Cleaner and non-carbon reduction for preparing high content VN alloy by direct nitriding of V₂O₃ in ammonia gas

2.2.1. NH₃ roasting experiments

The reduction and nitriding of V₂O₃ powder were performed in a tube furnace. A porcelain boat containing 1.5 g of V₂O₃ powder was placed inside the furnace. Ammonia gas was introduced at the beginning, and the temperature was increased from room temperature to 200°C at a rate of 10°C min^-1, followed by a 10-minute holding period. Subsequently, the temperature was further raised to the target roasting temperature at the same heating rate of 10°C/min. After a predetermined duration, the roasting temperature was gradually decreased. When the furnace temperature cooled down to room temperature, nitrogen gas was introduced to replace the ammonia. Finally, the sample was removed after ensuring that all ammonia had been completely discharged from the system. The preparation process of vanadium-nitrogen alloy in this study has been shown in Figure 1. Each experiment was conducted in triplicate, and the mean value was calculated to ensure the reliability and accuracy of the results.

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Figure 1.

The preparation process of vanadium-nitrogen alloy in this study.

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2.2.2. Analytical methods

The main elements analysis of the raw material V₂O₃ was analyzed via inductively coupled plasma optical emission spectrometry (ICP-OES) on an Optima-4300DV instrument from PerkinElmer. The vanadium content in VN products is determined according to the standard GB/T 24583.1-2019, whereas the nitrogen and oxygen contents are quantified using a nitrogen-oxygen analyzer. X-ray diffraction (XRD, Rigaku, Tokyo, Japan) was performed to characterize the phase composition of the products. The chemical states of the different elements in samples were analyzed by X-ray photoelectron spectroscopy (XPS) using a K-Alpha XPS system from Thermo Fisher Scientific. The microcosmic appearance of the products is characterized by SEM (Scanning Electron Microscope, JSM-IT300, JEOL, Tokyo, Japan) images and EDS (Energy Dispersive Spectrometer, Oxford Instruments, Oxford, UK).

3.1.1. Analysis of possible thermodynamic reactions

Preliminary thermodynamic assessment plays an important guiding role in the conduct of experiments [29]. In this study, the HSC software was employed to systematically analyze the thermodynamic behavior of the nitridation reaction of V₂O₃ in an ammonia medium [30-32]. In the V₂O₃-NH₃ calcination system, the possible chemical reactions have been shown in Table 2. According to the analysis in Table 2, these reactions mainly include the following three types: (1) Direct nitridation reaction between V₂O₃ and NH₃ (see Equation (1)); (2) Indirect nitridation reactions between V₂O₃ and nitrogen (see Equations (2)-(5)); and (3) Indirect nitridation reactions after hydrogen reduction (see Equations (6)-(10)). The Gibbs free energy changes (ΔG^θ) of Equations (1)-(10) in the temperature range of 0-1200K have been shown in Figure 2.

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Figure 2.

Changes in ΔG^θ for possible reactions in this study.

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It can be seen from Figure 2 that within the temperature range of 0-1200K, only Equations (3) and (10) among the 10 chemical reactions in Table 2 can occur spontaneously. Specifically, Equation (1) can only directly occur when the temperature is higher than 983K (about 710°C); Equation (2) can directly occur only when the temperature is higher than 457K (about 184°C). In contrast, the traditional nitrogen calcination system usually needs to operate at high temperatures of 1300-1500°C, while the ammonia calcination system can achieve the nitridation process of vanadium at lower temperatures. Moreover, ammonia can decompose into nitrogen and hydrogen at temperatures above 184°C. However, since Equations (3)-(5) and (7)-(9) are thermodynamically infeasible, from a thermodynamic perspective, the main reaction occurring in the V₂O₃-NH₃ calcination system is the direct nitridation of V₂O₃, with the products being VN and water vapor.

