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Synthesis of spherical metal oxide particles using homogeneous precipitation of aqueous solutions of metal sulfates with urea
Powder Technology 169 (2006) 33 – 40 www.elsevier.com/locate/powtec
Synthesis of spherical metal oxide particles using homogeneous precipitation of aqueous solutions of metal sulfates with urea Jan Šubrt ⁎, Václav Štengl, Snejana Bakardjieva, Lorant Szatmary Institute of Inorganic Chemistry, Academy of sciences of the Czech Republic, 250 68 Řež, Czech Republic Received 21 June 2005; received in revised form 4 July 2006; accepted 5 July 2006 Available online 12 September 2006
Abstract Spherical micron sized porous particles of goethite FeO(OH), boehmite AlO(OH), anatase TiO2, Zn(OH)2 and binary mixtures of these oxides have been synthesized by homogeneous precipitation from aqueous solution containing urea in the presence of corresponding metal sulfates. Metal (hydrous) oxide particles obtained show spherical morphology and consist of agglomerated randomly oriented nanocrystallites. Synthesized metal oxide hydroxides were characterized using Brunauer–Emmett–Teller (BET) surface area and Barrett–Joiner–Halenda porosity (BJH), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX). © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Homogeneous hydrolysis; Urea; Oxides; Precipitation
1. Introduction The preparation of nanosized metal oxide and hydrated metal oxide powders has been an area of active investigations. Precipitation from aqueous solution in the presence of urea has been described as an effective method to obtain metal (hydrous) oxide particles [1]. Precipitation of soluble metal salts by alkaline solution is commonly used for making oxide hydroxide of trivalent (Fe3+, Al3+, Cr3+) or tetravalent (Ti4+) metal ions. Sodium hydroxide or ammonium hydroxide is often used as precipitating agent. The production of small metal oxide particles from the starting nitrate water-soluble salts via homogeneous precipitation with urea is demonstrated as a promising approach to successful preparing of powders [2,3] with controlled particle size and shape. In contrast to heterogeneous precipitation with alkali or ammonia, during the homogeneous precipitation the precipitating agent is slowly formed in the reaction mixture, the slow reaction course allows ripening of particles during precipitation resulting in better crystallinity, often also to regular shape and size of particles. ⁎ Corresponding author. E-mail address: [email protected] (J. Šubrt). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.07.009
Kato et al. [4] prepared TiO2 spherical particles from a solution of TiOSO4 by homogeneous precipitation using urea as the precipitating agent at 70–90 °C. Uniform nanocrystalline iron hydrous oxide particles formed by hydrolysis of Fe2(SO4)2 with urea were prepared and reported by Šubrt et al. earlier [5– 8]. Some papers have been published recently [9–23] describing synthesis of various hydrated metal oxide nanoparticles (Al, Sc, Ti, Y, Zr, etc.) using homogeneous precipitation of water-soluble metal salts with urea as the precipitating agent. The number of papers is increasing significantly in the recent years showing growing interest in use of this simple and effective method to prepare metal oxide nanoparticles. De A.A. Soler-Illia et al. [1] have presented in detail the urea method as an adequate way to synthesize both amorphous and crystalline metal (hydrous) oxide uniform shape particles from an aqueous media. The decomposition of urea (CH4N2O) in aqueous solution is accompanied by slow and controlled supply of ammonia (NH3) and carbon dioxide (CO2) into solution. The smooth pH increase obtained by the degradation of urea in synchrony with the active release of OH− and CO32− ions, usually leads to the precipitation of metal hydrous oxide particles of controlled particle morphology. All microstructural parameters as particle shape and size, specific surface area and porosity are
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J. Šubrt et al. / Powder Technology 169 (2006) 33–40
quite sensitive towards pH, metal ion concentration, temperature and aging time. On heating the aqueous solution of urea (NH2)2CO, the stoichiometry of urea hydrolysis is described by Eq. (1) as: − ðNH2 Þ2 CO þ Hþ →2NHþ 4 þ HCO3
Sample no
Starting mixture [g]
Product composition [wt.%]
Surface area [m2/g]
Total pore volume [cm3/g]
Phase composition (XRD)
105
Ti: 30; Fe: 15 Ti: 30; Fe: 5 Ti: 30; Fe: 1 Ti: 1; Fe: 30 Ti: 5; Fe: 30 Ti: 15; Fe: 30 Ti: 30; Fe: 30 Al: 60; Fe: 30 Al: 30; Fe: 60 Al: 30; Fe: 30 Al: 60; Fe: 10 Al: 10; Fe: 60 Al: 60; Ti: 30 Al: 30; Ti: 60 Al: 30; Ti: 30 Al: 60; Ti: 10 Al: 10; Ti: 60 Zn: 30; Ti: 30 Zn: 30; Ti: 15 Zn: 15; Ti: 30 Zn: 30; Ti: 5 Zn: 5; Ti: 30 Al: 30; Zn: 30 Al: 30; Zn: 15 Al: 15; Zn: 30 Al: 30; Zn: 5 Al: 5; Zn: 30 Fe: 30; Zn: 30 Fe: 15; Zn: 30
Ti: 12.35; Fe: 33.19; S: 0.50 Ti: 48.63; Fe: 10.50; S: 0.56 Ti: 43.50; Fe: 2.11; S: 0.14 Ti: 1.52; Fe: 51.26; S: 2.08 Ti: 12.22; Fe: 39.36; S: 0.76 Ti: 29.19; Fe: 24.20; S: 0.24 Ti: 23.07; Fe: 24.45; S: 0.40 Al: 23.21; Fe: 12.00; S: 4.51 Al: 18.66; Fe: 43.18; S: 4.23 Al: 19.39; Fe: 28.22; S: 4.07 Al: 27.79; Fe: 7.56; S: 5.54 Al: 7.81; Fe: 43.63; S: 2.85 Al: 17.18; Ti: 27.27; S: 3.09 Al: 10.84; Ti: 35.62; S: 0.76 Al: 10.93; Ti: 36.34; S: 0.89 Al: 28.19; Ti: 10.10; S: 3.10 Al: 4.31; Ti: 34.46; S: 0.62 Zn: 21.50; Ti: 33.92; S: 0.99 Zn: 37.71; Ti: 21.10; S: 2.19 Zn: 15.84; Ti: 35.68; S: 0.39 Zn: 51.55; Ti: 6.11; S: 5.19 Zn: 10.19; Ti: 48.62; S: 0.15 Al: 13.06; Zn: 40.27; S: 5.82 Al: 6.13; Zn: 42.03; S: 6.06 Al: 17.99; Zn: 35.53; S: 6.23 Al: 2.06; Zn: 42.31; S: 6.97 Al: 30.30; Zn: 8.87; S: 3.29 Fe: 17.79; Zn: 33.20; S: 5.35 Fe: 7.04; Zn: 42.48; S: 4.50 Fe: 43.56; Zn: 15.51; S: 2.36
