See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/260296737 Composite materials based on nanoporous SiO2 matrices and squarylium dye ARTICLE in JOURNAL OF NON-CRYSTALLINE SOLIDS · APRIL 2014 Impact Factor: 1.77 · DOI: 10.1016/j.jnoncrysol.2014.01.052 CITATION 1 READS 99 10 AUTHORS, INCLUDING: Olga N Bezkrovnaya National Academy of Sciences of Ukraine 29 PUBLICATIONS 76 CITATIONS SEE PROFILE Igor Pritula Institute for Single Crystals, Kharkov, Natio… 80 PUBLICATIONS 200 CITATIONS SEE PROFILE Anna Plaksii Institute for Single Crystals of National Aca… 3 PUBLICATIONS 4 CITATIONS SEE PROFILE Available from: Igor Pritula Retrieved on: 10 February 2016
Journal of Non-Crystalline Solids 389 (2014) 11–16
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Composite materials based on nanoporous SiO2 matricesand 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 Institute for Single Crystals, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001 Kharkiv, Ukraineb Institute for Organic Chemistry, NAS of Ukraine, Murmanskaya 5, Kiev-94, 253660, Ukrainec Institute for Scintillation Material, SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin Avenue, 61001, Kharkiv, Ukraine
Article history:Received 4 November 2013Received in revised form 15 January 2014Available online xxxx
Keywords:Sol–gel SiO2 matrices;Nanopores;Transmission electron microscopy;Atomic force microscopy;Squarylium dyes;Luminescence spectra
Sol–gel SiO2 matrices with different contents of HF acid were synthesized. The matrices were annealed at 600 °Cand impregnated with squarylium dye SqD1 in methylmethacrylate (MMA) solvent. Xerogel nanoporousstructure of SiO2 matrices was found to contain 5–12 nm and 60–600 nm pores. The size of large pores increaseswith the concentration of HF acid. For the synthesis of SiO2 matrices the ratio n(HF/TEOS) = 0.25–0.38 is themost optimal. At other HF concentrations the formed matrices are prone to cracking at drying, annealing andimpregnation with the dye solution in MMA. The location of the maxima of the absorption and luminescencebands of SqD1 dye in the composite SiO2:SqD1:ММА is found to be influenced by the surface state of SiO2 xerogeldepending on the concentration of HF acid at the synthesis of the matrices.
Recent decade has seen increased interest in optical-quality nano-porous structures based on SiO2 synthesized by the sol–gel method(bulk monoliths [1–5], glasses [2,6–8], fibres [7] and films [9]) meantfor the creation of new optoelectronic devices. SiO2 matrices possesshigh transparency in the visible and near-IR spectral regions, and arecharacterized by high mechanical strength and bulk laser damagethreshold, as well as high absorption ability and chemical stability.Suchmatricesmay be used as a base for active solidmediawithminimallight scattering, good thermal conductivity and low temperature coeffi-cient of the change of the refractive index [1].
In a number of papers there is reported the obtaining of nanoporoussilicate matrices with isolated 5–15 nm PbS nanocrystals distributedin the matrix bulk. Such matrices are applied for the creation ofnarrow-band-gap semiconductors [10] and SiO2 matrices with LiIO3
nanocrystals for second-order nonlinear optics [11]. SiO2 matrices donot possess nonlinear optical (NLO) properties, but due to low opticallosses they serve as ideal matrices for nonlinear materials. Consideredin [12] is the use of the sol–gel method for the synthesis of three typesof nanocomposites with NLO properties: semiconductor-glass, metalcluster-glass, and organics-glass nanocomposites. Another topical prob-lem is the creation of laser media on the base of nanoporous SiO2
krovnaya).
vier B.V.
matrices with incorporated organic dyes which allow to obtain genera-tion of wide-band radiation while developing tuneable solid lasers[1,2,13,14].
Advance of laser technologies has aroused considerable interest inthe creation of optical limiting materials used for protecting solid-statesensors and eyes from intense laser beams [15–17]. Thereat, particularattention 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 behaviourof acid blue ethanol solutions under the influence of a low-power CWHe-Ne laser. In [15] the non-linear properties of polymethine andsquarylium dyes are investigated in the elastopolymeric material poly-urethane acrylate and in ethanol solutions.
