J. Phys. Chem. C 2008, 112, 3239-3246
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Layered Nanostructures of Delaminated Anatase: Nanosheets and Nanotubes Gregory Mogilevsky,† Qiang Chen,† Harsha Kulkarni,† Alfred Kleinhammes,† William M. Mullins,‡ and Yue Wu*,† Department of Physics & Astronomy and Curriculum in Applied and Materials Sciences, and Department of Mathematics, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3255 ReceiVed: August 28, 2007; In Final Form: December 6, 2007
Anatase, viewed traditionally as a three-dimensional TiO2 structure, has major applications in solar cells, hydrogen production, and catalysis. Here, we report the synthesis, characterization, and structural simulation of a layered nanosheet material that can be described as delaminated anatase along the [001] direction. Counting the outer and interlayer surfaces, such a nanosheet structure maximizes the areas of surfaces which are similar to anatase (001), believed to be the surface of central importance for the superior efficiency of anatase in various applications. Indeed, the important dissociative adsorption process of water predicted to occur on the anatase (001) surface is observed in such nanosheet material. It is also shown that the structure of titania nanotubes, an issue of debate for nearly a decade, can also be described by delaminated anatase along the [001] direction. Interestingly, the strains in such a curved tubular structure lead to major differences in surface chemistry such as water dissociation.
Introduction Layered structures are intriguing for their unique properties and applications such as Li intercalation in battery electrodes.1 Materials based on exfoliated layers often exhibit remarkable two-dimensional behaviors such as the manifestation of novel quantum phenomena of electronic states in graphene.2 Another fascinating aspect of layered structures is that they could form the structural basis for novel tubular structures such as the wellknown graphene-based carbon nanotubes.3 Titania is one of the most studied metal oxides stimulated by its broad range of applications such as solar cells and catalysis.4-8 Among the three mineral forms of TiO2, anatase is the one commonly found in titania nanocrystals.9 Anatase nanocrystals exhibit superior efficiency compared to rutile and brookite in various applications such as catalysis and solar cells.7,10-12 The structure of anatase, as shown in Figure 1a, is viewed traditionally as threedimensional, opposed to obviously two-dimensional structures such as graphite or MoS2. There is, however, structural anisotropy in anatase. The lengths of its apical bonds, all along [001], are 1.98 Å, while the equatorial bond lengths are 1.94 Å. Anisotropy like that could lead to a structure of delaminated anatase along the [001] direction, as illustrated in Figure 1b. Such layered structures of delaminated anatase would be very intriguing since external and internal surfaces of such layered materials are anatase (001) like. Theory suggests that the anatase (001) surface, although a minority surface (compared to the (101) surface), could play a crucial role in determining the chemical properties of nanocrystalline anatase such as dissociation of water.13 It follows that layered titania and titanate nanomaterials, based on delaminated anatase, could lead to superior performances in various important applications due to the larger effective (001) surface area than observed on anatase nanocrystals. * Corresponding author. E-mail:
[email protected]. † Department of Physics & Astronomy and Curriculum in Applied and Materials Sciences. ‡ Department of Mathematics.
Figure 1. Structural model of delaminated anatase along [001] direction. (a) Anatase unit cell. The apical bond length is 1.98 Å, and the equatorial bond length is 1.94 Å. The blue box shows where the unit cell is cleaved to achieve delamination. (b) Illustration of layers delaminated in [001] direction. The bridging O2c are highlighted in maroon to distinguish them from the regular O3c. (c, d) Delaminated structure viewed from different perspectives. The glide shift is 78° at interlayer spacing of 8.7 Å. The three major planes that are seen in XRD are shown in green (002), blue (103), and purple (101).