315.65
0.2703
An, Goe
308.79
0.2353
An
309.17
0.2291
An
281.88
0.2891
Goe
273.62
0.2078
Goe
328.86
0.2732
An, Goe
259.90
0.3110
An, Fh
0.0624
Amorphous
170.88
0.1960
Fh
116.36
0.1492
Fh
0.0362
AlH
251.27
0.2458
Fh
217.73
0.1857
An
349.99
0.2562
An
324.41
0.2028
An
112.41
0.1430
An
309.95
0.3023
An
336.79
0.2669
An, ZnH
123.71
0.1150
330.47
0.2796
An, ZnH, ZnSH An, ZnH
ð1Þ
At higher temperature in an acid environment the HCO3− ions will decompose to CO2 escaping to the atmosphere: 2HCO−3 →CO2 þ H2 O
Table 1 Properties of hydrated metal oxide binary mixtures prepared by homogeneous precipitation
106
ð2Þ
107
According to [1], the hydrolysis of urea takes place in two steps. The first one, the reversible formation of ammonium cyanate is followed by the irreversible hydrolysis of cyanate ions [24,25]. The concentration of HCO3− increases linearly with time and pseudo-zero order kinetics results. The kinetics and mechanism of the urea hydrolysis were described by Shaw and Bordeaux [25]. During the urea decomposition, NH4+ ions are slowly produced in the whole volume of the solution resulting in absence of H+ gradients in the solution during metal ions hydrolysis.
108 109 110 111 114 115 116
2. Experimental
117
2.1. Preparation of metal oxides
118
The samples of FeO(OH), AlO(OH) and TiO2, have been prepared by the following procedure: The weighed amounts of metal sulfates were suspended in 4 L of distilled water with 10 mL of 98% sulfuric acid. Urea was mixed with the above solution (see Table 1). The resulting strongly acidic solution was heated so, that the boiling point was reached after 1 h of heating. Then the solution was kept under stirring at the boiling temperature for 6 h until pH 7.0 was reached and the ammonia smell disappeared from the steam. After aging, the precipitates were separated from the mother liquor by filtration. Particles were washed with distilled water and dried at laboratory temperature for several hours. Fine powders of goethite FeO(OH), boehmite AlO(OH), and anatase TiO2 as well as their mixtures were obtained.
119 120 121 122 123 139 140 141 142
2.2. Sample characterization The XRD patterns were obtained using the Siemens D5005 powder diffractometer operating in the reflection mode with CuKα radiation (40 kV, 45 mA) and diffracted beam monochromator, using the step scan mode with the step of 0.02° 2Θ and 2.4 s/step. Diffraction peaks for both anatase and rutile powders were compared with reference to JCPDS diffraction files [26]. The crystallite size was calculated by Scherrer's equation (S = Kλ / (βcosθ)), where K, λ, β, and θ denote the Scherrer constant (= 0.89), wavelength (0.15405 nm) of CuKα radiation, integrated width of the peak profile, and Bragg angle of the diffraction peak, respectively. Transmission electron micrographs were obtained using a Philips 201 microscope operating at an accelerating voltage of 100 kV. The samples for electron microscopy were prepared using grinding and dispersing the powder in propanol and
143 145 146 147 148 149 150 151
152
Fe: 30; Zn: 15
74.175
22.182
40.981 432.41
0.0567 0.3267
27.632
0.1030
15.435
0.0698
43.377
0.1129
12.773
0.0553
32.338
0.0962
107.35 94.198
237.81
An, ZnH, ZnSH An
0.1565
Not determined Not determined Not determined Not determined Not determined Fh, ZnSH, ZSHH Fh, ZnSH
0.2945
Fh
0.2082
J. Šubrt et al. / Powder Technology 169 (2006) 33–40 Table 1 (continued) Sample no
Starting mixture [g]
Product composition [wt.%]
Surface area [m2/g]
Total pore volume [cm3/g]
Phase composition (XRD)
153
Fe: 30; Zn: 5 Fe: 5; Zn: 30
Fe: 50.87; Zn: 8.72; S: 2.66 Fe: 4.66; Zn: 65.84; S: 4.37
383.62
0.4820
Fh, ZnSH
26.53
0.0682
Not determined
154
An: anatase 89-4921 (C); Goe: goethite 81-0464 (C); Fh: ferrihydrite 29-0712 (N); AlH: Aluminium hydroxide 77-0114 (C); ZnH: Zinc hydroxide Zn(OH)2 12-0142 (D); ZnSH: Zinc Oxide Sulfate Hydrate Zn4O3(SO4)·7H2O 20-1436 (N); Zinc Sulfate Hydroxide Hydrate 6Zn(OH)2·ZnSO4·4H2O 11-0280 (N).
applying a drop of very dilute suspension on carbon coated grids. The suspensions were dried by slow evaporation at ambient temperature. The SEM micrographs were obtained using Philips XL 30 CP equipped with EDX, SE, BSE and Robinson detectors, the last allowing studies of uncoated non-conductive samples and observation of the original texture.
35
B.E.T. surface area of anatase and rutile powders was determined by means of Coulter SA 3100 surface area instrument on the basis of nitrogen adsorption measured at − 196 °C. The pore size distributions (pore diameter and pore volume) were measured from the N2 desorption isotherm using the cylindrical pore model (BJH) [27]. DTA–TG measurements were made using the NETZSCH STA 409 apparatus. A helium flow of 40 cc min− 1 and a heating rate of 10 K min− 1 were used. Study of annealing of samples was carried out in a muffle furnace at various temperatures, and the samples were heated for 3 h. 3. Results and discussion 3.1. Homogeneous precipitation of one-component systems It is well known [28] that slow hydrolysis of metal salts leads to crystalline nanoparticles of corresponding metal oxides. For
Fig. 1. Electron micrographs of TiO2 samples prepared by homogeneous precipitation of TiOSO4 aqueous solutions with urea. A, B — SEM images at different magnification; C, D — HRTEM images at different magnification; SAED image.
