Low-temperature hydrothermal synthesis of phase-pure rutile titania nanocrystals: Time temperature tuning of morphology and photocatalytic activity

In this paper, a simple and efficient methodology for the low-temperature synthesis of phase-pure nanocrystalline rutile TiO₂ with tuned morphology is reported. Control on morphology has been achieved by simple variation of the hydrothermal process, starting with titanium-tetrachloride without using mineralizers, additives or templating agents. The X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns showed no other phases of TiO₂ establishing the formation of phase-pure rutile titania in the entire temperature range of synthesis (40-150 °C) and most noticeably even at a considerably low temperature (40 °C). Fourier transform infrared (FT-IR) spectra strongly indicated the presence of hydroxyl group or surface adsorbed water and the thermogravimetry and differential thermo-gravimetry (TG-DTG) showed no phase change up to 1000 °C. A combination of reaction parameters (temperature, time) with a thorough transmission electron microscopy (TEM) study demonstrated the formation of phase-pure rutile titania nanocrystals as nano-rods, bunched nano-spindles or spherical nanoparticles depending on the hydrothermal reaction conditions. The photocatalytic activity of the synthesized nanocrystals has been successfully evaluated on the photodegradation of methyl orange (MO), a well-known pollutant azo-dye, as a model reaction.     

Oxidation of carbon monoxide over Cu- and Ag-NaY catalysts with aqueous hydrogen peroxide

A novel oxidation reaction of CO with aqueous H₂O₂ over Cu-NaY (2-15 wt%) and Ag-NaY (5-15 wt%) catalysts has been achieved at low temperatures (55-70 °C) using a flow mode system. The employed catalysts were prepared by the incipient wetness impregnation of NaY zeolite (Si/Al = 5.6, surface area = 910 m²/g) with an aqueous solution of known concentrations of copper acetate and silver nitrate. Solids were subjected to thermal treatment at 300-450 °C prior to catalytic measurements unless subjected to subsequent reduction with hydrogen at 350 °C. The physicochemical characterization of the catalysts was probed using X-ray diffraction (XRD), FT-IR and combined thermal analyses TGA-DrTGA. The XRD data indicated that, the Ag particles have an ordered location in the sodalite cavity and the center of a single six-ring. The FT-IR data also proved the presence of a new peak at 1385 cm⁻¹ that is assigned to Ag-coordinated with the framework.A slow induced oxidation of CO (induction period, t_ind) took place at the initial stage of the CO oxidation reaction after which the reaction obeyed first-order kinetics. The utilized metal ions are proposed to be reduced to lower oxidation states such as Cu⁺ and Ag⁰ during the first period of reaction, t_ind, where the reaction proceeded favorably on such sites. Such argument was evidenced by carrying out the oxidation reaction over H₂-reduced Cu10-NaY and Ag10-NaY catalysts. The reduction caused a decrease in the t_ind, giving an evidence that the lower oxidation states Cu⁺ and Ag⁰ are the active sites in the studied oxidation reaction. The enhancement in catalytic activity was interpreted in terms of the facile adsorption of CO on the low oxidation state species.     

Solvothermal synthesis of nanorods of ZnO, N-doped ZnO and CdO

ZnO nanorods with diameters in the 80-800 nm range are readily synthesized by the reaction of zinc acetate, ethanol and ethylenediamine under solvothermal conditions. The best products are obtained at 330 °C with a slow heating rate. Addition of the surfactant Triton^®-X 100 gave nanorods of uniform (300 nm) diameter. By adding a small amount of liquid NH₃ to the reaction mixture, N-doped ZnO nanorods, with distinct spectroscopic features are obtained. CdO nanorods of 80 nm diameter have been prepared under solvothermal conditions using a mixture of cadmium cupferronate, ethylenediamine and ethanol at 330 °C. Similarly, Zn_1−xCd_xO nanorods of a 70 nm diameter are obtained under solvothermal conditions starting with a mixture of zinc acetate, cadmium cupferronate, ethanol and ethylenediamine.     

