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Single-crystalline TiO2 nanorods: Highly active and easily recycled photocatalysts
Applied Catalysis B: Environmental 73 (2007) 166–171 www.elsevier.com/locate/apcatb
Single-crystalline TiO2 nanorods: Highly active and easily recycled photocatalysts Yuxiang Yu, Dongsheng Xu * State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China Received 12 December 2005; received in revised form 23 July 2006; accepted 25 July 2006 Available online 30 August 2006
Abstract Large-scale and single-crystalline anatase TiO2 nanorods were synthesized via hydrothermal reaction of H-titanate fibers. It was found that the pH plays an important role in the shape and crystal structures of the products by hydrothermal post-treatment of H-titanate fibers in acidic solutions. At pH 2–7, the pure anatase phase was formed and the samples reserved the rod-like morphology with a diameter of 20–200 nm, in contrast to the formation of particles with a mixture of rutile and brookite phases at pH 0. Photocatalytic experiments indicated that the obtained single-crystalline anatase TiO2 nanorods are highly active for photodegradation of organic pollutants. Furthermore, we demonstrated that these nanorod catalysts can be easily recycled without decrease of the photocatalytic activity. # 2006 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; TiO2; Nanorods; Single-crystal; Recycling
1. Introduction Since the discovery of the photoelectrochemical splitting of water on n-TiO2 electrodes [1], TiO2-based materials have been widely studied in photocatalysts [2–4], photosplitting water [5], and solar cells [6,7]. In particular, anatase TiO2 has been undoubtedly proved to be the most excellent photocatalyst because its relative cheapness, biological and chemical stability, non-photocorrosivity and superior photocatalytic activity [2–4,8]. In the field of photocatalysis, most of the studies have been done with spherical TiO2 nanocrystals, in which higher photocatalytic efficiency is observed with the large surface-to-volume ratio by decreasing the particle size [2– 4,9]. However, in a practical photocatalytic process, the separation of these finely powdered photocatalysts from solution after reaction could be very difficult, and meanwhile, a strong tendency to agglomerate into larger particles will result in a reduction of the photocatalytic activity. Although nanocrystalline TiO2 immobilized on large supports can improve the separated efficiency, it usually decreases the overall photo-
* Corresponding author. E-mail address: [email protected] (D. Xu). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.07.017
catalytic activity comparing to the dispersed TiO2 due to lowering of the surface-to-volume ratio and partial loss of the active surface sites of photocatalysts [10]. Continuing efforts have been made to find a kind of low cost, highly active and easily recycled TiO2-based photocatalysts [11– 18]. TiO2-based fibers with large aspect ratio can be separated easily from solution, which can overcome the disadvantages of the spherical TiO2 catalysts [13,14]. However, the photocatalytic activity is relatively low due to their poor crystallinity and less active surface sites. Although nanostructured porous fibers of H2Ti8O171.5H2O display highly photooxidative activity [17], a certain decrease for photocatalytic activity could be observed within three-time cycling use [17]. Consequently, the improvement of the photocatalytic activities as well as cycling use of photocatalysts is an important task in the development of TiO2 nanowire photocatalysts. Among various methods [13–16,19–30], hydrothermal synthesis has become one of the most powerful and promising strategy for preparing 1D TiO2 nanostructures. This approach involves generation of alkali titanate nanofibers by hydrothermal reaction in alkali solution, exchanging alkali ions with protons to form the H-titanates, and then thermal dehydration reactions in air at high temperature [28–30] or hydrothermal reactions [13–16] of the H-titanate fibers to produce TiO2
Y. Yu, D. Xu / Applied Catalysis B: Environmental 73 (2007) 166–171
nanofibers with different crystallographic phases such as brookite, monoclinic, anatase, and rutile. However, the requirement of good crystallinity is still a major problem in hydrothermal synthesis of anatase TiO2 nanowires with large aspect ratio. In this paper, we report on the large scale synthesis of single-crystalline TiO2 nanorods with large aspect ratio, employing a simple and low-cost method based on the hydrothermal reaction of H-titanates nanofibers. It was found that the pH in the solution plays an important role in the shape and crystal structures of the products by hydrothermal posttreatment of H-titanates nanofibers. These single-crystal TiO2 nanorod catalysts have higher photocatalytic activity, close to that of P25. Furthermore, the stability of the newly synthesized TiO2 nanorod catalysts was tested following the evolution of their photocatalytic activity for 15-time cycling use. No obvious decrease of the photocatalytic activity could be observed during this time.
