Heterostructure Arrays Research Articles

Effects of rapid thermal annealing in InGaN/GaN quantum disk-in-GaN nanowire arrays

The effects of rapid thermal annealing on the structural and optical properties of InGaN/GaN quantum disk-in-nanowire heterostructure arrays have been investigated. Temperature-dependent and time-resolved photoluminescence measurements were complemented by x-ray photoemission spectroscopy. It was observed that annealing, in general, increased the non-radiative recombination rate in the InGaN quantum disk regions and consequently lowered the internal quantum efficiency and reduced the average carrier lifetime. These are due to an increase in the density of surface states associated with –O and –OH bonds in the disk region and possible enhancement of non-radiative recombination due to In segregation.

Non-3d Metal Modulation of a 2D Ni-Co Heterostructure Array as Multifunctional Electrocatalyst for Portable Overall Water Splitting.

Portable water splitting devices driven by rechargeable metal-air batteries or solar cells are promising, however, their scalable usages are still hindered by lack of suitable multifunctional electrocatalysts. Here, a highly efficient multifunctional electrocatalyst is demonstrated, i.e., 2D nanosheet array of Mo-doped NiCo2 O4 /Co5.47 N heterostructure deposited on nickel foam (Mo-NiCo2 O4 /Co5.47 N/NF). The successful doping of non-3d high-valence metal into a heterostructured nanosheet array, which is directly grown on a conductive substrate endows the resultant catalyst with balanced electronic structure, highly exposed active sites, and binder-free electrode architecture. As a result, the Mo-NiCo2 O4 /Co5.47 N/NF exhibits remarkable catalytic activity toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), affording high current densities of 50 mA cm-2 at low overpotentials of 310 mV for OER, and 170 mV for HER, respectively. Moreover, a low voltage of 1.56 V is achieved for the Mo-NiCo2 O4 /Co5.47 N/NF-based water splitting cell to reach 10 mA cm-2 . More importantly, a portable overall water splitting device is demonstrated through the integration of a water-splitting cell and two Zn-air batteries (open-circuit voltage of 1.43 V), which are all fabricated based on Mo-NiCo2 O4 /Co5.47 N/NF, demonstrating a low-cost way to generate fuel energy. This work offers an effective strategy to develop high-performance metal-doped heterostructured electrode.

Plasma-induced surface reorganization of porous Co3O4-CoO heterostructured nanosheets for electrocatalytic water oxidation

Herein, porous Co3O4-CoO heterostructured nanosheets are constructed by plasma treatment. The density-functional theory calculations demonstrate that constructing Co3O4-CoO heterostructures modify the electronic structure to achieve enhanced electrical conductivity, but also boost the charge transfer to realize enhanced surface reactivity. Contributing to the short ion diffusion path, the rich electroactive surface sites and enhanced charge transfer capability, the resulting Co3O4-CoO nanosheet arrays possess the low overpotential of 270 mV at 10 mA cm−2 and the low Tafel slope of 49 mV dec−1. This work provides a novel strategy to construct heterostructured nanosheet arrays by surface reorganization for cost-effective OER electrocatalyst.

A one-pot "shielding-to-etching" strategy to synthesize amorphous MoS2 modified CoS/Co0.85Se heterostructured nanotube arrays for boosted energy-saving H2 generation.

In this study, a facile one-pot "shielding-to-etching" strategy has been designed for the synthesis of amorphous MoS2 modified CoS/Co0.85Se heterostructured nanotube arrays (a-MoS2/CoS/Co0.85Se HNTs) as a highly efficient electrocatalyst for boosted energy-saving H2 production, using Co(OH)x(CO3)y nanorods as the template. Interestingly, these multi-composition and hollow structured products were achieved in a controllable manner via only a one-pot synthesis, in which the nanorods with shielding layers were firstly obtained and then converted into nanotubes through further etching reaction. Benefiting from the combined nature of multiple-component hollow nanostructures, the electronic structure of the electrocatalyst is efficiently modulated, the electron transfer and ion diffusion pathways are effectively shortened and abundant active sites are created, which leads to excellent activity for urea oxidation and hydrogen evolution reactions with the as-prepared a-MoS2/CoS/Co0.85Se HNTs being employed as the electrodes. As a result, the whole urea electrolysis cell exhibits a driven voltage of only 1.42 V to achieve a current density of 10 mA cm-2, surpassing most reported transition-metal-based electrocatalysts. Moreover, a battery-assisted urea electrolyzer was assembled as well to demonstrate the feasibility of practical less-energy-intensive H2 generation. The method developed in this work is expected to broaden the way of designing and synthesizing multiple-component hollow nanostructures for various applications.