Synthesis of samarium sesquioxide from the thermal decomposition of samarium oxychloride

In this work, Sm₂O₃ was synthesized from the thermal decomposition of SmOCl (P4/nmm). The process was studied by gravimetry under flowing air between 600 °C and 950 °C. Reactants and products were identified and analyzed by X-ray diffraction and energy dispersive spectroscopy. From the results, the stoichiometry of the reaction was obtained. Between 800 °C and 950 °C, the successive formation of both cubic Sm₂O₃ (Ia3) and monoclinic Sm₂O₃ (C2/m) phases was observed. Below 800 °C, no transformation of cubic Sm₂O₃ to monoclinic Sm₂O₃ was detected. Quantification of phases was made using the Rietveld Method. Combined with the analysis of the evolution of the microstructure by scanning electron microscopy, an elemental analysis of the kinetics of the reaction was made and a reaction scheme was proposed.     

Photocatalytic properties of BiVO4 prepared by the co-precipitation method: Degradation of rhodamine B and possible reaction mechanisms under visible irradiation

Bismuth vanadate (BiVO₄) was synthesized by the co-precipitation method at 200 °C. The photocatalytic activity of the oxide was tested for the photodegradation of rhodamine B under visible light irradiation. The analysis of the total organic carbon showed that the mineralization of rhodamine B over a BiVO₄ photocatalyst (∼40% after 100 h of irradiation) is feasible. In the same way, a gas chromatography analysis coupled with mass spectroscopy revealed the existence of organic intermediates during the photodegradation process such as ethylbenzene, o-xylene, m-xylene, and phthalic anhydride. The modification of variables such as dispersion pH, amount of dissolved O₂, and irradiation source was studied in order to know the details about the photodegradation mechanism.     

CuO microflowers composed of nanosheets: Synthesis, characterization, and formation mechanism

CuO microflowers composed of nanosheets with tunable size have been fabricated through a simple reaction in ammonia solution at 90-180 °C, in which the nanosheets with zigzag edges were 20-40 nm in thickness and 500-800 nm in width. The high ammonia concentration, high ratio of CNH₃ to CCu2+ (R_ac) and elevated temperature were necessary for the formation of microflowers, and the ammonia concentration was critical for the morphology evolution of the particles. The fabrication mechanism of CuO microflowers based on the assembly of Cu(OH)₂ nanobelts was discussed. In addition, the CuO microflowers showed effective catalytic activity on the decomposition of ammonium perchlorate.     

Synthesis of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

Thermal stability and transport studies of (100 − 2x)TeO2-xAg2O-xWO3 (7.5 ≤ x ≤ 30) glass system

Differential scanning calorimetry (DSC), infrared (IR) and direct current (DC) conductivity studies have been carried out on (100 − 2x)TeO₂-xAg₂O-xWO₃ (7.5 ≤ x ≤ 30) glass system. The IR studies show that the structure of glass network consists of [TeO₄], [TeO₃]/[TeO₃₊₁], [WO₄] units. Thermal properties such as the glass transition (T_g), onset crystallization (T_o), thermal stability (ΔT), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl), heat capacity change (ΔC_p) and ratios C_pl/C_pg of the glass systems were calculated. The highest thermal stability (237 °C) obtained in 55TeO₂-22.5Ag₂O-22.5WO₃ glass suggests that this new glass may be a potentially useful candidate material host for rare earth doped optical fibers. The DC conductivity of glasses was measured in temperature region 27-260 °C, the activation energy (E_act) values varied from 1.393 to 0.272 eV and for the temperature interval 170-260 °C, the values of conductivity (σ) of glasses varied from 8.79 × 10⁻⁹ to 1.47 × 10⁻⁶ S cm⁻¹.     

Synthesis of cubic niobium nitride by reactive diffusion under nitrogen pressure

The synthesis of niobium nitride by reactive diffusion in a furnace at 1395-1475 °C and under nitrogen pressure in the range 2-25 MPa was investigated. In experiments, we used compacted Nb powder with a mean particle size of 43 μm. Phase transformations in the product as studied by electron probe microanalysis (EPMA) were found to proceed in the following order: Nb → α-Nb(N) → β-Nb₂N_1±x → γ-Nb₄N_3±x → δ-NbN_1±x. The size of niobium particles which could react with nitrogen to yield cubic niobium nitride was estimated (SEM analysis) from the dependence of the thickness Δ of the δ-NbN_1±x outer layer formed on the surface of Nb particles on the dwell time t_dw at 1460-1473 °C. It was shown that Δ grew nearly proportional to t_dw. At t_dw = 30 min and P(N₂) = 2 MPa, Δ was found to attain a value of about 15.5 μm. Prolonged heating (t_dw ≈ 60 min) was found to result in decomposition of the single-phase cubic niobium nitride into a two-phase (multiphase) product. This was confirmed by XRD data and magnetic measurements which showed the occurrence of two different critical temperatures T_c in the same sample. The maximum critical temperature T_c was found to attain a value of 15.6 K.     

