Dopant Valence Research Articles

High-performance single crystal Ni-rich cathode with regulated lattice and interface constructed by separated lithiation and crystallization calcination

Incorporating high valence dopants, such as W6+ and Mo6+ has been verified to be effective for tuning the microstructure and grain boundary of polycrystal Ni-rich cathode. However, the hindered consolidation of primary particles induced by dopants during lithiation calcination limits the utilization of those dopants to crystalize single-crystal Ni-rich cathodes with stabilized lattice and surface. Herein, high performance single crystal LiNi0.84Co0.11Mn0.05O2 cathode with Al3+ and W6+ regulated lattice and boundary phase was construed based on commercial process with two-step calcination process containing separated lithiation and crystallization. The introduction of appropriate amount of Al3+ in the first lithiation calcination of 6 h endows the bulk of crystalline with enhanced lattice stability, while the incorporation of W6+ with stoichiometrical LiOH in the secondary crystallization calcination of 6 h renders uniformly distributed surface layer without hampering the growth of single-crystal. With the Al3+ doped bulk lattice, W6+ doped subsurface region and hetero-epitaxially grown Li2WO4, the cathode infused by two-step calcination exhibits high discharge capacity, rate performance, and cycling stability. Specifically, the modified LiNi0.84Co0.11Mn0.05O2 exhibits exceptional capacity retention, maintaining 88.98% of its initial capacity after 200 cycles at a rate of 1 C within a voltage window of 2.7–4.3 V at a temperature of 25 °C in half-cell. This performance is markedly superior to the capacity retention of 72.96% observed for pristine cathode. Even when subjected to a stringent test after 200 cycles at the same rate, the modified cathode sustains an impressive capacity retention of 82.41% at an elevated cut-off voltage of 4.5 V and a temperature of 30 °C.

Modulating the Stoichiometry and Lithium Content to Boost the Electrochemical Performance of Li2ZnTi3O8 via High Valence Nb5+ Dopants

AbstractA series of anode materials with the general formula of Li2ZnTi3O8 (LZTO) and Li2‐xZnTi3‐xNbxO8 (x = 0.2, 0.4, and 0.5) (LZTNO) are designed. Different amount of high valence Nb5+ dopants not only reduces the content of lithium in LZTO but also alters the intrinsic characteristics of the composites, leading to optimized electrochemical performances. XRD, SEM, TEM, and XPS results suggest that Nb dopants are introduced successfully, but large amount of Nb5+ dopants (x > 0.2) result in the formation of ZnNb2O6 and TiO2. Owing to the small particle size and the improved structural stability, LZTNO‐2 sample exhibits the best electrochemical performance, and it can deliver a charge/discharge capacity of 182.7/181.2 mAh g−1 at 1 A g−1 after 500 cycles, which is much higher than the value of LZTO (100.41/100.41 mAh g−1). The experiments suggest that the introduction of high valence dopants can effectively modulate the stoichiometry and lithium content of the anode materials, making the available vacant sites for subsequent intercalation of lithium obviously extended. Such a strategy is expected to be feasibly applied to other anode materials to enhance their specific capacity and electrochemical performances.

Strategic co-doping of ceria for improved oxidation kinetics in solar thermochemical fuel production

La and Yb were incorporated as co-doping elements into Zr-doped ceria to investigate the influence of doping ions by means of ionic radius and valence on the ceria crystal lattice and potentially on thermodynamic and kinetic properties relevant to solar-driven two-step thermochemical CO2 splitting. Both Zr–La and Zr–Yb co-doped ceria clearly exhibited better reduction capability than ceria and partial molar enthalpy and entropy that were in between those of Zr-doped ceria and undoped ceria. In the case of CO2 splitting kinetics, Zr-doped ceria was the slowest in accordance with relevant literature, but La-doped samples were much faster than Yb-doped counterparts suggesting that extrinsic oxygen vacancy formation, even though important, does not primarily determine the CO2 splitting kinetics unlike the hypothesis postulated by previous works. This work elucidates the role of dopant's ionic radius, lattice constant and valence on thermodynamics and CO2 splitting kinetics via experimental demonstration, and provides a new doping strategy not based on the extrinsic oxygen vacancy formation but based on the dopant ionic radius and valence with an insight for the further doped ceria-based redox material discovery.

