ZnO Content Research Articles

A Comparative Study of ZnS–ZnO Nanocomposite Assembly for Photocatalytic Removal of Crystal Violet Dye

Herein, the effect of composition on the optical and photocatalytic performance of ZnS–ZnO composite is described. The composite samples are synthesized by chemical precipitation method using (NaOH + Na2S) mixture as the precipitating agent. The chemical composition of precipitating agent results in in situ formation of ZnO and ZnS. Lower content of NaOH in precipitating agent solution (1:3 by volume) exhibits the formation of ZnS, while, its higher concentration induces the phase separation with higher volume fraction of ZnO as observed from X‐ray diffraction patterns. Optical studies suggest the enhancement of visible absorption along with redshift in the absorption edge of composites as compared with bare ZnS nanoparticles (NPs). Moreover, the incorporation of ZnO with ZnS is also supported by optical emission spectra in which intense emission around 507 nm is associated with S‐species. ZnO encapsulation on ZnS NPs exhibits higher emission associated with O‐ defects as compared to S‐species. Since both are near‐UV excited semiconductors, both contribute toward the generation of charge carriers to produce radicals and hence photo‐oxidation of crystal violet dye. Further, little concentration of ZnO with ZnS NPs exhibits comparable photocatalytic efficiency (≈88%) that is observed with other ZnS–ZnO nanocomposite samples.

The Effect of Silver Content in ZnO–Ag Nanoparticles on Their Photochemical and Antibacterial Activity

The Effect of Silver Content in ZnO–Ag Nanoparticles on Their Photochemical and Antibacterial Activity

The Effect of Silver Content in ZnO–Ag Nanoparticles on Their Photochemical and Antibacterial Activity

The development of new materials with antibacterial properties is a promising direction in the field of nanotechnology. In this work, ZnO–Ag nanoparticles with a silver content of 0.1–50 at % have been fabricated by the exploding wire method. ZnO–Ag nanoparticles absorb visible light and destroy the model dye Rhodamine B. The introduction of silver into nanoparticles has made it possible to shift the main absorption edge to 1.59–2.74 eV. The determined optimal content of silver in nanoparticles of 12 at % has ensured the degree of Rhodamine B decoloration by 85% within 60 min of exposure to visible light and has completely stopped the growth of E. coli bacteria at a concentration of 15.6 µg/mL. In addition, nanoparticles containing 12 at % silver have sterilized a sample of natural water contaminated with microorganisms. The results obtained offer an efficient method for the synthesis of antibacterial nanocomposites with heterojunctions employing a high-performance technique for producing nanoparticles, namely, the exploding wire method.

Investigation of structural, morphological and photoluminescence properties in ZnO: Co2+ nanostructures prepared by solution combustion technique using Mimosa pudica leaf extract as a fuel

Novel solution combustion technique was equipped for the synthesis of ZnO:Co2+ (1–11 mol%) nanostructures using mimosapudica (MP) leaf extract as a fuel. The prepared sample exhibits highly crystalline structure showing mushroom like morphological features. This might be due to the experimental reaction took place at the time of synthesis and evident that morphology was highly dependant on concentration of leaf extract. The obtained product of ZnO:Co2+ through MP leaf extract was examined through several characterization techniques. The packing diagram obtained using Rietveld refinement data shows the formation of hexagonal wurtzite structure having crystallite size 43–68 nm, cell volume ∼ 47.58 Å3 and density of atoms ∼ 8.42 × 1022 atoms percm3. From FTIR spectra the absorption peak in the range 500–650 cm−1exhibits the Zn-O stretching frequency having the high orientation of ZnO wurtzite structure. The energy band gap increases with Co2+ ion concentration in ZnO. The Photoluminescence (PL) emission spectra was observed in blue and green region at 422–435 & 524 nm respectively due to the presence of Zinc and oxygen vacancies. The CIE coordinate value reflecting in blue and green region confirms that the ZnO:Co2+ can be used as a one of the component in production of artificial white light LEDs/solid state display applications.

