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.