Band Edge Alignment Research Articles

Hexagonal Janus Zn2XY (X=S, Se; Y[dbnd]Se, Te; X≠Y) monolayers: A high-performance photocatalyst for water splitting

Efficient photocatalytic water splitting is of holds paramount importance in addressing contemporary energy and environmental challenges. The emergence of symmetry-deficient Janus two-dimensional materials has opened new avenues for advancing the field of photocatalytic hydrogen production. Using first-principles calculations, we investigated a novel Janus Zn2XY (X = S, Se; YSe, Te; X≠Y) monolayer. The results demonstrate exceptional structural stability and favorable characteristics for water splitting, including direct band gaps and optimal band edge alignment. Spatially separating band edge states and the intrinsic vertical electric field effectively suppress the recombination rate of photogenerated electron-hole pairs. Strain engineering enhances the monolayer's capacity for visible light absorption. Furthermore, All Janus monolayers exhibit an impressive electron mobility of 103 cm2/V/s, with Janus Zn2STe and Zn2SeTe monolayers reaching 16.28% and 16.34% Solar-to-hydrogen (STH) efficiencies, respectively. These findings showcase the superior performance of Janus Zn2XY in water splitting and guide future experimental endeavors.

First-principles prediction of enhanced photocatalytic activity in La-X (X = Sc, V, Cr, Mn, Fe, Co, Ni, or Cu) Co-doped SrTiO3

The influence of La-X (X = Sc, V, Cr, Mn, Fe, Co, Ni, or Cu) co-doping on the electronic structure and photocatalytic activity of SrTiO3 system was investigated using the open-source software package Quantum Espresso (QE) based on density functional theory. Defect formation energy calculations showed an energy advantage for co-doped systems under Ti-poor and O-rich conditions. The La-V, La-Cr, La-Mn, La-Fe, and La-Ni co-doped systems showed magnetic properties with corresponding total magnetization of 0.08 µB, 2.97 µB, 3.30 µB, 1.00 µB and 1.00 µB, respectively. The La-X (X = V, Cr, Mn, Fe, Ni, or Cu) co-doped system exhibits photoabsorption activity in the visible region as compared to the pure SrTiO3. The La-X (X = Cr, Fe, Ni, or Cu) co-doped SrTiO3 exhibits a long carrier lifetime, especially the La-Fe co-doped system with a high effective mass ratio of holes to electrons of 5.79. However, the redox potential of the water decomposition reaction of the La-Cu co-doped system is too large, making it unsuitable for water splitting. In summary, the La-X (X = Cr, Fe, or Ni) co-doped systems exhibited intermediate states in the band structure and density of states, which better meet the specific requirements for photocatalysis compared to systems without intermediate states, showing superior photocatalytic potential. The band edge alignment of the three co-doped systems with water redox potentials demonstrated good matching. These findings suggest that La-X (X = Cr, Fe, or Ni) co-doped SrTiO3 is a promising candidate material for solar-driven water splitting.

A theoretical investigation on optoelectronic and photocatalytic behaviour of Janus X-Ga-Al-Y (X, Y[dbnd]S and Te) monolayers

The structural, electronic, and optical characteristics of hexagonal Janus X-Ga-Al-Y (X, YS and Te) monolayers have been assessed within the theoretical framework by using density functional theory. The negative cohesive energy of all monolayers confirms the energetic stability. The phonon dispersion diagram validates that all Janus X-Ga-Al-Y (X, YS and Te) monolayers are dynamically stable. The obtained indirect type band gap values of monolayers GaS, GaAlS2, GaAlTe2, SGaAlTe, and TeGaAlS are 3.25 eV, 2.87 eV, 2.45 eV, 2.25 eV, and 1.39 eV by utilizing Heyd-Scuseria-Ernzerhof functional respectively which indicates their semiconductor behaviour. All the monolayers possess the utmost absorption in the ultraviolet (UV) portion. Hence, X-Ga-Al-Y (X, YS and Te) monolayers have promising applications as UV absorbers, optoelectronic devices, and UV detectors. The band edge alignments of the X-Ga-Al-Y (X, YS and Te) monolayers also have been analyzed which suggests that X-Ga-Al-Y (X, YS and Te) monolayers are efficient as photocatalysts for hydrogen production.

