Jeff Secor

Oxidized g‐C3N4 Nanospheres as Catalytically Photoactive Linkers in MOF/g‐C3N4 Composite of Hierarchical Pore Structure

A unique composite of the copper-based metal-organic framework (Cu-benzene tricarboxylic acid (BTC)) with oxidized graphitic carbon nitride nanospheres is synthesized. For comparison, a hybrid material consisting of g-C3 N4 and Cu-BTC is also obtained. Their surface features are analyzed using Fourier transform infrared spectroscopy, X-ray diffraction, sorption of nitrogen, thermal analysis, scanning electron microscopy, photoluminescence, and diffuse reflectance UV-Vis spectroscopy. The results suggest that the formed nanospheres of oxidized g-C3 N4 act as linkers between the copper sites, playing a crucial role in the composite building process. Their incorporation to the Cu-BTC framework causes the development of new mesoporosity. Remarkable alterations in the optical properties, as a result of the coordination of oxygen containing functional groups of the oxidized graphitic carbon nitride to the copper atoms of the framework, suggest an increase in photoreactivity. On the other hand, for the hybrid material consisting of Cu-BTC and g-C3 N4 , the unaltered pore volume and optical properties support the formation of a physical mixture rather than of a composite. The tests on reactive adsorption and detoxification of G-series organophosphate nerve agent surrogate show the enhanced performance of the composite as catalysts and photocatalyst in visible light.

A conversion-based highly energy dense Cu2+ intercalated Bi-birnessite/Zn alkaline battery

Manganese dioxide (MnO2)–zinc (Zn) batteries are cheap and environmentally benign and have sufficient theoretical energy density to be used as an energy storage device for the grid; however, they have been relegated to primary systems, where the complete energy is delivered in a single discharge, due to the irreversibility of their active materials. Until recently, rechargeable MnO2–Zn batteries have only been able to cycle ∼10% of MnO2s theoretical 2-electron capacity (617 mA h g−1), thus delivering significantly reduced energy density. In a recent paper from our group, we reversibly accessed the full theoretical 2-electron capacity of MnO2 for >6000 cycles by using a layered polymorph of MnO2 mixed with bismuth oxide (Bi2O3) called Bi-birnessite (Bi–δ-MnO2) intercalated with Cu2+ ions. This discovery highlighted the possibility of achieving very high energy densities from inexpensive aqueous batteries; however, a full-cell demonstration with Zn as the anode was not studied. Here we report for the first time the effect of Zn anodes on the cycle life and energy density of a full cell, where we observe that 15% depth-of-discharge (DOD) of the Zns theoretical capacity (820 mA h g−1) creates a cell energy density of ∼160 W h L−1; however, this causes a drastic shape change and formation of irreversible zinc oxide (ZnO) at the anode, which ultimately causes cell failure after ∼100 cycles. A drop in energy density is also observed as a result of the interaction of dissolved Zn ions with the cathode, which forms a resistive Zn-birnessite compound in the early cycles, and then forms a highly resistive haeterolite (ZnMn2O4) in the later cycles, and ultimately causes cathode failure. A possible solution using a calcium hydroxide layer as a separator is presented, where the layer blocks the interaction of zinc ions through a complexing mechanism to obtain >900 cycles with >80% retention of MnO2 DOD.

Molecular Beam Epitaxial Growth and Properties of Bi2Se3 Topological Insulator Layers on Different Substrate Surfaces

Growth of high-quality Bi2Se3 films is crucial not only for study of topological insulators but also for manufacture of technologically important materials. We report a study of the heteroepitaxy of single-crystal Bi2Se3 thin films grown on GaAs and InP substrates by use of molecular beam epitaxy. Surface topography, crystal structure, and electrical transport properties of these Bi2Se3 epitaxial films are indicative of highly c-axis oriented films with atomically sharp interfaces.

Phonon renormalization and Raman spectral evolution through amorphous to crystalline transitions in Sb2Te3 thin films

A symmetry specific phonon mode renormalization is observed across an amorphous to crystalline phase transformation in thin films of the topological material Sb2Te3 using Raman spectroscopy. We present evidence for local crystalline symmetry in the amorphous state, eventhough, the q = 0 Raman selection rule is broken due to strong structural disorder. At crystallization, the in-plane polarized (Eg2) mode abruptly sharpens while the out-of-plane polarized (A1g) modes are only weakly effected. This effect unique to the Eg symmetry is exceptional considering that polarized spectra and comparison of the single phonon density of states between the amorphous and crystalline phases suggest that short range order of the amorphous phase is, on the average, similar to that of the crystalline material while electrical transport measurements reveal a sharp insulator-to-metal transition. Our findings point to the important role of anisotropic disorder affecting potential applications of topological and phase-change ba...

Errata: Time-resolved fluorescence and ultrafast energy transfer in a zinc (hydr)oxide–graphite oxide mesoporous composite

Abstract. Ultrafast energy decay kinetics of a zinc (hydr)oxide–graphite oxide (GO) composite is studied via time-resolved fluorescence spectroscopy. The time-resolved emission is spectrally decomposed into emission regions originating from the zinc (hydr)oxide optical gap, surface, and defect states of the composite material. The radiative lifetime of deep red emission becomes an order of magnitude longer than that of GO alone while the radiative lifetime of the zinc optical gap is shortened in the composite. An energy transfer scheme from the zinc (hydr)oxide to GO is considered.