Calculated Band Alignment Research Articles

Fabrication of a novel tin disulfide/vanadium carbide MXene (SnS2/V2CTx) photocatalytic heterojunction with the potential application for environmental remediation

Lately, researchers have been striving to develop efficient and economically viable multifunctional photocatalysts. This is in effort to address the currently faced energy and water crises. Herein, tin disulfide/vanadium carbide MXene (SnS2/V2CTx) heterojunction was prepared through facile self-assembly approach, followed by calcination for strong integration of the two materials (SnS2 and V2CTx). The prepared pristine and composite materials' structural characteristics, chemical composition, morphology, and charge transfer kinetics were studied using spectroscopic, microscopic techniques, and electrochemical impedance spectroscopy (EIS). The prepared SnS2/V2CTx composites exhibited improved optical and photoelectrochemical properties compared to the pristine materials. In particular, the energy bandgap (Eg) was reduced from 1.30 eV (SnS2) to 0.81 eV (SnS2/V2CTx). Additionally, the calculated band alignments exhibited superior redox potentials towards the generation of useful reactive oxygen species. Moreover, the formation of a heterostructure between SnS2 and V2CTx MXene presents a reproducible approach towards fabrication of efficient photocatalyst with combined synergistic catalytic attributes.

Oxidizing Role of Cu Cocatalysts in Unassisted Photocatalytic CO2 Reduction Using p-GaN/Al2O3/Au/Cu Heterostructures.

Photocatalytic CO2 reduction to CO under unassisted (unbiased) conditions was recently demonstrated using heterostructure catalysts that combine p-type GaN with plasmonic Au nanoparticles and Cu nanoparticles as cocatalysts (p-GaN/Al2O3/Au/Cu). Here, we investigate the mechanistic role of Cu in p-GaN/Al2O3/Au/Cu under unassisted photocatalytic operating conditions using Cu K-edge X-ray absorption spectroscopy and first-principles calculations. Upon exposure to gas-phase CO2 and H2O vapor reaction conditions, the composition of the Cu nanoparticles is identified as a mixture of CuI and CuII oxide, hydroxide, and carbonate compounds without metallic Cu. These composition changes, indicating oxidative conditions, are rationalized by bulk Pourbaix thermodynamics. Under photocatalytic operating conditions with visible light excitation of the plasmonic Au nanoparticles, further oxidation of CuI to CuII is observed, indicating light-driven hole transfer from Au-to-Cu. This observation is supported by the calculated band alignments of the oxidized Cu compositions with plasmonic Au particles, where light-driven hole transfer from Au-to-Cu is found to be thermodynamically favored. These findings demonstrate that under unassisted (unbiased) gas-phase reaction conditions, Cu is found in carbonate-rich oxidized compositions rather than metallic Cu. These species then act as the active cocatalyst and play an oxidative rather than a reductive role in catalysis when coupled with plasmonic Au particles for light absorption, possibly opening an additional channel for water oxidation in this system.

Computational study of the electronic structures and photocatalytic water splitting performances of Janus aluminium chalcogenide monolayers

Through extensive first-principles calculations based on density functional theory, we investigate the structural stabilities, electronic and photocatalytic properties of Janus aluminium chalcogenide monolayers (Al2XY, X/Y = S, Se, and Te). Our studies reveal that these Janus Al2SSe, Al2SeTe and Al2SeTe monolayers are dynamically, thermally, and mechanically stable, and exhibit indirect semiconductor characteristics with suitable band gaps of 1.84 - 2.80 eV. Moreover, according to the calculated band alignment and optical absorption spectra, we successfully identify that the Al2SeTe monolayer would be a highly effective photocatalysts due to the adequate driving force for the water oxidation/reduction reaction and excellent visible-light harvesting ability. Furthermore, we find that the band gaps, band edge positions, and optical absorption can be remarkably modified by applying 4 % tensile strain. These findings indicate that Janus aluminium chalcogenide monolayers hold great promise as highly effective photocatalysts for water splitting.

