Characteristics of Titanate Nanotube and the States of the Confined

Oct 30, 2008 - Dmitry V. Bavykin , Marina Carravetta , Alexander N. Kulak and Frank C. Walsh. Chemistry of Materials 2010 22 (8), 2458-2465. Abstract ...
0 downloads 0 Views 516KB Size
Download PDF
18474

J. Phys. Chem. C 2008, 112, 18474–18482

Characteristics of Titanate Nanotube and the States of the Confined Sodium Ions Junya Suetake,† Atsuko Y. Nosaka,*,† Kazunori Hodouchi,‡ Hiroshi Matsubara,‡ and Yoshio Nosaka*,† Department of Materials Science and Technology, and Analysis and Instrumentation Center, Nagaoka UniVersity of Technology, Kamitomioka, Nagaoka, 940-2188, Japan ReceiVed: August 3, 2008; ReVised Manuscript ReceiVed: September 23, 2008

The titanate nanotubes (TNTs) prepared by the reaction of titanium dioxide (TiO2) with NaOH were treated with acid (HNO3) solutions of various concentrations and/or heat treated at different temperatures. The properties of the treated TNTs were investigated in relation to the photocatalytic activities. The state and the role of Na ions confined in the TNTs were analyzed by means of 23Na and 1H NMR spectroscopy. It was revealed that there exist two states of Na ions, i.e. strongly and weakly interacting with the TNT system. The Na ions strongly bound to the interlayer of TNT play an important role to stabilize the tube structure. On the other hand, Na ions weakly interacting with the surface of the TNT suppress the photocatalytic activity but can be easily removed by a weak acid treatment with 50 mM HNO3 solution to lead the optimum photocatalytic activity. In addition, the Na ions on the surface of the TNT interfere with the crystalline transition to anatase on calcinations. Although the photocatalytic activity of TNT has been believed to be lower than that of conventional TiO2 photocatalysts of nanoparticles, it was also found that TNT showed specifically high photocatalytic activities, even higher than P25 TiO2 (Degussa P25), for some cationic reactant such as trimethylamine. 1. Introduction TiO2 nanoparticles have been widely utilized in photocatalytic or photochemical systems owing to the high stability and semiconductor characteristics to generate charge carriers such as electrons and holes by absorbing photon energies.1 TiO2based nanotubes with a highly specific surface area and ionexchangeable and photocatalytic abilities have been considered promising for extensive applications, such as Li-ion batteries,2 thin-layer electrodes,3 hydrophilic films,4 electron field emission,5 and dye-sensitized solar cells.6 Since the development of the unsophisticated hydrothermal synthesis procedure with a high yield by Kasuga et al.,7 titanate nanotubes (TNT) have attracted growing interest.8-18 The nanotube is derived from the alkaline treatment of TiO2 nanoparticles under a highly basic condition. It is believed that the nanotube structure is formed by scrolling the titanate nanosheet prepared by treating TiO2 with NaOH.12-15 The tube structure is not obtained when KOH or LiOH is used instead of NaOH.12-14 A large number of Na ions are removed in an electrodeposition process.16 Removal of Na ions from TNT by treatment with HCl solution changes the crystalline phases from titanate to anatase titania.16,17 These facts imply that Na ions must play important roles in the fabrication of the tube structure and stabilization of the crystalline phases. Photocatalyst is a useful application of TNT. To meet the practical use, various improvements have been attempted. To achieve a higher activity, hybridization of Pt,19 CdS,20 Ni,21 Ru, Pd, and Au22,23 in the inner hole of TNT has been conducted. To functionalize the TNT photocatalysts responsive to visible light, nitrogen doping24 and WO3 coating25 were employed. However, to develop TNTs with desired functions it is inevitable * Corresponding author. E-mail: [email protected]. † Department of Materials Science and Technology. ‡ Analysis and Instrumentation Center.

