Surface Electronic States Research Articles

Investigating carrier mobility in hollow and mesoporous ZnSe/ZnTe heterostructures: Microscopic observations of swift charge transfer and visible-light-driven dye decomposition

We report herein a one-pot hydrothermal approach to synthesize zinc selenide (ZnSe)/zinc telluride (ZnTe) heterostructures - a set of common cation-based photocatalysts. The crystal structure and phase purity of the heterostructures were verified by powder X-ray diffraction (PXRD) analysis, while field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques were employed to study their morphology, microstructures, and surface electronic states, respectively. Notably, a detailed XPS analysis was conducted to discern the chemical species present on the surface of the photocatalyst, providing valuable insights into the compound's stability in an aqueous medium. Furthermore, the synthesized hybrid structures were utilized to fabricate Schottky barrier diodes, enabling a study of their electrical and dielectric properties using the Spatial-charge-limited conduction (SCLC) mechanism. The key charge transport parameters for all the photocatalysts, namely, carriers' mobility and transit time, influencing the photo response and consequently, dye degradation were estimated. Notably, the ZnSe/ZnTe heterostructure, comprising 50 % ZnSe and 50 % ZnTe, exhibited the highest mobility (3.36 × 10-6 S/m in the dark, increasing to 1.70 × 10-5 S/m in light) and proved to be the most effective photocatalyst for degrading Rhodamine B (with up to ∼ 78 % degradation over 60 min of solar light irradiation) - underscoring the pivotal role of carriers' mobility in governing the photocatalytic activity. Moreover, the ZnSe/ZnTe heterostructure demonstrated a remarkable reduction in the photo-corrosion process, a key challenge affecting the photocatalytic activity of numerous materials. Our discussion highlighted the efficient separation of photoinduced electron-hole pairs in the ZnSe/ZnTe heterostructure, facilitated by the synergistic effects of ZnSe and ZnTe, leading to the achievement of the highest photocatalytic performance.

Surface modulation of transition-metal-doped MoS2@graphite felt for bifunctional catalysis in Zn-air batteries

Surface modulation is essential to bifunctional catalysis especially for energy devices. Herein, transition-metal-doped MoS2 is shown to have modulated interfacial states to facilitate bifunctional catalysis of the oxygen reduction and evolution reactions (ORR/OER) in Zn-air batteries. The uniform distribution, vertically aligned layer, and stable Fe doped MoS2 semiconducting nanoparticles produce promising bifunctional catalysis in the fabricated Zn-air batteries. Electrochemical assessment reveals that the surface electronic states and adsorption/desorption effects are crucial to the properties and stability of the bifunctional electrode in ORR/OER. The new findings divulge an effective strategy to modulate surface/interface states to produce high-performance bifunctional catalysts for the oxygen reduction/evolution reactions.

Elucidating the Surface Properties of Sr3PbO Inverse‐Perovskite Topological Insulator: A First‐Principles Study

The emergence of robust surface electronic states in topological insulators such as bulk Sr3PbO inverse‐perovskite makes them suitable candidates for spintronic devices and solid‐state quantum computers. Herein, the atomic structure, surface energetics, and electronic properties of Sr3PbO inverse‐perovskite Sr2O (SO)‐terminated and SrPb‐terminated (001) surfaces are examined using density functional theory. A comparison of the computed structural properties of SO‐termianted and SrPb‐terminated (001) surfaces reveals maximum surface rumpling and changes in interlayer distances for the SrPb‐terminated surface of Sr3PbO. However, the calculated surface energies indicate that both SO‐termianted and SrPb‐terminated (001) surfaces of Sr3PbO are energetically feasible, indicating that these surfaces can coexist in a polycrystalline sample of this material. Due to the presence of Pb in Sr3PbO, a comprehensive examination of the electronic structure of bulk and supercell slab structures of Sr3PbO by taking spin–orbit coupling effects into consideration is conducted. Noninsulating nature of electronic structure for the two possible (001) terminations of Sr3PbO is found. The domination of Pb‐6p states at the Fermi energy and the hole screening observed at the SrPb‐terminated surface of Sr3PbO support the p‐type nature observed in the experiment.

