Aligned ZnO Nanorod Arrays Research Articles

Fabrication of H2Gas Sensor Based on ZnO Nanarod Arrays by a Sonochemical Method

We report a simple method for fabricating ZnO gas sensors via a sonochemical route and their gas sensing properties. Vertically aligned ZnO nanorod arrays as a sensing material were synthesized on a Pt-electrode patterned alumina substrate under ambient conditions. The advantage of the proposed method is a high speed of processing. The gas sensor based on ZnO nanorod arrays with large specific surface area showed a high response to and a detection limit of 70 ppm at . Also, their response and recovery time were relatively short and a complete regeneration was observed. A mechanism for sensing gas on the surface of ZnO nanorods is proposed.

Controllable Growth of Highly Ordered ZnO Nanorod Arrays via Inverted Self-Assembled Monolayer Template

This article presents a facile and effective approach to the controllable growth of highly ordered and vertically aligned ZnO nanorod arrays on the GaN substrate via a hydrothermal route by using the TiO(2) ring template deriving from the polystyrene microsphere self-assembled monolayer. The size of TiO(2) ring template can be flexibly tuned from 50 to 400 nm for the 500 nm polystyrene microspheres by varying the time of reactive ion etching and the concentration of TiO(2) sol. As a result, the diameter of the individual ZnO nanorods can be potentially tuned over a wide range. The combination of several characterization techniques has demonstrated that the ordered ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays with high crystalline quality.

Buffer-Layer-Assisted Epitaxial Growth of Perfectly Aligned Oxide Nanorod Arrays in Solution

Epitaxy between materials with good lattice match requires perfect staking on the atomic scale and is thus often considered to be challenging to achieve by low-temperature solution-based hydrothermal growth. Inspired by the concept of the buffer layer in vapor-phase epitaxial growth, we present herein a novel synthetic protocol of employing ZnAl2O4 epilayers to hydrothermally grow perfectly aligned ZnO nanorod arrays in solution. Remarkable tunability of the density and dimensions of the nanorods can be realized by adjusting the roughness of the ZnAl2O4 layer and the nutrient solution concentration, which is in line with the classical nucleation theory and also confirms that the nanorod growth is mass-transport-limited. Furthermore, through nanopatterning of the ZnAl2O4 layer by electron beam lithography, shape-controlled vertical ZnO nanostructures at predesigned positions can be achieved. Finally, taking advantage of the epitaxial relationships, we adjusted the tilting orientations of the aligned nanorods by selecting sapphire substrates with different surface planes.

Improved performance of MEH-PPV/ZnO solar cells by addition of lithium salt

Abstract The paper reports the improved performance by addition of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) to poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in the hybrid solar cells consisting of MEH-PPV as an electron donor and vertically aligned ZnO nanorod array as an electron acceptor. Results show that, with increasing the weight ratio R of LiTFSI/MEH-PPV, the charge transfer efficiency at MEH-PPV/ZnO interface, the device short circuit current ( J sc ) and open circuit voltage ( V oc ) get increased for R ⩽ 2/10, but decreased when R > 2/10, resulting in a peak power conversion efficiency of η = 0.48% for R = 2/10 at AM 1.5 illumination (100 mW/cm 2 ). It is revealed that the increased J sc is due to the improved charge transfer between the MEH-PPV and ZnO as a result of the interaction between LiTFSI and MEH-PPV, while the increased V oc and the decreased charge recombination are attributed to the increased hole mobility of MEH-PPV; moreover, the decreased J sc and V oc at high R values are attributed to the morphology degradation in the active layer due to the high LiTFSI content.

