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Open AccessLetter

The Structure, Vibrational Spectra, and Thermal Expansion Study of AVO4 (A=Bi, Fe, Cr) and Co2V2O7

by Xiaoke He 1,*, Chenjun Zhang 2,3 and Ding Tian 1
1
School of Electric Power, North China University of Water Resources and Electric Power, 450045, China
2
National Research Center of Pumps, Jiangsu University, Zhenjiang, 212013, China
3
School of Mechanical Engineering, Luoyang Institute of Science and Technology, Luoyang, 471023, China
*
Author to whom correspondence should be addressed.
Materials 2020, 13(7), 1628; https://doi.org/10.3390/ma13071628
Received: 9 February 2020 / Revised: 23 March 2020 / Accepted: 27 March 2020 / Published: 1 April 2020

Abstract

Vanadate is an important functional material. It has been widely studied and applied in luminescence and photocatalysis. Vanadium compounds have been synthesized to investigate the thermal expansion properties and structure. Both BiVO4 and Co2V2O7 are monoclinic at room temperature, FeVO4’s crystal structure is triclinic, and CrVO4 is orthorhombic. The relatively linear, thermal-expansion, and temperature-dependent Raman spectroscopy results showed that the phase transition of BiVO4 occurred at 200 to 300 °C. The coefficient of thermal expansion (CTE) of Co2V2O7 was larger than that of the monoclinic structure BiVO4. The CTE of the tetragonal structure of BiVO4 was 15.27 × 10−6 °C−1 which was the largest CTE in our measurement results, and the CTE of anorthic structure FeVO4 was 2.84 × 10−6 °C−1 and was the smallest.
Keywords: vanadium; Raman spectroscopy; thermal expansion; crystal structure vanadium; Raman spectroscopy; thermal expansion; crystal structure

1. Introduction

Due to the multivalent of vanadium, vanadate has rich physical and chemical properties. Vanadate is a kind of important functional material; it has been widely studied and applied in luminescence and photocatalysis. The bandgap of BiVO4 is approximately 2.0 eV which means that it is a classic semiconductor. Bidmuth vanadate (BiVO4) is a polycrystalline compound, among which there are three kinds of crystal structures: monoclinic, orthorhombic, and tetragonal structure. The tetragonal structure has absorption band in the ultraviolet region, while the monoclinic structure has absorption band in the visible region as well as in the ultraviolet region. Vanadate (BiVO4) has emerged as a very photoanode for photoelectrochemical water splitting [1,2,3,4,5]. Although the hole/electron pair, produced by the excitation of BiVO4, has strong redox ability; it also has some disadvantages for practical application: a high electron hole recombination rate, low photocatalytic efficiency, small particles which are easy to lose and difficult to recover, etc. As an n-type semiconductor material, the bandgap of FeVO4 is approximately 1.9–2.7 eV. There are four crystal types of FeVO4, only triclinic structure material is easy to obtain [6]. The triclinic structure of FeVO4 remains up to approximately 3 GPa, and then a first-order phase transition to a new monoclinic with space group C2/m is observed [7]. As a kind of transition metal oxide, FeVO4 can be used as electrode materials for ion batteries and supercapacitors [8,9,10,11]. The compound materials of FeVO4/BiVO4 and FeVO4/V2O5 has higher than pure FeVO4 photocatalytic activity [12,13]. Chrome vanadate has three different crystal forms tetragonal, monoclinic, and orthorhombic structures [7]. The ambient-pressure stable polymorph of CrVO4 is orthorhombic space group D 2 h 17 -Cmcm-, with Z=4 at room temperature [14]. Cobalt vanadates and their composites have drawn a tremendous amount of attention because of their outstanding cycling stability [15,16]. The Co2V2O7 was recently reported to exhibit amazing magnetic field-induced magnetization plateaus and ferroelectricity, but its magnetic ground state remains ambiguous due to the fact of its structural complexity [17].
From the above discussion, we know that vanadium compounds have many structures, rich physical and chemical properties which makes them have potential application value in many aspects. Although there are many studies on the structure and application of vanadium compounds, the thermal expansion of materials has not been reported very intensively. It has been pointed out that ionic radius and electronegativity of the cations are important with respect to structure and phase transition temperature [18,19]. Herein, we have prepared some vanadate materials by a simple solid-phase sintering method, X-ray diffraction (XRD) was used to measure the structure of materials, Raman scattering was used to measure the lattice vibration, and dilatometers was used to measure the thermal expansion.

