ARTICLE pubs.acs.org/JPCC
Hydrothermal Growth of Layered Titanate Nanosheet Arrays on Titanium Foil and Their Topotactic Transformation to Heterostructured TiO2 Photocatalysts Changhua Wang,†,‡ Xintong Zhang,*,† Yanli Zhang,† Yan Jia,† Jikai Yang,† Panpan Sun,† and Yichun Liu*,† †
Centre for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, People’s Republic of China ‡ College of Chemistry and Biology, Beihua University, Jilin 132013, People’s Republic of China
bS Supporting Information ABSTRACT: Layered titanate nanostructures have been recognized as an important intermediate in preparing TiO2 nanostructures due to their availability in various morphologies, among which ultrathin nanosheets have attracted particular attention due to their unique properties arising from their high surface area and confined thickness in nanoscale. Herein, we report a simple approach to fabricate heterostructured TiO2 nanosheet array film by (1) hydrothermal growth of sodium trititanate nanosheet array on Ti foil in alkaline solution, (2) ion exchange of Na+ in titanate naosheet with H+, and (3) topotactical transformation into TiO2 nanosheet via thermal annealing. XRD and Raman as well as HRTEM analyses proved the formation of a novel anatase/TiO2(B) heterostructured nanosheet array with coherent interface between the two phases, which exhibited superior photocatalytic efficiency under UV illumination as compared with the relevant commercial products P25 and anode oxidized TiO2 nanotube film of the same thickness but with greater mass of photocatalyst. The high photocatalytic activity of the TiO2 nanosheet array film can be ascribed to the synergistic effect of large surface area and high crystallinity as well as heterojunction between TiO2(B) and anatase phase.
1. INTRODUCTION TiO2-based nanostructures with controllable morphologies, exposed crystal facets, surface defects or disorder, and phase composition have long been extensively studied for their attractive applications in photoelectrochemistry, photocatalysis, and lithium storage.17 Recently, layered titanates intercalated with alkali metal have emerged as promising precursors to TiO2 nanostructures.8 One reason is that the layered titanate nanostructures can be available in various forms, such as nanotubes, nanowires, nanobelts, and nanosheets. The other reason is that layered titanate family and TiO2 have similar structure features with TiO6 octahedra arranged in a zigzag configuration, and can be topotactically transformed into TiO2 phase while maintaining their original morphology. More interestingly, a less common phase, TiO2(B), which is hard to synthesize via other precursors can be obtained from layered titanate precursors, and thus not only well-known anatase/rutile heterostructure but also novel TiO2(B)/anatase heterostructure can be easily obtained via one-step annealing treatment rather than a complex fabrication process.9 There have been two wet-chemical approaches frequently reported for the synthesis of layered titanates. The first approach involves the treatment of TiO2 in alkaline solution with high concentration (e.g., as high as 10 M NaOH or KOH).10,11 After the pioneering work on TiO2-based nanotubes,10 this approach r 2011 American Chemical Society
has become popular for the preparation of one-dimensional layered titanate as well as TiO2 nanostructures in forms such as nanotubes and nanofibers. The second approach involves the hydrothermal treatment of Ti metal in forms of foil, mesh, and powders, in alkaline solution with relatively low concentration (e.g., low than 1 M NaOH).12 By this approach, highly oriented TiO2 nanoarrays could be obtained on supporting substrates in contrast to randomly dispersed nanostructures by the first one. Interestingly, not only one-dimensional nanotubes or nanowires but also two-dimensional nanosheet arrays can grow on the Ti substrate by the second approach. Among various TiO2 nanostructures obtained from layered titanates precursors, vertically aligned TiO2 nanosheets may show greater potential in solar energy conversion applications due to their greater light-harvesting capability and favorable morphology for further heterostructure fabrication.13,14 However, in contrast to the numerous reports about one-dimensional TiO2 nanostructures, the number of reports about TiO2 nanosheet is still very scarce, probably due to the lack of facile preparation methods. Moreover, in the few scattered reports about TiO2 nanosheet synthesized from layered titanates precursors, Received: August 23, 2011 Revised: October 12, 2011 Published: October 17, 2011 22276
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Figure 1. SEM images of samples synthesized in NaOH solution under hydrothermal treatment for different times: (a,b) 3 h; (c,d) 6 h; (e,f) 12 h. EDX analysis of the sample synthesized in NaOH solution under hydrothermal treatment for different times: (g) 3 h; (h) 6 h; (i) 12 h.
