Nanoscale Zinc Oxide Research Articles

The growth of nanoscale ZnO films by pulsed-spray evaporation chemical vapor deposition and their structural, electric and optical properties

Great interest in nanoscale thin films (sub-100 nm) has been stimulated by the developing demands of functional devices. In this paper, nanoscale zinc oxide (ZnO) thin films were deposited on glass substrates at 300 °C by pulsed-spray evaporation chemical vapor deposition. Scanning electron micrographs indicate uniform surface morphologies composed of nanometer-sized spherical particles. The growth kinetics and growth mode are studied and the relationship between the film thickness and the electric properties with respect to the growth mode is interpreted. X-ray diffraction shows that all ZnO films grown by this process were crystallized in a hexagonal structure and highly oriented with their c-axes perpendicular to the plane of the substrate. Optical measurements show transparencies above 85% in the visible spectral range for all films. The absorbance in the UV spectral range respects well the Beer–Lambert law, enabling an accurate optical thickness measurement, and the absorption coefficient was measured for a selected wavelength. The measured band gap energies exhibit an almost constant value of 3.41 eV for all films with different thicknesses, which attributed to the thickness-independent crystallite size.

Structural Isomerism and Thermodynamic Properties of the Methylzinc Alkoxide Molecules (MeZnOR)n (R = Me, tBu) and Cations [(MeZnOMe)n]+ (n = 3, 4) Studied by B3LYP and PCM Calculations

Zinc oxide is a semiconductor as well as a catalyst in important industrial processes, and alkylzinc alkoxide species of the type (RZnOR)(n) are precursors for the preparation of nanoscaled zinc oxide particles. In this work, the structures, thermodynamic properties, dipole moments, and molecular vibrations of substituted zinc oxide molecules and cations have been studied by density functional calculations at the B3LYP/6-31+G(2df,p) level. The neutral tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) are cubane-like species of T(d) symmetry in the gas phase, whereas the cation [(MeZnOMe)(4)](+) is approximately of C(3v) symmetry due to a Jahn-Teller distortion. For the neutral trimers (MeZnOMe)(3) and (MeZnO(t)Bu)(3) two structural isomers of almost identical energy were located on the potential energy surfaces: A closo-cluster with eight ZnO bonds is most stable both as isolated molecule and in a polarizable continuum, but a roof-shaped structure with seven ZnO bonds differs in enthalpy by less than 8 kJ mol(-1). In the case of the analogous cation [(MeZnOMe)(3)](+), the isomer with the roof-shaped Zn(3)O(3) skeleton is 31.7 kJ mol(-1) more stable in the gas phase than the cluster isomer. A particularly long Zn-C bond in the most stable cationic tri- and tetramers explains the loss of a methyl group, observed mass spectrometrically after electron impact ionization. The dissociation of the tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) into 4/3 trimers is endothermic and endergonic, but less so for the tert-butyl derivative compared to the all-methyl cluster. This destabilizing effect of tert-butyl substituents on the tri- and tetrameric clusters follows also from the larger internuclear distances between the non-hydrogen atoms in the structures of (MeZnO(t)Bu)(n) (n = 3 and 4) compared to the analogous all-methyl derivatives. Remarkable differences of the atomic charges of the carbon atoms of methyl or tert-butyl groups attached to oxygen have been found.

Fabrication of superhydrophobic surfaces with double-scale roughness

A surface with micro- and nano-scale ZnO (zinc oxide) structure was fabricated by alkaline hydrothermal method. The CA (contact angle) on this double-roughness surface was low (down to ∼ 0°), but after spin-coating Teflon, the CA was increased to ∼ 168°. By observing with SEM, a “rose” like ZnO crystal surface structure was found, which was responsible for both superhydrophilicity and superhydrophobicity. The prepared surface also showed high chemical stability even after immersing the sample in water for 15 days. The method is simply controllable, cost-effective, and has a wide range of potential applications such as self-cleaning superhydrophobic coating on large areas of different substrates.

