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Metastability of anatase: size dependent and irreversible anatase-rutile phase transition in atomic-level precise titania.

Satoh N, Nakashima T, Yamamoto K - Sci Rep (2013)

Bottom Line: To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection.Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states.The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

View Article: PubMed Central - PubMed

Affiliation: Photovoltaic Materials Unit, National Institute for Materials Science, Tsukuba, Ibaraki, Japan. SATOH.Norifusa@nims.go.jp

ABSTRACT
Since crystal phase dominantly affects the properties of nanocrystals, phase control is important for the applications. To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection. Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states. Therefore, we take into account the kinetic control polymerization of TiO6 octahedral units to explain how the crystal phase of the crystal-nucleus-size titania is dependent on which coordination sites, cis- or trans-, react in the TiO6 octahedra, suggesting possibilities for the synthetic phase control of nanocrystals. In short, the dendrimer templates give access to crystal nucleation chemistry. The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

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Schismatic protocol for preparation and thermal treatment of Q-size TiO2.To prevent molecular aggregation, we control the cast amount of the dendrimer templates assembling Ti(acac)Cl3 based on the molecular size and spread the solution on substrates. Thus, the size of the dendrimer templates determines the maximum dot density, ca. 1013 cm−2. The chemical conversions using hydrolysis and thermolysis provide rutile and anatase crystal phases, respectively. At the final process, combination of annealing at 500°C and UV-O3 cleaning eliminates the dendrimer template. The thermal treatment for the anatase Q-size TiO2 reveals that anatase phase irreversibly transforms into rutile phase at T(r) through the melting-like process.
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f1: Schismatic protocol for preparation and thermal treatment of Q-size TiO2.To prevent molecular aggregation, we control the cast amount of the dendrimer templates assembling Ti(acac)Cl3 based on the molecular size and spread the solution on substrates. Thus, the size of the dendrimer templates determines the maximum dot density, ca. 1013 cm−2. The chemical conversions using hydrolysis and thermolysis provide rutile and anatase crystal phases, respectively. At the final process, combination of annealing at 500°C and UV-O3 cleaning eliminates the dendrimer template. The thermal treatment for the anatase Q-size TiO2 reveals that anatase phase irreversibly transforms into rutile phase at T(r) through the melting-like process.

Mentions: Since phenylazomethine dendrimer (DPA) used in the following experiments can control the number of metal ions in the metal assembly process, it acts as a template to provide atomic-level precise Q-size TiO229. Dendrimer has attracted much attention as a template for nanoparticles because of the well-defined size with a single molecular weight as a branched polymer synthesized through the perfectly-regulated architecture303132. As a distinguished feature of DPA, metal chlorides coordinate with phenylazomethine units within DPA in a radial stepwise manner reflecting the layer-by-layer gradient in basicity of phenylazomethine units2933. Thus, DPA template assembles 14 and 30 equivalents of Ti(acac)Cl3, where acac is acetylacetonate, to produce the number-corresponding Q-size TiO2 (14TiO2 and 30TiO2) ca. 1.3 and 1.6 nm on substrate, respectively; the restriction of the component number also results in the small standard deviation (s.d.), 0.2 nm. These advantages in size control are suitable to discuss the size dependence for particles less than 2 nm. In the previous paper, the authors reported the size dependence in bandgap energy observed with OWG spectroscopy. The size dependence is practically described as follows: where E(r) is size-dependent bandgap energy; <$>\raster="rg1"<$> is Planck's constant; e is the charge on the electron; Ebulk is the bandgap in the bulk; μ′ is the semi-empirically adjusted reduced mass; and ε is the dielectric constant of the bulk semiconductor (see; the further explanation in Supplementary Fig. S1). The different chemical conversion procedures using hydrolysis and thermolysis provide the same size particles, but different E(r). Equation (2) explains that E(r) strictly senses the size of Q-size TiO2 and the difference in crystal structure. Recently, these results have been supported by first-principles simulations343536. The sensibility is beneficial to detect the crystal-nucleus-scale phenomena after the thermal treatment. Note that Q-size TiO2 is sintered in air at 500°C for 1 hour to remove the dendrimer templates, which is sufficient time to crystallize Q-size TiO2 and consume the unreacted precursors in the templates. To strengthen the correlation with other studies, herein, we first prepared 2 nm Q-size TiO2 using tetraphenylmethane core dendrimer (TPM-DPA) template; TPM-DPA is the biggest derivative in DPAs37, which has 60 binding sites for metal ions to form 60TiO2 ca. 2 nm in size. The 60 equiv. of Ti(acac)Cl3 within TPM-DPA transform into 60TiO2 on substrate using the previously reported procedure as well as 14TiO2 and 30TiO2, and then we observed size-dependent and irreversible phase transition of the anatase Q-size TiO2 accompanied with the size growth (Fig. 1)29. The results support our hypothesis and suggest that Q-size TiO2 maintains some parts of the bulk crystal structure even less than 2 nm in size.