Vertically Aligned ZnO Nanorod Arrays Coated with $\hbox{SnO}_{\bf 2}$/Noble Metal Nanoparticles for Highly Sensitive and Selective Gas Detection

Mimicking the biological olfactory receptor array that possesses large surface area for molecule capture, vertically aligned ZnO nanowire arrays, used as structural templates, were coated with SnO2/noble metal nanoparticles as active materials for fabrication of 3-D gas sensors. The gas sensors showed room-temperature responses to environmental toxic gases, such as NO2 and H2S, down to ppb level, which can be attributed to the large surface area of their 3D structure and catalytic behaviors of noble metals. A sensor array composed of three sensors with the different noble metals decoration (Pd, Pt, and Au) has shown capability to discriminate five different gases (H2S, NO2, NH3, H2, and CO) when using principal component analysis (PCA) incorporated the response speed as a discrimination factor. This study demonstrates a rational strategy to prepare sensing devices with 3-D structures for selective detection, which can be readily extended to other sensing materials that can be hardly grown as 3-D nanowire arrays.

Synthesis of Aligned ZnO Nanorod Array on Silicon and Sapphire Substrates by Thermal Evaporation Technique

High density ZnO nanorods were grown by thermal evaporation of Zn powder at 700°C on Si (100) and sapphire (0001) substrates at atmospheric pressure without adding any catalyst. The nanorods were characterizated in terms of their structural and optical properties. The nanorods grown on Si have a diameter of 350–400 nm and a length of 1.2 μm while those on sapphire have a diameter of 600–800 nm and a length of 2.5 μm. During the structural characterization, it is noticed that the rods grow along the (0002) plane with perfect hexagonal facet. The room temperature photoluminescence spectrum showed a strong UV emission peak at 385 nm with a weak green band emission, which confirms that nanorods have good optical properties. It is observed that the oxygen partial pressure plays an important role to control the shape and size of the nanorods in thermal evaporation growth technique.

Low temperature growth and properties of ZnO nanorod arrays

In this paper, well aligned ZnO nanorod arrays were synthesized by a simple hydrothermal route at a low temperature. The diameters of the as-synthesized products were 20–60 nm and the lengths were as much as several micrometers. The surfaces and tops of the nanorods were smooth. The as-grown nanorod arrays were investigated by x-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), photoluminescence (PL) spectroscopy and contact angle (CA) analysis. The as-grown nanorods were single crystalline structures with a wurtzite phase, and grew along the [0001] direction. The PL spectrum with only one strong peak at 383 nm shows good intrinsic emission.

Influence of the alignment of ZnO nanorod arrays on light extraction enhancement of GaN-based light-emitting diodes

ZnO nanorod arrays (ZNAs) were hydrothermally synthesized on the surface of patterned indium-doped tin oxide p-type contact (PIPC) of GaN-based light-emitting diodes (GaN-LEDs) for enhancing the light extraction efficiency (LEE). It was found that the alignment of the ZnO nanorod arrays in the grooves of the PIPC was poorer than these grown on the ridges of the PIPC. By comparing the light output of the GaN-LEDs with and without ZNAs grown in the grooves of PIPC, the influence of the alignment of ZNAs on the LEE of GaN-LEDs was revealed and investigated. Numerical analysis based on the finite difference of time domain (FDTD) method suggested that the poorer alignment of ZNAs grown on GaN-LEDs resulted in more energy reflected back into GaN-LEDs and lower light extraction efficiency.

Electronic Structure and Optical Properties of Vertically Aligned ZnO Nanorod Arrays Grown by Low-temperature Hydrothermal Method

ZnO nanorod arrays were grown on Si substrate utilizing low-temperature hydrothermal method.The X-ray diffraction(XRD) and scanning electron microscope(SEM) measurements were carried out to investigate the crystalline and morphology of the as-grown ZnO nanorods.Results indicate that the hexagonal prism ZnO nanorod arrays are well-aligned along the c-axis and distribute densely and uniformly on the substrate.The room temperature photoluminescence(PL) spectra demonstrate that high optical quality ZnO nanorod arrays could be achieved by this low temperature hydrothermal process.X-ray absorption near-edge spectroscopy(XANES) measurements at O K-edge were carried out with synchrotron radiation to investigate the local electronic structure of ZnO nanorod arrays with different diameter,and the electronic structure dependence on the diameter was discussed.In addition,the contrast of local electric structure for ZnO nanorods and thin film were also discussed.

