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Hydrothermal synthesis and characterization of TiO2 nanostructures prepared using different solvents
Materials Letters 220 (2018) 20–23
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Hydrothermal synthesis and characterization of TiO2 nanostructures prepared using different solvents B. Gomathi Thanga Keerthana, T. Solaiyammal, S. Muniyappan, P. Murugakoothan ⇑ MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India
a r t i c l e
i n f o
Article history: Received 14 January 2018 Received in revised form 23 February 2018 Accepted 25 February 2018 Available online 27 February 2018 Keywords: Semiconductors Nanocrystalline materials Crystal structure Electron microscopy Powder technology
a b s t r a c t Pure TiO2 nanostructures were synthesized by hydrothermal method using NaOH and KOH as solvents. Structural, optical and morphological analyses of TiO2 nanostructures were presented in this letter. Powder X-ray diffraction study was used to identify the anatase, rutile and titanate phase formation of TiO2 nanostructures. The presence of functional groups in TiO2 nanostructures was identified using Fourier transform infrared (FTIR) analysis. UV–vis-NIR reflectance spectra explain the absorption nature of titanate nanostructures and bandgap was calculated as 3.43 eV and 2.96 eV. Transmission electron microscopy (TEM) revealed the tube and rodlike morphologies of nano TiO2 synthesized using different solvents. The length and diameter of titanate nanotubes are 24 nm and 4 nm respectively. The length of the nanorod is 30 nm and the width is 4.5 nm. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is the most common compound of titanium, and is frequently used in several applications that include anticorrosion, photocatalysis, self-cleaning coatings, and solar cells. Nanostructured TiO2 materials with a dimension less than 100 nm have recently emerged. Such materials include spheroidal nanocrystallite and nanoparticles together with elongated nanotubes, nanosheets and nano fibers. TiO2 nanostructures can be synthesized using different methods. Hydrothermal and sol–gel methods are most common among them, and can be accompanied by surface directing agents, ultrasonic irradiation or microwaves. Hydrothermal method is regarded as a suitable synthetic approach as it has several advantages such as appropriate crystallization temperature, being environment-friendly, controllability of reaction conditions, low energy consumption and low cost. It has been noted that the resultant morphology of one-dimensional titanate nanostructures can get influenced by the base solution used in hydrothermal process [1]. The effect of solvents on crystalline phase and morphology of TiO2 nanostructures has been researched extensively. NaOH is attributed as most popular solvent among material preparation techniques. While the influence of NaOH on the crystalline phase and morphology of TiO2 nanostructures are well-documented, there is very limited research and conclusions on utilizing other alkali solution as solvents [2]. In the present ⇑ Corresponding author. E-mail address: [email protected] (P. Murugakoothan). https://doi.org/10.1016/j.matlet.2018.02.119 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
work, we have examined the effect of KOH solution, in addition to NaOH solution, on the morphology and crystal structure of TiO2 nanostructures obtained by hydrothermal method. It would be valuable to collect information and publish research on the effect of alkali solution on the preparation of TiO2 nanostructures.
2. Experimental 2.1. Synthesis of TiO2 nanostructures Pure TiO2 nanostructures were synthesized by hydrothermal method. Pure TiO2 powder (Degussa P25, 98%) with particle size 25 nm was used as starting material with crystalline structure of mixed anatase and Rutile (80:20). TiO2 powder of 0.5 g was first added to an aqueous solution of NaOH (10 M, 40 mL) and stirred vigorously for 30 min. Then the mixture was transferred to a 50 mL teflon-lined stainless steel autoclave for hydrothermal treatment at 200 °C for 48 h in a muffle furnace. After the reaction, the white precipitate was separated from autoclave, cooled at room temperature and washed with 0.1 M HCl acid solution and deionized water. This acid treatment carried out after alkaline treatment play an important role in controlling the amount of sodium ions remaining in the sample solution. The washing procedure was repeated until almost all the Na+ ions were removed [2,7]. Finally, the majority of the Na+ ions were removed and the resultant white precipitate after the centrifugation process was dried in an oven at 60 °C then calcined at 250 °C for 2 h. In order
21
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
1.0
2.2. Characterization
(a) (b)
550
0.6 0.4
4000
3. Results and discussion
1387 3000
2000
755
0.0
3428
0.2
1625
3272
Transmittance
0.8
The powder X-ray diffraction (PXRD) study was carried out using PANalytical X’Pert Pro diffractometer employing CuKa radiation (k = 1.5406 Å). The Shimadzu FTIR instrument with a resolution of 1.0 cm1 was used to identify the functional groups present in the sample. The UV–vis-NIR absorption spectrum was recorded in the wavelength range from 190 to 900 nm in the diffused reflectance spectrum (DRS) mode with BaSO4 as a reference material using LABINDIA Model UV 3092 spectrophotometer. Transmission electron microscopy (TEM) was employed to analyze the morphology and size of the product using JEOL/JEM 2100 transmission electron microscope.