The multilayered titania nanotube discovered by Kasuga et al. in 1998 is one of the layered titania nanomaterials14 that has generated a great deal of interest because of its unique morphology and potential applications.15-17 Comprehensive reviews in this intensely researched field by Chen17 et al. and Bavykin15 et al. summarize recent findings on titania and titanate based materials. Despite discoveries of many interesting proper-
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ties through nearly a decade of intensive investigation, it is surprising to note that an established structural model of such nanotubes remains missing.14-22 Here, we report the synthesis and characterization of a novel nanosheet material in bulk quantity directly from titania nanotubes under neutral pH condition. The X-ray diffraction (XRD) patterns of such nanosheet material retain most of the features seen in titania nanotubes, indicating that intrinsic structural properties, such as distance between layers and order within layers, are common to both. Therefore, understanding the structure of such nanosheet material provides direct insight into the structure of titania nanotubes. Simulation of the structure demonstrated that nanotubes and nanosheet material can be viewed as delaminated anatase along the [001] direction. Also, nanosheet material could possess unique properties associated with the anatase (001) surface. Indeed, through 1H NMR and thermogravimetric analysis (TGA), the theoretically predicted dissociative adsorption of water,13,23-26 which remains to be verified experimentally on anatase (001) surface, is positively identified in this nanosheet material. Experimental Section Preparation of Nanotubes. Titania nanotubes were synthesized from 32 nm anatase nanoparticles (Alfa Aesar) and NaOH pellets producing water washed nanotubes as previously described.27 Anatase nanoparticles (4 g) mixed with 160 mg of NaOH dissolved in 400 mL of distilled water were heated in a Teflon lined stainless steel autoclave at 140 °C and maintained at that temperature for 72 h. After cooling, the resulting material was distributed into centrifuge tubes by partially filling those and using distilled water to top off the tubes. After spinning at 4400 rpm for 5 min, the top solution was removed and the centrifuge tubes were refilled with fresh distilled water and placed in a Vortexer (Glas-Col) to mix for 10 min. Further spinning in the centrifuge followed by removal of the top solution completed the first cycle of water washing. The washing process was repeated 20-25 times until the resulting pH of the top solution reached 5-6. After reaching the desired pH, the top solution was discarded and the precipitant spread out in a Pyrex trough and dried overnight in an oven at 50 °C. The scanning electron microscopic (SEM) images of the material so prepared show long entwined strands of TiO2 nanotubes (Figure 2a). Preparation of Shortened Nanoscrolls. A 100 mg sample of synthesized nanotubes and 20 g of 100 µm diameter ZrO2 microbeads (Glenn-Mills) were mixed together with 65 mL of distilled water in a Teflon grinding vessel. Grinding took place in a bead beater (Glenn-Mills) for 45 min, with the grinding vessel being surrounded by an ice bath to keep the temperature from rising due to friction of the grinding media. After grinding, ZrO2 beads immediately sunk to the bottom. The supernatant solution was taken off with a pipet into centrifuge tubes. Centrifuge tubes were sonicated for 15 min to further disperse and separate all species. Then the species were spun for 5 min at 4400 rpm. The milky supernatant suspension contains cut nanotubes, whereas precipitant at the bottom of the centrifuge tubes contains mostly uncut nanotubes and residual grinding media. The supernatant formed a colloidal suspension that did not settle over days and was used for further characterization. Preparation of Nanosheets from Nanotubes. The same procedure as described above was employed to create the nanosheets, except for one crucial difference. The grinding vessel was not inserted into an ice bath and the temperature of the grinding process was allowed to rise to 100 °C. To ensure
Figure 2. SEM images of (a) nanotubes and (b) nanosheets.
that the temperature of the mixture in the grinding vessel remained at constant temperature, nanotubes and microbeads were mixed with boiling water prior to cutting. After grinding and spinning, the transparent supernatant again formed a longlasting colloidal suspension. The SEM image (Figure 2b) shows that the resulting material is a collection of aggregated flakes that were called “nanosheets”. The large area image shows no evidence of “stringlike” nanotubes. Sample Preparation for Characterization. For XRD, TGA, UV-vis, and NMR experiments, the supernatant solution was deposited on glass slides and dried at 50 °C. Incidentally, a uniform film formed on the glass slides after water evaporated with good adhesion properties not observed in the original uncut nanotubes. Dried sample then was scratched off the glass slides for characterization. For transmission electron microscopic (TEM), atomic force microscopic (AFM), and SEM experiments, the sample was left suspended in water, sonicated for 15 min, and dropped onto a 400 mesh lacey carbon grid for TEM, freshly cleaved mica surface for AFM, and a silicon wafer for SEM. Characterization Methods. XRD was performed using monochromatic Cu 40 kV/40 mA (λ ) 0.154 05 nm) radiation at 0.25°/min with a resolution of 0.02°. 1H NMR was conducted on a solid-state pulsed NMR spectrometer at 400 MHz. Magic angle spinning (MAS) was employed at a spinning rate of 20 kHz, and all spectra are based on TMS as the chemical shift reference. TEM and high-resolution TEM (HRTEM) were performed on JEOL-100CX-II and on JEOL-2010F, respec-
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Figure 4. Low-resolution TEM image of cut nanotubes after grinding in ice bath. Temperature of the grinding solution did not exceed room temperature.
