The first oxidation pathway of TiN
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of TiN upon oxidation at atomic scale, illustrating the complete phase transformation from cubic TiN to tetragonal anatase TiO2. Figure 1 displays time-resolved structural evolution of an individual TiN during thermal oxidation process, successfully achieved by the setup built on AC-ETEM platform (Supplementary Fig. 1). That allows direct observation of the dynamic oxidation reaction process at high temperature (range of RT – 900°C) in oxygen. In order to obtain reliable results, a static time of approximately 5–10 minutes is necessary to maintain image stability, due to obvious image shift and vibrating caused by the thermal expansion of the SiN film as changing the temperature and pressure. The starting time of the first stable image was set as t0. As shown in Fig. 1a, initial TiN phase nearly has no change during the static period, the orientation of such particle can be labeled as zone axis of [001] with two sets of crystal planes of (200) and (020), respectively, being perpendicular to each other. The inset in the lower right corner presents the corresponding fast Fourier transform (FFT) image of Fig. 1a. Figure 1g and 1h illustrates the schematic image of atomic structure of both TiN and anatase TiO2 phases, in where displays the atomic configuration and atomic radii, as well as the numbers of Ti observed along the [001] axis.
Under oxidation conditions of 450°C and 50 Pa oxygen pressure, after 15 minutes, 5–7 vacancies of titanium atoms (Fig. 1b) are identified along the (200) direction at the near surface region of the particle. Intriguingly, we discovered that titanium vacancies created a periodic array of defects with one row of titanium atom absence every three (200) crystal planes. In other words, there are two regular (200) planes between every two planes with vacancies, just as shown in the second step in Fig. 2e. With oxidation progressing, an increasing number of titanium vacancies (Fig. 1c) appeared along both the (200) and (020) directions, following the same periodicity. That implies more titanium-nitrogen bonds have been broken to subsequently form titanium-oxygen or titanium-oxygen-nitrogen bonds. The work by Janina et al31 also demonstrated that certain titanium atoms in TiN have a tendency to bond with oxygen during the initial oxidation process. This results in titanium departure from TiN crystal lattice sites with leaving vacancies at those sites. Theoretical calculation results confirm that the presence of titanium vacancies arranged in chains facilitate to reducing the internal energy and enhancing the structure stability of the oxide system, though this phenomenon has not been experimentally validated to date31. Thus, the presence of periodic atom row absence would largely contribute to the following oxidation proceeding and the initial structures characteristics of titanium-oxygen bond formation. The generation of titanium vacancies along the (200) direction during the oxidation process is not coincidental, we observed the same phenomenon in other regions (Supplementary Fig. 4) and along another zone axis of [110] as well, as shown in Fig. 1j. These above observations together prove TiN displays the highest oxidation activity along the crystal planes of {200} family compared to other crystal planes. To gain deeper insights, we conducted rigorous First-principles calculations to evaluate the migration barriers of titanium atoms along series of crystal planes (refer to Fig. 1h for the results). Consistently, our calculations are in constant with the notion that the influence of titanium migration along the (200) plane is comparatively minimal. These findings underscore the pivotal role played by the (200) crystal plane in the oxidation of TiN, highlighting its significance in this context.
Following one hour of oxidation treatment, a substantial influx of oxygen atoms penetrated the TiN lattice, resulting in pronounced lattice distortion. Consequently, it becomes unfeasible to clearly visualize atomic structure changes of TiN in this specific area (Fig. 1e). Nonetheless, drawing from the atomic structures gathered during the oxidation process depicted in Supplementary Fig. 5, we made rational deductions regarding the structural evolution throughout the entire oxidation process. The region depicted in Fig. 2a corresponds to the [001] axis of TiN, where titanium vacancies emerge along the (020) crystal plane. By employing line profiles, we compared the variations in atomic contrast across four distinct regions spanning. Ten columns of titanium atoms were selected in parallel to the (200) crystal plane, with each individual column denoted by uppercase letters A-J. We used the alignment of column D atoms to obtain the relative displacement of atoms in different regions. In the region without titanium vacancies (i.e., the 4# curve in Fig. 2c), titanium atoms are arranged strictly in accordance with atom sites in the lattice structure of TiN, and the spacing between every two titanium atoms is measured as 0.21 nm. In the other three curves, we observed partially missing atomic columns (i.e., columns B, E, and H), which corresponded to the generation of titanium vacancies. Wherein one titanium vacancy emerged every three atom columns, corroborating the phenomenon observed in Fig. 1. Additionally, we noticed that after vacancy formation, neighboring titanium atoms approached the vacancies, with columns A and C moving closer to their original positions in column B, reducing the distance between columns A and C. Employing First-principles calculations, we realized that the driving force is the difference in internal energy among the intermediate oxide structures that are subsequent to the creation of titanium vacancies (Fig. 2f). The simulation results demonstrate that after the formation of titanium vacancies, oxygen atoms tend to occupy the vacancy sites and form titanium-oxygen bonds with the titanium atoms on both sides of the vacancies, thereby reducing the distance between the two columns of atoms.
