Au ZnO Research Articles

X-ray-induced synthesis of Au nanoparticles on ZnO nanowire arrays

Abstract We report the synchrotron X-ray-induced synthesis of plasmonic Au nanoparticles (NPs) on ZnO nanowire (NW) arrays. By X-ray irradiation at room temperature, Au NPs were synthesized from an aqueous solution containing HAuCl4 and NaHCO3. Samples prepared in continuous and scanning exposure modes were examined by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy and Raman spectroscopy. In the continuous exposure mode, a match-like Au ZnO structure was dominantly observed. We found that the scanning mode is suitable for the synthesis of monodisperse Au NP-ZnO NW heterostructures. The average diameter of the Au NPs was approximately 11.8 nm. The peak intensity of the E2(high) mode doubled and the 2E2 peak can be clearly observed in Raman spectrum of Au NP-ZnO NW heterostructure. These changes are evidence of the surface plasmon resonance effect due to the Au NPs.

Sensitive and Rapid Ethanol Gas Sensor Based on Au–ZnO Microwires

Sensitive and Rapid Ethanol Gas Sensor Based on Au–ZnO Microwires

Fabrication of highly sensitive and selective room-temperature nitrogen dioxide sensors based on the ZnO nanoflowers

Herein, ZnO nanoflowers without any doping and modification were synthesized by hydrothermal method. The room temperature nitrogen dioxide sensor with high sensitivity, selectivity and long-term stability was prepared by combining ZnO nanoflowers with interdigital electrodes. The average diameter of the ZnO nanoflowers was about 0.9–1 μm. In addition, the gas sensing properties of the nitrogen dioxide gas sensors at room temperature were investigated. Compared with other ZnO sensors, the sensor based on the ZnO nanoflowers revealed outstanding sensing response to NO2, giving a response of 0.05–6 ppm. The sensor exhibited a lower detection limit of 0.05 ppm and had an ultra-high response to 1 ppm NO2 at room temperature. The response was only decreased by 16% of the initial value after 40 days of measurement. The Schottky barrier could be the key factor for the excellent NO2 sensing properties, which was formed between the Au electrodes and ZnO nanoflowers.

Effects of Au nanoparticles and ZnO morphology on the photocatalytic performance of Au doped ZnO/TiO2 films

Au doped ZnO nanocomposite films on TiO2 seeding layer (AuZ/T) were fabricated by hydrothermal processing and their photocatalytic performance was investigated. It could be found that the AuZ/T with micrometer(μm)-sized, lying ZnO bulks revealed optimal photocatalytic performance toward methyl orange under simulated sunlight, whose apparent degradation rate constant Kapp of 1.31 was about 20% higher compared to that of ZnO/TiO2 and 3 times higher compared to that of ZnO. The Au nanoparticles, TiO2 seeding layer and hydrothermal processing time imposed vital influence on the morphology of ZnO nanostructures, which played key roles in the formation of ZnO/TiO2 heterojunction and charge transfer (CT) inside it, as demonstrated by kinetics of transient photoluminescence (PL) decaying. The incorporation of Au nanoparticles not only induced the variations of ZnO crystallinity and reduction of ZnO band gap (Eg), but also generated the Schottky heterojunction of metal–semiconductor, which would be beneficial to the CT inside nanocomposite films and separation of photo-generated electron–hole pairs, as verified by the remarkable PL suppression. The mechanism responsible for photocatalysis enhancement, which was resulted from the hybrid effects of Au nanoparticles and the ZnO morphology was discussed in details.

Theoretical and Experimental Investigation of an All-Fiber Waveguide Coupled Surface Plasmon Resonance Sensor With Au–ZnO–Au Sandwich Structure

In this paper, we proposed an all-fiber waveguide coupled surface Plasmon resonance (SPR) sensor which was designed and fabricated with Au-ZnO-Au sandwich multi-layer structure. Based on the transfer matrix method (TMM), the theoretical modeling of the proposed sensor structure was investigated. The influence of the layer thickness on the transmission spectrum of the sensor was studied numerically, in order to optimize each layer thickness, leading to sensitivity enhancement in the infrared spectral range. Meanwhile, an experimental testbed for the refractive index measurement was implemented based on the proposed all-fiber waveguide coupled SPR sensor. Using this waveguide coupled SPR sensor, the refractive index sensing was realized with a sensitivity enhancement of 7%—131% compared with conventional SPR sensors.

