Breathable, water-repellent γ-MnOOH nest production by vapor-assisted conversion

Abstract

Bird nests, formed by slender, semi-flexible twigs, are lightweight, freestanding porous frameworks with excellent permeability to gases and liquids. Translating this concept to microscale inorganic materials promises a broad impact on engineering and biomedical applications. However, constructing freestanding porous architectures from rigid one-dimensional (1D) particles remains challenging due to the directional processing and sparse interparticle contacts. To overcome these limitations, an in situ route for growing high-aspect-ratio 1D crystals within powder compacts is developed, yielding abundant, randomly oriented interparticle contacts. Exposing MnCO₃ to H₂O₂/H₂O vapor at 200 °C drives vapor-assisted oxidative conversion to γ-MnOOH, producing hierarchical, nest-like porous frameworks. The resulting γ-MnOOH bodies exhibit high water permeability (≈140 L m⁻² h⁻¹) and rapid evaporation (0.45–0.56 L m⁻² h⁻¹ at 40 °C), and simple postsynthetic surface modification imparts water repellency (contact angle of 169°). Because oxyhydroxides are versatile precursors for ceramic oxides, this approach provides a platform for entangled, binder-free inorganic architectures. This establishes a scalable and straightforward route to breathable porous materials, guided by the design logic of nature.

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1. Introduction

Bird nests are a remarkable example of natural engineering, in which slender, semi-flexible twigs are intricately interwoven by frictional and geometrical interactions.^1–3 These structures form lightweight frameworks that remain permeable and resilient owing to their loosely entangled architecture.⁴ This bio-inspired principle suggests a route to freestanding, porous bodies in which structural integrity emerges from numerous non-directional contacts rather than from binders or sintered necks.⁵

Translating this concept to microscale inorganic materials would enable breathable, robust architectures for thermal management, acoustic damping, mass transport, and environmental interfaces.^6–9 Furthermore, employing biocompatible materials could provide cells with favorable microenvironments as biomedical scaffolds.¹⁰ However, constructing freestanding porous architectures from rigid one-dimensional (1D) particles remains a formidable challenge. While additive manufacturing and freeze casting can impose complex shapes or align anisotropic building blocks,^11–15 they inherently rely on directional deposition or templated freezing, which promotes mechanical anisotropy. Moreover, interparticle contacts are sparse and weak in the absence of mechanical interlocking.¹⁶

To overcome the limitations of directional processing and sparse interparticle contacts inherent to the prevailing methods, we propose an in situ route that generates abundant, randomly oriented contacts by growing high-aspect-ratio 1D crystals within powder compacts. This approach proceeds via the vapor-assisted oxidative conversion of manganese carbonate (MnCO₃) to manganese oxyhydroxide (γ-MnOOH) at 200 °C in H₂O₂/H₂O vapors (Fig. 1A). Growth within confined powder compacts produces randomly oriented, interwoven 1D crystals that self-organize into nest-like porous frameworks with hierarchical features (Fig. 1B). The resulting freestanding γ-MnOOH bodies exhibit high liquid permeability and rapid evaporation and can be rendered water-repellent via postsynthetic surface modification. Because oxyhydroxides serve as versatile precursors for subsequent transformations into ceramic oxides, this approach establishes a platform for entangled, binder-free inorganic architectures. Together, these results outline a simple, scalable route to breathable porous bodies guided by the design logic of nature.

Fig. 1 Schematic illustration of a vapor-assisted conversion strategy for fabricating a porous γ-MnOOH framework. (A) Formation of needle-like γ-MnOOH aggregates from spherical MnCO₃ particles. (B) Development of nest-like porous γ-MnOOH architectures within the MnCO₃ powder compact.

2. Experimental

2.1. Synthesis of spherical MnCO₃ powder

Spherical MnCO₃ particles were synthesized via a controlled precipitation method.¹⁷ Briefly, 0.52 mol of (NH₄)₂CO₃ (FUJIFILM Wako Pure Chemical) and 60 mL of 28% NH₄OH (FUJIFILM Wako Pure Chemical) were dissolved in water and diluted to a final volume of 500 mL. This carbonate solution was added dropwise to 500 mL of 0.5 M MnSO₄ (Kishida Chemical) under continuous stirring at a rate of 15 mL min⁻¹. After complete addition, the suspension was stirred for 1 h. The resulting MnCO₃ precipitate was collected by filtration, washed with water, and dried at 100 °C in an oven.

