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1 Counts (a.u.) Supporting Information Comprehensive Evaluation of CuBi 2 O 4 as a Photocathode Material for Photoelectrochemical Water Splitting Sean P. Berglund, * Fatwa F. Abdi, Peter Bogdanoff, Abdelkrim Chemseddine, Dennis Friedrich, and Roel van de Krol Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, Berlin, Germany Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, Berlin, Germany * address correspondence to sean.berglund@helmholtz-berlin.de CuBi 2 O 4 (kusachiite) Sn (cassiterite) 500 o C 450 o C 400 o C Theta (deg) Figure S1. XRD spectra for CuBi 2 O 4 photocathodes synthesized by drop-casting (30 mm, 3 layers) with annealing temperatures of 400, 450, and 450 o C as indicated. Red lines ( ) represent the reference pattern for CuBi 2 O 4 (kusachiite, PDF# ) and blue lines ( ) represent the reference pattern for Sn (cassiterite, PDF# ) with line lengths proportional to peak intensity. S1

2 Figure S2. Photographs and SEM images of CuBi 2 O 4 photocathodes synthesized by drop-casting (30 mm, 450 o C) with 1, 2, and 3 layers (drop-casting repeats) to produce different thicknesses: 1 layer from (a) top view, (b) top view, and (c) cross-sectional view. 2 layers from (d) top view, (e) top view, and (f) cross-sectional view. 3 layers from (g) top view, (h) top view, and (i) cross-sectional view. S2

3 Effective Thickness, d eff (nm) Measurements fit line fit line = b (Drop-Cast Layers) b = 97.4 nm R 2 = Drop-Cast Layers Figure S3. Effective thickness (d eff ) values determined by ICP-MS analysis of material dissolved from the center of CuBi 2 O 4 photocathodes synthesized with various drop-casting conditions: bare FTO (0 layers), 15 mm one time (0.5 layers), and 30 mm multiple repeats (1, 2, and 3 layers). The fit line illustrates the linear relationship between the effective thickness and number of drop-cast layers. S3

4 Reflectance Transflectance mm, 1 layer mm, 2 layers 30 mm, 3 layers Wavelength (nm) Figure S4. Transflectance spectra for CuBi 2 O 4 films synthesized on quartz substrates with various conditions resulting in different film thicknesses. Dashed lines are for frontside illumination (light incident on CuBi 2 O 4 side) and solid lines are for backside illumination (light incident on glass side) mm, 3 layers 30 mm, 2 layers 30 mm, 1 layer Wavelength (nm) Figure S5. Reflectance spectra for CuBi 2 O 4 films synthesized on quartz substrates with various conditions resulting in different film thicknesses. Dashed lines are for frontside illumination (light incident on CuBi 2 O 4 side) and solid lines are for backside illumination (light incident on glass side). S4

5 ( h ) 2 (ev 2 cm -2 ) ( h ) 1/2 (ev 1/2 cm -1/2 ) (a) 30 mm, 1 layer 30 mm, 2 layers 30 mm, 3 layers h (ev) 1x x x mm, 1 layer 30 mm, 2 layers 30 mm, 3 layers 4x x h (ev) (b) Figure S6. (a) Indirect and (b) direct bandgap Tauc plots for CuBi 2 O 4 films synthesized on quartz substrates with various conditions resulting in different film thicknesses. Plots were derived from transflectance measurements under backside illumination. S5

6 (cm 2 V -1 s -1 ) max (cm 2 V -1 s -1 ) 15 fit line = a + b(φσμ) max a = 12 cm 2 V -1 s -1 b = TRMC signal fit line 4x x x10 14 Laser Pulse (photons cm -2 ) Figure S7. Maximum TRMC signal vs. incident laser pulse intensity for a CuBi 2 O 4 film synthesized by drop-casting (30 mm, 3 layers, 450 o C) on a quartz substrate. 10 fit line = (ΦΣμ) max + C 1 e ( t t 0 τ 1 ) + C2 e (t t 0 τ 2 ) 05 1 = 32 ns TRMC signal fit line 2 = 819 ns x x10-6 Time (s) Figure S8. TRMC signal vs. time for a CuBi 2 O 4 film synthesized by drop-cast (30 mm, 3 layers, 450 o C) on a quartz substrate at an incident laser pulse intensity of 7.7x10 14 photons/cm 2. A fit line is included for an exponential decay with two time constants ( 1 and 2 ). S6

7 Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) with Ar bubbling without Ar bubbling Potential (V vs. RHE) Figure S9. Chopped (light/dark) linear sweep voltammetry scans (25 mv/s) for a CuBi 2 O 4 photocathode (30 mm, 2 layers, 450 o C) in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with and without Ar bubbling as indicated. Scans were performed under backside illumination frontside backside frontside with H 2 backside with H Potential (V vs. RHE) Figure S10. Chopped (light/dark) linear sweep voltammetry scans (25 mv/s) for a CuBi 2 O 4 photocathode (30 mm, 2 layers, 450 o C) in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with Ar bubbling or with H 2 added under frontside and backside illumination as indicated in the figure legend. S7

