ZnO rGO Research Articles

Thermal Defect Modulation and Functional Performance: A Case Study on ZnO–rGO Nanocomposites

Herein, a reduced graphene oxide–zinc oxide (rGO–ZnO) hybrid nanocomposite (1 wt% rGO) is synthesized and heat treated at different temperatures, aimed at modulating the intrinsic bulk/surface defects naturally present in nano‐ZnO. The correlation of both the dispersion of rGO within the metal oxide scaffold and the defects present on the semiconductor crystalline lattice with the photocatalytic performance toward the degradation of a molecular dye in water is investigated and discussed. It is shown that several processes compete to determine the catalytic skill of the nanocomposite, which can be enhanced by a simple thermal treatment at moderate temperatures.

Non-invasive oral cancer detection from saliva using zinc oxide—reduced graphene oxide nanocomposite based bioelectrode

Multifunctional materials with excellent biocompatibility and electron-transport properties are critical for the pursuit of point-of-care biosens-ing devices. The authors report the synthesis of zinc oxide–reduced graphene oxide (ZnO–rGO) nanocomposite for the fabrication of an electrochemical immunosensing test-bed for noninvasive onsite detection of oral cancer biomarker (interleukin-8, IL8). The immunosensor showed successful detection of IL8 at low concentration ranges, i.e., 100 fg/mL–5 ng/mL with a sensitivity of 12.46 ± 0.82 µA mL/ng and a detection limit of 51.53 ± 0.43 pg/mL. These results have been validated through in vitro investigations using real saliva samples spiked with IL8.

Experimental and density functional theory investigations of catechol sensing properties of ZnO/RGO nanocomposites

Catechol, an essential industrial feedstock is one of the major environmental pollutant which causes several health issues including skin irritation, eye damage and genetic defects. In this work, we have investigated the catechol sensing performance of ZnO/RGO composites for its quantitative detection. ZnO/RGO composite showed superior electron transfer kinetics, sensitivity (162.04 μA mM cm−2) and lower detection limit (47 nM) than that of bare ZnO nanoparticles. Further, the change in the zeta potential towards the lower negative demonstrated the strong electrostatic interaction between ZnO-RGO and catechol where it is converted as 1, 2 Benzoquinone through redox reactions. By using state of the art of Density Functional Theory simulations, we have demonstrated the interaction of catechol molecule on various ZnO clusters and RGO supported ZnO clusters. The strong bonding between p orbital of O atom of catechol and d orbital of Zn atom and its charge transfer facilitate the oxidation of catechol molecule. In the presence of RGO, the binding energy of catechol and charge transfer increases which qualitatively infer that RGO/ZnO exhibits higher sensitivity compared to bare ZnO and in consistency with the experimental observations.

ZnO nanorod array/reduced graphene oxide substrate with enhanced performance in photocatalytic degradation

A novel ZnO nanorod array/reduced graphene oxide (ZnO array/rGO) composite coating was synthesised via the combination of seed layers-growth and aqueous solution reaction for the formation of ZnO nanorod arrays, which was further combined with rGO using a simple water bath method. The results revealed that perpendicular arrays with hexagonal planes were successfully fabricated. The size and density of the arrays could be optimised by adjusting the reactant concentrations. The influence of the rGO fraction in the ZnO array/rGO composites on the photocatalytic degradation of the methylene blue under UV radiation was investigated. It was found that the photocatalytic performance could be significantly improved by adding rGO. The enhanced photocatalytic activities were attributed to the interactions between ZnO nanorod arrays and rGO nanosheets and subsequently effective separation of photo-generated electron-hole pairs via rGO as electron sinks. The ZnO array/rGO composites presented advantages including facile and green preparation, long photo-electron lifetime, easy particle agglomeration and recovery, and therefore is expected to be an ideal photocatalysts for the future utilisation.

