Highly Ordered Inverse Opal Structures Synthesized from Shape-Controlled Nanocrystal Building Blocks

Colloidal Templating in Catalyst Design for Thermocatalysis

Conventional catalyst preparative methods commonly entail the impregnation, precipitation, and/or immobilization of nanoparticles on their supports. While convenient, such methods do not readily afford the ability to control collective ensemble-like nanoparticle properties, such as nanoparticle proximity, placement, and compartmentalization. In this Perspective, we illustrate how incorporating colloidal templating into catalyst design for thermocatalysis confers synthetic advantages to facilitate new catalytic investigations and augment catalytic performance, focusing on three colloid-templated catalyst structures: 3D macroporous structures, hierarchical macro-mesoporous structures, and discrete hollow nanoreactors. We outline how colloidal templating decouples the nanoparticle and support formation steps to devise modular catalyst platforms that can be flexibly tuned at different length scales. Of particular interest is the raspberry colloid templating (RCT) method which confers high thermomechanical stability by partially embedding nanoparticles within its support, while retaining high levels of reactant accessibility. We illustrate how the high modularity of the RCT approach allows one to independently control collective nanoparticle properties, such as nanoparticle proximity and localization, without concomitant changes to other catalytic descriptors that would otherwise confound analyses of their catalytic performance. We next discuss how colloidal templating can be employed to achieve spatially disparate active site functionalization while directing reactant transport within the catalyst structure to enhance selectivity in multistep catalytic cascades. Throughout this Perspective, we highlight developments in advanced characterization that interrogate transport phenomena and/or derive new insights into these catalyst structures. Finally, we offer our outlook on the future roles, applications, and challenges of colloidal templating in catalyst design for thermocatalysis.

Metal organic framework-assisted copper-modified titania (Cu/TiO2) with abundant exposed active sites and highly accessible pore channels for an enhanced photo-generation of hydrogen

Metal-doping is a common strategy for establishing active sites on photocatalyst, but appropriately exposing them for maximized atomic utilization remains a great challenge in photocatalytic research. Herein, we propose a metal organic framework (MOF)-assisted approach to synthesis copper-modified titania (Cu-TiO2/Cu) photocatalyst with homogenously distributed and highly accessible active sites in its matrix. Significantly, an MOF precursor, namely NH2-MIL-125, with co-chelation of titania (Ti) and copper (Cu) was subjected to mild calcination, subsequently results in Cu-modified TiO2 with highly accessible channels to its inner surface. These channels provide not only a large reactive surface (>400 m2 g-1); they also enable facile modifying route for the pre-deposited Cu in prior to photoreaction. Specifically, NH3 treatment was applied to partially reduce deposited Cu ions (Cu+ and Cu2+) into Cu nanoparticles, where their interplays realize improved optical properties and charge separation during photoreactions. Furthermore, the NH3-induced Cu nanoparticles could also serve as the adsorptive site for H+, thereby enabling 5629 μmol h-1 g-1 H2 generation over the optimum photocatalyst of Cu20/TiO2/Cu500. Such performance is associated to 35.44 and 1.71-fold improvements compared to pure TiO2 (Cu0/TiO2) and untreated Cu-ion modified TiO2 (Cu20/TiO2), respectively. This work offers a new synthetic strategy for obtaining photocatalyst with evenly distributed and highly accessible active sites, thus improving the commensurability of photocatalytic H2 generation from the industrial perspective.

Controlling nanoparticle placement in Au/TiO2 inverse opal photocatalysts

Gold nanoparticle-loaded titania (Au/TiO2) inverse opals are highly ordered three-dimensional photonic structures with enhanced photocatalytic properties. However, fine control over the placement of the Au nanoparticles in the inverse opal structures remains challenging with traditional preparative methods. Here, we present a multi-component co-assembly strategy to prepare high-quality Au/TiO2 inverse opal films in which Au nanoparticles are either located on, or inside the TiO2 matrix, as verified using electron tomography. We report that Au nanoparticles embedded in the TiO2 support exhibit enhanced thermal and mechanical stability compared to non-embedded nanoparticles that are more prone to both leaching and sintering.

Chemical composition from photos: Dried solution drops reveal a morphogenetic tree

Under nonequilibrium conditions, inorganic systems can produce a wealth of life-like shapes and patterns which, compared to well-formed crystalline materials, remain widely unexplored. A seemingly simple example is the formation of salt deposits during the evaporation of sessile droplets. These evaporites show great variations in their specific patterns including single rings, creep, small crystals, fractals, and featureless disks. We have explored the patterns of 42 different salts at otherwise constant conditions. Based on 7,500 images, we show that distinct pattern families can be identified and that some salts (e.g., Na2SO4 and NH4NO3) are bifurcated creating two distinct motifs. Family affiliations cannot be predicted a priori from composition alone but rather emerge from the complex interplay of evaporation, crystallization, thermodynamics, capillarity, and fluid flow. Nonetheless, chemical composition can be predicted from the deposit pattern with surprisingly high accuracy even if the set of reference images is small. These findings suggest possible applications including smartphone-based analyses and lightweight tools for space missions.