3.1.2. Phase diagram analysis of the V₂O₃-NH₃ roasting system

Based on thermodynamic reaction analysis, a detailed phase diagram calculation was performed for the V₂O₃-NH₃ roasting system, and the results are presented in Figure 3. Under standard conditions, setting ΔG = 0 in Equation (1) leads to T = 983 K. If the influence of gas partial pressure is neglected, the reaction can proceed spontaneously at this temperature. However, when considering the effect of gas partial pressure, at a given temperature, assuming the actual value in the system [(PH₂O/P0)³/(PNH₃/P0)²] = 10ˣ, adjusting the parameter x such that the Gibbs free energy of the reaction becomes negative allows the Gibbs free energy of Equation (1) to be expressed as:

$\begin{array}{l}\Delta {\text{G}}_{(\text{1})}^{\theta }=\Delta {\text{G}}^{\theta }+\text{RTln}{\left({\text{P}}_{\text{H2O}}/{\text{P}}^0\right)}^{\text{3}}/{\left({\text{P}}_{\text{NH3}}/{\text{P}}^0\right)}^{\text{2}}=\text{142}0\text{46}.0\\ -\text{144}.\text{534T}+\text{RTln}{\left({\text{P}}_{\text{H2O}}/{\text{P}}^0\right)}^{\text{3}}/{\left({\text{P}}_{\text{NH3}}/{\text{P}}^0\right)}^{\text{2}}\end{array}$

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Figure 3.

(a) The equilibrium relationship between reaction temperature and gas partial pressure in the V₂O₃-NH₃ system and (b) the phase diagram of the NH₃-O-V ternary system.

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A gas partial pressure relationship diagram for Reaction (1) was constructed with lg[(P_H2O/P⁰)/(P_H2/P⁰)] as the abscissa and T as the ordinate, as illustrated in Figure 3(a). According to Equation (1), to reduce the reaction temperature for the nitrogenation of V₂O₃ with NH₃ to prepare VN, it is necessary to maximize the partial pressure of NH₃ while minimizing that of H₂O. As shown in Figure 3(a), decreasing the partial pressure of H₂O can significantly lower the reaction temperature and effectively promote the nitrogenation reaction.

The phase diagram of the NH₃-O-V system at high-temperature equilibrium is presented in Figure 3(b). From Figure 3(b), it can be observed that the significant condensed phases in the NH₃-O-V system may include V₂O₃, NH₃, VN, and H₂O. Notably, V₂O₃ and NH₃ cannot coexist in equilibrium, whereas VN and V₂O₃ can coexist. Additionally, NH₃, VN, and H₂O can coexist, as can NH₃ and H₂O.

Based on the aforementioned thermodynamic analysis, ammonia can directly nitrogenate V₂O₃ at a lower temperature to produce VN and water vapor. Compared with traditional techniques, this process eliminates the need for carbon participation and operates at a lower temperature, providing a distinct energy consumption advantage and aligning well with the dual-carbon strategy.

3.2.1. Effect of ammonia roasting conditions on product composition

According to the thermodynamic feasibility study in Section 3.1, the direct reduction and nitridation of V₂O₃ in an ammonia atmosphere is theoretically feasible. Recent studies have shown that the generation efficiency of active N atoms during the ammoniation process is a key factor affecting the purity of the product [33,34]. By optimizing the reaction temperature and ammonia flow rate, the decomposition of NH₃ into active N atoms was promoted, avoiding the reduction side reactions caused by excessive H₂, and the nitrogen content and purity of VN were improved [35]. In this study, V₂O₃ was used as the raw material, and verification experiments were conducted in an ammonia atmosphere in a tube furnace. The results have been shown in Figure 4.

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Figure 4.

Effect of ammonia roasting conditions on product composition (a) roasting temperature; (b) flow rate; (c) roasting time.

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The influence of roasting temperature on the V, O, and N content in the product was investigated under conditions of a 4-h roasting time and a flow rate of 70 mL min^-1. The results have been presented in Figure 4(a). As depicted in Figure 4(a), as the roasting temperature increases, the O content in the product decreases gradually, while the V and N content increase progressively. When the roasting temperature reaches 650°C, the nitrogen content in the sample has increased to 5.5%, indicating that the nitridation reaction begins at this temperature, which is consistent with the thermodynamic calculations reported in section 3.1.1. With further increases in roasting temperature, the nitridation reaction becomes increasingly complete. When the roasting temperature exceeds 800°C, the N content in the product exceeds 16%. Although increasing the roasting temperature further continues to enhance the N content, considering the initial objective of preparing VN16, the optimal roasting temperature is determined to be 800°C.

In this section, pure ammonia gas was employed as the roasting atmosphere, and the primary focus was placed on investigating the impact of ammonia gas flow rate on the roasting performance. Under the conditions of a 4 h roasting time and an 800°C roasting temperature, the effect of varying ammonia gas flow rates on the V, O, and N content in the product was examined. The results have been presented in Figure 4(b). As illustrated in Figure 4(b), with increasing ammonia gas flow rate, the O content in the product decreases progressively, while the V and N content increase gradually. When the ammonia gas flow rate exceeds 70 mL min^-1, the N content in the product surpasses 16%. Although further increasing the ammonia gas flow rate continues to enhance the N content, to minimize ammonia gas consumption and optimize process conditions, 70 mL min^-1 was ultimately determined as the optimal ammonia gas flow rate.

