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Effects of hydrothermal temperature and acid concentration on the transition from titanate to titania
Journal of Industrial and Engineering Chemistry 18 (2012) 1141–1148
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of hydrothermal temperature and acid concentration on the transition from titanate to titania Soonhyun Kim *, Minsun Kim, Sung-Ho Hwang, Sang Kyoo Lim Division of Nano-Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea
A R T I C L E I N F O
Article history: Received 22 September 2011 Accepted 10 January 2012 Available online 18 January 2012 Keywords: Nanostructure Hydrothermal Phase transformation Titanates Titania Photocatalytic
A B S T R A C T
Nanostructured titanates were prepared by alkaline hydrothermal processing. The titanates were transformed to the anatase particles at low concentrations of acid, whereas the rutile rods were produced from the titanates at high concentrations of acid. These changes were more retarded in the titanate nanofiber product than in titanate nanotubes product, which could be attributed to the stability and high crystallinity of the titanate nanofiber product prepared at a high hydrothermal temperature. The photocatalytic activities of the acid treated nanostructured titanates for MB degradation and gaseous CH3CHO oxidation were strongly dependent on the presence of the anatase crystalline phase. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Since TiO2-based nanotubes, or more precisely titanate nanotubes (TiNTs), were first fabricated by Kasuga et al. [1] using hydrothermal treatment, much effort has been spent to understand the mechanism of nanotubular titanate formation. It has been generally recognized that the nanotubes could be formed by the slow dissolution of raw TiO2 nanoparticles accompanied by the epitaxial growth and exfoliation of layered nanosheets of titanate and subsequent scrolling of the exfoliated nanosheet [2–4]. The alkaline hydrothermal treatment of TiO2 at an elevated temperature and pressure could provide an alternative route for the synthesis of nanostructured titanates, including nanotubes, nanosheet, nanorods/nanowires, and nanoribbons/nanobelts, via a sequence involving the dissolution of the initial TiO2 particle and the crystallization of the final product [5–7]. On the other hand, the alkaline hydrothermally prepared titanate nanostructured materials show relatively low photocatalytic activities. Although the lifetime of trapped electrons in titanate nanotubes is longer than that in TiO2 nanoparticles [8], the photocatalytic activity of as prepared titanate nanotubes was found to be smaller than that of the standard P25 catalyst [9], which could be attributed either to the impurities of sodium or to the moderate crystallinity of the as prepared titanate nanotubes. Sodium ions retained in titanate nanostructures after alkaline
* Corresponding author. Tel.: +82 53 785 3410; fax: +82 53 785 3439. E-mail address: [email protected] (S. Kim).
hydrothermal treatment could act as recombination centers and decrease the photocatalytic activity [5]. Several researchers have attempted to enhance photocatalytic activity through the phase transition of titanate nanotubes to anatase crystallites by calcination [10–13], H2O2 treatment [14] or hydrothermal treatment [15]. Recently, Zhu et al. reported the pathways for the phase transition from the titanate nanostructures to anatase or rutile titania polymorphs via simple wet chemical reactions at moderate temperatures [16]. The photocatalytic activity of anatase fibers transformed from the titanate fiber by weak acid treatment was higher than that of parent titanate fiber and commercial anatase TiO2. Therefore, the phase transition phenomenon of titanate nanostructures is likely of remarkable interest to the area of photocatalysis. The phase transition of titanate nanostructures could be strongly affected by the conditions of wet chemical reaction such as acid concentration. Moreover, the stability of titanate nanostructures could affect the phase transition of titanate nanostructures because a solid titanate fiber is more resistant to wet chemical treatments than a hollow titanate nanutube. Recently, Bavykin et al. reported that titanate nanotubes have a metastable nature, and they are intermediate species during the transformation of TiO2 to titanates [17]. However, the phase transitions of titanate nanostructures have not been extensively studied. More detailed investigations of the phase transition of titanate nanostructures are needed. Therefore, in this study, the effects of hydrothermal temperature and the acid concentration on the phase transition of titanate nanostructures were studied. Titanate tubes/fibers were prepared by alkaline hydrothermal treatment at different hydrothermal
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2012.01.015
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temperatures and by subsequent treatment with acid at relatively low temperatures under different acid concentrations. Their photocatalytic activities for the degradation of methylene blue (MB) in aqueous suspension and the oxidation of acetaldehyde in gas phase were compared. 2. Experimental 2.1. Syntheses of titanate nanotubes/nanofibers P25 was added to 600 ml of 10 M NaOH solution, and the mixture was hydrothermally reacted at 100 8C, 150 8C, or 200 8C for 48 h. Then, the precipitates were washed with distilled water until the washing solution reached pH 7 and were dried by freeze drying. The titanate powders are denoted as BHT100, BHT 150, and BHT200 according to the hydrothermal temperature, respectively. For the acid treatment, the titanates were added to 200 ml of HNO3 solution, and the mixture was refluxed at 80 8C for 24 h. Then, these precipitates were washed with distilled water and were dried by freeze drying. The acid treated titanates powders with 0.05 M, 0.1 M, and 2.65 M HNO3 are denoted as BHTXXX-AT0.05, BHTXXX-AT0.1, and BHTXXX-AT2.65, respectively. 2.2. Characterizations Transmission electron micrographs were obtained by using a high resolution transmission electron microscope (HR-TEM, JEM2200FS). X-ray diffraction (XRD) patterns were obtained with an Xray diffractometer (Rigaku D/MAX-2500, 18 kV) using Cu-Ka1 radiation. Zeta potentials were measured using an electrophoretic light scattering spectrophotometer (Zetasizer, Malvern). The Brunauer–Emmett–Teller (BET) surface areas were determined from nitrogen adsorption–desorption isotherms at 77 K (ASAP 2020 Micromeritics). The effective surface areas were estimated at a relative pressure (P/P0) ranging from 0.01 to 0.1. Diffuse reflectance UV–vis absorption spectra (DRS) of the powder
