Synthesis of Vertically Oriented Graphene Sheets or Carbon Nanowalls—Review and Challenges

The paper presents a review on the current methods for deposition of vertically oriented multilayer graphene sheets (often called carbon nanowalls—CNWs) on solid substrates. Thin films of CNWs are among the most promising materials for future applications in capacitors, batteries, electrochemical devices, and photovoltaics, but their application is currently limited by slow deposition rates and difficulties in providing materials of a desired structure and morphology. The review paper analyzes results obtained by various groups and draws correlations between the reported experimental conditions and obtained results. Challenges in this scientific field are presented and technological problems stressed. The key scientific challenge is providing the growth rate as well as morphological and structural properties of CNWs thin films versus plasma parameters, in particular versus the fluxes of reactive plasma species onto the substrate surface. The technological challenge is upgrading of deposition techniques to large surfaces and fast deposition rates, and development of a system for deposition of CNWs in the continuous mode.

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title: Constructing in-chip micro-supercapacitors of 3D graphene nanowall/ruthenium oxides electrode through silicon-based microfabrication technique
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title: Carbon nanowalls as platinum support for fuel cells
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title: Highly conductive Na-embedded carbon nanowalls for hole-transport-material-free perovskite solar cells without metal electrodes
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title: Synthesis of carbon nanowall by plasma-enhanced chemical vapor deposition method.
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title: Plasma techniques for nanostructured carbon materials synthesis. A case study: carbon nanowall growth by low pressure expanding RF plasma
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title: Effect of nitrogen configuration on carbon nanowall surface: Towards the improvement of electrochemical transduction properties and the stabilization of gold nanoparticles
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title: Fabrication of vertically aligned carbon nanowalls using capacitively coupled plasma-enhanced chemical vapor deposition assisted by hydrogen radical injection
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title: Controllable growth of vertically oriented graphene for high sensitivity gas detection
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title: Initial growth process of carbon nanowalls synthesized by radical injection plasma-enhanced chemical vapor deposition
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title: Synthesis of carbon nanowalls by plasma-enhanced chemical vapor deposition in a CO/H2 microwave discharge system
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title: Non-Catalytic Low-Temperature Synthesis of Carbon Nanofibers by Plasma-Enhanced Chemical Vapor Deposition in a CO/Ar/O2 DC Discharge System
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title: Effect of the aromatic precursor flow rate on the morphology and properties of carbon nanostructures in plasma enhanced chemical vapor deposition
abstract: Understanding the effects of the synthesis parameters on the morphology and electrochemical properties of nanocarbon layers is a key step in the development of application-tailored nanostructures. In this paper we used an aromatic carbon as a new kind of precursor for the synthesis of carbon based nanostructures by plasma enhanced chemical vapor deposition (PECVD). Complex precursor molecules enable a new degree of influence over the atomic structure of PECVD synthesized carbons. Here, we report on the synthesis and characterization of the nanostructures resulting from varied flow rates of p-xylene used as carbon precursor. By changing the flow rate of the precursor, three different morphologies with graphitic character were synthesized. The resulting structures were carbon nanofibers (CNF), freestanding carbon nanowalls (fCNW) and interconnected carbon nanowalls (iCNW), formed at flow rates of 3 ml h−1, between 1 and 3 ml h−1 and less than 1 ml h−1, respectively. Structural characterization by transmission electron microscopy and Raman spectroscopy indicate a lower defect density for the CNF in comparison to the CNW nanostructures. The electrochemical characterization of the oxygen reduction reaction onset potential and effective surface area feature a significantly higher onset at around −171 mV and an electrochemically active surface area of 0.76 μm−1 for the iCNW compared to −196 mV, 0.61 μm−1 and 0.22 μm−1 for the fCNW and CNF, respectively. The similarities in defect density and differences in activity observed for the iCNW and fCNF suggest that the kind of the defects determines the electrochemical properties. Thus, the iCNW was identified as the most appropriate morphology for further investigations.

title: Carbon nanowalls synthesis by means of atmospheric dcPECVD method
abstract: Carbon nanowalls (CNWs) were grown by atmospheric dc plasma enhanced chemical vapour deposition method. Maintaining plasma at atmospheric pressure give us possibility to synthetize CNWs at high rate. By using two different liquid carbon sources we can control CNWs density. Growth of sparsely distributed CNWs can be achieved by using ethanol, while hexane gives densely packed CNWs films.

