Bare ZnO Nanorods Research Articles

Self-Assembly of NiO-Coated ZnO Nanorod Electrodes with Core–Shell Nanostructures as Anode Materials for Rechargeable Lithium-Ion Batteries

A porous nickel hydroxide shell was self-assembled by hydrolysis of aqueous nickel chloride in the presence of hexagonal ZnO nanorod template at room temperature. The nickel hydroxide shell converted to cubic NiO after heat treatment at 500 °C. Galvanostatic charge and discharge results indicated that the NiO-coated ZnO nanorod electrode is capable of delivering a higher capacity than the bare ZnO nanorod and carbon-coated ZnO nanorod electrodes especially in high-rate charge and discharge processes. The presence of porous NiO shell might prevent the disintegration of ZnO nanorods because of the large volume change during charge and discharge. In addition, the porous NiO shell ensured good electrical contact of ZnO with the current collector and facilitated the charge transfer and transport of lithium ions. The amount of NiO coated on ZnO nanorods significantly affected the electrochemical performance of ZnO electrode toward lithium. Insufficient NiO shell would not provide enough electrical conductivity and protection against disintegration. When the NiO content was higher than 32.6 wt %, the electrode performance could be significantly improved.

Double-sided ZnO nanorod arrays on single-crystal Ag holed microdisks with enhanced photocataltytic efficiency

Novel hierarchical heterostructures of double-sided ZnO nanorod (NR) arrays grown on single-crystal Ag holed microdisks (HMDs) have been prepared through a two-step aqueous strategy including ZnO seed loading and the subsequent heteroepitaxial growth of ZnO NRs on Ag HMDs. By simply adjusting the synthetic parameters, ZnO NRs with variable NR diameters (20-200 nm), lengths (100-1.8 μm) and unusual shapes (concave, tubular and sharp tips) on Ag HMDs have been realized, which endows the Ag/ZnO heterostructures with versatile morphologies. The novel Ag/ZnO heterostructures consisting of integrated 1D semiconductor/2D metal nanostructured blocks with high specific surface area (SSA) and opened spatial architectures may promise important applications related to photoelectric fields. As expected, in photocatalytic measurements, the typical Ag HMD/ZnO NR heterostructure exhibits superior catalytic activity over other catalysts of bare ZnO NRs, ZnO NR arrays or heterostructured Ag nanowires (NWs)/ZnO NRs. The synergistic effect of the unique Ag HMD/ZnO NR heterostructures contributing to the high catalytic performance has been discussed in detail.

Cu-doping ZnO/ZnS nanorods serve as the photoanode to enhance photocurrent and conversion efficiency

Cu-doped ZnO/ZnS (Cu/ZnO/ZnS) nanorods were prepared via sol-gel and hydrothermal deposition routes. The target product can serve as the photoanode of the dye-sensitized solar cells. Four nanostructures (ZnO, Cu/ZnO, ZnO/ZnS and Cu/ZnO/ZnS nanorods) were successfully synthesized and characterized. For the Cu/ZnO/ZnS structure, both the Cu-doping and ZnS coating can enhance photocurrent and conversion efficiency compared with the bare ZnO nanorods. The promoting effect of the Cu-doping on the photoanode could be attributed to the formation of intermediate energy level that allows Cu/ZnO to be activated easily in the visible area. The ZnS compact layer retards the back transfer of electrons to the dye and electrolyte, decreasing the composite of them and increasing the final photocurrent. Hence, the Cu/ZnO/ZnS one obtains the maximum efficiency 1.40%, and the Jsc and Voc were 5.7mAcm^-^2 and 631mV, respectively.

Controlled synthesis of carbon nanostructures using aligned ZnO nanorods as templates

By combining in situ X-ray photoemission spectroscopy, ex situ high resolution transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy, we show that chemical vapor deposition (CVD) on vertically aligned ZnO nanorods can synthesize different carbon nanostructures (CNs), whose morphology is driven by the ZnO nanorods and whose dimensions and structures change as a function of the process temperature. The CNs range from amorphous carbon cups, completely covering the nanorods, to high density one-dimensional carbon nano-dendrites (CNDs), which start to appear like short hairs on the ZnO nanorods. The nanorods are partially etched when the process is done at 630–800°C, while they are completely etched at temperatures higher than 800°C. In the latter case, CNDs emerge from a porous carbon sponge formed at the substrate interface but they are preferentially aligned along the location of the pristine ZnO nanorods. When used as a chemiresisitor the CND–ZnO structures have a higher sensitivity to ammonia compared to chemiresistors made by bare ZnO nanorods, to other one-dimensional CNs, like carbon nanotubes or other metal/metal-oxides hybrid CNs.

