Direct synthesis of nanocrystalline oxide powders by wet-chemical techniques

In a recent period there is a great need for increasing the knowledge of tailoring the innovative procedures for the synthesis of electroceramic nanopowders and materials with improved quality for specific application. In order to produce electroceramics with desirable microstructure and properties, synthesis of stoichiometric, ultra-fine and agglomerate free powders with narrow size distributions is one of the most important steps. Within this scope, in the present paper we summarize our recent results on direct synthesis of some important perovskites and ferrites nanopowders by wet-chemical techniques.


I. Introduction
Considerable research effort directed to synthesis and characterization of electroceramic materials has been driven in recent years by their potential applications in the manufacture of a variety of microelectronic devices (transducers and actuators, infrared sensors, electro-optic switches, nonvolatile computer memories etc.) [1][2][3][4].It is well known that most properties of electroceramic materials change considerably when the crystalline size is decreased to the submicrometer regime and the composition is modified by doping.Thus, synthesis of stoichiometric, high-purity, ultra-fine and agglomerate free powders with narrow size distributions is the first and perhaps the most important step in producing electroceramics with desirable microstructure and properties.Several wet-chemical methods are available for synthesis of nanosize powder, such as sol-gel synthesis, hydrothermal reactions, coprecipitation, molten salt preparation and emulsion technique.These processes are based on com-plex solution chemistry, require little capital investment and enable the production of relatively large quantities of high purity powders of very fine particle size.However, only techniques for the direct synthesis of crystalline nanoparticles are promising, as they enable elimination of calcination step usually responsible for considerably particle coarsening and the hard agglomerate formation.Several papers based on low-temperature synthesis of crystalline perovskite [5][6][7][8] or ferrite [9][10][11] particles were published recently.In most of the works homogeneous distribution of cations on atomic scale was already maintained in solution, which enables reaction at relatively low temperature and formation of homogeneous and stoichiometric nanocrystalline particles.In the present paper we summarize our recent work on direct synthesis of some important perovskite and ferrite powders by wet-chemical techniques.

II. Perovskite nanopowders
Nanocrystalline titanate (SrTiO 3 , PbTiO 3 , La 2/3 TiO 3 , Bi 4 Ti 3 O 12 ) powders were synthesized using a two-step process: first by controlled hydrolysis of titanium-butoxide (Ti(OC 4 H 9 ) 4 , Fluka) with distilled water, and then by 128 V.V. Srdić et al. / Processing and Application of Ceramics 4 [3] (2010) [127][128][129][130][131][132][133][134] further reaction of the formed hydrated titania gel particles with corresponding cations under different reaction conditions in a strong alkaline solution (pH > 13) [12,13].The synthesis was conducted first by hydrolysis of Ti(OC 4 H 9 ) 4 dissolved in anhydrous ethanol with distilled water.The obtained white titania sol was added to NaOH aqueous solution under vigorous stirring to precipitate the titanium hydroxide gel.Then an aqueous solution containing Sr 2+ (from strontium nitrate (Sr(NO 3 ) 2 , Fluka), Pb 2+ (from lead acetate (Pb(CH 3 COO) 2 ×3H 2 O, Aldrich, USA), Bi 3+ (form bismuth(III)-nitrate pentahydrate (Bi(NO 3 ) 3 ×5H 2 O, Fluka) or La 3+ (from lanthanum nitrate (La(NO 3 ) 3 ×6H 2 O, Riedel-de-Haen) was slowly added to the prepared slurry, and the reaction between titanium hydroxide particles and corresponding ions was carried out at different temperatures (over a period of 60 min): i) up to 80°C at ambient pressure or ii) up to 165°C under the hydrothermal conditions.The precipitated powders were collected with a centrifuge and washed several times with distilled water to expel sodium ions and then with absolute ethanol to decrease pow-der agglomeration by removing free water and replace the particle surface hydroxyl with ethoxy groups.The washed powders were dried at 120°C for 1 day in air.

