ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 437, Part 2, pp. 120–123. © Pleiades Publishing, Ltd., 2011.Original Russian Text © V.P. Zubov, A.A. Ishchenko, P.A. Storozhenko, I.V. Bakeeva, Yu.O. Kirilina, G.V. Fetisov, 2011, published in Doklady Akademii Nauk, 2011, Vol. 437,No. 6, pp. 768–771.

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Nanocrystalline silicon (nc�Si) has a number ofunique optical and electrophysical properties [1],which, together with nontoxicity and relatively lowcost, make it quite promising for a wide diversity offields of science and technology. For the creation andefficient practical use of new nc�Si�based polymermaterials with useful properties, such as sun�protec�tive films [2] and coatings [3], photo� and electrolumi�nescent composites [1, 4], and light�fast pigments [5],an important synthetic problem is to insert thesenanosized particles into polymer matrices. In the sim�plest case, the role of a polymer matrix reduces toensuring necessary physicomechanical properties of acomposite (mechanical strength, elasticity, adhesionto a support, resistance to aggressive media, etc.). Inaddition, the matrix should prevent agglutination ofnanoparticles and create the conditions for uniformdistribution of nc�Si throughout the sample volume orfor its immobilization on the surface. Finally, if thematrix itself has special optical, electrical�conduction,or photoelectric properties, then the addition of thefunctional properties of nc�Si particles gives rise to amaterial with a set of new characteristics.

Such matrices can be polymer hydrogels, which arewidely used for solving scientific and applied problemsfor more than half a century [6]. The known methodsfor producing hydrogels, as well as all polymer gels, arebased on the creation of a three�dimensional networkby forming covalent or ionic bonds between macro�molecules. The cross�linking of polymer chains toform a spatial frame is possible with the participation

of hydrogen or coordination bonds, van der Waalsforces, and hydrophobic interactions. Agents thatcross�link macromolecules can also be nanoparticlesof various chemical natures [7–11]. The systems inwhich the dispersion medium is an organic polymer,the dispersed phase is inorganic inclusions, and thecomponents interact on the molecular level are calledhybrid materials (nanocomposites), and the corre�sponding hydrogels are called organic–inorganichybrid hydrogels.

In this work, we developed approaches to produc�ing organic–inorganic hybrid hydrogels based onpolyvinylpyrrolidone and the products of hydrolyticpolycondensation of tetramethoxysilane as structure�forming agents with the inclusion of nc�Si particles.We studied the spectral properties and potential appli�cation fields of such materials.

Particles of nc�Si were synthesized in an argonplasma in a closed gas cycle. The reactor was a plasmaevaporator�condenser operating in a low�frequencygas discharge. The initial raw material was a siliconpowder, which was fed to the reactor with a gas flowfrom a dispenser. In the reactor, the powder evapo�rated at 7000–10 000°C. At the outlet of the high�tem�perature plasma zone, the obtained vapor–gas mix�ture was subjected to abrupt cooling by gas jets,because of which the silicon vapor condensed to forman aerosol. The ready powder was collected by a tubu�lar cloth filter. From the filter, the collected readypowder was transferred in an inert atmosphere (in abox) to a sealed container or was moved to a microen�capsulation system, where the powder particle surfacewas coated with an inert weather�protective layer[3, 12].

The initial testing of the nc�Si properties was per�formed using complementary procedures [13]. Thenc�Si powder particles were visualized by transmissionelectron microscopy. The electron micrographs of0.1 wt % nc�Si samples were taken with a LEO912AB

CHEMISTRY

Organic–Inorganic Hybrid Hydrogels Containing Nanocrystalline Silicon

V. P. Zubova, A. A. Ishchenkoa, Corresponding Member of the RAS P. A. Storozhenkob, I. V. Bakeevaa, Yu. O. Kirilinaa, and G. V. Fetisovc

Received June 18, 2010

DOI: 10.1134/S0012500811040148

a Lomonosov Moscow State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 119571 Russia

b State Research Institute of Chemistry and Technology of Organoelement Compounds, sh. Entuziastov 38, Moscow, 111123 Russia

c Moscow State University,Moscow, 119991 Russia

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ORGANIC–INORGANIC HYBRID HYDROGELS 121

OMEGA electron microscope. A visual analysis of themicrographs showed that the nc�Si particles are spher�ical and have indistinct contours; probably, this is howthe particle shells look. The quality of the high�resolu�tion transmission microscopy images taken at variousmagnifications allowed us, using the UTHSCSAImage Tool program, to perform particle size classifi�cation, construct distributions functions for all thesamples studied, and determine the average nc�Si par�ticle diameter (27 ± 3 nm).

