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.