Strontium titanate powders
Direct synthesis of crystalline strontium titanate particles is based on the reaction between the hydrolyzed titanium alkoxide and Sr 2+ ions in a highly alkaline solution.In the first step, the highly porous titanium hydrous gel, exhibiting a fine, porous network structure and a very high surface area, 260 m 2 /g, is formed [12].In the second step, the amorphous titanium hydrous gel is infiltrated by aqueous Sr(NO 3 ) 2 solution, and soluble Sr 2+ ions are first attached to the surface of the porous titanium hydroxide gel and then incorporated into the gel structure, forming perovskite SrTiO 3 phase.
The SrTiO 3 powder synthesized at 20°C has a very high surface area (192 m 2 /g) and contains only traces of the perovskite phase, whereas sharp peaks of the crystalline cubic perovskite phase are present in the SrTiO 3 powder synthesized at 80°C (Fig. 1).Both samples contain SrCO 3 , as an impurity phase, which is unavoidable unless great care is taken to ensure that the precursors and the reaction environment are CO 2 -free [5,14].However, the intensity of the SrCO 3 peaks in the XRD pattern of the sample synthesized at 20°C is considerably higher than for the sample synthesized at 80°C.This was explained [12] by the fact that at 20°C incorporation of Sr 2+ ions into the porous gel structure is very slow, and they are mostly consumed in the much faster reaction with CO 2 .Thus, the powder synthesized at 20°C consists of an amorphous titania phase, small portion of SrTiO 3 and considerable amount of extremely fine SrCO 3 crystallites.On the other side, at higher synthesis temperature of 80°C the favourable processes are incorporation of Sr 2+ ions into the titanium hydrous gel structure, interaction with it and formation of the crystalline SrTiO 3 phase.As the reaction between Sr 2+ and the titania gel is relatively fast at 80°C, only a part of Sr 2+ ions is spent in the reaction with CO 2 , and the nanopowder with dominant perovskite phase and a   samples contain SrCO 3 , as an impurity phase, which is normal unless great care is taken to ensure that the precursors and the reaction environment are CO 2 -free [5,14].However, the intensity of the SrCO 3 peaks in the XRD pattern of the sample synthesized at 20°C is considerably higher than for the sample synthesized at 80°C.This was explained [12] by the fact that at 20°C incorporation of Sr 2+ ions into the porous gel structure is very slow, and they are mostly consumed in the much faster reaction with CO 2 .Thus, the powder synthesized at 20°C consists of an amorphous titania phase, small portion of SrTiO 3 and considerable amount of extremely fine SrCO 3 crystallites.On the other side, at higher synthesis temperature of 80°C the favourable processes are incorporation of Sr 2+ ions into the titanium hydrous gel structure, interaction with it and formation of the crystalline SrTiO 3 phase.
As the reaction between Sr 2+ and the titania gel is relatively fast at 80°C, only a part of Sr 2+ ions is spent in the reaction with CO 2 , and the nanopowder with dominant perovskite phase and a small amount of SrCO 3 is formed.The powders synthesized at 20°C and 80°C are agglomerated (Fig. 2), with the primary particle size in nanometer range 32 nm and 37 nm, respectively.
xx 4 9 4 in anhydrous ethanol with distilled water.The obtained white titania sol was added to NaOH aqueous solution under vigorous stirring to precipitate the titanium hydroxide gel.Then an aqueous solution containing Sr 2+ (from strontium nitrate (Sr(NO 3 ) 2 , Fluka), Pb 2+ (from lead acetate (Pb(CH 3 COO) 2 ×3H 2 O, Aldrich, USA), Bi 3+ (form bismuth(III)-nitrate pentahydrate (Bi(NO 3 ) 3 ×5H 2 O, Fluka) or La 3+ (from lanthanum nitrate (La(NO 3 ) 3 ×6H 2 O, Riedel-de-Haen) was slowly added to the prepared slurry, and the reaction between titanium hydroxide particles and corresponding ions was carried out at different temperatures (over a period of 60 min): i) up to 80°C at ambient pressure or ii) up to 165°C under the hydrothermal conditions.The precipitated powders were collected with a centrifuge and washed several times with distilled water to expel sodium ions and then with absolute ethanol to decrease powder agglomeration by removing free water and replace the particle surface hydroxyl with ethoxy groups.The washed powders were dried at 120°C for 1 day in air.