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instance, by hydrolysis of TiCl4 [29] stable aqueous colloid solutions of 4–6 nm Q-TiO2 (quantum sized) particles are formed. These solutions are stable in an acid medium, when negative surface charge stabilizes particles towards coagulation. By increasing pH this surface charge gradually disappears and the particles can aggregate. According to Soler-Illia [1], the
amorphous compounds are spherical, whereas those of crystalline solid phases show characteristic crystal habits. For amorphous solids, minimization of the surface energy leads to sphericity. The final particle shape depends on whether oriented or isotropic aggregation takes place. Among the various factors that control the aggregation, dispersive forces and electrostatic
Fig. 2. SEM micrographs of mixtures of hydrated metal oxides with different chemical compositions prepared by homogeneous precipitation of TiOSO4 aqueous solutions with urea. A: sample 108; B: sample 117; C: sample 123; D: sample 139; E: sample 145; F: sample 153.
J. Šubrt et al. / Powder Technology 169 (2006) 33–40
37
Fig. 3. SEM micrographs of annealed hydrated FeOOH sample prepared by homogeneous precipitation of Fe2(SO4)3 aqueous solutions with urea. A, B — the initial sample dried at room temperature, C, D — the sample annealed at 1000 °C for 2 h.
interparticle interactions are the most important. Oriented aggregation is a consequence of the dissimilar electrostatic charges that develop on the different crystal faces of the primary particles, which aggregate in the directions of minimum repulsion; this phenomenon occurs mainly when the overall particle charge is large. On the other hand, isotropic aggregation, which usually leads to the formation of spherical particles, takes place in the vicinity of the isoelectric point. Fig. 1 shows electron micrographs of TiO2 precipitate prepared according to the procedure described above. The SEM micrographs (Fig. 1A and B) show roughly spherical particles that are ∼ 2 μm in size. At higher magnifications on the TEM micrographs (Fig. 1C) small subunits forming the agglomerates can be observed. HRTEM image (Fig. 1D) shows small 4–5 nm anatase microcrystals forming the agglomerates. SEAD diffraction pattern on the Fig. 1E confirms the crystallinity of the sample. The specific surface area of this TiO2 sample determined from the B.E.T. N2 adsorption isotherm was 281 m2/g (generally varies between 200 and 500 m2/g). The porosity of the sample was found to be 0.25 cm3/g and consists mostly of micropores (82% of porosity belongs to pores with diameter b8 nm).
Similar results were observed studying precipitation of some other salts by urea under similar conditions. In the case of precipitation of Cr2(SO4)3 spherical aggregates of grimalddite CrO(OH) particles [15] are formed whereas Fe2(SO 4) 3 provides spheres consisting of small particles of goethite and/ or ferrihydrite [15] or schwertmannite [30]. All these materials are similar in shape of particles, exhibit high surface area (up to 1000 m2 /g) and pronounced porosity. In all materials the broadened XRD lines indicate a very small particle size of the primary crystallites lying in the range 4–6 nm. 3.2. Homogeneous precipitation of binary systems Using the same synthetic procedure we studied also homogeneous precipitation of aqueous solutions of selected binary salt mixtures in systems: Ti–Fe, Al–Fe, Ti–Al, Ti–Zn, Al–Zn, Fe–Zn. The composition of the starting salt mixtures and product properties is given in Table 1. We observed a similar reaction course like in the case of one-component system. The SEM micrographs of particles are shown in Fig. 2. As it can be concluded from these micrographs, also the morphology of the particles resembles significantly the products of precipitation in
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Fig. 4. SEM micrographs of annealed hydrated Ti–Fe oxide mixture (sample 110) prepared by homogeneous precipitation of mixtures of Fe2(SO4)3 and TiOSO4 aqueous solutions with urea. A — sample annealed at 300 °C for 2 h; B — sample annealed at 500 °C for 2 h; C — sample annealed at 800 °C for 2 h; D — the sample annealed at 1200 °C for 2 h.
one-component systems. The particles are spherical and composed of small nanocrystalline subunits. In most experiments, high surface area products with considerable high porosity originated. 3.3. Annealing the particles In order to observe changes occurring in the materials at elevated temperature, the obtained samples were annealed at temperatures 300, 500, 800, 1000 and 1200 °C. As can be seen in Fig. 3, showing the appearance of particles of hydrated iron oxides before and after annealing to 1000 °C, the shape of the particles did not change dramatically. Also the binary mixtures of hydrated metal oxides prepared by homogeneous precipitation of mixtures of metal sulfates did not show significant changes of spherical agglomerate appearance after annealing. The appearance of particles of the heated Ti–Fe hydroxide mixture is presented in Fig. 4. No apparent changes of particle shape were observed up to 800 °C, the spherical particles were not interconnected and their mutual separation was easily possible. At higher temperatures (1200 °C), Fig. 4D we can observe rather
large sub-micron particles characteristic of sharp edges. At lower temperatures the primary particles remain in the nanosize region. The XRD diagram (see Fig. 5) shows changes of particle size and phase composition during annealing. The starting mixture (Fig. 5A) shows only broad XRD peaks of anatase, and the calculated particle size corresponds to 4 nm. Up to 300 °C (Fig. 5B) we can observe only indistinctive narrowing of anatase peaks corresponding to the particle growth of this phase. Significant peak narrowing and formation of pseudobrookite phase took place between 500 and 800 °C (Fig 5C and D). At 800 °C the mixture consists of anatase and pseudobrookite, which is the only phase detected by XRD at 1200 °C (Fig. 5E). The behavior of other studied mixtures on heating was similar to that described for Fe–Ti mixtures. We observed formation of binary compounds in the case that these exist, if not, mixtures of particular components with bigger primary nanoparticles were observed after annealing. 4. Conclusions Homogeneous precipitation of aqueous solutions of some amphoteric metal salts using urea as the precipitation agent
J. Šubrt et al. / Powder Technology 169 (2006) 33–40
results in formation of micron size, roughly spherical particles of corresponding hydrated metal oxides. The spherical agglomerates are mostly crystalline and consist of very small randomly oriented primary nanosized crystallites. Specific surface area of the materials is rather high, depending on the chemical composition that varies between 13 and 432 m2/g, and the material is porous (0.036–0.482 cm3 /g). Precipitation
39
of mixtures of metal salts leads at the same conditions to formation of agglomerates with similar size and shape containing tightly mixed particular components. On heating up to 1000 °C, the resemblance of the spherical particles remains preserved and only negligible interparticle sintering occurs. Inside the spheres growth of crystallites was observed on heating, in the binary systems formation of mixed
Fig. 5. XRD diffractographs of annealed hydrated Ti–Fe oxide mixture (sample 110) prepared by homogeneous precipitation of mixtures of Fe2(SO4)3 and TiOSO4 aqueous solutions with urea. A — the initial sample dried at room temperature, B — sample annealed at 300 °C for 2 h; C — sample annealed at 500 °C for 2 h; D — sample annealed at 800 °C for 2 h; E — sample annealed at 1200 °C for 2 h.
