Ag-doped ZnO Research Articles

Characteristics of Ga and Ag-doped ZnO-based nanowires for an ethanol gas sensor prepared by hot-walled pulsed laser deposition

Pure ZnO and Ga (3 % w/w) and Ag (3 % w/w)-doped ZnO nanowires (NWs) have been grown by use of the hot-walled pulse laser deposition technique. The doping characteristics of Ga and Ag in ZnO NWs were analyzed by use of photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) and the results were compared with those for pure ZnO NWs. We also fabricated gas sensors by use of pure ZnO and Ga and Ag-doped ZnO NWs. Among the NW sensors, the Ag-doped NW sensor was most sensitive. We synthesized the NWs on sapphire substrates under different conditions, for example temperature, time, gas flow, and distance between target and substrate. The diameter and length of NWs were <100 nm and several microns, respectively. To analyze the effect of Ag doping on ZnO NWs, we investigated the near band edge emission by use of low-temperature PL and XPS. Significant changes in resistance and sensitivity were observed. When the sensors were used at 300 °C for detection of 1 ppm ethanol vapor, the sensitivity of the pure ZnO and the Ga and Ag-doped ZnO NW gas sensors was 97, 48, and 203 %, respectively.

The synergetic antibacterial activity of Ag islands on ZnO (Ag/ZnO) heterostructure nanoparticles and its mode of action

Sliver nanoparticles (NPs) possess wonderful antibacterial activity while ZnO is a promising photocatalyst. Although it has been reported that the photocatalytic activity of Ag/ZnO heterostructure nanoparticles (HNPs) was better than that of pure ZnO NPs, the existing comparisons on their antibacterial activity were all taken between Ag-doped ZnO HNPs and pure Ag NPs (or pure ZnO NPs), and were still controversial. Ag islands on ZnO, another type of Ag/ZnO HNPs, were synthesized by a two-step preparation and characterized in this work. The antibacterial activity of as-synthesized Ag/ZnO HNPs, especially with 5.0wt.% of Ag, against Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923) was enhanced greatly in contrast with that of the simple mixture of Ag and ZnO NPs. The enhancement was found to be mainly due to the obvious increase in the reactive oxidative species (especially superoxide) and the increased damage to plasmid DNA induced by Ag/ZnO HNPs.

Influence of Ag–S codoping on silver chemical states and stable p-type conduction behavior of the ZnO films

Abstract The effects of Ag–S codoping on chemical states of Ag doped in ZnO, as well as electrical and optical properties of the ZnO film were investigated by comparative study of undoped ZnO, Ag-doped ZnO (ZnO:Ag) and Ag–S codoped ZnO (ZnO:(Ag, S)) films. It was found that the Ag occupied the Zn site and showed only positive univalence ( Ag Zn 1 + ) in the ZnO:(Ag, S). Compared to the ZnO:Ag, the solid solubility of the Ag Zn 1 + is greatly enhanced in the ZnO: (Ag, S) with incorporation of S. Stable p-type conduction was observed and proved further in the ZnO:(Ag, S) films. Low temperature photoluminescence measurement for the ZnO:(Ag, S) indicated that there were two kinds of acceptors of Ag Zn 1 + – S O complex and Ag Zn 1 + in the ZnO:(Ag, S), moreover, the ionization energy of the Ag zn 1 + – S O was smaller than that of the Ag Zn 1 + . The stable p-type conduction of the ZnO:(Ag, S) was suggested to be attributed to the increase of the Ag Zn 1 + content induced by S doping and to the formation of the Ag Zn 1 + – S O complex acceptor.

Effects of Annealing on the Structural and Photoluminescent Properties of Ag-Doped ZnO Nanowires Prepared by Ion Implantation

ZnO nanowires deposited on Si substrates were prepared by thermal evaporation of a mixture of ZnO and carbon powder. Ag ions with an energy of 63 keV and a dose of 5 × 1015 ions/cm2 were implanted into the as-prepared ZnO nanowires. After ion implantation, the Ag-implanted ZnO nanowires were annealed in air at different temperatures from 600°C to 1000°C. Effects of ion implantation and thermal annealing on the structural and photoluminescent (PL) properties of the ZnO nanowires were investigated by transmission electron microscopy (TEM), selected area energy dispersive X-ray spectroscopy (SAEDX), X-ray diffraction (XRD), and fluorescence spectrophotometry. TEM, HR-TEM, and SAEDX analyses demonstrated that efficient doping of Ag was achieved by ion implantation and the subsequent annealing process. XRD patterns revealed that the hexagonal wurtzite structure of ZnO nanowires was maintained after ion implantation. Photoluminescent emissions of ZnO nanowires were decreased significantly by Ag implantation but could be recovered by thermal annealing. The mechanism of the influence of ion implantation and annealing on the PL intensity was assessed.

