**3.2 Adsorption configurations of PATP on PMNMs**

The three adsorption features feasible as PATP toward the surface of metal are: (i) formation of strong chemical bonds (Au▬S or Ag▬S bond) when the easy binding of thiol group with gold/silver takes place, (ii) simultaneous breaking of S▬H bond, and (iii) the formation of weak coordination bonds (Au▬N or Ag▬N bond) as the amino group moves closer to the surface of metal [33]. As a result, there is concurrent binding between the surface of metal and thiol as well as amino groups. On the metal surface, the top, bridge, and hollow sites possessing large adsorption energies can hold the sulfur atoms (**Figure 5A**). The adsorption configuration exposing a skewed angle with regard to the surface has been observed as the top/bridge sites hold the thiol group. In the case of hollow sites, the molecular

#### **Figure 5.**

*(A) One-end adsorption configuration at a top site, a bridge site, a hollow site, and double end configuration through the interaction between C2V point group of amino nitrogen binding and silver. (B) Simulated Raman spectra of PATP with different silver clusters using DFT theoretical methods (B3LYP/6-311 + G\*\*(C, N, S, and H)/LANL2DZ(Ag)) (reproduced with permission from Wu et al., published by ACS, 2009 [33]).*

symmetric axis of PATP can be visualized as perpendicular with respect to the surface. A skewed angle (about 60°) was observed with regard to the line normal to the surface.

The adsorption configurations of PATP on PMNMs can be easily understood from our DFT calculations [33]. The simulated Raman spectra of PATP adsorbed on various silver clusters have been shown in **Figure 5B**. The frequencies of intense peaks observed at 379, 630, 1001, 1071, 1167, 1336, 1476, and 1596 cm<sup>−</sup><sup>1</sup> can be related with the peak frequencies at 379, 630, 1010, 1080, 1181, 1334, 1489, and 1588 cm<sup>−</sup><sup>1</sup> . Here, the latter peak frequency values represent the PATP adsorbed (in aqueous acidic solution) on a rough silver electrode [31]. The strongest Raman peaks appear at 1071 and 1596 cm<sup>−</sup><sup>1</sup> . These peaks symbolize the absolutely symmetric mixed vibrational modes of C▬C and C▬S stretching and the C▬C bonds stretching parallel to C2 axis when we assume PATP with a symmetric point group of C2v. Here we assumed that the backbone of the adsorbed PATP has a local and approximate C2V symmetry of point group, as shown in **Figure 4A**. The totally symmetric in-plane bending vibration of C▬H bonds can be observed at 1181 as well as 1489 cm<sup>−</sup><sup>1</sup> . In the case of free as well as adsorption states, PATP has been anticipated to be in C2V point group. The four vibrational modes (1125, 1286, 1322, and 1426 cm<sup>−</sup><sup>1</sup> ) with low Raman intensity present in between 1100 and 1450 cm<sup>−</sup><sup>1</sup> possess b2 symmetry, which correspond to the asymmetric stretching of C▬C bonds and in-plane bending vibrations of C▬H bonds. Our results along with the study of vibrational analysis of free and adsorbed PATP revealed the deficit vibrational fundamental frequencies at about 1390 cm<sup>−</sup><sup>1</sup> . This shows the significant dissimilarity between experimental as well as theoretical Raman spectra [30, 31]. In spite of the photon-driven charge transfer Herzberg-Teller vibronic coupling, a likely intense peak of SERS cannot be offered by the chemical enhancement mechanism of PATP. The spectra of PATP adsorbed on silver, gold, as well as copper were investigated in various configurations in order to comprehend the vibronic coupling

**165**

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

driven charge transfer enhancement mechanism of adsorbed PATP.

**3.3 Surface catalytic coupling reactions on PMNMs**

as follows: (i) SERS peaks at 1140, 1390, and 1426 cm<sup>−</sup><sup>1</sup>

peaks (from1440 and 1080 cm<sup>−</sup><sup>1</sup>

enhancement in the SERS spectra of PATP. These conclusions lead to the initial hesitation for the strong SERS peaks only related to the earlier elucidated photo-

In analysis of low-lying excited states, a photo-driven charge transfer reaction takes place from PATP toward the surface of metal. This has been also revealed from our density functional theory (DFT) studies [34]. Additionally, the energies of lowlying excited states have been evaluated by using a molecule-metal cluster modeling system. The charge transfer energies for PATP-to-silver clusters (~2.28 eV) and PATP-to-gold clusters (~2.08 eV) were estimated in the case of PATP-Mn clusters, where n = 13. The energies of transition from Mn clusters to PATP for charge transfer excited states were additionally examined to be higher than 3.0 eV [35]. This reveals that in common SERS measurements, the incident photonic energies are lower when compared to the charge transfer energies from metal toward PATP. Furthermore, it has been found that the interband transition energies of gold are lower than the energies of charge transfer and the interband transition energies of silver are nearer to the energies of charge transfer. Thus, under the irradiation of visible light, the photon-driven charge transfer should take place from PATP towards metal surfaces. This direction of charge transfer has been revealed from our early DFT calculations. When the wavelength of laser increases, the maximum potential in the potentialdependent SERS intensity profile should travel toward the positive direction. Our theoretical results were in concordance with the results of theoretical studies from other groups [36, 37]. In contrast, our results were incoherent with the earlier proposed SERS results of PATP adsorbed on surfaces of various metals [30]. This above deviation deliberates the additional uncertainty on the SERS signal appeared.

