Reduced Graphene Oxide ZnO Research Articles

An Electrochemical Sensor Based on Reduced Graphene Oxide and ZnO Nanorods-Modified Glassy Carbon Electrode for Uric Acid Detection

In this paper, reduced graphene oxide–ZnO (RGO–ZnO) nanorods composite was prepared via a simple one-pot hydrothermal approach. The synthesized RGO–ZnO nanorods composite has been successfully applied for glassy carbon electrode (GCE) surface modification. The RGO–ZnO nanorods composite-modified GCE was applied for sensitive and selective determination of uric acid (UA). The biosensor exhibited a linear dependence on UA concentration ranging from 1 to 800 μM with a detection limit of 0.312 μM(S/N = 3). The proposed UA sensor also showed an excellent stability, reproducibility and anti-interference property.

Facile Synthesis of ZnO–Reduced Graphene Oxide Nanocomposites for NO2 Gas Sensing Applications

AbstractGraphene and its exotic forms have been widely recognized as exceptional materials for gas‐sensing applications because of their extraordinary electrical conductivity and large surface area to volume ratios. Herein, chemically reduced graphene oxide (rGO) and zinc oxide–reduced graphene oxide (ZrGO) nanocomposite powders have been successfully synthesized through a simple hydrolysis method followed by annealing in ambient N2 gas. The reduction of graphene oxide by hydrazine hydrate and the decoration of the graphene surface by ZnO nanoparticles have occurred during the synthesis process. The prepared samples were characterized by various microscopic techniques to explore the surface morphology and uniformity. Spectroscopic techniques were used to investigate the quality of the as‐synthesized powder samples as well as the extent of graphitization of the samples. Coil sensors with two Pt terminals and a heating element have been designed to extensively monitor the effect of temperature on the electrical and gas‐sensing properties of the rGO and ZrGO nanocomposite samples. The ZrGO nanocomposites possess better electrical and NO2 gas sensing properties than the pristine rGO. The ZrGO nanocomposite sensor exhibits a high response (ca. 32 %) for 50 ppm NO2 at relatively low temperature (50 °C). Our results suggest that the ZrGO nanocomposite material could be used to fabricate a new generation of low‐power portable NO2 sensors.

Label-free detection of zinc oxide nanowire using a graphene wrapping method

Zinc oxide nanowires (ZnO NWs) have been attempted to various applications, such as piezoelectric devices, energy harvesting devices, self-powered nanosensors, and biomedical devices. However, recent reports have shown the toxic effect of ZnO NWs. In this report, we described the detection of ZnO NWs, for the first time using reduced graphene oxide (RGO) wrapping method. By wrapping RGO to ZnO NW (RGO–ZnO NW), we are able to aggregate ZnO NWs and increase the sensing performance. The detection measurement is based on the resonance frequency shift derived from mass variation of RGO–ZnO NW adsorption on the DNA immobilized resonator. The resonator is able to detect ZnO NWs with detection limit of 100ngmL−1 which is 2 order below the fatal toxic concentration of ZnO NWs in Human Monocyte Macrophages (HMMs). Furthermore, the resonator is able to detect ZnO NWs in real tap water, showing the potential as ZnO NWs screening platform in real environmental aqua system.

ZnO-reduced graphene oxide nanocomposites as efficient photocatalysts for photocatalytic reduction of CO2

A series of zinc oxide/reduced graphene oxide nanocomposites (ZnO-rGO) were fabricated by a facile one-step hydrothermal method. The composite materials were characterized by X-ray diffraction (XRD), Transmission scanning electron microscopy (TEM), Energy dispersive X-ray analysis (EDX), X-ray photoelectron Spectroscopy (XPS), nitrogen adsorption-desorption isotherms, photoluminescence spectra (PL) and UV–vis diffuse reflectance (DRS). The photocatalytic performance of the ZnO-rGO composites was investigated towards photoreduction of CO2 to CH3OH. The yield of CH3OH on ZnO-rGO was five times as high as that on pure ZnO. The ZnO-rGO nanocomposites possess excellent reusing and cycling property.

ZnO nanoflowers with single crystal structure towards enhanced gas sensing and photocatalysis.

