A Review of Recent Progress on Silicon Carbide for
Photoelectrochemical Water Splitting
Jingxin Jian and Jianwu Sun*
1. Introduction
Hydrogen is recognized as a clean and renewable energy source and it only gives water as exhaust. Hydrogen-driven cars have been available on the market today. Production of hydrogen from solar water splitting is regarded as a much attractive route. With harvesting the abundant solar radiation and producing the separated gas products, photoelectrochemical (PEC) water splitting is a promising technology for producing a renewable, sustainable, environment-friendly hydrogen fuel which would support the future energy demand.[1–5]Therefore, it contributes to solve the current serious environmental problems of green-house gas CO2emission from the burning of fossil fuels, thus
mitigating climate change.
The idea of solar water splitting is inspired by natural photosyn-thesis, which perfectly utilizes the solar energy to convert CO2and
water into glucose.[6–8]It has been proven that solar water splitting is practical feasible, combining the photosystem II (water oxida-tion center) with the highly efficient hydrogenase (hydrogen evo-lution enzyme) to achieve overall water splitting.[9–12]In principle, the water splitting reaction is an uphill process that the water
molecule is broken down into oxygen and hydrogen (2H2O! H2þ O2). Then, a
min-imum energy of 1.23 eV is required due to the Gibbs free energy change (ΔG0) of 237.2 kJ mol1in thermodynamic, and fur-ther adding up with the energy losses due to kinetic overpotentials for the hydrogen evo-lution reaction (HER) and oxygen evoevo-lution reaction (OER) (Figure 1).[13,14] To have enough energy to split water, the semicon-ductor materials should absorb sunlight in the range of 400–800 nm efficiently, as this range covers the most part of solar spectrum (Figure 1).
The semiconductor materials, which capture photons to generate holes in the valence band (VB) and electrons in the conduction band (CB), are regarded as the key part of PEC water splitting configuration. In general, to accomplish an effective PEC water splitting, the semiconductor photoelectrode should meet certain essential cri-teria: 1) moderate bandgap which can efficiently absorb visible light, 2) ideal band positions that straddle the water redox poten-tials, 3) high carrier mobilities for the transport of photogener-ated charge carriers, 4) rapid charge transfer at the electrode/ electrolyte interface (high catalytic activity and gas evolution rate), and 5) long-term stability against corrosion in the aqueous elec-trolytes during PEC reaction. As shown in Figure 1, the theoreti-cal maximum solar-to-hydrogen (STH) conversion efficiency (right blue axis) and the related photocurrent (right dark-blue axis) are determined by the bandgap of semiconductor, assuming all captured solar photons are used for water splitting without any efficiency loss. Considering the required energy for water split-ting and the energy loss, for a single semiconductor material there is a hypothetical ideal bandgap of 2.03 eV for the theoretical maximum of the solar water splitting efficiency.[15,16]
Since the pioneering work of TiO2PEC water splitting system
by Fujishima and Honda in 1972,[17] semiconductor materials such as oxides, sulfides, phosphides, and silicon materials were widely studied as the photoelectrode for PEC water splitting (Figure 2).[18–28]However, the bandgaps of oxides are either too large (e.g., TiO2, ZnO, etc.) to absorb visible sunlight or their band
positions do not straddle the water redox potentials (e.g., Fe2O3,
BiVO4, WO3, etc.).[1,29]Furthermore, most materials suffer from
the instability issue. For instance, the Cu2O material has a suitable
bandgap, but the poor stability of Cu2O due to the decomposition
in aqueous solutions is a limiting factor for PEC water splitting application.[30,31] The II–VI and III–V materials (e.g., sulfide and phosphide) exhibit excellent sunlight absorption properties, but their improper band positions versus water redox potentials hamper the application of these materials in overall solar water
Dr. J. X. Jian, Dr. J. W. Sun
Department of Physics, Chemistry and Biology (IFM) Linköping University
Linköping 58183, Sweden E-mail: jianwu.sun@liu.se
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.202000111. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
DOI: 10.1002/solr.202000111
Solar water splitting based on semiconductor photoelectrodes is a promising route to convert solar energy into renewable hydrogen fuel. Since the pioneering work of photoelectrochemical (PEC) systems in 1972, a large variety of semi-conductors such as oxides, sulfides, phosphides, and silicon have been studied in the context of PEC water splitting configuration. Among them, silicon carbide (SiC) exhibits an excellent energy band structure that straddles the water redox potentials. In particular, cubic SiC (3C-SiC), with a suitable bandgap of 2.36 eV, is favorable for visible sunlight absorption. Recently, 3C-SiC has attracted much interest in PEC water splitting. In this review, the progress, challenges, and prospects of using SiC for PEC water splitting are summarized.
