Coaxial Heterostructures Research Articles

In situ growth of MnO2 on graphitized cellulose nanocrystal: A novel coaxial heterostructures with unblocked conductive networks as advanced electrode materials for supercapacitor

Manganese dioxide (MnO2) with high theoretical capacitance and natural abundance has attracted great attention but is still challenging for use in supercapacitors, due to the limited conductivity and lower electron and ion migration. Here, a facile in situ growth strategy is developed to prepare heterostructural graphitized cellulose nanocrystal (GCNC)/MnO2 with unblocked conductive networks through a hydrothermal reaction. The GCNC with highly ordered graphitic carbon layers as conductive support framework solves the agglomeration problem of MnO2 and increases effective specific surface areas in contact with electrolyte ions. Meanwhile, the stable connection of MnOC covalent bonds enhances the interface bonding, which effectively reduces the contact resistance at the interface of heterostructural GCNC/MnO2 and enhances the interface firmness. The resulting GCNC/MnO2 electrode shows a remarkable specific capacitance of 528.2 F g−1 at 0.5 A g−1. Furthermore, the symmetric supercapacitor based on GCNC/MnO2-15 mM exhibits high rate capability and excellent cycle stability of 100 % capacitance retention after 3000 cycles. This work provides a new path to designing high performance electrode material for sustainable energy technologies.

Boosting CO2 Hydrogenation to Methanol over Monolayer MoS2 Nanotubes by Creating More Strained Basal Planes.

The controlled creation, selective exposure, and activation of more basal planes while simultaneously minimizing the generation and exposure of edge sites are crucial for accelerating methanol synthesis from CO2 hydrogenation over MoS2 catalysts but remain a bottleneck. Here, we report a facile method to fabricate heteronanotube catalysts with single-layer MoS2 coaxially encapsulating the carbon nanotubes (CNTs@MoS2) through host-guest chemistry. Inheriting the long tubular structure of CNTs, the grown MoS2 nanotubes exhibit significantly more basal planes than bulk MoS2 crystals. More importantly, the tubular curvature not only promotes strain and sulfur vacancy (Sv) generation but also preferentially exposes more in-plane Sv while limiting edge Sv exposure, which is conducive to methanol synthesis. Both the strain and layer number of MoS2 can be easily and finely adjusted by altering CNT diameter and quantity of precursors. Remarkably, CNTs@MoS2 with monolayer MoS2 and maximum strain displayed methanol selectivity of 78.1% and methanol space time yield of 1.6 g gMoS2-1 h-1 at 260 °C and GHSV of 24000 mL gcat.-1 h-1, representing the best results to date among Mo-based catalysts. This study provides prospects for novel catalyst design by synthesizing coaxial tubular heterostructure to create additional catalytic sites and ultimately enhance conversion and selectivity.

A Breathable Fibrous Membrane with Coaxially Heterogeneous Conductive Networks toward Personal Thermal Management and Electromagnetic Interference Shielding.

The expeditious growth of wearable electronic devices has boomed the development of versatile smart textiles for personal health-related applications. In practice, integrated high-performance systems still face challenges of compromised breathability, high cost, and complicated manufacturing processes. Herein, a breathable fibrous membrane with dual-driven heating and electromagnetic interference (EMI) shielding performance is developed through a facile process of electrospinning followed by targeted conformal deposition. The approach constructs a robust hierarchically coaxial heterostructure consisting of elastic polymers as supportive "core" and dual-conductive components of polypyrrole and copper sulfide (CuS) nanosheets as continuous "sheath" at the fiber level. The CuS nanosheets with metal-like electrical conductivity demonstrate the promising potential to substitute the expensive conductive nano-materials with a complex fabricating process. The as-prepared fibrous membrane exhibits high electrical conductivity (70.38 S cm-1), exceptional active heating effects, including solar heating (saturation temperature of 69.7 °C at 1 sun) and Joule heating (75.2 °C at 2.9V), and impressive EMI shielding performance (50.11dB in the X-band), coupled with favorable air permeability (161.4mm s-1 at 200Pa) and efficient water vapor transmittance (118.9g m-2 h). This work opens up a new avenue to fabricate versatile wearable devices for personal thermal management and health protection.

