Nanoporous ZnO Research Articles

Beneficial synergetic effect of feedstock characteristics and reaction conditions on bio crude production from hydrothermal liquefaction of mixed residential waste

Hydrothermal Liquefaction (HTL) is a promising method for sustainable waste management and renewable energy production, converting mixed feedstocks into bio-crude, a precursor to various biofuels. A study focused on Mixed Residential Waste (MRW) as an HTL feedstock investigated temperature ranges (280 °C-360 °C) and residence times (30 min–90 min), achieving a maximum bio-crude yield of 39.16% at 340 °C and 75 min. Also, a thorough investigation of the synergistic relationships between all subcomponents of the MRW feedstock was conducted and concluded that the mixed waste (MW) feedstock samples containing a higher proportion of food and plastic wastes and MRW sample presented with a co-liquefaction percentage (CE%) of around 60% and 107% respectively for production of bio crude. Also, solvents such as ethanol, glycerol and aqueous phase (AQ) were tested for their potential as hydrothermal mediums and found that bio crude yield of 46.19% was obtained in case of AQ phase recirculation. Further, the quantitative and qualitative effect of usage of four different catalysts were tested individually and in combination with AQ phase recirculation and found that, although individually Nanoporous ZnO and Diatomaceous Earth (DE) yielded bio crude in the range of 46.86% and 42.68% respectively, when used in combination, DE cat–HTL with AQ resulted in maximum bio crude yield of 54.35%. Furthermore, qualitatively, the bio crude from DE cat–HTL with AQ presented with a high carbon and energy recovery percentage of 62.20% and 72.95% respectively and a high hydrocarbon content of 58.98%.

Accessibility and Mechanical Stability of Nanoporous Zinc Oxide and Aluminum Oxide Coatings Synthesized via Infiltration of Polymer Templates.

The conformal nanoporous inorganic coatings with accessible pores that are stable under applied thermal and mechanical stresses represent an important class of materials used in the design of sensors, optical coatings, and biomedical systems. Here, we synthesize porous AlOx and ZnO coatings by the sequential infiltration synthesis (SIS) of two types of polymers that enable the design of porous conformal coatings-polymers of intrinsic microporosity (PIM) and block co-polymer (BCP) templates. Using quartz crystal microbalance (QCM), we show that alumina precursors infiltrate both polymer templates four times more efficiently than zinc oxide precursors. Using the quartz crystal microbalance (QCM) technique, we provide a comprehensive study on the room temperature accessibility to water and ethanol of pores in block copolymers (BCPs) and porous polymer templates using polystyrene-block-poly-4-vinyl pyridine (PS75-b-P4VP25) and polymers of intrinsic microporosity (PIM-1), polymer templates modified by swelling, and porous inorganic coatings such as AlOx and ZnO synthesized by SIS using such templates. Importantly, we demonstrate that no structural damage occurs in inorganic nanoporous AlOx and ZnO coatings synthesized via infiltration of the polymer templates during the water freezing/melting cycling tests, suggesting excellent mechanical stability of the coatings, even though the hardness of the inorganic nanoporous coating is affected by the polymer and precursor selections. We show that the hardness of the coatings is further improved by their annealing at 900 °C for 1 h, though for all the cases except ZnO obtained using the BCP template, this annealing has a negligible effect on the porosity of the material, as is confirmed by the consistency in the optical characteristics. These findings unravel new potential for the materials being used across various environment and temperature conditions.

Toward Industrial Production of a High-Performance Self-Powered Ultraviolet Photodetector Using Nanoporous Al-Doped ZnO Thin Films.

Al-doped ZnO (AZO) thin films are effective n-type semiconductors for ultraviolet (UV) detection because of their low cost, high electron mobility, and high sensitivity to UV light, especially in the UVA spectrum. However, a reasonable compromise between performance (such as sensitivity, detectivity, and response time) and fabrication ease remains an obstacle to the practicability of AZO-based UV photodetectors. To address this issue, we propose an efficient strategy to achieve a large AZO photoactive area for outstanding performance, along with a facile sol-gel method. Consequently, the device exhibits a superb on/off ratio of >104, a high detectivity of 1.85 × 1012 Jones, and a fast response speed under 365 nm UVA illumination without external energy consumption. Hence, this study suggests a self-powered and high-performance nanoporous AZO-based UVA detector with an environmentally friendly scalable process that satisfies industrial production requirements for numerous practical UV-detection applications.

