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Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication.

Yu K, Bo Z, Lu G, Mao S, Cui S, Zhu Y, Chen X, Ruoff RS, Chen J - Nanoscale Res Lett (2011)

Bottom Line: 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).

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA. jhchen@uwm.edu.

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.PACS: Graphene (81.05.ue), Chemical vapor deposition (81.15.Gh), Gas sensors (07.07.Df), Atmospheric pressure (92.60.hv).

No MeSH data available.


Raman and XPS spectra. (a) Raman spectrum of CNWs (original and reduced) showing the presence of D and G bands as well as the overtone and combination mode features taken with 532 nm laser excitation. (b) The C1 s and (c) the O1 s XPS spectra of CNWs before and after thermal annealing. The as-grown CNWs contained many oxygen functional groups, while only a low fraction of hydroxyl groups remained after thermal reduction in H2 for 2 h at 900°C. The peak components (green curves) were analyzed with a Gaussian fit.
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Figure 3: Raman and XPS spectra. (a) Raman spectrum of CNWs (original and reduced) showing the presence of D and G bands as well as the overtone and combination mode features taken with 532 nm laser excitation. (b) The C1 s and (c) the O1 s XPS spectra of CNWs before and after thermal annealing. The as-grown CNWs contained many oxygen functional groups, while only a low fraction of hydroxyl groups remained after thermal reduction in H2 for 2 h at 900°C. The peak components (green curves) were analyzed with a Gaussian fit.

Mentions: Raman spectra showed D and G bands located at 1,347 and 1,584 cm-1, respectively (Figure 3a). The bulk graphite has a G peak at approximately 1,580 cm-1 [36], whereas a D peak at approximately 1,350 cm-1 is seen for defective graphite [37]. The position and shape of the G peak suggest that graphitized carbon was synthesized. The 2 D band (2,682 cm-1) suggests the presence of "graphene-like" materials. A very small 2D' band (approximately 3,233 cm-1) indicates the existence of the D' band that is however probably convoluted with the G band. The G peak for graphene sheets [38,39] occurs at approximately 1,580 cm-1, and this peak broadens and significantly shifts to 1,594 cm-1 for graphite oxide sheets [40,41]. The upshift of what we attribute as the G peak (to 1,584 cm-1) suggests a possibility of a high fraction of oxygen contained in the as-grown CNWs. In the growth of CNTs, it was stated that oxygen etches the carbon on the catalyst particle surface and thus promotes CNT growth [42]. We found that oxygen-containing radicals also appear to be essential for the growth of CNWs in our growth attempts. Hung et al. attributed the formation of nucleation sites for the growth of CNWs to the etching by oxygen-containing species [22]. In addition to using ethanol, we tried to synthesize CNWs with pure CH4 or with n-hexane vapor with Ar as the carrier gas, but no CNWs were observed. However, CNWs could be readily synthesized with CH4 and water vapor (again with Ar as the carrier gas), where the presence of C-OH groups was confirmed with optical emission spectroscopy (see Figure S-2 and S-3 in Additional file 1). The 1:2 O/C ratio in the ethanol precursor is perhaps too high to produce high-purity "graphene-like" material with the approach we have used, but we note the recent report of very carbon-pure graphene made from ethanol using a microwave plasma operated at low pressure [43]. It is likely that the oxygen radicals etch away carbon as it is deposited during the growth, which may explain broken edges and pinholes on the resulting CNW sheets.


Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication.

Yu K, Bo Z, Lu G, Mao S, Cui S, Zhu Y, Chen X, Ruoff RS, Chen J - Nanoscale Res Lett (2011)

Raman and XPS spectra. (a) Raman spectrum of CNWs (original and reduced) showing the presence of D and G bands as well as the overtone and combination mode features taken with 532 nm laser excitation. (b) The C1 s and (c) the O1 s XPS spectra of CNWs before and after thermal annealing. The as-grown CNWs contained many oxygen functional groups, while only a low fraction of hydroxyl groups remained after thermal reduction in H2 for 2 h at 900°C. The peak components (green curves) were analyzed with a Gaussian fit.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211258&req=5

Figure 3: Raman and XPS spectra. (a) Raman spectrum of CNWs (original and reduced) showing the presence of D and G bands as well as the overtone and combination mode features taken with 532 nm laser excitation. (b) The C1 s and (c) the O1 s XPS spectra of CNWs before and after thermal annealing. The as-grown CNWs contained many oxygen functional groups, while only a low fraction of hydroxyl groups remained after thermal reduction in H2 for 2 h at 900°C. The peak components (green curves) were analyzed with a Gaussian fit.
Mentions: Raman spectra showed D and G bands located at 1,347 and 1,584 cm-1, respectively (Figure 3a). The bulk graphite has a G peak at approximately 1,580 cm-1 [36], whereas a D peak at approximately 1,350 cm-1 is seen for defective graphite [37]. The position and shape of the G peak suggest that graphitized carbon was synthesized. The 2 D band (2,682 cm-1) suggests the presence of "graphene-like" materials. A very small 2D' band (approximately 3,233 cm-1) indicates the existence of the D' band that is however probably convoluted with the G band. The G peak for graphene sheets [38,39] occurs at approximately 1,580 cm-1, and this peak broadens and significantly shifts to 1,594 cm-1 for graphite oxide sheets [40,41]. The upshift of what we attribute as the G peak (to 1,584 cm-1) suggests a possibility of a high fraction of oxygen contained in the as-grown CNWs. In the growth of CNTs, it was stated that oxygen etches the carbon on the catalyst particle surface and thus promotes CNT growth [42]. We found that oxygen-containing radicals also appear to be essential for the growth of CNWs in our growth attempts. Hung et al. attributed the formation of nucleation sites for the growth of CNWs to the etching by oxygen-containing species [22]. In addition to using ethanol, we tried to synthesize CNWs with pure CH4 or with n-hexane vapor with Ar as the carrier gas, but no CNWs were observed. However, CNWs could be readily synthesized with CH4 and water vapor (again with Ar as the carrier gas), where the presence of C-OH groups was confirmed with optical emission spectroscopy (see Figure S-2 and S-3 in Additional file 1). The 1:2 O/C ratio in the ethanol precursor is perhaps too high to produce high-purity "graphene-like" material with the approach we have used, but we note the recent report of very carbon-pure graphene made from ethanol using a microwave plasma operated at low pressure [43]. It is likely that the oxygen radicals etch away carbon as it is deposited during the growth, which may explain broken edges and pinholes on the resulting CNW sheets.

Bottom Line: 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).

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA. jhchen@uwm.edu.

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.PACS: Graphene (81.05.ue), Chemical vapor deposition (81.15.Gh), Gas sensors (07.07.Df), Atmospheric pressure (92.60.hv).

No MeSH data available.