Electrodeposition of ZnO Nanorod Arrays on Transparent Conducting Substrates–a Review

Enhanced Field Emission and Low-Pressure Hydrogen Sensing Properties from Al-N-Co-Doped ZnO Nanorods

ZnO nanostructures show great potential in hydrogen sensing at atmospheric conditions for good gas adsorption abilities. However, there is less research on low-pressure hydrogen sensing performance due to its low concentration and in-homogeneous distributions under low-pressure environments. Here, we report the low-pressure hydrogen sensing by the construction of Al-N-co-doped ZnO nanorods based on the adsorption-induced field emission enhancement effect in the pressure range of 10-7 to 10-3 Pa. The investigation indicates that the Al-N-co-doped ZnO sample is the most sensitive to low-pressure hydrogen sensing among all ZnO samples, with the highest sensing current increase of 140% for 5 min emission. In addition, the increased amplitude of sensing current for the Al-N-co-doped ZnO sample could reach 75% at the pressure 7 × 10-3 Pa for 1 min emission. This work not only expands the hydrogen sensing applications to the co-doped ZnO nanomaterials, but also provides a promising approach to develop field emission cathodes with strong low-pressure hydrogen sensing effect.

Vertically Aligned Nanowires and Quantum Dots: Promises and Results in Light Energy Harvesting

The synthesis of crystals with a high surface-to-volume ratio is essential for innovative, high-performance electronic devices and sensors. The easiest way to achieve this in integrated devices with electronic circuits is through the synthesis of high-aspect-ratio nanowires aligned vertically to the substrate surface. Such surface structuring is widely employed for the fabrication of photoanodes for solar cells, either combined with semiconducting quantum dots or metal halide perovskites. In this review, we focus on wet chemistry recipes for the growth of vertically aligned nanowires and technologies for their surface functionalization with quantum dots, highlighting the procedures that yield the best results in photoconversion efficiencies on rigid and flexible substrates. We also discuss the effectiveness of their implementation. Among the three main materials used for the fabrication of nanowire-quantum dot solar cells, ZnO is the most promising, particularly due to its piezo-phototronic effects. Techniques for functionalizing the surfaces of nanowires with quantum dots still need to be refined to be effective in covering the surface and practical to implement. The best results have been obtained from slow multi-step local drop casting. It is promising that good efficiencies have been achieved with both environmentally toxic lead-containing quantum dots and environmentally friendly zinc selenide.

Controlled Growth of Semiconducting ZnO Nanorods for Piezoelectric Energy Harvesting-Based Nanogenerators

Zinc oxide (ZnO) nanorods have attracted considerable attention in recent years owing to their piezoelectric properties and potential applications in energy harvesting, sensing, and nanogenerators. Piezoelectric energy harvesting-based nanogenerators have emerged as promising new devices capable of converting mechanical energy into electric energy via nanoscale characterizations such as piezoresponse force microscopy (PFM). This technique was used to study the piezoresponse generated when an electric field was applied to the nanorods using a PFM probe. However, this work focuses on intensive studies that have been reported on the synthesis of ZnO nanostructures with controlled morphologies and their subsequent influence on piezoelectric nanogenerators. It is important to note that the diatomic nature of zinc oxide as a potential solid semiconductor and its electromechanical influence are the two main phenomena that drive the mechanism of any piezoelectric device. The results of our findings confirm that the performance of piezoelectric devices can be significantly improved by controlling the morphology and initial growth conditions of ZnO nanorods, particularly in terms of the magnitude of the piezoelectric coefficient factor (d33). Moreover, from this review, a proposed facile synthesis of ZnO nanorods, suitably produced to improve coupling and switchable polarization in piezoelectric devices, has been reported.

Substrate-Enabled Room-Temperature Electrochemical Deposition of Crystalline ZnMnO3

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials' composition, crystal structure and morphology are urgently needed. Here we report on the room-temperature electrodeposition of a ternary oxide based on earth-abundant metals, specifically, the defective cubic spinel ZnMnO₃ . In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO₄ ^- in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO₃ . Epitaxial growth of ZnMnO₃ on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2-dimensional morphology of the electrodeposit forming - after partial delamination from the substrate - twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder-free nanocomposite electrodes.

