Formation of Novel Bimetal Oxide In2V2O7 through a Shock Compression Method.

Bimetal oxides with a chemical formula of A2B2O7 have received much attention from plenty of research groups owing to their outstanding properties, such as electronic, optical, and magnetic properties. Among the abundant element combinations of cations A and B, some theoretically predicted compounds have not successfully been synthesized in experiments, such as In2Zr2O7, In2V2O7, etc. In this study, a novel tetragonal pyrochlore-like In2V2O7 nanopowder has been reported for the first time. In2O3 and VO2 powders mixed through ball milling were reacted to form In2V2O7 by shockwave loading. The recovered sample is investigated to be nanocrystalline In2V2O7 powder through various techniques, such as X-ray diffraction, scanning electron microscopy, X-ray energy spectrum analysis, and transmission electron microscopy. The formed In2V2O7 is indexed as a tetragonal cell with a = b = 0.7417 nm and c = 2.1035 nm. Moreover, the formation mechanism of In2V2O7 through a shock synthesis process is carefully analyzed based on basic laws of shockwave. The experimental results also confirm that a high shock temperature and high shock pressure are the two key factors to synthesize the In2V2O7 nanopowder. Our investigation demonstrates the high potential application of a shock-induced reaction on the synthesis of novel materials, including the preparation of new bimetal oxides.

Introduction

The human quest for new and exotic materials has given thrust to the research on materials science, urging researchers to focus on creating (exploring) materials with exotic properties by disturbing their original electronic structure using various external parameters such as pressure and/or temperature.[1]

Among plenty of materials, bimetal oxides are important functional semiconductor materials featuring low cost and outstanding properties owing to the coupling effect of two metal elements.[2−4] With respect to the bimetal oxide with a general formula of A2B2O7, coupling effects lead to their various applications,[5−8] including multiferroics,[9,10] radiation resistance,[11] high-temperature heating materials,[12] sensors,[13,14] fuel cell electrode materials,[14,15] nuclear waste solidification,[16] and photocatalytic materials.[17−20] Transition metal vanadate nanomaterials are important functional materials owing to their fascinating structures and outstanding properties, such as electronic,[21,22] optical,[21,23] catalytic,[24] and magnetic properties.[25,26]

Since Ye et al.[27] reported the high photocatalytic performance of monoclinic phase InVO4-III semiconductors, the indium vanadate materials have attracted the attention of many research groups. Because of their narrow band gap, suitable redox potential, and wide corresponding light range, indium vanadates, such as InVO4 and InVO3, possess good application prospect in photocatalytic decomposition of water to produce hydrogen,[27] for photocatalytic degradation of organic pollutants,[28] and as sensor materials.[29,30] Several methods have been applied to prepare indium vanadate materials, including a high-temperature solid-state method,[27] hydrothermal method,[31] sol–gel method,[32] spray pyrolysis,[33] electrospinning method,[34] etc. Through the above methods, InVO4 and InVO3 are obtained by mixing precursors and inducing a synthetic reaction at certain temperatures with normal pressure. Synthesis of indium vanadate at a high pressure has rarely been reported. Based on previous research studies on bimetal oxide with the A2B2O7 formula, the radii of In3+ and V4+ are suitable to synthesize In2V2O7 in theory.[5] However, in spite of the suitable ionic radii, the corresponding pyrochlore oxide could not be stabilized under normal pressure synthesis conditions. High pressure techniques have been used to prepare pyrochlore-type materials. For example, the formation pressure to synthesize Mn2V2O7 is 7 GPa[35] through a static high pressure method. However, the research corresponding to the synthesis of In2V2O7 has not been reported through a high-pressure synthesis method or other methods.

Shock compression technology features a high dynamic pressure, high temperature, high quenching rate, etc.[36] Thus, it is utilized to process materials with large volume compared with static compression technology. Through this technology, the shockwave acts on a target material to generate ultrahigh pressures (100–102 GPa) and temperatures (102–104 K) in microseconds, inducing multiple physical and chemical phenomena. Consequently, shock loading approaches have been widely applied in various material processing fields, such as metallic material hardening,[37−39] material doping,[40] synthesis of novel materials,[41−43] shock-induced phase transformation,[44,45] and shock activation of materials.[46] With respect to this method, several techniques have been developed to adjust the pressure and temperature during shock compression for the synthesis of materials, such as organic molecules using inorganic precursors,[47] nanodiamonds,[48] doped TiO2,[49] and doped graphene.[43] Recently, a shock-induced chemical reaction has been applied to prepare bimetal oxide materials, such as Gd2Zr2O7,[50] La2Ti2O7,[36] etc.

