- Email: [email protected]
Inductive heating hydrothermal synthesis of titanium dioxide nanostructures
Materials Letters 95 (2013) 59–62
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Inductive heating hydrothermal synthesis of titanium dioxide nanostructures S. Novaconi a,b,n, N. Vaszilcsin b a b
National Institute for Research and Development in Electrochemistry and Condensed Matter, Pl.Andronescu 1, 300254-Timisoara, Romania Politehnica University of Timisoara, P-ta.Victoriei 2, 300006-Timisoara, Romania
a r t i c l e i n f o
abstract
Article history: Received 10 August 2012 Accepted 21 December 2012 Available online 31 December 2012
Nanocrystalline powders of pure anatase and rutile/anatase mixture type of titania with different morphology were prepared through hydrothermal method using titanium tetrachloride in water/citric acid and n-butanol, at 200 1C in Teflon lined steel autoclave using inductive heating. The influences of hydrothermal treatment conditions on the formation features, phase composition, particle size and morphology of the products were investigated by X-ray diffraction and scanning electron microscopy. The results are compared with titanium dioxide powders obtained through classical hydrothermal synthesis, at same temperature. It was found that inductive heating hydrothermal processing results in formation of considerably low dimensional dispersion of titania nanostructures and the time for synthesis is also much smaller. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanoparticles TiO2 Hydrothermal Induction heating
1. Introduction Nanomaterials offer an extremely large area of practical applications, opening new perspectives regarding technical performances of solid-state devices. Hence, it is necessary to develop new suitable synthesis methods providing a more strictly control of the dimensionality, structure and properties of these materials. Currently, titanium dioxide is one of the most interesting nanostructured materials because they manifest great optical, electrical, photocatalytic and thermal properties. Such property are particularly important for their potential applications as electrode materials for solar cells, photocatalysts, wide band gap materials for gas sensing, pharmaceuticals, paints, and disinfectants [1–7]. Each application of titanium dioxide requires a specific crystalline structure and also a specific size. For example titanium dioxide nanocrystals with controlled properties can increase conversion efficiency in photo-electrochemical cells [8]. Presently, hydrothermal synthesis is normally applied for the preparation of different classes of inorganic nanomaterials [9]. Hydrothermal technique is defined as a heterogeneous chemical reaction in the presence of a solvent, aqueous or nonaqueous, above room temperature and at pressure 41 atm in a closed system [9]. The hydrothermal method is environmentally friendly because the reactions are carried out in a closed system [9, 10].
n Corresponding author at: National Institute for Research and Development in Electrochemistry and Condensed Matter, Pl.Andronescu 1, 300254-Timisoara, Romania, Tel.: þ40 256 494 413; fax: þ40 256 204 698. E-mail address: [email protected] (S. Novaconi).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.083
In the classical TiO2 hydrothermal synthesis method, the precursors are usually introduced in an autoclave heated in an electrical oven. Heating process takes place by radiation and air convection between oven and autoclave. Thus the temperature (180–220 1C) needed for crystallization inside the autoclave is performed in tens of minutes, even hours. In this paper we propose a method of nanomaterial hydrothermal synthesis through inductive heating of autoclave. As we now hydrothermal synthesis by induction heating is not reported so far, but some treatments are reported [11]. This rapid method can be applied for all hydrothermal synthesis in metal autoclave (lined or not) replacing conventional hydrothermal method with better results. The attractive features of this technique include efficient energy transfer and rapid processing of materials with cost reduction. Also this method can replace the fast microwave heating with two clear advantages; synthesis in metal autoclave, more suitable for high pressure and temperature, and possibility of using solvents with low dielectric constant where microwave heating is inefficient. In particular we synthesized through this method titania nanostructures with low dimensional distribution. The results are compared with the titania nanostructures obtained by classical hydrothermal method.
2. Material and methods Induction heating is a non-contact heating process which uses alternative current electricity to heat materials that are electrically conductive. This process is very efficient since the heat is actually generated inside the workpiece.
