Researcher:
Kahraman, Abdullah

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PhD Student

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Abdullah

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Kahraman

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Kahraman, Abdullah

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2019 - 201922020 - 20238

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    Pyridinic nitrogen induced compressed bilayer graphene for oxygen reduction reaction
    (Elsevier Sci Ltd, 2023) Cankaya, Mehmet; Titus, Charles James; Lee, Sang Jun; Nordlund, Dennis; Ogasawara, Hirohito; Tekin, Adem; Department of Computer Engineering;Department of Chemistry; Solati, Navid; Kahraman, Abdullah; Şimşek, Kaan; Kaya, Sarp; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Engineering; College of Sciences
    Despite the emergence of nitrogen-doped graphene as a noble-metal free electrocatalyst for oxygen reduction reaction, its participation in the electrochemical conversion mechanism is not well -established. In the present study, functionalities of the nitrogen species on the oxygen reduction activ-ity of bilayer graphene were investigated by combining atom-specific X-ray spectroscopy, Raman spec-troscopy, and density functional theory calculations with electrochemical activity tests in alkaline media. Among various nitrogen species, pyridinic nitrogen as the dominant species improved the electro-chemical activity of bilayer graphene, which was followed by graphene bilayers doped with graphitic nitrogen in majority. Polarization curves revealed a significantly high electrocatalytic oxygen reduction activity of the nitrogen-doped bilayer graphene where the pyridinic nitrogen was the major dopant. This improved activity was confirmed by the lowest overpotential and Tafel slope (78.9 mV/dec). The enhanced interaction of graphene bilayers doped with pyridinic nitrogen is shown to be the main reason for this improvement.(c) 2023 Elsevier Ltd. All rights reserved.
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    Accelerating water oxidation on BiVO4 photoanodes via surface modification with Co dopants
    (Royal Soc Chemistry, 2023) Osterbacka, Nicklas; Erdem, Emre; Wiktor, Julia; Department of Physics;Department of Chemistry; Barzgarvishlaghi, Mahsa; Kahraman, Abdullah; Usman, Emre; Sennaroğlu, Alphan; Kaya, Sarp; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences
    Despite the vast investigations on improving the photoelectrochemical performance of BiVO4 for water splitting, charge recombination in the near-surface region remains a challenge. In this study, we showed that the diffusion of Co2+ ions into the BiVO4 subsurface boosted the water oxidation activity and charge injection efficiency remarkably. The increase in the concentration of oxygen vacancies upon the incorporation of cobalt ions was shown by electron paramagnetic resonance (EPR) spectroscopy and confirmed by density functional theory (DFT) calculations. DFT calculations revealed that vanadium sites in the subsurface region were the most favorable sites for substitution with cobalt ions. Charge localization at surface oxygen vacancies was found less favorable in the presence of cobalt in the subsurface layer, eliminating surface recombination. This resulted in 4.25 times larger charge injection efficiency and 6.2 times higher photocurrent density at the potential of & SIM;0.6 V, as compared to pristine BiVO4. This enhancement was significantly larger as compared to CoOx-loaded BiVO4, indicating that the suppressed recombination at the surface and improved charge transfer kinetics obtained solely by CoOx deposition are not sufficient for enhanced activity of BiVO4. A longer charge carrier lifetime obtained upon cobalt incorporation was observed by transient absorption spectroscopy and verified the reduced rate of recombination.
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    Easy hydrogenation and dehydrogenation of a hybrid graphene and hexagonal boron nitride monolayer on platinum
    (Institute of Physics (IOP) Publishing, 2021) Pis, Igor; Nappini, Silvia; Magnano, Elena; Bondino, Federica; N/A; Department of Chemistry; N/A; Department of Chemistry; Panahi, Mohammad; Kaya, Sarp; Kahraman, Abdullah; PhD Student; Faculty Member; PhD Student; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Sciences; Graduate School of Sciences and Engineering; N/A; 116541; N/A