Effects of CeO2 Nanomaterial on Pseudomonas Aeruginosa Mediated Bronchiectasis Inflammation Response

There are many researches on the preparation of rare earth metals (REM) materials and bactericidal materials, but there are few researches on the preparation of REM oxides based on the laser evaporation (LE) method in the field of bactericidal. In this research, ZnO–CeO2 nanoparticles (NPs) were obtained by the LE method with 100 W single-mode CW CO2 laser as the light source and cerous carbonate and zinc acetate as the target materials. While physical characterization of the ZnO–CeO2 NPs, Staphylococcus aureus (SA), Bacillus subtilis var niger (BSVN), and pseudomonas aeruginosa (PAE) were undertaken as test objects to verify the bactericidal performance of the nanomaterial. A PAE mediated inflammation model of bronchiectasis rats was established, and 40 SD rats were screened and rolled into 4 groups: a control (Ctrl) group, a sham group, a model group without nano solution injection (Model group), and a model group injected with nano solution (Model +NPs group). The effects of nanomaterials on inflammation were analyzed. The results suggested that ZnO–CeO2 NPs were scattered rod particles with uniform morphological distribution. XRD tests revealed that with the increase of ZnO content, its characteristic peaks in the NPs also increased. The preparation of NPs exhibited good bactericidal performance against SA and BSVN, and increasing the concentration of NPs greatly inhibited the activity of PA. By comparing with the model group, the upregulated IL-17 and IL-6 were inhibited (P <0.01), while the downregulated IL-10 and TGF-β were reversed (P <0.01). It meant that the NPs alleviated the inflammation of bronchiectasis mediated by PA, and thus improved the lung function damaged by inflammation.

ELECTROSPINNING OF ANTIBACTERIAL CELLULOSE ACETATE NANOFIBERS

"Herein, it was aimed to achieve antibacterial cellulose acetate (CA) nanofiber production and characterization. Firstly, solution properties, such as viscosity, conductivity and surface tension, were determined. Secondly, CA/zinc oxide (ZnO) composite nanofibers were produced with optimum process parameters via the electrospinning method. Then, the electrospun nanofibers were characterized by SEM, EDX, DSC, TGA, XRD, air permeability and water vapor permeability testing. Lastly, antibacterial activity tests were carried out in accordance with the AATCC100 method, against Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922). According to the results, solution conductivity decreased and surface tension did not change with ZnO concentration. On the other hand, viscosity decreased significantly with the first addition of ZnO and then increased slightly with increasing ZnO concentration. Generally, fine (354–464 nm), uniform and beadless nanofibers were obtained. Average fiber diameter, air permeability and water vapor permeability increased with ZnO concentration. EDX analysis results verified the existence of ZnO in the structure of CA nanofibers. As a result of antibacterial studies, it was determined that the CA/zinc oxide (ZnO) composite nanofibers with the highest concentration of ZnO showed very good antibacterial activity against both S. aureus and E. coli bacterial strains."

Investigation of electrophysical, photo- and gas-sensitive properties of ZnO–SnO2 sol–gel films

Thin nanocomposite films based on tin dioxide with a low content of zinc oxide (0.5–5[Formula: see text]mol.%) were obtained by the sol–gel method. The synthesized films are 300–600[Formula: see text]nm thick and contains pore sizes of 19–29[Formula: see text]nm. The resulting ZnO–SnO2 films were comprehensively studied by atomic force and Kelvin probe force microscopy, X-ray diffraction, scanning electron microscopy, and high-resolution X-ray photoelectron spectroscopy spectra. The photoconductivity parameters on exposure to light with a wavelength of 470[Formula: see text]nm were also studied. The study of the photosensitivity kinetics of ZnO–SnO2 films showed that the film with the Zn:Sn ratio equal to 0.5:99.5 has the minimum value of the charge carrier generation time constant. Measurements of the activation energy of the conductivity, potential barrier, and surface potential of ZnO–SnO2 films showed that these parameters have maxima at ZnO concentrations of 0.5[Formula: see text]mol.% and 1[Formula: see text]mol.%. Films with 1[Formula: see text]mol.% ZnO exhibit high response values when exposed to 5–50[Formula: see text]ppm of nitrogen dioxide at operating temperatures of [Formula: see text]C and [Formula: see text]C.

