Synthesis of Soluble High Molar Mass Poly(Phenylene Methylene)-Based Polymers

Poly(phenylene methylene) (PPM) is a multifunctional polymer that is also active as an anticorrosion fluorescent coating material. Although this polymer was synthesized already more than 100 years ago, a versatile synthetic route to obtain soluble high molar mass polymers based on PPM has yet to be achieved. In this article, the influence of bifunctional bis-chloromethyl durene (BCMD) as a branching agent in the synthesis of PPM is reported. The progress of the reaction was followed by gel permeation chromatography (GPC) and NMR analysis. PPM-based copolymers with the highest molar mass reported so far for this class of materials (up to Mn of 205,300 g mol-1) were isolated. The versatile approach of using BCMD was confirmed by employing different catalysts. Interestingly, thermal and optical characterization established that the branching process does not affect the thermoplastic behavior and the fluorescence of the material, thus opening up PPM-based compounds with high molar mass for applications.

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.

The Role of Zn Ions in the Structural, Surface, and Gas-Sensing Properties of SnO2:Zn Nanocrystals Synthesized via a Microwave-Assisted Route

Although semiconducting metal oxide (SMOx) nanoparticles (NPs) have attracted attention as sensing materials, the methodologies available to synthesize them with desirable properties are quite limited and/or often require relatively high energy consumption. Thus, we report herein the processing of Zn-doped SnO2 NPs via a microwave-assisted nonaqueous route at a relatively low temperature (160 °C) and with a short treatment time (20 min). In addition, the effects of adding Zn in the structural, electronic, and gas-sensing properties of SnO2 NPs were investigated. X-ray diffraction and high-resolution transmission electron microscopy analyses revealed the single-phase of rutile SnO2, with an average crystal size of 7 nm. X-ray absorption near edge spectroscopy measurements revealed the homogenous incorporation of Zn ions into the SnO2 network. Gas sensing tests showed that Zn-doped SnO2 NPs were highly sensitive to sub-ppm levels of NO2 gas at 150 °C, with good recovery and stability even under ambient moisture. We observed an increase in the response of the Zn-doped sample of up to 100 times compared to the pristine one. This enhancement in the gas-sensing performance was linked to the Zn ions that provided more surface oxygen defects acting as active sites for the NO2 adsorption on the sensing material.

Chitin Nanofibrils from Fungi for Hierarchical Gel Polymer Electrolytes for Transient Zinc-Ion Batteries with Stable Zn Electrodeposition

Rechargeable batteries play an integral role toward carbon neutrality. Environmentally sustainable batteries should consider the trade-offs between material renewability, processability, thermo-mechanical and electrochemical performance, as well as transiency. To address this dilemma, we follow circular economy principles to fabricate fungal chitin nanofibril (ChNF) gel polymer electrolytes (GPEs) for zinc-ion batteries. These biocolloids are physically entangled into hierarchical hydrogels with specific surface areas of 49.5 m2 ·g-1 . Ionic conductivities of 54.1 mS·cm-1 and a Zn2+ transference number of 0.468 are reached, outperforming conventional non-renewable/non-biodegradable glass microfibre separator-liquid electrolyte pairs. Enabled by its mechanically elastic properties and large water uptake, a stable Zn electrodeposition in symmetric Zn|Zn configuration with a lifespan above 600 h at 9.5 mA·cm-2 is obtained. At 100 mA·g-1 , the discharge capacity of Zn/α-MnO2 full cells increases above 500 cycles when replacing glass microfiber separators with ChNF GPEs, while the rate performance remains comparable to glass microfiber separators. To make the battery completely transient, the metallic current collectors are replaced by biodegradable polyester/carbon black composites undergoing degradation in water at 70 °C. This work demonstrates the applicability of bio-based materials to fabricate green and electrochemically competitive batteries with potential applications in sustainable portable electronics, or biomedicine.