Synthesis, Characterization, Defect Chemistry, and FET Properties of Microwave-Derived Nanoscaled Zinc Oxide

The formation of nanoscale zinc oxide particles with an almost-monomodal size distribution synthesized by microwave heating of solutions of mononuclear zinc oximato or zinc acetylacetonato complexes in various alkoxyethanols is investigated. Transparent stable suspensions that contain these particles can be obtained from the zinc oximato precursor. Based on electron paramagnetic resonance (EPR) studies, a core/shell model with a finite surface shell thickness of 1.000 ± 0.025 nm is proposed for the ZnO nanoparticles. Field-effect transistor (FET) devices with these ZnO particles as the active semiconducting layer exhibited a charge carrier mobility of 0.045 cm2/(V s) and Ion/off current ratios of ∼460.000, with a threshold voltage of 8.78 V.

Surface Functionalization of Zinc Oxide by Carboxyalkylphosphonic Acid Self-Assembled Monolayers

Two carboxyalkylphosphonic acids (HOOC(CH(2))(n)P(O)(OH)(2), n = 2 for 3-PPA and n = 9 for 10-PDA) have been deposited onto 1D zinc oxide (ZnO) nanowires and bare ZnO wafers to form stable self-assembled monolayers (SAMs). The samples were systematically characterized using wettability, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). 3-PPA was bound to the ZnO surfaces mainly through the CO(2)H headgroup, and 10-PDA formed self-assembled monolayers on the nanoscaled ZnO surface through the PO(3)H(2) headgroups. To verify the potential utilization of the functionalized surfaces in the construction of biosensors or bioelectronics, IgG (immunoglobulin G) protein immobilization through SAM bridging was demonstrated. This work expands the application of phosphonic acid-based surface functionalization on sensing and optoelectronic devices.

Application of Flexible Field-Effect Transistor Device with Nanoscale Zinc Oxide as Active Semiconductor Material

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Nanosized zinc oxide particles induce neural stem cell apoptosis

Given the intensive application of nanoscale zinc oxide (ZnO) materials in our life,growing concerns have arisen about its unintentional health and environmentalimpacts. In this study, the neurotoxicity of different sized ZnO nanoparticles in mouseneural stem cells (NSCs) was investigated. A cell viability assay indicated that ZnOnanoparticles manifested dose-dependent, but no size-dependent toxic effectson NSCs. Apoptotic cells were observed and analyzed by confocal microscopy,transmission electron microscopy examination, and flow cytometry. All the resultssupport the viewpoint that the ZnO nanoparticle toxicity comes from the dissolvedZn2+ in the culture medium or inside cells. Our results highlight the need for caution during theuse and disposal of ZnO manufactured nanomaterials to prevent the unintendedenvironmental and health impacts.

A Printed and Flexible Field‐Effect Transistor Device with Nanoscale Zinc Oxide as Active Semiconductor Material

A family of single-source precursors for low temperature processing of uniform, transparent, and adherent ZnO thin films on various substrates is studied. They decompose cleanly under very mild processing conditions of 150 °C. The resulting ZnO thin films exhibit promising semiconducting properties when printed in a field-effect transistor (FET) device structure. This class of precursor compounds is compatible with existing printing technologies and allows printing of semiconductors on flexible polymer substrates under mild conditions.

Can in vitro assays substitute for in vivo studies in assessing the pulmonary hazards of fine and nanoscale materials?

