La-doped NaTaO3 Research Articles

Visible-light photooxidation of ciprofloxacin utilizing metal oxide incorporated sol-gel processed La-doped NaTaO3 nanoparticles: A comparative study

The supper dissemination of antibiotic waste in water resources has exponentially progressed the vital water and soil pollution that affect human health and the environment. Consequently, there have been several types of research anticipated for the green mineralization of such pollutants. Herein, we intended a surfactant-aided sol-gel formation of lanthanum-doped sodium tantalate (LNTO) nanocrystals. The synthesized 13 nm averaged-size perovskite LNTO nanocrystals were responsive to visible-light irradiation by incorporation of 4.4–5.2 nm oxide nanoparticles, namely Bi2O3, CdO, Fe2O3, and CuO at 4.0 wt% through coprecipitation. The formed nanomaterials unveiled mesostructured surface textures with specific surface areas of 199–229 m2 g−1. The obtained nanoceramics were employed for the mineralization of 10 ppm of ciprofloxacin antibiotic (CPF) as an emerging antibiotic waste in water under visible light irradiation. The CuO-incorporated LNTO exhibited the best photocatalytic oxidation of CPF after 120 min compared with other oxides with an excellent photoreaction rate of 0.0343 min−1 which is 49 times higher than the pure LNTO. The 2.0 gL−1 CuO/LNTO-dose achieved the full photooxidation of CPF at an oxidation speed of 0.0738 min −1 within just 1.0 h of visible light irradiation and magnificent regeneration ability. This enhanced activity of CuO/LNTO is regarded as significant light absorption and a bandgap energy reduction to 2.12 eV. Besides that, the heterojunction between CuO and LNTO amended the photogenerated carrier mobility and separation as concluded from the photoluminescence and photocurrent exploration. This comparative work suggests the proper design of low bandgap oxide decoration of solution-based perovskite oxide photocatalysts for promoting the visible-light mineralization of antibiotics in water.

Photoactivity enhancement of La-doped NaTaO3 nanocrystals by CuO decoration toward fast oxidation of ciprofloxacin under visible light

The use of antibiotics has increased over recent years owing to the spread of infections worldwide. The spread of this toxic matter, especially in water, has become crucial to public health and the environment. Accordingly, there has been extensive research, and numerous approaches have been proposed for the eco-friendly elimination of antibiotic contaminants. In this study, we designed a solution-based synthesis of La-doped NaTaO3 crystals, which become visible-light-responsive photocatalysts upon decoration with 1.0–4.0 wt% CuO nanoparticles. CuO/LNTO exhibited a mesoporous texture and large specific surface areas in the range 199–221 m2 g−1 compared to 230 m2 g−1 for pure LNTO. Tuning the CuO content in LNTO provides a material with a broad absorbance in the visible range and a prominent attenuation of the bandgap energy from 4.05 to 2.1 eV at CuO content of 3.0 wt%. CuO/LNTO was used for the photodegradation of ciprofloxacin as a nascent antibiotic model. The 2.0 gL−1 dose of 3%-CuO/LNTO accomplished total oxidation of CPF with photoreaction rate of 0.071 min−1 in water after 60 min visible light illumination and maintained substantial recyclability of five cycles. The high photooxidation activity of CuO/LNTO is ascribed to the formation of a heterojunction between CuO and LNTO. The heterojunction improved the photoinduced charge separation and mobility, as confirmed by the photoluminescence and photocurrent studies. This novel study proposes that the decorated sol-gel produced perovskite oxide nanostructures are promising photocatalysts is for the fast elimination of antibiotic contaminants under visible light.

(Invited) Core-Shell Double Doping for the Remarkable Photocatalytic Water Splitting on Ga2O3