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oxide phases was observed at comparatively low temperatures (above ∼600 °C). Acknowledgments This work was supported by the Grant Agency of the Czech Republic, (Project 104/04/0467) and Ministry of Education, Youth and Sport of the Czech Republic, (Project LC 0525). References [1] G. Soler-Illia, M. Jobbagy, R.J. Candal, A.E. Regazzoni, M.A. Blesa, Synthesis of metal oxide particles from aqueous media: the homogeneous alkalinization method, J. Dispers. Sci. Technol. 19 (1998) 207–228. [2] S. Sohn, Y. Kwon, Y. Kim, D. Kim, Synthesis and characterization of nearmonodisperse yttria particles by homogeneous precipitation method, Powder Technol. 142 (2004) 136–153. [3] A. Ookubo, K. Ooi, T. Tomita, New types of hydrous titanium-oxides obtained by homogeneous precipitation from (titanium(Iii) chloride+urea) solutions, J. Mater. Sci. 24 (1989) 3599–3604. [4] K. Kato, A. Tsuzuki, H. Taoda, Y. Torii, T. Kato, Y. Butsugan, Crystalstructures of TiO2 thin coatings prepared from the alkoxide solution via the dip-coating technique affecting the photocatalytic decomposition of aqueous acetic-acid, J. Mater. Sci. 29 (1994) 5911–5915. [5] S. Bakardjieva, J. Šubrt, V. Štengl, L.A. Perez-Maqueda, M.A. Alario Franco, Characterization of photocatalytically active TiO2 by electron microscopy, Proc. 5th Multinational Congress on Electron Microscopy, Lecce, Italy, 2001, pp. 463–464. [6] S. Bakardjieva, J. Šubrt, V. Štengl, E. Vecernikova, P. Bezdicka, Solid State Phenom. 90–91 (2003) 7–12. [7] V. Štengl, F. Peterka, S. Bakardzijeva, J. Šubrt, V. Balek, J. Jirkovský, L. Kavan, Development of spherical porous anatase and its application as photoactive material, Proc. The Sixth International Conference on TiO2 Photocatalytic Purification and Treatment of Water and Air, Niagara Falls, Ontario, Canada, 2001, pp. 109–110. [8] J. Šubrt, V. Štengl, S. Bakardjieva, V. Balek, J. Jirkovský, Characterization of microstructure changes during heating of hydrous titania layers, Proc. The 4th International Symposium on Photoreaction Control and Photofunctional Materials, Tsukuba, Ibaraki, Japan, 2001, pp. 186–187. [9] S. Bakardjieva, J. Šubrt, W. Stengl, E. Vecernikova, P. Bezdicka, Solid State Phenom. 90–91 (2003) 7–12. [10] X.C. Jiang, C.H. Yan, L.D. Sun, Z.G. Wei, C.S. Liao, Hydrothermal homogeneous urea precipitation of hexagonal YBO3:Eu3+ nanocrystals with improved luminescent properties, J. Solid State Chem. 175 (2003) 245–251. [11] M. Adachi-Pagano, C. Forano, J.P. Besse, Synthesis of Al-rich hydrotalcitelike compounds by using the urea hydrolysis reaction—control of size and morphology, J. Mater. Chem. 13 (2003) 1988–1993. [12] S. Yamabi, H. Imai, Synthesis of rutile and anatase films with high surface areas in aqueous solutions containing urea, Thin Solid Films 434 (2003) 86–93.
[13] K. Ada, Y. Sarikaya, T. Alemdaroglu, M. Onal, Thermal behaviour of alumina precursor obtained by the aluminium sulphate–urea reaction in boiling aqueous solution, Ceram. Int. 29 (2003) 513–518. [14] V. Štengl, J. Šubrt, S. Bakardjieva, R. Bezdicka, A. Kalendova, P. Kalenda, Solid State Phenom. 90–91 (2003) 115–119. [15] V. Štengl, J. Šubrt, P. Bezdicka, M. Marikova, S. Bakardjieva, Solid State Phenom. 90–91 (2003) 121–126. [16] Y. Wang, J.Y. Lee, Preparation of SnO2-graphite nanocomposite anodes by urea-mediated hydrolysis, Electrochem. Commun. 5 (2003) 292–296. [17] M. Abecassis-Wolfovich, H. Rotter, M.V. Landau, E. Korin, A.I. Erenburg, D. Mogilyansky, E. Gartstein, Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and lowtemperature supercritical drying, J. Non-Cryst. Solids 318 (2003) 95–111. [18] D.S. Seo, H. Kim, H.C. Jung, J.K. Lee, A. Lee, Synthesis and characterization of TiO2 nanocrystalline powder prepared by homogeneous precipitation using urea, J. Mater. Res. 18 (2003) 571–577. [19] M. Hirano, S. Okumura, Y. Hasegawa, M. Inagaki, Direct precipitation of spinel-type Zn(Fe, Ga)(2)O-4 solid solutions from aqueous solutions at 90 degrees C: influence of iron valence of starting salt on their crystallite growth, J. Solid State Chem. 168 (2002) 5–10. [20] R. Zanella, S. Giorgio, C.R. Henry, C. Louis, Alternative methods for the preparation of gold nanoparticles supported on TiO2, J. Phys. Chem., B 106 (2002) 7634–7642. [21] A.C. Tas, P.J. Majewski, F. Aldinger, Synthesis of gallium oxide hydroxide crystals in aqueous solutions with or without urea and their calcination behavior, J. Am. Ceram. Soc. 85 (2002) 1421–1429. [22] A.C. Tas, P.J. Majewski, F. Aldinger, Chemical synthesis of crystalline, pure or Mn-doped ZnGa2O4 powders at 90 degrees C, J. Mater. Res. 17 (2002) 1425–1433. [23] H.J. Song, J.H. Oh, S.C. Choi, J.C. Lee, Preparation and characterization of Ni ferrite powders by urea decomposition, Phys. Status Solidi A-Appl. Res. 189 (2002) 849–852. [24] J.A. Warner, F.B. Stitt, The conversion of ammonium cyanate into urea. Mechanism and kinetic salt effect, J. Am. Chem. Soc. 55 (1933) 4807–4812. [25] W.H.R. Shaw, J. Bordeaux, The decomposition of urea in aqueous media, J. Am. Chem. Soc. 77 (1955) 4729–4733. [26] JCPDS PDF-2, release 2001, ICDD Newton Square