Photon-driven Charge Transfer Research Articles

Determination of photon-driven charge transfer efficiency: Drawbacks, accuracy and precision of different methods using Hematite as case of study

The electrochemical properties of photoelectrodes must be measured accurately and precisely to enable better comparisons between different materials. Along with the flat band potential, the interfacial charge transfer efficiency, which is the ratio between charge transfer rate at the photoelectrode surface and rate of charge carrier generation in the photoelectrode, can be used to predict the current density response at a given photon flux and electrode potential. The most widely used techniques for measuring charge transfer efficiencies are Photo-Electrochemical Impedance Spectroscopy (PEIS), current density ratios in the presence and absence of hole/electron scavengers, chrono-amperometry and Intensity Modulated Photocurrent Spectroscopy (IMPS). Charge transfer efficiencies can be estimated from PEIS and IMPS spectra either by using raw data (graphically), by fitting equivalent electrical circuits or by computing the Distribution of Relaxation Times (DRT). However, these techniques have their own drawbacks and impracticalities, that require researchers to make a choice between measuring accurately or pragmatically. Hitherto, the theoretical and experimental details of these techniques have not been summarised collectively and comprehensively. Here, we report the benefits and drawbacks, the accuracy, precision and best experimental recommendations when employing different techniques for photon-driven charge transfer efficiency determination.

A Density Functional Theoretical Study on the Charge-Transfer Enhancement in Surface-Enhanced Raman Scattering.

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.

Role of the I2/I– Equilibrium in the Photocatalysis of Oxygen Reduction at Ag/AgI

Photocatalysis of the oxygen reduction reaction at a silver electrode coated with silver iodide is examined with particular attention to the role of the I2/I– equilibrium in the mechanism of catalysis. We demonstrate the dependence of the photovoltage on the standard reduction potentials for the O2/H2O and the I2/I– equilibria. Iodide, present only via photon-driven charge transfer from silver into the AgI film, oxidizes to iodine in coupling with the oxygen reduction reaction. The photovoltage at the cathode is found tied to that redox couple. The photovoltage is nearly pH-independent owing to a balance between proton consumption during oxygen reduction and the shifting ratio ([I2]/aI–2). The catalytic cycle is maintained by regeneration of AgI by the thermodynamically allowed reaction of iodine with silver.

Surface-enhanced Raman spectroscopic study of p-aminothiophenol

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.

Photon-driven charge transfer and Herzberg-Teller vibronic coupling mechanism in surface-enhanced Raman scattering of p-aminothiophenol adsorbed on coinage metal surfaces: A density functional theory study

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.

Photon-driven charge transfer and photocatalysis of p-aminothiophenol in metal nanogaps: a DFT study of SERS

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.

Adsorption and photon-driven charge transfer of pyridine on a cobalt electrode analyzed by surface enhanced Raman spectroscopy and relevant theories

Surface enhanced Raman spectroscopy (SERS) has been applied to study the interaction of pyridine with a cobalt electrode surface. A set of good quality SERS spectra was obtained by using an appropriate surface-roughening procedure and a highly sensitive confocal Raman microscope. The surface enhancement factor was about three orders of magnitude calculated from experimental data and analyzed by the relevant theories. The electromagnetic contribution accounts for two out of the three orders of magnitude mainly through the lightning-rod effect. The remaining enhancement originates from the photon-driven charge transfer mechanism. The SERS intensity-potential profile shows the existence of two charge transfer processes. One is the excitation of the metal 4s orbital to the mixing orbital of the metal 4px and π-type 3b1 of pyridine, and the other is from the metal 3d orbital to the π-type 2a2 orbital of pyridine. Furthermore, the spectral difference for the adsorbed pyridine on cobalt and silver surfaces indicates that the chemical interaction of pyridine with the former is considerably stronger than the latter.

Inverse photoemission of pyridine on “sers active” silver films

The unresolved a 2 and b 1 affinity levels of pyridine adsorbed on coldly-condensed silver films is found at 2.6±0.2 eV above E F and at 6.6±0.4 eV above E F for condensed pyridine. Resonant Raman scattering by “photon driven charge transfer” could therefore only be possible for directly adsorbed pyridine. This would explain the “first layer Raman enhancement”. Observed differences in the UPS spectra of pyridine on Ag(111) and on coldly-deposited Ag films are discussed.

On the nature of “sers active sites”

Active sites of short range enhancement in surface enhanced Raman scattering (SERS) of adsorbates on cold deposited, porous silver films have been envisioned either as “cavity sites with local electromagnetic resonances” or as “sites of atomic scale roughness with photon driven charge transfer between metal and adsorbate”. Whereas SERS of CO and N 2 is observed at low coverages, the Raman signal of physisorbed O 2 is below the noise level, even at monolayer coverage of the “internal surface” or “filling of the pores”. On the other hand, SERS of ethylene and pyridine is quenched by some percent of a monolayer of oxygen. The Raman signal of multilayer condensed O 2 is stronger for a “compact” film than for a porous film. These observations cannot be reconciled with the first hypothesis, but they are consistent with the second.