3D Printing of Ordered Mesoporous Silica Complex Structures

Ordered mesoporous silica materials gain high interest because of their potential applications in catalysis, selective adsorption, separation, and controlled drug release. Due to their morphological characteristics, mainly the tunable, ordered nanometric pores, they can be utilized as supporting hosts for confined chemical reactions. Applications of these materials, however, are limited by structural design. Here, we present a new approach for the 3D printing of complex geometry silica objects with an ordered mesoporous structure by stereolithography. The process uses photocurable liquid compositions that contain a structure-directing agent, silica precursors, and elastomer-forming monomers that, after printing and calcination, form porous silica monoliths. The objects have extremely high surface area, 1900 m2/g, and very low density and are thermally and chemically stable. This work enables the formation of ordered porous objects having complex geometries that can be utilized in applications in both the industry and academia, overcoming the structural limitations associated with traditional processing methods.

O rdered mesoporous silica are used in widespread technological applications requiring both functionality and high porosity such as catalysis, 1 selective adsorption, 2 separation, 3 and controlled drug release, 4 due to their tunable pore diameter (2−50 nm), uniform cylindrical structure, 5 high chemical and thermal stability, and high surface area. 6 Applications of these materials, however, are limited by the structural design of currently available methods.
One possible strategy to synthesize ordered mesoporous silica (OMS) is by blending silica-forming precursors with a liquid crystal template. 7,8 The template is based on concentrated surfactants that self-assemble into mesophase micelles. The condensation polymerization of silica precursors by a sol−gel process is confined by the template, forming a ceramic-like framework with an ordered orientation of channels (e.g., hexagonal, cubic, etc.). After condensation, an aging process takes place, making the formed silica framework stronger, though accompanied by shrinking of the obtained monolith. 9 After the aging process, the organic templates can be calcined, resulting in OMS monolith. Triblock copolymers such as poly(ethylene oxide)−poly(propylene oxide)−poly-(ethylene oxide) (PEO−PPO−PEO) surfactants are good templating agents because of their microstructural ordering properties, amphiphilic character, commercial availability, and biodegradability. Specifically, Pluronic F-127 is a unique surfactant because it has a high molecular weight and long hydrophilic chains, allowing the synthesis of mesoporous oxides with thick walls that mechanically stabilize the monolith. 10 3D printing technology is a powerful additive manufacturing (AM) approach for fast, accurate, and customized fabrication of objects with complex geometries, in the macro-, 11−15 meso-, 16,17 and microscales. 18−23 To the best of our knowledge, there are no reports yet on the formation of ordered mesoporous structures by printing precursor solutions, which result in ordered monolithic complex structures at high resolution. The 3D printing reports in this field were conducted by applying a direct ink writing (DIW) process, which is based on the extrusion of a viscous liquid and typically has low resolution and structural complexity. The printed compositions were based on either mesoporous particles dispersed in a liquid or a sol−gel composition. The first utilized the incorporation of mesoporous silica particles within a matrix, for biomedical applications, such as bone regeneration 24,25 and drug delivery. 26,27 The second approach used a sol−gel-based ink, composed of glycolated silanes and structure-directing agents, to fabricate hierarchical porous silica objects. 28 However, these objects do not have a very high surface area, only 100−700 m 2 /g, and their structural complexity and resolution are limited. It is therefore highly desired to develop new AM pathways for the fabrication of high-resolution, complex 3D objects, with ordered porous microstructures that enable very high surface area, while utilizing the beneficial features of silica such as high thermal and mechanical stabilities.
Here, we utilize digital light processing (DLP) printing, which is a stereolithography 3D printing technology which applies a patterned UV light to cure sequential 2D layers of UV-polymerizable ink to create OMS 3D objects. This technology is characterized by micrometer-resolution abilities, fast processing speed, and the capability of generating highly complex 3D objects from macro-to microscale without support materials. 29,30 The printed objects have unprecedented complexity while maintaining a highly ordered mesoporous structure of silica (OMS) monolith, with very low density and extremely high surface area due to the nanometric hierarchical structure. This state-of-the-art printing process bypasses the structural limitations associated with OMS monolith fabrication, a significant step toward new and improved reactors and thermal insulators.
