[en] Stabilizing the doped TiO₂ structure along with broad absorption in the visible regime is helpful in improving its efficiency under solar irradiations. With ab-initio calculations, the mono-doped, co-doped and tri-doped systems of indium (In), silver (Ag) and nitrogen (N) in the bulk of anatase TiO₂ are introduced. The geometrical and band structure of perfect TiO₂ is modified by doping foreign atoms. Impurity states in the form of In 5p, Ag 4d, N 2p are introduced in the band structure extending its absorption edge towards visible regime. Populated states are noticeable in the band gap of In, Ag, N tri-doped model which might be helpful in step wise migration of electrons between valence and conduction band. The optical absorption is tested via absorption coefficient and imaginary part of the dielectric function. The tri-doped model exhibit enhanced absorption in the visible regime predicting an efficient material for photoelectrochemical applications. (paper)

[en] The unwanted structural changes and isolated unoccupied states in the band structure of TiO₂ due to doping could be minimized by proper selection of dopants. A good combination of dopants would improve the optical response in visible regime and remove the isolated states from the band gap while keeping minimum structural distortion. Enhanced visible light absorption and effective utilization of photo-generated carriers would improve the photoactivity of TiO₂ under solar irradiations. Mono-doped, co-doped and tri-doped models of molybdenum (Mo), carbon (c), and nitrogen (N) are developed in the structure of anatase TiO₂ and the corresponding change in the geometrical structure, band structure, and optical response is noted. The Mo and C co-doping produced least structure modification when compared to a reference TiO₂ system. The Mo 3d states are mixed with Ti 3d states reducing the band gap of Mo-doped TiO₂. While shifting the Fermi level to the middle of the band gap, the C 2p states are introduced around the middle of the gap of C doped TiO₂. The impurity states are occupied and maximum reduction in the band gap is found in Mo, C codoped TiO₂. The addition of N in the Mo, C codoped TiO₂ modified the band structure by creating isolated states in the band gap. The photo-response of Mo, C, N tri-doped TiO₂ is prominent among the simulated models attributed to the step wise transition of electrons by the C 2p and N 2p states. The improved optical response allows more visible photons for absorptions but the isolated states may annihilate it. The Mo, C codoped system provided minimum structure changes with clear band structure and reasonable visible light absorption. (paper)

[en] Highlights: ► First principles calculation were carried out for Mo, N, and (Mo, N) co-doped TiO₂. ► The geometrical, electronic and optical properties of the doped models were evaluated. ► Mo and N co-doping introduce Mo 4d and N 2p states in the band gap. ► (Mo, N) co-doped TiO₂ have best electronic and optical properties among all models. - Abstract: Density functional theory calculations were performed in order to investigate the effect of (Mo, N) co-doping on the electronic and optical properties of anatase TiO₂. Comparative theoretical study is organized using different doping models including single Mo doping, single N doping in anatase TiO₂ and three different models of (Mo, N) co-doped TiO₂ regarding the position of the dopants with respect to each other. Mo doping in anatase TiO₂ reduced the band gap of pure TiO₂ from 2.12 eV to 1.90 eV by introducing Mo 4d state below the conduction band and shifted the Fermi level from the top of the valence band to the bottom of the conduction band which verifies the n-type doping nature of Mo in TiO₂. Isolated N 2p state was created above the top of the valence band due to the N doping and the band gap of N–TiO₂ was effectively reduced to about 0.78 eV; however the unoccupied N 2p states annihilate the electron–hole pairs which will limit the efficiency of N–TiO₂ in visible light photocatalytic activity. (Mo, N) co-doped TiO₂ has narrowed band gap of about 1.50 eV and simultaneous impurity states, one is above the top of the valence band (N 2p) and the other is just below the conduction band (Mo 4d). The introduction of Mo 4d state changes the character of N 2p states from unoccupied to occupied states which will lead to removal of the electron–hole recombination center and enhance the visible light photocatalytic activity. Furthermore, optical absorption coefficient spectra results show that (Mo, N) co-doping in anatase TiO₂ can have enhanced optical absorption in the visible region compared with that of N–TiO₂ and Mo–TiO₂, which is attributed to the reduced recombination centers. It is argued that (Mo, N) co-doped TiO₂ shows enhanced visible light photocatalytic activity due to the effective utilization of electron–hole pairs in the oxidation/reduction process.

[en] Calculations based on density functional theory (DFT) are performed to reveal the interplay between the yttrium dopant and oxygen vacancy in the structure of zirconia. Non-compensated and compensated yttrium stabilized zirconia (YSZ) systems are introduced. Oxygen deficient ZrO₂ models are also simulated to investigate the influence of oxygen vacancy on the structure and optoelectronic properties of the system. Comparing the structure of different models, the compensated YSZ systems provided better structure stability. Charge compensation is very crucial for stabilizing the structure of ZrO₂. Different compensated YSZ models are introduced and their structural, electronic and optical properties are compared. The compensated YSZ system having two Y³⁺ atoms at Zr⁴⁺ sites along with one oxygen vacancy (named as Zr₁₄Y₂O₃₁) provided very clear band gap. Very slight change in the optical response of Zr₁₄Y₂O₃₁ is found compared to pure zirconia. It is concluded that not only yttrium dopant nor oxygen vacancy is only responsible for the structure stability of ZrO₂. The synergistic effect due to yttrium dopant and oxygen vacancy leads to the improved stability, suitable band structure and non-modified optical response of zirconia. (paper)

