Synthesis, microstructure, and phase transition characteristics of Gd/Nd-doped nano VO2 powders

Bin Wang; Dandan Zhao; Jinjing Du; Linbo Li; Jun Zhu; Chao Wang

doi:10.1515/gps-2022-0055

Article Open Access

Synthesis, microstructure, and phase transition characteristics of Gd/Nd-doped nano VO₂ powders

Bin Wang , Dandan Zhao , Jinjing Du , Linbo Li , Jun Zhu and Chao Wang

Published/Copyright: May 31, 2022

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From the journal Green Processing and Synthesis Volume 11 Issue 1

Abstract

The nano VO₂ powders were prepared by hydrothermal synthesis. The effects of Gd and Nd element doping on the structure and phase transition temperature of VO₂ were studied. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) results showed that Gd element and Nd element will affect the structure of VO₂. Gd³⁺ and Nd³⁺ can occupy partial position of V⁴⁺ lattice and form solid solution, increasing the lattice parameters of VO₂. Both the doped and un-doped VO₂ powders exhibit a monoclinic structure at room temperature. Due to the lattice deformation caused by Gd or Nd doping, the aggregation of particles is prevented, and the grain is refined obviously. Differential scanning calorimetry curves showed that both Gd doping and Nd doping can reduce the phase transition temperature of VO₂(M). When the Gd doping concentration is 6 at%, the phase transition temperature can be reduced from 71.7°C to 60.3°C, and the infrared transmittance before and after the phase transition also changes significantly, reaching more than 40%. Nd doping is similar, and the phase transition temperature decreased to 55.6°C with the addition of 9 at% Nd.

Keywords: nano VO2 powder; hydrothermal synthesis; Gd/Nd doping; microstructure; phase transition

1 Introduction

To the best of our knowledge, there are about 20 kinds of vanadium oxides, each of which has different crystal structures and different properties [1,2]. According to incomplete statistics, at least eight kinds of vanadium oxides can achieve a mutual semiconductor-to-metal transition (SMT) in a temperature range from –147°C to 257°C. As representative vanadium oxide, the phase transition temperature of VO₂ is about 68°C. It is close to room temperature, which is a first-order phase transition [3,4].

VO₂ behaves as a semiconductor below the phase transition temperature, and as a metal above the phase transition temperature. Mutations with 4 or 5 orders of magnitude in the resistance (rate) can be obtained, accompanied by a sudden change in magnetic susceptibility, optical refractive index, transmittance, and reflectance during the SMT. Especially in the infrared and near-infrared bands, the optical transmittance changes most obviously [5,6,7]. These changes are because when the temperature is lower than the phase transition temperature, VO₂ structure transformed from tetragonal rutile structure to monoclinic rutile structure, in which the electron position of dⅡ orbital and the π* orbital is changed. It will result in a change in the electron motion from a continuous state to a discontinuous state [8,9]. The thermotropic phase-transition characteristics of VO₂ enable it to have high application value in the field of energy-saving windows [10,11]. But the phase transition temperature, ∼68°C, is still relatively high. To meet the requirements of the practical applications, a variety of approaches have been taken to decrease the phase transition temperature of VO₂ [12], such as element doping [13], electric fields [14], surface and interface engineering [15], preparation of VO₂ nanocomposites [16,17,18], and structural modification [19,20]. Among these strategies, element doping can lower SMT effectively. A series of studies on element doping have been carried out, mainly including: W [21,22,23,24,25], Mg [26], Mo [27,28], F [29,30], Zr [31], Sb [32], Cr [33], Nb [34], Al [35], etc.

It is well known that rare earth (RE)-doping has been used as an effective approach for modulating the microstructures and photoelectric performance of optoelectronic materials and photocatalytic materials. It was reported that Eu-doping decreases the SMT temperature of monoclinic VO₂ polycrystalline thin films from 68°C to 47.5°C due to substitution of trivalent Eu³⁺ ions in VO₂ lattice [36]. Similar decreasing phenomena in SMT temperature of VO₂ polycrystalline thin films with Y³⁺-doping are also observed [37].