Enhanced photocatalytic activities of ZnO dumbbell/reduced graphene oxide nanocomposites for degradation of organic pollutants via efficient charge separation pathway

In the present work, a well-defined dumbbell shaped ZnO was synthesized by hydrothermal method and ZnO dumbbell/reduced graphene oxide (ZnO/rGO) nanocomposites were prepared by a simple solution mixing method with the different rGO loading amount. The formation of nanocomposites, crystal structure, shape, size and optical properties of the ZnO/rGO nanocomposites were investigated using various analytical techniques. The photocatalytic degradation efficiency of synthesized materials was evaluated by the degradation of aqueous Methyl Orange (MO) and Methylene Blue (MB) under UV–Visible light irradiation. The results revealed that 3 wt% rGO loaded ZnO dumbbell (ZnO-3% rGO) exhibited the higher photocatalytic degradation efficiency than pure ZnO dumbbell and rGO. The enhancement in the photocatalytic dye degradation efficiency of ZnO-3% rGO is attributed to the efficient dye adsorption nature, a red shift in light absorption and inhibition of photo-excited electron-hole pair recombination rate. The photocatalytic dye degradation efficiency also evaluated for various catalyst dosage, dye concentration and pH of the solution. Moreover, it was demonstrated the stability of catalyst over a repeated cycle of dye treatment.

Room-temperature NO2 gas sensors based on rGO@ZnO1-x composites: Experiments and molecular dynamics simulation

Recently, room-temperature gas sensors have become very attractive due to the fact that they can be operated without heating, and thus simplifying the sensor design, reducing the fabrication cost, decreasing the power consumption and increasing the long-term stability. In this study, we propose a facile one-step hydrothermal method to prepare a composite of reduced graphene (rGO)/oxygen-deficient zinc oxide (ZnO1-x), which exhibits obvious room-temperature gas sensing response. X-ray diffraction, Raman and X-ray photoelectron spectroscopy demonstrate that the rGO@ZnO1-x composite is successfully synthesized and large numbers of dual donor defects, oxygen vacancy and zinc interstitial, are introduced into the composite. Field-emission scanning electron microscopy and transmission electron microscopy results reveal that many nanoscale p-n junctions are in-situ formed between ZnO nanosheets and rGO sheets. UV–vis spectra show that the light absorption of the rGO@ZnO1-x composite is red-shifted and extended to the whole visible light region in comparison. The fraction of rGO in the composites plays an important role in the sensing performance. 2.0% is the optimal proportion to obtain the best sensing properties in terms of sensitivity, response and recovery times. The rGO@ZnO1-x composite exhibits significant responses to ppb-level NO2 with white LED light stimulation at room temperature. The enhanced sensing properties can be attributed to three factors: light activation, synergistic effects between rGO and ZnO1-x, and high concentration in donor defects. Molecular Dynamics (MD) is used to quantitively simulate the adsorption process, and the results show that the incorporation of rGO decreases the adsorption energy of NO2. It means that more NO2 species would adsorb on the rGO@ZnO1-x composites, which greatly improves the sensitivity. A new gas sensing mechanism based on the MD calculated results and Langmuir adsorption model is used to explain the reason that the rGO@ZnO1-x composites have a much faster response and recovery process. In addition, the rGO@ZnO1-x sensor shows a weaker response to other interference gases.

Ultraviolet photodetector based on nanostructured ZnO-reduced graphene oxide composite

Due to the special structure and electrical properties of graphene, the ultraviolet (UV) photodetectors based on ZnO nanorods (NRs) were optimized by electrostatic self-assembly with reduced graphene oxide (rGO). UV photodetectors based on ZnO NRs or ZnO–rGO nanocomposites (NCs) with high responsibility, sensitivity and good stability were fabricated by spin-coating method simply, which may be used for reference in manufacture of some big size device. Moreover, our work proves that the composite materials of ZnO and graphene have good prospects in the field of UV photodetector.

Binding ZnO nanorods in reduced graphene oxide via facile electrochemical method for Na-ion battery

Alternating voltage electrochemical dispersion, as a green and facile electrochemical technique, has been successfully developed to design ZnO nanorods/reduced graphene oxide (ZnO/RGO) composite. The morphology of ZnO can be induced to form rod-like structure in this composite via the introduction of GO in the electrolyte solution. Meanwhile, GO can be reduced to RGO during the electrochemical process without adding extra reducing agent, which can enhance the electrical conductivity of ZnO/RGO composite and accommodate the volume change of ZnO. Moreover, ZnO nanorods are well embedded on the surface of RGO with the present of COZn bridge bonds. This COZn formed in the electrochemical preparation can be helpful to contact ZnO nanorods and RGO, which accelerate electron transport rate and enhance structural stability of ZnO/RGO electrode. As-prepared ZnO/RGO composite as an anode for sodium-ion batteries (SIBs) displays much higher capacity, rate performance and cycling behavior than those of pure ZnO material, especially shows ultra-long cycle life (91.9% capacity retention at 500 mA g−1 after 1000 cycles).

