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Studies on transformation of titanate nanotubes into nanoribbons
Materials Letters 63 (2009) 2615–2618
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Studies on transformation of titanate nanotubes into nanoribbons Yucong Peng a, Sushil K. Kansal b,⁎, Wenli Deng a,1 a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou, Guandong, China, 510640 University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh- 160014, India
a r t i c l e
i n f o
Article history: Received 9 July 2009 Accepted 3 September 2009 Available online 13 September 2009 Keywords: Titania Titanates Hydrothermal method Nanomaterials Semiconductors
a b s t r a c t High aspect ratio titanate nanostructures were synthesized by simple hydrothermal treatment and the nature of two distinct morphologies, hollow nanotubes and titanate nanoribbons was explored as a function of hydrothermal processing conditions. The samples were characterized by means of SEM, XRD and TEM. The specific surface area of the final products was determined by Brunauer-Emmett-Teller (BET) method. It has been found that hydrothermal temperature and the treatment duration have a strong effect on the morphological control of the resulting products. Transformation of nanotubes into nanoribbons was observed with increase in the treatment temperature from 180 °C to 200 °C which became more dense with further increase in the temperature from 200 °C to 220 °C and treatment duration from 12 h to 24 h. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, fabrication of nanomaterials with a controllable size and shape has been of great scientific and technological interest due to their unique properties and potential applications. Significant efforts have been taken in the synthesis of various 1D nanoscale materials over the past few years. Among a variety of materials high aspect ratio titanate nanostructures have been of special interest due to titanium dioxide's strong resistance to chemical and photocorrosion, its safety and low cost and biological harmlessness. They have remarkable applications in the field of photocatalysis, dye sensitized solar cells, gas sensors, electrodes for Li battery and so on [1–4]. On the basis of the pioneering work of Kasuga et al. research efforts on titanates were at first concentrated on structure elucidation of titanate nanotubes [5]. TiO2 nanotubes have been synthesized by various techniques correlated to high and low temperature techniques such as chemical vapor deposition, thermal evaporation and hydrothermal method [6]. However substantial research studies have indicated that hydrothermal route is a powerful and promising strategy for preparing 1D nanomaterials because of its advantages such as simple procedure and low cost [7–14]. It has been claimed that this process yields single crystal nanotubes with a uniform outer diameter of around 10 nm and a length of several 100 nanometers [15–17]. Although till date, most researchers have focused on the formation of these nanotubes, now a days, nonhollow titania nano objects are also being investigated by several groups [18–21]. However statements in the literature concerning the impact of the various preparation conditions ⁎ Corresponding author. Tel.: +91 172 2534920, +91 9876581564(mobile); fax: +91 172 2779173. E-mail addresses: [email protected] (S.K. Kansal), [email protected] (W. Deng). 1 Tel.: + 86 20 22236708. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.015
on the resulting structure are often conflicting and inconsistent. It was reported by Weng et al. that increasing the hydrothermal duration leads to the increase in nanotubes length until there is no further increase in the length if the treatment duration exceeds 24 h [22]. Yuan et al. reported that the optimum yield of nanotube formation is achieved at 150 °C, whereas the formation of high aspect ratio nanoribbons is observed only at temperature higher than 180 °C [23]. Recently Elsanuosi et al. synthesized titanate nanotubes and nanoribbons by hydrothermal treatment and found that hydrothermal temperature and treatment duration had a strong effect on the morphology control of the resulting products. They have suggested that more studies on the effect of hydrothermal conditions should be performed [24]. Taking all these factors into consideration, the present study has been focused on the investigations of the relationship between hydrothermal conditions, size and morphology of the high aspect ratio products. 