to elucidate the relationship among the photocatalytic activity, the confined Na ions, and the surface structure of TNT, which has not been fully understood yet. In the present study, the locations and the bindings of the Na ions in the TNT and the effect on the structure and the photocatalytic activity were investigated mainly by means of 23Na and 1H nuclear magnetic resonance (NMR) spectroscopy. To our knowledge, this is the first report employing 23Na NMR spectroscopy to analyze the characteristics of Na ions confined in TNT. It has been believed that TNT does not have a high level of photocatalytic ability as compared to TiO2 nanoparticles, as is the case for some reactants. However, we found that for the cationic reactant such as trimethylamine TNT showed specifically high photocatalytic activities, even higher than P25 TiO2 (Degussa P25). 2. Experimental Details 2.1. Materials. TNT was synthesized based on the procedure reported by Kasuga et al.7 as described in the following. A 2.5g-sample of P25 TiO2 (Nippon Aerosil, Degussa P25) was mixed with 80 mL of 10 M NaOH solution and the mixture was stirred for 10 min. The mixture was then placed in a 100 mL Teflon vessel and kept for 20 h at 150 °C. Since the present experiment was aimed at investigating the effect of the remaining Na ions, the obtained TNT/NaOH mixture was washed with a weak acid (0.1 mM HNO3) until the solution became pH 9, and then it was washed with pure water until the solution reached neutral pH and was used as “as-synthesized TNT”. The as-synthesized TNT powder was heat-treated in an alumina vessel for 1 h during which it was heated from 200 to 800 °C at the heating rate of 5 deg/min. Acid treatments were performed by dispersing TNT powders (2.8 g) in 50 mL of HNO3 aqueous solution of different concentrations (1 mM to 1 M) for 10 min to 20 h. Then the

10.1021/jp8069223 CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

States of Na Ions in Titanate Nanotubes precipitates were washed with water, and vaccuum-dried for 1 day at ambient temperature. As a standard anatase crystal, ST-01 TiO2 (Ishihara Sangyo) was used. 2.2. Characterization of Materials. TNTs prepared under different conditions were characterized as follows. The tube structure was determined by the transmission electron microscope (TEM) with a JEM-2010 (JEOL, Co.) operated at 200 kV. The crystalline phase was analyzed by X-ray diffraction (XRD) with an M03X-HF22 diffractometer (Mac Science) with Cu KR1 X-ray (154.056 pm). To determine the elements of the sample powders electron probe microanalysis (EPMA) was carried out with a WDX (wavelength dispersive X-ray spectroscopy) type EPMA (EPMA-1600, Shimadzu). Optical spectra at ultraviolet (UV) and visible (vis) regions were measured with a UV-vis-NIR spectrophotometer (UV3150, Shimadzu) equipped with an integration sphere (ISR3100) and the band gap energy of the specimen was obtained from a plot of (Rhν)1/2 as a function of hV, where the absorbance R was obtained from Kubelka-Munk transformation of the reflectance. BET surface area was measured by using a Flow Sorb 2300 (Micrometrics) with N2:He (3:7) mixture gas. Zeta potential was measured with an automatic electrophoresis analyzer (ZEECOM, Microtec, Inc.) by changing solution pH with HNO3 and KOH. 23Na NMR spectra of TNT and TNT suspension were measured at 105.5 MHz with a JNM-AL400 high-resolution spectrometer (JEOL), using a 5 mm o.d. quartz sample tube, in which 0.1 cm3 of sample powder or suspension was placed. MAS (magic angle spinning) was not used in the experiments. Chemical shift was referred to the signal for 0.1 mL of 0.1 M NaCl aqueous solution. A total of 1000 scans were usually accumulated with a repetition time of 4 s. 2.3. Evaluation of Photocatalytic Activity. Photocatalytic activity of TNT in the gas phase was evaluated with the decomposition of acetaldehyde. TNT powders (0.13 g) were placed in a 5-L polyvinyl fluoride bag, in which 3000 ppmv (parts per million in volume) of acetaldehyde was enclosed with saturated water vapor. The bag was UV irradiated with a 10-W black light (BLB, 1 mW/cm2). Acetaldehyde in the gas phase was analyzed with a gas chromatograph (GC-8AIT, Shimadzu) equipped with an APS-201 T60/80 column (20% flusin) with He as a carrier gas. On the other hand, photocatalytic activities of TNT in aqueous solution were evaluated with the decompositions of acetaldehyde and trimethylamine. The photocatalytic reaction was carried out in an NMR tube of 5-mm diameter. TNT powders of 5 mg were dispersed in 0.5 mL of D2O (99.9% Isotec Inc.) solution of 10 mM (M ) mol/dm3) reactant. 1H NMR spectra were measured with a JEOL EX-400 spectrometer at 400 MHz. The amount of the reactant adsorbed on the TNT surface was estimated from the difference of NMR signal intensities measured before and 12 h after the addition of TNT. The sample was photoirradiated with three 4-W black light bulbs surrounding the NMR sample tube under aerobic condition. The sample tube was rotated during the irradiation to maintain the powder suspension. The incident light with the wavelength range of 320-380 nm was about 1 mW/cm2 for each light bulb. To measure the concentration change of the reactant, the NMR spectra were recorded at intermissions in the total irradiation time of 2 h. The concentration of the reactant in the solution was estimated by taking the relative peak area to that of the external standard of DSS (2, 2-dimethyl-2-silapentane-5-sulfonate sodium salt) in a glass capillary.26,27

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18475

Figure 1. TEM pictures of as-synthesized TNT.