Enhanced Photocatalytic Performance of Al-Doped ZnO NPs-Reduced Graphene Oxide Nanocomposite for Removing of Methyl Orange Dye from Water Under Visible-Light Irradiation

In this work, a series of Al-doped ZnO nanoparticles-reduced graphene oxide (AZO–RGO) nanocomposites were successfully synthesized by loading AZO nanoparticles (AZO NPs) on the graphene oxide sheets via in situ and low temperature solvothermal method. Several techniques were utilized to characterize the resultant nanocomposites including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and FT-Infra Red (FT-IR) analyses. SEM and TEM studies showed that AZO NPs have been formed on RGO surface, confirming the formation of AZO–RGO nanocomposites. XPS, FT-IR, and XRD analyses revealed that the oxygen-containing functional groups can prepare as anchoring sites for capturing AZO NPs on RGO surface. Moreover, it was observed that these nanoparticles have wurtzite structure. The photo-catalysis results showed that the 5% AZO–RGO nanocomposite has a higher efficiency than that of pure ZnO and ZnO–RGO samples for removing methyl orange dye from water under visible light irradiation. The enhancement in the photocatalytic activity can be attributed to the increase of surface area of AZO–RGO nanocomposites in comparison with pure ZnO. Furthermore, the existing of Al dopants and RGO sheets in the prepared samples can effectively decrease the charge recombination process in the AZO–RGO nanocomposites.

Fabrication of white light LED photocatalyst ZnO–rGO heteronanosheet hybrid materials

ZnO–rGO nanosheets were fabricated using ZnO nanopowder, GO powder and NaOH via a very facile and scalable thermochemical approach. A systematic sample preparation, X-ray diffraction analysis and scanning electron microscopy were conducted to prove the formation of ZnO nanosheet–rGO nanosheet morphology. Moreover, X-ray photoelectron spectroscopy revealed п–п* satellites which is a sign of recovering of aromatic chains of GO while its reduction and ZnO nanosheet formation. The significant shift of Zn 2p3/2 to higher energies proved the hybridization of ZnO with rGO sheets. Transmission electron microscopy demonstrated there is no particle decoration of rGO sheets. The hybrid nanomaterials indicated a spectacular methyl blue degradation under a distant indoor LED light source. Impedance spectroscopy confirmed the enhancement of visible light photo-degradation could be due to enhancement of charge transfer through the catalyst and charge recombination hindrance. Impedance spectroscopy confirmed the enhancement of electron transfer through the structure of nanosheets is the main cause of enhancement of photocatalyst performance of the hybrid materials. BET/BJH analyses revealed an enhancement of surface area and pore size about 134 m2 g−1and 12 nm, respectively which confirms the visible light degradation of MB.

Sunlight driven photocatalytic reduction of 4-nitrophenol on Pt decorated ZnO-RGO nanoheterostructures

Low photocatalytic efficiency, rapid charge recombination and limited light absorption in wide band gap materials like ZnO severely hinder their practical applications. In order to overcome the above mentioned drawbacks and improve the photocatalytic efficiency, we have developed a series of Pt loaded ZnO-reduced graphene oxide (RGO) nanoheterostructures. The photocatalytic activity of these nanoheterostructures in different compositions, under sunlight irradiation, towards the reduction of 4-nitrophenol has been investigated. The prepared nanoheterostructures showed superior photocatalytic activity and composition having 5 wt% of RGO and 2 wt% of Pt in ZnO (ZPG5) was found to exhibit the highest activity. Control experiments were performed on ZnO-RGO (ZG) nanocomposites without Pt to determine the role of Pt loading. The rate constant of 4-nitrophenol reduction reaction obtained using ZPG5 nanoheterostructure was 0.4203 min−1, which is about 30 folds higher than the rate constant catalyzed using bare ZnO NR (0.0142 min−1) and 6 folds higher than ZG5. Strong enhancement in the photocatalytic activity could be evidenced for the Pt loaded ZnO-RGO nanoheterostructures under sunlight irradiation, which could be attributed to improved charge separation, enhanced charge transfer across the Schottky barrier between ZnO and Pt and the high ability of RGO to adsorb 4-nitrophenol. The developed nanoheterostructures can be used as highly efficient sunlight activated heterogeneous photocatalysts for several real life applications.

