Metal oxides have garnered significant attention in modern materials science because of their exceptional piezoelectricity, semiconductor properties, and optical performance, which enable diverse applications. Compared with conventional two-dimensional (2D) architectures, three-dimensional (3D) structural designs can enhance performance by increasing the energy density of energy-storage devices, improve the mechanical properties of materials, and achieve functionalities that are not inherent to base materials. However, conventional lithography-based fabrication is limited to simple 2D patterning, which restricts micro/nanodevice integration. Additive-manufacturing techniques such as selective laser sintering and direct ink writing have been employed for 3D metal-oxide fabrication; however, their resolution remains constrained (10?100 μm) by nozzle dimensions and material rheology. Hence, two-photon polymerization (TPP) combined with thermal sintering is proposed, which involves the utilization of metal-ion-doped photoresists to create submicron 3D oxides after the removal of organics. However, chemically active ions (e.g., Fe3?) inhibit polymerization by quenching free radicals. Meanwhile, alternative scaffold-absorption-based methods immerse hydrogel templates in metal salt solutions to adsorb ions without interfering with TPP. Nonetheless, ion absorption reduces the hydrogel swelling capacity, thus resulting in low metal loading and porous sintered structures. Consequently, a universally applicable, high-precision fabrication strategy is urgently required that balances material adaptability with dense, defect-free 3D metal-oxide nanoarchitectures. The development of such methods can facilitate advanced applications in photonics, flexible electronics, and energy storage by fully exploiting the unique properties of 3D metal oxides.

This study proposes a versatile and efficient method for fabricating 3D metal-oxide nanoarchitectures. Leveraging the coordination mechanism between metal ions and carboxyl groups within a polymer scaffold, the approach integrates multiple metal ions into TPP-printed 3D polymer templates. Subsequent thermal sintering removes the organic framework, thus yielding high-quality 3D metal-oxide architectures with sub-500 nm feature sizes (minimum of 410 nm). The key fabrication steps are as follows: 1) TPP-based femtosecond laser printing of 3D polymer scaffolds; 2) immersion in 0.1 mol/L metal salt solutions for 120 min to facilitate metal-carboxyl coordination and ion infiltration; and 3) high-temperature sintering to decompose the polymer and form dense metal-oxide structures. This study systematically investigates factors affecting metal-scaffold coordination, such as solution concentration and immersion duration, to optimize ion uptake and structural fidelity. Compared with conventional hydrogel-based adsorption methods, this coordination-driven strategy enhances the metal-loading capacity and minimizes the porosity of the final sintered structures. The proposed method demonstrates broad applicability across various metal oxides while circumventing polymerization inhibition caused by reactive ions (e.g., Fe3?) in direct TPP approaches.

To achieve high shape fidelity in sintered metal-oxide nanoarchitectures, a novel TPP photoresist with enhanced metal adsorption efficiency is developed in this study, which significantly improves the ion-loading capacity of the scaffold during immersion [Fig. 1(b)]. The polymer scaffold, which is fabricated via femtosecond laser TPP, enables efficient metal ion incorporation via a dual mechanism [Fig. 1(d)]. First, the dissociation of carboxylic acid groups (—COOH) in the polymer network generates negatively charged —COO^- groups, thus inducing electrostatic repulsion-driven swelling to facilitate metal-ion diffusion. Second, stable coordination bonds form between oxygen lone pairs in carboxylate groups and metal ions, thus ensuring robust chemical adsorption. This synergistic approach overcomes the limitations of conventional hydrogel-based methods and minimizes the porosity of the final structures. Benefiting from TPP submicron resolution and controlled shrinkage during sintering, the method affords a NiO “buckyball” architecture with features measuring 410 nm at the minimum [Fig. 1(e)]. Complex architectures, including CoO woodpile arrays and Ni_0.5Co_0.5O curved lattices, demonstrate the capability of this technique for fabricating intricate 3D geometries and multi-metal-oxide systems. The key process parameters are systematically optimized as follows: 1) The acrylic acid (AAc) content in the photoresist is optimized to balance the carboxyl density and structural integrity, thus maximizing the ion-loading rate (Fig. 2); 2) the laser energy and exposure time are calibrated to ensure scaffold fidelity, which is critical for preserving structural details after sintering (Fig. 4); and 3) immersion conditions are optimized to enhance coordination efficiency without compromising scaffold stability. Post-sintering characterization (Figs. 6 and 7) confirms the formation of phase-pure metal oxides, whereas mechanical tests reveal the exceptional mechanical properties of the CoO microstructures (Fig. 8).

This study presents a highly efficient and versatile method for fabricating 3D metal-oxide nanoarchitectures by integrating femtosecond-laser TPP with a metal-ion coordination mechanism. Unlike conventional nanofabrication techniques, our approach introduces metal ions into the polymer scaffold after printing, thereby effectively avoiding the interference of metal ions during polymerization. By optimizing the photoresist formulation and coordination reaction conditions, we achieve high-efficiency metal-ion loading. Subsequent thermal sintering removes the organic framework, thus successfully yielding 3D metal-oxide structures with features measuring 410 nm at the minimum. By decoupling the printing and metallization stages, the proposed method enables the fabrication of intricate architectures with diverse metal-oxide compositions while maintaining submicron precision. In particular, its application spans from single-component oxides (e.g., NiO and CoO) to multi-metal-oxide systems (e.g., Ni_0.5Co_0.5O), thus highlighting its broad material adaptability. The proposed technique provides a novel paradigm for creating 3D functional devices to be used in cutting-edge applications such as energy-storage devices, photodetectors, and semiconductor sensors.