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An ab initio evolutionary search algorithm was combined with density functional theory (DFT) calculations to predict a series of 2-D BxNy (1 < x/y ≤ 2). Particularly, B5N3 and B7N5 monolayers have sufficiently low formation enthalpy and excellent dynamic stability that make them promising for synthesis in experiments. Electronic structure calculations reveal that B5N3 and B7N5 monolayers possess an indirect band gap of 1.99 eV and a direct band gap of 2.40 eV, respectively. The calculated absorption coefficients for B5N3 and B7N5 monolayers are significantly improved in the low end of the visible region compared with that of 2-D h-BN. Moreover, our calculations reveal that both B5N3 and B7N5 monolayers have high electron carrier mobilities. The narrow band gaps, high carrier mobilities, strong near-ultraviolet absorption, and high synthesis possibility of B5N3 and B7N5 monolayers render them promising new materials for application in novel electronics and environmentally benign solar energy conversion.

Although tin monoxide (SnO) is an interesting compound due to its p-type conductivity, a widespread application of SnO has been limited by its narrow band gap of 0.7 eV. In this work, we theoretically investigate the structural and electronic properties of several SnO phases under high pressures through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (β-SnO), which possesses space group P21/c and a wide band gap of 1.9 eV, is more stable than α-SnO at pressures higher than 80 GPa. Moreover, a stable (space group P2/c) and a metastable (space group Pnma) phases of SnO appear at pressures higher than 120 GPa. Energy and topological analyses show that P2/c-SnO has a high possibility to directly transform to β-SnO at around 120 GPa. Our work also reveals that β-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase α-SnO for the phase transition path Pnma-SnO →β-SnO → α-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize β-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of β-SnO can be engineered in a low-pressure range (0–9 GPa) through a semiconductor-to-metal transition, while maintaining transparency in the visible light range.

First-principles calculations of the density of states (DOS) and partial charge distributions for BaSi2 revealed that the electronic states of the valence bands of BaSi2 are mainly derived from the Si 3p states of Si4 tetrahedra formed in BaSi2, while those of the conduction bands originate from the Si 3p states of Si4 and 5d states of the Ba atom. Thus, the molecular orbital diagram of BaSi2 was constructed. The origin of the large slopes at the band edges in the DOS of BaSi2, which results in a high optical absorption coefficient for the material, was discussed.

Iron(III) titanates are composed of earth-abundant elements and are attracting rapidly growing interest as highly promising candidates for solar-energy as well as optoelectronics applications. In this article, we report on the successful synthesis of pseudobrookite Fe2TiO5 thin films with RF sputtering followed by post-deposition annealing in air. The chemical composition, crystal structure, surface morphology, and optical properties of the films were characterized both experimentally and theoretically. The film was confirmed to be single phase and exhibited a highly crystalline orthorhombic structure with a preferential crystal orientation of (131) with excellent adhesion on both glass and silicon substrates, indicating that an epitaxial substrate is not required. Dielectric functions from spectroscopic ellipsometry and density functional theory (DFT) also provide a good understanding of the optical characteristics of the films.

A new visible light responsive photocatalyst Sn(II)-doped SrNb2O6 has been developed for water reduction and water oxidation reactions. First-principles calculations revealed that the Sn(II) dopant created a new occupied state in the band gap which resulted in photocatalytic activities under visible light irradiation.

We investigated the electronic structures and optical properties of doped anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ in the infrared region using first-principles density functional theory with the independent-particle approximation. By examining the dopants from groups V-B, VI-B, and VII-A of the Periodic Table of Elements, we found that the anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ doped with different dopants Nb, Ta, W, and F showed excellent plasmonic properties in the near-infrared region with nonmetal-to-metal crossover frequencies within a range of 2.0--2.7 \textmu{}m along the densest-plasma direction. Furthermore, we discussed the prospect of producing low-cost robust mid- to far-infrared plasmonic devices based on doped anatase $\mathrm{Ti}{\mathrm{O}}_{2}$. Our combined analysis with electromagnetic numerical simulation showed that the doped anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ exhibited well-defined surface plasmon polariton as well as localized surface plasmon resonances in the midinfrared region. We expect that this $n$-type doped anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ is a highly promising material for infrared plasmonic applications in harsh environments such as aqueous solutions and/or high temperature.

Tin is an earth abundant harmless element and its oxides (tin oxide) has been attracted researchers of a wide range of fields in environmentally benign materials development. Photocatalytic hydrogen production is one such promising application of tin oxide for several reasons such as stability in water solution and sufficiently negative band-edge position for the proton reduction. However, two major bulk phases SnO and SiO2 are not suitable for solar-driven photocatalysis reaction due to the fact that too negative valence band edge of SnO disables the oxidation reaction of water solution and too wide bandgap of SnO2 (~ 3.6 eV) hampers the absorption of visible light. Fortunately, bandgap or band-edge engineering is feasible by adjusting relative distance of lone pairs induced by divalent tin ions, Sn2 +, and the demonstration of this strategy is the main focus of this article. In this chapter, we introduce our two approaches toward the band-edge engineering of tin oxides. One is the formation of SnO nanosheets and modulation of band edges by application of strain. Thanks to the quantum confinement effect, SnO nanosheets have much wider bandgaps than of bulk SnO. The bandgap of SnO nanosheet depends both on the number of layers and biaxial strain and these two parameters were successfully used for the design of promising photocatalysts. Second, we seek the possibility of forming mixed valence tin oxides in which both divalent and tetravalent tin ions are coexisting. Using advanced computational technique of evolutional crystal structure search, we have identified stable tin oxides such as Sn3O4, that is, (Sn2 +)2(Sn4 +)O4, and Sn5O6, that is, (Sn2 +)2(Sn4 +)3O6, which possess suitable band-edge positions for visible-light-driven photocatalysis hydrogen evolution. Finally, we discuss experimental realization of Sn3O4 photocatalyst. From hydrothermal method, the mixed valent tin oxide Sn3O4 was successfully synthesized and it exhibits excellent performance for photocatalytic H2 evolution from water solution under visible light irradiation.

