A Combined Experimental and Theoretical Study on the Formation of

Article pubs.acs.org/JPCC

A Combined Experimental and Theoretical Study on the Formation of Crystalline Vanadium Nitride (VN) in Low Temperature through a Fully Solid-State Synthesis Route Eugenio F. de Souza,*,† Carlos A. Chagas,‡ Teodorico C. Ramalho,∥ Victor Teixeira da Silva,§ Daniel L. M. Aguiar,†,⊥ Rosane San Gil,⊥,¶ and Ricardo B. de Alencastro† †

Instituto de Química, Programa de PG em Química, Laboratorio de Modelagem Molecular-LABMMOL, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos No 149, CT, Bloco A, sala 609 Cidade Universitária, Ilha do Fundão, Rio de Janeiro - RJ 21941-909, Brazil ‡ Núcleo de Catálise, Programa de Engenharia Química, COPPE, Universidade Federal do Rio de Janeiro, Av. Horácio Macedo, 2030 Centro de Tecnologia - Bloco G - Sala G-115 Ilha do Fundão, Rio de Janeiro - RJ 21941-914, Brazil § Núcleo de Catálise, Programa de Engenharia Química, COPPE, Universidade Federal do Rio de Janeiro, Av. Horácio Macedo, 2030 Centro de Tecnologia - Bloco G - Sala G-116 Ilha do Fundão 21941-914, Rio de Janeiro - RJ 21941-914, Brazil ∥ Departamento de Química, Universidade Federal de Lavras, 37200-000 Lavras, Brazil ⊥ Instituto de Pesquisas de Produtos Naturais Walter Mors, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil ¶ Instituto de Quimica, Laboratorio de RMN de Solidos, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil S Supporting Information *

ABSTRACT: An efficient method of synthesis of the vanadium nitride (VN) at low temperature is evaluated, and a mechanism for the crystallization process is proposed in this paper. From the mixture of ammonium m-vanadate with guanidinium carbonate an intermediate, guanidinium m-vanadate (GmV), is produced. GmV decomposed and underwent interesting structural transformations with increasing temperatures. This process is studied by theoretical (periodic DFT calculations) and experimental (51V MAS NMR, XRD, FTIR, and elemental analysis) methods. It is proposed that GmV is first decomposed into reactive species, then through solid-state transformations it is converted into vanadium oxynitride (VOxN1−x) with varying stoichiometry, and, last, GmV transforms itself into crystalline NaCl-type structure vanadium nitride. The DFT calculations show that this transformation is energetically favorable, and the formation of a VOxN1−x solid solution is feasible.

1. INTRODUCTION When nitrogen atoms are inserted into the crystal lattice of transition metals, an interesting class of materials known as refractory transition metal nitrides can be formed.1 These materials usually crystallize in the rock-salt (NaCl-type) structure and are compounds of great practical and theoretical interest, characterized by a substantial gain in physical and chemical stability. They combine characteristics of both ceramics and metals, which makes them materials with unique physical and chemical properties.2 Particularly, vanadium nitrides present good thermal and electric conductivity1 as well as important catalytic properties. Vanadium nitride-based materials find many uses, having been applied to n-butane dehydrogenation, NH3 synthesis, and decomposition,4,5 and show high activity in hydrogenation, hydrogenolysis, and hydrotreatment (HDT) of hydrocarbons.6 On the other hand, the synthesis of vanadium nitride is usually difficult, requiring elevated temperatures and long reaction times. © 2013 American Chemical Society

Conventionally, they are prepared, among other methods,13−15,22 by self-propagating high-temperature synthesis (SHS),7 chemical vapor deposition,8 reduction−nitridation,9 direct reaction of vanadium metallic with nitrogen,1 carbothermal reduction and nitridation of vanadium oxides in N2,10 or temperature-programmed reactions.11,12 In a previous article, we reported recent experimental findings on the crystallization of vanadium nitride (VN) in the NaCl-type structure via a method named “guanidinium route”.30 However, the crystallization of vanadium nitride from a chemical intermediate (GmV) with raising temperatures was found to be a complex process regarding its real composition because competitive reactions could occur and continuous solid solutions, such as carbonitrides, carbides, and oxynitrides,1,3 for Received: August 27, 2013 Revised: November 18, 2013 Published: November 20, 2013 25659

