(Nanowerk Spotlight) The field of three-dimensional (3D) nanoprinting has long promised to revolutionize the fabrication of advanced materials and devices. This technology offers the potential to create intricate structures with nanoscale precision, opening up new possibilities in areas such as electronics, optics, energy storage, and sensing. However, progress in 3D nanoprinting of metal oxides has been hindered by several persistent challenges. These include limited material options, significant shape distortion during fabrication, and difficulties in creating heterogeneous structures combining multiple materials.
Metal oxides are a crucial class of materials with diverse properties that make them indispensable in many technological applications. Their unique characteristics, such as semiconductivity, piezoelectricity, and optical transparency, make them ideal for use in sensors, batteries, and various electronic and optoelectronic devices. The ability to craft these materials into precise 3D nanostructures could dramatically enhance their performance and enable entirely new functionalities.
Previous attempts at 3D nanoprinting metal oxides have faced significant hurdles. Some methods involved pyrolyzing organic templates infused with nanoparticles or metal ions, but these approaches struggled with cross-contamination when trying to create structures with multiple materials on the same substrate. Other techniques used colloidal nanocrystals but suffered from extremely slow manufacturing speeds due to the low concentration of nanocrystals in the printing medium. Perhaps the most promising prior approach involved using lipophilic resins doped with metal ions, but this method was limited by the low solubility of metal ions in the resin, restricting the range of metal oxides that could be produced.
Against this backdrop, researchers from Huazhong University of Science and Technology and Optics Valley Laboratory, both in China, have developed a new method for 3D nanoprinting of metal oxides that addresses many of these longstanding challenges. Their approach, detailed in a recent paper published in Advanced Materials (“3D Nanoprinting of Heterogeneous Metal Oxides with High Shape Fidelity”), centers on the creation of a novel type of photosensitive resin they call metal ion synergistic coordination water-soluble (MISCWS) resin.
Fabrication of 3D microstructures of metal oxides via TPP printing the MISCWS resins. a) Illustration of the preparation principle of the MISCWS resins. 1-Vinylimidazole effectively coordinates with metal ions dissolved in water (left), leading to macromolecular precipitates of metalorganic frameworks (middle). Hydrogen ions from acrylic acid convert the macromolecular precipitates into evenly dispersed micromolecular complexes (right), while facilitating the coordination between acrylate ions and metal ions by consuming hydrogen ions. b) Illustration of 3D printing principle of metal oxides. Under two-photon activation, acrylate and 1-vinylimidazole are bonded by the C─C bond (right), thereby introducing metal ions into the 3D microstructures (left and center). c–j) Microscopic images of the 3D microstructures of metal oxides: c) Body-centered microstructure of Cr2O3 consisting of over 1000 wire segments, d) Top-view pattern with a linewidth of 391 nm, e) Buckyball array structure of MnO2, f) Magnified image of (e), g) Gyroid structure of Co3O4, h) Magnified image of (g), and i,j) Spherical microlens structures of high-refractive-index ZnO. The scale bars in (d), (f), and (h) are 2 µm; all other scale bars are 5 μm. (Image: Reproduced with permission by Wiley-VCH Verlag)
The key innovation in this work is the discovery of a synergistic coordination mechanism between imidazole and acrylic acid that allows for the effective incorporation of various metal ions into a water-based resin. This mechanism enables the creation of stable, printable resins containing a much higher concentration of metal ions than previous methods. The researchers were able to achieve metal ion content within the 3D polymer structures of up to 30.5% by weight, which represents at least a 2.54-fold increase compared to values reported in earlier literature.
The MISCWS resin is composed of three main components: saturated metal salt solutions (typically derived from nitrates), coordination monomers (1-vinylimidazole and acrylic acid), and a water-soluble photoinitiator along with a water-soluble crosslinker. The researchers developed a custom water-soluble version of the common photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) to enable the photopolymerization process in an aqueous environment.
Using this novel resin system, the team was able to fabricate 3D nanostructures of various metal oxides including manganese dioxide (MnO2), chromium oxide (Cr2O3), cobalt oxide (Co3O4), and zinc oxide (ZnO). The printing process involves using two-photon polymerization (TPP) to selectively solidify the resin in desired patterns, followed by a pyrolysis step to convert the polymer structures into pure metal oxides.
One of the most significant advantages of this new method is the greatly reduced shape distortion of the final metal oxide structures. The higher metal ion content in the initial polymer structure means less shrinkage occurs during the pyrolysis step. The researchers observed linear shrinkage of only 30-55%, compared to up to 80% reported in previous works. This improved shape fidelity allows for the creation of more complex and precise 3D nanostructures.
The team demonstrated the capabilities of their technique by fabricating a range of intricate structures. These included a body-centered microstructure of Cr2O3 with feature sizes as small as 391 nm, a porous buckyball array structure of MnO2, and a gyroid structure of Co3O4. They also created arrays of smooth ZnO microlenses with individual sizes of 30.5 µm and surface roughness of only 4.1 nm.
Importantly, the method also enables the creation of heterogeneous structures combining multiple metal oxides. The researchers successfully fabricated 2D “Tai Chi” structures with two nested metal elements, 3D “Kater Ring” structures, and “Ring” structures incorporating four different metal oxides. This capability for multi-material printing is crucial for the development of advanced integrated microsystems.
To showcase the practical potential of their technique, the team fabricated a 3D ZnO microsensor for detecting nitrogen dioxide (NO2) gas. The 3D porous structure of the sensor greatly increased its surface area, resulting in exceptional sensitivity. The sensor achieved a maximum response of 1.113 million at 200 ppm NO2, surpassing the reported sensitivity of conventional 2D sensors by tenfold at equivalent concentrations. The sensor also demonstrated excellent selectivity, with its response to NO2 being at least four orders of magnitude higher than its response to other gases at 100 ppm.
EDS fluorescence maps of the 3D microstructures and heterogeneous structures of seven kinds of materials. a) 3D model of the buckyball structure. b–d) EDS fluorescence maps of Co3O4, Cr2O3, and ZnO, respectively. e) Tai Chi model. f–h) EDS fluorescence maps of MnO2 with NiO, Cr2O3 with Al2O3, and ZnO with MgO, respectively. i) Kater ring model. j,k) EDS fluorescence maps of Cr with Ni elements and Mn with Al elements of polymers, respectively. l) EDS fluorescence ring map of Mn, Ni, Cr, and Al elements of polymers. All scale bars are 10 µm. (Image: Reproduced with permission by Wiley-VCH Verlag)
The development of this new 3D nanoprinting method for metal oxides represents a significant advance in the field of nanofabrication. By enabling the creation of complex, multi-material 3D nanostructures with high shape fidelity, it opens up new possibilities for the design and manufacture of advanced functional devices. The technique could find applications in diverse areas such as high-performance sensors, micro-batteries, microelectronics, micro-optics, and integrated microsystems.
While the current work focused on a limited set of metal oxides, the underlying principles of the MISCWS resin system suggest that it could be extended to a wider range of materials. Future research may explore the application of this technique to other metal oxides, or even to different classes of inorganic materials.
As with any new technology, there are likely to be challenges in scaling up this process for industrial production. Issues such as manufacturing speed, cost-effectiveness, and long-term stability of the printed structures will need to be addressed. However, the fundamental advances demonstrated in this work provide a solid foundation for future development and optimization.
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