Band Edge Alignment Research Articles

Vertical Stacking of Copper Sulfide Nanoparticles and Molybdenum Sulfide Nanosheets for Enhanced Nonlinear Absorption.

The construction of p-n junctions is necessitated by applications which require effective charge separation. Here, a novel heterostructure (HS) of molybdenum sulfide (MoS2) and copper sulfide (Cu2-xS) was synthesized by chemical vapor deposition, with Cu2-xS nanoparticles vertically stacked on a MoS2 nanosheet. A well-defined epitaxial relationship between MoS2 and Cu2-xS is established, although the corresponding lattice mismatch is as large as 20%. The band-edge alignment is experimentally determined, indicating that the MoS2-Cu2-xS HS is a type II heterojunction. Photoluminescence quenching indicates effective charge separation in HS. The resultant HS shows enhanced nonlinear absorption in comparison with single-component MoS2 nanosheets and Cu2-xS nanoparticles.

Energetics and electronic structures of borders between MoS2 and WS2

We studied the energetics of two-dimensional in-plane heterostructures consisting of transition metal dichalcogenides, MoS2 and WS2, with respect to the border structure using density functional theory. A WS bond is energetically preferable at the border between MoS2 and WS2, so that the border tend to form WS bond as much as possible. We also found that the polarization at the zigzag border increases the total energy of the heterostructures. Competition between the bond formation energy and the polarization energy leads to chiral borders at which WS heterobonds are dominant. The border possesses type II band edge alignment with the band offsets of 0.16 and 0.28 eV for valence and conduction band edges, respectively.

Phosphorene-based van der Waals heterojunction for solar water splitting

As a clean and renewable future energy source, hydrogen fuel can be produced via solar water splitting. Two-dimensional (2D) black phosphorene (black-P) can harvest visible light due to the desirable band gap, which promises it as a metal-free photocatalyst. However, black-P can be only used to produce hydrogen since the oxidation potential of water locates lower than the position of the valence band maximum. To improve the photocatalytic performance of black-P, here, using black-P and blue phosphorene (blue-P) monolayers, we propose a 2D van der Waals (vdW) heterojunction. Theoretical results, including the band structures, density of states, Bader charge population, charge density difference, and optical absorption spectra, clearly reveal that the visible light absorption ability is obviously improved, and the band edge alignment of the proposed vdW heterojunction displays a typical type-II feature to effectively separate the photogenerated carriers. At the same time, the built-in interfacial electric field prevents the electron-hole recombination. These predictions suggest that the examined phosphorene-based vdW heterojunction is an efficient photocatalyst for solar water splitting.

Energy band edge alignment of anisotropic BiVO4 to drive photoelectrochemical hydrogen evolution

Bismuth vanadate (BiVO4) has been highlighted as the most efficient photoanode light absorber for the solar light-driven water oxidation reaction despite the lack of fundamental understanding of its poor performance. Though the BiVO4 performance has been significantly improved, the underlying reasons for this performance are not well understood. Here, anisotropically grown BiVO4 is designed to achieve band edge engineering induced by coupled crystal facets for hydrogen evolution reaction (HER), which is not possible with isotropic BiVO4 powder suspension. HER performances depending on the exposure area of the crystal facets in the anisotropic BiVO4 are investigated using electronic conduction and in situ X-ray absorption spectroscopy during HER. Our results reveal that the nature of the exposed area of the optimized coupled crystal facets in a BiVO4 photoelectrode critically dominates the surface charge density and conductivity, efficiently driving the HER. Sequentially, coupling between different crystal facets of BiVO4 photoelectrode led energy band edge to be upward. The results show that the energy band edge alignment tuned by coupled crystal facet can be applicable to other inorganic photocatalyst/photoelectrodes to promote efficient artificial photosynthetic reactions.

Tailoring of Interfacial Band Offsets by an Atomically Thin Polar Insulating Layer To Enhance the Water-Splitting Performance of Oxide Heterojunction Photoanodes.

