Tailoring of Low-Dimensional Titanate Nanostructures - The Journal of

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Tailoring of Low-Dimensional Titanate Nanostructures Jiquan Huang, Yongge Cao,* Meili Wang, Changgang Huang, Zhonghua Deng, Hao Tong, and Zhuguang Liu Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China ReceiVed: May 4, 2010; ReVised Manuscript ReceiVed: July 11, 2010

The accurate tailoring of low-dimensional titanate nanostructures, including crystal nanotubes, nanowires, and amorphous nanoparticles was achieved by the control of the hydrothermal reaction conditions. The compositions, crystal structures, and formation mechanisms of these titanate nanomaterials, as well as the transformation relationships among them, were studied. It was found that crystal titanate nanotubes/nanowires were obtained by the hydrothermal treatment of either titania or amorphous titanate in moderate NaOH solution (5-12 M) and the reaction was accelerated by the increase of either temperature or NaOH concentration, while amorphous titanate nanoparticles were synthesized by the treatment of either titania or titanate nanotubes/ nanowires in extreme NaOH condition (CNaOH > 15 M). 1. Introduction Nanoscale structures, including nanoparticles, nanotubes, nanowires, nanobelts, nanorods, nanosheets, and nanoflowers, have commanded considerable attention because of their unique atomic structures and unusual physicochemical behavior.1,2 It is well-known that properties of nanostructured materials depend strongly on their shapes and sizes. Therefore, the controlled growth of nanostructures with desired morphologies plays a key role in optimizing the material properties.3 Since the discovery of carbon nanotubes,4 the tailoring of traditional materials, such as TiO2, ZnO, ZnS, and SnO2, into nanomaterials with desired shapes has attracted a great deal of interest.1,3,5 Among all the nanomaterials, titania/titanate nanostructures are promising1,6 due to their attractive potential applications in catalysts,7 solar batteries,8,9 lithium ion batteries,10,11 hydrogen storage,12 and gas sensors.13 It has been found that their crystallinities, chemical stabilities, specific areas, and physical properties depend on the morphologies. Each type of titanate nanostructure may exhibit its own particular advantages because of their diversity. For example, titanate nanowires show better crystallinity and structure stability, and thus have been considered as intelligent absorbents for the removal of radioactive ions;14 titanate nanotubes show potential advantage as catalysts and electrodes for dye-sensitized solar cells owing to their larger specific surface, open mesoporous structure, and excellent adsorption capacity.7,15,16 Obviously, by tailoring their nanostructures, the unique properties of titanate nanomaterials can be adjusted to satisfy specific needs for applications in various fields. A key issue with regard to the envisaged and optimized applications of titanate nanomaterials is therefore to mainly focus on the control of morphologies, sufrace densities, and aspect ratios. In addition, the further understanding of the formation conditions and mechanisms of various titanate nanostructures, including the transformation relationships among these materials, would provide a more effective tool to tailor the morphologies and structures of titanate nanomaterials. Unfortunately, up to now, little information has been available on the transformation * To whom correspondence should be addressed. Telephone: +86-59183721039. Fax: +86-591-83713291. E-mail: [email protected].

details between various titanate nanostructures, and even the chemical compositions and the exact formation mechanisms of the titanate nanostructures are still under disputation.17,18 Therefore, the accurate tailoring of various titanate nanostructures for specific application remains one of the most prized objectives. In this paper, we focus mainly on the controlled synthesis of various titanate nanostructures including titanate nanotubes, titanate nanowires, and amorphous titanate spherical particles. The formation details for these nanostructures and the transformation relations among them were studied. It was found that the morphologies of the titanate nanomaterials can be tailored accurately by adjusting the hydrothermal parameters. 2. Experimental Section Various titanate nanostructures were synthesized by the hydrothermal process similar to that described by Kasuga et al.19 The synthesis process was performed as follows: 0.25 g of the TiO2 nanopowders was mixed with 20 mL of NaOH solution. The concentration of NaOH ranged from 2 to 25 M. The mixture was stirred for 0.5 h in a 25 mL autoclave and then held in an oven for the hydrothermal treatment. The hydrothermal temperatures ranged from 100 to 240 °C, while the duration ranged from 0.5 h to 40 days. After the hydrothermal reaction, the as-obtained precipitate was collected and washed with distilled water and ethanol several times and then dried in air at 70 °C for 20 h. Part of the as-obtained nanotubes and nanowires were treated hydrothermally in 20 M NaOH solution at 150 °C for 0.5-240 h, while part of the as-obtained amorphous titanate powders were treated hydrothermally in 8 M NaOH solution at 150 °C (or 200 °C) for 24-120 h. The precipitate was washed with distilled water and ethanol several times and then dried in air at 70 °C for 20 h. To obtained various titania nanostructures, the as-obtained precipitate, including titanate nanotubes, nanowires, and amorphous particles, was collected and washed with 0.1 M HCl aqueous solution, distilled water, and ethanol in order dozens of times. The washed samples were dried in air at 70 °C for

