Analytical Spectroscopy

Minute Paper (2009.10.25)- Antonio Campos
Title: Surface Mediated Visible-Light Photo-oxidation on Pure TiO2(001)
By: Hiroko Ariga et al
Journal: JACS
Link: Could not access JACS through my U account due to system outage…
DOI: 10.102/ja9066805

The authors show that the TiO2(001) face has photochemical activity under visible light exctiation. The band gap for bulk TiO2 is 3.0-3.2 eV (near UV, 3.0 eV = 414 nm), which would require UV light excitation to decompose formic acid. However, the authors report that it is possible to decompose formic acid into H20 and CO2 using a single crystal of TiO2 with broadband visible light in the presence of O2 making it possible to perform this decomposition using solar energy ( the major components of solar light are in the visible region). The techniques used in this paper are scanning tunneling microscopy (STM), electron energy loss spectroscopy (EELS), and density functional theory (DFT).

The authors note that it is possible to red shift the band gap towards visible light through doping, but there is not literature citing the photochemical activity of the surface of TiO2. The authors find through STM, the surface of TiO2 is active at about 2.3 eV (539 nm). Formic acid is shown to dissociatively adsorb as a formate ion on the TiO2(001) surface, where there is a 4 fold coordination axis of Ti4+. In order for the formic acid to decompose to H2O and CO2, the use of O2 was needed. The authors found that the reaction rate was proportional to the O2 pressure. Under an O2 pressure of 1x10-6 Pa and broadband visible light excitation (2.1-2.8 eV), the decomposition of formic acid behaved as it was would when exposed to UV light.

While experiments in this paper are reasonable, the authors do not conclusively prove the energy needed for this process to take place. In the experimental details, the authors conclude that the band gap is 2.6 eV on the TiO2(001) face, but use EELS to prove the energy of the band gap and state that EELS overestimates the band gap energy. They go on to say that because the EELS did not disprove the existence of a 2.6 eV band gap, the band gap that is possible is 2.6 eV. The authors leave out some of the explanation as to why they concluded that this was the band gap energy. Prior to this statement, the authors state that the photo-oxidation of formic acid does not take place below 2.1 eV. However, the DFT calculations on this system did corroborate the 2.6 eV band gap by comparing the calculation of TiO2(110) and TiO2(001).

From these promising results, a possible next experiment is the optimization of the ordering system in the TiO2 to yield crystals that are more uniformly ordered like the surface. If particles can be designed to photochemically decompose formic acid into H2O and CO2 using solar power, the use of the process could gain widespread use. Perhaps if the efficiency of this process is high, then you could conceivably use formic acid gathered from ants or carabid beetles to drive photosynthesis in a greenhouse. While, both ants and carabid beetles use formic acid as a defense mechanism, extracting the formic acid could prove to be a bit of an annoyance to the insects, using formic acid to drive photosynthesis would be an interesting use of these crystals. Another use of the crystal could be as a solar driven filter for toxic chemicals, if the authors could expand the physical properties of the crystals to include decompostion of some common toxic substances such as those found in cigarette smoke.