Metal Phosphorous Chalcogenide: A Promising Material for Advanced Energy Storage Systems

The development of efficient and affordable electrode materials is crucial for clean energy storage systems, which are considered a promising strategy for addressing energy crises and environmental issues. Metal phosphorous chalcogenides (MPX3 ) are a fascinating class of two-dimensional materials with a tunable layered structure and high ion conductivity, making them particularly attractive for energy storage applications. This review article aims to comprehensively summarize the latest research progress on MPX3 materials, with a focus on their preparation methods and modulation strategies. Additionally, the diverse applications of these novel materials in alkali metal ion batteries, metal-air batteries, and all-solid-state batteries are highlighted. Finally, the challenges and opportunities of MPX3 materials are presented to inspire their better potential in energy storage applications. This review provides valuable insights into the promising future of MPX3 materials in clean energy storage systems.

Optimization of Mass and Light Transport in Nanoparticle-Based Titania Aerogels

Aerogels composed of preformed titania nanocrystals exhibit a large surface area, open porosity, and high crystallinity, making these materials appealing for applications in gas-phase photocatalysis. Recent studies on nanoparticle-based titania aerogels have mainly focused on optimizing their composition to improve photocatalytic performance. Little attention has been paid to modification at the microstructural level to control fundamental properties such as gas permeability and light transmittance, although these features are of fundamental importance, especially for photocatalysts of macroscopic size. In this study, we systematically control the porosity and transparency of titania gels and aerogels by adjusting the particle loading and nonsolvent fraction during the gelation step. Mass transport and light transport were assessed by gas permeability and light attenuation measurements, and the results were related to the microstructure determined by gas sorption analysis and scanning electron microscopy. Mass transport through the aerogel network was found to proceed primarily via Knudsen diffusion leading to relatively low permeabilities in the range of 10-5-10-6 m2/s, despite very high porosities of 96-99%. While permeability was found to depend mainly on particle loading, the optical properties are predominantly affected by the amount of nonsolvent during gelation, allowing independent tuning of mass and light transport.

Superelastic Cobalt Silicate@Resorcinol Formaldehyde Resin Core-Shell Nanobelt Aerogel Monoliths with Outstanding Fire Retardant and Thermal Insulating Capability

The practical applications of resorcinol formaldehyde resin (RFR) aerogels are prevented by their poor mechanical properties. Herein, a facile template-directed method is reported to produce macroscopic free-standing cobalt silicate (CS)@RFR core-shell nanobelt aerogels that display superelastic behavior and outstanding thermal insulating and fire-resistant capability. The synthesis relies on the polymerization of RFR on pre-formed CS nanobelts which leads to in situ formation of hydrogel monoliths that can be transformed to corresponding aerogels by a freeze-drying method. The composite nanobelt aerogel can withstand a compressive load of more than 4000 times of its own weight and fully recover after the removal of the weight. It can also sustain 1000 compressive cycles with 6.9% plastic deformation and 91.8% of the maximum stress remaining, with a constant energy loss coefficient as low as 0.16, at the set strain of 30%. The extraordinary mechanical properties are believed to be associated with the structural flexibility of the nanobelts and the RFR-reinforced joints between the crosslinked nanobelts. These inorganic-organic composite aerogels also show good thermal insulation and excellent fire-proof capability. This work provides an effective strategy for fabricating superelastic RFR-based aerogels which show promising applications in fields such as thermal insulation, energy storage, and catalyst support.

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.

Conducting ITO Nanoparticle-Based Aerogels—Nonaqueous One-Pot Synthesis vs. Particle Assembly Routes

Indium tin oxide (ITO) aerogels offer a combination of high surface area, porosity and conductive properties and could therefore be a promising material for electrodes in the fields of batteries, solar cells and fuel cells, as well as for optoelectronic applications. In this study, ITO aerogels were synthesized via two different approaches, followed by critical point drying (CPD) with liquid CO2. During the nonaqueous one-pot sol–gel synthesis in benzylamine (BnNH2), the ITO nanoparticles arranged to form a gel, which could be directly processed into an aerogel via solvent exchange, followed by CPD. Alternatively, for the analogous nonaqueous sol–gel synthesis in benzyl alcohol (BnOH), ITO nanoparticles were obtained and assembled into macroscopic aerogels with centimeter dimensions by controlled destabilization of a concentrated dispersion and CPD. As-synthesized ITO aerogels showed low electrical conductivities, but an improvement of two to three orders of magnitude was achieved by annealing, resulting in an electrical resistivity of 64.5–1.6 kΩ·cm. Annealing in a N2 atmosphere led to an even lower resistivity of 0.2–0.6 kΩ·cm. Concurrently, the BET surface area decreased from 106.2 to 55.6 m2/g with increasing annealing temperature. In essence, both synthesis strategies resulted in aerogels with attractive properties, showing great potential for many applications in energy storage and for optoelectronic devices.