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.

3 + 2] Cycloaddition promoted by zinc oxide nanoparticles anchored on reduced graphene oxide using green solvent

ABSTRACTZnO anchored on RGO-catalyzed [3 + 2] cycloaddition of nitriles and sodium azide in the synthesis of 5-substituted 1H-tetrazoles in water is an efficient and simple protocol with low catalyst loading under reflux condition. It is applied to a wide range of substrates with an excellent yield, simple workup procedure, recovery, and reusability of catalyst with consistent activity and high turnover number. It is proposed that the functional groups present in RGO surface are effective for preventing the aggregation of catalytically active ZnO species during the reaction. Moreover, the excellent performance of ZnO–RGO nanocomposite is ascribed to the excellent dispersity of catalyst in water, hydrophilic nature of the RGO for the accumulation of organic substrates in water, and the “Breslow effect.”

Enhancement of photocatalytic performance in sonochemical synthesized ZnO–rGO nanocomposites owing to effective interfacial interaction

Nanocomposites composed of two or more components with desirable performance have attracted tremendous attention, mainly due to the synergic effect between the components. The effective combination of ZnO and reduced graphene oxide would lead to ameliorate the photocatalytic performance. To enhance applicability of semiconductor photocatalytic, the composites used should be good interfacial contact governed by suitable particle size distribution. Herein we aim to fabricate the different crystallize size of ZnO nanoparticles (NPs) in ZnO–reduced graphene oxide (ZnO–rGO) nanocomposites by sonochemical synthesis and subsequent facile drying treatment method. The Zn precursor, Zn(Ac)2, with a plenty of functional groups, was used as a starting source for both reduction of graphene oxide and formation of ZnO on rGO sheets through chemical bonds without the addition of hazardous reducing agents. LiOH was chosen as an assistive reagent to enhance the complete reaction between Zn(Ac)2 and GO in the formation of ZnO–rGO nanocomposites. More remarkably, drying condition has the great influence on the crystallize size of ZnO NPs in as-prepared ZnO–rGO nanocomposites. It is found that ZnO–rGO nanocomposites dried at −50 °C (freeze drying) show the highest photocatalytic efficiency in the degradation of rhodamine B (RhB) as compared to ZnO–rGO nanocomposites by other drying conditions under visible-light irradiation. Correlating the crystallize size obtained by different drying temperatures with the photocatalytic activity, it is probed that the smaller crystallize size in ZnO–rGO nanocomposites enhances the interfacial contact and a chemical bonding between rGO and ZnO NPs leading to the effective separation of electrons and holes. In addition, the O 2 ·− anion was determined to be the main active oxidant by free radicals trapping experiment and a photodegradation mechanism of ZnO–rGO nanocomposites over rhodamine B (RhB) was proposed based on our observations.

Highly sensitive graphene oxide functionalized ZnO nanowires for ammonia vapour detection at ambient temperature

Zinc oxide nanowires were deposited by successive ionic layer adsorption and reaction (SILAR) method on a glass substrate. Graphene oxide powders were electrochemically exfoliated from graphite. The presence of peaks at 20.42°, 22.69°, 25.61° and 43° in the XRD pattern confirms the formation of reduced graphene oxide. Raman peak ratio (ID/IG) was found to be 1.051 for the synthesized powder and it confirms the formation of reduced graphene oxide. Finally, zinc oxide nanowires (ZnO NWs) were successfully functionalized with reduced graphene oxide (rGO). The basic characterization studies such as morphological, structural, and compositional analyses were carried out for rGO/ZnO NWs. It confirmed the formation of ZnO NWs and rGO functionalization over it. The sensing response of ZnO and rGO/ZnO NWs towards the ammonia vapour were studied at ambient temperature (32°C). The obtained results revealed that the sensitivity and selectivity of the ZnO NWs were increased after functionalization. The adsorption mechanism was studied through FTIR and the corresponding results were discussed.

