**3. Photoinduced charge transfer of aromatic nitro and amino molecules on PMNMs**

To illustrate the reaction mechanism, the electroreduction of p-nitrobenzene to aminobenzene derivatives on the surface of noble metals was monitored by SERS. The essential uses of p-substituted aniline as intermediates can be seen in pharmaceuticals and dyes industries. The formation of p-substituted aniline as a result of electrochemical reduction of p-substituted nitrobenzene was revealed from the vanished Raman peak at about 1338 cm<sup>−</sup><sup>1</sup> (corresponding to the symmetric stretching vibration of nitro group in p-substituted nitrobenzene) and appearance of new peaks. Whereas, the mechanistic pathway for the electrochemical polymerization of aminobenzene or aniline was also studied by SERS [21]. Gold colloidal nanoparticles, silver nanofilms, and copper nanoparticles have improved the adsorption and photochemical processes of substituted amino- and nitrobenzenes [22–24]. The reaction mechanisms and the equivalent kinetics are however essential to comprehend the photochemical reactions. All the vibrational frequencies witnessed by our DFT calculations are not highly sensitive to the functional groups such as ▬OH, ▬COOH, ▬SH, ▬CN, and ▬NO2, which are substituted at para positions of nitrobenzene and aminobenzene. All these substituted aminobenzenes and nitrobenzenes on nanoscale noble metals were chemically transformed to azobenzene derivatives after the photochemical reaction (**Figure 2A**). However, the SERS related to the N〓N stretching modes of para-substituted azobenzene exposes an apparently high sensitivity [25]. In addition, the electron density of para functional groups decides the relative intensities of the peaks. The peak appeared at around 1430 cm<sup>−</sup><sup>1</sup> is weaker compared to the peak at around 1390 cm<sup>−</sup><sup>1</sup> in the case of electron-donating functional groups (hydroxyl, amine, and thiol). For electron-withdrawing functional groups (nitro, nitrile, and carboxylic acid), the peak detected at about 1390 cm<sup>−</sup><sup>1</sup> is however weaker than the peak at about 1430 cm<sup>−</sup><sup>1</sup> (**Figure 2B)**.

We have given the possible photochemical reaction mechanism for the reduction and oxidation of substituted aromatic nitro and amino compounds into corresponding azobenzene derivatives (**Figure 3A** and **B**). The substituted amino molecules transformation into azobenzene derivatives in the presence of light involves two steps: 1. Oxidation of amino molecules, and 2. Coupling of two oxidized amino molecules into disubstituted azobenzenes, whereas, the reduction of amino-substituted nitro molecules transformation into azobenzenes in the presence of light involves three steps: 1. Reduction of nitro molecules, 2. Condensation of reduced nitromolecules to oxadia-ziridine, 2,3-bis (4-mercaptophenyl), and 3. Further oxidation of Oxadia-ziridine can convert 2,3-bis (4-substituted phenyl) into disubstituted azobenzene. The experimental results from the oxidation/reduction of different para-substituted aminobenzene/nitrobenzene into para-substituted azobenzenes nevertheless depends on the experimental conditions along with the laser powers

**161**

**Figure 3.**

**Figure 2.**

above mentioned SERS molecules.

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

employed for Raman excitation, electrolyte pH as well as adsorption that promotes the electroreduction of nitro groups into aromatic amines or azo compounds. Thus, numerous studies have been developed for studying the surface reactions of the

*Schematic representation of reaction mechanism for the photochemical reduction of X-ArNO2 (A) and* 

*oxidation of X-ArNH2 (B) (X = ▬OH, ▬COOH, ▬SH, ▬CN, and ▬NO2).*

*(A) Schematic representation for photochemical transformation of substituted amino and nitrobenzenes to azobenzene derivatives. (B) Raman spectra of azobenzene derivatives with various para functional groups simulated by PW91PW91/6-311 + G(d, p): (a) ▬SH, (b) ▬COOH, (c) ▬NH2, (d) ▬NO2, (e) ▬OH, and* 

*(f) ▬CN. (reproduced with permission from Zhao et al., published by RSC, 2012 [25]).*

Among other substituted aromatic nitro compounds, SERS has been employed widely to study the photochemical as well as electrochemical reduction of p-nitrobenzoic acid (PNBA) and p-nitro-thiophenol (PNTP) [26]. The decrease in the intensities of several original peaks and appearance of several new peaks take place in the course of cathode polarization/laser irradiation. Nonetheless, the detailed study for the above spectral alterations is still contentious. The appeared new peaks