samples were obtained using a spectrophotometer (Varian Cary 100) equipped with a diffuse reflectance accessory. 2.3. Photocatalytic activity measurements The photocatalytic activity of the samples for MB was determined in aqueous suspensions. Each photocatalyst powder was dispersed in distilled water at 0.5 g/L and was sonicated for 30 s. A desired amount of MB stock solution was added to the suspension, and it was stirred for 30 min in the dark to allow the equilibrium adsorption of the substrate on the photocatalyst surface. A 300 W Xe arc lamp (Oriel) was used as a light source, and the light beam was passed through a 10 cm IR water filter and a UV cutoff filter (l > 295 nm) and focused onto a 90 ml cylindrical quartz reactor with a window (40 mm in diameter). The sample aliquots were intermittently withdrawn by a 1 ml syringe during UV irradiation and were filtered through a 0.45 mm PTFE filter (Milipore). The MB concentrations were monitored using UV–vis spectrophotometer (Varian) at a wavelength of 665 nm. The photocatalytic oxidation of gaseous CH3CHO was determined in a closed circulated stainless steel reactor (volume 150 cm3), which could be divided into two parts (upper and bottom) by a control valve, similar to an experiment previously reported [18]. The gases used were CH3CHO (300 ppmv in N2) as a CH3CHO standard, O2 (99.9999%), and Ar (99.9999%) as a carrier gas. The concentration of CH3CHO and O2 was 30–35 ppmv and 20%, respectively. Each photocatalyst was deposited on a quartz plate sample holder with a 40 mm diameter. A 0.5 ml amount of photocatalyst suspension, finely ground powder in distilled water, was pipetted on the center of the quartz plate sample holder. After the dispersion of suspension, the sample holder was dried at 100 8C for 3 h. Each photocatalyst weight in the sample holder was about 5 mg. First, the mixed gas passed through the empty upper reactor, and the concentration of CH3CHO in the exit stream was monitored until it attained a constant value; the gas was then circulated by means of the pump. Next, the circulated gas was passed through
Fig. 1. HR-TEM images of BHT100 (a–c) and BHT200 (d–f).
S. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1141–1148
the bottom reactor by controlling the valve so that it came into contact with the surface of the photocatalyst placed in the bottom reactor. After adsorption equilibrium with the surface of the photocatalyst was established in the dark, the catalyst was illuminated with UV light. The distance between the sample and the lamp was 20 cm, and a cut-off filter (l < 295 nm) was used. The removal of CH3CHO was monitored using a gas chromatograph (GC, HP6890) that was equipped with a Porapak-Q column, a flame ionization detector (FID), a CO2 methanizer (Ni catalyst), and a gassampling valve. 3. Results and discussion 3.1. Phase transition from titanate to titania Fig. 1 shows the HR-TEM images of the BHT 100 and BHT 200. The morphologies of BHT100 and BHT200 were very different. BHT 100 comprised hollow tubes, whereas BHT200 consisted of solid fibers, and the degree of crystallinity of BHT200 was higher than that of BHT100. Fig. 1b shows that BHT100 had a lattice fringe of ca. 0.70 nm, which corresponds to the interplanar distance of hydrogen titanate. Generally, the nanotubes have a distance between layers of approximately 0.72 nm in the protonated form. In Fig. 1c, the weak lattice fringe of 0.19 nm of the BHT100
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corresponds to the (0 2 0) plane of titanates [6]. On the other hand, BHT200 had a lattice fringe of 0.35–0.37 nm, which corresponds to the (1 1 0) plane of titanates [6]. These results imply that low crystalline titanate nanotubes were produced at relatively low temperature, whereas high crystalline titanate nanofibers were produced at relatively high temperature, which could be attributed the rate of crystallization of the nanosheets [5]. Under alkaline hydrothermal conditions, nanosheets could be converted to nanotubes by scrolling at a relatively low temperature (100– 150 8C), whereas nanofibers or nanoribbons could be produced from the nanosheets at temperatures above 170 8C. At high temperatures, the rate of crystallization was large enough. Therefore, the nanosheet became too thick and rigid to bend before curving could occur. The TEM images in Fig. 2a–l show the morphology of acid treated BHT100, BHT150 and BHT200 according to the acid concentration. As the acid concentration was increased, the morphology of titanates changed from nanotubes/nanofibers to nanoparticles/nanorods. On the other hand, as the hydrothermal temperature increased, the change in morphology hardly occurred. For BHT100, in a dilute HNO3 (0.05 M) solution, the almost all nanotubes were converted to the nanoparticles, whereas for BHT200, a slight change from nanofibers to nanoparticles was observed in the 0.1 M HNO3 solution. The difference seems to be
Fig. 2. TEM images of acid treated titanates: (a) BHT100, (b) BHT150, (c) BHT200, (d) BHT100-AT0.05, (e) BHT150-AT0.05, (f) BHT200-AT0.05, (g) BHT100-AT0.1, (h) BHT150AT0.1, (i) BHT200-AT0.1, (j) BHT100-AT2.65, (k) BHT150-AT2.65, and (l) BHT200-AT2.65.
