Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication
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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.
PACS: Graphene (81.05.ue), Chemical vapor deposition (81.15.Gh), Gas sensors (07.07.Df), Atmospheric pressure (92.60.hv)
KeywordsHRTEM Reduce Graphene Oxide Monolayer Graphene Oxygen Functional Group Carbon Nanowalls
Graphene possesses many extraordinary properties and has been the subject of intense scientific interest [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Exceptional values have been reported of: ballistic electron mobility (>200,000 cm2/V-s for particular samples) [13, 14], high thermal conductivity (5,000 W/m-K) , Young's modulus (approximately 1,100 GPa), fracture strength (125 GPa) , and a high specific surface area (approximately 2,600 m2/g) relevant to electrical energy storage .
"Carbon nanowalls" (CNWs), also referred to as "carbon nanoflakes", are two-dimensional "graphitic" platelets that are typically oriented vertically on a substrate. An individual CNW has been reported to have a few stacked layers ("graphitic") with typical lateral dimensions of several micrometers . CNWs might exhibit similar properties as graphene. The sharp edges and vertical orientation make CNWs a potential field emission material [18, 19, 20]. The high surface area of CNWs could be ideal for catalyst support. Recently, CNWs have been tested for use in Li-ion batteries  and electrochemical capacitors . CNWs can also be used as a template for loading other nanomaterials; and the resulting hybrid nanostructures are potentially useful for various applications [23, 24, 25].
CNWs were discovered by Wu et al.  and since then they have been grown using various low-pressure processes. Initially, substrates were sputter-coated with transition metals as catalysts and the growth of CNWs was typically carried out in a microwave plasma-enhanced chemical vapor deposition (MPECVD) system . Only a few studies of CNW growth using low-pressure, low-voltage, high-current dc PECVD have been conducted . The growth parameters were very similar to those used for PECVD growth of carbon nanotubes (CNTs), but the pressure used in the reactor chamber was much lower (≤1 Torr) [17, 26, 27, 28, 29, 30, 31]. There have been a number of studies focused on understanding the CNW growth mechanism and thus targeting control of the growth process [22, 26, 32, 33]. Nevertheless, to our knowledge, no CNW growth has been reported at atmospheric pressure.
Here, we report on the synthesis of CNWs using dc PECVD at atmospheric pressure by taking advantage of the high electric field generated in a pin-plate dc glow discharge. In general, PECVD processes for the material growth can occur at a relatively lower temperature due to the significant contribution from energetic electrons to cracking down precursor species. Prior studies using low-pressure PECVD systems to grow CNWs mainly rely on the increased mean free path (mfp) of electrons in vacuum to obtain energetic electrons needed for the decomposition of carbon precursors. The electric field generated in the low-pressure PECVD system is generally low. By using a pair of asymmetric discharge electrodes, i.e., a sharpened tungsten tip as cathode and a planar substrate as anode, a highly enhanced electric field about two to three orders of magnitude higher than that in the previous MPECVD system is generated near the tungsten tip so that the mfp of electrons can be lowered or the system pressure can be elevated (e.g., to atmospheric pressure) to generate similar energetic electrons.
Our method does not require a sealed reactor, which presents a path for continuous line production of CNWs. An atmospheric-pressure process to replace the vacuum process should also reduce the product cost. A recent study on the high cost of modern vacuum deposition methods highlighted the need for atmospheric synthesis . The as-grown CNWs were decorated with oxygen-containing functional groups. By thermal annealing in H2, most oxygen functional groups can be effectively eliminated. In addition, most of the product CNWs are non-aggregated with large surface area, which makes the product readily useful for various applications such as sensing and catalysis. This is in contrast to stacked CNWs that require additional dispersion, such as through ultrasonication, to obtain individual CNWs. To illustrate the advantage of our growth method, CNWs deliberately grown between metal electrodes were used for detection of low-concentration gases including NO2 and NH3, thereby demonstrating a one-step gas sensor fabrication process.
The plasma reactor consists of a quartz tube that houses a tungsten needle cathode, a grounded graphite rod anode, and a dc high negative voltage supply (EMCO 4100N; up to -10 kV) to drive the dc glow discharge. Argon was used as the plasma gas. A tube furnace (TF55035 A-1, Lindberg/BLUE M, Asheville, USA) was used to heat the reactor. Silicon wafers, stainless steel plates, and Cu plates were used as substrates. The substrates were mounted on the top of the graphite rod; no metals were added as potential catalysts.
