ZnO Nanotetrapods Research Articles

Photoluminescence-based biosensor for the detection of antibodies against SARS-CoV-2 virus proteins by ZnO tetrapod structure integrated within microfluidic system

This paper reports on development of an optical biosensor for the detection of antibodies against SARS-CoV-2 virus proteins in blood serum. ZnO nanotetrapods with high surface area and stable room temperature photoluminescence (PL) were selected as transducers. Structure and optical properties of the ZnO tetrapods have been studied by XRD, SEM and Raman spectroscopy. Crystallinity, dimensions and emission peaks of the ZnO tetrapods were determined. The ZnO tetrapods were fixed on glass chip. Silanization of ZnO tetrapods surface resulted in forming of functional surface groups suitable for the immobilization of bioselective layer. Two types of recombinant proteins (rS and rN) have been used to form bioselective layer on the surface of the ZnO tetrapods. Flow through microfluidic system, integrated with optical system, has been used for the determination of antibodies against SARS-CoV-2 virus proteins present in blood samples. The SARS-CoV-2 probes, prepared in PBS solution, have been injected into the measurement chamber with a constant pumping speed. Steady-state photoluminescence spectra and photoluminescence kinetics have been studied before and after injection of the probes. The biosensor signal has been tested to anti-SARS-CoV-2 antibodies in the range of 0.001 nM–1 nM. Control measurements have been performed with blood serum of healthy person. ZnO-SARS-CoV-2-rS and ZnO-SARS-CoV-2-rN biosensors showed high stability and sensitivity to anti-SARS-CoV-2 antibodies in the range of 0.025–0.5 nM (LOD 0.01 nM) and 0.3–1 nM (LOD 0.3 nM), respectively. Gibbs free energy of interaction between ZnO/SARS-CoV-2-rS and ZnO/SARS-CoV-2-rN bioselective layers with anti-SARS-CoV-2 antibodies showed −35.5 and −21.4 kJ/mol, respectively. Average detection time of biosensor integrated within microfluidic system was 15–20 min. The detection time and pumping speed (50 μL/min) were optimized to make detection faster. The developed system and ZnO-SARS-CoV-2-rS nanostructures have good potential for detection of anti-SARS-CoV-2 antibodies from patient's probes.

Transport studies in piezo-semiconductive ZnO nanotetrapod based electronic devices

ZnO nanotetrapods (ZnO NTs) with a non-centrosymmetric crystal structure consisting of four 1-D arms interconnected together through a central crystalline core, introduce interesting piezoelectric semiconducting responses in nanorods in the bent state. Considering the widespread applications of nanotetrapods in semiconductor devices, it becomes very crucial to establish a coupled model based on piezoelectric and piezotronic effects to investigate the carrier transport mechanism, which is being reported here in detail for the first time. In this work, we established a multiphysics coupled model of stress-regulated charge carrier transport by the finite element method (FEM), which considers the full account of the wurtzite (WZ) and zinc blende (ZB) regions as well as the spontaneous polarization dependence and the dependence of the material properties on the arm orientation. It is discovered that the forward gain of ZnO NT in the lateral force working mode is almost 50 % higher than that in the nanorod or in the normal force working mode while the reverse current is reduced to negligible. Through the simulation calculations and corresponding analysis, it is confirmed that the developed piezoelectric polarization charges are able to regulate the transport and distribution of carriers in ZnO crystal, which lays a theoretical foundation for the application of piezo-semiconductive ZnO NT devices in advanced technologies.

Understanding the piezoelectric response of ZnO nanotetrapods: Detailed numerical calculations

The complex tetrapod shape of zinc oxide nanostructure, which is constructed from four one-dimensional arms interconnected together via a central core, is a special 3D geometry with multifunctional applications in advanced technologies. The ZnO hexagonal wurtzite crystal lattice with a non-centrosymmetric structure introduces interesting piezoelectric property in nanorods in the bent state, which has been well reported and utilized in piezo- and tribo-electric nanogenerator applications. Considering the broad technological relevance of tetrapods, it is important to understand the piezoelectric response of zinc oxide tetrapods under different conditions. In this study, we explicate the fundamental mechanical and electrical properties of ZnO nanotetrapods (ZnO NTs) through a detailed finite element method analysis. On this basis, the effects of shape factors (including length, height, and aspect ratio) as well as connection strength and packing density on the deformation and piezoelectric potential of ZnO NTs are examined, offering guidance for the fabrication of ZnO NTs. This theoretical model and numerical simulation provide an avenue for further piezoelectric and piezotronic research of ZnO NTs.