*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*

#### **Figure 2.**

*Nanoplasmonics*

**on PMNMs**

around 1430 cm<sup>−</sup><sup>1</sup>

1430 cm<sup>−</sup><sup>1</sup>

the peak detected at about 1390 cm<sup>−</sup><sup>1</sup>

(**Figure 2B)**.

the spherical silver nanoparticle possessing a diameter of 25 nm. When the size of metal nanoparticles increases, a considerable increase in the probability of radiation procedure was witnessed unlike nanoparticles with smaller size. In the case of PMNMs, as the lifetime of SPR gets longer, a larger probability in the distribution of hot carriers (high energy) was observed. The lifetime of SPR is longer for spherical silver nanoparticles relative to gold nanoparticles based on the respective interband

**3. Photoinduced charge transfer of aromatic nitro and amino molecules** 

the symmetric stretching vibration of nitro group in p-substituted nitrobenzene) and appearance of new peaks. Whereas, the mechanistic pathway for the electrochemical polymerization of aminobenzene or aniline was also studied by SERS [21]. Gold colloidal nanoparticles, silver nanofilms, and copper nanoparticles have improved the adsorption and photochemical processes of substituted amino- and nitrobenzenes [22–24]. The reaction mechanisms and the equivalent kinetics are however essential to comprehend the photochemical reactions. All the vibrational frequencies witnessed by our DFT calculations are not highly sensitive to the functional groups such as ▬OH, ▬COOH, ▬SH, ▬CN, and ▬NO2, which are substituted at para positions of nitrobenzene and aminobenzene. All these substituted aminobenzenes and nitrobenzenes on nanoscale noble metals were chemically transformed to azobenzene derivatives after the photochemical reaction (**Figure 2A**). However, the SERS related to the N〓N stretching modes of para-substituted azobenzene exposes an apparently high sensitivity [25]. In addition, the electron density of para functional groups decides the relative intensities of the peaks. The peak appeared at

To illustrate the reaction mechanism, the electroreduction of p-nitrobenzene to aminobenzene derivatives on the surface of noble metals was monitored by SERS. The essential uses of p-substituted aniline as intermediates can be seen in pharmaceuticals and dyes industries. The formation of p-substituted aniline as a result of electrochemical reduction of p-substituted nitrobenzene was

is weaker compared to the peak at around 1390 cm<sup>−</sup><sup>1</sup>

is however weaker than the peak at about

case of electron-donating functional groups (hydroxyl, amine, and thiol). For electron-withdrawing functional groups (nitro, nitrile, and carboxylic acid),

We have given the possible photochemical reaction mechanism for the reduction and oxidation of substituted aromatic nitro and amino compounds into corresponding azobenzene derivatives (**Figure 3A** and **B**). The substituted amino molecules transformation into azobenzene derivatives in the presence of light involves two steps: 1. Oxidation of amino molecules, and 2. Coupling of two oxidized amino molecules into disubstituted azobenzenes, whereas, the reduction of amino-substituted nitro molecules transformation into azobenzenes in the presence of light involves three steps: 1. Reduction of nitro molecules, 2. Condensation of reduced nitromolecules to oxadia-ziridine, 2,3-bis (4-mercaptophenyl), and 3. Further oxidation of Oxadia-ziridine can convert 2,3-bis (4-substituted phenyl) into disubstituted azobenzene. The experimental results from the oxidation/reduction of different para-substituted aminobenzene/nitrobenzene into para-substituted azobenzenes nevertheless depends on the experimental conditions along with the laser powers

(corresponding to

in the

transition energy values (3.2 and 2.3 eV) [19, 20].

revealed from the vanished Raman peak at about 1338 cm<sup>−</sup><sup>1</sup>

**160**

*(A) Schematic representation for photochemical transformation of substituted amino and nitrobenzenes to azobenzene derivatives. (B) Raman spectra of azobenzene derivatives with various para functional groups simulated by PW91PW91/6-311 + G(d, p): (a) ▬SH, (b) ▬COOH, (c) ▬NH2, (d) ▬NO2, (e) ▬OH, and (f) ▬CN. (reproduced with permission from Zhao et al., published by RSC, 2012 [25]).*

#### **Figure 3.**

*Schematic representation of reaction mechanism for the photochemical reduction of X-ArNO2 (A) and oxidation of X-ArNH2 (B) (X = ▬OH, ▬COOH, ▬SH, ▬CN, and ▬NO2).*

employed for Raman excitation, electrolyte pH as well as adsorption that promotes the electroreduction of nitro groups into aromatic amines or azo compounds. Thus, numerous studies have been developed for studying the surface reactions of the above mentioned SERS molecules.

