Search

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes results selected from on-going studies of earth-abundant electrocatalysts for the reactions that underpin artificial photosynthesis: nickel-molybdenum alloys for the hydrogen evolution reaction, calcium birnessite as a heterogeneous analogue for the oxygen-evolving complex in natural photosynthesis, and single-crystalline copper in relation to the carbon dioxide reduction reaction.

Synthetic routes have been developed that allow attachment of a variety of functional groups to etched, single-crystal InP surfaces. Benzyl halides, alkyl halides, silyl halides, and esters reacted readily with InP to yield covalently attached overlayers on the semiconductor surface. High-resolution X-ray photoelectron spectroscopy (XPS) revealed that the functionalization chemistry was consistent with the reactivity of surficial hydroxyl groups. Analysis of the XP spectra of the (111)B-oriented (P-rich) face in ultrahigh vacuum revealed signals ascribable to a monolayer of oxidized P atoms on the etched (111)B InP surface. The lack of reactivity of the (111)A-oriented (In-rich) face with these same functionalization reagents is therefore attributed to the difference in the nucleophilicity and acidity of the In and P oxides that are present on the (111)A and (111)B faces, respectively. The coverage of benzylic groups obtained through functionalization of (111)B-oriented InP with benzyl halides was estimat...

Fermi's golden rule is used to formulate rate expressions for charge transfer of delocalized carriers in a nondegenerately doped semiconducting electrode to localized, outer-sphere redox acceptors in an electrolyte phase. If the charge-transfer rate constant is known experimentally, these rate expressions allow computation of the value of the electronic coupling matrix element between the semiconducting electrode and the redox species. This treatment also facilitates comparison between charge-transfer kinetic data at metallic and semiconducting electrodes in terms of parameters such as the electronic coupling to the electrode, the attenuation of coupling with distance into the electrolyte, and the reorganization energy of the charge-transfer event. Within this framework, rate constant values expected at representative semiconducting electrodes have been evaluated from experimental data for charge transfer from Au electrodes to ferrocene-terminated thiols, to Ru(NH3)53+/2+-terminated thiols, and through bl...

Frumkin corrections for semiconductor electrodes have been evaluated in both depletion and accumulation conditions. In conjunction with the Gouy-Chapman-Stern model, a finite difference approach was used to calculate the potential drop in a depleted semiconductor and in the compact and diffuse layers of the contacting solution as a function of the potential applied to the solid/liquid interface. At potentials greater than 30 mV positive of the flat-band potential Efb the potential frop across the solution accounts for less than 3% of the total potential drop across an n-type semiconductor of dopant density 1 × 1015 cm−3 in a methanolic solution of 1.0 M LiCl. Under these conditions, the concentration of a non-adsorbing, dipositively-charged redox species at the outer Helmholtz plane does not vary from its concentration in the bulk of the solution by more than 2%. This relatively small concentration gradient and potential drop across the Helmholtz layer combine to produce negligible Frumkin correction terms for kinetic data at depleted semiconductor electrodes compared to those for metallic electrodes at the same applied potential relative to the potential of zero charge. Under accumulation conditions, the potential drop across the solution is more significant, and the concentration of redox species at the surface can be as much as twice as great as that in the bulk of the solution. However, these conditions require an applied potential of −1 V relative to Efb. Additionally, under all conditions that were simulated, the correction to the driving force used to evaluate the heterogeneous rate constant does not exceed 2% of the uncorrected heterogeneous rate constant.

Photoconductivity decay data have been obtained for NH_4F_((aq))-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene^(+/0) (Fc^(+/0)), [bis(pentamethylcyclopentadienyl)iron]^(+/0) (Me_(10)Fc^(+/0)), iodine (I_2), or cobaltocene^(+/0) (CoCp_2^(+/0)). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc^(+/0), I_2/I^-) or relatively negative (CoCp_2^(+/0)) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N_2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me_(10)Fc^(+/0)) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si−OCH_3 bonds, produced by reaction of H-terminated Si with CH_3OH−Fc^(+/0), showed lower surface recombination velocities in contact with N_(2(g)) or in contact with CH_3OH−Me_(10)Fc^(+/0) solutions than did NH_4F_((aq))-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH_3OH−Fc^(+/0)-treated surfaces showed lower surface recombination velocities than surfaces containing Si−I bonds, which were formed by the reaction of H-terminated Si surfaces with CH_3OH−I_2 or THF−I_2 solutions. These results can all be consistently explained through reference to the electrochemistry of Si/liquid contacts. In conjunction with prior measurements of the near-surface channel conductance for p^+−n−p^+ Si structures in contact with CH_3OH−Fc^(+/0) solutions, the data reveal that formation of an inversion layer (i.e., an accumulation of holes at the surface) on n-type Si, and not a reduced density of surface electrical trap sites, is primarily responsible for the long charge carrier lifetimes observed for Si surfaces in contact with CH_3OH or THF electrolytes containing I_2 or Fc^(+/0). Similarly, formation of an accumulation layer (i.e., an accumulation of electrons at the surface) consistently explains the low effective surface recombination velocity observed for the Si/CH_3OH−CoCp_2 and Si/CH_3CN−CoCp_2 contacts. Detailed digital simulations of the photoconductivity decay dynamics for semiconductors that are in conditions of inversion or depletion while in contact with redox-active electrolytes support these conclusions.

Photoconductivity decay lifetimes have been obtained for NH₄F_(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of CH₃OH or tetrahydrofuran (THF) containing either ferrocene^(+/0) (Fc^(+/0)), bis(pentamethylcyclopentadienyl) Fe^(+/0), or I₂. Si surfaces in contact with electrolytes having Nernstian redox potentials >0 V versus the standard calomel electrode exhibited low effective surface recombination velocities regardless of the different surface chemistries, whereas those exposed only to N₂(g) ambients or to electrolytes containing mild oxidants showed differing rf photoconductivity decay behavior depending on their different surface chemistry. The data reveal that formation of an inversion layer, and not a reduced density of electrical trap sites on the surface, is primarily responsible for the long charge-carrier lifetimes observed for Si surfaces in contact with CH₃OH or THF electrolytes containing I₂ or Fc^(+/0).

A two-step, chlorination/alkylation procedure has been used to convert the surface Si–H bonds on NH4F(aq)-etched (111)-oriented Si wafers into Si–alkyl bonds of the form Si–CnH2n + 1 (n>=1). The electrical properties of such functionalized surfaces were investigated under high-level and low-level injection conditions using a contactless rf apparatus. The charge carrier recombination velocities of the alkylated surfaces were