ZnO Nanotubes Research Articles

Scalable ZnO nanotube arrays grown on CVD-graphene films

We report the growth of wafer-scale arrays of individually position-controlled and vertically aligned ZnO nanotube arrays on graphene deposited by chemical vapor deposition (CVD-graphene). Introducing two-dimensional layered materials such as graphene as a growth buffer has recently been suggested for growing nanomaterials on traditionally incompatible substrates. However, their growth has been restricted to small areas or had limited controllability. Here, we study the distinct growth behavior of ZnO on CVD-graphene that makes the selective area growth of individual nanostructures on its surface difficult, and propose a set of methods to overcome this. The resulting nanotube arrays, as examined by scanning electron microscopy and transmission electron microscopy, exhibited uniform morphologies and high structural quality over a large area and could be prepared on a broad variety of substrates, including amorphous, metallic, or flexible substrates.

Effect of variable spacing on performance of plate heat exchanger using nanofluids

This paper presents details of an experimental investigation into the effect of different spacings (ΔX = 2.5 mm, 5.0 mm, 7.5 mm and 10.0 mm) in plate heat exchanger (PHE) on the basis of its combined energetic and exergetic performance by using various nanofluids, i.e., TiO2, Al2O3, ZnO, CeO2, hybrid (Cu+Al2O3), graphene nanoplate (GNP) and multi-walled carbon nanotube (MWCNT). On the basis of experiment data, various energetic and exergetic performance parameters have been evaluated and their inter-relationship has been discussed. The optimum heat transfer characteristics in the nanofluids and their exergetic performance have been found to be achieved with a spacing of ΔX = 5.0 mm. Based on these data, it has been found that the MWCNT/water nanofluid, with a spacing of ΔX = 5 mm in PHE, has the maximum heat transfer coefficient, which is 53% higher compared to water at 0.75 vol % (optimum). Nanofluids significantly improve heat transfer capacity with a nominal rise in pressure drop at 0.75 vol %. This study will help to understand the process of heat transfer augmentation by using various nanofluids in the PHE on the basis of energetic and exergetic performance of the system.

Screw-dislocation-driven growth of ZnO nanotubes seeded by self-perpetuating spirals during hydrothermal processing

We report the effects of precursor concentration on the characteristics of ZnO nanostructures during hydrothermal processing. Self-perpetuating surface spirals are fabricated at concentrations of 0.25 and 0.5 M, with samples grown at concentrations of 0.05 and 0.125 M exhibiting ZnO nanorods. This can be explained by a change in the growth mode from an initial columnar growth to a screw-dislocation-driven growth with decreased supersaturation. The screw dislocations nucleate at the V-shaped valleys of the columnar boundaries during the intermediate stage. We demonstrate that continuous screw-dislocation-driven growth leads to the formation of ZnO nanotubes having Burger’s vectors of 1.45 nm.

Effects of the surface morphologies of ZnO nanotube arrays on the performance of amperometric glucose sensors

Amperometric glucose sensors have been fabricated with glucose oxidase (GOx) immobilized on ZnO nanotubes (ZnONTs) by physical adsorption. The ZnONTs were formed through selective dissolution of ZnO nanorods (ZnONRs) which were hydrothermally synthesized on Au cylindrical spiral (AuCS) electrodes. With the etching temperature regulated, the surface morphologies of the ZnONT arrays were tailored and their effects on the performance of the corresponding glucose sensors were investigated. It is found that at 65°C the as-prepared ZnONT arrays show a Gaussian rough surface with larger surface area and better hydrophilicity, and have an effective solid–liquid interface with GOx solution, which further results in desirable GOx immobilization. Therefore favorable performance of the ZnONT-based glucose sensor was obtained, such as the sensitivity 2.63 μA/(mMcm2), linear range 0–6.5mM, low detection limit 8μM (S/N=3) and Michaelis-Menten constant 5.24mM, which are superior to that of the ZnONR-based one. Moreover, the ZnONT-based glucose sensor exhibits good long-term stability, and excellent anti-interference ability to uric acid and ascorbic acid. The results can also be used for the performance optimization and the process standardization of other ZnONT-based amperometric biosensors.