Figure 3. HRTEM image of nanotubes. Image shows interlayer spacing of 8.0 Å.
tively. Atomic force microscopy was performed on a Topometrix Explorer II type AFM in noncontact mode. SEM was performed on a Hitachi S-470 instrument operated at 2.0 kV. UV-vis was performed in reflection mode on a Shimadzu ISR-3100 with an integrating sphere, and thermogravimetric analysis (TGA) was performed on Perkin-Elmer Pyris 1 TGA. Simulation. Using Accelrys Materials Studio, the anatase unit cell was preloaded. The unit cell was cleaved in the (001) plane and delaminated along the [001] direction as shown in Figure 1. Simulated XRD is done through the Reflex Powder Diffraction module of Materials Studio. The model structure is set to be much larger in the a and b crystallographic directions and quite short (∼10%) in the c direction for the Scherrer-Debye calculations. Results and Discussion Morphology Analysis. The layered morphology of titania nanotubes is well established.15-17 Figure 3 shows a transmission electron microscopic (TEM) image of the synthesized titania nanotubes with inner diameter of 4-5 nm and outer diameter of 9-10 nm. The tubular wall consists of several layers of atomic structure with interlayer spacing of about 8.0 Å. Typically, the number of layers on one side is one more than on the other side of the wall as expected from a scroll structure.18 In the process of cutting the long synthesized nanotubes with ball milling in water at 100 °C, it was discovered that the shortened nanotubes (about 75 nm long) transform efficiently into nanosheets in bulk quantity. Nanosheet formation can be described as a result of nanotubes fracturing and unrolling in the ball-milling process. When they are cut to 75-100 nm in length, the nanotubes unroll and water dissociates on the (001)like surface of the unrolled tubes (see NMR results below). When the grinding takes place in an ice bath, keeping the ballmilling temperature from rising to 100 °C, the nanotubes are cut but do not convert to two-dimensional sheets. They retain the hollow tube shape but are limited to 75-150 nm in length (Figure 4). Physical grinding alone does not convert the nanotubes into sheets. The grinding must take place under the elevated temperature of 100 °C.28
Unlike the uncut titania nanotubes, such nanosheets form a colloidal suspension in water that can be easily harvested and dried. Figure 5 shows images of such a material taken with highresolution TEM (Figure 5a,b) and AFM (Figure 5c). The TEM micrograph (Figure 5a) shows an array of thin sheets or sheetlike structures that appear to be amorphous. However, when examined at higher magnificationsFigure 5b shows the enlarged area indicated by the rectangle in Figure 5aslattice fringes 0.38 nm apart become clearly visible. Those lattice fringes are expected to be apparent through a top view of a (001) surface of anatase or delaminated anatase as indicated in Figure 1b. The observed lattice fringes cover extended areas indicating a flat surfacesno evidence for tubelike material is found in any TEM micrograph of nanosheet material. Further evidence of crystalline structure is provided by the selected area electron diffraction pattern (SAED) shown in the inset of Figure 5a and taken with the electron beam covering the area encircled by the broken line. The elliptical shape of the diffraction pattern indicates that a preferred orientation exists in the indicated area. In addition, the bright dot pattern highlighting the oval rings indicates that the material within the encircled area contains larger crystals beside the polycrystalline material producing the rings. In the AFM image the color contrast indicates height differences with elevated areas producing brighter colors. The image shows a clear indication of plateaus, as demonstrated by the scan taken along the bright yellow line in Figure 5c and the obtained height profile displayed in Figure 5d. The observed plateau has a height of 1.4 nm and is approximately 230 nm wide. Together, TEM and AFM microscopies show that the produced sheets are thin and are made up of mostly crystalline material. The observed separation of the lattice fringes of 0.38 nm strongly suggests that the characterized surface resembles the (001) surface of anatase and supports the assumption that the layers are produced through delamination of anatase as described in Figure 1. The measured plateau height of 1.4 nm indicates that sheets containing not more than two layers exist in the material. Structural Analysis. X-ray diffraction (XRD) measurements support the conclusion that the sample is composed of sheets containing a few layers. Figure 6 shows XRD spectra of nanotubes and nanosheet material. The major difference between the two materials is the broad peak centered on 2θ ∼ 28° that
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Figure 5. TEM and AFM images of the nanosheets. (a) TEM of a typical plateau of nanosheets that contains several nanosheets aggregated together. Inset: Selected area diffraction of the circled area shows that the aggregates have semicrystalline structure and have a preferred orientation. (b) HRTEM image of area marked with rectangle in (a). Lattice fringes are observed with a spacing of 3.8 Å. Scale bars on (a) and (b) read 50 nm and 5 nm, respectively. (c) AFM image of a plateau with line analysis (d) showing dimensions.
