Development and Properties of Surfactant‐Free Water‐Dispersible Cu2ZnSnS4 Nanocrystals: A Material for Low‐Cost Photovoltaics

A simple, yet novel hydrothermal method has been developed to synthesize surfactant‐free Cu₂ZnSnS₄ nanocrystal ink in water. The environmentally friendly, 2–4 nm ultrafine particles are stable in water for several weeks. Detailed X‐ray diffraction (XRD) and high‐resolution transmission electron microscopy revealed the formation of single‐crystalline‐kesterite‐phase Cu₂ZnSnS₄. Elemental mapping by scanning electron microscopy/energy dispersive spectrometry corroborated the presence of all four elements in a stoichiometric ratio with minor sulfur deficiency. Finally, Raman spectroscopy ruled out the possible presence of impurities of ZnS, Cu₂SnS₃, SnS, SnS₂, Cu_2?xS, or Sn₂S₃, which often interfere with the XRD and optical spectra of Cu₂ZnSnS₄. X‐ray photoelectron spectroscopic studies of the as‐synthesized samples confirmed that the oxidation states of the four elements match those of the bulk sample. Optical absorption analyses of thin film and solution samples showed high absorption efficiency (>10⁴ cm^?1) across the visible and near‐infrared spectral regions and a band gap E_g of 1.75 eV for the as‐synthesized sample. A non‐ohmic asymmetric rectifying response was observed in the I–V measurement at room temperature. The nonlinearity was more pronounced for this p‐type semiconductor when the resistance was measured against temperature in the range 180–400 K, which was detected in the hot‐point probe measurement.     

Rapid Synthesis of Mesoporous TiO2 for the Photoanodes in Dye Sensitized Solar Cells

Mesoporous TiO₂ microspheres with high specific surface areas were synthesized by means of a facile one‐step microwave hydrothermal process without using any template. The mesoporous materials were rapidly achieved using TiCl₄, urea and ammonium sulphate at comparatively low microwave power (400 W) for 8 min irradiation. The morphology and microstructure of the as‐prepared products were characterized by field emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer‐Emmett‐Teller (BET) surface area analysis. Structural characterization indicates that the TiO₂ microspheres display mesoporous structure. The average pore sizes and BET surface areas of the spheres were 5.3 nm and 222 m²g^?1, respectively. The mesoporous nanocrystals synthesized at 160 °C for 8 min were then used to prepare the photoanode for dye sensitized solar cells (DSSCs). A high power conversion efficiency of 5.72% was achieved from the mesoporous TiO₂ based photoanode, representing about 25.7% improvement over the efficiency of P25 photoanode.     

Catalytic Activities of Au6, , and Clusters for CO oxidation: A density functional study

We have studied the CO oxidation over neutral, anionic, and cationic gold hexamer clusters using density functional theory which elucidates the effect of cluster charge state on the catalytic activity. Herein, we have considered the conventional bimolecular Langmuir–Hinshelwood mechanism with coadsorbed CO and O₂ at the neighboring sites in all the clusters. Among the three clusters, entails lower barriers during the various steps of the oxidation mechanism. The stability of all the species including the transition states with respect to the interacting species in indicates no thermal activation. Our study suggests better catalytic activity of as compared to the neutral and cationic counterparts. © 2014 Wiley Periodicals, Inc.     

A theoretical investigation on the kinetics and reactivity of the gas-phase reactions of ethyl chlorodifluoroacetate with OH radical and Cl atom at 298 K

The mechanism, kinetics, and thermochemistry of the gas-phase reactions of CF₂ClC(O)OCH₂CH₃,ethyl chlorodifluoroacetate (ECDFA) with the OH radical and Cl atom are investigated. Geometry optimization and frequency calculations have been performed at the MPWB1K/6-31+G(d,p) level of theory and energetic information is refined by using G2(MP2) theory. Transition states are searched on the potential energy surface of reaction channels and each of the transition states is characterized by the presence of only one imaginary frequency. Connections of the transition states between designated local minima are confirmed by intrinsic reaction coordinate calculation. Theoretically calculated rate constants at 298 K using the Canonical Transition State Theory are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions as working chemical reactions, the standard enthalpies of formation for CF₂ClC(O)OCH₂CH₃, CF₂ClC(O)OCH₂CH₂, and CF₃C(O)OCHCH₃ are also reported for the first time. The hydrogen abstraction occurs mainly from –CH₂ group. The T1 diagnostic calculation suggests that the multi-reference character is not an issue for such systems. The estimated atmospheric life time of ECDFA is expected to be around 24 days.     

