Time-Resolved Spectroscopy of Defect-Induced Effects on Photocarrier Dynamics in SrTiO3 Powder
Authors: Yamakata Akira , Kato Kosaku 
Published: December 2020

Photocatalytic water splitting has attracted much attention as an environment-friendly way to produce hydrogen gas as energy resources. Photocatalysts are usually used in powder form in view of their low cost and large surface area. Powder particles are rich in surface defects. It has been widely believed that defects work as electron-hole recombination centers and decrease the photocatalytic activity. Nevertheless, in fact, photocatalytic reactions proceed well with powders. Moreover, it has been reported that introducing more defects can sometimes elongate the photocarrier lifetime and improve the photocatalytic activity. However, the effects of powder defects on photocarrier dynamics have not been fully understood yet. Herein, we studied the effects of defects on photocarrier recombination processes by comparing powder and a defect-less single-crystalline SrTiO3. We also compared photocarrier dynamics in undoped and Na-doped SrTiO3 to investigate the roles of defects on photocatalytic reaction.
A broadband time-resolved absorption spectroscopy was conducted to observe the photocarrier dynamics. In the case of the single-crystalline SrTiO3, most of the photoexcited electrons survive as free electrons without being trapped by defects. The absorptions by free electrons hardly decayed in the picosecond region. However, the absorption decayed almost single-exponentially in the nanosecond region, and the intensity decreased to less than one-hundredth the original value by 150 ns. On the other hand, in the case of the powder SrTiO3, a large portion of the photogenerated carriers are trapped by defects. The free and trapped electrons decayed rapidly even within a few picoseconds, where trapped electrons decreased more rapidly than free electrons. This result indicates that defects accelerate the recombination just after the excitation. However, the electrons in the powder decayed not in a single-exponential but a multi-exponential manner, and their decay decelerated gradually. Especially, the decay of the trapped electrons became slower than that of free electrons within ~200 ps. Some of the photocarriers survive into the millisecond region. When the decay of electrons in the single-crystalline and powder SrTiO3 were compared, the lifetime of photocarriers in the single crystal was longer than that in powder in a few nanoseconds. However, the photocarrier lifetime in powder became longer in the nanoseconds to milliseconds region than that in the single crystal.
These results suggest that defects in powders initially accelerate the photocarrier recombination, but later they decelerate it. This dualistic property of defects in powder is considered to depend on whether electrons and holes are trapped in the vicinity or far away from each other. Powders contain a lot of defects, and most of the photoexcited carriers in powder are captured by these defects. Since the charge separation is not proceeded enough just after the excitation, electrons and holes will tend to be captured when in the vicinity of one another. The closely trapped electrons and holes can hop to approach each other. Therefore, the recombination will be accelerated in the picosecond region. However, once the electrons and holes escape from this initial recombination process, they can spread and get separated by longer distances. In this case, the electrons and holes need to travel long distances by repeated hopping or tunneling for recombination to come closer. This makes the photocarrier lifetime in powder longer than that in single crystal after the nanosecond region.
To further investigate the effects of defects in photocatalysts, we next compared the transient absorption of 4 mol% Na-doped SrTiO3 with that of undoped SrTiO3. We found that the trapped electrons in Na-doped SrTiO3 have longer lifetime than those in undoped one, and the number of surviving electrons in microseconds increased ~15 times. It is well known that substitution of cations by lower valence cations increases oxygen vacancies to keep the charge neutrality. When Na ions are doped into SrTiO3, Sr ions tend to be substituted by Na ions. This results in increasing the number of oxygen vacancies, which works as electron trapping sites. Moreover, the decay of trapped electrons was accelerated by introducing O2 gas. This indicates that the trapped electrons can still keep the reactivity with reactant molecules. Finally, we measured the photocatalytic activities of these photocatalysts. The rate of hydrogen evolution from pure water increased by ~200 times by Na-doping. This drastic enhancement could be accounted for by the increase of trapped electrons.
The results obtained in this study demonstrate that the trapped electrons are contributing to enhance the steady-state water splitting reactions. This confirms that appropriate defect control is important on improving the photocatalytic activity, because the long photocarriers lifetime increases the probability of photocarriers to meet with the reactant molecules.