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    Tuning the Band Gap in Titanium Dioxide Thin Films by Surfactant-Mediated Confinement and Patterning of Gold Nanoparticles

    Year: 2017

    Journal: J. Phys. Chem. C, Volume 121, OCT 5, page 21311–21323

    Authors: Mukherjee, Smita; Das, Pradip Sekhar; Choudhuri, Madhumita; Datta, Alokmay; Ghosh, Jiten; Saha, Biswajit; Koshmak, Konstantin; Nannarone, Stefano; Mukhopadhyay, Anoop Kr.

    Organizations: CSIR; DST; ICTP

    Au nanoparticle (NP)-decorated titanium dioxide (TiO2) thin films (Au-TO), prepared by a unique surfactant-assisted 2D self-assembling technique of Au NP layering onto a titanium dioxide matrix (TO) with molecular level control, in conjunction with a classic sol-gel route, showed a significant decrease in the optical band gap (Delta E-g = similar to 0.6 eV) of Au-TO films compared to the conventional sol-gel-prepared pristine counterpart. Strong dependence on surfactant type and deposition temperature of the 2D Au NP layer was observed upon a band gap decrease of the films. Unlike spin-coated Au NP overlayers on TiO2, which resulted in Au NP agglomeration, in this modified inverted Langmuir-Schaefer (MILS) technique, the organic surfactant induced 2D patterned confinement of Au NP layers on TiO2, causing an increase in the active surface area of the Au NP-TiO2 interface. Results of X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy indicated changes in crystal structure as well as in electronic states at the O k absorption edge for Au NP-surfactant patterned films, changes being maximum for films showing a maximum band gap decrease. A temperature evolution of morphology of Au-TO films and their Au NP-surfactant monolayer counterpart at the air-water interface, by ellipsometric imaging and Brewster angle microscopy, respectively, revealed that Au NP patterning was induced by surfactant and varied with the temperature of the Au NP-surfactant monolayer, the band gap decrease being closely associated with a change in the active Au NP-TiO2 surface area. The maximum band gap decrease was observed for Au-TO films having tree- or finger-like 2D Au NP patterns, clearly indicating that an increase in active Au-TiO2 surface area causes enhancement in structural changes and hence a greater band gap decrease of the system. Results demonstrate the potential of the MILS technique of surfactant-aided Au NP-patterned confinement in semiconducting oxides, in band gap engineering of the latter.
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