# Advancements in nitridation of TiO2 layers: Mechanisms, techniques, and applications for TiN thin films

Victor Vallejo-Otero, Nicolas Crespo-Monteiro, Emilie Gamet, Nadège Ollier, Christophe Donnet, Arnaud Valour, Yves Jourlin

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Victor Vallejo-Otero, Nicolas Crespo-Monteiro, Emilie Gamet, Nadège Ollier, Christophe Donnet, et al. Advancements in nitridation of TiO2 layers: Mechanisms, techniques, and applications for TiN thin films. Journal of the European Ceramic Society, 2025, 45, 10.1016/j.jeurceramsoc.2025.117330. hal-04984130

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Review article

# Advancements in nitridation of TiO₂ layers: Mechanisms, techniques, and applications for TiN thin films

Victor Vallejo-Otero*, Nicolas Crespo-Monteiro, Emilie Gamet, Nadège Ollier, Christophe Donnet, Arnaud Valour, Yves Jourlin

Université Jean Monnet Saint-Étienne, CNRS, Institut D'Optique Graduate School, Laboratoire Hubert Curien UMR5516, Saint-Étienne F-42023, France

<figure><img src="image_2.png" /></figure>

ARTICLE INFO

Keywords:

Tin

TiO2

Nitriding

Thin films

A B S T R A C T

Thin films of titanium nitride (TiN) are extensively utilized across various industrial sectors due to their exceptional mechanical and tribological properties, as well as their high chemical and thermal stability. Additionally, their plasmonic behavior makes them highly promising for optical metasurfaces and innovative plasmonic materials mainly in the visible (around 600–780 nm) and in the near-infrared (NIR) range wavelength according to the permittivity values giving rise to metallic behavior in the considered wavelength range. The layers of TiN can be produced by using reactive deposition technique such as chemical vapor deposition or physical vapor deposition but the process of thermal nitridation by gas is also largely used, in particular to coat wide surfaces. Usually, this process involves heating titanium or titanium alloys at high temperature in a nitrogen atmosphere to form TiN. However, the process of forming TiN by nitriding titanium oxide (TiO₂) is a promising alternative that offers many advantages (the possibility to use sol-gel and a control over the nitrogen incorporation) but is not yet well understood and documented. In this review, the nitriding mechanisms of TiO₂ by ammonolysis and by carbothermal nitriding will be presented and compare. Different nitridation processes will then be described and the electrical, mechanical, optical and plasmonic properties of the nitride TiN films will be compared to the other deposition techniques. The applications and the perspectives of this type of nitridation such as formation of TiN metasurfaces or sustainable process of elaboration of TiN will be presented as well.

1. Introduction

Nitridation is a surface treatment created in the early 1910s by Adolph Machlet and Adolph Fry [1]. At the time the nitridation was primarily performed on steel in order to improve both the mechanical properties (hardness and toughness) [2] and the chemical resistance (corrosion resistance) [3]. The principle of this technique was to heat up the steel at high temperature so that the ammonia gas injected into the oven would decompose. The nitrogen produced by this decomposition was incorporated in the steel surface by diffusion. Nowadays, this process has expended by using more recent technologies such as plasma nitriding [4-7] or laser nitriding [8-11]. According to the book of Mittemeijer et al. [12] the nitriding process involves the introduction of nitrogen into the surface of a component. Usually this introduction and diffusion is helped by an elevated temperature but not necessarily [13-15]. It is important to understand the difference between nitridation technique and other deposition technique allowing the formation of

nitrided films such as PVD (physical vapor deposition) [16-19] and CVD (chemical vapor deposition) [20-24]. For these techniques, the association between the nitrogen and the component to be nitrided occurs at a gaseous phase, the nitrides formed are then deposited at the surface of a substrate.

Nowadays a wide variety of metals and alloys are nitrided [25-31]. The motivation behind these surface treatments is still mostly for the mechanical improvement and the chemical resistance [32-36]. However, new applications of this nitridation process have been developed. For example we can mention production of biocompatible surfaces [37, 38], semiconductor [39,40] and optical devices [41,42]. Among all the metals and alloys nitrided, the titanium (Ti) is one of the material subjected to the most studies regarding its nitridation [8,31,43-47]. The interest mainly come from the properties that this material gains after its nitridation into titanium nitride (TiN) such as its low resistivity, high reflectance in the infrared spectral range, good corrosion resistance, good chemical inertness, good thermal stability (melting temperature

<footnote>* Corresponding author.
E-mail address: victor.vallejo.otero@univ-st-etienne.fr (V. Vallejo-Otero).</footnote>
2930 °C), and high hardness (~30 GPa) [48–51]. Furthermore the titanium can be nitrided in a wide variety of ways from the laser nitriding [8,52,53], to plasma nitriding [6,7,54], to solid-solid diffusion [55] and even automotive waste firing [56]. Another aspect that makes the study of the nitridation of titanium very fruitful is that the reaction mechanisms and the properties obtained, largely differ as a function of the titanium source [57–60]. The presence of hydrogen during the nitriding of pure titanium makes the treated surface brittle [61]. In a titanium alloy the elements present will affect its nitridation and mechanisms of internal diffusion and segregation will occur [32,45,62]. With titanium oxide (TiO₂), a mechanism of reduction has to be associated to the nitridation in order to form TiN [63,64]. The nitriding of TiO₂ instead of Ti (pure or alloys) offers possibilities to tune properties as a function of the nitridation parameters. For example, through nitridation, TiO₂ can be doped in nitrogen leading to an improvement of its photocatalytic properties [65–67]. By further nitriding titanium oxide, the crystalline structure will be modified to form titanium oxynitride (TiOₓNᵧ) which has photonic properties [68,69] and widely used for its photo-electrochemical [70], optical [71] and bioactive [72] properties. Finally, with a suitable process, TiN can be obtained by nitriding [42, 73–75].

TiO₂ can be produced by oxidation of titanium or titanium alloys [76,77], by deposition vacuum based processes such as ALD (Atomic Layer Deposition) [78], PVD [79] or CVD [80] or by using the sol-gel route [81–83]. It permits the elaboration of films and powders with an advanced control over the size of grains/particles, the thickness and morphology [84–90]. It also provide an easy way to micro-structure the deposited TiO₂ layers using optical lithography techniques like nano-imprint [91–93] or inkjet printing [94–96] for example. Studies have already benefited from these properties at their advantage to structure TiN layers [27,42,97,98] or to produce TiN with specific morphology [99,100]. The main focus of this article concerns the nitridation of TiO₂ to form TiN. The formation of TiN from TiO₂ has a wide range of applications in various fields (mechanic, electronic, optic, medical ...) such as wear-resistant coatings [101], diffusion barriers [102], plasmonic metasurfaces [103] or biocompatible surfaces [104]. The main advantage of the use of the nitridation of TiO₂ to form TiN films is the possibility to control the morphology of TiN through this process. Indeed, using different structuration processes it is possible to easily structure TiO₂ in different morphology such as micro-nanostructured thin films, nanofibers or nanowires for example [75,105,106]. Such process also allows the reduction of the process flow and large and cost-efficient structuration via nano-imprint [75,105, 106]. Once these structures are made, the TiO₂ can be nitridated to obtain the same structures in TiN. The possibility to control the morphology of TiN have interest for plasmonic, microelectronic applications. In their paper Crespo-Monteiro et al. [98] present a process to obtain TiN grating through the structuration of TiO₂ xerogel and its nitridation. This paper authors demonstrate that such microstructured layers could be used as plasmonic devices such as plasmonic biosensors or gas sensors. Moon et al. [43] have also developed monolayer films of nitridated TiO₂ hollow shells which can be exploited as a supercapacitor electrode. On another note, the doping in nitrogen of TiO₂ films through the same kind of nitridation process is also widely developed and have applications through its photocatalytic properties [67,107,108]. The goal of this study is to present the current state of the art of TiO₂ nitridation to form TiN films. The authors will describe the nitridation mechanism of TiO₂, addressing gas behavior, reduction, and nitridation, along with the key reaction parameters involved in both ammonolysis and carbothermal nitriding. They will then present and compare various thermal nitridation processes of TiO₂, providing detailed descriptions of each. The discussion will also include a comparison of the properties of TiN produced by nitridation and sputtering processes. Finally, the paper will conclude with an analysis of the applications and future perspectives of TiN thin films produced by thermal nitridation.

This work is primary focused on the production of thin films but

some papers and data from the nitridation of TiN powders have also been used to complement it. The results of these studies have therefore been used with caution to suit the scope of this article.

## 1.1. Nitridation mechanism of TiO₂

Since its invention, nitridation has developed considerably, and new methods are now widely used. In order to better understand the range of techniques used, nitridation processes are generally divided into three main categories:

- **The thermal nitridation by gas** is the oldest and most used and known nitridation technique [52,98,109,110]. The principle is to heat up a workpiece in gaseous atmosphere containing nitrogen to allow the introduction and diffusion of nitrogen atoms into the titanium surface. The mechanisms to form TiN from titanium sources (titanium, titanium alloys or titanium oxide) by nitridation have been widely studied, but a lot of them still needs to be understood especially the reduction-nitridation mechanisms to form TiN from TiO₂.

- **The nitridation by molten salted bath (salt bath nitriding).** This process, mostly used in the industry for its low cost and high scalability, involves immersing the work piece into a molten salt bath containing nitrogen-based salts at high temperature (600–700°C). Then, the nitrogen diffuses into the titanium source to form TiN. This technique allows the production of TiN with uniform thickness even on complex shapes and also for very thick layers (several microns). However, the main limitation is the environmental impact of such process. Indeed, the by-products and salts of these reactions are often toxic and must be treated. Some researchers have developed processes that enable this nitration using more environmentally friendly products such as potassium nitrate [111].

- **The nitridation by plasma.** Also known as ion nitriding, glow discharge nitriding or plasma ion nitriding. In this process the reactive gas (nitrogen usually) is ionized by continuous or pulsed direct current [112] to form a nitrogen plasma that will diffuse into the workpiece. To improve the diffusion, the surface can be heated by an external source [113] or simply by the heat generated from the collisions and the kinetic energy of plasma species [114]. Even though this technology is more and more developed, it still presents issues. Indeed, such a treatment can lead to an inhomogeneity and a degradation of the surface (arcing damage, edge effect and hollow cathode effect on the workpiece surface) [115].