2. Experimental and Methods

The AVO4 and Co2V2O7 were synthesized by a solid-state method from Fe2O3 (≥99.0%), Bi2O3 (≥99.0%), Cr2O3 (≥99.0%), Co2O3 (≥99.5%), and V2O5 (≥99.0%). The raw materials were mixed according to stoichiometric amounts (1:1) of A2O3 and 2%–5% excess V2O5 of desirable material except (in order to compensate for the loss in the sintering process) and ground in a mortar for 2 h. Then, alcohol was poured over the raw material and grind again until dry. Lastly, it was pressed into tablets with a length of approximately 7 mm and a diameter of approximately 6 mm, followed by sintering at 750 °C for 4 h and cooling naturally to room temperature.
The XRD measurements were carried out with an X’Pert PRO X-ray Diffractometer (Bruker D8, Bruker, Karlsruhe, Germany). Raman spectroscopy (Renishaw MR-2000 Raman spectrometer, Gloucestershire, UK) with a TMS 94 heating/freezing stage with an accuracy of ±0.1 °C was used to characterize the vibrational property of lattice. The linear thermal expansion coefficients were measured on dilatometers (LINSEIS DIL L76, Linseis, Selb, Germany), with heating and cooling rates of 5 °C/min.

3. Results and Discussion

3.1. Crystal Structure Analysis

Figure 1 shows the XRD patterns of as-prepared materials. Figure 1a is the pattern of BiVO4, all diffraction peaks corresponded to BiVO4 (PDF No. 01-083-1699) which means that the material crystals were in monoclinic structure space group I2/b, with Z = 4. The lattice constants of BiVO4 were a = 5.196 Å, b = 5.094 Å, c = 11.703 Å and α = β = 90°, γ = 90.380°. Figure 1b is the pattern of CrVO4, the primary diffraction peaks corresponded to CrVO4 (PDF No. 00-038-1376, space group Amam) except for weak peaks indicated as “∇” for secondary phase Cr2O3 and “∗” for secondary phase V2O5 which could relate to the fact that the reaction time was much shorter than that reported in the literature (122 h). There was a second and third phase which could lead to some situations, such as internal stress, many cracks on the tablet, etc. The material crystals in orthorhombic structure with space group Amam from the primary diffraction peaks. The lattice constants of CrVO4 are a = 5.567 Å, b = 8.210 Å, c = 5.975 Å. The pattern of FeVO4 is shown in Figure 1c. As seen, the diffraction peaks are corresponding to FeVO4 (PDF No. 00-038-1372, space group P-1) which crystal in anorthic structure. The lattice constants of FeVO4 are a = 6.720 Å, b = 8.059 Å, c = 9.256 Å, and α = 96.7°, β = 106.4°, γ = 101.6°. Figure 1d is the pattern of Co2V2O7, all diffraction peaks corresponded to Co2V2O7 (PDF No. 01-070-1189) with lattice constants a = 6.595 Å, b = 8.380 Å, and c = 9.470 Å which means that the material crystals were in monoclinic structure space group P21/c, with Z = 4.
To visualize the coordination number associated with the structural transitions, crystal structures of monoclinic (BiVO4, Co2V2O7), triclinic (FeVO4), and orthorhombic (CrVO4) systems with polyhedral representation were drawn using a VESTA software as shown in Figure 2 (the “atomic coordinates” were obtained come from the joint conferences on pervasive computing (JCPC) references). The BiVO4 crystal structure was monoclinic. From Figure 2a,b, Bi and V atoms occupied the symmetry position 4e, and O atoms occupied 8f. The distance between the V and O atoms was evenly distributed (approximately 1.68 Å and 1.785 Å); however, the distance of the Bi and O was very variable. There are six symmetry VO4 tetrahedras and asymmetrical BiO6 octahedra in one primitive cell of BiVO4. The CrVO4 crystal structure was orthorhombic (Figure 2c,d). The distance between the V and O atoms (approximately 1.63342 Å and 1.70978 Å) was shorter than that of BiVO4, and the distances between the Cr and O atoms were 1.98422 Å and 2.04868 Å. It can be seen that there are four CrO6 octahedra around each tetrahedron; however, each CrO6 octahedron is not only connected with six tetrahedron vertices, but also connected with two other octahedron edges. The FeVO4 crystal structure was triclinic Figure 2d. For FeVO4, there were 18 symmetrical inequivalent atoms in a one-unit cell, and all the atoms occupied the symmetry position 2i (Figure 2e). The total number of atoms in a unit cell was 36. The distance between the V atom and the O atom was different. The unit cell contained three asymmetrical inequivalent VO4 tetrahedra, two asymmetrical inequivalent FeO6 octahedra, and one FeO5 polyhedron [20] (Figure 2f). The CoV2O7 crystal structure was monoclinic Figure 2g, the total number of atoms in a unit cell was 44. The distance between the V atom and the O atom was different. The unit cell contained three symmetrical VO4 of each tetrahedron and six asymmetrical CoO6 octahedra (Figure 2h). It can be seen that each VO4 tetrahedron was connected to four CoO6 octahedra by the O atom; however, each CrO6 octahedron was connected by the O atom to six VO4 tetrahedras and shared a common edge with two other octahedral.