Among other substituted aromatic nitro compounds, SERS has been employed widely to study the photochemical as well as electrochemical reduction of p-nitrobenzoic acid (PNBA) and p-nitro-thiophenol (PNTP) [26]. The decrease in the intensities of several original peaks and appearance of several new peaks take place in the course of cathode polarization/laser irradiation. Nonetheless, the detailed study for the above spectral alterations is still contentious. The appeared new peaks are in concordance with the observed SERS results of aromatic amines adsorbed on silver nanostructures [27]. Thus, some studies equate the new peaks to aromatic amines as reduction products. For example, Sun et al. described that, the product formed can be both aniline species and azo compound or individual aniline species/ azo compound. They proposed the charge transfer mechanism where the transfer of charge between silver island films and p-nitrobenzoic acid arose from the laser excitation [28]. The high-power laser irradiation also resulted in the formation of p, p-azodibenzoic acid through reductive coupling reaction [29].

p-aminothiophenol (PATP) is a molecule possessing thiol group at the para position of aniline. It can form self-assembled monolayer (SAM) on the surface of nanometals. Additionally, PATP has been targeted as a significant surface probe molecule in the areas of SERS and nanoscience. An exceptional and sturdy SERS signal has been displayed by PATP. However, the study of enhancement mechanism is still challenging since/in 1990s. An intense potential-dependent peak of SERS for three excitation wavelengths (488, 514.5, and 633 nm) was seen at 1430 cm<sup>−</sup><sup>1</sup> [30]. As the excitation wavelength increases, the potential analogous to the maximum Raman peak intensity proceeds toward the direction of more negative potential [30]. This reveals that these light irradiations can perform the charge transfer from the PMNMs to adsorbed surface species. From DFT calculations of metallic cluster models, we have proposed a clear theoretical pathway for the charge transfer mechanism from PATP to PMNMs and PMNMs to PNTP in (**Figure 4**). In the course of incident light excitation on the surface of rough/colloidal silver and gold, the participation of surface plasmon and charge transfer processes have been described. At this juncture, for oxidation and reduction reactions, the metal nanoparticles act as sink of electrons and electron source. The Fermi level of PMNMs which exists in between HOMO and LUMO of PNTP or PATP (adsorbate) can be altered by the applied potential. The resonant charge transfer can occur as the energy difference between the ground state ψg and the photon-driven charge transfer excited state ψCT equates the exciting radiation energy. Thus, the charge transfer takes place from the molecule to the

#### **Figure 4.**

*(A) Schematic representation of charge transfer from metal to molecule: an electron excitation from the Fermi level of PMNMs to the LUMO of aromatic nitro compounds. (B) Charge transfer from molecule to metal: an electron excitation from the HOMO of aromatic amine to the Fermi level of PMNMs.*

**163**

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

surface of silver for X–ArNH2 and from silver surface to molecule for X–ArNO2. In both cases, X symbolizes the thiol functional group at the para position. Later, the excited surface complex possibly will go through one of the two dissimilar deexcitations as follows: (i) based on the reverse charge transfer back to ground state which is followed by a radiative process, where, this deexcitaion (purely physical) comprises of either Raman scattering or fluorescence emission and (ii) photochemical reaction. Here, a neutral ArNO2H radical can be formed by the excited nitro radical anion, which takes up proton from proton donors present in solution. Further reactions take place from the so formed neutral ArNO2H radical. The neutral ArNH radical formed from the excited amine radical cation by giving out a proton. This ArNH radical can undergo nitrogen-nitrogen coupling reaction (dimerization), indicating the molecules are strongly adsorbed on the electrode surface (**Figure 2**). Thus, the charge transfers from molecule (PATP) to metal nanoparticles and metal nanoparticles to molecule (PNTP) occur. In fact, the experimentally observed charge transfer should be responsible to a process from silver electrode to adsorbed DMAB species,

**3.1 Effect of pH toward photochemical reaction on PATP-adsorbed PMNMs**

In order to give a clear explanation of the effect of pH, adsorption of molecules for surface catalytic coupling reactions, and oxygen-assisted photocatalytic reactions on PMNMs, we have chosen only para-aminothiophenols (PATPs) as representative compounds. The SERS signals are highly sensitive to the pH of the electrolyte and applied potential. It is evident from the previous SERS measurements of the adsorption of PATP on the surface of rough silver and gold electrodes, reported by Hill and Wehling [31]. The property of SERS significantly varies from acidic to alkaline medium. For the case of anodic and cathodic polarization, SERS spectra of PATP show substantial variations in alkaline solution. In contrast to alkaline medium, a good reversibility behavior (in intensity of SERS) was seen in acidic medium based on changing potentials. As the applied potentials move toward the negative direction, the

