ZnO rGO Research Articles

Novel electrochemical synthesis of N-doped ZnO–rGO films for the photoelectrocatalytic degradation of antibiotics

Novel electrochemical synthesis of N-doped ZnO–rGO films for the photoelectrocatalytic degradation of antibiotics

Binary Ce-doped-ZnO/rGO composite as excellent photocatalyst for bromothymol blue dye degradation

Organic contaminants pose a grave danger to both the surroundings and public health, which emphasizes the urgent need for efficient decomposition techniques. Building a composite and adding a cocatalyst can both enhance the transfer of photogenerated electrons, which are useful strategies to increase photocatalytic efficiency. This study sought to synthesize cerium–doped ZnO–rGO nanoparticles with varying concentrations of Ce for visible–light photocatalytic degradation of bromothymol blue dye in water. The synthesized photocatalysts were characterized using transmission electron microscopy, x–ray diffraction, ultraviolet–visible spectrophotometry, energy dispersive spectroscopy, and scanning electron microscopy. The results disclosed that the photoactivity of the 0.6% Ce–ZnO–rGO was the highest and degraded 92% of bromothymol blue dye under visible light within 180 min (k = 1.072 ×10–2 min–1) compared to the pristine ZnO which only degraded the dye by about 22% (k = 1.15 ×10−3 min−1). The band gap of the pure ZnO was estimated to be 3.09 eV and that of the best performing composite (0.6% Ce) to be 2.81 eV which we suggest are responsible for the improved photodegradation. The pseudo–first–order photocatalytic kinetic has a strong correlation value of 0.99. The Ce–ZnO–rGO composites offer a lot of potential for cleaning the environment.

ZnO nanorod and rGO based nanocomposite decorated with Au nanoparticles for enhanced UV photodetection

A comprehensive analysis of a hydrothermally prepared Au nanoparticle decorated nanocomposite of reduced graphene oxide (rGO) and a ZnO nanorod (NR) for possible use as a UV photodetector is presented. The effect of rGO loading in ZnO and incorporation of Au decoration in the best combination of rGO/ZnO for possible enhancement of a photocatalytic effect are experimented. X-ray diffraction, scanning electron microscopy, energy diffraction, and UV-Vis spectroscopy are done for morphological, structural, and optical attribute analysis of the prepared materials. An increase in photocurrent is seen from 2.19 nA to 6.14 mA in dark and UV environments (370 nm) at 5 V bias voltage for the Au decorated nanocomposite, which is found to be better among the experimented composites. The responsivity and detectivity of the Au decorated nanocomposite are analyzed with the increase in incident UV light intensities. The findings are analyzed, and an explanation of the detailed UV photodetection mechanism is outlined in this paper.

La–doped ZnO–rGO nanocomposites: Synthesis and characterization as photocatalyst for effective degradation of bromothymol blue dye in water

Several techniques for eliminating organic dyes and pesticides present in water and wastewater have been developed to lessen their negative environmental effects. The use of semiconductor photocatalysts to break down organic pollutants like dyes and insecticides has garnered a lot of interest recently. In this work, we have prepared simple, quick, and high–yielding La–doped ZnO–rGO nanoparticles that can be used for photocatalytic degradation. The prepared nanoparticles were characterized by various methods including morphological, optical, and photocatalytic properties using cutting–edge technologies like Transmission electron microscopy, energy dispersive spectroscopy, ultraviolet–visible spectroscopy, X–ray diffraction and scanning electron microscopy. The study investigated the photocatalytic action of La–doped ZnO–rGO nanocomposites under visible light irradiation and bromothymol blue as a module dye. According to the findings, La–doped ZnO–rGO nanoparticles demonstrated discernible photocatalytic performance, with the 0.6% La–doped ZnO–rGO exhibiting the best degradation percentage of 81% in 210 min and a rate constant of 1.158 × 10−3 min−1.