2.2. Vapor-assisted conversion of MnCO₃ powder

The as-prepared MnCO₃ powders were subjected to vapor-phase treatment using either water or H₂O₂ (FUJIFILM Wako Pure Chemical) as the vapor source. In a typical procedure, 5 mL of water or a 5–10% H₂O₂ solution was placed at the bottom of a PTFE-lined stainless-steel autoclave (internal volume: 50 cm³). A PTFE support with a recessed top was used to hold 0.5 g of MnCO₃ powder above the liquid phase to avoid direct contact. The sealed autoclave was heated to 200 °C and maintained for 6 h. After cooling to room temperature, the treated powders were washed with water and ethanol and dried at 100 °C.

2.3. Fabrication of nest-like porous γ-MnOOH architecture

Nest-like porous γ-MnOOH architectures were fabricated via vapor-phase treatment of MnCO₃ powder compacts. The compacts were prepared via hydrothermal hot-pressing.¹⁸ Wet MnCO₃ powder containing 15 mass% water was loaded into a cylindrical mold (inner diameter: 20 mm) and compressed at 100 MPa using PTFE-sealed pistons. The mold was externally heated to 120 °C and maintained for 2 h. After cooling, the compacts were demolded, dried, and polished to a thickness of 1.5 mm. Vapor-phase treatment was then performed at 200 °C for up to 16 h in a PTFE-lined autoclave (internal volume: 250 cm³) using 20 mL of water or a 5–30% H₂O₂ solution. The compacts were placed on a support to avoid direct contact with the liquid. After treatment, the samples were dried at 100 °C. Hydrophobic modification was performed by vapor-phase silanization. The solid sample and trimethoxy(methyl)silane (KBM-13, Shin-Etsu Chemical) were placed in a steel-lined autoclave without direct contact and exposed to silane vapor at 100 °C for 2.5 h.

2.4. Characterization

Phase identification was performed using powder X-ray diffraction (XRD, D2 PHASER, Bruker AXS) with Cu Kα radiation at 30 kV and 10 mA. Data were collected with a step size of 0.02° (2θ) and a counting time of 0.5 s step⁻¹. Phase quantification was performed semi-quantitatively using the reference intensity ratio method with the DIFFRAC.EVA software (ver. 5.2.0.5, Bruker AXS). Morphological and microstructural analyses were conducted using scanning electron microscopy (SEM, JSM-6010LA, JEOL) and transmission electron microscopy (TEM, JEM-2100F, JEOL). Three-dimensional (3D) microstructural imaging was obtained using focused ion beam SEM (FIB-SEM, NB5000, Hitachi High-Tech), and 3D reconstructions were generated using Avizo software (ver. 2023.2). The pore size distributions were measured by mercury intrusion porosimetry (AutoPore V, Micromeritics), with the pore sizes calculated using the Washburn equation. The surface tension and contact angle of mercury were set to 485 mN m⁻¹ and 130°, respectively. The surface functional groups were analyzed using Fourier-transform infrared spectroscopy (FTIR, IRSpirit, Shimadzu) in attenuated total reflectance mode. Contact angle measurements were performed by vertically dispensing a 10 µL droplet at 25 °C and capturing an image after 30 s, following the method described by Alkawareek et al.¹⁹ The oil slurry used in the three-phase separation was prepared by mixing a lipophilic dye (Oil Red O, FUJIFILM Wako Pure Chemical) dissolved in linseed oil (FUJIFILM Wako Pure Chemical) with hydrophobically modified spherical Fe₃O₄ particles.

