Porous ZnO Nanosheets Research Articles

Facile Hydrothermal Synthesis of Two-Dimensional Porous ZnO Nanosheets for Highly Sensitive Ethanol Sensor

Two-dimensional porous ZnO nanosheets were synthesized by a facile hydrothermal method for ethanol gas-sensing application. The morphology, composition, and structure of the synthesized materials were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and high-resolution transmission electron microcopy. Results showed that the synthesized ZnO materials were porous nanosheets with a smooth surface and a thickness of 100 nm and a large pore size of approximately 80 nm. The as-prepared nanosheets, which had high purity, high crystallinity, and good dispersion, were used to fabricate a gas sensor for ethanol gas detection at different operating temperatures. The porous ZnO nanosheet gas sensor exhibited a high response value of 21 toward 500 ppm ethanol at a working temperature of 400°C with a reversible and fast response to ethanol gas (12 s/231 s), indicating its potential application. We also discussed the plausible sensing mechanism of the porous ZnO nanosheets on the basis of the adopted ethanol sensor.

Photocatalytic producing dihydroxybenzenes from phenol enabled by gathering oxygen vacancies in ultrathin porous ZnO nanosheets

As an energy-efficient and environmental friendliness method, solar sunlight-driven photo-oxidation catalysis process for organic chemicals synthesis has gained enormous attention, but still faces huge challenge in developing highly-efficient photocatalysts material. Two-dimensional materials engineering and surface defect engineering of photocatalysts both provide an effective strategy to improve the catalytic activity. Inspired by this pathway, we design and synthesize ultrathin porous ZnO nanosheets featuring abundant oxygen vacancies specific to producing dihydroxybenzenes based on a photocatalytic oxidation process. Several valid characterizations had been employed to discern the structural character of the obtained model catalyst, revealing that the resultant ZnO sheets afford an average thickness of 3 nm, and abundant surface porosity, thereby contributing to the rich oxygen vacancies. Such a structure could generate a synergistic effect to enhance the optical absorption and improve the transportation rate of photogenerated charge carriers from the materials design. As expected, the specific ultrathin ZnO nanosheets exhibited a greatly-improved photocatalytic activity for oxidation of phenol to dihydroxybenzenes (31.5% conversion & almost 76.7% selectivity of DHB), near 3 and 4 times higher, respectively than its counterparts that one with few oxygen vacancies and Bulk-ZnO. Impressively, the obtained catalyst showed durable catalytic activity without any activity loss during the five recycling. Finally, the feasible oxidation mechanism was proposed and testified by the controlled scavenger experiments. This study provides a novel reference on how to design high-performance photocatalytic material.

Synthesis of NiO-decorated ZnO porous nanosheets with improved CH4 sensing performance

Aimed at improving the methane (CH4) sensing performance, this paper presents the preparation of pure and NiO decorated ZnO porous nanosheets (PNSs, with the size of 200 nm) through a hydrothermal route combined with post-treatment, in which the pre-prepared Zn5(CO)3(OH)6 was employed as sacrificial template. The specific surface areas of ZnO decorated with NiO nanoparticles is higher than that of pure ZnO. The introduction of NiO on ZnO PNSs not only increases the specific surface areas of ZnO/NiO PNSs, but also could promote the formation of p-n heterojunction at the interface of NiO and ZnO. The sensing results indicate that the sensor based on NiO-decorated ZnO PNSs exhibit an enhanced sensing performance in the comparison with pure ZnO PNSs at the working temperature of 340 °C, such as higher response and good long-term stability within 30 days, as well as the capacity of response-recovery. These results demonstrate that the potential application for CH4 detection of ZnO/NiO PNSs. Besides, the improved sensing mechanism of ZnO/NiO was also discussed in the paper.