3. Results and Discussion 3.1. Characterization of As-Synthesized TNT. Figure 1 shows the TEM pictures of the as-synthesized TNT. The tube structures of 10 nm in outer diameter, 6 nm in inner diameter, and hundreds nanometers in length are observed. The layer structure of mostly 3-fold can be recognized. No particulate structure of the TiO2 precursor was observed in every view field, indicating that the whole TiO2 was converted to TNT by hydrothermal synthesis. The yield of TNT from 2.50 g of TiO2 was 2.82 g after vacuum drying, due to the involvement of Na atoms and hydrated water in TNT. EPMA analysis showed that the as-synthesized TNT contains Na atoms by about 1 wt %. UV-vis spectra were measured to determine the band gap energy (Eg) of the TNT prepared. From the plot of (Rhν)1/2 vs hν, the Eg of TNT was calculated to be 3.40 eV (365 nm), which is consistent with the reported value.24 As a reference, the Eg of anatase TiO2 (ST-01) was measured to be 3.25 eV (385 nm). Figure 2 shows the XRD patterns of the as-synthesized TNT. The XRD pattern of the as-synthesized TNT (Figure 2a) agrees well with that appearing in references,28 where the indexes were determined based on the calculation for a scrolled titanate nanosheet (H2Ti3O7). Since the XRD pattern is similar to that of H2Ti2O5 · H2O (Figure 2a′) of ICDD No.00-47-0124, the formation of titanate structure was suggested.29 3.2. The Effect of Heat Treatment. As shown in Figure 2a, the XRD patterns of the as-synthesized TNT did not contain the peak of anatase nor rutile as indicated also by the TEM observations. Curves b-f in Figure 2 show the change of the XRD patterns on heat treatments of the as-synthesized TNT up to 800 °C. No significant change in the XRD patterns up to 400 °C (Figure 2a-c) was observed, indicating that the nanotube structure held up to 400 °C. It is notable that the peak around

18476 J. Phys. Chem. C, Vol. 112, No. 47, 2008

Suetake et al.

Figure 2. XRD patterns of (a) as-synthesized TNT, calcined at (b) 200, (c) 400, (d) 500, (e) 600, and (f) 800 °C for 1 h. T and A stand for the peak of titanate and anatase, respectively. Bars in (a′) are the reported patterns for H2Ti2O5 · H2O.

10°, which is attributed to the diffraction between titanate layers, shifted slightly to a higher value with an increase of temperature. After detailed analysis, the 2θ values were obtained for (a) 9.6°, (b) 10.1°, and (c) 10.6°, which correspond to the interlayer distances of 0.92, 0.88, and 0.83 nm, respectively. This would mean that the tube structure shrank with increasing calcination temperature, probably due to the vaporization of the water molecules confined in the as-synthesized TNT. The XRD pattern changed significantly at 500 °C (Figure 2d), resulting from the crystalline phase transition from titanate to anatase.29,30 Although the crystalline phase was mostly anatase at this temperature, some peaks of titanate still remained at about 11° and 30°. On the TEM image of the as-synthesized TNT calcined at 500 °C in Figure 3A the crystallites of two different shapes could be recognized. One is the well-crystallized rectangular crystal of about 30 nm whose interplaner space is 0.35 nm as shown in Figure 3B, which agrees well with that of 0.352 nm of the anatase (101) plane. Another is the rod-like particles with 7-nm width and about 100 nm in length, which might correspond to the transitional form from the tube structure to the anatase crystallite. When as-synthesized TNT was calcined at 600 °C, and at 800 °C, the XRD pattern became extremely complicated (Figure 2e,f), indicating the drastic change in the form, which has not been analyzed yet. In the literature, the heat treatment of NaTNT usually results in transformation of nanotubes to Na2Ti6O13 nanorod at 600 °C.31-33 Figure 4 shows the changes of the BET surface area of TNT and P25 TiO2 with increasing the calcination temperatures. Before calcinations, the surface area of the as-synthesized TNT was 210 m2/g, while that of P25 TiO2 having a diameter of about 20 nm was 49 m2/g. The large surface area of the assynthesized TNT is comparable to 300 m2/g of commercially available anatase nanocrystallite (ST-01 TiO2) with a diameter of 9 nm. The surface area of TNT decreased with an increase of calcination temperature. At 500 °C, where the crystalline phase transition from titanate to anatase occurs, a substantial decrease in the surface area was observed. And at 600 °C the surface area became almost the same as that of P25 TiO2, whose surface area did not change within the temperature range

Figure 3. TEM pictures of TNT calcined at 500 °C for 1 h. Panel B is the enlargement of the marked part in panel A.

