Tunable green syngas generation from CO2 and H2O with sunlight as the only energy input

Abstract

The carbon-neutral synthesis of syngas from CO₂ and H₂O powered by solar energy holds grand promise for solving critical issues such as global warming and the energy crisis. Here we report photochemical reduction of CO₂ with H₂O into syngas using core/shell Au@Cr₂O₃ dual cocatalyst-decorated multistacked InGaN/GaN nanowires (NWs) with sunlight as the only energy input. First-principle density functional theory calculations revealed that Au and Cr₂O₃ are synergetic in deforming the linear CO₂ molecule to a bent state with an O-C-O angle of 116.5°, thus significantly reducing the energy barrier of CO₂RR compared with that over a single component of Au or Cr₂O₃. Hydrogen evolution reaction was promoted by the same cocatalyst simultaneously. By combining the cooperative catalytic properties of Au@Cr₂O₃ with the distinguished optoelectronic virtues of the multistacked InGaN NW semiconductor, the developed photocatalyst demonstrated high syngas activity of 1.08 mol/g_cat/h with widely tunable H₂/CO ratios between 1.6 and 9.2 under concentrated solar light illumination. Nearly stoichiometric oxygen was evolved from water splitting at a rate of 0.57 mol/g_cat/h, and isotopic testing confirmed that syngas originated from CO₂RR. The solar-to-syngas energy efficiency approached 0.89% during overall CO₂ reduction coupled with water splitting. The work paves a way for carbon-neutral synthesis of syngas with the sole inputs of CO₂, H₂O, and solar light.

Keywords: Au@Cr2O3; multistacked InGaN/GaN nanowire; photocatalytic CO2 reduction; tunable syngas synthesis.

Fig. 1.

(A) Schematic of Mg-doped InGaN/GaN (active/capping layer) NW structures grown on a GaN NW template. (B) A 45°-tilted SEM image of InGaN/GaN NWs grown on Si (111) substrate. (C) Room temperature PL spectrum of the as grown InGaN/GaN NWs. (D) High-angle annular dark field STEM image showing InGaN/GaN NW. (E) EDX line scan showing the distribution of In, Ga, and N along the blue line in (D). a.u., arbitrary units.

Fig. 2.

(A) High-angle annular dark field STEM image of Au@Cr₂O₃-decorated InGaN/GaN NW. (Scale bar: 200 nm.) (B) STEM-EDX elemental mapping images of Ga, In, N, Au, and Cr with overall mapping. (Scale bars: 200 nm.) (C) TEM image of Au@Cr₂O₃/NW sample. The inset is HRTEM image of core/shell Au@Cr₂O₃ on NW surface. (Scale bars: C, 20 nm; Inset, 5 nm.) High-resolution XPS of (D) Au 4f and (E) Cr 2p of Au@Cr₂O₃-decorated InGaN/GaN NW sample. a.u., arbitrary units.

Fig. 3.

(A) Schematic illustration of overall CO₂RR over Au@Cr₂O₃-decorated InGaN/GaN NW. (B) The output gas evolution on NW with various cocatalysts. The inset shows a typical NW sample used for these experiments. (C) The output gas evolution with various Au@Cr₂O₃ ratios on multistacked InGaN/GaN NWs. (D) Dependence of STS on Au/Cr₂O₃ ratios. (E) Time courses of H₂, O₂, and CO evolution over Au@Cr₂O₃-decorated InGaN/GaN NW. The dashed lines indicate the evacuation of photoreactor and restart of the test. A sample of ∼3 cm² in surface area (active GaN/InGaN catalyst weight ∼0.5 mg) and 16-sun illumination were employed in the experiment.

Fig. 4.

Photocatalytic activity of Au@Cr₂O₃-decorated InGaN/GaN NWs under concentrated Xe lamp light illumination using AM 1.5-G filter (FA) and 400-nm long-pass filter (400LP). (A) The output of H₂, O₂, and CO gas evolution comparison between AM 1.5-G and 400-nm filters. (B) Repeated cycles of photocatalytic gas evolution from CO₂ reduction using 400LP.

Fig. 5.

Side views of the optimized configurations of CO₂ adsorbed on (A) Au(111), (B) Cr₂O₃(0001), and (C) Au₄/Cr₂O₃. Calculated free energy diagrams for (D) CO₂ reduction and (E) HER on different substrates. Au, gold; Cr, blue; C, brown; and O, red.