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Figure 1. SEM images of hydrothermal products obtained by treating TiO2 in 8 M NaOH solution at 180 °C for (a) 3, (b) 12, (c) 40, and (d) 80 h.

20 h and then calcined in air at temperatures ranging from 300 to 950 °C for 2 h. The morphological observation of the resulting samples was performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The phase structure was analyzed by X-ray diffraction (XRD). The concentration of Ti(IV) in alkaline solution was determined by the Ultima2 inductively coupled plasma (ICP) optical emission spectrometer. Fourier transform infrared (FTIR) spectra were measured on a Perkin-Elmer IR spectrophotometer using a KBr pellet technique. The chemical compositions of various titanate nanostructures were determined by the Ultima2 ICP optical emission spectrometer. Before the detection, the samples were washed with distilled water to pH ∼8 and then dried at 70 °C for 20 h. Thermal analysis was performed by thermogravimetric (TG) analysis on a Netzsch STA449C apparatus. Before the detection, the samples were washed with dilute HCl solution to pH ∼3 and then distilled water to pH ∼7. Finally, the washed samples were dried at 70 °C for 20 h. 3. Results and Discussion 3.1. Effect of Temperature and NaOH. The sequence of events for the formation of various crystal titanate nanostructures was studied. Figure 1 shows the SEM images of the hydrothermal products obtained by treating TiO2 in 8 M NaOH solution at 180 °C for different durations. The morphologies of the products obtained in 8 M NaOH solution at 240 °C, and in 12 M NaOH solution at 180 °C, are presented in Figure S1 in the Supporting Information. As shown in Figure 1a, at the beginning (t ) 3 h), the transformation of TiO2 particles into titanate nanosheets is observed. As the reaction advances (t ) 12 h), pure titanate nanotubes are formed (Figure 1b). For the reaction lasting 40 h, quite a number of titanate nanowires are observed besides the nanotubes (Figure 1c). By undergoing 80 h reaction, the final product, nanowire, is obtained, as shown in Figure 1d. It is obvious that the evolution of the products obeys the proposed “TiO2 precursor f nanosheets f nanotubes f nanowires” transformation mechanism.17 Further studies suggest that this transformation process can be accelerated by the increase of either hydrothermal temperature or the concentration of NaOH (CNaOH), as shown in Figure S1 (see the Supporting Information) and Figure 2. Figure 2 shows the critical treatment duration needed for the transformation of nanotubes into nanowires at different temperatures. Less time is needed for this transformation if the temperature or concentration of NaOH is increased. The inset in Figure 2 shows the plots of ln t versus 1/T under 8 and 12 M NaOH solution. The relationship between ln t and 1/T was found to follow the linear equations

ln t ) -22.15 + 11874.18/T

CNaOH ) 8 M

(1)

Figure 2. Time needed for the transformation of nanotubes into nanowires at different hydrothermal temperatures. Inset shows the linear fitting plotted as ln t versus 1/T.

ln t ) -21.63 + 10893.62/T

CNaOH ) 12 M

(2) Bavykin et al. studied the relationship between the concentration of the dissolved Ti(IV) (CTi(IV)) in 10 M alkali solution and temperature.20 They found that CTi(IV) increased as the temperature was increased and it could be approximated by a linear fitting as ln CTi(IV) ) A - B/T. Accordingly, under NaOH solution with a set concentration, with increasing temperature, CTi(IV) increases while the viscosity of the alkali solution decreases. As a result, the mass transfer capability in the reaction system is improved and the reaction is thereby accelerated, as shown in eqs 1 and 2. From the viewpoint of grain growth kinetics, the successive appearances of titanate nanotubes and nanowires are two kinetic products of the hydrothermal reaction,17 and the growth kinetics of the titanate products can be stated as the following equation:21 n n Dwire - Dtube ) k0tme(-Ea/RT)