This study presents the synthesis of novel bimetal oxide, In2V2O7 nanopowder, through a shock-induced chemical reaction using In2O3 and VO2 powders as precursors. The experimental results demonstrate that a high shock pressure and high shock temperature are the two critical factors to improve the formation of In2V2O7 through this method.

Experimental Procedures

In the experiments, a copper recovery device was designed with three circumferentially uniformly distributed sample holes with a diameter of 10 mm and depth of 1 mm (see Figure ). The precursor powder, a mixture of In2O3 and VO2 powders with a mole ratio of 1:2, was activated through ball milling treatment for 6 h. The corresponding conditions include a milling ball material of zirconia, milling speed of 750 r/min, and ball milling medium of 20 g of ethanol and helium gas. Subsequently, the activated precursor powder was filled into the sample holes with a certain initial density (ρ00, identified as the ratio of precursor mass to sample hole volume) and blocked by a copper plug.

Figure 1

(a, b) Schematic of shock compression assembly. 1: detonator, 2: detonation lens and main charge, 3: flyer and flyer cover, 4: PVC surpport, 5: sample recovery device, 6: copper plug, 7: steel base, 8: momentum-trap blocks, and 9: precursor.

Then, the sample recovery device with a flyer, a main charge, and other devices was assembled, as shown in Figure . The detonation lens and main charge (a mixture of RDX and a polymer binder with a detonation velocity of 8315 m/s) were applied to accelerate the copper flyer. Then, the flyer impacts on the sample recovery device to generate a high temperature and high pressure in the precursor to synthesize In2V2O7. In this experiment, the different initial relative densities of precursor powder (ρ00/ρ, in which ρ is the theoretical density of the precursor powder) were used to adjust the shock temperature owing to the adiabatic process during shock loading. In general, a low relative density leads to a stronger adiabatic compression, inducing a higher shock temperature. In addition, the impact velocity (v) was used to adjust the shock pressure. The corresponding experimental conditions are listed in Table . After shock experiments, the shock-treated samples were recovered for further characterization.

Table 1

Experimental Conditions for Shock Loading on Precursor Powdersa

no.	ball mill time (h)	ρ00 (g·cm–3)	ρ00/ρ	v (km·s–1)	p (GPa)	IIn2O3:Inew
1	6	5.178	0.91	2.2	54.7	557:100
2		3.755	0.66	2.2	54.7	79:100
3		2.333	0.41	2.2	54.7	65:100
4	6	5.235	0.92	3.2	90.3	1042:100
5		3.642	0.64	3.2	90.3	21:100
6		2.219	0.39	3.2	90.3	0:100

ρ00 is the initial density, ρ00/ρ is the initial relative density, v is the impact velocity, p is the shock pressure. IIn2O3:Inew is the intensity ratio of the XRD peak at 31.8° (strongest peak of the new phase) to that of In2O3.

Considering the 1 mm thickness of precursor powder in the sample holes, the shockwave reflects at the device–precursor interface and precursor–plug interface repeatedly to increase the pressure and temperature in the precursor powder and induce the chemical reaction during the shock process.

The impact velocity (umax) values were calculated according to the Gürney equation[51]where D is the detonation velocity of the explosive, ρ0 is the density of the explosive, l is the height of the main charge, ρM is the flyer density, and d is the flyer thickness.

In the calculation of shock pressure, the mixed precursors were treated as a thin wafer (1 mm thick) enclosed by copper. After the impact induced by the shock-driven flyer, the generated shockwave reflects at the upper and lower interface between the thin precursor wafer and surrounding copper repeatedly, leading to the increase in shock pressure in the precursor. After multiple reflections, the shock pressure in the precursor reaches the pressure at the interface between the copper box and flyer. The shock pressure values were calculated based on the impedance match method. The detailed calculation process has been explained in (36) and (51). The corresponding experimental parameters and calculated results are listed in Table .