Curvature-controlled band alignment transition in 1D van der Waals heterostructures

The effect of curvature on the band alignment of one-dimensional (1D) van der Waals (vdW) transition metal dichalcogenide (TMDC) heterostructures is studied by comprehensive first-principles calculations. We find that, as the diameter of a TMDC nanotube decreases, the combined effect of curvature-induced flexoelectricity and circumferential tensile strain causes a rapid lowering of the conduction band minimum, whereas the valence band maximum exhibits an initial lowering before rising. As individual TMDC nanotubes form coaxial heterostructures, the concerted effect of diameter-dependent band-edge levels and intertube coupling via flexovoltage can result in a transition of intertube band alignment from Type II to Type I in multiple heterostructural systems, including large-diameter MoSe2@WS2, MoTe2@MoSe2, and MoTe2@WS2 heterostructures. These results lay down a foundation for the rational design of 1D vdW heterostructures.

Coaxial heterostructure formation of highly crystalline graphene flakes on boron nitride nanotubes by high-temperature chemical vapor deposition

We develop a high-temperature chemical vapor deposition of highly crystalline graphene on the surface of boron nitride nanotubes (BNNTs). The growth of few-layer graphene flakes on BNNT templates was confirmed by scanning transmission electron microscopy. Based on an investigation of the effect of growth temperature and growth time on defect density, graphene with relatively high crystallinity was obtained at 1350 °C. The absence of undesirable alterations in the BNNT lattice during graphene growth was verified by multiple analyses. The high-temperature growth of heterolayers would assist in the advancement of nanodevices that coaxially combine graphene and boron nitride.

Nitrogen-doped carbon nanotube in-situ loaded LiNbO3 anode with high capacitance contribution for lithium-ion capacitors

LiNbO3 is an intercalation material with high theoretical specific capacity, which is promising for lithium-ion capacitor (LIC) anode. However, the poor charge transfer dynamics makes LiNbO3 unable to match the capacitive cathode in rate performance and cycle stability, resulting in poor electrochemical performance of the device. Herein, through the integrated process of in-situ nitrogen-doping with polypyrrole as the nitrogen source and hydrothermal deposition, the coaxial heterostructure of ultrafine LiNbO3 nanoparticles and nitrogen-doped carbon nanotubes (NCNT) was realized. The large lithium-ion accessible surface and the bonding with the conductive channel significantly enhance the pseudocapacitive behavior of LiNbO3 (capacitance contribution of 78.5% at 1 mV s−1). After 1000 cycles at a current density of 2 A g−1, NCNT@LiNbO3 still has a specific capacity of 154.9 mAh g−1 (89.8%). Thus, the assembled NCNT@LiNbO3//AC devices can charge and discharge up to 10000 cycles at a current density of 2 A g−1, and the capacity retention rate can still reach 90.3%, and can provide an energy density of 142 Wh kg−1. This work provides ideas for the interface and structure design of nanocomposite electrode materials.

Selenium-doped carbon nanotubes/nickel selenide coaxial nanocables for energy storage

Coaxial nanocable heterostructures consisting of highly conductive carbon nanomaterial core and active transition-metal compound sheath are promising potential candidates for advanced energy storage materials. Herein, the multi-functional selenium‐doped carbon nanotubes/nickel selenide coaxial nanocables with chemical bonding interface are prepared by using an electroless deposition combined with the following chemical vapor reaction process, in which the nickel layer is first deposited onto the carbon nanotube core and subsequently transformed in-situ into the nickel selenide sheath by the controlled selenization reaction at elevated temperature. Such coaxial structure provides the large active material/electrolyte contact area, and the fast diffusion and transfer of mass and charge in the electrochemical process, thus which endows superior supercapacitive and good lithium storage performances. The coaxial nanocables exhibit a high specific capacity of 340.0 mAh g −1 at 1 A g −1 as an electrode for supercapacitor, and a lithium storage capacity of 600.0 mAh g −1 at 0.1 A g −1 as an anode for lithium-ion batteries. A supercapacitor based on the nanocables delivers a high energy density of 70.0 Wh kg −1 at 803.0 W kg −1 and long cycling stability with 65.0% capacitance retention after 8000 cycles at 10 A g −1 . • Preparing multi-functional Se-CNTs@NiSe nanocables with chemical bonding interface. • Graphene/NiSe nanoribbons are derived due to splitting of coaxial nanocables. • Nanocables show the excellent supercapacitive and lithium storage performances.