Preparation and characterization of electrodeposited Ag-doped ZnO inverse opals with a smooth surface

Ag-doped ZnO inverse opal structures were prepared by electrochemical deposition using polystyrene colloidal crystal templates. The Ag-doped ZnO structures had granular morphology, but this morphology became smooth using a three-dimensional (3D) porous template in the same electrolyte. The Ag-doped ZnO structures were characterized using energy-dispersive X-ray spectroscopy, and the lattice parameter was found to increase compared with undoped ZnO as verified by X-ray diffraction. Strong reflection photonic stop bands centered at 495 and 681 nm were detected from inverse opals templated from microspheres having diameters of 285 and 370 nm. The filling ratio of Ag-doped ZnO inverse opals using the template was lower than that of the uniform film growth of undoped ZnO. Moreover, photochemical cell analysis revealed that doped ZnO inverse opals with n- and p-type conductivities were successfully formed using electrolytes with different silver ion concentrations.

Graphene–Ag/ZnO nanocomposites as high performance photocatalysts under visible light irradiation

Visible-light-responsive Graphene-Ag/ZnO nanocomposites were fabricated using a facile, one-pot, nontoxic solvothermal process for the photodegradation of organic dyes. During the solvothermal process reduction of graphene oxide and loading of Ag-doped ZnO nanoparticles on two-dimensional graphene sheets were achieved. Electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, BET surface area measurements, X-ray photoelectron spectroscopy and powder X-ray diffraction were used to confirm that the Ag-doped ZnO nanoparticles as randomly dispersed and effectively decorated on graphene sheets via covalent bonds between Zn and C atoms. Optical properties studied using UV-vis diffuse reflectance spectroscopy confirmed that the absorption edge of Ag-doped ZnO shifted to visible-light region with the incorporation of graphene. The as-synthesized Graphene-Ag/ZnO nanocomposites showed unprecedented photodecomposition efficiency compared to the Ag-doped ZnO, pristine ZnO and commercial ZnO under visible-light. The textile mill effluent containing organic substances was also treated using photocatalysis and the reduction in the chemical oxygen demand (COD) of the treated effluent revealed a complete destruction of the organic molecules along with colour removal. This dramatically enhanced photoactivity of the composite which is attributed to retarded charge recombination rate, great adsorption of dyes, enhanced visible light absorption and fast transfer processes. This research has the potential to provide new avenues for the in situ fabrication of the Graphene-Ag/ZnO composites as highly efficient photocatalysts. © 2013 Elsevier B.V. All rights reserved.

A selective ethanol gas sensor based on spray-derived Ag–ZnO thin films

A simple and cost-effective spray pyrolysis technique was employed to synthesize silver-doped zinc oxide (Ag–ZnO) thin films on the glass substrates from aqueous solutions of zinc acetate and silver nitrate precursors at 450 °C. The effects of Ag doping on structural, morphological, and gas-sensing properties of films were examined. The X-ray diffraction spectra of the Ag–ZnO films showed the polycrystalline nature having hexagonal crystal structure. Scanning electron microscopy (SEM) images of the pure ZnO films revealed the uniform distribution of the spherical grains (~80 nm size). Tiny Ag nanoparticles are clearly visualized in the SEM of Ag–ZnO films. The investigation of the effect of Ag doping on the gas-sensing properties of the Ag–ZnO revealed that the 15 % Ag-doped ZnO sample has the highest gas sensitivity (85 %) and excessive Ag doping in ZnO degraded the gas sensitivity. A possible mechanism of Ag–ZnO-based sensor sensitivity to the target gas is also proposed.

Solar photocatalytic activity of sol–gel prepared Ag-doped ZnO thin films

Pure and silver doped ZnO thin films over glass substrate were prepared by sol–gel spin-coating method. Prepared films were calcined at different temperatures. X-ray diffraction and UV–Vis spectroscopy were used to characterize the prepared samples. The photocatalytic activity under solar irradiation of the prepared thin films was tested for the degradation of three azo reactive dyes namely Reactive Red 195, Reactive yellow 145 and Reactive orange 122 as an organic pollutant. 6 wt.% Ag doping ratio showed the highest photocatalytic activity. Various operational parameters such as pH of the solution and initial concentration of the dye have been investigated.

