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Hydroxides as precursors of nanocrystalline oxides
NanoSmcmrcd
Pergamon PI1 SO%59773(99)00190-7
HYDROXIDES
AS PRECURSORS
X. Bokhimi’,
A. Morales’,
Materials,
Vol. 12, pp. 589492, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-see front matter
OF NANOCRYSTALLINE M. Portilla*,
OXIDES
and A. Garcia-Ruiz3
1. Institute of Physics, UNAM, A. P. 20-364, 01000 Mexico D. F., Mexico. 2. Faculty of Chemistry, UNAM, A. P. 70-197,01000 Mexico D. F., Mexico. 3. UPIICSA-COFAA, IPN, TC No. 950 Esq. Resina, 08400 Mexico D. F., Mexico. Abstract. The formation of nanocrystalline oxides by hydrolysis is analyzed. X-ray powder d&i-action was used to measure atom distribution; when it was crystalline the structure was refined by the Rietveld method. Three systems were studied: zirconia, titania and magnesia; they were prepared by using the sol-gel technique. During hydrolysis, the proportions of aqua, hydroxo and 0x0 bindings determined the initial atom distribution and its evolution with temperature. In zirconia, the amorphous structure was fixed by aquo bondings; this structure was obtained in titania samples only at high water to alkoxide molar ratios. In magnesia system, synthesis ions also aflected the concentration of these bindings. 01999 Acta Metallurgica I~C.
INTRODUCTION
Nanostructured oxides prepared chemically have hydroxides as intermediate precursor phases; for example, in the sol-gel technique where it is a consequence of hydrolysis. When a transition metal ion Mz’ is dissolved in water, it becomes solvated by the surrounding water molecules and gives rise to the [M-OH21Z+ ion, which has a partial covalent bond between metal and oxygen atoms. Some charge transfers from the filled 3ar bonding orbital of the water molecule to the empty d orbitals of the transition metal ion [l]. This increases the positive partial charge on hydrogen and causes water molecules to become more acidic. Depending on the electron transfer, the following reactions occur: [M-OH21Z+ =, [M-OH] (’ -‘) + + H+ 2 [M-O] (’ - 2,+ + 21-1’ If N (coordination) is the number of water molecules covalently bound to one cation M”, the formula for any hydrolyzed ion can be written as [MONH~N-A] (z-A)+, where A is defined as the molar ratio of hydrolysis. When A = 0, the result is an aquo-ion [M(OH&]” with only aquo ligands (-OH2) , while for A=2N it is an oxo-ion [Ti&] ( 2N- ’ )- with only 0x0 ligands (=O). If O< A < 2N, the ion can be either an hydroxo complex [M(OHh] ( ’ - N ) + (A=N) with only hydroxo ligands (-OH), an oxo-hydroxo complex [MOx(OHh-x] (N + x - ‘) - (A > N), or an (z-A)+ (A < N). Therefore, 0x0, hydroxo, and aquo hydroxo-aquo complex [M(OI-&(OH& _ A] ligands coexist in hydrolysis. When hydrolyzed samples are annealed, they dehydroxylize, changing the proportion of the different ligands.
590
FOURTH INTERNATIONAL
CONFERENCE
ON NAN~STRUCTURED
MATERIALS
In the present paper we report the temperature evolution of hydrolyzed zirconium, titanium, and magnesium until they form the corresponding nanocrystalline oxide.
EXPERIMENTAL Samples were prepared by using the sol-gel technique; details about synthesis are published elsewhere [2-51. Atom distributions were measured by x-ray powder difiaction with CL& radiation; crystalline structures were refined with the Rietveld technique [6].
RESULTS
AND DISCUSSION
Zirconia. Samples annealed below 300°C were amorphous (Fig. 1A). The basic element building this structure is the zirconyl group [Zr,(OH),(OH&]*‘, which has 8 hydroxo-bridges and 16 aquo bindings [7]. Since the group has sixteen sites at which condensation takes place, polymeric growth can proceed in many different paths. Polymerization occurs by olation (hydroxo binding) between tetramers to produce oligomers, composed of a few zirconyl groups, distributed at random. When samples were annealed, aquo bindings were transformed into hydroxo bonds, oligomers ordered partially and the samples crystallized. After heating samples at 400°C they were composed of nanocrystalline tetragonal and monoclinic phases (Fig. 1B); for the refinement, these phases were modeled with the corresponding tetragonal and monoclinic structures of zirconia [5]; the tetragonal structure was significantly zirconium deficient (4.8 %). When samples were annealed at higher temperatures, the tetragonal phase was transformed into the monoclinic one (Fig. 1C). The large zirconium deficiency can be explained by assuming that the system was not only composed of zirconium and oxygen, but of zirconium, oxygen, and hydrogen atoms. Oxygen atoms occupied the same sites that they do in tetragonal zirconia, but hydrogen was associated to some of them, forming the hydroxyl groups that were stabilizing the tetragonal symmetry [5]. A simple analysis of electrical charge equilibrium implies that empty zirconium sites must be in hydroxyls neighborhood.
