Interaction of H2S with α-Fe2O3(0 0 0 1) surface

The atomic-scale structural changes in an α-Fe₂O₃ (hematite) (0 0 0 1) surface induced by sulfidation and subsequent oxidation processes were studied by X-ray photoemission spectroscopy, LEED, and X-ray standing wave (XSW) measurements. Annealing the α-Fe₂O₃(0 0 0 1) with a H₂S partial pressure of 1 × 10⁻⁷ Torr produced iron sulfides on the surface as the sulfur atoms reacted with the substrate Fe ions. The oxidation state of the substrate Fe changed from 3+ to 2+ as a result of the sulfidation. The XSW measured distance of the sulfur atomic-layer from the unrelaxed substrate oxygen layer was 3.16 Å. The sulfide phase consisted of three surface domains identified by LEED. Formation of the two-dimensional FeS₂ phase with structural parameters consistent with an outermost layer of (1 1 1) pyrite has been proposed. Atomic oxygen exposure oxidized the surface sulfide to a sulfate () and regenerated the α-Fe₂O₃(0 0 0 1) substrate, which was indicated by a (1 × 1) LEED pattern and the re-oxidization of Fe to 3+.     

Surface studies of 2,4-pentanedione on γ-Al2O3/NiAl (1 0 0) and NiAl (1 0 0)

The chemical compound 2,4-pentanedione (Hacac) has been shown to etch the oxidized metal surfaces metals such as copper and nickel, but not their unoxidized surfaces. Here it is shown that on the γ-Al₂O₃/NiAl (1 0 0) surface (oxidized NiAl (1 0 0)) etching of aluminum occurs at 170 K and 750 K. Reflection-absorption infrared spectroscopy (RAIRS) is used to show that Hacac binds to both the clean, metallic and oxidized surfaces, but decomposition and combustion products dominate on the metallic surface and no etching occurs. The binding process that involves a deprotonation reaction of the enol species was identified by redshift in the carbonyl peaks and the appearance of an Al-H peak observed in the IR spectrum. The implication of these results is that there is both an unusual low temperature and high temperature etching of the alumina by bound acac.     

Structural and photoluminescence properties of SnO2:Ga films deposited on α-Al2O3 (0 0 0 1) by MOCVD

Gallium-doped tin oxide (SnO₂:Ga) films have been prepared on α-Al₂O₃ (0 0 0 1) substrates at 500 °C by the pulse mode metalorganic chemical vapor deposition (MOCVD) method. The relative amount of Ga (Ga/(Ga+Sn) atomic ratio) varied from 3% to 15%. Post-deposition annealing of the films was carried out at different temperatures for 1.5 h in ambient atmosphere . The structural, electrical, optical and photoluminescence (PL) properties of the films have been investigated as a function of annealing temperature. All the films have the rutile structure of pure SnO₂ with a strong (2 0 0) preferred orientation. A single ultraviolet (UV) PL peak near 337.83 nm was observed at room temperature for the 3% Ga-doped as-grown film and near 336 nm for the 15%-doped film, which can be ascribed to electron transition from the oxygen vacancy and interstitial Ga³⁺ donor levels to the acceptor level formed by the substitution of Ga³⁺ for the Sn site. After annealing, the luminescence spectra have changed a little bit, which is being discussed in detail.     

Highly thermal stable transparent conducting SnO2:Sb epitaxial films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Antimony-doped tin oxide (SnO₂:Sb) single crystalline films have been prepared on α-Al₂O₃ (0 0 0 1) substrates by metal organic chemical vapor deposition (MOCVD). The antimony doping was varied from 2% to 7% (atomic ratio). Post-deposition annealing of the SnO₂:Sb films was carried out at 700-1100 °C for 30 min in atmosphere ambient. The effect of annealing on the structural, electrical and optical properties of the films was investigated in detail. All the SnO₂:Sb films had good thermal stability under 900 °C, and the 5% Sb-doped SnO₂ film exhibited the best opto-electrical properties. Annealed above 900 °C, the 7% Sb-doped SnO₂ film still kept high thermal stability and showed good electrical and optical properties even at 1100 °C.     

