Gold clay from self-assembly of 2D microscale nanosheets

The study addresses the challenge of scaling nano/microscale materials with well-defined shapes into free-standing, three-dimensional macroscopic metals. While natural clays—composed of microscale 2D silicate sheets—can be shaped and retain structure even at high temperatures, artificial metal clays made from nanoparticles tend to collapse or lose detail upon dehydration or high-temperature processing. The authors aim to create an artificial "gold clay" composed of 2D {111}-oriented microscale nanosheets that self-assemble into oriented films, are soft and highly plastic at room temperature, can be molded into complex free-standing metallic architectures with high resolution, maintain structural detail upon annealing, and exhibit high electrical conductivity with stress/strain-dependent behavior. This approach is inspired by the interlaced packing and strong interplate interactions of natural clays and leverages controlled growth within succinic acid surfactant bilayers followed by interfacial self-assembly.

Prior work has achieved shape-controlled synthesis of metal nanocrystals and nanoparticle self-assembly, but scaling to free-standing macroscopic metals remains difficult. A notable prior report demonstrated moldable gold clay from self-assembled nanoparticles; however, nanoparticle-derived clays often collapse or lose detail, especially above ~300 °C. Natural clays, by contrast, are built from microscale 2D hexagonal silicate sheets (~2 µm) that interlace strongly and retain shape even when dehydrated or calcined. Carboxylic groups in surfactants and polymers can act as reducing and stabilizing agents for nanoparticle synthesis, and ligands with carboxylate groups preferentially bind to {111} metal facets to stabilize anisotropic growth. These insights motivate using dodecenylsuccinic acid (DSA) bilayer membranes to template anisotropic 2D growth and to promote oriented assembly akin to natural clays.

Synthesis of gold nanosheets: Dodecenylsuccinic acid (DSA) compounds (C12, C14, C18 alkyl chains) were prepared by hydrolysis of the respective succinic anhydrides. DSA was dissolved in water to 1–1.2 wt% and equilibrated above its Krafft point (e.g., C12-DSA at 53 °C, pH ~3.3) to form lamellar bilayer membrane structures with 100–200 nm water layers sandwiched between bilayers. HAuCl4 solution (2 × 10−3 M; typically 500 µL added to 5 mL DSA solution; Au3+/CDSA molar ratios <0.006 overall and varied up to 4.0 × 10−3 for growth studies) was gently mixed and the mixture incubated at 53 °C without stirring. Color evolved from the water/air interface to the bulk within ~15 min, indicating nucleation and growth. Growth proceeded via progressive nucleation of small nanoparticles followed by anisotropic growth of a subset into 2D nanosheets. DSA carboxylate-rich bilayers acted as soft templates and likely served as mild reducing/stabilizing agents, promoting preferential binding/growth along {111} facets. Reaction time controlled lateral size; ~1.5 h produced ~5 µm plates. Control experiments at room temperature (below Krafft point) or with too-short alkyl chain DSA (e.g., C4) did not yield nanosheets, confirming the necessity of bilayer formation for anisotropic growth. Self-assembly at liquid–liquid interface: After synthesis (e.g., Au3+/CDSA = 5.7 × 10−3), adding an immiscible organic solvent—most effectively ethyl acetate (also dichloromethane)—triggered immediate self-assembly of nanosheets (with some nanoparticles) into a thin layer at the water/organic interface. Other solvents (acetone, hexane, toluene) did not trigger assembly. The assemblies persisted at the interface despite agitation and formed spherical interfacial aggregates as water volume decreased, remaining until solvents were removed. DSA partially extracted into the organic phase aided interfacial tension reduction and mobile 2D assembly. Interfacial films were transferred by pipette onto substrates (e.g., Si) to form ~300 nm thick films (3D laser scanning microscopy). As-prepared films comprised predominantly nanosheets of several µm diameter and ~10 nm thickness (aspect ratio ~300), with some residual nanoparticles. Collection of gold clay: Solvents (ethyl acetate and water) were drawn off by pipette from the bottom while maintaining the assembly at the interface; upon complete removal of solvents, the nanosheets formed an aggregate (gold clay). As-prepared gold clay contained ~40 wt% solvent; residual DSA remained on nanosheet surfaces (EDX Au:(C+O) ~97.3%:2.7%). The clay could be used immediately or after brief drying. Characterization: UV–Vis–NIR spectroscopy monitored LSPR evolution with Au3+/CDSA ratio. Morphologies were imaged by SEM and AFM; thickness and topography by 3D laser scanning microscopy; crystallinity by high-resolution TEM and SAED (lattice spacings 2.4 Å; hexagonal SAED with {220} and 1/3(422) reflections) and XRD (strong Au(111) peak at 38.1° with Au(200) 44.3°, Au(220) 64.6°, Au{311} 77.5°). Elemental mapping by TEM-EDX confirmed surface C and O from DSA. Mechanical properties were assessed by compressive stress–strain tests (Tensilon EZ-LX) at room temperature, including after mild annealing at 70 °C (0.5 h, 6 h). Specific surface area measured by BET (~1.49 m2 g−1). Thermal behavior assessed by TGA; patterning demonstrated by molding against a coin relief and by letterpress-like printing on paper. Annealing tests: Patterned gold clay architectures were annealed at 20–450 °C to assess dimensional stability, color/hardness changes, and retention of structural detail.

