3.5 Defect engineering
Defect engineering, beyond doping, involves native point defects such as
vacancies and interstitials occurring naturally during material
synthesis, influencing catalytic, electrical, and optical properties.
These defects are now recognized as a strategy to enhance photoactivity
in heterogeneous catalysis. Classified based on dimensional space,
defects include 0D (point defects), 1D (line defects), 2D (interface
defects), and 3D (bulk defects).[89-93] Their
roles encompass both geometric and electronic effects, acting as
adsorption sites due to their high energy state and influencing
electronic structures. Defects, with their dynamic structures, improve
activation and diffusion in stable CO2 reactions.
Additionally, they alter electronic structures to affect adsorption
energy, steering reaction pathways in processes like CO2reduction. The controlled fabrication of desirable defects is crucial
for successful defect engineering, enhancing CO2reduction kinetics, and minimizing catalysis-recombination trade-offs
without needing external catalytic
entities.[94-98]
Dong et al. investigated grain boundary (GB) oxidation in Cu−Ag thin
films and its impact on the selectivity of CO and CH4production.[99] They developed a photocathode
incorporating a Cu−Ag thin-film cocatalyst and p-type Si to enhance the
efficiency of PEC CO2RR, as revealed in Figure 7A,B. The
electron beam evaporation method facilitated the direct growth of Cu
thin films on the substrate, with the grain size easily controlled by
adjusting the thickness through nucleation and growth processes. It was
observed that oxygen from the surrounding air permeated the Cu thin film
through gaps between the Ag islands, leading to the oxidation of the Cu,
particularly at the unstable GBs of uncoordinated Cu atoms.
Consequently, smaller Cu grains with a higher GB density were prone to
oxidation, which compromised the catalytic activity of the Cu−Ag
thin-film catalyst. In contrast, a relatively thick Cu layer (≥80 nm)
with a larger grain size effectively prevented oxidation, resulting in
catalytic properties comparable to those of bulk Cu−Ag catalysts.
Optimizing the Cu (100 nm) − Ag (3 nm) thin-film catalyst revealed a
bifunctional characteristic. This catalyst could selectively produce
both CO (FECO of 79.8%) and CH4(FECH4 of 59.3%) at potentials of −1.0 and −1.4 Vvs. RHE, respectively, as shown in Figure 7C,D. Furthermore,
introducing a novel PEC architecture comprising the patterned Cu−Ag thin
film, a SiO2 passivation layer, and a p-Si photocathode,
significantly improved the selectivity of CO and CH4under light illumination (100 mW cm−2). Cheng et al.
explored the synthesis of CdS NPs featuring controllable S-vacancies
encapsulated within a Zeolitic imidazolate framework-8 (ZIF-8) and
utilized as a precursor for nitrogen-doped porous carbon (NCP) through a
two-step process.[100] Initially, CdS NPs were
stabilized with polyvinylpyrrolidone (PVP), followed by the deposition
of a ZIF-8 shell onto their surface, resulting in a core-shell structure
with the CdS NPs as the core and ZIF-8 as the shell. The subsequent
control of S vacancies in the CdS and the pyridinic N content in the NCP
was achieved through pyrolysis at various temperatures. The CdS/NCP
catalysts, serving as cathodes with a TiO2 nanotube
array photoanode, enhanced the conductivity and stability of TMS-based
catalysts and facilitated CO2 reduction. Based on the
pyrolysis temperature, the tunable S vacancies in the CdS/NCP samples
led to enhanced selectivity towards CH3OH, which is
attributed to the synergistic impact of S-vacancies in CdS and the
pyridinic N content in nitrogen-doped porous carbon. The resulting
CdS/NCP material, with its porous structure, abundant S-vacancies, high
pyridinic N content, and enhanced conductivity, proved effective as a
cathode catalyst for CO2 reduction. This synergistic
effect was precisely controlled by adjusting the temperature
(i.e. , 300, 500, and 700 °C) during the thermal treatment of the
hybrid material. Notably, the sample treated at 500 °C (CdS/NCP-500
catalyst) exhibited a high conversion rate (3052
nmol·h–1·cm–2) with a selectivity
of 77.3% towards CH3OH. Tu et al. investigated the
mechanisms governing the impact of nitrogen vacancies on
photocatalysis.[101] They synthesized
g-C3N4 nanosheets with adjustable
nitrogen vacancies as photocatalysts for both H2evolution and CO2 photoreduction. Combining experimental
analyses with DFT calculations, the study found that nitrogen vacancies
in g-C3N4 triggered the formation of
mid-gap states below the conduction band edge, with the depth of these
states increasing proportionally to the number of nitrogen vacancies.