Plasmonic control of solar-driven CO2 conversion at the metal/ZnO interfaces

Molecular-level understanding of the solar-driven CO2 conversion is of importance to design high-efficiency artificial photosynthetic systems for rebalancing the global carbon cycle. Herein, some physical insights into the surface plasmon resonance (SPR) mediated CO2 photoreduction were demonstrated with metal (Au, Ag, and Pd)/3D porous ZnO nanosheets (NSs). Such plasmonic photocatalysts were designed elaborately to expose the polar {001} facet, based on the physical prototype of field-field coupling, in order to benefit chemical polarization and activation of the inert molecule. Among these plasmonic metals, gold was found to be not only more effective for promoting the solar-driven CO2 conversion, but unique for producing the higher hydrocarbon, C2H6. A 10-fold enhanced conversion efficiency and a quantum efficiency of 1.03% were achieved on Au/ZnO NSs at ca. 80% selectivity to hydrocarbons under solar light irradiation. The characterization results indicated that the metal-semiconductor interaction enables the electron-phonon decoupling to generate more amounts of energetic electrons in the excited ZnO NSs by a proposed pathway, called the SPR energy transfer induced interband transition that promotes the semiconductor photoexcitation, kinetically accelerating the conversion. Density functional theory calculations revealed that the field-field coupling greatly intensifies the surface polarization for adsorbates, charging negatively the C atom of CO2 and making OCO bond bent, along with the electrophilic attack by two competitive paths, leading to the concomitance of CO and CH4. The loading of plasmonic metal nanoparticles alters the molecular paths of CO2 conversion by tuning thermodynamically the first dehydroxylation step, consequently the product selectivity. Especially Au plasmon, it enables the CO hydrogenation path, making CH4 faster.

Pierced ZnO nanosheets via a template-free photopolymerization in microemulsion

Multilevel porous ZnO nanosheets have been synthesized from the precursor solution containing monomer and photoinitiator by a photopolymerization method in water-in-oil (W/O) microemulsion, avoiding the introduction of a template. The radical-mediated photopolymerization is a cross-linking polymerization process induced by the UV light to motivate the photoinitiator from the ground state to the excited state, which makes it possible to realize the rapid transformation of liquid monomers into solid polymer. After calcination to remove the solid polymer, pierced ZnO nanosheets show a well-defined interconnected architecture with large specific surface area and various pore sizes from mesopores to macropores. Additionally, the morphology and size of pores can be adjusted by changing the components of W/O microemulsion. Merited by the unique porous structure and high specific surface area, the pierced ZnO nanosheets demonstrate intended performance as an anode of lithium ion batteries. Moreover, this strategy is a well-designed and attractive method for producing other porous nanomaterials.

Composites of Donor-π-Acceptor type configured organic compound and porous ZnO nano sheets as corrosion inhibitors of copper in chloride environment

Donor-π-Acceptor (D-π-A) type compounds are very interesting for corrosion study since they have unique structural configuration. Their high chemical reactivity promotes an easy bonding with the metals, which helps in corrosion inhibition. In this work, ethyl-2-cyano-3-(4-(dimethylamino) phenyl) acrylate (ECDPA) (a D-π-A type compound) is used to restrict the copper loss in 1 M HCl. The impedance analysis portrays that ECDPA saves 75% of copper at its maximum concentration in HCl solution. To further increase the inhibition efficiency, ECDPA-ZnO composites are developed. The ZnO is synthesized hydrothermally and calcined at two different temperatures (300 °C and 500 °C). The morphology of synthesized ZnO changes drastically with calcination; however, the main morphological difference in ZnO at 300 °C (Z3) and ZnO at 500 °C (Z5) is increment of porosity. Hence, two composites are made to analyze the change in corrosion behavior of the composites. The composites are designated as: ECDPA-ZnO at 300 °C to EZ3 and ECDPA-ZnO at 500 °C to EZ5. The characterizations of ZnO, ECDPA and ECDPA-ZnO composites have been performed by UV–visible spectroscopy, FTIR spectroscopy, HRTEM, HRSEM, EDAX and XRD techniques. The corrosion inhibition efficiencies of EZ3 and EZ5 are achieved as 78% and 81%. This fact suggests that ZnO increases the adsorption of ECDPA over copper surface, which is confirmed by Langmuir adsorption analysis and electrochemical atomic force microscopy (EC-AFM) measurements.

The detection of ethylene using porous ZnO nanosheets: utility in the determination of fruit ripeness

Porous ZnO nanosheets exhibit superior sensitivity in ethylene detection and present different intensity responses to bananas at different maturity stages.

2D nanosheet-assembled Pd[sbnd]ZnO microflowers for acetone sensor with enhanced performances

Hierarchical microflowers architecture assembled with 2D porous ZnO nanosheets were fabricated by a one-step solvothermal method and subsequent annealing process. A series of PdZnO have been produced by injecting various volumes of Pd(NO3)2·2H2O solution. The ZnO and PdZnO microflowers are applied to fabricate gas sensors to investigate their sensing characteristics for various gases at different operating temperatures. The 0.05 wt% PdZnO exhibit the highest selective enhancement of acetone (165%) than ammonia (0.9%), methanal (38%), methanol (71%), H2 (35%) and CO (15%). In addition, it possesses the lower operating temperature and shorter response/recovery times (11s, 5s) toward acetone gas compared with the pure ZnO microflowers.