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2.3. Photocatalytic experiments Photocatalytic reactions for degradation of Rhodamine B (RhB) were carried out in a Pyrex beaker with 90 mm in diameter and 50 mm in height. An 8 W U-type UV lamp with a maximum emission at 254 nm was used as the UV resource. Before irradiation, 50 mg TiO2 particles were dispersed in 100 mL 8 mg/L RhB aqueous solution in an ultrasonic bath to form a suspension and the suspension was magnetically stirred for 15 min in dark. At regular irradiation time intervals, the dispersion was sampled (1 mL) and centrifuged to separate the TiO2 particles. The residual RhB concentration was detected using a Lambda 45 UV/Vis spectrometer (Perkin-Elmer instruments). For cycling use experiments, after the nanorods were separated from solution by sedimentation for 2 h, we removed the upper clear solution, and then the nanorods were dispersed in 100 mL RhB aqueous solution for another cycling use.
2. Experimental 3. Results and discussion 2.1. Synthesis of the anatase TiO2 nanowires 3.1. Material characteristics Sodium titanate (Na-titanate) nanotubes/nanorods were synthesized by a hydrothermal reaction between NaOH solution and titania particles [31–35]. Washing the products with dilute HCl promotes complete exchange of Na+ by H+ to form layered hydrogen titanate (H-titanate) nanofibers [35]. In a typical procedure, 1 g commercial TiO2 was added into a 50 mL Teflon vessel then filled with 10 M NaOH aqueous solution up to 80% of the total volume. The hydrothermal reaction was carried out at 200 8C for 24 h, and then naturally cooled to room temperature, producing white Na2Ti3O7xH2O precipitates. These white precipitates were isolated from the solution by centrifugation and sequentially washed with deionized water for several times and dried at 70 8C for 10 h. For ion-exchange, the sodium titanate nanofibers were immerged into a 0.1 M HNO3 solution for 6 h, washed with deionized water for several times until the pH value of the solution was about 7, and then dried at 70 8C for 10 h. The obtained H-titanate nanofibers were added into a 100 mL Teflon vessel then filled dilute HNO3 aqueous solution with different pH values up to 80% of the total volume and maintained at 180 8C for 24 h. Finally, the products were isolated from the solution by centrifugation and sequentially washed with deionized water for several times, and dried at 70 8C for 10 h.
Fig. 1a shows the XRD pattern of the as-prepared H-titanate fibers, which exhibits features similar to the hydrogen titanates with a monoclinic lattice (C2/m). Under hydrothermal treatment, the H-titanate fibers convert to different TiO2 phases, depending on the pH concentration of the solution. In this case, the H-titanate fibers were dispersed in a HNO3 solution with a designated pH value and kept at 180 8C for 1 day. At pH 2–7, the pure anatase phase was formed (Fig. 1c–e), whereas the diffraction peaks from both the rutile and the
2.2. Characterization Crystallographic phases of the samples were characterized by a powder X-ray diffractometry (XRD, D/MAX-PC 2500 with Cu Ka radiation). The size and morphology of the prepared products were measured by transmission electron microscopy (TEM) (JEOL 200CX TEM, 160 kV), selected area electron diffraction (SAED) and higher-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30, 300 kV). Surface area was determined by using BET method based on N2 adsorption with a Micromeritics ASAP 2010 Analyze.
Fig. 1. XRD patterns of the H-titanate nanofibers (a) and the samples obtained by hydrothermal reaction in HNO3 solution with different pH: (b) 0, (c) 2, (d) 4, and (e) 7. (*) Rutile peaks; (&’’) TiO2-B peaks.
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brookite phases were observed in the products with hydrothermal treatment at pH 0 (Fig. 1b). The morphological and structural development of the products by hydrothermal treatments in acidic solution with different pH was further investigated by TEM. The as-prepared H-titanate nanofibers are 40–300 nm in thickness and up to tens of micrometers in length (Fig. 2a). The SAED pattern (the inset of Fig. 2b) and the HRTEM image (Fig. 2e) reveal that the nanofibers are well crystallized and grew along the [0 1 0] direction of the monoclinic H-titanate crystal. Fig. 2c show the TEM images of the products obtained by hydrothermal reaction of H-titanate nanofibers at pH of 2, in which the samples reserved the rod-like morphology with a diameter of 20– 200 nm and a length up to tens micrometers. Fig. 2d gives a typical TEM image of a single nanorod obtained at pH 2, showing that the nanorod is continuous and the surface is relatively smooth. The diffraction pattern (inset of Fig. 2d) and the HRTEM image (Fig. 2f) of this nanorod indicate a singlecrystalline structure. By measuring the lattice fringes of the HRTEM image and from the ED patterns, we found that the {0 0 1} lattice fringes are parallel to the axis of the nanorod and the nanorod grows along the [1 0 0] direction of single crystalline anatase TiO2. Furthermore, we found that the pH of the solution largely affected the morphologies of the products. At pH in a range of 4–7, the samples also reserved the rod-like morphology, but
Fig. 2. TEM images of H-titanate nanofibers (a and b) and the samples obtained by hydrothermal reaction in HNO3 solution at pH 2 (c and d). Inset in (b) and (d): the corresponding SAED patterns of the nanorod. The HRTEM images of the same samples are shown in (e) and (f), respectively.
