Structure and Dynamics of Bimodal Colloidal Dispersions in a Low-Molecular-Weight Polymer Solution

We present an experimental study of the structural and dynamical properties of bimodal, micrometer-sized colloidal dispersions (size ratio ≈ 2) in an aqueous solution of low-molecular-weight polymer (polyethylene glycol 2000) using synchrotron ultra-small angle X-ray scattering (USAXS) and USAXS-based X-ray photon correlation spectroscopy. We fixed the volume fraction of the large particles at 5% and systematically increased the volume fraction of the small particles from 0 to 5% to evaluate their effects on the structure and dynamics. The bimodal dispersions were homogenous through the investigated parameter space. We found that the partial structure factors can be satisfactorily retrieved for the bimodal colloidal dispersions using a Percus–Yevick hard-sphere potential when the size distributions of the particles were taken into account. We also found that the partial structure factor between the large particles did not exhibit a significant variation with increasing volume fraction of the small particles, whereas the isothermal compressibility of the binary mixture was found to decrease with increasing volume fraction of the small particles. The dynamics of single-component large-particle dispersion obey the principles of de Gennes narrowing, where the wave vector dependence of the interparticle diffusion coefficient is inversely proportional to the interparticle structure factor. The dynamics of the bimodal dispersions demonstrate a strong dependence on the fraction of small particles. We also made a comparison between the experimental effective dynamic viscosity of the bimodal dispersion with the theoretical predictions, which suggest that the complex mutual interactions between the large and small particles have a strong effect on the dynamic behaviors of bimodal dispersions

Aerosol Synthesis of Vanadium Oxide-Carbon Hybrid Nanoparticle Clusters for High-Performance Lithium Extraction via Electrochemical Deionization

An aerosol-based synthetic approach is demonstrated for the development of vanadium oxide-carbon hybrid nanoparticle clusters (VOx-C-NPCs) used for the fabrication of a unique electrochemical deionization (ECDI) cell. Hybrid ECDI cells are constructed with a positive electrode of pseudocapacitive VOx-C-NPCs and a negative electrode of battery-like silver–carbon nanoparticle clusters (Ag-C-NPCs). The reversible extraction of Li+ ions and the storage of Cl– ions are conducted through Faradaic reactions on VOx-C-NPCs and Ag-C-NPCs, respectively. The surface of the VOx-C-NPC electrodes can be successfully protected by an anion-doped polypyrrole (PPy) film, by which the structural stability of VOx-C-NPC, in terms of the salt adsorption capacity (SAC) retention, is enhanced to 92%. Remarkably high SAC values of the hybrid ECDI cells are achievable in comparison to the reported values in the field: up to 27.5 mg/g for the bare VOx-C//Ag-C cell and 49.3 mg/g for the PPy-protected VOx-C//Ag-C cell. This work provides a prototype study for the rapid and continuous production of high-performance VOx-C-NPCs using aerosol-based synthesis supported by complementary material characterization. The mechanistic understanding of the material synthesis and the corresponding ECDI process shows promise for achieving an optimal deionization performance in terms of SAC and stability

Gas-Phase Synthesis of Ni–CeO_<i>x</i> Hybrid Nanoparticles and Their Synergistic Catalysis for Simultaneous Reforming of Methane and Carbon Dioxide to Syngas

We develop a new Ni–CeO_<i>x</i> hybrid nanoparticle as a high-performance catalyst for dry reforming of methane with carbon dioxide (DRM) using a facile gas-phase synthetic approach. The crystallites of Ni and CeO₂ were homogeneously self-assembled as a hybrid nanostructure, creating a large amount of Ni–Ce–O interface for a strong metal–support interfacial interaction. Chemical composition and oxidation state of the hybrid nanostructure were tunable directly in the aerosol state. The results show that the starting temperature of catalysis by Ni-based catalysts reduced by ∼150 °C through the hybridization with CeO₂ followed by a direct H₂ reduction in the aerosol state. In comparison to Ni-only nanoparticle, the Ni–CeO_<i>x</i> hybrid nanoparticle showed stable and high conversion for CH₄ and CO₂ with a remarkably turnover frequency at low temperature (0.1 s^–1 at 450 <b>°</b>C). The amount of coke formation greatly reduced by 18×, whereas the H₂/CO ratio was constant at ∼0.8 by a 1.5× of the increase of CO₂/CH₄ ratio. The work demonstrated a facile route for controlled synthesis of Ni–CeO_<i>x</i> hybrid nanoparticles with a very high catalytic activity and stability of DRM. The findings of this study can shed light on the mechanism of Ni–Ce–O synergistic catalysis, which can be especially useful for methane-based energy applications