Size-controlled, high optical quality ZnO nanowires grown using colloidal Au nanoparticles and ultra-small cluster catalysts

Size-controlled ZnO nanowires were grown by eclipse pulsed laser deposition using Au colloids and ultra-small (sub 2 nm) Au101 and Au9 clusters as catalysts, tethered to c-plane sapphire substrates using aminopropyltrimethoxysilane (APTMS). A strong correlation was observed between ZnO nanowire tip diameter and Au catalyst size, with no apparent lower limit to the size of Au nanoparticles able to catalyze nanowire growth. All ZnO nanowires produced intense ultraviolet photoluminescence with almost no visible defect band emission that is commonly observed in ZnO-based materials, including nanowires. A correlation was also observed between the quality of the ultraviolet photoluminescence (in terms of decreasing exciton line widths and surface-related emission) and decreasing ZnO nanowire diameter, with the highest quality emission obtained using Au9 atomically precise cluster catalysts, used here as catalysts for ZnO growth for the first time.

Development of ALD Coatings for Harsh Environment Applications.

Atomic layer deposition (ALD) is a well-known technique for the fabrication of ultrathin and highly conformal barrier coatings which have extensively been used for the protection of electronic devices in open atmospheric conditions. Here, we extend the scope for the application of low-temperature-deposited plasma-enhanced ALD barrier coatings for the protection of devices in a variety of chemical environments. The chemical stability tests were conducted in 3.5% NaCl, sea water, HCl (pH 4), and H2SO4 (pH 4) solutions for ALD Al2O3, HfO2, TiO2, and ZrO2, deposited at 100 °C on TiO2-coated Au and ALD ZnO (photoactive)-coated Si substrates. Using electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) study, various aspects of the barrier properties and performance of ALD films in harsh chemical environments were explored. We demonstrate that the combined approach involving EIS and PL provides unique insights into the suitability of ALD films as barriers in harsh environments involving ionic solutions. The observations from EIS and PL tests are supported by the X-ray photoelectron spectroscopy analysis of ALD materials. Of the materials tested, ALD TiO2 and ZrO2 were found to be the most stable, chemically, in all four solutions, whereas TiO2 was a better permeation barrier.

Self-Assembled Ordered Three-Phase Au-BaTiO3 -ZnO Vertically Aligned Nanocomposites Achieved by a Templating Method.

Complex multiphase nanocomposite designs present enormous opportunities for developing next-generation integrated photonic and electronic devices. Here, a unique three-phase nanostructure combining a ferroelectric BaTiO3 , a wide-bandgap semiconductor of ZnO, and a plasmonic metal of Au toward multifunctionalities is demonstrated. By a novel two-step templated growth, a highly ordered Au-BaTiO3 -ZnO nanocomposite in a unique "nanoman"-like form, i.e., self-assembled ZnO nanopillars and Au nanopillars in a BaTiO3 matrix, is realized, and is very different from the random three-phase ones with randomly arranged Au nanoparticles and ZnO nanopillars in the BaTiO3 matrix. The ordered three-phase "nanoman"-like structure provides unique functionalities such as obvious hyperbolic dispersion in the visible and near-infrared regime enabled by the highly anisotropic nanostructures compared to other random structures. Such a self-assembled and ordered three-phase nanocomposite is obtained through a combination of vapor-liquid-solid (VLS) and two-phase epitaxy growth mechanisms. The study opens up new possibilities in the design, growth, and application of multiphase structures and provides a new approach to engineer the ordering of complex nanocomposite systems with unprecedented control over electron-light-matter interactions at the nanoscale.

Growth Condition-Oriented Defect Engineering for Changes in Au⁻ZnO Contact Behavior from Schottky to Ohmic and Vice Versa.

