**5. Summary and prospects**

As the molecules from liquid/gas phase proceed toward the surface of metal, considerable variations in the optical physical and chemical characteristics are observed. SERS as well as surface photocatalytic chemical reactions explain the above characteristic variations on PMNMs. Herein, the oxidation and reduction reactions of PATP and PNTP on the surface of PMNMs were described. In addition, the reaction mechanisms based on these aromatic compounds were investigated in the region of visible light, where the surface plasmon resonance appears on nanostructures of noble metals. The photo-driven charge transfer and the interfacial chemical reactions take place because of the hot electrons and holes generated from SPR relaxation. The hot electrons actuate the surface-activated oxygen species, which result in the surface catalytic coupling reaction in gas/solid interfaces. Relative to the activation barrier of gold nanostructures, the activation barrier of silver nanostructures is smaller. It is evident from the DFT calculations in simulated SERS and chemical reaction pathways. These calculations and SERS measurements describe the reason for the occurrence of highly selective and rapid oxidation coupling reaction of PATP Relative to photooxidation of PATP on PMNMs, the photoreduction of PNTP is apparently slower. A steady decrease in the SERS signal of the symmetric stretching vibration (nitro group) was observed in the photoreduction reaction. The emphatic evidences for the photooxidation of adsorbed PATP

**171**

*Surface Plasmon Enhanced Chemical Reactions on Metal Nanostructures*

on metal nanomaterials is inadequate compared to the evidences for the conversion of PNTP to DMAB. The main reason is the former reaction is much faster than the latter one. The primary outcomes for the photoreduction and photooxidation processes have been given through theoretical as well as experimental investigations. In addition, the effect of pH, power of laser, incident wavelengths, and oxygen

The study of the effect of enhancement mechanism resulting from surface plasmon resonance on the thermodynamic and dynamics of chemical reactions of metals in nanoscale dimensions is an interesting area. The SPR can enhance the local surface optical electric field on the surface of PMNMs, which in turn increases the charge transfer probability between metal and molecules. The chemical reactions on the surface were actuated by the hot electrons and holes generated from the SPR relaxation. The processes result in surface transient active species inclusive of anions with negative charge and free radicals carrying positive charge. However, there is lack of evidence available for the initial reaction steps taking place on the

The major concern for hot electrons can be observed only in the photochemical reactions on the surface of PMNMs. A contemporary consideration of hot electrons as well as the hot holes is essential for a complete system of chemical reaction. For the transformation of light energy in-to chemical energy, a stable and reliable system can be created by PMNMs. The hot holes were attracted to the surface of PMNMs after the completion of PATP photooxidation. But, the retention of hot electrons reduces the surface oxidation species as well as hydrated protons and water molecules into hydrogen gas. The hot electrons present on the surface of PMNMs moreover result in hydrated electrons, which reduces the oxygen species in solution phase. Here, the oxidation reactions are actuated by the hoarded hot holes on the surface of PMNMs. In addition, water molecules in aqueous solution can be oxidized into oxygen gas in the presence of hot holes. On the basis of SPR effect, these photochemical surface reactions actuate chemical processes for the transformation of light energy to chemical energy. The above mentioned surface plasmonmediated chemical reactions, can find promising applications in the enhancement of light energy efficiency in photocatalytic fuel cells and dye-sensitized and new

This work was financially supported by the National Science Foundation of

assisted on photoreduction/photooxidation were tried to be made clear.

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

surface of PMNMs.

perovskite solar cells.

**Acknowledgements**

**Conflict of interest**

China (21533006 and 21373172).

The authors declare no conflict of interest.

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

*Nanoplasmonics*

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.

As the molecules from liquid/gas phase proceed toward the surface of metal, considerable variations in the optical physical and chemical characteristics are observed. SERS as well as surface photocatalytic chemical reactions explain the above characteristic variations on PMNMs. Herein, the oxidation and reduction reactions of PATP and PNTP on the surface of PMNMs were described. In addition, the reaction mechanisms based on these aromatic compounds were investigated in the region of visible light, where the surface plasmon resonance appears on nanostructures of noble metals. The photo-driven charge transfer and the interfacial chemical reactions take place because of the hot electrons and holes generated from SPR relaxation. The hot electrons actuate the surface-activated oxygen species, which result in the surface catalytic coupling reaction in gas/solid interfaces. Relative to the activation barrier of gold nanostructures, the activation barrier of silver nanostructures is smaller. It is evident from the DFT calculations in simulated SERS and chemical reaction pathways. These calculations and SERS measurements describe the reason for the occurrence of highly selective and rapid oxidation coupling reaction of PATP Relative to photooxidation of PATP on PMNMs, the photoreduction of PNTP is apparently slower. A steady decrease in the SERS signal of the symmetric stretching vibration (nitro group) was observed in the photoreduction reaction. The emphatic evidences for the photooxidation of adsorbed PATP

**5. Summary and prospects**

**170**

on metal nanomaterials is inadequate compared to the evidences for the conversion of PNTP to DMAB. The main reason is the former reaction is much faster than the latter one. The primary outcomes for the photoreduction and photooxidation processes have been given through theoretical as well as experimental investigations. In addition, the effect of pH, power of laser, incident wavelengths, and oxygen assisted on photoreduction/photooxidation were tried to be made clear.

The study of the effect of enhancement mechanism resulting from surface plasmon resonance on the thermodynamic and dynamics of chemical reactions of metals in nanoscale dimensions is an interesting area. The SPR can enhance the local surface optical electric field on the surface of PMNMs, which in turn increases the charge transfer probability between metal and molecules. The chemical reactions on the surface were actuated by the hot electrons and holes generated from the SPR relaxation. The processes result in surface transient active species inclusive of anions with negative charge and free radicals carrying positive charge. However, there is lack of evidence available for the initial reaction steps taking place on the surface of PMNMs.

The major concern for hot electrons can be observed only in the photochemical reactions on the surface of PMNMs. A contemporary consideration of hot electrons as well as the hot holes is essential for a complete system of chemical reaction. For the transformation of light energy in-to chemical energy, a stable and reliable system can be created by PMNMs. The hot holes were attracted to the surface of PMNMs after the completion of PATP photooxidation. But, the retention of hot electrons reduces the surface oxidation species as well as hydrated protons and water molecules into hydrogen gas. The hot electrons present on the surface of PMNMs moreover result in hydrated electrons, which reduces the oxygen species in solution phase. Here, the oxidation reactions are actuated by the hoarded hot holes on the surface of PMNMs. In addition, water molecules in aqueous solution can be oxidized into oxygen gas in the presence of hot holes. On the basis of SPR effect, these photochemical surface reactions actuate chemical processes for the transformation of light energy to chemical energy. The above mentioned surface plasmonmediated chemical reactions, can find promising applications in the enhancement of light energy efficiency in photocatalytic fuel cells and dye-sensitized and new perovskite solar cells.
