In the development of oxynitride photocatalysts, thermal ammonolysis of a metal oxide precursor has often been conducted by varying the reaction conditions (e.g., temperatures, reaction times, and ammonia gas flow rates) to obtain high-quality oxynitride particles that efficiently function as photocatalysts. However, this approach may suffer from undesirable changes in the physicochemical properties of the resulting oxynitride, leading to the lowering of the photocatalytic activity. Here, we show that it is possible to control the photocatalytic activity of Ruddlesden–Popper metastable layered oxynitride K2LaTa2O6N, obtained from the Dion–Jacobson phase KLaTa2O7 through a topochemical ammonolysis reaction, by controlling the quality of the KLaTa2O7 template. During the ammonolysis of KLaTa2O7, in the presence of K2CO3, to K2LaTa2O6N, the structural properties (e.g., degree of crystallinity and particle size) of the oxide precursor were replicated in the resulting oxynitride. Namely, the use of KLaTa2O7, possessing a higher degree of crystallinity, led to larger K2LaTa2O6N particles being formed. By increasing the crystallinity of KLaTa2O7, the photocatalytic activity of the resulting K2LaTa2O6N for H2 evolution was improved for reaction in aqueous NaI solution under visible light irradiation. This improvement in performance was due to the longer lifetime of the photogenerated mobile electrons in high-crystallinity K2LaTa2O6N compared with that in the low-crystallinity analogue, as confirmed by femtosecond transient absorption spectroscopy. However, the photocatalytic activity of K2LaTa2O6N derived from well-grown larger KLaTa2O7 particles was an order of magnitude lower than that of the best-performing material. Physicochemical measurements revealed that the large K2LaTa2O6N particles contained a relatively high density of anionic defects on the surface, which shortened the lifetime of the photogenerated charge carriers, leading to lower photocatalytic activity.
Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p–n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p–n junction of CdS–(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS–(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p–n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.
We report THz detection with GeTe/Sb2Te3 multi-layered topological insulator. Photoconductive-type detector device was fabricated and THz-induced change of the current was observed when the bias voltage was applied and THz pulse was irradiated.
Exploiting of unique quantum states in topological insulators is expected to realize new applications of THz technology. Among various kinds of topological materials, GeTe/Sb 2 Te 3 , that is multilayered system consist of topological insulator and normal insulator, has been received remarkable attention. In this study, we investigated THz transmission and emission property in interfacial phase change memory material [(GeTe) 2 /(Sb 2 Te 3 )1]n and Ge 2 Sb 2 Te 5 alloy. In addition, to understand the basic property of material, we also used GeTe and Sb 2 Te 3 . We observed pronounced differences between multilayered system and alloy.
Abstract Photocatalytic water splitting by solar light is an environment‐friendly means for generating hydrogen as energy resources. For practical use, photocatalysts with higher activity are desired. Recently it was found that the photocatalytic activity of SrTiO 3 is remarkably improved by Na‐doping. However, why Na‐doping enhances the activity has not been well understood. In this work, we found that Na‐doping in SrTiO 3 introduces new mid‐gap states. Transient absorption measurements revealed that photoexcited electrons were trapped into these states within ∼20 ps after excitation. These trapped electrons have longer lifetime than those in undoped SrTiO 3 , and number of surviving electrons in microseconds increased ∼15 times. These electrons are trapped at the mid‐gap states, but still keep reactivity with reactant molecules. Furthermore, they were effectively captured by H 2 ‐evolution cocatalyst, indicating that they can participate in steady‐state reactions. This work emphasizes the important role of electron‐trapping states introduced by doping on photocatalytic activity.
Pulverization followed by annealing treatment improved the activity of BiVO 4 although the annealing treatment had a negative impact on the non-milled sample.
Photocatalytic water splitting reaction attracts considerable attention owing to their potential application to generate H2 gas from H2O by using solar energy. However, further activity enhancement is indispensable for industrial use. Various approaches have been adopted to improve its activity; however, as these defects and impurities are the main obstacles that reduce quantum efficiency (QE), the fabrication of fine crystals with low defects and impurities has been essential for activity improvement. Here, we found that Zn and Ca core–shell double doping in polycrystalline β-Ga2O3 photocatalysts is very effective to enhance the photocatalytic activity, and QE reaches 71% under 254 nm illumination. Time-resolved IR absorption spectroscopy and first-principles calculations revealed that Zn and Ca create shallow mid-gap states, and electron trapping at these states prevent the electron–hole recombination. STEM–EDS mapping analysis demonstrate that Ca is doped uniformly in the bulk, but Zn is doped on the surface. These findings herein indicate that the induced concentration gradient of the dopants effectively inhibit the recombination in the bulk and at the surface and assist the diffusion of trapped electrons from the bulk to the surface, thereby accelerating reactions at the surface. These cooperative effects provide an attractive strategy to enhance the photocatalytic activity, which can be applied to many other photocatalysts including rough polycrystalline powders. This method requires neither the fabrication of fine single crystals nor the precise control of the co-catalyst loading.
We demonstrate terahertz pulse generation from silver nanoparticle ink, originally developed for printed electronics, under irradiation by femtosecond laser pulses. Using metal nanoparticle ink, metallic nanostructures can be easily made in a large area without lithographic techniques. Terahertz pulses were emitted from the baked ink, having spontaneously formed nanostructures of ∼100 nm. From the results of the baking temperature dependence and the polarization measurement, the terahertz generation is attributed to the nonlinear polarization induced by the enhanced local fields around these nanostructures. This study paves the way for the future development of terahertz emitters which have resonances in both the near-infrared light and the terahertz wave, by combining micrometer-scale structures drawn by an inkjet printer and nanometer-scale structures formed during the baking process.
The fabrication of heterojunctions with different band gap semiconductors is a promising approach to increase photoelectrochemical (PEC) activity. The PEC activity is determined by the charge separation; hence, the behaviors of charge carriers at the junctions should be elucidated. However, it has been quite challenging since the distinction of carriers located in different layers has been extremely hard. In this work, we succeeded in the identification of the individual electron- and hole-transfer kinetics at CoOx/BiVO4/SnO2 double heterojunctions by measuring transient absorption (TA) from the visible to mid-IR region: we found that the absorption peaks of electrons and holes depend on the materials. From the change in spectral shape after the selective photoexcitation of BiVO4, it was confirmed that electrons excited in the BiVO4 rapidly transferred to the SnO2 layer after ∼3 ps, but the holes remained in the BiVO4 and further transferred to CoOx in a few picoseconds. As a result, recombination of charge carriers was suppressed and 2.4 and 3.6 times a large amount of carriers are surviving at 5 μs on BiVO4/SnO2 and CoOx/BiVO4/SnO2, respectively, compared to bare BiVO4. For such picosecond-rapid and effective charge separation, the previously well proposed sole intralayer or interlayer charge separation mechanism is not enough. Hence the synergetic effect of these two mechanisms, the band-bending-assisted charge transfer across the heterojunction, is proposed. The enhanced PEC activity of CoOx/BiVO4/SnO2 electrodes was reasonably explained by this synergistic charge separation kinetics. This fundamental knowledge of charge carrier dynamics will be beneficial for the design of superior solar energy conversion systems.