Out-of-plane polarization modulated band alignments in β-In2X3/α-In2X3 (X = S and Se) vdW heterostructures

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures is an effective strategy to overcome the intrinsic disadvantages of individual 2D materials. Herein, by employing first-principles calculations, the electronic structures and potential applications in the photovoltaic field of the β-In2X3/α-In2X3 (X = S and Se) vdW heterostructures have been systematically unraveled. Interestingly, the band alignments of β-In2S3/α-In2S3, β-In2Se3/α-In2Se3, and β-In2Se3/α-In2S3 heterostructures can be transformed from type-I to type-II by switching the polarization direction of α-In2X3 layers. It is highlighted that the light-harvesting ability of the β-In2X3/α-In2X3 vdW heterostructures is significantly higher than the corresponding monolayers in nearly the entire visible light region. Interestingly, type-II β-In2S3/α-In2Se3↓ heterostructure can achieve the power conversion efficiency of 17.9%, where the α-In2Se3 layer acts as a donor and the β-In2S3 layer displays as the acceptor. The present research not only provides an in-depth understanding that the out-of-plane polarization of α-In2X3 monolayers can efficiently modulate the band edge alignment of the β-In2X3/α-In2X3 vdW heterostructures, but also paves the way for the application of these heterostructures in the field of photovoltaics and optoelectronics.

Impact of Strain on Electronic and Optical Properties of MgClBr Monolayer: First-principle Calculation

We have introduced a novel halide monolayer, MgClBr, and performed a comprehensive analysis of its structural, and optoelectronics properties using first-principles calculations. The electronic band structure revealed that MgClBr material has a wide “Γ-Γ” direct band gap. The monolayer's dynamic, thermal, and energetic stability is verified through phonon spectra, AIMD calculation, and cohesive energy calculation, respectively. The MgClBr exhibited a 6.08 eV band gap from HSE06 hybrid functional, slightly larger than MgBr2 but smaller than MgCl2 monolayers. The Strain effects on the optoelectronic characteristics are examined. The MgClBr monolayer shows a band gap of 4.23 eV for −10% and 4.30 eV for + 10% strain, remaining stable under these conditions. Notably, the MgClBr monolayer has favourable band-edge alignment for oxidation–reduction processes involved in water splitting at pH zero. Additionally, the monolayer exhibited significant absorption in extreme UV regions, suggesting its potential as a material for optoelectronic nanodevices like UV-emitters and detectors, electrically insulators and non-reflective overlay material.

Using orbital sensitivity analysis to pinpoint the role of orbital interactions in thermoelectric power factor

The modification of the electronic band structure is of critical importance for thermoelectric materials whose heat to electricity power generation is related to carrier effective mass and alignment of semiconductor band edges. One approach to optimize the electronic band structure is by modification of orbital interactions through doping or alloying. While the current ab-initio quantum chemical calculations can help us to investigate orbital components of electronic bands, they reveal little information on the relative tunability of electronic states and transport properties with respect to orbital interactions. In this work, we present a method, based on a symmetry-adapted tight-binding model and sensitivity analysis, that can pinpoint the role of orbital interactions in determining electronic band structure and transport properties. As an application, a systematic theoretical analysis is used to show how the power factor of PbTe can/cannot be improved by playing with interatomic orbital interactions. The orbital interaction sensitivity analysis introduced in this work can easily be applied to other compounds or properties.

