Tunable Band Edge Research Articles

Rational Design of Metal Oxide‐Based Heterostructure for Efficient Photocatalytic and Photoelectrochemical Systems

AbstractSolar‐driven photo‐to‐chemical conversion is an interesting approach for energy harvesting and storage with high sustainability. To achieve high photo‐to‐chemical conversion efficiency, it is important to develop cost‐effective and stable catalysts with high activity. Metal oxides provide an interesting platform for the development of efficient catalysts owing to their abundance, high stability, and tunable band edges. Their performance highly depends on the rational design of heterostructures with engineered electronic structures, modified charge migration behavior, tailored interfacial properties, and amplified electromagnetic fields. All these enable the achievement of efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Herein, a recent study on rationally designed metal oxide heterostructures for enhancing photo‐to‐chemical energy conversion via photocatalytic and photoelectrochemical systems is reviewed. The approaches to enhance their conversion efficiency are as follows: 1) surface modification via loading of plasmonic metals and other photosensitizers; 2) surface regulation, including morphology, defect, and dopants; 3) interfacial assembly with metal oxides, other metal compounds, and metal‐free materials. Moreover, the underlying reaction mechanisms of metal oxide‐based heterostructures and how they affect the energy conversion efficiency are discussed. Finally, the challenges and perspectives on the development of metal oxide‐based heterostructures and associated photo‐to‐chemical devices are presented.

Fabrication and characterization of gallium nitride thin film deposited on a sapphire substrate for photoelectrochemical water splitting applications

Gallium nitride (GaN) is of special interest as photoanode material in the photoelectrochemical (PEC) water splitting applications due to its excellent thermodynamically stable structure, tunable band edge potentials, and chemical stability. In this paper, we deposited a high-quality GaN thin film on a sapphire (Al2O3) substrate using a low-cost chemical vapor deposition technique. The X-ray diffraction (XRD) study confirmed the presence of the hexagonal wurtzite structure of GaN thin film. Raman spectra confirmed the existence of E2 (low), E2 (high) and A1 (LO) phonon mode in GaN thin film which are in agreement with XRD analysis. Field emission scanning electron microscopy (FESEM) and Atomic force microscopy (AFM) measurements revealed a smooth surface morphology and stepped-trace like structure formation of GaN with an average height of 3.3 Å. The functional groups were identified using Fourier transform infrared (FTIR) spectroscopy. The room temperature photoluminescence (PL) studies show near band-edge emission (NBE) of 3.21 eV. The thermogravimetry analysis (TGA) revealed that GaN thin film is thermally stable up to a temperature of 894 °C and a definitive mass loss was observed above 900 °C. Further, the photoelectrochemical (PEC) water splitting measurements showed that the GaN thin film exhibited a photocurrent density of 0. 099 mA/cm2 and a photoconversion efficiency of 0.73% at 0.59 V vs. RHE.

Broadband-absorbing polycyclic aromatic hydrocarbon composite films on topologically complex substrates

Abstract Reactive vapor deposition (RVD) allows researchers to coat a diverse set of textiles with robust films of conjugated macromolecules and build conformally-integrated nanostructured electronics on textiles. However, the number of precursors amenable to RVD are limited, which in turn limits the variety of device architectures and optoelectronic functions that can be integrated with textile substrates. Here, we develop a dehydrogenative coupling reaction that can be used to create uniform, conformal and patternable polycyclic aromatic hydrocarbon (PAH) coatings on textiles using RVD protocols. PAHs possess large absorption coefficients, tunable band edges, high charge carrier mobilities and ambipolarity, meaning that a number of photonic and optoelectronic devices can be elaborated from the coatings reported here. We discuss, in particular, a fluoranthene/perifluoranthene composite PAH coating created using RVD, which exhibits strong light absorption over a large wavelength range and accompanying intermolecular energy transfer, qualifying it as a competitive photoactive material for conformally integrated, textile-based solar energy harvesters.

Band Edge Tuning of Two-Dimensional Ruddlesden–Popper Perovskites by A Cation Size Revealed through Nanoplates

Current understanding of the effects of various A-site cations on the photophysical properties of halide perovskites (APbI3) is limited by the compositional tunability. Here we report the synthesis...

