Comparison between synthesis techniques to obtain ZnO nanorods and its effect on dye sensitized solar cells

This paper reports additive-free, reproducible, low-temperature solution-based process for the preparation of crystalline ZnO nanorods by homogeneous precipitation from zinc acetate. Also, ZnO nanorod structured dye sensitized solar cells using ruthenium dye (Z907) have been fabricated and characterized. The formation and growth of zinc oxide nanorods are successfully achieved. We analyzed three different synthesis method using solution phase, autoclave and microwave. The calcination effects on the morphology of ZnO nanorods are also investigated. Analysis of ZnO nanorods shows that calcination at lower temperature is resulted in a nanorod growth. Additive-free, well-aligned ZnO nanorods are obtained with the length of 330–558 nm and diameters of 14–36 nm. The XRD, SEM, and PL spectra have been provided for the characterization of ZnO nanorods. Microwave-assisted ZnO nanostructured dye sensitized solar cell devices yielded a short-circuit photocurrent density of 6.60 mA/cm², an open-circuit voltage of 600 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 2.35% under standard AM 1.5 sun light.     

Hydrothermal synthesis of ZnO nanorod arrays with the addition of polyethyleneimine

ZnO nanorod arrays were synthesized on glass substrates coated by a ZnO seeding layer via a hydrothermal technique by adding polyethyleneimine (PEI) to the growth solution. The XRD and SEM results show that the ZnO nanorods have the single crystal wurtzite structure with the (0 0 2) direction normal to the substrates. In 50 ml growth solution of 0.05 M zinc salts, the average diameter of the nanorods was reduced drastically from 300 nm to 40 nm with the PEI amount increasing from 0 ml to 6 ml. The diameter distribution and the polycrystalline layer at the bottom of the nanorods were improved. Longer nanorods were obtained by prolonging the growth time. Based on the results, possible mechanisms that PEI adsorbs on the non-polar facets of ZnO nanorods and its coordination to zinc ions were proposed to elucidate the effect of PEI on reducing the diameters and improving the morphologies of nanorods.     

Preparation of nanostructured ZnO nanorods in a hydrothermal-electrochemical process

An array of ZnO nanorods, each nanorod being covered with a shell of porous ZnO was prepared in two steps by hydrothermal-electrochemical processes. The growth of ZnO nanorods was achieved in a zinc nitrate and hexamethylenetetramine aqueous solution on fluorine-doped tin oxide film. The porous ZnO shell was grown from a similar solution in the presence of eosin Y as nanostructuring agent. A dye-sensitized solar cell was assembled using as photoanode an eosin Y sensitized ZnO nanorod/shell layer.     

Synthesis of size-tunable ZnO nanorod arrays from NH3·H2O/ZnNO3 solutions

Well-aligned crystalline ZnO nanorod arrays were fabricated via an aqueous solution route with zinc nitrate and ammonia as precursors. Dip-coating was firstly utilized to form a ZnO film on ITO substrate as a seed layer for subsequent growth of ZnO nanorods. The effects of NH₃·H₂O/ZnNO₃ molar ratio, ZnNO₃ concentration, growth temperature and time on nanorod morphology were respectively investigated. It was found that the size of nanorod is mainly determined by the molar ratio and concentration. XRD demonstrates that ZnO nanorods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. SEM confirms that ZnO nanorods grew up perpendicular to the substrate. The diameter and length were tunable in a broad range from 80 nm to 500 nm and 250 nm up to 8 μm, respectively. The aspect ratio changed from 3 to 17 mainly dependent on composition of the aqueous solution.     

Flame synthesis of zinc oxide nanocrystals

Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counter-flow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ～99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 μm. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ～0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed.     

Preparation and characterization of copper nanoparticles/zinc oxide composite modified electrode and its application to glucose sensing

We report a new method for selective detection of d(+)-glucose using a copper nanoparticles (Cu-NPs) attached zinc oxide (ZnO) film coated electrode. The ZnO and Cu-NPs were electrochemically deposited onto indium tin oxide (ITO) coated glass electrode and glassy carbon electrode (GCE) by layer-by-layer. In result, Cu-NPs/ZnO composite film topography was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. SEM and AFM confirmed the presence of nanometer sized Cu-NPs/ZnO composite particles on the electrode surface. In addition, X-ray diffraction pattern revealed that Cu-NPs and ZnO films were attached onto the electrode surface. Indeed, the Cu-NPs/ZnO composite modified electrode showed excellent electrocatalytic activity for glucose oxidation in alkaline (0.1 M NaOH) solution. Further, we utilized the Cu-NPs/ZnO composite modified electrode as an electrochemical sensor for detection of glucose. This glucose sensor showed a linear relationship in the range from 1 × 10^? 6 M to 1.53 × 10^? 3 M and the detection limit (S/N = 3) was found to be 2 × 10^? 7 M. The Cu-NPs/ZnO composite as a non-enzymatic glucose sensor presents a number of attractive features such as high sensitivity, stability, reproducibility, selectivity and fast response. The applicability of the proposed method to the determination of glucose in human urine samples was demonstrated with satisfactory results.     

