Titanium dioxide (TiO2) polymorphs have recently gained a lot of attention in dye-sensitized solar cells (DSSCs). The brookite polymorph, among other TiO2 polymorphs, is now becoming the focus of research in DSSC applications, despite the difficulties in obtaining it as a pure phase experimentally. The current theoretical study used different nonmetals (C, S and N) and (C-S, C-N and S-N) as dopants and co-dopants, respectively, to investigate the effects of mono-doping and co-doping on the electronic, structural, and optical structure properties of (210) TiO2 brookite surfaces, which is the most exposed surface of brookite. The results show that due to the narrowing of the band gap and the presence of impurity levels in the band gap, all mono-doped and co-doped TiO2 brookite (210) surfaces exhibit some redshift. In particular, the C-doped, and C-N co-doped TiO2 brookite (210) surfaces exhibit better absorption in the visible region of the electromagnetic spectrum in comparison to the pure, S-doped, N-doped, C-S co-doped and N-S co-doped TiO2 brookite (210) surfaces.

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First-principle calculations were used to investigate the structural, optoelectronic, elastic and thermodynamic properties of Br-doped CsPbI3 perovskite material using GGA-PBE, SCAN, and LDA functionals. The computed lattice parameters are consistent with the experimental and theoretical calculations, reported in the literature. The band structure along with the electronic density of states indicated that CsPbI3-xBrx (x = 0, 1, 2, 3) materials are semiconductors with direct band gaps, as projected using the three functionals. The energy band gap of CsPbI3 was tuned by replacing I ions with Br ions, resulting in CsPbI2Br, CsPbBr2I, and CsPbBr3 materials. These perovskite materials were found to be mechanically stable, ductile in nature and elastically anisotropic. The results of optical parameters such as absorption coefficients, refractive index, optical conductivity, optical reflectivity, electron energy loss, and extinction coefficients were calculated and analysed. The thermodynamic parameters including heat capacity, and Debye temperature were calculated. The direct band gap and energy-dependent optical parameters especially the absorption coefficient in the infrared and visible region of these perovskites’ materials suggest that they might be candidates for potential use in photovoltaic solar cells and optoelectronic applications.

Abstract. Titanium dioxide (TiO2) polymorphs are widely used in many energy-related applications due to their peculiar electronic and physicochemical properties. The electronic structures of brookite TiO 2 surfaces doped with transition metal ruthenium have been investigated by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke-Ernzerhof (PBE) to describe the exchange-correlation functional. All calculations were carried out with CASTEP (Cambridge Sequential Total Energy Package) code in Materials Studio of Accelrys Inc. The surface structures of Ru doped TiO 2 were constructed by cleaving the 1 × 1 × 1 optimized bulk structure of brookite TiO2. The results indicate that Ru doping can narrow the band gap of TiO2, leading to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. The theoretical calculations could provide meaningful guide to develop more active photocatalysts with visible light response. IntroductionIn the last decades, properties for titanium dioxide (TiO 2 ) polymorphs have been the subject of many experimental and computational studies, the most common polymorphs being the minerals rutile, anatase, and brookite [1]. A large number of applications of TiO 2 in materials science is, almost without exception, ultimately a result of the facile electron-transfer processes that occur at the interface between the semiconductor and adsorbed molecules [1][2][3][4][5][6][7]. When photons are excited, TiO 2 is a good electron and hole donor and can, therefore promote photocatalytic processes at its interface [3]. Its photocatalytic properties, in addition to its abundance, low cost, stability, and low toxicity, are the basis for its use in solar cells [4]. However, brookite is the rarest of the natural occurring TiO 2 polymorphs, and it is the most difficult phase to prepare in the laboratory [1]. As a result, the properties of pure brookite are poorly known. TiO 2 can only show photocatalytic activity under ultraviolet (UV) light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5%), in contrast to visible light for a major part of solar energy (approximately 45%), but can be photosensitized by the adsorption of chromophores that, when excited, inject electrons into the TiO 2 conduction band [8]. In spite of a large number of publications on pure and doped TiO 2 many aspects of

The most popular semiconductor in dye-sensitized solar cells (DSSCs) is titanium dioxide (TiO2) because of its low cost, non-toxicity, and good stability. However, the DSSCs still have low efficiency due to the low light absorption of TiO2 in the visible region. Understanding the properties of TiO2 can contribute to improving the efficiency of DSSCs. In this study, we use density functional theory to investigate the electronic and optical properties of TiO2 brookite (210) surface mono-doped and co-doped with 4d transition metals, silver, and molybdenum. Our results show that the band gap energy of brookite (210) surface is 3.514 eV, which reduces to 1.143 eV and 0.183 eV when doped with Ag and Mo, respectively. However, doping with both Ag and Mo yielded a band gap of 0.387 eV. The results suggest the presence of Ag and Mo 4d states below the conduction band minimum, which could be responsible for the narrowing of the band gap on brookite (210) surface. Both mono-doped and co-doped brookite (210) surfaces have higher visible light absorbance compared to the undoped brookite (210) surface and extends to the near-infrared region.

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The adsorption and interaction mechanisms of gaseous molecules on ZnO surfaces have received considerable attention because of their technological applications in gas sensing. The adsorption behavior of NH3 and NO2 molecules on undoped and Sn-doped ZnO (101) surfaces was investigated using density functional theory. The current findings revealed that both molecules adsorb via chemisorption rather than physisorption, with all the adsorption energy values found to be negative. The calculated adsorption energy revealed that the adsorption of the NH3 molecule on the bare ZnO surface is more energetically favorable than the adsorption of the NO2 molecule. However, a stable adsorption configuration was discovered for the NO2 molecule on the surface of the Sn-doped ZnO surface. Furthermore, the adsorption on the undoped surface increased the work function, while the adsorption on the doped surface decreased. The charge density redistribution showed charge accumulation and depletion on both adsorbent and adsorbate. In addition, the density of states and band structures were studied to investigate the electronic behavior of NH3 and NO2 molecules adsorbed on undoped and Sn-doped ZnO (101) surfaces.

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