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Efficient electrocatalysts for hydrogen evolution reaction are key to realize clean hydrogen production through water splitting. As an important family of functional materials, transition metal oxides are generally believed inactive towards hydrogen evolution reaction, although many of them show high activity for oxygen evolution reaction. Here we report the remarkable electrocatalytic activity for hydrogen evolution reaction of a layered metal oxide, Ruddlesden−Popper-type Sr 2 RuO 4 with alternative perovskite layer and rock-salt SrO layer, in an alkaline solution, which is comparable to those of the best electrocatalysts ever reported. By theoretical calculations, such excellent activity is attributed mainly to an unusual synergistic effect in the layered structure, whereby the (001) SrO-terminated surface cleaved in rock-salt layer facilitates a barrier-free water dissociation while the active apical oxygen site in perovskite layer promotes favorable hydrogen adsorption and evolution. Moreover, the activity of such layered oxide can be further improved by electrochemistry-induced activation. Water may serve as a renewable hydrogen fuel source to replace fossil fuels, although such electrolysis requires highly active catalysts. Here, authors explore Ruddlesden−Popper oxides as hydrogen evolution electrocatalysts that feature an unusual synergistic effect to promote high activity.

The state-of-the-art active HER catalysts in acid media (e.g., Pt) generally lose considerable catalytic performance in alkaline media mainly due to the additional water dissociation step. To address this issue, synergistic hybrid catalysts are always designed by coupling them with metal (hydro)oxides. However, such hybrid systems usually suffer from long reaction path, high cost and complex preparation methods. Here, we discover a single-phase HER catalyst, SrTi 0.7 Ru 0.3 O 3-δ (STRO) perovskite oxide highlighted with an unusual super-exchange effect, which exhibits excellent HER performance in alkaline media via atomic-scale synergistic active centers. With insights from first-principles calculations, the intrinsically synergistic interplays between multiple active centers in STRO are uncovered to accurately catalyze different elementary steps of alkaline HER; namely, the Ti sites facilitates nearly-barrierless water dissociation, Ru sites function favorably for OH* desorption, and non-metal oxygen sites (i.e., oxygen vacancies/lattice oxygen) promotes optimal H* adsorption and H 2 desorption. Efficient electrocatalysts are crucial for the sustainable hydrogen production as an alternative clean fuel. Here, the authors explore a single-phase perovskite oxide as a high-performance hydrogen evolution electrocatalyst via super-exchange induced atomic scale synergistic active sites.

Single Pt atom catalysts are key targets because a high exposure of Pt substantially enhances electrocatalytic activity. In addition, PtRu alloy nanoparticles are the most active catalysts for the methanol oxidation reaction. To combine the exceptional activity of single Pt atom catalysts with an active Ru support we must overcome the synthetic challenge of forming single Pt atoms on noble metal nanoparticles. Here we demonstrate a process that grows and spreads Pt islands on Ru branched nanoparticles to create single-Pt-atom-on-Ru catalysts. By following the spreading process by in situ TEM, we found that the formation of a stable single atom structure is thermodynamically driven by the formation of strong Pt–Ru bonds and the lowering of the surface energy of the Pt islands. The stability of the single-Pt-atom-on-Ru structure and its resilience to CO poisoning result in a high current density and mass activity for the methanol oxidation reaction over time. PtRu nanoparticles are the state-of-the-art catalysts for methanol electrooxidation—the anodic reaction in direct methanol fuel cells. Now, a method of dispersing single Pt atoms over Ru nanoparticles is presented and monitored in situ, thereby boosting the catalytic performance in the methanol oxidation reaction.

Versatile superstructures composed of nanoparticles have recently been prepared using various disassembly methods. However, little information is known on how the structural disassembly influences the catalytic performance of the materials. Here we show how the disassembly of an ordered porous La 0.6 Sr 0.4 MnO 3 perovskite array, to give hexapod mesostructured nanoparticles, exposes a new crystal facet which is more active for catalytic methane combustion. On fragmenting three-dimensionally ordered macroporous (3DOM) structures in a controlled manner, via a process that has been likened to retrosynthesis, hexapod-shaped building blocks can be harvested which possess a mesostructured architecture. The hexapod-shaped perovskite catalyst exhibits excellent low temperature methane oxidation activity ( T 90% =438 °C; reaction rate=4.84 × 10 −7  mol m −2  s −1 ). First principle calculations suggest the fractures, which occur at weak joints within the 3DOM architecture, afford a large area of (001) surface that displays a reduced energy barrier for hydrogen abstraction, thereby facilitating methane oxidation. Disassembly of three-dimensionally ordered materials generates nanoparticles with new structural and physicochemical properties. Here the authors show a fragmentation strategy applied to a perovskite material leading to nanostructures with improved catalytic activity in the methane combustion.

