(1) Infrared spectroscopy: When electromagnetic radiation interacts with material molecules, its energy is equivalent to the difference in vibration or rotation energy of the molecules, causing the molecules to transition from low energy levels to high energy levels. As a result, certain specific wavelengths of electromagnetic radiation are absorbed by the material molecules. The intensity of radiation at different wavelengths is measured to obtain the infrared absorption spectrum. After absorbing infrared radiation, the molecules undergo transitions in vibration and rotation energy levels. Therefore, infrared spectroscopy is also known as molecular vibration rotation spectrum. In short, the generation of infrared spectra is due to the absorption of light energy, which causes vibrations in the molecular dipole moment to change.
(2) Raman spectroscopy: When light is irradiated onto a substance, photons collide with electrons inside the molecule. If a inelastic collision occurs, a portion of the energy of the photons is transferred to the electrons, and the frequency of the scattered light is not equal to the frequency of the incident light. This scattering is called Raman scattering, and the resulting spectrum is called Raman spectroscopy. In short, the generation of Raman spectroscopy is due to the comprehensive scattering effect of light generated after monochromatic light irradiation, which causes vibrations in the molecular polarization rate to change.
（2） Activity discrimination
(1) Mutual exclusion rule
Any molecule with a symmetrical center, if its molecular vibration is Raman active, then its infrared absorption is inactive. On the contrary, if it is infrared active, then Raman is inactive.
(2) Mutual permission rules
Molecules without a symmetrical center have active Raman and infrared spectra (with a few exceptions).
(3) Mutual prohibition rules
For the vibration of a few molecules, both Raman and infrared spectra are inactive (such as ethylene molecules).
（3） Testing instruments
1) Infrared spectroscopy
(1) Dispersion type infrared spectrometer: Similar to UV visible spectrophotometer, it consists of a light source, monochromator, absorption cell, detector, and recording system. Using prisms or gratings as dispersive elements, due to the use of slits, the energy of such dispersive instruments is strictly limited, the scanning time is long, and the sensitivity, resolution, and accuracy are low.
(2) Fourier transform infrared spectrometer: without dispersive elements, mainly composed of a light source, Michelson interferometer, detector, computer, etc. Compared to dispersive infrared spectrometers, it has the advantages of high resolution, high wavenumber accuracy, fast scanning rate, wide spectral range, and high sensitivity.
2) Raman spectroscopy
(1) Dispersion type laser Raman spectrometer: mainly composed of a sample chamber, laser, monochromator, detector, etc.
(2) Fourier transform near-infrared laser Raman spectrometer: mainly composed of a sample chamber, laser light source, Michelson interferometer, filter group, detector, etc.
(3) Laser micro Raman spectrometer: The incident laser is focused on a small part of the sample through a microscope, and the magnified image is directly observed using devices such as a camera tube and monitor, so as to align the laser point with the micro area without interference from surrounding substances, and accurately obtain the Raman spectrum of the irradiated part.
（4） Differences and Similarities
1) Similarities: For a given chemical bond, its infrared absorption frequency is equal to its Raman shift, both representing the energy of the first vibrational level. Therefore, for a given compound, the infrared absorption wave number and Raman shift of certain peaks are exactly the same, and both the infrared absorption wave number and Raman shift are in the infrared region, reflecting the structural information of the molecule. Raman spectroscopy, like infrared spectroscopy, is also used to detect the vibrational and rotational energy levels of material molecules.
2) Different points
(1) Fundamental difference: Infrared spectroscopy is absorption spectroscopy, while Raman spectroscopy is scattering spectroscopy.
(2) Infrared is easier to measure and has a stronger signal, but the Raman signal is weaker. However, Raman spectra are generally clearer and overlapping bands are rarely seen, making spectral analysis more convenient.
(3) Infrared spectroscopy uses infrared light (especially mid infrared light), while Raman spectroscopy can choose from visible light to near-infrared light.
(4) Infrared spectroscopy is commonly used to study the asymmetric vibrations of polar groups, while Raman spectroscopy is commonly used to study the symmetric vibrations of non-polar groups and skeletons.
(5) Raman spectroscopy can measure aqueous solutions (water has weak Raman scattering), while infrared spectroscopy is not suitable for measuring aqueous solutions.
(6) Raman spectroscopy does not require special sample preparation, while infrared spectroscopy requires sample preparation.
(7) Raman spectroscopy can be measured in glass containers or capillaries, but infrared spectroscopy cannot be measured in glass containers.
(8) Raman spectroscopy and infrared spectroscopy often complement each other, with strong infrared and weak Raman. Infrared weak, Raman strong.
(9) Infrared spectroscopy is more effective in identifying organic compounds, while Raman spectroscopy provides more comprehensive information on inorganic compounds.