Transition metal sulfide and nickel-iron layered double hydroxide nanohybrids for promising alkaline seawater oxidations

Direct seawater electrolysis is of great significance to produce green hydrogen using sustainable resources which can be applicable for water-scarce regions. Herein, a nanohybrid consisted of transition metal sulfide and layered double hydroxide is designed with aims to modify unfavorable surface electronic state of nickel–iron LDH (NF-LDH) for seawater oxidation. Specifically, Mo-doped NiS2 is deposited onto the self-standing two dimentional NF-LDH nanosheets, (Ni,Mo)S2//NF-LDH 300, using an e-beam evaporator. Physical deposition technique. The designed (Ni, Mo)S2//NF-LDH 300 catalyst proves a superior catalytic activity and stability for seawater oxidation under alkaline electrolyte. The deposited transition sulfide layer induces an improved charge transfer as well as reaction kinetics for the (Ni, Mo)S2//NF-LDH 300. Further, adsorptions of chloride anions at the surface is substantially suppressed due to the deposited transition metal sulfide effectively preventing chloride corrosions. The (Ni, Mo)S2//NF-LDH 300 demonstrates practical level of performance for alkaline seawater oxidation, shedding lights on our catalyst design strategy for direct seawater electrolysis.

Electric field-dependent photoexcited spin-polarized electron transport in Bi2Se3 topological insulator

Three-dimensional (3D) topological insulators (TIs) exhibit spin-polarized surface states in which the spin of electrons is locked to their momentum. The helical surface states can be explored from circularly polarized light-induced spin photocurrent, a phenomenon called circular photogalvanic effect (CPGE). In this work, we fabricate a TI transistor with the Bi2Se3 channel layer synthesized using vapor deposition. The photocurrent response of Bi2Se3 TI is characterized under horizontal and longitudinal electric fields. CPGE and linear photogalvanic effect (LPGE) contribute to the surface state photocurrent at room temperature. The longitudinal electric field provides kinetic energy to the electrons so that the transition of electrons to higher energy bands in momentum space occurs. Under a photon excitation with the energy far above the TI bandgap, we observed a photocurrent difference between left and right circularly polarized light excitation. The photocurrent variation with gate voltage (longitudinal field) is also investigated. Adjusting the Fermi level with the gate bias leads to changes in the population of bulk state carriers and spin electrons in surface states. By shifting the gate bias toward negative, the CPGE current increases because of the reduced scattering with bulk carriers. Our work reveals that longitudinal and horizontal electric fields can manipulate the helical spin-polarized photocurrent of Bi2Se3. From the asymmetry of circularly polarized photoconductive differential current (CPDC) under the horizontal field, we found evidence of spin-polarized electron transition to other conduction band valleys at room temperature under a high-energy photon excitation.

Microenvironment tailoring mediated oxygen-vacancy defect engineering strategy facilitating O2 and alcohol activation on HEOs with superior catalytic efficiency

Achieving selective oxidation of biomass-derived benzyl alcohol under a mild environment to prepare high-value-added oxygenates is an attractive goal but remains a significant challenge in the absence of qualifying catalysts. Herein, we propose a microenvironment modulation mediated oxygen-vacancy defect engineering for piloting the benzyl alcohol oxidation on catalyst MW-HEO. Collective characterizations indicate that the contemporary tactic can modulate the catalyst's pore structure and surface electronic states and form massive oxygen vacancies. Both density functional theory (DFT) simulations and experimental results conclusively reveal that the oxygen vacancies contribute to the O2 activation and forming active radicals, thus facilitating the deeper reactant activation and enabling the subsequent oxidation reactions to proceed feasibly. This work lays a solid foundation for microenvironment regulation-mediated oxygen vacancy defect engineering for the high-efficiency oxidation catalysis of benzyl alcohol and a series of alcohol derivatives.

Fermi Level Pinning in Concentrated Light-Induced Band Edge Tuning Maximizes Photocatalytic Solar-to-Hydrogen Efficiency.

