**3.3 Surface catalytic coupling reactions on PMNMs**

We observed that the mentioned charge transfer mechanism cannot be suitable for all the experimental studies. Here, two main key contradictions were observed as follows: (i) SERS peaks at 1140, 1390, and 1426 cm<sup>−</sup><sup>1</sup> appeared in Raman excitation wavelengths from 488 to 1064 nm for PATP-adsorbed silver and gold surfaces [32, 38]. The same SERS peaks were also acquired in the nanocavity between silver nanoparticle and smooth gold substrate that employs the wavelength of 1064 nm [32]. The UV-Visible absorption peak at 295 nm (π → π\* transition) of PATP in methanol solution resembles transition energy of about 4.20 eV, whereas, SERS peaks arising from photo-driven charge transfer are in the range of 1.16–2.54 eV (corresponding to the incident photonic energy), which would contradict the predicted charge transfer transition. Therefore, if the charge transfer enhancement mechanism was categorized as a resonance-like Raman scattering process, this is the inconsistent large energy gap between the intramolecular excited state and the charge transfer excited state [39–41]. (ii) Another important contradiction is the pH effect. In acidic solutions, the reversible behavior of intensity ratios of SERS peaks (from1440 and 1080 cm<sup>−</sup><sup>1</sup> ) was described with respect to the potentials applied in the earlier studies. Some studies used isomerization to elucidate the reversibility nature with applied potentials. In the case of alkaline solutions, the elucidation of irreversible behavior is not possible. In addition, the correlation between the reversible and irreversible nature in both acidic as well as basic solutions was not simply explained on the basis of charge transfer mechanism.

Motivated by the experimental results, the probable surface species have been reassessed. We have proposed three different surface species for adsorption of PATP on the surface of PMNMs (**Figure 6A**). (i) PATP was oxidized to 4′-mercapto-4-aminodiphenylamine by increasing the potential anodically at PATP-adsorbed gold and platinum metal surfaces. Conversely, a quite different simulated Raman

#### **Figure 6.**

*(A) The three possible reaction pathways of PATP adsorbed on nanostructured metal surfaces after the photochemical reactions (I), (II), and (III). A theoretical adsorption configuration of DMAB adsorbed on rough metal surfaces (IV). (B) SERS spectrum of synthesized DMAB (a), simulated Raman spectrum of DMAB on two silver clusters using DFT (PW91PW91/6-311 + G\*\*(C, N, S, and H)/LANL2DZ(Ag) level (b)), and SERS spectrum from PATP adsorbed on silver nanoparticles measured with the excitation at 632.8 nm (c).*

spectrum was observed for the PATP adsorbed on silver electrodes [30]. (ii) The formation of p,p′-diaminobenzenedisulfuride was observed for PATP adsorbed on silver films, wherein the disulfide compound was believed to form from its corresponding azo compound [42]. However, the disulfide bond will be shattered because of the strong Ag-S bond on silver surfaces [43]. (iii) Chemical transformation of PATP to DMAB (p,p′-dimercaptoazobenzene) was due to the surface catalytic coupling reaction on noble metal surfaces. We suggested that PATP adsorbed on noble metal surfaces can transform to DMAB under irradiation of visible laser based on our DFT and experimental results [33, 44].

DMAB complex was calculated on the basis of static polarizability derivatives. Moreover, DMAB complex and DMAB-Agn complexes were also obtained by a single-end configuration, but the observed Raman spectra were found to be very similar [33, 34]. In particular, the vibrations of azo (N〓N) and benzene ring result in the strong Raman peaks. The theoretical and experimental results from Sun and Xu support the above interpretations [45, 46]. Considerable influence of Ag▬S vibrational frequencies was also observed by the corresponding strong Raman peaks because of the localization interaction of sulfur and silver clusters. Furthermore, the active Raman modes (Ag and Bg) are the irreducible representations for symmetric center trans DMAB to C2h. The strong Raman peaks at 1130, 1390, and 1440 cm<sup>−</sup><sup>1</sup> thus mainly appeared from azo group and benzene ring symmetric vibrations of DMAB [(44)]. The peak at 1130 cm<sup>−</sup><sup>1</sup> resembles C▬N symmetric stretching vibration and the peaks at 1390 and 1440 cm<sup>−</sup><sup>1</sup> show the mixed vibrations of the N〓N bond stretching and the C▬H in-plane symmetric bending [33, 44]. By our DFT calculations, we concluded that proper functional is very important to predict the N〓N bond distance, since the positions of latter two strong peaks are crucial. Theoretical frequencies were in good agreement with the experimental frequencies when the PW91PW91 functional was combined with the triple-zeta Gaussian basis set 6-311 + G\*\*, whereas, the theoretically and experimentally observed vibrational frequencies are pointedly overestimated when B3LYP functional was used. Two different N〓N bond distances for DMAB were calculated to be 1.273 Å (by PW91PW91/6-311 + G\*\*) and 1.256 Å (by B3LYP/6-311 + G\*\*) [33]. Other reports