The presented process for the fabrication of complex objects with porous structures is based on the formation of new ink compositions, 3D printing, aging, and calcination, as schematically shown in Figure 1. The ink solutions are composed of a templating agent, silica precursor, and polymerizable monomers. The templating agent, F-127, leads to the formation of a hexagonal liquid crystalline structure, composed of rodlike micelles (Figure 1a,b). When the silica precursor, tetraethyl orthosilicate (TEOS), is added, a sol−gel process begins to form a silica skeleton around the rodlike micelles, by hydrolysis and condensation reactions (Figure 1c). For enabling the photopolymerization process by the DLP printing, the above solution is mixed with a UV-curable monomer composition that was previously reported by us, which leads to highly elastic polymer upon UV exposure. 31 The UV-curable composition is a mixture of epoxy aliphatic acrylate monofunctional monomer, aliphatic urethane diacrylate cross-linker, and a photoinitiator (PI). During the UV irradiation, the PI initiates a localized photopolymerization of the organic monomers, corresponding to the desired computer-aided design (CAD) model (Figure 2a), to form an elastomeric polymeric organic network that confines the ordered silica−surfactant network ( Figure 2b). The role of the stretchable organic network is crucial for preventing the deformation of the ordered microstructure, occurring later during the sol−gel aging The calcined 3D-printed porous objects were visualized by SEM ( Figure S1) and STEM. Figure 2e,f,h−i shows the hexagonal morphology, with an average pore size of 5.7 ± 0.3 nm and an average silica wall thickness of 6.3 ± 0.3 nm. An energy-dispersive X-ray spectroscopy (EDS) analysis confirmed that the final monolith contained only Si and O atoms (Figure 2g,j), in agreement with thermal gravimetric analysis (TGA) showing no weight loss after heating the calcined monoliths to 950°C, as expected ( Figure S2).
To further study the mesoporous printed structures, N 2 adsorption−desorption measurements and small-angle X-ray scattering (SAXS) measurements were performed. The surface area was calculated from the BET curves (Figure 3a) which showed a type IV isotherm according to the IUPAC classification of SBA-15 mesoporous structures. The BET surface area of the OMS objects was calculated to be 1900 ± 80 m 2 /g, which is about twice the highest value reported so far for monoliths prepared from solutions 32 in using Pluronic F-127 as a template, and comparable to the highest surface area for OMS powders obtained by any other surfactant. 33 The pore size distribution was analyzed by Barrett−Joyner− Halenda (BJH) desorption plots, showing an average size of 5.6 ± 0.3 nm (Figure 3b), in agreement with the observations in the STEM images (Figure 2f). The low density of the objects, 0.26 ± 0.02 g/mL, is a result of both the mesopores within the ordered structure and the macropores occurring during the fabrication process. The contribution of the macrosize pores (seen in Figure S1) to the object volume was calculated as follows: from the BJH desorption results, the total volume of the nanometric pores is 2.7 ± 0.2 cm 3 /g. The volume of the objects used for density measurements was 3.8 ± 0.3 cm 3 /g, and the volume of the silica is 0.37 cm 3 /g. Therefore, the volume of the macropores is 0.7 ± 0.4 cm 3 /g, which means that approximately 78% of the volume of the objects results from the presence of the nanopores. SAXS  (Figure 3c). The (1, 1) peak was not observed because it was localized at a minimum of the form factor of a long cylinder. We attribute the high surface area of this material to the high order of the pores and high pore density. To evaluate the best ink composition for the fabrication of OMS, objects with various ratios of liquid-crystal solution to elastomer were prepared and measured by SAXS. Figure 4a shows scattering curves from the ordered mesoporous structure with 50%, 45%, and 60% elastomer. Figure 4b shows line-shape analysis of the first correlation peak, extracting the hexagonal lattice parameter as well as the average domain size (the length over which positional correlations are maintained) of the ordered structure. 34 The largest ordered domain size was obtained with 50% elastomer. Two hexagonal phases coexisted when the ratio was 40% or 70%. These samples had a small bump at very low q, arising from a loose arrangement with a correlation distance of about 30 nm. The low-order pore arrangement can be seen from the TEM images presented in Figure S3.
