Browsing by Author "Ülker, E."

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Open Access
Cobalt borophosphate on nickel foam as an electrocatalyst for water splitting
(Elsevier BV, 2022-06-10) Ülker, E.; Akbari, Sina Sadigh; Karadas, Ferdi
One of the most critical steps in the transition to carbon-free energy systems is sustainable hydrogen evolution from water. In this research, a cobalt borophosphate crystalline compound consisting of phosphate and borate anions was synthesized with a solid-state reaction. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray Photoelectron (XPS) was employed to investigate the structure, composition, and morphology of Co3BPO7. Electrocatalytic performances of the catalyst towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) have been investigated on nickel foam (NF) electrode in 1.0 M KOH (pH 13.6) by linear sweep voltammetry, chronopotentiometry, cyclic voltammetry, and electrochemical impedance spectroscopy. For OER, the catalyst exhibits an overpotential of 230 mV at 10 mA cm−2 with a Tafel slope of 130 mV dec−1, which is comparable to that of the benchmark RuO2 electrocatalyst, and 220 mV overpotential for a current density of 10 mA cm−2 with a Tafel slope of 147 mV dec−1 for HER process. Long-term chronoamperometry and multiple cyclic voltammetric experiments indicate the catalyst is stable throughout both HER and OER processes. Electrochemical experiments and characterization studies performed on the pristine and post-catalytic electrode indicate that the catalyst is robust under alkaline electrocatalytic conditions (pH 13.6).
Open Access
A cyanide-based coordination polymer for hydrogen evolution electrocatalysis
(Springer New York LLC, 2018) Alsaç, Elif Pınar; Ülker, E.; Nune, Satya Vijaya Kumar; Karadaş, Ferdi
Abstract: Research on H2 production has recently been directed to the development of cost-efficient and robust heterogeneous catalysts for hydrogen evolution reaction (HER). Given the promising catalytic activities of several cobalt-based systems and the robustness of Prussian blue analogues in harsh catalytic processes including water oxidation, a Co-Co Prussian blue analogue was investigated as a HER catalyst for the first time. Co-Co Prussian Blue modified fluorine doped tin oxide (FTO) electrode demonstrated a significant HER activity with an onset overpotential of 257 mV, a Tafel slope of 80 mV dec−1, and a turnover frequency of 0.090 s−1 at an overpotential of 250 mV. Comparative XPS, Infrared, and XRD studies performed on pristine and post-catalytic electrodes confirm the stability of the catalyst.
Open Access
Effect of cobalt doping on photocatalytic water splitting activity of NiTi-layered double hydroxide
(Royal Society of Chemistry, 2022-03-17) Samuei, Sara; Akbari, Sina Sadigh; Ülker, E.; Karadaş, Ferdi
Metal doping has been used as an effective strategy to tune the energy levels of semiconductors. Herein, we dope NiTi layered double hydroxide (NiTi-LDH) with cobalt to prepare a ternary LDH, CoNiTi-LDH, to enhance its photocatalytic performance towards both water oxidation and hydrogen evolution. A CoNiTi-LDH with smaller plate sizes and a higher degree of order is obtained, which allows the band gap to shrink from 2.7 eV to 2.4 eV. CoNiTi-LDH exhibits a photocatalytic water oxidation activity of 366 μmol g−1 h−1, which is more than two times higher than NiTi-LDH (166 μmol g−1 h−1). We observed that appropriate energy levels of CoNiTi-LDH allow it to be an efficient photocatalyst also for hydrogen evolution. We performed detailed characterization studies to elucidate the effect of Co-doping on photocatalytic activity.
Open Access
Enhancing oxygen evolution catalytic performance of nickel borate with cobalt dopingand carbon nanotubes
(Wiley, 2023-02-16) Enez, S.; Karani Konuksever, V.; Samuei, S.; Karadaş, Ferdi; Ülker, E.
Open Access
Experimental evaluation of impact ionization coefficients in Al xGa1-xN based avalanche photodiodes
(AIP Publishing LLC, 2006) Tut, T.; Gökkavas, M.; Bütün, B.; Bütün, S.; Ülker, E.; Özbay, Ekmel
The authors report on the metal-organic chemical vapor deposition growth, fabrication, and characterization of high performance solar-blind avalanche photodetectors and the experimental evaluation of the impact ionization coefficients that are obtained from the photomultiplication data. A Schottky barrier, suitable for back and front illuminations, is used to determine the impact ionization coefficients of electrons and holes in an AlGaN based avalanche photodiode. © 2006 American Institute of Physics.
Open Access
Fabrication of 15- $\mu$ m pitch $640{\rm ×}512$ InAs/GaSb type-II superlattice focal plane arrays
(Institute of Electrical and Electronics Engineers Inc., 2019) Oğuz, Fikri; Arslan, Y.; Ülker, E.; Bek, A.; Özbay, Ekmel
We present the fabrication of large format 640 × 512, 15-μm pitch, mid-wave infrared region (MWIR) InAs/GaSb type-II superlattice (T2SL) focal plane array (FPA). In this report, the details of device design and fabrication processes are withheld adhering to the common practice of most of the manufactures and developers because of the strategic importance; however, information about fabrication processes of T2SLs FPA is presented to a certain extent. Comparison of etching techniques, passivation materials and methods, and substrate thinning (mechanical and chemical) is given besides of details regarding the standard ohmic contact and indium (In) bump formations. Morphological investigations of fabrication step are included. Large area pixels, 220 μm × 220 μm, fabricated by different etching methods and passivation materials/methods are compared in terms of dark current levels. Wet passivation with (NH 4 ) 2 S is discussed in terms of morphological investigations, and dark current results are compared with untreated samples. Large area pixel level characterizations as well as image level benchmarking of mechanical and chemical substrate thinning are reported. Effect of GaSb substrate on device performance and the way of reducing stress of In bumps are revealed. The importance of complete substrate removal is demonstrated through FPA images.
