Patterning of Nanoparticle-Based Aerogels and Xerogels by Inkjet Printing

Investigation of the Photocatalytic Hydrogen Production of Semiconductor Nanocrystal-Based Hydrogels

Destabilization of a ligand-stabilized semiconductor nanocrystal solution with an oxidizing agent can lead to a macroscopic highly porous self-supporting nanocrystal network entitled hydrogel, with good accessibility to the surface. The previously reported charge carrier delocalization beyond a single nanocrystal building block in such gels can extend the charge carrier mobility and make a photocatalytic reaction more probable. The synthesis of ligand-stabilized nanocrystals with specific physicochemical properties is possible, thanks to the advances in colloid chemistry made in the last decades. Combining the properties of these nanocrystals with the advantages of nanocrystal-based hydrogels will lead to novel materials with optimized photocatalytic properties. This work demonstrates that CdSe quantum dots, CdS nanorods, and CdSe/CdS dot-in-rod-shaped nanorods as nanocrystal-based hydrogels can exhibit a much higher hydrogen production rate compared to their ligand-stabilized nanocrystal solutions. The gel synthesis through controlled destabilization by ligand oxidation preserves the high surface-to-volume ratio, ensures the accessible surface area even in hole-trapping solutions and facilitates photocatalytic hydrogen production without a co-catalyst. Especially with such self-supporting networks of nanocrystals, the problem of colloidal (in)stability in photocatalysis is circumvented. X-ray photoelectron spectroscopy and photoelectrochemical measurements reveal the advantageous properties of the 3D networks for application in photocatalytic hydrogen production.

Development and Characterizations of Novel Aqueous-Based Ceramic Inks for Inkjet Printing

Stable rheological properties of ceramic ink are a key requirement for inkjet printing (IJP), which should be satisfied in terms of the Reynolds and Weber numbers. In this paper, the reverse microemulsion was introduced for the synthesis of monodispersed nanosized ceramic powders, and the average size was less than 100 nm. A comparison of two different dispersants, i.e., polyacrylic ammonium (PAANH₄) and polyacrylic aid (PAA), revealed that the former exerted a good dispersion effect on the ceramic ink. The sedimentation ratio, zeta potential, surface tension, viscosity, and density of the inks were measured, and the Reynolds and Weber numbers, as well as Z value, were calculated. A stable, homogeneous, and high solid loading (20 wt%) ceramic ink could be achieved after aging for a period of 72 h. Finally, the ceramic inks showed the desired printable property in the inkjet printing process. Combining inkjet printing technology with a sintering process, Ni-Mn-O films have the potential to monitor temperature and humidity parameters for intelligent wearable devices.

Temperature and Composition Dependent Optical Properties of CdSe/CdS Dot/Rod-Based Aerogel Networks

Employing nanocrystals (NCs) as building blocks of porous aerogel network structures allows the conversion of NC materials into macroscopic solid structures while conserving their unique nanoscopic properties. Understanding the interplay of the network formation and its influence on these properties like size‐dependent emission is a key to apply techniques for the fabrication of novel nanocrystal aerogels. In this work, CdSe/CdS dot/rod NCs possessing two different CdSe core sizes were synthesized and converted into porous aerogel network structures. Temperature‐dependent steady‐state and time‐resolved photoluminescence measurements were performed to expand the understanding of the optical and electronic properties of these network structures generated from these two different building blocks and correlate their optical with the structural properties. These investigations reveal the influence of network formation and aerogel production on the network‐forming nanocrystals. Based on the two investigated NC building blocks and their aerogel networks, mixed network structures with various ratios of the two building blocks were produced and likewise optically characterized. Since the different building blocks show diverse optical response, this technique presents a straightforward way to color‐tune the resulting networks simply by choosing the building block ratio in connection with their quantum yield.

Morphology effect on zinc oxide quantum photoinitiators for radical polymerization

Semiconductor nanocrystal based photoinitiators, quantum PIs, are a viable alternative to organic photoinitiators demonstrating unique advantages, including a broad and tunable excitation window, limited migration, and more. Aiming towards efficient quantum PIs with tunable properties, a deeper understanding of the relationships between the nanoparticle properties and their efficiency is required. Herein, we studied the morphological effect on ZnO nanocrystals functioning as photoinitiators in both water-based and solvent-free formulations by comparing rod and pyramidal shaped particles of similar volumes and nearly identical surface area. Superior polymerization performances are measured for the nanorods. Photocatalytic characterization including oxygen consumption and reactive oxygen species formation as well as dyes reduction and oxidation, also showed enhanced activities for the nanorods. The different performances were attributed to the anisotropic nanorod morphology which is beneficial for charge separation as well as to the presence of a reactive [0001] facet in the nanorods, which is known to increase the adsorption of molecular oxygen and anionic molecules, thus affecting the catalytic activity. These observations, along with the higher photoinitiation efficiency of the ZnO nanorods, bring them closer to functionality as photoinitiators in numerous photopolymerization applications.