Fermi level induced band edge alignment and band bending in Ag3PO4/Cu2O p-n heterojunction for proficient photocatalytic applications

Fabrication of binary p-n heterojunction and the band alignment at the interface of the individual semiconductor photocatalyst has been extensively studied to verify superior charge separation and migration capability in photocatalytic applications. Here, a simple wet chemical followed by a hydrothermal fabrication strategy was introduced for the development of the binary p-n Ag3PO4/Cu2O heterostructures. The tunable band structure of individual semiconductors with the work function (φ), witnessed a band bending at the space charge region. The bending at the interface induces a carrier concentration gradient and manifests a rectifying current transport diode. The fabricated p-n heterostructures and carrier migration between n-type Ag3PO4 and p-type Cu2O were verified by different morphological and physicochemical methods. The band banding at the interface, leading to a narrow depletion region, favors the tunneling of electron-hole pairs through a Z-scheme carrier transport mechanism. The electron-hole pair movement has further been confirmed by considering the band edge position after contact, photocatalytic scavengers, and the radical trapping experiment. The Ag3PO4/Cu2O p-n heterojunction photocatalyst manifested a 737.4 μmol g−1h−1 of H2 generation and 91% endosulfan degradation efficiency, these are 12 and 9 times higher than that of pure Ag3PO4. The p-n heterojunction photocatalyst displayed a higher current density with electron-hole migration efficiency, synergistically enhancing the catalytic activity through the interfacial space charge junction.

Screening Multi-layered Two-dimensional Cd-chalcogenide Structures as Potential Candidates for Photocatalysis.

In recent years, clean and sustainable energy generation by photocatalytic water splitting has gained enormous attention from researchers. Two-dimensional Cd-based structures play a central role in the research of semiconductor-based photocatalysis. Here, a few layers of cadmium monochalcogenides (CdX; X=S, Se, and Te) are theoretically investigated using density functional theory (DFT). For their potential applicability in photocatalysis, it is proposed that they are exfoliated from the wurtzite structure with an electronic gap that depends on the thickness of the proposed systems. Our calculations address a long-standing doubt about the stabilities of free-standing CdX monolayers (ML). Induced buckling removes the acoustic instabilities in 2D planar hexagonal CdX structures (due to interlayer interactions) that depend on the number of neighboring atomic layers present. All studied (and stable) systems have an electronic gap of >1.68 eV, calculated with hybrid functionals (HSE06). A band-edge alignment plot about the water's oxidation-reduction potential is constructed, and a potential energy surface is constructed for the hydrogen evolution reaction. Our calculations suggest that the chalcogenide site is most favorable for hydrogen adsorption, and the energy barrier falls within the experimentally achievable limits.

Predictions of two-dimensional biphenylene GeS and its seven counterparts and possible applications

Very recently, a two-dimensional (2D) biphenylene carbon network, named as bi-C here, has received many attentions. In this work, we identified a 2D biphenylene GeS, named as bi-GeS, and its seven counterparts. For bi-GeS, examination of the stability shows that it meets the requirements of dynamical, thermal and mechanical stabilities. Analyses of the electronic properties suggest that bi-GeS monolayer, bilayer and trilayer retain semiconducting properties with suitable band edge alignments of water splitting. In particular, distinct charge separations are found in bi-GeS bilayer and trilayer. Besides, our studies bore out that its seven counterparts, including bi-P, bi-As, bi-GeSe, bi-SnS, bi-GaS, bi-GaSe and bi-InSe, are also dynamically stable. Further studies revealed that they also have opportune redox levels of water splitting. Our results tremendously enriched 2D biphenylene family and provided many potential maquettes for the photocatalytic application.

Engineering two-dimensional SnC/HfSSe heterojunction as a direct Z-scheme photocatalyst for water splitting hydrogen evolution

Developing robust water-splitting photocatalyst remains a pivotal challenge for the conversion of solar energy to hydrogen fuel. Herein, we report SnC/HfSSe heterojunctions to be a competitive direct Z-scheme photocatalysts with HfSSe acting as an oxidant and SnC as a reductant for water-splitting hydrogen generation by employing the first-principles calculations. The photogenerated electron–hole migration logically follows the Z-scheme path due to the large interlayer built-in electric field, small interlayer bandgap, proper band edge alignment, and high interlayer carrier recombination. The overpotential is appropriate to the oxygen evolution reaction (OER), while the overpotential is slightly large to the hydrogen envolution reaction (HER) so that the incident photon energy above the band gap of the components is required for the HER. The heterojunctions have excellent visible light absorption, exhibiting the remarkable red shift and stronger light absorption with respect to the two components of the heterojunction. Moreover, the heterojunction exhibits an outstanding solar-to-hydrogen (STH) efficiency as high as 27% in extremely acidic solution.