dx.doi.org/10.1021/jp410885u | J. Phys. Chem. C 2013, 117, 25659−25668

The Journal of Physical Chemistry C

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resonance frequency) on a Bruker Avance III 400 spectrometer. Single excitation pulse experiments were carried out using central transition +1/2 → −1/2 selective pulse (π/8) of 1.11 μs and 500 kHz of spectral width. For each ONE of the samples, a 4 mm CPMAS Bruker probehead was used to spin the ZrO2 rotors at 10, 12, and 13.5 kHz, in order to obtain the isotropic resonance. The experiments were performed at room temperature. In order to get well-resolved spectra, the number of scans were set to 10 000, with repetition times set to 0.5 s. A 0.16 M VOCl3 solution was used as a external reference for 51V chemical shifts (0 ppm), and the magic angle was fitted using potassium bromide. All spectra were processed using the TopSpin software, through baseline corrections, phasing, and Fourier transformations. Further characterization details may be found in ref 30. 2.2. Computational Details. All calculations were performed using the plane-wave pseudopotential total energy method by means of the density functional theory (DFT), as implemented in the Quantum Espresso suite of programs.31 All phases representing the solid state transformations considered here have cubic NaCl-type structure, with space group FCC No. 225.32,33 Geometry optimization (the atomic basis vectors and the atomic positions) for the cubic supercells of VOxN1−x (x = 0, 1, 2, 3, and 4) solid solutions were carried out using the Broyden−Fletcher−Goldfarb−Shano (BFGS) algorithm. The Kohn−Sham electronic states were expanded in plane waves up to a kinetic energy cutoff of 60 Ry (1 Ry = 13.605 eV or 1312.749 kJ mol−1) and 480 Ry for the charge density cutoff. The Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation was used to calculate the exchange and correlation contributions.29 Integrals over the Brillouin zone were calculated by sums of 12 × 12 × 12 Monkhorst−Pack34 kmesh grid, using Marzari−Vanderbilt cold-smearing scheme35 with a broadening of 0.01 Ry. Electron−ion interactions were described using Ultrasoft pseudopotentials in the Vanderbilt form. Spin-polarization effects were not taken into account in the present work since tests (not presented here) were carried out and showed convergence to a solution with negligible spinpolarization. Self-consistency was achieved when the force applied on each atom was less than 10−3 Ry/bohr, and the variation in the total energies between two consecutive iterations was on the order of 10−4 Ry. The electronic density of states (DOS) was calculated within the tetrahedron method using a 15 × 15 × 15 mesh.

example, could be formed. Therefore, some questions about the vanadium nitride crystallization pathway, as the possible structural and chemical composition of the materials and its transformations during the process, remained unclear. Characterization methods such as infrared spectroscopy (IR), solidstate magic-angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), among others, were used to monitor the decomposition of formed intermediates and to shed light on the composition dependence on the structural properties of the material during the transformations that take place in the crystallization of vanadium nitride, also providing clues on the mechanisms for the in situ preparation of a great variety of transition metal nitrides and/or carbides at low temperatures for technological and academic applications. Furthermore, we also performed plane-wave density functional theory (DFT) calculations in order to study the electronic structure and the thermodynamic stability of the proposed structures. Finally, in the present paper, we have proposed a possible pathway for the crystallization process of vanadium nitride, which in turn could be extensible to similar systems.

2. METHODS 2.1. Experimental Section. Most of the experimental details were described elsewhere.30 In brief, a mixture of NH4VO3 (AMV) and guanidinium carbonate (GC) ensuring a molar ratio vanadate:guanidinium of 1:3 was prepared by mixing the solid reactants. This process was followed by a pretreatment in which the mixture (∼500 mg) was gradually heated (10 °C/min) from ambient to reaction temperature (150 °C) and kept constant for 12 h. In a “U”-shaped quartz reactor coupled to a furnace the temperature was linearly raised at the rate of 10 °C/min from ambient up to 300 °C. After that, a final temperature (400, 500, 600, 700, or 800 °C) was set and kept fixed for 4 h. The samples in the passivated form were analyzed by XRD using a Rigaku instrument (Miniflex model) operated at 30 kV and 15 mA. The angular interval of 10°−100° was varied in 0.05° steps using a 1 s counting time per step. The diffraction data were refined by means of Rietveld method using FullProf Suite software package and graphical interface WinPLOTR. (This methodology is based on the simulation of the entire diffractometric pattern using structural parameters of the constituent phases.) The lattice parameters were determined by the refinement, as specified in Table 1. The FT-IR spectra of the samples were recorded using a Nicolet Magna-IR 760 spectrometer in the range of 4000−400 cm−1 with the samples in the form of KBr pellets. The XPS analysis was carried out using a UNI-SPECS UHV spectrometer. 51V MAS NMR measurements were performed in the magnetic field H0 = 9.4 T (ν0 = 105.2 MHz for 51V

3. RESULTS AND DISCUSSION 3.1. Material Characterization. In a previous work, we showed that the intermediate (GmV), possibly formed through a partial double-displacement solid state reaction (eq 1), presents distinct characteristics of the reagents (NH3VO4 and GC). We also investigated the local properties of GmV, and our results showed good correlation between the DFT clustermodel calculations and experimental observations.30 It was suggested that an incomplete chemical reaction takes place in a “solute (NH3VO4)−solvent (GC)” interface. Our data also indicated that the chemical stability of GmV might be attributed to the strong interactions (H-bonds) formed between the guanidinium and vanadate species. A small amount of volatile salt ammonium carbonate [(NH4)2CO3] (XRD pattern is shown in the Supporting Information) was detected, and it supports our early assumption: the double displacement mechanism, with an incomplete reaction.

