ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 5, pp. 487–490. © Pleiades Publishing, Inc., 2006. Original Russian Text © Yu.F. Kargin, S.N. Ivicheva, E.Yu. Buslaeva, T.B. Kuvshinova, V.D. Volodin, G.Yu. Yurkov, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 5, pp. 547–550.
Preparation of Bismuth Nanoparticles in Opal Matrices through Reduction of Bismuth Compounds with Supercritical Isopropanol Yu. F. Kargin, S. N. Ivicheva, E. Yu. Buslaeva, T. B. Kuvshinova, V. D. Volodin, and G. Yu. Yurkov Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia e-mail: [email protected]
Received October 19, 2005; in final form, November 21, 2005
Abstract—Bismuth nanoparticles have been produced in pores of opal matrices by reducing bismuth salts and oxide compounds with supercritical isopropanol. According to transmission electron microscopy data, the diameter of the SiO2 spheres in the opal matrices is about 260 nm, and that of the bismuth nanoparticles does not exceed 80 nm. DOI: 10.1134/S0020168506050074
There is wide research interest in nanoparticles as structural units of novel materials because they offer properties missing in bulk solids and allow one to control the lattice parameters, atomic dynamics, and thermal, electrical, and magnetic properties of materials. All of these effects are related to the small size of nanoparticles and are highly dependent on their surface condition and nanoparticle–matrix interaction. Isolated nanoparticles differ in properties from clusters forming nanosystems. In connection with this, synthesis methods play an important role in determining the qualitative characteristics of nanoparticles and nanostructures. Isolated nanoparticles can be prepared by a variety of processes [1–5]. Yurkov et al.  have recently proposed a technique for producing isolated bismuth-containing nanoparticles (in oxide, chloride, or oxychloride form) via thermal decomposition of appropriate salts in a high-temperature solution of high-pressure polyethylene. They have shown that the composition of the bismuth-containing nanoparticles stabilized in a polyethylene matrix can be varied, e.g., using wellknown reactions (with KOH, O2 , H2O2 , and Cl2) and also reactions of supercritical (SC) isopropanol with the nanoparticles. Nanoparticles can also be stabilized on the surfaces of various microgranules, including SiO2 . In this paper, we report the preparation of bismuth nanoparticles in pores of three-dimensional opal matrices through the reduction of bismuth compounds with SC isopropanol. An opal matrix, made up of monodisperse SiO2 spheres in a cubic close-packed arrangement, may function as a set of nanoreactors, determining the
length scale of the chemical processes in its pores and the physicochemical properties of the resulting substances. Bismuth compounds are widely used in the production of ferroelectric materials, scintillators, semiconductors, and superconductors. When filling nanoscale pores in three-dimensional dielectric opal matrices, bismuth may exhibit unusual electrical properties. The process we used to produce bismuth–opal nanocomposites involved four main steps: (1) synthesis of an opal matrix, (2) impregnation of the matrix with a concentrated bismuth nitrate solution, (3) heat treatment of the salt-impregnated matrix, and (4) reduction of the decomposition products of bismuth nitrate in the pores of the opal matrix to metallic bismuth with SC isopropanol. The chemical and phase compositions of the bismuth–opal nanocomposites were determined by x-ray diffraction (XRD) (CuKα radiation, Geigerflex diffractometer), differential thermal analysis (DTA) (Netzsch STA 409 thermal analyzer), chemical analysis, and x-ray microanalysis (Cameca MS-46). The size of bismuth-containing nanoparticles was determined by transmission electron microscopy (TEM) on a JEOL JEM-100B. The specimen was dispersed in an alcoholin-water solution by sonication, and the dispersion was applied to a copper grid coated with poly(vinyl formal) and carbon. Opal matrices were prepared by hydrolytic polycondensation of tetraethyl orthosilicate (TEOS) in an alcohol–ammonia medium [6, 7], followed by sedimentation of the resultant silica spheres with no thermal or mechanical influences. TEOS hydrolysis may lead to the formation of different polymer chains, which would
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broaden the size distribution of the primary SiO2 particles. In connection with this, high purity (fine purification) of the starting reagents is critical. The opal structure was hardened by thermal or hydrothermal treatment . According to TEM results, the diameter of the SiO2 spheres in opal matrices was 200 to 260 nm, depending on the synthesis and hardening conditions. The pore size was no greater than 50–80 nm (Fig. 1). The matrices were impregnated with concentrated (0.5–2 M) Bi(NO3)3 solutions in the presence of polyatomic alcohols (mannitol and glycerol). The resultant basic bismuth nitrate was then thermally decomposed to give fine-particle bismuth oxide. After impregnation with Bi(NO3)3 solutions, the opal samples were dried in air at room temperature. Next, the opal + Bi(NO3)3 samples were heat-treated at 450°C for 2 h. According to XRD and chemical analysis data, the heat-treated samples consisted of opal and x-ray amorphous Bi2O3 . The DTA curves of such samples showed no thermal events up to 800°C. Nevertheless, the presence of bismuth silicates in the samples heat-treated at 720°C for 30 min or after DTA scans provides indirect evidence that the pores of the opal matrix contained x-ray amorphous Bi2O3 . According to XRD results (Fig. 2), the solid-state reaction between amorphous Bi2O3 and opal yielded a mixture of Bi4Si3O12 (eulytite) and Bi2SiO5
Fig. 1. TEM micrograph (480 × 500 nm) of an opal–bismuth nanocomposite; the diameter of the SiO2 spheres is 260 nm, and that of the bismuth nanoparticles is about 80 nm; 60000×.
