The sub-bandgap light-heat synergism induced electronic transition and their role in photocatalysis were revealed by means of in situ diffusion reflectances and (photo) conductances.
The present research showed that the non-heating effect of plasmonic absorption caused a great increase in the acetone dehydrogenation over Ag nanocubes in high selectivity at low temperatures.
The catalytic reduction of p-nitrophenol (4-NP)
to 4-aminopyridine (4-AP) over Au nanoparticles can be increased by
light illumination. Whether this is caused by the plasmonic effect
remains unclear. The present research carried out a careful examination
of the effects of light illumination and temperature on the catalytic
conversion of 4-NP to 4-AP over Au nanorods. It was seen that light
illumination has no effect on the apparent activation energy; this
indicates that the catalytic mechanism is unchanged and the activity
increase cannot be attributed to the effect of hot electrons. Based
on the simulation of finite-difference time domain, the theoretical
analysis also showed that plasmonic heating cannot play a major role.
Thermographic mapping showed that the temperature of water solutions
shows an increase under light illumination. By taking this temperature
increase into consideration, the light-induced increase of the 4-NP
to 4-AP conversion can agree well with dark catalysis, which cannot
be attributed to the plasmonic effects of the Au nanorods.
In situ diffusion reflectances reveal the trapping-filling effect in the electron transfer from TiO2 to O2 and Laplace transform was developed to derive the broadened apparent barrier energy distribution.
On-line optical absorptions were monitored under steady light illuminations to study the electron relaxations happening through the transfer from nano-TiO2 to O2, which are found to be slow and dispersive. A quasi-equilibrium (QE) theory and Monte Carlo simulations are developed to model the electron transfer, and they give good fittings to the early stage electron relaxations (over 70%). It is shown that the electron QE population at traps is kept during the whole electron relaxations. The slow kinetics is attributed to both the low probability (ptr) for an electron transferring to an O2 from a trap and the multi-trapping transport. The dispersive feature is ascribed to the dynamic decrease in the quasi-Fermi level (EF). The electron transfer rate constants just after the termination of light illuminations are taken out from the QE model fittings to analyze the relaxation kinetics. It is found that O2 amounts mainly affect the electron transfer by changing ptr; light intensities and temperatures mainly affect the electron transfer by changing the multi-trapping transport. The difference between the conduction band edge and the EF is the thermal barrier of the electron transfer from TiO2 to O2. The apparent activation energy (Eapp) of the electron transfer, determined from the absorption decays measured at different temperatures, is smaller than the real thermal barrier because of the decrease of EF with temperatures. The disagreement between the simulations and the later stage relaxations is not caused by the none-QE electron distribution at deep traps, and additional deep traps with a different distribution should also contribute to the electron relaxations.
A quasi-equilibrium (QE) theoretical model is proposed to fit the slow dispersive electron relaxation of nano-TiO2 that occurs through the transfer to O2. The electron relaxation is obtained from measurement of photoinduced absorptions. By including both the traps with exponential and Gaussian distributions with respect to the energy, the electron relaxation is fully fitted with the QE model. A Monte Carlo simulation is also realized to fit the electron relaxation, which agrees well with the QE model. It is revealed that the kinetics of the electron transfer from TiO2 to O2 contains both contributions from the exponential and Gaussian traps. Their distributions are obtained from the QE model fitting. The dispersion factor of the exponential traps is ∼0.65 and the trap density is high. The Gaussian traps locate ∼0.4 eV below the conduction band and have narrow distribution. The density of the Gaussian traps is more than three orders of magnitude lower than that of the exponential traps. Despite the low density, the Gaussian traps have an important effect on the electron relaxation. The distributions of the thermal barriers for the electron relaxation are obtained for both relaxations contributed by the exponential and Gaussian traps, based on which the kinetics equations are proposed. The Gaussian trap contributed relaxation accords with mono-exponential kinetics, while the relaxation contributed from the exponential traps involves exponentially distributed weights. The apparent activation energy, kinetic time constants, and pre-exponential factor can be obtained.
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