title: Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication
abstract: Carbon nanowalls (CNWs), two-dimensional "graphitic" platelets that are typically oriented vertically on a substrate, can exhibit similar properties as graphene. Growth of CNWs reported to date was exclusively carried out at a low pressure. Here, we report on the synthesis of CNWs at atmosphere pressure using "direct current plasma-enhanced chemical vapor deposition" by taking advantage of the high electric field generated in a pin-plate dc glow discharge. CNWs were grown on silicon, stainless steel, and copper substrates without deliberate introduction of catalysts. The as-grown CNW material was mainly mono- and few-layer graphene having patches of O-containing functional groups. However, Raman and X-ray photoelectron spectroscopies confirmed that most of the oxygen groups could be removed by thermal annealing. A gas-sensing device based on such CNWs was fabricated on metal electrodes through direct growth. The sensor responded to relatively low concentrations of NO2 (g) and NH3 (g), thus suggesting high-quality CNWs that are useful for room temperature gas sensors. Graphene (81.05.ue), Chemical vapor deposition (81.15.Gh), Gas sensors (07.07.Df), Atmospheric pressure (92.60.hv)

title: Synthesis of carbon nanowalls from a single-source metal-organic precursor
abstract: In this work, the deposition of carbon nanowalls (CNWs) by inductively coupled plasma enhanced chemical vapor deposition (ICP-PECVD) is investigated. The CNWs are electrically conducting and show a large specific surface area, which is a key characteristic to make them interesting for sensors, catalytic applications or energy-storage systems. It was recently discovered that CNW films can be deposited by the use of the single-source metal-organic precursor aluminium acetylacetonate. This precursor is relatively unknown in combination with the ICP-PECVD deposition method in literature and, thus, based on our previous publication is further investigated in this work to better understand the influence of the various deposition parameters on the growth. Silicon, stainless steel, nickel and copper are used as substrate materials. The CNWs deposited are characterized by scanning electron microscopy (SEM), Raman spectroscopy and Auger electron spectroscopy (AES). The combination of bias voltage, the temperature of the substrate and the substrate material had a strong influence on the morphology of the graphitic carbon nanowall structures. With regard to these results, a first growth model for the deposition of CNWs by ICP-PECVD and aluminium acetylacetonate is proposed. This model explains the formation of four different morphologies (nanorods as well as thorny, straight and curled CNWs) by taking the surface diffusion into account. The surface diffusion depends on the particle energies and the substrate material and thus explains the influence of these parameters.

title: Process-specific mechanisms of vertically oriented graphene growth in plasmas
abstract: Applications of plasma-produced vertically oriented graphene nanosheets (VGNs) rely on their unique structure and morphology, which can be tuned by the process parameters to understand the growth mechanism. Here, we report on the effect of the key process parameters such as deposition temperature, discharge power and distance from plasma source to substrate on the catalyst-free growth of VGNs in microwave plasmas. A direct evidence for the initiation of vertical growth through nanoscale graphitic islands is obtained from the temperature-dependent growth rates where the activation energy is found to be as low as 0.57 eV. It is shown that the growth rate and the structural quality of the films could be enhanced by (a) increasing the substrate temperature, (b) decreasing the distance between the microwave plasma source and the substrate, and (c) increasing the discharge power. The correlation between the wetting characteristics, morphology and structural quality is established. It is also demonstrated that morphology, crystallinity, wettability and sheet resistance of the VGNs can be varied while maintaining the same sp3 content in the film. The effects of the substrate temperature and the electric field in vertical alignment of the graphene sheets are reported. These findings help to develop and optimize the process conditions to produce VGNs tailored for applications including sensing, field emission, catalysis and energy storage.

title: Vertically-oriented graphene nanowalls: Growth and application in Li-ion batteries
abstract: Vertically-oriented graphene nanowalls (VGNWs) have been successfully synthesized under different experimental conditions (such as flow rate of argon, growth temperature, plasma power as well as deposition time) on copper foils in the reactive methane and argon environment using a capacitively coupled plasma-enhanced horizontal tube furnace deposition system. It is shown that the morphology and structure of VGNWs can be effectively tailored by experimental conditions. Furthermore, the growth mechanism of VGNWs produced in the plasma-based approach has been investigated. The synthesized VGNWs are used as anode materials in Li-ion batteries and the batteries show outstanding cycle efficiency (~99%). The specific capacity of coin-type cells can be improved by adjusting the experimental parameters and reaches 400 mAh g−1 in the first charge-discharge cycle. This work is particularly important for the development of an advanced process for the synthesis and application of carbon-based nanomaterials.

