Three-dimensional (3D) microprinting is considered a next-generation manufacturing process for the production of microscale components; however, the narrow range of suitable materials, which include mainly polymers, is a critical issue that limits the application of this process to functional inorganic materials. Herein, we develop a generalised microscale 3D printing method for the production of purely inorganic nanocrystal-based porous materials. Our process was designed to solidify all-inorganic nanocrystals via immediate dispersibility control and surface linking in the nonsolvent linker bath and thereby created multibranched gel networks. The process works with various inorganic materials, including metals, semiconductors, magnets, oxides, and multi-materials, not requiring organic binders or stereolithographic equipment. Filaments with a diameter of sub-10 µm are printed into designed complex 3D microarchitectures, which exhibit full nanocrystal functionality and high specific surface areas comparable to those of typical aerogels. This approach enables the manufacture of a very broad range of functional inorganic materials.
Article
3D microprinting of inorganic porous materials by chemical linking-induced solidification of nanocrystals
https://doi.org/10.21203/rs.3.rs-2580380/v1
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Additive manufacturing, commonly known as 3D printing technology, allows the production of materials with customised shapes and dimensions1,2. In particular, micro-stereolithography techniques3,4 and direct ink writing5 can revolutionise the manufacturing of microscale components for various applications, including micromechanics, microelectronics, and biomedical systems by enabling the creation of previously inaccessible 3D architectures6–10. Micro-stereolithography is based on multiphoton absorption and is achieved using a projection lens, while direct ink writing employs microneedles; however, these processes critically rely on organic or polymer-based resin inks to ensure photocurability or rheological printability in the optical lithography or ink writing process, respectively, which limit the range of printable materials and the intrinsic functionality of the printed objects. Nanomaterial-based polymer resin-free printing techniques were recently developed by introducing photocurable molecules to the nanomaterial surfaces11,12. Sun et al. achieved the stereolithographic 3D nanoprinting of quantum dots by exploiting photoexcitation-driven chemical bonding12. Despite these recent successes, existing processes for inorganic nanomaterials or composites are only applicable to specific photo-active materials and require special lithographic equipment. Thus, new processes are required to diversify and expand the range of inorganic materials suitable for 3D microprinting.
Colloidal nanocrystals have emerged as versatile inorganic building blocks of functional 2D and 3D solids with tailorable physicochemical properties13–15, while advances in synthetic methodology have enabled the preparation of colloidal particles of almost any inorganic functional material16–18. In particular, the assembly of colloidal nanocrystals into macroscopic solid gels introduces a new class of inorganic porous material with high surface areas and low densities, whose electronic, magnetic, and optical properties originate from building blocks19–21, and can thus be tailored to a wide variety of applications, including energy storage and conversion22,23, catalysis24,25, adsorbent26, filter27, and electrodes28, and sensors29. Herein, we demonstrate a generalised method to achieve high-resolution wet 3D microprinting of inorganic porous materials by the direct writing of purely inorganic colloidal nanocrystal ink in the linker-containing nonsolvent bath (Fig. 1a). This process is applicable to a diverse range of materials and produces crystalline nanostructures with high structural integrity. Further, the process is tailored toward the creation of purely inorganic materials with 3D microarchitectures and does not require any polymer resins. The inorganic ligand-capped nanocrystal inks were solidified by the instant interconnection of the nanocrystals through solvent polarity change and surface linking during the ink extrusion, which affords a multibranched gel network (Fig. 1b). The dimension-controlled printing of microscale filaments creates complex 3D architectures, which retain the functionalities of the primary nanocrystals and exhibit high specific surface areas comparable to those of existing nanocrystal-based and sol-gel-processed aerogels. Further, our method produced multiple materials using mixed nanocrystal inks or sequential printing.
Wet 3d Microprinting Of Inorganic Nanocrystals
The developed wet 3D microprinting method involves three steps: i) preparation of a negatively charged nanocrystal inks, ii) controlled solidification of the extruded nanocrystals in the nonsolvent bath containing linker ions, and iii) supercritical drying of the wet-state of printed objects into the solid-state 3D architectures. Inorganic ligand (i.e., tetrathiomolybdate; MoS42−)-capped nanocrystals dispersed in polar solvent (i.e., N-methylformamide (NMF)) were exploited as printing inks, which are extruded into nonsolvent with lower dielectric constant (i.e., butanol) containing linker ions. Instantaneous solvent-to-nonsolvent mixing during the ink extrusion instantly reduces the diserpsiblity of negatively charged nanocrystals, resulting in agglomeration of nanocrystals. Systematic studies on the combination of solvent (NMF) and nonsolvent with respect to their solubility parameters and dielectric constants reveal that the optimum ranges are essential to flocculate nanocrystals (Supplementary Fig. 1 and Supplementary Table 1). When the differences in solubility parameters of nonsolvent with NMF exceeded certain limits, their phases are separated. Also, the nonsolvent with high dielectric constant solubilized the negatively charged nanocrystals. Under the optimum ranges of both characteristics of nonsolvent, the instant agglomeration of nanocrystals was achieved (Supplementary Fig. 2, 3 and Supplementary Table 1).