Bottom-Up Design of a Green and Transient Zinc-Ion Battery with Ultralong Lifespan

Transient batteries are expected to lessen the inherent environmental impact of traditional batteries that rely on toxic and critical raw materials. This work presents the bottom-up design of a fully transient Zn-ion battery (ZIB) made of nontoxic and earth-abundant elements, including a novel hydrogel electrolyte prepared by cross-linking agarose and carboxymethyl cellulose. Facilitated by a high ionic conductivity and a high positive zinc-ion species transference number, the optimized hydrogel electrolyte enables stable cycling of the Zn anode with a lifespan extending over 8500 h for 0.25 mA cm^-2 - 0.25 mAh cm^-2 . On pairing with a biocompatible organic polydopamine-based cathode, the full cell ZIB delivers a capacity of 196 mAh g^-1 after 1000 cycles at a current density of 0.5 A g^-1 and a capacity of 110 mAh g^-1 after 10 000 cycles at a current density of 1 A g^-1 . A transient ZIB with a biodegradable agarose casing displays an open circuit voltage of 1.123 V and provides a specific capacity of 157 mAh g^-1 after 200 cycles at a current density of 50 mA g^-1 . After completing its service life, the battery can disintegrate under composting conditions.

Improving the Corrosion Protection of Poly(phenylene methylene) Coatings by Side Chain Engineering: The Case of Methoxy-Substituted Copolymers

This work aims to improve the corrosion protection features of poly(phenylene methylene) (PPM) by sidechain engineering inserting methoxy units along the polymer backbone. The influence of side methoxy groups at different concentrations (4.6% mol/mol and 9% mol/mol) on the final polymer properties was investigated by structural and thermal characterization of the resulting copolymers: co-PPM 4.6% and co-PPM 9%, respectively. Then, coatings were processed by hot pressing the polymers powder on aluminum alloy AA2024 and corrosion protection properties were evaluated exposing samples to a 3.5% w/v NaCl aqueous solution. Anodic polarization tests evidenced the enhanced corrosion protection ability (i.e., lower current density) by increasing the percentage of the co-monomer. Coatings made with co-PPM 9% showed the best protection performance with respect to both PPM blend and PPM co-polymers reported so far. Electrochemical response of aluminum alloy coated with co-PPM 9% was monitored over time under two "artificially-aged" conditions, that are: (i) a pristine coating subjected to potentiostatic anodic polarization cycles, and (ii) an artificially damaged coating at resting condition. The first scenario points to accelerating the corrosion process, the second one models damage of the coating potentially occurring either due to natural deterioration or due to any accidental scratching of the polymer layer. In both cases, an intrinsic self-healing phenomenon was indirectly argued by the time evolution of the impedance and of the current density of the coated systems. The degree of restoring to the "factory conditions" by co-polymer coatings after self-healing events is eventually discussed.

Robust Antibacterial Activity of Xanthan-Gum-Stabilized and Patterned CeO2-x-TiO2 Antifog Films

Increased occurrence of antimicrobial resistance leads to a huge burden on patients, the healthcare system, and society worldwide. Developing antimicrobial materials through doping rare-earth elements is a new strategy to overcome this challenge. To this end, we design antibacterial films containing CeO_2-x-TiO₂, xanthan gum, poly(acrylic acid), and hyaluronic acid. CeO_2-x-TiO₂ inks are additionally integrated into a hexagonal grid for prominent transparency. Such design yields not only an antibacterial efficacy of ∼100% toward Staphylococcus aureus and Escherichia coli but also excellent antifog performance for 72 h in a 100% humidity atmosphere. Moreover, FluidFM is employed to understand the interaction in-depth between bacteria and materials. We further reveal that reactive oxygen species (ROS) are crucial for the bactericidal activity of E. coli through fluorescent spectroscopic analysis and SEM imaging. We meanwhile confirm that Ce³⁺ ions are involved in the stripping phosphate groups, damaging the cell membrane of S. aureus. Therefore, the hexagonal mesh and xanthan-gum cross-linking chains act as a reservoir for ROS and Ce³⁺ ions, realizing a long-lasting antibacterial function. We hence develop an antibacterial and antifog dual-functional material that has the potential for a broad application in display devices, medical devices, food packaging, and wearable electronics.