Fig. 3. TEM images of the samples obtained by hydrothermal reaction in HNO3 solution at pH of 7 (a–b) and 0 (c). Inset in (b): the corresponding SAED patterns of the nanorod. The HRTEM image of the sample prepare at pH 7 is shown in (d).
the surfaces of the products are rough (Fig. 3a). A highermagnification TEM image of the nanorod prepared at pH 7 reveals a porous rod-like structure with aggregated nanocrystals (Fig. 3b). The regular ED pattern in the inset of Fig. 3b, which can be indexed to the [0 1 0] zone axis of anatase TiO2 with a [1 0 0] direction along the wire axis, implies that the nanocrystals in this nanorod have the same orientation. The HRTEM image of this nanorod in Fig. 3d further confirms that there were some crystal interfaces between these nanocrystals and the orientation of these nanocrystals is limited. At higher H+ concentrations, the products may lose the rod-like morphology and result in nonspherical particles (Fig. 3c). The common features of titanate and anatase are that they are composed of zigzag ribbons of edge-sharing TiO6 octahedra [13,36]. Viewing from [0 1 0] direction of a titanate lattice, zigzag chain share edges at one level in line group, and each group is joined above and below similar groups by further edge sharing (Fig. 4a), whereas TiO6 octahedra share edges along the [1 0 0] direction in an anatase lattice (Fig. 4b). A topochemical reaction process has been proposed to explain the phase transition from H-titanate to anatase [13]. By reaction with the acid, the H-titanate nanowires change to anatase through a H+ catalytic dehydration reaction between Ti–OH and HO–Ti. Meanwhile, this dehydration is accompanied by an in situ rearrangement of the structural units (the zigzag chain). Under moderate conditions, this conversion is preferred for retaining the rod-like morphology, with a [1 0 0] orientation along the wire axis (Fig. 2d). At higher H+ concentrations, the Ti–OH should be protonized to generate Ti–OH2+. Because of the repulsion of the same charges between Ti–OH2+ and +H2O–Ti, the dehydration hardly occurs and TiO6 octahedra prefer to share corner with each other, resulting in TiO2 particles with rutile (Fig. 4c) and brookite (Fig. 4d) structures. In addition, due to lack of the protons, it is not easy to dehydrate the Ti–OH groups at low H+ concentrations. The formation of the anatase nanorods composing of aggregated nanocrystals (Fig. 3b) may
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Fig. 4. Crystal structures of: (a) H-titanate, (b) anatase TiO2, (c) rutile TiO2, and (d) brookite TiO2.
be originated by the incomplete dehydration of the Ti–OH groups. 3.2. Photooxidative degradation of rhodamine B Furthermore, we have investigated the photocatalytica activity of both the H-titanate nanofibers and the samples, which obtained from a hydrothermal reaction of H-titanate at different pH, with the photocatalytic oxidative decomposition of RhB under UV irradiation as a test reaction, as illustrated in Fig. 5. The single-crystalline anatase nanorods (Fig. 2c) exhibit a higher activity than both the porous nanorods with aggregating of anatase nanocrystals (Fig. 3a) and the particles
with a mixture of rutile and brookite (Fig. 3c). For comparison, the photocatalytic activity of Degussa P25, a widely used standard photocatalyst, is also given in Fig. 5. The activity of the single-crystalline anatase nanorods is very close to that of commercial P25. To quantitatively evaluate the photocatalytic activities of catalysts, we studied the kinetics of RhB photodegradations over the above samples. In aqueous RhB solution, the photocatalytic decomposition reaction with suspended TiO2 approximately obey the first-order kinetics and the photocatalytic reaction can be simply described by dC/dt = kC, where C is the concentration of RhB, and k denotes the overall degradation rate constant. The photocatalytic activity has been defined as the overall degradation rate constant of the catalysts. By plotting ln(C0/C) as a function of time through regression, we obtained for each sample the k (min 1) constant from the slopes of the simulated straight lines, as listed in Table 1. Surface area is one of the vital factors in the determining of the photocatalytic activity of the catalysts. In Table 1, we can see that the surface area of the samples obtained from pH 2, 4 and 7 Table 1 Structures and photocatalytic properties of the samples
Fig. 5. Photocatalytic activity of the samples: H-titanate nanofibers, anatase nanorods obtained at different pH, and Degussa P-25 for RhB degradation. The catalyst concentration was 0.05 g/L and the initial concentration (c0) of RhB was 8 mg/L.
