Pressure-induced phase transitions of TiO₂ nanosheets with high reactive {001} facets

Abstract

The high pressure phase transition behaviors of anatase TiO₂ nanosheets with high reactive {001} facets were studied using in situ synchrotron X-ray diffraction and Raman spectroscopy. The phase transition from starting anatase phase to a low ordered baddeleyite structure was found upon compression. Upon decompression, the low ordered baddeleyite structure transforms into an α-PbO₂ phase. The obtained bulk modulus for TiO₂ nanosheets is 317 (10) GPa, which shows ultralow compressibility compared with those of nanoparticles and bulks. We suggest that the enhanced bulk modulus for the TiO₂ nanosheets can be attributed to their ultrafine thickness along the [001] direction with fewer “soft” empty O6 octahedra distributed in the TiO₂ nanosheets than in other nanostructures and bulks. TiO₂ nanosheets retain their original morphology after being released to ambient pressure. These results indicate that the sheet-like morphology with exposed {001} facets plays important roles in the high pressure phase transition of TiO₂ nanosheets.

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Introduction

As a novel functional nanomaterial, anatase TiO₂ nanosheets with high reactive {001} facets have attracted much attention because of their potential applications in photocatalysis, lithium ion batteries, and solar cells.^1–5 Both experimental and theoretical studies found that the {001} facets of anatase TiO₂ are more reactive than the thermodynamically stable {101} facets. Roy et al.⁴ reported that TiO₂ nanosheets exhibited excellent photocatalytic activity by adjusting the optimum ratio of the high energy {001} and low energy {101} facets. Ultra-thin TiO₂ nanosheets showed a high H₂ evolution rate (7381 μmol hg⁻¹) under UV-vis light irradiation because of their exposed reactive {001} facets and high crystallinity.⁵ Further, N and S co-doped TiO₂ nanosheets were synthesized and showed superior photocatalytic performance under visible light irradiation.³ These studies indicated that the excellent performance of TiO₂ nanosheets depends on their unique nanostructure, which provides new insight for investigating physicochemical properties of TiO₂ nanomaterials.

Recently, nanosize and morphology effects on high pressure phase transition behaviors of TiO₂ have been widely studied.^6–14 Previous studies showed different structural stabilities and phase transition process between nanomaterials with unique morphologies and conventional TiO₂ nanoparticles or bulks. These included size-dependent pressure-induced phase transitions that were found in anatase TiO₂ nanoparticles, which have been explained by a critical grain size mechanism.^7,8,11 In addition, Lee et al. studied the compressibility of TiO₂ nanoparticles with different shapes (rod- and rice-like) and found that the shape shows obvious modulation effects on compression of the nanoparticles.¹³ Recently, we further studied morphology-tuned phase transitions in TiO₂ nanomaterials under high pressure.^9,10,15 The direct phase transition from anatase to α-PbO₂ phase was found in TiO₂ nanowires, and unique pressure-induced amorphization and polyamorphisms were observed in TiO₂–B nanoribbons, these are attributed to their particular one-dimensional nanostructures.^9,10 These studies suggest that the high pressure behaviors of nanomaterials largely depend on their nanoscale morphology. However, previous studies focused most on sphere-like nanoparticles^{7,8,11,12,14,16} and quasi-one dimensional nanomaterials (nanorods, nanowires, nanobelts, and so on).^9,10,17–19 There is limited amount of high pressure studies on nanosheets with special exposed facets. TiO₂ nanomaterials with unique exposed facets usually show excellent performance, which provides a method for tailoring the properties of nanomaterials.^2–5 In addition, it is also an interesting subject for investigating the high pressure behaviors of nanomaterials with significant one-dimensional confinement. Therefore, anatase TiO₂ nanosheets with exposed {001} facets would be a typical model for high pressure study of nanomaterials.

Here, we report the high pressure study of TiO₂ nanosheets using synchrotron X-ray diffraction and Raman spectroscopy. The starting anatase phase transforms into a low ordered baddeleyite structure upon compression. Upon decompression, the low ordered baddeleyite structure transforms into the α-PbO₂ phase. A significantly enhanced bulk modulus (∼317 GPa) for the TiO₂ nanosheets was obtained. We suggest that the sheet-like morphology with exposed {001} facets plays important roles in the high pressure phase transitions of TiO₂ nanosheets.

