Tackling Disorder in γ-Ga2 O3.

Ga2 O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2 O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga2 O3 presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga2 O3 . The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.

Toward Standardized Photocatalytic Oxygen Evolution Rates Using RuO2@TiO2 as a Benchmark.

Quantitative comparison of photocatalytic performances across different photocatalysis setups is technically challenging. Here, we combine the concepts of relative and optimal photonic efficiencies to normalize activities with an internal benchmark material, RuO2 photodeposited on a P25-TiO2 photocatalyst, which was optimized for reproducibility of the oxygen evolution reaction (OER). Additionally, a general set of good practices was identified to ensure reliable quantification of photocatalytic OER, including photoreactor design, photocatalyst dispersion, and control of parasitic reactions caused by the sacrificial electron acceptor. Moreover, a method combining optical modeling and measurements was proposed to quantify the benchmark absorbed and scattered light (7.6% and 81.2%, respectively, of λ = 300-500 nm incident photons), rather than just incident light (≈AM 1.5G), to estimate its internal quantum efficiency (16%). We advocate the adoption of the instrumental and theoretical framework provided here to facilitate material standardization and comparison in the field of artificial photosynthesis.

Synthesis of Three-Layer Perovskite Oxynitride K2Ca2Ta3O9N·2H2O and Photocatalytic Activity for H2 Evolution under Visible Light.

Substitution of oxide anions (O2-) in a metal oxide for nitrogen (N3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

An Artificial Z-Scheme Constructed from Dye-Sensitized Metal Oxide Nanosheets for Visible Light-Driven Overall Water Splitting.

Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h-1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Two-Dimensional Perovskite Oxynitride K2 LaTa2 O6 N with an H+ /K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution.

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

Synthesis of a Layered Niobium Oxynitride, Rb2NdNb2O6N·H2O, Showing Visible-Light Photocatalytic Activity for H2 Evolution.

Two-dimensional (2D) layered oxynitrides are promising candidates as visible-light-driven photocatalysts, but the actual examples are rare because of the difficulty in synthesizing the 2D oxynitrides. Here a phase-pure layered perovskite, Rb2NdNb2O6N·H2O, that belongs to a tetragonal P4/ mmm space group was successfully synthesized by thermal ammonolysis of a mixture of layered RbNdNb2O7 and Rb2CO3, as revealed by synchrotron X-ray diffraction, elemental analyses, and atomic-scale electron microscopy observation. The synthesized Rb2NdNb2O6N·H2O had an absorption edge at around 500 nm and a sufficiently high conduction-band potential to allow for proton reduction. With modification by a platinum cocatalyst, Rb2NdNb2O6N·H2O became photocatalytically active for H2 evolution in the presence of triethanolamine as an electron donor under visible light (λ > 400 nm).

Photocatalytic overall water splitting on Pt nanocluster-intercalated, restacked KCa2Nb3O10 nanosheets: the promotional effect of co-existing ions.

Promotional effects of co-existing ions on overall water splitting into H2 and O2 have been studied in bulk-type semiconductor photocatalysts (e.g., TiO2), but such an effect remains unexplored in two-dimensional nanosheet photocatalysts. Here we examined the effect of co-existing ions on the photocatalytic water splitting activity of Pt nanocluster-intercalated KCa2Nb3O10 nanosheets. Interestingly, not only anions, as usually observed in bulk-type photocatalysts, but also cations had a significant influence on the photocatalytic performance. The rates of H2 and O2 evolution over Pt/KCa2Nb3O10 as well as the product stoichiometry were improved in the presence of NaI. I- ions were found to effectively suppress undesirable backward reactions, consistent with the previous work by Abe et al. (Chem. Phys. Lett., 2003, 371, 360-364). On the other hand, Na+ ions in the reaction solution were exchanged for K+ in the interlayer space of KCa2Nb3O10 during the water splitting reaction, which promoted interlayer hydration and consequently improved photocatalytic performance.

Undoped Layered Perovskite Oxynitride Li2 LaTa2 O6 N for Photocatalytic CO2 Reduction with Visible Light.

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Structures, electron density and characterization of novel photocatalysts, (BaTaO2N)1-x(SrWO2N)x solid solutions.