Table 1. Structural and Statistical Parameters Obtained by Refinement Using the Rietveld Methoda sample (°C) 500 600 700 800

a0 (Å) 4.105 4.121 4.130 4.137

(4.161) (4.139) (4.133) (4.119)

Vm (Å3) 69.219 70.028 70.445 70.848

(72.038) (71.235) (70.564) (69.894)

χ2 3.85 2.18 2.19 1.66

a

Numbers in parentheses are computed with DFT. a0 = lattice parameter, Vm = cell volume, and χ2 = statistical factor of adjustment. 25660



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local environment of vanadium atoms in a nearly regular local tetrahedral coordination and to isolated tetrahedral vanadium sites with small chemical shift anisotropy.60,71−73 These features are in accordance with our model, since this scenario suggests that GmV derives from the breaking of the tetrahedral VO4 chains into discrete units by simultaneous interaction (H-bond) with guanidinium species.30 We are aware of the limitations of our proposed model, which represents only an idealized Hbonded structure of a complex chemical composition, but we believe that the model is adequate for our purposes because the GmV properties are considered to be essentially local, as observed in the MAS NMR results. Although we are still currently evaluating the performance of different density functionals and of basis sets for our system, preliminary GIAO NMR calculations indicate a good agreement with experimental data. In fact, a small deviation between theory and experiment for the vanadium atom was obtained at relatively low computational costs, for instance at the B3LYP/6-31G* level of theory. A detailed NMR theoretical work will be the target of our next study. It is also interesting to note that the resonance peak at ∼-613 ppm was detected only at higher spin rates (13.5 kHz), indicating the presence of vanadium atoms related to V2O5-like structures, interpreted as the combination of the narrow peak at −∼ 613 ppm and a broader peak overlapping it (only visible at high spinning speed). These features are also associated with the reduction of the “(NH4)2O” units, as suggested by Brown and co-workers,63 who considered the AMV as having a (NH4)2O·V2O5 composition. Moreover, the less intense peak located at ∼632 ppm is in agreement with our assumptions, since it may be assigned to vanadium atoms distributed mainly in tetrahedral oxygen environment, although a certain amount of octahedral vanadium may be present as well, as observed by other authors.72,73 Figure 2 depicts the XRD patterns and shows the phase evolution of GmV. The diffraction pattern clearly shows that

(Gua)2 CO3(s) + 2NH4VO3 (s) → 2GmV(s) + CO3(NH4)2 (s)

(1)

The thermal behavior of GmV was also analyzed by TG/ DTG.30 The initial mass loss (until ∼168 °C) was believed to be due to the desorption of water and other gases, while intense DTG peaks at ∼200 and ∼310 °C were assigned to the decomposition and structural transformations of GC and of NH4VO3. On the other hand, the peak at ∼410 °C was linked to the (key step) migration of nitrogen and oxygen atoms forming the first V−N bonds.17 51 V is an extremely suitable nucleus for MAS NMR analyses, mainly due to its high natural abundance (∼99.76%) and rather small quadrupolar interactions.60,61 Even if small amounts of vanadium are present and/or slight structural changes occur, it can be detected by MAS NMR spectroscopy. Taking advantage of these features and in order to identify the species that may be formed in the solid-state reaction, 51V MAS NMR analysis of the GmV sample was carried out. As seen in Figure 1, 51V MAS

Figure 1. MAS NMR spectra of the GmV sample at 9.4 T recorded at different spinning speeds.

NMR spectra shows that the chemical reaction forms at least five environmentally distinct vanadium atoms. One important change that occurred after the chemical reaction was the absence of a peak at ∼−670 ppm, corresponding to the vanadium atoms in the NH4VO3 (AMV) crystal structure.62 Therefore, we assume that during the partial chemical reaction the amount of unreacted AMV underwent structural transformations as a function of the temperature, in agreement with many investigations on this subject,63−67 enabling some insights into why GmV shows distinct characteristics of the reagents.30 It is known that in inert atmospheres AMV decomposes in a first stage into the intermediate ammonium bivanadate ((NH4)2V4O11) (ABV), evolving to structures containing vanadate species as a function of the temperature. 67 Accordingly, the resonance at ∼−586 ppm may be assigned to the chemical environments in which vanadium atoms are present in structures with pyramidal association,69,70 consisting mainly of V4O11 layers, as found in the ABV structure.67 Our assumption is furthermore supported by the studies of Lamure and Colin,68 who established that ABV is only formed in small sample masses and temperatures below 180 °C. Despite our proposed model presenting only one 51V environment, the peaks located at ∼596 and ∼−602 ppm are mostly attributable to typical values related to distortions of the symmetry of the