One-dimensional covalent organic framework—Carbon nanotube heterostructures for efficient capacitive energy storage

Covalent organic frameworks (COFs) with redox-active moieties are potential capacitive energy storage materials. However, their performance is limited by their poor electrical conductivity and sluggish ion diffusion in their nanopores. Herein, we report coaxial one-dimensional van der Waals heterostructures (vdWHs) comprised of a carbon nanotube (CNT) core and a pyrene–pyridine COF shell synthesized by an in situ wrapping method. The coaxial structure allows efficient electronic interaction between the CNT core and COF shell and improves the electrical conductivity significantly. It also improves electrolyte ion accesses to redox-active pyridine groups in the COF, resulting in excellent capacitive energy storage performance with a high specific capacitance of ∼360 F g−1, an excellent rate capability of ∼80%, and a good stability of 92% capacitance retention after 20 000 charge/discharge cycles. Our strategy opens the door to create other multi-dimensional vdWHs for various potential applications.

Enhanced Sodium‐Ion Storage Performance of a 2D MoS2 Anode Material Coated on Carbon Nanotubes

AbstractMolybdenum disulfide (MoS2) is considered a promising anode material for sodium‐ion batteries (SIBs). However, it suffers from poor kinetics and rapid capacity decay. In this work, a coaxial heterostructure composed of MoS2 nanosheets coated on carbon nanotubes (CNTs) is reported, where the MoS2 layers are less stacked at the surface of the CNTs substrate with an enhanced interlayer spacing, which is prepared through a simple one‐pot hydrothermal preparation. The MoS2@CNTs composite material is proven to have superior rate performance as an anode material for SIBs, which delivers 508, 418, 359, 305, 258, and 183 mAh/g at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A/g, respectively, together with excellent cycling stability to retain 360 mAh/g after 400 cycles at 0.5 A/g, owing to the robust combination of MoS2 and CNT components. This work offers a way to synthesize heterostructured composite materials with great performance superiority for large‐scale energy storage systems.

One-Dimensional van der Waals Heterostructures as Efficient Metal-Free Oxygen Electrocatalysts.

Two-dimensional covalent organic frameworks (2D-COFs) may serve as an emerging family of catalysts with well-defined atomic structures. However, the severe stacking of 2D nanosheets and large intrinsic bandgaps significantly impair their catalytic performance. Here, we report coaxial one-dimensional van der Waals heterostructures (1D vdWHs) comprised of a carbon nanotube (CNT) core and a thickness tunable thienothiophene-pyrene COF shell using a solution-based in situ wrapping method. Density functional theory calculations and operando and ex situ spectroscopic analysis indicate that carbon-sulfur regions in thienothiophene groups in the COF serve as an active catalytic site for oxygen reduction and evolution reactions. The coaxial structure enables n-doping from the CNT core to the COF shell, which is controllable by varying COF shell thickness. The charge transfer from CNTs lowers COF's bandgap and work function, which reduces the charge transfer barrier between the active catalytic sites and adsorbed oxygen intermediates, resulting in dramatically enhanced catalytic activity. The 1D vdWHs were applied as a bifunctional oxygen electrocatalyst in rechargeable zinc-air batteries, delivering a high specific capacity of 696 mAh gZn-1 under a high current density of 40 mA cm-2 and excellent cycling stability. The 1D vdWHs based on the coaxial structure of 2D COF wrapped around CNT cores may be further used as versatile building units to create multidimensional vdWHs for exploring fundamental physics and chemistry as well as practical applications in electrochemistry, electronics, photonics, and beyond.