It is interesting to note that the reduction-nitridation reaction to form TiN from TiO₂ is largely achieved using thermal nitriding by gas (global heating [98,110,116] or local heating notably by laser [52,117]), to our knowledge, there are just a few studies using other nitriding methods [104,118]. The following part will describe the different mechanisms of this reaction. However, the laser nitriding will not be addressed because the mechanism to form TiN from TiO₂ is not well documented yet. In order to describe the various mechanisms reported in the literature, a synthesis of different articles on the subject has been conducted. Among the various processes, two nitridation-reduction methods stand out. The first is ammonolysis nitridation-reduction, and the second is carbothermal nitridation-reduction. First, the different mechanisms of ammonolysis will be described, then they will be compared to those of carbothermal nitridation-reduction.

### 1.1.1. Gas behavior

1.1.1.1. Ammonia gas. The nitridation of TiO₂ to form TiN requires a reducing agent in order to substitute the oxygen with the nitrogen. Among the various possibilities, one of the most described process is the nitridation of TiO₂ with NH₃ [42,64,97,119]. In this case, the ammonia
gas acts both as a reducing agent and a nitrogen source. This nitridation technique was first developed in 1987 by Kamiya *et al.* [100] to nitride TiO₂ fibers prepared from a TiO₂ sol-gel. It has then been adapted to nitride xerogel TiO₂ films [120] in 1990.

The first step of the nitridation of TiO₂ with NH₃ is the dissociation of the ammonia [43,100,120–123] (cf. Eq. 1). This reaction of dissociation is presented by Cooper *et al.* [124] in conditions detailed next as such:

$$2\text{NH}_3 \rightarrow \text{N}_2 + 3\text{H}_2 \quad (1)$$

However, studies such as that by of Ye *et al.* [125] or Odochian *et al.* [126] indicate that this mechanism of decomposition leads to the creation of a wide range of molecular or radical nitrogen-containing intermediates of ammonia pyrolysis such as NH₂, NH, N, N₂Hₓ. As shown in the Table 1, multiple mechanisms of dissociation of the NH₃ occur, simultaneously forming by-products (other than N₂ and H₂). These species also take part in this reduction nitridation process, promoting the introduction of nitrogen into titania, according to Chen *et al.* [127].

Optimizing ammonia dissociation leads to an improvement of the nitridation process due to the high presence of H₂ and N₂. Multiple parameters have an impact on the decomposition of NH₃. The first is the reaction temperature: the higher the temperature, the faster and more advanced the dissociation (from NH₃ to N₂ and H₂) will occur (cf. Eq. 1). As it can be seen Fig. 1A and B, the rate of formation of N₂ at 800°C nearly reaches 2 times the rate of its formation at 750°C.

Slycke *et al.* [128] conclude that the ideal conditions for NH₃ decomposition are high temperature and low pressure, as shown in Fig. 2A, where dissociation is optimized at a total gas pressure of 0.1 atm and a temperature above 326°C. The second parameter that will impact the dissociation rate will be the induction time corresponding to the duration of reaction at a given temperature. Odochian *et al.* [126] have carried out a comprehensive study analyzing this parameter [126]. As it can be seen in Fig. 2B, the longer the reaction the more the dissociation progresses. However, it can also be seen that after a long induction duration (65–70 s), the concentration of N₂, H₂ and NH₃ seems to stabilize. Thus, by extrapolating the curves depicted in Fig. 2B, it can be suggested that the complete decomposition of NH₃ into N₂ and H₂ required a much longer induction duration.

<table><caption>**Table 1**
The mechanism of thermal decomposition of NH₃ [127].</caption><thead><tr><th>No.</th><th>Reaction</th><th>A, 1 mol<sup>-1</sup>s<sup>-1</sup></th><th>m</th><th>E<sub>a</sub>, kJ/mol</th></tr></thead><tbody><tr><td>1</td><td>NH<sub>3</sub> + M[NH<sub>3</sub>; N<sub>2</sub>; H<sub>2</sub>] = NH<sub>2</sub> + H + M</td><td>2.20 × 10<sup>13</sup></td><td>0</td><td>391.34</td></tr><tr><td>2</td><td>NH<sub>3</sub> + NH = 2NH<sub>2</sub></td><td>3.16 × 10<sup>11</sup></td><td>0</td><td>112.08</td></tr><tr><td>3</td><td>NH<sub>2</sub> + NH = NH<sub>3</sub> + N</td><td>1.00 × 10<sup>10</sup></td><td>0</td><td>8.37</td></tr><tr><td>4</td><td>NH<sub>3</sub> + NH<sub>2</sub> = N<sub>2</sub>H<sub>3</sub> + H<sub>2</sub></td><td>1.00 × 10<sup>8</sup></td><td>0.5</td><td>90.43</td></tr><tr><td>5</td><td>NH + H<sub>2</sub> = NH<sub>2</sub> + H</td><td>1.00 × 10<sup>11</sup></td><td>0</td><td>84.03</td></tr><tr><td>6</td><td>NH + H = N + H<sub>2</sub></td><td>3.20 × 10<sup>10</sup></td><td>0</td><td>1.36</td></tr><tr><td>7</td><td>N<sub>2</sub>H<sub>3</sub> + M[N<sub>2</sub>; NH<sub>3</sub>; N<sub>2</sub>H<sub>4</sub>] = NH<sub>2</sub> + NH + M</td><td>5.00 × 10<sup>13</sup></td><td>0</td><td>251.21</td></tr><tr><td>8</td><td>N<sub>2</sub>H<sub>3</sub> + M[N<sub>2</sub>; NH<sub>3</sub>; N<sub>2</sub>H<sub>4</sub>] = N<sub>2</sub>H<sub>2</sub> + H + M</td><td>1.00 × 10<sup>14</sup></td><td>0</td><td>138.16</td></tr><tr><td>9</td><td>N<sub>2</sub>H<sub>2</sub> + M[N<sub>2</sub>; H<sub>2</sub>] = NNH + H + M</td><td>5.00 × 10<sup>13</sup></td><td>0</td><td>209.34</td></tr><tr><td>10</td><td>NNH + M[N<sub>2</sub>; H<sub>2</sub>] = N<sub>2</sub> + H + M</td><td>1.00 × 10<sup>10</sup></td><td>0.5</td><td>12.81</td></tr><tr><td>11</td><td>N<sub>2</sub>H<sub>2</sub> + NH<sub>2</sub> = NH<sub>3</sub> + NNH</td><td>8.80 × 10<sup>-5</sup></td><td>4.05</td><td>-6.75</td></tr><tr><td>12</td><td>N<sub>2</sub>H<sub>2</sub> + H = NNH + H<sub>2</sub></td><td>85</td><td>2.63</td><td>-0.963</td></tr><tr><td>13</td><td>2NH<sub>2</sub> + act. site(s) = N<sub>2</sub>H<sub>4</sub> + act. site(s)</td><td>k = 4.23 × 10<sup>13</sup></td><td>1 mol<sup>-1</sup>s<sup>-1</sup></td><td></td></tr><tr><td>14</td><td>N<sub>2</sub>H<sub>4</sub> + M[N<sub>2</sub>; NH<sub>3</sub>; N<sub>2</sub>H<sub>4</sub>] = 2NH<sub>2</sub> + M</td><td>5.00 × 10<sup>11</sup></td><td>0</td><td>251.21</td></tr><tr><td>15</td><td>N<sub>2</sub>H<sub>4</sub> + M[N<sub>2</sub>; NH<sub>3</sub>; N<sub>2</sub>H<sub>4</sub>] = N<sub>2</sub>H<sub>3</sub> + H + M</td><td>1.00 × 10<sup>12</sup></td><td>0</td><td>266.28</td></tr><tr><td>16</td><td>N<sub>2</sub>H<sub>4</sub> + NH = NH<sub>2</sub> + N<sub>2</sub>H<sub>3</sub></td><td>1.00 × 10<sup>6</sup></td><td>1.5</td><td>8.37</td></tr><tr><td>17</td><td>N<sub>2</sub>H<sub>4</sub> + H = N<sub>2</sub>H<sub>3</sub> + H<sub>2</sub></td><td>7.00 × 10<sup>9</sup></td><td>0</td><td>10.47</td></tr><tr><td>18</td><td>N<sub>2</sub>H<sub>4</sub> + N = N<sub>2</sub>H<sub>3</sub> + NH</td><td>1.00 × 10<sup>7</sup></td><td>1</td><td>8.37</td></tr><tr><td>19</td><td>N<sub>2</sub>H<sub>4</sub> + NH<sub>2</sub> = N<sub>2</sub>H<sub>3</sub> + NH<sub>3</sub></td><td>1.80 × 10<sup>3</sup></td><td>1.71</td><td>-5.78</td></tr><tr><td>20</td><td>NH<sub>3</sub> + H = NH<sub>2</sub> + H<sub>2</sub></td><td>5.42 × 10<sup>2</sup></td><td>2.4</td><td>41.53</td></tr><tr><td>21</td><td>2 H + act. site(s) = H<sub>2</sub> + act. site(s)</td><td>k = 3.51 × 10<sup>14</sup></td><td>1 mol<sup>-1</sup>s<sup>-1</sup></td><td></td></tr></tbody></table>

By observing the data gathered by Ristig *et al.* [129] on the decomposition of NH₃ (Table 2), it can be seen that each studies have very different reaction conditions (variation of temperature, catalyst material used and reaction time mainly). It is particularly noted that the conditions with the highest conversion rates (100 % or close to 100 %) exhibit the longest reaction times. These data confirm that to achieve complete decomposition of NH₃ into N₂ and H₂, long reaction time are required (45 h minimum according to Table 2). Furthermore, the type of catalyst material has a significant impact on the reaction. Ristig *et al.* [129] stated that a reliable comparison of ammonia decomposition performance from the multitude of available literature studies or comprehensive reviews is complicated because of significant discrepancies in the experimental procedures. Indeed, the level of detail in the description of these procedures as well as disparities in the analysis and presentation of the results makes difficult the accurate comparison of each process. However, it is clear from this study that to achieve a complete reaction, very specific conditions need to be gathered (long reaction time and use of appropriate catalyst for this dissociation like the Cobalt and the ruthenium [130]). These conditions are never fulfilled for an ammonia gas nitridation, therefore during any of these nitridation processes, NH₃ is not entirely decomposed. The reaction gases are a mixture of N₂, H₂, undecomposed NH₃ and other gases such as NH, NH₂, N₂H₃ ...