Investigation of Photoelectrochemical Performance under the Piezoelectric Effect Based on Different Zinc Oxide Morphologies

Recently, the piezoelectric effect has been widely used in photoelectrochemical (PEC) water splitting, and the morphology of the piezoelectric material is a critical factor affecting the piezo-photoelectrochemical water splitting performance. Herein, we explored the mechanism of the piezo-photoelectrochemical performance of zinc oxide (ZnO) that is affected by the morphology. Firstly, three different ZnO nanostructures (nanosheets, nanorods, and nanospheres) were synthesized by the electrodeposition, hydrothermal, and sol-gel methods, respectively. Then, the measurements of PEC water splitting performance under the piezoelectric effect revealed a 3-fold increase for the ZnO nanosheets, a 1.4-fold increase for the nanorods, and a 1.2-fold increase for the nanospheres compared to no piezoelectric effect. Finally, finite element simulation showed that nanosheets generated the highest piezoelectric potential (0.6 V), followed by nanorods (0.2 V), and nanospheres the lowest (0.04 V). Thus, among the three morphologies, the ZnO nanosheets exhibited a great improvement in PEC performance under the piezoelectric effect. The great improvement is due to the non-axial vertical homogeneous growth of the ZnO nanosheets, subjecting them to the highest effective deformation stress, which enables the ZnO nanosheets to produce the highest piezoelectric potential to accelerate the carrier separation and limit the recombination of photoelectrons and holes. This work serves as a guide for developing various photoelectrodes that are used in piezo-photoelectrochemical water splitting.

Nanomaterials as Photocatalysts-Synthesis and Their Potential Applications

Increasing demand for energy and environmental degradation are the most serious problems facing the man. An interesting issue that can contribute to solving these problems is the use of photocatalysis. According to literature, solar energy in the presence of a photocatalyst can effectively (i) be converted into electricity/fuel, (ii) break down chemical and microbial pollutants, and (iii) help water purification. Therefore, the search for new, efficient, and stable photocatalysts with high application potential is a point of great interest. The photocatalysts must be characterized by the ability to absorb radiation from a wide spectral range of light, the appropriate position of the semiconductor energy bands in relation to the redox reaction potentials, and the long diffusion path of charge carriers, besides the thermodynamic, electrochemical, and photoelectrochemical stabilities. Meeting these requirements by semiconductors is very difficult. Therefore, efforts are being made to increase the efficiency of photo processes by changing the electron structure, surface morphology, and crystal structure of semiconductors. This paper reviews the recent literature covering the synthesis and application of nanomaterials in photocatalysis.

Electrochemical Immunosensor for Detection of H. pylori Secretory Protein VacA on g-C3N4/ZnO Nanocomposite-Modified Au Electrode

A g-C₃N₄/ZnO (graphitic carbon nitride/zinc oxide) nanocomposite-decorated gold electrode was employed to design an antigen-antibody-based electrochemical biosensor to detect Helicobacter pylori specific toxin, vacuolating cytotoxin A (VacA). The thermal condensation method was used to synthesize the g-C₃N₄/ZnO nanocomposite, and the nanocomposite was deposited electrochemically on a gold electrode. The morphology as well as the structure of the synthesized nanocomposite were confirmed by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, and Fourier transform infrared techniques. The nanocomposite efficiently increased the sensor performance by amplifying the signals. EDC-NHS chemistry was exploited for attachment of VacA antibodies covalently with the g-C₃N₄/ZnO-modified gold electrode. This modified electrode was exploited for immunosensing of H. pylori-specific VacA antigen. The immunosensor was stable for up to 30 days and exhibited good sensitivity of 0.3 μA^-1 ng mL^-1 in a linear detection range of 0.1 to 12.8 ng mL^-1. Apart from this, the fabricated sensor showed unprecedented reproducibility and remarkable selectivity toward the H. pylori toxin VacA. Thus, the highly sensitive immunosensor is a desirable platform for H. pylori detection in practical applications and clinical diagnosis.