Strategic enhancement of oxygen defects in ZnO from ZnS for water splitting to generate green electricity by hydroelectric cell

The green energy produced by the material device Hydroelectric Cell possesses a huge potential to transform the energy generation by water splitting. It is an efficient, clean and green electricity generating device accomplishing the ambitious Net Zero target. The present study reports a facile and unique process to obtain highly defective and nanoporous ZnO for water splitting at room temperature by strategic heat treatment of ZnS and lithium substituted ZnS. The creation of defects enhanced microstrain and dislocation density in ZnO has been confirmed by X-ray diffraction analysis. Surface and lattice defects presence have been distinctly evidenced by photoluminescence (PL) spectroscopy. The oxygen vacancy in ZnO and Li-ZnO has been also confirmed by the g value of 2.06 and 2.09 calculated from electron paramagnetic resonance (EPR) measurements. As soon as water is sprinkled on the Hydroelectric cell, the surface defects chemidissociate water into H3O+ and OH− ions. Hydronium ions trapped in nanopores create a high electric potential that sustains water splitting. Due to redox reactions at electrodes of Hydroelectric cell, voltage and electric current is generated. The fabricated hydroelectric cells (4 cm2 area) of pristine ZnO, ZnS converted ZnO, and Li-ZnO exhibited short circuit current, ISC 5.6 mA, 15 mA and 53 mA respectively by instant water splitting. Innovative method of oxygen defects creation in ZnO by heat treatment of bulk ZnS is an environment friendly, novel and simple non-photocatalytic technique to generate green electricity by water splitting at room temperature.

Effect of Au Doping on ZnO Nanoporous Structure for H2S Gas Sensing

This research mainly constitutes the fabrication of semiconductor gas sensors using high Power pulse magnetron sputtering (HiPIMS) technology to deposit ZnO thin films. As a doping substance, different thicknesses of gold plating is deposited on the surface of ZnO thin films. Various thicknesses of gold used in the study are 3.3 nm, 6.6 nm, 10 nm, 13.3 nm and 16.6 nm. The structural properties of thin films were identified by X-ray- diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). EDS analysis conveyed that gold particles filled the pores of nanoporous ZnO structures, which increased the films surface area and enhancing the gas sensing response. Various concentrations of H2S gas is used to test the gas sensing properties and the results proved the Au doped ZnO thin film sensor has enhanced sensing than the pure ZnO thin film sensor. 10 nm thickness doped Au has prime sensing properties with the operating temperature of 300 °C. Other sensing properties such as repeatability, selectivity and effect of humidity are tested and presented the data which shows the Au doping ZnO superiority over pure ZnO thin films.

Nanoporous ZnO structure prepared by HiPIMS sputtering for enhanced ozone gas detection

A high performance ozone gas sensor is fabricated using high power impulse magnetron sputtering (HiPIMS) process with nanoporous ZnO as the sensing material which is deposited on micro electro mechanical systems (MEMS) device. The morphology and nanostructure properties of ZnO were tested with XRD, SEM, TEM, EDS and XPS. As the sensor is tested at different temperatures, the sensor showed high sensing response at 200 °C operating temperature and with 400 °C annealed sample of nanoporous ZnO sensor. Low concentrations of ozone were chosen to obtain best possible sensing response characteristics ranging from 50 ppb to 1000 ppb. At 1000 ppb, sensor recorded a high 638% sensing response at 200 °C optimal temperature. Selectivity test and effects of relative humidity data is presented in the study, where sensor had high selectivity towards O3 gas and the sensing response showed decreasing trend with increase in relative humidity. The reason behind sensing mechanism of the nanoporous ZnO sensor is explained due to the adsorption and desorption phenomenon on the surface of ZnO sensor.