Open Access
High performance 15-μm pitch 640 × 512 MWIR InAs/GaSb type-II superlattice sensors
(IEEE, 2021-11-18) Oğuz, Fikri; Ülker, E.; Arslan, Y.; Nuzumlali, Ö. L.; Bek, Alpan; Özbay, Ekmel
We report the high performance of Mid-wave Infrared Region (MWIR) InAs/GaSb Type-II Superlattice (T2SL) sensors with $640\times512$ format and 15- $\mu \text{m}$ pixel pitch at both Focal Plane Array (FPA) and pixel level. The p-intrinsic-Barrier-n epilayer structure is adopted for this study, which is grown on 620 ± $30~ \mu \text{m}$ thick GaSb substrate and highly-doped GaSb cap layer at the top structure. The mesa type pixels with sizes of $220\,\,\mu \text{m}\,\,\times 220\,\,\mu \text{m}$ have dark currents $7.8\times10$ −12 A at 77 K both of which are equivalent to state-of-the-art values for Type-II Superlattice sensors. The various passivation techniques to lower the dark current are applied and the results are given in terms of dark current. Electro-optical measurements yielded comparable results to literature. After gathering data and optimizing the fabrication conditions, the FPA of 15- $\mu \text{m}$ pitch having $4.92~ \mu \text{m}$ cut-off wavelength ( $\lambda _{\mathrm {c}}$ ) shows 1.6 A/W peak responsivity, Noise Equivalent Temperature Difference (NETD) of 22.6 mK with optics of f/2.3, quantum efficiency larger than 65% and 99.75% operability. The acquired images by using aforementioned FPA device is presented in this paper. With the reduction of dark current, an encouraging imaging performance is obtained which shows the potential of the Type-II Superlattice detectors in 3 rd generation infrared sensors.
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
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.
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Preparation and capacitance properties of graphene quantum Dot/NiFe−layered double‐hydroxide nanocomposite
(Wiley-VCH Verlag, 2021-01) Samuei, S.; Rezvani, Z.; Shomali, A.; Ülker, E.; Karadaş, Ferdi
A new composite from graphene quantum dots (GQDs) and NiFe layered double hydroxide was successfully prepared by the coprecipitation method under optimal conditions. The nanoparticles of the composite were analyzed by X‐ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA) to obtain the structure, composition and morphology information. Also, the electrochemical properties were investigated by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The nanocomposite displays a specific capacitance of 712.7 F g−1 and excellent cycle life after 2500 cycles by applying 10 A g−1 of the current density in 1 M KOH electrolyte, which confirms that the nanocomposite has superb capacitance retention (∼94.8 %) and can be used as a capable supercapacitor. Furthermore, this study provides a desirable procedure for the preparation of novel nanocomposites based on graphene quantum dots, which can be used in energy storage/conversion devices.
Open Access
Tuning the electronic properties of prussian blue analogues for efficient water oxidation electrocatalysis: experimental and computational studies
(Wiley-VCH Verlag, 2018) Alsaç, Elif Pınar; Ülker, E.; Nune, Satya Vijaya Kumar; Dede, Y.; Karadaş, Ferdi
Although several Prussian Blue analogues (PBAs) have been investigated as water oxidation catalysts, the field lacks a comprehensive study that focuses on the design of the ideal PBA for this purpose. Here, members of a series of PBAs with different cyanide precursors have been investigated to study the effect of hexacyanometal groups on their electrocatalytic water oxidation activities. Cyclic voltammetric, chronoamperometric, and chronopotentiometric measurements have revealed a close relationship between the electron density of electroactive cobalt sites and electrocatalytic activity, which has also been confirmed by infrared and XPS studies. Furthermore, pH-dependent cyclic voltammetry and computational studies have been performed to gain insight into the catalytic mechanism and electronic structure of cyanide-based systems to identify possible intermediates and to assign the rate-determining step of the target process.
Open Access
Water oxidation electrocatalysis with a cobalt ‐ borate ‐ based hybrid system under neutral conditions
(Wiley-VCH Verlag, 2018) Turhan, Emine A.; Nune, Satya Vijaya Kumar; Ülker, E.; Şahin, U.; Dede, Y.; Karadaş, Ferdi
The development of new water oxidation electrocatalysts that are both stable and efficient, particularly in neutral conditions, holds great promise for overall water splitting. In this study, the electrocatalytic water oxidation performance of a new cobalt-based catalyst, Co3(BO3)2, with a Kotoite-type crystal structure is investigated under neutral conditions. The catalyst is also hybridized with CNTs to enhance its electrocatalytic properties. A remarkable increase in catalytic current along with a significant shift in the onset overpotential is observed in Co3(BO3)2@CNT. Additionally, CNT addition also greatly influences the surface concentration of the catalyst: 12.7 nmol cm−2 for Co3(BO3)2@CNT compared with 3.9 nmol cm−2 for Co3(BO3)2. Co3(BO3)2@CNT demands overpotentials of 303 and 487 mV to attain current densities of 1 and 10 mA cm−2, respectively, at pH 7. Electrochemical and characterization studies performed over varying pH conditions reveal that the catalyst retains its stability over a pH range of 3-14. Multi-reference quantum chemical calculations are performed to study the nature of the active cobalt sites and the effect of boron atoms on the activity of the cobalt ions.