PA, U.S.A. [27] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distribution in porous substances. I. Computations from nitrogern isotherms, J. Am. Chem. Soc. 73 (1951) 373–380. [28] H. Uchida, S. Hirao, T. Torimoto, S. Kuwabata, T. Sakata, H. Mori, H. Yoneyama, Preparation and properties of size-quantized TiO2 particles immobilized in polyvinylpyrrolidinone gel films, Langmuir 11 (1995) 3725–3729. [29] K. Macounova, H. Krysova, J. Ludvik, J. Jirkovsky, Kinetics of photocatalytic degradation of diuron in aqueous colloidal solutions of Q-TiO2 particles, J. Photochem. Photobiol. A-Chem. 156 (2003) 273–282. [30] J. Šubrt, J. Boháček, V. Štengl, T. Grygar, P. Bezdička, Uniform particles with a large surface area formed by hydrolysis of Fe–2(SO4)(3) with urea, Mater. Res. Bull. 34 (1999) 905–914.
Synthesis of spherical metal oxide particles using homogeneous precipitation of aqueous solutions of metal sulfates with urea Jan Šubrt ⁎, Václav Štengl, Snejana Bakardjieva, Lorant Szatmary Institute of Inorganic Chemistry, Academy of sciences of the Czech Republic, 250 68 Řež, Czech Republic Received 21 June 2005; received in revised form 4 July 2006; accepted 5 July 2006 Available online 12 September 2006
Abstract Spherical micron sized porous particles of goethite FeO(OH), boehmite AlO(OH), anatase TiO2, Zn(OH)2 and binary mixtures of these oxides have been synthesized by homogeneous precipitation from aqueous solution containing urea in the presence of corresponding metal sulfates. Metal (hydrous) oxide particles obtained show spherical morphology and consist of agglomerated randomly oriented nanocrystallites. Synthesized metal oxide hydroxides were characterized using Brunauer–Emmett–Teller (BET) surface area and Barrett–Joiner–Halenda porosity (BJH), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX). © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Homogeneous hydrolysis; Urea; Oxides; Precipitation
1. Introduction The preparation of nanosized metal oxide and hydrated metal oxide powders has been an area of active investigations. Precipitation from aqueous solution in the presence of urea has been described as an effective method to obtain metal (hydrous) oxide particles [1]. Precipitation of soluble metal salts by alkaline solution is commonly used for making oxide hydroxide of trivalent (Fe3+, Al3+, Cr3+) or tetravalent (Ti4+) metal ions. Sodium hydroxide or ammonium hydroxide is often used as precipitating agent. The production of small metal oxide particles from the starting nitrate water-soluble salts via homogeneous precipitation with urea is demonstrated as a promising approach to successful preparing of powders [2,3] with controlled particle size and shape. In contrast to heterogeneous precipitation with alkali or ammonia, during the homogeneous precipitation the precipitating agent is slowly formed in the reaction mixture, the slow reaction course allows ripening of particles during precipitation resulting in better crystallinity, often also to regular shape and size of particles. ⁎ Corresponding author. E-mail address: [email protected] (J. Šubrt). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.07.009
Kato et al. [4] prepared TiO2 spherical particles from a solution of TiOSO4 by homogeneous precipitation using urea as the precipitating agent at 70–90 °C. Uniform nanocrystalline iron hydrous oxide particles formed by hydrolysis of Fe2(SO4)2 with urea were prepared and reported by Šubrt et al. earlier [5– 8]. Some papers have been published recently [9–23] describing synthesis of various hydrated metal oxide nanoparticles (Al, Sc, Ti, Y, Zr, etc.) using homogeneous precipitation of water-soluble metal salts with urea as the precipitating agent. The number of papers is increasing significantly in the recent years showing growing interest in use of this simple and effective method to prepare metal oxide nanoparticles. De A.A. Soler-Illia et al. [1] have presented in detail the urea method as an adequate way to synthesize both amorphous and crystalline metal (hydrous) oxide uniform shape particles from an aqueous media. The decomposition of urea (CH4N2O) in aqueous solution is accompanied by slow and controlled supply of ammonia (NH3) and carbon dioxide (CO2) into solution. The smooth pH increase obtained by the degradation of urea in synchrony with the active release of OH− and CO32− ions, usually leads to the precipitation of metal hydrous oxide particles of controlled particle morphology. All microstructural parameters as particle shape and size, specific surface area and porosity are
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J. Šubrt et al. / Powder Technology 169 (2006) 33–40
quite sensitive towards pH, metal ion concentration, temperature and aging time. On heating the aqueous solution of urea (NH2)2CO, the stoichiometry of urea hydrolysis is described by Eq. (1) as: − ðNH2 Þ2 CO þ Hþ →2NHþ 4 þ HCO3
Sample no
Starting mixture [g]
Product composition [wt.%]
Surface area [m2/g]
Total pore volume [cm3/g]
Phase composition (XRD)
105
