Surface Electronic States Research Articles

Regulating surface wettability and electronic state of molybdenum carbide for improved hydrogen evolution reaction

Molybdenum carbide (Mo2C) is a cost-effective transition metal carbides (TMCs) electrocatalyst for hydrogen evolution reaction (HER) due to its electronic structure similar to Pt-group noble metal. Herein, we report that an effective strategy of regulating the surface wettability and electronic state of Mo2C by ammonia and hydrothermal co-treatment to enhance its HER activity. The electrochemical results demonstrate that Mo2C undergoing ammonia and hydrothermal co-treatment (Mo2C-wh) exhibits remarkably improved electrocatalytic HER activity as compared to the pristine Mo2C (Mo2C-p) catalyst. The activity-structure relationship studies manifest that ammonia and hydrothermal co-treatment increases the content of hydroxyl group and pyridinic-N, thus endowing Mo2C-wh with lower charge transfer resistance, larger electrochemical active surface area and higher surface wettability. DFT calculations reveal that ammonia and hydrothermal co-treatment enhances the Mo 3d-band center and reduces the hydrogen adsorption free energy. These changes in electronic states of Mo sites and physical properties of Mo2C positively contribute to the improvement of electrocatalytic HER activity.

Plasmon-enhanced photocatalysis using gold nanoparticles encapsulated in nanoscale molybdenum oxide shell

Using solar energy to enhance the transformation rate of organic molecules is a promising strategy to advance chemical synthesis and environmental remediation. Plasmonic nanoparticles responsive to sunlight show great promise in the catalysis of chemical reactions. In this work, we used a straightforward wet-chemistry method to synthesize plasmonic octahedral gold nanoparticles (NPs) coated with thin molybdenum oxide (MoO3−x ), Au@MoO3−x NPs, which exhibited strong surface plasmon resonance in a broad wavelength range. The synthesized Au@MoO3−x NPs were characterized by UV–vis, SEM, TEM, EDS, XPS, and the electrochemical technique of cyclic voltammetry (CV). The catalytic performance of Au@MoO3−x NPs under visible light irradiation was investigated using the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction. The presence of a thin capping layer of MoO3−x on our Au NPs contributed to the broadening of their range of absorption of visible light, resulting in a stronger intra-particle plasmonic resonance and the modulation of surface energy and electronic state. Accordingly, the kinetics of plasmon photocatalytic transformation of 4-NP to 4-AP was significantly accelerated (by a factor of 8.1) under visible light, compared to uncapped Au NPs in the dark. Our as-synthesized Au@MoO3−x NPs is an example that the range of plasmonic wavelengths of NPs can be effectively broadened by coating them with another plasmon-active (semiconducting) material, which substantially improves their plasmonic photocatalytic performance. Meanwhile, the synthesized Au@MoO3−x NPs can be used to accelerate the transformation of organic molecules under visible light irradiation.

The Role of M3+ Substitutional Doping (M = In, Sb, Bi) in the Passivation of the α-CsPbI3(100) Surface

Photovoltaic applications based on perovskite materials still face several challenges to deliver high power conversion efficiency, in particular the chemical passivation of surfaces, which is required to remove surface electronic states. In this work, we propose a surface passivation route for the α-CsPbI3(100) surface through substitutional doping at the Pb sites by trivalent cations such as In, Sb, or Bi. Our ab initio investigation was based on density functional theory calculations. We found a preferential doping position at surface octahedrons by more than 0.35 eV/dopant in comparison with inner layers as well as a bandgap decrease due to surface states and optical bandgaps wider than fundamental ones as a result of dark transitions. The surface passivation with Bi atoms demonstrates to be the most effective one, with enhanced sunlight absorption when compared to the 3D bulk α-CsPbI3. Furthermore, we changed the doping concentration and demonstrate the possibility to combine trivalent cations with other passivating mechanisms. Therefore, our computational results disclose a promising route for saturating perovskite surfaces.

2H-MoS 2 Modified Nitrogen-Doped Hollow Mesoporous Carbon Spheres as the Efficient Catalytic Cathode Catalyst for Aprotic Lithium-Oxygen Batteries

2H-MoS ₂ Modified Nitrogen-Doped Hollow Mesoporous Carbon Spheres as the Efficient Catalytic Cathode Catalyst for Aprotic Lithium-Oxygen Batteries

Thermally Accelerated Surface Polaron Hopping in Photoelectrochemical Water Splitting.