Photocatalytic enhancement mechanisms for novel g-C3N4/PVK nanoheterojunction

The interactions between monolayer graphitic carbon nitride (g-C3N4) and conjugated polymer poly(9-vinylcarbazole) (PVK) have been explored. We investigated the enhanced photocatalytic mechanisms for the novel g-C3N4/PVK nanoheterojunction covering the state-of-the-art of DFT by performing rigorous DFT calculations combined with van der Waals corrections (GGA + vdW). The calculated band alignment between g-C3N4 monolayer and PVK monomer clearly reveals that the conduction band minimum and the valence band maximum of g-C3N4 monolayer are higher than those of the conjugated polymer PVK. This predicted band alignment ensures the photogenerated electrons easily migrate from the g-C3N4 monolayer to the PVK monomer, and will lead to high hydrogen-evolution reaction activity. The charge transfer between g-C3N4 monolayer and PVK results in a polarized field within the interface region, which will benefit the separation of photogenerated carriers. The calculated density of electronic states, Lowdin charge transfer and charge density difference certify that this proposed layered nanoheterojunction is an excellent light-harvesting semiconductor. These findings indicate that the conjugated polymer PVK is a promising candidate as a non-noble metal co-catalyst for g-C3N4 photocatalysis. It also provides useful information for understanding the observed enhanced photocatalytic mechanisms in experiments.

Density Functional Theory Study on the Enhancement Mechanism of the Photocatalytic Properties of the g-C3N4/BiOBr(001) Heterostructure.

The van der Waals heterostructures fabricated in two semiconductors are currently attracting considerable attention in various research fields. Our study uses density functional theory calculations within the Heyd–Scuseria–Ernzerhof hybrid functional to analyze the geometric structure and electronic structure of the g-C3N4/BiOBr(001) heterojunction in order to gain a better understanding of its photocatalytic properties. The calculated band alignments show that g-C3N4/BiOBr can function as a type-II heterojunction. In this heterojunction, the electrons and holes can effectively be separated at the interface. Moreover, we find that the electronic structure and band alignment of g-C3N4/BiOBr(001) can be tuned using external electric fields. It is also noteworthy that the optical absorption peak in the visible region is enhanced under the action of the electric field. The electric field may even improve the optical properties of the g-C3N4/BiOBr(001) heterostructure. Given the results of our calculations, it seems that g-C3N4/BiOBr(001) may be significantly superior to visible light photocatalysis.

Photocatalytic and photoelectrochemical activities of strained BiVO4

We theoretically investigate the impact of strain on the photocatalytic and photoelectrochemical activities of BiVO4. Through first-principles hybrid functional calculations, the electronic and structural properties of three strained BiVO4 such monoclinic scheelite, tetragonal scheelite, and tetragonal zircon, are examined. For most of the BiVO4 polymorphs, the dependence of the out-of-plane lattice constant upon biaxial strain and of the bandgap upon in-plane biaxial and hydrostatic strains shows good linearity. Combined with calculated band alignment, we assess the impact of strain on the photocatalytic and photoelectrochemical activities of the material. Tensile strain shifts upward the positions of the conduction band edge in monoclinic scheelite and tetragonal zircon close to or even higher than the H+/H2 level. These suggest that strain plays a crucial role in the experimentally observed performance of BiVO4-based systems for hydrogen production via water splitting.

Tunable electronic and optical properties of a BAs/As heterostructure by vertical strain and external electric field.

Based on the first-principles method, we investigated the electronic properties of a BAs/arsenene (As) van der Waals (vdW) heterostructure and found that it has an intrinsic type-II band alignment with a direct band gap of 0.25 eV, which favors the separation of photogenerated electrons and holes. The band gap can be effectively modulated by applying vertical strain and external electric field, displaying a large alteration in the band gap via the strain and experiencing an indirect-to-direct band gap transition. Moreover, the band gap of the heterostructure varies almost linearly with external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and optical absorption reveal that the BAs/As heterostructure could present an excellent light-harvesting performance. The absorption strength can be tuned mainly by interlayer coupling, while external electric field shows clear regulating effects on the absorption strength and absorption edge.

