Visible light scattering study at 470, 550, and 660 nm of components of mineral dust aerosol: Hematite and goethite

Iron oxides, usually in the form of hematite or goethite, comprise an important component of atmospheric mineral dust aerosol. Because these minerals are strong visible absorbers they play a critical role in determining the overall impact of dust aerosol on climate forcing. In this work, results from light scattering measurements from hematite and goethite dust aerosol are presented for three visible wavelengths, λ=470, 550, and 660 nm. We observe important systematic differences in the scattering between these different iron oxide samples, as well as significant wavelength dependence across the visible region of the spectrum. Aerosol size distributions are measured simultaneously with the light scattering, enabling a rigorous comparison between theoretical light scattering models and experimental data. Theoretical simulations of the scattering are carried out using both Mie and T-Matrix theories. Simulations are in reasonably good agreement with experimental data for hematite; thus, our data offer a useful check on tabulated optical constants for hematite. However, simulations show very poor agreement for goethite. The poor agreement in the goethite case is likely the result of particle shape effects related to the rod-like morphology of the goethite particles. This study demonstrates how particle mineralogy and morphology play an important role in dictating the optical properties of mineral dust aerosol, a major component of tropospheric dust.     

Multi-competitive interaction of As(III) and As(V) oxyanions with Ca(2+), Mg(2+), PO(3-)(4), and CO(2-)(3) ions on goethite

Complex systems, simulating natural conditions like in groundwater, have rarely been studied, since measuring and in particular, modeling of such systems is very challenging. In this paper, the adsorption of the oxyanions of As(III) and As(V) on goethite has been studied in presence of various inorganic macro-elements (Mg(2+), Ca(2+), PO(3-)(4), CO(2-)(3)). We have used 'single-,' 'dual-,' and 'triple-ion' systems. The presence of Ca(2+) and Mg(2+) has no significant effect on As(III) oxyanion (arsenite) adsorption in the pH range relevant for natural groundwater (pH 5-9). In contrast, both Ca(2+) and Mg(2+) promote the adsorption of PO(3-)(4). A similar (electrostatic) effect is expected for the Ca(2+) and Mg(2+) interaction with As(V) oxyanions (arsenate). Phosphate is a major competitor for arsenate as well as arsenite. Although carbonate may act as competitor for both types of As oxyanions, the presence of significant concentrations of phosphate makes the interaction of (bi)carbonate insignificant. The data have been modeled with the charge distribution (CD) model in combination with the extended Stern model option. In the modeling, independently calculated CD values were used for the oxyanions. The CD values for these complexes have been obtained from a bond valence interpretation of MO/DFT (molecular orbital/density functional theory) optimized geometries. The affinity constants (logK) have been found by calibrating the model on data from 'single-ion' systems. The parameters are used to predict the ion adsorption behavior in the multi-component systems. The thus calibrated model is able to predict successfully the ion concentrations in the mixed 2- and 3-component systems as a function of pH and loading. From a practical perspective, data as well as calculations show the dominance of phosphate in regulating the As concentrations. Arsenite (As(OH)(3)) is often less strongly bound than arsenate (AsO(3-)(4)) but arsenite responses less strongly to changes in the phosphate concentration compared to arsenate, i.e., deltalogc(As(III))/deltalogc(PO(4)) approximately 0.4 and deltalogc(As(V))/deltalogc(PO(4)) approximately 0.9 at pH 7. Therefore, the response of As in a sediment on a change in redox conditions will be variable and will depend on the phosphate concentration level.     

Bias-Free Solar-Driven Syngas Production: A Fe2O3 Photoanode Featuring Single-Atom Cobalt Integrated with a Silver-Palladium Cathode

Photoelectrochemical syngas production from aqueous CO₂ is a promising technique for carbon capture and utilization. Herein, we demonstrate the efficient and tunable syngas production by integrating a single-atom cobalt-catalyst-decorated α-Fe₂O₃ photoanode with a bimetallic Ag/Pd alloy cathode. A record syngas production activity of 81.9 μmol cm⁻² h⁻¹ (CO/H₂ ratio: ≈1 : 1) was achieved under artificial sunlight (AM 1.5 G) with an excellent durability. Systematic studies reveal that the Co single atoms effectively extract the holes from Fe₂O₃ photoanodes and serve as active sites for promoting oxygen evolution. Simultaneously, the Pd and Ag atoms in bimetallic cathodes selectively adsorb CO₂ and protons for facilitating CO production. Further incorporation with a photovoltaic, to allow solar light (>600 nm) to be utilized, yields a bias-free CO₂ reduction device with solar-to-CO and solar-to-H₂ conversion efficiencies up to 1.33 and 1.36 %, respectively.     

Enhanced photoelectrochemical performance of Ti-doped hematite thin films prepared by the sol-gel method

Ti-doped α-Fe₂O₃ thin films were successfully prepared on FTO substrates by the sol-gel route. Hematite film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The XRD data showed α-Fe₂O₃ had a preferred (1 1 0) orientation which belonged to the rhombohedral system. Interestingly, the grains turned into worm-like shape after annealed at high temperature. The IPCE could reach 32.6% at 400 nm without any additional potential vs. SCE. Titanium in the lattice can affect the photo electro chemical performance positively by increasing the conductivity of the thin film. So the excited electrons and holes could live longer, rather than recombining with each other rapidly as undoped hematite. And the efficient carrier density on the Ti-doped anode surface was higher than the undoped anode, which contribute to the well PEC performance.     

Magnetic properties of magnetite prepared by ball-milling of hematite with iron

Magnetic properties of magnetite powder prepared by ball-milling of stoichiometric mixture of hematite and iron in an inert atmosphere are reported. Hysteresis loops, isothermal remanence acquisition curves and temperature dependence of magnetic susceptibility measurements are used to characterise this material and to examine the effects of heating in air and in an argon atmosphere. Ball-milling of hematite with iron during periods ranging from 30 min up to almost 5 h yields magnetite which exhibits high magnetic hardness, characterised by coercive force three times higher than that typical for single-domain natural magnetites. However, the magnetite produced is unstable upon heating in air, reoxidising almost completely to hematite. Heating in an argon atmosphere causes enhancement of typical magnetic parameters, but decreases the magnetic hardness.     

Low Catalyst Loading Enhances Charge Accumulation for Photoelectrochemical Water Splitting

Solar water oxidation is a critical step in artificial photosynthesis. Successful completion of the process requires four holes and releases four protons. It depends on the consecutive accumulation of charges at the active site. While recent research has shown an obvious dependence of the reaction kinetics on the hole concentrations on the surface of heterogeneous (photo)electrodes, little is known about how the catalyst density impacts the reaction rate. Using atomically dispersed Ir catalysts on hematite, we report a study on how the interplay between the catalyst density and the surface hole concentration influences the reaction kinetics. At low photon flux, where surface hole concentrations are low, faster charge transfer was observed on photoelectrodes with low catalyst density compared to high catalyst density; at high photon flux and high applied potentials, where surface hole concentrations are moderate or high, slower surface charge recombination was afforded by low-density catalysts. The results support that charge transfer between the light absorber and the catalyst is reversible; they reveal the unexpected benefits of low-density catalyst loading in facilitating forward charge transfer for desired chemical reactions. It is implied that for practical solar water splitting devices, a suitable catalyst loading is important for maximized performance.