As-grown Zinc Oxide Research Articles

Environmental-related applications of ZnO nanopowders: Photocatalytic activity and photoluminescence response to ethanol

Zinc oxide (ZnO) and copper-doped zinc oxide (ZnO:Cu) nanopowders were synthesized via solvothermal methods using methanol and hexamethylenetetramine (HMTA). Undoped ZnO nanopowders underwent calcination in O2-rich and H2-rich atmospheres at 600 °C. Samples were studied by scanning electron microscopy, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), and Raman, photoluminescence (PL) and UV–vis absorbance spectroscopies. The doping and calcinations led to a reduction of the optical bandgap of the nanopowders, while their structure remained hexagonal wurtzite with some changes in lattice parameters and average nanoparticle sizes. The Cu2+ doping led to a BET surface area and violet PL component increase. Samples were also examined for environmental related applications, namely as photocatalyzers for dye degradation and as ethanol optical sensors. For photocatalytic activity in methylene blue degradation under UV, H2-rich calcined powders excelled, with a rate constant of −0.076 min−1, surpassing −0.057 min−1 (ZnO:Cu), −0.056 min−1 (O2-rich calcination), and −0.041 min−1 (as-grown ZnO). We propose that this improvement can be attributed to the formation of ZnO/Zn interfaces stemming from the reduction of surface interstitial zinc by H2 during calcination, in addition to the observed reduction of the ZnO bandgap. The doped nanopowders PL also showed excellent response when exposed to ethanol vapor.

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120K when subjected to a sub-bandgap (405nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

UV Sensor Based on Surface Acoustic Waves in ZnO/Fused Silica.

Zinc oxide (ZnO) thin films have been grown by radio frequency sputtering technique on fused silica substrates. Optical and morphological characteristics of as-grown ZnO samples were measured by various techniques; an X-ray diffraction spectrum showed that the films exhibited hexagonal wurtzite structure and were c-axis-oriented normal to the substrate surface. Scanning electron microscopy images showed the dense columnar structure of the ZnO layers, and light absorption measurements allowed us to estimate the penetration depth of the optical radiation in the 200 to 480 nm wavelength range and the ZnO band-gap. ZnO layers were used as a basic material for surface acoustic wave (SAW) delay lines consisting of two Al interdigitated transducers (IDTs) photolithographically implemented on the surface of the piezoelectric layer. The Rayleigh wave propagation characteristics were tested in darkness and under incident UV light illumination from the top surface of the ZnO layer and from the fused silica/ZnO interface. The sensor response, i.e., the wave velocity shift due to the acoustoelectric interaction between the photogenerated charge carriers and the electric potential associated with the acoustic wave, was measured for different UV power densities. The reversibility and repeatability of the sensor responses were assessed. The time response of the UV sensor showed a rise time and a recovery time of about 10 and 13 s, respectively, and a sensitivity of about 318 and 341 ppm/(mW/cm2) for top and bottom illumination, respectively. The ZnO/fused silica-based SAW UV sensors can be interrogated across the fused silica substrate thanks to its optical transparency in the UV range. The backlighting interrogation can find applications in harsh environments, as it prevents the sensing photoconductive layer from aggressive environmental effects or from any damage caused by cleaning the surface from dust which could deteriorate the sensor's performance. Moreover, since the SAW sensors, by their operating principle, are suitable for wireless reading via radio signals, the ZnO/fused-silica-based sensors have the potential to be the first choice for UV sensing in harsh environments.

