ZnO Nanosheets Research Articles

Enhancing Triboelectric Nanogenerator Performance with Metal–Organic-Framework-Modified ZnO Nanosheets for Self-Powered Electronic Devices and Energy Harvesting

Enhancing Triboelectric Nanogenerator Performance with Metal–Organic-Framework-Modified ZnO Nanosheets for Self-Powered Electronic Devices and Energy Harvesting

Nanosheet Zinc Oxide Synthesized by Solution-Immersion Method for Triboelectric Nanogenerator

Most global problems are being solved by using sustainable energy harvesting technologies to retain the social ecosystem in great condition. The triboelectric nanogenerator (TENG) which is a renewable energy harvesting device, collects the waste mechanical energy from its surroundings and performs the electric signal conversion. TENG have garnered increased attention in recent years by offering prospective use in energy harvesting technology. Particularly, there is a need for flexible energy conversion that serves as a power supply for portable electronic equipment. In this study, ZnO nanosheet thin film prepared on the flexible conductive aluminium foil through a low temperature immersion technique was used to generate electrical energy. The effect of heat treatment on ZnO nanosheet thin film was also investigated on the surface morphology, strutural properties and nanogerator performance. The high density of interconnected ZnO nanosheet were observed before and after heat treatment as confirmed by FESEM studies. The analysis using XRD confirmed that ZnO nanosheet thin film was successfully deposited on the aluminium foil. Additionally, the ZnO nanosheet thin films improved significantly with heat treatment, enhancing their crystalline quality. The triboelectric nanogenerator (TENG) was successfully constructed in contact and separation mode using Kapton tape on top and ZnO nanosheet thin film on the bottom to generate electricity by a force of hand pressing. The output electrical voltage of the device doubled from around 2 V to 4 V after underwent the heat treatment. This study provides an essential insight into fabrication of TENG using the ZnO nanosheet thin film through a clean and effective method for nanogenerator applications.

One-Step Dry Coating of Hybrid ZnO-WO3 Nanosheet Photoanodes for Photoelectrochemical Water Splitting with Composition-Dependent Performance.

In this study, the potential of zinc oxide (ZnO), tungsten oxide (WO3), and their composites (ZnO-WO3) as photoanodes for photoelectrochemical (PEC) water splitting was investigated. ZnO-WO3 nanocomposites (NCs) were deposited on fluorine-doped tin oxide substrates at room temperature using a one-step dry coating process, the nanoparticle deposition system, with no post-processes. Different compositions of ZnO-WO3 NCs were optimized to enhance the kinetics of the PEC water-splitting reaction. Surface morphology analysis revealed the transformation of microsized particle nanosheets (NS) powder into nanosized particle nanosheets (NS) across all photoanodes. The optical characteristics of ZnO-WO3 photoanodes were scrutinized using diffuse reflectance and photoluminescence emission spectroscopy. Of all the hybrid photoanodes tested, the photoanode containing 10 wt.% WO3 exhibited the lowest bandgap of 3.20 eV and the lowest emission intensity, indicating an enhanced separation of photogenerated carriers and solar energy capture. The photoelectrochemical results showed a 10% increase in the photocurrent with increasing WO3 content in ZnO-WO3 NCs, which is attributed to improved charge transfer kinetics and carrier segregation. The maximum photocurrent for a NC, i.e., 10 wt.% WO3, was recorded at 0.133 mA·cm-2 at 1.23V vs. a reversible hydrogen electrode (RHE). The observed improvement in photocurrent was nearly 22 times higher than pure WO3 nanosheets and 7.3 times more than that of pure ZnO nanosheets, indicating the composition-dependence of PEC performance, where the synergy requirement strongly relies on utilizing the optimal ZnO-WO3 ratio in the hybrid NCs.