Sol–gel template synthesis and characterization of aligned anatase-TiO2 nanorod arrays with different diameter

Template-based growth has been demonstrated as an attractive method for the synthesis of oxide nanorod arrays. In this paper, TiO₂ nanorod arrays in anodic alumina membranes have successfully been synthesized by an improved sol–gel template method. TiO₂ nanorods were obtained to injection of prepared TiO₂-sol from the sol–gel process and then the samples were immediately immersed into boiling TiO₂-sol solutions. Aligned TiO₂ nanorod arrays were polycrystalline anatase-phase after annealing at 400 °C for 2 h with diameters about 160–250, 80–130 and 40–70. A lateral shrinkage approximately 15–20% was found in TiO₂ nanorods with respect to the template pore diameter. This size difference is likely due to the volume shrinkage caused by removing residual materials and densification during annealing.     

Enhanced H2S gas sensing properties of multiple-networked Pd-doped SnO2-core/ZnO-shell nanorod sensors

Pd-doped SnO₂-core/ZnO-shell nanorods were synthesized by using a three-step process: thermal evaporation of Sn powders in an oxygen atmosphere, atomic layer deposition of ZnO, and Pd diffusion followed by annealing. The sensitivity of the multiple networked SnO₂-core/ZnO-shell nanorod sensor to H₂S gas was found to be improved further significantly by Pd doping. The Pd-doped SnO₂-core/ZnO-shell nanorod sensor showed sensitivities of 6.4, 15.4, and 36.2% at H₂S concentrations of 20, 50, and 100 ppm at room temperature. The sensitivity of the nanorods was improved by more than 10 times at a H₂S concentration of 100 ppm. The sensitivity enhancement of the SnO₂-core/ZnO-shell nanorods by Pd doping may be attributed to the spillover effect, active reaction site generation, and the enhancement of chemisorption and dissociation of gas.     

Hierarchical nanostructures of ZnO obtained by spray pyrolysis

A two step spray pyrolysis deposition method was applied in order to grow ZnO nanorod core/ZnO shell hierarchical nanostructures with various surface morphologies, such as a highly organised platelet network on the side facets of the ZnO rod and bundles of nanoneedles on the top plane of the rod. First, well-shaped ZnO nanorods with lengths of ca. 1 μm and average diameters of 150–300 nm were deposited from zinc chloride (ZnCl₂·2H₂O) aqueous solutions onto TCO/glass substrates. Then, zinc acetate (Zn(CH₃COO)₂·2H₂O) solution was pulverised over the surface of the sprayed ZnO nanorods at a growth temperature of approximately 330 °C within 6–10 min. The obtained structures were characterised by high resolution SEM, UV–VIS and XRD. To estimate the surface areas and photocatalytic ability of the bare rods and hierarchical structures, their adsorption ability and activity of photocatalytic oxidation of doxycycline were measured. It was found that the surface area of hierarchical structures comprised of a network of platelets is at least 4 times larger than that of a bare rod. The structural and morphological properties of sprayed hierarchical structures largely depend on the spraying rate of the zinc acetate solution and on the ZnO nanorod top plane shape.     

Glucose sensor using periodic nanostructured hybrid 1D Au/ZnO arrays

Hybrid 1D nanostructured Au/ZnO arrays were created by heat treatment of a spin-coated zinc acetate-PVA-Au(III) layer on surface relief grating and functioned as an electrochemical and optical D(+)-glucose sensor due to electrochemical oxidation between hybrid nanostructures and D(+)-glucose. The morphology and chemical composition of 1D Au/ZnO hybrid arrays were characterized by means of AFM, SEM, EDAX, and XPS. Electrochemical and optical sensitivities by the addition of D(+)-glucoses on 1D Au/ZnO arrays were investigated using Cyclic voltammetry and UV–vis-NIR spectra in the medical concentration ranges of 0.5, 2.0, and 8.0 mM.     

