Ugur Unal

CdTe quantum dot sensitized hexaniobate nanoscrolls and their photoelectrochemical properties

We have constructed hybrid quantum dot-layered niobate films with an electrophoretic deposition method. The structure and photoelectrochemical behaviour were demonstrated.

Sonochemical synthesis and electrochemical characterization of α-nickel hydroxide: precursor effects

Sonochemical degradation of urea was employed to synthesize alpha-nickel hydroxide from different nickel salts. Utilization of ultrasound yielded products with properties significantly different than the products obtained by thermal degradation of urea. The effect of intercalating chloride, nitrate, acetate, and sulfate anions on morphology and electrochemical performance was studied. The sulfate-intercalated sample had the smallest interlayer spacing when obtained by the sonochemical method, contradicting all the previous thermal synthesis results. The specific capacitance trend also differed from the literature values, and the value for the sulfate-intercalated sample was larger than that of acetate- and nitrate-intercalated samples. Ultrasonic synthesis increased the specific capacitance of the sulfate-intercalated sample significantly. This sample was also the most reversible and had the highest charge efficiency.

Single step synthesis of (α-Fe2O3) hematite films by hydrothermal electrochemical deposition

A single step electrodeposition of α-Fe2O3 films under hydrothermal conditions without post-annealing requirement is described. Primary attention is paid to understand the effects of synthesis conditions, such as temperature, precursor concentration, pH, and time on the structure and morphology of the films. Moreover, the photoelectrochemical properties of hematite films grown by hydrothermal-electrochemical deposition (HED) are also discussed. It is discovered that HED enables the production of crystalline α-Fe2O3 phase without thermal annealing as opposed to electrodepositions reported at ambient temperature. Photoelectrochemical studies demonstrated that better performance can be obtained with the films prepared at higher pH. A net photocurrent density of 23.6 μA cm−2 is obtained in 0.1 M NaOH under 1 sun conditions at 1.23 V vs. NHE with the film prepared from a bath containing 0.05 M FeCl2 and 0.2 M NaCH3COO via hydrothermal-electrodeposition with a photocurrent onset potential of 0.96 V vs. NHE.

Synthesis and Photoluminescence Properties of Niobate Layered Oxides Intercalated with Rare Earth Ions by Electrostatic Self-Assembly Methods

The niobate layered oxides intercalated with Tb3+ or Eu3+ ions were prepared by the electrostatic self-assembly deposition method. The Tb3+ ions in the interlayer exhibited green luminescence by energy transfer from the host NbO6 layer to the guest Tb3+ ions, which was based on the niobate nanosheet band gap excitation. The Tb3+ emission intensity decreased with decreasing interlayer water molecules, indicating that the presence of interlayer water molecules is inevitable for the Tb3+ emission assigned to the energy transfer. On the other hand, the emission intensity of Eu3+ in the interlayer was weaker than that of Tb3+.

Discovery of Superior Cu-GaOx-HoOy Catalysts for the Reduction of Carbon Dioxide to Methanol at Atmospheric Pressure

Catalytic conversion of carbon dioxide to liquid fuels and basic chemicals by using solar‐derived hydrogen at, or near, ambient pressure is a highly desirable goal in heterogeneous catalysis. If realized, this technology could lead to a more sustainable society together with decentralized power generation. A novel class of holmium‐containing multi‐metal oxide Cu catalysts, discovered through the application of high‐throughput methods, is reported. In particular, ternary systems of Cu‐GaOx‐HoOy>Cu‐CeOx‐HoOy∼Cu‐LaOx‐HoOy supported on γ‐Al2O3 exhibited superior methanol production (10×) with less CO formation than previously reported catalysts at 1 atm pressure. Holmium was shown to be highly dispersed as few‐atom clusters, suggesting that the formation of trimetallic sites could be the key for the promotion of methanol synthesis by Ho.

Core–Shell Type Ionic Liquid/Metal Organic Framework Composite: An Exceptionally High CO2/CH4 Selectivity

Here, we present a new concept of a core-shell type ionic liquid/metal organic framework (IL/MOF) composite. A hydrophilic IL, 1-(2-hydroxyethyl)-3-methylimidazolium dicyanamide, [HEMIM][DCA], was deposited on a hydrophobic zeolitic imidazolate framework, ZIF-8. The composite exhibited approximately 5.7 times higher CO2 uptake and 45 times higher CO2/CH4 selectivity at 1 mbar and 25 °C compared to the parent MOF. Characterization showed that IL molecules deposited on the external surface of the MOF, forming a core (MOF)-shell (IL) type material, in which IL acts as a smart gate for the guest molecules.

