Band Edge Alignment Research Articles

First-principles study on the strain-mediated g-C3N4/blue phosphorene heterostructures for promising photocatalytic performance

Well-designed two-dimensional heterogeneous photocatalysts have attracted significant attention due to the enhancement in visible-light absorption and effective charge separation. In this paper, the electronic and optical properties of g-C3N4/BlueP (blue phosphorene) heterojunction under varying strains were investigated systematically by first-principles calculations. The results showed that the type transformation of g-C3N4/BlueP heterojunction can be achieved by suitable biaxial strain. The CBM was found to be composed of g-C3N4 as electron acceptor, while the VBM was contributed by BlueP as electron donor which solved the problem of high electron–hole recombination of type-I heterostructures. The band gap and band edge alignment under −6% to −8% compressive biaxial strain could satisfy the REDOX (reduction-oxidation) potential of photolysis water. A wide optical response range and good absorbance were also observed for the heterostructure under strain, which improved the solar utilization rate compared with individual g-C3N4 and BlueP.

Computational Study of Novel Semiconducting Sc2CT2 (T = F, Cl, Br) MXenes for Visible-Light Photocatalytic Water Splitting

Seeking candidate photocatalysts for photocatalytic water splitting, via visible light, is of great interest and importance. In this study, we have comprehensively explored the crystal structures, electronic properties, and optical absorbance of two-dimensional (2D) Sc2CT2 (T = F, Cl, Br) MXenes and their corresponding photocatalytic water splitting, under the visible-light region, by first-principles calculations. Herein, we have proposed that 2D Sc2CT2 MXenes can be fabricated from their layered bulk compounds, alternatively to the traditional chemical etching method. Creatively, we proposed Sc2CT2 (T = F, Br) as new materials; the band edge alignments of Sc2CF2 can be tuned to meet the water redox potentials at pH = 8.0. It is highlighted that Sc2CF2 shows outstanding optical spectra harvested under visible-light wavelength regions, and efficient separation of photo-induced electrons and holes in different zones. These present results provide eloquent evidence and open a new door on the photocatalysis applications of such novel semiconducting MXenes.

Rational Design of Two-Dimensional Binary Polymers from Heterotriangulenes for Photocatalytic Water Splitting.

On the basis of first-principles calculations, we report the design of three two-dimensional (2D) binary honeycomb-kagome polymers composed of B- and N-centered heterotriangulenes with a periodically alternate arrangement as in hexagonal boron nitride. The 2D binary polymers with donor-acceptor characteristics are semiconductors with a direct band gap of 1.98-2.28 eV. The enhanced in-plane electron conjugation contributes to high charge carrier mobilities for both electrons and holes, about 6.70 and 0.24 × 103 cm2 V-1 s-1, respectively, for the 2D binary polymer with carbonyl bridges (2D CTPAB). With appropriate band edge alignment to match the water redox potentials and pronounced light adsorption for the ultraviolet and visible range of spectra, 2D CTPAB is predicted to be an effective photocatalyst/photoelectrocatalyst to promote overall water splitting.

Achieving simultaneous Cu particles anchoring in meso-porous TiO2 nanofabrication for enhancing photo-catalytic CO2 reduction through rapid charge separation

A facile solvo-thermal approach was successfully employed to prepare titanium oxide (TiO2) nano-aggregates with simultaneous copper particles anchoring. The as-synthesized composite could convert CO2 into CH4 and CO products under simulated solar irradiation. The impact of copper loading amounts on the photo-reduction capability was evaluated. It was found proper amount of Cu loading could enhance the activity of CO2 photo-reduction. As a result, the optimal composite (TiO2-Cu-5%) consisting of TiO2 supported with 5% (mole ratio) Cu exhibits 2.2 times higher CH4 yield and 3 times higher CO yield compared with pure TiO2. Conduction band calculated from the band gap and valence X-ray photoelectron spectroscopy (XPS) indicated TiO2 nano-aggregates have suitable band edge alignment with respect to the CO2/CH4 and CO2/CO redox potential. Furthermore, with involving of Cu particles, an efficient separation of photo-generated charges was achieved on the basis of photocurrent response and photoluminescence spectra results, which contributed to the improved photo-catalytic performance. The present work suggested that the Cu-decorated TiO2 could serve as an efficient photo-catalyst for solar-driven CO2 photo-reduction.

