Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion

Engineering tunable graphene–semiconductor interfaces while simultaneously preserving the superior properties of graphene is critical to graphene-based devices for electronic, optoelectronic, biomedical, and photoelectrochemical applications. Here, we demonstrate this challenge can be surmounted by constructing an interesting atomic Schottky junction via epitaxial growth of high-quality and uniform graphene on cubic SiC (3C-SiC). By tailoring the graphene layers, the junction structure described herein exhibits an atomic-scale tunable Schottky junction with an inherent built-in electric field, making it a perfect prototype to systematically comprehend interfacial electronic properties and transport mechanisms. As a proof-of-concept study, the atomic-scale-tuned Schottky junction is demonstrated to promote both the separation and transport of charge carriers in a typical photoelectrochemical system for solar-to-fuel conversion under low bias. Simultaneously, the as-grown monolayer graphene with an extremely high conductivity protects the surface of 3C-SiC from photocorrosion and energetically delivers charge carriers to the loaded cocatalyst, achieving a synergetic enhancement of the catalytic stability and efficiency.

The Raman spectra showed the good crystalline quality of the as-grown graphene on 3C-SiC substrate. In Supporting Figure 2, we observed the G and 2D peaks in all graphene/3C-SiC heterojunctions, which are characteristics of graphene. The G peak located around 1600 cm −1 is the first-order Raman scattering process in the first Brillouin zone, while the 2D band located from 2730 cm −1 to 2750 cm −1 originates from the second-order scattering. 1 Besides these typical peaks, a double-band structure including two bands peaking around 1360 and 1600 cm −1 were observed, corresponding to the buffer layer underlying the graphene on hexagonal polytypes. 2,3 The 2D peak of 1L/3C-SiC shows a single Lorentzian shape with a full width at half maximum (FWHM) of ~42 cm -1 , suggesting the presence of monolayer graphene. As for 2L/3C-SiC and 4L/3C-SiC, their 2D peaks exhibited asymmetric shape, but was blueshifted along with an increase of FWHM to ~70 cm -1 . This typical change of 2D peak is explained by the evolution of the electronic band with increasing the number of graphene layer. 4 The position of Raman 2D peaks in the as-grown multilayer-graphene/3C-SiC samples correspond well to the 2D peaks of graphene grown on 4H-or 6H-SiC and measured with the same laser wavelength in other studies. 5 The charge transfer density modulations at the interface could be possibly reflected from the XPS measurement. To this end, a sophisticated time-dependent XPS technique (also known as XPS depth profiling analysis) with continuous Ar + sputtering (0.02 nm/s) was adopted to investigate the chemical states of Si atoms both on the interface and in the bulk. With an Ar + sputtering time of 10 s, we were able to observe the Si 2p XPS spectra for Si atoms mainly on the surface (Supporting Figure 3a). As compared with 3C-SiC, the intense peak around 101.1 eV (typical of Si-C bonds) of BL/3C-SiC, shifted to higher binding energies. This is intriguing binding energy shifting after the growth of buffer layer on 3C-SiC was due to the localized charge decrease, agreeing well with the charge transfer from 3C-SiC to buffer layer as theoretically predicted (Figure 2a). When the Ar + sputtering time was lengthened to 100 s, corresponding to a bulk detection length of 10 nm, the XPS peak indexed to Si-C bonds of BL/3C-SiC remained almost unchanged as compared with that of 3C-SiC (Supporting Figure 3b). Intuitively, charge exchange mainly occurred at the interface, suggesting the electronic states of Si atoms within 3C-SiC were not possibly disturbed in the bulk.  Supporting Figure 5a shows the energy band diagram for freestanding graphene and 3C-SiC.
Work function (Φ 3C-SiC ) of 3C-SiC was calculated to be 4.11 eV. χ SiC the electron affinity of 3C-SiC, which is calculated to be ~4.00 eV. 6 In theory, difference between conduction potential (E CB ) and Fermi potential (E F ) should be around E CB -E F = 0.11 eV. To validate the theoretical value of the difference between E CB and E F , we measured doping concentration (N D ) of the n-type 3C-SiC to be around 7.5 × 10 15 cm -3 . According to E CB -E F = kT × ln(N C /N D ), the experimental value of E CB -E F is around 0.12 eV, close to the theoretical one. Here, the effective conduction band density of states (N C ) at room temperature for 3C-SiC is 1.54 × 10 19 cm -3 . (e) Schematic illustration of built-in electric field among graphene layers. 3C-SiC is an indirect bandgap (E g = 2.36 eV) semiconductor, which results in a relatively larger light penetration depth. For instance, it is reported that the absorption coefficient (α) of 3C-SiC is around ~100 cm -1 at 2.4 eV, which gives rise to the light penetration depth (1/α) of ~100 µm. 9 However, the width of space charge region and carrier diffusion length ) are much smaller than the light penetration depth. Here, μ is the carrier mobility, is the carrier lifetime, k B T/q is the product of the Boltzmann constant and the temperature divided by the electron charge, ε s is the dielectric constant of the semiconductor, ε 0 is the permittivity in vacuum, V b is the built-in potential and N D is the donor concentration.
Using the N D of 7.5 × 10 15 cm -3 , the width of space charge region W is smaller than 0.3 µm if we assume Vb < 1V. Using the highest carrier lifetime (τ = 8.6 µs) reported in our previous work, 10 we estimate that the value of L D is smaller than 10 µm if assuming µ<5 cm 2 /Vs (We determined this mobility value of ~5 cm 2 /Vs by the Hall-effect measurement). Therefore, the maximum value of (W+L D ) is ~ 10.3 µm, which is much smaller than the light penetration depth (~100 µm at 2.4 eV).
This    Figure S14e). Binding energies of these peaks matched well with the characteristics of Fe 3+ in FeOOH. [13][14][15] Moreover, the Fe 2p XPS spectrum also showed two small satellite peaks (sat) located at higher binding energies, ∼719.0 eV for Fe 2p 3/2 , and ∼733.2 eV for Fe 2p 1/2 , which were S14 due to the charge-transfer or shakeup processes associated with Fe 3+ . 16  CoOOH nanoparticles were in situ grown on the 3C-SiC and graphene/3C-SiC photoanodes through a facile photoelectrochemical method. 19 It should be pointed out that for this comparison, we cut the same 3C-SiC substrate into two pieces, one for the deposition of CoOOH and the other one S15 for growth of monolayer graphene and subsequent deposition of CoOOH. In a typical three electrode cell, 3C-SiC was used as the working electrode, Ag/AgCl as the reference electrode, and Pt mesh as and H 2 evolution, which show high dependency on the photoanodes used in distinct PEC systems.
Apparently, high photovoltage benefited the multi-electron CO 2 reduction and suppressed the two-electron proton reduction reaction. S17