In the context of plasma nitriding N₂-N₂ gas mixtures are mainly used for the introduction of nitrogen into TiO₂ matrix [104,118,130]. During this process, H₂ will be ionized to form very reactive hydrogen, ideal for the reduction of TiO₂. This process is used for two main applications, the first one is the doping of TiO₂ with N atoms for catalytic application mainly [130–133] and the second is to produce TiN from TiO₂ by plasma nitridation [104,118,130]. However, in the context of thermal nitridation using N₂-H₂, mixed gas is much less efficient if not useless [58,130, 134]. Liu *et al.* [58] explained that, unlike standard H₂ and N₂, the hydrogen and nitrogen produced by NH₃ decomposition are more reactive, leading to a more efficient production of TiN from TiO₂. This is supported by Saillard *et al.* [134], who state that thermal nitridation cannot be achieved with an N₂-H₂ gas mixture. According to their findings, the nitridation using N₂-H₂ only occurs in the presence of plasma, since the reducing properties of H₂ are attributed to active hydrogen atoms rather than H₂ molecules. Furthermore, nascent nitrogen formed by the dissociation of NH₃ is known to be highly reactive and to promote the diffusion into surfaces [135–137]. These matters explain why thermal nitridation with H₂-N₂ are much less efficient than with NH₃. Using other reductant such as carbon-based gas can also improve the reactivity.

### 1.1.1.2. Carbon-based gas.
In the case of the carbothermal nitridation, a carbon source is used as the reducing agent while a nitrogen gas (mainly N₂ gas) is used as nitrogen source [56,63,75,109,110,116,138,139]. In the literature, different carbon sources are used to reduce TiO₂. The use of solid carbon to reduce powdered TiO₂ is the most widely documented technique [63,109,116,138,140–143]. Three different gases are used for this reaction: C₂H₂ [144–147], C₂H₄ [148] and CH₄ [57,109,149–151].

As demonstrated in numerous studies, the final products of carbon-based gas decomposition, whatever the gas used, are carbon and hydrogen [152–154]:

$$\mathrm{CH_4} \rightarrow \mathrm{C} + 2\mathrm{H_2} \quad (2)$$

$$\mathrm{C_2H_2} \rightarrow 2\mathrm{C} + \mathrm{H_2} \quad (3)$$

$$\mathrm{C_2H_4} \rightarrow 2\mathrm{C} + 2\mathrm{H_2} \quad (4)$$

One challenging aspect of this gas decomposition is the state of the carbon. Indeed, unlike for the ammonia dissociation, a solid will be formed: carbon. As explained in the paper by Cetinkaya *et al.*, the carbon produced by the pyrolysis of carbon-based gas, here CH₄, coats TiO₂. In the case of TiO₂ films, a thin layer of carbon is deposited on TiO₂ [144].
<figure><img src="image_3.png" /><figcaption>Fig. 1. Rate of N₂ (A) and H₂ (B) formation versus contact time at 700, 750 and 800°C [64].</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. (A) Fractional degree of (equilibrium) dissociation for gaseous ammonia, q, as a function of the temperature at three different levels for the total gas pressure [128]. (B) Kinetic curves of NH₃ consumption and hydrogen and nitrogen formation at 800°C [126].</figcaption></figure>

<table><caption>**Table 2**
Decomposition conditions of NH3 of different studies [129].</caption><thead><tr><th>Catalyst material</th><th>Temperature [K]</th><th>GHSV [Lgcat<sup>-1</sup>h<sup>-1</sup>]</th><th>Flow Rate [Lh<sup>-1</sup>]</th><th>Conversion [%]</th><th>Productivity [kgH<sub>2</sub>kgcat<sup>-1</sup>h<sup>-1</sup>]</th><th>Reaction Time [h]</th></tr></thead><tbody><tr><td>Co@SiO<sub>2</sub> [127]</td><td>600</td><td></td><td>15</td><td>71</td><td>12</td><td>4</td></tr><tr><td>Co@La-MgO [128]</td><td>550</td><td>22</td><td></td><td>100</td><td>3</td><td>45</td></tr><tr><td>Co@MWCNTs [129]</td><td>500</td><td>6</td><td></td><td>75</td><td></td><td>20</td></tr><tr><td>Ru@K-CNTs [130]</td><td>500</td><td>150</td><td></td><td>29</td><td>6</td><td>10</td></tr><tr><td>Ru@K-CNTs [96]</td><td>500</td><td></td><td>3</td><td>100</td><td>8</td><td>200</td></tr><tr><td>Ru@MgO-CNTs [95]</td><td>450</td><td>60</td><td></td><td>100</td><td></td><td>200</td></tr><tr><td>Ni<sub>2</sub>Al<sub>2</sub>O<sub>4</sub> [102]</td><td>600</td><td>36</td><td></td><td>93</td><td></td><td>70</td></tr><tr><td>Ni<sub>3</sub>MoN [131]</td><td>525</td><td>6000</td><td></td><td>98</td><td></td><td>30</td></tr></tbody></table>

The study by Hamdani et al. [155] also states that during reaction, depending on the growth of carbon nuclei, a variety of carbon structures can be formed. These different structures ultimately have an impact on the reductive reactivity of the carbon layer [154]. Thus, different parameters will have an impact on the decomposition of the carbon-based gas and on the reactivity of the final deposited carbon, among which, the temperature has a huge impact. As expected, an increase in decomposition temperature leads to an increase in carbon formation and a decrease in gas yield [155,156]. The study by Pérez et al. confirmed that high temperature and low pressure lead to high methane conversion [157] as does NH₃ dissociation. As it can be seen Fig. 3, increasing the temperature above 500°C leads to a significant increase in methane conversion and lower pressure also yield better result. In the same way, as with NH₃, higher gas residence time will lead to higher carbon yield [154,156].

### 1.1.2. Reduction and nitridation

#### 1.1.2.1. Reduction of titanium oxide.
As previously mentioned, that produce titanium nitride by thermal gas nitriding, nitridation of TiO₂ requires the reduction of titanium oxide unlike other titanium sources.

<figure><img src="image_5.png" /><figcaption>Fig. 3. Equilibrium conversion of methane with temperature and pressure [157].</figcaption></figure>
The reasons are multiple, including the oxidation state. First, the Ti present in the non-reduced TiO₂ exhibits an oxidation states of (Ti⁴⁺) compatible with the oxidation state of the oxygen (O²⁻) but much less so with that of nitrogen (N³⁻). Numerous studies show the reduction of oxidation states from (4⁺) to (3⁺) with the reduction of titanium oxide, this reduction is also synonymous of the formation of oxygen vacancies alloying the substitution with nitrogen [66,127,138]. This means that a reduction of TiO₂ seems to be required for the nitriding to occur.

The second reason is the saturation of the TiO₂ matrix. Indeed, without reduction of the TiO₂ matrix, nitrogen does not substitute with oxygen and is limited to a certain atomic percentage. Numerous studies have worked on the doping of titania films with nitrogen to improve the photocatalytic properties [67,107,108]. These studies usually conclude that around 6 at% is the saturation concentration for nitrogen in a titania matrix. Lenarduzzi et al. explain this phenomenon by a decrease in the nitrogen diffusion in the titania matrix: indeed, nitrogen atoms being small, they can move in the matrix through the interstices and settle in these areas, but the more nitrogen atoms there are in the matrix, the more it will be deformed. The slight change in matrix structure makes it more difficult the introduction of nitrogen, thus the diffusion of both oxygen and nitrogen hinders, stabilization of the TiO₂ doped phase as it is. According to Nambu et al. [66], this saturation also indicates that the recombination of nitrogen atoms integrated into TiO₂ and their release as N₂ from TiO₂ occurs once the nitrogen concentration in TiO₂ exceeds a specific threshold (around 6 %). Thus, without reduction of

the TiO₂ matrix, no substitution occurs and nitrogen is introduced only in limited amount. It is now clear that to form TiN from TiO₂, a reduction must occur before any nitriding reaction happens. In the next section the reduction and nitriding mechanism will be described in detail.

1.1.2.2. Ammonolysis. Once NH₃ has been decomposed into active H₂ and N₂ (Eq. 1), these gases diffuse into the coating and react with TiO₂. Liu et al. [58] made a comprehensive and extensive work describing the reaction path observable during the nitridation of TiO₂ with NH₃. Two different cases have been described in their study. The first one occurs at temperature below 1000°C, and TiO₂ reacts with decomposed NH₃ gas directly to form TiNₓO₁₋ₓ (Eq. 5), as described in the study of Gou et al. [158].

$$ \mathrm{TiO_2} + \frac{2-x}{3}N_2 + (2-x)\mathrm{H_2} \rightarrow \mathrm{TiN_{1-x}O_x} + (2-x)\mathrm{H_2O} + \frac{1+x}{6}N_2 \quad (5) $$

Based on the studies analyzing phase transformation during nitridation below 1000°C, two key observations can be proposed. First, this reaction pathway is confirmed by the coexistence of TiO₂ and TiNₓO₁₋ₓ, as noted by Gou et al. and Chen et al. [127]. Indeed, after the reaction at temperature below 1000°C, they have been able to detect both TiO₂ and TiNₓO₁₋ₓ in the final product, meaning that TiO₂ is directly nitrided. Secondly, the phase of TiO₂ present before the transformation into TiN varies with the temperature. Indeed, Chen et al. exhibit the presence of anatase phase before the formation of TiN for nitridation at 750°C, while

<figure><img src="image_6.png" /><figcaption>Fig. 4. Summary of nitridation paths from literature.</figcaption></figure>
Liu et al. and Gou et al. observed a rutile phase at 900°C and 1000°C respectively. Thus, for temperature below around 800°C, the TiO₂ anatase directly transforms into TiN (Fig. 4a). Above 800°C, the TiO₂ first forms rutile before TiN formation (Fig. 4b).

