(Invited) P-Type 3C-SiC Photocathode for Solar to Hydrogen Energy Conversion
Authors: Kato Masashi 
Published: February 2020

Solar to hydrogen conversion technology attracts attention as renewable energy generation. In general, to perform solar to hydrogen conversion, photocatalytic materials are employed. However, most of photocatalytic materials cannot absorb visible light resulting in low conversion efficiency and degrade during operation. Thus, we need to find new photocatalytic materials which have both high conversion efficiencies and resistance to corrosion.
Silicon carbide (SiC) is a chemically stable and photocatalytic semiconductor material. SiC can be grown by different atomic configurations resulting in a large number of polytypes. Among SiC polytypes, 3C-SiC has the smallest band gap (2.3 eV), and thus 3C-SiC is capable to absorb significant part of solar light. In addition, conductive type of SiC can be controlled by doping. Photocatalytic materials with p-type conduction operate as cathode and reduce hydrogen in water. Cathodic operation prevents oxidation of photocatalytic materials. Therefore, p-type 3C-SiC will be an efficient and durable photocatalyst.
However, bulk crystal growth of 3C-SiC is very difficult because this polytype is stable only in growth at much lower temperature than sublimation of SiC, which is a typical condition for bulk SiC growth. This difficulty makes 3C-SiC growth only by chemical vapor deposition (CVD) on Si or 4H- or 6H-SiC substrates. Such the heteroepitaxial growth induces large number of structural defects or polytype mixing owing to lattice mismatch or inheritance of the substrate polytype.
For our recent 3C-SiC photocathode fabrication, we employed p+-type 4H-SiC substrates with the (0001) Si-face inclined 0.7° to <1120> direction. Growth on the 4H-SiC substrates with quasi on-axis Si-face prevents polytype mixing and p+-type conductivity of the substrate facilitates ohmic contact formation on the substrate side. We performed 3C-SiC growth on the substrates by CVD with silane and propane as source gases and in an Al acceptor doping condition. A photograph of the grown crystal is shown in Fig. 1(a). After the growth, we fabricated ohmic contact on the substrate side of the crystals. The 3C-SiC surface is active surface as the photocathode. In addition, we formed Pt or Pd cocatalysts on the 3C-SiC surface to enhance hydrogen generation by the photocathodes. The schematic of the photocathode structure is shown in Fig. 1(b).
We measured photocurrent-potential characteristics for the photocathodes in 1 M H2SO4 aqueous solution using chopped solar simulator light with 100 mW/cm2. The results are shown in Fig. 1(c). For bare surface, cathodic photocurrent was observed below 0.6 V vs SCE, and it reached -1.4 A/cm2 at -1 V vs SCE. On the other hand, with cocatalysts, photocurrents were observed at higher potential, and in the case of with Pt cocatalysts, it was observed from 0.9 V vs SCE. In addition, at -1 V vs SCE, the photocurrent was -2.3 A/cm2.
In solar to hydrogen conversion, the Faraday efficiencies of the photocathodes were near to 100% and the photocathodes were not degraded during experiments. Solar to hydrogen conversion efficiency for the photocathode with Pt cocatalysts was 0.52 %. Considering such a high efficiency and possibility of further improvement, SiC is a very promising material for the solar to hydrogen conversion technology.

Figure 1