High-performance self-powered ultraviolet photodetectors based on mixed-dimensional heterostructure arrays formed from NiO nanosheets and TiO2 nanorods

The morphology and geometry of semiconductors will affect the photoresponse performance of mixed-dimensional heterojunction photodetectors (PDs).

Three-Dimensional Heterostructured NiCoP@NiMn-Layered Double Hydroxide Arrays Supported on Ni Foam as a Bifunctional Electrocatalyst for Overall Water Splitting.

Herein, the rational design and preparation of three-dimensional heterostructured NiCoP@NiMn-layered double hydroxide arrays supported on Ni foam (NiCoP@NiMn LDH/NF) is reported as a new bifunctional water-splitting electrocatalyst with high performance. Prepared with facile hydrothermal reactions and phosphorization, the NiCoP@NiMn LDH/NF is simultaneously highly active toward oxygen evolution reaction (OER) (100, 300, and 600 mA cm-2 at overpotentials of 293, 315, and 327 mV, respectively) and hydrogen evolution reaction (HER) (100, 200, and 300 mA cm-2 at overpotentials of 116, 130, and 136 mV, respectively). Interestingly, with cell voltages of 1.519, 1.642, 1.671, and 1.687 V at 10, 100, 200, and 300 mA cm-2, respectively, for overall water splitting, this electrocatalyst achieves 95.2% faradaic efficiency for OER, suggesting a relatively high contribution of water splitting in the apparent current in spite of the existence of partial catalyst oxidation. The heterostructure arrays supported on Ni foam have some advantages, acting as a bifunctional water-splitting electrocatalyst: (1) heterostructured NCoP@NiMn LDH combines the intrinsic properties of individual NiCoP (excellent activity for HER) and NiMn LDH (high activity for OER) via the effective interface engineering between the two phases; (2) the NiCoP core material serves as a fast electron transfer channel to enhance the electrode's electrical conductivity; and (3) Ni foam with a three-dimensional-network structure as a support is beneficial to exposing more active sites and ensures efficient gas bubble release and electron/mass transfer.

Field Emission Properties of Molybdenum Nanoparticles Decorated ZnO Nanorod Arrays.

Precisely controlled dimensions of heterostructured ZnO nanorod arrays were grown on micropatterned Au films supported by Si substrate using chemical vapor deposition (CVD). The field emission properties were attributed to pointed nanorods, thickness of catalyst, preferential growth, density, morphology of ZnO and Molybdenum (Mo) decorated ZnO nanorod arrays (Mo/ZnO). The selective restrained heterostructure approach resulted in excellent control over periodicity, location and density of ZnO nanorod arrays. Overall, field emission properties of bare ZnO nanorod arrays showed a low turn-on field of ~4.7 V/μm and a high field enhancement factor (β) ~1686 to 7.3 V/μm and (β) ~807 for Mo/ZnO. It was also found that the field emission properties were significantly influenced by densely decorated Mo nanoparticles on as-grown ZnO nanorod arrays.

