Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide

Chemically modified graphene sheets are obtained through solvothermal reduction of colloidal dispersions of graphite oxide in various solvents. Reduction occurs at relatively low temperatures (120-200 °C). Reaction temperature, the self-generated pressure in the sealed reaction vessel and the reducing power of the solvent influences the extent of reduction of graphite oxide sheets to modified graphene sheets.     

Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy

Several nanometer-thick graphene oxide films deposited on silicon nitride-on silicon substrates were exposed to nine different heat treatments (three in Argon, three in Argon and Hydrogen, and three in ultra-high vacuum), and also a film was held at 70 °C while being exposed to a vapor from hydrazine monohydrate. The films were characterized with atomic force microscopy to obtain local thickness and variation in thickness over extended regions. X-ray photoelectron spectroscopy was used to measure significant reduction of the oxygen content of the films; heating in ultra-high vacuum was particularly effective. The overtone region of the Raman spectrum was used, for the first time, to provide a “fingerprint” of changing oxygen content.     

Preparation of carbon-based transparent and conductive thin films by pyrolysis of silylated graphite oxides

Thin films of silylated graphite oxide were obtained from a chloroform/cyclohexane dispersion of n-hexadecylamine-intercalated silylated graphite oxide by a casting method at a low temperature. Carbon-based thin films were obtained from the pyrolysis of the resulting films under a reduced pressure at 500 °C or higher temperatures. The resulting samples were well adhered to the substrate because of the presence of silicon containing species as a “glue”. The resistivity decreased with an increase in the film thickness or a decrease in the transparency. Based on the data obtained for the samples prepared from graphite with different particle sizes and graphite oxide with different oxygen contents, the conduction of the electrons within each carbon sheet seemed important for large film thickness and conduction through the boundary seemed important when the film thickness was small. A low sheet resistance of 3.7 kΩ/sq for 80% of transmittance was achieved, when graphite oxide with a lower oxygen content was prepared from graphite with smaller particle sizes and the precursor film was heated at 500 °C. At 900 °C, it further decreased to a value of 700 Ω/sq.     

Production of graphene from graphite oxide using urea as expansion-reduction agent

Graphene sheets were produced from graphite oxide using a simple two-step process. First, graphite oxide (GO) is well mixed with an expansion-reduction agent, such as urea, that decomposes upon heating to release reducing gases. Second, the mix is heated in an inert gas environment (e.g. N₂) for a very short time to a moderate temperature (ca. 600 °C). Reaction temperature selection should be consistent with the decomposition temperature of the expansion-reduction agent. Upon cooling, graphene can readily be collected as the solid byproduct. Graphene samples were characterized by XRD, TEM, EELS, SEM, Raman Spectroscopy and the GO and urea mixtures decomposition-reduction process studied by TGA/DSC analysis. This graphene generation process is rapid, inexpensive and easy to scale up.     

Changes in electrical and microstructural properties of microcrystalline cellulose as function of carbonization temperature

AC and DC electrical measurements were made to better understand the thermal conversion of microcrystalline cellulose to carbon. This study identifies five regions of electrical conductivity that can be directly correlated to the chemical decomposition and microstructural evolution of cellulose during carbonization. In Region I (250-350 °C), a decrease in overall AC conductivity occurs due to the loss of the polar oxygen-containing functional groups from cellulose molecules. In Region II (400-500 °C), the AC conductivity starts to increase with heat treatment temperature due to the formation and growth of conducting carbon clusters. In Region III (550-600 °C), a further increase of AC conductivity with increasing heat treatment temperature is observed. In addition, the AC conductivity demonstrates a non-linear frequency dependency due to electron hopping, interfacial polarization, and onset of a percolation threshold. In Region IV (610-1000 °C), a frequency independent conductivity (DC conductivity) is observed and continues to increase with heat treatment due to the growth and further percolation of carbon clusters. Finally in Region V (1200-2000 °C), the DC conductivity reaches a plateau with increasing heat treatment temperature as the system reaches a fully percolated state.     

The production of graphene nanosheets decorated with silver nanoparticles for use in transparent, conductive films

Aggregation and restacking of graphene nanosheets (GNS) can be efficiently inhibited by decorating the silver nanoparticles on the surface of GNS to form GNS/silver (GNS-Ag) composites, which can construct high transparent and electrically conductive thin films. Silver nanoparticles act as a useful nanospacer and conductor, which not only increase the interlayer distance but also improve the electrical conductivity between layers. A two-step reduction process using sodium borohydride and ethylene glycol was also demonstrated reducing graphene oxide to GNS efficiently. The GNS-Ag composite films showed a maximum sheet resistance of 93 Ω□⁻¹, while maintaining up to 78% light transmittance, which was two order of magnitude lower than that of GNS (8.2 × 10³ Ω□⁻¹, 81%), and the value of DC conductivity to optical conductivity ratio was 13.5 instead of 0.25.     

