I. INTRODUCTION
Titanate-based nanotubes can be easily prepared using a hydrothermal synthesis method (Kasuga et al., Reference Kasuga, Hiramatsu, Hoson, Sekino and Niihara1998; Tian et al., Reference Tian, Voigt, Liu, Mckenzie and Xu2003; Xu et al., Reference Xu, Vanamu, Nie, Konishi, Yeredla, Phillips and Wang2006). Owing to the high specific surface area, the nanotubes can be used as adsorbent (Bavykin et al., Reference Bavykin, Lapkin, Plucinski, Friedrich and Walsh2005) and heterogeneous catalysts. The nanotube can be a potential photocatalyst because of its unique electrical polarity between inner- and outer surfaces (Xu et al., Reference Xu, Vanamu, Nie, Konishi, Yeredla, Phillips and Wang2006). Recent studies also show that nano-gold particles supported on titanate nanotubes and used as a heterogeneous catalyst may enhance low-temperature CO oxidation (Ntho et al., Reference Ntho, Anderson and Scurrell2008; Sikuvhihulu et al., Reference Sikuvhihulu, Coville, Ntho and Scurrell2008; Mendez-Cruz et al., Reference Mendez-Cruz, Ramirez-Solis and Zanella2011). It is important to understand the stability of the nanotubes at high temperature and high pressure in order to assure long-term performance of titanate nanotube-based materials. In this paper, we report systematic studies of the nanotubes at elevated temperatures and pressures.
II. SAMPLES AND EXPERIMENTAL METHODS
The Na-titanate nanotubes were prepared by adding 1 g of P25 powder to a 60 ml 10 M NaOH solution and annealing at 150 °C in digestion bomb for 20 h (Kasuga et al., Reference Kasuga, Hiramatsu, Hoson, Sekino and Niihara1998; Xu et al., Reference Xu, Vanamu, Nie, Konishi, Yeredla, Phillips and Wang2006). The products were washed using distilled water until the water became neutral. Transmission electron microscope (TEM) analyses were carried out using a JEOL FEG-2010TEM with attached Oxford Instruments' X-ray energy-dispersive spectroscopy (EDS) system. In order to analyze thermal stability of the nanotubes, the synthesized Na-titanate nanotubes were annealed at different temperatures (400, 450, 500, and 800 °C) in air for different time periods. All X-ray diffraction (XRD) analyses of the nanotube samples and heated nanotube samples were carried out using a Rigaku Rapid II XRD system with a two-dimensional image plate (MoKα radiation). The high pressure, in situ XRD experiments were carried out using a diamond anvil cell technique and energy-dispersive X-ray diffraction (EDXRD) at X-17C of NSLS of Brookhaven National Laboratory. We used bulk metallic glass as a gasket to avoid the interference of gasket XRD peaks with a samples' pattern.
III. RESULTS AND DISCUSSION
TEM images show that the sample is dominated by Na-titanate nanotubes that range from ~50 nm to several hundreds of nm (Figure 1). Diameters of nanotubes are very uniform. Inner diameters range from ~4 to ~5 nm. Outer diameters range from ~7 to ~10 nm. A high-resolution TEM image [Figure 1(B)] shows a smooth inner surface and some terraces on the outer surface. The structure of the nanotube matches that of Na2Ti3O7 phase well, although X-ray energy-dispersive spectra show the Na/Ti ratio is lower than 2:3. The nanotubes or rolled sheets of the Na-titanate contain less Na with respect to flat and ideal Na2Ti3O7. Results from Raman spectroscopy analyses show that nanotubes dried at 200 °C still contain OH in structure. It is proposed that H+ may be incorporated into the structures of the nanotubes. The chemical formula for the H-bearing titanate nanotubes may be expressed as (Na,H)2Ti3O7. A fast Fourier transform (FFT) pattern of the nanotube [Figure 2(B) insert] shows broad streaking (h00 and 110) reflections and a relatively sharp (020) reflection. The streaking is a result of the thin wall (only 3–5 repetitions) of the nanotube. The nanotube's elongation direction is b-axis.
Figure 1. (Color online) (A) Bright-field TEM image showing Na-titanate nanotubes. (B) High-resolution TEM image showing a nanotube and its FFT pattern (insert at up-left corner). Indexing is based on a Na2Ti3O7 phase with monoclinic symmetry although the number of Na for the nanotubes is less than 2 per formula, i.e. (Na, H+)2Ti3O7.
Figure 2. (Color online) XRD patterns of annealed products of the synthesized Na-titanate nanotubes at different temperatures for different times: (A) heated at 400 °C for 1, 8, and 16 h; (B) heated at 450 °C for 1, 8, 16, and 32 h; (C) heated at 500 °C for 1, 8, and 16 h; (D) heated at 600 °C for 1, 8, and 16 h; (E) heated at 800 °C for 1, 8, and 16 h; (F) heated at 450 °C for 1 h and then the product was soaked in water for 24 h at room temperature. “Initial” refers to the synthesized nanotubes at room temperature.
XRD patterns show that (001) peaks of (Na, H)2Ti3O7 nanotubes are slightly shifted toward higher angle after the sample was annealed at 400, 450, and 500 °C for 1 h (Figure 1). Decreasing d 001 spacing may be caused by dehydration of the nanotubes. At high temperature (e.g. 800 °C), the (Na, H)2Ti3O7 nanotubes transformed into Na2Ti6O13 completely within 1 h [Figure 2(E)]. Anatase appears as the sample is annealed for 8 h or longer at 800 °C [Figure 2(E)].