An important factor in the performance of photoelectrochemical water splitting is the band edge alignment of the photoelectrodes for efficient transport and transfer of photogenerated carriers. Many studies for improving charge transfer ability between the electrode and the electrolyte have been reported, while research to improve charge transfer at the interface of the photoactive semiconductor and the conducting substrate is largely lacking. Here, we demonstrate that the water-splitting performance of an oxide heterostructured photoelectrode can be increased 6-fold by inserting an atomically thin polar LaAlO3 interlayer compared with that of an oxide heterostructure without an insertion to modify interfacial band offsets. The electrically lowered Schottky barrier is driven by the atomically thin layer, and the charge transfer resistance between the oxides is reduced by up to 2 orders of magnitude upon insertion of LaAlO3, a wide-gap (5.6 eV) insulator. We show that the critical thickness of the polar layer for enhancing the charge transfer is 3 unit cells. The dipole moment from the polar sheets of LaAlO3 introduces an internal electric field, which modifies the effective band offsets in the device. This work serves as a proof of concept that photoelectrochemical performance can be improved by manipulating the band offsets of the heterostructure interface, suggesting a new design strategy for heterostructured water-splitting photoelectrodes.

Band edge alignment for tuning interfacial charge transfer: a case study of NaTaO3 as photoelectron platform by anchoring CdTe quantum dots

CdTe quantum dots (QDs) were anchored on NaTaO3 nanocubes with the aim into tuning the charge kinetics for spatial separation of the photogenerated electrons and holes by modifying the relative potential of the conduction band of CdTe QDs. In this work, CdTe QDs (particle size 2–3 nm) and NaTaO3 were synthesized via reflux and hydrothermal reaction. Thioglycolic acid (TGA) acted as both a stabilizer and linker molecules during the synthesis of CdTe QDs and NaTaO3/CdTe heterostructure. On accounts of density functional theory (DFT) predictions, electrons can transfer from NaTaO3 to CdTe due to the difference of the Fermi level between two semiconductors, which will establish a built-in electric field at semiconductor interfaces, accelerating the charge separation kinetics between CdTe and NaTaO3. In response, electronic structure tunable CdTe QDs were surface engineered on NaTaO3 nanocubes to enhance the visible light (VL) harvesting capability. By carefully controlling the fine nature of CdTe QDs, photogenerated electrons in CdTe can be efficiently injected into the conduction band of NaTaO3, leading to spatial charge separation between CdTe and NaTaO3. This could be affirmed by applied bias photo to current efficiency (ABPE), incident photocurrent responses as well as electrochemical impedance curves. With well-defined crystallinity, electronic structure, and interfacial contact between NaTaO3 and CdTe, the optimized photocatalytic activity toward hydrogen production over NaTaO3/CdTe heterostructure achieved an evolution rate of 56 μmol·g−1·h−1, which is far surpassed than that of pristine NaTaO3 and CdTe.

Insulating titanium oxynitride for visible light photocatalysis

We propose a systematic approach to obtain various forms of insulating titanium oxynitrides Ti$_{n}$N$_{2}$O$_{2n-3}$ and we conduct a detailed study on its $n=2$ case, Ti$_{2}$N$_{2}$O. We study the energetics and the electronic structures of Ti$_{2}$N$_{2}$O and compare these results with those of pristine and nitrogen-doped TiO$_{2}$ within the framework of the density-functional theory (DFT) and the GW approximation. We find that Ti$_{2}$N$_{2}$O is semiconducting with the calculated band-gap of 1.81 eV, which is significantly smaller than those of pristine TiO$_{2}$ rutile (3.14 eV) or anatase (3.55 eV). Furthermore, the reduction of the band-gap of Ti$_{2}$N$_{2}$O is realized not by lowering of the conduction-band minimum but by rising the valence-band maximum. Therefore the proposed Ti$_{2}$N$_{2}$O has suitable band-edge alignment for water-splitting photocatalysis. Finally, total energy calculations indicate that Ti$_{2}$N$_{2}$O is potentially easier to synthesize than nitrogen-doped TiO$_{2}$. Based on these results, we propose Ti$_{2}$N$_{2}$O as a promising visible-light photocatalytic material.

Strain-Tunable Visible-Light-Responsive Photocatalytic Properties of Two-Dimensional CdS/g-C₃N₄: A Hybrid Density Functional Study.

