1. Titanium Dioxide Sensitized with Porphyrin Dye as a Photocatalyst for the Degradation of Water Pollutants
Kevin Reyes, A.S. & Ivana Jovanovic, Ph.D.
Department of Chemistry, New York City College of Technology, CUNY
Abstract
Methods
Results
TiO2 was sensitized with a porphyrin, Tetra(4-carboxyphenyl) porphyrin(TCPP) and used as photocatalyst under visible light for degradation of aqueous solutions of methylene blue (MB) dye. The quantification of the degradation of MB was assessed by UV-Vis spectroscopy. The results of this procedure showed the decrease in concentration of MB over time when reaction is exposed to visible light. The TiO2-TCPP complex was tested as well for the recovery of metals from the AgNO3 aqueous solution. The reduction of silver cations to silver metal was observed through formation of black precipitate.
Preparation of  TiO2 -TCPP complex:
16mg of TCPP was dissolved in
30 mL of ethanol and mixed with
400mg of TiO2. The mixture was
stirred overnight away from the
light to produce TiO2 coated with
TCPP. The mixture was centrifuged
to separate solid TiO2-TCPP complex,
and washed with ethanol and water
three times. The brownish/pinkish
solid of TiO2-TCPP complex was let
to dry for 24h (fig 2).[2]
The photocatalytic activity of TiO2 under UV and visible light:
TiO2 - TCPP complex as photocatalyst under visible light:
30mg of TiO2 -TCPP and 3.0mM MB solution was let to equilibrate for 1h in the dark, then irradiated with the visible light lamp for 2 h. A control test was preformed - same sample left in the dark for 2h. The UV-Vis spectrum of MB was observed over the course of 2h. The irradiated solution was left in the sunlight for 3 days, and the dark solution was kept in the dark (resulting solutions fig 6).
TiO2 -TCPP as photocatalyst for precipitation of metals:
30mg of TiO2-TCPP was suspended in water. And mixed
with 5 ml 1mM AgNO3 and 1 ml of 2-propanol[3] and
irradiated under visible light for 2h, then left in the
sunlight for 3 days (fig 7). The black precipitate
was formed indication of the reduction of Ag+ to Ag 0. A C A B B D
Fig. 2. A. The TiO2-TCPP complex powder seen to have a brown color as opposed to B. the white color of solid TiO2.
Introduction
Fig. 4. Absorption spectra of 1.5mM MB monitored over 2h of irradiation with A. UV lamp (660nm band decreased) and C. UV-Vis lamp (660nm decreased very little) when TiO2 as photocatalyst was used. B. and D. The linearized percentage of initial concentration over time. A B
Two 1.5mM aq. solutions of MB were mixed with 30mg of TiO2 and left for 30 min in the dark to equilibrate. One was irradiated with a UV lamp and 2nd with visible spectrum LED lamp for 2h (fig 3). Every 30 min during the irradiation period, an aliquot was taken from each, solution, filtered and examined by UV-Vis spectrophotometer. A C A B C
Fig. 1.
A. The tetragonal crystalline structure of TiO2, the red molecules being oxygen and the gray molecules being titanium.
B. The molecular structure of
Tetra(4-carboxyphenyl) porphyrin (TCPP) C. Rendition of TCPP attaching to the surface of a TiO2 crystal.[2] B D
Fig. 3. A. The UV light set-up with the TiO2-MB solution (right), and MB only solution (left). B. The visible light set-up with the MB only solution(front), the TiO2-MB solution (back left), and TiO2-TCPP-MB solution(back right).
Fig. 5. Absorption spectra of  3.0mM MB monitored over 2h of irradiation with A. UV-Vis lamp and C. in dark (no light) using TiO2-TCPP as photocatalyst. 660nm band decreased over time in both samples, significant difference was not observed.   B. and D. The linearized percentage of initial concentration over time.
Titanium dioxide is a non-toxic, white, crystalline, semiconductor that has applications across many industries, from its use as a food additive, to solar cell panels. It is useful as a photocatalyst under UV wavelengths due to its electronic properties. More importantly, when treated with porphyrins, it is observed that the photocatalytic properties of TiO2 shift to be more sensitive under the visible light spectrum. This molecular complex (fig 1) becomes a cost-effective candidate for the degradation of water pollutants. When excited, the TiO2 valence electrons have enough energy to produce highly reactive oxidizing species(ROS) such as hydroxyl(•OH) and superoxide(O2•-) radical anions that can promote the degeneration of water pollutants.[1]
After 3 days in sunlight and dark A B
Fig. 6. A. TiO2-TCPP in 3.0mM MB solution left in sunlight. Solution colorless, no band at 660 nm was observed.  B. TiO2-TCPP in 3.0mM MB solution left in the dark; purple color is indication of TCPP in solution, confirmed with the absorption spectra showing bands of porphyrin.
Conclusion
TiO2 sensitized with TCPP was observed to be a viable photocatalyst for the degradation of methylene blue in an aqueous solution, as well as a viable reducing agent for cationic metals from an aqueous solution. Further research is required to examine the adsorption effect of MB to TiO2 to explain similar result obtained in the light and dark samples. This research can be expanded to the degradation of different “pollutants”. Further applications can be for the reduction of metal cations to form metal nanoparticles which have applications across many scientific fields.
References
[1] Pelaez, M. et al. Applied Catalysis B: Environmental 2012, 125, 331–349.
[2] Wang, H. et al. Materials Research Bulletin 2014, 57, 311–319.
[3]Pearson, Andrew, et al.  Langmuir, vol. 27, no. 15, 2011, pp. 9245–9252.
Fig .7. Precipitation of Ag metal
Acknowledgements
Special thanks to Professor Hamid Norouzi, Director of Undergraduate Research, and the Emerging Scholars Program at the New York City College of Technology.