Creation of optical limiting devices for protecting sensitive optical el-ements involves the use of active components with high reverse satura-ble absorption (RSA) [15]. The mechanism of RSA is based on inducedabsorption from excited singlet or triplet states of the medium. This isone of low-threshold and effective physicalmechanisms leading to non-linear attenuation of radiation intensity. The necessary condition for theexistence of RSAmechanism in dyemolecules is the excess of the cross-section of absorption from the excited state over that of linear absorp-tion from the ground state at the pumping wavelength [15,16,18,19].High values of RSA in the visible spectral region are characteristic of or-ganic dyes including polymethine and squarylium which have highlypolarizable π-electron systems [15,17,18]. Symmetric squarylium dyesare essentially linear molecules with identical donor groups at bothends bound with the central acceptor group C4O2, and hence they
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have no permanent dipole moments and exhibit high nonlinearities. Inparticular, polymer matrices based on polymethylmethacrylate dopedwith squarylium dyes possess highly nonlinear optical response [19].In solid matrix of polyurethane [15] photostability of squarylium dyeis higher in comparison with that of ethanol solutions, therefore itseems interesting to introduce such a dye in solid SiO2 matrix for creat-ing optical limiting materials.
As shown in a number of papers [15,20], the media with low viscos-itymay shownon-radiative decay caused by internal rotation of the C–Cbonds between the central C4O2 group and electron-donor end frag-ments in the excited state. For squarylium dyes non-radiative excited-state decay may be inhibited by hydrogen bonds between the donorand acceptor parts of the dyemolecule [15]. The presence of a hydroxylgroup in the ortho-position of aromatic donor groups in squaryliumdyemolecule such as SqD1 (Fig. 1), leads to the formation of hydrogenbonds between these hydroxyl groups and the polar oxygen atoms ofthe central ring C4O2 [15,20].
The structure of porous SiO2 matrices meant for dye incorporation isto allow penetration of the solvent into their bulk and to remain undis-turbed at saturation with the dye solvent. The pore size in the matricescan be controlled by technological conditions of the obtaining of thema-terial taking into account the size of the objects to be incorporated. Itshould be noted that the micro-environment of the dye molecules in-corporated into SiO2 matrices differs from that in the solution. This isdue to the interaction of the dye with the pore surface. The characterof the distribution of the pores in the bulk of nanoporous materialsbased on SiO2was investigated by themethods of scanning electronmi-croscopy [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 ma-trices with different porosity, as well as on the conditions of theobtaining of composite SiO2:SqD1:ММАmaterials and on their spectralproperties. It is shown that the use of HF acid with different concentra-tions in the process of sol–gel synthesis of SiO2matrices leads to the for-mation of not only nanometric (up to 12 nm [2,9]), but alsomuch largerpores (200 nm and more). The size of the latter is defined by the con-centration of HF acid.
2. Experimental
2.1. Materials and synthesis of SiO2 matrices
The silica gelwas synthesizedusing tetraethoxysilane (TEOS;Aldrich),additionally purified ethyl alcohol, HF (40%, Aldrich), twice distilledwater. In the capacity of active molecules there was applied squaryliumdye 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 hy-drolysis with the addition of nitric acid as a reaction catalyst [2]. Ethanoland TEOSwere beingmixed during 30 minutes. Then therewere addedtwice distilled water, a few drops of nitric acid, and different quantitiesof hydrofluoric acid (HF), thereat themolar ratio n(HF/TEOS)was variedfrom0.08 to 0.76. The resultingmixturewas being stirred during 2 h. Thesynthesized sol was poured into plastic cuvettes, the latter were hermet-ically 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 roomtemperature and at 60 °С during the next 7–10 days. The solid xerogelswere shaped as parallelepipeds with the dimensions 0.5 × 0.5 × 1.5 cm.To raise their mechanical strength, the samples were annealed inair at temperatures up to 600 °С, the rate of temperature rise anddrop was 80 °С/hr. The density and porosity of the samples of SiO2
matrices were determined by the method of hydrostatic weighingand from their geometric size and weight. The annealed sampleswere being impregnated with the solutions of squarylium dye SqD1(Fig. 1) in methylmethacrylate during 3 days and then polymerizedat 45–50 °С during 7 days.