Titania. At low temperatures, the atom distribution depended on hydrolysis conditions. The strongly hydrolyzed titanium, which was achieved at large water to alkoxide molar ratios, was amorphous (Fig. 2A). Since a high water to alkoxide molar ratio produces insoluble giant molecules [S], the observed amorphous structure can be explained as a non-correlated mixture of these molecules. Their structure is unknown; but it must contain many aquo bindings. At low and medium water to alkoxide molar ratios, samples contained only nanocrystalline anatase and brookite (Fig.2B). FTIR studies shows that they have many hydroxyls in their crystalline structures [4]; the number of hydroxyls decreases with temperature, while the phases transform into rutile. After refining crystalline structures, we observed that anatase and brookite were titanium deficient; this was produced by the presence of hydroxyls in the crystalline structures.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
591
TiO,
Fig. I Sol-gel zirconia annealed at deferent temperatures. Upper tick marks correspond to tetragonal zirconia; lower, to monoclinic zirconia.
Fig. 2 Sol-gel titania prepared with a large (A) and a small (B) water to alkoxide molar ratio, and doped with platinum (C). Upper tick marks correspond to anatase; middle, to rutile; and lower, to brookite.
Rutile can also be produced at low temperatures (Fig. 2C), this occurred by doping the sample with platinum [9], which catalyzes the generation of 0x0 bindings. Magnesia. During synthesis, not only water to alkoxide molar ratio determined the atom distribution, but also all involved ions (Figs. 3 and 4) [3]. When only hydroxyls were present, the starting phase was nanocrystalline MgO (Fig. 3A); this group favors the formation of 0x0 bindings. When carboxyl was the group, the initial phase was glushinskite (C2Mg04.2H20) (Fig. 3B); this phase was partially transformed into nanocrystalline brucite and periclase when the sample was annealed at 300°C in air (Fig. 3C). Ammonium ions gave rise to an amorphous structure (Fig. 4A); this behavior changed when platinum was added to the solution [4]; it promoted the formation of hydroxo bindings, producing nanocrystalline brucite (Fig. 4B).
CONCLUSIONS
The aquo, hydroxo, and 0x0 bindings, generated during hydrolysis, determined the initial atom distribution and its crystallization in nanostructured phases. The proportion of each binding depended on the system and the ions present in the sample preparation. A large proportion of aquo bindings favored the formation of amorphous structures. These bindings are easily transformed at low temperatures, generating nanocrystalline phases. When samples were annealed, the produced crystalline oxides were rich in hydroxyls that stabilized their structure and gave rise to cation vacancies.
592
FOURTH
II
I
I,
40
I
4
50
INTERNATIONAL
I,
II
I
60
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
,
70
Two Theta (degree)
40
50
60
70
Two Theta (degree)
Fig. 3 MgOprepared with d@erent hydroIyis catalysts: (A) HCI, (B) and (C) acetic acid Upper tick marks correspond to brucite; middle, to MgO and lower to giushinskite.
Fig. 4 Sol-gel MgO: (A) non-doped, (B) doped with platinum. Tick marks correspond to brucite.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
9.
Livage, J., Hem-y, M. and Sanchez, C., Prog SolidState Chem., 1988, 18,259. Bokhimi, Morales, A., Lbpez, T. and G6mez, R., J. SolidState Chem., 1995, 115,411. Bokhimi, Aceves A., Novaro, O., L6pez, T. and G&nez, R., J. Phys. Chem., 1995, 99, 14403. Shchez, E., Lbpez, T., G6mez, R., Bokhimi, Morales, A. and Novaro, O., J. Solid State Chem., 1996, 122,309. Bokhimi, X., Morales, A., Novaro, O., Portilla, M., Lbpez, T., Tzompanzi, F. and G6mez, R., J Solid State Chem., 1998, 135,28. “The Rietveld Method”, Edited by R. A. Young, Oxford University Press, N. Y, USA, 1993. Turrillas, X., Barnes, P., Tarling, S. E., Jones, S. L., Norman, C. J. and Ritter, C., J Mater. Sci. Lett., 1993, 12,223. Bradley, D. C., Gaze, R. and Wardlaw W., J: Chem. Sot., 1955,3977. Sanchez, E., L6pez, T., G6mez, R., Bokhimi, Morales, A. and Novaro, O., J. Solid State Chem., 1996, 122,309.
