Simultaneous optimization of colloidal stability and interfacial charge transfer efficiency in photocatalytic Pt/CdS nanocrystals

Colloidal stability and efficient interfacial charge transfer in semiconductor nanocrystals are of great importance for photocatalytic applications in aqueous solution since they provide long-term functionality and high photocatalytic activity, respectively. However, colloidal stability and interfacial charge transfer efficiency are difficult to optimize simultaneously since the ligand layer often acts as both a shell stabilizing the nanocrystals in colloidal suspension and a barrier reducing the efficiency of interfacial charge transfer. Here, we show that, for cysteine-coated, Pt-decorated CdS nanocrystals and Na2SO3 as hole scavenger, triethanolamine (TEOA) replaces the original cysteine ligands in situ and prolongs the highly efficient and steady H2 evolution period by more than a factor of 10. It is shown that Na2SO3 is consumed during H2 generation while TEOA makes no significant contribution to the H2 generation. An apparent quantum yield of 31.5%, a turnover frequency of 0.11 H2/Pt/s, and an interfacial charge transfer rate faster than 0.3 ps were achieved in the TEOA stabilized system. The short length, branched structure and weak binding of TEOA to CdS as well as sufficient free TEOA in the solution are the keys to enhancing colloidal stability and maintaining efficient interfacial charge transfer at the same time. Additionally, TEOA is commercially available and cheap, and we anticipate that this approach can be widely applied in many photocatalytic applications involving colloidal nanocrystals.

“Acid-spike” effect in spurs/tracks of the low/high linear energy transfer radiolysis of water : potential implications for radiobiology and nuclear industry

Résumé : Les ions hydronium (H3O + ) sont formés, à temps courts, dans les grappes ou le long des trajectoires de la radiolyse de l'eau par des rayonnements ionisants à faible transfert d’énergie linéaire (TEL) ou à TEL élevé. Cette formation in situ de H3O + rend la région des grappes/trajectoires du rayonnement temporairement plus acide que le milieu environnant. Bien que des preuves expérimentales de l’acidité d’une grappe aient déjà été signalées, il n'y a que des informations fragmentaires quant à son ampleur et sa dépendance en temps. Dans ce travail, nous déterminons les concentrations en H3O + et les valeurs de pH correspondantes en fonction du temps à partir des rendements de H3O + calculés à l’aide de simulations Monte Carlo de la chimie intervenant dans les trajectoires. Quatre ions incidents de différents TEL ont été sélectionnés et deux modèles de grappe/trajectoire ont été utilisés : 1) un modèle de grappe isolée "sphérique" (faible TEL) et 2) un modèle de trajectoire "cylindrique" (TEL élevé). Dans tous les cas étudiés, un effet de pH acide brusque transitoire, que nous appelons un effet de "pic acide", est observé immédiatement après l’irradiation. Cet effet ne semble pas avoir été exploré dans l'eau ou un milieu cellulaire soumis à un rayonnement ionisant, en particulier à haut TEL. À cet égard, ce travail soulève des questions sur les implications possibles de cet effet en radiobiologie, dont certaines sont évoquées brièvement. Nos calculs ont ensuite été étendus à l’étude de l'influence de la température, de 25 à 350 °C, sur la formation in situ d’ions H3O + et l’effet de pic acide qui intervient à temps courts lors de la radiolyse de l’eau à faible TEL. Les résultats montrent une augmentation marquée de la réponse de pic acide à hautes températures. Comme de nombreux processus intervenant dans le cœur d’un réacteur nucléaire refroidi à l'eau dépendent de façon critique du pH, la question ici est de savoir si ces fortes variations d’acidité, même si elles sont hautement localisées et transitoires, contribuent à la corrosion et l’endommagement des matériaux.

Detailed analysis of a conservative difference approximation for the time fractional diffusion equation

Diffusion equations that use time fractional derivatives are attractive because they describe a wealth of problems involving non-Markovian Random walks. The time fractional diffusion equation (TFDE) is obtained from the standard diffusion equation by replacing the first-order time derivative with a fractional derivative of order α ∈ (0, 1). Developing numerical methods for solving fractional partial differential equations is a new research field and the theoretical analysis of the numerical methods associated with them is not fully developed. In this paper an explicit conservative difference approximation (ECDA) for TFDE is proposed. We give a detailed analysis for this ECDA and generate discrete models of random walk suitable for simulating random variables whose spatial probability density evolves in time according to this fractional diffusion equation. The stability and convergence of the ECDA for TFDE in a bounded domain are discussed. Finally, some numerical examples are presented to show the application of the present technique.

Micropolar flow in a porous channel with high mass transfer

Two dimensional flow of a micropolar fluid in a porous channel is investigated. The flow is driven by suction or injection at the channel walls, and the micropolar model due to Eringen is used to describe the working fluid. An extension of Berman's similarity transform is used to reduce the governing equations to a set of non-linear coupled ordinary differential equations. The latter are solved for large mass transfer via a perturbation analysis where the inverse of the cross-flow Reynolds number is used as the perturbing parameter. Complementary numerical solutions for strong injection are also obtained using a quasilinearisation scheme, and good agreement is observed between the solutions obtained from the perturbation analysis and the computations.

Determination of preventive maintenance lead time using hybrid analysis

The time for conducting Preventive Maintenance (PM) on an asset is often determined using a predefined alarm limit based on trends of a hazard function. In this paper, the authors propose using both hazard and reliability functions to improve the accuracy of the prediction particularly when the failure characteristic of the asset whole life is modelled using different failure distributions for the different stages of the life of the asset. The proposed method is validated using simulations and case studies.

Effective Cell-Centred Time-Domain Maxwell's Equations Numerical Solvers

This research work analyses techniques for implementing a cell-centred finite-volume time-domain (ccFV-TD) computational methodology for the purpose of studying microwave heating. Various state-of-the-art spatial and temporal discretisation methods employed to solve Maxwell's equations on multidimensional structured grid networks are investigated, and the dispersive and dissipative errors inherent in those techniques examined. Both staggered and unstaggered grid approaches are considered. Upwind schemes using a Riemann solver and intensity vector splitting are studied and evaluated. Staggered and unstaggered Leapfrog and Runge-Kutta time integration methods are analysed in terms of phase and amplitude error to identify which method is the most accurate and efficient for simulating microwave heating processes. The implementation and migration of typical electromagnetic boundary conditions. from staggered in space to cell-centred approaches also is deliberated. In particular, an existing perfectly matched layer absorbing boundary methodology is adapted to formulate a new cell-centred boundary implementation for the ccFV-TD solvers. Finally for microwave heating purposes, a comparison of analytical and numerical results for standard case studies in rectangular waveguides allows the accuracy of the developed methods to be assessed.