    Understanding the fundamental steps of adsorption and controlled release of hydrogen in two-dimensional (2D) materials is of relevance for applications in nanoelectronics requiring tuning the physical properties or functionalization of the material, hydrogen storage and environmental sensors. Most applications demand that hydrogen adsorption and desorption can be controlled at room temperature. Here we report an element-specific study on the hydrogenation and dehydrogenation, in a low coverage regime, of a quasi-free standing 2D heterostructure (h-BNG) in the form of coexisting lateral domains of isostructural hexagonal boron nitride (h-BN) and graphene (Gr) on Pt(111). At very low hydrogen coverage a selective and partial hydrogenation of the Gr domains is observed in h-BNG. At the same time no changes are detected in the h-BN domains, indicating a preferential hydrogenation of Gr rather than h-BN domains. At higher coverage, hydrogenation of both Gr and h-BN domains is detected. A thermally facile hydrogen release from h-BN domains near room temperature is observed. Furthermore, the hybrid h-BNG 2D heterostructure enables also a much easier H-2 thermal release from Gr domains when compared with a full Gr monolayer grown on the same Pt(111) substrate. These results suggest that the presence of coexisting hydrogenated h-BN domains could destabilize C-H bonds in Gr.
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    A comprehensive study on the characteristic spectroscopic features of nitrogen doped graphene
    (Elsevier, 2019) Ogasawara, Hirohito; N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Solati, Navid; Mobassem, Sonia; Kahraman, Abdullah; Kaya, Sarp; PhD Student; PhD Student; PhD Student; Faculty Member; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 116541
    Despite significant methodical improvements in the synthesis of N-doped graphene, there are still unsolved questions regarding the control of content and the configuration of nitrogen species in graphene honeycomb network. A cross-examination of X-ray photoelectron spectroscopy and Raman spectroscopy findings indicates that the nitrogen dopant amount is graphene thicknesses dependent, but the various nitrogen dopant coordination can be obtained on both double- and few-layer graphene. Characteristic defect features (D') appearing in Raman spectra upon N-doping is sensitive to nitrogen dopant coordination, graphitic-pyridinic/nitrilic species and therefore the doping level can be identified. Pyridinic and nitrilic nitrogen as primary species turn graphene to p-type semiconductor after a mild thermal treatment.
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    Modifying the electron-trapping process at the BiVO4 surface states via the TiO2 overlayer for enhanced water oxidation
    (Amer Chemical Soc, 2021) N/A; N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Usman, Emre; Barzgarvishlaghi, Mahsa; Kahraman, Abdullah; Solati, Navid; Kaya, Sarp; Master Student; PhD Student; PhD Student; Researcher; Faculty Member; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; N/A; 116541
    BiVO4 is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition. With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 VRHE increased from 0.64 to 1.1 mA-cm(-2) under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO2 overlayer can be used to fine tune the surface properties of BiVO4 and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.
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    The fast-track water oxidation channel on BiVO4 opened by nitrogen treatment
    (Amer Chemical Soc, 2020) N/A; N/A; N/A; N/A; Department of Physics; Department of Chemistry; Department of Physics; Department of Chemistry; Kahraman, Abdullah; Barzgarvishlaghi, Mahsa; Toker, Işınsu Baylam; Sennaroğlu, Alphan; Kaya, Sarp; PhD Student; PhD Student; PhD Student; Faculty Member; Faculty Member; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; N/A; 23851; 116541
    BiVO4 is one of the most promising photoanode materials for water-splitting systems. Nitrogen incorporation into a BiVO4 surface overcomes the known bottleneck in its charge-transfer kinetics into the electrolyte. We explored the role of nitrogen in the surface charge recombination and charge-transfer kinetics by employing transient photocurrent spectroscopy at the time scale of surface recombination and water oxidation kinetics, transient absorption spectroscopy, and X-ray photoelectron spectroscopy. We attributed the activity enhancement mechanism to the accelerated V5+/V4+ redox process, in which incorporated nitrogen suppresses a limiting surface recombination channel by increasing the oxygen vacancies.
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    Increasing charge separation property and water oxidation activity of BiVO 4 photoanodes via a postsynthetic treatment
    (Amer Chemical Soc, 2020) N/A; N/A; Department of Chemistry; Department of Chemistry; Barzgarvishlaghi, Mahsa; Kahraman, Abdullah; Kaya, Sarp; PhD Student; PhD Student; Faculty Member; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); N/A; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 116541
    Postsynthetic treatments of BiVO4 photoanodes have recently shed light on better understanding and improving the photoanodes for water splitting. We demonstrate that a mild heat treatment of BiVO4 under O-2 flow at 200 degrees C improves its water oxidation activity. Charge separation and charge injection efficiencies increase along with the decreased charge transfer resistances across the BiVO4/electrolyte interface. The depletion region width and the band bending have shown to increase after annealing, while charge carrier density remains unchanged. Transient photocurrent measurements further confirm the reduced charge carrier recombination as a result of enlarged band bending. The surface states decrease and the fraction of vanadium ions increases in the surface region after the heat treatment. Since the surface of the BiVO4 photoanodes is generally vanadium deficient, it is suggested that such a treatment can improve the BiVO4 photoanodes performance prepared by other methods as well.