Controlling particle size and structure is challenging in the synthesis of composite metal oxide nanoparticles. In this study, first-principles calculations based on density functional theory were ...

A large single crystal of CuI was fabricated by the temperature difference method using NH4I aqueous solution. To explore the efficient crystal growth condition, we calculate the phase diagram of c...

The present study comprehensively investigated the metal sulfide Cu2ZnSnS4 (CZTS) as a visible-light-active photocatalyst for selective CO2 reduction on the basis of theoretical calculation and experimental evaluation of a highly pure CZTS film electrode. For our first-principles calculation, the Heyd–Scuseria–Ernzerhof functional and a slab model with a sufficiently thick vacuum layer for the crystal phase were used to determine the accurate band gap value and to discuss the proper positions of its conduction and valence bands, respectively. Consequently, CZTS could be expected to have sufficient high conduction band level to drive CO2 reduction under visible light irradiation. We fabricated a highly pure CZTS thin film via spin-coating, and elemental analysis indicated that the film consisted of almost stoichiometric chemical composition of CZTS. Our pristine CZTS film electrode functioned as a p-type photocathode in aqueous media bubbled with CO2, and it could generate carbon monoxide (CO) in addition ...

The quasi-particle band structure, carrier mobility, and optical response of atomic-thin TiO2 nanosheets were accurately predicted with the many-body perturbation theory of G0W0+BSE calculations. The lepidocrocite-type TiO2 exhibits an unexpected direct band gap of 5.3 eV, different from the retaining indirect band gap character in anatase-type TiO2 nanosheet. Because of the dispersive valence band maxima from the strong overlap between O-2p orbitals, an extremely high hole mobility of 1069 cm2 V–1 s–1 was proposed for the lepidocrocite-type TiO2 nanosheet. Including the electron–hole exchange of excitonic effect, our simulations well reproduce the experimental absorption spectra implying a huge exciton binding energy. The strong anisotropic exciton originates from the crystal dependent effective mass in the lepidocrocite-type TiO2 nanosheet due to the asymmetric orbital overlaps. The strongly bound exciton still renders a sufficiently high potential for hydrogen production from aqueous solution, although...

The effects of Sr substitution in BaSi2, a promising photoabsorber for thin-film solar cells, were examined computationally and experimentally. The calculations showed that Ba0.5Sr0.5Si2 with Sr atoms at the A1 site (A1-Sr-BSS) is more stable than that with Sr atoms at the A2 site (A2-Sr-BSS). Both materials are indirect bandgap semiconductors. The indirect and direct bandgaps of A1-Sr-BSS are larger than those of BaSi2, while those of A2-Sr-BSS are smaller. The indirect bandgap of A1-Sr-BSS is 0.01 eV higher than that of BaSi2, while that of A2-Sr-BSS is 0.06 eV lower. This trend is attributable to the difference in the electronic properties around the conduction band minimum, which is determined by the type of atoms (Ba or Sr) that occupy the A1 site. Single-crystal X-ray diffraction and diffuse reflectance measurements of Ba1-xSrxSi2 (0.0 ≤ x ≤ 0.8) revealed that the Ba atoms at the crystallographic site A1 are preferentially substituted by Sr atoms and that the bandgap, Eg, decreases with x (1.24 eV at x = 0 and 1.15 eV at x = 0.65). The preferential occupation of the A1 site by Sr atoms is consistent with the computational result that A1-Sr-BSS is more stable than A2-Sr-BSS. However, the experimentally observed decrease in Eg with x is closer to the decrease of −0.06 eV in the indirect bandgap of A2-Sr-BSS than to the 0.01 eV increase in the case of A1-Sr-BSS. The reason for this discrepancy is discussed.

Two-dimensional (2D) boron structures, in which boron atoms arrange in a 2D manner, have attracted great attention for their potential applications in nanoelectronic devices. To improve the stability of 2D boron and find new functionalities, we predict a series of sandwich-shaped 2D structures of IrBx (8 ≤ x ≤ 16) through employing an ab initio evolutionary structure search. It is demonstrated that the stability of 2D boron sheets is greatly improved with the introduction of Ir atoms. Among various compositions, it turns out that 2D IrB14 is thermodynamically more stable than a mixture of the known IrB9 compound and α-boron. Moreover, we have studied the gas sensitivity of 2D-IrB14 to the adsorption of CO and CO2 molecules. It has been found that CO gas molecules bind chemically to boron atoms, whereas CO2 molecules do not. Upon CO adsorption, there is a charge transfer from surface atoms to CO molecules. Hence, acceptor states are formed above the Fermi energy, indicating that IrB14 is a promising novel 2D gas sensor material.