Ti: 30; Fe: 15 Ti: 30; Fe: 5 Ti: 30; Fe: 1 Ti: 1; Fe: 30 Ti: 5; Fe: 30 Ti: 15; Fe: 30 Ti: 30; Fe: 30 Al: 60; Fe: 30 Al: 30; Fe: 60 Al: 30; Fe: 30 Al: 60; Fe: 10 Al: 10; Fe: 60 Al: 60; Ti: 30 Al: 30; Ti: 60 Al: 30; Ti: 30 Al: 60; Ti: 10 Al: 10; Ti: 60 Zn: 30; Ti: 30 Zn: 30; Ti: 15 Zn: 15; Ti: 30 Zn: 30; Ti: 5 Zn: 5; Ti: 30 Al: 30; Zn: 30 Al: 30; Zn: 15 Al: 15; Zn: 30 Al: 30; Zn: 5 Al: 5; Zn: 30 Fe: 30; Zn: 30 Fe: 15; Zn: 30
Ti: 12.35; Fe: 33.19; S: 0.50 Ti: 48.63; Fe: 10.50; S: 0.56 Ti: 43.50; Fe: 2.11; S: 0.14 Ti: 1.52; Fe: 51.26; S: 2.08 Ti: 12.22; Fe: 39.36; S: 0.76 Ti: 29.19; Fe: 24.20; S: 0.24 Ti: 23.07; Fe: 24.45; S: 0.40 Al: 23.21; Fe: 12.00; S: 4.51 Al: 18.66; Fe: 43.18; S: 4.23 Al: 19.39; Fe: 28.22; S: 4.07 Al: 27.79; Fe: 7.56; S: 5.54 Al: 7.81; Fe: 43.63; S: 2.85 Al: 17.18; Ti: 27.27; S: 3.09 Al: 10.84; Ti: 35.62; S: 0.76 Al: 10.93; Ti: 36.34; S: 0.89 Al: 28.19; Ti: 10.10; S: 3.10 Al: 4.31; Ti: 34.46; S: 0.62 Zn: 21.50; Ti: 33.92; S: 0.99 Zn: 37.71; Ti: 21.10; S: 2.19 Zn: 15.84; Ti: 35.68; S: 0.39 Zn: 51.55; Ti: 6.11; S: 5.19 Zn: 10.19; Ti: 48.62; S: 0.15 Al: 13.06; Zn: 40.27; S: 5.82 Al: 6.13; Zn: 42.03; S: 6.06 Al: 17.99; Zn: 35.53; S: 6.23 Al: 2.06; Zn: 42.31; S: 6.97 Al: 30.30; Zn: 8.87; S: 3.29 Fe: 17.79; Zn: 33.20; S: 5.35 Fe: 7.04; Zn: 42.48; S: 4.50 Fe: 43.56; Zn: 15.51; S: 2.36
315.65
0.2703
An, Goe
308.79
0.2353
An
309.17
0.2291
An
281.88
0.2891
Goe
273.62
0.2078
Goe
328.86
0.2732
An, Goe
259.90
0.3110
An, Fh
0.0624
Amorphous
170.88
0.1960
Fh
116.36
0.1492
Fh
0.0362
AlH
251.27
0.2458
Fh
217.73
0.1857
An
349.99
0.2562
An
324.41
0.2028
An
112.41
0.1430
An
309.95
0.3023
An
336.79
0.2669
An, ZnH
123.71
0.1150
330.47
0.2796
An, ZnH, ZnSH An, ZnH
ð1Þ
At higher temperature in an acid environment the HCO3− ions will decompose to CO2 escaping to the atmosphere: 2HCO−3 →CO2 þ H2 O
Table 1 Properties of hydrated metal oxide binary mixtures prepared by homogeneous precipitation
106
ð2Þ
107
According to [1], the hydrolysis of urea takes place in two steps. The first one, the reversible formation of ammonium cyanate is followed by the irreversible hydrolysis of cyanate ions [24,25]. The concentration of HCO3− increases linearly with time and pseudo-zero order kinetics results. The kinetics and mechanism of the urea hydrolysis were described by Shaw and Bordeaux [25]. During the urea decomposition, NH4+ ions are slowly produced in the whole volume of the solution resulting in absence of H+ gradients in the solution during metal ions hydrolysis.
108 109 110 111 114 115 116
2. Experimental
117
2.1. Preparation of metal oxides
118
The samples of FeO(OH), AlO(OH) and TiO2, have been prepared by the following procedure: The weighed amounts of metal sulfates were suspended in 4 L of distilled water with 10 mL of 98% sulfuric acid. Urea was mixed with the above solution (see Table 1). The resulting strongly acidic solution was heated so, that the boiling point was reached after 1 h of heating. Then the solution was kept under stirring at the boiling temperature for 6 h until pH 7.0 was reached and the ammonia smell disappeared from the steam. After aging, the precipitates were separated from the mother liquor by filtration. Particles were washed with distilled water and dried at laboratory temperature for several hours. Fine powders of goethite FeO(OH), boehmite AlO(OH), and anatase TiO2 as well as their mixtures were obtained.
119 120 121 122 123 139 140 141 142
2.2. Sample characterization The XRD patterns were obtained using the Siemens D5005 powder diffractometer operating in the reflection mode with CuKα radiation (40 kV, 45 mA) and diffracted beam monochromator, using the step scan mode with the step of 0.02° 2Θ and 2.4 s/step. Diffraction peaks for both anatase and rutile powders were compared with reference to JCPDS diffraction files [26]. The crystallite size was calculated by Scherrer's equation (S = Kλ / (βcosθ)), where K, λ, β, and θ denote the Scherrer constant (= 0.89), wavelength (0.15405 nm) of CuKα radiation, integrated width of the peak profile, and Bragg angle of the diffraction peak, respectively. Transmission electron micrographs were obtained using a Philips 201 microscope operating at an accelerating voltage of 100 kV. The samples for electron microscopy were prepared using grinding and dispersing the powder in propanol and
143 145 146 147 148 149 150 151
152
Fe: 30; Zn: 15
74.175
22.182
40.981 432.41
0.0567 0.3267
27.632
0.1030
15.435
0.0698
43.377
0.1129
12.773
0.0553
32.338
0.0962
107.35 94.198
237.81
An, ZnH, ZnSH An
0.1565
Not determined Not determined Not determined Not determined Not determined Fh, ZnSH, ZSHH Fh, ZnSH
0.2945
Fh
0.2082
J. Šubrt et al. / Powder Technology 169 (2006) 33–40 Table 1 (continued) Sample no
Starting mixture [g]
Product composition [wt.%]
Surface area [m2/g]
Total pore volume [cm3/g]
Phase composition (XRD)
153
Fe: 30; Zn: 5 Fe: 5; Zn: 30
Fe: 50.87; Zn: 8.72; S: 2.66 Fe: 4.66; Zn: 65.84; S: 4.37
383.62
0.4820
Fh, ZnSH
26.53
0.0682
Not determined
154
An: anatase 89-4921 (C); Goe: goethite 81-0464 (C); Fh: ferrihydrite 29-0712 (N); AlH: Aluminium hydroxide 77-0114 (C); ZnH: Zinc hydroxide Zn(OH)2 12-0142 (D); ZnSH: Zinc Oxide Sulfate Hydrate Zn4O3(SO4)·7H2O 20-1436 (N); Zinc Sulfate Hydroxide Hydrate 6Zn(OH)2·ZnSO4·4H2O 11-0280 (N).
