Density Functional Theory Study on the Structural and Electronic Properties of Low Index Rutile Surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 Composite Systems

Enhanced Sensing Performance of SnX Ti1-X O2 -TiX Sn1-X O2 Core-Shell Heterostructure via Increasing the Density of Unsaturated Sn and Ti Atoms

The strategy of combining different semiconductor materials is adjudged an effective approach to improve the sensing performances of semiconductor materials. However, the specific synergistic mechanism for the excellent gas-sensitive performances of composite materials has not been elucidated. Herein, a facile solvothermal method is employed to synthesize SnX Ti1-X O2 -TiX Sn1-X O2 core-shell heterostructures using SnCl4 •5H2 O and tetrabutyl titanate (TBOT) as raw materials. When the molar ratio of SnCl4 •5H2 O/TBOT is 1.8/3.0, the afforded composite exhibited the highest gas sensing performances compared with other composites prepared with other molar ratios. The enhanced sensing performance is attributed to the simultaneous incorporation of Sn and Ti ions into each other's lattice, leading to an increase in the density of unsaturated Sn and Ti atoms on the surface. Ultimately, more oxygen vacancies are formed by the unsaturated Sn and Ti atoms, which benefits electron capture and the redox reaction of adsorbed gases. Thus, the concept of increased unsaturated metal atoms and oxygen vacancy resulting from the doping of different metal ions into each other's lattice has deepened the understanding of gas sensing and the catalytic reaction mechanisms. The lattice synergy of different metals provides a pathway for the design of advanced gas-sensing materials and catalysts.

A method of calculating surface energies for asymmetric slab models

Many essential chemical processes, such as adsorption and catalysis, take place at the surface of a solid material. Hence, accurately determining the energy of a solid surface provides crucial information about the material's potential utility for such processes. The standard method of calculating surface energy yields good approximations for solids that, upon cleavage, expose identical surface terminations (symmetric slabs) but suffers critical shortcomings when applied to the multitude of materials that expose atomically different terminations (asymmetric slabs) due to the inaccurate assumption that the two terminations exhibit exactly the same energy. A more rigorous method of calculating the individual energetic contributions of the two terminations of a cleaved slab was pursued in 2018 by Tian and colleagues, however the approach's accuracy is weakened by a similar assumption that frozen asymmetric terminations contribute exactly the same energy. Herein, a novel technique is presented. The method expresses the slab's total energy in terms of the energy contributions of the top (A) and bottom (B) surfaces in both the relaxed and frozen states. Total energies for different combinations of these conditions are obtained through a series of density-functional-theory calculations alternately optimizing different parts of the slab model. The equations are then solved for the individual surface energy contributions. The method shows improvement over the previously-established approach by exhibiting greater precision and internal consistency, while also providing additional information about the contributions of frozen surfaces.

Effects of oxygen pressure on the morphology and surface energetics of β-PbO2: insight from DFT calculations

For over a century, lead dioxide (PbO₂) has been investigated in lead-acid batteries and extensively utilized in a variety of applications. Identifying the surface properties and equilibrium morphology of β-PbO₂ (rutile phase) particles is mandatory for industrial utilization and surface engineering. Using density-functional calculations within the generalized gradient approximation revised for solids (PBEsol), we investigate a variety of surface properties of β-PbO₂. The surface energies of low-Miller-index stoichiometric surfaces are firstly determined, and the (110) surface is found to be the most thermodynamically stable. The relative energetics of these surfaces are represented by a Wulff construction which shows an acicular shape, mostly dominated by the (110) and (100) surfaces. Besides, we investigate the surface chemistry of β-PbO₂ under reduction and oxidation conditions as a function of oxygen pressure, finding that most surfaces except for (100) and (110) are likely to be oxidized. Under oxygen pressure at 1 atm and oxygen-rich limit, the (101) surface is the most thermodynamically stable, dominating the Wulff construction with pyramidal shapes. Our results indicate that the growth conditions that cause non-stoichiometry of the surface could modify the equilibrium Wulff shape of β-PbO₂. Our predicted Wulff shapes and dominant facets agree with the experimental results in which the pyramidal shape of the β-PbO₂ grains has often been observed with the (101) preferred orientation.

