Anatase-rutile phase transformation of titanium dioxide bulk material: a DFT + U approach

Autonomous materials synthesis via hierarchical active learning of nonequilibrium phase diagrams

Autonomous experimentation enabled by artificial intelligence offers a new paradigm for accelerating scientific discovery. Nonequilibrium materials synthesis is emblematic of complex, resource-intensive experimentation whose acceleration would be a watershed for materials discovery. We demonstrate accelerated exploration of metastable materials through hierarchical autonomous experimentation governed by the Scientific Autonomous Reasoning Agent (SARA). SARA integrates robotic materials synthesis using lateral gradient laser spike annealing and optical characterization along with a hierarchy of AI methods to map out processing phase diagrams. Efficient exploration of the multidimensional parameter space is achieved with nested active learning cycles built upon advanced machine learning models that incorporate the underlying physics of the experiments and end-to-end uncertainty quantification. We demonstrate SARA’s performance by autonomously mapping synthesis phase boundaries for the Bi₂O₃ system, leading to orders-of-magnitude acceleration in the establishment of a synthesis phase diagram that includes conditions for stabilizing δ-Bi₂O₃ at room temperature, a critical development for electrochemical technologies.

The Hubbard-U correction and optical properties of d0 metal oxide photocatalysts

We report a systematic investigation of individual and multisite Hubbard-U corrections for the electronic, structural, and optical properties of the metal titanate oxide d⁰ photocatalysts SrTiO₃ and rutile/anatase TiO₂. Accurate bandgaps for these materials can be reproduced with local density approximation and generalized gradient approximation exchange-correlation density functionals via a continuous series of empirically derived U^d and U^p combinations, which are relatively insensitive to the choice of functional. On the other hand, lattice parameters are much more sensitive to the choice of U^d and U^p, but in a systematic way that enables the U^d and U^p corrections to be used to qualitatively gauge the extent of self-interaction error in the electron density. Modest U^d corrections (e.g., 4 eV-5 eV) yield the most reliable dielectric response functions for SrTiO₃ and are comparable to the range of U^d values derived via linear response approaches. For r-TiO₂ and a-TiO₂, however, the U^d,p corrections that yield accurate bandgaps fail to accurately describe both the parallel and perpendicular components of the dielectric response function. Analysis of individual U^d and U^p corrections on the optical properties of SrTiO₃ suggests that the most consequential of the two individual corrections is U^d, as it predominately determines the accuracy of the dominant excitation from O-2p to the Ti-3d t_2g/e_g orbitals. U^p, on the other hand, can be used to shift the entire optical response uniformly to higher frequencies. These results will assist high-throughput and machine learning approaches to screening photoactive materials based on d⁰ photocatalysts.

Room-Temperature Ferromagnetism in Transition Metal Embedded Borophene Nanosheets

Exploring two-dimensional (2D) materials with room-temperature ferromagnetism and large perpendicular magnetic anisotropy is highly desirable but challenging. Here, through first-principles calculations, we propose a viable strategy to achieve such materials based on transition metal (TM) embedded borophene nanosheets. Due to electron deficiency, the commonly existent hexagon boron vacancies in various borophene phases serve as intrinsic anchor points for electron-rich transition metals, which not only adsorb strongly upon the vacancies but also favor to be embedded into the vacancies, forming 2D planar hybrid nanosheets. The adsorption-to-embedding transition is feasible thermodynamically and kinetically, owing to its exothermic nature and relatively small kinetic barriers. After embedding, phase transition is further proposed to obtain diverse structures of TM embedded borophenes with versatile magnetic properties. Based on the example of χ₃ phase borophene, several ferromagnetic TM embedded borophene nanosheets with high Curie temperature and large perpendicular magnetic anisotropy have been predicted.

Bandgap reduction of photocatalytic TiO2 nanotube by Cu doping

We performed the electronic structure calculations of Cu-doped TiO₂ nanotubes by using density functional theory aided by the Hubbard correction (DFT + U). Relative positions of the sub-bands due to the dopants in the band diagram are examined to see if they are properly located within the redox interval. The doping is found to tune the material to be a possible candidate for the photocatalyst by making the bandgap accommodated within the visible and infrared range of the solar spectrum. Among several possibilities of the dopant positions, we found that only the case with the dopant located at the center of nanotube seems preventing from electron-hole recombinations to achieve desired photocatalytic activity with n-type behavior.