Aerosol-Based Self-Assembly of a Ag–ZnO Hybrid Nanoparticle Cluster with Mechanistic Understanding for Enhanced Photocatalysis

A gas-phase-controlled synthetic approach is demonstrated to fabricate Ag–ZnO hybrid nanostructure as a high-performance catalyst for photodegradation of water pollutants. The degradation of rhodamine B (RhB) was used as representative, which were tested and evaluated with respect to the environmental pH and the presence of dodecyl sulfate corona on the surface of the catalyst. The results show that a raspberry-structure Ag–ZnO hybrid nanoparticle cluster was successfully synthesized via gas-phase evaporation-induced self-assembly. The photodegradation activity increased significantly (20×) by using the Ag–ZnO hybrid nanoparticle cluster as a catalyst. A surge of catalytic turnover frequency of ZnO nanoparticle cluster (>20×) was observed through the hybridization with silver nanoparticles. The dodecyl sulfate corona increased the photocatalytic activity of the Ag–ZnO hybrid nanoparticle cluster, especially at the acidic and neutral pH environments (maximum 6×), and the enhancement in catalytic activity was attributed to the improved colloidal stability of ZnO-based nanoparticle cluster under the interaction with RhB. Our work provides a generic route of facile synthesis of the Ag–ZnO hybrid nanoparticle cluster with a mechanistic understanding of the interface reaction for enhancing photocatalysis toward the degradation of water pollutants

Quantitative Attachment and Detachment of Bacterial Spores from Fine Wires through Continuous and Pulsed DC Electrophoretic Deposition

We demonstrate the uniform attachment of bacterial spores electrophoretically onto fine wires in liquids and subsequently quantitatively detached back into suspension. It was found that the use of a pulsed voltage method resulted in a uniform coverage of spores and prevented visible bubble formation resulting from water electrolysis which tended to dislodge the spores from the wires. By monitoring the electrophoretically derived current, this method could also be used to quantitatively measure the surface charges on spores and the deposition rate. The method is generic and should be applicable to the deposition of any charged biological material (e.g., spores, bacteria, viruses) onto metal surfaces

Metal–Organic Framework Colloids: Disassembly and Deaggregation

We demonstrate a high-resolution method as an efficient tool to <i>in situ</i> characterize partially reversible assembly and aggregation of metal–organic framework (MOF) colloids. Based on the gas-phase electrophoresis, the primary size and the degree of aggregation of the MOF-525 crystals are tunable by pH adjustment and mobility selection. These findings allow for the further size control of MOF colloids and prove the capability of semiquantitative analysis for the MOF-based platforms in a variety of aqueous formulations (e.g., biomedical applications)

Mechanistic Study of Gas-Phase Controlled Synthesis of Copper Oxide-Based Hybrid Nanoparticle for CO Oxidation

We report a systematic study of gas-phase controlled synthesis of copper oxides-based hybrid nanoparticles for catalytic CO oxidation. The complementary physical, spectroscopic, and microscopic analyses were conducted to obtain a better understanding of the material properties, including particle size, crystallinity, elemental composition, and oxidation state. Results showed that the synthesized nanoparticles exhibited highly durable catalytic activity and stability, also the particle size, crystallite size, and chemical composition were tunable by choosing suitable chemical compositions of precursors and temperatures. The crystallite size of CuO influenced the reducibility of CuO by CO and the subsequent catalytic activity of CO oxidation. The hybridization process of CeO₂ and CuO induces the formation of new active sites at the Cu–Ce–O interface, which enhances reproducibility of CuO and the catalytic activity. However, the reproducibility of CuO and catalytic activity were considerably decreased when CeO₂ was replaced with the inert Al₂O₃. This work describes a prototype method to form highly pure and well-controlled hybrid nanocatalysts, which can be used to establish the correlation of material properties versus reducibility and subsequent catalytic activity for energy and environmental applications

Surface PEGylation of Silver Nanoparticles: Kinetics of Simultaneous Surface Dissolution and Molecular Desorption