Samples
pH
component
Shape
SBET (m2/g)
T1 A7 A4 A2 A0 P25
7 4 2 0 –
H2Ti3O7xH2O Anatase Anatase Anatase Rutile, brookite Anatase (80%), rutile (20%)
Rods Rods Rods Rods Particles Particles
34.1 47.0 45.6 39.2 16.2 52.0
a
Overall degradation rate constant.
ka (min 1) 0.005 0.064 0.067 0.094 0.044 0.099
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(Fig. 6b and c), while the aqueous suspension of P25 was still turbid even after 2 h (Fig. 6d). Fig. 7 shows photodegradation of RhB over the single-crystalline anatase nanorod catalysts with 15-time cycling uses and indicates no obvious decrease for photocatalytic activity. This fact that the nanowire photocatalysts with high photocatalytic activity can be easily recovered by sedimentation will greatly promote their industrial application to eliminate the organic pollutants from wastewater. Fig. 6. Sedimentation for 1 h in aqueous suspensions of H-titanate nanofibers (a), anatase nanorods obtained at pH of 2 (b) and 7 (c), P25 (d), and anatase nanorods after 15 cycling uses (e).
HNO3 solution is 39.2, 45.6 and 47.0 m2/g, respectively. Interestingly, the activity of the single-crystalline anatase nanorods reached 94% of the P25, although their surface area is only 85% of the P25. The difference in the photocatalytic activity between the nanorods with aggregating of anatase nanocrystals and the single-crystalline anatase nanorods can be explained by the large numbers of crystal interfaces in the porous nanorods and hence increasing the holes and electrons recombination probability. 3.3. Long-term stability Cycling use as well as maintaining high activity of photocatalysts is a critical issue towards long-term photocatalytic applications. Ultra-fine catalyst powders with sizes of several nanometers present a superior activity due to the large surface-to-volume ratios. However, in a practical photocatalytic process, the separation of these powder photocatalysts from solution after reaction could be very difficult, and meanwhile, the tendency to agglomerate into larger particles will result in a reduction of the photocatalytic activity during the cycling use. Fibril photocatalysts have taken an advantage over spherical powder catalysts for separating the catalyst from solution by filtration or sedimentation [14,17]. In our experimental, the anatase nanorod catalysts with lengths at scale of micrometers sedimentated from an aqueous suspension in less than 1 h
Fig. 7. Photocatalytic activity of the TiO2 nanorods obtained at pH 2 for RhB degradation with three times of cycling uses. Inset: overall degradation rate constant k vs. times of cycling uses for RhB degradation of TiO2 nanorod photocatalysts.
4. Conclusion In summary, large-scale single-crystalline anatase TiO2 nanorods have been synthesized by controlling the reaction temperature and the acidic concentration through a hydrothermal reaction of H-titanate fibers. The obtained single-crystalline anatase TiO2 nanorods are highly active for photodegradation of organic pollutants. Due to their large aspect ratios, these nanorod photocatalysts have taken an important advantage over spherical powder catalysts for separating the catalyst from solution by sedimentation. Furthermore, we have demonstrated these nanorod catalysts can be easily recycled without decrease of the photocatalytic activity. A combination of their unique features of low-cost, single-crystallinity, easy separation and recycling use, and highly photocatalytic activity suggests that these single-crystalline anatase TiO2 nanorods will provide possibility to future industrial applications in environmental pollutants cleaning up. Acknowledgements We thank Prof. Y.F. Zhu from Tsinghua University, China for helpful discussion. This work is supported by NSFC (Grant No. 20433010, 20525309, 20521201), MSTC (MSBRDP, Grant No. 2006CB806402, and CNPC Innovation Fund). References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] D.S. Bhatkhande, V.G. Pangarkar, A. Beenackers, ACM. J. Chem. Technol. Biotechnol. 77 (2001) 102. [3] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357. [4] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [5] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [6] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [7] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [8] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. A 1 (2000) 1. [9] T.Y. Peng, D. Zhao, K. Dai, W. Shi, K. Hirao, J. Phys. Chem. B 109 (2005) 4947. [10] A. Haarstrick, O.M. Kut, E. Heinzle, Environ. Sci. Technol. 30 (1996) 817. [11] S. Parra, S.E. Stanca, I. Guasaquillo, K.P. Thampi, Appl. Catal. B 51 (2004) 107. [12] M.C. Hidalgo, D. Bahnemann, Appl. Catal. B 61 (2005) 259. [13] H.Y. Zhu, X.P. Gao, Y. Lan, D.Y. Song, Y.X. Xi, J.C. Zhao, J. Am. Chem. Soc. 126 (2004) 8380. [14] 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. [15] H.Y. Zhu, J.A. Orthman, J.Y. Li, J.C. Zhao, G.J. Churchman, E.F. Vansant, Chem. Mater. 14 (2002) 5037.
