Surface Properties Of ZnO Research Articles

Biogenic Preparation of ZnO Nanostructures Using Leafy Spinach Extract for High-Performance Photodegradation of Methylene Blue under the Illumination of Natural Sunlight.

To cope with environmental pollution caused by toxic emissions into water streams, high-performance photocatalysts based on ZnO semiconductor materials are urgently needed. In this study, ZnO nanostructures are synthesized using leafy spinach extract using a biogenic approach. By using phytochemicals contained in spinach, ZnO nanorods are transformed into large clusters assembled with nanosheets with visible porous structures. Through X-ray diffraction, it has been demonstrated that leafy spinach extract prepared with ZnO is hexagonal in structure. Surface properties of ZnO were altered by using 10 mL, 20 mL, 30 mL, and 40 mL quantities of leafy spinach extract. The size of ZnO crystallites is typically 14 nanometers. In the presence of sunlight, ZnO nanostructures mineralized methylene blue. Studies investigated photocatalyst doses, dye concentrations, pH effects on dye solutions, and scavengers. The ZnO nanostructures prepared with 40 mL of leafy spinach extract outperformed the degradation efficiency of 99.9% for the MB since hydroxyl radicals were primarily responsible for degradation. During degradation, first-order kinetics were observed. Leafy spinach extract could be used to develop novel photocatalysts for the production of solar hydrogen and environmental hydrogen.

Studying stearic acid interaction with ZnO/SiO2 nanoparticles with tailored morphology and surface features: A benchmark for better designing efficient ZnO-based curing activators

The interaction between activators (ZnO) and co-activators (stearic acid, SA) represents a key step in the vulcanization process, to generate Zn(II) intermediate complexes that enhance the reaction kinetic and promote the shortening of sulfur bridges, leading to highly cross-linked materials. To understand the influence of the structural, morphological and surface properties of ZnO in the reactivity with SA, in this work, pure ZnO nanoparticles (NPs) and ZnO NPs anchored on SiO2 (ZnO/SiO2) were prepared through soft chemistry techniques. Tailoring of morphology and surface features was accomplished through a fine control of the synthetic parameters, as demonstrated by the careful characterization of the activators. Then, the interaction of pure ZnO and ZnO/SiO2 activators with SA in the absence of rubber was investigated by using Differential Scanning Calorimetry (DSC). The occurrence of Zn(II)-SA complexes with different thermal stability and structural properties was assessed by a comprehensive thermogravimetric and spectroscopic survey. The generation and the chemical structure of these specific vulcanization intermediates was related to the peculiar characteristics of the ZnO/SiO2 systems and, in turn, to their ability in imparting faster kinetics and higher curing efficiency to isoprene rubber nanocomposites, compared to bare ZnO.These results, besides proposing a valid benchmark for achieving further insights on the interaction of stearic acid with activators, pave the way to provide specific protocols for a better design and implementation of ZnO-based materials able to effectively enhance the rubber vulcanization process, with significant economic and environmental advantages.

Dynamic chemical processes on ZnO surfaces tuned by physisorption under ambient conditions

The catalytic properties of non-reducible metal oxides have intrigued continuous interest in the past decades. Often time, catalytic studies of bulk non-reducible oxides focused on their high-temperature applications owing to their weak interaction with small molecules. Hereby, combining ambient-pressure scanning tunneling microscopy (AP-STM), AP X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations, we studied the activation of CO and CO2 on ZnO, a typical non-reducible oxide and major catalytic material in the conversion of C1 molecules. By visualizing the chemical processes on ZnO surfaces at the atomic scale under AP conditions, we showed that new adsorbate structures induced by the enhanced physisorption and the concerted interaction of physisorbed molecules could facilitate the activation of CO and CO2 on ZnO. The reactivity of ZnO towards CO could be observed under AP conditions, where an ordered (2 × 1)–CO structure was observed on ZnO(101¯0). Meanwhile, chemisorption of CO2 on ZnO(101¯0) under AP conditions was also enhanced by physisorbed CO2, which minimizes the repulsion between surface dipoles and causes a (3 × 1)–CO2 structure. Our study has brought molecular insight into the fundamental chemistry and catalytic properties of ZnO surfaces under realistic reaction conditions.