Hierarchical Nanocellulose-Based Gel Polymer Electrolytes for Stable Na Electrodeposition in Sodium Ion Batteries

Sodium ion batteries (NIBs) based on earth-abundant materials offer efficient, safe, and environmentally sustainable solutions for a decarbonized society. However, to compete with mature energy storage technologies such as lithium ion batteries, further progress is needed, particularly regarding the energy density and operational lifetime. Considering these aspects as well as a circular economy perspective, the authors use biodegradable cellulose nanoparticles for the preparation of a gel polymer electrolyte that offers a high liquid electrolyte uptake of 2985%, an ionic conductivity of 2.32 mS cm^-1 , and a Na⁺ transference number of 0.637. A balanced ratio of mechanically rigid cellulose nanocrystals and flexible cellulose nanofibers results in a mesoporous hierarchical structure that ensures close contact with metallic Na. This architecture offers stable Na plating/stripping at current densities up to ±500 µA cm^-2 , outperforming conventional fossil-based NIBs containing separator-liquid electrolytes. Paired with an environmentally sustainable and economically attractive Na₂ Fe₂ (SO₄ )₃ cathode, the battery reaches an energy density of 240 Wh kg^-1 , delivering 69.7 mAh g^-1 after 50 cycles at a rate of 1C. In comparison, Celgard in liquid electrolyte delivers only 0.6 mAh g^-1 at C/4. Such gel polymer electrolytes may open up new opportunities for sustainable energy storage systems beyond lithium ion batteries.

Studies on the Interaction of Poly(phenylene methylene) with Silver(I) and Hexacarbonylchromium(0)

Complexes of poly(phenylene methylene) (PPM) with silver(I) ions and tricarbonylchromium(0) moieties, respectively, were synthesized. ¹³C NMR spectra indicate interaction of phenylene groups with silver(I) and chromium(0), and peak broadening implies dynamic behavior of the silver(I) complexes, with all phenylene groups temporarily involved in coordination, in contrast to the chromium complexes. About 5–10% of the phenylene groups are coordinated to metal atoms. ¹H NMR and IR spectra, in the case of chromium(0), and the solubility of silver salts in the presence of PPM provide further evidence of coordination. The complexes are soluble in chloroform, but the silver complexes decay in tetrahydrofuran (second-order kinetics were observed in an example). The photoluminescence (fluorescence) of PPM is maintained upon complexation, although coordination of silver(I) seems to favor the so-called blue phase of PPM relative to the green phase by a factor of approximately two in PL spectra. The pronounced absorption of the tricarbonylchromium(0) units interferes with the blue phase, which almost disappears at a concentration of 50 mg/mL in PLE spectra, whereas the emission maximum of the green phase is hardly affected. This leads to a confinement of the emitted wavelength range of PPM. Thus, the perceived optical emission of PPM can be modified by coordinated entities.

The Importance of the Macroscopic Geometry in Gas-Phase Photocatalysis

Photocatalysis has the potential to make a major technological contribution to solving pressing environmental and energy problems. There are many strategies for improving photocatalysts, such as tuning the composition to optimize visible light absorption, charge separation, and surface chemistry, ensuring high crystallinity, and controlling particle size and shape to increase overall surface area and exploit the reactivity of individual crystal facets. These processes mainly affect the nanoscale and are therefore summarized as nanostructuring. In comparison, microstructuring is performed on a larger size scale and is mainly concerned with particle assembly and thin film preparation. Interestingly, most structuring efforts stop at this point, and there are very few examples of geometry optimization on a millimeter or even centimeter scale. However, the recent work on nanoparticle-based aerogel monoliths has shown that this size range also offers great potential for improving the photocatalytic performance of materials, especially when the macroscopic geometry of the monolith is matched to the design of the photoreactor. This review article is dedicated to this aspect and addresses some issues and open questions that arise when working with macroscopically large photocatalysts. Guidelines are provided that could help develop novel and efficient photocatalysts with a truly 3D architecture.

