Homogeneous solution assembled Turing structures with near zero strain semi-coherence interface

The study addresses the challenge of charge recombination in thin-film devices (e.g., inorganic perovskite and organic solar cells, and photoelectrochemical cells), which limits efficiency and device lifetime. Bulk heterojunction and hierarchical heterojunction strategies improve charge separation but suffer from interface issues like lattice mismatch and strain, especially over large areas. Nature inspires interface architectures with abundant, smooth interfaces (e.g., Turing patterns) arising from reaction–diffusion with activator–inhibitor species of different diffusivities. While Turing-type structures have been realized in heterogeneous systems (e.g., interfacial polymerization membranes, diffusion-driven cation exchange on nanostructures), an inorganic Turing structure (spots/stripes) from a homogeneous solution has not been realized due to similar diffusion coefficients of small molecules. The purpose is to realize and understand Turing structures in homogeneous solutions for spinel ferrite films, achieving near-zero strain semi-coherent interfaces to enhance built-in fields and charge separation.

Prior work has increased charge separation via bulk heterojunctions and hierarchical heterojunctions (core–shell, layered), but fabrication challenges and lattice mismatch limit scalability and reproducibility. Turing-type patterns have shown utility across disciplines and in materials: e.g., Turing-structured polyamide membranes via aqueous–organic interfacial polymerization with superior desalination; diffusion-driven Ag2Se formation on CoSe2 nanobelts in heterogeneous solvents yielding high OER efficiency. However, these examples are in heterogeneous environments; homogeneous-solution formation of inorganic Turing structures remained unreported due to diffusion-coefficient similarity among small molecules. This study builds on reaction–diffusion theory and HSAB concepts to create sufficient diffusivity contrast in a homogeneous medium.

• Materials synthesis: Atomic-level Turing interface films were fabricated by spray pyrolysis from methanolic solutions containing zinc acetate and iron(III) 2,4-pentanedionate (Fe[C5H7O2]3) at defined Zn:Fe ratios (e.g., 1:3), sprayed onto FTO at 400 °C, followed by calcination (600 °C 1 h, then 700 °C 10 min). Analogous procedures produced Mg:Fe and Cu:Fe systems (using Mg/Zn/Cu acetates), Zn-doped α-Fe2O3, as well as powders via drying and calcination. Conventional dual-phase interface films were prepared by electrophoretic deposition; non-Turing dual-phase films used FeCl3 or Fe(NO3)3 instead of Fe[C5H7O2]3.
• Characterization: XRD with Rietveld refinement for phase ratios; XPS for surface composition; SEM/TEM/HAADF-STEM for morphology and atomic interfaces; SAED; geometric phase analysis (GPA) for strain (εxx, εxy, εyy); atom probe tomography (APT) for 3D composition; optical microscopy for in situ droplet evolution; AFM/KPFM and conductive AFM; PL/TRPL; PEIS; OCP transients and TPV; ICP for ion concentrations; diffusion coefficients via diaphragm cell and NMR DOSY.
• Mechanistic chemistry: HSAB-driven re-coordination assessed via DFT (Gaussian16, B3LYP/6-311G(d,p), SMD methanol) to evaluate molecular electrostatic potentials, frontier orbitals, ionization energies/electron affinities to estimate chemical hardness. Demonstrated re-coordination exchange in mixed solutions (Fe[C5H7O2]3 with Zn(CH3COO)2) yielding Zn[C3H7O2]2 and Fe(CH3COO)3, affecting diffusivities.
• Diffusion dynamics: 2D DOSY NMR measured anion diffusion (CH3COO− ~4× faster than [C3H7O2]−). Diaphragm-cell diffusion quantified orders-of-magnitude differences in systems using FeCl3, Fe(NO3)3, or Fe[C5H7O2]3 at various Zn:Fe ratios.
• MD simulations: NPT (298 K, 10 ns) modeled mixed solutions (e.g., Fe(NO3)3 + Zn(CH3COO)2; Fe[C5H7O2]3 + Zn(CH3COO)2) showing reduced Zn2+ mobility due to strong affinity with [C3H7O2]− and enhanced Fe3+ mobility in coordinated systems, yielding large activator/inhibitor diffusivity contrast.
• Numerical simulation: Reaction–diffusion equations (predator–prey style) solved via finite differences in MATLAB to generate Turing spots/stripes under parameter regimes matching experimental diffusivity ratios, showing pattern evolution with time and sensitivity to diffusion-coefficient disparity and initial conditions.
• PEC testing: Three-electrode measurements in 1 M NaOH under AM 1.5G; J–V curves, Faradaic efficiencies by GC; ηsep and ηinj via hole scavenger (Na2SO3); band alignment by Mott–Schottky; TPV dynamics; KPFM under 405 nm illumination; C-AFM current mapping.

