Fabrication, characterization, and optoelectronic properties of layer-by-layer films based on terpyridine-modified MWCNTs and ruthenium(III) ions

A novel full-conjugated 4-(2,2′:6′,2″-terpyrid-4′- yl) benzenediazonium tetrafluoroborate (diazo-tpy) was synthesized and used for surface modification of materials, such as quartz wafers, ITO glass, silicon, and multiwalled carbon nanotubes (MWCNTs). Under UV irradiation, the diazonium group of diazo-tpy is decomposed and the residual terpyridine group is covalently anchored to the surface of substrates. The obtained tpy-modified MWCNTs (tpy-MWCNTs) have good solubility in common organic solvents. TGA and HRTEM analyses confirmed that terpyridine groups have been symmetrically grafted on MWCNTs. The thickness of the tpy-modified monolayer is about 2.3 nm, which is approximately 2 times the axial length of the 4-(2,2′:6′, 2″-terpyrid-4′-yl)phenyl group. The introduction of terpyridine groups on the surface of MWCNTs provides a coordination site to complex with metal ions. Multilayer films were fabricated from tpy-MWCNTs and ruthenium ions [Ru(III)] via the layer-by-layer self-assembled (LBL SA) technique on the tpy-modified quartz wafer, ITO glass, or silicon. The UV-vis results indicate that (1) Ru(III)-tpy-MWCNT SA multilayer films are successfully formed based on the coordination interaction between ruthenium ions and terpyridine groups, and (2) a progressive assembly occurred regularly with almost an equal amount of deposition in each cycle. The SEM image showed a highly covered Ru(III)-tpy-MWCNT film on the substrate. Moreover, the optoelectronic conversion was also studied by assembling Ru(III)-tpy-MWCNT multilayer films on ITO substrates. Under illumination, the LBL SA films on ITO showed an effective photoinduced charge transfer because of their conjugated structure and the ITO current density changed with the number of bilayers. As the number of bilayer increases, the photocurrent intensity increases and reaches its maximum (∼65 nA/cm²) at six bilayers. These results allow us to design novel materials for applications in optoelectronic devices by using LBL SA techniques.