Design and Fabrication of a Tip‐Like ZnO Nanotube Array Structure with Condensate Microdrop Self‐Propelling Function

AbstractHere, we present a type of tip‐like nanotube array with condensate microdrop self‐propelling (CMDSP) function. Based on the basic principle that condensate microdrops can self‐remove via coalescence‐released weak surface energy as long as interface adhesion induced energy dissipation is sufficiently low, we design tip‐like nanotubes with extremely low solid–liquid interface adhesion. To verify the feasibility of this idea, we propose a strategy of preferential etching of six corners and the interior of ZnO nanopencils to achieve tip‐like nanotubes. Our studies demonstrate that this nanostructure can be achieved by immersing ZnO nanopencils in 0.006 m HCl aqueous solution at 80 °C for 3 h. After silane modification, the resulting nanostructure shows CMDSP function. This work offers a new avenue for developing CMDSP functional films, which can find a variety of technological applications such as moisture self‐cleaning, antifrosting and enhancing heat transfer.

Photoluminescent properties of electrochemically synthetized ZnO nanotubes

ZnO nanotubes were prepared by a sequential combination of electrochemical deposition, chemical attack and regeneration. ZnO nanocolumns were initially electrodeposited on conductive substrates and then converted into nanotubes by a process involving chemical etching and subsequent regrowth. The morphology of these ZnO nanocolumns and derived nanotubes was monitored by Scanning Electron Microscopy and their optical properties was studied by photoluminescence spectroscopy. Photoluminescence were measured as a function of temperature, from 6 to 300K, for both nanocolumns and nanotubes. In order to study the behaviour of induced intrinsic defect all ZnO films were annealed in air at 400°C and their photoluminescent properties were also registered before and after annealing. The behaviour of photoluminescence is explained taking into account the contribution of different point defects. A band energy diagram related to intrinsic defects was proposed to describe the behaviour of photoluminescence spectra.

Surface-conducted field emission electron sources with ZnO emitters of different morphologies

Surface-conducted field emission electron sources using ZnO nanostructures as surface-conducted emitters have been successfully fabricated, where the linear cathode and gate electrodes were interdigitated and paralleled on the same plane and ZnO emitters with different morphologies were deposited in the gap between the cathode and gate electrode. Field emission investigations indicate that the turn-on voltage of the fabricated SCFE electron sources with ZnO nanorods, ZnO nanotubes, and ZnO nanocones modifies from 260 V to 220 V at the anode voltage of 2 kV. The emission efficiency of SCFE electron source with ZnO nanocones is as large as 43.6%, which is much higher than that of SCFE electron sources with ZnO nanorods and nanotubes at the gate voltage of ∼300 V. The whole surface emission of the fabricated electron source with ZnO nanocones on the fluorescent screen is more homogeneous than that of the electron sources with ZnO nanorods and nanotubes.

High-response and selective hydrogen sensing properties of porous ZnO nanotubes

Hydrogen gas is utilized in various industrial fields, and its utility in modern life is becoming a reality since the development of hydrogen fuel cell vehicles. However, hydrogen gas is extremely explosive at room temperature and requires cautious handling. Herein, a ZnO nanotube sensor was fabricated via a three step process; this sensor exhibited high response and selectively detected hydrogen gas owing to its large surface area and grain boundaries. The response of this sensor was quantified as 139.11 when 1000 ppm H2 gas was supplied to the sensor. A porous ZnO nanotube sensor was fabricated and the mechanism by which enhanced hydrogen sensing is achieved was evaluated.

Magnesium-doped zinc oxide nanorod–nanotube semiconductor/p-silicon heterojunction diodes

Nanostructured zinc oxide material is usable in electronic device applications such as light-emitting diodes, heterojunction diode, sensors, solar cell due to its interesting electrical conductivity and optical properties. Magnesium-doped zinc oxide nanorod (NR)–nanotube (NT) films were grown by microwave-assisted chemical bath deposition to fabricate ZnO-based heterojunction diode. It is found that ZnO hexagonal nanorods turn into hexagonal nanotubes when the Mg doping ratio is increased from 1 to 10 %. The values of the optical band gap for 1 % Mg-doped ZnO NR and 10 % Mg-doped ZnO NT films are found to be 3.14 and 3.22 eV, respectively. The n-ZnO:Mg/p-Si heterojunction diodes were fabricated. The diodes exhibited a rectification behavior with ideality factor higher than unity due to the presence of surface states in the junction and series resistance. The obtained results indicate that Mg doping improves the electrical and optical properties of ZnO.

