CC BY
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Structural hierarchy of NH4V3O7 particles prepared under hydrothermal conditions
G. S. Zakharova1'2, Y. Liu3, I. S. Popov1, A.N. Enyashin1
1 Institute of Solid State Chemistry UB RAS, Ekaterinburg, Russia 2Kirchhoff Institute for Physics, University of Heidelberg, Germany 3Institute of Materials Science and Engineering, Wuhan University of Technology, PR China
PACS 61.50.Ah, 61.72.Hh, 71.20.Ps DOI 10.17586/2220-8054-2015-6-4-583-592
Despite having simple stoichiometry, NH4V3O7 still remains an odd compound with poorly resolved structure among the series of known ammonium vanadates. Here, a new hydrothermal synthesis of the product with explicit NH4V3O7 stoichiometry is evaluated. Intricate microstructure of the product is revealed as an aggregate of spherical microparticles consisting of microplatelets via scanning electron microscopy. To further guide the characterization of the NH4V3O7 phase, X-ray diffraction analysis and first-principle calculations were carried out to refine the structure at an atomistic level and to predict electronic properties. The results suggest a complex structural hierarchy with consequent nanodomain organization of prepared NH4V3O7 microplatelets.
Keywords: Ammonium vanadates, Hydrothermal synthesis, Microplatelets, Nanodomains, DFT calculations.
Received: 22 June 2015
1. Introduction
Mixed-valence Vanadium oxides and several of their derivatives form a wide class of functional materials for catalysts, Li-ion batteries, chemosensors, electronic and optical devices [1-3]. A rich variation of V4+ and V5+ content, the types of coordination polyhedra and their possible arrangements permit a very large variety of possible V-O frameworks and serve as great opportunity to design new advanced crystalline structures. The chemical nature of V-O frameworks enables hydrothermal (i.e. solvothermal) synthesis as the most outstanding and cost-effective method with widely varying experimental conditions for their fabrication [4,5].
The electroneutrality of a charged V-O framework can be compensated using appropriate amount of intercalated metal or complex cations, such as ammonium [6-9]. Particularly, to date, a few compounds in the family of ammonium polyvanadates are known - NH4V4O10 [9-12], NH4V4O14 [13], (NH4)2V3O8 [14], NH4V3O7 [15]. Large NH+-ions stabilize the internal pillarlike cavities within the vanadate frameworks, leading to enhanced diffusion rate of lithium ions. The latter is a necessary attribute of the cathode material for high capacity rechargeable Li-ion batteries.
Diammonium trivanadate (NH4)2V3O8 crystallizes as a fresnoite structure and attracts much attention due to its magnetic properties [16,17]. Ammonium vanadium bronze, NH4V4O10 (or (NH4)0 5V2O5) has a monoclinic structure. It was suggested as a potential electrode material for high capacity Li-ion batteries because of its good cyclic stability [11]. NH4V4O10 showed a discharge capacity of 197.5 mAh/g remaining after 11 cycles and excellent cycling stability with the capacity retention of 81.9% after 100 cycles at 150 mAh/g [10]. Bicationic vanadium bronzes, (NH4)0.83Na0.43V4O10-0.26H2O [18] and (NH4)0.25Na0.14V2O5 [19], exhibit advanced
electrochemical properties, which might presumably be attributed to the modulation of the lattice parameters due to the co-intercalation of different cations.
Despite its simple stoichiometry, NH4V3O7 still remains an odd compound with a poorly resolved structure among the known ammonium vanadates. This compound has been synthesized hydrothermally using NH4VO3, CuCO3Cu(OH)2 and NH4F as precursors [15]. Yet, under the reaction conditions chosen, NH4V3O7 was prepared in a mixture with (NH4)2V4O9 phase, which challenges some credibility of subsequent crystallographic and conductivity measurements.