Coaxial nanofibers of nickel/gadolinium oxide/nickel oxide as highly effective electrocatalysts for hydrogen evolution reaction

Cost-effective, active and stable electrocatalysts are crucial for hydrogen production via electrocatalytic water splitting. Here, we describe the preparation of novel nanofibers (NF) made of Ni/Gd2O3/NiO heterostructures by electrospinning. The fabricated materials showed high electrocatalytic performance for hydrogen evolution reaction (HER) with onset potential values of 89 mV, which are very close to those of platinum (Pt). NiO chemical and electronic properties were successfully optimized in Ni/Gd2O3/NiO coaxial heterostructures; NiO NFs doped with Gd3+ significantly enhanced its electrical conductivity and promoted HER reaction kinetics. These NFs offer the distinct advantages of long-term durability and readiness for hydrogen production via HER, and also better performance than benchmark Pt catalysts. The successful fabrication of these metal oxide NFs and nanostructures may represent a new approach for the rational synthesis of efficient HER catalysts.

Development of a new electrochemical sensing platform based on MoO3-polypyrrole nanowires/MWCNTs composite and its application to luteolin detection

In this paper, a novel sensing platform based on MoO3-polypyrrole nanowires /MWCNTs (MoO3-PPy NWs/MWCNTs) composite was developed for electrochemical detection of luteolin, a widely used antitumoral agent. One dimensional (1D) MoO3 NWs were synthesized by hydrothermal method, which shows a large specific surface area. PPy coating on the surface of MoO3 NWs not only increases the electrical conductivity of MoO3, but also prevents the aggregation of MoO3 NWs. MWCNTs serves as electrical conducting wires that connect MoO3-PPy NWs coaxial heterostructure NWs, which is beneficial to promote the transfer of electrons and the redox performance of catalyst. Due to the excellent synergic effect between MoO3-PPy NWs and MWCNTs, the sensor based on MoO3-PPy NWs/MWCNTs can detect luteolin with low limit of detection (LOD) down to 0.03 nM. Additionally, outstanding long-term stability, excellent selectivity and prominent reproducibility were obtained. Furthermore, luteolin in real samples was detected by the proposed sensor with good performance. The sensor based on MoO3-PPy NWs/MWCNTs provides a wide linear response ranging from 0.1 nM to 10.0 μM and a low limit of detection (LOD) of 0.03 nM (S/N = 3) for luteolin. The method has been applied to detect luteolin in chrysanthemum tea with recoveries of 96.00–108.20% and RSDs of 3.39–4.02% (n = 4).

Enhanced In-Plane Thermal Conductance of Thin Films Composed of Coaxially Combined Single-Walled Carbon Nanotubes and Boron Nitride Nanotubes.

Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are one-dimensional materials with high thermal conductivity and similar crystal structures. Additionally, BNNTs feature higher thermal stability in air than CNTs. In this work, a single-walled carbon nanotube (SWCNT) film was used as a template to synthesize a BNNT coating by the chemical vapor deposition (CVD) method to form a coaxial heterostructure. Then, a contact-free steady-state infrared (IR) method was adopted to measure the in-plane sheet thermal conductance of the as-synthesized film. The heterostructured SWCNT-BNNT film demonstrates an enhanced sheet thermal conductance compared with the bare SWCNT film. The increase in sheet thermal conductance shows a reverse relationship with SWCNT film transparency. An enhancement of over 80% (from ∼3.6 to ∼6.4 μW·K-1·sq-1) is attained when the BNNT coating is applied to an SWCNT film with a transparency of 87%. This increase is achieved by BNNTs serving as an additional thermal conducting path. The relationship between the thermal conductance increase and transparency of the SWCNT film is studied by a structured modeling of the SWCNT film. We also discuss the effect of annealing on the thermal conductance of SWCNTs before BNNT growth. Along with the preservation of high electrical conductance, the enhanced thermal conductance of the heterostructured SWCNT-BNNT films makes them a promising building block for thermal and optoelectronic applications.

