The aim of the present work is synthesis of ferroelectric nanomaterials, in the K2O-Nb2O5-SiO2 system via solgel method and studying the processes of formation and structure of the synthesized ferroelectric nanomaterials. The structure of synthesized materials has been studied by means of the following methods: EDS, XRD, FT-IR, SEM and AFM. The results obtained showed that the structure of the investigated compositions does not depend on the niobium content and all the samples keep their amorphous nature at room temperature. The surface structure shows random distribution of different kinds of aggregates with dimensions about 200–500 nm. The presence of a hybrid nanostructure with well-deﬁ ned nanounits having special geometry is clearly observed

Aiying Wu

Bisserka Samuneva

Georgi Chernev

Isabel M. Miranda Salvado

Paula Vilarinho

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47Processing and Application of Ceramics  3 [1-2] (2009) 47–50* Corresponding author: tel: +359 2 8163 373fax: +359 2 8685 488, e-mail: georgi_chernev@yahoo.comSynthesis and structure of nanomaterials in the systemK2O-Nb2O5-SiO2Georgi Chernev1, Bisserka Samuneva1, Isabel M. Miranda Salvado2, Paula Vilarinho2,Aiying Wu21University of Chemical Technology and Metallurgy 1756 Soﬁ  a, Bulgaria2Department of Ceramic and Glass Technology, CICECO, University of Aveiro, 3810-193 Aveiro, PortugalReceived 15 September 2008; received in revised form 27 February 2009; accepted 2 April 2009AbstractThe aim of the present work is synthesis of ferroelectric nanomaterials, in the K2O-Nb2O5-SiO2 system via sol-gel method and studying the processes of formation and structure of the synthesized ferroelectric nanomateri-als. The structure of synthesized materials has been studied by means of the following methods: EDS, XRD, FT-IR, SEM and AFM. The results obtained showed that the structure of the investigated compositions does not depend on the niobium content and all the samples keep their amorphous nature at room temperature. The surface structure shows random distribution of different kinds of aggregates with dimensions about 200–500 nm. The presence of a hybrid nanostructure with well-deﬁ  ned nanounits having special geometry is clearly observed.Keywords: sol-gel synthesis, ferroelectric nanomaterials, K2O-Nb2O5-SiO2 systemI. IntroductionThe sol–gel preparation method [1,2] is being in-creasingly used to produce novel materials with inter-esting physical and chemical properties. Some of these materials are of great technological interest because they combine novel chemical properties with controlla-ble porosity and large surface areas. Silica-based mate-rials are some of the most important sol-gel formed ma-terials, especially with the addition of secondary oxides (e.g. TiO2, ZrO2) which introduce different surface acid sites, making them ideal catalysts. Niobium compounds and materials are now inter-esting and important catalysts for various reactions. Although there are few differences in electronegativi-ty and ionic radius between Nb and its neighbours (V, Zr, Mo) in the periodic table of elements, it is intriguing that the catalytic behaviours of niobium compounds are quite different from those of the surrounding elements’ compounds. Thus, the research and development on the catalytic application of niobium compounds have been very active for the last 20 years [3]. The development of studies within synthesis, characterization and appli-cation of Nb-containing catalysts was possible thanks to the great progress of the spectroscopic as well as the other physical techniques which allow the analysis of Nb state in the solids [4].Properties of MOx-SiO2 binary materials are highly dependent on the structure and dispersion of the two ox-ides. Recently the relatively little studied Nb2O5-SiO2 binary system has been generating signiﬁ  cant interest for a range of chemical processes including