splitting. Also, most III–V semiconductors are not stable during the PEC reaction because these materials either dissolve or form a thin oxidefilm (photocorrosion) which prevents electron transfer across the semiconductor–electrolyte interface.[32–34]To date, it is still very challenging tofind a semiconductor which could meet all these requirements to split water efficiently.[2,16]
Silicon carbide (SiC) is an environment-friendly, earth-abundant element, chemically stable and industrially manufac-tured semiconductor material, which has attracted considerable interest in solar fuels.[35,36]In contrast to the extensively studied PEC semiconductors such as Si, Fe2O3, and BiVO4, the band
positions of SiC ideally straddle the water redox potentials, indi-cating that the photogenerated carriers have enough energy to overcome the energetic barrier of water reduction and/or water oxidation.[37]In this review, we summarize the progress,
challenges, and prospect of using SiC for PEC water splitting. To make a conveniently parallel comparison of PEC results, we converted most of the potentials versus reference electrodes (e.g., potential vs Ag/AgCl,VAg/AgCl, and potential vs saturated
calomel electrode, VSCE) in the literature into potential versus
reversible hydrogen electrode (VRHE) when the defined reference
electrode and pH value of the electrolyte solution were given. We also list the light source and light density which were given.
2. SiC Polytypes and Growth
SiC is a stable binary compound made by the earth-abundant ele-ments of silicon and carbon. The building block of SiC displays a tetrahedron structure, where a silicon/carbon atom bonds to four carbon/silicon atoms, respectively. The Si─C bond has a bond length of 1.89 Å and is 88% covalent and 12% ionic. SiC has more than 250 polytypes according to the crystal structures, such as the hexagonal SiC (4H- and 6H-SiC) and cubic SiC (3C-SiC). As shown in Figure 3, the SiC crystal structures are defined by layers stacked in different sequences. The atoms of those layers are arranged in three configurations, A, B, or C. For instance, the 4H-SiC unit cell has ABCB stacking and the 6H-SiC unit cell is stacked as ABCACB. The cubic 3C-SiC, also called β-SiC, has ABC stacking. As shown in Figure 3 and 6-SiC, 4H-SiC and 3C-SiC can have two polar faces, namely, the Si-face and C-face. Now, the 4H- and 6H-SiC wafers have been commercially available. Due to their large bandgaps of 3.23 eV (4H-SiC) and 3.02 eV (6H-SiC), they can only absorb the ultraviolet part of the solar spectrum. In contrast, 3C-SiC exhibits a relatively small bandgap of 2.36 eV, which is highly desirable for sunlight absorp-tion (Table 1). As a result, it has been demonstrated that 3C-SiC exhibits a better photoelectrochemical performance than 6H-SiC and 4H-SiC.[43] From the bandgap characteristic of 3C-SiC,
a maximum STH conversion efficiency of 10% and a related photocurrent density (jph) of 8.13 mA cm2 under standard
AM1.5G illumination are expected in theoretical terms (Figure 1). However, the 3C-SiC has not been well studied in PEC water splitting due to the great challenge of growth of high-quality 3C-SiC.[44]And the reported efficiencies of 3C-SiC-based photoelectrodes were far below the predicted value.
To date, the commercial 4H- and 6H-SiC single crystals have been developed using the physical vapor transport (PVT) method and chemical vapor deposition (CVD) method. However, the
PVT growth of high-quality 3C-SiC remains a huge challenge due to its metastable nature.[45]The 3C-SiC grown on Si exhibits a large number of defects due to the mismatch at 3C-SiC/Si inter-face (20% lattice constant mismatch and 8% thermal expansion coefficient mismatch between 3C-SiC and Si (see Figure 4A).[46]
Jingxin Jian received B.S. degree in biochemistry from Wuhan University in 2011 and Ph.D. degree from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences in 2017. After 2 years (2017– 2019) as a postdoctoral fellow, he worked as principal research engineer at Department of Physics, Chemistry and Biology (IFM), Linköping University. His research interests focus on solar-to-fuel conversion by artificial photosynthesis systems, including photocatalytic hydrogen evolution and
photoelectrochemical water splitting.
