Sol-gel ZnO Research Articles

Ultraviolet response of ZnO nanorod/Sb-doped ZnO thin film homojunction structure prepared by all-chemically wet process

In this work, ZnO nanorod/Sb-doped ZnO thin film structure was fabricated and its relevant properties were investigated. The device was prepared by Sb-doped ZnO sol-gel spin coating technique onto ITO substrate annealed in nitrogen atmosphere followed by the hydrothermal process to obtain ZnO nanorod. The crystal structure demonstrates (002) plane dominant hexagonal wurtzite ZnO without phase impurity. The deterioration in the crystal structure of Sb-doped ZnO due to the Sb is incorporated in the ZnO lattice. The element component in each layer is confirmed by depth profile measurement. The low transmittance in the visible region is due to high reflection at the surface of the ZnO nanorod. The electrical measurement confirms that ZnO nanorod/Sb-doped ZnO thin film has p-n homojunction properties under dark conditions and 18 times higher photovoltage than the Sb-doped/undoped ZnO double layer device under UV LED irradiation due to enhanced photon harvesting of ZnO nanorod structure.

Enhanced Photocatalytic Performance under Ultraviolet and Visible Light Illumination of ZnO Thin Films Prepared by Modified Sol-Gel Method.

Semiconductor oxides are frequently used as active photocatalysts for the degradation of organic agents in water polluted by domestic industry. In this study, sol-gel ZnO thin films with a grain size in the range of 7.5-15.7 nm were prepared by applying a novel two-step drying procedure involving hot air treatment at 90-95 °C followed by conventional furnace drying at 140 °C. For comparison, layers were made by standard furnace drying. The effect of hot air treatment on the film surface morphology, transparency, and photocatalytic behavior during the degradation of Malachite Green azo dye in water under ultraviolet or visible light illumination is explored. The films treated with hot air demonstrate significantly better photocatalytic activity under ultraviolet irradiation than the furnace-dried films, which is comparable with the activity of unmodified ZnO nanocrystal powders. The achieved percentage of degradation is 78-82% under ultraviolet illumination and 85-90% under visible light illumination. Multiple usages of the hot air-treated films (up to six photocatalytic cycles) are demonstrated, indicating improved photo-corrosion resistance. The observed high photocatalytic activity and good photo-corrosion stability are related to the hot air treatment, which causes a reduction of oxygen vacancies and other defects and the formation of interstitial oxygen and/or zinc vacancies in the films.

Sol-gel derived ZnO film as a gas sensor: Influence of UV processing versus a thermal annealing

While preparing oxide layers as gas sensors by a sol-gel approach, a high-temperature annealing makes a challenge to apply in numerous applications like flexible electronics with a heavy influence on the oxide microstructure. Therefore, its replacing by UV irradiation combined with a mild heating as “photo-annealing” paves the way to develop soft protocols when designing oxide-based gas sensors bearing a fine nanocrystallinity. Herein, we consider hierarchical sol-gel derived ZnO films which were a subject of conventional annealing and photoannealing to compare their gas-sensor performance when exposed to alcohol vapors. It is found that films obtained by photoannealing have an X-ray amorphous character, in contrast to ones being thermally annealed; although, the hierarchical organization of both samples revealed by SEM is almost identical. The DFTB modeling performed for ZnO crystal exposed to alcohol molecules and water has indicated the chemiresistive effect to be enhanced with a molecular weight of analytes. These observations were validated in experiment with sol-gel ZnO layers which exhibited an alcohol response in sub-ppm concentration range down to 10 ppb. To selectively compare the impact of various alcohols, we successfully applied a linear-discriminant analysis to the vector signal of the on-chip multisensor array.

