Effect of Interfacial SiOx Defects on the Functional Properties of Si-Transition Metal Oxide Photoanodes for Water Splitting

The transfer of photogenerated charges through interfaces in heterojunction photoanodes is a key process that controls the efficiency of solar water splitting. Considering Co3O4/SiOx/Si photoanodes prepared by physical vapor deposition as a representative case study, it is shown that defects normally present in the native SiOx layer dramatically affect the onset of the photocurrent. Electron paramagnetic resonance indicates that the signal of defects located in dangling bonds of trivalent Si atoms at the Si/SiOx interface vanishes upon vacuum annealing at 850 °C. Correspondingly, the photovoltage of the photoanode increases to ≈500 mV. Similar results are obtained for NiO/SiOx/Si photoanodes. Photoelectrochemical analysis and impedance spectroscopy (in solution and in the solid state) indicate how the defect annealing modifies the Co3O4/SiOx/Si junction. This work shows that defect annealing at the solid–solid interface in composite photoanodes strongly improves the efficiency of charge transfer through interfaces, which is the basis for effective solar-to-chemical energy conversion.

The solid lines are the best fit to the experimental data.The obtained of the double layer capacitance are the same within the experimental uncertainty (5%), as expected.Considering that the geometric area is 0.2826 cm 2 and the reference value for a flat Co 3 O 4 layer (C = 20 µF/cm 2 , from R. van de Krol, M. Gratzel, 'Photoelectrochemical hydrogen production', Springer, 2012), the electrochemically active surface area is ≈3.5 times the geometric one.
Figure S 1: (a,b) SEM images of Si/SiO x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 .(c) Grazing incidence X-ray Diffraction pattern (incident angle ω = 1°) of Si/SiO x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 photoanodes (spectra vertically shifted for clarity).The only crystalline phase is Co 3 O 4 , the average crystal size (Debye Scherrer analysis) is about 25 nm in both cases.The spurious scattering contribution from the substrate is limited to the shaded part.(d) XPS spectra for the Si/SiO x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 photoanodes in the binding energy region of Co 2p levels (spectra vertically shifted for clarity).The very same features typical of Co 3 O 4 are present in both cases [9].(e) Tafel plot of p ++ -Si/Co 3 O 4 measured in 1M KOH. 2 photocurrent (mA/cm 2 ) Figure S 5: XPS spectrum in the Si 2p electronic energy range for Si/SiO x,RT substrate and best fit curve obtained considering only the Si 0 (green) and Si +4 (orange) contributions.

Figure S 6 :Figure S 7 :
Figure S 6: Results of the linear fit (space charge region-scr and quasi-neutral region-qnr) of the solid state ln(J)-V curves measured for the Si/SiO x,850 /Co 3 O 4 photoanode.Each region has been modelled according to the diode equation:

Figure S 8 :
Figure S 8: Equivalent electrical circuit used for fitting the EIS data.Two Constant Phase Elements (CPE) are used, whose impedance is Z CP E =1/[C ef f (j ω) n ].The two R-CPE loops in series describe the solid-electrolyte interface (R 1 -CPE 1 , resonant at low frequency) and the solid-solid interface (R 2 -CPE 2 , resonant at high frequency).

Figure S 9 :FigureFigure S 12 :
Figure S 9: Fitting results, according to the model circuit of Figure S8 of the EIS spectra recorded at the open circuit potential in 1M KOH in dark condition.Uncertainties are in the last digit.
x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 photoanodes (spectra vertically shifted for clarity).The only crystalline phase is Co 3 O 4 , the average crystal size (Debye Scherrer analysis) is about 25 nm in both cases.The spurious scattering contribution from the substrate is limited to the shaded part.(d) XPS spectra for the Si/SiO x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 photoanodes in the binding energy region of Co 2p levels (spectra vertically shifted for clarity).The very same features typical of Co 3 O 4 are present in both cases [9].(e) Tafel plot of p ++ -Si/Co 3 O 4 measured in 1M KOH. 2 Estimation of the electrochemically active surface area for the Si/SiO x,850 /Co 3 O 4 and Si/SiO x,RT /Co 3 O 4 photoanodes, obtained by CV curves recorded at different scan rate.