Oxidation mechanism and ferryl domain formation on the α-Fe2O3 (0 0 0 1) surface

Recent experimental evidence calls for a reinterpretation of the oxidized structure in chemically distinct domains of the hematite (0 0 0 1) surface as the ferryl (FeO) termination rather than the bulk terminated O₃-Fe-Fe-R structure. Although this interpretation is consistent with experimental data and ab initio thermodynamics calculations, it raises serious questions about how molecular oxygen can be dissociated on a surface where reactive iron centers are slightly more than 5 Å apart. Here, we propose a novel cooperative bimolecular mechanism that provides a reasonable pathway for the formation of the unusual ferryl surface termination and should be readily reversible, which is important for understanding the function of hematite surfaces as an oxidation catalyst.     

Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Surface and redox chemistry of Sm0.95Ce0.05Fe1 − xNixO3 − δ perovskites

A new series of perovskite materials with formula Sm_0.95Ce_0.05Fe_1 − xNi_xO_3 − δ (0 ≤ x ≤ 0.10) has been prepared by sol-gel combustion via a citrate precursor route. X-ray diffraction data showed that materials prepared by this method had a single orthorhombic phase belonging to the Pnma (62) space group. The study of powders sintered in air and in reducing atmospheres reveals that these materials do not show phase separation in air (up to 1350 °C) nor under 5% v/v H₂/N₂ (up to 700 °C), but a phase separation of Sm₂O₃ does occur at and above 800 °C under 5% v/v H₂/N₂ without deterioration of the perovskite phase. The surfaces of all the powders (fresh, in-situ reduced and ex-situ reduced) were Sm rich, and multiple oxidation states for Fe were observed. XP analysis of in-situ reduced samples (800 °C and above) shows that metallic Fe forms in all nickel doped materials except x = 0.07. The surface oxygen vacancies and percentages of lattice and adsorbed oxygen for this series of Ni doped materials were determined and the oxygen recapturing ability is explained in terms of the multiple oxidation states of Fe.     

Oxygen relaxation and phase transition in GdBaCo2O5 + δ oxide

Electrical conductivity, internal friction techniques and dilatometer have been used to investigate the oxygen relaxation, phase transition and thermal expansion behavior of GdBaCo₂O_5 + δ. The main electronic charge carriers in GdBaCo₂O_5 + δ are electronic holes, which could be assigned to the formation of Co⁴⁺. The oxygen exchange kinetics intensely depends on oxygen partial pressure and is also closely related to temperature. Both electrical conductivity and internal friction give rise to an abnormal at about 75 °C, which are related to the insulator-metal transition occurring in GdBaCo₂O_5 + δ. One large relaxation internal friction peak, due to the motion of oxygen within Gd-O plane, is also found in the oxide. The average thermal expansion coefficient (TEC) of GdBaCo₂O_5 + δ is about 21.4 × 10⁻⁶ K⁻¹ between 500 °C and 900 °C.     

Transparent conducting Sn-doped Ga1.4In0.6O3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Sn-doped Ga_1.4In_0.6O₃ films have been prepared on α-Al₂O₃ (0 0 0 1) substrates by the metalorganic chemical vapor deposition (MOCVD) method. The Sn-doping was varied from 0% to 7% (atomic ratio). Polycrystalline films with resistivity of 4.9 × 10⁻³Ω cm, carrier concentration of 5.9 × 10¹⁹ cm⁻³ and Hall mobility of 21.4 cm² v⁻¹ s⁻¹ was obtained at 5 at.% of Sn concentration. The average transmittance for the Sn-doped Ga_1.4In_0.6O₃ films in the visible range was over 90%. The bandgap of the films varies from 3.85 to 4.21 eV.     

