Bundle-Type Columnar Cu₂O Photoabsorbers with Vertical Grain Boundaries Fabricated Using Instant Strike-Processed Metallic Seeds and Their Enhanced Photoelectrochemical Efficiency

Abstract

The design to control grain boundaries (GBs) causing recombination losses as planar defects in the absorbing layers is an essential strategy for the development of highly efficient photoelectrochemical (PEC) reaction systems. We propose a method to design a progressive bundle-type columnar structure as an alternative to single crystals with the least defects, where the GBs are parallel to the charge transport movement and the electric field direction. An instant strike bias (0.01 s and −1 V vs Ag/AgCl) in the same electrolytes induces the formation of island-shaped metallic Cu nanoparticles in the initial stage. These act as seed crystals for controlling Cu 2 O growth evolution, resulting in dramatically highdensity Cu 2 O nuclei. This is followed by the deposition of bundle-type columnar Cu 2 O with longitudinal GBs, contrary to the typical randomly crystallized Cu 2 O. Metallic Cu seeds with a stronger electric field than that of the exposed indium tin oxide (ITO) region provide selective crystallization sites for Cu 2 O growth along the ⟨111⟩ ionic bonding. Despite the instant strike interval, the p-type Cu 2 O photoelectrodes retained an outstanding photocurrent density of 5.2 mA cm −2 at 0 V RHE and an onset potential of 0.7 V RHE because of the highly improved charge transport and transfer efficiencies inside Cu 2 O induced by effectively suppressing charge scattering in the GBs. The impedance measurements depict the overall decrease in the total resistance, charge-transfer resistance, and the photoabsorber/electrode interface resistance of the PEC systems with the introduction of the instant strike process.

Photoelectrochemical properties of single-grain hematite films grown by electric-field-assisted liquid phase deposition

“…One would expect that the films with a thickness of one grain would show better PEC properties than that whose thickness includes several grains as the interfaces between the grains can act as sites for charge carrier scattering and electron–hole pair recombination. 30,31 However, scattering and electron–hole pair recombination occur in the film with fewer interfaces, as evidenced by the PEC properties of our samples. Additionally, the crack area percentages for the EA-LPD and C-LPD samples (9% and 15.5%, respectively) suggest that more hematite covers the surface in the EA-LPD samples, which increases the light absorption efficiency.…”

“…They found that increasing the grain size from ∼40 to ∼100 nm causes a ten-fold increase in photoconductivity. Choi et al 31 developed a bundle-type columnar Cu 2 O photocathode with fewer GBs than its conventional polycrystalline film via the electrochemical deposition technique. The former structure showed better PEC properties than the latter due to the suppression of charge carrier scattering and electron–hole recombination.…”