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Carbon monoxide is the key intermediate in the electrochemical CO2 reduction reaction and determines the overpotentials and selectivities for C1 and C2 products on copper electrodes. The kinetic mo...

A micro-kinetic model combining electrochemical rate theory and first-principles simulation is developed to study the influences of solution pH and electrode potential on reaction rate, reaction pathways, and product distribution of electrocatalytic CO2 conversion. Two critical issues involved in electrochemical reaction mechanism are investigated: 1) competing concerted and sequential proton-electron transfer pathways, 2) competing thermodynamics-controlled and kinetics-controlled pathways. Our results show that the electrochemical reduction of CO2 to CO and HCOOH adopts a thermodynamics-controlled CPET mechanism at low pH, while follows a kinetics-controlled SPET mechanism at high pH. The electrocatalytic activity and selectivity can be effectively modulated by manipulating of solution pH and electrode potential. It is demonstrated that HCOOH is the main product at low overpotential while CO becomes the main product at high overpotential. In addition, increasing pH is conducive to improving the Faradic efficiency of HCOOH production and suppressing the hydrogen evolution reaction.

The synthesis of Co-doped Mn

The electrochemical reduction of CO2 is a promising route for converting intermittent renewable energy into storable fuels and useful chemical products. A theoretical investigation of the reaction mechanism and kinetics is beneficial for understanding the electrocatalytic activity and selectivity. In this report, a kinetic model based on Marcus theory is developed to compute the potential-dependent reaction barrier of the elementary concerted proton–electron transfer steps of electrochemical CO2 reduction reactions, different from the previous hydrogen atom transfer model. It is found that the onset potentials and rate-determining steps for CO and CH4 formation are determined by the first and third concerted proton–electron transfer steps C1 and C3. The influence of binding energy, electrode potential, and reorganization energy on the computed reaction barriers of the C1 and C3 reactions is discussed. In general, the calculated reaction barrier shows a quadratic relationship with the applied electrode potential. Specifically, the reaction barrier is merely determined by the reorganization energy at equilibrium potential. The present kinetic model is applied to compare the electrocatalytic activities in the electrochemical reduction of CO2 on various copper crystal surfaces. Among the four studied copper single-crystal surfaces, Cu(211) exhibits the best electrocatalytic activity for CO formation and CH4 formation due to its low onset potential and overpotential.

Global development has been heavily reliant on the overexploitation of natural resources since the Industrial Revolution. With the extensive use of fossil fuels, deforestation, and other forms of land-use change, anthropogenic activities have contributed to the ever-increasing concentrations of greenhouse gases (GHGs) in the atmosphere, causing global climate change. In response to the worsening global climate change, achieving carbon neutrality by 2050 is the most pressing task on the planet. To this end, it is of utmost importance and a significant challenge to reform the current production systems to reduce GHG emissions and promote the capture of CO2 from the atmosphere. Herein, we review innovative technologies that offer solutions achieving carbon (C) neutrality and sustainable development, including those for renewable energy production, food system transformation, waste valorization, C sink conservation, and C-negative manufacturing. The wealth of knowledge disseminated in this review could inspire the global community and drive the further development of innovative technologies to mitigate climate change and sustainably support human activities., Graphical abstract, Public summary • Carbon neutrality may be achieved by reforming current global development systems to minimize greenhouse gas emissions and increase CO2 capture • Harnessing the power of renewable and carbon-neutral resources to produce energy and other fossil-based alternatives may eliminate our dependence on fossil fuels • Protecting natural carbon sinks and promoting CO2 capture, utilization, and storage are conducive to mitigating climate change • This review presents the current state, opportunities, challenges, and perspectives of technologies related to achieving carbon neutrality