Synthesis and applications of TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes

In recent years, TiO2/ZnO hybrid nanostructures have been attracting the interest of the scientific community due to their excellent photoelectrochemical properties. The main advantage of TiO2/ZnO hybrid nanostructures over other photocatalysts based on semiconductor materials lies in their ability to form heterojunctions in which the valence and conduction bands of both semiconductors are intercalated. This factor produces a decrease in the band gap and the recombination rate and an increase in the light absorption range. The aim of this review is to perform a revision of the main methods to synthesise TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes. Electrochemical synthesis methods provide an easy, fast, and highly efficient route to carry out the synthesis of nanostructures such as nanowires, nanorods, nanotubes, etc. They allow us to control the stoichiometry, thickness and structure mainly by controlling the voltage, time, temperature, composition of the electrolyte, and concentration of monomers. In addition, a study of the most promising applications for TiO2/ZnO hybrid nanostructures has been carried out. In this review, the applications of dye-sensitised solar cell, photoelectrocatalytic degradation of organic compounds, photoelectrochemical water splitting, gas sensors, and lithium-ion batteries have been highlighted.

Characterization, Luminescence and Optical Resonant Modes of Eu-Li Co-Doped ZnO Nano- and Microstructures

ZnO nano- and microstructures co-doped with Eu and Li with different nominal concentrations of Li were grown using a solid vapor method. Different morphologies were obtained depending on the initial Li content in the precursors, varying from hexagonal rods which grow on the pellet when no Li is added to ribbons to sword-like structures growing onto the alumina boat as the Li amount increases. The changes in the energy of the crystallographic planes leading to variations in the growth directions were responsible for these morphological differences, as Electron Backscattered Diffraction analysis shows. The crystalline quality of the structures was investigated by X-ray diffraction and Raman spectroscopy, showing that all the structures grow in the ZnO wurtzite phase. The luminescence properties were also studied by means of both Cathodoluminescence (CL) and Photoluminescence (PL). Although the typical ZnO luminescence bands centered at 3.2 and 2.4 eV could be observed in all cases, variations in their relative intensity and small shifts in the peak position were found in the different samples. Furthermore, emissions related to intrashell transitions of Eu3+ ion were clearly visible. The good characteristics of the luminescent emissions and the high refraction index open the door to the fabrication of optical resonant cavities that allow the integration in optoelectronic devices. To study the optical cavity behavior of the grown structures, µ-PL investigations were performed. We demonstrated that the structures not only act as waveguides but also that Fabry–Perot optical resonant modes are established inside. Quality factors around 1000 in the UV region were obtained, which indicates the possibility of using these structures in photonics applications.

Electrochemical Synthesis of Zinc Oxide Nanostructures on Flexible Substrate and Application as an Electrochemical Immunoglobulin-G Immunosensor

Immunoglobulin G (IgG), a type of antibody, represents approximately 75% of serum antibodies in humans, and is the most common type of antibody found in blood circulation. Consequently, the development of simple, fast and reliable systems for IgG detection, which can be achieved using electrochemical sandwich-type immunosensors, is of considerable interest. In this study we have developed an immunosensor for human (H)-IgG using an inexpensive and very simple fabrication method based on ZnO nanorods (NRs) obtained through the electrodeposition of ZnO. The ZnO NRs were treated by electrodepositing a layer of reduced graphene oxide (rGO) to ensure an easy immobilization of the antibodies. On Indium Tin Oxide supported on Polyethylene Terephthalate/ZnO NRs/rGO substrate, the sandwich configuration of the immunosensor was built through different incubation steps, which were all optimized. The immunosensor is electrochemically active thanks to the presence of gold nanoparticles tagging the secondary antibody. The immunosensor was used to measure the current density of the hydrogen development reaction which is indirectly linked to the concentration of H-IgG. In this way the calibration curve was constructed obtaining a logarithmic linear range of 10–1000 ng/mL with a detection limit of few ng/mL and good sensitivity.

Original Approach to Synthesize TiO2/ZnO Hybrid Nanosponges Used as Photoanodes for Photoelectrochemical Applications

In the present work, TiO2/ZnO hybrid nanosponges have been synthesized for the first time. First, TiO2 nanosponges were obtained by anodization under hydrodynamic conditions in a glycerol/water/NH4F electrolyte. Next, in order to achieve the anatase phase of TiO2 and improve its photocatalytic behaviour, the samples were annealed at 450 °C for 1 h. Once the TiO2 nanosponges were synthesized, TiO2/ZnO hybrid nanosponges were obtained by electrodeposition of ZnO on TiO2 nanosponges using different temperatures, times, and concentrations of zinc nitrate (Zn(NO3)2). TiO2/ZnO hybrid nanosponges were used as photoanodes in photoelectrochemical water splitting tests. The results indicate that the photoelectrochemical response improves, in the studied range, by increasing the temperature and the Zn(NO3)2 concentration during the electrodeposition process, obtaining an increase in the photoelectrochemical response of 141% for the TiO2/ZnO hybrid nanosponges electrodeposited at 75 °C with 10 mM Zn(NO3)2 for 15 min. Furthermore, morphological, chemical, and structural characterization was performed by Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive X-Ray spectroscopy (EDX), Raman Confocal Laser Spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), and Grazing Incidence X-Ray Diffraction (GIXRD).

