Abstract
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 (VRHE) 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 VRHE). We further demonstrate that the optimized nanoporous photoanode exhibits an enhanced light-harvesting efficiency (LHE) of over 93%, a high charge-separation efficiency (Φsep) of 38%, and a high charge-injection efficiency (Φox) of 91% for water oxidation at 1.23 VRHE, which are significantly outperforming those its planar counterpart (LHE = 78%, Φsep = 28%, and Φox = 53% at 1.23 VRHE). 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.
Highlights
IntroductionPhotoelectrochemical (PEC) water splitting is a promising approach to convert the intermittent solar radiation into a renewable, storable, and clean chemical energy in the form of hydrogen (H2).[1−9] To accomplish an efficient solar-to-hydrogen conversion in the PEC cell, the semiconductor photoelectrodes should meet certain criteria: (i) moderate band gap that can efficiently absorb visible sunlight to generate electrons and holes with enough energy to overcome the energetic barrier of water splitting, (ii) ideal band positions that straddle the water redox potentials, (iii) efficient carrier separation and transport before recombination, (iv) high activity for water splitting with low overpotential, and (v) long-term stability against corrosion in aqueous electrolytes.[10−12] To date, there is no cost-effective single material which satisfies all of these requirements for solar water splitting.[6,10−12] Most of the extensively studied materialsReceived: January 10, 2021 Accepted: February 10, 2021 Published: February 19, 2021ACS Nano www.acsnano.orgArticle such as Si, TiO2, Fe2O3, BiVO4, WO3, ZnO, II−VI, and III−V semiconductors exhibit either a too large band gap to harvest visible sunlight (e.g., TiO2, ZnO, and so on) or unmatched band positions that are not able to oxidize or reduce water (e.g., Si, Felee2cOtr3o,lyBteiV.1O3−41,8 and WO3, etc.), or In this regard, cubic a poor stability in the silicon carbide (3C-SiC)has a relatively small band gap of 2.36 eV, which is close to the hypothetical ideal band gap (2.03 eV) of a single material for a maximum of the solar water splitting efficiency.[19]
Cm−2 illumination, which is 3.3 times higher than that of its planar We further demonstrate that the optimized nanoporous photoanode exhibits an enhanced light-harvesting efficiency (LHE) of over 93%, a high charge-separation efficiency (Φsep) of
We demonstrated that, with coating of Ni:FeOOH as the OER cocatalyst and the protection layer, the resulting nanoporous photoanodes significantly enhanced lightharvesting efficiency, charge-separation efficiency, and chargeinjection efficiency for water oxidation, improving the overall PEC water splitting performance
Summary
Photoelectrochemical (PEC) water splitting is a promising approach to convert the intermittent solar radiation into a renewable, storable, and clean chemical energy in the form of hydrogen (H2).[1−9] To accomplish an efficient solar-to-hydrogen conversion in the PEC cell, the semiconductor photoelectrodes should meet certain criteria: (i) moderate band gap that can efficiently absorb visible sunlight to generate electrons and holes with enough energy to overcome the energetic barrier of water splitting, (ii) ideal band positions that straddle the water redox potentials, (iii) efficient carrier separation and transport before recombination, (iv) high activity for water splitting with low overpotential, and (v) long-term stability against corrosion in aqueous electrolytes.[10−12] To date, there is no cost-effective single material which satisfies all of these requirements for solar water splitting.[6,10−12] Most of the extensively studied materialsReceived: January 10, 2021 Accepted: February 10, 2021 Published: February 19, 2021ACS Nano www.acsnano.orgArticle such as Si, TiO2, Fe2O3, BiVO4, WO3, ZnO, II−VI, and III−V semiconductors exhibit either a too large band gap to harvest visible sunlight (e.g., TiO2, ZnO, and so on) or unmatched band positions that are not able to oxidize or reduce water (e.g., Si, Felee2cOtr3o,lyBteiV.1O3−41,8 and WO3, etc.), or In this regard, cubic a poor stability in the silicon carbide (3C-SiC)has a relatively small band gap of 2.36 eV, which is close to the hypothetical ideal band gap (2.03 eV) of a single material for a maximum of the solar water splitting efficiency.[19]. 3C-SiC, as a photoelectrode material, has not been well studied due to a lack of high-quality materials.[22] Recently, Kato and co-workers reported that a photocathode fabricated using the p-type 3C-SiC epilayer grown on 4H-SiC by chemical vapor deposition and coated with Pt nanoparticles exhibited a promising water reduction performance, which achieved a solarto-hydrogen (STH) conversion efficiency of 0.52%.23. They further demonstrated that the 3C-SiC p−n junction photocathode coated with Pt showed the STH efficiency of 0.72%.24.
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