Fast and simple fabrication of a large transparent chemically-converted graphene film by spray-coating

We report the fast, low-cost, simple fabrication of a large chemically-converted graphene (CCG) films by spray deposition of graphene oxide (GO)-hydrazine dispersions. The GO-hydrazine dispersion was prepared by mixing GO dispersion with excess amount of hydrazine monohydrate. By spray deposition on preheated substrate, the creation of the thin film and the reduction of GO to CCG were carried out simultaneously. The prepared CCG films had a low sheet resistance of 2.2 × 10³ Ω □⁻¹ and a high transmittance of 84% at a wavelength of 550 nm. Atomic force microscope images clearly showed continuous films resulting from the overlap of graphene sheets and a uniform surface morphology with root mean square about 1 nm.     

Low temperature elastic properties of chemically reduced and CVD-grown graphene thin films

We have measured internal friction and shear modulus of both reduced graphene oxide and chemical-vapor deposited graphene films measuring as thin as 5 nm. Graphene oxide films were deposited from solutions by spin-coating, and graphene films were synthesized by chemical-vapor deposition (CVD) on Ni thin films. In both cases, these films were transferred from their host substrate into a water bath, then re-deposited onto to a high-Q single crystal silicon mechanical double-paddle oscillator. A minimal thickness dependence of both internal friction and shear modulus was found within the experimental uncertainty for reduced graphene oxide films varying thickness from 5 to 90 nm. The internal friction of all films exhibits a temperature independent plateau below 10 K. The values of the plateaus are similar for both the reduced graphene oxide films and CVD graphene films, and they are as high as the universal “glassy range” where the tunneling states dominated internal friction of amorphous solids lies. This result shows that from a mechanical loss point of view, both graphene oxide and CVD graphene films have high and similar level of disorder. Raman measurements performed on the same samples show higher structure order in CVD graphene films than in graphene oxide films. Our results suggest that internal friction probes different sources of disorder from those by Raman, and the disorder is not directly related to the existence of C–O binding in the graphene oxide films. The shear modulus averages 53 GPa after subtracting Young's modulus component from the vibration mode used in experiments.     

Synthesis of graphene on silicon carbide substrates at low temperature

A method for the synthesis of millimeter-scaled graphene films on silicon carbide substrates at low temperatures (750 °C) is presented herein. Ni thin films were coated on a silicon carbide substrate and used to extract the substrate’s carbon atoms under rapid heating. During the cooling stage, the carbon atoms precipitated on the free surface of the Ni and formed single-layer or few-layer graphene. The result shows that the number of graphene layers might be further controlled by appropriate process conditions. In contrast to the epitaxial graphene synthesis on single crystal silicon carbide, the graphene prepared here are continuous over the entire Ni-coated area, and can be stripped from the substrate much more easily for further characterization. The large-scaled, low temperature and transferable features of our method suggest the potential for future graphene-based applications.     

Surface chemistry of Ni induced graphite formation on the 6H-SiC (0 0 0 1) surface and its implications for graphene synthesis

Ni films ranging in thickness from 0.4 nm to 50 nm were deposited by evaporation onto terraced SiC (0 0 0 1) substrates at room temperature and annealed at 700 °C. The resulting changes in surface composition and morphology were characterized using Auger electron spectroscopy and atomic force microscopy. In all cases, graphitic films dominate the surface chemistry. There appears to be three different thickness dependent morphology regimes. For the thinnest Ni films (0.4 nm), there is a uniform carbon-overlayer. For slightly thicker Ni films (0.6-9.6 nm), clustering and platelet formation are observed, and for still thicker films (50 nm), the platelets give way to hillocks. Within the platelet regime, there is a critical thickness at which surface roughening occurs. These results reveal a potential parametric window in which graphene may be produced and harvested.     

