Special phase transformation and crystal growth pathways observed in nanoparticles†

Phase transformation and crystal growth in nanoparticles may happen via mechanisms distinct from those in bulk materials. We combine experimental studies of as-synthesized and hydrothermally coarsened titania (TiO(2)) and zinc sulfide (ZnS) with thermodynamic analysis, kinetic modeling and molecular dynamics (MD) simulations. The samples were characterized by transmission electron microscopy, X-ray diffraction, synchrotron X-ray absorption and scattering, and UV-vis spectroscopy. At low temperatures, phase transformation in titania nanoparticles occurs predominantly via interface nucleation at particle–particle contacts. Coarsening and crystal growth of titania nanoparticles can be described using the Smoluchowski equation. Oriented attachment-based crystal growth was common in both hydrothermal solutions and under dry conditions. MD simulations predict large structural perturbations within very fine particles, and are consistent with experimental results showing that ligand binding and change in aggregation state can cause phase transformation without particle coarsening. Such phenomena affect surface reactivity, thus may have important roles in geochemical cycling

Role of Surface Area, Primary Particle Size, and Crystal Phase on Titanium Dioxide Nanoparticle Dispersion Properties

Characterizing nanoparticle dispersions and understanding the effect of parameters that alter dispersion properties are important for both environmental applications and toxicity investigations. The role of particle surface area, primary particle size, and crystal phase on TiO2 nanoparticle dispersion properties is reported. Hydrodynamic size, zeta potential, and isoelectric point (IEP) of ten laboratory synthesized TiO2 samples, and one commercial Degussa TiO2 sample (P25) dispersed in different solutions were characterized. Solution ionic strength and pH affect titania dispersion properties. The effect of monovalent (NaCl) and divalent (MgCl2) inert electrolytes on dispersion properties was quantified through their contribution to ionic strength. Increasing titania particle surface area resulted in a decrease in solution pH. At fixed pH, increasing the particle surface area enhanced the collision frequency between particles and led to a higher degree of agglomeration. In addition to the synthesis method, TiO2 isoelectric point was found to be dependent on particle size. As anatase TiO2 primary particle size increased from 6 nm to 104 nm, its IEP decreased from 6.0 to 3.8 that also results in changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, TiO2 nanoparticle IEP was found to be insensitive to particle crystal structure

Mineralogical attenuation for metallic remediation in a passive system for mine water treatment

Passive systems with constructed wetlands have been consistently used to treat mine water from abandoned mines. Long-term and cost-effective remediation is a crucial expectation for these water treatment facilities. To achieve that, a complex chain of physical, chemical, biological, and mineralogical mechanisms for pollutants removal must be designed to simulate natural attenuation processes. This paper aims to present geochemical and mineralogical data obtained in a recently constructed passive system (from an abandoned mine, Jales, Northern Portugal). It shows the role of different solid materials in the retention of metals and arsenic, observed during the start-up period of the treatment plant. The mineralogical study focused on two types of materials: (1) the ochre-precipitates, formed as waste products from the neutralization process, and (2) the fine-grained minerals contained in the soil of the wetlands. The ochre-precipitates demonstrated to be poorly ordered iron-rich material, which gave rise to hematite upon artificial heating. The heating experiments also provided mineralogical evidence for the presence of an associated amorphous arsenic-rich compound. Chemical analysis on the freshly ochre-precipitates revealed high concentrations of arsenic (51,867 ppm) and metals, such as zinc (1,213 ppm) and manganese (821 ppm), indicating strong enrichment factors relative to the water from which they precipitate. Mineralogical data obtained in the soil of the wetlands indicate that chlorite, illite, chlorite–vermiculite and mica–vermiculite mixedlayers, vermiculite, kaolinite and goethite are concentrated in the fine-grained fractions (<20 and <2 μm). The chemical analyses show that high levels of arsenic (up to 3%) and metals are also retained in these fractions, which may be enhanced by the low degree of order of the clay minerals as suggested by an XRD study. The obtained results suggest that, although the treatment plant has been receiving water only since 2006, future performance will be strongly dependent on these identified mineralogical pollutant hosts.Fundação para a Ciência e a Tecnologia (FCT

An oxalate cathode for lithium ion batteries with combined cationic and polyanionic redox

