**1. Introduction**

Surface plasmon resonance (SPR) is the phenomenon resulting from the resonant oscillation of conduction electrons present at the interface between negative and positive permittivity of plasmonic metal nanomaterials (PMNMs) that can be induced by incident light. The electric field of the incident light and the free electrons present in PMNMs results in SPR, which yields strong intraband transition [1]. Thus, the above physical process permits the PMNMs to collect the incident light and focus the collected energy near the surface of PMNMs, which in turn transforms the light energy into energy associated with excited charge carriers. The features of PMNMs such as size, shape, and aggregation of nanoparticles as well as the permittivity of surrounding medium decide the intensity of SPR and resonant energy [2]. Several novel surface reactions, namely radiative and non-radiative relaxations, are initiated by the effect of SPR. The influence of SPR leads to the

generation of hot electrons and holes, which are usually termed as hot carriers [3]. These hot carriers can stimulate chemical reactions to the molecules which are present vicinal to the PMNMs [4]. The exceptional characteristics of PMNMs have been extensively employed in numerous applications such as photovoltaic cells, plasmonic sensors, fuel cell fluorescence enhancement as well as local spectroscopies including surface-enhanced Raman spectroscopy (SERS) [5].

Among other analytical techniques, SERS has been deliberated as a commanding analytical technique for the study of surfaces and interfaces. The phenomenon of SERS was initially witnessed in pyridine adsorbed on a rough silver electrode surface in 1974 [6]. Recently, SERS has been utilized as an in situ potential powerful technique that offers unusual surface sensitivity and information regarding molecular fingerprint, to investigate the transient surface species as well as reaction mechanisms. Based on the unique features, SERS has been widely used in medical diagnosis, single molecule detection, pesticide analysis, safety inspections, and identification [7]. Notably, plasmonic metal nanomaterials (such as Ag, Au, and Cu), rough metal films, and gaps between metal surfaces and metal nanomaterials are the widely used SERS platforms for promoting photochemical reactions [8]. The molecules comprising of mostly aromatic or unsaturated bonds are generally targeted for SERS measurements. The Raman signal of the surface species present vicinal to the plasmonic PMNMs can be considerably promoted by the enhanced electromagnetic field, when excited with the appropriate light. Later, the electrochemical SERS had been also employed extensively to investigate the electrochemical processes (interfacial structures, surface reactions, and adsorption of species on electrodes at different potentials) [9]. This measurement has been performed in noble transition nanometals as well as single crystal surfaces for the chemical transformation of aromatic molecules in recent years [10].

From the results of former studies, substituted and unsubstituted aromatic amines as well as nitro compounds have been employed as demonstrative models for aromatic compounds. It is also evident from the earlier studies that an azo species p,p-dimercaptoazobenzene (DMAB) can be selectively resulted from the adsorbed substituted or unsubstituted p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) on the surface of PMNMs [11]. The PMNMs, pH of the solution, substrate, wavelength, and power of irradiation as well as the environmental conditions can have a strong impact on the above-mentioned conversion [12]. The study of this reaction mechanism is still a challenging issue. The complete thermodynamic and kinetic data are also required for the above system. Therefore, the interpretation of reaction mechanisms, which assist in the optimization of experimental conditions and increasing yield as well as selectivity, can be given by theoretical studies on model reactions.

In this chapter, we have given a brief explanation of SPR and its rapid relaxation of nanometals toward surface hot carriers in Section 2. Then, charge transfer mechanism (metal-to-molecule and molecule-to-metal) and the surface catalytic coupling/condensation of aromatic amines and aromatic nitro compounds on noble metal nanomaterials have been discussed through theoretical and experimental results. Since the experimental conditions are highly dependent on the effect of pH and adsorption of molecule, one of the aromatic amines (PATP) has been taken as a model compound to discuss clearly the effect of pH and adsorption of molecule on the nanometals in Section 3. As DMAB can be selectively resulted from the adsorbed PATP and PNTP on the surface of plasmonic nanostructures during the photochemical reaction, the reported SERS results from chemically synthesized DMAB with experimentally derived DMAB from PATP and PNTP have also been discussed in Section 3. Aerobic oxidation-assisted aromatic amine based on nanoplasmonic photocatalysts will be discussed in Section 4. Finally, the reaction mechanisms and future research prospective have been given Sections 5.

**159**

**Figure 1.**

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

accurate relaxation time has been calculated as about 10 fs [14].

polarization of lasers can also determine the process of relaxation [17].

The size, shape, and dielectric constant of the medium of environment in which

the single metal nanoparticles are present determine the frequency of SPR. The SPR lifetime is around 10 fs as the 2.2-eV incident photonic energy falls on gold nanospheres (**Figure 1B**). This results in higher energy photogeneration (around 2.0 eV) of hot electrons compared to the Fermi level, but the energy of the hot hole (lower than the Fermi level) was estimated to be around 1.0 eV [19]. The extinction spectrum of a spherical silver nanoparticle (diameter = 15 nm) shows 380 nm as its SPR frequency. A considerable decrease in the lifetime of hot carriers was observed as the size of silver nanoparticles increased [20]. For instance, in the excitation light, the major distribution of hot carriers was seen surrounding the Fermi level for

*(A) SPR of a spherical PMNM excited by visible light. (B) Normalized distribution of hot electrons and hot holes on Au slab with thicknesses: (i) 10 nm, (ii) 20 nm, and (iii) 40 nm with incident photonic energy of* 

