Discovery and Characterization of TiSeS-156 and 1T-(TiSeS)2: New Ordered Phases with Enhanced Properties for Optoelectronic and Photocatalytic Applications

Titanium-based solid solution systems, including TiS₂–TiSe₂, has exceptional physical properties with potential applications in electronics, optoelectronics, and energy-harvesting devices. The investigations into ordered phases within the TiS₂–TiSe₂ system remains relatively uncharted territory. The disordered nature of these materials has posed challenges in achieving uniformity and consistency in their properties, which can be critical for practical applications. Disordered phases often result in unpredictable and less stable material behaviors, which can limit their functionality and reliability in technological applications. Moreover, the intrinsic randomness in atomic distribution in disordered phases impedes the precise control over material properties that is necessary for the development of high-performance devices. Recognizing these challenges, Dr. Yue Lou from the Aurora New Energy Materials Research Institute in Hong Kong and Professor Ping Lou from Anhui University conducted a systematic investigation of the ordered phases of the TiS₂–TiSe₂ system. They used advanced computational techniques, such as the particle swarm optimization (CALYPSO) algorithm to uncover new, stable configurations that could offer superior and more predictable properties compared to their disordered counterparts. The new work is now published in Physical Chemistry Chemical Physics.

The CALYPSO algorithm identified stable ordered phases within the TiS2–TiSe₂ system and allowed them to explore a vast array of potential structures by optimizing the atomic configurations to minimize the total energy. That process resulted in identifying two previously unreported ordered phases: TiSeS-156 and 1T-(TiSeS)₂ which were found to possess distinct stacking patterns of S–Ti–Se Janus layers, characterized by a specific arrangement of atoms that differs from the disordered phases. The researchers calculated their phonon dispersions using density functional theory to ascertain the dynamic stability of these new structures, the absence of negative phonon frequencies in the entire Brillouin zone for both TiSeS-156 and 1T-(TiSeS)₂ confirmed their dynamic stability and indicated that the materials remain stable at low temperatures and under thermal vibrations. Additionally, the researchers found that the maximum phonon frequencies of TiSeS-156 and 1T-(TiSeS)₂ were intermediate between those of their parent compounds, 1T-TiS₂ and 1T-TiSe₂, which confirmed the unique structural properties.

They calculated the formation energies of TiSeS-156 and 1T-(TiSeS)₂   to evaluate their chemical stability. These values were found to be negative, indicating that the formation of these ordered phases from elemental Ti, S, and Se is exothermic and, therefore, thermodynamically favorable. Dr. Yue Lou and Professor Ping Lou also assessed the mechanical stability of the new structures by calculating their elastic constants (Cij) and showed that TiSeS-156 and 1T-(TiSeS)₂ met all the mechanical stability criteria, confirming their robustness. The bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio were also computed. These mechanical properties indicated that both ordered phases were brittle, similar to their disordered counterparts, but with improved anisotropy. This anisotropy, measured by the elastic anisotropy ratio, suggested that these materials could be tailored for specific mechanical applications where directional properties are crucial. Moreover, the electronic properties of the identified phases were explored through DFT+U calculations. The band structures revealed that both TiSeS-156 and 1T-(TiSeS)₂ are narrow-gap semiconductors with indirect band gaps. This finding was crucial for potential applications in optoelectronics and photovoltaics, where narrow-gap semiconductors are often desirable. The optical conductivity of these materials was also investigated and found the optical properties were similar to those of 1T-TiS₂ and 1T-(TiSeS)₂ which make them effective in optoelectronic devices, harnessing their semiconducting properties for efficient light absorption and emission. The thermal properties of TiSeS-156 and 1T-(TiSeS)₂  were examined by calculating their specific heat capacities (Cv) at various temperatures. The Cv values for both materials increased with temperature and approached the Dulong–Petit limit at high temperatures, indicating their stability and predictable thermal behavior. These properties are vital for applications requiring thermal management, such as in electronic devices where heat dissipation is a critical factor. To further validate the thermal stability of the new structures, quantum molecular dynamics simulations were performed. These simulations involved subjecting the materials to thermal fluctuations at 300 K for extended periods and the authors showed only minor fluctuations in temperature and energy, with no significant bond fractures or distortions which confirmed the robustness of TiSeS-156 and 1T-(TiSeS)₂ under thermal stress. The thermal stability enhances their suitability for real-world applications where materials must withstand varying temperature conditions. One of the most significant findings of Dr. Yue Lou and Professor Ping Lou was the Janus layer structure of the new phases. The S–Ti–Se layers in TiSeS-156 and 1T-(TiSeS)₂ created an intrinsic electrostatic dipole due to the asymmetry in atomic composition. This unique feature was shown to enhance photocatalytic activity by facilitating the separation of photogenerated electron-hole pairs, thereby improving the efficiency of photovoltaic devices and photocatalysts. The study highlighted that these Janus layers provide a pathway for preparing high-performance monolayer and multilayer materials with tailored properties for specific technological applications.

In conclusion, Yue Lou and Professor Ping Lou successfully identified new ordered phases, TiSeS-156 and 1T-(TiSeS)₂ that exhibited more predictable and stable properties compared to the traditionally studied disordered phases which make them suitable for developing materials with specific and reliable characteristics.  Janus layers have intrinsic electrostatic dipoles due to their asymmetric structure, which can dramatically enhance properties such as photocatalytic activity and charge separation efficiency in photovoltaic applications. Moreover, the findings of narrow band gap and semiconducting nature of TiSeS-156 and 1T-(TiSeS)₂ make them ideal candidates for optoelectronic devices and photovoltaic cells. Furthermore, the Janus layer structure improves the efficiency of photocatalytic processes, which are essential for applications like water splitting and environmental purification. Additionally, another important implication is the demonstration of predictable thermal properties and high stability under thermal stress make these materials suitable for electronic devices where effective thermal management is critical. This could lead to the development of more reliable and durable electronic components.

Acknowledgement: The prototype of stable ordered phases of solid solution 1T-TiSeS has been created by Aurora New Energy for further evaluation and improvement for the eventual application.