only single phase of anatase or mixed phase of anatase and rutile is concerned. Almost no reports before have dealt with TiO2(B)/ anatase nanosheet heterostructure, although such kind of heterostructure should have promising applications in photocatalysis. On the basis of the above consideration, we report herein the synthesis of perpendicularly aligned TiO2 nanosheet array on Ti foil by a simple hydrothermal treatment of Ti foil in alkaline medium, followed by proton exchange and thermal annealing. HRTEM, XRD, and Raman analyses reveal that TiO2(B)/anatase heterojunction with coherent interface is formed in the nanosheet after calcinations at 550 °C for 2 h. The bicrystalline TiO2 nanosheets film exhibits better photocatalytic activity for rhodamine B degradation, in comparison with standard Degussa P25 nanoparticles film as well as anatase TiO2 nanotube film of the same thickness but much greater mass, mainly due to its twodimensional open structure, high specific surface area, and efficient charge separation promoted by TiO2(B)/anatase heterojunction.
2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. All reagents were of analytical grade and used without further purification. Sodium hydroxide (NaOH), hydrochloric acid (HCl), and acetone (CH3COCH3) were obtained from Beijing Chemical Reagent Co. Ti foil (99.9% purity) was obtained from Nilaco. Deionized water was used in all experiments. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku, D/max-2500 X-ray diffractometer. Raman spectra were recorded on a Jobin-Yvon HR800 instrument with an Ar+
laser source of 488 nm wavelength in a macroscopic configuration. Surface morphology of samples was observed with a Hitachi S-4800 field emission scanning electron microscope (FESEM). High-resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100 instrument working at an acceleration voltage of 200 kV. UVvis diffusion reflectance (DR) spectra of the samples were recorded on a Lambda 900 UVvisNIR spectrophotometer (Perkin-Elmer) and BaSO4 was used as reference. Photoluminescence (PL) spectra of photocatalysts were recorded with a Jobin Yvon HR800 micro-Raman spectrometer using a 325 nm line from a HeCd laser. 2.2. Sample Preparation. Ti foils (3.5 1.5 0.1 cm) were cleaned by sonication in a detergent solution, acetone, and 2-propanol, respectively, and rinsed with deionized water and finally dried in a nitrogen gas flow. Then, the dried Ti foil was put into a Teflon-lined stainless steel autoclave loaded with 15 mL of 5 M NaOH solution. The autoclave was put in a preheated electric oven at 180 °C for different time under autogenously pressure and static conditions, and then air-cooled to room temperature. After hydrothermal processing, the sample was washed with deionized water several times and immersed into 0.6 M hydrochloric acid for 24 h. Finally, the as-washed sample was calcinated at 350, 550, and 750 °C for 2 h, respectively. As reference samples, anodized TiO2 nanotube films were prepared on Ti foils by the reported method, and TiO2 (Degussa P25) nanoparticle films were prepared on Ti foils by the doctorblade method. These samples were prepared by the same size and similar film thickness as TiO2 nanosheet film for good comparison. 22277
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Figure 2. (a,b) TEM and HRTEM images of samples synthesized in NaOH solution under hydrothermal treatment for 3 h. (c,d) TEM and HRTEM images of sample synthesized in NaOH solution under hydrothermal treatment for 12 h.
2.3. Photocatalysis Experiments. Photocatalytic activity of the prepared samples was evaluated by degradation of rhodamine B (RB) solution at ambient temperature. Typically, the film sample was immersed in a 10 mL of RB aqueous solution with a concentration of 10 mg/L in a rectangular cell (3.5 cm (width) 6.5 cm (length) 1.0 cm (height)). The test cell containing film catalyst and RB solution was placed under a Hayashi LA-410 light source, which emitted UV light in the range of 320400 nm. By adjusting the power output and the distance between the sample and the light source, the light intensity was maintained at 10 mW cm2, as measured with a UV power meter. The solution was placed in the dark for 30 min to obtain a good dispersion and establish adsorptiondesorption equilibrium between the organic molecules and the catalyst surface. Decrease in the concentration of dye solution was measured with a spectrophotometer at λ = 554 nm at given reaction intervals. Photocatalytic activity of the reference P25 film as well as anodized TiO2 nanotube film was also measured by the same method.
