Hierarchical Porous ZnO Research Articles

Laser-induced in-situ electrohydrodynamic jet printing of micro/nanoscale hierarchical structure

With the rapid advancement of micro/nanoscale devices, there is a growing demand for hierarchical micro/nano porous-structure, particularly in fields of high-performance sensors, electrochemical energy storage, and photocatalysis. The fabrication methods for hierarchical micro/nano-porous structures have been limited by complex processes and high costs, making further development and application difficult. In this paper, a novel strategy of laser-induced in-situ electrohydrodynamic jet (E-Jet) printing of hierarchical micro/nano-porous structures was proposed. Based on the mechanism of high-energy laser beam induction on the jet, it successfully fabricated hierarchical porous ZnO structures from nanoscale to dozens of microns. The jet size focusing and solidification behavior were analyzed by combining experimental and simulative exploration. The resultant effects of the thermal field, flow field, and laser field on the spatial temperature distribution and the jetting morphology were examined. Furthermore, the laser-induced influence on the morphology of the printed micro/nano-porous ZnO structures was explored. Meanwhile, the performance of micro/nano-porous ZnO photoelectric sensors printed by E-Jet under different laser powers was investigated. The laser-induced in-situ E-Jet printing method provided an innovative pattern for the high-resolution additive manufacturing of hierarchical porous structures, demonstrating its potential for applications in advanced material and high-performance devices.

High transition kinetics enabling high Zn utilization for high-capacity and long-lifespan nickel-zinc battery

Aqueous nickel-zinc (Ni-Zn) battery is one promising grid energy storage device owing to its high theoretical energy density, high safety and low cost. However, the large-scale commercialization of Ni-Zn battery is significantly hindered by its low practical energy density and poor cycle lifespan caused by the low reversibility and transition kinetics between the metallic Zn and ZnO with low Zn utilization. To address these issues, a type of hierarchical porous Indium-doping ZnO micro-bowls (In-ZnO) is constructed through an in-situ chemical corrosion strategy. The abundant voids and pores facilitate the ionic transport, and simultaneously accommodate the volumetric expansion during the electrochemical transition between metallic Zn and ZnO. Density functional theory calculations further reveal that In-ZnO achieves the enhanced electrical/ionic conductivity and improved adsorption to OH–, accelerating the solid–liquid transitions of Zn-Zn(OH)42- and Zn(OH)42--ZnO, as well as the solid–solid transition of Zn-ZnO. Simultaneously, the Indium-doping inhibits the hydrogen evolution and passivation. Ascribed to the synergetic effect of the hierarchical porous structure and Indium-doping, the In-ZnO electrode demonstrates ultra-stable capacity retention of 98.6% over 1000 cycles at 9 A g-1 under 92.3% Zn utilization, and high capacity of 590.2 mAh g-1 even at high current density of 36 A g-1.

Enhanced Gas Sensing Performance of CuO-ZnO Composite Nanostructures for Low-Concentration NO2 Detection

The detection of nitrogen dioxide (NO2) is essential for safeguarding human health and addressing environmental sustainability. That is why, in the last decades, gas sensors have been developed to detect NO2 to overcome these hazards. This study explores the use of a novel CuO-ZnO composite synthesized through a polyol and sol–gel technique to enhance gas sensing performance. The CuO-ZnO composite offers the advantage of a synergic combination of its properties, leading to improved sensitivity, selectivity, and low detection limit. The innovative polyol technique employed in this research enables the controlled synthesis of hierarchical CuO and porous ZnO structures. The composite formation is achieved using the sol–gel method, resulting in CuO-ZnO composites with different ratios. The structural, morphological, and optical properties of the materials have been characterized using FESEM, X-ray diffraction, and UV-vis spectroscopy. Gas sensing experiments demonstrate enhanced performance, particularly in sensitivity and selectivity for NO2, even at low concentrations. The composites also exhibit improved baseline stability compared to pristine CuO and ZnO. This study explains the influence of humidity on gas sensing properties by examining interactions between water molecules and sensor surfaces. Notably, the developed CuO-ZnO composite displays excellent selectivity towards NO2, attributed to favorable bonding characteristics and acid-base properties. Overall, this research contributes to advancing gas sensor technology, providing a promising potential for sensitive and selective NO2 detection, thereby addressing critical needs for human health and environmental protection.