Here, we demonstrate a concentrated light-induced band edge tuning effect in photocatalytic hydrogen production. This band movement along with Femi level pinning leads to two distinct catalytic behaviors upon irradiation flux increase. Specifically, the concentration of the light promotes more long-lived carriers bound to the surface electronic states, progressively boosting energy conversion efficiency to a maximum value. Afterward, efficiency diminishes gradually due to poor carrier transfer. This work offers critical insights into efficient and economical photocatalytic hydrogen production.

Stacking and spin order in a van der Waals Mott insulator 1T-TaS2

Quasi-two-dimensional charge density wave system of 1T-TaS2 has attracted recent interest due to topological excitations, emergent superconductivity, ultrafast synaptic functionality, and the possibility of a quantum spin liquid state. While electron correlation has been known to be essential in this system, the nature of its insulating phase is currently under debate. Here, we reinvestigate the origin of the insulating band structures of the 1T-TaS2 surface using density-functional theory calculations to consider the recently-raised issues such as interlayer coupling, surface effect, and interlayer spin ordering. We identify four distinct electronic states of the surface layer such as a 2D Mott phase, a strongly-coupled antiferromagnetic insulator, a weakly-coupled ferromagnetic insulator, and a small-gap semiconductor, depending on types of the surface termination and the interlayer spin configuration. These distinct surface electronic states explain the different sizes of spectroscopic band gaps observed in scanning tunneling microscopy, revealing the complexity of the interlayer charge and spin couplings in layered correlated materials.

Antiferromagnetic topological insulator with selectively gapped Dirac cones

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.

Synergetic Regulation of Interface Defects and Carriers Dynamics for High-Performance Lead-Free Perovskite Solar Cells.

Severe nonradiative recombination and open-circuit voltage loss triggered by high-density interface defects greatly restrict the continuous improvement of Sn-based perovskite solar cells (Sn-PVSCs). Herein, a novel amphoteric semiconductor, O-pivaloylhydroxylammonium trifluoromethanesulfonate (PHAAT), is developed to manage interface defects and carrier dynamics of Sn-PVSCs. The amphiphilic ionic modulators containing multiple Lewis-base functional groups can synergistically passivate anionic and cationic defects while coordinating with uncoordinated Sn2+ to compensate for surface charge and alleviate the Sn2+ oxidation. Especially, the sulfonate anions raise the energy barrier of surface oxidation, relieve lattice distortion, and inhibit nonradiative recombination by passivating Sn-related and I-related deep-level defects. Furthermore, the strong coupling between PHAAT and Sn perovskite induces the transition of the surface electronic state from p-type to n-type, thus creating an extra back-surface field to accelerate electron extraction. Consequently, the PHAAT-treated device exhibits a champion efficiency of 13.94% with negligible hysteresis. The device without any encapsulation maintains 94.7% of its initial PCE after 2000h of storage and 91.6% of its initial PCE after 1000h of continuous illumination. This work provides a reliable strategy to passivate interface defects and construct p-n homojunction to realize efficient and stable Sn-based perovskite photovoltaic devices.

Activation of peroxymonosulfate for ciprofloxacin degradation by a novel oxygen-deficient CoTiO3/TiO2NTs/Ti composite catalytic membrane:Electron transfer pathway and underlying mechanism