[en] The synthesis and characterization of Dy-doped Lu₁Gd₂Ga₂Al₃O₁₂ are reported in this article. Solid-state reaction method is used to synthesize the material. X-ray diffraction and scanning electron microscopy characterization techniques are used to study the phase and structure of the synthesized material. Luminescence, which is the main property of the phosphor material, is characterized by UV- and X-ray-induced luminescence spectroscopy. Lu₁Gd₂Ga₂Al₃O₁₂: Dy³⁺ phosphor shows its highest emission spectra in blue and yellow regions. A combination of yellow and blue gives us white light, displayed by chromaticity diagram for this phosphor. Hence, this phosphor may be used in white-light-emitting diodes. The absorption spectra of our material match well with spectral curve of LEDs. Therefore, it may be used in LEDs applications. (author)

[en] Using the hydrothermal method, a P-doped TiO₂ nano-catalyst is prepared for widening the application spectrum of TiO₂. The synthesized samples are investigated using XRD, TEM, and UV–visible absorption spectra. A P-doped TiO₂ system is simulated and calculations for geometrical structure, electronic and optical properties are performed based on density functional theory. Comparison of the electronic band structure of anatase TiO₂ before and after doping verified that doping tuned the band structure. XRD patterns revealed that pure anatase phase is the only phase in case of pure and doped samples. TEM observations reveal spherical morphology. The P doped TiO₂ experimentally as well as theoretically responded to visible light confirming the band structure findings. Photocatalytic activity of the doped samples drastically improved compared to bare TiO₂. (paper)

[en] With ab-initio calculations, the geometrical structure, electronic band structure, optical and thermodynamic properties of pure and Ce doped La₂Zr₂O₇ are calculated. Doping Ce at Zr site modified the intrinsic band gap of La₂Zr₂O₇. However, increasing the Ce doping concentration has no considerable effect on the band structure. With the increase in Ce doping concentration, the absorption spectra of the doped La₂Zr₂O₇ is shifted to higher wavelengths and the peaks intensity is reduced. Some of the peaks associated with the intrinsic La₂Zr₂O₇ are disappeared and new peaks are introduced in 3.40% Ce doped La₂Zr₂O₇. Perturbations in the phonons density of states are reduced with the increasing Ce doping concentration. The La₁₆Zr₁₃Ce₃O₅₆ system with 3.40% of Ce doping provided the most stable phonons curve among the simulated systems. Interesting behavior is observed in the thermal conductivity of the doped models with the increasing temperature. The thermal conductivity of La₁₆Zr₁₅CeO₅₆ model increases compared to the pure La₂Zr₂O₇. However, further increase in the Ce doping concentration drastically reduced the thermal conductivity values in the range 373–1673 K. The La₁₆Zr₁₃Ce₃O₅₆ system (3.4% Ce) provided the lowest thermal conductivity attributed to the high concentration of point defects affecting the phonon scattering. (paper)

[en] The electronic band structure of TiO₂ could be modulated by dopants to harvest the major part of the solar spectrum. A cationic pair of dopant comprising of iron and lanthanum is studied in the lattice of anatase TiO₂ and modifications in the geometrical structure, band structure and optical response are observed. Ferrum and lanthanum doping reduced the band gap of TiO₂ along with inducing visible light absorption. Isolated states due to Fe are created above top of the valence band in Fe@TiO₂. In case of La@TiO₂, the La states successfully mixed with the valence and conduction band reducing the band gap. Substitutional La at Ti site in Fe@TiO₂ changed the nature of Fe 3d in the band structure and strongly contributed to the narrowing of band gap. The Fe and La co-doped TiO₂ possess reduced band gap with superior optical response in the visible regime. The Fe and La as cationic pair of dopant synergistically stabilize the system with the improved optical response explaining the experimental observation of enhanced photoactivity. (paper)

[en] To understand the improved stability and reduced thermal conductivity of 20% CeO₂ stabilized ZrO₂ coatings compared to pure zirconia, different Ce doped ZrO₂ systems are modeled. The Ce insertion in ZrO₂ lattice elongates the lattice parameters and the elongation is proportional to the Ce doping concentration. Moreover, the cell volume linearly increases with the Ce doping level. X-ray diffraction analysis confirmed the Zr_0.84Ce_0.16O₂ phase, describing the substitutional Ce doped at Zr sites. Ce doping modified the electronic band structure of ZrO₂ without creating any isolated states in the band gap. The Ce₄Zr₁₂O₃₂ modeled system with Ce doping concentration of 8.33% provided the stable phonon density of states curves compared to the pure and doped systems. Ce doping reduced the thermal conductivity of ZrO₂. Increasing Ce doping level further reduced the thermal conductivity reaching the maximum reduction at 8.33% doping concentration. Further increase in the Ce level has no effective reduction in the thermal conductivity values. The reduced thermal conductivity is attributed to the enhanced phonon scattering due to the substitutional Ce⁴⁺ dopant and optimal doping concentration. The experimental observation could be satisfactorily explained by the calculations results. (paper)

[en] With indium and nitrogen as a pair of dopants, different substitutional defects are induced in the structure of anatase TiO₂. Along with substitutional point defects, oxygen vacancy is created for charge compensation/stabilization of the defect induced system. The In 5p states associated with the indium are coupled with the O 2p states narrowing the band gap of TiO₂, and inducing visible light absorption. Nitrogen doping in TiO₂ created isolated N 2p stated above the valence band maximum. With respect to the position of dopants, three different In, N co-doped models are introduced. Moreover, the In, N compensated co-doped configuration is also developed. The In 5p and N 2p states of the dopants are strongly coupled with the host O 2p states reducing the band of TiO₂. The indium and nitrogen doped 5.477 Å apart from each other in the lattice of TiO₂ (anatase) provided maximum absorption in the visible regime among the codoped models. The theoretical findings can explain the experimental observations and widen the applications spectrum of TiO₂. (paper)