Zinc Oxide Surface Research Articles

Chemical Protection of Material Morphology: Robust and Gentle Gas-Phase Surface Functionalization of ZnO with Propiolic Acid.

Chemical functionalization of ZnO surface with an alkyne functional group was successfully achieved by exposing ZnO nanopowder to gas-phase propiolic acid in vacuum, which left the alkyne group available for subsequent chemical modification via the azide-alkyne cycloaddition "click" reaction with benzyl azide. The highly selective formation of a bidentate carboxylate linkage and the reaction of benzyl azide were confirmed by solid-state nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Most importantly, scanning electron microscopy revealed that the surface morphology was perfectly preserved by this gas-phase modification, as opposed to the alternative protocols based on liquid phase processing. This simple and precise design can serve as a universal method for the modular functionalization of zinc oxide surface following the initial surface preparation and be further applied to thin films, nanostructures, and powders, where preserving surface morphology during chemical modification is especially important.

Electroless-plated Ni pattern with catalyst printing on indium–gallium–zinc oxide surface

Electroless plated metals have been used for wiring and electrodes in the manufacture of electronic devices. To obtain plated patterns, etching and photoresist are generally used. However, through catalyst patterning by printing, we can obtain metal patterns without etching and photoresists by electroless plating. Solution-processed indium–gallium–zinc oxide (IGZO) has received significant attention for showing high performance and ease of preparation in air atmosphere. In this study, we prepared an electroless plated pattern by catalyst printing as electrodes of IGZO TFT. There are few reports on the application of plated metal electrodes prepared by catalyst printing to the source and drain electrodes of IGZO TFT. The prepared IGZO TFT exhibits a typical current–voltage (I–V) curve. The plated electrodes caused many problems such as performance degradation. However, our result showed that the plated metal electrodes can drive IGZO TFT. In addition, we confirm plated metal growth into the catalyst layer by cross sectional scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) of the plated Ni. We discuss the relevance of the measured work function (WF) of the electrode materials and the performance of IGZO TFT.

The Relationship between the Mechanism of Zinc Oxide Crystallization and Its Antimicrobial Properties for the Surface Modification of Surgical Meshes.

Surgical meshes were modified with zinc oxide (ZnO) using a chemical bath deposition method (CBD) at 50 °C, 70 °C, or 90 °C, in order to biologically activate them. Scanning electron microscopy (SEM), mass changes, and X-ray diffraction measurements revealed that at low temperatures Zn(OH)2 was formed, and that this was converted into ZnO with a temperature increase. The antimicrobial activity without light stimulation of the ZnO modified Mersilene™ meshes was related to the species of microorganism, the incubation time, and the conditions of the experiment. Generally, cocci (S. aureus, S. epidermidis) and yeast (C. albicans) were more sensitive than Gram-negative rods (E. coli). The differences in sensitivity of the studied microorganisms to ZnO were discussed. The most active sample was that obtained at 90 °C. The mechanism of antimicrobial action of ZnO was determined by various techniques, such as zeta potential analysis, electron paramagnetic resonance (EPR) spectroscopy, SEM studies, and measurements of Zn(II) and reactive oxygen species (ROS) concentration. Our results confirmed that the generation of free radicals was crucial, which occurs on the surface of crystalline ZnO.

All-Aluminum Thin Film Transistor Fabrication at Room Temperature.

Bottom-gate all-aluminum thin film transistors with multi conductor/insulator nanometer heterojunction were investigated in this article. Alumina (Al2O3) insulating layer was deposited on the surface of aluminum doping zinc oxide (AZO) conductive layer, as one AZO/Al2O3 heterojunction unit. The measurements of transmittance electronic microscopy (TEM) and X-ray reflectivity (XRR) revealed the smooth interfaces between ~2.2-nm-thick Al2O3 layers and ~2.7-nm-thick AZO layers. The devices were entirely composited by aluminiferous materials, that is, their gate and source/drain electrodes were respectively fabricated by aluminum neodymium alloy (Al:Nd) and pure Al, with Al2O3/AZO multilayered channel and AlOx:Nd gate dielectric layer. As a result, the all-aluminum TFT with two Al2O3/AZO heterojunction units exhibited a mobility of 2.47 cm2/V·s and an Ion/Ioff ratio of 106. All processes were carried out at room temperature, which created new possibilities for green displays industry by allowing for the devices fabricated on plastic-like substrates or papers, mainly using no toxic/rare materials.