Gate-Induced Trans-Dimensionality of Carrier Distribution in Bilayer Lateral Heterosheet of MoS2 and WS2 for Semiconductor Devices with Tunable Functionality

Using the metal–organic chemical vapor deposition technique, we synthesized a bilayer lateral heterostucture of MoS2 and WS2, each of which layers consist of alternately arranged nanostrips of MoS2 and WS2. Transmission electron microscope images exhibit a checked pattern contrast, reflecting the three distinct interlayer metal arrangements of Mo/Mo, W/Mo (Mo/W), and W/W stacking. Theoretical calculations based on the density functional theory elucidated that the bilayer lateral heterostructure of MoS2 and WS2 exhibits complexed type-II band edge alignments depending on the interlayer metal arrangement: The conduction band edge is located at the Mo atom in a Mo/Mo sector, while the valence band edge is located at the W atom in a W/W sector. According to the complexed band edge alignment, the field-induced carrier injection behavior in the bilayer heterosheet composed of MoS2 and WS2 strips shows gate-induced trans-dimensionality, where the accumulated carrier distributions continuously vary from zero- to two-dimensional based on the applied gate voltage. Tunable carrier dimensionality can be applicable for wide areas of electronics, such as quantum dot arrays with tunable dot–dot interaction.

Quantitative Evaluation of the Carrier Separation Performance of Heterojunction Photocatalysts: The Case of g-C3N4/SrTiO3

Heterojunction photocatalysts are of great interest in the energy and environmental fields, because of their potential to significantly increase the efficiency of harvesting solar energy. Advances in design have been hampered by the continued use of only qualitative analyses. Quantitative evaluation of the carrier separation performance is urgently needed for the design and application of heterojunction photocatalysts. Taking the g-C3N4/SrTiO3 heterojunction as an example, we address the conventional energy band and electronic structure issues by first-principles analysis. After interface coupling, the band edge alignment reverses from that of the respective isolated states of the heterojunction components, suggesting new ways of thinking about the catalytic mechanism of the heterojunction. More significantly, we show the carrier separation performance of heterojunction photocatalysts can be quantitatively predicted by the nonadiabatic molecular dynamics method, enabling more precisely directed research and promoting the quantified design and application of heterojunction photocatalysis, making a contribution of great scientific significance.

Janus Ga2SeTe and In2SeTe nanosheets: Excellent photocatalysts for hydrogen production under neutral pH

In the past few years, Janus nanosheets have attracted much interest according to their specific structure and considerable potential to address the energy and environmental issues. Herein, the electronic, optical and photocatalytic properties of two-dimensional Janus Ga2SeTe and In2SeTe have been studied using ab-initio computations based on the density functional theory. The obtained results show that these nanomaterials exhibit a semiconductor behavior with direct and moderate bandgaps using hybrid HSE06 functional. Subsequently, the understudied compounds present suitable optical conductivity, absorption, transmission and reflectivity for water splitting under the ultraviolet–visible light irradiation. Interestingly, the band edge positions of Janus Ga2SeTe and In2SeTe excellently straddle the redox potentials of water under neutral pH. Additionally, the free energy values for the formation of H2 from H adsorbed on the Ga2SeTe and In2SeTe compounds are respectively 1.304eV and 0.976eV at pH = 7. More excitingly, the present study proposes strain engineering approach to improve the photocatalytic performance of the Janus Ga2SeTe and In2SeTe monolayers. Specifically, the investigated semiconductors show more appropriate band edge alignment and better hydrogen evolution reaction activity under biaxial tensile strain, which fulfil the water splitting requirements at neutral pH conditions. Our findings conclude that the Janus Ga2SeTe and In2SeTe nanosheets are promising candidates for photocatalytic hydrogen production.