Waste-cleaning waste: synthesis of ZnO porous nano-sheets from batteries for dye degradation.

This paper describes a clean approach of waste-cleaning waste. Two-dimensional (2D) ZnO porous nano-sheets were synthesized from end-of-life zinc-carbon batteries via a simple homogeneous precipitation-calcination route, and the as-synthesized product was applied as photocatalyst for the purpose of photodegradation of methylene blue/MB aqueous solution under UV-Vis irradiation. Precipitation at ambient temperature resulted in the formation of the crystalline phase of zinc hydroxide nitrate hydrate [Zn5(OH)8(NO3)2(H2O)2], which was then transformed to ZnO through calcination. FE-SEM studies revealed the resulting ZnO had the morphology of porous nano-sheets with thickness of up to 100nm. The photocatalytic activity tests proved that the batteries-derived ZnO porous nano-sheets can be a promising candidate for photodegradation of organic pollutant in industrial waste water. The results presented here confirm a possibility of utilization waste batteries as a resource of photodegrading MB in wastewater treatment, hereby an opportunity to deliver environmental benefits. Graphical abstract.

Defects driven photoluminescence property of Sm-doped ZnO porous nanosheets via a hydrothermal approach

Samarium doped ZnO porous nanosheets were prepared via a hydrothermal approach and well characterized by X-ray diffraction (XRD), Raman spectroscopy, transition electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and UV–vis absorption spectroscopy. With increasing the Sm3+ doping concentrations, the crystallization of the thin nanosheets with irregular porosities became worse and the size of the porosities in the nanosheets gradually diminished due to the obstructions of Zn–O–Sm and the generated volatile gas. It was also found that the Sm3+ ions not only acted as the donors sinks to modulate the defects in host, but also expanded the visible light response of the ZnO. The slight redshift of UV NBE (near-band edge) emission as compared to undoped one indicated the band gap of the doped nanosheets became narrower due to the combination of the formed impurity band and the strong exchange interactions between electrons. The kinds of the defects as the deep-level-defect luminescent centers in DLE (deep-level emission) varied with the Sm3+ doping concentrations, and the detail change of the defects was also discussed in detail. These findings would be useful for the material design and defect modification for optical applications.

2D Porous ZnO Nanosheets: One Pot Synthesis with Low Turn-on Field

Low turn-on field of 2.3 V/µm was found for the emission current density of 10 µA/cm2 from 2D porous ZnO nanosheets. High current density of 0.76 mA/cm2 was drawn at an applied field of 4.1 V/µm. The observed low turn-on field of porous ZnO nanosheets has been found to be superior to the other ZnO nanostructures reported in the literature. Also, the emission current stability over a period of 3 hr is found to be better. The field emission current density-applied field (J-E) and current-time (I-t) measurements were carried out in all metal field emission microscope by using ‘close proximity’ (also termed as ‘planar diode’). The porous ZnO nanosheets were synthesized by Chemical Bath Deposition (CBD) method at room temperature followed by annealing at 200 oC. The annealed ZnO nanosheets were subjected to structural and morphological analysis prior to the field emission studies. The XRD spectrum of the as-synthesized product reveals formation of crystalline hexagonal phase of ZnO. Simple synthesis route with superior field emission properties indicate the possible use of porous ZnO nanosheets for micro/nanoelectronic devices.

Amorphous carbon-coated ZnO porous nanosheets: Facile fabrication and application in lithium- and sodium-ion batteries

Rational design and facile synthesis of ZnO-based electrode materials for highly reversible and high-rate lithium storage still remain a significant challenge. Herein, carbon-coated ZnO (abbreviated as C-ZnO) 2 dimensional (2D) porous nanosheets are successfully achieved by a scalable hydrothermal method followed by a chemical vapor deposition process. The ultrathin ZnO nanosheets possessing net-like structures are uniformly coated by a layer of amorphous carbon ∼5 nm in thickness. The amorphous carbon coating can not only effectively improve the electrical conductivity and durability of the ZnO nanosheets but also act as a buffer to accommodate the large volume strain during cycling. Furthermore, the porous C-ZnO nanosheets afford both large electrode/electrolyte contact interfaces and short Li+/Na+ diffusion paths. As anode materials of lithium- and sodium-ion batteries (LIBs and SIBs), the C-ZnO structured composites exhibit excellent electrochemical performances. The C-ZnO electrodes exhibit high reversible capacities of 851 mAh g−1 after 50 cycles at 100 mA g−1 and 804 mAh g−1 after 200 cycles at 1000 mA g−1 for LIBs. Significantly, the prepared C-ZnO can be a promising anode material for lithium ion battery.