Nondestructive functionalization of monolayer black phosphorus using Lewis acids: A first-principles study

Functionalization of layered black phosphorus is valuable for engineering its chemical properties and enhancing the chemical stability. Here we report density functional theory calculations, through which we show that the phosphorus surface can be decorated by a few Lewis acids with stable interfacial binding, which preserves the band gap of monolayer black phosphorus with tunable band edge energies. The strong interfacial binding is mediated by a Lewis acid-base chemistry between the lone pair electrons in phosphorous and the electron accepting Lewis acids. We further show that magnetic moment can be introduced into phosphorus using the Lewis acids such as FeCl3. Furthermore, the surface modification can be responsive using the acylation chemistry. Our work thus shows that the chemical stability and electronic properties of black phosphorus can be finely tuned because of its rich chemistry.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.

Edge functionalized graphene nanoribbons with tunable band edges for carrier transport interlayers in organic-inorganic perovskite solar cells.

Organic based graphene nanoribbons (GNRs) can be good candidates as carrier extraction interlayers for organic/inorganic hybrid perovskite solar cells, owing to the possibility of tuning the band edge energy levels through varying the width and the type of edge functionalization. By using the density functional theory (DFT) method, the electronic structures of H or F edge functionalized armchair type GNRs on MAPbI3(001) are calculated. It is shown that the electronic structure of H- or F-passivated GNRs is almost undisrupted by the non-covalent interaction with the PbI2 surface layer of MAPbI3(001), thereby one can tune the width and edge chemistry of GNRs to enhance the carrier extraction or blocking. Especially all H-GNRs five to ten carbon atoms wide exhibit good matching for hole extraction, while F-GNRs require a specific width for electron extraction. Exploiting the unzipping synthesis of carbon nanotubes in the solution phase, our result provides a facile strategy for efficient carrier extraction.

The effect of silver oxidation on the photocatalytic activity of Ag/ZnO hybrid plasmonic/metal-oxide nanostructures under visible light and in the dark

A new synergetic hybrid Ag/ZnO nanostructure was fabricated which is able to cause photocatalytic degradation (in high yields) of methylene blue under visible light as well as in the dark. In this nanostructure, ZnO was synthesized using the arc discharge method in water and was coupled with Ag via a chemical reduction method. X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy results confirmed the existence of defects in ZnO in the hybrid nanostructures; these defects act as electron traps and inhibit the recombination of electron-hole pairs. The absorption edge of the hybrid nanostructure shifts toward the visible region of the spectrum due to a combination of the Ag plasmonic effect and the defects in ZnO. Band-edge tuning causes effective visible light absorption and enhances the dye degradation efficiency of Ag/ZnO nanostructures. Silver oxidation in the hetero-structure changed the metal-semiconductor interface and suppressed the plasmonic enhancement. Nevertheless, the synthesized Ag/ZnO decomposed methylene blue in visible light, and the silver oxidation only affected the catalytic activity in the dark. This work provides insight into the design of a new and durable plasmonic-metal oxide nanocomposite with efficient dye degradation even without light illumination.

A Critical Review on Enhancement of Photocatalytic Hydrogen Production by Molybdenum Disulfide: From Growth to Interfacial Activities.

Ultrathin 2D molybdenum disulfide (MoS2 ), which is the flagship of 2D transition-metal dichalcogenide nanomaterials, has drawn much attention in the last few years. 2D MoS2 has been banked as an alternative to platinum for highly active hydrogen evolution reaction because of its low cost, high surface-to-volume ratio, and abundant active sites. However, when MoS2 is used directly as a photocatalyst, contrary to public expectation, it still performs poorly due to lateral size, high recombination ratio of excitons, and low optical cross section. Besides, simply compositing MoS2 as a cocatalyst with other semiconductors cannot satisfy the practical application, which stimulates the pursual of a comprehensive insight into recent advances in synthesis, properties, and enhanced hydrogen production of MoS2 . Therefore, in this Review, emphasis is given to synthetic methods, phase transitions, tunable optical properties, and interfacial engineering of 2D MoS2 . Abundant ways of band edge tuning, structural modification, and phase transition are addressed, which can generate the neoteric photocatalytic systems. Finally, the main challenges and opportunities with respect to MoS2 being a cocatalyst and coherent light-matter interaction of MoS2 in photocatalytic systems are proposed.