Jianwu Sun received his B.S. degree in 2003 from Northwest University and Ph.D. degree in 2008 from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, China. From 2008 to 2010, he worked as an Assistant Professor in Xiamen University, China. From 2009 to 2013, he conducted postdoctoral research at CNRS-Université Montpellier II, France and Linköping University, Sweden, respectively. From 2013 to 2014, he was permanently employed as a Senior Researcher in IMEC, Belgium. In 2015, he joined Linköping University as an Associate Professor. His current research focuses on semiconductor materials and physics for energy applications including solar energy conversion
(photoelectrochemical water splitting and CO2conversion),
Ga2O3solar-blind photodetectors, and transistors.
Figure 1. The solar spectrum and the theoretical maximum conversion efficiency and photocurrent as a function of material bandgap.
There has been a large effort focused on the growth of 3C-SiC on Si substrate by CVD method.[47,48]Nishino et al. reported that a buffer layer of SiC on Si substrates was introduced so as to improve the crystalline quality of 3C-SiC.[47]To reduce antiphase boundary (APB) and stacking-fault (SF) simultaneously, Nagasawa et al. grew the 3C-SiC on undulant-Si in which the surface is covered with continuous slopes oriented in the (110) and (1 1 0) directions.[48]
Hexagonal SiC (4H-SiC and 6H-SiC) substrates are regarded as promising substrates for the growth of 3C-SiC because the in-plane lattice mismatch between 3C-SiC and its hexagonal counterparts is below 0.1%, and these SiC polytypes have similar thermal expansion coefficient. Therefore, nominally on-axis hexagonal SiC was commonly used as substrates for the growth of 3C-SiC epilayers (Figure 4B).[49,50] However, this growth method always results in the formation of double-positioning boundaries (DPBs) defects, which is a fundamental problem
and ascribed to the fact that 3C-SiC has two types of stacking sequence with a 60 rotational change (i.e., ABC and CBA) (Figure 4B). Essentially, DPBs are boundary defects where oppo-site rotational variants meet, which significantly deteriorate the crystalline quality and have a strongly negative impact on the electronic properties of the material. Since the DPBs are formed at the interface of 3C-SiC/hexagonal SiC, it is a great challenge to control the nucleation of 3C-SiC and to grow a single-domain 3C-SiC on on-axis hexagonal SiC substrates.
Recently, Jokubavicius et al. demonstrated that high-quality, DPB-free single-domain 3C-SiC can be grown by the sublimation method using off-axis hexagonal SiC as substrates.[45,51,52]This growth method uses 4off-axis 4H-SiC as substrates and controls 3C-SiC nucleation on the terraces followed by a lateral enlarge-ment of 3C-SiC domains along the step-flow direction. High-quality thick (1 mm) single-domain 3C-SiC materials have been successfully demonstrated. After polishing away the 4H-SiC sub-strates and the interfacial layer, a 300μm free-standing 3C-SiC samples can be obtained (Figure 4C). Very recently, we have also demonstrated that using the same growth method, the high-quality C-face 3C-SiC with thickness of1 mm can be grown over a large single domain without DPBs. Furthermore, the C-face 3C-SiC exhibits a smoother surface than the Si-face 3C-SiC. High-resolution X-ray diffraction (HRXRD) and low-temperature photoluminescence results evidence that C-face 3C-SiC can reach the same high crystalline quality as the Si-face 3C-SiC.[53]
Figure 5 shows some representative atomic force microscopy (AFM), XRD, and absorption results of 3C-SiC grown on off-axis 4H-SiC substrate.[54]As shown in the AFM topography image, the surface of Si-face 3C-SiC is atomically smooth, illustrating regular steps with a terrace width of50 nm and a step height of1.5 nm (Figure 5A).[54]In the XRD pattern, only two sharp diffraction peaks were observed, which are assigned to the (111) and (222) reflections of 3C-SiC, respectively (Figure 5B).[54]The high-resolution XRDω-scan rocking curve of (111) Bragg reflec-tions is symmetric with a full width at half maximum (FWHM) value of 27 arcsec (see inset of Figure 5B). The HRXRD result evidences that the crystalline quality of 3C-SiC is comparable with the commercial hexagonal SiC (4H- and 6H-SiC), which exhibits a FWHM value of ω-rocking curve in the range of 10–40 arcsec.[54,55]The absorption spectra and related Tauc plot of this free-standing 3C-SiC showed a rather sharp band-edge absorption and an optical bandgap of 2.36 eV, which is consistent with the bandgap of 3C-SiC (Figure 5C).[40]
3. Hexagonal SiC for PEC Water Splitting
Hexagonal SiC is a commercially available semiconductor with a similar bandgap of TiO2, which has attracted considerable
inter-est in solar conversion. In 1979, after the pioneering work of TiO2 semiconductor for PEC water splitting, Honda’s group
investigated the SiC material for solar conversion application.[56]
The SiC exhibited the highest yield of solar fuels after 7 h illumination than that of other materials (WO3, TiO2,
ZnO, CdS, and GaP), due to its more negative conduction band, which is thus more favorable for the photogenerated electron to reduce CO2.