Expanding Selectivity Functionality of a ZnO Nanotetrapod-Based Volatile Organic Compound Sensor Using Au Nanoparticle Decoration

Gas sensors with multiselectivity are drawing increasing attention and sensors with temperature-tuned dual selectivity have been developed. Because Au nanoparticles (NPs) can lower the activation barrier for sensing reactions, herein, Au NP decoration was used to expand the selectivity range of a ZnO nanotetrapod (NTP)-based sensor. A temperature-tuned triselectivity sensor based on Au NP–ZnO NTPs was developed. The sensor exhibited enhanced selectivity for formaldehyde when the operation temperature was <200 °C, ethanol when the operation temperature was between 200 and 340 °C, and acetone when the operation temperature was >400 °C. Additionally, Au NP decoration increased the magnitude of response and decreased the optimal detection temperature of the sensor. This work demonstrates that Au NP decoration is an efficient approach for improving the performance of semiconductor-based gas sensors with temperature-tunable selectivity.

Fast and one-step synthesis of small ZnO nano-tetrapods Using CO2 laser in ambient air: physical properties

In this study, a simple laser ablation method was developed to synthesize small nano-tetrapods as a fast and one-step process. The used laser was a CO2 laser with 10.6 μm of wavelength with proper thermal penetration depth for performing the ZnO tetrapods in the interaction with Zn target in ambient air. The samples were produced in two different morphologies: nanorods and nano-tetrapods, in two states. The optical, structural, and morphological properties were studied by performing room-temperature scanning electron microscopy (FESEM), dynamic light scattering (DLS), x-ray diffraction (XRD), photoluminescence (PL), and UV-visible analysis. The FESEM analysis shows that the diameters of the produced ZnO nano-tetrapods are approximately 10–13 nm by arm length of about 250 nm with excellent morphologies. The XRD results indicated that the nanoparticles are in phase with pure ZnO and both morphologies have the Hexagonal wurtzite. Two high peaks in the PL spectrum were observed on UV and visible spectrums, a peak in the red wavelength was demonstrated that has been rarely found for ZnO nanostructures and show the capability of produced particle to use in lightning industry. UV–vis spectral of dispersed particles in deionized water determined that nanorods and nano-tetrapods have peaks at 374 nm and 370 nm, respectively. The spectrums showed produced nanostructures have a broad absorption band from ultra-violet to visible regions, leading the decreases the absorption when the wavelength increases slowly to the visible spectrum, that show capability to apply in photocatalytic processes. Therefore, results show the capability of the introduced method to access two different morphologies of ZnO and significantly the nano-tetrapods structure with well physical and morphological properties.

A rule for operation temperature selection of a conductometric VOC gas sensor based on ZnO nanotetrapods

The performance of conductometric gas sensors depends critically on the operation temperature. However, it remains unclear how the characteristics of gas may require a shift in the operation temperature to achieve the best detection performance. In this work, we studied the effects of temperatures on the selectivity and response of a ZnO nanotetrapod (NTP)-based sensor using four common volatile organic compounds (VOCs). Our experiments revealed that the optimal detection temperatures for formaldehyde and acetone are the lowest and highest, respectively, among formaldehyde, ethanol, methanol and acetone. Selectivity shifts from formaldehyde to ethanol before and after 260 ℃. More interestingly, temperatures for optimizing the response magnitude and selectivity to formaldehyde are different: 260 ℃ and 100 ℃. A theoretical explanation is proposed that is primarily associated with chemical bonds and electrons involved in gas redox reactions. Ultimately, we proposed a temperature selection rule based on the molecular structure of the target gas to compromise between selectivity and response magnitude. For maximizing the response magnitude, the gas with a larger molecular structure requires a higher temperature. For optimizing the selectivity, low temperatures favor selectivity to smaller molecules with fewer and weaker bonds. This work might facilitate multifunctional gas sensor development.

Influence of the Crystal Structure of the Nucleus on the Morphology of t-ZnO Tetrapods

A new approach to simulating the morphology of hierarchical nanocrystals has been proposed on the example of ZnO nanotetrapods. Within this approach, twinning is considered as a point of symmetric growth bifurcation under unstable conditions. The electronic structure has been calculated for elementary ZnO clusters of different symmetries by the molecular dynamics method to reveal the processes of symmetry transformations occurring under nonequilibrium conditions during the forming of the tetrapod nucleus.