Effect of ZnO surface morphology on its electrochemical performance

The purpose of this paper is to bridge the gap between ZnO surface morphology and its electrochemical performance. For this reason, ZnO nanowires (NWs) of different length were synthesized using an electrochemical method. Then, the electrochemical performance of the synthesized ZnO surfaces was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical analysis results revealed that the increase of ZnO NW length contributes to the retrogression of electrochemical performance. Indeed, the electrochemical performance is mainly related to the wettability behavior of the ZnO nanowire surfaces. When the ZnO NWs length increases, the surface become more hydrophobic, therefore, charge transfers between the electrode/electrolyte decrease. To improve the electrochemical performance of ZnO, we propose a new strategy combining NWs and microsheets (μSs) for further improving the morphology. Finally, the surfaces based on the double structure of ZnO provide good propagation of charge at the surface, good transfer in the electrode, good stability, and excellent scanning ability. In the present work we intend to pave the way for achieving high electrochemical performance ZnO-based layers.

The purpose of this paper is to bridge the gap between ZnO surface morphology and its electrochemical performance.

Low-Cost Electrodeposition of Size-Tunable Single-Crystal ZnO Nanorods

In this paper we report a low cost, simple, electrochemical method for large-area growth of single crystal ZnO nanorods. The method utilizes a metallic zinc foil as the source of the necessary zinc ions for ZnO growth on indium-doped tin oxide (ITO) glass slides. The method is thoroughly discussed and investigated varying all the parameters involved. The resulting ZnO nanorods are highly oriented along c-axis and densely packed, while their length and diameter can be tuned by varying the growth parameters. Two different types of seed layers on the ITO glass slides are tested. A seed layer made by spin coating of ZnO nanoparticles results in a twofold increase of the ZnO nanorod surface density as compared with a ZnO thin film seed layer by physical vapor deposition. Additionally, the effect of oxygen supply during electrodeposition was investigated as a crucial regulatory parameter not only for the geometrical and topological characteristics of the ZnO nano-arrays but for their physical properties as well.

Unravelling the Structure of a Medium-Sized Metalloid Gold Nanocluster and its Filming Property

Unravelling the structure of thiolated metalloid gold nanoclusters in the medium-sized range by single crystal X-ray crystallography (SCXC) is challenging. Herein, we successfully synthesized a novel Au₆₇ (SR)₃₅ nanocluster, and unravelled its single crystal structure by SCXC, which features a mix-structured Au₄₈ kernel protected by one Au₄ (SR)₅ staple and fifteen Au(SR)₂ staples. Unprecedentedly, this structure can be thermally induced to aggregate into larger nanoparticles and self-deposit to form a gold nanoparticles film onto the walls of a vial or other substrates such as quartz, mica or ceramic, which can be developed into a facile, substrate-universal and scalable filming method. The film exhibits high sensitivity, uniformity and recyclability as a surface-enhanced Raman scattering (SERS) substrate and can be applied for detecting multiple organic pollutants.

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm2  V-1 s-1 . The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (VZn ) with one or more oxygen vacancies (VO ) in larger structural entities. Such VO -VZn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.

Synthesis of Vertically Aligned ZnO Nanorods Using Sol-gel Seeding and Colloidal Lithography Patterning

Different ZnO nanostructures can be grown using low-cost chemical bath deposition. Although this technique is cost-efficient and flexible, the final structures are usually randomly oriented and hardly controllable in terms of homogeneity and surface density. In this work, we use colloidal lithography to pattern (100) silicon substrates to fully control the nanorods' morphology and density. Moreover, a sol-gel prepared ZnO seed layer was employed to compensate for the lattice mismatch between the silicon substrate and ZnO nanorods. The results show a successful growth of vertically aligned ZnO nanorods with controllable diameter and density in the designated openings in the patterned resist mask deposited on the seed layer. Our method can be used to fabricate optimized devices where vertically ordered ZnO nanorods of high crystalline quality are crucial for the device performance.