Good electrical conductivity and high electron mobility of the sorbent materials are prerequisite for electrocatalytically switchable CO 2 capture. However, no conductive and easily synthetic sorbent materials are available until now. Here, we examined the possibility of conductive graphitic carbon nitride (g-C 4 N 3 ) nanosheets as sorbent materials for electrocatalytically switchable CO 2 capture. Using first-principle calculations, we found that the adsorption energy of CO 2 molecules on g-C 4 N 3 nanosheets can be dramatically enhanced by injecting extra electrons into the adsorbent. At saturation CO 2 capture coverage, the negatively charged g-C 4 N 3 nanosheets achieve CO 2 capture capacities up to 73.9 × 10 13  cm −2 or 42.3 wt%. In contrast to other CO 2 capture approaches, the process of CO 2 capture/release occurs spontaneously without any energy barriers once extra electrons are introduced or removed and these processes can be simply controlled and reversed by switching on/off the charging voltage. In addition, these negatively charged g-C 4 N 3 nanosheets are highly selective for separating CO 2 from mixtures with CH 4 , H 2 and/or N 2 . These predictions may prove to be instrumental in searching for a new class of experimentally feasible high-capacity CO 2 capture materials with ideal thermodynamics and reversibility.

We employ ıtab-initio calculations to investigate the charge density waves in single-layer NbSe\\(_2\\), and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer NbSe\\(_2\\) in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavoured, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favour the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favour one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favour the reduction of the symmetry of the charge density distribution.

The electronic properties of interphase boundaries are of basic importance for most materials, particularly when those properties deviate strongly from the bulk behavior. We introduce a mechanism that can result in metallicity at stoichiometric interphase boundaries between semiconductors based on the idea of polar catastrophe, which is usually considered only in the context of heterostructures. To this end, we perform ab initio calculations within density functional theory to investigate the electronic states at stoichiometric SnO/SnO2 (110) interphase boundaries. In this system, one would not expect polar catastrophe to have a role according to state-of-the-art theory because the interface lacks formal charge discontinuity. However, we observe the formation of a hole gas between the semiconductors SnO and SnO2 . To explain these findings, we provide a generalized theory based on the idea that the charge density discontinuity between SnO and SnO2 , a consequence of lattice mismatch, drives a polar catastrophe scenario. As a result, SnO/SnO2 (110) interphase boundaries can develop metallicity depending on the grain size. The concept of metallicity due to polar catastrophe induced by charge density discontinuity is of general validity and applies to many interphase boundaries with lattice mismatch.

The suitability of silicon for micro and sub-micro electronic devices is being challenged by the aggressive and continuous downscaling of device feature size. New materials with superior qualities are continually sought-after. In this thesis, defects are examined in two sets of silicon alternate materials; germanium (Ge) and III-V semiconductors. Point defects are of crucial importance in understanding and controlling the properties of these electronic materials. Point defects usually introduce energy levels into the band gap, which influence the electronic performance of the material. They are also key in assisting mass transport. Here, atomistic scale computational methods are employed to investigate the formation and migration of defects in Ge and III-V semiconductors. The behaviour of n-type dopants coupled to a vacancy in Ge (known as E-centres) is reported from thermodynamic and kinetic points of view, revealing that these species are highly mobile, consequently, a strategy is proposed to retard one of the n-dopants. Further, the electronic structure of Ge is examined and the changes induced in it due to the application of different types of strain along different planes and directions. The results obtained agree with established experimental values regarding the bands transition from indirect to direct under biaxial strain. This is used to support further predictions, which indicate that a moderate strain parallel to the [111] direction can efficiently transform Ge into a direct band gap material, with a band gap energy useful for technological applications. Vacancies and antisites in III-V semiconductors have been studied under various growth and doping conditions. Results presented in this thesis help predict and explain the stability of some defects over a range of growth conditions. This, together with knowledge of the kinetics of migration of Ga and As/Sb vacancies is used to explain the disparities in self-diffusion between GaAs and GaSb.

Topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin-orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSH phase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.