Nanoporous Cubic Silicon Carbide Photoanodes for Enhanced Solar Water Splitting

Cubic silicon carbide (3C-SiC) is a promising photoelectrode material for solar water splitting due to its relatively small band gap (2.36 eV) and its ideal energy band positions that straddle the water redox potentials. However, despite various coupled oxygen-evolution-reaction (OER) cocatalysts, it commonly exhibits a much smaller photocurrent (<∼1 mA cm^-2) than the expected value (8 mA cm^-2) from its band gap under AM1.5G 100 mW cm^-2 illumination. Here, we show that a short carrier diffusion length with respect to the large light penetration depth in 3C-SiC significantly limits the charge separation, thus resulting in a small photocurrent. To overcome this drawback, this work demonstrates a facile anodization method to fabricate nanoporous 3C-SiC photoanodes coupled with Ni:FeOOH cocatalyst that evidently improve the solar water splitting performance. The optimized nanoporous 3C-SiC shows a high photocurrent density of 2.30 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (V_RHE) under AM1.5G 100 mW cm^-2 illumination, which is 3.3 times higher than that of its planar counterpart (0.69 mA cm^-2 at 1.23 V_RHE). We further demonstrate that the optimized nanoporous photoanode exhibits an enhanced light-harvesting efficiency (LHE) of over 93%, a high charge-separation efficiency (Φ_sep) of 38%, and a high charge-injection efficiency (Φ_ox) of 91% for water oxidation at 1.23 V_RHE, which are significantly outperforming those its planar counterpart (LHE = 78%, Φ_sep = 28%, and Φ_ox = 53% at 1.23 V_RHE). All of these properties of nanoporous 3C-SiC enable a synergetic enhancement of solar water splitting performance. This work also brings insights into the design of other indirect band gap semiconductors for solar energy conversion.