Tunable covalent benzo-heterocyclic rings constructed using two-dimensional conjugated polymers for visible-light-driven water splitting.

Developing highly efficient, stable, and cost-effective two-dimensional (2D) conjugated polymers (CPs) for overall water splitting (OWS) is critical for producing clean and renewable hydrogen energy, yet it remains a great challenge. Here, we designed eight 2D CPs through the topological assembly of diacetylene and benzene-derived molecular linkers that can offer active sites for hydrogen and oxygen evolution reactions, and explored their structural, electronic, optical, and photocatalytic OWS properties by performing first-principles computations. It is shown that incorporating benzo-heterocyclic rings into CPs can significantly modulate the electronic structures of CPs and broaden the spectral absorption, suitable for visible-light-driven OWS. Remarkably, through a range of screening criteria, including stability, electronic band structures, band edge alignments, and photocatalytic activity, we found that CP-4 based on diacetylene and benzotrifuran can spontaneously trigger the OWS in a neutral environment under its own light-induced bias, eliminating the need for sacrificial agents or cocatalysts. Specifically, the HER active site is primarily located at diacetylene moieties, while the OER active site is mainly concentrated on the benzo-heterocyclic rings. Moreover, the ideal STH efficiency for OWS on CP-4 was estimated to be 13.87%, highlighting its potential as a prospective photocatalyst for large-scale industrial OWS. Our findings open a door to the rational design of novel polymer photocatalysts for OWS.

Improving photocatalytic hydrogen production by switching charge kinetics from type-I to Z-scheme via defective engineering.

By providing the spatial separation of the active sites and retaining high oxidative and reducing capacity, the direct Z-scheme heterostructure is considered the most potential structure for yielding photo-electric response. However, challenges still exist in the directional transfer of charge carriers between two semiconductors in direct Z-scheme structures. In this regard, by constructing the Vzn defect and p-n junction, a direct Z-scheme ZnxCd1-xS@ZnS-NiS heterostructure was obtained for the regulated electronic structure, which ensured high-yield hydrogen properties. The Zn vacancy in the partially-coated ZnS shell led to the Vzn energy level, and the addition of NiS led to the p-n structure, which caused a drastic downshift of the band edge potentials in comparison to that of pristine CdS. This variation gave rise to a staggered band edge alignment between ZnxCd1-xS and NiS, resulting in the variation of charge transfer kinetics from type-I to direct Z-scheme. Through careful characterization, it was found that the optimal photocatalytic hydrogen precipitation activity reached 16 683.6 μmol g-1 h-1, which was 70 times that of CdS, and this improvement was considered to form a spatial barrier, providing a clear direction and path for carrier transmission.

Growth-control of hexagonal CdS-decorated ZnO nanorod arrays with low-temperature preheating treatment for improved properties and efficient photoelectrochemical applications.

The limitations of oxide semiconductor-based solar cells in achieving high energy conversion efficiencies have prompted incessant research efforts towards the creation of efficient heterostructures. Despite its toxicity, no other semiconducting material can fully replace CdS as a versatile visible light-absorbing sensitizer. Herein, we explore the aptness of preheating treatment in the successive ionic layer adsorption and reaction (SILAR) deposition technique and improve the understanding of the principle and the effects of a controlled growth environment on thus-formed CdS thin films. Single hexagonal phases of nanostructured cadmium sulfide (CdS)-sensitized zinc oxide nanorods arrays (ZnO NRs) have been developed without the support of any complexing agent. The influences of film thickness, cationic solution pH and post-thermal treatment temperature on the characteristics of binary photoelectrodes have been investigated experimentally. Interestingly, the preheating-assisted deposition of CdS, which is rarely applied for the SILAR technique, resulted in improved photoelectrochemical performance similar to the post-annealing effect. The X-ray diffraction pattern revealed that optimized ZnO/CdS thin films were polycrystalline with high crystallinity. Examination of the morphology of the fabricated films via field emission scanning electron microscopy showed that film thickness and medium pH altered the growth mechanism of nanoparticles, thereby changing their particle sizes, which had a significant influence on the film's optical behavior. The effectiveness of CdS as a photosensitizer and the band edge alignment for ZnO/CdS heterostructures were evaluated using ultra-violet visible spectroscopy. Facile electron transfer in the binary system as evidenced in electrochemical impedance spectroscopy Nyquist plots, therefore, promotes higher photoelectrochemical efficiencies from 0.40% to 4.30% under visible light illumination as compared with the pristine ZnO NRs photoanode.