By means of a hybrid density functional, we comprehensively investigate the energetic, electronic, optical properties, and band edge alignments of two-dimensional (2D) CdS/g-CN heterostructures by considering the effect of biaxial strain and pH value, so as to improve the photocatalytic activity. The results reveal that a CdS monolayer weakly contacts with g-CN, forming a type II van der Waals (vdW) heterostructure. The narrow bandgap makes CdS/g-CN suitable for absorbing visible light and the induced built-in electric field between the interface promotes the effective separation of photogenerated carriers. Through applying the biaxial strain, the interface adhesion energy, bandgap, and band edge positions, in contrast with water, redox levels of CdS/g-CN can be obviously adjusted. Especially, the pH of electrolyte also significantly influences the photocatalytic performance of CdS/g-CN. When pH is smaller than 6.5, the band edge alignments of CdS/g-CN are thermodynamically beneficial for oxygen and hydrogen generation. Our findings offer a theoretical basis to develop g-CN-based water-splitting photocatalysts.

Role of Dimensionality for Photocatalytic Water Splitting: CdS Nanotube versus Bulk Structure.

Using state-of-the-art density functional theoretical calculations, we have modelled a facetted CdS nanotube (NT) catalyst for photocatalytic water splitting. The overall photocatalytic activity of the CdS photocatalyst has been predicted based on the electronic structures, band edge alignment, and overpotential calculations. For comparisons, we have also investigated the water splitting process over bulk CdS. The band edge alignment along with the oxygen evolution reaction/hydrogen evolution reaction (OER/HER) mechanism studies help us find out the effective overpotential for the overall water splitting on these surfaces. Our study shows that the CdS NT has a highly stabilized valence band edge compared to that of bulk CdS owing to strong p-d mixing. The highly stabilized valence band edge is important for the hole-transfer process and reduces the risk of electron-hole recombination. CdS nanotube requires less overpotential for water oxidation reaction than the bulk CdS. Our findings suggest that the efficiency of the water oxidation/reduction process further improves in CdS as we reduce its dimensionality, that is going from bulk CdS to one-dimensional nanotube. Furthermore, the stabilized valence band edge of CdS nanotube also improves the photostability of CdS, which is a problem for bulk CdS.

Dual Liquid Junction Photoelectrochemistry: Part II. Open-Circuit Photovoltage Variations Due to Surface Chemistry, Interfacial Dipoles, and Non-Ohmic Junctions at Back Contacts

Dual-liquid-junction photoelectrochemistry and finite-element computational modeling quantified the effect on open-circuit photovoltage, Voc, of varying barrier heights at the back, traditionally ohmic contact to a semiconductor. Variations in experimental back-contact barrier heights included changes in the redox potential energy of the contacting phase afforded by a series of nonaqueous, metallocene-based redox couples that demonstrate facile, one-electron transfer and dipole-based band edge shifts due changes in the chemical species at the semiconductor surface. Variation in semiconductor surface chemistry included hydrogen-terminated Si(111) as well as methyl-terminated Si(111) that yields a shift in band-edge alignment of ∼0.3 eV relative to hydrogen termination. While methylation of n-Si improves Voc values at rectifying contacts, methylation at an ohmic contact has a deleterious impact on Voc values. We discuss the present experimental and computational results in the context of non-ideal semiconductor contacts.

WS2/BSe van der Waals type-II heterostructure as a promising water splitting photocatalyst

The structure, electronic properties, charge transfer, band edge alignments and optical properties of WS2/BSe heterostructures are investigated by using first-principle calculations. Results imply that WS2/BSe heterostructures are semiconductor with an indirect energy gap (Egap) of 1.73 eV. Additionally, it has a feature of type-II band alignment, which contributes to spatial separation of photo-generated electron-hole pairs in WS2/BSe heterostructures. Charge transfer takes place from BSe to WS2 monolayers, which extends lifetime of the separated photo-induced charge carriers. Most importantly, band edges of the heterostructure are found to meet hydrogen evolution reaction (HER) in acid solutions (0 < pH < 7). Moreover, compared with only one for WS2 and BSe monolayers, the optical-absorption strength of the heterostructure is significantly enhanced and WS2/BSe heterostructures larges the range of absorption to 717 nm in the visible light region, improving the potential photocatalytic efficiency. These results explain the underlying mechanism of the enhanced photocatalytic activity of WS2/BSe heterostructures. Thus WS2/BSe heterostructures can be applied to 2D heterostructured photocatalysts.