Sputter-Deposited Nano-porous ZnO Electrode for Highly Efficient Optoelectronic and Solid-State Energy Storage Devices

Sputter-Deposited Nano-porous ZnO Electrode for Highly Efficient Optoelectronic and Solid-State Energy Storage Devices

Enhanced photovoltage production from Canna dyes with surface passivation of ZnO based dye sensitized solar cells

Among the vast number of floras, Canna lily Red (CR) and Canna lily Yellow (CY) flower extracts were used as photosensitizers for natural dye-sensitized solar cells (NDSSC) with ZnO photoanode material. This study incorporates a blocking or compact layer (CL) below the nanoporous ZnO semiconducting metal oxide to reduce the device's recombination rate. A noticeable increment of J SC after adding a compact layer is observed for both the dyes; that is for CR dye from 0.43 to 0.47 mA/cm 2 , and for CY dye, 0.38 to 0.44 mA/cm 2 is obtained with a consistent high V OC of 0.54 to 0.60 V. Moreover, the device's charge collection efficiency has been enhanced from ∼ 93 to ∼ 97 % for CR dye and from ∼ 89 to ∼ 96 % for CY dye-loaded films after adding a compact layer. The enhancement in J SC , V OC , and fill factor ( FF ) is credited to increased electron lifetime, increased diffusion length, reduced series resistance ( Rs ), and exclusively enhanced recombination resistance among the FTO/electrolyte interface due to the compact layer. Despite visible defect states in the ZnO photoanode, a consistent photovoltage of 0.54 V to 0.6 V significantly manifests enhanced photovoltage production of Canna dyes in the device. The effect of compact layers on photovoltaic parameters is well established through the EIS study. • Canna dyes utilized as photosensitizers with ZnO photoanode in DSSC. • The rate of recombination and back transfer of charge carriers was reduced. • V OC and J SC are tunable with electrochemical impedance spectroscopy. • Photovoltage of 0.55V - 0.6V production is the iconic feature of these canna dyes.

Synthesis and Characterization of Nanoporous ZnO Films by Controlling the Zn Sublimation by Using ZnO/Zn Precursor Films.

A reliable process for the formation of nanoporous ZnO films supported on amorphous quartz and (100) silicon substrates via the processing of ZnO/Zn precursor films is reported. The process is based on the sublimation mechanism of Zn implemented in a novel ZnO/Zn precursor film to produce a nanoporous film. A scanning electron microscopy analysis of the nanoporous ZnO films’ surfaces revealed the presence of ZnO nano-features with round tips; in contrast, the nanoporous ZnO films supported on (100) Si substrates showed hexagonal nut-like nanostructures. The crystallite size of the nanoporous ZnO films decreased as the sublimation temperature was increased. X-ray photoelectron spectroscopy studies demonstrated that formations of oxygen vacancies were produced during the processing stages (as the main structural lattice defects in the ZnO nanoporous films). The analysis of the photoluminescence response confirmed that the active deep-level centers were also related to the oxygen vacancies generated during the thermal processing of the ZnO/Zn precursor films. Finally, a qualitative mechanism is proposed to explain the formation of nanoporous ZnO films on quartz and crystalline Si substrates. The results suggest that the substrates used have a strong influence on the nanoporous ZnO structures obtained with the Zn-sublimation-controlled process.

Dark nanostructured ZnO films formed by anodic oxidation as photoanodes in photoelectrochemical water splitting

Zinc oxide, due to its unique properties is very often considered as an alternative for titanium dioxide commonly used in photoelectrochemical and photocatalytic applications. In this paper, we present, for the first time, a complex characterization of the photoelectrochemical activity of dark ZnO layers formed by anodic oxidation of metallic Zn in alkaline electrolyte. A special emphasis was put on finding any relations between anodizing duration, morphology of the obtained layers, their defective nature, and photoelectrochemical activity. The research strategy included detailed analysis of morphology of the nanostructured ZnO, their physicochemical characterization (e.g., absorption and luminescence properties), and finally, extensive photoelectrochemical measurements. It was found that dark nanoporous ZnO layers are formed during anodization of Zn at the potential of 2 V in 1 M NaOH. However, if the process is longer than 15 min, the surface of the anodically generated film is gradually covered with a layer of ultra-thin nanowires. The highly defective nature of ZnO films was confirmed using photoluminescence measurements, and the number of defects increases significantly with time of anodization. On the other hand, further thermal treatment in air at 200 °C resulted in the gradual elimination of defects, especially those responsible for visible emission. The best photoelectrochemical performance was achieved for the oxide films with a thickness of c.a. 1.6 µm for both as-anodized and annealed samples. The formation of “lawn” nanowires on the surface of the nanoporous film did not cause any significant improvement in the photoelectrochemical properties of the materials.

