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Preparation and characterization of TiO2 bulk porous nanosolids
Materials Letters 59 (2005) 1962 – 1966 www.elsevier.com/locate/matlet
Preparation and characterization of TiO2 bulk porous nanosolids Hongyan Xua, Xiulin Liua, Mei Lia, Zhi Chena, Deliang Cuia,T, Minhua Jianga, Xianping Mengb, Lili Yuc, Chengjian Wangc b
a State Key Lab of Crystal Materials, Shandong University, Jinan 250100, PR China Bureau of Planning, National Natural Science Foundation of China, Beijing 100085, PR China c School of Physics and Microelectronics, Shandong University, Jinan 250100, PR China
Received 24 February 2004; received in revised form 20 January 2005; accepted 7 February 2005 Available online 17 March 2005
Abstract A novel TiO2 bulk porous nanosolid (also called a bTiO2 nanospongeQ) was prepared by a solvothermal hot-press (SHP) technique, using TiO2 nanoparticles (average particle size of 15 nm) and various solvents as the starting materials. The pore diameters of the nanosolids were rather uniform, and the maximum value of pore volume and specific surface area were 0.249 cm3/g and 59.455 m2/g, respectively. The pore volume and diameter of TiO2 bulk porous nanosolid could be controlled by changing either the type or the amount of the solvent used in the experiment. No residual solvents could be detected by FTIR analysis, which means that the solvents had entirely escaped from the samples during the process of preparing TiO2 bulk porous nanosolids. D 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 nanoparticles; Bulk porous nanosolid; Solvothermal hot-press
1. Introduction Because of the high chemical stability and high photocatalytic efficiency, TiO2 nanoparticles have been widely used in the degradation of environmental pollutants [1–6]. Up to now, almost all the TiO2 nanomaterials have been used in powder form, which is dispersed in the reaction solutions. Although the catalytic efficiency of TiO2 nanoparticles improves as the particle size decreases, it is very difficult to separate TiO2 nanoparticles from the products. This results not only in cost increases, but also in secondary environmental pollution. In order to overcome these disadvantages, the possibility of using TiO2 porous membranes as the photo-catalyst has been intensively investigated [7,8]. It is well known that porous structures are favorable for improving catalytic efficiency. The catalytic effects of TiO2 porous membranes on organic compounds have been reported [9–14]. In addition, new methods for preparing TiO2 porous membranes have also been develT Corresponding author. Fax: +86 531 8361856. E-mail address: [email protected] (D. Cui). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.036
oped [15]. The sol–gel method is most commonly applied. Because of the large number of micro-pores in TiO2 porous membranes prepared by the sol–gel method, the solution can easily be diffused into the pores; the effective surface area of TiO2 membrane is increased, and the catalytic efficiency is improved accordingly [15]. For example, with monomer droplets as templates, Zhang et al. prepared TiO2 porous micro-spheres by a reversed suspension polymerization and sol–gel method [16]. Kozuka et al. prepared TiO2 porous membranes from Ti(OC2H5)4 solutions containing HPC (hydroxypropylcellulose) and HNO3 [17]. Kato et al. also prepared TiO2 porous films on quartz glass plates from alkoxide solutions containing an organic polymer by a dip-coating technique [18]. However, it is usually necessary to sinter the TiO2 porous membranes (films) at rather high temperature for the practical applications. This process will result in the disappearance of small pores and the increase of pore volume in TiO2 porous membrane, which diminishes the catalytic efficiency. A bulk porous nanosolid (nanosponge) is a new intermediate state between nanoparticles and nanoceramics. The nanosolid is a solid constructed of nanoparticles
H. Xu et al. / Materials Letters 59 (2005) 1962–1966
1963
mental protection, preparation of sensors and the development of new composite materials, etc. To our knowledge, there has been no report on either the preparation of TiO2 bulk porous nanosolid or the investigation on its properties until now. In this paper, we have prepared TiO2 bulk porous nanosolid by a unique solvothermal hot-press (SHP) method, and the samples are characterized by XRD, FTIR, TEM and thermal analysis methods. Furthermore, the relative compressive strength, pore volume and specific surface area of the samples have been measured.