0-D/3-D heterojunction composite constructed by decorating transition metal oxide nanoparticle on peony-like ZnO hierarchical microstructure for improved photodegradation of palm oil mill effluent

A series of transition metal oxide nanoparticles decorated on peony-like hierarchical porous ZnO (MO/ZnO, where MO = CuO, WO3 and Nb2O5) was synthesized using a precipitation-ultrasonic method. The X-ray diffraction, energy dispersive X-ray and fourier-transform infrared spectroscopy findings indicated the existence of both MO and ZnO in the composites. The field-emission scanning electron microscope and transmission electron microscope observations revealed that the MO nanoparticles were well dispersed onto peony-like ZnO hierarchical microstructure. Furthermore, the enhanced visible light absorption abilities of MO/ZnO were confirmed by UV–vis diffuse reflectance spectroscopy. The photocatalytic effectiveness of MO/ZnO was investigated by palm oil mill effluent (POME) degradation under UV irradiation. Among them, Nb2O5/ZnO composite exhibited the best photodegradation efficiency (91.7%) and the highest rate constant (0.0102 min−1) after 240 min irradiation. Moreover, a complete decolorization of POME was observed after 30 min irradiation using Nb2O5/ZnO composite. The phytotoxic effects of POME against the Vigna radiata seeds also reduced greatly after photocatalytically treated using Nb2O5/ZnO composite. Meantime, the product revealed great photocatalytic stability with the degradation efficiency retained more than 80% after 4 cycles. The radical trapping experiment demonstrated that hydroxyl radical and photogenerated hole played major role in POME degradation. The outstanding photoactivity of Nb2O5/ZnO composite can be explicated by efficient charge separation between Nb2O5 and ZnO where the evidence can be obtained from photoluminescence and terephthalic acid-photoluminescence probing analyses. The present study to construct these heterojunction composites can provide an innovative view to design highly effective photocatalytic system for the treatment of industrial wastewater.

Enhanced ethanol gas sensing properties of hierarchical porous SnO2-ZnO microspheres at low working temperature

In this work, the hierarchical porous SnO2-ZnO microspheres have been successfully synthesized by hydrothermal method followed by calcination. The structure, chemical composition, specific surface area and morphology of as-synthesized samples were detailed characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results indicate that the response of hierarchical porous SnO2-ZnO microspheres-based sensor toward 100 ppm ethanol is 74 at low working temperature (225 °C), which is higher than that of pure hierarchical porous ZnO microspheres-based sensor (39). Moreover, the hierarchical porous SnO2-ZnO microspheres-based sensor presents fast response and recovery time (4 s, 6 s), good selectivity, repeatability and stability. Therefore, the sensor based on hierarchical porous SnO2-ZnO microspheres will be a potential candidate for ethanol detection.

Enhancing triethylamine sensing of ZIF-derived ZnO microspheres arising from cobalt doping and defect engineering

Reasonable doping is beneficial to the generation of defects, which is a feasibility strategy to improve the ZnO sensing performance. Herein, we presented an in situ self-sacrificing template strategy for fabricating Co doped h-ZnO core-shell structures (h-ZnO/ZnCox) with different defect contents, pyrolyzing hierarchical porous ZnO (h-ZnO) sub-microspheres coated by zeolite imidazolate frameworks (h-ZnO/ZIF-ZnCox). The investigations of X-ray photoelectron (XPS), photoluminescence (PL) and Raman spectra indicate that donor defects include zinc interstitial (Zni) and oxygen vacancy (VO) in h-ZnO/ZnCox can be tuned by Co dopant (x = 0–30%). Resultantly, the h-ZnO/ZnCox exhibits a significantly enhanced response and selectivity towards triethylamine (TEA), beyond the undoped h-ZnO, and 15% Co-doped h-ZnO (h-ZnO/ZnCo15%) conducts the maximum responses of 1020 to 50 ppm TEA at 573 K, in the top set for the similar type of sensors. Further, the sensing mechanism of h-ZnO/ZnCox is elaborated, possibly resulting from abundant active oxygen species conversed from more oxygen adsorbed which corresponds to cobalt doping generating rich donor-related defects and additional electrons in h-ZnO/ZnCo15%.