Double Layer Growth of ZnO Nanorods by a Low Temperature Solution Method: Synthesis and Photoluminescence Properties

In the present work, double layer Zinc Oxide (ZnO) nanorods (NRs) were fabricated hydrothermally and their photoluminescence (PL) properties were investigated. Two different recipes and their combination were used to obtain double layer vertically well-aligned ZnO NRs. These recipes include polyethylenimine (PEI) and citrate as additives in the growth solution resulting long NRs with a broad defect emission and relatively short NRs with a near band-edge ultraviolet (UV) emission, respectively. Double layer growth of long-long (LL), long-short (LS) and short-short (SS) ZnO NRs were considered. Grown samples were annealed in a forming gas atmosphere for a better quality NR structure. LL ZnO NRs (long ZnO NRs grown on a long ZnO NR layer) with a 65 µm thickness showed a broad yellow-orange PL emission and no any near band-edge UV emission was observed. LS ZnO NRs representing short ZnO NRs grown on a long ZnO NR layer (LS1= 36 µm, LS2= 48 µm and LS3= 44 µm) showed an enhanced near band-edge UV emission when compared to that of the long ZnO NRs. The UV intensity was found to decrease with the increase in thickness of the NRs in LS samples. Finally, SS ZnO NR sample, (short ZnO NRs grown on a short ZnO NR layer) which has a thickness of 33 µm, displayed a stronger near band-edge UV emission with a negligible broad emission than that of as-grown SS ZnO NRs (UV peak intensity ratio 59). This study should be important for applications where longer NRs with enhanced PL properties are strictly required.

Decoration of Ag Nanoparticle on ZnO Nanowire by Intense Pulsed Light and Enhanced UV Photodetector

Zinc oxide (ZnO) nanowires (NWs) are wide-bandgap semiconductors that absorb ultraviolet (UV) radiation. Various post-treatment processes have been studied to improve the optical properties of the as-grown ZnO NWs. Among them, Ag nanoparticles (NPs) effectively improved the optical properties on the surface of the ZnO NWs. In this study, ZnO NWs were synthesized via the hydrothermal synthesis method. ZnO NWs were decorated with Ag NPs on the surface of the ZnO NWs in a silver nitrate (AgNO3) aqueous solution by intense pulsed light (IPL) irradiation. Ag NPs were successfully decorated under the following conditions: aqueous AgNO3 solution of 100 nM, an energy of 1 J/cm2, and an exposure time of 8 ms. The responsivity and sensitivity of the ZnO NW UV photodetectors increased by 7.43 and 3.37 times, respectively. The IPL process makes it possible to decorate Ag NPs in a simple manner within an extremely short time.

Optical, Structural, and Crystal Defects Characterizations of Dip Synthesized (Fe-Ni) Co-Doped ZnO Thin Films.

Sol-gel technique is used to synthesize as-grown zinc oxide (ZnO) and iron-nickel (Fe-Ni) co-doped ZnO thin films deposited on glass substrates using dip coating technique. The structural properties and crystal imperfections of as-prepared thin films are investigated. We performed the structural analysis of films using X-ray diffraction (XRD). The XRD analysis reveal that the as-prepared films exhibit wurtzite structure. Furthermore, XRD-line profile analysis is performed to study the correlation between structural properties and imperfections of the nanocomposite thin films. The crystallite size and microstrains parameters are predicted using the Williamson–Hall method. We found that the crystallites size increases as the co-doped (Fe-Ni) concentration is increased. However, microstrains of the nanocomposite films decreases as (Fe-Ni) concentration is increased. The optical properties of the (Fe-Ni) co-doped nanocomposite films are investigated by performing UV-Vis (250 nm–700 nm) spectrophotometer measurements. We found that as the (Fe-Ni) concentration level is steadily increased, transmittance of the undoped ZnO thin films is decreased. Remarkably, refractive index of undoped ZnO thin films is found to exhibit values extending from 1.55 to1.88 that would increase as (Fe-Ni) concentration is increased.