Highly effective Fe-doped TiO2 nanoparticles for removal of toxic organic dyes under visible light illumination

This article addresses the synthesis of Fe3+ doped TiO2 nanoparticles with variations of molar concentrations of Fe3+ and their adequate use as potential photocatalysts for Photocatalysis applications. Synthesized photocatalysts were characterized thoroughly by different analytical techniques in terms of morphological, chemical, structural, crystalline, optical, electronic structure, surface area etc properties. The occurrence of red shift phenomenon of the energy band gap attributes to the transfer of charges and transition between the d electrons of dopant and conduction band (CB) or valence band (VB) of TiO2. The doping of Fe3+ ions generates more trap sites for charge carriers with the surface trap sites. Thorough experimental conclusions revealed that the Fe3+ ions necessarily regulate the catalytic property of TiO2 nanomaterial. The obtained total degradation efficiency rate of Methylene Blue (MB) was 93.3% in the presence of 0.1 M Fe3+ in the host material and for Malachite Green Oxalate the efficiency was 100% in the presence of 0.05 M and 0.1 M Fe3+in the host material. In both the cases the total visible light irradiation time was 90 min. The adsorption properties of the photocatalysts have been also performed in a dark for 90 min in the presence of MB dye. However, till now there are hardly reported photocatalysts which shows complete degradation of these toxic organic dyes by visible light driven photocatalysis. of potential values of valence and conduction band shows the production of active oxidizing species for hydrogen yield and the possible mechanism of the Schottky barrier has been proposed. A schematic diagram of visible light driven Photocatalysis has been pictured showing degradation activity of Fe3+-TiO2 catalysts sample.

Mo-doped PdCu nanoparticles as high-performance catalysts for oxygen reduction reactions.

The instability of palladium-based binary alloys hinders their wide application in the oxygen reduction processes. Here, we prepared Mo-doped PdCu nanoparticles with controllable dopant content and valence. Further research has revealed that Mo, particularly Mo5+, may effectively suppress the oxidation of Pd and Cu, optimize the oxygen binding of Pd, and increase catalytic activity and stability. In particular, Mo-PdCu-1/C with the highest Mo5+ content shows the best oxygen reduction reaction (ORR) mass activity (1.20 A mg-1Pd), which is 4.8 times higher than that of PdCu/C. It also exhibits outstanding stability, retaining 80.8% of the original mass activity after 20 000 cycles. This study clearly explains the mechanism by which Mo doping affects the performance and provides a reference for further optimization of catalyst performance for fuel cell industrialization.

A look into donor–acceptor compensation in ZnO thin films driven by dopant valence

A look into donor–acceptor compensation in ZnO thin films driven by dopant valence

Oxidation kinetics of La and Yb incorporated Zr-doped ceria for solar thermochemical fuel production in the context of dopant ionic radius and valence

The influence of ionic radii and valence of dopants in Ce0.9LaxYbyZr0.1−x−yO2−δ (x = 0, 0.05, 0.1, y = 0, 0.05, 0.1) on the oxidation kinetics were investigated by thermogravimetric analysis in synthetic air and were compared to undoped ceria. Samples co-doped with Zr–La and Zr–Yb exhibited moderate oxidation kinetics that were slower than undoped ceria, but much faster than 10mol% Zr-doped ceria. The extrinsic oxygen vacancy induced by the trivalent dopants improves the kinetics at oxidation temperatures below 700 °C, where the diffusion, and not the surface exchange reaction is the limiting factor. A smaller ionic radius of the substituent (i.e. r(Yb3+) < r(La3+)) in the co-doped ceria tends to facilitate lower activation energy resulting in slightly faster oxidation kinetics at temperatures below 700 °C. In contrast, additional extrinsic vacancies are rather obstructive at high temperatures (i.e. T> 700 °C) due to a change of rate limiting mechanism from bulk oxygen diffusion to surface exchange reaction. Overall, the valence of the dopant rather than the ionic radius seems to determine the oxidation kinetics primarily, and additional La or Yb doping on Zr-doped ceria is appealing especially when the applications are focused on low temperature reactions.