3.2. Thermal Expansion Property

Figure 3 shows the relative linear thermal expansion of BiVO4, FeVO4, and Co2V2O7. It was found that the samples have different relative linear thermal expansion. For BiVO4, there was a thermal expansion inflection point at about 237 °C which means that the material occurs phase transition at the temperature. The coefficients of thermal expansion (CTEs) were calculated as the average linear thermal expansion coefficient in terms of the slope of thermal expansion versus temperature. The CTEs were measured to be (4.664 ± 0.005) × 10−6 °C−1 (25–235 °C) and (15.40 ± 0.002) × 10−6 °C−1 (240–550 °C). For FeVO4, there was a gradual change thermal expansion at approximately 400 °C, the CTE of FeVO4 was obtained to be (2.751 ± 0.004) × 10−6 °C−1 from 20 to 350 °C and (5.245 ± 0.005) ×10−6 °C−1 from 400 to 600 °C. Although both vanadium and iron are variable metals and thermal expansion is related to valence states [21], we prepared and measured the material in an air atmosphere, vanadium and iron should remain stable in the highest valence state. So, there should be no chemical expansion here. The thermal expansion of Co2V2O7 was stable below 500 °C, and the CTE was (9.230 ± 0.004) × 10−6 °C−1 from 20 to 500 °C. The inflection point above 500 °C is due to the softening of glass state above 500 °C which can be explained by the fact that Co2V2O7 goes from the crystalline form to a glassy one. This phenomenon indicates that material intelligent stability exists with below 500 °C. Though the structure of Co2V2O7 is similar to BiVO4, their CTE is very different. This could come from the different ionic radius of Co3+ (63 pm) and Bi3+ (108 pm). The ionic radius of Co3+ (63 pm) equals that of Fe3+ (64 pm); however, they had the largest difference in CTE in this study. This was due to the different structures.
Raman spectroscopy was applied to further demonstrate the existence of crystal. Raman spectra collected at room temperature is shown in Figure 4. The Raman spectra of BiVO4, CrVO4, FeVO4, and Co2V2O7 were in agreement with literature [10,20,22,23] and the spectra did not show the characteristic bands of V2O5. Hence, no effort was taken to consider the product selectivity in this work. For BiVO4, the primitive cell contained 28 atoms (Figure 2) and, in principle, 81 vibrational modes were expected. The band at approximately 828 cm−1 corresponded to stretching modes of V–O bonds, and there was no splitting which means that degeneracy occurs in the symmetric stretching vibration of VO4 tetrahedron. The strongest peak of CrVO4 and FeVO4 was much higher, whereas the stretching modes of V–O give rise to intense bands, the difference in electronegativity of these metal (Bi, Co, Cr, and Fe). The Raman bands of FeVO4 were much more than that of BiVO4, CrVO4 and Co2V2O7, because the structure of FeVO4 is triclinic, and all vibrations are nondegenerate. The 36 atoms in the unit cell had 105 vibrational modes among which 54 optical modes were Raman active Ag modes, 51 were infrared active Au modes [23]. For all Raman spectra, the stretching modes of V–O combining M–O and V–O occurred above 650 cm−1, and bending modes together with stretching modes appeared in the 630–420 cm−1 region. The lower wavenumber bands were external modes from lattice, translational, and vibrational motions [24,25]. From a crystalline perspective, all catalysts were composed of V–O polyhedrons and other metal–oxygen polyhedrons.
In order to study the sudden change in thermal expansion of BiVO4, the Raman spectroscopy dependent temperature of BiVO4 is shown in Figure 5. The Raman bands became weaker and weaker with the increasing temperature which reflects the increase of the degree of disordering of the crystal structure. The relative intensity of 368 and 324 cm−1 had obvious change at 200 °C, and they disappeared at 300 °C; meanwhile, there was a new band at approximately 345 cm−1. There was not only one change. The bands at 127 and 211 cm−1 gradually became a wave packet; meanwhile, the 703 cm−1 band disappeared, and the 828 cm−1 band moved to 815 cm−1, this might be caused by the bond expansion and weakening. All these mean that there was a phase transition between 200 °C and 300 °C. There was no change in the Raman spectra above 300 °C. The high temperature Raman spectra were in agreement with tetragonal structure which means that BiVO4 crystal tetragonal as well [26]. Compared with Figure 3, we found that materials with high symmetry have larger CTE. The Raman band at 368 and 324 cm−1 could inhibit the thermal expansion of material. It means that the thermal expansion property was related to the structure of the material.