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because the frameworks of titanate nanostructures produced at high hydrothermal reaction temperatures are stronger and more stable, as shown in Fig. 1. At a high concentration of acid treatment, the nanotubes/nanofibers and the nanoparticles disappeared, and the nanorods were observed. These morphological changes should be related to the crystal phase transition. The phase transitions were also observed with the XRD patterns, as shown in Fig. 3. Before acid treatment, BHT100 showed one diffraction peak at 158, corresponding to the (020) plane, which was broader, indicating a poor crystallinity. This result may be attributed to the lack of hydrogen titanate framework formation at this temperature. The XRD patterns of BHT150 and BHT200 exhibited additional peaks appearing at 2u = 11, 25, 30, 34.5, 39, and 498, which are similar to those of the hydrogen titanates, such as H2Ti3O7xH2O [6,16] and H2Ti2O5H2O [19]. With acid treatment, the phase transition from titanate to Fig. 4. Phase transition diagram from titanate to titania as a function of the hydrothermal temperature and acid concentration.
anatase or rutile occurred, and the anatase or rutile formation was significantly affected by the acid concentration. At low acid concentrations, the anatase phase was formed, whereas at high acid concentration, the rutile phase was produced. The changes in diffraction peaks according to the increase of the acid concentration indicated that the phase transitions readily occurred in the following order: BHT100 > BHT150 > BHT200. This result indicates that the change in morphology and the crystallinity due to the acid treatment was strongly dependent on the crystallinity of the parent titanate tubes/fibers. The simple phase transition diagram, which is based on these results, is shown in Fig. 4. The
Fig. 3. XRD patterns of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
Fig. 5. (a) N2 adsorption and desorption and pore size distributions of acid treated BHT100, BHT150, and BHT200 and (b) BET surface area.
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crystal phase and the morphology of titanate–titania nanomaterials are affected by the hydrothermal temperature and the subsequent acid concentration. These observations are consistent with the results of Zhu et al. [16] who reported that titanate fibers prepared at lower hydrothermal temperature had more defects and a less rigid crystal structure so that the phase change was easier. Similarly, Nakahira et al. also reported that the Na/Ti ratio of TiO2-derived titanate nanotubes had much effect on the thermal behavior and that Na-containing TiO2-derived titanate nanotubes were thermally stable [20]. It is obvious that hydrogen titanate nanostructured materials prepared by alkaline hydrothermal reaction could be transformed to anatase or rutile titania by acid or heat treatments and the phase transitions should be affected by the stability of titanate nanostructured materials which were influenced by the hydrothermal reaction temperature.
Fig. 6. DRS absorption spectra of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
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The BET surface area should be affected by the hydrothermal temperature and the subsequent acid concentration. Fig. 5 shows the N2 adsorption and desorption and the pore size distributions of BHT100, BHT150, and BHT200. Both BHT100 and BHT150 exhibited type IIb isotherms, which are indicative of the presence of non-rigid slit-shaped pores [21]. The BJH pore size analysis shows that pore volume for BHT200 sample is smaller than that in samples BHT100 and BHT150. Table 1 shows the physicochemical properties, including the BET surface area and pore volume. The surface area of untreated BHT100, BHT150, and BHT 200 were 128.87, 109.54, and 25.1 m2/g, respectively. The surface area was drastically reduced with increasing hydrothermal temperature,
Fig. 7. Time-profiled MB degradation under UV-irradiated suspension of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
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Table 1 Physicochemical properties of acid treated titanates. Acid treated
Crystal phasea
Morphologyb
BET surface area (m2/g)
Pore volume (cm3/g)
BE (eV)c
PEMB (vs. P25)d
PECH3 CHO (vs. P25)e
BHT100
None AT0.05 AT0.1 AT2.65
Am, T A A, R R
NT NP NP, NR NR
128.87 47.55 61.21 96.83
0.317 0.137 0.134 0.131
3.78 3.08 3.10 3.17
0 0.31 0.21 0.23
0.15 1.41 0.70 0.10
BHT150
None AT0.05 AT0.1 AT2.65
T T, A A, R R
NT, NF NF, NP NP NR
109.54 78.54 94.13 92.19
0.265 0.223 0.253 0.177
3.81 3.44 3.20 3.14
0.01 0.33 0.18 0.02
0.07 0.58 0.72 0.20
BHT200
None AT0.05 AT0.1 AT2.65
T T, A T, A R
NF NF NF, NP NR
25.1 38.85 39.17 95.47
0.058 0.110 0.107 0.152
3.88 3.42 3.47 3.12
0.05 0.22 0.31 0.03
0.04 0.54 0.76 0.09
a b c d e
Crystal phase from Fig. 3. Am, amorphous; T, titanates; A, anatase; R, rutile. Morphology from Fig. 2. NT, nanotubes; NF, nanofibers; NP, nanoparticles; NR, nanorods. Estimated using the plot of [F(R1)hv]0.5 versus hv. Photocatalytic efficiency for MB degradation: PEMB (vs. P25) = (the change of MB concentration for 30 min on sample)/(the change of MB concentration for 30 min on P25). Photocatalytic efficiency for CH3CHO degradation: PECH3 CHO (vs. P25) = (kobs for sample)/(kobs for P25).
which could be strongly related to the morphological changes from hollow tubes to solid ribbons and the increase of the crystallinity. On the other hand, the surface areas of the acid treated samples were also strongly dependent on the parent titanate tubes/fibers. The surface areas of both BHT100 and BHT150, consisting of the hollow tubes, were reduced by the acid treatment, which was due to the destruction of the tube structures. Interestingly, the surface area of the acid treated samples increased with increasing acid concentration despite the formation of the rutile phase of titania, which was more stable and has generally lower surface area. The increasing surface area with increasing acid concentration seems to be due to the effect of etching by acid. Diffused reflectance UV–vis spectra were measured to compare the electronic band structures (Fig. 6). The diffused reflectance UV– vis spectrum of the acid treated hydrogen titanates were red shifted from that of untreated BHT100, BHT150, and BHT200. This result may be because the band gaps of the titania phases, such as anatase and rutile, are smaller than those of the titanates [16]. The degree of shift could be strongly related with the change of the crystalline phase of the samples. On the basis of the assumption that titanates and titania are semiconductors with an indirect band gap, band gap energy (Eg) was determined by the extrapolation of the linear portion of the alternative plot of [(Kubelka–Munk)E]1/2 against E, where E is the incident photon energy. In Table 1, the band gaps of the samples are roughly divided into three ranges, which could be correlated with the crystal phase of the samples. The band gap range of the samples with anatase or rutile titania particles, mixing of anatase and titanates, and titanates only were about 3.1 0.1, 3.4 0.1, and 3.8 0.1 eV, respectively.