2. Experimental Fig. 1. XRD patterns of TiO2 nanoparticles (a) and sample S-4 (b).
forming pores and channels. Bulk porous nanosolids possess both the high reactivity of nanoparticles and the ideal strength of nanoceramics. The major differences between bulk porous nanosolids and conventional porous materials are as follows: Firstly, the surfaces of the pores or channels are the surfaces of nanoparticles. As a result, the reactivity of a bulk porous nanosolid is much higher than that of conventional porous materials. Secondly, unlike conventional micro- and meso-porous materials, bulk porous nanosolids are strong and tough, which makes them easy to recover and recycle. Thirdly, because of the large number of pores and channels, combined with the highly reactive surface, the conductivity of a TiO2 bulk porous nanosolid is very sensitive to changes in its environment. This advantage makes TiO2 bulk porous nanosolids ideal candidates for the fabrication of chemical sensors with high sensitivity. It can be expected that TiO2 bulk porous nanosolids will be widely used in chemical engineering, environ-
The starting materials used in the experiments were: TiO2 nanoparticles (with average particle size of 15 nm, purchased from Mingri Nanometer Materials, used as received), deionized water, hydrogen peroxide (30%), anhydrous ethanol (A.R.) and N, N-Dimethyl formamide (DMF, A.R.). In order to investigate the effects of amount and type of solvent on pore diameter distribution of TiO2 bulk porous nanosolid, two series of experiments were conducted: (1) Six 2-g portions of TiO2 nanoparticles were placed in six 50-ml beakers, and then 1.0, 1.2, 1.4, 1.8, 2.0 and 2.2 ml deionized water were added into the six beakers, respectively. After thorough grinding in an agate mortar for 30 min, the mixtures were mounted into six SHP (solvothermal hot-press) autoclaves constructured according to Ref. [19]. The autoclaves were pressurized to 90 MPa, at the same time, the temperature was raised to 200 8C at a rate of 2 8C/min and kept constant for 180 min. When the autoclaves were cooled to room temperature, samples S-1, S-2, S3, S-4, S-5 and S-6 were obtained.
Fig. 2. TEM photos and SAED patterns of (a) TiO2 nanoparticles and (b) bulk porous nanosolid (sample S-4).
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H. Xu et al. / Materials Letters 59 (2005) 1962–1966
(2) Four 2-g portions of TiO2 nanoparticles were placed in four beakers, and 1.6 ml deionized water, 1.6 ml hydrogen peroxide, 1.6 ml anhydrous ethanol and 1.6 ml DMF were added, respectively. Similar to the above, TiO2 nanoparticles pastes were mounted into four SHP autoclaves after grinding for 30 min. All the other experimental procedures were the same as the above experiments. The samples thus prepared were denoted samples S-7, S-8, S-9 and S-10. XRD patterns of the samples were recorded on a Rigaku D/max-gA X-ray diffractometer with Cu Ka radiation, the scanning speed was 48/min. The morphology and selective area electron diffraction (SAED) patterns of the samples were observed with a Hitachi H-800 transmission electron microscope (TEM). The IR absorption spectra of the samples were collected on a Nicolet NEXUS-670 FTIR spectrometer, the spectral resolution was 4.00 cm 1 and wave number precision was 0.01 cm 1. The pore volume and specific surface area of the samples were examined on a Quantachrome Pore Master-60 mercury porosimetry at 20 8C. The compressive strength of the samples was examined with an INSTRON 6658 materials strength testing instrument, the sizes of the samples were all the same, namely, 20 mm in diameter and 4 mm in thickness.
3. Results and discussion The XRD patterns of TiO2 nanoparticles and TiO2 bulk porous nanosolid (sample S-4) verified that during the solvothermal hot-press treatment process, no changes in the phase and particle size of TiO2 nanoparticles can be detected. In Fig. 1, the particle size calculated from the full width of the half maximum (FWHM) of XRD peaks is 15 nm for both TiO2 nanoparticles and sample S-4. All the diffraction peaks in Fig. 1(a and b) can be attributed to anatase TiO2, and no difference can be found between the XRD patterns of TiO2 nanoparticles and sample S-4. This result shows that TiO2 nanoparticles have been constructed into a bulk nanosolid without phase transformation or changes in particle size.