DFT band alignment of polar and nonpolar GaN/MgGeN2, ZnO/MgGeN2 and GaN/ZnO heterostructures for optoelectronic device design

We calculated the band alignment among MgGeN2, ZnO and GaN compounds using density functional theory (DFT) calculations. The norm-conserving pseudopotentials approach was performed to determine the band gap of each compound. The band offsets at the interfaces of GaN/MgGeN2 and ZnO/MgGeN2 were calculated from the differences in the bulk band edges including the interfacial dipole-potentials. Moreover, the effect of strain was also investigated to govern the lattice mismatches between MgGeN2 and substrates (i.e. GaN and ZnO). We found that the band alignment of nonpolar heterojunctions is type-I for GaN/MgGeN2 and type-II for ZnO/MgGeN2 respectively. From these results, the band offsets of GaN/ZnO were extracted by using transitivity rule and confirmed with the direct calculation. Moreover, the polar band offsets shift from nonpolar ones and change the band alignment of ZnO/MgGeN2 with (Mg,Ge)-O interface to type-I. These calculated band alignments present the potential of using MgGeN2 in optoelectronic applications as an electron blocking layer for ZnO-based UV-LED and a quantum-well barrier for ZnO and GaN-based UV laser diode.

Structural, electronic, and optical properties of pristine and bilayers of hexagonal III-V binary compounds and their hydrogenated counterparts

Density functional theory calculations were performed to investigate the structural, electronic, and optical properties of two-dimensional hexagonal III-V binary compounds (h-MX, M = B, Al, Ga, and X = N, P and As), the fully hydrogenated counterparts (MXH2) and the MX/MXH2 van der Waals (vdW) heterostructures. The results show that hydrogenation can change the stability and electronic band dispersion of hexagonal III-V binary compounds, of which the fully hydrogenated h-AlN becomes dynamically unstable. The full hydrogenation induces an indirect-to-direct bandgap transition for hexagonal BN, GaN, GaP, GaAs, and the band gaps become tunable by applying the biaxial strain.These characteristics are conducive to design of strain-based 2D materials for application in nanoelectronic and optoelectronic devices. Furthermore, the different stacking pattern MX/MXH2 vdW heterostructures have been investigated. Of which, BN/BNH2, GaN/GaNH2, GaP/GaPH2 and GaAs/GaAsH2 were found to have the typical type-II band alignments, which can facilitate the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the GaP/GaPH2 and GaAs/GaAsH2 vdW heterostructures possess great potential to be high-performance optoelectronic device.

Tunable electronic and optical properties of SnC/BAs heterostructure by external electric field and vertical strain

Based on the first-principles method, we investigate the electronic structure of SnC/BAs van der Waals (vdW) heterostructure and find that it has an intrinsic type-II band alignment with a direct band gap of 0.22 eV, which favors the separation of photogenerated electron–hole pairs. The band gap can be effectively modulated by applying vertical strain and external electric field, displaying a large alteration of band gap via the strain and experiencing an indirect-to-direct band gap transition. Moreover, the band gap of the heterostructure varies almost linearly with external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and the optical absorption reveal that the SnC/BAs heterostructure could present an excellent light-harvesting performance. Our designed heterostructure is expected to have great potential applications in nanoelectronic devices and photovoltaics and optical properties.

Band offset modulation in Si-EuO heterostructures via controlled interface formation

Combining first-principles calculations and experiment, we investigate the atomic and electronic structure of the Si/EuO interface. We consider the thermodynamic stability of interface structures with different levels of oxidation to identify the most probable configuration. By comparing the calculated band alignment and core-level shifts with measured values, we validate the theoretically constructed interface model. We find that the band offset can be tuned by altering the relative energy positions of the Si and EuO conduction bands via interface oxidation, which can be used to tune this materials system for specific applications in spintronics.

Atomic structure and band alignment at Al2O3/GaN, Sc2O3/GaN and La2O3/GaN interfaces: A first-principles study

The atomic structures, chemical bonding and band alignment at trivalent oxides X2O3 (where X = Al, Sc and La) and GaN interface are studied based on the density functional supercell calculations. The insulating interfaces with small roughness and a clean bandgap are built based on the electron counting rule. The results prove that GaO bonds dominate the interfacial chemical bonding for all the interfaces, and the calculated oxide/GaN band alignment consistent with the experimental values. All the oxides are proved to have the type-I band alignment with GaN with hybrid functional calculation. For the Al2O3 interface, the calculated valence band offset is 1.17 eV, while that for the Sc2O3 and La2O3 interface are 0.81 eV and 0.95 eV, respectively. The calculated conduction band offsets are all larger than 1 eV, and as large as 1.8 eV for the Al2O3 interface. The theoretically calculated band alignments indicate that the studied trivalent oxides Al2O3, Sc2O3 and La2O3 are all suitable gate insulators for GaN-based MOSFET applications.

