Colloidal Nanocrystal Frameworks

Dynamic Coassembly of Amphiphilic Block Copolymer and Polyoxometalates in Dual Solvent Systems: An Efficient Approach to Heteroatom-Doped Semiconductor Metal Oxides with Controllable Nanostructures

Dynamic coassembly of block copolymers (BCPs) with Keggin-type polyoxometalates (POMs) is developed to synthesize heteroatom-doped tungsten oxide with controllable nanostructures, including hollow hemispheres, nanoparticles, and nanowires. The versatile coassembly in dual n-hexane/THF solvent solution enables the fomation of poly(ethylene oxide)-b-polystyrene (PEO-b-PS)/POMs (e.g., silicotungstic acid, H₄SiW₁₂O₄₀) nanocomposites with different morphologies such as spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles, which can be readily converted into diverse nanostructured metal oxides with high surface area and unique properties via in situ thermal-induced structural evolution. For example, uniform silicon-doped WO₃ (Si-WO₃) hollow hemispheres derived from coassembly of PEO-b-PS with H₄SiW₁₂O₄₀ were utilized to fabricate gas sensing devices which exhibit superior gas sensing performance toward acetone, thanks to the selective gas-solid interface catalytic reaction that induces resistance changes of the devices due to the high specific surface areas, abundant oxygen vacancies, and the Si-doping induced metastable ε-phase of WO₃. Furthermore, density functional theory (DFT) calculation reveals the mechanism about the high sensitivity and selectivity of the gas sensors. On the basis of the as-fabricated devices, an integrated gas sensor module was constructed, which is capable of real-time monitoring the environmental acetone concentration and displaying relevant sensing results on a smart phone via Bluetooth communication.

Insights into ZnO-based doped porous nanocrystal frameworks

Colloidal nanocrystals play a vital role in several applications. The doping of cations in the nanocrystal matrix enhances the optical, electrical, and magnetic properties. The number and well-defined distribution of the dopant are crucial to protect the nanocrystal from clustering. The XRD, XPS, and XAS instruments reveal the change in the lattice parameters, chemical states, and local coordination environment information. In addition of detecting the position and distribution of the dopant, the 4D-STEM detector mode gathers all types of real-space atomic-resolution images by collecting all diffraction datasets from each electron probe with high-speed and efficient detection. Dopant-host ligand type, reactions conditions, and reaction time optimization during synthesis are critical for the host and dopant reactivity balance. Pearson's hard/soft acids/bases theory would be a base for balancing the solubility of the dopant-host in the given solvents/surfactant. In addition, tuning the colloidal nanocrystals to secondary structures, which enhances the mass-/ions transport, can contribute a combination of properties that do not exist in the original constituents.

Insight into nanocrystal synthesis: from precursor decomposition to combustion

Nanotechnology-based synthesis of nanoscale materials has appealed to the attention of scientists in the modern scientific community. In the bottom-up approach, atoms start to aggregate/agglomerate and form nuclei within the minimum and maximum supersaturation range. Once nuclei are generated above the critical-free energy/radius, the growth is initiated by obeying the LaMar model with a slight extra simple growth by diffusion advancement. The in situ real-time liquid phase analysis using STEM, AFM, and XAS techniques is used to control precursor decomposition to the nanocrystal formation process and should be a non-stoppable technique. Solution combustion synthesis (SCS) is a time-/energy-efficient self-sustained process that produces mass-/ion transport active porous materials. SCS also permits the synthesis of evenly distributed-doped and hybrid-nanomaterials, which are beneficial in tuning crucial properties of the materials. The growth and development of nanocrystals, dehydrating the sol in the presence of a surfactant or/and fuel results in combustion once it arrives at the ignition temperature. Besides, the kinetic and thermodynamics controlled architecture-directing agent-assisted SCS offers colloidal nanocrystal framework formation, which is currently highly applicable for energy devices. This short review provides insightful information that adds to the existing nanocrystal synthesis process and solution combustion synthesis and recommends future directions in the field.

The LaMar model visualizes the process of nanocrystal formation. The solution combustion synthesis approach is a noble methodology resulting in highly stable and ordered porous nanomaterials.

