Nanotube Heterostructures Research Articles

Homogeneous CdTe quantum dots-carbon nanotubes heterostructures

The development of homogeneous CdTe quantum dots-carbon nanotubes heterostructures based on electrostatic interactions has been investigated. We report a simple and reproducible non-covalent functionalization route that can be accomplished at room temperature, to prepare colloidal composites consisting of CdTe nanocrystals deposited onto multi-walled carbon nanotubes (MWCNTs) functionalized with a thin layer of polyelectrolytes by layer-by-layer technique. Specifically, physical adsorption of polyelectrolytes such as poly (4-styrene sulfonate) and poly (diallyldimethylammonium chloride) was used to deagglomerate and disperse MWCNTs, onto which we deposited CdTe quantum dots coated with mercaptopropionic acid (MPA), as surface ligand, via electrostatic interactions. Confirmation of the CdTe quantum dots/carbon nanotubes heterostructures was done by transmission and scanning electron microscopies (TEM and SEM), dynamic-light scattering (DLS) together with absorption, emission, Raman and infrared spectroscopies (UV–vis, PL, Raman and FT-IR). Almost complete quenching of the PL band of the CdTe quantum dots was observed after adsorption on the MWCNTs, presumably through efficient energy transfer process from photoexcited CdTe to MWCNTs.

Enhanced Photoelectrocatalytic and Photoelectrochemical Properties by High-Reactive TiO2/SrTiO3 Hetero-Structured Nanotubes with dominant {001} Facet of Anatase TiO2

High-reactive TiO2 crystals with controllable facets have been widely investigated in photocatalysis, photoelectrocatalysis, solar energy conversion and other related fields due to their unique structure-dependent properties. In this work, high-reactive TiO2/SrTiO3 (TSr) hetero-structured nanotubes with dominant TiO2 {001} facet are fabricated in a Sr(OH)2 solution by varying the hydrothermal time, utilizing anodizing TiO2 nanotubes on a Ti substrate as both a “structure-directed” template and an initial reagent to obtain these hetero-structured composites. Based on XRD, FE-SEM with EDS, FE-TEM, XPS and Raman characterization, the high-reactive TSr hetero-structured nanotubes with dominant {001} facet of anatase TiO2 are definitely synthesized and well-dispersed, crystallized SrTiO3 are acquired on the surface of TiO2 nanotubes after the hydrothermal treatment. Compared with the reference TiO2 nanotubes, transient photocurrent (I-t) and open circuit voltage (V-t) measurements indicate that the photoelectrochemical properties of these resulting composites are distinctly enhanced by controlling the hydrothermal time to regulate SrTiO3 coverage on the surface of nanotubes. In particular, photoelectrocatalytic (PEC) degradation of “target molecules” MB are further implemented to evaluate their PEC activities, and these data reveal the significant improvement of their corresponding activities over the reference TNT2, and this TiO2/SrTiO3 composite with the hydrothermal time of 30min (TSr3) possesses the highest PEC rate of 99.93% for MB after the degradation of 20min. These above results are attributed to the high-reactive TiO2 {001} facet of TiO2/SrTiO3 interior and the hetero-structured interfaces by SrTiO3 generated on TiO2 nanotubes in a hydrothermal process. In addition, the mechanism of PEC degradation is also simply discussed in comparison to the relevant photocatalytic (PC) and electrocatalytic (EC) reaction. This work demonstrates that a synergetic mechanism between high-reactive {001} facet of anatase TiO2 and hetero-structured characteristics contributes to enhancing their PEC activities and according photoelectrochemical properties, which can be extended to other binary compounds with high-reactive facets.

Rational design, high-yield synthesis, and low thermal conductivity of Te/Bi2Te3 core/shell heterostructure nanotube composites

Te/Bi2Te3 core/shell heterostructure nanotube (NT) composites with rough serrated interfaces, and hollow structures were designed and synthesized to enhance phonon scattering. A conventional two-step solution phase method was used to synthesize the mass products in high-yield. The external diameter and wall thickness of the NTs measured approximately 250nm and 60nm, and their lengths ranged from 7μm to 9μm. The hexagonal phase Te core, rhombohedral phase Bi2Te3 shell, hollow structure, and saw-toothed interface were accurately determined through X-ray diffraction, scanning electron microscopy and transmission emission microscopy. Relatively stable and low thermal conductivities (from 0.43Wm−1K−1 to 0.46Wm−1K−1) were obtained when the temperatures increased from 300K to 400K (near room temperatures). Finally, this research systematically examined the formation mechanisms based on the in situ growth method and the phonon scattering occurring in both the hollow structures and on the saw-toothed interface.