Electron-hole separation is a main challenge that limits the energy efficiency of photoelectrochemical water splitting for hydrogen fuel production. Surface polaron states with an energy level distribution near the conduction band are highly efficient charge separation passageways to massively accept or transfer the photogenerated electrons. Here, we found that the charge separation via surface polaron states could be further enhanced by heating (<100 °C) to accelerate the electron mobility of surface polaron states. As a result of heating from 30 to 70 °C, the saturated photocurrent increased about 34.5% under 1 sun and 18.3% under 10 suns from heat-induced increase in electron flux of surface polaron states. The heat-sensitive surface-state electron transfer provides a new heat-photoelectricity coupling mechanism to guide the design of new photoanodes that are available for complementary multienergy systems with high energy efficiency.

Activation of 2D titanate nanosheet photocatalysts by nitrogen doping and solution plasma processing.

Novel two-dimensional (2D) oxides are of great interest for photocatalysis because of their superlative physical features, namely, large surface areas, short charge diffusion pathways, high crystallinity and easy surface modification. However, most 2D oxides suffer from weak visible light absorption and severe photogenerated carrier recombination. Nitrogen doping can successfully narrow the bandgap of 2D oxides but can hardly improve the charge separation. In this work, we pre-dope nitrogen into 2D titanate nanosheets (HTiO), followed by surface processing with solution plasma. By dual modification of nitrogen doping and solution plasma processing (SPP), the modified 2D titanate nanosheets (N-HTiO-SPP) display broad absorption extending to the visible light region and the healing of oxygen vacancies brought about by nitrogen doping. Compared with HTiO and nitrogen doped titanate (N-HTiO), a higher removal rate and mineralization rate towards the photocatalytic degradation of acetaldehyde were achieved over N-HTiO-SPP under solar light. This work provides a powerful way to activate 2D wide bandgap semiconductors for enhanced photocatalytic activity.

Sensors on the surface acoustic waves for intelligent systems

The work is aimed at the study of surface processes on the dynamically deformed adsorbed surface of semiconductors, which will be used as a sensitive substrate in radiometric temperature sensors. The choice of semiconductors with a zinc blende structure is explained by the sensitivity of such electronic subsystem to the deformation of the crystal lattice, which can be caused by the self-consistent redistribution of defects, inconsistency of the parameters of the crystal lattice, or external factors, for example, the influence of mechanical or electric fields. Based on established regularities of the influence of the concentration and type of adsorbed atoms on the spectrum of surface electronic states and the distribution of electron density on the dynamically deformed adsorbed surface of a single crystal, the development of a new class of intelligent sensors with increased accuracy of measuring the concentration of adsorbed atoms and temperature on surface acoustic waves is proposed. Such a new approach is based on the self-consistent effect of the deformation of the crystal lattice on the dispersion law and the spectral width of the phonon mode, the electric charge density, and the energy displacement of the edges of the allowed zones. It is calculated the temperature-concentration coefficient of the resonance frequency of the surface acoustic wave and the regularities of its change depending on the concentration of adsorbed atoms are established. The relevance of this research is determined both by the needs of fundamental research and by applied aspects of development, optimization and cost reduction of the process of designing and creating devices, the functioning of which is carried out on surface acoustic waves.

Engineering quantum states and electronic landscapes through surface molecular nanoarchitectures

Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scattering and confinement. Both atomic manipulation and supramolecular principles provide access to custom-designed molecular superlattices, which tailor the surface electronic landscape and influence fundamental chemical and physical properties at the nanoscale. Herein, we review the confinement of surface state electrons focusing on their interaction with molecule-based scaffolds created by molecular manipulation and self-assembly protocols under ultrahigh vacuum conditions. Starting from the quasi-free 2D electron gas present at the (111)-terminated surface planes of noble metals, we illustrate the enhanced molecule-based structural complexity and versatility compared to simple atoms. We survey low-dimensional confining structures in the form of artificial lattices, molecular nanogratings or quantum dot arrays, which are constructed upon appropriate choice of their building constituents. Whenever the realized (metal-)organic networks exhibit long-range order, modified surface band structures with characteristic features emerge, revealing intriguing physical properties, such as discretization, quantum coupling or energy and effective mass renormalization. Such collective electronic states can be additionally modified by positioning guest species at the voids of open nanoarchitectures [...].