title: Direct synthesis of vertical graphene nanowalls on glass substrate for thermal management
abstract: Vertical few layered graphene (VG) nanoflakes were directly synthesized on traditional glass substrate by the plasma-assisted chemical vapor deposition, where the extremely low growing temperature (<560 °C) and short time (<30 min) were involved. The VG microstructure could be well-controlled by tuning the plasma power, growth time and growth temperature. Raman, TEM, and SEM were used to determine the quality and single size of the VG. Because of the unique three-dimensional structure with high surface area, the VG network can provide superior thermal management capabilities, which has been demonstrated by the scalable frost removal. We believed that this hybrid system consisting of vertical graphene nanowalls and glass substrates has great potentials for future transparent 'green-warmth' constructions.

title: Morphology, Microstructure, and Hydrogen Content of Carbon Nanostructures Obtained by PECVD at Various Temperatures
abstract: Carbon nanostructures were obtained by acetylene injection into an argon plasma jet in the presence of hydrogen. The samples were synthesized in similar conditions, except that the substrate deposition temperatures were varied, ranging from 473 to 973 K. A strong dependence of morphology, structure, and graphitization upon was found. We obtained vertical aligned carbon nanotubes (VA-CNTs) at low temperatures as 473 K, amorphous carbon nanoparticles (CNPs) at temperatures from about 573 to 673 K, and carbon nanowalls (CNWs) at high temperatures from 773 to 973 K. Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, elastic recoil detection analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy were used to substantiate the differences in these material types. It is known that hydrogen concentration modifies strongly the properties of the materials. Different concentrations of hydrogen-bonded carbon could be identified in amorphous CNP, VA-CNT, and CNW. Also, the H : C ratios along depth were determined for the obtained materials.

title: Synthesis and electrical characterization of n-type carbon nanowalls
abstract: Nitrogen-incorporated carbon nanowalls are prepared by microwave plasma-enhanced chemical vapor deposition using acetylene and methane. n-type conduction in the nanowalls is confirmed by Hall- and Seebeck-effect measurements. We show that increasing the amount of C2 radicals by adding Ar enables catalyst-free growth of nanowalls at a high rate up to about 1 μm/min and reduces the deposition temperature (TD) down to around 650 °C. A substrate pretreatment using diamond powder results in a composite of nanowalls and nanocrystalline diamond films, suggesting that the nanowall growth is limited by gas-phase conditions rather than surface conditions. The low conductivity nanowalls for low TD exhibit thermal activation in the Arrhenius plot, indicative of semiconducting conduction, while the high conductivity nanowalls for high TD are almost temperature independent, indicative of quasimetallic conduction. The high conductivity is attributed to a global increase in the sp2 cluster size and crystallinity, which i...

title: Direct synthesis of multi-layer graphene film on various substrates by microwave plasma at low temperature
abstract: We introduce a possible route for vertically standing multi-layer graphene films (VMGs) on various substrates at low temperature by electron cyclone resonance microwave plasma. VMG films on various substrates, including copper sheet, glass and silicon oxide wafer, were analyzed by studying their structural, electrical, and optical properties. The density and temperature of plasma were measured using Cylindrical Langmuir probe analysis. The morphologies and microstructures of multi-layer graphene were characterized using field emission scattering electron microscope, high resolution transmission electron microscope, and Raman spectra measurement. The VMGs on different substrates at the same experimental conditions synthesized the wrinkled VMGs with different heights. In addition, the transmittance and electrical resistance were measured using ultra-violet visible near-infrared spectroscopy and 4 probe point surface resistance measurement. The VMGs on glass substrate obtained a transmittance of 68.8% and sheet resistance of 796 Ω/square, whereas the VMGs on SiO 2 wafer substrate showed good sheet resistance of 395 Ω/square and 278 Ω/square. The results presented herein demonstrate a simple method of synthesizing of VMGs on various substrates at low temperature for mass production, in which the VMGs can be used in a wide range of application fields for energy storage, catalysis, and field emission due to their unique orientation.

title: Synthesis of vertically aligned carbon nanoflakes by hot-wire chemical vapor deposition: Influence of process pressure and different substrates
abstract: The vertically aligned carbon nanoflakes thin films were synthesized on Si (100) and alkali free borosilicate glass substrates at relatively low substrate temperature of 400 °C, without any catalyst or surface pre-treatment, using hot-wire chemical vapor deposition (HWCVD) technique. Raman Spectroscopy studies reveal that the crystallinity of the films increases with increase in process pressure. Our studies show that HWCVD is a versatile technique for the synthesis of these films at low temperature compared to plasma enhanced CVD on both semiconducting Si as well as insulating glass substrates.