Selective and efficient solid-phase microextraction of polycyclic aromatic hydrocarbons in water by robust two-dimensional zinc oxide nanosheets grown on a superelastic nickel-titanium alloy fiber prior to determination by HPLC-UV

Oriented zinc oxide nanosheets (ZnONSs) were directly grown on pretreated nickel-titanium alloy (NiTi) fiber substrates without a traditional seeding layer of ZnO by electrochemical deposition for the first time. The fiber coatings were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. Direct growth of ZnONSs on the NiTi fiber substrate was dependent on the type of zinc salt. The adsorption performance of the ZnONSs coatings was evaluated using representative aromatic compounds as model analytes together with high performance liquid chromatography with ultraviolet detection. The as-prepared fiber shows higher extraction capability for the selected polycyclic aromatic hydrocarbons (PAHs) than for ultraviolet filters in water samples, and better extraction selectivity for PAHs. For this purpose, the important experimental parameters were optimized for the extraction of PAHs. Under the optimized conditions, the calibration curves are linear in the range of 0.03-200 μg L-1 with correlation coefficients greater than 0.999. Limits of detection ranged from 0.011 μg L-1 to 0.082 μg L-1. Intra-day and inter-day relative standard deviations (RSDs) of the developed method with a single fiber ranged from 2.69% to 4.18% and from 4.44% to 5.40%, respectively. RSDs for the fiber-to-fiber reproducibility varied between 5.57% and 7.66%. The developed method was successfully applied for selective preconcentration and determination of trace PAHs in five real water samples. Relative recoveries varied from 84.5% to 104% with RSDs between 1.65% and 8.30%. Furthermore, the as-prepared fiber is highly stable.

Arrays Formation of Zinc Oxide Nano-Objects with Varying Morphology for Sensor Applications

The regularities and features of the formation of arrays of zinc oxide nano-objects with varying morphology are determined by CO₂ laser processing with intensification of diffusion processes in the solid state of Cu–Zn metallic materials which are selectively oxidizable. In the process of laser treatment in air using the synergy of heat exposure and vibrations induced by laser with a force fundamental frequency of 100 Hz, the brass surface of samples is oxidized mainly with the generation of ZnO nanowires. The condition for intensification is the local non-stationary deformation caused by sound waves induced by laser. Upon the initiation of the processes of exfoliation of the initially formed layers on the material surface, apart from a disordered structure, a structure was formed in the central region containing two-dimensional objects made of zinc oxide with characteristic thicknesses of 70–100 nm. Such arrays can provide the potential to create a periodic localized electric field applying direct current, this allows the production of electrically switched diffraction gratings with a variable nature of zones. It has been established that during laser pulse-periodic irradiation on brass, the component of the metal alloy, namely, zinc, will oxidize on the surface in the extent that its diffusion to the surface will be ensured. During laser pulse-periodic heating under conditions of the experiment, the diffusion coefficient was 2–3 times higher than from direct heating and exposure to a temperature of 700 °C. The study of the electrical resistance of the created samples by the contact probe method was performed by the four-point probe method. It was determined that the specific electrical resistance at the center of the sample was 30–40% more than at the periphery. To determine the possibility of using the obtained material based on zinc oxide for the creation of sensors, oxygen was adsorbed on the sample in an oxygen–argon mixture, and then the electrical resistance in the central part was measured. It was found that the adsorbed oxygen increases the electrical resistivity of the sample by 70%. The formation of an oxide layer directly from the metal substrate can solve problem of forming an electrical contact between the gas-sensitive oxide layer and this substrate.

Flexible ZnO-mAb nanoplatforms for selective peripheral blood mononuclear cell immobilization

Cancer is the second cause of death worldwide. This devastating disease requires specific, fast, and affordable solutions to mitigate and reverse this trend. A step towards cancer-fighting lies in the isolation of natural killer (NK) cells, a set of innate immune cells, that can either be used as biomarkers of tumorigenesis or, after autologous transplantation, to fight aggressive metastatic cells. In order to specifically isolate NK cells (which express the surface NKp30 receptor) from peripheral blood mononuclear cells, a ZnO immunoaffinity-based platform was developed by electrodeposition of the metal oxide on a flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate. The resulting crystalline and well-aligned ZnO nanorods (NRs) proved their efficiency in immobilizing monoclonal anti-human NKp30 antibodies (mAb), obviating the need for additional procedures for mAb immobilization. The presence of NK cells on the peripheral blood mononuclear cell (PBMCs) fraction was evaluated by the response to their natural ligand (B7-H6) using an acridine orange (AO)-based assay. The successful selection of NK cells from PBMCs by our nanoplatform was assessed by the photoluminescent properties of AO. This easy and straightforward ZnO-mAb nanoplatform paves the way for the design of biosensors for clinic diagnosis, and, due to its inherent biocompatibility, for the initial selection of NK cells for autotransplantation immunotherapies.