The microstructure change caused by doping can obviously adjust the physical properties of vanadium oxide, while the symmetric local structure can help to reduce the thermal barrier of SMT, thus reducing the SMT temperature [38]. Considering the high symmetry of cubic Gd₂O₃ and Nd₂O₃ lattice [39], it can be inferred that Gd³⁺ or Nd³⁺ may be a promising rare earth dopant. Based on this motivation, we explored the influence of Gd³⁺ doping and Nd³⁺ doping on the structure and SMT properties of VO₂ powders.

2 Experimental

10 mmol oxalic acid (C₂H₂O₄·2H₂O, analytically pure) and 5 mmol vanadium pentoxide (V₂O₅, analytical reagent) powders were mixed with 80 mL of deionized water. The mixture was continuously stirred at 50°C until the suspension liquid turned into dark blue. Then 10 mL of urea precipitant (H₂NCONH₂, analytically pure, 0.15 mol‧L⁻¹) and a certain proportion of Gd(NO₃)₃ (analytically pure) or Nd(NO₃)₃ (analytically pure) were added to the dark blue suspension liquid. After sufficient stirring, the mixed suspension was transferred to a closed polytetrafluoroethylene reactor with heat preservation at 190°C for 60 h. After the completion of the reaction, the product was filtered and washed to obtain VO₂(B) precursor. The precursor VO₂(B) was placed in a tube furnace with heat treatment at 600°C for 5 h under nitrogen atmosphere and then cooled down to ambient temperature in the furnace. The product was dispersed with ultrasonic treatment to obtain nano VO₂(M) powder. The effect of Gd doping and Nd doping on the microstructure and phase transition temperature of VO₂ powder was investigated. Gd atomic ratio of 2%, 4%, and 6% and Nd/V atomic ratio of 3%, 6%, and 9% were designed to conduct the comparative study, respectively.

X-ray diffractometer (D8 ADVANCE A25, Cu Kα as the source of radiation, output power was 3 kW) was employed to analyze the phase composition. The chemical valences of elements in the powders were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Scanning electron microscope (SEM, JSM-6390A) was applied to observe the microstructure. The TEM and HRTEM images of the powders were obtained by using high-resolution transmission electron microscopy (HRTEM, JEM–2100F STEM/EDS). Laser particle size analyzer (Mastersizer 2000) was used to test the particle size distribution of the nano powder particles. Synchronous thermal analyzer (STA449F3) and Fourier transform infrared spectrometer (Thermo Fisher Scientific IS5) was used to analyze the phase transition characteristics of the nano VO₂(M) powders.

3 Results and discussion

3.1 Effect of Gd/Nd doping on the phase and structure of VO₂

Gd/Nd-doped VO₂(M) powders were prepared with Gd(NO₃)₃ or Nd(NO₃)₃ as dopant. X-ray diffraction (XRD) results of different samples are shown in Figure 1. All the Gd-doped and Nd-doped samples exhibited obvious monoclinic VO₂ structure, which indicates that moderate Gd doping or Nd doping does not significantly change the crystal structure of VO₂(M). After Gd³⁺ or Nd³⁺ is incorporated, it can form a substitutional solid solution with VO₂.

Figure 1

XRD analysis of VO₂(M) powders doped with different dopants: (a) Gd³⁺ and (b) Nd³ ⁺.