Multivalent metal ion linkers play the role of creating covalent bonds with the surface inorganic ligands among nanocrystals in agglomerates which enhance the overall structural entanglement (Supplementary Fig. 4), eventually leading to the robust solidification. Since the thiomolybdate anion ligand (i.e., a soft Lewis base) has chemical affinity to metal ion linkers that act as soft Lewis acids30,31, soft Lewis acids of multivalent coordinating ions, including Au3+, Pt4+, and Fe2+ but not limited, were chosen to serve as metal linkers (e.g. Au3+ linker for Au and Ag nanocrystals, Pt4+ for FePt nanocrystals, and Fe2+ for CdSe and Fe3O4 nanocrystals). The linking of the nanocrystals was optimised using the control parameters of the concentrations of both the metal ion linkers and the nanocrystals. When the concentrations of either the nanocrystals or the coordinating linker ions exceeded certain limits, the nozzle was easily clogged, causing discontinuous ink deposition (Supplementary Fig. 5). In contrast, decreasing their concentrations “too far” caused the dissolution of nanocrystals into the linker bath, and we could not obtain the defined filaments by printing (Supplementary Fig. 5). In turn, under optimum conditions, defined 3D inorganic filaments with multibranched porous nanocrystal networks were printed by our process.
A model system consisting of thiomolybdate-capped Ag nanocrystal building blocks was studied. The electrokinetic ξ-potentials changed from − 50.7 to − 10.1 mV during the deposition, indicating that the nanocrystals lost their surface charges upon coordination of the metal ion linkers (Supplementary Fig. 6). Moreover, the peaks in the S 2p region of the X-ray photoelectron spectroscopy (XPS) spectrum showed a peak shift to higher binding energies upon ligation to metal ions, while the Ag 3d peaks did not change (Supplementary Fig. 7). This result confirms that no reaction occurred between the metal ions and the nanocrystals occurs, but rather, the nanocrystals were bridged by linking the surface thiometallate ligands with metal ions.
The system used to print the inorganic nanocrystals consisted of a micropipette nozzle connected to a syringe-type reservoir containing negatively charged inorganic nanocrystals, which were extruded using a pneumatic pressure controller. The stage mounted with the linker-containing solidification bath was moved along the x-, y-, and z-axes in a pre-designed model (Supplementary Fig. 8). The diameters of the filaments were precisely controlled from 8 to 49 µm in the wet state by adjusting the nozzle diameter; thus, the method demonstrates microscale printability. The wet filaments were further dried by supercritical CO2 drying, effectively generating extremely microporous materials without substantial structural shrinkage or distortion, as shown in the optical microscopy (OM) images (Fig. 1c, d). Scanning electron microscopy (SEM) images (Fig. 1f–i) confirmed that the dried filaments constructed from Ag nanocrystals showed uniform linewidths with controllable diameters ranging from 7 to 44 µm (Fig. 1e). High-resolution (HR) SEM images clearly revealed extremely porous and multibranched networks in all samples, which showed characteristics of typical aerogels (Fig. 1f–i, insets).
This ability to print microscale inorganic filaments enables the construction of complex 3D architectures in diverse ranges of materials. For example, a 3D cubic lattice structure was built via the layer-by-layer deposition of 32 Ag filament layers in a single pass (Fig. 2a, Supplementary Fig. 9, Supplementary Video 1). The printing method showed high precision, while the printed 3D architectures exhibited excellent structural fidelity and were consistent with the design model (Fig. 2a, inset). The 3D-printed filaments maintained a circular cross section with a uniform diameter (Fig. 2b, c, Supplementary Fig. 9). Energy dispersive X-ray spectroscopy (EDS) maps showed that Ag nanocrystals, S-based ligands, and Au-based linkers were confined to the patterned layers (Fig. 2d). Lattice structures were printed using various materials, including metallic Au, magnetic FePt and Fe3O4, and semiconducting CdSe nanocrystals (Fig. 2e–h). SEM images and EDS maps confirm that all samples show high structural and compositional integrity and uniformity (Supplementary Fig. 10). Various 3D objects were printed, including a CdSe-based pyramid (Fig. 2i, Supplementary Video 2) and a FePt-based hexagonal prism (Fig. 2j, Supplementary Video 3), which all showed excellent lateral and vertical shape fidelity. Moreover, a large square lattice pattern with dimensions of 1.2 mm \(\times\) 1.2 mm was constructed by the layer-by-layer printing of 8 Ag filament layers (Fig. 2k), thereby demonstrating the feasibility of the printing process over a millimetre-scale area. These examples of complex 3D architectures built from various nanocrystals clearly demonstrate the microscale 3D printability of the method and the wide range of applicable materials.