In recent work, we evaluate a new hydrothermal synthesis of NH4V3O7 compound, which allowed the isolation of a product with explicit NH4V3O7 stoichiometry and with a morphology consisting of microplatelets which were assembled into spherical particles. Such a high texture does not enable us to refine the unit cell parameters. Yet, as a guide for further interpretation of the NH4V3O7 phase, first-principle calculations were carried out to confirm the structure at an atomistic level and to predict its electronic properties. Our calculations reveal that, fabricated NH4V3O7 microplatelets should have a nanodomain structure.
2. Experimental part 2.1. Chimie douce synthesis
All chemical reagents were purchased from Sigma Aldrich and used without further purification. Ammonium metavanadate NH4VO3 was used as precursor and citric acid C6H8O7 was used as a mild reductant. The synthesis procedure was as follows: NH4VO3 powder was dissolved with stirring in deionized water. Then, an appropriate amount of saturated aqueous citric acid was added drop-wise until 4<pH<5.5 is achieved. The homogenous solution was placed into a teflon-lined stainless steel autoclave and maintained at 180 °C for 48 hours. After cooling to room temperature, the obtained black sediment was filtered, washed with deionized water and air-dried at 50 °C.
2.2. Characterization techniques
The morphology of the powder and elemental analysis were studied by scanning electron microscope Nano-SEM (FEI) with integrated energy-dispersive X-ray microspectrometer for analysis (EDX). The product was characterized by powder X-ray diffraction (XRD) by means of Shimadzu diffractometer XRD-7000 S using Cu Ka radiation. Thermogravimetry (DSC-TG) was carried out using analyzer DTA 409 PC/PG (Netzsch). The samples were heated at a rate of 10 K/min up to 800 °C under N2.
2.3. Computational details
The spin-polarized calculations of NH4V3O7 compound were performed within the framework of the density-functional theory (DFT) [20] using the SIESTA 2.0 implementation [21,22]. The exchange-correlation potential within the Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof parametrization was used [23]. The core electrons were treated within the frozen core approximation, applying norm-conserving Troullier-Martins pseudopotentials [24]. The valence electrons were taken as 3d34s24p0 for V, 2s22p4 for O, 2s22p3 for N and 1s1 for H. The pseudopotential core radii were chosen as 2.34 aB for V3d and V4s, 2.50 aB for V4p states, 1.45 aB for all O states, 1.04 aB for all N states, and 0.15 aB for His states. In all calculations, a double-Z polarized basis set was used. The k-point mesh was generated by the method of Monkhorst and Pack [25]. The real-space grid used for the numeric integrations was set to correspond to the energy cutoff of 300 Ry. For k-point sampling, a cutoff of 10 A was used [26]. All calculations were performed using variable-cell and atomic
position relaxations, with convergence criteria corresponding to the maximum residual stress of 0.1 GPa for each component of the stress tensor, and the maximum residual force component of 0.05 eV/A.
The optimized geometry was used to calculate the XRD spectra for the radiation wavelength Л = 1.5406 A (nickel-filtered CuKa radiation). XRD spectra of nanosized NH4V3O7 were calculated in Debye approximation as for ensemble of monodisperse nanoparticles. The smearing of reflection profiles was approximated with correction for the isotropic atomic temperature factor and with regard to the instrumental line broadening [27].
3. Results and Discussion 3.1. Scanning microscopy
The microstructure of synthesized ammonium trivanadate (NH4V3O7) was characterized using SEM method. The SEM data reveal an insignificant dependence of the compound's morphology on the variation of pH value in the primary reaction mixture. The samples fabricated at pH 4 consist mainly of the spherical-like particles with 3-8 ^m diameters (Fig. 1a). In turn, these microparticles are assembled of stochastically oriented microplatelets with thickness of 50 - 200 nm and with the characteristic edge lengths up to 3 ^m (Fig. 1b).
(a) (b)
Fig. 1. SEM images of NH4V3O7 powder fabricated from the precursor solution with pH = 4
The morphology of NH4V3O7 samples isolated from the less acidic precursor solutions (pH < 4) is enriched by the separate single microplatelets. A pH value of 5.5 leads to the formation of square-like microplatelets stochastically aggregated into the particles with the diameters of 20 - 30 ^m. These platelets have the larger thickness and lengths up to 250 -950 nm and 5-15 ^m, respectively.