is addressed below. Both materials share the same intrinsic structuress stacking arrangement perpendicular to the layers and the same order within layers. For the same reason, mulitwalled carbon nanotubes and turbostratic graphite, a tubular and a layered material, display the same XRD spectra. The layer distance is inferred from the (002) reflection at 2θ ) 10.2° that translates to a separation of 8.7 Å and is depicted in Figure 1c. Other reflection planes are indexed according to simulation results discussed below. The fringe separation of 8 Å observed in the TEM micrograph is well explained by invoking the ultrahigh vacuum environment the technique requires.29,32 Surface moieties might desorb and the layer separation would shrink. All XRD peaks shown in Figure 6 are broadened beyond instrumental resolution, indicating finite size effects. Of interest is the sheet thickness that can be estimated from the full width at half-maximum (fwhm) of the (002) peak using the Scherrer formula:30
∆d )
0.9λ B(θ) cos(θ)
where ∆d is the thickness of the sheet perpendicular to the layer plane, λ (1.5418 Å) is the X-ray wavelength, θ is the diffraction angle, B(θ) is the fwhm of the peak in radians, and 0.9 is the Scherrer constant. A fwhm of 2.1° taking the instrumental resolution into account translates to a sheet thickness of 38 Å. On average each sheet holds five layers, confirming the thinness of the produced sheets. The major difference in the XRD spectra of nanotubes and nanosheets is the broad peak of 16° fwhm centered on 2θ ∼ 28°. The width of that peak masking many overlapping lines is characteristic of short-range order in amorphous regions. The peak could therefore be attributed to the amorphous nature of the glass substrate used in the XRD measurements since the sample of nanosheets was less than 1 mg. An alternative explanation is suggested by the simulation results also displayed in Figure 6. If the XRD simulation is based on sheets of limited size (3 nm × 3 nm × 1 nm), all peaks broaden and a broad peak underlying the crystalline spectrum appears centered around 28° (see below). The observed XRD pattern is different from that of earlier reported titanate nanoribbons and lepidocrocite-type titania nanosheets obtained via different routes of synthesis.31,32 The
Layered Nanostructures of Delaminated Anatase
Figure 6. XRD patterns of anatase, nanosheets, nanotubes, and simulation based on delaminated anatase structure. XRD patterns and simulation based on the delaminated anatase model described in the text. Blue line shows the simulation for a model sheet that is only constrained in the c direction. Dotted black line shows spectra broadened due to finite size constraints in all crystallographic directions (3 nm × 3 nm × 1 nm). Broadened peak seen in the measured XRD of nanosheets may be attributed to limited size of individual crystallites or the background glass slide.