Light-Addressable Electrodes for Dynamic and Flexible Addressing of Biological Systems and Electrochemical Reactions

In this review article, we are going to present an overview on possible applications of light-addressable electrodes (LAE) as actuator/manipulation devices besides classical electrode structures. For LAEs, the electrode material consists of a semiconductor. Illumination with a light source with the appropiate wavelength leads to the generation of electron-hole pairs which can be utilized for further photoelectrochemical reaction. Due to recent progress in light-projection technologies, highly dynamic and flexible illumination patterns can be generated, opening new possibilities for light-addressable electrodes. A short introduction on semiconductor–electrolyte interfaces with light stimulation is given together with electrode-design approaches. Towards applications, the stimulation of cells with different electrode materials and fabrication designs is explained, followed by analyte-manipulation strategies and spatially resolved photoelectrochemical deposition of different material types.

Graphene Oxide-Assisted Morphology and Structure of Electrodeposited ZnO Nanostructures

In this paper, ZnO electrodeposition was studied with the presence of graphene oxide (GO) exploited as a possible structure-directing agent. The effect of deposition potential and duration on the morphology and structure of ZnO was analyzed. The morphology and structure of the hybrids was analyzed by Raman spectroscopy, X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM). The Raman results indicate a successful modification of ZnO with GO sheets and a hybridization threshold of 10 mg L⁻¹ by the evolution of the defect related band of ZnO at 580 cm⁻¹. The morphology results show that a low GO content only slightly influences the morphology and orientation of ZnO nanostructures while a high content as 10 mg L⁻¹ changes the morphology in nanoplates and growth orientation to lateral. The results show that while GO participated in the deposition reaction, it has a two-fold role, also by structure-controlling ZnO, indicating that the approach is valid for the use of GO as a structure-directing agent for the fabrication of ZnO nanostructures by electrodeposition with varying morphologies and orientations.

Synthesis, structure and optimized field emission properties of highly oriented ZnO/Pt core–shell nanorods on a Zn substrate

The current density of as-fabricated Pt15/ZnO NR emitters was about 1900 times that of the bare ZnO emitters.

Bioinspired ZnO-Based Solar Photocatalysts for the Efficient Decontamination of Persistent Organic Pollutants and Hexavalent Chromium in Wastewater

Biomimetic/bioinspired engineering and sulfidation processes are effective strategies for improving the visible light-driven photocatalytic performance of ZnO photocatalysts. A facile electrodeposition process in high oxygen-flux conditions was used to synthesize well-defined fractal micro/nanoferns, consequently increasing the photocatalyst’s light-trapping capability and the accessible active surface. Next, a simple sulfidation process was used to form a thin layer of ZnS, producing ZnO@ZnS core@shell micro/nanoferns, thereby tuning the optoelectronic properties and extending the photoresponse to the visible region. The ZnO@ZnS micro/nanoferns exhibited clear superiority over other ZnO photocatalysts in the photooxidation of persistent organic pollutants (POPs) and the photoreduction of Cr(VI). Their excellent photocatalytic performance allowed the photodegradation under UV-filtered sunlight of nearly 97% of methylene blue after 60 min; the mineralization of >98% of a mixture of methylene blue, 4-nitrophenol, and rhodamine-B after 210 min; and the removal of nearly 65% of Cr(VI) after 180 min. In addition, the ZnO@ZnS micro/nanoferns demonstrated a good ability to decontaminate an inorganic-organic bipollutant system, with promising potential to leverage synergistic effects. Finally, these micro/nanoferns presented great recyclability and reusability for both photooxidation and photoremediation processes. These findings support that sulfidation and biomimetic engineering can be a superior route for designing efficient sunlight-driven ZnO-photocatalysts for water decontamination.