In contrast to n-type transparent conducting oxides (TCOs), very few p-type TCOs have been discovered to date. In this work, we predicted two new tin-germanate-based compounds (Sn2GeO4) as potential p-type TCOs with exceptionally low hole effective masses and wide band gap energies. Through detailed electronic structure calculations, we revealed that the Sn2GeO4 compounds possess sufficiently wide band gaps (3.24 eV) for transparency, and very small effective masses (0.46) for hole carriers due to the influence of divalent Ge2+. Our study shows the potential for band engineering of Sn2+ and Ge2+ cations with (n − 1)d10ns2 electronic configurations as p-type TCOs; this could expand the materials family for the future design and development of p-type TCOs.

Herein, the electronic and optical properties of TiO2(010) nanosheet solid solutions with transition metal oxycarbides, nitrides, and oxynitrides, (TiO2)2/3(M2O3C)1/3 (M = Nb or Ta), (TiO2)2/3(MN2)1/3 (M = W or Mo), and (TiO2)2/3(MOE)1/3 (M = W, Mo, E = C, and M = Nb, Ta, E = N) are systematically investigated. Forming a solid solution is a viable way to realize visible-light absorption and a direct band gap. The electron affinity of a solid-solution nanosheet closely depends on the energy level of the transition metal (M) d states; i.e., the hybridization of the M d states and Ti–O antibonding states introduces new bonding states, leading to a downward shift of the conduction band minimum. Meanwhile, the ionization potentials of these solid solutions are relatively low because of the introduction of high-lying occupied C/N 2p states, which lift the valence band maximum upward above that of the pristine TiO2. The modulation of band edges effectively narrows the band gaps of the solid solutions, except for...

We performed systematic study on electronic structures and photoelectrochemical (PEC) properties of ilmenite-type MTiO3 (M = Mn, Fe, Co, and Ni) thin-film photoanodes for water oxidation reactions. Single-phase MTiO3 films were grown on α-Al2O3 substrates by using pulsed-laser deposition. Formation of the ilmenite-type structures and oxidation states of M2+Ti4+O3 were revealed by X-ray diffraction and X-ray absorption spectroscopy, respectively. The PEC performance in water was investigated by linear sweep voltammetry under Xe-lamp illumination and incident photon to conversion efficiency measurements under monochromatic illumination. We found that NiTiO3 acted as the most effective photoanode among MTiO3. The mechanism of the different PEC performance was discussed from the viewpoint of the electronic structures investigated by photoemission spectroscopy and band calculation based on density functional theory. We revealed that the stronger hybridization between O 2p and M 3d states was responsible for the more efficient photoanodic activity among ilmenite-type MTiO3.

Recently, the successful synthesis of two-dimensional (2D) boron on metallic surfaces has motivated great interest in improving the stability of 2D boron to allow the realization of promising materials. Through the use of an ab initio evolutionary search algorithm, we have discovered a series of TiBx (2 ≤ x ≤ 16) structures that consist of earth-abundant titanium and boron atoms in 2D arrangements. These structures are greatly stabilized by electron transfer from Ti to B, therefore, leading to much better stability than the 2D boron sheets proposed so far. In particular, TiB12 has a low enough energy to make it competitive to a mixture of the well-known TiB2 compound and a 2D α-boron sheet and exhibits a quasi-Dirac point with a 0.02 eV gap. Interestingly, the work function and conductivity of this 2D TiB12 material are calculated to be tunable through the application of biaxial strain. The possibility of synthesis and novel electronic properties expected for 2D TiB12 render it a promising new 2D material for nanoelectronic applications.

Although graphitic carbon nitride (g-C3N4) is a promising photofunctional material, its structure is poorly understood. Here, we present a systematic study of stable crystal structures of g-C3N4 by ab initio evolutionary searching. It was discovered that off-plane distortion of heptazine units is a characteristic of the most stable structure, which explains a known discrepancy between the lattice parameters determined by X-ray diffraction (XRD) patterns and the planar structures modeled in previous studies. A phase transition from a metastable phase to the global minimum phase provides a reasonable explanation for the observed red shift in photoabsorption edges upon high-temperature annealing. The recently suggested salt-melt synthesis for g-C3N4 is subject to the contamination of hydrogen, chlorine, and lithium according to our detailed analysis of the crystal structures of C6N9H3-Li3Cl and C6N9H3-LiCl in comparison with the measured XRD patterns of these samples. Finally, a viable synthesis pathway for ...

Barium disilicide (BaSi2), composed of abundant and inexpensive elements, is a potential absorber material for thin-film solar cells. In this study, density-functional theory calculations show that BaSi2 belongs to a Zintl phase with a mixed character of covalent units of tetrahedral Si4 with an ionic nature described by (2Ba2+)(Si4)4−. The molecular orbital diagram is elucidated based on the electronic structures, suggesting that the charge transfer transition from Si p to Ba d greatly enhances optical absorption. A large photoabsorption coefficient is confirmed using advanced excited state calculations that include excitonic effects. The ionization potential of BaSi2 is smaller than that of silicon or germanium, suggesting that the band edge positions are suitable for p-type conductivity and type II heterojunctions for BaSi2/Si or BaSi2/Ge. The chemical potential window for the stable growth of a stoichiometric BaSi2 film is very narrow, and the stability of native defects is investigated under realistic growth conditions. Si vacancy, Ba substituted for Si antisite, and Si interstitial defects are predominant but do not cause the generation of a significant number of carriers. The calculated Fermi level is pinned in the middle of the band gap for the entire silicon chemical potential range and a wide growth temperature range, indicating the feasibility of bipolar doping, which is advantageous for fabricating p–n junctions.

