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.