Life cycle assessment of facile microwave-assisted zinc oxide (ZnO) nanostructures

The life cycle assessment of several zinc oxide (ZnO) nanostructures, fabricated by a facile microwave technique, is presented. Key synthesis parameters such as annealing temperature, varied from 90°C to 220°C, and microwave power, varied from 110W to 710W, are assessed. The effect of these parameters on both the structural characteristics and the environmental sustainability of the nanostructures is examined. The nanostructures were characterized by means of X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), ultraviolet–visible spectroscopy (UV–Vis), Photoluminescence (PL) and Brunauer–Emmett–Teller (BET) analysis. Crystalline size was found to be 22.40nm at 110W microwave power, 24.83nm at 310W, and 24.01nm at 710W. Microwave power and synthesis temperature were both directly proportional to the surface area. At 110W the surface area was 10.44m2/g, at 310W 12.88m2/g, and at 710W 14.60m2/g; while it was found to be 11.64m2/g at 150°C and 18.09m2/g at 220°C. Based on these, a life cycle analysis (LCA) of the produced ZnO nanoparticles was carried out, using the ZnO surface area (1m2/g) as the functional unit. It was found that the main environmental weaknesses identified during the production process were; (a) the use of ethanol for purifying the produced nanomaterials and (b) the electricity consumption for the ZnO calcination, provided by South Africa's fossil-fuel dependent electricity source. When the effect of the key synthesis parameters on environmental sustainability was examined it was found that an increase of either microwave power (from 110 to 710W) or synthesis temperatures (from 90 to 220°C), results in higher sustainability, with the environmental footprint reduced by 27% and 41%, respectively. Through a sensitivity analysis, it was observed that an electricity mix based on renewable energy could improve the environmental sustainability of the nanoparticles by 25%.

Illumination intensity dependent photoresponse of ultra-thin ZnO/graphene/ZnO heterostructure

A heterostructure of zinc oxide (ZnO) and single layer graphene is fabricated by sandwiching a transferred graphene between two thin ZnO films (∼20 nm each). ZnO thin films were grown using decomposition of Zn(acac)2 and spin-coating technique. Graphene transfer route with PMMA temporary carrier and metal etching process was used to transfer high quality commercial graphene from copper foil on the zinc oxide surface on glass. This novel and ultra-thin heterostructure (∼40 nm) is sensitive for UV illumination and works as a photodetector (PD). In this device, both positive and negative photoconductivity (PC) were observed depends on illumination intensity and spectrum of incident light. Relatively long response and recovery times obtained in ZnO/G/ZnO structure are related to the metastable defect states of ZnO and its interfaces with graphene and/or silver contacts. The obtained results show that the transferred single layer graphene sheet between thin ZnO films could be a novel route for improvement properties this low-cost metal oxide.

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer.

In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitates charge extraction from the BHJ active layer but also minimizes the energy loss within the charge transport processes. In addition, the organic-inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturation current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy, but effective, approach to reach high performance inverted PSCs.

Probing the Relative Photoinjection Yields of Monomer and Aggregated Dyes into ZnO Crystals.

