Photoelectrochemical (PEC) water splitting is a promising technology for the production of renewable solar hydrogen from water. A method of fabricating photoelectrodes from semiconductor particles for efficient water splitting was investigated using LaTiO 2 N as a test case. When a Ti conductor electrode with the back side covered with LaTiO 2 N particles was constructed by radio-frequency magnetron sputtering, the LaTiO 2 N photoelectrode generated a remarkable photoanodic current and evolved O 2 . The insertion of a Ta or Nb interlayer between the Ti conductor and the LaTiO 2 N particles enhanced the photocurrent. This method of fabricating photoelectrodes from semiconductor particles enables the use of simple powder semiconductors in solar energy conversion systems.

Tantalum oxynitride photoanode is fabricated and modified with calcium ferrite to form a heterojunction anode for a photoelectrochemical water splitting cell. The synthesized powders are loaded sequentially to the transparent conducting glass by electrophoretic deposition, which is advantageous to form a uniform layer and a junction structure. X-ray diffraction, UV-vis diffuse reflectance spectroscopy, scanning electron microscopy, and impedance spectroscopy analysis are conducted to investigate the structural, morphological, and electrochemical characteristics of the anode. The introduction of CaFe2O4 overlayer onto TaON electrode increases the photocurrent density about five times at 1.23 V vs reversible hydrogen electrode without any co-catalyst. Impedance spectroscopy analysis indicates that the junction formation increased photocurrent density by reducing the resistance to the transport of charge carriers and thereby enhancing the electron-hole separation. This photocurrent generation is a result of the overall water splitting as confirmed by evolution of hydrogen and oxygen in a stoichiometric ratio. From the study of different junction configurations, it is established that the intimate contact between TaON and CaFe2O4 is critical for enhanced performance of the heterojunction anode for photoelectrochemical water oxidation under simulated sun light.

Femtosecond to second time-resolved visible to mid-infrared absorption spectroscopy was applied to investigate the behavior of photogenerated electrons and holes on a Pt- or CoO x -loaded LaTiO2N photocatalyst. CoO x -loaded catalyst exhibits the highest activity for water oxidation under visible light (<600 nm) excitation, and the quantum efficiency reaches up to ∼30%. Transient absorption spectra suggest that most of the photoexcited electrons in LaTiO2N lose activity by deep trapping in the mid-gap states created at 0.74 eV (6000 cm–1) below the conduction band. In this case, Pt loading was not so effective for H2 evolution because the loaded Pt could not effectively capture the trapped electrons from LaTiO2N. The electron transfer was slow, proceeding in 0–100 μs, and was thus ineffective. However, in the case of CoO x loading, we have clearly observed, for the first time, that the holes are captured rapidly by CoO x in a few picoseconds, and the lifetimes of electrons are dramatically prolonged to the second region. This implies that the photogenerated holes and electrons are separated effectively in CoO x and LaTiO2N, respectively. Furthermore, the electron trap becomes shallower, its depth decreasing from 0.74 eV (6000 cm–1) to 0.49 eV (4000 cm–1) upon CoO x loading, suggesting that the reactivity of the trapped electrons increases. These perturbations of electrons and holes are what cause the dramatic increase in photocatalytic activity. We expected that coloading of Pt and CoO x would further increase the activity, but it was significantly reduced. It was demonstrated that the undesirable process of recombination is accelerated under high loading and coloading.

Photon upconversion (PUC) via triplet-triplet annihilation (TTA) from near-infrared (NIR) to blue photons could have important applications especially to bio-imaging and drug delivery accompanied by photochemical reaction. The fundamental challenges in achieving this has been the large anti-Stokes shift combined with the need to efficiently sensitize within the biological transparency window (700-900 nm). This calls for materials combinations with minimal energy losses during sensitization and minimal energy requirements to drive efficient TTA. Here, we demonstrate efficient PUC converting from NIR energy to blue photons using the commercially available material 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) as the annihilator. With a conventional triplet sensitizing system, TIPS-Ac performed TTA efficiency of 77 ± 3 % despite a relatively small driving force, compared to conventional TTA material converting from NIR to blue, for the TTA of less than 0.32 eV. Combined with Pt(II) meso-Tetraphenyl Tetrabenzoporphine (PtTPBP), which is a heavy atom triplet sensitizer that directly generates triplets upon NIR photon excitation, the resulting system allowed for an anti-Stokes shift of 1.03 eV. Our results highlight the use of direct triplet generation via NIR excitation as a useful path to achieving large anti-Stokes shift and also show that high TTA efficiencies can be achieved even in the absence of large driving energies for the TTA process.