Synthesis Of Heterostructures Research Articles

Scalable synthesis of heterostructure molybdenum and nickel sulfides nanosheets for efficient hydrogen generation in alkaline electrolyte

Hydrogen generation by electrochemical water splitting with the renewable power source is one of the most promising pathways for green and sustainable society. The challenge is to explore a simple and cost-effective method to synthesize earth-abundant and noble-metal-free electrocatalysts with reliable hydrogen evolution reaction (HER) activity. Herein, we report a facile and scalable method to synthesize hybrid MoS2 and nickel sulfides (NiS/NiS2/Ni3S4) nanosheets (Mo-Ni-S NSs) with highly efficient HER activity in alkaline media. The hybrid phase and NSs allow large exposure of edge sites of MoS2 and construct highly active hetero-interfaces between MoS2 and NiSx and sufficient durability in alkaline solution. Mo-Ni-S NSs with optimal composition presents a low overpotential of 83 mV to reach a current density of 10 mA cm−2 in 1.0 M KOH, which is comparable to the performance of recent reported highly active MoS2 based electrocatalysts. This work sheds light on the facile synthesis of multiple heterostructures as efficient electrocatalysts for scalable application in energy chemistry.

Layered Insulator/Molecule/Metal Heterostructures with Molecular Functionality through Porphyrin Intercalation.

Intercalation of molecules into layered materials is actively researched in materials science, chemistry, and nanotechnology, holding promise for the synthesis of van der Waals heterostructures and encapsulated nanoreactors. However, the intercalation of organic molecules that exhibit physical or chemical functionality remains a key challenge to date. In this work, we present the synthesis of heterostructures consisting of porphines sandwiched between a Cu(111) substrate and an insulating hexagonal boron nitride ( h-BN) monolayer. We investigated the energetics of the intercalation, as well as the influence of the capping h-BN layer on the behavior of the intercalated molecules using scanning probe microscopy and density functional theory calculations. While the self-assembly of the molecules is altered upon intercalation, we show that the intrinsic functionalities, such as switching between different porphine tautomers, are preserved. Such insulator/molecule/metal structures provide opportunities to protect organic materials from deleterious effects of atmospheric environment, can be used to control chemical reactions through spatial confinement, and give access to layered materials based on the ample availability of synthesis protocols provided by organic chemistry.

Sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets.

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material.

Polarization-dependent epitaxial growth and photocatalytic performance of ferroelectric oxide heterostructures

Epitaxial heterostructures are of particular interest owing to their fascinating properties for wide applications in energy, environment and electronic devices. The understanding of epitaxial growth in solution phase, however, remains a fundamental challenge to realize the rational synthesis of heterostructures. Here we report that anatase TiO2 can epitaxially grow on the selective polar surface of single-crystal and single-domain ferroelectric PbTiO3 nanoplates. The interplay of ferroelectric polar surface and corresponding ion adsorptions on them has been revealed experimentally and theoretically to determine the epitaxial growth mode, giving rising to zero-dimensional (0D)/two-dimensional (2D) and 2D/2D heterostructures. A combination of experimental and theoretical calculations indicate that the resulting heterostructures adopt a polarization-dependent photocatalytic performance under visible light irradiation, including hole-based photodegradation and electron-based hydrogen evolution reaction (HER) of water splitting. Such findings allow investigation of the potential of ferroelectric polarization towards tuning epitaxial growth and functionality of heterostructures.

Single source precursor driven phase selective synthesis of Au-CuGaS2 heteronanostructures: an observation of plasmon enhanced photocurrent efficiency.