Figure 4. Changes in the BET surface area for the as-synthesized TNT and P25 TiO2 on calcinations at various temperatures.

measured (25-600 °C). This would suggest the collapse of the tube structure of TNT. Thus the large difference in the surface area before calcinations for TNT and P25 would be attributed to the tube structure of TNT. The surface area for both TNT and P25 decreased on further calcinations at higher temperatures (800-1000 °C), indicating the growth of the crystallites. 3.3. The Effect of Acid Treatment. Since TNT is formed by rolling the titanate nanosheet having negative charges with Na ions, the neutralization process of TNT/NaOH suspension is considered to affect the structure of the TNT. Curves a-d in Figure 5 show the XRD patterns of TNT after washing with HNO3 solution of different concentrations. After treatment with HNO3 of low concentration (10 mM), the XRD pattern of TNT (Figure 5b) is almost the same as that of as-synthesized TNT (Figure 5a), indicating that the tube structure holds. After treatment with HNO3 of higher concentration (100 mM), distinct

States of Na Ions in Titanate Nanotubes

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18477

Figure 5. XRD patterns of TNT treated with HNO3 of different concentrations and calcined at various temperatures after treatment with 100 mM HNO3: (a) before HNO3 treatment with face index for comparison; treatment with (b) 10, (c) 100, and (d) 1 M HNO3; calcined at (e) 300, (f) 400, and (g) 500 °C after treatment with 100 mM HNO3. Peaks marked with A stand for anatase.

changes in the XRD pattern were observed (Figure 5c). Peaks (211) and (113) characteristic of the tube structure disappeared and the (200) peak became broad. With treatment with a higher concentration of HNO3 (1 M), the XRD pattern remained almost unchanged as shown in Figure 5d. After the 100 mM HNO3 treated TNT was heated above 400 °C, peaks (110) and (020) characteristic of the tube structure (Figure 5a) were replaced by peaks of (101) and (200) characteristic of the anatase structure (Figure 5f,g), respectively. A crystalline phase of nanotubular TiO2(B) has been reported between the TNT and anatase structure.35 After treatment at as high as 800 °C, the anatase structure was still the main crystalline phase in the XRD patterns, which is consistent with the report by Tsai and Teng.36 These observations clearly indicate that treatment with the acid of higher concentration induces the change of the crystalline structure of TNT as was observed for the heat treatment above 500 °C. After treatment with HNO3 of the higher concentration (1 M), the (200) peak of TNT almost disappeared (Figure 5d), suggesting that further structural change should take place. A similar change in the crystalline phases has been recently reported for the TNT washed with HCl solutions of different pH,17 where the titanate structure observed at pH 6.3 changed to the anatase nanotube at pH 1.6. Hydrothermal treatments (at 175 °C) of TNT at various pH values were also reported to fabricate a single crystalline anatase,29 where the titanate structure held at pH 4.0, but at a lower pH of 2.2 the anatase nanorod was formed. Thus the reported observations36 and the present results strongly suggest that the removal of Na ions from TNT should affect the conversion of the tube structure to anatase crystallite. HCl often has been used for exchanging Na+ for proton. Clshowed a similar effect on the acid treatment to that of NO3-, as compiled by Tsai and Teng.16 H2SO4 also would be applicable for the acid treatment.34 Thus the present findings in the acid treatment likely hold for every kind of acid, such as HCl and H2SO4. The interlayer spaces of TNT must be highly anionic since a high concentration of alkaline should be used to prepare TNT. Because of the electrostatic repulsion, these anions, even Cl-, would not penetrate into the interlayer of the tube.