2. Experimental details 2.1. Sample preparation TiO2 nanotubes/nanoribbons were prepared using a chemical process similar to that described by Kasuga et al. [5]. In a typical preparation procedure, 1 g of TiO2 nanopowder (average size 32 nm) was added to 100 mL of 10 M NaOH and stirred for 30 min in a beaker. The mixture was then transferred into a teflon-lined stainless steel autoclave of 100 mL capacity till about 80% of its total volume, aging the suspension at different temperatures (180 °C, 200 °C and 220 °C) and time durations (12 h and 24 h). After the autoclave was naturally cooled to room temperature, the obtained sample was subsequently filtered and washed with distilled water to get its pH neutral. Then acid washing was done with 0.1 M HCl for 6 h. Subsequently the solution was washed
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several times until the pH value of the solution reached to 7, and then the sample was dried at 70 °C for 6 h. 2.2. Phase structure and morphological characterization The phase structure, morphology and composition of the as-prepared titainia nanotubes/nanoribbons were examined by using various techniques like scanning electron microscopy (SEM, Philips FEI-XL30) operating at 25 kV and by X-ray powder diffraction (XRD, Panlytical XpertPRO diffractometer with CuKα radiation, λ =0.1542 nm, 40 kV, 40 mA). The specific surface area of the as-prepared nanostructure was determined by Brunauer-Emmett-Teller (BET) method by using nitrogen adsorption data at 473 K on a Micromeritics Analyzer ASAP 2020. 3. Results and discussion
Increasing the treatment temperature to 200 °C revealed that at 12 h, some nanotubes were formed with diameters of about 10 nm and lengths up to several hundreds of nanometers (Fig. 1c). The formation of few nanoribbons was also observed beside the nanotubes, which became more dense with increase in treatment duration from 12 h to 24 h (Fig. 1c-d). The nanoribbons had widths ranging from 50 to 200 nm and lengths of several micrometers. At higher temperature of 220 °C (Fig. 1e-f), SEM observations showed that dense nanoribbons had already been formed at 12 h. At longer treatment duration of 24 h, bundles of very long and wide nanoribbons were noticed having lengths of several tens of micrometers and width variations between 50 nm and 500 nm. It is clear from these results that the hydrothermal treatment duration and treatment temperature has a strong effect on the morphological features of the resulting products. We can see that with increase in the treatment duration, more dense pure nanoribbons would be formed. The temperature has the similar effect i.e. at higher temperature, dense nanoribbons are formed in shorter duration.
3.1. SEM analysis Fig. 1 shows the SEM images of the samples treated at 180 °C, 200 °C and 220 °C for different hydrothermal durations. Fig. 1a shows that the morphology of the products obtained by treating the sample at 180 °C for 12 h was anomalistic. After 24 h hydrothermal treatment duration, formation of some one dimensional structures could be seen (Fig. 1b).
3.2. XRD analysis To observe the crystallographic changes of the sample due to the change in the morphology from nanotubes to naoribbons, analysis with XRD was made to all products and Fig. 2a-g shows the patterns obtained.
Fig. 1. SEM images of the samples treated at different temperatures and different durations (a) 180 °C 12 h; (b) 180 °C 24 h; (c) 200 °C 12 h; (d) 200 °C 24 h; (e) 220 °C 12 h; (f) 220 °C 24 h.
Y. Peng et al. / Materials Letters 63 (2009) 2615–2618
2617
Table 1 BET surface area (SBET) and pore volume of synthesized nanostructures. Sample 180 °C, 180 °C, 200 °C, 200 °C, 220 °C, 220 °C,
12 h 24 h 12 h 24 h 12 h 24 h
Surface area (m2/ g)
Pore volume (cm3/ g)
41.3 59.2 55.3 163.9 93.5 116.8
0.006 0.010 0.010 0.033 0.017 0.023
was found to be corresponding to titanate crystals which are expected to arise due to the reaction of TiO2 with the concentrated NaOH solution or during the washing process with HCl, where residual Na+ ions are replaced by H+ ions. It was also observed that sharp peaks arise corresponding to titanate at higher treatment durations as shown in Fig. 2 (e, f and g) depending on the treatment duration. The crystallite was mainly composed of H-titanate i.e. H2Ti3O7 crystal phase (a = 1.602 nm, b= 0.375 nm, c = 0.919 nm; JCPDS card No.47-0561). From these results we can predict that increase in the hydrothermal treatment duration and temperature might improve the crystallinity of the product. 3.3. TEM analysis
Fig. 2. XRD of the starting material and products :(a) Anatase TiO2, (b) Titanate 180 °C 12 h; (c) 200 °C 12 h; (d)220 °C 12 h; (e) 180 °C 24 h; (f) 200 °C 24 h; (g) 220 °C 24 h.
