Atomic scale study of surface orientations and energies of Ti₂O₃ crystals

Titanium oxide compounds such as TiO₂, Ti₂O₃, and TiO show a variety of interesting functional properties. They have been widely applied in optical devices, electrochemical energy storage, catalysts, and beauty care products. Ti₂O₃ has a thermodynamically higher energy phase than TiO₂ and TiO. The phase, microstructure, morphology, and stoichiometry have a large impact on its functional properties. Crystallographically, Ti₂O₃ possesses the Al₂O₃-type corundum structure for which Ti³⁺ is octahedrally coordinated by six oxygen anions and the oxygen sites are 4-fold coordinated.¹ In Ti₂O₃, one third of the possible octahedrally coordinated cation sites are vacant.² There are two different Ti-Ti distances as shown in Fig. 1. One large covalent radius of titanium is ∼2.99 Å, while the other bond radius is 2.58 Å.² The short bond radius indicates a strong electron-electron correlation effect, which is related to the semiconducting gap.³ The temperature variation can influence the short Ti-Ti distance due to the thermal expansion effect. The electronic structure of the Ti 3d orbitals shows t_2g and e_g like features.

The bulk Ti₂O₃ shows semiconducting behavior at 450 K with a small bandgap of ∼0.1 eV.^4,5 Ti₂O₃ exhibits a second order semiconductor-to-metal transition upon heating,^6,7 which involves a likely band-crossing mechanism.⁸ Ti₂O₃ exhibits strong acid and alkali corrosion resistance.⁹ In addition, Ti₂O₃ shows interesting thermoelectric properties that can be enhanced by the chemical substitutions of the titanium cation.⁶ The Cr-doped Ti₂O₃ films show giant negative magnetoresistance as reported by Wang et al.¹⁰ 

Ti₂O₃ can be prepared by the sol-gel method⁹ and pulsed laser deposition.¹¹ Researchers have found that Ti₂O₃ can form as an intermediate phase as a result of electron bombardment of TiO₂.¹² Veremchuk et al. reported that Ti₂O₃ can be made from a mixture of powders of rutile/anatase with titanium metal using the spark-plasma-sintering technique. Reduction of TiO₂ can trigger the formation of the Ti₂O₃ phase as reported by Chae et al.¹³ In addition, the Ti₂O₃ phase was also found in a new refractory mineral within a chondrule from Allende meteorite.¹⁴ 

For electrochemical application of titanium oxide as a coating layer on a cathode, it is necessary to clarify the structural nature of the coating layer. Ti₂O₃ has been identified following the coating layer formation. Understanding the role of Ti₂O₃ as a coating layer for the cathode material critically depends on the comprehensive characterization of the surface faceting planes and the chemical terminations of these faceting planes. Here, we report the detailed atomic level characterization of the microstructure of Ti₂O₃ nanocrystals by using a combination of atomic scanning transmission electron microscopy (STEM) imaging and density functional theory (DFT) calculations, intending to identify the faceting planes and the terminating chemical species on these surface planes, which will provide insight for elucidating the interaction of these surfaces with the liquid under electrochemical conditions.

The Ti₂O₃ samples used in our study are the parasitic phase found in the process of producing TiO₂-coating on cathode materials of Li-ion battery materials. For preparation of TiO₂ coated samples, pristine cathode powder was dispersed in N-methyl-2-pyrrolidone (NMP) and then Ti(OBu)₄ was dripped into the suspension. The suspension was homogeneously stirred and exposed to air at 60 °C for 24 h to accelerate and ensure the complete hydrolysis of Ti(OBu)₄. Finally, the precursors from above were heated to 450 °C and kept at this temperature for 5 h to form the TiO₂-coated sample. We believe that Ti₂O₃ is produced due to incomplete oxidation during the production. The microstructure analysis of the samples was carried out using a probe aberration-corrected Titan microscope (FEI, Hillsboro, Oregon, USA) operated at 300 kV. The atomic models of the Ti₂O₃ are built using VESTA software.

As shown in Fig. 2, the atomic scale STEM high angle annular dark field (HAADF) image is an incoherent Z-contrast image, in which the higher contrast atomic columns indicate a higher average atomic number. It contains valuable composition information at the atomic scale. The Ti columns show high contrast, while the O atomic columns are not visible in the HAADF image. The atomic scale HAADF image in Fig. 2(a) corresponds well to the atomic model of the Ti₂O₃ imaged along the [42-1] zone in Fig. 2(b). The dark columnar region as pointed by the red arrow in Fig. 2(a) is the large spacing between the (012) Ti atomic planes. The distance between these (012) planes is around ∼2.3 Å as labeled in Fig. 2(a). The neighboring Ti columns show alternating short (∼2.1 Å) and long (3.3 Å) distances in the horizontal directions and zigzag arrangement (shift by ∼0.5 Å) in the vertical directions as shown in Fig. 2(b). The atomic arrangement is consistent with the corundum crystal structure.