Tungsten-modified barium tantalum oxynitride is a new visible-light photocatalyst for water oxidation. In the present work, novel barium tantalum strontium tungsten oxynitride solid solutions, (BaTaO2N)1-x(SrWO2N)x, with a cubic Pm3[combining macron]m perovskite-type structure (x = 0.01 and 0.02) have been prepared by heating oxide precursors under an ammonia flow. These (BaTaO2N)1-x(SrWO2N)x catalysts exhibited photocatalytic water oxidation activity under visible light irradiation. The crystal structure, electron-density distribution, and optical properties of (BaTaO2N)1-x(SrWO2N)x (x = 0, 0.01, and 0.02) have been studied using synchrotron X-ray powder diffraction, Rietveld analysis, the maximum-entropy method (MEM), and UV-Vis reflectance measurements. The lattice parameters of (BaTaO2N)1-x(SrWO2N)x decreased linearly with increasing SrWO2N content x. The minimum electron density (MED) at the (Ta,W)-(O,N) bond, determined by the MEM analysis of (BaTaO2N)1-x(SrWO2N)x, increased with x, as supported by DFT-based calculations. These results indicate the formation of (BaTaO2N)1-x(SrWO2N)x solid solutions and enhanced covalent bonding due to the stronger W-N bond. The MED of the (Ta,W)-(O,N) bond was higher than that of (Ba,Sr)-(O,N), indicating that the (Ta,W)-(O,N) bond is more covalent. The presence of nitrogen in (BaTaO2N)1-x(SrWO2N)x was confirmed by the occupancy factor refined using neutron diffraction data and by the weight gain observed by thermogravimetric analysis in air. UV-Vis reflectance spectra and DFT calculations indicated that (BaTaO2N)1-x(SrWO2N)x contains W5+ cations with a [Xe] 4f14 5d1 electron configuration and exhibits a more n-type semiconducting character compared with BaTaO2N, which could improve the photocatalytic water oxidation activity under visible-light irradiation.

Inert Layered Silicate Improves the Electrochemical Responses of a Metal Complex Polymer.

A chemically inert, insulating layered silicate (saponite; SP) and an iron(II)-based metallo-supramolecular complex polymer (polyFe) were combined via electrostatic attraction to improve the electrochromic properties of polyFe. Structural characterization indicated that polyFe was intercalated into the SP nanosheets. Interestingly, the redox potential of polyFe was lowered by combining it with SP, and the current was measurable despite the insulating nature of SP. X-ray photoelectron spectroscopy showed that the decrease in the redox potential observed in the SP-polyFe hybrid was caused by the electrostatic neutralization of the Fe cation in polyFe by the negative charge on SP. Electrochemical analyses indicated that electron transfer occurred through electron hopping across the SP-polyFe hybrid. Control experiments using a metal complex composed of Fe and two 2,2':6',2''-terpyridine ligands (terpyFe) showed that SP contributes to the effective electron hopping. This modulation of the electrochemical properties by the layered silicates could be applied to other electrochemical systems, including hybrids of the redox-active ionic species and ion-exchangeable adsorbents.

Microwave Effects on Co-Pi Cocatalysts Deposited on α-Fe2O3 for Application to Photocatalytic Oxygen Evolution.

We analyze the effects of microwave applied in the process of photoelectrochemical deposition of cobalt-based cocatalysts, Co-Pi, onto well-orientated flat α-Fe2O3 thin films, which were fabricated by pulsed laser deposition. As compared with conventional heating, microwave significantly affects the morphology, chemical composition, and photocatalytic activity of Co-Pi/α-Fe2O3 composite. A significant enhancement in photocurrent related to photocatalytic water oxidation is achieved by the Co-Pi catalyst prepared under microwave irradiation. This, along with its interfacial electron-transfer properties, is studied by means of electrochemical impedance spectroscopy.

Synthesis and photocatalytic activity of K2CaNaNb3O10, a new Ruddlesden-Popper phase layered perovskite.