Influence of Seed Layer on Surface Morphology of ZnO Thin Films Grown by SILAR Method

Zinc oxide (ZnO) nanostructured seed layer was grown by successive ionic layer adsorption and reaction (SILAR) method on glass substrate. The as-prepared nanostructured seed layer was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) for its structure and surface morphology. XRD results showed a ZnO seed layer growth oriented along the (100) direction. Study of surface morphology showed ZnO nanocrystal clusters with hexagonal shape. ZnO nanorods (NRs) have been grown over the as-prepared ZnO nanostructured seed layer by immersing seed layer substrate in a chemical bath. It has been found that the morphology of the nanostructured seed layer is a key influencing factor for the growth of vertical ZnO NRs. In our growth method, we were successful in growing vertical NRs with diameter of about 100–240[Formula: see text]nm with perfect hexagonal shape. The transmittance studies were carried out to analyze the optical bandgap of the as-grown ZnO nanorods.

Endurance degradation of solution-processed ZnO polycrystalline film-based resistive switching memory

Metal oxide-based resistive switching memory cell has potential applications in the next generation of nonvolatile memory devices due to its simple structure, fast reading/writing speed, and good endurance. Zinc oxide (ZnO) is one of excellent resistive switching candidate materials owing to its low-cost, simple preparation process and significant resistive switching characteristics. In this work, homogeneous ZnO precursor solution is spin-coated on indium tin oxide (ITO)-coated glass substrate to deposit a smooth and dense film composed by ZnO nanoparticles with the hexagonal wurtzite structure. Atomic force microscopy images reveal the average grain size of ZnO polycrystalline nanoparticle is about 23.7 nm. An optical band gap of about 3.3 eV is derivate from the ultraviolet-visible light absorption spectra. Current-voltage ( I - V ) curves of Ag/ZnO/ITO memory cells exhibit reversible low-voltage bipolar resistive switching characteristics. Their switching voltages ( V Set/ V Reset) are lower than ± 0.4 V, and the ON/OFF resistance ratio is 103–104 at the read voltage of −0.1 V, which are superior to resistive switching performance of other ZnO polycrystalline films prepared via similar solution methods. However, the memory cell can only maintain 60 stable I - V cycles. After that, the low resistance state gradually degrades to the high resistance state, and the resistive switching memory window completely disappears after 100 I - V cycles. X-ray photoelectron spectra of Zn 2p and O 1s core-levels for ZnO film before and after I - V measurement demonstrate that as-grown film is oxygen-defective ZnO x I - V measurement. Under sustained electric field stress, some drifted oxygen ions combine with oxygen vacancies and change to localized lattice oxygen atoms, this partially remedies the oxygen-deficiency of as-grown non-stoichiometric ZnO x

Synthesis of Ag Nanoparticle-Decorated ZnO Nanorods Adopting the Low-Temperature Hydrothermal Method

Vertically aligned and highly dense Zinc oxide (ZnO) nanorods (NRs) have been successfully synthesized by a two-step hydrothermal method and decorated by silver (Ag) nanoparticles (NPs) via a dip coating technique. Absorption spectra indicate the presence of metal Ag NPs. The photoluminescence (PL) spectrum of as-grown ZnO nanorods shows ultra violet (UV) emission centered around 390 nm and a sharp defect-related emission peak around 580 nm. The presence of Ag NPs on the ZnO NRs shows a significant red shift in PL peak position in the visible region and a complete quenching of UV emission. The changes in UV–Vis and PL spectra of ZnO NRs decorated with Ag metal NPs are studied and discussed.

Simulation study on piezoelectric characteristics of two‐dimensional ZnO nanodiscs

Two-dimensional (2D) zinc oxide (ZnO) nanostructures have emerged as an attractive approach for constructing high-performance piezoelectric nanogenerators (NGs) because of their high rate of surface area to volume and good mechanical durability for applications in energy conversion devices. However, there has been to date no comprehensive study on piezoelectric characteristics of 2D ZnO nanostructures. In this paper, 2D nanodiscs have been prepared by aqueous solution method on flexible indium tin oxide coated polyethylene terephthalate substrates. Also, the piezoelectric characteristics are statistically demonstrated and analysed on the as-grown 2D ZnO nanodiscs by applying different external force with the finite element method (COMSOL Multiphysics). Besides, the size-dependent simulation analysis of piezopotentials is also conducted for further optimising output performance of NGs based on 2D ZnO nanostructures. Also, the simulation will provide a guideline for designing high-performance piezo-nanodevices.

