Evaporation-Induced Crumpling of Graphene Oxide Nanosheets in Aerosolized Droplets: Confinement Force Relationship

A possible solution to solve the restacking issue of graphene oxide (GO) nanosheets during large-scale production is to turn the two-dimensional (2D) nanosheets into three-dimensional (3D) crumpled balls that have excellent compressive properties but still maintain high free volumes. An aerosol-based process has been proven to be a rational method for this purpose, in which, the crumpling phenomenon, however, has hitherto remained unclear. Here we present a detailed understanding of the crumpling of GO nanosheets by a systematic investigation conducted in aerosolized droplets by means of in-line (e.g., scanning mobility particle sizer) and off-line (e.g., electron microscopy) measurements. Correlations between the confinement force and various parameters, such as evaporation rate and precursor concentration were established to derive a universally applicable equation. Both calculation and experimental results revealed that the evaporation rate plays an important role in controlling the crumpling process

Elemental mercury oxidation in an electrostatic precipitator enhanced with in situ soft X-ray irradiation

<div><p>Corona discharge based techniques are promising approaches for oxidizing elemental mercury (Hg⁰) in flue gas from coal combustion. In this study, in-situ soft X-rays were coupled to a DC (direct current) corona-based electrostatic precipitator (ESP). The soft X-rays significantly enhanced Hg⁰ oxidation, due to generation of electrons from photoionization of gas molecules and the ESP electrodes. This coupling technique worked better in the positive corona discharge mode because more electrons were in the high energy region near the electrode. Detailed mechanisms of Hg⁰ oxidation are proposed and discussed based on ozone generation measurements and Hg⁰ oxidation behavior observations in single gas environments (O₂, N₂, and CO₂). The effect of O₂ concentration in flue gas, as well as the effects of particles (SiO₂, TiO₂, and KI) was also evaluated. In addition, the performance of a soft X-rays coupled ESP in Hg⁰ oxidations was investigated in a lab-scale coal combustion system. With the ESP voltage at +10 kV, soft X-ray enhancement, and KI addition, mercury oxidation was maximized.</p><p>Implications: <i>Mercury is a significant-impact atmospheric pollutant due to its toxicity. Coal-fired power plants are the primary emission sources of anthropogenic releases of mercury; hence, mercury emission control from coal-fired power plant is important. This study provides an alternative mercury control technology for coal-fired power plants. The proposed electrostatic precipitator with in situ soft X-rays has high efficiency on elemental mercury conversion. Effects of flue gas conditions (gas compositions, particles, etc.) on performance of this technology were also evaluated, which provided guidance on the application of the technology for coal-fired power plant mercury control.</i></p></div

Rapid Formation of Metal–Organic Frameworks (MOFs) Based Nanocomposites in Microdroplets and Their Applications for CO₂ Photoreduction

A copper-based metal–organic framework (MOF), [Cu₃(TMA)₂(H₂O)₃]_<i>n</i> (also known as HKUST-1, where TMA stands for trimesic acid), and its TiO₂ nanocomposites were directly synthesized in micrometer-sized droplets via a rapid aerosol route for the first time. The effects of synthesis temperature and precursor component ratio on the physicochemical properties of the materials were systematically investigated. Theoretical calculations on the mass and heat transfer within the microdroplets revealed that the fast solvent evaporation and high heat transfer rates are the major driving forces. The fast droplet shrinkage because of evaporation induces the drastic increase in the supersaturation ratio of the precursor, and subsequently promotes the rapid nucleation and crystal growth of the materials. The HKUST-1-based nanomaterials synthesized via the aerosol route demonstrated good crystallinity, large surface area, and great photostability, comparable with those fabricated by wet-chemistry methods. With TiO₂ embedded in the HKUST-1 matrix, the surface area of the composite is largely maintained, which enables significant improvement in the CO₂ photoreduction efficiency, as compared with pristine TiO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy analysis suggests that the performance enhancement was due to the stable and high-capacity reactant adsorption by HKUST-1. The current work shows great promise in the aerosol route’s capability to address the mass and heat transfer issues of MOFs formation at the microscale level, and ability to synthesize a series of MOFs-based nanomaterials in a rapid and scalable manner for energy and environmental applications

