3.4 Nanostructure engineering
Nanostructure engineering is an effective method for enhancing the
performance of photocathodes by manipulating the dimensions and
morphology of photocathode materials at the nanometer
scale.[73,74] Notable examples of these
nanostructures include nanorods, nanowires, dendritic structures,
core/shell structures, as well as highly nanoporous and hollow
structures. The benefits of employing such nanostructures are manifold.
They facilitate enhanced light absorption through scattering and reduce
bulk recombination. Additionally, these structures increase the specific
surface area, leading to a corresponding increase in active
sites.[75]
Furthermore, nanostructure engineering involves tuning the
crystallographic orientations. Semiconductor nanocrystals typically
exhibit anisotropic characteristics, composed of face-dependent
electronic structures, adsorption energy/reactive sites, and
photocorrosion resistance, attributable to different atomic
configurations and coordination on various crystal
facets.[76-81] Overall, synthesizing
nanostructures with a high aspect ratio improves light harvesting and
charge separation in the semiconductor bulk.[43]Additionally, nanostructures that expose selective facets, favorable for
CO2 reduction kinetics, enhance charge separation at the
surface.
For example, Liu et al. fabricated a photocathode for PEC
CO2RR to CO by combining InP nanopillar arrays with
Au-TiO2 interfaces
(Au-TiO2/InP).[82] In the
fabrication process, they used inductively coupled plasma reactive ion
etching (ICP-RIE) on Au/SiOx-masked InP wafers to
synthesize InP NPs. However, ICP-RIE led to plasma-induced surface
defects on the InP NPs, resulting in a reduced minority carrier lifetime
compared to planar InP. To remedy this, a dilute HCl solution was used
to remove the plasma-damaged layer, leading to fewer surface defects in
the InP NPs and improved minority carrier lifetime, as shown in Figure
6A. Thus, these treatments, aimed at eliminating surface defects,
contributed to an increased surface area, reduced light reflection, and
minimized carrier recombination losses, ultimately enhancing
light-harvesting efficiency. The PEC performance of the nanostructured
Au-TiO2/InP photocathodes demonstrated an onset
potential of +0.3 V vs . RHE and an FECO of 84.2%
at −0.11 V vs . RHE in a CO2-purged 0.1M
KHCO3 solution under simulated 1 sun illumination, as
illustrated in Figure 6B,C. Cai et al. constructed a core/shell
structured nanosheet array of porous ZnO@ZnSe photocathode on FTO
glass.[83] The synthesis involved a
dissolution-recrystallization process, serving as a template for ZnSe
growth through an anion exchange reaction with a Se2-source after cultivating the ZnO Ns array on the FTO glass coated with a
ZnO seed layer. The optimal ZnSe loading demonstrated that while higher
loading increased light absorption intensity and active site density,
excessive ZnSe created additional surface defect states. Consequently,
ZnO-10@ZnSe-3, with an appropriate loading amount, exhibited an onset
potential of +0.39 V vs . RHE in a CO2-saturated
0.5M NaHCO3 electrolyte, a photocurrent of 1.17 mA
cm–2 at +0.11 V vs . RHE, and an
FECO of 52.9% at −0.4V vs . RHE under visible
light irradiation. Hu et al. developed a photocathode for PEC
CO2 conversion into CO featuring uniformly dispersed Au
NPs with Au (111)/Au (200) boundaries on the p-Si surface
(b-Au1/Si).[84] Initially, small,
consistently sized Au seeds were distributed on the p-Si surface using a
chemical deposition (CD) method. Subsequent electrodeposition ensured
continuous growth of the Au above the CD seeds, maintaining a
homogeneous distribution, as depicted in Figure 6D. DFT calculations
suggested that the presence of Au (111)/Au (200) boundaries
substantially decreased the energy barrier for forming the*COOH intermediate during CO2reduction. As a result, the b-Au1/Si photocathode
achieved an impressive photocurrent density of −13.1 mA
cm–2 at −1.0V vs. RHE with an
FECO of 82.2% at −0.4 V vs. RHE, and it
maintained remarkable operational stability for over a week, as shown in
Figure 6E,F.
Mubarak et al. prepared a 3D nanoporous structured TiO2NPs on a thin Ti foil photocathode for CO2 reduction to
HCOOH; the process involved a chemical treatment with
H2O2, followed by calcination at
elevated temperatures ranging from 400 to 800
℃.[85] The resulting 300 to 500 nm thick 3D
nanoporous layer on the Ti-foil surface displayed significant porosity.
This structure enhanced the photon conversion efficiency of
TiO2 NPs, increasing photon absorption per unit surface
area and strengthening the electrochemical reaction capabilities due to
the large specific surface area. The photocathode produced HCOOH as the
primary product in PEC CO2RR after more than 25 h of
chronoamperometric electrolysis and achieved an FEHCOOHof 64% with an HCOOH yield of 165 μmol cm–2h–1 at −1.3 V vs . Ag/AgCl. Paul et al.designed a photocathode using a morphology-controlled synthesis of Ag
NPs distributed onto WO3 nanorods
(Ag/WO3-NR) for PEC CO2RR to
HCOO–.[86] The one-pot
fabrication process employed cetyltrimethylammonium bromide (CTAB) as a
structure-directing agent for forming WO3 nanorods.
Critical parameters such as CTAB concentration, reflux time, and
temperature were pivotal in determining the nanorod morphology. The
Ag/WO3-NR photocathode exhibited a significant current
density of 0.4 mA cm–2 for HCOO–production and achieved a rate of 31.7 mmol h–1.
Gurudayal et al. constructed a photocathode for PEC
CO2RR to C2+ products, utilizing a
back-illuminated n-type Si absorber covered with an Ag-supported
dendritic Cu catalyst.[87] The synthesis of the
Ag-supported dendritic Cu catalyst involved the evaporation of Ag,
followed by high-rate electrodeposition of Cu, resulting in a highly
porous structure with nanocactus-like morphology and featuring a
dendritic Cu structure on the pyramid-shaped Ag. The Ag-supported
dendritic Cu catalyst exhibited a high electrochemically active surface
area, enabling the photocathode to operate at a high current density.
Therefore, the photocathode produced C2+ products,
including C2H4,
C2H5OH, and
C3H7OH, in CO2-saturated
0.1M CsHCO3 under simulated one sun illumination.
Additionally, the photocathode maintained over 60% FE for hydrocarbon
and oxygenated products, primarily C2H4,
C2H5OH, and
C3H7OH, for several days under simulated
diurnal illumination. Kempler et al. utilized high loadings of Cu
integrated onto Si microwire arrays
(n+p–Si μW/Cu) for PEC
CO2RR to
C2H4.[88] The Si
microwire array structure could diminish trade-offs between catalyst
loading and light absorption intensity. A Si photocathode with Cu
electrodeposited onto the vertical sidewalls of high-aspect-ratio
microwires was designed to minimize parasitic absorption by the
catalyst, as revealed in Figure 6G, demonstrating a
|Jph| exceeding 25 mA
cm–2 before and after 48 h of PEC
CO2RR, resulting in the production of
C2H4 at more positive potentials under
1-sun illumination. Thus, the
n+p–Si μW/Cu photocathode revealed
a maximum |JC2H4| of 2.1 \(\pm\) 0.2
mA cm–2 at −0.44 V vs . RHE and a maximum
|JCH4| of 2.9 \(\pm\) 0.7 mA
cm–2 at −0.62 V vs . RHE, as shown in Figure
6H,I.