Transient Batteries: A Promising Step Towards Powering Green Electronics

Transient electronics is an emerging class of innovative technology wherein electronic devices undergo controlled degradation processes after a period of stable operation, leaving no toxic products behind. This technology offers exciting opportunities in research areas of green electronics, temporary biomedical implants, data-secure hardware systems, and many others. However, one major challenge with these devices is their rigid and bulky batteries that contain toxic chemicals and are not at all degradable. So, to realize autonomous and self-sufficient transient electronics, the development of transient batteries is a pre-requisite. This review provides an overview of the advancements in the field of transient batteries, their materials, output performance, transience behaviour, and a few potential applications.

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.

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.

Degradation Behavior, Biocompatibility, Electrochemical Performance, and Circularity Potential of Transient Batteries

Transient technology seeks the development of materials, devices, or systems that undergo controlled degradation processes after a stable operation period, leaving behind harmless residues. To enable externally powered fully transient devices operating for longer periods compared to passive devices, transient batteries are needed. Albeit transient batteries are initially intended for biomedical applications, they represent an effective solution to circumvent the current contaminant leakage into the environment. Transient technology enables a more efficient recycling as it enhances material retrieval rates, limiting both human and environmental exposures to the hazardous pollutants present in conventional batteries. Little efforts are focused to catalog and understand the degradation characteristics of transient batteries. As the energy field is a property-driven science, not only electrochemical performance but also their degradation behavior plays a pivotal role in defining the specific end-use applications. The state-of-the-art transient batteries are critically reviewed with special emphasis on the degradation mechanisms, transiency time, and biocompatibility of the released degradation products. The potential of transient batteries to change the current paradigm that considers batteries as harmful waste is highlighted. Overall, transient batteries are ready for takeoff and hold a promising future to be a frontrunner in the uptake of circular economy concepts.

Colloidal Nanocrystals: A Toolbox for Materials Chemistry

Colloidal nanocrystals are the ideal building blocks for the fabrication of functional materials. Using various assembly, patterning or processing techniques, the nanocrystals can be arranged with unprecedented flexibility in 1-, 2- or 3-dimensional architectures over several orders of length scales, providing access to ordered or disordered, porous or non-porous, and simple as well as hierarchical structures. Careful selection of colloidal nanocrystals allows the properties of the final materials to be predefined. Moreover, by combining different nanocrystals, these properties can be fine-tuned for a specific application, opening up fascinating opportunities to create new materials for energy storage and conversion, catalysis, photocatalysis, biomedicine or optics. Indeed, functional materials made of preformed nanoparticles have been realized for metals, polymers, semiconductors, and ceramics, as well as for composites and organic-inorganic hybrids. In this review article, we introduce some concepts for the fabrication of colloidal nanocrystals and their assembly into dense and porous 3-dimensional structures. Porosity is a particularly important material property that strongly influences its application potential. Therefore, we pay special attention to this aspect and compare porous materials synthesized from nanoparticles with those from molecular routes. An additional focus is set on the degree of structural order that can be achieved on different length scales.

Stable Na Electrodeposition Enabled by Agarose-Based Water-Soluble Sodium Ion Battery Separators

Developing efficient energy storage technologies is at the core of current strategies toward a decarbonized society. Energy storage systems based on renewable, nontoxic, and degradable materials represent a circular economy approach to address the environmental pollution issues associated with conventional batteries, that is, resource depletion and inadequate disposal. Here we tap into that prospect using a marine biopolymer together with a water-soluble polymer to develop sodium ion battery (NIB) separators. Mesoporous membranes comprising agarose, an algae-derived polysaccharide, and poly(vinyl alcohol) are synthesized via nonsolvent-induced phase separation. Obtained membranes outperform conventional nondegradable NIB separators in terms of thermal stability, electrolyte wettability, and Na⁺ conductivity. Thanks to the good interfacial adhesion with metallic Na promoted by the hydroxyl and ether functional groups of agarose, the separators enable a stable and homogeneous Na deposition with limited dendrite growth. As a result, membranes can operate at 200 μA cm^-2, in contrast with Celgard and glass microfiber, which short circuit at 50 and 100 μA cm^-2, respectively. When evaluated in Na₃V₂(PO₄)₃/Na half-cells, agarose-based separators deliver 108 mA h g^-1 after 50 cycles at C/10, together with a remarkable rate capability. This work opens up new possibilities for the use of water-degradable separators, reducing the environmental burdens arising from the uncontrolled accumulation of electronic waste in marine or land environments.