Authors acknowledge financial support from the National Natural Science Foundation of China (51822210), the Australian Research Council (ARC) for its support through Discover Project (DP 140100193),Shenzhen Peacock Plan (KQJSCX20170331161244761), the Program for Guangdong Innovative and Entrepreneurial Teams (No. 2017ZT07C341), and the Development and Reform Commission of Shenzhen Municipality for the development of the “Low-Dimensional Materials and Devices” discipline.The growing demand for advanced lithium-ion batteries calls for the continued development of high-performance positive electrode materials. Polyoxyanion compounds are receiving considerable interest as alternative cathodes to conventional oxides due to their advantages in cost, safety and environmental friendliness. However, polyanionic cathodes reported so far rely heavily upon transition-metal redox reactions for lithium transfer. Here we show a polyanionic insertion material, Li2Fe(C2O4)2, in which in addition to iron redox activity, the oxalate group itself also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution. The current study gives oxalate a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.Publisher PDFPeer reviewe

The timescales of mineral redox reactions

Redox-reactive minerals can serve as electron donors or acceptors for abiotic reactants or microbial metabolic processes, and hence can play important roles in terrestrial and aquatic environments, particularly if their reaction rates are comparable to those of other biogeochemical processes. Under such circumstances, their reactions can control metal and contaminant bioavailability and change the permeability of soils and sediments. While the thermodynamic driving force for a mineral redox reaction is frequently a good predictor of relative rates of reaction, there are many examples in which kinetic factors limit reaction rate. Understanding the thermodynamic and kinetic aspects of mineral reaction rates, and their sensitivity to environmental conditions such as temperature or pH, is important for anticipating the biogeochemical evolution of natural environments subjected to change. Achieving this goal requires knowledge of the reaction pathway, the timescales of intermediate steps, and the lifetimes of metastable reaction states. Mineral redox reactions proceed through a combination of steps that can include electron and proton transfer, the breaking or formation of bonds, and mineral dissolution or phase transformation. The combination of conventional kinetics approaches, newly-developed ultrafast time-resolved methods and molecular simulation can provide elucidation of such complex reaction pathways. This chapter summarizes key concepts in mineral and interfacial redox reactions with illustrations from the rich geochemistry of iron and ironbearing minerals

The timescales of mineral redox reactions

Redox-reactive minerals can serve as electron donors or acceptors for abiotic reactants or microbial metabolic processes, and hence can play important roles in terrestrial and aquatic environments, particularly if their reaction rates are comparable to those of other biogeochemical processes. Under such circumstances, their reactions can control metal and contaminant bioavailability and change the permeability of soils and sediments. While the thermodynamic driving force for a mineral redox reaction is frequently a good predictor of relative rates of reaction, there are many examples in which kinetic factors limit reaction rate. Understanding the thermodynamic and kinetic aspects of mineral reaction rates, and their sensitivity to environmental conditions such as temperature or pH, is important for anticipating the biogeochemical evolution of natural environments subjected to change. Achieving this goal requires knowledge of the reaction pathway, the timescales of intermediate steps, and the lifetimes of metastable reaction states. Mineral redox reactions proceed through a combination of steps that can include electron and proton transfer, the breaking or formation of bonds, and mineral dissolution or phase transformation. The combination of conventional kinetics approaches, newly-developed ultrafast time-resolved methods and molecular simulation can provide elucidation of such complex reaction pathways. This chapter summarizes key concepts in mineral and interfacial redox reactions with illustrations from the rich geochemistry of iron and ironbearing minerals

Coordination Structure of Adsorbed Zn(II) at Water−TiO2Interfaces

The local structure of aqueous metal ions on solid surfaces is central to understanding many chemical and biological processes in soil and aquatic environments. Here, the local coordination structure of hydrated Zn(II) at water-TiO{sub 2} interfaces was identified by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy combined with density functional theory (DFT) calculations. A nonintegral coordination number of average {approx}4.5 O atoms around a central Zn atom was obtained by EXAFS analysis. DFT calculations indicated that this coordination structure was consistent with the mixture of 4-coordinated bidentate binuclear (BB) and 5-coordinated bidentate mononuclear (BM) metastable equilibrium adsorption (MEA) states. The BB complex has 4-coordinated Zn, while the monodentate mononuclear (MM) complex has 6-coordinated Zn, and a 5-coordinated adsorbed Zn was found in the BM adsorption mode. DFT calculated energies showed that the lower-coordinated BB and BM modes were thermodynamically more favorable than the higher-coordinated MM MEA state. The experimentally observed XANES fingerprinting provided additional direct spectral evidence of 4- and 5-coordinated Zn-O modes. The overall spectral and computational evidence indicated that Zn(II) can occur in 4-, 5-, and 6-oxygen coordinated sites in different MEA states due to steric hindrance effects, and the coexistence of different MEA states formed the multiple coordination environments