Tuning the emission color and temperature range of dual-mode luminescent thermometer by dopant valence states control

Dual emitting materials are of great interest for luminescence thermometry. The operating temperature range, the thermal sensitivity as well as the emission color are crucial parameters, which define the competitiveness of the thermal sensor. In this work, we propose a new and convenient method for the modulation of emission color range and sensitivity through the control of the dopant oxidation states and crystal field variations. To this end, we investigated inorganic phosphors in which two valence states of Mn luminescent centers can be stabilized. When inserted in octahedral environment, Mn4+ ions present a red luminescence (2E → 4A2 transition), whereas Mn2+ ions exhibit a green luminescence in tetrahedral environment (4T1 → 6A1 transition). Due to the different behavior of Mn4+ and Mn2+ luminescence properties according to the temperature, the resulting emission color varies from red to green and can be exploited for visual temperature detection. We demonstrate the proportion of Mn4+ and Mn2+ ions can be finely tuned, owing to a thermally driven Mn4+ to Mn2+ reduction, which allows to modulate color change at a specific temperature. Two isostructural materials were investigated, SrMgAl10O17:Mn4+,Mn2+ and BaMgAl10O17:Mn4+,Mn2+, and can be envisioned as competitive ratiometric and colorimetric luminescent thermometer, with tunable properties.

Estimating the Single-Element Concentration of Intercalated Insulators for the Emergence of Superconductivity.

To predict whether a compound will superconduct and to predict its transition temperature T c prior to measurement have always been desires of the materials science community. Matthias was first to report the necessary conditions for the occurrence of superconductivity in elements, compounds, and alloys in terms of density (valence electrons per atom). This current report is motivated by somewhat similar empirical observations concerning the importance of valence electrons per unit cell; more specifically, dopant valence electrons per unit cell within intercalated insulators. In this article, though not exhaustive, a representative list of 40 superconductors will be used to show that the onset of superconductivity (insulator-superconductor boundary) within intercalated insulators can easily be modeled, almost exactly, by the ideal gas law equation. Given this observation, in contrast to Matthias, interactions are semiclassically accounted for to ultimately determine the single-element onset concentration needed to bring about superconductivity within many intercalated insulators known to date. The 13 compounds which were previously intercalated and will be discussed include inorganics, TiSe2, C60, YBa2Cu3O6, IrTe2, Bi2Se3, MoS2, ZrNCl, HfNCl, BP (black phosphorus), HoTe3, and Y2Te5, and organics, C22H14 and C14H10. In essence, the overall objective of this report is to offer a slightly different viewpoint on superconductivity, led by empirical observations, which seemingly leads to predictable experimental outcomes. If newly discovered materials further validate this approach to intercalated superconductors, with minor refinements, a route to purposefully designing superconductors may be accessible through onset conditions outlined in this article.

Dopant Induced-Modulation in Reducing Ability of Cerium in Doped Ceria System and Its Effect on Oxy-Ion Conductivity: Core Study by XPS and XANES Probes

