Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal–Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes

Laminar membranes comprising graphene oxide (GO) and metal–organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H₂TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0 L m⁻² h⁻¹ bar⁻¹ and high anionic dye rejection (>99% for methyl blue).     

Graphene oxide: the mechanisms of oxidation and exfoliation

Graphene oxides (GOs) with large sheets and more perfect aromatic structure were prepared by a novel modified Hummers method. We demonstrated that the graphite did not need to be oxidized to such a deep degree as described in Hummers method because the space distance increased little when the oxidation proceeded to a certain extent and the obtained graphite oxides (GTOs) could be fully exfoliated to single layers with high thermal stability. The oxidation mechanism and chemical structure model of GO were proposed by analyzing the evolution of the functional groups with oxidation proceeded based on thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The layer spacing calculated by molecular dynamics simulations coincided with the X-ray diffraction results. Furthermore, the size distribution and thickness of GOs were also studied. The results confirmed that the GOs prepared by the modified method were fully exfoliated to uniform single layers, and this method may be important for efficient exfoliation of GTO to GO and large-scale production of graphene.     

Size-specified graphene oxide sheets: ultrasonication assisted preparation and characterization

The preparation of graphene oxide (GO) sheets with specified size was developed by simply controlling the time of ultrasonication to the large-size GO (LGO) sheets. The LGO sheets were synthesized by choosing large parent graphite, mild oxidation condition and a two-step centrifugation. The different-sized GO samples prepared under different ultrasonication times, are characterized by Scanning electron microscopy, X-ray photoelectron spectroscopy, Ultraviolet–visible spectroscopy, and X-ray diffraction. It is found that the size of the GO sheets, which has a Gaussian distribution, decreases from 231 to 17 μm² as the ultrasonication time increases. Moreover, the ultrasonication not only can exfoliate and break GO sheets, but also increase the oxidation degree of GO sheets, especially when the GO sheets have a weak oxidation degree. It is reasonable to believe that the size of GO sheets is closely correlated to the C–O content, which enables the size of GO sheets to be controlled. Our work demonstrates that ultrasonication is an important method to control the size and the oxidation degree of GO sheets, to a certain extent.     

Facile and size-controllable preparation of graphene oxide nanosheets using high shear method and ultrasonic method

ABSTRACT

The lateral size of the graphene oxide (GO) nanosheets could be controlled by preparation method, and a simple and effective strategy to adjust the lateral size of GO nanosheets by selecting suitable method is presented. The high shear method was introduced to produce GO nanosheets, and the GO nanosheets (few micrometres) prepared by high shear method is about one order of magnitude larger than GO nanosheets (few hundred nanometres) obtained by ultrasonic method, as evidenced by atomic force microscopy. The FTIR, XPS and Raman analysis revealed that there are no distinct differences in composition and functional groups between the GO nanosheets produced by high shear method and ultrasonic method. The cavitation in the procedure of ultrasonic method is favourable for GO exfoliation, but it also could result in damage to GO nanosheets. The shearing force in the process of high shear method is effective for GO delamination with minimal fragmentation. The results indicated that the high shear method proposed in this paper is an efficient exfoliation means to produce single-layer GO nanosheets.     

Influence of graphene oxide with different degrees of oxidation on the conductivity of graphene/poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) composites

A modified Hummers method was used to synthesize graphene oxide (GO) with different degrees of oxidation. Single-factor and orthogonal experiments were designed to analyze the influence of the oxidant dose on the properties of the GO. The GO series were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, and zeta potential measurements. We synthesized RGO/PEDOT:PSS composites by an in situ polymerization with the GO. Through analysis of the specific conductance of the composites, we determined the optimum conditions for forming RGO/PEDOT:PSS which can improve the specific conductance to 114.06 S/cm. Thus, by changing the oxidation degree of GO we were able to improve the conductivity of RGO/PEDOT:PSS composites.     