From XRD patterns (peak b and c), we observe that at lower treatment temperature of 180 °C (12 h and 24 h), titanate crystals have not yet been formed completely. At higher treatment temperature of 200 °C, peak d
The morphological and structural development of the products and their identification was further investigated by TEM analysis. Fig. 3a-c shows the TEM micrograph of the sample treated at 180 °C, 200 °C and 220 °C respectively. Fig. 3a shows the products to be fine hollow open ended tubes with a uniform diameter along their length. The titanate nanotubes appear short and thin. Fig. 3b shows that at higher temperature of 200 °C, the length and width of nanotubes increases and some nanotubes transform into nanoribbons. By increasing the treatment temperature to 220 °C (Fig. 3c), formation of bundles of very long, wide and dense titanate was observed indicating the transformation of nanotubes into nanoribbons.
Fig. 3. TEM micrographs of the nanotubular titanate products obtained by hydrothermal process with 12 h reaction time: (a) 180 °C 100 k; (b) 200 °C 100 k; (c) 220 °C 19 k.
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3.4. Specific surface area
Acknowledgments
Specific surface areas of the samples were determined from the N2 absorption data measured at liquid nitrogen temperature using Brunauer-Emmett-Teller technique. Table 1 shows the surface area of samples obtained at various hydrothermal conditions. It can be seen that specific surface area of the reaction product increases with increase in the hydrothermal treatment duration. The cumulative pore volume exhibits a similar trend as a function of time. The observed phenomenon can be interpreted in full agreement with the TEM images presented in Fig. 3. We suggest that the increase of the specific surface area with treatment duration corresponds to the formation of hollow titanate nanotubes from the anatase starting material and thereafter transformation of nanotubes into nanoribbons. Increase in the treatment temperature from 180 °C to 220 °C seems to have a similar effect as that of treatment duration.
This work was supported financially by the State Key Development Program for Basic Research of China (2009CB930604) and the Natural Science Foundation of Guangdong Province (B6080100).
4. Conclusions We have concluded that hydrothermal duration and temperature have a strong impact on the morphological structure of the synthesized product; it plays a major role in the conversion of nanotubes into nanoribbons. Formation of nanotubes occurred at a temperature of 180 °C, whereas at temperature range between 180 °C to 200 °C, the formation of few nanoribbons was observed and subsequently dense nanoribbons were formed at 220 °C. Similar effect was observed when the treatment duration was varied. So we predict that optimizing the hydrothermal temperature and treatment duration may lead to a better control of product morphology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Kansal SK, Singh M, Sud D. J Hazard Mater 2007;141:581. Zhu K, Neale NR, Miedaner A, Frank AJ. Nano Lett 2007;7:69. Albu SP, Ghicov A, Macak JM, Hahn R, Schmuki P. Nano Lett 2007;7:1286. Mor GK, Gavalho MA, Varghese OK, Pishko MV, Grimes CA. J Mater Res 2004;19:628. Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K. Langmuir 1998;14:3160. Yu H, Yu J, Cheng B, Zhou MJ. Solid State Chem 2006;179:349. Wang W, Li Y. J Am Chem Soc 2002;124:2880. Xu A, Fang Y, You L, Liu H. J Am Chem Soc 2003;125:1494. Liu B, Zeng HC. J Am Chem Soc 2003;125:4430. Li Y, Wang J, Deng Z, Wu Y, Sun X, Yu D, et al. J Am Chem Soc 2001;123:9904. Cao M, Hu C, Peng G, Qi Y, Wang E. J Am Chem Soc 2003;125:4982. Wang X, Li Y. Angew Chem Int Ed Engl 2002;41:4790. Li Y, Ding Y, Wang Z. Mater Des 1999;11:847. Sun X, Chen X, Li Y. Inorg Chem 2002;41:4996. Zhang S, Peng LM, Chen Q, Du GH, Dawson G, Zhou WZ. Phys Rev Lett 2003;91:256103. Yao B. Appl Phys Lett 2003;82:281. Bavykin DV, Parmon VN, Lapkin AA, Walsh FCJ. Mater Chem 2004;14:3370. Armstrong AR, Armstrong G, Canales J, Bruce PG. J Power Sources 2005;146:501. Wu JM, Shih HC, Wu WT. Chem Phys Lett 2005;413:490. Xu CY, Zhang Q, Zhang H, Zhen L, Tang J, Qin LCJ. Am Chem Soc 2005;127:11584. Xiong CR, Balkus KJ. Chem Mater 2005;17:5136. Weng LQ, Song HS, Hodgson S, Baker A, Yu J. J Eur Ceram Soc 2006;26:1405. Yuan ZY, Su BL. Colloids Surf A 2004;241:173. Elsanousi A, Elssfah EM, Zhang J, Lin J, Song HS, Tang C. J Phys Chem C 2007;111:14353.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Studies on transformation of titanate nanotubes into nanoribbons Yucong Peng a, Sushil K. Kansal b,⁎, Wenli Deng a,1 a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou, Guandong, China, 510640 University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh- 160014, India
a r t i c l e
i n f o