A facile, coverage controlled deposition of Au nanoparticles on ZnO nanorods by sonochemical reaction for enhancement of photocatalytic activity

This study reports the synthesis of Au/ZnO heterostructured nanorods by a facile sonochemical method and their improved photocatalytic activity. A simple and fast deposition of gold nanoparticles on ZnO nanorods was performed without any surfactants or additives. Through various analyses (SEM, XRD, TEM, and EELS), we confirmed the formation of a highly crystalline Au/ZnO nanostructure; several tens of nanometer-sized polycrystalline Au nanoparticles were uniformly deposited on the surface of ZnO nanorods. The size and the density of Au nanoparticles could be controlled by the modulation of the concentration of the gold ion precursor and the solvent compositions. Formation of smaller Au nanoparticles with a lower surface coverage was induced at a lower concentration of the gold ion source with a solvent composition ratio of 1:4 of D.I. water to ethanol. Compared with bare ZnO nanorods, Au/ZnO heterostructures exhibited enhanced photocatalytic activity in the photodegradation of an organic dye, which is caused by the Au nanoparticles acting as an electron sink and lowering the local work function.

CuO/ZnO Heterostructured Nanorods: Photochemical Synthesis and the Mechanism of H2S Gas Sensing

This study reports on the structural properties of CuO/ZnO nanorods and the mechanism by which bundles of these nanorods are able to sense H2S gas. The CuO/ZnO nanorods were prepared by deposition of CuO nanostructures on the hydrothermally grown ZnO nanorods using a photochemical method. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to confirm that the heterogeneous nanostructure of the CuO/ZnO nanorods was highly crystalline. The H2S gas sensing properties of CuO/ZnO nanorod bundles was evaluated in air containing dilute H2S gas at sensing temperatures (Ts) ≤ 500 °C. The response of CuO/ZnO nanorod sensors to H2S gas was enhanced compared to that of bare ZnO nanorods. The heterostructured nanorods showed an exponential increase in sensing tendency with Ts. X-ray photoelectron spectroscopy analysis results indicated that the enhanced response of CuO/ZnO nanorod sensors is due to the chemical conversion of CuO into CuS upon H2S exposure, w...

A Quasi‐Liquid Iontronic–Electronic Light‐Harvesting Hybrid Photodetector with Giant Response

A heterostructure formed by a layer of agarose gel drop-cast on a hydrothermally grown layer of ZnO nanorods on fluorine-doped tin oxide (FTO)-coated glass is examined for photoresponse with a top platinum tip contact. This ionic-gel-based hybrid device shows three orders of magnitude higher photocurrent as compared to the case of bare ZnO nanorods film.

Electrochemical properties of TiO2 encapsulated ZnO nanorod aggregates dye sensitized solar cells

Dye sensitized solar cells based on TiO2 encapsulated ZnO nanorod (NR) aggregates were fabricated and electrochemical performance was analyzed using impedance spectroscopy as a function of forward bias voltage. Charge transfer properties such as electron life time (τn), electron diffusion coefficient (Dn) and electron diffusion length (Ln) were calculated in order to ensure the influence of TiO2 layer over the ZnO NR aggregates. It is found that the short circuit current density (Jsc=5.8mAcm−2), open circuit potential (Voc=0.743V), fill factor (FF=0.57) and conversion efficiency are significantly improved by the introduction of TiO2 layer over ZnO photoanode. A power conversion efficiency of about 2.48% has been achieved for TiO2/ZnO cell, which is higher than that of bare ZnO NR aggregate based cells (1.73%). The formation of an inherent energy barrier between TiO2 and ZnO films and the passivation of surface traps on the ZnO film caused by the introduction of TiO2 layer increase the dye absorption and favor the electron transport which may be responsible for the enhanced performance of TiO2/ZnO cell.

Effect of oxygen-related surface adsorption on the efficiency and stability of ZnO nanorod array ultraviolet light-emitting diodes

Ultraviolet light-emitting diodes using MgZnO-coated and bare ZnO nanorod arrays as active layers were manufactured. Both types were exposed to ambient air over a 1-yr period to assess their stability. By monitoring the electroluminescence evolution with air-exposure time and comparing the changes of electroluminescence and x-ray photoelectron spectra before and after vacuum desorption, it is concluded that surface-adsorbed O2 and OH− species, as acceptor and donor surface states, quench ultraviolet electroluminescence, and favor undesirable surface-mediated nonradiative and deep-level recombination. The MgZnO coating prevents surface adsorption, and so the coated nanorod device shows higher efficiency and stability than the uncoated one.