Characterization of the fullerene end-functionalized ZnO nanotube: A computational study

The electronic and field emission properties of the fullerene end-functionalized zinc oxide nanotube (ZnONT) are investigated by density functional theory (DFT) to search for novel field emitter nano material. The interaction energies of ZnONT/fullerenes complexes gradually increase, with increasing the nanotube lengths which indicate that ZnONTs with higher lengths could improve the stability of the complexes. The band gaps of connected construction of fullerene molecules with ZnONTs gradually reduced by increasing the tube length, but were not sensitive to the tubes diameter. It is found that the ionization potentials of ZnONT/fullerenes complexes mainly decrease compared to that of pristine nanotube in the presence of 0, 0.002, 0.004[Formula: see text]a.u. electric field. The reduction of the ionization potential means the enhancement of the field emission properties of ZnONT/fullerenes complexes compared with simple ZnONT and fullerene molecules. The calculations show that the combination of ZnONT with fullerene molecules indeed improves the field emission by controlling the tube size and electric field strength.

Single-pot ZnO nanostructure synthesis by chemical bath deposition and their applications

ZnO is commonly synthesized using many different techniques. Among them, the chemical bath deposition (CBD) technique is appealing as it is low cost and environmentally friendly, owing partly to the low temperature conditions used. An exploration of the variety of ZnO nanostructures that can be synthesized using this technique is presented in this study. ZnO fibers, nanorods, cauliflowers, nanorod balls, nanoforest, nanopencils, ellipsoids and nanotubes are synthesized using CBD by using different additives and conditions that alter the growth of the ZnO nanostructures. The application of these nanostructures in a wide variety of devices is analyzed.

Atomic Layer Deposition on Block Copolymer Membranes with Gyroidal Nanopores Toward Periodically Nanostructured Vapor Sensors: Nanotubes versus Nanorods

3D gyroidal networks of ZnO nanorods or nanotubes are synthesized by replicating block copolymer (BCP) templates with gyroidal nanopores via atomic layer deposition (ALD). The generation of 3D ZnO nanorods or nanotubes depends on the thicknesses of deposited ZnO layers, which can be easily and precisely controlled by adjusting ALD cycles. The as‐obtained ZnO nanostructures have porosities as high as 77% for gyroidal nanorods and 86% for gyroidal nanotubes due to their unique interconnected structures and rough surfaces. Both gyroidal ZnO nanostructures are used as vapor sensors to detect ethanol with different concentrations. Gyroidal ZnO nanotubes show higher ethanol sensitivity than nanorods because of the inherently interconnected nanotubes and larger porosity. Gyroidal ZnO nanotubes can also act as formaldehyde sensors with the sensitivity as high as 42 to 50 ppm formaldehyde, which is much higher than previously reported sensors based on ZnO nanorods or nanotubes.

Site specific interaction of aromatic amino acids with ZnO nanotubes: A density functional approach

The fast growing application of inorganic nanostructures for biomedical purposes has led to an increasing demand for further studies to obtain deeper understanding on the interactions between biomolecules and nanostructures at the atomic level. In this regard, a group of amino acids (AA) are the subject of the present study; in which density functional theory (DFT) is employed to investigate the interaction between aromatic amino acids (AAs) and Zinc oxide single walled nanotubes (SWNT) to identify the effective parameters for the biomedical applications. The structural changes and site specific binding nature of the bio-conjugated system are examined in detail. The results show that nanotube’s curvature is an effective factor in the magnitude of binding energy, since by decrease of nanotubes’ curvature the binding energy increases. Examination of the interactions between ZnO SWNTs and different binding sites of aromatic AAs indicates that the charge-solvent structure involving aromatic ring site of AAs is energetically more favorable. In this configuration the bonding is mediated by hybridization of π orbitals in aromatic ring and the d orbitals of Zinc atoms. The results of this study support the possible capability of ZnO SWNTs in detection of amino acids which shows the promising application of ZnO SWNTs in biosensing purposes.

Exploring the sensitivity of ZnO nanotubes to tyrosine nitration: A DFT approach

Due to association of protein tyrosine nitration (PTN) with development of some serious human disorders and diseases, in this paper, the possible applications of ZnO-based nanobiosensors in nitrated tyrosine (nTyr) detection were explored within the density functional framework. With this motivation, the interaction of nTyr with ZnO single walled nanotubes via all possible active sites of nTyr was investigated. The results show the tendency of nTyr to interact through its nitro site (forming nitro-site configuration) with ZnO SWNTs as it has the highest binding energy; while, the charge–solvent configuration involving the interaction of nTyr's phenolic ring has the second place in terms of binding energy magnitude. Regardless of which active site contributes in interaction, the binding energies exhibit an ascending trend with decrease of SWNTs' curvature. Electronic properties analysis indicates that nTyr interaction via its nitro group results in formation of some flat bands inside the band gap region leading to significant reduction of overall band gap energy. Similar behavior is also observed in charge–solvent configuration but the band gap energy is larger. These red shifts are mainly attributed to contribution of 2p orbitals of species present in nTyr. Also, the hybridization of 3d orbital of Zn atom with 2p orbitals of nitro group atomic species is found responsible for bonding formation in bioconjugated system possessing the highest binding energy. Comparison of the electronic band structure of ZnO SWNT–Tyr with that of ZnO SWNT–nTyr indicates the sensitivity of ZnO SWNTs toward tyrosine nitration hence, a considerable change in its optical spectra is expectable. This introduces ZnO SWNTs as a promising candidate for PTN detection.

