Using TiO₂ as a Conductive Protective Layer for Photocathodic H₂ Evolution

Abstract

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO(2) layer on top of a thin Ti metal layer may be used to protect an n(+)p Si photocathode during photocatalytic H(2) evolution. Although TiO(2) is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H(2) evolution conditions. This behavior is due to the fortunate alignment of the TiO(2) conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H(2) evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H(2) evolution) was also significantly enhanced by the antireflective properties of the TiO(2) layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe(2+)/Fe(3+) redox couple was used to help elucidate details of the band diagram.

The Impact of Different Si Surface Terminations in the (001) n-Si/NiOx Heterojunction on the Oxygen Evolution Reaction (OER) by XPS and Electrochemical Methods

“…Sun and He 9,10 used NiO x layers instead of TiO 2 , thus foregoing the need of an additional catalyst layer, and Mei 11 employed an Ir | IrO x structure instead. While Seger et al 12,13 found that a thin metal layer on the silicon prior to metal oxide deposition resulted in improved performance by reducing the oxidation of the silicon substrate.…”

Enhanced photoelectrochemical water-splitting performance of semiconductors by surface passivation layers

“…49 Seger et al has used a 100 nm TiO 2 layer protection onto an n + p Si photocathode by sputter deposition method. 79 Aer annealing, the protected photocathode had a stable HER photocurrent for 72 h in 1 M HClO 4 electrolyte. Although TiO 2 is a n-type semiconductor, a metallic conductor behavior was found due to the alignment of the Si conduction band with the TiO 2 conduction band and the hydrogen evolution potential.…”

Mo3S4Clusters as an Effective H2Evolution Catalyst on Protected Si Photocathodes

“…9 Previous research has shown that when using moderately doped TiO 2 , anodic electron transfer cannot occur at potentials much more anodic H + /H 2 redox potential. 9 In Figure 3 the 5 nm Pt/n + p Si shows anodic current due It should be noted that different Si wafers were used in Figure 1 and Figure 3. Each Si wafer has a slightly different photovoltage, thus accounting for the small difference in onset potentials between Figure 1 and Figure 3.…”

“…[6][7][8] However, the rectifying properties of this film may restrict the ability to electrodeposit catalysts onto the protected photoabsorber surface. 9,10 Even though the TiO 2 protection layer can be detrimental to certain electrodeposition procedures, it may also open up a new avenue to help improve the stability of molecular catalysts.…”