**4. Aerobic oxidation-assisted aromatic amine on PMNMs photocatalysts**

The mechanism for the electrochemical reduction of aromatic nitro compounds into aromatic amines at electrode interface has been reported by Haber in 1898 [53]. The catalytic oxidation of aniline to azobenzene on TiO2-reinforced gold nanoparticles (in the presence of oxygen atmosphere) was proposed by Grirrane et al. [54]. After that, several oxidation and reduction reactions of aminobenzene and nitrobenzene have been reported to form azobenzene. However, limited number of mechanisms (which strongly depend on experimental conditions) have been investigated as silver/gold nanomaterials based on photochemical and photoelectrochemical processes under visible light [55]. The respective absorption bands for PATP and PNTP occur around 300 and 326 nm revealing the absence of absorption peaks of the above two molecules in visible light region [30]. Interestingly, many studies reported that the surface reactions of PATP and PNTP chemically transformed into DMAB in the presence of visible light regions using SERS. Therefore, the adsorbed and photochemical reacted PATP and PNTP molecules on PMNMs needed for the new pathways under SERS investigation.

The activation of O2 is found to be one of the most significant steps in the case of catalytic oxidation reactions. The activation of O2 into two weakly-bonding oxygen atoms has been achieved by employing silver anions and gold clusters [56]. The binding nature of negative metal clusters with oxygen is stronger, compared to neutral metal clusters. The surface plasmon-mediated injection of an excited electron from the optically excited silver nanoparticles toward adsorbate can activate O2 into two weakly-bonding oxygen atoms as reported by Christopher et al. [17, 57]. Oxygen adsorption on the surface of plasmonic nanostructures may involve direct as well as indirect charge transfer reactions. On the other hand, no clear evidences are available to describe that either the direct or indirect charge transfer reactions played the significant part [58]. The study of activation mechanism of oxygen is the challenging part, because of the catalytic oxidation reaction taking place on the surface of plasmonic metal nanoparticles, which involves the direct participation of O2 in metal-oxygen battery systems.

The renowned surface plasmon resonance has been displayed in **Figure 7A**. After the optical excitation of surface plasmon, there are two probable channels of relaxation. The process of scattering (radiative decay into photons) or absorption (non-radiative decay into electron-hole pairs) may result in surface plasmon damping (**Figure 7B**). When the volume of nanoparticle increases, the ratio of scattering to absorption gets increased on the basis of discrete dipole approximation (DDA) simulation and Mie theory calculation [59]. Aimed at the nanoparticles in very small size, the generation of electron-hole pair acts as the foremost pathway to decay. In order to demonstrate the dynamics of excited electrons and holes as well as energy levels, several theoretical models were developed. Based on first-principle TD-DFT calculations [60], we have recently studied the photoinduced excitation of small metal clusters. The evaluated TD-DFT results are in concordance with the distribution results of plasmonic carriers in gold and silver nanoparticles, which were acquired from the quantum equation of motion for the density matrix (18). Because

**169**

composites [20].

**Figure 7.**

**Figure 8.**

*adsorbed oxygen molecules.*

on the surface of silver nanoparticles [60].

*mechanism for PATP adsorbed PMNMs in the solid/liquid interfaces.*

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

of their higher energy, hot electrons will extend further away from the nanoparticle surface than an equilibrium electron distributed at the Fermi level. As shown in **Figure 7C**, the electron acceptor (oxygen adsorbed on a nanoparticle surface) located nearer can accept the hot electron tunneling to its unoccupied orbitals. This effective reaction is equivalent to the process occurring in the metal-semiconductor

*(A) Schematic representation of surface oxidation coupling reaction for PATP-adsorbed PMNMs by surface oxygen species activated by hot electrons under the excitation by visible light. (B) The hot-hole oxidization* 

*(A) Schematic representation of coherent optical excitation of SPR on noble metal surfaces. (B) Hot electronhole pairs created by non-radiative decay of SPR. (C) Surface plasmon-induced hot electron injection to* 

