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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.

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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 pressure through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (A-SnO), which possesses space group P21/c and a wide band gap of 1.9 eV, is more stable than l-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 t-SnO at around 120 GPa. Our work also reveals that h-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase o-SnO for the phase transition path Pnma-SnO ®u-SnO ® g-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize h-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of -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.

The existence of a topological Dirac nodal loop is predicted for hydrogenated monolayer boron sheets with a nonsymmorphic symmetry. The three-center two-electron bonds in boron compounds restrict the electronic system to be insulating or semimetallic with a Dirac nodal loop. Two types of electronic structures are distinguished by the Z_2 topological index and confirmed by first-principles total-energy calculations. The topological taxonomy, developed in this research, can be applied to other two-dimensional materials and to seek novel Dirac fermions.

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.

Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an average yield of 42.3% at room temperature. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair dist...

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.

Diffusion of A-site cations in A-site-deficient BaZrO3 and SrZrO3 perovskite oxides was investigated by first-principles nudged elastic band calculations. We found an energy barrier for surface diffusion that was significantly lower than that for bulk diffusion (3.41 eV vs 2.41 eV for Ba diffusion in Ba-deficient BaZrO3 and 3.07 eV vs 1.44 eV for Sr diffusion in Sr-deficient SrZrO3), which suggests unusually rapid cation diffusion at low temperatures in nanomaterials with a large proportion of surface-exposed atoms. The lower mobility of Ba in BaZrO3 compared to that of Sr in SrZrO3 explains why the proportion of A-site vacancies in BaZrO3 nanoparticles is much greater than that in SrZrO3 nanoparticles, as previously observed during nanoparticle formation in which an A-site-deficient nucleus gradually incorporated A-site cations. Finally, thermal and nanoscale effects on the lattice parameter were studied by carrying out high-temperature X-ray diffraction measurements on BaZrO3 nanoparticles synthesized i...

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.

Photoreduction of CO2 for fuel production is considered to be an ultimate solution to today's energy crisis. Platinum (Pt) particles are known to promote photocatalysis reactions when loaded on the surface of titanium dioxide (TiO2). In this study, we investigate the initial step of the reduction process of CO2 with water, i.e., the formation of formate, HCOO, from surface bound CO2 and H2O on rutile TiO2(110) in terms of energetics of initial and final states using density functional theory calculations. To understand the role of a Pt cocatalyst, chemisorption energies of HCOO and OH on TiO2(110) are investigated with and without a Pt cluster. It is revealed that free electrons provided by the Pt cluster dramatically decrease the chemisorption energy thanks to the electron transfer from high lying Pt states to unoccupied valence states induced by the adsorbates, which facilitates ionization of HCOO- and OH- on the TiO2 surface near the Pt cluster. Direct adsorption of HCOO and OH on the surface of the Pt cluster is also energetically favored.

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.

Metal/TiO2 interfaces have been extensively studied because of their importance in electronic devices, electrochemical cells, and photocatalysis. In this article, we present our studies on electronic structures for anatase TiO2(001)/fcc-metal(001) (metal = Pt, Pd, or Au) interfaces using first-principles calculations. It is demonstrated that the Schottky barrier height depends on the metal work function and significantly decreases at an interface with strong adhesion between the metal and TiO2. The sizable reduction of the barrier height is a consequence of dipole formation at the interface due to electron transfer from TiO2 to the metal. The formation of dipoles at the Pt/TiO2 interface is supported by our experimental results for a core-level binding-energy shift in Pt clusters loaded on the surface of TiO2. Differences in the bonding and antibonding characters of metal–O bonds for the three metals are discussed based on the projected densities of states given by our density-functional theory calculations.

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

Mesoporous Pd nanoparticles (MPNs) enclosed by high-index facets have been successfully prepared by taking advantage of successive oxygen adsorption and desorption caused by the oxidative etching effect. The as-prepared MPNs exhibit excellent performance toward formic acid electro-oxidation, which is due to the synergetic effect between the diffusion-feasible tubular mesochannels and the high index facets.

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.

SrGe 2 and BaGe 2 were characterized for structural, electronic and optical properties by means of diffuse reflectance and first-principles density functional theory. These two germanides crystallize in the BaSi 2 -type structure, in which Ge atoms are arranged in tetrahedral configuration. The calculation indicates a charge transfer from Sr (or Ba) atoms to Ge atoms along with the formation of covalent bonds among Ge atoms in Ge tetrahedral. The computational results confirm that these two germanies are Zintl phase described as Sr 2 Ge 4 (or Ba 2 Ge 4 ), which are characterized by positively charged [Sr 2 (or Ba 2 )] 2.59+ and negatively charged [Ge 4 ] 2.59− units acting as cation and anion, respectively. These compounds are indirect gap semiconductors with band gap estimated to be E g = 1.02 eV for BaGe 2 and E g = 0.89 eV for SrGe 2 which are in good agreement with our experimental measured values ( E g = 0.97 eV for BaGe 2 and E g = 0.82 eV for SrGe 2 ). Our calculations demonstrate that the band gaps are narrowed by application of hydrostatic pressure; the pressure coefficients are estimated to be −10.54 for SrGe 2 and −10.06 meV/GPa for BaGe 2 . Optical properties reveal that these compounds have large absorption coefficient (∼7.5 × 10 4 cm −1 at 1.5 eV) and the estimated high frequency (static) dielectric constant are, e ∞ ( e 0 ) ≈ 12.8(20.97) for BaGe 2 and e ∞ ( e 0 ) ≈ 14.27(22.87) for SrGe 2 .

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.

Two vanadates, Ag2Sr(VO3)4 and Sr(VO3)2, have been studied as visible-light-driven water oxidation photocatalysts with the help of density-functional theory calculations. Our computational results for the density of states and partial charge densities implied that Ag2Sr(VO3)4 and Sr(VO3)2 possess desirable electronic structures for the water oxidation reaction, i.e., the valence band (VB) maximum of Ag2Sr(VO3)4 consists of multiple orbitals of Ag d and O p, while Sr(VO3)2 has a broad VB associated with oxygen non-bonding states. We have experimentally demonstrated that these vanadates efficiently oxidize water to O2 under irradiation of visible light in the presence of the sacrificial agent.

Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1-x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d-d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1-x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps.

Novel intermetallic TaPt3 nanoparticles (NPs) are materialized, which exhibit much higher catalytic performance than state-of-the-art Pt3Sn NPs for electrooxidation of ethanol. In situ infrared-reflection-absorption spectroscopy (IRRAS) elucidates that the TaPt3 NPs cleave the C–C bond in ethanol at lower potentials than Pt NPs, efficiently promoting complete conversion of ethanol to CO2. Single-cell tests demonstrate the feasibility of the TaPt3 NPs as a practical energy-extraction catalyst for ethanol fuels, with more than two times higher output currents than Pt-based cells at high discharge currents.