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80–150nm and length ~2μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375nm and a broad defect-related emission band centered at ~596nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50s. The enhancement factor reached a maximum value for the Au sputtering time of 50s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.

ZnO nanorod/CdS nanocrystal core/shell-type heterostructures for solar cell applications

ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method.Single-crystalline ZnO nanorod arrays were first electrochemically grown onSnO2:F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, usingthe successive ion layer adsorption and reaction (SILAR) technique, to form core/shellnanocable architectures. Structural, morphological and optical properties of the nanorodheterojunctions were investigated. The results indicate that CdS single-crystalline domainswith a mean diameter of about 7 nm are uniformly and conformally covered on the surfaceof the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nmcorresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed.The dispersion in this value may originate in quantum confinement inside thenanocrystalline material. The appearance of both edges corresponds with the separation ofZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. Thephotovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrayshas been investigated as solar cell photoanodes in a photoelectrochemical cellunder white illumination. In comparison with bare ZnO nanorod arrays, a 13-foldenhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.

A novel gas ionization sensor using Pd nanoparticle-capped ZnO

A novel gas ionization sensor using Pd nanoparticle-capped ZnO (Pd/ZnO) nanorods as the anode is proposed. The Pd/ZnO nanorod-based sensors, compared with the bare ZnO nanorod, show lower breakdown voltage for the detected gases with good sensitivity and selectivity. Moreover, the sensors exhibit stable performance after more than 200 tests for both inert and active gases. The simple, low-cost, Pd/ZnO nanorod-based field-ionization gas sensors presented in this study have potential applications in the field of gas sensor devices.

Synergistic effects of SPR and FRET on the photoluminescence of ZnO nanorodheterostructures

Solution-grown ZnO nanorods (NRs) were successfully conjugated withCdSe/ZnS quantum dots (QDs) and Ag nanoparticles (NPs) to suppress intrinsicdefect emission and to enhance band-edge emission at the same time. First,high-density and high-crystallinity ZnO NRs of diameter 80–90 nm and length 1.2–1.5 µm were grown on glass substrates using a low-temperature seed-assisted solution method. Theas-synthesized ZnO NRs showed sharp photoluminescence (PL) band-edge emission centered at ∼ 377 nm together with broad defect emission in the range of ∼ 450–800 nm. The ZnO NRs were decorated with CdSe/ZnS QDs and Ag NPs, respectively, bysequential drop-coating. The PL of CdSe/ZnS NR conjugates showed that ZnO band-edge emission decreased by 73.8% due tofluorescence resonance energy transfer (FRET) and charge separation between ZnO andCdSe/ZnS by type II energy band structure formation. On the other hand, Ag NR conjugates showed increased band-edge emission (by 25.8%) and suppressed defectemission compared to bare ZnO NRs. A possible energy transfer mechanism to explain theimproved PL properties of ZnO NRs was proposed based upon the combined effects ofFRET and surface plasmon resonance (SPR).

Synthesis and photoluminescence properties of the ZnO@SnO2 core–shell nanorod arrays

The ZnO@SnO2 core–shell nanorod arrays have been synthesized. As the cores, ZnO nanorod arrays were first prepared by aqueous chemical growth method. Then using a simple liquid-phase deposition method, SnO2 was deposited on the ZnO nanorod arrays. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to characterize the morphologies and structures of the products. Photoluminescence properties were also investigated. It was found that the ZnO@SnO2 core–shell nanorod arrays showed enhanced UV and green emissions when compared with the bare ZnO nanorod arrays.

Effect of the TiO 2 shell thickness on the dye-sensitized solar cells with ZnO–TiO 2 core–shell nanorod electrodes

Well-aligned ZnO nanorod arrays with high aspect ratio have been grown on FTO substrate by hydrothermal process. FTO substrate was pre-modified with ZnO thin film as a seed layer by sol–gel coating. TiO 2 thin shells with different thicknesses were grown on the ZnO nanorods by chemical vapor deposition method. Sample characterization was performed by X-ray diffraction (XRD), electron microscopy (FESEM, TEM, and SAD), X-ray photoelectron spectroscopy (XPS) and UV–vis Spectroscopy. XRD and TEM analyses indicated a wurtzite structured with high crystallinity and confirmed that each individual ZnO nanorod was a single crystal. The photoelectrochemical experiments were performed in a sandwich type two-electrode cell and I– V curves were obtained. Result of solar cell testing showed that addition of TiO 2 shells to ZnO nanorod significantly increased the J SC , V OC , fill factor and efficiency relative to devices without TiO 2 shells. Overall cell efficiency jumped from 0.45% for bare ZnO nanorod array to 0.92% for 14 nm thick TiO 2 shells on ZnO which showed a twofold increase. The fill factor increased from 0.37 to 0.60, showing a 62% improvement. For a shell with thickness of 21 nm, V OC increased by about 150 mV to 0.68 V. The results showed that it is possible to fabricate core–shell cells of higher efficiency by using nanorod arrays and other morphologies with larger surface area.