Strontium titanate powders
Direct synthesis of crystalline strontium titanate particles is based on the reaction between the hydrolyzed titanium alkoxide and Sr 2+ ions in a highly alkaline solution.In the first step, the highly porous titanium hydrous gel, exhibiting a fine, porous network structure and a very high surface area, 260 m 2 /g, is formed [12].In the second step, the amorphous titanium hydrous gel is infiltrated by aqueous Sr(NO 3 ) 2 solution, and soluble Sr 2+ ions are first attached to the surface of the porous titanium hydroxide gel and then incorporated into the gel structure, forming perovskite SrTiO 3 phase.
The SrTiO 3 powder synthesized at 20°C has a very high surface area (192 m 2 /g) and contains only traces of the perovskite phase, whereas sharp peaks of the crystalline cubic perovskite phase are present in the SrTiO 3 powder synthesized at 80°C (Fig. 1).Both samples contain SrCO 3 , as an impurity phase, which is normal unless great care is taken to ensure that the precursors and the reaction environment are CO 2 -free [5,14].However, the intensity of the SrCO 3 peaks in the XRD pattern of the sample synthesized at 20°C is considerably higher than for the sample synthesized at 80°C.This was explained [12] by the fact that at 20°C incorporation of Sr 2+ ions into the porous gel structure is very slow, and they are mostly consumed in the much faster reaction with CO 2 .Thus, the powder synthesized at 20°C consists of an amorphous titania phase, small portion of SrTiO 3 and considerable amount of extremely fine SrCO 3 crystallites.On the other side, at higher synthesis temperature of 80°C the favourable processes are incorporation of Sr 2+ ions into the titanium hydrous gel structure, interaction with it and formation of the crystalline SrTiO 3 phase.
As the reaction between Sr 2+ and the titania gel is relatively fast at 80°C, only a part of Sr 2+ ions is spent in the reaction with CO 2 , and the nanopowder with dominant perovskite phase and a small amount of SrCO 3 is formed.The powders synthesized at 20°C and 80°C are agglomerated (Fig. 2), with the primary particle size in nanometer range 32 nm and 37 nm, respectively.was slowly added to the prepared slurry, and the reaction between titanium hydroxide particles and corresponding ions was carried out at different temperatures (over a period of 60 min): i) up to 80°C at ambient pressure or ii) up to 165°C under the hydrothermal conditions.The precipitated powders were collected with a centrifuge and washed several times with distilled water to expel sodium ions and then with absolute ethanol to decrease powder agglomeration by removing free water and replace the particle surface hydroxyl with ethoxy groups.The washed powders were dried at 120°C for 1 day in air.

Strontium titanate powders
Direct synthesis of crystalline strontium titanate particles is based on the reaction between the hydrolyzed titanium alkoxide and Sr 2+ ions in a highly alkaline solution.In the first step, the highly porous titanium hydrous gel, exhibiting a fine, porous network structure and a very high surface area, 260 m 2 /g, is formed [12].In the second step, the amorphous titanium hydrous gel is infiltrated by aqueous Sr(NO 3 ) 2 solution, and soluble Sr 2+ ions are first attached to the surface of the porous titanium hydroxide gel and then incorporated into the gel structure, forming perovskite SrTiO 3 phase.
The SrTiO 3 powder synthesized at 20°C has a very high surface area (192 m 2 /g) and contains only traces of the perovskite phase, whereas sharp peaks of the crystalline cubic perovskite phase are present in the SrTiO 3 powder synthesized at 80°C (Fig. 1).Both samples contain SrCO 3 , as an impurity phase, which is normal unless great care is taken to ensure that the precursors and the reaction environment are CO 2 -free [5,14].However, the intensity of the SrCO 3 peaks in the XRD pattern of the sample synthesized at 20°C is considerably higher than for the sample synthesized at 80°C.This was explained [12] by the fact that at 20°C incorporation of Sr 2+ ions into the porous gel structure is very slow, and they are mostly consumed in the much faster reaction with CO 2 .Thus, the powder synthesized at 20°C consists of an amorphous titania phase, small portion of SrTiO 3 and considerable amount of extremely fine SrCO 3 crystallites.On the other side, at higher synthesis temperature of 80°C the favourable processes are incorporation of Sr 2+ ions into the titanium hydrous gel structure, interaction with it and formation of the crystalline SrTiO 3 phase.
As the reaction between Sr 2+ and the titania gel is relatively fast at 80°C, only a part of Sr 2+ ions is spent in the reaction with CO 2 , and the nanopowder with dominant perovskite phase and a small amount of SrCO 3 is formed.The powders synthesized at 20°C and 80°C are agglomerated (Fig. 2), with the primary particle size in nanometer range 32 nm and 37 nm, respectively.small amount of SrCO 3 is formed.The powders synthesized at 20 and 80°C are agglomerated (Fig. 2), with the primary particle size in nanometer range 32 nm and 37 nm, respectively.