Risk evaluations for nanomaterials require the generation of hazard data as well as exposure assessments. Most of the validated nanotoxicity studies have been conducted using in vivo experimental designs. It would be highly desirable to develop in vitro pulmonary hazard tests to assess the toxicity of fine and nanoscale particle-types. However, in vitro evaluations for pulmonary hazards are known to have limited predictive value for identifying in vivo lung toxicity effects. Accordingly, this study investigated the capacity of in vitro screening studies to predict in vivo pulmonary toxicity of several fine or nanoparticle-types following exposures in rats. Initially, complete physicochemical characterization of particulates was conducted, both in the dry and wet states. Second, rats were exposed by intratracheal instillation to 1 or 5 mg/kg of the following particle-types: carbonyl iron, crystalline silica, amorphous silica, nanoscale zinc oxide, or fine zinc oxide. Inflammation and cytotoxicity endpoints were measured at 24 h, 1 week, 1 month and 3 months post-instillation exposure. In addition, histopathological analyses of lung tissues were conducted at 3 months post-exposure. Pulmonary cell in vitro studies consisted of three different culture conditions at 4 different time periods. These included (1) rat L2 lung epithelial cells, (2) primary rat alveolar macrophages, and (3) alveolar macrophage—L2 lung epithelial cell co-cultures which were incubated with the same particles as tested in the in vivo study for 1, 4, 24, or 48 h. Cell culture fluids were evaluated for cytotoxicity endpoints and inflammatory cytokines at the different time periods in an attempt to match the biomarkers assessed in the in vivo study. Results of in vivo pulmonary toxicity studies demonstrated that instilled carbonyl iron particles produced little toxicity. Crystalline silica and amorphous silica particle exposures produced substantial inflammatory and cytotoxic effects initially, but only the crystalline silica variety produced sustained and progressive inflammatory and cytotoxic responses, leading to the development of pulmonary fibrosis. Exposures to nanoscale or fine-sized zinc oxide particles produced potent but typical “metal fume fever”-like reversible inflammation/cytotoxic effects which were resolved by 1-month postinstillation exposure. In contrast to the in vivo results, using cytotoxicity and inflammation endpoints, in vitro effects to the various particle-types were difficult to gauge, owing to the number of variables that were studied (i.e., cell-types, time-course, dose response (including particle overload doses)), and various endpoints (e.g., cytotoxicity = LDH, MTT; inflammation/cytokines = MIP-2). For instance, none of the in vitro endpoints could mimic a transient inflammatory/cytotoxic response—as was measured following exposures to amorphous silica, or fine or nanoscale zinc oxide particles. We conclude that current in vitro cell culture systems do not accurately forecast the pulmonary hazard responses of instilled particle-types. It seems clear that in vitro cellular systems will need to be further developed, standardized, and validated (relative to in vivo effects) in order to provide useful screening data on the relative toxicity of inhaled particles.

Synthesis, characterization, and photocatalytic activities of titanate nanotubes surface-decorated by zinc oxide nanoparticles

Nanoscaled zinc oxide (ZnO) particles with different amounts are coated on titanate nanotubes (TNTs) by a facile chemical method at room temperature. The characterizations of XPS, TEM, XRD and UV–vis spectra confirm that pure hexagonal wurtzite ZnO nanoparticles with an average size of about 9 nm are distributed on the surfaces of TNTs evenly and attached strongly. The photocatalytic activities of the ZnO–TNTs nanocomposite are superior to those of P25, ZnO, TNTs and ZnO–anatase TiO 2 (TNP) nanocomposite in the oxidation of rhodamine B under UV light irradiation. A comparison of the photocatalytic activities between different catalysts is discussed. Furthermore, we also find that the ZnO–TNT nanocomposite shows very favorable recycle use potential, because they have a high sedimentation rate and their photocatalytic activity is only slightly decreased even after five times of repeated uses.

