3.1 Plasmonic noble metal cocatalysts
Plasmonic noble metal cocatalysts incorporate noble metals like Ag and Au, known for their abundance of free-mobility electrons.[28] Surface plasmon resonance in these cocatalysts originates from the collective oscillations of nanostructures and nanogaps under intense electromagnetic radiation.[29-31] Such resonance, characterized by incident light interacting with the cocatalyst, enhances the redistribution and conversion of light energy through the re-emission of plasmon-induced light, non-radiative decay to excited carriers (hot electrons and holes), and thermal effects over specific timescales.[28]
Regarding PEC reactions, the size effect of plasmonic materials becomes significant, with plasmonic noble metal nanostructures improving light absorption, catalytic activity, selectivity, and efficiency.[32-36] Hence, plasmonic noble metal cocatalysts, by exciting surface plasmons, could harness broad-spectrum sunlight, producing high-energy hot carriers that facilitate PEC CO2RR.
For example, Liu et al. designed a CuBi2O4 inverse opal photocathode modified with plasmonic Ag nanoparticles (Ag NPs) using a sacrificial template method (CuBi2O4IOs-Ag).[37] The 3D-ordered structure of CuBi2O4 inverse opal enabled higher mass transfer rates and light harvesting efficiency. Furthermore, incorporating Ag NPs significantly enhanced the surface charge distribution by forming an ohmic contact with CuBi2O4. The CuBi2O4 IOs-Ag photocathodes showed notable improvements in selectivity for CO production, achieving a faradaic efficiency for CO (FECO) of 92% at 0.2 Vvs. reversible hydrogen electrode (RHE), which is 1.6 times greater than that of the pristine CuBi2O4 thin film. Wang et al. prepared n+p Si coated with a TiO2 interlayer and coupled it with plasmonic Au NPs to fabricate photocathodes (Au/TiO2/n+p Si) for PEC CO2RR to CO.[38] A schematic illustration of the synthesis method is depicted in Figure 2A. Initially, a TiO2 layer was deposited on the micro-pyramid Si surface using an ALD process, after which Au NPs were fabricated on top of TiO2 layer through an electrodeposition method. The Au/TiO2/n+p Si photocathodes exhibited an onset potential of +0.24 V vs. RHE, a maximum FECO of 86%, and a partial photocurrent density for CO of −5.52 mA cm–2 at −0.8 V vs. RHE, as shown in Figure 2B-D. Additionally, these photocathodes demonstrated superior long-term operational stability for CO production under continuous illumination for 20 h, as shown in Figure 2E. Density Functional Theory (DFT) calculations indicated that the synergistic effect of Au NPs and TiO2 enhanced CO2adsorption and expedited the generation of the *COOH intermediate and *CO desorption from active sites. This research group further investigated the localized surface plasmon resonance (LSPR) effect of Au on the TiO2 layer, which contributed to increased activity and selectivity for CO production by utilizing hot electrons generated in Au NPs. Bharath et al. designed photocathodes by integrating plasmonic Ag NPs with TiO2/RGO (Ag-TiO2/RGO) via a hydrothermal method followed by microwave irradiation.[39] In this composite, Ag NPs not only absorbed visible light but also acted as efficient electron scavengers, thus enhancing PEC performance for CO2reduction. PEC measurements revealed that the Ag-TiO2/RGO photocathodes achieved a notable total photocurrent density of 23.5 mA cm–2 and exhibited low resistance of 125 Ω in a CO2-saturated 1.0M KOH solution under UV-vis light illumination. Furthermore, the Ag-TiO2/RGO photocathodes displayed a CH3OH yield of 85 μmol L–1cm–2, a QE of 20%, and a faradaic efficiency for CH3OH (FECH3OH) of 60.5% at an onset potential of −0.7 V vs. Ag/AgCl. Bharath et al. also fabricated photocathodes featuring plasmonic-Au and RGO-incorporated α-Fe2O3 nanorods (Au/α-Fe2O3/RGO) aiming for highly selective CH3OH production.[40]The synergistic effects among the size-dependent properties of α-Fe2O3, the plasmonic nature of Au, and the chemical interactions of Au, RGO, and α-Fe2O3 nanorods resulted in a higher band gap for Au/α-Fe2O3/RGO (2.60 eV). This band gap allowed the composite to absorb more intensely in the high-energy range of the visible spectrum, enabling the efficient use of photogenerated electrons and reducing e/h+ pair recombination effects. Consequently, the Au/α-Fe2O3/RGO photocathodes exhibited an impressive photocurrent density of −31.5 mA cm–2 and achieved a maximum CH3OH yield of 43 μmol L–1 cm–2, as shown in Figure 2F,G. Additionally, the photocathodes attained QE and FECH3OH of 21.5% and 91%, respectively, at −0.6 Vvs. SCE in a CO2-saturated 0.1M KOH electrolyte under illumination.
Li et al. developed a photocathode for PEC CO2RR by dispersing Ag NPs onto a Cu-modified mesoporous TS-1 zeolite (Ag/Cu-TS-1).[41] The fabrication process involved an ion exchange method followed by an in-situ photodeposition method, resulting in the synthesis of Cu-TS-1 via ion exchange and the highly dispersed Ag NPs onto Cu-TS-1 through in-situ photodeposition. The Ag/Cu-TS-1 photocathode showed exceptional light absorption and efficient separation of e/h+pairs, enhancing CO2RR. This improvement was attributable to the heterostructure of Cu2O/CuO and the LSPR effect of the Ag NPs. In PEC performance terms, the Ag/Cu-TS-1 photocathode demonstrated conversion of CO2 into CH3OH and C2H5OH at rates of 5.64 and 2.62 μmol cm–2h–1, respectively, at −0.6 V vs. RHE in a CO2-saturated 0.1M KHCO3 electrolyte. Zhang et al. constructed a plasmonic Ag-adorned Cu2O nanowire (Cu2O/Ag) photocathode for PEC CO2RR targeting C2+products.[42] The LSPR effect of Ag contributed to both enhanced rapid separation of e/h+ pairs and improved surface catalytic reactions for C2+ product generation. To substantiate the LSPR effect in the Cu2O/Ag photocathode, the ultraviolet-visible (UV-vis) absorption spectrum was used to confirm increased light absorption due to the presence of Ag NPs. Additional in-situ attenuated total reflection infrared spectroscopy (ATR-IR) results indicated that incorporating Ag NPs improved the formation and adsorption of the CH3O* intermediate. PEC tests showed the Cu2O/Ag photocathode achieved a faradaic efficiency for CH3COOH (FECH3COOH) of 47.7%, with a generation rate of 212.7 μmol cm–2h–1 at −0.7 V vs. RHE under illumination.