The second reaction pathway presented in the study of Lui et al. [58] concern temperature above 1000°C. In this case, successive reductions occur before the substitution of oxygen with nitrogen. According to Lui et al. [58], a range of substoichiometric titanium oxides are formed by TiO₂ reduction. These substoichiometric titanium oxide phases are called Magnéli phases, defined as TiₙO₁ₙ-₁ (where n is between 4 and 10) [159,160]. There is a consensus regarding the formation of sub-stoichiometric of titanium oxides as intermediate phases during nitridation process above 1000°C as such:

$$TiO_2 \rightarrow Ti_nO_{2n-1} \rightarrow TiN_{1-x}O_x \rightarrow TiN \quad (6)$$

According to Djaoued et al. [122], Liu et al. [58] and other studies [100,120,121], the reduction step from TiO₂ to a substoichiometric titanium oxides which allows substitution of oxygen by nitrogen is not direct and proceeds through successive steps that overlap (cf. Eqs. 7–9). Even though these studies agree regarding the process flow, there is disagreement regarding the last titanium oxide appearing before the apparition of the TiN₁-xOₓ at temperature above 1000°C. Djaoued et al. [122] claimed that TiO₂ is reduced to Ti₂O₃ at 1100°C (Eq. 7 and Fig. 4c). Meanwhile, Liu et al. [58] observed the formation of TiN₁-xOₓ in the presence of Ti₄O₇ and also noted that increasing the nitridation temperature can result in more reduced phases, such as Ti₃O₅ at 1200°C (Eq. 8 and Fig. 4d). On another side Gou et al. [158] claimed that at 1100°C only one reduced state of TiO₂ seems to appear (Ti₉O₁₇) before the formation of the TiN₁-xOₓ phase (Eq. 9 and Fig. 4e).

$$TiO_2 \xrightarrow{1100^{\circ}\text{C}} Ti_9O_{17} \rightarrow Ti_8O_{15} \rightarrow \dots \rightarrow Ti_4O_7 \rightarrow Ti_3O_5 \rightarrow Ti_2O_3 \rightarrow TiN_{1-x}O_x \quad (7)$$

$$TiO_2 \xrightarrow{1100^{\circ}\text{C}} Ti_9O_{17} \rightarrow Ti_8O_{15} \rightarrow \dots \rightarrow Ti_4O_7 \xrightarrow{1200^{\circ}\text{C}} Ti_3O_5 \rightarrow TiN_{1-x}O_x \quad (8)$$

$$TiO_2 \xrightarrow{1100^{\circ}\text{C}} Ti_9O_{17} \rightarrow TiN_{1-x}O_x \quad (9)$$

However, these studies need to be put into perspective: the determination of the stoichiometry in a phase can be a difficult feat. The precise identification of each titanium sub-oxide as presented in the studies seems rather unreliable and need to be discussed. Furthermore, as we have seen previously, the presence of catalyzing species may potentially influence these results, although they are often not considered. Currently researchers seem to agree on the differences below and above 1000°C, but the nitridation reaction path is not yet fully understood (sub-oxide formation during TiO₂ reduction).

The minimum temperature for the appearance of the TiN phase has been studied [74,120,161–163]. Table 3 shows the temperature of the TiN appearance obtained in different studies [74,120,161–163].

Titanium oxynitride formation is said to begin at temperature between 600 and 700°C [74,120,161–163], more specially the study of Romero-Gómez et al. [163] tried to spot the limit temperature to produce TiN as the main crystalline phase. According to them, NH₃ treatment at 650 °C leads to a retention of the anatase phase doped with nitrogen atoms while at 660°C the first TiN crystalline structure is formed. These reaction temperatures are surely impacted by the other parameters such as pressure, heating rate and presence of catalyzer, which explains the differences [164,165]. Fig. 4 is a summary of all possible reactions occurring during the nitriding of TiO₂ with ammonia gas as a function of temperature. It also shows all the possible phase transformations described in the literature.

<table><caption>**Table 3**
Temperature of first appearance of TiN according to the literature.</caption><thead><tr><th>Reference</th><th>[120]</th><th>[74]</th><th>[161]</th><th>[163]</th><th>[162]</th></tr></thead><tbody><tr><td>Temperature</td><td>600°C</td><td>675°C</td><td>700°C</td><td>660°C</td><td>600°C</td></tr></tbody></table>

It is important to note that during the nitriding process, titanium nitride initially forms as an oxynitride TiN₁-xOₓ and get closer to a pure TiN, provided a sufficient reaction time [43,58,122,158,166]. However, the titanium oxynitride remains poorly understood, as it can exist in either crystalline [167,168] or amorphous forms [169,170]. Scarce literature suggests that TiNₓOᵧ generally forms as a complex mixture of TiN and various oxides [171,172] (Fig. 5).

Crystallographic studies frequently identify a face-centered cubic NaCl-type phase, corresponding to a solid solution between TiN and TiO [173–175]. The primary distinction between TiO and TiN lies in their lattice parameters, with TiN having a lattice constant of 4.24 Å and TiO exhibiting a slightly larger value of 4.29 Å. This minimal difference makes it challenging to distinguish between the two structures, as noted by Chappé et al. [175], who propose that the oxynitride film consists of a weakly crystallized face-centered cubic (f.c.c.) phase of TiN₁-xOₓ and an amorphous TiO₂:N phase. Similarly, Mucha et al. [174] describe the titanium oxynitride structure as a slightly deformed cubic phase due to the lattice mismatch between TiO and TiN (Fig. 6). This cubic phase remains stable up to a composition of TiN₀.₆O₀.₄ (20 at% oxygen) before additional oxide crystalline phases, such as a hexagonal phase, emerge [176,177].

Detecting oxygen in titanium nitride layers is challenging due to the limited sensitivity of X-ray diffraction (XRD) [175]. Techniques such as Rutherford Backscattering Spectroscopy (RBS) and X-ray Photoelectron Spectroscopy (XPS) are more effective for detecting oxygen in TiN, as suggested by Vasile et al. [178]. Oxygen incorporation in TiN coatings is inevitable, with measured levels ranging from 2 % to 15 %, depending on deposition methods [178,179]. While some studies consider this oxygen as contamination, others classify it as intrinsic to the material, defining it as TiN rather than TiNₓO₁ₓ [178,180,181]. This inherent oxygen presence is attributed to the strong affinity between titanium and oxygen [178,181,182]. Oxygen from native substrate oxidation can also diffuse into the film during high-temperature processing, further complicating the elimination of residual oxygen. As Guillot [172] notes, the presence of oxygen within TiN layers is unavoidable, regardless of the deposition technique. Currently, no parameter in the literature clearly differentiates between TiNₓO₁ₓ from TiN, as achieving perfectly stoichiometric TiN remains unattainable. However, optimizing reaction temperature and duration can help minimize oxygen content and surface degradation. As it will be discussed in the next section, carbothermal nitriding, while similar to ammonia gas nitriding, presents distinct limitations and advantages.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Ternary phase diagram of Ti-O-N [172].</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. Unit cell crystal structures of (a) TiN, (b) TiN<sub>0.5</sub>O<sub>0.5</sub> with Ti the black dot, N the red dot, O the yellow dot [174].</figcaption></figure>

1.1.2.3. Carbothermal nitridation. Based on the data on nitridation using carbon-based gases and their decomposition, it can be concluded that at temperatures above 1000°C, the primary contributor to the TiO<sub>2</sub> reduction is the solid carbon deposited. Thus, as most studies described it [138,139,143,183], the consecutive reduction using solid carbon and nitridation using N<sub>2</sub> gas can be defined as in Eqs. 10–13.

Similar to the nitridation with NH<sub>3</sub>, the phases to which TiO<sub>2</sub> transforms prior to TiN are difficult to determine. Some studies on the formation of these phases agree that the first suboxide formed is Ti<sub>5</sub>O<sub>9</sub> [139,183]. However, in the study of Woo et al. [143], the first phase that appeared is stated to be Ti<sub>4</sub>O<sub>7</sub>. The main difference that could explain the absence of Ti<sub>5</sub>O<sub>9</sub> is the reaction temperature during reduction. Indeed, in the study of Woo et al. [143] the reaction temperature is 1500°C whereas in the two other studies it is between 1100 and 1200°C. As we have seen previously in the case of nitriding with NH<sub>3</sub>, temperature has a strong impact on the compounds formed. It is also likely that at a temperature as high as 1500°C, the reduction is probably so fast that the Ti<sub>5</sub>O<sub>9</sub> phase is skipped or unperceivable. Another point of contention regarding the phases formed is the last phase appearing before the apparition of TiN. Even though most studies agree that the last suboxide appearing before the formation of the first TiN phase is Ti<sub>3</sub>O<sub>5</sub> [109, 138–140], the discussion about the formation of Ti<sub>2</sub>O<sub>3</sub> is still ongoing. Indeed, some researchers are working on the formation of TiC by carbothermal reduction of TiO<sub>2</sub> [57,149,184]. In these studies, TiO<sub>2</sub> suboxide of as reduced as Ti<sub>2</sub>O<sub>3</sub> is formed before the TiC starts appearing. Thus, as the reduction nitridation process using carbon being very similar, the absence of Ti<sub>2</sub>O<sub>3</sub> is unexpected. This phenomenon has been explained in two different ways by Berger [185] and Rezan et al. [139]. In his study, Berger showed that subjecting Ti<sub>2</sub>O<sub>3</sub> to a nitrogen environment at 1300°C for 60 minutes led to its transformation into Ti<sub>3</sub>O<sub>5</sub> and a "cubic phase" TiO<sub>x</sub>N<sub>y</sub>, even in the absence of carbon. This observation would imply that Ti<sub>2</sub>O<sub>3</sub> exhibits high instability in nitrogen, resulting in its decomposition into Ti<sub>3</sub>O<sub>5</sub> along with the formation of a cubic phase. On the other hand, Rezan et al. [139] claimed that this could be explained by the kinetics of reaction where nitriding would be much faster than the reduction. Indeed, during a carbothermal reduction of TiO<sub>2</sub> without a nitrogen atmosphere, the slowest reaction appears to be the conversion of Ti<sub>2</sub>O<sub>3</sub> to TiO<sub>x</sub>C<sub>y</sub>. However, in the presence of nitrogen the conversion of Ti<sub>2</sub>O<sub>3</sub> to TiO<sub>x</sub>C<sub>y</sub>N<sub>z</sub> is much faster making the conversion of Ti<sub>3</sub>O<sub>5</sub> to Ti<sub>2</sub>O<sub>3</sub> the slowest. Thus, before forming TiO<sub>x</sub>C<sub>y</sub>N<sub>z</sub>, the Ti<sub>3</sub>O<sub>5</sub> oxide is much more persistent than Ti<sub>2</sub>O<sub>3</sub> making the latter difficult to detect through characterization. Once again, it seems difficult to be certain that these phases can be accurately detected.

$$5\text{TiO}_2 + C \rightarrow \text{Ti}_5\text{O}_9 + \text{CO}_{\text{(g)}} \quad (10)$$

$$4\text{Ti}_5\text{O}_9 + C \rightarrow 5\text{Ti}_4\text{O}_7 + \text{CO}_{\text{(g)}} \quad (11)$$

$$3\text{Ti}_4\text{O}_7 + C \rightarrow 4\text{Ti}_3\text{O}_5 + \text{CO}_{\text{(g)}} \quad (12)$$

$$2\text{Ti}_3\text{O}_5 + 10\text{C} + 3\text{N}_{2(\text{g})} \rightarrow 6\text{TiN} + 10\text{CO}_{\text{(g)}} \quad (13)$$

Although Eqs. 10–13 are a good description of the carbothermal nitriding reaction, by-products created during the process have an impact on it. As shown from the previous reactions (cf. Eqs. 2–4;10–13), two by-products are formed through this process: H<sub>2</sub> and CO gas. Firstly, studies have shown that the H<sub>2</sub> produced during the decomposition of carbon-based gas acts as a reducing agent along carbon solid deposited [138,149,151]. Zhang et al. [149] described the consecutive reductions that occur (cf. Eqs. 14–17).

$$5\text{TiO}_2 + \text{H}_2 \rightarrow \text{Ti}_5\text{O}_9 + \text{H}_2\text{O} \quad (14)$$

$$4\text{Ti}_5\text{O}_9 + \text{H}_2 \rightarrow 5\text{Ti}_4\text{O}_7 + \text{H}_2\text{O} \quad (15)$$

$$3\text{Ti}_4\text{O}_7 + \text{H}_2 \rightarrow 4\text{Ti}_3\text{O}_5 + \text{H}_2\text{O} \quad (16)$$

$$2\text{Ti}_3\text{O}_5 + \text{H}_2 \rightarrow 3\text{Ti}_2\text{O}_3 + \text{H}_2\text{O} \quad (17)$$

However, the nitridation process using H<sub>2</sub> instead of solid carbon is known to be thermodynamically much less efficient, as explained by Li et Riley [138]. So, if the partial pressure in H<sub>2</sub> of the reaction is too high, it is likely that the reaction will be slowed.