Triboelectric Nanogenerators Based on Nanostructured Layers of Zinc Oxide Deposited on Carbon Fabric

In this work, to obtain textile triboelectric layers for wearable flexible triboelectric nanogenerators (TENGs), we used two modes of growing nanostructured zinc oxide (ZnO) arrays on a carbon fabric (CF) using the automatic Successive Ionic Layer Adsorption and Reaction (SILAR) method. To produce a CF/ZnO_nr triboelectric textile with an array of intergrown short ZnO nanorods, we used a pre-coating of carbon fibers with ZnO seed layers. When the ZnO layer was fabricated by automatic SILAR on bare carbon fabric, we obtained the CF/ZnO_ns textile with an array of interconnected ZnO nanosheets 50–100 nm thick. As a proof of concept, we developed and tested two prototypes of flexible vertical contact–separation mode CF/ZnO_nr/PET/ITO and CF/ZnO_ns/PET/ITO TENGs, in which a gap was involuntarily formed between the smooth PET layer and the woven carbon textile coated with nanostructured ZnO films. In pressing tests with a force of ~5 N (pressure ~33 kPa), the CF/ZnO_ns/PET/ITO TENG created a higher open-circuit voltage up to 30 V and a higher maximum surface charge density of 1.3 μC/m2. In the successive press–release tests, this TENG showed an output voltage of 3.6 V, a current density of 1.47 μA/cm2, and a power density of 1.8 µW/cm2, confirming its effectiveness.

ZnNiCo layered double hydroxide@Mxene with 2D/2D hierarchical structure derived from micro-sized ZIF-8/67 anchored to Mxene for asymmetric supercapacitors

Compared with other power storage technologies such as batteries, supercapacitors can attain higher power density, but the situation of lower energy density requires improvement. In this study, in order to maintain the stability of the template structure, zinc ions in the template of ZnO nanosheets were partially dissolved and combined with 2-methylimidazole (2-MI) to form ZnO@ZIF-8 on the residual ZnO by controlling the addition amount of organic ligands. Then the remaining ZnO was continued to be dissolved with 2-MI, and the released zinc ions and the co-added cobalt ions were further integrated with the organic complex to form ZIF-8/67, which can be anchored to two-dimensional monolayer Mxene. Finally, ZIF-8/67 can be etched with nickel nitrate and cobalt nitrate to obtain ZnNiCo LDH@Mxene (ZNC@M) with 2D/2D hierarchical structure. The specific capacitance of ZNC@M can achieve 2011F g−1 (1 A g−1). Meanwhile, it has excellent rate performance (10 A g−1 maintained at 68.6 %) and cyclic stability (maintained at 92.8 % after 5000 cycles). Potential window and energy density of the asymmetric supercapacitor (ASC) were obviously improved while composites are applied as cathode and activated carbon (AC) as anode, and KOH solutions with potassium ferricyanide as electrolyte. When the power density of the assembled device is 800–8000 W kg−1, the energy density is maintained from 65.6 Wh kg−1 to 37.3 Wh kg−1. In addition, the density of states (DOS), project density of states (PDOS), and energy band changes before and after the addition of Mxene were discussed by using density functional theory (DFT) calculation. The above conclusions confirm that Mxene is beneficial to promote the electron transfer state of ZnNiCo LDH. Therefore, this work demonstrates that the anchoring of micro-sized materials is a powerful approach for the preparation of two-dimensional hierarchical structures.

Preparation of ZnO Nanosheet Array and Research on ZnO/PANI/ZnO Ultraviolet Photodetector.

High-performance ultraviolet photodetectors have important scientific research significance and practical application value, which has been the focus of researchers. In this work, we have constructed a highly photosensitive UV photodetector with a unique "sandwich" structure, which was mainly composed of two layers of ZnO nanosheet arrays and one layer of polyaniline (PANI). The results showed that the UV current of ZnO/PANI devices was 100 times higher than that of pure ZnO devices under the same UV irradiation time. At a 365 nm wavelength, the device had excellent photocurrent responsiveness and photoconductivity. This high performance was attributed to the large specific surface area of ZnO nanosheets and the p-n junction formed between P-type PANI nano-porous film and N-type ZnO nanosheets. This provides a solid theoretical basis for the application of ZnO nanosheets in ultraviolet detection, and possesses significance for the development of ultraviolet photodetectors.