Pergamon PI1 SO%59773(99)00190-7
HYDROXIDES
AS PRECURSORS
X. Bokhimi’,
A. Morales’,
Materials,
Vol. 12, pp. 589492, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-see front matter
OF NANOCRYSTALLINE M. Portilla*,
OXIDES
and A. Garcia-Ruiz3
1. Institute of Physics, UNAM, A. P. 20-364, 01000 Mexico D. F., Mexico. 2. Faculty of Chemistry, UNAM, A. P. 70-197,01000 Mexico D. F., Mexico. 3. UPIICSA-COFAA, IPN, TC No. 950 Esq. Resina, 08400 Mexico D. F., Mexico. Abstract. The formation of nanocrystalline oxides by hydrolysis is analyzed. X-ray powder d&i-action was used to measure atom distribution; when it was crystalline the structure was refined by the Rietveld method. Three systems were studied: zirconia, titania and magnesia; they were prepared by using the sol-gel technique. During hydrolysis, the proportions of aqua, hydroxo and 0x0 bindings determined the initial atom distribution and its evolution with temperature. In zirconia, the amorphous structure was fixed by aquo bondings; this structure was obtained in titania samples only at high water to alkoxide molar ratios. In magnesia system, synthesis ions also aflected the concentration of these bindings. 01999 Acta Metallurgica I~C.
INTRODUCTION
Nanostructured oxides prepared chemically have hydroxides as intermediate precursor phases; for example, in the sol-gel technique where it is a consequence of hydrolysis. When a transition metal ion Mz’ is dissolved in water, it becomes solvated by the surrounding water molecules and gives rise to the [M-OH21Z+ ion, which has a partial covalent bond between metal and oxygen atoms. Some charge transfers from the filled 3ar bonding orbital of the water molecule to the empty d orbitals of the transition metal ion [l]. This increases the positive partial charge on hydrogen and causes water molecules to become more acidic. Depending on the electron transfer, the following reactions occur: [M-OH21Z+ =, [M-OH] (’ -‘) + + H+ 2 [M-O] (’ - 2,+ + 21-1’ If N (coordination) is the number of water molecules covalently bound to one cation M”, the formula for any hydrolyzed ion can be written as [MONH~N-A] (z-A)+, where A is defined as the molar ratio of hydrolysis. When A = 0, the result is an aquo-ion [M(OH&]” with only aquo ligands (-OH2) , while for A=2N it is an oxo-ion [Ti&] ( 2N- ’ )- with only 0x0 ligands (=O). If O< A < 2N, the ion can be either an hydroxo complex [M(OHh] ( ’ - N ) + (A=N) with only hydroxo ligands (-OH), an oxo-hydroxo complex [MOx(OHh-x] (N + x - ‘) - (A > N), or an (z-A)+ (A < N). Therefore, 0x0, hydroxo, and aquo hydroxo-aquo complex [M(OI-&(OH& _ A] ligands coexist in hydrolysis. When hydrolyzed samples are annealed, they dehydroxylize, changing the proportion of the different ligands.
590
FOURTH INTERNATIONAL
CONFERENCE
ON NAN~STRUCTURED
MATERIALS
In the present paper we report the temperature evolution of hydrolyzed zirconium, titanium, and magnesium until they form the corresponding nanocrystalline oxide.
EXPERIMENTAL Samples were prepared by using the sol-gel technique; details about synthesis are published elsewhere [2-51. Atom distributions were measured by x-ray powder difiaction with CL& radiation; crystalline structures were refined with the Rietveld technique [6].
RESULTS
AND DISCUSSION
Zirconia. Samples annealed below 300°C were amorphous (Fig. 1A). The basic element building this structure is the zirconyl group [Zr,(OH),(OH&]*‘, which has 8 hydroxo-bridges and 16 aquo bindings [7]. Since the group has sixteen sites at which condensation takes place, polymeric growth can proceed in many different paths. Polymerization occurs by olation (hydroxo binding) between tetramers to produce oligomers, composed of a few zirconyl groups, distributed at random. When samples were annealed, aquo bindings were transformed into hydroxo bonds, oligomers ordered partially and the samples crystallized. After heating samples at 400°C they were composed of nanocrystalline tetragonal and monoclinic phases (Fig. 1B); for the refinement, these phases were modeled with the corresponding tetragonal and monoclinic structures of zirconia [5]; the tetragonal structure was significantly zirconium deficient (4.8 %). When samples were annealed at higher temperatures, the tetragonal phase was transformed into the monoclinic one (Fig. 1C). The large zirconium deficiency can be explained by assuming that the system was not only composed of zirconium and oxygen, but of zirconium, oxygen, and hydrogen atoms. Oxygen atoms occupied the same sites that they do in tetragonal zirconia, but hydrogen was associated to some of them, forming the hydroxyl groups that were stabilizing the tetragonal symmetry [5]. A simple analysis of electrical charge equilibrium implies that empty zirconium sites must be in hydroxyls neighborhood.