NEW INSIGHTS ON THE MECHANISM OF SEMICONDUCTOR NANOSTRUCTURES FORMED DURING VAPOR TRANSPORT AT ATMOSPHERIC PRESSURE

Co-deposition of two types of high aspect ratio nanostructured ZnO involving nanowires and nanotetrapods with a uniform structure were carried out through thermal evaporation and vapor transportation of a mixture of highly pure ZnO and graphite powders in 1:1 weight ratio. The mixture was heated at 1100 oC under various flow rates of N2 and air mixture. The surface morphology and structural characteristics of synthesized ZnO nanostructured materials revealed a highly crystalline structure with an average diameter of about 30 nm and length of several micrometers. The mechanism of co-deposition of ZnO nanowires and nanotetrapods during vapor transport at atmospheric pressure was proposed.

Separation of Viscous Oil Emulsions Using Three-Dimensional Nanotetrapodal ZnO Membranes

The steam-assisted gravity-drainage (SAGD) method has emerged as among the leading methods of enhanced oil recovery and is predicated on the injection of steam within the wellbore followed by extraction of emulsions of viscous oil and water. The emulsions are stabilized by endogenous surfactants, necessitating extensive processing such as addition of chemical de-emulsifiers and slow gravity-based separation methods. Here, we show that a hierarchically textured membrane exhibiting orthogonal wettability, specifically, superoleophilic but superhydrophobic behavior, allows for effective separation of the water and viscous oil fractions of SAGD emulsions. The membrane is constructed by integrating ZnO nanotetrapods onto stainless steel meshes using a conformal amorphous SiO2 layer and is both mechanically resilient and thermally robust. The intrinsic surface energy characteristics of the ZnO tetrapods as well as their three-dimensional texture when arrayed atop the stainless steel mesh substrates contribute to the observed differential wettability between water and oil. Water content in permeated bitumen is reduced to as low as 0.69 vol % through a single-pass filtration step with the further advantage of eliminating silt particles. The permeation temperature and water content are tunable based on modulation of the mesh size and ZnO loading. The membranes allow for operation at SAGD temperatures in excess of 130 °C, thereby enabling the thermal disruption of hierarchical emulsions. The membrane-based separation of SAGD emulsions under realistic process conditions paves the way for entirely new process designs for recovering dry viscous oil.

NH₄OH-Oriented and pH-Dependent Growth of ZnO Nanostructures via Microwave-Assisted Growth Method.

We proffer NH4OH-oriented and pH-dependent growth of ZnO nanostructures via a microwaveassisted growth method. The fabrication of ZnO nanorods (ZNRs), nanoflowers (ZNFs), nanostars (ZNSs), and nanotetrapods (ZNTs) is presented. NH4OH was used as a mineralizer to change the solution pH for nanostructure growth, where temperature and other variables were fixed. Because of an efficient heat transfer and facile growth of nanostructures, a domestic microwave oven was used to facilitate the nanostructure growth in the span of just 10-15 min. The results showed that the growth of ZnO nanostructures was dependent upon the number of growth units and ZnO nuclei present in the solution, which ultimately depend upon the pH of the solution. At the outset, without the addition of NH4OH, the pH of the solution was ~6.8 and the ZNRs were formed in the solution or on a seeded substrate which persisted in the pH range of ~6.8-9. An abrupt change in the shapes and the types of the nanostructures was observed when the pH was boosted beyond 10. A transition from ZNRs to ZNFs was observed at pH 10 and ZNFs were formed at pH 11. The solution gave birth to ZNSs and ZNTs when the pH was further raised to 12 and 13, respectively.

Direct growth of ZnO tetrapod on glass substrate by Chemical Vapor Deposition Technique

This research demonstrates the growth of ZnO tetrapod structure on glass substrate for different types of flow gas and at different growth temperatures. The study on the morphological structure and electrical properties of ZnO thin film growth by Chemical Vapour Deposition (CVD) technique showed that the optimum growth temperature was obtained at 750°C with ZnO nanotetrapod morphological structure. Introducing Nitrogen gas flow during the growth process exhibited leg-to-leg linking ZnO tetrapods morphology. The electrical properties of ZnO tetrapods film were measured by using two point probes and it shows that, the sample growth in Ar and O2 atmosphere have better I-V characteristic.

