AVS 62nd International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS+EN-TuA

Paper SS+EN-TuA8
Improving Hematite-Based Solar Water Splitting by Surface Modification with Sn, Ti, and FeOOH

Tuesday, October 20, 2015, 4:40 pm, Room 112

Session: Photocatalysis, Photochemistry, and Chirality at Surfaces
Presenter: Anjli Patel, Drexel University
Authors: A.M. Patel, Drexel University
A.J. Abel, Drexel University
I. Garcia-Torregrosa, Utrecht University, Netherlands
B. Opasanont, Drexel University
J.B. Baxter, Drexel University
Correspondent: Click to Email
Photoelectrochemical (PEC) water splitting with hematite (α-Fe2O3) photoanodes presents a promising route to sustainable energy production due to hematite’s favorable bandgap, chemical stability, and widespread abundance. However, limitations arise from sluggish oxygen evolution reaction (OER) kinetics at the hematite-electrolyte interface, requiring a significant overpotential to induce photocatalysis. We report on the effects of Sn, Ti, and FeOOH surface treatments on hematite photoanodes to improve PEC performance by overcoming surface kinetic limitations. Thin film hematite photoanodes were fabricated by successive ionic layer adsorption and reaction (SILAR) of FeOOH on F:SnO2–coated glass substrates, followed by annealing at 450 C to induce phase transition of FeOOH into hematite. Subsequent annealing at 775 C caused diffusion of Sn from the F:SnO2 substrate through the hematite, resulting in 0.5at% Sn concentration at the photoanode surface. Current-voltage testing revealed that the presence of Sn in the hematite film significantly reduced the photocurrent onset potential, suggesting improved hole injection efficiency. Electrochemical impedance spectroscopy (EIS) revealed a reduction in the surface state charge transfer resistance (Rct,ss) by 2 orders of magnitude, supporting the importance of interfacial kinetics. Hematite photoanodes doped with up to 10% Ti were also prepared by incorporating titanium into the SILAR deposition bath. Ti doping decreased the onset potential by 600 mV and significantly increased the plateau photocurrent density from 0.01 mA/cm2 at 1.23 V vs. RHE for undoped hematite to nearly 0.6 mA/cm2 for Ti-doped photoanodes. EIS showed that Ti-doping reduced the Rct,ss at applied potentials ranging from 0.8 to 1.6 V vs. RHE, indicating a possible catalytic effect on the OER reaction at the photoanode surface. FeOOH films were deposited on the hematite photoanodes by an additional SILAR step, which reduced the photocurrent onset potential and increased the plateau photocurrent density by 20%. Unlike Ti, the FeOOH surface treatment exhibited little to no effect on the Rct,ss, suggesting that FeOOH does not directly catalyze the OER. However, both the FeOOH treatment and Ti doping significantly increased the peak surface state capacitance, which may be attributed to an increase in density of charged states at the hematite surface, resulting in higher plateau photocurrent. Together, these treatments yield photocurrents that are 3x larger than previous reports using SILAR-deposited planar hematite films, offering promising opportunities to overcome challenges in PEC water splitting with hematite photoanodes.