layered nature of titania nanotubes and nanosheets is revealed by the low angle XRD peak at 2θ ) 10.2°, which corresponds to an interlayer spacing of 8.7 Å. Since simulation of the XRD pattern of a layered nanosheet structure is much easier than that of multiwalled nanotubes and nanoscrolls, the availability of bulk quantity nanosheets that were made by breaking up and unrolling nanotubes with similar XRD patterns helps a great deal in solving the structures of the nanotubes. Naturally, several layered structures of titanate have been considered as the underlying structures of nanotubes, but each was found with certain deficiencies.15-19,33-35 Some of these models provide reasonable peak indexing, but satisfactory XRD patterns based on such structures were not provided. Although the XRD patterns of titania nanotubes and nanosheets are very different from that of anatase as shown in Figure 6, previous Raman study shows that the local structures of titania nanotubes upon intercalation of certain organic molecules such as hydroquinone are nearly identical to that of anatase. 36 Removal of intercalated hydroquinone in water at room temperature completely restores the Raman spectrum of pure titania nanotubes. Since no global structural changes are observed by XRD and TEM upon addition and removal of hydroquinone, this suggests that the local structure of nanotubes is very similar to that of anatase. It was also shown previously that the XRD patterns of some titania nanotubes convert to that of anatase at low pH values and some change reversibly back to that of nanotubes at higher pH values.37,38 All these facts indicate a close relationship between the structures of titania nanotubes and anatase. So far, although some anatase-based structures have been mentioned for titania nanotubes,20-22 no direct connection between the XRD patterns of titania nanotubes and anatase-based structures have been established. Simulation of Structure. Consider the structural model of anatase delaminated along the [001] direction with interlayer spacing of 8.7 Å shown in Figure 1. Here, the layers can be
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3243 viewed as lying in the (001) plane and stacked along the [001] direction with a glide symmetry. Each layer is shifted half a unit cell in the [100] and [010] directions, repeating every two layers. Figure 1d shows that the angle of the glide between the layers in this model is about 78°, which agrees with previous TEM observation.35 The simulated XRD pattern of such a model structure is shown in Figure 6. It is evident that it agrees very well with the observed XRD pattern of nanotubes. The experimentally observed broadening of XRD peaks and the appearance of a broad line centered at 28° is well reproduced using finite size flakes as the base for the simulation. Surface Structures. As shown in Figure 1b, all four Ti atoms in each unit cell of delaminated anatase are fivefold coordinated (Ti5c) rather than sixfold as in anatase. Of the eight oxygen atoms per unit cell, four remain threefold coordinated as in anatase and four are twofold coordinated (O2c) bridging oxygens. It was shown previously that 40% of Ti atoms in titania nanotubes are undercoordinated.22 Unlike titanate structural models where a large number of defects have to be invoked to account for such large number of undercoordinated Ti, the delaminated anatase structure provides a natural explanation for this observation. Hydration would restore the sixfold coordination for some of the Ti atoms and stabilize the layered structure. Clearly, it is very important to investigate the state of hydration of such a layered structure. This would be particularly informative since the (001) surface stands out in dissociative adsorption of water compared to other anatase surfaces based on theoretical predictions.13,23-26 The hydrated (001) surface is shown to be energetically stable against reconstruction.13,26 In contrast, water adsorption on other anatase surfaces such as the commonly found (101) surface is nondissociative. Although theoretical predictions of water dissociation processes differ somewhat in detail in several studies, the essence is the same. Figure 7a shows a scheme of dissociative adsorption of H2O where the Ti5cO2c bond is broken by forming two terminal Ti5c-OH hydroxyls (Ti is coordinated with four framework oxygens plus OH).13,23 Such dissociative H2O adsorption is predicted to be a favorable process up to a coverage of θ ) 0.5 (number of adsorbed water molecules versus the number of surface Ti). Figure 7b shows a slightly different scenario. Here, after breaking the Ti5c-O2c bond, H binds to O, forming a Ti5c-OH hydroxyl, but the OH group is attached to an adjacent Ti5c site, forming a Ti6c-OH hydroxyl, leaving behind a fourfold coordinated Ti.24 These studies show that a mixture of dissociative and nondissociative adsorptions occurs at higher coverage (e.g., above θ ) 0.5). A somewhat different scheme of dissociative adsorption is shown in Figure 7c. Here, dissociative H2O adsorption is not accompanied by Ti5c-O2c bond breaking. The OH group attaches to a Ti5c site, forming a terminal Ti6c-OH hydroxyl, and H binds to adjacent bridging oxygen, forming a bridging oxygen hydroxyl.25 Surface Chemistry. 1H nuclear magnetic resonance (NMR) under magic angle spinning (MAS) at spinning rate of 20 kHz is employed to evaluate the state of adsorbed water. Figure 8 shows the 1H NMR MAS spectra of nanosheets and nanotubes under various drying conditions. Two peaks are clearly resolved in an as-synthesized nanosheet sample kept in a desiccator for 1 day: a broad low-field peak at 4.8 ppm with fwhm of about 4.4 ppm and a narrow high-field peak at 1.2 ppm with fwhm of 0.9 ppm. The total number of protons measured is x ) 0.63, defined as TiO2‚xH2O. The ratio of the number of protons of the low-field peak at 4.8 ppm versus that of the high-field peak at 1.2 ppm is 1:1. Based on previous studies, the 1.2 ppm peak is attributed to basic terminal hydroxyl and the broad peak at
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Figure 8. 1H NMR MAS spectra of nanosheets and nanotubes at various stages of dehydration. 1H NMR MAS spectra of nanosheets (blue) and nanotubes (red) dried in desiccator for 1 and 3 days along with spectra of samples annealed at 100 °C under N2 flow. The plots are all scaled to per unit of TiO2 weight. NMR-determined water contents are indicated as x values defined by TiO2‚xH2O. For nanosheets the low-field (toward left) and high-field (toward right) peak intensities (with sum ) 2x representing the total number of protons) are also indicated in the figure.