With prolonged oxidation time, the amounts of titanium vacancies gradually increase, leading to the generation of titanium vacancies on the other crystal planes of (020) between the adjacent two vacancy columns (Fig. 2b). Through image contrast comparisons in Fig. 2d, we observed that newly generated titanium vacancies were arranged with one vacancy existing in every three titanium atoms. These newly formed titanium vacancies result in the formation of a regularly arranged Z-shaped Tetris block (as shaped as formed in the third step in Fig. 2e) morphology between two long chains of titanium vacancies (The structure of this Z-shaped Tetris block can be observed more intuitively in Supplementary Fig. 8b).
Additionally, we compare the system energies of two potential intermediate structures based on the current vacancy formation pattern (Fig. 2g). The results indicate a lower system energy for the Z-shaped structure compared to the square structure, inasmuch as a Z-shaped intermediate structure prefers forming during oxidation. The presence of titanium vacancies on both sides of the vacancy chain is correlated, and this mechanism of vacancy formation leads to the formation of staggered atomic pairs, as depicted in the third step of Fig. 2e. The structure within the green dashed box in Fig. 1e is identical to that in Fig. 2b, suggesting that these two areas experience the same oxidation process. Subsequent to the formation of the Z-shaped intermediate structure, we further observed the presence of an atomic arrangement along the anatase [001] axis within the oxide (Supplementary Fig. 5c). Structurally, the uniform dispersion of atoms within the plane leads to the transition from the Z-shaped intermediate structure to the atomic arrangement along the anatase [001] axis, effectively alleviating internal stress. Hence, we propose that during the mid-to-late stages of the oxidation process, the oxide optimally adjusts atomic distances to relieve the internal stresses generated during oxidation. This adaptive mechanism leads to the development of a transitional structure resembling the fourth step depicted in Fig. 3e, ultimately resulting in the transformation into anatase. This indicates that there is a fixed correspondence between TiN and anatase, i.e., TiN [001] corresponds to anatase [001] after oxidation.
However, we observed some areas where the oxidation paths were unclear, such as the green dashed circle in Fig. 1c, where no long chains of titanium vacancies along the (200) or (020) directions formed. Similar phenomena have been observed in the oxidation process of other particles (see Supplementary Fig. 6), which may be attributed to different modes of titanium vacancy formation. Furthermore, certain areas exhibited low signal-to-noise ratios, posing challenges in determining the detailed oxidation pathways.
The second oxidation pathway of TiN
To enhance our comprehensive understanding of the oxidation process, we refined the reaction conditions to promote milder conditions, resulting in decelerated oxidation rates and prolonged reaction durations. This adjustment facilitated the acquisition of additional structural insights during the oxidation process. Figure 3 presents a series of HAADF-STEM images of TiN particles in the oxidation process, taken at reducing the reaction conditions (Temperature: 400°C, and O2 pressure of 20 Pa). Figure 3a illustrates atomic structure of TiN observed along the [001] axis, while Supplementary Fig. 7 provides a magnified HAADF image revealing a clearly visible grain boundary. The adjacent grains exhibit same zone axis of [001] with a grain boundary angle of 3o. Prior to the observation of this area, different regions of the sample have undergone varying degrees of oxidation, and long chains of titanium vacancies aligned along (200) have generated at the right edge of grain 2. However, the oxidation behavior at the left edge of the grain 2 and the edges of the grain 1 exhibit distinct characteristics compared to the previous observations, e.g. arrangement of titanium vacancies are consistent with those shown in the green circle in Fig. 1c. It suggests that no difference in the atomic structure evolution and phase transformation even though altered the reaction conditions to a milder status. After 55 minutes of oxidation, four distinct oxide structures were identified on the TiN, depicted in Fig. 3c, and Supplementary Fig. 8 sequentially displays the magnified HAADF images, which include the long chains of titanium vacancies and the staggered atomic pairs structure. Here named these distinct structures as the first oxidation path (Path I). The other two oxide structures are categorized as the second oxidation path (Path II) since they are absence in the Path I. Under varying reaction conditions, both oxidation pathways occurred, suggesting that temperature and oxygen pressure are not determining factors for the distinct oxidation pathways in TiN. The structure observed in the grain 1 should be the initial structure in the Path II, whereas the structure at the left edge of the grain 2 emerged later in the Path II (the supporting evidence will be discussed later).
