3.3 Junction engineering
A heterojunction is typically defined as a structure composed of two or
more different semiconductors with a contacting interface, distinct band
energy levels, a matching crystal lattice, and similar thermal expansion
coefficients.[61] Heterojunctions can be
classified into type-I, type-II (including p–n junctions), and Schottky
barrier junctions (metal-semiconductor). Semiconductor heterojunctions
often involve interfacing two semiconductor materials with differing
Fermi-level energies, creating a built-in electric field that promotes
the separation of electrons and holes upon light excitation.
In the case of type-I heterojunctions, two semiconductors with
overlapping band structures typically exhibit one semiconductor with a
more negative CB position and a more positive VB position than the other
semiconductor. Conversely, in type-II heterojunctions, the two
semiconductors have staggered band structures where electrons transition
from a more negative CB to a less negative CB, and holes move in the
opposite direction. In type-II heterojunctions, photogenerated electrons
and holes are efficiently separated, allowing for
e–/h+ pairs to be excited using a
greater number of photons.[61]
Schottky barrier junctions create band bending near the
semiconductor-metal catalyst interface, facilitating electron transfer
from the semiconductor to the metal. Nevertheless, this section focused
solely on semiconductor heterojunctions, as Schottky barrier junctions
overlap with concepts such as cocatalysts, and there is extensive prior
research in this domain.
For example, Quyang et al. prepared a photocathode tailored for PEC
CO2RR to HCOOH, consisting of Bi-modified 1D
ZnO/α-Fe2O3 nanotubes (1D Bi@ZFO
NTs).[62] The formation of an n-n heterojunction
between the narrow bandgap of α-Fe2O3and the wide bandgap of ZnO was instrumental in enhancing charge
transfer, establishing an internal electric field conducive to driving
the transfer of photoexcited charges, as illustrated in Figure 5A.
Additionally, depositing Bi onto the
ZnO/α-Fe2O3 heterojunction potentially
increased the carrier concentration at the electrode surface, thereby
enhancing the efficiency of photogenerated charge separation.
Consequently, the Bi@ZFO NTs photocathode indicated a low onset
potential of −0.53 V vs. RHE, a low Tafel slope of 101.2 mV
dec–1, and achieved a high FEHCOOH of
61.2% at −0.65 V vs. RHE, maintaining stability over 4 h under
visible light, as shown in Figure 5B,C. Jiang et al. reported a
photocathode incorporating CuO onto graphitic carbon nitride
(g-C3N4) supported on carbon paper
(CuO/g-C3N4/carbon paper) for PEC
CO2RR to
CH3OH.[63] Establishing a type-Ⅰ
heterojunction facilitated photogenerated electron transfer from the CB
of g-C3N4 to the CB of CuO, which has a
relatively less negative CB energy level under illumination.
Concurrently, favorable hole transfer occurred from the VB of
g-C3N4 to the VB of CuO, which held a
lower VB energy. As a result, the
CuO/g-C3N4/carbon paper photocathode
exhibited a high IPCE, an increased photocurrent response, and a
remarkable FE of 75% and QE of 8.9%, respectively, for
CH3OH production. Pan et al. presented a photocathode
consisting of a Cu catalyst decorated with flower-like
CeO2 NPs and CuO NPs, functioning as n-type and p-type
semiconductors, respectively.[64] The formation of
a p-n heterojunction with CeO2 NPs/CuO NPs facilitated
the synergistic migration of photoexcited electrons and holes, leading
to exceptional PEC performance for CO2 reduction. Thus,
the CeO2 NPs/CuO NPs/Cu photocathode revealed a
CH3OH yield rate of 3.44 μmol cm–2h–1 and exhibited a high FECH3OH of
approximately 60% at – 1.0 V vs. saturated calomel electrode
(SCE) under visible light irradiation. Zheng et al. utilized Zinc
phthalocyanine (ZnPc) integrated with carbon nitride nanosheets as a
photocathode for PEC CO2RR to
CH3OH.[65] The aligned energy
bands between ZnPc and carbon nitride facilitated electron transfer and
reduced recombination of e–/h+pair. Simultaneous exposure to light and an external voltage ensured
that reductive electrons were generated not only through light
excitation but also by the external voltage. This augmented the transfer
rate of photogenerated electrons from the CB of carbon nitride to the
LUMO of ZnPc. Consequently, the ZnPc/carbon nitride photocathode
exhibited a predominant CH3OH product with a yield of 13
μmol L–1 after 8 h at −1.0 V vs. SCE. Tarek et
al. developed a heterostructured
CdS-CuFe2O4 photocathode to convert
CO2 into
CH3OH.[66] The
CdS–CuFe2O4 photocathode exhibited a
higher IPCE of 12.09% compared to
CuFe2O4 with an IPCE of 7.28% at 470
nm, illustrating effective visible light absorption during PEC
CO2RR. Within the
CdS–CuFe2O4 heterojunction, the CB of
CdS served as the site for CO2 reduction, capturing
photogenerated electrons originating from
CuFe2O4, while water oxidation occurred
at the VB of CuFe2O4. Accordingly, PEC
performance indicated that the
CdS–CuFe2O4 photocathode revealed an
FECH3OH of 72% and a QECH3OH of 16.9%
and recorded a maximum CH3OH yield of 23.8 μmol
L–1 cm–2 in
CO2-saturated 0.1M NaHCO3 electrolyte.
