Location of Hole and Electron Traps on Nanocrystalline Anatase TiO₂

The defect photoluminescence from TiO₂ nanoparticles in the anatase phase is reported for nanosheets which expose predominantly (001) surfaces and compared to that from conventional anatase nanoparticles which expose mostly (101) surfaces. Also reported is the weak defect photoluminescence of TiO₂ nanotubes, which we find using electron backscattered diffraction to consist of walls which expose (110) and (100) facets. The nanotubes exhibit photoluminescence that is blue-shifted and much weaker than that from conventional TiO₂ nanoparticles. Despite the preponderance of (001) surfaces in the nanosheet samples, they exhibit photoluminescence similar to that of conventional nanoparticles. We assign the broad visible photoluminescence of anatase nanoparticles to two overlapping distributions: hole trap emission associated with oxygen vacancies on (101) exposed surfaces, which peaks in the green, and a broader emission extending into the red which results from electron traps on undercoordinated titanium atoms, which are prevalent on (001) facets. The results of this study suggest how morphology of TiO₂ nanoparticles could be optimized to control the distribution and activity of surface traps. Our results also shed light on the mechanism by which the TiCl₄ surface treatment heals traps on anatase and mixed-phase TiO₂ films and reveals distinct differences in the trap-state distributions of TiO₂ nanoparticles and nanotubes. The molecular basis for electron and hole traps and their spatial separation on different facets is discussed

Manganese Doping of Magnetic Iron Oxide Nanoparticles: Tailoring Surface Reactivity for a Regenerable Heavy Metal Sorbent

A method for tuning the analyte affinity of magnetic, inorganic nanostructured sorbents for heavy metal contaminants is described. The manganese-doped iron oxide nanoparticle sorbents have a remarkably high affinity compared to the precursor material. Sorbent affinity can be tuned toward an analyte of interest simply by adjustment of the dopant quantity. The results show that following the Mn doping process there is a large increase in affinity and capacity for heavy metals (i.e., Co, Ni, Zn, As, Ag, Cd, Hg, and Tl). Capacity measurements were carried out for the removal of cadmium from river water and showed significantly higher loading than the relevant commercial sorbents tested for comparison. The reduction in Cd concentration from 100 ppb spiked river water to 1 ppb (less than the EPA drinking water limit of 5 ppb for Cd) was achieved following treatment with the Mn-doped iron oxide nanoparticles. The Mn-doped iron oxide nanoparticles were able to load ∼1 ppm of Cd followed by complete stripping and recovery of the Cd with a mild acid wash. The Cd loading and stripping is shown to be consistent through multiple cycles with no loss of sorbent performance

Sodium Ion Insertion in Hollow Carbon Nanowires for Battery Applications

Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g^–1 and 82.2% capacity retention over 400 charge–discharge cycles between 1.2 and 0.01 V (vs Na⁺/Na) at a constant current of 50 mA g^–1 (0.2 C). Excellent cycling stability is also observed at an even higher charge–discharge rate. A high reversible capacity of 149 mAh g^–1 also can be obtained at a current rate of 500 mA g^–1 (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials

Surface-Driven Sodium Ion Energy Storage in Nanocellular Carbon Foams

Sodium ion (Na⁺) batteries have attracted increased attention for energy storage due to the natural abundance of sodium, but their development is hindered by poor intercalation property of Na⁺ in electrodes. This paper reports a detailed study of high capacity, high rate sodium ion energy storage in functionalized high-surface-area nanocellular carbon foams (NCCF). The energy storage mechanism is surface-driven reactions between Na⁺ and oxygen-containing functional groups on the surface of NCCF. The surface reaction, rather than a Na⁺ bulk intercalation reaction, leads to high rate performance and cycling stability due to the enhanced reaction kinetics and the absence of electrode structure change. The NCCF makes more surface area and surface functional groups available for the Na⁺ reaction. It delivers 152 mAh/g capacity at the rate of 0.1 A/g and a capacity retention of 90% for over 1600 cycles