Thermally stimulated currents (TSCs) of layered organic photoconductors (OPCs) were measured by excitation of the bisazo pigment to generate carriers. The various TSCs were measured varying the condition of irradiation (temperature, electric field) and methods for current measurement (electric field). The amount of collected carriers was measured with TSC and discussed in terms of carrier generation mechanism. Generation, recombination and separation processes of ion-pairs were analyzed by comparing temperature and/or electric field dependence on carrier generation efficiencies of asymmetrical azo pigment based sample and of an unsymmetrical azo pigment based sample. Observed differences originating from carrier generation materials (CGMs) and carrier transport materials (CTMs) were discussed. The difference in sensitivity among CGMs combined with CTMs is mainly determined by the process of initial ion-pair (or charge transfer state) formation at the interface between CGM and CTM.
Electron-transfer proteins (ETPs) play important roles in biological functions such as photosynthesis, respiration, nitrogen fixation and others. In response to electron acceptance or donation of an ETP, atoms around the active center in it reorganize by an amount of energy λ. The reorganization energy λ is an important physical quantity determined by each ETP, regulating electron-transfer reactions by it. Let us measure currents through an ETP in scanning tunneling microscopy (STM) with and without photoirradiation. It is pointed out that a difference in the threshold bias voltage for rise of the current becomes a direct measure of λ. Simultaneously, the redox potential is obtained. Such measurements for various ETPs would provide a fundamental basis for understanding their functions in biological organisms.
Reductive energy generated by using photocatalysts under UV light can be stored in coupled energy storage materials, and the stored energy can be used in dark. TiO2 and SrTiO3 are used as photocatalysts and WO3 and MoO3 as energy storage materials. The energy storage is possible not only in electrolytes but also in non-electrolytic media including pure water and humid air. The mechanism of the energy storage by photocatalysts and charge-discharge behavior are discussed as well as their applications. Energy storage photocatalysts retain their anti-corrosion effect and anti-bacterial effect even after the UV light is turned off.
Research on the water splitting into H2 and O2 using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis, is introduced. It was found that the H2 evolution took place on a Pt-SrTiO3(Cr-Ta-doped) photocatalyst using an I− electron donor under the visible light irradiation. And the Pt-WO3 photocatalyst showed an excellent activity of the O2 evolution using an IO3− electron acceptor under visible light. Both H2 and O2 evolution was observed with a stoichiometric ratio (H2/O2 = 2) for more than 250 h under visible light using a mixture of the Pt-WO3 and the Pt-SrTiO3(Cr-Ta-doped) powders suspended in NaI aqueous solution. It is the first finding that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I−/IO3− redox mediator. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.
N-doped titanium oxide photocatalyst is found to be activated by visible light irradiation as well as ultraviolet irradiation. We evaluated its crystal structure, N states in the lattice, composition, and surface morphology using X-ray diffiraction (XRD), X-ray photoemission spectroscopy (XPS), and scanning probe microscopy (SPM). As a result, it is found that the materials have rutile or anatase crystal structure with nitrogen doped into substitutional sites of oxygen in TiO2, which contributes band gap narrowing. It is also confirmed that XPS measurement was influenced by surface treatment such as Ar ion bombardment.
New visible-light-driven photocatalysts have been developed by doping of transition metal ions into photocatalyst materials, TiO2, SrTiO3, and ZnS, which have wide band gaps. TiO2 co-doped with chromium and antimony ions and Pt-loaded SrTiO3 co-doped with chromium and antimony or tantalum ions evolved O2 and H2 from water containing sacrificial reagents, respectively. In these photocatalysts, the visible-light-responses were due to the electron donor levels in the band gaps formed by doped transition metal ions. Moreover, maintaining of the charge balance in the titanate photocatalysts by co-doping of Cr3+ with high-valent ions (Sb5+ and Ta5+) played an important role for suppression of recombination between photogenerated electrons and holes, resulting in showing high photocatalytic activities. On the other hand, transition metal ion-doped ZnS photocatalysts efficiently evolved H2 from the water containing sacrificial reagents even without Pt co-catalysts.
The analyses of the wavelength-dependent photocatalytic activity by taking action spectra are useful for the evaluation of an active crystalline phase of semiconductor photocatalysts that contain two or more polymorphic crystalline phases and/or the different chemical compositions of different photoabsorption characteristics. The action spectra of photocatalytic reactions of H2 evolution, acetic acid decomposition, and Ag deposition were measured by using powdered titanium (IV) oxide (TiO2) photocatalysts of anatase-rutile mixtures. The discrimination of anactive crystalline phase in powdered photocatalysts was performed by the comparison between the action spectra of photocatalytic reactions and the diffuse reflectance spectra of TiO2 powders. The active phase was different depending on the kinds of photocatalytic reactions even if the same mixed TiO2 powders were used as photocatalysts; the photocatalytic activity of H2 evolution was not so different in the anatase and rutile phases included in the mixed TiO2 powders, while anatase or rutile phases were more active for the acetic acid decomposition or Ag deposition reactions, respectively.
Behavior of the electrons and holes photogenerated in TiO2 particles was observed by time-resolved infrared absorption spectroscopy in the presence of oxygen, water, and methanol vapor. The electrons photogenerated by a band-gap excitation displayed a structureless, broad absorption of IR light from 3000 to 900 cm−1, which was assigned to the intra-conduction-band transition and/or excitation from mid-gap traps to the conduction band. This electron-induced absorption was probed as a function of time delay after the photoexcitation. Electron decay caused by the recombination with holes and by the charge-transfer reactions with adsorbates were kinetically analyzed. The electron decay was accelerated in the presence of oxygen gas due to an electron-capture reaction at the interface, whereas was decelerated in methanol vapor due to an effective hole-capture by methoxy groups. On platinized TiO2 particles exposed to water vapor, a hole-capture reaction completed within 2 μs after band-gap excitation, whereas the electron-capture reaction occurred in 10 μs or later. These results demonstrate the effectiveness of this method to identify individual steps of photo-induced reactions at interfaces.