Acceptor-doped ceria systems are potential materials forelectrolytes in intermediate temperature solid oxide fuel cells (IT-SOFCs), but the reduction of Ce4+ to Ce3+ in the ceria-based electrolytes limits such functionality. Reduction is accompanied by the release of one electron + oxygen vacancy; leading to electronic contribution and making short circuiting critical. Suppressing the reducing ability of cerium is thus a core challenge in optimizing its use. We developed acceptor (Sm3+, Gd3+) singly and doubly doped ceria, as valence and size of dopants are key in altering ionic conductivity. The oxidation state of dopant and host is revealed by XPS spectroscopy and XANES. XPS confirms the existence of Sm in two states, while Gd exists in one. The existence of Ce in Ce3+ and Ce4+ states are confirmed by both techniques, which also show that Ce4+ content in Sm-Gd doubly doped system is more than singly doped, and pure ceria, i.e. reduction ability of cerium in ceria is found to be suppressed by Sm and Gd double dopant. Effects of doping on the geometric and electronic structure have been studied using first-principles density functional theory. Ionic conductivity of a doubly doped system is enhanced by one order with suppression of reduction of cerium. The study shows acceptor multiple dopants in ceria system have potential to suppress reducing ability of cerium and improve ionic conductivity at intermediate temperature (400 °C–700 °C).

The mechanism of alkali doping in CsPbBr3: A first-principles perspective

All-inorganic CsPbBr3 perovskite doped with alkali metal atoms has been attracting increasing attention due to its superior optoelectronic properties. However, there still exists significant uncertainty regarding the doping mechanism. One view of the mechanism is that alkali metal atoms tend to substitute Cs in CsPbBr3 crystals. Another view is that Li and Na tend to intercalate into interstitial sites because their radii are much smaller than that of Cs. To elucidate the doping mechanism, it is necessary to investigate the point defects physics of alkali metal elements in CsPbBr3. In this work, by using first-principles calculations we find that alkali metal atoms energetically prefer to substitute for Cs or Pb atoms in CsPbBr3 crystals under different chemical potential conditions. To determine the alkali metal atoms doping site, one should consider the chemical potential of synthesis conditions, the dopant valence states, and atomic radii. Notably, alkali metal atoms doping mainly introduces shallow levels, which is helpful for improving the p-type conductivity of CsPbBr3.

Influence of dopant valence on the thermochromic properties of VO2 nanoparticles

The effect of valence of dopants on thermochromic properties of VO2 nanoparticles are investigated. In the present work, VO2 nanoparticles were synthesized by hydrothermal method and subsequently spin coated on a glass substrate. Dopants with various valences can break the structural symmetry of the V4+-V4+ chains, which results in various degrees of lattice distortion and changes in the crystal structure of VO2. Therefore, the transition temperature (Tc) can be associated with the valence of transition metal dopants, and it decreases with the increase of elements’ valence. Meanwhile, various valence results in the changeable optical band gap (Eg). The doping of lower valence elements results in narrowing of Eg, thus leading to enhancements in both luminous transmittance (Tlum) and solar energy modulation ability (ΔTsol). The phase transition temperature is significantly reduced by 20.2 °C for 1% doping of W. The Zr-doped film exhibits the highest Tlum and ΔTsol of 61.4% and 10.3%, respectively. The Mg-doped film has shown Tlum and ΔTsol of 59.4% and 9.5%, respectively, which is slightly inferior to Zr-doped sample. It can be deduced that W/Zr and W/Mg co-doped VO2 films can improve Tc, Tlum, and ΔTsol, simultaneously, which can provide a potential solution for manufacturing smart windows.

Remarkable effects of dopant valency – a comparative study of CaBaCo3.96Cr0.04O7 and CaBaCo3.96Ni0.04O7

Distinctive effects of dopant valency is discussed using an impurity level 1% doping each of Cr3+ and Ni2+ in CaBaCo4O7. Through a comparative study of the magnetic and dielectric properties of magnetoelectric CaBaCo3.96Cr0.04O7 and CaBaCo3.96Ni0.04O7, we highlight that Cr doping does not significantly alter the properties in spite of differences in magnetic spin and ionic radius compared to Co3+ and Co2+ while Ni doping induces spectacular changes. Particularly, about 4 times larger electric polarization change (~650 μC/m2 at 5.6 kV/cm) is observed in CaBaCo3.96Ni0.04O7 compared to CaBaCo3.96Cr0.04O7 and a stronger magnetoelectric coupling leading to a polarization change of ~12% in 15 T magnetic field, below 40 K. Further, magnetodielectric effects hint to competing magnetic phases over the temperature range of magnetic transitions. The article discusses the observed disparity in the light of a possible site selective doping of Cr3+ and Ni2+ in the triangular and kagomé layers, respectively, of the parent compound.