Electron transport bilayer with cascade energy alignment based on Nb2O5–Ti3C2 MXene/TiO2 for efficient perovskite solar cells

Nb2O5–Ti3C2 exhibits beneficial band-edge alignment, favoring carrier migration with a higher density of extracted charges and increased electron lifetime.

Strain engineering the electronic properties of the type-II CdO/MoS2 van der Waals heterostructure

The structural stability, electronic properties, band edge alignment and photocatalytic water splitting characteristics of the CdO/MoS2 heterostructure have been systematically investigated by first-principles calculations. The results show that the CdO/MoS2 heterostructure is a stable indirect bandgap (1.35 eV) semiconductor with a type-II band alignment and a large built-in electric field pointing from MoS2 to CdO, which greatly hinders the recombination of photogenerated carriers. Meanwhile, the CdO/MoS2 heterostructure with large carrier mobility can withstand the large biaxial strains. The band edge positions of the CdO/MoS2 heterostructure can be modulated by biaxial strain engineering. These results provide a theoretical basis for the application of the CdO/MoS2 heterostructure in photovoltaic and other electronic devices.

Robust broken-gap MoTe2/ZrS2 van der Waals heterostructure

Two-dimensional van der Waals (vdW) heterostructures with several possibilities of band edge alignment are promising candidates in the design of semiconductor junction devices with good electrostatic control. We predict a robust broken-gap band alignment for the ${\mathrm{MoTe}}_{2}/{\mathrm{ZrS}}_{2}$ vdW heterostructure and uncover the underlying mechanism. Our first-principles calculations reveal that upon formation of the vdW heterostructure a small amount of charge is transferred between the layers which sits on the central transition metal atoms and is responsible for the band edge alignment. Moreover, the lattice-mismatch-induced in-plane strain and the interfacial polarization are shown to play no role in band alignment. However, we find a weak sensitivity of the electronic structure and band edge alignment of the vdW heterostructures to an external normal-to-plane strain or electric field. Our findings are helpful in designing heterojunction devices with desired electronic structures.

Electronic and photocatalytic properties of ZnO/GaTe heterostructure from first principles calculations

Constructing a novel van der Waals (vdW) heterostructure by integrating diverse two-dimensional (2D) materials is significant to generate the more fascinating performances. In this present contribution, the electronic and photocatalytic properties of ZnO/GaTe heterostructure have been explored using first-principles calculations. The results reveal that the interaction between layers is dominated by the vdW forces. It displays an intrinsic type-II band alignment and a direct band gap of 1.730 eV under hybrid HSE06 functional. The valence band top and conduction band bottom are located at ZnO and GaTe layers, respectively, which guarantees the spatial separation of photogenerated electron-hole pairs and improves the photocatalytic efficiency. Moreover, the band edges all span the energy levels of both the reduction and oxidation potentials of H2O, indicating that its better photocatalytic performance. The modulated photocatalytic properties under varied pH circumstances imply that an acid environment is more favorable to the water splitting process. Furthermore, the band edge alignment exhibits the significant adjustability through varying vertical and in-plane biaxial strains. These theoretical findings indicate that the ZnO/GaTe heterostructure can offer an alternative strategy to design the efficient photocatalysts for water splitting.