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Raman technology is used as molecular fingerprints to identify specific molecules, predict chemical concentrations in mixtures, and analyze molecular structures. This is possible due to the high selectivity, unique chemical characteristics in the Raman spectrum, and the fact that the intensity of the Raman peak is proportional to the concentration of the relevant compound.
The non-destructive nature of Raman technology and the fact that the test does not require sample preparation make Raman spectroscopy the preferred technique for analysis in real-world applications. Raman technology has advanced the development of portable Raman systems with laboratory-level performance.
For example, in the rapid identification of unknown compounds, raw material verification, quantitative analysis of mixtures, and identification of mixtures, Raman has been widely used by chemical companies, pharmaceutical companies, and safety and security personnel worldwide.
In addition, the peak intensity ratio between the two Raman characteristic peaks can also provide practical data on the crystallinity of the material, the disorder of the material structure, and the phase transition.
This article discusses portable Raman spectroscopy for the online characterization of carbon black. Due to the unique data contained in the peak ratio between the D band and G band of sp2 carbon materials, Raman spectroscopy can be an effective test for characterizing carbon black materials.
This research uses a portable Raman spectrometer to highlight the versatility of Raman spectroscopy and the potential impact of advanced portable Raman spectrometers on various industries.
The carbon black material is graphite with an amorphous structure, but its crystallinity is lower than that of graphite. It is often used as a reinforcing filler for automobile tires and other rubber products, as well as paints, pigments and carbon paper.
Due to the lack of clear standard testing techniques and the complexity of the material structure, material characterization is relatively limited to traditional testing. These tests include particle size estimates based on the surface area value, surface area, iodine adsorption value used to evaluate carbon black grades, and dibutyl phthalate (DBP) absorption value to determine the relative amount of oil that carbon black can absorb.
Although there are some technologies that can display the structure of carbon black materials at the atomic level-such as high-performance imaging analysis or X-ray diffraction analysis-these technologies do not allow fast and convenient online or online real-time-molecular level time testing may have Helps evaluate, control and monitor the carbon black manufacturing process.
The carbon microstructure has a high degree of Raman activity, making Raman very suitable for testing carbon materials with various crystal structures. Graphite has a hexagonal plane of carbon atoms, and there are four carbon atoms in a single unit cell. The different planes are connected by rotation or translation around the axis of symmetry.
For the single crystal graphite symmetry group D6h, one of the vibration modes, E2g, has strong Raman activity and is associated with the Raman peak of 1582 cm-1 (G band).
Therefore, graphite with a high degree of single crystallinity, called highly ordered pyrolytic graphite (HOPG), only shows a Raman peak at 1582 cm-1. Figure 1 illustrates the graphite structure of the E2g mode associated with the G band. For carbon black materials with an amorphous microcrystalline structure, another peak appears near 1350 cm-1 (D-band).
It has been determined that the peak at 1350 cm-1 is due to the disorder of the structure near the edge of the crystallites causing the symmetrical structure to deform.
Figure 1. E2g vibration mode of carbon atoms in a layer of graphite
ID/IG Raman peak intensity ratio can be used to describe the degree of disorder of graphite materials. Researchers also believe that for grain sizes greater than 2 nm4, ID/IG is inversely proportional to the grain size of carbon black materials.
Figure 2 shows the Raman spectra of three different carbon black samples. G-band and D-band are mentioned, which were collected with 532 nm laser excitation using the portable Raman spectrometer i-Raman Plus.
Figure 2 Raman spectra of D-band and G-band carbon black materials
The carbon black materials currently on the market use B&W Tek’s portable Raman spectrometer i-Raman Plus® for Raman spectroscopy. The laser excitation wavelength is 532 nm and the spectral resolution is 4.5 cm-1.
The portable video microscope sampling system is integrated with the portable i-Raman Plus system to precisely focus the laser on the sample surface. The picture of the experimental device is shown in Figure 3.