Acetone detection at low temperature using a gas nano-sensor based on ZnO flakes loaded with metal organic-skeleton ZIF-8

Timely detection of gases that are toxic, flammable, polluting, or otherwise harmful is essential for human health. Unfortunately, gas sensors based on a single metal oxide suffer from low sensitivity and poor selectivity, which is suitable neither for practical applications nor industrial development. Therefore, there is an urgent need to develop a gas sensor with excellent sensitivity and good selectivity. In the study reported here, heterostructured ZIF-8/ZnO nanosheets were prepared as gas-sensitive materials by a simple hydrothermal method, and the self-stacking of ZnO nanosheets was lessened by the loading effect of ZIF-8 for improved gas sensitivity when detecting acetone. The results show that the sensitivity of the ZIF-8/ZnO sheet structure reaches 78.6 for 50 ppm acetone at 100 °C, which is nearly four times that of pure ZnO sheets. Meanwhile, with short response and recovery times (9 s and 4 s, respectively) and excellent selectivity and stability, the ZIF-8/ZnO sheet structure is suitable for practical applications. This enhancement effect is attributed to the supporting filtering effect of ZIF-8, which is carried in between the ZnO nanosheets and acts as an electron transport channel, reducing the accumulation of ZnO nanosheets and accelerating the electron transport rate. This provides a new idea for the future detection of acetone gas.

Observation of Superfluorescence of ZnO Nanosheets

ZnO nanostructures are attractive candidates as source media for realizing optoelectronic devices. Based on the comparison of cathodoluminescence images and scanning electron microscope morphology images, the mixing color images of ZnO nanosheets indicate that the emission of the ZnO nanosheet is emitted from its flat surface. In time‐resolved photoluminescence, a sharp emission pulse is observed after the delay time τD of 56 ps relative to zero time at the threshold excitation power, the pulse peak intensity exhibits a superlinear increase proportional to P2.7 (P is the excitation power density), the reduction of τD is proportional to , oscillation is demonstrated in the decay trace at high excitation power, and the time interval of two oscillation emissions is 36 ps. Experimental results indicate that superfluorescence (SF) is demonstrated at room temperature and it is assigned to coherent dipole states in the population version localized to approximately 3 nm thick ZnO nanosheet. SF of coherent dipole states is characterized by the delay time τD—the unique feature of SF and the Burnham–Chiao ringing behavior. Experimental observation of SF of ZnO nanosheets may have potential value for the investigation of room temperature many‐body quantum phenomena, coherent control, and applications for high‐performance optoelectronic devices.

Enhanced photocatalytic and antimicrobial properties of undoped and aluminum-doped zinc oxide nanosheets synthesized via novel laser-assisted chemical bath technique

Enhanced photocatalytic and antimicrobial properties of undoped and aluminum-doped zinc oxide nanosheets synthesized via novel laser-assisted chemical bath technique

Strong Interface Interaction of ZnO Nanosheets and MnSx Nanoparticles Triggered by Light over Wide Ranges of Wavelength to Enhance Their Removal of VOCs