title: Low temperature growth of graphene film by microwave assisted surface wave plasma CVD for transparent electrode application
abstract: Here, we report the synthesis of a graphene film on Cu foil by microwave assisted surface wave plasma chemical vapor deposition (MW-SWP-CVD) at a low pressure and temperature using a hydrocarbon gas. Raman spectroscopy and transmission electron microscopy (TEM) studies clearly show deposited carbon films that consist of few layer graphene sheets. Deposition of graphene films with a few layers structure was achieved at a temperature as low as 240 °C. In the nucleation and growth process, carbon radicals are formed with hydrogen termination in the surface wave plasma, which absorb continuously on the Cu surface to form sp2 carbon and eventually a graphene structure. The transparency and conductivity characteristics of the transferred graphene films show suitability for flexible electronics applications.

title: Low-temperature plasma enhanced chemical vapour deposition of carbon nanotubes
abstract: Vertically aligned carbon nanotubes were selectively grown at temperatures as low as 120 °C by plasma enhanced chemical vapour deposition. We investigated the effects of acetylene, ethylene and methane as carbon source gases together with ammonia as an etchant and nickel as catalyst material. The diluted acetylene plasma gave the highest nanotube growth rate and showed the most intense C 2 Swan bands in optical emission. The activation energy for the growth rate was found to be 0.23 eV, much less than for thermal chemical vapour deposition (1.2–1.5 eV). This suggests growth occurs by surface diffusion of carbon on nickel. The result allows more cost-effective nanotube production, direct growth of nanotubes onto low-temperature substrates like plastics, and could facilitate carbon nanotube integration into sensitive nanoelectronic devices.

title: Pecvd synthesis of 2d nanostructured carbon material
abstract: Carbon nanowalls were synthesised by downstream deposition in an expanding radiofrequency argon plasma beam discharge injected with acetylene and hydrogen. The obtained layers have been investigated by Scanning and Transmission Electron Microscopy, Selected Area Electron Diffraction, and Raman Spectroscopy. We present the influence of the substrate type on the material characteristics and report on the possibility to obtain free standing carbon nanowall layers. Also, the importance of pressure values and argon carrier mass flow rate during the deposition process is discussed in relation to the quality of the nanostructured material.

title: Vertical growth of carbon nanowalls using rf plasma-enhanced chemical vapor deposition
abstract: Two-dimensional carbon nanostructures (carbon nanowalls) were fabricated by capacitively coupled, radio-frequency (rf) plasma-enhanced chemical vapor deposition (PECVD) employing fluorocarbon/hydrogen (H) mixtures. Correlation between carbon nanowall growth and fabrication conditions, such as the carbon source gases, was investigated. In addition, H atom density in the plasma was measured using vacuum ultraviolet absorption spectroscopy (VUVAS), in order to discuss the growth mechanism of carbon nanowalls.

title: Growth of Carbon Nanowalls on a SiO2 Substrate by Microwave Plasma-Enhanced Chemical Vapor Deposition
abstract: We investigated the growth process of carbon nanowalls (CNWs) on a SiO2 substrate by microwave plasma-enhanced chemical vapor deposition (MPECVD). It is revealed that the CNWs are grown at the fine-textured structure on the SiO2 and the growth process does not require the catalyst. The CNW initially has a semicircular shape. The height, thickness, and mesh size increase with growth time. It is found that the height of CNWs as a function of time obeys the square root law. Extremely high growth rate, approximately 10 µm/h, is achieved, in contrast to previous studies.

title: DC plasma enhanced growth of oriented carbon nanowall films by HFCVD
abstract: Two dimensional graphitic carbon structures, commonly referred to as carbon nanowalls (CNW), are raising increasing interest in the scientific community. Their surface area is theoretically twice that of the closed boundary structures, such as carbon nanotubes, making them extremely attractive for chemical and biosensor applications. In this work CNW, with maximum longitudinal dimension ranging from 10 to 200 nm and wall thickness lower than 5 nm, have been grown in a HFCVD reactor on Si substrates. The growth precursors consisted of methane diluted into He noble gas. The effect of a DC plasma on the growth rate and film morphology was explored. The experimental setup consisted of a two grid system which allowed to vary independently the plasma voltage and current density on the substrate surface. An increase of growth rate was observed as the film thickness increased from a few nanometers to about 200 nm when the substrate current density was increased from 0 to 3.5 mA/cm 2 for 30 min.