Under the conditions of a 4-h roasting time and an 800°C roasting temperature, the effect of varying ammonia gas flow rates on the V, O, and N content in the product was examined. The results have been presented in Figure 4(c). As shown in Figure 4(c), with increasing ammonia gas flow rate, the O content in the product decreases gradually, while the V and N content increase progressively. When the ammonia gas flow rate exceeds 70 mL min^-1, the N content in the product surpasses 16%. Further increasing the ammonia gas flow rate continues to enhance the N content. Table 3 compares the key process parameters and performance metrics between this study and existing methods for VN synthesis, demonstrating that the proposed approach enables low-temperature, carbon-free, and highly efficient synthesis of VN. Currently, the synthesis of vanadium nitride alloys is primarily achieved through several established methods, including arbothermal reduction, magnesiothermic reduction followed by nitridation, and precursor-based approaches [15, 17, 18, 20 and 36].

Furthermore, a preliminary environmental advantage of the proposed route was quantified by comparing it with conventional carbothermal reduction. The most significant difference lies in CO₂ emissions: while the carbothermal process theoretically generates approximately 0.8 kg of CO₂ per kg of VN (In fact, due to the excessive addition of carbon and incomplete combustion, more CO₂ needs to be released), the ammonia nitridation process produces no direct CO₂ emissions from its chemical reactants. This stark contrast underscores the potential of our method as a cleaner alternative aligned with carbon reduction goals.

3.2.2. Estimation of ammonia utilization efficiency

An estimation of the ammonia utilization efficiency was conducted based on the stoichiometry of the dominant reaction (Eq. 1) and the standard experimental conditions (1.5 g V₂O₃, 800°C, 4 h, 70 mL min^-1 NH₃ flow). The theoretical amount of NH₃ required for the complete conversion of 1.5 g V₂O₃ to VN is approximately 496 mL. In contrast, the total volume of NH₃ supplied over the 4-h duration was 16.8 L. Consequently, the ammonia utilization efficiency under these conditions is estimated to be only about 3.0%.

This low efficiency is common in laboratory-scale gas-solid reaction systems where a significant excess of the gaseous reactant is employed. The high flow rate serves critical purposes: (1) it maintains a high NH₃/V₂O₃ ratio, shifting the reaction equilibrium towards the products according to Le Chatelier’s principle; (2) it facilitates the rapid removal of H₂O(g), a reaction product that would otherwise inhibit the forward reaction; and (3) it ensures a uniform atmosphere and efficient heat transfer within the tube furnace. While this demonstrates the feasibility of the process, the low utilization rate highlights a key area for improvement in potential industrial applications. Future work will focus on optimizing the flow rate, exploring staged feeding, or investigating the feasibility of recycling the exhaust gas to enhance ammonia utilization and economic viability.

3.4.1. SEM-EDS analysis

To study the roasting mechanism of VN under the intervention of ammonia, the surface morphology of the raw material V₂O₃ and the products at different roasting temperatures was analyzed first. The surface morphology of the raw material V₂O₃ (see Figure 6(a)) and the products obtained at different roasting temperatures (see Figure 6(b-f)) was examined by SEM-EDS, as shown in Figure 6. Notably, the overall particle size and block-like morphology remain largely unchanged across the temperature range from 650°C to 800°C. This morphological stability is attributed to the high melting point of V₂O₃ (∼2000°C) and the solid-state, topotactic nature of the direct nitridation process, which proceeds without particle melting or significant structural reconstruction under the applied conditions (≤850°C).

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Figure 6.

(a) SEM-EDS analysis of raw materials and (b-f) vanadium-nitrogen alloy products obtained under different roasting temperatures.