The design of new functional metal-semiconductor heteronanostructures with improved photovoltaic efficiencies has drawn significant attention because of their unprecedented properties and potential applications. Herein, we report a phase selective synthesis of ternary CuGaS2 (wurtzite and tetragonal) by simple solution based thermal decomposition of a new binuclear single molecular precursor [Ga(acda)3Cu(PPh3)2]NO3 (acda = 2-aminocyclopentene-1-dithiocarboxylic acid, PPh3 = triphenylphosphine) where the phase selectivity has been achieved easily by changing the combination of surface active agents. Furthermore, we have extended our approach to develop a well-controlled synthetic strategy for the preparation of a Au-CuGaS2 heteronanocomposite with both the phases. A detailed microscopic study reveals that during heterostructure synthesis, an epitaxial junction has been formed at the interface of ternary CuGaS2 and metallic Au. To find out the influence of this epitaxial connectivity on the properties, we have studied the photocurrent and photoresponse behavior of the material and compared them with that of bare CuGaS2. For both the wurtzite and tetragonal phases, the Au-CuGaS2 twin structure exhibits a plasmon enhanced superior charge transport ability and an abruptly high photocurrent density compared to that of pure CuGaS2. Due to efficient charge separation by strong plasmon-exciton coupling at the interface, Au-CuGaS2 can be used as a potential candidate for photoelectrochemical applications.

The Standard Model of the Heterostructure for Microwave Devices

A methodology of the selection of the initial materials, architecture and synthesis of heterostructures based on domestic materials and technologies as applied to specific types of microwave components needs to be developed. As the nomenclature expands, the requirements on energy consumption, overall dimensions and weight, frequency range, noise, values of working temperatures, and other characteristics of microwave components have significantly increased. Specific examples of power amplifiers for various applications (wireless communication systems and location systems) are considered. It is shown that in order to implement such developments, it is necessary to apply modern methods of multilevel computer modeling using various methods of optimization and widely use the tested technical solutions. The final result of this development is the creation of a set of basic physical models of the heterostructures, including those based on the solution of optimized problems by choosing the initial material, substrate material, layer composition, their sequences, layer thicknesses, impurity contents, and their distribution by the layer thickness. All this makes it possible to form an acceptable level of mechanical stresses and high values of the electrophysical characteristics in the heterostructure. The initial data set in the form of a library of basic models of the heterostructures will make it possible to significantly accelerate the development of various microwave components and optoelectronic components in the system of instrument and technological design and improve the characteristics of the devices and economic rates.

Nanostar morphology of plasmonic particles strongly enhances photoelectrochemical water splitting of TiO2 nanorods with superior incident photon-to-current conversion efficiency in visible/near-infrared region

Photoelectrochemical (PEC) water splitting with TiO2 photoanodes is a promising technique for converting solar energy into a clean chemical energy. The key drawbacks of titania involve still a low PEC performance and its large band gap energy allowing absorption only in the UVregion. Plasmonic nanostructures such as Au and Ag represent a powerful tool towards the light harvesting enhancement with the efficiency dependent on their size and loading. Here, the unique nanostar morphology of plasmonic particles is presented as a new principal factor allowing the significant improvement of the incident photon-to-current conversion efficiency (IPCE) and the extension of light absorption over the broadband UV–Vis–NIR region. A plasmonic metal/semiconductor heterostructure synthesis is based on ex situ deposition of multispiked gold nanostars (Au-NSs) onto hydrothermally grown TiO2 nanorods. Compared to bare TiO2 nanorods, the Au-NSs-decorated TiO2 exhibits 350% and ca. 20% increase in the photocurrent density under visible light and simulated sunlight irradiation, respectively. Importantly, a significant enhancement of IPCE over the Vis-NIR region is observed with single-phase Au-NSs, the value two times higher compared to the spherical morphology. This is largely attributed to the long-wavelength plasmon resonances of Au-NSs and their ability to promote surface plasmon resonance (SPR)-mediated hot electron transfer.

One-pot synthesis of bismuth silicate heterostructures with tunable morphology and excellent visible light photodegradation performances

Construction of a heterostructure to prolong the life of electron-hole pairs is a very important approach to endow it with excellent photodegradation performances. Particularly, one-pot synthesis of heterostructures with the same component but different crystal structures to form a proper band gap is still challenging. Herein, bismuth silicate (BSO) heterostructures are synthesized using a one-pot hydrothermal approach without adding any other inorganic components. The crystal phase, morphology, surface state, and photochemical properties of the BSO materials are precisely tuned to fabricate two kinds of bismuth silicate heterostructures: rod-like Bi2SiO5/Bi12SiO20 and flower-like Bi2SiO5/Bi4Si3O12 heterostructures. Thanks to the two heterostructures and clean surface, the optimized BSO material exhibits a highly active photocatalytic performance with a remarkable cycling stability. It photodegrades Rhodamine B under visible light irradiation as fast as 15min with the reaction rate constants k and ks to be 0.399min−1 and 0.698min−1Lm−2, respectively, which is up to 189 times faster than reported.