potential for heterogeneous catalysis [5–7]. The use of this sys-tem includes methanol oxidation, ethanol dehydrogen-ation, partial methane oxidation, selective adsorption and use as a biosensor after immobilising mediator spe-cies. Despite the chemical interest in this system there is still relatively little known about the structural ba-sis of these properties. For the Nb2O5-SiO2 system it is still unclear as to whether catalytic properties are relat-ed to surface niobium atoms that are actually in the sili-ca framework, or a separate surface dispersed phase, or a mixture of both of these effects.Several reports on hydrothermal synthesis of KNbO3 have been documented recently [8–12]. One issue ad-dressed is the large variations in the amount of reaction products, whose structure phase and morphology are great-48G. Chernev et al. / Processing and Application of Ceramics 3 [1-2] (2009) 47–50ly different, using the same precursor reagents and equip-ment. The studies of the reactions between sodium and/or potassium carbonates and niobium oxide by diffusion cou-ples between 500°C and 700°C have been published [13].In the last years, some attempts have been made to produce transparent glass ceramics exhibiting second har-monic generation (SHG). More recently transparent glass ceramics with remarkable SHG efﬁ  ciency were synthe-sized in the K2O-Nb2O5-SiO2 (KNS) system [14,15].The aim of the present work is synthesis of ferro-electric nanomaterials, with composition K2O-Nb2O5-SiO2, via sol-gel method and studying the processes of formation and structure of the synthesized ferroelectric nanomaterials.II. ExperimentalThe ferroelectric K2O-Nb2O5-SiO2 materials with dif-ferent composition (Nb2O5 : SiO2 ratio of 1:10 and 1:20) have been obtained using sol-gel method. The precur-sors used are as follows: tetraethylortosilicate, niobium (V) ethoxide and potassium ethoxide. A poly-step pro-cedure has been applied at strictly controlled conditions in order to obtain the desired nanostructred materials. A small amount of 0.1 N HCl is added, to increase hydroly-sis rate. No phase separation is observed before and after the gelation point. The main steps of the synthesis have been described and published previously [16].The structure of the investigated materials has been studied using the following methods: X-ray diffraction (XRD) (Philips, PW1730/10), Furrier tran  sforming-infrared (FT-IR) spectroscopy (MATSON 7000), Ra-man spectroscopy (Bruker RFS-100/S), scanning elec-tron microscope (SEM) (Philips - 515), atomic force microscopy (AFM) (NanoScope Tapping ModeTM) and roughness analysis (RA).III. Results and discussionxperimentaXRD patterns of the prepared samples (Fig. 1) ex-hibit a diffuse maximum at about 2θ = 24°, which is typical for the amorphous silica. It should be empha-sized that regardless of the niobium content all samples keep their amorphous nature at room temperature. FTIR and Raman spectra of dried gel are shown in Figs. 2 and 3. As expected, the analysis of these data gives detailed information about the siloxane backbone structure and the distribution of hydroxyl groups, as well. The characteristic intensive band at ~1064 cm–1 is relat-ed to the presence of Si-O-Si. The shoulder at 950 cm–1 can be assigned to the Si-O(-Nb) bonds. For the T-Nb2O5 phase three different stretching modes should be expect-ed: the collinear  Nb-O-Nb (850 cm–1), the Nb3O (470 cm–1) and the bridging Nb-O-Nb (650 cm–1). The band at 1430 cm-1 is assigned to C-O-H vibrations. The char-acteristic bands at around 3450 cm-1 and at 1620 cm-1 as-signed to H-O-H vibration can also be detected. The band at 807 cm–1 is related to symmetric vibration of two sili-Figure 1. XRD pattern of the dried gel K2O-Nb2O5-SiO2 system (Nb2O5:SiO2 ratio 1:10)Figure 2. FT-IR spectra of the dried gel K2O-Nb2O5-SiO2 system (Nb2O5:SiO2 ratio 1:10)Figure 3. Raman spectra of the dried gel K2O-Nb2O5-SiO2 