Morphology Tuned Pressure Induced Amorphization in VO₂(B) Nanobelts

Abstract

Pressure-induced amorphization (PIA) has drawn great attention since it was first observed in ice. This process depends closely on the crystal structure, the size, the morphology and the correlated pressurization environments, among which the morphology-tuned PIA remains an open question on the widely concerned mesoscale. In this work, we report the synthesis and high-pressure research of VO₂(B) nanobelts. XRD and TEM were performed to investigate the amorphization process. The amorphization pressure in VO₂(B) nanobelts(~30 GPa) is much higher than that in previous reported 2D VO₂(B) nanosheets(~21 GPa), the mechanism is the disruption of connectivity at particular relatively weaker bonds in the (010) plane. These results suggest a morphology-tuned pressure-induced amorphization, which could promote the fundamental understanding of PIA.

1. Introduction

Amorphous materials without long-range order present unique physical properties compared to their crystalline counterparts, which is the prime motivation behind research on materials science. Since the first observation of transforming crystalline materials into amorphous phase by applying pressure (pressure-induced amorphization, PIA), PIA has attracted considerable interest because of the disordering process and diverse properties of amorphous phase, and this phenomenon has been observed in many materials like H₂O, TiO₂, SiO₂, Y₂O₃ and CaAl₂Si₂O₈ [1,2,3,4,5,6]. From a certain point of view, PIA is a unique kind of pressure-induced phase transition, whose process depends closely on the crystal structure, the size, the morphology and the correlated pressurization environments of the materials. For example, size-dependent amorphization has been found in Y₂O₃ [5]. Apart from this, non-hydrostatic induced amorphization has been observed in B₄C [7] and MAPbBr₃ [8]. Recent high-pressure study on several nanomaterials with different morphology show morphology-tuned high-pressure phase transitions. For example, a different phase transition routine has been found in one-dimensional (1D) Si [9], AlN [10], ZnS [11], ZnO [12], SnO₂ [13], BaTiO₃ [14], LiMn₂O₄ [15] and Zn₂SnO₄ [16] in contrast to their bulk materials or two-dimensional (2D) nanomaterials. For morphology-tuned amorphization, although one-dimensional TiO₂ nanorods and nanorices show different onset pressures in pressure-induced amorphization revealed by synchrotron WAXS at atomic scale [17], open questions remain regarding the amorphization and morphology of typical oxide materials as a function of pressure on the widely concerned mesoscale. Determining the amorphization process is one of the crucial parameters required for understanding nanomaterials properties as the structure-function relationship, while the morphology-tuned pressure-induced amorphization remains unclear for the functional materials.

VO₂ is a representative polymorphic material with various conventional structures at ambient conditions, including tetragonal VO₂(A), monoclinic VO₂(M₁) and VO₂(B). Besides, VO₂ nanomaterials with different morphology have been synthesized in recent years, providing an ideal model for investigating the morphology effect on the structure transitions of nanomaterials [18,19,20,21,22]. Recent high-pressure study on VO₂(M1) suggest a M₁→Mx transition in bulk materials [23], and a transition sequence of M₁→M’₁→X up to 61.2 GPa in nanobelts [24], but no PIA is observed. For VO₂(A) phase, nanorods transform into an amorphous state at ~32 GPa [25], but lack of contrast in other initial morphology. VO₂(B) is an important polymorph at ambient condition owning a monoclinic symmetry C2/m and a layered structure composed of edge and corner-sharing VO6 octahedra [19]. Recently, PIA has been found in 2D VO₂(B) nanosheets at ~21.1 GPa [26], inspired by this result, it is reasonable to explore the phase transition process in VO₂(B) nanomaterials, and further clarify the potential morphology-tuned amorphization. The study of morphology effect on the PIA of VO₂ could potentially allow a more comprehensive understanding of the order–disorder transformation mechanism. This nanomaterial has a preferred orientation and a large L/D ratio (length/diameter ratio). Rods along axial direction can be regarded as micrometer material, while rods along radial direction can be regarded as nanomaterial. This could lead to an anisotropy of pressure effect between axial direction and radial direction, and finally result in various transition processes. An example is how the high mechanical stability of ZnS nanobelts results in an explosive mechanism for the wurtzite-to-sphalerite phase transformation, which is totally different with that in bulk and nanoparticle materials [11]. The strain energy of Si nanowires also generates a lower phase transition pressure (~2 GPa) and a larger bulk modulus (25%) than bulk Si [9].