Solution-Processed Bilayered ZnO Electron Transport Layer for Efficient Inverted Non-Fullerene Organic Solar Cells

Organic solar cells (OSCs) are becoming increasingly popular in the scientific community because of their many desirable properties. These features include solution processability, low weight, low cost, and the ability to process on a wide scale using roll-to-roll technology. Enhancing the efficiency of photovoltaic systems, particularly high-performance OSCs, requires study into not only material design but also interface engineering. This study demonstrated that two different types of OSCs based on the PTB7-Th:IEICO-4F and PM6:Y6 active layers use a ZnO bilayer electron transport layer (ETL). The ZnO bilayer ETL comprises a ZnO nanoparticle (ZnO NP) and a ZnO layer created from a sol-gel. The effect of incorporating ZnO NPs into the electron transport layer (ETL) was studied; in particular, the effects on the electrical, optical, and morphological properties of the initial ZnO ETL were analyzed. The ability of ZnO films to carry charges is improved by the addition of ZnO nanoparticles (NPs), which increase their conductivity. The bilayer structure had better crystallinity and a smoother film surface than the single-layer sol-gel ZnO ETL. This led to a consistent and strong interfacial connection between the photoactive layer and the electron transport layer (ETL). Therefore, inverted organic solar cells (OSCs) with PTB7-Th:IEICO-4F and PM6:Y6 as photoactive layers exhibit improved power conversion efficiency and other photovoltaic properties when using the bilayer technique.

Therapeutic Potential of Sol-Gel ZnO Nanocrystals: Anticancer, Antioxidant, and Antimicrobial Tri-Action.

Zinc oxide nanocrystals (ZnO NCs) hold great promise in nanomedicine with fascinating multifunctional properties. We investigated the therapeutic potential of sol-gel synthesized ZnO NCs with crystal sizes of 52.65 and 25.11 nm, focusing on their anticancer effects on HepG2 and HT29 cells, antioxidant properties, and antimicrobial activity. Both samples displayed a hexagonal wurtzite ZnO structure, wherein the crystal sizes diminished with lower calcination temperatures according to X-ray diffraction. The scanning electron microscopy analysis revealed that lowering the calcination temperature resulted in a decrease in the grain size of the ZnO NCs, as expected. This reduction in grain size combined with a decrease in crystal size resulted in a significant 40% reduction in the reflectance of the ZnO NCs in UV-vis-NIR spectroscopy. It was also observed that the ZnO NCs calcined at higher temperatures exhibited larger particle sizes with a reduced surface area mean of 69.30 μm and a stable negative zeta potential of -11.2 mV. In contrast, the ZnO NCs calcined at lower temperatures exhibited a larger surface area mean of 34.56 μm and a positive zeta potential of +10 mV. In both cell lines, the cytotoxic potential was found to be higher in HepG2 cells. Specifically, when ZnO nanocrystals (NCs) with a crystal size of 52.65 nm were used, the lowest cell viability was observed at a concentration of 5.74 μg/mL. Based on oxidative stress index values, a lower crystal size of ZnO NCs displayed greater effectiveness in HT29 cells, while a higher crystal size of ZnO NCs had pronounced effects in HepG2 cells. Moreover, both ZnO NCs exhibited significant antimicrobial activity against Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and Candida parapsilopsis fungus. These findings emphasize sol-gel ZnO NCs' potential as versatile agents in nanomedicine, spurring research on targeted cancer therapies and antimicrobial innovations.

Influence of crystallographic facet orientations of sol-gel ZnO on the photocatalytic degradation of p-nitrophenol in water

Influence of crystallographic facet orientations of sol-gel ZnO on the photocatalytic degradation of p-nitrophenol in water

Synthesis, characterization, and photocatalytic activities of green sol-gel ZnO nanoparticles using Abelmoschus esculentus and Salvia officinalis: A comparative study versus co-precipitation-synthesized nanoparticles