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Despite this macroscopic uniformity, EDS mapping and quantitative analysis provide compelling evidence for a stepwise, diffusion-controlled nitridation mechanism. The evolution of elemental distribution, particularly nitrogen, reveals a clear progression. At the initial stage (650°C), nitrogen is sparsely distributed on the particle surfaces with minimal spatial overlap with vanadium, indicating limited surface adsorption and negligible bulk diffusion. As the temperature increases to 700°C and 750°C, the nitrogen signal intensifies and extends progressively into the particle interior, resulting in markedly improved colocalization with vanadium. By 800°C, nitrogen and vanadium exhibit a highly uniform spatial distribution (>90% overlap), confirming complete conversion throughout the entire particle volume. This sequential transition, from surface-localized to core-penetrating nitrogen incorporation, directly reflects the inward propagation of a diffusion-limited reaction front.

This interpretation is further supported by quantitative EDS and elemental analyzer data (Figure 4), which show a continuous decrease in oxygen content from 28.3 wt.% to 10.54 wt.% and a concomitant increase in nitrogen content from 5.5 wt.% to 16.29 wt.% over the same temperature range. The nearly linear inverse correlation between O and N concentrations is characteristic of a kinetically controlled, progressive substitution process, inconsistent with an instantaneous or homogeneous transformation. Together, these results establish a coherent and robust mechanistic picture of stepwise nitridation governed by solid-state diffusion.

3.4.2. XRD analysis

XRD, as an important method for characterizing crystal structures and their evolution laws, has been widely applied in various fields such as materials science, chemistry, biology, medicine, ceramics, metallurgy, and mineral resources [37,38]. To investigate the phase transformation of V₂O₃ under various roasting conditions, XRD analysis was performed on the roasting products. The results have been presented in Figure 7. The XRD patterns of the roasting products at different temperatures have been shown in Figure 7(a). As depicted in Figure 7(a), within the experimental temperature range, no other vanadium-containing phases were detected by XRD analysis except for V₂O₃ and VN. After 800°C, no diffraction peaks corresponding to V₂O₃ were observed, indicating that the predominant reaction was the direct nitridation of ammonia gas. Higher temperatures promoted a more complete reaction, which is consistent with the SEM-EDS findings. The XRD patterns of the products obtained at different ammonia flow rates have been illustrated in Figure 7(b). As shown in Figure 7(b), within the experimental ammonia flow rate range, no other vanadium-containing phases were detected by XRD analysis except for V₂O₃ and VN. No diffraction peaks corresponding to V₂O₃ were observed when the flow rate exceeded 70 mL min^-1. The XRD patterns of the products obtained at different roasting times have been displayed in Figure 7(c). As indicated in Figure 7(c), within the experimental roasting time range, no other vanadium-containing phases were detected by XRD analysis except for V₂O₃ and VN. No diffraction peaks corresponding to V₂O₃ were observed when the roasting time exceeded 4 h. Within the experimental roasting time range, no other vanadium-containing phases were detected, suggesting that the reaction mechanism involved the direct nitridation of V₂O₃ without the formation of additional vanadium-containing phases.

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Figure 7.

XRD analysis of vanadium-nitrogen alloy products obtained under (a) different roasting temperatures (b) ammonia flow rates and (c) roasting times..

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It can also be known from Figure 7 that in the process of preparing VN by using V₂O₃ as the precursor and NH₃ as the nitrogen source through the reduction nitridation reaction, the reaction temperature has a significant regulatory effect on the phase composition and crystal structure of the product. The XRD refinement results at each temperature have been shown in Table 5. As shown in Table 5, the Rwp values in the experiment fluctuated within the range of 2.357 to 2.501, all far below the 5% threshold for high-quality refinement [39].

During the temperature ramp from 650°C to 850°C, the phase composition of the products exhibits a clear and systematic evolution. At 650°C, V₂O₃ dominates the product phase with a content of 88.6 wt.%, while VN accounts for only 11.4 wt.%, indicating limited NH₃ decomposition and sluggish nitrogen diffusion under these conditions. As a result, the nitridation reaction proceeds minimally, and the precursor V₂O₃ remains largely untransformed. Upon increasing the temperature to the 700–750°C range, a pronounced shift occurs: the V₂O₃ content sharply decreases from 44.5 wt.% to 27.4 wt.%, while VN increases correspondingly from 55.5 wt.% to 72.6 wt.%. This interval represents the onset of rapid nitridation, driven by enhanced thermal energy that promotes NH₃ decomposition into reactive nitrogen species and facilitates N atom diffusion into the V₂O₃ lattice, thereby accelerating the O–N anion exchange process. At 800°C, residual V₂O₃ is reduced to merely 6.4 wt.%, with VN reaching 93.6 wt.%, signifying near-complete conversion. Finally, at 850°C, V₂O₃ is no longer detectable, and VN constitutes 100 wt.% of the product, establishing 850°C as the threshold temperature for full phase transformation in this system.