Hydrogen‐Assisted Synthesis of Large‐Size 2D Bismuth Telluride Flakes for Broadband Photodetection up to 2 µm

AbstractBroadband photodetection including optical communication wavelength band is of great significance because of the great potential for various civilian and military applications. Topological insulators with unique surface electronic states have attracted more and more attention in broadband photodetection. Among them, due to its suitable band gap (0.16–1.36 eV), ultra‐high mobility, and excellent air stability, 2D bismuth telluride (Bi2Te3) is regarded as a promising candidate. However, the growth of large‐size Bi2Te3 is challenging because of the poor chemical reactivity of tellurium (Te) element. Here, large‐scale (240 µm, ten times larger than the current record) Bi2Te3 flakes with controllable thickness (one to few layers) are successfully synthesized by H2‐assisted chemical vapor deposition. Thanks to the strong reducibility and the passivation on the substrate of H2, the growth rate is as high as 48 µm min−1, which is ten times faster than that reported. Moreover, the Bi2Te3‐photodetector exhibits broadband detection (520–2000 nm), high responsivity of 252 A W−1, excellent detectivity of 5.1 × 1011 cm Hz1/2 W−1, and fast response time (τrise/τdecay ≈ 41/38 ms), implying potential applications in broadband optoelectronics. More importantly, this work may pave a way for the rapid growth of other novel large‐scale 2D materials.

Surface Electrons: Theory and Electrohydrodynamic Applications

The paper presents the theory of surface electronic states induced by a high-voltage field and considers applications of the theory of surface electrons to electrohydrodynamic flows. Theoretical considerations were carried out within the framework of a one-electron quantum mechanical model. It is shown that surface electrons initiate the injection of negative charg-es from the cathode into a liquid dielectric containing an electron-withdrawing impurity, for example, molecular iodine. Comparison of the theoretic and experimental data is satisfacto-ry

Strengthening the metal-support interaction over Pt/SiO2-TiO(OH)2 by defect engineering for efficient dehydrogenation of dodecahydro-N-ethylcarbazole

Liquid organic hydrogen carriers (LOHC) are considered as the promising candidates for large-scale hydrogen storage. The N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) system has received increasing attention due to its excellent comprehensive properties in the hydrogen storage candidates. In this work, the Pt/SiO2-TiO(OH)2 catalysts are synthesized and show good activity, selectivity and stability for 12H-NECZ dehydrogenation reaction. Among them, the 2.5 wt% Pt/SiO2-TiO(OH)2 catalyst shows the best dehydrogenation performance with 5.75 wt% H2 release amount and 98 % selectivity of NECZ. Combined with XRD, HRTEM, ATR-FTIR, XPS and EPR analysis, it is found that the oxygen vacancy concentration in Pt/SiO2-TiO(OH)2 is significantly enhanced due to the formation of Si-O-Ti species compared with Pt/SiO2 and Pt/TiO(OH)2, which strengthens the SMSI (strong metal-supporting interaction) effect between metal Pt and support SiO2-TiO(OH)2. The optimized d-band center of catalyst is beneficial for accelerating the sluggish kinetics of the rate-limiting step of the whole dehydrogenation reaction. Furthermore, compared with the results by replacing SiO2 with Al2O3 or CeO2, it is indicated that the introduction of oxides mainly plays the role of regulating the surface electronic state of TiO(OH)2 and increasing the oxygen vacancy concentration.

Interface Metal Oxides Regulating Electronic State around Nickel Species for Efficient Alkaline Hydrogen Electrocatalysis.

Heterogeneous electrocatalysis typically depends on the surface electronic states of active sites. Modulating the surface charge state of an electrocatalysts can be employed to improve performance. Among all the investigated materials, nickel (Ni)-based catalysts are the only non-noble-metal-based alternatives for both hydrogen oxidation and evolution reactions (HOR and HER) in alkaline electrolyte, while their activities should be further improved because of the unfavorable hydrogen adsorption behavior. Hereto, Ni with exceptional HOR electrocatalytic performance by changing the d-band center by metal oxides interface coupling formed in situ is endowed. The resultant MoO2 coupled Ni heterostructures exhibit an apparent HOR activity, even approaching to that of commercial 20% Pt/C benchmark, but with better long-term stability in alkaline electrolyte. An exceptional HER performance is also achieved by the Ni-MoO2 heterostructures. The experiment results are rationalized by the theoretical calculations, which indicate that coupling MoO2 with Ni results in the downshift of d-band center of Ni, and thus weakens hydrogen adsorption and benefits for hydroxyl adsorption. This concept is further proved by other metal oxides (e.g., CeO2 , V2 O3 , WO3 , Cr2 O3 )-formed Ni-based heterostructures to engineer efficient hydrogen electrocatalysts.