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.

Multifunctional Batteries: Flexible, Transient, and Transparent

The primary task of a battery is to store energy and to power electronic devices. This has hardly changed over the years despite all the progress made in improving their electrochemical performance. In comparison to batteries, electronic devices are continuously equipped with new functions, and they also change their physical appearance, becoming flexible, rollable, stretchable, or maybe transparent or even transient or degradable. Mechanical flexibility makes them attractive for wearable electronics or for electronic paper; transparency is desired for transparent screens or smart windows, and degradability or transient properties have the potential to reduce electronic waste. For fully integrated and self-sufficient systems, these devices have to be powered by batteries with similar physical characteristics. To make the currently used rigid and heavy batteries flexible, transparent, and degradable, the whole battery architecture including active materials, current collectors, electrolyte/separator, and packaging has to be redesigned. This requires a fundamental paradigm change in battery research, moving away from exclusively addressing the electrochemical aspects toward an interdisciplinary approach involving chemists, materials scientists, and engineers. This Outlook provides an overview of the different activities in the field of flexible, transient, and transparent batteries with a focus on the challenges that have to be faced toward the development of such multifunctional energy storage devices.

A Sodium-Ion Battery Separator with Reversible Voltage Response Based on Water-Soluble Cellulose Derivatives

The development of efficient, safe, and environmentally friendly energy storage systems plays a pivotal role in moving toward a more sustainable society. Sodium-ion batteries (NIBs) have garnered considerable interest in grid energy storage applications because of the abundance of sodium, low cost, and suitable redox potential. However, NIB technology is still in its infancy, especially with regard to separators. Here we develop a novel separator based on renewable water-soluble cellulose derivatives. Carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) are cross-linked to afford large-specific-surface-area membranes upon nonsolvent-induced phase separation (NIPS). Long-term galvanostatic cycling in a symmetric Na/Na cell configuration shows an impressive reversible voltage response with a square wave shape of the polarization even after 250 h of cycling, indicating remarkably stable Na plating and stripping with Na dendrite growth suppression. This novel membrane is evaluated as a separator in Na₃V₂(PO₄)₃/Na half-cells. After 10 cycles at C/10, the cellulosic separator delivers a capacity of 74 mA·h·g^-1 with a 100% Coulombic efficiency compared to that of 61 mA·h·g^-1 and 96% obtained for Whatman GF/D as a commercially available separator. Our work provides novel cues for the development of biomass-derived porous membranes to function as battery separators, surpassing the performance of commercially available separators based on fossil resources in terms of capacity retention, Coulombic efficiency, homogeneous plating/stripping of Na, and dendrite growth suppression. These separators, which may be extended to other battery systems, are expected to play a significant role in developing sustainable energy storage systems.

SnS/N-Doped carbon composites with enhanced Li+ storage and lifetime by controlled hierarchical submicron- and nano-structuring

Hollow and dense hierarchical SnS microspheres coated with a nitrogen doped carbon layer were synthesised, tested and compared as anodes in lithium ion battery half cells.

Towards fast-charging technologies in Li+/Na+ storage: from the perspectives of pseudocapacitive materials and non-aqueous hybrid capacitors

Since the discovery of the pseudocapacitive behavior in RuO₂ by Sergio Trasatti and Giovanni Buzzanca in 1971, materials with pseudocapacitance have been regarded as promising candidates for high-power energy storage. Pseudocapacitance-involving energy storage is predominantly based on faradaic redox reactions, but at the same time the charge storage is not limited by solid-state ion diffusion. Besides the search for pseudocapacitive materials, their implementation into non-aqueous hybrid capacitors stands for the strategy to increase power density by a rational design of the battery structure. Composed of a battery-type anode and a capacitor-type cathode, such devices show great promise to integrate the merits of both batteries and capacitors. Today, the availability of fast-charging technologies is of fundamental importance for establishing electric vehicles on a mass scale. Therefore, from the perspective of materials and battery design, understanding the basics and the recent developments of pseudocapacitive materials and non-aqueous hybrid capacitors is of great importance. With this goal in mind, we introduce here the fundamentals of pseudocapacitance and non-aqueous hybrid capacitors. In addition, we provide an overview of the latest developments in this fast growing research field.