In this paper, ZnO nanoflowers (NFs) were fabricated by thermal decomposition in an organic solvent and their application in gas sensors and photocatalysis was investigated. These single crystal ZnO NFs, which were observed for the first time, with an average size of ∼60 nm and were grown along the {100} facet. It was suggested that oleylamine used in the synthesis inhibited the growth and agglomeration of ZnO through the coordination of the oleylamine N atoms. The NFs exhibited excellent selectivity to acetone with a concentration of 25 ppm at 300 °C because they had a high specific surface area that provided more active sites and the surface adsorbed oxygen species for interaction with acetone. In addition, the ZnO NFs showed enhanced gas sensing response which was also ascribed to abundant oxygen vacancies at the junctions between petals of the NFs. Furthermore, ZnO-reduced graphene oxide (RGO) composites were fabricated by loading the ZnO NFs on the surface of the stratiform RGO sheet. In the photodegradation of rhodamine B tests, the composite revealed an enhanced photocatalytic performance compared with ZnO NFs under UV light irradiation.

Reduced graphene oxide-ZnO self-assembled films: tailoring the visible light photoconductivity by the intrinsic defect states in ZnO.

ZnO is a wide direct bandgap semiconductor; its absorption can be tuned to the visible spectral region by controlling the intrinsic defect levels. Combining graphene with ZnO can improve its performance by photo-induced charge separation by ZnO and electronic transport through graphene. When reduced graphene oxide-ZnO is prepared by a hydrothermal method, the photophysical studies indicate that oxygen vacancy defect states are healed out by diffusion of oxygen from GO to ZnO during its reduction. Because of the passivation of oxygen vacancies, the visible light photoconductivity of the hybrid is depleted, compared to pure ZnO. In order to overcome this reduction in photocurrent, a photoelectrode is fabricated by layer-by-layer (LBL) self-assembly of ZnO and reduced graphene oxide. The multilayer films are fabricated by the electrostatic LBL self-assembly technique using negatively charged poly(sodium 4-styrene sulfonate)-reduced graphene oxide (PSS-rGO) and positively charged polyacrylamide-ZnO (PAM-ZnO) as building blocks. The multilayer films fabricated by this technique will be highly interpenetrating; it will enhance the interaction between the ZnO and rGO perpendicular to the electrode surface. Upon illumination under bias voltage defect assisted excitation occurs in ZnO and the photogenerated charge carriers can transfer to graphene. The electron transferred to graphene sheets can recombine in two ways; either it can recombine with the holes in the valence band of ZnO in its bilayer or the ZnO in the next bilayer. This type of tunnelling of electrons from graphene to the successive bilayers will result in efficient charge transfer. This transfer and propagation of electron will enhance as the number of bilayers increases, which in turn improve the photocurrent of the multilayer films. Therefore this self-assembly technique is an effective approach to fabricate semiconductor-graphene films with excellent conductivity.

Enhanced photocatalytic efficiency of AuPd nanoalloy decorated ZnO-reduced graphene oxide nanocomposites

Possible mechanism involved for the degradation of R6G dye upon UV irradiation by using ZnO–RGO–M (M = Au, Pd, AuPd) nanocomposite.

Photoinduced superhydrophilicity and high photocatalytic activity of ZnO–reduced graphene oxide nanocomposite films for self-cleaning applications

A series of ZnO–reduced graphene oxide (ZnO–rGO) nanocomposite films were developed on glass substrate by a sol–gel technique. The deposited films were highly transparent and showed enhanced photocatalytic activity with increasing rGO content. Significantly, the films showed surface hydrophilicity and photoinduced superhydrophilicity that enhanced with the increase of rGO/ZnO ratio. The enhancement of photocatalytic activity of ZnO–rGO films with increasing rGO/ZnO ratio is attributed to the enhancement of their efficient charge separation owing to electrons injection from the conduction band of ZnO to graphene, the decrease of average particle size, and the increase of superhydrophilicity. The promising characteristics (inexpensive, transparent, photoinduced superhydrophilic, and highly photocatalytically active) make the ZnO–rGO films have the potential use in self-cleaning and various indoor applications.