Figure 2. XRD patterns of decomposition of guanidinium m-vanadate with increasing temperatures.

treatment of the precursor obtained through the guanidinium route can give bulk VN. The results also indicate that calcination of the precursor is a necessary condition for preparing vanadium nitride. At 400 °C no crystalline phase has been clearly identified. From 500 to 800 °C, an increase in crystallinity is observed. Evidences of the formation of VN at 25661



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amount of vanadium oxynitride, as also observed by Lu and coworkers in a recent article.36 The additional O 1s component centered at 531.4 (eV) (Figure 3D) may be ascribed to OC− N bonds.27 Regarding the possible mechanism of crystallization, the formation of the early V−N bonds (400−410 °C) is likely due to the simultaneous formation of vanadium oxide and reactive carbon nitride species (C3N3+, C2N2+, C3N2+) which we consider as being important mediators in the crystallization of vanadium nitride by acting as reducing agents.26 Recently, Sui and Lu37 have reported the synthesis of carbon nitride species at ambient pressure and temperatures below 500 °C from a single inorganic precursor (TiC0.3N0.7). In this study, the authors showed that the mass loss started at about 300 °C with complete decomposition of the carbon nitride material occurring at 650 °C, confirming the low thermal stability of the material. This is a strong indication that in our case the oxygen atoms of the vanadium oxide species are reduced and displaced by nitrogen and carbon atoms from the decomposition of the carbon nitride species, in a dynamical process dependent on the temperature and facilitated by the lower diffusion barrier of carbon.38 The FTIR spectra of the samples treated at 400, 500, 600, and 700 °C also support our assumptions, as seen in Figure 4.

lower temperatures were found and the presence of a highly crystalline Fm3m (NaCl-type) VN phase at 800 °C was observed. Table 1 presents the principal crystalline structure parameters obtained by refinement and adjustment of the statistical indicators of quality. As the sample prepared at 400 °C did not exhibit diffraction peaks it was not refined. In general, the results displayed reasonable correspondence between experimental (the simulated curves can be seen in the Supporting Information) and theoretical data for the lattice parameters (see also Figure 6). Furthermore, a relatively low specific area (Sg) of ∼25 m2 g−1 has been found. Although this result was expected, because the sample was prepared via a precursor method, we firmly believe that the formation of a thin layer of oxide after passivation might play a role on the analysis by screening the measurement of Sg. Therefore, experiments are now in progress in order to evaluate the role of the passivation over such a measurement, and the results will be published elsewhere. 3.2. On the Crystallization Process with Increasing Temperature. In order to shed some more light on the vanadium nitride crystallization process, XPS analyses were carried out. The sample thermally treated at 400 °C (Figure 3A) shows the C 1s component of the XP spectrum divided

Figure 4. FTIR spectra of GmV treated at 400, 500, 600, and 700 °C.

In the sample treated at 400 °C, some major peaks may be attributed to the vanadium oxide phases, namely, 970 cm−1 [(υ VO (asym)], 810 cm −1 [(υ V−O (VO 2 )], 640 cm −1 [(υV−O(V2O3)], and 490 cm−1 [υV−O(puckered ring)]. The peaks around 1630 cm−1 may also be assigned to the vanadium oxides.24 Absorptions observed at 1200−1500 cm−1 and the less intense ones located at 3000−3500 cm−1 were attributed to a small amount of unreacted organic material, mainly NH2 scissoring and NH in-plane bending, and to the stretching modes of the N−H bond, respectively. In the higher temperature samples (500, 600, and 700 °C), the absorption at 990 cm−1 is characteristic of the υV−N (fundamental) of the nanostructured vanadium nitride.23 We were also able to identify an extra peak around 1620 cm−1 mostly attributable to an intermediate phase related to such structures as VNpO (0.1 < p < 0.86) also formed through oxidation of the VN surface.24 Finally, Figure 5 depicts the 51V MAS NMR spectra performed in three different spinning speeds (10, 12, and 13.5 kHz) for the sample thermally treated at 400 °C. Because of the formation of the first identifiable V−N chemical bonds,

Figure 3. XP spectra of the C 1s (A), N 1s (B), V 2p3/2 (C), and O 1s (D) regions for the sample prepared at 400 °C.

into two main regions (284.3 and 288.3 eV), which indicates the presence of the doubly coordinated carbon atoms (284.3 eV), and the sp3 C−N bonds, characteristic of regions above 287.0 eV, which, in turn, may be correlated with CNx and C3N4-type structures.21,25,26 Moreover, the N 1s peak component centered at 398.7 and at 399.8 eV (Figure 3B) may be assigned to the doubly and triply coordinated nitrogen atoms that are also characteristic of C3N4 structures. Although not identified by XRD patterns (see Figure 3), the surface structure analysis of the V 2p3/2 region (517.3, 516.4, and 515.5 eV) (Figure 3C) and of the O 1s region (530.3 and 529.7 eV) (Figure 3D) supports the existence of a complex mixture of vanadium oxides.19 Furthermore, the less intense V 2p3/2 component located at 514.7 eV can be related to formation of the first V−N−O bonds, which reveals the presence of small 25662