3. RESULTS AND DISCUSSION 3.1. Formation of Sodium Trititanate Nanostructures. Figure 1 shows the SEM images of the samples synthesized in NaOH solution under hydrothermal treatment for different time. After hydrothermal reaction for 3 h, the film displays porous network morphology (Figure 1a). Magnified image shows that the network consists of scrolled-up nanotubes, and their diameters are ca. 10 nm (Figure 1b). Since the diameters of the titanate nanotubes are very small, they tend to bend or enlace with other nanotubes as their length increases.15 Therefore, the titanate film looks like a porous network structure from the top view. The cross-section image shows that the oriented nanotubes are
perpendicularly grown on the Ti foil, and the thickness of the film is about 2 μm (Figure 1b, inset). When the hydrothermal reaction time is increased to 6 h, some curved nanosheet aggregates are formed on the surface of porous network film as shown in Figure 1c. Magnified image shows that the thickness of the nanosheet is about 10 nm and the diameter of the nanotube remains unchanged (Figure 1d). The cross-section image shows that nanosheet aggregates are sitting on the layer of oriented nanotubes (Figure 1d, inset), and the thickness of the film is about 3 μm. With further increase of the hydrothermal reaction time to 12 h, the morphology of the sample changes significantly. As shown in Figure 1e, the surface of the film is uniformly composed of nanosheet aggregates with high density. Magnified image shows that the thickness of the nanosheet is about 10 nm (Figure 1f). The cross-section image shows that the film thickness is increased to about 6 μm (Figure 1f, inset). EDS analyses for all the above-mentioned samples (Figure 1gi) reveal atomic ratio of Na:Ti = 18:28, 15:32, and 7:64 for samples obtained with hydrothermal reaction times of 3, 6, and 12 h, respectively, indicating that the composition of the titanates are different depending on the hydrothermal reaction time. The titanate nanostructures are further investigated with TEM and HRTEM. As shown in Figure 2a, the sample displays uniform nanotube morphology with one end open after hydrothermal treatment for 3 h. The walls of the titanate nanotubes are several layers, and the distance between the adjoining layers is 0.76 nm (Figure 2b). Such lattice fringe reveals the presence of stacked polyanion sheets, which are made of interconnected [TiO6] octahedral. With the increase of reaction time to 12 h, as shown in Figure 2c, nanosheet instead of nanotube is indeed observed. Lattice spacing of d = 0.19 nm can be assigned to the (020) planes of the layered sodium titanate crystal (Figure 2d). 22278
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Figure 3. (a) XRD pattern of samples synthesized in NaOH solution under hydrothermal treatment for different times; (b) Raman spectra of samples synthesized in NaOH solution under hydrothermal treatment for different times.
The hydrothermally grown titanate nanotubes/nanorods were reported to have various structure and composition, for example, A2Ti3O7, A2Ti2O4(OH)2, and lepidocrocite-type AxTi2x/40x/4O4, as well as ATi3O6(OH) 3 2H2O (A = Na and/or H; 0 = vacancy), due to the abundant chemistry of Ti in alkaline medium.11a,1619 In order to identify the structure of our titanate nanosheet array, we did XRD and Raman characterizations in addition to HRTEM. The characteristic diffraction at ca. 10° in XRD results (Figure 3a) can be indexed as the typical layered titanate structures. The peaks at 48.1° can be satisfactorily assigned to the (020) plane of the monoclinic crystal structure of the sodium trititanate phaseNa2Ti3O7, respectively (JCPDS No.72-0148). The assignment is further supported by Raman spectra, in which the as-synthesized titanates are identical to that of sodium trititanate reported by Riss et al.20 It should be noted that the diffraction peak at ca. 10° gradually shifts to lower angles with the increase of hydrothermal reaction time from 3 to 12 h, indicating the increase in the distance between polyanion sheet composed of interconnected [TiO6] octahedral. The change of intersheet distance is probably caused by the morphology evolution of sodium titanate nanostructures. That is, in sodium titanate nanotubes, compressive strain exists in interconnected [TiO6] octahedral layer, leading to smaller distance between polyanion sheets. On the contrary, in sodium