A quantitative study of the stability of silver nanoparticles (AgNPs) conjugated with thiolated polyethylene glycol (SH-PEG) was conducted using gas-phase ion-mobility and mass analyses. The extents of aggregation and surface dissolution of AgNPs, as well as the amount of SH-PEG adsorption and desorption, were able to be characterized simultaneously for the kinetic study. The results show that the SH-PEG with a molecular mass of 6 kg/mol (SH-PEG6K) was able to adsorb to the surface of AgNP to form PEG6K-HS-AgNP conjugates, with the maximum surface adsorbate density of ∼0.10 nm^–2. The equilibrium binding constant for SH-PEG6K on AgNPs was calculated as ∼(4.4 ± 0.9) × 10⁵ L/mol, suggesting a strong affinity due to thiol bonding to the AgNP surface. The formation of SH-PEG6K corona prevented PEG6K-HS-AgNP conjugates from aggregation under the acidic environment (pH 1.5), but dissolution of core AgNPs occurred following a first-order reaction. The rate constant of Ag dissolution from PEG6K-HS-AgNP was independent of the starting surface packing density of SH-PEG6K on AgNP (σ₀), indicating that the interactions of H⁺ with core AgNP were not interfered by the presence of SH-PEG6K corona. The surface packing density of SH-PEG6K decreased simultaneously following a first-order reaction, and the desorption rate constant of SH-PEG6K from the conjugates was proportional to σ₀. Our work presents the first quantitative study to illustrate the complex mechanism that involves simultaneous aggregation and dissolution of core AgNPs in combination with adsorption and desorption of SH-PEG. This work also provides a prototype method of coupled experimental scheme to quantify the change of particle mass versus the corresponding surface density of functional molecular species on nanoparticles

Temperature-Programmed Electrospray–Differential Mobility Analysis for Characterization of Ligated Nanoparticles in Complex Media

An electrospray–differential mobility analyzer (ES-DMA) was operated with an aerosol flow-mode, temperature-programmed approach to enhance its ability to characterize the particle size distributions (PSDs) of nanoscale particles (NPs) in the presence of adsorbed and free ligands. Titanium dioxide NPs (TiO₂-NPs) stabilized by citric acid (CA) or bovine serum albumin (BSA) were utilized as representative systems. Transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry were used to provide visual information and elemental-based PSDs, respectively. Results show that the interference resulting from electrospray-dried nonvolatile salt residual nanoscale particles (S-NPs) could be effectively reduced using the thermal treatment process: PSDs were accurately measured at temperatures above 200 °C for CA-stabilized TiO₂-NPs and above 400 °C for BSA-stabilized TiO₂-NPs. Moreover, TEM confirmed the volumetric shrinkage of S-NPs due to thermal treatment and also showed that the primary structure of TiO₂-NPs was relatively stable over the temperature range studied (i.e., below 700 °C). Conversely, the shape factor for TiO₂-NPs decreased after treatment above 500 °C, possibly due to a change in the secondary (aggregate) structure. S-NPs from BSA-stabilized TiO₂-NPs exhibited higher global activation energies toward induced volumetric shrinkage than those of CA-stabilized TiO₂-NPs, suggesting that activation energy is dependent on ligand size. This prototype study demonstrates the efficacy of using ES-DMA coupled with thermal treatment for characterizing the physical state of NPs, even in a complex medium (e.g., containing plasma proteins) and in the presence of particle agglomerates induced by interaction with binding ligands

Quantifying Surface Area of Nanosheet Graphene Oxide Colloid Using a Gas-Phase Electrostatic Approach

We demonstrate a new, facile gas-phase electrostatic approach to successfully quantify equivalent surface area of graphene oxide (GO) colloid on a number basis. Mobility diameter (<i>d</i>_p,m)-based distribution and the corresponding equivalent surface area (SA) of GO colloids (i.e., with different lateral aspect ratios) were able to be identified by electrospray-differential mobility analysis (ES-DMA) coupled to a condensation particle counter (CPC) and an aerosol surface area analyzer (ASAA). A correlation of SA ∝ <i>d</i>_p,m^2.0 was established using the ES-DMA-CPC/ASAA, which is consistent with the observation by the 2-dimensional image analysis of size-selected GOs. An ultrafast surface area measurement of GO colloid was achieved via a direct coupling of ES with a combination of ASAA and CPC (i.e., measurement time was 2 min per sample; without size classification). The measured equivalent surface area of GO was ∼202 ± 7 m² g^–1, which is comparable to Brunauer–Emmett–Teller (BET) surface area, ∼240 ± 59 m² g^–1. The gas-phase electrostatic approach proposed in this study has the superior advantages of being fast, requiring no elaborate drying process, and requiring only a very small amount of sample (i.e., <0.01 mg). To the best of our knowledge, this is the first study of using an aerosol-based electrostatic coupling technique to obtain the equivalent surface area of graphene oxide on a number basis with a high precision of measurement