The characteristics of the surface and interface of nanocomposites are important for exerting multi-functional properties and widening interdisciplinary applications. These properties are mainly depending on the electronic structures of materials. Some key factors, such as the surface, interface, grain boundaries, and defects take vital roles in the contribution of desired properties. Due to the excellent sensitivity of the QCM (quartz crystal microbalance) device, the surface and interface features of the nanocomposite were studied with the aid of the gas-response of the sensors (Sensor’s Gas-Sensitivity) in this work. To make full use of the visible light and part of NIR, a ZnO/MnSx nanocomposite was constructed using hydrothermal synthesis for narrowing the bandgap width of wide bandgap materials. The results indicated that the absorbance of the resulting nanocomposite was extended to part of the NIR range due to the introduction of impurity level or defect level, although ZnO and MnS belonged to wide bandgap semiconductor materials. To explore the physical mechanism of light activities, the photoconductive responses to weak visible light (650 nm, etc.) and NIR (near-infrared) (808 nm, 980 nm, and 1064 nm, etc.) were studied based on interdigital electrodes of Au on flexible PET (polyethylene terephthalate) film substrate with the casting method. The results showed that the on/off ratio of ZnO/MnSx nanocomposite to weak visible light and part of NIR light were changed by about one to five orders of magnitude, with changes varying with the amount of MnSx nanoparticle loading due to defect-assisted photoconductive behavior. It illustrated that the ZnO/MnSx nanocomposite easily produced photo-induced free charges, effectively avoiding the recombination of electrons/holes because of the formation of strong built-in electrical fields. To examine the surface and interface properties of nanocomposites, chemical prototype sensor arrays were constructed based on ZnO, ZnO/MnSx nanocomposite, and QCM arrays. The adsorption response behaviors of the sensor arrays to some typical volatile compounds were examined under a similar micro-environment. The results exhibited that in comparison to ZnO nanosheets, the ZnO nanosheets/MnSx nanocomposite increased adsorption properties to some typical organic volatile compounds significantly. It would have good potential applications in photo-catalysts, self-cleaning films, multi-functional coatings, and organic pollutants treatment (VOCs) of environmental fields for sustainable development. It provided some reference value to explore the physical mechanism of materials physics and photophysics for photo-active functional nanocomposites.

Interfacial Modification of a Zn Anode by Amorphous Se toward Long-Life and Hydrogen Evolution-Free Aqueous Rechargeable Batteries.

The Zn anode in rechargeable aqueous Zn-ion batteries suffers from hydrogen gas evolution and dendrite growth, which are critical issues that make the battery impractical. Here, the Zn anode performance is greatly improved by coating an amorphous selenium overlayer with a simple chemical bath reaction process. The reduction of SeO32- ions by the Zn metal leads to the formation of an amorphous Se layer, and the optimal reaction time that determines the thickness of the Se coating as well as the Zn anode performance is found to be 2 h. The symmetric cell using Zn@Se exhibits improved rate performance and an ultralong cycle life of 4500 h after being tested at 1 mA cm-2 and 1 mAh cm-2, respectively. Compared to the bare Zn anode, the Zn@Se anode leads to a larger Zn2+ transference number and reduced charge transfer resistance. The button-type MnO2∥Zn@Se full cell exhibits higher capacity and a much longer cycle life compared to the counterpart using a bare Zn anode. Also, pouch-type MnO2∥Zn@Se full cells with a high capacity of 9.7 mAh cm-2 are made to demonstrate the inhibition of hydrogen evolution and practical applications. It is found that the in situ formation of an amorphous ZnO nanosheet network induced by the amorphous Se overlayer plays a key role in enhancing the Zn anode performance.

The impact of titanium doping upon the sensing capability of ZnO nanosheets in detecting phosgene

The impact of titanium doping upon the sensing capability of ZnO nanosheets in detecting phosgene

Hydrothermal synthesis of ZnO Crystals: Diverse morphologies and characterization of the photocatalytic properties

Zinc oxide (ZnO) microstructures including the ZnO nanorods (R-ZnO), ZnO nanopillars (C-ZnO), and multistage microspheres composed of ZnO nanosheets (M-ZnO) have been synthesized using a two-step hydrothermal approach by adjusting the concentrations of the growth solution. Scanning Electron Microscopy (SEM) was employed to observe the surface morphology of the fabricated samples. X-ray diffraction (XRD) was utilized for the structure characterization, which the results reveal a hexagonal wurtzite ZnO structure in all three samples with different microstructures. X-ray Photoelectron Spectroscopy (XPS) was used to investigate their defects, indicating that the oxygen vacancy ratios were 34.3 %, 21.8 %, and 51.5 % respectively for R-ZnO, C-ZnO, and M-ZnO. The band gaps of R-ZnO, C-ZnO, and M-ZnO, determined through Ultraviolet–Visible Absorption Spectroscopy (UV–Vis), were found to be 3.35 eV, 3.6 eV, and 3.19 eV, respectively. Finally, the photocatalytic performances of the prepared ZnO samples were investigated under simulated sunlight irradiation by observing the degradation of methyl orange, which the M-ZnO samples obtained a superior photocatalytic efficiency of 79 %.