The IR spectra were recorded with a NicoletImpact 410 Fourier�transform IR spectrometer thatoperated in the wavelength range 2.5–25 µm (4000–400 cm–1) and was equipped with a diffuse reflectanceunit for recording the spectra of powder materials. Forthese studies, a thin powder layer several micrometersthick was placed in a specially constructed cell withwindows made from polished silicon plates. The IRspectra of the nc�Si particles contain intense absorp�tion bands at 461, 799, 978, 1072, and 1097 cm–1.These intense bands are indicative of the formation ofeither the phase SiO2 or the phase SiOx (x = 1.5–2),which, according to our concepts of the synthesismechanism, forms on the surface of the nanoparticles.

The Raman spectra were recorded on a Jobin YvonT�64000 triple Raman spectrometer system withargon laser excitation (λ = 514.5 nm). The Ramanband of the nc�Si powder is shifted toward lower fre�quencies by 1.5–2.5 cm–1 with respect to the band ofsingle�crystalline silicon. Moreover, the Raman peakof the powder silicon is, on the average, 25% broaderthan the single�crystalline silicon band, the width ofwhich is ~4 cm–1. The average nc�Si particle diameter

can be calculated using a model of spatial phononconfinement and light scattering in a spherical crystal�lite. Using a published procedure [13], the nc�Si par�ticle core size can be found to be 17 ± 2 nm.

The transmission spectra of these composite mate�rials within the range 200–850 nm were recorded witha Specord M 40 spectrometer (Carl Zeiss Jena). Thesamples were measured according to two procedures.In the first of them, the collimated transmission wasmeasured in the direction parallel to that of the proberadiation incident on the sample, which correspondedto the measurement of the optical density at the obser�vation wavelength (Fig. 1a). In the second, the trans�mission spectra were recorded using an integratingsphere in which a test sample was placed. In this case,the photodetector collected not only the light thatpassed directly through the sample layer, but also theradiation scattered into the solid angle 2π. The proce�dure for spectral analysis of nc�Si was described indetail previously [13].

The X�ray diffraction analysis of the obtained nc�Sipowder was performed using a Guinier�type focusingtransmission diffractometer with a Huber G670 cam�era with a bent Ge(111) primary�beam monochroma�tor, which cuts out the line (λ = 1.5405981 Å) of

characteristic radiation of an X�ray tube with a copperanode. The X�ray powder diffraction pattern withinthe diffraction angle range 2θ from 3° to 100° wasrecorded using an IP detector—an optical memoryplate curved along the circumference of the chamber.The phase composition of the samples was estimatedby comparing the X�ray powder diffraction patternswith each other and with standard patterns. The parti�

Kα1

80

200λ, nm

T, %

400 600 800 1000

70

60

50

40

30

20

10

0

12

3(а)

20 40 60 80 100 120 140 160

0.020

0.015

0.010

0.005

0

P(d

), r

el. u

nit

s

(b)

d, nm

Fig. 1. (a) Transmission spectra of a polyvinylpyrrolidone–nc�Si/SiO2 film at nanosilicon concentrations [nc�Si] of (1) 0.5,(2) 1.0, and (3) 1.5 wt % and (b) the nc�Si crystallite size distribution density function obtained by modeling and fitting using thecomplete profile of the X�ray powder diffraction pattern.

122

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ZUBOV et al.

cle size distribution was determined by modeling theparticle microstructure [14] and constructing thecomplete profile of the diffraction pattern with con�sideration of the instrumental function of the diffrac�tometer and the log�normal particle size distribution.The parameters of the model were refined by fitting itby nonlinear least squares to the complete profile ofthe experimental diffraction pattern (Fig. 1b) usingthe PM2K program.