• Successful synthesis of single-crystalline Au nanosheets within DSA bilayer membranes; nanosheets have diameters of several micrometres (~5 µm typical after 1.5 h) and thickness ~10 nm (aspect ratio ~300). High-resolution TEM shows 2.4 Å lattice spacing; SAED exhibits hexagonal symmetry with {220} and 1/3(422) reflections, indicating preferred {111} orientation with stacking faults; XRD shows a dominant Au(111) peak (38.1°) over Au(200), Au(220), and Au{311}.
• Interfacial self-assembly at the water–ethyl acetate (or dichloromethane) interface produces transferable, relatively flat films (~300 nm thick) composed of predominantly nanosheets. Assemblies preferentially orient parallel; orientation stronger than in randomly packed control without interfacial self-assembly.
• Residual DSA remains on nanosheet surfaces post-assembly: EDX mapping shows Au:(C+O) ≈ 97.3%:2.7% by atom weight percentage.
• Mechanical properties: Gold clay exhibits plastic deformation with significant hysteresis under compression. Stiffness increases with strain, showing densification/hardening at high strain (e.g., up to ~459 kPa at ~60–70% strain before annealing; after 70 °C annealing, stiffness increases up to ~1726 kPa at similar strain). Energy dissipation (area between loading/unloading) increases from ~0.71 × 10^4 to ~2.96 × 10^4 kJ m−3 after mild annealing (70 °C, 6 h). Despite hardening, materials remain extremely soft relative to bulk Au (E ≈ 79 × 10^6 kPa), i.e., about six orders of magnitude softer.
• Morphology under compression transitions from loose, dark-brown aggregates to compact, fused, shiny surfaces; SEM shows reduced gaps and fewer grain boundaries with increasing strain, consistent with densification and improved interplate connectivity.
• Electrical properties: Without thermal treatment, resistivity decreases markedly with compressive strain. Example: from ~5 × 10^−5 Ω·m (as-prepared, thickness ~1.17 mm) to 3.2 × 10^−7 Ω·m after pressing to ~0.09 mm (≈ three orders of magnitude improvement). The compressed clay’s resistivity is within an order of magnitude of bulk gold (2.44 × 10^−8 Ω·m) and comparable to or better than some sputtered/evaporated Au films. Conductivity enhancement correlates with stress/strain-induced morphological compaction/fusion and nanoparticle-assisted nanosheet connectivity.
• Patterning and structural fidelity: Gold clay can be molded at room temperature into free-standing metallic architectures (e.g., flower relief, letters) that replicate features with tens of micrometre height differences (e.g., stripe width 100 µm, height 50 µm). Structures can be peeled free-standing without thermal sintering.
• Thermal stability of patterns: Up to 250 °C, patterns and dimensions remain largely unchanged while color becomes more golden and hardness increases (consistent with gradual decomposition of ~3 wt% DSA above ~200 °C). Above 350 °C, overall dimensions shrink anisotropically by ~10% (especially thickness); above 450 °C slight curvature appears but structural details remain visible and hardness approaches bulk gold.
• Specific surface area of porous assemblies ~1.49 m^2 g^−1; catalytic activity is lower than typical noble metal nanoparticles, likely due to DSA blocking active sites.
• Process robustness: Various DSA chain lengths (C12, C14, C18) yield Au nanomaterials; anisotropic nanosheet growth requires bilayer formation above the Krafft point; certain organic solvents (ethyl acetate, dichloromethane) specifically trigger interfacial assembly.