Biomass-derived hierarchical porous ZnO microtubules for highly selective detection of ppb-level nitric oxide at low temperature

• Porous ZnO hierarchical structure was simply prepared by using hemp fiber as biotemplate. • Hemp fiber template plays a vital role in regulating microstructure and performance of ZnO-600. • The ZnO-600 sensor exhibits fast response, high sensitivity and selectivity to trace NO at 92 °C. • The detection limit of 5 ppb NO for ZnO-600 material is the lowest in reported ZnO-based sensors. Highly sensitive and selective detection of nitric oxide (NO) has recently attracted much attention due to its environmental pollution and biological role. Herein, hierarchical porous ZnO microtubules were simply and massively prepared through zinc salt immersion and air calcination using hemp fiber as biotemplate. The effect of different calcination temperature on the microstructure and gas-sensing performance was investigated. The ZnO-600 microtubules calcined at 600 °C are assembled from the cross-linking nanoparticles with good crystallinity. Especially, the multi-pores and diverse nano-channels are in favor of the quick diffusion and adsorption of target gas, enabling ZnO-600 material to present excellent gas-sensing performance towards trace NO. At low working temperature of 92 °C, the sensor fabricated by ZnO-600 microtubules to NO exhibits high response (10 ppm, S = 78.54), fast response-recovery, good selectivity as well as satisfactory stability and anti-humidity. Meanwhile, this sensor represents the lowest detection limit of 5 ppb (S = 1.22) among all reported ZnO-based sensors. Moreover, the gas-sensing mechanism for ZnO-600 material is demonstrated to be surface adsorption control model.

Degradation mechanism of microcystin-LR by Bi2WO6/ZnO/biochar composites

In this study, hierarchical porous ZnO photocatalysts were successfully prepared using bionic material rice husk as biological template. Bi2WO6/pure ZnO and Bi2WO6/hierarchical porous ZnO catalysts with different Bi2WO6 doping amounts were prepared via hydrothermal synthesis. The results show that the specific surface area of Bi2WO6/hierarchical porous ZnO catalyst with rice husk as biological template was 131.8505 m2/g, which was 101 m2/g higher than that of Bi2WO6/pure ZnO catalyst without rice husk as biological template. In addition, the hierarchical porous ZnO photocatalyst with rice husk template increased the degradation efficiency of microcystin (MC-LR) by 10.60% compared with commercially available ZnO photocatalyst. When the optimum doping amount was 0.2 mol/mol, the degradation efficiency of MC-LR by 0.2Bi2WO6/hierarchical porous ZnO was 79.0%, which was higher than that of the composite catalyst without rice husk template and single photocatalyst. This study provides a new idea for the degradation of MC-LR by using green photocatalyst.

Pd-based catalysts promoted by hierarchical porous Al2O3 and ZnO microsphere supports/coatings for ethyl acetate highly active and stable destruction

Developing economical and active materials is of great significance for VOC purification. Here, hierarchical porous Al2O3 and ZnO microspheres (Al2O3-pm and ZnO-pm) were synthesized by a facile hydrothermal strategy. The urchin-like Al2O3-pm and flower-like ZnO-pm possess high specific surface area (especially; external surface area) obviously boost the dispersion of Pd with 29.3 % and 30.1 % over Pd/Al2O3-pm and Pd/ZnO-pm, respectively, over 3.4 times higher than those of commercial Al2O3- and ZnO-supported counterparts. Pd/Al2O3-pm possesses excellent activity and CO2 yield in ethyl acetate (EA) degradation, with TOF reaches 7.76 × 10−3 s−1 at 160 °C under GHSV of 50,000 h-1. Moreover, Pd/Al2O3-pm exhibits satisfied performance in EA-contained binary VOCs oxidation and has high long-term stability under both dry and humid conditions. Both Pd sites and Brønsted acid sites participated in reaction process and initially react with EA to form ethylene and ethanol, respectively. Larger amount Brønsted acid sites over Pd/Al2O3-pm promote ethanol formation and C-C cleavage, resulting in different CO2 yields and EA activation mechanisms. The coating greatly enhances Pd dispersion over Pd supported monolithic catalyst, endowing its desired activity and stability even with a much lower Pd loading. This work promotes the potential application of noble-metal-based monolithic materials in VOC degradation.