Managing the degree of exfoliation and number of graphene layers using probe sonication approach

We have demonstrated a fast, versatile, and scalable approach to synthesize high-quality few layer graphene sheets with low defect ratio and high crystallinity produced from exfoliation of graphite flakes in DMF by using probe sonication. The effect of sonication time on degree of exfoliation and number of graphene layers has been fully investigated. The degree of exfoliation of graphene sheets as a function of sonication time has been successfully analyzed by XRD, UV-Vis spectroscopy, TEM, and BET studies. The morphological changes at different sonication times have also been observed by SEM. A structural and defect characterization of graphene sheets has been discussed in detail by Raman spectroscopic technique. The shift in position of 2D Raman band and its de-convolution provided information about formation of multi to few layer graphene sheets with sonication. Moreover, Raman results are highly consistent with TEM studies as per number of graphene layers is concerned.     

Preparation and properties of polystyrene nanocomposites with graphite oxide and graphene as flame retardants

In this study, graphite oxides (GOs) with different oxidation degrees and graphene nanosheets were prepared by a modified Hummers method and thermal exfoliation of the prepared GO, respectively. Polystyrene (PS)/GO and PS/graphene nanocomposites were prepared via melt blending. X-ray diffraction results showed that GOs and graphene were exfoliated in the PS composites. It could be observed from the scanning electron microscope images that GOs and graphene were well dispersed throughout the matrix without obvious aggregates. Dynamic mechanical thermal analysis suggested that the storage modulus for the PS/GO1 and PS/graphene nanocomposites was efficiently improved due to the low oxygen content of GO1 and the elimination of the oxygen groups from GO. The flammability of nanocomposites was evaluated by thermal gravimetric analysis and cone calorimetry. The results suggested that both the thermal stability and the reduction in peak heat release rate (PHRR) decreased with the increasing of the oxygen groups in GOs or graphene. The optimal flammability was obtained with the graphene (5 wt%), in which case the reduction in the PHRR is almost 50 % as compared to PS.     

不同氧化程度氧化石墨烯/聚乙烯醇气凝胶对亚甲基蓝的吸附

氧化石墨烯(GO)对水中染料有着优异的吸附性能,但其氧化程度对复合材料吸附性能和机制的影响还未被充分研究。采用Hummer法,制备3种不同氧化程度的氧化石墨烯,与聚乙烯醇(PVA)复合得到三种GO/PVA气凝胶。利用红外光谱(FTIR)、元素分析(EA)和热重分析(TG)分析了3种GO的氧化程度;以亚甲基蓝(MB)为模拟污染物,通过静态吸附实验考察了GO氧化程度对GO/PVA气凝胶在不同溶液pH、吸附时间、初始浓度下对MB吸附性能的影响。通过吸附动力学模型、吸附等温线模型和吸附热力学模型探究了GO氧化程度对GO/PVA气凝胶吸附机制的影响。研究结果表明:GO/PVA气凝胶对MB的吸附行为受pH影响较小;提高GO的氧化程度可以明显提升GO/PVA气凝胶的吸附容量和吸附速度,GO氧化程度的提高增加了气凝胶上的吸附位点,有利于吸附。此外,GO氧化程度对GO/PVA气凝胶的吸附机制无明显影响。      

Reinforcing polyamide 1212 with graphene oxide via a two-step melt compounding process

In this study, graphene oxide (GO) was synthesized from graphite powders via Hummers’ method. Polyamide 1212 (PA1212)/GO composites were prepared via a two-step melt compounding process. First, GO concentrates were prepared via solution coagulation. In this method, a GO solution was mixed with an ethanol-soluble polyamide solution. The resulting product was melt-compounded with a PA1212 matrix. This method enabled GO nanosheets to be well-dispersed in a PA1212 matrix. GO, which functioned as a nucleation template, exhibited heterogeneous nucleation effect in the PA1212 matrix because of its large specific surface area. The mechanical properties of the oriented PA1212/GO composites improved efficiently compared with those of pure PA1212. Crystal orientation degree and crystallinity in the composites increased slightly when GO was added after drawing. The composites’ reinforcing effect was mainly attributed to GO nanosheet alignment. These nanonsheets functioned as the nuclei to reinforce the entire oriented crystals.     

Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

以氧化石墨烯为氧化介质制备石墨烯/聚苯胺导电复合材料

采用超声辅助Hummers法制得厚度约为1 nm的氧化石墨烯, 以其为氧化介质与苯胺反应合成了石墨烯/聚苯胺(RGO/PANI)导电复合材料。利用AFM、SEM、XRD和FTIR对反应所得产物进行了表征。结果表明: 苯胺在略高于室温的酸性水溶液中可以对氧化石墨烯(GO)进行还原, 而苯胺自身则被氧化石墨烯中大量的含氧基团氧化并发生聚合反应, 最终生成RGO/PANI导电复合材料, 当苯胺用量为1 mL, 氧化石墨烯用量为0.1 g, 在水浴温度为70 ℃下剧烈搅拌24 h时, 获得的RGO/PANI复合材料导电性最佳, 约为10 S/cm。     

Sulfur and Nitrogen Co‐Doped Graphene for Metal‐Free Catalytic Oxidation Reactions

Sulfur and nitrogen co‐doped reduced graphene oxide (rGO) is synthesized by a facile method and demonstrated remarkably enhanced activities in metal‐free activation of peroxymonosulfate (PMS) for catalytic oxidation of phenol. Based on first‐order kinetic model, S–N co‐doped rGO (SNG) presents an apparent reaction rate constant of 0.043 ± 0.002 min^?1, which is 86.6, 22.8, 19.7, and 4.5‐fold as high as that over graphene oxide (GO), rGO, S‐doped rGO (S‐rGO), and N‐doped rGO (N‐rGO), respectively. A variety of characterization techniques and density functional theory calculations are employed to investigate the synergistic effect of sulfur and nitrogen co‐doping. Co‐doping of rGO at an optimal sulfur loading can effectively break the inertness of carbon systems, activate the sp²‐hybridized carbon lattice and facilitate the electron transfer from covalent graphene sheets for PMS activation. Moreover, both electron paramagnetic resonance (EPR) spectroscopy and classical quenching tests are employed to investigate the generation and evolution of reactive radicals on the SNG sample for phenol catalytic oxidation. This study presents a novel metal‐free catalyst for green remediation of organic pollutants in water.     

Conjugated‐Polyelectrolyte‐Functionalized Reduced Graphene Oxide with Excellent Solubility and Stability in Polar Solvents

Conjugated‐polyelectrolyte (CPE)‐functionalized reduced graphene oxide (rGO) sheets are synthesized for the first time by taking advantage of a specially designed CPE, PFVSO₃, with a planar backbone and charged sulfonate and oligo(ethylene glycol) side chains to assist the hydrazine‐mediated reduction of graphene oxide (GO) in aqueous solution. The resulting CPE‐functionalized rGO (PFVSO₃‐rGO) shows excellent solubility and stability in a variety of polar solvents, including water, ethanol, methanol, dimethyl sulfoxide, and dimethyl formamide. The morphology of PFVSO₃‐rGO is studied by atomic force microscopy, X‐ray diffraction, and transmission electron microscopy, which reveal a sandwich‐like nanostructure. Within this nanostructure, the backbones of PFVSO₃ stack onto the basal plane of rGO sheets via strong π–π interactions, while the charged hydrophilic side chains of PFVSO₃ prevent the rGO sheets from aggregating via electrostatic and steric repulsions, thus leading to the solubility and stability of PFVSO₃‐rGO in polar solvents. Optoelectronic studies show that the presence of PFVSO₃ within rGO induces photoinduced charge transfer and p‐doping of rGO. As a result, the electrical conductivity of PFVSO₃‐rGO is not only much better than that of GO, but also than that of the unmodified rGO.     