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Single-crystalline TiO2 nanorods: Highly active and easily recycled photocatalysts Yuxiang Yu, Dongsheng Xu * State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China Received 12 December 2005; received in revised form 23 July 2006; accepted 25 July 2006 Available online 30 August 2006
Abstract Large-scale and single-crystalline anatase TiO2 nanorods were synthesized via hydrothermal reaction of H-titanate fibers. It was found that the pH plays an important role in the shape and crystal structures of the products by hydrothermal post-treatment of H-titanate fibers in acidic solutions. At pH 2–7, the pure anatase phase was formed and the samples reserved the rod-like morphology with a diameter of 20–200 nm, in contrast to the formation of particles with a mixture of rutile and brookite phases at pH 0. Photocatalytic experiments indicated that the obtained single-crystalline anatase TiO2 nanorods are highly active for photodegradation of organic pollutants. Furthermore, we demonstrated that these nanorod catalysts can be easily recycled without decrease of the photocatalytic activity. # 2006 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; TiO2; Nanorods; Single-crystal; Recycling
1. Introduction Since the discovery of the photoelectrochemical splitting of water on n-TiO2 electrodes [1], TiO2-based materials have been widely studied in photocatalysts [2–4], photosplitting water [5], and solar cells [6,7]. In particular, anatase TiO2 has been undoubtedly proved to be the most excellent photocatalyst because its relative cheapness, biological and chemical stability, non-photocorrosivity and superior photocatalytic activity [2–4,8]. In the field of photocatalysis, most of the studies have been done with spherical TiO2 nanocrystals, in which higher photocatalytic efficiency is observed with the large surface-to-volume ratio by decreasing the particle size [2– 4,9]. However, in a practical photocatalytic process, the separation of these finely powdered photocatalysts from solution after reaction could be very difficult, and meanwhile, a strong tendency to agglomerate into larger particles will result in a reduction of the photocatalytic activity. Although nanocrystalline TiO2 immobilized on large supports can improve the separated efficiency, it usually decreases the overall photo-
* Corresponding author. E-mail address: [email protected] (D. Xu). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.07.017
catalytic activity comparing to the dispersed TiO2 due to lowering of the surface-to-volume ratio and partial loss of the active surface sites of photocatalysts [10]. Continuing efforts have been made to find a kind of low cost, highly active and easily recycled TiO2-based photocatalysts [11– 18]. TiO2-based fibers with large aspect ratio can be separated easily from solution, which can overcome the disadvantages of the spherical TiO2 catalysts [13,14]. However, the photocatalytic activity is relatively low due to their poor crystallinity and less active surface sites. Although nanostructured porous fibers of H2Ti8O171.5H2O display highly photooxidative activity [17], a certain decrease for photocatalytic activity could be observed within three-time cycling use [17]. Consequently, the improvement of the photocatalytic activities as well as cycling use of photocatalysts is an important task in the development of TiO2 nanowire photocatalysts. Among various methods [13–16,19–30], hydrothermal synthesis has become one of the most powerful and promising strategy for preparing 1D TiO2 nanostructures. This approach involves generation of alkali titanate nanofibers by hydrothermal reaction in alkali solution, exchanging alkali ions with protons to form the H-titanates, and then thermal dehydration reactions in air at high temperature [28–30] or hydrothermal reactions [13–16] of the H-titanate fibers to produce TiO2
Y. Yu, D. Xu / Applied Catalysis B: Environmental 73 (2007) 166–171
nanofibers with different crystallographic phases such as brookite, monoclinic, anatase, and rutile. However, the requirement of good crystallinity is still a major problem in hydrothermal synthesis of anatase TiO2 nanowires with large aspect ratio. In this paper, we report on the large scale synthesis of single-crystalline TiO2 nanorods with large aspect ratio, employing a simple and low-cost method based on the hydrothermal reaction of H-titanates nanofibers. It was found that the pH in the solution plays an important role in the shape and crystal structures of the products by hydrothermal posttreatment of H-titanates nanofibers. These single-crystal TiO2 nanorod catalysts have higher photocatalytic activity, close to that of P25. Furthermore, the stability of the newly synthesized TiO2 nanorod catalysts was tested following the evolution of their photocatalytic activity for 15-time cycling use. No obvious decrease of the photocatalytic activity could be observed during this time.
