Coaxial Nanorod Research Articles

Arrays of ZnO/MoS2 nanorods and MoS2 nanotubes for spectral enhancement and self-cleaning

Structure and heterojunction of substrates are widely regarded as critical factors to improve Raman detection and recyclibility, but the exact roles are rarely explored for the mixed contributions. Herein, the arrays of ZnO/MoS2 coaxial nanorods (NRs) and MoS2 nanotubes (NTs) were successfully constructed for the surface-enhanced Raman scattering (SERS). The influences of NR and NT length on the morphology, light absorption and emission, as well as SERS performance were systematically investigated. Both the arrays grown for 40 min displayed optimal SERS performance, in which MoS2 NT array owned stronger enhancement with a detection limit of 5 × 10−7 M than that of 1 × 10−6 M for ZnO/MoS2 NR array. The NTs also effectively enhanced photocatalysis, in which rhodamine 6G pollutant was completely degraded within 140 min under ultraviolet light, surpassing that by the NRs. The analysis revealed that band alignment and structure characteristic of the specimens played critical roles in the spectral enhancement and self-cleaning, while the nanostructure surpassed heterojunction as it supplied void interior for extending light propagation and more contact area for adsorbing pollutants. These results would contribute to a better understanding of the Raman enhancement and could be extended to the design of SERS substrates to harness their potential application.

Spatial Decoupling of Redox Chemistry for Efficient and Highly Selective Amine Photoconversion to Imines.

Light-driven primary amine oxidation to imines integrated with H2 production presents a promising means to simultaneous production of high-value-added fine chemicals and clean fuels. Yet, the effectiveness of this strategy is generally limited by the poor charge separation of photocatalysts and uncontrolled hydrogenation of imines to secondary amines. Herein, a spatial decoupling strategy is proposed to isolate redox chemistry at distinct sites of photocatalysts, and CoP core-ZnIn2S4 shell (CoP@ZnIn2S4) coaxial nanorods are assembled as the proof-of-concept photocatalyst. Directional and ultrafast carrier separation occurs between the CoP core and the ZnIn2S4 shell, as confirmed by in situ X-ray photoelectron spectroscopy, surface photovoltage spectroscopy, and transient absorption spectroscopy analyses. Toward the photoconversion of model substrate benzylamine to N-benzylbenzaldimine, CoP@ZnIn2S4 exhibits a 48-time higher production rate and >99% selectivity when compared to ZnIn2S4 (ca. 20% selectivity), and the detailed reaction mechanism has been verified by in situ diffuse reflectance infrared Fourier transform spectroscopy.

Spontaneous polarization of ferroelectric heterostructured nanorod arrays for high-performance photoelectrochemical cathodic protection

Corrosion is a critical global problem for the steel industry and a grand challenge toward Net-Zero clean growth. TiO2 nanorods (TNRs) have been developed for photoelectrochemical cathodic protection (PCP) of coupled metal. However, the performance was hindered by their wide bandgap, weak visible-light response, and low separation efficiency of photocarriers. In this work, a unique multilayered coaxial nanorod array (TNRs/BTO/Ag NPs) structure has been proposed to enhance ‘hot’ electrons injection via inserting a ferroelectric semiconductor BaTiO3 (BTO) nanolayer between TNRs and Ag nanoparticles (Ag NPs). The spontaneous ferroelectric BTO nanolayer built a polar charge-induced electric field (PEF), resulting in a strong spontaneous polarization. Furthermore, the as-proposed structure can negatively shift the surface potential of the coupling 304 stainless steel by 0.80 V relative to its corrosion potential under simulated solar light illumination which is the record PCP potential among reported works. Based on comprehensive electrochemical and theoretical studies, the origin of the enhanced photo-generated carriers’ separation and depletion to the coupled metals is attributed to a synergic effect between PEF (the spontaneous polarization of BTO) and internal electric field (IEF) resulting from the cascade energy band alignment. Various promising applications can be envisioned from as-proposed multilayered coaxial nanorod array photoelectrodes.