A facile one-step hydrothermal synthesis of NiO/ZnO heterojunction microflowers for the enhanced formaldehyde sensing properties

NiO/ZnO heterojunction microflowers were successfully synthesized by using a facile one-step hydrothermal process followed by calcination. The structural features were mainly characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It is found that the obtained products showed flower-like structures assembled by a number of ZnO porous nanosheets, and which were successfully retained after introducing NiO. The formaldehyde gas sensing properties were systematically investigated between the pure and NiO/ZnO microflowers. NiO/ZnO microflower sensor exhibited the enhanced response and lower operating temperature compared to pure one. Especially, the 3 mol% NiO/ZnO microflower sensor exhibited the highest responses compared with others at a relatively lower operating temperature of 200 °C. In addition, fast response and recovery, good reproducibility and stability, and good selectivity to formaldehyde were also obtained, indicating the formation of NiO/ZnO heterojunction in favor of improving the sensing properties. Meanwhile, the sensing enhancement mechanism of the NiO/ZnO microflowers was also discussed, which is related to the formation of hetrojunction at interface and the high catalytic activity of NiO.

Significant Enhancement of Photocatalytic Reduction of CO2 with H2O over ZnO by the Formation of Basic Zinc Carbonate.

Electron-hole pair separation efficiency and adsorption performance of photocatalysts to CO2 are the two key factors affecting the performance of photocatalytic CO2 reduction with H2O. Distinct from conventional promoter addition, this study proposed a novel approach to address these two issues by tuning the own surface features of semiconductor photocatalyst. Three ZnO samples with different morphologies, surface area, and defect content were fabricated by varying preparation methods, characterized by XRD, TEM, and room-temperature PL spectra, and tested in photoreduction of CO2 with H2O. The results show that the as-prepared porous ZnO nanosheets exhibit a much higher activity for photoreduction of CO2 with H2O when compared to ZnO nanoparticles and nanorods attributed to the existence of more defect sites, that is, zinc and oxygen vacancies. These defects would lower the combination rate of electron-hole pair as well as promote the formation of basic zinc carbonate by Lewis acid-base interaction, which is the active intermediate species for photoreduction of CO2. ZnO nanoparticles and ZnO nanorods with few defects show weak adsorption for CO2 leading to the inferior photocatalytic activities. This work provides new insight on the CO2 activation under light irradiation.

Trimethylamine Sensors Based on Au-Modified Hierarchical Porous Single-Crystalline ZnO Nanosheets.

It is of great significance for dynamic monitoring of foods in storage or during the transportation process through on-line detecting trimethylamine (TMA). Here, TMA were sensitively detected by Au-modified hierarchical porous single-crystalline ZnO nanosheets (HPSCZNs)-based sensors. The HPSCZNs were synthesized through a one-pot wet-chemical method followed by an annealing treatment. Polyethyleneimine (PEI) was used to modify the surface of the HPSCZNs, and then the PEI-modified samples were mixed with Au nanoparticles (NPs) sol solution. Electrostatic interactions drive Au nanoparticles loading onto the surface of the HPSCZNs. The Au-modified HPSCZNs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrum (EDS), respectively. The results show that Au-modified HPSCZNs-based sensors exhibit a high response to TMA. The linear range is from 10 to 300 ppb; while the detection limit is 10 ppb, which is the lowest value to our knowledge.

Chlorobenzene sensor based on Pt-decorated porous single-crystalline ZnO nanosheets

Detection of chlorobenzene is a challenge for metal oxide gas sensors because of its stable molecule structure. In this work, Pt-decorated porous single-crystalline ZnO nanosheets (PSC ZnO NSs) were successfully synthesized via a one-pot hydrothermal method followed by a liquid phase reduction process. The morphology and structure of the as-prepared sample were well characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive spectrum (EDS) and ICP-MS analysis. From gas-sensing test, it can be found that the decoration of Pt nanoparticles largely enhanced the sensing performance of the PSC ZnO NSs. The Pt-decorated PSC ZnO NSs showed good response to low concentrations of chlorobenzene from 10 to 500ppb. In contrast, the Au and Ag-decorated PSC ZnO NSs had still no response to 500ppb of chlorobenzene. The enhancement sensing mechanism of Pt nanoparticle decoration has also been discussed in detail.