Electrochemical reduction of nitrogen to produce ammonia at moderate conditions in aqueous solutions holds great prospect but also faces huge challenges. Considering the high selectivity of Au-based materials to inhibit competitive hydrogen evolution reaction (HER) and high activity of transition metals such as Fe and Mo toward the nitrogen reduction reaction (NRR), it was proposed that Au-based alloy materials could act as efficient catalysts for N2 fixation based on density functional theory simulations. Only on Mo3 Au(111) surface the adsorption of N2 is stronger than H atom. Thermodynamics combined with kinetics studies were performed to investigate the influence of composition and ratio of Au-based alloys on NRR and HER. The binding energy and reorganization energy affected performance for the initial N2 activation and hydrogenation process. By considering the free-energy diagram, the computed potential-determining step was either the first or the fifth hydrogenation step on metal catalysts. The optimum catalytic activity could be achieved by adjusting atomic proportion in alloys to make all intermediate species exhibit moderate adsorption. Free-energy diagrams of N2 hydrogenation via Langmuir-Hinshelwood mechanism and hydrogen evolution via Tafel mechanism were compared to reveal that the Mo3 Au surface showed satisfactory catalytic performance by simultaneously promoting NRR and suppressing HER. Theoretical simulations demonstrated that Au-Mo alloy materials could be applied as high-performance electrocatalysts for NRR.

Organic carbonyl compounds are regarded as promising candidates for next-generation rechargeable batteries due to their low cost, environmentally benign nature, and high capacity. The carbonyl utilization is a key issue that limits the practical specific capacity of multi-carbonyl compounds. In this work, a combination of thermodynamic computation and electronic structure analysis is carried out to study the influence of carbonyl type and carbonyl number on the electrochemical performance of a series of multi-carbonyl compounds by using density functional theory (DFT) calculations. By comparing discharge profiles of six tetraone compounds with different carbonyl sites, it is demonstrated that pentacene-5,7,12,14-tetraone (PT) with para-dicarbonyl and pyrene-4,5,9,10-tetraone (PTO) with ortho-dicarbonyl undergo four-lithium transfer while the other four compounds with meta-dicarbonyl fragments show only two-lithium transfer during the discharge process. By further increasing the carbonyl number, the electrochemical performance of molecules with similar para-dicarbonyl sites to PT can not be strongly improved. Among all the studied multi-carbonyl compounds, triphenylene-2,3,6,7,10,11-hexaone (TPHA) and tribenzo[f,k,m]tetraphen-2,3,6,7,11,12,15,16-octaone (TTOA) with similar ortho-dicarbonyl sites to PTO exhibit the best electrochemical performance due to simultaneous high specific capacity and high discharge voltage. Our results offer evidence that conjugated multiple-carbonyl molecules with ortho-dicarbonyl sites are promising in developing high energy-density organic rechargeable batteries.

The electrochemical reduction of N2 is a promising reaction candidate for the ammonia synthesis process. Density functional theory simulations are carried out to study the reaction thermodynamics and kinetics for a better understanding of the catalytic performance of Fe, Mo, Rh, and Ru electrodes. The distal pathway is the most likely reaction pathway for nitrogen reduction on transition metal surfaces according to the computed reaction free energies. The onset potential of nitrogen reduction on Fe(110) (-0.49 V) and Mo(110) (-0.52 V) is determined by the hydrogenation of NH to NH2, which is more positive than the onset potential on the Ru(0001) (-0.76 V) and Rh(111) (-0.98 V) surfaces attributed to the hydrogenation of N2 to NNH. In particular, the initial hydrogenation of N2 on Mo(111) is a spontaneous process due to the strong interaction of N2 and NNH with the Mo(110) surface. Electronic structure analyses including Bader charge analysis and projected crystal orbital Hamilton populations are performed to interpret the difference in adsorption energy of key intermediates on the four metal surfaces. It is found that both N2 and NNH species have the strongest interaction with Mo(110) leading to the initial activation of N2 on the Mo(110) surface being a spontaneous process. A kinetic model based on the Marcus theory is applied to calculate the potential-dependent reaction barrier of electrochemical hydrogenation steps of the N2 reduction reaction. The rate-determining step is the fifth hydrogenation step *NH → *NH2 on Fe(110) and Mo(110) surfaces, and the first hydrogenation step *N2 → *NNH on Rh(111) and Ru(0001) surfaces. The predicted electrocatalytic activity from the potential-dependent rate constant of the rate-determining step on the four metal electrodes decreases in sequence: Fe(110) > Mo(110) > Ru(0001) > Rh(111).