Printed aerogels: chemistry, processing, and applications

As an extraordinarily lightweight and porous functional nanomaterial family, aerogels have attracted considerable interest in academia and industry in recent decades. Despite the application scopes, the modest mechanical durability of aerogels makes their processing and operation challenging, in particular, for situations demanding intricate physical structures. "Bottom-up" additive manufacturing technology has the potential to address this drawback. Indeed, since the first report of 3D printed aerogels in 2015, a new interdisciplinary research area combining aerogel and printing technology has emerged to push the boundaries of structure and performance, further broadening their application scope. This review summarizes the state-of-the-art of printed aerogels and presents a comprehensive view of their developments in the past 5 years, and highlights the key near- and mid-term challenges.

Recent Advances in Wearable Devices for Non-Invasive Sensing

The development of wearable sensors is aimed at enabling continuous real-time health monitoring, which leads to timely and precise diagnosis anytime and anywhere. Unlike conventional wearable sensors that are somewhat bulky, rigid, and planar, research for next-generation wearable sensors has been focused on establishing fully-wearable systems. To attain such excellent wearability while providing accurate and reliable measurements, fabrication strategies should include (1) proper choices of materials and structural designs, (2) constructing efficient wireless power and data transmission systems, and (3) developing highly-integrated sensing systems. Herein, we discuss recent advances in wearable devices for non-invasive sensing, with focuses on materials design, nano/microfabrication, sensors, wireless technologies, and the integration of those.

Control over Structure and Properties in Nanocrystal Aerogels at the Nano-, Micro-, and Macroscale

The assembly of individual colloidal nanocrystals into macroscopic solvogels and aerogels introduced a new exciting type of material into the class of porous architectures. In these so-called nanocrystal gels, the structure and properties can be controlled and fine-tuned to the smallest details. Recently it was shown that by employing nanocrystal building blocks for such gel materials, the interesting nanoscopic properties can be conserved or even expanded to properties that are available neither in the nanocrystals nor in their respective bulk materials. In general, the production of these materials features the wet-chemical synthesis of stable nanocrystal colloids followed by their carefully controlled destabilization to facilitate arrangement of the nanocrystals into highly porous, interconnected networks. By isolation of the synthesis of the discrete building blocks from the assembly process, the electronic structure, optical properties, and structural morphology can be tailored by the myriad of procedures developed in colloidal nanocrystal chemistry. Furthermore, knowledge and control over the structure–property correlation in the resulting gel structures opens up numerous new ways for extended and advanced applications. Consequently, the amount of different materials converted to nanocrystal-based gel structures is rising steadily. Meanwhile the number of methods for assembly initiation is likewise increasing, offering control over the overall network structure and porosity as well as the individual nanocrystal building block connection. The resulting networks can be dried by different methods to obtain highly porous air-filled networks (aerogels) or applied in their wet form (solvogels). By now a number of different applications profiting from the unique advantages of nanocrystal-based gel materials have been realized and exploited in the areas of photocatalysis, electrocatalysis, and sensing.

In this Account, we aim to summarize the efforts undertaken in the structuring of nanocrystal-based network materials on different scales, fine-tuning of the individual building blocks on the nanoscale, the network connections on the microscale, and the macroscale structure and shape of the final construct. It is exemplarily demonstrated how cation exchange reactions (at the nanoscale), postgelation modifications on the nanocrystal networks (microscale), and the structuring of the gels via printing techniques (macroscale) endow the resulting nanocrystal gel networks with novel physicochemical, mechanical, and electrocatalytic properties. The methods applied in the more traditional sol–gel chemistry targeting micro- and macroscale structuring are also reviewed, showing their future potential promoting the field of nanocrystal-based aerogels and their applications.

