Self-Assembled Preparation of Porous Nickel Phosphide Superparticles with Tunable Phase and Porosity for Efficient Hydrogen Evolution.

Self-assembly of colloidal nanoparticles enables the easy building of assembly units into higher-order structures and the bottom-up preparation of functional materials. Nickel phosphides represent an important group of catalysts for hydrogen evolution reaction (HER) from water splitting. In this paper, the preparation of porous nickel phosphide superparticles and their HER efficiencies are reported. Ni and Ni2 P nanoparticles are self-assembled into binary superparticles via an oil-in-water emulsion method. After annealing and acid etching, the as-prepared Ni-Ni2 P binary superparticles change into porous nickel phosphide superparticles. The porosity and crystalline phase of the superparticles can be tuned by adjusting the ratio of Ni and Ni2 P nanoparticles. The resulting porous superparticles are effective in driving HER under acidic conditions, and the modulation of porosity and phase further optimize the electrochemical performance. The prepared Ni3 P porous superparticles not only possess a significantly enhanced specific surface area compared to solid Ni-Ni2 P superparticles but also exhibit an excellent HER efficiency. The calculations based on the density functional theories show that the (110) crystal facet exhibits a relatively lower Gibbs free energy of hydrogen adsorption. This work provides a self-assembly approach for the construction of porous metal phosphide nanomaterials with tunable crystalline phase and porosity.

Oriented Porous NASICON 3D Framework via Freeze-Casting for Sodium-Metal Batteries.

Sodium-metal batteries are promising candidates for low-cost, large-format energy storage systems. However, sodium-metal batteries suffer from high interfacial resistance between the electrodes and the solid electrolyte, leading to poor electrochemical performance. We demonstrate a sodium superionic conductor (NASICON) with an oriented porous framework of sodium aluminum titanium phosphate (NATP) fabricated by the freeze-casting technique, which shows excellent properties as a solid electrolyte. Using X-ray computed tomography, we confirm the uniform low-tortuosity channels present along the thickness of the scaffold. We infiltrated the porous NATP scaffolds with sodium vanadium phosphate (NVP) cathode nanoparticles achieving mass loadings of ∼3-4 mg cm-2, which enables short sodium ion diffusion path lengths. For the resulting hybrid cell, we achieved a capacity of ∼90 mAh g-1 at a specific current of 50 mA g-1 (∼300 Wh kg-1) for over 100 cycles with ∼94% capacity retention. Our study offers valuable insights for the design of hybrid solid electrolyte-cathode active material structures to achieve improved electrochemical performance through low-tortuosity ion transport networks.

Gas-Phase Nitrogen Doping of Monolithic TiO2 Nanoparticle-Based Aerogels for Efficient Visible Light-Driven Photocatalytic H2 Production.

The development of visible light-active photocatalysts is essential for increasing the conversion efficiency of solar energy into hydrogen (H2). Here, we present a facile method for nitrogen doping of monolithic titanium dioxide (TiO2) nanoparticle-based aerogels to activate them for visible light. Plasma-enhanced chemical vapor deposition at low temperature enables efficient incorporation of nitrogen into preformed TiO2 aerogels without compromising their advantageous intrinsic characteristics such as large surface area, extensive porosity, and nanoscale properties of the semiconducting building blocks. By balancing the dopant concentration and the defects, the nitridation improves optical absorption and charge separation efficiency. The nitrogen-doped TiO2 nanoparticle-based aerogels loaded with palladium (Pd) nanoparticles show a significant enhancement in visible light-driven photocatalytic H2 production (3.1 mmol h-1 g-1) with excellent stability over 5 days. With this method, we introduce a powerful tool to tune the properties of nanoparticle-based aerogels after synthesis for a specific application, as exemplified by visible light-driven H2 production.

Synthesis of Cu3N and Cu3N-Cu2O multicomponent mesocrystals: non-classical crystallization and nanoscale Kirkendall effect.

Mesocrystals are superstructures of crystallographically aligned nanoparticles and are a rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. This study reports the first synthesis of Cu3N mesocrystals employing structure-directing agents with a subtle tuning of the reaction parameters. Detailed structural characterizations carried out with a combination of transmission electron microscopy techniques (HRTEM, HAADF-STEM-EXDS) reveal that Cu3N mesocrystals form by non-classical crystallization, and variations in their sizes and morphologies are traced back to distinct attachment scenarios of corresponding mesocrystal subunits. In the presence of oleylamine, the mesocrystal subunits in the early reaction stages prealign in a crystallographic fashion and afterwards grow into the final mesocrystals, while in the presence of hexadecylamine the subunits come into contact through misaligned attachment, and subsequently, to some degree, realign in crystallographic register. Upon prolonged heating both types of mesocrystals undergo chemical conversion processes resulting in structural and morphological changes. A two-step mechanism of chemical conversion is proposed, involving Cu3N decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent/heterostructured Cu3N-Cu2O mesocrystals, which subsequently convert into Cu2O nanocages. It is anticipated that combining nanostructured Cu3N and Cu2O in a mesocrystalline and hollow morphology will provide a platform to expand their application potential.