Cyanine dyes, often used in dye-sensitized solar cells (DSSCs), form a range of molecular species from monomers to large H and J aggregates in both solution and when adsorbed at a photoelectrode surface. To determine the relative capability of the different dye species to inject photoexcited electrons into a wideband gap oxide semiconductor, sensitization at a single-crystal zinc oxide surface was studied by simultaneous attenuated reflection (ATR) ultraviolet-visible (UV-vis) absorption and photocurrent spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface with simultaneous photocurrent spectroscopy to identify the contribution of the various dye forms to photocurrent signal. We study the dye 2,2'-carboxymethylthiodicarbocyanine bromide that is particularly prone to aggregation both in solution and at the surface of sensitized oxide semiconductors.

Surface Engineering of Nanostructured ZnO Surfaces

Zinc Oxide (ZnO) nanostructures represent promising substrate materials for numerous applications, ranging from new generation solar cells, to bio‐ and chemical sensors. This interest is due to particular physical and chemical properties and highly active surface areas. Despite showing promising perspectives, the performances of ZnO nanomaterials are affected by several drawbacks, including a poor chemical stability against reaction solutions and intrinsic defects that are still preventing their integration into product applications. To overcome some of these limitations, different functionalization strategies have been introduced to engineer the ZnO surface properties. The synthetic methods used for surface functionalization are here discussed and analyzed with respect to the morphology of the agent to be anchored and to the functionalization approach. These approaches range from continuous coatings, to low‐dimensional ligands like nanoparticles, photosensible dyes, quantum dots and organic compounds.

Acetone gas sensing mechanism on zinc oxide surfaces: A first principles calculation

Semiconducting metal oxide gas sensors have attracted growing interest as a result of their outstanding performance in the bio and industrial applications. Nevertheless, the sensing mechanism is yet not totally understood. In this study, we extensively investigate the adsorption mechanism of acetone molecule on ZnO-based thin film sensors by performing ab initio density functional theory calculations and employing quantum molecular dynamic simulations. Since the sensitivity of a metal oxide sensor is exceedingly depends on molecular oxygen exposure and operating temperature, we explore the competitive adsorption of acetone and oxygen molecule on the most stable orientation of ZnO surface (101̅0) at different temperatures. Results indicate that at elevated temperatures acetone gains required thermal energy to remove preadsorbed oxygen molecule from the surface in a competitive process. We will show that this competition is responsible for the resistive switching behavior in the ZnO-based gas sensors.

Surface defect modification of ZnO quantum dots based on rare earth acetylacetonate and their impacts on optical performance

The surface defect modification has an important effect on the application of ZnO quantum dots, and it has gained much progress in recently years, propelled by the development of additives. Our research efforts are directed toward developing a new surface modification additive RE(AcAc)3 (RE=Ce, Dy, Tb) to achieve fine ZnO QDs and adjust their surface properties. RE(AcAc)3/ZnO QDs nanostructured materials have been designed and prepared, and particular emphasis has been given to the relation between the surface modification and optical properties. The effects of RE(III) acetylacetonate modification on the FT-IR, TEM images and photoluminescence (PL) spectra were investigated, and the surface defect modification principle and effect were discussed in details. The band gap (Eg) was also calculated to prove the surface modification effect. For the RE(AcAc)3/ZnO QDs complex materials, stable linkage occurs because of the affinity of COOH from acetylacetonate anionic ligand to zinc oxide surfaces, with attachment to the zinc oxide by hydrogen bonding between the protons of the hydroxyl groups on the surface of ZnO QDs and the π–system of acetylacetone.

Light backscattering (e.g. reflectance) by ZnO nanorods on tips of plastic optical fibres with application for humidity and alcohol vapour sensing

Relative humidity and alcohol vapour sensing by light backscattering was demonstrated by zinc oxide (ZnO) nanorods grown on tips of plastic optical fibres. Synthesis of ZnO nanostructures was accomplished by the hydrothermal method. A new technique was proposed to extract the backscattered light by ZnO nanostructures. The system was shown experimentally to sense moisture and alcohol vapours. Three ranges of the visible spectrum blue (450–500 nm), green (500–570 nm) and red (610–700 nm) were studied in experiments. Spectral reflectance based on backscattering from ZnO surface structures was used to measure the amount of light backscattered by ZnO nanorods. Results showed that light in the red visible range of the spectrum had higher sensitivity and reflectivity compared with blue and green spectral ranges.

