•Solution processing of ITO/TiO2/ BiFeO3/poly(3-hexylthiophene) solar cell.•X-ray diffraction reveals polycrystalline BiFeO3 film with rhombohedral texture.•Solar cell produced 0.05 mA cm−2 and an open circuit voltage of −0.46 V.We report the fabrication of solar cells by spin-coating and annealing sol–gel layers of TiO2 and BiFeO3 on indium oxide coated glass substrates. A layer of poly(3-hexylthiophene) was spin coated onto the inorganic layers to form hybrid organic–inorganic solar cells. X-ray powder diffraction revealed the BiFeO3 layers are polycrystalline with rhombohedral texture and average crystallite size about 20 nm. Scanning electron microscopy images showed the films are granular and microporous. Irradiation with visible light resulted in a photocurrent of 0.05 mA cm−2 and an open circuit voltage of −0.46 V.

Nitrogen-doped TiO2 nanoparticles have been synthesized using sol–gel methods and subsequently fluorinated at room temperature by aging in acidic solutions of NaF. The nanoparticles were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, UV–vis, and IR diffuse reflectance spectroscopy. After aging at room temperature in NaF solutions, the Ti–OH groups on the surface of the TiO2 nanoparticles were replaced by Ti–F bonds, which resulted in a decrease of the point of zero charge from pH 5.4 to 2.8. Most importantly, the nitrogen dopants were retained after the fluorination process, and the amorphous nanoparticles were partially converted into the anatase phase. Annealing the photocatalysts resulted in a decrease of both the nitrogen and fluoride atomic concentration. Diffuse reflectance spectra show an increase in absorbance above 400 nm after annealing the F,N-doped TiO2, which suggests the formation of color centers. The photoactivity of the F,N-doped and N-doped TiO2 catalysts were evaluated by monitoring by the decolorization of methylene blue with visible light. Mass spectrometric analysis revealed that methylene blue undergoes successive demethylation, and more importantly, the rate of decolorization depends on the fluoride concentration. These results show the importance of a two-step synthesis method to independently control the nitrogen and fluoride concentration.

Tunneling spectroscopy was used to characterize a series of junctions with para-substituted (SH, F, Cl, Br, and I) benzoic acid monolayers positioned between aluminum/aluminum oxide surfaces and Pb electrodes. The tunneling conductance spectra revealed several effects that can be attributed to the size and reactivity of the para-substituent. First, as the size of the halogen substituent increased from fluoro to iodo, the extent of Pb penetration through the monolayer to the substrate was diminished. The extent of penetration was observed as a shift in the conductance minimum toward higher bias voltage. In the case of the 4-fluoro substituent, there was significant penetration of Pb to the bottom interface, and the conductance minimum was observed near 140 mV. In the case of the 4-iodo substituent, there was minimal Pb penetration, and the conductance minimum was ∼20 mV. Second, when the para-substituent was a thiol group, the deposition of lead results in Pb–S bond formation, which was confirmed by the absence of the S–H stretching mode in inelastic tunneling spectroscopy. The effect of Pb–S bond formation was to inhibit Pb penetration and invert the barrier height, which resulted in a shift in the minimum conductance to a negative bias. These results highlight the importance of tunneling spectroscopy to better understand contacting details at the metal–molecule interface.