Rita Toth

Photonic light trapping in self-organized all-oxide microspheroids impacts photoelectrochemical water splitting

Thin films involving an oxide heterojunction are increasingly employed as electrodes for solar water splitting in photoelectrochemical cells. Hematite (α-Fe2O3) and tungsten oxide form an attractive heterojunction for this purpose. A major limitation of this strategy is the short charge carrier diffusion length in hematite. Ultra-thin films were implemented to address this low conductivity issue. Nevertheless, such ultrathin films do not absorb light efficiently. The present study explores light trapping strategies to increase the optical path length of photons in hematite. Vesicle suspensions were developed to obtain thin films composed of a microspheroid array with a tungsten oxide core and a nanometer sized hematite overlayer. This bottom-up approach allows a fine control of the spheroid dimensions at the micrometric to the submicrometric scale. Using the finite difference time domain method, light propagation inside the microstructures was quantitatively simulated. The simulation results were coupled to an analysis of the photoelectrochemical response of the films. Experiments and simulation show quantitative agreement and bring important insights into the relationship between the interaction of light with the microstructure and the photoanode performance.

Dynamic control and information processing in the Belousov–Zhabotinsky reaction using a coevolutionary algorithm

We propose that the behavior of nonlinear media can be controlled dynamically through coevolutionary systems. In this study, a light-sensitive subexcitable Belousov-Zhabotinsky reaction is controlled using a heterogeneous cellular automaton. A checkerboard image comprising of varying light intensity cells is projected onto the surface of a catalyst-loaded gel resulting in rich spatiotemporal chemical wave behavior. The coevolved cellular automaton is shown to be able to either increase or decrease chemical activity through dynamic control of the light intensity within each cell in both simulated and real chemical systems. The approach is then extended to construct a number of simple logical functions.

Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation

Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe(2+) species which become oxidized toward Fe(3+) during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.

Maze solving using fatty acid chemistry

This study demonstrates that the Marangoni flow in a channel network can solve maze problems such as exploring and visualizing the shortest path and finding all possible solutions in a parallel fashion. The Marangoni flow is generated by the pH gradient in a maze filled with an alkaline solution of a fatty acid by introducing a hydrogel block soaked with an acid at the exit. The pH gradient changes the protonation rate of fatty acid molecules, which translates into the surface tension gradient at the liquid-air interface through the maze. Fluid flow maintained by the surface tension gradient (Marangoni flow) can drag water-soluble dye particles toward low pH (exit) at the liquid-air interface. Dye particles placed at the entrance of the maze dissolve during this motion, thus exhibiting and finding the shortest path and all possible paths in a maze.

Wave initiation in the ferroin-catalysed Belousov-Zhabotinsky reaction with visible light

The initiation of chemical reaction–diffusion waves by visible light of wavelength λ=633 nm from a 20 mW He–Ne laser in the ferroin-catalysed BZ reaction on a polysulfone membrane is repor ted. With low loading of the catalyst on the membrane, oxidation waves can be initiated from the resting steady state and in the recovering tail of a wave. With high loading, waves can only be initiated in the ‘vulnerable’ region behind an existing wavefront. The mechanism of this initiation is discussed in terms of the photoreduction of the metal–ligand catalyst and expressed in terms of a modified Oregonator model. These new observations are in contrast to the inhibitory effect of visible light in the light-sensitive Ru-catalysed BZ system.

Simple Collision-Based Chemical Logic Gates with Adaptive Computing

We present a method that is capable of implementing information transfer without any rigidly controlled architecture using the light-sensitive Belousov-Zhabotinsky (BZ) reaction system. Chemical wave fragments are injected into a subexcitable area and their collisions result in annihilation, fusion or quasi-elastic interactions depending on their initial positions. The fragments of excitation both pre and post collision possess a considerable freedom of movement when compared to previous implementations of information transfer in chemical systems. We propose that the collision of such wave fragments can be controlled automatically through adaptive computing. By extension, forms of unconventional computing, i.e., massively parallel non-linear computers, can be realised by such an approach. In this study we present initial results from using a simple evolutionary algorithm to design Boolean logic gates within the BZ system. [Article copies are available for purchase from InfoSci-on-Demand.com]

Maze solving using temperature-induced Marangoni flow

The pH-induced Marangoni flow has been recently shown to be of use for analog computing of topological problems, such as maze solving. Here we show that the temperature-induced Marangoni flow can also be used to find the shortest path in a maze filled with a hot solution of a fatty acid, where the temperature gradient is created by cooling down the exit of the maze. Our method utilizes the fact that the temperature-induced Marangoni flow can transport dye particles at the liquid–air interface added to the entrance of the maze which subsequently dissolve in water during their motion revealing the most likely paths. The most intense flow is maintained through the shortest path which is, therefore, marked by the most intense color of the dissolved dye particles.

A facile nonpolar organic solution process of a nanostructured hematite photoanode with high efficiency and stability for water splitting

We present a facile nonpolar organic solution synthesis and processing method for water splitting photoanodes based on iron oxide with a relatively low sintering temperature (550 °C). The photocurrent of these photoanodes can reach 3.3 mA cm−2 at 1.23 V vs. RHE with high IPCE and stability, which makes them highly attractive candidate photoanodes for hydrogen generation.

Flow-driven instabilities in the Belousov–Zhabotinsky reaction: Modelling and experiments

The development of propagating patterns arising from the differential flow of reactants through a tubular reactor is investigated. The results from a series of experimental runs, using the BZ reaction, are presented to show how the wavelength and propagation speed of the patterns depend on the imposed flow velocity and the concentration of BrO3− in the inflow. A model for this system, based on a two-variable Oregonator model for the BZ reaction, is considered. A stability analysis of the model indicates that the mechanism for pattern formation is through a convective instability. Numerical simulations confirm the existence of propagating patterns and are in reasonable agreement with the experimental observations.

Flow-distributed oscillation patterns in the Oregonator model

The conditions under which chemical patterns corresponding to “flow-distributed oscillations ” are formed are determined analytically for the Oregonator model of the Belousov–Zhabotinsky reaction. These analytical results are confirmed by numerical computation and are also used to predict typical values for the critical flow velocity and how the wavelength varies with the concentrations of the major reactants.