583
to assess the effect of different solvents, a similar experimental procedure was carried out using KOH as a solvent.
1000
Wavenumber (cm-1)
X-ray diffraction analysis was carried out to determine the phase formation. Fig. 1(a) and (b) gives the typical XRD pattern of TiO2 nanostructures for the solvents NaOH and KOH respectively. The distinctive TiO2 peaks are found for the sample prepared using NaOH solvent at 2h angles 25.6°, 27.5°, 30.1°, 34.2°, 35.1°, 37.8°, 39.2°, 41.5°, 44.1°, 46.5°, 48.3°, 54.6° and 62.6° could be assigned to the mixed phase of anatase, rutile and titanate phases
of pure TiO2 nanostructures (1 0 1), (1 1 1), (2 1 0), (2 0 3), (1 0 3),
(0 0 4), (2 0 0), (3 1 1), (2 1 0), (3 1 3), (2 0 0), (5 0 1) and (2 0 4) crystal planes (JCPDS 21-1272, 21-1276, 31-1329) respectively [3]. The distinctive peaks are found for the sample synthesized using KOH solvent at 2h angles 19.5°, 23.8°, 29.1°, 31.9°, 33.9°, 41.2° and 47.5° corresponding to the titanate phase (0 0 2), (1 1 0), (3 1 0), (1 1 2), (2 0 3), (4 0 3) and (0 2 0) crystal planes (JCPDS 74-0275). The peak observed at 54.3° can be assigned to the (2 1 1) anatase phase of TiO2. The FTIR study was used to analyse the presence of functional groups in the material. FTIR spectra of the synthesized samples using different solvents, such as NaOH and KOH are shown in Fig. 2. The bands in the range 3000–3550 cm1 were attributed to stretching vibration of H2O molecule which adsorbed on the surface of the samples. The band observed in both the samples at 1625 cm1 could be assigned to bending mode of water molecule [4]. The distinct broad bands in 400–800 cm1 region were assigned to Ti–O and Ti–O–Ti skeletal frequency region. From the earlier report it is observed that the samples of titania nanotubes
Fig. 2. FTIR spectrum of TiO2 nanostructures prepared from (a) NaOH and (b) KOH.
begin to appear as a new peak at 1387 cm1 [5]. It is probable that during protonation of the surface, TiO2 in the presence of solvents forms Ti–OH2+ groups which confirms the formation of titanate nanotubes. The UV–vis-NIR diffuse reflectance spectroscopy was carried out to study the optical absorption of TiO2 nanostructures and their corresponding reflectance spectra are shown in Fig. 3(a). The bandgap energy of the TiO2 nanostructures was estimated from the Kubelka –Munk plot as shown in Fig. 3(b). The energy bandgap was obtained by extrapolating the linear part of the square root of the modified Kubelka – Munk function at the y- axis versus photon energy (hm) at the x– axis.
FðRÞ ¼
ð1 RÞ2 2R
where, R is the reflectance. The calculated bandgap energy of TiO2 nanostructures synthesized using NaOH and KOH were found to be 3.43 eV and 2.96 eV respectively. The higher bandgap of these sample compared to bulk may be due to decrease in size of the sample called size effect. The transmission electron microscopy (TEM) analysis was carried to obtain the microscopic structural information of the samples. TEM images of the TiO2 nanostructures and their
Fig. 1. XRD patterns of TiO2 nanostructures synthesized from (a) NaOH and (b) KOH.
22
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
Fig. 3. Reflectance spectra (a) and Kubelka-Munk plot (b) for TiO2.
Fig. 4. a and c TEM images and 4 b and d SAED patterns of synthesized TiO2 nanostructures using NaOH and KOH respectively.
corresponding selected area electron diffraction (SAED) patterns are shown in the Fig. 4 (a–d). From the TEM images, the hydrothermal treatment with NaOH solvent produced tube –like morphology and with KOH solvent produced rod-like morphology. TEM results show that the nanotubes have approximate outer diameter of 4 nm, inner diameter approximately 2 nm and length of 24 nm. The width and length of the synthesized nano rod was measured to be 4.5 nm and 30 nm respectively. From the SAED pattern of both the samples, the interplanar d-spacing of (1 1 2), (0 0 4) which corresponds to NaOH, and (1 0 3) and (2 1 1) which corresponds to KOH produce same anatase phase of TiO2. The SAED pattern con-
firms the anatase phase of nanotube and nanorod, and the dotted rings indicate the TiO2 nanostructures are crystalline in nature [6]. These results are well matched with the XRD results.
