Sustainable and Low-Cost Electrodes for Photocatalytic Fuel Cells

Water pollutants harm ecosystems and degrade water quality. At the same time, many pollutants carry potentially valuable chemical energy, measured by chemical oxygen demand (COD). This study highlights the potential for energy harvesting during remediation using photocatalytic fuel cells (PCFCs), stressing the importance of economically viable and sustainable materials. To achieve this, this research explores alternatives to platinum cathodes in photocathodes and aims to develop durable, cost-effective photoanode materials. Here, zinc oxide nanorods of high density are fabricated on carbon fiber surfaces using a low-temperature aqueous chemical growth method that is simple, cost-efficient, and readily scalable. Alternatives to the Pt cathodes frequently used in PCFC research are explored in comparison with screen-printed PEDOT:PSS cathodes. The fabricated ZnO/carbon anode (1.5 × 2 cm2) is used to remove the model pollutant used here and salicylic acid from water (30 mL, 70 μM) is placed under simulated sunlight (0.225 Sun). It was observed that salicylic acid was degraded by 23 ±0.46% at open voltage (OV) and 43.2 ± 0.86% at 1 V with Pt as the counter electrode, degradation was 18.5 ± 0.37% at open voltage (OV) and 44.1 ± 0.88% at 1 V, while PEDOT:PSS was used as the counter electrode over 120 min. This shows that the PEDOT:PSS exhibits an excellent performance with the full potential to provide low-environmental-impact electrodes for PCFCs.

Metal Oxide Nanostructures Enhanced Microfluidic Platform for Efficient and Sensitive Immunofluorescence Detection of Dengue Virus

Rapid and sensitive detection of Dengue virus remains a critical challenge in global public health. This study presents the development and evaluation of a Zinc Oxide nanorod (ZnO NR)-surface-integrated microfluidic platform for the early detection of Dengue virus. Utilizing a seed-assisted hydrothermal synthesis method, high-purity ZnO NRs were synthesized, characterized by their hexagonal wurtzite structure and a high surface-to-volume ratio, offering abundant binding sites for bioconjugation. Further, a comparative analysis demonstrated that the ZnO NR substrate outperformed traditional bare glass substrates in functionalization efficiency with 4G2 monoclonal antibody (mAb). Subsequent optimization of the functionalization process identified 4% (3-Glycidyloxypropyl)trimethoxysilane (GPTMS) as the most effective surface modifier. The integration of this substrate within a herringbone-structured microfluidic platform resulted in a robust device for immunofluorescence detection of DENV-3. The limit of detection (LOD) for DENV-3 was observed to be as low as 3.1 × 10-4 ng/mL, highlighting the remarkable sensitivity of the ZnO NR-integrated microfluidic device. This study emphasizes the potential of ZnO NRs and the developed microfluidic platform for the early detection of DENV-3, with possible expansion to other biological targets, hence paving the way for enhanced public health responses and improved disease management strategies.

A visible-light photodetector based on heterojunctions between CuO nanoparticles and ZnO nanorods

Optoelectronic devices have various applications in medical equipment, sensors, and communication systems. Photodetectors, which convert light into electrical signals, have gained much attention from many research teams. This study describes a low-cost photodetector based on CuO nanoparticles and ZnO nanorods operating in a wide range of light wavelengths (395, 464, 532, and 640 nm). Particularly, under 395 nm excitation, the heterostructure device exhibits high responsivity, photoconductive gain, detectivity, and sensitivity with maximum values of 1.38 A·W-1, 4.33, 2.58 × 1011 Jones, and 1934.5% at a bias of 2 V, respectively. The sensing mechanism of the p-n heterojunction of CuO/ZnO is also explored. Overall, this study indicates that the heterostructure of CuO nanoparticles and ZnO nanorods obtained via a simple and cost-effective synthesis process has great potential for optoelectronic applications.

Surface characterizations and methylene blue pollutant removal efficiency of ZnO nanorods/biochar hybrids

In this study, the integration of carbon nanotube (CNT), graphene, and biochar (BC) with zinc oxide nanorods (ZnO NRs) was investigated for efficient water pollutant removal. Two types of ZnO NRs/BC hybrids (BC on top and bottom of ZnO NRs) were synthesized and compared to other carbon material-based ZnO NRs combinations. Methylene blue (MB) adsorption efficiency was evaluated for various carbon material-based ZnO NRs composites, revealing good performance in ZnO NRs/BC hybrids, particularly with BC on top. The adsorption efficiency reached an impressive 61.79% for ZnO NRs/BC, surpassing other configurations. MB removal by ZnO NRs/BC fitted well with pseudo-first-order kinetics and the rate constants of MB adsorption is 9.19 × 10-2 1/min (R2  = 0.9237). Surface characterizations revealed a distinctive distribution of BC grains, with denser aggregation observed on top of ZnO NRs. This unique distribution contributed to higher MB adsorption rates, substantiated by Fourier transform infrared spectroscopy (FTIR) analysis that showcased stronger MB adsorption in ZnO NRs/BC hybrids. Notably, the enhanced MB adsorption rates were attributed to the population of BC grains. This research establishes ZnO NRs/BC composites as promising candidates for effective water pollutant removal. The developed materials can be combined with the existed conventional wastewater treatment systems to further purify the water quality. PRACTITIONER POINTS: ZnO NRs/BC hybrids achieve a remarkable 61.79% efficiency in removing MB pollutants, surpassing other carbon materials. MB removal using BC-based materials follows pseudo-first-order kinetics. BC grains exhibit unique distribution patterns on ZnO NRs, with densely packed grains atop contributing to higher MB removal. FTIR analysis confirms increased MB-related bond vibration, supporting the effectiveness of ZnO NRs/BC hybrids for water pollutant removal.

Engineering the defect distribution in ZnO nanorods through laser irradiation

In recent years, defect engineering has shown great potential to improve the properties of metal oxide nanomaterials for various applications thus received extensive investigations. While traditional techniques mostly focus on controlling the defects during the synthesis of the material, laser irradiation has emerged as a promising post-deposition technique to further modulate the properties of defects yet there is still limited information. In this article, defects such as oxygen vacancies are tailored in ZnO nanorods through nanosecond (ns) laser irradiation. The relation between laser parameters and the temperature rise in the ZnO due to laser heating was established based on the observation in the SEM and the simulation. Raman spectra indicated that the concentration of the oxygen vacancies in the ZnO is temperature-dependent and can be controlled by changing the laser fluence and exposure time. This is also supported by the absorption spectra and the photoluminescence spectra of ZnO NRs irradiated under these conditions. On the other hand, the distribution of the oxygen vacancies was studied by XPS depth profiling, and it was confirmed that the surface-to-bulk ratio of the oxygen vacancies can be modulated by varying the laser fluence and exposure time. Based on these results, four distinctive regimes containing different ratios of surface-to-bulk oxygen vacancies have been identified. Laser-processed ZnO nanorods were also used as the catalyst for the photocatalytic degradation of rhodamine B (RhB) dye to demonstrate the efficacy of this laser engineering technique.

Kirigami Photopiezo Catalysts for Self-Sustainable Environment Remediation

Photo(electro)-piezo catalysis has emerged as one of the most effective strategies for sustainable environmental remediation. While various (nano)materials have been investigated for enhancing the intrinsic properties related to the interfacial band structure, increasing the efficiency by integration of materials with rational design for stress-strain applications has not yet been considered. Herein, we introduce kirigami strain engineering to photopiezo catalysts for enhancing efficiency by increasing the magnitude of applied strain and density of bends. Macroscale stretching motion is converted into localized bending by a pliable kirigami structure using similar or even lower input energy, which can be easily modulated by natural waves. The kirigami structure leads to a significant enhancement (∼250%) in the degradation of dyes, and we discovered the significant contribution of the oxygen reduction pathway in the charge-transfer mechanism, which corresponds to the observed enhancement. The photopiezo catalytic effects of kirigami were further highlighted by the small water reservoir test, showing its feasibility in nature for self-sustainable environmental remediation that can be modulated using motions of winds, waves, and life vibrations.