Doped samarium oxide xerogels for oxidative coupling of methane—Effects of high-valence dopants at very low concentrations

The effects of high-valance dopants on the catalytic properties of samarium oxide xerogels were investigated at concentrations of 0.1 and 1.0 % (by mol) in the oxidative coupling of methane (OCM). Gd, Y, Zr, and V dopants were selected to examine the influence of oxidation states between +3 and +5 on the OCM performance. Even at these low loadings, the high-valance dopants were observed to have a significant impact on the OCM reaction. At the lowest loading, 0.1 mol %, and below 700 °C, all dopants improved the activity over that of the pure Sm2O3 xerogel, and most also improved the selectivity. In particular, the ethylene yield was significantly improved over these catalysts between 500 and 700 °C. However, the stability of the doped catalysts compared to the undoped Sm2O3 xerogel were inferior above 700 °C, and the higher concentration (1.0 mol %) resulted in catalysts with a lower stability. Time-on-stream experiments revealed that the 0.1 mol % high valence dopants improved the stability of the Sm2O3 xerogel at 700 °C. As a result of the higher stability, the doped catalysts retain more of the original specific surface area and appear to stabilize the more active cubic Sm2O3 phase compared with the undoped catalyst. Therefore, the doped catalysts have a higher number of available basic sites during reaction. This study reveals that high-valence dopants have potential to improve the low temperature (500–700 °C) activity of OCM catalysts. However, the concentrations must be kept very low, as dopants that increase the activity and selectivity at concentrations of 0.1 mol % can result in inferior catalysts at 1.0 mol %, and temperatures above 700 °C must be avoided or rapid deactivation can occur.

Effects of low molar concentrations of low-valence dopants on samarium oxide xerogels in the oxidative coupling of methane

The effects of low-valence dopants on the catalytic properties of samarium oxide xerogel catalysts were investigated in the oxidative coupling of methane (OCM). More specifically, very low concentrations (0.1 and 1.0 % by mol) of transition metal (Ag, Ni, and Cu) and traditional alkali metal (Li and K) dopants were investigated. At these low loadings, it was shown that transition metal dopants have potential to improve the activity and selectivity over an undoped Sm2O3 xerogel, but these dopants can only outperform alkali metal dopants under certain conditions. Even at a concentration of 0.1 mol %, the dopants significantly increased the number of basic sites compared with the pure Sm2O3 xerogel. However, no trend is evident between the number or strength of the basic sites and the activity or selectivity in the methane coupling reaction. The XRD data reveal a lattice expansion upon addition of the low valence dopants, which is consistent with substitutional doping and the formation of oxygen vacancies due to charge compensation. In most cases the majority of the dopant stayed in the lattice during reaction. The dopants were also shown to influence the Sm2O3 structure, and the dopants that were more effective in suppressing the transformation from cubic to monoclinic Sm2O3 in general resulted in the more active and selective catalysts. While the Ag- and Ni-dopants could outperform the alkali metal doped catalysts in narrow temperature ranges, the best performing catalysts were still the K-doped Sm2O3 catalysts, as the 1.0 % K catalyst exhibited the highest activity at the lowest temperature (500 °C) and the 0.1 % K-doped catalyst was the most stable during extended operation. These results indicate that transition metal dopants, at low concentrations, can positively affect the activity and selectivity of a methane coupling catalyst, such as Sm2O3, and suggests that there may be benefits to other OCM catalyst systems from traditionally non-selective dopants, as long as the concentrations are kept very low and stability issues are addressed.