titanate nanosheets, compressive strain in interconnected [TiO6] octahedral layer is gradually relaxed, so the distance between polyanion sheets gradually becomes larger. Raman spectra collected from above samples display Raman features that are consistent with those reported for the trititanate nanostructures (shown in Figure 3b). The band regions of 100800 cm1 are related to TiO in sodium trititanate with different stretching vibrations in TiO6 octahedral. The band around 900 cm1 is assigned to TiONa symmetrical stretching mode with very short TiO distance in titanate structures.2022 The peak located around 900 cm1 shifts downward by 14 cm1 from nanotubes to nanosheets, which can be caused by the relax of compressive strain existed in nanotubes. This is well consistent with the above inference in XRD analysis. 3.2. Formation Mechanism of Sodium Titanate Nanostructures. The formation of sodium trititanate can be described by eq 1. 3Ti þ xNaOH þ ð7 xÞH2 O f Nax H2x Ti3 O7 þ 6H2 v
ð0 < x < 2Þ
ð1Þ
The x value in the sample obtained with a hydrothermal reaction time of 3, 6, and 12 h, respectively, is estimated to be 1.93, 1.41, and 0.33, respectively, by the EDS measurement above
(Figure 1). Therefore, the corresponding chemical molecule formula can be approximately denoted as Na1.93H0.07Ti3O7, Na1.41H0.59Ti3O7, and Na0.33H1.67Ti3O7, respectively. Correlating the morphology evolution with composition change of titanate nanostructures suggests that the content of sodium has a significant influence on the morphology evolution from nanotube to nanosheet. The possible formation process of sodium titanate nanosheet is described as following: at the beginning, the reaction solution is highly alkaline. Alkaline treatment of Ti metal induces delamination into sodium trititanate nanosheets intermediates, then nanosheets tend to curl and scroll to multiwalled nanotubes due to the balance between the difference in surface energy and the lattice strain induced by bending. In this step, highly alkaline concentration is necessary to stabilize the nanotube. However, as the reaction proceeds, the concentration of sodium hydroxide gradually decreases. Consequently, more protons tend to intercalate between the stacked polyanion sheets. The removal of Na+ leads to the weakened interaction between neighboring TiO6 layers, as observed in the above Raman analysis, making titanate nanotubes no longer stable. So the loosening may promote the possibility of unscrolling nanotube back to flat nanosheets. Finally, large-sized sodium titanate nanosheets tend to grow via Ostwald ripening-assisted oriented attachment crystal growth mechanism, similar to that proposed in formation of titanate nanowires from titanate nanotubes.23 In fact, the oriented attachment crystal growth could be indeed supported by the HRTEM image (Figure S1 in the Supporting Information) of the intermediate product taken after hydrothermal reaction time for 6 h. To prove the formation mechanism of nanosheet titanate mentioned above, we also performed two comparative experiments: (1) Repeated hydrothermal treatment of Ti foil in fresh 5 M NaOH solution for 3 h. The treatment was repeated for four times and therefore the total reaction time was also 12 h. As shown in Figure 4a,b, tubular titanate was grown on Ti foil instead of nanosheet. (2) Hydrothermal treatment of Ti foil in 1 and 10 M NaOH solution for 12 h, respectively. As expected, the sample obtained at 1 M NaOH solution displays nanosheet features, whereas (Figure 4c) the sample obtained at 10 M NaOH solution takes on one-dimensional morphology (Figure 4d). Both the comparative experiments above indicate that the decrease of sodium content intercalated between stacked polyanion sheets is indeed necessary for the formation of sodium titanate nanosheet. 3.3. Transformation of Sodium Titanate Nanosheets to TiO2 Nanosheets. After complete ion exchange and further annealing treatment, highly ordered TiO2 nanosheet array films with large-area uniformity and good reproducibility can be obtained 22279
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Figure 4. (a,b) Low and high magnified SEM images of sample synthesized in 5 M NaOH solution under hydrothermal treatment for 3 h; such treatment is repeated for 4 times. (c) SEM image of sodium titanate sample synthesized in 1 M NaOH for 12 h. (d) SEM images of sodium titanate sample synthesized in 10 M NaOH for 12 h.