3D-Printed Quantum Dot Nanopixels

The pixel is the minimum unit used to represent or record information in photonic devices. The size of the pixel determines the density of the integrated information, such as the resolution of displays or cameras. Most methods used to produce display pixels are based on two-dimensional patterning of light-emitting materials. However, the brightness of the pixels is limited when they are miniaturized to nanoscale dimensions owing to their limited volume. Herein, we demonstrate the production of three-dimensional (3D) pixels with nanoscale dimensions based on the 3D printing of quantum dots embedded in polymer nanowires. In particular, a femtoliter meniscus was used to guide the solidification of liquid inks to form vertically freestanding nanopillar structures. Based on the 3D layout, we show high-density integration of color pixels, with a lateral dimension of 620 nm and a pitch of 3 μm for each of the red, green, and blue colors. The 3D structure enabled a 2-fold increase in brightness without significant effects on the spatial resolution of the pixels. In addition, we demonstrate individual control of the brightness based on a simple adjustment of the height of the 3D pixels. This method can be used to achieve super-high-resolution display devices and various photonic applications across a range of disciplines.

Revealing the Correlation of the Electrochemical Properties and the Hydration of Inkjet-Printed CdSe/CdS Semiconductor Gels

The mobility of charge carriers across a semiconductor-nanoparticle-based 3D network (i.e., a gel) and the interfacial transfer of the charge carriers across the nanoparticle network/electrolyte boundary are elementary processes for applications in the fields of sensing and energy harvesting. The automated manufacturing of electrodes coated with porous networks can be realized by inkjet printing. By simultaneous printing of CdSe/CdS dot-in-rod-shaped nanorods (NRs) and the destabilization reagent, CdSe/CdS gel-network-coated electrodes can be obtained. In this work, the charge carrier mobility of the electrons and the holes within the porous CdSe/CdS nanorod gel network is investigated via photoelectrochemistry. Using linear sweep voltammograms (LSVs) and intensity-modulated photocurrent spectroscopy (IMPS), it is shown that the electron is moving within the tip-to-tip-connected CdSe/CdS NR gel structure, while the holes are trapped in the CdSe seed of the semiconductor heterostructures. Furthermore, the preparation process of gel structures is related to the elementary mechanism of hydration, which can be shown via photoelectrochemical long-term studies.

A Versatile Route to Assemble Semiconductor Nanoparticles into Functional Aerogels by Means of Trivalent Cations

3D nanoparticle assemblies offer a unique platform to enhance and extend the functionality and optical/electrical properties of individual nanoparticles. Especially, a self-supported, voluminous, and porous macroscopic material built up from interconnected semiconductor nanoparticles provides new possibilities in the field of sensing, optoelectronics, and photovoltaics. Herein, a method is demonstrated for assembling semiconductor nanoparticle systems containing building blocks possessing different composition, size, shape, and surface ligands. The method is based on the controlled destabilization of the particles triggered by trivalent cations (Y³⁺ , Yb³⁺ , and Al³⁺ ). The effect of the cations is investigated via X-ray photoelectron spectroscopy. The macroscopic, self-supported aerogels consist of the hyperbranched network of interconnected CdSe/CdS dot-in-rods, or CdSe/CdS as well as CdSe/CdTe core-crown nanoplatelets is used to demonstrate the versatility of the procedure. The non-oxidative assembly method takes place at room temperature without thermal activation in several hours and preserves the shape and the fluorescence of the building blocks. The assembled nanoparticle network provides longer exciton lifetimes with retained photoluminescence quantum yields, that make these nanostructured materials a perfect platform for novel multifunctional 3D networks in sensing. Various sets of photoelectrochemical measurements on the interconnected semiconductor nanorod structures also reveal the enhanced charge carrier separation.

Exploiting kinetics for assembly of multicomponent nanoparticle networks with programmable control of heterogeneity

Kinetic control of metal chalcogenide nanoparticle oxidative assembly is realized by varying the redox potential of the chalcogenide, structure (wurtzite vs. zinc blende), and ligand chain length. This knowledge is exploited to form two-component (ZnS + CdSe) hybrid aerogels with minimal heterobonding (phase-segregated) or maximal heterobonding (intimately mixed).

Nanocrystal Aerogels with Coupled or Decoupled Building Blocks

The influence of interparticle contact in nanoparticle-based aerogel network structures is investigated by selectively connecting or isolating the building blocks inside of the network, thereby coupling and decoupling them in regards to their optical and electronic properties. This is achieved by tuning the synthesis sequence and exchanging the point of shell growth and the point of particle assembly, leading to two distinctly different structures as examined by electron microscopy. By thorough examination of the resulting optical properties of the generated structures, the clear correlation between nanoscopic/microscopic structure and macroscopic optical properties is demonstrated. Temperature-dependent measurements and effective mass approximation calculations support our findings.