Fabrication and characterization of TiO2: ZnO thin films as electron transport material in perovskite solar cell (PSC)

In developing quicker, smaller, and more efficient optoelectronic devices, composite thin films are becoming progressively crucial. One reason for using composite thin films is to improve materials optical and electrical properties. Thermal Evaporation is a type of physical vapor deposition that can produce thin films with well-controlled nanostructures that are pure, homogeneous, and conformal. The prime focus of this study was to fabricate TiO2 and composite TiO2:ZnO thin films on ITO-coated glass substrates by thermal Evaporation. The goal of this study was to investigate the optical, structural, and electrical properties of deposited thin films. The optical properties were assessed using UV–Vis and Photoluminescence spectroscopy, and FTIR spectroscopy was used to identify the fingerprint of materials. The crystal structure of deposited thin films was investigated using X-ray diffraction. The changes in morphology induced by adding ZnO content to TiO2 were examined using optical microscopy. SEM investigation corroborated the granular-like structure of thin films, whereas EDX demonstrated the doping and composition of ZnO concentration in TiO2. The electrical measurements were performed using the Van der Pauw technique. Adding ZnO into TiO2 improved transmission, photoconductivity, and optical bandgap to generate a viable electron transport material (ETM) for efficient perovskite solar cells (PSC). XRD results revealed the grain size increase after adding ZnO into TiO2. The transmission was increased to 95%, while the bandgap was decreased from 3.8 eV to 3.68 eV. It was also revealed that after integrating ZnO into TiO2, the generated thin films have stronger visible photoluminescence peaks, and the material's conductivity has been improved. The results indicate that TiO2:ZnO could be employed as an ETM in PSC to improve productivity.

Impact of ZnO on structural and electrochemical properties of silver spinel ferrites for asymmetric supercapacitors

Supercapacitors are receiving great scientific attention as energy storage devices due to their rapid charge/discharge rates, high power densities, and high stability. Silver spinel ferrite (AgFe2O4) and its composites AgFe2O4@ZnO (5 & 10 %) have been synthesized by using the hydrothermal method. XRD showed that the prepared samples have a cubic spinel structure. The crystallite sizes of the samples vary from 20 to 27 nm. TEM results showed that all ferrite particles had a spherical morphology. In the UV–vis absorption spectrum, the estimated Eg was 4.9 eV for AgFe2O4 and Eg increased with higher ZnO concentration. The specific capacity for AgFe2O4@10 %ZnO is 585.4 C/g which is higher than 497 C/g for AgFe2O4@5%ZnO and 394.2 C/g for AgFe2O4. For making an asymmetric device AgFe2O4@10 %ZnO//AC, activated carbon was chosen as the negative electrode and AgFe2O4@10 %ZnO as the positive electrode. With this device, a specific capacity of 171.3 C/g was attained. AgFe2O4@10 %ZnO was found to have a power density of 720 W/kg and an energy density of 28 Wh/kg. This work indicates that a mixture of silver spinel ferrite composite (AgFe2O4@10 %ZnO) may be a more suitable electrode material for supercapacitor applications.

Biodegradation of synthetic PVP biofilms using natural materials and nanoparticles

Abstract Biodegradable nanofilms from polyvinyl pyrrolidone (PVP), carboxymethyl cellulose (CMC), citric acid (CA), glycerol (G), and zinc oxide nanoparticles (ZnO-NPs) were prepared using different ZnO concentrations and different electron beam irradiation doses, enabling crosslinking formation. The prepared films were characterized by X-ray diffractometer, Fourier transform infrared spectroscopy, thermogravimetric analyser, and transmission electron microscopy. The swelling percentage of PVP:CMC films was ordered in the sequence of composition ratio 1:2 > 1:1 > 2:1. Results showed decrease in swelling capacity accompanied by increase in gelation percentage of (PVP:CMC)/CA/G)/ZnO nanofilms as the irradiation dose increased up to 20 kGy. The tensile strength of (PVP:CMC) films increased by the incorporation of ZnO-NPs and increasing the irradiation dose. The thermal stability of the prepared (PVP:CMC)/CA/G/ZnO nanofilms was enhanced as the irradiation dosage increased. The water vapour transmission rate of the irradiated films was decreased. The biodegradability of the prepared nanofilms was monitored during 16 weeks and it exceeded 65% weight loss from the original blank weight. Moreover, the nanofilms exhibit antimicrobial activity against fungi, Gram-negative, and Gram-positive bacteria. The broad antimicrobial activity spectrum of the prepared nanofilms increased as the concentration of ZnO-NPs increased. These results suggested that (PVP:CMC)/CA/G/ZnO nanofilms can serve as biodegradable materials in various applications characterized by antimicrobial activity.