(3)

where Dwire and Dtube are the average particle sizes of titanate nanowires and nanotubes, respectively, t is the time needed for the transformation of nanotubes into nanowires at a selected temperature T, m is the time exponent, n is the growth exponent, Ea is the activation energy, R is the gas constant, and k0 is a preexponential constant. It was found that the morphology of nanotubes (as well as nanowires) depends slightly on the synthesis conditions (e.g., T and CNaOH). That means all nanotubes, regardless of which synthesis temperature/CNaOH is used, show the same morphology, i.e., a length of several hundred nanometers, an outer diameter of about 7-10 nm, an inner diameter of about 4 nm, and an SBET value of about 250 m2/g. Similarly, all of the nanowires synthesized under different conditions have similar average widths of about 100 nm, lengths of several tens of micrometers, and SBET values of about 15 m2/g. In other words, the values of

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Figure 3. Dependence of dissolved Ti(IV) on (a) concentration of NaOH and (b, c) treatment duration. The gray area in (b) and (c) represents the time frame for the transformation from nanotubes into nanowires.

Dwire and Dtube can be assumed to be constants. Therefore, eq 3 can be rewritten as follows: n n k0tme(-Ea/RT) ) Dwire - Dtube ) const

(4)

or

ln t )

C0 Ea 1 + m mR T

(

(

C0 ) ln

n n Dwire - Dtube k0

))

(5)

Equation 5 suggests that the time needed for the transformation of nanotubes into nanowires decreases as the temperature increases. From comparison among eqs 1, 2, and 5, it was found that the value of Ea decreases obviously (Ea(12 M)/Ea(8 M) ) 0.92) while m remains almost unchanged (m12 M/m8 M ) 1.02) as CNaOH increases from 8 to 12 M. The decrease of Ea leads to a smaller t at a set temperature (as shown in eq 5 and Figure 2); i.e., less hydrothermal duration is needed for the transformation of nanotubes into nanowires as the concentration of NaOH increases. This acceleration of reaction with increasing concentration of NaOH may be affiliated with the mass transfer capability. Figure 3a shows the relationship between the concentration of the dissolved Ti(IV) (CTi(IV)) in alkali solution and the concentration of NaOH. It was found that CTi(IV) increases as CNaOH is increased and can be approximated by a linear fitting stated as ln CTi(IV) ) A + BCNaOH. A higher concentration of dissolved Ti(IV) means an improved mass transfer capability. Consequently, the reaction was accelerated with increasing concentration of NaOH. Figure 3b,c displays the dependence of the concentration of dissolved Ti(IV) on the treatment duration. CTi(IV) reaches a maximum value while the system is undergoing the nanotubenanowire transition. This increase in CTi(IV) may be related to the Ostwald ripening (OR). During the transformation of nanotubes into nanowires, part of the small nanotubes may redissolve and act as fuel for the growth of bigger nanowires.22

Figure 4. TEM images of hydrothermal products obtained by treating TiO2 in 20 M NaOH solution at (a) 150 °C for 12 h and (b) 210 °C for 144 h.