Comparison of the Degradation Effect of Methylene Blue for ZnO Nanorods Synthesized on Silicon and Indium Tin Oxide Substrates

In the context of ZnO nanorods (NRs) grown on Si and indium tin oxide (ITO) substrates, this study aimed to compare their degradation effect on methylene blue (MB) at different concentrations. The synthesis process was carried out at a temperature of 100 °C for 3 h. After the synthesis of ZnO NRs, their crystallization was analyzed using X-ray diffraction (XRD) patterns. The XRD patterns and top-view SEM observations demonstrate variations in synthesized ZnO NRs when different substrates were used. Furthermore, cross-sectional observations reveal that ZnO NRs synthesized on an ITO substrate exhibited a slower growth rate compared to those synthesized on a Si substrate. The as-grown ZnO NRs synthesized on the Si and ITO substrates exhibited average diameters of 110 ± 40 nm and 120 ± 32 nm and average lengths of 1210 ± 55 nm and 960 ± 58 nm, respectively. The reasons behind this discrepancy are investigated and discussed. Finally, synthesized ZnO NRs on both substrates were utilized to assess their degradation effect on methylene blue (MB). Photoluminescence spectra and X-ray photoelectron spectroscopy were employed to analyze the quantities of various defects of synthesized ZnO NRs. The effect of MB degradation after 325 nm UV irradiation for different durations can be evaluated using the Beer-Lambert law, specifically by analyzing the 665 nm peak in the transmittance spectrum of MB solutions with different concentrations. Our findings reveal that ZnO NRs synthesized on an ITO substrate exhibited a higher degradation effect on MB, with a rate of 59.5%, compared to NRs synthesized on a Si substrate, which had a rate of 73.7%. The reasons behind this outcome, elucidating the factors contributing to the enhanced degradation effect are discussed and proposed.

Randomly Disassembled Nanostructure for Wide Angle Light Extraction of Top-Emitting Quantum Dot Light-Emitting Diodes

The quantum dot light-emitting diode (QLED) represents one of the strongest display technologies and has unique advantages like a shallow emission spectrum and superior performance based on the cumulative studies of state-of-the-art quantum dot (QD) synthesis and interfacial engineering. However, research on managing the device's light extraction has been lacking compared to the conventional LED field. Moreover, relevant studies on top-emitting QLEDs (TE-QLEDs) have been severely lacking compared to bottom-emitting QLEDs (BE-QLEDs). This paper demonstrates a novel light extraction structure called the randomly disassembled nanostructure (RaDiNa). The RaDiNa is formed by detaching polydimethylsiloxane (PDMS) film from a ZnO nanorod (ZnO NR) layer and laying it on top of the TE-QLED. The RaDiNa-attached TE-QLED shows significantly widened angular-dependent electroluminescence (EL) intensities over the pristine TE-QLED, confirming the effective light extraction capability of the RaDiNa layer. Consequently, the optimized RaDiNa-attached TE-QLED achieves enhanced external quantum efficiency (EQE) over the reference device by 60%. For systematic analyses, current-voltage-luminance (J-V-L) characteristics are investigated using scanning electron microscopy (SEM) and optical simulation based on COMSOL Multiphysics. It is believed that this study's results provide essential information for the commercialization of TE-QLEDs.

Changes to Material Phase and Morphology Due to High-Level Molybdenum Doping of ZnO Nanorods: Influence on Luminescence and Defects

The influence of Mo on the electronic states and crystalline structure, as well as morphology, phase composition, luminescence, and defects in ZnO rods grown as free-standing nanoparticles, was studied using a variety of experimental techniques. Mo has almost no influence on the luminescence of the grown ZnO particles, whereas shallow donors are strongly affected in ZnO rods. Annealing in air causes exciton and defect-related bands to drop upon Mo doping level. The increase of the Mo doping level from 20 to 30% leads to the creation of dominating molybdates. This leads to a concomitant drop in the number of formed ZnO nanorods.

Magnetically retrievable Fe(3)O(4)@SiO(2)@ZnO piezo-photocatalyst: Synthesis and multiple catalytic properties

The piezo-/photocatalytic effects of ZnO have been in the limelight because of their great potential in environmental remediation and energy conversion. However, the poor recyclability of the suspended catalysts can cause inevitable secondary pollution, which is one of the major issues that limit the practical application of these materials. To address this problem, a magnetically retrievable Fe₃O₄@SiO₂@ZnO nanocomposite was designed and successfully synthesized by multi-step reactions. The ZnO nanorods were vertically grown on the surface of the magnetic Fe₃O₄@SiO₂ microspheres, while SiO₂ served as an insulator to protect the inner core and to inhibit charge transfer across the core/shell interface. The Fe₃O₄@SiO₂@ZnO nanocomposite can be easily collected and separated by using a magnetic field. Along with the good recyclability, the material also exhibited high efficiencies in piezocatalytic, photocatalytic and piezo-photocatalytic dye degradation processes. The rate constant of piezo-photocatalysis reached 95.9 × 10^-3 min^-1, which was 2.2 and 6.1 times that of the individual piezocatalysis and photocatalysis, respectively. The present result confirmed the existence of a synergetic effect between piezo- and photocatalytic processes. Hereby, we demonstrated that incorporation of a magnetic carrier is a feasible strategy to achieve retrievable and highly efficient piezo-/photocatalyst.

ZnO Nanorods Create a Hypoxic State with Induction of HIF-1 and EPAS1, Autophagy, and Mitophagy in Cancer and Non-Cancer Cells

Nanomaterials are gaining increasing attention as innovative materials in medicine. Among nanomaterials, zinc oxide (ZnO) nanostructures are particularly appealing because of their opto-electrical, antimicrobial, and photochemical properties. Although ZnO is recognized as a safe material and the Zn ion (Zn²⁺) concentration is strictly regulated at a cellular and systemic level, different studies have demonstrated cellular toxicity of ZnO nanoparticles (ZnO-NPs) and ZnO nanorods (ZnO-NRs). Recently, ZnO-NP toxicity has been shown to depend on the intracellular accumulation of ROS, activation of autophagy and mitophagy, as well as stabilization and accumulation of hypoxia-inducible factor-1α (HIF-1α) protein. However, if the same pathway is also activated by ZnO-NRs and how non-cancer cells respond to ZnO-NR treatment, are still unknown. To answer to these questions, we treated epithelial HaCaT and breast cancer MCF-7 cells with different ZnO-NR concentrations. Our results showed that ZnO-NR treatments increased cell death through ROS accumulation, HIF-1α and endothelial PAS domain protein 1 (EPAS1) activation, and induction of autophagy and mitophagy in both cell lines. These results, while on one side, confirmed that ZnO-NRs can be used to reduce cancer growth, on the other side, raised some concerns on the activation of a hypoxic response in normal cells that, in the long run, could induce cellular transformation.

Fabrication of Well-Aligned ZnO Nanorods with Different Reaction Times by Chemical Bath Deposition Method Applying for Photocatalysis Application

Zinc oxide nanorods were grown on an aluminum-doped zinc oxide seeds layer using the chemical bath deposition method. The effects of growth reaction time on the structural, optical, and photocatalytic properties of zinc oxide nanorods were investigated. It was clearly observed that the growth direction of zinc oxide nanorods were dependent on the crystallinity of the as-deposited aluminum-doped zinc oxide seed layer. The crystallinity of the obtained zinc oxide nanorods was improved with the increase in reaction times during the chemical bath deposition process. The mechanism of zinc oxide nanorod growth revealed that the growth rate of nanorods was influenced by the reaction times. With increasing reaction times, there were much more formed zinc oxide crystalline stacked growth along the c-axis orientation resulting in an increase in the length of nanorods. The longest nanorods and the high crystallinity were obtained from the zinc oxide nanorods grown within 5 h. The optical transmittance of all zinc oxide nanorods was greater than 70% in the visible region. Zinc oxide nanorods grown for 5 h showed the highest degradation efficiency of methyl red under ultraviolet light and had a high first-order degradation rate of 0.0051 min^-1. The photocatalytic mechanism was revealed as well.