Barriers to carriers: faults and recombination in non-stoichiometric perovskite scintillators

Tuning the efficiency and speed of charge carrier recombination in inorganic scintillators can potentially improve their performance in diverse applications. Recent work suggests that this maybe be achieved via a two-phase scintillator AB that naturally phase separates into A-rich and B-rich domains. In addition, a favorable electronic structure and band-edge alignment such that the charge carriers are confined or are thermodynamically driven to preferentially accumulate in one of the two domains, might lead to an improved radiative recombination rate. Here, we use density functional theory computations and ab initio molecular dynamics (AIMD), including non-adiabatic molecular dynamics (NAMD) simulations, to examine an alternative phase structure and its potential impact on recombination. Using a model perovskite SrTiO_3 system with one-, two- and three-dimensional Ruddlesden–Popper (RP) phases, we demonstrate that RP faults induce band structure changes in the material that can act as barriers to carrier transport. Our AIMD/NAMD simulations indicate competing effects of a lower mean free path (potentially enhancing the desired radiative recombination and overall scintillating efficiency) and faster non-radiative recombination (undesired) due to enhanced electron–phonon coupling in the faulted system. Full exploitation of such a rational design approach would require tuning of the effective scintillation efficiency by varying the perovskite chemistry using appropriate arrangements of RP faults in the bulk material. Finally, other effects, such as the tendency of point defects to segregate at the interface, that might affect the overall performance, are briefly discussed. We expect the basic results found here to apply to other nanostructured scintillators.Graphical

Synthesis of ZnS-CuS-Bi nanonail heterostructures and funnel mechanism of their photocatalytic activity

Nanonail shaped ZnS-CuS-Bi three component nanoparticles are synthesized via a simple one-pot colloidal synthesis route by thermal decomposition of metal-thiolate precursors. A thiol serves as the sulfur source and a phase directing capping agent promotes selective anisotropic growth of the nanocrystals in a noncoordinating solvent. After the nucleation and growth of a CuS rod, reactive Cu ions serve as catalytic seeds for the nucleation and growth of ZnS and Bi at the CuS nanorod tips. Thereby the obtained ZnS-CuS-Bi nanocrystals form a chain of two semiconductors of decreasing band gap and a metallic Bi nanoparticle. The three components absorb light in different spectral regions enabling efficient light harvesting. Furthermore, the band edge alignment of ZnS and CuS promote photogenerated electron funneling towards the Bi catalyst particle, which promotes charge carrier separation, effectively channeling the catalytic activity. The photocatalytic performance is assessed at the example of the photodegradation of the organic dye Rhodamine B, and shows excellent performance rendering these nanonails as inexpensive, non-toxic and efficient photocatalyst to remedy environmental pollution.

δ-SnS: An Emerging Bidirectional Auxetic Direct Semiconductor with Desirable Carrier Mobility and High-Performance Catalytic Behavior toward the Water-Splitting Reaction.

We propose a novel two-dimensional SnS allotrope (monolayer δ-SnS) based on an auxetic δ-phosphorene configuration using first-principles calculations. This monolayer appears to have outstanding stability as revealed by its energetic, kinetic, thermodynamic, and mechanic calculations, and it can withstand temperatures as high as 900 K. Monolayer δ-SnS is a wide direct-bandgap (2.354 eV) semiconductor, and its electron mobility is as high as ∼1.25 × 103 cm2 V-1 s -1, higher than that of monolayer KTlO (∼450 cm2 V-1 s-1) and MoS2 (∼200 cm2 V-1 s-1). Optical absorption spectra, reaching up to the order of ∼105 cm-1, are obviously excellent in the visible-light region, suggesting efficient harvesting of solar radiation. Because of its unique atomic motif, monolayer δ-SnS presents an unusual bidirectional auxetic effect: a high negative in-plane Poisson's ratio (-0.048 and -0.068), which is larger than those for many recently reported two-dimensional auxetic materials, e.g., black phosphorene (-0.027), borophene (-0.04), and monolayer penta-B2N4 (-0.02). The bandgap and band edge can be substantially manipulated under strain to meet the requirement of the water-splitting reaction. Particularly, when pH = 7, suitable band-edge alignments and small overpotentials of the photocatalytic OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) appear, endowing monolayer δ-SnS with great potential as an efficient visible-light-driven bifunctional photocatalyst for water splitting.