3. Results and discussion

3.1. Vapor-assisted conversion of MnCO₃ to interwoven γ-MnOOH nanorods

Hydrothermal vapor treatment of MnCO₃ in the presence of H₂O₂ induces γ-MnOOH formation via simultaneous decarboxylation, hydration, and oxidation. Manganese oxides catalyze liquid H₂O₂ decomposition, rapidly releasing oxygen at particle surfaces.^20,21 Consistently, MnCO₃ reacted immediately upon contact with liquid H₂O₂ (Fig. S1). To avoid premature reactions, MnCO₃ and H₂O₂ were spatially separated in a sealed autoclave (Fig. 1A). Spherical MnCO₃ particles (median diameter ≈2 µm) exposed to H₂O₂/H₂O vapor generated from 5% aqueous H₂O₂ at 200 °C developed surface protrusions within 0.5 h (Fig. 2A). After 1 h, they were converted into spherical aggregates composed of needle-like γ-MnOOH crystals and remained morphologically stable for 6 h. Powder XRD showed the progressive conversion of MnCO₃ to γ-MnOOH with Mn₃O₄ as a minor by-product (Fig. S2). Increasing the H₂O₂ concentration to 10% accelerated the conversion without altering the aggregate morphology (Fig. S3). In contrast, replacing H₂O₂ with water vapor markedly suppressed the formation of γ-MnOOH while facilitating the production of Mn₃O₄, yielding a mixture of spherical MnCO₃, needle-like γ-MnOOH, and polyhedral Mn₃O₄ (Fig. S4).

Fig. 2 Morphological evolution and nanoscale features of γ-MnOOH produced via vapor-assisted conversion. (A) SEM image sequence showing the transformation of spherical MnCO₃ in 5% H₂O₂ vapor. (B) TEM images of needle-like γ-MnOOH after 2 h in 10% H₂O₂ vapor: (a) overview, (b) branched nanorods, and (c) lattice-resolved tip.

The reaction proceeds at the solid–gas interface, where γ-MnOOH nucleates and grows by consuming Mn²⁺ from MnCO₃. The progressive evolution from smooth MnCO₃ spheres to surface-nucleated γ-MnOOH nanorods (Fig. 2A) indicates an interface-mediated oxidative dissolution-reprecipitation mechanism, in which Mn²⁺ ions released at the MnCO₃ surface are oxidized and reprecipitate as γ-MnOOH; the preserved spherical aggregate morphology is consistent with a surface-confined reaction front advancing inward, rather than a bulk solid-state transformation. The crystals exhibited unidirectional elongation and freely interwove (Fig. 2B(a)). High-resolution TEM indicates preferential growth along 〈101〉 while exposing facets indexed as (1̄11) (Fig. S5), behavior commonly observed in liquid-phase syntheses.^22–24 Spontaneous branching occurs during vapor-induced growth (Fig. 2B(b)), consistent with reports on hydrothermal γ-MnOOH.^25–27 We infer that rapid tip-directed growth of sword-like nanorods along 〈101〉 is followed by tip-to-tip fusion to form branches.²⁷ Moiré fringes at the branch bases suggest slight lattice rotations or interplanar mismatches. Continuous H₂O₂ supply sustains rapid crystal growth, and the exposed (111)-type facets at rod tips (Fig. 2B(c)) are conducive to twin-plane formation at branching interfaces.²⁷ A poorly crystalline Mn-oxide layer (≈3 nm) was also observed on particle surfaces (Fig. S5), likely arising from electron-beam-induced oxidation during TEM imaging.²⁷ Under the synthesis conditions (200 °C, elevated pressure), such surface activity may influence Mn-oxide stability and phase evolution.

Under hydrothermal vapor conditions, γ-MnOOH formation proceeds via coupled redox and decarboxylation reactions. Vapor-phase H₂O₂ oxidizes Mn²⁺ in MnCO₃ while decomposing into hydroxyl radicals and ions:

MnCO₃ + H₂O₂ → HO˙ + OH⁻ + [Mn³⁺⋯CO₃²⁻] (1)

The bracketed species denote the transient complex at the active site. The carbonate ion donates oxygen to the lattice and is released as CO₂, whereas hydroxide ions yield γ-MnOOH as follows:

OH⁻ + [Mn³⁺⋯CO₃²⁻] → γ-MnOOH + CO₂ (2)