Preparation of ZnO flower/reduced graphene oxide composite with enhanced photocatalytic performance under sunlight

Reduced graphene oxide (RGO) coated ZnO flower was synthesized by a simple one-pot hydrothermal method. The morphology and properties of ZnO/RGO composites were characterized by UV–vis spectroscopy, Raman spectroscopy, SEM, TEM, EDX, XRD, photoluminescence spectroscopy and photocurrent measurement. It was found that the ZnO flowers were effectively covered with RGO sheets and formed a network structure. The photocatalytic activities of the as-prepared composites were investigated by photodegrading methylene blue under both UV light and sunlight. Results show that the ZnO/RGO composites exhibited a remarkably enhanced photocatalytic efficiency compared to pure ZnO flowers. The cause of the enhanced photocatalytic performance could be ascribed to the synergistic effect between ZnO flowers and RGO sheets. Moreover, we found that the content of graphene oxide introduced into composite material was a crucial factor for its improved photocatalytic performance.

ZnO–graphene composites as practical photocatalysts for gaseous acetaldehyde degradation and electrolytic water oxidation

A facile, green one-pot hydrothermal method has been developed to prepare ZnO-reduced graphene oxide (rGO) composites with controllable rGO content. Time-resolved photoluminescence spectra revealed that an increased electron-transfer rate constant was observed for ZnO–rGO with increasing rGO contents, suggesting that an increased number of photoexcited charge carriers were separated and available for photocatalysis utilization. The photocatalytic properties of the ZnO–rGO composites were investigated by using gaseous acetaldehyde (CH3CHO), a typical volatile organic compound (VOC), as the test pollutant. This is the first time that ZnO–rGO composites have been used as photocatalysts for gaseous CH3CHO degradation. Compared to pure ZnO, ZnO–rGO composites with suitable rGO contents (1.0 and 3.0wt%) displayed significantly enhanced photocatalytic activity in both CH3CHO degradation and CO2 generation. This enhancement was ascribed to the rGO support that can promote the carrier utilization efficiency of ZnO by readily accepting the photoexcited electrons from ZnO. As a result, abundant photogenerated holes remained on the ZnO and were available for participation in the CH3CHO photodegradation. Furthermore, the photoactivity of ZnO–rGO toward electrolytic water oxidation was evaluated. The results showed that ZnO–rGO composites achieved 50% increase in water oxidation current over pure ZnO under white light illumination. The demonstration from this work may facilitate the use of ZnO–rGO composites in photodegradation of VOCs as well as for photoelectrochemical applications, which is imperative from the environmental and energetic points of view.

Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid

This paper scrutinizes the fabrication of a chemiresistive type of acetylene (C2H2) gas sensor by synthesizing a silver (Ag)-loaded zinc oxide (ZnO)-reduced graphene oxide (Gr) hybrid via a facile chemical route. The as-synthesized hybrid was characterized in detail in terms of its structural, morphological and compositional properties. The physical properties of the hybrid exhibited a well-structured crystalline nature and mixed phases of Ag, Gr, and ZnO. The morphological characterization revealed that particle-like nanostructures of the ZnO and Ag mixer were well distributed and closely affixed onto the surface of thin-layer reduced graphene oxide sheets. At an optimum temperature of 150°C, the 3wt% Ag-loaded ZnO–Gr hybrid showed preferential detection of acetylene gas with a response value of 21.2 for 100ppm gas concentrations. The fabricated sensor showed a low detection limit of 1ppm, fast response and recovery times of 25s and 80s, respectively, and good repeatability. Experimental results also showed that the synthesized hybrid had a negligible relative humidity (RH) effect up to 31% RH, and then deteriorated significantly with increasing RH concentrations. After detailed examination, we conclude that an Ag-loaded ZnO–Gr hybrid could be an effective means of fabricating high-performance practical C2H2 sensors.