The electrochemical reduction of CO

Organic redox compounds have become the emerging electrode materials for rechargeable lithium ion batteries. The high electrochemical performance provides organic electrode materials with great opportunities to be applied in electric energy storage devices. Among the different types of organic materials, conjugated carbonyl compounds are the most promising type at present, because only they can simultaneously achieve, high energy density, high cycling stability, and high power density. In this research, a series of heteroatom substituted anthraquinone (AQ) derivatives were designed theoretically so that the high theoretical capacity of AQ remained. The discharge and charge mechanism as well as the thermodynamic and dynamic properties of AQ and its derivatives were investigated using first-principles density functional theory. Using heteroatom substitution, both the thermodynamic and dynamic properties of AQ as cathode materials could be largely improved. Among these conjugated carboxyl compounds, BDOZD and BDIOZD with a simultaneously high theoretical capacity and high working potential exhibit the largest energy density of about 780 W h kg−1, which is 41% larger than that of AQ. The PQD with the smallest value of λITo gives the largest charge transfer rate constant, which is about four times as large as the prototype molecule, AQ. The most interesting finding is that the lithium ion transfer plays a very important role in influencing both the discharge potential and electrochemical charge transfer rate. The present study illustrates that theoretical calculations provide a highly effective way to discover potential materials for use with rechargeable lithium ion batteries.

The synthesis of aromatic azo compounds from anilines and nitrobenzene usually requires environmentally unfriendly transition metal catalysts. Here, we propose that azobenzene oligomers are formed in surface-enhanced Raman experiments assisted by surface plasmon resonance. The photocatalytic polymerization of p-dinitrobenzene (DNB) and p-phenylenediamine (PDA) to the corresponding azo-like oligomers on silver surfaces is studied by the surface-enhanced Raman spectroscopy and density functional theory. First, the normal Raman and surface Raman spectra of DNB and PDA are simulated and measured. The simulated normal Raman spectra are in good agreement with their solid-state Raman spectra. However, the simulated surface Raman spectra of DNB-Ag10 and PDA-Ag10 complexes are significantly different from the experimental surface-enhanced Raman spectra. We propose that DNBs has a reductive coupling and PDAs undergo an oxidative coupling reaction during SERS experiments. Secondly, the Raman spectra of the possible NN coupling oligomers of DNB and PDA are simulated. They are compared with the SERS spectra of DNB and PDA adsorbed on silver surfaces. Especially, the Raman spectra of p,p′-dinitroazobenzene and p,p′-diaminoazobenzene are in good agreement with the SERS spectra of DNB and PDA adsorbed on silver surface. Finally, two reaction mechanisms are proposed to explain the surface photocatalysis of DNB and PDA. In the solution, the nitro group of DNB can be reduced by the excited hot electrons. DNBs are then converted to p,p′-dinitroazobenzene through multiple proton and electron transfers. In the air and in the presence of O2, O2 is firstly activated through surface plasmon resonance. PDAs are then converted to p,p′-diaminoazobenzene by the generated active oxygen species. Copyright © 2016 John Wiley & Sons, Ltd.

p-Aminothiophenol (PATP) is an important probe molecule in surface-enhanced Raman spectroscopy. The unique and strong SERS signals of PATP distinguished from its normal Raman spectrum were considered as a signal of an existing charge transfer mechanism. Recent theoretical and experimental studies demonstrate that PATP undergoes surface catalytic coupling reaction to produce an aromatic azo species p,p′-dimercaptoazobenzene (DMAB), which should be responsible for the abnormal signals in the observed SERS spectra of PATP. In this work, three aminothiophenol derivatives with different substitute position and conjugation degree between the amino group (−NH2) and mercapto group (−SH) were chosen to study the effects of substituent including adsorption orientation effect and conjugation effect on the reactivity of photoinduced surface catalytic coupling reactions. A combined SERS and DFT study indicated that no surface reactions occurred for compound C1 and compound C2, while compound C3 was converted to the co...

