O. The origin of such 2D peak is from double resonance transitions resulting in production of two phonons with opposite momentum  . Inset of Figure 5 shows the intensity ratio of the D band to G band, which essentially gives the information about the oxidation degree. It is evident that ID/IG ratio decreases with milling time which is due to decrease in oxygen related groups in GO. In order to confirm and get more insights pertinent to the presence of various oxygen related functional groups in parent and BMGO samples, FTIR measurements were carried out.
Figure 6(a) represents the FTIR spectra of parent GO and various BMGO samples. Obtained characteristic peaks of graphene oxide by FTIR spectroscopy are consistent with the existing literature   . Figures 6(b)-(f) represent various oxygen related groups say C=O (1725 cm−1), C-O (1065 cm−1), defective OH group (1390 cm−1), bending OH group (652 cm−1) and stretching OH group (3406 cm−1) respectively. From the figure it is evident that the peak intensity of aforementioned groups diminishes with the applied compressive stress. It is believed that band gap of GO (2 - 5 eV) varies depending on the oxygen groups that are present. As we stated above, decrease in oxygen related groups may allow engineering the band gap values with the applied compressive stress. In order to look into above aspect, wavelength versus absorption spectrum was recorded by UV Vis NIR Spectrometer in the UV VIS region (200 nm to 800 nm). 1.2 mg of GO (0 hr BMGO) sample was dissolved in 25 ml of DI water and the same is followed for other samples (10 hr BMGO and 20 hr BMGO) in order to obtain absorption spectra.
Figure 7(a) shows the normalized absorbance vs. wavelength spectrum for all the ball milled GO samples. It is evident that the maximum absorption peak prevails around 230 nm due to π-π* transition of the aromatic carbon-carbon (C=C) bonds and shoulder peak nearly at 301 nm obtained due to n-π* transitions of epoxide (C-O-C) bonds   . It is also evident from inset of the figure that peak position shifts to higher wavelengths, hinting a reduction in band gap upon milling time. The relation between incident photon energy and absorption coefficient can be defined as
Figure 5. Raman spectrum of all BMGO (0 hr, 10 hr and 20 hr) samples. Inset shows the variation of ID/IG ratio of BMGO samples with ball milling time.
Figure 6. (a) FTIR plots of all graphene oxide samples; (b)-(f) represents the peaks and their behaviour of various oxygen related groups say C=O (1725 cm−1), C-O (1065 cm−1), defective OH group (1390 cm−1), bending OH group (652 cm−1) and stretching OH group (3406 cm−1), respectively.
where A is the constant, Eg is the band gap of the material and n is the exponent depend on the type of transition. As GO consists direct band gap values, we used n = 1/2. Using the above relation and Tauc plots   , we extracted the band gap values by extrapolating the linear region of E vs. (αE)2 to the energy axis. As shown in inset of Figure 7(b), the band gap decreases with the applied compressive stress and this result is consistent with our FTIR measurements. From the measurements of Raman spectra, FTIR and UV-Vis-NIR measurements we believe that indeed there exists a marginal variation of optical properties upon wet ball milling due to decrease in oxygen related groups. The variation of band gap value from 4.06 eV (0 hrs) to 3.92 eV (20 hrs) can be attributed to the de-oxidation compared with the parent compound upon ball milling.
In summary, we have demonstrated the effect of compressive stress on the structural and optical properties of graphene oxide through wet ball milling technique. Williamson-Hall method infers a decrease in average crystallite size upon ball milling. Raman studies infer decrease in defects of GO sample upon wet ball milling. In
Figure 7. (a) Wavelength vs normalized absorption spectrum of all graphene oxide samples. Inset shows clear shift of absorption spectrum towards higher wavelengths; (b) Tauc plots of all graphene oxide samples. Inset shows variation of band gap with ball milling time.
addition to Raman spectra, FTIR and UV-Vis-NIR measurements together support the fact of reduction in oxygen functional groups in GO during wet ball milling. The shift in absorbance peak to higher wavelength values upon ball milling indicates a decrease in band gap. From the Tauc plots it is evident that there exists a marginal variation of band gap value from 4.06 eV (0 hr BMGO) to 3.92 eV (20 hr BMGO). Such variation is due to de-oxidation of GO. From the above, it can be understood that indeed there exists a complex interplay between reduction in average crystallite size and decrease in oxygen related functional groups which results in a marginal change in optical properties. The present results would indeed be helpful in developing GO based optoelectronic devices and sensors for biological applications.
We acknowledge Indian Institute of Technology Hyderabad for the financial support.
Cite this paper
M. Venkat Narayana,S. Narayana Jammalamadaka, (2016) Tuning Optical Properties of Graphene Oxide under Compressive Strain Using Wet Ball Milling Method. Graphene,05,73-80. doi: 10.4236/graphene.2016.52008
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