Concurrently, hydroxyl radicals further oxidize adjacent MnCO₃:

HO˙ + MnCO₃ → OH⁻ + [Mn³⁺⋯CO₃²⁻] (3)

Overall:

2MnCO₃ + H₂O₂ → 2γ-MnOOH + 2CO₂ (4)

The proposed involvement of HO˙ is inferred from established literature on H₂O₂ decomposition over manganese oxides^20,21 and represents a working mechanistic hypothesis; direct verification remains a subject for future work. In the absence of H₂O₂, Mn²⁺ can be oxidized by molecular oxygen in water vapor.²⁸ Owing to its limited oxidizing power, this pathway favors Mn₃O₄ and the coexistence of Mn²⁺ and Mn³⁺ ions. In contrast, H₂O₂ vapor promotes further oxidation and hydration, ultimately producing γ-MnOOH.

This vapor-phase route directly generates nanoscale 1D materials from solid precursors, offering a simple alternative to template-assisted and liquid-phase hydrothermal methods. Conventional vapor-phase crystal growth often relies on metal substrates to establish locally high pH and ionic gradients.^29,30 In contrast, our method uses only carbonate particles and H₂O₂/H₂O vapor to drive transformation and growth. The resulting structures—spherical aggregates of randomly interwoven γ-MnOOH nanorods—faithfully preserve the morphology of the parent MnCO₃ particles, revealing a self-organized pathway to freestanding porous frameworks.

3.2. Self-organized nest-like γ-MnOOH frameworks via vapor-assisted conversion

Building on the observation that MnCO₃ particles convert into 1D γ-MnOOH aggregates under vapor exposure, we treated MnCO₃ powder compacts to realize nest-like porous architectures. The compacts were produced by a hydrothermal hot-pressing strategy that leverages interfacial dissolution–precipitation, thereby avoiding conventional high-temperature sintering and enabling binder-free consolidation with minimal water.¹⁸ Under optimized conditions, the spherical morphology of MnCO₃ was preserved during consolidation (Fig. S6).

Upon exposure to H₂O₂/H₂O vapor generated from 5–30% aqueous H₂O₂ at 200 °C for 12 h, MnCO₃ pellets underwent phase conversion to γ-MnOOH (Fig. 3A). Single-phase γ-MnOOH was obtained at 5–10% H₂O₂ vapor, whereas higher concentrations promoted further oxidation and dehydration, yielding Mn-oxide by-products (β-MnO₂ and α-Mn₂O₃). Under water vapor, Mn₃O₄ was the dominant product. Surface analysis revealed that water vapor disrupted the spherical MnCO₃ morphology, whereas H₂O₂ vapor promoted the anisotropic 1D growth of γ-MnOOH (Fig. 3B, upper). The resulting γ-MnOOH microrods (length ≈5 µm) formed interlocked networks with occasional branching. At 30% H₂O₂, angular nanoparticles (350–750 nm) appeared alongside the transition from rod-like γ-MnOOH to nanowire structures. Nanowires were formed via tip-directed elongation and, in some cases, by the longitudinal splitting of γ-MnOOH microrods (Fig. S7). Structurally, γ-MnOOH comprises chains of corner-sharing Mn³⁺O₆ octahedra forming proton-containing 1 × 1 rhombohedral tunnels,³¹ whereas β-MnO₂ adopts a rutile-type framework of Mn⁴⁺O₆ octahedra that is structurally related to γ-MnOOH, facilitating preferential formation via oxidative decomposition.^32–34 Accordingly, the nanowires derived from γ-MnOOH were identified as β-MnO₂, and the angular nanoparticles were assigned to cubic α-Mn₂O₃.

Fig. 3 Nest-like porous γ-MnOOH produced by vapor-assisted conversion: fabrication and characterization. (A) XRD patterns of samples treated with water or 5–30% H₂O₂ vapor. (B) SEM images of MnCO₃ powder compacts after vapor treatment: (upper) surface regions after 12 h at varying H₂O₂ concentrations; (lower) interior regions for 2 and 12 h at 5% H₂O₂ vapor. (C) Time-series XRD patterns of samples exposed to 5% H₂O₂ vapor. (D) FIB-SEM 3D reconstruction of the nest-like porous γ-MnOOH architecture obtained after 16 h at 5% H₂O₂.