4. Summary

Vanadium compounds were synthesized to investigate the thermal expansion properties and structure. The CTE of Co2V2O7 was bigger than monoclinic structure BiVO4 which means that the thermal expansion property was related to the ionic radius of metals. The CTE of the tetragonal structure of BiVO4 was 15.27 × 10−6 °C−1 which was the biggest CTE in our measurement results, and the CTE of tetragonal structure FeVO4 was 2.84 × 10−6 °C−1 which was the smallest. This indicates that the thermal expansion property was related to the structure of the material.

Author Contributions

Conceptualization, X.H. and C.Z.; methodology, X.H.; software, D.T.; validation, X.H., C.Z. and D.T.; formal analysis, D.T.; investigation, X.H.; resources, X.H.; data curation, C.Z.; writing—original draft preparation, X.H.; writing—review and editing, X.H.; visualization, X.H.; supervision, X.H.; project administration, X.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Henan Province Education “The 13th Five-Year Plan” Project of Year 2016, grant number 2016-JKGHA-0017, and the Open Fund of Key Laboratory of Water-saving Agriculture of Henan Province, grant number FIRI2016-19-01.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction pattern of (a) BiVO4, (b) CrVO4, (c) FeVO4, and (d) Co2V2O7.
Figure 1. X-ray diffraction pattern of (a) BiVO4, (b) CrVO4, (c) FeVO4, and (d) Co2V2O7.
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Figure 2. Crystal structure of (a,b) BiVO4, (c,d) CrVO4, (e,f) FeVO4, and (g,h) Co2V2O7.
Figure 2. Crystal structure of (a,b) BiVO4, (c,d) CrVO4, (e,f) FeVO4, and (g,h) Co2V2O7.
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Figure 3. Relative length change of BiVO4, FeVO4, and Co2V2O7.
Figure 3. Relative length change of BiVO4, FeVO4, and Co2V2O7.
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Figure 4. Raman spectra of BiVO4, FeVO4, CrVO4, and Co2V2O7.
Figure 4. Raman spectra of BiVO4, FeVO4, CrVO4, and Co2V2O7.
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Figure 5. Raman spectra of BiVO4 at temperature of 18, 50, 100, 200, 300, 400, and 600 °C.
Figure 5. Raman spectra of BiVO4 at temperature of 18, 50, 100, 200, 300, 400, and 600 °C.
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