Coaxial Ni3S2@CoMoS4/NiFeOOH nanorods for energy-saving water splitting and urea electrolysis

High efficiency and energy-saving electrochemical hydrogen production has always been a research challenge, mainly due to the limitation of the sluggish kinetics of anodic reactions. Excellent performance depends largely on the clever design of nano-architectures and smart hybridization of active components. It is also very important to establish the relationship between structure and performance of materials. Form perspectives of chemical composition and nanostructure, we developed a novel heterostructure of Ni3S2@CoMoS4/NiFeOOH coaxial nanorods on NF scaffold, in which the two-dimensional CoMoS4 nanoplates and ultrathin NiFeOOH nanosheets vertically coil around the Ni3S2 nanorods by hydrothermal reaction combined with electrodepostion process. Such hierarchical nanorods can provide the heterointerface with highly open surface, ensuring the maximization of synergistic interaction. This heterostructure results in prominent bifunctional activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water electrolysis systems, and HER coupled with urea oxidation reaction in urea electrolysis devices. It exhibits very low cell voltages for alkaline water splitting (1.732 V) and urea electrolysis (1.660 V) to afford a current density of 100 mA cm−2 in the two-electrode system as well as excellent long-term stability. Our work provides a new construction for combining various active materials to competitive bifunctional electrocatalysts applied in energy-relative electrochemical devices.

Two-step preparation and characterization of ZnO Core–Si shell coaxial nanorods

This study compares and analyzes the key characteristics of the ZnO core–Si shell coaxial nanorod (Si@ZnO NR) heterostructures that were grown on semi-insulating, (100)-oriented Si substrates at Si layer temperatures of 450, 500, 550 and 600 °C for optoelectronic device applications. The Si@ZnO NRs were prepared in two separate stages. Vapor-phase transport produced the ZnO NRs, whereas rapid thermal chemical vapor deposition formed the Si layers. Results obtained from X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and Raman tests indicate that the ZnO NRs were single crystals with a zincblende configuration. The findings also reveal that the Si layer was polycrystalline in nature, poly-Si, with a wurtzite configuration. The present research is beneficial to the optoelectronic devices such as light-emitting diodes, solar cells and photodetectors in the UV–infrared range using the Si@ZnO coaxial nanostructures.

Charge Transport between Coaxial Polymer Nanorods and Grafted All-Inorganic Perovskite Nanocrystals for Hybrid Organic Solar Cells with Enhanced Photoconversion Efficiency

The versatile optoelectronic properties of perovskite nanocrystals (NCs) have provided a strong surge for their utilization in different classes of solar cells, with organic photovoltaic systems being no exception. In an unprecedented approach, a hybrid solar cell with CsPbBr1.5I1.5 NCs strategically grafted on poly(3-hexylthiophene-2,5-diyl) (P3HT) nanorods (NRs) is shown to have a photoconversion efficiency of 9.72 ± 0.4%, with only 1.5 wt % NCs. The improvement is twice more than the P3HT:PCBM reference devices (4.09 ± 0.2%). The choice of NC composition is validated by density functional theory calculations, which show decent charge carrier mobility in CsPbBr1.5I1.5, besides having better stability than CsPbI3, making CsPbBr1.5I1.5 NCs a suitable contender for hybrid device architecture. A trivial blending of the NCs in the P3HT:PCBM matrix results in their nonuniform distribution, escalating charge carrier trapping, albeit maintaining a device efficiency of 8.07 ± 0.3% with 1 wt % NCs. Uniform NC grafting is propitious over inhomogeneous blending because CsPbBr1.5I1.5 NCs not only act as additional light harvesters, but their chemical grafting onto the P3HT NRs improves the charge transport by creating better charge percolation pathways. The higher crystallinity of the P3HT NRs than P3HT also helps in reducing the trap states.

Facile fabrication of coaxial Ge/CNT nanorod anode for lithium-ion battery

In this work, CNTs are directly grown on the Ni-foam without deliberately using any catalyst and barrier layer. The CNTs coated Ni-foam is subsequently utilized as the substrate for Ge deposition, which formed a coaxial Ge/CNT nanorod structure and is evaluated as anode for lithium-ion battery. Due to the high conductivity contributed by CNTs and the strain-releasing structure derived from the coaxial nanorod, both of the rate capability and the cyclability of the anode are improved. As a result, at a high current density of 50 A/g, the anode delivered a specific capacity of 800 mAh/g. And no apparent capacity decay is detected after cycling for 300 cycles at 10 A/g.