However, it can also be seen from Figure 1 that Gd/Nd doping affects the microstructure of VO₂. For un-doped samples, the angular position of the strongest Bragg peak, the (011)-peak, is located at 27.72°. After doping 2, 4, and 6 at% Gd in the powder samples, the angular position is shifted to 27.83°, 27.89°, and 27.96°, respectively. These small shifts to higher diffraction angles indicate the decrease in the crystal lattice spacing according to Bragg’s equation:

(1) d = λ / 2 sin θ ,

where d – spacing, λ – wavelength of X-ray, and θ – diffraction angle. With the increase in Nd doping amount, the diffraction angle also changed slightly. After doping 3, 6, and 9 at% Nd in the powder samples, the angular position is shifted to 27.92°, 27.94°, and 28.10°, respectively. The main reason is that the ion radii of Gd³⁺ and Nd³⁺ are larger than V⁴⁺. The main reason is that the ion radii of Gd³⁺ and Nd³⁺ are larger than V⁴⁺. After replacing V⁴⁺ with Gd³⁺ or Nd³⁺, the lattice spacing of (011) plane is compressed, resulting in the diffraction peak deviation to a higher angle.

In order to verify whether the Gd/Nd atoms existed in the M-phase VO₂ lattice, XPS characterizations were performed on the 4 at% Gd-doped and 6 at% Nd-doped powder samples, and the results are shown in Figures 2 and 3. Figure 2a shows four elements including C, V, Gd, and O in 4 at% Gd-doped powder sample, indicating that the obtained powder product is Gd-doped VO₂. A peak appears in the O1s region with the major peak at 530.17 eV, which is assigned to O^2– ions in the V–O bonding. The V2p_3/2 peak splits into two peaks at 517.37 eV [V(V)] and 516.07 eV [V(IV)], as shown in Figure 2b. The peak of V⁵⁺ was stronger than that of V⁴⁺, which was mainly due to the exposure of powder materials in the air, leading to the oxidation of some particle surfaces, while XPS detected the main surface composition of the particle materials.

Figure 2

XPS spectra for the 4 at% Gd-doped VO₂(M) powder samples: (a) survey spectrum, (b) core-level spectrum for V 2p, and (c) core-level spectrum for Gd 4d.

Figure 3

XPS spectra for the 6 at% Nd-doped VO₂(M) powder samples: (a) survey spectrum, (b) core-level spectrum for V 2p, and (c) core-level spectrum for Nd 3d.

Two peaks of Gd 4d are shown in Figure 2c at positions of 141.17 and 142.07 eV, respectively, which is corresponding to Gd 4d, and it indicates that the valence state of Gd³⁺ exists in the Gd-doped VO₂ powder.

The O and V peaks of Nd-doped sample were similar to those in Figure 2. In Figure 3c, there are two obvious peaks, which are binding energy 983.77 and 1,003.0 eV, respectively, corresponding to Nd 3d. It indicates that the valence state of Nd³⁺ exists in the Nd-doped VO₂ powder.

Figure 4 is a topographical view of nano VO₂ powders with different dopants. The influence of heat treatment on the micro-structure morphology was researched. It can be seen from Figure 4a that the sample without heat treatment exhibits a typical VO₂(B) nanoneedle morphology. After heat treatment at 600°C for 5 h, the particle size of the sample increased significantly, showing a micron-scale rod shape, as shown in Figure 4b. This is mainly because the V element diffusion among the grains promotes the interparticle fusion and grain boundary migration during the high-temperature heat treatment process, thereby the particle size is increased. It can also be seen from Figure 4b–e that Gd doping can play a certain role in refining grains. After 4 and 6 at% Gd doping, the original shapes of VO₂ are changed into the spheroidal shape, and the particle sizes are reduced to about 200 nm. The same phenomenon was observed for 6 at% Nd-doped sample, which can be seen from Figure 4f. This is probably due to the more lattice deformation of recrystallization caused by Gd/Nd doping during the heat treatment process. It will provide more chances to generate new nucleation, thus creating appropriate conditions for grain refinement. In addition, Gd–O and Nd–O bonding strength are higher than that of V–O bond, the diffusion of V atom is inhibited, thus the grain growth is prevented to a certain extent.

Figure 4

SEM images of nano VO₂(M) powders doped with different dopants: (a) 0 at% without heat treatment, (b) 0 at%, heated at 600°C for 5 h, (c) 2 at% Gd, (d) 4 at% Gd, (e) 6 at% Gd, and (f) 6 at% Nd.