Large Landau-level splitting in a tunable one-dimensional graphene superlattice probed by magnetocapacitance measurements

The unique zero energy Landau Level of graphene has a particle-hole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zero-energy Landau level inside the {\it bulk} can lead to many interesting physics, such as quantum spin Hall effect, Dirac like singular points of the chiral edge modes, and others. However, so far the obtained splitting with high-magnetic field even on a hBN substrate are not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a large gap zero-energy Landau level splitting ($\sim$ 150 meV) with the usage of a one-dimensional (1D) superlattice potential. We have created tunable 1D superlattice in a hBN encapsulated graphene device using an array of metal gates with a period of $\sim$ 100 nm. The Landau level spectrum is visualized by measuring magneto capacitance spectroscopy. We monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landau level spitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multi-functionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future.

Synthesis of band gap-tunable alkali metal modified graphitic carbon nitride with outstanding photocatalytic H2O2 production ability via molten salt method

Band gap-tunable alkali metal modified graphitic carbon nitride was prepared by a molten salt method. X-ray diffraction, N2 isothermal sorption, ultraviolet-visible spectroscopy, scanning electron microscope, X-ray photoelectron spectroscopy and photoluminescence were used to characterize the obtained catalysts. The photocatalytic H2O2 production ability of as-prepared catalyst was investigated. The results indicate that K+ and Na+ are doped into g-C3N4 lattice simultaneously by the molten salt method. Alkali metal modification not only promotes the specific surface area, visible light absorption and separation of electron-hole pairs, but tunes the conduction band and valence band edge positions of as-prepared catalysts by controlling the weight ratio of eutectic salts to melamine. The tunable band edge positions result in the photocatalytic H2O2 production from “single channel pathway” to “two channel pathway”, leading to the promoted H2O2 production ability.

High Br– Content CsPb(ClyBr1–y)3 Perovskite Nanocrystals with Strong Mn2+ Emission through Diverse Cation/Anion Exchange Engineering

The unification of tunable band edge (BE) emission and strong Mn2+ doping luminescence in all-inorganic cesium lead halide perovskite nanocrystals (NCs) CsPbX3 (X = Cl and Br) is of fundamental importance in fine tuning their optical properties. Herein, we demonstrate that benefiting from the differentiation of the cation/anion exchange rate, ZnBr2 and preformed CsPb1- xCl3: xMn2+ NCs can be used to obtain high Br- content Cs(Pb1- x- zZn z)(Cl yBr1- y)3: xMn2+ perovskite NCs with strong Mn2+ emission, and the Mn2+ substitution ratio can reach about 22%. More specifically, the fast anion exchange could be realized by the soluble halide precursors, leading to anion exchange within a few seconds as observed from the strong BE emission evolution, whereas the cation exchange instead generally required at least a few hours; moreover, their exchange mechanism and dynamics process have been evaluated. The Mn2+ emission intensity could be further varied by controlling the replacement of Mn2+ by Zn2+ with prolonged ion exchange reaction time. White light emission of the doped perovskite NCs via this cation/anion synergistic exchange strategy has been realized, which was also successfully demonstrated in a prototype white light-emitting diode (LED) device based on a commercially available 365 nm LED chip.

Growth of gallium nitride nanowires on sapphire and silicon by chemical vapor deposition for water splitting applications

Gallium nitride (GaN) nanowires (NWs) are one of the most promising candidates for photoelectrode materials due to their tunable band edge potentials and high stability in electrolytes. In this study, GaN NWs were grown on sapphire (Al2O3) (002) and silicon (Si) (111) substrate by chemical vapor deposition (CVD) method. High quality of GaN NWs on sapphire and silicon were confirmed by powder X-ray diffraction (XRD). Structural characterization of the synthesized NWs were performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The images revel the pristine and smooth surface of GaN NWs. Further, optical properties of GaN NWs were investigated at room temperature photoluminescence emission. The photocurrent density of GaN/Si NWs is found to be higher than that of GaN/Al2O3 NWs. The GaN/Si NWs having large surface area, are grown in a much simpler method. GaN/Si NWs are potential candidate for hydrogen energy generation, due to their enhanced water splitting efficiency by utilizing solar energy.