Formation of Nitrogen-Containing Organic Aerosol during Combustion of High-Sulfur-Content Coal

Carbonaceous aerosols, including organic carbon aerosols and black carbon aerosols, are produced by the combustion of pulverized coal even under fuel-lean conditions. These carbonaceous aerosols can be particularly hazardous to human health. In this study, the chemical compositions and formation pathways of organic aerosols emitted during the combustion of high-sulfur-content coals were investigated. It was found that nitrogen-containing organic matter contains a significant proportion of organic aerosol mass from the combustion of high-sulfur-content coals, which is not the case for organic aerosols generated during the combustion of low-sulfur-content coals. The formation of organic aerosols was significantly enhanced when higher-sulfur-content coal was burned. A strong correlation between organic aerosol mass and the sulfate concentration was observed. It is proposed that acidic sulfate particles absorb the nitrogen-containing organic volatiles produced by coal pyrolysis onto the particle phase through acid–base neutralization reactions

Aerosol Synthesis of Self-Organized Nanostructured Hollow and Porous Carbon Particles Using a Dual Polymer System

A facile method for designing and synthesizing nanostructured carbon particles via ultrasonic spray pyrolysis of a self-organized dual polymer system comprising phenolic resin and charged polystyrene latex is reported. The method produces either hollow carbon particles, whose CO₂ adsorption capacity is 3.0 mmol g^–1, or porous carbon particles whose CO₂ adsorption capacity is 4.8 mmol g^–1, although the two particle types had similar diameters of about 360 nm. We investigate how the zeta potential of the polystyrene latex particles, and the resulting electrostatic interaction with the negatively charged phenolic resin, influences the particle morphology, pore structure, and CO₂ adsorption capacity

Iron Mesh-Based Metal Organic Framework Filter for Efficient Arsenic Removal

Efficient oxidation from arsenite [As­(III)] to arsenate [As­(V)], which is less toxic and more readily to be adsorbed by adsorbents, is important for the remediation of arsenic pollution. In this paper, we report a metal organic framework (MIL-100­(Fe)) filter to efficiently remove arsenic from synthetic groundwater. With commercially available iron mesh as a substrate, MIL-100­(Fe) is implanted through an in situ growth method. MIL-100­(Fe) is able to capture As­(III) due to its microporous structure, superior surface area, and ample active sites for As adsorption. This approach increases the localized As concentration around the filter, where Fenton-like reactions are initiated by the Fe²⁺/Fe³⁺ sites within the MIL-100­(Fe) framework to oxidize As­(III) to As­(V). The mechanism was confirmed by colorimetric detection of H₂O₂, fluorescence, and electron paramagnetic resonance detection of ·OH. With the aid of oxygen bubbling and Joule heating, the removal efficiency of As­(III) can be further boosted. The MIL-100­(Fe)-based filter also exhibits satisfactory structural stability and recyclability. Notably, the adsorption capacity of the filter can be regenerated satisfactorily. Our results demonstrate the potential of this filter for the efficient remediation of As contamination in groundwater

Enhanced Water Photolysis with Pt Metal Nanoparticles on Single Crystal TiO₂ Surfaces

Two novel deposition methods were used to synthesize Pt-TiO₂ composite photoelectrodes: a tilt-target room temperature sputtering method and aerosol-chemical vapor deposition (ACVD). Pt nanoparticles (NPs) were sequentially deposited by the tilt-target room temperature sputtering method onto the as-synthesized nanostructured columnar TiO₂ films by ACVD. By varying the sputtering time of Pt deposition, the size of deposited Pt NPs on the TiO₂ film could be precisely controlled. The as-synthesized composite photoelectrodes with different sizes of Pt NPs were characterized by various methods, such as SEM, EDS, TEM, XRD, and UV–vis. The photocurrent measurements revealed that the modification of the TiO₂ surface with Pt NPs improved the photoelectrochemical properties of electrodes. Performance of the Pt-TiO₂ composite photoelectrodes with sparsely deposited 1.15 nm Pt NPs was compared to the pristine TiO₂ photoelectrode with higher saturated photocurrents (7.92 mA/cm² to 9.49 mA/cm²), enhanced photoconversion efficiency (16.2% to 21.2%), and increased fill factor (0.66 to 0.70). For larger size Pt NPs of 3.45 nm, the composite photoelectrode produced a lower photocurrent and reduced conversion efficiency compared to the pristine TiO₂ electrode. However, the surface modification by Pt NPs helped the composite electrode maintain higher fill factor values