4. Conclusion Pure TiO2 nanostructures were successfully synthesized using NaOH and KOH by hydrothermal method. Powder XRD analysis confirmed the crystalline phase formation of TiO2 nanostructures. FTIR spectrum showed the functional groups present in the
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
synthesized nanostructures. UV–vis-NIR reflectance spectra revealed the bandgap energies of the samples of TiO2. The TiO2 nanotube showed higher bandgap energy compared to the TiO2 nanorod. TEM images give the length of the tube and rod, diameter of the tube and confirm the formation of TiO2 nanostructures. These materials might be suitable for photocatalytic applications. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2018.02.119.
23
References [1] Dmitry V. Bavykin, Jens M. Friedrich, Frank C. Walsh, Adv. Mater 18 (2006) 2807–2824. [2] Saeid Zanganeh, Amir Kajbafvala, Navid Zanganeh, Roya Molaei, M.R. Bayati, H. R. Zargar, S.K. Sadrnezhaad, Adv. Powder Technol 22 (2011) 336–339. [3] M. Qamar, C.R. Yoon, H.J. Oh, N.H. Lee, K. Park, D.H. Kim, K.S. Lee, W.J. Lee, S.J. Kim, Catal. Today 131 (2008) 3–14. [4] A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa, J. Soria, J. Catal. 2 (2001) 413–420. [5] C.T. Nam, W.D. Yang, L.M. Duc, et al., Bull. Mater. Sci. 36 (2013) 779–788. [6] L. Cui, K.N. Hui, K.S. Hui, S.K. Lee, W. Zhou, Z.P. Wan, Chi-Nhan Ha Thuc, Mater. Lett. 75 (2012) 175–178. [7] Srimala Sreekantan, Lai Chin Wei, J. Alloys Compd. 490 (2010) 436–442.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Hydrothermal synthesis and characterization of TiO2 nanostructures prepared using different solvents B. Gomathi Thanga Keerthana, T. Solaiyammal, S. Muniyappan, P. Murugakoothan ⇑ MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India
a r t i c l e
i n f o
Article history: Received 14 January 2018 Received in revised form 23 February 2018 Accepted 25 February 2018 Available online 27 February 2018 Keywords: Semiconductors Nanocrystalline materials Crystal structure Electron microscopy Powder technology
a b s t r a c t Pure TiO2 nanostructures were synthesized by hydrothermal method using NaOH and KOH as solvents. Structural, optical and morphological analyses of TiO2 nanostructures were presented in this letter. Powder X-ray diffraction study was used to identify the anatase, rutile and titanate phase formation of TiO2 nanostructures. The presence of functional groups in TiO2 nanostructures was identified using Fourier transform infrared (FTIR) analysis. UV–vis-NIR reflectance spectra explain the absorption nature of titanate nanostructures and bandgap was calculated as 3.43 eV and 2.96 eV. Transmission electron microscopy (TEM) revealed the tube and rodlike morphologies of nano TiO2 synthesized using different solvents. The length and diameter of titanate nanotubes are 24 nm and 4 nm respectively. The length of the nanorod is 30 nm and the width is 4.5 nm. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is the most common compound of titanium, and is frequently used in several applications that include anticorrosion, photocatalysis, self-cleaning coatings, and solar cells. Nanostructured TiO2 materials with a dimension less than 100 nm have recently emerged. Such materials include spheroidal nanocrystallite and nanoparticles together with elongated nanotubes, nanosheets and nano fibers. TiO2 nanostructures can be synthesized using different methods. Hydrothermal and sol–gel methods are most common among them, and can be accompanied by surface directing agents, ultrasonic irradiation or microwaves. Hydrothermal method is regarded as a suitable synthetic approach as it has several advantages such as appropriate crystallization temperature, being environment-friendly, controllability of reaction conditions, low energy consumption and low cost. It has been noted that the resultant morphology of one-dimensional titanate nanostructures can get influenced by the base solution used in hydrothermal process [1]. The effect of solvents on crystalline phase and morphology of TiO2 nanostructures has been researched extensively. NaOH is attributed as most popular solvent among material preparation techniques. While the influence of NaOH on the crystalline phase and morphology of TiO2 nanostructures are well-documented, there is very limited research and conclusions on utilizing other alkali solution as solvents [2]. In the present ⇑ Corresponding author. E-mail address: [email protected] (P. Murugakoothan). https://doi.org/10.1016/j.matlet.2018.02.119 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
work, we have examined the effect of KOH solution, in addition to NaOH solution, on the morphology and crystal structure of TiO2 nanostructures obtained by hydrothermal method. It would be valuable to collect information and publish research on the effect of alkali solution on the preparation of TiO2 nanostructures.
