Insulator Layer Engineering toward Stable Si Photoanode for Efficient Water Oxidation

Abstract

The ultrathin insulator layer in the silicon metal–insulator–semiconductor (MIS) photoanode plays important roles in determining the performance of water oxidation. We report that an insulator oxide electroforming phenomenon that occurred during long-term oxygen evolution reaction testing under constant external voltage is the primary reason for the degradation of the silicon MIS photoanode. Compared with TiO2, ZrO2 exhibits high electro-reduce resistance, which minimizes the cation transport in the dielectric film, thereby significantly enhancing the stability of silicon MIS photoanodes. Silicon passivated with an ultrathin ZrO2 film deposited by atomic layer deposition (ALD) exhibits high stability (>100 h) under alkaline conditions (PH = 14). Combined with a NiFe catalyst, the NiFe/ZrO2/n-Si photoanode exhibits improved OER activity with a low onset potential (∼0.96 V versus RHE), a high photocurrent density (26.6 mA·cm–2) at 1.23 V versus RHE, and a record high saturated current density of 36.4 mA·cm–2.

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

“…These include SiO 2 ,28–41 TiO 2 ,42–47 HfO 2 ,48,49 Al 2 O 3 ,42,50–53 SrTiO 3 ,54 and ZrO 2. 55 Initially, MIS systems were motivated by their ability to improve the chemical stability of semiconductors, but recently many design strategies have been identified to improve the efficiency of these systems. In general, a large barrier height, low resistance, and ideal interfaces with minimal defects are necessary to achieve high efficiencies and photovoltages in MIS systems.…”

Atomic Layer Deposition for the Photoelectrochemical Applications

“…During OER operation, TiO 2 deposited on photoanode degrades. [ 121–123 ] Electron conductivity strongly depends on the TiO 2 structure. [ 122 ] Amorphous compact layers deposited at moderate temperatures (100–200 °C) are almost insulators.…”

“…Chemically stable, refractory oxide 1–3 nm thin films as HfO 2 [ 129 ] and ZrO 2 [ 121 ] have been used as a protection layer for photoanodes, using Ni or NiFe as OER catalyst, respectively. A comparison of only 2 nm thin TiO 2 and ZrO 2 , shows that TiO 2 of such low thickness is damaged by oxide electro‐reduction and Ti dissolution caused under OER conditions.…”

“…On the other hand, Zr atoms generated from the electro‐reduction of ZrO 2 react with Si atoms diffused from Si substrate to form a Zr‐Si layer between Si and ZrO 2 layer, preventing further corrosion of ZrO 2 . [ 121 ]…”

Enhancing photocatalytic performance and solar absorption by schottky nanodiodes heterojunctions in mechanically resilient palladium coated TiO2/Si nanopillars by atomic layer deposition

“…Moreover, Si-NPls are quite attractive structures that have shown wider absorption and enhanced performance due to the clear increment of their active surface. 11,12 However, as photoanode, bare Si suffers from fast degradation and corrosion in aqueous environments, which dramatically limits its operation time 13 Several strategies based on the introduction of protective coatings to hinder surface passivation have been proposed to extend the lifetime of Si based photoanodes 14,15 Additional studies have shown the advantages of TiO2 or ZnO coatings on the performance of Si-NPls, [16][17][18] due to their optical tunability, 6 mechanical reinforcement, 10,19 and high photoactive performance. 6,10 Recently, photoactive junctions of precious metals and semiconducting materials have shown enhanced carrier transport capabilities of photogenerated electrons which could provide a more efficient catalytic performance for water splitting [20][21][22][23][24][25][26] .…”