In dual-templated sol-gel synthesis of three-dimensionally ordered hierarchical (macroporous plus mesoporous) metal oxides, a polymer-sphere colloidal crystal acts as a hard template for the macropore skeleton while a block copolymer such as Pluronic P123 soft-templates mesopores in the walls. A 2012 study of SiO2, Al2O3 and TiO2 prepared by this route found that structural differences between the three oxides trace back to the precursor hydrolysis rate, but could only infer this qualitatively, from differences between the final structures. The open problem is to establish, quantitatively and predictively, how precursor hydrolysis/condensation kinetics — as tuned by the adjustable synthesis parameters that control them in sol-gel chemistry (for example pH, temperature, precursor concentration, and acid catalyst and complexing-agent amounts) — govern the simultaneous ordering of both the macropore skeleton and the mesoporous walls, and to identify the mechanism coupling hydrolysis rate to ordering at both length scales. Reviews through 2020 still catalog per-oxide empirical recipes rather than such a predictive framework.

Background

Hierarchical porous materials combine pores of at least two size regimes — mesopores (roughly 2-50 nm) supplying surface area, and macropores (larger than about 50 nm) supplying mass transport — in one connected solid; for oxides such as silica, alumina and titania this combination underpins catalysis, photocatalysis, separation and energy-storage applications. A standard route to 3D ordered hierarchical oxides is dual templating: an opal-like colloidal crystal of monodisperse polymer spheres is infiltrated with a sol-gel precursor solution and later burned out, leaving an inverse opal (also called a three-dimensionally ordered macroporous, 3DOM, skeleton), while an amphiphilic block copolymer co-dissolved in the same solution — typically Pluronic P123, a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer — self-assembles on solvent evaporation (evaporation-induced self-assembly, EISA) into lyotropic liquid-crystal mesophases that template ordered mesopores in the skeleton walls. The inorganic network forms by hydrolysis and condensation of molecular precursors (metal alkoxides or salts): hydrolysis replaces alkoxide groups with hydroxyls, and condensation links the resulting species into an extended oxide network. The two templating events must succeed in one pot, and their kinetics are dominated by precursor hydrolysis, whose rate differs by orders of magnitude between metals — silicon alkoxides hydrolyze slowly and controllably, whereas titanium, aluminum and zirconium alkoxides react so fast that they precipitate rather than replicate the templates. The methodological lineage was established by co-templating with liquid-crystalline surfactant phases and polymer colloids (Sel, Kuang, Thommes and Smarsly, Langmuir 2006), and colloidal-crystal templating of inverse opals and hierarchical structures was systematized in the review of Stein, Li and Denny (Chemistry of Materials 2008); well-resolved hierarchical Ce-Zr mixed oxides and TiO2/SiO2 or TiO2/ZrO2 composites had been demonstrated before 2012. Against this background, Yang, Ma and Tang (Advanced Materials Research 2012) prepared hierarchical SiO2, Al2O3 and TiO2 with a colloidal-crystal hard template plus P123, and observed that the three oxides emerged with different pore structures, attributing the differences to the precursors' hydrolysis rates — a conclusion reached only by comparing final structures, with no kinetic measurements. Later reviews of ordered mesoporous metal oxides (Ren, Ma and Bruce, Chemical Society Reviews 2012; Gu and Schüth, Chemical Society Reviews 2014) codified the practical countermeasures to fast hydrolysis — acid catalysis, chelating ligands, pre-hydrolyzed sols — but present them as per-material, empirically tuned choices. At the mechanism level, in-situ time-resolved small-angle X-ray scattering has resolved how EISA builds mesostructure in supported films (Grosso et al., Advanced Functional Materials 2004) and has measured titania sol-gel condensation in free sols (Pattier, Henderson, Brotons and Gibaud, Journal of Physical Chemistry B 2010), yet these studies do not address the coupled situation inside a colloidal-crystal template. Reviews of hierarchically porous materials as late as 2017 (Yang, Chen, Li, Rooke, Sanchez and Su, Chemical Society Reviews 2017) and 2020 (Wu, Li, Fu and Su, National Science Review 2020) still frame the synthesis-structure relationship as an empirical design challenge, and confined synthesis of well-ordered macro/mesoporous titania was still described as 'a challenge' in 2019 (Wang et al., Chemical Physics 2019). What is missing, and what the source's observation points to, is a quantitative account linking precursor hydrolysis/condensation kinetics — and the adjustable synthesis parameters that control them — to the degree of ordering achieved at both template length scales, in place of a recipe re-derived empirically for each oxide.

Problem Statement

For oxides synthesized by dual templating — infiltration of a polymer-sphere colloidal crystal (hard template) with a solution containing a metal alkoxide or salt precursor plus Pluronic P123 (soft template), followed by drying, gelling and template removal — establish quantitatively how the hydrolysis and condensation kinetics of the precursor determine the degree of ordering achieved simultaneously at the two template length scales: the fidelity of the 3D ordered macropore (inverse-opal) skeleton and the ordering of the mesopores in its walls, with ordering characterized by standard metrics (e.g. small-angle X-ray scattering peak positions and q-ratios for mesopore symmetry, electron-microscopy assessment of macropore periodicity and wall integrity, surface area and pore-size distributions, and retention of ordering after template removal and calcination). The kinetics are to be connected to the adjustable synthesis parameters that control them in sol-gel chemistry — for example pH, temperature, precursor concentration, and acid catalyst and complexing-agent amounts — so that the account goes beyond the source's post-hoc comparison of final structures and supports prediction. The problem has two linked deliverables: (1) a quantitative parameter-to-kinetics-to-ordering relationship for this materials class; and (2) the mechanistic explanation of how precursor hydrolysis rate couples to the self-assembly and templating processes at both length scales — why too-fast hydrolysis degrades replication and ordering, and in what kinetic window both the colloidal-crystal replication and the P123-directed mesostructure formation succeed. The route of investigation is left open: in-situ time-resolved scattering or spectroscopy, kinetic/assembly modeling, and systematic synthesis-kinetics-structure experimentation are each admissible ways of establishing the relationship. Extending the account beyond the SiO2, Al2O3 and TiO2 systems studied in the source is a motivation for making it predictive — hydrolysis rates differ by orders of magnitude between metals — rather than a required demonstration on any particular further oxide. The overarching aim is predictive parameter selection: for a target oxide, to be able to choose synthesis-parameter values in advance that yield hierarchical porosity ordered at both scales, judged by the characterization metrics listed above.