Effect of Ce3+ and Tb3+ codoping on luminescence properties of Na3SrMg11(PO4)9:Eu2+ phosphors

Eu2+-doped Na3SrMg11(PO4)9 has been identified as an efficient blue phosphor recently. The emission properties can be significantly improved by introducing Ce3+ ions into Na3SrMg11(PO4)9:Eu2+ phosphor. It is also witnessed that the emission intensity of Eu2+ band increases firstly, and then decreases with the increasing of Tb3+ concentration. Spectral analysis indicates that the luminescence properties of Eu2+ are controlled via the energy transfers between Eu2+ and Ce3+/Tb3+ and the changes in the valence state of Eu ions. Codoping Ce3+/Tb3+ ions facilitates the reduction of Eu3+ in the Na3SrMg11(PO4)9 host. Not only could the effective concentration of Eu2+ luminescence center be increased, but also the non-radiative transition probability between Eu2+ and Eu3+ could be reduced, leading to an enhancement of Eu2+ luminescence. The strategy of codoping Ce3+/Tb3+ ions as charge compensation agent suggested herein would also work for other phosphors containing mixed valence of dopants.

Transition-metal doped, magnesium oxide-supported terbium oxides as catalysts for the oxidative coupling of methane

The effects of transition metal dopants on the catalytic properties of magnesia-supported terbium oxide catalysts in the oxidative coupling of methane were investigated. A few transition metals were included based on their oxidation states relative to Tb in TbO1.81 (where Tb is between Tb3+ and Tb4+). Ni and Fe were selected as low valence dopants (LVDs) and Zr and V were considered high valence dopants (HVDs) in the study. The effect of dopant concentration was also investigated, between 0.2 and 1.0 % by weight (wt%), since high loadings could result in segregation of the transition metal (TM) dopants, while low loadings may not yield observable effects. At the higher loading (1.0 wt%) it appears that the oxidation state relative to that of Tb is of minor importance, and the redox properties of the dopant have a larger impact. At this loading, the best performing catalysts were the Ni- and Zr-doped TbOx/n-MgO, while the selectivities of the Fe- and V-doped catalysts toward OCM products were very low, particularly at the lower temperatures (<600 °C). The facile redox reactions that allow Fe and V to easily change oxidation state are likely detrimental to the OCM reaction, particularly at higher concentrations where segregation of the transition metal oxide may occur at the surface. In contrast, at the lower loading (0.2 wt%) all doped catalysts performed better than the undoped TbOx/n-MgO. At this loading, it appears that the LVDs resulted in better performing OCM catalysts, as the Ni- and Fe-doped TbOx/n-MgO both had higher activities and selectivities compared with the Zr- and V-doped catalysts.The characterization data revealed that even at the low concentration of 0.2 wt%, the transition metals markedly alters the properties of the doped TbOx/n-MgO catalysts. For example, the number of basic sites was significantly increased by adding 0.2 wt% of Fe or Zr to the TbOx/n-MgO catalyst and this led to an improvement in the performance of the catalysts in the OCM reaction. The dopants also affected the oxidation state of Tb in the TbOx lattice as well as the dispersion of TbOx on the surface of the MgO. Both XPS and XRD data indicated that the TbO1.81, which was present on the surface of the fresh catalysts, in most cases was reduced to Tb2O3 during reaction. However, there were no evident trends to indicate that LVDs promote, while HVDs suppress, surface reduction of the host oxide, i.e. TbOx. The lack of trends may in part be due to the presence of the MgO support, which results in a complex catalyst system. Nevertheless, the data revealed that even low loadings of a transition metal dopant can positively affect the activity and selectivity of a TbOx/n-MgO catalyst.