ZnO nanorods loading with fatty amine as composite PCMs device for efficient light-to-thermal and electro-to-thermal conversion

Phase change materials (PCMs) with ideal light-to-thermal conversion efficiency play an important role in solar energy storage and conversion. Hence, we report the fabrication of a novel composite PCMs (CPCMs) device based on ZnO nanorods deposited indium tin oxide (ITO) glass loading with fatty amines. ZnO nanorods were deposited on the ITO glass using a three-electrode electrodeposition method, and 1-Hexadecylamine (HDA) was loaded on the ITO glass via spin-coating, followed by spraying polypyrrole (ppy) on the surface of CPCM device to improve thermal conductivity and solar absorption. The as-prepared CPCM device exhibits excellent light-to-thermal conversion efficiency, achieving a high conversion efficiency of 90.2% obtained at 1sun owing to its high light absorption (80%), enhanced thermal conductivity (improved by 57.8%), and the unique vertical aligned nanorods structure which could significantly decrease tortuosity, thereby reducing thermal route and lowering thermal response time. Furthermore, the electro-to-thermal conversion efficiency of the CPCMs device has also been investigated and the results show that it can reach up to 69.8% under a low voltage of 5 V, indicating that the CPCM device has a high potential in the field of electro-to-thermal conversion. Based on the benefits listed above, the CPCM device may serve an ideal platform for a wide range of solar energy storage and conversion applications.

Boosting the Photoelectrochemical Performance of Au/ZnO Nanorods by Co-Occurring Gradient Doping and Surface Plasmon Modification

Band bending modification of metal/semiconductor hybrid nanostructures requires low-cost and effective designs in photoelectrochemical (PEC) water splitting. To this end, it is evinced that gradient doping of Au nanoparticles (NPs) inwards the ZnO nanorods (NRs) through thermal treatment facilitated faster transport of the photo-induced charge carriers. Systematic PEC measurements show that the resulting gradient Au-doped ZnO NRs yielded a photocurrent density of 0.009 mA/cm² at 1.1 V (vs. NHE), which is 2.5-fold and 8-fold improved compared to those of Au-sensitized ZnO and the as-prepared ZnO NRs, respectively. The IPCE and ABPE efficiency tests confirmed the boosted photoresponse of gradient Au-incorporated ZnO NRs, particularly in the visible spectrum due to the synergistic surface plasmonic effect of Au NPs. A gradient Au dopant profile promoted the separation and transfer of the photo-induced charge carriers at the electrolyte interface via more upward band bending according to the elaborated electrochemical impedance spectroscopy and Kelvin probe force microscopy analyses. Therefore, this research presents an economical and facile strategy for preparing gradient plasmonic noble NP-incorporated semiconductor NRs, which have excellent potential in energy conversion and storage technologies.

Preparation of BiOI-Functionalized ZnO Nanorods for Ppb-Level NO2 Detection at Room Temperature

Light activation is an effective method to improve sensor performance at room temperature (RT). This work realized the effective detection of trace-level NO₂ at RT under visible light by combining ZnO with the excellent photocatalyst BiOI. A 1.5 atom % BiOI-ZnO-based sensor under 520 nm light exhibited optimal sensing properties with the maximum responses (13.9 to 1 ppm NO₂), fast response/recovery time (66 s/47 s to 1 ppm), and a low detection limit of 25 ppb (theoretically 0.34 ppb). In the meantime, the sensor also possessed excellent selectivity, repeatability, and stability. The excellent properties were attributed to the high concentration of oxygen vacancies and the prolonged lifetime of photogenerated carriers. In addition, the observed photovoltaic effect of the sensor at RT indicated that the sensor held application prospects in the photovoltaic self-power field.

Green microwave synthesis of ZnO and CeO(2) nanorods for infectious diseases control and biomedical applications

Control of Infectious diseases such as; bacteria and viruses, has become a globally critical issue, since the appearance of COVID-19 virus in 2020. In addition to the microbial resistance of the currently available therapeutic drugs as well as, its prolonged side effects make its use is of health care concern. Green nanotechnology approach is a promising solution for controlling such infectious diseases and many biomedical purposes. In the present study, green synthesis approach based on microwave-assisted hydrothermal method is an innovative and environmentally friendly method for preparation of bioactive CeO₂ and ZnO nanorod structured materials using Olea europaea (O. e.) leaf plant extract as a natural medicinal capping agent for controlling the shape and size of nano-products. The optical and structural analyses of the obtained nanorod-structures are characterized using; TEM, FTIR, XRD, SBET analyses and particle size analyzer. The green-synthesized ZnO and CeO₂ nanorods display an average crystallite size of approximately 15 and 5 nm, respectively. The antimicrobial activity of ZnO and CeO₂ nanorods compared with the traditional hydrothermal methods, was examined on six clinical pathogens including; (E. coli Serratia sp., S. aureus, Bacillus subtilis, Streptococcus mutant, and MRSA). The results indicated superior antimicrobial and anti-tumor activities towards hepatocellular carcinoma cell lines (IC₅₀ = 117.24 and 103.50 μg mL^-1 for ZnO and CeO₂ and LD₅₀ > 3000 mg kg^-1). This demonstrates that the green microwave process is a promising approach for the synthesis of effective ZnO and CeO₂ nanomaterials applied for many biomedical applications.

Beetle-Inspired Dual-Directional Janus Pumps with Interfacial Asymmetric Wettability for Enhancing Fog Harvesting

Fog-harvesting devices (FHDs) have been widely explored and applied to alleviate the shortage of fresh water. However, during the fog collection process, how to maintain a balance between fog capture and water removal behaviors to enhance the water collection rate still remains a challenge. Herein, inspired by the Stenocara beetle, we combined a beetle-like Janus surface and the conventional cross-sectional Janus structure together, developed a simple spray-and-dry strategy to obtain three types of biomimetic asymmetric meshes, and explored the working modes for atmospheric fog collection. The surface wettability could be carefully controlled, and various asymmetric meshes with different water transportation behaviors were obtained. Through a detailed study of the fog collection process, we concluded that there existed three main working modes: Janus mode, hybrid mode, and Janus and hybrid mode. It was noted that the dual-directional Janus pump with the Janus and hybrid working mode balanced the fog capture and water removal ability and exhibited the highest water collection rate of 2478.73 mg m^-2 h^-1, which was 2.61 times more than that of the corresponding superhydrophilic mesh. Furthermore, the prepared dual-directional Janus pump showed superior mechanical durability and antibacterial ability. In general, this work was considered instrumental in the reasonable design of biomimetic asymmetric meshes and could provide references for efficient atmospheric fog harvesting.

Mixed Dimensional ZnO/WSe2 Piezo-gated Transistor with Active Millinewton Force Sensing

This work demonstrates a mixed-dimensional piezoelectric-gated transistor in the microscale that could be used as a millinewton force sensor. The force-sensing transistor consists of 1D piezoelectric zinc oxide (ZnO) nanorods (NRs) as the gate control and multilayer tungsten diselenide (WSe₂) as the transistor channel. The applied mechanical force on piezoelectric NRs can induce a drain-source current change (ΔI_ds) on the WSe₂ channel. The different doping types of the WSe₂ channel have been found to lead to different directions of ΔI_ds. The pressure from the calibration weight of 5 g has been observed to result in an ∼30% I_ds change for ZnO NRs on the p-type doped WSe₂ device and an ∼-10% I_ds change for the device with an n-type doped WSe₂. The outcome of this work would be useful for applications in future human-machine interfaces and smart biomedical tools.

