Chiral Mesostructured ZnIn2S4 Photocatalyst

• The paper introduces a CMZI photocatalyst achieving a record 962 μmol·g⁻¹·h⁻¹ acetic acid yield with 97.3% selectivity via spin-polarized C–C coupling.
• It employs a 3D chiral framework with ordered sulfur sites on (102) facets, enhancing CO₂ adsorption and stabilizing key reaction intermediates.
• The catalyst design leverages chirality-induced spin selectivity and site-specific reactivity to streamline hydrogenation, paving the way for scalable multicarbon synthesis.

A chiral mesostructured ZnIn₂S₄ (CMZI) photocatalyst is a functional inorganic material designed for highly selective and efficient photocatalytic reduction of CO₂ to acetic acid. Distinguished by its chiral framework and ordered sulfur sites on the (102) crystal facets, CMZI achieves a record acetic acid yield of 962 μmol·g⁻¹·h⁻¹ and selectivity of 97.3%, exceeding prior benchmarks by a factor of ten. This performance is attributed to the cooperative interaction between chirality-induced spin polarization and the site-specific reactivity of sulfur atoms, promoting C–C coupling via triplet-state intermediate stabilization and facilitating streamlined hydrogenation steps. The theoretical and experimental results position CMZI as a template for next-generation catalysts that enable scalable, efficient synthesis of multicarbon products through precise manipulation of electron spin and reactive site chemistry (Cui et al., 21 Sep 2025).

1. Structural and Chemical Design of CMZI

The chiral mesostructured ZnIn₂S₄ catalyst is architected with a three-dimensional mesoscale chiral framework. The chirality arises from an ordered arrangement of the lattice, leading to macroscopic asymmetry and spin-dependent charge transport properties. Key design aspects include:

• Chiral Mesostructure: Left-handed (L-CMZI) and right-handed (D-CMZI) polymorphs exhibit mirror asymmetry.
• Sulfur Sites on (102) Facets: Surface analysis and DFT calculations point to S atoms on the (102) crystal facets as the dominant active sites for adsorption and chemical transformation.
• Particle Morphology: The mesostructured nature enables enhanced photonic and electronic coupling, with increased surface area and accessible active sites relative to conventional ZnIn₂S₄.

The configuration ensures the co-localization of chiral and electronic effects, providing both spin-selective charge transport and catalytic activity.

2. Mechanistic Overview: Spin Polarization and ³OCCO Intermediate Stabilization

The mechanism underpinning CMZI’s enhanced C–C coupling efficiency is founded on chirality-induced spin selectivity (CISS):

• CISS Effect: The chiral framework generates asymmetric spin–orbit coupling, initiating spin polarization of photoexcited electrons. Experimental techniques such as magnetic tip CAFM and circularly polarized transient absorption confirm selective transfer of spin-up (L-CMZI) or spin-down (D-CMZI) electrons.
• Triplet OCCO Formation: CO₂ is first photoreduced to *CO. Two adjacent *CO species on the surface dimerize to form an OCCO intermediate, which—due to CISS—exists preferentially in a triplet configuration (³OCCO).
• Stabilization Dynamics: The parallel spin orientation at the catalyst interface reduces electron–electron repulsion and stabilizes the ³OCCO intermediate, as dictated by the Pauli exclusion principle. In achiral photocatalysts, the singlet OCCO (¹OCCO) is destabilized and prone to dissociation, limiting C–C coupling and multicarbon product selectivity.

The consequence is a metastable ³OCCO intermediate, which directly increases the probability and yield of downstream hydrogenation into acetic acid.

3. Surface Chemistry: Function of Sulfur Sites on (102) Facets

High CMZI yield and selectivity derive from the energetics and reactivity of S atoms on the (102) facets:

• Adsorption Energies: DFT calculations report enhanced affinity for CO binding at S sites, as quantified by Bader charge analysis.
• Reaction Pathway Selectivity: S sites preferentially catalyze the exothermic hydrogenation of *CO to *CHO rather than *COH, enabling a low-activation energy route for acetic acid formation. The pathway is further favored compared to ethanol production, with the final *CH₃CO → *CH₃COOH transformation possessing a minimal energy barrier (0.64 eV).
• Intermediates and Progression: Stabilization and sequential hydrogenation of OCCO-based intermediates (*OCCO⁻, *HOCCO, *CH₂CO, *CH₃COO) at S sites facilitates efficient acetic acid release.

The integration of S site reactivity with spin-polarized intermediates streamlines multistep reduction cycles and suppresses competitive side reactions.

4. Theoretical Models and Reaction Sequence

Calculation and modeling details are central to elucidating the reaction energetics:

• Adsorption Energy Formula:

$$E_{\text{ads}}(X) = -\left(E_{X/\text{slab}} - E_{\text{slab}} - E_X\right)$$

• Gibbs Free Energy for Elementary Steps:

$G = E + \text{ZPE} - T\,S$

where $E$ is the DFT energy, ZPE is zero-point vibrational energy, $T$ the temperature (typically 298.15 K), and $S$ the entropy.

• Overall Photocatalytic Reaction:

$$CO_2 + 8\, H^+ + 8\, e^- \rightarrow CH_3COOH + H_2O$$

A plausible implication is that the triplet pathway lowers overall kinetic barriers and shifts reactant flux toward acetic acid rather than competitive alcohol or hydrocarbon products.

5. Performance Metrics and Yield-Selectivity Relationship

Chiral mesostructured ZnIn₂S₄ displays metrics significantly surpassing previous benchmarks:

Catalyst	Acetic Acid Yield (μmol·g⁻¹·h⁻¹)	Selectivity (%)
CMZI (this work)	962	97.3
Best prior	≤96.2	Unspecified

The tenfold improvement in acetic acid yield (relative to established literature) is coupled with state-of-the-art selectivity, underscoring the advantage afforded by the chiral/S-site synergy.

6. Impact for Catalytic Strategies and Multicarbon Product Synthesis

CMZI offers strategic insight into photocatalyst design for sustainable CO₂ conversion:

• Spin Control for Pathway Steering: Engineering chirality in inorganic matrices introduces deterministic spin dynamics, which can be leveraged to selectively stabilize intermediates central to multicarbon product formation.
• Scalable Synthesis Template: CMZI’s mechanistic features—demonstrated via high-yield acetic acid production—can be generalized to other chiral, site-engineered photocatalysts for diverse chemical targets.
• Industrial Relevance: Translation of CMZI architecture to commercial-scale systems could provide energy-efficient and selective synthesis routes for value-added chemicals directly from CO₂, aligning with both carbon economy and chemical feedstock demands.

This suggests that future catalyst development should systematically integrate spin-dependent frameworks and facet-specific chemical design to maximize both throughput and efficiency in photocatalytic CO₂ reduction.

7. Current Research Trajectory and Prospective Applications

Recent advances exemplified by CMZI highlight:

• Rational Catalyst Design: Precise control of chirality and facet chemistry emerges as a routine method for tuning spin-based reaction networks.
• CO₂ Utilization Technologies: Adoption of CMZI-like catalysts may facilitate scalable production of acetic acid and analogous multicarbon products, with direct implications for climate-responsive chemical engineering.
• Expansion into Other Reaction Classes: The mechanistic principles—spin polarization, facet engineering, and site-specific catalysis—may extend to transformations beyond acetic acid synthesis, including ethanol, ethylene, or longer-chain organics.

A plausible implication is that chiral mesostructured semiconductors are poised to redefine selectivity control in heterogeneous photochemistry, with the CMZI system serving as an instructive case paper for future innovation.