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Figure 5. MAS NMR spectra at 9.4 T of the thermally treated sample (400 °C) recorded at different spinning speeds.

atoms, the former model, V1N1 (VOxN1−x; x = 0), representing the final product and the latter one, V1O1 (VOxN1−x; x = 1) representing the possible initial step of the crystallization process. This assumption is supported by our experimental findings as well as by the literature.37 To represent the possible stable structures, the total energy was calculated for models with different number of oxygen and nitrogen atoms. This approximation was successfully applied by Abadias and coworkers39 in order to systematically investigate the composition dependence of the structure, elastic, and electronic properties of the Ti1−xZrxN system. Other authors have also applied this calculation method with considerable success.39−45 Convergence tests were carried out analyzing the dependence of the plane-wave cutoff energy on the total energy and the Monkhorst−Pack grid of k-points. These tests showed that the parameters used here (see section 2.2) are sufficient to ensure that relative energies converged and that the theoretical results are reliable. Since, to the best of our knowledge, there is only few experimental and theoretical data concerning the lattice constants for the VOxN1−x compositions (0 < x < 1),46,47 the results reported here are predictions. For instance, lattice constants were reported for bulk vanadium oxyinitride with different compositions: for VO1.20N0.11, a = 4.076 Å and for VO0.12N0.93, a = 4.124 Å.49 With respect to the vanadium oxynitride phases, Kafizas et al.47 described the synthesis of films with gradating phases to a range of cubic VOxN1−x (0.23 < x < 0.53) using a triple source of metal and nonmetal precursors (VCl4, H2O, and NH3) to synthesize numerous phases and compositions for their final product (VO xN 1−x). Moreover, beyond VO xN 1−x solid solutions they have observed bulk mixed oxide phases (V2O3−V8O15, for instance) via XRD analysis. In our case, it was not possible to identify clearly the diffraction peaks, at 400 °C, relative to oxides (Figure 2); in higher temperatures peaks assigned to the Fm3m were unique in the XRD pattern. Thus, we assume the existence of a complex mixture of vanadium oxides given that our 51V MAS NMR spectra (Figure 5) identified only one isotropic peak which encompasses all possible vanadium sites. On the other hand, Elwin and Parkin46 also employed an ACVD-based technique in order to

we believe that this system represents a key structure in the VN crystallization process. Interestingly, marked changes in the shape may be observed if we compare the 51V MAS NMR spectra of GmV (Figure 1) and the spectra of the sample treated at 400 °C (Figure 5), both obtained at the same spinning speeds. In the first one (Figure 1), the presence of five resonance peaks representing five distinct environments experimented by vanadium atoms may be noted, while in the latter (Figure 5), we see that only one isotropic peak is present. These differences suggest dramatic structural transformations occurring during the thermal treatment, and it corroborates the amorphous nature of the key structure, also shown by the XRD diffraction patterns (see Figure 2). The spectra depicted in Figure 5 are characterized by one broad isotropic resonance for 51 V located at 656 ppm, which, despite the system’s noncrystalline nature, can be associated with structures containing vanadium in octahedral sites, for example, those present in NH4VO(SO4)2, KVO(SO4)2, and RbVO(SO4)2.60,70 Analyzing these spectra (Figure 5), we suggest that the structure does not present long-range order due to the thermal transformations, also characteristic of amorphous systems. Despite this fact, according to the literature it is possible to preserve to some extent the short-range chemical ordering even in the amorphous phase.28 In the present case, this might mean a variety of chemical environments detected only by the single and broad resonance, encompassing all possible vanadium sites. In turn, such a complex structure underwent continuous thermal transformations with raising temperatures increasing its crystallinity and thus forming vanadium nitride in the end of the process. 3.3. Theoretical and Experimental Evidences. The possible mechanism of the vanadium nitride crystallization was investigated via DFT calculations. Because at the initial step of the V−N bond formation (∼400 °C), the exact structure of the system under study was not well-defined, since a complex mixture of vanadium oxide and carbon nitride species was detected, the choice of a proper computational model proved to be particularly difficult. The computational model described here was based on the NaCl-type structure of vanadium nitride with some amounts of nitrogen atoms replaced by oxygen 25663