Boosting photo charge carrier transport properties of perovskite BaSnO3 photoanodes by Sr doping for enhanced DSSCs performance

BaSnO3 (BSO) could be becoming an important next generation electron transport material in semiconductor technology due to its abnormal electrical properties even at room temperature. In this study, the strontium (Sr) is doped into BSO semiconductor via the facile co-precipitation method and then used as electron transporter in dye sensitized solar cells (DSSCs) is reported towards improving the power conversion efficiency (PCE). The physiochemical properties changes in the BSO systems by Sr doping assisted for an expeditious photo charge carrier transportation by making a desirable conduction band edge alignment with neighbouring TiO2 layer when use in BSO/TiCl4 treatment/TiCl4 scattering layer based photoanodes is confirmed by impedance studies. So, our new photoelectrode composition, 5% Sr: BSO/1 min TiCl4 treatment/TiO2 scattering layer yields one of the highest PCE by the outcome of 16.95 mA cm−2 (Jsc), 0.63 (FF), and 0.82 V (Voc), which is 8.83%. This result shows superior PCE among all our early reports of photoanodes such as BSO, BSO/scattering layer, BSO/TiCl4 treatment, BSO/TiCl4 treatment/scattering layer and 3% Fe: BSO/TiCl4 treatment/scattering layer. On the basis of recent reports, a gradual upsurge in the efficiency of BSO nanostructures based photoanodes under easily reproducible scientific/rational approach will facilitate an astonishing possibility to enhancing the PCE which confidently reinforce the probability of implementation in practical life and is indispensable for balancing the future non-renewable energy dearth.

Monolayer, Bilayer, and Heterostructures of Green Phosphorene for Water Splitting and Photovoltaics

We report the results of density functional theory-based calculations on monolayer and bilayer green phosphorene and their heterostructures with MoSe2. Both monolayer and bilayer green phosphorene are direct band gap semiconductors and possess anisotropic carrier mobility as high as 104 cm2/V/s. In bilayers, the pressure of about 9 GPa induces the semiconductor–metal transition. Moreover, the band gap depends strongly on the thickness of the films and the external electric field. By employing strain engineering under suitable solution conditions, monolayer and AC-stacked bilayer green phosphorene offer the band edge alignments which can be used for water splitting. The upper limit of the power conversion efficiencies for monolayer and AB- and AC-stacked bilayer green phosphorene heterostructures with MoSe2 is calculated to be 18–21%. Our results show the possibility of green phosphorene to be used as a photocatalytic and photovoltaic material in energy-related applications.

Effect of Water Adsorption on the Interfacial Structure and Band Edge Alignment of Anatase TiO2(001)/Water by First-Principles Molecular Dynamics

The generation of hydrogen by photoelectrochemical (PEC) water splitting is a promising pathway to produce renewable and scalable carbon-free energy. To improve the performance of PEC cells, it is essential to understand the structural and electronic properties of the photoelectrode/electrolyte heterogeneous interfaces. By performing first-principles molecular dynamics (FPMD) simulations, we revealed the dynamic structure, such as water dissociation and adsorption and hydrogen bond network, and the band edge alignment of the TiO2/water interfaces. Mixed molecular and dissociative water adsorption is adopted at the (001)/water interface with a water dissociation percentage of ∼25% stabilized at ambient conditions, in contrast to the unstable water dissociation at the (101)/water interface. The different interfacial structures for the two surfaces lead to their distinct proton transfer processes. The calculated band edge alignment across the interfaces shows good agreement with the experimental data and exp...

Enhancing the Photocatalytic Performance of MXenes via Stoichiometry Engineering of Their Electronic and Optical Properties.

Combining both density functional theory and the cluster expansion method, we investigate 3 binary MXene alloy systems of semiconducting Ti2CO2, Zr2CO2, and Hf2CO2, where the transition metals substitute one another (i.e., Ti2(1- x)Zr2 xCO2, Ti2(1- x)Hf2 xCO2, and Zr2(1- x)Hf2 xCO2). We show that this group of MXene alloys forms the solid-solution phase across all compositions. Special quasirandom structures are generated to model the solid-solution phase of these alloys, using which we demonstrate how their structural, mechanical, electronic, and optical properties are tuned via stoichiometry engineering. These alloys exhibit outstanding mechanical strength and stability. They possess indirect band gaps of 1.25-1.80 eV. For Ti2(1- x)Zr2 xCO2 and Ti2(1- x)Hf2 xCO2, they display higher absorbance in the solar spectrum than their constituent Zr2CO2 and Hf2CO2, respectively. Most of the MXene alloys also show appropriately aligned band edges for water splitting. We predict the Ti2(1- x)Zr2 xCO2 alloy with x = 0.2778 to be the most promising water-splitting photocatalyst among the MXenes studied here, outperforming its constituents, Ti2CO2 and Zr2CO2, when solar absorbance performance and band-edge alignments are simultaneously considered. This work demonstrates that alloying can be used to effectively tune photocatalytic performance.