Characterization of Ti/SnO2 Interface by X-ray Photoelectron Spectroscopy

The Ti/SnO₂ interface has been investigated in situ via the technique of x-ray photoelectron spectroscopy. Thin films (in the range from 0.3 to 1.1 nm) of titanium were deposited on SnO₂ substrates via the e-beam technique. The deposition was carried out at two different substrate temperatures, namely room temperature and 200 °C. The photoelectron spectra of tin and titanium in the samples were found to exhibit significant differences upon comparison with the corresponding elemental and the oxide spectra. These changes result from chemical interaction between SnO₂ and the titanium overlayer at the interface. The SnO₂ was observed to be reduced to elemental tin while the titanium overlayer was observed to become oxidized. Complete reduction of SnO₂ to elemental tin did not occur even for the lowest thickness of the titanium overlayer. The interfaces in both the types of the samples were observed to consist of elemental Sn, SnO₂, elemental titanium, TiO₂, and Ti-suboxide. The relative percentages of the constituents at the interface have been estimated by curve fitting the spectral data with the corresponding elemental and the oxide spectra. In the 200 °C samples, thermal diffusion of the titanium overlayer was observed. This resulted in the complete oxidation of the titanium overlayer to TiO₂ upto a thickness of 0.9 nm of the overlayer. Elemental titanium resulting from the unreacted overlayer was observed to be more in the room temperature samples. The room temperature samples showed variation around 20% for the Ti-suboxide while an increasing trend was observed in the 200 °C samples.

Ferromagnetic alloy for high-efficiency photovoltaic conversion in solar cells: first-principles insights when doping SnO2 rutile with coupled Eu-Gd

From results of first-principles all-electron full-potential augmented spherical-wave calculations within a generalized gradient approximation, a materials design for half-metallic ferromagnetic semiconductors based on (Eu,Gd)-doped SnO2 rutile is proposed. Moreover, their half-metallic ferromagnetic properties are homogenous and energetically stable for different crystallographic directions. Therefore, the interatomic exchange interaction between the spins of double impurity ions is a long-range ferromagnetic interaction that is sharply weakened when the distance between Eu-Gd increases. The double impurities most likely substitute adjacent Sn sites and result in strong ferromagnetic interactions by p-f hybridization between rare earth 4f and Op states. There is great interest in the configuration that has the lowest energy difference, where the double impurity substitutes the nearest neighbor Sn sites along the z-axis of SnO2 rutile. Generalized gradient approximation GGA and GGA+U calculations were performed. According to our revPBE-GGA calculations, the ferromagnetic compound is capable of absorbing 96% from the visible light. Furthermore, the transport properties at room temperature ensure excellent electrical conductivity, low thermal conductivity, and the most optimal figure of merit (ZT), which leads to high thermoelectric performance. As the latter are closely related to free flow charge carriers, we can subsequently predict that the ferromagnetic alloy will be able to be a great power source for highly effective photovoltaic conversion in solar cells. Further experimentation will be necessary to obtain confirmation of our ab initio predictions.

First Principles Study of Bonding Mechanisms at the TiAl/TiO2 Interface

The adhesion properties of the TiAl/TiO2 interface are estimated in dependence on interfacial layer composition and contact configuration using the projector augmented wave method. It is shown that a higher value of the work of separation is obtained at the interface between the Ti-terminated TiAl(110) surface and the TiO2(110)O one than at that with the Al-terminated alloy. An analysis of structural and electronic factors dominating the chemical bonding at the interfaces is carried out. It is shown that low bond densities are responsible for low adhesion at both considered interfaces, which may affect the spallation of oxide scale from the TiAl matrix.

Synthesis of coral-globular-like composite Ag/TiO2-SnO2 and its photocatalytic degradation of rhodamine B under multiple modes

Taking cetyltrimethylammonium bromide (CTAB) as the template and using TiO₂ as the substrate, coral-globular-like composite Ag/TiO₂-SnO₂ (CTAB) was successfully synthesized by the sol-gel combined with a temperature-programmed treatment method. X-ray diffraction, scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, SEM combined with X-ray energy dispersive spectroscopy, and N₂ adsorption-desorption tests were employed to characterize samples' crystalline phase, chemical composition, morphology and surface physicochemical properties. Results showed that composites not only had TiO₂ anatase structure, but also had some generated SnTiO₄, and the silver species was metallic Ag⁰. Ag/TiO₂-SnO₂ (CTAB) possessed a coral-globular-like structure with nanosheets in large quantities. The photocatalytic activity of Ag/TiO₂-SnO₂ (CTAB) had studied by degrading organic dyes under multi-modes, mainly using rhodamine B as the model molecule. Results showed that the coral-globular-like Ag/TiO₂-SnO₂ (CTAB) was higher photocatalytic activity than that of commercial TiO₂, Ag/TiO₂-SnO₂, TiO₂-SnO₂ (CTAB), and TiO₂-SnO₂ under ultraviolet light irradiation. Moreover, Ag/TiO₂-SnO₂ (CTAB) composite can significantly affect the photocatalytic degradation under multi-modes including UV light, visible light, simulated solar light and microwave-assisted irradiation. Meanwhile, the photocatalytic activity of Ag/TiO₂-SnO₂ (CTAB) was maintained even after three cycles, indicating that the catalyst had good usability.