Experimental section

TiO₂ nanosheets were synthesized by a hydrothermal method as described in the previous report.²⁰ In a typical synthetic procedure, Titanium butoxide (Ti(OC₄H₉)₄, 15 ml) and hydrofluoric acid solution (1.8 ml, 40 wt%) were mixed in a Teflon-lined autoclave with a capacity of 50 ml, and then kept at 180 °C for 24 h. After hydrothermal reaction, white products were collected and washed with ethanol and deionized water several times, and then dried in an oven at 80 °C for 24 h. The morphology of the sample was characterized using high resolution transmission electron microscope (HRTEM, JEM-2200FS). High pressure XRD experiments were performed at the X17C beamline of Brookhaven National Laboratory. The incident wavelength of the beam is 0.4066 Å with a beam size of 25 × 30 μm². The high pressure was generated by a symmetric diamond-anvil cell (DAC). A 4 : 1 mixture of methanol and ethanol was used as the pressure transmitting medium. The pressure was determined from the shift of the ruby fluorescence line. High pressure Raman spectra were collected using a Renishaw inVia Raman spectrometer with an Ar-ion laser (λ = 514.5 nm). The recovered sample from high pressure was transferred to a carbon-coated copper grid for HRTEM observation.

Results and discussion

TEM and HRTEM images show the morphology and crystal structure of our sample. As shown in Fig. 1a, the sample is a typical sheet like nanostructure with a rectangular shape. The side length and the thickness of the TiO₂ nanosheets are ca. 20–40 nm and 5–8 nm, respectively. The HRTEM image (Fig. 1b) shows that the lattice spacing is 0.351 nm, corresponding to (101) planes of anatase TiO₂. Meanwhile, Fig. 1c demonstrates the lattice spacing of 0.236 nm, corresponding to (001) planes of anatase TiO₂. Based on the TEM/HRTEM results, it can be concluded that the TiO₂ nanosheets are dominated by {001} facets and their percentage is estimated to be 70% statistically. This indicates that ultrafine thickness (5–8 nm) along the [001] direction is a crystal feature for the TiO₂ nanosheets with a high percentage of {001} facets.

Fig. 1 TEM (a) and HRTEM (b, c) images of TiO₂ nanosheets.

The selected XRD patterns of TiO₂ nanosheets are shown in Fig. 2. All diffraction reflections at ambient pressure can be indexed to the anatase structure with lattice constants of a = b = 3.789 Å and c = 9.515 Å, consistent with the previous results.⁶ Upon compression, the anatase phase was found to persist to 12.3 GPa. At 14.6 GPa, the anatase phase starts to transform into a baddeleyite structure. With further increasing pressure, the broadening of the diffraction peaks for the anatase phase become very obvious and the intensity of these peaks weaken significantly. Above 22.8 GPa, the anatase phase transforms into the baddeleyite structure completely. However, the peak intensities of the baddeleyite phase do not increase further after 18.5 GPa. The XRD peaks broaden gradually upon further compression. Eventually, these weakened and broadened peaks exist up to 35.8 GPa. This indicates that a low-ordered baddeleyite structure or a partial amorphous phase form during the phase transition. Upon decompression, the high pressure structure transforms into the α-PbO₂ phase (as shown in Fig. 2b). This is in good agreement with previous studies.^6,9,11,12 We suggest that the reverse transition of the α-PbO₂ phase to the anatase phase must be sluggish and a high kinetic barrier exists.

Fig. 2 Selected X-ray diffraction patterns of TiO₂ nanosheets under high pressures. (a) Compression; (b) decompression. The diffraction peaks for baddeleyite phase are marked as B.