Synthesis of nano-porous ZNO as pigment for white silicone thermal control coating and investigation of structure, optical and thermal properties

PurposeThe harsh environment of space, especially radiation of direct solar rays, can potentially raise the temperature of the spacecraft to harmful levels. Thermal control coatings (TCCs) fix the thermal condition of the spacecraft acceptable for its components. This is possible by diffusely reflecting all effective ultraviolet (UV), visible (VIS) and near infrared (IR) (NIR) wavelengths of solar radiation and emmition of IR energy. The most commonly used TCCs have used ZnO as a pigment, but absorption of the UV light by ZnO pigment can change the ideal condition of these TCCs. The aim of his study is the using the porous ZnO particles as pigment to prevent the UV absorption.Design/methodology/approachTo enhance the efficiency of these coatings, in the present study, nano-porous zinc oxide particles were synthesized and used as pigments for white TCCs.FindingsThe results revealed that the proposed TCC (TPZ), Thermal control coating with porous ZnO had better reflection (scattering) and emittance properties in comparison with the coating using ZnO as a pigment (TZ coating); so this coating had a solar absorptance value equal to 0.141, whereas this value for TZ was 0.150. Furthermore, TPZ showed higher thermal emittance (0.937) in comparison with TZ (0.9). These changes were because of the improvement in the refractive index, shape and surface area of the pigments. The general trend of the scattering coefficients for the prepared coating, as calculated from the Kubelka–Munk equation, showed that scattering was more efficient in the UV region, as compared with the TCC containing ZnO pigments.Originality/valueThis type of pigment for the first time is evaluated in TCCs.

Transformation of Wurtzite ZnO to a New Triclinic Nanoporous ZnO Phase via Hydrothermal Treatment with Metformin for Designing Proton Conducting Material.

Zinc oxide is one of the most widely studied semiconductor metal oxides, which predominantly crystallizes as hexagonal wurtzite and often cubic zinc-blende phases. Here we report the transformation of the highly stable wurtzite ZnO to a new triclinic phase NZO-2 by using metformin as a template during post-synthesis hydrothermal treatment. This crystalline phase of the material NZO-2 has been identified through the refinement of the powder XRD data. NZO-2 possesses porous rod like particle morphology consisting of the self-assembly of 3-7 nm size spherical nanoparticles and interparticle nanoscopic voids spaces. NZO-2 has been surface phosphorylated and the resulting material displayed good proton conductivity. Further, NZO-2 displayed ultra-low band gap of 1.74 eV, thereby responsible for red emission under high energy laser excitation and this may open new opportunities in optoelectronic application of ZnO.

Pyrolysis Degradation of Cellulose over Highly Effective ZnO and ZnO−CuO Nanocatalysts

AbstractPyrolysis of lignocellulosic biomass with the use of appropriative catalysts can lead to the production of high yields of fuels ‐ bio‐oils. Here, zinc oxide ‐ copper oxide (ZnO−CuO) nanocatalysts were synthesized by solvothermal synthesis. High‐angle annular dark‐field imaging scanning transmission electron microscopy (HAADF‐STEM), high‐resolution transmission electron microscopy (HRTEM), and energy‐dispersive X‐ray spectroscopy (EDXS) results suggested that ZnO−CuO nanoparticles (D=23±5 nm) exhibit porous nanostructure. The pyrolytic degradation of cellulose using pyrolysis‐gas chromatography‐mass spectrometry (Py‐GC/MS) unit has been studied over ZnO and ZnO−CuO nanocatalysts at the temperature range 400–800 °C. The activation energy of ZnO−CuO (67.21 and 70.04 kJ/mol) was lower by 30 kJ/mol from the activation energy of clean ZnO and the calculated rate constants showed that the cellulose pyrolytic reaction is faster using ZnO−CuO catalyst. Nanoporous ZnO−CuO shifted the products maximum towards lower temperatures (<500 °C), reduced the content of aldehydes at 400–500 °C and enhanced the overall product composition and bio‐oil yield. Porous structure of ZnO nanocatalysts had a significant effect on the product selectivity and reaction mechanism of cellulose pyrolysis.