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2.3. Photocatalytic experiments Photocatalytic reactions for degradation of Rhodamine B (RhB) were carried out in a Pyrex beaker with 90 mm in diameter and 50 mm in height. An 8 W U-type UV lamp with a maximum emission at 254 nm was used as the UV resource. Before irradiation, 50 mg TiO2 particles were dispersed in 100 mL 8 mg/L RhB aqueous solution in an ultrasonic bath to form a suspension and the suspension was magnetically stirred for 15 min in dark. At regular irradiation time intervals, the dispersion was sampled (1 mL) and centrifuged to separate the TiO2 particles. The residual RhB concentration was detected using a Lambda 45 UV/Vis spectrometer (Perkin-Elmer instruments). For cycling use experiments, after the nanorods were separated from solution by sedimentation for 2 h, we removed the upper clear solution, and then the nanorods were dispersed in 100 mL RhB aqueous solution for another cycling use.
2. Experimental 3. Results and discussion 2.1. Synthesis of the anatase TiO2 nanowires 3.1. Material characteristics Sodium titanate (Na-titanate) nanotubes/nanorods were synthesized by a hydrothermal reaction between NaOH solution and titania particles [31–35]. Washing the products with dilute HCl promotes complete exchange of Na+ by H+ to form layered hydrogen titanate (H-titanate) nanofibers [35]. In a typical procedure, 1 g commercial TiO2 was added into a 50 mL Teflon vessel then filled with 10 M NaOH aqueous solution up to 80% of the total volume. The hydrothermal reaction was carried out at 200 8C for 24 h, and then naturally cooled to room temperature, producing white Na2Ti3O7xH2O precipitates. These white precipitates were isolated from the solution by centrifugation and sequentially washed with deionized water for several times and dried at 70 8C for 10 h. For ion-exchange, the sodium titanate nanofibers were immerged into a 0.1 M HNO3 solution for 6 h, washed with deionized water for several times until the pH value of the solution was about 7, and then dried at 70 8C for 10 h. The obtained H-titanate nanofibers were added into a 100 mL Teflon vessel then filled dilute HNO3 aqueous solution with different pH values up to 80% of the total volume and maintained at 180 8C for 24 h. Finally, the products were isolated from the solution by centrifugation and sequentially washed with deionized water for several times, and dried at 70 8C for 10 h.
Fig. 1a shows the XRD pattern of the as-prepared H-titanate fibers, which exhibits features similar to the hydrogen titanates with a monoclinic lattice (C2/m). Under hydrothermal treatment, the H-titanate fibers convert to different TiO2 phases, depending on the pH concentration of the solution. In this case, the H-titanate fibers were dispersed in a HNO3 solution with a designated pH value and kept at 180 8C for 1 day. At pH 2–7, the pure anatase phase was formed (Fig. 1c–e), whereas the diffraction peaks from both the rutile and the
2.2. Characterization Crystallographic phases of the samples were characterized by a powder X-ray diffractometry (XRD, D/MAX-PC 2500 with Cu Ka radiation). The size and morphology of the prepared products were measured by transmission electron microscopy (TEM) (JEOL 200CX TEM, 160 kV), selected area electron diffraction (SAED) and higher-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30, 300 kV). Surface area was determined by using BET method based on N2 adsorption with a Micromeritics ASAP 2010 Analyze.
Fig. 1. XRD patterns of the H-titanate nanofibers (a) and the samples obtained by hydrothermal reaction in HNO3 solution with different pH: (b) 0, (c) 2, (d) 4, and (e) 7. (*) Rutile peaks; (&’’) TiO2-B peaks.
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brookite phases were observed in the products with hydrothermal treatment at pH 0 (Fig. 1b). The morphological and structural development of the products by hydrothermal treatments in acidic solution with different pH was further investigated by TEM. The as-prepared H-titanate nanofibers are 40–300 nm in thickness and up to tens of micrometers in length (Fig. 2a). The SAED pattern (the inset of Fig. 2b) and the HRTEM image (Fig. 2e) reveal that the nanofibers are well crystallized and grew along the [0 1 0] direction of the monoclinic H-titanate crystal. Fig. 2c show the TEM images of the products obtained by hydrothermal reaction of H-titanate nanofibers at pH of 2, in which the samples reserved the rod-like morphology with a diameter of 20– 200 nm and a length up to tens micrometers. Fig. 2d gives a typical TEM image of a single nanorod obtained at pH 2, showing that the nanorod is continuous and the surface is relatively smooth. The diffraction pattern (inset of Fig. 2d) and the HRTEM image (Fig. 2f) of this nanorod indicate a singlecrystalline structure. By measuring the lattice fringes of the HRTEM image and from the ED patterns, we found that the {0 0 1} lattice fringes are parallel to the axis of the nanorod and the nanorod grows along the [1 0 0] direction of single crystalline anatase TiO2. Furthermore, we found that the pH of the solution largely affected the morphologies of the products. At pH in a range of 4–7, the samples also reserved the rod-like morphology, but
Fig. 2. TEM images of H-titanate nanofibers (a and b) and the samples obtained by hydrothermal reaction in HNO3 solution at pH 2 (c and d). Inset in (b) and (d): the corresponding SAED patterns of the nanorod. The HRTEM images of the same samples are shown in (e) and (f), respectively.