Surface-enhanced Raman Spectroscopy (SERS) as a unique spectroscopic tool has been broadly used to detect surface reactions, yet normal SERS detection and theoretical simulation cannot satisfy the urgent need of revealing detailed reaction mechanisms. Here, we introduce an isotopic SERS method to demonstrate the occurrence of surface chemical reactions theoretically and experimentally. Isotopic effects on Raman vibrational frequency and intensity due to the change of reduced masses and the vibrational coupling were verified by DFT simulations and its corresponding isotopic SERS measurements. Taking surface catalytic coupling reactions of PATP/PNTP to DMAB on silver surfaces as the modeling system, we first created chemically identical yet vibrationally distinct isotopologues of redox species by artificial isotopic substitution. Our theoretical and experimental results both showed the isotopic effects have obviously different features in the fundamental frequencies and the relative intensities in the Raman...

Recent studies demonstrated that aromatic amines and aromatic nitro compounds could be converted to the corresponding azo species during surface-enhanced Raman experiments. It is very interesting to study the reaction mechanism for molecules that contain both an amino group and a nitro group, nitroaniline isomers. DFT calculations are applied to study the surface-enhanced Raman scattering and plasmonic photocatalysis of nitroaniline isomers on silver surfaces. The normal Raman and surface Raman spectra of nitroaniline isomers are first simulated and compared with experimental results. The calculated Raman spectra of o-nitroaniline (ONA), m-nitroaniline (MNA), and p-nitroaniline (PNA) correspond to their solid-state Raman spectra. However, the simulated surface Raman spectra of nitroaniline–silver complexes are significantly different from the experimental SERS spectra. According to the theoretical simulation, the appearance of new peaks in the SERS experiments of nitroaniline is attributed to the formatio...

The surface plasmon catalytic selective aerobic oxidation of aromatic amines to aromatic azo compounds in metal/molecule/metal junctions was explored by density functional theory. The overall reaction could be divided into the initial plasmon-induced oxygen activation and the subsequent photothermal-driven dehydrogenation process. The activation of oxygen on silver and gold surfaces is proposed through a surface plasmon-mediated hot electron injection mechanism at solid/gas interface. Resonance absorption of incident light by metal nanostructures generates energetic electron–hole pairs. Time-dependent density functional theory calculations illustrate that the excited hot electrons created on metal surfaces can transfer to the antibonding 2π* orbital of adsorbed oxygen, which facilitates the dissociation of O2 on metal surfaces. Silver shows better catalytic performance for the oxygen activation due to its stronger surface plasmon resonance absorption intensity and higher hot electron energy level. Aromati...

The chemical enhancement due to ground-state charge transfer (GSCT) and photon-driven charge transfer (PDCT) in surface-enhanced Raman scattering (SERS) has been investigated by density functional theory. Para-substituted thiophenol derivatives adsorbed on silver and gold surfaces are selected as model systems to evaluate the chemical enhancement factor. By changing the functional groups on thiophenol, we are allowed to modulate the chemical interactions between the thiophenol and the metal cluster in both ground state and charge transfer excited state. Both off-resonance and pre-resonance SERS spectra are simulated to calculate the chemical enhancement factors. The GSCT enhancement factor, EFGSCT , shows a roughly linear relationship to (ωTP /ωM-TP )4 , where ωTP denotes the HOMO-LUMO gap of free molecule, and ωM-TP denotes the energy difference between the HOMO of the molecule and the LUMO of the metal. The PDCT enhancement factor, EFPDCT , is governed by the energy difference between the incident light energy and the excitation energy to the CT excited state. EFPDCT first increases and then decreases with the increase of incident light energy.