Synthesis of few-layered MoS₂ nanosheet-coated electrospun SnO₂ nanotube heterostructures for enhanced hydrogen evolution reaction.

In this work, we report the fabrication of low crystalline, few-layered MoS₂ nanosheet-coated electrospun SnO₂ nanotube (MoS₂/SnO₂) heterostructures with three-dimensional configurations by electrospinning combined with a one-step solvothermal approach. The morphologies and compositions of the as-prepared hybrid nanotubes were characterized by field-emission scanning electron microscopy, transmission electron microscopy, ICP-AES, BET method, X-ray diffraction and X-ray photoelectron spectroscopy. Results show that SnO₂ nanotubes are uniformly covered by sheet-like MoS₂ subunits on both outer and inner surfaces. The electrocatalytic activity of MoS₂/SnO₂ heterostructures towards a hydrogen evolution reaction was examined using linear sweep voltammetry and AC impedance measurements. It is shown that the MoS₂/SnO₂ modified electrode exhibits excellent catalytic activity for hydrogen evolution with low overpotential, a small Tafel slope and high current density.

3C-SiC nanocrystals/TiO2 nanotube heterostructures with enhanced photocatalytic performance

p-type ultrathin 3C-SiC nanocrystals are coated on heat-treated n-type TiO2 nanotube arrays formed by electrochemical etching of Ti sheets to produce heterostructured photocatalysts. Depending on the amounts of 3C-SiC nanocrystals on the TiO2 nanotubes, photocatalytic degradation of organic species can be enhanced. The intrinsic electric field induced by the heterojunction promotes separation of the photoexcited electrons-holes in both the TiO2 nanotubes and 3C-SiC nanocrystals. Hence, holes can more effectively travel to the surface of 3C-SiC nanocrystals and there are more electrons on the surface of TiO2 nanotubes consequently forming more •O2− and •OH species to degrade organic molecules.

The preparation of tubular heterostructures based on titanium dioxide and silica nanotubes and their photocatalytic activity

Tubular heterostructures based on titanium dioxide (TiO2) and silica nanotubes (SNTs) with high photocatalytic activity have been successfully obtained by a simple combination of an electrospinning technique and a solvothermal process. The as-prepared products were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and N2 absorption-desorption experiments. The results confirmed that SNTs with high specific surface area were obtained by efficiently controlling the phase separation and solvent evaporation during the process of electrospinning and calcination, and TiO2 was successfully grown on the SNT substrates. The obtained titanium dioxide/silica nanotubes (TiO2/SNTs) heterostructures showed high photocatalytic activities to degrade Rhodamine 6G because of the formation of heterostructures, which might improve the separation of photogenerated electrons and holes. Furthermore, the TiO2/SNTs heterostructures could be easily recycled without an evident decrease in photocatalytic activity.

Modeling transport properties of single-wall carbon nanotube between metal and graphene electrodes

In this article, transport properties of a single-wall carbon nanotube having chirality (4, 0), connected to copper, gold and graphene electrodes, have been investigated. Electronic structures have been analyzed using the density functional theory calculator. Transport properties of the devices formed by interfacing of single-wall carbon nanotube and electrodes are computed using the nonequilibrium Green’s functional method coupled with density functional theory approach. Contact effects due to metal–carbon nanotube interface have significant impact on transport properties of charge carriers. Therefore, transport properties have been critically analyzed with the help of transmission spectra, differential conductance and conductance plots and I–V characteristics. From these results, it has been concluded that formulation of the electrode material is not the single-factor responsible for creating conduction problems. Along with the electrode material, interface position of metal–carbon nanotube configuration and heterostructure contacts are also responsible in influencing transport phenomenon. In this way, varying the contact geometry is found to play an important role in integrated circuit interconnect technologies for exploring the transport properties. Gold is best among all three simulated contact electrodes, and for low-power applications, graphene can be next choice over an applied bias range of −1 to +1 V. Atomistix software tool has been used to analyze transport properties of interconnect models.