Effect of reaction solvent on hydroxyapatite synthesis in sol–gel process

Synthesis of hydroxyapatite (HA) through sol–gel process in different solvent systems is reported. Calcium nitrate tetrahydrate (CNTH) and diammonium hydrogen phosphate (DAHP) were used as calcium and phosphorus precursors, respectively. Three different synthesis reactions were carried out by changing the solvent media, while keeping all other process parameters constant. A measure of 0.5 M aqueous DAHP solution was used in all reactions while CNTH was dissolved in distilled water, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) at a concentration of 0.5 M. Ammonia solution (28–30%) was used to maintain the pH of the reaction mixtures in the 10–12 range. All reactions were carried out at 40 ± 2°C for 4 h. Upon completion of the reactions, products were filtered, washed and calcined at 500°C for 2 h. It was clearly demonstrated through various techniques that the dielectric constant and polarity of the solvent mixture strongly influence the chemical structure and morphological properties of calcium phosphate synthesized. Water-based reaction medium, with highest dielectric constant, mainly produced β-calcium pyrophosphate (β-CPF) with a minor amount of HA. DMF/water system yielded HA as the major phase with a very minor amount of β-CPF. THF/water solvent system with the lowest dielectric constant resulted in the formation of pure HA.

Morphology and crystal structure control of α-Fe2O3 films by hydrothermal-electrochemical deposition in the presence of Ce3+ and/or acetate, F− ions

Hydrothermal-electrochemical growth of Hematite (α-Fe2O3) thin films in the presence of Ce3+ and/or CH3COO− and F− ions is reported. Primary attention is paid to understanding the synergistic effect of temperature and additive ions on the growth of Hematite particles. The literature describes the shape-controlled electrodeposition of iron oxide films, but these reports involve low-temperature depositions (<100 °C) that require a post-annealing step to obtain the Hematite phase. We studied size and shape control of Hematite crystals by introducing capping agents during hydrothermal-electrochemical deposition (HED) at 130 °C under autonomous pressure. When HED is used together with growth-modifying species such as CH3COO− and/or Ce3+, Hematite thin films with various structures including spherical, rhombohedral and platelet-like particles were obtained which were crystallized directly in the α-Fe2O3 phase without thermal annealing. In contrast, either Lepidocrocite or an amorphous phase forms with low-temperature electrodeposition under otherwise identical conditions. Addition of F− into the deposition solution induces formation of bundles that are composed of square planar Akaganeite particles even during HED. The aspect ratio of the Akaganeite particles is controlled with co-addition of Ce3+ ions. This variety of nanostructures gives us the opportunity to evaluate the photoelectrochemical performance of the α-Fe2O3 films based on particle size and shape. It was demonstrated that porous, square planar α-Fe2O3 with a high aspect ratio shows the highest performance.

Thermally-induced phase changes in electrophoretically deposited titanate and niobate layered oxides

Thin films of Cs x Ti ( 2 - x / 4 ) □ x / 4 O 4 (CsTiO), K 4 Nb 6 O 1 7 (KNbO), and their proton-exchanged forms, i.e., H x Ti ( 2 - x / 4 ) □ x / 4 O 4 (HTiO) and H 4 Nb 6 O 1 7 (HNbO), were prepared using the electrophoretic deposition technique. The amine- and thiol-intercalated HTiO and HNbO films were prepared by exfoliation of powders in aqueous ethylamine and (mercaptoethyl)amine hydrochloride solutions, respectively. The heat-induced phase transformation of these films was investigated. Evidently, the CsTiO and thiol-intercalated HTiO films underwent phase transformation at relatively high temperatures due to the cations within the interlayer. CsTiO and HTiO films lost their layered structure and transformed, in turn, into the anatase and rutile phases with increasing temperature. However, the intercalated samples exhibited unidentified phases at in-between temperatures, eventually transforming to TiO 2 . The KNbO film transformed into a layered KNb 3 O 8 structure, while the HNbO lost its layered structure completely to form Nb 2 O 5 . Thus, the phase change depended on the modification of the interlayers, and the heat treatment resulted in thin films with new crystal structures for the amine- and thiol-intercalated samples.

A New Approach for the Preparation, Exfoliation, and Intercalation Properties of Niobium Layered Sulfide Material

We have developed the new process that can synthesize a layered niobium sulfide under heating a layered oxide with H2S/N2 gas mixture. It was confirmed that K0.34(H2O)xNbS2 was obtained as a single phase and high crystallinity under heating K4Nb6O17 · 3H2O with the gas at 750°C for 10 h and washing with distilled water. The layered sulfide had the large plate‐like shapes that have 50 µm wide length on average. Potassium ions in the interlayer of K0.34(H2O)yNbS2 could be exchanged to hydronium ions by stirring 2M H2SO4. It was also confirmed that tetrabutylammonium could intercalate in interlayer of proton exchanged form. In addition, colloidal solution was obtained after ultrasonication of proton exchanged form. According to the AFM image, the thickness of sulfide nanosheets had 4.2 Å, which roughly corresponded with the thickness of NbS2 single layer.