Recent Progress in Photocatalytic CO2 Reduction Over Perovskite Oxides

Photocatalytic CO2 reduction has attracted widespread attention in recent years, due its potential to reduce anthropogenic CO2 emissions from fossil fuel combustion whilst also yielding fuels and valuable chemical feedstocks. Among the various semiconductor photocatalysts capable of driving CO2 reduction under direct sunlight, perovskite oxides are arguably the most promising due to their high catalytic activity, good stability, long charge diffusion lengths, and compositional flexibility that allows precise bandgap and band edge tuning. This review aims to showcase recent advances in the design of perovskite oxides and their derivatives for photocatalytic CO2 reduction, placing particular emphasis on structure modulation, defect engineering, and interface construction as rational approaches for enhancing solar‐driven CO2 conversion to CH4, CO, and other valuable oxygenates (CxHyOz).

Improvement of efficiency in graphene/gallium nitride nanowire on Silicon photoelectrode for overall water splitting

Gallium nitride (GaN) nanowires are one of the most promising photoelectrode materials due to their high stability in acidic and basic electrolytes, and tunable band edge potentials. In this study, GaN nanowire arrays (GaN NWs) were prepared by molecular beam epitaxy (MBE); their large surface area enhanced the solar to hydrogen conversion efficiency. More significantly, graphene was grown by chemical vapor deposition (CVD), which enhanced the electron transfer between NWs for water splitting and protected the GaN NW surface. Structural characterizations of the prepared composite were performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocurrent density of Gr/GaN NWs exhibited a two-fold increase over pristine GaN NWs and sustained water splitting up to 70min. These improvements may accelerate possible applications for hydrogen generation with high solar to hydrogen conversion efficiency.

The Effect of Anisotropic Physical Properties of Cu(InxGa1-x)(Se,S)2 (CIGS) Single Crystal Photocatalysts on Water Splitting

The photoelectrochemical splitting of water to produce H2 and O2 is an attractive process for the large-scale production of clean, recyclable H2. The field of solar water splitting has gradually divided into either using conventional materials as thin films with stabilization layers or developing new materials that are themselves stable in electrolyte solutions under illumination. Divergent from these paths, we are investigating semiconductors in their single crystal form in order to take advantage of the physical properties unique to crystalline materials that are likely to cause a significant impact on photoactivity. Due to crystalline anisotropy, the crystallographic orientation of the exposed electrode surface is anticipated to affect the overall efficiency of the photocatalytic water-splitting process and the photostability of the material. Moderate band gap ternary chalcopyrites are a promising class of semiconductors for photoelectrochemical water splitting. Specifically, copper-indium-gallium dichalcogenides (diselenide or disulfide) [Cu(InxGa1-x)(Se,S)2]—abbreviated as CIGS—appear interesting due to their ideal, tunable light-absorption properties and band edge positions. To date, CIGS research has focused solely on thin film of nanoparticle materials. Electrodes based on these materials have demonstrated activity as photocathodes in photoelectrochemical cells (PEC) for water splitting. The role of crystallographic quality of CIGS in single crystal form has yet to be examined in PEC water splitting studies. The tetragonal crystal lattice structure of CIGS (Figure 1) gives a degree of asymmetry to the crystals, generating anisotropic properties in different lattice planes. Anisotropic variability has been shown to affect the optical, magnetic, and thermal expansion properties of CIGS. These differences in physical properties are expected to generate a crystal face dependence on the photoelectrochemical response of CIGS. In this presentation, we will report on the solid-state synthesis of oriented CIGS single crystal electrodes, along with the effects of crystallographic orientation on the efficiency of water-splitting. Figure 1: Tetragonal crystal structure of CIGS chalcopyrite semiconductors. View along the c-axis (left); view along the a-axis (right). Cu atoms are copper, In/Ga atoms are teal, and Se/S atoms are yellow. Figure 1

Enhancing Perovskite Solar Cell Performance by Interface Engineering Using CH3NH3PbBr0.9I2.1 Quantum Dots.