treatment, the surfaces of hydrogen titanates were positively shifted. The pH of the zero point of charge (pHZPC) of BHT150 was measured to be ca. pH 3.5, 4.4, and 5.9, respectively. The pHZPC of BHT150-AT0.05 and BHT150-AT2.65 were slightly shifted to higher pH because the surface negative charges were partly neutralized by the phase transition from titanate to anatase or rutile. Therefore, the strong adsorption of MB on the as prepared titanates, BHT100, BHT150, and BHT200, was due to the negatively charged surface. Although the change of the MB concentration in the UV irradiated titanates suspension could not be determined, there was no color change of test solution, which implies that the MB was not photocatalytically degraded on the titanates. On the other hand, the acid treated titanates could degrade the MB under UV irradiation, which seems to be due to the formation of the anatase phase. It is known that the anatase phase exhibits higher photocatalytic activity. At a high concentration of acid, the photocatalytic degradation of MB was negligible due to the rutile phase formation. The photocatalytic oxidations of gaseous CH3CHO were also investigated, as shown in Fig. 9. The simultaneous CO2 production due to CH3CHO oxidation was also observed (data not shown). The results were similar to the photocatalytic degradation of MB. The photocatalytic oxidation of gaseous CH3CHO on both the untreated titanate samples and the strong acid treated rutile samples was negligible. However, the weak acid treated samples, which have
3.2. Photocatalytic activities The adsorption and the photocatalytic degradation of MB were investigated. Fig. 7 shows the degradation of MB under a UV irradiated suspension of acid treated titanates according to the acid concentration. The differences among the initial MB concentrations were caused by the adsorbed amount of MB. Because the MB concentration was measured by a UV–vis spectrophotometer after filtration, which was needed for the removal of suspended particles, the adsorbed amount of MB was not reflected in the absorption spectra. The adsorption of cationic MB could be strongly affected by the surface charge of samples. Fig. 8 shows the variation of the zeta potentials of BHT150, BHT150-AT0.05, and BHT150-AT2.65 M in water as a function of pH. With acid
Fig. 8. Zeta potentials of BHT150 and acid treated BHT150 as a function of solution pH.
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Fig. 10. Photocatalytic activities of MB (a) and CH3CHO (b) degradation.
4. Conclusions
Fig. 9. Effects of acid concentration for the photocatalytic degradation of gaseous CH3CHO on acid treated BHT100 (a), BHT150 (b), and BHT200 (c) under UV irradiation in a closed circulation system after dark adsorption.
the anatase phase, exhibited the efficient photocatalytic activities for gaseous CH3CHO oxidation. The photocatalytic activities for MB degradation and CH3CHO oxidation are compared with those on P25 in Fig. 10. The MB degradation on all samples were slower than that on P25, whereas the CH3CHO oxidations on acid treated samples containing anatase phase were comparable with that on P25. The BHT100-AT0.05, which is composed of anatase phase only, showed the best photocatalytic activities. The titanate tube structures exhibited a high surface area and did not strongly affect the photocatalytic activities. The photocatalytic activities were dependent on the presence of anatase particles.
Titanate tubes/fibers were hydrothermally prepared and were subsequently treated with acid at relatively low temperatures to investigate the effects of the hydrothermal temperature of titanate tube/fiber formation and the acid concentration of the post-acid treatment. The morphologies and the crystalline phase were strongly affected by the hydrothermal temperature and the subsequent acid concentration. For the formation of titanate tubes/fibers from P25, low crystalline titanate hollow nanotubes were produced at relatively low hydrothermal temperatures, whereas the high crystalline solid titanate solid nanofibers were produced at relatively high temperatures. By further acid treatment at 80 8C, the titanate tubes/fibers were transformed to anatase particles at a low concentration of acid, whereas the rutile rods were produced from the titanate tubes/fibers at a high concentration of acid. These changes in morphology and crystallinity due to acid treatment were strongly dependent on the crystallinity of the parent titanate tubes/ fibers and the concentration of acid. The photocatalytic activities for MB degradation and gaseous CH3CHO oxidation were strongly dependent on the presence of the anatase crystalline phase. Acknowledgment This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (11-NB03). It was also supported by the technology innovation program of the Ministry of Knowledge Economy of Korea (Industrial strategic technology development program, No 10034046). References [1] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160.
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T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307. A. Nakahira, T. Kubo, C. Numako, Inorg. Chem. 49 (2010) 5845. D.V. Bavykin, J.M. Friedrich, F.C. Walsh, Adv. Mater. 18 (2006) 2807. D.V. Bavykin, F.C. Walsh, Titanate and Titania Nanotubes: Synthesis, Properties and Applications, Royal Society of Chemistry, Cambridge, 2010. K. Kiatkittipong, C. Ye, J. Scott, R. Amal, Cryst. Growth Des. 10 (2010) 3618. D.R. Zhang, C.W. Kim, Y.S. Kang, J. Phys. Chem. C 114 (2010) 8294. T. Tachikawa, S. Tojo, M. Fujitsuka, T. Sekino, T. Majima, J. Phys. Chem. B 110 (2006) 14055. H.H. Ou, C.H. Liao, Y.H. Liou, J.H. Hong, S.L. Lo, Environ. Sci. Technol. 42 (2008) 4507. J.G. Yu, H.G. Yu, B. Cheng, X.J. Zhao, Q.J. Zhang, J. Photochem. Photobiol. A 182 (2006) 121. H.L. Kuo, C.Y. Kuo, C.H. Liu, J.H. Chao, C.H. Lin, Catal. Lett. 113 (2007) 7.
