Tuning Octahedral Tilting by Doping to Prevent Detrimental Phase Transition and Extend Carrier Lifetime in Organometallic Perovskites.

As one of the most promising materials for next-generation solar cells, organometallic perovskites have attracted substantial fundamental and applied interest. Using first-principles quantum dynamics calculations, we show that octahedral tilting plays an important role in stabilizing perovskite structures and extending carrier lifetimes. Doping the material with (K, Rb, Cs) ions at the A-site enhances octahedral tilting and the stability of the system relative to unfavorable phases. The stability of doped perovskites is maximized for uniform distribution of the dopants. Conversely, aggregation of dopants in the system inhibits octahedral tilting and the associated stabilization. The simulations also indicate that with enhanced octahedral tilting, the fundamental band gap increases, the coherence time and nonadiabatic coupling decrease, and the carrier lifetimes are thus extended. Our theoretical work uncovers and quantifies the heteroatom-doping stabilization mechanisms, opening up new avenues to enhancing the optical performance of organometallic perovskites.

Well-Dispersed Ruthenium in Mesoporous Crystal TiO2 as an Advanced Electrocatalyst for Hydrogen Evolution Reaction.

TiO2 mesoporous crystal has been prepared by one-step corroding process via an oriented attachment (OA) mechanism with SrTiO3 as precursor. High resolution transmission electron microscopy (HRTEM) and nitrogen adsorption-desorption isotherms confirm its mesoporous crystal structure. Well-dispersed ruthenium (Ru) in the mesoporous nanocrystal TiO2 can be attained by the same process using Ru-doped precursor SrTi1- xRu xO3. Ru is doped into lattice of TiO2, which is identified by HRTEM and super energy dispersive spectrometer (super-EDS) elemental mapping. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) suggest the pentavalent Ru but not tetravalent, while partial Ti in TiO2 accept an electron from Ru and become Ti3+, which is observed for the first time. This Ru-doped TiO2 performs high activity for electrocatalytic hydrogen evolution reaction (HER) in alkaline solution. First-principles calculations simulate the HER process and prove TiO2:Ru with Ru5+ and Ti3+ holds high HER activity with appropriate hydrogen-adsorption Gibbs free energies (Δ GH).

The Role of Thermal Fluctuations and Vibrational Entropy: A Theoretical Insight into the δ-to-α Transition of FAPbI3.

Formamidinium lead iodide as a typical organometal perovskite has attracted considerable interest due to its suitable electronic structure. However, the intrinsic mechanisms of its unwanted δ-to-α phase transition remain elusive. By combined first-principles calculations, lattice dynamics analysis, and molecular dynamics simulations, we assign the α phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emerging as a dynamically unstable maximum in the system's potential energy landscape. Finite-temperature Gibbs free energy calculations confirm that the δ-to-α transition should be considered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, phonon thermal property calculations indicate that the driving force of the process is the vibrational entropy difference. These results point out the dynamical nature of the α phase and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for the successful uptake of organometal perovskites in commercial applications.

Charge-Transfer-Promoted High Oxygen Evolution Activity of Co@Co9S8 Core-Shell Nanochains.

Co@Co9S8 nanochains with core-shell structures are prepared by a direct-current arc-discharge technique and followed sulfurization at 200 °C. The nanochains, which consist of uniform nanospheres connecting each other, can range up to several micrometers. The thickness of Co9S8 shell can be changed by regulating the sulfurization time. In this heterostructure of Co@Co9S8, Co nanochains function as a conductive network and can inject electrons into Co9S8, which manipulates the work function of Co9S8 and makes it more apposite for catalysis. The density functional theory calculation also reveals that coupling with Co can significantly reduce the overpotential needed to drive the oxygen evolution process. On the basis of the exclusive structure, Co@Co9S8 nanochains have shown high catalytic activity in the oxygen evolution reaction. Co@Co9S8 reaches an overpotential of 285 mv at 10 mA cm-2, which is much lower than that of Co nanochains (408 mV) and Co9S8 (418 mV). Co@Co9S8 also shows higher catalytic activity and robustness compared to state-of-the-art noble-metal catalyst RuO2.

Near-Infrared Light Manipulated Chemoselective Reductions Enabled by an Upconversional Supersandwich Nanostructure.

Core-satellite is one of the most powerful superstructures since it leads to enhanced or completely new properties through compatible combination of each component. Here we create a novel ceria-based core-shell-satellite supersandwich structure with near-infrared (NIR) light manipulated catalytic activity by integrating the upconversion luminescent and catalytic functionality of CeO2 nanoparticles. Specifically, lanthanide-doped octahedral CeO2 nanoparticles (o-CeO2) are coated with silica layer (o-CeO2@SiO2) to enhance their luminescence intensity. The pH-dependent catalytic active cubic CeO2 nanoparticles (c-CeO2) are then assembled on the surface of o-CeO2@SiO2 to form the supersandwich structure (o-CeO2@SiO2@c-CeO2) following a classic chemical reaction. The upconversion quantum yield of o-CeO2 in this nanostructure can be nearly doubled. Furthermore, under NIR light irradiation, the o-CeO2@SiO2@c-CeO2 supersandwich structure based composite catalyst displays superior catalytic activity in selective reduction of aromatic nitro compounds to corresponding azo compounds, and the composite photocatalyst can be easily recycled for several times without significant loss of catalytic activity. This strategy may serve as a universal method for the construction of multifunctional nanostructures and shed light on the green chemistry for chemical synthesis.