S2 S1. Contents This document contains a description of the experimental and modeling/simulation methods utilized in this work (Sections S2 and S3), additional scanning-electron micrographs (Section S4), analyses of the elemental composition and structure of the photoelectrodeposits (Section S5), voltammetric data (Section S6), and a list of associated references (Section S7). S3 S2. Experimental Methods Materials and Chemicals Buffered HF improved etchant (Transene), poly(methyl methacrylate)

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... PMMA, Microchem), methyl isobutyl ketone (99.9 %, VWR), isopropanol (99.5 %, VWR), Ti (99.995 %, Kurt J. Lesker), Au (99.999 %, Materion), In (99.999 %, Alfa Aesar), Ga (99.999 %, Alfa Aesar), H2SO4 (ACS Reagent, J. T. Baker), SeO2 (99.4 %, Alfa Aesar), and TeO2 (99+ %, Sigma-Aldrich) were used as received. H2O with a resistivity ≥ 18.2 MΩ cm (Barnstead Nanopure System) was used throughout. Au-coated n + -Si(100) (< 0.005 Ω cm, Asdoped, 525 ± 25 µm thick, single-side polished, Addison Engineering) was used as a substrate for deposition. Flash-Dry Ag Paint (SPI Supplies), EP21ARHTND Epoxy (MasterBond) and nitrocellulose-based nail polish were used to assemble the working electrodes. Substrate Preparation n + -Si wafers were rinsed with acetone and isopropanol. A bilayer resist for electron-beam lithography composed of poly(methyl methacrylate) (PMMA) was then applied by spin-coating. First, a base layer of 495 PMMA A4 was applied at 4000 rpm for 1.0 min and baked at 180 °C for 3.0 min. Second, an overlayer of 950 PMMA A4 was applied at 4000 rpm for 1.0 min and baked at 180 °C for 3.0 min. Next, a VISTEC EBPG 5000+ electronbeam pattern generator was used to pattern the resist into a square lattice of 150 nm diameter circles on an 8 μm pitch. Pattern generation was effected using a 100 kV accelerating voltage, 300 μm aperture, 5 nA current, and a dose of 1000 μC cm -2 . After electron-beam pattern generation, the substrates were immersed in mixture of methyl isobutyl ketone : isopropanol in a 1 : 3 volume ratio for 1.0 min at room temperature, to develop the pattern and produce arrays of holes. Next, the substrates were etched with buffered HF for 1.0 min, rinsed with H2O, dried under a stream of N2(g), and then immediately transferred to a CHA Mark 40 electron-beam metal evaporator with a base pressure of 10 -6 torr. A 5 nm thick Ti adhesion layer was first