Supporting Information Oxygen Intercalated CuFeO 2 Photocathode Fabricated by Hybrid Microwave Annealing for Efficient Solar Hydrogen Production Youn Jeong Jang, Yoon Bin Park, Hyo Eun Kim, Yo Han Choi, Sun Hee Choi, Jae Sung Lee * Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea Division of Advanced Nuclear Engineering, Pohang University of Science and Technology(POSTECH), Pohang 790-784, South Korea Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
Figure S1. A: X-ray diffraction (XRD) patterns of copper iron oxide annealed at different conditions; as spin-coated (CFO 450 Air), and as annealed under Ar flow at 500 o C (CFO 500 Ar), 600 o C (CFO 600 Ar), and 700 o C (CFO 700 Ar) for 10 h. * represents peaks of SnO 2 from FTO substrate. The insets are photographs of films to show their transparency. B: Powder XRD of copper iron oxide calcined at 450 o C in air and delafossite CuFeO 2 annealed 700 o C in Ar with references CuFeO 2 (JCPDS no.01-075-2146, red), Fe 2 O 3 (JCPDS no. 01-085-0599, blue), and Cu 2 O (JCPDS no. 01-078-2076, green). The powder samples were prepared using the same precursor solution of film fabrication. C: X-ray photoelectron spectra of Sn 3d for CFO 600 Ar and CFO 700 Ar samples
Figure S2. Cu K-edge (A) and Fe K-edge (B) XANES spectra of CuFeO 2 electrodes annealed at different temperatures under Ar flow.
Figure S3. k 3 -weighted Fourier transforms of Fe K-edge EXAFS functions for annealed CuFeO 2 photocathodes: (A) FT magnitude, (B) imaginary function.
Figure S4 High resolution scanning electron microscopy (SEM) images showing surface morphologies of the films: (A) Copper iron oxide thin film as spin-coated and annealed at 450 in air, (B) annealed in Ar flow at 500 o C, (C) annealed in Ar flow at 600 o C and (D) annealed in Ar flow at 700 o C for 10 h.
Figure S5. Current(J) potential(v) curves of the photocathodes measured under the chopped illumination of the simulated 1 sun in Ar-purged electrolyte.
Figure S6. A: X-ray diffraction (XRD) patterns, B: Depth profiled ratios of Cu 2+ /(Cu 2+ + Cu + ) from X-ray photoelectron spectra (XPS) of Cu 2p, C: Scheme of heating mechanism using hybrid microwave annealing, 37 D,E,F: XPS of Cu 2p for bare CFO 600 and HMA and CTA treated CFO, respectively..
Figure S7. XRD patterns of copper iron oxide post-treated by HMA with a graphite susceptor. The inset is a blow-up of the XRD pattern showing characteristic peaks of spinel CuFe 2 O 4 as highlighted with red.
Figure S8. UV-Vis Diffuse Reflectance Spectra (UV-Vis DRS) (A) and the calculated absorbed photon flux relative to illuminated 1 sun (B) of unannealed CFO, post-hybrid microwave annealing (HMA) and post-conventional thermal annealing (CTA).
Figure S9. A: Linear sweep voltammetry of Pt, NiFe LDH, NiFe LDH/RGO on a Ni foam in 1 M NaOH electrolyte.
Figure S10. SEM images showing morphologies of NiFe layered double hydroxide (NiFe; A1) and NiFe/reduced graphene oxide composite (NiFe/RGO; B1). Bottom SEM images show surface morphologies of HMA-treated CuFeO 2 films deposited with NiFe (HMA-NiFe; A2) and NiFe/RGO (HMA-NiFe/RGO; B2)
Figure S11. IPCE results for CFO, CTA, HMA and HMA-NiFe/RGO. The inset is the integrated photocurrent based on IPCE.
Figure S12. A: XRD, XPS of Cu (B), Fe (C) and Ni (D) for before/after HMA-NiFe/RGO.
Figure S13. A: Hydrogen (blue) and oxygen (red) evolutions from water splitting over bare CFO photocathode during chronoamperemetry test (black) under 1 sun illumination in Arpurged electrolyte for 1 h. B: Calculated faradaic efficiencies for H 2 and O 2, and the H 2 /O 2 stoichiometry over CFO, XPS of Cu (C) and Fe (D) for before/after tested CFO.
Table S1. Summary of performances of reported copper based photocathodes for solar hydrogen generation Electrode Photocurrent [macm -2 ] Potential [V vs. RHE] Electrolyte Light Source Reference CuFeO 2-0.3 0.4 CuFeO 2 -HMA - 1.3 0.4 CuFeO 2 - HMA/NiFe LDH- RGO -2.4 0.4 Cu 2 O/AZO/TiO 2 /Pt ca. -1.0 0.3 1 M NaOH Ar 1 M Na 2 SO 4 - N 2 This work This work This work 8 CuGaSe 2 /CdS/Pt ca. -3.3 0.4 0.1 M Na 2 SO 4 12 CuFeO 2-25 μacm -2 0.4 1 M NaOH 1 sun 18 ca. -1.51 0.35 1 M NaOH-O 2 CuFeO 2-0.98 0.4 1 M NaOH-O 2 CuFeO 2 / CuAlO 2-1.95 0.4 1 M NaOH-O 2 19 CuFeO 2 ca. -0.3 0.4 1 M NaOH-N 2 ca. -0.9 0.4 1 M NaOH-O 2 20 CuO-HMA ca. -0.75 0.3 0.5 M Na 2 SO 4 31 CuFeO 2 /CuO ca. -0.8 0.4 1 M KHCO 3 - N 2 42