Microstructural Characteristics
Low-resolution and HRSEM images of the printed structures reveal porous microstructures with interconnected networks of extremely thin wire-like structures with numerous bifurcations (Fig. 3a, b). The typical dimensions of these multibranched wire-like structures are on the same size scale as the diameter of the original nanocrystals (5–10 nm), which demonstrates that the printed structures were formed directly from the original colloidal nanocrystal building blocks without the formation of any kind of secondary structures. A diverse range of pore sizes was observed, ranging from a few nanometres to several hundreds of nanometres. TEM images (Fig. 3c, Supplementary Fig. 11) show that the nanocrystals were not merged, but rather maintained their primary shapes and sizes, which supports the hypothesis that the solidification of nanocrystals is induced by the surface linking reaction. The random orientation of the nanocrystals in the chains, as seen in the HRTEM images, resulted in ring-like electron diffraction patterns (Fig. 3d). X-ray diffraction (XRD) spectra of the as-synthesized nanocrystals and printed structures were essentially identical in terms of the observed peak widths and positions (Fig. 3e, Supplementary Fig. 12), further suggesting the prefect or near-perfect persistence of the nanocrystals through the entire process. Thermogravimetric analysis of the 3D-printed Au showed total weight losses of less than 5%, even at temperatures as high as 700°C (Supplementary Fig. 13), considerably lower than the 20 ~ 30% weight loss typically observed in organic-capped nanocrystal-based aerogels. This result further illustrates the stability of the printed inorganic aerogels.
The specific surface area and porosity of the printed structures were evaluated using nitrogen adsorption/desorption isotherms. All samples exhibited typical type II isotherms with type H3 hysteresis loops similar to those of known nanocrystal-based aerogels at high relative pressures (Fig. 3f, Supplementary Fig. 14). The pore size distribution plots of the printed structures confirm the broad pore size distribution (almost independent of the materials), indicating that the formation of pores was induced by the surface bridging of nanocrystals and was therefore independent of the characteristics of the nanocrystal cores (Fig. 3f inset, Supplementary Fig. 15). The printed Fe3O4 and Au samples showed the highest (279 m2 g− 1) and lowest (54.2 m2 g− 1) Brunauer-Emmett-Teller (BET) surface areas, respectively, which is in good agreement with those of reported nanocrystal-based aerogels. The mole-based surface areas of the printed materials are very high because the molecular weights of these materials are significantly higher than those of well-known silica aerogels. The silica-equivalent surface areas of the 3D-printed structures ranged from ~ 300 to ~ 600 m2 g− 1 (Fig. 3g, Supplementary Table 2), which are comparable to those of nanocrystal- and oxide-based aerogels32–39.
Diverse Functionality Of Printed Materials
The solidification of the inorganic nanocrystals induced by the surface-linking reaction effectively conserved the primary functionalities of the nanocrystals in the 3D-printed structures. For example, the 3D-printed Fe3O4 exhibited superparamagnetic properties like those of the as-synthesised Fe3O4 nanocrystals, with a blocking temperature of 28 K, and in good agreement with the preserved nano-structural characteristics observed in the SEM and TEM images (Supplementary Fig. 15). The magnetization curves also confirm the conservation of the magnetic properties of Fe3O4 nanocrystals, showing that the saturation magnetization and coercive fields of printed and as-synthesized nanocrystals at 2 K and 300 K were essentially identical (Fig. 3h). As another example, the 3D-printed wet and dried CdSe structures retained the excitonic features of the CdSe semiconductor nanocrystals. Even though a small red-shift and peak broadening were observed in the absorption spectrum owing to electronic coupling among nanocrystals in the connected networks, the printed CdSe still exhibit a relatively sharp onset, indicating that the quantum confinement effect of the nanocrystals is retained in the printed structure (Supplementary Fig. 16). The luminescent properties of the printed CdSe were recovered by the photooxidation treatment, although the luminescent properties of the inorganic CdSe nanocrystals in the ink were initially quenched via the surface exchange with the inorganic thiometallate ligands, which introduced numerous surface traps that cause non-radiative recombination. Our previous work described that the photooxidation of inorganic ligand-capped CdSe nanocrystals enhances their luminescent properties by the passivation of surface dangling bonds via surface oxidation40. The printed CdSe exhibits relatively sharp emission along with a small blue-shift and peak broadening in the photoluminescent (PL) spectrum (Fig. 3i). This is coincident with the absorption spectrum, which shows the peak shift and broadening, indicating that the PL properties originate from the band edges rather than the surface trap states. The blue shifts observed in the CdSe solid were attributed to deep surface oxidation, which reduced the size of the CdSe core. As a third example, the electronic coupling between metal nanocrystals in the percolating porous structure was responsible for its favourable electrical transport properties (Fig. 3j, Supplementary Fig. 17). The 3D-printed and CO2-dried Au sample exhibited an electrical conductivity of 4.0\(\times {10}^{1}\) S m− 1, which is within in the reported range of the reported conductivities of porous metal materials41. Drying the wet Au sample under ambient conditions induced a transformation into a denser xerogel with a significantly enhanced electrical conductivity of 1.9 \(\times {10}^{3}\) S m− 1. Heat treatment at 600°C further increased the electrical conductivity to 1.4 \(\times {10}^{6}\) S m− 1, which is within the range of the conductivities of micro-patterned electrodes in microelectronic systems42.