(shown in Figure S3 in the Supporting Information). Meanwhile, the effect of calcination temperatures on the morphology of TiO2 nanosheets is studied. Over a range of calcination temperatures from 350 to 550 °C, the shapes of TiO2 nanosheets remain invariant regardless of the calcination temperatures (Figure 5ad), confirming the high thermal stability of TiO2 nanosheet. However, when calcined at 750 °C, the surface morphology of calcined TiO2 nanosheets has a great change. The surface of nanosheet becomes rough as shown in Figure 5e, and higher magnification image reveals that these nanosheets contain aggregates of nanoparticles with diameters of 2030 nm (Figure 5f). The morphology of TiO2 nanosheet calcinated at 350750 °C for 2 h was further analyzed by the TEM and HRTEM. Figure 6ac shows the TEM image of three as-calcinated samples. In all the images, some nanocavities are distributed uniformly on the surface of nanosheet and can be observed from the light dots on the dark body of the nanosheet. Such nanocavities may be created by the dehydration process during phase transformation. The size and quantity of nanocavities increase with annealing temperature. Figure 6d shows the HRTEM image of sample calcinated at 350 °C. It can be observed that two different regions, marked by anatase and TiO2(B), are presented in the nanosheet. In region of anatase phase, the lattice spacing of 0.20 nm coincides with the crystal plane (200) of anatase phase TiO2 (Figure 6e). However, such lattice fringes are not very clear and do not spread over the entire region, revealing that the crystallinty is not good enough. In the region of TiO2(B), the lattice spacing of 0.31 nm coincides with the crystal plane (002) of phase of TiO2(B) (Figure 6e). Therefore, heterojunction forms between anatase and TiO2(B) phase in the nanosheet. When the calcination temperature increases to 550 °C, likewise, both lattice fringes attributed to (200) plane of anatase phase and (002) plane of TiO2(B) phase can be observed (Figure 6f,g). The clear lattice fringes attributed
to the (200) plane of anatase phase indicate the high crystallinity of anatase phase. In the inset of Figure 6g is shown the fast Fourier transform (FFT) image corresponding to the sample calcined at 550 °C. Combining HRTEM with FFT analysis helps us to determine the orientation relationship between anatase and TiO2(B) phase to be [011]anatase//[110]TiO2(B). With further increase of temperature to 750 °C, the nanosheet was transformed to single-phase anatase, as evidenced by the clear lattice fringes belonging to anatase (101) crystal plane (Figure 6h). The corresponding FFT image shown in the inset of Figure 6h reveals the high crystallinty of anatase phase. The TiO2 nanosheet film was further characterized by XRD. Figure 7a shows XRD patterns of samples calcinated at different temperatures. In the pattern of sample calcinated at 350 °C, diffraction peaks attributed to trititanate have disappeared and peaks attributed to anatase appear at 2θ = 25.3° and 48.1°. Diffraction peaks corresponding to TiO2(B) phase are not detected, probably resulting from the small grain size and the overlapping with those of anatase phase. Samples calcined at higher temperature displayed narrower diffraction peaks, indicating the improved crystallinity with calcination temperature. Raman spectroscopy was further employed to analyze the phase composition of TiO2 nanosheet films. Figure 7b shows the Raman spectra of the nanosheet film. The film calcinated at 350 °C shows Raman bands belonging to both TiO2(B) and anatase phase. The nonoverlapping diagnostic bands around 250 and 450 cm1 indeed prove the existence of TiO2(B) phase. The other bands are attributed to anatase phase, including bands at 144 cm1 (Eg), 197 cm1 (Eg), 399 cm1 (B1g), 513 cm1 (A1 g + B1g), and 628 cm1 (Eg). The coexisted anatase and TiO2(B) phases can also be observed in the film calcinated at 550 °C. However, the bands (250 and 450 cm1) of TiO2(B) decrease significantly relative to those of anatase, indicating that 22280
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Figure 5. SEM images of obtained TiO2 nanosheets array film calcinated at different temperatures: (a,b) 350 °C; (c,d) 550 °C; (e,f) 750 °C.
phase transition from TiO2(B) to anatase phase occur and the content of TiO2(B) phase in the composite decreases. The sample calcinated at 750 °C only displays Raman bands corresponding to anatase phase only, confirming the complete phase transition from TiO2(B) to anatase phase at 750 °C. By comparing the thermal stability of the present TiO2 nanosheet with that synthesized via normal solgel method, the temperature of anataserutile phase transformation can be increased from approximately 550 to 750 °C. Here, since no dopants have been introduced in the fabrication process which may enhance the thermal stability of the anatase phase, we believe the presence of TiO2(B) phase and subsequent TiO2(B)-to-anatase transformation play key role in the suppressing formation of rutile phase at high temperature. 3.4. Photocatalytic Activity of TiO2 Nanosheets Film. The photocatalytic activity of the samples was evaluated by photocatalytic degradation decolorization of RB aqueous solution under UV light irradiation. Under dark conditions without light illumination, the RB concentration has little change for every measurement using various TiO2 nanosheet samples. Also, illumination in the absence of TiO2 nanosheet film does not result in the photocatalytic decolorization of RB. Therefore, the decolorization of RB aqueous solution is caused by photocatalytic reactions on TiO2 nanosheet surface under the UV illumination. Figure 8a shows the comparison of photocatalytic activity of the TiO2 nanosheet film after calcination at different temperatures.