Characteraction of Ag-doped ZnO thin film synthesized by sol–gel method and its using in thin film solar cells

Pure 2% and 4% Ag-doped ZnO thin films have been synthesized on glass substrates by sol–gel method. The structure, morphology and optical properties of the samples have been studied by X-ray diffractometer (XRD), scanning probe microscope, UV–vis spectrophotometer, respectively. The XRD result shows that the pure ZnO has a wurtzite hexagonal structure, no phase segregation is observed. The surface morphology of pure ZnO thin film shows that the grains are growing preferentially along the c-axis orientation perpendicular to the substrates. The transmittance spectra reveal that all samples have high transmittance above 90% in visible region. With Ag doping content increase, a red shift is observed. The performance of Ag-doped ZnO films using in thin film solar cells are simulated. The results show that 4% Ag-doped ZnO thin film can greatly improve the absorption of the cells. Compare to pure ZnO, solar cell's energy conversion efficiency improvement of 2.47% is obtained with 4% Ag doped ZnO thin film.

Influence of ZnO buffer layer on microstructure and Raman scattering of ZnO:Ag film on Si substrate

Pure ZnO and Ag-doped ZnO (ZnO:Ag) films were deposited on ZnO buffer layer and p-Si (100) substrate by radio frequency reactive magnetron sputtering. ZnO buffer layer was deposited at lower power. The microstructures of the films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis techniques. The results indicated that ZnO:Ag film had a stronger preferred orientation toward the c-axis, a more uniform grain size and smoother surface after using ZnO buffer layer, and the residual strain was reduced. In addition, stronger Raman scattering was observed from ZnO:Ag film on ZnO buffer layer than that on Si substrate. It was because the ZnO buffer layer effectively relaxed the partial stress induced by the large lattice mismatch between the film and Si. Two additional local vibrational modes (LVMs) at 245 and 495.8cm−1 induced by Ag dopant in ZnO:Ag films were observed by Raman spectra, corresponding to Ag atoms located at O sites (LVM of Zn–Ag) and Ag atoms in the films. It meant that the LVMs could act as an indication of Ag incorporation into ZnO:Ag film.

Highly stable non-polar p-type Ag-doped ZnO thin films grown on r-cut sapphire

Non-polar p-type Ag-doped ZnO thin films were grown on r-cut sapphire substrates by pulsed laser deposition. Films deposited at a substrate temperature above 250°C were shown to be (112¯0) oriented (a-plane) according to X-ray diffraction, and maintained p-type behavior after one year according to Hall-effect measurements. Hole carrier concentrations lie in the range of 3.1×1016–5.3×1017cm−3, mobilities in the range of 0.3–1.5cm2V−1s−1, and resistivities in the range of 43–136Ωcm. Optical transmittance measurements indicate a decrease in band gap of ∼0.07–0.08eV for the p-type films. Characteristic stacking fault features were observed along the (011¯2) plane in the p-type films by transmission electron microscopy.

Enhancement of p-type conduction in Ag-doped ZnO thin films via Mg alloying: The role of oxygen vacancy

ZnO, ZnMgO, Ag-doped ZnO (ZnO:Ag), and Ag-doped ZnMgO (ZnMgO:Ag) thin films have been prepared by pulsed laser deposition. All the films have a preferred orientation with the c-axis perpendicular to the substrates. Hall-effect measurements indicate that the ZnO:Ag film exhibits p-type conduction, but obviously worse than that of the ZnMgO:Ag film. A comparative study of p-type ZnO:Ag and ZnMgO:Ag films using photoluminescence and x-ray photoelectron spectroscopy measurements shows that the enhanced p-type conduction in ZnMgO:Ag film is closely related to the increase of the activation energy of the intrinsic donors and the suppression of charge-compensating oxygen-related defects after Mg incorporation.