3D Porous Pyramid Heterostructure Array Realizing Efficient Photo‐Electrochemical Performance

AbstractDirect photo‐electrochemical (PEC) water splitting is of great practical interest for developing a sustainable energy systems, but remains a big challenge owing to sluggish charge separation, low efficiency, and poor stability. Herein, a 3D porous In2O3/In2S3 pyramid heterostructure array on a fluorine‐doped tin oxide substrate is fabricated by an ion exchange–induced synthesis strategy. Based on the synergistic structural and electronic modulations from density functional theory calculations and experimental observations, 3D porous In2O3/In2S3 photoanode by the protective layer delivers a low onset potential of ≈0.02 V versus reversible hydrogen electrode (RHE), the highest photocurrent density of 8.2 mA cm−2 at 1.23 V versus RHE among all the In2S3 photoanodes reported to date, an incident photon‐to‐current efficiency of 76% at 400 nm, and high stability over 20 h for PEC water splitting are reported. This work provides an alternative promising prototype for the design and construction of novel heterostructures in robust PEC water splitting applications.

Deterministic Assembly of Arrays of Lithographically Defined WS2 and MoS2 Monolayer Features Directly From Multilayer Sources Into Van Der Waals Heterostructures

Abstract One of the major challenges in the van der Waals (vdW) integration of two-dimensional (2D) materials is achieving high-yield and high-throughput assembly of predefined sequences of monolayers into heterostructure arrays. Mechanical exfoliation has recently been studied as a promising technique to transfer monolayers from a multilayer source synthesized by other techniques, allowing the deposition of a wide variety of 2D materials without exposing the target substrate to harsh synthesis conditions. Although a variety of processes have been developed to exfoliate the 2D materials mechanically from the source and place them deterministically onto a target substrate, they can typically transfer only either a wafer-scale blanket or one small flake at a time with uncontrolled size and shape. Here, we present a method to assemble arrays of lithographically defined monolayer WS2 and MoS2 features from multilayer sources and directly transfer them in a deterministic manner onto target substrates. This exfoliate–align–release process—without the need of an intermediate carrier substrate—is enabled by combining a patterned, gold-mediated exfoliation technique with a new optically transparent, heat-releasable adhesive. WS2/MoS2 vdW heterostructure arrays produced by this method show the expected interlayer exciton between the monolayers. Light-emitting devices using WS2 monolayers were also demonstrated, proving the functionality of the fabricated materials. Our work demonstrates a significant step toward developing mechanical exfoliation as a scalable dry transfer technique for the manufacturing of functional, atomically thin materials.

Enhanced ethanol sensing performance using Co3O4–ZnSnO3 arrays prepared on alumina substrates

Abstract In this work, novel n-type heterostructures of ZnSnO 3 modified p-type Co3O4 (Co3O4–ZnSnO3 ) nanowire arrays were fabricated in-situ on flat alumina substrates via a simple hydrothermal method without seed layers. SEM and TEM characterizations revealed that the Co 3O4 nanowire arrays were fully covered by ZnSnO3 nanosheets. The gas-sensing measurement revealed that the Co3O4–ZnSnO3 composite arrays showed the highest response 5.57 (Rg/Ra) towards ethanol, which was 2.04 times higher than that of Co3O4 nanowire arrays (Rg/Ra = 2.73). Meanwhile, the enhanced sensing mechanism of Co3O4–ZnSnO3 composite arrays was also discussed, which could be explained by the special heterojunction structure arrays that offered an additional modulation in resistance. Our studies shed a new light to design p-n heterostructure arrays in-situ sensing device by a facile method. Moreover, the as-designed Co 3O4–ZnSnO3 composite arrays have potential application for fabricating high performance ethanol sensors.

Heterostructure arrays of NiMoO4 nanoflakes on N-doping of graphene for high-performance asymmetric supercapacitors

Supercapacitors, as a clean and sustainable power supply, have been a popular subject of research in the field of energy storage owing to their excellent electrochemical performance. As an important part of supercapacitors, electrode materials and structures have been continuously designed and improved. This study presents a facile synthesis of an N-doping of graphene/NiMoO4 (NG/NiMoO4) heterostructure for electrochemical supercapacitors. By combining excellent conductivity, large specific surface area of graphene, and reversible redox of metal oxide, this hybrid composite exhibits an ultrahigh specific capacitance of 1913 F/g at 1 A/g. Using NG/NiMoO4 as positive electrode and active carbon as negative electrode, an asymmetric supercapacitor was assembled. The device exhibits a high capacitance of 62.5 F/g at 0.5 A/g, a maximum energy density of 22.2 Wh/kg at the power density of 400 W/kg, and excellent cycling stability (94.2% retention after 5000 cycles). The results demonstrate that the NG/NiMoO4 composite is a promising electrode material for supercapacitors in energy storage applications.