Facile synthesis of poly(Safranine T)/Reduced graphene oxide nanocomposite for supercapacitors with wide potential window in aqueous neutral electrolyte

A facile method is introduced for incorporating reduced graphene oxide (rGO) into poly(safranine T) (PST) films. First, ST-functionalized GO (ST/GO) was obtained via the absorption of ST on GO in pH 7.0 phosphate buffer solution. Then rGO/PST composite was synthesized by the electropolymerization of ST and the subsequent electrochemical reduction of GO. The as-prepared PST/rGO composite films are characterized using scanning electron microscope, X-ray diffraction, and Fourier transform–infrared spectroscopy. PST/rGO composites possess a microporous structure, which creates enormous amount of pores, and therefore provides larger interfaces for charge carrier. The properties of electrochemical capacitance for PST/rGO composites have also been investigated with cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. The experimental results manifest that the PST/rGO composite showed high capacitance (293.2 F g^?1) at 20-mV s^?1 CV scan and an excellent cycling stability (8.3% drop after 1000 cycles) in 0.1 M Na₂SO₄ electrolyte.     

An electrochemical route to graphene oxide

Large-scale graphene oxide (GO) with adjustable resistivity was synthesized from graphite via an electrochemical method using KCl solution as an effective electrolyte. During the exfoliation process, electrostatic force intercalates chloride ions between the expanded graphite layers on the anode. These chloride ions form small gas bubbles between the graphite layers in the electrochemical reaction. It is believed that the gas bubbles expand the gap between graphite sheets and produce a separating force between adjacent graphene layers. This separating force overcomes the Van der Waals force between adjacent sheets and exfoliates graphene layers from the starting graphite. Because the graphene is electrochemically oxidized by chorine during the exfoliation, the exfoliated GO sheets are hydrophilic and easily dispersed in the electrolyte solution. The GO solution prepared by the electrochemical exfoliation can be simply sprayed or spin-coated onto any substrate for device applications. The measured average thicknesses of a monolayer, bilayer, and trilayer exfoliated GO on SiO2 substrate were 1.9, 2.8, and 3.9 nm, respectively. It was observed that the measured resistance of the exfoliated GO sheets increases due to electrochemical oxidation in the solution. This electrochemical approach offers a low-cost and efficient route to the fabrication of graphene based devices.     

L-半胱氨酸还原氧化石墨烯的研究

采用改进Hummers法合成氧化石墨,在水中超声分散获得氧化石墨烯水溶胶,并加入L-半胱氨酸于95℃进行回流反应后得到还原氧化石墨烯。采用X射线衍射仪、傅里叶变换红外光谱仪和热重-差热扫描仪等探讨氧化石墨烯还原前后的结构与性能变化。结果表明,L-半胱氨酸能有效还原氧化石墨烯,且还原后的氧化石墨烯在乙醇中有较好的分散性,其所制薄膜的导电率为500S/m。由此法制备的石墨烯有望广泛应用于电子、光电、电容器和传感器等器件中。     

氧化石墨烯/聚氨酯杂化膜的原位聚合构建及气体渗透性

采用Hummers法制备了3种不同氧化程度的氧化石墨烯(GO),通过聚氨酯(PU)单体(4,4’-二异氰酸苯酯(MDI)和1,4-丁二醇(BDO))与GO的原位聚合构建了GO/PU杂化膜。利用XRD、Raman、FTIR和TEM等表征了GO的结构;探讨了GO填充量对GO/PU杂化膜的形貌和CO2、N2渗透性的影响。结果表明:3种不同氧化程度的GO均呈完全剥离状态,为半透明片状结构;随着氧化程度的增加,拉曼D峰与G峰的相对强度比分别为0.947、1.103和1.245;GO的氧化程度对GO在溶剂和杂化膜中的分散性有较大影响,氧化程度越高,分散性越好。GO/PU杂化膜的CO2、N2渗透系数及CO2/N2渗透选择因子均随GO填充量的增加先增大后减小;当中等氧化程度的GO(M-GO)与(MDI+BDO)的质量比为1.0%时,M-GO/PU杂化膜的CO2渗透系数为63.6×10-13 cm3(STP)/(cm·Pa·s),其中STP表示标准温度及压力,CO2/N2渗透选择因子可达48.5;填充适量的GO能显著提高GO/PU杂化膜的CO2渗透性及CO2/N2渗透选择性。     