title: Synthesis and characterization of carbon nanowalls on different substrates by radio frequency plasma enhanced chemical vapor deposition
abstract: A radio frequency plasma enhanced chemical vapor deposition system was used for the successful growth of thin vertical carbon nanowalls, also known as vertical graphene, on various substrates. Transmission electron microscopy studies confirmed the presence of vertical graphene walls, which are tapered, typically consisting of 10 layers at the base tapering off to 2 or 3 layers at the top. The sides of the walls are facetted at quantized angles of 30° and the facetted sides are usually seamless. Growth occurs at the top open edge which is not facetted. Hydrogen induced etching allows for nucleation of branch walls apparently involving a carbon onion-like structure at the root base. Characterization by a superconducting quantum interference device showed magnetic hysteresis loops and weak ferromagnetic responses from the samples at room temperature and below. Temperature dependence of the magnetization revealed a magnetic phase transition around T  = 50 K highlighting the coexistence of antiferromagnetic interactions as well as ferromagnetic order.

title: Efficient and controllable growth of vertically oriented graphene nanosheets by mesoplasma chemical vapor deposition
abstract: Owing to the unique nanoarchitecture and graphene in nature, the vertically oriented graphene nanosheets (VGN) possesses many promising properties and holds great potential for various applications. Although the plasma-enhanced chemical vapor deposition (PECVD) techniques have been the effective and widely used approaches, they remain the challenge for the controllable and efficient growth of VGN for the mass and intended applications. Also, each method provides limited structures. Herein, for the first time, we report a synthesis of VGN with various controllable structures and fast growth rates by mesoplasma CVD at Ar-H2-CH4 gas mixture. Five types of structures, including cauliflower-like, petal-like, quasi-maze-like, maze-like, and floc-like structures are synthesized by simply adjusting the growth parameters. The growth dynamics and structure transition of VGN are analyzed from the aspect of the deposition precursor density and the H/C ratio. Even at no H2 addition, VGN is synthesized at mesoplasma condition and an extremely high rate of 18.08 μm/min is obtained, which is at least one order of magnitude faster than any other methods reported. This work provides a new route to increase the growth rate of VGN and also demonstrates the advantages of mesoplasma CVD for the growth of high quality films with high efficiency.

title: Understanding growth of carbon nanowalls at atmospheric pressure using normal glow discharge plasma-enhanced chemical vapor deposition
abstract: Synthesis of carbon nanowalls (CNWs) at atmospheric pressure is realized by using a negative normal glow discharge, which differs from prior low-pressure plasma-enhanced chemical vapor deposition methods for CNW growth and holds great potential for mass-production of CNWs. The glow discharge is characterized for electrical and optical properties. The resulting CNW structures are analyzed using transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy. The CNW growth process is examined for effects of discharge regime, discharge duration (growth time), hydroxyl radicals (water vapor), and local current density. Possible CNW growth mechanism and critical process parameters to the CNW growth and evolution are identified. OH radicals play an essential role in the initial nucleation process, but excess OH radicals accelerate the oxidation of CNWs. For a fixed growth time, there exists an optimum feed gas relative humidity and an optimum current density (∼40% and ∼9.17 A/m2, respectively, in the current work) for the growth of large and less defective CNWs with a high degree of crystallinity. Results obtained in this study can be used to tailor final properties of the as-grown CNWs for various applications.

title: Solution plasma exfoliation of graphene flakes from graphite electrodes
abstract: Graphene flakes were successfully produced by a simple method named a solution plasma process. The plasma was generated between two carbon electrode tips which were immersed in distilled water. The production approach is a continuous energy accumulation induced by focusing of plasma streamers on the surface of electrodes. This focused energy produces thermal gradient zones where a proper energy level is produced. Concisely, the energy is enough to extract graphene layers from the graphite structure but not enough to break covalent C–C bonds within the graphene sheets. Optical emission spectroscopy results showed the decomposition of carbon from the graphite electrodes with different intensities for the carbon species. Transmission electron microscopy images confirmed the hexagonal honeycomb structure of the graphene flakes. Furthermore, broadening of the 2D band in the Raman spectra revealed that the graphene flakes were disordered multilayers.

