Hole-Selective CoO_x/SiO_x/Si Heterojunctions for Photoelectrochemical Water Splitting

Abstract

Cobalt oxide (CoO x ), an earth-abundant and low-cost oxygen evolving catalyst (OEC), has notable advantages as a top protection layer of photoanodes for solar-driven water oxidation because of its desirable durability. However, cobalt oxides exist as various phases, such as Co(II)O, Co2(III)O3, Co3(II,III)O4, and the (photo)­electrochemical properties of CoO x are significantly governed by its phase. Atomic layer deposition (ALD) is a suitable method to form a multifunctional layer for photoelectrochemical (PEC) water splitting because it allows direct growth of a conformal high-quality film on various substrates as well as facile control over its chemical phases by adjusting the deposition conditions. Here, a well-controlled CoO x /SiO x /n-Si heterojunction prepared by ALD is demonstrated for solar-driven water splitting. The phase of the ALD CoO x films can be easily controlled from CoO to Co3O4 by varying the deposition temperature. In addition, this systematic study reveals that its energetic as well as electrochemical properties are changed significantly with the phase. Whereas CoO grown at 150 °C produces high photovoltage by building desirable hole-selective heterojunctions with n-Si, Co3O4 formed at 300 °C has a better catalytic property for water oxidation. To address this competitive correlation, we developed a double-layered (DL) ALD CoO x film that has advantages of both CoO and Co3O4. The DL ALD CoO x /SiO x /Si heterojunction photoanode produces a photocurrent density of 3.5 mA/cm2 without a buried junction and maintains a saturating current density of 32.5 mA/cm2 without noticeable degradation during 12 h in 1 M KOH under a simulated 1 sun illumination.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar‐Driven Water Splitting

“…In addition, it was reported that the role of cobalt oxide in PEC water splitting can be engineered by ALD temperature. Oh et al observed that CoO grown at a low temperature (150 °C) could be used to build a hole‐selective heterojunction with n‐Si, and Co 3 O 4 formed at a high temperature (300 °C) acted as an efficient catalyst for the OER . On this basis, they constructed a SiO x /n‐Si photoanode loaded with a double‐layered ALD CoO x film (CoO and Co 3 O 4 ), which exhibited a J sc of 3.5 mA cm −2 , a value twofold higher than that of the CoO‐modified SiO x /n‐Si photoanode.…”

“…The use of ALD for electrocatalyst synthesis was recently summarized in a book chapter . These features make ALD a promising technique to deposit various electrocatalysts, e.g., Pt NPs, cobalt oxide, nickel oxide, and molybdenum sulfide, on the surface of different semiconductors to enhance the PEC water‐splitting performance.…”

“…ALD enables conformal deposition of a range of transition metal oxides such as CoO x , and NiO x , which can be used as effective catalysts in PEC water splitting. Moreover, the crystal structure and stoichiometry of the deposited oxide layers can be tuned by adjusting the deposition conditions.…”

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

“…222 In the eld of electrocatalysis various suldes, phosphides, nitrides, oxides and (oxy)hydroxides are common catalysts, which are deposited on a substrate that serves as the electrode and they enhance either or both the activity and stability (for example, due to higher stability than the Si substrate in a strong alkaline environment). [223][224][225][226][227] A common starting high-specic-surface-area metallic substrate is nickel foam (NF), on which gas-phase electrodeposition and a variety of solvothermal reactions are used to deposit the metallic catalyst. Various redox catalysts were deposited on NF such as binary Ni-based materials, e.g., Ni 2 P, 228 Mo-doped Ni 2 P, 229 ternary amorphous tungsten-doped Ni x P, 228 or multiphase nickel suldes, 230 taking advantage of the abundant Ni surface.…”

Ion Sputtering–Assisted Double‐Side Interfacial Engineering for CdIn2S4 Photoanode toward Improved Photoelectrochemical Water Splitting

“…Nevertheless, these benefits are counterbalanced by various disadvantages, including low conductivity, serious charge recombination originating from high structural and surface defects, and sluggish OER kinetics . To overcome these issues, a number of strategies have been employed, such as design of nanostructured architecture, intrinsic and extrinsic doping, and heterojunction construction . These strategies can significantly suppress the recombination that occurs in the bulk, thus improving the PEC activity.…”