Fe-doped Material Research Articles

Acceleration mechanisms of Fe and Mn doping on CO2 separation of CaCO3 in calcium looping thermochemical heat storage

The doping strategy with dark metallic oxide has proven effective in improving optical absorptions and heat storage performances of calcium-based materials for the direct solar-driven thermochemical energy storage system, but the microscopic mechanisms of accelerated decomposition of CaCO3 during heat storage process are still unclear. Carbide slag as an industrial waste with low cost and high CaO content is considered as a potential calcium-based precursor for large-scale thermochemical energy storage. Herein, the novel Fe-doped and Mn-doped calcium-based materials were synthesized from carbide slag and their optical absorption properties and heat storage performances were determined in the experiment. The optimum decomposition temperatures of CaCO3 during heat storage process decreased 10.5 °C and 18.6 °C due to Fe doping and Mn doping, respectively. The acceleration mechanisms by Fe doping and Mn doping for enhancing the CO2 separation of CaCO3 in the calcination stage of the heat storage process were investigated by density functional theory (DFT) calculations. The structural parameters, partial density of states, electron differential densities and energy barriers during CO32– dissociation in heat storage process on the doped CaCO3 and undoped CaCO3 surfaces were compared to clarify the effects of Fe doping and Mn doping on the CaCO3 decomposition. The energy barriers of Fe-doped material and Mn-doped material are 1.68 eV and 1.42 eV, respectively, which are 29.4% and 40.3% lower than that of undoped material. This work helps to understand the microscopic mechanisms of accelerated CaCO3 decomposition by Fe and Mn during heat storage process.

Strongly facet-dependent activity of iron-doped β-nickel oxyhydroxide for the oxygen evolution reaction.

Iron (Fe)-doped β-nickel oxyhydroxide (β-NiOOH) is a highly active, noble-metal-free electrocatalyst for the oxygen evolution reaction (OER), with the latter being the bottleneck in electrochemical water splitting for sustainable hydrogen production. The mechanisms underlying how the Fe dopant modulates this host material's water electro-oxidation activity are still not entirely clear. Here, we combine hybrid density functional theory (DFT) and Hubbard-corrected DFT to investigate the OER activity of the most thermodynamically favorable (and therefore, expected to be the majority) crystallographic facets of β-NiOOH, namely (0001) and (101̄0). By considering active sites involving both oxidation and reduction of the transition-metal active center during the redox cycle on these two different facets, we show that six-fold-lattice-coordinated Fe in β-NiOOH is redox inactive towards both oxidation and reduction while five-fold-lattice-coordinated Fe in β-NiOOH does exhibit redox activity. However, the determined redox activity of Fe (or lack of it) is not indicative of good (or bad) performance as a dopant on these two facets. Three of the four active sites investigated (oxo and hydroxo sites on (0001) and a hydrated site on (101̄0)) exhibit only a marginal (<0.1 V) decrease or increase in the thermodynamic overpotential upon doping with Fe. Only one of the redox-active sites investigated, the hydroxo site on (101̄0), exhibits a large attenuation in the thermodynamic overpotential upon doping (to ∼0.52 V from 0.86 V), although the doped overpotential is larger than that observed experimentally for Fe-doped NiOOH. Thus, although pure β-NiOOH facets containing four-, five-, or six-fold lattice-coordinated Ni sites have roughly equal OER activities, yielding similar OER onset potentials (shown in A. Govind Rajan, J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2020, 142, 3600-3612), only those facets containing four-fold lattice-coordinated Fe (e.g., as shown in J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2019, 141, 693-705) would be active under analogous conditions for the Fe-doped material. It follows that, while undoped β-NiOOH demonstrates a roughly facet-independent oxygen evolution activity, the activity of Fe-doped β-NiOOH strongly depends on the crystallographic facet. Our study further motivates the investigation of strategies for the selective growth of facets with low iron coordination number to enhance the water splitting activity of Fe-doped β-NiOOH.