Sandwich-type double-layer piezoelectric nanogenerators based on one- and two-dimensional ZnO nanostructures with improved output performance

Piezoelectric nanogenerators (PENGs) have attracted great interest owing to their broad range application in environmental mechanical energy harvesting to power small electronic devices. In this study, novel flexible and high-performance double-layer sandwich-type PENGs based on one-dimensional (1-D) and two-dimensional (2-D) zinc oxide (ZnO) nanostructures and Ni foam as the middle layer have been developed. The morphology and structure of 1- and 2-D ZnO nanostructures have been studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). To investigate the effect of structural design on the piezoelectric performance, single-layer PENGs were also fabricated. The piezoelectric output of all prepared PENGs were evaluated under different human impacts at various forces and frequencies. The double-layer designed PENGs showed a two times larger voltage output compared to the single-layer PENGs, and the use of Ni foam as middle-layer and of 2-D ZnO nanosheets (compared to 1-D nanorods) was also found to increase the performance of the designed PENGs. The working mechanism of the prepared PENGs is also discussed. The design of nanogenerators as double-layer sandwich structures instead of two integrated single-layer devices reduces the overall preparation time and processing steps and enhances their output performance, thus opening the gate for widening their practical applications.

Piezoelectric polarization induced by dual piezoelectric materials ZnO nanosheets/MoS2 heterostructure for enhancing photoelectrochemical water splitting

Zinc oxide (ZnO) has a broad range of applications in piezo-photoelectrochemical water splitting. However, the narrow light absorption range and high photogenerated carrier recombination efficiency make ZnO somewhat limited in applying piezo-photoelectrochemical water splitting. Heterogeneous structure construction is a superior handle to these two drawbacks. Herein, few-layer molybdenum disulfide (MoS2) nanospheres are compounded on ZnO nanosheets (NSs) to form a dual-piezoelectric-material heterojunction of ZnO NSs/MoS2. The photocurrent density of ZnO NSs/MoS2 reaches 0.68 mA/cm2 at 1.23 V vs. RHE under ultrasonic vibrations. It is 2.4 times higher than that of ZnO NSs under ultrasonic vibrations. The efficient piezo-photoelectrochemical performance is attributed to increased absorption range and polarization field. On the one hand, the narrow band gap of the few-layer MoS2 widens the light absorption range of ZnO. On the other hand, compared to pure ZnO NSs, ZnO NSs/MoS2 has an enhanced polarization field under ultrasonic vibrations due to the piezoelectric properties of dual piezoelectric materials, which dramatically accelerates the electron transfer and suppresses the recombination of between electrons and holes. This work provides a new approach to constructing photoelectrodes with effective piezoelectric photocatalytic properties.

Understanding the Plasmonic Effect of Enhanced Photodegradation with Au Nanoparticle Decorated ZnO Nanosheet Arrays under Visible Light Irradiation.

Plasmonic-enhanced photocatalysis using visible light is considered a promising strategy for pollution photodegradation. However, there is still a lack of comprehensive and quantitative understanding of the underlying mechanisms and interactions involved. In this study, we employed a two-step process to fabricate arrays of ZnO nanosheets decorated with Au nanoparticles (Au-ZnO NS). Various characterization techniques were used to examine the morphological, structural, and chemical properties of the fabricated Au-ZnO NS array. Furthermore, we systematically investigated the photocatalytic degradation of methyl orange under visible light irradiation using Au-ZnO NS arrays prepared with varying numbers of photochemical reduction cycles. The results indicated that as the number of photochemical reduction cycles increased, the photodegradation efficiency initially increased but subsequently decreased. Under visible light irradiation, the Au-ZnO NS array obtained via four cycles of photochemical reduction exhibits the highest photocatalytic degradation rate of methyl orange 0.00926 min-1, which is six times higher than that of the ZnO NS array. To gain a better understanding of the plasmonic effect on photodegradation performance, we utilized electromagnetic simulations to quantitatively investigate the enhancement of electric fields in the Au-ZnO NS array. The simulations clearly presented the nonlinear dependencies of electric field intensity on the distribution of Au nanoparticles and the wavelength of radiation light, leading to a nonlinear enhancement of hot electron injection and eventual plasmonic photodegradation. The simulated model, corresponding to four cycles of photochemical reduction, exhibits the highest electric field intensity at 550 nm, which can be attributed to its strong plasmonic effect. This work provides mechanistic insights into plasmonic photocatalysts for utilizing visible light and represents a promising strategy for the rational design of high-performance visible light photocatalysts.