To further understand the element distribution of the powder samples, elemental mapping analyses were performed, as shown in Figures 5 and 6. It is clearly seen that V, Gd, and O elements are distributed uniformly in the sample, which suggested that the Gd element is well dispersed into VO₂ lattice. The Nd element is also mostly dispersed in the sample, but some segregation for Nd can be observed obviously. The reason for the uneven compositions can be derived from the above given analysis. This indicates that although Nd can also be doped into VO₂ lattice, the doping uniformity is not as good as that of Gd.

Figure 5

Elements distribution on the Gd-doped VO₂ powder: (a) SEM and (b–d) distribution of V, Gd, and O.

Figure 6

Elements distribution on the Nd-doped VO₂ powder: (a) SEM and (b–d) distribution of V, Nd, and O.

Particle size distribution diagrams of VO₂ powder samples doped with different dopants are shown in Figure 7. It can be seen that the particle size of each sample is substantially normally distributed. The average particle size of the un-doped samples was 898.6 nm. After adding 2, 4, and 6 at% Gd doping, the average particle size of VO₂ powder samples dropped to 733.8, 332.4, and 167.3 nm, respectively. These particle size distribution data are similar to the SEM results of Figure 4. After adding 3, 6, and 9 at% Nd doping, the average particle size of VO₂ powder samples dropped to 656.3, 296.2, and 263.8 nm, respectively. The results of the particle size analysis show that Gd or Nd doping has an obvious effect of grain refinement. Among all the doping level, the effect of grain refinement at 6 at% Gd doping is more obvious. Smaller average particle size and narrower distribution range can be obtained, which indicate that the particle size is relatively uniform.

Figure 7

Particle size distribution of nano Gd/Nd-doped VO₂ powders: (a) Gd and (b) Nd.

In order to further analyze the lattice structure of the nano VO₂ powder samples, the TEM test was performed on the sample with 6 at% Gd doping, and the results are shown in Figure 8. It can be seen from Figure 8a that the shape of the particles is mainly spherical-like particles, the particle size is substantially less than 300 nm, and the particle size distribution range is wide, which is consistent with the observation by SEM. In Figure 8b, the clear lattice planes can be seen, indicating a good crystallinity of the obtained nano VO₂ powder. The interplanar distance of (011) plane is 0.321 nm, which is in accordance with the HRTEM image of VO₂(M). Energy spectrum analysis was carried out for different regions in Figure 8a, and the results were shown in Figure 8c. Besides the copper elements in the micro-grid of copper mesh, the main elements in different regions were only Gd, O, and V, indicating that the nano VO₂ powder prepared by this method had little impurity content.

Figure 8

(a) TEM, (b) HRTEM images, and (c) EDS for the 6 at% Gd-doped VO₂ powders.

3.2 Effect of Gd/Nd doping on phase transformation of VO₂

Differential scanning calorimetry (DSC) was applied to investigate the Gd/Nd concentrations on the phase transition temperature of VO₂(M) powder samples, and the results are shown in Figure 9.

Figure 9

DSC curves of VO₂(M) powders doped with different dopants: (a) Gd and (b) Nd.

It can be seen from Figure 9 that obvious endothermic peaks correspond to the phase transitions for all the tested powder samples during environmental heating process. The phase-transition temperature decreases with increase in the Gd doping amount. When adding 2 at% Gd, the phase-transition of VO₂(M) reduced from 71.7°C to 65.9°C. Further increasing Gd concentration up to 6 at%, the phase-transition temperature dropped down to 60.3°C. Nd doping also plays a role in reducing the phase-transition temperature of VO₂(M). When adding 6 and 9 at% Nd, the phase-transition of VO₂(M) reduced to 58.8°C and 55.6°C, respectively. Analysis of its reasons, mainly because the Gd or Nd doping changes the energy level structure of VO₂. The introduction of impurity elements and the difference in radius of Gd/Nd and V ions destroy the stable phase structure combining V⁴⁺–V⁴⁺. With increase in the Gd³⁺ or Nd³⁺ amount, the lattice deformation is more serious, which will result in reducing the phase-transition temperature of nano VO₂ powder.