The technique of activating peroxymonosulfate (PMS) by enriching oxygen vacancies (Vo) in heterogeneous catalysts has gained extensive attentions in the removal of emerging contaminants, e.g., antibiotics from wastewater. In this study, a Vo-CoTiO3/TiO2NTs/Ti (Vo-CTNs) catalytic membrane with rich oxygen vacancies was prepared and employed for the activation of PMS in degrading ciprofloxacin (CIP) in a continuous flow reactor. The averaged degradation efficiency of CIP in the Vo-CTNs/PMS system could reach 97.3 %, significantly higher than that in the CoTiO3/TiO2NTs/Ti (CTNs) system (91.1 %). Quenching experiments and electron paramagnetic resonance analysis collectively confirmed the presence of •OH, SO4•−, O2•− and 1O2 during the CIP degradation process. The results of the electrochemical experiments demonstrated that in the presence of oxygen vacancies, the mutual conversion between Co2+ and Co3+ was facilitated, resulting in a lower transfer resistance. At an initial CIP concentration of 10 mg/L and a membrane flux of 600 LMH, the degradation efficiency and mineralization rate of CIP could reach 95.2 % and 51.1 % after 12 h of operation, respectively. Meanwhile, limited leaching of cobalt ions (52 μg/L) from the catalytic membrane was observed during treatment. Theoretical calculations revealed that Vo engineering can modulate the surface electronic states, thereby enhancing binding energy and electron transfer for PMS activation. The degradation of CIP is mainly achieved through the ring opening reactions in the piperazine, quinolone, and cyclopropyl groups attacked by reactive oxygen species. In this work, we built cobalt-based composite nanomaterials to promote effective electron transfer and minimize leaching of cobalt ions, and providing a new method for activating PMS in a continuous flow system by enriching the oxygen vacancies in the catalytic membrane. This method has potentials to be applied in treating wastewater containing antibiotics such as CIP.

Constructing built-in electric field in 3D core-shell p-n heterostructure of NiFe-LDH nanosheets modified Cr-doped Ni3S2 nanofibers for efficient and stable oxygen evolution reaction

Designing and creating effective, dependable, and conveniently accessible electrocatalysts for the oxygen evolution process is necessary to increase the efficiency of hydrogen production by water electrolysis. In this study, utilizing hydrothermal and electrodeposition methods, we have successfully synthesized a unique 3D Cr-Ni3S2 @NiFe-LDH/NF core-shell p-n heterostructure with a built-in electric field for use as an oxygen evolution reaction (OER) electrocatalyst. By constructing a built-in electric field into the p-n junction, the electronic state of the NiFe-LDH surface can be modified, resulting in an increase in hole concentration and enhanced adsorption of hydroxyl groups from the electrolyte. This facilitates OER reaction kinetics and improves electrocatalytic performance. The Cr-Ni3S2 @NiFe-LDH/NF-120 electrocatalyst exhibits a low overpotential of only 190 mV at a current density of 10 mA cm−2, with a Tafel slope of 33.4 mV dec−1. Remarkably, the overpotential is low, remaining at just 270 mV even at 100 mA cm−2. By operating continuously for 40 h at room temperature in a 1.0 M KOH solution while maintaining a current density of 100 mA cm−2, the exceptional stability was demonstrated. Our research provides a solid framework for the creation of extremely stable and effective 3D core-shell heterojunction electrocatalysts, which can greatly improve OER performance.

2D Atomic Layers for CO2 Photoreduction.

Artificial photosynthesis can convert carbon dioxide into high value-added chemicals. However, due to the poor charge separation efficiency and CO2 activation ability, the conversion efficiency of photocatalytic CO2 reduction is greatly restricted. Ultrathin 2D photocatalyst emerges as an alternative to realize the higher CO2 reduction performance. In this review, the basic principle of CO2 photoreduction is introduced, and the types, advantages, and advances of 2D photocatalysts are reviewed in detail including metal oxides, metal chalcogenides, bismuth-based materials, MXene, metal-organic framework, and metal-free materials. Subsequently, the tactics for improving the performance of 2D photocatalysts are introduced in detail via the surface atomic configuration and electronic state tuning such as component tuning, crystal facet control, defect engineering, element doping, cocatalyst modification, polarization, and strain engineering. Finally, the concluding remarks and future development of 2D photocatalysts in CO2 reduction are prospected.

Surface restructuring Prussian blue analog-derived bimetallic CoFe phosphides by N-doped graphene quantum dots for electroactive hydrogen evolving catalyst