3d Printing Of Multi-material Architectures
Porous structures with multiple material compositions were 3D-printed by sequentially printing different nanocrystal inks, or by printing mixed inks containing different nanocrystals. A combination of Au and CdSe nanocrystals was selected as a model system because metal-semiconductor hybrid systems have broad applications in photonics, optoelectronics, and photocatalysis43. A lattice structure with alternating Au and CdSe layers was obtained via the sequential printing of Au and CdSe nanocrystal inks (Fig. 4a). SEM and EDS images showed good separation between the vertical Au and horizontal CdSe layers, without merging at the junctions (Fig. 4b–d). The printed lattice of the mixed Au-CdSe yielded a homogeneous composition of Au and CdSe phases, as confirmed by SEM and EDS images (Fig. 4e–h). The microstructural morphology of the highly porous network was similar to that of single nanocrystal-based samples (Fig. 4f inset). The STEM-EDS maps and high angle annular dark field-scanning TEM (HAADF-STEM) images showed that the Au and CdSe nanocrystals were isolated in the porous multibranched network (Fig. 4i). The absorption spectrum and XRD pattern confirmed that the Au and CdSe nanocrystals were retained in the composite structure, without atomic-scale structural rearrangements or changes in the electronic spectra (Fig. 4j, k). The BET surface area (132 m2 g− 1) was similar to the average value of the Au and CdSe printed structures, reflecting its homogeneity (Fig. 4l). These results demonstrate the straightforward 3D printing of multiple materials with multifunctional and synergistic features using our method.
We report the development of a high-precision 3D printing method for the production of sub-10 µm inorganic porous architectures by the ink writing of inorganic nanocrystals in a linker-containing nonsolvent bath. Controlling dispersibility and the subsequent surface linking reaction of the nanocrystals enabled their instant connection during the ink extrusion to produce high-fidelity 3D microarchitectures. This chemical strategy enables the 3D microprinting of a diverse range of functional materials, including metals, semiconductors, magnets, oxides, and so-called “multi-materials”, without the use of organic binders or special lithographic equipment. By incorporating low-dimensional materials, such as 1D and 2D materials and molecular clusters, we are confident that our process can be extended for printing an even broader range of inorganic materials with 3D micro-architectures and diverse functionalities. This approach conserves the microstructural porosity and properties of the building blocks in the 3D-printed inorganic structures, and will, we suggest, enable their wider implementation in thermal, catalytic, electronic, optical, and magnetic components, and in the manufacture of micro-inorganics for microelectronics, micromechanics, and biomedical systems.
