2.3 Raman
Raman spectroscopy utilizing the inelastic scattering of light was first developed in
1912 by the Indian physicist C. V. Raman [23]. Although Raman can provide the
fingerprint spectrum interference-free from water, the tiny Raman cross-section area
limits its broad application in chemical analysis for a long period since its invention.
The integration of Fourier transform technology, charge-coupled device (CCD)
detectors, compact spectrographs, effective laser rejection filters, near-infrared
lasers, and small computers greatly push forward to the applicability of Raman
spectroscopy in chemical analysis [24]. The enhancement of the inelastic scattering
probability can be further achieved from resonance Raman (RR) scattering, where
the incident laser is near an electronic transition of the molecule of interest, increasing the signal by an additional factor of 10
2
–10
6 . The surface-enhanced Raman
spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) allow the
possibility of the single-molecule detection [25], which use plasmonically enhanced
Raman scattering to characterize the chemical information on single molecules.
Regarding the reaction tracking, the priority of Raman technology based on the
molecular vibration and rotation lies in the fingerprint spectra interference-free from
water. However, Raman spectroscopy usually gives rise to strong signals of inorganic catalyst instead of surface chemical probes, preventing it from the application
in tracing the transformation path of the surficial species.
Currently, SERS has mainly used for in situ monitoring the kinetics of noble
metal (Au, Ag)-catalyzed photocatalytic reaction on the basis of the surface plasmon
resonance (SPR) effect [26]. In this way, SPR has the dual function of activating the
chemical reaction and enhancing the Raman signal of surface species. O 2 have been
found to be involved into the SPR-catalyzed oxidation reaction, but the activation
mechanism/path of oxygen molecules is still obscure due to the extremely short
lifetime of SPR-induced hole–electron pair. Using SPR-assisted selective oxidation
of p-aminothiophenol (PATP) as a demonstration, SERS was used for the in situ
tracking of the O 2 evolution (Fig. 2.13). Both experiments and DFT calculations
reveal that O 2 were activated by accepting an electron from a metal nanoparticle
under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The
anion was then transformed to Au or Ag oxides or hydroxides on the surface to
oxidize the surface species, which was also supported by the heating effect of the
SPR (Fig. 2.14).
Using the PATP oxidation SPR-catalyzed by Ag nanoparticle as a model reaction, a radical-capturer-assisted SERS has been used as an in situ tracking technique
to explore the primary active species determining the reaction path [27]. Hole is
revealed to be directly responsible for the oxidation of PATP to DMAB and O 2
functions as an electron capturer to form isolated hole. The oxidation degree of
PATP can be further enhanced through a joint utilization of electron capturers of
AgNO 3 and atmospheric O 2 , producing p-nitrothiophenol (PNTP) within 10 s due to
the improved hole–electron separation efficiency (Fig. 2.15).
2.3 Raman
33
Raman spectroscopy utilizing the inelastic scattering of light was first developed in
1912 by the Indian physicist C. V. Raman [23]. Although Raman can provide the
fingerprint spectrum interference-free from water, the tiny Raman cross-section area
limits its broad application in chemical analysis for a long period since its invention.
The integration of Fourier transform technology, charge-coupled device (CCD)
detectors, compact spectrographs, effective laser rejection filters, near-infrared
lasers, and small computers greatly push forward to the applicability of Raman
spectroscopy in chemical analysis [24]. The enhancement of the inelastic scattering
probability can be further achieved from resonance Raman (RR) scattering, where
the incident laser is near an electronic transition of the molecule of interest, increasing the signal by an additional factor of 10
2
–10
6 . The surface-enhanced Raman
spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) allow the
possibility of the single-molecule detection [25], which use plasmonically enhanced
Raman scattering to characterize the chemical information on single molecules.
Regarding the reaction tracking, the priority of Raman technology based on the
molecular vibration and rotation lies in the fingerprint spectra interference-free from
water. However, Raman spectroscopy usually gives rise to strong signals of inorganic catalyst instead of surface chemical probes, preventing it from the application
in tracing the transformation path of the surficial species.
Currently, SERS has mainly used for in situ monitoring the kinetics of noble
metal (Au, Ag)-catalyzed photocatalytic reaction on the basis of the surface plasmon
resonance (SPR) effect [26]. In this way, SPR has the dual function of activating the
chemical reaction and enhancing the Raman signal of surface species. O 2 have been
found to be involved into the SPR-catalyzed oxidation reaction, but the activation
mechanism/path of oxygen molecules is still obscure due to the extremely short
lifetime of SPR-induced hole–electron pair. Using SPR-assisted selective oxidation
of p-aminothiophenol (PATP) as a demonstration, SERS was used for the in situ
tracking of the O 2 evolution (Fig. 2.13). Both experiments and DFT calculations
reveal that O 2 were activated by accepting an electron from a metal nanoparticle
under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The
anion was then transformed to Au or Ag oxides or hydroxides on the surface to
oxidize the surface species, which was also supported by the heating effect of the
SPR (Fig. 2.14).
Using the PATP oxidation SPR-catalyzed by Ag nanoparticle as a model reaction, a radical-capturer-assisted SERS has been used as an in situ tracking technique
to explore the primary active species determining the reaction path [27]. Hole is
revealed to be directly responsible for the oxidation of PATP to DMAB and O 2
functions as an electron capturer to form isolated hole. The oxidation degree of
PATP can be further enhanced through a joint utilization of electron capturers of
AgNO 3 and atmospheric O 2 , producing p-nitrothiophenol (PNTP) within 10 s due to
the improved hole–electron separation efficiency (Fig. 2.15).
2.3 Raman
33