applying a drop of very dilute suspension on carbon coated grids. The suspensions were dried by slow evaporation at ambient temperature. The SEM micrographs were obtained using Philips XL 30 CP equipped with EDX, SE, BSE and Robinson detectors, the last allowing studies of uncoated non-conductive samples and observation of the original texture.
35
B.E.T. surface area of anatase and rutile powders was determined by means of Coulter SA 3100 surface area instrument on the basis of nitrogen adsorption measured at − 196 °C. The pore size distributions (pore diameter and pore volume) were measured from the N2 desorption isotherm using the cylindrical pore model (BJH) [27]. DTA–TG measurements were made using the NETZSCH STA 409 apparatus. A helium flow of 40 cc min− 1 and a heating rate of 10 K min− 1 were used. Study of annealing of samples was carried out in a muffle furnace at various temperatures, and the samples were heated for 3 h. 3. Results and discussion 3.1. Homogeneous precipitation of one-component systems It is well known [28] that slow hydrolysis of metal salts leads to crystalline nanoparticles of corresponding metal oxides. For
Fig. 1. Electron micrographs of TiO2 samples prepared by homogeneous precipitation of TiOSO4 aqueous solutions with urea. A, B — SEM images at different magnification; C, D — HRTEM images at different magnification; SAED image.
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J. Šubrt et al. / Powder Technology 169 (2006) 33–40
instance, by hydrolysis of TiCl4 [29] stable aqueous colloid solutions of 4–6 nm Q-TiO2 (quantum sized) particles are formed. These solutions are stable in an acid medium, when negative surface charge stabilizes particles towards coagulation. By increasing pH this surface charge gradually disappears and the particles can aggregate. According to Soler-Illia [1], the
amorphous compounds are spherical, whereas those of crystalline solid phases show characteristic crystal habits. For amorphous solids, minimization of the surface energy leads to sphericity. The final particle shape depends on whether oriented or isotropic aggregation takes place. Among the various factors that control the aggregation, dispersive forces and electrostatic
Fig. 2. SEM micrographs of mixtures of hydrated metal oxides with different chemical compositions prepared by homogeneous precipitation of TiOSO4 aqueous solutions with urea. A: sample 108; B: sample 117; C: sample 123; D: sample 139; E: sample 145; F: sample 153.
J. Šubrt et al. / Powder Technology 169 (2006) 33–40
37
Fig. 3. SEM micrographs of annealed hydrated FeOOH sample prepared by homogeneous precipitation of Fe2(SO4)3 aqueous solutions with urea. A, B — the initial sample dried at room temperature, C, D — the sample annealed at 1000 °C for 2 h.
interparticle interactions are the most important. Oriented aggregation is a consequence of the dissimilar electrostatic charges that develop on the different crystal faces of the primary particles, which aggregate in the directions of minimum repulsion; this phenomenon occurs mainly when the overall particle charge is large. On the other hand, isotropic aggregation, which usually leads to the formation of spherical particles, takes place in the vicinity of the isoelectric point. Fig. 1 shows electron micrographs of TiO2 precipitate prepared according to the procedure described above. The SEM micrographs (Fig. 1A and B) show roughly spherical particles that are ∼ 2 μm in size. At higher magnifications on the TEM micrographs (Fig. 1C) small subunits forming the agglomerates can be observed. HRTEM image (Fig. 1D) shows small 4–5 nm anatase microcrystals forming the agglomerates. SEAD diffraction pattern on the Fig. 1E confirms the crystallinity of the sample. The specific surface area of this TiO2 sample determined from the B.E.T. N2 adsorption isotherm was 281 m2/g (generally varies between 200 and 500 m2/g). The porosity of the sample was found to be 0.25 cm3/g and consists mostly of micropores (82% of porosity belongs to pores with diameter b8 nm).
Similar results were observed studying precipitation of some other salts by urea under similar conditions. In the case of precipitation of Cr2(SO4)3 spherical aggregates of grimalddite CrO(OH) particles [15] are formed whereas Fe2(SO 4) 3 provides spheres consisting of small particles of goethite and/ or ferrihydrite [15] or schwertmannite [30]. All these materials are similar in shape of particles, exhibit high surface area (up to 1000 m2 /g) and pronounced porosity. In all materials the broadened XRD lines indicate a very small particle size of the primary crystallites lying in the range 4–6 nm. 3.2. Homogeneous precipitation of binary systems Using the same synthetic procedure we studied also homogeneous precipitation of aqueous solutions of selected binary salt mixtures in systems: Ti–Fe, Al–Fe, Ti–Al, Ti–Zn, Al–Zn, Fe–Zn. The composition of the starting salt mixtures and product properties is given in Table 1. We observed a similar reaction course like in the case of one-component system. The SEM micrographs of particles are shown in Fig. 2. As it can be concluded from these micrographs, also the morphology of the particles resembles significantly the products of precipitation in
38
J. Šubrt et al. / Powder Technology 169 (2006) 33–40
Fig. 4. SEM micrographs of annealed hydrated Ti–Fe oxide mixture (sample 110) prepared by homogeneous precipitation of mixtures of Fe2(SO4)3 and TiOSO4 aqueous solutions with urea. A — sample annealed at 300 °C for 2 h; B — sample annealed at 500 °C for 2 h; C — sample annealed at 800 °C for 2 h; D — the sample annealed at 1200 °C for 2 h.