Disturbances in growth, oxidative stress, energy reserves and the expressions of related genes in Daphnia magna after exposure to ZnO under thermal stress

The toxicological effects of metal contamination are influenced by the ambient temperature. Therefore, global warming affects the toxicity of metal contamination in aquatic ecosystems. ZnO is widely used as a catalyst in many industries, and causes contamination in aquatic ecosystems. Here, we investigated the effects of ZnO concentration under elevated temperature by observing growth, oxidative stress, energy reserves and related gene expression in exposed Daphnia magna. Body length and growth rate increased in neonates exposed to ZnO for 2 days but decreased at 9 and 21 days under elevated temperature. ZnO concentration and elevated temperature induced oxidative stress in mature D. magna by reducing superoxide dismutase (SOD) activity and increasing malondialdehyde (MDA) levels. In contrast, juveniles were unaffected. Carbohydrate, protein and caloric contents were reduced throughout development in D. magna treated with ZnO and elevated temperature in all exposure periods (2, 9 and 21 days). However, lipid content also decreased in mature D. magna treated with ZnO cultured under elevated temperature, while that of juveniles showed an increase in lipid content. Therefore, energy was perhaps allocated to physiological processes for detoxification and homeostasis. Moreover, expression patterns of genes related to physiological processes changed under elevated temperature and ZnO exposure. Taken together, our results highlight that the combination of temperature and ZnO concentration induced toxicity in D. magna. This conclusion was confirmed by the Integrated Biological Response (IBR) index. This study shows that changes in biological levels of organization could be used to monitor environmental change using D. magna as a bioindicator.

Influence of ZnO (Nano)Rods on the Dielectric Properties of DGEBF Epoxy Exposed to Progressive Artificial Weathering for VHF/Early UHF Applications

An as-produced epoxy material reinforced with ZnO (nano)rods for outdoor use was produced for the purpose of enclosing VHF/early UHF transceiving elements. Three different ZnO (nano)rod percentages were dispersed (0.1 wt%, 0.5 wt% and 2 wt%) in epoxy samples and were consequently exposed to an artificial weathering environment for a total period of 2880 h. SEM and XRD characterizations were followed by gravimetric, colorimetric and dielectric measurements that were conducted every 576-h AWE treatment intervals and were complemented by FTIR, UV-Vis, contact angle and gloss measurements. The composite exhibited favourable dielectric behaviour: ZnO presence inflicted minimal impact to the composites’ loss tangent, while dielectric permittivity was reduced for low ZnO content (0.1 wt%/0.5 wt%), especially until the 1152-h interval, while it suffered an increase for 2 wt% samples that was maintained from the beginning until the end of the experiment. Hence, the low-k (4–5.2) constructed material, compared to bare epoxy resin, maintained moderate loss tangent levels (0.55–0.7) in the VHF/early UHF spectrum, throughout the full AWE treatment, and also attained reduced dielectric permittivity, thus exhibiting moderate radio frequency transparency, together with enhanced endurance against environmental fatigue.

Green Fabrication, Characterization of Zinc Oxide Nanoparticles Using Plant Extract of Momordica charantia and Curcuma zedoaria and Their Antibacterial and Antioxidant Activities.