3D Printed Scaffolds for Monolithic Aerogel Photocatalysts with Complex Geometries.

Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocatalysis, such as the gas flow through and the ultraviolet light penetration into the aerogel and to customize its geometric shape to a continuous gas flow reactor. Using photocatalytic methanol reforming as a model reaction, it is shown that the optimization of these parameters leads to an increase of the hydrogen production rate by a factor of three from 400 to 1200 µmol g-1 h-1 . The rigid scaffolds also enhance the mechanical stability of the aerogels, lowering the number of rejects during synthesis and facilitating handling during operation. The combination of nanoparticle-based aerogels with 3D printed polymeric scaffolds opens up new opportunities to tailor the geometry of the photocatalysts for the photocatalytic reaction and for the reactor to maximize overall performance without necessarily changing the material composition.

Porous Silica Microspheres with Immobilized Titania Nanoparticles for In-Flow Solar-Driven Purification of Wastewater.

In this paper, inorganic silica microspheres with interconnected macroporosity are tested as a platform for designing robust and efficient photocatalytic systems for a continuous flow reactor, enabling a low cost and straightforward purification of wastewater through solar-driven photocatalysis. The photocatalytically active microspheres are prepared by wet impregnation of porous silica scaffolds with Trizma-functionalized anatase titania (TiO2) nanoparticles (NPs). NPs loading of 22 wt% is obtained in the form of a thin and well-attached layer, covering the external surface of the microspheres as well as the internal surface of the pores. The TiO2 loading leads to an increase of the specific surface area by 26%, without impacting the typically interconnected macroporosity (≈60%) of the microspheres, which is essential for an efficient flow of the pollutant solution during the photocatalytic tests. These are carried out in a liquid medium for the decomposition of methyl orange and paracetamol. In addition to photocatalytic activity under continuous flow, the microspheres offer the advantage that they can be easily removed from the reaction medium, which is an appealing aspect for industrial applications. In this work, the typical issues of TiO2 NPs photocatalysts are circumvented, without the need for elaborate chemistries, and for low availability and expensive raw materials.

Unveiling Critical Insight into the Zn Metal Anode Cyclability in Mildly Acidic Aqueous Electrolytes: Implications for Aqueous Zinc Batteries.

The cost benefit and inherent safety conferred by the energy-dense metallic zinc anode and mildly acidic aqueous electrolytes are critical to aqueous zinc batteries' (AZBs) large-scale energy-storage ambition. Aggressive research efforts in the past five years have resulted in the discovery of several high-energy positive (cathode) host materials, but understanding of the Zn anode rechargeability and any influence of the electrolyte, which are critical for AZBs' practical development, remains limited. As we unravel here, under realistic test conditions, when parameters are set keeping practical applications in mind, Zn anode cycling appears vulnerable to dendritic failure in all common AZB electrolytes. While 3 M ZnSO4 delivers the best overall performance for the Zn anode cycling, viability of the oxidatively stable "water in salt" electrolyte appears gravely restricted. Defying the general understanding of metal electrodeposition, a high current density is found to dramatically prolong the Zn cycling lifetime, achieving >8000 cycles at 20 mA cm-2 for 1 mAh cm-2 capacity in 3 M ZnSO4. High current also allows prolonged cycling at capacities of 2 and 4 mAh cm-2. Such a striking improvement in lifetime on going from low to high currents is further confirmed through Zn|Zn0.25V2O5 and Zn|LiMn2O4 full-cell studies with practical electrode loading. Not surprisingly, all the parameters influence the cycled Zn morphology, which in turn dictates the propensity for short-circuit. These findings not only divulge previously unanticipated insight into the Zn anode cycling and electrolyte performance in AZBs but also offer a rational basis to gauge their practical development.

Composites of Copper Nanowires in Polyethylene: Preparation and Processing to Materials with NIR Dichroism.