The aging process of the sol−gel also affected the order of the pores due to the change of the micelles' arrangements at various temperatures. To investigate the optimal aging conditions, printed hybrid objects with inks consisting of 50% of elastomer were aged at several temperatures between −18 and 60°C, prior to calcination. The absence of high harmonic correlation peaks in the SAXS data at 60, 4, and −18°C (Figure 4c) shows that the level of order was lower compared with the samples which were aged at 40 and 22°C, suggesting that there is a narrow range of temperature that supports higher order. Aging at 4°C showed two adjacent peaks, corresponding to two arrangements of pores. As only the first harmonics of both arrangements were detected, the lattices' symmetry could not be determined. However, they are likely to be hexagonal phases, as observed at 40 and 22°C and revealed from the TEM images of the sample ( Figure S4). Figure 4d presents the nearest neighbor distance between the pores, as a function of the aging temperature, assuming that the organization of the hollow cylinders is hexagonal.
The liquid crystal template is a key factor in determining the order of the final monolith. Figure 5a presents the cryo-TEM image of the template before the silica precursor addition, visualizing large arrays of long wormlike mesophase micelles. Image analysis of the micelles shows a wall to wall thickness of 8.2 ± 0.4 nm. Scattering curves at low (Figure 5b) and high ( Figure 5c) q values from the micellar solution, measured in our wide-angle X-ray scattering (WAXS) setup, 35 reveal and confirm the hexagonal phase of elongated F-127 micelles, with a lattice size of 7.98 nm in agreement with the image analysis data. The main peaks visible at the wide-angle signal are characteristic of the PEO monoclinic crystalline phase 36,37 with lattice parameters of a = 0.804 nm, b = 1.30 nm, c = 1.95 nm, and β = 125.4°. After the addition of TEOS, the center-tocenter lattice spacing is 14.3 ± 0.5 nm, and the arrangement of the pores is hexagonal ( Figure S5). In summary, we described a 3D printing process to fabricate SBA-15 silica composed of highly ordered, nanometric pores. The 3D printing of these inorganic monoliths enables the construction of complex shaped objects, which have chemical resistance, 38 are stable at elevated temperatures, and have very low density with an extremely high surface area. This opens the way for many applications such as catalysis, adsorption, separation, and thermal insulators in which the performance of materials with high surface area can be improved by shaping them into specific structures, such as catalytic plug-flow reactors. 39 An additional field in which the presented approach can be utilized is bioprinting, for example, in the fabrication of bioceramic scaffolds for bone regeneration. 40,41 Although we Nano Letters pubs.acs.org/NanoLett Letter report here on mesoporous silica, the same approach can be applied for a variety of other ceramic materials. We expect that the same type of compositions can be utilized in two-photon printing, to yield micron-size objects with submicron features. Experimental Section. Ink Preparation. The sol−gel solution was prepared as follows: 14 wt % Pluronic F-127 (Sigma-Aldrich) was slowly added to 33 wt % warm (35°C) ethanol (ethanol absolute, ACROS Organics) under stirring. After the full dissolution of the Pluronic F-127, 7 wt % TDW (tripled distilled water) and 1 wt %, 2 M HCl solution (Sigma-Aldrich) were added, and the solution was stirred for another 1 min at 35°C. Then, the solution was cooled down to room temperature (RT) under stirring for 15 min, and 45 wt % of TEOS (tetraethoxysilane, 98%, Alfa Aesar) was added slowly into the solution. For the preparation of the UV-curable stretchable ink, aliphatic urethane diacrylate (Ebecryl 8413, Allnex), epoxy aliphatic acrylate (Ebecryl 113, Allnex), and TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide, IGM)  Nano Letters pubs.acs.org/NanoLett Letter with a weight ratio of 48.5:48.5:6, respectively, were mixed for 1 h at 50°C until a homogeneous solution was obtained. Before printing, the sol−gel solution and the stretchable ink were mixed for 5 min at different wt % ratios (usually 50:50). Sample Fabrication. A predesigned CAD model was 3Dprinted using a DLP 3D printer (Freeform PICO 2, Asiga). This printer operates by a UV-LED light source (385 nm), with a light intensity of 30 mW/cm 2 . The printer bath was filled with the ink, and the exposure was set to be 6 s for a layer thickness of 200 μm. After 3D printing, the structures were kept in an open vessel at temperatures from −18 to 60°C for 1 week. Then, the aged samples were calcined at 700°C (heating profile, 1°C/min to 480°C for 4 h, then 1°C/min to 700°C for 2 h).
