TiNF and Related Analogues of TiO2 : A Combined Experimental and Theoretical Study.

Aliovalent anion substitution in inorganic materials brings about marked changes in properties, as exemplified by N,F-codoped metal oxides. Recently, complete substitution of oxygen in ZnO by N and F was carried out to generate Zn2 NF. In view of the important properties of TiO2 , we have attempted to prepare TiNF by employing an entirely new procedure involving the reaction of TiN with TiF4 . While the reaction at low temperature (450 °C) yields TiNF in the anatase phase, reaction at a higher temperature (600 °C) yields TiNF in the rutile phase. This is interesting since the anatase phase of TiO2 also transforms to the rutile phase on heating. The lattice parameters of TiNF are close to those of the parent oxide. Partial substitution of oxygen in TiO2 by N and F reduces the band gap, but complete substitution increases the value comparable to that of the oxide. We have examined properties of N,F-codoped TiO2 , and more interestingly N,F-codoped Ti3 O5 , both with lower band gaps than the parent oxides. A detailed first-principles calculations has been carried out on structural and electronic properties of N,F-TiO2 and the TiNF phases. This has enabled us to understand the effects of N,F substitution in TiO2 in terms of the crystal structure, electronic structure and optical properties.

Zn2NF and Related Analogues of ZnO.

Substitution of aliovalent N(3-) and F(-) anions in place of O(2-) in ZnO brings about major changes in the electronic structure and properties, the composition, even with 10 atomic percent or less of the two anions, rendering the material yellow colored with a much smaller band gap. We have examined the variation of band gap of ZnO with progressive substitution of N and F and more importantly prepared Zn2NF which is the composition one obtains ultimately upon complete replacement of O(2-) ions. In this article, we present the results of a first complete study of the crystal and electronic structures as well as of properties of a stable metal nitride fluoride, Zn2NF. This material occurs in two crystal forms, tetragonal and orthorhombic, both with a band gap much smaller than that of ZnO. Electronic structures of Zn2NF as well as ZnO0.2N0.5F0.3 investigated by first-principles calculations show that the valence bands of these are dominated by the N (2p) states lying at the top. Interestingly, the latter is a p-type material, a property that has been anticipated for long time. The calculations predict conduction and valence band edges in Zn2NF to be favorable for water splitting. Zn2NF does indeed exhibit good visible-light-induced hydrogen evolution activity unlike ZnO. The present study demonstrates how aliovalent anion substitution can be employed for tuning band gaps of materials.

Generation of Hydrogen by Visible Light-Induced Water Splitting with the Use of Semiconductors and Dyes.

Photosynthesis that occurs in plants involves both the oxidation of water and the reduction of carbon dioxide. Plants carry out these reactions with ease, by involving electron-transport chains. In this article, hydrogen generation by the reduction of water in the laboratory by using semiconductor nanostructures through artificial photosynthesis is examined. Dye-sensitized photochemical generation of hydrogen from water is also discussed. Hydrogen generation by these means has great technological relevance, since it is an environmentally friendly fuel. The way in which oxygen can be generated by the oxidation of water using metal oxide catalysts is also shown.

Structural and magnetic diversity based on different imidazolate linkers in Cu(II)-azido coordination compounds.

This Article reports the syntheses, structural characterization, and magnetic studies of four different Cu(II)-azido compounds based on imidazole or substituted imidazole ligand. The compounds, [Cu2(µ1,1-N3)2(EtimiH)4(ClO4)2] (1) (EtimiH = 2-ethylimidazole), [Cu2(µ-Meimi(-))(MeimiH)2(µ1,1-N3)2(µ1,3-N3)]n (2) (MeimiH = 2-methylimidazole; µ-Meimi(-) is the bridging mononegative anion of 2-methylimidazole), [Cu2(µ-imi(-))(imiH)2(µ1,1-N3)2(µ1,3-N3)]n (3), and [{Cu2(µ1,1-N3)2(µ1,3-N3)(µ-imi(-))(imiH)3}·H2O]n (4) (imiH = imidazole; µ-imi(-) = bridging mononegative anion of imidazole), have been synthesized by the self-assembly of Cu(II) salts, azide ion, and the corresponding imidazole bridging ligands. By changing the substitution on the second linker (imidazole or substituted imidazole) and varying synthetic conditions, diverse structural and magnetic features have been achieved in compounds 1-4. Compound 1 has a double end-on azido bridged dinuclear core, while the other compounds (2-4) have 2D networks. Compound 2 and 3 contain 1D chains with alternate µ1,1-N3 and µ-Meimi(-) bridging, and such chains are further connected through a µ1,3-N3 bridge to result in the formation of the 2D network. Compound 4 is a novel 2D coordination polymer consisting of a zigzag 1D coordination chain having (µ1,1-N3)2, µ-imi(-), and (µ1,3-N3)2 bridging groups and the chains undergo bridging through a µ1,3-N3 group resulting in the 2D network. Temperature dependent magnetic measurements show diverse magnetic properties of 1-4. Such versatile magnetic behaviors have been correlated to the respective bridging mode of azide and the corresponding imidazole bridging ligands.