In recent years, the rapid increase in the resistance of microorganisms to antibiotics has produced major health issues. Novel applications for these compounds have been developed by integrating modern technologies such as nanotechnology and material science with the innate antibacterial activity of metals. The current study demonstrated the synthesis of zinc oxide nanoparticles (ZnO NPs) from Momordica charantia and Curcuma zedoaria plant extracts, as well as their antibacterial properties. The synthesis of ZnO NPs was confirmed via UV-visible spectroscopy, showing clear peaks at 375 and 350nm for M. charantia and C. zedoaria, respectively. Scanning electron microscopy (SEM) analysis revealed crystals of irregular shapes for the majority of the nanoparticles synthesized from both plants. The existence of ZnO NPs was confirmed using X-ray diffraction while the particle size was calculated using Scherrer's equation, which was 19.65 for C. zedoaria and 17.02 for M. charantia. Different functional groups were detected through Fourier transform infrared spectroscopy analysis. The antibacterial activity of the ZnO NPs at three different concentrations (250, 500, and 1000µg/ml) was assessed against three different bacterial strains, i.e., Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and Pseudomonas aeruginosa (P. aeruginosa), using disc diffusion methods. The ZnO nanoparticles showed promising antibacterial activity against bacterial strains. For C. zedoaria, the highest growth inhibition was observed at a concentration of 1000µg/ml, which was 18, 19, and 18mm as compared to antibiotics (15, 11, and 15.6mm) against E. coli, P. aeruginosa, and S. aureus, respectively. Similarly, at 1000µg/ml of NPs, M. charantia showed the highest growth inhibition (18, 15, and 17mm) as compared to antibiotics (15, 11, and 14.6mm) against E. coli, P. aeruginosa, and S. aureus, respectively. In conclusion, compared to pure plant extract and antibiotics, ZnO NPs at a higher concentration (1000µg/ml) exhibited a significant difference in zone of inhibition against all the bacterial strains. Different concentrations of ZnO using M. charantia and C. zedoaria caused increments in the scavenging of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals and 2,2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The nanoparticles extracted using C. zedoaria exhibited higher antioxidant activity than M. charantia. Greenly synthesized ZnO nanoparticles have remarkable antibacterial properties and antioxidant activity, making them a promising contender for future pharmaceutical application.

Catalytic behavior of a ZnO/TiO2 composite in the synthesis of polycarbonate diol.

ZnO/TiO2 catalysts with different ZnO contents have been prepared through equal volume impregnation method, characterized by XRD, SEM, Py-IR, ICP, XPS, NH3-TPD and N2 adsorption/desorption, and evaluated in the synthesis of polycarbonate diol (PCDL) through transesterification. The results showed that titanium zinc oxide formed in these catalysts, and the content of acidic sites varied with the ZnO content, and ZnO/TiO2 (10%) has the highest acid amount. The ZnO/TiO2 (20%) with medium acidic sites showed the highest catalytic activity. The synthesis process of polycarbonate glycol was also optimized. Under the optimal reaction conditions, the yield of PCDL was 72.5%, and the M n reached 4829 g mol-1 with a PDI of 1.6.

Intermediate electron trap levels generation and enhanced carrier concentration in ZnO by strontium and molybdenum co-doping: an effective approach for dye degradation

Intermediate electron trap levels generation and enhanced carrier concentration in ZnO by strontium and molybdenum co-doping: an effective approach for dye degradation

Studies on optical and electronic characteristics of ternary zinc-tellurite glasses with varying Zn doping concentration

• Ternary Zn-Te base glasses with different mole percent of ZnO concentration were prepared by melt and quench method. • Changes in the glass composition were found to alter the characteristics of ternary Zn-Te base glasses. • With increasing ZnO concentration from 5% to 20% density increases from 4.918 to 5.196 gm/cc. • With the addition of Zinc oxide to ternary Zn-Te glass matrix, the refractive index increases. • High R.I. and good optical allowance recommend the candidature of synthesized ternary glasses for various photonic devices. Ternary zinc-tellurite glasses of batch composition x%-(ZnO)-(90-x)%(TeO 2 )-10 %Na 2 O with × = 5, 10, 15 and 20 %, were synthesized by customary melting and quenching method. The morphological, physical, and optical characteristics of the material were investigated. Differential scanning calorimetry was done for glass transition temperature, whereas X-ray diffraction and, Scanning Electron Microscopy (SEM) -Energy Dispersive X-ray Analysis (EDAX or EDS), was done for information of surface morphology and batch composition. Structural information is gathered by Raman spectra. Physical parameters as density, oxygen packing density (OPD), Zn 2+ concentration, poleron radius and inter atomic distance have been calculated. Optical properties such as coefficient of absorption, optical energy band gap, refractive index and Urbach’s energy were calculated. The effect of ZnO concentration on physical and optical parameter has been studied.