High response methane sensor based on Au-modified hierarchical porous nanosheets-assembled ZnO microspheres

Detecting methane (CH4) gas is very important owing to its widely use as a fossil fuel with an explosive hazard in modern society. In this study, a CH4 gas sensor was prepared, which is based on hierarchical porous nanosheets-assembled Au/ZnO microspheres with diameters of 10–12 μm. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the textural properties and morphology of the nanocomposites. In addition, the gas sensing properties of the sensors basedon pure ZnO and Au/ZnO nanomaterials were systematically investigated. By modifier Au nanoparticles, the characteristics of ZnO based sensor for CH4 detection were significantly enhanced. Especially, the Au/ZnO based sensor with an Au content of 1.0 at.% exhibited the best sensing performances (4.16) of 100 ppm CH4 at 250 °C. Furthermore the sensor also exhibited a good linear relation (10–2000 ppm CH4 concentration), excellent repeatability, satisfactory stability and a low detection limit of detection (1.83 ppm). The possible enhanced sensing mechanism could be attributed to the synergism between the unique hierarchical porous nanosheets-assembled microspheres structure and the sensitizing actions utilized by the Au nanoparticles.

Preparation of black hierarchical porous ZnO and its photocatalytic activity under visible light

Black hierarchical porous ZnO was prepared by oxidation of nano-porous Zn/ZnO sample obtained from the Al-Zn alloy etched in an NaOH aqueous solution, and was characterized by XRD, SEM, TEM, BET, EDS and UV–Vis spectrophotometer measurements. The results showed that the ZnO appeared black in the sunlight and its reflectivity was approximately 19.5% in the wavelength range from 370 nm to 700 nm. The Black ZnO sample exhibited a hierarchical porous structure consisting of nanosheets with uniformly distributed mesopores, that gives rise to a high specific surface area of about 36 m2/g. Compared to the ZnO nanopowders and TiO2 (P25), black porous ZnO exhibited excellent photocatalytic activity in the degradation of a methyl orange solution under visible light.

WO<sub>3</sub>/Nb<sub>2</sub>O<sub>5</sub> Nanoparticles-Decorated Hierarchical Porous ZnO Microspheres for Enhanced Photocatalytic Degradation of Palm Oil Mill Effluent and Simultaneous Production of Biogas

Conventionally, palm oil mill effluent (POME) was treated using open ponding system, which nevertheless long retention times and large treatment areas were required. In this report, heterogeneous photocatalysis was used to degrade the POME and simultaneously assessed the biogas formation. Characterization of the chemically prepared hierarchical porous ZnO microspheres showed that wurtzite was the predominant crystalline phase with a band gap energy of 3.22 eV. Moreover, the as-prepared ZnO were assembled by large numbers of interleaving nanosheets and formed an open porous structure. Under UV irradiation, the as-prepared ZnO demonstrated photocatalytic property on POME degradation. The WO3 and Nb2O5 decorated ZnO photocatalysts (WO3/ZnO and Nb2O5/ZnO) with improved photocatalytic performances were also prepared using a simple and rapid way. Significantly, in the presence of WO3/ZnO and Nb2O5/ZnO composites, the degradation of POME achieved 68.3% and 91.7%, respectively after 240 min irradiation. Interestingly, the assessment of the biogas formation showed that the photocatalytic reactions over Nb2O5/ZnO and WO3/ZnO composites generated higher amount of biogas products (CH4 + CO2) compared to that of ZnO. The photocatalytic enhancement was attributed to the high separation efficiency of photogenerated electron–hole pairs based on the formation of heterojunction structures between the WO3/Nb2O5 and ZnO. The observed findings also revealed that the photocatalytic technology using hierarchical WO3/ZnO and Nb2O5/ZnO composites had the potential to efficiently treat wastewater.

Insights into charge transfer and solar light photocatalytic activity induced by the synergistic effect of defect state and plasmon in Au nanoparticle-decorated hierarchical 3D porous ZnO microspheres

This study provides insights into the charge transfer and solar light photocatalytic activity induced by the synergistic effect between defect state and plasmon in Au nanoparticle-decorated hierarchical 3D porous ZnO microspheres. Photoluminescence emission shows that the photogenerated electrons of ZnO are efficiently transferred from its defect level to the conduction band (CB) induced through the coupling action of the defect state and plasmon. Compared with the pure ZnO microsphere photocatalysts, the fabricated Au/ZnO microsphere photocatalysts show remarkably enhanced activity for the decomposition of methylene blue dyes under solar-light illumination. Moreover, the photocatalytic performance of the Au/ZnO microspheres was manipulated by tuning the size of the Au nanoparticles (NPs). When decorated with 10 nm Au NPs, the Au/ZnO microsphere photocatalyst achieves a rate constant of 0.09846 min−1, 26 and 8.5 times higher than those of the pure ZnO microsphere photocatalysts (0.00378 min−1) and the Au/ZnO microsphere photocatalysts decorated with 30 nm Au NPs (0.01154 min−1), respectively.. The enhanced photocatalytic performance of the Au/ZnO microsphere photocatalysts is ascribed to the defect state-plasmon coupling-induced efficient transfer of photogenerated electrons from the defect level to the CB of ZnO. This work enhances the understanding of the defect state-plasmon coupling-induced enhanced photocatalytic activity in metal/semiconductor heterostructure photocatalysts.