A Bronsted acid-promoted, unprecedented formal (3 + 2) annulation strategy for the synthesis of multisubstituted furan-3-carbothioates is reported. This transformation represents the first regioselective annulation of α-oxo ketene dithio-acetals as 1,3-bis-nucleophiles in a cascade manner. The choice of isoindoline-1,3-dione-derived propargyl alcohols is crucial to the uncommon annulation mode between an alkyne-type bis-electrophile and a 1,3-bis-nucleophile under metal-free conditions. The scale-up of the synthesis and several interesting transformations of an as-synthesized product were further investigated. A Nazarov-like cyclization is proposed for the ring-closure process according to the experimental observations.

Conjugated carbonyl compounds have become the most promising type of organic electrode materials for rechargeable Li-ion batteries because only they can achieve simultaneously high energy density, high cycling stability, and high power density. In this work, we have performed first-principles density functional theory (DFT) calculations to explore the fundamental rules of how the electronic structure and redox properties of a typical conjugated carbonyl compound, phenanthrenequinone (PQ), are modified by adjusting the heteroaromatic building blocks. Such a molecular design strategy allows for the improvement in discharge potential while the specific capacity remains nearly unchanged. The correlation between the electronic structures and redox properties for the designed PQ derivatives is systematically discussed. It is demonstrated that the discharge potential of the PQ derivatives depends strongly on the frontier orbital levels, the electric potential, and the Li-bonding configurations. The electrostatic potential (ESP) maps show visible displays of molecular electric structures and can be applied to understand how the redox properties of the PQ derivatives are modified by the heteroaromatic building blocks.

Nanoscale noble metals can exhibit excellent photochemical and photophysical properties, due to surface plasmon resonance (SPR) from specifically collective electronic excitations on these metal surfaces. The SPR effect triggers many new surface processes, including radiation and radiationless relaxations. As for the radiation process, the SPR effect causes the significant focus of light and enormous enhancement of the local surface optical electric field, as observed in surface-enhanced Raman spectroscopy (SERS) with very high detection sensitivity (to the single-molecule level). SERS is used to identify surface species and characterize molecular structures and chemical reactions. For the radiationless process, the SPR effect can generate hot carriers, such as hot electrons and hot holes, which can induce and enhance surface chemical reactions. Here, we review our recent work and related literature on surface catalytic-coupling reactions of aromatic amines and aromatic nitro compounds on nanostructured noble metal surfaces. Such reactions are a type of novel surface plasmon-enhanced chemical reaction. They could be simultaneously characterized by SERS when the SERS signals are assigned. By combining the density functional theory (DFT) calculations and SERS experimental spectra, our results indicate the possible pathways of the surface plasmon-enhanced photochemical reactions on nanostructures of noble metals. To construct a stable and sustainable system in the conversion process of the light energy to the chemical energy on nanoscale metal surfaces, it is necessary to simultaneously consider the hot electrons and the hot holes as a whole chemical reaction system.

The electroreduction of p-nitrothiophenol (PNTP) on gold and silver electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman/IR spectral simulations has allowed us to reveal the reaction mechanism and reaction products of electroreduction of PNTP on metal electrodes. First, thermodynamic calculations were carried out to calculate the standard electrode potentials of PNTP and its possible intermediates. The potential energy curves of PNTP reduction as a function of the applied potential are obtained on the basis of the calculated standard electrode potentials of the elementary electrochemical reactions. Second, surface vibrational spectral simulation was performed to provide theoretical assignments of reaction products for the in situ Raman/IR experimental studies of electroreduction of PNTP. The most interesting finding in the reaction product identified by IR spectroscopy is PATP; however, Raman spectroscopy shows that the main ...