Investigating the magnetic properties of rGO, and rGO-ZnO nanocomposite

In the present work, magnetic properties of the graphene oxide (GO), reduced graphene oxide (rGO), rGO-ZnO hybrid (RGZ) nanocomposite and ZnO nanorods have been investigated. A simple hydrolysis method is used to synthesize (RGZ) nanocomposite and ZnO nanorods. The structural and the morphological studies are performed using XRD, TEM, FTIR, TGA and FESEM. Magnetic properties of the samples have been investigated through SQUID magnetometer. The as synthesized GO, rGO samples show diamagnetic behavior with weak ferromagnetic component at room temperature (RT) while at low temperatures (10 K) these samples exhibit paramagnetic behavior with small ferromagnetic signature. As synthesized ZnO nanorods exhibit weak ferromagnetism at RT. The RGZ nanocomposite exhibits weak ferromagnetic behavior at RT and the magnetization value is found greater than that of ZnO. Moreover, RGZ nanocomposite shows higher value of magnetization than GO, rGO and pure ZnO samples at low temperatures. Further, we have investigated the temperature dependent magnetization (M−T) in zero field cooled (ZFC) and field cooled (FC) conditions for ZnO nanorods, GO, rGO and rGO-ZnO samples at 100 Oe applied magnetic field. The possible mechanisms for magnetic behavior of rGO-ZnO composite are discussed and described in this manuscript.

A comparative study: Green synthesis and evaluation of ZnO-GO and ZnO-RGO nanocomposites for antibacterial applications

In this study, ZnO-GO, and ZnO-ROG nanocomposites were synthesized by the green chemistry method to study their antibacterial activity. XRD pattern and XPS confirmed that the nanocomposites successfully were synthesized. The UV–vis spectroscopy results showed the bonding between ZnO nanoparticles and GO sheets can engineer the semiconductor band gap and shift the absorption edge to longer wavelengths as compared to pure ZnO. The inhibition rate of ZnO nanoparticles, GO, and RGO sheets after 12 h of contact with E. coli were 84 %, 81 %, and 73 %, respectively. Inhibition rate for Z-1GO, Z-5GO, Z-10GO, Z-1RGO, Z-5RGO, and Z-10RGO nanocomposites were 89 %, 92 %, 94 %, 85 %, 89 %, and 91 % (under dark conditions). Enhanced inhibition rate in nanocomposites may be due to the simultaneous activation of mechanisms of dissolution of Zn2+ ions from ZnO and physical interaction between the sharp edges of GO or RGO sheets and E. coli.

Highly sensitive detection for acetic acid based on biomorphic ZIF-8 and MOF dual-template-derived ZnO–rGO nanocomposites

ZIF-8-derived ZnO and rGO nanocomposites based on biomorphic cotton templates are synthesized. The low-cost and reproducible biotemplate method provides a new idea for designing gas sensors with excellent performance.

Tailored ZnO-ZnS heterostructure enables a rational balancing of strong adsorption and high catalytic activity of polysulfides for Li-S batteries

Electrocatalytic conversion of polysulfides is an effective strategy to alleviate shuttle effects for Li-S batteries, but achieving the synergistic effects between strong adsorption and high catalytic activity with a single catalyst is still challenging. Herein, we report the rational balancing of strong adsorption and high catalytic activity on the tailored ZnO-ZnS/reduced graphene oxide (rGO) heterostructure to achieve significantly enhanced conversion of polysulfides. By integrating the merits of adsorptive ZnO, catalytic ZnS and conductive rGO, the ZnO-ZnS/rGO heterostructures could enable an unrestricted adsorption-diffusion-conversion of polysulfides. Moreover, by controlling the sulfurization degree, the ZnO-ZnS/rGO heterostructure with ZnO/ZnS ratio of 7:3 performs the optimized overall performance, indicative of a trade-off between adsorption and catalytic activity. When served as functional coatings onto the separators, the ZnO-ZnS/rGO heterostructures could greatly promote the redox kinetics and retrain the polysulfides shuttling. The pure sulfur cathode with the 7ZnO-3ZnS/rGO modified separator delivers a high initial specific capacity of 1186 mAh g −1 at 0.5 C and a low capacity fading of 0.06% per cycle over 500 cycles at 1 C. Even with high sulfur loading of 7.6 mg cm −2 , a high reversible specific capacity of 671 mAh g −1 at 1 C can still be retained. This work presents a rational method to tailor the heterostructure catalysts for boosting electrocatalytic conversion of polysulfides towards high-performance Li-S batteries.

