Energy-Resolved Measurement of Electron Traps in Metal-Oxide Particulate Photocatalysts By Newly Developed Reversed Double-Beam Photoacoustic Spectroscopy

Metal-oxide photocatalysts such as titanium(IV) oxide (titania) have long been called "semiconductor photocatalysts" presumably because they show n-type semiconducting properties of dark-current rectification and anodic photocurrent when they are used in the form of electrodes.  However, there seems no influence of such semiconducting properties on their photocatalytic activities when used in a particle form.  Then, why should metal-oxide photocatalysts be of semiconductor?  Based on the results and discussion on the metal-oxide photocatalysis, the authors came across a hypothesis that electron traps liberated by electron detachment from donor impurity levels below the bottom of conduction band (CB) in n-type semiconductors.  It is expected that deep traps may enhance recombination of photoexcited electrons and positive holes and shallow traps may enhance electron migration through trap–detrap mechanism.  Therefore, energy-resolved measurement of trap density is required to design highly active photocatalysts.

It is well known that photoirradiation of metal oxide particles or a single crystal in the presence of electron donors and in the absence of electron acceptors such as oxygen induces electron accumulation.  The energy levels in particulate titania samples have been estimated by a chemical titration method to be 100–200 meV below the CB bottom,¹ which is consistent with the hypothesis, though the energy resolution is not sufficient and alternative easier measurement technique should be developed.

A group of the authors has developed double-beam photoacoustic spectroscopy (DB-PAS) by which the total density of electron traps can be estimated in relatively short time of measurement.²  In DB-PAS system, two beams, modulated probe light and continuous pump light, are used to detect photoabsorption at the wavelength of the probe light induced by photochemical reaction induced by pump light; change in photoabsorption intensity, even for opaque samples such as powder, under photoirradiation can be measured and thus total density of electron accumulation in titania particles have been measured.  Recently, we have developed a new PAS system, reversed DB-PAS (RDB-PAS) modifying DB-PAS to make energy-resolved electron trap measurements possible.

In RDB-PAS, a titania sample was filled in a PA cell, methanol-saturated argon was flowed and the cell was tightly sealed.  The detection-light source was an LED (625 nm) and the output intensity was modulated by a digital function generator at 80 Hz.  In addition to the modulated light, continuous scanning monochromatic light was also irradiated for direct excitation from valence band to electron traps of a sample.  The acquired spectra were differentiated from long-wavelength side.  RDB-PA signal from titania samples was observed at the wavelength longer than the rise of PA spectra corresponding to the band gap (3.2 eV and 3.0 eV for anatase and rutile respectively).  It is suggested that RDB-PA signal corresponds to the accumulation of electrons into electron traps by direct excitation from valence band to electron traps.  Increase in RDB-PA signal was not observed when platinum (Pt) was loaded on titania due to electron transfer to Pt.  Derivative RDB-PA spectra of TIO-2 (anatase) and CR-EL (rutile) were shown in Fig. 1, where broad peaks at ca.0—0.45 eV and 0—0.25 eV, respectively, below the bottom of conduction band were observed.  It is presumed that derivative RDB-PA spectra correspond to the energy-resolved distribution of the density of electron traps in titania photocatalyst powders.

[1] Phys. Chem. Chem. Phys., 5, 778 (2003). 

[2] J. Phys. Chem. C, 111, 11927 (2007).