Size and Structure Matter: Enhanced CO₂ Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO₂ Single Crystals

A facile development of highly efficient Pt-TiO₂ nanostructured films via versatile gas-phase deposition methods is described. The films have a unique one-dimensional (1D) structure of TiO₂ single crystals coated with ultrafine Pt nanoparticles (NPs, 0.5–2 nm) and exhibit extremely high CO₂ photoreduction efficiency with selective formation of methane (the maximum CH₄ yield of 1361 μmol/g-cat/h). The fast electron-transfer rate in TiO₂ single crystals and the efficient electron–hole separation by the Pt NPs were the main reasons attributable for the enhancement, where the size of the Pt NPs and the unique 1D structure of TiO₂ single crystals played an important role

Surface Engineered CuO Nanowires with ZnO Islands for CO₂ Photoreduction

Large arrays of massively parallel (10⁸ cm^–2) CuO nanowires were surface engineered with dense ZnO islands using a few pulsed cycles of atomic layer deposition (ALD). These nanowires were subjected to UV–vis radiation-based CO₂ photoreduction under saturated humidity (CO₂ + H₂O mixture) conditions. We monitored CO₂ to CO conversion, indicating the viability of these nanostructures as potential photocatalysts. High-resolution transmission electron microscopy and atomic force microscopy indicated an island growth mechanism of ZnO epitaxially depositing on pristine, single crystal CuO nanowire surface. Photoluminescence and transient absorption spectroscopy showed a very high density of defects on these ZnO islands which trapped electrons and enhanced their lifetimes. Peak CO conversion (1.98 mmol/g-cat/hr) and quantum efficiency (0.0035%) were observed in our setup when the ZnO islands impinged each other at 1.4 nm (8 cycles of ALD) diameter; at which point ZnO island perimeter lengths maximized as well. A mechanism whereby simultaneous H₂O oxidation and CO₂ reduction occurred in the active perimeter region between CuO nanowire and ZnO islands is proposed to explain the observed photoconversion of CO₂ to CO

Engineered Crumpled Graphene Oxide Nanocomposite Membrane Assemblies for Advanced Water Treatment Processes

In this work, we describe multifunctional, crumpled graphene oxide (CGO) porous nanocomposites that are assembled as advanced, reactive water treatment membranes. Crumpled 3D graphene oxide based materials fundamentally differ from 2D flat graphene oxide analogues in that they are highly aggregation and compression-resistant (i.e., π–π stacking resistant) and allow for the incorporation (wrapping) of other, multifunctional particles inside the 3D, composite structure. Here, assemblies of nanoscale, monomeric CGO with encapsulated (as a quasi core–shell structure) TiO₂ (GOTI) and Ag (GOAg) nanoparticles, not only allow high water flux via vertically tortuous nanochannels (achieving water flux of 246 ± 11 L/(m²·h·bar) with 5.4 μm thick assembly, 7.4 g/m²), outperforming comparable commercial ultrafiltration membranes, but also demonstrate excellent separation efficiencies for model organic and biological foulants. Further, multifunctionality is demonstrated through the in situ photocatalytic degradation of methyl orange (MO), as a model organic, under fast flow conditions (<i>t</i>_res < 0.1 s); while superior antimicrobial properties, evaluated with GOAg, are observed for both biofilm (contact) and suspended growth scenarios (>3 log effective removal, <i>Escherichia coli</i>). This is the first demonstration of 3D, crumpled graphene oxide based nanocomposite structures applied specifically as (re)­active membrane assemblies and highlights the material’s platform potential for a truly tailored approach for next generation water treatment and separation technologies