1. Schematic diagram of the fabrication of BiZn_x/Si photoelectrocathode and CO₂ reduction process

2. (a) Physical picture and (b) schematic model of photoelectrochemical cell for CO₂ reduction

3. SEM images of (a) planar-Si, (b) Si treated with NaOH solution, (c) Bi/Si, (d) BiZn₁/Si, (e) BiZn₂/Si, and (f) BiZn₃/Si with (g-i) elemental mappings of BiZn₂/Si

4. (a) XRD patterns of Bi/Si, BiZn₁/Si, BiZn₂/Si, and BiZn₃/Si, and (b) UV-Vis reflectivity spectra of Planar-Si, Si-T, Bi/Si, BiZn₁/Si, BiZn₂/Si and BiZn₃/Si

5. (a) High resolution Bi4f XPS spectra of Bi/Si, BiZn₁/Si, BiZn₂/Si and BiZn₃/Si, (b) ratios of Bi³⁺/Bi⁰ for BiZn₁/Si, BiZn₂/Si, and BiZn₃/Si, and (c) high resolution Zn2p XPS spectrum for BiZn₂/Si

6. (a) Transient photocurrent curves of Si-T, Bi/Si, BiZn₁/Si, BiZn₂/Si, and BiZn₃/Si, (b) LSV curves of untreated Si (Planar Si), Si-T, Bi/Si, and BiZn₂/Si in Ar or CO₂-saturated 0.5 mol·L^-1 KHCO₃ aqueous solution, (c) FE histograms of CO₂ reduction productions and (d) HCOOH current densities for Bi/Si, BiZn₁/Si, BiZn₂/Si, and BiZn₃/Si, (e) electrochemical impedance spectra (EIS) of Si-T, Bi/Si and BiZn₂/Si, and (f) stability of BiZn₂/Si during 10 h test

7. PEC and EC CO₂ reduction performances of BiZn₂/Si

8. (a) ECSA lines of Bi/Si, BiZn₁/Si, BiZn₂/Si, and BiZn₃/Si, and (b) oxidation LSV curves of PEC and EC CO₂ reduction for BiZn₂/Si in 0.1 mol·L^-1 KOH aqueous solution

9. Mechanismic schematic of formation process of HCOOH on the photocathode BiZn₂/Si

S1. TEM and HRTEM images of (a-d) Bi/Si and (e-h) BiZn₂/Si

S2. Elemental distribution mapping of BiZn₂/Si

S3. i-t curves of (a) Bi/Si, (b) BiZn₁/Si, (c) BiZn₂/Si and (d) BiZn₃/Si

S4. (a-c) Images of retention time H₂, CO and O₂, (d) H¹ NMR spectrum of liquid product

S5. FE for PEC CO₂ to CO, H₂, HCOOH productions over BiZn₂/Si, Bi/Si and Zn/Si

S6. ¹H NMR spectrum of the liquid phase products of BiZn₂/Si in CO₂-saturated 0.5 mol·L^-1 KOH and Ar-saturated 0.5 mol·L^-1 KHCO₃ solutions for photoelechemical reduction of CO₂

S7. (a) Photocurrent curves of BiZn₂/Si, Bi/Si and Si-T, (b) PL spectra of BiZn₂/Si, Bi/Si and Si-T

S8. Cycling stability runs of BiZn₂/Si in CO₂-saturated 0.5 mol·L^-1 KHCO₃ solution

S9. (a) XRD patterns of BiZn₂/Si before and after stability test, (b) typical SEM image of BiZn₂/Si after stability test

S10. CV curves for (a) Bi/Si, (b) BiZn₁/Si, (c) BiZn₂/Si and (d) BiZn₃/Si at different scan rates

S11. LSV curves for BiZn₁/Si, BiZn₂/Si and BiZn₃/Si

S12. FT-IR spectra of BiZn₂/Si before and after photoelectrochemical CO₂ reduction reaction

S13. Oxidation LSV curves of BiZn₂/Si, Bi/Si and Si-T

Table 1. Bi/Zn molar ratios of different samples in ICP results

Table 2. Faraday efficiency and current density comparison of different photo/electrocatalysts