Effect of precursor dopant valence state on the photocatalytic performance of Mo3+- or Mo5+-Doped TiO2 thin films

Mo3+- or Mo5+-doped TiO2 sol-gel thin films (≤0.100 mol%) were spin coated on fused silica glass substrates and annealed at 450 °C for 2 h. The effect of the valence of the dopant precursor is significant to the nanostructural development and resultant properties. Mo3+ or Mo5+ doping yields converse trends with doping level, thus reflecting the competing influences of lattice distortion and destabilization (dominating Mo3+ doping) and nucleation and recrystallization (dominating Mo5+ doping). Mo doping results in oxidation of Mo3+ and reduction of Mo5+, both of which alter to Mo(5−x)+, as well as oxidation of Ti3+ to Ti(4+x)+; all of these result in VO•• annihilation. Although the absorption edges were largely indistinguishable, Mo3+ doping causes a red shift and Mo5+ doping causes a blue shift. These data suggest that the performances are controlled by the synergistic effects of crystallinity, surface area, and band gap, with the latter's exhibiting the dominant effect. This suggests that the defect structure governs the photocatalytic performance but also that the defect chemistry at these low annealing temperatures is indicative of nonequilibrium conditions, thereby explaining the significance of the dopant valence.

Yellow Emission from Low Coordination Site of Sr2 SiO4 :Eu2+ , Ce3+ : Influence of Lanthanide Dopants on the Electron Density and Crystallinity in Crystal Site Engineering Approach.

Lanthanide doping through a crystal site engineering approach tunes the emission wavelength suitable for LED applications, but weak emission from low coordination sites remains a huge challenge. Herein the use of a sensitizer is reported to enhance the emission strength and unravel the crystallinity and phase, as this approach demands a large amount of dopants. Doping of Eu2+ ions at SrO10 and SrO9 sites of Sr2 SiO4 (S2 S), respectively, tunes the emission from green to yellow and controlled doping of a Ce3+ sensitizer quadruples the quantum efficiency of yellow emission. Remarkably, doping of Eu2+ at the SrO9 site produces polycrystals, whereas co-doping of Ce3+ and Eu2+ at the same site produces single crystals. DFT calculations further delineate the underlying changes wherein strong interaction of dopant with its neighbours determines the electron density, and thus the crystallinity and phase, rather than usual explanation of aliovalent conditions, which is further substantiated by TEM results. Irrespective of dopant valence, use of large amounts of dopants and their interaction with the host is responsible for the crystallinity and phase change (α'-S2 S to β-S2 S). The XPS valence band spectra experimentally evidences the changes in bonding nature of O 2p and O 2s orbitals of silicate and its electron density, due to doping at the two sites. In short, the outcomes resulting from this work could be extended for the development of other two-coordination site lanthanide-doped materials and crystallization of inorganic materials.

C-, N-, S-, and F-Doped Anatase TiO2 (101) with Oxygen Vacancies: Photocatalysts Active in the Visible Region

Anatase TiO2 presents a large bandgap of 3.2 eV, which inhibits the use of visible light radiation (λ &gt; 387 nm) for generating charge carriers. We studied the activation of TiO2 (101) anatase with visible light by doping with C, N, S, and F atoms. For this purpose, density functional theory and the Hubbard U approach are used. We identify two ways for activating the TiO2 with visible light. The first mechanism is broadening the valence or conduction band; for example, in the S-doped TiO2 (101) system, the valence band is broadened. A similar process can occur in the conduction band when the undercoordinated Ti atoms are exposed on the TiO2 (101) surface. The second mechanism, and more efficient for activating the anatase, is to generate localized states in the gap: N-doping creates localized empty states in the bandgap. For C-doping, the surface TiO2 (101) presents a “cleaner” gap than the bulk TiO2, resulting in fewer recombination centers. The dopant valence electrons determine the number and position of the localized states in the bandgap. The formation of charge carriers with visible light is highly favored by the oxygen vacancies on TiO2 (101). The catalytic activity of C-doping using visible radiation can be explained by its high absorption intensity generated by oxygen vacancies on the surface. The intensity of the visible absorption spectrum of doped TiO2 (101) follows the order: C &gt; N &gt; F &gt; S dopant.