Agglomeration of copper nanowires (aspect ratios on the order of 1000) in polyethylene, commonly a major problem, could be prevented by modification of the nanowires with a surface layer of oleylamine. Nanocomposite films were prepared by mixing nanowire dispersions in organic solvents with polyethylene solutions followed by casting, drying, and sometimes hot pressing. Orientation of the copper nanowires by solid-state drawing of the composites at elevated temperatures led to preferential alignment of the nanowires in the drawing direction. This arrangement gave rise to a uniform dichroism in the near-infrared (NIR) region, which is uncommon in the case of the hitherto reported dichroic nanocomposites. The NIR dichroism is ascribed to the high aspect ratio of the metal wires. Hence, drawing of isotropic nanocomposites with metal wires may serve for the manufacture of NIR polarization filters.

3D printing of sacrificial templates into hierarchical porous materials.

Hierarchical porous materials are widespread in nature and find an increasing number of applications as catalytic supports, biological scaffolds and lightweight structures. Recent advances in additive manufacturing and 3D printing technologies have enabled the digital fabrication of porous materials in the form of lattices, cellular structures and foams across multiple length scales. However, current approaches do not allow for the fast manufacturing of bulk porous materials featuring pore sizes that span broadly from macroscopic dimensions down to the nanoscale. Here, ink formulations are designed and investigated to enable 3D printing of hierarchical materials displaying porosity at the nano-, micro- and macroscales. Pores are generated upon removal of nanodroplets and microscale templates present in the initial ink. Using particles to stabilize the droplet templates is key to obtain Pickering nanoemulsions that can be 3D printed through direct ink writing. The combination of such self-assembled templates with the spatial control offered by the printing process allows for the digital manufacturing of hierarchical materials exhibiting thus far inaccessible multiscale porosity and complex geometries.

Oxide versus Nonoxide Cathode Materials for Aqueous Zn Batteries: An Insight into the Charge Storage Mechanism and Consequences Thereof.

Aqueous Zn-ion batteries, which are being proposed as large-scale energy storage solutions because of their unparalleled safety and cost advantage, are composed of a positive host (cathode) material, a metallic zinc anode, and a mildly acidic aqueous electrolyte (pH ≈ 3-7). Typically, the charge storage mechanism is believed to be reversible Zn2+ (de)intercalation in the cathode host, with the exception of α-MnO2, for which multiple vastly different and contradicting mechanisms have been proposed. However, our present study, combining electrochemical, operando X-ray diffraction, electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and in situ pH evolution analyses on two oxide hosts-tunneled α-MnO2 and layered V3O7·H2O vis-à-vis two nonoxide hosts-layered VS2 and tunneled Zn3[Fe(CN)6]2, suggests that oxides and nonoxides follow two dissimilar charge storage mechanisms. While the oxides behave as dominant proton intercalation materials, the nonoxides undergo exclusive zinc intercalation. Stabilization of H+ on the hydroxyl-terminated oxide surface is revealed to facilitate the proton intercalation by a preliminary molecular dynamics simulation study. Proton intercalation for both oxides leads to the precipitation of layered double hydroxide (LDH)-Zn4SO4(OH)6·5H2O with a ZnSO4/H2O electrolyte and a triflate anion (CF3SO3-)-based LDH with a Zn(SO3CF3)2/H2O electrolyte-on the electrode surface. The LDH precipitation buffers the pH of the electrolytes to a mildly acidic value, sustaining the proton intercalation to deliver large specific capacities for the oxides. Moreover, we also show that the stability of the LDH precipitate is crucial for the rechargeability of the oxide cathodes, revealing a critical link between the charge storage mechanism and the performance of the oxide hosts in aqueous zinc batteries.

Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure-Property Relations.

In food-grade emulsions, particles with an appropriate surface modification can be used to replace surfactants and potentially enhance the stability of emulsions. During the life cycle of products based on such emulsions, they can be exposed to a broad range of pH conditions and hence it is crucial to understand how pH changes affect stability of emulsions stabilized by particles. Here, we report on a comprehensive study of the stability, microstructure, and macroscopic behavior of pH-controlled oil-in-water emulsions containing silica nanoparticles modified with chitosan, a food-grade polycation. We found that the modified colloidal particles used as stabilizers behave differently depending on the pH, resulting in unique emulsion structures at multiple length scales. Our findings are rationalized in terms of the different emulsion stabilization mechanisms involved, which are determined by the pH-dependent charges and interactions between the colloidal building blocks of the system. At pH 4, the silica particles are partially hydrophobized through chitosan modification, favoring their adsorption at the oil-water interface and the formation of Pickering emulsions. At pH 5.5, the particles become attractive and the emulsion is stabilized by a network of agglomerated particles formed between the droplets. Finally, chitosan aggregates form at pH 9 and these act as the emulsion stabilizers under alkaline conditions. These insights have important implications for the processing and use of particle-stabilized emulsions. On one hand, changes in pH can lead to undesired macroscopic phase separation or coalescence of oil droplets. On the other hand, the pH effect on emulsion behavior can be harnessed in industrial processing, either to tune their flow response by altering the pH between processing stages or to produce pH-responsive emulsions that enhance the functionality of the emulsified end products.

Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes.

We report a solution-phase approach to the synthesis of crystalline copper nanowires (Cu NWs) with an aspect ratio >1000 via a new catalytic mechanism comprising copper ions. The synthesis involves the reaction between copper(II) chloride and copper(II) acetylacetonate in a mixture of oleylamine and octadecene. Reaction parameters such as the molar ratio of precursors as well as the volume ratio of solvents offer the possibility to tune the morphology of the final product. A simple low-cost spray deposition method was used to fabricate Cu NW films on a glass substrate. Post-treatment under reducing gas (5% H2 + 95% N2) atmosphere resulted in Cu NW films with a low sheet resistance of 24.5 Ω/sq, a transmittance of T = 71% at 550 nm (including the glass substrate), and a high oxidation resistance. Moreover, the conducting Cu NW networks on a glass substrate can easily be transferred onto a polycarbonate substrate using a simple hot-press transfer method without compromising on the electrical performance. The resulting flexible transparent electrodes show excellent flexibility ( R/ Ro < 1.28) upon bending to curvatures of 1 mm radius.

Demonstration of cellular imaging by using luminescent and anti-cytotoxic europium-doped hafnia nanocrystals.

Luminescent nanoparticles are researched for their potential impact in medical science, but no materials approved for parenteral use have been available so far. To overcome this issue, we demonstrate that Eu3+-doped hafnium dioxide nanocrystals can be used as non-toxic, highly stable probes for cellular optical imaging and as radiosensitive materials for clinical treatment. Furthermore, viability and biocompatibility tests on artificially stressed cell cultures reveal their ability to buffer reactive oxygen species, proposing an anti-cytotoxic feature interesting for biomedical applications.

Nano-Sized Structurally Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors.

A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered Ni xFe yO z@rGO delivers a capacity of 388 mAh g-1 at 5 A g-1, which is 6 times that of the crystalline sample. Disordered Ni xFe yO z@rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory calculations reveal a smaller volume expansion during Li+ insertion for the structurally disordered Ni xFe yO z nanoparticles, and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

Early Dynamics and Stabilization Mechanisms of Oil-in-Water Emulsions Containing Colloidal Particles Modified with Short Amphiphiles: A Numerical Study.

Emulsions stabilized by mixtures of particles and amphiphilic molecules are relevant for a wide range of applications, but their dynamics and stabilization mechanisms on the colloidal level are poorly understood. Given the challenges to experimentally probe the early dynamics and mechanisms of droplet stabilization, Brownian dynamics simulations are developed here to study the behavior of oil-in-water emulsions stabilized by colloidal particles modified with short amphiphiles. Simulation parameters are based on an experimental system that consists of emulsions obtained with octane as the oil phase and a suspension of alumina colloidal particles modified with short carboxylic acids as the continuous aqueous medium. The numerical results show that attractive forces between the colloidal particles favor the formation of closely packed clusters on the droplet surface or of a percolating network of particles throughout the continuous phase, depending on the amphiphile concentration. Simulations also reveal the importance of a strong adsorption of particles at the liquid interface to prevent their depletion from the droplet surface when another droplet approaches. Strongly adsorbed particles remain immobile on the droplet surface, generating an effective steric barrier against droplet coalescence. These findings provide new insights into the early dynamics and mechanisms of stabilization of emulsions using particles and amphiphilic molecules.

Synthesis of a rare-earth doped hafnia hydrosol: Towards injectable luminescent nanocolloids.

A major obstacle in the introduction of luminescent nanoparticles (NPs) for medical applications is that quantum dots, the most widely studied luminescent materials, despite being biologically safe after coating with a bioshell, still contain a toxic core mostly consisting of semi-conductor NPs, which are not approved by regulatory agencies. Here we point to a potential solution of this problem by using rare-earth (RE) doped hafnia NPs. Hafnia is approved for medical injections as an effective means for the treatment of radiosensitive and radioresistant tumors and can significantly decrease potential toxicity of RE ions. As a step towards the achievement of this goal we describe the development of a bio-friendly method for the preparation of a stable doped hafnia hydrosol with an isoelectric point (IEP) of 8.2, which shows high fluorescence and biocompatibility in regular coagulant tests and cytotoxic assays.