Green synthesis of spongy Nano-ZnO productive of hydroxyl radicals for unconventional solar-driven photocatalytic remediation of antibiotic enriched wastewater

Herein, novel green/facile approach to synthesize spongy defective zinc oxide nanoparticles (ZnONPs) is presented using for the first time pomegranate seeds molasses as a green capping fuel/reducing mediator during an aqueous solution combustion process. The developed ZnONPs is characterized by UV–Vis. Spectrophotometry and fluorimetry, XRD, Raman spectroscopy, SEM, TEM and BET. Interestingly, pomegranate seeds molasses within a viable content of bio-capping molecules reveal a defective nanoporous ZnO NPs of smaller particle size, greater pore size/volume, and higher surface area compared to the bulky non-biogenic ZnONPs. Moreover, the biosynthesized defective ZnONPs showed narrowed band gap and higher absorption of visible photons that breed higher density of hydroxyl radicals (•OH) under Solar-illumination. Even further, the bulk ZnO and the biosynthesized ZnO photocatalysts were examined in photodegrading flumequine (FL) antibiotic. The bulk ZnO gives 41.46% photodegradation efficiency compared to 97.6% for the biosynthesized ZnO. In highly acidic or highly alkaline media, FL photodegradability is greatly retarded. Scavenging experiment infers considerable contribution of holes over electrons in photodegradation reaction. The biosynthesized ZnO shows high durability in FL photodegradation after four reusing cycles. These promising findings highlight new insights for biogenic synthesis of tuned size/controlled morphology semiconductor NPs relevant to environmental remediation applications.

Impedance Spectroscopy Analysis of Structural Defects in Sputtered ZnO Films

AbstractThe degradation of sputtered columnar ZnO layers under DC polarization was studied by using electrochemical impedance spectroscopy and electron microscopy. It was found that the structure of the as‐deposited ZnO film was dense at the nanoscale. An equivalent circuit model including de Levie impedance accounted for the localized propagation of microscale cracks towards the copper substrate. This generates a capacitance (CZnO) that represents the crack surface area in contact with the electrolyte. CZnO is small enough not to be obscured by the double layer capacitance at the top of the layers and increases with increasingly negative potential and time. These results were compared to nanoporous ZnO layers that behave differently and exhibit a large CZnO. The combination of in situ EIS analysis with the ex situ structural information provided by electron microscopy proved to be an efficient methodology to characterize very different microstructures of conductive coatings.

Synthesis of Porous ZnO Films on Quartz Substrates by Thermal Oxidation and the Oxidant Atmosphere Effect

The synthesis and characterization of porous ZnO films using a two stage process by thermal oxidation using a bilayer precursor film (ZnO/Zn) consisting of a Zn film covered with a ZnO nanofilm formed on quartz substrates is reported. The Zn films of 50 nm were grown by DC sputtering method at 300K. In the first stage bilayer precursor films (BPF) of ZnO/Zn were produced by growing a ZnO nanofilm on Zn films by thermal oxidation at 350 °C by 30 min in N2 atmosphere containing 5 ppm of O2, and in the second stage the BPFs were oxidized at 800 °C for one hour either in dry N2, dry or wet air with 42% of humidity. The produced porous ZnO films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Vis spectroscopy measurements. The results revealed the role of the oxygen content and the relevance of the humidity content in the processing atmosphere. When the BPF was oxidized in N2 with low oxygen content nanoporous ZnO films of wurtzite phase with its c-axis perpendicular to the plane of the substrate were produced. When rich oxygen content oxidation atmospheres were used, either in dry or wet air, nanoporous ZnO films with three main crystallite orientations (100), (002) and (101), were produced. The optical transmittance characteristics at the band edge region were strongly influenced by the humidity content but induce the formation of reproducible nanoporous ZnO films with sizes of ≈ 10 nm.

