Squaraine-sensitized composite of a reduced graphene oxide/TiO2 photocatalyst: π–π stacking as a new method of dye anchoring

Abstract

We synthesized a near infra-red (NIR)-absorbing squaraine dye (VJ-S) showing strong absorption and emission maxima at 684 and 704 nm, respectively, with a high molar extinction coefficient (ε) of 1.277 × 10⁵ M⁻¹ cm⁻¹ and a band gap of 1.77 eV. Its oxidation and reduction potentials were found to be 0.889 and −0.795 V, respectively, with HOMO and LUMO levels of −5.21 and −3.53 eV, respectively. We also prepared the self-assembled core/shell nanocomposite r-NGOT, where TiO₂ is the core and reduced nano-sized graphene oxide (r-NGO) is the shell. When VJ-S was anchored on r-NGOT, it showed π–π stacking with r-NGO, which is confirmed by Fourier-transformed infrared spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and electron energy loss spectroscopy. The optical absorption spectrum of the VJ-S/r-NGOT nanocomposite measured with diffuse reflectance UV/visible absorption spectroscopy covers the whole range of visible light wavelengths up to 800 nm. The photocatalytic activity of VJ-S/r-NGOT at visible light wavelengths (λ > 420 nm) is much higher than that of r-NGOT alone.