Enhancement of solar cell performance of p-Cu2O/n-ZnO-nanotube and nanorod heterojunction devices

Nanostructured thin films are appealing to solar light harvesting and high-efficiency photovoltaic devices. A new Cu2O/ZnO heterojunction solar cell architecture has been developed with ZnO nanotubes (NTs) and nanorods (NRs), on a 500nm thick, n-type ZnO seed layer, covered by a 2.5–3µm thick, p-type Cu2O layer to provide the full built-in potential across the junction area, all done by electrodeposition method. This architecture is used to improve the light harvesting and charge collection efficiency by taking advantage of the high surface area junction and direct charge transfer pathway of these one-dimensional ZnO nanostructures. In addition, we calculate the optimized thickness of p-type Cu2O film of 2.5–3µm to maximize their open-circuit voltages. Post-annealing of the p-type Cu2O nanostructured film at 200°C is found to provide better crystallinity and a higher conductivity for additional improvement in the performance. Further optimization of the Cu2O film thickness according to their charge carrier concentration and absolute permittivity values also assures the full built-in potential across the junction interface. Finally, the electrodeposited tubular ZnO nanostructures are grown on the top of a sufficiently thick ZnO seed layer, which prevents any leakage pathway and increases the junction area, and consequently increases their short-circuit current values. The open-circuit voltages obtained for Cu2O/ZnO-NT (0.66V) and Cu2O/ZnO-NR heterojunction devices (0.71V) represent the highest values reported to date for this type of electrodeposited Cu2O/ZnO solar cells. Interestingly, the short-circuit current density of the Cu2O/ZnO-NT cell (2.40mA/cm2) is twice that of the Cu2O/ZnO-NR cell (1.12mA/cm2) with a photon-to-electron conversion efficiencies of 0.8% and 0.4%, respectively. These results illustrate the advantage of single-step electrodeposition of ZnO nanotubes, which provide a larger interfacial area and a much lower defect density than previously reported nanotubes obtained by etching ZnO nanorods. The new device architecture also offers a minimum leakage path and reduced recombination loss expected in typical nanostructure-based photovoltaics. This study demonstrates a promising approach to fabricating low-cost metal oxide nanostructured thin-film solar cells by a scalable, efficient electrochemical method for cost reduction, process simplification, and performance improvement.

Structural, Electronic, Vibrational, and Topological Analysis of Single-Walled Zinc Oxide Nanotubes

Single-walled armchair and zigzag ZnO nanotubes (SWZnONTs) have been studied via periodic computational simulations based on density functional theory with the B3LYP, HSE06, PBE0, and PWGGA functional and all-electron basis set. The influence of the diameter of the nanotubes was carried out with respect to the bond length (Zn–O), bond angles (Zn–O–Zn), energy strain, band gap, density of states (DOS), band structures, vibrational analysis, and topological analysis of the electron density according to the quantum theory of atoms in molecules applied to the solid state. Its nanotubes properties were compared with the ZnO bulk and (0001) monolayer surface. The topological analysis, infrared and Raman spectra, and its vibrational modes at increasing diameter are reported for the first time. Owing to these analysis, both chiralities with large diameter can be used interchangeably in semiconductor applications. These theoretical models can be extended to study further issues, such as the effects caused by the addition of dopant and the interaction of molecules inside and/or outside of the nanotube.

Vacuum template synthesis of multifunctional nanotubes with tailored nanostructured walls.

