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Background: Medicinal plants are used by many ethnic groups as a source of medicine for the treatment of various ailments in both humans and domestic animals. These plants produce secondary metabolites that have antimicrobial properties, thus screening of medicinal plants provide another alternative for producing chemical fungicides that are relatively non-toxic and cost-effective. Materials and methods: Leaf extracts of selected South African plant species (Bucida buceras, Breonadia salicina, Harpephyllum caffrum, Olinia ventosa, Vangueria infausta and Xylotheca kraussiana) were investigated for activity against selected phytopathogenic fungi (Aspergillus niger, Aspergillus parasiticus, Colletotricum gloeosporioides, Penicillium janthinellum, P. expansum, Trichoderma harzianum and Fusarium oxysporum). These plant fungal pathogens causes major economic losses in fruit industry such as blue rot on nectaries and postharvest disease in citrus. Plant species were selected from 600 evaluated inter alia, against two animal fungal pathogens (Candida albicans and Cryptococcus neoformans). Antioxidant activity of the selected plant extracts were investigated using a qualitative assay (2, 2-diphenyl-1-picrylhydrazyl (DPPH)). Bioautography assay was used to determine the number of antifungal compounds in plant extracts. Results: All plant extracts were active against the selected plant phytopathogenic fungi. Moreover, Bucida buceras had the best antifungal activity against four of the fungi, with minimum inhibitory concentration (MIC) values as low as 0.02 mg/ml and 0.08 mg/ml against P. expansum, P. janthinellum, T. harzianum and F. oxysporum. The plant extracts of five plant species did not possess strong antioxidant activity. However, methanol extract of X. kraussiana was the most active radical scavenger in the DPPH assay amongst the six medicinal plants screened. No antifungal compounds were observed in some of the plant extracts with good antifungal activity as shown in the microdilution assay, indicating possible synergism between the separated metabolites. Conclusion: The results showed that acetone was the best extractant. Furthermore, our findings also confirm the traditional use of Breonadia salicina and demonstrate the potential value of developing biopesticides from plants.

We have performed a genetic algorithm search on the tight-binding interatomic potential energy surface (PES) for small TiN (N = 2-32) clusters. The low energy candidate clusters were further refined using density functional theory (DFT) calculations with the PBEsol exchange-correlation functional and evaluated with the PBEsol0 hybrid functional. The resulting clusters were analysed in terms of their structural features, growth mechanism and surface area. The results suggest a growth mechanism that is based on forming coordination centres by interpenetrating icosahedra, icositetrahedra and Frank-Kasper polyhedra. We identify centres of coordination, which act as centres of bulk nucleation in medium sized clusters and determine the morphological features of the cluster.

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We have applied density functional theory calculations with Hubbard corrections (DFT+U) to investigate the structural, electronic, and redox properties of Ti-substituted zirconia (111) surfaces. The calculations show that titanium dopants are likely to accumulate at the oxide surface, where an isolated dopant is 0.25 eV more stable than in the bulk. We have investigated in detail the relative distribution of dopants and oxygen vacancies at the surface and report the most stable configurations for each composition. It is found that the formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. The analysis of the electronic structure of the doped and reduced surfaces reveals that, when an O vacancy is created around an isolated Ti dopant, a Ti4+ → Ti2+ reduction takes place, with the reduced cation in a high-spin configuration. However, if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a +3 oxidation state with an additional decrease in the vacancy formation energy.

Ab initio density functional theory was used to investigate the adsorption of oxygen molecule on the nickel-rich pentlandite (110) surface, which is important for mineral extraction. The three most reactive adsorption sites: Fe-top, Ni-top, and fcc-hollow have been considered. Firstly, the non-adsorbed pentlandite surface reflects the Ni atoms relaxing inwards. Consequently, their electronic structure showed high Fe 3d-orbital contribution than the Ni 3d-orbitals at the EF (indicating that the Fe atoms are more reactive than Ni). Secondly, the O2-adsorbed surface predicted lowest adsorption energy for Fe-top (-1.902 eV), as a more spontaneous reaction is likely to occur than on fcc-hollow (-1.891 eV) and Ni-top (-0.040 eV) sites, suggesting Fe preferential oxidation. The density of states indicates that the O2 show prevalence of electrons in the πp* antibonding orbitals, and are reduced to zero states at the valence band on metal-bonded oxygen (O1). The πp* orbital is observed to reside just above the EF for Fe-top and fcc-hollow site, while on Ni-top is half-occupied for both metal-bonded oxygen (O1) and terminal oxygen (O2). Finally, the isosurface charge density difference showed electron (charge) depletion on Ni/Fe metals and accumulation on the O2 molecule. Bader analysis indicated that the oxidized Fe and Ni atoms adopt more positive charge, while O2 on Fe-top atoms possesses more negative charge than on Ni-top, resulting with O1 possessing a smaller charge than O2 atom.

First-principle electronic structure investigations of transition-metal sulfides Co 9 S 8 and related alloys with the unique structure of pentlandite are carried out using density-functional theory within the local-density approximation. The total-energy calculations for Co 9 S 8 and (Fe,Ni) 9 S 8 alloys have been computed and we predict equilibrium lattice parameters that are on average 1% smaller than in the experiment. The heats of formation have been calculated, the theoretical prediction for Co 9 S 8 being in excellent agreement with that available in the Thermocalc database. The predicted heat of formation for the Fe 5 Ni 4 S 8 alloy is very close to Co 9 S 8 , reflecting the fact that the Fermi level is found to fall in a pseudogap for an average number of valence electrons per atom e/aϭ7.58. Furthermore, we determined the individual bond energies for Co 9 S 8 and Co 8 S 8 to stress the contribution of the octahedral metal cobalt to the stability of the Co 9 S 8 phase.