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, 14109 Berlin, Germany Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 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 20 25 30 35 40 45 50 55 2 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# 00-042-0334) and blue lines ( ) represent the reference pattern for Sn (cassiterite, PDF# 00-046-1088) with line lengths proportional to peak intensity. S1
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
Effective Thickness, d eff (nm) 300 200 Measurements fit line fit line = b (Drop-Cast Layers) b = 97.4 nm R 2 = 0.9184 100 0 0 1 2 3 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
Reflectance Transflectance 1.0 0.8 0.6 0.4 30 mm, 1 layer 0.2 30 mm, 2 layers 30 mm, 3 layers 0.0 300 400 500 600 700 800 900 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). 0.3 0.2 0.1 30 mm, 3 layers 30 mm, 2 layers 30 mm, 1 layer 0.0 300 400 500 600 700 800 900 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
( h ) 2 (ev 2 cm -2 ) ( h ) 1/2 (ev 1/2 cm -1/2 ) 700 600 500 400 300 200 100 0 (a) 30 mm, 1 layer 30 mm, 2 layers 30 mm, 3 layers 3.0 2.5 2.0 1.5 h (ev) 1x10 11 8x10 10 6x10 10 30 mm, 1 layer 30 mm, 2 layers 30 mm, 3 layers 4x10 10 2x10 10 0 3.0 2.5 2.0 1.5 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
(cm 2 V -1 s -1 ) max (cm 2 V -1 s -1 ) 15 fit line = a + b(φσμ) max 10 05 00 a = 12 cm 2 V -1 s -1 b = -4.9 10-19 TRMC signal fit line 4x10 14 6x10 14 8x10 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 00 0.0 5.0x10-7 1.0x10-6 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
Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) -0.25-0.50 with Ar bubbling without Ar bubbling 0.4 0.6 0.8 1.0 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. 0.25-0.25-0.50-0.75-1.00-1.25 frontside backside frontside with H 2 backside with H 2 0.4 0.6 0.8 1.0 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
Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE -0.05 frontside illumination backside illumination -0.10-0.15-0.25 (a) 1 2 3 Drop-Cast Layers frontside illumination backside illumination -0.50-0.75-1.00 (b) 1 2 3 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
Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE Photocurrent Density (ma/cm 2 ) at 0.6 V vs. RHE -0.05 frontside illumination backside illumination -0.10-0.15 (a) 400 450 500 Annealing Temperature ( o C) -0.25 frontside illumination backside illumination -0.50-0.75-1.00 (b) 400 450 500 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
OCP (V vs. RHE) Current Density (ma/cm 2 ) 0.25 Ar bubbling without Ar bubbling with H 2-0.25-0.50 0.0 0.2 0.4 0.6 0.8 1.0 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. -0.25 H 2 bubbling 0.25 0.50 0.75 Ar bubbling 1.00 bubbling 1.25 0 25 50 75 100 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
Current Density (ma/cm 2 ) Current Density (ma/cm 2 ) 1.0 0.5 0.0-0.5-1.0-1.5 H 2 bubbling Ar bubbling bubbling 0.0 0.5 1.0 1.5 2.0 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. 5.0 2.5 0.0 bubbling Ar bubbling H 2 bubbling -2.5-5.0-2 -1 0 1 2 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
APCE (%) IPCE (%) 70 60 50 40 30 20 10 0 backside with H 2 frontside with H 2 backside, Pt on surface frontside, Pt on surface backside frontside 400 500 600 700 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. 90 80 70 60 50 40 30 20 10 0 backside with H 2 frontside with H 2 backside, Pt on surface frontside, Pt on surface backside frontside 400 500 600 700 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
I (na) 0.75 0.50 0.25 Power Density (mw/cm 2 ) 1.00 frontside (quartz/electrolyte/quartz) backside (FTO) 400 500 600 700 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 5 0-5 -10-15 -6-3 0 3 6 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
Current Density (ma/cm 2 ) Impedance (Ohm) 10 5 Real Imaginary 10 4 10 3 10 2 10 1 10 0 10-1 10 0 10 1 10 2 10 3 10 4 10 5 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). 0.05-0.05 dark light dark light dark -0.10-0.15-0.20 CuBi 2 O 4 with Ar bubbling CuBi 2 O 4 /Pt with Ar bubbling 0 20 40 60 80 100 120 140 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