Light-activated gas sensing activity of ZnO nanotetrapods enhanced by plasmonic resonant energy from Au nanoparticles

High response and low working temperature are two essential requirements for promising chemi-resistive gas sensors, considering the issues of practical usage, safety, stability, energy conservation, etc, while traditional sensors usually work at high temperatures. One typical approach to solve this drawback is using photon activated process, but its overall activation response is not high enough for room-temperature operation. Utilization of localized surface plasmon resonance (LSPR) can increase light absorption and energetic carrier generation. Based on this, we developed a high-response and room-temperature sensor by decorating 3-dimensional-structured ZnO nanotetrapods (ZnO NTPs) with Au nanoparticles (Au NPs) through a facile sputtering-annealing process. Compared to pure ZnO NTPs, this hybrid nanostructure enhanced detection of volatile vapor components including ethanol, formaldehyde, acetone and methanol under 6 mW/mm2 white light illumination at room temperature. Representatively, the sensing response increased remarkably from 5.5 to 62 towards 500 ppm ethanol vapor. It also exhibits decent long-term stability and a better sensing performance than a commercial thermal-activated alcohol sensor operating at an optimal temperature of 370 °C. The reported LSPR assisted approach holds strong potential for practical devices and pave a new way for the development of room-temperature gas sensor with high-performance.

Biomimetic Plastronic Surfaces for Handling of Viscous Oil

Unconventional deposits such as extra heavy oil and bitumen represent a steadily increasing proportion of extracted fuels. The rheological properties of viscous crude oil represents a formidable impediment to their extraction, transportation, and processing and have necessitated considerable retooling and changes to process design. In this work, we demonstrate that highly textured inorganic substrates generated by depositing ZnO nanotetrapods onto periodically ordered stainless steel mesh substrates exhibit viscous oil contact angles exceeding 150° as well as enable the facile gliding of viscous oil. Such functionality is derived as a result of multiscale texturation and porosity achieved within these substrates, which are characterized by trapping of plastronic air pockets at the solid/liquid interface. Further reduction of the surface energy has been achieved by constituting a helical highly ordered self-assembled monolayer of a perfluorinated phosphonic acid on the ZnO surfaces. Such structures are strongly ejected upon immersion in water with water contact angles in excess of 160°. The functionalized substrates demonstrate remarkable superoleophobic behavior toward viscous crude oil with contact angles reaching 156° and are furthermore stable to temperatures of 290 °C. The remarkable results evidenced here hold promise for deployment of these constructs in the handling of viscous oil in order to reduce losses associated with transportation from railroad cars, pipelines, and other oil-handling equipment.

Facile synthesis of Sn doped ZnO nanotetrapods for detection of relatively non-lethal volatiles

While there’s a perpetual buzz around zinc oxide superstructures for their unique optical features, the versatile material has been constantly utilized to manifest tailored electronic properties through rendition of distinct morphologies. And yet, the unorthodox approach of implementing the hierarchical structures of ZnO for volatile sensing applications has ample scope to accommodate new unconventional morphologies. Likewise, this article presents self-catalytic synthesis of Sn-doped ZnO nanotetrapods on Si (100) substrates through thermal evaporation–condensation method, and their subsequent deployment for volatile sensing. In particular, the sensors were utilized to detect molecules of acetone and ammonia below their permissible exposure limits which returned sensitivities of around 80% and 50% respectively. The influence of Sn concentration on the growth, microstructural and optical properties of the nanoprisms along with its role in augmenting the sensing parameters has been detailed. The features for the nanoprisms include a length of few micrometers along with a diameter ranging from 300 to 500nm. High resolution microscopic images confirmed the hexagonal crystallography for the nanoprisms, while SAED pattern asserted the single crystalline nature. An estimate of the sensing parameters against dispensed target molecules highlighted the potential for the nanoprisms as an effective volatile sensing material.

Orthogonal Wettability of Hierarchically Textured Metal Meshes as a Means of Separating Water/Oil Emulsions

The removal of submicrometer‐sized oil droplets from water remains a key challenge in engineering the separation of emulsions and has emerged as an urgent imperative given the increasing use of unconventional extractive processes. In this work, the authors demonstrate that a substrate with hierarchical texturation shows pronounced differences in the wettability of water and hexadecane, thereby, facilitating the separation of these two disparate liquids at room temperature and pressure. The multiscale textured substrates are assembled using a facile and readily scalable process, wherein ZnO nanotetrapods are spray‐deposited onto a steel mesh with micron‐sized features. Separation efficiencies well over 99% are accessible by simply flowing emulsions across these hierarchically textured surfaces.

Flexible Light-Emitting Nanocomposite Based on ZnO Nanotetrapods.