Device Performance Related to Amphiphilic Modification at Charge Separation Interface in Hybrid Solar Cells with Vertically Aligned ZnO Nanorod Arrays

This paper reports the chemical modification effects at charge separation interface on the performance of the hybrid solar cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as an electron donor (D) and vertically aligned ZnO nanorod arrays as an electron acceptor (A). Results show that, with increasing the modification time Ts for grafting dye Z907 onto the ZnO surface from 0 to 8 h, the charge transfer efficiency at the MEH-PPV/ZnO interface keeps increasing, the short circuit current (Jsc) increases and reaches a peak value at Ts = 8 h, but the open circuit voltage (Voc) increases within Ts = 1−3 h and reduces with further increasing Ts up to ≥6 h. By controlling the Ts, a peak power conversion efficiency of η = 0.61% at AM 1.5 illumination (100 mW/cm2) is obtained for Ts = 6 h. It is revealed that the Z907 modification mainly contributes to the enhanced Jsc by increasing the charge separation efficiency as a result of the improved electronic coupling property at th...

A catalyst-free and facile route to periodically ordered and c-axis aligned ZnO nanorod arrays on diverse substrates

In this work we present a method for the deposition of periodically ordered, c-axis aligned ZnO nanorod arrays. By using chemical bath deposited films in conjunction with silica templating through nanosphere monolayers, masks suitable for high temperature deposition are created. A vapour phase transport technique is then used to deposit ordered arrays, quickly and inexpensively in a manner ideal for low cost, scalable and reproducible growth on a diverse range of substrates.

Electrochemical growth of vertically aligned ZnO nanorod arrays on oxidized bi-layer graphene electrode

Vertically aligned ZnO nanorod arrays were directly grown on flexible and transparent oxidized bi-layer graphene electrodes by seedless electrochemical deposition. Oxidized defects on the graphene surface induce epitaxial growth of highly dense single crystal ZnO nanorods. The diameter, length as well as morphology of the nanorods can be effectively controlled by adjusting reaction conditions.

DNA-based fabrication of density-controlled vertically aligned ZnO nanorod arrays and their SERS applications

We described a novel seeded synthesis of vertically aligned ZnO nanorod arrays on a DNA network surface. Zinc ions were first adsorbed onto the DNA network and then heated to yield layers of ZnO seeds. The DNA network plays an important role not only as a substrate for the adsorbing of zinc ions, but also as a fine template for density controllable synthesis of high-quality ZnO seeds. The density of ZnO seeds was controlled by the adsorption times of zinc ions. The final density of ZnO nanorods was determined mainly by that of ZnO seeds. The ZnO nanorod arrays were used as templates for making silver-coated ZnO nanocomposites, which were applied as substrates in surface-enhanced Raman scattering (SERS) measurements. Two typical probe molecules, R6G and 4-ATP were used to test the SERS activity of the ZnO/Ag composites and the results indicated good Raman activity on the substrate.

Effects of Substrate on the Structure, Morphology and Optical Properties of Ver-tically Aligned ZnO Nanorod Arrays Grown by Low-temperature CBD Method

Effects of Substrate on the Structure, Morphology and Optical Properties of Ver-tically Aligned ZnO Nanorod Arrays Grown by Low-temperature CBD Method

Dynamic Characterization of Hybrid Solar Cells Based on Polymer and Aligned ZnO Nanorods by Intensity Modulated Photocurrent Spectroscopy

For characterization of polymer-based solar cells with vertically aligned ZnO nanorod arrays (ZnO-NAs) by intensity modulated photocurrent spectroscopy (IMPS), a dynamic IMPS model is developed, where the structure-related charge generation and transport dynamics are considered. The model describes the IMPS responses affected by the phase shift ϕn(ω) due to the exciton diffusion property (ω0) and the structure-related device ideality factor N, the electron diffusion coefficient De, the exciton dissociation rate S, and the device structure (e.g., nanorod length d and interspacing l). The main expectations of the model are confirmed by the experimental data of the polymer/ZnO-NA cells with d = 180−650 nm, offering mechanistic information on the structure-related charge generation, charge transport, and device performance. The presence of the ϕn(ω) makes IMPS response not spiral into the origin and the phase angle in its Bode plot not tend to 90°; the d-dependent direct diffusion (DD) and diffusion-reflectio...