The production of H2 gas from water using solar energy with photocatalysts is a promising method to realize a sustainable society. As H2 is clean and free from CO2 emissions, photocatalysts are expected to address many problems such as the energy crisis, environmental pollution, and global warming. However, the efficiency of photocatalysis is still not sufficient for industrial use; hence, further activity enhancement is required. The efficiency of photocatalysis is determined by the competition between electron-hole pair recombination and the rate of charge transfer to the reactant molecules. Therefore, in order to improve the activity, the recombination should be suppressed and the rate of charge transfer to the reactant molecules should be enhanced. It has long been assumed that the fabrication of fine crystals with low defects and impurities is essential for improving photocatalytic activity, since defects and impurities have previously been believed to accelerate the recombination process. However, the discovery of activity enhancement of NaTaO3 photocatalysts by La3+ doping, has led to the development of doping as a new methodology to increase the photocatalytic activity. This method has been applied to SrTiO3, Ga2O3, and TiO2, etc. However, the role of these dopants, except for band gap narrowing for visible absorption, is still a controversial issue. Several mechanisms have been proposed, such as the change in surface morphology to enhance the charge separation, the formation of a band-gap gradient, an increase in the n-type or p-type conductivity, and a reduction of oxygen vacancies (VO), etc., but their actual role still remains unclear.Recently, a study by Sakata et al.,[1] succeeded in upgrading the world record of the highest quantum efficiency (QE) of water-splitting photocatalysts; a remarkably high QE of ~71% was achieved under 254 nm light illumination by doping Zn and Ca in Ga2O3. To our knowledge, this remarkably high QE surpasses the highest value of La-doped NaTaO3 with a QE of 56% at 270 nm, reported in 2000.[2] Previous studies have already reported that surface doping of Ga2O3 with divalent cations such as Zn or Ca increases its activity. However, the combination of surface Zn-doping and bulk Ca-doping further enhanced the activity. Double doping is often applied to prevent VO formation and/or to fix the valence number of doped cations by allowing for charge balance within the material. However, in the present case, Zn2+ and Ca2+ have the same valence number; hence, it is expected that some co-operative effects are present between Zn and Ca. This double-doping technique could be applied to many other photocatalysts, but the underlying mechanism of the enhancement of QE by Zn and Ca double-doping still remains uncertain.In this work, we have unraveled the mechanism of the enhancement of the activity of Ga2O3 induced by Zn and Ca double-doping. Visible-to-mid-IR transient absorption spectroscopy (TAS) was adapted to elucidate the effects of dopants on the behavior of photogenerated charge carriers as this technique is quite effective in differentiating the behaviors of free electrons and electrons trapped at the defects on photocatalysts, such as TiO2, Fe2O3, NaTaO3, SrTiO3, LaTiO2N, and Ga2O3. Free electrons in conduction band (CB) and trapped electrons at the defects contribute to transient absorption (TA) signals at different wavenumbers;[3] hence, the behavior of these electrons can be separately investigated. Furthermore, the depth of the electron traps can be estimated from the absorption energy of the trapped electrons. Based on the TAS results, we found that the lifetime of photoelectrons increases due to surface Zn-doping and bulk Ca-doping, but the combination of Zn and Ca doping further increased the electron lifetime. It is also elucidated that the lifetime-extension of photocarriers is responsible for the formation of shallow mid-gap states by Zn and Ca doping; electron-trapping at these traps reduces the probability of interacting with holes, and thus, the lifetime of electrons increases further. Notably, most electrons are trapped at these mid-gap states, but they have a higher reactivity with water than those in the undoped sample. The mechanism of drastic activity enhancement by Zn and Ca double doping is discussed based on the TAS results, first-principles density functional theory calculations, and scanning transmission electron microscopy with EDS analysis.[1] Y. Sakata, T. Hayashi, R. Yasunaga, N. Yanaga, H. Imamura, Chem. Commun., 51 (2015) 12935.[2] A. Kudo, H. Kato, Chem. Phys. Lett., 331 (2000) 373.[3] A. Yamakata, M. Kawaguchi, N. Nishimura, T. Minegishi, J. Kubota, K. Domen, J. Phys. Chem. C, 118 (2014) 23897.

La-doped NaTaO3 nanoparticles: Sol-gel synthesis and synergistic effect of CdO decoration toward efficient visible-light degradation of ciprofloxacin in water