Simultaneously Enhanced Efficiency and Stability of Perovskite Solar Cells with TiO2@CdS Core–Shell Nanorods Electron Transport Layer

AbstractBoth demands of stability and efficiency constitute the gargantuan barriers of perovskite solar cells (PSCs), hampering the academic communities and industrial production. Herein, it is demonstrated that, after depositing optimized CdS shell layers on TiO2 nanorod arrays (NAs) by a simple and rapid chemical bath method at room temperature, the efficiency and stability of PSCs are simultaneously enhanced. The PSCs based on TiO2/CdS core–shell NAs achieve higher power conversion efficiency up to 17.71%, compared to 15.93% of the pristine TiO2 NAs‐based cells. Especially, the stability of the PSCs is prominently improved after optimized CdS layer modification without encapsulation. The significant enhancement of both efficiency and stability are mainly ascribed to that the type‐II structure of TiO2/CdS coaxial nanorods can suppress recombination, and the oxygen vacancies on TiO2 surfaces are not directly contacted with perovskite layers. The surface modification on TiO2 NAs opens up an alternative approach toward improving the performance and stability of PSCs.

Heterostructured nanorod array with piezophototronic and plasmonic effect for photodynamic bacteria killing and wound healing

Bacteria induced infectious diseases have threated the lives and health of millions people each year. Nanosized titanium dioxide (TiO2) have been developed for photodynamic bacterial killing with minimal drug resistance, but the efficiency was restricted by their large band gap, limited light-absorption region, and rapid electron-hole recombination. In this work, we rationally fabricated a TiO2/BTO/Au multilayered coaxial heterostructured nanorod array by inserting a ferroelectric semiconductor barium titanate (BaTiO3) nanolayer between TiO2 nanorod and gold nanoparticles (AuNPs). The TiO2/BTO/Au heterostructure showed greatly enhanced reactive oxygen species (ROS) (superoxide (O2•-) and hydroxyl radicals (•OH)) generation, and incident photo-electron conversion efficiency (IPCE) in UV/visible light region. On the basis of experimental observations, the detailed photodynamic mechanism of the enhanced ROS generation was proposed, mainly ascribed to the piezophototronic effect and plasmonic property of the nanorod array. The nanorod array was used as an antibacterial coating to kill gram-negative bacterium E. coli and gram-positive bacterium S. Aureus with an antibacterial efficiency up to 99.9% under simulated sunlight. It also showed an efficient promotion of infectious wound regeneration in mice with S. aureus infected dermal wounds.

Entropy-driven self-assembly of chiral nematic liquid crystalline phases of AgNR@Cu2O hyper branched coaxial nanorods and thickness-dependent handedness transition

The chiral nematic liquid crystalline phase (CNLCP) of noble metal nanorods induces a strong chiroptical response due to their intrinsic physical and chemical properties. Here, we demonstrate that the formation of CNLCP of Ag nanorods (AgNRs) originates from their bent-shape and is the result of purely entropic effects. The chirality of the liquid crystalline phase of AgNR@Cu2O hyper branched coaxial nanorods (HBCNRs) can be switched from left-handed to right-handed by increasing Cu2O thickness. It is proposed that the increase of coating thickness decreases the curvature of nanorods, which induces variation of the twist constant (K2) and bend elastic constant (K3). The increased thickness also changes the direction of director with respect to the helical axis. In addition, hydrogen bonds can break the CNLCP, which can be attributed to their stronger effection compared to van der Waals forces and electrostatic interactions. In contrast to the variation of coating thickness, the surface morphology, constituents of the hybrid building blocks and polarity of the solvents do not play important roles in the handedness transition of the liquid crystalline phase. Furthermore, the results presented here give insight into the structure–property relationship and our strategy provides guidance for the synthesis of other inorganic chiral suprastructures driven by entropic effects.

Effect of Si Growth Temperature on Fabrication of Si-ZnO Coaxial Nanorod Heterostructure on (100) Si Substrate

The realization and application of optoelectronics, photonics, and sensing, such as in solar diode sensors and photodiodes, which are potentially useful from ultraviolet to infrared light sensing, is dramatically advanced when ZnO is integrated into semiconductor nanostructures, especially when compatible with mature silicon technology. Here, we compare and analyze the fundamental features of the Si-ZnO coaxial nanorod heterostructures (Si@ZnO NRs) grown on semi-insulating (100)-oriented Si substrates at growing temperatures of 500°C, 600°C, 650°C, and 700°C of the Si layer for device applications. ZnO NRs were grown by a vapor phase transport, and Si layers were made by rapid thermal chemical vapor deposition. X-ray diffraction, field emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy, and Raman experiments showed that ZnO NRs were single crystals with a wurtzite structure, while the Si layer was polysilicon with a zincblende structure. Furthermore, FESEM revealed that Si shell thickness of the Si@ZnO NRs increases with increasing growing temperatures of Si from 500°C to 700°C.