BackgroundThe development of green chemistry methods involving plant-based nanoparticle synthesis presents an affordable and eco-friendly approach for wastewater treatment and color removal. This study aimed to synthesize ZnO nanoparticles using the sol-gel method with Salvia officinalis and Abelmoschus esculentus plants, examining their photocatalytic efficiency for organic dye removal. MethodsTo compare the properties of ZnO nanoparticles, another type of ZnO-NPs was synthesized using the co-precipitation method. The characterization of synthesized nanoparticles was performed using thermogravimetric analysis (TGA-DTG), X-ray diffraction (XRD), Dynamic Light Scattering (DLS), Zeta potential (ZP), field emission scanning electron microscopy (FE-SEM), Energy Dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), and UV–Vis spectrophotometry. ResultsBased on XRD results, the average crystalline size of nanoparticles was calculated using the Debye-Scherer equation for synthesized nanoparticles using the S. officinalis at 22.99 nm and for the A. esculentus at 29.79 nm, and for the co-precipitation method at 18.83 nm. The FE-SEM images showed spherical ZnO nanoparticles. Photocatalytic properties of ZnO-NPs were investigated for remove of methylene blue organic dye in the presence of UV light. The pH 10 was identified as the best pH, which had the highest percentage of color degradation. All three types of nanoparticles were tested by up to 360 min to optimize the dyeing time. For A. esculentus, the highest percentage of color removal occurred in the first 90 min (41.0 %), for S. officinalis nanoparticles between 75 and 90 min (86.9 %), and for chemically synthesized nanoparticles between 30 and 45 min (100 %). ConclusionsIn conclusion, the best MB dye degradation capacity belonged to co-precipitation ZnO nanoparticles followed by S. officinalis and A. esculentus nanoparticles.

Spectroscopic study on the influence of post-processing annealing on ZnO films produced with a sol-gel method

Zinc oxide is still one of the most crucial wide bandgap semiconductors for electronics. It is highly used because of its ease of production, doping, and low cost. However, in the production of zinc oxide materials, it is necessary to consider the formation of native defects, which strongly influence the material's properties. Therefore, it is essential to understand the behavior in thin films during the annealing process.The presented work shows the appearance of oxygen and zinc vacancies during annealing at specific temperatures and times. Samples annealed in an oxygen-rich atmosphere change their structural and chemical properties. Specific defects may be detected using Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. In addition, energy bandgap, defects energy, and the thermal dependence of luminescence emission were analyzed as the primary indicators of the changes during the sintering.

Effect of Pb doping on the structural, optical and electrical properties of sol–gel ZnO nanoparticles

Pure and Pb-doped ZnO NPs were synthesized using the sol–gel method. The structural and morphological properties were investigated via XRD, FTIR and FE-SEM. A detailed study of the effect of Pb on the properties of ZnO NPs ascertained many interesting results. XRD analysis showed that doped and undoped samples have hexagonal wurtzite structures and the average grain size increases from 21.2 to 27.8 nm with doping. Morphological analysis of undoped and Pb-doped ZnO NPs indicated that they are composed of quasi-spherical particles. Optical and electrical characteristics were also evaluated by UV–Vis spectroscopy and electrical measurements. These studies revealed that the Pb doping resulted in a blue shift of the absorption edge and thus increasing the gap band, from 3.39 to 3.53, and the electrical resistance. Based on these characteristics, the possible use of the sol–gel synthesized Pb–ZnO for conductometric gas sensing, was also discussed.

Improved optoelectrical properties for organic solar cells by introducing silicon quantum dots via eco-friendly and simple process

In this study, we synthesized eco-friendly silicon quantum dots (SiQDs) with absorption ≤400 nm and light emission of 440 nm and 500–550 nm. SiQDs were introduced into ZnO (sol-gel), an electron transport layer (ETL) of organic solar cells (OSCs) with an inverted structure.The introduction of SiQDs led to a smoother and more uniform surface morphology of the ETL. SiQDs improved the carrier mobility of the device, and increased carrier transport by reducing carrier recombination and interfacial resistance. Furthermore, SiQDs were found to convert ultraviolet (UV) light to visible light via light conversion effect. They contributed to improved JSC and FF in OSCs, and improved the power conversion efficiency (PCE) from 14.48 % to 15.31 % for the PM6:BTP-eC9 device (JSC: 24.42 mA cm−2 to 24.87 mA cm−2, FF: 73.05 %–75.23 %) and from 15.43 % to 16.3 % for the PM6:L8-BO device (JSC: 23.37 mA cm−2 to 24.57 mA cm−2, FF: 75.95 %–76.77 %).SiQD-doped ZnO, with eco-friendly synthesized SiQDs, not only improved the efficiency with the ETL of OSCs, but also demonstrated good stability.