Figure 3. The stacking sequence of three most commonly SiC polytypes, 6H-, 4H- and 3C-SiC.
Table 1. Material properties of three most commonly SiC polytypes (Carrier mobilities are collected from the published experimental data.).[38–42]
Polytypes 4H-SiC 6H-SiC 3C-SiC
Bandgap [eV] 3.23 3.02 2.36
Electron mobility [cm2V1s1] 1000 450 1000
Hole mobility [cm2V1s1] 120 100 126
Saturated electron velocity (107cm s1) 2.2 2.0 2.7
Figure 2. The bandgap and band position of some semiconductors investigated for solar conversion.
In 1997, Lauermann et al. investigated the electrochemical properties of single-crystalline p- and n-type 6H-SiC materials.[57]
The Mott–Schottky results of p- and n-type 6H-SiC under both acidic and alkaline conditions confirmed that the band position of 6H-SiC straddles the water redox potentials. Under 400 mW cm2white light (4-Sun) illumination, the 6H-SiC pho-toanode exhibited ajphof0.17 mA cm2at 1.23VRHEand an
onset potential (Eonset) of0 VRHEin natural Na2SO4solution.
Akikusa et al. found that the PEC water splitting performance of p-type 6H-SiC could efficiently improve by surface treatments with aqua-regia-HF solution (HNO3:HCl:HF¼ 1:3:4) and
elec-trodeposition of Pt cocatalyst.[58] As shown in Figure 6A, the Eonsetof 6H-SiC photocathodes were significantly positive-shifted
from 1.0 VRHE to 1.3 VRHE after surface treatments and
deposition of Pt. The similar plateau photocurrents of about 0.13 mA cm2were obtained at 0.2V
RHE, indicating that this
limitingjphis only dependent on the number of photogenerated
charge carriers. The optimized 6H-SiC photocathode with Pt co-catalyst exhibits a photoconversion efficiency (PCE) of 0.17% at 0.45 VRHEunder 50 mW cm2illumination (Figure 6B).
Moreover, combining this p-type 6H-SiC photocathode with a n-type TiO2photoanode, a close-circuit potential of 0.23 V and
ajphof 0.05 mA cm2were obtained under 50 mW cm2
illumi-nation, indicating that the p-SiC/n-TiO2system can drive water
photoelectrolysis without external bias.
In 2009, van Drop investigated the p-type 4H-SiC for PEC water splitting and hydrogen storage (Figure 7A).[59]After
elec-trodeposition of Pd cocatalyst, the 4H-SiC/Pd photocathode
Figure 4. A) Schematic growth of 3C-SiC on Si substrate. The lattice constant mismatch occurred at the Si/3C-SiC interface. B) Schematic growth of 3C-SiC on 4H-SiC or 6H-SiC substrate. The mechanism illustration of the formation of a DPB. C) Sublimation growth of 3C-SiC on 4off-axis 4H-SiC substrates. After polishing the 4H-SiC substrate, free-standing 3C-SiC was obtained (Reproduced with permission.[45] Copyright 2015, American
Chemical Society).