Deep Ultraviolet Luminescence Due to Extreme Confinement in Monolayer GaN/Al(Ga)N Nanowire and Planar Heterostructures.

We present experimental results confirming extreme quantum confinement in GaN/AlxGa1-xN (x = 0.65 and 1.0) nanowire and planar heterostructures, where the GaN layer thickness is of the order of a monolayer. The results were obtained from temperature- and excitation-dependent and time-resolved photoluminescence measurements. In the GaN/AlN nanowire heterostructure array sample, the measured emission peak at 300 K is ∼5.18-5.28 eV. This is in excellent agreement with the calculated optical gap of 5.23 eV and 160-260 meV below the calculated electronic gap of 5.44 eV, suggesting that the observed emission is excitonic in nature with an exciton binding energy of ∼160-260 meV. Similarly, in the monolayer GaN/Al0.65Ga0.35N planar heterostructure, the measured emission peak at 300 K is 4.785 eV and in good agreement with the calculated optical gap of 4.68 eV and 95 meV below the calculated electronic gap of 4.88 eV. The estimated exciton binding energy is 95 meV and in close agreement with our theoretical calculations. Excitation-dependent and time-resolved photoluminescence data support the presence of excitonic transitions. Our results indicate that deep-ultraviolet excitonic light sources and microcavity devices can be realized with heterostructures incorporating monolayer-thick GaN.

Bifunctional NiFe layered double hydroxide@Ni3S2 heterostructure as efficient electrocatalyst for overall water splitting

The tailored construction of non-noble metal bifunctional electrocatalysts for high-efficiency oxygen/hydrogen evolution reactions (OER/HER) is vital for the development of electrochemical energy conversion. Herein, we report a powerful combined wet chemical method to fabricate a novel binder-free NiFe layered double hydroxide@Ni3S2 (NiFe LDH@Ni3S2) heterostructure as an efficient bifunctional electrocatalyst for overall water splitting. The hydrothermal-synthesized NiFe LDH nanosheets are uniformly coated on the Ni3S2 nanosheet skeleton forming 3D porous heterostructure arrays. By virtue of its synergistic advantages, including its binder-free characteristics, increased catalysis sites and structural stability, the as-obtained NiFe LDH@Ni3S2/NF electrode exhibits low overpotentials of 184 and 271 mV at 20 mA cm−2 for HER and OER in 1 M KOH, respectively. Notably, a low operation potential of 1.74 V at a current density of 20 mA cm−2 is achieved for overall water splitting with a stable cycling life. In addition, the intimate composite structure and sensitive interface of NiFe LDH@Ni3S2 are responsible for the good electrocatalytic activity with a low Tafel slope, fast reaction kinetics and high stability. The versatile fabrication protocol and heterostructure interface engineering provide a new way to construct other bifunctional and cost-effective electrocatalysts for electrocatalysis.

In situ growth of Co3O4@NiMoO4 composite arrays on alumina substrate with improved triethylamine sensing performance