Lead titanate powders
Direct synthesis of crystalline lead titanate particles is based on the reaction between the hydrolyzed titanium alkoxide and Pb 2+ ions (from an aqueous Pb(CH 3 COO) 2 solution) in a highly alkaline solution [12].It is believed that the relatively soluble Pb 2+ ions easy cover the entire surface of the porous titanium hydroxide gel, than slowly incorporate into the gel structure and finally form Ti-O-Pb bonds.If the reaction is carried out at low temperature (20 and even 80°C) there is not enough energy for the nucleation of PbTiO 3 phase, and the as-synthesized powders are completely XRD amorphous (Fig. 3).However, the amorphous structure can be directly transformed into the perovskite PbTiO 3 phase after calcination at around 500°C [12].Both powders (synthesized at 20 and 80°C) consist of very fine amorphous particles with the average size of about 20 nm (Fig. 4a) and have very high surface areas (149 and 147 m 2 /g, respectively).
Crystalline phase was directly synthesized in the reaction of Pb 2+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions (Fig. 3).Thus, an intermediate crystalline phase was obtained at 135 and 150°C, whereas perovskite PbTiO 3  xx particles is based on the reaction between the hydrolyzed titanium alkoxide and Pb 2+ ions (from an aqueous Pb(CH 3 COO) 2 solution) in a highly alkaline solution [12].It is believed that the relatively soluble Pb 2+ ions easy cover the entire surface of the porous titanium hydroxide gel, than slowly incorporate into the gel structure and finally form Ti-O-Pb bonds.If the reaction is carried out at low temperature (20°C and even 80°C) there is not enough energy for the nucleation of PbTiO 3 phase, and the as-synthesized powders are completely XRD amorphous (Fig. 3).However, the amorphous structure can be directly transformed into the perovskite PbTiO 3 phase after calcination at around 500°C [12].Both powders (synthesized at 20°C and 80°C) consist of very fine amorphous particles with the average size of about 20 nm (Fig. 4a) and have very high surface areas (149 m 2 /g and 147 m 2 /g, respectively).
Crystalline phase was directly synthesized in the reaction of Pb 2+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions (Fig. 3).Thus, an intermediate crystalline phase was obtained at 135°C and 150°C, whereas perovskite PbTiO 3 phase was formed at 165°C.Pure perovskite PbTiO 3 phase can be also formed in the hydrothermal reaction conducted at 150°C, but for conditions provide enough energy, necessary not only for rearrangement of the titania gel structure by incorporating of Pb 2+ ions initially adsorbed on its surfaces, but also for nucleation of crystalline lead titanate phases.xx particles is based on the reaction between the hydrolyzed titanium alkoxide and Pb 2+ ions (from an aqueous Pb(CH 3 COO) 2 solution) in a highly alkaline solution [12].It is believed that the relatively soluble Pb 2+ ions easy cover the entire surface of the porous titanium hydroxide gel, than slowly incorporate into the gel structure and finally form Ti-O-Pb bonds.If the reaction is carried out at low temperature (20°C and even 80°C) there is not enough energy for the nucleation of PbTiO 3 phase, and the as-synthesized powders are completely XRD amorphous (Fig. 3).However, the amorphous structure can be directly transformed into the perovskite PbTiO 3 phase after calcination at around 500°C [12].Both powders (synthesized at 20°C and 80°C) consist of very fine amorphous particles with the average size of about 20 nm (Fig. 4a) and have very high surface areas (149 m 2 /g and 147 m 2 /g, respectively).
Crystalline phase was directly synthesized in the reaction of Pb 2+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions (Fig. 3).Thus, an intermediate crystalline phase was obtained at 135°C and 150°C, whereas perovskite PbTiO 3 phase was formed at 165°C.Pure perovskite PbTiO 3 phase can be also formed in the hydrothermal reaction conducted at 150°C, but for conditions provide enough energy, necessary not only for rearrangement of the titania gel structure by incorporating of Pb 2+ ions initially adsorbed on its surfaces, but also for nucleation of crystalline lead titanate phases.xx particles is based on the reaction between the hydrolyzed titanium alkoxide and Pb 2+ ions (from an aqueous Pb(CH 3 COO) 2 solution) in a highly alkaline solution [12].It is believed that the relatively soluble Pb 2+ ions easy cover the entire surface of the porous titanium hydroxide gel, than slowly incorporate into the gel structure and finally form Ti-O-Pb bonds.If the reaction is carried out at low temperature (20°C and even 80°C) there is not enough energy for the nucleation of PbTiO 3 phase, and the as-synthesized powders are completely XRD amorphous (Fig. 3).However, the amorphous structure can be directly transformed into the perovskite PbTiO 3 phase after calcination at around 500°C [12].Both powders (synthesized at 20°C and 80°C) consist of very fine amorphous particles with the average size of about 20 nm (Fig. 4a) and have very high surface areas (149 m 2 /g and 147 m 2 /g, respectively).
Crystalline phase was directly synthesized in the reaction of Pb 2+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions (Fig. 3).Thus, an intermediate crystalline phase was obtained at 135°C and 150°C, whereas perovskite PbTiO 3 phase was formed at 165°C.Pure perovskite PbTiO 3 phase can be also formed in the hydrothermal reaction conducted at 150°C, but for conditions provide enough energy, necessary not only for rearrangement of the titania gel structure by incorporating of Pb 2+ ions initially adsorbed on its surfaces, but also for nucleation of crystalline lead titanate phases.phase was formed at 165°C.Pure perovskite PbTiO 3 phase can be also formed in the hydrothermal reaction conducted at 150°C, but for somewhat longer period of 180 minutes (Fig. 5).The obtained results confirm that only hydrothermal conditions provide enough energy, necessary not only for rearrangement of the titania gel structure by incorporating of Pb 2+ ions initially adsorbed on its surfaces, but also for nucleation of crystalline lead titanate phases.
Depending on processing conditions as-synthesized powders can have different morphology, starting from spherical nanoparticles (Fig. 4a), to platelet-like structure (Fig. 4c) or even a complex structures involving titanate particles covered with one-dimensional nanostructured elements (Fig. 4b).