The evolution of the lattice parameter a of VN with temperature provides direct insight into the structural maturation of the crystalline product. VN crystallizes in the face-centered cubic (fcc) structure, with a reference lattice parameter of 4.1398 Å. However, at 650°C, the refined lattice parameter is only 4.08357 Å, significantly contracted due to a high concentration of nitrogen vacancies resulting from incomplete nitridation. With increasing temperature, nitrogen incorporation improves, leading to progressive filling of vacancy sites and a concomitant expansion of the lattice: a reaches 4.11572 Å at 700°C, 4.12746 Å at 750°C, 4.13061 Å at 800°C, and 4.13391 Å at 850°C, a final deviation of just 0.14% from the standard value. Concurrently, the unit cell volume increases from 68.096 ų to 70.645 ų. This continuous trend demonstrates a marked enhancement in lattice integrity and stoichiometric fidelity toward the ideal VN (1:1) composition, reflecting improved crystallinity and structural stability at higher synthesis temperatures. The minor residual discrepancy in lattice parameter is likely attributable to residual microstrain or trace-level impurity doping within the crystal lattice, common features in solid-state reaction systems, and consistent with typical experimental observations.

3.4.3. FTIR analysis

FTIR analysis is a physical method that analyzes and identifies the structure of substances by their characteristic absorption of infrared radiation. It is widely used in fields such as chemistry, materials science, and environmental monitoring [40-43]. To investigate the direct reduction mechanism of vanadium trioxide under ammonia intervention, infrared spectroscopy analysis was performed on calcined samples at various temperatures, and the results are presented in Figure 8. As shown in the FTIR spectra, the absorption peak at 531 cm^-1 corresponds to the V-O-V bond, the peak at 989 cm^-1 is characteristic of the V-O bond, the peak at 989 cm^-1 is attributed to the V=O bond, the peak at 3421 cm^-1 originates from the hydroxyl bond of adsorbed water molecules, and the peak at 1636 cm^-1 arises from the H-O-H bond [44-46]. From the FTIR spectra of VN in samples calcined at different temperatures, it can be observed that characteristic absorption peaks of V-N bonds emerge at 1050 cm^-1 and 1090 cm^-1. However, in samples calcined at 650°C to 750°C, the characteristic absorption peaks of V-N bonds are not prominent. As the roasting temperature increases, the characteristic absorption peaks of V-N bonds gradually become more distinct, indicating that the transformation mechanism during the roasting process under ammonia involves the conversion of V-O-V and V=O bonds into V-N bonds, which corresponds to the substitution of O by N from NH₃. It is noteworthy that the V–N characteristic vibrational peak of the VN synthesized in this study differs significantly from that of pure-phase VN reported in the literature. This deviation arises because the as-synthesized VN is an oxynitride solid solution containing 10.54 wt.% oxygen. Due to the higher electronegativity of oxygen compared to nitrogen, the incorporation of O atoms induces a redistribution of electron density within the V–N bonds, resulting in an increased bond force constant. This strengthening of the bonding interaction leads to a blue shift in the vibrational frequency, shifting the characteristic peak to a higher wavenumber-consistent with established trends in doped and non-stoichiometric VNs.

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Figure 8.

FTIR analysis of raw materials and vanadium-nitrogen alloy products obtained under different roasting temperatures.

