Sylvie Rangan

Controlled Synthesis of Water-Dispersible Faceted Crystalline Copper Nanoparticles and Their Catalytic Properties

We report a solution-phase synthetic route to copper nanoparticles with controllable size and shape. The synthesis of the nanoparticles is achieved by the reduction of copper(II) salt in aqueous solution with hydrazine under air atmosphere in the presence of poly(acrylic acid) (PAA) as capping agent. The results suggest that the pH plays a key role for the formation of pure copper nanoparticles, whereas the concentration of PAA is important for controlling the size and geometric shape of the nanoparticles. The average size of the copper nanoparticles can be varied from 30 to 80 nm, depending on the concentration of PAA. With a moderate amount of PAA, faceted crystalline copper nanoparticles are obtained. The as-synthesized copper nanoparticles appear red in color and are stable for weeks, as confirmed by UV/Vis and X-ray photoemission (XPS) spectroscopy. The faceted crystalline copper nanoparticles serve as an effective catalyst for N-arylation of heterocycles, such as the C--N coupling reaction between p-nitrobenzyl chloride and morpholine producing 4-(4-nitrophenyl)morpholine in an excellent yield under mild reaction conditions. Furthermore, the nanoparticles are proven to be versatile as they also effectively catalyze the three-component, one-pot Mannich reaction between p-substituted benzaldehyde, aniline, and acetophenone affording a 100% conversion of the limiting reactant (aniline).

Electrochemical Performance of Acid-Treated Nanostructured LiMn1.5Ni0.5O4 − δ Spinel at Elevated Temperature

A surface treatment process based on a mild acidic solution was utilized to stabilize the surface of the LiMn 1.5 Ni 0.5 O 4-δ (LMNO) spinel cathode material and to improve its elevated temperature performance. To characterize the failure mechanism of the LMNO spinel at an elevated temperature (60°C), the effect of the Mn 3+ content and the charge/discharge state storage conditions were studied. It was shown that the existence of Mn 3+ is necessary for an improved elevated temperature performance. It was also identified that one of the main degradation mechanisms at an elevated temperature was the systematic impedance rise rather than the intrinsic capacity loss. The results of the charged state storage at 60°C demonstrated the worst condition for the spinel materials; however, the surface-treated materials presented an improved elevated temperature cycling and a much less impedance increase than the untreated spinel after 4 weeks of storage at 60°C. X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to characterize the effect of surface treatment on the crystal structure and morphology of the acid-treated material.

Symmetry-breaking charge transfer in a zinc chlorodipyrrin acceptor for high open circuit voltage organic photovoltaics.

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

GeOx interface layer reduction upon Al-gate deposition on a HfO2∕GeOx∕Ge(001) stack

The metallization of HfO2∕Ge by Al at room temperature was studied using photoemission and inverse photoemission. Upon deposition, Al reduces the GeOx interfacial layer between Ge and HfO2, and a thin Al2O3 layer is formed at the Al∕HfO2 interface. The band alignment across the Al∕HfO2∕Ge stacks is also addressed.

Energy level alignment of polythiophene/ZnO hybrid solar cells

Energy level alignment at interfaces is critical for fundamental understanding and optimization of organic photovoltaics (OPV) as band offsets of the donor and acceptor materials largely determine the open circuit voltage (Voc) of the device. Using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS), we examined the correlation between energy level alignment and photovoltaic properties of a model bilayer hybrid solar cell incorporating electrodeposited polythiophene (e-PT) films on ZnO planar substrates. The electrolyte anion (BF4−, PF6−, ClO4− or CF3SO3−) in the electrodeposition solution was found to have a strong influence on the e-PT film morphology and adhesion, the energy level alignment at the interface, and ultimately the Voc of the photovoltaic devices.