Array-based sensors are considered as potential candidates for gas detection due to their low cost and great miniaturization potential. However, the fabrication process of array-based sensors is complex and time-consuming since it usually contains the formation and growth processes of seed layers on the surface of alumina substrates. In addition, the gas-sensing materials for the fabrication of array-based sensors are mainly confined to n-type semiconductors such as ZnO, TiO2 and WO3. In this work, a kind of p-type heterostructures arrays composed of NiMoO4 nanosheets and Co3O4 nanowire (Co3O4@NiMoO4) was fabricated in-situ on flat alumina substrates via a simple hydrothermal method without seed layers. SEM and TEM characterizations revealed that the Co3O4 nanowire arrays were fully covered with NiMoO4 nanosheets. The gas-sensing measurements revealed that the Co3O4@NiMoO4 composite arrays showed the highest response (Rg/Ra = 17.12) towards 100 ppm trimethylamine at its optimal operating temperature of 250 °C. This response value was 3.91 times higher than that of Co3O4 arrays (Rg/Ra = 4.39) and 9.25 times higher than that of NiMoO4 nanosheets (Rg/Ra = 1.85) at their optimal operating temperatures of 250 and 350 °C, respectively. Meanwhile, the enhanced sensing mechanism of the Co3O4@NiMoO4 composite arrays was also discussed. It could be explained by the special heterojunction structure of the Co3O4@NiMoO4 composite arrays, which offered a high surface area and an additional modulation in resistance. Our studies shed a new light to design p-type heterostructure arrays in-situ for fabricating sensing device by a facile method. Moreover, the as-designed Co3O4@NiMoO4 composite arrays are potential candidates in the fabricating of high performance trimethylamine sensors.

Hetero-structure arrays of MnCo2O4 nanoflakes@nanowires grown on Ni foam: Design, fabrication and applications in electrochemical energy storage

In this work, we report a two-step synthesis of hierarchical heterostrtuture based MnCo2O4 nanoflakes@PPy nanowires arrays on Ni foam using facile hydrothermal method and subsequent annealing process. The physico-chemical and surface properties of the MnCo2O4 electrodes were investigated through XRD, FESEM, Raman, BET and XPS analysis. Owing to the exclusive hetero-structure arrays and the synergetic effect of MnCo2O4@PPy electrode displays higher specific capacitance of 2933 Fg-1 than bare MnCo2O4 (2144 Fg-1). Moreover, the heterostructure MnCo2O4@PPy electrode exhibits 86.7% capacitance retention after 5000 cycles at 20Ag-1, whereas the bare MnCo2O4 electrode is only 80%. This can be attributed to the surface of MnCo2O4 offers a conductive network for rapid electron transfer by coupling of PPy matrix, which support the transport of electrons from PPy to MnCo2O4, benefiting for improving the electrochemical performance. Further, the asymmetric supercapacitor cell (ASC) is fabricated using MnCo2O4/NF@PPy and Activated carbon (acetylene black: carbon black: PVDF) as the positive and negative electrodes. The devices showed high energy density (78.5 Whkg−1 at 1121 Wkg-1), and superior stability (94.3% capacitance retention after 5000 cycles at 20 Ag-1).

Strain-Engineered van der Waals Interfaces of Mixed-Dimensional Heterostructure Arrays.

van der Waals (vdWs) heterostructures have provided a platform for nanoscale material integrations and enabled promise for use in optoelectronic devices. Because of the ultrastrength of two-dimensional materials, strain engineering is considered as an effective way to tune their band structures and further tailor the interface performance of vdWs heterostructures. However, the less-constrained vdWs interfaces make the traditional strain technique via lattice-mismatched growth infeasible. Here, we report a strategy to construct mixed-dimensional heterostructure arrays with periodically strain-engineered vdWs interfaces utilizing one-dimensional semiconductor-induced nanoindentation. Using monolayer MoS2 (1L-MoS2)/ZnO heterostructure arrays as a model system, we demonstrate inhomogeneous built-in strain gradient at the heterointerfaces ranging from 0 to 0.6% tensile. Through systematic optical characterization of the hybrid structures, we verify that strain can improve the interfacial charge transfer efficiency. Consequently, we observe that the photoluminescence (PL) emission of 1L-MoS2 at strained interfaces is dramatically quenched more than 50% with respect to that at unstrained interfaces. Furthermore, we confirm that the strain-optimized interfacial carrier behavior is attributed to the reduction of interfacial barrier height, which originated from the strain-dependent Fermi level of 1L-MoS2. These results demonstrate that strain provides another degree of freedom in tuning the vdWs interface performance and our method developed here should enable flexibility in achieving more sophisticated vdWs integration via strain engineering.

