Visible Light Activity of TiO2 Induced by Organic Surface Complexes and Metal Ion Doping

Abstract

Titanium dioxide has been extensively studied for a great variety of photocatalytic processes because of its superior properties such as high photo-oxidation power, excellent chemical stability, non-toxicity, and low material cost. The photocatalytic reactions on TiO2 are initiated by bandgap (Eg) excitation and the subsequent created electron and hole pairs in conduction band and valence band, respectively. In general, the photoactivity of TiO2 is mostly limited to only UV region because of its wide bandgap that requires the excitation light wavelength range shorter than ca. 400 nm, which is a major drawback. Therefore, we developed various strategies to expand the photoactivity of TiO2 to the visible light region, as visible light accounts for 45% of incident solar energy. In the first topic, we investigated synergic effects of co-dopants of nitrogen and niobium in TiO2 and its photocatalytic activities under visible light. In the second topic, we studied enhanced visible light photocatalysis through a surface-complex charge-transfer between Nb-doped TiO2 and fullerol. In the third topic, the curcumin, an active ingredient of turmeric powder, as a TiO2 visible light-sensitizer was explored using photocatalytic and photoelectrochemical methods. Finally, in the fourth topic, squaraine dye-sensitized composite of reduced graphene oxide and TiO2 photocatalyst was investigated. 1. Nitrogen and niobium co-doped TiO2 was synthesized by a simple sol–gel method and its visible light photocatalytic activities were compared with bare, N-, and Nb-doped TiO2. The synthesized photocatalysts were characterized by XRD, DRS, FT-IR, XPS, and EDX. TiO2 co-doped with N and Nb can have unique properties distinguished from the singly doped TiO2 in the aspects of the dopant-induced charge distribution, vacancy sites, structural stability, defect energy levels, and optical absorption. The most notable is that the visible light absorption by (N,Nb)-TiO2 was significantly higher than either of N- or Nb-TiO2 sample. The doped samples exhibited a slight change in the band gap compared with bare TiO2, which could be confirmed by Tauc plot, Mott-Schottky plot, and the electronic structure calculation. The co-doping of N and Nb in TiO2 induced the creation of the mid-gap levels and Ti3+ states that enhance the visible light absorption. The photocatalytic activities were compared for the photocatalytic degradation of 4-chlorophenol, oxidation of iodide, and reduction of chromate in the aqueous phase under visible light. For all tested substrates, (N,Nb)-TiO2 exhibited the markedly higher activities than either N-TiO2 or Nb-TiO2. A similar trend was also observed for the photocurrent generation under visible light irradiation. 2. Visible light photocatalysis by TiO2 nanoparticles modified with both fullerol complexation and Nb-doping demonstrated an enhanced performance. Nb-doped TiO2 was firstly prepared by a conventional sol–gel method, and subsequently fullerol was adsorbed on the surface of Nb–TiO2. The physicochemical and optical properties of as-prepared fullerol/Nb-TiO2 were analyzed by various spectroscopic methods. The adsorption of fullerol on Nb–TiO2 surface increased the visible light absorption through a surface-complex charge-transfer mechanism. Nb-doping enhanced the charge transport and induced the Ti cation vacancies that retarded the recombination of photo-generated charge pairs by trapping the electrons injected from the HOMO level of fullerol. Due to the advantage of simultaneous modification of fullerol and Nb-doping, the visible light photoactivity of fullerol/Nb–TiO2 was more enhanced than either Nb–TiO2 or fullerol/TiO2. The photocatalytic activities of fullerol/Nb–TiO2 were all higher than bare TiO2 and singly modified TiO2 under visible light. A similar result was also confirmed for their photoelectrochemical behavior: the electrode made of fullerol/Nb–TiO2 exhibited an enhanced photocurrent under visible light. On the other hand, the decay of open-circuit potential of the fullerol/Nb–TiO2 electrode after turning off the visible light was markedly slower than either that of Nb–TiO2 or fullerol/TiO2, which implies the retarded recombination of photo-generated charge pairs on fullerol/Nb–TiO2. In addition, the electrochemical impedance spectroscopic data supported that the charge transfer resistance is lower with the fullerol/Nb–TiO2 than either Nb–TiO2 or fullerol/TiO2. This specific combination of the bulk and surface modifications of titanium dioxide might be extended to other cases of bulk + surface combined modifications. 3. The use of curcumin, an active ingredient of turmeric powder, as a TiO2 photo-sensitizer has been investigated using photocatalytic and photoelectrochemical investigations. Due to its visible light absorption and strong surface complexation, curcumin-sensitized TiO2 composite was used for photocatalytic reduction of chromate and phosphomolybdate ions. Using various spectroscopic methods in corroboration with photoelectrochemical and electrochemical impedance measurements, the visible light activity of curcumin-TiO2 was compared with TiO2 sensitized by traditional ruthenium complex in the presence of ethanol as electron donor. Due to the strong thermodynamic driving force between curcumin LUMO and TiO2 conduction band levels, curcumin-TiO2 demonstrated more efficient interfacial electron transfer compared to RuL3-TiO2. This resulted in enhanced photocatalytic and photoelectrochemical properties of curcumin-sensitized TiO2. Also, the higher pH stability of curcumin-TiO2 compared to RuL3-TiO2 was confirmed by correlating adsorption isotherms of both sensitizers in aqueous TiO2 suspensions and pH-dependent photocatalytic chromate reduction efficiencies. These results confirm the practical viability of using commercial curcumin as an efficient, eco-friendly, cheap and stable TiO2 photo-sensitizer. 4. We synthesized a near infra-red absorbing squaraine dye (VJ-S) showing strong absorption and emission maxima at 684 and 704 nm, respectively, with a high molar extinction coefficient (ɛ) of 1.277 × 105 M-1cm-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 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 is much higher than that of r-NGOT alone.