Table 1 Pore volume, surface area and pore diameter of TiO2 bulk porous nanosolid S-1 Ratio of solvent/TiO2 (w/w) Total pore volume (cm3/g) Total surface area (m2/g) Primary pore radius (nm)
0.5
0.2438
S-2 0.6
0.2017
S-3 0.7
0.2486
S-4 0.9
0.2963
S-5 1.0
0.2729
S-6
8–13
8–13
8–13
8–13
Although there are no obvious differences in the particle size of TiO2 nanoparticles and bulk nanosolid, some differences were found by comparing their selective area electron diffraction (SAED) patterns. Fig. 2 shows the TEM photos and SAED patterns of both TiO2 nanoparticles and sample S-4. Compared with TiO2 bulk nanosolid, the diffraction rings of TiO2 nanoparticles are much more diffusive, which indicates that the crystallinity of TiO2 bulk nanosolid is much better than that of the TiO2 nanoparticles. Furthermore, all the diffraction rings in these two SAED patterns can be attributed to anatase TiO2. This phenomenon indicates that during the SHP process, some re-crystallization may have taken place, resulting in improved of crystallinity. Generally, the total pore volume, pore diameter distribution and the specific surface area are the most important parameters for the characterization of a bulk porous nanosolid. Therefore, samples S-1, S-2, S-3, S-4, S-5 and S-6 were characterized using a mercury porosimeter to compare the effect of the amount of water used in the synthesis. The porosity measurement results (Table 1) reveal that there are large numbers of pores or channels in TiO2 bulk nanosolid. Under the same processing temperature conditions, the pore volume and specific surface area of TiO2 bulk porous nanosolid increase slightly with the increasing amount of water. When this amount increases to 0.9 w/w, the pore volume and surface area are the maximal, namely, 0.2963 cm3/g and 52.6249 m2/g, respectively. However, when the amount of water exceeds this value, both the pore volume and specific surface area gradually decrease.
1.1
0.2410
43.6424 47.1720 50.0073 52.6249 50.1775 50.1284 8–13
Fig. 3. Pore diameter distribution of TiO2 bulk porous nanosolid S-4.
8–13
Table 2 Surface area, pore volume and primary pore diameter of TiO2 bulk porous nanosolid Solvent Total pore volume (cm3/g) Total surface area (m2/g) Primary pore diameter (nm)
S-7
S-8
S-9
S-10
H2 O 0.2017 47.1720 8–13
H2O2 0.2015 52.1782 7–10
C2H5OH 0.2401 59.1457 8–13
DMF 0.2490 59.4550 7–10
H. Xu et al. / Materials Letters 59 (2005) 1962–1966
1965
suggests that the solvents have entirely escaped from the samples during the preparation of TiO2 bulk porous nanosolid. The results of thermal analysis of the samples show that no weight loss is found even if they are heated to 750 8C. This phenomenon also proves that no residual solvents remain in the TiO2 bulk porous nanosolids. The compressive strength of the samples is also affected by increasing the amount of solvent. The compressive strength of sample S-4 is almost three times that of S-2, and the strength of S-10 is two times that of S-2. This phenomenon indicates that the compressive strength of TiO2 bulk porous nanosolids increases with the increase of the amount of water. Fig. 4. FTIR spectra of TiO2 nanoparticles and TiO2 bulk porous nanosolids. (a) TiO2 nanoparticles, (b), (c) and (d) are TiO2 bulk porous nanosolids prepared with water, C2H5OH and H2O2, respectively.
At the same time, the primary pore diameters of the samples remain unchanged while varying the amount of water. We found that the primary pore diameters of all six samples are the same, 8–13 nm. As a representative, the pore diameter distribution of sample S-4 is shown in Fig. 3. It is obvious that the pore diameters are distributed in a rather narrow region. Besides the amount of water, the type of solvent is another key factor that has important influence on the specific surface area, pore volume and diameter of TiO2 bulk porous nanosolid. In the comparative experiments, H2O, H2O2, ethanol and DMF were used as the solvents. Table 2 shows the measurement results of pore volume, pore diameter distribution and specific surface area of the corresponding TiO2 bulk porous nanosolids. Obviously, when the ratio of solvent/TiO2 nanoparticles (w/w) is the same, i.e. 0.6, both the pore volume and specific surface area of TiO2 bulk porous nanosolid changed because of the nature of the solvent. From the data in Table 2, it can be seen that the sample prepared with DMF as the solvent has the highest pore volume and specific surface area values, 0.2490 cm3/g and 59.4550 m2/g, respectively. From the experimental results, we also find that the specific surface area of the sample increases with the increase of solvent molecular weight. However, the primary pore diameters distribute in the region of 7–13 nm for all the four samples. As a nondestructive detection method, Fourier transform infrared spectroscopy (FTIR) is used to detect the adsorbed impurities on the surface of pores. Fig. 4 shows the FTIR spectra of the samples prepared by using different solvents. In this figure, the peak at 3449 cm 1 comes from the stretching mode vibrations of –OH, while the peak at 1634 cm 1 is attributed to the bending vibrations of adsorbed H2O molecules. The very broad band at 775 to 600 cm 1 can be ascribed to the vibration of Ti–O bond in anatase. No solvent signal was detected by the FTIR method, which
4. Conclusions TiO2 bulk porous nanosolid with uniform pore diameter and high thermal stability has been prepared by a novel solvothermal hot-press (SHP) method. The specific surface area, pore volume and diameter distribution and the compressive strength can be adjusted by changing either the kind or the amount of solvent. Because of its high porosity and high reactivity, TiO2 bulk porous nanosolid should find wide applications in environmental protection, fabrication of sensors and development of micro-cavity optoelectronic devices.