Effects of different edge contacts on the photocatalytic and optical properties of blue phosphorene/arsenene lateral heterostructures

Recent advances in the synthesis and characterization techniques of atomic-scale thicknesses of crystals have enabled the fabrication of various two-dimensional lateral heterostructures (LHSs). In this paper, we systematically investigate the geometrical structures, electronic properties, band alignments, and optical properties of phosphorene/arsenene LHSs using first-principles calculations based on density functional theory. First, we constructed four stable phosphorene/arsenene LHSs with different edge contacts, named A–Z, Z–A, A–A and Z–Z phosphorene/arsenene LHSs. We then investigated the electronic structures of the different edge contacts. Interestingly, the calculated electron localization functions show that the most stable six-membered rings and previously reported five-membered and seven-membered rings are formed at the interface of the blue phosphorene/arsenene LHSs. Moreover, the band edge alignments and optical absorption properties show that A–A and Z–A phosphorene/arsenene LHSs have band-edge conditions suitable for photocatalytic water decomposition, which provides a possibility for their application in the area of photocatalytic water decomposition. These results provide a route for applications of these heterostructures in future optoelectronic and semiconductor devices, and also pave the way for the design of new blue phosphorene/arsenene LHSs and the exploration of their potential applications in optoelectronic devices.

The rise of Nb-, Ta-, and Bi-based oxides/chalcogenides for photocatalytic applications

Niobates, tantalates, and bismuthates have recently attracted attention as compounds to be used in photonics and energy-harvesting applications as substitutes for the relatively unstable lead-halide perovskites. Having the photocatalytic reaction for H2 production and CO2 conversion in mind, we perform first-principles materials design to determine the potential for electrochemistry of certain Nb-, Ta-, and Bi-based oxides/chalcogenides: NaMO(3−x)Qx (M = Nb, Ta and Q = S, Se), and ABiX2 and ABiX3 (A = Na, K and X = O, S). The analysis of the effective masses, alongside the determination of the actual electronic dimensionality, reveals that several compounds exhibit reduced electronic dimensionality despite possessing a higher structural dimensionality. The electrochemical performances of all compounds are then investigated by computing the band-edge alignment with respect to various redox levels, against the relevant photocatalysis reactions in aqueous media at different pH's. Our work highlights the effect of chalcogen incorporation in the electrochemical performance of this class of materials, and proposes some of the Bi-based chalcogenides, namely KBiO2 and KBiS2, as efficient photocatalysts over a wide range of pH conditions.

Van der Waals heterostructures based on SiC-BS: A promoted photocatalytic properties for water splitting

Motivated by the previous study of electronic properties of the two-dimensional (2D) hexagonal structure of group III-VI binary monolayers, this work further investigates the band edge alignment, charge transfer and strain response based on the density functional theory (DFT). We here combine monolayer SiC and BS through van der Waals (vdW) interaction to obtain a new heterostructure form with well-defined type-II alignment. Their potential applications for an efficient use of light have also been systematic studied. This work proves that SiC-BS heterostructures can fully meet all ideal criteria and have much better mobility priorities than that of pristine structures. The results indicate that the applied electric field can regulate the band alignment between type-I and type-II, while the applied strain can maintain three heterostructures in a relatively wide range of type-II character to effectively separate light induced carriers in space.