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Chemicals
Gold(Ⅲ) chloride trihydrate (HAuCl4·3H2O, 99.9% trace metal basis, Aldrich), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, ACS reagent 37.5% Pt basis, Aldrich), iron(Ⅱ) chloride tetrahydrate (FeCl2·4H2O, puriss. p.a. 99%, Aldrich), silver nitrate (AgNO3, 99% ACS reagent, Aldrich), copper(Ⅱ) acetylacetonate (Cu(acac)2, 99.9% trace metal basis, Aldrich), platinum(Ⅱ) acetylacetonate (Pt(acac)2, 97%, Aldrich), iron(Ⅲ) acetylacetonate (Fe(acac)3, 99.9%, Aldrich), cadmium oxide (CdO, 99.99% trace metal basis, Aldrich), selenium powder (Se, 200 mesh 99.999% metal basis, Alfa Aesar), borane tert-butylamine complex (TBAB, 97%, Aldrich), 1,2,3,4-tetrahydronaphthalene (tetralin, 97%, Alfa Aesar), butylamine (99.5%, Aldrich), palmitic acid (99%, Aldrich), 1,2-hexadecanediol (90%, Aldrich), oleic acid (OA, 90%, Aldrich), oleyl amine (OLAm, 70%, Aldrich), trioctylphosphine (TOP, 90%, Aldrich), 1-octadecene (ODE, 90%, Aldrich), dioctyl ether (99%, Aldrich), phenyl ether (99%, Acros organics), ammonium tetrathiomolybdate (ATTM, 99.97%, Aldrich), nitrosyl tetrafluoroborate (NOBF4, 95%, Aldrich), trichloro(1H,1H,2H,2H-perfluorooctly)silane (PFOCTS, 97%, Aldrich), (3-aminopropyl)triethoxysilane (APTES, 99%, Aldrich), N-methylformamide (NMF, 99%, Aldrich), N,N-dimethylformamide (DMF, 99.8%, Aldrich), hexane (anhydrous 95%, Aldrich), toluene (anhydrous 99.8%, Aldrich), ethyl acetate (EA, 99.5%, SAMCHUN), tetrahydrofuran (THF, 99.5%, SAMCHUN), acetonitrile (anhydrous 99.8%, Sigma), N-methyl-2-pyrrolidone (NMP, anhydrous 99.5%, Sigma), dimethyl sulfoxide (DMSO, 99.9%, Sigma), dichloromethane (DCM, 99.8%, Sigma), chloroform (99.5%, SAMCHUN), pentanol (99%, Sigma), hexanol (98%, Sigma), cyclohexane (99%, Sigma), octane (95%, SAMCHUN), 1-butanol (BtOH, 99%, SAMCHUN), ethanol (99.5%, SAMCHUN), methanol (99.5%, SAMCHUN), 2-propanol (IPA, 99.5%, SAMCHUN), and acetone (99.5%, SAMCHUN). All the chemicals were used as received, without further purification.
Synthesis of colloidal nanocrystal inks
Au nanocrystals. Au nanocrystals were synthesised using a modified method based on a previous report44. 6 nm-sized Au seed nanocrystals were synthesised first. HAuCl4 (0.2 g), tetralin (10 mL), and OLAm (10 mL) were mixed into three-neck round bottom flask at room temperature under N2 flow and vigorous magnetic stirring. TBAB (0.5 mmol), tetralin (1 mL), and OLAm (1 mL) were mixed via sonication for 1 h at room temperature. The solution was then injected into the HAuCl4 solution and stirred for 1 h at room temperature. Acetone was added to the mixture, and centrifuged (7500 rpm, 5 min) to collect 6 nm Au seeds. 8 nm Au nanocrystals were synthesized using the synthesised Au seeds. HAuCl4 (0.1 g) was dissolved in ODE (10 mL) and OLAm (10 mL) at room temperature under N2 flow in three-neck round bottom flask. 6 nm Au seeds (30 mg) were added to a solution and the reaction solution was heated to 80 °C for 12 min and kept at this temperature for 2 h. Afterward, the heat source was removed, and the product solution was allowed to cool to room temperature. Ethanol was added to the mixture and centrifuged (7500 rpm, 5 min) to remove any unreacted residue. The Au nanocrystals were then dissolved in hexane.
Ag nanocrystals. Ag nanocrystals were synthesised using a modified method based on a previous report45. AgNO3 (1.7 g), Cu(acac)2 (0.2 g), and toluene (3.468 mL) were mixed into three-neck round bottom flask at room temperature. Butylamine (1.352 mL) and palmitic acid (0.5 g) were added to this mixed solution. The reaction mixture was heated to 110 °C for 17 min and kept at this temperature for 2 h. Afterward, the heat source was removed, and the product solution was allowed to cool to room temperature. Methanol was added to the mixture, and centrifuged (7500 rpm, 5 min) to remove any unreacted residue. The Ag nanocrystals were then dissolved in hexane.
FePt nanocrystals. FePt nanocrystals were synthesised using a modified method based on a previous report46. Pt(acac)2 (0.196 g), Fe(acac)3 (0.177 g), reducing agent 1,2-hexadecanediol (1.292 g), OA (0.16 mL), OLAm (0.164 mL), and dioctyl ether (20 mL) were mixed into three-neck round bottom flask at room temperature in glove box. The reaction mixture was heated to reflux at 286 °C and kept at this temperature for 30 min. Afterward, the heat source was removed, and the product solution was allowed to cool to room temperature. Ethanol was added to the mixture and centrifuged (7500 rpm, 5 min) to remove any unreacted residue. The FePt nanocrystals were then dissolved in hexane.