Figure 6. (A) 23Na NMR spectra for the TNT treated with HNO3 solution of various concentrations. As-synthesized TNT (dot-dash line), and TNT treated with 10 mM (broken line), 100 mM (dot line), and 1 M (solid line) HNO3. (B) 23Na NMR spectra of the as-synthesized TNT powders suspended in 10 mM HNO3 aqueous solution. (C) Change in the 23Na NMR spectra after storing in the ambient condition in the dark. The broken lines are the spectra of the TNT measured immediately after the treatment with 10 and 100 mM HNO3 (same as broken and dot lines in panel A). Solid lines are those measured for TNT powders treated with 10 and 100 mM HNO3 after long-term storage (>10 days). (For 100 mM treatment, the broken and solid lines are overlapped due to the lack of change.)

3.4. NMR Analyses of TNT. To investigate the state and the effect of Na ions confined in the TNT, 23Na NMR spectra of TNT powders were measured. The 23Na NMR spectrum of the as-synthesized TNT presented a single broad line as shown in Figure 6A. On treatment with 10 mM HNO3 the signal intensity was substantially reduced due to the removal of Na ions and concomitantly the peak was significantly broadened and shifted upfield. With an increase of the concentration of HNO3, the signal intensities decreased while the chemical shift was unchanged, and after treatment with 1 M HNO3 the signal was barely observed. This observation indicated that on treatment with 1 M HNO3, almost all the Na ions observable with NMR are removed from TNT, where the structural change took place as observed by XRD (Figure 5c). Since the TNT treated with 100 mM HNO3 for 10 min showed a similar spectrum to that treated for 20 h, all the HNO3 treatments were performed for 10 min. On the analysis of the line shape of the as-synthesized TNT, the apparent single signal was found to consist of two components: a sharp peak at -2 ppm and a broad signal at -10 ppm with a line width at half-peak height of 800 Hz and 3 kHz, respectively. The sharp peak was suppressed and the broad peak remained in the system after treatment with 10 mM HNO3. This

18478 J. Phys. Chem. C, Vol. 112, No. 47, 2008 fact means that the sharp peak corresponds to Na ions which interact weakly with the TNT and are easily suppressed, while the broad peak corresponds to the Na ions strongly associated with TNT. Since the tube structure is lost with the disappearance of the broad peak after treatment with 1 M HNO3, the latter Na ions must play a key role to stabilize the tube structure. Thus Na ions of the sharp signal may bind weakly to the surface of TNT with high mobility. On the other hand, Na ions of the broad peak would strongly bind to TNT, most probably being incorporated into the interlayers of TNT with restricted mobility. To understand the state of the Na ions in TNT more precisely, 23Na NMR spectra were measured for TNT in aqueous suspension where the volume of the suspension was the same as the volume of powders. Figure 6B shows a 23Na NMR spectrum of the as-synthesized TNT suspended in 10 mM HNO3 solution. Upon line shape analysis, the signal was found to consist of three components. In addition to the broad and sharp components observed for as-synthesized TNT in powder form, an additional sharper peak was recognized at 0 ppm, which is attributable to the free Na ions in aqueous solution. This observation means that the Na ions bound weakly to the surface of TNT exchange slowly with the free Na ions dissolved from TNT in solution. Such a reversible Na+-H+ exchange within the layered titanate nanostructure was also suggested by Riss et al.37 Since the spectral feature did not change for a week, the equilibrate state would be established rapidly. The chemical shift and the line width of the other broad and sharp peaks are the same as those for the as-synthesized TNT powders and could be attributed to Na ions bound to TNT, most probably at the outside and the interlayer space. In general, water molecules are adsorbed on the solid surface of TiO2. Therefore, the Na ions of the sharp peak at -2 ppm observed for the as-prepared TNT powder would be attributed to the Na ions in the equilibrated state of the exchange of the Na ions interacting at the outside of TNT with those in the physisorbed water layers on the TNT surface. Actually, when the physisorbed water is eliminated by vacuum drying from the 10 mM HNO3 treated TNT powders, only the broad peak was observed immediately after vacuum drying (for 1 day at room temperature) as shown by the dotted line in Figure 6C. When it was stored in ambient air for 10 days, the relatively sharp Na peak (solid line) appeared as the water molecules in the air were readsorbed on the TNT surface as reported for TiO2 nanocrystallites.38 On the other hand, for the TNT treated with 100 mM HNO3, the spectral feature did not change and only the broad peak was observed even after storage for 1 month. The recovery of the physisorbed water layers was observed for these two TNT specimens by 1H NMR spectroscopy as shown in Figure 7. 1H NMR signals of water molecules adsorbed on the HNO3 treated TNT powders which were measured immediately after drying the powders were represented as dotted lines, while those with solid lines were measured after storing for 8 months under ambient condition. The increase in the 1H NMR intensities indicates that the water molecules were equally readsorbed on the surface of both TNTs as equilibrated with the water vapor in the air. The low field shift of the water signal for the 100 mM treated TNT compared to that for the 10 mM treated TNT would imply that the more acidic surface of TNT was formed after treatment with the higher concentration of HNO3. These observations obviously indicate that the Na ions weakly bound at the surface of the TNT exchange with free Na ions in the water phase rapidly in the NMR time scale, but the Na ions in the interlayers do not undergo such exchange because

Suetake et al.