Preparation and characterization of the Cu, Fe co-doped Bi2Ti2O7/EG-g-C3N4 material for organic model pollutants removal under direct sun light irradiation

The following hindrances should be overcome to enhance the efficacy of photo-catalysts such as a poor-visible light response, high charge-carrier recombination rate, less surface area, and inappropriate band-edge alignment for photo-induced redox reaction. This study reported, visible light responsive Cu, Fe-co-doped Bi2Ti2O7/EG-g-C3N4 catalyst for photo-removal of organic pollutants. Series of characterization tools are used to testify and demonstrate the properties of formed material. The photo-removal efficiency of Cu, Fe-co-doped Bi2Ti2O7/EG-g-C3N4 hybrids are estimated to be 69.5 and 83.69% for ciprofloxacin and Rhodamine B organic molecules. Free radical trapping experiments were carried to predict active radical's participating in the reaction. The possible reaction mechanism was explained based on band edge potentials and trapping experiment results. This reporting Cu, Fe-co-doped Bi2Ti2O7/EG-g-C3N4 catalyst showed good durability. This work gives a new insight into coupling tactics of Cu, Fe-co-doped Bi2Ti2O7 and g-C3N4 material and which will helpful for designing efficient photocatalytic materials.

Physicochemical characterization and catalytic performance of Fe doped CuS thin films deposited by the chemical spray pyrolysis technique

The thin films Fe doped copper sulphide Cu1−xFexS (CFS) (x = 0.01, 0.03, 0.05, and 0.07) were elaborated by spray pyrolysis deposition technique. The characterization by XRD and SEM of the thin films shows a Covellite CuS single phase without formation of other phases. The structure is a simple hexagonal with unit cell dimension, a = b = 3.79 A and c = 16.34 A. The Analysis of the UV–Vis spectra reveals that the energy band gap has been decreased from 2.47 to 1.98 eV with the increase of Fe concentration. The absorption coefficients of CFS films have increased from 1.155 × 105 to 1.712 × 105 cm−1. It has demonstrated that a right band gap with a right band edge alignment at a pH value for Fe-doped CFS can boost the material application as a photocatalyst for the visible light. According to this study, CFS (0.07) thin films for a pH = 3 solutions is a promising material for photocatalysis application for water splitting to hydrogen-oxygen production. Nevertheless, we demonstrate that the formation of straddling gap heterostructure for CuS and CFS for a pH solution between 7 and 8 induces the production of oxygen and hydrogen.

Structure and Band Alignment of InP Photocatalysts Passivated by TiO2 Thin Films

Composite materials made by a low-band-gap semiconductor covered by a thin oxide layer are increasingly used in solar cells and photocatalysis. InP/TiO2 heterojunctions show high efficiency because of visible light absorption by the InP component and an efficient migration of photogenerated electrons toward TiO2. The passivating TiO2 film also guarantees stability in humid environments. In this work, we investigate the nature of InP/TiO2 by means of density functional theory calculations, showing that due to strong interface chemical bonds, the formation of an InP/TiO2 junction is a favorable process. Irrespective of the anatase TiO2 surface in contact with InP, (101) or (001), the system behaves as a type II junction, where the TiO2 band edges are lower in energy than InP ones, providing a rationalization for the observed electron–hole separation. We also considered explicitly the role of the TiO2 thin layer thickness, finding that a favorable junction is retained for every thickness of the titania passivating film. The highest efficiency is predicted for film thicknesses of 2–3 nm as this provides a good compromise between favorable band edge alignment and efficient transfer of the photogenerated electrons to the surface.

Ionic gate spectroscopy of 2D semiconductors

Reliable and precise measurements of the relative energy of band edges in semiconductors are needed to determine band gaps and band offsets, as well as to establish the band diagram of devices and heterostructures. These measurements are particularly important in the field of two-dimensional materials, in which many new semiconducting systems are becoming available through exfoliation of bulk crystals. For two-dimensional semiconductors, however, commonly employed techniques suffer from difficulties rooted either in the physics of these systems, or of technical nature. The very large exciton binding energy, for instance, prevents the band gap to be determined from a simple spectral analysis of photoluminescence, and the limited lateral size of atomically thin crystals makes the use of conventional scanning tunneling spectroscopy cumbersome. Ionic gate spectroscopy is a newly developed technique that exploits ionic gate field-effect transistors to determine quantitatively the relative alignment of band edges of two-dimensional semiconductors in a straightforward way, directly from transport measurements (i.e., from the transistor electrical characteristics). The technique relies on the extremely large geometrical capacitance of ionic gated devices that -- under suitable conditions -- enables a change in gate voltage to be directly related to a shift in chemical potential. Here we present an overview of ionic gate spectroscopy, and illustrate its relevance with applications to different two-dimensional semiconducting transition metal dichalcogenides and van der Waals heterostructures.