CdSe nanocrystals. CdSe nanocrystals were synthesised using a modified method based on a previous report47. CdO (0.255 g), OA (3.11 mL), and ODE (35 mL) were mixed into three-neck round bottom flask at room temperature in glove box. The solution was heated to 180 °C for 31 min and kept at this temperature for 1 h to form a clear solution under N2 atmosphere. Subsequently, the solution was heated up to 250 °C for 14 min. TOP-Se solution of Se (0.051 g), TOP (0.3 mL), and ODE (5 mL) were mixed and stirred more than 1 h in glove box. The TOP-Se solution was then injected into the CdO solution and stirred for 2 min. Afterward, the heat source was removed, and the product solution was allowed to cool to room temperature. Methanol, acetone, IPA were added to the mixture and centrifuged (7500 rpm, 5 min) to remove any unreacted residue. The CdSe nanocrystals were then dissolved in hexane.
Fe3O4 nanocrystals. Fe3O4 nanocrystals were synthesised using a modified method based on a previous report48. Fe(acac)3 (0.7063 g), 1,2-hexadecanediol (2.5844 g), OA (1.91 mL), OLAm (1.975 mL), and phenyl ether (20 mL) were mixed into three-neck round bottom flask at room temperature in glove box. The reaction mixture was heated to 200 °C for 35 min under flow of N2, and then kept at this temperature for 30 min. Subsequently, the mixture was heated to 265 °C for 13 min, then kept at this temperature for 30 min. The black-brown mixture was cooled to room temperature by removing the heat source. Ethanol was added to the mixture, and centrifuged (6000 rpm, 5 min) to remove any unreacted residue. The Fe3O4 nanocrystals were dissolved in hexane.
Synthesis of inorganic ligands-capped nanocrystal inks. Negative charges on the surfaces of nanocrystals were introduced by the ligand exchange process. All ligand exchange reactions were performed in a N2-filled glovebox using a typical two-phase ligand-exchange strategy49. ATTM was exploited as an inorganic ligand to replace the existing organic ligands. ATTM (0.9 g) was dissolved in NMF (30 mL) and stirred for 10 min. Then, 10 mL of nanocrystals containing hexane solution (30 mg mL-1) was added to a vial containing 30 mL of ATTM solution (30 mg mL-1). The mixture was vigorously stirred until the phase transfer of nanocrystals from the upper hexane phase to the bottom NMF phase was completed. After ligand exchange, the upper hexane phase was discarded, and the bottom layer of ATTM-capped nanocrystals was collected by the addition of IPA (210 mL). The purification step was repeated two times to collect the ATTM-capped nanocrystals.
For the case of Fe3O4 nanocrystals, the two-phase ligand exchange process cannot be adopted directly. The ligand stripping process was conducted first to remove the existing organic ligands50. NOBF4 (0.9 g) was dissolved in DMF (30 mL) and stirred for 10 min. To form an immiscible two-phase mixture, 10 mL of nanocrystals in hexane (30 mg mL-1) was added to a vial containing 30 mL of NOBF4 solution (30 mg mL-1). The immiscible two-phase mixture was vigorously stirred until the phase transfer of nanocrystals from the upper hexane phase to the bottom NOBF4 phase is completed. After ligand stripping, the upper hexane phase was discarded, and the bottom solution was collected by the addition of toluene (210 mL). The purification step was repeated two times to remove unreacted NOBF4 species. The stripped nanocrystals were redispersed in 30 mL of ATTM stock solution (30 mg mL-1) and stirred overnight. The solution was precipitated by the addition of IPA (210 mL) to collect the ATTM-capped Fe3O4 nanocrystals.
The ATTM- capped nanocrystals were dispersed in NMF to form stable colloidal solution with concentration of 25 mg mL-1 (Ag), 50 mg mL-1 (Au), 45 mg mL-1 (FePt), 50 mg mL-1 (CdSe), and 70 mg mL-1 (Fe3O4). A colloidal solution of ATTM-capped nanocrystals was exploited as 3D printing ink without any organic additives.
Hansen solubility parameter(HSP) difference (Ra) calculations
The HSPs indicate the cohesive energy density of a chemical resulting from the interactions of a given solvent molecule51. The energy needed to break all the cohesive bonds involves dispersion force, permanent dipole-permanent dipole forces, and hydrogen bonding. Thus, the total solubility parameter can be calculated in eq 1
Where δt (MPa1/2) is the solubility parameter and δd, δp, and δh are the dispersion force, dipole interaction force, and hydrogen bonding force term, respectively. Ra is the difference between the HSPs of two materials, given by eq 2
A smaller Ra indicates that the HSP of the two materials are likely to be miscible.