Enhanced Ultraviolet Emission from Hydrothermally Grown ZnO Nano-Grass on Si Substrate

Highly crystalline one-dimensional zinc oxide (ZnO) nano-grass was grown on silicon (Si) substrate by a modified hydrothermal method. A predominantly c-oriented ZnO nano-grass with an average diameter of 40–60 nm and length of 1.5–2.0 μm was obtained. From the photoluminescence (PL) measurement, we observed a defect-free, intense ultraviolet emission of as-grown ZnO nano-grass, confirming the absence of singly ionized oxygen vacancies. The absence of green deep-level emission in the PL spectrum further implies a high crystallinity of as-grown ZnO nano-grass. The high-quality ZnO nano-grass has potential applications in single nanowire-based light-emitting diodes, solar cells, themoresistive sensing, photocatalysis, ultraviolet photodetectors, optical switches, waveguides and nano-lasers.

A Simple, One-step, Seedless Hydrothermal Growth of ZnO Nanorods on Printed Circuit Board Substrate

Abstract: High quality, high density, and well-aligned zinc oxide (ZnO) nanorods have been synthesized on cost-effective printed circuit board (PCB) substrates via a simple, seedless, one-step, low-temperature hydrothermal method based galvanic cell structure. It was found that the outer diameters of the ZnO nanorods range from 50 nm to 400 nm. The as-grown ZnO nanorods prefer to grow along the c axis. The morphologies of the ZnO nanorods were investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD). The crystallinity properties were characterized by Raman spectroscopy and photoluminescence (PL) spectroscopy.

Zinc Oxide Nanorods Grown on Printed Circuit Board for Extended-Gate Field-Effect Transistor pH Sensor

Zinc oxide (ZnO) nanorods (NRs) were grown directly on printed circuit boards with a 35-μm-thick copper layer using a seedless galvanic-cell hydrothermal process. The hexagonal structure of the synthesized ZnO NRs was observed by scanning electron microscopy. The microstructural characteristics of the as-grown ZnO NRs were investigated by x-ray diffraction analysis, revealing preferred (002) growth direction. Raman and photoluminescence spectra confirmed the high crystalline quality of the ZnO NRs. As-grown ZnO NRs were then grown for 7 h using the galvanic effect for use as the pH membrane of an extended-gate field-effect transistor pH sensor (pH-EGFET). The current–voltage characteristics showed sensitivity of 15.4 mV/pH and 0.26 (μA)1/2/pH in the linear and saturated region, respectively. Due to their cost effectiveness, low-temperature processing, and ease of fabrication, such devices are potential candidates for use as flexible, low-cost, disposable biosensors.

A High-Sensitivity Gas Sensor Toward Methanol Using ZnO Microrods: Effect of Operating Temperature

In the present work, zinc oxide (ZnO) microrods with the average diameter of 350 nm have been synthesized on fluorine doped tin oxide (FTO) substrate using a hydrothermal reaction process at a low temperature of 90°C. The methanol gas sensing behaviour of as-synthesized ZnO microrods have been studied at different operating temperatures (100–300°C). The gas sensing results show that the ZnO microrods exhibit excellent sensitivity, selectivity, and stability toward methanol gas at 300°C. The as-grown ZnO microrods sensor also shows the good sensitivity for methanol even at a low operating temperature of 100°C. The ultra-high sensitivity of 4.41 × 104% [gas sensitivity, S g = (I g − I a)/I a × 100%] and 5.11 × 102% to 100 ppm methanol gas at a temperature of 300°C and 100°C, respectively, has been observed. A fast response time of 200 ms and 270 ms as well as a recovery time of 120 ms and 1330 ms to methanol gas have also been found at an operating temperature of 300°C and 100°C, respectively. The response and recovery time decreases with increasing operation temperature of the sensor.