In contrast to surfaces continuously exposed to H₂O₂ vapor, internal conversion and crystal growth proceeded gradually. Near-surface MnCO₃ decomposes rapidly at early stages, yielding γ-MnOOH and transient Mn₃O₄ (Fig. 3C); continuous H₂O₂ supply converts Mn₃O₄ onward to γ-MnOOH.³⁵ Inside, spherical MnCO₃ persisted owing to preferential H₂O₂ consumption at the surface (Fig. 3B, lower). The surface evolved into a rod-like framework that enhanced vapor penetration, while fibrous γ-MnOOH nucleated on the remaining spherical cores. With prolonged exposure, these features transformed into spherical aggregates composed of 1D γ-MnOOH. Notably, the internally formed γ-MnOOH exhibited smaller minor axes than the surface-grown crystals, indicating differences in the growth kinetics arising from gradients in vapor permeability and H₂O₂ concentration. As H₂O₂ is consumed during MnCO₃ conversion and γ-MnOOH growth, its concentration decreases in depth. Early stage fibrous γ-MnOOH increases vapor access and promotes the development of microfibers and rod-like γ-MnOOH surrounding spherical nanofiber aggregates. FIB-SEM reveals a seamless internal architecture in which spherical nanofiber aggregates are embedded within a microfiber framework (Fig. 3D and Movie S1), yielding a hierarchical, nest-like porous structure that emerges spontaneously from spatially varying growth rates.

The pore architecture of the nest-like γ-MnOOH was quantified using mercury intrusion porosimetry. The pore size distributions of the samples treated with 5 and 10% H₂O₂ vapor exhibited two key trends (Fig. 4A): (i) the average pore size increased with the H₂O₂ concentration and (ii) the distributions narrowed with longer exposure. The porosity, pore surface area, and average pore size were further analyzed (Fig. 4B). Assuming a bulk density of ≈1.53 g cm⁻³ (geometric determination), the porosity remained broadly constant (58–64%), consistent with the values obtained using the Archimedes method in water (58–65%). In contrast, the pore surface area and pore size were strongly concentration-dependent, with higher H₂O₂ concentrations accelerating the conversion of MnCO₃ to γ-MnOOH and promoting more uniform crystal growth, yielding narrower pore distributions and more uniform pores enclosed by interwoven γ-MnOOH nanorods. As growth proceeded, the γ-MnOOH aspect ratios decreased, reducing the pore surface area and increasing the average pore size.

Fig. 4 Pore structure and vapor-phase conversion of nest-like γ-MnOOH. (A) Pore size distributions for samples exposed for 8–16 h to 5 and 10% H₂O₂ vapor. (B) Quantitative analysis of porosity, pore surface area, and mean pore diameter. (C) Schematic of the vapor-phase conversion of spherical MnCO₃ to nest-like porous γ-MnOOH via solid–gas interfacial reactions.

Overall, the nest-like porous architecture forms via the vapor-phase conversion of spherical MnCO₃ particles, producing aggregates that retained the parent spherical morphology (Fig. 4C). Gradients in the H₂O₂ vapor concentration and exposure time between the surface and interior introduce a temporal offset in growth, resulting in a hierarchical framework composed of a microfiber skeleton interlaced with spherical aggregates of γ-MnOOH nanorods. This outcome is governed by the interplay between vapor transport, reaction kinetics, and morphological evolution. The initial packing density of the MnCO₃ compact is also expected to influence this outcome. Higher packing density steepens the H₂O₂ concentration gradient and deepens the structural heterogeneity between the surface and interior, whereas lower packing density permits more uniform vapor penetration. In this study, hydrothermal hot-pressing¹⁸ was employed to achieve reproducible packing with preserved spherical morphology (Fig. S6), ensuring consistent packing density across samples (relative density ≈57%).