Fabrication and Performance of UV Photodetector of ZnO Nanorods Decorated with Al Nanoparticles

In this work, localized surface plasmon resonance (LSPR) mediated by aluminum nanoparticles (Al NPs) was investigated to enhance the ultraviolet (UV) response of the zinc oxide nanorods (ZnO NRs) grown by the hydrothermal method. The ZnO NRs were characterized by scanning electron microscope, energy dispersive spectroscopy, X-ray diffractometer, Raman spectrometer, ultraviolet-visible spectrophotometer and fluorescence spectrometer. The results show that the morphology and crystalline structure of the ZnO NRs could not be changed before and after decoration with Al NPs, but the absorption rates in the UV range and the photoluminescence (PL) properties were improved. The photo-to-dark current ratio of ZnO NRs with Al NPs was about 447 for 325 nm UV light (5 mW/cm²) at 3.0 V bias, with the sensitivity increasing from 9.5 to 47.8, and the responsivity increasing from 53 to 267 mA/W.

Electrical and Optoelectronic Properties Enhancement of n-ZnO/p-GaAs Heterojunction Solar Cells via an Optimized Design for Higher Efficiency

In this study, we report the fabrication of high quality AZO/NRs-ZnO/n-ZnO/p-GaAs heterojunction via a novel optimized design. First of all, the electrical proprieties of gallium arsenide (GaAs) substrates were enhanced via an optimized gettering treatment that was based on a variable temperature process (VTP) resulting in an obvious increase of the effective minority carrier lifetime (τ_eff) from 8.3 ns to 27.6 ns, measured using time-resolved photoluminescence (TRPL). Afterward, the deposition of a zinc oxide (ZnO) emitter was optimized and examined in view of its use both as a light trapping layer (antireflection) and as the n-type partner for the p-type (GaAs) substrate. Nanorod-shaped ZnO was grown successfully on top of the emitter, as an antireflective coating (ARC), to further boost the absorption of light for a large broadband energy harvesting. The interface state of the prepared heterojunction is a key parameter to improve the prepared heterojunction performance, thus, we used laser ablation to create parallel line microgroove patterns in the GaAs front surface. We studied the effect of each step on the performance of the n-ZnO/GaAs heterojunction. The results demonstrate a significant improvement in V_oc, J_sc, fill factor (FF), and an obvious enhancement in the I-V characteristics, exhibiting good diode properties, giving rise to the photovoltaic conversion efficiency (η) from 8.31% to 19.7%, more than two times higher than the reference.

Uncovering the Role of Surface-Attached Ag Nanoparticles in Photodegradation Improvement of Rhodamine B by ZnO-Ag Nanorods

ZnO nanorods decorated with metal nanoparticles have sparked considerable interest in recent years thanks to their suitability for a wide range of applications, such as photocatalysis, photovoltaics, antibacterial activity, and sensing devices. In this study, we prepared and investigated the improved solar-light-assisted photocatalytic activity of ZnO nanorods (NRs) decorated with Ag nanoparticles (NPs) using a conventional rhodamine B (RB) dye as a model water pollutant. We showed that the presence of Ag NPs on the surface of ZnO NRs significantly increases the degradation rate of RB dye (~0.2432 min^-1) when compared to bare ZnO NRs (~0.0431 min^-1). The improved photocatalytic activity of ZnO-Ag was further experimentally tested using radical scavengers. The obtained results reveal that ˙OH and ˙O₂^- radicals are main active species involved in the RB dye photodegradation by ZnO-Ag NRs. It was concluded that efficient charge separation plays a major role in photocatalytic activity improvement.

Understanding the Enhancement Mechanism of ZnO Nanorod-based Piezoelectric Devices through Surface Engineering

ZnO is a typical piezoelectric semiconductor, and enhancing the piezoelectric output of ZnO-based devices is essential for their efficient applications. Surface engineering is an effective strategy to improve the piezoelectric output of ZnO-based devices, but its unclear regulation mechanism leads to a lack of reasonable guidance for device design. In this work, the regulation effect of the barrier layer in ZnO-based piezoelectric devices is systematically investigated from the carrier perspective through surface engineering, resulting in a significant improvement (nearly 10-fold) in the output performance of piezoelectric devices. The regulation mechanism of the ZnO-Cu₂O p-n heterojunction devices on piezoelectric output is revealed in terms of built-in electric field, depletion layer width, and junction capacitance. These findings facilitate further insight into the enhancement mechanism of the piezoelectric output of ZnO-based devices and provide reasonable ideas for efficient device design.

Formation of a ZnO nanorods-patterned coating with strong bactericidal capability and quantitative evaluation of the contribution of nanorods-derived puncture and ROS-derived killing

To endow Ti-based orthopedic implants with strong bactericidal activity, a ZnO nanorods-patterned coating (namely ZNR) was fabricated on Ti utilizing a catalyst- and template-free method of micro-arc oxidation (MAO) and hydrothermal treatment (HT). The coating comprises an outer layer of ZnO nanorods and a partially crystallized inner layer with nanocrystalline TiO₂ and Zn₂TiO₄ embedded amorphous matrix containing Ti, O and Zn. During HT, Zn²⁺ ions contained in amorphous matrix of the as-MAOed layer migrate to surface and react with OH⁻ in hydrothermal solution to form ZnO nuclei growing in length at expense of the migrated Zn²⁺. ZNR exhibits intense bactericidal activity against the adhered and planktonic S. aureus in vitro and in vivo. The crucial contributors to kill the adhered bacteria are ZnO nanorods derived mechano-penetration and released reactive oxygen species (ROS). Within 30 min of S. aureus incubation, ROS is the predominant bactericidal contributor with quantitative contribution value of ∼20%, which transforms into mechano-penetration with prolonging time to reach quantitative contribution value of ∼96% at 24 h. In addition, the bactericidal contributor against the planktonic bacteria of ZNR is relied on the released Zn²⁺. This work discloses an in-depth bactericidal mechanism of ZnO nanorods.

(1)

A templates and catalysts-free method is used to fabricate ZnO nanorods on Ti

(2)

ZnO nanorods-arrayed coating shows intense broad-spectrum bactericidal activity

(3)

Main bactericidal contributor of ZnO nanorods to adhered bacteria is mechano-puncture

(4)

Main bactericidal contributor of ZnO nanorods to planktonic bacteria is released Zn²⁺

Recyclable, Antibacterial, Isoporous Through-Hole Membrane Air Filters with Hydrothermally Grown ZnO Nanorods

Reusable, antibacterial, and photocatalytic isoporous through-hole air filtration membranes have been demonstrated based on hydrothermally grown ZnO nanorods (NRs). High-temperature (300~375 °C) stability of thermoset-based isoporous through-hole membranes has enabled concurrent control of porosity and seed formation via high-temperature annealing of the membranes. The following hydrothermal growth has led to densely populated ZnO NRs on both the membrane surface and pore sidewall. Thanks to the nanofibrous shape of the grown ZnO NRs on the pore sidewall, the membrane filters have shown a high (>97%) filtration efficiency for PM2.5 with a rather low-pressure (~80 Pa) drop. The membrane filters could easily be cleaned and reused many times by simple spray cleaning with a water/ethanol mixture solution. Further, the grown ZnO NRs have also endowed excellent bactericidal performance for both Gram-positive S. aureus and Gram-negative S. enteritidis bacteria. Owing to the wide bandgap semiconductor nature of ZnO NRs, organic decomposition by photocatalytic activity under UV illumination has been successfully demonstrated. The reusable, multifunctional membrane filters can find wide applications in air filtration and purification.

