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This work investigates the effect of thermal annealing on the crystalline structure and optical properties of hydrothermal titanate nanotubes (TNTs). It shows that subjecting TNTs to air at 400 • C can be used to form a junction between TNTs and TiO 2 nanoparticles. Furthermore, the TNT microstructure is transformed from monoclinic to anatase phase by the annealing process. As a result of the self-junction formation, the visible photocatalytic activity in the degradation of methylene blue is enhanced in the thermal annealed sample in which the pseudo-first-order model fits well with the kinetic reaction rates of the TNTs and thermal TNTs with values of 0.0034 and 0.0082 min −1 , respectively. Insights into the improvement in photocatalytic activity are revealed by ultraviolet-visible diffuse reflectance and photoluminescence emission spectroscopies.

In this study, we synthesized graphene oxide (GO) by modified low temperature Hummer’s method and in situ hydrothermally grown TiO­2 nanotube (TNT) onGO sheet. Transmission electron microscopic (TEM) images showed the homogeneous formation of TNT with the mean diameter of ~8 nm and the co-existence of TNT and GO in the composite sample. X-ray differaction pattern of GO indicated the successful fabrication. The UV-vis measurement with methylene blue indicated the improvement of physical adsorption of the composite samples. Keywords TNTs, GO, physical adsorption, composite References [1] C. Dette et al., “TiO¬2 Anatase with a Bandgap in the Visible Region,” Nano Lett., vol. 14, no. 11, pp. 6533–6538, 2014.[2] A. Ibhadon and P. Fitzpatrick, “Heterogeneous Photocatalysis: Recent Advances and Applications,” Catalysts, vol. 3, no. 1, pp. 189–218, 2013.[3] Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Letters, vol. 11, no. 7. pp. 2865–2870, 2011.[4] C. L. Wong, Y. N. Tan, and A. R. Mohamed, “A review on the formation of titania nanotube photocatalysts by hydrothermal treatment,” Journal of Environmental Management, vol. 92, no. 7. pp. 1669–1680, 2011.[5] Z. Bo et al., “Synthesis and stabilization of small Pt nanoparticles on TiO2partially masked by SiO2,” Appl. Catal. A Gen., vol. 551, pp. 122–128, 2018.[6] K. Bubacz, B. Tryba, and A. W. Morawski, “The role of adsorption in decomposition of dyes on TiO 2 and N-modified TiO 2 photocatalysts under UV and visible light irradiations,” Materials Research Bulletin, vol. 47, no. 11. pp. 3697–3703, 2012.[7] M. Faraji and N. Mohaghegh, “Ag/TiO2-nanotube plates coated with reduced graphene oxide as photocatalysts,” Surf. Coatings Technol., vol. 288, pp. 144–150, 2016.[8] L. C. Sim, and K. H. Leong, “Graphene oxide and Ag engulfed TiO2 nanotube arrays for enhanced electron mobility and visiblelight-driven photocatalytic performance,” Journal of Materials Chemistry A, vol. 2, no. 15. pp. 5315–5322, 2014.[9] C.-Y. Tsai, C.-W. Liu, C. Fan, H.-C. Hsi, and T.-Y. Chang, “Synthesis of a SnO 2 /TNT Heterojunction Nanocomposite as a High-Performance Photocatalyst,” J. Phys. Chem. C, vol. 121, no. 11, pp. 6050–6059, 2017.[10] S. Gayathri, M. Kottaisamy, and V. Ramakrishnan, “Facile microwave-assisted synthesis of titanium dioxide decorated graphene nanocomposite for photodegradation of organic dyes,” AIP Adv., vol. 5, no. 12, 2015.[11] H. Tao, X. Liang, Q. Zhang, and C. T. Chang, “Enhanced photoactivity of graphene/titanium dioxide nanotubes for removal of Acetaminophen,” Appl. Surf. Sci., vol. 324, pp. 258–264, 2015.[12] M.Z. Wang , F. X. Liang , B. Nie , L.H. Zeng , L. X. Zheng , Peng Lv , Y. Q. Yu , C. Xie, Y. Y. Li, “TiO2 Nanotube Array/Monolayer Graphene Film Schottky Junction Ultraviolet Light Photodetectors." Part. Part. Character. Syst., vol. 30, 7, pp. 630-636, 2013. [13] J. Yu, T. Ma, and S. Liu, “Enhanced photocatalytic activity of mesoporous TiO 2 aggregates by embedding carbon nanotubes as electron-transfer channel,” Phys. Chem. Chem. Phys., vol. 13, no. 8, pp. 3491–3501, 2011.[14] H. L. Poh, F. Šaněk, A. Ambrosi, G. Zhao, Z. Sofer, and M. Pumera, “Graphenes prepared by Staudenmaier, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties,” Nanoscale, vol. 4, no. 11, p. 3515, 2012.[15] P. B. Arthi G and L. BD, “A Simple Approach to Stepwise Synthesis of Graphene Oxide Nanomaterial,” J. Nanomed. Nanotechnol., vol. 06, no. 01, 2015.[16] M. Faraldos and A. Bahamonde, “Environmental applications of titania-graphene photocatalysts,” Catal. Today, vol. 285, pp. 13–28, 2017.

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In this work, we successfully fabricated homogeneous hydrothermal titanate nanotubes (TNTs) coated with Ag nanoparticles (NPs) and elucidated the role of Ag NPs on local surface plasmonic resonance, surface-enhanced Raman scattering, and the enhanced photocatalytic activity of TNT/Ag nanocomposite. The results showed that the photodegradation process reached equilibrium in just ~5 min for the TNT/Ag nanocomposite, which was much shorter than that of the TNT sample (~90 min). TEM micrographs showed that Ag NPs were well dispersed on the walls of the nanotubes. XRD patterns and Raman spectra indicated that the TNTs were in the monoclinic structure of H2Ti3O7. Furthermore, Raman active modes of the TNTs were significantly enhanced in the TNT/Ag sample, which was attributed to surface-enhanced Raman spectroscopy. The enhanced photocatalytic activity of the TNT/Ag sample was explained by UV-vis diffuse reflectance spectroscopy and photoluminescence emission spectroscopy, which showed local surface plasmonic resonance-induced visible light absorption enhancement and effective charge separation, respectively.