As a crucial stage of electrochemical water splitting, hydrogen evolution reaction (HER) favour catalyst to attain rapid kinetics for its broader application, alternating Pt in the acidic environment. Transition metal phosphides (TMPs) are one kind of earth-abundant, nonprecious-based catalyst which has been classified as a viable alternative and active for HER. While the performance remains inferior to Pt which primarily targets durability under high current density, pinpointing the reconfiguration strategy would be critical to their catalytic competency. Herein, we reported engineered N-doped graphene quantum dots (NGQD) on the surface of bimetallic CoFe phosphide (CoFeP) derived from cobalt iron Prussian blue analogue (CoFePBA) as an efficient HER. By introducing the NGQD, the surface architect and electronic state of the transition metal are altered through an adjusted electronic configuration and thus, improving the electrocatalytic activity for HER. The X-ray absorption spectroscopy (XAS) highlighting the role of NGQD, which successfully induced the electron density of Co atoms, further expedites its conductivity and electroactivity. The optimized NGQD/CoFeP substantially surpasses an overpotential of 70 mV (vs. RHE) at the current density of 10 mA cm−2 in 0.5 M H2SO4. Furthermore, the NGQD/CoFeP maintains its exceptional stability under an extremely high current density of 600 mA cm−2 after 12 h of continuous operation. Our findings show that NGQD/CoFeP might demonstrate as a viable alternative to the conventional Pt electrocatalyst in commercial water splitting for hydrogen generation.

Quantitative Determination the Role of the Intrabandgap States in Water Photooxidation over Hematite Electrodes.

The intrabandgap states on the hematite (α-Fe2O3) electrodes are believed to play an important role in water photooxidation. Yet, it is not fully understood how the intrabandgap states are involved in the reaction. In this work, the intraband-gap states in water photooxidation on α-Fe2O3 electrodes are investigated by a combination of multiple (photo-) electrochemical techniques and operando spectroscopic methods. Two kinds of surface states are observed on the electrodes during water photooxidation, and their roles are quantitatively determined by the correlation with the steady-state photocurrent. It is demonstrated that the intrinsic electronic surface state close to the conduction band can act only as the recombination center for the photocarriers. However, the photogenerated surface state closer to the valence band is revealed to be the reactant in the rate-determining step in oxygen evolution reaction. These findings may be beneficial to elucidate the actual function of the surface states and provide insights into the kinetic and mechanism studies of water photooxidation on the α-Fe2O3 electrodes.

Synergistic Enhancement Mechanism of High Electron Density and Localized Surface Plasmons for Strong Light‐Matter Interactions

Abstract2D layered semiconductor materials hold the potential to address the crucial technical challenges of poor signal uniformity and low repeatability of traditional metal plasmonic nanostructures in sensing, which hinder quantitative detection and analysis in molecular detection. However, the ultra‐low light absorption efficiency and electronic state density intrinsically result in their inferior sensitivity compared with those of conventional metal plasmonic nanostructures. Therefore, the integration of the classical back‐gate modulation strategy and surface plasmon resonance into 2D heterostructures is proposed. This integration aims to control the interaction between the surface sensing material of the heterostructure and target molecules from the perspective of surface electronic state density and optical absorption efficiency. The sensitivity (≈10−12 m) of as‐designed MoS2/graphene/Ag heterojunction is comparable to that of advanced metal plasmonic nanostructures, and the corresponding enhancement factor (EF, 1.71 × 104) is much higher than that of most heterojunction‐based and metal‐semiconductor coupled devices. This study uncovers the internal mechanism of the surface plasmon effect and high electronic state density in enhancing light‐matter interactions, and provides an alternative avenue for high‐performance surface‐enhanced devices.

Perovskite-type tellurium-doped nickel stannate nanorods for highly sensitive electrochemical detection of arsenic drug roxarsone in food and environmental samples

The synthesis of perovskite-type tellurium-doped nickel stannate nanorods (TNS NRs) using surfactant-assisted hydrothermal followed by annealing processes is reported for the first time in this paper. Various analytical techniques were utilized to characterize the surface morphology, crystallinity, electronic states, and elemental compositions. The XRD patterns and FE-SEM/HR-TEM analysis demonstrated that TNS NRs have a tetragonal rutile phase with nanorod morphology. The TNS NRs modified electrode demonstrated a significantly improved electrocatalytic performance in the detection of organoarsenic drug roxarsone (ROX). The modified TNS NRs electrode demonstrated a low reduction potential (−0.53 V vs. Ag/AgCl) and a high reduction peak current response compared to previously reported sensors. The exceptional electrocatalytic performance of TNS NRs was primarily attributable to their nanorod morphology and high surface area, which allowed for a large number of active sites with a rapid electron transfer rate, making them optimal for ROX detection. Under optimal conditions, the proposed sensor performed well analytically, with a detection limit of 0.8 nM and a large recovery finding of 99.9% (n = 3). In addition, the sensor's durability was demonstrated by its remarkable repeatability (RSD 2.1%), reproducibility, and 30-day storage capacity. In light of this, the present work shows a chemically viable method for designing electrocatalysts with tailored architecture and paves the way for the development of sustainable materials for environmental protection.