A new three-layer perovskite oxide with the Ruddlesden-Popper (R-P) phase, K2CaNaNb3O10, and its protonated form were synthesised and their photocatalytic performance was compared to that of KCa2Nb3O10 or the protonated form with the Dion-Jacobson (D-J) structure in terms of H2 and O2 evolution. K2CaNaNb3O10 exhibited a higher activity for O2 evolution than KCa2Nb3O10 when IO3- was used as an electron acceptor. However, protonated KCa2Nb3O10 worked more efficiently than protonated K2CaNaNb3O10 when Fe3+ was used as an electron acceptor. In both cases, it is likely that the stronger affinity of water with the interlayer contributed to higher performance. The activity of the D-J material for H2 evolution was much lower when 1-propanol was used as an electron donor than when methanol was used. In contrast, the R-P phase exhibited a similar activity regardless of the electron donor. These results indicate that the interlayer space acts as an oxidation site, so that better access of the electron donor to the interlayer is an important factor that enhances photocatalytic activity.

Photocatalytic Water Oxidation over Metal Oxide Nanosheets Having a Three-Layer Perovskite Structure.

Metal oxide nanosheets having a three-layer perovskite structure were studied as photocatalysts for water oxidation in the presence of IO3 (-) as a reversible electron acceptor. This work examined the effects of the lateral dimensions and composition of the nanosheets as well as metal oxide co-catalysts deposited on the restacked nanosheets. Depositing metal oxides capable of promoting reduction reactions on the nanosheets were found to promote the water oxidation activity. In contrast, the lateral dimensions and the degree of crystallinity of the nanosheets had little effect on the activity. Experimental results demonstrated that the reduction of IO3 (-) is the rate-limiting step in this reaction and that nanosheets with less distorted structures are advantageous with regard to increasing both light absorption and the mobility of photoexcited charge carriers.

Emission spectroscopy of a ruthenium(ii) polypyridyl complex adsorbed on calcium niobate lamellar solids and nanosheets.

Ru(ii) tris-diimine complexes are known to exhibit emission at around 630 nm as a result of (1)MLCT photoexcitation. The emission is quenched in the presence of a suitable semiconductor solid due to electron injection from the excited state of a Ru(ii) complex to the conduction band of the adjacent semiconductor. Here we investigated emission quenching behaviour of Ru(II){(4,4'-(CH3)2-bpy)2(4,4'-(CH2PO3H2)2-bpy)} (bpy = 2,2'-bipryridine) adsorbed on HCa2Nb3O10 solids having an ordered lamellar structure or a disordered nanostructure. Even though electron injection from the excited state of the Ru complex to the conduction band of nanostructured HCa2Nb3O10 is thermodynamically less favorable than that of layered HCa2Nb3O10, faster electron injection was observed using nanostructured HCa2Nb3O10. Experimental results highlighted that electron injection from the excited Ru complex takes place not only in the conduction band of HCa2Nb3O10 but also mid-gap states whose density is strongly dependent on both the morphological feature and the preparation method of HCa2Nb3O10.

Intercalation of highly dispersed metal nanoclusters into a layered metal oxide for photocatalytic overall water splitting.

Metal nanoclusters (involving metals such as platinum) with a diameter smaller than 1 nm were deposited on the interlayer nanospace of KCa2 Nb3 O10 using the electrostatic attraction between a cationic metal complex (e.g., [Pt(NH3 )4 ]Cl2 ) and a negatively charged two-dimensional Ca2 Nb3 O10 (-) sheet, without the aid of any additional reagent. The material obtained possessed eight-fold greater photocatalytic activity for water splitting into H2 and O2 under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of Pt nanoclusters with diameters smaller than 1 nm for photocatalytic overall water splitting. This material shows the greatest efficiency among nanosheet-based photocatalysts reported to date.

Perovskite oxide nanosheets with tunable band-edge potentials and high photocatalytic hydrogen-evolution activity.

Perovskite nanosheets of HCa(2-x)Sr(x)Nb3O10 and HCa2Nb(3-y)Ta(y)O10 with controlled band-edge potentials were prepared. They worked as highly efficient heterogeneous photocatalysts for H2 evolution from a water/methanol mixture under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa2Nb2TaO10 nanosheets, recording an apparent quantum yield of approximately 80% at 300 nm, which is the highest value for a nanosheet-based photocatalyst reported to date.