Lanthanum titanate powders
La 2/3 TiO 3 nanopowders were prepared in the reaction between the hydrolyzed titanium alkoxide and La 3+ ions in a highly alkaline solution.XRD presented in Fig. 6 shown that the powder synthesized at 20°C (and even at 80°C) consists of almost pure La(OH) 3 phase.Formation of La(OH) 3 phase is favourable even under the hydrothermal conditions, and at 110°C only a small amount of perovskite phase is obtained (indicated by arrows in Fig. 6).This that, like the 3 system, incorporation of La 3+ ions in the titanium hydrous gel structure is impeded by the formation of an impurity phase.On the other side, different behaviour between strontium and lead in the process of titanate formation may be found in the nature of these elements, because Sr, an alkaline earth metal, reacts easily with CO 2 and Sr-O bond does not possess a partial covalent character like Pb-O bond.

Bismuth titanate powders
Bismuth titanate powders (Bi 4-x A x Ti 3 O 12 , where A = La or Ce and x ≤ 1) were synthesized by aforementioned sol-gel process modified in a way that bismuth, cerium or lanthanum precursors were first dissolved in nitric acid and then mixed with hydrated titinia gel particles.All as-synthesized powders consist of very fine particles with the size in the nanometer range and with relatively high values of specific surface areas (~160 m 2 /g).Since X-ray diffraction data revealed that assynthesized powders are amorphous, thermal analyses were performed in order to determine the crystallization temperature required to obtain bismuth titanate phase, Fig. 7.The first exothermic peak on the DTA curve at around 290°C can be attributed to decomposition of residual organics and nitrates.The second exothermic effect, observed around 520°C for the pure bismuth titanate powder (sample Bi 4 Ti 3 O 12 ), can be attributed to crystallization of bismuth titanate [15].After calcination at 600°C /1h a rapid drop in the specific surface area in the Bi 4 Ti 3 O 12 powders was observed (from 160 to 15.5 m 2 /g) and crystalline nanoparticles were obtai- Depending on processing conditions assynthesized powders can have different morphology, starting from spherical nanoparticles (Fig. 4a  Depending on processing conditions assynthesized powders can have different morphology, starting from spherical nanoparticles (Fig. 4a ned.It is interesting to note that XRD peaks at 2θ angles of ~32°, ~40° and ~58° are unseparated, indicating that the calcined unmodified powder has tetragonal structure, Fig. 8. Since tetragonal Bi 4 Ti 3 O 12 is stable only above Curie temperature, the presence of tetragonal structure at room temperature could be due to kinetics [16] and/or size effect [17].Micrographs of the calcined powder (Fig. 9) confirm that powder consists of agglomerates of fine equiaxial-shaped particles, however, sinterability of the obtained particles are good and almost fully dense ceramics can be processed even after uneasily pressing and pressure-less sintering [13].
Crystalline phase was directly synthesized in the reaction of Bi 3+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions at 160°C (Fig. 10).Since the reaction time was short, the characteristics XRD peaks are very broad and not defined well.However, work of Xu et al. [18] showed that with prolonged hydrothermal reaction (for 30 or even 50 hours) fully crystallized bismuth-titanate particles can be formed.This additionally confirmed that even the more complex perovskite structure can be directly synthesised by wet-chemical techniques.

III. Ferrite nanopowders
Pure zinc ferrite (ZnFe 2 O 4 ) and doped zinc ferrite (Zn 1-x In x Fe 2 O 4 and ZnY x Fe 2-x O 4 ) nanoparticles were prepared by a low-temperature chemical co-precipitation method using aqueous solutions of nitrate precursors [19].Stoichiometric amounts of Fe(NO 3 ) 3 ×9H 2 O (Merck) and Zn(NO 3 ) 2 ×6H 2 O (Merck) were dissolved in distilled water, mixed with appropriate amount of aqueous InCl 3 or YCl 3 solution and precipitated with a hydroxide solution (NH 4 OH or NaOH).The co-precipitation reaction was carried out at different temperatures up to 95°C for 60 minutes under continuous stirring.The precipitates were separated from the slurry by centrifuging and washed a number of times with distilled water and then with absolute ethanol.The formed nanoparticles were dried at 120ºC for 1 day and finally dry milled in a mortar.