[12] F. Cesano, S. Bertarione, M.J. Uddin, G. Agostini, D. Scarano, A. Zecchina, J. Phys. Chem. C 114 (2010) 169. [13] C.H. Lin, J.H. Chao, C.H. Liu, J.C. Chang, F.C. Wang, Langmuir 24 (2008) 9907. [14] M.A. Khan, H.T. Jung, O.B. Yang, J. Phys. Chem. B 110 (2006) 6626. [15] Y.B. Mao, S.S. Wong, J. Am. Chem. Soc. 128 (2006) 8217. [16] H.Y. Zhu, Y. Lan, X.P. Gao, S.P. Ringer, Z.F. Zheng, D.Y. Song, J.C. Zhao, J. Am. Chem. Soc. 127 (2005) 6730. [17] D.V. Bavykin, A.N. Kulak, F.C. Walsh, Cryst. Growth Des. 10 (2010) 4421. [18] S. Kim, S.K. Lim, Appl. Catal. B: Environ. 84 (2008) 16. [19] J. Suetake, A.Y. Nosaka, K. Hodouchi, H. Matsubara, Y. Nosaka, J. Phys. Chem. C 112 (2008) 18474. [20] A. Nakahira, T. Kubo, C. Numako, ACS Appl. Mater. Int. 2 (2010) 2611. [21] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solid, Academic Press, Marseille, 1999.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of hydrothermal temperature and acid concentration on the transition from titanate to titania Soonhyun Kim *, Minsun Kim, Sung-Ho Hwang, Sang Kyoo Lim Division of Nano-Bio Technology, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea
A R T I C L E I N F O
Article history: Received 22 September 2011 Accepted 10 January 2012 Available online 18 January 2012 Keywords: Nanostructure Hydrothermal Phase transformation Titanates Titania Photocatalytic
A B S T R A C T
Nanostructured titanates were prepared by alkaline hydrothermal processing. The titanates were transformed to the anatase particles at low concentrations of acid, whereas the rutile rods were produced from the titanates at high concentrations of acid. These changes were more retarded in the titanate nanofiber product than in titanate nanotubes product, which could be attributed to the stability and high crystallinity of the titanate nanofiber product prepared at a high hydrothermal temperature. The photocatalytic activities of the acid treated nanostructured titanates for MB degradation and gaseous CH3CHO oxidation were strongly dependent on the presence of the anatase crystalline phase. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Since TiO2-based nanotubes, or more precisely titanate nanotubes (TiNTs), were first fabricated by Kasuga et al. [1] using hydrothermal treatment, much effort has been spent to understand the mechanism of nanotubular titanate formation. It has been generally recognized that the nanotubes could be formed by the slow dissolution of raw TiO2 nanoparticles accompanied by the epitaxial growth and exfoliation of layered nanosheets of titanate and subsequent scrolling of the exfoliated nanosheet [2–4]. The alkaline hydrothermal treatment of TiO2 at an elevated temperature and pressure could provide an alternative route for the synthesis of nanostructured titanates, including nanotubes, nanosheet, nanorods/nanowires, and nanoribbons/nanobelts, via a sequence involving the dissolution of the initial TiO2 particle and the crystallization of the final product [5–7]. On the other hand, the alkaline hydrothermally prepared titanate nanostructured materials show relatively low photocatalytic activities. Although the lifetime of trapped electrons in titanate nanotubes is longer than that in TiO2 nanoparticles [8], the photocatalytic activity of as prepared titanate nanotubes was found to be smaller than that of the standard P25 catalyst [9], which could be attributed either to the impurities of sodium or to the moderate crystallinity of the as prepared titanate nanotubes. Sodium ions retained in titanate nanostructures after alkaline
* Corresponding author. Tel.: +82 53 785 3410; fax: +82 53 785 3439. E-mail address: [email protected] (S. Kim).
hydrothermal treatment could act as recombination centers and decrease the photocatalytic activity [5]. Several researchers have attempted to enhance photocatalytic activity through the phase transition of titanate nanotubes to anatase crystallites by calcination [10–13], H2O2 treatment [14] or hydrothermal treatment [15]. Recently, Zhu et al. reported the pathways for the phase transition from the titanate nanostructures to anatase or rutile titania polymorphs via simple wet chemical reactions at moderate temperatures [16]. The photocatalytic activity of anatase fibers transformed from the titanate fiber by weak acid treatment was higher than that of parent titanate fiber and commercial anatase TiO2. Therefore, the phase transition phenomenon of titanate nanostructures is likely of remarkable interest to the area of photocatalysis. The phase transition of titanate nanostructures could be strongly affected by the conditions of wet chemical reaction such as acid concentration. Moreover, the stability of titanate nanostructures could affect the phase transition of titanate nanostructures because a solid titanate fiber is more resistant to wet chemical treatments than a hollow titanate nanutube. Recently, Bavykin et al. reported that titanate nanotubes have a metastable nature, and they are intermediate species during the transformation of TiO2 to titanates [17]. However, the phase transitions of titanate nanostructures have not been extensively studied. More detailed investigations of the phase transition of titanate nanostructures are needed. Therefore, in this study, the effects of hydrothermal temperature and the acid concentration on the phase transition of titanate nanostructures were studied. Titanate tubes/fibers were prepared by alkaline hydrothermal treatment at different hydrothermal
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2012.01.015