Stable GaSe-Like Phosphorus Carbide Monolayer with Tunable Electronic and Optical Properties from Ab Initio Calculations.

On the basis of density functional theory (DFT) calculations, we propose a stable two-dimensional (2D) monolayer phosphorus carbide (PC) with a GaSe-like structure, which has intriguing electronic and optical properties. Our calculated results show that this 2D monolayer structure is more stable than the other allotropes predicted by Tománek et al. [Nano Lett., 2016, 16, 3247–3252]. More importantly, this structure exhibits superb optical absorption, which can be mainly attributed to its direct band gap of 2.65 eV. The band edge alignments indicate that the 2D PC monolayer structure can be a promising candidate for photocatalytic water splitting. Furthermore, we found that strain is an effective method used to tune the electronic structures varying from direct to indirect band-gap semiconductor or even to metal. In addition, the introduction of one carbon vacancy in such a 2D PC structure can induce a magnetic moment of 1.22 µB. Our findings add a new member to the 2D material family and provide a promising candidate for optoelectronic devices in the future.

CsTaWO6 as photoelectron platform by carbon nitride sensitizing toward enhanced charge carrier separation and photocatalytic activity

A series of CsTaWO6/g-C3N4 composite photocatalysts were successfully prepared aiming to regulate the charge kinetics for spatial separating the photoinduced electrons and holes under visible light irradiation, which was ascribed to the matched band edge alignment of CsTaWO6 and g-C3N4. Careful characterization on the as-prepared photocatalysts indicated the co-existent of g-C3N4 and CsTaWO6 in the composite. Because of electronic structure match and fine interfacial contact, CsTaWO6 anchored with g-C3N4 can enhance the visible light harvesting capacity and the photocatalytic activity toward hydrogen evolution was greatly improved with an optimized evolution rate of 7.5 μmol·g−1·h−1 and an excellent photocatalytic stability was achieved. The enhancement of photocatalytic activity was attributed to the fact that photoinduced electrons in the conduction band of g-C3N4 can be efficiently injected into the conduction band of CsTaWO6, leading to spatial charge separation between g-C3N4 and CsTaWO6, which can be confirmed by incident photocurrent responses and electrochemical impedance curves.

(Invited) Light Absorption Enhancement Using Graphene Quantum Dots in Dye-Sensitized Solar Cells and Photoelectrochemical Water Splitting