Mining waste acting as a precursor of environmental stress in sediments

Abstract Waste generated by mineral extraction is globally associated with environmental disturbances due to its deleterious effect on water resources. However, research focused on the influence of mine tailings resulting from the extraction of semi-precious stones on fluvial systems is still incipient in the environmental literature. From this perspective, this study quantified the average concentrations of major oxides present in the fine fractions of the sediment samples from the Várzea river, in the State of Rio Grande do Sul, southern Brazil, using wavelength dispersive X-ray fluorescence spectrometry. This region is acknowledged as the largest rock amethyst mining area in the world. Additionally, geochemical indices were established to characterize potential sources of production, maturity, degree of weathering, and sediment pollution. To evaluate the influence of mine tailings on the Várzea river sediments, the contents of Al2O3, Fe2O3, MnO, P2O5, CaO, SiO2, K2O, CuO, ZnO, and TiO2 major oxides present in sediment samples were determined and compared to the local background values; the values varied significantly (p < 0.05), classifying them as polluted and medium polluted. Also, the sediment samples with evident characteristics of extreme chemical weathering consist mainly of clay minerals and mafic igneous rocks, and similarities were found between sediment samples and tailings from the mineral extraction zone. The Principal Component Analysis and the cluster analysis also suggest the existence of three distinct mineral oxide groups, differentiating the zones leaving and upstream the mining zone from the other sampling points.

Effect of ZnO nanoparticles on structure and magnetic properties of Bi2O3-B2O3: Cr2O3 glasses

Three glass samples are synthesized by traditional melt-quenching method. The composition of the samples and their labeling is as follows: 29Bi2O3–70B2O3–1Cr2O3 (BBCZ1), 29Bi2O3–70B2O3–0.5Cr2O3–0.5ZnO (BBCZ2) and 29Bi2O3–70B2O3–1ZnO (BBCZ3). The X-ray diffraction spectra of the samples have displayed two halos at small glancing angles of 2θ (≈27o and 45°), due to absence of long-range periodic network. The significant bands have been observed in FTIR spectra of the samples attributed to [BO3] units at about 1360 cm−1 and 1210 cm−1; [BO4] units at about 1010 cm−1; and bending vibrations of OBO linkages at 635 cm−1. The semi glass forming units of [BiO6] units are identified at about 490 cm−1. The weak vibrations of ZnO bond of [ZnO4] units and/or (CrOCr) chains have been noticed at around 430 cm−1. The bands at 2970 cm−1, 2080 cm−1, 1950 cm−1 and 1740 cm−1 are observed in the spectra, which can be attributed to presence of moisture (H2O) in the glasses. The EPR spectra of the samples BBCZ1 and BBCZ2 have shown two resonance signals one at g ≈ 4.7 in low field and another at g ≈ 1.98 in high field. These resonance signals arise because of paramagnetic Cr3+ ions in the glasses. The structure and magnetic properties of three samples developed have been investigated by changing the concentrations of Cr2O3 and ZnO. Accordingly, these glasses would catch hold of possible applications as optical elements, moisture sensors and magnetic sensors etc.

Enhancement of electrical conductivity of Zn4Sb3 by addition of ZnO particles

We have investigated the effect of the addition of 50 nm ZnO particles on the thermoelectric material Zn4Sb3 from the view point of the electrical conduction. The conductivity of Zn4Sb3 reached its maximum value upon the addition of 0.3 wt. % of ZnO particles. This value was four times larger than that of the undoped sample. The mobility also displayed the same trend as the conductivity, indicating that the change in the conductivity of Zn4Sb3 by the addition of ZnO particles was attributable to the ZnO content dependence of the mobility.

Zinc borate glasses: properties, structure and modelling of the composition-dependence of borate speciation.

The short-range order of binary zinc borate glasses, xZnO-(1-x)B2O3, has been quantitatively described as a function of ZnO content over the entire glass forming range for the first time, to the best of our knowledge. Multiple spectroscopic techniques (11B NMR, Raman, infrared) reveal detailed structural information regarding borate speciation and network connectivity, and a new model for quantifying the molar fractions of short-range order units is proposed. A consistent thermal history dependence for the fraction of tetrahedral boron (N4) is well accounted for by the proposed model. The model predicts density within 0.1% of experimental values and N4 to within 1% of NMR values. The intermediate character of four-coordinated zinc in borate glasses of this series is evident by the far infrared profiles and the glass transition temperature behavior, which decreases non-monotonically with increasing ZnO content.