Structure and photoluminescence properties of epitaxial SnO2 films grown on α-Al2O3 (0 1 2) by MOCVD

SnO₂ thin films have been successfully deposited on α-Al₂O₃ (0 1 2) substrates by metalorganic chemical vapor deposition (MOCVD) in the temperature range 500-700 °C. The films were epitaxially grown in the tetragonal SnO₂ phase and were (1 0 1) oriented. In-plane orientation relationship [0 1 0]SnO₂||[1 0 0]Al₂O₃ and [1 0 1?]SnO₂||[1? 2? 1]Al₂O₃ was determined between the film and substrate. Photoluminescence (PL) spectra measured at room temperature revealed that the film grown at 700 °C showed an intense ultra-violet (UV) PL peak at 333 nm, which was a band-edge emission peak in SnO₂ films. At a temperature of 13 K, a new broad PL band centered at about 480 nm was observed. The corresponding PL mechanisms are discussed in detail.     

Characterization of YSr2Fe3O8 − δ as electrode materials for SOFC

YSr₂Fe₃O_8 − δ was prepared by traditional solid state reaction method and characterized by X-ray diffraction, ac impedance, dc conductivity, dilatometry and thermogravimetric analysis for possible use in solid oxide fuel cells (SOFCs). YSr₂Fe₃O_8 − δ crystallizes with tetragonal symmetry in the space group P4/mmm and found to be stable at high temperatures under H₂ and air. Four probe dc electrical conductivity measurements show that the conductivity increases up to 745 K and then decreases with temperature; the highest conductivity σ_745K = 43.5 S cm^− 1. The n-type conductivity at low oxygen partial pressure (pO₂) changes to p-type at high pO₂. Polarization behavior was investigated measuring the ac impedance response in symmetrical cell arrangements in air with YSZ and GDC electrolytes. Cathodic area specific resistance (ASR) varies with firing temperature. The lowest area specific resistance was observed with a GDC electrolyte fired at 1000 °C. In case of YSZ, ASR increases and in case of GDC, ASR decreases in air when electrode firing temperature decreases. At 800 °C ASRs are 0.20 Ω cm² and 0.65 Ω cm² with GDC and YSZ electrolytes, respectively, in air. Fuel cell measurements with symmetrical electrodes were performed using a thin YSZ electrolyte under H₂ at anode and air at cathode, show that the power density is about 0.035 W/cm² at 900 °C.     

Rovibrational spectroscopy of the Fermi-interacting ν4 = 1 and ν3 = ν6 = 1 levels of DCF3

The high-resolution infrared spectrum of deuterated fluoroform (DCF₃) was studied in the 700 and 1200 cm⁻¹ regions, with the aim of assigning and analyzing the ν₄ CF₃ asymmetric stretching vibration. The Fermi-type anharmonic coupling between the ν₄ = 1 and ν₃ = ν₆ = 1 rovibrational levels, already mentioned in an early work of Ruoff et al. [Spectrochimica Acta Part A 31A (1975) 1099-1100], was studied here for the first time under high resolution. Assignments in the ν₃ + ν₆/ν₄ band system were confirmed and extended by the identification of the ν₃ + ν₆ − ν₆ and ν₄-ν₆ bands in the 700 cm⁻¹ region, the latter being enhanced near the Fermi crossings of the studied levels. Data from both the hot and difference bands were included in the analysis. The close separation of the studied vibrational levels of about 14.8 cm⁻¹ produces a large variety of resonance crossings which involve levels with . Besides the Fermi () and Coriolis () resonances, they were accounted for by inclusion of additional higher-order ( and ) interaction terms between the vibrational states. The least-squares fit of more that 16,000 vibration-rotation transitions provides a quantitative reproduction of data in all bands.     

The rotational spectra of the υ8 = υ9 = 1 and υ6 = υ7 = 1 interacting vibrational states of nitric acid (HNO3)

The analysis of the rotational spectrum of HNO₃ has been extended to include the υ₈ = υ₉ = 1 state at 1205.7 cm⁻¹ and the υ₆ = υ₇ = 1 state at 1223.4 cm⁻¹. Based on 78-519 GHz data, the assignments in the 8¹9¹ vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241-243]. A new microwave analysis is also reported for the 6¹7¹ vibrational state. A simultaneous analysis takes into account the localized ΔK_a = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8¹9¹ and 6¹7¹ states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm⁻¹. The rotational constants agree well with those predicted by vibrational-rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO₃ in the 8.3 μm region.     