Following the complete transformation of the sample into titanium dioxide (Fig. 3e), we subjected the oxide to heating in an oxygen environment. As a result, we observed a morphological reconstruction without any further phase transformation that consistent with previous reports32–34. Figure 3e illustrates the presence of two atomic steps consisting of anatase (101) crystal planes on the left edge of the oxide (Supplementary Fig. 9). However, these atomic steps exhibit instability under the current environmental conditions. In Fig. 3e to 3f, the outermost atomic step progressively diminishes from bottom to top. Subsequently, in Fig. 3g, the following outermost atomic step initiates its disappearance from the top. Finally, in Fig. 3h, the outermost atomic step completely vanishes, leading to the formation of a regular oxide shape. We conducted First-principles calculations to assess the system energy of oxide configurations characterized by varying numbers of steps. The results demonstrate that a reduction in the number of steps contributes to the establishment of more stable systems (Supplementary Fig. 10). Consequently, the oxide exhibits a tendency to adopt flat surfaces. In the following sections, we elucidate the second oxidation pathway (Path II) of TiN by meticulously tracing the atomic structural evolution of the right edge of grain 1 throughout the oxidation process.
Figure 4a presents the atomic structure of an intermediate phase in the Path II, three distinct regions that encompassed crystal planes of (200) and (020) to accurately deduce the structure evolution. These regions are selected to include both vacancy-containing and vacancy-free areas labeled with dashed blue, orange and green, respectively (Fig. 4a). In Fig. 4g, the 1# curve displays changes in atomic contrast for the repeating units along (200) after oxidation, every three atoms with relatively strong contrast are surrounded by two atoms with weaker contrast. Each repeating unit is separated by a titanium vacancy and adjacent planes of repeating units are arranged in a staggered pattern. The related atom configuration is shown in the first step of Fig. 4j, with atoms of weaker contrast represented in lighter colors. We observed a significant reduction in atomic spacing at where containing vacancies compared to regions without vacancies. The average interplanar spacings of the (200) and (020) planes decreased from 0.21 nm to 0.19 nm after the vacancies emerged, as depicted in Fig. 4g and 4h. First-principles calculations indicated that continuous infiltration of oxygen atoms into the TiN lattice resulted in the formation of bonds with the surrounding titanium atoms. These titanium-oxygen bonds lead to a reduction in the interatomic distance, therefore, a general decrease in atomic spacing is observed in the vacancy-rich regions (Supplementary Fig. 9).
As shown in Fig. 4b-d, in Path II, as the oxidation process unfolds, the presence of titanium vacancies and a general reduction in atomic spacing lead to a gradual decrease in the angle between the (200) and (020) crystal planes. This phenomenon can be attributed to internal strains induced by structural changes or phase transformations. When the (200) crystal plane tilts to a certain degree, a portion of the atoms are subjected to compressive strains in their vicinities, causing them to deviate from their original lattice positions, as highlighted in the red box during the second step of Fig. 4j. As the (200) crystal plane continues to rotate, two atoms from the upper and lower rows approach closer, subsequently moving far away from their initial positions due to lattice compression, as indicated by the red box in the third step of Fig. 4j.
