Woo-Sik Jung

Reaction intermediate(s) in the conversion of β-gallium oxide to gallium nitride under a flow of ammonia

Abstract The process of conversion of β-gallium oxide (β-Ga 2 O 3 ) to gallium nitride (GaN) under a flow of ammonia was monitored by XRD, IR, and 71 Ga magic-angle spinning (MAS) NMR spectroscopy. It is most likely that the conversion of β-Ga 2 O 3 to GaN does not proceed through Ga 2 O but stepwise via amorphous gallium oxynitrides (GaO x N y ) as intermediates.

Synthesis of aluminum nitride by a modified carbothermal reduction and nitridation method using basic dicarboxylate Al(III) complexes Al(OH)(Cn+2H2nO4).xH2O (n = 3, 6, 8)

Abstract Aluminum nitride (AlN) particles and whiskers were synthesized by using basic dicarboxylate Al(III) complexes Al(OH)(Cn+2H2nO4)·xH2O (n =3, 6, 8) as a precursor. AlN was obtained by calcining the glutarate complex (n=3, AG) without further additions of carbon source under flowing nitrogen in the temperature range 1200–1500°C and then burning out the residual carbon. In contrast, for suberate (n=6, ASu) and sebacate (n=8, ASe) complexes additional carbon was required for their complete conversion to AlN. The process of conversion of AG to AlN was investigated by 27Al magic-angle spinning (MAS) nuclear magnetic resonance, infrared spectroscopy, and X-ray diffraction. The complex began to decompose at ca. 400°C and then turned into γ-alumina at temperature above 600°C. Finally, the γ-alumina was converted to AlN without γ-α alumina phase transformation. The morphology of AlN powders was very similar to that of the precursor, indicating that conversion of alumina to AlN during the carbothermal reduction and nitridation does not proceed through gaseous intermediates but through solid-state Al–oxynitride compounds. The AlN powders obtained consisted of ultrafine particles or mixtures of particles and whiskers.

Synthesis of aluminium nitride by the reaction of aluminium sulfide with ammonia

Abstract Aluminium nitride (AlN) powder was prepared by the solid–gas reaction of aluminium sulfide (Al 2 S 3 ) with ammonia. The process of conversion of Al 2 S 3 to AlN was investigated by XRD and 27 Al magic-angle spinning NMR spectroscopy. The formation of AlN commenced at ca. 550°C, which is the lowest temperature in the AlN formation through the solid–gas and liquid–gas reactions.

Preparation of aluminium nitride powder from a (hydroxo)(succinato)aluminium(III) complex

Aluminium nitride (AIN) powder was prepared by using a (hydroxo)(succinato)aluminium(III) complex as a new precursor. The AIN powder was obtained by calcining the complex without mixing any carbon source under a flow of nitrogen in the temperature range 1200–1500 °C and then burning out the residual carbon. The complex began to decompose at ca. 400 °C and turned into γ-alumina at temperatures >600 °C. Carbothermal reduction and nitridation of the γ-alumina, resulting in the formation of AIN, commenced at ca. 1200 °C without γ–α alumina transition. A short time at the higher calcination temperature was more likely to favour complete nitridation than a long time at the lower calcination temperature. The AIN powders prepared were ultrafine, and their particle size depended on the calcination temperature and duration of calcination.

Growth mechanism of aluminum nitride whiskers prepared from a (glutarato)(hydroxo)aluminum(III) complex

Abstract Aluminum nitride (AlN) whiskers were synthesized by calcining an Al(OH)(glutarate)·xH2O complex without mixing any catalyst under a flow of nitrogen in the temperature range 1150–1600 °C and then burning out the residual carbon (by the so-called modified carbothermal reduction and nitridation method). At the very low reaction temperature of 1150 °C, the whiskers with a bead-necklace structure were obtained, showing the beginning step in formation of the whisker and with increasing the temperature, the morphology of the whisker became the shape of a six-sided prismatic needle with the axis of [0001] direction. It is concluded from the change of morphologies with temperature that AlN whiskers grow through both the primary nanometer-sized AlN particle-to-particle connection and the solid diffusion of the particle, the latter being operative at relatively high temperature.

Protonation and stability constants for Co2+, Ni2+, Cu2+ and Zn2+ of two open-chain hexadentate N6 ligands containing two pyridyl groups: Crystal structures of their Ni(II) complexes

Abstract Two open-chain hexadentate N6 ligands containing two pyridyl groups, 1,13-bis(2-pyridyl)-2,5,9,12-tetraazatridecane (bpytd) and 1,14-bis(2-pyridyl)-2,6,9,13-tetraazatetradecane (bpytt), have been synthesized as their tetrahydrochloride salts and their proton association constants and stability constants of Co(II), Ni(II), Cu(II) and Zn(II) chelates have been determined by potentiometry. The stability constants are higher than those for analogous hexadentate N6 ligands containing one pyridyl moiety, which are in turn higher than those for analogous aliphatic N6 ligands. The Ni(II) complexes [Ni(bpytd)](ClO4)2 · H2O (1) and [Ni(bpytt)](ClO4)2 (2) have been synthesized and their structures have been determined by three-dimensional X-ray diffraction methods. Their geometries at nickel are six-coordinate pseudo-octahedra, with pyridyl nitrogen atoms being mutually cis for 1 and trans for 2.