Star-shaped ZnO/Ag hybrid nanostructures for enhanced photocatalysis and antibacterial activity

Zinc oxide (ZnO) particles with a star-shaped morphology have been synthesized by a novel and simple room-temperature method and decorated with silver nanoparticles (SNPs) for enhanced photocatalysis and bactericide applications. The presence of thiourea during the precipitation of ZnO in alkaline conditions allowed the control of morphological features (e.g. average size and shape) and the surface functionalization with thiocyanate ions (SCN−). SNPs were deposited into the ZnO surface by a photoreduction method and their sizes could be easily controlled by changing the ZnO/AgNO3 ratio. The presence of SCN− on the semiconductor surface prevents uncontrollable growth of Ag nanoparticles into different morphologies and high degrees of polydispersity. XRD, SEM, TEM, FTIR, UV-vis-NIR and PL were employed for characterizing the structure, morphology and optical properties of the as-obtained pure and hybrid nanostructures. Finally, the hybrid ZnO/Ag particles have shown plasmon-enhanced performance for applications in photocatalysis and antibacterial activity compared to the pure ZnO counterpart. In this work, evaluation of the photodegradation of an aqueous methylene blue solution under UV-A irradiation and the antibacterial activity toward 4 bacterial strains, including Gram-positive bacteria Staphylococcus aureus (ATCC 43300, ATCC 25923 and ATCC 33591) and Gram-negative bacteria Pseudomonas aeruginosa (ATCC 27853).

Modified photoluminescence and photodetection characteristics of chemically grown SnO coated ZnO nanoneedles

The authors report on the synthesis of tin oxide (SnO) coated zinc oxide (ZnO) needlelike nanostructures and their modified light emission and detection features. The formation of SnO phase on ZnO surface has been revealed from energy dispersive x-ray analysis and secondary ion mass spectrometry studies. The luminescence response of the SnO-coated ZnO nanoneedles gets lowered compared to that of bare ZnO and is assigned to the lowering of radiative emission due to the occurrence of charge-carrier separation. Again in the heterostructured system, due to SnO led surface passivation, the band-edge emission becomes prominent and defect-related emission gets lowered. The photoconductivity response is found to be significantly enhanced for the SnO-ZnO heterostructured material formed with lower Sn:Zn molar ratio. The enhancement of photocurrent has been understood in the light of carrier separation and carrier multiplication processes occurring at the SnO-ZnO heterojunctions.

Oxygen reduction at electrodeposited ZnO layers in alkaline solution

Zinc oxide (ZnO) layers were electrodeposited from an aqueous nitrate bath at 62°C on copper substrates. At −0.9V (vs. saturated calomel reference electrode), the growth rate is 600nmmin−1 . In the early stages of the deposition, the layers are porous. At longer deposition times, the surface becomes dense and rough. The wurtzite crystalline structure is confirmed by XRD measurements and the chemical composition of the ZnO surface was assessed by EDX and XPS. The oxygen reduction reaction (ORR) was investigated at room temperature in a 10−3M KOH solution with KCl as supporting electrolyte. The ORR onset potential is found to be much larger than that of platinum taken as reference electrocatalyst. Rotating ring-disk electrode experiments evidence a negligible production of hydrogen peroxide as intermediate product of the reaction. The latter follows thus a direct four-electron pathway at pH ∼11.