Surface and Structural Investigation of a MnOx Birnessite‐Type Water Oxidation Catalyst Formed under Photocatalytic Conditions

Catalytically active MnOx species have been reported to form in situ from various Mn-complexes during electrocatalytic and solution-based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnOx catalysts form without supports in situ under photocatalytic conditions. Our most active (4)MnOx catalyst (∼0.84 mmol O2  mol Mn(-1) s(-1)) forms from a Mn4O4 bearing a metal-organic framework. (4)MnOx is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR-TEM as a disordered, layered Mn-oxide with high surface area (216 m(2) g(-1)) and small regions of crystallinity and layer flexibility. In contrast, the (S)MnOx formed from Mn(2+) salt gives an amorphous species of lower surface area (80 m(2) g(-1)) and lower activity (∼0.15 mmol O2  mol Mn(-1) s(-1)). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p3/2 core levels detects enriched Mn(3+) and Mn(2+) content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.

Band gap of epitaxial in-plane-dimerized single-phase NbO2 films

We demonstrate the epitaxial growth of high quality crystalline phase-pure NbO2 films on various oxide substrates using molecular beam epitaxy. Using a combination of reflection high-energy electron diffraction and x-ray diffraction we show that the films grow with the pseudo-rutile (100) orientation out of plane. The band gap of the NbO2 films is determined to be at least 1.0 eV using a combination of x-ray photoelectron spectroscopy and inverse photoelectron spectroscopy, in conjunction with hybrid density functional calculations of the density of states.

Ionic liquid ultrathin films at the surface of Cu(100) and Au(111)

Monolayer to multilayer ultrathin films of the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide have been prepared on Au(111) and Cu(100) surfaces using physical vapor deposition. The ion-surface interactions are studied using a combination of scanning tunnel microscopy, as well as ultraviolet and x-ray photoemission spectroscopies. It is found that the IL does not decompose at the surface of the metals, and that the IL interaction with the Cu(100) surface is much stronger than with the Au(111) surface. As a consequence, STM imaging at room temperature results in more stable imaging at the monolayer coverage on Cu(100) than on Au(111), and work function measurements indicate a large interface dipole upon deposition of a monolayer of IL on Cu. Additional IL depositions on the two surfaces result in two distinct behaviors for the IL core levels: a gradual energy shift of the core levels on Au and a set of two well defined monolayer and multilayer core level components found at fixed energies on Cu, due to the formation of a tightly bound monolayer. Finally, it is proposed that the particularly strong cation-Cu interaction leads to stabilization of the anion and prevents its decomposition at the surface of Cu(100).

XeF2-induced removal of SiO2 near Si surfaces at 300 K: An unexpected proximity effect.

XeF2 interaction with SiO2/Si stacks has been investigated to understand the role of Si in proximity of SiO2 during XeF2 exposures of Si/SiO2 stacks. In situ Fourier transform infrared absorption spectroscopy, using a custom-made reaction cell compatible with high XeF2 pressures, reveals that, while pure SiO2 is not etched by XeF2, the oxide in SiO2/Si stacks is effectively removed when XeF2 has access to the silicon, i.e., when the Si in close proximity to the oxide is etched. Thick oxides (∼1–2 μm) are removed if sample edges are accessible, while thinner oxides (50–100 nm) are removed without requiring edge access. This unexpected SiO2 removal is found to be due to the formation of reactive fluorine species (XeF and F) evolved by the reaction of XeF2 with Si, which can, subsequently, etch SiO2. Calculations based on density functional theory provide critical insight into the underlying energetics and reaction pathways controlling XeF2 etching of both Si and SiO2.

Adsorption Geometry and Energy Level Alignment at the PTCDA/TiO2(110) Interface

The adsorption geometry and energy alignment at the PTCDA/TiO2(110) interface are investigated using a combination of experimental and theoretical approaches. The energy alignment is determined experimentally from the occupied and unoccupied states electronic structure measured using X-ray and UV photoemission and inverse photoemission, respectively. Two possible adsorption geometries compatible with previous studies, a flat geometry and a tilted geometry, were explored using DFT techniques, in order to obtain theoretical STM images and energy alignment at the interface. Both STM images simulation and resulting energy alignment point to a tilted geometry for PTCDA on TiO2(110).