Supporting Information for Adv

Supporting Information for Adv. Energy Mater., DOI: 10.1002/aenm.201902985

Tailored TiO2 Protection Layer Enabled Efficient and Stable Microdome Structured p-GaAs Photoelectrochemical Cathodes Shiyao Cao, Zhuo Kang, Yanhao Yu, Junli Du, Lazarus German, Jun Li, Xiaoqin Yan,* Xudong Wang,* and Yue Zhang* Tailored TiO2 Protection Layer Enabled Efficient and Stable Micro-Dome Structured

p-GaAs Photoelectrochemical Cathodes

Shiyao Cao†, Zhuo Kang†, Yanhao Yu, Junli Du, Lazarus German, Jun Li, Xiaoqin Yan*, Xudong

Wang*, Yue Zhang*

Figure S1 (a) optical image of p-GaAs wafer with patterned photoresist and Au on the surface. (b) optical image of patterned p-GaAs wafer. (c) larger area SEM image of patterned p-GaAs wafer.

Figure S2 (a) Top view SEM image of patterned p-GaAs-Pt electrode (b) EDS characterizations of highlight regions in blue dashed boxes of Figure S2a.

Figure S3 Schematics of the FDTD simulation models of (a) planar GaAs (top) and patterned

GaAs (bottom) illuminated with a 580 nm wavelength planar light,(b) planar GaAs (top) and patterned GaAs (bottom) illuminated with a 850 nm wavelength planar light with transverse magnetic mode polarization.

Figure S4 UV-Vis reflection spectra of planar p-GaAs and micro-dome structured p-GaAs.

Figure S5 TEM sample prepared by the Focused Ion Beam (FIB), (a) TiO2 protection layer

grown under 300℃ for 300 cycles on p-GaAs. (b) 300 cycles under 150 ℃

Figure S6 Atomic force microscope image and thickness measurement of (a) TiO2 protection layer grown under 300℃ for 300 cycles. (b) TiO2 protection layer grown under 150℃ for 300 cycles.

Figure S7 EIS Nyquist plots of p-GaAs-Pt and p-GaAs-300C TiO2-Pt photocathode (a) without and (b) with illumination of 100 mW/cm2.

Figure S8 Chronoamperometry (CA) of planar p-GaAs and patterned p-GaAs-Pt photocathode.

Figure S9. Morphological analysis of the surface stability of the photocathodes. Scanning electron microscope images of (a) p-GaAs-Pt without any protection layer before any electrochemistry and (b), after photoelectrochemistry.(c), The p-GaAs-300C-Pt surface before and (d), after photoelectrochemistry. (e) Regional enlarged view of p-GaAs-Pt after photoelectrochemistry.

Figure S10. Morphological analysis of the surface stability of the photocathodes. Scanning electron microscope images of (a)Patterned p-GaAs-Pt without any protection layer before any electrochemistry and (b), after photoelectrochemistry. (c), The patterned p-GaAs-300C-Pt surface before and (d), after photoelectrochemistry.

Figure S11 Morphological analysis of the surface stability of the photocathodes. Scanning electron microscope images of (a) p-GaAs-50 amorphous-Pt photocathode before and (b), after photoelectrochemistry. (c) p-GaAs-300 amorphous-Pt photocathode surface before and (d), after photoelectrochemistry. (e) p-GaAs-50C-Pt photocathode surface before and (f), after photoelectrochemistry.

Figure S12 Schematic of XPS depth profile of (a) unprotected GaAs and (b) GaAs protected by the best parameter protection layer.

Figure S13 The XPS full scan spectra of the (a) unprotected p-GaAs photocathode after photoelectrochemistry and (b) 300 cycle TiO2 deposited under 300 ℃ crystal protection layer protected p-GaAs after photoelectrochemistry.

Figure S14 Full XPS spectra of the unprotected p-GaAs photocathode after photoelectrochemistry (a) As 3d, (b) Ga 2p3/2 and (c) Ga 3d.