Revisiting the BaBiO3 semiconductor photocatalyst: synthesis, characterization, electronic structure, and photocatalytic activity.

This article revisits the properties of BaBiO3 examined extensively in the last two decades because of its electronic properties as a superconductor and as a semiconductor photocatalyst. Solid-state syntheses of this bismuthate have often involved BaCO3 as the barium source, which may lead to the formation of BaBiO3/BaCO3 heterostructures that could have an impact on the electronic properties and, more importantly, on the photocatalytic activity of this bismuthate. Accordingly, we synthesized BaBiO3 by a solid-state route to avoid the use of a carbonate; it was characterized by XRD, SEM, and EDX, while elemental mapping characterized the composition and the morphology of the crystalline BaBiO3 and its thin films with respect to structure, optoelectronic, and photocatalytic properties. XPS, periodic DFT calculations, and electrochemical impedance spectroscopy ascertained the electronic and electrical properties, while Raman and DRS spectroscopies assessed the relevant optical properties. The photocatalytic activity was determined via the degradation of phenol in aqueous media. Although some results accorded with earlier studies, the newer electronic structural data on this bismuthate, together with the photocatalytic experiments carried out in the presence of selective radical trapping agents, led to elucidating some of the mechanistic details of the photocatalytic processes that previous views of the BaBiO3 band structure failed to address or clarify. Analytical refinement of the XRD data inferred the as-synthesized BaBiO3 adopted the C2/m symmetry rather than the I2/m structure reported earlier, while Tauc plots from DRS spectra yielded a bandgap of 2.05 eV versus the range of 1.1-2.25 eV reported by others; the corresponding flatband potentials were 1.61 eV (EVB) and - 0.44 eV (ECB). The photocatalytic activity of BaBiO3 was somewhat greater than that of the well-known Evonik P25 TiO2 photocatalyst under comparable experimental conditions.

Ab Initio Insight into Mechanisms of Ozone Interaction with a Surface of Dehydrated Nanocrystalline TiO2.

Density functional theory (DFT) study of ozone adsorption on dehydrated nanocrystalline TiO2 is presented. Singlet and triplet binding modes of ozone to the oxide's titanium cations are considered. In both the modes, monodentate and bidentate ozone complexes are formed. According to DFT, the triplet monodentates are the most stable species. The formation of monodentate ozone adsorption complexes is in-line with an earlier interpretation of infrared (IR) spectroscopic data on ozone adsorption on an anatase surface. However, the computed difference in the fundamental vibrational frequencies (ν1 - ν3) of ozone in the triplet monodentates is significantly larger than the corresponding IR value. This discrepancy is resolved by demonstrating that the triplet monodentates readily decompose, realizing molecular oxygen that is consistent with published experimental data. The predicted energy barrier of the dissociative adsorption is less than 2 kcal/mol. In contrast, the computed difference in the fundamental vibrational frequencies (ν1 - ν3) of adsorbed ozone in the singlet bidentates perfectly agrees with the experiment.

Effect of Composition on the Optical and Photocatalytic Properties of Visible Light Responsive Materials Bi26-xMgxO40.

We report the synthesis and the crystal and electronic structure as well as the optical and photocatalytic properties of novel photoactive materials of the general formula Bi26-xMgxO40. Two compounds with compositions of Bi24.28(3)Mg1.72(3)O40 and Bi24.05(3)Mg1.95(3)O40 are synthesized using the pyrolytic method. According to X-ray diffraction analysis, the materials are monocrystalline species. Their electronic bandgaps determined from Tauc plots are 2.41 eV [Bi24.28(3)Mg1.72(3)O40] and 2.69 eV [Bi24.05(3)Mg1.95(3)O40]. Keeping in mind that optical bandgaps are typically larger than their electronic counterparts, we find that the bismuthate bandgaps match well that of Bi24Mg2O40 (2.26 eV) predicted by density functional theory. Apparently, the synthesized bismuthates are indirect bandgap semiconductors just like Bi24Mg2O40. Both materials demonstrate nearly identical luminescence spectra. Their luminescence emission at 620 nm is most efficiently excited by 365 nm light. The materials' photocatalytic properties are evaluated in a visible light-induced photocatalytic phenol degradation reaction. Rather low activity of both compounds is detected. However, Bi24.05(3)Mg1.95(3)O40 is ∼2 times more photocatalytically active than Bi24.28(3)Mg1.72(3)O40, which is associated with a higher Bi5+ content in the former.