We observed that the mentioned charge transfer mechanism cannot be suitable for all the experimental studies. Here, two main key contradictions were observed

) was described with respect to the potentials

tion 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

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

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

simply explained on the basis of charge transfer mechanism.

appeared in Raman excita-

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

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

*Nanoplasmonics*

the surface.

**Figure 5.**

1588 cm<sup>−</sup><sup>1</sup>

well as 1489 cm<sup>−</sup><sup>1</sup>

and 1426 cm<sup>−</sup><sup>1</sup>

peaks appear at 1071 and 1596 cm<sup>−</sup><sup>1</sup>

symmetric axis of PATP can be visualized as perpendicular with respect to the surface. A skewed angle (about 60°) was observed with regard to the line normal to

*H)/LANL2DZ(Ag)) (reproduced with permission from Wu et al., published by ACS, 2009 [33]).*

peaks observed at 379, 630, 1001, 1071, 1167, 1336, 1476, and 1596 cm<sup>−</sup><sup>1</sup>

The adsorption configurations of PATP on PMNMs can be easily understood from our DFT calculations [33]. The simulated Raman spectra of PATP adsorbed on various silver clusters have been shown in **Figure 5B**. The frequencies of intense

*(A) One-end adsorption configuration at a top site, a bridge site, a hollow site, and double end configuration through the interaction between C2V point group of amino nitrogen binding and silver. (B) Simulated Raman spectra of PATP with different silver clusters using DFT theoretical methods (B3LYP/6-311 + G\*\*(C, N, S, and* 

related with the peak frequencies at 379, 630, 1010, 1080, 1181, 1334, 1489, and

metric mixed vibrational modes of C▬C and C▬S stretching and the C▬C bonds stretching parallel to C2 axis when we assume PATP with a symmetric point group of C2v. Here we assumed that the backbone of the adsorbed PATP has a local and approximate C2V symmetry of point group, as shown in **Figure 4A**. The totally symmetric in-plane bending vibration of C▬H bonds can be observed at 1181 as

anticipated to be in C2V point group. The four vibrational modes (1125, 1286, 1322,

possess b2 symmetry, which correspond to the asymmetric stretching of C▬C bonds and in-plane bending vibrations of C▬H bonds. Our results along with the study of vibrational analysis of free and adsorbed PATP revealed the deficit

dissimilarity between experimental as well as theoretical Raman spectra [30, 31]. In spite of the photon-driven charge transfer Herzberg-Teller vibronic coupling, a likely intense peak of SERS cannot be offered by the chemical enhancement mechanism of PATP. The spectra of PATP adsorbed on silver, gold, as well as copper were investigated in various configurations in order to comprehend the vibronic coupling

vibrational fundamental frequencies at about 1390 cm<sup>−</sup><sup>1</sup>

. Here, the latter peak frequency values represent the PATP adsorbed (in aqueous acidic solution) on a rough silver electrode [31]. The strongest Raman

. In the case of free as well as adsorption states, PATP has been

) with low Raman intensity present in between 1100 and 1450 cm<sup>−</sup><sup>1</sup>

. These peaks symbolize the absolutely sym-

can be

. This shows the significant

**164**

enhancement in the SERS spectra of PATP. These conclusions lead to the initial hesitation for the strong SERS peaks only related to the earlier elucidated photodriven charge transfer enhancement mechanism of adsorbed PATP.

In analysis of low-lying excited states, a photo-driven charge transfer reaction takes place from PATP toward the surface of metal. This has been also revealed from our density functional theory (DFT) studies [34]. Additionally, the energies of lowlying excited states have been evaluated by using a molecule-metal cluster modeling system. The charge transfer energies for PATP-to-silver clusters (~2.28 eV) and PATP-to-gold clusters (~2.08 eV) were estimated in the case of PATP-Mn clusters, where n = 13. The energies of transition from Mn clusters to PATP for charge transfer excited states were additionally examined to be higher than 3.0 eV [35]. This reveals that in common SERS measurements, the incident photonic energies are lower when compared to the charge transfer energies from metal toward PATP. Furthermore, it has been found that the interband transition energies of gold are lower than the energies of charge transfer and the interband transition energies of silver are nearer to the energies of charge transfer. Thus, under the irradiation of visible light, the photon-driven charge transfer should take place from PATP towards metal surfaces. This direction of charge transfer has been revealed from our early DFT calculations. When the wavelength of laser increases, the maximum potential in the potentialdependent SERS intensity profile should travel toward the positive direction. Our theoretical results were in concordance with the results of theoretical studies from other groups [36, 37]. In contrast, our results were incoherent with the earlier proposed SERS results of PATP adsorbed on surfaces of various metals [30]. This above deviation deliberates the additional uncertainty on the SERS signal appeared.
