6. Temperature and gas exposure effects on graphene Raman spectra

The Raman spectra of graphene were also recorded at varying temperatures (30–200C) using the Ventacon heated cell and the 780-nm laser with the DXR Raman spectrometer. The graphene samples were all on a silicon/SiO2 substrate and subjected to consecutive heating/ cooling cycles between 30 and 200C in a sealed chamber. Figure 13 is the spectrum collected at 30C. As discussed earlier, the G-band at 1598 cm<sup>1</sup> originates from intraplanar stretching, while the peak at 2703 cm<sup>1</sup> corresponds to the 2D band. The latter band is due to a secondorder two-phonon process that is highly dispersive. It was discovered that this band can be used in estimating the number of layers of a graphene sample. Based on the analysis for this study, the graphene sample(s) in this experiment proved to be largely single-layered. A third band around 3078 cm<sup>1</sup> was also present among the Raman spectra collected. It was tracked

and is probably due to the substrate or glass surface of the microscope stage. Another possibility is that it is either an overtone Raman band of graphene or a C-H stretch benzene ring vibration. Further analysis is needed to determine with certainty the precise origin of the

Raman Spectroscopy of Graphitic Nanomaterials http://dx.doi.org/10.5772/intechopen.72769 173

Raman spectra of the graphene sample(s), under similar heating/cooling cycles was also performed simultaneously with exposure to gaseous (H2O, NO, SO2, NO2). This analysis allowed us to search for any possible patterns in the response of the graphene as its temperature was increased before gas exposure. Each Raman spectral acquisition was analyzed with

Plots of the Raman shift, light intensity, and peak width vs. temperature (in the range 30–150C) were recorded before exposure and after being exposed to a specific gas. These plots (Figures 14–17) were made for all four vapor and gases of interest (H2O, NO, SO2,

The plots of the Raman shift, light intensity, and peak width over the temperature range 24.0–

Figure 16. Raman spectra illustrating the sensing of NO2 gas on graphene as a function of temperature in the range 26.0– 150.0C. [top to bottom: Sample at 26.2C (pristine graphene before heating/pre-exposure), 151.2C (after heating/preexposure), 31.4C (after cooling/pre-exposure), 28.9C (last of gas exposure), 150.0C (after heating/post-exposure), and

Figure 17. Raman spectra illustrating the sensing of SO2 gas on graphene as a function of temperature in the range 24.0– 137.0C. [top to bottom: Sample at 24.3C (pristine graphene before heating/pre-exposure), 149.0C (after heating/preexposure), 29.3C (after cooling/pre-exposure), 27.7C (last of gas exposure), 135.6C (after heating/post-exposure), and

150C before and after being exposed to NO are shown in Figure 18 [16].

spectral feature at 3078 cm<sup>1</sup>

31.1C (after cooling/post-exposure, respectively].

32.4C (after cooling/post-exposure, respectively].

NO2).

.

regards to band frequency, band intensity, and peak width.

Figure 13. Raman spectrum of graphene at 30C.

Figure 14. Raman spectra illustrating the sensing of water vapor and humidity effects on graphene as a function of temperature in the range 24.0–150.0C [top to bottom: Sample at 24.0C (pristine graphene before heating/pre-exposure), 150.5C (after heating/pre-exposure), 29.3C (after cooling/pre-exposure), 26.4C (last of gas exposure), 149.0C (after heating/post-exposure), and 28.5C (after cooling/post-exposure, respectively].

Figure 15. Raman spectra illustrating the sensing of NO gas on graphene as a function of temperature in the range 24.0– 150.0C. [top to bottom: Sample at 24.8C (pristine graphene before heating/pre-exposure), 139.2C (after heating/preexposure), 27.7C (after cooling/pre-exposure), 27.5C (last of gas exposure), 146.2.0C (after heating/post-exposure), and 28.9C (after cooling/post-exposure, respectively].

and is probably due to the substrate or glass surface of the microscope stage. Another possibility is that it is either an overtone Raman band of graphene or a C-H stretch benzene ring vibration. Further analysis is needed to determine with certainty the precise origin of the spectral feature at 3078 cm<sup>1</sup> .

used in estimating the number of layers of a graphene sample. Based on the analysis for this study, the graphene sample(s) in this experiment proved to be largely single-layered. A third band around 3078 cm<sup>1</sup> was also present among the Raman spectra collected. It was tracked

Figure 14. Raman spectra illustrating the sensing of water vapor and humidity effects on graphene as a function of temperature in the range 24.0–150.0C [top to bottom: Sample at 24.0C (pristine graphene before heating/pre-exposure), 150.5C (after heating/pre-exposure), 29.3C (after cooling/pre-exposure), 26.4C (last of gas exposure), 149.0C (after

Figure 15. Raman spectra illustrating the sensing of NO gas on graphene as a function of temperature in the range 24.0– 150.0C. [top to bottom: Sample at 24.8C (pristine graphene before heating/pre-exposure), 139.2C (after heating/preexposure), 27.7C (after cooling/pre-exposure), 27.5C (last of gas exposure), 146.2.0C (after heating/post-exposure), and

heating/post-exposure), and 28.5C (after cooling/post-exposure, respectively].

Figure 13. Raman spectrum of graphene at 30C.

172 Raman Spectroscopy

28.9C (after cooling/post-exposure, respectively].

Raman spectra of the graphene sample(s), under similar heating/cooling cycles was also performed simultaneously with exposure to gaseous (H2O, NO, SO2, NO2). This analysis allowed us to search for any possible patterns in the response of the graphene as its temperature was increased before gas exposure. Each Raman spectral acquisition was analyzed with regards to band frequency, band intensity, and peak width.

Plots of the Raman shift, light intensity, and peak width vs. temperature (in the range 30–150C) were recorded before exposure and after being exposed to a specific gas. These plots (Figures 14–17) were made for all four vapor and gases of interest (H2O, NO, SO2, NO2).

The plots of the Raman shift, light intensity, and peak width over the temperature range 24.0– 150C before and after being exposed to NO are shown in Figure 18 [16].

Figure 16. Raman spectra illustrating the sensing of NO2 gas on graphene as a function of temperature in the range 26.0– 150.0C. [top to bottom: Sample at 26.2C (pristine graphene before heating/pre-exposure), 151.2C (after heating/preexposure), 31.4C (after cooling/pre-exposure), 28.9C (last of gas exposure), 150.0C (after heating/post-exposure), and 32.4C (after cooling/post-exposure, respectively].

Figure 17. Raman spectra illustrating the sensing of SO2 gas on graphene as a function of temperature in the range 24.0– 137.0C. [top to bottom: Sample at 24.3C (pristine graphene before heating/pre-exposure), 149.0C (after heating/preexposure), 29.3C (after cooling/pre-exposure), 27.7C (last of gas exposure), 135.6C (after heating/post-exposure), and 31.1C (after cooling/post-exposure, respectively].

Figure 18. Plots showing the change in Raman frequency shift, light intensity, and peak width, over a temperature range (30–150C), before and after being exposed to NO [16].