Changes of Morphological, Optical, and Electrical Properties Induced by Hydrogen Plasma on (0001) ZnO Surface

Plasma provides specific adjustment of solid‐state surface properties offering an alternative to high temperature treatment. Herein, hydrogen plasma treatment of monocrystalline (0001) ZnO surface is studied in an inductively coupled plasma reactor with reduced capacitively coupled plasma mode. The crucial role of electrical grounding of the sample holder for plasma etching and related changes in the morphology, optical, and electrical properties of surfaces exposed to electron and ion bombardment are explained. The effects on the chemical composition of the surface are analyzed by X‐ray photoelectron spectroscopy (XPS), optical properties by photoluminescence spectroscopy, topography, roughness, and surface measurements by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). All methods show altered ZnO surface properties before and after plasma treatment strongly depending on the electrical potential of the holder.

Facile solution combustion synthesized, Li doped ZnO nanostructures for removal of abiotic contaminants

ZnO is a versatile photocatalytic material, yet it needed further modifications to explore it on the commercial scale. In the present study, photoactive Li doped ZnO nano-materials with different Li content were synthesized by adopting a facile low-temperature solution combustion approach. Li was introduced to improve ZnO characteristics correspond to photocatalytic efficiency. The effect of lithium doping on morphology, crystallinity, chemical state, and optical properties were investigated by utilizing various advanced techniques. The characterization analysis confirms the presence of thermally stable, well-crystalline wurtzite ZnO with a low bandgap and porous nature. Organic dye, Cibacron Red (CR), and a pesticide, Triclopyr (TC) were taken as model pollutants to evaluate the photodegradation performance of all samples under UV light source. Li doped ZnO nanomaterials containing 5 mol% Li attained the highest photocatalytic activity for the removal of both pollutants. The results of the study demonstrate that a strong electronic interaction between Li and ZnO results in the improvement of charge transferability and retards their recombination. The alteration in optical and surface properties of ZnO due to the substitution of Li into ZnO lattice was another reason for the superior activity of Li doped ZnO.

Comparative structural and electrochemical study of spherical ZnO with different tap density and morphology as anode materials for Ni/Zn secondary batteries

In this work, we compare the behavior of five kinds of high-density spherical ZnO with different surface morphology as anode materials for nickel/zinc rechargeable batteries to investigate the effects of microstructure and surface properties of ZnO on the electrochemical performance of zinc anodes. For the first time, five spherical ZnO samples with tap densities of 1.62, 2.00, 2.38, 2.83, and 3.12 g cm−3 have been successfully developed via a complexing co-precipitation method by adjusting the composition of alkaline solution and pH value. It is discovered that an enhanced volume specific capacity and more stable cycling performance can be achieved for the anode with spherical ZnO (3.12 g cm−3). Nevertheless, the as-obtained ZnO sample with higher tap density shows slightly worse high-rate performance and lower discharge capacity. Interestingly, as confirmed by the CV and Tafel polarization test, with the increase of ZnO’s tap density, the zinc anodes exhibit better reversibility and higher corrosion resistance. Consequently, the denser solid spherical structure of spherical ZnO is beneficial to effectively adjust its “dissolution-deposition” process, significantly suppressing the outward growth of zinc dendrites and enhancing the batteries’ cycling stability.

Influence of PDA Coating on the Structural, Optical and Surface Properties of ZnO Nanostructures.

Polydopamine (PDA) is a new biocompatible material, which has prospects in biomedical and sensor applications. Due to functional groups, it can host wide range of biomolecules. ZnO nanostructures are well known templates for optical sensors and biosensors. The combination of ZnO and PDA results in a change of optical properties of ZnO–PDA composites as a shift of photoluminescence (PL) peaks and PL quenching. However, to date, the effect of the PDA layer on fundamental properties of ZnO–PDA nanostructures has not been studied. The presented paper reports on optical and surface properties of novel ZnO–PDA nanocomposites. PDA layers were chemically synthesized on ZnO nanostructures from different solution concentrations of 0.3, 0.4, 0.5 and 0.7 mg/mL. Structure, electronic and optical properties were studied by SEM, Raman, FTIR, diffuse reflectance and photoluminescence methods. The Z-potential of the samples was evaluated in neutral pH (pH = 7.2). The response of the samples towards poly-l-lysine adsorption, as a model molecule, was studied by PL spectroscopy to evaluate the correlation between optical and surface properties. The role of the PDA concentration on fundamental properties was discussed.