Prior to the growth, the substrate was brought to 700°C and held at that temperature for 10 min in an Ar/H2 flow (1% H2 by volume) of 500 standard cubic centimeters per minute (sccm). The two discharge electrodes were separated by a distance of 1.0 cm. Then the Ar/H2 flow was switched to an Ar/ethanol flow (1,000 sccm) through an ethanol bubbler. The dc glow discharge was ignited at a dc voltage of 3.3 kV. Once the dc plasma was formed, the voltage between the electrodes immediately dropped to 2.2 kV, and the current was about 1.3 mA, yielding a total plasma power of 2.9 W.
The plasma was typically left on for 15 min. Then, the plasma was turned off and the system was cooled down to room temperature with a flow of Ar/H2 only. Throughout the process, the reactor pressure was maintained at one atmosphere. The reactor temperature was measured as close to 700°C (the preset furnace temperature) using a thermocouple. This suggests that the energy dissipated in the dc glow discharge was non-thermal (electrons were preferentially heated by the plasma) and heavy species (e.g., gas molecules, atoms, radicals, and ions) were not substantially heated by the plasma. After the plasma was turned off, a layer of black, powder-like material could be seen on the substrate. In order to reduce oxygen functional groups decorated on the as-grown CNWs, the CNWs were thermally annealed at 900°C in H2 flow (1,000 sccm) for 2 h at atmospheric pressure.
Scanning electron microscopy (SEM) analysis of the as-grown samples was performed with a Hitachi S-4800 SEM having a stated resolution of 1.4 nm at 1 kV acceleration voltage. Transmission electron microscopy (TEM) was performed with a Hitachi H 9000 NAR TEM, which has a stated point resolution of 0.18 nm at 300 kV in the phase contrast, high-resolution TEM (HRTEM) imaging mode. In order to perform TEM characterizations, the as-grown CNWs were wetted with ethanol and contact-transferred to lacey carbon-coated TEM grids or bare Cu grids. A confocal Raman system, which is composed of a TRIAX 320 spectrograph, liquid nitrogen-cooled CCD (CCD 3000), and "spectrum one" CCD controller (all manufactured by HORIBA Jobin Yvon), was used to record the Raman spectra of the samples with an excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS, Omicron NanoESCA probe, Omicron NanoTechnology GmbH, Taunusstein, Germany) was used to analyze the chemical composition as well as the nature of the chemical bonds in the as-grown CNWs.
Gold-interdigitated electrodes with both finger width and inter-finger spacing of about 1 μm and thickness of 50 nm were fabricated using an e-beam lithography process (Raith 150 lithography tool, 30 kV) on an Si wafer with a top layer of thermally-formed SiO2 (thickness of 200 nm). Sensor current was measured using a Keithley 2602 source meter.
Results and discussion
The 2 D peak is a signature of graphitic carbon in the graphene-like materials . The Raman spectrum obtained from the as-grown CNWs exhibits a peak centered at 2,682 cm-1 (Figure 3a, pink curve), indicating that the analyzed region consists of considerable amount of graphene or oxygenated graphene. After thermal annealing, the 2 D peak shifted to 2,675 cm-1 (Figure 3a, olive curve). This trend is in agreement with literature. The 2 D peaks were reported at 2,861 cm-1 for monolayer graphene oxide , and 2,700 cm-1 for monolayer graphene . For monolayer reduced graphene oxide, the 2 D peak was found around 2,700 cm-1 or below 2,700 cm-1[44, 46, 47]. The 2 D band is very sensitive to the number of layers in the sample. Figure 3a shows single Lorentzian profiles of the few-layered graphene sheets, which are different from the case of few-layered graphene sheets generated by micromechanical cleavage of graphite . The reason is that an ordered stacking (i.e., ABAB stacking) and therefore an electronic coupling do not occur in all region of a CNW sheet .
The D peak and 2D' peak are attributed to the structural disorder in the CNW sheets . The intensity of the D band is at least partly a consequence of the high fraction of open edges and pinholes within the CNWs (Figure 2a) . The disorder-induced combination mode (D + G) at about 2,920 cm-1 was also observed. For comparison of the relative intensity of each peak, the Raman spectra were normalized. Both of the G peaks intensities before and after reduction were fixed at 1 (Figure 3a). The band area ratios I(2D)/I(G) increased from 0.79 to 0.81 after thermal reduction. This change indicates a slight increase of sp2 carbon domain. The band area ratios I(D)/I(G) decreased from 1.73 to 1.63 after thermal reduction. The reducing I(D)/I(G) indicates a decreasing degree of disordered carbon. The ratio of the intensity of the G band to that of the D band I(G)/I(D) is directly related to the in-plane crystallite size La (nanometers) = 19.2 (I(G)/I(D)), and an increase of La from 11.1 to 11.8 nm was obtained .
