Synthetic Insights into Surface Functionalization of Si(111)–R Photoelectrodes: Steric Control and Deprotection of Molecular Passivating Layers

We report the utility of controlled spacing of molecular monolayers on Si(111) surfaces by the use of sterically bulky silanes. The steric bulk of a 3,5-diphenolic linker of type Ph–diO–SiR₃ (R = hexyl, phenyl, ^<i>i</i>Pr)as well as the smaller Ph–diOMeis shown to control the surface coverage on Si(111). The para substituent was also changed from −F (small) to −OTf (triflate, large) to modulate the conformation of a selected bulky silane (SiR₃; R = hexyl) to further control the steric environment of the monolayer. The surface coverage values are found to vary systematically from 57 → 21 → 15 → 11% for the series CH₃ → hexyl → ^<i>i</i>Pr → phenyl. Substitution at the para position (F → OTf) decreased the packing density for R = hexyl to as low as 8% (from 21%). The molecular coverage was also found to control the rate and extent of surface oxidation when unfunctionalized sites were allowed to oxidize. Following attachment, facile deprotection of the silanes was achieved by treatment with BBr₃ to afford the diphenolic −OH groups. To electronically characterize the monolayers, voltammetry was performed in contact with liquid Hg to determine the barrier height, which was decreased by 70 mV as the coverage is increased. This study provides a synthetic rationale for controlling the packing density of surface linkers using electroless chemistry at semiconductor interfaces, thus providing further tunability and functionality of photoelectrochemical devices

Hybrid Organic/Inorganic Band-Edge Modulation of <i>p</i>‑Si(111) Photoelectrodes: Effects of R, Metal Oxide, and Pt on H₂ Generation

The efficient generation of dihydrogen on molecularly modified <i>p</i>-Si­(111) has remained a challenge due to the low barrier heights observed on such surfaces. The band-edge and barrier height challenge is a primary obstruction to progress in the area of integration of molecular H₂ electrocatalysts with silicon photoelectrodes. In this work, we demonstrate that an optimal combination of organic passivating agent and inorganic metal oxide leads to H₂ evolution at photovoltages positive of RHE. Modulation of the passivating R group [CH₃ → Ph → Naph → Anth → Ph­(OMe)₂] improves both the band-edge position and Δ<i>V</i> (<i>V</i>_onset – <i>V</i>_{<i>J</i>_max}). Subsequent atomic layer deposition (ALD) of Al₂O₃ or TiO₂ along with ALD-Pt deposition results in to our knowledge the first example of a positive H₂ operating potential on molecularly modified Si(111). Mott–Schottky analyses reveal that the flat-band potential of the stable Ph­(OMe)₂ surface approaches that of the native (but unstable) hydride-terminated surface. The series resistance is diminished by the methoxy functional groups on the phenyl unit, due to its chemical and electronic connectivity with the TiO₂ layer. Overall, judicious choice of the R group in conjunction with TiO₂|Pt effects H₂ generation on <i>p</i>-Si­(111) photoelectrodes (<i>V</i>_oc = 207 ± 5.2 mV; <i>J</i>_sc = −21.7 mA/cm²; ff = 0.22; η_H₂ = 0.99%). These results provide a viable hybrid strategy toward the operation of catalysts on molecularly modified <i>p</i>-Si­(111)

Steric Spacing of Molecular Linkers on Passivated Si(111) Photoelectrodes

Surfaces with high photoelectrochemical and electronic quality can be prepared by tethering small molecules to single-crystalline Si(111) surfaces using a two-step halogenation/alkylation method (by Lewis and co-workers).− We report here that the surface coverage of custom-synthesized, phenyl-based molecular linkers can be controlled by varying the steric size of R-groups (R = CH₃, C₆H₁₁, 2-ethylhexyl) at the periphery of the linker. Additionally, the linkers possess a para triflate group (−O₂SCF₃) that serves as a convenient analytical marker and as a point of covalent attachment for a redox active label. Quantitative X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface coverage systematically varies according to the steric size of the linker: CH₃ (6.7 ± 0.8%), CyHex (2.9 ± 1.2%), EtHex (2.1 ± 0.9%). The stability of the photoelectrochemical cyclic voltammetry (PEC-CV) behavior was dependent on an additional methylation step (with CH₃MgCl) to passivate residual Si(111)–Cl bonds. Subsequently, the triflate functional group was utilized to perform Pd-catalyzed Heck coupling of vinylferrocene to the surface-attached linkers. Ferrocene surface coverages measured from cyclic voltammetry on the ferrocene-functionalized surfaces Si(111)–<b>8</b><b>a</b>/CH₃–Fc (R = CH₃) and Si(111)–<b>8</b><b>c</b>/CH₃–Fc (R = 2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a ∼1:1 conversion of surface triflate groups to vinyl-Fc sites. The surface defect densities of the linker/CH₃ modified surfaces are dependent on the coverage and composition of the organic layer. Surface recombination velocity (SRV) measurements indicated that <i>n</i>-Si­(111)–<b>8</b><b>a</b>/CH₃ and the ferrocene coupled <i>n</i>-Si­(111)–<b>8</b><b>a</b>/CH₃–Fc exhibited relatively high surface carrier lifetimes (4.51 and 3.88 μs, respectively) and correspondingly low <i>S</i> values (3880 and 4510 cm s^–1). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated <i>n</i>-Si­(111)–CH₃ surface

Silicon Photoelectrode Thermodynamics and Hydrogen Evolution Kinetics Measured by Intensity-Modulated High-Frequency Resistivity Impedance Spectroscopy

We present an impedance technique based on light intensity-modulated high-frequency resistivity (IMHFR) that provides a new way to elucidate both the thermodynamics and kinetics in complex semiconductor photoelectrodes. We apply IMHFR to probe electrode interfacial energetics on oxide-modified semiconductor surfaces frequently used to improve the stability and efficiency of photoelectrochemical water splitting systems. Combined with current density-voltage measurements, the technique quantifies the overpotential for proton reduction relative to its thermodynamic potential in Si photocathodes coated with three oxides (SiO_<i>x</i>, TiO₂, and Al₂O₃) and a Pt catalyst. In pH 7 electrolyte, the flatband potentials of TiO₂- and Al₂O₃-coated Si electrodes are negative relative to samples with native SiO_<i>x</i>, indicating that SiO_<i>x</i> is a better protective layer against oxidative electrochemical corrosion than ALD-deposited crystalline TiO₂ or Al₂O₃. Adding a Pt catalyst to SiO_<i>x</i>/Si minimizes proton reduction overpotential losses but at the expense of a reduction in available energy characterized by a more negative flatband potential relative to catalyst-free SiO_<i>x</i>/Si