The dissolution of small crystals leads to the increase of CTi(IV). After the transformation, the crystallinity of the titanate is improved and the surface area is decreased.17 Consequently, CTi(IV) decreases with increasing hydrothermal duration. As discussed above, the formation of both nanotubes and nanowires can be accelerated by increasing alkaline concentration. However, the product will become amorphous titanate spherical particles if CNaOH is further increased to a extreme high value, such as 20 M, as shown in Figure 4 and Figure S2 in the Supporting Information. Figure 4 and Figure S2 in the Supporting Information show TEM images of the hydrothermal products obtained by treating TiO2 in 20 M NaOH solution at different temperatures for different durations, in which only amorphous particles can be observed. The formation of amorphous particles is thus suggested to be determined only by the concentration of NaOH, and independent of the temperature or duration. Additionally, it was found that the BET specific surface area (SBET) of the amorphous titanate reaches 400 m2/g, which is far higher than that of titanate nanowires (10-30 m2/g), and even higher than that of titanate nanotubes (about 250 m2/g). The reason for the formation of amorphous titanate is suggested to be associated with the concentration of dissolved Ti(IV) and the viscosity of the solution. Apparently, the solution will be very viscous if the NaOH concentation is extremely high. Simultaneously, CTi(IV) increases greatly with increased CNaOH, as shown in Figure 3a. The concentration of Ti(IV) is increased from 7.52 × 10-6 to 3.75 × 10-4 M, while CNaOH increases

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Figure 5. XRD patterns of titanate (a) nanotubes and (b) nanowires. The standard diffraction pattern of H2Ti2O5 · H2O from JCPDS (No. 47-0124) is provided at the bottom of (a), and the diffraction pattern of NaTi2O4(OH) modeled by Peng et al.24 is provided at the bottom of (b).

Figure 6. FTIR spectra of titanate nanotubes, titanate nanowires, and amorphous titanate.

from 2 to 15 M after the hydrothermal treatment at 180 °C for 24 h. As a result, a great deal of Ti(IV) ion exists in the viscous solution during the hydrothermal treatment. In this case, the nucleation density should be very high, and the diffusion of the atoms is restricted. Accordingly, the formation of amorphous particles is inevitable. 3.2. Composition and Structure Determination. The XRD patterns of nanotubes and nanowires were comparatively studied. Figure 5a shows a representative XRD pattern of nanotubes, and the standard diffraction pattern of H2Ti2O5 · H2O, which is identical to the protonic titanate H2Ti2O4(OH)2,23 is provided at the bottom of the figure. The observed diffraction peaks at 2θ ) 9.8°, 23.9°, 28.0°, 48.0°, and 61.9° correspond well with those of lattice planes (200), (110), (310), (020), and (002) of orthorhombic H2Ti2O5 · H2O (H2Ti2O4(OH)2). This indicates that the nanotube structure might be assigned to layered A2Ti2O4(OH)2 (A ) Na and/or H). Figure 5b shows a representative XRD pattern of nanowires. The crystal structure of nanowires is quite different from that of nanotubes. More diffraction peaks are observed, and the intensity of the peaks increases. The XRD pattern of the nanowires is in remarkable agreement with that of monoclinic NaTi2O4(OH).24 ICP detection indicates that the mole ratio of Na to Ti (Na/Ti) is 0.47 for nanowires, which further confirms the above result. The comparative study of the XRD patterns of nanotubes and nanowires suggests that the morphological transformation from nanotubes into nanowires is accompanied by the improvement in crystallinity and structural transformation from orthorhombic A2Ti2O4(OH)2 into monoclinic NaTi2O4(OH). The surface chemistry of the crystal nanotubes, nanowires, and amorphous titanate was examined by FTIR spectroscopy, as shown in Figure 6. The band located at about 1630 cm-1 is associated with the H-O-H deformation mode.25 The band at about 900 cm-1 is attributed to the four coordinated Ti-O stretching modes that involving nonbridging oxygen atoms interacting with Na+.19,25 The band at about 680 cm-1 is related

to the vibrations of bridging oxygen atoms coordinated with Na ions polymerizing the Ti-O-Na clusters.26 The band at about 480 cm-1 can be attributed to the Ti-O-Ti vibrations of the interconnected octahedra that are the rigid units.25 Therefore, it is deduced from the FTIR spectra that these titanate nanostructures have the following chemical bonds: Ti-O-Ti, Ti-O-Na, surface Ti-OH, and Ti-O terminal bonds. However, in the 1000-400 cm-1 spectral region, it was found that the intensity of the vibrations decreases with the decrease of crystallinity. The vibrations at about 900, 680, and 470 cm-1 are very weak for the amorphous titanate particles. This may be caused by the specific structural unit and the molecular arrangement within the clusters of the amorphous materials.27 The structural information is easily lost, because of the adding up of the interatomic distance information of different clusters and the intercluster boundaries. Protonated titanate can be obtained by extracting the interlayered Na+ ions in sodium titanate with diluted hydrochloric acid or other inorganic acid aqueous solution.28-30 TiO2 can be otained by the calcination of protonated titanate.31 The overall reactions can be explained as follows:

NaxTiyO2y(OH)x · nH2O + xH+ f HxTiyO2y(OH)x · nH2O + xNa+

(6)

heat at T1

HxTiyO2y(OH)x · nH2O f HxTiyO2y(OH)x + nH2O

(7) heat at T2

HxTiyO2y(OH)x f yTiO2 + xH2O

(8)

where the first step (6) represents the protonated process. The second step (7) is the desorption of absorbed water by calcining the protonated titanate at temperature T1, and the third step (8) corresponds to the decomposition of the titanate into titania at a higher temperature T2 (T2 > T1). By the measurement of the weight loss under different temperatures, we can obtain the values of x, y, and n, and thereby the chemical composition of the sodium titanate. The dehydration processes of the protonated titanate nanotubes, nanowires, and amorphous particles were studied by thermogravimetric analysis (TGA). The dehydration process can be divided into two stages for all three samples, according to the TGA traces, as shown in Figure 7. Below 180 °C, the weight loss is mainly attributed to the desorption of adsorbed water (see chemical reaction eq 7). The weight losses in this step are 16.8%, 17.7%, and 7.3% for amorphous titanate, titanate

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Figure 7. TGA traces of the titanate nanotubes, titanate nanowires, and amorphous titanate.

Figure 8. XRD patterns of samples obtained by the calcination of (a) amorphous titanate, (b) titanate nanotubes, and (c) titanate nanowires at 500 °C for 2 h, or (d) nanowires at 600 °C for 2 h.

nanotubes, and titanate nanowires, respectively. Above 180 °C (180-550 °C), the weight loss is due to the decomposition of the titanate into titania and water (see chemical reaction eq 8). The weight losses in this step are 7.6%, 7.8%, and 9.3% for amorphous titanate, titanate nanotubes, and titanate nanowires, respectively. On the basis of the mass loss detection, the chemical compositions of the protonated samples can be described as H2Ti2.27O5.54 · 2.2H2O for amorphous titanate, H2Ti2.15O5.3 · 2.3H2O for titanate nanotubes, and H2Ti2.02O5.04 · 0.8H2O for titanate nanowires. Besides, ICP detection indicates that the mole ratio of Ti to Na (Ti/Na) is 1.06 for amorphous titanate, which is consistent with the TGA data. Combining the XRD, TGA, and ICP data, it is proposed that the chemical compositions of these hydrothermal products are Na2Ti2O5 · xH2O for amorphous titanate, Na2Ti2O5 · nH2O for titanate nanotubes, and NaTi2O4(OH) · yH2O for titanate nanowires. According to the TGA data (Figure 7), the structure transformation from protonated titanate into crystal titania was completed at about 480 °C. This phase evolution was examined by XRD detection. The three protonated titanate nanomaterials were calcined at 500 °C for 2 h, and the XRD pattern is shown in Figure 8. Both amorphous titanate particles and orthorhombic H2Ti2O5 nanotubes transform into the pure phase of anatase titania, while the monoclinic HTi2O4(OH) nanowire transforms into monoclinic TiO2(B) (at 500 °C) and thereafter anatase (at 600 °C). The XRD results suggest that Na+ ions had been substituted successfully and fully by H+ before the thermogravimetric analysis, and thus imply the accuracy of the above composition analysis based on the TGA results. Moreover, the transformation of titanate into TiO2(B) may further demonstrate the reliability of the proposed crystal structure of NaTi2O4(OH) for nanowires, due to the same monoclinic structure. Additionally, owing to its relatively open structure and low density, TiO2(B) shows advantages over other TiO2 polymorphs and has been considered as an excellent host for Li intercalation. Therefore, it is promised that the obtained single-phase TiO2(B)