To improve the interfacial charge transfer that is crucial to the performance of perovskite solar cells, the interface engineering in a device should be rationally designed. Here we have developed an interface engineering method to tune the photovoltaic performance of planar-heterojunction perovskite solar cells by incorporating MAPbBr3-xIx (MA = CH3NH3) quantum dots (QDs) between the MAPbI3 perovskite film and the hole-transporting material (HTM) layer. By adjustment of the Br:I ratio, the as-synthesized MAPbBr3-xIx QDs show tunable fluorescence and band edge positions. When the valence band (VB) edge of MAPbBr3-xIx QDs is located below that of the MAPbI3 perovskite, the hole transfer from the MAPbI3 perovskite film to the HTM layer is hindered, and hence, the power conversion efficiency decreases. In contrast, when the VB edge of MAPbBr3-xIx QDs is located between the VB edge of the MAPbI3 perovskite film and the highest occupied molecular orbital of the HTM layer, the hole transfer from the MAPbI3 perovskite film to the HTM layer is well-facilitated, resulting in significant improvements in the fill factor, short-circuit photocurrent, and power conversion efficiency.

Perovskite Quantum Dots Modeled Using ab Initio and Replica Exchange Molecular Dynamics

Organometal halide perovskites have recently attracted tremendous attention at both the experimental and theoretical levels. Much of this work has been dedicated to bulk material studies, yet recent experimental work has shown the formation of highly efficient quantum-confined nanocrystals with tunable band edges. Here we investigate perovskite quantum dots from theory, predicting an upper bound of the Bohr radius of 45 A that agrees well with literature values. When the quantum dots are stoichiometric, they are trap-free and have nearly symmetric contributions to confinement from the valence and conduction bands. We further show that surface-associated conduction bandedge states in perovskite nanocrystals lie below the bulk states, which could explain the difference in Urbach tails between mesoporous and planar perovskite films. In addition to conventional molecular dynamics (MD), we implement an enhanced phase-space sampling algorithm, replica exchange molecular dynamics (REMD). We find that in simulati...

In situ synthesis of binary cobalt–ruthenium nanofiber alloy counter electrode for electrolyte-free cadmium sulfide quantum dot solar cells

A facile, low-cost and low-temperature fabrication approach of counter electrode is essential for pursuing robust photovoltaic devices. Herein, we develop a hydrothermal in situ growth of Cobalt–Ruthenium (Co–Ru) alloy nanofiber electrode for quantum dot solar cell (QDSC) applications. Colloidal CdS QDs with tunable absorption band edge are synthesized and used as light absorber. After optimizing the QDs with the highest photoluminescence quantum yield accompanied by considerable solar light absorption ability, QDSC based on Co–Ru alloy electrode delivers a much higher power conversion efficiency than its counterparts, i.e., either pure Co or Ru metal electrodes. In detail, Co–Ru alloy electrode exhibits high specific area, excellent electrical behavior, intimate interface contact, and good stability, thus leading to notable improved device performances. The impressive robust function of Co–Ru alloy with simple manufacturing procedure highlights its potential applications in robust QDSCs.

Role of Hole Conductors in Quantum Dot and Organometal Perovskite Based Solid State Solar Cells

Semiconductor quantum dots (QDs) have the potential to design high efficiency solid-state QDSCs and ETA (Extremely Thin Absorber) solar cells. Tunable band gap and band edge through size quantization, large intrinsic dipole moments, and high extinction coefficients make them suitable light absorbing materials. A number of semiconductors have been explored in the design of solid state solar cells. These include CdS, CdSe, In2S3, and Sb2S3. Of particular interest is Sb2S3 which has a band gap of 1.7 − 1.8 eVallowing for light absorption throughout the visible range. Organometal perovskites such as CH3NH3PbI3 are the new semiconducting materials that have led to advances in thin film solar cells. However, hole transport remains a limiting factor in maximizing power conversion efficiency.Using Sb2S3 (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. We have employed transient absorption spectroscopy to probe the hole transfer between Sb2S3 and CuSCN. In the Sb2S3 absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S−•) species. This trapped hole is transferred from the Sb2S3 absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. Similarly the hole transport properties in (CH3NH3PbI3) and hole conductor copper iodide has been probed by impedence spectroscopy. Charge separation in Sb2S3 and (CH3NH3PbI3) based solar cells which provide new insight into the design of new architectures for higher efficiency devices will be discussed. ACKNOWLEDGMENT. The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533.