Antiferroelectric phase transition in pyrochlore-like (Dy1− xCax)2Ti2O7 − δ (x = 0, 0.1) high temperature conductors

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy_1 − xCa_x)₂Ti₂O_7 − δ (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy₂Ti₂O₇ leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.     

Polymorphism and high-temperature conductivity of Ln2M2O7 (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores

We present a comparative analysis of the order─disorder transitions in Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) pyrochlore-like compounds and solid solutions existing in the Ln₂O₃─MO₂ systems. In the range ~ 600─1200 °C, Ln₂Ti₂O₇ (Ln = Sm─Lu) and Ln₂Zr₂O₇ (Ln = Sm─Gd) undergo ordering transitions, F? → PI → P, which culminate in the formation of an ideal pyrochlore structure, P, existing between 1100 and 1300 °C. Above 1300 °C, Ln₂Ti₂O₇ (Ln = Gd─Lu), Ln₂Zr₂O₇ (Ln = Sm─Gd) and Ln₂Hf₂O₇ (Ln = Eu─Tb) exist as oxygen-ion-conducting phases, PII, disordered in both the oxygen and cation sublattices. Ionic conductivity data for Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) synthesized at 1600-1670 °C indicate that the highest conductivity in these systems is typically offered by nominally stoichiometric (Ln:M = 1:1), disordered Ln₂M₂O₇ (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores containing anti-structure pairs (Ln_M^' + M_Ln^•) and oxygen vacancies (V_O^••) on the 48f (O2) site. The highest conductivity of Yb₂Ti₂O₇, in which the cations have the smallest radii among the lanthanides and Group IVa metals, seems to be due to the increased role of the geometric factor in the Ln₂Ti₂O₇ (Ln = Sm-Lu) pyrochlores with predominantly covalent metal─oxygen bonding M-O (Ti-O). The ion transport parameters in these materials are determined primarily by the relationship between the sizes of the mobile oxygen ions and conduction channels.     

Anisotropic characteristics of a-plane GaN films grown on γ-LiAlO2 (3 0 2) substrates by MOCVD

Single crystalline wurtzite a-plane GaN films were deposited on (3 0 2) LiAlO₂ (LAO) substrates by metal organic chemical vapor deposition (MOCVD). The high resolution X-ray diffraction (HRXRD) results and selected area electron diffraction (SAED) patterns in cross section indicated that the crystallographic orientation between LAO and wurtzite GaN was: [3 0 2]_LAO parallel to , parallel to and [0 1 0]_LAO parallel to [0 0 0 1]_GaN, the mismatches were −4.43%, −2.86% and −0.31%, respectively. When the incident beam parallel (or perpendicular) to the [0 0 0 1] direction of GaN, the FWHM values of ω-scans reached the minimum (or maximum). The a-GaN film was found to have steps along direction and strips coalesced parallel to [0 0 0 1] direction. The PL intensity of the emission peak around 364 nm reduced a lot when the polarization changed from E⊥c to E||c.     

Catalysis of the hydrogen oxidation reactions by Sr-doped LaMn1 − yCryO3 ± δ oxides

Sr-doped and Sr-free La_1 − xSr_xMn_1 − yCr_yO_3 ± δ (LSMC, x(Sr) = 0-0.2, y(Cr) = 0.4-0.6) perovskite-type oxides were synthesized and evaluated as single phase anodes for use in intermediate temperature solid oxide fuel cell applications. Their thermo-chemical and chemical stabilities were investigated in hydrogen at high temperatures and correlated with their oxygen non-stoichiometry (3 ± δ), determined by permanganate titration. The catalytic activity towards hydrogen oxidation was examined as a function of oxide sintering time, operating temperature, and the Sr and Cr contents, using a Pt mesh current collector. While all of the perovskite oxides studied here showed some irreversible performance degradation with time under both open circuit and anodically polarized conditions, La_0.9Sr_0.1Mn_0.6Cr_0.4O_3.03 (LSMC9164), sintered at 1200 °C for 10 h, was found to be the most catalytically active and also the most stable.     