Photoelectrochemical Lithium Extraction from Waste Batteries

The amount of global hybrid-electric and all electric vehicle has increased dramatically in just five years and reached an all-time high of over 10 million units in 2022. A good deal of waste lithium (Li)-containing batteries from dead vehicles are invaluable unconventional resources with high usage of Li. However, the recycle of Li by green approaches is extremely inefficient and rare from waste batteries, giving rise to severe environmental pollutions and huge squandering of resources. Thus, in this mini review, we briefly summarized a green and promising route-photoelectrochemical (PEC) technology for extracting the Li from the waste lithium-containing batteries. This review first focuses on the critical factors of PEC performance, including light harvesting, charge-carrier dynamics, and surface chemical reactions. Subsequently, the conventional and PEC technologies applying in the area of Li recovery processes are analyzed and discussed in depth, and the potential challenges and future perspective for rational and healthy development of PEC Li extraction are provided positively.

Evoking C2+ production from electrochemical CO2 reduction by the steric confinement effect of ordered porous Cu2O

Selective conversion of carbon dioxide (CO2) to multi-carbon products (CO2-to-C2+) at high current densities is in essential demand for the practical application of the resultant valuable products, yet it remains challenging to conduct due to the lack of efficient electrocatalysts. Herein, three-dimensional ordered porous cuprous oxide cuboctahedra (3DOP Cu2O-CO) were designed and synthesized by a molecular fence-assisted hard templating approach. Capitalizing on the merits of interconnected and uniformly distributed pore channels, 3DOP Cu2O-CO exhibited outstanding electrochemical CO2-to-C2+ conversion, achieving faradaic efficiency and partial current density for C2+ products of up to 81.7% and -0.89 A cm-2, respectively, with an optimal formation rate of 2.92 mmol h-1 cm-2 under an applied current density of -1.2 A cm-2. In situ spectroscopy and simulation results demonstrated that the ordered pores of 3DOP Cu2O-CO can effectively confine and accumulate sufficient *CO adsorption during electrochemical CO2 reduction, which facilitates efficient dimerization for the formation of C2+ products. Furthermore, the 3DOP structure induces a higher local pH value, which not only enhances the C-C coupling reaction, but also suppresses competing H2 evolution.

Evaporation-Induced Self-Assembly of Metal Oxide Inverse Opals: From Synthesis to Applications

Inverse opals (IOs) are highly interconnected three-dimensional macroporous structures with applications in a variety of disciplines from optics to catalysis. For instance, when the pore size is on the scale of the wavelength of visible light, IOs exhibit structural color due to diffraction and interference of light rather than due to absorption by pigments, making these structures valuable as nonfading paints and colorants. When IO pores are in an ordered arrangement, the IO is a 3D photonic crystal, a structure with a plethora of interesting optical properties that can be used in a multitude of applications, from sensors to lasers. IOs also have interesting fluidic properties that arise from the re-entrant geometry of the pores, making them excellent candidates for colorimetric sensors based on fluid surface tension. Metal oxide IOs, in particular, can also be photo- and thermally catalytically active due to the catalytic activity of the background matrix material or of functional nanoparticles embedded within the structure.

Evaporation-induced self-assembly of sacrificial particles has been developed as a scalable method for forming IOs. The pore size and shape, surface chemistry, matrix material, and the macroscopic shape of the IO, as well as the inclusion of functional components, can be designed through the choice of deposition conditions such as temperature and humidity, types and concentrations of components in the self-assembly mixture, and the postassembly processing. These parameters allow researchers to tune the optical, mechanical, and thermal transport properties of IOs for optimum functionality.

In this Account, we focus on experimental and theoretical studies to understand the self-assembly process and properties of metal oxide IOs without (bare) and with (hybrid) plasmonic or catalytic metal nanoparticles incorporated. Several synthetic approaches are first presented, together with a discussion of the various forces involved in the assembly process. The visualization of the deposition front with time-lapse microscopy is then discussed together with analytical theory and numerical simulations to determine the conditions needed for the deposition of a continuous IO film. Subsequently, we present high-resolution scanning electron microscopy (SEM) of assembled colloids over large areas, which provides a detailed view of the evolution of the assembly process, showing that the organization of the colloids is initially dictated by the meniscus of the evaporating suspension on the substrate, but that gradually all grains rotate to occupy the thermodynamically most favorable orientation. High-resolution 3D transmission electron microscopy (TEM) is then presented together with analysis of the wetting of the templating colloids by the matrix precursor to provide a detailed picture of the embedding of metallic nanoparticles at the pore–matrix interface. Finally, the resulting properties and applications in optics, wetting, and catalysis are discussed, concluding with an outlook on the future of self-assembled metal-oxide-based IOs.