The X-ray diffraction (XRD) patterns of the recovered samples were recorded by operating a BRUKER D8 at a working voltage of 40 kV and a working current of 40 mA with Cu Kα radiation. The phase analysis and content of recovered samples were investigated through XRD results. The microstructures and morphology of samples were characterized by using a Tecnai G2 F30 transmission electron microscope (TEM) with an accelerating voltage of 300 kV and a HITACHI S4800 scanning electron microscope (SEM), respectively. The chemical composition was further probed by XPS analysis using a Thermo ESCALAB 250 Xi spectrometer with a monochromatic AlK (1486.6 eV) X-ray source. The obtained XPS spectra were fit with nonlinear Shirley background correlation.

Results and Discussion

Figure shows the XRD patterns of the ball-milled precursor (M0) and the recovered Nos. 1–6 samples, revealing the presence of In2O3, VO2, and copper and several XRD peaks of a new phase. The copper XRD peaks in Nos. 2–6 samples are from the copper fragments of the copper sample recovery device. The XRD pattern of the ball-milled precursor presents characteristic peaks at 21.5, 27.8, 30.5, 35.5, 37.8, 39.8, 45.6, 51.0, and 60.7°, showing the presence of In2O3 and VO2. It indicates that ball milling treatment can hardly lead to the chemical reaction of In2O3 and VO2 under the conditions in these experiments.

Figure 2

XRD patterns of ball-milled precursor powder and recovered Nos. 1–6 samples.

With respect to the XRD results of recovered samples, besides the peaks of In2O3, VO2, and copper, several XRD peaks are observed at 14.6, 29.4, 36.3, 39.3, 53.9, 59.7, 62.6, 66.8, and 77.2°. These peaks are not matched with the known phases, implying the formation of a new phase through the shock loading process. Note that the peak positions and relative intensity values of the corresponding peaks are the same for the recovered samples (Nos. 2, 3, 5, and 6 samples, see Table ), suggesting that the formed new phase should be the same in these shock experiments.

Table 2

Peak Positions and Relative Intensities from XRD Patterns of Nos. 2, 3, 5, and 6 Samples

	no. 2		no. 3		no. 5		no. 6
peak number	2θ (°)	I (%)	2θ (°)	I (%)	2θ (°)	I (%)	2θ (°)	I (%)
1					14.63	7.52	14.59	8.68
2	29.47	49.90	29.47	51.30	29.50	51.23	29.44	49.84
3	31.79	100.00	31.83	100.00	31.81	100.00	31.86	100.00
4	36.33	50.11	36.35	50.61	36.32	49.21	36.36	52.89
5	39.29	21.92	39.32	22.01	39.33	21.76	39.38	21.86
6	46.71	6.34	46.62	6.03	46.58	6.23	46.66	6.59
7	53.88	30.39	53.91	30.20	53.69	29.89	53.94	30.55
8	59.79	21.14	59.77	21.05	59.75	20.88	59.81	21.38
9	61.29	6.35	61.25	6.19	61.22	6.12	61.22	6.59
10	62.76	22.41	62.69	22.38	62.62	21.98	62.65	22.51
11	66.81	6.58	66.71	6.68	66.68	6.65	66.75	6.75
12	77.19	7.09	77.23	7.21	77.12	7.11	77.29	7.23

The XRD pattern of No. 1 sample presents two new peaks at 29.4 and 31.8°, implying the formation of the new phase. The XRD patterns of Nos. 1–3 samples show that the increase in shock temperature leads to more new XRD peaks with higher intensities in the XRD patterns. Based on the XRD patterns, the intensity ratio of the strongest peak of In2O3 to that of the new phase (peak at 31.8°) (see Table ) is used to estimate the content of the new phase in recovered samples approximately. The intensity ratios of recovered samples suggest that the decrease in the initial relative density (corresponding to a higher shock temperature) can enhance the new phase content in recovered samples. In addition, under the same initial relative density, a higher impact velocity (corresponding to a higher shock pressure) also increases the new phase content in recovered samples by comparing Nos. 1–3 samples and Nos. 4–6 samples. With respect to the XRD pattern of No. 6 sample, there are only XRD peaks of copper and the new phase, implying that, at higher shock pressures (90 GPa) and temperatures, all precursor reacts to form the new phase compound.