Xu et al. prepared a 2D heterojunction of
TiO2/Ti3CN MXene as a photocathode for
PEC CO2RR, synthesized using a simple hydrothermal
oxidation method.[67] The 2D
TiO2/Ti3CN heterojunction, with its
large specific surface area, exceptional light absorption ability, and
abundant Ti3+ species, facilitated the efficient
generation and migration of e–/h+pairs. To confirm the impact of Ti3+, electron
paramagnetic resonance (EPR) spectra were obtained, as depicted in
Figure 5D. The EPR spectra demonstrated widespread detection of
Ti3+ species, proving beneficial for trapping charge
carriers and mitigating the recombination of
e–/h+ pairs. Furthermore, DFT
calculations suggested that the 2D
TiO2/Ti3CN heterojunction could
spontaneously adsorb CO2 molecules and stabilize key
intermediates crucial for HCOOH production, as shown in Figure 5E.
Consequently, the novel PEC system, composed of
Pd@TiO2/Ti3CN||SCE||BiVO4,
effectively produced HCOO–, CH3OH,
and C2H5OH with a remarkable formation
rate of 45.6 μM cm–2 h–1, as
depicted in Figure 5F. Lu et al. developed a photocathode for PEC
CO2RR to C2H5OH,
comprising arrays of 0D/1D CuFeO2/CuO nanowire
heterojunction arrays synthesized through an in-situ
method.[68] Due to the comparable energy band gaps
of CuO and CuFeO2, the photogenerated electrons
originating from the CB of CuFeO2 migrated to the
surface of CuO, while photogenerated holes derived from the VB of CuO
moved to CuFeO2. This arrangement suppressed the
recombination of e–/h+ pairs under
the built-in electric field of the heterojunction. The PEC performance
of the CuFeO2/CuO nanowire photocathode exhibited an
impressive faradaic efficiency for
C2H5OH (FEC2H5OH) of
66.73% at −0.6 V vs. Ag/AgCl. Zhang et al. designed a
CuFeO2/TNNTs photocathode, incorporating
high-temperature-durable n-type Nb-doped TiO2 nanotube
arrays (TNNTs) and p-type CuFeO2 for PEC
CO2RR.[69] Initially, TNNTs were
synthesized through anodic oxidation on TiNb alloy sheets. Subsequently,
CuFeO2/TNNTs were constructed by coating precursor
solution on TNNTs, followed by annealing in an argon atmosphere. The
high heat stability of TNNTs preserved the well-maintained structure of
regular nanotube arrays. Additionally, TNNTs exhibited semiconductor
properties comparable to those of n-type TiO2, enabling
their integration with p-type CuFeO2 to form a p-n
heterojunction. As a result, the CuFeO2/TNNTs
photocathode exhibited high light absorption and accelerated carrier
transport due to a suitable band gap and the presence of the p-n
heterojunction. In addition, the CuFeO2/TNNTs
photocathode demonstrated an outstanding photocurrent of 80 μA
cm–2, resulting in
C2H5OH production at a rate of 3.3
μmol/5h·cm–2. Wang et al. presented a photocathode,
designated as BCW-X, which was achieved by depositing a
Bi2WO6/BiOCl heterojunction onto an
F-SnO2 substrate through an in-situ hydrothermal
process.[70] Notably, the exposed pristine (101)
crystal plane of BiOCl transformed into the (112) plane in the
heterojunction, facilitated by the excellent compatibility between the
(112) planes of BiOCl and the (113) planes of
Bi2WO6. Simultaneously, the
heterojunctions in BCW-X maintained a 2D layered structure, thereby
enhancing the efficiency of e–/h+pair separation. PEC experiments were conducted in a
BCW-6|KHCO3|BiVO4 PEC
cell under illumination from an Xe lamp, with an external voltage
ranging from −0.6 to 1.1 V. The
BCW-6|KHCO3|BiVO4system achieved a C2H5OH yield rate of
600 μmol h–1 g–1 with an
exceptional selectivity of 80% at −1.0 V.
Wang et al. reported, for the first time, a
g-C3N4/ZnTe type-II heterojunction
photocathode for PEC CO2RR to
C2H5OH.[71] This
heterojunction facilitated the efficient separation of photoexcited
e–/h+ pairs and promoted electron
transfer from ZnTe to g-C3N4, driven by
the establishment of an interfacial internal electric field (IEF)
created between the two semiconductors. The
g-C3N4/ZnTe photocathode displayed a
remarkable C2H5OH production rate of
17.1 μmol cm–2 h–1 at −1.1 Vvs. Ag/AgCl. Furthermore, DFT calculations suggested that the
collaboration between ZnTe, with high CO2 adsorption
ability, and g-C3N4, rich in pyridinic
N, played a role as a CO-producing site, achieving the C–C coupling
process through the adsorption of CO with proton-coupled electron
transfer. Wang et al. utilized 3D C/N-doped heterojunctions of
Znx:Coy@Cu as a photocathode for PEC
CO2RR to paraffin products.[72]The Znx:Coy@Cu photocathode consisted of
p-type semiconductor Co3O4 and n-type
semiconductor ZnO on Cu foam, forming heterojunctions with various
active sites that led to outstanding C–C coupling for paraffin product
generation. Upon exposure to light irradiation in a PEC cell, the
Znx:Coy@Cu photocathode generated
photoexcited e–/h+ pairs, which
were rapidly separated by the built-in electric field, leading to
enhanced mobilities of charge carriers, as illustrated in Figure 5G.
Electrons could migrate from the CB of p-type
Co3O4 to the CB of n-type ZnO, while
holes were either trapped by electrons from the PEC cell circuit or
reacted with OH. Therefore, the high concentration of photoelectrons was
captured by protons on the surface, resulting in the formation of
abundant active hydrogen atoms capable of converting multiple
CO2 molecules into paraffin products. The
Znx:Coy@Cu photocathode demonstrated its
optimal PEC performance by achieving a paraffin yield rate of 325 μg
h–1 at −0.4V vs. SCE, all while avoiding the
release of H2, as observed in Figure 5H.