Edge Epitaxy of Two-Dimensional MoSe2 and MoS2 Nanosheets on One-Dimensional Nanowires.

Rational design and synthesis of heterostructures based on transition metal dichalcogenides (TMDs) have attracted increasing interests because of their promising applications in electronics, catalysis, etc. However, the construction of epitaxial heterostructures with an interface at the edges of TMD nanosheets (NSs) still remains a great challenge. Here, we report a strategy for controlled synthesis of a new type of heterostructure in which TMD NSs, including MoS2 and MoSe2, vertically grow along the longitudinal direction of one-dimensional (1D) Cu2-xS nanowires (NWs) in an epitaxial manner. The obtained Cu2-xS-TMD heterostructures with tunable loading amount and lateral size of TMD NSs are achieved by the consecutive growth of TMD NSs on Cu2-xS NWs through gradual injection of chalcogen precursors. After cation exchange of Cu in Cu2-xS-TMD heterostructures with Cd, the obtained CdS-MoS2 heterostructures retained their original architectures. Compared to the pure CdS NWs, the CdS-MoS2 heterostructures with 7.7 wt % loading of MoS2 NSs exhibit the best performance in the photocatalytic hydrogen evolution reaction with a H2 production rate up to 4647 μmol·h-1·g-1, about 58 times that catalyzed with pure CdS NWs. Our synthetic strategy opens up a new way for the controlled synthesis of TMD-based heterostructures, which could have various promising applications.

Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WxMo1-xS2) and Graphene with Superior Catalytic Performance for Hydrogen Evolution.

Large-area (∼cm2) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and WxMo1-xS2 alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec-1 and 96 mV onset potential (at current density of 10 mA cm-2) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W0.4Mo0.6S2 alloys are far more efficient than WS2 and MoS2 by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the WxMo1-xS2 alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H2; with the lowest energy barrier occurring for the W0.4Mo0.6S2 alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/W0.4Mo0.6S2 produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.

Sol–gel synthesis of SnO2/CdS heterostructures using various Cd:S molar ratio solutions and its application in photocatalytic degradation of organic dyes

Synthesis of heterostructures of large band gap metal oxides coupled with chalcogenide semiconductors is an emerging area of research. They have been reported to be used as effective photocatlysts for purification of water in recent years. It can converts toxic organic pollutants into less or nontoxic compounds. In this present work we have reported synthesis of SnO2/CdS heterostructures by two-step sol–gel route: In the first step, the synthesis of nano sized SnO2 particles by sol gel method, and in the second step synthesis of of SnO2/CdS heterostructures with CdS solution, in various molar ratios of cadmium and sulphur. Obtained samples were characterized by structural, morphological, electrical and optical studies. The photocatalytic behavior of the samples were also were studied. The samples demonstrate high photo catalytic degradation of methylene blue than individual SnO2 due to effectual charge separation. Increasing sulfur content in the solution introduces an effective charge separation in the grown heterostructures resulted in diminished PL intensity. For heterostructures with higher sulfur content, photo catalytic performance reaches upto 96% photo degradation of methylene blue as compare to bare SnO2 (26%) under the influence of UV and visible light irradiation. Obtained heterostructures are highly suitable for various industrial applications.