Zinc ferrite nanoparticles
The zinc-ferrite powders, synthesized at 20°C and 60°C with NH 4 OH, are amorphous (Fig. 11) and crystallize after calcination at temperature between 550 and 600°C.However, both powders calcined at 600°C consist of mixture of the spinel ZnFe 2 O 4 phase and the pure oxide phases.If the precipitation was carried out in presence of NaOH at 20°C the obtained powder is also amorphous (Fig. 11) confirming that the direct synthesis of crystalline ZnFe 2 O 4 particles is not possible in the presence of NH 4 OH or at room temperature.On the other hand, in presence of NaOH, crystalline powders, having the average crystallite size of about 3-4 nm, can be directly synthesized at 80°C (Fig. 11).Corresponding HRTEM im- Crystalline phase was directly synthesized in the reaction of Bi 3+ ions with the titanium hydrous gel at high temperature, i.e. under the hydrothermal conditions at 160°C (Fig. 10).Since we used short reaction time the characteristics XRD peaks are very broad and not defined well.However, work of Xu et al. [18] showed that with prolonged hydrothermal reaction (for 30 or even 50 hours) fully crystallized bismuth-titanate particles can be formed.This additionally confirmed that even the more complex perovskite structure can be directly synthesised by wet-chemical techniques.

III. Ferrite nanopowders
Pure zinc ferrite (ZnFe 2 O 4 ) and doped zinc ferrite (Zn 1-x In x Fe 2 O 4 and ZnY x Fe 2-x O 4 ) nanoparticles were prepared by a low-temperature chemical coprecipitation method using aqueous solutions of nitrate precursors [19].Stoichiometric amounts of Fe(NO 3 ) 3 9H 2 O (Merck) and Zn(NO 3 ) 2 6H 2 O (Merck) were dissolved in distilled water, mixed with appropriate amount of aqueous InCl 3 or YCl 3 solution and precipitated with a hydroxide solution (NH 4 OH or NaOH).The co-precipitation reaction was carried out at different temperatures up to 95°C for 60 minutes under continuous stirring.The precipitates were separated from the slurry by centrifuging and washed a number of times with distilled water and then with absolute ethanol.The formed nanoparticles were dried at 120ºC for 1 day and finally dry milled in a mortar.

Zinc ferrite nanoparticles
The zinc-ferrite powders, synthesized at 20°C and 60°C with NH 4 OH, are amorphous (Fig. 11) and crystallize after calcination at temperature between 550 and 600°C.However, both powders calcined at 600°C consist of mixture of the spinel ZnFe 2 O 4 phase and the pure oxide phases.If the precipitation was carried out in presence of NaOH at 20°C the obtained powder is also amorphous (Fig. 11) confirming that the direct synthesis of crystalline ZnFe 2 O 4 particle is not possible in the presence of NH 4 OH or at room temperature.On the other hand, in presence of NaOH, crystalline powders, having the average crystallite size of about 3-4 nm, can be directly synthesized at 80°C (Fig. 11).Corresponding HRTEM image (Fig. 12) implies that as-synthesized particles are uniform in size, well crystallized and agglomerated.With the increase of synthesis temperature to 95°C the singlephase cubic spinel phase is formed too, but the corresponding peaks are more sharp and narrow.
The lattice parameter of the as-prepared ZnFe 2 O 4 nanoparticles (in presence of NaOH and at 80°C) is found to be a larger than the bulk zinc ferrite value [20], suggesting that they have a different crystal structure in terms of the distribution of the constituent cations over two lattice sites of the spinel structure compared to the bulk material [21].These structural Crystalline phase was directly synthesized in the reaction of Bi 3+ ions with the titanium hydrous gel at conditions at 160°C (Fig. 10).Since we used short reaction time the characteristics XRD peaks are very broad and not defined well.However, work of Xu et al. [18] showed that with prolonged hydrothermal reaction (for 30 or even 50 hours) fully crystallized bismuth-titanate particles can be formed.This additionally confirmed that even the more complex perovskite structure can be directly synthesised by wet-chemical techniques.

III. Ferrite nanopowders
Pure zinc ferrite (ZnFe 2 O 4 ) and doped zinc ferrite (Zn 1-x In x Fe 2 O 4 and ZnY x Fe 2-x O 4 ) nanoparticles were prepared by a low-temperature chemical coprecipitation method using aqueous solutions of nitrate precursors [19].Stoichiometric amounts of Fe(NO 3 ) 3 9H 2 O (Merck) and Zn(NO 3 ) 2 6H 2 O (Merck) were dissolved in distilled water, mixed with appropriate amount of aqueous InCl 3 or YCl 3 solution and precipitated with a hydroxide solution (NH 4 OH or NaOH).The co-precipitation reaction was carried out at different temperatures up to 95°C for 60 minutes under continuous stirring.The precipitates were separated from the slurry by centrifuging and washed a number of times with distilled water and then with absolute ethanol.The formed nanoparticles were dried at 120ºC for 1 day and finally dry milled in a mortar.