The presence of nitrogen vacancies was pivotal in exciting electrons
into mid-gap states, enabling absorption of longer-wavelength photons
and minimizing recombination losses of
e–/h+ pairs. The trapped electrons
were efficiently transferred to a Pt cocatalyst. Additionally, the
nitrogen vacancies induced a unique chemical environment on the surface,
leading to highly dispersed ultrasmall Pt NPs (1−2 nm) on
g-C3N4. The optimally doped
photocatalyst with the ideal nitrogen vacancy density showed markedly
improved photocatalytic activities, an 18-fold increase in
H2 evolution, and a four-fold enhancement for
CO2 reduction to CO. However, an excessive increase in
nitrogen vacancies resulted in diminished photocatalytic activity due to
the deeper mid-gap states acting as recombination sites for
photogenerated e–/h+ pairs, thus
reducing overall efficiency.
Kan et al. ingeniously devised and crafted a
p-Si/n-ZnOv/p-Cux O
heterostructure, incorporating a ZnOv -derived
Cux O defect level that exhibits a remarkable
capability for selective PEC CO2RR to
C2H5OH at low
biases.[102] The p-n-p band alignment was pivotal
in confining and accumulating multiple electrons within the conduction
band of n-type ZnOv, facilitated by a built-in electric
field. Simultaneously, the shallow ZnOv defect level
(vEZnOv) allowed electrons to escape from the confined
well and reach the CuxO (ECux O,
0.05 V vs. RHE). These tunneling defect energy levels on
CuxO closely aligned with those required for the
CO2 to C2H5OH reduction,
contributing to the heterostructure’s exceptional selectivity for PEC
CO2RR towards C2H5OH at
low biases, accompanied by an outstanding FE. In contrast, control
samples, including p-Si/p-CuxO and
p-Si/n-ZnOv, necessitated higher overpotentials to
overcome larger energy barriers, resulting in distinct
CO2 reduction selectivity towards CH4and HCOO–, respectively. The transfer of
photoelectrons in the Si/ZnOv/CuxO
system was facilitated by a built-in electric field of approximately 0.6
V through a leaky tunnel formed in the defect levels of
ZnOv and CuxO, allowing for a close
matching of energy levels and enabling the selective conversion of
CO2 into C2H5OH. The
optimized potential and functional interface contributed to achieving an
impressive FE exceeding 60% for PEC reduction of CO2 to
C2H5OH under 0 V vs. RHE.
In addition to the prerequisites for efficient charge carrier transfer,
studies have indicated that defect engineering plays a crucial role in
regulating catalytic activity and CO2 reduction
selectivity based on the surface state of the photocatalyst.
Nevertheless, the specific impact of sulfur vacancies on the transfer of
photogenerated e–/h+ pairs and the
reaction mechanism during CO2 reduction remains unclear.
Zhou et al. implemented a heat treatment strategy on
Cu2ZnSnS4/CdS (CZTS/CdS) photocathodes,
achieving simultaneous optimization of interface charge transfer and
surface S vacancy engineering.[103] These
advancements significantly contributed to the enhanced overall PEC
CO2RR performance and manageable selectivity. Heat
treatment improved the CZTS/CdS heterojunction interface by promoting
elemental inter-diffusion between Cd in CdS and Cu/Zn in CZTS, as shown
in Figure 7E. This resulted in a more favorable p-n junction with an
enlarged built-in potential, prolonged carrier lifetime, and suppressed
charge recombination. Additionally, defects on the surface of CdS could
be modulated through heat treatment in different atmospheres. Heat
treatment in air replenished intrinsic S vacancies on the CZTS/CdS
surface with oxygen, enhancing CO2 and CO adsorption
capability, as observed in Figure 7F, which leads to improved
CO2 reduction activity and higher selectivity toward
CH3OH / C2H5OH.
Conversely, heat treatment in N2 generated more S
vacancies on the surface, facilitating surficial CO desorption and
higher CO selectivity. By combining heterojunction design and
modification of catalyst surface properties through a simple heat
treatment strategy, this work established a new approach to designing
photocathodes for high-performance PEC CO2RR activity
with controlled selectivity. Table 2 lists the performance of
interface-engineered photocathodes for PEC CO2RR.