还原温度对氧化石墨烯的湿度-甲醛交叉敏感性能的影响

以氧化石墨烯溶胶为前体,通过旋涂工艺制备薄膜型气敏元件,在低温80～180℃下进行热处理,获得系列不同还原程度的还原氧化石墨烯气敏元件,采用XRD、AFM、FT-IR、XPS对样品的层结构、薄膜厚度及含氧官能团变化属性进行表征,将气敏薄膜元件在相对湿度为11.3%～93.6%的范围内进行预湿处理,并测定元件对甲醛气氛的敏感性能。结果表明：随热还原处理温度的升高,氧化石墨烯的结构逐渐向类石墨结构转变,含氧官能团逐渐脱失,缺陷增多,薄膜的方块电阻呈数量级地减小,从41 MΩ减小至928 Ω;经不同湿度预处理的气敏元件置于甲醛气氛中产生了水分子与甲醛分子的竞争吸附,从而导致电阻的明显变化;在10^?4甲醛气氛下,未还原或热还原温度较低的气敏元件适用于低、高湿环境下甲醛气氛的气敏测试,最大灵敏度为69.1%,而还原温度适中的元件则适用于中湿环境的甲醛测试,最大灵敏度为80.3%。     

Characterization of indium tin oxide (ITO) thin films prepared by a sol-gel spin coating process

Indium tin oxide (ITO) thin films were prepared by a sol-gel spin coating method, fired, and then annealed in the temperature range of 450-600°. The XRD patterns of the thin films indicated the main peak of the (2 2 2) plane and showed a higher degree of crystallinity with an increase in the annealing temperature. Upon annealing the films at 500 and 600°, two binding energy levels of Sn⁴⁺ ion of 486.9 eV and 486.6 eV, respectively, were measured in the XPS spectra. The ITO film that was annealed at 600° contained two oxidation states of Sn, Sn²⁺ and Sn⁴⁺, and it had a higher sheet resistance based on a rather low doping concentration of Sn⁴⁺. The film that was annealed at 500° and subsequently treated with 0.1 N HCl solution for 40 s showed a sheet resistance of 225 Ω/square. The surface treatment by the acidic solution diminished the RMS (root mean square) roughness value and the residual carbon content (XPS peak intensity of carbon) of the ITO films. It seems that the acid-cleaning of the ITO thin films led to a decrease of the surface roughness and sheet resistance.     

Preparation of graphene nanosheet/polymer composites using in situ reduction-extractive dispersion

Graphene nanosheet/polymer composites were prepared using in situ reduction-extractive dispersion technology. The morphology and microstructure of the composites were examined by scanning electron and optical microscopy. The results indicate that graphene nanosheets from the reduction of graphite oxide are about 5 nm thick and 1-3 μm in diameter. Reduction-extractive dispersion technology can effectively promote the dispersion of graphene nanosheets and consequently an excellent conductive network is formed in the matrix. The percolation threshold of the composite is about 0.15 vol.%. When the graphene nanosheet content is lower than 1.5 vol.%, the conductivity of the composites is 3-5 orders of magnitude higher than that of composites filled with graphite nanosheets from expanded graphite.     

A simple two-step electrochemical synthesis of graphene sheets film on the ITO electrode as supercapacitors

In this study, a simple and controllable two-step electrochemical process is described for the synthesis of graphene sheets (GS) film on a cleaned indium tin oxide (ITO) sheet electrode. Namely, the main procedures involve the electrophoretic deposition (EPD) of graphene oxide (GO) film onto ITO electrode and the subsequent in situ electrochemical reduction (ECR) of GO to generate GS film. X-ray photoelectron spectroscopy (XPS) measurement demonstrates that most of the oxygen-containing functional groups in GO film have been removed after ECR. By electrochemical measurements, the maximum specific capacitance of the prepared GS film electrode was calculated to be 156 F g⁻¹, besides, the capacitance retention of the material remained 78% after 400 times of cycling, showing a promising prospect as supercapacitor materials.     

The effect of graphitization temperature on the structure of helical-ribbon carbon nanofibers

Structural rearrangement of helical-ribbon carbon nanofibers (CNFs) was studied as a function of graphitization temperature. The as-produced nanofibers are composed of a helical ribbon of graphene spiralled about and angled to the fiber axis. The discrete layers of graphene ribbon overlap each other forming the helical-ribbon in contrast to the discontinuous cones of the more common stacked-cup CNF morphology. After heat treatment to 2400 °C and above, the CNFs were completely free of residual metal catalyst inclusions, principally nickel used in their synthesis, and other functionalities. The formation of loops at the graphene edges was also observed. Heat treatment through the temperature range 1500-2800 °C resulted in a relatively minor contraction in interlayer spacing d₀₀₂ from 0.3381 to 0.3363 nm. This was attributed to the highly graphitic character of the as-produced CNFs. However, there were significant increases in the crystallite thickness L_c through this temperature range. In addition, heat treatment above 2400 °C induced a marked change of the nanofiber morphology from circular to faceted polygonal cross-section resulting from the re-ordering of the turbostratic, curved graphene layers to regions of planar graphene layers with 3-dimensional graphitic structure (AB stacking).     