Zinc ferrite nanoparticles
The zinc-ferrite powders, synthesized at 20°C and 60°C with NH 4 OH, are amorphous (Fig. 11) and crystallize after calcination at temperature between 550 and 600°C.However, both powders calcined at 600°C consist of mixture of the spinel ZnFe 2 O 4 phase and the pure oxide phases.If the precipitation was carried out in presence of NaOH at 20°C the obtained powder is also amorphous (Fig. 11) confirming that the direct synthesis of crystalline ZnFe 2 O 4 particle is not possible in the presence of NH 4 OH or at room temperature.On the other hand, in presence of NaOH, crystalline powders, having the average crystallite size of about 3-4 nm, can be directly synthesized at 80°C (Fig. 11).Corresponding HRTEM image (Fig. 12) implies that as-synthesized particles are uniform in size, well crystallized and agglomerated.With the increase of synthesis temperature to 95°C the singlephase cubic spinel phase is formed too, but the corresponding peaks are more sharp and narrow.
The lattice parameter of the as-prepared ZnFe 2 O 4 nanoparticles (in presence of NaOH and at 80°C) is found to be a larger than the bulk zinc ferrite value [20], suggesting that they have a different crystal structure in terms of the distribution of the constituent cations over two lattice sites of the spinel structure Crystalline phase was directly synthesized in the 3+ conditions at 160°C (Fig. 10).Since we used short reaction time the characteristics XRD peaks are very broad and not defined well.However, work of Xu et al. [18] showed that with prolonged hydrothermal reaction (for 30 or even 50 hours) fully crystallized bismuth-titanate particles can be formed.This additionally confirmed that even the more complex perovskite structure can be directly synthesised by wet-chemical techniques.

III. Ferrite nanopowders
Pure zinc ferrite (ZnFe 2 O 4 ) and doped zinc ferrite (Zn 1-x In x Fe 2 O 4 and ZnY x Fe 2-x O 4 ) nanoparticles were prepared by a low-temperature chemical coprecipitation method using aqueous solutions of nitrate precursors [19].Stoichiometric amounts of Fe(NO 3 ) 3 9H 2 O (Merck) and Zn(NO 3 ) 2 6H 2 O (Merck) were dissolved in distilled water, mixed with appropriate amount of aqueous InCl 3 or YCl 3 solution and precipitated with a hydroxide solution (NH 4 OH or NaOH).The co-precipitation reaction was carried out at different temperatures up to 95°C for 60 minutes under continuous stirring.The precipitates were separated from the slurry by centrifuging and washed a number of times with distilled water and then with absolute ethanol.The formed nanoparticles were dried at 120ºC for 1 day and finally dry milled in a mortar.

Zinc ferrite nanoparticles
The zinc-ferrite powders, synthesized at 20°C and 60°C with NH 4 OH, are amorphous (Fig. 11) and crystallize after calcination at temperature between 550 and 600°C.However, both powders calcined at 600°C consist of mixture of the spinel ZnFe 2 O 4 phase and the pure oxide phases.If the precipitation was carried out in presence of NaOH at 20°C the obtained powder is also amorphous (Fig. 11) confirming that the direct synthesis of crystalline ZnFe 2 O 4 particle is not possible in the presence of NH 4 OH or at room temperature.On the other hand, in presence of NaOH, crystalline powders, having the average crystallite size of about 3-4 nm, can be directly synthesized at 80°C (Fig. 11).Corresponding HRTEM image (Fig. 12) implies that as-synthesized particles are uniform in size, well crystallized and agglomerated.With the increase of synthesis temperature to 95°C the singlephase cubic spinel phase is formed too, but the corresponding peaks are more sharp and narrow.
The lattice parameter of the as-prepared ZnFe 2 O 4 nanoparticles (in presence of NaOH and at 80°C) is found to be a larger than the bulk zinc ferrite value [20], suggesting that they have a different crystal structure in terms of the distribution of the constituent cations over two lattice sites of the spinel structure age (Fig. 12) implies that as-synthesized particles are uniform in size, well crystallized and agglomerated.With the increase of synthesis temperature to 95°C the single cubic spinel type is formed too, but the corresponding peaks are more sharp and narrow.
The lattice parameter of the as-prepared ZnFe 2 O 4 nanoparticles (in presence of NaOH and at 80°C) is found to be a larger than the bulk zinc ferrite value [20], suggesting that they have a different crystal structure in terms of the distribution of the constituent cations over two lattice sites of the spinel structure compared to the bulk material [21].These structural features will have a major impact on the magnetic properties of the studied system.And indeed, magnetic behaviour of the as-prepared ZnFe 2 O 4 , Fig. 13, has shown that in these nanoparticles, redistribution of cations occurred.Raman and Mössbauer measurements additionally confirmed the inversion of cations.
As-synthesized zinc ferrite nanoparticles show the characteristic hysteresis at 5 K with a high value of magnetization (M s = 63.2 emu/g) which is a clear sign of ferrimagnetic ordering that originates from the difference in the magnetic moments of the ions at the octahedral and tetrahedral lattice sites, and thus directly reflects the distribution of the magnetic Fe 3+ and nonmagnetic Zn 2+ ions between the two sublattices.On the other hand, the hysteresis completely vanishes at the temperature of 300 K (Fig. 13), while the S shape of the curve imply on the presence of single domain nanoparticles in superparamagnetic state.The obtained values of magnetization are much higher that the magnetization of bulk zinc ferrite [22,23], which together with the structural characteristics and high specific surface area (around 220 m 2 /g), implies that nanoscaling in this system modifies the magnetic properties enabling the design of the magnetic ceramics with significantly improved properties compared to the bulk-counterparts.