8 θ, deg
Fig. 2. XRD pattern of the opal + Bi(NO3)3 sample heat-treated at 720°C in air for 2 h. INORGANIC MATERIALS
PREPARATION OF BISMUTH NANOPARTICLES IN OPAL MATRICES
Fig. 4. Photograph (15 × 15 mm) of an opal sample containing Bi4Si3O12 and Bi2SiO5 (Fig. 2) after reduction with SC isopropanol; dark areas represent Bi-containing zones.
Fig. 3. TEM micrograph (500 × 750 nm) of an opal–bismuth silicate nanocomposite; the diameter of the SiO2 spheres is 260 nm, and that of the Bi4Si3O12 and Bi2SiO5 nanoparticles (light areas) is from several nanometers to 20–30 nm; 60000×.
(metastable layered bismuth silicate), in full accord with earlier data . Bismuth silicate nanoparticles were formed on the surfaces of the SiO2 spheres (Fig. 3) and ranged in size from several nanometers to 20–30 nm. The experimental procedure we used and the reactions of SC isopropanol with different oxides were described earlier [10, 11]. The reduction of Bi2O3 in opal matrices to metallic bismuth with SC isopropanol was performed at temperatures no higher than 300°C. After exposure to SC i-PrOH, the samples differed in appearance. The reduction of the opal + Bi(NO3)3 and opal + Bi2O3(x-ray amorphous) samples led to the formation of uniformly colored bismuth–opal nanocomposites. Their color, dark violet to brown and green, probably depended on the size of the SiO2 spheres. The reduction of the opal–bismuth silicate composites (after heat treatment at 720°C) led to the formation of Bi– opal nanocomposites with a nonuniform Bi distribution (Fig. 4) in the form of rounded zones containing bismuth nanoparticles. INORGANIC MATERIALS
The bismuth in the pores of opal matrices was detected by a sulfide test. Since the solubility product of Bi2S3 is 1.6 × 10–72, the reaction involved (leading to the precipitation of black bismuth sulfide powder) does not require to dissolve or grind the sample. XRD results confirmed the presence of metallic bismuth in the opal matrix. According to x-ray microanalysis data, the Bi content of the nanocomposites was within 23 wt %. As seen in Fig. 1, the diameter of the bismuth nanoparticles in the pores of the opal matrix does not exceed 80 nm. Thus, metallic bismuth nanoparticles can be produced in three-dimensional opal matrices by reducing various bismuth compounds (nitrate, oxide, or silicates) with SC isopropanol. ACKNOWLEDGMENTS We are grateful to K.G. Kravchuk for his assistance in performing reduction with supercritical isopropanol. This work was supported by the Russian Foundation for Basic Research (grant no. 05-03-32721) and the Russian Academy of Sciences (project TsB-216: Controlled Synthesis of Substances with Tailored Properties and Fabrication of Related Functional Materials). REFERENCES 1. Pomogailo, A.D., Rozenberg, A.S., and Uflyand, I.E., Nanochastitsy metallov v polimerakh (Metal Nanoparticles in Polymers), Moscow: Khimiya, 2000.
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2. Gusev, A.I., Nanokristallicheskie materialy: metody polucheniya i svoistva (Nanocrystalline Materials: Preparation and Properties), Yekaterinburg, 1998. 3. Lehn, J.M., Supramolecular Chemistry: Concepts and Perspectives, Weinheim: VCH, 1995. 4. Yurkov, G.Yu., Astaf’ev, D.A., Gorkovenko, M.Yu., et al., Compositional Modification of Bismuth-Containing Nanoparticles in a Polyethylene Matrix, Zh. Neorg. Khim., 2005, vol. 50, no. 9, pp. 1402–1407. 5. Gubin, S.P., Yurkov, G.Yu., and Kataeva, N.A., Microgranules and Nanoparticles on Their Surfaces, Neorg. Mater., 2005, vol. 41, no. 10, pp. 1159–1175 [Inorg. Mater. (Engl. Transl.), vol. 41, no. 10, pp. 1017–1032]. 6. Deniskina, N.D., Kalinin, D.R., and Kazantseva, L.K., Blagorodnye opaly, prirodnye i sinteticheskie (Natural and Synthetic Noble Opals), Novosibirsk: Nauka, 1987. 7. Ströber, W., Fink, A., and Bohn, E., Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range, J. Colloid Interface Sci., 1968, vol. 26, pp. 62−69.
8. Samoilovich, L.A. and Ivicheva, S.N., Effect of Heat Treatment on the Morphology, Structure, and Phase Composition of Synthetic Noble Opal, Doklady XII vsesoyuznogo soveshchaniya po eksperimental’noi mineralogii (Proc. XII All-Union Conf. on Experimental Mineralogy), Miass, 1991, p. 117. 9. Kargin, Yu.F., Endrzheevskaya, V.Yu., and Skorikov, V.M., Solid-State Reactions between Bismuth and Germanium (Silicon) Oxides, Izv. Akad. Nauk SSSR, Neorg. Mater., 1991, vol. 27, no. 3, pp. 530–533. 10. Buslaeva, E.Yu., Kargin, Yu.F., Kravchuk, K.G., et al., Reaction of α-Bi2O3 with Supercritical Isopropanol, Zh. Neorg. Khim., 2001, vol. 46, no. 3, pp. 380–383. 11. Buslaeva, E.Yu., Kravchuk, K.G., Kargin, Yu.F., and Gubin, S.P., Reactions of MnO2 , Mn2O3 , α-Bi2O3 , and Bi12Ti1 – xMnxO20 with Supercritical Isopropanol, Neorg. Mater., 2002, vol. 38, no. 6, pp. 706–710 [Inorg. Mater. (Engl. Transl.), vol. 38, no. 6, pp. 582–585].