Influence of Ti and Fe doping on the structural and electrochemical performance of LiCo0.6Ni0.4O2 cathode materials for Li-ion batteries

The combination of lithium cobalt oxide (LCO) and lithium nickel oxide (LNO) property for Li-ion batteries (LIB) brings a very promising cathode material, LiCo1−xNixO2 with a high specific reversible capacity and good cycling behaviour. Nonetheless, high toxic Co content and an instability of Li+/Ni2+ interaction in LiCo1−xNixO2 crystal structure paved the way for some modification for the development of this potential material. In this research, the self-propagating combustion method is used to reduce 40% Co content of LCO by replacing it with 40% Ni content resulting in cathode material with the stoichiometry of LiCo0.6Ni0.4O2 (LCN). To improve the stability of the LiCo0.6Ni0.4O2 structure, 5% of Ti and Fe was substituted at the Co site of the LCN material. The effect of these different cation substitutions (Ti4+ and Fe3+) on the structural and electrochemical performance of layered LiCo0.6Ni0.4O2 cathode materials was investigated. Rietveld refinement revealed that Fe doped material has the longest atomic distance Li–O in the structure that allow better Li+ diffusion during intercalation/deintercalation to give an excellent electrochemical performance (138 mAhg−1). After 50th cycle, it is found that the discharge cycling for Ti and Fe substituted materials were improved by more than 5% compared to pristine material. Both Ti and Fe doped materials were also having less than 13% of capacity fading indicates that the substitution of some Co with Ti and Fe are stable and can retain their electrochemical properties.

Ab initio study of electronic and optical properties of [formula omitted] doped anatase [formula omitted] (1 0 1) surface

In this paper we investigated the effects of Fe -doping of the anatase TiO2 (1 0 1) surface on the crystal structure, electronic and optical properties, and impurity formation energy by means of density functional theory (DFT). The calculations were performed by the SIESTA DFT code and were carried out by using Troullier-Martins pseudopotentials for the 12-electron valence configuration (3s23p63d24s2) of Ti atom, 6-electron valence configuration (2s22p4) of O atom and 8-electron valence configuration (3d64s2) of Fe atom. We used a double- ζ basis set including polarization functions. All calculations were spin-polarized. The mechanism of narrowing the band gap and increasing the photocatalytic activity in the visible light region, of the doped TiO2 is discussed by investigating the density of state. The band gap decreases as the concentration of the dopant increases. The Partial Density of States (PDOS) is not the same in the case of spin-up state or spin-down state. Enhanced optical absorption, for light polarized in the z direction (parallel to the surface normal) is clearly observed for Fe doped as compared to the pure anatase TiO2 and the optical absorption is found to increase with the increase in the Fe concentration. The DFT results indicate that the source of the increasing photocatalytic activity in the visible light region of the Fe doped material is due to the introduction of additional electronic states within the band gap.Since the Fe atoms are more stable in Ti substitutional lattice positions for the entire range of Fermi energy EF over the band gap, only this substitutional position is considered.We hope that our results will highlight a route to improved electronic and optical properties of anatase TiO2 for industrial applications.

Enhancement of CO2 Adsorption and Catalytic Properties by Fe-Doping of [Ga2(OH)2(L)] (H4L = Biphenyl-3,3',5,5'-tetracarboxylic Acid), MFM-300(Ga2).