Figure 5. A) AFM topography image (5 5 μm2) of free-standing Si-face 3C-SiC(111); B) XRD pattern of Si-face 3C-SiC, the inset shows the HRXRD
ω-rocking curve of Si-face 3C-SiC (111) reflection with a FWHM of 27 arcsec. C) Absorption spectrum and Tauc plot (inset) of free-standing 3C-SiC. (Reproduced with permission.[54]Copyright 2019, Royal Society of Chemistry).
displayed a positive-shiftedEonsetof1.1 VRHEand an enhanced
jphof5.3 mA cm2at 0VRHEin 0.3MKOH solution under the
illumination of UV light from a Hg arc-lamp (500 W) (Figure 7B). Under a steady illumination, the 4H-SiC/Pd photocathode exhibited a limitingjphpeaks of5.3 mA cm2and a stabilized
jph of 0.16 mA cm2 at 0.2 VRHEwhen short-circuited to a
Pt counter electrode. From the ratio of the detected H2product
and the passed charges, the author suggested that 33% of the produced H2 was stored at this system due to the
potentio-dynamically evolution of H2from the SiC electrode.
The previous works showed that the integration of an efficient cocatalyst on the SiC photoelectrode can be an efficient strategy to improve its PEC water performance, whereas only the noble metal of Pt and Pd was applied as water-reduction cocatalyst. In 2015, Digdaya et al. introduced an earth-abundant Ni-Mo cocatalyst on a SiC/TiO2 heterojunction photocathode.[60] This
p–i–n heterojunction photocathode was fabricated by atomic layer deposition (ALD) of an n-type amorphous TiO2 onto a
p-type/intrinsic (p/i) amorphous SiC (a-SiC) on an fluorine doped tin oxide substrate (Figure 8A). The p–i–n heterojunction
structure increases the photovoltage from 0.5 to 0.8 V under open-circuit condition. Under 100 mW cm2 simulated AM1.5G illumination, the (p/i) a-SiC/(n) TiO2/Ni-Mo
photocath-ode exhibited a significantly positive-shifted Eonset of 0.8VRHE
and dramatically increased jph of 8.3 mA cm2 at 0VRHE,
(Figure 8B).[60] In addition, after coating an amorphous TiO2
layer, the a-SiC-based photocathode showed a high stability for 12 h of operation in the PEC measurements. This enhanced water splitting performance of a-SiC/TiO2/Ni-Mo photocathode
indicated that p–n heterojunction structure and earth-abundant cocatalyst could efficiently enhance the photovoltage and catalytic activity of SiC photoelectrode, which gives a promising route to form a multijunction system for highly efficient bias-free solar water splitting devices.
Recently, our group investigated the PEC water splitting performance of 6H-SiC photoanodes with different polar faces, namely, Si- and C-faces. The Si- and C-face 6H-SiC epi-layers (30 μm) samples were grown by CVD on Si- and C-face 6H-SiC substrates, respectively.[61]As shown in Figure 9, theoretical investigation of water adsorption showed that C-face performs an
Figure 6. A)J–V curves of 6H-SiC, 6H-SiC after surface treatment (6H-SiC*), 6H-SiC after surface treatment and electrodeposition of Pt cocatalyst (6H-SiC*/Pt). B) Photoconversion efficiency of 6H-SiC*/Pt photocathode. Under 50 mW cm2Xe-Hg lamp illumination, in 0.5MH
2SO4electrolyte
solution. (Reproduced with permission.[58]Copyright 2002, Elsevier).
Figure 7. A) Illuminated p-type 4H-SiC photocathode short-circuited to Pt counter electrode to split water. The hydrogen generated was stored to a considerable extent in the solid. B)J–V curves of 4H-SiC and 4H-SiC/Pd photocathode in 0.3 KOH electrolyte soltuion, under the illumination of UV light from a Hg arc-lamp (500 W). (Reproduced with permission.[59]Copyright 2009, Wiley-VCH).