Preparation of ultra-fine cobalt-nickel manganite powders and ceramics derived from mixed oxalate

Co_0.30Ni_0.66Mn_2.04O₄ negative temperature coefficient ceramics were derived from mixed oxalate Co_0.30Ni_0.66Mn_2.04(C₂O₄)₃·nH₂O. The mixed oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. An ultra-fine Co_0.30Ni_0.66Mn_2.04O₄ powder was obtained by calcining the mixed oxalate in air at 800 °C for 3 h. The oxide powder compact was sintered at a relatively low temperature of 1100 °C for 5 h, achieving a relative density of ∼98%. The specific resistivity ρ_25 °C and the thermal constant B_25/85 °C were 765.2 Ω cm and 3604 K, respectively. The resistance drift after aging at 150 °C for 500 h was 1.5%.     

Synthesis, crystal structure, dielectric properties, and potential use of nanocrystalline complex perovskite ceramic oxide Ba2ErZrO5.5

A new member belongs to Ba₂REZrO_5.5 (RE = Rare-Earth) perovskites, viz. Ba₂ErZrO_5.5, is synthesized as nanocrystals using a combustion process. Unlike the other Ba₂REZrO_5.5 perovskites, which are cubic, Ba₂ErZrO_5.5 crystallizes in tetragonal structure having space group P4/mnc (#128). Phase purity and ultrafine morphology of Ba₂ErZrO_5.5 powders were examined using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), Fourier transform of infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) specific surface area measurements, and transmission electron microscopy (TEM). Nanocrystals of Ba₂ErZrO_5.5 was sintered at ∼1500 °C for 4 h; whereas coarse-grained powders synthesized through solid state reaction could not be sintered even at 1700 °C for prolonged duration. XRD pattern of Ba₂ErZrO_5.5 was refined for atomic coordinates, lattice parameters, occupancies, and thermal factors using Rietveld analysis of XRD. Dielectric constant (?′) of Ba₂ErZrO_5.5 at 10 MHz is 21.62 ± 2 and dielectric loss (tan δ) is 5 × 10⁻³ at room temperature. Similar to the other Ba₂REZrO_5.5 perovskites, Ba₂ErZrO_5.5 is also showed chemically stability with YBa₂Cu₃O_7−δ (YBCO) and Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) superconductors. Ba₂ErZrO_5.5 can possibly be used as a substrate for high temperature superconductor (HTS) films, or be used as an insulator in the active superconductor-insulator-superconductor (SIS) structures.     

Hydrothermal synthesis and study of an inorganic-organic hybrid vanadate of a nickel(II) coordination complex with pyrazine, Ni3(C4H4N2)3(V8O23)

The three-dimensional hybrid compound Ni₃(C₄H₄N₂)₃(V₈O₂₃) has been synthesized by mild hydrothermal methods under autogenous pressure at 170 °C. The structure of the phase is stable until 380 °C. The removal of the pyrazine molecules from the structure induces its collapse. The IR spectrum shows the vibration modes of the pyrazine molecule and those of the [VO₄]³⁻ groups. A UV-visible spectrum shows the characteristic bands of the Ni(II) d⁸-high-spin cation in a slightly distorted octahedral coordination. Magnetic measurements indicate the existence of antiferromagnetic interactions that can be fitted with a chain model to give g = 2.31, J/k = −5.3, and zJ′/k = −5.5.     

Piezoelectric and ferroelectric properties of K0.5Na0.5NbO3 ceramics synthesized by spray drying method

Potassium-sodium niobate was synthesized at 800 °C for 1 h using dried precursors in a powder form obtained by the spray drying method. Different samples were sintered from 1060 to 1120 °C for 2 h reaching a relative density as high as 96% of the theoretical value. Piezoelectric and ferroelectric properties were studied for these samples and some of the most prominent results are: k_p, d₃₁, 2P_r, and 2E_C of 0.36, 39 pC/N, 29 μC/cm² and 16.5 kV/cm, respectively, for the sample sintered at 1080 °C. The methodology presented in this study can be used to synthesize submicrometer powders.     