It can be seen that the calcination temperatures have a great influence on the photocatalytic activity of the TiO2 nanosheet. The order of activity of the samples is TNS-550 > TNS-750 > TNS350. For TiO2 photocatalysts, factors such as surface area and crystallinity have a major influence on the photocatalytic activity. The larger the surface area is, the more active sites and photocatalytic reaction centers for the adsorption of reactant molecules, which is in favor of the activity. Poorly crystalline defects in samples always lead to the recombination of electrons and holes at defect positions. To obtain high photocatalytic activity, it is necessary to have a high surface area and good cryatallinity. Certainly, there is a need to find a compromise because the increase in crystallinity reduces the surface area. To evaluate the reactivity of the samples quantitatively, the apparent reaction rates (k) of RB degradation are calculated. The calculated values of k for TNS-350, TNS-550, and TNS-750 are 0.0034, 0.0071, and 0.0041 min1, respectively. Meanwhile, to further compare the real surfaces of the catalyst, k is normalized to the specific surface area, referred to ks. The specific surface area is estimated by a dye adsorption method without destroying the TiO2 nanosheet film sample.24 The detail of the measurement is described in the Supporting Information. According to the specific surface area analysis above, the surface area ratio of STNS‑350:STNS‑550: STNS‑750 = 1:0.8158:0.4885. When normalized to the specific surface area, the reaction rate (ks) ratio of STNS‑350 to STNS‑550 to 22281
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Figure 6. (ac) TEM images of TiO2 nanosheets calcinated at different temperatures. (d) HRTEM image of a single TiO2 nanosheet calcinated at 350 °C. (e) Magnified HRTEM image of the junction region in (d). (f) HRTEM image of a single TiO2 nanosheet calcinated at 550 °C. (g) Magnified HRTEM image of the junction region in (f). (h) HRTEM image of a single TiO2 nanosheet calcinated at 750 °C. Yellow dashed line denotes the interface between anatase and TiO2(B) phase. Red circle denotes the magnified junction region.
STNS‑750 is 1:2.6006:2.5162. Therefore, it can be deduced that the difference in reactivity far exceeds that expected from simple consideration of surface area. Considering the factor of crystallinity, it is not surprising that sample TNS-550 exhibits higher activity than TNS-350 because of its higher crystallinty. It is interesting to find that TNS-750 with best crystallinity exhibits lower activity than that of TNS-550. We believe that the higher activity of TNS-550 partly benefits from its unique bicrystalline framework. As observed from the HRTEM image of sample TNS-550, coherent interface exists between anatase and
TiO2(B) phase. Recently, it has been reported that the heterojunction composed of anatase phase and TiO2(B) phase does accelerate photostimulated electronhole separation and consequently improves the photocatalytic activitly. Zhu et al.9a proved that, in the photocatalysis with a shell of anatase phase on the fibrous core of TiO2(B) phase, the difference in the band edges of the two phases promotes migration of the photogenerated holes from anatase phase to the TiO2(B) phase. Lu et al.9b also propose that, in bicrystalline dehydrated nanoribbon with alternate structure of TiO2(B) and anatase phase, the holes remaining in 22282
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Figure 7. (a) XRD patterns of TiO2 nanosheets samples calcinated at different temperatures. (b) Raman spectra of TiO2 nanosheets samples calcinated at different temperatures.