Structural and Electrical Characterization of Pure and Al-Doped ZnO Nanorods

Pure and Al-doped (3 at.%) ZnO nanorods were prepared by two-step synthesis. In the first step, ZnO thin films were deposited on silicon wafers by spin coating; then, ZnO nanorods (NR) and Al-doped ZnO NR were grown using a chemical bath method. The structural properties of zincite nanorods were determined by X-ray diffraction (XRD) and corroborated well with the morphologic properties obtained by field-emission gun scanning electron microscopy (FEG SEM) with energy-dispersive X-ray spectroscopy (EDS). Morphology results revealed a minute change in the nanorod geometry upon doping, which was also visible by Kelvin probe force microscopy (KPFM). KPFM also showed preliminary electrical properties. Detailed electrical characterization of pure and Al-doped ZnO NR was conducted by temperature-dependent current–voltage (I–V) measurements on Au/(Al)ZnO NR/n-Si junctions. It was shown that Al doping increases the conductivity of ZnO NR by an order of magnitude. The I–V characteristics of pure and Al-doped ZnO NR followed the ohmic regime for lower voltages, whereas, for the higher voltages, significant changes in electric conduction mechanisms were detected and ascribed to Al-doping. In conclusion, for future applications, one should consider the possible influence of the geometry change of (Al)ZnO NRs on their overall electric transport properties.

Solar-Driven Photoelectrochemical Performance of Novel ZnO/Ag2WO4/AgBr Nanorods-Based Photoelectrodes

Highly efficient photoelectrochemical (PEC) water oxidation under solar visible light is crucial for water splitting to produce hydrogen as a source of sustainable energy. Particularly, silver-based nanomaterials are important for PEC performance due to their surface plasmon resonance which can enhance the photoelectrochemical efficiency. However, the PEC of ZnO/Ag2WO4/AgBr with enhanced visible-light water oxidation has not been studied so far. Herein, we present a novel photoelectrodes based on ZnO/Ag2WO4/AgBr nanorods (NRs) for PEC application, which is prepared by the low-temperature chemical growth method and then by successive ionic layer adsorption and reaction (SILAR) method. The synthesized photoelectrodes were investigated by several characterization techniques, emphasizing a successful synthesis of the ZnO/Ag2WO4/AgBr heterostructure NRs with excellent photocatalysis performance compared to pure ZnO NRs photoelectrode. The significantly enhanced PEC was due to improved photogeneration and transportation of electrons in the heterojunction due to the synergistic effect of the heterostructure. This study is significant for basic understanding of the photocatalytic mechanism of the heterojunction which can prompt further development of novel efficient photoelectrochemical-catalytic materials.

Integration of ZnO nanorods with MOS capacitor for self-powered force sensors and nanogenerators

In this work, we present a novel force-sensing device with zinc oxide nanorods (ZnO NRs) integrated with a metal-oxide-semiconductor (MOS) capacitor and encapsulated with Kapton tape. The details of the fabrication process and working principle of the integrated ZnO NRs-MOS capacitor as a force sensor and nanogenerator have been discussed. The fabricated ZnO-MOS device is tested for both the open-circuit and resistor-connected mode. For an input force in the range of 1-32 N, the open-circuit output voltage of the device is measured to be in the range of 60-100 mV for different device configurations. In the resistor-connected mode, the maximum output power of 0.6 pW is obtained with a 1 MΩ external resistor and input force of 8 N. In addition, the influence of different seed layers (Ag and ZnO) and the patterning geometry of the ZnO nanorods on the output voltage of ZnO-MOS device have been investigated by experiments. An equivalent circuit model of the device has been developed to study the influence of the geometry of ZnO NRs and Kapton tape on the ZnO-MOS device voltage output. This study could be an example of integrating piezoelectric nanomaterials on traditional electronic devices and could inspire novel designs and fabrication methods for nanoscale self-powered force sensors and nanogenerators.

Formamidinium Lead Iodide Perovskite Films with Polyvinylpyrrolidone Additive for Active Layer in Perovskite Solar Cells, Enhanced Stability and Electrical Conductivity

In this paper, we studied the influence of polyvinylpyrrolidone (PVP) as a stabilization additive on optical and electrical properties of perovskite formamidinium lead iodide (FAPI) polycrystalline thin films on ZnO nanorods (ZNR). FAPI (as an active layer) was deposited from a single solution on ZNR (low temperature processed electron transport layer) using a one-step method with the inclusion of an anti-solvent. The role of PVP in the formation of the active layer was investigated by scanning electron microscopy and contact angle measurements to observe the effect on morphology, while X-ray diffraction was used as a method to study the stability of the film in an ambient environment. The effect of the PVP additive on the optical and electrical properties of the perovskite thin films was studied via photoluminescence, UV-Vis measurements, and electrical impedance spectroscopy. We have demonstrated that PVP inclusion in solution-processed perovskite FAPI thin films prevents the degradation of the film in an ambient atmosphere after aging for 2 months. The inclusion of the PVP also improves the infiltration of FAPI perovskite into ZnO nanostructures, increases electrical conductivity and radiative recombination of the photo-generated charge carriers. These results show promising information for promoting PVP stabilized FAPI perovskites for the new generation of photovoltaic devices.

Effects of Seed-Layer N2O Plasma Treatment on ZnO Nanorod Based Ultraviolet Photodetectors: Experimental Investigation with Two Different Device Structures

The crystalline quality of ZnO NR (nanorod) as a sensing material for visible blind ultraviolet PDs (photodetectors) critically depends on the SL (seed layer) material of properties, which is a key to high-quality nanocrystallite growth, more so than the synthesis method. In this study, we fabricated two different device structures of a gateless AlGaN/GaN HEMT (high electron mobility transistor) and a photoconductive PD structure with an IDE (interdigitated electrode) pattern implemented on a PET (polyethylene terephthalate) flexible substrate, and investigated the impact on device performance through the SL N₂O plasma treatment. In case of HEMT-based PD, the highest current on-off ratio (~7) and spectral responsivity R (~1.5 × 10⁵ A/W) were obtained from the treatment for 6 min, whereas the IDE pattern-based PD showed the best performance (on-off ratio = ~44, R = ~69 A/W) from the treatment for 3 min and above, during which a significant etch damage on PET substrates was produced. This improvement in device performance was due to the enhancement in NR crystalline quality as revealed by our X-ray diffraction, photoluminescence, and microanalysis.

Rapid synthesis of microwave-assisted zinc oxide nanorods on a paper-based analytical device for fluorometric detection of l-dopa

l-dopa is a catecholamine neurotransmitter used to treat Parkinson's disease. This paper presents a low-cost paper-based biosensor aimed at enhancing the convenience of monitoring l-dopa concentrations. ZnO nanorods (ZnO-NRs) were synthesized on papers in less than 90 min using a microwave-assisted hydrothermal method. The ZnO-NRs amplify green fluorescence signals to enhance the detection sensitivity of l-dopa, best measured at excitation/emission wavelengths of 475/537 nm. We systematically characterized the effect of reaction conditions on the corresponding fluorescence enhancements. The proposed ZnO NRs-paper biosensor presented a ∼3-fold increase in green fluorescence compared to unmodified papers. The linear range of detection for l-dopa was 25-2000 nM, with a limit of detection of 24 nM, which meets the clinical requirements for the monitoring of l-dopa in Parkinson's patients.

Ultraviolet Radiation Sensor Based on ZnO Nanorods/La3Ga5SiO14 Microbalance

The possibility of creating resonant ultraviolet (UV) sensors based on the structure of ZnO nanorods/La₃Ga₅SiO₁₄ microbalance (LCM) has been investigated. The principle of sensor operation is based on the desorption of oxygen from the surface of ZnO nanorods upon irradiation with UV light and an increase in the concentration of charge carriers that leads to an increase in the capacitance of the structure of ZnO nanorods/LCM. It has been shown that UV radiation intensity affects the resonance oscillation frequency of the LCM sensor. After the end of irradiation, the reverse process of oxygen adsorption on the surface of ZnO nanorods occurs, and the resonance frequency of the sensor oscillations returns to the initial value.