Chemical bonding and band alignment at X2O3/GaN (X = Al, Sc) interfaces

The chemical bonding and the band alignment at Al2O3/GaN and Sc2O3/GaN interfaces are studied using density functional supercell calculations. Using bonding models based on the electron counting rule, we have created the insulating interfaces with a small roughness and a clean bandgap. Ga-O bonds dominate the interfacial chemical bonding at both interfaces. The calculated band alignment agrees with the experimental values. For the Al2O3 interface, the calculated valence band offset is 1.17 eV using hybrid functionals, while that for the Sc2O3 interface is 0.81 eV. The conduction band offsets for both are larger than 1 eV, and is as large as ∼2 eV for the Al2O3 interface. The calculated band alignments indicate that Al2O3 and Sc2O3 are both suitable insulators for GaN-based MOSFET applications.

Stacking effect on electronic, photocatalytic and optical properties: A comparison between bilayer and monolayer SnS

In this work, the stacking-dependent optoelectronic performances of a bilayer SnS were explored based on density functional theory (DFT). The results demonstrated that an AB-stacking induces an indirect-to-direct transition, a feature that is capable of vanquishing electron transition impediment from an intrinsic indirect monolayer SnS. An anisotropic and small carrier effective mass exists in all the stacking models, among which the AB-stacking with the lowest value favors high carrier mobility. Calculated band alignments are indicative of acceptable and adjustable photocatalytic activity for all the stacking models, unlike the monolayer SnS. The AB-stacking configuration possesses the strongest redox power, which facilitates it to be a potential candidate for photocatalytic water splitting. Additionally, the AB-stacking does effectively improve optoelectronic properties. The study demonstrated that layer-stacking is an availably adjustable method in the fields of sunlight-driven photocatalysis for nano-optoelectronic devices.

The electronic and transport properties of edge contact borophane-MoSe2 heterojunction: A first principles study

First principles calculations are carried out to study the effect of contact configurations on the electronic and transport properties of borophane-MoSe2 heterojunction. Four different contact configurations between borophane and MoSe2 are designed and the most stable configurations are determined. Calculations show that metallic behavior is appeared in semiconductor MoSe2 as a result of junction formation with borophane. The metallic states are mainly located at the contact interface and produced by d states of Mo atoms, p states of B and Se atoms. The calculated band alignments indicate that n-type Schottky contacts are formed between borophane and MoSe2. The different current-voltage behaviors of two represented configurations indicate the contact configurations are important for the electronic transport properties of borophane-MoSe2 heterojunction.

Epitaxial engineering of polar ε-Ga2O3 for tunable two-dimensional electron gas at the heterointerface

We predict the formation of a polarization-induced two-dimensional electron gas (2DEG) at the interface of ε-Ga2O3 and CaCO3, wherein the density of the 2DEG can be tuned by reversing the spontaneous polarization in ε-Ga2O3, for example, with an applied electric field. ε-Ga2O3 is a polar and metastable ultra-wide band-gap semiconductor. We use density-functional theory (DFT) calculations and coincidence-site lattice model to predict the region of epitaxial strain under which ε-Ga2O3 can be stabilized over its other competing polymorphs and suggest promising substrates. Using group-theoretical methods and DFT calculations, we show that ε-Ga2O3 is a ferroelectric material where the spontaneous polarization can be reversed through a non-polar phase by using an electric field. Based on the calculated band alignment of ε-Ga2O3 with various substrates, we show the formation of a 2DEG with a high sheet charge density of 1014 cm−2 at the interface with CaCO3 due to the spontaneous and piezoelectric polarization in ε-Ga2O3, which makes the system attractive for high-power and high-frequency applications.

