Trapped State Sensitive Kinetics in LaTiO2N Solid Photocatalyst: With and without Cocatalyst Loading

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by defects which can both enhance photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers is also very crucial to the performance of the photocatalyst [1]. Similarly, addition of cocatalyst is also known to enhance the photocatalytic activity of a material by promoting efficient charge separation.

In this work we report on the charge carrier dynamics in a visible-light responsive (oxy) nitride photocatalyst: LaTi₂ON with and without the presence of CoO_x cocatalyst. For femtosecond diffuse reflectance measurements, an amplified 150 fs Ti:Sa laser with OPA for pump (λ_exc= 500nm & 580nm) and white light continuum and IR light for probes are used [1]. Synthesis of LaTiO₂N and LaTiO₂N/CoO_x powder are described elsewhere [2].

Figure 1a shows the effect of change of excitation wavelength on the dynamics of the energetically shallow trapped charge carriers of LaTiO₂N powder (with and without CoO_x) probed at 880 nm. As is evident from the figure, shifting the excitation wavelength to longer λ_exc 580 nm, the kinetics of the relaxation from the shallow to deep traps slows down for unloaded LaTiO₂N. This hints at the existence of energetically distributed trapped states in unloaded LaTiO₂N powder which play an interesting role in the charge carrier kinetics making it excitation wavelength dependent. Possibly, direct trap filling of deep traps for higher λ_exc= 580nm slows down the relaxation process of charge carriers from shallow trap states. Kinetics of mobile conduction band electrons, which are observed by the IR probe, however remains unaffected to excitation wavelength changes.

Interestingly, co-catalyst loading is found to have a suppressing effect on this slowing down of this relaxation of shallow trapped electrons to deep-trapped states as well as a decrease in TA intensity of the shallow trapped carriers.  As can be seen in Figure 1b, the larger the amount of the cocatalyst, the faster is the decay and smaller the TA intensity. Thus, it is evident that cocatalyst loading has an impact on the carrier dynamics but, a better knowledge of the cocatalyst distribution, size etc. is necessary to strongly comment on the observation. At this point we lack this information.

However, based on literature we can very loosely comment at this stage that probably due to charge transfer from the photocatalyst to the cocatalayst (TA data implies in <1ps which is very fast), the decay becomes less sensitive to λ_exc.  Infact, a previous study on Co modified single crystalline LaTiO₂N reports ~27% quantum efficiency at 440nm [2]. CoO_x deposition was found to prolong the lifetime of mobile carriers (2000 cm^-1) to a time scale of 1 second.

Acknowledgement: This work is supported by “Research Project for Future Development: Artificial Photosynthetic Chemical Process (ARPChem)” (METI, Japan: 2012-2022).

Reference

[1] Tamaki, Y. Hara, K. Katoh, R. Tachiya, M. Furube, A., J. Phys. Chem. C, 2009, 113, 11741.  [2] Zhang, Y, Yamakata, A, Maeda, K, Moriya, Y, Takata, T, Kubota, J, Teshima, K, Oishi, S, Domen, K., J. Am. Chem. Soc. 2012, 134, 8348.