The expanded use and disposal of antibiotics in water has led to health and environmental problems. However, photocatalysis is an efficient way to make progress toward the safe destruction of this emergent waste. Here, we present the photocatalytic degradation of ciprofloxacin (CP), an emerging antibiotic probe, over sol-gel synthesized La-doped NaTaO3 (LNTO) nanoparticles. We also studied the synergistic influence of adding a low amount (1.0–4.0 wt %) of CdO nanocrystals to the LNTO to enhance the photocatalytic performance under visible light. The introduction of CdO at only 3.0–4.0 wt% assisted the visible light response of the LNTO by reducing the bandgap energy (Eg) from 4.1 to approximately 2.6 eV and maintaining the high surface area of the mesostructured surface at 188 m2 g−1. Additionally, the 3% CdO-loaded LNTO demonstrated complete photodegradation of CP within 90 min and a 732-fold increase in the degradation rate compared to that of pristine LNTO. The obtained CdO-modified LNTO could also be reused five times with negligible reduction in performance. The results reflect the significant contribution of CdO to the promotion of photoinduced charge separation. This study demonstrates the use of perovskite-based photocatalyst modification with simple oxides to promote sustainable visible-light degradation of such antibiotic waste in water.

Visible light-driven photodegradation of ciprofloxacin over sol-gel prepared Bi2O3-modified La-doped NaTaO3 nanostructures

The rise of antibiotic waste becomes an urgent problem for humankind and the environment. The emission of this waste in water impacts the health of the population and decreases antibiotics’ efficacy. Consequently, numerous studies and tactics have been applied for spotless and harmless removal of these substances. Here, we describe a sol-gel amalgamation of visible-light responsive nanocrystalline La-doped NaTaO3 perovskites (LNTO) modified with a low quantity of Bi2O3 (BO) at x = 1.0–4.0 wt% to make x%BO/LNTO nanocomposites. The obtained BO/LNTO displayed mesoporous surfaces with significant surface areas (178–198 m2 g−1). Additionally, the proper ratio of BO to LNTO demonstrates wide harvesting in the visible range and a notable drop in the bandgap energy (Eg) to 2.73 eV from 4.12 eV in neat LNTO. The prepared nanocomposites were employed for the photocatalytic degradation of ciprofloxacin (CIP) as an emergent antibiotic target. 3%BO/LNTO achieved complete photodegradation of CIP after 120 min of visible-light irradiation while maintaining high reusability. The high photocatalytic performance of BO/LNTO is due to the close junction of BO and LNTO, which improves the photoinduced charge transfer and Eg suppression. The present study suggests the application of modified perovskites for improved photoelimination of antibiotics.

The role of the shell in core-shell-structured La-doped NaTaO3 photocatalysts.

NaTaO3, a semiconductor with a perovskite structure, has long been known as a highly active photocatalyst for overall water splitting when appropriately doped with La cations. A profound understanding of the surface feature and why and how it may control the water splitting activity is critical because redox reactions take place at the surface. One surface feature characteristic of La-doped NaTaO3 is a La-rich layer (shell) capping La-poor bulk (core). In this study, we investigate the role of the shell in core-shell-structured La-doped NaTaO3 through systematic chemical etching with an aqueous HF solution. We find that the La-rich shell plays a role in electron-hole recombination, electron mobility and water splitting activity. The shallow electron traps populating the La-rich shell trap the photoexcited electrons, decreasing their mobility. The shallowly trapped electrons remain reactive and are readily available on the surface to be extracted by the cocatalysts for the reduction reaction evolving H2. The presently employed chemical etching method also confirms the presence of a La concentration gradient in the core that regulates the steady-state electron population and water splitting activity. Here, we successfully reveal the nanoarchitecture-photoactivity relationship of core-shell-structured La-doped NaTaO3 that thereby allows tuning of the surface features and spatial distribution of dopants to increase the concentration of photoexcited electrons and therefore the water splitting activity. By recognizing the key factors that control the photocatalytic properties of a highly active catalyst, we can then devise proper strategies to design new photocatalyst materials with breakthrough performances.

La-doped NaTaO3 perovskite nanocrystals supported with α-Fe2O3 for sustainable visible-light-driven elimination of ciprofloxacin in water

