Microscopic autofluorescence spectral imaging of chloroplasts in maize mesophyll cells using near-infrared laser excitation has previously shown that a photosystem I spectral component exhibits an intensity similar to that of photosystem II at ~294 K when a continuous-wave laser at 800-820 nm is used. To establish the generality of this phenomenon, chloroplasts in Parachlorella kessleri cells (P. kessleri) were studied. A continuous-wave laser at 785 nm promoted photosystem-I-specific fluorescence in P. kessleri chloroplasts. The difference in chlorophyll fluorescence peak wavelengths between P. kessleri and maize correlated well with those observed at cryogenic temperatures. To further clarify the nature of anti-Stokes fluorescence, we studied chloroplasts in acetone-treated P. kessleri cells (in a medium containing 15% (v/v) acetone) by microscopic fluorescence and absorption spectra on a cell-by-cell basis. A continuous-wave laser at 785 nm led to significant fluorescence from acetone-treated cells, which was attributed to aggregation of chlorophylls. Anti-Stokes fluorescence spectral imaging thus seems to be effective for detection of lowest-energy trap states that are only weakly fluorescent.

Heterogeneous photocatalytic water splitting under the irradiation of sunlight is an attractive method for generating hydrogen from water. While the photocatalytic mechanism has been extensively studied, most of the experimental studies have been performed with an ensemble of photocatalyst particles with various sizes, morphologies, and secondary structures. To gain a deeper understanding of the mechanism of photocatalysis, it is indispensable to clarify how the geometric structure of photocatalyst affects the kinetics of photogenerated carriers and redox reactions. In this study, the hole decay characteristics and photocatalytic activity of BiVO4, a promising photocatalyst for oxygen evolution with visible light, have been investigated with single-particle transient absorption microscopy. Upon irradiation with 527 nm light, well-faceted nonaggregated crystallites show fast hole decay and little reactivity for Fe3+ reduction. In contrast, aggregated particles with grain boundaries between small primary crystallites show slower hole decay and higher reactivity for Fe3+ reduction than the nonaggregated crystallites. This behavior is increasingly pronounced as the secondary particle size of aggregated crystallite increases. This indicates that grain boundaries in aggregated particles do not work as recombination centers but play an important role in elongation of carrier lifetime and thus in enhancing the reactivity of photocatalyst through trap–detrap processes.

Cocatalysts are usually needed to improve photoconversion efficiency in water splitting with heterogeneous photocatalysts. Here, we show that NiO x (0 < x < 1) nanoparticles loaded on a layered perovskite, BaLa4Ti4O15 (BLT), serve as an effective electron sink. We have measured the time profiles of transient absorption (TA) of BLT with and without loading NiO x cocatalyst in air and in water in a wide wavelength range from 400 nm to 4 μm upon excitation with a femtosecond pulse at 266 nm. TA at 4 μm indicates that photogenerated electrons are rapidly transferred to cocatalyst within 1 ps. The time profile of TA at 402 nm from sub-μs to 10 ms contributed by surface photoholes is affected by loading of cocatalyst and redox reactions with water. Fittings of the decay profiles of TA at 402 nm with a trap–detrap kinetic model indicate that the oxidation of water appreciably starts at around 1 ms after the pump pulse, while the reduction of water takes place prior to oxidation in much early time domains. This implies that the redox reactions take place under substantial imbalance between electron and hole densities.