Browsing by Author "Karadas, F."

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Open Access
Catalytic properties of vanadium diselenide: a comprehensive study on ıts electrocatalytic performance in alkaline, neutral, and acidic media
(American Chemical Society, 2017) Ghobadi, T. G. U.; Patil, B.; Karadas, F.; Okyay, Ali Kemal; Yilmaz, E.
Here, we report the synthesis of vanadium diselenide (VSe2) three-dimensional nanoparticles (NPs) and two-dimensional (2D) nanosheets (NSs) utilizing nanosecond pulsed laser ablation technique followed by liquid-phase exfoliation. Furthermore, a systematic study has been conducted on the effect of NP and NS morphologies of VSe2 in their catalytic activities toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline, neutral, and acidic conditions. Research on VSe2 clearly demonstrates that these morphologies do not have a significant difference for ORR and OER; however, a drastic effect of morphology was observed for HER. The ORR activity of both NSs and NPs involves ∼2.85 numbers of electrons with the Tafel slope of 120 mV/dec in alkaline and neutral pH. In alkaline solution, NPs are proved to be an efficient catalyst for OER with an onset potential 1.5 V; however, for HER, NSs have a better onset potential of −0.25 V. Moreover, the obtained NPs have also better catalytic activity with a 400 mV anodic shift in the onset potential compared to NSs. These results provide a reference point for the future application of VSe2 in energy storage and conversion devices and mass production of other 2D materials.
Open Access
Differential charging in SiO2/Si systems as determined by XPS
(American Chemical Society, 2004) Karadas, F.; Ertas, G.; Süzer, Şefik
The Si2p binding and the SiKLL kinetic energy difference between the SiO2 layer and Si substrate is shown to be influence by application of external voltage bias to the sample holder due to the differential charging as was already reported earlier (Ulgut, B.; Suzer, S. J. Phys. Chem. B 2003, 107, 2939). The cause of this bias induced (physical)-shift is now proven to be mostly due to partial neutralization by the stray electrons within the vacuum system by (i) introducing additional stray electrons via a filament and following their influence on the measured binding energy as a function of the applied voltage, (ii) measuring and Auger parameter. It is also shown that citrate-capped gold nanoclusters deposited on the SiO2/Si system experience differential charging similar to that of the oxide layer rather than the silicon substrate.
Open Access
A metal dicyanamide cluster with high CO2/N2 selectivity
(Elsevier B.V., 2016) Tekin, A.; Karalti, O.; Karadas, F.
A new microporous metal dicyanamide cluster, Co(hmt)(dca)2 (hmt: hexamethylenetetramine, dca: dicyanamide), with accessible N-donor sites exhibits high CO2/N2 selectivity, 83 at 295 K and 1 bar, for a mixture with a 15:85 CO2 to N2 ratio. Adsorption studies show that the use of hmt and dca moieties as building blocks for solid adsorbents can enhance the CO2:surface interactions due to N atoms available inside the pores, which is confirmed by X-ray single crystal studies.
Open Access
Metal dicyanamides as efficient and robust water-oxidation catalysts
(Wiley Blackwell, 2017) Nune, S. V. K.; Basaran, A. T.; Ülker, E.; Mishra, R.; Karadas, F.
Non-oxide cobalt-based water-oxidation electrocatalysts have received attention recently for their relative ease of preparation, they are stable both in acidic and basic media, and they have higher turnover frequencies than cobalt oxides. Recent studies show that one of the main bottlenecks in the implementation of non-oxide systems to water splitting is the low number of active metal sites, which is in the order of nmol cm−2. Herein, a new series of non-oxide water-oxidation catalysts has been introduced to the field. Cobalt dicyanamides are observed to have around four times higher surface active sites and better catalytic performances than cyanide-based systems. Long-term catalytic studies (70 h) at an applied potential of 1.2 V and electrochemical studies performed in solutions in pH values of 3.0–12.0 indicate that the compounds are robust and retain their structures even under harsh conditions. Moreover, the addition of Ni impurities to cobalt dicyanamides is a feasible method to improve their catalytic activities.
Open Access
Molecular catalysts for artificial photosynthesis: general discussion
(Royal Society of Chemistry, 2017) Wang, M.; Artero, V.; Hammarström, L.; Martinez, J.; Karlsson, J.; Gust, D.; Summers, P.; Machan, C.; Brueggeller, P.; Windle, C. D.; Kageshima, Y.; Cogdell, R.; Tolod, K. R.; Kibler, A.; Apaydin, D. H.; Fujita, E.; Ehrmaier, J.; Shima, S.; Gibson, E.; Karadas, F.; Harriman, A.; Inoue, H.; Kudo, A.; Takayama, T.; Wasielewski, M.; Cassiola, F.; Yagi, M.; Ishida, H.; Franco, F.; Kang, S. O.; Nocera, D.; Li C.; Fonzo, F. D.; Park, H.; Sun, L.; Setoyama, T.; Kang, Y. S.; Ishitani, O.; Shen, J. R.; Son, H. J.; Masaoka, S.