Dielectric constants of solvents were obtained in the ref. 52.
Wet 3D microprinting process of inorganic nanocrystals
Preparation of micronozzles, substrates, and linker containing solidification baths. The borosilicate glass capillaries were cleaned by rinsing with methanol, acetone, and IPA under sonication for 5 min each. The borosilicate glass capillaries were pulled to prepare a nozzle with a pipette puller (P-1000, Sutter Instruments). The pipette-pulling parameters, such as heat, pull, velocity, time, delay, and pressure, were tuned to fabricate diameter- and morphology-controlled glass pipettes. The pre-pulled glass pipettes were O2-plasma treated for surface hydrophilisation. Hydrophobic surface treatment was performed by a PFOCTS self-assembled monolayer (SAM) deposition step using a vapour-phase technique. The trichlorosilane-based head groups reacted with the hydroxyl group on the substrate to form a stable covalent bond. The PFOCTS SAM-coated glass pipettes were thermally treated at 120 °C for 20 min and rinsed with hexane to remove the unreacted PFOCTS species. The Si wafers were cleaned by rinsing with methanol, acetone, and IPA under sonication for 5 min each. The clean Si substrates were O2-plasma treated for surface hydrophilisation. Hydrophilic surface treatment was performed via APTES SAM deposition using a vapour-phase technique. The triethoxysilane-based head groups reacted with the hydroxyl group on the substrate to form a stable covalent bond. The APTES SAM-coated substrates were thermally treated at 100 °C for 30 min and rinsed with toluene to remove the unreacted APTES species. Finally, the linker baths were prepared by dissolving HAuCl4, H2PtCl6, and FeCl2·4H2O in 1-butanol to obtain 0.5–1 mM solution.
Wet 3D microprinting procedure. The printing machine consisted of a micronozzle connected to a dispenser (Ultimus 2, Nordson EFD) and a three-axis (x,y,z) stepping motor nanostage (Aerotech). The inorganic ligand-capped nanocrystals were loaded into a PFOCTS SAM-coated micronozzle, and the inks were pneumatically driven through a micronozzle at 2.1–100 kPa. The APTES SAM-coated Si substrate was attached to a glass petri dish and placed on a three-axis (x,y,z) nanostage. The distance between the micronozzle and the substrate was fixed at 10 μm. After controlling the distance, the linker bath solution was poured into a glass petri dish. Their positions and moving speeds were accurately controlled in real time using a motion composer software (A3200, Aerotech). The stage was translated at a speed of 1.2 mm s-1 during printing and the overall process was monitored using a side-view charge coupled device (CCD) camera (MicroPublisher 5.0 RTV, QImaging). Also, the videos showing the printing process were recorded using a CCD camera (Supplementary Video 1-3).
Supercritical drying. The printed object was solvent-exchanged with fresh butanol at 25 °C several times. After a complete exchange of the solvent, the printed wet state objects were transferred into a supercritical fluid extractor (SFT-110XW, Supercritical Fluid Technologies Inc.) with an excess amount of butanol to prevent the evaporation of the solvents. The chamber was flushed with liquid CO2 to exchange the butanol. To convert the liquid CO2 to the supercritical state, the vessel was pressurised and heated to 1800 psi and 60 °C, respectively. The butanol-CO2 mixture was extracted continuously through the exit of the vessel until all solvents were removed.
Characterisations
Microscopy analyses. The dimension and microstructure of the printed inorganic nanocrystal-based porous materials were imaged using OM and SEM, respectively. Optical imaging was performed using an OM (BX51M, Olympus). SEM (including tilted views) and EDS mapping image were collected using a field-effect SEM (Nova NanoSEM, FEI and SU7000, Hitachi High-Tech) with a 10 kV (SEM image) and 20 kV (EDS mapping image) electron beam. The CCD images and videos were obtained using a CCD camera (MicroPublisher 5.0 RTV, QImaging). The TEM images were obtained at 200 kV using a JEOL-2100 microscope (JEOL). HR-TEM, HAADF-STEM imaging, and spectral imaging based on STEM-EDS were performed at 200 kV using a JEM-2100F microscope (JEOL). For analysis, the printed objects were crushed and suspended in methanol by ultrasound for 15–120 s, depending on their dispersing ability.
N2 adsorption/desorption analysis. The sample porosities were determined using an N2 sorption analyser (BELSORP-Max, BEL) operated at 77 K. Prior to the measurements, the sample surfaces were evacuated at 70 °C for 12 h under vacuum conditions to clean the surfaces. The specific surface areas of the samples were calculated using the BET equation, while their pore size distributions were derived from the adsorption branches of the isotherms using the BJH (Barrett-Joyner-Halenda) method. The silica equivalent surface areas were calculated by the relative density method53. Here, the density of silica is assumed to be an average density of quartz (2.65 mg cm-3), tridymite (2.31 mg cm-3), and cristobalite (2.33 mg cm-3), 2.43 mg cm-3.