Realization of efficient p-type conductivity in wide gap oxides is a challenging task partly due to the localized nature of non-bonding oxygen 2p states of which the valence band maximum consists. In this study, effects of anisotropic strain on an accepter level of Al-doped anatase TiO2 are investigated using LDA+U calculations. Strain engineering effectively increases the cation states in the valence band maximum of TiO2. It is demonstrated that a deep acceptor level induced by substitutional Al for Ti is turned into a delocalized shallow level under a tensile strain of as much as 8%. This effect is confirmed by the analysis of thermodynamic transition level which is largely shifted from 0.8 eV above to below the valence band maximum, being a shallow acceptor, as the tensile strain is increased.

Band-gap engineering of oxide materials is of great interest for optoelectronics, photovoltaics, and photocatalysis applications. In this study, electronic structures of perovskite oxynitrides, LaTiO2 N and SrNbO2 N, and solid solutions, (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x , are investigated using hybrid density functional calculations. Band gaps of LaTiO2 N and SrNbO2 N are much smaller than that of SrTiO3 owing to the formation of a N 2p band, which is higher in energy than the O 2p band. The valence- and conduction-band offsets of SrTiO3 /LaTiO2 N and SrTiO3 /SrNbO2 N are computed, and the adequacy for H2 evolution is analyzed by comparing the positions of the band edges with respect to the standard hydrogen electrode (SHE). The band gap of (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x solid solutions are also discussed.

Finding a highly efficient, selective and economic approach for electrochemical reduction of aqueous carbon dioxide is a great challenge in realizing an artificial system for a sustainable carbon cycle. Novel mesoporous palladium–copper bimetallic electrocatalysts with superior activity and high faradaic efficiencies (FEs) are reported for the first time. The mesoporous nanostructure provides a roughened surface which is abundant in active sites and promotes selective conversion of CO2 to CO. First-principles calculations exhibit that Pd atoms on the catalyst surface serve as reactive centers and highly selective CO formation is attributed to the geometric and electronic effects within the palladium–copper bimetallic alloys. The CO2 and COOH* intermediate adsorption ability and the CO desorption ability on Pd atoms are effectively enhanced in the presence of Cu. Our results provide wide ranging implications for further improving the design and preparation of CO2 reduction electrocatalysts.

We studied the formation of Ba1−xSrxZrO3 (0 ≤ x ≤ 1) nanoparticles under highly super-saturated conditions, using supercritical water. It is known that B-site Zr in the perovskite structure plays a dominant role at the nucleation stage, with high nucleation rates under supercritical conditions; this is due to the significantly lower solubility of Zr, compared with A-site ions (i.e., Zr precipitates faster, and A-site ions are taken up into particle after the Zr nucleation). However, in this study, it was found that A-site Ba and Sr significantly influenced the particle size, the A-site deficiency rate, and the surface-OH density of the nanoparticles. The differences in particle size suggested that the ripening or coalescence that occurred after the nucleation stage was dominant in determining the particle size, even under highly super-saturated conditions such as those realized in the supercritical hydrothermal synthesis. The characteristic nanostructure formed in the supercritical water was analyzed in detail; variables such as the A-site deficiency rate and the surface-OH density were investigated. The existence of vacancies at the A-site was confirmed using X-ray absorption fine structure, and a highly defective structure was obtained, particularly when the Ba content was high. The surface state of the nanoparticles was also studied using X-ray photoelectron spectroscopy and first-principles calculations, with the aim of understanding the differences in particle size, and the effects of the A-site deficiencies; the amount of surface-OH corresponded to the A-site deficiency rate, and had an inverse relationship with the particle size.

The effects of in-plane biaxial strain on the electronic structure of a photofunctional material, single-layer SnS2, were systematically investigated using hybrid density functional calculations. The bonding diagram for the band gap was firstly proposed based on the crystal orbital overlap population analysis. The conduction band-edge of single-layer SnS2 is determined by the anti-bonding interaction between Sn-5s and S-3p orbitals, while the valence band-edge comes from the anti-bonding between the neighboring S atoms. It is found that the compressive strain not only decreases the indirect band gap of single-layer SnS2, but also effectively promotes the band-edges of the conduction band to realize the overall water splitting. Besides, the dispersion of the valence band of single-layer SnS2 becomes weaker with increasing tensile strain which is beneficial for the photo-excitation through direct transitions.

The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid density functional calculations. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concentrations, giving rise to free electrons with carrier densities from 1011 to 1019 cm-3 under oxygen-rich and oxygen-poor conditions, respectively. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chemical potential. Even under extreme oxygen-rich conditions, the Fermi level, which is determined from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band maximum, indicating that p-type conductivity can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type conductivity of TiO2. At a typical annealing temperature and under a typical oxygen partial pressure, the carrier concentration is found to be approximately 5 × 1013 cm-3.

In situ high-energy X-ray diffraction measurements were conducted to elucidate the dynamics of barium zirconate (BaZrO 3 ) nanoparticle formation in supercritical water. Time-resolved experiments of approximately milliseconds were achieved using a continuous-flow reactor and high-energy X-ray diffraction, which allows us to probe the sample in a stainless-steel tube. The size and structure of BaZrO 3 in the early stage of crystallization under supercritical conditions (400 °C, 30 MPa) and on the order of milliseconds (46–66 ms) were successfully observed. An increase in the lattice parameter of BaZrO 3 owing to a decrease in the number of defect sites in the time range was observed for the first time. A first-principles calculation confirmed the relationship between Ba defects and the lattice parameter and supported the formation mechanism of BaZrO 3 in which nuclei, consisting of mainly zirconium, absorb the barium ion in supercritical water during crystallization.