Al2O3, Al doped ZnO and SnO2 encapsulation of randomly oriented ZnO nanowire networks for high performance and stable electrical devices

Two-dimensional randomly oriented nanowire (NW) networks, also called nanonets (NNs), have remarkable advantages including low-cost integration, good reproducibility and high sensitivity, which make them a promising material for electronic devices. With this work, we focus on the study of ZnO NNs as channel materials in field effect transistors (FETs). In our process, ZnO NWs were assembled in NNs by the liquid filtration method and were integrated in transistors, with the bottom-gate configuration, using simple technological steps. Non-encapsulated devices exhibited state of the art performances but their stability toward air exposure was poor. Using a proper encapsulation of the nanonets, with cheap, abundant and non-toxic oxides, we demonstrate our ability not only to stabilize their electrical properties, but also to enhance performance to values never reach before for ZnO NW-based transistors. Our best FETs exhibit a low Off-current while maintaining a very good On-current, which results in a I _on/I _off ratio exceeding 10⁶ for a drain voltage of 5 V. The behavior of these ZnO NN-based FETs was studied for three different encapsulation materials, alumina (Al₂O₃), tin oxide (SnO₂) and Al-doped ZnO (AZO). These results prove that ZnO NNs are highly promising materials for an easy and low-cost integration into FETs.

First-Principles Investigation of the Structure and Properties of Au Nanoparticles Supported on ZnO

We present a first-principles investigation of the structure, stability, and reactivity of Au nanoparticles (NPs) supported on ZnO. The morphologies of supported Au NPs are predicted using the formation energy of Au surfaces and the adhesion energy between Au and the dominant ZnO surfaces exposed on ZnO tetrapods. We show how Zn interstitials (a stable intrinsic defect in ZnO) are attracted toward the Au/ZnO interface and in the presence of oxygen can lead to the encapsulation of Au by ZnO, an effect that is observed experimentally. We find that O₂ molecules absorb preferentially at the perimeter of the NP in contact with the ZnO support. These results provide atomistic insight into the structure of ZnO-supported Au NPs with relevance to CO oxidation.

Selective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method

High sensitivity and selectivity detection of NO at room temperature has always been full of challenges. In this work, a kind of porous ZnO with coralline-like nanostructure was prepared by a rapid and simple solid-state synthesis strategy, using zinc acetate and oxalic acid as precursors. Structural analysis and morphological investigations of the ZnO powder showed that it has a large specific surface area (32.75 m² g^-1) and many nanometer-sized channels between ZnO nanoparticles. This is beneficial to the adsorption and desorption of NO, which is an important reason for the selective detection of NO by the ZnO powder at room temperature. So based on the ZnO powder, a gas sensor was fabricated and its gas-sensing properties were investigated. It exhibited outstanding response (23.59) and fast response time (331 s) to 40 ppm of NO at room temperature (21 ± 2 °C). As the relative humidity study changed from 17% to 80% at 10 ppm of NO, the sensitivity of the sensor changed little, only decreased from 1.43 to 1.12. The stability study was also carried out. Under the concentration of 5 ppm of NO, the relative standard deviation was 0.33% within 8 days, which indicates that the obtained sensor is suitable for practical application.

Deposition Time and Annealing Effects of ZnO Seed Layer on Enhancing Vertical Alignment of Piezoelectric ZnO Nanowires

Well aligned crystalline zinc oxide (ZnO) nanowires (NWs) on ZnO/Au/Ti/Si substrates were grown by so-called “hydrothermal synthesis”. ZnO seed layers with different thicknesses ranging from 5 to 100 nm, achieved by controlling the deposition time, were prepared by radio-frequency sputtering, followed by a post-annealing treatment in air at 400 °C. The effects of deposition time and annealing treatment of ZnO seed layers on the subsequent growth of ZnO NWs were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The experimental results reveal that the quality and growth behaviors of ZnO NWs are strongly dependent on both the thickness and the heat treatment of the ZnO seed layers. This work is an optimization step of an easy, cost-effective, and industrially scalable process flow recently developed for the fabrication of a high performance, nanocomposite-based stretchable nanogenerator (SNG) on polydimethylsiloxane (PDMS) substrate. The morphological improvement of hydrothermally grown ZnO NWs may therefore lead to higher performance SNGs for the targeted application of mechanical energy harvesting, in order to supply flexible and wearable electronics.