Conductometric acetone vapor sensor based on the use of gold-doped three-dimensional hierarchical porous zinc oxide microspheres.

A versatile nanoprobe for acetone vapor was designed and fabricated. It is based on the use of gold-doped three-dimensional (3D) hierarchical porous zinc oxide microspheres (Au/ZnO HPMSs). The nanoprobe was synthesized by annealing zinc hydroxide carbonate precursor (obtained by a hydrothermal method) doped with gold nanoparticles. The resulting products possess a 3D open framework structure built of 2D porous nanosheets with a nanoporous wormhole-like shape. The microspheres doped with 0.5mol% gold display a good selectivity towards acetone. The conductometric nanoprobe, typically operated at a voltage of 5V, can detect sub-ppm levels of acetone, and the detection limit is as low as 0.2ppm. The response (at a level of up to 100ppm of acetone at 325°C) was high (74 ± 1.9), and the response and recovery time are 6 and 3s, respectively. This superior performance is ascribed (a) to the hierarchical porous ZnO architecture that warrants a large surface area; and (b) to the presence of gold nanoparticles that facilitate the chemisorption and dissociation of gas molecules. Graphical abstract Gold-doped 3D hierarchical porous ZnO microspheres (Au/ZnO HPMSs) architectures assembled by interconnected 2D porous nanosheets structures. The resistive sensor using these Au/ZnO HPMSs demonstrates outstanding acetone vapor sensing behaviors and 0.2ppm detection limits.

Hierarchical porous ZnO films synthesized by sol–gel method using triethylenetetramine stabilizer

Hierarchically porous ZnO films are synthesized through a novel sol–gel method using triethylenetetramine (TETA) as a stabilizer. Highly crystalline hierarchical porous ZnO thin films with different pore sizes are synthesized in various reaction mediums by altering solvents, sol concentration, the molar ratio of TETA to zinc acetate dehydrate (ZAD), and withdrawal speed. FESEM images show the formation of a hierarchical porous film with spherical and agglomerated particles in the optimized condition of ethanol as the solvent, sol concentration of 0.2 M, the molar ratio of [TETA]:[ZAD] = 2, and withdrawal speed of 30 mm/min. Increasing the solvent chain length, increasing the sol concentration, decreasing the TETA content, or increasing the withdrawal speed increases the particle size and porosity. XRD data, UV–Vis, and PL spectroscopy indicate high crystallinity with appropriate optical properties for the optimized sample. The hierarchical porous ZnO film synthesized by this method can be utilized as a photoelectrochemical electrode in devices such as photocatalysis and photoelectrochemical water splitting.

Preparation of Nb2O5-decorated hierarchical porous ZnO microspheres with enhanced photocatalytic degradation of palm oil mill effluent

In the present work, Nb2O5-decorated hierarchical porous ZnO microspheres (ZnO/Nb2O5) were successfully prepared through a facile surfactant-free method. The as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV–Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy and Brunauer–Emmett–Teller surface area analyses. Under UV light irradiation, the ZnO/Nb2O5 composites degraded palm oil mill effluent (POME) efficiently and demonstrated much higher photocatalytic activity compared to those of pure ZnO and Nb2O5. The enhanced photocatalytic degradation performance of ZnO/Nb2O5 composites was attributed to the high charge separation efficiency and hydroxyl radical generation ability as verified by the photoluminescence spectra. Phytotoxicity test upon the POME degradation over ZnO/Nb2O5 photocatalysis considerably decreased through the measurement of radicle lengths of Vigna radiata. Moreover, the ZnO/Nb2O5 composites were reused several times without appreciable loss of activity. This work revealed that the as-prepared ZnO/Nb2O5 composites have great potential for practical applications in the field of wastewater treatment.