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A source of AC electricity is used to drive a large alternating current through a work coil. The alternating magnetic field induces a current flow in the conductive workpiece (eddy currents), heating it. Frequency used in induction heating systems forces the alternating current to flow in a thin layer over the surface of the workpiece (skin effect). The penetration depth can be calculated from the following equation [12]:
d ¼ ðpf msÞ1=2 where d-penetration depth (m), f-frequency (Hz), m-magnetic permeability (H/m) and s-electrical conductivity (S/m) The skin effect increases the effective resistance of the metal to the passage of the alternating current increasing the heating effect. For ferrous metals an additional heating mechanism takes place at the same time. The intense alternating magnetic field repeatedly magnetizes and de-magnetizes the iron crystals. This rapid flipping of the magnetic domains causes considerable friction and heating inside the material. For this reason ferrous materials lend themselves more easily to heating by induction. In this work we used Teflon lined steel autoclave, 160 mm long and 60 mm diameter, with an internal volume of 100 cm3. The schematic diagram is shown in Fig. 1. Alternative current signal given by the variable frequency generator (ATEN8603B) and amplified by a power amplifier in bridge mode (EUROPOWER BEHRINGER EP4000) is applied across the parallel resonant circuit. The work coil resonates at operating frequency by means of a capacitor placed in parallel with it. The parallel resonance also magnifies the current through the work coil. The copper (3 mm) work coil has 50 turns and 75 mm diameter (inductance of 110 mH measured with Tegam3550 Impedance/LCR Meter). At the operating frequency of 11 kHz
the power factor correction (PFC) capacitor placed in parallel with coil has 2.2 mF/450 V and 15 kvar. Compared with classical resistive heating process the inductive heating is much faster, operating temperature of 200 1C is achieved in only 5–7 min. The surface temperature of autoclave is not uniform at heating, thermal conduction of autoclave material is unable to compensate rapid heating through induced currents. Temperature gradients are significantly lower after reaching the work parameters, when only heat loss should be compensated. The feedback system, through temperature controller (OMROM E5CN) cuts off the frequency control of the amplifier when the temperature achieved preset value. Phase-pure anatase TiO2 nanocrystallites were produced directly from a titanium tetrachloride (TiCl4) aqueous solution using citric acid (C6H8O7) as additive. TiO2 synthesis has been realized by forced hydrolysis in hydrothermal condition at 200 1C starting from 0.45 M TiCl4 to 0.5 M citric acid in distilled water with the employment of both; conventional and inductive heating. Mixed phases TiO2 nanocrystallite were synthesized in solvothermal condition at 200 1C using 0.5 M TiCl4 in n-butanol (C4H9OH) using also both heating method. The precursors were initially mixed and continuous stirred on a hot plate at 60 1C for 30 min and then introduced in a Teflon lined steel autoclave, under a fill degree of 80%. The inductive heating hydrothermal synthesis (IHHS) is conducted with a setup as above, for 30 min. Conventional hydrothermal synthesis (CHS) was performed in a twin autoclave, at the same temperature, heated for 3 h in an electric oven. After the synthesis reactions, the autoclave was forced cooled with water to room temperature. The resulting powders was filtered, washed with distilled water, ethanol and acetone several times and dried at 80 1C in a conventional drying oven. The phase structures of the prepared samples was investigated by X-ray diffraction using PANalytical X’PertPRO MPD Difract˚ Scherrer equation: ometer with CuKa radiation l ¼1.5406 A. d ¼ 0:9l=ðbcos yÞ was used to estimate grain average sizes of crystallites, where b is the half height width of the reflection peak at 2y and l is the wavelength of the radiation. Powder morphology was observed using a Zeiss Evo 50 XVP Scanning Electron Microscope with LaB6 cathode.
3. Results and discussion
Fig. 1. Schematic diagram of inductive heating system, thermal image (FLIR i5o) after 3 min of inductive heating; temperature gradient across autoclave.
All of the diffraction peaks, for powders synthesized in citric acid, are well consistent with the standard spectrum (JCPDS no.:84–1286), indicating that they were anatase-type titania (Fig. 2((a) and (b)) The average grain crystallites sizes estimated with Scherrer equation using (1 0 1) and (2 0 0) reflection are about 35 nm for aTiO2 obtained by CHS-method and 18 nm for a-TiO2 obtained by IHHS-method. For TiO2 powders obtained in n-butanol, XRD patterns reveal that, independent of the synthesis method used, rutile is the main crystallographic phase (Fig. 2((c) and (d)). The rutile/anatase ratio is about 5/1 for powders prepared by CHS-method and about 6/1 for powders prepared by IHHS-method. The reference line patterns correspond to the JCPDS no:89–0553 and no:86–1156. Refinement was performed using 9009083 CIF file [13]. Crystallinity of the powder appears to be related more to the reaction time than the synthesis method. When reaction time
S. Novaconi, N. Vaszilcsin / Materials Letters 95 (2013) 59–62
61
Fig. 2. X-ray diffraction pattern of TiO2 powders obtained in water/citric acid: (a) CHS-method; (b) IHHS-method and in n-butanol (c) CHS-method; (d) IHHS-method.