8 Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE frontside illumination backside illumination (a) Drop-Cast Layers frontside illumination backside illumination (b) Drop-Cast Layers Figure S11. Average photocurrent density for CuBi 2 O 4 photocathodes (30 mm, 450 o C) at 0.6 V vs. RHE under frontside and backside illumination vs. the number of drop-cast layers. Photocurrent density was obtained from linear sweep voltammetry scans (25 mv/s) in (a) 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) and (b) 0.3 M K 2 SO 4, and 0.2 M phosphate buffer : 30% H 2 in a volume ratio of 4 : 1 (ph 6.30). The error bars represent ± one standard deviation for measurements of different photocathodes. S8

9 Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE frontside illumination backside illumination (a) Annealing Temperature ( o C) frontside illumination backside illumination (b) Annealing Temperature ( o C) Figure S12. Average photocurrent density for CuBi 2 O 4 photocathodes (30 mm, 2 layers) at 0.6 V vs. RHE under frontside and backside illumination vs. the annealing temperature used during synthesis. Photocurrent density was obtained from linear sweep voltammetry scans (25 mv/s) in (a) 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) and (b) 0.3 M K 2 SO 4, and 0.2 M phosphate buffer : 30% H 2 in a volume ratio of 4 : 1. The error bars represent ± one standard deviation for measurements of different photocathodes. S9

10 OCP (V vs. RHE) Current Density (ma/cm 2 ) 0.25 Ar bubbling without Ar bubbling with H Potential (V vs. RHE) Figure S13. Linear sweep voltammetry scans (25 mv/s) for a Pt electrode in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with Ar bubbling, without Ar bubbling, and with H 2 added as indicated in the figure legend H 2 bubbling Ar bubbling 1.00 bubbling Time (sec) Figure S14. Open-circuit potential (OCP) measurements for a Pt cylinder electrode in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with H 2, Ar, or bubbling as indicated. The reference electrode was Ag/AgCl (saturated KCl) and the counter electrode was a Pt mesh with excess surface area. S10

11 Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) H 2 bubbling Ar bubbling bubbling Potential (V vs. RHE) Figure S15. Three-electrode cyclic voltammetry scans (25 mv/s) for a Pt cylinder electrode in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with H 2, Ar, or bubbling as indicated. The reference electrode was Ag/AgCl (saturated KCl) and the counter electrode was a Pt mesh with excess surface area bubbling Ar bubbling H 2 bubbling Applied Bias (V) Figure S16. Two-electrode cyclic voltammetry scans (25 mv/s) for a Pt cylinder electrode 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with H 2, Ar, or bubbling as indicated. The counter electrode was a Pt mesh with excess surface area. S11

12 APCE (%) IPCE (%) backside with H 2 frontside with H 2 backside, Pt on surface frontside, Pt on surface backside frontside Wavelength (nm) Figure S17. IPCE measurements for CuBi 2 O 4 and CuBi 2 O 4 /Pt photocathodes (30 mm, 2 layers, 450 o C) conducted in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with Ar bubbling or with H 2 added as indicated in the figure legend. Measurements were conducted under frontside and backside illumination as indicated backside with H 2 frontside with H 2 backside, Pt on surface frontside, Pt on surface backside frontside Wavelength (nm) Figure S18. APCE measurements for CuBi 2 O 4 and CuBi 2 O 4 /Pt photocathodes (30 mm, 2 layers, 450 o C) conducted in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with Ar bubbling or with H 2 added as indicated in the figure legend. Measurements were conducted under frontside and backside illumination as indicated. S12

13 I (na) Power Density (mw/cm 2 ) 1.00 frontside (quartz/electrolyte/quartz) backside (FTO) Wavelength (nm) Figure S19. Power spectra used for IPCE measurements for frontside and backside illumination as indicated. 15 FTO/CuBi 2 O 4 /FTO 10 Au/CuBi 2 O 4 /Au V (V) Figure S20. Solid-state I-V measurements across FTO/CuBi 2 O 4 /FTO and Au/CuBi 2 O 4 /Au interfaces made by synthesizing CuBi 2 O 4 thin films (30 mm, 6 layers, 450 o C) within scribed channels on FTO and Au coated glass substrates S13

14 Current Density (ma/cm 2 ) Impedance (Ohm) 10 5 Real Imaginary Frequency (Hz) Figure S21. Electrochemical impedance spectroscopy (EIS) at 1.09 V vs. RHE for a CuBi 2 O 4 photocathode (30 mm, 2 layers, 450 o C) in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) dark light dark light dark CuBi 2 O 4 with Ar bubbling CuBi 2 O 4 /Pt with Ar bubbling Time (min) Figure S22. Constant potential measurement at o.6 V vs. RHE for CuBi 2 O 4 and CuBi 2 O 4 /Pt photocathodes (30 mm, 2 layers) in the dark and light. Measurements were done in 0.3 M K 2 SO 4 and 0.2 M phosphate buffer (ph 6.65) with Ar bubbling. S14