2. Experimental 2.1. Synthesis of TiO2 nanostructures Pure TiO2 nanostructures were synthesized by hydrothermal method. Pure TiO2 powder (Degussa P25, 98%) with particle size 25 nm was used as starting material with crystalline structure of mixed anatase and Rutile (80:20). TiO2 powder of 0.5 g was first added to an aqueous solution of NaOH (10 M, 40 mL) and stirred vigorously for 30 min. Then the mixture was transferred to a 50 mL teflon-lined stainless steel autoclave for hydrothermal treatment at 200 °C for 48 h in a muffle furnace. After the reaction, the white precipitate was separated from autoclave, cooled at room temperature and washed with 0.1 M HCl acid solution and deionized water. This acid treatment carried out after alkaline treatment play an important role in controlling the amount of sodium ions remaining in the sample solution. The washing procedure was repeated until almost all the Na+ ions were removed [2,7]. Finally, the majority of the Na+ ions were removed and the resultant white precipitate after the centrifugation process was dried in an oven at 60 °C then calcined at 250 °C for 2 h. In order
21
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
1.0
2.2. Characterization
(a) (b)
550
0.6 0.4
4000
3. Results and discussion
1387 3000
2000
755
0.0
3428
0.2
1625
3272
Transmittance
0.8
The powder X-ray diffraction (PXRD) study was carried out using PANalytical X’Pert Pro diffractometer employing CuKa radiation (k = 1.5406 Å). The Shimadzu FTIR instrument with a resolution of 1.0 cm1 was used to identify the functional groups present in the sample. The UV–vis-NIR absorption spectrum was recorded in the wavelength range from 190 to 900 nm in the diffused reflectance spectrum (DRS) mode with BaSO4 as a reference material using LABINDIA Model UV 3092 spectrophotometer. Transmission electron microscopy (TEM) was employed to analyze the morphology and size of the product using JEOL/JEM 2100 transmission electron microscope.
583
to assess the effect of different solvents, a similar experimental procedure was carried out using KOH as a solvent.
1000
Wavenumber (cm-1)
X-ray diffraction analysis was carried out to determine the phase formation. Fig. 1(a) and (b) gives the typical XRD pattern of TiO2 nanostructures for the solvents NaOH and KOH respectively. The distinctive TiO2 peaks are found for the sample prepared using NaOH solvent at 2h angles 25.6°, 27.5°, 30.1°, 34.2°, 35.1°, 37.8°, 39.2°, 41.5°, 44.1°, 46.5°, 48.3°, 54.6° and 62.6° could be assigned to the mixed phase of anatase, rutile and titanate phases
of pure TiO2 nanostructures (1 0 1), (1 1 1), (2 1 0), (2 0 3), (1 0 3),
(0 0 4), (2 0 0), (3 1 1), (2 1 0), (3 1 3), (2 0 0), (5 0 1) and (2 0 4) crystal planes (JCPDS 21-1272, 21-1276, 31-1329) respectively [3]. The distinctive peaks are found for the sample synthesized using KOH solvent at 2h angles 19.5°, 23.8°, 29.1°, 31.9°, 33.9°, 41.2° and 47.5° corresponding to the titanate phase (0 0 2), (1 1 0), (3 1 0), (1 1 2), (2 0 3), (4 0 3) and (0 2 0) crystal planes (JCPDS 74-0275). The peak observed at 54.3° can be assigned to the (2 1 1) anatase phase of TiO2. The FTIR study was used to analyse the presence of functional groups in the material. FTIR spectra of the synthesized samples using different solvents, such as NaOH and KOH are shown in Fig. 2. The bands in the range 3000–3550 cm1 were attributed to stretching vibration of H2O molecule which adsorbed on the surface of the samples. The band observed in both the samples at 1625 cm1 could be assigned to bending mode of water molecule [4]. The distinct broad bands in 400–800 cm1 region were assigned to Ti–O and Ti–O–Ti skeletal frequency region. From the earlier report it is observed that the samples of titania nanotubes
Fig. 2. FTIR spectrum of TiO2 nanostructures prepared from (a) NaOH and (b) KOH.