3.3. Transport and wetting control in nest-like architectures enabling three-phase separation

The functional advantages of nest-like porous architectures arise from intricately interwoven branches that promote liquid permeability and vapor/gas transport. When the constituent material is intrinsically hydrophobic, water-repellency emerges without additional texturing.³⁶ Motivated by this structure-function interplay, we evaluated the liquid permeation behavior, evaporation performance, wetting control via hydrophobic treatment, and application potential of nest-like γ-MnOOH compacts.

We assembled a simple filtration setup using a fabricated nest-like γ-MnOOH compact (ϕ20 × 1.5 mm) as a membrane (Fig. S8) and assessed the liquid permeability under vacuum filtration at a transmembrane pressure of 20 kPa. The flux was measured for water, ethanol, and 2-propanol (Fig. 5A). The average flux values for water and ethanol were approximately 140 and 115 L m⁻² h⁻¹, respectively, whereas that for 2-propanol was lower at ≈41 L m⁻² h⁻¹. Notably, the water flux exceeds that of ceramic nanomembranes.^37,38 Given prior reports that surface polarity in pores larger than 100 nm does not significantly influence solvent transport,³⁹ the dominant factor governing flux is likely the liquid viscosity. Indeed, the viscosities of water, ethanol, and 2-propanol at 25 °C are 0.92, 1.08, and 2.06 mPa s, respectively.³⁹ The corresponding decrease in flux with increasing viscosity indicates that interactions with the particle surfaces during permeation were negligible.

Fig. 5 Transport and wetting control in nest-like γ-MnOOH frameworks. (A) Filtration-based liquid permeability for water, ethanol, and 2-propanol. (B) Water evaporation at a controlled temperature. (C) Hydrophobic modification and wetting response: (a) floating, (b) droplet rolling, (c) vertical drop rebound, and (d) contact angles for water–ethanol mixtures. (D) Three-phase (water/oil/solid) separation of an oil slurry using hydrophobic nest-like γ-MnOOH.

In addition to enabling rapid water permeation, the porous structure accelerates evaporation. We determined the water content gravimetrically for samples loaded with 200 µL of water in a constant-temperature chamber maintained between 25 and 40 °C (Fig. 5B). For water droplets placed on a Petri dish, only 14% of the water evaporated after 1 h at 25 °C. In contrast, when water was impregnated into the porous architecture, it wetted the surface of γ-MnOOH particles. The enlarged thin-film area, together with capillary self-pumping along the particle arrays, markedly enhanced evaporation.⁴⁰ At 40 °C, water evaporation ranged from 70% to 88%, corresponding to an estimated flux of 0.45–0.56 L m⁻² h⁻¹. The 3D, interconnected pore network should facilitate efficient vapor transport by enhancing diffusion and promoting convective flow.⁴¹ Accordingly, forced convection can further increase the evaporation rate.

Nest-like γ-MnOOH compacts were readily rendered hydrophobic via silane coupling by leveraging surface hydroxyl groups. Exposure to trimethoxy(methyl)silane vapor introduced methyl groups onto the particle surfaces. FTIR spectroscopy revealed weak bands attributable to the Si–CH₃ group: a C–H bending mode at 1276 cm⁻¹ and a stretching mode at 780 cm⁻¹ (Fig. S9). The formation of Si–CH₃ groups weakened hydrogen bonding between surface hydroxyls, shifting the hydrogen-bond bands centered at 2565 cm⁻¹ and the combination band with Mn–O at 1940 cm⁻¹ to higher wavenumbers.⁴² Owing to the resulting hydrophobicity, the porous architecture floated on water (Fig. 5C(a)). When slightly tilted, water droplets rolled off (Fig. 5C(b)), and vertically dropped droplets rebounded without adhesion (Fig. 5C(c) and Movie S2). The hydrophobic nest-like γ-MnOOH architectures exhibited a static water contact angle of 169°, indicative of a superhydrophobic character (Fig. 5C(d)). The 1D-grown γ-MnOOH particles produced a rough surface that supported a Cassie–Baxter-type “lotus effect”,^43,44 resulting in an excellent water-repellent surface. Because the contact angle is governed by interfacial energies at the solid–liquid–gas boundaries and depends on the liquid surface tension (σ), we investigated wetting using water–ethanol mixtures. Relative to pure water (σ = 72.8 mN m⁻¹ at 20 °C), the surface tension of the mixtures decreases asymptotically with increasing ethanol content (pure ethanol: 22.3 mN m⁻¹).⁴⁵ In response, the contact angle decreased approximately linearly up to 30 vol% ethanol, while a hydrophobic state with a contact angle of ≈117° was retained. At ethanol contents exceeding 35 vol%, droplets began to penetrate the substrate; at 40 vol% they fully infiltrated.