We have previously demonstrated the formation oforganic–inorganic hybrid hydrogels based on linearwater�soluble organic polymers (poly(N�vinylpyrroli�done), poly(N�vinylcaprolactam)) and inorganicnanoparticles (products of hydrolytic polycondensa�tion of tetramethoxysilane) [8, 10, 11]. A gel networkin such systems forms because of the formation of sta�ble hydrogen bonds between surface HO groups of sil�ica and O=C groups in substituents in chains ofpoly(N�vinylamides). Since the surface layers of thenc�Si particles may contain HO groups resulting fromsilicon oxidation, these particles can be expected toalso participate in the formation of bonds with hydro�philic polymer chains and become nodes in organic–inorganic hybrid hydrogels.

The experiments confirmed that, at an nc�Si parti�cle concentration in an aqueous polyvinylpyrrolidone

( = 1.3 × 106) solution of 2–5 wt %, a organic–inorganic hybrid hydrogel can be obtained. This is ahighly colored dark material without satisfactoryphysicomechanical properties, which completelyabsorbs light in the UV range. To obtain homogeneoustransparent films with necessary physicomechanical

wM

and optical characteristics, a procedure for producinga mixed�type organic–inorganic hybrid hydrogel wasproposed. In this case, the base�forming particles weresilica particles produced by sol–gel transformations oftetramethoxysilane in a polyvinylpyrrolidone solu�tion, and the particles coparticipating in the gel for�mation were nc�Si particles.

The initial mixture was an aqueous polyvinylpyr�rolidone solution to which various amounts of anaqueous dispersion of nc�Si particles (0.5–1.5 wt %)and tetramethoxysilane were added. In the course ofgel formation, neither aggregation of nc�Si particles,nor phase separation of components was observed.The properties of the uniform (homogeneous)organic–inorganic hybrid hydrogel (table; Fig. 2)depended on both polymer and tetramethoxysilaneconcentrations and were hardly affected by varying theconcentration of nc�Si particles. The physicomechan�ical characteristics of the organic–inorganic hybridhydrogel were measured in both uniaxial tension andiniaxial compression. Young’s modulus Е was calcu�lated by a standard equation [16].

The measurement results in Fig. 2 show that, withincreasing polyvinylpyrrolidone and tetramethoxysi�lane concentrations, Young’s modulus of the gelincreases and the equilibrium degree of swelling αdecreases, which is indicative of an increase in theeffective gel cross�linking density.

Conversely, the optical properties of the organic–inorganic hybrid hydrogel changed strongly after add�ing nc�Si particles. The organic–inorganic hybridhydrogel without nc�Si particles was optically trans�

8 12 16

30

α,

g w

ater

/g g

el

(b)28

26

24

22

20

18

[PVP], wt %10 14 18

E, Pa(а)4000

0

3000

2000

1000

Fig. 2. (a) Uniaxial tension Young’s modulus and (b) equilibrium degree of swelling of the polyvinylpyrrolidone–SiO2 organic–inorganic hybrid hydrogel versus polyvinylpyrrolidone (PVP) concentration at a tetramethoxysilane concentration in the reactionmixture of 6.2 wt %. Different symbols represent the results of testing of different series of specimens.

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ORGANIC–INORGANIC HYBRID HYDROGELS 123

parent within the wavelength range 200–1000 nm,whereas after addition of nc�Si particles the materialhad uniform yellow�brown color, the intensity andshade of which depended on the nc�Si particle con�centration. Figure 1a presents the transmission spectraof the organic–inorganic hybrid hydrogel produced atvarious nc�Si particle concentrations.

Obviously, the nc�Si particles strongly influencelight absorption, and it increases with increasing nc�Siparticle concentration. To study the optical character�istics, the organic–inorganic hybrid hydrogel was pro�duced on quartz glass plates. After water evaporation,the forming films had high adhesion to the support andhigh mechanical strength.