Metal–organic frameworks (MOFs) are usually synthesized using a single type of metal ion, and MOFs containing mixtures of different metal ions are of great interest and represent a methodology to enhance and tune materials properties. We report the synthesis of [Ga2(OH)2(L)] (H4L = biphenyl-3,3′,5,5′-tetracarboxylic acid), designated as MFM-300(Ga2), (MFM = Manchester Framework Material replacing NOTT designation), by solvothermal reaction of Ga(NO3)3 and H4L in a mixture of DMF, THF, and water containing HCl for 3 days. MFM-300(Ga2) crystallizes in the tetragonal space group I4122, a = b = 15.0174(7) Å and c = 11.9111(11) Å and is isostructural with the Al(III) analogue MFM-300(Al2) with pores decorated with −OH groups bridging Ga(III) centers. The isostructural Fe-doped material [Ga1.87Fe0.13(OH)2(L)], MFM-300(Ga1.87Fe0.13), can be prepared under similar conditions to MFM-300(Ga2) via reaction of a homogeneous mixture of Fe(NO3)3 and Ga(NO3)3 with biphenyl-3,3′,5,5′-tetracarboxylic acid. An Fe(III)-based material [Fe3O1.5(OH)(HL)(L)0.5(H2O)3.5], MFM-310(Fe), was synthesized with Fe(NO3)3 and the same ligand via hydrothermal methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn21 with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å and incorporates μ3-oxo-centered trinuclear iron cluster nodes connected by ligands to give a 3D nonporous framework that has a different structure to the MFM-300 series. Thus, Fe-doping can be used to monitor the effects of the heteroatom center within a parent Ga(III) framework without the requirement of synthesizing the isostructural Fe(III) analogue [Fe2(OH)2(L)], MFM-300(Fe2), which we have thus far been unable to prepare. Fe-doping of MFM-300(Ga2) affords positive effects on gas adsorption capacities, particularly for CO2 adsorption, whereby MFM-300(Ga1.87Fe0.13) shows a 49% enhancement of CO2 adsorption capacity in comparison to the homometallic parent material. We thus report herein the highest CO2 uptake (2.86 mmol g–1 at 273 K at 1 bar) for a Ga-based MOF. The single-crystal X-ray structures of MFM-300(Ga2)-solv, MFM-300(Ga2), MFM-300(Ga2)·2.35CO2, MFM-300(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), and MFM-300(Ga1.87Fe0.13)·2.0CO2 have been determined. Most notably, in situ single-crystal diffraction studies of gas-loaded materials have revealed that Fe-doping has a significant impact on the molecular details for CO2 binding in the pore, with the bridging M–OH hydroxyl groups being preferred binding sites for CO2 within these framework materials. In situ synchrotron IR spectroscopic measurements on CO2 binding with respect to the −OH groups in the pore are consistent with the above structural analyses. In addition, we found that, compared to MFM-300(Ga2), Fe-doped MFM-300(Ga1.87Fe0.13) shows improved catalytic properties for the ring-opening reaction of styrene oxide, but similar activity for the room-temperature acetylation of benzaldehyde by methanol. The role of Fe-doping in these systems is discussed as a mechanism for enhancing porosity and the structural integrity of the parent material.

Aging in the relaxor and ferroelectric state of Fe-doped (1-x)(Bi1/2Na1/2)TiO3-xBaTiO3 piezoelectric ceramics

Aging of piezoelectric properties was investigated in lead-free (1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 doped with 1at. % Fe. The relaxor character of the un-poled material prevents macroscopic aging effects, while in the field-induced ferroelectric phase aging phenomena are similar to those found in lead zirconate titanate or barium titanate. Most prominent aging effects are the development of an internal bias field and the decrease of switchable polarization. These effects are temperature activated, and can be explained in the framework of defect complex reorientation. This picture is further supported by electron paramagnetic resonance spectra indicating the existence of (FeTi′−VO••)• defect complexes in the Fe-doped material.

Conductivity of Fe2O3-doped YSZ

Effect of 2 mol.% Fe2O3 addition on conductivity of yttria stabilized zirconia ceramics (YSZ, 6–12 mol.% Y2O3) was established. For Fe doped material, it was shown that conductivity demonstrates maximum at 10 mol.% Y2O3, and its value increases by 1.4 times. Conductivity maximum corresponds with the minimal volume of crystal lattice. Discussion of results was performed in terms of the model that assumes the control effect of hydrostatic pressure on oxygen ions diffusion.

Zirconia Ceramics Effect of Fe2O3 Addition on Conductivity of YSZ Ceramics

Effect of 2 mol. % Fe2O3 addition on conductivity of yttria stabilized zirconia ceramics (YSZ, 6-12 mol.% Y2O3) was established. For Fe doped material, it was shown that conductivity demonstrates maximum at 10 mol. % Y2O3, and its value increases by 1.4 times. It was shown using X-ray phase analysis that the material was predominantly in the cubic phase within the concentration range. Discussion of results was performed in terms of the model that assumes the control effect of hydrostatic pressure on oxygen ions diffusion.