Preparation, structure and dielectric properties of Ba4LaMNb3O15 (M = Ti, Sn ) ceramics

Two new cation-deficient hexagonal perovskites Ba₄LaMNb₃O₁₅ (M = Ti, Sn) ceramics were prepared by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The Ba₄LaTiNb₃O₁₅ has high dielectric constant of 52, high quality factors (Q) 3500 (at 4.472 GHz), and temperature variation of resonant frequency (τ_f) +93 ppm °C⁻¹ at room temperature; Ba₄LaSnNb₃O₁₅ has dielectric constant of 39 with high Q value of 2510 (at 5.924 GHz), and τ_f −29 ppm °C⁻¹.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Synthesis of flower-like Boehmite (AlOOH) via a simple solvothermal process without surfactant

Boehmite (AlOOH) with hierarchical flower-like structures was synthesized by the solvothermal reaction of AlCl₃·6H₂O in the presence of ethanol and toluene at 200 °C for 24 h. The product was characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that boehmite with flower-like nanostructures, which aggregated together by the weak hydrogen bonds, was formed through dissolution-deposition process of boehmite microcrystals and the toluene has a great effect on the morphology of product in the reaction system. Meanwhile, the γ-Al₂O₃ was also obtained by calcination of above product at 500 °C for 2 h, and the flower-like morphology kept no change. The surface area of γ-Al₂O₃ powder was determined to be 166.8 m²/g by N₂ adsorption measurement. The possible formation mechanism of flower-like boehmite nanostructures was proposed and discussed.     

Mass production of multi-wall carbon nanotubes by metal dusting process with high yield

Carbon nanotube materials were synthesized over Fe-Ni nanoparticles generated during disintegration of the surface of alloy 304L under metal dusting environment. The metal dusting condition was simulated and optimized through exposing stainless steel samples during long term repetitive thermal cycling in CO/H₂ = 1/1, total gas flow rate 100 cm³/min, at 620 °C for 300 h. After reaction, surface morphology of the samples and also carbonaceous deposition which had grown on sample surfaces were examined by stereoscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results revealed that multi-wall carbon nanotubes could be formed over nanocatalyst generated on the alloy surface by exploiting metal dusting process. By optimization of reaction parameters the yields of carbon nanotube materials obtained were 700-1000%. Also it has been shown herein that the amount of carbon nanotube materials remarkably increases when the reaction time is extended up to 300 h, indicating a possibility of the mass production by this easy method.     

Deposition and characterization of BN/Si(0 0 1) using tris(dimethylamino)borane

Boron nitride thin films could be deposited on Si(0 0 1) by chemical vapor deposition (CVD) at atmospheric pressure using a single source precursor. IR absorption spectra of films deposited between 750 and 1000°C using B[N(CH₃)₂]₃ (tris(dimethylamino)borane, TDMAB) as the boron and nitrogen source showed a peak absorption at ∼1360 cm⁻¹ characteristic of the in-plane vibrational mode seen in h-BN. It was noted that the mode at 800 cm⁻¹ is very weak. The observed growth rate varied exponentially with temperature in the range 850-900°C. Ellipsometry measurements were used to investigate the thickness and optical constant of the films. The refractive index, slightly lower than the bulk material, is close to 1.65-1.7 depending on the surface morphology of the films. The surface morphology of thin layers has been observed by atomic force microscopy with an increase of the surface roughness from 0.3 to 3.5 nm as the growth temperature increases from 800 to 950°C.     

Nanoparticles of La0.8Ca0.2Fe0.8Ni0.2O3−δ perovskite for solid oxide fuel cell application

Polycrystalline samples of La_0.8Ca_0.2Fe_0.8Ni_0.2O_3−δ (LCFN) with perovskite type structure have been prepared by combustion, freeze drying, citrate-gel process and liquid mix method. The analysis of X-ray powder diffraction indicated that the samples were single phase and crystallized in an orthorhombic (space group, Pnma no. 62) structure.Transmission electron microscopy (TEM) analysis on the synthesized powder at 600 °C by liquid mix method showed clusters of 150 nm formed by nanoparticles of 20 nm. Electrochemical performance of LCFN cathodes, which are used for intermediate temperature solid oxide fuel cells, were investigated. The polarization resistance was studied using two different electrolytes: Y-doped zirconia (YSZ) and Sm-doped ceria (SDC). The dc four-probe measurement exhibits a total electrical conductivity, over 100 S cm⁻¹ at T ≥ 600 °C, pointing out that strontium can be substituted for the cheaper calcium cation without destroying the electrochemical properties. Experimental results indicate that nanoparticles have more advantages in terms of smaller particle size and better electrochemical performance.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.