Figure 7. 1H NMR spectra showing the recovery of adsorbed water on TNT. The 1H NMR spectra of TNT treated with 10 (black lines) and 100 mM (gray lines) HNO3. The spectra immediately after vacuum drying and those after storage of 8 months under the ambient condition are represented with dot and solid lines, respectively.

such Na ions are firmly incorporated in the system. On treatment with 100 mM HNO3, almost all the loosely bound Na ions would be removed. The tightly bound Na ions would play a key role in stabilizing the tube structure. 3.5. The Effect of Heat Treatment of the Acid-Treated TNT. To make the role of the Na ions in the phase transition more clear, the effect of heat treatments of the HNO3-treated TNT on the phase transition was investigated. As stated above, the XRD pattern of the TNT treated with 100 mM HNO3 indicated that a phase transition proceeds to some extent. Although on calcinations of the TNT treated with 100 mM HNO3 at 300 °C no change was observed (Figure 5e), the XRD pattern clearly presented the peaks characteristic of anatase at 400 °C (Figure 5e) and the patterns became more clear at 500 °C (Figure 5f). Thus after pretreatment with HNO3, the thermal transition from nanotube to anatase occurred between 300 and 400 °C. Taking into account that the transition did not take place for the as-synthesized TNT at 400 °C (Figure 2c) but at 500 °C (Figure 2c), it seems likely that the removal of Na ions makes the transition more feasible. Thus, this observation suggests that the existence of Na ions at the surface of the TNT interferes with the crystalline transition to anatase. 3.6. The Photocatalytic Activities. For the application to photocatalysts, it is prerequisite to prepare the tube structure with optimum photocatalytic activity. The above results implied that both the heat and acid treatments evidently affected the structure of TNT. To elucidate the relation of such structural changes induced by the treatments with the photocatalytic activities, the photocatalytic activities of the HNO3-treated and heat-treated TNTs were measured for the decomposition of acetaldehyde in the air. Figure 8A presents the decomposition profiles of acetaldehyde on the UV irradiation for the assynthesized TNT and P25 TiO2, which is widely used as a standard photocatalyst for comparison. The initial slope was used to determine the decomposition as a measure of photocatalytic activity. The reaction rate for the as-synthesized TNT is significantly lower than that for P25 TiO2, although the surface area is much larger than that of P25 (as-synthesized TNT: 210 m2/g; P25 TiO2: 49 m2/g). The low activity of TNT may be partly attributed to the difference in the absorption threshold for an efficient photoexcitation. Since P25 TiO2 contains rutile crystallite of Eg ) 3.0 eV (413 nm), the absorbance at the excitation wavelength of around 365 nm for BLB is larger than that of TNT, which shows the absorption threshold at 365 nm. Then, the photocatalytic activity of TNT may increase when the same amounts of photons were absorbed.

States of Na Ions in Titanate Nanotubes

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18479

Figure 9. The effects of the heat treatments on the photocatalytic activities for the as-synthesized TNT (solid bar) and the TNT treated with 100 mM HNO3 (dotted bar). A and T represent the crystalline phases of anatase and titanate of the samples, respectively.

Figure 8. (A) Change in the concentration (ppm in volume) of acetaldehyde by photocatalytic reactions for as-synthesized TNT (9) and P25 TiO2 (b). (B) The Na content measured by NMR spectroscopy and the decomposition rates of acetaldehyde are plotted as a function of the HNO3 concentrations treated. The treatment with 0 mM HNO3 stands for the as-synthesized TNT.