This chapter discusses the computational design of inorganic photoelectrochemical (PEC) materials. The electronic structure of photoanodes and photocathodes significantly affect photoabsorption, carrier transport, and water redox properties. Adjusting the band edge positions with respect to redox potentials is, therefore, an important task for the design of photocatalysts for PEC application. We present our recent attempts on the band edge engineering of semiconductor photocatalysts using density functional theory (DFT) calculations. First, we discuss the effects of doping on visible light absorption as well as introduction of conductive carriers. Second, we demonstrate how the band edge positions are controlled by biaxial strain. Third, we present our studies on an evolutional crystal structure search for predicting novel photofunctional materials. These results demonstrate that modern DFT-based computational materials science is a powerful tool for finding promising PEC materials.

The main aim of this study is to scrutinize promising plasmonic materials by understanding their electronic structure and correlating them to the optical properties of selected refractory materials. For this purpose, the electronic and optical properties of the conductive ceramics TiC, ZrC, HfC, TaC, WC, TiN, ZrN, HfN, TaN, and WN are studied systematically by means of first-principles density functional theory. A full ab initio procedure to calculate plasma frequency from the electronic band structure is discussed. The dielectric functions are calculated by including electronic interband and intraband transitions. Our calculations confirm that transition metal nitrides, such as TiN, ZrN, and HfN, are the strongest candidates, exhibiting performance comparable to that of conventional noble metals in the visible to the near-infrared regions. On the other hand, carbides are not suitable for plasmonic applications because they show very large losses in the same regions. From our calculated dielectric functions, the scattering and absorption efficiencies of nanoparticles made of these refractory materials are evaluated. It is revealed that TiN and TaC are the best candidate materials for applications in photothermal energy conversion over a broad spectral region. Furthermore, quality factors for localized surface plasmon resonance and surface plasmon polaritons are calculated to compare quantitative performances, and ZrN and HfN are found to be comparable to conventional plasmonic metals such as silver and gold, Comment: 9 pages, 7 figures and 1 table

A series of Sn(Nb 1− x Ta x ) 2 O 6 solid solutions were successfully synthesized as novel visible-light-active photocatalysts. The characterizations of powder X-ray diffraction and UV–vis absorption spectrum indicate that these solid solutions possess continuous changes in the crystallography features and optical band gaps, and are therefore a group of continuous solid solutions. Their photocatalytic properties were evaluated via methylene blue (MB) photodegradation. The Sn(Nb 0.8 Ta 0.2 ) 2 O 6 sample showed the best photocatalytic performance, full mineralization ability, and an excellent stability. After 120 min of irradiation, the degradation and mineralization percentages of MB over Sn(Nb 0.8 Ta 0.2 ) 2 O 6 reached 99.5% and 98.9%, respectively; in addition, the degradation ratio remained at 97% after five-cyclic test. The further study on the active species and the theoretical calculation based on density functional theory helped to clarify the mechanism of the photodegradation reaction. On this basis, we propose the strategies to design high-performance photocatalysts employed in hole-dominated and electron-dominated photocatalytic applications, respectively.

Silver orthophosphate (Ag3PO4) is known as a highly active visible-light sensitized photocatalyst, yet its doping effects on electric properties have not been well understood. Using hybrid density-functional calculations, we study possibilities for n-type and p-type doping in Ag3PO4. It is found that a sulfur substituted for phosphorus (SP) has a relatively low formation energy (high solubility) and acts as a shallow donor in any growth conditions examined. Whereas, a substitutional silicon at phosphorus site (SiP) is a deep acceptor and its solubility is low, indicating that p-type conductivity is unlikely to occur by Si doping. Our results suggest that sulfur doping is a promising approach for the realization of n-type Ag3PO4.

The effects of interlayer lone-pair interactions on the electronic structure of SnO are explored using density-functional theory. Our comprehensive study reveals that the band gap of SnO opens with the increase in the interlayer Sn-Sn distance. The effect is rationalized by the character of band edges which consist of bonding and anti-bonding states from interlayer lone pair interactions. The band edges for several nanosheets and strained double-layer SnO are estimated. We conclude that the double-layer SnO is a promising material for visible-light driven photocatalysts for hydrogen evolution.

For the facet engineering of brookite TiO2, the surface atomic structure is known but the electronic structure has been rarely studied to date. Herein, we investigated both the surface atomic and electronic structure of brookite TiO2 with various facets exposed. Theoretical calculations reveal that the {121} surface contains more undercoordinated Ti atoms and a higher surface energy than that of the {211} surface, and the experimental results show that brookite TiO2 nanorods exposed with majority {121} facets (T121) have a lower valence band (VB) potential; those above-mentioned superior properties enable T121 to show excellent performance in RhB photodegradation. Nevertheless, the electronic structure analyzed from the Density of State (DOS) plots revealed that the electron density is dispersed in the bulk for TiO2 covered with a {121} surface, indicating that the electrons might be more reluctant to migrate from bulk to surface, which might be the reason for the poor H2 productivity of T121. In contrast, brookite TiO2 nanosheets exposed with dominant {211} facets (T211) exhibited a much higher conduction band (CB) potential resulting in a much higher H2 evolution rate (801 μmol h−1) in photocatalytic water splitting. Accordingly, combining the analyses of the surface atomic structure and electronic band structure, it is suggested that, for brookite TiO2, the {121} surface is beneficial for photocatalytic oxidation reactions while the {211} surface can facilitate the photocatalytic reduction process.