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lattice parameter of the VN model was found to be 4.119 Å while for VO (considering both VN and VO models in the NaCl-type structure) a value of 4.172 Å was found. In the case of the VN model, our results deviate by less than 0.5% from experiment (4.137 Å),50 while for the VO model, our DFT calculations are in excellent agreement with the calculated value (4.19 Å), previously reported by Kresse et al.52 Furthermore, the lattice parameter calculated by structure refinement of the sample thermally treated at 700 °C (4.132 Å) shows excellent agreement with our DFT calculated value (4.133 Å) for the VO0.25N0.75 model, which reinforces the idea that the formation of vanadium nitride via the present route is preceded by solidstate transformations involving oxynitride species.22 Although slight differences between the calculated and experimental lattice parameters may be observed, the variation of these values is nearly linear with the composition, following Vegard’s law.59 As observed by Vegard in what nowadays is known as “Vegard’s law”, the lattice constant of a continuous substitutional solid solution is dependent on its composition (size of added atoms or ions), which means that the lattice constant is equivalent to the composition-weighted average of the lattice constants of the pure compounds. It is worth noticing that our models did not consider the existence of vacancies in the crystalline lattice, and in spite of their simplicity they reproduce fairly well the corresponding experimental data. The apparent discrepancy between the experimental and theoretical data can be clarified if one considers that the existence of vacancies formed due to the diffusion of oxygen atoms in raising temperatures would result in the reduction of the unit-cell size and, in turn, would lead to slightly overestimated theoretical parameters, except in the VN model which was slightly underestimated (∼0.48%). Moreover, our models did not take into account the presence and diffusion of carbon atoms (see Table 2) from the crystalline lattice to the exterior, which also contributes to the gradual increase as observed in the experimental parameters.

synthesize vanadium oxynitride coatings but utilized a double source of precursors (VOCl3 and NH3). Their work stressed that the composition of the vanadium oxynitride films are highly dependent on the experimental conditions and that the incorporation of oxygen in the films is a serious problem which can alter some properties of the final product. In our methodology, the source of oxygen atoms is the vanadate species, and there is no further contact of the samples (except during passivation) with external oxygen. Furthermore, in our mechanism, we propose that the oxygen atoms are expelled during heating via diffusion processes and that the formation of vanadium nitride is dependent on the temperature. Finally, we would like to stress that the CVD-based methods constitute a class of very advanced techniques and are extremely important for the area of materials science research, being characterized by quite elaborate synthesis methods. It should be kept in mind that each synthesis method has its advantages and drawbacks, and careful validation is necessary in order to identify a particular method (or a combination thereof) that affords reliable results from the academic or technological point of view. Turning now to the theoretical side, a previous study using the density functional methods as well as the all-electron scalar-relativistic linear muffin-tin orbital (LMTO) theory and dealing with the structural properties of nonstoichiometric vanadium oxynitrides, VOxN1−x, was carried out by Lumey and Dronskowski.48 In their interesting paper, the main goal was to provide a systematic study, based on the structures of polymorphs with composition AB2, of the energy−volume− pressure relationship and phase transitions in order to predict the energetically most stable oxynitride phase as well as to propose a high-pressure synthesis of stoichiometric VON. However, in their study, they used only the VO 0.5N0.5 composition model. In our case, we have used at least five different composition models in order to (a) evaluate the dependence of the lattice parameters of the continuous substitutional solid solutions with its composition, (b) assess the energetic stability of the solid-state transformations due to the substitution of the oxygen by the nitrogen atom, and finally (c) shed light on the electronic structure of the models. As our synthesis route was carried out in ambient pressure, this variable and its variations were not taken into account. Thus, the models we used, despite their simplicity, were able to provide information on the structures involved in the complex solid-state transformations occurring during the synthesis of the Fm3m VN structure. The calculated and experimental lattice parameters of the VOxN1−x structures are shown in Figure 6. The computed

Table 2. Carbon, Nitrogen, and Hydrogen Contents of the Samples in Function of the Increase in Temperature temp (°C)

C (wt %)

N (wt %)

H (wt %)

400 500 600 700 800

11.06 7.88 2.78 2.13 1.92

21.81 18.47 15.29 14.71 16.82

0.64 0.33 0.12 0.01 0.01

In order to access the stability of the VOxN1−x structures, the formation energies in function of composition, EF(x), were calculated as follows: E F(x) = E VOx N1−x − [(xE VN) + (1 − x)E VO)]

Herein, EVOxN1−x, EVN, and EVO are the zero absolute temperature total energy of the optimized VOxN1−x, VN (x = 0), and VO (x = 1) NaCl-type structures, respectively. Moreover, the influence of pressure was not considered due to its low influence on the condensed matter phases. Figure 7 shows the formation energy for the VOxN1−x system as a function of composition. Thermodynamically, the calculated formation energy is the Gibbs free energy of mixing, calculated at T = 0 K. As seen in Figure 7, it is energetically favorable to form the VOxN1−x solid solution through the entire composition range, from VO and VN, which agrees with our