The contribution of CO in the reduction mechanism has been discussed in multiple studies [63,109,140,142,183,186]. Thanks to these studies, a description on how carbon monoxide interacts with titanium oxide and carbon can be presented. After its formation, the carbon monoxide will act as a reducer of titanium oxide and contributes to the formation of titanium nitride phases [63,183,187] as it can be seen in Eqs. 18–21.

$$5\text{TiO}_2 + \text{CO}_{\text{(g)}} \rightarrow \text{Ti}_5\text{O}_9 + \text{CO}_{2(\text{g})} \quad (18)$$

$$4\text{Ti}_5\text{O}_9 + \text{CO}_{\text{(g)}} \rightarrow 5\text{Ti}_4\text{O}_7 + \text{CO}_{2(\text{g})} \quad (19)$$

$$3\text{Ti}_4\text{O}_7 + \text{CO}_{\text{(g)}} \rightarrow 4\text{Ti}_3\text{O}_5 + \text{CO}_{2(\text{g})} \quad (20)$$

$$2\text{Ti}_3\text{O}_5 + 10\text{CO}_{\text{(g)}} + 3\text{N}_{2(\text{g})} \rightarrow 6\text{TiN} + 10\text{CO}_{2(\text{g})} \quad (21)$$

The Magneli phases written above are those suspected to appear in this case scenario. It is still very difficult to have a clear idea regarding this matter. However, these reactions still need to be further analyzed to have a better understanding of their mechanism.

As can be seen, once CO reacts with titanium oxide, carbon dioxide is formed (cf. Eqs. 18–21). This new gas is reduced by the carbon present to reform CO through Boudouard's reaction [142,186,187]:

$$\text{CO}_{2(\text{g})} + C \rightarrow 2\text{CO}_{\text{(g)}} \quad (22)$$

Once reduced, CO reacts again with titanium oxide and favors the reduction (Eq. 22). Even though this mechanism seems to have a positive impact on the nitridation process, a high partial pressure of CO will tend to consume a lot of solid carbon, the main reducing agent. As previously stated, the carbothermal reduction of titanium oxide is strongly affected by the gaseous atmosphere. Chai et al. [142] have experimentally verified that CO slowed down the rate of carbothermal reduction of TiO<sub>2</sub> into Ti<sub>3</sub>O<sub>5</sub>. This observation is confirmed by the study of Maitre et al. [186], in which they show the negative effect of confinement. Indeed, in an open reactor, carbon monoxide can easily be carried away while the partial pressure of CO tends to be high in a confined reactor. The latter configuration leads to a decrease of reaction rate. Fig. 7 summarizes the carbothermal nitriding mechanism of described here. Another harmful mechanism that can occur during this process is the deposition of the carbon layer. We have seen previously that the decomposition of carbon-based gas leads to the formation of a carbon layer and that this carbon layer usually is the main source of carbon for reduction [148–150]. However, some studies [149,151] have shown that this mechanism is positive for the reduction process to a certain extent. During the reduction using carbon-based gas,
<figure><img src="image_9.png" /><figcaption>Fig. 7. Illustration of the carbothermal nitridation mechanisms.</figcaption></figure>

decomposition of the gas and reduction using the solid carbon occur at the same time. If the reduction rate is faster than the decomposition rate, the quantity of carbon will lack to pursue the reduction slowing it down. But if the decomposition rate is faster than the reduction rate, the thickness of the carbon layer will increase, blocking the surface. Thus, both the outward carbon monoxide and the inward nitrogen will be blocked, hindering the reaction. According to Zhang et al. [151], the latter mechanism occurs at temperature above 1350°C. As it can be seen in Table 4 most of the carbothermal nitridation processes do not exceed a reaction temperature of 1200°C, which is consistent to all the harmful mechanism we have seen above.

It is also interesting to note that the formation of TiN from a TiO₂ sol-gel is common [27,42,100,110,120,122]. However, the impact on the reduction of the carbonaceous species present in these films is rarely discussed, whereas it probably has consequences during the firing process [122]. As the reaction mechanisms have been described in more detail, the parameters affecting these nitriding reactions will be described in the following part.

### 1.1.3. Reaction parameters

#### 1.1.3.1. Parameters affecting the ammonolysis nitridation.
In the literature, authors have studied the thermal nitridation of TiO₂ using ammonia with different reaction parameters. Table 4 presents the different parameters of nitridation of TiO₂ found in the literature. As presented before, the most studied parameter is the reaction temperature [74,120,122,161,188]. The higher the temperature, the further and the faster the reaction.

The two main reasons are better decomposition of the NH₃ as stated before, and the improvement of diffusion at higher temperatures. The duration of reaction is also an important parameter that mainly controls the stoichiometry of the titanium oxynitride produced. Studies by Jiménez et al. [74], Liu et al. [58] and Dinh et al. [64] demonstrate that the nitrogen content in the resulting products gradually increases with extended reaction time. As Liu et al. [58] present in their paper, in the point of view of the crystal structure, for a set temperature the increase of nitridation time of TiO₂ lead to the formation of a TiNₓO₁₋ₓ with x close to 0. The Fig. 8 presents the XRD diffractogram of TiO₂ nitridated at 900°C for 1 h, 3 h, 5 h and 8 h. In the Fig. 8 A, it can be seen that the increase in nitriding time lead to the disappearance of any TiO₂ phases (anatase or rutile). Unlike for 1 h and 3 h no phase of TiO₂ appears in XRD for 5 h and 8 h treatment, only the cubic phase of TiNₓO₁₋ₓ which shows a better advancement of the reaction for long nitriding. According to the authors, the peaks presented in Fig. 8B are attributable to TiNₓO₁₋ₓ. The Fig. 8B exhibits the shift of these peak to the left, closer to the perfect lattice parameter of TiN. The longer the reaction time is, the closer the XRD diffractogram is to lead to the perfect lattice parameter of TiN. Such influence of the reaction duration is also observable in the study of Li et al. [190]. Both studies conclude that as nitriding is a gas-solid reaction it is controlled by the inward diffusion of nitrogen and the outward diffusion of oxygen. Prolonging the nitriding time allows the oxygen content in TiNₓO₁₋ₓ to gradually decrease while the nitrogen content increase explaining the shift of the XRD peak and allowing the obtention of a TiNₓO₁₋ₓ with x close to 1. Extending the duration proved beneficial for both the reduction and nitrogen diffusion processes.

In their study Liu et al. [58] have computed the ratio of titanium,

<table><caption>**Table 4**

Reaction parameters of thermal nitridation of TiO₂ using ammonia gas.</caption><thead><tr><th>Titanium source</th><th>Reactive gas</th><th>Gas flow/ pressure (ml/min)</th><th>Temperature of reaction (°C)</th><th>Heating rate (°C/min)</th><th>Duration of reaction (h)</th><th>Thickness (nm)</th><th>Resistivity (Ω.m)</th><th>Hardness</th><th>Ref</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> xerogel</td><td>NH<sub>3</sub></td><td>4</td><td>675–1000°C</td><td>150</td><td>0.5</td><td>120</td><td>1.4 × 10<sup>-3</sup><br>2.4 × 10<sup>-4</sup></td><td>/</td><td>[74]</td></tr><tr><td>TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>200–300</td><td>500–1100°C</td><td>5 or 20</td><td>5</td><td>30–80</td><td>/</td><td>/</td><td>[122]</td></tr><tr><td>TiO<sub>2</sub>/TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>67</td><td>1200°C</td><td></td><td>1</td><td>/</td><td>/</td><td>/</td><td>[189]</td></tr><tr><td>TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>200</td><td>600–1100°C</td><td>5</td><td>5</td><td>140–300</td><td>/</td><td>/</td><td>[120]</td></tr><tr><td>TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>800</td><td>600–1100°C</td><td>5</td><td>2</td><td>85–165</td><td>/</td><td>/</td><td>[161]</td></tr><tr><td>TiO<sub>2</sub> xerogel</td><td>NH<sub>3</sub></td><td>1000</td><td>/</td><td>Nearly instantaneous</td><td>10 min</td><td>50</td><td>1.5 × 10<sup>-4</sup></td><td>10.5 GPa</td><td>[42]</td></tr><tr><td>TiO<sub>2</sub> xerogel</td><td>NH<sub>3</sub></td><td>80</td><td>900°C</td><td>1, 10 or 70</td><td>1</td><td>60–70</td><td>2 × 10<sup>-4</sup></td><td>/</td><td>[164]</td></tr><tr><td>TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>/</td><td>1100°C</td><td>5</td><td>5</td><td>/</td><td>/</td><td>/</td><td>[121]</td></tr><tr><td>TiO<sub>2</sub> anatase</td><td>NH<sub>3</sub></td><td>/</td><td>800–1000</td><td>5</td><td>/</td><td>/</td><td>/</td><td>/</td><td>[188]</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>Fig. 8. XRD patterns of samples roasted for different hours at 900°C (A and B) [58].</figcaption></figure>

nitrogen and oxygen after different nitriding processes of TiO₂. The data presented in the Table 5 confirmed that the increase of temperature and nitriding time leads to an overall advancement of the nitriding process. Thus, a nitriding at 1200°C for 8 h leads to a titanium nitride as: TiN₀.₉₇O₀.₀₃.