system (Nb2O5:SiO2 ratio 1:10)49G. Chernev et al. / Processing and Application of Ceramics 3 [1-2] (2009) 47–50Figure 4. SEM image (a) and EDS analysis (b) of the ferroelectirc nanomaterial in the system K2O-Nb2O5-SiO2 (Nb2O5:SiO2 ratio 1:10)Figure 5. AFM images and RMS roughness of ferroelectirc nanomaterials in the systems K2O-Nb2O5-SiO2 (Nb2O5:SiO2 ratio 1:10)Figure 6. AFM images and RMS roughness of ferroelectirc nanomaterials in the systems K2O-Nb2O5-SiO2 (Nb2O5:SiO2 ratio 1:20)50G. Chernev et al. / Processing and Application of Ceramics 3 [1-2] (2009) 47–50con atoms about their bridged oxygen. The bands at 815 cm–1 and 870 cm–1 can be related to the Nb–O stretching modes of distorted NbO6 octahedra sharing a corner with SiO4 tetrahedra. Particularly, they can be related to the Nb-O(-Nb) and Nb-O(-Si), respecti  vely. Actually, the ni-obium is dispersed within the siloxane network mainly as NbO4 tetrahedra (band at 770 cm–1). FTIR and Raman spectra of the niobium contai  ning dried gels exhibit some differences depending on the ni-obium amount. The highest absorption band gradually shifts toward lower frequencies going from 1082 cm–1 to 1064 cm–1 suggesting that the portion of Si-O-Si is pro-gressively inﬂ  uenced by the presence of niobium. In addi-tion, as the Nb content increases, NbO6 octahedra (bands at 815, 870 and 950 cm–1) and cluster (band at 680 cm–1) start to appear, while the tetrahedra disappear.The surface structure of the ferroelectrics in the K2O-Nb2O5-SiO2 system observed by SEM is shown in Fig. 4a. The random distribution of different kinds of aggre-gates with dimensions about 200–500 nm have been ob-served. In all studied samples presence of Si, Nb and K cations are conﬁ  rmed by EDS analysis (Fig. 4b). The presence of a hybrid nanostructure with well-deﬁ  ned nanounits having special geometry and their ag-gregates, formed by self-organizing processes, is clear-ly observed by AFM studies (Figs. 5 and 6). The size of nanoparticles calculated from AFM images are about 12–21 nm (Figs. 5a and 6a). In the same ﬁ  gures the height distribution proﬁ  les of surfaces roughness were shown. The histograms of the surface height distribu-tion proﬁ  les (Figs. 5b and 6b), obtained from AFM im-ages, showed that all synthesized samples had surfaces with irregularities of quite small height. IV. ConclusionsThe XRD pattern of the dried gel shows that re-gardless of the niobium content all samples keep their amorphous nature at room temperature. FTIR and Ra-man spectra give detailed information about the silox-ane backbone structure and the distribution of hydroxyl groups, as well. The presence of a hybrid nanostructure with well-deﬁ  ned nanounits having special geometry and their aggregates, formed by self-organizing proc-esses, is clearly observed by AFM studies. The surface morphology and structure of nanobuilding blocks in each synthesized ferroelectric nanomaterials is differ-ent and depends on its chemical composition.Acknowledgment: The support from the Bulgarian National Science Fund, Contract VU-TN-102/2005 is greatly acknowledged.ReferencesC.J. Brinker, G.W. Scherer, 1.   Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Aca-demic, San Diego, 1990.L. Hench, J. West, “The Sol-Gel Process”,  2.  Chem. Rev., 90 (1990) 33–72.K. Tanabe, “Catalytic application of niobium com- 3. pounds”, Catal. Today, 78 (2003) 65–77.M. Ziolek 4.  , “Niobium-containing catalysts - the state of the art”, Catal. Today, 78 (2003) 47–64.K.O. Drake, D. Carta, L.J. Skipper, F.E. Sowrey, R.J.  5. Newport, M.E. Smith, “A multinuclear solid sta-te NMR study of the sol-gel formation of amorpho-us Nb2O5-SiO2 materials”, Solid State Nucl. Magn. Reson., 27 (2005) 28–36.M. Francisco, Y. 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Synthesis and structure of nanomaterials in the system K2O-Nb2O5-SiO2

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