Figure 6. Calculated equilibrium lattice constant for VOxN1−x models. 25664



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elemental analysis (Table 2) of the thermally treated samples confirms the existence of both carbon and nitrogen in all samples. From 500 to 800 °C, the C and N contents fall considerably. So, the presence of small quantities of O or C atoms is a plausible scenario, and increasing temperatures (up to 800 °C) might cause its release into the gas phase and its migration to the surface, a possibility confirmed by XPS analysis, while N atoms would be “trapped” in the bulk, then completing the formation of vanadium nitride. The atomic structure of a solid solution is important for understanding its properties.52,53 When the nature of the bonding present is similar in the constituent phases, the analysis of the density of states (DOS) may give us useful information to understand the nature of this bonding. The total DOSs of the various models considered here were examined and are presented in Figure 8, which shows the DOS of the NaCl-type structure of VOxN1−x models for x = 1, 0.75, 0.50, 0.25, and 0. In all cases, the Fermi level is located at the origin of the energy axis (represented by the red line). As observed in Figure 8, the electronic spectra can be split into two main bands. The analysis of the partial DOS (PDOS, Supporting Information) shows that the low-energy bands originate in the O 2p state in the V1N1 (VOxN1−x; x = 1) model while the next band, located between −4 and 2 eV, can be associated with the V 4d state, preserved around the Fermi level, suggesting an unstable system since a very weak V 4d−O 2p interaction may be observed. As seen in Figure 8B, substitution of oxygen by nitrogen causes a small decrease in the DOS value around the Fermi level region, suggesting a more stable system. Furthermore, one notices a loss of intensity in the low-energy band as well as the appearance of an additional peak around −5 eV, due to the O 2p−N 2p interaction. Closest to the Fermi level, the occupied state still remains localized mainly on the V 4d state with small interactions of such state with N 2p and O 2p states. Going from VO0.75N0.25 (VOxN1−x; x = 0.75) to the VO0.5N0.5 (VOxN1−x; x = 0.50) model, as shown in Figure 8C, the next step of our proposed path of the crystallization of vanadium nitride, a strong hybridization of the N 2p−O 2p and

Figure 7. Calculated formation energy for VOxN1−x models.

experimental findings. We observe that the VO0.75N00.25 (the one corresponding to the first stage of the oxygen release) model represents the most thermodynamically stable system (EF(0.25) = −36.3 kJ/mol), revealing a clear tendency to release oxygen atoms and form the NaCl-type VN. For comparison purposes the EF(x) for the VOxC1−x system, simulating the formation of an oxycarbide structure, was also calculated (Supporting Information). Contrary to the VOxN1−x system, the calculated formation energy of the VOxC1−x system is positive in the whole composition range. Therefore, according to our calculations for the chemical driving force (EF(x)), this system should be unstable and decompose into VO and VC (vanadium carbide). This situation (formation of VC) was not observed, in agreement with our experiments. Moreover, Ivanshenko and co-workers32 showed via DFT calculations that for cubic-based VOxN1−x and VCxN1−x small amounts of oxygen and carbon admixtures do not stabilize NaCl-VN when compared to WC-VN. They concluded also that only carbon contents larger than 34% (x > 0.34) could stabilize the NaCl-based VN structure. In our case, the

Figure 8. Calculated DOSs for VOxN1−x models. The vertical line indicates the Fermi energy. 25665



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passivation and exposure of the sample to air were also detected. The XPS data of the N 1s (397.1 eV) agree well with the reported values for vanadium nitride.17,18 We were also able to identify contributions (398.5 and 400.8 eV) which may be attributed to C−N bonded nitrogen.20 The XPS analysis of the C 1s level indicates the presence of graphitic carbon with a BE of 284.3 eV, suggesting that no carbidic carbon (C 1s BE of 282.4 ± 0.2 eV)4 is present in the sample.21 No side reactions besides crystallization were observed, and diffraction peaks relative to vanadium oxides cannot be identified, suggesting that the bulk is mostly attributable to VN. Moreover, the lattice parameter calculated by refinement (a = 4.132 Å) is in good agreement with the value reported in the literature for VN (a = 4.137 Å, JCPDS card, No. 78-1315).