References

  1. Renchun Yang, Fengyun Ma, Dingxing Tang, 'Template Synthesis to Fabrication of 3D Ordered Hierarchical Materials', Advanced Materials Research 602-604 (2012) 1355-1358, DOI 10.4028/www.scientific.net/amr.602-604.1355, https://doi.org/10.4028/www.scientific.net/amr.602-604.1355
  2. Ozlem Sel, Daibin Kuang, Matthias Thommes, Bernd Smarsly, 'Principles of Hierarchical Meso- and Macropore Architectures by Liquid Crystalline and Polymer Colloid Templating', Langmuir 22 (2006) 2311-2322, DOI 10.1021/la0520841, https://doi.org/10.1021/la0520841
  3. Andreas Stein, Fan Li, Nicholas R. Denny, 'Morphological Control in Colloidal Crystal Templating of Inverse Opals, Hierarchical Structures, and Shaped Particles', Chemistry of Materials 20 (2008) 649-666, DOI 10.1021/cm702107n, https://doi.org/10.1021/cm702107n
  4. Nicole D. Petkovich, Stephen G. Rudisill, Luke J. Venstrom, Daniel B. Boman, Jane H. Davidson, Andreas Stein, 'Control of Heterogeneity in Nanostructured Ce1-xZrxO2 Binary Oxides for Enhanced Thermal Stability and Water Splitting Activity', The Journal of Physical Chemistry C 115 (2011) 21022-21033, DOI 10.1021/jp2071315, https://doi.org/10.1021/jp2071315
  5. Xiufang Chen, Xinchen Wang, Xianzhi Fu, 'Hierarchical macro/mesoporous TiO2/SiO2 and TiO2/ZrO2 nanocomposites for environmental photocatalysis', Energy & Environmental Science 2 (2009) 872, DOI 10.1039/b904012a, https://doi.org/10.1039/b904012a
  6. Yu Ren, Zhen Ma, Peter G. Bruce, 'Ordered mesoporous metal oxides: synthesis and applications', Chemical Society Reviews 41 (2012) 4909, DOI 10.1039/c2cs35086f, https://doi.org/10.1039/c2cs35086f
  7. David Grosso, Florent Cagnol, Galo J. de A. A. Soler-Illia, Eduardo L. Crepaldi, Heinz Amenitsch, Aline Brunet-Bruneau, Alexandre Bourgeois, Clement Sanchez, 'Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly', Advanced Functional Materials 14 (2004) 309-322, DOI 10.1002/adfm.200305036, https://doi.org/10.1002/adfm.200305036
  8. Bruno Pattier, Mark Henderson, Guillaume Brotons, Alain Gibaud, 'Study of Titanium Oxide Sol-Gel Condensation Using Small Angle X-ray Scattering', The Journal of Physical Chemistry B 114 (2010) 5227-5232, DOI 10.1021/jp9086305, https://doi.org/10.1021/jp9086305
  9. Dong Gu, Ferdi Schüth, 'Synthesis of non-siliceous mesoporous oxides', Chemical Society Reviews 43 (2014) 313-344, DOI 10.1039/c3cs60155b, https://doi.org/10.1039/c3cs60155b
  10. Zhen-Xing Li, Ming-Ming Li, 'Highly Ordered Hierarchical Macroporous-Mesoporous Alumina with Crystalline Walls', Catalysis Letters 146 (2016) 1712-1717, DOI 10.1007/s10562-016-1795-5, https://doi.org/10.1007/s10562-016-1795-5
  11. Minghui Sun, Chen Chen, Lihua Chen, Baolian Su, 'Hierarchically porous materials: Synthesis strategies and emerging applications', Frontiers of Chemical Science and Engineering 10 (2016) 301-347, DOI 10.1007/s11705-016-1578-y, https://doi.org/10.1007/s11705-016-1578-y
  12. Xiao-Yu Yang, Li-Hua Chen, Yu Li, Joanna Claire Rooke, Clément Sanchez, Bao-Lian Su, 'Hierarchically porous materials: synthesis strategies and structure design', Chemical Society Reviews 46 (2017) 481-558, DOI 10.1039/c6cs00829a, https://doi.org/10.1039/c6cs00829a
  13. Ruicong Wang, Kun Lan, Beibei Liu, Yifeng Yu, Aibing Chen, Wei Li, 'Confinement synthesis of hierarchical ordered macro-/mesoporous TiO2 nanostructures with high crystallization for photodegradation', Chemical Physics 516 (2019) 48-54, DOI 10.1016/j.chemphys.2018.08.025, https://doi.org/10.1016/j.chemphys.2018.08.025
  14. Liang Wu, Yu Li, Zhengyi Fu, Bao-Lian Su, 'Hierarchically structured porous materials: synthesis strategies and applications in energy storage', National Science Review 7 (2020) 1667-1701, DOI 10.1093/nsr/nwaa183, https://doi.org/10.1093/nsr/nwaa183