Simultaneous and sensitive detection of multiple small biological molecules by microfluidic paper-based analytical device integrated with zinc oxide nanorods

In this work, ZnO nanorods (ZnO NRs) with different sizes were hydrothermally grown on the surface of Whatman filter paper for the fabrication of a microfluidic paper-based device (μPAD) for the simultaneous detection of glucose and uric acid. As dual enzymatic reaction was employed for the colorimetric detection in this μPAD, the presence of ZnO NRs promoted the enzyme immobilization thus significantly enhancing the colorimetric signal. The coffee ring effect was effectively conquered by the uniform distribution of ZnO NR as well as a specialized double-layered μPAD design. Meanwhile, two color indicators with distinct colors were used to provide complementary results to better quantify the concentration of the analytes by naked eye. As a result, two linear calibration curves were obtained for the detection of glucose (0.01-10 mmol L^-1) and uric acid (0.01-5 mmol L^-1), along with a LOD of 3 μmol L^-1 for glucose and 4 μmol L^-1 for uric acid, respectively. The practical usefulness of the proposed μPAD was further validated by the simultaneous analysis of glucose and uric acid in serum samples and urine samples.

Structure Dependent Product Selectivity for CO2 Electroreduction on ZnO Derived Catalysts

Electrochemical conversion of CO2 is an attractive alternative to releasing it to the atmosphere. Catalysts derived from electroreduction of metal oxides are often more active than when starting with metallic phase catalyst. The origin of this effect is not yet clear. Using ZnO nanorods, we show that the initial structure of the oxide as well as the electrolyte medium have a profound impact on the structure of the catalytic active Zn phase, and thereby the selectivity of the catalysts. ZnO nanorods with various aspect ratios were electrochemically reduced in different electrolytes leading to metallic Zn with different structures; a sponge-like structure, nanorods and nanoplates. The sponge-like Zn produced syngas with H2 : CO=2, and some formate, the nanorods produced only syngas with H2 : CO=1, while Zn nanoplates exhibited 85 % selectivity towards CO. These results open a pathway to design new electrocatalysts with optimized properties by modifying the structure of the starting material and the electroreduction medium.

Fabrication and Characterization of High-Quality UV Photodetectors Based ZnO Nanorods Using Traditional and Modified Chemical Bath Deposition Methods

Ultraviolet (UV) photodetectors (PDs) based on high-quality well-aligned ZnO nanorods (NRs) were fabricated using both modified and conventional chemical bath deposition (CBD) methods. The modified chemical bath deposition (M-CBD) method was made by adding air bubbles to the growth solution during the CBD process. The viability and effectiveness of M-CBD were examined by developing UV PDs based on ZnO NRs. The ZnO nano-seed layer was coated on a glass substrate utilizing radiofrequency (RF) sputtering. The impact of the different growth-times on morphology, growth rate, crystal structure, and optical and chemical properties were investigated systematically using different characterization techniques, such as field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, UV–VIS double beam spectrometer, and energy dispersive X-ray analysis (EDX), respectively. The Al/ZnO UV PDs based on ZnO nanorods were fabricated with optimum growth conditions through the two methods of preparation. This study showed that the synthesized ZnO NRs using the M-CBD method for different growth times possess better properties than the conventional method under similar deposition conditions. Despite having the highest aspect ratio and growth rate of ZnO NRs, which were found at 4 h growth duration for both methods, the aspect ratio of ZnO NRs using the M-CBD technique was comparatively higher than the conventional CBD method. Besides, the UV PDs fabricated by the M-CBD method at 5 V bias voltage showed high sensitivity, short response time, quick recovery time, high gain, low dark current, and high photocurrent compared with the UV PD device fabricated by the conventional CBD method.

Green synthesis of stable antioxidant, anticancer and photocatalytic activity of zinc oxide nanorods from Leea asiatica leaf

The present work reveals zinc oxide nanorods (ZnO NRs) obtained via green synthesis approach utilizing phytochemicals present in the Leea asiatica plant. The synthesized ZnO NRs was confirmed by various characterization methods like X-ray diffraction, FTIR, SEM-EDX, UV-vis, TEM and Raman spectra analysis. The obtained nanorods show less particle size with a distinctive high surface area. The photodegradation activity of obtained ZnO NRs shows 81.3 % of dye degradation at high concentration whereas 42.9 % in low concentration and also antioxidant test shows a better result for free radical scavenging. To understand the safety of the ZnO NRs zebrafish embryos was utilized as a model and found to be less toxic after assessment. Furthermore, to understand the anticancer activity of ZnO NRs it was assessed against breast cancer cell line through MTT, XTT, NRU, and LDH assay. The ZnO NRs exposed against breast cancer cell line (MCF-7) had resulted in necrosis and apoptosis which was further confirmed by genotoxicity assay. Also, the molecular mechanisms behind the necrosis and apoptosis are also discussed to show the involvement of ZnO NRs towards breast cancer cells (MCF-7). The present study on ZnO NRs gives a wide spectrum understanding of using ZnO NRs as a potential drug for human breast cancer by showing its physiological pathways as well as a novel way to synthesize ZnO NRs using Leea asiatica plant extract.

Antibacterial, Antiviral, and Self-Cleaning Mats with Sensing Capabilities Based on Electrospun Nanofibers Decorated with ZnO Nanorods and Ag Nanoparticles for Protective Clothing Applications

The COVID-19 pandemic has clearly shown the importance of developments in fabrication of advanced protective equipment. This study investigates the potential of using multifunctional electrospun poly(methyl methacrylate) (PMMA) nanofibers decorated with ZnO nanorods and Ag nanoparticles (PMMA/ZnO-Ag NFs) in protective mats. Herein, the PMMA/ZnO-Ag NFs with an average diameter of 450 nm were simply prepared on a nonwoven fabric by directly electrospinning from solutions containing PMMA, ZnO nanorods, and Ag nanoparticles. The novel material showed high performance with four functionalities (i) antibacterial agent for killing of Gram-negative and Gram-positive bacteria, (ii) antiviral agent for inhibition of corona and influenza viruses, (iii) photocatalyst for degradation of organic pollutants, enabling a self-cleaning protective mat, and (iv) reusable surface-enhanced Raman scattering substrate for quantitative analysis of trace pollutants on the nanofiber. This multi-functional material has high potential for use in protective clothing applications by providing passive and active protection pathways together with sensing capabilities.

Surface Modification of Basalt Fibres with ZnO Nanorods and Its Effect on Thermal and Mechanical Properties of PLA-Based Composites

The composites based on basalt fibres and poly(lactic acid) (PLA) show promising applications in biomedical and automotive fields, but their mechanical performance is still largely hindered by poor interfacial properties. Zinc oxide nanorods have been successfully used to tune the PLA/basalt fibre interface by growing them on commercially available basalt fabrics. The hierarchical fibres significantly enhanced the mechanical properties of PLA-based composites, especially their flexural strength and stiffness. These values are 26% and 22% higher than those of unmodified basalt/PLA composites, and 24% and 34% higher than those of glass/PLA composites used as a baseline. The increase in tensile and flexural properties hinges on the mechanical interlocking action promoted by ZnO nanorods and on the creation of a compact transcrystallinity structure. A degradation of PLA matrix was detected but it was positively counteracted by the better interfacial stress transfer. This study offers a novel approach for modifying the fibre–matrix interface of biocomposites intended for high-performance applications.