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    Roles of charge carriers in the excited state dynamics of BiVO4 photoanodes
    (Amer Chemical Soc, 2019) N/A; N/A; N/A; N/A; Department of Physics; Department of Chemistry; Department of Physics; Department of Chemistry; Kahraman, Abdullah; Barzgarvishlaghi, Mahsa; Toker, Işınsu Baylam; Sennaroğlu, Alphan; Kaya, Sarp; PhD Student; PhD Student; PhD Student; Faculty Member; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; N/A; 23851; 116541
    Photogenerated charge carrier dynamics of BiVO4 have been investigated by ultrafast transient absorption spectroscopy (TAS), and a numerical modeling has been applied to reveal the origins of the dynamical behavior. The numerical model, based on rate equations, presents the possibility of both photogenerated hole and electron absorption dynamics below 500 nm, as opposed to the generally suggested photogenerated hole absorption mechanism. The investigations done in the ultrafast time regime show that the positive transient absorption peak at 470 nm exhibits inverse behavior as compared to the broad-band feature represented at 550 nm under anodic bias, in the presence of a hole scavenger and at increasing excitation pump power. A combination of TAS findings under various conditions with the numerical modeling reveals that both electron and hole absorption are possible in the spectral region above 500 nm whereas electron absorption at the excited state is the dominant process at shorter wavelengths. Moreover, the major changes in transient absorption response take place in the ultrafast time scale, and overall recombination dynamics is a reflection of the ultrafast recombination mechanism.
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    Charge transfer controlled hydrogenation of graphene on an electronically modified Pt(111) surface
    (Pergamon-Elsevier Science Ltd, 2020) Balkan, Timucin; Pis, Igor; Bondino, Federica; N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Panahi, Mohammad; Solati, Navid; Kahraman, Abdullah; Kaya, Sarp; PhD Student; PhD Student; PhD Student; Faculty Member; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 116541
    The interfaces of graphene and hydrogenated graphene with 3d atom embedded Pt(111) [Pt-3d-Pt(111), 3d: Fe, Co] substrates have comparatively been investigated utilizing X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and temperature-programmed XPS (TPXPS) and desorption (TPD). 3d atoms in the subsurface layer of Pt(111) change the electronic properties of graphene via modifying its p-doping level. Hydrogenation makes graphene pinned to the Pt(111) and Pt-3d-Pt(111) substrates and induces surface segregation of 3d atoms from the subsurface layer into the interface of HGr and Pt-3d-Pt(111). Such a mechanism changes the desorption energetics of hydrogen significantly.
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    The significance of the local structure of cobalt-based catalysts on the photoelectrochemical water oxidation activity of BiVO4
    (Pergamon-Elsevier Science Ltd, 2021) Harfouche, Messaoud; Ogasawara, Hirohito; N/A; N/A; N/A; N/A; N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Barzgarvishlaghi, Mahsa; Kahraman, Abdullah; Apaydın, Sinem; Usman, Emre; Aksoy, Dilan; Balkan, Timuçin; Munir, Shamsa; Kaya, Sarp; PhD Student; PhD Student; Master Student; Master Student; PhD Student; Other; Researcher; Faculty Member; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; N/A; College of Sciences; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); N/A; N/A; N/A; N/A; N/A; N/A; N/A; 116541
    The local structures of the water oxidation catalysts play an important role in reaction kinetics and the performance of the photoanodes. In this study, we deposited cobalt-based catalysts on nanoporous BiVO4 with controlled thicknesses by atomic layer deposition (ALD). Despite the similar oxidation states of cobalt in all depositions, different water oxidation activities in neutral pH conditions were observed. A dramatic photocurrent raise, lowered kinetic overpotential, and smaller charge transfer resistance across the photoanode/electrolyte interface were achieved when a uniform ultrathin Co(OH)(2) layer was formed on BiVO4. Photocurrent density for water oxidation showed a 95% enhancement at 0.6 V vs. RHE when the catalyst was in the form of Co(OH)(2), while an 80% increase was obtained for CoO. Ideal coordination of Co(OH)(2) on hydroxylated BiVO4 surface assists the charge transfer between the electrolyte and BiVO4 without increasing surface recombination. The results of this study emphasize the importance of controlling the local structure of the catalysts in the performance of the water splitting photoanodes.