The electro-oxidation of p-aminothiophenol (PATP) on gold electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman and infrared (IR) spectral simulations has allowed us to reveal the electro-oxidation mechanism and reaction products of PATP on gold electrodes in acidic, neutral, and basic solutions. PATP can be first oxidized to the radical cation PATP(NH2•+) or the neutral radical PATP(NH•) depending on the pH of aqueous solutions, and this is the rate-determining step. The radical cation or neutral radical can then transform to the dimerized products through a radical coupling reaction. In the acidic medium, the radical cation reacts with its resonance hybrid through a N–C4 coupling to form 4′-mercapto-N-phenyl-1,4-quinone diimine (D1), which can further undergo hydrolysis to yield 4′-mercapto-N-phenyl-1,4-quinone monoimine (D2). In the neutral medium, the neutral radical reacts with its resonance hybrid through the N–C2(6) cou...

Electrooxidation of hypophosphite (H2PO2−) on Ni electrode was investigated at the molecular level by external-reflection Fourier-transform infrared spectroscopy (FTIR), surface-enhanced infrared absorption spectroscopy with attenuated total reflection (ATR-SEIRAS), and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). The results of external-reflection FTIR demonstrated that H2PO2− could be oxidized to HPO32− at significantly low potentials (E

Surface plasmon resonances (SPRs) have been found to promote chemical reactions. In most oxidative chemical reactions oxygen molecules participate and understanding of the activation mechanism of oxygen molecules is highly important. For this purpose, we applied surface-enhanced Raman spectroscopy (SERS) to find out the mechanism of SPR-assisted activation of oxygen, by using p-aminothiophenol (PATP), which undergoes a SPR-assisted selective oxidation, as a probe molecule. In this way, SPR has the dual function of activating the chemical reaction and enhancing the Raman signal of surface species. Both experiments and DFT calculations reveal that oxygen molecules were activated by accepting an electron from a metal nanoparticle under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The anion was then transformed to Au or Ag oxides or hydroxides on the surface to oxidize the surface species, which was also supported by the heating effect of the SPR. This work points to a promising new era of SPR-assisted catalytic reactions.

This work uses the potential energy curve (pec) and density of states (dos) methods to calculate the cyclic voltammogram for a redox adsorbate that undergoes a simple one-electron reversible redox ...

This work explores the electronic energy of CdSe nanoparticles as a function of nanoparticle (NP) size and capping ligand. Differential pulse voltammetry was used to determine the valence band edge of CdSe NPs that are capped with three different ligands (aniline, thiophenol, and phenylphosphonic acid), and the experimental values are compared with DFT calculations. These results show how the energy position and the size-dependent behavior of the energy bands of CdSe can be modulated by the chemical nature of the capping ligand. The computations underscore how the nature of the highest lying filled states of the nanoparticle can change with ligand type and how this can explain differences between previously reported size-dependent data on similar systems. The findings show that both the ligand and quantum confinement effects should be accounted for in modeling size-dependent effects for different NP–ligand systems.

NSF of China[20973143, 21021002, 91027009]; National Basic Research Programs[2007CB815303, 2009CB930703]; Xiamen University[2010121020]; HPC of Xiamen University

NSF of China[20973143, 91027009]; Ministry of Science and Technology of China[2007CB815303, 2009CB930703]; HPC of Xiamen University; [2010121020]

Taking advantage of the unique capacity of surface plasmon resonance, plasmon-enhanced heterogeneous catalysis has recently come into focus as a promising technique for high performance light-energy conversion. This work performs a theoretical study on the reaction mechanism for conversions of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to aromatic azo species, p,p'-dimercaptoazobenzene (DMAB). In the absence of O2 or H2, the plasmon-driven photocatalysis mechanism (hot electron-hole reactions) is the major reaction channel. In the presence of O2 or H2, the plasmon-assisted surface catalysis mechanism (activated oxygen/hydrogen reactions) is the major reaction channel. The present results show that the coupling reactions of PATP and PNTP strongly depend on the solution pH, the irradiation wavelength, the irradiation power, and the nature of metal substrates as well as the surrounding atmosphere. The present study has drawn a fundamental physical picture for understanding plasmon-enhanced heterogeneous catalysis.