Randomly-grown high-dielectric-constant ZnO nanorods for near-field enhanced Raman scattering

We investigate the dependence of the size parameter in the Mie scattering theory on the near-field enhanced Raman scattering properties for high dielectric constant ZnO nanorods grown randomly by PLD (pulsed laser deposition). High Raman signals of Rhodamine 6G (R6G) at 532 nm excitation wavelength were observed with nanorods of 400 nm average diameter. This experimental result was explained theoretically by the size parameter described in the Mie scattering theory, not by surface plasmon polaritons. This was also confirmed by the near-field distribution calculated by the FDTD (Finite-Difference Time Domain) method. The ZnO nanorods with 400 nm average diameter can detect as low as 1 μM of R6G. This near-field enhancement factor is equivalent to that with 10-nm-thick gold-coated ZnO nanorods (nanoshells) with an average core diameter of 100 nm. Controlling the diameter of bare ZnO nanorods is effective for obtaining large enhancement factors without an additional process of gold thin film coating on them.

ZnO and TiO 2 1D nanostructures for photocatalytic applications

ZnO and TiO 2 1D nanostructures (nanorods and nanotubes) were prepared by low-cost, low-temperature, solution-based methods and their properties and photocatalytic performance were studied. ZnO nanorod samples with titania and alumina shells were also prepared by solution-based methods, and their properties and photocatalytic performance were compared to that of bare ZnO nanorods. We found that ZnO and TiO 2 exhibited comparable photocatalytic performance. Faster dye degradation under simulated solar illumination was observed for ZnO, while under UV illumination faster degradation was observed for TiO 2. ZnO nanorods with titania shells exhibited inferior photocatalytic performance, while for alumina shells the performance was similar to bare ZnO. Reasons for observed differences are discussed, and the effect of the shell on photocatalytic activity is attributed to the changes in native defects at the ZnO surface/shell interface.

Improvement of chemical stability of aqueous solution grown ZnO nanorods by aminosilane modification

AbstractAiming at stable biosensing operation, aqueous‐grown ZnO nanorods on sapphire substrates were examined their chemical stability against the immersion in HCl solution. Density and diameter of the ZnO nanorods were remained but their height was shortened when immersed with aminosilane modification while bare ZnO nanorods were completely disappeared after 5 min immersion, indicating that the aminosilane modification is useful to improve the chemical stability of ZnO nanolods even though the membrane by the silanezation process is not fully stable to HCl solution. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solution Based Synthesis of ZnO/CdS Composite Nanorod Array Film and Its Photoelectric Properties

Well-aligned ZnO/CdS composite nanorod array film was grown on an indium tin oxide(ITO) substrate by two-step chemical solution deposition method.The effects of CdS deposition time on the crystal structure,morphology,and photoelectric performance of the ZnO/CdS composite film were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),ultraviolet-visible absorption spectroscopy(UV-Vis),photoluminescence spectroscopy(PL),and surface photovoltage spectroscopy(SPS).Results showed that the absorbance of the composite film extended into the visible region compared with the bare ZnO nanorod arrays.SPS also showed a new response region corresponding to the absorption spectrum.This result indicated a remarkable photoelectric conversion efficiency improvement in the visible region.We also found that the SPS response intensity of the composite film decreased gradually above 383 nm with an increase in CdS deposition time.However,the SPS response intensity increased below 383 nm.We interpreted this phenomenon using two distinct photoinduced charge generation and transfer mechanisms.

Interfacial Dynamics of Perylene Derivatives Attached to Metal Oxide Particle and Nanorod Films

AbstractPhoto-induced electron transfer and recombination was investigated for prototype molecular absorbers covalently bound to metal-oxide films. Two perylene derivatives with different anchor/bridge groups, a propionic and an acrylic acid, were adsorbed on both bare ZnO nanorods and TiO2 coated nanorods and the dynamics of the systems compared.