Illumination effects on the ferroelectric properties of zinc oxide films grown by DC-unbalanced magnetron sputtering

We study the illumination effect on the ferroelectric properties of zinc oxide (ZnO) film grown by DC-unbalanced magnetron sputtering. We focus on the P–E hysteresis response of the as-grown ZnO (ag-ZnO) and annealed-ZnO (ann-ZnO) films under dark and light conditions. The measurement of ferroelectric properties is performed by driving a positive voltage on the top-side of the films. Under the dark condition, a strong P–E response is observable on the ann-ZnO film due to the structural enhancement. The value of electrical coercivity for ferroelectric polarization is strongly related to the light illumination. The illumination treatment changed the P–E hysteresis of the ZnO films from symmetric to asymmetric. We found that higher energy illumination promotes a higher electric coercivity. These results confirmed that ferroelectric properties could be effectively tailored by tuning the energy of the light source. This interrelated electrical and optical properties is an important phenomenon to design a new light-induced non-volatile device application.

Investigation of the Influence of the As-Grown ZnO Nanorods and Applied Potentials on an Electrochemical Sensor for In-Vitro Glucose Monitoring

The influence of the as-grown zinc oxide nanorods (ZnO NRs) on the fabricated electrochemical sensor for in vitro glucose monitoring were investigated. A direct growth of ZnO NRs was performed on the Si/SiO2/Au electrode, using hydrothermal and sol-gel techniques at low temperatures. The structure, consisting of a Si/SiO2/Au/GOx/Nafion membrane, was considered as a baseline, and it was tested under several applied potential 0.1–0.8 V. The immobilized working electrode, with GOx and a nafion membrane, was characterized amperometrically using a source meter Keithely 2410, and an electrochemical impedance Gamry potentiostat. The sensor exhibited the following: a high sensitivity of ~0.468 mA/cm2 mM, a low detection limit in the order of 166.6 µM, and a fast and sharp response time of around 2 s. The highest sensitivity and the lowest limit of detection were obtained at 0.4 volt, after the growth of ZnO NRs. The highest net sensitivity was obtained after subtracting the sensitivity of the baseline, and it was in the order of 0.315 mA/cm2·mM. The device was tested with a range of glucose concentrations from 1–10 mM, showing a linear line from 3–8 mM, and the device was saturated after exceeding high concentrations of glucose. Such devices can be used for in vitro glucose monitoring, since glucose changes can be accurately detected.

Low-temperature rapid syntheses of high-quality ZnO nanostructure arrays induced by ammonium salt

We have developed a simple ammonium ion-assisted hydrothermal method for the fast preparation of high quality Zinc Oxide (ZnO) nanorod arrays. Ammonium salts were introduced into typically hydrothermal growth solutions formed from Zinc acetate (ZnAc2) and hexamethylenetetramine (HMTA). Scanning electron microscope (SEM), X-ray diffractometer (XRD), High resolution transmission electron microscopic (HRTEM) and photoluminescence (PL) measurements revealed that the growth rate of ZnO nanorods was promoted by adding ammonium salts and the as-grown ZnO nanostructure arrays showed remarkably low defect density. Upon addition of ammonium salt to the hydrothermal reaction solution, complex Zn(NH3)42+ was formed by Zn2+ capturing NH3 molecules dissociated from HMTA. The improvement of growth rate and optical property of the ZnO nanostructure arrays was attributed to positively charged Zn(NH3)42+ attracting oxygen rich OH- in the growth sites of ZnO for hydrolysis reaction and simultaneously inhibiting the defect. Our results demonstrated that ammonium salt can act as a new paradigm to control morphology and quality of the ZnO microstructures.