We propose that aromatic nitro and amine compounds undergo photochemical reductive and oxidative coupling, respectively, to specifically produce azobenzene derivatives which exhibit characteristic Raman signals related to the azo group. A photoinduced charge transfer model is presented to explain the transformations observed in para-substituted ArNO(2) and ArNH(2) on nanostructured silver due to the surface plasmon resonance effect. Theoretical calculations show that the initial reaction takes place through excitation of an electron from the filled level of silver to the lowest unoccupied molecular orbital (LUMO) of an adsorbed ArNO(2) molecule, and from the highest occupied molecular orbital (HOMO) of an adsorbed ArNH(2) molecule to the unoccupied level of silver, during irradiation with visible light. The para-substituted ArNO(2)(-)˙ and ArNH(2)(+)˙ surface species react further to produce the azobenzene derivatives. Our results may provide a new strategy for the syntheses of aromatic azo dyes from aromatic nitro and amine compounds based on the use of nanostructured silver as a catalyst.

National Science Foundation of China (NSFC)[20973142]; NSFC Innovation Group of Interfacial Electrochmistry[21021002]; National Project 985 of High Education; New Faculty Starting Package of Xiamen University

When p-aminothiophenol (PATP) is used as a probe molecule and adsorbs on silver and gold nanogaps, a significant change of relative SERS intensities can be observed. Our DFT calculations show that surface photocatalytic coupling reactions yield a new surface species of p,p′-dimercaptoazobenzene (DMAB) causing the significant change in the SERS spectra.

p-aminothiophenol (PATP) is an important molecule for surface-enhanced Raman spectroscopy (SERS). It can strongly interact with metallic SERS substrates and produce very strong SERS signals. It is a molecule that has often been used for mechanistic studies of the SERS mechanism as the photon-driven charge transfer (CT) mechanism is believed to be present for this molecule. Recently, a hot debate over the SERS behavior of PATP was triggered by our finding that PATP can be oxidatively transformed into 4,4'-dimercaptoazobenzene (DMAB), which gives a SERS spectra of so-called "b2 modes". In this perspective, we will give a general overview of the SERS mechanism and the current status of SERS studies on PATP. We will then demonstrate with our experimental and theoretical evidence that it is DMAB which contributes to the characteristic SERS behavior in the SERS spectra of PATP and analyze some important experimental phenomena in the framework of the surface reaction instead of the contribution "b2 modes". We will then point out the existing challenges of the present system. A clear understanding of the reaction mechanism for nitrobenzene or aromatic benzene will be important to not only understand the SERS mechanism but to also provide an economic way of producing azo dyes with a very high selectivity and conversion rate.

The chemical enhancement effects in surface-enhanced Raman scattering of p-aminothiophenol (PATP, it is also called p-mercaptoaniline or p-aminobenzenthiol) adsorbed on coinage metal surfaces with single thiol end or trapped into metal-molecule-metal junctions with both thiol and amino groups have been studied by density functional theory (DFT). We focus on the influence of photon-driven charge transfer (PDCT) and chemical bonding interaction (ground-state charge transfer) on the intensity enhancement and frequency shift in the surface Raman spectra of PATP. For comparison, the electronic structures and transitions of free PATP are studied first. The simulated pre-resonance UV Raman spectra illustrate that b(2) modes can be selectively enhanced via vibronic coupling. The fundamentals of all the b(2) modes in the frequency range of 1000 to 1650 cm(-1) are assigned in detail. For PATP adsorbed on coinage metals, the time-dependent-DFT calculations indicate that the low-lying CT excited state arises from the π bonding orbital of molecule to the antibonding s orbital of metallic clusters. Our results further show that the PDCT resonance-like Raman scattering mechanism enhances the totally symmetric vibrational modes and the NH(2) wagging vibration. Finally, the effect of chemical bonding interaction is also investigated. The amino group binding to metals gives a characteristic band of the NH(2) wagging mode with the large blueshift frequency and an intense Raman signal.