Comparison of optical properties of Si and ZnO/CdTe core/shell nanowire arrays

The systematic computations of the short-circuit current density have been performed for Si and ZnO/CdTe core shell nanowire arrays of 1 μm height in order to optimize the structural morphology in terms of nanowire diameter and period. It is found that the best structural configuration for Si leading to the ideal short-circuit current density of 19.6 mA/cm² is achieved for a nanowire diameter and period of 315 nm and 350 nm, respectively. In case of ZnO/CdTe, the ideal short circuit current density is of 24.0 mA/cm², the nanowire diameter and period is of 210 nm and 350 nm, respectively. It is shown that the optimal configuration is more compact in the case of Si nanowire arrays than in the case of ZnO/CdTe nanowire arrays. Since Si has a smaller absorption coefficient than CdTe, a larger amount of material is needed and thus more compact nanowire arrays are required. It is also revealed that core–shell nanowire arrays made of ZnO/CdTe more efficiently absorb light than that of Si, making this device a good candidate for the next generation of nanostructured solar cells.     

Ordered multiphase polymer nanocomposites for high-performance solid-state supercapacitors

Multilayered ordered nanostructures were fabricated by assembling in-situ grown polyaniline nanowire arrays with graphene oxide nanosheets. As-fabricated nanostructure was subsequently impregnated with the (H₃PO₄–Nafion)/polyvinyl alcohol solution to create a multiphase composite, which was used as a solid-state supercapacitor where graphene oxide/polyaniline nanowires served as electrode and (H₃PO₄–Nafion)/polyvinyl alcohol served as solid electrolyte. The ordered polyaniline (PANI) nanostructures facilitated the charge transfer and resulted in the specific capacitance of 83 F/g even if the discharge current was 5 A/g. The efficient charge transportation and electrode–electrolyte interaction resulted in small equivalent series resistance as low as 5.83 Ω, and thus outstanding electrochemical performance. The charge transfer resistance was much smaller than other commonly used solid-state electrolyte and almost negligible. As a result only 7% capacitance loss was found when the frequency increased from 100 to 1000 Hz. The energy density was as high as 26.5 Wh/kg while the power density was ∼3600 W/kg. The energy storage performance was also very stable since 82% specific capacitance was maintained after 1000 cycles.     

Controlled synthesis of ZnO branched nanorod arrays by hierarchical solution growth and application in dye-sensitized solar cells

We demonstrate the controlled synthesis of ZnO branched nanorod arrays on fluorine-doped SnO₂-coated glass substrates by the hierarchical solution growth method. In the secondary growth, the concentration of Zn(NO₃)₂/hexamethylenetetramine plays an important role in controlling the morphology of the branched nanorod arrays, besides that of diaminopropane used as a structure-directing agent to induce the growth of branches. The population density and morphology of the branched nanorod arrays depend on those of the nanorod arrays obtained from the primary growth, which can be modulated though the concentration of Zn(NO₃)₂/hexamethylenetetramine in the primary growth solution. The dye-sensitized ZnO branched nanorod arrays exhibit much stronger optical absorption as compared with its corresponding primary nanorod arrays, suggesting that the addition of the branches improves light harvesting. The dye-sensitized solar cell based on the optimized ZnO branched nanorod array reaches a conversion efficiency of 1.66% under the light radiation of 1000 W/m². The branched nanorod arrays can also be applied in other application fields of ZnO.     

Effect of annealing on the magnetic properties of solution synthesized Zn1−xMnxO nanorods

Mn-doped ZnO nanorods with ～30 nm in diameter and ～200 nm in length were synthesized by a seed-mediated solution method. The structures, magnetic properties, as well as the annealing effect were characterized by transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum and physical properties measurement system. Magnetic properties measurement revealed that the Zn_0.97Mn_0.03O nanorods exhibited ferromagnetism with a saturation magnetization of 0.005 emu g^?1 and a coercivity of 110 Oe at 305 K. After annealing the samples at 900 °C for 2 h in air, the nanorods were transformed into nanoparticle aggregates. The coercivity and saturation magnetization increased obviously. Detailed analyses proved that a phase-separation process was happened at the high temperature. In this process, most of the particles preserved the wurtzite ZnO structure, while a few small ones evolved into spinel-structured particles. The increasing of the ferromagnetism of the annealed sample is attributed to the formation of secondary phase Zn_xMn_3?xO_4.     