a mixing with the V 4d states are clearly observed. Although the region just above the Fermi level is relatively dominated by V 4d states, a decrease in the DOS value around the Fermi level suggests an increase in the chemical stability of the system and indicates a covalent bond nature. Despite the insignificant difference in the Fermi energy (0.034 eV) between VO0.75N0.25 and VO0.50N0.50, the literature shows that the decrease in the DOS around the Fermi level is caused by a reduction in the valence electron concentration (VEC),32 which can be linked to the increase in the chemical stability of the VO0.50N0.50 system and also to the negative formation energy. Considering the VO0.25N75 (VOxN1−x; x = 0.25) system as shown in Figure 8D, which hypothetically represents the last but one stage in the crystallization process, we see that the DOS near the Fermi level is less dominated by the metallic character originated from V 4d states and characterized mainly by considerable hybridization between V 4d−N 2p and V 4d− O 2p (with less overlap), O 2p−N 2p, and less intense mixing of the V 4s−O 2p states. Such configuration suggests the formation of partial covalent bonds among constituents, characterizing a stable structure. Conversely, as observed experimentally by elemental analysis (Table 2), temperatureprogrammed surface reaction (TPSR), and mass spectrometry analysis,30 the carbon content falls drastically with a raise in the temperature, followed by the concomitant formation of high intensity ion fragments of carbon (m/z = 28 (CO), 44 (CO2)) and nitrogen (m/z = 30 (NO), 46 (NO2)) oxides. That is despite the fact that the hypothetic V0.25N0.75 system represents a thermodynamically stable system; in our synthesis route it would decompose as a function of the increase in temperature, the whole process ending with the formation of highly crystalline vanadium nitride with a NaCl-type structure around 800 °C. As seen in Figure 8E, the electronic DOS suggests that for V1N1 (VOxN1−x; x = 1) the bonds are formed mainly between the vanadium and nitrogen atoms (V 4d−N 2p), resembling the early transition metal (d-block) nitrides and carbides54,55 and in turn resembling noble metals electronic properties.56 It is important to mention that the present theoretical work deals only with idealized structures, since our models did not take into account crystal defects and vacancies. As we known, transition metal nitrides and carbides are stable over a wide range of composition, belonging to a class of essentially nonstoichiometric compounds.3,57 So, the final product synthesized here would likely have a nonperfect stoichiometric structure, with small concentration of vacancies, according to the elemental (Table 2) and XPS analyses (Figure 3). Recent DFT studies32,58 revealed that NaCl-based structures induce vanadium vacancies and cause the formation of additional peaks in the region of the “s” and “p−d” bands, assigned to nonbonding nitrogen states, while vacancies in the nitrogen sublattice induce the formation of additional states below the Fermi level, resulting in a small decrease in the DOS at the Fermi level, which in theory increases its chemical stability. A detailed periodic DFT study on the electronic structure of bulk, surface, and catalytic properties of stoichiometric and nonstoichiometric vanadium nitrides using large supercells is now in progress, and it will be the target for future works. A further confirmation by XPS30 analysis shows that the binding energies (BE) of the V 2p3/2 core level (513.4 and 514.4 eV) in the sample prepared at 800 °C are in excellent agreement with the literature.18 The contributions from the oxides19 (516.6 and 517.3 eV) formed on the surface after

4. CONCLUSIONS In this study, we describe an important reaction intermediate (GmV) formed by the reaction between NH4VO3 and GC.51V MAS NMR spectrosopy provided information on local properties of GmV, which is composed mainly by vanadium in a tetrahedral oxygen environment, H-bonded to guanidinium species. Furthermore, we show that the thermal treatment of GmV yields a complex mixture of vanadium oxide and carbonitride (CxNy) species, which in turn will suffer a solidstate transformation to vanadium nitride. The solid-state conversion occurs through several steps as the temperature increases. First, GmV is decomposed into reactive vanadium oxide and carbonitride (CxNy) species, which in turn react by exchanging oxygen and nitrogen, being converted into vanadium oxynitride (VOxN1−x) with varying stoichiometry dependence on temperature and then transformed into the final product, a NaCl-type structure vanadium nitride. The DFT calculations show that this transformation is energetically favorable and the formation of a VOxN1−x solid solution is feasible. The XPS results show that the surface of the sample obtained at 800 °C consists mainly of a mixture of vanadium oxides and an amount of non-carbidic carbon. This work also shows that the proposed route is an effective and easy method for preparing vanadium nitride and provides clues for the synthesis of other metal nitrides. The present study also gives insights into an interesting solid-state transformation and encourages the use of combined theoretical and experimental investigations.

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ASSOCIATED CONTENT

S Supporting Information *

Multipurpose unit used in the solid-state synthesis; diffraction pattern of ammonium carbonate at 150 and 400 °C; calculated formation energy for VOxC1−x; partial DOS (PDOS) for the VOxN1−x models; diffraction patterns and Rietveld refinement of the thermally treated samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel (21) 2562-7132; fax (21) 2562-7132; e-mail eugeniofs@ iq.ufrj.br. Notes

The authors declare no competing financial interest. 25666



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ACKNOWLEDGMENTS

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REFERENCES

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The work was supported by Coordenaçaõ de Aperfeiçoamento ́ Superior (CAPES), Conselho Nacional de de Pessoal de Nivel ́ Desenvolvimento Cientifico e Tecnológico (CNPq), and Fundaçaõ de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). The authors are acknowledged to Dr. Carlos A. Perez for helpful discussions on the Rietveld analysis. E. F. Souza is especially thankful to Prof. Adriano S. Martins for fruitful discussions.

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