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temperatures and by subsequent treatment with acid at relatively low temperatures under different acid concentrations. Their photocatalytic activities for the degradation of methylene blue (MB) in aqueous suspension and the oxidation of acetaldehyde in gas phase were compared. 2. Experimental 2.1. Syntheses of titanate nanotubes/nanofibers P25 was added to 600 ml of 10 M NaOH solution, and the mixture was hydrothermally reacted at 100 8C, 150 8C, or 200 8C for 48 h. Then, the precipitates were washed with distilled water until the washing solution reached pH 7 and were dried by freeze drying. The titanate powders are denoted as BHT100, BHT 150, and BHT200 according to the hydrothermal temperature, respectively. For the acid treatment, the titanates were added to 200 ml of HNO3 solution, and the mixture was refluxed at 80 8C for 24 h. Then, these precipitates were washed with distilled water and were dried by freeze drying. The acid treated titanates powders with 0.05 M, 0.1 M, and 2.65 M HNO3 are denoted as BHTXXX-AT0.05, BHTXXX-AT0.1, and BHTXXX-AT2.65, respectively. 2.2. Characterizations Transmission electron micrographs were obtained by using a high resolution transmission electron microscope (HR-TEM, JEM2200FS). X-ray diffraction (XRD) patterns were obtained with an Xray diffractometer (Rigaku D/MAX-2500, 18 kV) using Cu-Ka1 radiation. Zeta potentials were measured using an electrophoretic light scattering spectrophotometer (Zetasizer, Malvern). The Brunauer–Emmett–Teller (BET) surface areas were determined from nitrogen adsorption–desorption isotherms at 77 K (ASAP 2020 Micromeritics). The effective surface areas were estimated at a relative pressure (P/P0) ranging from 0.01 to 0.1. Diffuse reflectance UV–vis absorption spectra (DRS) of the powder
samples were obtained using a spectrophotometer (Varian Cary 100) equipped with a diffuse reflectance accessory. 2.3. Photocatalytic activity measurements The photocatalytic activity of the samples for MB was determined in aqueous suspensions. Each photocatalyst powder was dispersed in distilled water at 0.5 g/L and was sonicated for 30 s. A desired amount of MB stock solution was added to the suspension, and it was stirred for 30 min in the dark to allow the equilibrium adsorption of the substrate on the photocatalyst surface. A 300 W Xe arc lamp (Oriel) was used as a light source, and the light beam was passed through a 10 cm IR water filter and a UV cutoff filter (l > 295 nm) and focused onto a 90 ml cylindrical quartz reactor with a window (40 mm in diameter). The sample aliquots were intermittently withdrawn by a 1 ml syringe during UV irradiation and were filtered through a 0.45 mm PTFE filter (Milipore). The MB concentrations were monitored using UV–vis spectrophotometer (Varian) at a wavelength of 665 nm. The photocatalytic oxidation of gaseous CH3CHO was determined in a closed circulated stainless steel reactor (volume 150 cm3), which could be divided into two parts (upper and bottom) by a control valve, similar to an experiment previously reported [18]. The gases used were CH3CHO (300 ppmv in N2) as a CH3CHO standard, O2 (99.9999%), and Ar (99.9999%) as a carrier gas. The concentration of CH3CHO and O2 was 30–35 ppmv and 20%, respectively. Each photocatalyst was deposited on a quartz plate sample holder with a 40 mm diameter. A 0.5 ml amount of photocatalyst suspension, finely ground powder in distilled water, was pipetted on the center of the quartz plate sample holder. After the dispersion of suspension, the sample holder was dried at 100 8C for 3 h. Each photocatalyst weight in the sample holder was about 5 mg. First, the mixed gas passed through the empty upper reactor, and the concentration of CH3CHO in the exit stream was monitored until it attained a constant value; the gas was then circulated by means of the pump. Next, the circulated gas was passed through
Fig. 1. HR-TEM images of BHT100 (a–c) and BHT200 (d–f).
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the bottom reactor by controlling the valve so that it came into contact with the surface of the photocatalyst placed in the bottom reactor. After adsorption equilibrium with the surface of the photocatalyst was established in the dark, the catalyst was illuminated with UV light. The distance between the sample and the lamp was 20 cm, and a cut-off filter (l < 295 nm) was used. The removal of CH3CHO was monitored using a gas chromatograph (GC, HP6890) that was equipped with a Porapak-Q column, a flame ionization detector (FID), a CO2 methanizer (Ni catalyst), and a gassampling valve. 3. Results and discussion 3.1. Phase transition from titanate to titania Fig. 1 shows the HR-TEM images of the BHT 100 and BHT 200. The morphologies of BHT100 and BHT200 were very different. BHT 100 comprised hollow tubes, whereas BHT200 consisted of solid fibers, and the degree of crystallinity of BHT200 was higher than that of BHT100. Fig. 1b shows that BHT100 had a lattice fringe of ca. 0.70 nm, which corresponds to the interplanar distance of hydrogen titanate. Generally, the nanotubes have a distance between layers of approximately 0.72 nm in the protonated form. In Fig. 1c, the weak lattice fringe of 0.19 nm of the BHT100
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corresponds to the (0 2 0) plane of titanates [6]. On the other hand, BHT200 had a lattice fringe of 0.35–0.37 nm, which corresponds to the (1 1 0) plane of titanates [6]. These results imply that low crystalline titanate nanotubes were produced at relatively low temperature, whereas high crystalline titanate nanofibers were produced at relatively high temperature, which could be attributed the rate of crystallization of the nanosheets [5]. Under alkaline hydrothermal conditions, nanosheets could be converted to nanotubes by scrolling at a relatively low temperature (100– 150 8C), whereas nanofibers or nanoribbons could be produced from the nanosheets at temperatures above 170 8C. At high temperatures, the rate of crystallization was large enough. Therefore, the nanosheet became too thick and rigid to bend before curving could occur. The TEM images in Fig. 2a–l show the morphology of acid treated BHT100, BHT150 and BHT200 according to the acid concentration. As the acid concentration was increased, the morphology of titanates changed from nanotubes/nanofibers to nanoparticles/nanorods. On the other hand, as the hydrothermal temperature increased, the change in morphology hardly occurred. For BHT100, in a dilute HNO3 (0.05 M) solution, the almost all nanotubes were converted to the nanoparticles, whereas for BHT200, a slight change from nanofibers to nanoparticles was observed in the 0.1 M HNO3 solution. The difference seems to be
Fig. 2. TEM images of acid treated titanates: (a) BHT100, (b) BHT150, (c) BHT200, (d) BHT100-AT0.05, (e) BHT150-AT0.05, (f) BHT200-AT0.05, (g) BHT100-AT0.1, (h) BHT150AT0.1, (i) BHT200-AT0.1, (j) BHT100-AT2.65, (k) BHT150-AT2.65, and (l) BHT200-AT2.65.