Decoration of Zinc Oxide Nanorods into the Surface of Activated Carbon Obtained from Agricultural Waste for Effective Removal of Methylene Blue Dye

Zinc oxide (ZnO) nanorods incorporated activated carbon (AC) composite photocatalyst was synthesized using a hydrothermal process. The AC was prepared from lapsi (Choerospondias axillaris) seed stone, an agricultural waste product, found in Nepal by the chemical activation method. An aqueous suspension of AC with ZnO precursor was subjected to the hydrothermal treatment at 140 °C for 2 h to decorate ZnO rods into the surface of AC. As-obtained ZnO nanorods decorated activated carbon (ZnO/AC) photocatalyst was characterized by various techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. Results showed that highly crystalline hexagonal ZnO nanorods were effectively grown on the surface of porous AC. The photocatalytic property of the as-prepared ZnO/AC composite was studied by degrading methylene blue (MB) dye under UV-light irradiation. The ZnO/AC composite showed better photocatalytic property than that of the pristine ZnO nanorods. The enhanced photocatalytic performance in the case of the ZnO/AC composite is attributed to the combined effects of ZnO nanorods and AC.

Enhanced Energy Harvesting Ability of ZnO/PAN Hybrid Piezoelectric Nanogenerators

Miniaturization of energy conversion and storage devices has attracted remarkable consideration in the application of wearable electronics. Compared with film-based flexible electronics, fiber-based wearable electronics (e.g., nanogenerators and sensors made from electrospun nanofibers) are more appealing and promising for wearables. However, there are two bottlenecks, a low power output and poor sensing capability, limiting the application of piezoelectric nanofibers. Herein, we integrated zinc oxide nanorods (ZnO NRs) to a less known piezoelectric polymer, polyacrylonitrile (PAN) nanofiber, forming a ZnO/PAN nanofabric, which significantly improved the pressure sensitivity and vibrational energy harvesting ability by about 2.7 times compared with those of the pristine PAN nanofiber, and the maximum output power density of ∼10.8 mW·m^-2 is achieved. Noteworthily, the ZnO/PAN nanofabric showed a power output about twice of the one made of ZnO and polyvinylidene fluoride. It was revealed that the integration of ZnO NRs clearly improved the planar zigzag conformation in microstructures of the PAN nanofiber. Further, successful demonstrations of a mechanically robust pressure sensor and wearable power source confirm the potential applications in human activity monitoring and personal thermal management, respectively.

Synthesis of ZnO Nanorod Film Deposited by Spraying with Application for Flexible Piezoelectric Energy Harvesting Microdevices

Industry 4.0 and the Internet of Things have significantly increased the use of sensors and electronic products based on flexible substrates, which require electrical energy for their performance. This electrical energy can be supplied by piezoelectric vibrational energy harvesting (pVEH) devices. These devices can convert energy from ambient mechanical excitations into electrical energy. In order to develop, these devices require piezoelectric films fabricated with a simple and low-cost process. In this work, we synthesize ZnO nanorod film by a solvothermal method and deposit by spraying on ITO (indium-tin-oxide)/PET (polyethylene terephthalate) flexible substrate for a pVEH microdevice. The results of the characterization of the ZnO nanorod film using X-ray diffraction (XRD) confirm the typical reflections for this type of nanomaterial (JCPDS 36-145). Based on transmission electron microscopy (TEM) images, the size of the nanorod film is close to 1380 nm, and the average diameter is 221 ± 67 nm. In addition, the morphological characteristics of the ZnO nanorod film are obtained using atomic force microscopy (AFM) tapping images. The pVEH microdevice has a resonant frequency of 37 Hz, a generated voltage and electrical power of 9.12 V and 6.67 μW, respectively, considering a load resistance of 107.7 kΩ and acceleration of 1.5 g. The ZnO nanorod film may be applied to pVEH microdevices with flexible substrates using a low-cost and easy fabrication process.

SERS Effect on Spin-Coated Seeding of Tilted Au-ZnO Nanorods for Low-Cost Diagnosis

Uniformly parallel Au-coated ZnO nanorods have previously been shown to amplify local Raman signals, providing increased sensitivity to disease markers in the detection of inflammation and cancer. However, practical and cost-effective fabrication methods of substrates for surface-enhanced Raman spectroscopy (SERS) fail to produce highly uniform surfaces. Here, the feasibility of Raman enhancement on less-uniform substrates is assessed. ZnO nanorod structures were fabricated by hydrothermal synthesis, starting from spin-coated seed substrates. Following analysis, the nanostructures were coated with Au to create stochastically variant substrates. The non-uniformity of the fabricated Au-coated ZnO nanorod structures is confirmed morphologically by FE-SEM and structurally by X-ray diffraction, and characterized by the angular distributions of the nanorods. Monte Carlo finite element method simulations matching the measured angular distributions and separations predicted only moderate increases in the overall Raman enhancement with increasing uniformity. Highly variant substrates exhibited approximately 76% of the Raman enhancement of more uniform substrates in simulations and experiments. The findings suggest that, although highly inhomogeneous Au-coated ZnO nanorod substrates may not attain the same Raman enhancement as more uniform substrates, the relaxation of fabrication tolerances may be economically viable.

Selective Detection of Nano-Sized Diagnostic Markers Using Au-ZnO Nanorod-Based Surface-Enhanced Raman Spectroscopy (SERS) in Ureteral Obstruction Models

Background

This study investigated the diagnosis of renal diseases using a biochip capable of detecting nano-sized biomarkers. Raman measurements from a kidney injury model were taken, and the feasibility of early diagnosis was assessed.

Materials and Methods

Rat models with mild and severe unilateral ureteral obstructions were created, with the injury to the kidney varying according to the tightness of the stricture. After generating the animal ureteral obstruction models, urine was collected from the kidney and bladder.

Results and Discussion

After confirming the presence of renal injury, urine drops were placed onto a Raman chip whose surface had been enhanced with Au-ZnO nanorods, allowing nano-sized biomarkers that diffused into the nanogaps to be selectively amplified. The Raman signals varied according to the severity of the renal damage, and these differences were statistically confirmed.

Conclusion

These results confirm that ureteral stricture causes kidney injury and that signals in the urine from the release of nano-biomarkers can be monitored using surface-enhanced Raman spectroscopy.

Graphene Quantum Dot-Sensitized ZnO-Nanorod/GaN-Nanotower Heterostructure-Based High-Performance UV Photodetectors

The fabrication of a superior-performance ultraviolet (UV) photodetector utilizing graphene quantum dots (GQDs) as a sensitization agent on a ZnO-nanorod/GaN-nanotower heterostructure has been realized. GQD sensitization displays substantial impact on the electrical as well as the optical performance of a heterojunction UV photodetector. The GQD sensitization stimulates charge carriers in both ZnO and GaN and allows energy band alignment, which is realized by a spontaneous time-correlated transient response. The fabricated device demonstrates an excellent responsivity of 3.2 × 10³ A/W at -6 V and displays an enhancement of ∼265% compared to its bare counterpart. In addition, the fabricated heterostructure UV photodetector exhibits a very high external quantum efficiency of 1.2 × 10⁶%, better switching speed, and signal detection capability as low as ∼50 fW.

New Insights towards High-Temperature Ethanol-Sensing Mechanism of ZnO-Based Chemiresistors

In this work, we investigate ethanol (EtOH)-sensing mechanisms of a ZnO nanorod (NRs)-based chemiresistor using a near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). First, the ZnO NRs-based sensor was constructed, showing good performance on interaction with 100 ppm of EtOH in the ambient air at 327 °C. Then, the same ZnO NRs film was investigated by NAP-XPS in the presence of 1 mbar oxygen, simulating the ambient air atmosphere and O₂/EtOH mixture at the same temperature. The partial pressure of EtOH was 0.1 mbar, which corresponded to the partial pressure of 100 ppm of analytes in the ambient air. To better understand the EtOH-sensing mechanism, the NAP-XPS spectra were also studied on exposure to O₂/EtOH/H₂O and O₂/MeCHO (MeCHO = acetaldehyde) mixtures. Our results revealed that the reaction of EtOH with chemisorbed oxygen on the surface of ZnO NRs follows the acetaldehyde pathway. It was also demonstrated that, during the sensing process, the surface becomes contaminated by different products of MeCHO decomposition, which decreases dc-sensor performance. However, the ac performance does not seem to be affected by this phenomenon.