is increased the peak broadening decreases indicating the increasing of both average crystallite size and crystallinity. Average crystallites sizes estimated with Scherrer equation using (1 1 1) and (1 0 1) reflection are about 25 nm for r-TiO2 obtained by CHS-method and 15 nm for r-TiO2 obtained by IHHSmethod. The microscopic (SEM) images of TiO2 powders obtained in citric acid through CHS-method and IHHS-method, shown in Fig. 3. indicate that the particles had mixed morphology with some elongated crystals and some quasi spherical crystals randomly disposed.
The grain size for TiO2, results from the analysis of SEM image, for about 200 particles, with ScionImage, reveals that the particles obtained by IHHS-method are smaller than particles obtained by CHS-method and in good agreement with dimension results from the XRD pattern. It can also be observed that the particle size distribution obtained through IHHS-method is smaller than the particle size distribution obtained through CHS-method. This can be attributed to the heating process, much faster, and reaching the required temperature in the autoclave in less time. Because of this, the premature nucleation and transitory processes are almost avoided, and the crystallization is taking place in isothermal conditions.
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Fig. 3. SEM Image of TiO2 powders prepared in water/citric acid: (a) CHS-method; (b) IHHS-method; dimensional distribution.
4. Conclusions Nanosized TiO2 powders were successfully synthesized via hydrothermal method through inductive heating and compared with powder obtained through classical hydrothermal method. The features of this technique include efficient energy transfer and rapid processing of materials with cost reduction and energy saving. Heating process is much faster and required temperature is achieved in less time. The premature nucleation and transitory processes are almost avoided, and the crystallization is taking place in isothermal conditions. TiO2 nanoparticle synthesized in water/citric acid is anatase pure type titania. The dimension of nanocrystallites obtained by IHHS-method is smaller than particles obtained by CHS-method. Significant is the fact that dimensional dispersion is lower for particles obtained through IHHS method, making them of interest for targeted applications like electrode for dye solar cells. For TiO2 powders obtained in n-butanol, independent of the synthesis method used, rutile is the main crystallographic phase. Micrometer size spherical agglomerations consisting of acicular crystals were obtained through both methods. Apparently, shape, size and the rutile/anatase ratio depends very little on the synthesis method. References
Fig. 4. SEM Image of TiO2 powders prepared in n-butanol: (a) CHS-method; (b) IHHS-method.
Fig. 4((a) and (b)) shows the SEM photographs of the TiO2 samples obtained through CHS-method and IHHS-method, in n-butanol. SEM image of synthesized powders show spherical granule formed by acicular crystals (typical for rutile) in agglomerated clusters, with some much smaller irregular shape particle at surface (probable anatase). Apparently, shape and size depends very little on the synthesis method. Acicular spherical agglomerations have micrometer size for both cases, slightly lower for particles synthesized by IHHS-method.
[1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13]
O’Regan B, Gratzel M. Nature 1991;353:737–40. Gratzel M. Chem Lett 2005;34(1):8–13. Fujishima A, Rao TN, Tryk DA. Electrochim Acta 2000;45:4683–90. Sunada K, Kikuchi Y, Hashimoto K, Fujishima A. Environ Sci Technol 1998;32:726–8. Tang H, Prasad K, Sanjines R, Levy F. Sens Actuators, B 1995;26–27:71–5. Akbar SA, Younkman LB. J Electrochem Soc 1997;144:1750–3. Savage NO, Akbar SA, Dutta PK. Sens Actuators B, 2001;72:239–48. Nakade S, Saito Y, Kubo W, Kitamura T, Wada Y, Yanagida SJ. Phys Chem Earth Part B 2003;107:8607–11. Byrappa K, Yoshimura M. Handbook of hydrothermal technique: a technology for crystal growth and materials processing. New York: William Andrew Pub; 2001. Demazeau G. J Mater Chem 1999;9:15–8. Devillers S, Barthelemy B, Delhalle J, Mekhalif ZACS. ACS Appl Mater Interfaces 2011;3(10):4059–66. Shen LC, Kong JA. Applied electromagnetism. 2nd ed. Boston: PWS pub; 1987. Wyckof RWG. Crystal structures, 1. NY: Interscience Publishers; 1963 239–444.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Inductive heating hydrothermal synthesis of titanium dioxide nanostructures S. Novaconi a,b,n, N. Vaszilcsin b a b