Fig. 3. TEM images of the samples obtained by hydrothermal reaction in HNO3 solution at pH of 7 (a–b) and 0 (c). Inset in (b): the corresponding SAED patterns of the nanorod. The HRTEM image of the sample prepare at pH 7 is shown in (d).
the surfaces of the products are rough (Fig. 3a). A highermagnification TEM image of the nanorod prepared at pH 7 reveals a porous rod-like structure with aggregated nanocrystals (Fig. 3b). The regular ED pattern in the inset of Fig. 3b, which can be indexed to the [0 1 0] zone axis of anatase TiO2 with a [1 0 0] direction along the wire axis, implies that the nanocrystals in this nanorod have the same orientation. The HRTEM image of this nanorod in Fig. 3d further confirms that there were some crystal interfaces between these nanocrystals and the orientation of these nanocrystals is limited. At higher H+ concentrations, the products may lose the rod-like morphology and result in nonspherical particles (Fig. 3c). The common features of titanate and anatase are that they are composed of zigzag ribbons of edge-sharing TiO6 octahedra [13,36]. Viewing from [0 1 0] direction of a titanate lattice, zigzag chain share edges at one level in line group, and each group is joined above and below similar groups by further edge sharing (Fig. 4a), whereas TiO6 octahedra share edges along the [1 0 0] direction in an anatase lattice (Fig. 4b). A topochemical reaction process has been proposed to explain the phase transition from H-titanate to anatase [13]. By reaction with the acid, the H-titanate nanowires change to anatase through a H+ catalytic dehydration reaction between Ti–OH and HO–Ti. Meanwhile, this dehydration is accompanied by an in situ rearrangement of the structural units (the zigzag chain). Under moderate conditions, this conversion is preferred for retaining the rod-like morphology, with a [1 0 0] orientation along the wire axis (Fig. 2d). At higher H+ concentrations, the Ti–OH should be protonized to generate Ti–OH2+. Because of the repulsion of the same charges between Ti–OH2+ and +H2O–Ti, the dehydration hardly occurs and TiO6 octahedra prefer to share corner with each other, resulting in TiO2 particles with rutile (Fig. 4c) and brookite (Fig. 4d) structures. In addition, due to lack of the protons, it is not easy to dehydrate the Ti–OH groups at low H+ concentrations. The formation of the anatase nanorods composing of aggregated nanocrystals (Fig. 3b) may
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Fig. 4. Crystal structures of: (a) H-titanate, (b) anatase TiO2, (c) rutile TiO2, and (d) brookite TiO2.
be originated by the incomplete dehydration of the Ti–OH groups. 3.2. Photooxidative degradation of rhodamine B Furthermore, we have investigated the photocatalytica activity of both the H-titanate nanofibers and the samples, which obtained from a hydrothermal reaction of H-titanate at different pH, with the photocatalytic oxidative decomposition of RhB under UV irradiation as a test reaction, as illustrated in Fig. 5. The single-crystalline anatase nanorods (Fig. 2c) exhibit a higher activity than both the porous nanorods with aggregating of anatase nanocrystals (Fig. 3a) and the particles
with a mixture of rutile and brookite (Fig. 3c). For comparison, the photocatalytic activity of Degussa P25, a widely used standard photocatalyst, is also given in Fig. 5. The activity of the single-crystalline anatase nanorods is very close to that of commercial P25. To quantitatively evaluate the photocatalytic activities of catalysts, we studied the kinetics of RhB photodegradations over the above samples. In aqueous RhB solution, the photocatalytic decomposition reaction with suspended TiO2 approximately obey the first-order kinetics and the photocatalytic reaction can be simply described by dC/dt = kC, where C is the concentration of RhB, and k denotes the overall degradation rate constant. The photocatalytic activity has been defined as the overall degradation rate constant of the catalysts. By plotting ln(C0/C) as a function of time through regression, we obtained for each sample the k (min 1) constant from the slopes of the simulated straight lines, as listed in Table 1. Surface area is one of the vital factors in the determining of the photocatalytic activity of the catalysts. In Table 1, we can see that the surface area of the samples obtained from pH 2, 4 and 7 Table 1 Structures and photocatalytic properties of the samples
Fig. 5. Photocatalytic activity of the samples: H-titanate nanofibers, anatase nanorods obtained at different pH, and Degussa P-25 for RhB degradation. The catalyst concentration was 0.05 g/L and the initial concentration (c0) of RhB was 8 mg/L.