A polyaniline-coated ZnS/ZnO/FTO photoelectrode for improving photocorrosion prevention and visible light absorption

To enhance the absorption of visible light for wide-bandgap semiconductors, methods such as sensitizing with nanoparticles or quantum dots and bandgap engineering using dopants have been reported.

Synthesis of zinc oxide nanorods or nanotubes on one side of a microcantilever

Cantilevers are really promising sensitive sensors despite their small surface. In order to increase this surface and consequently their sensitivity, we nanostructured them with zinc oxide (ZnO) nanorods or nanotubes having a diameter of approximately 100 nm and a length of 1 µm. The nanostructure growth was first optimized on a silicon wafer and then transferred to the cantilevers. The ZnO nanorods were grown in an autoclave. The centre of the nanorods was dissolved in order to obtain nanotubes. The dissolution conditions were optimized in order to have the longest etching depth. After 1.25 h in a dissolution solution containing 0.75 wt% of NH_3(aq) and 0.75 wt% of cetyltrimethyl ammonium bromide, the longest etching depth was obtained. After the transfer of the syntheses to the cantilevers, nanorods/nanotubes grew on both sides of the cantilever, which prevents the reflection of the laser allowing the resonance frequency measurement. A masking procedure was developed in order to avoid the growth on one face of the cantilever of zinc oxide nanostructures. As far as the authors are concerned, for the first time, zinc oxide nanotubes were synthesized on only one face of cantilevers with optical readout.

Control of the growth of electrodeposited zinc oxide on FTO glass

Effect of four parameters on the ZnO growth onto FTO.

Characterization of electrodeposited undoped and doped thin ZnO passive films on zinc metal in alkaline HCO3−/CO32− buffer solution

Electrochemical characterization of anodically grown thin ZnO films on pure zinc metal was studied in pH 9.2 bicarbonate/carbonate buffer solution. The different undoped passive films were formed potentiostatically in pH 9.2 borate buffer solution at processing anodic voltage (V_a) of −1.04, −1.02, −1.0 and −0.99 V (vs. Ag/AgCl). While, various doped ZnO films were fabricated by anodizing the metal at a fixed potential of −1.00 V in the same borate buffer solution containing different amounts of LiCl or InCl₃. The electrochemical and semiconducting properties of all formed films were investigated using chronoamperometric measurements, EIS and Mott–Schottky analysis supported by scanning electron microscopy. The impedance results showed a direct correlation between V_a and the value of either total resistance (R_f) of undoped passive film or its thickness (δ_f). It is evident that anodization can afford better conditions for forming thicker compact passive films with more advanced barrier properties. On the other hand, R_f decreases with increasing Li-doping level in the oxide film, and increases in case of In-doping. Interestingly, R_f values of the doped films are always lower when compared to its value for the undoped film grown at −1.00 V, likely due to possible change in the film microstructure upon doping. For both undoped and doped ZnO films, Mott–Schottky plots reveals unintentional n-type conductivity with high electron density. Moreover, with increasing dopant level in ZnO host materials, Mott–Schottky analysis revealed a parallel correlation between charge carrier donor concentration (N_D) and the passive film thickness (δ_f), where the trend of their values are to decrease for Li⁺-doped and to increase for In³⁺-doped films.

The trend of charge carrier density (N_D) and film thickness (δ_f) dependence on the parameter is indicated on each arrow for undoped, Li-doped, and In-doped ZnO semiconductor materials.

Three-Dimensional ZnO Hierarchical Nanostructures: Solution Phase Synthesis and Applications

Zinc oxide (ZnO) nanostructures have been studied extensively in the past 20 years due to their novel electronic, photonic, mechanical and electrochemical properties. Recently, more attention has been paid to assemble nanoscale building blocks into three-dimensional (3D) complex hierarchical structures, which not only inherit the excellent properties of the single building blocks but also provide potential applications in the bottom-up fabrication of functional devices. This review article focuses on 3D ZnO hierarchical nanostructures, and summarizes major advances in the solution phase synthesis, applications in environment, and electrical/electrochemical devices. We present the principles and growth mechanisms of ZnO nanostructures via different solution methods, with an emphasis on rational control of the morphology and assembly. We then discuss the applications of 3D ZnO hierarchical nanostructures in photocatalysis, field emission, electrochemical sensor, and lithium ion batteries. Throughout the discussion, the relationship between the device performance and the microstructures of 3D ZnO hierarchical nanostructures will be highlighted. This review concludes with a personal perspective on the current challenges and future research.