Effect of Fe and Fe–Ba substitution on the piezoelectric and dielectric properties of lead zirconate titanate ceramics

Polycrystalline samples of Fe and Fe–Ba doped lead zirconate titanate (PZT) ceramics near the morphotrophic phase boundary have been synthesized by a solid-state reaction technique. Preliminary X-ray analysis of the compound confirms that there is no change in the crystal structure of PZT on co-doping with Fe and Ba. The maximum mechanical quality factor Q m was found to be 1000 for Fe doped material and 880 for Fe–Ba doped material. The electromechanical coupling factor for Fe and Fe–Ba doped samples were 0.535 and 0.495 respectively. The corresponding values for the piezoelectric charge constant d 33 were 135 and 250 pC/N respectively. These results are discussed in terms of position occupied by dopants in to the lattice and their corresponding microstructures. These Fe–Ba doped PZT materials could be likely candidates for high power ultrasonic and underwater SONAR transducer systems.

3C–6H phase transition in BaTiO3 induced by Fe ions: an electron paramagnetic resonance study

X-ray diffraction (XRD) patterns and electron paramagnetic resonance (EPR) powder spectra (9 and34 GHz) of BaTiO3+0.04 BaO+0.5xFe2O3 (0.0005≤x≤0.02) ceramics were studied to investigate the development of the hexagonal phase(6H modification) of Fe-doped material in dependence upon doping levelx and sinteringtemperature Ts. Three axially symmetric EPR spectra assigned toFe3+ ions substituted at Ti lattice sites corresponding to the different distorted octahedra intetragonal and hexagonal modification are observed at room temperature. Thepresence of a hexagonal phase is shown by the XRD pattern and the EPR spectra.The 6H modification begins to occur at a nominal Fe concentration of betweenx = 0.005 and 0.01 and increases with increasing sintering temperature.BaTiO3 ceramicswith x = 0.02 sintered at Ts = 1400 °C is hexagonal. As discussed in the case of other 3d ions we propose the Jahn–Teller(JT) distortion as the driving force for the cubic–hexagonal transition at hightemperatures whereas a sufficiently high concentration of oxygen vacancies isthe second condition for this transformation process. In the case of Fe-dopedBaTiO3 a minimumconcentration of Fe4+ in the ceramic grains could trigger the transmutation into the hexagonal phase.

Investigation of compensation defect centres in semi-insulating InP crystals

Laplace transform photoinduced transient spectroscopy (LTPITS) has been applied to study defect, centres in Fe-doped and undoped semi-insulating (SI) InP. A high resistivity (∼ 2 x 10 7 Ω cm) of the latter wins achieved by annealing at 950 °C for 40 h under a phosphorus overpressure. It is shown that shallow donors in this material have an activation energy of 10 meV and are mainly compensated with deep acceptors characterised by activation energies of 350 and 470 meV with respect to the bottom of the conduction band. In the Fe-doped material, the shallow donors are compensated with Fe-related deep acceptors having activation energies of 590 and 640 meV.

The effect of various dopants on the dielectric properties of barium strontium titanate

Various acceptor and donor dopants were incorporated into Ba 0.60Sr 0.40M x Ti 1− x O 3 and their effect on dielectric properties for the material's use as a phase shifting device was studied. The best combination of high ϵ, low tan δ, and largest Δϵ with applied bias ΔV, was found in the Fe-doped material. The addition of 1 mol% Ba or Sr to the Fe-doped material was seen to further improve the properties. Other dopants studied included Mn, Bi, Ga, Y, and Nb.

