Supporting Information for:

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1 Supporting Information for: Charge-Transfer through Ultrathin Film TiO 2 on n-si(111) Photoelectrodes: Experimental and Theoretical Investigation of Electric Field-Enhanced Transport with a Non-Aqueous Redox Couple Hark Jin Kim 1, Kara L. Kearney 2,3, Luc H. Le 1 Zachary J. Haber 2, Angus A. Rockett 2,3 and Michael J. Rose 1 * 1 Department of Chemistry, The University of Texas at Austin, Welch Hall, 105 E 24 th St, Austin, TX 78712, USA 2 Department of Materials Science and Engineering, University of Illinois, 1304 West Green Street, Urbana, IL, 61801, USA 3 International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka , Japan these authors contributed equally Table of Contents Figure S1. Raman spectra of Si-CH 3 a-tio 2 and Si-CH 3 c-tio 2 Figure S2. CV for Si-CH 3 a-tio 2 (60 cycles) post-deposition heat-treatment Figure S3. AFM images of Si-CH 3 a-tio 2 and Si-CH 3 c-tio 2 Figure S4. SEM images of Si-CH 3 a-tio 2 and Si-CH 3 c-tio 2 Figure S5. Thickness dependent XPS spectra of Si-CH 3 a-tio 2 and Si-CH 3 c-tio 2 Figure S6. Circuit model used for EIS fitting Figure S7. EIS plots for Si-CH 3 a-tio 2 and Si-CH 3 c-tio 2 Figure S8. Thickness dependent CVs for Si-CH 3 a-tio 2 Pt and Si-CH 3 c-tio 2 Pt Page S2 S3 S4 S5 S6 S7 S8 S9 S1

2 Figure S1. Raman spectra of amorphous (red) and crystalline (blue) TiO 2 films grown on Si(111) CH 3. The peaks at 145 and 520 cm -1 are consistent with previously reported Raman data for anatase TiO 2 and the silicon substrate, respectively. 1-2 S2

3 Photocurrent density(µa/cm 2 ) Potential (V vs. Ag) Figure S2. Cyclic voltammogram for a-tio 2 cycles (~21 Å) post-deposition heat-treatment at 240 C for 10 min. This result indicates that the post-heat treatment does not induce the phase transition to crystalline. S3

4 (a) nm (b) nm 100nm 100nm 0.00 nm 0.00 nm (c) nm (d) nm 100nm 100nm 0.00 nm 0.00 nm Figure S3. Atomic force microscopy images. a) topography view of n-si(111)-ch 3 a-tio 2, b) friction image of n-si(111)-ch 3 a-tio 2, c) topography view of n-si(111)-ch 3 c-tio 2 and d) friction image of n-si(111)-ch 3 c-tio 2. As shown, both films consist of titania particles atop one another. While the amorphous film has larger particles, the crystalline film appears to be nanocrystalline. Furthermore, it was observed that the amorphous film showed some variation between scans, and the crystalline film was more regular. This observation is consistent with poorly anchored particles in the amorphous film, but it should be noted that the cantilever did not make a permanent mark in the film, suggesting a certain extent of elasticity in the surface. Also, it is notable that no pinholes were observed in either film. S4

5 (a (b Figure S4. SEM images of (a) Si(111) CH3 c-tio2 and (b) Si(111) CH3 a-tio2. Both samples were prepares with 60 ALD cycles of TiO2. S5

6 (a) a-tio 2-20 Si 2p (b) c-tio 2-20 Si 2p Intensity (a.u.) a-tio 2-40 a-tio 2 a-tio 2-80 Intensity (a.u.) c-tio 2-40 c-tio 2 c-tio 2-80 (c) Intensity (a.u.) Binding energy (ev) a-tio 2-20 a-tio 2-40 a-tio 2 a-tio 2-80 Ti 2p (d) Intensity (a.u.) Binding energy (ev) c-tio 2-20 c-tio 2-40 c-tio 2 c-tio 2-80 Ti 2p Binding energy (ev) Binding energy (ev) Figure S5. X-ray photoelectron spectra of Si 2p and Ti 2p for the photoelectrodes depending on TiO 2 ALD cycles. (a) and (c) are for amorphous TiO 2, (b) and (d) are for crystalline TiO 2. S6

7 CPE CH 1 3 CPE 2 R s R 1 R 2 W Si(111) TiO 2 Electrolyte Figure S6. Circuit used for fitting of EIS plot in Figure 5. S7

8 (a) Z (ohm) Exp. for n-si CH 3 a-tio 2 Fitted Exp. for n-si CH 3 c-tio 2 Fitted Frequency (Hz) (b) Phase angle (deg.) Frequency (Hz) Figure S7. Comparison of experimental and fitted results for a-tio 2 (red) and c-tio 2 (blue) by using equivalent circuit of R(CPE,R)(CPE,RW) shown in Figure S6. S8

9 Current density (ma cm -2 ) (a)... a-tio 2-10 Pt... a-tio 2-20 Pt... a-tio 2-30 Pt... a-tio 2-40 Pt... a-tio 2-50 Pt... a-tio 2 Pt... a-tio 2-70 Pt... a-tio 2-80 Pt... a-tio 2-90 Pt... a-tio Pt... a-tio Pt... a-tio Pt Potential (V vs. Ag) Potential (V vs. Ag) Figure S8. Cyclic voltammograms for TiO 2 -(x) Pt at varying thicknesses of (a) a- TiO 2 and (b) c-tio 2. Current density (ma cm -2 ) (b)... c-tio Pt... c-tio Pt... c-tio Pt... c-tio Pt... Pt... c-tio 2-10 Pt... c-tio 2-20 Pt... c-tio 2-30 Pt... c-tio 2-40 Pt... c-tio 2-50 Pt... c-tio 2 Pt... c-tio 2-70 Pt... c-tio 2-80 Pt... c-tio 2-90 Pt... c-tio Pt S9

10 References 1 J.D. Ye, P. Parkinson, F.F. Ren, S.L. Gu, H.H. Tan, and C. Jagadish, Optics Express, 2012, 20, X. Pan, X.L. Wang, H.L. Xiao, C.M. Wang, C. Feng, L.J. Jiang, H.B. Yin, H. Chen, Journal of Crystal Growth, 2011, 331, S10