However, the study of Dinh *et al.* [64] explained that extensive temperature or reaction time tends to degrade the final product. Indeed, it is visible in Fig. 9 that the sample nitrided at 800°C for 5 hours resembles the one treated at 900°C for 2 hours, while the one treated at 900°C for 5 hours shows a significantly higher pore density. Thus, the degradation is clearly observable.

It is also stated that NH₃ treatment at a lower temperature for a longer treatment time could provide a similar effect to the treatment at a higher temperature for a shorter treatment time. In the study of Wei *et al.* [161], a comparison of the surfaces and cross-sections of the nitrided films (at temperatures between 900°C and 1000°C for 2 hours) was carried out by Scanning Electron Microscopy (SEM) (Fig. 10). An increase in grain size and roughness can be seen, and the appearance of pores is observable at 1100°C, confirming the claims of Dinh *et al.* [64]. Furthermore, the increase in roughness and grain size is also confirmed by the Atomic Force Microscopy (AFM) study of Trapalis *et al.* [188]. In this article [188], it can be seen that Roat-Mean Square (RMS) roughness and grain dimension are respectively 1 nm and 100 nm at 800°C and at 6 nm and 275 nm at 1000°C.

Other parameters have also been studied, such as the heating rate. In the study by Keddie *et al.* [164], faster heating during the nitridation process leads to faster and better nitridation of TiO₂ into TiN. According to them, when a fast heating rate is used, the nitridation reaction precedes densification and the oxide film transforms fully into nitride. On the other hand, a slow heating rate delays nitridation. Sintering of the resulting oxynitride creates a barrier, resulting in a concentration gradient. This may explain why the use of an Rapid Thermal Annealing (RTA) oven allows a fast and “close to stoichiometric” nitridation [27,42,

<table><caption>**Table 5**
Calculated formula of samples reacted under various nitriding conditions [58].</caption><thead><tr><th>Temperature (°C)</th><th>Time (h)</th><th>TiN<sub>x</sub>O<sub>1-x</sub></th></tr></thead><tbody><tr><td>900</td><td>3</td><td>TiN<sub>0.48</sub>O<sub>0.52</sub></td></tr><tr><td>900</td><td>5</td><td>TiN<sub>0.59</sub>O<sub>0.41</sub></td></tr><tr><td>900</td><td>8</td><td>TiN<sub>0.66</sub>O<sub>0.34</sub></td></tr><tr><td>1000</td><td>3</td><td>TiN<sub>0.62</sub>O<sub>0.38</sub></td></tr><tr><td>1000</td><td>5</td><td>TiN<sub>0.66</sub>O<sub>0.34</sub></td></tr><tr><td>1000</td><td>8</td><td>TiN<sub>0.78</sub>O<sub>0.22</sub></td></tr><tr><td>1100</td><td>3</td><td>TiN<sub>0.70</sub>O<sub>0.30</sub></td></tr><tr><td>1100</td><td>5</td><td>TiN<sub>0.77</sub>O<sub>0.23</sub></td></tr><tr><td>1100</td><td>8</td><td>TiN<sub>0.83</sub>O<sub>0.17</sub></td></tr><tr><td>1200</td><td>3</td><td>TiN<sub>0.91</sub>O<sub>0.09</sub></td></tr><tr><td>1200</td><td>5</td><td>TiN<sub>0.95</sub>O<sub>0.05</sub></td></tr><tr><td>1200</td><td>8</td><td>TiN<sub>0.97</sub>O<sub>0.03</sub></td></tr></tbody></table>

98]. Alhussain *et al.* [191] investigated the influence of ammonia gas exposure, by studying both isothermal and non-isothermal nitridations (at 900°C). For one sample, ammonia was introduced at 900°C (isothermal nitridation) and for the other sample the ammonia was introduced at the beginning of the heating process i.e. at ambient temperature (non-isothermal nitridation). The isothermal nitridation result shows highly agglomerated particles with irregular shapes while non-isothermal nitridation result shows smaller particles that are almost identical to those of the starting material (anatase TiO₂ powder). The authors explain this phenomenon by the doping of TiO₂ with nitrogen before the nitridation. Indeed, nitrogen doping is said to suppress the movement of ions in TiO₂ and the sintering of TiO₂ particles. Thus, by introducing ammonia at the beginning, the nitrogen is introduced into TiO₂, thus stabilizing the structure, whereas if the ammonia is introduced after TiO₂, the particles will have had the time to change shape and sinter. Studies regarding the influence of initial crystalline state of TiO₂ on the nitridation have been carried out [192–194]. These studies conclude that the crystallinity of TiO₂ doesn't seems to have a significant impact on the nitriding process. The only slit impact can be seen on the morphology of TiN produced.

The densification during the nitridation process has a significant influence on the nitriding. According to the literature the density goes from 4.2 g.cm⁻³ for the rutile and 3.78 g.cm⁻³ for the anatase to 5.22 g.cm⁻³ for the TiN [74]. This shrinkage represents a volume contraction of about 20–30 %. As nitridation is a gas-solid reaction the gas penetration in the layer play a significant role in the nitridation process. Nitrogen atoms diffuses inward while oxygen diffuse outward. In their study Jiménez *et al.* [74] claim that the reduction nitridation kinetic of TiO₂ with NH₃ follows a parabolic law. Unlike for the titanium nitridation the rate-limiting factor doesn't seems to be the inward diffusion of nitrogen but rather the outward diffusion of oxygen occurring during reduction [74,100]. At the beginning of the reaction the gas reacts with the solid at the surface but the further the reaction advances the deeper the reaction will take place. The dense TiN layer at the surface acts as an oxygen barrier hindering the outward diffusion of the oxygen. Thus, such densification slows down the reaction. Another consequence of the volume contraction is the possible appearance of cracks and porosity. The study of Szekely and Evans [195] gives great input regarding the impact of the film structure on the diffusion behavior during the reaction. In this study, a pore model with a moving reaction front is used to describe the gas-solid reaction. According to this model the increase in porosity density by increase of pores number and/or size leads to the increase of gas penetration thus promoting the nitridation reaction. However, such degradation of the TiN film has notorious impact on the properties of the layer. In their paper Berthod *et al.* [97] have observed the formation of micro-cracks reducing of the conduction and the optical performances leading to losses for plasmonic applications. Valour *et al.* [42] compared the resistivity of their TiN film obtained through the
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** SEM images of TiO₂ nanorods treated under NH₃ at different temperatures and treatment durations: (a) 800 °C for 2 h, (b) 800 °C for 5 h, (c) 900 °C for 2 h, and (d) 900 °C for 5 h [64].</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Surface and cross-section FE-SEM micrographs of thin films of TiN prepared at different nitridation temperatures in ammonia gas [161].</figcaption></figure>

nitridation of a xerogel of TiO₂ with the literature and observed a difference of one order of magnitude (1.51 × 10⁻⁴ Ω·cm for them and 2.8 × 10⁻⁵ Ω·cm for the PVD deposition) [196]. They attribute this difference to the inherent porosity observed for the sol-gel route which leads to the degradation of the electrical conduction.

#### 1.1.3.2. Parameters impacting the carbothermal nitridation.

In order to compare the different carbothermal nitriding process, data have been gathered in Table 6. As can be seen, processes using films, pellets and powders are compared in order to obtain as much data as possible. According to the studies by White et al. [140] and Maitre et al. [186], reactivity is directly linked to the carbon size: White et al. [140] stated that an increase of the surface area of carbon leads to a decrease in carbon reactivity. Similarly, Maitre et al. [186] claim that the
<table><caption>**Table 6**
Different carbothermal nitridation parameters.</caption><thead><tr><th>Titanium source</th><th>Carbon/reducing agent source and quantity</th><th>Nitrogen source</th><th>Reaction temperature</th><th>Heating rate (°C/min)</th><th>Reaction time</th><th>Form</th><th>Final phases present</th><th>Ref</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> anatase</td><td>Activated carbon<br/>C/TiO<sub>2</sub> = 2:1</td><td>95 % N<sub>2</sub>/5 % H<sub>2</sub> at<br/>200 ml/min</td><td>1200°C</td><td>&gt; 1400°C/min</td><td>20 min</td><td>Powder</td><td>TiN</td><td>[183]</td></tr><tr><td>TiO<sub>2</sub> from additive</td><td>CH<sub>4</sub> from plastic waste pyrolysis</td><td>1 L/min N<sub>2</sub> gas</td><td>1150°C</td><td>Instantaneous</td><td>40 min</td><td>Film</td><td>TiN</td><td>[56]</td></tr><tr><td>TiO<sub>2</sub> anatase or rutile</td><td>Activated carbon<br/>C/TiO<sub>2</sub> = 2:1</td><td>0.1 L/min N<sub>2</sub> gas</td><td>1150°C</td><td>5 °C/min</td><td>4 h</td><td>Powder</td><td>TiN</td><td>[140]</td></tr><tr><td>TiO<sub>2</sub> anatase or rutile</td><td>Lamp black or pyrolysis of sucrose<br/>C/TiO<sub>2</sub> = 2:1</td><td>95 % N<sub>2</sub>/5 % H<sub>2</sub> at<br/>2–4 cm<sup>3</sup>/s</td><td>1100–1500°C</td><td>4 °C/min</td><td>1 h</td><td>Powder</td><td>TiN</td><td>[138]</td></tr><tr><td>TiO<sub>2</sub>-graphite mixture</td><td>C/TiO<sub>2</sub> = 4.5:1</td><td>1.5 L/min N<sub>2</sub> gas</td><td>1200–1500°C</td><td>2 K/min</td><td>3 h</td><td>Pellet</td><td>TiO<sub>x</sub>CyN<sub>z</sub></td><td>[109]</td></tr><tr><td>TiO<sub>2</sub> rutile</td><td>Carbon black<br/>C/TiO<sub>2</sub> = 2:1</td><td>1-atm of N<sub>2</sub> gas</td><td>900–1100°C</td><td>40 °C/min</td><td>45 min</td><td>Powder</td><td>TiN</td><td>[63]</td></tr></tbody></table>

finest powders of carbon allow a better reactivity. In the case of carbothermal reduction using carbon-based gas, the shape and size of solid carbon, hence the reactivity, are directly impacted by the pyrolysis parameters according to Hamdani *et al.* [155]. Finally, according to Ruiz *et al.* [156] for the same decomposition parameters, solid carbon produced from ethylene is more reactive towards oxygen than that produced from acetylene. It is probable that other carbon-based gases such as methane have a different reactivity, but to our knowledge, no studies have been published on this subject. As it can be seen in Table 6, the large majority of papers producing TiN from carbothermal nitridation of TiO₂ are studying powders. As explained previously, the principal reducer is the carbon layer deposited and produced by the decomposition. However, Maitre *et al.* [186] explain that a good distribution of the carbon and TiO₂ is required for such a solid-solid reaction to be efficient. In the case of powders, TiO₂ and carbon powders distribution can be optimized. For films after decomposition of the carbon-based gas, a carbon layer is deposited over the TiO₂ film. Expect if the carbon is introduced through the use of a sol-gel no mix is possible. The reaction is then pursued by diffusion mechanisms [144] leading to carbon pollution of the final TiN phase.