Fig. 3 shows the pressure-dependent variations of the relative lattice parameters (a/a₀, c/c₀) for the TiO₂ nanosheets in the range of 0–14.6 GPa. The c-axis length is more compressible than the a-axis length. The anisotropic compressibility between the a-axis and c-axis is a structural character of the anatase TiO₂ crystal, which shows more occupied TiO6 and less empty O6 octahedra on the a-axis than on the c-axis. In the pressure range of 0–12.3 GPa, the relative lattice parameters (a/a₀, c/c₀) show monotonic decreases with increasing pressure. However, an abnormal increase for the relative lattice parameters occurs at 14.6 GPa. It is possibly derived from the crystal lattice distortion during the phase transition from anatase to baddeleyite phase. Moreover, compared with anatase TiO₂ nanoparticles and bulks, we found that the slope of c/c₀ in the range of 0–8 GPa is less than those of nanoparticles and bulks.^9,13,25 This indicates that the c-axis of TiO₂ nanosheets is less compressible than those of TiO₂ bulks and nanoparticles with other morphology. Previous studies have explained the anisotropic compressibility along different directions in terms of the population of the hard TiO6 and soft O6 octahedra.^13,25 Park and his coauthors¹³ suggested that the nanoparticle shape affects the bulk compressibility and explained it by the differences in the relative pollution of the soft empty O6 octahedral units within different nanocrystals. Therefore, the less compressibility of TiO₂ nanosheets might be attributed to the obvious nanoconfinement of c-axis in our case, which results in fewer soft empty O6 octahedral units in TiO₂ nanosheets.

Fig. 3 Pressure dependence of relative lattice parameters (a/a₀, c/c₀) of TiO₂ nanosheets. The squares and circles represent the a/a₀ and c/c₀ for the anatase TiO₂ nanosheets, respectively.

The evolution of the volume with pressure is shown in Fig. 4. The pressure–volume data of the anatase phase was fitted to the third-order Birch–Murnaghan equation of state. The bulk modulus (B₀) of the anatase TiO₂ nanosheets was determined to be 317 (10) GPa with the first derivative (B^′₀) being fixed at 4. The bulk modulus is much higher than those of nanoparticles (180–240 GPa)^6–8,21,22 but similar to that of the rice-shape nanoparticles (319 (20) GPa).¹³ This indicates that the sheet like morphology enhances significantly the bulk modulus for TiO₂ nanosheets. For an anatase TiO₂ crystal, there are more “soft” empty O6 octahedra on the c-axis than on the a-axis.¹³ The lowest dimension (5–8 nm) of the TiO₂ nanosheets is along the [001] orientation (c-axis). Thus, there are fewer “soft” empty O6 octahedra distributed in the TiO₂ nanosheets than in other TiO₂ nanoparticles. It is reasonable to explain the enhanced bulk modulus for TiO₂ nanosheets can be attributed to their sheet like morphology, which shows significant nanosize effects because of their ultrafine thickness along the [001] direction.

Fig. 4 Pressure–volume diagram of TiO₂ nanosheets.

Raman measurements were performed on TiO₂ nanosheets with selected spectra depicted in Fig. 5. As shown in Fig. 5a, the typical Raman modes of anatase TiO₂ at ambient pressure were observed at 141 cm⁻¹ (E_g), 194 cm⁻¹ (B_1g), 394 cm⁻¹ (B_1g), 514 cm⁻¹ (A_1g and B_1g), and 635 cm⁻¹ (E_g), consistent with previous studies.¹¹ An additional weak Raman band at 490 cm⁻¹ occurs at 16.7 GPa, which can be attributed to the baddeleyite phase. It shows that the baddeleyite phase did form. The band intensities of the baddeleyite structure increase first and then decrease gradually with increasing pressure. Above 32 GPa, no obvious Raman bands can be observed. This likely indicates that the sample became highly disordered or partly amorphous. Upon decompression, the high pressure structure persists down to 9.7 GPa, and then transforms into the α-PbO₂ phase at lower pressure. When the pressure is released to ambient pressure, the bands of the α-PbO₂ phase are still broadened dramatically with weakened intensities. Obviously, the quenched sample has the α-PbO₂ structure with poor crystallinity.

Fig. 5 Raman spectra of TiO₂ nanosheets upon compression (a) and decompression (b).

The morphology change of the TiO₂ nanosheets during the compression–deconpression cycle was investigated by TEM/HRTEM. As shown in Fig. 6a, the quenched sample still has rectangular sheet-like morphology but with some distortion. Fig. 6b shows the lattice spacing of 0.286 nm, corresponding to (111) planes of the α-PbO₂ phase. It is obvious that TiO₂ nanosheets with a high pressure α-PbO₂ phase were obtained. In our previous studies, we have found the morphologies of TiO₂–B nanoribbons, anatase nanowires, and nanoporous particles possessed excellent stability during the compression–decompression cycles.^9,10,15 These results demonstrate that high pressure α-PbO₂ structural TiO₂ nanomaterials with various morphologies can be obtained by high pressure treatment of the counterpart starting materials. This opens the possibility to obtain other novel functional nanomaterials with high pressure structures.

