Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides

Ordered mesoporous transition metal oxides have attracted considerable research attention due to their unique properties and wide applications. The preparation of these materials has been reported in the literature using soft and hard templating pathways. Compared with soft templating, hard templating, namely, nanocasting, is advantageous for synthesizing rigid mesostructures with high crystallinity and has already been applied to numerous transition metal oxides such as Co₃O₄, NiO, Fe₂O₃, and Mn₃O₄. However, nanocasting is often complicated by the multiple steps involved: first, the preparation of ordered mesoporous silica as the hard template, then infiltration of the metal precursor into the pores, and finally, formation of the metal oxide and removal of the hard template. In this paper, we provide a complete protocol that covers the preparation of most widely used ordered mesoporous silica templates (MCM-41, KIT-6, SBA-15) and the nanocasting process for obtaining ordered mesoporous metal oxides, with emphasizing cobalt oxide as an example. Characterization of the products is presented, and the factors that can potentially affect the process are discussed

Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation

The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation–calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co₃O₄ are obtained. Co₃O₄ incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N₂ sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co₃O₄ with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co₃O₄, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst

Influence of Fe Doping on Structure and Water Oxidation Activity of Nanocast Co₃O₄

Herein, we demonstrate that a perfect replication of a desired composition is not only related to the degree of interconnectivity of the double gyroid ordered mesoporous silica template, there is also an enormous effect from the nature of precursor and its composition. For the first time, the symmetry of ordered mesoporous Co₃O₄ was tuned with iron doping by using the same batch of cubic ordered mesoporous silica (KIT-6) as a hard template. Nanocasting of the pure Co₃O₄ results in a negative replica of the silica template that has a monomodal pore size distribution and a dense coupled structure, while incorporation of a small amount of iron lowers the mesostructural symmetry and alters the pore system of the replica. The effect of this remarkable observation was further investigated for electrochemical water oxidation where superior catalytic activities were observed when Co₃O₄ was doped with small amounts of iron. Furthermore, iron incorporated Co₃O₄ indicated comparable water oxidation activity with noble metal and cobalt based electrocatalysts. This kind of abundant transition metal based mesostructured material has the potential to be used as promising electrocatalysts for water oxidation

Impacts of Geometry, Symmetry, and Morphology of Nanocast Co₃O₄ on Its Catalytic Activity for Water Oxidation

Herein, we report a systematic study on the synthesis of ordered mesoporous Co₃O₄ nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N₂ sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co₃O₄. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co₃O₄ replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co₃O₄ replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co₃O₄ replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co₃O₄ shows superior activity to the cubically ordered one

Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction

Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co₃O₄ spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm² with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni­(OH)₂ species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry

Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Cesium Lead Iodide Perovskite Quantum Dots

The outstanding performance of halide perovskites in optoelectronic applications can be partly attributed to their high absorption coefficient and long carrier lifetime, which are also desirable for photocatalysts. Herein, we report that cesium lead iodide perovskite quantum dots (CsPbI₃ QDs) can be used as catalysts to promote the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene under visible light illumination while preserving the quantum dot in the desirable cubic crystal phase. Simultaneously, the generated conducting poly­(3,4-ethylenedioxythiophene), PEDOT, encapsulates and stabilizes the morphology of the CsPbI₃ QDs. The photocatalytic polymerization clearly depends on the concentration of the CsPbI₃ QDs, and the CsPbI₃ QDs maintain the desirable perovskite phase when the concentration of the QD increases. Molecular oxygen and 1,4-benzoquinone can serve as electron acceptors during the photocatalytic polymerization reaction. When molecular oxygen is used, the structure of the CsPbI₃ QD transforms from cubic to orthorhombic, while usage of 1,4-benzoquinone preserves the cubic phase of CsPbI₃ QD. This novel approach enables the one-step formation of CsPbI₃/PEDOT composite, which could be promising for the preparation of novel optoelectronic materials and high performance devices

Pseudomorphic Transformation of Organometal Halide Perovskite Using the Gaseous Hydrogen Halide Reaction

Halide exchange is a facile method of adjusting the band gap and optimizing the performance of organometal halide perovskite. During the halide exchange processes, preserving the crystallinity and morphology of highly crystalline materials will be desirable for preparing novel materials with outstanding performance. In this study, the gasous hydrogen halides were used as reactants for halide exchange processes. The mutual conversions among three halides for condense films were realized. Moreover, perovskite inverse opals and perovskite single crystals were also adopted as substrates to illustrate the morphology preservation and crystallinity preservation, respectively. Powder X-ray diffraction and UV–vis diffuse reflectance spectra demonstrated the segregation when smaller ions were substituted by larger ions. Scanning electron microscopy displayed the direct evidence for morphology preservation during the transformation. For the first time, single crystal X-ray diffraction confirmed the single-crystal-to-single-crystal transformation from bromide to chloride analogy, which demonstrated that the presented method can preserve the crystalline framework of large-sized perovskite during the halide exchange

Design of Ordered Mesoporous Composite Materials and Their Electrocatalytic Activities for Water Oxidation

The controlled synthesis of a series of ordered mesoporous composite materials via solid–solid reaction of ordered mesoporous Co₃O₄ with various transition metal precursors is reported. This versatile methodology allows preparation of a range of composites with precisely controllable material compositions. The textural parameters of the heterostructured compounds are highly dependent on the oxidation state of the dopant. Electrocatalytic activities of the prepared materials were investigated as oxygen evolution catalysts for the electrolysis of water. Among the ordered mesoporous composite materials, Co₃O₄–CuCo₂O₄ shows a significant enhancement for electro-catalytic water splitting with a lower onset potential and higher current density. Following these results, a series of ordered mesoporous composite materials based on cobalt and copper oxides with different atomic ratios were prepared through a nanocasting route. Enhanced electrocatalytic performance was obtained for all composite samples in comparison with Co₃O₄

Standardized Benchmarking of Water Splitting Catalysts in a Combined Electrochemical Flow Cell/Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) Setup

The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma–optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO₂ and NiCoO₂ used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers)

Dendritic Cells Pulsed with Leukemia Cell-Derived Exosomes More Efficiently Induce Antileukemic Immunities

<div><p>Dendritic cells (DCs) and tumor cell-derived exosomes have been used to develop antitumor vaccines. However, the biological properties and antileukemic effects of leukemia cell-derived exosomes (LEXs) are not well described. In this study, the biological properties and induction of antileukemic immunity of LEXs were investigated using transmission electron microscopy, western blot analysis, cytotoxicity assays, and animal studies. Similar to other tumor cells, leukemia cells release exosomes. Exosomes derived from K562 leukemia cells (LEX_K562) are membrane-bound vesicles with diameters of approximately 50–100 μm and harbor adhesion molecules (<i>e.g.</i>, intercellular adhesion molecule-1) and immunologically associated molecules (<i>e.g.</i>, heat shock protein 70). In cytotoxicity assays and animal studies, LEXs-pulsed DCs induced an antileukemic cytotoxic T-lymphocyte immune response and antileukemic immunity more effectively than did LEXs and non-pulsed DCs (<i>P</i><0.05). Therefore, LEXs may harbor antigens and immunological molecules associated with leukemia cells. As such, LEX-based vaccines may be a promising strategy for prolonging disease-free survival in patients with leukemia after chemotherapy or hematopoietic stem cell transplantation.</p></div