The two other main parameters studied in the literature are the reaction temperature and the duration of reaction. Regarding the temperature, it is a consensus that the reduction rate and extent of reduction greatly increase with the temperature [57,139,150,151]. In the same way, increasing the duration of the reaction would also lead to a better reduction [141,142,144]. However, unlike the use NH₃, two competing reactions occur at the same time. When titanium oxide is sufficiently reduced, both carbon and nitrogen atoms can occupy the oxygen vacancies. According to thermodynamic calculations of Li and Riley [138], the reaction temperature of carbothermal nitridation should not exceed 1600°C as the formation of TiC will be more favored than TiN. Thus, at this temperature, carbon can even substitute the nitrogen, forming TiC instead of the TiN already formed. This phenomenon is confirmed by White *et al.* [140]: they stated that at high temperature (1500°C), carbon atoms are incorporated into the nitride lattice, replacing up to 50% of the nitrogen content, forming a carbonitride. It is also claimed that at lower temperature this substitution can also occurs on certain conditions (long reaction time, low quantity of nitrogen associated with a high quantity of carbon). Therefore, carbothermal nitridation needs to be treated with much more care than ammonia nitridation. Even though the reaction using carbothermal reduction is more efficient [138,140], this reaction is limited both in temperature and in reaction time.

Similar to nitridation using ammonia, the final titanium nitride phase will always contain oxygen contamination. As explained before, whatever the synthesis technique, TiN will always be contaminated with oxygen forming TiOₓNᵧ. Concerning carbothermal nitridation, while most authors present their final product as TiN in Table 5, it is more likely that the final product is a solid solution of TiO, TiC, and TiN as presented in the studies of Rezan *et al.* and Zhang *et al.* [109,139,149]. It

has been proven that carbothermal reduction does not achieve complete reduction even after a 5 h treatment of at 1200°C [149]. Furthermore, high temperature and long reaction time will undoubtedly lead to an increase of oxygen or nitrogen substitution with carbon. By analyzing the crystal structure of TiO, TiC and TiN [197–199], it can be seen that each of these phases has the same crystal structure (Fm$\bar{3}$m) with very similarly lattice parameters (4.29 Å, 4.33 Å and 4.24 Å respectively). Thus the crystal structure of the solid solution obtained [138,149] is most likely to look like the oxynitride presented by Mucha *et al.* [174]: It seems that these authors use TiN instead of TiOₓCyNz for simplicity, but it's important to clarify this distinction to avoid confusion. With the mechanisms involved in nitriding now explained, a review of the processes and a comparison of the performance of the TiN thin film produced will be presented in the next section.

## 1.2. Description and comparison of processes of thermal nitridation of TiO₂

### 1.2.1. Description of the processes

In the case of the nitriding of TiO₂ to form TiN thin films, the titanium oxide layer is most commonly deposited through a sol-gel route. The first paper discussing about this process is by of Kamiya *et al.* [120]. In this paper, TiO₂ xerogel film is deposited before being annealed at 500°C, leading to an anatase porous thin film. The TiO₂ film is then heated under ammonia atmosphere at a heating rate of 5 °C/min to temperatures up to 1100°C during 5 h. Jiménez *et al.* [74] used a very similar process but further improved the heating process. In their study, authors use RBS and Nuclear Reaction Analysis (NRA) to identify the composition of the films at different temperatures and heating times to exhibit the optimal parameters. They concluded that their optimal condition was a nitriding at 1000°C for 30 min (the heating rate is no mention in their work). They used resistivity measurement as a quality factor: low resistivity implies that oxygen content is minimal and that the film is not too degraded by porosity. Under these conditions, a TiN thin film with a resistivity of 240 μΩ.cm is obtained. However, a residual porosity of the TiN film due to the outward diffusion of oxygen is present.

Valour *et al.* [27,42,98] have shown the possibility of using a RTA oven to nitride thin TiO₂ films. The heating process of the nitridation is carried in a RTA oven working with high intensity infrared light flash. Unlike the previous studies, the heating rate is extremely high, which, as explained previously, promotes almost complete transformation of TiO₂ to TiN. This good conversion to TiN is presented in their paper has been demonstrated by using Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-Ray Spectroscopy (EDS). The other main advantage of such technique is the duration of the process, about 10–15 min including the heating rate. However, the problem of film deterioration remains the same as the other processes: densification and outward diffusion of oxygen lead to formation of cracks and porosities (Fig. 11).
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Cross-sectional HAADF STEM of the TiN thin film, annular dark field (ADF), and element mapping images for the N, Ti, and O elements in the TiN layer using Electron Energy Loss Spectroscopy (EELS) technique [27].</figcaption></figure>

Finally, in these studies, a photo-structurable TiO₂ xerogel is used in order to easily obtain structured TiN thin films. Indeed, the sol-gel is a complex TIPT-BzAc (tetraisopropylorthotitanate and benzoylacetone). When exposed to UVA light, this complex is degraded and becomes insoluble in different solvent (alcohol, chloroform, acetone, etc.). By controlling the illuminated and non-illuminated areas, TiO₂ structures are created. After nitriding the final product is structured TiN (Fig. 12).

In their study, Yoshimoto *et al.* [200] use a nitridation reaction similar as the previous one, but with a different TiO₂ source. Anodic oxidation of a titanium plate is carried out in an electrolytic bath to obtain a 10 µm thick layer mixing TiO₂ and Ti₃O₅. The plate is then placed in a crucible with a powder containing iron, Al₂O₃ and graphite. The iron is used for its catalytic effect on carbon (enhancing the carbon movement in graphite). Al₂O₃ prevents the powder from sintering and finally the graphite is the carbon source that reduces titanium oxide. The filled crucible is placed in an electric furnace and heated at 1100°C for 10 h with a 500 ml/min flow of nitrogen gas. As seen previously, reduction of the oxide phase with a carbon-based reducing agent leads to the formation of CO gas. The authors used CO formation to monitor the advancement of the carbonitriding reaction. However, a harmful phenomenon was exhibited. The accumulation of CO gas (generated during the reaction) near the interface between the film and the plate, leads to the formation of cracks on the porous surface (Fig. 9). Elemental analysis

of the film shows the presence of carbon and residual oxygen. Thus, this process does not allow a complete oxygen elimination. (Fig. 13)

The study by Yin *et al.* [56] presents another more sustainable way to produce TiN from TiO₂ through nitridation. Indeed, instead of relying on the sol-gel route to deposit TiO₂ on a surface, automotive plastics waste is used. In the automotive industry, TiO₂ is used as an additive in plastic for UV and thermal stability purposes. Furthermore, as these plastics are carbon-bearing materials, they are predominantly composed of polypropylene and polyethylene. Liberation of molecules such as CO and CH₄ occurs at high temperature, ideal for the reduction of TiO₂ as showed in the previous part. Thus, in order to produce a thin film of TiN, the authors coat automotive plastics waste on a steel substrate. The sample is then inserted in a furnace preheated at 1150°C for 20 min with a nitrogen gas flow of 1 L/min and finally placed in a cold zone with a nitrogen atmosphere for another 15 min to minimize sample oxidation. The reaction equations of carboreduction and nitridation are the same as presented in the previous part (Eqs. 2, 10–13). As it can be seen in the

<figure><img src="image_14.png" /><figcaption>**Fig. 13.** SEM micrograph of the surface of an anodized titanium plate heat-treated at 1373 K for 3.6 ks in a nitrogen flow using a powder mixture [200].</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Photographs of structured TiO₂ thin films after UV exposure through photomasks with a slight purple color in reflection (A, D) and photographs of the same structured films after nitridation with a burnished color in reflection (B, E) or with a grayish color in transmission (C, F). The thickness of the microstructured TiO₂ sol-gel is around 300 nm and the obtained thickness of the microstructured TiN layer is around 70 nm, which explains the partial transmittance when viewing the microstructured substrate in transmission. [7]</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 14. (a) Shiny gold TiN coated steel and (b) a reference steel sample, heat treated under the same condition, in isolation. (C) C, O, N, Ti depth distribution on the surface of an ASR treated steel sample [56].</figcaption></figure>

Fig. 14A and B, this process permits the production of a gold-colored TiN. However, the TiN layer produced from this process is limited. The characterization presented in this paper shows that the TiN produced is largely polluted by carbon and oxygen (Fig. 14 C) respectively 5–10% and 15–20% after etching. Furthermore, the thickness of the film obtained is about 18 nm.

In order to improve the process, thermal nitriding is more and more associated with plasma. In this process, nitrogen-containing gas is ionized to form a plasma around the surface to be nitrided. Simultaneously the sample is heated to promote diffusion of nitrogen ions into the surface. This process is currently mainly used to nitride titanium and titanium alloys. The main interest of using plasma is the possibility to greatly reduce the nitriding temperature due to plasma reactivity's [32, 45,134]. In their paper, Domínguez-Meister et al. [45], present a plasma nitriding process for titanium which permits the formation of a TiN layer of 50 µm at temperature below 600°C in about 5 h in a nitrogen atmosphere. Apart from the lower nitriding temperature, the plasma allows the use of gases other than NH₃. Indeed, N₂ gas can be ionized to form a highly reactive plasma. Shen et al. [201] have shown that plasma nitridation using N₂-NH₃ gas mixture gives better results than NH₃ gas with the same nitridation temperature and time. According to the authors, N₂-NH₃ gas mixture provide a higher nitrogen concentration for diffusion than NH₃, leading to thicker TiN layers. Saillard et al. [134] also show that unlike with a simple thermal nitriding which required NH₃, TiO₂ native layer can be reduced using a N₂-H₂ mixture with a plasma process. So far, few researchers have been studying the plasma nitriding of TiO₂, the published work mainly concern nitrogen doping of TiO₂ using nitrogen plasma for photocatalytic applications [202,203].