Open Access
Molten salt assisted self-assembly: synthesis of mesoporous LiCoO2 and LiMn2O4 thin films and investigation of electrocatalytic water oxidation performance of lithium cobaltate
(Wiley-VCH Verlag, 2018) Saat, G.; Balci, F. M.; Alsaç, E. P.; Karadas, F.; Dağ, Ömer
Mesoporous thin films of transition metal lithiates (TML) belong to an important group of materials for the advancement of electrochemical systems. This study demonstrates a simple one pot method to synthesize the first examples of mesoporous LiCoO2 and LiMn2O4 thin films. Molten salt assisted self-assembly can be used to establish an easy route to produce mesoporous TML thin films. The salts (LiNO3 and [Co(H2O)6](NO3)2 or [Mn(H2O)4](NO3)2) and two surfactants (10-lauryl ether and cethyltrimethylammonium bromide (CTAB) or cethyltrimethylammonium nitrate (CTAN)) form stable liquid crystalline mesophases. The charged surfactant is needed for the assembly of the necessary amount of salt in the hydrophilic domains of the mesophase, which produces stable metal lithiate pore-walls upon calcination. The films have a large pore size with a high surface area that can be increased up to 82 m2 g−1. The method described can be adopted to synthesize other metal oxides and metal lithiates. The mesoporous thin films of LiCoO2 show promising performance as water oxidation catalysts under pH 7 and 14 conditions. The electrodes, prepared using CTAN as the cosurfactant, display the lowest overpotentials in the literature among other LiCoO2 systems, as low as 376 mV at 10 mA cm-2 and 282 mV at 1 mA cm-2.
Open Access
A novel synthetic route for the preparation of an amorphous Co/Fe prussian blue coordination compound with high electrocatalytic water oxidation activity
(American Chemical Society, 2016) Aksoy, M.; Nune, S. V. K.; Karadas, F.
Co/Fe Prussian Blue coordination networks have recently been investigated for heterogeneous water oxidation catalysis. Despite their robustness and stability in both acidic and neutral media, the relatively low current density obtained is their main drawback as a result of their low surface concentration. A novel synthetic approach was employed using a pentacyanometalate-based metallopolymer for the preparation of amorphous Co/Fe coordination polymers to overcome this problem. The surface concentration was improved approximately 7-fold, which also resulted in an increase in the catalytic activity. A current density of 1 mA·cm-2 was obtained only at = 510 mV, while the same current density could be obtained at higher overpotentials (>600 mV) with conventional Prussian Blue analogues. IR, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy studies were performed to investigate the stability of electrodes before and after the electrocatalytic process. The results of this study indicate that the rich and diverse chemistry of pentacyanometalates makes them potential candidates for application in heterogeneous water oxidation catalysis.
Open Access
One-dimensional copper (II) coordination polymer as an electrocatalyst for water oxidation
(Wiley-VCH Verlag, 2017) Mishra, R.; Ülker, E.; Karadas, F.
Although cobalt-based heterogeneous catalysts are the central focus in water oxidation research, interest in copper-based water oxidation catalysts has been growing thanks the great abundance of copper and its biological relevance. Several copper oxides have recently been reported to be active catalysts for water oxidation. In this study, a heterogeneous copper-based water oxidation catalyst that is not an oxide has been reported for the first time. Single-crystal XRD studies indicate that the compound is a one-dimensional coordination compound incorporating copper paddle-wheel units connected through phosphine dioxide ligands. The catalyst exhibits an onset potential of 372 mV at pH 10.2, whereas an overpotential of only 563 mV is required to produce a current density of 1 mA cm−2. In addition to cyclic voltammetric and chronoamperometric studies, an investigation into the effect of pH on the catalytic activity and the robustness of the catalyst using long-term bulk electrolysis (12 h) is presented.
Open Access
X-ray-induced production of gold nanoparticles on a SiO2/Si system and in a poly(methyl methacrylate) matrix
(American Chemical Society, 2005) Karadas, F.; Ertas, G.; Ozkaraoglu, E.; Süzer, Şefik
Prolonged exposure to X-rays of HAuCl4 deposited from an aqueous solution onto a SiO2/Si substrate or into a poly(methyl methacrylate) (PMMA) matrix induces reduction of the Au3+ ions to Au0 and subsequent nucleation to gold nanoclusters as recorded by X-ray photoelectron spectroscopy. The corresponding major oxidation product is determined as chlorine {HAuCl4(ads) + X-rays -Au(ads) + (3/2)Cl 2(ada) + HCl(ads)}, which is initially adsorbed onto the surface but eventually diffuses out of the system into the vacuum. The reduced gold atoms aggregate (three-dimensionally) into gold nanoclusters as evidenced by the variation in the binding energy during X-ray exposure, which starts as 1.3 eV but approaches a value that is 0.5 eV higher than that of the bulk gold. The disappearance of the oxidation product (Cl2p signal) and the growth of the nanoclusters (related to the measured binding energy difference between the Si2p of the oxide and Au4f of the reduced gold) exhibit first-order kinetics which is approximately 3 times slower than the reduction of Au3+, indicating that both of the former processes are diffusion controlled. Similarly, gold ions incorporated into PMMA can also be reduced and aggregated to gold nanoclusters using 254 nm deep UV irradiation in air evidenced by UV - vis - NIR absorption spectrocopy.