The broader absorption bands observed in the region of visible light are due to the influence of sturdy surface plasmon resonance. The generation of hot electronhole pairs by the process of irradiation relaxation depends on the lifetime (10–20 fs) of localized SPR. The oxygen molecules can be reduced into active oxygen species (under oxygen atmosphere) by the hot electrons (reducing nature) produced at the surface of metal. Thus, adsorbed PATP on silver nanoparticles can be oxidized by the active oxygen atoms (**Figure 8A**). Here, the oxidation of amine group results in the formation of respective imine/free radical. Later, hydroazobenzene was formed due to dimerization. Further oxidation leads to the formation of adsorbed DMAB

In the absence of active oxygen species, the hot-hole oxidation mechanism for the conversion of PATP into DMAB has been displayed in **Figure 8B**. The presence of interfacial defects/small clusters lengthens the lifetimes of hot electrons or holes on the surface of PMNMs. The energy of the hot hole at interfaces and the energy position of the occupied orbital were alike for the adsorption of PATP on the surface of metals [25, 60]. Thus, PATP can form a cation free radical upon oxidation. The

*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 7.**

*Nanoplasmonics*

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

**4. Aerobic oxidation-assisted aromatic amine on PMNMs photocatalysts**

The activation of O2 is found to be one of the most significant steps in the case of catalytic oxidation reactions. The activation of O2 into two weakly-bonding oxygen atoms has been achieved by employing silver anions and gold clusters [56]. The binding nature of negative metal clusters with oxygen is stronger, compared to neutral metal clusters. The surface plasmon-mediated injection of an excited electron from the optically excited silver nanoparticles toward adsorbate can activate O2 into two weakly-bonding oxygen atoms as reported by Christopher et al. [17, 57]. Oxygen adsorption on the surface of plasmonic nanostructures may involve direct as well as indirect charge transfer reactions. On the other hand, no clear evidences are available to describe that either the direct or indirect charge transfer reactions played the significant part [58]. The study of activation mechanism of oxygen is the challenging part, because of the catalytic oxidation reaction taking place on the surface of plasmonic metal nanoparticles, which involves the direct participation of O2 in

The renowned surface plasmon resonance has been displayed in **Figure 7A**. After the optical excitation of surface plasmon, there are two probable channels of relaxation. The process of scattering (radiative decay into photons) or absorption (non-radiative decay into electron-hole pairs) may result in surface plasmon damping (**Figure 7B**). When the volume of nanoparticle increases, the ratio of scattering to absorption gets increased on the basis of discrete dipole approximation (DDA) simulation and Mie theory calculation [59]. Aimed at the nanoparticles in very small size, the generation of electron-hole pair acts as the foremost pathway to decay. In order to demonstrate the dynamics of excited electrons and holes as well as energy levels, several theoretical models were developed. Based on first-principle TD-DFT calculations [60], we have recently studied the photoinduced excitation of small metal clusters. The evaluated TD-DFT results are in concordance with the distribution results of plasmonic carriers in gold and silver nanoparticles, which were acquired from the quantum equation of motion for the density matrix (18). Because

The mechanism for the electrochemical reduction of aromatic nitro compounds into aromatic amines at electrode interface has been reported by Haber in 1898 [53]. The catalytic oxidation of aniline to azobenzene on TiO2-reinforced gold nanoparticles (in the presence of oxygen atmosphere) was proposed by Grirrane et al. [54]. After that, several oxidation and reduction reactions of aminobenzene and nitrobenzene have been reported to form azobenzene. However, limited number of mechanisms (which strongly depend on experimental conditions) have been investigated as silver/gold nanomaterials based on photochemical and photoelectrochemical processes under visible light [55]. The respective absorption bands for PATP and PNTP occur around 300 and 326 nm revealing the absence of absorption peaks of the above two molecules in visible light region [30]. Interestingly, many studies reported that the surface reactions of PATP and PNTP chemically transformed into DMAB in the presence of visible light regions using SERS. Therefore, the adsorbed and photochemical reacted PATP and PNTP molecules on PMNMs

reaction mechanisms will be discussed in the next section.

needed for the new pathways under SERS investigation.

metal-oxygen battery systems.