ZnO has the built-in characteristics of both ionic and covalent compound semiconductors, which makes the metal–ZnO carrier transport mechanism quite intricate. The growth mechanism-centric change in ZnO defect density and carrier concentration also makes the contact formation and behavior unpredictable. This study investigates the uncertainty in Au–ZnO contact behavior for application-oriented research and the development on ZnO nanostructures. Herein, we explain the phenomenon for how Au–ZnO contact could be rectifying or non-rectifying. Growth method-dependent defect engineering was exploited to explain the change in Schottky barrier heights at the Au–ZnO interface, and the change in device characteristics from Schottky to Ohmic and vice versa. The ZnO nanorods were fabricated via aqueous chemical growth (ACG) and microwave-assisted growth (MAG) methods. For further investigations, one ACG sample was doped with Ga, and another was subjected to oxygen plasma treatment (OPT). The ACG and Ga-doped ACG samples showed a quasi-Ohmic and Ohmic behavior, respectively, because of a high surface and subsurface level donor defect-centric Schottky barrier pinning at the Au–ZnO interface. However, the ACG-OPT and MAG samples showed a more pronounced Schottky contact because of the presence of low defect-centric carrier concentration via MAG, and the removal of the surface accumulation layer via the OPT process.

Synthesis of ZnO–Au composite microspheres by one-pot polyol method and its application as a photo-catalyst

The growth of metal mixed semiconductor particles was known to enhance their photocatalytic properties. In this study, a brand-new and simple one-pot polyol method was successfully used to synthesize metal gold embedded in ZnO nanoparticles (NPs), termed ZnO–Au microspheres. The physicochemical properties of ZnO–Au microspheres were analyzed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy measurements, UV–Vis absorption espectroscopy and Fourier transform infrared espectroscopy. Applied to degradation of methylene blue under visible light, the ZnO–Au microspheres photocatalyst showed better photocatalytic activity than pure ZnO NPs. The reusability of ZnO–Au microspheres was also examined.

Facile synthesis of ZnO morphological evolution with tunable growth habits: Achieving superior gas-sensing properties of hemispherical ZnO/Au heterostructures for triethylamine

In this paper, we focused on the hydrothermal synthesis of ZnO structures with varied morphologies by finely adjusting the ratio of water to glycol, indicating the polarity of the solution affects the growth habit of the crystal. It showed that the precise control in growth rates of different ZnO crystal faces in mixed solution is the key factor to control the morphologies of ZnO samples. Compared to pure ZnO, the response value of the hemispherical ZnO/Au to 100 ppm triethylamine (TEA) has been improved to 104.8 at the operating temperature of 260 °C, and the response and recovery time are 5 and 2 s, respectively. The enhancing mechanism of as-prepared ZnO/Au hemispheres can be ascribed to the combination of the tuning of surface electron transfer, microstructures, and the electronic and chemical sensitization of Au nanoparticles, along with the formation of Schottky junction between Au nanoparticles and ZnO hemispheres. The excellent sensing performance of ZnO/Au composite hemispheres enables them to function as promising materials for actual application in the monitoring and detecting of TEA.

The interplay of Au nanoparticles and ZnO nanorods for oxygen-promoted, base-free, complete formaldehyde reforming into H2 and CO2

Au nanoparticles on ZnO nanorods is a sustainable catalyst for oxygen-promoted, base-free, complete formaldehyde reforming into H2 and CO2 at low temperatures. The strong metal-support interaction (SMSI) between Au and ZnO is suggested to be responsible for this catalytic feature partially due to the presence of surface cationic Au+ species formed during high temperature treatment in air. in-situ EPR and resonance Raman analysis show that a two-electron reduction pathway is involved to produce hydrogen peroxide as the active species and CH bond cleavage is the rate-determining step.

Fluorescent ZnO-Au Nanocomposite as a Probe for Elucidating Specificity in DNA Interaction.