Article history: Received 9 July 2009 Accepted 3 September 2009 Available online 13 September 2009 Keywords: Titania Titanates Hydrothermal method Nanomaterials Semiconductors
a b s t r a c t High aspect ratio titanate nanostructures were synthesized by simple hydrothermal treatment and the nature of two distinct morphologies, hollow nanotubes and titanate nanoribbons was explored as a function of hydrothermal processing conditions. The samples were characterized by means of SEM, XRD and TEM. The specific surface area of the final products was determined by Brunauer-Emmett-Teller (BET) method. It has been found that hydrothermal temperature and the treatment duration have a strong effect on the morphological control of the resulting products. Transformation of nanotubes into nanoribbons was observed with increase in the treatment temperature from 180 °C to 200 °C which became more dense with further increase in the temperature from 200 °C to 220 °C and treatment duration from 12 h to 24 h. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, fabrication of nanomaterials with a controllable size and shape has been of great scientific and technological interest due to their unique properties and potential applications. Significant efforts have been taken in the synthesis of various 1D nanoscale materials over the past few years. Among a variety of materials high aspect ratio titanate nanostructures have been of special interest due to titanium dioxide's strong resistance to chemical and photocorrosion, its safety and low cost and biological harmlessness. They have remarkable applications in the field of photocatalysis, dye sensitized solar cells, gas sensors, electrodes for Li battery and so on [1–4]. On the basis of the pioneering work of Kasuga et al. research efforts on titanates were at first concentrated on structure elucidation of titanate nanotubes [5]. TiO2 nanotubes have been synthesized by various techniques correlated to high and low temperature techniques such as chemical vapor deposition, thermal evaporation and hydrothermal method [6]. However substantial research studies have indicated that hydrothermal route is a powerful and promising strategy for preparing 1D nanomaterials because of its advantages such as simple procedure and low cost [7–14]. It has been claimed that this process yields single crystal nanotubes with a uniform outer diameter of around 10 nm and a length of several 100 nanometers [15–17]. Although till date, most researchers have focused on the formation of these nanotubes, now a days, nonhollow titania nano objects are also being investigated by several groups [18–21]. However statements in the literature concerning the impact of the various preparation conditions ⁎ Corresponding author. Tel.: +91 172 2534920, +91 9876581564(mobile); fax: +91 172 2779173. E-mail addresses: [email protected] (S.K. Kansal), [email protected] (W. Deng). 1 Tel.: + 86 20 22236708. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.015
on the resulting structure are often conflicting and inconsistent. It was reported by Weng et al. that increasing the hydrothermal duration leads to the increase in nanotubes length until there is no further increase in the length if the treatment duration exceeds 24 h [22]. Yuan et al. reported that the optimum yield of nanotube formation is achieved at 150 °C, whereas the formation of high aspect ratio nanoribbons is observed only at temperature higher than 180 °C [23]. Recently Elsanuosi et al. synthesized titanate nanotubes and nanoribbons by hydrothermal treatment and found that hydrothermal temperature and treatment duration had a strong effect on the morphology control of the resulting products. They have suggested that more studies on the effect of hydrothermal conditions should be performed [24]. Taking all these factors into consideration, the present study has been focused on the investigations of the relationship between hydrothermal conditions, size and morphology of the high aspect ratio products. 2. Experimental details 2.1. Sample preparation TiO2 nanotubes/nanoribbons were prepared using a chemical process similar to that described by Kasuga et al. [5]. In a typical preparation procedure, 1 g of TiO2 nanopowder (average size 32 nm) was added to 100 mL of 10 M NaOH and stirred for 30 min in a beaker. The mixture was then transferred into a teflon-lined stainless steel autoclave of 100 mL capacity till about 80% of its total volume, aging the suspension at different temperatures (180 °C, 200 °C and 220 °C) and time durations (12 h and 24 h). After the autoclave was naturally cooled to room temperature, the obtained sample was subsequently filtered and washed with distilled water to get its pH neutral. Then acid washing was done with 0.1 M HCl for 6 h. Subsequently the solution was washed
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Y. Peng et al. / Materials Letters 63 (2009) 2615–2618