Semiconductor photocatalysts that can produce hydrogen from water splitting are of great interest. Among the various possibilities, nitrogen (N)-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ is a promising candidate for hydrogen evolution under visible-light irradiation. In this study, hybrid density-functional calculations are employed to investigate the stability and impact of nitrogen impurities on the electronic and optical properties in $\mathrm{SrTi}{\mathrm{O}}_{3}$. We find that the substitutional N on O site (${\mathrm{N}}_{\mathrm{O}}$) is a deep acceptor in $\mathrm{SrTi}{\mathrm{O}}_{3}$. Moreover, ${\mathrm{N}}_{\mathrm{O}}$ predominates over other N-related defects under equilibrium growth conditions in $n$-type $\mathrm{SrTi}{\mathrm{O}}_{3}$. Our results reveal that ${\mathrm{N}}_{\mathrm{O}}$ gives rise to visible-light absorption in agreement with experimental observations. In addition, we find that hydrogen can bind to ${\mathrm{N}}_{\mathrm{O}}$, forming ${\mathrm{N}}_{\mathrm{O}}\ensuremath{-}{\mathrm{H}}_{i}$ complex which leads to a blueshift in the optical absorption. Other N configurations can also contribute to optical absorption in the visible range. The vibration frequencies of different N configurations are provided to support identification using vibrational spectroscopy techniques.

Although compositional tuning of metal nanoparticles (NPs) has been extensively investigated, possible control of the catalytic performance through bulk-structure tuning is surprisingly overlooked. Here we report that the bulk structure of intermetallic ZrPt3 NPs can be engineered by controlled annealing and their catalytic performance is significantly enhanced as the result of bulk-structural transformation. Chemical reduction of organometallic precursors yielded the desired ZrPt3 NPs with a cubic FCC-type structure (c-ZrPt3 NPs). The c-ZrPt3 NPs were then transformed to a different phase of ZrPt3 with a hexagonal structure (h-ZrPt3 NPs) by annealing at temperatures between 900 and 1000 °C. The h-ZrPt3 NPs exhibited higher catalytic activity and long-term stability than either the c-ZrPt3 NPs or commercial Pt/C NPs toward the electro-oxidation of ethanol. Theoretical calculations have elucidated that the enhanced activity of the h-ZrPt3 NPs is attributed to the increased surface energy, whereas the stabi...

Air pollution by automobile exhaust is becoming a global challenge; it is not limited to developing countries where traffic volumes are rapidly increasing.[1] Small vehicles, such as motorcycles and scooters with an engine displacement of 100 mL or less,[2] are one of the major causes for such air pollution; only a small percentage of these vehicles is equipped with exhaust purifiers.[3] Current exhaust purifiers comprise catalysts based on platinum-group metals (PGMs), such as Pt, Pd, and Rh.[4–7] The supply of PGMs may not be reliable for meeting the increasing demands for exhaust purification since deposits of PGMs in the Earth's crust are limited and localized.[8] Furthermore, PGM-based catalysts are not suited to the low-temperature exhaust of small vehicles (

Most transparent conducting oxides (TCOs) exhibit n-type conductivity and are difficult to dope into p-type. Therefore, the development of efficient p-type TCOs is challenging. ZnRh2O4 spinel has been recognized as a potential p-type TCOs. However, the source of its p-type conductivity has not been elucidated. In this study, we used hybrid density functional calculations to investigate the energetics and electronic properties of native defects in ZnRh2O4, including vacancies, interstitials, and cation antisites. We found that all acceptor-type defects including Zn vacancies, Zn antisites, and Rh vacancies acted as deep centers. Charge neutrality analysis suggested that undoped ZnRh2O4 may behave as a p-type semiconductor with hole concentrations of 1018–1019 cm−3 under the extreme O-rich/Rh-poor growth condition in which ZnRh has a low formation energy and acts as the major source of hole carriers. However, under realistic growth conditions, the experimentally determined hole concentration significantly exceeds that which is calculated. Our results suggest that native point defects are unlikely to be responsible for the high hole concentrations observed in ZnRh2O4 spinel.

Copper iodide (CuI) is an attractive transparent p-type semiconductor, and we investigated the relationship between the optical property and native defects in CuI using experimental and theoretical studies. To exclude neither surface impurity nor interface strain, we used well-defined CuI single crystals with native defects, i.e., Cu-rich CuI and I-rich CuI, as well as highly pure CuI, which were prepared by post-annealing treatment of the CuI single crystal under controlled atmosphere. The optical absorption and photoluminescence (PL) properties of these samples were then carefully evaluated. Consequently, two absorption signals (AB1: 2.9 eV, AB2: 2.7 eV) and two PL peaks (PL1: 2.9 eV, PL2: 1.8 eV) were observed. The AB1, AB2, and PL1 signals were obvious under I-rich conditions, whereas the PL2 signal was dominant in the Cu-rich sample. To discuss the origin of these absorption and PL signals, we calculated the absorption and emission energies of defects in CuI using the density function theory (DFT). As a result, AB1 and PL1 are assigned to the transition and recombination between copper vacancy (VCu) and conduction band, while PL2 is assigned to the recombination from the iodine vacancy (VI) to the valence band. Most interestingly, AB2 is presumed to be due to the transition from the valence band to antisites of iodine substituted for copper (ICu), which can be reasonably explained by the off-center model of substituted iodine ions. This work will contribute to developing and understanding opto-electrical devices using CuI.Copper iodide (CuI) is an attractive transparent p-type semiconductor, and we investigated the relationship between the optical property and native defects in CuI using experimental and theoretical studies. To exclude neither surface impurity nor interface strain, we used well-defined CuI single crystals with native defects, i.e., Cu-rich CuI and I-rich CuI, as well as highly pure CuI, which were prepared by post-annealing treatment of the CuI single crystal under controlled atmosphere. The optical absorption and photoluminescence (PL) properties of these samples were then carefully evaluated. Consequently, two absorption signals (AB1: 2.9 eV, AB2: 2.7 eV) and two PL peaks (PL1: 2.9 eV, PL2: 1.8 eV) were observed. The AB1, AB2, and PL1 signals were obvious under I-rich conditions, whereas the PL2 signal was dominant in the Cu-rich sample. To discuss the origin of these absorption and PL signals, we calculated the absorption and emission energies of defects in CuI using the density function theory (DFT). A...