In this work, we report the interaction of a fluorescent ZnO–Au nanocomposite with deoxyribonucleic acid (DNA), leading to AT-specific DNA interaction, which is hitherto not known. For this study, three natural double-stranded (ds) DNAs having different AT:GC compositions were chosen and a ZnO–Au nanocomposite has been synthesized by anchoring a glutathione-protected gold nanocluster on the surface of egg-shell-membrane (ESM)-based ZnO nanoparticles. The ESM-based bare ZnO nanoparticles did not show any selective interaction toward DNA, whereas intrinsic fluorescence of the ZnO–Au nanocomposite shows an appreciable blue shift (Δλmax = 18 nm) in the luminescence wavelength of 520 nm in the presence of ds calf thymus (CT) DNA over other studied DNAs. In addition, the interaction of the nanocomposite through fluorescence studies with single-stranded (ss) CT DNA, synthetic polynucleotides, and nucleobases/nucleotides (adenine, thymine, deoxythymidine monophosphate, deoxyadenosine monophosphate) was also undertaken to delineate the specificity in interaction. A minor blue shift (Δλmax = 5 nm) in the emission wavelength at 520 nm was observed for single-stranded CT DNA, suggesting the proficiency of the nanocomposite for discriminating ss and ds CT DNA. More importantly, fluorescence signals from the nano-bio-interaction could be measured directly without any modification of the target, which is the foremost advantage emanated from this study compared with other previous reports. The AT base-pair-induced enhancement was also found to be highest for the melting temperature of CT DNA (ΔTmCT = 6.7 °C). Furthermore, spectropolarimetric experiments followed by calorimetric analysis provided evidence for specificity in AT-rich DNA interaction. This study would lead to establish the fluorescent ZnO–Au nanocomposite as a probe for nanomaterial-based DNA-binding study, featuring its specific interaction toward AT-rich DNA.

Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles.

Metal-semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au-ZnO particles (Pt-Au-ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au-ZnO particles (Au@Au-ZnO). An epitaxial connection at the Au-ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on-off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation properties of Pt-Au-ZnO hybrids compared to Au-ZnO. In contrast, Au-ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au-ZnO (Au, ∼5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi-level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au-ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au-ZnO. Pt-Au-ZnO hybrids showed better photodegradation and mineralization properties compared to both Au-ZnO and Au@Au-ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt-Au-ZnO, Au@Au-ZnO, and Au-ZnO were 84, 66, and 39%, respectively. In the interest of effective metal-semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired.

The structural, electronic and optical properties of Au–ZnO interface structure from the first-principles calculation

The interface structure, electronic and optical properties of Au–ZnO are studied using the first-principles calculation based on density functional theory (DFT). Given the interfacial distance, bonding configurations and terminated surface, we built the optimal interface structure and calculated the electronic and optical properties of the interface. The total density of states, partial electronic density of states, electric charge density and atomic populations (Mulliken) are also displayed. The results show that the electrons converge at O atoms at the interface, leading to a stronger binding of interfaces and thereby affecting the optical properties of interface structures. In addition, we present the binding energies of different interface structures. When the interface structure of Au–ZnO gets changed, furthermore, varying optical properties are exhibited.

Surface plasmon-driven photoelectrochemical water splitting of aligned ZnO nanorod arrays decorated with loading-controllable Au nanoparticles

In this paper, we designed a series of well-aligned ZnO nanorod arrays decorated with loading-controllable Au nanoparticles and studied their surface plasmon-driven photoelectrochemical (PEC) water splitting performances. The PEC water splitting ability of Au-ZnO nanorod arrays was evaluated under illumination with λ > 420 nm light. These nanorod arrays show remarkable PEC water splitting performances and achieve the highest photocurrent density of 30 μA cm−2 at 0.8 V versus Ag/AgCl. Furthermore, the PEC performance for heterogenous nanorod arrays can be effectively adjusted by controlling loading amounts of Au nanoparticles. We experimentally demonstrate that the Au-ZnO nanorod arrays show enhanced visible light absorption ability. The superior PEC performance of Au-ZnO nanorod arrays is attributed to the synergistic effects of plasmonic Au nanoparticles, ZnO semiconductor and Schottky barrier built in heterogenous nanorod array. This work provides a facile strategy to manipulate the PEC water splitting activity of Au-ZnO hybrid nanostructures by simply controlling the loading amounts of metallic Au nanoparticles. Furthermore, our research offers a potentially efficient strategy for the design and fabrication of new types of plasmonic-metal/semiconductor hybrid nanostructures with a plasmonic-enhanced PEC water splitting activity under the visible light, which are as valuable photocatalysts for solar-to-chemical/electrical energy conversion.