For the acid treatments with different concentrations of HNO3, the photocatalytic activity increased with increasing concentrations of HNO3 up to 100 mM. However, it decreased after treatment with 1 M HNO3 where the tube structure was considered to be lost as was indicated by XRD (Figure 5d). This result suggests that the treatment with a specific concentration of HNO3 between 10 mM and 1 M should provide a nanotube state with an optimum photocatalytic activity. Then the concentration dependence of photocatalytic activity was examined in more detail. Figure 8B shows the photocatalytic activity together with the amount of Na ions in TNT as a function of the treated HNO3 concentration. The amount of Na ions was estimated from the peak area of the 23Na NMR signal. With an increase of HNO3 concentration of the treatment up to about 50 mM, the photocatalytic activity increased with a drastic decrease in Na content, but at the higher concentrations the photocatalytic activity started to decrease, while the Na content continued to decrease gradually. The pH of 50 mM HNO3 solution was 2.4. Since the isoelectric point (IEP) of TNT is pH 2.4, Na ions at the surface must be removed with 50 mM HNO3, but the strength of the acid is not high enough to remove Na ions strongly associated with the interlayer of TNT. The increase of the photocatalytic activity up to 50 mM HNO3 would result from the removal of surface Na ions which suppress the photocatalytic activity. But the further removal of Na ions from TNT makes the tube structure unstable, resulting in the decrease in the photocatalytic activity. Then, the treatment with the acid solution whose pH is equal to the IEP would be suitable to prepare the stable TNT as photocatalysts with high activity. Riss et al.37 have reported recently the relation between Na content and the intensity of photoluminescence. They repeated washing TNT with 0.1 M HCl and measured the decrease of Na content with EDX (energy dispersive X-ray spectroscopy). They observed the decrease of photoluminescence intensity with

the decrease of Na/Ti ratio and suggested that it was caused by the protonation of octahedral O ions of the [TiO6] subunit of titanate. In general, the increase of photoluminescence means the elongation of the lifetime of the excited state owing to the suppression of the recombination of photoinduced electron-hole pairs. Then, the decrease in the photocatalytic activity observed from 50 mM to 100 mM HNO3 treatment in Figure 8B could be attributed to the protonation of the interlayer Ti-O unit to shorten the excited state. 3.7. The Effect of the Calcination on the Photocatalytic Activity. The effect of the calcination on the photocatalytic activity was examined for the as-synthesized TNT and the TNT treated with 100 mM HNO3. On the calcination at 500 °C where the crystalline phase changes from the tube structure (T) to anatase (A) (Figure 2d), the as-synthesized TNT showed an increase in photocatalytic activity as shown in Figure 9. The further heat treatments at 600 and 800 °C did not cause significant increase in the decomposition rates because of the decrease of the anatase phase as shown by the XRD patterns e and f in Figure 2. On the other hand, the TNT treated with 100 mM HNO3 showed significant increase in the activity on the calcinations at 400 and 500 °C, where it became anatase as shown by curves f and g in Figure 5. These TNT showed a high decomposition rate, which is close to that for P25 TiO2 (437 ppm/h). This observation suggests that Na ions weakly bound to the outside of the TNT should suppress the photocatalytic activity when the crystal phase became the anatase crystallites. In the process of phase change Na ions may be introduced into the crystallites and induce crystalline defects leading to the reduction of the photoexcited state. 3.8. Photocatalytic Decomposition of Cationic Reactant in Solution. Figure 10 shows zeta potential of as-synthesized TNT as a function of pH of the solution. The surface of the TNT is acidic and negatively charged since the isoelectric point (IEP) was 2.4. In the presence of 10 mM trimethylamine (TMA), the surface negative charges are partly neutralized and the IEP became 2.6 as shown in Figure 10. This observation indicates that positively charged trimethylamine is adsorbed on the surface of the as-synthesized TNT. For comparison, we measured the IEP of P25 TiO2 particles. The IEP was 6.2 and no shift in the IEP was observed on addition of trimethylamine, indicating that trimethylamine is barely adsorbed on P25 TiO2 particles. The adsorption and the decomposition of trimethylamine were investigated by means of 1H NMR spectroscopy. The 1H NMR peak of trimethylamine was observed at 2.91 ppm from DSS as a singlet signal. The concentration of trimethylamine in D2O

18480 J. Phys. Chem. C, Vol. 112, No. 47, 2008

Suetake et al.

Figure 10. Zeta potential in 1 mM KNO3 solution of the as-synthesized TNT (0) and that adsorbed with trimethylamine (TMA) (9).

Figure 12. The amount of adsorption (open bar) and photocatalytic decomposition rate (hatched bar) of (A) acetaldehyde and (B) trimethylamine (TMA) for the TNTs treated with HNO3 and P25 TiO2.