To date, the emergence of antibiotic waste in water has become an important issue for humans and other living organisms. The spread of such waste will influence human health and the effectiveness of such antibiotics. Accordingly, several research approaches are offered for the clean and safe elimination of such waste. Herein, we present a facile synthesis of visible-light-activated La-doped NaTaO3 perovskite nanocrystals (LNTO) supported with small amounts (1.0–4.0 wt %) of Fe2O3 (FO) nanoparticles to produce x%FO@LNTO nanocomposites. These FO@LNTO samples exhibited a mesostructured crystalline surface with a high surface area (200–230 m2 g−1). Additionally, the loading of FO nanoparticles onto LNTO resulted in a broad visible-light absorption and reduction of the bandgap (Eg) to 2.35 eV compared to 4.40 eV for the parent LNTO. The produced nanocomposite was tested for the photodegradation of ciprofloxacin (CIP) as an emerging probe of antibiotic waste in water. The 3% FO@LNTO displayed a total elimination of 10 ppm CIP after 90 min of visible-light illumination with sustainable recyclability. This fascinating action of FO@LNTO is referred to as the formation of a close heterojunction between FO and LNTO that decreases the Eg value and enhances photocharge separation. This work implies the use of perovskite-based photocatalysts for reasonable elimination of antibiotic waste in water.

Enhance photocatalytic of hydrogen production from water-glycerol solution over RuO2-loaded

Study of hydrogen production from aqueous glycerol solution on RuO2-loaded LaNaTaO3 has been studied. This research aim is to investigate the influence of co-catalyst RuO2 and glycerol as a sacrificial reagent on the photocatalytic process of hydrogen production. La-doped NaTaO3 was prepared via sol-gel route. The RuO2 was loaded onto the LaNaTaO3 surface by impregnation method. The sample was characterization by Scanning Electron microscopy (SEM) and X-ray Diffractometer (XRD). From XRD spectrum shows that the La-NaTaO3 photocatalyst has high crystallinity. The SEM analysis indicates that the size of La-NaTaO3 photocatalyst is ranging from 100 to 250 nm without agglomeration and RuO2 as a co-catalyst is well loaded into the La-NaTaO3 surface. The rate of hydrogen production increased significantly with a glycerol concentration. Loading RuO2 on La-NaTaO3 photocatalyst is proved to increase the rate of hydrogen production of 3.2 times with the addition of glycerol as a sacrificial reagent. The highest hydrogen production activity was obtained for 0.3 wt.% ruthenium loading. The role of co-catalyst RuO2 and glycerol as a sacrificial reagent has enhanced the performance of La-NaTaO3 photocatalyst since both play an active role in the separation efficiency of electrons and holes and the reaction of hydrogen formation.

Stability of La dopants in NaTaO3 photocatalysts

Doping NaTaO3 with La typically results in increased photocatalytic activity. The photocatalytic activity of La-doped NaTaO3 should therefore be sensitive to the photostability of the La species being doped. Unfortunately, the photostability of the La species in NaTaO3 is not fully known. Herein, we perform an in-depth study on the photostability of the La species. Advanced characterization techniques, including X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS), are employed to clarify the chemical state and local structure of the La species in NaTaO3. Our findings show that the examined La-doped NaTaO3 can be effectively reused for six consecutive runs without a significant loss of its photocatalytic activity for the decomposition of recalcitrant organic compounds under UV light. The reusability of La-doped NaTaO3 is proposed to be due to the photostability of the La species. The La species are photostable because their oxidation state is unaltered and their structure in the host NaTaO3 is well preserved.

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is ...

Highly Active NaTaO3 -Based Photocatalysts for CO2 Reduction to Form CO Using Water as the Electron Donor.

Doped NaTaO3 (NaTaO3 :A, where A=Mg, Ca, Sr, Ba, or La) has arisen as a highly active photocatalyst for CO2 reduction to simultaneously form CO, H2 , and O2 using water as the electron donor when used with an Ag cocatalyst, under UV irradiation, and with 1 atm (0.1 MPa) of CO2 . The ratio of the number of reacted electrons/holes was almost unity, indicating that water was consumed as the electron donor. A liquid-phase reduction method for loading of the Ag cocatalyst was superior to photodeposition and impregnation methods. The Ag cocatalyst-loaded NaTaO3 :Ba was the most active photocatalyst in water with no required additives. The addition of bases, such as hydrogencarbonate, was effective to enhance the CO formation for Mg-, Ca-, Sr-, Ba-, and La-doped NaTaO3 photocatalysts with an Ag cocatalyst. Ca- and Sr-doped NaTaO3 photocatalysts showed especially high activity along with the Ba-doped photocatalyst in the aqueous NaHCO3 solution. The selectivity for the CO formation [CO/(CO+H2 )] on Ca-, Sr-, and Ba-doped NaTaO3 photocatalysts with Ag cocatalyst reached around 90 %.