The crystal facet-dependent gas sensing properties of ZnO nanosheets: Experimental and computational study

Herein, we focused on the effects of exposed crystal planes on the gas sensing property of ZnO. For this purpose, we designed and synthesized two porous ZnO nanosheets with different exposed crystal facets (0001) and (101¯0) by a facile hydrothermal routes. The characterization results show that both the porous nanosheets have a near specific surface area about 7.5m2/g, thickness about 100nm, diameter about 5μm and pore size of tens of nanometers. However, their dominating exposed crystal facets are (0001) and (101¯0), respectively. When employed them as sensing materials in gas sensors, porous ZnO nanosheets with dominating exposed (0001) facet exhibit a superior sensitivity than the (101¯0) one. The enhanced gas response is attributed to a large amount of oxygen vacancy defects and unsaturated dangling bonds existing in the ZnO nanosheets with exposed crystal facet (0001), which is favorable for the adsorption of gas molecular on the sensor surface and result in improvement of the gas response. Finally, the calculation of the chemisorption energy of oxygen on ZnO crystal facets also proves the reactive-facet-enhanced gas sensitivity.

Single-crystalline porous ZnO nanosheet frameworks for efficient fully flexible dye-sensitized solar cells

The growth of single-crystalline porous ZnO nanosheet frameworks (ZnO NSFs) was performed with a simple hydrothermal approach. The highly transparent poly(3,4-ethylenedioxythiophene) (PEDOT) film was prepared by an in-situ electrodeposition method on flexible ITO-PET substrate. Moreover, ZnO NSFs and PEDOT were made as photoanodes and counter electrodes (CE) for assembling fully flexible dye-sensitized solar cells (DSSCs), respectively. The interconnected nanostructures of ZnO NSF can deliver direct electron transport paths resulting in reduced recombination rate and accelerated electron transport. The PEDOT/ITO-PET exhibits outstanding catalytic ability in the reaction of I‾/I3‾ redox couple. A power conversion efficiency of 2.97% obtained from the fully flexible DSSCs, showed a comparable photovoltaic performance to the device with Pt CE. The highly photovoltaic performance can be mainly ascribed to the unique structure of porous ZnO NSFs and the outstanding catalytic properties of PEDOT CE.

Growth of porous ZnO single crystal hierarchical architectures with ultrahigh sensing performances to ethanol and acetone gases

Three-dimensional (3D) porous ZnO hierarchical architectures have been fabricated through annealing the zinc hydroxide carbonate precursor, which was obtained by a one-pot hydrothermal process with the assistance of carbon nano-fiber and cetyltrimethyl ammonlum bromide (CTAB). The ZnO hierarchical architectures are assembled from porous single crystal ZnO nanosheets. Experiment evidence reveals that carbon nano-fiber plays an important role in the formation of 3D hierarchical architectures. The growth mechanism of the 3D porous ZnO hierarchical architectures has been proposed based on control experiments. Gas sensing test results show that as prepared 3D porous ZnO hierarchical architectures exhibit superior sensing performance to ethanol and acetone. The sensor response to 100ppm ethanol and acetone are 340 and 362 at the optimum operating temperature of 370°C and 300°C, respectively. The superior gas sensing performances are attributed to the unique 3D porous hierarchical architectures structure, the single crystal nature and the exposed polar crystal faces of ZnO nanosheet.

Large scale synthesis of hexagonal simonkolleit nanosheets for ZnO gas sensors with enhanced performances

Hexagonal simonkolleite· (Zn5(OH)8Cl2·H2O) nanosheets were synthesized via a one-pot hydrothermal method. The precursors can be converted to porous ZnO nanosheets consisting of nanoparticles by annealing at higher temperature, and the final ZnO products can inherit the hexagonal shapes of the precursors. The gas sensing properties of the precursors and ZnO nanosheets were also studied systematically. It was found that the annealing temperature can dramatically affect the gas sensing performances of the devices for the tunable microstructures and compositions of the materials. After annealed at 500°C, the ZnO nanosheets exhibit excellent sensing performances to ethanol of 100ppm.