Controllability of defect and impurity properties in oxides is an important issue in various oxide applications such as the high-permittivity (high-κ) gate insulating oxides for field-effect transistors (FETs). The structure of high-κ/Si interface is affected by the formation of defects in high-κ oxides. For example, the growth of a SiOx interfacial layer at HfO2/Si is promoted by the formation oxygen vacancies in HfO2 [1,2]. While, a silicate layer La2(1−x)Si3x/2O3 is formed at La2O3/Si interface. The origin of the different interfacial structures has been recently clarified [3]. Our comparative study for a silicon impurity in La2O3 and HfO2 reveal that the stability of silicon at substitutional sites strongly depends on its coordination number. When substituted for lanthanum, a silicon atom fits comfortably in La2O3 thanks to the formation of a SiO4 tetrahedral structure (Fig. 1a). In addition, the substitutional silicon acts as a donor impurity in La2O3, increasing oxygen content in the oxide. This contributes to the formation of lanthanum silicate at the La2O3/Si interface. By contrast, a Si atom is unstable in HfO2 because it is coordinated with six oxygen atoms (Fig. 1b), which results in precipitation of SiOx layer at the HfO2/Si. This leads to an important concept that donor impurities in an oxide turn it into an oxygen absorber. Based on the above concept, we have suggested a scavenging strategy for the redundant SiOx layer at the HfO2/Si [4]. A recent requirement for the use of a high-κ oxide in FETs imposes a new challenge in achieving an atomically abrupt oxide/silicon interface without the formation of a SiOx interfacial layer. Our first-principles calculations exhibit doping HfO2 with donors significantly increases the oxygen concentration in HfO2, indicating a great possibility of scavenging oxygen atoms from the neighboring SiOx layer (Fig. 2). This prediction has been validated by our experimental results for Ta2O5-x deposition on a HfO2/SiO2/Si stack. The thickness of the SiOx layer was significantly reduced as Ta atoms diffuse into HfO2. This impurity-driven interfacial reaction will be a key technology in oxide applications. The success of our theoretical design of oxides is not limited in the high-κ gate insulators. We have recently exported the defect-impurity physics to the design of photocatalytic materials, which possesses a great potential for environmental remediation and fuel production [5]. In this talk, I will also talk about recent progresses in understanding of photocatalytic activities in nitrogendoped TiO2 [6] and Chromium-doped SrTiO3 [7]. This work was supported by the Japan Science and Technology Agency (JST) Precursory Research for Embryonic Science and Technology (PRESTO) program.

In this study, for the first time, {111} facet exposed anatase TiO2 single crystals are prepared via both F– and ammonia as the capping reagents. In comparison with the most investigated {001}, {010}, and {101} facets for anatase TiO2, the density functional theory (DFT) calculations predict that {111} facet owns a much higher surface energy of 1.61 J/m2, which is partially attributed to the large percentage of undercoordinated Ti atoms and O atoms existed on the {111} surface. These undercoordinated atoms can act as active sites in the photoreaction. Experimentally, it is revealed that this material exhibits the superior electronic band structure which can produce more reductive electrons in the photocatalytic reaction than those of the TiO2 samples exposed with majority {010}, {101}, and {001} facets. More importantly, we demonstrate that this material is an excellent photocatalyst with much higher photocatalytic activity (405.2 μmol h–1), about 5, 9, and 13 times that of the TiO2 sample exposed with do...

The electronic structures of highly active Ag-based oxide photocatalysts Ag3AsO4 and Ag3PO4 are studied by hybrid-density functional calculations. It is revealed that Ag3AsO4 and Ag3PO4 are indirect band gap semiconductors. The Hartree–Fock mixing parameters are fitted for experimental band gaps of Ag3AsO4 (1.88 eV) and Ag3PO4 (2.43 eV). The smaller electron effective mass and the lower valence band edge of Ag3AsO4 are likely to be responsible for the superior photocatalytic oxidation reaction to Ag3PO4. The comparable lattice constant and analogous crystal structure between the two materials allow the opportunities of fine-tuning the band gap of Ag3AsxP1−xO4 using a solid-solution approach. The development of Ag3AsxP1−xO4 should be promising for the discovery of novel visible-light sensitized photocatalysts.

