No abstract

Photoinduced electron transfer (ET) reactions between amines and a series of coumarins have been investigated using fluorescence-quenching measurements in aqueous P123 triblock copolymer micellar solutions. Fluorescence spectral characteristics and fluorescence anisotropy measurements indicated a nearly similar microenvironment for all of the coumarins used in P123 micelles. Substantial quenching of coumarin fluorescence in the presence of amines has been observed. The quenching rates (k(q)(TR)) are largely reduced in the P123 micelle as compared to those in other micelles (sodium dodecyl sulfate (SDS), Triton-X 100 (TX-100), cetyl trimethyl ammonium bromide (CTAB), and dodecyl trimethyl ammonium bromide (DTAB)), which is probably due to larger coumarin-amine separations in the micellar phase. The k(q)(TR) values, when plotted against free energy changes (DeltaG degrees), follow a Marcus predicted bell-shaped correlation. The estimated activation energy for the ET reactions follow an inverse bell-shaped correlation with DeltaG degrees. Present results indicate that the appearance of Marcus inversion is primarily related to the changes in the activation barrier, as predicted from the Marcus ET theory. As the k(q)(TR) values are higher than the estimated bimolecular diffusional rate constant, the role of reactant diffusion on the quenching kinetics in the P123 micelle is negligible. The appearance of Marcus inversion at unexpectedly lower exergonicity has been rationalized on the basis of slow solvent relaxation and by the application of the two-dimensional ET (2DET) theory. Critical analysis of the present results establishes that the inversion in the ET rates at high exergonicity is not due to the alteration in the diffusion parameters of the reactants, as has been suggested in some recent reports. Instead, it is evident that the inversion in quenching rates at high exergonicity is due to the alteration in the activation barrier for the ET reactions.

Ultrafast photoinduced intermolecular electron transfer (ET) dynamics involving 7-aminocoumarin derivatives as electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelle as electron acceptor has been investigated to understand the role of separation and orientation of reactants on micellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understood to give the ultrafast ET component with the shortest time constant in the range of approximately 4 ps, in CPC micelles with pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecond time scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafast ET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micelles are significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observed ET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shaped correlation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at a relatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafast ET reaction, as envisaged from two-dimensional ET (2DET) model. Along with the ultrafast ET component, there are also slower ET components in these systems, which are attributed to those close-contact donor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improper orientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shifting of Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by using surfactant molecule as an intrinsic reactant.

have been explored to this end [ 7,8 ] and in particular cobalt-based oxides and their binary oxides with nickel are reported to be potential candidates. [ 9 ] For a favorable OER, the M  OH bond strength should be moderate enough as per the Sabatier's principle of balanced intermediate adsorption in catalysis. [ 4,10,11 ] Cobalt in Co 3 O 4 spinel occupies two different sites, tetrahedral for Co 2+ and both tetrahedral and octahedral for Co 3+ . [ 12 ] Incorporation of nickel in the Co 3 O 4 spinel structure results in the octahedral substitution, which is reported to improve the electronic conductivity and effective surface area and add benefi cial effect on oxygen evolution kinetics. [ 13,14 ] Additionally, nickel-based oxides and hydroxides that are prepared by wet chemical routes are found to produce hierarchical nanostructures, and as a consequence provide high surface area for catalytic reactions. [ 15,16 ] Microstructuring of materials into thin fi lms, [ 17 ] 3D cages, [ 18 ] nanorod, [ 19 ] wires, [ 20 ] and porous structures [ 21 ] are known to enhance the active surface area. The oxygen vacancies and nonstoichiometry introduced during fabrication and annealing processes can further improve the conductivity and lower the hydroxyl adsorption energy. [ 22,23 ] Rational design of morphology is a promising approach to promote material's performance. Building hierarchical hollow structures thus holds promise for more effi cient electrocatalyst which can render large surface area, better electronic conductivity and porosity for electrochemical processes. [ 14,24 ] Great efforts have been devoted to develop such porous and hollow nanostructure, which can not only enhance the desired activity, but also impart new functionalities. [ 24,25 ] Recently, metal-organic framework (MOF) has been demonstrated as excellent precursors and templates for fabrication of 3D structures of metal oxides for highly effi cient supercapacitors, Li ion batteries, and oxygen reduction reactions. [ 26 ] Following similar fabrication methodology, MOF derived coreshell structured NiCo 2 O 4 -Co 3 O 4 has been prepared and used to study supercapacitor and catalytic oxygen evolution behavior. [ 18 ] There is, however, a need for systematic investigations to arrive at an optimum nickel-cobalt oxide for OER based on this fabrication route. Such investigations assume importance in realizing a competitive OER catalyst of this class for use in solar to hydrogen conversion devices.In this paper, we report a simple template based fabrication route for non-stoichiometric Ni  Co metal oxide nanocages with mesoporous structure, and rich in Ni and Co redox centers, and Nonstoichiometric Ni x Co 3− x O 4− y 3D nanocages are fabricated through metalorganic framework template route and their electrocatalytic oxygen evolution reaction (OER) characteristics have been investigated. Substitution of Ni in Co 3 O 4 spinel structure improves the intrinsic catalytic activity. Enhanced OER activity stems from the presence of nonstoichiometry and low co...

A facile hydrothermal method is adopted for the synthesis of hierarchical flowerlike nickel sulfide nanostructure materials and their composite with carbon quantum dot (NiS/C-dot) composite. The composite material exhibited good performance for electrochemical energy-storage devices as supercapacitor with a specific capacity of 880 F g –1 at a current density of 2 A g –1 . The material remained stable up to the tested 2000 charge–discharge cycles. Carbon quantum dots of size 1.3 nm were synthesized from natural sources and the favorable electronic and surface property of C-dots were utilized for improvement of the supercapacitor performance of NiS. The results from Tafel analysis, double-layer capacitance, and the impedance measurement reveal that the incorporation of C-dots inside the NiS matrix has improved the charge-transfer process, which is mainly responsible for the enhancement of the supercapacitive property of the composite materials.