Reversible cation exchange on macroscopic CdSe/CdS and CdS nanorod based gel networks

Optical properties of NIR photoluminescent PbS nanocrystal-based three-dimensional networks

The assembly of nanocrystals (NCs) into three-dimensional network structures is a recently established strategy to produce macroscopic materials with nanoscopic properties. These networks can be formed by the controlled destabilization of NC colloids and subsequent supercritical drying to obtain NC-based aerogels. Even though this strategy has been used for many different semiconductor NCs, the emission of NC-based aerogels is limited to the ultraviolet and visible and no near-infrared (NIR) emitting NC-based aerogels have been investigated in literature until now. In the present work we have optimized a gelation route of NIR emitting PbS and PbS/CdS quantum dots (QDs) by means of a recently established gel formation method using trivalent ions to induce the network formation. Thereby, depending on the surface ligands and QDs used the resulting network structure is different. We propose, that the ligand affinity to the nanocrystal surface plays an essential role during network formation, which is supported by theoretical calculations. The optical properties were investigated with a focus on their steady-state and time resolved photoluminescence (PL). Unlike in PbS/CdS aerogels, the absorption of PbS aerogels and their PL shift strongly. For all aerogels the PL lifetimes are reduced in comparison to those of the building blocks with this reduction being especially pronounced in the PbS aerogels.

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.

Electrochemical gelation of quantum dots using non-noble metal electrodes at high oxidation potentials

Relative to conventional chemical approaches, electrochemical assembly of metal chalcogenide nanoparticles enables the use of two additional levers for tuning the assembly process: electrode material and potential. In our prior work, oxidative and metal-mediated pathways for electrochemical assembly of metal chalcogenide quantum dots (QDs) into three-dimensional gel architectures were investigated independently by employing a noble-metal (Pt) electrode at relatively high potentials and a non-noble metal electrode at relatively low potentials, respectively. In the present work, we reveal competition between the two electrogelation pathways under the condition of high oxidation potentials and non-noble metal electrodes (including Ni, Co, Zn, and Ag), where both pathways are active. We found that the electrogel structure formed under this condition is electrode material-dependent. For Ni, the major phase is oxidative electrogel, not a potential-dependent mixture of oxidative and metal-mediated electrogel that one would expect. A mechanistic study reveals that the metal-mediated electrogelation is suppressed by dithiolates, a side product from the oxidative electrogelation, which block the Ni electrode surface and terminate metal ion release. In contrast, for Co, Ag, and Zn, the electrode surface blockage by dithiolates is less effective than for Ni, such that metal-mediated electrogelation is the primary gelation pathway.

Colloidal Zn3X2 (X = P, As) quantum dots with metal salts and their transformation into (InyZn1-y)3X2via cation-exchange reactions

We synthesized uniform Zn₃X₂ (X = P, As) quantum dots (QDs) for the first time using a stable, environmentally friendly zinc precursor instead of an organometallic precursor such as Me₂Zn or Et₂Zn, and controlled the QD size from about 2.0 nm to 6.0 nm. Moreover, tetragonal Zn₃P₂ and Zn₃As₂ QDs were transformed into zinc blende (In_yZn_1-y)₃P₂ and (In_yZn_1-y)₃As₂ QDs via the In³⁺ cationic-exchange reaction. To ensure the cation exchange reaction, we controlled reaction conditions, and confirmed it with various analytical methods. The replacement of Zn²⁺ by In³⁺ in the Zn₃X₂ QDs did not lead to changes in the particle size, but altered the optical properties and structure. In addition, we presented Cd₃P₂ and Cd₃As₂, which have the same tetragonal structure as the Zn₃X₂ QDs, through the same cationic exchange reaction.

Atomically dispersed Pb ionic sites in PbCdSe quantum dot gels enhance room-temperature NO2 sensing

Atmospheric NO2 is of great concern due to its adverse effects on human health and the environment, motivating research on NO2 detection and remediation. Existing low-cost room-temperature NO2 sensors often suffer from low sensitivity at the ppb level or long recovery times, reflecting the trade-off between sensor response and recovery time. Here, we report an atomically dispersed metal ion strategy to address it. We discover that bimetallic PbCdSe quantum dot (QD) gels containing atomically dispersed Pb ionic sites achieve the optimal combination of strong sensor response and fast recovery, leading to a high-performance room-temperature p-type semiconductor NO2 sensor as characterized by a combination of ultra-low limit of detection, high sensitivity and stability, fast response and recovery. With the help of theoretical calculations, we reveal the high performance of the PbCdSe QD gel arises from the unique tuning effects of Pb ionic sites on NO2 binding at their neighboring Cd sites.

Aerogelation of Polymer-Coated Photoluminescent, Plasmonic, and Magnetic Nanoparticles for Biosensing Applications

Macroscopic materials with nanoscopic properties have recently been synthesized by self-assembling defined nanoparticles to form self-supported networks, so-called aerogels. Motivated by the promising properties of this class of materials, the search for versatile routes toward the controlled assembly of presynthesized nanoparticles into such ultralight macroscopic materials has become a great interest. Overcoating procedures of colloidal nanoparticles with polymers offer versatile means to produce aerogels from nanoparticles, regardless of their size, shape, or properties while retaining their original characteristics. Herein, we report on the surface modification and assembly of various building blocks: photoluminescent nanorods, magnetic nanospheres, and plasmonic nanocubes with particle sizes between 5 and 40 nm. The polymer employed for the coating was poly(isobutylene-alt-maleic anhydride) modified with 1-dodecylamine side chains. The amphiphilic character of the polymer facilitates the stability of the nanocrystals in aqueous media. Hydrogels are prepared via triggering the colloidally stable solutions, with aqueous cations acting as linkers between the functional groups of the polymer shell. Upon supercritical drying, the hydrogels are successfully converted into macroscopic aerogels with highly porous, open structure. Due to the noninvasive preparation method, the nanoscopic properties of the building blocks are retained in the monolithic aerogels, leading to the powerful transfer of these properties to the macroscale. The open pore system, the universality of the polymer-coating strategy, and the large accessibility of the network make these gel structures promising biosensing platforms. Functionalizing the polymer shell with biomolecules opens up the possibility to utilize the nanoscopic properties of the building blocks in fluorescent probing, magnetoresistive sensing, and plasmonic-driven thermal sensing.

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.