3.2. Thermogravimetric analysis
In order to determine the stoichiometry and the thermal stability of NH4V3O7 samples, DSC study was performed. The data of the mass loss measurements under an inert atmosphere allowed us to adjust the explicit NH4V3O7 stoichiometry for the prepared samples. Upon the
heating of NH4V3O7 powder in a stream of nitrogen, the mass loss is observed as a single stage process, which is finalized at 422 °C (Fig. 2). The decomposition of the samples is an endothermic process with the minimum corresponding to the temperature at 369 °C. In general, the decomposition of NH4V3O7 can be described according to the reaction
6NH4V3O7 ^ I8VO2 + 4NH3 f +N2 f +6H2O.
Am, %
exo
200 400 600
t, °c
Fig. 2. TG and DSC thermogravimetric curves of NH4V3O7 powder decomposition under a stream of nitrogen
3.3. X-ray diffraction
The structure and the lattice type of experimentally observed NH4V3O7 phase have not been validated, yet. The former study after Trombe et al did not describe the texture of the samples in detail and no X-ray diffractogram was quoted [15]. Despite the presence of an admixture of an (NH4)2V4O9 phase, the lattice parameters of NH4V3O7 were ascribed there to the crystal structure with its own monoclinic type and with lattice parameters of a = 12.198 A, b = 3.7530 A, c = 13.178 A, p = 100.532°, Z = 4 (ICSD 417589). This lattice was represented as a stack of V3O7 layers, consisting of sextuple ribbons of distorted VO6 octahedra and intercalated by ammonium cations (Fig. 3a).
XRD measurements of our samples prepared at a pH level of 4.0 - 5.5 give evidence that the crystal structure may also be described as a monoclinic phase. Yet, noticeably different lattice parameters are found: a = 12.247(5) A, b = 3.4233(1) A, c = 13.899(4) A, p = 89.72(3)°,
Fig. 3. Polyhedral models for DFT optimized supercells (Z = 16) of two polytypic NH4V3O7 forms: (a) the most stable polytype I; (b) hypothetical and less stable polytype II (the views along b-axes are shown). Ball-and-stick model (c)demonstrates a single needle-like nanoparticle of polytype I with characteristic size 6 nm x 2 nm x 100 nm in a, b, c directions, respectively.
Table 1. Lattice parameters of NH4V3O7 compound concerning to experimental X-ray diffraction data in the recent work and in the work [15] versus the data for two polytypes from DFT calculations.
NH4V3O7 Z a,  b,  c,  ß, 0 V, 3
exp. [15] 4 12.198 3.753 13.178 100.5 593.1
exp. here 4 12.247 3.423 13.899 89.72 582.3
calc. here polytype I 4 12.211 3.832 12.886 97.8 597.5
calc. here polytype II 4 16.652 3.841 10.204 88.8 652.5
V = 582.3(4) A3 (Table 1). At pH values > 5.5, the product possesses an admixture of NH4V4Oio compound (JCPDS 031-0075) and was not considered henceforth.
Our data suggest that the NH4V3O7 compound, as prepared in a recent work, should have a layered structure composed of (V3O7)-layers with an orientation within (101) planes.
Regretably, further XRD structure refinement of our highly textured NH4V3O7 microparticles did allow a detailed view of the internal structure of the lattice. The discrepancy between our data and the previously-obtained data [15] may be attributed either to the poor characterization of highly textured samples or to the formation of a new polymorph. Hence, the first-principle calculations were employed to explore both options and to judge the crystal motif of NH4V3O7.
3.4. Possible polytypism of NH4V3O7
Many layered compounds are inclined to a rich polytypism due to unrestrained combination of layers' stacking. To explain the discrepancy between the recent data and earlier data [15], we surmised that another possible crystalline structure, based on the same type of V3O7, layers may exist. Like the crystal motif suggested by Trombe et al. [15] (hereafter polytype I, Fig. 3 a), the hypothetical NH4V3O7 polymorphic modification can be based on the same type of V3O7 layers, yet, with every second layer shifted on b/2 along [010] direction (polytype II, Fig. 3b). To validate the structure of both polytypes, optimization of their geometry has been carried out and their relative stability has been analyzed using DFT calculations.