Metastability of anatase: size dependent and irreversible anatase-rutile phase transition in atomic-level precise titania.

Satoh N, Nakashima T, Yamamoto K - Sci Rep (2013)

Schismatic protocol for preparation and thermal treatment of Q-size TiO2.To prevent molecular aggregation, we control the cast amount of the dendrimer templates assembling Ti(acac)Cl3 based on the molecular size and spread the solution on substrates. Thus, the size of the dendrimer templates determines the maximum dot density, ca. 1013 cm−2. The chemical conversions using hydrolysis and thermolysis provide rutile and anatase crystal phases, respectively. At the final process, combination of annealing at 500°C and UV-O3 cleaning eliminates the dendrimer template. The thermal treatment for the anatase Q-size TiO2 reveals that anatase phase irreversibly transforms into rutile phase at T(r) through the melting-like process.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3675453&req=5

f1: Schismatic protocol for preparation and thermal treatment of Q-size TiO2.To prevent molecular aggregation, we control the cast amount of the dendrimer templates assembling Ti(acac)Cl3 based on the molecular size and spread the solution on substrates. Thus, the size of the dendrimer templates determines the maximum dot density, ca. 1013 cm−2. The chemical conversions using hydrolysis and thermolysis provide rutile and anatase crystal phases, respectively. At the final process, combination of annealing at 500°C and UV-O3 cleaning eliminates the dendrimer template. The thermal treatment for the anatase Q-size TiO2 reveals that anatase phase irreversibly transforms into rutile phase at T(r) through the melting-like process.
Mentions: Since phenylazomethine dendrimer (DPA) used in the following experiments can control the number of metal ions in the metal assembly process, it acts as a template to provide atomic-level precise Q-size TiO229. Dendrimer has attracted much attention as a template for nanoparticles because of the well-defined size with a single molecular weight as a branched polymer synthesized through the perfectly-regulated architecture303132. As a distinguished feature of DPA, metal chlorides coordinate with phenylazomethine units within DPA in a radial stepwise manner reflecting the layer-by-layer gradient in basicity of phenylazomethine units2933. Thus, DPA template assembles 14 and 30 equivalents of Ti(acac)Cl3, where acac is acetylacetonate, to produce the number-corresponding Q-size TiO2 (14TiO2 and 30TiO2) ca. 1.3 and 1.6 nm on substrate, respectively; the restriction of the component number also results in the small standard deviation (s.d.), 0.2 nm. These advantages in size control are suitable to discuss the size dependence for particles less than 2 nm. In the previous paper, the authors reported the size dependence in bandgap energy observed with OWG spectroscopy. The size dependence is practically described as follows: where E(r) is size-dependent bandgap energy; <$>\raster="rg1"<$> is Planck's constant; e is the charge on the electron; Ebulk is the bandgap in the bulk; μ′ is the semi-empirically adjusted reduced mass; and ε is the dielectric constant of the bulk semiconductor (see; the further explanation in Supplementary Fig. S1). The different chemical conversion procedures using hydrolysis and thermolysis provide the same size particles, but different E(r). Equation (2) explains that E(r) strictly senses the size of Q-size TiO2 and the difference in crystal structure. Recently, these results have been supported by first-principles simulations343536. The sensibility is beneficial to detect the crystal-nucleus-scale phenomena after the thermal treatment. Note that Q-size TiO2 is sintered in air at 500°C for 1 hour to remove the dendrimer templates, which is sufficient time to crystallize Q-size TiO2 and consume the unreacted precursors in the templates. To strengthen the correlation with other studies, herein, we first prepared 2 nm Q-size TiO2 using tetraphenylmethane core dendrimer (TPM-DPA) template; TPM-DPA is the biggest derivative in DPAs37, which has 60 binding sites for metal ions to form 60TiO2 ca. 2 nm in size. The 60 equiv. of Ti(acac)Cl3 within TPM-DPA transform into 60TiO2 on substrate using the previously reported procedure as well as 14TiO2 and 30TiO2, and then we observed size-dependent and irreversible phase transition of the anatase Q-size TiO2 accompanied with the size growth (Fig. 1)29. The results support our hypothesis and suggest that Q-size TiO2 maintains some parts of the bulk crystal structure even less than 2 nm in size.

Bottom Line: To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection.Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states.The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

View Article: PubMed Central - PubMed

Affiliation: Photovoltaic Materials Unit, National Institute for Materials Science, Tsukuba, Ibaraki, Japan. SATOH.Norifusa@nims.go.jp

ABSTRACT
Since crystal phase dominantly affects the properties of nanocrystals, phase control is important for the applications. To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection. Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states. Therefore, we take into account the kinetic control polymerization of TiO6 octahedral units to explain how the crystal phase of the crystal-nucleus-size titania is dependent on which coordination sites, cis- or trans-, react in the TiO6 octahedra, suggesting possibilities for the synthetic phase control of nanocrystals. In short, the dendrimer templates give access to crystal nucleation chemistry. The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

Show MeSH
Related in: MedlinePlus