Design of Improved Acetone Gas Sensors Based on ZnO Doped rGO Nanosheets

To meet future social and environmental objectives for diagnosis of human diseases has offered to develop the proficient gas sensors devices with higher selectivity and sensitivity. For the production of ZnO doped reduced graphene oxide (ZnO doped rGO) nanocomposite, a one-pot hydrothermal approach. The morphological, structural and composition of nanocomposite were investigated to confirm ZnO nanoparticle effectively doped on rGO nanosheets. The nanocomposite has exhibited a superior acetone sensing characteristics. Furthermore, the nanocomposite has a high selectivity for acetone vapour. These findings emphasise advantageous synergistic effects among ZnO and excellent rGO sheet substrate properties.

Reversible Switching from P- to N-Type NO2 Sensing in ZnO Rods/rGO by Changing the NO2 Concentration, Temperature, and Doping Ratio

Gas sensors based on hybrid materials of reduced graphene oxide/metal oxide semiconductors are an effective way to improve sensor performance. In this paper, rGO (reduced graphene oxide) was uniformly and compactly anchored on the surface of presynthesized ZnO rods using a facile hydrothermal synthesis. The ZnO rods and reduced graphene oxide combined to form an np-type heterojunction structure for use in a nitrogen dioxide (NO2) sensor. Compared with pure ZnO rods and rGO, the ZnO rod/rGO composites display abnormal sensing behavior in the form of reversible transitions from p- to n-type sensing induced by changes in the NO2 gas concentration (C) and temperature (T). Based on familiar phase diagrams, a binary (T–C) transition diagram was created in terms of the gas-sensing response, which can be directly used to design and control p–n transitions. In addition, we also found that as the doping ratios (c) change, the semiconductor type of the composite also changes. Finally, the mechanisms underlying the NO2-sensing transitions are described based on the dual conduction path model: electron transfer in the internal region and chemical reactions at the surface. This reversible p–n transition of ZnO/rGO materials upon NO2 sensing is interesting and holds great potential for the efficient detection of NO2.

Facile synthesis of nanostructured ZnO–rGO based graphene and its application in wastewater treatment

In the present work, graphene oxide GO is prepared by chemical exfoliation of graphite using Hummer’s method. A facile and green synthesis of ZnO–rGO nanocomposite is performed using aloe vera plant extract. The characterization tools (XRD, FTIR, FESEM, HrTEM, AFM) proved the formation of single phase of ZnO–rGO nanocomposite. Since the environmental contamination caused by Cd(II) ion is a world issue, it has a harmful effect, especially on the human health and environment. Subsequently, our goal in this work is to find an accurate method for detection and adsorption of toxic Cd(II). ZnO and ZnO–rGO nanocomposites are prepared for removing Cd(II), and on behalf of increasing its removal efficiency, GO is added. The results showed a great improvement in removal efficiency reached up to 90% at pH 6 after 90 min.