Our DFT results are encouraging, showing that the crystal structure of polytype I has the lowest total energy and is the most stable. However, the energy of polytype II is only on 0.14 eV/NH4V3O7 higher than that of polytype I. Such a minor energy difference suggests the existence of at least these two polytypes or even a number of intermediate NH4V3O7 polytypes. The presence of numerous random dislocations along [010] direction may be not excluded, too.
The accurate first-principle calculations permit crystallographic parameter determination for both NH4V3O7 polytypes and comparison of them with experimental data (Table 1). Both experimental datasets do not reproduce the crystallographic properties found for hypothetical polytype II. Yet, the lattice parameters of the most stable NH4V3O7 polytype I may be reliably attributed to and can be found in fair agreement with former experimental values of Trombe et al. [15]. The largest deviation between calculated and these experimental lattice parameters does not exceed 2%. Thus, our experimental data cannot be seemingly assigned to the most stable crystalline phase of NH4V3O7.
A more detailed insight into the structure of our highly textured samples can be performed by the comparison of experimental X-ray diffractograms with those simulated using the geometries resulted from DFT calculations. Again, theoretical diffractogram of polytype II does not reveal any similarity with the experimental data (Fig. 4). Nonetheless, theoretical diffractogram of polytype I also showed a remarkable difference. Particularly, it contains very strong reflexes missing on our experimental diffractogram at angles 20 = 24°, 46° and, in general, it has a more fine profile. Such a mismatch is a clear indication of poor crystallinity for our NH4V3O7 product. We may surmise that the microplatelets, assembling the spherical particles of our product, should have their own internal organization at the nanoscale; e.g., every microplatelet could be an aggregate of nanoparticles or could have a nanodomain structure.
In addition, X-ray diffraction spectra of NH4V3O7 samples have been simulated as for the sets of monodisperse free-standing nanoparticles or nanodomains of polytype I. Routine fitting has been performed for a wide range of sizes and for several possible morphologies (compact 0D particles, 2D films, 1D needles). Despite the simplicity of all of these models, neglecting the lattice strain and possible surface reconstructions, an evident coincidence with our experimental data has been found for the case of needle-like nanoparticles with characteristic size 6 nm x 2 nm x ~100-500 nm along a, b, c directions, respectively (Fig. 3c). Some of the peaks on the profile of simulated XRD spectrum may be found as slightly shifted to the lower angles 20, since our DFT calculations may overestimate interlayer distances.
FIG. 4. Comparison between X-ray diffractograms for NH4V3O7 compound as observed for the samples fabricated in this work and as theoretically predicted for the monocrystals of two NH4V3O7 polytypic forms (I and II) and for a single needle-like nanodomain of polytype I with characteristic size of 6 nm x 2 nm x ~100 nm along a, b, c directions, respectively.
Thus, DFT calculations suggest that experimental XRD data, as obtained from our highly textured samples, should be not treated as for the monocrystal. The structural hierarchy of NH4V3O7 compound can be drastically enriched at the nanolevel. The NH4V3O7 microplatelets may have interim nanodomain structure, as the grains of polytype I and numerous low-energy dislocations, as the grain boundaries. As well, they might be assembled of free-standing NH4V3O7 nanoneedles. Further investigation by means of high-resolution electron microscopy could resolve the structural hierarchy of our samples in greater detail.
3.5. Electronic properties of NH4V3O7
DFT calculations enable us to give prior information about the electronic properties of NH4V3O7 compound. The electronic density of states calculated for the most stable polytype I is visualized in Fig. 5. The studied compound, NH4V3O7, should be a magnetic semiconductor with a band gap of ^0.83 eV. The bottom of splitted conduction band has a dominant V3d-character. The top of valence band is also formed by V3d-states with an admixture of O2p-states, while the remaining wide part of the band at —2 ... —6 eV is composed predominantly of O2p-states. The deep and His states.