Acknowledgements This project was supported by the NSFC (Contract No.50272036, 90101016, 90206042), Key Project of Science and Technology Research Program of Ministry of Education (KPSTRP) and the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP). We thank Dr. Pamela Holt for helpful discussions.
References [1] S.K. Zheng, T.M. Wang, G. Xiang, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 187 (2002) 479. [2] G.P. Fotou, S. Vemury, S.E. Pratsinis, Chem. Eng. Sci. 49 (1994) 4939. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [4] J.S. Kim, T.K. Lee, Korean J. Chem. Eng. 18 (2001) 935. [5] D. Robert, J.V. Weber, J. Mater. Sci. Lett. 18 (1999) 97. [6] T. Kasuga, M. Hiramatsu, M. Hirano, A. Hoson, J. Mater. Res. 12 (1997) 607. [7] T. Tsuru, T. Kan-no, T. Yoshioka, M. Asaeda, Catal. Today 82 (2003) 41 – 48. [8] A. Wold, Chem. Mater. 5 (3) (1993) 280. [9] M. Schneider, M. Maciejewski, J. Catal. 149 (1994) 326. [10] Krishnankutty-Nair P. Lumar, Klass Keizer, J. Mater. Chem. 3 (9) (1993) 917. [11] L.S. Li, Y.M. Chen, Thin Solid Films 284/285 (1996) 592. [12] Z. Zhu, L.Y. Tsung, J. Phys. Chem. 99 (1995) 15945.
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[13] Krishnankutty-Nair P. Kumar, Appl. Catal., A Gen. 119 (1994) 163. [14] Q.Y. Xu, M.A. Arderson, J. Ceram. Soc. 76 (8) (1993) 2093. [15] Q. Dai, Y. Guo, C.W. Yuan, R.B. Xi, B.P. Lin, Chin. J. Catal. 20 (3) (1999) 317 – 320. [16] B.L. Zhang, B.S. Chen, K.Y. Shi, S.J. He, et al., Appl. Catal., B 40 (2003) 253 – 258.
[17] H. Kozuka, Y. Takahashi, G.L. Zhao, T. Yoko, Thin Solid Films 358 (2000) 172 – 179. [18] K. Kato, A. Tsuzuki, Y. Yorii, et al., J. Mater. Sci. 30 (1995) 837 – 841. [19] X.L. Liu, H.Y. Xu, X.P. Meng, et al., Chin. Sci. Bull. (2004) (revised).