Li‐O2 Batteries: Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries (Adv. Energy Mater. 24/2019)

In article number 1900788, Zhiwei Zhang, Longwei Yin, and co-workers demonstrate a 3D hierarchical NiCo2S4@NiO heterostructure array on carbon paper for Li-O2 battery cathodes. A built-in interfacial potential between NiCo2S4 and NiO can drastically enhance interfacial charge transfer kinetics. Furthermore, intrinsic LiO2-affinity characteristics of NiCo2S4 and NiO play a synergistic role in constructing a low-impedance Li2O2/cathode contact interface. The resulting hybrids exhibit superior electrocatalytic activity toward both ORR and OER.

Introduction of Ti3+ ions into heterostructured TiO2 nanotree arrays for enhanced photoelectrochemical performance

The mixed-phase heterostructured TiO2 nanotree arrays were synthesized by a secondary hydrothermal route, and Ti3+ ions were introduced into the arrays for enhanced photoelectrochemical performance. The results show that the arrays consist of heterostructures with anatase and rutile phases. XPS results indicate that during the hydrogenation process, part of Ti4+ ions transform to Ti3+ ions. Although the band gap shifts by only 0.1 eV, introduced defects and Ti3+ ions can greatly increase the efficiency of electron-hole separation. Photocurrent of the heterostructured TiO2 nanotree arrays with introduced Ti3+ ions increases by 41 times, and the photocurrent density enhances from 0.0055 to 0.2272 mA/cm2.

Highly Efficient Photoinduced Enhanced Raman Spectroscopy (PIERS) from Plasmonic Nanoparticles Decorated 3D Semiconductor Arrays for Ultrasensitive, Portable, and Recyclable Detection of Organic Pollutants.

Semiconductor materials have become competitive candidates for surface-enhanced Raman scattering (SERS) substrates; however, their limited SERS sensitivity hinders the practical applications of semiconductors. Here, we develop a hybrid substrate by integrating anatase/rutile TiO2 heterostructure with dense plasmonic hotspots of Ag nanoparticle (AgNPs) for efficient photoinduced enhanced Raman spectroscopy (PIERS). The PIERS mechanism is systematically investigated by means of a portable Raman instrument. When ultraviolet (UV) light irradiates the substrate, the TiO2-Ag hybrid arrays produce remarkable charge-transfer enhancement, which can be ascribed to the highly efficient charge separation driven by heterojunction and transfer from TiO2 heterostructure to AgNPs. This platform allows for the rapid detection of multifold organic species, including malachite green (MG), crystal violet (CV), rhodamine 6G (R6G), thiram, and acephate, and as high as 27.8-fold enhancement over the normal SERS is achieved, representing the highest PIERS magnification up to the present time. The intensive PIERS enhancement makes it ultrasensitively detect analyte concentration of an order of magnitude lower than that of SERS method. The improved sensitivity and resolution can be readily realized by simple UV irradiation, which represents a major advantage of our PIERS methodology. Besides, the integration of uniform TiO2 heterostructure arrays with AgNPs generates superior signal reproducibility with relative standard deviation (RSD) value of less than 14%. In addition, the detected molecules on the substrate can be eliminated by photocatalytic degradation after PIERS measurements by using UV irradiation, which makes the substrate reusable for 15 cycles. The ultrahigh sensitivity, superior reproducibility, and excellent recyclability displayed by our platform may provide new opportunities in field detection analysis coupled with a portable Raman instrument.

Free-standing porous Ni2P-Ni5P4 heterostructured arrays for efficient electrocatalytic water splitting

Constructing heterointerfaces in heterostructures could effectively enlarge the electroactive sites and enhance the interfacial charge transfer, and thus improve the electrocatalytic performances. Herein, free-standing porous Ni2P-Ni5P4 heterostructured arrays are successfully prepared through in situ phosphating Ni(OH)2 arrays by simply tuning the reaction temperatures. Contributing from the interfacial coupling effects of two phases, large surface areas, highly conductive support of carbon cloth substrates and unique free-standing arrays, Ni2P-Ni5P4 heterostructured arrays show the enhanced kinetics and electrocatalytic performances for the hydrogen evolution reaction, oxygen evolution reaction and overall water splitting. Our research might offer insight into constructing heterophase junctions for efficient overall water splitting.