Rational design of MoS2/g-C3N4/ZnIn2S4 hierarchical heterostructures with efficient charge transfer for significantly enhanced photocatalytic H2 production

The rational design of hierarchical heterojunction photocatalysts with efficient spatial charge separation remains an intense challenge in hydrogen generation from photocatalytic water splitting. Herein, a noble-metal-free MoS2/g-C3N4/ZnIn2S4 ternary heterostructure with a hierarchical flower-like architecture was developed by in situ growth of 3D flower-like ZnIn2S4 nanospheres on 2D MoS2 and 2D g-C3N4 nanosheets. Benefiting from the favorable 2D-2D-3D hierarchical heterojunction structure, the resultant MoS2/g-C3N4/ZnIn2S4 nanocomposite loaded with 3 wt% g-C3N4 and 1.5 wt% MoS2 displayed the optimal hydrogen evolution activity (6291 μmol g−1 h−1), which was a 6.96-fold and 2.54-fold enhancement compared to bare ZnIn2S4 and binary g-C3N4/ZnIn2S4, respectively. Structural characterizations reveal that the significantly boosted photoactivity is closely associated with the multichannel charge transfer among ZnIn2S4, MoS2, and g-C3N4 components with suitable band-edge alignments in the composites, where the photogenerated electrons migrate from g-C3N4 to ZnIn2S4 and MoS2 through the intimate heterojunction interfaces, thus enabling efficient electron-hole separation and high photoactivity for hydrogen evolution. In addition, the introduction of MoS2 nanosheets highly benefits the improved light-harvesting capacity and the reduced H2-evolution overpotential, further promoting the photocatalytic H2-evolution performance. Moreover, the MoS2/g-C3N4/ZnIn2S4 ternary heterostructure possesses prominent stability during the photoreaction process owing to the migration of photoinduced holes from ZnIn2S4 to g-C3N4, which is deemed to be central to practical applications in solar hydrogen production.

2D Honeycomb-Kagome Polymer Tandem as Effective Metal-Free Photocatalysts for Water Splitting.

On the basis of first-principles calculations, the potential of applying 2D honeycomb-kagome polymers made of heteroatom-centered triangulene derivatives to photocatalyze water splitting is explored. The designed 2D polymers possess indirect bandgaps in the range of 1.80-2.84eV and show pronounced light absorption in the ultraviolet and visible region of the solar spectrum. With suitable band edge alignment, the examined N- and B-center polymers can generate sufficient photon-excited electrons and holes to activate the hydrogen and oxygen evolution reactions, respectively. The combination of lattice-inherent band features (flat bands) with chemical functionalization (potential shift due to heteroatoms) makes it possible to construct tandem cells with suppressed electron/hole recombination for effective overall water splitting. In addition, there is a potential difference between the half-electrodes that can be utlized to power​ auxiliary components in self-sufficient photocatalyzers.

Role of P in improving V-doped SrTiO3 visible light photocatalytic activity for water splitting: A first – Principles study

In this work, the +3-valent P-doped SrTiO3 has been shown for the first time to be theoretically achievable and to improve the photocatalytic performance of SrTiO3 in co-doping with metal ions. The effects of P and V doping on the electronic structure, optical properties and photocatalytic activity of the SrTiO3 system were systematically investigated using a first-principles calculation method. The results show that when V is singly doped with SrTiO3, although the band gap width is greatly reduced and the optical absorption performance is significantly enhanced, it is difficult to photocatalytic split water for hydrogen production due to the high VBM. When one electron provided by +3-valent P as a donor is transferred near +5-valent V, the orbital hybridization between V-3d and O-2p will lead to a suitable reduction in the band gap width of SrTiO3. The band edge alignment respect to water redox potentials proves that its band edges match well with the redox potentials of water. It was also found that P and V co-doping could reduce the complexation of photogenerated carriers and create a new optical absorption peak in the visible range. The calculated results imply that the P and V co-doping could improve the visible light photocatalytic activity of perovskite SrTiO3.

NaNbO3/NaTaO3 Superlattices: Cation-Ordering Improved Band-Edge Alignment for Water Splitting and CO2 Photocatalysis.