2.2. Characterization
The absorption spectra of the samples were recorded by a spectro-photometer 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 pho-tometric reproducibility being ±0.001 Å and b 0.001 Å, respectively.The measurement of the luminescence spectra was realized on a fluo-rimeter FluoroMax-4 (Horiba Jobin Yuon, USA), thereat the accuracyand repeatability were 0.5 nm and 0.1 nm, respectively, the integrationtime varied from 0.001 to 160 sec. The Fourier transforms infrared spec-tra of the crystals and of the powders were recorded at room tempera-ture in 400–4000 cm−1 region using Spectrum One PerkinElmer with aresolution of 1 cm−1 by the KBr pellet technique. For preparation of thepellets therewere used equalweighed samples each containing thema-trix substance (0.0005 g) and 0.3 g of KBr. The size of SiO2 nanoparticlesforming the xerogel structure was checked on a transmission electronmicroscope (EM-125) with an accelerating voltage of 100 kV. The sam-ples were prepared according to the standard procedure by placing thereplicas from the inner cleavage of thematrices on coppermeshes coat-ed with thin carbon film with subsequent drying. The surface micro-relief was investigated by means of an atomic force microscope SolverP47H PRO (Russia). AFM data are qualitative for all the dimensions ina grey scale: dark and light tones represent the low and high features,respectively.
The linear attenuation coefficient of X-ray passed through the sam-ple (μ, cm−1) was determined using a DRON–ЗМ diffractometer inСоKα1 – radiation (λ = 1.54051 Å [28,29]). The measurements of μwere realized on specially prepared plane-parallel sampleswith a thick-ness of 2 mm cut out from SiO2 matrices along the directions a (thehorizontal direction) and z (the vertical one) of the sample. The valueof 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, theintensity of the X-ray beam transmitted through the sample, d, thethickness of the investigated sample. The error at determination of μ in-cludes those of determination of I, I0 and d. The intensities I and I0 weremeasured with an error of 0.5%, for d the error was 1%. The error ofdetermination 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 isfound 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, 0.85and 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 syn-thesis 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 trans-mittance of the samples at 600 nm is 71%, 51% and 25%, respectively(Fig. 3a). For the matrices synthesized at the same concentration of ni-tric acid, but without HF acid, the considered value amounts to 80% at600 nm (its open porosity is 21%). At the same time, the transmittance
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.
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of the matrices synthesized at n(HF/TEOS) = 0.38 (at 600 nm) whichhave been immersed into ethylene glycol during 24 h till saturation ofthe pores with the solvent, increases from 51% to 80% (at 600 nm)
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 atn(HF/TEOS) = 0.38 and saturated with ethylene glycol (b). The matrices were annealedat temperatures up to 600оС.
(Fig. 3b). Addition of HF acid in the process of synthesis of the matricesincreases their open porosity. Low light transmittance is evidentlycaused 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 whichsize (400–700 nm and more) is comparable with the incident lightwavelength.
At HF concentrations lesser than n(HF/TEOS) = 0.25 the obtainedmatrices have low porosity and high density, that results in their de-struction at annealing or impregnation with methylmethacrylate andimpedes penetration of active molecules into the matrix bulk. At HFconcentration higher than n(HF/TEOS) = 0.38 the matrices have largepores leading to the formation of cracks in the samples in the processof drying and annealing.
As is known, the quality of silicate sol–gel matrices essentially de-pends on the composition of the initial reaction mixtures, the synthesisand drying conditions. The use of the acid as a catalyst results in the for-mation of a three-dimensional network consisting of SiO2 nanoparticles.The density of this network, its compression degree and the pore sizecan be regulated by changing the acidity of the medium in the courseof sol–gel synthesis [30–33]. According to [30], due to the rise of the cat-alyst basicity the diameter of the silica gel pores grows from 0.5 nm to4 nm. HF which is a weak acid is used for reducing the influence ofthe acidity of the medium on the structure of the formed three-dimensional networkwhile synthesizing SiO2 sol. As a result, the poros-ity of the material changes [2,9,34]. The density of bulk SiO2 matricesstudied in [2] was 0.76 and 0.56 g/cm3, their porosity was 16.47% and77.44% at n(HF/TEOS) = 0.15 and 0.30, respectively. According to liter-ature data, the characteristic pore size for SiO2 films is 2–10 nm [9];1.4–18.4 nmat the increase of the ratio n(HF/TEOS) from 0 to 0.12 in sil-ica membranes [34]; 9.04 nm and 30.02 nm at n(HF/TEOS) = 0.15 and0.30, respectively, in bulk SiO2 matrices [2].