Exploiting the differences in surface tension that govern liquid permeability, we further demonstrated the three-phase (water/oil/solid) separation of an oil slurry using a hydrophobically modified, nest-like γ-MnOOH compact (Fig. 5D). We prepared a slurry by mixing hydrophobized spherical Fe₃O₄ particles (≈1 µm in diameter) with red-dyed oil, producing a black slurry. Upon introduction into water, the slurry settled and accumulated on the hydrophobic pellet. The Fe₃O₄ spheres were retained on the pellet surface (Fig. S10), whereas only the oil permeated the porous network and was extracted. This proof-of-concept separation indicates potential utility for collecting or recovering microplastic–oil–dispersant aggregates that form in marine environments.^46–48 By tailoring the surface chemistry of porous architectures and tuning liquid properties (e.g., viscosity and surface tension), diverse applications based on selective permeation, evaporation, and water repellency are enabled. Beyond environmental purification, potential uses include solar-driven seawater desalination⁴⁹ and adaptive insulation materials responsive to ambient conditions.⁵⁰ The freestanding pellets maintained structural integrity without cracking or disintegration during filtration (20 kPa), evaporation testing, and three-phase separation, demonstrating that the physical entanglement of high-aspect-ratio nanorods provides sufficient cohesion for practical use, although formal mechanical testing remains a subject for future work. No degradation of the porous framework was observed under the mild operating conditions employed here; for more chemically demanding environments, thermal transformation of γ-MnOOH into MnO₂ or Mn₂O₃ offers a route to enhanced chemical stability while retaining the porous architecture.

4. Conclusion

In summary, we established a bio-inspired synthetic strategy to realize freestanding, breathable porous architectures in which structural robustness arises from the extensive entanglement of high-aspect-ratio γ-MnOOH nanorods. Vapor-assisted oxidative conversion drives tip-directed elongation and spontaneous branching, yielding binder-free hierarchical nest-like frameworks directly from MnCO₃ precursors. These architectures combine high water permeability (≈140 L m⁻² h⁻¹), rapid evaporation (0.45–0.56 L m⁻² h⁻¹ at 40 °C under static conditions), and superhydrophobicity (contact angle of 169° after surface modification), enabling efficient three-phase (water/oil/solid) separation. This scalable platform, rooted in the design logic of nature, offers a versatile route to oxyhydroxide-derived functional materials for applications ranging from thermal management to biocompatible scaffolds.

Author contributions

Takahiro Kozawa: conceptualization, funding acquisition, investigation, methodology, project administration, resources, supervision, validation, visualization, writing – original draft, writing – review & editing. Kayo Fukuyama: investigation. Minoru Osada: writing – review & editing. Hiroya Abe: funding acquisition, methodology, supervision, writing – original draft, writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting the findings of this study are available within this article and its supplementary information (SI). Supplementary information is available: photos, characterization including XRD, crystalline phase ratio, SEM, TEM, and FTIR, filtration device, and movies for FIB-SEM and hydrophobic nature. See DOI: https://doi.org/10.1039/d6ma00109b.

Acknowledgements

This work was funded by the Environmental Research and Technology Development Fund (JPMEERF20211R04), Environmental Restoration and Conservation Agency (ERCA), Grand-in-Aid for Scientific Research KAKENHI (24K21675, 24K01180), Japan Society for the Promotion of Science (JSPS), and the Cooperative Research Project of Design & Engineering by Joint Inverse Innovation for Materials Architecture (DEJI²MA), MEXT. This work was partially conducted through the Joint Usage/Research Program of the Institute of Materials and Systems for Sustainability, Nagoya University.

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