Tuning electronic and optical properties of arsenene/C3N van der Waals heterostructure by vertical strain and external electric field

Searching for new van der Waals (vdW) heterostructure with novel electronic and optical properties is of great interest and importance for the next generation of devices. By using first-principles calculations, we show that the electronic and optical properties of the arsenene/C3N vdW heterostructure can be effectively modulated by applying vertical strain and external electric field. Our results suggest that this heterostructure has an intrinsic type-II band alignment with an indirect bandgap of 0.16 eV, facilitating the separation of photogenerated electron–hole pairs. The bandgap in the heterostructure can be tuned from 0–0.35 eV via the strain, experiencing an indirect-to-direct bandgap transition. Moreover, the bandgap of the heterostructure varies linearly with respect to a moderate external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and the optical absorption reveal that the arsenene/C3N heterostructure could present excellent light-harvesting performance. Our designed vdW heterostructure is expected to have great potential applications in nanoelectronic devices and photovoltaics.

Hybrid density functional study on the mechanism for the enhanced photocatalytic properties of the ultrathin hybrid layered nanocomposite g-C3N4/BiOCl

To investigate the origin of the high photocatalytic performance of experimentally synthesized g-C3N4/ BiOCl, we studied its geometry structure, electronic structure, and photocatalytic properties by means of hybrid density-functional theory (DFT). The calculated band alignment of g-C3N4 and few-layer BiOCl sheets clearly shows that g-C3N4/ BiOCl is a standard type-II nanocomposite. The density of states, Bader charge, partial charge density, charge density difference, and the effective masses show that electron-hole pair can be effectively separated in the g-C3N4/BiOCl interface. The calculated absorption coefficients indicate an obvious redshift of the absorption edge. The band gap of g-C3N4/BiOCl can be modulated by external electric field, and a semiconductor-semimetal transition is observed. The type-II vdW heterostructure is still maintained during the changes of external electric field. Especially, when the electric field reaches to +0.7 V/Å, the impurity states have been eliminated with the band gap of 2.3 eV. An analysis of optical properties shows that the absorption coefficient in the visible-light region is enhanced considerably as the electric-field strength increases. Our calculation results suggest that the ultrathin hybrid layered g-C3N4/BiOCl nanocomposite may have significant advantages for visible-light photocatalysis.

Determination of Band Alignment in the Synergistic Catalyst of Electronic Structure-Modified Graphitic Carbon Nitride-Integrated Ceria Quantum-Dot Heterojunctions for Rapid Degradation of Organic Pollutants

We engineered novel heterojunction ceria (CeO2) QDs decorated on the surfaces of graphitic carbon nitride (g-C3N4) nanosheets by a facile in situ hydrothermal synthetic route. Using core-level/valence-band X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy, and work function measurements of the materials, we constructed the energy band alignment at the heterojunction. The band alignment has a Type-II alignment between organic (g-C3N4) and inorganic (CeO2 QDs) semiconductors junction with valence/conduction band offsets (VBO/CBO) of −0.07/–0.31 eV. The calculated band alignment parameters of the heterojunction were compared with the experimental values of g-C3N4/CeO2 QD composite and a new energy band diagram was proposed for the electronic structure-modified g-C3N4/CeO2 QDs heterojunction. The newly constructed heterojunction is formed by carbon-vacancy-promoted g-C3N4 coupled to lower defect-mediated (oxygen vacancies) CeO2, as determined by high-resolution XPS analysis. Moreover, t...

Tunable Structural, Electronic, and Optical Properties of Layered Two-Dimensional C2N and MoS2 van der Waals Heterostructure as Photovoltaic Material

The nitrogenated porous two-dimensional (2D) material C2N has been successfully synthesized using a simple wet-chemical reaction, which provides a high-performance way to produce such 2D materials with novel electronic and optical properties. In this work, density functional theory (DFT) calculations were performed to investigate the structural, electronic, and optical properties of the layered C2N/MoS2 van der Waals (vdW) heterojunction. The C2N/MoS2 heterojunction was found to have a direct band gap of 1.30 eV and to present the typical type-II heterojunction feature, facilitating the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the C2N/MoS2 heterojunction should exhibit good light-harvesting properties. The vertical strain can effectively tune the electronic properties and optical absorption of the C2N/MoS2 heterojunction by changing the interaction between the pz orbital of C2N and the dz2 orbital of MoS2. The mo...