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because the frameworks of titanate nanostructures produced at high hydrothermal reaction temperatures are stronger and more stable, as shown in Fig. 1. At a high concentration of acid treatment, the nanotubes/nanofibers and the nanoparticles disappeared, and the nanorods were observed. These morphological changes should be related to the crystal phase transition. The phase transitions were also observed with the XRD patterns, as shown in Fig. 3. Before acid treatment, BHT100 showed one diffraction peak at 158, corresponding to the (020) plane, which was broader, indicating a poor crystallinity. This result may be attributed to the lack of hydrogen titanate framework formation at this temperature. The XRD patterns of BHT150 and BHT200 exhibited additional peaks appearing at 2u = 11, 25, 30, 34.5, 39, and 498, which are similar to those of the hydrogen titanates, such as H2Ti3O7xH2O [6,16] and H2Ti2O5H2O [19]. With acid treatment, the phase transition from titanate to Fig. 4. Phase transition diagram from titanate to titania as a function of the hydrothermal temperature and acid concentration.
anatase or rutile occurred, and the anatase or rutile formation was significantly affected by the acid concentration. At low acid concentrations, the anatase phase was formed, whereas at high acid concentration, the rutile phase was produced. The changes in diffraction peaks according to the increase of the acid concentration indicated that the phase transitions readily occurred in the following order: BHT100 > BHT150 > BHT200. This result indicates that the change in morphology and the crystallinity due to the acid treatment was strongly dependent on the crystallinity of the parent titanate tubes/fibers. The simple phase transition diagram, which is based on these results, is shown in Fig. 4. The
Fig. 3. XRD patterns of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
Fig. 5. (a) N2 adsorption and desorption and pore size distributions of acid treated BHT100, BHT150, and BHT200 and (b) BET surface area.
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crystal phase and the morphology of titanate–titania nanomaterials are affected by the hydrothermal temperature and the subsequent acid concentration. These observations are consistent with the results of Zhu et al. [16] who reported that titanate fibers prepared at lower hydrothermal temperature had more defects and a less rigid crystal structure so that the phase change was easier. Similarly, Nakahira et al. also reported that the Na/Ti ratio of TiO2-derived titanate nanotubes had much effect on the thermal behavior and that Na-containing TiO2-derived titanate nanotubes were thermally stable [20]. It is obvious that hydrogen titanate nanostructured materials prepared by alkaline hydrothermal reaction could be transformed to anatase or rutile titania by acid or heat treatments and the phase transitions should be affected by the stability of titanate nanostructured materials which were influenced by the hydrothermal reaction temperature.
Fig. 6. DRS absorption spectra of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
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The BET surface area should be affected by the hydrothermal temperature and the subsequent acid concentration. Fig. 5 shows the N2 adsorption and desorption and the pore size distributions of BHT100, BHT150, and BHT200. Both BHT100 and BHT150 exhibited type IIb isotherms, which are indicative of the presence of non-rigid slit-shaped pores [21]. The BJH pore size analysis shows that pore volume for BHT200 sample is smaller than that in samples BHT100 and BHT150. Table 1 shows the physicochemical properties, including the BET surface area and pore volume. The surface area of untreated BHT100, BHT150, and BHT 200 were 128.87, 109.54, and 25.1 m2/g, respectively. The surface area was drastically reduced with increasing hydrothermal temperature,
Fig. 7. Time-profiled MB degradation under UV-irradiated suspension of acid treated BHT100 (a), BHT150 (b), and BHT200 (c) according to the acid concentration.
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Table 1 Physicochemical properties of acid treated titanates. Acid treated
Crystal phasea
Morphologyb
BET surface area (m2/g)
Pore volume (cm3/g)
BE (eV)c
PEMB (vs. P25)d
PECH3 CHO (vs. P25)e
BHT100
None AT0.05 AT0.1 AT2.65
Am, T A A, R R
NT NP NP, NR NR
128.87 47.55 61.21 96.83
0.317 0.137 0.134 0.131
3.78 3.08 3.10 3.17
0 0.31 0.21 0.23
0.15 1.41 0.70 0.10
BHT150
None AT0.05 AT0.1 AT2.65
T T, A A, R R
NT, NF NF, NP NP NR
109.54 78.54 94.13 92.19
0.265 0.223 0.253 0.177
3.81 3.44 3.20 3.14
0.01 0.33 0.18 0.02
0.07 0.58 0.72 0.20
BHT200
None AT0.05 AT0.1 AT2.65
T T, A T, A R
NF NF NF, NP NR
25.1 38.85 39.17 95.47
0.058 0.110 0.107 0.152
3.88 3.42 3.47 3.12
0.05 0.22 0.31 0.03
0.04 0.54 0.76 0.09
a b c d e
Crystal phase from Fig. 3. Am, amorphous; T, titanates; A, anatase; R, rutile. Morphology from Fig. 2. NT, nanotubes; NF, nanofibers; NP, nanoparticles; NR, nanorods. Estimated using the plot of [F(R1)hv]0.5 versus hv. Photocatalytic efficiency for MB degradation: PEMB (vs. P25) = (the change of MB concentration for 30 min on sample)/(the change of MB concentration for 30 min on P25). Photocatalytic efficiency for CH3CHO degradation: PECH3 CHO (vs. P25) = (kobs for sample)/(kobs for P25).
