Photo-excited charge carriers play a vital role in photocatalysts and photovoltaics, and their dynamic processes must be understood to improve their efficiencies by controlling them. The photo-excited charge carriers in photocatalytic materials are usually trapped to the defect states in the picosecond time range and are subject to recombination to the nanosecond to microsecond order. When photo-excited charge carrier dynamics are observed via refractive index changes, especially in particulate photocatalytic materials, another response between the trapping and recombination phases is often observed. This response has always provided the gradual increase of the refractive index changes in the nanosecond order, and we propose that the shallowly trapped charge carriers could still diffuse and be trapped to other states during this process. We examined various photocatalytic materials such as TiO2, SrTiO3, hematite, BiVO4, and methylammonium lead iodide for similar rising responses. Based on our assumption of surface trapping with diffusion, the responses were fit with the theoretical model with sufficient accuracy. We propose that these slow charge trapping processes must be included to fully understand the charge carrier dynamics of particulate photocatalytic materials.
Photocatalytic water splitting is
a promising method to obtain
clean hydrogen in the near future, and a variety of materials for
this purpose have been examined. The charge carrier dynamics have
been studied for understanding the underlying mechanism of the interfacial
reactions, but inhomogeneous reactions due to the particulate materials
have been an obstacle for the distinction of types of charge carriers.
Herein, we demonstrated a strategy to differentiate the types of charge
carriers by measurement of local charge carrier dynamics and categorization
of carrier types on a photocatalytic surface. The local charge carrier
dynamics were obtained by the home-built pattern-illumination time-resolved
phase microscopy method. Two of the photocatalytic materials, Rh-doped
SrTiO3 and Mo-doped BiVO4, which are used in
particulate photocatalyst sheets for Z-Scheme overall water splitting,
were examined. From the distribution maps of charge carrier types
and their dependence on the scavengers, the local charge carriers
relevant to the water splitting were spatially separated and extracted
and provided local information on the surface-trapped charge carriers.
Photo-excited charge carrier dynamics in photocatalytic materials with rough surfaces have been studied via measurements using pattern-illumination time-resolved phase microscopy. Optimal defocusing is necessary for the phase-contrast detection of the refractive index change due to the photo-excited charge carriers. The signal enhancement of the phase-change was explained theoretically and experimentally. The optical phase variation due to the transmission of a rough surface is coupled with the quadratic phase term in Fresnel diffraction, and a slight defocusing can convert the phase image to the corresponding amplitude image. The phase-contrast image due to the photo-excited charge carriers is also enhanced by the defocusing. The explanation was supported by wave optics calculation, and the enhancement was demonstrated for two types of TiO2 substrates with different roughnesses.
The transient electromagnetic fields due to electrostatic discharge (ESD) between charged metals have wide-band frequency spectra up to the microwave region, which give a serious malfunction to high-tech information devices. For the above ESD fields, we previously analyzed them, using the finite-difference time-domain (FDTD) method, and showed that the metals enhance the field level according to the metal dimension. From the standpoint of reducing such the ESD fields, the electromagnetic fields caused by the spark between the metals with ferrite core attachments were investigated. The FDTD method was also used to compute the ESD fields. A FDTD algorithm for the magnetic field inside the ferrite core was newly derived. The results show that the cores attached near the spark gap considerably reduce the magnetic field level, which is also confirmed experimentally.
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