Figure 8. (a) Degradation curves of RB over TiO2 nanosheets calcinated at different temperatures. (b) PL spectra of different samples. (c) Dependence of the PL intensity on different samples in a 5 104 M basic solution of terephthalic acid after 45 min UV light irradiation. (d) Degradation curves of RB over TiO2 nanosheets calcinated at 550 °C, P25 and TNT.
anatase phase prefer to migrate toward TiO2(B) phase and the photoexcited electrons in the conduction band of TiO2(B) favor the migration to the anatase phase. To demonstrate the positive role of anatase/TiO2(B) heterojunction played in the separation of electronhole pairs, we characterized photoluminescence of the three TiO2 nanosheet array samples. The photoluminescence emission mainly results from the recombination of excited electrons and holes, and the lower photoluminescence intensity indicates the decrease in recombination rate. Figure 8b shows the photoluminescence spectra of different samples. As can be seen, TNT-750 film has the highest photoluminescence intensity followed by TNS-350 and TNS-550. Bandband photoluminescence intensities mainly depend on the extent of photoinduced charge carrier separation. Expectedly, TNS-350 and TNS-550 with the presence of TiO2(B) and anatase display lower photoluminescence
intensity than TNS-750, confirming that heterojunction between TiO2(B) and anatase phase can efficiently decrease the recombination rate of photogenearated electrons and holes. Lastly, the lower photoluminescence intensity of TNS-550 than that of TNS-350 is due to the higher crystallinty of each phase. Therefore, the lowest photoluminescence intensity and the highest photocatalytic activity of TNS-550 can be reasonably explained on the basis of efficient charge separation at the coherent TiO2(B)/anatase interface. To further prove the better catalytic activity of the TiO2 nanosheet film calcinated at 550 °C, we have investigated the activity of the samples by measuring the yield of active hydroxyl radicals (OH 3 ), the most important oxidative species in photocatalysis reaction under UV irradiation. Figure 8c displays the fluorescence spectra of the UV light irradiated nanosheet film sample calcinated at different temperatures in 10 mL of the 22283
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The Journal of Physical Chemistry C 5 104 M terephthalic acid (TA) aqueous solution with a concentration of 2 103 M NaOH at different irradiation times. The results show that the linear relationships between fluorescence intensity and irradiation time are found for all samples. The unique fluorescence peak at 425 nm originated from 2-hydroxyterephthalic acid produced by the reaction of TA with OH 3 in basic solution.25 Usually, photoluminescence (PL) intensity is proportional to the amount of produced hydroxyl radicals and PL intensity has a positive relation with photocatalytic activity. It can be easily seen that, at a fixed time (30 min), TiO2 nanosheet film calcinated at 550 °C has the strongest PL intensity, implying highest photocatalytic activity. This is well in accordance with the above photoluminescence studies results. In combination with the above analysis, it can be concluded that the photocatalytic activity of the present nanosheet film is governed by the synergistic effect of surface area and crystallinity as well as TiO2(B)anantase heterojunction. We also compared the photocatalytic activity of TNS-550 with Degussa P25 film and anodized polycrystalline TiO2 nanotube (TNT) films of the same thickness (see SEM images in Figure S2 in the Supporting Information). The degradation curves are shown in Figure 8d. The rate constants of samples of P25 and TNT films are 0.004 59 and 0.003 63 min1, respectively. After normalization to the specific surface area, the ks ratio of TNS-550 to P25 to TNT is 1:0.2547:0.3171. Accordingly, TNS-550 film displays higher activity than both P25 and TNT, demonstrating the great potential for practical application of TNS film.
4. CONCLUSIONS In conclusion, we have reported a novel approach to fabricate heterostructured TiO2 nanosheet array film on Ti foil. The vertically aligned nanosheets with thickness ranges from 10 to 20 nm are successfully grown on the Ti substrate. Annealing treatment imposes significant effect on the phase composition and morphology of TiO2 nanosheet and the calcinations of precursor H-trititanate nanosheet at 550 °C produce bicrystalline composite nanosheet with TiO2(B) phase and anatase phase; also, coherent interface between the two phases are formed. The orientation relationship between anatase and TiO2(B) phase was determined to be [011]anatase//[110]TiO2(B). Photocatalytic tests show that TiO2 nanosheet array film possesses considerably high photocatalytic activity under UV light illumination, due to the synergistic effect of large surface area and high crystallinity as well as heterojunction between TiO2(B) and anatase phase. The present nanosheet array film can serve as optoelectronic functional materials that can be used in photovoltaic cells and heterogeneous photocatalysis. ’ ASSOCIATED CONTENT
bS
Supporting Information. Surface area evaluation of TiO2 nanosheets; HRTEM image of the sodium titanate intermediate product; SEM images of P25 film and TNT films; and low magnified SEM image of typical TiO2 nanosheet array film annealed at 550 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-431-85099772. Fax: +86-431-85099772. E-mail: xtzhang@ nenu.edu.cn (X.Z.);
[email protected] (Y.L.).
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