One-step, low temperature synthesis of reduced graphene oxide decorated with ZnO nanocrystals using galvanized iron steel scrap

Production of a ZnO–rGO composite, using a novel one-pot method consisting in continuously flowing argon into a GO aqueous suspension heated at 80 °C, in the presence of galvanized iron steel scrap is presented. FTIR shows the complete disappearance of GO functional groups and only the C=C band remained, indicating extensive GO reduction. Raman spectra indicated sp2 character increase after reaction and the presence of the E2h mode of ZnO. SEM showed submicron crystals identified by XRD as ZnO in the hexagonal phase, while TEM images indicate ZnO nanoparticles decorate mainly the rGO borders. Optical band gap of 3.5 eV corresponding to ZnO, and optical transitions at 4.1 and 5.5 eV related with n → π and π → π* were observed. Electrochemical characterization by cyclic voltammetry shows an specific capacitance of 4.7 F g−1 at a scan rate of 5 mVs−1, which drops to ca. 0.8 F g−1 at 200 mVs−1. By electrochemical impedance spectroscopy, the relaxation time was ca. 5 ms. The proposed mechanism for the materials‘ synthesis includes Zn dissolution from scrap, galvanic displacement of oxygen moieties at the GO sheet, Zn deposition onto the carbon surface, and further oxidation and growth of ZnO nanocrystals.

Performance Improvement of n-ZnO/p-rGO Heterojunction Based Room Temperature Hydrogen Gas Sensor

This letter reports a room-temperature hydrogen gas sensor based on the nanohybrid structure of ZnO nanorods (NRs) and reduced graphene oxide (rGO) with better sensitivity. In this nanohybrid structure, ZnO NRs were grown up by the chemical bath deposition technique followed by the electrochemical exfoliation of rGO layer. The collaborative hybridization of these two dissimilar sensing elements revealed much better response at room temperature, which could hardly be achieved by the individual ones. The sensing was carried out at room temperature at ~40% RH, with hydrogen as a target gas with fastest response and recovery times (17.02 and 27.06 s), which have not been achieved earlier. The physics behind it is the high carrier mobility of rGO. A very high response magnitude (586.93% at 100 ppm) was found due to the increased number of gas-interaction sites and availability of free surface energy provided by ZnO NRs and rGO. A comparative discussion elucidating the role of ZnO NRs-rGO junctions has also been presented correlating the experimental findings.

Novel ZnO tetrapod-reduced graphene oxide nanocomposites for enhanced photocatalytic degradation of phenolic compounds and MB dye

In the present work, a bunch of ZnO tetrapod-reduced graphene oxide (ZTPG) nanocomposites (NCs) were synthesized via a simple and economical in-situ approach using high temperature refluxing method. The phase purity, morphology, optical and photocatalytic properties of NCs were studied comprehensively. Various weight percentage of rGO were proposed for efficient separation of electron hole pairs and improved photodegradation of phenolic compounds and methylene blue (MB) dye. The photocatalytic degradation of 4-chlorophenol (4-CP) and MB dye in aqueous system was performed for the assessment of photodegradation productivity of various ZTPG NCs with illumination of UV-light. The utmost mineralization after photodegradation attained about 94.8% for 4-CP over ZTPG-5 with UV-light exposure for 180 min. Similarly, the efficiency was also found 98.05% for MB dye photodegradation after 90 min UV-light introduction. The improved photocatalytic performance of ZTPG-5 NCs attributed to the enhanced adsorption capacity of 4-CP and MB dye over nanocomposite photocatalyst, effectual suppression of the recombination rate of electron hole pairs by synergistic influence among ZnO tetrapod and rGO sheet.

Minimizing electron-hole pair recombination through band-gap engineering in novel ZnO-CeO2-rGO ternary nanocomposite for photoelectrochemical and photocatalytic applications.