Preparation and characterization of TiO2 bulk porous nanosolids Hongyan Xua, Xiulin Liua, Mei Lia, Zhi Chena, Deliang Cuia,T, Minhua Jianga, Xianping Mengb, Lili Yuc, Chengjian Wangc b
a State Key Lab of Crystal Materials, Shandong University, Jinan 250100, PR China Bureau of Planning, National Natural Science Foundation of China, Beijing 100085, PR China c School of Physics and Microelectronics, Shandong University, Jinan 250100, PR China
Received 24 February 2004; received in revised form 20 January 2005; accepted 7 February 2005 Available online 17 March 2005
Abstract A novel TiO2 bulk porous nanosolid (also called a bTiO2 nanospongeQ) was prepared by a solvothermal hot-press (SHP) technique, using TiO2 nanoparticles (average particle size of 15 nm) and various solvents as the starting materials. The pore diameters of the nanosolids were rather uniform, and the maximum value of pore volume and specific surface area were 0.249 cm3/g and 59.455 m2/g, respectively. The pore volume and diameter of TiO2 bulk porous nanosolid could be controlled by changing either the type or the amount of the solvent used in the experiment. No residual solvents could be detected by FTIR analysis, which means that the solvents had entirely escaped from the samples during the process of preparing TiO2 bulk porous nanosolids. D 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 nanoparticles; Bulk porous nanosolid; Solvothermal hot-press
1. Introduction Because of the high chemical stability and high photocatalytic efficiency, TiO2 nanoparticles have been widely used in the degradation of environmental pollutants [1–6]. Up to now, almost all the TiO2 nanomaterials have been used in powder form, which is dispersed in the reaction solutions. Although the catalytic efficiency of TiO2 nanoparticles improves as the particle size decreases, it is very difficult to separate TiO2 nanoparticles from the products. This results not only in cost increases, but also in secondary environmental pollution. In order to overcome these disadvantages, the possibility of using TiO2 porous membranes as the photo-catalyst has been intensively investigated [7,8]. It is well known that porous structures are favorable for improving catalytic efficiency. The catalytic effects of TiO2 porous membranes on organic compounds have been reported [9–14]. In addition, new methods for preparing TiO2 porous membranes have also been develT Corresponding author. Fax: +86 531 8361856. E-mail address: [email protected] (D. Cui). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.036
oped [15]. The sol–gel method is most commonly applied. Because of the large number of micro-pores in TiO2 porous membranes prepared by the sol–gel method, the solution can easily be diffused into the pores; the effective surface area of TiO2 membrane is increased, and the catalytic efficiency is improved accordingly [15]. For example, with monomer droplets as templates, Zhang et al. prepared TiO2 porous micro-spheres by a reversed suspension polymerization and sol–gel method [16]. Kozuka et al. prepared TiO2 porous membranes from Ti(OC2H5)4 solutions containing HPC (hydroxypropylcellulose) and HNO3 [17]. Kato et al. also prepared TiO2 porous films on quartz glass plates from alkoxide solutions containing an organic polymer by a dip-coating technique [18]. However, it is usually necessary to sinter the TiO2 porous membranes (films) at rather high temperature for the practical applications. This process will result in the disappearance of small pores and the increase of pore volume in TiO2 porous membrane, which diminishes the catalytic efficiency. A bulk porous nanosolid (nanosponge) is a new intermediate state between nanoparticles and nanoceramics. The nanosolid is a solid constructed of nanoparticles
H. Xu et al. / Materials Letters 59 (2005) 1962–1966
1963
mental protection, preparation of sensors and the development of new composite materials, etc. To our knowledge, there has been no report on either the preparation of TiO2 bulk porous nanosolid or the investigation on its properties until now. In this paper, we have prepared TiO2 bulk porous nanosolid by a unique solvothermal hot-press (SHP) method, and the samples are characterized by XRD, FTIR, TEM and thermal analysis methods. Furthermore, the relative compressive strength, pore volume and specific surface area of the samples have been measured.
2. Experimental Fig. 1. XRD patterns of TiO2 nanoparticles (a) and sample S-4 (b).
forming pores and channels. Bulk porous nanosolids possess both the high reactivity of nanoparticles and the ideal strength of nanoceramics. The major differences between bulk porous nanosolids and conventional porous materials are as follows: Firstly, the surfaces of the pores or channels are the surfaces of nanoparticles. As a result, the reactivity of a bulk porous nanosolid is much higher than that of conventional porous materials. Secondly, unlike conventional micro- and meso-porous materials, bulk porous nanosolids are strong and tough, which makes them easy to recover and recycle. Thirdly, because of the large number of pores and channels, combined with the highly reactive surface, the conductivity of a TiO2 bulk porous nanosolid is very sensitive to changes in its environment. This advantage makes TiO2 bulk porous nanosolids ideal candidates for the fabrication of chemical sensors with high sensitivity. It can be expected that TiO2 bulk porous nanosolids will be widely used in chemical engineering, environ-
The starting materials used in the experiments were: TiO2 nanoparticles (with average particle size of 15 nm, purchased from Mingri Nanometer Materials, used as received), deionized water, hydrogen peroxide (30%), anhydrous ethanol (A.R.) and N, N-Dimethyl formamide (DMF, A.R.). In order to investigate the effects of amount and type of solvent on pore diameter distribution of TiO2 bulk porous nanosolid, two series of experiments were conducted: (1) Six 2-g portions of TiO2 nanoparticles were placed in six 50-ml beakers, and then 1.0, 1.2, 1.4, 1.8, 2.0 and 2.2 ml deionized water were added into the six beakers, respectively. After thorough grinding in an agate mortar for 30 min, the mixtures were mounted into six SHP (solvothermal hot-press) autoclaves constructured according to Ref. [19]. The autoclaves were pressurized to 90 MPa, at the same time, the temperature was raised to 200 8C at a rate of 2 8C/min and kept constant for 180 min. When the autoclaves were cooled to room temperature, samples S-1, S-2, S3, S-4, S-5 and S-6 were obtained.