Surface engineering of ZnO electrocatalyst by N doping towards electrochemical CO2 reduction

The discovery of efficient, selective, and stable electrocatalysts can be a key point to produce the large-scale chemical fuels via electrochemical CO2 reduction (ECR). In this study, an earth-abundant and nontoxic ZnO-based electrocatalyst was developed for use in gas-diffusion electrodes (GDE), and the effect of nitrogen (N) doping on the ECR activity of ZnO electrocatalysts was investigated. Initially, a ZnO nanosheet was prepared via the hydrothermal method, and nitridation was performed at different times to control the N-doping content. With an increase in the N-doping content, the morphological properties of the nanosheet changed significantly, namely, the 2D nanosheets transformed into irregularly shaped nanoparticles. Furthermore, the ECR performance of ZnO electrocatalysts with different N-doping content was assessed in 1.0 M KHCO3 electrolyte using a gas-diffusion electrode-based ECR cell. While the ECR activity increased after a small amount of N doping, it decreased for higher N doping content. Among them, the N:ZnO-1 h electrocatalysts showed the best CO selectivity, with a faradaic efficiency (FECO) of 92.7% at −0.73 V vs. reversible hydrogen electrode (RHE), which was greater than that of an undoped ZnO electrocatalyst (FECO of 63.4% at −0.78 VRHE). Also, the N:ZnO-1 h electrocatalyst exhibited outstanding durability for 16 h, with a partial current density of −92.1 mA cm−2. This improvement of N:ZnO-1 h electrocatalyst can be explained by density functional theory calculations, demonstrating that this improvement of N:ZnO-1 h electrocatalyst comes from (i) the optimized active sites lowering the free energy barrier for the rate-determining step (RDS), and (ii) the modification of electronic structure enhancing the electron transfer rate by N doping.

ZnO/GaN n-n heterojunction porous nanosheets for ppb-level NO2 gas sensors

The development of NO2 gas sensors with lower operating temperature and good thermal stability is always a challenge to solve. In this work, 2D loose porous ZnO/GaN heterogeneous nanosheets were synthesized by ultrasonic mixing, in which ZnO nanosheets and GaN nanoparticles were prepared by hydrothermal method and solid-state pyrolysis, respectively. BET characterization result showed that the specific surface area of loose porous ZnO nanosheets was 57.24 m2/g, and XPS tests proved that the formation of heterogeneous interface between ZnO and GaN can effectively increase the number of negative oxygen ion species on the surface. Noticeably, 15 mol % ZnO/GaN composites (Ga:Zn) exhibited the highest response of 134.5–2 ppm of NO2 at 125 °C and lower limit of detection of 20 ppb. Furthermore, it showed outstanding selectivity, repeatability and long-term stability. The excellent gas-sensing properties can be attributed to the loose porous structure of ZnO and the formation of n-n heterojunction between ZnO and GaN. The gas-sensing mechanism of ZnO/GaN composites to NO2 was also discussed in detail.

Enhancing Dielectric, Ferroelectric and Antibacterial Properties of Siloxene Nanosheets by Wet Chemical Deposition of Flower-like Spherical ZnO Nanosheets

Enhancing Dielectric, Ferroelectric and Antibacterial Properties of Siloxene Nanosheets by Wet Chemical Deposition of Flower-like Spherical ZnO Nanosheets

Photochemical Synthesis of Benzimidazoles Promoted by Palladium-Decorated ZnO Nanosheets

Photochemical Synthesis of Benzimidazoles Promoted by Palladium-Decorated ZnO Nanosheets