Here we demonstrate the morphology-tuned pressure-induced amorphization in one dimensional VO₂(B) nanobelts using in situ X-ray diffraction and TEM measurements. The crystal-amorphous transition is completed at ~30 GPa, much higher than ~21 GPa in 2D VO₂(B) nanosheets [26], and the unique morphology-tuned amorphization mechanism has been carefully discussed. These results are different from previous work on VO₂(B) nanosheets, providing not only an experimental understanding of order–disorder transformation mechanism, but also a synthetic guideline for functional amorphous materials from their crystalline phases.

2. Experimental

2.1. Preparation of VO₂(B) Nanobelts

VO₂(B) nanobelts were synthesized by a hydrothermal method using V₂O₅ as the source of vanadium and oxalic acid (H₂C₂O₄ 2H₂O) as a reducing agent. All the chemical reagents in our experiments were of analytical grade and used without further purification. In a typical reaction, a certain amount of V₂O₅ (0.18 g) and oxalic acid (H₂C₂O₄ 2H₂O) (0.252 g) were added into 100 mL distilled water at room temperature to form a suspension solution. The suspension was continuously stirred until a clear orange transparent solution was formed, after which the solution was placed in a 50 mL Teflon cup with a filling ratio 0.6 and then heated in a sealed autoclave with a stainless steel shell at 180 °C for 1 day.

After hydrothermal treatment, the autoclave was cooled down naturally to room temperature. The resulting black precipitates were collected by centrifuge tubes, washed with distilled water and dehydrated alcohol for several times by centrifuging, and then dried in vacuum freeze dryer for 2 days.

2.2. Morphology and Crystal Structure Characterizations

The morphology and crystal structure of VO₂(B) nanobelts were characterized using Transmission Electron Microscope (TEM, 200 KV, HITACHI, H-8100IV), High Resolution Transmission Electron Microscope (HRTEM, 80 KV, JOEL, JEM-2200FS), Raman (Renishaw in Via) with 514.5 nm laser excitation and X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15418 nm). High pressure measurements were generated by a diamond anvil cell (DAC)In situ angle-dispersive synchrotron XRD (ADXD) measurements under high pressures were carried out at 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF). The wavelength is 0.6199 Å. The X-ray beam size is 26 × 8 µm². The position and orientation of the detector to the X-ray beam were calibrated using the diffraction pattern of a CeO₂ standard. The culet size of DAC is ~400 μm in diameter. The gasket was T301 stainless steel that was pre-indented to ~20 GPa. A hole with a diameter of ~150 μm was drilled to serve as the sample chamber by a laser drilling system at the center of the gasket indent. The powder was pre-pressed into a thin pellet and cut into flakes with proper sizes for DACs. Then, the tiny flake was loaded into a DAC along with a tiny ruby ball. Argon was used as the pressure-transmitting medium (PTM), which was loaded by cryogenic loading method. The background scattering was collected at each pressure point by shining the X-ray beam on the empty area inside the sample chamber, which only passed through the pressure medium and two diamond anvils. The exposure time was set at 30 s. One-dimensional (1D) XRD patterns were obtained by integrating the 2D patterns along the azimuth angle from 0° to 360° with the Dioptas software [27] The resulting diffraction patterns were refined using the GSAS package [28]. The pressures were determined from the pressure dependent shift of the R1 line fluorescence of ruby [29]. For the HRTEM of the recovered sample, the powder samples were dispersed in ethyl alcohol and picked up using a holey carbon grid. HRTEM experiments were carried out using the Field Emission Electron Microscope (JEM-2200FS) in Jilin University. An accelerating voltage of 80 kV was used to minimize the possible electron irradiation effect on the sample structure.