Fig. 2. TEM photos and SAED patterns of (a) TiO2 nanoparticles and (b) bulk porous nanosolid (sample S-4).
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H. Xu et al. / Materials Letters 59 (2005) 1962–1966
(2) Four 2-g portions of TiO2 nanoparticles were placed in four beakers, and 1.6 ml deionized water, 1.6 ml hydrogen peroxide, 1.6 ml anhydrous ethanol and 1.6 ml DMF were added, respectively. Similar to the above, TiO2 nanoparticles pastes were mounted into four SHP autoclaves after grinding for 30 min. All the other experimental procedures were the same as the above experiments. The samples thus prepared were denoted samples S-7, S-8, S-9 and S-10. XRD patterns of the samples were recorded on a Rigaku D/max-gA X-ray diffractometer with Cu Ka radiation, the scanning speed was 48/min. The morphology and selective area electron diffraction (SAED) patterns of the samples were observed with a Hitachi H-800 transmission electron microscope (TEM). The IR absorption spectra of the samples were collected on a Nicolet NEXUS-670 FTIR spectrometer, the spectral resolution was 4.00 cm 1 and wave number precision was 0.01 cm 1. The pore volume and specific surface area of the samples were examined on a Quantachrome Pore Master-60 mercury porosimetry at 20 8C. The compressive strength of the samples was examined with an INSTRON 6658 materials strength testing instrument, the sizes of the samples were all the same, namely, 20 mm in diameter and 4 mm in thickness.
3. Results and discussion The XRD patterns of TiO2 nanoparticles and TiO2 bulk porous nanosolid (sample S-4) verified that during the solvothermal hot-press treatment process, no changes in the phase and particle size of TiO2 nanoparticles can be detected. In Fig. 1, the particle size calculated from the full width of the half maximum (FWHM) of XRD peaks is 15 nm for both TiO2 nanoparticles and sample S-4. All the diffraction peaks in Fig. 1(a and b) can be attributed to anatase TiO2, and no difference can be found between the XRD patterns of TiO2 nanoparticles and sample S-4. This result shows that TiO2 nanoparticles have been constructed into a bulk nanosolid without phase transformation or changes in particle size.
Table 1 Pore volume, surface area and pore diameter of TiO2 bulk porous nanosolid S-1 Ratio of solvent/TiO2 (w/w) Total pore volume (cm3/g) Total surface area (m2/g) Primary pore radius (nm)
0.5
0.2438
S-2 0.6
0.2017
S-3 0.7
0.2486
S-4 0.9
0.2963
S-5 1.0
0.2729
S-6
8–13
8–13
8–13
8–13
Although there are no obvious differences in the particle size of TiO2 nanoparticles and bulk nanosolid, some differences were found by comparing their selective area electron diffraction (SAED) patterns. Fig. 2 shows the TEM photos and SAED patterns of both TiO2 nanoparticles and sample S-4. Compared with TiO2 bulk nanosolid, the diffraction rings of TiO2 nanoparticles are much more diffusive, which indicates that the crystallinity of TiO2 bulk nanosolid is much better than that of the TiO2 nanoparticles. Furthermore, all the diffraction rings in these two SAED patterns can be attributed to anatase TiO2. This phenomenon indicates that during the SHP process, some re-crystallization may have taken place, resulting in improved of crystallinity. Generally, the total pore volume, pore diameter distribution and the specific surface area are the most important parameters for the characterization of a bulk porous nanosolid. Therefore, samples S-1, S-2, S-3, S-4, S-5 and S-6 were characterized using a mercury porosimeter to compare the effect of the amount of water used in the synthesis. The porosity measurement results (Table 1) reveal that there are large numbers of pores or channels in TiO2 bulk nanosolid. Under the same processing temperature conditions, the pore volume and specific surface area of TiO2 bulk porous nanosolid increase slightly with the increasing amount of water. When this amount increases to 0.9 w/w, the pore volume and surface area are the maximal, namely, 0.2963 cm3/g and 52.6249 m2/g, respectively. However, when the amount of water exceeds this value, both the pore volume and specific surface area gradually decrease.