Synthesis and characterization of coaxial SnO2–SiO x core–shell nanorods

Free-standing coaxial SnO2–SiO x core–shell nanorods were synthesized in a kinetically controlled manner by the chemical vapor transport method. The free-standing SnO2–SiO x core–shell nanorods appear with uniform morphology exhibiting smooth surface, diameters of 100–200 nm, and length of 1 μm. The low oxygen partial pressure and high reaction temperature are advantageous to form the (101) surface, which decreases the (101) crystal surface energy. The [101] growth direction in our process also satisfies the lowest energy principle in thermodynamics due to change of (101) surface energy. A heterogeneous nucleation site was provided by the SiO x dismutation reaction, and the formation of core–shell (SnO2–SiO x ) structure should result from the phase separation energy. The free-standing coaxial SnO2–SiO x heterostructured nanorods grow by modification of the mass transfer coefficient. As-grown depositions show shrimp-like, flower-like, and worm-like morphologies under a higher mass transfer coefficient. Formation of the free-standing core–shell structure depends on the vertical growth mechanism.

Plasmon transmission through excitonic subwavelength gaps.

We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction.

Synthesis and characterization of Si/ZnO coaxial nanorod heterostructure on (100) Si substrate

One-dimensional ZnO nanorods were grown vertically on a (100) Si substrate using a vapor phase transport method. Following the fabrication of ZnO nanorods, Si layers were deposited by rapid thermal chemical vapor deposition (RTCVD) directly on the ZnO nanorod/Si (100) substrate. Field emission scanning electron microscopy revealed that a Si/ZnO nanorod coaxial heterostructure were synthesized vertically oriented along the (002) plane on a Si substrate. X-ray diffraction, Energy dispersive X-ray and Raman spectroscopy revealed that the ZnO nanorods were single crystals with a hexagonal structure, and grew with a c-axis orientation perpendicular to the Si substrate, whereas the Si layer was poly-silicon with cubic structure. These results demonstrated the Si/ZnO nanorod coaxial heterostructure were synthesized successfully on a (100) Si substrate and the ZnO nanorod enables the synthesis of a vertically grown well-aligned Si/ZnO coaxial nanorod heterostructure.

B21-P-05Characterization of InxGa1-xAs/InAs Coaxial Nanorod Grown on Graphene Layers by Catalyst-Free Molecular Beam Epitaxy

One dimensional (1-D) semiconductor nanostructures are emerging as new candidates for the future electronic and optoelectronic devices due to their excellent physical properties, e.g. long lifetime and high efficiency, and various functionalities, e.g. flexibility and transparency [1,2]. Molecular beam epitaxy (MBE) provide accurate control over the various growth parameters, thereby allowing the growth of high-quality 1-D nanostructures with very clean and sharp interfaces [3]. Here, we grow InxGa1-xAs/InAs coaxial nanorod on graphene layers using catalyst-free MBE and investigate the chemical composition and crystal structure of nanorods using transmission electron microscope. Cross-sectional analysis shows that InxGa1-xAs coating layers have uniform thickness and cover the entire surface of the InAs core nanorods without any interfacial layers.

Catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on graphene layers using molecular beam epitaxy

We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. The graphene layers used as the substrate were prepared by chemical vapor deposition and transferred onto SiO2/Si substrates. InAs nanorods and their heterostructures were grown vertically on the graphene layers; electron microscopy images revealed uniform distributions for their diameter, length and density. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction. Nanorods with InAs cores and InxGa1−xAs shells have been grown catalyst-free on graphene using molecular beam epitaxy by scientists in Korea. Inorganic semiconductor nanorods on graphene layers are promising for producing electronic and optoelectronic devices that are simultaneously flexible and have long lifetimes and high efficiencies. Now, researchers at Seoul National University led by Gyu-Chul Yi have realized catalyst-free molecular beam epitaxy growth of InAs/InxGa1−xAs coaxial nanorods on graphene. This is an important step as catalyst-free molecular beam epitaxy can produce ultrapure single crystals and multilayered heterostructures with precisely-controlled thickness and chemical compositions. Analysis showed that the cores and shells were clearly defined with clean interfaces and had uniform compositions and thicknesses. This demonstration opens up the path for monolithic integration of semiconductor coaxial nanorod heterostructures on two-dimensional materials. We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction.