Figure 7. Three different schemes of dissociative adsorption of H2O on anatase (001) surface. (a) H2O dissociates into a proton and a hydroxyl group by breaking the bridging O-Ti bond. The hydroxyl group settles on the Ti site forming a terminal hydroxyl, while the proton completes the newly formed nonbridging O site, also forming a terminal hydroxyl. The resulting configuration leaves all Ti fivefold coordinated. (b) H2O dissociates as in (a). However, instead of settling the OH on the newly created Ti4c site, the OH settles on the adjacent Ti5c site, making it sixfold coordinated and leaving behind a Ti4c site. Unlike in (a), two very different terminal hydroxyls are formed. (c) H2O dissociates where the hydroxyl settles on the fivefold coordinated Ti site, making it sixfold coordinated, while the proton settles on bridging oxygen forming a bridging hydroxyl.
4.8 ppm is typical of incorporated molecular water.39,40 By drying the sample in the desiccator for 3 days, 75% of the 4.8 ppm peak intensity and 21% of the intensity of the terminal hydroxyl peak (now shifted slightly to 1.1 ppm) disappeared (Figure 8). Annealing the nanosheet sample at 100 °C under N2 gas flow for 12 h causes further intensity reduction of the terminal hydroxyl peak along with a further upfield shift to 1.0 ppm (with broader line width of fwhm ) 1.5 ppm). Instead of the 5.0 ppm peak, a low-field peak appears now at 7.2 ppm, which could be attributed to the acidic bridging hydroxyl protons.39,40 The total proton content defined by x and peak intensities are indicated in Figure 8. Clearly, terminal hydroxyl groups dominate in nanosheets at a coverage of θ ) 0.33. Here, the terminal hydroxyls are mostly without accompanying bridging hydroxyls. Therefore, the scheme illustrated in Figure 7a is likely to be the dominant dissociation pathway in nanosheets. At higher coverage (e.g., θ ) 0.63), further dissociative adsorption of water occurs but a large fraction of
the adsorbed water is in the form of molecular water. The appearance of the 7.2 ppm peak indicates that annealing at 100 °C might lead to structural reconstruction where some bridging hydroxyls are created. Figure 8 also shows 1H NMR MAS spectra of titania nanotubes. It is very interesting to note that the well-defined peak at 1.2 ppm observed in nanosheets is missing in nanotubes and the incorporated molecular water peak at 4.6 ppm is much stronger than that of nanosheets. It is clear that the majority of adsorbed water molecules are nondissociative in nanotubes at θ ) 0.37. There are also two small peaks with about equal intensities appearing at 2.1 and 6.9 ppm, consistent with terminal hydroxyl and bridging hydroxyl groups observed in titania, respectively.39,40 The equal intensities of these two peaks are consistent with the dissociation process illustrated in Figure 7c. It is interesting to note the small but significant difference in the chemical shifts of the terminal hydroxyl peaks, 1.2 ppm in nanosheets and 2.1 ppm in nanotubes. The difference in chemical shift indicates that the terminal hydroxyls in nanosheets are more basic than in nanotubes. Based on the difference in the number of oxygen coordination of Ti, the Ti cation in Ti5c-OH shown in Figure 4a is less positive than that of Ti6c-OH shown in Figure 7c. Thus, the terminal hydroxyl Ti5c-OH is more basic in nanosheets with smaller proton chemical shifts than that of Ti6c-OH in nanotubes. The major differences in dissociative adsorption of water could arise from strains in nanotube structures with possible connections to the formation of tubular structures. NMR measurements make a clear distinction between nanotube and nanosheet surfaces: the former contain molecular water physically adsorbed on the surface while the latter dissociates water and incorporates hydroxyl groups on the surface. Thus nanosheets form a hydrated anatase or titanate structure, while nanotubes are a form of titania. The dissociation of water is expected by theory to be promoted on the (001) surface of anatase. Thus NMR results strongly indicate that chemically the surfaces of nanosheets resemble an (001) surface of anatase. Water Desorption Characteristics Observed by TGA. The mechanism for stabilizing the delaminated anatase nanosheet
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J. Phys. Chem. C, Vol. 112, No. 9, 2008 3245 on whether the band gap is direct (n ) 2) or indirect (n ) 1/2). Data analysis shows that the UV-vis spectra of both nanotubes and nanosheets are described by n ) 2. Figure 10 shows that the band gap energy Eg changes significantly from 3.5 eV in nanotubes to 4.0 eV in nanosheets. Such a substantial increase of band gap has also been observed previously in lepidocrocitebased titania nanosheets41 and attributed to exciton trapping along the direction perpendicular to the layer plane; i.e., size quantization is induced by the layer thickness. A shift of ∆Eg of approximately 1.4 eV in lepidocrocite-based titania nanosheets was seen as evidence for a predominantly single-layer (0.75 nm) composition of the material. Thus an increase of the band gap energy ∆Eg by 0.5 eV as observed for nanosheets in this work strongly indicates that the produced sheet structure contains only a few layers. Conclusions
Figure 9. TGA measurements for nanoscrolls and nanosheets. Nanoscrolls (black line) exhibit a weight loss from ambient temperature to 150 °C that corresponds to molecular water desorbing from the surface. Nanosheets (red line) also exhibit the low-temperature drop along with the drop at 350 °C that corresponds to hydroxyls desorbing.