Huang et al. nanowires may display simultaneously Li ion transport and electronic conductivity and the level of each can be controlled accurately by the control of the degree of intercalation.6d 3.3. Transformation between Nanowires/Nanotubes and Amorphous Particles. As mentioned above, the hydrothermal treatment of titania in moderate NaOH solution (e.g., CNaOH ) 5-12 M) results in the formation of titanate nanotubes/ nanowires, while only amorphous titanate particles can be obtained if the concentration of NaOH is ultrahigh (e.g., CNaOH ) 20 M; see Figure 4 and Figure S2 in the Supporting Information). In this section, we will further investigate the relationship between crystal nanotubes/nanowires and amorphous particles. Crystal titanate nanotubes and/or nanowires, instead of TiO2 powders, were put into the NaOH solution (CNaOH ) 20 M) and then hydrothermally treated at 150 °C for different durations. The TEM images of the resultant products are shown in Figure 9. It was found that both titanate nanowires and nanotubes were transformed into amorphous particles after hydrothermal treatment in 20 M NaOH solution (Figure 9c,d). Figure 9a-c displays the transformation from crystal nanowires into amorphous particles. Most of the nanowires had transformed into amorphous particles on the fourth day (Figure 9a), and the transformation was completed by increasing the treatment duration to 6 days (Figure 9c). Figure 9b shows the TEM image of a remnant nanowire which is undergoing fracture. The HRTEM image (bottom inset in Figure 9b) and the SAED pattern (top inset in Figure 9b) of this nanowire clearly show that the core remained still crystalline while the surface had been transformed into amorphous phase. Apparently, the transformation from nanowires into small amorphous particles is conducted by the eroding of NaOH. The nanowires were surrounded by NaOH during the hydrothermal treatment. The surface was eroded and transformed into amorphous particles, and the internal area was then exposed to the NaOH solution. As a result, the whole nanowire was eroded by NaOH gradually and should fully transform into amorphous particles finally. The formation mechanism of amorphous particles from titanate nanowires is similar to that from crystal titania. Generally, the chemical reaction equations for the formation of titanate nanotubes/nanowires can be explained as follows:

2TiO2 + 2NaOH f 2Na+ + Ti2O52- + H2O

(9)

2Na+ + Ti2O52- + H2O S Na2Ti2O4(OH)2

(10)

Na2Ti2O4(OH)2 f NaTi2O4(OH) + NaOH

(11)

NaTi2O4(OH) S Na+ + Ti2O4(OH)-

(12)

where the first step (9) represents the dissolution of titania precursor into Ti(IV), probably in the form Ti2O52-. The second step (10) represents the equilibrium of dissolution-crystallization for dissolved Ti(IV) and solid titanate nanotubes (and nanosheets). The third step (11) corresponds to the transformation of nanotubes into nanowires, while the fourth step (12) represents the dissolution-crystallization equilibrium between solid nanowires and dissolved Ti(IV). In a moderate alkaline solution, the dissolved Ti(IV) recrystallized into solid titanate nanosheets and thereafter nanotubes (formula 10) and, finally, nanowires (formula 11). However, the concentration of dissolved Ti(IV) in the solution increases

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Figure 9. TEM images of hydrothermal products obtained by treating titanate nanowires in 20 M NaOH solution at 150 °C for (a, b) 4 days and (c) 6 days (c) or (d) titanate nanotubes in 20 M NaOH solution for 1 h. Insets are HRTEM images (a, b) and SAED patterns (b-d) of each.

4. Summary

Figure 10. TEM images of hydrothermal products obtained by treating amorphous titanate particles in 8 M NaOH solution at (a) 150 or (b) 200 °C for 3 days. Insets are (a) HRTEM image and (b) SAED pattern.

In conclusion, various titanate nanostructures were tailored accurately by controlling the reaction conditions. Crystal titanate nanotubes and nanowires are formed under moderate NaOH condition (5-12 M) and the reaction for the formation of both nanotubes and nanowires is accelerated by the increase of temperature or the concentration of NaOH, whereas higher alkali concentration (CNaOH > 15 M) promotes the formation of amorphous titanate particles. Further study shows that titanate nanotubes/nanowires will transform into amorphous titanate particles if they are treated with high concentration of NaOH solution. Titanate nanotubes/nanowires are obtained by treating amorphous particles in moderate NaOH solution. The reaction of titania with NaOH results in the formation of various titanate nanostructures, while the latter will transform into titania after acid treatment and then calcination. We expect that these investigations will be helpful for the further understanding of various titanate nanomaterials, and the accurate tailoring of these nanostructures will be beneficial to their particular uses in different domains.