which could be strongly related to the morphological changes from hollow tubes to solid ribbons and the increase of the crystallinity. On the other hand, the surface areas of the acid treated samples were also strongly dependent on the parent titanate tubes/fibers. The surface areas of both BHT100 and BHT150, consisting of the hollow tubes, were reduced by the acid treatment, which was due to the destruction of the tube structures. Interestingly, the surface area of the acid treated samples increased with increasing acid concentration despite the formation of the rutile phase of titania, which was more stable and has generally lower surface area. The increasing surface area with increasing acid concentration seems to be due to the effect of etching by acid. Diffused reflectance UV–vis spectra were measured to compare the electronic band structures (Fig. 6). The diffused reflectance UV– vis spectrum of the acid treated hydrogen titanates were red shifted from that of untreated BHT100, BHT150, and BHT200. This result may be because the band gaps of the titania phases, such as anatase and rutile, are smaller than those of the titanates [16]. The degree of shift could be strongly related with the change of the crystalline phase of the samples. On the basis of the assumption that titanates and titania are semiconductors with an indirect band gap, band gap energy (Eg) was determined by the extrapolation of the linear portion of the alternative plot of [(Kubelka–Munk)E]1/2 against E, where E is the incident photon energy. In Table 1, the band gaps of the samples are roughly divided into three ranges, which could be correlated with the crystal phase of the samples. The band gap range of the samples with anatase or rutile titania particles, mixing of anatase and titanates, and titanates only were about 3.1 0.1, 3.4 0.1, and 3.8 0.1 eV, respectively.
treatment, the surfaces of hydrogen titanates were positively shifted. The pH of the zero point of charge (pHZPC) of BHT150 was measured to be ca. pH 3.5, 4.4, and 5.9, respectively. The pHZPC of BHT150-AT0.05 and BHT150-AT2.65 were slightly shifted to higher pH because the surface negative charges were partly neutralized by the phase transition from titanate to anatase or rutile. Therefore, the strong adsorption of MB on the as prepared titanates, BHT100, BHT150, and BHT200, was due to the negatively charged surface. Although the change of the MB concentration in the UV irradiated titanates suspension could not be determined, there was no color change of test solution, which implies that the MB was not photocatalytically degraded on the titanates. On the other hand, the acid treated titanates could degrade the MB under UV irradiation, which seems to be due to the formation of the anatase phase. It is known that the anatase phase exhibits higher photocatalytic activity. At a high concentration of acid, the photocatalytic degradation of MB was negligible due to the rutile phase formation. The photocatalytic oxidations of gaseous CH3CHO were also investigated, as shown in Fig. 9. The simultaneous CO2 production due to CH3CHO oxidation was also observed (data not shown). The results were similar to the photocatalytic degradation of MB. The photocatalytic oxidation of gaseous CH3CHO on both the untreated titanate samples and the strong acid treated rutile samples was negligible. However, the weak acid treated samples, which have
3.2. Photocatalytic activities The adsorption and the photocatalytic degradation of MB were investigated. Fig. 7 shows the degradation of MB under a UV irradiated suspension of acid treated titanates according to the acid concentration. The differences among the initial MB concentrations were caused by the adsorbed amount of MB. Because the MB concentration was measured by a UV–vis spectrophotometer after filtration, which was needed for the removal of suspended particles, the adsorbed amount of MB was not reflected in the absorption spectra. The adsorption of cationic MB could be strongly affected by the surface charge of samples. Fig. 8 shows the variation of the zeta potentials of BHT150, BHT150-AT0.05, and BHT150-AT2.65 M in water as a function of pH. With acid
Fig. 8. Zeta potentials of BHT150 and acid treated BHT150 as a function of solution pH.
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Fig. 10. Photocatalytic activities of MB (a) and CH3CHO (b) degradation.
4. Conclusions
Fig. 9. Effects of acid concentration for the photocatalytic degradation of gaseous CH3CHO on acid treated BHT100 (a), BHT150 (b), and BHT200 (c) under UV irradiation in a closed circulation system after dark adsorption.
the anatase phase, exhibited the efficient photocatalytic activities for gaseous CH3CHO oxidation. The photocatalytic activities for MB degradation and CH3CHO oxidation are compared with those on P25 in Fig. 10. The MB degradation on all samples were slower than that on P25, whereas the CH3CHO oxidations on acid treated samples containing anatase phase were comparable with that on P25. The BHT100-AT0.05, which is composed of anatase phase only, showed the best photocatalytic activities. The titanate tube structures exhibited a high surface area and did not strongly affect the photocatalytic activities. The photocatalytic activities were dependent on the presence of anatase particles.
Titanate tubes/fibers were hydrothermally prepared and were subsequently treated with acid at relatively low temperatures to investigate the effects of the hydrothermal temperature of titanate tube/fiber formation and the acid concentration of the post-acid treatment. The morphologies and the crystalline phase were strongly affected by the hydrothermal temperature and the subsequent acid concentration. For the formation of titanate tubes/fibers from P25, low crystalline titanate hollow nanotubes were produced at relatively low hydrothermal temperatures, whereas the high crystalline solid titanate solid nanofibers were produced at relatively high temperatures. By further acid treatment at 80 8C, the titanate tubes/fibers were transformed to anatase particles at a low concentration of acid, whereas the rutile rods were produced from the titanate tubes/fibers at a high concentration of acid. These changes in morphology and crystallinity due to acid treatment were strongly dependent on the crystallinity of the parent titanate tubes/ fibers and the concentration of acid. The photocatalytic activities for MB degradation and gaseous CH3CHO oxidation were strongly dependent on the presence of the anatase crystalline phase. Acknowledgment This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (11-NB03). It was also supported by the technology innovation program of the Ministry of Knowledge Economy of Korea (Industrial strategic technology development program, No 10034046). References [1] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160.
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