Solution-processed Ag-doped ZnO nanowires grown on flexible polyester for nanogenerator applications

The integration of ZnO nanowire-based energy harvesting devices into flexible polyesters or clothes would have a significant effect on the energy harvesting building block for harvesting the mechanical energy from human motions. Moreover, the demonstration of high output power via a doping process opens an important method for enhancing the output power. Here, we report solution-based synthesis of Ag-doped ZnO nanowires on flexible polyester substrates without using any high temperature annealing processes. Along with the structural and optical characteristics of the Ag-doped ZnO nanowires, we demonstrate the efficient features of Ag-doped nanogenerators through the measurement of a sound-driven piezoelectric energy device with an output power of 0.5 μW, which is nearly 2.9 times that of a nanogenerator with un-doped ZnO NWs. This finding could provide the possibility of high output nanogenerators for practical applications in future portable/wearable personal displays and motion sensors.

Investigation on the fabrication of Ag-doped ZnO nanorods by hydrothermal method

Ag-doped ZnO nanorods with different morphologies and optical properties are synthesized by hydrothermal method on the DC magnetron-sputtered Al-doped ZnO (AZO) seed layers. The influences of the molar ratio of Ag ions to Zn ions (RAg/Zn) and the AZO seed layer on the structural and optical properties of the Ag-doped ZnO nanorods are carefully studied by using X-ray diffractometry, scanning electron microscopy, spectrophotometry, EDS spectrum, etc. The changes in the microstructure and optical property of Ag-doped ZnO nanorods are closely related to the change in the average head-face dimension induced by Ag doping as RAg/Zn increases, owing to the different relative proportions of Ag ions doped in ZnO nanorods resulting from the different particle sizes and densities of the seed layers. The photoluminescence intensity in the visible region for the ZnO nanorods growing on the 15 min-sputtered AZO is stronger than that of the ZnO nanorods growing on the 10 min-sputtered AZO seed layer at the same RAg/Zn, which results from the increased defects in ZnO. More point defects caused by Ag doping are produced as RAg/Zn increases, resulting in the broadening of PL envelope in the visible region. The microstructure of pure ZnO nanorod is related to the seed layer thickness-related degree of crystallinity and particle size.

Effects of S on solid solubility of Ag and electrical properties of Ag-doped ZnO films grown by radio frequency magnetron sputtering

Effects of S on solid solubility of Ag and electrical properties of Ag-doped ZnO films are investigated by experimental and first-principles studies. It is found that S alloying in ZnO can increase solid solubility of Ag substituting for Zn (AgZn) due to that Ag binds more easily with S than O, leading to that Ag–S codoped ZnO films gradually transform from n- to p-type conductivity with increasing S content. The low-temperature photoluminescence measurement and first-principles calculation indicate that there are two kinds of AgZn and AgZn–nSO acceptors in the p-type Ag–S codoped ZnO and the AgZn–nSO acceptor has lower formation energy and acceptor transition energy than AgZn. The p-type conduction of the Ag–S codoped ZnO comes dominantly from contribution of the AgZn–nSO acceptor.

Ag-doped ZnO nanorods synthesized by two-step method

A two-step method is adopted to synthesize Ag-doped ZnO nanorods. A ZnO seed layer is first prepared on a glass substrate by thermal decomposition of zinc acetate. Ag-doped ZnO nanorods are then assembled on the ZnO seed layer using the hydrothermal method. The influences of the molar percentage of Ag ions to Zn ions (RAg/Zn) on the structural and optical properties of the ZnO nanorods obtained are carefully studied using X-ray diffractometry, scanning electron microscopy and spectrophotometry. Results indicate that Ag ions enter into the crystal lattice through the substitution of Zn ions. The (002) c-axis-preferred orientation of the ZnO nanorods decreases as RAg/Zn increases. At RAg/Zn > 1.0%, ZnO nanorods lose their c-axis-preferred orientation and generate Ag precipitates from the ZnO crystal lattice. The average transmissivity in the visible region first increases and then decreases as RAg/Zn increases. The absorption edge is first blue shifted and then red shifted. The influence of Ag doping on the average head face, and axial dimensions of the ZnO nanorods may be optimized to improve the average transmissivity at RAg/Zn < 1.0%.