A novel ZnO-CeO2-rGO (ZCG) ternary nanocomposite with varying ZnO/CeO2 weight proportions was synthesized by a hydrothermal process for photoelectrochemical water splitting and photocatalytic application. XRD diffraction peaks of ZCG nanocomposites displayed the patterns of ZnO and CeO2 nanoparticles, and SEM revealed irregular flake-like particles, which were uniformly decorated on the rGO matrix. Increase in the intensity ratio of D and G bands from Raman spectra revealed changes in oxygen bonding in the ZnO-rGO (ZG) and ZCG nanocomposites. The shift in the band edge positions and the decrease in the band gap with increase in the cerium oxide content in ZCG composites were observed from UV-Vis and Mott-Schottky plots. XPS results showed that Ce3+ fraction increased with an increase in the cerium oxide content in ZCG nanocomposites. The ZCG3 (85:15) nanocomposite exhibited decreased electron-hole recombination rate as evidenced from the photoluminescence and electrochemical impedance spectroscopy Nyquist plots. The characteristic frequency in Bode's plot shifted to a lower frequency for the ZCG3 electrode demonstrating low interfacial charge transfer resistance, and ZCG3 photoelectrode displayed a higher photocurrent density of 0.69mA/cm2 at 1.5V compared with other photoelectrode. The optimized and highly efficient ZCG3 nanocomposite exhibited improved photocatalytic degradation of methylene blue (MB) with a reaction rate constant of 0.0201min-1. Combination of defects in the form of Ce3+ ion and surface oxygen vacancies coupled with rGO as the electron acceptor improved the charge carrier density and carrier transport in addition to the formation Schottky-type junction and the presence of an internal electric field.

Microwave-Assisted Synthesis of ZnO-rGO Core-Shell Nanorod Hybrids with Photo- and Electro-Catalytic Activity.

The unique two-dimensional structure and surface chemistry of reduced graphene oxide (rGO) along with its high electrical conductivity can be exploited to modify the electrochemical properties of ZnO nanoparticles (NPs). ZnO-rGO nanohybrids can be engineered in a simple new two-step synthesis, which is both fast and energy-efficient. The resulting hybrid materials show excellent electrocatalytic and photocatalytic activity. The structure and composition of the as-prepared bare ZnO nanorods (NRs) and the ZnO-rGO hybrids have been extensively characterised and the optical properties subsequently studied by UV/Vis spectroscopy and photoluminescence (PL) spectroscopy (including decay lifetime measurements). The photocatalytic degradation of Rhodamine B (RhB) dye is enhanced using the ZnO-rGO hybrids as compared to bare ZnO NRs. Furthermore, potentiometry comparing ZnO and ZnO-rGO electrodes reveals a featureless capacitive background for an Ar-saturated solution whereas for an O2 -saturated solution a well-defined redox peak was observed using both electrodes. The change in reduction potential and significant increase in current density demonstrates that the hybrid core-shell NRs possess remarkable electrocatalytic activity for the oxygen reduction reaction (ORR) as compared to NRs of ZnO alone.

Biocide mechanism of highly efficient and stable antimicrobial surfaces based on zinc oxide-reduced graphene oxide photocatalytic coatings.

Highly efficient photoactive antimicrobial coatings were obtained using zinc oxide-reduced graphene oxide nanocomposites (ZnO-rGO). Their remarkable antibacterial activity and high stability demonstrated their potential use for photoactive biocide surfaces. The ZnO-rGO nanocomposites were prepared by the sol-gel technique to create photocatalytic surfaces by spin-coating. The coatings were deeply characterised and several tests were performed to assess the antibacterial mechanisms. rGO was homogeneously distributed as thin sheets decorated with ZnO nanoparticles. The surface roughness and the hydrophobicity increased with the incorporation of graphene. The ZnO-rGO coatings exhibited high activity against the Gram-positive bacterium Staphylococcus aureus. The 1 wt% rGO coated surfaces showed the highest antibacterial effect in only a few minutes of illumination with up to 5-log reduction in colony forming units, which remained essentially free of bacterial colonization and biofilm formation. We demonstrated that these coatings impaired the bacterial cells due to cell membrane damage and intracellular oxidative stress produced by the photogenerated reactive-oxygen species (ROS). The enhancement of the ZnO photocatalytic performance upon rGO incorporation is due to the increased detected generation of hydroxyl radicals, attributed to the reduction of electron-hole pair recombination. This intimate contact between both components also conveyed stability against zinc leaching and improved the coating adhesion.