The angle between the (020) and (200) crystal planes can reach a minimum of 69o, and when a certain number of titanium atoms have completed their migration, the angle between them stops decreasing and gradually starts to increase. This is evident in Fig. 4d, where the angle between the two crystal planes in that region returns to 76o when the titanium atoms selected in the second and third steps of Fig. 4j complete their migration. Over time, the atomic structure with staggered atomic pairs undergoes a relaxation process and evolves into those belonging to anatase TiO2 of [001] axis (Supplementary Fig. 10), in coincidence with the rotation returning to 90o (as depicted in Fig. 4e and 4f). It is worth mentioning that in Fig. 4i, the atomic spacing does not change in structural relaxation process; rather, it just leads to changes in the relative positions of the atoms. From this, it is evident that the correspondence between TiN and TiO2 observed in the Path I that also applies to the Path II, meaning that an epitaxy relationship between TiN [001] and anatase [001].
The influence of curvature on the oxidation pathway
In summary, we observed two main oxidation pathways during the transformation process from TiN to anatase TiO2. Intermediate oxidation structures for both oxidation pathways have been observed in the oxide under various reaction conditions, including different temperatures and oxygen pressures. Hence, it can be concluded that temperature and oxygen pressure do not dictate the TiN reaction pathway. According to the analysis above, we find out that the surface curvature plays a significant role in determining the pathways via influencing structure evolution during oxidation process. With relatively flat edges, just like the area where the red sphere is located at Fig. 5c, the oxidation proceeding mainly depends on the Path I. For example, this surface edges of particles in Fig. 1, as opposed to the flatness observed in the left edge of grain 2 in Fig. 3. Conversely, the Path II is more prevalent in regions with curved edges, just like the area where the blue sphere is located at Fig. 5c. Motivated by these experimental findings, we delve into a detailed analysis through first-principle calculations to uncover the intricate influence of curvature on oxidation pathways
Figure 5a demonstrates that when the curvature radius of the grain exceeds 4.5 nm, the oxidation pathway tends to follow the Path I. Conversely, as curvature radii are smaller than 4.5 nm, the Path II becomes more favorable (The curvature radius at this location is defined based on the radius of the circumscribed circle in the respective region, as shown in Supplementary Fig. 11). The curvature of the materials would generate both axial and tangential stresses35, which significantly influences both the oxidation rate and the morphology of the oxide layer36–39. These tensile stresses are found along the axial direction and compressive stresses are along the tangential direction. As the curvature radius decreases, the material experiences an increasing compressive stress, which’s magnitude is directly proportional to the strain with a constant composition32. Due to the limitations of simulation dimensions and the periodic boundary conditions, it is not feasible to simulate an entire spherical structure. Therefore, we employ lateral strain tension to mimic the curvature of the material. Figure 5b displays a relatively limited response to applied strain along Path I, while a noticeable reduction in system energy was observed with the application of positive strains exceeding 2% along Path II. As a result, the greater tendency to follow the Path II is regulated by the smaller surface curvature radii of TiN nanoparticles, because of elevated stress status upon oxidation.
The effect of (200) crystal plan on the oxidation resistance of TiN
Building on the insights gained from the atomic-level oxidation processes, we now shift our focus to the influence of the (200) crystal plane on the oxidation of bulk TiN. In both pathways, oxidation predominantly initiates along the (200) crystal plane. Accordingly, two distinct titanium nitride materials were engineered: one with a distinct orientation along the (200) crystal plane, and the other with an absence of discernible orientation. This tailored design seeks to elucidate the impact of the (200) crystal plane on the oxidation dynamics of titanium nitride. As observed in Supplementary Fig. 13, under identical thermal treatment conditions, the TiN material with a (200) preferred orientation demonstrates increased susceptibility to oxidation. Post heat treatment, detectable diffraction peaks indicative of titanium oxide were evident in the former, while the latter, with a random orientation, exhibited an absence of such distinctive peaks. Upon elevating the heat treatment temperature to 600 oC and maintaining it for 40 minutes, detectable diffraction peaks indicative of titanium oxide emerge in the randomly oriented TiN samples. Significantly, at this juncture, the intensity of the titanium oxide diffraction peaks in the (200) preferred orientation sample triples that of its randomly oriented counterpart. This emphasizes the potential for enhancing the antioxidative performance of TiN through careful control of its orientation.