Spin selection in atomic-level chiral metal oxide for photocatalysis

Photocatalysis, aiming to mimic the efficiency of natural photosynthesis, faces challenges in carrier separation and water oxidation. Natural photosynthesis utilizes spin-dependent steps; for example, Photosystem II acts as a spin filter for oxygen evolution, favoring triplet-state O2 formation. Electron transfer in Photosystem I is also highly spin-dependent. While many studies focus on improving photo(electro)catalysis, manipulating spin-dependent properties remains crucial. Previous research has explored regulating spin configurations and spin-dependent electron transfer in catalysts, often by tuning coordination structures or valence states of transition metals, or by applying external magnetic fields. A more universal approach involves using spin filters, such as chiral structures, which exhibit the chiral-induced spin selectivity (CISS) effect. This effect arises as the electron's velocity and spin direction change, acting as an effective magnetic field that controls electron spin and breaks spin energy level degeneracy. While the CISS effect has been observed in chiral molecules, its application in chiral inorganic materials for photocatalysis remains largely unexplored. This study focuses on fabricating chiral ZnO photocatalysts to investigate the impact of chiral structures on photocatalytic activity.

Extensive research has focused on enhancing photocatalysis by manipulating the spin degree of freedom. Studies have highlighted the importance of spin configurations and orbital interactions in electrocatalysts like NixFe1−xOOH and FeN4 for oxygen evolution and reduction reactions, respectively. Methods include creating magnetic ordering structures or applying external magnetic fields to generate spin-polarized electrons. In photocatalysis, researchers have manipulated spin-polarized electrons in Ti-defected TiO2 and regulated cobalt spin states in COF-367-CoII to improve activity and selectivity. Magnetic field promotion of photocatalytic water splitting has also been explored. However, these approaches often rely on fine-tuning specific metal centers. Utilizing chiral structures as universal spin filters offers a potentially more versatile approach. While chiral molecules have shown promise in enhancing spin selectivity, directly incorporating chirality into inorganic oxides and understanding its effects on photocatalysis remains a challenge. This work addresses this gap by investigating chiral ZnO.

Chiral ZnO photocatalysts were synthesized using an amino acid-induced self-assembly method. L- and D-methionine, possessing mirror-image symmetric transmitted circular dichroism (TCD) spectra, were used as chiral inducers for asymmetric coordination with Zn2+ ions. Hydrothermal synthesis followed by pyrolysis produced L- and D-ZnO, with achiral DL-ZnO synthesized for comparison. Various characterization techniques were employed: XRD, SEM, TEM, EDX, XPS, UV-Vis absorption spectroscopy, Mott-Schottky plots, and TCD spectroscopy to confirm the successful synthesis of chiral ZnO and to analyze its structure, composition, and optical properties. The chiral structure at both nano- and atomic levels was confirmed via TEM and SAED analyses. Spin-polarized CPL emission tests and MCD spectroscopy were used to investigate spin polarization in chiral ZnO, with mc-AFM used to evaluate spin-selective charge transport. Photoelectrochemical measurements, including linear sweep voltammetry (LSV) and incident photon-to-current efficiency (IPCE) measurements, were conducted to assess the photoelectrocatalytic water oxidation activity. Photocatalytic oxygen evolution and RhB photodegradation experiments were performed to evaluate the photocatalytic activity of the chiral and achiral ZnO. Transient-absorption spectroscopy (TAS) was used to monitor charge carrier dynamics. Rotating ring-disk electrode (RRDE) measurements and H2O2 detection using o-tolidine were performed to investigate electron transfer numbers and H2O2 formation. Finally, in situ DMPO-trapped EPR experiments were used to measure the concentration of OH radicals.

The chiral ZnO exhibited hierarchical chirality at both the nanoscale and atomic levels, confirmed by various characterization methods. The chiral structure acted as an effective spin filter, inducing spin polarization of photoexcited carriers, as evidenced by spin-polarized CPL emission, MCD spectroscopy, and mc-AFM. The spin polarization resulted in significantly extended charge carrier lifetimes, as revealed by transient-absorption spectroscopy (TAS). The chiral ZnO showed a 2.4 (1.2) times longer carrier lifetime and a ~5.5 fold reduction in singlet byproduct (H2O2) formation compared to the achiral ZnO. Consequentially, the chiral ZnO demonstrated significantly enhanced photocatalytic activity, with L- and D-ZnO exhibiting 1.9-2.0 times higher O2 production and 2.0-2.5 times higher RhB photodegradation rates than the achiral ZnO. The enhanced activity is attributed to the spin-polarized carriers, which promoted triplet oxygen formation and inhibited the formation of singlet byproducts like H2O2, aligning with spin conservation principles. The physical mixture of L-ZnO and D-ZnO showed similar activity to individual chiral ZnO samples, suggesting that spin selection occurs within individual particles.

The findings demonstrate that manipulating the spin properties of metal oxides via the introduction of inherent chiral structures can significantly boost photocatalytic performance. The enhanced activity stems from the CISS effect, leading to spin-polarized carriers with extended lifetimes and preferential formation of triplet products. This work highlights a previously unexplored approach for enhancing photocatalysis, providing a versatile strategy beyond traditional methods focusing on specific metal centers. The success of this approach opens new avenues for designing and synthesizing highly efficient photocatalysts. The results support the fundamental importance of spin considerations in designing efficient photocatalytic materials and processes.

This study successfully fabricated chiral ZnO photocatalysts exhibiting enhanced photocatalytic activity due to chirality-induced spin selectivity. The chiral structure acted as a spin filter, leading to spin-polarized carriers with extended lifetimes and increased triplet product formation. The improved activity highlights the importance of considering spin effects in photocatalyst design. Future research should focus on expanding this approach to other materials and exploring strategies for further enhancing spin polarization and photocatalytic performance.

The study focused on ZnO as a model system. While the findings demonstrate the potential of the chiral approach, further investigations are needed to assess the generalizability of this strategy to other semiconductor materials. The synthesis method may need optimization for scalability and cost-effectiveness. A more comprehensive understanding of the underlying mechanisms involved in spin-dependent charge separation and reaction kinetics would enhance the applicability of this approach.