Light-induced oxygen sensing using ZnO/GO based gas sensor

The hydrothermal process was used to prepare Mn3O4/x%GO nanocomposites (NC’s) having different ratios of the Mn3O4 nanoparticles (NP’s) on the surface of graphene oxide (GO) sheet. SEM image showed that the Mn3O4 NP’s were distributed over the surface of GO sheet. HRTEM images exhibited the lattice fringe arising from the (101) plane of the Mn3O4 NP’s having the interplanar d-spacing of 0.49 nm decorating on the surface of GO. The electronic absorption spectra of Mn3O4/x%GO NC’s also show broad bands from 250 to 550 nm. These bands arise from the d–d crystal field transitions of the tetrahedral Mn3+ species and indicate a distortion in the crystal structure. Photo-catalytic activity of spinel ferrite Mn3O4 NP’s by themselves was low but photo-catalytic activity is enhanced when the NP’s are decorating the GO sheet. Moreover, the Mn3O4/10%GO NC’s showed the best photo-catalytic activity. This result comes from the formation of Mn–O–C bond that confirm by FT-IR. This bond would facilitate the transfer of the photoelectrons from the surfaces of the NP’s to the GO sheets. PL emission which is in the violet–red luminescent region shows the creation of defects in the fabricated Mn3O4 NP’s nanostructures. These defects create the defect states to which electrons in the VB can be excited to when the CB. The best degradation efficiency was achieved by the Mn3O4 NP’s when they were used to decorate the GO sheets in the Mn3O4/10%GO NC’s solution.HighlightsLattice fringe of Mn3O4 with an interplanar d-spacing of 0.49 nm for (101) plane.Photocatalytic activity of spinel ferrite Mn3O4 nanoparticles by itself is low.Number of photoelectrons created depends on number of Mn3O4 on a given area of GOThe bonding of the Mn3O4 to the GO sheet would be though a Mn–O–C junction.The degradation processes were accelerated by Mn3O4/10%GO nanocompositesGraphic abstract

Large-area sheets are highly desirable for fundamental research and technological applications of graphene. Here we introduce a modified chemical exfoliation technique to prepare large-area graphene oxide (GO) sheets. The maximum area of the GO sheets obtained can reach ∼40000 μm(2). We found that the GO area is strongly correlated with the C-O content of the graphite oxide, which enables the area of the synthesized GO sheets to be controlled. By simply changing oxidation conditions, GO sheets with an average area of ca. 100-300, ca. 1000-3000, and ∼7000 μm(2) were selectively synthesized. For transparent conductive film applications, thin GO films were fabricated by self-assembly on a liquid/air interface and reduced by HI acid. We found that the sheet resistance of the reduced GO (rGO) films decreases with increasing sheet area at the same transmittance because of the decrease in the number of intersheet tunneling barriers. The rGO film made from GO sheets with an average area of ∼7000 μm(2) shows a sheet resistance of 840 Ω/sq at 78% transmittance, which is much lower than that (19.1 kΩ/sq at 79% transmittance) of a rGO film made from small-area GO sheets of ca. 100-300 μm(2), and comparable to that of graphene films grown on Ni by chemical vapor deposition.

Graphene oxide (GO) is a promising precursor to produce graphene-family nanomaterials for various applications. Their potential health and environmental impacts need a good understanding of their cellular interactions. Many factors may influence their biological interactions with cells, and the lateral dimension of GO sheets is one of the most relevant material properties. In this study, a model bacterium, Escherichia coli ( E. coli ), was used to evaluate the antibacterial activity of well-dispersed GO sheets, whose lateral size differs by more than 100 times. Our results show that the antibacterial activity of GO sheets toward E. coli cells is lateral size dependent. Larger GO sheets show stronger antibacterial activity than do smaller ones, and they have different time- and concentration-dependent antibacterial activities. Large GO sheets lead to most cell loss after 1 h incubation, and their concentration strongly influences antibacterial activity at relative low concentration (<10 μg/mL). In contrast, when incubating with small GO sheets up to 4 h, the inactivation rate of E. coli cells continues increasing. The increase of small GO sheet concentration also results in persistent increases in their antibacterial activity. In this study, GO sheets with different lateral sizes are all well dispersed, and their oxidation capacity toward glutathione is similar, consistent with X-ray photoelectron spectroscopy and ultraviolet-visible absorption spectroscopy results. This suggests the lateral size-dependent antibacterial activity of GO sheets is caused by neither their aggregation states, nor oxidation capacity. Atomic force microscope analysis of GO sheets and cells shows that GO sheets interact strongly with cells. Large GO sheets more easily cover cells, and cells cannot proliferate once fully covered, resulting in the cell viability loss observed in the followed colony counting test. In contrast, small GO sheets adhere to the bacterial surfaces, which cannot effectively isolate cells from environment. This study highlights the importance of tailoring the lateral dimension of GO sheets to optimize the application potential with minimal risks for environmental health and safety.