Investigations on the thermo-chemical stability and electrical conductivity of K-doped Ba3 − xKxCaNb2O9 − δ (x = 0.5, 0.75, 1, 1.25)

In this paper, we report the synthesis, crystal structure and electrical transport properties of new K-doped Ba₃CaNb₂O₉ (BCN) and investigate their chemical stability in H₂O and pure CO₂ at elevated temperature. The powder X-ray diffraction (PXRD) of Ba_2.5K_0.5CaNb₂O_9 − δ, Ba_2.25K_0.75CaNb₂O_9 − δ, Ba₂KCaNb₂O_9 − δ, and Ba_1.75K_1.25CaNb₂O_9 − δ showed the formation of a single-phase double perovskite (A₃BB^/₂O₉)-like cell with a lattice constant of a ∼ 2a_p (where a_p is a simple perovskite cell of ∼ 4 Å). Perovskite-like structure was found to be retained after treating with CO₂ at 700 °C and also after boiling H₂O for 120 h. The lattice constant of CO₂ and H₂O treated samples was found to be comparable to that of the corresponding as-prepared compound. The total electrical conductivity of all the investigated K-doped BCN increases with increasing K content in BCN in various atmospheres, including air, dry H₂, wet N₂ and wet H₂. The electrical conductivity in dry and wet H₂ atmospheres was found to be higher than that of air in the temperature range of 300-700 °C, while in wet N₂ a slightly lower value was observed. Among the compounds investigated in the present study Ba_1.75K_1.25CaNb₂O_9 − δ showed the highest total electrical conductivity of 1 × 10^− 3 S/cm in dry H₂ at 700 °C with an activation energy of 1.28 eV in the temperature range of 300-700 °C.     

Non-universal dielectric relaxation in SrFeO3 − δ

SrFeO_3 − δ compound is prepared by the thermal decomposition method followed by ball milling. Analysis of Mössbauer spectrum and X-ray diffraction study proves the presence of multi-phase nature, i.e., Sr₈Fe₈O₂₃ and Sr₄Fe₄O₁₁ phases at room temperature. Furthermore, the Mössbauer spectrum at room temperature evidenced the presence of major Fe^3.5+ which is the resultant of equal contributions of Fe⁴⁺ and Fe³⁺. The Nyquist plot at all measured temperatures (80–230 K) suggests that the dielectric response is well associated with single relaxation time (exponential parameter, n∼1$n\sim 1$) i.e., the Debye-type. Modulus analysis exhibits the non-universal dielectric behaviour (stretched exponential parameter, β>1$\beta >1$) below 230 K and the Debye-type responds (β∼1$\beta \sim 1$) at and above 230 K. The Debye-type behaviour exhibited by SrFeO_2.81 at around room temperature in its defect state offers a new opening for this material for multifunctional applications.     

Fabrication, sintering and electrical properties of cobalt oxide doped Gd0.1Ce0.9O2 − δ

Ceria-based electrolytes have been widely researched in intermediate-temperature solid oxide fuel cell (SOFC), which might be operated at 500~600 °C. Gd0.1Ce0.9O1.95(GDC10) powders were prepared by a modified chemical co-precipitation process with Gd(NO3)3 and Ce(NO3)3 as precursors, and ammonia and hydrogen peroxide as precipitants. The precursors of GDC10 were fired at 350 °C for 2 h, then the fluorite structure cerias were identified by X-ray diffraction. The powders are well crystallized, with the size about 5 nm and surface area of 148.3 m2/g. Loading 1mol% cobalt oxide as additive, the GDC10 were succeeded to densify at 950 °C by liquid phase sintering mechanism. The grain size of 1CoGDC10 is small, about 100 nm. The electrical conductivity of samples sintered at 950 °C is about 0.01S/cm at 600 °C. The existence of cobalt oxide and smaller grain size of 1CoGDC10 don't affect the electrical conductivity.