Fig. 6 TEM (a) and HRTEM (b) images of TiO₂ nanosheets after being released from 35.8 GPa to ambient pressure.

It has been found that both size and morphology play important roles in the high pressure phase transitions. For TiO₂ nanomaterials, size-dependent selective phase transitions have been found in anatase TiO₂ nanoparticles,^7,8,11 morphology-tuned phase transition behaviors have been discovered in one-dimensional TiO₂–B nanoribbons and anatase TiO₂ nanowires.^9,10 The size of nanoparticles and their growth orientations dominate structural stabilities and phase transition behaviors largely. In this study, the TiO₂ nanosheets are typical two dimensional nanostructures with the lowest dimension in thickness (5–8 nm). The thickness size is less than the critical diameters for nucleation and growth of the baddeleyite phase (∼12 nm) and α-PbO₂ phase (∼50 nm) proposed by experiments,^7,8,11 but is larger than the critical diameter of the baddeleyite phase (∼4 nm) and lower than that of the α-PbO₂ phase (∼15 nm) that were calculated by Hearne et al.¹¹ Therefore, in our case, the anatase to low-ordered baddeleyite phase transition in TiO₂ nanosheets may be dominated by the nanosize effects in thickness. We propose that the small thickness size precludes the nucleation and growth of baddeleyite upon compression, which results in a low ordered or poor crystalline baddeleyite phase forming under high pressure.

With decreasing particle size, surface energy plays a significant role in the structural stability.^11,12,18,23 It has been found that the more reactive {001} facets have a higher surface energy (0.9 J cm⁻²) than that of the thermodynamically stable {101} facets (0.44 J cm⁻²) in anatase TiO₂.^24,25 Thus, the total surface energy of the TiO₂ nanosheets with a large percentage of reactive {001} facets is much higher than that of the conventional nanoparticles. Previous studies have indicated that a high surface energy leads to an increase of the phase transition pressure for TiO₂ nanoparticles.^7,8,11,26 According to this view, the phase transition pressure of TiO₂ nanosheets should be higher than that of TiO₂ nanoparticles. However, the phase transition pressure (12.3–14.6 GPa) for the TiO₂ nanosheets is lower than that of nanoparticles.²⁶ Obviously, the unique phase transition behaviors cannot be explained by the surface energy difference, but are likely to be dominated by the significant nanoconfinement effects along [001] direction. We suggest that the unique nanosheet-like morphology with exposed {001} facets not only influences on the compressibility but also the phase transition process.

Conclusions

In summary, we have studied the high pressure phase transition behaviors of anatase TiO₂ nanosheets with reactive {001} facets using in situ synchrotron X-ray diffraction and Raman spectroscopy. TiO₂ nanosheets transform from the starting anatase phase to the low ordered baddeleyite phase at ∼14.6 GPa directly. Upon decompression, the low ordered baddeleyite structure transforms into the α-PbO₂ phase. We suggest that the small thickness size precludes the nucleation and growth of baddeleyite upon compression, which leads to a low ordered baddeleyite phase forming under high pressure. The enhanced bulk modulus (B₀ = 317 (10) GPa) of the anatase TiO₂ nanosheets was obtained, which can be attributed to the significant nanosize effects in c-axis ([001] direction) that result in fewer “soft” empty O6 octahedra distributed in the TiO₂ nanosheets than in other nanostructures. In addition, α-PbO₂ phase TiO₂ nanosheets were obtained after a compression–decompression cycle. Our results show that the sheet-like morphology with exposed {001} facets plays important roles in the high pressure behaviors of TiO₂ nanosheets.

Acknowledgements

This work was supported financially by the National Basic Research Program of China (2011CB808200), the NSFC (10979001, 51025206, 51032001, 21073071, and 11004075), and the Cheung Kong Scholars Programme of China. This research was partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 06-49658.

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