Modification of the ZnO(0001) Surface by Adsorption of Hydrogen Atoms

Abstract The electronic and chemical properties of ZnO surfaces are greatly affected by the adsorption of H atoms. The interaction of H atoms with ZnO(0001) and ZnO(000-1) surfaces has been investigated by temperature programmed desorption (TPD) methods. When the ZnO(000-1) surface is exposed to atomic H at 200 K, one desorption peak of H2 is observed at 455 K (α-peak). In the case of ZnO(0001), two desorption peaks of H2 are observed at 255 K (γ-peak) and 455 K (α-peak). As H exposure increases, the intensity of the γ-peak increases as the α-peak is attenuated. The α-peak is assigned as the hydrogen adsorbed on O-sites and the γ-peak is attributed to hydrogen adsorbed on Zn clusters on ZnO. Zn clusters are produced only on the ZnO(0001) surface as surface O-H bonds are formed.

Dye wastewater treatment enabled by piezo-enhanced photocatalysis of single-component ZnO nanoparticles.

Conventionally, composite materials are usually employed as a catalyst in piezo-photocatalytic dye wastewater treatment. Here, we report the synthesis of ZnO nanoparticles, as a single-component catalyst, by surfactant-assisted precipitation in which the size of ZnO nanoparticles (20–100 nm) can be simply controlled by the use of Tween80 as a surfactant. Although, ZnO nanoparticles exhibited appreciable photocatalytic activities for the degradation of methylene blue (MB) dye, upon the addition of a mechanical force, the photocatalytic dye degradation efficiency was substantially improved. Furthermore, we postulated that the surface properties of ZnO play an important role in charge transfer phenomena based on photoluminescence results together with functional groups on the surface of ZnO. In addition, application of single-component ZnO in piezo-promoted photocatalytic degradation of cationic and anionic dyes was accomplished. Our results regarding the behaviour of single-component ZnO nanoparticles under vibrational energy in addition to their conventional solar harvesting can provide a promising strategy for developing photocatalysts for practical wastewater treatment.

Switching the Electronic Properties of ZnO Surfaces with Negative T‐Type Photochromic Pyridyl‐dihydropyrene Layers and Impact of Fermi Level Pinning

AbstractRemote control of the electronic energy levels by external stimuli such as light will enable optoelectronic devices with improved or additional functionalities. Here, it is demonstrated that the electronic properties of ZnO interfaced with negative T‐type photoswitches, that is, pyridyl‐dihydropyrene (Py‐DHP), can indeed be photomodulated. The process of forward switching of Py‐DHP with green light from an isomer with a low energy gap to an isomer with a wider one is followed by a thermally activated backward transfer. Using photoemission spectroscopy and density functional theory modeling, it is shown that Py‐DHP ring closure/opening reactions result in a reversible shift of frontier occupied molecular levels by 0.7 eV with respect to the Fermi level. Notably, in both molecular configurations, the energy level alignment at ZnO/Py‐DHP interfaces is governed by a Fermi level pinning at the lowest unoccupied molecular level. Moreover, upon switching, an increase in the ionization energy for Py‐DHP multilayers compared to that of a monolayer is observed. This is attributed to a different preferred molecular orientation in monolayer versus multilayers. The results show that a dynamic tuning of the energy level alignment at inorganic/organic interfaces by external stimuli is feasible and will aid the development of photoprogrammable optoelectronic devices.

Interfacial Modifier Having Julolidine for the Cathode Buffer Layer in PTB-7:PC70BM Based Inverted Organic Photovoltaic Cells.