3. Results and Discussion

The crystal structure and phase composition of the obtained products were revealed by XRD. As shown in Figure 1a, all the diffraction peaks match well with the standard XRD pattern of a monoclinic phase (space group: C2/m, JCPDS No. 81-2392) and no peaks of any other vanadium oxide phases or impurities were detected. The strong and sharp diffraction peaks suggest high crystallinity. The lattice parameters of our sample refined by GSAS are a = 12.166 Å, b = 3.655 Å, c = 6.423 Å, β = 105.78°, which are in good agreement with JCPDS No. 81-2392 (a = 12.09 Å, b = 3.702 Å, c = 6.433 Å, β = 106.6°) and previous work [21,30]. Raman spectra of as-prepared VO₂(B) nanobelts are shown in Figure 1b. The peaks located at 193, 270, 313, 405, 512, 699 and 904 cm⁻¹ are attributed to Ag, Bg, Ag, Ag, Bg, Bg and B2g modes, respectively. Our Raman results match well with previous work [31,32,33].

From Figure 1c we can see that all the as-prepared products are belt-like morphology. The diameters of the belts are in a short range between 140 nm to 240 nm, and the length of the nanobelts are estimated to be several micrometers. In Figure 1d, the selected-area electron diffraction (SAED) pattern (inset) taken from the individual nanobelt demonstrates that the synthesized samples are single-crystal. In Figure 1e, the HRTEM image of the individual nanobelt of Figure 1d clearly exhibits lattice fringes, demonstrating that the synthesized nanobelts are single-crystal with a preferred [010] orientation. Our results correlate well with previous work in VO₂(B) nanowires and nanorods [34,35,36].

The crystal structure of VO₂(B) belongs to a space group of C2/m, which consists of a three-dimensional framework of VO6 octahedra. The VO₂(B) structure can be considered as formed by two identical layers of atoms along b, and the second layer is shifted with respect to the first one by 1/2, 1/2, 0. The pressure evolution of the XRD patterns of VO₂(B) nanobelts are shown in Figure 2a. All the Bragg peaks shift to larger angles, showing the shrinkage of the VO₂(B) lattice. The diffraction peaks of argon appear at 2.7 GPa and last till the highest pressure 40 GPa, which is similar to Santilla’n’s work in Mn₂O₃ [37]. Upon compression to 20 GPa the diffraction peaks of the sample broaden dramatically, the intensity of these diffraction peaks decrease sharply and several weak peaks disappear without occurring any new peak. With increasing pressure to 29.6 GPa, the disappearance of VO₂(B) peaks signifies that the sample transformed into an amorphous state completely. This amorphous state lasts till the highest pressure 40 GPa. Upon decompression, the peaks of Ar disappear and no new peak emerges when the pressure is released to ambient condition, suggesting an irreversible amorphization. This PIA is similar to the previous study in VO₂(B) nanosheets [26] and high pressure structural transition of VO₂(A) in our previous work [25], but totally different with the crystal-crystal structural transition in VO₂(M1) [23,24,38]. It is noticeable that the amorphization pressure in this work is significantly higher than that in VO₂(B) nanosheets [26], similar to that in VO₂(A) nanorods. Furthermore, we have refined the bulk modulus of VO₂(B) nanobelts by the Birch–Murnaghan equation, bulk modulus B₀ = 132.9 GPa, pressure derivative B₀′ = 4, and the zero-pressure volume V₀ = 274.3 ų, the observed bulk modulus is slightly higher than that in nanosheets (B₀ = 129.1 GPa, B₀′ = 4).