Graphene Nanomesh by ZnO Nanorod Photocatalysts

Local photodegradation of graphene oxide sheets at the tip of ZnO nanorods was used to achieve semiconducting graphene nanomeshes. The chemically exfoliated graphene oxide sheets, with a thickness of approximately 0.9 nm, were deposited on quartz substrates. Vertically aligned ZnO nanorod arrays with diameters of 140 nm and lengths of <1 microm were grown on a glass substrate by using a hydrothermal method. The graphene oxide sheets were physically attached to the tip of the ZnO nanorods by assembling the sheets on the nanorods. UV-assisted photodegradation of the graphene oxide sheets (with dimension of approximately 5x5 microm) at a contact place with the ZnO nanorods resulted in graphene nanomeshes with a pore size of approximately 200 nm. The graphene nanomeshes prepared by using the photocatalytic property of the ZnO nanorods contained smaller oxygen-containing carbonaceous bonds and higher defects as compared to the as-prepared graphene oxide sheets. When chemical reduction of the graphene nanomeshes by hydrazine was used, the oxygen bonds of the nanomeshes more decreased while their graphitization increased. Based on X-ray photoelectron spectroscopy at low binding energies, the graphene nanomeshes reduced by hydrazine exhibited as a broad sheet a p-type semiconductor with an approximately 1.2 eV energy gap between the valence band and the Fermi level.

Formamide driven synthesis of well-aligned ZnO nanorod arrays on glass substrate

A cost effective, low-temperature approach has been developed for the large-area deposition of ZnO nanorod/nanotube arrays on a ZnO coated glass substrate by the natural oxidation of zinc metal in formamide/water mixtures. The two-step seed deposition and wet-chemical approach exhibited well-controlled growth of highly oriented and densely packed ZnO nanorod/nanotube arrays with large-area homogeneity and uniform morphologies. In order to investigate the quality and alignment of ZnO nanorod arrays grown on the ZnO seed layer coated substrate, three different methods of ZnO coating have been deposited by ultrahigh vacuum evaporation system, DC sputtering and RF sputtering, respectively. Our results showed that the ZnO seed layer grown by RF sputtering resulted in high quality ZnO nanorod arrays.

Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays

Vertically aligned ZnO nanorod arrays with different heights are grown on the ZnO seeded indium tin oxide substrate by cathodic electrochemical deposition from zinc nitrate at two temperatures of 60 °C and 80 °C. As-grown ZnO nanorods exhibit wurzite crystal structure and their heights can be well controlled by different deposition times. The fluorination coating tends to induce a superhydrophobicity of ZnO nanorods, i.e., the maximal value of contact angle: 166.9°. The super water repellency can be attributed to the fact that an air layer is confined in the nanorod arrays, and thus leads to water droplets sitting on the ZnO surfaces, referring as Cassie state. Interestingly, their water contact angles are found to vary with the heights of ZnO nanorods, ranged from 99.8 to 746 nm. The superhydrophobicity of ZnO surfaces can be well predicted by a proposed model that is capable of determining the wetted fraction of ZnO pillars. This satisfactory result would shed one light on how the variation of rod height would induce the superhydrophobic behavior of ZnO nanorod arrays.

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm −1, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.

Growth of vertically aligned ZnO nanorod arrays as antireflection layer on silicon solar cells

In this work we investigated the effects of growth time, spin-coating rates, and solution concentration on the reflection properties of the solution-grown ZnO nanorod arrays. The vertically aligned nanorod arrays were deposited on the surface of the Si solar cells as the antireflection (AR) layer. We found that the nanorod morphology, controlled through synthetic chemistry, has a great effect on the AR layer performance. We also demonstrated that the light harvest efficiency of the solar cells was greatly improved from 10.4% to 12.8% by using the vertically aligned ZnO nanorod arrays as the AR layer on poly-Si solar cells.