Cu2O/TiO2 heterostructure nanotube arrays prepared by an electrodeposition method exhibiting enhanced photocatalytic activity for CO2 reduction to methanol

Cu2O/TiO2 composite nanotube arrays demonstrating enhanced photocatalytic performance were synthesized using an electrodeposition method to impregnate the p-type Cu2O into the n-type titanium dioxide nanotube arrays (TNTs). The morphological results confirmed that the TNTs are wrapped by the Cu2O nanoparticles and the UV–Vis absorption spectra showed that the Cu2O/TNTs display a better ability for visible light absorption compared to the pure TNTs. CO2 photocatalytic reduction experiments carried out by using Cu2O/TNT nanocomposites proved that Cu2O/TNTs exhibit high photocatalytic activity in conversion of CO2 to methanol, while pure TNT arrays were almost inactive. Furthermore, Cu2O/TNTs also exhibited augmented activity in degradation of target organic pollutant like acid orange (AO) under visible light irradiation. The ultra enhanced photocatalytic activity noticed by using Cu2O/TNTs in CO2 reduction and degradation of organic pollutant could be attributed to the formation of Cu2O/TiO2 heterostructures with higher charge separation efficiency.

Design of bristle-like TiO2–MWCNT nanotubes to improve the dielectric and interfacial properties of polymer-based composite films

TiO2–MWCNT heterostructure nanotubes have been fabricated via a solvent-thermal method. Energy dispersive spectrometer and X-ray diffraction analysis revealed that the nanotubes are composed of C, Ti and O elements and TiO2 only contains the tetragonal anatase phase. SEM and TEM results show that most of the TiO2 bristles are vertically studded on the surface of the MWCNT. Subsequently, TiO2–MWCNT/polyarylene ether nitrile (PEN) composite films were prepared in order to investigate the effect of TiO2–MWCNT on the PEN matrix. SEM images exhibit that there is strong interfacial adhesion between the PEN matrix and fillers owing to the special bristle-like structure. Thermal analysis results show that TiO2–MWCNT/PEN composite films possess excellent thermal properties endowed by the PEN matrix. Besides, the dielectric constant of the composite films increases from 4 to 109 at 100 Hz when the TiO2–MWCNT loading reaches 8 wt%. Rheology measurements reveal that there is an obvious difference between the rheological percolation threshold and the electrical percolation threshold.

Characterization of individual multifunctional nanoobjects with restricted geometry

Anisotropic nanostructures such as nanowires (NWs)/nanotubes and nanometer-sized islands are the subject of intense research efforts due to several interesting phenomena observed to occur in confined geometries with a high aspect ratio and/or reduced size. Understanding and controlling the mechanical and electromechanical behaviors of these nanostructures are crucial for the practical implementation of such building blocks as active or passive components in nanodevices. Recent progresses toward the characterization of individual one-dimensional (NWs and nanotubes) and two-dimensional (islands) functional nanoscale objects of piezoelectric and ferroelectric (BaTiO3, Bi4Ti3O12 (BiT), and NaNbO3) and multiferroics (BiFeO3 and Bi2FeCrO6 (BFCO)) are presented. These nanoobjects have been characterized using scanning probe microscopy-based techniques, combined with various modulation schemes for enhanced sensing capabilities. The most remarkable results include: ferroelectricity of NaNbO3/Nb2O5 nanotube heterostructures, negative piezoelectric coefficient in highly anisotropic BiT mesoscopic rods, and preservation of the multiferroic character of epitaxial BFCO nanoislands.

CuO Necklace: Controlled Synthesis of a Metal Oxide and Carbon Nanotube Heterostructure for Enhanced Lithium Storage Performance

Metal oxide and nanocarbon heterostructures provide a powerful strategy to materials innovation because judicious combination of different materials may afford collective properties that are otherwise unattainable. However, design and fabrication of spatially patterned heterostructures has been still challenging. Here, we develop a facile approach to synthesize a CuO and carbon nanotube necklace-like heterostructure in solution for enhanced lithium storage performance. Observations confirm that propagative sidewall alkylcarboxylation of carbon nanotubes in liquid ammonia allows the CuO nanospheres to grow on the functional defect sites introduced by the functionalization. Such a method does not require surfactants and is highly scalable. The resulting materials show high lithium ion storage capacities (stable at 500 mAh g–1) and enhanced cycling performance as a lithium-ion battery anode. The improved performance is attributed to the necklace-like nanostructure, which provides an electrical channel as well as excess room to accommodate the volume changes of CuO during electrochemical cycling.