Samples
pH
component
Shape
SBET (m2/g)
T1 A7 A4 A2 A0 P25
7 4 2 0 –
H2Ti3O7xH2O Anatase Anatase Anatase Rutile, brookite Anatase (80%), rutile (20%)
Rods Rods Rods Rods Particles Particles
34.1 47.0 45.6 39.2 16.2 52.0
a
Overall degradation rate constant.
ka (min 1) 0.005 0.064 0.067 0.094 0.044 0.099
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(Fig. 6b and c), while the aqueous suspension of P25 was still turbid even after 2 h (Fig. 6d). Fig. 7 shows photodegradation of RhB over the single-crystalline anatase nanorod catalysts with 15-time cycling uses and indicates no obvious decrease for photocatalytic activity. This fact that the nanowire photocatalysts with high photocatalytic activity can be easily recovered by sedimentation will greatly promote their industrial application to eliminate the organic pollutants from wastewater. Fig. 6. Sedimentation for 1 h in aqueous suspensions of H-titanate nanofibers (a), anatase nanorods obtained at pH of 2 (b) and 7 (c), P25 (d), and anatase nanorods after 15 cycling uses (e).
HNO3 solution is 39.2, 45.6 and 47.0 m2/g, respectively. Interestingly, the activity of the single-crystalline anatase nanorods reached 94% of the P25, although their surface area is only 85% of the P25. The difference in the photocatalytic activity between the nanorods with aggregating of anatase nanocrystals and the single-crystalline anatase nanorods can be explained by the large numbers of crystal interfaces in the porous nanorods and hence increasing the holes and electrons recombination probability. 3.3. Long-term stability Cycling use as well as maintaining high activity of photocatalysts is a critical issue towards long-term photocatalytic applications. Ultra-fine catalyst powders with sizes of several nanometers present a superior activity due to the large surface-to-volume ratios. However, in a practical photocatalytic process, the separation of these powder photocatalysts from solution after reaction could be very difficult, and meanwhile, the tendency to agglomerate into larger particles will result in a reduction of the photocatalytic activity during the cycling use. Fibril photocatalysts have taken an advantage over spherical powder catalysts for separating the catalyst from solution by filtration or sedimentation [14,17]. In our experimental, the anatase nanorod catalysts with lengths at scale of micrometers sedimentated from an aqueous suspension in less than 1 h
Fig. 7. Photocatalytic activity of the TiO2 nanorods obtained at pH 2 for RhB degradation with three times of cycling uses. Inset: overall degradation rate constant k vs. times of cycling uses for RhB degradation of TiO2 nanorod photocatalysts.
4. Conclusion In summary, large-scale single-crystalline anatase TiO2 nanorods have been synthesized by controlling the reaction temperature and the acidic concentration through a hydrothermal reaction of H-titanate fibers. The obtained single-crystalline anatase TiO2 nanorods are highly active for photodegradation of organic pollutants. Due to their large aspect ratios, these nanorod photocatalysts have taken an important advantage over spherical powder catalysts for separating the catalyst from solution by sedimentation. Furthermore, we have demonstrated these nanorod catalysts can be easily recycled without decrease of the photocatalytic activity. A combination of their unique features of low-cost, single-crystallinity, easy separation and recycling use, and highly photocatalytic activity suggests that these single-crystalline anatase TiO2 nanorods will provide possibility to future industrial applications in environmental pollutants cleaning up. Acknowledgements We thank Prof. Y.F. Zhu from Tsinghua University, China for helpful discussion. This work is supported by NSFC (Grant No. 20433010, 20525309, 20521201), MSTC (MSBRDP, Grant No. 2006CB806402, and CNPC Innovation Fund). References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] D.S. Bhatkhande, V.G. Pangarkar, A. Beenackers, ACM. J. Chem. Technol. Biotechnol. 77 (2001) 102. [3] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357. [4] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [5] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [6] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [7] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [8] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. A 1 (2000) 1. [9] T.Y. Peng, D. Zhao, K. Dai, W. Shi, K. Hirao, J. Phys. Chem. B 109 (2005) 4947. [10] A. Haarstrick, O.M. Kut, E. Heinzle, Environ. Sci. Technol. 30 (1996) 817. [11] S. Parra, S.E. Stanca, I. Guasaquillo, K.P. Thampi, Appl. Catal. B 51 (2004) 107. [12] M.C. Hidalgo, D. Bahnemann, Appl. Catal. B 61 (2005) 259. [13] H.Y. Zhu, X.P. Gao, Y. Lan, D.Y. Song, Y.X. Xi, J.C. Zhao, J. Am. Chem. Soc. 126 (2004) 8380. [14] 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. [15] H.Y. Zhu, J.A. Orthman, J.Y. Li, J.C. Zhao, G.J. Churchman, E.F. Vansant, Chem. Mater. 14 (2002) 5037.
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