Interface modification of MoS2/ZnO heterojunction for controllable electron flow and fabrication of high-performance UV photodetectors with fast response/recovery speed

Hetero-structured thin films, MoS2/ZnO thin films, were fabricated by adding MoS2 films under the ZnO thin films via sol-gel spin-coating method; ZnO nanoparticles were observed on the surface of sol-gel spin-coated ZnO thin films. Although ZnO nanoparticles were observed on the surface of hetero-structured thin films, the surface area of the hetero-structured thin films was higher than ZnO thin films owing to the high surface roughness of the MoS2 films. Three diffraction peaks were observed in the hetero-structured thin films at 31.74°, 34.38°, and 36.22°, which indicates heterostructure was polycrystalline, and the heterostructure was formed successfully. When the UV photodetector was fabricated by using hetero-structured thin films, the enhanced values of photocurrent, photoresponsivity and photosensitivity were observed. In addition, the hetero-structured thin films exhibited a faster response/recovery speed because the move of electrons and holes during UV illumination. Thus, it was concluded that the interface modification of the MoS2/ZnO heterojunction is an effective solution for overcoming the drawbacks of ZnO-based UV photodetectors. This approach can be considered a novel method for high-performance optoelectronics with fast response speed applicable to various fields.

ZnO/TiO2 Composite Thin-Film Photocatalysts for Gas-Phase Oxidation of Ethanol

The aim of this project is to investigate the photocatalytic activity of ZnO/TiO2 composite films for the gas-phase oxidation of ethanol. Pure TiO2, pure ZnO, and their composites were formulated using a sol-gel synthesis method, and the resulting powders were cast and dried as thin films in a flat-plate ultraviolet light-emitting diode (UV LED) reactor. P25 TiO2 and commercially available ZnO were also used for comparison. The structural, morphological, and optical characteristics of the materials were characterized. The photocatalytic oxidation of ethanol vapors in air after 24 h of reactor operation was used to assess the relative photocatalytic activity of the ZnO/TiO2 composite films. Our results show that as ZnO content increased in the coupled semiconductor materials, the apparent photocatalytic activity decreased. In this study, pure ZnO (both sol-gel and purchased) had the least photocatalytic activity for vapor-phase ethanol oxidation in our test apparatus. For gas-phase photocatalysis, surface area was shown to be a critical feature for photocatalytic activity. However, the inherent photocatalytic activity of the materials was not decoupled from the effects of surface area in this study.

Insecticide Monitoring in Cattle Dip with an E-Nose System and Room Temperature Screen-Printed ZnO Gas Sensors

Taktic, an Amitraz-based insecticide, is commonly used in sub-Saharan Africa to treat cattle for ticks. Due to misuse in rural dipping pools, some ticks are showing resistance to Taktic. This work presents a low-cost e-nose with commercial sensors to monitor Taktic levels in dipping pool water. The device shows distinctly different measurements for the odours of air, distilled water, farm water, and four levels of Taktic insecticide in farm water. A naive Bayes algorithm with a Gaussian distribution is trained on the data and a validation set achieves a 96.5% accuracy. This work also compares two sol-gel ZnO nanoparticle solutions with an off-the-shelf ZnO nanoparticle ink for use as active material in chemiresistive gas sensors to be employed in an e-nose array. The ZnO solutions are screen-printed onto gold electrodes, auto-sintered with a built in heater, and used with UV illumination to operate as low-power, room temperature gas sensors. All of the screen-printed ZnO sensors show distinct changes in resistance when exposed to Taktic vapours under room temperature and humidity conditions. The custom room temperature ZnO gas sensors fabricated via facile and low-cost processes are suitable for future integration in a point-of-need microsystem for the detection of Taktic in water.

Optical, luminescence, photocurrent and structural properties of sol-gel ZnO fibrous structure thin films for optoelectronic applications: A combined experimental and DFT study

Zinc oxide is an important oxide semiconductor, with excellent physical and chemical properties that are used in different applications fields. In this research paper, ZnO thin films were synthesized using different types of solvent by sol-gel spin coating technique. The effect of solvent on the structural, morphological, photocurrent and optical properties of the films has been studied. X-ray diffraction revealed the high crystallinity of films in the wurtzite structure with a c-axis preferred orientation. SEM micrographs revealed that all films exhibit a fibrous structure, where the more homogeneous and dense surfaces were observed for ZnO-2-PrOH and ZnO–EtOH films. The optical properties analysis shows that the films have high transparency. Different structural and optical parameters were influenced by the type of solvent, where ZnO-2-PrOH film displays an improvement in structural and optical properties. The photoluminescence PL spectrum revealed three emissions, two green emissions and a UV emission at 390 nm. ZnO-2-PrOH film exhibits a high intensity ratio of UV to visible (IUV/Ivis), a best UV response, and photocurrent properties. Finally, structural and optical parameters were also calculated by first-principles DFT calculations, discussed, and compared to those experimental.