3D printing of concentrated emulsions into multiphase biocompatible soft materials.

3D printing via direct ink writing (DIW) is a versatile additive manufacturing approach applicable to a variety of materials ranging from ceramics over composites to hydrogels. Due to the mild processing conditions compared to other additive manufacturing methods, DIW enables the incorporation of sensitive compounds such as proteins or drugs into the printed structure. Although emulsified oil-in-water systems are commonly used vehicles for such compounds in biomedical, pharmaceutical, and cosmetic applications, printing of such emulsions into architectured soft materials has not been fully exploited and would open new possibilities for the controlled delivery of sensitive compounds. Here, we 3D print concentrated emulsions into soft materials, whose multiphase architecture allows for site-specific incorporation of both hydrophobic and hydrophilic compounds into the same structure. As a model ink, concentrated emulsions stabilized by chitosan-modified silica nanoparticles are studied, because they are sufficiently stable against coalescence during the centrifugation step needed to create a bridging network of droplets. The resulting ink is ideal for 3D printing as it displays high yield stress, storage modulus and elastic recovery, through the formation of networks of droplets as well as of gelled silica nanoparticles in the presence of chitosan. To demonstrate possible architectures, we print biocompatible soft materials with tunable hierarchical porosity containing an encapsulated hydrophobic compound positioned in specific locations of the structure. The proposed emulsion-based ink system offers great flexibility in terms of 3D shaping and local compositional control, and can potentially help address current challenges involving the delivery of incompatible compounds in biomedical applications.

Colloidal Nanocrystal-Based BaTiO3 Xerogels as Green Bodies: Effect of Drying and Sintering at Low Temperatures on Pore Structure and Microstructures.

Although aerogels prepared by the colloidal assembly of nanoparticles are a rapidly emerging class of highly porous and low-density materials, their ambient dried counterparts, namely xerogels, have hardly been explored. Here we report the use of nanoparticle-based BaTiO3 xerogels as green bodies, which provide a versatile route to ceramic materials under the minimization of organic additives with a significant reduction of the calcination temperature compared to that of conventional powder sintering. The structural changes of the xerogels are investigated during ambient drying by carefully analyzing the microstructure at different drying stages. For this purpose, the shrinkage was arrested by a supercritical drying step under full preservation of the intermediate microstructure, giving unprecedented insight into the structural changes during ambient drying of a nanoparticle-based gel. In a first step, the large macropores shrink because of capillary forces, followed by the collapse of residual mesopores until a dense xerogel is obtained. The whole process is accompanied by a volume shrinkage of 97% and a drop in surface area from 300 to 220 m2 g-1. Finally, the xerogels are sintered, causing another shrinkage of up to 8% with a slight increase in the average pore and crystal sizes. At temperatures higher than 700 °C, an unexpected phase transition to BaTi2O5 is observed.

Pickering and Network Stabilization of Biocompatible Emulsions Using Chitosan-Modified Silica Nanoparticles.

Edible solid particles constitute an attractive alternative to surfactants as stabilizers of food-grade emulsions for products requiring a long-term shelf life. Here, we report on a new approach to stabilize edible emulsions using silica nanoparticles modified by noncovalently bound chitosan oligomers. Electrostatic modification with chitosan increases the hydrophobicity of the silica nanoparticles and favors their adsorption at the oil-water interface. The interfacial adsorption of the chitosan-modified silica particles enables the preparation of oil-in-water emulsions with small droplet sizes of a few micrometers through high-pressure homogenization. This approach enables the stabilization of food-grade emulsions for more than 3 months. The emulsion structure and stability can be effectively tuned by controlling the extent of chitosan adsorption on the silica particles. Bulk and interfacial rheology are used to highlight the two stabilization mechanisms involved. Low chitosan concentration (1 wt % with respect to silica) leads to the formation of a viscoelastic film of particles adsorbed at the oil-water interface, enabling Pickering stabilization of the emulsion. By contrast, a network of agglomerated particles formed around the droplets is the predominant stabilization mechanism of the emulsions at higher chitosan content (5 wt % with respect to silica). These two pathways against droplet coalescence and coarsening open up different possibilities to engineer the long-term stabilization of emulsions for food applications.

3D Printing of Emulsions and Foams into Hierarchical Porous Ceramics.

Bulk hierarchical porous ceramics with unprecedented strength-to-weight ratio and tunable pore sizes across three different length scales are printed by direct ink writing. Such an extrusion-based process relies on the formulation of inks in the form of particle-stabilized emulsions and foams that are sufficiently stable to resist coalescence during printing.