Gas Sensing of NiO‐SCCNT Core–Shell Heterostructures: Optimization by Radial Modulation of the Hole‐Accumulation Layer

AbstractHierarchical core–shell (C–S) heterostructures composed of a NiO shell deposited onto stacked‐cup carbon nanotubes (SCCNTs) are synthesized by atomic layer deposition (ALD). A film of NiO particles (0.80–21.8 nm in thickness) is uniformly deposited onto the inner and outer walls of the SCCNTs. The electrical resistance of the samples is found to increase of many orders of magnitude with the increasing of the NiO thickness. The response of NiO–SCCNT sensors toward low concentrations of acetone and ethanol at 200 °C is studied. The sensing mechanism is based on the modulation of the hole‐accumulation region in the NiO shell layer upon chemisorption of the reducing gas molecules. The electrical conduction mechanism is further studied by the incorporation of an Al2O3 dielectric layer at NiO and SCCNT interfaces. The investigations on NiO–Al2O3–SCCNT, Al2O3–SCCNT, and NiO–SCCNT coaxial heterostructures reveal that the sensing mechanism is strictly related to the NiO shell layer. The remarkable performance of the NiO–SCCNT sensors toward acetone and ethanol benefits from the conformal coating by ALD, large surface area of the SCCNTs, and the optimized p‐NiO shell layer thickness followed by the radial modulation of the space‐charge region.

A highly active and durable electrocatalyst for large current density hydrogen evolution reaction

Splitting water under large current density is essential for efficient large-scale production and commercial utilization of hydrogen. However, the performance of the available electrocatalysts for hydrogen evolution reaction (HER) is far from satisfactory under large current density in alkaline electrolyte. Here we report a remarkably active and durable electrocatalyst, long and dense MoS2/Ni3S2 co-axial heterostructure nanowires on nickel foam (NF). Notably, it requires only 182 and 200 mV overpotential to achieve large current density of 500 and 1000 mA cm−2, respectively, in alkaline solution, which are far superior to those of Pt/C-NF (281 and 444 mV) and the reported best non-noble metal catalysts (191 and 220 mV). The physical origin for this extraordinary HER performance is analyzed, which provides a useful guide for structure design of electrocatalysts to further improve their performance.

MoO3@PEDOT coaxial heterostructure nanobelts by in situ polymerization with enhanced electrochromic performance

Electrochromic materials have shown great application prospects in electronic display and information storage. In this work, we synthesized MoO3@PEDOT coaxial heterostructure nanobelts by a simple and green in situ polymerization method and investigated their electrochromic properties. This MoO3@PEDOT nanobelts take internal MoO3 nanobelts as mechanical support and growth templates to make the externally PEDOT covered uniformly on the outside. This unique 1D nano-structure has large surface area, provides large number of accessible electroactive sites for high color contrast. It was found that the electrochromic composite films prepared by spin coating MoO3@PEDOT coaxial heterostructure nanobelts on ITO exhibited a high contrast of 31.8%, and colored efficiency up to 86.57 cm2·C−1. It turned out the MoO3@PEDOT coaxial heterostructure nanobelts can improve PEDOT electrochromic properties greatly and will be a suitable choice for smart window application.

Synthesis, characterization and dye-sensitized solar cell application of Zinc oxide based coaxial core-shell heterostructure

In the present work, the ZnO/CdS coaxial core–shell heterostructure has been synthesized on indium doped tin oxide glass substrate using chemical solution methods and further it is used as photoanode in the DSSC application. Four samples with different concentration of sulphur source were prepared. The approximate crystallite size of 22 nm for ZnO and 18 nm for CdS has been obtained as analyzed from x-ray diffraction data. FESEM and HRTEM images confirm the formation of CdS shell on ZnO nanorods with a width below 100 nanometer. UV-visible spectroscopy and photoluminescence spectroscopy were used to examine the optical response revealing the fact that bandgap reduction and a small redshift takes place for core–shell structure as compared to bare ZnO nanorods array. The J-V characterization was performed to study the photovoltaic response. Improved efficiency was observed for core–shell ZnO/CdS heterostructure (3.66%) as compared to bare ZnO nanorods based DSSCs (0.70%). This improvement is due to increased current density caused by reduced recombination of photogenerated electron hole pairs. This reduced recombination is achieved by two factors, one is the formation of type two core–shell heterostructure and second is increasing sulphur concentration in shell structure.