Evaluation of reduced graphene oxide/ZnO effect on properties of PVDF nanocomposite films

In this work, we report the preparation and characterization of PVDF/reduced graphene oxide (RGO)-ZnO nanocomposite films via synthesis of reduced graphene oxide-ZnO nanocomposite by solution casting method. Different weight ratios of RGO-ZnO nanocomposites were produced and compounded with PVDF by solution mixing. The compounding was followed by solution casting and drying to form nanocomposite films. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction techniques (XRD), revealed an obvious α to β-phase transformation compared to pure PVDF which a maximum content of 83% for β-phase was calculated by using FTIR techniques. The morphology of the nanocomposites was investigated using scanning electron microscope (SEM). SEM micrographs showed a decrease in size of spherulitic crystal structure of PVDF/RGO-ZnO nanocomposites compared with pure PVDF which has an obvious relation with β-phase enhancement. The thermal behaviors of PVDF/RGO-ZnO nanocomposite films have been investigated by employing thermogravimetric analysis (TGA). Since the interface between PVDF and the nanoparticles has an important role in the nucleation of the polymer phase, thermogravimetric analysis was used in order to identify and quantify the interface region and to correlate it with the β-phase content. It is found that an intimate relation exists between the size of the interface region and the piezoelectric β-phase formation that depends on the RGO-ZnO content. The interface value and the β-phase content increase with increasing RGO-ZnO. The PVDF nanocomposites showed higher thermal stability than the pure polymer.

Structurally nanocrystalline-electrically single crystalline ZnO-reduced graphene oxide composites.

ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.

Enhancing NO2 gas sensing performances at room temperature based on reduced graphene oxide-ZnO nanoparticles hybrids

NO2 gas sensor has been constructed using reduced graphene oxide-ZnO nanoparticles (ZnO-rGO) hybrids as sensing materials. Most importantly, the sensor exhibits higher sensitivity, shorter response time and recovery time than those of the sensor based on rGO, indicating that the sensing performances for NO2 sensing operating at room temperature have been enhanced by introduction of ZnO nanoparticles into rGO matrix.

A facile route to reduced graphene oxide–zinc oxide nanorod composites with enhanced photocatalytic activity

In this study, highly efficient photocatalysts were synthesized by the successful hybridization of ZnO nanorods (ZnO NRs) with reduced graphene oxide (RGO) by a facile and green hydrothermal process. The photocatalytic experimental results indicated that all the RGO–ZnO NR composite photocatalysts exhibited enhanced photocatalytic activity for the degradation of Rhodamine B (RhB) under UV-light irradiation. It is found that the degree of photocatalytic activity enhancement strongly depends on the mass ratio of RGO in the composites; the highest photocatalytic activity which equals to 6.8 times as that of pristine ZnO NRs can be obtained when the loading amount of RGO is 4.0wt.% of RGO. The enhancement of photocatalytic activity can be attributed to the synergistic effect of the effective inhibition of the recombination of photo-generated electron–hole pairs and the elevated absorption ability for dyes, due to the strong electronic interaction between ZnO NRs and RGO nanosheets. Considering the facile and green method for the reduction of GO, the present investigation can be further employed to fabricate more graphene-based composites for various environmental and energy-related applications.

The Role of Ionic Electrolytes on Capacitive Performance of ZnO-Reduced Graphene Oxide Nanohybrids with Thermally Tunable Morphologies

In the present work, the role of the reaction temperatures on the morphologies of zinc oxide-reduced graphene oxide (ZnO-RGO) nanohybrids and their supercapacitive performance in two different aqueous electrolytes (1.0 M KCl and Na2SO4) were investigated. The ZnO-RGO nanohybrids were synthesized at two different temperatures (ca. 95 and 145 °C) by solvothermal method and labeled as ZnO-RGO-1 and ZnO-RGO-2, respectively. The structure and composition of ZnO-RGO nanohybrids were confirmed by means of X-ray diffraction, electron microscopes (scanning and transmission), X-ray photoelectron, photoluminescence, and Raman spectroscopy. These results show that the temperature allows a good control on loading and morphology of ZnO nanoassemblies in ZnO-RGO nanohybrids and at elevated temperature of 145 °C, ZnO nanoassemblies break and get completely embedded into RGO matrices. The electrochemical performance of ZnO-RGO nanohybrids was examined by cyclic voltammograms (CVs), galvanostatic charge-discharge (chronopotentiometry) and electrochemical impedance spectroscopy (EIS) in 1.0 M KCl and Na2SO4 aqueous electrolytes respectively. Combining the EIS and zeta potential behavior, a direct link between the charge transfer resistance and electrical double layers is established which is responsible for excellent capacitive performance of ZnO-RGO-2. The ZnO-RGO-2 displays high specific capacitance (107.9 F/g, scan rate = 50 mVs(-1)) in 1.0 M KCl and exhibits merely 4.2% decay in specific capacitance values over 200 cycles.