several times until the pH value of the solution reached to 7, and then the sample was dried at 70 °C for 6 h. 2.2. Phase structure and morphological characterization The phase structure, morphology and composition of the as-prepared titainia nanotubes/nanoribbons were examined by using various techniques like scanning electron microscopy (SEM, Philips FEI-XL30) operating at 25 kV and by X-ray powder diffraction (XRD, Panlytical XpertPRO diffractometer with CuKα radiation, λ =0.1542 nm, 40 kV, 40 mA). The specific surface area of the as-prepared nanostructure was determined by Brunauer-Emmett-Teller (BET) method by using nitrogen adsorption data at 473 K on a Micromeritics Analyzer ASAP 2020. 3. Results and discussion
Increasing the treatment temperature to 200 °C revealed that at 12 h, some nanotubes were formed with diameters of about 10 nm and lengths up to several hundreds of nanometers (Fig. 1c). The formation of few nanoribbons was also observed beside the nanotubes, which became more dense with increase in treatment duration from 12 h to 24 h (Fig. 1c-d). The nanoribbons had widths ranging from 50 to 200 nm and lengths of several micrometers. At higher temperature of 220 °C (Fig. 1e-f), SEM observations showed that dense nanoribbons had already been formed at 12 h. At longer treatment duration of 24 h, bundles of very long and wide nanoribbons were noticed having lengths of several tens of micrometers and width variations between 50 nm and 500 nm. It is clear from these results that the hydrothermal treatment duration and treatment temperature has a strong effect on the morphological features of the resulting products. We can see that with increase in the treatment duration, more dense pure nanoribbons would be formed. The temperature has the similar effect i.e. at higher temperature, dense nanoribbons are formed in shorter duration.
3.1. SEM analysis Fig. 1 shows the SEM images of the samples treated at 180 °C, 200 °C and 220 °C for different hydrothermal durations. Fig. 1a shows that the morphology of the products obtained by treating the sample at 180 °C for 12 h was anomalistic. After 24 h hydrothermal treatment duration, formation of some one dimensional structures could be seen (Fig. 1b).
3.2. XRD analysis To observe the crystallographic changes of the sample due to the change in the morphology from nanotubes to naoribbons, analysis with XRD was made to all products and Fig. 2a-g shows the patterns obtained.
Fig. 1. SEM images of the samples treated at different temperatures and different durations (a) 180 °C 12 h; (b) 180 °C 24 h; (c) 200 °C 12 h; (d) 200 °C 24 h; (e) 220 °C 12 h; (f) 220 °C 24 h.
Y. Peng et al. / Materials Letters 63 (2009) 2615–2618
2617
Table 1 BET surface area (SBET) and pore volume of synthesized nanostructures. Sample 180 °C, 180 °C, 200 °C, 200 °C, 220 °C, 220 °C,
12 h 24 h 12 h 24 h 12 h 24 h
Surface area (m2/ g)
Pore volume (cm3/ g)
41.3 59.2 55.3 163.9 93.5 116.8
0.006 0.010 0.010 0.033 0.017 0.023
was found to be corresponding to titanate crystals which are expected to arise due to the reaction of TiO2 with the concentrated NaOH solution or during the washing process with HCl, where residual Na+ ions are replaced by H+ ions. It was also observed that sharp peaks arise corresponding to titanate at higher treatment durations as shown in Fig. 2 (e, f and g) depending on the treatment duration. The crystallite was mainly composed of H-titanate i.e. H2Ti3O7 crystal phase (a = 1.602 nm, b= 0.375 nm, c = 0.919 nm; JCPDS card No.47-0561). From these results we can predict that increase in the hydrothermal treatment duration and temperature might improve the crystallinity of the product. 3.3. TEM analysis
Fig. 2. XRD of the starting material and products :(a) Anatase TiO2, (b) Titanate 180 °C 12 h; (c) 200 °C 12 h; (d)220 °C 12 h; (e) 180 °C 24 h; (f) 200 °C 24 h; (g) 220 °C 24 h.
From XRD patterns (peak b and c), we observe that at lower treatment temperature of 180 °C (12 h and 24 h), titanate crystals have not yet been formed completely. At higher treatment temperature of 200 °C, peak d
The morphological and structural development of the products and their identification was further investigated by TEM analysis. Fig. 3a-c shows the TEM micrograph of the sample treated at 180 °C, 200 °C and 220 °C respectively. Fig. 3a shows the products to be fine hollow open ended tubes with a uniform diameter along their length. The titanate nanotubes appear short and thin. Fig. 3b shows that at higher temperature of 200 °C, the length and width of nanotubes increases and some nanotubes transform into nanoribbons. By increasing the treatment temperature to 220 °C (Fig. 3c), formation of bundles of very long, wide and dense titanate was observed indicating the transformation of nanotubes into nanoribbons.