**167**

1430 cm<sup>−</sup><sup>1</sup>

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

by gas electron diffraction showed comparable result with the experimental value (1.260(8) Å) of azobenzene [47, 48]. **Figure 6B** shows the theoretically simulated Raman spectrum of DMAB (b) is in a good agreement with experimental spectra of DMAB interacted with two silver clusters (a). But for DMAB, such a Raman spectrum for PATP adsorbed on silver cannot be obtained even under the consideration of the photon-driven charge transfer mechanism. Duan and Luo reported that experimentally observed SERS peaks of DMAB adsorbed on silver surfaces are the same as the theoretically observed SERS peaks for DMAB adsorbed on silver surfaces [49]. This was considered due to the tension effect of the adsorption on

The intramolecular resonance effect can be contributed to DMAB adsorbed metallic nanostructures since DMAB is a dye molecule, which owns its absorption band in the visible region. We also observed that charge transfer takes place from silver to DMAB molecule in the low-lying excited states. Recently, we reported the resonance-like enhancement effect of photo driven charge transfer mechanism and intramolecular electronic transition [33, 34]. As the molecular orbitals are mainly distributed in the DMAB azo group (>C▬N〓N▬C<), azo Raman peaks at 1130,

lecular resonance energy, relative to other Raman peaks at 1078 and 1596 cm<sup>−</sup><sup>1</sup>

Because of surface plasmon resonance and intramolecular resonance effects, SERS peak of DMAB appeared very easily after the oxidation of PATP through the surface catalytic coupling reaction. Based on the deep studies, the SERS spectrum of DMAB follows the surface-enhanced resonance Raman spectroscopy in the visible region. This study also revealed that the enormous enhancement effect can be observed in the SERS spectrum of PATP adsorbed on silver or gold nanostructures. The key factors such as acidity and applied potentials influence the stability of DMAB at electrochemical interfaces [50]. Therefore, DMAB can be converted into PATP in acidic solution, due to the reversibility of applied potentials in acidic solution [51]. Even though, DMAB is more stable in the alkaline solution, it can be reduced into

The charge transfer mechanism was also engaged to study the Raman spectra of DMAB at silver/gold surfaces and PATP on silver or gold substrates. In the case of DMAB, the photon-driven charge transfer is from metal (silver or gold) to DMAB, whereas, photo-driven charge transfer occurs from PATP to metals under visible light for PATP adsorbed on silver or gold substrates. Therefore, charge transfer directions are opposite for low-lying excited states of PATP and DMAB. Moreover, the reversibility or irreversibility of DMAB is strongly dependent on acidity of aqueous solutions. Kim and coworkers studied the abnormal SERS peaks arising from the view of CT mechanism in various conditions such as pH, rotation, temperature, and reducing agent as follows [52]. They observed SERS peaks at 1130, 1390, and

in an acidic solution with pH = 3. SERS signals of the ▬NO2 symmetric

after 30 min. It was believed that the

stretching gradually disappeared when PNTP was adsorbed on a rotation platform with 3000 circles per minute modified with silver nanoparticles. They visualized

strong peaks appeared only from the PATP, and not from PNTP. They concluded that no photochemical reaction occurred for PATP at the boundary of ice and silver nanoparticles at liquid nitrogen temperature (77 K) on the basis of nonexistence of reaction for PNTP at the same boundary [52]. Even when strong reductants such as NaBH4 exist, they still observed abnormally strong SERS peaks. It may be assumed that such a significant SERS band should be observed only in the SERS spectra of

The reaction mechanisms and their dynamics of PATP and PNTP are very different. The activation energy of the rate determination steps for PATP and PNTP on

strongly match with the photonic energies to the intramo-

.

