Convenient chirality transfer from organics to titania: construction and optical properties

Abstract

Polyethyleneimine (PEI) complexed with chiral d- (or l-) tartaric acid (tart) in water can self-organize into chiral and crystalline PEI/tart assemblies. It has been previously confirmed that the complexes of PEI/tart could work as catalytic/chiral templates to induce the deposition of SiO₂ nanofibres with optical activity but without outwards shape chirality such as helices. In this work, we found that the templating functions of PEI/tart were still effective to prompt the deposition of TiO₂ to form chiral PEI/tart@TiO₂ hybrid nanofibres under aqueous and room temperature conditions within two hours. Furthermore, the co-deposition of TiO₂ and SiO₂ was also fulfilled to yield chiral PEI/tart@TiO₂/SiO₂ nanofibres. These TiO₂-containing hybrid nanofibres showed non-helical shapes on the length scale; however, chiroptical signals with mirror relation around the UV-Vis absorption band of TiO₂ remarkably appeared on their circular dichroism (CD) spectra. By means of the protocols of XRD, TEM, SEM, UV-Vis, CD and XPS, structural features and thermoproperties of the chiral TiO₂ and SiO₂/TiO₂ were investigated.

Scheme 1. Synthetic procedures for chiral TiO₂- (top) and TiO₂/SiO₂-related products (bottom) by the hydrolysis and condensation of titanium bislactate (TiLact) and tetramethoxy silane (TMOS) in the presence of PEI/tart complexes resulted from polyethyleneimine (PEI) and tartaric acid (tart).

Fig. 1. (a) XRD patterns for d-PEI/tart, d-PEI/tart@TiO₂ (before calcination), and d-TiO₂-X (after calcination, calcination temperature X = 500, 600, 700 and 800 °C); (b) TG curve for d-PEI/tart@TiO₂.

Fig. 2. SEM image for (a) d-PEI/tart@TiO₂, (b) l-PEI/tart@TiO₂, (c) d-TiO₂-500 and (d) d-TiO₂-800; (e and f) the TEM images for d-TiO₂-500.

Fig. 3. (a) XRD patterns for d-PEI/tart@TiO₂/SiO₂ (red line), d-TiO₂/SiO₂-500 (green line, inset) and d-TiO₂/SiO₂-800 (magenta line); (b) the TG curve for d-PEI/tart@ TiO₂/SiO₂.

Fig. 4. SEM image for (a) d-PEI/tart@TiO₂/SiO₂, (b) l-PEI/tart@TiO₂/SiO₂, (c) d-TiO₂/SiO₂-500 and (d) d-TiO₂/SiO₂-800; (e and f) the TEM images for d-TiO₂/SiO₂-800; (g) the elemental mapping of d-TiO₂/SiO₂-800 (green for Ti, blue for Si, red for O).

Fig. 5. The DRCD (top) and UV-Vis absorption (bottom) spectra for (a) TiO₂-related samples and (b) TiO₂/SiO₂-related samples (solid lines for d-form samples and dotted lines for l-form ones; red lines for the hybrids; green and magenta lines for the samples calcined at 500 and 800 °C, respectively).

Fig. 6. XPS spectra of the samples of d-TiO₂ (black line), d-TiO₂/SiO₂ (red line), and achiral aTiO₂ (green line) calcined at 500 °C.