Ultrafast carrier dynamics of conformally grown semi-polar (112[combining macron]2) GaN/InGaN multiple quantum well co-axial nanowires on m-axial GaN core nanowires.

The growth of semi-polar (112[combining macron]2) GaN/InGaN multiple-quantum-well (MQW) co-axial heterostructure shells around m-axial GaN core nanowires on a Si substrate using MOCVD is reported for the first time. The core GaN nanowire and GaN/InGaN MQW shells are grown in a two-step growth sequence of vapor-liquid-solid and vapor-solid growth modes. The luminescence and carrier dynamics of GaN/InGaN MQW coaxial nanowires are studied by photoluminescence, cathodoluminescence, and low temperature time-resolved photoluminescence (TRPL). The emission is tuned from 430 nm to 590 nm by increasing the InGaN QW thickness. The non-single exponential decay measured by low-temperature TRPL was attributed to the indium fluctuations in the InGaN QW. The ultrafast radiative lifetime was measured from 14 ps to 26 ps with different emission wavelengths at a very high internal quantum efficiency up to 68%. An ultrafast carrier lifetime was assigned to the growth of the InGaN QW on semi-polar (112[combining macron]2) growth facet and the improved carrier collection efficiency due to the radial growth of the GaN/InGaN MQW shells. Such an ultrafast carrier dynamics of NWs provides a meaningful active medium for high speed optoelectronic applications.

Resolving ZnO-based coaxial core-multishell heterostructure by electrical scanning probe microscopy

Coaxially periodic ZnO/ZnMgO core-multishell nanowire (NW) heterostructures were grown via a metal organic chemical vapor deposition method. We investigated their electrical properties via the application of two locally resolved electrical scanning probe microscopy techniques, i.e., scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), following a planarization process. As a result, ZnO and ZnMgO layers can be unambiguously distinguished by both techniques on NWs with diameters <1 μm and the smallest layer thickness of 10 nm, where a higher free carrier concentration along with a low resistivity is revealed for the ZnO regions in comparison to ZnMgO portions, as expected. This work demonstrates the high capability of SCM/SSRM as supplementary and effective tools for probing local electrical properties within functional complex quasi-1D heterostructures.

Water splitting over core-shell structural nanorod CdS@Cr2O3 catalyst by inhibition of H2-O2 recombination via removing nascent formed oxygen using perfluorodecalin

One-dimensional CdS@Cr2O3 core/shell nanorod catalyst was prepared via one-step photodeposition for photocatalytic water splitting to hydrogen. The Cr2O3 shell was homogeneously coated on the surface of CdS/Pt core, forming coaxial heterostructure. This one-dimensional core/shell heterostructural assembly facilitates the high efficient photogenerated charge separation and transfer from CdS to Cr2O3 under visible light illumination, which suppressed the photocorrosion of CdS. Moreover, with the help of artificial blood component perfluorodecalin (PFDL), the nascent formed oxygen was removed from the reaction mixture, which further inhibited the oxygen induced photocorrosion, finally the catalyst exhibited 126.6μmol·g−1catalyst H2 formation in 2h without sacrifice reagent addition under visible light irradiation. No significant decay of activity was observed in 8h. The present results showed core-shell heterostructural CdS@Cr2O3 could be one of candidates for photocatalytic water splitting catalysts and oxygen capture reagent PFDL could be used for removing nascent oxygen to inhibit the H2-O2 recombination reverse reaction and enhance hydrogen evolution activity.