Crystal orientation-dependent etching and trapping in thermally-oxidised Cu2O photocathodes for water splitting

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu2O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. −8.6 mA cm−2 and −7 mA cm−2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (VRHE), respectively, in the champion sample with an onset potential of 0.92 VRHE and a fill factor of 44%. An applied bias photon-to-current efficiency of 3.6% at 0.56 VRHE was obtained, which represents a new record for Cu2O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu2O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu2O/Ga2O3 interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu2O sample during the ALD process, while the reduction from Cu2O to Cu is not favourable.

Packaging of the photocathodes. After the ALD process, a 100nm gold layer was deposited onto the backside of the Cu 2 O sheet by a sputter coater (Safematic CCU-010). A copper wire was connected to the gold layer by silver paste. Finally, the Cu 2 O was masked and sealed by opaque epoxy, exposing only the working area.
The working area was measured with ImageJ software.
Catalyst loading. Ruthenium oxide (RuO x ) or platinum (Pt) were used as the hydrogen evolution reaction (HER) catalyst. RuO x was photoelectrodeposited under one sun illumination with a constant current density of 28.3 µA cm -2 from a 1.3 × 10 -3 M potassium perruthenate (KRuO 4 , Alfa Aesar) solution, as described in the literature. 2 The deposition time was 15 minutes. Pt catalyst was sputtered onto the surface of the fabricated device, monitored by a quartz microbalance to control the thickness.
The overall fabrication procedures are showed in Fig. S1.

PEC, ABPE, IPCE, and EIS measurement of Cu 2 O photocathodes
Where J is the photocurrent density, V is the applied potential, and P is the light intensity (100mW cm -2 ). (3) Where J is the photocurrent density, P is the light intensity at each light wavelength, is the wavelength of the monochromatic light. The calculated photocurrents based on the IPCE are slightly lower than the measured photocurrents. One reason for an underestimation is due to the low signal in the UV and blue regions due to low photon flux from the monochromator, as can be seen in the quasi-stochastic response in those regions in

Device fabrication and measurement for CV and DLCP
A copper wire was connected to the TiO 2 layer by silver paste instead of loading a catalyst onto the photocathode device. The CV and DLCP were measured with a potentiostat (Biologic SP-200) in a twoelectrode configuration. For CV measurement, the DC bias (V DV ) was scanned from -1.5 V to 0.5 V, and the amplitude of the AC bias was 5 mV. The frequency of the AC bias was scanned from 1kHz to 1MHz.
To calculate the carrier densities from CV (N CV ), the following equation (4) was used: Where is the elementary charge, is the dielectric constant of the Cu 2 O (taken as 7.6 4 ), is the 0 permittivity of free space, and is the area of the diode.
For the DLCP measurement, the DC bias was from -1 V to 0.5 V, while the amplitudes of the AC biases were ranging from 20 mV to 300 mV. An additional offset DC voltage was applied for each AC bias = 0 + 1 + 2 ( ) 2 + … By fitting with a quadratic function to obtain C 0 and C 1 , the carrier density from DLCP (N DL ) could be calculated by: The profiling distance from the junction barrier for both CV (using C in equation (4)) and DLCP (using 〈 〉 C 0 in equation (5)) is given by the equation (7).       (g, h). In normal data process, a quadratic function was used to fit C and V AC to get C 0 = 0 + 1 + 2 ( ) 2 and C 1 . (Normally, C 0 is positive, C 1 is negative) However, in the unetched sample, some concave curves will result a positive C 0 and C 1 , which will make the calculated N DL negative. In these cases, a linear fitting was used instead of a quadratic = 0 + 1 fitting to get C 0 and C 1 .       k) The morphology of etch pit for these nine crystals. The orientation indexes of (c) to (k) are [5,2,9], [4,3,13], [5,1,10], [4,2,7], [4,1,5], [6,2,7], [4,1,12], [5,2,5], and [4,2,9] respectively.      The red circle showed where the STEM-EDS was performed.