1.1
0.2410
43.6424 47.1720 50.0073 52.6249 50.1775 50.1284 8–13
Fig. 3. Pore diameter distribution of TiO2 bulk porous nanosolid S-4.
8–13
Table 2 Surface area, pore volume and primary pore diameter of TiO2 bulk porous nanosolid Solvent Total pore volume (cm3/g) Total surface area (m2/g) Primary pore diameter (nm)
S-7
S-8
S-9
S-10
H2 O 0.2017 47.1720 8–13
H2O2 0.2015 52.1782 7–10
C2H5OH 0.2401 59.1457 8–13
DMF 0.2490 59.4550 7–10
H. Xu et al. / Materials Letters 59 (2005) 1962–1966
1965
suggests that the solvents have entirely escaped from the samples during the preparation of TiO2 bulk porous nanosolid. The results of thermal analysis of the samples show that no weight loss is found even if they are heated to 750 8C. This phenomenon also proves that no residual solvents remain in the TiO2 bulk porous nanosolids. The compressive strength of the samples is also affected by increasing the amount of solvent. The compressive strength of sample S-4 is almost three times that of S-2, and the strength of S-10 is two times that of S-2. This phenomenon indicates that the compressive strength of TiO2 bulk porous nanosolids increases with the increase of the amount of water. Fig. 4. FTIR spectra of TiO2 nanoparticles and TiO2 bulk porous nanosolids. (a) TiO2 nanoparticles, (b), (c) and (d) are TiO2 bulk porous nanosolids prepared with water, C2H5OH and H2O2, respectively.
At the same time, the primary pore diameters of the samples remain unchanged while varying the amount of water. We found that the primary pore diameters of all six samples are the same, 8–13 nm. As a representative, the pore diameter distribution of sample S-4 is shown in Fig. 3. It is obvious that the pore diameters are distributed in a rather narrow region. Besides the amount of water, the type of solvent is another key factor that has important influence on the specific surface area, pore volume and diameter of TiO2 bulk porous nanosolid. In the comparative experiments, H2O, H2O2, ethanol and DMF were used as the solvents. Table 2 shows the measurement results of pore volume, pore diameter distribution and specific surface area of the corresponding TiO2 bulk porous nanosolids. Obviously, when the ratio of solvent/TiO2 nanoparticles (w/w) is the same, i.e. 0.6, both the pore volume and specific surface area of TiO2 bulk porous nanosolid changed because of the nature of the solvent. From the data in Table 2, it can be seen that the sample prepared with DMF as the solvent has the highest pore volume and specific surface area values, 0.2490 cm3/g and 59.4550 m2/g, respectively. From the experimental results, we also find that the specific surface area of the sample increases with the increase of solvent molecular weight. However, the primary pore diameters distribute in the region of 7–13 nm for all the four samples. As a nondestructive detection method, Fourier transform infrared spectroscopy (FTIR) is used to detect the adsorbed impurities on the surface of pores. Fig. 4 shows the FTIR spectra of the samples prepared by using different solvents. In this figure, the peak at 3449 cm 1 comes from the stretching mode vibrations of –OH, while the peak at 1634 cm 1 is attributed to the bending vibrations of adsorbed H2O molecules. The very broad band at 775 to 600 cm 1 can be ascribed to the vibration of Ti–O bond in anatase. No solvent signal was detected by the FTIR method, which
4. Conclusions TiO2 bulk porous nanosolid with uniform pore diameter and high thermal stability has been prepared by a novel solvothermal hot-press (SHP) method. The specific surface area, pore volume and diameter distribution and the compressive strength can be adjusted by changing either the kind or the amount of solvent. Because of its high porosity and high reactivity, TiO2 bulk porous nanosolid should find wide applications in environmental protection, fabrication of sensors and development of micro-cavity optoelectronic devices.
Acknowledgements This project was supported by the NSFC (Contract No.50272036, 90101016, 90206042), Key Project of Science and Technology Research Program of Ministry of Education (KPSTRP) and the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP). We thank Dr. Pamela Holt for helpful discussions.
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