Thus, in this work, we proposed a new approach tothe insertion of nc�Si into matrices of mixed organic–inorganic hybrid hydrogels based on linear water�sol�uble organic polymers, in which the structure�formingagent is silica nanoparticles and the optical propertiesare ensured by the presence of a given amount of nc�Siparticles. Obviously, this approach can also be used forinserting other (fluorescent, magnetic, etc.) nanopar�ticles into polymer matrices to obtain functional com�posite systems with controlled properties.

ACKNOWLEDGMENTS

We thank V.N. Bagratashvili and A.P. Sviridov(Institute of Problems of Laser and Information Tech�nologies, Russian Academy of Sciences), and also toA.A. Rybaltovskii (Research Institute of NuclearPhysics, Moscow State University) and K.V. Zaitseva

(Lomonosov Moscow State Academy of Fine Chemi�cal Technology) for their help in spectral experimentsand discussion of the results obtained.

This work was supported by the Russian Founda�tion for Basic Research (project nos. 09–02–12325�ofi_M and 10–02–92000�NNS_a).

REFERENCES

1. Nanosilicon, Kumar, V., Ed., Elsevier, 2008.2. Application for RF Patent No. 2009145013, December

4, 2009.3. Bagratashvili, V.N., Tutorskii, I.A., Belogorokhov, A.I.,

et al., Dokl. Phys. Chem., 2005, vol. 405, part 1,pp. 240–243 [Dokl. Akad. Nauk, 2005, vol. 405, no. 3,pp. 360–363].

4. Nanostructured Materials. Processing, Properties, andApplications, Koch, C.C., Ed., New York: WilliamAndrew, 2009.

5. Application for RF Patent No. 2009146715, December16, 2009.

6. Biorelated Polymers and Gels, Okano, T., Ed., SanDiego: Academic, 1998.

7. Averochkina, I.A., Papisov, I.M., and Matvienko, V.N.,Vysokomol. Soedin., Ser. A, 1993, vol. 35, no. 12,pp. 1986–1990.

8. Loos Wouter, Verbrugge Sam, Du Prez Filip E., et al.,Macromol. Chem. Phys., 2003, vol. 204, no. 1, pp. 98–103.

9. Bakeeva, I.V., Kolesnikova, Yu.A., Kataeva, N.A.,et al., Izv. Akad. Nauk, Ser. Khim., 2008, no. 2,pp. 329–336.

10. Kirilina, Yu.O., Bakeeva, I.V., Bulychev, N.A., andZubov, V.P., Vysokomol. Soedin. Ser. B, 2009, vol. 51,no. 4, pp. 705–713.

11. Bakeeva, I.V., Ozerina, L.A., Ozerin, A.N., andZubov, V.P., Vysokomol. Soedin. A, 2010, vol. 52, no. 5,pp. 776–786.

12. Tutorskii, I.A., Belogorokhov, A.I., Ishchenko, A.A.,and Storozhenko, P.A., Kolloidn. Zh., 2005, vol. 67,no. 4, pp. 5441–5447.

13. Rybaltovskii, A.O, Ishchenko, A.A., Koltashev, V.V.,et al., Metodika eksperimental’nogo issledovaniya spek�tral’nykh kharakteristik vodno�emul’sionnykh kompozit�nykh sred, soderzhashchikh nanochastitsy kremniya(Procedure of Experimental Study of Spectral Charac�teristics of Water–Emulsion Composite Media Con�taining Silicon Nanoparticles), Certificate of the Met�rologic Expertise of the State Service of Standard Ref�erence Data GSSSD ME 131�2007, June 12, 2007.

14. Scardi, P., Z. Kristallogr., 2008, vol. 27, Suppl.,pp. 101–111.

15. Kuleznev, V.N. and Shershnev, V.A., Khimiya i fizikapolimerov (Chemistry and Physics of Polymers), Mos�cow: KolosS, 2007.

Effect of the composition of the polyvinylpyrrolidone–wa�ter–tetramethoxysilane mixture on uniaxial tension Young’smodulus of the organic–inorganic hybrid hydrogel

Initial concentration of aqueous polyvinylpyr�

rolidone solution

Tetramethoxysilane concentration

in reaction mixture E, kPa

wt %

6 3.9 0.7

5.4 1.1

8 4.2 1.3

7.2 1.7

10 3.9 2.3

6.4 4

9.0 10