Interface Structure, Band Alignment, and Built-In Potentials at LaFeO_{3}/n-SrTiO_{3} Heterojunctions

Enhancement of multilayer lithium storage in a β12-borophene/graphene heterostructure with built-in dipoles

The combination of borophene with a supporting metallic layer is beneficial in stabilizing its structure and promoting its application in energy storage. Here, through first-principles calculations, we screen a β12-borophene/graphene (β12-B/G) heterostructure with superior structural integrity, strong interlayer binding, and high thermodynamic stability among different B/G heterostructures. Besides, it is noteworthy that β12-B/G has been recently synthesized, further opening the possibility of expanding its use in energy storage. Then the selected target is systematically investigated as an anode material for lithium-ion batteries (LIBs). Compared with each monolayer component, multiple lithium-ion adsorption is achieved in the β12-B/G heterostructure, resulting in an ultra-high theoretical specific capacity of 2267 mA h g-1. In addition, a lower diffusion energy barrier indicates faster electron transport and lithium-ion diffusion in the β12-B/G heterostructure. Notably, the multilayer lithium adsorption avoids the formation of dendritic deposits, as evidenced by complete ionization of the cationic layers. Moreover, the disparity in the work functions of the individual layers gives rise to a built-in dipole in β12-B/G, further enhancing the multilayer lithium storage and ion migration. All these results suggest that the construction of borophene-based heterostructures with built-in dipoles is a feasible way to design high-performance LIB anode materials.

Multiple roles for LaFeO3 in enhancing the Photoelectrochemical performance of WO3

For photoelectrochemical (PEC) water splitting, constructing heterojunctions and loading co-catalysts are effective means to realizing sufficient light absorption, effective photogenerated carrier separation and fast charge transport. However, during implementation, the PEC performance of the catalyst is affected by both parasitic light absorption and reflection and the change in energy band structure due to the creation of new interfaces. Herein, in order to minimize the effect of recombination of photogenerated electron-hole pairs on the catalyst PEC performance due to the nascent interface arising from the co-catalyst compounding, WO3 and Ni/Co co-doped LaFeO3 (LFO) are constructed as heterojunctions, in which NiCo-LFO acts both as a part of the heterojunction to enhance photogenerated carrier separation and a co-catalyst to enhance the conductivity and modulate the surface state density at the catalyst-electrolyte interface. The current density of NiCo-LFO/WO3 reaches 3.92 mA cm-2, which is more than 7 times that of LFO/WO3. This work provides a reference for the efficient water splitting of B-site doped, especially the co-doped perovskite oxide as multifunctional roles integrated with conventional photoelectrodes.

Oxidic 2D Materials

The possibility of producing stable thin films, only a few atomic layers thick, from a variety of materials beyond graphene has led to two-dimensional (2D) materials being studied intensively in recent years. By reducing the layer thickness and approaching the crystallographic monolayer limit, a variety of unexpected and technologically relevant property phenomena were observed, which also depend on the subsequent arrangement and possible combination of individual layers to form heterostructures. These properties can be specifically used for the development of multifunctional devices, meeting the requirements of the advancing miniaturization of modern manufacturing technologies and the associated need to stabilize physical states even below critical layer thicknesses of conventional materials in the fields of electronics, magnetism and energy conversion. Differences in the structure of potential two-dimensional materials result in decisive influences on possible growth methods and possibilities for subsequent transfer of the thin films. In this review, we focus on recent advances in the rapidly growing field of two-dimensional materials, highlighting those with oxidic crystal structure like perovskites, garnets and spinels. In addition to a selection of well-established growth techniques and approaches for thin film transfer, we evaluate in detail their application potential as free-standing monolayers, bilayers and multilayers in a wide range of advanced technological applications. Finally, we provide suggestions for future developments of this promising research field in consideration of current challenges regarding scalability and structural stability of ultra-thin films.

The origins of charge separation in anisotropic facet photocatalysts investigated through first-principles calculations

It was recently discovered that the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can be completed on the {110} and {001} facets, respectively, of a 18-facet SrTiO₃ mono-crystal. The effective charge separation is attributed to the facet junction at the interface between two arbitrary anisotropic crystal planes. Theoretical estimation of the built-in potential at the facet junction can greatly improve understanding of the mechanism. This work employs density functional theory (DFT) calculations to investigate such potential at the (110)/(100) facet junction in SrTiO₃ crystals. The formation of the facet junction is verified by a calculated work function difference between the (110) and (100) planes, which form p-type and n-type segments of the junction, respectively. The built-in potential is estimated at about 2.9 V. As a result, with the ultra high built-in potential, electrons and holes can effectively transfer to different anisotropic planes to complete both photo-oxidative and photo-reductive reactions.