**168**

*(A) Schematic representation of coherent optical excitation of SPR on noble metal surfaces. (B) Hot electronhole pairs created by non-radiative decay of SPR. (C) Surface plasmon-induced hot electron injection to adsorbed oxygen molecules.*

#### **Figure 8.**

*(A) Schematic representation of surface oxidation coupling reaction for PATP-adsorbed PMNMs by surface oxygen species activated by hot electrons under the excitation by visible light. (B) The hot-hole oxidization mechanism for PATP adsorbed PMNMs in the solid/liquid interfaces.*

of their higher energy, hot electrons will extend further away from the nanoparticle surface than an equilibrium electron distributed at the Fermi level. As shown in **Figure 7C**, the electron acceptor (oxygen adsorbed on a nanoparticle surface) located nearer can accept the hot electron tunneling to its unoccupied orbitals. This effective reaction is equivalent to the process occurring in the metal-semiconductor composites [20].

The broader absorption bands observed in the region of visible light are due to the influence of sturdy surface plasmon resonance. The generation of hot electronhole pairs by the process of irradiation relaxation depends on the lifetime (10–20 fs) of localized SPR. The oxygen molecules can be reduced into active oxygen species (under oxygen atmosphere) by the hot electrons (reducing nature) produced at the surface of metal. Thus, adsorbed PATP on silver nanoparticles can be oxidized by the active oxygen atoms (**Figure 8A**). Here, the oxidation of amine group results in the formation of respective imine/free radical. Later, hydroazobenzene was formed due to dimerization. Further oxidation leads to the formation of adsorbed DMAB on the surface of silver nanoparticles [60].

In the absence of active oxygen species, the hot-hole oxidation mechanism for the conversion of PATP into DMAB has been displayed in **Figure 8B**. The presence of interfacial defects/small clusters lengthens the lifetimes of hot electrons or holes on the surface of PMNMs. The energy of the hot hole at interfaces and the energy position of the occupied orbital were alike for the adsorption of PATP on the surface of metals [25, 60]. Thus, PATP can form a cation free radical upon oxidation. The

intermediate formed can give out a proton to neutral imine free radical in alkaline/ neutral solution. This can be further dimerized and oxidized for the formation of DMAB, parallel to the oxidization reaction based on surface-active oxygen species [61]. Because of the SPR effect, a larger value of rate constant has been exposed by the hot-hole oxidization mechanism unlike the direct charge transfer process.

The experimental environments decide the hot-hole oxidation mechanism for the adsorption of PATP on the surfaces of noble PMNMs. Various investigations have been reported for the generation of electron-hole pairs, which prompts the chemical reactions on the surface of semiconductors. Once the electron-hole pairs got separated, the electrons (present in conduction band) involve in the process of reduction and the holes (present in valence band) are responsible for initiating oxidation. An apparent decrease in the lifetimes of hot electrons and holes was observed as the persistent distribution of energy band seen around the Fermi level on the PMNMs [62]. On the surface of noble PMNMs, the lifetime of hole is frequently found to be in femtoseconds [63]. The lifetime of excitation electrons (associated with its excitation energy) in the excited sp-band is lower than the lifetime of d-band holes [64]. The involvement of hole in the chemical processes has been deliberated in limited reports. The relationship of excitation wavelengths with the pH values has been presented by our results. The conversion of PATP into DMAB based on the oxidation coupling reaction is a multistep reaction comprising of electron/hole transfer as well as proton transfer steps. The values of applied potential, pH of solution, and incident photonic energy decide the chemical potentials of hot electrons, holes as well as protons in the electrochemical SERS measurements [60]. As the incident photonic energy increases, an increase in energies of hot electrons and holes was observed. In contrast, a decrease in their lifetimes with regard to the Fermi level was observed. When the pH value of the solution increased, a decrease in the chemical potential of protons present in the interfacial solution phase was observed. The increase in the energy of hot-hole pairs as well as the value of pH supports the oxidation of PATP into DMAB with loss of two electrons and two protons for each surface reactant. This is complicated but interesting.