On the high response towards TEA of gas sensors based on Ag-loaded 3D porous ZnO microspheres

In this paper, an ultra-high response and fast response/recovery triethylamine (TEA) gas sensor is fabricated by growing Ag nanoparticles on the surface of hierarchical 3D porous ZnO microspheres (Ag-ZnO) via a green and facile precipitation method. The morphological characterizations reveal that the hierarchical 3D ZnO microspheres are assembled by large numbers of thin and porous nanosheets. The sizes of the microspheres are 3–5 μm and the thickness of these porous nanosheets is about 20 nm. Gas sensing measurements show that the Ag-ZnO gas sensor exhibits superior TEA sensing performance. Its response to 100 ppm TEA is 6043, which is 14.8 times higher than that of pure ZnO sensor. Besides, its optimal working temperature decreases from 230 to 183.5 °C compared with the pure one. And the Ag-ZnO gas sensor also exhibits fast response/recovery rate (about 1 s and 60 s, respectively). Furthermore, the Ag-ZnO gas sensor shows no obvious degradation through a testing period of 30 days. The superior TEA sensing performances could be attributed to the structural advantage of hierarchical 3D porous microsphere, as well as the synergistic effects of Ag nanoparticles and ZnO microspheres.

Enhanced photoelectrochemical water splitting in hierarchical porous ZnO/Reduced graphene oxide nanocomposite synthesized by sol-gel method

Today the utilization of solar energy to split water and its conversion to hydrogen and oxygen has been considered as a powerful way to solve the environmental crisis. Hierarchical porous nanostructured ZnO and ZnO/reduced graphene oxide (rGO) composite photoanodes are synthesized by innovated sol-gel method using triethylenetetramine (TETA) as a stabilizer. The hierarchical porous ZnO structure containing large agglomerates each consisting of tiny nanoparticles are formed. The X-ray diffraction analysis and Raman spectroscopy confirm the in-situ reduction of graphene oxide sheets during synthesis and formation of ZnO/rGO nanocomposite. Although the band gap and transmittance of the porous nanocomposites do not dramatically change by rGO addition, the main photoluminescence peak quenches entirely showing prolonging exciton lifetime. The ZnO/rGO porous structure achieved remarkably improved current density (1.02 mA cm−2 at 1.5 V vs. Ag/AgCl) in 1 wt% rGO, up to 12 times higher compared to the bare ZnO (0.09 mA cm−2 at 1.5 V vs. Ag/AgCl), which attributes to positive role of ZnO hierarchical porous structure and rGO electron separation/transportation. These findings provide new insights into the broad applicability of this methodology for promising future semiconductor/graphene composite in the field of photoelectrochemical water splitting.

Hierarchical porous ZnO microflowers with ultra-high ethanol gas-sensing at low concentration

Hierarchical porous and non-porous ZnO microflowers have been successfully fabricated by hydrothermal method. Their crystal structure, morphology and gas-sensing properties were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical gas sensing intelligent analysis system (CGS). Compared with hierarchical non-porous ZnO microflowers, hierarchical porous ZnO microflowers exhibited ultra-high sensitivity with 50 ppm ethanol at 260 °C and the response is 110, which is 1.8 times higher than that of non-porous ZnO microflowers. Moreover, the lowest concentration limit of hierarchical porous ZnO microflowers (non-porous ZnO microflowers) to ethanol is 0.1 (1) ppm, the response value is 1.6 (1).

A facile route for fabrication of hierarchical porous Nb2O5/ZnO composites with enhanced photocatalytic degradation of palm oil mill effluent

A facile route was demonstrated for the synthesis of hierarchical porous ZnO microspheres based on a surfactant-free chemical solution method. The as-synthesized ZnO were assembled by large numbers of interleaving nanosheets and formed an open porous structure. Under UV irradiation, the as-synthesized ZnO exhibited photocatalytic property on the degradation of palm oil mill effluent (POME) solution. The Nb2O5 decorated ZnO photocatalysts (Nb2O5/ZnO) with enhanced photocatalytic performances were also synthesized via a facile and rapid route. Compared with ZnO, the Nb2O5(3 wt%)/ZnO exhibited the best POME degradation and colour removal efficiencies of 91.7% and 100%, respectively after 240 min irradiation. Phytotoxicity of the POME after Nb2O5/ZnO photocatalysis was significantly reduced via the measurement of radicle lengths of Vigna radiata. The observed results demonstrated the photocatalytic technology using hierarchical Nb2O5/ZnO composites had the potential to effectively purify wastewater.