Self-assembled organic nanowires for high power density lithium ion batteries.

The electroactive organic materials are promising alternatives to inorganic electrode materials for the new generation of green Li-ion batteries due to their sustainability, environmental benignity, and low cost. Croconic acid disodium salt (CADS) was used as Li-ion battery electrode, and CADS organic wires with different diameters were fabricated through a facile synthetic route using antisolvent crystallization method to overcome the challenges of low electronic conductivity of CADS and lithiation induced strain. The CADS nanowire exhibits much better electrochemical performance than its crystal bulk material and microwire counterpart. CADS nanowire with a diameter of 150 nm delivers a reversible capability of 177 mAh g(-1) at a current density of 0.2 C and retains capacity of 170 mAh g(-1) after 110 charge/discharge cycles. The nanowire structure also remarkably enhances the kinetics of croconic acid disodium salt. The CADS nanowire retains 50% of the 0.1 C capacity even when the current density increases to 6 C. In contrast, the crystal bulk and microwire material completely lose their capacities when the current density merely increases to 2 C. Such a high rate performance of CADS nanowire is attributed to its short ion diffusion pathway and large surface area, which enable fast ion and electron transport in the electrode. The theoretical calculation suggests that lithiation of CADS experiences an ion exchange process. The sodium ions in CADS will be gradually replaced by lithium ions during the lithiation and delithiation of CADS electrode, which is confirmed by inductively coupled plasma test.

FDE-vdW: A van der Waals inclusive subsystem density-functional theory.

We present a formally exact van der Waals inclusive electronic structure theory, called FDE-vdW, based on the Frozen Density Embedding formulation of subsystem Density-Functional Theory. In subsystem DFT, the energy functional is composed of subsystem additive and non-additive terms. We show that an appropriate definition of the long-range correlation energy is given by the value of the non-additive correlation functional. This functional is evaluated using the fluctuation-dissipation theorem aided by a formally exact decomposition of the response functions into subsystem contributions. FDE-vdW is derived in detail and several approximate schemes are proposed, which lead to practical implementations of the method. We show that FDE-vdW is Casimir-Polder consistent, i.e., it reduces to the generalized Casimir-Polder formula for asymptotic inter-subsystems separations. Pilot calculations of binding energies of 13 weakly bound complexes singled out from the S22 set show a dramatic improvement upon semilocal subsystem DFT, provided that an appropriate exchange functional is employed. The convergence of FDE-vdW with basis set size is discussed, as well as its dependence on the choice of associated density functional approximant.

Crystal and Electronic Structures of New Two Dimensional 3-NH3-PyPbX4 Haloplumbate Materials.

In this study, we explored both the crystal and electronic structures of new synthesized materials 3-NH3-PyPbX4 (X = Br, I). Both compounds are isostructural, and they crystallize in the monoclinic space group P21/c, with four formula units in the unit cell. According to the analysis of their electronic structures, both compounds are direct semiconductors with direct transitions between valence and conduction bands occurring at the k-points A, Z, and at about half of the distance between the k-points D/D1 and D1/E. An inspection of DOS reveals that, in both perovskites, the highest energy VBs are comprised mainly of electronic states of halogen anions, while the lowest states in the conduction band originate from lead orbitals. In addition, there are two flat bands composed of electronic states of carbon and nitrogen originating from the organic subsystems and presumably corresponding to the π* orbitals of 3-NH3-C5H6N cations. Both materials demonstrate a broad luminescence emission. Two mechanisms of the radiative relaxation based on either self-trapped excitons or on charge transfers between inorganic and organic subsystems are discussed.

Effective Method for the Determination of the Unit Cell Parameters of New MXenes.

MXenes are of great practical interest. While the physical properties of such a well-known MAX phase as Ti3AlC2 and the Ti3C2 MXene that is based on it have been widely studied, it is extremely important to study the properties of new four-component MAX-phases and the MXenes based on them. To do this, first, it is necessary to characterize the obtained materials. In this work, the Ti3-xNbxC2 MXene was characterized. Since the material is fairly new, there are no crystallographic data for such systems in the international databases. We proposed a method for the determination of the main unit cell parameters of the new Ti3-xNbxC2 MXene, which was based on a combination of the DFT method, TEM studies, and an X-ray diffraction analysis.