order to describe the above character, an isomerization of aromatic and quinonoidic configurations was postulated for PATP adsorbed on the surface of metal electrodes in acidic medium. Under negative applied potential (−1.4 V vs. Ag|AgCl) in pure sodium disulfide, the quinonoidic configuration was proposed in alkaline medium to support the above hypothesis. Based on the adsorption of PATP in the nanoscale cavity between the substrate (gold) and silver nanoparticles, the isomerization and charge transfer mechanism are given concurrently to the SERS mechanism by Zhou et al. [32]. Till now, we know these changes of SERS are closely associated with the

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

) retained virtually in the alkaline medium. In

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

as proposed from previous DFT calculations [25].

SERS peaks (1130, 1390, and 1440 cm<sup>−</sup><sup>1</sup>

surface catalytic coupling reaction of PATP to DMAB.

**3.2 Adsorption configurations of PATP on PMNMs**

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

*Nanoplasmonics*

are in concordance with the observed SERS results of aromatic amines adsorbed on silver nanostructures [27]. Thus, some studies equate the new peaks to aromatic amines as reduction products. For example, Sun et al. described that, the product formed can be both aniline species and azo compound or individual aniline species/ azo compound. They proposed the charge transfer mechanism where the transfer of charge between silver island films and p-nitrobenzoic acid arose from the laser excitation [28]. The high-power laser irradiation also resulted in the formation of p,

p-aminothiophenol (PATP) is a molecule possessing thiol group at the para position of aniline. It can form self-assembled monolayer (SAM) on the surface of nanometals. Additionally, PATP has been targeted as a significant surface probe molecule in the areas of SERS and nanoscience. An exceptional and sturdy SERS signal has been displayed by PATP. However, the study of enhancement mechanism is still challenging since/in 1990s. An intense potential-dependent peak of SERS for three excitation wavelengths (488, 514.5, and 633 nm) was seen at 1430 cm<sup>−</sup><sup>1</sup>

the excitation wavelength increases, the potential analogous to the maximum Raman peak intensity proceeds toward the direction of more negative potential [30]. This reveals that these light irradiations can perform the charge transfer from the PMNMs to adsorbed surface species. From DFT calculations of metallic cluster models, we have proposed a clear theoretical pathway for the charge transfer mechanism from PATP to PMNMs and PMNMs to PNTP in (**Figure 4**). In the course of incident light excitation on the surface of rough/colloidal silver and gold, the participation of surface plasmon and charge transfer processes have been described. At this juncture, for oxidation and reduction reactions, the metal nanoparticles act as sink of electrons and electron source. The Fermi level of PMNMs which exists in between HOMO and LUMO of PNTP or PATP (adsorbate) can be altered by the applied potential. The resonant charge transfer can occur as the energy difference between the ground state ψg and the photon-driven charge transfer excited state ψCT equates the exciting radiation energy. Thus, the charge transfer takes place from the molecule to the

*(A) Schematic representation of charge transfer from metal to molecule: an electron excitation from the Fermi level of PMNMs to the LUMO of aromatic nitro compounds. (B) Charge transfer from molecule to metal: an* 

*electron excitation from the HOMO of aromatic amine to the Fermi level of PMNMs.*

[30]. As

p-azodibenzoic acid through reductive coupling reaction [29].

**162**

**Figure 4.**

surface of silver for X–ArNH2 and from silver surface to molecule for X–ArNO2. In both cases, X symbolizes the thiol functional group at the para position. Later, the excited surface complex possibly will go through one of the two dissimilar deexcitations as follows: (i) based on the reverse charge transfer back to ground state which is followed by a radiative process, where, this deexcitaion (purely physical) comprises of either Raman scattering or fluorescence emission and (ii) photochemical reaction. Here, a neutral ArNO2H radical can be formed by the excited nitro radical anion, which takes up proton from proton donors present in solution. Further reactions take place from the so formed neutral ArNO2H radical. The neutral ArNH radical formed from the excited amine radical cation by giving out a proton. This ArNH radical can undergo nitrogen-nitrogen coupling reaction (dimerization), indicating the molecules are strongly adsorbed on the electrode surface (**Figure 2**). Thus, the charge transfers from molecule (PATP) to metal nanoparticles and metal nanoparticles to molecule (PNTP) occur. In fact, the experimentally observed charge transfer should be responsible to a process from silver electrode to adsorbed DMAB species, as proposed from previous DFT calculations [25].