Furthermore, Table and Figure also show the results of normalized peak intensity of the new phase with the peak at 31.86° as the standard XRD peak, revealing the high consistency of the relative intensity of corresponding peaks. Based on the above XRD results, it is assumed that the shock loading leads to the chemical reaction of In2O3 and VO2 under a high shock temperature and pressure. The higher temperature and pressure enhance the yield of the formed new phase. Under proper conditions (No. 6 sample), the mixed precursor reacted completely to form the pure new phase.

Figure 3

Relative peak intensities from XRD patterns.

Typical TEM images of No. 6 sample (Figure a) show quasi-spherical nanoparticles in sizes of 50–100 nm. The corresponding high-resolution TEM (HRTEM) (Figure b) further reveals the nanocrystalline structure with high crystallinity. The corresponding d-spacing values from Figure b are 3.04 and 2.48 Å, respectively, which are in accordance with the XRD peaks at 2θ = 29.4 and 36.3° of the new phase. It also confirms the presence of new phase nanoparticles in No. 6 sample. Furthermore, the inset of Figure a presents the SAED patterns of the recovered samples, showing several ring-like diffraction patterns with dispersed bright spots, which indicate the presence of highly crystalline multiple crystals in the recovered samples. The TEM results are in good agreement with the XRD results, indicating the formation of the new phase induced by shock loading. Typical SEM images of No. 6 sample powder (Figure c,d) reveal the presence of agglomeration of formed new phase nanoparticles, which are mostly in a quasi-spherical shape with a size of 50–100 nm. In addition, the observed quasi-spherical particles imply that the melting phenomenon is generated by the shock-induced high temperature.

Figure 4

TEM and SEM imagines of No. 6 sample. (a) TEM and (b) HRTEM images of No. 6 sample. (c) SEM and (d) high-magnification SEM images of No. 6 sample.

For further characterization, EDS is applied to confirm the elemental ratio of the new phase. The dot mapping EDS (see Figure a) shows that three elements (In, V, and O) are uniformly distributed in recovered samples. The corresponding general elemental ratio of In:V:O is 18.31:17.87:63.82, which is consistent with the element ratio of the precursor powder (In:V:O = 2:2:7). Considering that the majority of No. 6 sample is the formed new phase, the element ratio of In:V:O in the formed compound is supposed to be 2:2:7. Furthermore, all spot EDS results (see Figure b and Table ) present that the element ratio of In:V:O in formed particles is 2:2:7 approximately, showing the elemental uniform distribution in the formed new phase. Consequently, the new phase revealed in the above characterization results is supposed to be one compound nanopowder with a chemical formula of In2V2O7, which is different from the previous indium vanadates, e.g., InVO4 and InVO3.

Figure 5

EDS results of No. 6 sample: (a) dot-mapping mode results and (b) spot mode results.

Table 3

EDS Results of No. 6 Sample

	percentage (%)
element	dot-mapping	spot 1	spot 2	spot 3	spot 4	spot 5	spot 6
In	18.31	19.31	18.00	19.06	19.5	19.19	17.95
V	17.87	19.37	19.19	18.77	17.04	18.56	17.52
O	63.82	61.32	62.81	62.17	63.46	62.25	64.53
total	100	100	100	100	100	100	100

Figure presents the typical In 3d, V 2p, and O 1s XPS spectra of No. 6 sample. The corresponding peaks appeared at approximately 444.7 eV in In 3d spectrum, 517.1 eV in V 2p spectrum, and 530.0 eV in O 1s spectrum, showing the presence of indium vanadate in No. 6 sample. Moreover, based on the XPS results, the atomic ratio of In:V:O is 1:0.98:3.52, confirming that the new phase is In2V2O7. The XPS results also imply the element composition and ratio, which support the identification of In2V2O7 in the recovered samples.

Figure 6

XPS spectra of No. 6 sample: (a) In 3d spectrum, (b) V 2p spectrum, and (c) O 1s spectrum.

Based on the recorded XRD data, this new phase can be indexed as a pyrochlore-like tetragonal cell with a = b = 0.7417 nm, c = 2.1035 nm, and Vcell = 1.157 nm3 using the X-cell program in Material Studio. The calculated cell parameters of the new phase are different from those of indium vanadates, e.g., InVO4 and InVO3, also indicating that a new structure of indium vanadate formed under shockwave loading.