ordinary hydrogen bond (HB) or hydrogen-like bond interaction with water molecule and the Si-face splits the water molecule into two fragments of H─ and HO─ bonding with two adjacent Si atoms. This result indicated that the exposed surfaces display different atomic and electronic structures, which remarkably influence water adsorption, hydroxylation, and water oxidation behaviors. On the Si-face, the theoretical calculations suggested that the terminal Si atoms catalyzed barrierless O─H breaking with a facile proton exchange and migration characters
(Figure 9C). Compared with the Si-face, high energy was required for the proton-coupled electron transfer (PCET) at the C-face. Consequently, this mechanistically shifted the rate-limiting step of water oxidation from sluggish PCET on the C-face to a more energy-favorable charge transfer on the Si-face, which leads to the different PEC water splitting performance of Si- and C-face SiC samples (Figure 9). Under the illumination of monochromatic light (410 nm, 30 mW cm2), the Si-face 6H-SiC photoanode exhibited a remarkably negative-shifted Eonset of
Figure 8. A) Schematic structure of the (p/i) a-SiC/(n) TiO2photocathode. B) TheJ–V curves of the (p/i) a-SiC and the isolated (p/i) a-SiC/(n) TiO2
photocathodes without catalyst and with Ni-Mo catalyst under simulated AM1.5G illumination using 0.5Mpotassium hydrogen phthalate electrolyte solution at pH 4. For comparison, the Pt catalyzed (p/i) a-SiC/(n) TiO2is also shown. (Reproduced with permission.[60]Copyright 2015, Royal Society of
Chemistry).
Figure 9. Theoretical investigation of water adsorption on the A) C-face and B) Si-face of 6H-SiC. The yellow and blue iso-surfaces represent charge accumulation and depletion in the space along with water adsorption, respectively. The iso-value is 0.005 au. C) Proton exchange and migration on the Si-face. Red dashed lines show the ordinary HB or hydrogen-like bond. D) Schematic illustration of water oxidation kinetics increases due to the different pathways of electron transfer (ET) and proton transfer (PE) on the C- and Si-face of 6H-SiC. E)J–V curves of C- and Si-face 6H-SiC photoanodes in 0.5M
pH 6.8 Na2SO4solution under 410 nm monochromatic light. Inset shows the schematic illustration of the PEC cell. (Reproduced with permission.[61]
0.10 VRHEcompared withEonsetof 0.56 VRHEfor the C-face
6H-SiC (Figure 9F). This proof-of-concept investigation could open up new possibilities to design sophisticated photoelectrodes for an efficient solar water splitting cell via surface engineering.
Nanostructured photoelectrode surface is highly desirable for efficient PEC water splitting due to its large active surface area, efficient light absorption, and significantly reduced distance for charge transport. Recently, we reported a facile approach to fabricate the nanoporous 6H-SiC photoanode with a conformal coating of Ni-FeOOH nanorods as a water oxidation cocatalyst (Ni-FeOOH/PSC18) (Figure 10A).[62]Such a nanoporous
photoa-node exhibits a significantly improved PEC water splitting perfor-mance with enhanced photocurrent and a low onset potential of 0VRHE. Under 1 sun illumination, the porous Ni-FeOOH/PSC18
photoanode exhibits an Eonset of 0VRHE and a high jph of
0.684 mA cm2 at 1VRHE, which is 342 times higher than that
of the planar 6H-SiC photoanode with Ni-FeOOH cocatalyst (Ni-FeOOH/SC) (Figure 10B). Moreover, the porous photoanode shows a maximum applied bias photon-to-current efficiencies (ABPE) of 0.58% at a very low bias of 0.36VRHE, distinctly
outper-forming the planar counterpart. The impedance measurements demonstrate that the porous photoanode possesses a significantly reduced charge transfer resistance, which explains the dramati-cally enhanced PEC water splitting performance. Moreover, the dendritic porous structure with the large surface area could increase the photoanode/electrolyte interface area for water split-ting. The reported approach here can be widely used to fabricate other nanoporous semiconductors for solar energy conversion.
4. Cubic SiC for PEC Water Splitting
Compared with the wide bandgap hexagonal SiC, 3C-SiC exhibits the smallest bandgap of 2.36 eV among all SiC polytypes. Due to the indirect bandgap nature, 3C-SiC has a low absorption coef fi-cient. This indicates that a thick 3C-SiCfilm is required to fully absorb the sunlight. For instance, the n-type 3C-SiC with the
nitrogen-doping concentration of 6.9 1016cm3 exhibited an absorption coefficient of 250 cm1 at 496 nm.[63] This means, at least, a 40μm thick of 3C-SiC was required to absorb 63% of 496 nm light according to the light penetration depth. As mentioned earlier, it is still a great challenge to grow a thick high-quality 3C-SiCfilm.