The julolidine based interfacial modifier (IM-J) for cathode buffer layer following the "donor-acceptor" design concept with julolidine substituent as an electron donating moiety was incoporated to improve the surface properties of ZnO. Simple treatment of metal oxide type cathode buffer materials with organic interfacial modifier induces the enhanced photovoltaic performance and could effectively overcome several interfacial problems in inverted organic photovoltaic cells (I-OPVs). We studied on the coverage of IM-J on ZnO surface with variation of solution concentrations to reduce charge recombination and macroscopic phase separation. At the optimum condition, ZnO/IM-J (0.05 w/v%), IM-J significantly decreased the surface tension (46.1 mN/m) and improved surface morphology (RMS roughness: 0.61 nm). As a result, compared to the unmodified ZnO based device, the ZnO/IM-J based I-OPVs showed significantly improved power conversion efficiency (PCE) from 7.41 to 8.07% due to the increased photocurrent density (Jsc) and fill factor (FF). It is concluded that IM-J is one of the promising candidates for controlling electronic property of ZnO buffer layer in inverted organic photovoltaic cells. Also, our interfacial modified system can be utilized in other optoelectronic devices.

The electrical, elemental, optical, and surface properties of Si-doped ZnO thin films prepared by thermionic vacuum arc

The purpose of this work is to study the properties of Si-doped ZnO (SZO) thin films, which were prepared using the non-reactive thermionic vacuum arc technique. The analysis of the elemental, optical, and surface properties of ZnO:Si thin films was carried out using energy dispersive x-ray spectroscopy, UV–VIS spectrophotometry, atomic force microscopy, and scanning electron microscopy, respectively. The current–voltage measurement was employed in order to study the electrical properties of the films. The effect of Si doping on the physical properties of ZnO films was investigated. The film thicknesses were measured as 55 and 35 nm for glass and PET substrates, respectively. It was clearly observed from the x-ray diffraction results that the Si and ZnO peaks were present in the coated SZO films for all samples. The morphological studies showed that the deposited surfaces are homogenous, dense, and have a uniform surface, with the existence of some cracks only on the glass substrate. The elemental composition has confirmed the existence of Zn, Si, and O elements within the prepared films. Using a UV–VIS spectrophotometer, the optical parameters such as transmittance, absorbance, refractive index, and reflectance were calculated. It should be noted that the transparency and refractive indices obtained from the measurements decrease with increasing Si concentration. The obtained optical bandgap values using transmittance spectra were determined to be 3.74 and 3.84 eV for the glass and PET substrates, respectively. An increase in the bandgap results demonstrates that the Si doping concentration is comparable to the pure ZnO thin films. The current versus voltage curves revealed the ohmic nature of the films. Subsequently, the development and fabrication of excellent transparent conducting electrodes enabled the appropriate use of Si-doped ZnO thin films.

High performance planar perovskite solar cells by ZnO electron transport layer engineering

ZnO as electron extraction layer in photovoltaic devices has many advantages, including high mobility and low processing temperature. However, it has been underutilized in perovskite solar cells due to the reported instabilities of perovskite layers deposited on ZnO resulting in poor device performance. Herein, we modify the ZnO layer by incorporating Cs or Li dopants in its bulk and depositing a self-assembled monolayer on its surface. This combined approach of engineering both the bulk and surface properties of ZnO results in significant improvements in the performance of planar MAPbI3 perovskite solar cells with a maximum power conversion efficiency of 18%, accompanied by a reduction in hysteresis and a significant enhancement of the device stability. Our work makes engineered solution-processed ZnO layers a practical alternative to TiO2 as electron extraction layers in perovskite solar cells, while also eliminating the need for high temperature sintering steps from the device fabrication.

An improvement of dye-sensitized solar cell by acid treatment on modified ZnO photoanode

ABSTRACTAn acid treatment of ZnO films was presented using HCl and HNO3 to improve the ZnO surface properties for applying a photoanode in dye-sensitized solar cell. Scanning electron microscope (SEM) image showed that the ZnO films changed after the treatment. Pore area and pore density were increased and a significant decrease in reflectance was observed. The treated films were used as photoanode of dye-sensitized solar cell and efficiency was enhanced. The maximum efficiencies were 0.73% and 0.89% for optimal HCl and HNO3 treatment compared with the non-treated photoanode of 0.64%. The efficiency was enhanced directly correlated to fill factor achievement. The achievement of fill factor was caused by an increase of pore area and pore density which lead an improvement of photoanode/dye/electrolyte interface. These results implied that HCl and HNO3 have potential to improve porous property of ZnO film for dye-sensitized solar cell application.