Electronic Band Structure and Surface States in Dirac Semimetal LaAgSb2.

LaAgSb is a Dirac semimetal showing charge density wave (CDW) order. Previous angle-resolved photoemission spectroscopy (ARPES) results suggest the existence of the Dirac-cone-like structure in the vicinity of the Fermi level along the –M direction. This paper is devoted to a complex analysis of the electronic band structure of LaAgSb by means of ARPES and theoretical studies within the ab initio method as well as tight binding model formulation. To investigate the possible surface states, we performed the direct DFT slab calculation and the surface Green function calculation for the (001) surface. The appearance of the surface states, which depends strongly on the surface, points to the conclusion that LaSb termination is realized in the cleaved crystals. Moreover, the surface states predicted by our calculations at the and X points are found by ARPES. Nodal lines, which exist along the X–R and M–A paths due to crystal symmetry, are also observed experimentally. The calculations reveal other nodal lines, which originate from the vanishing of spin–orbit splitting and are located at the X–M–A–R plane at the Brillouin zone boundary. In addition, we analyze the band structure along the –M path to verify whether Dirac surface states can be expected. Their appearance in this region is not confirmed.

Engineering the electronic structure of high performance FeCo bimetallic cathode catalysts for microbial fuel cell application in treating wastewater

The development of high-performance, strong-durability and low-cost cathode catalysts toward oxygen reduction reaction (ORR) is of great significance for microbial fuel cells (MFCs). In this study, a series of bimetallic catalysts were synthesized by pyrolyzing a mixture of g-C3N4 and Fe, Co-tannic complex with various Fe/Co atomic ratios. The initial Fe/Co atomic ratio (3.5:0.5, 3:1, 2:2, 1:3) could regulate the electronic state, which effectively promoted the intrinsic electrocatalytic ORR activity. The alloy metal particles and metal-Nx sites presented on the catalyst surface. In addition, N-doped carbon interconnected network consisting of graphene-like and bamboo-like carbon nanotube structure derived from g-C3N4 provided more accessible active sites. The resultant Fe3Co1 catalyst calcined at 700 °C (Fe3Co1-700) exhibited high catalytic performance in neutral electrolyte with a half-wave potential of 0.661 V, exceeding that of the commercial Pt/C (0.6 V). As expected, the single chamber microbial fuel cell (SCMFC) with 1 mg/cm2 loading of Fe3Co1-700 catalyst as the cathode catalyst afforded a maximum power density of 1425 mW/m2, which was 10.5% higher than commercial Pt/C catalyst with the same loading (1290 mW/m2) and comparable to the Pt/C catalyst with 2.5 times higher loading ( 1430 mW/m2). Additionally, the Fe3Co1-700 also displayed better long-term stability over 1100 h than the Pt/C. This work provides an effective strategy for regulating the surface electronic state in the bimetallic electro-catalyst.

Topological electronic bands in crystalline solids

Topology is now securely established as a means to explore and classify electronic states in crystalline solids. This review provides a gentle but firm introduction to topological electronic band structure suitable for new researchers in the field. I begin by outlining the relevant concepts from topology, then give a summary of the theory of non-interacting electrons in periodic potentials. Next, I explain the concepts of the Berry phase and Berry curvature, and derive key formulae. The remainder of the article deals with how these ideas are applied to classify crystalline solids according to the topology of the electronic states, and the implications for observable properties. Among the topics covered are the role of symmetry in determining band degeneracies in momentum space, the Chern number and topological invariants, surface electronic states, two- and three-dimensional topological insulators, and Weyl and Dirac semimetals

Temperature dependence of the hyperfine parameters on Fe3O4(111) surfaces

The Verwey transition on Fe3O4(111) surface was investigated using nuclear resonant x-ray scattering (NRS). Measurements were conducted below (85 K) and above (297 and 500 K) the Verwey transition temperature. The internal magnetic field estimated from the NRS frequency spectra agreed with that of bulk Fe3O4 at all measured temperatures. A clear change in the intensity ratio of the two peaks in the frequency spectra was observed across the Verwey transition. The results indicate that despite the presence of the peculiar surface electronic states which makes the changes in the electronic properties across the Verwey transition temperature blurred, the hyperfine parameters of the (111) surface show a clear change across the Verwey transition temperature.