title: Synthesis of graphene nanosheets from petroleum asphalt by pulsed arc discharge in water
abstract: The synthesis of graphene nanosheets has been achieved by pulsed arc discharge in water with petroleum asphalt as a carbon source. The synthesized graphene nanosheets were examined by Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy and atomic force microscopy. The results indicate that most of as-synthesized graphene nanosheets have 2–5 layers, and small amounts of them have sheets in the range of 5–10 layers. The optical emission spectroscopy was used to monitor the arc discharge process, which indicates that the released hydrogen from asphalt may play a key role in the formation of graphene.

title: Rapid growth of micron-sized graphene flakes using in-liquid plasma employing iron phthalocyanine-added ethanol
abstract: Giant graphene flakes on the micron scale were synthesized and grown in plasmas in liquid-phase pure ethanol with added iron phthalocyanine (FePc) in a solvent. At atmospheric pressure, plasmas were generated in the gas phase filled with Ar and in the liquid phases comprising bubbles and liquid solutions. In the mixture of FePc in ethanol, nanographene sheets aggregated to form giant graphene flakes, as confirmed by the D, G, and 2D bands in the corresponding Raman spectra. Therefore, a bottom-up approach of graphite synthesis from pure ethanol with additives and a catalyst was realized by in-liquid plasma processing.

title: Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications
abstract: Graphene has attracted growing interest in the past few years. Growing vertically oriented graphene sheets with a designed pattern is practically attractive for device applications based on graphene. Here we report a patterned synthesis of vertical graphene nanosheets using plasma-enhanced chemical vapor deposition. Both experimental and modeling results suggest that the electric field distribution above the substrate material plays a key role in the graphene coverage. Vertical graphene patterns can thus be designed through artificially designing the surface electric field distribution. A field-effect transistor (FET) sensor device has been demonstrated for detection of low-concentration gases using vertically patterned graphene sheets bridging a metal electrode gap.

title: Preparation and electron field emission of carbon nanowall by Cat-CVD
abstract: Carbon nanowall (CNW) films have been prepared by catalytic chemical vapor deposition using only CH4 gas. In the SEM images of the samples prepared at a substrate heating temperature, Ts, of 500 °C, the wall structures on the substrates were observed. In those prepared at Ts of 400 °C, however, the wall structure was not observed. The electron field emission from the CNW films was observed at small electric field. The wall width and height increased when the Raman intensity ratio, ID/IG, decreased. Then, the threshold electric field decreased. The smallest threshold electric field was 2.6 V/μm in this work.

title: Effect of hydrogen dilution in preparation of carbon nanowall by hot-wire CVD
abstract: Carbon nanowall films prepared on the stainless steel substrates by hot-wire chemical vapor deposition using CH 4 with different hydrogen dilution ratios and structure variation in the CNWs against hydrogen dilution have been studied. In the scanning electron microscope images, the wall height and width in the samples prepared with the hydrogen dilution ratio, H 2 /(CH 4 +H 2 ), between 10% and 25% were larger than that prepared without hydrogen dilution. In the Raman spectra for the samples prepared with the H 2 /(CH 4 +H 2 ) below 25%, the intensity ratio of the G peak to the D peak, I G /I D , increased with increasing the H 2 /(CH 4 +H 2 ). In the samples prepared with the H 2 /(CH 4 +H 2 ) over 25%, the wall size and the I G /I D decreased.

title: Oxygen atom loss coefficient of carbon nanowalls
abstract: Extremely high values of atomic oxygen loss coefficient on carbon nanowall (CNW) surface are reported. CNW layers consisting of interconnected individual nanostructures with average length of 1.1 μm, average thickness of 66 nm and surface density of 3 CNW/μm 2 were prepared by plasma jet enhanced chemical-vapor deposition using C 2 H 2 /H 2 /Ar gas mixtures. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectrometry (RS) as well as X-ray photoelectron spectroscopy (XPS). The surface loss coefficient was measured at room temperature in a flowing afterglow at different densities of oxygen atoms supplied from inductively coupled radiofrequency O 2 plasma. The RF generator operated at 13.56 MHz and different nominal powers up to 900 W corresponding to different O-atom density in the afterglow up to 1.3 × 10 21  m −3 . CNW and several different samples of known coefficients for heterogeneous surface recombination of neutral oxygen atoms have been placed separately in the afterglow chamber and the O-atom density in their vicinity was measured with calibrated catalytic probes. Comparison of measured results allowed for determination of the loss coefficient for CNWs and the obtained value of 0.59 ± 0.03 makes this material an extremely effective sink for O-atoms.