*2.22 eV (reproduced with permission from Govorov et al., published by Elsevier, 2014 [18]).*

**2. Surface plasmon resonance effect on PMNMs and its fast relaxations**

electrons, me = effective mass, and e = charge unit of electrons [13]. The plasmon resonance frequencies of metals such as gold, silver, and copper in bulk were found to be 9.0, 9.0, and 7.9 eV, respectively. Noticeably, the energy of interband transition is lower compared to the transition energies. This leads to the retardation of intraband transition, which in turn results in a fairly large damping constant. The fairly

Compared to gold, silver, and copper bulk metals, the frequency of SPR gradually shifts to the longer wavelength as the size of metals gets decreased under the surface effect. The lower excitation energy of silver nanostructures hinders the interband pathway, but considerably unusual optical characteristics are displayed by the intraband transition [15]. **Figure 1A** shows the SPR of a spherical PMNMs excited by visible light and the absorption and scattering of light on the noble PMNMs are primarily decided by the effect of SPR. The SPR effect is vital for the formation of sub-wavelength area (hot spot), which results from the conversion of far-field light irradiation into near-field photonic energy [16]. The probing molecules can display the effect of SERS/actuate the reactions of surface photochemistry, as they are adsorbed on the sub-wavelength area. The existence of SPR effect can be observed on transition metals unlike noble metals (copper, gold, and silver). The shorter lifetime of SPR is however due to the following reasons: (i) the radiative relaxation based on the photon emission or (ii) non-radiative relaxation via producing hot carriers. The property of metals, size of nanostructures, energy and the

The phenomenon of collective excitation of free electrons present in the noble metals when subjected to an external field generates plasmon resonance. For bulk metals, the density of the free electrons decides the characteristic frequency (ωp)

1/2e; here n = density of conducting

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

of plasmon resonance, that is, ωp = (4πn/me)

*Nanoplasmonics*

generation of hot electrons and holes, which are usually termed as hot carriers [3]. These hot carriers can stimulate chemical reactions to the molecules which are present vicinal to the PMNMs [4]. The exceptional characteristics of PMNMs have been extensively employed in numerous applications such as photovoltaic cells, plasmonic sensors, fuel cell fluorescence enhancement as well as local spectrosco-

Among other analytical techniques, SERS has been deliberated as a commanding analytical technique for the study of surfaces and interfaces. The phenomenon of SERS was initially witnessed in pyridine adsorbed on a rough silver electrode surface in 1974 [6]. Recently, SERS has been utilized as an in situ potential powerful technique that offers unusual surface sensitivity and information regarding molecular fingerprint, to investigate the transient surface species as well as reaction mechanisms. Based on the unique features, SERS has been widely used in medical diagnosis, single molecule detection, pesticide analysis, safety inspections, and identification [7]. Notably, plasmonic metal nanomaterials (such as Ag, Au, and Cu), rough metal films, and gaps between metal surfaces and metal nanomaterials are the widely used SERS platforms for promoting photochemical reactions [8]. The molecules comprising of mostly aromatic or unsaturated bonds are generally targeted for SERS measurements. The Raman signal of the surface species present vicinal to the plasmonic PMNMs can be considerably promoted by the enhanced electromagnetic field, when excited with the appropriate light. Later, the electrochemical SERS had been also employed extensively to investigate the electrochemical processes (interfacial structures, surface reactions, and adsorption of species on electrodes at different potentials) [9]. This measurement has been performed in noble transition nanometals as well as single crystal surfaces for the chemical

From the results of former studies, substituted and unsubstituted aromatic amines as well as nitro compounds have been employed as demonstrative models for aromatic compounds. It is also evident from the earlier studies that an azo species p,p-dimercaptoazobenzene (DMAB) can be selectively resulted from the adsorbed substituted or unsubstituted p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) on the surface of PMNMs [11]. The PMNMs, pH of the solution, substrate, wavelength, and power of irradiation as well as the environmental conditions can have a strong impact on the above-mentioned conversion [12]. The study of this reaction mechanism is still a challenging issue. The complete thermodynamic and kinetic data are also required for the above system. Therefore, the interpretation of reaction mechanisms, which assist in the optimization of experimental conditions and increasing yield as well as

In this chapter, we have given a brief explanation of SPR and its rapid relaxation

of nanometals toward surface hot carriers in Section 2. Then, charge transfer mechanism (metal-to-molecule and molecule-to-metal) and the surface catalytic coupling/condensation of aromatic amines and aromatic nitro compounds on noble metal nanomaterials have been discussed through theoretical and experimental results. Since the experimental conditions are highly dependent on the effect of pH and adsorption of molecule, one of the aromatic amines (PATP) has been taken as a model compound to discuss clearly the effect of pH and adsorption of molecule on the nanometals in Section 3. As DMAB can be selectively resulted from the adsorbed PATP and PNTP on the surface of plasmonic nanostructures during the photochemical reaction, the reported SERS results from chemically synthesized DMAB with experimentally derived DMAB from PATP and PNTP have also been discussed in Section 3. Aerobic oxidation-assisted aromatic amine based on nanoplasmonic photocatalysts will be discussed in Section 4. Finally, the reaction

mechanisms and future research prospective have been given Sections 5.

pies including surface-enhanced Raman spectroscopy (SERS) [5].

transformation of aromatic molecules in recent years [10].

selectivity, can be given by theoretical studies on model reactions.

**158**