Figure 10. UV-vis spectra of nanotubes and nanosheets. Inset shows plots based on the equation (Rhν)2 ) A(hν - Eg), from which the band gap energies of nanotubes and nanosheets are determined.
structure comes from hydroxyl groups derived from dissociated water where the Ti5c-O2c bond is broken and two terminal hydroxyls form. Thermogravimetric analysis (TGA) allows for one to observe these hydroxyl groups through high-temperature water desorption. Figure 9 shows the TGA curves for both nanotubes and nanosheets. As reported previously, nanotubes exhibit water desorption from ∼50 to ∼150 °C that corresponds mostly to adsorbed molecular water. A similar weight dropoff is also seen in nanosheets. However, nanosheets also exhibit an additional component at 350 °C that corresponds to hydroxyl groups as observed by NMR. TGA results are in perfect agreement with NMR observations. Band Gap Information through Optical Properties. The major differences in the state of hydration between nanosheets and nanotubes could also have a direct influence on their physical properties. Figure 10 shows the UV-vis spectra of nanotubes and nanosheets. Optical absorption is described by (Rhν)n ) A(hν - Eg), where R is the absorption coefficient, hν is the photon energy, and A is a constant. The value of n depends
A nanosheet material is produced through processing of titania nanotubes under neutral pH condition. The structure of such nanosheets exhibits an XRD pattern similar to that of titania nanotubes. Structural simulation shows that the XRD pattern of such nanosheets can be simulated very well by a structural model of delaminated anatase along the [001] direction. It should be pointed out that this does not imply a direct physical delamination process. Intercalation of Na under high pH conditions plays an essential role in the delamination process as theory also suggests such a separation upon intercalation.42 This result indicates that the basic structure of titania nanotubes can also be described by delaminated anatase along the [001] direction. However, the rolling of the delaminated layers with strains led to significant differences in surface chemistry such as dissociation of water. NMR experiments reveal clearly that water adsorbs dissociatively on nanosheets in excellent agreement with theoretical predictions of dissociative water adsorption on the (001) anatase surface. In contrast, titania nanotubes do not show such a significant dissociative water adsorption. TGA results are also in excellent agreement with such differences in water adsorption between nanosheets and nanotubes. The nanosheet material with abundant (001)-like anatase surface could be superior in catalytic applications compared to conventional titania nanomaterials. Acknowledgment. We gratefully acknowledge the support of NSF under Contract No. DMR 0513915 and ARO under Contract No. DAAD19-03-1-0326. We are grateful to Prof. LuChang Qin, Dr. Qi Zhang, and Letian Lin for their expertise and help with TEM observations. References and Notes (1) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359-367. (2) Geim, A. K.; Novoselov, K. S. Nat. Mat. 2007, 6, 183-191. (3) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (4) Gra¨tzel, M. Nature 2001, 414, 338-344. (5) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (6) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991-1022. (7) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735-758. (8) Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. (9) Zhang, H.; Banfield, J. F. J. Mater. Chem. 1998, 8, 20732076. (10) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. ReV. 1996, 25, 61-69. (11) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49-68. (12) Kavanetal, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716-6723.
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