Morphology-Controlled Synthesis of Hexagonal Boron Nitride Crystals by Chemical Vapor Deposition

Synthesis of hexagonal boron nitride (hBN) crystals with a controlled morphology is a major challenge due to various kinetic and thermodynamic factors at the edge. Supplying BN building blocks by heating precursor ammonia borane at a temperature of 90 °C produced triangular crystals, whereas, at higher heating (130 °C), hBN crystals with a hexagonal morphology were observed. The shape of crystals could also be modulated from hexagonal to triangular under a continuous reduced supply of BN building blocks. We attributed these phenomena to a different growth mechanism dependent on the concentration of BN radicals in the growth region. With a low supply of BN building blocks, crystal growth is the edge attachment limited, producing triangles, whereas, under higher concentration of BN building blocks, crystal growth is limited by diffusion producing hexagons. The presence of alternating B and N terminated vertices in hexagonal hBN crystals offers the possibility of hBN/graphene in-plane heterostructure synthesis with B–C and N–C bonding at the interface, which is not possible with N terminated triangular crystals. Supplying the controlled amount of BN building blocks to synthesize triangular and hexagonal crystals and modulation from hexagons to triangles is a major achievement (in the present work) due to single parameter control over the experiment.

(Invited) Realizing 2D Materials Via MOCVD

This manuscript presents a brief review of previous works related to synthesis of 2D materials via metal-organic chemical vapor deposition. Additionally, synthesis of heterostructures based on tungsten diselenide and epitaxial graphene are discussed.

Metal Oleate Induced Etching and Growth of Semiconductor Nanocrystals, Nanorods, and Their Heterostructures.

Unexpected etching of nanocrystals, nanorods, and their heterostructures by one of the most commonly used metal precursors, metal oleates, is reported. Zn oleate is shown to etch CdS nanorods anisotropically, where the length decreases without a significant change in the diameter. Sodium oleate enhances the etch rate, whereas oleic acid alone does not cause etching, indicating the importance of the countercation on the rate of oleate induced etching. Subsequent addition of Se precursors to the partially etched nanorods in Zn oleate solution can lead to epitaxial growth of CdSe particles rather than the expected ZnSe growth, despite an excess amount of Zn precursors being present. The composition of this epitaxial growth can be varied from CdSe to ZnSe, depending on the amount of excess oleic acid or the reaction temperature. Similar tuning of composition can be observed when starting with collinear CdSe/CdS/CdSe rod/rod/rod heterostructures and spherical CdS (or CdSe/CdS core/shell) nanocrystals. Conversion of collinear rod/rod/rod structures to barbells and interesting rod growth from nearly spherical particles among other structures can also result due to the initial etching effect of metal oleates. These observations have important implications on our understanding of nanocrystal heterostructure synthesis and open up new routes to varying the composition and morphology of these materials.

Plateau-Rayleigh Crystal Growth of Nanowire Heterostructures: Strain-Modified Surface Chemistry and Morphological Control in One, Two, and Three Dimensions.

One-dimensional (1D) structures offer unique opportunities for materials synthesis since crystal phases and morphologies that are difficult or impossible to achieve in macroscopic crystals can be synthesized as 1D nanowires (NWs). Recently, we demonstrated one such phenomenon unique to growth on a 1D substrate, termed Plateau-Rayleigh (P-R) crystal growth, where periodic shells develop along a NW core to form diameter-modulated NW homostructures with tunable morphologies. Here we report a novel extension of the P-R crystal growth concept with the synthesis of heterostructures in which Ge (Si) is deposited on Si (Ge) 1D cores to generate complex NW morphologies in 1, 2, or 3D. Depositing Ge on 50 nm Si cores with a constant GeH4 pressure yields a single set of periodic shells, while sequential variation of GeH4 pressure can yield multimodulated 1D NWs with two distinct sets of shell periodicities. P-R crystal growth on 30 nm cores also produces 2D loop structures, where Ge (Si) shells lie primarily on the outside (inside) of a highly curved Si (Ge) core. Systematic investigation of shell morphology as a function of growth time indicates that Ge shells grow in length along positive curvature Si cores faster than along straight Si cores by an order of magnitude. Short Ge deposition times reveal that shells develop on opposite sides of 50 and 100 nm Si cores to form straight 1D morphologies but that shells develop on the same side of 20 nm cores to produce 2D loop and 3D spring structures. These results suggest that strain mediates the formation of 2 and 3D morphologies by altering the NW's surface chemistry and that surface diffusion of heteroatoms on flexible freestanding 1D substrates can facilitate this strain-mediated mechanism.