Fig. 3. TEM micrographs of the nanotubular titanate products obtained by hydrothermal process with 12 h reaction time: (a) 180 °C 100 k; (b) 200 °C 100 k; (c) 220 °C 19 k.
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3.4. Specific surface area
Acknowledgments
Specific surface areas of the samples were determined from the N2 absorption data measured at liquid nitrogen temperature using Brunauer-Emmett-Teller technique. Table 1 shows the surface area of samples obtained at various hydrothermal conditions. It can be seen that specific surface area of the reaction product increases with increase in the hydrothermal treatment duration. The cumulative pore volume exhibits a similar trend as a function of time. The observed phenomenon can be interpreted in full agreement with the TEM images presented in Fig. 3. We suggest that the increase of the specific surface area with treatment duration corresponds to the formation of hollow titanate nanotubes from the anatase starting material and thereafter transformation of nanotubes into nanoribbons. Increase in the treatment temperature from 180 °C to 220 °C seems to have a similar effect as that of treatment duration.
This work was supported financially by the State Key Development Program for Basic Research of China (2009CB930604) and the Natural Science Foundation of Guangdong Province (B6080100).
4. Conclusions We have concluded that hydrothermal duration and temperature have a strong impact on the morphological structure of the synthesized product; it plays a major role in the conversion of nanotubes into nanoribbons. Formation of nanotubes occurred at a temperature of 180 °C, whereas at temperature range between 180 °C to 200 °C, the formation of few nanoribbons was observed and subsequently dense nanoribbons were formed at 220 °C. Similar effect was observed when the treatment duration was varied. So we predict that optimizing the hydrothermal temperature and treatment duration may lead to a better control of product morphology.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Kansal SK, Singh M, Sud D. J Hazard Mater 2007;141:581. Zhu K, Neale NR, Miedaner A, Frank AJ. Nano Lett 2007;7:69. Albu SP, Ghicov A, Macak JM, Hahn R, Schmuki P. Nano Lett 2007;7:1286. Mor GK, Gavalho MA, Varghese OK, Pishko MV, Grimes CA. J Mater Res 2004;19:628. Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K. Langmuir 1998;14:3160. Yu H, Yu J, Cheng B, Zhou MJ. Solid State Chem 2006;179:349. Wang W, Li Y. J Am Chem Soc 2002;124:2880. Xu A, Fang Y, You L, Liu H. J Am Chem Soc 2003;125:1494. Liu B, Zeng HC. J Am Chem Soc 2003;125:4430. Li Y, Wang J, Deng Z, Wu Y, Sun X, Yu D, et al. J Am Chem Soc 2001;123:9904. Cao M, Hu C, Peng G, Qi Y, Wang E. J Am Chem Soc 2003;125:4982. Wang X, Li Y. Angew Chem Int Ed Engl 2002;41:4790. Li Y, Ding Y, Wang Z. Mater Des 1999;11:847. Sun X, Chen X, Li Y. Inorg Chem 2002;41:4996. Zhang S, Peng LM, Chen Q, Du GH, Dawson G, Zhou WZ. Phys Rev Lett 2003;91:256103. Yao B. Appl Phys Lett 2003;82:281. Bavykin DV, Parmon VN, Lapkin AA, Walsh FCJ. Mater Chem 2004;14:3370. Armstrong AR, Armstrong G, Canales J, Bruce PG. J Power Sources 2005;146:501. Wu JM, Shih HC, Wu WT. Chem Phys Lett 2005;413:490. Xu CY, Zhang Q, Zhang H, Zhen L, Tang J, Qin LCJ. Am Chem Soc 2005;127:11584. Xiong CR, Balkus KJ. Chem Mater 2005;17:5136. Weng LQ, Song HS, Hodgson S, Baker A, Yu J. J Eur Ceram Soc 2006;26:1405. Yuan ZY, Su BL. Colloids Surf A 2004;241:173. Elsanousi A, Elssfah EM, Zhang J, Lin J, Song HS, Tang C. J Phys Chem C 2007;111:14353.