Vitamin C-assisted synthesis and gas sensing properties of coaxial In2O3 nanorod bundles

In2O3 nanorod bundles with well-designed morphology, hierarchical nanostructure and large specific surface area were quickly prepared via microwave hydrothermal method in the presence of vitamin C. The observations of field emission electron microscopy and transmission electron microscopy revealed that the as-prepared In2O3 samples were constructed from lots of well-aligned nanorods with average diameter of about 50nm. To demonstrate their potential application in detecting harmful gases, these bundle-like In2O3 samples were applied to fabricate gas sensor and their sensing properties were examined. It was found that the sensors based on such novel In2O3 nanorod bundles exhibited high response and excellent selectivity during detecting NO2 gas at the operating temperature of 100°C.

Photovoltaic device based on TiO2 rutile/anatase phase junctions fabricated in coaxial nanorod arrays

Phase-junctions formed in mixed crystalline phases of semiconductor based photocatalysts (TiO2 and Ga2O3) shows enhanced photocatalytic activity owing to efficient separation of photogenerated charges. However, the phase junction effect on charge separation has not been directly verified yet. In this study, we fabricated a prototype photovoltaic device based on TiO2 rutile/anatase coaxial nanorod arrays (NRAs) to demonstrate charge separation at the interface of polymorphic crystal phases. The device—FTO/rutile NRAs/anatase/ITO shows an ordinary photovoltaic response (open-circuit voltage Voc: 154mV, short-circuit current density Jsc: 1.76mA/cm2), contrasting with photo resistor behavior of two TiO2 single phase devices of core–core (FTO/rutile NRAs/ITO) and shell–shell (FTO/anatase/ITO). Experimental evidences suggest that the built-in electric field at the interface of rutile/anatase phase junctions in the FTO/rutile NRAs/anatase/ITO device provides the direct driving force for efficient separation of photogenerated charges. The demonstrated strategy of fabricating phase-junction photovoltaic devices may inspire further investigations on new device converting solar into electrical energy and highlight the key role of the phase junction interface in the properties of the mix-phase photocatalysts.

Enhanced supercapacitive performance on TiO2@C coaxial nano-rod array through a bio-inspired approach

A coaxial core–shell structured TiO2@carbon nano-rod arrays (TiO2@C NRAs) was constructed and tested as a supercapacitor electrode in this study. Dopamine was self-assembled into a polydopamine nano-rod arrays within the TiO2 nano-tube arrays (TiO2 NTAs) and was used as a precursor for the carbon nano-rod generation. Electrochemical measurements showed that the electrodes made with coaxial TiO2@C NRAs exhibited substantially higher electrochemical performances such as larger areal capacitance and faster charge/discharge capability than those of the pristine TiO2 NTA electrode and porous active carbon materials. Benefiting from the enhanced electrical conductivity and increased surface area of the coaxial nano-rod structure, the prepared coaxial TiO2@C NRAs electrode achieved a remarkable areal capacitance of 40.75mFcm−2 at a current density of 0.2mAcm−2, which was significantly higher than that of the pristine TiO2 NTAs (0.31mFcm−2).

Core-shell Ti@Si coaxial nanorod arrays formed directly on current collectors for lithium-ion batteries.

Silicon is a promising candidate to replace the dominantly used carbon as the anode material for lithium ion batteries (LIBs). Si has the highest theoretical capacity (4200 mA·h/g) and is one of the most abundant elements. Unfortunately, Si has the issues of huge volume variation upon dis/charge cycling and low conductivity, leading to poor cycling and rate performances. Designing special nanostructures and improving conductivity and integration of Si electrodes could dramatically enhance their performance. Here, we introduce a novel strategy to integrate the core-shell nanorod arrays of Ti@Si on Ti foil with good conductivity as an additive-free electrode. The Ti core functions as a stable metallic support for the Si shell and dramatically reduces the diffusion length. The as-prepared core-shell nanorod arrays of Ti@Si on Ti foil, without any postsynthesis treatment, as electrodes demonstrated reversible capacity of 1125 mA·h/g over at least 30 cycles with highly improved Coulombic efficiency.