A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations. Computational analysis suggested that tin monosulfide (SnS) would be more efficient than SnS2 as a photocathode for hydrogen production because of the low ionization potential and weak ionic character of SnS. To test this experimentally, nanoparticles of SnS were loaded onto a mesoporous electrode using a wet chemical method, and the bandgap of the synthesized SnS quantum dots was found to be tunable by adjusting the number of successive ionic layer adsorption and reaction (SILAR) cycles, which controls the magnitude of the quantum confinement effect. Efficient hydrogen production was achieved when the bandgap of SnS was wider than that of the bulk form.

Colorful rutile TiO2 was prepared by heating Ti2O3 at 550–900 °C to develop novel visible-light-sensitive and eco-friendly photocatalysts for environmental remediation under visible-light irradiation. The colors of the prepared samples, which ranged from grayish green to yellowish off-white via yellow differed from the reported colors of reduced TiO2, such as blue and black. The TiO2 prepared in this study was characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and UV–Visible spectroscopy. These measurements showed that the TiO2 contained Ti3+-interstitial sites. The TiO2 was sensitive to visible light, and calculation of the band diagram demonstrated that this visible-light absorption is caused mainly by formation of Ti3+-interstitial sites in rutile TiO2. Among the prepared samples, the TiO2 prepared by heating Ti2O3 at 700 °C shows the highest photocatalytic activity under visible-light irradiation. In addition, the sample was further and mildly ground using a bead-milling machine. The ground sample possessed higher surface area and better photocatalytic activity.

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

The behavior of holes in the valence band of BaTiO3 is investigated using hybrid density-functional calculations. We find that holes tend to self-trap, localizing on individual O atoms and causing local lattice distortions, forming small hole-polarons. This takes place even in the absence of intrinsic defects or impurities. The self-trapped hole (STH) is more energetically favorable than the delocalized hole in the valence band. The calculated emission peak energy corresponding to the recombination of a conduction band electron with a STH can explain the observed photoluminescence at low temperatures. The stability of the STH, its migration barrier, and the related emission peak are then compared to those of SrTiO3.

The conduction mechanism of HfSiON stacked-gate dielectric films after a spike annealing is investigated. FETs fabricated using the Damascene process is used to evaluate the electronic characteristics of a stacked-gate dielectric film. The effect of an additional spike annealing after the TiN gate electrode fabrication on the conductivity is analyzed. The electron current over the entire range of gate voltage is increased, while the hole current remains unchanged. The TAT and Schottky analyses derive a consistent energy level of the intermediate trap centers contributing to the electron conduction. The average lifetime is improved by annealing. However, the spike annealing does not change the slope of the TDDB Weibull plots. The average lifetime is improved by annealing. From NBTI analyses, the threshold voltages displayed linearly shifts with increasing injection charge concentration, both with and without the spike annealing. The slope of the threshold voltage shift is defined as a degradation rate.

HfO2-based oxide is a promising gate insulator for the next generation of the field effect transistors. Yet it is accompanied by a problematic effect, that is, the Fermi level of a metal electrode deposited on HfO2 is pinned at a certain position regardless of the type of the metal. On the other hand, this effect, the so-called Fremi level pinning, does not occur in a metal/La2O3/Si gate stack. This is puzzling because both oxides have similar band gaps and dielectric constants. We suggest that the high solubility of Si in La2O3, and thereby the formation of a silicate layer LaxSiyOz between La2O3 and the Si substrate plays important role in the suppression of electron transfers from La2O3 to the metal electrode: The charge neutrality is satisfied at the ionic/ionic interface La2O3/LaxSiyOz, and thus there is no need to exchange electrons with outside. At the ionic/covalent interface HfO2/SiO2, however, the charge neutrality is insufficient and electrons are readily transferred into the metal electrode causing the Fermi level pinning.

We have succeeded in imaging the leakage sites of hafnium silicate gate dielectrics of metal-oxide-semiconductor field-effect transistors (MOSFETs) by using electron-beam-induced current (EBIC) method. Leakage sites of p-channel MOSFETs were identified as bright spots under appropriate reverse bias condition when the electron beam energy is high enough to generate carriers in the silicon substrate. Most of the leakage sites were observed in the peripheries of shallow trench isolation. These results suggest that some process induced defects are the cause of leakage in these MOSFETs. Our observation demonstrates the advantage of EBIC characterization for failure analysis of high-k MOSFETs.

Ionicity has been raised as an important factor in discussing defect formation in Hf-based oxides. It has been elucidated from our first-principles calculations, that the stability of defects is dominated by Coulomb interactions between charged defects and surrounding ions. For instance, the formation energy of positively charged oxygen vacancies (VO+2) is markedly decreased when they are coupled with substitutional N atoms at O sites, as the nominal charges of N3- are negatively greater than those of the oxygen ions O2-. Our computational results have further revealed that the ionic character of Hf causes generation of a low-lying Si dangling bond level in Si doped HfSiOx. These results suggest that one must take into account the ionicity of the Hf-based compounds, which possess totally different properties from the conventional gate insulator SiO2 in terms of defect formation.

Leakage current characteristics with significantly reduced dispersion were achieved for HfSiON gate dielectric film by selecting TiN gate electrode with low reactivity, compared to the conventional poly-Si gate. The breakdown lifetime and its characteristic homogeneity were also enormously improved. With intensive study on these electrical characteristics, it was concluded that the concentration of oxygen vacancy was significantly reduced, giving rise to the homogeneity improvement. Moreover, we have shown an explanation for the dielectric breakdown in terms of oxygen transport in the Hf-based dielectric film.