However, as Ryu et al. [204] explained the use of N₂-H₂ gas for plasma nitridation open new pathways for preparation of thin films such as titanium oxynitride layer with a reduced bandgap. According to them, this would make a beneficial heterojunction structure for enhanced photoresponsive performances. Thus, the use of plasma, currently well developed for the nitriding of titanium and titanium alloys, is under development for TiO₂. This would allow nitriding at lower temperature and the use of N₂-H₂ mixture. Plasma nitriding is a legitimate candidate to both improve the quality of the films and eliminating the need for ammonia making the nitriding of TiO₂ more sustainable.

### 1.2.2. Comparison of the properties of TiN

The plasmonic properties of TiN has been extensively studied in the last years [50,205–207]. The films exhibiting plasmonic properties are mainly produced by CVD or PVD. However, as presented before authors have also demonstrated promising results of plasmonic properties of nitride films of TiO₂ [42,193]. The FoM evaluates the metallic behavior of a film in regards as metallic properties and losses in the visible and near IR wavelength range since it considers both metallic properties (ε') and losses (ε''). It is defined as:

$$ FoM = \frac{\epsilon'^2}{\epsilon''} \qquad (23) $$

The higher this factor is for a given wavelength, the better the plasmonic properties, meaning that the enhancement by the electric field gives rise to localized or non-localized resonances (plasmon modes) with high efficiencies. Among the most commonly used materials in plasmonic, gold can be mentioned [50]. Fig. 15 presents the figure of merit for TiN film deposited by molecular beam epitaxy (MBE) [50], TiN obtained by nitriding [193], and gold film [50]. In the infrared range, gold film exhibits the highest FoM values, reaching 1500. In comparison, deposited TiN film displays values around 70 and nitrided TiN film around 10. The differences between TiN film deposited and nitrided can be explained by the higher roughness, porosity and oxygen contamination of the TiN film [208–210] produced by nitriding processes. Thus, improvement in the nitriding process still needs to be made to get closer to conventional methods.

Table 7 presents different properties of TiN films. TiN films obtained using sputtering deposition technique have better properties overall. Indeed, for similar thickness, resistivity is more than two times smaller and hardness is two times higher. These data also show that nitriding by carbonitriding process leads to film contaminated with higher percentages of both carbon and oxygen.

As explained before the nitriding process parameters have a significant impact on the oxygen and nitrogen ratio of the films. Several studies have shown that this ratio influence the properties of these films. According to different studies the increase in oxygen in titanium nitride films leads to the increase of the resistivity [214,215]. In term of mechanical properties, Graciani et al. [216] have computed that an

<figure><img src="image_17.png" /><figcaption>Fig. 15. Figure of merit of gold [50], TiN deposited by MBE [50] and TiN obtained by nitriding of TiO₂ [193].</figcaption></figure>
<table><caption>**Table 7**
Comparison of TiN properties from TiO₂ nitridation and deposition technique.</caption><thead><tr><th></th><th>Ref</th><th>Elaboration methods</th><th>Thickness (nm)</th><th>Resistivity (μΩ.cm)</th><th>Hardness (GPa)</th><th>Reflectance at 2000 nm</th><th>Contamination percentages</th></tr></thead><tbody><tr><td rowspan="13">TiO₂ nitriding</td><td>[161]</td><td>Ammonolysis</td><td>165</td><td></td><td></td><td></td><td>10 % Oxygen</td></tr><tr><td>[164]</td><td>Ammonolysis</td><td>60–70</td><td>200</td><td></td><td></td><td>2 % Oxygen</td></tr><tr><td>[74]</td><td>Ammonolysis</td><td>80–100</td><td>240</td><td></td><td></td><td></td></tr><tr><td>[193]</td><td>Ammonolysis</td><td>50–60</td><td></td><td></td><td>78 %</td><td></td></tr><tr><td>[163]</td><td>Ammonolysis</td><td>350</td><td>95</td><td></td><td></td><td></td></tr><tr><td>[121]</td><td>Ammonolysis</td><td>600</td><td></td><td></td><td>80 %</td><td></td></tr><tr><td>[42]</td><td>Ammonolysis</td><td>50–60</td><td>150</td><td>10.5 ± 0.5</td><td>70 %</td><td></td></tr><tr><td>[211]</td><td>Ammonolysis</td><td>100</td><td></td><td></td><td></td><td></td></tr><tr><td>[200]</td><td>Carbonitridating</td><td>7000</td><td></td><td></td><td>13</td><td>2–5 % Carbon</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>10–15 % Oxygen</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>5–10 % Carbon</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>15–20 % Oxygen</td></tr><tr><td>[56]</td><td>Carbonitridating</td><td>18</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">TiN deposition</td><td>[212]</td><td>Reactive Sputtering</td><td>30–50</td><td></td><td></td><td></td><td></td></tr><tr><td>[208]</td><td>Reactive Sputtering</td><td>100</td><td>43</td><td>24</td><td></td><td></td></tr><tr><td>[213]</td><td>Reactive Sputtering</td><td>70</td><td></td><td></td><td>88 %</td><td></td></tr></tbody></table>

increase in of oxygen leads to a decrease in the bulk modulus of the titanium nitride films. Furthermore, nanohardness measurements have also been done by Chappé *et al.* [175] showing the softening effect of oxygen on titanium nitride films. The impact of the composition of the titanium oxynitride on the plasmonic properties have also been investigated. Both the study of Ali *et al.* [217] and Odusanya *et al.* [218] conclude that an increase of nitrogen and a decrease of oxygen leads to a better plasmonic behavior of the TiN films. Finally, it has been shown that the color of titanium nitride varies greatly as a function of its stoichiometry. Fig. 16 from the study of Guo *et al.* [219] shown the color difference between gold, TiN and TiN<sub>0.28</sub>. it is clear that with a lower quantity of nitrogen the film presents a more brownish color. According to Chappé *et al.* [175], the dark golden titanium nitride becomes dark green with increase of oxygen content. It then shifted gradually to lighter green and finally transparent yellow-green for titanium oxides.

## 2. Conclusions and future outlooks

The mechanism of reduction nitridation of TiO₂ is an intricate phenomenon that still needs to be studied for a better understanding and process control. This review gathers some of the knowledge produced regarding gas thermal reduction nitridation of TiO₂ to form TiN. Two of the main processes have been investigated: thermal nitridation of TiO₂ by ammonolysis and carbothermal nitridation of TiO₂. It was seen although these two processes have very similar mechanisms, the use of carbon as reducing agent has both advantages and drawback as compared to the use of ammonia. For example, the carbon is a better reducer than ammonia for this reaction (leading to possible faster reaction). However, its use leads to much higher contamination risks. Furthermore, high temperature leads to a competition between the formation of TiN and TiC, which is not the case when ammonia gas is

used. As it has been seen, different processes of nitriding of TiO₂ have been developed in the recent years. Some of the authors mainly focused on the development of sustainable processes to produce TiN thin films, while other have worked with sol-gel to permit an easy and rapid structuring of the TiN film for example. A comparison of TiN obtained by nitriding of TiO₂ and TiN obtained by reactive sputtering shows clear differences in their properties. Indeed, nitriding still leads to lower film quality than conventional deposition techniques such as PVD and CVD. It was also found that the nitriding of TiO₂ is mainly done by ammonolysis with furnace that can consume energy extensively. However, this process still has plenty of applications. For example, the possibility to structure the TiN films allows to use it for optical and plasmonic applications [27,42,98], and the process can be used to coat non-planar surfaces which could be very challenging for other deposition methods [91,220]. Producing structured TiN films inside a cylinder would be very difficult using conventional deposition techniques such as PVD and CVD but it is achievable using this process [221,222]. In addition, nitridation not only allows the production of TiN thin films, but also allows TiO₂ to be doped with nitrogen to improve photocatalytic properties [65–67].

As it has been seen, new annealing techniques, such as RTA nitridation, permit an optimization of the nitridation process even though these studies are on the premises [27,42,193]. Plasma nitriding offers a promising alternative for nitriding titanium and titanium alloys by ionizing nitrogen-containing gas into plasma around the surface while heating the sample. This method significantly reduces the nitriding temperature due to the plasma's high reactivity and allows the use of gases like N₂ instead of NH₃. Domínguez-Meister et al. [45] demonstrated the formation of a 50 µm TiN layer below 600°C in 5 hours using nitrogen plasma. Shen et al. [201] showed that an N₂-NH₃ gas mixture produces better results than NH₃ alone, leading to thicker TiN layers. Saillard et al. [134] revealed that N₂-H₂ plasma can reduce the TiO₂ native layer without needing NH₃. While research on plasma nitriding of TiO₂ is limited, it shows potential for producing titanium oxynitride layers with improved photocatalytic properties, as noted by Ryu et al. [204]. Plasma nitriding is a viable candidate for improving film quality, lowering nitriding temperature, and reducing reliance on ammonia, making the process more sustainable. Furthermore, other more sustainable annealing techniques have been used to nitride titanium such as solar energy concentrated by a Fresnel lens. In their study, Rodriguez et al. [223] were able to produce TiN through nitridation of Ti₆Al₄V alloy by a solar nitriding process in about 15 min. This type of technique can and will surely be adapted to TiO₂ as more sustainable sciences are developing. Going further down the sustainability path, researchers are currently working on processes to substitute the ammonia gas with urea [224,225]. This replacement would allow to get rid of the toxicity of ammonia while at the same time upcycling human waste. Researchers have worked, and are still working, on making this process more and

<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Comparison of the colors of a stoichiometric titanium nitride (TiN), substoichiometric (TiN<sub>0.28</sub>) layer and a gold (Au) layer.</figcaption></figure>
more sustainable, using less pollutant reactant and less energy-demanding heating process.

## CRediT authorship contribution statement

**CRESPO-MONTEIRO Nicolas:** Writing – review & editing, Validation, Supervision, Project administration. **VALLEJO-OTERO Victor:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Conceptualization. **OLLIER Nadège:** Writing – review & editing, Supervision, Conceptualization. **GAMET Emilie:** Validation, Supervision, Project administration, Funding acquisition. **DONNET Christophe:** Writing – review & editing, Validation, Supervision. **JOURLIN Yves:** Writing – review & editing, Supervision, Project administration, Funding acquisition. **VALOUR Arnaud:** Validation, Resources.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors acknowledge the French National Research Agency (ANR) for financial support in the framework of project NITRURATION (ANR-21-CE08–0042–01), and the Centre National de la Recherche Scientifique CNRS (French RENATECH+, nano-SaintEtienne plateform).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jeurceramsoc.2025.117330.

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