Band gap tuning of the wide band gap semiconductor, TiO2, has great importance due to its versatile properties in solar cell applications. Visible light activity of TiO2 can enhance the efficiency of the third-generation solar cells by acting itself as light absorbing material. In this study, we demonstrate a surface structure modification and band gap tuning of TiO2 nanotube arrays (TNTA) by anodization accompanied by a short-term, quick annealing process. This TNTA shows absorption both in the UV and entire visible range (400–700 nm, an increase by 350%). The effective band gap is found to be 1.7 eV. Through a detailed analysis we show that the significantly enhanced visible light absorption in the entire visible range is due to the substitutional and interstitial carbon atoms on the surface which introduces a structural disorder and localized states at the surface compared to the bulk. Based on the results from photoemission spectra, the probable band gap modeling shows a band bending at the surface. This behavior points to the excessive electronic conduction at the surface which has both merits and demerits in the applications of TNTAs in photocatalytic and photovoltaics in terms of surface recombination. This is confirmed by a solar cell device fabrication and testing.

A controlled, one-step ammonolysis method with three different Fe/Ga ratios (70:30, 50:50, and 30:70) was used to synthesize functional nanocomposite materials consisting of 30−40 nm particles of γ′-Fe4N Fe4−x Ga x N phases in a GaN and Fe-doped GaN phase matrix. The γ′-Fe4N, Fe4−x Ga x N, and GaN phases were confirmed by the Rietveld analysis of the X-ray diffraction patterns, and the Fe-doped GaN phase was confirmed from Mössbauer spectroscopy and magnetization measurements. The magnetization of the superparamagnetic nanoparticles was expectedly reduced with the Ga incorporation in the samples. The coexistence of antiferromagnetic Fe4−x Ga x N and the ferromagnetic γ′-Fe4N phases resulted in an exchange bias effect (hysteresis loop shift of 28 Oe at 5 K) and a gradual magnetic phase transition from 250 to 55 K. Mössbauer spectroscopic analysis showed a hyperfine magnetic field distribution that the ab initio calculations duly supported. Ab initio calculations of the equilibrium lattice and Mössbauer hyperfine parameters were also performed.