Semiempirical modeling of a three sublayer photoanode for highly efficient photoelectrochemical water splitting: Parameter and electrolyte optimizations

Abstract

Below we present semiempirical modeling of conceptually new three-sublayer photoanode, composed of Absorber, Grading and Barrier sublayers, for highly efficient photoelectrochemical water dissociation. The modeling resulted into Absorber (Sub-A) made of Cd 0.55 Zn 0.45 O due to its favorable positions of the band extrema to the water splitting potentials and a band gap ~2.0 eV. The Grading layer (Sub-G) was composed of Cd x Zn 1-x O with a gradual decrease of x across the profile, changing from 0.2 to 0.55, aiming to photon absorption from 2.0 to 3.0 eV. At the same time, Sub-G furnishes the profile with an implanted electrical field that improves the hole-transport. The electronBarrier layer (Sub-B) deposited above the Sub-A, was engineered to provide 1 eV high barrier in the conduction band. It comprised of a 50 nm thick Ni 0.4 Cd 0.6 O film with E g~3 .0 eV with a valence band aligned to the one of the Sub-A, providing a barrier-free hole-flow. In this paper, we provide evidence that the proposed three-sublayer concept clearly represents a new paradigm for an improved efficiency for photocatalytic water dissociation. The highest photocatalytic activity of the optimized profile was achieved with an optimized electrolyte: 87% 1M K 2 HPO 4 and 13% 1M Na 2 SO 3 (known to act as a hole scavenger or sacrificial agent) at pH=10. A noteworthy feature of this study is that under optimized profile parameters and customized electrolyte conditions the photocurrent yields increased from ~0.05 mA/cm 2 to ~20 mA/cm 2 at +1.2 V for visible light. The observed Incident Photon-to-Current Efficiency (IPCE) was about 50% at a photon energy of 3 eV.

Controlling electrical and optical properties of wurtzite CdxZn1−xO with high Cd contents via native defects manipulation by low-temperature annealing

“…[3][4][5][6][7] It has also been reported that wurtzite (WZ) structured Cd x Zn 1−x O alloys have a large bandgap tunability, from 3.4 (ZnO) to ∼1.8 eV (x ∼ 0.7), [8][9][10][11][12][13][14][15][16][17] covering a wide spectral range from the ultraviolet (UV) to the visible and, hence, extending the functionalities of ZnO-based materials to light absorbing layers in solar cells and photoelectrochemical cells for water splitting. 18,19 However, since ZnO and CdO have different stable crystal structures, i.e., wurtzite and rock salt (RS), respectively, WZ-Cd x Zn 1−x O alloys are only stable up to x ∼ 0.4-0.7, depending on the growth conditions. On the other hand, RS-Cd x Zn 1−x O with x > 0.7 have been shown to have an intrinsic bandgap larger than that of CdO (up to ∼2.5 eV for x ∼ 0.7).…”

Numerical calculation of visible light absorption enhancement of CdSe-quantum dot-sensitized TiO2 nanorod periodic array as photoanode

“…Numerous works have been recently conducted on photoanodes for solar water splitting and optical devices. These photoanodes are based on the semiconductor photoanode, usually TiO 2 or ZnO [1][2][3][4][5][6][7][8][9][10][11][12]. One-dimensional (1D) metal oxide semiconductors such as TiO 2 and ZnO have been applied as photocatalysts over the past decades because of their availability, photostability, and non-toxic nature [13][14][15].…”