*DOI: http://dx.doi.org/10.5772/intechopen.89606*

silver surfaces.

1390, and 1440 cm<sup>−</sup><sup>1</sup>

PATP at more cathodic or negative potentials.

the strong peaks at 1130, 1390, and 1430 cm<sup>−</sup><sup>1</sup>

PATP and DMAB adsorbed on noble metals.

#### *Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures DOI: http://dx.doi.org/10.5772/intechopen.89606*

*Nanoplasmonics*

**Figure 6.**

*632.8 nm (c).*

spectrum was observed for the PATP adsorbed on silver electrodes [30]. (ii) The formation of p,p′-diaminobenzenedisulfuride was observed for PATP adsorbed on silver films, wherein the disulfide compound was believed to form from its corresponding azo compound [42]. However, the disulfide bond will be shattered because of the strong Ag-S bond on silver surfaces [43]. (iii) Chemical transformation of PATP to DMAB (p,p′-dimercaptoazobenzene) was due to the surface catalytic coupling reaction on noble metal surfaces. We suggested that PATP adsorbed on noble metal surfaces can transform to DMAB under irradiation of visible laser

*(A) The three possible reaction pathways of PATP adsorbed on nanostructured metal surfaces after the photochemical reactions (I), (II), and (III). A theoretical adsorption configuration of DMAB adsorbed on rough metal surfaces (IV). (B) SERS spectrum of synthesized DMAB (a), simulated Raman spectrum of DMAB on two silver clusters using DFT (PW91PW91/6-311 + G\*\*(C, N, S, and H)/LANL2DZ(Ag) level (b)), and SERS spectrum from PATP adsorbed on silver nanoparticles measured with the excitation at* 

DMAB complex was calculated on the basis of static polarizability derivatives. Moreover, DMAB complex and DMAB-Agn complexes were also obtained by a single-end configuration, but the observed Raman spectra were found to be very similar [33, 34]. In particular, the vibrations of azo (N〓N) and benzene ring result in the strong Raman peaks. The theoretical and experimental results from Sun and Xu support the above interpretations [45, 46]. Considerable influence of Ag▬S vibrational frequencies was also observed by the corresponding strong Raman peaks because of the localization interaction of sulfur and silver clusters. Furthermore, the active Raman modes (Ag and Bg) are the irreducible representations for symmetric center trans DMAB to C2h. The strong Raman peaks at 1130, 1390, and 1440 cm<sup>−</sup><sup>1</sup> thus mainly appeared from azo group and benzene ring symmetric vibrations

N〓N bond stretching and the C▬H in-plane symmetric bending [33, 44]. By our DFT calculations, we concluded that proper functional is very important to predict the N〓N bond distance, since the positions of latter two strong peaks are crucial. Theoretical frequencies were in good agreement with the experimental frequencies when the PW91PW91 functional was combined with the triple-zeta Gaussian basis set 6-311 + G\*\*, whereas, the theoretically and experimentally observed vibrational frequencies are pointedly overestimated when B3LYP functional was used. Two different N〓N bond distances for DMAB were calculated to be 1.273 Å (by PW91PW91/6-311 + G\*\*) and 1.256 Å (by B3LYP/6-311 + G\*\*) [33]. Other reports

resembles C▬N symmetric stretching

show the mixed vibrations of the

based on our DFT and experimental results [33, 44].

of DMAB [(44)]. The peak at 1130 cm<sup>−</sup><sup>1</sup>

vibration and the peaks at 1390 and 1440 cm<sup>−</sup><sup>1</sup>

**166**

by gas electron diffraction showed comparable result with the experimental value (1.260(8) Å) of azobenzene [47, 48]. **Figure 6B** shows the theoretically simulated Raman spectrum of DMAB (b) is in a good agreement with experimental spectra of DMAB interacted with two silver clusters (a). But for DMAB, such a Raman spectrum for PATP adsorbed on silver cannot be obtained even under the consideration of the photon-driven charge transfer mechanism. Duan and Luo reported that experimentally observed SERS peaks of DMAB adsorbed on silver surfaces are the same as the theoretically observed SERS peaks for DMAB adsorbed on silver surfaces [49]. This was considered due to the tension effect of the adsorption on silver surfaces.