Fully Integrated Design of a Stretchable Solid-State Lithium-Ion Full Battery

A solid-state lithium-ion battery, in which all components (current collector, anode and cathode, electrolyte, and packaging) are stretchable, is introduced, giving rise to a battery design with mechanical properties that are compliant with flexible electronic devices and elastic wearable systems. By depositing Ag microflakes as a conductive layer on a stretchable carbon-polymer composite, a current collector with a low sheet resistance of ≈2.7 Ω □^-1 at 100% strain is obtained. Stretchable electrodes are fabricated by integrating active materials with the elastic current collector. A polyacrylamide-"water-in-salt" electrolyte is developed, offering high ionic conductivity of 10^-3 to 10^-2 S cm^-1 at room temperature and outstanding stretchability up to ≈300% of its original length. Finally, all these components are assembled into a solid-state lithium-ion full cell in thin-film configuration. Thanks to the deformable individual components, the full cell functions when stretched, bent, or even twisted. For example, after stretching the battery to 50%, a reversible capacity of 28 mAh g^-1 and an average energy density of 20 Wh kg^-1 can still be obtained after 50 cycles at 120 mA g^-1 , confirming the functionality of the battery under extreme mechanical stress.

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.

Titania-Cellulose Hybrid Monolith for In-Flow Purification of Water under Solar Illumination

In this work, we report a versatile approach for the development of an in-flow purification water system under solar illumination. Cellulose nanofibrils (CNFs) were impregnated with TiO₂ nanoparticles using water as a solvent to obtain hybrid CNF/TiO₂ monoliths with 98% porosity. The opposite surface potential enables an electrostatically induced direct conjugation between TiO₂ and CNFs. Scanning electron microscopy analysis of the surface morphology of the CNF/TiO₂ monolith shows a homogeneous dense coating of titania nanoparticles onto the interconnected nanofibril network, providing a Brunauer-Emmett-Teller surface area of about 80 m²·g^-1 for the hybrid monolith. Furthermore, compression tests reveal a good shape recovery after unloading, thanks to the highly flexible and mechanically stable three-dimensional structure. Finally, the CNF-based hybrids were tested as catalysts for the decomposition of organic pollutants under solar illumination. The tests were performed using a continuous flow reactor with a customized holder, allowing the solution to pass through the monolith. The results reveal a good photocatalytic activity and a long-term stability of the hybrid CNF/TiO₂ monolith toward the decomposition of methyl orange and paracetamol. These features provide a proof of concept for the applicability of the hybrid CNF/TiO₂ monoliths for in-flow purification of water under solar illumination, not only for model dyes but also for organic pollutants of high practical relevance.

Synthesis of High Molar Mass Poly(phenylene methylene) Catalyzed by Tungsten(II) Compounds

Poly(phenylene methylene)s (PPMs) with high molar masses were isolated by polymerization of benzyl chloride catalyzed with tungsten(II) compounds and subsequent fractionation. Four different tungsten(II) catalysts were successfully exploited for the polymerization, for which a strict temperature profile was developed. The PPMs possessed roughly a trimodal molar mass distribution. Simple fractionation by phase separation of 2-butanone solutions allowed to effectively segregate the products primarily into PPM of low molar mass (M_n = 1600 g mol⁻¹) and high molar mass (M_n = 167,900 g mol⁻¹); the latter can be obtained in large quantities up to 50 g. The evolution of the trimodal distribution and the monomer conversion was monitored by gel permeation chromatography (GPC) and ¹H NMR spectroscopy, respectively, over the course of the polymerization. The results revealed that polymerization proceeded via a chain-growth mechanism. This study illustrates a new approach to synthesize PPM with hitherto unknown high molar masses which opens the possibility to explore new applications, e.g., for temperature-resistant coatings, fluorescent coatings, barrier materials or optical materials.

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/ R_o < 1.28) upon bending to curvatures of 1 mm radius.