From the pressure dependence of the d value of three selected peaks (001), (020) and (200) upon compression in Figure 2b, we can find a nonlinear variation of (020) peak at 12 GPa in contrast to the linear dependence of (001) peak and (200) peak. The low compressibility of (020) planes above 12 GPa reveals that the movement of the atoms along the b axis becomes difficult. The pressure transmitting medium in this experiment is argon, whose highest hydrostatic pressure differs from ~12 GPa [39] to ~30 GPa [40] in the previous research, if the aeolotropy of pressure effect by the [010] orientation is derived from the non-hydrostatic effect at higher pressures, similar discontinuity should also be observed in the [001] and [100] orientation, but Figure 2b shows a linear dependence of (001) peak and (200) peak. Thus, the aeolotropy of pressure effect by the [010] orientation at ~12 GPa has no relationship with the non-hydrostatic effect of PTM, and it is reasonable to speculate that this aeolotropy is related to the unique crystal structure, especially the orientation of the VO6 polyhedra in the crystal lattice.

The crystal structure of VO₂(B) is shown in Figure 3. There are two identical layers of atoms along b in a unit cell, the second layer is shifted with respect to the first one by 1/2, 1/2, 0, and VO6 octahedra are connected with each other by corners or edges. For the circled regions in Figure 3b, the large empty spaces are relatively easier to be compressed than the solid octahedra. On the other hand, the connection between corner-shared VO6 octahedra is much weaker than the connection between edge-shared VO6 octahedra. From the mechanical-stability point of view, breaking the connection of octahedra in the (010) plane is much easier than that in the (100) plane and the (001) plane [41]. Therefore, the coherence along the [010] direction may break first at a critical pressure.

For the various compressibility in three dimensions of VO6 octahedra, we can find that the structure of VO₂(B) is formed by two identical layers of atoms along the b axis, the VO6 octahedra along the b axis are all edge-shared, and the VO6 octahedra arrangement is relatively dense. While along the a axis and c axis, there are some corner-shared VO6 octahedra, and the VO6 octahedra arrangement is relatively sparse. This can explain the discontinuity of (020) peak in Figure 2b, indicating a lower compressibility along the b axis than the other two axes.

To further reveal the structure change of VO₂(B) nanobelts during amorphization, we carried out high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analysis for the pristine sample and samples released from 15 and 30 GPa, respectively. For the sample released from 15 GPa, there is a small amount of amorphous segments in the crystalline lattice in contrast to the pristine one, as marked by rectangles in Figure 4b. Moreover, many diffraction points become weak and disappear and an amorphous halo emerges in Figure 4e. Hence, in the regions marked by rectangles, the local structure of sample is expected at the beginning stage of amorphization. For the sample released from 30 GPa, no crystalline lattice is observed in Figure 4c and only an amorphous halo emerges in Figure 4f. This indicates that the amorphization was completed and the sample transformed to the amorphous state totally at 30 GPa. The whole process is clear and in good agreement with XRD data.

Combining the above data, we propose that the amorphization is possibly induced by the disruption of connectivity between the octahedra in the (010) plane. From Figure 1d,e, we can find the nanobelts in our experiments own a [010] growth orientation, the amount of edge-shared VO6 octahedra along this orientation, in other words, along the b axis, is much larger than that along the a axis and c axis, making a higher amorphization pressure than nanosheets, indicating a morphology-tuned pressure-induced amophization.

4. Conclusions

In summary, observations of PIA in VO₂(B) nanobelts have been performed using high pressure XRD and HRTEM analysis. The initial monoclinic phase transformed into an amorphous form at ~30 GPa. Upon decompression, the amorphous retained until the ambient condition. We propose that the PIA is due to the disruption of connectivity at particular, relatively weaker bonds in the (010) plane, which is helpful and powerful to probe into the implications on the nature of morphology-tuned amorphization. Further, such pressure-induced amorphous nanomaterials hold great promise for numerous applications in the future, and the corresponding synthetic guideline would motivate us to discover more pressure-induced amorphous materials.