3.2. Study of porosity in SiO2 matrices by transmission electronmicroscopy,atomic force microscopy and X-ray methods
TEM study of the synthesized SiO2 matrices shows that theirstructure is formed by chains of 6–12 nm nanoparticles, as well as bysmall (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. Thesmall pores may be rounded (at HF concentrations of 0.38–0.76 moleper 1 mole of TEOS) or elongated (when these concentrations diminishto 0.20–0.08 mole per 1 mole of TEOS).
The analysis of the obtained data shows that with the rise of the con-centration of HF at the synthesis of the matrices the size of the formedSiO2 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 1mole of TEOS the pore size is 60–140 nmand 100–200 nm, respectively.HF acid possessesweak alkalinity and regulates the rates of TEOShydro-lysis and condensation. As a result, the SiO2 particles formed in thexerogel network are larger in comparison with the ones obtainedwhile using a strong acid as a catalyst [9].
The concentration of HF also influences the formation of branchedstructure consisting of SiO2 nanoparticles in the samples. Due to highconcentrations of the catalyst HF (which shortens the period of gelation),TEOS does not undergo the complete stage of hydrolysis and condensa-tion, and such a branched structure is not formed. It is known that withthe rise of the HF/TEOS molar ratio the surface area of the samples de-creases, while the pore volume and the pore radii increase [2,34]. Thesurface area (m2/g) of bulk SiO2 matrices is 827 m2/g, 679 m2/g and216m2/g at the ratio n(HF/TEOS) equal to 0.09, 0.15 and0.30, respective-ly [2]. As shown in [9], SiOF thin film contains natural Si-O tetrahedron,destroyed F-O tetrahedron, and bridge oxide, which lead to relaxationof the network structure leavingmore space andmore pores in the films.
The method of АFM allowed to reveal the presence of pores with asize of 100–300 nm at n(HF/TEOS) = 0.08 and 300–600 nm and largerat n(HF/TEOS) = 0.38 (Fig. 5). The pore sizes are confirmed by the
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).
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profiles of AFM images. Thus, SiO2 matrices are porous xerogel struc-tures which consist of small (5–12 nm) and large (60–600 nm) pores.The size of the pores increases with the content of HF.
Fig. 5.AFM image of the surface of SiO2matrices synthesized at differentmolar ratio n(HF/TEOS): 0
To estimate the character of pore distribution in the matrices wemeasured linear attenuation coefficient μ of monochromatic СоKα1 ofincident X-ray. This coefficient characterizes relative reduction of the
.08 (a) and 0.38 (c), and the corresponding surface profiles: n(HF/TEOS): 0.08 (b) and0.38 (d).
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).
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intensity of the beam transmitted through a layer of absorber. As seenfrom the data presented in Fig. 6, the coefficient μ diminishes with therise of the concentration of HF, due to the increase of the porosity ofthe formed matrices. Thereat, if the concentration of HF is lower than0.38 mole per a mole of TEOS, the coefficient μ has different values forthe samples of the same matrices cut out from it along the directionsа and z (Fig. 6). This testifies that the pores are extended along the di-rection z. If the matrix density decreases the coefficient μ has thesame value for the directions а and z, and diminishes practically byhalf 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 anopen porosity of 74.7% and the same density along the directions аand z. For the samples cut out from different parts in the bulk of thesame SiO2 matrix, the linear coefficients of X-radiation attenuation μhave 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 inmethylmethacrylate (SiO2:SqD1:MMA) were synthesized at n(HF/TEOS) ratios equal to 0.08, 0.38 and 0.76, the dye concentration variedfrom 7.1 · 10−6 М to 1.5 · 10−4 М. The absorption band maximumof 1 · 10−5 М SqD1 in methylmethacrylate, ethanol and SiO2 sole is640 nm, 641 nm and 646 nm, respectively (Fig. 7). The observed insig-nificant bathochromic shift of the absorption maximum of SqD1 in SiO2
with respect to that in the solution is caused by changes in the micro-environment of SqD1 dye and by intensification of the interactionbetween its molecules and the medium. Typical absorption and lumi-nescence spectra of SiO2:SqD1:ММА composites are presented inFigs. 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 nmand 671 nm, respectively. It is to be noted that the increase of the con-centration of HF acid up to n(HF/TEOS) = 0.76 at the synthesis of thematrices gives rise to the shift of the absorption maximum of SiO2:SqD1:ММА composites to 634 nm. Thereat, the luminescence maximaof these composites synthesized at n(HF/TEOS) = 0.08, 0.38 and 0.76are 675 nm, 675 nm and 653 nm, respectively (Fig. 8b).