Characterization. Shrinkage and the density were measured by a caliber for printed cube models with a size of 5 × 5 × 5 mm 3 , after printing, aging, and calcination.
TGA measurements of the objects were performed with a TGA/DSC1 stare system Mettler−Toledo in the range 25− 950°C at a heating rate of 1°C/min under air.
Small-/wide-angle X-ray scattering (SAXS/WAXS) measurements were performed using the Kα photons with an energy of 8 keV (λ = 1.54 Å) and X-ray beam size defined by two scatterless slits whose opening were set to be 1 × 1 mm 2 and 600 × 600 μm 2 . Azimuthal integration of the 2D scattering pattern was performed using FIT2D. 42 Before each measurement, silver behenate was used as a standard to determine the sample-to-detector distance (about 1850 mm for the SAXS setup and 450 mm for the WAXS setup). 43 Before measurements, samples were prepared as powders and introduced into a thin quartz capillary (1.5 mm in diameter and a wall thickness of 0.01 mm) to minimize beam absorption by the sample holder. The scattering signal of an empty capillary was measured and used as a background measurement. The background-subtracted data were analyzed using our homedeveloped cutting edge data analysis software X+ 44 and D+. 45 Whereas D+ computes exactly, without approximation, the scattering of the ordered structure, according to the parameters introduced in the model (radius of the pores, D spacing, and domain size), X+ uses the Warren's approximation 34 in order to calculate the average domain size of the organized hollow cylinders.
Direct imaging of the samples was performed by electron microscopy. The slicing of the printed object for the STEM imaging was performed by a focused ion beam (FIB) instrument (Helios 460F1, FEI), after selective coating of the surface-of-interest with a layer of platinum to prevent damaging of the sample. STEM imaging was performed using a Themis-Z instrument equipped with a superX EDX probe (Thermo Fisher Scientific). TEM images were obtained with the use of a high-resolution transmission scanning electron microscope (Tecnai F20 G2), and SEM images were obtained with the use of an extra-high-resolution scanning electron microscope (XHR Magellan 400L). The imaging of the micelle solution in their native, aqueous environment was performed using cryogenic transmission electron microscopy (cryo-TEM). In this method, a drop (2.5 μL) of the solution was deposited on a glow-discharged TEM grid (300 mesh Cu Lacey substrate, Ted Pella, Ltd.). A Vitrobot Mark IV (FEI) instrument was used to blot the excess liquid in a controlled environment (temperature and humidity) and to vitrify the specimens by a rapid plunging into liquid ethane precooled with liquid nitrogen. The vitrified samples were examined at −177°C using an FEI Tecnai 12 G 2 TWIN TEM instrument operated at 120 kV and equipped with a Gatan 626 cold stage. TIA (Tecnai Imaging & Analysis) software was used to record the images in low-dose mode by a 4K × 4K FEI Eagle CCD (charge-coupled device) camera.
The specific surface area and pore size distribution were measured with the use of a N 2 adsorption−desorption apparatus (Micromeritics ASAP 2020), at −196°C. All samples were degassed under vacuum at 120°C for 10 h before analysis. The surface area was calculated using the BET equation, over acquired adsorption data in the P/P 0 range 00.2−1. The pore distribution was analyzed using the BJH method, over the acquired desorption data points.
SEM images, TEM images of objects with varies elastomer percentages, TEM images of objects aged at different aging temperatures, cryo-TEM of the liquid crystal template with the silica precursor, and TGA of the objects (PDF) Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) program.