Orbital-Resolved Imaging of the Adsorbed State of Pyridine on GaP(110) Identifies Sites Susceptible to Nucleophilic Attack

Abstract

Artificial photosynthesis by photoelectrocatalytic CO2 reduction is dependent, as is natural photosynthesis, on interfacial electron transfer to couple light excitation energy to reaction centers. For heterogeneous systems, in the context of frontier orbital theory artificial reaction centers are defined through the interactions of filled and empty orbitals within a few electronvolts of the Fermi energy of the adsorbate complex. Here we report a scanning tunneling microscopy (STM) and density functional theory investigation of the orbital-resolved adsorption state defining the dative bonding interaction between a III–V semiconductor surface [GaP(110)] and a N-containing heteroaromatic (pyridine). This system was selected for its relevance to photoelectrocatalysis utilizing heteroaromatic cocatalysts, which has been reported to yield highly selective CO2 reduction to fuels. By examining the distribution of unoccupied molecular orbitals, we show that STM images can be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This indicates that STM can be used to explore the local reaction centers of adsorbed ambidentate electrophiles and nucleophiles relevant to artificial photosynthesis, and more broadly to generate critical mechanistic information for various heterogeneous acid–base reactions

Hydride Shuttle Formation and Reaction with CO2on GaP(110)

“…Such ah ypothesis is supportedf urther both by recent predictions from our group [16] and measurements by the Bocarsly group. [11][12][13] These results support the hypothesis of DHP* formation by Py* reduction and the need for an adsorbed co-catalyst (e.g.,D HP*) to shuttle electrons from the surface to CO 2 .T he hypothesis of Py* reduction to DHP* is also supported by the moderate value of the computed reduction potentialf or this process to occur (À0.71 Vv s. the saturated calomel electrode, SCE), [8,9] which lies well below the GaP CB min (À1.76 Vvs. [19] If the CO 2 reduction mechanism in this system were homogeneous,t hen this observation could be explained only by as ubstantial differencei nt he energy of the photoexcited electrons provided by the two surfaces.…”

“…Several adsorptionf ree energy studies found that Py and DHP favorably bind to the GaP surface, whereas CO 2 does not. [11][12][13] These results support the hypothesis of DHP* formation by Py* reduction and the need for an adsorbed co-catalyst (e.g.,D HP*) to shuttle electrons from the surface to CO 2 .T he hypothesis of Py* reduction to DHP* is also supported by the moderate value of the computed reduction potentialf or this process to occur (À0.71 Vv s. the saturated calomel electrode, SCE), [8,9] which lies well below the GaP CB min (À1.76 Vvs. SCE computed for the solvated GaP(110) surface and À1.68 Vv s. SCE computed for the solvated reconstructed GaP(111)s urface).…”

“…[10,16] This model consists of a2 3s upercell with af ivelayer thickness. [10,12,[14][15][16] In this work, we used the climbingimage nudged-elastic-band (CI-NEB) method [50] to identify transition-state structures (see below), which we were not able to converge with mirrored adsorbates. In our previous studies, we placed adsorbates on both sides of the slab to prevent the development of artificial dipoles.…”

“…[1] Alternatively, Py* might form simply through Py adsorption froms olution, based on previous adsorption free energy predictions and experiments showingt hat Py favorably adsorbs on this surface, [11][12][13] and H À *m ight form by dissociative adsorption of water and reduction by GaP surface electrons. PyH + is present in significantc oncentrationi ns olution, given that its pK a of 5.2 is nearly equal to the aqueous solution's pH of 5.3, in the experimental system of Bocarsly and co-workers.…”

“…PyH + is present in significantc oncentrationi ns olution, given that its pK a of 5.2 is nearly equal to the aqueous solution's pH of 5.3, in the experimental system of Bocarsly and co-workers. [1] Alternatively, Py* might form simply through Py adsorption froms olution, based on previous adsorption free energy predictions and experiments showingt hat Py favorably adsorbs on this surface, [11][12][13] and H À *m ight form by dissociative adsorption of water and reduction by GaP surface electrons. In fact, ap revious combined experimental-computational study of the GaP(110) surface exposed to water showed that partial water dissociation is thermodynamically favored on this surface at room temperature, which yields adsorbed protons that developh ydride character.…”