Figure 11. (A) Photocatalytic decompositions of acetaldehyde (0) and trimethylamine (TMA) (]) with as-synthesized TNT in D2O solution represented by the change in the reactant concentration with the irradiation time of UV light. Decrease in the concentration at the irradiation time of 0 min corresponds to the adsorption of the reactant on the TNT surface. (B) Photocatalytic decomposition of trimethylamine with the TNTs treated with HNO3 and P25 TiO2: 0, as-synthesized TNT; 2, TNT treated with 50 mM HNO3 (pH 2.4); [, TNT treated with 1 M HNO3; and O, P25 TiO2.

solution was calculated from the integrated peak area of the signal by using the DSS signal as a standard. Figure 11A shows the adsorption and the decomposition of trimethylamine and acetaldehyde with 5 mg as-synthesized TNT powders suspended in 0.5 mL of D2O solution. The decrease in the concentration before the illumination corresponds to the adsorption of the reactant on the TNT surface. The amounts of reactants adsorbed per unit surface area were calculated to be 1.09 and 0.43 µmol/m2 for trimethylamine and acetaldehyde, respectively. The decomposition rate was calculated from the slope in this figure. Figure 11B shows the effect of HNO3 treatments of the assynthesized TNT on the adsorption and the photocatalytic activities. For comparison the effect was tested for the same amount of P25 TiO2. For the as-synthesized TNT, the amount of adsorption for trimethylamine was notably higher by the

factor of 15 as compared to that on the P25 TiO2. Though the surface area of P25 TiO2 is much smaller than that of TNT, the amount of adsorption per surface area (0.33 µmol/m2) for P25 TiO2 is still lower by the factor of 1/3. On acid treatment of the as-synthesized TNT with 50 mM HNO3 (pH 2.4), the adsorption was slightly increased, while with 1 M HNO3 treatment the adsorption decreased significantly. The amount of adsorption and the decomposition rate were summarized in panels A and B of Figure 12. Though the surface area of TNT was larger than that of P25 TiO2, the amount of acetaldehyde adsorbed was comparable as shown in Figure 12A. However, the decomposition rate with the TNT photocatalysts was significantly smaller than that with P25 TiO2. This observation is consistent with the decomposition in air as described above. In marked contrast to the case of acetaldehyde, the amount of adsorption and the decomposition rate of trimethylamine with the as-synthesized TNT were higher than those with P25 TiO2. The difference in the photocatalytic activities for trimethylamine would be responsible for the large difference in the adsorption ability for the cationic species. As shown in Figure 12B, the decomposition rate of trimethylamine was increased for the pH 2.4 treated TNT as compared to nontreated (as-synthesized) TNT, while the adsorption was almost the same. This observation suggests that by removing surface Na ions the surface of the TNT should change to reduce the deactivation process such as the recombination of electrons and holes. It has been generally believed that TNT does not possess a high level of photocatalytic ability as compared to TiO2 nanoparticles. Actually in the present study the photocatalytic decomposition of acetaldehyde is much lower than that of P25

States of Na Ions in Titanate Nanotubes

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18481 the TNT caused the destruction of the tube structure because of the lack of the interlayer Na ions and reduced the photocatalytic activity. Thus, the removal of the surface Na ions in TNT could enhance the photocatalytic activity as long as the tube structure holds. Consequently, we found that treatment with the solution with pH equal to the IEP showed the maximum decomposition rate. In addition, the Na ions on the surface of the TNT interfere with the crystalline transition to anatase on calcinations. It was also found that because of its negative charge, TNT showed specifically high photocatalytic activities, even higher than P25 TiO2, for some cationic reactant such as trimethylamine. Thus, the negative charge of TNT provides a potential function of the TNT photocatalysts besides the tube structure, which can take some reactants as cocatalyst into the inner hole. Acknowledgment. The authors thank Dr. Masahiro Miyauchi for the initiation of this work. This study was partially supported by a Grant-in-Aid on Priority Areas from the Ministry of Education, Culture, Science and Technology (MEXT), Japan. References and Notes

Figure 13. Plausible states of Na ions and TNT treated with HNO3 of different concentrations. (A) For the as-synthesized TNT or TNT treated with weak acid (pH >2.4), there exist two states of Na ions: one is the Na ions strongly bound to the interlayer of the TNT which stabilizes the tube structure, and another is the Na ions weakly interacting with the surface of the TNT which decrease the photocatalytic activity. (B) The treatment with HNO3 solution having the same pH as the IEP of TNT (pH 2.4) eliminates only the surface Na ions leaving the interlayer Na ions with preserving the tube structure. With this treatment the maximum photocatalytic activity is attained. (C) Further removal of Na ions from the interlayer of TNT with strong acid (pH