Rigid, porous objects and surfactants serve as powerful templates for the formation of mesoporous and macroporous materials. When both types of template are combined in a single synthesis, materials with intricate architectures and hierarchical porosity can be obtained. In this tutorial review, we explain how to conduct syntheses with both soft and hard templates; moreover, we describe methods to control the final structure present in the templated material. Much of the foundation for multiple templating lies in the study of materials made with only one type of template. To establish a foundation in this area, a description of hard and soft templating is given, delving into the templates available and the steps required for effective templating. This leads into an extended discussion about materials templated with both hard and soft templates. Through the use of recent examples in the literature, we aim to show the diversity of structures possible through multiple templating and the advantages these structures can provide for a wide range of applications. An emphasis is placed on how various factors-such as the type of template, type of precursor, heat-treatment temperature, confinement within a small space, and template-template interactions-impact morphology.

Finding pathways to renewable generation of fuels is a crucial step toward mitigating the ecological impacts of fossil fuel combustion. A renewable fuel requires a sustainable energy input and abundant feedstocks. One promising route is through the use of concentrated solar energy to drive the thermochemical splitting of H 2 O and CO 2 . The splitting of H 2 O generates hydrogen fuel and oxygen. Additionally, splitting both H 2 O and CO 2 generates syngas (CO and H 2 ) that can be converted into hydrocarbon fuels through the FischerÀTropsch process. Direct splitting of H 2 O and CO 2 in a single step is extremely endergonic (ΔG H 2 > 0 at T < 4700 K; ΔG CO > 0 at T < 3200 K), and it is difficult to separate the product gases at the required temperature. 1,2 Therefore, direct "one-step" splitting remains impractical.Metal oxides can be used to circumvent the challenges of one step thermochemical fuel production by breaking the process into two steps. 3 First, the solid metal oxide is thermally reduced at high temperatures (>1200 °C), releasing O 2 . The reactive material is then reoxidized by H 2 O or CO 2 at lower temperatures, producing H 2 or CO. This cyclic process allows for lower requisite temperatures for splitting, permits recycling of the metal oxide, and provides intrinsic separation of product gases (O 2 in one step and H 2 or CO in the other). 4,5 The most commonly investigated two-step metal oxide cycles are the zinc (ZnOÀZn) and ferrite (FeOÀFe 3 O 4 ) cycles. However, problems are encountered in both systems. Notably, facile recombination of zinc vapor with O 2 during thermal reduction occurs in the zinc cycle, 6,7 and an inert zirconium oxide phase is needed to stabilize the active materials in the ferrite cycle. 8,9 Cerium oxide is an attractive alternative for solar thermochemical fuel production (eqs 1À3) from metal oxides. Both water splitting and CO 2 splitting with CeO 2 have been investigated, first in catalytic systems 10,11 and then as a solar thermochemical process. 5,12À17 CeO 2 has found use in automotive three-way catalysis and other catalytic systems, due to its ability to reversibly store and release lattice oxygen. 18À20 This mechanism occurs due to the partial reduction of the Ce 4+ cations in CeO 2 to Ce 3+ , and it results in the formation of nonstoichiometric, cubic phases via the formation of oxygen vacancies without significant reorganization of the lattice. 20,21 When compared to other metal oxide materials for fuel production, CeO 2 has a higher melting point (2400 °C), improved thermal stability, and lack of crystal reordering phase transitions in the operating temperature range. 16 CeO 2 h CeO 2Àδ þ 0:5δO 2

The oxidation of three-dimensionally ordered macroporous (3DOM) CeO2 (ceria) by H2O and CO2 at 1100 K is presented in comparison to the oxidation of nonordered mesoporous and sintered, low porosity ceria. 3DOM ceria, which features interconnected and ordered pores, increases the maximum H2 and CO production rates over the low porosity ceria by 125% and 260%, respectively, and increases the maximum H2 and CO production rates over the nonordered mesoporous cerium oxide by 75% and 175%, respectively. The increase in the kinetics of H2O and CO2 splitting with 3DOM ceria is attributed to its enhanced specific surface area and to its interconnected pore system that facilitates the transport of reacting species to and from oxidation sites.

Two-step thermochemical cycling was achieved using CeO2 with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO2. The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO2 and nonordered macroporous (NOM) CeO2. Aggregated CeO2 nanoparticles with feature sizes similar to those in 3DOM CeO2 were prepared by fragmenting 3DOM CeO2 into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO2 were tested in thermochemical cycles using an infrared furnace. CeO2 was reduced at ∼1200 °C, and the reduced CeO2−δ materials were reoxidized under CO2 at ∼850 °C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO2. Additionally, 3DOM CeO2 provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling.

Solid-contact ion-selective electrodes (SC-ISEs) can exhibit very low detection limits and, in contrast to conventional ISEs, do not require an optimization of the inner filling solution. This work shows that subnanomolar detection limits can also be achieved with SC-ISEs with three-dimensionally ordered macroporous (3DOM) carbon contacts, which have been shown recently to exhibit excellent long-term stabilities and good resistance to the interferences from oxygen and light. The detection limit of 3DOM carbon-contacted electrodes with plasticized poly(vinyl chloride) as membrane matrix can be improved with a high polymer content of the sensing membrane, a large ratio of ionophore and ionic sites, and conditioning with a low concentration of analyte ions. This permits detection limits as low as 1.6×10−7 M for K+ and 4.0×10−11 M for Ag+.