Manipulating the ion-transfer kinetics and interface stability for high-performance zinc metal anodes

We report a new class of Zn anodes modified by a three-dimensional nanoporous ZnO architecture (Zn@ZnO-3D), which can accelerate the kinetics of Zn2+ transfer and deposition, inhibit dendrite growth, and reduce the side-reactions.

Facile fabrication of ZnO/C nanoporous fibers and ZnO hollow spheres for high performance gas sensor

The porous and hollow nanostructure materials have drawn consideration attention due to their broader use. Herein, the ZnO/Carbon nanoporous fibers (ZnO/C nanoporous fibers) and ZnO hollow sphere were synthesized simultaneously by using activated carbon fibers (ACF) as an efficiently template. Gas sensors were made based on ZnO/C nanoporous fibers, ZnO hollow sphere, Ag doped ZnO hollow spheres (Ag/ZnO) and Pt decorated ZnO hollow spheres (Pt/ZnO), respectively. Studies found that ZnO/C nanoporous fibers possess better gas-sensing properties than ZnO hollow sphere, the responses of the ZnO/C nanoporous fibers to 100 ppm ethanol and acetone are 53.222 and 59.273 at the optimum operating temperature. The Ag/ZnO and Pt/ZnO exhibited significantly enhanced gas sensing performance, including 4.58 times and 8.35 times higher responsivity to ethanol than ZnO hollow spheres. Furthermore, the four types of gas sensors exhibit higher sensing properties than other ZnO based gas sensors. Hence, our results should be useful for creating high efficiency gas sensing devices.

Synthesis and characterization of nanoporous ZnO and Pt/ZnO thin films for dye degradation and water splitting applications

In this paper, nanoporous ZnO and Pt/ZnO films were synthesized by a combination of simple spray pyrolysis and DC sputtering techniques at different flow rates and deposition times. X-ray diffraction, contact angle, and spectrophotometric measurements showed the growth of hexagonal ZnO Wurtzite phase with hydrophilic nature and high transparency in the visible region. By sputtering Pt nanoparticles on ZnO surfaces, the absorption edge is redshifted from UV region to the visible light region. The optical band gap of Pt/ZnO films is decreased from 3.74 to 1.86 eV as the deposition time increased to 4 min. The Pt/ZnO films exhibited higher photocatalytic activity and stability than pure ZnO films for methylene blue photodegradation. From the photoelectrochemical water splitting measurements, the current density of 4 min Pt/ZnO film is one hundred times greater than the value of pure ZnO film. The incident photon-to-current conversion efficiency at 470 nm was for the first time 76.2%.

Conversion Reaction of Nanoporous ZnO Forstable Electrochemical Cycling of Binderless Simicroparticle Composite Anode

Binderless, additive-less Si electrode design is developed where a nanoporous ZnO matrix is coated on a Si microparticle electrode to accommodate extreme Si volume expansion and facilitate stable electrochemical cycling. The conversion reaction of nanoporous ZnO forms an ionically and electrically conductive matrix of metallic Zn embedded in Li2O that surrounds the Si microparticles. Upon lithiation, the porous Li2O/Zn matrix expands with Si preventing extensive pulverization while Zn serves as active material to form LixZn to further enhance capacity. Electrodes with Si mass loading of 1.5 mg/cm2 was fabricated and high initial capacity of ~3,900 mAh/g was achieved with excellent reversible capacity of ~1,500 mAh/g (areal capacity ~1.7 mAh/cm2) beyond 200 cycles. A high first cycle coulombic efficiency was obtained owing to the conversion reaction of nanoporous ZnO, which is a notable feature in comparison to conventional Si anodes. Ex situ analyses confirmed that the nanoporous ZnO coating maintained the coalescence of SiMPs throughout extended cycling. Therefore, the Li2O/Zn matrix derived from conversion reacted nanoporous ZnO acted as an effective buffer to lithiation induced stresses from volume expansion and served as a binder-like matrix that contribute to the overall electrode capacity and stability.