Graphene nanosheets prepared by low-temperature exfoliation and reduction technique toward fabrication of high-performance poly(1-butene)/graphene films

Thermal exfoliation and reduction of graphene oxide (GO) were performed to prepare graphene nanosheets at 300 °C under the ambient atmosphere without any supplementary conditions. The microstructure and morphology of the resulting graphene nanosheets were characterized with scanning electron microscopy, transmission electric microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The composite films based on poly(1-butene) (PB) and graphene nanosheets were prepared successfully through solution blending and compression molding. The morphological investigation suggested that the graphene nanosheets with nanoscale thickness achieved a homogeneous dispersion in the PB matrix. The composite films exhibited a sharp transition from insulating state to the conducting one with a low percolation threshold, followed by a high electrical conductivity at graphene content higher than 1.6 vol %. The composite films also achieved high dielectric constant with low dielectric loss due to the effective electrical conductive path established by graphene nanosheets in a local range. Moreover, the mechanical evaluation demonstrated that a considerable reinforcement was achieved for the composite films due to the strong interfaces between the graphene nanosheets and PB matrix. The introduction of graphene nanosheets not only enhanced the nucleation capability and crystallinity of PB domain but also improved the thermal stability of the composite films. In addition, the composite films showed an increase in storage modulus and a decrease in loss factors due to the incorporation of graphene nanosheets.     

Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves

Rapid and mild thermal reduction of graphene oxide (GO) to graphene was achieved with the assistance of microwaves in a mixed solution of N,N-dimethylacetamide and water (DMAc/H₂O). The mixed solution works as both a solvent for the produced graphene and a medium to control the temperature of the reactive system up to 165 °C. Fourier transform infrared spectrometry, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermogravimetric analysis confirmed the formation of graphene under this mild thermal reduction condition. The reduction time is found to be in the scale of minutes. The as-prepared graphene can be well dispersed in DMAc to form an organic suspension, and the suspension is stable for months at room temperature. The conductivity of graphene paper prepared by the microwave reduced product is about 10⁴ times than that of GO paper.     

Effects of thermal annealing on the optical properties of Ar ion irradiated ZnS films

Ar-ion-implantation to a dose of 1×10¹⁷ ions/cm² was performed on cubic ZnS thin films with (111) preferred orientation deposited on fused silica glass substrates by vacuum evaporation. After ion implantation, ZnS films were annealed in flowing argon at different temperatures from 400 to 800 °C. The effects of ion implantation and post-thermal annealing on the structural and optical properties of ZnS films were investigated by X-ray diffraction (XRD), photoluminescence (PL) and optical transmittance measurements. XRD reveals that the diffraction peaks recover at ∼500 °C. The optical transmittances show that the bandgap of ZnS films blueshifts when annealed below 500 °C, and redshifts when annealed above 500 °C. PL results show that the intrinsic defect related emissions decrease with increasing annealing temperature from 400 to 500 °C, and increase with increasing annealing temperature from 500 to 800 °C. The observed PL emissions at 414 and 439 nm are attributed to the transitions of Zn_i→V_Zn and V_S→VBM, respectively.     

A method for the catalytic reduction of graphene oxide at temperatures below 150 °C

Anhydrous AlCl₃ was used to increase the reducing ability of sodium borohydride (NaBH₄) for removing oxygen functional groups on graphene oxide (GO) at a reaction temperature below 150 °C, which provided an extendable, mild, and controllable route for large-scale production of graphene. The influences of reducing temperature and reducing time on the electrical conductivity of reduced GO were examined. Structural evolution during the reduction of GO was studied by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and elemental analysis.     

Studies of interfacial layers between 4H-SiC (0 0 0 1) and graphene

The region between epitaxial graphene and the SiC substrate has been investigated. 4H-SiC (0 0 0 1) samples were annealed in a high temperature molecular beam epitaxy system at temperatures between 1100 and 1700 °C. The interfacial layers between the pristine SiC and the graphene layers were studied by X-ray photoelectron spectroscopy. Graphene was found to grow on the SiC surface at temperatures above 1200 °C. Below this temperature, however, sp³ bonded carbon layers were formed with a constant atomic Si concentration. C1s and Si2p core level spectra of the graphene samples suggest that the interface layer we observe has a high carbon concentration and its thickness increases during the graphitization process. A significant concentration of Si atoms is trapped in the interface layer and their concentration also increases during graphitization.