Despite great recent progress, the processing of graphene oxide (GO) sheets in polymers are often one of the most challenging steps in fabricating graphene/polymer nanocomposites. The challenge is how to achieve high levels of dispersion and reduction of GO simultaneously, without any residual reducing agents in the composites. In this work, microwave irradiation is applied as a remote source to in situ reduce GO sheets embedded in the poly (vinyl alcohol) (PVA) matrix, which process maximizes the advantage of GO's solvent processability. The active response of GO to microwave irradiation allows for the uniform and selective heating of GO, leading to a minutes‐quick reduction of GO without physical damage to the polymer matrix. Significant improvements in the mechanical properties, electrical conductivities and glass transition temperatures (Tgs) of GO/PVA nanocomposites are achieved, and possible reasons for the improvements are also discussed. POLYM. COMPOS., 40:170–178, 2019. © 2017 Society of Plastics Engineers

Excellent photocatalytic performance of Zn(1 − x)MgxO/rGO nanocomposites under natural sunlight irradiation and their photovoltaic and UV detector applications

Effects of probe and bath ultrasonic treatments on graphene oxide structure

The properties and applications of graphene-based materials largely depend on the lateral size and size distribution of graphene and its derivatives sheets. In this work, a novel size separation method was developed for graphene oxide (GO) sheets using a multi-layer filtering of silica gel column. GO sheets were separated into three groups with narrowed size distributions after filtering from the multi-scale holes that were formed due to the packing of irregular silica gel particles. This economical and continuous-operational separation method shows promising future to realize the size separation in industrial sector. The size separation mechanism was considered that the size of GO sheets affected the flow speed, owing to the friction force between the GO sheet and silica gel particle surface. In addition, the related reduced graphene oxide (rGO) was obtained via the separated GO. The size impact of rGO sheets on the electrochemical properties were studied using cyclic voltammetry (CV), galvanostat charge/discharge (GC/D) and electrochemical impedance spectroscopy (EIS).

Graphene oxide (GO) membranes become emerging efficient filters for molecular or ionic separation due to their well-defined two-dimensional nanochannels formed by closely spaced GO sheets and tunable physicochemical properties. The stability of GO membranes in aqueous solutions is a prerequisite for their applications. Here we show a novel and easy strategy for fabricating GO membranes with strong stability in aqueous solutions and controllable lamellar spacing by simply doping with partially reduced graphene oxide (prGO) sheets. With our prGO-doping strategy, the interlayer stabilizing force in GO membranes is enhanced due to the weakened repulsive hydration and enhanced π-π attraction between GO sheets; as a result, the fabricated GO membranes are featured with controllable lamellar spacing and extraordinary stability in water or even strong acid and base solutions as well as strong mechanical properties, which will expand the application scope of GO membranes and provide ever better performances in their applications with aqueous solution environments.

Polystyrene particles “armoured” with nanosized graphene oxide (GO) sheets have been prepared by aqueous miniemulsion polymerization of styrene, exploiting the amphiphilic properties of GO in the absence of conventional surfactants. The nanoscale GO sheets were prepared from graphite nanofibers of diameter approximately 100 nm based on a novel procedure, thus effectively ensuring the absence of larger sheets. Polymerization proceeded to high conversion with minor coagulation, with final number‐average particle diameters of approximately 500 nm, but relatively broad particle size distributions. Scanning electron microscopy analysis revealed particles with a textured surface, consistent with the expected morphology. Interestingly, analysis of GO sheets recovered from the polymerization revealed that the GO sheets are partially reduced during the polymerization—approximately 50% of the initial carboxyl groups of the GO were lost, consistent with some loss in colloidal stability at high conversion. The overall approach offers a convenient and attractive synthetic route to novel graphene‐based polymeric nanostructures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Graphene oxide (GO), the oxidized state of graphene, has a similar chemical structure to graphene, except for the existence of many defects and oxygen-containing functional groups. GO could be photoreduced under light irradiation, and it is crucial to study and understand the changes of chemical structure and photoluminescence during this photoreduction process. Herein, by using single-molecule optical microscopy, the photoreduction of three different GO sheets were monitored in situ. The photoluminescence from highly oxidized GO sheets changed continuously under light irradiation, and many blinking spots showed up on this GO sheet surface. The appearance of these blinking events is highly related to the stacking of the GO sheets. Much less blinking could be observed on stacked GO sheets. It was found that the reduction speed of GO in water is much higher than that in air, and much less blinking could be observed when GO was reduced in water. Moreover, this is also highly related to the chemical structures of the GO sheets used for photoreduction. No blinking could be observed on less oxidized GO sheets. This work provides an effective approach for in situ study of the optical properties of GO sheets, boosting a deeper understanding of GO photoreduction. Moreover, this method could also be extended for in situ studying the optical properties of other materials.