• Successful formation of Turing structures (spots/stripes) from homogeneous solutions by exploiting HSAB-driven re-coordination to create large diffusivity contrasts between activator and inhibitor species. ZnFe2O4–α-Fe2O3 dual-phase films display characteristic staggered microstructures and abundant smooth interfaces.
• APT confirms pervasive ZnFe2O4 and α-Fe2O3 interfacial regions without parasitic phases; Turing patterns are not due to simple phase separation.
• Diffusion control: CH3COO− diffuses ~4× faster than [C3H7O2]− (DOSY). Re-coordination in Fe[C5H7O2]3 + Zn(CH3COO)2 reduces Zn2+ (activator) diffusion via coordination with [C3H7O2]− and releases Fe3+ (inhibitor) to diffuse faster; MD reveals order-of-magnitude differences in Zn2+ diffusion between Fe(NO3)3 and Fe[C5H7O2]3 systems and strong Zn2+–[C3H7O2]− affinity.
• Interface crystallography and strain: HAADF-STEM and GPA show α-Fe2O3(001)/ZnFe2O4(111) semi-coherent interfaces with near-zero interfacial strain (εxx, εxy, εyy ≈ 0), contrasting larger strain in conventional dual-phase films. A β-Fe2O3 intermediate forms during pyrolysis and transforms to α-Fe2O3 upon annealing, maintaining low lattice mismatch and semi-coherence.
• Droplet evolution correlates with pattern formation: Larger diffusivity differences yield multiple nucleation centers (spots) that coalesce into stripes as differences decrease; minimal differences yield smooth films without Turing patterns.
• Photoelectrochemical performance: Turing interface film (Zn:Fe=1:3) achieves 2.56 mA cm−2 at 1.6 V vs RHE under 1 sun without cocatalysts; Faradaic efficiencies: O2 93.1%, H2 95.8%. Charge separation efficiency ηsep reaches 44.1% at 1.6 VRHE, among the highest for iron-based films. Compared to conventional dual-phase and non-Turing dual-phase films, the Turing film shows a 49-fold and 5-fold enhancement in charge separation efficiency at 1.6 V vs RHE, respectively.
• Transient and interfacial measurements: OCP shows accelerated decay and larger photovoltage; carrier lifetime during decay ~0.8 s vs ~2.6 s for ZnFe2O4, indicating strong built-in fields; TPV reveals enhanced fast drift (<1e−5 s) and interparticle diffusion (>1e−4 s) separation, with a distinct slow-process peak due to interfacial fields; KPFM under illumination shows positive CPD and localized current bursts (C-AFM), evidencing nanoscale charge separation. PEIS shows lowest interfacial resistance for Turing films.
• Generality and scalability: Similar volcanic performance trends vs α-Fe2O3 content; consistent results across fabrication methods (spray, drop-cast, spin-coat) and substrates; extension to Zn-doped α-Fe2O3, CuFe2O4, and MgFe2O4; demonstration of large-area (17×15 cm) films for green H2 production and wastewater purification driven by silicon solar cells (1.5 V).

The work addresses the central challenge of forming inorganic Turing structures in homogeneous solutions by engineering activator–inhibitor diffusivity contrast through HSAB-driven re-coordination. This induces diffusion-driven instability during film formation, yielding spot/stripe architectures. The resulting semi-coherent interfaces with near-zero strain minimize dangling bonds and defects, enhancing interface contact and built-in electric fields. These fields promote efficient charge separation and lower recombination, as corroborated by OCP, TPV, KPFM, and PEIS. Numerical reaction–diffusion simulations reproduce experimental pattern evolution, linking chemistry-controlled diffusivities to Turing space conditions. The strategy is general across spinel ferrites and scalable, suggesting a broad route to interface-engineered materials for energy conversion.

The study demonstrates that homogeneous-solution processing can yield inorganic Turing structures with near-zero-strain semi-coherent interfaces by leveraging re-coordination chemistry to create sufficient diffusion coefficient disparity. Atomic-level characterization confirms abundant, smooth α-Fe2O3/ZnFe2O4 interfaces with minimal strain, which substantially enhance built-in electric fields and charge separation, boosting PEC performance (e.g., 2.56 mA cm−2 at 1.6 V vs RHE; ηsep 44.1%). The approach is versatile (extended to CuFe2O4, MgFe2O4, Zn-doped α-Fe2O3) and scalable to large areas, offering a pathway to mitigate charge recombination in thin-film energy devices. Future work could expand ligand–metal combinations guided by HSAB to tailor diffusion kinetics, explore additional semiconductor systems, and optimize interface crystallography for targeted electronic properties.