The work demonstrates that mimicking natural clay architectures—using microscale, thin, interlaced 2D building blocks—enables moldable, free-standing metallic materials. Controlled anisotropic growth of single-crystal Au nanosheets within DSA bilayer membranes yields platelets that, upon interfacial self-assembly, form {111}-oriented arrays with strong interplate adhesion, aided by residual carboxylate-containing DSA. This architecture underlies the observed macroscopic plasticity and moldability at room temperature, allowing high-resolution replication of complex features without thermal sintering, addressing a key challenge in scaling nanostructures into macroscopic free-standing metals. Stress/strain-dependent electrical conductivity arises from compression-induced densification and surface fusion that reduce grain boundaries and inter-plate gaps, thereby enhancing electron transport. The resulting resistivity approaches that of bulk gold while maintaining plastic deformation, offering a tunable trade-off between softness and conductivity. Thermal treatments further harden the structures with minimal loss of structural detail up to moderate temperatures, suggesting compatibility with device processing. Overall, the findings provide a simple chemical route to 2D metallic nanostructures and a self-assembly strategy to fabricate soft, conductive, free-standing metal architectures with microscale resolution.

The authors introduce a simple, scalable route to fabricate a moldable gold clay by growing single-crystalline Au nanosheets within DSA bilayer membranes and self-assembling them at a liquid–liquid interface into {111}-oriented aggregates. The resulting materials are extremely soft relative to bulk gold, plastically deformable at room temperature into free-standing, high-fidelity metallic architectures, and exhibit stress/strain-tunable conductivity that approaches bulk values upon compression. Patterns remain stable and harden upon moderate thermal annealing while retaining detail even at higher temperatures. Potential future directions implied by the work include leveraging the method to produce free-standing metallic architectures for flexible electronics, devices, sensors, and thin metal foils, and further exploration of stress/strain-responsive electrical properties for sensing applications.

• Residual organic binder (DSA) remains on nanosheet surfaces (~3 wt% by TGA; EDX indicates Au:(C+O) ≈ 97.3%:2.7%), which is advantageous for plasticity/molding but blocks catalytic sites and reduces catalytic efficiency relative to typical noble metal nanoparticles.
• Anisotropic nanosheet growth requires formation of DSA bilayer structures above the Krafft point (e.g., ~53 °C for C12-DSA); at lower temperatures or with too-short alkyl chains, only nanoparticles form.
• Only specific organic solvents (ethyl acetate, dichloromethane) effectively trigger interfacial self-assembly; other solvents tested did not.
• Upon high-temperature annealing (>350 °C), samples exhibit ~10% anisotropic shrinkage (especially in thickness) and slight curvature above 450 °C, although structural details are retained.
• Electrical resistivity of compressed clay, while greatly improved, remains higher than bulk gold by about an order of magnitude.