At low temperature (400 °C), the nanotubes are stable and show no change after the slight shift of the broad (001) diffraction peak. It is proposed that water molecules may be in the “interlayer-like” positions associated with H (Figure 3). Dehydration of interlayer-like water resulted in shrinking of d 001 spacing from 9.91 to 8.54 Å. Rehydration of the dried sample resulted in expansion of d 001 spacing from 8.54 to ~9 Å. Slight increase in d 001 spacing indicates that the nanotubes were partially rehydrated (especially at the nanotube edges) at room temperature (Figure 3).
Figure 3. (Color online) Schematic diagram of the crystal structure change in synthesized nanotubes during the dehydration and rehydration processes. The amount of water in this diagram does not necessarily represent the stoichiometric ratio of water in the crystal structure. The (100) spacing difference is exaggerated in order to show differences among crystal structures.
When the temperature is raised to 450 °C, the transformation begins after the sample has annealed for 16 h. An intermediate phase similar to the Na0.23TiO2 phase appears, although titanium has a valance state of +4 in the oxidizing environment. The intermediate phase appears even sooner at 500 °C [Figure 2(C)]. The final transformation product of (Na,H)2Ti3O7 is Na2Ti6O13. The intermediate phase similar to Na0.23TiO2 occurs at low temperature (Figure 2, B-16, B-32, C-8, and C-16 h) but not at high temperature [Figure 2(E)].
The structural evolution of Na2Ti3O7 to Na2Ti6O13 has been studied by Liu, H. et al. (Reference Liu, Yang, Zheng, Ke, Waclawik, Zhu and Frost2010). According to their paper, the synthesized Na2Ti3O7 sample transformed into Na2Ti6O13 when the crystals were annealed at 300 °C for 4 h under air. Their reported temperature is much lower than our observed transformation temperature. The nanotube structure may increase the stability of H-bearing Na-titanate [(Na,H)2Ti3O7]. In their paper, the synthesized Na2Ti3O7 product shows platy fibrous morphology and sharp peaks in the XRD pattern indicating the well-crystallized structure. If the intermediate product is ignored, the phase transformation of Na-titanate [(Na,H)2Ti3O7] nanotubes can be represented as follows:
$${\rm 2}\, {{\rm \lpar Na}\comma \; \, {\rm H\rpar }_{\rm 2}}{\rm T}{{\rm i}_{\rm 3}}{{\rm O}_{\rm 7}} \to {\rm N}{{\rm a}_{\rm 2}}{\rm T}{{\rm i}_{\rm 6}}{{\rm O}_{{\rm 13}}} + {{\rm H}_{\rm 2}}{\rm O}$$The phase stability of the nanotubes at high pressure was also investigated using a diamond anvil cell technique and EDXRD at X-17C of NSLS. We used bulk metallic glass as a gasket to avoid the interference of gasket XRD peaks with a samples' pattern. In the EDXRD patterns, we focus on two peaks at ~2.3 Å (400 and 401 diffraction peaks) and one peak at ~1.9 Å (020 diffraction peak) (Figure 4). The peak at ~1.9 Å is relatively sharp because the nanotube's elongation direction is b-axis [Figures 1(A), 2(B) and 4). Intensity of the 1.9 Å peak decreases as the pressure increases (Figure 4). The peak disappears at ~29.5 GPa. Intensity of the 2.3 Å peak increases as the pressure increases to 15 GPa and higher (Figure 4), which indicates that the nanotubes collapse at about 15 GPa. Further increase in the pressure resulted in flatter nanotube and a stronger 400 diffraction peak. The nanotubes finally transformed into an amorphous phase at about 30 GPa. The nanotubes were kept in an amorphous state while further compressed to 50 GPa according to the XRD observation (Figure 4). It was reported that the microparticle and nanocrystalline anatase (TiO2) started transforming to a baddeleyite structure at ~12–18 GPa. Additional Na may have kinetically hindered recrystallization of high-pressure TiO2 phases after the nanotubes were crushed in our experiments.
Figure 4. (Color online) In situ EDXRD patterns of the nanotubes under different pressures.
Both single-wall carbon nanotubes and multi-wall carbon nanotubes collapse at pressure of ~10 GPa (Chesnokov et al., Reference Chesnokov, Nalimova, Rinzler, Smalley and Fischer1999; Chen et al., Reference Chen, Wang, Tang, Xie and Jin2001; Venkateswaran et al., Reference Venkateswaran, Brandsen, Schlecht, Rao, Richter, Loa, Syassen and Eklund2001). The multi-wall carbon nanotubes were also reported to be in an amorphous state at 10–20 GPa by in situ XRD observation. The titanate nanotube is mechanically stronger than carbon nanotubes under static compression. Ionic bonding between neighboring Ti–O octahedral sheets through Na+ is the major reason for the observed mechanical strength under static compression.
ACKNOWLEDGEMENTS
This work was supported by the NASA Astrobiology Institute (N07-5489). The authors acknowledge Brookhaven National Laboratory for using a beam line facility at NSLS. The authors also thank Mr. Nicholas Levitt for providing useful suggestions.