Density functional study on structures, stabilities, and electronic properties of size-selected Pd n Si q (n = 1–7 and q = 0, +1, −1) clusters

The density functional theory (DFT) calculations within the framework of generalized gradient approximation have been employed to systematically investigate the geometrical structures, stabilities, and electronic properties of Pd _n Si ^q (n = 1–7 and q = 0, +1, ?1) clusters and compared them with the pure ${\text{Pd}}_{n + 1}^{q}$ (n = 1–7 and q = 0, +1, ?1) clusters for illustrating the effect of doping Si atom into palladium nanoclusters. The most stable configurations adopt a three-dimensional structure for both pure and Si-doped palladium clusters at n = 3–7. As a result of doping, the Pd _n Si clusters adopt different geometries as compared to that of Pd _n+1. A careful analysis of the binding energies per atom, fragmentation energies, second-order difference of energies, and HOMO–LUMO energy gaps as a function of cluster size shows that the clusters ${\text{Pd}}_{4}^{ + }$ , ${\text{Pd}}_{4}$ , ${\text{Pd}}_{8}^{ - }$ , ${\text{Pd}}_{5} {\text{Si}}^{0, + , - }$ , and ${\text{Pd}}_{7} {\text{Si}}^{0, + , - }$ possess relatively higher stability. There is enhancement in the stabilities of palladium frameworks due to doping with an impurity atom. In addition, the charge transfer has been analyzed to understand the effect of doped atom and compared further.     

Plasma-Maser Instability of Bernstein Mode in Presence of Magnetohydrodynamic Turbulence

A theoretical study is made on the amplification mechanism of electrostatic Bernstein mode wave in presence of kinetic Alfven wave turbulence in a magnetized plasma on the basis of plasma-maser interaction. It is shown that a test high frequency electrostativ Bernstein mode wave is unstable in the presence of low frequency kinetic Alfven wave turbulence. The growth rate of the Bernstein wave vanishes only in an unmagnetized plasma. Because of the universal existence of the kinetic Alfven waves in large scale plasmas, the results have potential importance in space and astrophysical radiation processes.     

Orthogonal interaction of bernstein mode with ion-acoustic wave through plasma maser effect

Interaction of Bernstein mode wave with ion-acoustic turbulence is treated as the plasma maser effect. Ion-acoustic turbulence is considered as resonant mode, propagating orthogonal to the test Bernstein mode. The external magnetic field, in the direction of ion-acoustic turbulence, is playing key role in transferring energy in upconversion process from resonant low frequency wave to the high frequency wave through modulated electric field. It is shown that the direct coupling term and the associated reverse absorbtion effect do not contribute individually to the growth or damping to the Bernstein mode. Only the polarization coupling term is found to be effective in the energy upconversion process as the external magnetic field is providing momentum in the direction of propagation of Bernstein mode. The polarization coupling term is identified to be the dominant upconversion factor in the plasma maser effect.     

Modeling of heterogeneous catalysts based on silica and zeolites by the hybrid quantum chemical embedded cluster method

The review is dedicated to the elaboration and application of hybrid quantum mechani-cal/molecular mechanical methods for heterogeneous catalytic systems, including single atoms and clusters of transition metals immobilized on covalent oxide supports. The following issues are considered: (1) elaboration of the hybrid covEPE method for modeling of covalent sys-tems of the zeolite and silicate types, (2) computations of the properties of atoms and small titanium, rhodium, iridium, and gold clusters localized in cavities or embedded in the zeolite framework, and (3) computations of small silver and tantalum clusters anchored at the dehydr-oxylated and hydroxylated silica surfaces. The calculations were performed by the density functional theory (DFT) with the Becke—Perdew (BP) exchange-correlation potential.     

A New On-fluorescent Probe for Manganese (II) Ion

A new fluorescent probe for Mn²⁺ ion, (6E)-N-((E)-1,2-diphenyl-2-(pyridin-2-ylimino)ethylidene)pyridin-2-amine (L), has been synthesized from benzil and 2-amino pyridine and characterized. In 1:1 (v/v) CH₃CN:H₂O (pH 4.0, universal buffer) L exhibits fluorescent intensity with emission peak at λ_max 360 nm on excitation with photons of 310 nm. Fluorescent intensity of L increases distinguishingly on interaction with Mn²⁺ ion compared to metal ions—Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺ and Ag⁺ individually or all together. The enhancement in fluorescent intensity is due to snapping of photoinduced electron transfer (PET) prevailed in free L. Fluorescence and UV/visible spectral data analysis shows that binding stoichiometry between Mn²⁺ and L is 1:1 with log β?≈?3.0. Both L and its Mn²⁺ complex were optimised using density functional theory (DFT) and vibrational frequency calculations confirm that both are at local minima on the potential energy surfaces.     

Congruences for generalized Frobenius partitions with 4 colors

We present some congruences involving the functions c?₄(n) and which denote, respectively, the number of generalized Frobenius partitions of n with 4 colors and 4-order generalized Frobenius partitions of n with 4 colors.