Ultrasound effect: Preparation of PbS/TiO2 heterostructure nanotube arrays through successive ionic layer adsorption and the reaction method

PbS-modified TiO2 heterostructure nanotube arrays (NAs) were synthesised by successive ionic layer adsorption and the reaction method. Ultrasound (PbS(u)/TiO2 NAs) has an important function in the formation of PbS nanoparticles. Compared with non-ultrasound PbS/TiO2 NAs, PbS(u)/TiO2 NAs have small PbS nanoparticles uniformly distributed on both the outside and inside rather than the top surface of TiO2 NAs. The average size of the PbS nanoparticles in PbS(u)/TiO2 NAs ranges from 10 to 15nm. PbS/TiO2 NAs and PbS(u)/TiO2 NAs both exhibit high absorption in the 300–1000nm region. Moreover, a stronger separate efficiency of photogenerated charge carriers is found in PbS(u)/TiO2 NAs than that in PbS/TiO2 NAs.

Photoelectrochemical and photocatalytic properties of Ag-loaded BaTiO3/TiO2 heterostructure nanotube arrays

BaTiO3/TiO2 (BT) heterostructure nanotube arrays were fabricated by in situ hydrothermal method using TiO2 nanotubes as both template and reactant. Compared with pure TiO2 nanotube arrays, the BT heterostructures exhibited enhanced photocurrent under UV light irradiation. For further improving the photoelectrochemical performance, Ag nanoparticles were loaded on the surface of BT heterostructure by two different photo-reduction (Ag/BT-P) and chemical reduction (Ag/BT-C) methods. The results showed that the Ag nanoparticles on Ag/BT-C were uniform and dispersed homogeneously, but the Ag nanoparticles on Ag/BT-P were very large, which resulted in the tailored and integrated nanotube structure destroyed. The electrochemical impedance spectra (EIS) indicated that the impedance arc radius of Ag/BT-C was much smaller than Ag/BT-P and the pure BT nanotube arrays, indicating that the enhanced charge carrier separation was achieved on Ag/BT-C. In addition, the Ag/BT-C nanotube arrays exhibited a higher photocatalytic activity for methylene blue (MB) degradation.

Photoelectrochemical and charge transfer properties of SnS/TiO2 heterostructure nanotube arrays

SnS/TiO2 heterostructure nanotube arrays (NTAs) were synthesized through a successive ionic layer adsorption and reaction process. The photoelectrochemical (PEC) property of SnS/TiO2 heterostructure NTAs was evaluated in liquid junction PEC solar cells. The results revealed that SnS/TiO2 heterostructure NTAs exhibited PEC property with an open-circuit voltage of 1.08 V, a short-circuit current density of 1.55 mA/cm2, and a conversion efficiency of 0.75% under 1 sun illumination. These values are higher than those of pristine SnS nanoparticles and TiO2 NTAs. Surface photovoltage spectroscopy and phase spectroscopy measurements were conducted to verify the formation of heterojunction and probe charge transfer between SnS and TiO2 NTAs. The PEC property and surface photovoltaic response enhancement of SnS/TiO2 heterostructure NTAs can be attributed to the enhanced electron-hole separation due to the effect of interfacial electric field in SnS/TiO2 heterostructure NTAs.

Metal-Lined Semiconductor Nanotubes for Surface Plasmon-Mediated Luminescence Enhancement

Highly efficient solid-state light-emitting devices require semiconductor architectures equipped with high quantum efficiency and integratability on conductive substrates. Surface plasmon (SP)-mediated luminescence enhancement has been considered as one of the most promising solutions, because SP resonance can greatly improve the radiative recombination rate and be achieved using metal entities compatible with the electrode fabrication process. Nevertheless, metal/semiconductor heterostructures have had several fabrication-compatible issues due to metal as a potential contaminant of the semiconductor. We present here a simple fabrication scheme for a metal-lined semiconductor nanotube heterostructure, in which a metal layer is selectively formed on the inner wall of the semiconductor nanotube. The Ag-lining process in a ZnO nanotube resulted in 7.5-fold enhancement of the photoluminescence intensity at 11 K. This SP fabrication technique looks promising for highly efficient solid-state lighting based on semiconductor nanostructures without detrimental effects.