begin to appear as a new peak at 1387 cm1 [5]. It is probable that during protonation of the surface, TiO2 in the presence of solvents forms Ti–OH2+ groups which confirms the formation of titanate nanotubes. The UV–vis-NIR diffuse reflectance spectroscopy was carried out to study the optical absorption of TiO2 nanostructures and their corresponding reflectance spectra are shown in Fig. 3(a). The bandgap energy of the TiO2 nanostructures was estimated from the Kubelka –Munk plot as shown in Fig. 3(b). The energy bandgap was obtained by extrapolating the linear part of the square root of the modified Kubelka – Munk function at the y- axis versus photon energy (hm) at the x– axis.
FðRÞ ¼
ð1 RÞ2 2R
where, R is the reflectance. The calculated bandgap energy of TiO2 nanostructures synthesized using NaOH and KOH were found to be 3.43 eV and 2.96 eV respectively. The higher bandgap of these sample compared to bulk may be due to decrease in size of the sample called size effect. The transmission electron microscopy (TEM) analysis was carried to obtain the microscopic structural information of the samples. TEM images of the TiO2 nanostructures and their
Fig. 1. XRD patterns of TiO2 nanostructures synthesized from (a) NaOH and (b) KOH.
22
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
Fig. 3. Reflectance spectra (a) and Kubelka-Munk plot (b) for TiO2.
Fig. 4. a and c TEM images and 4 b and d SAED patterns of synthesized TiO2 nanostructures using NaOH and KOH respectively.
corresponding selected area electron diffraction (SAED) patterns are shown in the Fig. 4 (a–d). From the TEM images, the hydrothermal treatment with NaOH solvent produced tube –like morphology and with KOH solvent produced rod-like morphology. TEM results show that the nanotubes have approximate outer diameter of 4 nm, inner diameter approximately 2 nm and length of 24 nm. The width and length of the synthesized nano rod was measured to be 4.5 nm and 30 nm respectively. From the SAED pattern of both the samples, the interplanar d-spacing of (1 1 2), (0 0 4) which corresponds to NaOH, and (1 0 3) and (2 1 1) which corresponds to KOH produce same anatase phase of TiO2. The SAED pattern con-
firms the anatase phase of nanotube and nanorod, and the dotted rings indicate the TiO2 nanostructures are crystalline in nature [6]. These results are well matched with the XRD results.
4. Conclusion Pure TiO2 nanostructures were successfully synthesized using NaOH and KOH by hydrothermal method. Powder XRD analysis confirmed the crystalline phase formation of TiO2 nanostructures. FTIR spectrum showed the functional groups present in the
B. Gomathi Thanga Keerthana et al. / Materials Letters 220 (2018) 20–23
synthesized nanostructures. UV–vis-NIR reflectance spectra revealed the bandgap energies of the samples of TiO2. The TiO2 nanotube showed higher bandgap energy compared to the TiO2 nanorod. TEM images give the length of the tube and rod, diameter of the tube and confirm the formation of TiO2 nanostructures. These materials might be suitable for photocatalytic applications. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2018.02.119.
23
References [1] Dmitry V. Bavykin, Jens M. Friedrich, Frank C. Walsh, Adv. Mater 18 (2006) 2807–2824. [2] Saeid Zanganeh, Amir Kajbafvala, Navid Zanganeh, Roya Molaei, M.R. Bayati, H. R. Zargar, S.K. Sadrnezhaad, Adv. Powder Technol 22 (2011) 336–339. [3] M. Qamar, C.R. Yoon, H.J. Oh, N.H. Lee, K. Park, D.H. Kim, K.S. Lee, W.J. Lee, S.J. Kim, Catal. Today 131 (2008) 3–14. [4] A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa, J. Soria, J. Catal. 2 (2001) 413–420. [5] C.T. Nam, W.D. Yang, L.M. Duc, et al., Bull. Mater. Sci. 36 (2013) 779–788. [6] L. Cui, K.N. Hui, K.S. Hui, S.K. Lee, W. Zhou, Z.P. Wan, Chi-Nhan Ha Thuc, Mater. Lett. 75 (2012) 175–178. [7] Srimala Sreekantan, Lai Chin Wei, J. Alloys Compd. 490 (2010) 436–442.