(10) Infrared spectroscopy analysis: three elements (absorption frequency, intensity, peak shape). In addition to the three elements, Raman spectroscopy analysis also includes depolarization degree.
3、 Example analysis
（1） Example 1 [1]
Literature Introduction: The author synthesized nitrogen doped carbon nanotubes encapsulating Fe3C nanoparticles through a one-step pyrolysis method（ Fe3C@NCNTs ）Compare and characterize materials synthesized at different temperatures. The degradation of antibiotic sulfamethoxazole (SMX) by activated persulfate (PS) using synthetic catalysts was investigated and the reaction mechanism was elucidated.
Figure 1: Synthesis at 600-1000 ℃ Fe3C@NCNTs Raman spectroscopy
Figure 2: Synthesis at 600-1000 ℃ Fe3C@NCNTs FTIR Infrared Spectroscopy
Analysis: The author characterized the structural characteristics and degree of graphitization of the synthesized material through Raman spectroscopy (Figure 1). The D band reflects sp3 defects in carbon (such as amorphous carbon layers, edges, etc. in graphene), while the G band reflects the E2g vibration of sp2 hybridized graphitized carbon atoms. So, the higher the strength ratio of D and G bands to ID/IG values, the higher the degree of defects and the lower the degree of graphitization. Firstly, all samples exhibit two distinct peaks in the D band (~1350 cm-1) and G band (~1588 cm-1). Based on the diagram, it can be further concluded that, Fe3C@NCNTs-800 The lowest ID/IG value (0.86) indicates that it has the most complete sp2 hybrid structure, which may be related to its conductivity. The 2D peak (2705 cm-1) in Raman spectroscopy is another noteworthy feature of graphene materials, and its position and shape can distinguish the layering situation. All baked at temperatures between 700-1000 ℃ Fe3C@NCNTs The samples all have broad 2D peaks, indicating the presence of a few layer structure. In summary, when the pyrolysis temperature is low or high, carbon nanotubes with good sp2 structure cannot be formed, which requires appropriate calcination temperature to form a complete graphene structure. Oxygen functional groups were also detected from the FTIR infrared spectrum (Figure 2). When the calcination temperature increased from 600 ℃ to 1000 ℃, except for the C=C peak, the other four peaks weakened, indicating that unstable O and N species decomposed with the evolution of graphene structure.
（2） Example 2 [2]
Literature Introduction: The author prepared graphene loaded CuO materials and investigated the influence of graphene on the structure, optical, and phonon properties of CuO nanostructures. Study and discuss the changes in nanostructure morphology, grain size, and bandgap of composite materials under different graphene loadings, and directly compare the properties of the prepared materials by treating dye wastewater.
Figure 3 Graphene loaded CuO material
(a) FTIR infrared spectroscopy; (b) Phonon spectrum. (Note: "C" refers to CuO, and the number in "GCXX" represents the mass fraction of CuO precursor in composite materials)
图4
(a) Graphene loaded CuO Raman spectroscopy; (b) The phonon band positions and phonon intensities of the G band and (c) 2D band; (d) Enhancement factors for G-bands and 2D bands
Analysis: The phonon properties of synthesized nanomaterials were investigated by analyzing FTIR infrared spectra and Raman spectra. It can be concluded that an increase in grain size leads to an enhancement of spin phonon interactions in CuO, which is influenced by strong spin phonon interactions, resulting in relaxation of the selection rule; The observed multiple phonons in CuO are due to the morphology modification of CuO nanostructures, which leads to relaxation of the selection rule; The presence of graphene has a slight impact on the position and intensity of each phonon band in CuO.
（3） Example 3 [3]
Literature Introduction: This article reviews the current status, advantages, and disadvantages of Raman spectroscopy for detecting microplastics (<20 μ m), and compares it with other detection techniques such as infrared spectroscopy.
Figure 5 Polypropylene
(a) Raman image (left) and infrared image (right); (b) Raman spectroscopy (left) and infrared transmission spectroscopy (right)
Analysis: In order to verify the effectiveness of infrared spectroscopy and Raman imaging, the author purified and filtered marine microplastic samples with a size<400 μ m. A 1000 × 1000 μ m filter was selected for Raman imaging and FTIR transmission imaging analysis, and the results of the two spectral measurements were compared, including the number, type, and size of detectable microplastics, spectral quality, processing, and measurement time. Figure 5b shows the spectrum of small (15-20 μ m) polypropylene (PP) particles, with clear Raman light (left) contrasting with weak and noisy infrared spectra (right). For infrared spectroscopy, the particle size is close to the lateral resolution and diffraction limit of infrared microscopy, resulting in a lower signal-to-noise ratio. It is worth noting that the detected infrared spectrum may come from a mixture of the target analyte and adjacent particles. In contrast, the Raman spectrum of the same particle shows a better PP spectrum, although the Raman signal intensity is relatively low, the signal-to-noise ratio is significantly improved.
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