A laser power of about 40 mW was used to collect Raman spectra at room temperature with an incorporation time of 120 seconds.
Due to the low efficiency of the Raman phenomenon (10-8), it is very important that the dark noise and readout noise of the CCD detector in the Raman spectrometer remain much lower than the Raman signal. i-Raman Plus uses a back-thin CCD detector cooled by TE to -2ºC.
Compared with the traditional front-illuminated CCD with a quantum efficiency of about 50%, the quantum efficiency of the back-illuminated CCD can be as high as 90%. The photons enter from the back of the CCD, and the Si substrate is etched into a thin layer to improve the active light area of the CCD.
This can radically improve quantum efficiency by reducing photon loss. The TE cooling of the CCD device effectively reduces the dark noise: for every 7ºC decrease in device temperature, the dark noise will be halved.
Cooling the detector allows a longer incorporation time, such as 120 seconds in the experiment conducted in this study. This significantly increases the detection limit and enables some applications with low light levels.
The software BWSpec™ is used for data analysis, including peak analysis and baseline correction. The baseline correction function in BWSpec can eliminate any fluorescence that may appear in the Raman signal. The function is based on a novel adaptive iterative weighted penalty least squares (airPLS) algorithm.
Using the airPLS algorithm, the weight of the sum of square error (SSE) is iteratively modified between the fitted baseline and the original signal, while the weight of the SSE is trimmed using the difference between the previously fitted baseline and the original signal. The Raman spectra of carbon black before and after airPLS baseline correction are shown in Figure 4.
Figure 4. Raman spectra of carbon black before and after baseline correction
After the baseline is corrected, the peak analysis function of the software can be used to easily obtain the Raman peak intensity. Then the ratio of the Raman peak intensity of the G band and the D band can be calculated. The Raman spectra of three different carbon black materials after baseline correction are shown in Figure 5.
Figure 5. Raman spectra of carbon black samples after different ID/IG baseline corrections: red is C1, blue is C2, and green is C3.
The G-band and D-band peak positions, peak ratio ID/IG and peak intensity of the three samples are calculated and shown in Table 1. A very interesting feature of the D band is that the position of the D band is not affected by the wavelength of the excitation laser.
The results show that when the laser excitation is shifted from 488 nm to 647 nm, the D-band peak position shifts from 1360 cm-1 to 1330 cm-1.
As shown in the experiment, when using 532 nm laser excitation, the peak position of D band is about 1337 cm-1. For samples C1 and C2, the ID/IG ratio is less than 1, indicating that these carbon black materials have a small degree of disorder, which is within the typical range of graphite rod (C1) and graphite powder (C2).
The ID/IG ratio of sample C3 is greater than 1, indicating that the sample has a higher degree of disorder.
Table 1. D-band and G-band peak information of three carbon black samples excited by 532 nm laser
The high sensitivity of the Raman spectrometer provides high-quality spectra of carbon black materials with unique D-band and G-band, so that the correlation between structure and Raman spectra can be established.
The G band illustrates the level of order of graphite in its single crystal form. The existence of the D band is related to the disorder degree of the crystal structure with reduced sp2 carbon symmetry.
The ratio of ID/IG can describe carbon black materials from many aspects, including:
1) Estimation of the particle size of carbon black materials
2) Disorder of carbon black
3) Batch consistency when multiple measurements are taken at different locations of the material
Because portable Raman spectrometers can provide excellent spectra, carbon black materials can be analyzed online or online. This will facilitate carbon black manufacturing process control and process monitoring.
Unlike large bench-top laboratory-grade Raman spectrometers that cannot be easily placed online or online real-time analysis, a new generation of portable Raman spectrometers show great potential in industrial applications and will play a more important role in industrial applications. The near future.
This information is derived from, reviewed and adapted from materials provided by B&W Tek.
For more information on this source, please visit B&W Tek.
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