Outstanding Fill Factor in Inverted Organic Solar Cells with SnO2 by Atomic Layer Deposition.

Transport layers are of outmost importance for thin-film solar cells, determining not only their efficiency but also their stability. To bring one of these thin-film technologies toward mass production, many factors besides efficiency and stability become important, including the ease of deposition in a scalable manner and the cost of the different material's layers. Herein, highly efficient organic solar cells (OSCs), in the inverted structure (n-i-p), are demonstrated by using as electron transport layer (ETL) tin oxide (SnO2) deposited by atomic layer deposition (ALD). ALD is an industrial grade technique which can be applied at the wafer level and also in a roll-to-roll configuration. A champion power conversion efficiency (PCE) of 17.26% and a record fill factor (FF) of 79% are shown by PM6:L8-BO OSCs when using ALD-SnO2 as ETL. These devices outperform solar cells with SnO2 nanoparticles casted from solution (PCE 16.03%, FF 74%) and also those utilizing the more common sol-gel ZnO (PCE 16.84%, FF 77%). The outstanding results are attributed to a reduced charge carrier recombination at the interface between the ALD-SnO2 film and the active layer. Furthermore, a higher stability under illumination is demonstrated for the devices with ALD-SnO2 in comparison with those utilizing ZnO.

Nanopatterned rGO/ZnO:Al seed layer for vertical growth of single ZnO nanorods

In this work, we demonstrate a novel low-cost template-assisted route to synthesize vertical ZnO nanorod arrays on Si (100). The nanorods were grown on a patterned double seed layer comprised of reduced graphene oxide (rGO) and Al-doped ZnO nanoparticles. The seed layer was fabricated by spray-coating the substrate with graphene and then dip-coating it into a Al-doped ZnO sol–gel solution. The growth template was fabricated from a double-layer resist, spin-coated on top of the rGO/ZnO:Al seed layer, and patterned by colloidal lithography. The results show a successful chemical bath deposition of vertically aligned ZnO nanorods with controllable diameter and density in the nanoholes in the patterned resist mask. Our novel method can presumably be used to fabricate electronic devices on virtually any smooth substrate with a thermal budget of 1 min at 300 °C with the seed layer acting as a conductive strain-relieving back contact. The top contact can simply be made by depositing a suitable transparent conductive oxide or metal, depending on the specific application.

Enhanced performance of inverted polymer solar cells by adding benzyl viologen dichloride into ZnO electron transport layer

An organic-inorganic hybrid electron transport layer (ETL) is developed using benzyl viologen dichloride (BV–2Cl) as a dopant for the preparation of sol-gel ZnO precursor solution. The inverted polymer solar cells (I–PSCs) based on PTB7-Th:PC71BM were prepared by introducing hybrid ZnO:BV-2Cl as ETL, and the effect of BV-2Cl on the photovoltaic performance of PSCs was closely investigated. Consequently, the device with hybrid ZnO:BV-2Cl (1% wt%) as ETL achieves the power conversion efficiency (PCE) of 9.31%, which is 16.5% more efficient than the device using pure ZnO as ETL. As the results indicate, hybrid ZnO:BV-2Cl films can modulate the interfacial compatibility of the hydrophilic surface of conventional ZnO films, hence enhancing charge transport and exciton dissociation. Our results indicate that using BV-2Cl as a dopant for the preparation of ZnO ETL is a facile and efficient method of fabricating high-performance I–PSCs.

High-Performance Colloidal Quantum Dot Photodiodes via Suppressing Interface Defects.