Role of effective carrier mass in the photocatalytic efficiency of La-doped NaTaO3

In this work, a close relationship between the electronic structure of La-doped NaTaO3 and its photocatalytic activity is demonstrated. La doping of NaTaO3 increases the band gap and, for orthorhombic NaTaO3, changes it into an indirect band gap material. Furthermore, the present calculations reveal a dependence of the effective carrier mass in La-doped NaTaO3 on the La doping concentration. The carrier mass reaches its minimum value ca 4mol% doping concentration, increasing at higher and lower concentrations. Since both the presence of an indirect band gap and a low effective carrier mass are beneficial to catalytic efficiency, such dependence explains a previously reported dependence of the photocatalytic efficiency of La-doped NaTaO3 on the La doping concentration.

The influence of oxygen vacancies and La doping on the surface structure of NaTaO3

Doping of the semiconductor photocatalyst NaTaO3 with La is known to produce stepped features on the surfaces. Similar stepped-surface features can be produced in the undoped material by repeated high temperature calcination. In this work, density functional theory calculations for both La-doped and undoped NaTaO3 bulk and surfaces have been carried out to explore the hypothesis that La doping and high T calcination produce a common effect that favors the formation of stepped surfaces. It is found that La doping and oxygen vacancies lead to similar zig-zag distortions in the surface structure. It is suggested that this zig-zag distortion may induce the formation of the stepped-surface structures.

Pd/NiO core/shell nanoparticles on La0.02Na0.98TaO3 catalyst for hydrogen evolution from water and aqueous methanol solution

Novel Pd/NiO core/shell nanoparticles (NPs) have been synthesized by a simple impregnation method with low temperature processing, in a ‘green’, scalable process using nontoxic chemicals. The cocatalyst consisting of a Pd core and a NiO shell formed simultaneously on the surface of La-doped NaTaO3 photocatalyst. The Pd core both induces migration of photogenerated electrons from the bulk of the La0.02Na0.98TaO3 and transfers electrons to the NiO shell. Without the NiO shell, Pd NPs show negligible H2 production from water splitting, due to the rapid reaction between hydrogen and oxygen on the surface. On the other hand, the NiO shell allows the permeation of hydrogen and enables hydrogen reduction on Pd. The incorporation of NiO shell onto Pd remarkably enhances the photocatalytic performance of La-doped NaTaO3 for hydrogen production from pure water. In addition, the core/shell structure can significantly enhance the stability of Pd during the photocatalytic reaction. Similar concepts could be extended to other applications, where the catalytic activity and stability are of concerns. The formation mechanism of the core/shell photocatalyst is proposed based on the high resolution transmission electron microscopy (HRTEM) images and X-ray absorption near-edge structure (XANES) analyses.

The effect of Au cocatalyst loaded on La-doped NaTaO3 on photocatalytic water splitting and O2 photoreduction

Photocatalytic water splitting over La-doped NaTaO3 (NaTaO3:La) was improved by loading with Au cocatalyst which works as H2 evolution sites. NaTaO3:La loaded with Au by an impregnation method showed higher and more steady activity for water splitting than that by a photodeposition method. This difference in the activity for water splitting was related to the O2 photoreduction on the loaded Au cocatalyst, which is one of the backward reactions of water splitting. The impregnated spherical Au cocatalyst suppressed the O2 photoreduction by photogenerated electrons more efficiently than the photodeposited hemispherical Au cocatalyst, because the perimeter of Au/NaTaO3:La interface which produces activated O2 molecules was smaller in the impregnated Au than the photodeposited Au.

Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen

57 Water splitting using a photocatalyst for direct solar hydrogen production from sunlight and water, has been regarded as an important approach to artificial photosynthesis.1,2 Solar hydrogen, which is the simplest solar fuel, can then be converted to various energy carriers such as organic hydrides, methanol, methane, and ammonia for transportation and storage. Thus, energy systems based on solar hydrogen can lead to a stable, secure, and ecologically-friendly society. Although solar fuel production using electricity from photovoltaic cells and concentrated solar thermal power is an existing technology, direct synthesis of solar fuels by artificial photosynthesis has more scalability in terms of system cost. To split water into hydrogen and oxygen, 1.23 V of energy is required, corresponding to a 2 electron reaction. An energy of 1.23 V is equivalent to a photon energy of 1000 nm, so that the visible light (400-800 nm in wavelength) that represents half of the solar energy spectrum can be thermodynamically used for water splitting. The challenge is to obtain an efficient photocatalyst that has an absorption edge in longer wavelength and the ability to split water. Highly efficient oxide photocatalysts, such as La-doped NaTaO3, and Zn-doped Ga2O3, have been reported with quantum efficiencies of over 50%.3-5 However, these oxides only absorb ultraviolet (UV) light with wavelengths shorter than 300 nm. In the solar spectrum at the earth’s surface, such short UV light is not present, so that these oxides cannot operate efficiently under solar irradiation. Most oxides have band gaps wider than the UV energy because of the deeper O 2p potential of the valence band. The valence band of nitrides is composed of the shallower N 2p potential, and thus their band gaps are generally narrower than those of oxides. For photocatalytic water splitting, photoexcited electrons in the conduction band reduce water to evolve hydrogen, and photogenerated holes in the valence band oxidize water to evolve oxygen. Therefore, the potential of the conduction band should be shallower than the reversible hydrogen potential and the potential of the valence band should be deeper than the reversible oxygen potential. In this article, the properties of oxynitride and nitride photocatalysts for water splitting are described. Surface modification of semiconductor photocatalyst particles with hydrogenor oxygen-evolving cocatalysts is an indispensable technique in the study of photocatalytic water splitting. In fact, hydrogen-evolution cocatalysts are required Photocatalytic Water Splitting Using Oxynitride and Nitride Semiconductor Powders for Production of Solar Hydrogen

Green fabrication of La-doped NaTaO3 via H2O2 assisted sol–gel route for photocatalytic hydrogen production

Crystalline NaTaO3 nanoparticles doped with different concentrations of La3+ have been synthesized via a H2O2-assisted sol–gel route. In this reaction, TaCl5 is dissolved in aqueous H2O2 solution to form a stable transparent Ta-peroxo complex solution. The formation of tantalum-peroxo complexes and their chelation by citric acid enables a better control of crystal growth. X-ray diffraction and scanning/transmission electron microscopy provide useful information about crystallinity and morphology of the samples. The substitution of La3+ ions in the NaTaO3 lattice is verified by crystallographic simulation (CaRIne Crystallography version 3.1). These results indicate La3+ ions occupy the Na+ ions sites, which agrees very well with the experimental data. The optimal content of La3+ ions effectively increases crystallinity without agglomeration, contributing to efficient charge separation and preventing recombination between photogenerated electrons and holes. The highest photocatalytic H2 production of 1.43mmolh−1 is obtained for a 2.0mol% La-doped NaTaO3 sample. The NaTaO3 nanoparticles produced using this facile, environmentally friendly ‘green process’ have better crystallinity, smaller size and higher photocatalytic activity.

A Novel Photodeposition Method in the Presence of Nitrate Ions for Loading of an Iridium Oxide Cocatalyst for Water Splitting

Abstract IrO2 cocatalysts for water splitting were photodeposited from aqueous solution of (NH4)2[IrCl6] or Na3[IrCl6] in the presence of nitrate ions on La-doped NaTaO3 (denoted as NaTaO3:La) photocatalysts, while metallic iridium particles were deposited in the absence of nitrate ions. The IrO2-loaded NaTaO3:La showed the activity for water splitting 2.5 times higher than the nonloaded NaTaO3:La. In contrast, the steady activity was not obtained for the metallic iridium-loaded photocatalyst, because the back reaction quickly proceeded.

New aspects of heterogeneous photocatalysts for water decomposition

Several new photocatalysts for overall water splitting are described. Under UV light irradiation (270 nm), La-doped NaTaO3 modified with NiO decomposed water into H2 and O2 with extremely high quantum efficiency. Under an optimized condition, the apparent quantum efficiency, which was estimated with numbers of irradiated photons and evolved H2 molecules, reached 56%. New stable photocatalytic materials containing elements with d10 electronic configuration such as In3+ Sn4+ and Sb5+ were developed for overall water splitting. Some mesoporous oxides were proved to be effective photocatalysts. (Oxy)nitrides of some early transition metals, i.e., Ta, Nb and Ti, were found to be stable materials having potentials for H2 and O2 evolutions under visible light irradiation (⪯600 nm). The electronic structures of these photocatalysts are also discussed based on DFT calculation.