Recent advances in the synthesis of hierarchically mesoporous TiO2 materials for energy and environmental applications

Because of their low cost, natural abundance, environmental benignity, plentiful polymorphs, good chemical stability and excellent optical properties, TiO₂ materials are of great importance in the areas of physics, chemistry and material science. Much effort has been devoted to the synthesis of TiO₂ nanomaterials for various applications. Among them, mesoporous TiO₂ materials, especially with hierarchically porous structures, show great potential owing to their extraordinarily high surface areas, large pore volumes, tunable pore structures and morphologies, and nanoscale effects. This review aims to provide an overview of the synthesis and applications of hierarchically mesoporous TiO₂ materials. In the first section, the general synthetic strategies for hierarchically mesoporous TiO₂ materials are reviewed. After that, we summarize the architectures of hierarchically mesoporous TiO₂ materials, including nanofibers, nanosheets, microparticles, films, spheres, core-shell and multi-level structures. At the same time, the corresponding mechanisms and the key factors for the controllable synthesis are highlighted. Following this, the applications of hierarchically mesoporous TiO₂ materials in terms of energy storage and environmental protection, including photocatalytic degradation of pollutants, photocatalytic fuel generation, photoelectrochemical water splitting, catalyst support, lithium-ion batteries and sodium-ion batteries, are discussed. Finally, we outline the challenges and future directions of research and development in this area.

Printed gas sensors

This review presents the recent development of printed gas sensors based on functional inks.

Guiding kinetic trajectories between jammed and unjammed states in 2D colloidal nanocrystal-polymer assemblies with zwitterionic ligands

Mesostructured matter composed of colloidal nanocrystals in solidified architectures abounds with broadly tunable catalytic, magnetic, optoelectronic, and energy storing properties. Less common are liquid-like assemblies of colloidal nanocrystals in a condensed phase, which may have different energy transduction behaviors owing to their dynamic character. Limiting investigations into dynamic colloidal nanocrystal architectures is the lack of schemes to control or redirect the tendency of the system to solidify. We show how to solidify and subsequently reconfigure colloidal nanocrystal assemblies dimensionally confined to a liquid-liquid interface. Our success in this regard hinged on the development of competitive chemistries anchoring or releasing the nanocrystals to or from the interface. With these chemistries, it was possible to control the kinetic trajectory between quasi-two-dimensional jammed (solid-like) and unjammed (liquid-like) states. In future schemes, it may be possible to leverage this control to direct the formation or destruction of explicit physical pathways for energy carriers to migrate in the system in response to an external field.

High-Throughput Preparation of Metal Oxide Nanocrystals by Cathodic Corrosion and Their Use as Active Photocatalysts

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (H₂WO₄) nanoplates, titanium oxide (TiO₂) nanowires, and symmetric star-shaped bismuth vanadate (BiVO₄) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline H₂WO₄ and BiVO₄ particles and amorphous TiO₂ were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO₄ particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Nearest-neighbour nanocrystal bonding dictates framework stability or collapse in colloidal nanocrystal frameworks

Block copolymers serve as architecture-directing agents for the assembly of colloidal nanocrystals into a variety of mesoporous solids. Here we report the fundamental order-disorder transition in such assemblies, which yield, on one hand, ordered colloidal nanocrystals frameworks or, alternatively, disordered mesoporous nanocrystal films. Our determination of the order-disorder transition is based on extensive image analysis of films after thermal processing. The number of nearest-nanocrystal neighbours emerges as a critical parameter dictating assembly outcomes, which is in turn determined by the nanocrystal volume fraction (f_NC). We also identify the minimum f_NC needed to support the structure against collapse.

Ordering in Polymer Micelle-Directed Assemblies of Colloidal Nanocrystals

Assembly of presynthesized nanocrystals by block copolymer micelles can be rationalized by the incorporation of nanocrystals into micellar coronas of constant width. As determined by quantitative analysis using small-angle X-ray scattering, high loading of small nanocrystals yields composites exhibiting order on two length scales, whereas intermediate loading of nanocrystals larger than the coronal width produces single nanocrystal networks. The resulting structures obey expectations of thermodynamically driven assembly on the nanocrystal length scale, whereas kinetically frozen packing principles dictate order on the polymer micelle length scale.