Comparative toxicity of several metal oxide nanoparticle aqueous suspensions to Zebrafish (Danio rerio) early developmental stage

With the emergence of manufactured nanomaterials, it is urgent to carry out researches on their potential environmental impacts and biological effects. To better understand the potential ecotoxicological impacts of metal oxide nanoparticles released to aquatic environments, the zebrafish 96-h embryo-larval bioassay was used to assess and compare the developmental toxicities of nanoscale zinc oxide (nZnO), titanium dioxide (nTiO2) and alumina (nAl2O3) aqueous suspensions. Toxicological endpoints such as zebrafish embryos or larvae survival, hatching rate and malformation were noted and described within 96 h of exposure. Meanwhile, a comparative experiment with their bulk counterparts (i.e., ZnO/bulk, TiO2/bulk and Al2O3/bulk) was conducted to understand the effect of particle size on their toxicities. The results showed that: (i) both nZnO and ZnO/bulk aqueous suspensions delayed zebrafish embryo and larva development, decreased their survival and hatching rates, and caused tissue damage. The 96-h LC50 of nZnO and ZnO/bulk aqueous suspensions on the zebrafish survival are 1.793 mg/L and 1.550 mg/L respectively; and the 84-h EC50 on the zebrafish embryo hatching rate are 2.065 mg/L and 2.066 mg/L respectively. Serious tissue ulceration was found on zebrafish larvae exposed to nZnO and ZnO/bulk aqueous suspensions. (ii) In contrast, neither nTiO2 and TiO2/bulk nor nAl2O3 and Al2O3/bulk showed any toxicity to zebrafish embryos and larvae under the same experimental condition. It revealed that the metal oxide nanoparticles with different chemical composition have different zebrafish developmental toxicities. (iii) Exposures of nTiO2, nZnO and nAl2O3 produced toxic effects on zebrafish embryos and larvae, which was not different from the effects caused by exposing to their bulk counterparts. This is the first study about the developmental toxicity of metal oxide nanoparticles, and the results demonstrate that nZnO is very toxic to zebrafish embryos and larvae, which highlights the need to evaluate the potential eco-toxicity of these manufactured nanomaterials (MNMs).

Nanoscale ZnO and Al‐Doped ZnO Coatings on ZnS:Ag Phosphors and their Cathodoluminescent Properties

Blue‐emitting ZnS:Ag phosphors were coated with nanoscale zinc oxide (ZnO) and Al‐doped ZnO (AZO) and the cathodoluminescence (CL) performance of the coated phosphors was investigated. The nanoscale coating was obtained by controlling the hydrolysis reaction of ZnCl2 alcoholic solution using diethylamine as a gradual OH− former, and Al doping was performed to increase the electrical conductivity of ZnO. The coatings were composed of either nanosized particles or a continuous nano layer, and the AZO‐coated phosphor had a more uniformly covered surface. The coated phosphors exhibited improved aging behavior under CL excitation and the AZO coating was more effective at suppressing the degradation, possibly due to its uniformity and high conductivity.

Ultrasensitive DNA sequence detection using nanoscale ZnO sensor arrays

We report that engineered nanoscale zinc oxide structures can be effectively used for theidentification of the biothreat agent, Bacillus anthracis by successfully discriminating itsDNA sequence from other genetically related species. We explore both covalent andnon-covalent linking schemes in order to couple probe DNA strands to the zinc oxidenanostructures. Hybridization reactions are performed with various concentrations of targetDNA strands whose sequence is unique to Bacillus anthracis. The use of zinc oxidenanomaterials greatly enhances the fluorescence signal collected after carrying outduplex formation reaction. Specifically, the covalent strategy allows detection of thetarget species at sample concentrations at a level as low as a few femtomolar ascompared to the detection sensitivity in the tens of nanomolar range when using thenon-covalent scheme. The presence of the underlying zinc oxide nanomaterials iscritical in achieving increased fluorescence detection of hybridized DNA and,therefore, accomplishing rapid and extremely sensitive identification of the biothreatagent. We also demonstrate the easy integration potential of nanoscale zinc oxideinto high density arrays by using various types of zinc oxide sensor prototypesin the DNA sequence detection. When combined with conventional automaticsample handling apparatus and computerized fluorescence detection equipment, ourapproach can greatly promote the use of zinc oxide nanomaterials as signal enhancingplatforms for rapid, multiplexed, high-throughput, highly sensitive, DNA sensorarrays.