碳纤维-氧化石墨烯/环氧树脂复合材料的制备及表征

采用改进的Hummers法制备了五种具有不同氧化程度的氧化石墨烯（GO）。借助元素分析、X射线光电子能谱及FTIR红外光谱测试对所制备GO的组成及结构衍变进行了表征。利用光学显微镜对不同GO在固化剂异佛尔酮二胺（IPDA）中的分散状态进行观察,并将分散效果最佳的GO试样用于改性碳纤维/环氧树脂（CF/EP）复合材料。结果表明,随着氧化剂用量及反应时间的增加,GO的氧化程度也随之增加。在氧化程度较低时,GO表面官能团主要以羰基、羧基和酚羟基为主。随着氧化程度的不断提高,GO表面官能团主要为醚、环氧和醇羟基结构。GO在IPDA中的分散状态与其氧化程度密切相关,氧化程度最低和最高的GO均出现明显的聚集现象。另外,GO表面在分散过程中可被IPDA化学改性。在EP基体中加入分散效果最佳的GO（0.2wt%）后,与CF/EP复合材料相比,CF-GO/EP复合材料的弯曲强度、层间剪切强度和Ⅱ型层间断裂韧性分别提高了14%、17%和14%。     

An investigation of the electrical transport properties of graphene-oxide thin films

The electrical transport properties of graphene-oxide (GO) thin films were investigated. The GO was synthesized by a modified Hummers method and was characterized by X-ray diffraction and UV-visible spectroscopy. The thin film of GO was made on a Si/SiO₂ substrate by drop-casting. The surface morphology of the GO film was analyzed by using scanning electron microscopy and atomic force microscopy techniques. Temperature dependent resistance and current-voltage measurements were studied using four-terminal method at various temperatures (120, 150, 175, 200, 250 and 300 K) and their charge transport followed the 3D variable range hopping mechanism which was well supported by Raman spectra analysis. The presence of various functional groups in GO were identified by using high resolution X-ray photo electron (XPS) and Fourier transform infra red (FT-IR) spectroscopic techniques. Graphene-oxide thin film field effect transistor devices show p-type semiconducting behavior with a hole mobility of 0.25 cm² V⁻¹ s⁻¹ and 0.59 cm² V⁻¹ s⁻¹ when measured in air and vacuum respectively.     

Effect of reduced graphene oxide (rGO) on structural,optical, and dielectric properties of Mg(OH)2/rGO nanocomposites

In this paper, we report the synthesis of Mg(OH)₂ NPs and Mg(OH)₂–rGO nanocomposites (NCs) by microwave assisted co-precipitation method. The crystal phase, structural morphology and functional groups of the as-synthesized samples were analyzed by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR). Raman spectroscopy was used to study the defects in the samples. Raman spectroscopy and the SEM results validate the growth of Mg(OH)₂ NPs on the rGO nanosheets. The chemical composition of the prepared samples was analyzed by EDAX. Optical properties of the as-synthesized samples were studied by UV–visible spectroscopy and the energy band gap was calculated by Tauc relation which shows a decrease in band gap with an increase in the amount of Graphene Oxide (GO) in the NCs. The dielectric properties were studied as a function of frequency over a range of 50 Hz to 5 MHz at room temperature. The value of dielectric constant decreases with an increase in frequency, this could be due to the existence of a polarization process at the border of the rGO sheets and Mg(OH)₂ NPs. The value of dielectric loss shows a decreasing trend with an increase in frequency whereas the larger value of AC conductivity in Mg(OH)₂–rGO NCs as compared to Mg(OH)₂ NPs approves the restoration of sp² network in the graphene sheets.