Flexible, light-emitting materials have shown promise in a wide range of applications. Here, we develop an inverse soft-lithography process for embedding zinc oxide nanotetrapods (ZnO NTP) uniformly and nondestructively into a host matrix. The crystalline NTPs were synthesized using a catalyst-free, environmentally friendly chemical vapor transport method. The fluorescent emission of the ZnO NTPs was measured before and after the embedding process. Cyclical mechanical bend tests (N > 100) were performed. The emission of the nanomaterial remains throughout.

Enhanced piezo/solar-photocatalytic activity of Ag/ZnO nanotetrapods arising from the coupling of surface plasmon resonance and piezophototronic effect

Ag/ZnO nanotetrapods are synthesized in mass production via a simple thermal-evaporation/hydrothermal route, and Ag nanoparticles are randomly coated on ZnO nanotetrapods. Ag/ZnO nanotetrapods can co-use the solar and mechanical energy to degrade various organic pollutants, and the solar-photocatalytic activity is significantly enhanced by the piezo-assistance. For instance, under ultrasonic stimulation (200W) and solar illumination (500W), Ag/ZnO nanotetrapods can completely degrade methyl orange (MO) within 25min. The high piezo/solar-photocatalytic efficiency of Ag/ZnO nanotetrapods can be ascribed to the coupling of surface plasmon resonance and piezophototronic effect in the solar-photocatalytic process. The localized surface plasmon resonance effect of Ag nanoparticles can increase the visible light absorption. Ag/ZnO interface can facilitate the interfacial charge transfer and induce the separation of photo-induced charge carriers. The piezoelectric field originated from the deformation of ZnO nanotetrapods can further enhance the separation of photo-induced electron/hole pairs. Our results imply that Ag/ZnO nanotetrapods have great potentials of using sustainable energy in the nature for environmental remediation.

Tuning aspect ratio of hierarchical ZnO nanotetrapod

Fine-tuning the aspect ratio is crucial for practical application and optimizing various properties of nanostructure. In this work, a mixed metal (Zn) and oxide (ZnO + C) powder was adopted as precursor to obtain initial high reagent vapour pressure and subsequent low reagent vapour pressure, and one kind of hierarchical ZnO nanotetrapod with legs composed of thick primary nanorod and thin secondary nanowire was successfully synthesized. Two growth stages controlled by the reagent supersaturation ratio were proposed to unveil the underlying growth mechanism of this hierarchical structure. Further, a series of comparative experiments were undertaken to validate the growth mode and explore the growth strategy tailoring the aspect ratio. Our results suggest one accessible approach to manipulate the aspect ratio, and the growth mode has some promises for preparing hierarchical nanoblocks with specific geometry demanding.

Nanoscale mapping of plasmon and exciton in ZnO tetrapods coupled with Au nanoparticles.

Metallic nanoparticles can be used to enhance optical absorption or emission in semiconductors, thanks to a strong interaction of collective excitations of free charges (plasmons) with electromagnetic fields. Herein we present direct imaging at the nanoscale of plasmon-exciton coupling in Au/ZnO nanostructures by combining scanning transmission electron energy loss and cathodoluminescence spectroscopy and mapping. The Au nanoparticles (~30 nm in diameter) are grown in-situ on ZnO nanotetrapods by means of a photochemical process without the need of binding agents or capping molecules, resulting in clean interfaces. Interestingly, the Au plasmon resonance is localized at the Au/vacuum interface, rather than presenting an isotropic distribution around the nanoparticle. On the contrary, a localization of the ZnO signal has been observed inside the Au nanoparticle, as also confirmed by numerical simulations.

Structural and Optical Properties of ZnO Nanotetrapods Deposited by Thermal Chemical Vapor Deposition with Different Gas Flow Rate

ZnO nanotetrapod with different oxygen flow rate was prepared by thermal chemical vapor deposition. We have successfully deposited ZnO nanotetrapod on synthesis Zn powder using double furnace with argon (Ar) and oxygen (O2) gas as source material. In this study, we report the effect of different gas flow rate (5 sccm to 15 sccm) on structural and optical properties of the ZnO nanotetrapod. The morphology of ZnO nanotetrapods were analyzed by field emission scanning electron microscope (FE-SEM). It exhibits the length of the nanotetrapods arm decrease with increased of flow rate and diameter of nanotetrapod in range 30 nm to 90 nm. The optical properties were determined through XRD and photoluminescence with 2θ (30o to 80o) and wavelength 350 nm to 620 nm respectively. PL spectra show that the UV emission centred at 380 nm while yellow-orange emission centred at 540 nm.