Figure 11. Schematic drawing depicting the transformation relations between amorphous titanate particles, titanate nanotubes/nanowires, and titania.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (20901079) and the National Basic Research Program (2007CB936700).

rapidly with increasing CNaOH, as shown in Figure 3a. The high CTi(IV) implies that crystal titanate is apt to redissolve into the viscous solution with extremely high CNaOH. This leads to abundant dissolved Ti(IV) and results in the formation of amorphous particles. The treatment of titanate nanotubes/nanowires in NaOH with high concentration leads to the formation of amorphous titanate. Contrarily, the hydrothermal treatment of amorphous titanate in moderate NaOH solution (5 M < CNaOH < 12 M) results in the formation of titanate nanotubes/nanowires, as shown in Figure 10. Nanotubes were obtained by treating amorphous titanate particles in 8 M NaOH solution at 150 °C for 3 days (Figure 10a), while nanowires were obtained at a higher temperature of 200 °C after the same treatment duration (Figure 10b). The formation of nanotubes/nanowires under moderate alkali condition implies that the amorphous titanate can dissolve into Ti(IV) and then recrystallize into solid titanate crystal. A schematic drawing depicting the evolution between crystal titanate nanotubes/nanowires and amorphous titanate particles is shown in Figure 11. In addition, further study implies that the morphology of the crystal product (nanotubes or nanowires) obtained by the hydrothermal treatment of amorphous titanate is determined by the concentration of NaOH and temperature. We will discuss the dependence of the products on the hydrothermal condition in another article, and part of the results are shown in Figure S3 (see the Supporting Information).

Supporting Information Available: Figure S1 showing TEM images of products obtained by hydrothermal treatment of TiO2 in 8 M and 12 M NaOH solutions at 240 °C; Figure S2 showing TEM images of hydrothermal products obtained by treating TiO2 in 20 M NaOH solution at 120 °C; Figure S3 showing TEM images of products obtained by hydrothermal treatment of amorphous titanate particles in NaOH solutions at 160 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wu, D.; Liu, J.; Zhao, X. N.; Li, A. D.; Chen, Y. F.; Min, N. B. Chem. Mater. 2006, 18, 547. (b) Ma, R. Z.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115. (2) (a) Lin, Z. H.; Yang, Z. S.; Chang, H. T. Cryst. Growth Des. 2008, 8, 351. (b) Ren, Z. H.; Xu, G.; Liu, Y.; Wei, X.; Zhu, Y. H.; Zhang, X. B.; Lv, G. L.; Wang, Y. W.; Zeng, Y. W.; Du, P. Y.; Weng, W. J.; Shen, G.; Jiang, J. Z.; Han, G. R. J. Am. Chem. Soc. 2010, 132, 5572. (c) Sun, X. H.; Lam, S.; Sham, T. K.; Heigl, F.; Jurgensen, A.; Wong, N. B. J. Phys. Chem. B 2005, 109, 3120. (3) (a) Shi, L.; Xu, Y. M.; Li, Q. Cryst. Growth Des. 2009, 9, 2214. (b) Shi, L.; Xu, Y. M.; Li, Q. J. Phys. Chem. C 2009, 113, 1795. (4) Ijima, S. Nature 1991, 56, 354. (5) (a) Elias, J.; Levy-Clement, C.; Bechelany, M.; Michler, J.; Wang, G. W.; Wang, Z.; Philippe, L. AdV. Mater. 2010, 22, 1. (b) Vayssieres, L.; Graetzel, M. Angew. Chem., Int. Ed. 2004, 43, 3666. (6) (a) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (b) Viana, B. C.; Ferreira, O. P.; Filho, A. G. S.; Filho, J. M.; Alves, O. L. J. Braz. Chem. Soc. 2009, 20, 167. (c) Huang, J. Q.; Huang, Z.; Guo, W.; Wang, M. L.; Cao, Y. G.; Hong, M. C. Cryst. Growth Des. 2008, 8, 2444. (d) Armstrong, A. R.; Armstrong, G.; Canales,

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