Influence of annealing temperature on morphological, optical and UV detection properties of ZnO nanowires grown by chemical bath deposition

Chemical solution deposition is a low-temperature method with possibly the lowest-cost for growing zinc oxide (ZnO) nanowires (NWs) on an amorphous glass substrate. A ZnO seed layer is needed to initialize the uniform growth of oriented nanowires. In the present work, we deposited the seed layer onto the glass substrate using radio frequency magnetron sputtering system. The ZnO NWs were fabricated on the seeded substrate by immersing them in a chemical bath containing zinc nitrate and hexamethylenetetramine (HTMA) aqueous solution. Several properties of as-grown ZnO NWs and annealed ZnO NWs at different temperatures in pure oxygen atmosphere were investigated. Well-defined ZnO nanowires almost perpendicular to substrate surface were observed through field emission scanning electron microscopy micrographs. X-ray diffraction measurements showed that all ZnO samples possessed a typical wurtzite structure with high crystallinity and no other impurity phases. The results of UV–Visible spectroscopy and room temperature photoluminescence implied that increasing the annealing temperature could have a significant influence on the performance of samples. The ZnO-based interdigitated metal–semiconductor–metal (MSM) was produced using the thermal evaporation of silver contacts. The results of current–voltage and transient response measurements demonstrate that despite using the low-cost thin film techniques, high responsivity was obtained.

Vertically aligned zinc oxide nanowires electrodeposited within porous polycarbonate templates for vibrational energy harvesting

A piezoelectric nanogenerator has been fabricated using a simple, fast and scalable template-assisted electrodeposition process, by which vertically aligned zinc oxide (ZnO) nanowires were directly grown within a nanoporous polycarbonate (PC) template. The nanowires, having average diameter 184 nm and length 12 μm, are polycrystalline and have a preferred orientation of the [100] axis parallel to the long axis. The output power density of a nanogenerator fabricated from the as-grown ZnO nanowires still embedded within the PC template was found to be 151 ± 25 mW m−3 at an impedance-matched load, when subjected to a low-level periodic (5 Hz) impacting force akin to gentle finger tapping. An energy conversion efficiency of ∼4.2% was evaluated for the electrodeposited ZnO nanowires, and the ZnO–PC composite nanogenerator was found to maintain good energy harvesting performance through 24 h of continuous fatigue testing. This is particularly significant given that ZnO-based nanostructures typically suffer from mechanical and/or environmental degradation that otherwise limits their applicability in vibrational energy harvesting. Our template-assisted synthesis of ZnO nanowires embedded within a protective polymer matrix through a single growth process is thus attractive for the fabrication of low-cost, robust and stable nanogenerators.

Effect of Zinc Nitrate Concentration on the Optical and Morphological Properties of ZnO Nanorods for Photovoltaic Applications.

We report the effect of zinc nitrate (ZN) concentration on the growth of zinc oxide (ZnO) nanorods and their optical and morphological properties. As prepared ZnO nanorods on glass substrate were characterized using field emission scanning electron microscopy (FE-SEM), ultra violet-visible (UV-Vis), Raman and Photo-luminescence (PL) spectroscopy. FE-SEM results show that the nanorods were obtained for the 0.033 and 0.053 M concentration of ZN. As the ZN concentration increased from 0.033 M to 0.053 M, the diameter of the nanorods was increased. It indicated that the diameter of the nanorods was affected by the ZN concentration. The Raman spectra of nanorods show only one peak at 438 cm(-1) corresponding to E2(high) high mode, which means that ZnO nanorods grown perpendicularly on the glass substrate, i.e., the ZnO nanorod arrays are highly c-axis oriented. Room-temperature PL spectrum of the as-grown ZnO nanorods reveals a near-band-edge (NBE) emission peak and defect induced green light emission. The green light emission band at -579 nm might be attributed to surface oxygen vacancies or defects. The UV-visible measurements reflect that the total transmittance for the as grown ZnO nanorods is over 80%. The simple technique presented in this study to grow ZnO nanorods on a glass substrate can be helpful for making the cost effective photovoltaic devices.