The initial PEC study of 3C-SiC photoelectrode was based on the 6–12 μm-thick 3C-SiC epilayers grown on a highly doped n-type Si wafer.[57] The measuredflat potentials of n-type 3C-SiC from the Mott–Schottky curves were 0.1 to 0.8 VRHEat
different pH conditions, indicating that the band positions of 3C-SiC straddle the water redox potentials whether in an acidic or alkaline solution. However, the 3C-SiC photoanodes showed a rather low PEC water splitting performance due to its low crystal quality. We have pointed out in the previous section that 3C-SiC grown on Si always shows very defective layer due to large mismatch of 3C-SiC and Si. The 3C-SiC showed an Eonsetof
0.6 VRHE and jph of 0.11 mA cm2 at 1.23VRHE under
400 mW cm2white light illumination in 0.1MNa2SO4
electro-lyte solution (pH¼ 7). This PEC performance is even worse than that of the 6H-SiC counterpart at the same conditions. Similar to most other photoanodes, the n-type 3C-SiC photoanode also suf-fered from the anodic corrosion problem due to the surface oxi-dation during PEC reaction.
So far, there were very limited reports on using 3C-SiC as a photoanode due to lack of high-quality 3C-SiC material. In our previous works, we have demonstrated that state-of-the-art qual-ity n-type 3C-SiCfilm can be grown by the sublimation technique on 4off-axis 4H-SiC substrate.[51]Moreover, to overcome the
SiC surface oxidation issue, deposition of surface protection layer on 3C-SiC is necessarily required. As it is well known, the water oxidation at the photoanode involves a four-electron process with high energy barriers, which was more energetically sluggish and thus was regarded as the bottleneck for the PEC water splitting. Therefore, it is highly desirable to integrate an efficient water-oxidation cocatalyst on the 3C-SiC photoanode to improve its PEC water performance and stability.
Figure 10. A) Schematic diagram of the porous 6H-SiC photoanode with Ni-FeOOH co-catalyst for PEC water splitting. B)J–V curves of the planar Ni-FeOOH/SC and porous Ni-FeOOH/PSC18 photoanodes under AM1.5G 100 mW cm2illumination. (Reproduced with permission.[62]Copyright
Recently, we have demonstrated that the PEC performance of 3C-SiC photoanodes can be significantly improved by deposition of a cheap and earth-abundant water-oxidation cocatalyst of the mixed nickel–iron oxyhydroxide (Ni:FeOOH) onto the 3C-SiC photoanode (Figure 11).[64] Under AM1.5G 100 mW cm2
illumination, the 3C-SiC/Ni:FeOOH photoanode exhibited a very low Eonset of 0.2VRHE and a high jph of 1.15 mA cm2 at
1.23VRHE, distinctly outperforming the 3C-SiC counterpart
(Figure 11). The Faraday efficiency (ηF) of Ni:FeOOH-coated
3C-SiC was obtained to be 98% for H2evolution and 80% for
O2evolution, and thisηFof O2evolution could further improve
to 86% by the formation of a thick Ni:FeOOH layer. These results provide new insights for the development of 3C-SiC photoanode toward efficient solar fuel generation.
To further improve the PEC performance of 3C-SiC, we depos-ited a visible-light optically transparent, p-type NiO nanoclusters on 3C-SiC to form a p–n junction photoanode. Moreover, NiO is also a promising water oxidation cocatalyst. With coating the NiO nanoclusters on 3C-SiC surface, we have demonstrated a signi-ficantly enhanced photovoltage and photocurrent together with a substantial decrease in theEonset in a PEC water splitting cell
(Figure 12).[54]Under AM1.5G 100 mW cm2illumination, the
NiO-coated 3C-SiC photoanode exhibits a high photovoltage of 1.0 V, a very lowEonsetof 0.20 VRHE, and a dramatically high
jphof 1.01 mA cm2at 0.55VRHEfor PEC water splitting. Thisjph
was 33.6 times higher than that of the bare 3C-SiC photoanode (0.03 mA cm2 at 0.55V
RHE). Notably, the 3C-SiC/NiO
photo-anode shows a much steeper increase in photocurrent, which almost reaches a plateau value at a very low potential of 0.55VRHE, resulting in a highfill factor of 57%. The open-circuit
potential and valence band position measurements evidenced the formation of the 3C-SiC/NiO p–n heterojunction, which promotes the separation of photogenerated carriers. The electro-chemical impedance analysis confirmed that the 3C-SiC/NiO photoanode facilitates the charge transfer across the photoanode/ electrolyte interface for water oxidation.