Theoretical investigation of the adsorption behaviors of CO and CO2 molecules on the nitrogen-doped TiO2 anatase nanoparticles: Insights from DFT computations

Over the past years, an interest has arisen in resolving the problems of the increased carbon monoxide and carbon dioxide emissions, leading to the serious air pollution and many detrimental effects. A convenient solution would be a process that could utilize metal oxide nanoparticles such as TiO2 to control the concentration of atmospheric pollutants. The chemisorption of CO and CO2 molecules over the semiconductor titanium dioxide (TiO[Formula: see text] is such a process. In this way, density functional theory (DFT) calculations were performed to investigate CO and CO2 adsorptions on undoped and N-doped TiO2 anatase nanoparticles. The supercell approach is conducted to construct the considered nanoparticles and the adsorption of COx molecule was simulated by use of these chosen nanoparticles. By including van der Waals (vdW) interactions between COx molecule and TiO2 nanoparticle, we found that both CO and CO2 molecules can bind strongly to the N-doped nanoparticles. The adsorption on the five-fold coordinated titanium site of TiO2 nanoparticles including the bond lengths, bond angles, adsorption energies, density of states (DOSs), Mulliken population analysis and molecular orbitals has been broadly studied in this work. Based on the obtained results, it can be concluded that the adsorption on the N-doped nanoparticle is more energetically favorable than the adsorption on the pristine one, representing the higher tendency of N-doped nanoparticles for COx detention, compared to the undoped ones. Therefore, the results indicate that the N-doped TiO2 would be an ideal COx gas sensor in the environment.

Preparation of TiO2/SnO2 thin films by sol-gel method and periodic B3LYP simulations

Titanium dioxide (TiO2) thin films are grown by the sol-gel dip-coating technique, in conjunction with SnO2 in the form of a heterostructure. It was found that the crystalline structure of the most internal layer (TiO2) depends on the thermal annealing temperature and the substrate type. Films deposited on glass substrate submitted to thermal annealing until 550 °C present anatase structure, whereas films deposited on quartz substrate transform to rutile structure at much higher temperatures, close to 1000 °C, unlike powder samples where the phase transition takes place at about 780 °C. When structured as rutile, the oxide semiconductors TiO2/SnO2 have very close lattice parameters, making the heterostructure assembling easier. The SnO2 and TiO2 have their electronic properties evaluated by first-principles calculations by means of DFT/B3LYP. Taking into account the calculated band structure diagram of these materials, the TiO2/SnO2 heterostructure is qualitatively investigated and proposed to increase the detection efficiency as gas sensors. This efficiency can be further improved by doping the SnO2 layer with Sb atoms. This assembly may be also useful in photoelectrocatalysis processes.

Nanomaterials – the Next Great Challenge for Qsar Modelers

In this final chapter a new perspective for the application of QSAR in the nanosciences is discussed. The role of nanomaterials is rapidly increasing in many aspects of everyday life. This is promoting a wide range of research needs related to both the design of new materials with required properties and performing a comprehensive risk assessment of the manufactured nanoparticles. The development of nanoscience also opens new areas for QSAR modelers. We have begun this contribution with a detailed discussion on the remarkable physical–chemical properties of nanomaterials and their specific toxicities. Both these factors should be considered as potential endpoints for further nano-QSAR studies. Then, we have highlighted the status and research needs in the area of molecular descriptors applicable to nanomaterials. Finally, we have put together currently available nano-QSAR models related to the physico-chemical endpoints of nanoparticles and their activity. Although we have observed many problems (i.e., a lack of experimental data, insufficient and inadequate descriptors), we do believe that application of QSAR methodology will significantly support nanoscience in the near future. Development of reliable nano-QSARs can be considered as the next challenging task for the QSAR community.