Microfabrication by Two-Photon Lithography, and Characterization, of Sio2/Tio2 Based Hybrid and Ceramic Microstructures

Journal of Sol-Gel Science and Technology https://doi.org/10.1007/s10971-020-05355-3 REVIEW PAPER: NANO-STRUCTURED MATERIALS (PARTICLES, FIBERS, COLLOIDS, COMPOSITES, ETC.) Microfabrication by two-photon lithography, and characterization, of SiO2/TiO2 based hybrid and ceramic microstructures 1 1 2 1 1 1 Anne Desponds ● Akos Banyasz ● Gilles Montagnac ● Chantal Andraud ● Patrice Baldeck ● Stephane Parola Received: 5 May 2020 / Accepted: 25 June 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract High resolution fabrication using two-photon lithography is extensively studied for a large range of materials, from polymer to inorganics. Hybrid materials including a sol–gel step have been developed since two decades to increase mechanical or optical properties in particular on silicon-based materials. Among the metal oxide, few studies have been dedicated to titanium and, because of the high reactivity of titanium precursors, obtaining a resin with a high part of titanium is challenging. Indeed, resins for two-photon lithography have to be stable for the processing time and titanium precursors are more difficult to operate due to their higher reactivity and often require extreme working conditions in order to control the 1234567890();,: 1234567890();,: chemical processes. Here, we propose a method, working at ambient conditions, to print submicronic structures of organic–inorganic hybrids with a large proportion of titanium and ceramics using high resolution two-photon process. The material obtained and its evolution during the pyrolysis at 600 and 1000 °C are characterized. We show that TiO2/SiO2-based microceramics can be obtained after the pyrolysis of the microstructures. The respective roles of the two chemical reactions involved in this lithography process, sol–gel condensation and radical photopolymerization, are highlighted. Graphical Abstract Keywords Microfabrication ● Ceramics ● Sol–gel ● Hybrids ● Two-photon polymerization * Anne Desponds 2 Laboratoire de Géologie, CNRS UMR 5276, Ecole Normale [email protected] Supérieure de Lyon, Université de Lyon 1, 46 allée d’Italie, 69364 * Stephane Parola Lyon, France [email protected] 1 Laboratoire de Chimie, CNRS UMR 5182, Ecole Normale Supérieure de Lyon, Université de Lyon 1, 46 allée d’Italie, 69364 Lyon, France Journal of Sol-Gel Science and Technology Highlights ● 3D printing using two-photon absorption processes for the fabrication of high resolution at the submicronic scale of ceramic components. ● Printing TiO2-based ceramics of various compositions, controlling the resolution, structures, and shape after annealing is an important challenge and open perspective to large range of applications. ● Two chemical reactions concomitantly involved during the process are critical, sol–gel condensation and radical photopolymerization. ● Sol–gel approaches toward high resolution 3D printed ceramics is an extremely promising method. 1 Introduction addition reactions possible when coordination of the metal is not complete [25]. Consequently, this metal center is very The patterning of sub-micrometric structures with high sensitive to reaction with ambient H2O. To handle the three-dimensional (3D) spatial resolution is in continual titanium precursor and permit two-photon lithography resin, development to meet and improve the requirements of these compounds are stabilized by MAA to form less applications in various domains (automotive, aerospace, reactive clusters. These reactions between alkoxides and defense, healthcare, consumer, industrial automation, ligands have been particularly studied [26]. It has been robotics, telecommunications). Most of the processes used highlighted that the clusters formation are due to partial to obtain the microdevices are composed of several steps hydrolysis, thus making possible the incorporation of such including chemical and/or physical treatments [1], and the compounds as building blocks for hybrid inorganic–organic utilization of molds or masks, especially with inorganic matrix [27]. However, even under cluster form, the system materials [2, 3]. The multiphotonic lithography technic, also remains very reactive. The addition of a silicon precursor called “direct writing lithography”, may replace these mixed makes it possible to reach a manageable composition but processes which are difficult and complicated, through a resins including a molar ratio Ti/Si superior to 1 are not fabrication by a 3D printing process involving reactions of achievable in these conditions, and the microstructures radical photopolymerization. The two-photon lithography, presented eventually a cracked surface aspect [10]. More based on a two-photon absorption process, has attracted a recently, it has