Facial development of high performance room temperature NO2 gas sensors based on ZnO nanowalls decorated rGO nanosheets

A highly sensitive NO2 gas sensor based on ZnO nanowalls decorated rGO nanosheets was fabricated using a thermal reduction and soft solution process. The highly developed interconnected microporous networks of ZnO nanowalls were anchored homogeneously on the surface of reduced graphene oxide (rGO). Sensors fabricated with heterojunction structures achieved a higher response (S=9.61) and shorter response-recovery (25s, 15s) behavior at room temperature to 50ppm level NO2 effectively in contrast to those sensors based on net ZnO nanowalls or rGO layers. The stability and selectivity of ZnO/rGO heterojunction were carried out. Meanwhile, the effects of humidity on ZnO/rGO heterojunction gas sensor were investigated. The more preferable sensing performance of ZnO/rGO heterojunction to NO2 was discussed. It can be surmised that this NO2 gas sensor has potential for use as a portable room temperature gas sensor.

Multifunctional ZnO nanorod-reduced graphene oxide hybrids nanocomposites for effective water remediation: Effective sunlight driven degradation of organic dyes and rapid heavy metal adsorption

We demonstrate the multi-functionality engineering on nanocomposite by combining one dimensional (1D) ZnO nanorod (NR) and two dimensional (2D) reduced graphene oxide (rGO) for efficient water remediation. Nano-engineered ZnO NR-rGO nanocomposites show efficient water remediation in terms of degradation of organic dyes and removal of heavy metal ions. Herein, we report on the fabrication of ZnO NR-rGO nanocomposite via a facile template-free hydrothermal route with an aim to improve the visible photocatalytic efficiency of the ZnO NR based nanocomposites. The structural and morphological features reveal that the rGO sheets are attached on the ZnO NRs and form a hybrid composite assembly. The surface enabled ZnO NR-rGO nanocomposites were used to degrade organic dye molecules (methylene blue (MB), methyl orange (MO) and rhodamine B (RhB)) under visible irradiation and adsorb Cu (II) and Co (II) ions from water through an adsorption process. The nanocomposite containing 7.5wt% rGO and ZnO NRs shows a 4-fold enhancement in the visible photocatalytic activity and effective removal of Cu (II) and Co (II) ions from aqueous solution respectively. The photocatalytic performance is discussed in detail with respect to interaction between ZnO NRs and rGO sheets, light-harvesting properties of the nanocomposites. The effective experimental adsorption data also fit very well with the pseudo-second-order model which reveals the surface adsorption of metal ions. The results provide insight into a new method utilize for both visible photo degradation and adsorption for the removal of various wastewater pollutants. Construction of hybrid form of nanostructures delivers the effective catalytic properties with tunable functionalities for the water remediation.

Hydrothermal synthesis of ZnO flower-reduced graphene oxide composite for electrochemical determination of ascorbic acid

ABSTRACTIn this work, reduced graphene oxide coated zinc oxide flower (ZnO–RGO) nanocomposite has been prepared via a simple one-pot hydrothermal synthesis method. The morphology and properties of the proposed ZnO–RGO were characterized using SEM, Raman and UV–VIS spectroscopy. Then, an ascorbic acid (AA) electrochemical sensor was prepared based on a glassy carbon electrode (GCE) modified with the ZnO–RGO nanocomposite. An advanced performance was recorded on the ZnO–RGO/GCE compared with that of the bare GCE, ZnO/GCE, and RGO/GCE. The proposed electrochemical sensor exhibited a wide linear detection from 5 μM to 2 mM with a low detection limit of 1.2 μM. Further, the proposed AA electrochemical sensor showed a good repeatability, reproducibility and stability.

Ultrasensitive and low detection limit of nitrogen dioxide gas sensor based on flower-like ZnO hierarchical nanostructure modified by reduced graphene oxide

Hierarchical rGO/ZnO hybrids with a flower-like morphology of ZnO and flexible rGO sheets were synthesized by a facile solution-processed method. The structures and morphologies of the hybrids were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The gas sensing properties of hierarchical rGO/ZnO hybrids toward nitrogen dioxide were studied via a static system. The response of rGO/ZnO hybrids to 50ppb NO2 was 12, which was seven times higher than that of pristine ZnO at 100°C. The limit of detection could be achieved as low as 5ppb. The enhanced sensor response was attributed to the presence of local p-n heterojunctions between rGO sheets and hierarchical structure of ZnO.

Photocatalytic Activity of ZnO–rGO Nanocomposite: Synthesis and Characterization

Photocatalytic Activity of ZnO–rGO Nanocomposite: Synthesis and Characterization