27Al Magic-angle spinning nuclear magnetic resonance spectroscopic study of the conversion of basic dicarboxylate aluminium(III) complexes to alumina and aluminium nitride

The process of conversion of hydrated basic dicarboxylate aluminium(III) complexes Al(OH)(succinate)xH2O and Al(OH)(adipate)xH2O to alumina and aluminium nitride (AlN) has been investigated by 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The powdered succinate complex was calcined under various atmospheres such as N2, argon and air. The 27Al MAS NMR spectra for the calcined materials at 500C all showed three peaks at delta; 6, 33 and 63. The relative intensities of these peaks varied with increasing temperature and were also dependent on the calcination atmosphere. The 27Al NMR signal of AlN at delta; 114 was observed for the sample calcined in an N2 atmosphere at 1150C whilst in all spectra of samples calcined under an N2 atmosphere at 1150C there were no detectable signals other than those of gamma;-alumina and AlN. The finding that the ratio of the relative intensities of AlO6 and AlO4 groups in gamma;-alumina changes with temperature suggests that the carbothermal reduction and nitridation of alumina proceeds through intermediates with empirical formulae of the type AlOxNy. The degree of nitridation at each reaction temperature for the succinate complex was higher than that for the adipate complex.

Kinetics and mechanism of deoxygenatin of (N-salicylidene-2-aminophenolato)-(ethanol) dioxomolybdenum(VI) by thionyl chloride

Abstract The kinetics of deoxygenation of [MoO2(Sap)EtOH] (Sap2− = N-salicylidene-2-aminophenolate dianion) by thionyl chloride have been studied in acetonitrile and a mixture of tetrahydrofuran and acetonitrile (v/v = 9:1) by spectrophotometry. Under the conditions [MoO2(Sap)EtOH] ⪡ [SOCl2], the reaction of formation of the product MoO(Sap)Cl2 consists of two consecutive steps. The pseudo-first order rate constants of the first-step reaction depended on [SOCl2] 1 2 and were significantly affected by the dielectric constant of solvent used, whereas those of the second-step reaction were independent of [SOCl2]. A mechanism is proposed for the deoxygenation, where the reactant reacted initially with MoO2(Sap)EtOH is not SOCl2, but SOCl+ or Cl− ions.

Improved Efficiency of Dye-Sensitized Solar Cell Using Graphene-Coated Al2O3-TiO2 Nanocomposite Photoanode

Dye-sensitized solar cells (DSSCs) were fabricated using graphene-coated Al2O3 (GCA)-TiO2 nanocomposite electrodes. The GCA-TiO2 pastes were prepared by simply blending the GCA particles with TiO2 particles without any complex treatments. The DSSC with 1 wt.% GCA-TiO2 exhibited much better overall energy conversion efficiency, when compared to the DSSCs with TiO2 alone and with carbon nanotube-TiO2. It can be attributed to efficient electron transport through the GCA-TiO2 to the transparent conducting oxide, resulting in significantly enhanced electron lifetime.

Kinetics and mechanism of ligand exchange reaction in tetrakis(acetylacetonato)cerium(IV) in C2D6 and CD3CN

Abstract The kinetics of ligand exchange between tetrakis(acetylacetonato)cerium(IV) [Ce(acac)4] and free acetylacetone (Hacac) in C6D6 and CD3CN have been studied by the 1H NMR line-broadening method. The observed first-order rate constant kobsd depends on the concentration of Hacac in the enol form, [Hacac]enol, as follows: 1/kobsd = q+r/[Hacac]enol in C6D6 and kobsd = s+t [Hacac]enol inCD3CN. Addition of dimethyl sulphoxide (DMSO) in C6D6 retarded the exchange rate such that 1/kobsd = u+v [DMSO]. The exchange rate was not influenced by addition of H2O in C6D6. The deuterium isotope effect on the rate was relatively small. These results were explained by the mechanism, where the exchange reaction proceeds through two parallel rate-determining steps, following the formation of a nine-coordinate adduct complex (Ce(acac)4Hacac. The two steps are the proton transfer from coordinated Hacac to leaving acac and the ring opening of acac in the adduct complex. It is likely that the mechanism proposed in this study is also valid for the exchange reactions of acac in M(acac)4 complexes (M = Hf4+, Th4+ and U4+).