The terminating planes of the crystal depend on the surface energy of the crystal surfaces. Often, the exposed surfaces are important in determining the functional properties of the materials used as catalysts, sensors, and optical devices. By comparing the atomic resolution STEM-HAADF images in Figs. 3(a) and 3(b) and the atomic model in Fig. 3(c), the termination planes can be determined. The atomic model in Fig. 3(c) shows the (012) and (1–20) atomic planes using blue and red lines. As shown in Fig. 3, the Ti₂O₃ crystals are viewed along the [42-1] zone axis in the STEM. The (012) and (1–20) surfaces are observed frequently as the termination planes in Ti₂O₃. The (012) surfaces have a large Ti-plane spacing of ∼2.3 Å as shown in Fig. 2(a).

Figure 4(a) shows the atomic scale image of the Ti₂O₃ crystal in the [110] zone. The nearest Ti columns show dumbbell arrangement and the dumbbells are separated by 1.5 Å as shown by the blue arrows in the corresponding atomic model of the Ti₂O₃ structure in Fig. 4(b). The neighboring parallel dumbbell rows are shifted by ∼1.35 Å as measured from the STEM image in Fig. 4(a), which is also shown in the atomic model in Fig. 4(b). The (001) and (−114) planes are frequently observed as the terminating planes in Ti₂O₃ crystals in our experiments. In the atomic model shown in Fig. 4(b), the (001) plane is shown by the blue line, and the (−114) plane is shown by the red line.

The surface terminations are often very important in determining the chemical properties of the materials. In order to understand and fully appreciate the findings of electron microscopy analysis, we calculated the surface energies of these terminating planes in Ti₂O₃ using density functional theory (DFT) as implemented in the SIESTA code.¹⁵ The STEM experimental images cannot identify the oxygen atoms due to their minimum contrast. Therefore, we consider all the possible terminations of (001), (012), (−114), and (1–20) surfaces of Ti₂O₃. The calculated cleavage energies, relaxation energies, and surface energies are summarized in Table I. The broken Ti-O bond was necessary to create the surfaces. The cleavage energy is related to the number of bonds breaking per unit area on the cleavage surface. The relaxation energy is defined as the energy difference after relaxation with respect to the ideal surface. As shown in Fig. 5, all surfaces have multiple termination surfaces of different atomic configurations. For example, the (012) surface can be terminated by O, Ti, or TiO depending on where the cleavage occurs. The (012) surfaces terminated by O and Ti are complementary mutually. The (012) surface terminated by the TiO layer is complementary mutually with itself, while the (−114) surfaces can be terminated by O and TiO type I and II surfaces as cleaved at different atomic configuration surfaces. In the (001) surface with O termination, the surface energy is very low ∼1.02 J/m². However, the same (001) surface with Ti termination has a very high surface energy of 4.95 J/m². Therefore, we can conclude that the (001) surface observed in our experimental images should have oxygen termination. For (012) surfaces, there are three possibilities: O termination, Ti termination, and TiO termination as shown in Fig. 5(b). The surface with TiO termination shows the lowest surface energy of 1.6 J/m². In the case of the (−114) surface, the surface with O termination shows the lowest surface energy of 1.53 J/m² compared to 2.36 and 2.42 J/m² for the surfaces with TiO type I and II terminations. As for the case of (1–20) surface terminations, the surface with O-type I termination shows the lowest surface energy of 1.98 J/m². In conclusion, the (001) O-termination, (012) TiO-termination, (−114) O-termination, and (1–20) O-type I terminations have low surface energies. We also performed further surface energy calculations on other low index termination surfaces such as (100) and (110) surfaces. Their surface energies are generally much higher than that of the (012) surface, which may be the reason why we did not observe these (100) and (110) surfaces during TEM examination.

It can be seen from Fig. 5 that, at the (001) and (1–20) surfaces, the numbers of bonds per unit area are large, and therefore, a higher amount of energy is needed to break the Ti-O bonds along these two planes. Thus, these surfaces have the largest cleavage energies of 5.21 and 6.60 J/m², respectively. The (012)-TiO terminated surface shows the smallest cleavage energy and very low surface energies of 1.60 J/m². Therefore, we can conclude that the (012) surface with TiO-termination is the most stable and prevalent surface of Ti₂O₃.

We have found the formation of the Ti₂O₃ parasitic phase during the synthesis of TiO₂ coating on the cathode materials. Detailed STEM structural analysis along different zone axes has revealed the atomic configurations of the dominating termination planes of the Ti₂O₃ crystal. In combination with the density functional theory calculation of the surface energies of these surfaces, it has been found that the (001) O-termination, (012) TiO-termination, (−114) O-termination, and (1–20) O-type I terminations possess the low surface energies. These observations provide fundamental data for exploring the physical and chemical processes that are governed by the interface controlled process, typically such as the solid state interphase layer in rechargeable batteries.

See supplementary material for detailed calculation description and additional TEM images of these particles.

This work was supported by the start-up funding from the Southern University of Science and Technology (Project No. Y01256127). Part of the work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract No. DE-AC05-76RLO1830. Z.W. was financially supported by the National Natural Science Foundation of China (11474047).