Synthesis, characterization, optoelectronic and photocatalytic properties of Sr2Bi2O5/SrCO3 and Sr3Bi2O6/SrCO3 heterostructures with varying SrCO3 content.

We report on the structure and properties of novel Sr2Bi2O5/SrCO3 and Sr3Bi2O6/SrCO3 heterostructures with SrCO3 content varying from 10 wt% to 90 wt%. Formation of the heterostructures is succeeded via sintering technique and according to XRD, SEM, and EDX studies has resulted in highly crystalline materials with well-defined carbonate-bismuthate interfaces. In the reaction of photodegradation of phenol in aqueous solution the heterostructures demonstrated photocatalytic activity exceeding that of TiO2. It is demonstrated that their photocatalytic action is due to the presence of type II semiconductor heterojunctions. The content of SrCO3 ensuring utmost photocatalytic activity is explained based on numerical modeling of the heterostructures' formation using 'Unity' development platform. In addition, the heterostructures show photoluminescence. Its mechanism proposed here implies permeability of the materials' heterojunctions to electrons. This work describes new materials that can be used for photodegradation of organic pollutants. Besides, it extends available principles of design of photocatalytically active heterostructures.

Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3 Single Crystals.

We present an optical study of MAPbBr3 single crystal grown from solution. The crystal Pm3m symmetry was confirmed by electron backscatter diffraction. Our major attention was focused on optical effects related to the excitonic states in MAPbBr3. Photoluminescence temperature dependence of narrow exciton resonance showed encouragingly low inhomogeneous broadening Γ ≈ 0.5 meV that allows one to distinguish the signals from free excitons and those arising from recombination of excitons localized on defects. Excitonic origin of the resonance was proved by its superlinear pump intensity dependence, in contrast to the linear behavior of the defect-assisted recombination bands. For the first time, the phonon replicas originating from free exciton recombination accompanied by partial energy transfer to the phonons were observed in high-resolution PL spectra and confirmed by independent low-temperature Raman scattering experiments. In turn, low-temperature, low-frequency Raman scattering studies let us resolve the structure of the low-frequency phonon spectrum.

Calculating hyperfine couplings in large ionic crystals containing hundreds of QM atoms: subsystem DFT is the key.

We present an application of the linear scaling frozen density embedding (FDE) formulation of subsystem DFT to the calculation of isotropic hyperfine coupling constants (hfcc's) of atoms belonging to a guanine radical cation embedded in a guanine hydrochloride monohydrate crystal. The model systems range from an isolated guanine to a 15,000 atom QM/MM cluster where the QM region is comprised of 36 protonated guanine cations, 36 chlorine anions, and 42 water molecules. Our calculations show that the embedding effects of the surrounding crystal cannot be reproduced by small model systems nor by a pure QM/MM procedure. Instead, a large QM region is needed to fully capture the complicated nature of the embedding effects in this system. The unprecedented system size for a relativistic all-electron isotropic hfcc calculation can be approached in this work because the local nature of the electronic structure of the organic crystals considered is fully captured by the FDE approach.

Interaction between n-Alkane Chains: Applicability of the Empirically Corrected Density Functional Theory for Van der Waals Complexes.

The geometries, interaction energies, and vibrational frequencies of a series of n-alkane dimers up to dodecane have been calculated using density functional theory (DFT) augmented with an empirical dispersion energy term (DFT-D). The results obtained from this method for ethane to hexane dimers are compared with those provided by the MP2 level of theory and the combined Gaussian-3 approach with CCSD(T) being the highest correlation method [G3(CCSD(T))]. Two types of dimer isomers have been studied. The most stable isomers have the two carbon chains in parallel planes, whereas the second ones have the two carbon chains in the same plane. Butane is found to be the shortest carbon chain to form dimers with similar properties, that is, a constant average distance between the monomer carbon skeletons, a similar increment per CH2 unit for the dimer interaction energy, and comparable dimer symmetric stretching frequencies. The values and trends obtained from the DFT-D approach agree very well with those obtained from MP2 for the geometries and vibrational frequencies and from the G3(CCSD(T)) method for the energies, validating the use of DFT-D for the study of large hydrocarbon complexes.