Wilbert Mtangi

Bio: Wilbert Mtangi is an academic researcher from University of Pretoria. The author has contributed to research in topics: Schottky barrier & Deep-level transient spectroscopy. The author has an hindex of 14, co-authored 27 publications receiving 679 citations. Previous affiliations of Wilbert Mtangi include Chinhoyi University of Technology & Weizmann Institute of Science.

Control of Electrons’ Spin Eliminates Hydrogen Peroxide Formation During Water Splitting

Abstract: The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

Role of the Electron Spin Polarization in Water Splitting

Abstract: We show that in an electrochemical cell, in which the photoanode is coated with chiral molecules, the overpotential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. The hydrogen evolution efficiency is studied comparing seven different organic molecules, three chiral and four achiral. We propose that the spin specificity of electrons transferred through chiral molecules is the origin of a more efficient oxidation process in which oxygen is formed in its triplet ground state. The new observations are consistent with recent theoretical works pointing to the importance of spin alignment in the water-splitting process.

Analysis of temperature dependent I―V measurements on Pd/ZnO Schottky barrier diodes and the determination of the Richardson constant

Abstract: Temperature dependent current–voltage ( I – V ) and Hall measurements were performed on Pd/ZnO Schottky barrier diodes in the range 20–300 K The apparent Richardson constant was found to be 860 × 10 - 9 A K - 2 cm - 2 in the 60–160 K temperature range, and mean barrier height of 050 eV in the 180–300 K temperature range After barrier height inhomogeneities correction, the Richardson constant and the mean barrier height were obtained as 167 A K - 2 cm - 2 and 061 eV in the temperature range 80–180 K, respectively A defect level with energy at 012 eV below the conduction band was observed using the saturation current plot and ( 011 ± 001 ) eV using deep level transient spectroscopy measurements

Lithium and electrical properties of ZnO

Abstract: Hydrothermal grown n-type ZnO samples have been investigated by deep level transient spectroscopy (DLTS), thermal admittance spectroscopy (TAS), temperature dependent Hall effect (TDH) measurements, and secondary ion mass spectrometry (SIMS) after thermal treatments up to 1500 °C, in order to study the electrical properties of samples with different lithium content. The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Ni, and Fe with concentrations up to ∼5×1017 cm−3. The Li concentration was reduced from ∼1017 cm−3 in as-grown samples to ∼1015 cm−3 for samples treated at 1500 °C, while the concentration of all the other major impurities appeared stable. The results from DLTS and TAS displayed at least five different levels having energy positions of Ec−20 meV, Ec−55 meV, Ec−0.22 eV, Ec−0.30 eV, and Ec−0.57 eV (Ec denotes the conduction band edge), where the Ec−55 meV level is the dominant freeze out level for conduction electrons in samples treated at temperatures <1300 °C, wh...

Chiral molecules and the electron spin

Abstract: The electron’s spin is essential to the stability of matter, and control over the spin opens up avenues for manipulating the properties of molecules and materials. The Pauli exclusion principle requires that two electrons in a single spatial eigenstate have opposite spins, and this fact dictates basic features of atomic states and chemical bond formation. The energy associated with interacting electron clouds changes with their relative spin orientation, and by manipulating the spin directions, one can guide chemical transformations. However, controlling the relative spin orientation of electrons located on two reactants (atoms, molecules or surfaces) has proved challenging. Recent developments based on the chiral-induced spin selectivity (CISS) effect show that the spin orientation is linked to molecular symmetry and can be controlled in ways not previously imagined. For example, the combination of chiral molecules and electron spin opens up a new approach to (enantio)selective chemistry. This Review describes the theoretical concepts underlying the CISS effect and illustrates its importance by discussing some of its manifestations in chemistry, biology and physics. Specifically, we discuss how the CISS effect allows for efficient long-range electron transfer in chiral molecules and how it affects biorecognition processes. Several applications of the effect are presented, and the importance of controlling relative spin orientations in multi-electron processes, such as electrochemical water splitting, is emphasized. We describe the enantiospecific interaction between ferromagnetic substrates and chiral molecules and how it enables the separation of enantiomers with ferromagnets. Lastly, we discuss the relevance of CISS effects to biological electron transfer, enantioselectivity and CISS-based spintronics applications. Chiral molecules can filter electrons according to their spin. This chiral-induced spin selectivity (CISS) effect can have important applications, such as in spintronics and in enantioseparation. This Review describes the CISS effect, its mechanism and its fascinating applications.

The added value of small-molecule chirality in technological applications

Abstract: Chirality is a fundamental symmetry property; chiral objects, such as chiral small molecules, exist as a pair of non-superimposable mirror images. Although small-molecule chirality is routinely considered in biologically focused application areas (such as drug discovery and chemical biology), other areas of scientific development have not considered small-molecule chirality to be central to their approach. In this Review, we highlight recent research in which chirality has enabled advancement in technological applications. We showcase examples in which the presence of small-molecule chirality is exploited in ways beyond the simple interaction of two different chiral molecules; this can enable the detection and emission of chiral light, help to control molecular motion, or provide a means to control electron spin and bulk charge transport. Thus, we demonstrate that small-molecule chirality is a highly promising avenue for a wide range of technologically oriented scientific endeavours. Although it is a fundamental property of many small molecules, chirality is not widely exploited in materials applications as its benefits are not widely recognized — indeed, the need for stereoselective synthesis may be seen as a disadvantage. In this Review, we highlight recent research in which chirality has had an enabling impact in technological applications.

Alternative strategies in improving the photocatalytic and photoelectrochemical activities of visible light-driven BiVO4: a review

Abstract: The research interest on bismuth vanadate (BiVO4) has heightened over the past decade due to its proven high activity for water oxidation and organic degradations under visible light. Although metal doping and water-oxidation cocatalyst loading have been widely demonstrated to be useful to overcome the poor electron transport and slow water oxidation kinetics of BiVO4, the efficiency of this material is still greatly limited by poor charge separation. Various efforts directed at modifying the surface and bulk properties to improve the performance of BiVO4-based materials have therefore been developed, including crystal facet engineering, coupling with graphitic carbon material, annealing treatment, and nanoscaling. This review aims to provide insights into the most recent progress in these strategies in regard to their influences on the charge separation, transport, and transfer aspects of BiVO4, all of which are crucial to govern photochemical conversion efficiency. Understanding of these charge kinetics in relation to the properties of BiVO4 is of fundamental importance for rational design of BiVO4 with optimum structures, which may serve as a general guideline for the fabrication of metal oxide photocatalysts.