Surface state evolution induced by magnetic order in axion insulator candidate EuIn2As2

Gapping of Dirac surface states through time reversal symmetry breaking may realize the axion insulator state in condensed matter. Despite tremendous efforts, only a few material systems fall in this category of intrinsic magnetic topological insulators (TI). Recent theoretical calculations proposed the antiferromagnetic EuIn$_2$As$_2$ to be a topologically non-trivial magnetic insulator with gapped surface states. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) complemented with density-functional theory (DFT) calculations and modelling to probe the surface electronic states in EuIn$_2$As$_2$. We find a spin-orbit induced bulk gap of ~120 meV located only a few meV above the Fermi energy, within which topological surface states reside. Temperature dependent measurements provide evidence of the partial gapping (~40 meV) of the surface states at low temperatures below the AFM order, which decreases with increasing temperature but remains finite above $T_N$

Carrier properties of Bi(111) grown on mica and Si(111)

A comparison is presented between properties of Bi(111) films grown by Stranski-Krastanov epitaxy on Si(111) 7 $\ifmmode\times\else\texttimes\fi{}$ 7 and by van der Waals epitaxy on mica substrates. Thin film morphologies and electronic transport properties of Bi(111) films of variable thickness are investigated for each growth method. Atomic force micrographs for Bi(111) films on mica reveal clearly defined triangular regions consisting of layered steps with height 0.4 nm, corresponding to the Bi(111) bilayer height. Variable-temperature electronic transport measurements show the existence of a quantum confinement-induced energy gap in the film interiors, resulting in a semimetal-to-semiconductor transition. Magnetotransport analysis in a three-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provides a detailed study of densities, mobilities, and mean-free paths of the three carrier types. Improved electronic transport properties are found in Bi(111) films of higher thickness on mica compared to on Si(111), a likely result of the largely strain-free van der Waals epitaxial growth.

Breakdown of bulk-projected isotropy in surface electronic states of topological Kondo insulator SmB6(001)

The topology and spin-orbital polarization of two-dimensional (2D) surface electronic states have been extensively studied in this decade. One major interest in them is their close relationship with the parities of the bulk (3D) electronic states. In this context, the surface is often regarded as a simple truncation of the bulk crystal. Here we show breakdown of the bulk-related in-plane rotation symmetry in the topological surface states (TSSs) of the Kondo insulator SmB6. Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB6(001)-p(2 × 2) surface showed that TSSs are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry maintained in the bulk. This result emphasizes the important role of the surface atomic structure even in TSSs. Moreover, it suggests that the engineering of surface atomic structure could provide a new pathway to tailor various properties among TSSs, such as anisotropic surface conductivity, nesting of surface Fermi contours, or the number and position of van Hove singularities in 2D reciprocal space.

Passivation of Mid-Gap Electronic States at Calcium Aluminosilicate Glass Surfaces upon Water Exposure: An Ab Initio Study.

When a clean glass surface is exposed to humid air, a thin water layer forms on the hydrophilic surface. Using ab initio molecular dynamics, we simulate the changes in the electronic structure of a CaO-Al2O3-SiO2 glass model upon vacuum fracture and subsequent exposure to H2O. When the glass is fractured, dangling bonds form, which lower the band gap of the surface by ∼1.8 eV compared to the bulk value due to mid-gap surface states. When H2O adsorbs onto the vacuum-fractured surface, the band gap increases to a value closer to that of the bulk band gap. Using two different hydroxylation methods, we find that the calculated band gap of the glass surface depends on the hydroxylation state. Surfaces with ∼4.5 OH/nm2 have smaller band gaps due to unfilled surface states, and surfaces with ∼2.5 OH/nm2 have larger band gaps with no apparent unfilled surface states. The resulting changes in the electronic structure, quantified by electron affinity and work function values, are hypothesized to play an important role in the electrostatic charge transfer based on the principles of surface state theory, which posit that the density of electronic surface states determines the amount of electronic charge transfer to or from material surfaces.