A promising photoelectrochemical sensor based on a ZnO particle decorated N-doped reduced graphene oxide modified electrode for simultaneous determination of catechol and hydroquinone

This paper describes the two-step synthesis of a nitrogen-doped reduced graphene oxide–ZnO (N-doped RGO–ZnO) nanocomposite and its photo electrochemical application.

ZnO nanorods inserted graphene sheets with improved supercapacitive performance

In this paper, we demonstrate a facile one-pot approach for the synthesis of reduced graphene oxide-ZnO (Gr–ZnO) hybrids. Structural analysis revealed a homogeneous distribution of ZnO nanorods are inserted in graphene nanosheets, formed a sandwiched architecture. The Gr–ZnO material demonstrates superior electrochemical performance with high specific capacitance (140Fg−1 even at the scan rate of 500mVs−1) and long–term cycle stability (94% capacitance retention over 2000 cycles). The improved supercapacitance property of these materials could be ascribed to the increased the specific surface of graphene sheets by ZnO nanorods inserted. These results demonstrate the potential of the Gr–ZnO hybrid nanocomposites as an electrode in high-performance supercapacitors.

An in situ gelatin-assisted hydrothermal synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic performance under ultraviolet and visible light

A simple route for preparation of ZnO nanosphere–reduced graphene oxide (RGO) composites by an in situ gelatin-assisted hydrothermal synthesis was investigated. The chemical composition, morphology and structural features of the ZnO and ZnO–RGO samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy (XPS), thermogravimatric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption–desorption specific surface area measurements (BET) and diffuse reflectance absorption spectroscopy (DRS). The results showed that ZnO nanoparticles were well dispersed on the RGO nanosheets which serve as ZnO nanocrystal growing supports and the morphology controller. It was first found that through controlling the incorporated contents of RGO with the assistance of gelatin in hydrothermal synthesis, the morphology of ZnO, for example nanoflowers, nanorods and nanospheres, could be controlled. The ZnO–RGO composites were used as photocatalysts for degradation of methylene blue (MB) under ultraviolet (UV) and visible light irradiation. The results suggested that RGO improves the MB degradation performance under both UV and visible light irradiation, and the kinetic rate constants were both improved to 3 times higher by RGO incorporation. It was found that the mechanism of photocatalysis for ZnO–RGO composites under visible light irradiation is different from that under UV light. Under UV light, RGO serves as an electron reservoir to capture or shuttle photogenerated electrons from the ZnO and reduces electron–hole pair recombination; while under visible light, RGO behaves as a photosensitizing semiconductor and electron transformer to ZnO. Moreover, the large surface area and excellent electron transport of RGO also have some positive effects on the improvement of the photocatalytic performance of the composites.

A Multi‐synergistic Platform for Sequential Irradiation‐Activated High‐Performance Apoptotic Cancer Therapy

Artificial hyperthermia is an emerging technique to induce apoptotic cancer cell death. However, achieving effective hyperthermic apoptosis is often difficult, as cells typically acquire resistance to thermal stress. With the aid of sequential irradiation, highly integrated nanoassemblies based on reduced graphene oxide–ZnO nanoparticles–hyaluronic acid (rGo‐ZnO‐HA) can serve as a multi‐synergistic platform for targeted high‐performance apoptotic cancer therapy. The surface engineering of ZnO/graphene hybrid with multifunctional HA biomacromolecules simultaneously confers the system colloidal stability, biocompatibility, and a cancer cell targeting ability. After receptor‐mediated endocytosis, enzyme‐mediated fluorescence activation helps track cellular uptake and provides truly molecular imaging. Furthermore, the reactive oxygen species (ROS) generated by ZnO/rGo under light illumination can effectively sensitize cancer cells to the subsequent NIR laser‐induced apoptotic hyperthermia. In particular, photo modulation of cellular ROS to sensitize cells provides a novel approach to increase the efficacy of hyperthermic apoptosis. These findings suggest that a powerful apoptotic therapeutic platform could be achieved based on the multi‐synergistic platform.