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

Abstract

The efficient generation of dihydrogen on molecularly modified p-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 ΔV (Vonset - VJmax). 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 p-Si(111) photoelectrodes (Voc = 207 ± 5.2 mV; Jsc = -21.7 mA/cm(2); ff = 0.22; ηH2 = 0.99%). These results provide a viable hybrid strategy toward the operation of catalysts on molecularly modified p-Si(111).

Energetic Tug‐of‐War between Pt and Leaky TiO2: Positive and Negative Effects on the Function of Molecularly‐Modified p‐Si(111)|TiO2|Pt Photocathodes

“…Meanwhile, Pt deposition shifted the secondary electron cutoff energy higher to 16.69 eV. Consistent with the shallow escape depth of photoexcited electrons in UPS, the rank order of these shifts in secondary electron cutoff are in agreement with the literature regarding the relative work functions of pSi‐DMO (5.07 eV), TiO 2 (4.8–5.0 eV)„ and Pt (5.64 eV) . Therefore, simple equilibration between the semiconductor and metal does not explain the changes observed electrochemically.…”

“…Electron and hole conduction through the TiO 2 conduction and defect bands, respectively, allows TiO 2 to be a versatile overlayer, used for both photocathodes and photoanodes . However, these modes of conduction also represent possible losses due to the permeability of the overlayer to both majority and minority carriers .…”

“…In this work, we seek to understand the role of ultrathin (<2 nm) leaky TiO 2 as well as probe mechanisms of photovoltage loss at the Si|molecular|TiO 2 heterojunction. We previously reported that even small amounts of TiO 2 deposited on the surface can have considerable effects on the interfacial energetics . We deposit TiO 2 onto molecularly functionalized p ‐type Si(111) to limit complicating silicon oxide at this interface.…”

Photo-assisted electrodeposition of MoSxfrom ionic liquids on organic-functionalized silicon photoelectrodes for H2generation

“…Fig. 11,12,45,46 In this work, it was reported that the greatest extent of band bending occurred on the p-Si(111)|H electrodes as compared with other surface moieties ( Table 2). As shown in Fig.…”

“…12 A boron doped, Cz grown p-Si(111) wafer (single-side-polished, 450 AE 25 mm thick, Virginia Semiconductor Inc.) with 1.4-9 U-cm resistivity was used for organic functionalization of the surface. 12 A boron doped, Cz grown p-Si(111) wafer (single-side-polished, 450 AE 25 mm thick, Virginia Semiconductor Inc.) with 1.4-9 U-cm resistivity was used for organic functionalization of the surface.…”