The investigation of Ag/ZnO interface system by first principle: The structural, electronic and optical properties

Ag/ZnO interfaces have been investigated for both of Zn-termination and O-termination by the first principle based on density functional theory. Our calculations demonstrate that the Ag atoms go inward from the Ag/ZnO interface, and the Zn and O atoms are all move outward bulk in the Zn-termination interface, and the changes are just opposite for O-termination. These behaviors are in agreement with the other studies in literatures. Furthermore, an expansion situation is observed in the first two Zn-O bilayer and first three Ag monolayers for both of Zn-termination and O-termination interfaces by comparing with the pure ZnO(0001) and Ag(111) surfaces. Moreover, the valence-band both of O-2p and Zn-3d states of Ag/ZnO interface gradual close to Femi level as the Zn, O atoms locate at the deeper layer for Zn-termination, but it is the other way round for O-termination. Calculated absorption spectrum indicates that the absorption intensity of Zn-termination interface is stronger than that of O-termination in the lower energy range (visible light region). These properties of ZnO surfaces are also evaluated for comparison with interfaces.

The effect of the preparation procedure on the morphology, texture and photocatalytic properties of ZnO

This paper focuses on the modification of ZnO properties by changes in the preparation procedures, which determine the surface and photocatalytic properties. A diversity of morphologies, structural/textural and surface properties of ZnO was achieved by using three different synthetic procedures: the sol-gel method, the precipitation method with the use of oxalic acid, and a solvent evaporation procedure. The degree of crystallinity, particle size, porosity, band gap energy, and the presence and types of defect sites were measured and correlated to the preparation procedure and photocatalytic activity toward Rhodamine B (RhB) degradation. It was found that the preparation method and the use of a re-dispersion route strongly influence the ZnO morphology. In turn, the morphology determines the nature and density of surface defects. The highest photocatalytic activity towards RhB decomposition is exhibited by ZnO prepared by the precipitation method. The reaction rate is strongly affected by the crystallinity of the ZnO.

Catalytic potential of highly defective (211) surfaces of zinc blende ZnO.

Zinc blende (ZB) ZnO has gained increasing research interest due to its favorable properties and its stabilization on the nanoscale. While surface properties are important on the nanoscale, the studies on ZB ZnO surface properties are rare. Here we have performed first principles calculations of the energies and structures of ZB and wurtzite (WZ) ZnO surfaces. Our results indicate that, among the four surfaces parallel to the polar axes, such as (101̄0) and (112̄0) of the WZ phase and (110) and (211) of the ZB phase, the polar (211) surface has substantially lower surface vacancy formation energies than the others, which makes ZB ZnO promising for catalytic applications. Our results also imply that the stabilization of ZB ZnO on the nanoscale is due to some mechanisms other than surface energies.

Development of a julolidine-based interfacial modifier for efficient inverted polymer solar cells

We demonstrate the julolidine moiety is an excellent electron donating building block for developing interfacial modifiers to improve the ZnO surface properties in inverted polymer solar cells.

Study the Effect of Molar Concentration on the Optical and Surface Properties of ZnO Thin Films Prepared by Spray Pyrolysis

Study the Effect of Molar Concentration on the Optical and Surface Properties of ZnO Thin Films Prepared by Spray Pyrolysis

Improved Exciton Dissociation at Semiconducting Polymer:ZnO Donor:Acceptor Interfaces via Nitrogen Doping of ZnO.

Exciton dissociation at the zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) interface as a function of nitrogen doping of the zinc oxide, which decreases the electron concentration from approximately 1019 cm−3 to 1017 cm−3, is reported. Exciton dissociation and device photocurrent are strongly improved with nitrogen doping. This improved dissociation of excitons in the conjugated polymer is found to result from enhanced light-induced de-trapping of electrons from the surface of the nitrogen-doped ZnO. The ability to improve the surface properties of ZnO by introducing a simple nitrogen dopant has general applicability.