A three-step vacuum procedure for the fabrication of vertical TiO2 and ZnO nanotubes with three dimensional walls is presented. The method combines physical vapor deposition of small-molecules, plasma enhanced chemical vapor deposition of inorganic functional thin films and layers and a post-annealing process in vacuum in order to remove the organic template. As a result, an ample variety of inorganic nanotubes are made with tunable length, hole dimensions and shapes and tailored wall composition, microstructure, porosity and structure. The fabrication of multishell nanotubes combining different semiconducting oxides and metal nanoparticles is as well explored. This method provides a feasible and reproducible route for the fabrication of high density arrays of vertically alligned nanotubes on processable substrates. The emptying mechanism and microstructure of the nanotubes have been elucidated through SEM, STEM, HAADF-STEM tomography and energy dispersive X-ray spectroscopy. In this article, as a proof of concept, it is presented the straightforward integration of ZnO nanotubes as photoanode in a photovoltaic cell and as a photonic oxygen gas sensor.

Periodic density functional theory study of structural and electronic properties of single-walled zinc oxide and carbon nanotubes

Periodic density functional theory calculations with the B3LYP hybrid functional and all-electron Gaussian basis set were performed to simulate the structural and electronic properties as well as the strain and formation energies of single-walled ZnO nanotubes (SWZnONTs) and Carbon nanotubes (SWCNTs) with different chiralities as functions of their diameters. For all SWZnONTs, the band gap, strain energy, and formation energy converge to ~4.5eV, 0.0eV/atom, and 0.40eV/atom, respectively. This result suggests that the nanotubes are formed more easily from the surface than from the bulk. For SWCNTs, the strain energy is always positive, while the formation energy is negative for armchair and zigzag nanotubes, therefore suggesting that these types of nanotubes can be preferentially formed from the bulk. The electronic properties of SWCNTs depend on the chirality; all armchair nanotubes are metallic, while zigzag and chiral nanotubes can be metallic or semiconducting, depending on the n and m vectors.

Phase and composition selective superior cholesterol sensing performance of ZnO@ZnS nano-heterostructure and ZnS nanotubes

Development of efficient enzyme-immobilized amperometric biosensor using nanostructure inorganic materials to estimate different biomolecules is an important research area in present frontier research. Here, the controlled synthesis of phase selective zinc oxide@zinc sulfide, zinc blend(ZB)/wurtzite (WZ), (ZnO@ZnSZB/ZnSWZ) through substitution reaction of corresponding ZnO nanorod using potassium ethyl xanthogenate as sulfur source under hydrothermal/solvothermal condition as well as corresponding ZnS nanotube through acetic acid leaching and evaluation of cholesterol sensing performance of the synthesized nanostructured modified electrode is presented. Potassium ethyl xanthogenate is used for controlled substitution reaction (sulfidation). Both the ZnO@ZnSZB nano-heterostructure and ZnSZB nanotubes based modified biosensor showed excellent amperometric cholesterol-sensing performance, over bare ZnO, ZnO@ZnSWZ heterostructure and ZnSWZ nanotube. ZnSZB nanotube showed highest sensitivity (598mAM−1cm−2) with a LOD of 0.44mM, whereas ZnO@ZnSZB nano-heterostructure showed reasonably good sensitivity 293mAM−1cm−2) with very low LOD of 0.08mM. The ZnSZB nanotube exhibits maximum sensitivity due to its relatively high surface area and easy electron transfer through its tubular channel. ZnO@ZnSZB nano-heterostructure showed low LOD due to its modified band structure which facilitate the electron transport to the detector. The sensing performance is superior to that of other literature reported sensors.

Optical properties of pure and TM-doped single-walled ZnO nanotubes (8,0) (TM = V and Co) by first principles calculations

In this paper, some optical properties of pure and transition metal-doped (TM = Co and V) single-walled ZnO nanotubes (8,0) (SWZnONT(8,0)) such as, real and imaginary parts of the dielectric function, optical conductivity, refractive index and optical reflectivity, were investigated. The calculations have been performed within framework of the density functional theory (DFT) using the full potential linearized augmented plane wave (FP-LAPW) and the generalized gradient approximation (GGA). The results show that, optical properties of SWZnONT(8,0) are anisotropic, especially at low energies and this anisotropy at low energies increases with doping of V in SWZnONT(8,0) while the Co-doped SWZnONT(8,0) behaves like pure SWZnONT(8,0). Doping of ZnO nanotubes has a significant impact on the value of the dielectric constant, so that due to the presence of V atom, the dielectric constant is increased up to three times. Study of the imaginary part of the dielectric function and optical conductivity showed that the important energy range for absorption processes and optical transitions is low energy range to 15 eV. The optical transitions have been studied based on band structure and density of states. The results of the optical reflectivity showed that these nanotubes are transparent in a wide energy ranges which provide them for using in transparent coatings. In addition, due to the reported magnetic properties for V- and Co-doped ZnO nanotubes, these nanotubes are suitable for using in spintronics and magneto-optic devices.