Fig. 5. Total and partial densities-of-states (DOS) for two polytypic NH4V3O7 phases. DFT calculations.
Analysis of the Mulliken charge distribution indicates that two groups of V atoms can be distinguished, with the charges +1.11 and +1.14 respectively. These groups differ in the environment of their second coordination shell. The first group of four V atoms is placed within the middle of a V3O7 sextuple ribbon as VO6 octahedra with shared edges. The second group of two V atoms is placed at the edges of this ribbon as VO6 octahedra sharing their vertices with equivalent VO6 octahedra of the neighboring ribbon. The coordination polyhedra of the latter group have a heavily distorted geometry, with one of V-O distances at 2.44 A, the largest of the group. The spin-polarization calculations indicate a magnetization of the compound with the spin density redistribution at the V atoms. The estimated magnetic moments within aforementioned groups of V atoms were found to be 2.40 and 1.73 aB.
The charge distribution among the O atoms is wider, yet, a few groups of atoms can be distinguished, depending mainly on the coordination number: -0.42, -0.49 ... -0.60 and -0.54 ... -0.66 for O atoms within vanadyl groups, for bridging double and for triple coordinated O atoms, respectively. The charges on every N and every H atom are equal to -0.68 and +0.28, respectively, which amounts to the total formal charge of NH+ cation as equal to +0.44.
The results of DFT calculations for polytype II show that the overall qualitative picture of the density of states is preserved (Fig. 5). Both considered NH4V3O7 polytypes should be magnetic semiconductors. In general, the charge distribution and the DOS picture of both polytypes explicitly correspond to the formation of covalent V-O and N-H bonding within anionic V3O7 framework and ammonium NH4 cations. The relative position of dominating V3d-O2p and N2p-H1s overlaps in the valence band gives evidence for a highly versatile V3O7 network for redox reactions as well as a low ability of NH4 cations for reduction, e.g. using alkali metal atoms.
4. Summary
In summary, a facile chimie douce route was evaluated to produce highly textured product of pure ammonium trivanadate NH4V3O7 from the corresponding metavanadate NH4VO3 as precursor and citric acid C6H8O7 as a mild reductant. The intricate structure of the product was characterized by the combination of experimental SEM, XRD and computational DFT techniques, which uncover a complex structural hierarchy of synthesized NH4V3O7.
SEM data has revealed the microstructure as an aggregation of spherical-like particles with the diameters of ~30 ^m, assembled of stochastically oriented nanoplatelets with thicknesses of 50 - 200 nm and edge lengths up to 2 ^m. Yet, the values of the lattice parameters derived using recent XRD data and upon assumption of crystalline NH4V3O7 were found to be quite different from previous experimental and recent DFT data. Assuming a domain-like organization of synthesized NH4V3O7, X-ray diffractograms have been routinely simulated for a wide range of the size and for several possible domain morphologies. Indeed, it suggests even a more deep organization of NH4V3O7 microparticles at the nanoscopic level. Most likely, the lattice of NH4V3O7 studied in this work tends towards the formation of low-energy dislocations or twinning along [010] direction, which is a prerequisite for the emergence of needle-like nanodomains or a very rich polytypism.
A further study of NH4V3O7 by means of high-resolution electron microscopy could prove the structural hierarchy in more detail. Though, relying on DFT calculations, we predict that the electronic and chemical properties of layered NH4V3O7 would be not altered, even by possible domain-like organization. Two most prominent NH4V3O7 polytypes should be magnetic semiconductors with band gaps of ~0.8 eV. Irrespective of their lattice arrangement, the anionic V3O7 framework should be highly versatile in redox reactions, while NH+ cations should demonstrate a low ability for reduction, e.g. with alkali metal atoms.
Acknowledgments
The support from the Ministry of Science and Education of Russian Federation (unique project identifier RFMEF161314X0002) is gratefully acknowledged.
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