Rational fabrication of Ti0.69Zr0.31O2 modified Au-loaded halloysite core-shell catalyst for highly efficient reduction of 4-nitrophenol and dye pollutants

Catalytic reduction method can reductively degrade some highly toxic and harmful pollutants and transform them into useful chemical resources with low toxicity and easy biodegradability, owning the advantages of environmental friendliness, economic effectiveness and mild conditions. This study reported the rational fabrication of the HNTs-Au@Ti0.69Zr0.31O2 hollow tubular core-shell catalyst for facile reductive degradation of aqueous 4-nitrophenol and azo dye pollutants. The catalyst employed the interfacial reaction strategy for aminopropyl-functionalization of halloysite nanotubes as core, amino electrostatic adsorption and in situ redction of ultrasmall Au nanoparticles with good dispersibility, and sol-gel deposition of Ti0.69Zr0.31O2 shell. The obtained HNTs-Au@Ti0.69Zr0.31O2 catalyst manifested superior catalytic performance and atom utilization efficiency of Au active sites for reduction of 4-nitrophenol, methylene blue and methyl orange in comparison to those control samples. The integration of TiO2 and ZrO2 within Ti0.69Zr0.31O2 shell enriched the porosity of the catalyst, and generated the extraordinary interfacial chemical effect on Au NPs with improved surface electronic state. The interlayer-embedded Au nanoparticles were effectively solidified and avoided from agglomeration and loss, where the reactive electron migration rate from BH4- to target pollutants upon Au surfaces was elevated for prominent catalytic reaction rate. The unique structural characteristics and synergistic enhancement effect of catalyst components created excess catalytic active sites at the hybrid interface surrounded Au nanoparticles due to the strong metal-carrier chemical effect, and contributed to the construction of the high-performance catalytic system, which endowed the HNTs-Au@Ti0.69Zr0.31O2 catalyst with remarkable catalytic capability, reusability and structural stability. The reaction mechanism and structure-property relationship were finally proposed.

Investigation of the effect of different types of SiN layers and cap-GaN on the surface electronic states of AlGaN/GaN heterostructures with 2DEG using X-ray and UV photoelectron spectroscopy

Determining the effect of different crystalline and amorphous passivation layers on the electronic states and surface properties of III-nitride heterostructures is an important task from the point of view of improving the performance of modern microwave and power electronic devices. In the present work the effect of in situ deposited gallium nitride and silicon nitride passivation layers on the surface electronic states of AlGaN/GaN heterostructures with a two-dimensional electron gas is investigated using the X-ray, ultraviolet and angle-resolved ultraviolet photoelectron spectroscopy (XPS, UPS, ARUPS) techniques. A thorough analysis of the experimental UPS spectra was carried out, the primary photoelectrons were extracted from the experimental spectra and the contribution of the electron states on the surface in the energy range of the AlGaN bandgap was identified. It was found that the density of states on the surface decreases by about an order of magnitude in the AlGaN bandgap during the formation of a silicon nitride layer. For the first time, it was found that a significant decrease in the density of surface states (up to a factor of 6) is observed even at extremely low thickness of the SiN ordered layer formation with the (√3 × √3)R30° superstructure.

Electronic states in a quantum Beltrami surface

In this paper, we investigate the influence of the geometry in the electronic states of a quantum Beltrami surface. We have considered an electron governed by the spinless stationary Schrödinger equation constrained to move on the Beltrami surface due to a confining potential from which the Da Costa potential emerges. We investigate the role played by the geometry and orbital angular momentum on the electronic states of the system.