As shown in Figure b, the formation mechanism of shock-synthesized In2V2O7 is discussed as below. The In2O3 powder and VO2 powder were mixed and activated using ball milling. Then, the mixed precursor powder was impacted by the shock-driven flyer. Shock loading leads to adiabatic compression in precursor powder, generating a high temperature of a few thousands of K and a high pressure of dozens of GPa. Consequently, the chemical reaction of precursors is triggered by this extreme condition to form tetragonal pyrochlore-like In2V2O7 powder. The short duration (microseconds) of shock loading inhibits the growth of the In2V2O7 crystal to form nanocrystals. In addition, the high cooling rate during unloading is conducive to preserve the formed nanocrystalline In2V2O7. Thus, the majority of recovered In2V2O7 nanoparticles are in sizes of 50–100 nm. The corresponding chemical reaction formula is shown in Figure b. Furthermore, the quasi-spherical morphology indicates the formation process in which the melted In2V2O7 at a high pressure and high temperature cools down immediately and nucleates to form In2V2O7 nanoparticles. It is also reported in previous investigation on the shock synthesis of materials.[36]

During the shock compression process, the relative density of precursor powder (ρ00/ρ) and flyer velocity are the two main parameters to adjust the shock temperature and pressure, respectively. The decrease in ρ00/ρ enhances the adiabatic compression process during shock loading and increases the shock temperature accordingly, which is conducive to promote the synthetic reaction by increasing the activity of the precursor. With the ρ00/ρ at 90%, the XRD patterns of the recovered sample only show two peaks of In2V2O7, indicating that the majority of recovered samples (Nos. 1 and 4 samples) are the precursor. As the ρ00/ρ decreases to 40%, the corresponding XRD pattern shows 12 In2V2O7 XRD peaks with a higher intensity, indicating that the majority of the recovered sample is In2V2O7. Thus, the high shock pressure is one key factor to prepare tetragonal pyrochlore-like In2V2O7 in this study.

Moreover, a high pressure is another key factor to synthesize In2V2O7. As the flyer velocities increase, the shock pressure increases from 55 GPa (Nos. 1–3 experiments) to 90 GPa (Nos. 4–6 experiments). The corresponding yield of In2V2O7 also increases accordingly. Under conditions of a relative density of 40% and a shock pressure of 90 GPa, the yield of In2V2O7 is close to 100%. In previous studies, the pressure condition to synthesize InVO4 is usually normal pressure, but the formation pressure of pyrochlore-like transition metal vanadate with a formula of A2V2O7 is usually high.

Our study suggests that the shock method possesses high potential to synthesize bimetal oxide nanopowder, especially metastable bimetal oxides. Furthermore, the shock method is an ideal approach for doping and activation of nanopowders, implying the potential one-step synthesis of doped In2V2O7 powder in the further investigation.

Conclusions

In this study, we obtained novel indium vanadate with a formula of In2V2O7 and tetragonal pyrochlore-like structure through a shock-induced chemical reaction. The calculated average XRD parameters of shock-synthesized In2V2O7 are listed in Table . The crystal size of formed In2V2O7 is in the range of 20–30 nm approximately. Our study demonstrates that shock pressure and temperature are the two critical factors to form tetragonal pyrochlore-like In2V2O7 nanopowder. The increase in shock pressure and temperature improves the yield of In2V2O7. Under conditions at a relative density of 20% and shock pressure of 90 GPa, the yield of In2V2O7 is close to 100%. This study proves this feasible approach to synthesize new bimetal oxides with a formula of A2B2O7.

Table 4

XRD Parameters of Formed IVO

	In2V2O7
peak number	2θ (°)	d (Å)	I (%)
1	14.59	6.066	8.68
2	29.44	3.031	49.84
3	31.86	2.808	100.00
4	36.36	2.468	52.89
5	39.38	2.285	21.86
6	46.66	1.945	6.59
7	53.94	1.696	30.55
8	59.81	1.545	21.38
9	61.22	1.512	6.59
10	62.65	1.481	22.51
11	66.75	1.399	6.75
12	77.29	1.234	7.23