Graphene quantum dots (GQDs) have received great interest in the past few years due to its unique properties such as the quantum confinement and edge effects when their sizes are down to 10 nm. These new physical properties can induce size-dependent bandgap, and unique optical and electronic properties, which make them excellent materials for photovoltaics and photoelectochemical (PEC) devices. In a dye-sensitized solar cell (DSSC), light harvesting capability of dyes could be enhanced by Förster resonance energy transfer (FRET) phenomena, wherein energy is transferred from an excited donor fluorophore to a suitable acceptor dye molecule. The strict requirement that the overlap of the emission spectrum of donor and absorption spectrum of acceptor, which also requires proper band edge alignment between the them, significantly limits the variety of donors and acceptors suitable for the FRET-based DSSCs. In our DSSC work, graphene quantum dots were co-sensitized with N719 dyes and we successfully explored that the GQDs is a suitable donor material for the N719 dyes. From the emission and time decay spectra analysis, the FRET efficiency was measured to be 27 %. The power conversion efficiency (ƞ) of the standard DSSC is 6.12 % with a short-circuit current density (Jsc ) of 12.74 mAcm-2 and open-circuit voltage (Voc ) of 760 mV. The co-sensitization of GQD greatly enhances the ƞ up to 7.96 % with a Jsc of 16.54 mAcm-2 and Voc of 770 mV. The co-sensitization of GQDs improved both the Jsc and ƞvalues by ~ 30 %. The FRET enhanced the overall light absorption capacity of the N719 dye and hence photocurrent generation of the DSSC. Moreover, the co-sensitization of GQDs increase the TiO2/dye/electrolyte interface resistance, which suppresses the charge recombination and leads to increased Voc value. For PEC water splitting, higher light absorption can also be achieved by utilizing light-matter interplaymechanisms, such as, surface plasmon resonance (SPR), plasmonicresonance energy transfer (PRET) and exciton−plasmon interactions (EPI). The optical properties of the semiconductor and metal nanoparticles (SNPs-MNPs) are strongly modified viaEPI comparing to single NPs. Therefore, we have utilized the unique combination of GQDs and AuNPs (SNPs-MNPs) to effectively enhance PEC performance of TiO2 nanorods (TNRs). The AuNP-GQD decorated TNR photoelectrode has achieved a superior PEC performance with a photocurrent density of 1.75 mAcm-2 at 1.23 V vs RHE, which is one of higher value obtained in PEC measurement using TiO2 nanorods due to an efficient utilization of solar light. The experimental analysis has indicated that the AuNPs has contributed to the PRET / SPR-mediated hot electron injection into TiO2 and the GQDs contributed to the improved electron injection into the TiO2. Moreover, the significantly improved PEC water splitting efficiency is attributed to the synergistic effect due to EPI and/or FRET between AuNPs and GQDs in addition to the good catalytic property of the AuNPs and the GQDs.

Favourable band edge alignment and increased visible light absorption in β-MoO3/α-MoO3 oxide heterojunction for enhanced photoelectrochemical performance

Optimum band gap values, favourable band edge positions and stability in the electrolyte are critical parameters required for a semiconductor to have efficient photoelectrode properties. The present investigation carried out on the phase pure α & β MoO3 thin film shows that the low bandgap β-MoO3 possesses a mis-alignment with the water oxidation potential, while a more suitable band alignment is observed for the comparatively large bandgap α-MoO3. Both experimental and DFT calculations show that the valence edge of the orthorhombic (α-MoO3) phase is located at a higher energy (0.9 eV higher in VB-XPS and 1 eV higher in the DOS plots) than the monoclinic (β-MoO3) phase, while the conduction edge value is roughly at the same energy level (−2.5 eV) in both polymorphs. Based on the above investigations, an all oxide heterojunction comprising of β-MoO3/α-MoO3 is found to be suitable for improved PEC performance due to favourable energy band diagram and increased visible light absorption in β-MoO3. Significantly higher cathodic photocurrent is observed for the β-MoO3/α-MoO3 (1.6 mA/cm2 at applied bias of −0.3VRHE under simulated 1 sun irradiation) as compared to the very low anodic response in β-MoO3 (∼1.0 nA/cm2) and α-MoO3 (32 μA/cm2).

New nickel-based hybrid organic/inorganic metal halide for photovoltaic applications.

In this work, we have synthesized and fabricated solar cells with the hybrid metal halide compounds with the general formula ABX3, where the A cation is methylammonium, the B cation is nickel, and the X anion is chlorine or a mixture of chlorine and iodine. We obtained experimental evidence that this material is a semiconductor with an orthorhombic crystalline structure which pertains to the space group Cmcm. The bandgap can be modulated from 1.4 eV to 1.0 eV by changing the chlorine anion to iodine. Therefore, we were able to obtain solar cells with efficiencies up to 0.16% with the CH3NH3NiCl2I composition. We have also studied by means of first-principles calculations, taking into account van der Waals dispersive forces, the ground state properties of these materials such as their crystal structure and formation and decomposition energies. We have found that these energies are lowered by the lighter mass anion, and the calculated decomposition energies show that only CH3NH3NiCl3 is stable with respect to the most probable decomposition pathway. The electronic band structure and band edge alignments have been calculated using quasiparticle effects through the GW0 approximation; these materials show an indirect bandgap with the valence band maxima at -6.93 and -5.49 eV with respect to vacuum and the conduction band minima at -5.62 and -4.60 eV with respect to vacuum for CH3NH3NiCl3 and CH3NH3NiI3, respectively. This work provides a pathway to explore new hybrid A+B2+X3--type semiconductor materials.