to asymmetric stretching of Si-O-Si bonds [36]. The observed shift of the stretching vibration of Si-O-Si to higher frequencies caused by increased
Fig. 8. Normalized absorption (a) and luminescence (b) spectra of SiO2 matrices, synthesized at different molar ratio n(HF/TEOS): 0.76 (1), 0.38 (2), 0.08 (3) and saturated with 1.2 · 10−5 М SqD1 dye in ММА. SiO2:SqD1:MMA composite was excited at λex = 590 nm (1), λex = 640 nm (2) and λex = 640 nm (3).
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concentrations of HF acid will crack at drying, annealing or impregnation with methylmethacrylate. At a porosity of 50% and higher the density of the distribution of the matrices pores along the horizontal and vertical directions is the same. The properties of SqD1 dye in the matrices pores are influenced by its micro-environment. The observed hypochromic shift of the absorption and luminescence band maxima of SqD1 dye in the matrices synthesized at (n(HF/TEOS) = 0.76) with respect to the maxima in the matrices synthesized at n(HF/TEOS) =0.38–0.08 is caused by diminution of the quantity of ОН-groups on the xerogel surface.
Acknowledgments The authors are grateful to Dr. D. Sophronov for measurement of the FTIR-spectra (Institute for Single Crystals, NAS of Ukraine). Fig. 9. FTIR-spectra of pure SiO2 matrices synthesized at n(HF/TEOS): 0.08 (1), 0.38 (2), 0.76 (3). The matrices were annealed at temperatures up to 600оС.
References strengthening of the network in the matrices [37]. The peak around 960 cm−1 corresponds to the Si-O(H) stretching vibration [35,38]. As seen from Fig. 9, the ratio of the intensities of the absorption bands at ~ 1200–1080 cm−1 increases in comparison with that of the band with a peak at 961 cm−1 with the rise of the HF concentration. There is observed the decrease in the intensity of the absorption corresponding to Si-O(H) stretching vibration as against that of Si-O(− Si) stretching vibration. This is caused by the diminution of the number of-OH groups on the silica surface with simultaneous strengthening of the network of silica. Moreover, some part of OH-groups on the surface of SiO2 xerogel nanoparticles may be replaced by fluorine ions. The temperature of Si-F bond breaking may be as high as 700 °C, therefore during the annealing at temperatures up to 600 °C fluorine ions remain on the xerogel surface. As a result, the micro-environment of the dye molecules in the pores changes, and the interaction between the dye and the surface reduces. It is known that with the rise of the concentration of HF acid at the synthesis of the matrices their porosity increases, and the surface area diminishes due to the formation of large pores [2]. The influence of the xerogel porosity and of the presence of nanometric and larger pores reveals itself in the decreased quantity of the absorbed molecules of SqD1 dye in the matrices at the rise of their porosity. At n(HF/TEOS) = 0.08, 0.38 and 0.76 (and a porosity of 32.3%, 54.6% and 74.7%) the concentration of SqD1 dye molecules adsorbed in the matrices pores is 6.2 · 10−5 mole/dm3, 3.7 · 10−5 mole/cm3 and 2.6 · 10−5 mole/cm3, respectively. The matrices were saturated with SqD1 dye solution in ММА containing 1.2 · 10−5 М of the dye. The content of the dye in the matrix per unit volume is larger in comparison with the one in the solution, since the dye molecules are concentrated in the matrix. An essential diminution of the dye concentration in the matrices with a porosity of 74.7% and 54.6% in comparison with that in the matrices with a porosity of 32.3% is due to predominance of large pores with a size of several hundreds nanometers reducing the xerogel surface area. Thus, for the formation of the composite material (SiO2:SqD1:MMA) the matrices synthesized at the value of n(HF/TEOS) ranging between 0.25 to 0.38. are most preferable. 4. Conclusions It is shown that at the synthesis of SiO2 matrices using HF acid there is created a porous xerogel structure formed by both nanometric (5–12 nm) and considerably larger (60–600 nm) pores. The size of the large pores increases with the concentration of HF acid. To obtain composite materials based on SiO2 matrices annealed at temperatures up to 600 °С and higher, their synthesis is to be realized at the ratio n(HF/ TEOS) ranging between 0.25 and 0.38. The matrices formed at other
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