Doped zinc-ferrite nanoparticles
In addition, we have investigated the possibility of introducing different cations (In 3+ and Y 3+ ) in the spinel structure using the same one-step coprecipitation route.The results of X-ray analyses confirmed the nanosized nature and spinel type structure of the investigated samples, Figs. 14 and 15, which shows that the direct synthesis at low temperature can be applied even to obtain a complex oxide system (e.g.Zn 1-x In x Fe 2 O 4 and ZnY x Fe 2- x O 4 ).It was found that yttrium and indium have a quite different influence on the structural characteristics of zinc ferrites.Addition of yttrium causes an increase of crystallite size, while increase of indium content influence the broadening of the XRD peaks, leading to decrease of crystallite size, Fig. 15.The reason for this is likely to be found in the synthesis of ferrite nanoparticles, where the addition of Y 3+ ions in the solution of Fe 3+ and Zn 2+ ions increases the reaction rate and stimulates the formation of highly crystalline zinc ferrite nanopowders.On the other hand, indium inhibits the phase transformation, resulting in a small crystallinity of zinc ferrite nanopowders doped with indium.However, what is interesting to note is that the addition of indium and yttrium has a positive effect on the particle size distribution, making it more uniform, as confirmed by TEM results shown in Fig. 16.
Raman and Mössbauer spectroscopy studies carried out on the indium-and yttrium-zinc ferrite nanoparticles implied on the possible cation distribution between the tetrahedral and octahedral sites and formation of the partially inversed spinel.The study of the magnetic properties showed that hysteresis loops do not saturate even in the presence of high magnetic fields, which confirmed the superparamagnetic and single domain nature of samples.These observations imply that, besides the particle size, composition (e.g.addition of yttrium and indium) causes significant structural rearrangements which in turn induce changes in magnetic behavior of the investigated nanoparticulate systems.

IV. Conclusions
Nanocrystalline ceramic powders have been prepared by using chemical synthesis routes under specific conditions.Our research showed that direct synthesis is promising concept in producing very high purity, high-quality and high surface area nanopowders without further calcination steps.Nanoceramic powders -single phase or two phase mixtures of SrTiO The study of the magnetic properties showed that hysteresis loops do not saturate even in the presence of high magnetic fields, which confirmed the superparamagnetic and single domain nature of samples.These observations imply that, besides the particle size, composition (e.g.addition of yttrium and indium) causes significant structural rearrangements which in turn induce changes in magnetic behavior of the investigated nanoparticulate systems.

IV. Conclusions
Nanocrystalline ceramic powders have been fabricated by using chemical synthesis routes under specific conditions.Our research showed that direct synthesis is promising concept in producing very high purity, high-quality and high surface area nanopowders without further calcinations steps.Nanoceramic powders -single phase or two phase mixtures of SrTiO The study of the magnetic properties showed that hysteresis loops do not saturate even in the presence of high magnetic fields, which confirmed the superparamagnetic and single domain nature of samples.These observations imply that, besides the particle size, composition (e.g.addition of yttrium and indium) causes significant structural rearrangements which in turn induce changes in magnetic behavior of the investigated nanoparticulate systems.

IV. Conclusions
Nanocrystalline ceramic powders have been fabricated by using chemical synthesis routes under specific conditions.Our research showed that direct synthesis is promising concept in producing very high purity, high-quality and high surface area nanopowders without further calcinations steps.Nanoceramic powders -single phase or two phase mixtures of SrTiO synthesis is promising concept in producing very high purity, high-quality and high surface area synthesis is promising concept in producing very high purity, high-quality and high surface area usefulness of the chemical nanoparticle approach for advanced materials fabrication and the high potential of these materials for variety of applications.