PbS colloidal quantum dot (CQD) infrared photodiodes have attracted wide attention due to the prospect of developing cost-effective infrared imaging technology. Presently, ZnO films are widely used as the electron transport layer (ETL) of PbS CQDs infrared photodiodes. However, ZnO-based devices still suffer from the problems of large dark current and low repeatability, which are caused by the low crystallinity and sensitive surface of ZnO films. Here, we effectively optimized the device performance of PbS CQDs infrared photodiode via diminishing the influence of adsorbed H2O at the ZnO/PbS CQDs interface. The polar (002) ZnO crystal plane showed much higher adsorption energy of H2O molecules compared with other nonpolar planes, which could reduce the interface defects induced by detrimentally adsorbed H2O. Based on the sputtering method, we obtained the [002]-oriented and high-crystallinity ZnO ETL and effectively suppressed the adsorption of detrimental H2O molecules. The prepared PbS CQDs infrared photodiode with the sputtered ZnO ETL demonstrated lower dark current density, higher external quantum efficiency, and faster photoresponse compared with the sol-gel ZnO device. Simulation results further unveiled the relationship between interface defects and device dark current. Finally, a high-performance sputtered ZnO/PbS CQDs device was obtained with a specific detectivity of 2.15 × 1012 Jones at -3 dB bandwidth (94.6 kHz).

Enhanced Removal of Organic Dyes Using Co-Catalytic Ag-Modified ZnO and TiO2 Sol-Gel Photocatalysts

Zinc oxide and titanium dioxide semiconductor photocatalysts have been widely utilized in the last few decades for water treatment because of their high photocatalytic efficiency. Recently, a lot of researchers have focused on the improvement of the photocatalytic properties of catalysts through modifying and co-modifying them with different metals and nonmetals. These co-catalytic ions improve the photocatalytic activity of ZnO and TiO2 by reducing its energy band gap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants. In this study, we prepared semiconductor films that were surface-modified with Ag co-catalyst layers via the photo-fixation of Ag (I) ions with varied concentrations (10−2–10−4 M) in the water phase under UV illumination for the first time. The photocatalytic behavior was evaluated by the degradation of malachite green and methylene blue under UV and visible light irradiation. The ZnO/Ag and TiO2/Ag samples showed a faster degradation of malachite green compared to methylene blue due to the formation of stable intermediates by the reaction of OH radicals with the triarylmethane dye (C=C bond) during the photocatalysis. The co-catalytic-silver-modified films had a higher photocatalytic efficiency in comparison with the pure nanostructures. The dye photodegradation rate constants increased in the following order: pure films < films modified with Ag, 10−4 M < films modified with Ag10−3 M < films modified with Ag10−2 M. The Ag modification and the heterojunction of the composites contributed to trapping and transfer of the electrons. Therefore, the photogenerated charges had a longer lifetime, resulting in a strengthened photocatalytic ability of the ZnO/Ag and TiO2/Ag films.

Chemical VOC sensing mechanism of sol-gel ZnO pellets and linear discriminant analysis for instantaneous selectivity.

This work reports on the integration of ZnO pellets for use as a virtual sensor array (VSA) of volatile organic compounds (VOCs). ZnO pellets consist of nano-powder prepared using a sol-gel technique. The microstructure of the obtained samples was characterized by XRD and TEM methods. The response to VOCs at different concentrations was measured over a range of operating temperatures (250-450 °C) using DC electrical characterization. The ZnO based sensor showed a good response towards ethanol, methanol, isopropanol, acetone and toluene vapors. We note that the highest sensitivity (0.26 ppm-1) is obtained with ethanol while the lowest one (0.041 ppm-1) corresponds to methanol. Consequently, the limit of detection (LOD) estimated analytically reached 0.3 ppm for ethanol and 2.0 ppm for methanol at an operating temperature of 450 °C. The sensing mechanism of the ZnO semiconductor was developed on the basis of the reaction between the reducing VOCs with the chemisorbed oxygen. We verify through the Barsan model that mainly O- ions in the layer react with VOC vapor. Furthermore, dynamic response was investigated to construct mathematical features with distinctly different values for each vapor. Basic linear discrimination analysis (LDA) shows a good job of separating two groups by combining features. In the same way we have shown an original reason embodying the distinction between more than two volatile compounds. With relevant features and VSA formalism, the sensor is clearly selective towards individual VOCs.