The intramolecular resonance effect can be contributed to DMAB adsorbed metallic nanostructures since DMAB is a dye molecule, which owns its absorption band in the visible region. We also observed that charge transfer takes place from silver to DMAB molecule in the low-lying excited states. Recently, we reported the resonance-like enhancement effect of photo driven charge transfer mechanism and intramolecular electronic transition [33, 34]. As the molecular orbitals are mainly distributed in the DMAB azo group (>C▬N〓N▬C<), azo Raman peaks at 1130, 1390, and 1440 cm<sup>−</sup><sup>1</sup> strongly match with the photonic energies to the intramolecular resonance energy, relative to other Raman peaks at 1078 and 1596 cm<sup>−</sup><sup>1</sup> . Because of surface plasmon resonance and intramolecular resonance effects, SERS peak of DMAB appeared very easily after the oxidation of PATP through the surface catalytic coupling reaction. Based on the deep studies, the SERS spectrum of DMAB follows the surface-enhanced resonance Raman spectroscopy in the visible region. This study also revealed that the enormous enhancement effect can be observed in the SERS spectrum of PATP adsorbed on silver or gold nanostructures. The key factors such as acidity and applied potentials influence the stability of DMAB at electrochemical interfaces [50]. Therefore, DMAB can be converted into PATP in acidic solution, due to the reversibility of applied potentials in acidic solution [51]. Even though, DMAB is more stable in the alkaline solution, it can be reduced into PATP at more cathodic or negative potentials.

The charge transfer mechanism was also engaged to study the Raman spectra of DMAB at silver/gold surfaces and PATP on silver or gold substrates. In the case of DMAB, the photon-driven charge transfer is from metal (silver or gold) to DMAB, whereas, photo-driven charge transfer occurs from PATP to metals under visible light for PATP adsorbed on silver or gold substrates. Therefore, charge transfer directions are opposite for low-lying excited states of PATP and DMAB. Moreover, the reversibility or irreversibility of DMAB is strongly dependent on acidity of aqueous solutions.

Kim and coworkers studied the abnormal SERS peaks arising from the view of CT mechanism in various conditions such as pH, rotation, temperature, and reducing agent as follows [52]. They observed SERS peaks at 1130, 1390, and 1430 cm<sup>−</sup><sup>1</sup> in an acidic solution with pH = 3. SERS signals of the ▬NO2 symmetric stretching gradually disappeared when PNTP was adsorbed on a rotation platform with 3000 circles per minute modified with silver nanoparticles. They visualized the strong peaks at 1130, 1390, and 1430 cm<sup>−</sup><sup>1</sup> after 30 min. It was believed that the strong peaks appeared only from the PATP, and not from PNTP. They concluded that no photochemical reaction occurred for PATP at the boundary of ice and silver nanoparticles at liquid nitrogen temperature (77 K) on the basis of nonexistence of reaction for PNTP at the same boundary [52]. Even when strong reductants such as NaBH4 exist, they still observed abnormally strong SERS peaks. It may be assumed that such a significant SERS band should be observed only in the SERS spectra of PATP and DMAB adsorbed on noble metals.

The reaction mechanisms and their dynamics of PATP and PNTP are very different. The activation energy of the rate determination steps for PATP and PNTP on silver surfaces was calculated to be 5 and 12 kcal/mol, respectively. It indicates that the PATP oxidation reaction rate is quite fast so that an early photochemical reaction cannot be identified. The lack of thermodynamic properties and kinetic information for these reactions are observed in certain studies, which deal with the reaction mechanisms. Our opinions regarding the above issue as well as the anticipated reaction mechanisms will be discussed in the next section.