XPS studies reveal the nature of the carbon and oxygen bonds present in the samples (Figure 3b,3c). The XPS peaks were decomposed with a Gaussian fit. Analysis of the CNWs shows a significant reduction of oxygen functional groups after thermal annealing in H2 for 2 h at 900°C. Briefly, the as-grown CNWs contained non-oxygenated ring C (71.1%), sp3 C hybridized to C (C-C, 18.5%), C in C-OH bonds (9.1%), the carboxylate carbon (O = C-OH, 1.1%), and carbonyl carbon (<0.2%). After thermal annealing, only a small fraction of C in C-OH (1.7%) remained in the CNWs. C in C = C and C-C bonds increased to 72.8% and 25.5%, respectively. The O1 s spectra showed similar reduction of O - the peak weakened after reduction in H2 (Figure 3c). However, the accurate determination of every O-containing group after the thermal reduction is quite challenging due to the insufficient signal-to-noise ratio. Positions of carbon-related and oxygen-related peaks in the XPS spectra are consistent with those of oxidized graphene reported recently . The reduction of oxygen functional groups suggested by the XPS spectra is consistent with the Raman data.
Although a fraction of surface area of the CNW may be covered with oxygen groups, there are well-crystallined graphitic regions (sp2 carbon) in the CNW. Figure 4e is an HRTEM image from another CNW sample and shows two regions (arrowed) with well-defined fringes implying the good crystallinity of the CNW. The diffractogram (the inset in Figure 4e) of the red-squared region in Figure 4e gives a set of hexagonal spots, suggesting the possible monolayer nature of the region. We further inspected the squared area in Figure 4e by performing Fourier filtering. A filtered image with atomic resolution is shown in Figure 4f. The "honeycomb-like" carbon rings in Figure 4f clearly illustrate that the CNW consists of monolayer graphene. The length of the C-C bond in graphene is 0.142 nm , resulting in a hexagon with a width of 0.25 nm. We analyzed the intensity profile (Figure 4g) along the red dashed line in Figure 4e. The hexagon width measured from the intensity outline in Figure 4g is about 0.246 nm, which is in good agreement with the expected value of 0.25 nm. Our HRTEM analysis indicates the existence of monolayer graphene in the product CNWs.
Upon the introduction of NO2, the sensor current went up, i.e., the conductance of the sensor increased (Figure 5b, red curve). Upon exposure to NH3, the sensor current went down, i.e., the conductance of the sensor decreased (Figure 5b, blue curve). Thus, the CNW film behaves like a p-type semiconductor, similar to graphene exposed to air. NO2 is a strong oxidizer with electron-withdrawing power ; therefore, electron transfer from the CNWs to adsorbed NO2 leads to increased hole concentration and enhanced electrical conduction in the CNW network. Likewise, the absorbed NH3 molecules donate electrons to CNW and neutralize holes partially in the CNW, which results in a lower sensor current in the device. The sensing behavior of the as-grown CNW is consistent with a typical graphene or reduced graphene oxide gas sensor .
In summary, we have demonstrated a new path to low-cost production of CNWs on Si, stainless steel, and Cu substrates with a dc PECVD system operated at atmospheric pressure. SEM, HRTEM, Raman spectroscopy, and XPS reveal that the as-grown CNW material has a significant fraction of chemically functionalized mono- and few-layer graphene, with patches of O-containing functional groups; however, most of the O-containing functional groups can be removed by thermal annealing. Our atmospheric pressure process can be readily scaled up for large area growth through the use of an array of tungsten needle cathodes. A gas sensing device based on as-produced CNW film responds to low-concentration NO2 or NH3 in a similar fashion as sensing devices based on graphene or reduced graphene oxide. Therefore, a simple one-step gas sensor fabrication process has been demonstrated.
This work was supported by the US NSF (CMMI-0900509), the US DOE (DE-EE0003208), and We Energies. The authors thank H. A. Owen for technical support with SEM and R. Arora for technical support with Raman, M. Gajdardziska-Josifovska for providing TEM access, D. Robertson for technical support with TEM, and L. E. Ocola for assistance in the electrode fabrication. The SEM imaging was conducted at the EML of UWM. The TEM characterization was carried out at the UWM HRTEM Laboratory. The e-beam lithography was performed at the Center for Nanoscale Materials of Argonne National Laboratory, which is supported by the US Department of Energy (DE-AC02-06CH11357).
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