Plasmon-mediated charge dynamics and photoactivity enhancement for Au-decorated ZnO nanocrystals

Correlations among Au content, SPR-mediated charge transfer and electromagnetic response, and the resultant photoactivity enhancement for ZnO–Au nanocrystals were established.

Characterization of spatial manipulation on ZnO nanocomposites consisting of Au nanoparticles, a graphene layer, and ZnO nanorods

Three types of ZnO-based nanocomposites were fabricated consisting of 80-nm Au nanoparticles (NPs), a graphene layer, and ZnO nanorods (NRs). To investigate interactions between the ZnO NRs and Au nanoparticle, multiple material analysis techniques including field-emission scanning electron microscopy (FESEM), surface contact angle measurements, secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopic characterizations were performed. Results indicate that incorporating a graphene layer could block the interaction between the ZnO NRs and the Au NPs. Furthermore, the Raman signal of the Au NPs could be enhanced by inserting a graphene layer on top of the ZnO NRs. Investigation of these graphene-incorporated nanocomposites would be helpful to future studies of the physical properties and Raman analysis of the ZnO-based nanostructure design.

Theoretical modeling of a coupled plasmon waveguide resonance sensor based on multimode optical fiber

A coupled plasmon waveguide resonance (CPWR) sensor based on metal/dielectric-coated step index multimode optical fiber is proposed. Theoretical simulations using the four-layer Fresnel equations based on a bi-dimensional optical fiber model were implemented on four structures: Ag–ZnO, Au–ZnO, Ag–TiO2 and Au–TiO2. By controlling the thickness of dielectric layer, we managed to manipulate the CPWR resonance wavelengths. When a CPWR resonance dip is in the short wavelength region, it is insensitive to the change of surrounding refractive index (SRI) and can be used as a reference to improve the sensing accuracy of surface plasmon resonance (SPR) mode. With the increase of the thickness of the dielectric layer, the CPWR resonance dips shift to longer wavelength and the corresponding sensitivities increase. When the 1st CPWR resonance wavelength is near 1550 nm and SRI is around 1.333, the sensitivities of four structures reach 1360.61 nm/RIU, 1375.76 nm/RIU, 1048.48 nm/RIU and 1015.15 nm/RIU, respectively. The values are close to that of the conventional SPR optical fiber sensor while the spectral bandwidths of the optical fiber CPWR sensors are narrower.

Photodegradation pathway of rhodamine B with novel Au nanorods @ ZnO microspheres driven by visible light irradiation

A series of Au–ZnO photocatalysts were successfully synthesized from ZnO microspheres impregnated with Au nanorods by the seed-mediated method, and their photocatalytic activity of degradation of rhodamine B (RhB) was investigated. The nanocomposite catalyst exhibited high photocatalytic activity and degraded 92% of RhB solution under visible light irradiation in 330 min. The enhancement of photocatalytic effects was mainly ascribed to the surface plasmon resonance effect of Au nanorods; therefore, Au–ZnO spheres can absorb resonant photons and transfer the electron to the conduction band (CB) of ZnO leading to the separation of electrons and holes under visible light. Meanwhile, the photocatalytic performance was beneficial from the flower-like porous structure of ZnO, which enhances adsorption of the dye molecules and dissolved oxygen on the catalyst surface and facilitates the electron/hole transfer. Furthermore, the degradation pathway was proposed on the basis of the intermediates during the photodegradation process using liquid chromatography analysis coupled with mass spectroscopy (LC–MS). The degradation mechanism of pollutant is ascribed to the superoxide radicals (·O2−), which is the main oxidative species for the N-deethylated degradation of RhB. Moreover, the Au–ZnO photocatalysts demonstrated excellent photostability after five cycles. This work provides a facile and effective approach for removal of organic dyes under visible light and thus can be potentially used in the environmental purification.