Enhanced photodegradation by RGO/ZnO core-shell nanostructures

RGO/ZnO core-shell nanostructures were synthesized by a simple sol-gel method and then heated to 500℃at 4 × 10−3 Torr to remove moisture. The photodegradation efficiency and its related rate constant (k) were evaluated by using it in the degradation of methylene blue (MB, C16H18N3ClS) solution. The RGO/ZnO CSNs has a high photodegradation efficiency, which is attributable to the increased specific surfaces area, enhanced absorption of the visible light (λ = 380−780 nm), the defect levels of VO2- and Zn2+i which supply the extra electrons, highly effective charge-carrier transfer, and localized surface plasmon resonance (LSPR) at the interface between dielectric ZnO and electric RGO, resulting in the effective absorbance of MB molecules and an increase in the electron-hole lifetime, and a consequent increase in photodegradation efficiency. A critical loading of 800 ppm RGO was identified and found to provide an efficiency of higher than 93 % in 15 min under visible radiation.

Zinc oxide: reduced graphene oxide nanocomposite film for heterogeneous photocatalysis

The nanocomposite film based on zinc oxide nanostructures and reduced graphene oxide bilayer (ZnO/rGO) was synthesized, characterized and tested for the photodegradation of model organic dye (methyl orange) in water. Specifically, the nanorods of zinc oxide were obtained by the hydrothermal methods on the surface of rGO bilayers deposited on the quartz surface. The kinetics of dye photodegradation was studied via measurement of variation of the optical density at the maximum observed for the dye at 465 nm. The photodegradation efficiency of methyl orange (MO) was found to be increased from 70 to 85% when the nanocomposite material was used instead of pure ZnO nanostructures. The reaction rate constants calculated using the first-order approximation were equal to 7.2 × 10−3 min−1, 1 × 10−3 min−1 and 1.4 × 10−2 min−1 for ZnO nanorods, rGO bilayer, and ZnO/rGO nanocomposite, respectively. Hence, the rate constants clearly indicate that ZnO and rGO are functioning in a synergistic manner in the fabricated nanocomposite photocatalyst. It is necessary to point that the sample keeps its integrity after multiple experiments that is important for practical applications. The obtained results evidently demonstrate potential of the nanocomposite films based on ZnO nanostructures and rGO bilayers for production of the efficient catalysts.

Assessment of Antioxidant Activity of Pure Graphene Oxide (GO) and ZnO-Decorated Reduced Graphene Oxide (rGO) Using DPPH Radical and H2O2 Scavenging Assays

In this work, zinc oxide-decorated graphene oxide (ZnO–rGO) was successfully synthesized with a fast reflux chemical procedure at 100 °C. An equal mass ratio of graphene oxide (GO) and zinc acetate was used as starting materials dissolved, respectively, in ultrapure distilled water and dimethylformamide (DMF). Particularly, pure GO was synthesized using Hummers modified protocol by varying the mass ratio of (graphite:potassium permanganate) as follows: 1:2, 1:3, and 1:4, which allow us to obtain six types of pure and decorated samples, named, respectively, GO1:2, GO1:3, GO1:4, ZnO–rGO1:2, ZnO–rGO1:3, and ZnO–rGO1:4 using reflux at 100 °C. X-ray diffraction, FTIR, and Raman spectroscopy spectra confirm the formation of wurzite ZnO in all ZnO-decorated samples with better reduction of GO in ZnO–rGO1:4, confirming that a higher degree of graphene oxidation allows better reduction during the decoration process with ZnO metal oxide. Antioxidant activity of pure and zinc oxide-decorated graphene oxide samples were compared using two different in vitro assays (DPPH radical and H2O2 scavenging activities). Considerable in vitro antioxidant activities in a concentration-dependent manner were recorded. Interestingly, pristine GO showed more elevated scavenging efficiency in DPPH tests while ZnO-decorated GO was relatively more efficient in H2O2 antioxidant assays.