one-component systems. The particles are spherical and composed of small nanocrystalline subunits. In most experiments, high surface area products with considerable high porosity originated. 3.3. Annealing the particles In order to observe changes occurring in the materials at elevated temperature, the obtained samples were annealed at temperatures 300, 500, 800, 1000 and 1200 °C. As can be seen in Fig. 3, showing the appearance of particles of hydrated iron oxides before and after annealing to 1000 °C, the shape of the particles did not change dramatically. Also the binary mixtures of hydrated metal oxides prepared by homogeneous precipitation of mixtures of metal sulfates did not show significant changes of spherical agglomerate appearance after annealing. The appearance of particles of the heated Ti–Fe hydroxide mixture is presented in Fig. 4. No apparent changes of particle shape were observed up to 800 °C, the spherical particles were not interconnected and their mutual separation was easily possible. At higher temperatures (1200 °C), Fig. 4D we can observe rather
large sub-micron particles characteristic of sharp edges. At lower temperatures the primary particles remain in the nanosize region. The XRD diagram (see Fig. 5) shows changes of particle size and phase composition during annealing. The starting mixture (Fig. 5A) shows only broad XRD peaks of anatase, and the calculated particle size corresponds to 4 nm. Up to 300 °C (Fig. 5B) we can observe only indistinctive narrowing of anatase peaks corresponding to the particle growth of this phase. Significant peak narrowing and formation of pseudobrookite phase took place between 500 and 800 °C (Fig 5C and D). At 800 °C the mixture consists of anatase and pseudobrookite, which is the only phase detected by XRD at 1200 °C (Fig. 5E). The behavior of other studied mixtures on heating was similar to that described for Fe–Ti mixtures. We observed formation of binary compounds in the case that these exist, if not, mixtures of particular components with bigger primary nanoparticles were observed after annealing. 4. Conclusions Homogeneous precipitation of aqueous solutions of some amphoteric metal salts using urea as the precipitation agent
J. Šubrt et al. / Powder Technology 169 (2006) 33–40
results in formation of micron size, roughly spherical particles of corresponding hydrated metal oxides. The spherical agglomerates are mostly crystalline and consist of very small randomly oriented primary nanosized crystallites. Specific surface area of the materials is rather high, depending on the chemical composition that varies between 13 and 432 m2/g, and the material is porous (0.036–0.482 cm3 /g). Precipitation
39
of mixtures of metal salts leads at the same conditions to formation of agglomerates with similar size and shape containing tightly mixed particular components. On heating up to 1000 °C, the resemblance of the spherical particles remains preserved and only negligible interparticle sintering occurs. Inside the spheres growth of crystallites was observed on heating, in the binary systems formation of mixed
Fig. 5. XRD diffractographs of annealed hydrated Ti–Fe oxide mixture (sample 110) prepared by homogeneous precipitation of mixtures of Fe2(SO4)3 and TiOSO4 aqueous solutions with urea. A — the initial sample dried at room temperature, B — sample annealed at 300 °C for 2 h; C — sample annealed at 500 °C for 2 h; D — sample annealed at 800 °C for 2 h; E — sample annealed at 1200 °C for 2 h.
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oxide phases was observed at comparatively low temperatures (above ∼600 °C). Acknowledgments This work was supported by the Grant Agency of the Czech Republic, (Project 104/04/0467) and Ministry of Education, Youth and Sport of the Czech Republic, (Project LC 0525). References [1] G. Soler-Illia, M. Jobbagy, R.J. Candal, A.E. Regazzoni, M.A. Blesa, Synthesis of metal oxide particles from aqueous media: the homogeneous alkalinization method, J. Dispers. Sci. Technol. 19 (1998) 207–228. [2] S. Sohn, Y. Kwon, Y. Kim, D. Kim, Synthesis and characterization of nearmonodisperse yttria particles by homogeneous precipitation method, Powder Technol. 142 (2004) 136–153. [3] A. Ookubo, K. Ooi, T. Tomita, New types of hydrous titanium-oxides obtained by homogeneous precipitation from (titanium(Iii) chloride+urea) solutions, J. Mater. Sci. 24 (1989) 3599–3604. [4] K. Kato, A. Tsuzuki, H. Taoda, Y. Torii, T. Kato, Y. Butsugan, Crystalstructures of TiO2 thin coatings prepared from the alkoxide solution via the dip-coating technique affecting the photocatalytic decomposition of aqueous acetic-acid, J. Mater. Sci. 29 (1994) 5911–5915. [5] S. Bakardjieva, J. Šubrt, V. Štengl, L.A. Perez-Maqueda, M.A. Alario Franco, Characterization of photocatalytically active TiO2 by electron microscopy, Proc. 5th Multinational Congress on Electron Microscopy, Lecce, Italy, 2001, pp. 463–464. [6] S. Bakardjieva, J. Šubrt, V. Štengl, E. Vecernikova, P. Bezdicka, Solid State Phenom. 90–91 (2003) 7–12. [7] V. Štengl, F. Peterka, S. Bakardzijeva, J. Šubrt, V. Balek, J. Jirkovský, L. Kavan, Development of spherical porous anatase and its application as photoactive material, Proc. The Sixth International Conference on TiO2 Photocatalytic Purification and Treatment of Water and Air, Niagara Falls, Ontario, Canada, 2001, pp. 109–110. [8] J. Šubrt, V. Štengl, S. Bakardjieva, V. Balek, J. Jirkovský, Characterization of microstructure changes during heating of hydrous titania layers, Proc. The 4th International Symposium on Photoreaction Control and Photofunctional Materials, Tsukuba, Ibaraki, Japan, 2001, pp. 186–187. [9] S. Bakardjieva, J. Šubrt, W. Stengl, E. Vecernikova, P. Bezdicka, Solid State Phenom. 90–91 (2003) 7–12. [10] X.C. Jiang, C.H. Yan, L.D. Sun, Z.G. Wei, C.S. Liao, Hydrothermal homogeneous urea precipitation of hexagonal YBO3:Eu3+ nanocrystals with improved luminescent properties, J. Solid State Chem. 175 (2003) 245–251. [11] M. Adachi-Pagano, C. Forano, J.P. Besse, Synthesis of Al-rich hydrotalcitelike compounds by using the urea hydrolysis reaction—control of size and morphology, J. Mater. Chem. 13 (2003) 1988–1993. [12] S. Yamabi, H. Imai, Synthesis of rutile and anatase films with high surface areas in aqueous solutions containing urea, Thin Solid Films 434 (2003) 86–93.
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