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3.4.4. XPS analysis

XPS is primarily employed to determine the elemental composition and chemical states present on the surface of a material [47]. Figure 9 shows the wide-scan XPS spectra of V₂O₃ and its calcination products obtained at different temperatures. The results confirm that V, N, and O are the primary constituents of all calcination products. Notably, the peak intensity corresponding to nitrogen gradually increases with increasing temperature, indicating enhanced nitrogen incorporation into the material. To further elucidate the transformation mechanism of VN in an ammonia medium, XPS analysis was performed on N and V elements at various roasting temperatures, with the results presented in Figure 10. Figure 10(a) represents the high-resolution spectrum of N [48]. Based on the fitting results of Figure 10(a), it is evident that as the roasting temperature increases, the proportion of oxidized N in the calcined samples progressively decreases. At elevated temperatures, the predominant form of N in the vanadium-nitrogen alloy product exists in a low-valence state. Figure 10(b) illustrates the high-resolution XPS spectrum of V at different roasting temperatures. During the V2p spectral deconvolution, rigorous constraints were applied to ensure analytical accuracy: the spin-orbit splitting between V2p₃/₂ and V2p₁/₂ was fixed at 7.2 eV, with a fixed area ratio of 2:1. The binding energy positions for each vanadium oxidation state (V⁵⁺, V⁴⁺, V³⁺) were referenced to established literature values and constrained within well-defined ranges. Furthermore, the full width at half maximum (FWHM) of all component peaks was constrained to a consistent range (1.0–1.8 eV) to maintain physical plausibility and avoid overfitting [49]. From Figure 10(b), after spectral fitting and peak separation, it can be observed that V primarily exists in three distinct chemical environments in the samples after cooling at different temperatures: V⁵⁺, V⁴⁺, and V³⁺. According to the literature [50-52], these correspond to V-O, V-O(N), and V-N, respectively. Further area integration of the XPS analysis for the three forms of V-2p was conducted to determine the proportions of vanadium in these three chemical environments, with the results shown in Figure 11.

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Figure 9.

Wide scan XPS spectra analysis of raw materials and vanadium-nitrogen alloy products under different temperatures.

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Figure 10.

The XPS analysis of the (a) N element and (b) V element in the roasting samples under different temperatures.

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Figure 11.

Quantitative calculation and fitting analysis of vanadium elements in different valence states.

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As depicted in Figure 11, with increasing roasting temperature, the proportion of V-N in the calcined material gradually increases, while the proportion of V-O progressively decreases. The proportion of V-O(N) exhibits a trend of initially increasing and subsequently decreasing. This suggests that the substitution of O by N occurs gradually. Initially, V-O(N) forms, and as the roasting temperature rises, the substitution ratio of N gradually increases, leading to the conversion of the V-O(N) bond into V-N. Consequently, the proportion of V-O(N) demonstrates a trend of first increasing and then decreasing.

The direct nitridation of V₂O₃ to VN in an ammonia atmosphere follows a well-defined stepwise solid-state diffusion mechanism, with the structural and chemical evolution pathways comprehensively validated through the synergistic integration of XRD, XPS, SEM-EDS, and FTIR characterizations. Thermodynamically, ammonia (NH₃) decomposes into reactive nitrogen species above 184°C, and the direct nitridation reaction (V₂O₃ + 2NH₃ → 2VN + 3H₂O) becomes thermodynamically spontaneous at ≥710°C, initiating the phase transformation. The process proceeds via progressive substitution of oxygen (O) atoms in V₂O₃ by nitrogen (N) from NH₃. FTIR analysis reveals the gradual weakening of V–O–V (531 cm⁻¹) and V=O (989 cm⁻¹) vibrational modes, accompanied by the emergence and intensification of characteristic V–N stretching peaks at 1050 cm⁻¹ and 1090 cm⁻¹. XPS further confirms the formation of a V–O(N) intermediate state corresponding to V⁴⁺: its relative fraction initially increases during partial O/N exchange due to lattice distortion, then decreases as nitrogen incorporation completes and the system transitions fully to V–N bonding (V³⁺), reflecting a continuous reduction in vanadium oxidation state from V⁵⁺ (in V–O) to V³⁺ (in V–N). Concurrently, SEM-EDS mapping shows that the macroscopic block-like morphology of V₂O₃ remains preserved, attributable to its high melting point (∼2000°C), while nitrogen gradually diffuses from the particle surface toward the core, with increasing spatial colocalization between N and V signals. A schematic diagram of the mechanism in this study has been shown in Figure 12. XRD refinement quantitatively verifies the continuous phase evolution: V₂O₃ content decreases from 88.6 wt.% at 650°C to complete disappearance by 850°C, while VN increases to 93.6 wt.% at 800°C and reaches full phase purity (100 wt.%) at 850°C. Moreover, the VN lattice parameter expands from 4.08357 Å to 4.13391 Å, approaching the standard value of 4.1398 Å, indicating progressive filling of nitrogen vacancies and enhanced crystallinity. Collectively, these results establish that the nitridation is a surface-initiated, inward-propagating solid-state diffusion process: the chemical transformation advances stepwise from the exterior to the interior through sequential O–N substitution and transient intermediate formation, while the macroscopic particle architecture remains structurally intact, precluding liquid-phase formation or disruptive morphological reconstruction.

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Figure 12.

Schematic diagram of the mechanism in this study.

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