Recent advances in hydrothermal synthesis of facet-controlled CeO2-based nanomaterials

CeO₂-based nanomaterials have received tremendous attention due to their variety of applications. This paper is focused on the recent advances in facet-controlled CeO₂-based nanomaterials by the hydrothermal method. CeO₂-based nanomaterials with controllable size and exposed facets can be prepared by adjusting the reaction parameters. Moreover, doping and loading metals can improve the oxygen storage capacity (OSC) of CeO₂ and its catalytic activity. Various research studies on catalytic applications such as CO oxidation, water-gas shift reaction (WGSR), decomposition of hydrocarbons, and photocatalytic reaction have been carried out to exhibit the high potential of facet-controlled CeO₂ nanomaterials. This review will provide readers with various ideas for facet-controlled CeO₂-based nanomaterials.

Radiation Chemistry Provides Nanoscopic Insights into the Role of Intermediate Phases in CeO2 Mesocrystal Formation

The role of intermediate phases in CeO₂ mesocrystal formation from aqueous Ce^III solutions subjected to γ‐radiation was studied. Radiolytically formed hydroxyl radicals convert soluble Ce^III into less soluble Ce^IV. Transmission electron microscopy (TEM) and X‐ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO₂ mesocrystal evolution following the oxidation to Ce^IV: (1) formation of hydrated Ce^IV hydroxides, serving as intermediates in the liquid‐to‐solid phase transformation; (2) CeO₂ primary particle growth inside the intermediate phase; (3) alignment of the primary particles into “pre‐mesocrystals” and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of “mineral bridges”. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.

Surfactant-mediated morphology evolution and self-assembly of cerium oxide nanocrystals for catalytic and supercapacitor applications

Surfactant plays a remarkable role in determining the growth process (facet exposition) of colloidal nanocrystals (NCs) and the formation of self-assembled NC superstructures, the underlying mechanism of which, however, still requires elucidation. In this work, the mechanism of surfactant-mediated morphology evolution and self-assembly of CeO2 nanocrystals is elucidated by exploring the effect that surfactant modification has on the shape, size, exposed facets, and arrangement of the CeO2 NCs. It is directly proved that surfactant molecules determine the morphologies of the CeO2 NCs by preferentially bonding onto Ce-terminated {100} facets, changing from large truncated octahedra (mostly {111} and {100} exposed), to cubes (mostly {100} exposed) and small cuboctahedra (mostly {100} and {111} exposed) by increasing the amount of surfactant. The exposure degree of the {100} facets largely affects the concentration of Ce3+ in the CeO2 NCs, thus the cubic CeO2 NCs exhibit superior oxygen storage capacity and excellent supercapacitor performance due to a high fraction of exposed active {100} facets with great superstructure stability.

One-Pot Hydrothermal Synthesis of Victoria Green (Ca3Cr2Si3O12) Nanoparticles in Alkaline Fluids and Its Colour Hue Characterisation

One-pot hydrothermal preparation of Ca₃Cr₂Si₃O₁₂ uvarovite nanoparticles under alkaline conditions was investigated for the first time. The experimental parameters selected for the study considered the concentration of the KOH solvent solution (0.01 to 5.0 M), the agitation of the autoclave (50 rpm), and the nominal content of Si⁴⁺ (2.2–3.0 mole). Fine uvarovite particles were synthesised at 200 °C after a 3 h interval in a highly concentrated 5.0 M KOH solution. The crystallisation of single-phase Ca₃Cr₂Si₃O₁₂ particles proceeded free of by-products via a one-pot process involving a single-step reaction. KOH solutions below 2.5 M and water hindered the crystallisation of the Ca₃Cr₂Si₃O₁₂ particles. The hydrothermal treatments carried out with stirring (50 rpm) and non-stirring triggered the crystallisation of irregular anhedral particles with average sizes of 8.05 and 12.25 nm, respectively. These particles spontaneously assembled into popcorn-shaped agglomerates with sizes varying from 66 to 156 nm. All the powders prepared by the present method exhibited CIE-L*a*b* values that correspond to the Victoria green colour spectral space and have a high near infrared reflectance property. The particle size and structural crystallinity are factors affecting the Victoria pigment optical properties, such as CIE-L*a*b* values, green tonality, and near-infrared reflectance.

Mechanisms of the surface reaction and crystal growth of cerium oxide by supercritical hydrothermal treatment with carboxylic acids

The surface reaction and particle growth mechanisms of cerium oxide and decanoic acid revealed in this study.

Synthesis of low- and high-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles for improved activity and stability in electrocatalysis

Driven by the quest for future energy solution, faceted metal nanoparticles are being pursued as the next generation electrocatalysts for renewable energy applications. Thanks to recent advancement in solution phase synthesis, different low- and high-index facets on metal nanocrystals become accessible and are tested for specific electrocatalytic reactions. This minireview summarises the key approaches to prepare nanocrystals containing the most catalytically active platinum group metals (Pt, Pd, Ru, Ir and Rh) exposed with low- and high-index facets using solution phase synthesis. Electrocatalytic studies related to the different facets are highlighted to emphasise the importance of exposing facets for catalysing these reactions, namely oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation including methanol (MOR) and ethanol oxidation reactions (EOR), formic acid oxidation reaction (FAOR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The future outlook discusses the challenges and opportunities for making electrocatalysts that are even more active and stable.