National Institute for Research and Development in Electrochemistry and Condensed Matter, Pl.Andronescu 1, 300254-Timisoara, Romania Politehnica University of Timisoara, P-ta.Victoriei 2, 300006-Timisoara, Romania
a r t i c l e i n f o
abstract
Article history: Received 10 August 2012 Accepted 21 December 2012 Available online 31 December 2012
Nanocrystalline powders of pure anatase and rutile/anatase mixture type of titania with different morphology were prepared through hydrothermal method using titanium tetrachloride in water/citric acid and n-butanol, at 200 1C in Teflon lined steel autoclave using inductive heating. The influences of hydrothermal treatment conditions on the formation features, phase composition, particle size and morphology of the products were investigated by X-ray diffraction and scanning electron microscopy. The results are compared with titanium dioxide powders obtained through classical hydrothermal synthesis, at same temperature. It was found that inductive heating hydrothermal processing results in formation of considerably low dimensional dispersion of titania nanostructures and the time for synthesis is also much smaller. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanoparticles TiO2 Hydrothermal Induction heating
1. Introduction Nanomaterials offer an extremely large area of practical applications, opening new perspectives regarding technical performances of solid-state devices. Hence, it is necessary to develop new suitable synthesis methods providing a more strictly control of the dimensionality, structure and properties of these materials. Currently, titanium dioxide is one of the most interesting nanostructured materials because they manifest great optical, electrical, photocatalytic and thermal properties. Such property are particularly important for their potential applications as electrode materials for solar cells, photocatalysts, wide band gap materials for gas sensing, pharmaceuticals, paints, and disinfectants [1–7]. Each application of titanium dioxide requires a specific crystalline structure and also a specific size. For example titanium dioxide nanocrystals with controlled properties can increase conversion efficiency in photo-electrochemical cells [8]. Presently, hydrothermal synthesis is normally applied for the preparation of different classes of inorganic nanomaterials [9]. Hydrothermal technique is defined as a heterogeneous chemical reaction in the presence of a solvent, aqueous or nonaqueous, above room temperature and at pressure 41 atm in a closed system [9]. The hydrothermal method is environmentally friendly because the reactions are carried out in a closed system [9, 10].
n Corresponding author at: National Institute for Research and Development in Electrochemistry and Condensed Matter, Pl.Andronescu 1, 300254-Timisoara, Romania, Tel.: þ40 256 494 413; fax: þ40 256 204 698. E-mail address: [email protected] (S. Novaconi).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.083
In the classical TiO2 hydrothermal synthesis method, the precursors are usually introduced in an autoclave heated in an electrical oven. Heating process takes place by radiation and air convection between oven and autoclave. Thus the temperature (180–220 1C) needed for crystallization inside the autoclave is performed in tens of minutes, even hours. In this paper we propose a method of nanomaterial hydrothermal synthesis through inductive heating of autoclave. As we now hydrothermal synthesis by induction heating is not reported so far, but some treatments are reported [11]. This rapid method can be applied for all hydrothermal synthesis in metal autoclave (lined or not) replacing conventional hydrothermal method with better results. The attractive features of this technique include efficient energy transfer and rapid processing of materials with cost reduction. Also this method can replace the fast microwave heating with two clear advantages; synthesis in metal autoclave, more suitable for high pressure and temperature, and possibility of using solvents with low dielectric constant where microwave heating is inefficient. In particular we synthesized through this method titania nanostructures with low dimensional distribution. The results are compared with the titania nanostructures obtained by classical hydrothermal method.
2. Material and methods Induction heating is a non-contact heating process which uses alternative current electricity to heat materials that are electrically conductive. This process is very efficient since the heat is actually generated inside the workpiece.