Atomic Layer Deposition of Zinc Oxide: Diethyl Zinc Reactions and Surface Saturation from First-Principles

Zinc oxide thin films grown via atomic layer deposition have been under intense research for the past few years. Here, we present a comprehensive density functional theory study on the atomic layer deposition of zinc oxide. The adsorption of diethyl zinc and subsequent surface reactions are studied on an ideal (100) ZnO surface as well as on a stepped surface to compare ideal and nonideal surface structures. Our results show that diethyl zinc adsorbs and reacts rapidly on the surface to form monoethyl zinc. Our calculations also show that the initial ligand-exchange reactions are preferred on the planar surface over the step surface. Further reaction from monoethyl zinc to adsorbed zinc atoms has a high reaction barrier. We present two surface structures for the saturated zinc oxide surface at the end of the diethyl zinc pulse corresponding to low and high temperature approximations that are in good agreement with the experiments.

Effect of surface films on tribologically induced metallurgical transformations of steel in oil lubricated contacts

This study was initiated with the aim to assess the influence of boundary films on the tribologically induced metallurgical transformations and deformations of metals in lubricated contacts and their effect on wear. For this, the tribological behavior of a carbon steel contact was studied in two commercially available oils expected to form boundary films of quite different chemistry and structure. Surface analysis by AES and XPS revealed that under the investigated conditions the first oil led to the formation of an iron–zinc oxide surface film while the second to a calcium–carbon–oxygen rich film. The results show that, without influencing the coefficient of friction, the nature of the formed films significantly affected the metallurgical transformations (characterized by electron microscopy of focused ion beam cross sections) occurring in the near surface region of the metal and the corresponding wear response. The effect of boundary films on wear was attributed to their capability to influence the plastic flow of the nano-grained structures generated in the studied tribological contacts.

ZnO hollow nano-baskets for mineralization of cationic dye

The hollow nano-baskets (HNBs) of zinc oxide (ZnO) were synthesized by a simple low temperature solution method and used for the degradation of rhodamine 6G (Rh6G) dye. Well-defined hollow nano-baskets morphology of ZnO was observed which displayed a band gap of ~3.26eV. The photocatalytic degradation results revealed that the synthesized ZnO-HNBs exhibited a rapid degradation rate of ~97% of Rh6G-dye within ~90min. The mineralization of Rh6G-dye was examined before and after the photocatalytic reaction by a mass spectroscopy of the dye. The rapid and high degradation of Rh6G-dye might be attributed to ZnO hollow nanostructures via improved catalyst contact with dye by the inner and external surface of ZnO.

A Surfactants Walk to Work: Modes of Action of Citrate Controlling (10–10) and (000–1) Zinc Oxide Surface Growth from Solution

AbstractMolecular dynamics simulations unravel the association of citrate to (10–10) and (000–1) growth fronts of zinc oxide and demonstrate an unexpected mobility of the surfactant. Citrate association to perfectly planar (10–10) and (000–1) ZnO‐ethanol interfaces was found to be favored over dissociation by 1.5–2 eV hence suggesting strongly bound, immobilized surfactants. However, intramolecular stress prevents binding of all three carboxyl groups to planar surfaces and the typical arrangement is that of two carboxyl‐Zn contacts (including two salt bridges each) whilst the remaining carboxyl group is dangling into the solvent. As a consequence, the surfactant exhibits a “walking” mechanism to move along the surface by exchanging the role of its carboxyl groups. This finding has strong implications for the role of citrate during crystal growth as illustrated by a recently developed simulation scheme based on hundreds of individual Zn2+ and OH– ion association steps. In particular, for the (10–10) surface – which grows via formation of ridges embedded by {10–10} faces – these simulations show how citrate ions move towards steps and bind to the growth front by additional 4 eV per surfactant molecule.

Zinc oxide surface: a versatile nanoplatform for solvent-free synthesis of diverse isatin derivatives

Multicomponent reactions performed on the surface of nanostructured zinc oxide gave 3,3-bis(indolyl)indolin-2-one and xanthene derivatives with excellent yields. Both Lewis acidic (Zn2+) and basic (O2−) sites on the surface of zinc oxide were utilized to perform the aforementioned transformations. The significance of surface catalysis was further proved by performing the experiment with surface masked zinc oxide. The developed zinc oxide nanocatalyst was reusable up to five times without significant loss in its activity.