Highly Sensitive Biomolecular Fluorescence Detection Using Nanoscale ZnO Platforms

Fluorescence detection is currently one of the most widely used methods in the areas of basic biological research, biotechnology, cellular imaging, medical testing, and drug discovery. Using model protein and nucleic acid systems, we demonstrate that engineered nanoscale zinc oxide structures can significantly enhance the detection capability of biomolecular fluorescence. Without any chemical or biological amplification processes, nanoscale zinc oxide platforms enabled increased fluorescence detection of these biomolecules when compared to other commonly used substrates such as glass, quartz, polymer, and silicon. The use of zinc oxide nanorods as fluorescence enhancing substrates in our biomolecular detection permitted sub-picomolar and attomolar detection sensitivity of proteins and DNA, respectively, when using a conventional fluorescence microscope. This ultrasensitive detection was due to the presence of ZnO nanomaterials which contributed greatly to the increased signal-to-noise ratio of biomolecular fluorescence. We also demonstrate the easy integration potential of zinc oxide nanorods into periodically patterned nanoplatforms which, in turn, will promote the assembly and fabrication of these materials into multiplexed, high-throughput, optical sensor arrays. These zinc oxide nanoplatforms will be extremely beneficial in accomplishing highly sensitive and specific detection of biological samples involving nucleic acids, proteins and cells, particularly under detection environments involving extremely small sample volumes of ultratrace-level concentrations.

Polystyrene nanocomposite materials—Preparation, mechanical, electrical and thermal properties, and morphology

AbstractThis work prepared polystyrene resin nanocomposites with antistatic properties, by melt‐blending polystyrene with nanoscale zinc oxide. The effect of nanoscale zinc oxide on the electrical and physical characteristics of polystyrene nanocomposites was investigated. Two kinds of nanoscale powders, spherical zinc oxide (s‐ZnO) and zinc oxide whisker (w‐ZnO), were selected. The coupling agents, vinyltriethoxysilane and phenyltriethoxysilane, were utilized to improve the compatibility between nanopowders and polystyrene resin. Adding spherical zinc oxide and zinc oxide whisker improved the antistatic characteristic of materials. The surface resistivities of s‐ZnO and w‐ZnO nanocomposite were significantly reduced, by modification with vinyltriethoxysilane and phenyltriethoxysilane. Adding zinc oxide nanopowder increased the flexural modulus and reduces flexural strength. Silane coupling agent improved the flexural properties of nanocomposite. The glass transition temperature and thermal degradation temperature of zinc oxide/polystyrene nanocomposite increased with ZnO content. Treatment with silane increased the glass transition temperature and thermal degradation temperature of composite. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 508–515, 2006

Hydrogen in bulk and nanoscale ZnO

Zinc oxide (ZnO) is a wide-band gap semiconductor that has attracted resurgent interest as an electronic material for a range of applications. In our work, we have focused on the properties of hydrogen donors in ZnO. In bulk, single-crystal ZnO doped with hydrogen or deuterium, infrared (IR) absorption peaks corresponding to O–H and O–D stretch modes were observed at 3326.3 and 2470.3 cm −1, respectively, at liquid-helium temperatures. IR spectroscopy using polarized light showed that the O–H dipoles are aligned along an angle 111° to the c-axis. The slight negative shift of the O–H mode as a function of hydrostatic pressure suggests that the hydrogen resides in an anti-bonding orientation. The O–H complexes are unstable, perhaps due to the formation of “hidden” H 2 molecules. As dimensions approach the nanoscale, the vastly increased surface-to-volume ratio leads to interesting phenomena. At moderate annealing temperatures (350 °C), hydrogen interacts with the nanoparticles, resulting in a dramatic increase in electrical conductivity, free-carrier absorption, and IR reflectivity.