Improved InP regrowth properties in metalorganic vapor phase epitaxy by addition of CCl4

Selective and planar regrowth of InP around stripe mesas up to 5.5 μm height formed with reactive ion etching in InP substrates has been made with metalorganic vapor phase epitaxy by adding CCl4 to the process gases. CCl4 seems to prevent nucleation on the phosphorous-rich {111}B lattice planes and on the silicon-nitride cap at the top of the mesa and hence permits reproducible regrowth with desired morphologies for integration of optoelectronic circuits at mesas oriented along the [110] direction. The addition of CCl4 does not influence dopant capability or the semi-insulating properties of Fe-doped material, for which a resistivity of 1×109 Ω cm has been obtained.

The low-temperature photochromic response of bismuth germanium oxide

Exposing the photorefractive bismuth germanium oxide (BGO) at low temperatures to 2.5–3.3 eV light produces photochromic absorption bands. In both undoped and Fe-doped material the absorption consists of overlapping bands at 1.5, 2.2, 2.7, and 3.1 eV. The 1.5- and 2.2-eV components are relatively weaker in Fe-doped BGO. In undoped BGO the photochromic bands anneal together above 200 K. Doping with iron adds an anneal stage in the 110–200 K range that matches the recovery of Fe3+. Optical bleaching with 1.5-eV light was much less efficient than with visible band light. Thus, the 1.5-eV photochromic band may be an internal transition of the center responsible for the 2.2-eV band. In BGO:Al the aluminum electronically compensates the deep donor center responsible for the yellow coloration observed in undoped crystals. Photoexcitation at 9 K produces overlapping absorption bands at 1, 1.38, and 2.45 eV. All three bands have major anneal stage in the 80–100 K range. They also bleach out together when the sample is exposed to either infrared or visible range light. The [AlO4]0 center which causes the coloration observed in smoky quartz is a plausible model for the visible range photochromic center in BGO:Al.

Photoluminescence study of heat-treated InP

Low-temperature photoluminescence is used to study optical transitions in InP which arise from moderate-temperature annealing or dielectric encapsulation. A total of seven shallow emission lines appear in annealed or encapsulated InP that are not present in unprocessed material. These transitions are easily observed in undoped substrates. They are less intense or absent in Fe-doped material, and are not seen in p-type material. It is found that dielectric capping with SiO2 enhances the formation of several transitions, while phosphosilicate glass or Si3N4 effectively suppresses their formation. Surprisingly, some of these recombination centers are found to extend many microns into the substrate after SiO2 deposition at 350 °C or after annealing at temperatures as low as ∼400 °C. These centers appear to be related to previously observed processing-induced carrier concentration changes. Possible origins of the transitions are discussed.

Substrate dependence of InP m.e.s.f.e.t. performance

There is evidence to suggest that Fe outdiffuses, during the growth, into the epitaxial films prepared by vapour-phase epitaxy at 650°C. Field-effect transistors on Fe-doped material show substantial looping that was absent on Cr-doped material and exhibit about 2 dB worse noise figure at about 7 GHz. Experiments with low 1015 S-doped InP grown on Sn-, Cr- and Fe-doped substates indicate that such outdiffusion is typically about 5 μm. Saturation velocity levels in the m.e.s.f.e.t. channel are about 1.7 × 107 cm/s and 1.3 × 107 cm/s, associated with Cr and Fe doped substrates, respectively.

Effect of Oxygen Partial Pressure on the Creep of Polycrystalline Al2O3 Doped with Cr, Fe, or Ti

Steady‐state creep was studied in hot‐forged polycrystalline Al2O3 (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm2 at 1375° to 1525°C under O2 partial pressures of 0.88 to 10−10 atm. Except in the 10‐μm Fe‐doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of Po2 on the creep rate indicated that increased concentration of a divalent (Fe2+) or quadrivalent (Ti4+) impurity in solid solution enhances the creep rate of polycrystalline Al2O3. The activation energies for the creep of Fe‐ and Ti‐doped Al2O3 samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr‐doped material (114 kcal/mol). Taking into account the effects of Po2, temperature, and grain size, it was concluded that the steady‐state creep of transition‐metal‐doped Al2O3 is controlled by cation lattice diffusion.