60
S. Novaconi, N. Vaszilcsin / Materials Letters 95 (2013) 59–62
A source of AC electricity is used to drive a large alternating current through a work coil. The alternating magnetic field induces a current flow in the conductive workpiece (eddy currents), heating it. Frequency used in induction heating systems forces the alternating current to flow in a thin layer over the surface of the workpiece (skin effect). The penetration depth can be calculated from the following equation [12]:
d ¼ ðpf msÞ1=2 where d-penetration depth (m), f-frequency (Hz), m-magnetic permeability (H/m) and s-electrical conductivity (S/m) The skin effect increases the effective resistance of the metal to the passage of the alternating current increasing the heating effect. For ferrous metals an additional heating mechanism takes place at the same time. The intense alternating magnetic field repeatedly magnetizes and de-magnetizes the iron crystals. This rapid flipping of the magnetic domains causes considerable friction and heating inside the material. For this reason ferrous materials lend themselves more easily to heating by induction. In this work we used Teflon lined steel autoclave, 160 mm long and 60 mm diameter, with an internal volume of 100 cm3. The schematic diagram is shown in Fig. 1. Alternative current signal given by the variable frequency generator (ATEN8603B) and amplified by a power amplifier in bridge mode (EUROPOWER BEHRINGER EP4000) is applied across the parallel resonant circuit. The work coil resonates at operating frequency by means of a capacitor placed in parallel with it. The parallel resonance also magnifies the current through the work coil. The copper (3 mm) work coil has 50 turns and 75 mm diameter (inductance of 110 mH measured with Tegam3550 Impedance/LCR Meter). At the operating frequency of 11 kHz
the power factor correction (PFC) capacitor placed in parallel with coil has 2.2 mF/450 V and 15 kvar. Compared with classical resistive heating process the inductive heating is much faster, operating temperature of 200 1C is achieved in only 5–7 min. The surface temperature of autoclave is not uniform at heating, thermal conduction of autoclave material is unable to compensate rapid heating through induced currents. Temperature gradients are significantly lower after reaching the work parameters, when only heat loss should be compensated. The feedback system, through temperature controller (OMROM E5CN) cuts off the frequency control of the amplifier when the temperature achieved preset value. Phase-pure anatase TiO2 nanocrystallites were produced directly from a titanium tetrachloride (TiCl4) aqueous solution using citric acid (C6H8O7) as additive. TiO2 synthesis has been realized by forced hydrolysis in hydrothermal condition at 200 1C starting from 0.45 M TiCl4 to 0.5 M citric acid in distilled water with the employment of both; conventional and inductive heating. Mixed phases TiO2 nanocrystallite were synthesized in solvothermal condition at 200 1C using 0.5 M TiCl4 in n-butanol (C4H9OH) using also both heating method. The precursors were initially mixed and continuous stirred on a hot plate at 60 1C for 30 min and then introduced in a Teflon lined steel autoclave, under a fill degree of 80%. The inductive heating hydrothermal synthesis (IHHS) is conducted with a setup as above, for 30 min. Conventional hydrothermal synthesis (CHS) was performed in a twin autoclave, at the same temperature, heated for 3 h in an electric oven. After the synthesis reactions, the autoclave was forced cooled with water to room temperature. The resulting powders was filtered, washed with distilled water, ethanol and acetone several times and dried at 80 1C in a conventional drying oven. The phase structures of the prepared samples was investigated by X-ray diffraction using PANalytical X’PertPRO MPD Difract˚ Scherrer equation: ometer with CuKa radiation l ¼1.5406 A. d ¼ 0:9l=ðbcos yÞ was used to estimate grain average sizes of crystallites, where b is the half height width of the reflection peak at 2y and l is the wavelength of the radiation. Powder morphology was observed using a Zeiss Evo 50 XVP Scanning Electron Microscope with LaB6 cathode.
3. Results and discussion
Fig. 1. Schematic diagram of inductive heating system, thermal image (FLIR i5o) after 3 min of inductive heating; temperature gradient across autoclave.
All of the diffraction peaks, for powders synthesized in citric acid, are well consistent with the standard spectrum (JCPDS no.:84–1286), indicating that they were anatase-type titania (Fig. 2((a) and (b)) The average grain crystallites sizes estimated with Scherrer equation using (1 0 1) and (2 0 0) reflection are about 35 nm for aTiO2 obtained by CHS-method and 18 nm for a-TiO2 obtained by IHHS-method. For TiO2 powders obtained in n-butanol, XRD patterns reveal that, independent of the synthesis method used, rutile is the main crystallographic phase (Fig. 2((c) and (d)). The rutile/anatase ratio is about 5/1 for powders prepared by CHS-method and about 6/1 for powders prepared by IHHS-method. The reference line patterns correspond to the JCPDS no:89–0553 and no:86–1156. Refinement was performed using 9009083 CIF file [13]. Crystallinity of the powder appears to be related more to the reaction time than the synthesis method. When reaction time
S. Novaconi, N. Vaszilcsin / Materials Letters 95 (2013) 59–62
61
Fig. 2. X-ray diffraction pattern of TiO2 powders obtained in water/citric acid: (a) CHS-method; (b) IHHS-method and in n-butanol (c) CHS-method; (d) IHHS-method.