Radio-luminescence spectral features and fast emission in hafnium dioxide nanocrystals

In this work, we investigate the optical properties of hafnium dioxide nanocrystals, upon X-ray irradiation, looking for spectral evolution following thermal treatments in air up to 1000 °C that modify the crystal size as well as their point defect concentrations. Radio-luminescence measurements from 10 K up to room temperature reveal a rich and evolving picture of the optical features. A complete spectral analysis of the broad luminescence spectra reveals the presence of several emission components in the visible and UV regions. The lower energy components peaking at 2.1, 2.5, and 2.9 eV are characterized by a thermal quenching energy of 0.08 eV, while the corresponding value for the UV bands at 4.1 and 4.7 eV is close to 0.23 eV. We tentatively assign the components ranging from 2 to 3 eV to the presence of optically active defects of an intrinsic nature, together with the occurrence of titanium impurities; conversely, the bands at higher energies are likely to be of an excitonic nature. The comparison with previous photo-luminescence studies allows evidencing characteristic differences between the features of luminescence emissions caused by intra-centre excitation and those occurring under ionizing irradiation. Finally, scintillation measurements in the visible range reveal the existence of a fast decay in the nanosecond time scale for the smallest hafnia nanocrystals. This study offers a clear description of HfO2 luminescence characteristics upon excitation by X-rays and can lead to a better comprehension of the structure-property relationship at the nanoscale in metal oxides.

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.

Nonaqueous Sol-Gel Synthesis of Anatase Nanoparticles and Their Electrophoretic Deposition in Porous Alumina

Titanium dioxide (TiO₂) nanoparticles were synthesized by nonaqueous sol-gel route using titanium tetrachloride and benzyl alcohol as the solvent. The obtained 4 nm-sized anatase nanocrystals were readily dispersible in various polar solvents allowing for simple preparation of colloidal dispersions in water, isopropyl alcohol, dimethyl sulfoxide, and ethanol. Results showed that dispersed nanoparticles have acidic properties and exhibit positive zeta-potential which is suitable for their deposition by cathodic electrophoresis. Aluminum substrates were anodized in phosphoric acid in order to produce porous anodic oxide layers with pores ranging from 160 to 320 nm. The resulting nanopores were then filled with TiO₂ nanoparticles by electrophoretic deposition. The influence of the solvent, the electric field, and the morphological characteristics of the alumina layer (i.e., barrier layer and porosity) were studied.

Multicomposite Nanostructured Hematite-Titania Photoanodes with Improved Oxygen Evolution: The Role of the Oxygen Evolution Catalyst

We present a sol-gel processed hematite-titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm² at 1.23 V_RHE under simulated AM 1.5 G illumination (100 mW/cm²) thanks to the addition of an amorphous cocatalyst with the nominal composition Fe₂₀Cr₄₀Ni₄₀O _x . To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states (C _SS) compared to the bare photoanode, the function of the catalyst-photoanode interface resembles that of a p-n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states (R _ct,ss) was unchanged at potentials between 0.8 and 1.1 V_RHE after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated.

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.

Synthesis of aerogels: from molecular routes to 3-dimensional nanoparticle assembly

Colloidal nanocrystals are extensively used as building blocks in nanoscience, and amazing results have been achieved in assembling them into ordered, close-packed structures. But in spite of great efforts, the size of these structures is typically restricted to a few micrometers, and it is very hard to extend them into the macroscopic world. In comparison, aerogels are macroscopic materials, highly porous, disordered, ultralight and with immense surface areas. With these distinctive characteristics, they are entirely contrary to common nanoparticle assemblies such as superlattices or nanocrystal solids, and therefore cover a different range of applications. While aerogels are traditionally synthesized by molecular routes based on aqueous sol-gel chemistry, in the last few years the gelation of nanoparticle dispersions became a viable alternative to improve the crystallinity and to widen the structural, morphological and compositional complexity of aerogels. In this Review, the different approaches to inorganic non-siliceous and non-carbon aerogels are addressed. We start our discussion with wet chemical routes involving molecular precursors, followed by processing methods using nanoparticles as building blocks. A unique feature of many of these routes is the fact that a macroscopic, often monolithic body is produced by pure self-assembly of nanosized colloids without the need for any templates.