Spin selection in atomic-level chiral metal oxide for photocatalysis

Photosynthesis relies on spin-dependent steps in both water oxidation and electron transport. In Photosystem II, high-spin Mn4 sites act as spin filters, extracting electrons with the same spin orientation to form spin-aligned •OH intermediates, which favor triplet-state O2 formation and suppress singlet byproducts like H2O2. Electron transfer in Photosystem I is also highly spin-dependent due to spin angular momentum conservation, Hund’s rule, and Pauli exclusion principle, and parallel alignment of spin and momentum suppresses backscattering. Artificial photosynthesis systems (photo/electrocatalysis) remain limited by inefficient charge separation/migration and sluggish water oxidation, motivating manipulation of spin properties in photocatalysts. Prior strategies tune spin via transition-metal coordination, oxidation state, magnetic ordering, or external magnetic fields. A more universal approach is to use spin filters based on chiral-induced spin selectivity (CISS), demonstrated in chiral biomolecules and inorganic materials, enabling spin-selective transport and long-range electron transfer. However, chiral macromolecule–semiconductor assemblies can suffer from instability, conductivity barriers, and blocked active sites. Directly constructing chiral oxides is attractive but underexplored, and the impact of nanoscale chirality on charge transfer and oxygen evolution in photocatalysis has not been addressed. This study fabricates chiral ZnO via asymmetric coordination with chiral methionine to test whether intrinsic chirality can induce spin polarization, prolong carrier lifetime, suppress singlet byproducts (e.g., H2O2), and enhance photocatalytic oxygen evolution and pollutant degradation relative to achiral ZnO.

Research has highlighted spin configuration and orbital interactions as key descriptors in oxygen electrocatalysis, with materials such as NixFe1−xOOH and FeN4 showing spin-dependent activity. Spin-polarized channels can be established via magnetic ordering or external magnetic fields, improving interfacial electron transfer. In photocatalysis, spin effects have enhanced charge separation (e.g., spin-polarized Ti-defected TiO2, magnetic-field-boosted Fe3O4/N-TiO2 systems) and selectivity (e.g., Co spin-state regulation in COF-367-CoII). The CISS effect in chiral molecules and inorganic materials enables spin-selective transport and chiroptical responses, though quantitative links between optical response and CISS magnitude remain challenging. Chiral molecule assemblies on semiconductors (TiO2, Fe3O4) have accelerated oxygen evolution, but issues of stability, conductivity barriers, and active site shielding persist, motivating inherently chiral oxide semiconductors as spin filters for photocatalysis.

Synthesis of chiral ZnO: An amino acid-induced self-assembly method was used. Typically, 2 mmol of L-, D-, or DL-methionine was dissolved in 30 mL water, followed by dissolving 3 mmol zinc acetate at 0 °C. Activated FTO glass was immersed; 1 mmol ammonium carbonate was added and stirred for 30 min. The mixture with FTO was hydrothermally treated in a 50 mL Teflon-lined autoclave at 120 °C for 2 h. After washing and drying (60 °C), films were calcined at 550 °C for 6 h to yield L-, D-, or DL-ZnO films. Powder samples were collected by centrifugation from the same reaction and similarly calcined. Structural characterization: XRD verified wurtzite ZnO phase; SEM/TEM showed curved ultrathin nanosheets (~1.35 ± 0.3 µm film thickness) with 0.26 nm lattice fringes ((002) plane). EDX mapping confirmed uniform Zn/O distribution; BET indicated similar surface area and mesoporosity. XPS showed Zn2+ (Zn 2p1/2 at 1046.03 eV, 2p3/2 at 1022.98 eV) and O 1s components (lattice at 530.5 eV, adsorbed oxygen at 532.1 eV). UV-vis absorption/Mott–Schottky/XPS VB indicated similar band edges with 380 nm absorption edge (Eg ~3.08 eV). Chirality characterization: SAED and tilt-series TEM established helically bent nanoplates (left-handed for L-ZnO; right-handed for D-ZnO) with regional zone-axis misalignment requiring tilts of 4.28° and 8.55°. HRTEM/FFT showed atomic-level rotational distortion (anticlockwise ~−4.0° in L-ZnO; clockwise ~+3° in D-ZnO), absent in DL-ZnO. Transmitted circular dichroism (TCD) spectra exhibited mirror-image responses for L- and D-ZnO; after water infiltration to remove scattering-based optical activity, intrinsic absorption-based OA peaked near 380 nm with opposite sign for L and D. Spin-polarization measurements: Circularly polarized luminescence (CPL) under 325 nm excitation probed spin-dependent recombination; L-ZnO emitted left-CPL (positive CPL), D-ZnO the opposite; DL-ZnO showed none. Magnetic circular dichroism (MCD) under ±1.6 T fields showed symmetric positive (L) and negative (D) signals near the band edge independent of field direction; DL-ZnO showed no MCD signal; VSM showed nonmagnetic behavior, ruling out static spin splitting. Magnetic conductive-probe AFM (mc-AFM) measured J–V with tip magnetized up/down: L-ZnO exhibited higher current for down-magnetized tip and lower for up; D-ZnO showed the opposite; DL-ZnO showed no dependence. Spin polarization P = (Jup − Jdown)/(Jup + Jdown) ×100% was −85% (L) and +71.6% (D) at 5 V. Photoelectrochemistry: Three-electrode setup (CHI 660E) with 300 W Xe lamp (AM 1.5, 100 mW cm−2), 0.5 M Na2SO4 (pH ~6.8). LSV at 10 mV s−1; potentials converted to RHE. IPCE measured; ABPE evaluated. RRDE: ZnO inks (5 mg in water/ethanol + Nafion) drop-cast on glassy carbon disk with Pt ring; operated in 0.5 M Na2SO4 at 1000 rpm under irradiation; electron transfer number calculated. H2O2 detection: Photoelectrochemical oxidation at 1.23 V vs RHE for 3 h in 0.5 M Na2SO4 (pH 6.5); mixed electrolyte with o-tolidine and measured absorbance at 436 nm. Photocatalytic tests: Oxygen evolution in a closed top-irradiation Pyrex reactor (280 mL) with 20 mg catalyst decorated with CoOx, in 100 mL H2O with 0.01 M AgNO3 sacrificial agent; Ar purged; 300 W Xe lamp; 15 °C; O2 quantified by GC (TCD). RhB degradation: 20 mg ZnO in 100 mL RhB (200 µM); dark equilibration; Xe lamp irradiation; aliquots every 20 min for UV–vis analysis. Transient absorption spectroscopy: Femtosecond pump–probe (340 nm pump, ~50 µJ cm−2, spot ~150 µm) recorded spectra and decay kinetics; tri-exponential fitting extracted lifetimes.