Au nanoparticle-decorated ZnO nanorods as fluorescent non-enzymatic glucose probe

ZnO nanorods (NRs) synthesized by a hydrothermal method and decorated with Au nanoparticles (NPs) were used for fluorescent non-enzymatic glucose detection. The detection is based on the photoluminescence (PL) quenching of ZnO NRs/Au NPs (at 382 nm under 325 nm excitation) exposed to glucose. The sensor exhibits a high sensitivity of (22 ± 2) % mM^-1 (defined as percentage change of the PL peak intensity per mM) and a limit of detection (LOD) as low as 0.01 mM, along with an excellent selectivity and a short response time (less than 5 s). In comparison with a fluorescent non-enzymatic ZnO nanostructure-based glucose sensor, the addition of Au NPs significantly enhances the sensitivity. This is attributed to the surface plasmon resonance, which increases not only the photoluminescence intensity but also the photo-oxidation property of the ZnO NRs. Thus, ZnO NRs/Au NPs can act as an efficient photocatalyst for glucose detection. Most importantly, the probe is applicable to glucose detection in human blood serum. The outstanding performance of the material and its cost-effectiveness allow for potential application in single-use, noninvasive glucose devices.Graphical abstract A sensitive non-enzymatic fluorescent glucose probe-based ZnO nanorod decorated with Au nanoparticles.

Modeling and Piezoelectric Analysis of Nano Energy Harvesters

The expedient way for the development of microelectromechanical systems (MEMS) based devices are based on two key steps. First, perform the simulation for the optimization of various parameters by using different simulation tools that lead to cost reduction. Second, develop the devices with accurate fabrication steps using optimized parameters. Here, authors have performed a piezoelectric analysis of an array of zinc oxide (ZnO) nanostructures that have been created on both sides of aluminum sheets. Various quantities like swerve, stress, strain, electric flux, energy distribution, and electric potential have been studied during the piezo analysis. Then actual controlled growth of ZnO nanorods (NRs) arrays was done on both sides of the etched aluminum rod at low-temperature using the chemical bath deposition (CBD) method for the development of a MEMS energy harvester. Micro creaks on the substrate acted as an alternative to the seed layer. The testing was performed by applying ambient range force on the nanostructure. It was found that the voltage range on topside was 0.59 to 0.62 mV, and the bottom side was 0.52 to 0.55 mV. These kinds of devices are useful in low power micro-devices, nanoelectromechanical systems, and smart wearable systems.

Flower-Like ZnO Nanorods Synthesized by Microwave-Assisted One-Pot Method for Detecting Reducing Gases: Structural Properties and Sensing Reversibility

In this work, flower-like ZnO nanorods (NRs) were successfully prepared using microwave-assisted techniques at a low temperature. The synthesized NRs exhibited a smooth surface and good crystal structure phase of ZnO. The sharp peak of the XRD and Raman spectrum confirmed the high crystallinity of these ZnO NRs with a pure wurtzite structure. The nanorods were ~2 μm in length and ~150 nm in diameter, respectively. The electron diffraction pattern confirmed that the single crystal ZnO nanorods aligned along the [001] plane. The NRs were applied to fabricate a gas sensor for reducing gases such as CH4, CO, and H2. The sensor showed a good performance and sensitivity toward the target gases. However, its response toward CH4 and CO was higher compared to H2 gas. Although the operating temperature was varied from room temperature (RT) up to 350°C, the sensor did not show a response toward any of the target gases in the range of RT-150°C, but dramatic enhancement of the sensor response was observed at 200°C, and up to higher temperatures. This behavior was ascribed to the activity of the smooth surface and the reactivity of surface oxygen species with the targeted gases. The sensor response was measured at various gas concentrations, where the calibration curve was shown. The gas sensing mechanism was described in terms of the reaction of the gases with the transformed oxygen species on the surface of the oxides.

Ultraviolet Photodetection Based on High-Performance Co-Plus-Ni Doped ZnO Nanorods Grown by Hydrothermal Method on Transparent Plastic Substrate

A growth scheme at a low processing temperature for high crystalline-quality of ZnO nanostructures can be a prime stepping stone for the future of various optoelectronic devices manufactured on transparent plastic substrates. In this study, ZnO nanorods (NRs) grown by the hydrothermal method at 150 °C through doping of transition metals (TMs), such as Co, Ni, or Co-plus-Ni, on polyethylene terephthalate substrates were investigated by various surface analysis methods. The TM dopants in ZnO NRs suppressed the density of various native defect-states as revealed by our photoluminescence and X-ray photoelectron spectroscopy analysis. Further investigation also showed the doping into ZnO NRs brought about a clear improvement in carrier mobility from 0.81 to 3.95 cm2/V-s as well as significant recovery in stoichiometric contents of oxygen. Ultra-violet photodetectors fabricated with Co-plus-Ni codoped NRs grown on an interdigitated electrode structure exhibited a high spectral response of ~137 A/W, on/off current ratio of ~135, and an improvement in transient response speed with rise-up and fall-down times of ~2.2 and ~3.1 s, respectively.

Seedless Hydrothermal Growth of ZnO Nanorods as a Promising Route for Flexible Tactile Sensors

Hydrothermal growth of ZnO nanorods has been widely used for the development of tactile sensors, with the aid of ZnO seed layers, favoring the growth of dense and vertically aligned nanorods. However, seed layers represent an additional fabrication step in the sensor design. In this study, a seedless hydrothermal growth of ZnO nanorods was carried out on Au-coated Si and polyimide substrates. The effects of both the Au morphology and the growth temperature on the characteristics of the nanorods were investigated, finding that smaller Au grains produced tilted rods, while larger grains provided vertical rods. Highly dense and high-aspect-ratio nanorods with hexagonal prismatic shape were obtained at 75 °C and 85 °C, while pyramid-like rods were grown when the temperature was set to 95 °C. Finite-element simulations demonstrated that prismatic rods produce higher voltage responses than the pyramid-shaped ones. A tactile sensor, with an active area of 1 cm², was fabricated on flexible polyimide substrate and embedding the nanorods forest in a polydimethylsiloxane matrix as a separation layer between the bottom and the top Au electrodes. The prototype showed clear responses upon applied loads of 2-4 N and vibrations over frequencies in the range of 20-800 Hz.

Observation of CO Detection Using Aluminum-Doped ZnO Nanorods on Microcantilever

An oscillating piezoresistive microcantilever (MC) coated with an aluminum (Al)-doped zinc oxide (ZnO) nanorods was used to detect carbon monoxide (CO) in air at room temperature. Al-doped ZnO nanorods were grown on the MC surface using the hydrothermal method, and a response to CO gas was observed by measuring a resonant frequency shift of vibrated MC. CO gas response showed a significant increase in resonant frequency, where sensitivity in the order of picogram amounts was obtained. An increase in resonant frequency was also observed with increasing gas flow rate, which was simultaneously followed by a decrease in relative humidity, indicating that the molecular interface between ZnO and H₂O plays a key role in CO absorption. The detection of other gases of carbon compounds such as CO₂ and CH₄ was also performed; the sensitivity of CO was found to be higher than those gases. The results demonstrate the reversibility and reproducibility of the proposed technique, opening up future developments of highly sensitive CO-gas detectors with a fast response and room temperature operation.