The tunable electronic, electrical and optical properties of graphene oxide (GO) sheets were investigated using a controlled reduction by 500 keV Ar+-ion irradiation. The carbon to oxygen ratio of the GO sheets upon the ion beam reduction has been estimated using resonant Rutherford backscattering spectrometry analyses and its effect on the electrical and optical properties of GO sheets has been studied using sheet resistance measurements and photoluminescence (PL) measurements. The restoration of sp2-hybridized carbon atoms within the sp3 matrix is found to be increases with increasing the Ar+-ion fluences as evident from Fourier transform infrared, and x-ray absorption near-edge structure measurements. The decrease in the number of disorder-induced local density of states (LDOSs) within the π–π* gap upon the reduction causes the shifting of PL emission from near infra-red to blue region and decreases the sheet resistance. The improved electrical and optical properties of GO sheets were correlated to the decrease in the number of LDOSs within the π–π* gap. Our experimental investigations suggest ion beam irradiation is one of an effective approaches to reduce GO to RGO and to tailor its electronic, electrical and optical properties.

Optimization for size separation of graphene oxide sheets by flow/hyperlayer field-flow fractionation

Morphology-controlled synthesis of CdSe microspheres on graphene oxide sheets and their photocatalytic properties

A method for preparing three-dimensional (3D) carbon-based architectures consisting of mesoporous carbon spheres intercalated between graphene sheets is demonstrated in this paper. Colloidally dispersed negatively charged graphene oxide (GO) sheets strongly interacted with positively charged mesoporous silica spheres (MSS) to form a MSS–GO composite. The MSS were then used as template for replicating mesoporous carbon spheres (MCS) via a chemical vapor deposition process, during which the GO sheets were reduced to reduced graphene oxide (RGO). Removal of the silica spheres left behind a 3D hierarchical porous carbon architecture with slightly crumpled graphene sheets intercalated with MCS. The 3D carbon structure contained a low amount of oxygen (3.2% of atomic ratio of O/C) than a RGO sample (10.1%), which was prepared by using the chemical reduction method with hydrazine as the reducing agent. Thermal annealing of the 3D carbon structure in ammonia atmosphere further reduced the O/C atomic ratio to 1.6%. The capacitive performance of the samples as supercapacitor electrodes was investigated using the cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy techniques. The 3D carbon structure showed a substantially lower equivalent series resistance and a higher power capability than the RGO electrode. In addition, the 3D carbon electrode exhibited an excellent electrochemical cyclability with 94% capacitance retention after 1000 cycles of galvanostatic charge–discharge. The method demonstrated in this work opens up a new route to the preparation of 3D graphene-based architectures for supercapacitor applications.

The association of cellular toxicity with the physiochemical properties of graphene‐based materials is largely unexplored. A fundamental understanding of this relationship is essential to engineer graphene‐based nanomaterials for biomedical applications. Here, an in vitro toxicological assessment of graphene oxide (GO) and reduced graphene oxide (RGO) and in correlation with their physiochemical properties is reported. GO is found to be more toxic than RGO of same size. GO and RGO induce significant increases in both intercellular reactive oxygen species (ROS) levels and messenger RNA (mRNA) levels of heme oxygenase 1 (HO1) and thioredoxin reductase (TrxR). Moreover, a significant amount of DNA damage is observed in GO treated cells, but not in RGO treated cells. Such observations support the hypothesis that oxidative stress mediates the cellular toxicity of GO. Interestingly, oxidative stress induced cytotoxicity reduces with a decreasing extent of oxygen functional group density on the RGO surface. It is concluded that although size of the GO sheet plays a role, the functional group density on the GO sheet is one of the key components in mediating cellular cytotoxicity. By controlling the GO reduction and maintaining the solubility, it is possible to minimize the toxicity of GO and unravel its wide range of biomedical applications.