Recently, p-type 3C-SiC as a photocathode for PEC water reduction has been studied. Ichikawa et al. reported that coating of Pt and Pd nanoparticles on the p-type 3C-SiC photocathode
Figure 11. J–V curves of of 3C-SiC and 3C-SiC/Ni:FeOOH photoanodes under AM1.5G 100 mW cm2illumination. The inset shows the schematic diagram of the Ni:FeOOH-coated 3C-SiC photoanode and Pt counter elec-trode for PEC water splitting. (Reproduced with permission.[64]Copyright
2019, Wiley-VCH).
Figure 12. Schematic diagram of the NiO-coated 3C-SiC photoanode for PEC water splitting. The inset shows theJ–V curves of 3C-SiC and 3C-SiC/NiO photoanodes under 100 mW cm2AM1.5G illumination. (Reproduced with permission.[54]Copyright 2019, Royal Society of Chemistry).
can improve its PEC performance.[65]In their works, 30μm-thick Al-doped (NA< 1 1015cm3) p-type 3C-SiC sample was grown
on a p-type 4H-SiC substrate by CVD. With coating of Pt and Pd nanoparticles on 3C-SiC surface, theJ–V characteristics of 3C-SiC/Pt and 3C-SiC/Pd photocathodes exhibited the increased jphand positive-shiftedEonset(Figure 13A). In the two-electrode
systems with RuO2 as counter electrode, the 3C-SiC/Pt and
3C-SiC/Pd photocathodes reported the ABPE of 0.52% and 0.46% at the applied potentials of0.78 and 0.84 V, respectively (Figure 13B). It should be noted that the p-type 3C-SiC photo-cathode demonstrated a rather large on-set potential (1.23 VRHE),
which confirms the desirable band positions of 3C-SiC versus water redox potentials. In this review, we highlight some recent representative results of both n-type and p-type 3C-SiC for PEC water oxidation and reduction, which clearly demonstrate the promising properties of 3C-SiC for solar water splitting.
5. Conclusion
As a semiconductor with earth-abundant elements and promis-ing material properties, SiC has attracted considerable interest in solar water splitting in the past decades. In particular, the energy band positions of SiC ideally straddled the water redox potential, indicating that the photogenerated carriers have enough energy to overcome the energetic barrier of water reduction and/or water oxidation. In this work, we briefly review the recent progress of SiC for PEC water splitting.
Among SiC polytypes, 3C-SiC has a smallest bandgap of 2.36 eV, which is very promising material for PEC water split-ting, but the PEC study of 3C-SiC has long been hampered by the lack of high-quality material. Recently, it has been demon-strated that high-quality 3C-SiC can be grown by sublimation technique. We showed that the PEC water splitting performance of 3C-SiC can be significantly enhanced by coupling with an efficient water oxidation cocatalyst. Furthermore, the formation of nanostructured NiO/3C-SiC p–n heterojunction is demon-strated to significantly improve the PEC water oxidation
performance. The p-type 3C-SiC with coating of water reduction cocatalysts also showed a very promising performance for PEC water reduction. In a future perspective, the growth of large-scale high-quality 3C-SiC (both n-type and p-type), the integration of a cheap and efficient cocatalyst, and the formation of heterojunc-tion photoelectrode can be efficient strategies to fabricate an efficient and sustainable solar water splitting cell.
Acknowledgements
This work was supported by the Swedish Research Council (Vetenskapsrådet, grant nos. 621-2014-5461; 2018-04670), the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS, grant no. 2016-00559), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT, grant no. CH2016-6722), the ÅForsk Foundation (grant no. 19-311), and the Stiftelsen Olle Engkvist Byggmästare (grant no. 189-0243).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
photoanode, photocathode, photoelectrochemical, silicon carbide, water splitting
Received: February 29, 2020 Revised: April 15, 2020 Published online: May 15, 2020
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