Postsynthesis of h-BN/Graphene Heterostructures Inside a STEM.

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers.

Synthesis of Epitaxial Films Based on Ge–Si–Sn Materials with Ge/GeSn, Ge/GeSiSn, and GeSn/GeSiSn Heterojunctions

Results of investigations into the synthesis of heterostructures based on Ge–Si–Sn materials by the method of low-temperature molecular beam epitaxy are presented. The formation of epitaxial films during structure growth has been controlled by the reflection high-energy electron diffraction method. Films with Ge/GeSn, Ge/GeSiSn, and GeSn/GeSiSn heterojunctions are grown with Sn content changing from 2 to 10 % at temperatures in the interval 150–350°С. The stressed state, the composition, and the lattice parameter are studied by the x-ray diffraction method using Omega-scan curves and reciprocal space maps. A tensile strain in the Ge film during Ge/Ge0.9Sn0.1/Si structure growth has reached 0.86%.

Nanoscale mixing during double-flame spray synthesis of heterostructured nanoparticles

The combination of two nanoparticle-producing flame reactors to a double-flame (DF) spray pyrolysis process is an attractive method for the high-temperature gas-phase synthesis of multicompound materials and heterostructures. It allows separate control of particle growth in the individual flames up to the intersection or mixing point where the formation of heterostructures takes place. The effect of mixing of the aerosol streams on the process temperature and product characteristics is investigated based on the example of Pt on TiO2. Temperatures were determined by Fourier-transform infrared spectroscopy and thermocouple measurements along with computational fluid dynamics, while the degree of mixing was investigated based on surface area, Pt-dispersion measurements, and transmission electron microscopy image analyses. The quadrat method in combination with the variation coefficient was used to quantify the uniformity of the Pt cluster distribution on the TiO2 support. For high intersection distances of the two flame jets and small intersection angles, nonuniform mixing of the compounds and the formation of large Pt particles are observed. For small intersection distances and large angles, a uniform Pt distribution was achieved. Based on these findings, process design rules were established which can be transferred to other material systems.

Vertical heterostructure of two-dimensional MoS₂ and WSe₂ with vertically aligned layers.

Two-dimensional (2D) layered materials consist of covalently bonded 2D atomic layers stacked by van der Waals interactions. Such anisotropic bonding nature gives rise to the orientation-dependent functionalities of the 2D layered materials. Different from most studies of 2D materials with their atomic layers parallel to substrate, we have recently developed layer vertically aligned 2D material nanofilms. Built on these developments, here, we demonstrate the synthesis of vertical heterostructure of n-type MoS2 and p-type WSe2 with vertically aligned atomic layers. Thin film of MoS2/WSe2 vertical structure was successfully synthesized without significant alloy formation. The heterostructure synthesis is scalable to a large area over 1 cm(2). We demonstrated the pn junction diode behavior of the heterostructure device. This novel device geometry opens up exciting opportunities for a variety of electronic and optoelectronic devices, complementary to the recent interesting vertical heterostructures with horizontal atomic layers.

Chemically synthesized heterostructures of two-dimensional molybdenum/tungsten-based dichalcogenides with vertically aligned layers.

We demonstrate a one-step, facile chemical synthesis of heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) with vertically aligned 2D layers. Specifically, by using a chemical vapor transport method, we synthesize molybdenum (Mo)/tungsten (W)-based disulfide and diselenide heterostructures, such as MoS2/WS2 and MoSe2/WSe2. Detailed structural characterizations reveal that the 2D layers of two different TMDCs are vertically standing/aligned exposing the edge sites of the layers. The TMDC heterostructure materials exhibit a high uniformity of structural/chemical heterogeneity over a large area (>1 cm(2)) and show anisotropic carrier transport properties. These materials with unique heterostructures have great implications for both technological applications and fundamental sciences.