Titania. At low temperatures, the atom distribution depended on hydrolysis conditions. The strongly hydrolyzed titanium, which was achieved at large water to alkoxide molar ratios, was amorphous (Fig. 2A). Since a high water to alkoxide molar ratio produces insoluble giant molecules [S], the observed amorphous structure can be explained as a non-correlated mixture of these molecules. Their structure is unknown; but it must contain many aquo bindings. At low and medium water to alkoxide molar ratios, samples contained only nanocrystalline anatase and brookite (Fig.2B). FTIR studies shows that they have many hydroxyls in their crystalline structures [4]; the number of hydroxyls decreases with temperature, while the phases transform into rutile. After refining crystalline structures, we observed that anatase and brookite were titanium deficient; this was produced by the presence of hydroxyls in the crystalline structures.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
591
TiO,
Fig. I Sol-gel zirconia annealed at deferent temperatures. Upper tick marks correspond to tetragonal zirconia; lower, to monoclinic zirconia.
Fig. 2 Sol-gel titania prepared with a large (A) and a small (B) water to alkoxide molar ratio, and doped with platinum (C). Upper tick marks correspond to anatase; middle, to rutile; and lower, to brookite.
Rutile can also be produced at low temperatures (Fig. 2C), this occurred by doping the sample with platinum [9], which catalyzes the generation of 0x0 bindings. Magnesia. During synthesis, not only water to alkoxide molar ratio determined the atom distribution, but also all involved ions (Figs. 3 and 4) [3]. When only hydroxyls were present, the starting phase was nanocrystalline MgO (Fig. 3A); this group favors the formation of 0x0 bindings. When carboxyl was the group, the initial phase was glushinskite (C2Mg04.2H20) (Fig. 3B); this phase was partially transformed into nanocrystalline brucite and periclase when the sample was annealed at 300°C in air (Fig. 3C). Ammonium ions gave rise to an amorphous structure (Fig. 4A); this behavior changed when platinum was added to the solution [4]; it promoted the formation of hydroxo bindings, producing nanocrystalline brucite (Fig. 4B).
CONCLUSIONS
The aquo, hydroxo, and 0x0 bindings, generated during hydrolysis, determined the initial atom distribution and its crystallization in nanostructured phases. The proportion of each binding depended on the system and the ions present in the sample preparation. A large proportion of aquo bindings favored the formation of amorphous structures. These bindings are easily transformed at low temperatures, generating nanocrystalline phases. When samples were annealed, the produced crystalline oxides were rich in hydroxyls that stabilized their structure and gave rise to cation vacancies.
592
FOURTH
II
I
I,
40
I
4
50
INTERNATIONAL
I,
II
I
60
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
,
70
Two Theta (degree)
40
50
60
70
Two Theta (degree)
Fig. 3 MgOprepared with d@erent hydroIyis catalysts: (A) HCI, (B) and (C) acetic acid Upper tick marks correspond to brucite; middle, to MgO and lower to giushinskite.
Fig. 4 Sol-gel MgO: (A) non-doped, (B) doped with platinum. Tick marks correspond to brucite.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
9.
Livage, J., Hem-y, M. and Sanchez, C., Prog SolidState Chem., 1988, 18,259. Bokhimi, Morales, A., Lbpez, T. and G6mez, R., J. SolidState Chem., 1995, 115,411. Bokhimi, Aceves A., Novaro, O., L6pez, T. and G&nez, R., J. Phys. Chem., 1995, 99, 14403. Shchez, E., Lbpez, T., G6mez, R., Bokhimi, Morales, A. and Novaro, O., J. Solid State Chem., 1996, 122,309. Bokhimi, X., Morales, A., Novaro, O., Portilla, M., Lbpez, T., Tzompanzi, F. and G6mez, R., J Solid State Chem., 1998, 135,28. “The Rietveld Method”, Edited by R. A. Young, Oxford University Press, N. Y, USA, 1993. Turrillas, X., Barnes, P., Tarling, S. E., Jones, S. L., Norman, C. J. and Ritter, C., J Mater. Sci. Lett., 1993, 12,223. Bradley, D. C., Gaze, R. and Wardlaw W., J: Chem. Sot., 1955,3977. Sanchez, E., L6pez, T., G6mez, R., Bokhimi, Morales, A. and Novaro, O., J. Solid State Chem., 1996, 122,309.