Synthesis and photocatalytic activity of silane-coated and UV-modified nanoscale zincoxide

A new nanoporous material consisting of nano-ZnO particles bridged by –O–Si–O–Si–O– units hasbeen synthesized and characterized. The two-step synthesis involves a precipitation of zinc oxide froma 1 mol l−1 methanolic zinc acetate solution with methanolic KOH in the presence of alkoxyalkylsilanesas coating agents and a UV irradiation of the obtained material in aqueous suspension withair supply. This procedure leads to a nanoscale ZnO network with a high surface area(130 m2 g−1), a better chemical stability than other types of ZnO and a remarkable photocatalytic activity.The photocatalytic mineralization of tetrachloroethylene occurs in the presence of this material atpH = 8 even faster than with the well known and most appreciatednano-TiO2 (anatase).

Zinc Oxide Nanoparticles with Defects

AbstractZinc oxide in the form of nanoscale materials can be regarded as one of the most important semiconductor oxides at present. However, the question of how chemical defects influence the properties of nanoscale zinc oxide materials has seldom been addressed. In this paper, we report on the introduction of defects into nanoscale ZnO, their comprehensive analysis using a combination of techniques (powder X‐ray diffraction (PXRD), X‐ray absorption spectroscopy/extended X‐ray absorption fine structure (XAS/EXAFS), electron paramagnetic resonance (EPR), magic‐angle spinning nuclear magnetic resonance (MAS‐NMR), Fourier‐transform infrared (FTIR), UV‐vis, and photoluminescence (PL) spectroscopies coupled with ab‐initio calculations), and the investigation of correlations between the different types of defects. It is seen that defect‐rich zinc oxide can be obtained under kinetically controlled conditions of ZnO formation. This is realized by the thermolysis of molecular, organometallic precursors in which ZnO is pre‐organized on a molecular scale. It is seen that these precursors form ZnO at low temperatures far from thermodynamic equilibrium. The resulting nanocrystalline ZnO is rich in defects. Depending on conditions, ZnO of high microstructural strain, high content of oxygen vacancies, and particular content of heteroatom impurities can be obtained. It is shown how the mentioned defects influence the electronic properties of the semiconductor nanoparticles.

Polystyrene nanocomposite materials: Preparation, morphology, and mechanical, electrical, and thermal properties

AbstractIn this study, we prepared polystyrene (PS) resin nanocomposites with antistatic properties by melt‐blending PS with nanoscale zinc oxide (ZnO). The effects of nanoscale ZnO on the electrical and physical characteristics of the PS nanocomposites were investigated. Two kinds of nanoscale powders, spherical zinc oxide (s‐ZnO) and zinc oxide whisker (w‐ZnO), were selected. The coupling agents vinyltriethoxysilane (VTES) and phenyltriethoxysilane (PTES) were used to improve the compatibility between the nanopowders and PS resin. The addition of s‐ZnO and w‐ZnO improved the antistatic characteristics of the materials. The surface resistivities of the s‐ZnO and w‐ZnO nanocomposites were significantly reduced by modification with VTES and PTES. The addition of ZnO nanopowder increased the flexural modulus and reduced the flexural strength. The silane coupling agents improved the flexural properties of the nanocomposites. The glass‐transition temperatures and thermal degradation temperatures of the ZnO/PS nanocomposites increased with ZnO content. Treatment with silane increased the glass‐transition temperatures and thermal degradation temperatures of the composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2266–2273, 2005

Formation of cellular films consisting of wurtzite-type zinc oxide nanosheets by mediation of phosphate anions

We successfully prepared a wide variety of wurtzite-type zinc oxide films exhibiting columnar, cellular and densely packed morphologies in a simple aqueous solution system containing phosphate anions. As the phosphate concentration increased, the shape of crystalline units composing the films varied from hexagonal needles into seaweed-like sheets. A novel type of open cellular structures was obtained by assembly of nanoscale zinc oxide sheets covered with phosphate. Specific adsorption of phosphate anions on (001) of the wurtzite-type crystal flattened the crystal grains, and then induced the structural evolution into a cellular form. A blue shift of the absorption edge suggested that the quantum size effect occurred in the nanoscale platy crystals composing the cellular films.