Such shifts in the absorption and luminescence band maxima maybe due to changes in the dye micro-environment in the pores. To esti-mate the state of the xerogel surface we measured the IR spectra ofthe matrices synthesized at different HF concentrations.
As is known, the surface of pores in Er2O3-SiO2 xerogels is character-ized by the presence of Si–OH stretching vibration free silanol groups;moreover, the residual porosity and incomplete densification of thesamples are defined by the dopant concentration [6]. The structure of
Fig. 6. Dependences of the linear attenuation coefficient μ of X-radiation passing throughthe investigated part of SiO2matrices, on themolar ratio of the synthesis precursors n(HF/TEOS) and the porosity of annealed matrices (Po).
silica consists of assemblies of Si-O-Si rings of various sizes in whicheach Si-O-Si belongs to a cyclic structure [35]. The bands between1080 cm−1 and 1200 cm−1 correspond to Si-O(−Si) vibrations. Thepeaks observed at ~1100 cm−1 correspond to stretching vibration ofSi-O(−Si), belonging to a more linear and less cross-linked struc-ture [35]. For the synthesized matrices there was observed the shift ofthe 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 asymmet-ric stretching of Si-O-Si bonds [36]. The observed shift of the stretchingvibration of Si-O-Si to higher frequencies caused by increased
Fig. 8. Normalized absorption (a) and luminescence (b) spectra of SiO2 matrices, synthe-sized at different molar ratio n(HF/TEOS): 0.76 (1), 0.38 (2), 0.08 (3) and saturated with1.2 · 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).
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.
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strengthening of the network in the matrices [37]. The peak around960 cm−1 corresponds to the Si-O(H) stretching vibration [35,38].
As seen from Fig. 9, the ratio of the intensities of the absorptionbands at ~ 1200–1080 cm−1 increases in comparison with that of theband 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 corre-sponding to Si-O(H) stretching vibration as against that of Si-O(−Si)stretching vibration. This is caused by the diminution of the numberof-OH groups on the silica surface with simultaneous strengthening ofthe network of silica.
Moreover, some part of OH-groups on the surface of SiO2 xerogelnanoparticles may be replaced by fluorine ions. The temperature ofSi-F bond breaking may be as high as 700 °C, therefore during the an-nealing at temperatures up to 600 °Cfluorine ions remain on the xerogelsurface. As a result, the micro-environment of the dye molecules in thepores changes, and the interaction between the dye and the surfacereduces.
It is known that with the rise of the concentration of HF acid at thesynthesis of thematrices their porosity increases, and the surface area di-minishes due to the formation of large pores [2]. The influence of thexerogel porosity and of the presence of nanometric and larger pores re-veals itself in the decreased quantity of the absorbed molecules ofSqD1 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 con-centration of SqD1 dye molecules adsorbed in the matrices pores is6.2 · 10−5mole/dm3, 3.7 · 10−5mole/cm3 and 2.6 · 10−5mole/cm3, re-spectively. The matrices were saturated with SqD1 dye solution inММАcontaining 1.2 · 10−5 М of the dye. The content of the dye in the matrixper unit volume is larger in comparison with the one in the solution,since the dyemolecules are concentrated in thematrix. An essential dim-inution 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 of32.3% is due to predominance of large pores with a size of severalhundreds nanometers reducing the xerogel surface area. Thus, for theformation of the composite material (SiO2:SqD1:MMA) the matricessynthesized 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 thereis created a porous xerogel structure formed by both nanometric(5–12 nm) and considerably larger (60–600 nm) pores. The size of thelarge pores increases with the concentration of HF acid. To obtain com-posite materials based on SiO2 matrices annealed at temperatures up to600 °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
concentrations of HF acid will crack at drying, annealing or impregnationwith methylmethacrylate. At a porosity of 50% and higher the density ofthe distribution of thematrices pores along the horizontal and vertical di-rections is the same. The properties of SqD1 dye in thematrices pores areinfluenced by its micro-environment. The observed hypochromic shift ofthe absorption and luminescence bandmaxima of SqD1 dye in thematri-ces synthesized at (n(HF/TEOS) = 0.76) with respect to the maxima inthe matrices synthesized at n(HF/TEOS) =0.38–0.08 is caused by dimi-nution of the quantity of ОН-groups on the xerogel surface.
Acknowledgments
The authors are grateful to Dr. D. Sophronov for measurement of theFTIR-spectra (Institute for Single Crystals, NAS of Ukraine).
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