Tunable differential conductance of single wall C/BN nanotube heterostructure

The transport properties and differential conductance of the heterostructures constructed by (5,5) single wall carbon nanotube (SWCNT) and (5,5) single wall boron nitride nanotube (SWBNNT) are investigated using density functional theory in combination with non-equilibrium Green's functions. We find that the transmission conductance of (5,5) BN/C nanotube heterostructure is not only continually depressed as the BNNT region increases but also the drop of the conductance is uniform in the energy window (-1.43eV, 1eV), which leads to linear I-V dependence for the systems when the bias is within this energy range. Moreover, the differential conductance linearly decreases when n ≤ 3 but exponentially decreases when n ≥ 3 for (5,5)(BN) n /C heterostructure. Such tunable differential conductance of (5,5) BN/C nanotube heterostructure mainly derives from the blockage of the transport channels induced by the semiconductive BN segment.

Effect of mono-vacancy on transport properties of zigzag carbon- and boron-nitride-nanotube heterostructures

On the basis of first-principles density functional theory and non-equilibrium Green's function technique, we have investigated the effects of a mono-vacancy on the electronic transport properties of the carbon nanotube/boron nitride nanotube heterostructures. The results show that the electronic transport properties are strongly dependent on the position of the mono-vacancy, and the negative differential resistance and rectifying performances can be strengthened or weakened alternately with the position change of the mono-vacancy. Moreover, the performance change is more significant when the mono-vacancy occurs on the carbon nanotube part. These interesting phenomena are explained in terms of the evolution of the transmission spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analysis.

Electrospinning directly synthesized metal nanoparticles decorated on both sidewalls of TiO2 nanotubes and their applications

The hybrid structure of nanoparticle-decorated nanotubes has the advantage of both large specific surface areas of nanoparticles and anisotropic properties of nanotubes, which is desirable for many applications. In this study, Ag nanoparticles on highly porous TiO2 nanotubes (NTs) on both internal and external sidewalls (Ag@TiO2@Ag NTs) are directly synthesized by emulsion electrospinning and thermal evaporation for the first time. The Ag@TiO2@Ag NT heterostructures, which have large surface-to-volume ratios, improved electrical conductivity, and an excellent material synergetic effect, demonstrate excellent electrochemical properties and superior photocatalytic performance. The new method for the synthesis of Ag@TiO2@Ag NT heterostructures can be applied to fabricate various types of other novel metal nanoparticles (for example Au and Pt) on highly porous nanotubes on both internal and external sidewalls. The possible growth mechanism and reasons for excellent electrochemical properties and superior photocatalytic performance were discussed in detail.

Formation of MS–Ag and MS (M = Pb, Cd, Zn) nanotubes via microwave-assisted cation exchange and their enhanced photocatalytic activities

A facile microwave-assisted cation-exchange reaction route has been developed to synthesize a series of MS-Ag (M = Pb, Cd, Zn) heterostructure nanotubes (HSNTs) and MS nanotubes. As a demonstration, the as-prepared PbS-Ag HSNTs exhibit significantly enhanced photocatalytic activity for degradation of Congo red and reduction of Cr(VI).

Electrospun In2O3/α-Fe2O3 heterostructure nanotubes for highly sensitive gas sensor applications

In2O3/α-Fe2O3 (IFO) heterostructure nanotubes, with cubic-In2O3 nanocrystals randomly distributed on the surface of α-Fe2O3 nanotube-based backbones, were successfully prepared through a facile single-capillary electrospinning method. The morphologies of the resulting electrospun products were characterized by scanning electron microscopy and transmission electron microscopy. The crystal structures and components were determined by X-ray diffraction and energy-dispersive X-ray spectra, respectively. The results show that the morphology and structure of the products could be tailored by changing the content of indium nitrate in the precursor solutions and the calcination temperatures. Moreover, the IFO-0.1 (In/Fe = 1 : 9, molar ratio) nanotubes based sensor shows a high sensitivity to ethanol with a fast response/recovery time, and the performance is stable at a low operating temperature, which gives a potential application in ethanol gas sensors. A possible gas sensing mechanism based on the role of In2O3 in such a heterostructure is also discussed in detail.