The effect of ZnO coating on LiMn2O4 cycle life in high temperature for lithium secondary batteries

The surface of the spinel LiMn₂O₄ was modified with zinc oxide by a chemical process to improve its electrochemical performance at high temperatures. The physical properties of the prepared products have been investigated by thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-rays analysis (EDAX). The charge/discharge of the materials was carried at 1 mA/cm² in the range of 3.0 and 4.4 V at 55 °C. The discharge capacity of ZnO-coated LiMn₂O₄ (117 mAh/g) showed only 3% loss of the initial capacity (121 mAh/g) over 60 cycles. The cycle ability improvement of the spinel LiMn₂O₄ coated with ZnO is demonstrated at high temperatures. From the analysis of electrochemical impedance spectroscopy (EIS), the improvement of cycle ability may be attributed to the suppression on the formation of the passivation film and the reduction of Mn dissolution, which result from the modifying the surface of the spinel LiMn₂O₄ with zinc oxide.     

Effects of temperature on ZnO hybrids grown by metal-organic chemical vapor deposition

The growth of three-dimensional ZnO hybrid structures by metal-organic chemical vapor deposition was controlled through their growth pressure and temperature. Vertically aligned ZnO nanorods were grown on c-plane of sapphire substrate at 600 °C and 400 Torr. ZnO film was then formed in situ on the ZnO nanorods at 100, 600, and 700 °C and 10 Torr. High-resolution X-ray diffraction measurements showed that the ZnO film on the nanorods/sapphire grew epitaxially, and that the ZnO film/nanorods hybrid structures had well-ordered wurtzite structures. The hybrid ZnO structure was shown to be about 3–5 μm by field-emission scanning electron microscopy. The hybrid formed at 600 °C showed better crystalline quality those formed at 100 °C or 700 °C. These structures have potential applicability as nanobuilding blocks in nanodevices.     

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.     

2D SnO2 nanorod networks templated by garlic skins for lithium ion batteries

SnO₂ nanorods with 2D network structure prepared via an effective method of using the skins of garlic bulbs as templates are examined as anode materials for Li-ion battery. The samples are composed of SnO₂ nanorods with length of ca. 80 nm and possess high BET surface area of 163.25 m² g⁻¹. The synthesized SnO₂ electrode shows a high specific capacity (ca. 620 mAh g⁻¹), an excellent cycling stability and a good rate capability but low initial coulombic efficiency. The nanorods characteristics of SnO₂ ensure the fast Li-ion diffusion in the electrode. The loose network structure provides the electrode with a sufficient void space, which sufficiently alleviates the mechanical stress caused by volume change, and thus avoids the electrode to pulverize in the charge–discharge process. Therefore, the obtained SnO₂ electrode shows a good cyclic stability and a high rate performance.     

Sensitization of ZnO nanorods with CdSe quantum dots

We have optimized the low-temperature growth of aligned arrays of zinc oxide nanorods of controlled length and diameter on conductive substrates. Varying the solution concentration and growth time, we were able to tune the nanorod diameter and length in the ranges 40–600 nm and 0.5–15 μm, respectively. The grown zinc oxide nanorods were photosensitized with CdSe quantum dots (QDs) in an oleic shell, which was replaced by pyridine. We studied the optical and transport properties of the ZnO nanorod arrays, with and without CdSe QDs on their surface. The current-voltage characteristics of the ZnO nanorod arrays with CdSe QDs are significantly influenced by illumination with light at a wavelength under the absorption band of the QDs, which points to effective interaction between the QDs and ZnO matrix.     

Influence of pH and HPC concentration on the synthesis of zinc oxide photocatalyst particle from zinc-dust waste by hydrothermal treatment

This work aimed to use the waste zinc-dust from a hot-dip galvanizing plant for the synthesis of nanosized ZnO photocatalyst powder via hydrothermal treatment. ZnO particles with different morphologies and sizes were obtained by varying the solution pH (8–12) and the amount of hydroxypropyl cellulose (HPC) dispersant (0–0.15% (w/v)) under hydrothermal treatment at 170 °C for 8 h. The influence of the preparation conditions on the properties of resultant ZnO particles were evaluated by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, laser light scattering and Brunauer–Emmett–Teller analyses. The solution pH affected the crystallinity, particle morphology and specific surface area of the obtained ZnO, which in turn influenced its photocatalytic activity. The addition of the optimum amount of HPC (0.1% (w/v)) in the starting solution acted as a dispersant to reduce ZnO particle agglomeration but had the opposite effect at higher levels. Moreover, ZnO nanorods with various aspect ratios and a diameter and length range of 20–70 nm and 100–400 nm, respectively, were obtained depending on the amount of added HPC. The photocatalytic activity of the synthesized ZnO powder was improved by the addition of the optimal amount of HPC, and correlated to the particle dispersion and specific surface area.