Oxidative degradation of acid orange 7 using Ag-doped zinc oxide thin films

Ag-doped ZnO thin films with preferred c-axis orientation along (002) have been prepared by spray pyrolysis technique in aqueous medium on to the corning glass substrates. The effect of Ag-doping on to the photoelectrochemical, structural, morphological, optical, luminescence, electrical and thermal properties has been investigated. XRD and Raman study indicates that the films have hexagonal (wurtzite) crystal structure. The effect of Ag loading on the photocatalytic activity of Ag-doped ZnO in the degradation of azo dye is studied and results are compared with pure ZnO. The results show that the rate of degradation of azo dye over Ag-doped ZnO is much higher as compared to pure ZnO. Ag doping in ZnO is highly effective and can significantly enhance the photocatalytic degradation and mineralization of azo dye. The enhancement of photocatalytic activity of Ag-doped ZnO thin films is mainly due to their smaller crystallite size and capability for reducing the electron–hole pair recombination. Kinetic parameters have been investigated in terms of a first order rate equation. The rate constant (−k) for this heterogeneous photocatalysis is evaluated as a function of the initial concentration of original species. Substantial reduction in azo dye is achieved as analyzed from COD and TOC studies.

Theoretic insights into the Ag doping in monolayer and bilayer ZnO armchair nanoribbons: edge effect and position-dependent properties

One-dimensional nanoribbons, thickness of which is smaller than width, exhibit special confinement and edge effects far from uniform in the cross-section. By means of density functional theory, we investigated the influence of the edges and doping positions on the electronic structure of Ag-doped ZnO armchair nanoribbons (ZnOANRs). Ag doping in monolayer and bilayer ZnOANRs (m- and b-ZnOANRs) have all been examined. The results indicated that there is no significant difference on the stability of Ag doping in different positions of both m- and b-ZnOANRs, but exhibits very different electronic properties directly related to the different doping positions. The depth of the acceptor states has essential relationship with the interaction between host O 2p and Ag 4d states in the acceptor. Ag substituting Zn atoms at the inner region of m-ZnOANRs could create shallow acceptors with small hole effective masses, benefit for p-type conduction. Ag doping in the inner region of b-ZnOANRs would create shallow acceptors but larger hole effective masses. The difficulty of hole mobility could be improved by increasing the Ag doping concentration in b-ZnOANRs. The discussion about the mechanism and the suggestion about the achievement in the experiment are interesting and timely.

Enhanced Raman scattering and nonlinear conductivity in Ag-doped hollow ZnO microspheres

Hollow spherical ZnO particles doped with Ag were synthesized with a two-step oxidation and sublimation furnace annealing process. Ag nanoparticle precipitates, as observed by transmission electron microscopy, were present in the polycrystalline ZnO matrix at Ag concentrations below 0.02 mol%, significantly below the 0.8 mol% solubility limit for Ag in ZnO. Enhanced Raman scattering of ZnO phonon modes is observed, increasing with Ag nanoparticle concentration. A further enhancement in Raman scattering due to resonance effects was observed for LO phonons excited by 2.33-eV photons as compared with Raman scattering under 1.96-eV excitation. Room-temperature photoluminescence spectra showed both a near-band-edge emission due to free exciton transitions and a mid-gap transition due to the presence of singly ionized oxygen vacancies. ZnO:Ag particles were measured electrically in a packed column and in monolithic form, and in both cases displayed nonlinear current–voltage characteristics similar to those previously observed in sintered ZnO:Ag monoliths where Ag-enhanced disorder at grain boundaries is thought to control current transport. We demonstrate therefore that Ag simultaneously modifies the electrical and optical properties of ZnO particles through the introduction of vacancies and other defects.

Photocatalytic degradation of phosphamidon using Ag-doped ZnO nanorods

The photocatalytic degradation of the organo-phosphorous pesticide phosphamidon at low concentration in aqueous solution on Ag-doped ZnO nanorods was investigated. Nanosized Ag-doped ZnO rods were synthesized by using a microwave assisted aqueous method. High molecular weight polyvinyl alcohol was used as a stabilizing agent. Composition and structure were investigated using energy-dispersive X-ray spectroscopy (EDAX) and X-ray diffraction (XRD). The XRD pattern reveals that ZnO nanorods are of hexagonal wurtzite structure. The average crystallite size calculated from Scherrer's relation was found to be 30 nm. The effects of catalyst loading, pH value, and initial concentration of phosphamidon on the photocatalytic degradation efficiency using Ag-doped ZnO nanorods as a photocatalyst have been discussed. The results revealed that Ag-doped ZnO nanorods with a diameter of 30 nm showed highest photocatalytic activity at a surface density of 1 g dm−3. The catalyst doped with 0.2 mol% Ag is effective for the degradation of phosphamidon with visible light. This opens a new possibility to decompose pesticides that are present in wastewater.