is increased the peak broadening decreases indicating the increasing of both average crystallite size and crystallinity. Average crystallites sizes estimated with Scherrer equation using (1 1 1) and (1 0 1) reflection are about 25 nm for r-TiO2 obtained by CHS-method and 15 nm for r-TiO2 obtained by IHHSmethod. The microscopic (SEM) images of TiO2 powders obtained in citric acid through CHS-method and IHHS-method, shown in Fig. 3. indicate that the particles had mixed morphology with some elongated crystals and some quasi spherical crystals randomly disposed.
The grain size for TiO2, results from the analysis of SEM image, for about 200 particles, with ScionImage, reveals that the particles obtained by IHHS-method are smaller than particles obtained by CHS-method and in good agreement with dimension results from the XRD pattern. It can also be observed that the particle size distribution obtained through IHHS-method is smaller than the particle size distribution obtained through CHS-method. This can be attributed to the heating process, much faster, and reaching the required temperature in the autoclave in less time. Because of this, the premature nucleation and transitory processes are almost avoided, and the crystallization is taking place in isothermal conditions.
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Fig. 3. SEM Image of TiO2 powders prepared in water/citric acid: (a) CHS-method; (b) IHHS-method; dimensional distribution.
4. Conclusions Nanosized TiO2 powders were successfully synthesized via hydrothermal method through inductive heating and compared with powder obtained through classical hydrothermal method. The features of this technique include efficient energy transfer and rapid processing of materials with cost reduction and energy saving. Heating process is much faster and required temperature is achieved in less time. The premature nucleation and transitory processes are almost avoided, and the crystallization is taking place in isothermal conditions. TiO2 nanoparticle synthesized in water/citric acid is anatase pure type titania. The dimension of nanocrystallites obtained by IHHS-method is smaller than particles obtained by CHS-method. Significant is the fact that dimensional dispersion is lower for particles obtained through IHHS method, making them of interest for targeted applications like electrode for dye solar cells. For TiO2 powders obtained in n-butanol, independent of the synthesis method used, rutile is the main crystallographic phase. Micrometer size spherical agglomerations consisting of acicular crystals were obtained through both methods. Apparently, shape, size and the rutile/anatase ratio depends very little on the synthesis method. References
Fig. 4. SEM Image of TiO2 powders prepared in n-butanol: (a) CHS-method; (b) IHHS-method.
Fig. 4((a) and (b)) shows the SEM photographs of the TiO2 samples obtained through CHS-method and IHHS-method, in n-butanol. SEM image of synthesized powders show spherical granule formed by acicular crystals (typical for rutile) in agglomerated clusters, with some much smaller irregular shape particle at surface (probable anatase). Apparently, shape and size depends very little on the synthesis method. Acicular spherical agglomerations have micrometer size for both cases, slightly lower for particles synthesized by IHHS-method.
[1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13]
O’Regan B, Gratzel M. Nature 1991;353:737–40. Gratzel M. Chem Lett 2005;34(1):8–13. Fujishima A, Rao TN, Tryk DA. Electrochim Acta 2000;45:4683–90. Sunada K, Kikuchi Y, Hashimoto K, Fujishima A. Environ Sci Technol 1998;32:726–8. Tang H, Prasad K, Sanjines R, Levy F. Sens Actuators, B 1995;26–27:71–5. Akbar SA, Younkman LB. J Electrochem Soc 1997;144:1750–3. Savage NO, Akbar SA, Dutta PK. Sens Actuators B, 2001;72:239–48. Nakade S, Saito Y, Kubo W, Kitamura T, Wada Y, Yanagida SJ. Phys Chem Earth Part B 2003;107:8607–11. Byrappa K, Yoshimura M. Handbook of hydrothermal technique: a technology for crystal growth and materials processing. New York: William Andrew Pub; 2001. Demazeau G. J Mater Chem 1999;9:15–8. Devillers S, Barthelemy B, Delhalle J, Mekhalif ZACS. ACS Appl Mater Interfaces 2011;3(10):4059–66. Shen LC, Kong JA. Applied electromagnetism. 2nd ed. Boston: PWS pub; 1987. Wyckof RWG. Crystal structures, 1. NY: Interscience Publishers; 1963 239–444.