X-ray diffraction analysis. The XRD patterns were obtained using a high-power X-ray diffractometer (D/MAX2500V/PC, Rigaku) equipped with Cu Kα radiation and operated at 40 kV and 200 mA.
ζ-potential analysis. The ζ-potential data were collected using a Zetasizer Nano ZS instrument (Malvern). Inorganic ligand-capped Ag nanocrystals were measured before and after exposure to Au3+, Pt4+, and Fe2+ linker solution, respectively.
X-ray photoelectron spectroscopy. XPS spectra were acquired using an X-ray photoelectron spectrometer (ESCALAB 250XI, Thermo Fisher Scientific) with a monochromatic Al Kα X-ray source (1,486.6 eV). All XPS spectra were corrected with adventitious C 1s peak at 284.8 eV. For analysis, inorganic ligand-capped Ag nanocrystals were printed on the Si substrate filled with butanol, Au3+, Pt4+, and Fe2+ linker solutions, respectively. After drying with a supercritical fluid, all the samples were kept in a glove box to prevent oxidation before analysis.
Thermal stability analysis. The thermal stability of the printed Au nanocrystal-based porous material was investigated by thermogravimetric analysis (TGA Q500, TA Instruments) in the temperature range of 25–700 °C at a heating rate of 10 °C min-1 under a nitrogen atmosphere.
Magnetic property measurement. The magnetic property was measured by using a superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM, Quantum Design).
Optical property measurement. The UV-vis absorption spectra were measured using a UV-vis spectrophotometer (UV-2600, Shimadzu). The photoluminescence spectra were investigated using a UV-Vis-NIR spectrophotometer (Cary 5000) at room temperature. For analysis, wet state CdSe was prepared by extruding inorganic ligand-capped CdSe nanocrystals (1 mL) into the 0.5 mM concentration of Fe2+ linker solution (50 mL). The subsequent supercritical drying process can produce the dried state CdSe powder. The obtained dried state CdSe was suspended in NMF by ultrasound for 120 s. To investigate the specific oxidation effect on the optical properties, CdSe wet state and dried state dispersions were treated under UV irradiation under the air atmosphere for 24 h.
Electrical property measurement. The electrical conductivity was measured by a four-point van der Pauw method (Keithley 2,400 multimeter controlled by Lab trace 2.0 software, Keithley Instrument, Inc.). For analysis, inorganic ligand-capped Au nanocrystal inks (5 µl) were casted on the Si substrate filled with 0.5 mM Au3+ linker solution (2 mL) followed by supercritical drying process to produce the Au aerogel film. For the preparation of xerogel films, the solvent was evaporated in ambient conditions. After complete evaporation of the solvent, a homogeneous film was formed on the substrate. The Au xerogel film was heat treated at 600 °C for 1 h with a ramping rate of 5 °C min-1 under H2 (99.999%) in a tube furnace.
Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgments
We acknowledge the Nano·Material Technology Development Program (NRF-2018M3A7B8060697), the mid-career researcher program (NRF-2022R1A2C3009129 and NRF-2021R1A2C2007495), and the Creative Materials Discovery Program (NRF-2020M3D1A1110502) through the National Research Foundation of the Republic of Korea (NRF) funded by Ministry of Science and ICT, and the UNIST Research Fund (1.220024.01).
Author contributions
M.S. and J.S.S. designed the experiments, analysed the data, and wrote the paper. M.S., Y.K., B.V.C., S.E.Y., D.H.G., R.S.R. carried out the synthesis and basic characterisation of materials. M.S., D.S.B., and S.H.J. performed the characterisation of porosity. S.L. and J.-W.Y. performed the characterisation of magnetic properties. All authors discussed the results and edited and commented on the manuscript.
Competing interests
Ulsan National Institute of Science and Technology (UNIST) has filed a patent, PCT/KR2022/003817 (inventors: M.S., Y.K., and J.S.S.) that covers the wet 3D microprinting chemistry and methods reported in this article.
Yes there is potential Competing Interest.
Ulsan National Institute of Science and Technology (UNIST) has filed a patent, PCT/KR2022/003817 (inventors: M.S., Y.K., and J.S.S.) that covers the wet 3D microprinting chemistry and methods reported in this article.
- SupplementaryInformation.pdf
- SupplementaryVideo1.mov
Supplementary Video 1
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Supplementary Video 2
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Supplementary Video 3