Perovskite oxide heterostructures have been extensively investigated for their excellent photocatalytic properties. Here, through hybrid density functional theory calculations, we systematically investigate the formation of NaNbO3-NaTaO3 (NBO-NTO) heterostructures. The sequential cations replacement in the superlattices reveals the Nb-Ta ratio range that allows the effective formation of heterostructures, which occurs through a spontaneous polarization mechanism induced by the electrostatic potential discontinuity in the interface. The resulting cation ordering is responsible for the sawtooth-like potential distribution that spatially separates valence and conduction charges and reduces the heterostructure bandgap. The symmetric NBO5/NTO5 junction has the smallest bandgap (2.50 eV) whose transitions are associated with Nb 5dxy orbitals on the interfacial plane. Such a relaxation mechanism provides the heterostructure with anisotropic optical properties and interface absorption peaks closer to the visible light spectrum. The phenomena strongly suggest the use of these heterostructures in photocatalytic reactions, supported by their coherent band-edge alignment with both water splitting and CO2 reforming potentials.

New honeycomb-like M-based (M = C, Si, Ge and Sn) monochalcogenides polymorphs: An extended family as isoelectronic photocatalysts of Group-VA for water splitting

Employing density functional theory (DFT), designing isoelectronic counterparts of phosphorene, the lattice-structure-dependent geometric, energetic, stable, electronic and photocatalytic performances of phosphorene-like group-IV monochalcogenides are investigated systematically. For each monolayer, three possible atomic motifs were considered, namely, the δ-phosphorene-like, ε-phosphorene-like and γ-phosphorene-like structures, in which we choose the binary monolayers constituted by same numbers of group IV and group VI atoms. The binding energy, phonon and molecular dynamics simulations confirm that most of those two-dimensional (2D) group IV-VI systems are energetically, dynamically and thermodynamically stable synchronously. In electronics, most of them present semiconducting behaviors, among which ten monolayers are direct bandgap semiconductors. Interestingly, we note that many of those indirect semiconducting systems possess rather small difference between their direct and indirect bandgaps. Hence, there is great feasibility of making those indirect monolayers experience an indirect-to-direct bandgap transition by suitable modulations. In fact, under fairly small uniaxial load (⩽0.05), our results predict that as many as seven indirect semiconductors experience an indirect-to-direct bandgap transition, which greatly enrich the members of the 2D direct bandgap family. Moreover, the band-edge alignments of all 2D semiconductors reveal several sheets can be served as an intrinsic photocatalyst for application in the photocatalytic water splitting. Particularly, for δ-GeO, δ-SnO and δ-GeS monolayers, high visible light absorption, desirable carriers mobility, strong stability in aqueous and acidic environments make them efficient photocatalysts.

Stable Unbiased Photo-Electrochemical Overall Water Splitting Exceeding 3% Efficiency via Covalent Triazine Framework/Metal Oxide Hybrid Photoelectrodes.

Photo-electrochemical (PEC) water splitting systems using oxide-based photoelectrodes are highly attractive for solar-to-chemical energy conversion. However, despite decades-long efforts, it is still challenging to develop efficient and stable photoelectrodes for practical applications. Here, thin layers of covalent triazine frameworks (CTF-BTh) containing a bithiophene moiety are conformably deposited onto the surfaces of a Cu2 O photocathode and a Mo-doped BiVO4 photoanode via electropolymerization to construct new hybrid photoelectrodes, successfully addressing the efficiency and stability issues. The CTF-BTh possesses a suitable band structure to form favorable band edge alignment with each metal oxide, creating a p-n junction and a staggered type-II heterojunction with Cu2 O and Mo-doped BiVO4 , respectively. Thus, the as-fabricated hybrid photoelectrodes exhibit substantially increased PEC performances. Meanwhile, the CTF-BTh film also serves as an effective corrosion-resistant overlayer for both photoelectrodes to inhibit photocorrosion and enable long-term operation for 150 h with only ≈10% loss in photocurrent densities. Furthermore, a stand-alone unbiased PEC tandem device comprising CTF-BTh-coated photoelectrodes exhibits 3.70% solar-to-hydrogen conversion efficiency. Even after continuous operation for 120 h, the efficiency can still retain at 3.24%.