however being demonstrated that, under an high level of interest due to its high spatial resolution, argon environment without oxygen, the proportion of tita- making possible the production of microdevices and nium precursor could be increased [22]. microstructured surface. This technique was explored in the We propose here a simple route, in easily controllable 1990s and the 2000s [4–7]. Many materials are compatible conditions, to obtain ceramic microstructures with high with the two-photon lithography such as photopolymers [8], titanium dioxide content from sol–gel-based resins. The inorganic–organic material composed of silica precursors principle is to create mixed composite oligomers during built up in organic network [9], and its derivatives including the patterning of the microstructures, either by pre- titanium [10], zirconium [11, 12], or metal hybrids [13]. hydrolysis and aging condensation of the alkoxide pre- Full inorganic condensation sol–gel process has also been cursor, followed by two-photon polymerization, or by developed but remains hitherto limited to silica-based simultaneous two-photon controlled inorganic condensa- materials [14, 15]. To obtain full inorganic materials, the tion and two-photon organic photopolymerization. We fabrication in a polymeric matrix including inorganics fol- have noted that the type of titanium precursor has an lowed by a debinding of the organic matter is a route that influence on the stability of the resin. In particular, the has been explored for SiCN microstructures [16], SiO2/ZrO2 replacement of the traditionally used titanium isoprop- [17], or SiOC [18]. oxide by the titanium butoxide allows us to increase the The incorporation of TiO2 in such micromaterials has molar ratio Ti/Si up to 2.3 in the resin in our experimental been widely studied as this metal oxide presents excellent conditions. Infrared spectroscopy allows characterization optical properties (high refractive index) and electronic of the two chemical reactions involved during the transi- properties (semiconductor) [19, 20]. Similar to the tion from resin to micromaterial. On the one hand, the ORMOCER, inorganic–organic resins including Titania sol–gel condensation occurs, creating preform network precursors have been studied for photonics [10, 21] pressure and depending on the aging time, this reaction is more or sensors [22] and biological [23, 24] applications. However, less advanced. On the other hand, the radical photo- contrary to the silica precursors, the titanium is much more polymerization permits to connect the preform sol–gel sensitive to the environment because of (1) its stronger network, and thus to form the micromaterial. electrophile character and (2) its coordination number The fabricated microstructures of composite inorganic– which is superior to its valence, making nucleophilic organic hybrid can then be calcined to remove the organic and Journal of Sol-Gel Science and Technology thus lead to inorganic microstructures with increased optical 0.5, 1, and 3) are considered (Table 1). The reaction med- and mechanical properties. Such process toward Titania-based ium is stirred for 1 h. ceramics remains barely studied. Here is reported the two- Simultaneously, in another amber vial, the titanium pre- photon fabrication of TiO2-based microceramics from such cursor solution is prepared: a solution of Ti(OiPr)4 (TiP) at hybrids. 70 wt% in acetone is prepared and the MAA is added slowly under stirring, with a molar ratio equal to 1 (Table 1). The reaction is stirred for 1 h. 2 Experimental Finally, the resin is prepared in a third amber vial. The silicon solution is added to the photoinitiator, Michler’s 2.1 Chemicals ketone, (0.15 mmol). The titanium precursor solution is then dropped under stirring. The molar proportion of the TiP 3-(Trimethoxysilyl)propyl methacrylate 98% (MAPTMS), solution added to the MAPTMS solution is tuned between 4.4-Bis(dimethylamino)benzophenone 98% (Michler’s samples according to the following molar ratios Ti/Si: 0.4, ketone) are purchased from Sigma-Aldrich. Titanium iso- 0.7, 1, 1.5, 2.3 (Table 1). i propoxide Ti(O Pr)4 (TiP) and titanium butoxide Ti(OBu)4 (TiB) are purchased from Acros. Methacrylic acid (MAA) 2.3 Photopolymerizable resins: MAPTMS/TiB/MAA stabilized with 250 ppm MEHQ is bought from Alfa. The photobase “N-methylnifedipine ethyl ester derivative” is In an amber vial, the silicon precursor solution is prepared: synthetized in our lab and the synthesis method is available a solution of MAPTMS at 80 wt% in acetone is prepared, elsewhere [15]. Concentrated hydrochloric acid 37% (HCl), and a solution of diluted HCl (0.1 M) is added to hydrolyze and acetone 99% are purchased from VWR. The water the silane. The molar ratio H2O/MAPTMS is maintained (H2O) used

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