In order to investigate the change in the transmission properties of Gd/Nd-doped VO₂ before and after the phase transition, infrared transmittance tests were performed on 6% Gd-doped VO₂ (T _c1 = 60.3°C) and 6% Nd-doped VO₂ (T _c2 = 58.8°C). The test temperatures were 25°C and 65°C, separately. The obtained results are shown in Figure 10.

Figure 10

Infrared transmittance of VO₂ powders: (a) with 6 at% Gd doping and (b) with 6 at% Nd doping.

It can be clearly seen in Figure 10a that the infrared absorption spectra of 6 at% Gd-doped VO₂ samples at 25°C and 65°C is quite different. 25°C is lower than T _c1, and its infrared transmittance is higher, with the highest infrared transmittance reaching about 60%. 65°C is higher than T _c1, the infrared transmittance is basically maintained within 10%. Under the two conditions, the change in infrared transmittance ΔT is more than 50%. This indicates that under the condition of 65°C, VO₂ underwent a phase transition and was transformed into a metallic phase, which had a strong reflection on infrared light and reduced the transmittance. At 65°C, the 6 at% Nd-doped sample also underwent metallic phase transformation, and the infrared transmittance is generally less than 30%. Compared with the condition of 25°C, the infrared transmittance is reduced by about 30%.

4 Conclusion

For VO₂(M) powders doped with Gd(NO₃)₃ and Nd(NO₃)₃, Gd³⁺ and Nd³⁺ will occupy some certain positions in V⁴⁺ lattice, forming substitutional solid solution and increasing the lattice parameters of VO₂.
The lattice deformation of VO₂ caused by Gd or Nd doping will prevent the particles from accumulating, thus refining the grains.
Due to the difference in ion radius, Gd or Nd doping will destroy the stable phase structure of V⁴⁺–V⁴⁺, thus reducing the phase transition temperature.
With increase in the doping amount, the phase transition temperatures of VO₂(M) powders decrease gradually. With the introduction of 6 at% Gd or 9 at% Nd into VO₂(M) powders, the phase transition temperature can be reduced from 71.7°C to 60.3°C and 55.6°C, respectively.

Funding information: This work is supported by the Natural Science Foundation of Shaanxi Province (No. 2019JQ–761 and No. 2020JQ–681), the Scientific and Technological Innovation Team Project of Shaanxi Innovation Capability Support Plan (2022TD–30), the Foundation of Shaanxi Educational Committee (no. 20JK0702), and the Regional Innovation Capability Guidance Plan of Shaanxi Province (No. 2022QFY10–05 and No. 2022QFY10–03).
Author contributions: Bin Wang and Dandan Zhao: writing – original draft, writing – review and editing, conception, and design of the study; Jinjing Du, Linbo Li, and Jun Zhu: writing – original draft, methodology, analysis, and interpretation of data; Chao Wang: writing – original draft and methodology.
Conflict of interest: Authors state no conflict of interest.

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Received: 2021-09-15

Revised: 2022-04-25

Accepted: 2022-05-02

Published Online: 2022-05-31

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/gps-2022-0055

Keywords for this article

nano VO2 powder; hydrothermal synthesis; Gd/Nd doping; microstructure; phase transition

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Synthesis, microstructure, and phase transition characteristics of Gd/Nd-doped nano VO2 powders

Article

Abstract

1 Introduction

2 Experimental

3 Results and discussion

3.1 Effect of Gd/Nd doping on the phase and structure of VO2

3.2 Effect of Gd/Nd doping on phase transformation of VO2

4 Conclusion

References

Articles in the same Issue

Articles in the same Issue

Articles in the same Issue

Synthesis, microstructure, and phase transition characteristics of Gd/Nd-doped nano VO₂ powders

3.1 Effect of Gd/Nd doping on the phase and structure of VO₂

3.2 Effect of Gd/Nd doping on phase transformation of VO₂