Extracting band edge profiles at semiconductor heterostructures from hard-x-ray core-level photoelectron spectra

Internal electric fields that underpin functioning of multi-component materials systems and devices are coupled to structural and compositional inhomogeneities associated with interfaces in these systems. Hard-x-ray photoelectron spectroscopy is a valuable source of information on band-edge profiles, governed by the distribution of internal fields, deep inside semiconductor thin films and heterojunctions. However, extracting this information requires robust and physically meaningful decomposition of spectra into contributions from individual atomic planes. We present an approach that utilizes the physical requirements of a monotonic dependence of the built-in electrostatic potential on depth and continuity of the potential function and its derivatives. These constraints enable efficient extraction of band-edge profiles and allow one to capture details of the electronic structure, including determination of the signs and magnitudes of the band bending as well as the valence band offsets. The utility of this approach to generate quantitative insight into the electronic structure of complex materials is illustrated for epitaxial [Formula: see text] on intrinsic Si(001).

Tuning the Electronic Structure of LaNiO3 through Alloying with Strontium to Enhance Oxygen Evolution Activity

The perovskite oxide LaNiO3 is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO3 films (La1- x Sr x NiO3) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at x = 0.5 comparable to the state-of-the-art catalyst Ba0.5Sr0.5Co0.8Fe0.2O3- δ . By combining X-ray photoemission and X-ray absorption spectroscopies with density functional theory, it is shown that an upward energy shift of the O 2p band relative to the Fermi level occurs with increasing x in La1- x Sr x NiO3. This alloying step strengthens Ni 3d-O 2p hybridization and decreases the charge transfer energy, which in turn accounts for the enhanced OER activity.

Heterocarbides Reinforced Electrochemical Energy Storage

The feasibility of transition metal carbides (TMCs) as promising high-rate electrodes is still hindered by low specific capacity and sluggish charge transfer kinetics. Improving charge transport kinetics motivates research toward directions that would rely on heterostructures. In particular, heterocomposing with carbon-rich TMCs is highly promising for enhancing Li storage. However, due to limited synthesis methods to prepare carbon-rich TMCs, understanding the interfacial interaction effect on the high-rate performance of TMCs is often neglected. In this work, a novel strategy is proposed to construct a binary carbide heteroelectrode, i.e. incorporating the carbon-rich TMC (M=Mo) with its metal-rich TMC nanowires (nws) via an ingenious in situ disproportionation reaction. Results show that the as-prepared MoC-Mo₂ C-heteronanowires (hnws) electrode could fully recover its capacity after high-rates testing, and also possesses better lithium accommodation performance. Kinetic analysis verified that both electron and ion transfer in MoC-Mo₂ C-hnws are superior to those of its singular counterparts. Such improvements suggest that by taking utilization of the interfacial component interactions of stoichiometry tunable heterocarbides, the electrochemical performance, especially high-rate capability of carbides, could be significantly enhanced.

Influence of LaFeO3 Surface Termination on Water Reactivity

The polarity of oxide surfaces can dramatically impact their surface reactivity, in particular, with polar molecules such as water. The surface species that result from this interaction change the oxide electronic structure and chemical reactivity in applications such as photoelectrochemistry but are challenging to probe experimentally. Here, we report a detailed study of the surface chemistry and electronic structure of the perovskite LaFeO₃ in humid conditions using ambient-pressure X-ray photoelectron spectroscopy. Comparing the two possible terminations of the polar (001)-oriented surface, we find that the LaO-terminated surface is more reactive toward water, forming hydroxyl species and adsorbing molecular water at lower relative humidity than its FeO₂-terminated counterpart. However, the FeO₂-terminated surface forms more hydroxyl species during water adsorption at higher humidity, suggesting that adsorbate-adsorbate interactions may impact reactivity. Our results demonstrate how the termination of a complex oxide can dramatically impact its reactivity, providing insight that can aid in the design of catalyst materials.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films

The effect of A- and B-site cation deficiency on the optical absorption spectrum is presented for a series of LaFeO_3−δ epitaxial films providing insights into the relationship between defect chemistry and electronic structure in this semiconducting perovskite oxide.