• Intrinsic chirality in ZnO was realized by asymmetric coordination with L- or D-methionine, yielding hierarchical chirality: helically bent nanoplates and atomic-level rotational distortion; DL-ZnO was achiral.
• Chiroptical properties: L- and D-ZnO showed mirror-image TCD and absorption-based OA peaks near 380 nm; CPL indicated spin-dependent recombination (L-ZnO emitted left-CPL; D-ZnO right-CPL); DL-ZnO showed no CPL.
• MCD spectroscopy revealed positive (L) and negative (D) symmetric signals independent of magnetic field direction, consistent with dynamic CISS-induced spin polarization near the band edge; samples were nonmagnetic (no hysteresis).
• mc-AFM demonstrated strong spin-selective transport: at 5 V, spin polarization P was −85% (L-ZnO) and +71.6% (D-ZnO); DL-ZnO showed no spin selectivity.
• Photoelectrochemical performance (1 sun): Photocurrent density at 1.23 V vs RHE increased from 0.21 mA cm−2 (DL) to 0.43 (L) and 0.40 (D) mA cm−2. IPCE at 370 nm: 36% (L), 27% (D) vs 16% (DL). ABPE at 0.76 V vs RHE: 0.175–0.19% (L/D) vs 0.11% (DL).
• Photocatalytic oxygen evolution (powders) and RhB degradation: O2 production enhanced by 2.0× (L) and 1.9× (D) vs DL; RhB pseudo-first-order rate constants increased by 2.5× (L) and 2.0× (D) vs DL. Physical mixture L+D matched single-component performance, indicating spin selection within single particles.
• Transient absorption: Average carrier lifetimes increased from 1764.8 ps (DL) to 4218.9 ps (L; 2.4×) and 2173.5 ps (D; 1.2×); long-lived τ3 contribution increased from 31.3% (DL) to 40.6% (L) and 37.3% (D), indicating suppressed recombination due to spin polarization.
• Reaction pathway selectivity: RRDE showed higher electron transfer numbers for L/D-ZnO, indicating suppression of 2e− H2O2 pathway. o-Tolidine assay revealed ~5.5-fold reduction of H2O2 signal for L/D vs DL. In situ EPR (DMPO–•OH) showed ~2× higher •OH signal after 8 min for L/D vs DL. These confirm spin-polarized holes favor triplet O2 formation and suppress singlet byproducts (H2O2).

The chiral potential in ZnO acts as an effective spin filter (CISS), aligning spin orientations of photoexcited carriers. Spin polarization inhibits electron–hole recombination by enforcing spin angular momentum conservation, leading to significantly extended carrier lifetimes and enhanced long-lived components that are crucial for interfacial charge transfer. At the reaction interface, spin-polarized holes promote spin-allowed pathways, favoring triplet O2 formation and suppressing singlet byproduct (H2O2) formation, thereby increasing faradaic efficiency for oxygen evolution and improving photocatalytic degradation rates. The consistent chiroptical (CPL, MCD) and spin-transport (mc-AFM) signatures, along with kinetic (TAS) and product-selectivity (RRDE, o-tolidine, EPR) evidence, collectively demonstrate that intrinsic chirality in a nonmagnetic oxide semiconductor can regulate spin to boost both charge dynamics and surface reaction kinetics. This establishes a generalizable route to leverage spin for enhancing photocatalytic processes.

The study demonstrates that introducing intrinsic chirality into ZnO via amino acid–assisted synthesis produces strong chiral-induced spin selectivity, generating spin-polarized carriers without magnetic ordering. Chiral L- and D-ZnO exhibit prolonged carrier lifetimes (2.4× and 1.2× vs DL), suppressed H2O2 formation (~5.5× lower), and substantially improved performance: up to 2.0× higher O2 evolution, 2.5× higher RhB degradation rates, and enhanced PEC figures of merit (photocurrent, IPCE, ABPE). These results show that atomic- and mesoscale chirality can serve as an effective design principle to manipulate spin in metal-oxide photocatalysts, offering a new avenue to improve carrier dynamics and spin-dependent surface reactions. Future research should focus on developing universal strategies to fabricate a broad range of chiral photocatalysts and exploring their applicability across diverse photo(electro)catalytic reactions.

The authors note that developing a universal strategy to fabricate chiral photocatalysts is a key issue for further work; explicit additional limitations are not discussed.