Browsing by Author "Ghobadi, T. G. U."

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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
Controlling luminescent silicon nanoparticle emission produced by nanosecond pulsed laser ablation: role of interface defect states and crystallinity phase
(Royal Society of Chemistry, 2016) Ghobadi, T. G. U.; Ghobadi, A.; Okyay, T.; Topalli K.; Okyay, Ali Kemal
In this study, we provide a systematic study on the origin of green and blue emission from luminescent silicon nanoparticles (Si-NPs) synthesized in water using a nanosecond pulsed laser ablation methodology. Here we report a direct one-step process to make ultra-small Si-NPs (∼3 nm mean size) by utilizing spiral beam scanning. In each ablation cycle, this scanning scheme collects generated nanoparticles towards the center and ablates them in subsequent cycles. Therefore, the resultant Si-NPs can reach very small sizes in a short time with high uniformity in their size distribution. Further, we investigate the effect of laser fluence on the emission properties of the obtained nanoparticles. For this aim, two different values of 60 mJ cm−2 and 30 mJ cm−2 laser fluences are employed to achieve green and blue emitting Si-NPs, respectively. Our results show that the emission spectrum for both blue and green Si-NPs has two main peaks at 426 nm and 520 nm, but their relative intensity ratios are different for these two cases. The findings of this study suggest that the blue emission originates from oxide related surface defects at the Si/SiOx interface, while the green emission stems from grain boundaries existing at the NP surface. We found that these two types of disorders can be controlled by tuning the laser power. © The Royal Society of Chemistry.
Open Access
Examination of gas and solid products during the preparation of activated carbon using phosphoric acid
(Academic Press, 2018) Yagmur, E.; Inal, I. I. G.; Gokce, Y.; Ghobadi, T. G. U.; Aktar, T.; Aktas, Z.
Activating agents play significant roles in the preparation of activated carbon (AC) from biomasses and their wastes, which are widely used in AC production. Application methods are also important for the production process. Products give remarkable ideas regarding the method and heat treatment process. The activated carbon was produced from waste tea in accordance with either the conventional method or microwave energy pretreated method using phosphoric acid (H3PO4) as activating agent. The yields of the activated carbons were 51.8% for conventional method and 46.0% for microwave pretreated method. The acid suppressed the formation of tar and promoted the amount of solid and aromatic structure accordance to sp2 hybridisation. Additionally, the waste tea was directly carbonised (without H3PO4) and the yield was 36.3%. Major gas (H2, CH4, C2H6, C2H4, CO2 and CO) products obtained during heat treatment process in a conventional furnace were examined in terms of quantity and quality. The solid products were characterised in terms of surface area, pore size and surface properties. The result of gas analysis showed that phosphoric acid affected formation of activated carbon mechanism and significant reactions occurred during microwave pretreatment process.
Open Access
Impact of Li2O/metal mole ratio on lithium-ion battery anode performance
(Electrochemical Science Group, 2018) Kunduraci, M.; Ghobadi, T. G. U.; Yilmaz, E.
In this study the electrochemical impact of Li2O/metal mole ratio on the cycle life of lithium-ion battery anode materials is demonstrated. For this purpose, nanostructured layered LiNi1/3Mn1/3Co1/3O2 (LiNMC) and spinel LiMn1.5Ni0.5O4 (LiMNO) materials, traditionally known as cathode materials, are evaluated as anode materials and compared against their lithium-free versions NMC (Ni:Mn:Co=1:1:1) and MNO (Mn:Ni=3:1). The Li2O/metal ratio in fully lithiated states are 2.0 for lithium containing (LiNMC and LiMNO) and 1.3 for lithium-free (NMC and MNO) samples. Battery tests show that capacity fading of lithium containing samples is 3 to 4 times larger than lithium-free samples. The differences in the electronic conductivities and voltages profiles of lithium containing and lithium-free anode materials are suggested to be the origin of such electrochemical disparity.
Open Access
Improved lithium-ion battery anode performance via multiple element approach
(Elsevier, 2018) Ghobadi, T. G. U.; Kunduraci, M.; Yilmaz, Eda
In this work, single (Co3O4), binary (Co3O4/ZnO) and ternary (Co3O4/ZnO/NiO) nanomaterials were successfully synthesized by Pechini method followed by a calcination step. Electrochemical lithium storage capabilities of the anode materials were studied. The results showed that the best capacity retention and lowest voltage hysteresis was achieved with ternary material. The ternary material showed a first cycle charge capacity of 649 mAh/g at 70 mA/g and maintained 83% of this capacity after 39 cycles. The results demonstrated the positive impact of multiple element strategy on the cycle life of anode materials.
Open Access
Nanosecond pulsed laser ablated sub-10 nm silicon nanoparticles for improving photovoltaic conversion efficiency of commercial solar cells
(Institute of Physics Publishing Ltd., 2017) Rasouli, H. R.; Ghobadi, A.; Ghobadi, T. G. U.; Ates, H.; Topalli, K.; Okyay, Ali Kemal
In this paper, we demonstrate the enhancement of photovoltaic (PV) solar cell efficiency using luminescent silicon nanoparticles (Si-NPs). Sub-10 nm Si-NPs are synthesized via pulsed laser ablation technique. These ultra-small Si nanoparticles exhibit photoluminescence (PL) character tics at 425 and 517 nm upon excitation by ultra-violet (UV) light. Therefore, they can act as secondary light sources that convert high energetic photons to ones at visible range. This down-shifting property can be a promising approach to enhance PV performance of the solar cell, regardless of its type. As proof-of-concept, polycrystalline commercial solar cells with an efficiency of ca 10% are coated with these luminescent Si-NPs. The nanoparticle-decorated solar cells exhibit up to 1.64% increase in the external quantum efficiency with respect to the uncoated reference cells. According to spectral photo-responsivity characterizations, the efficiency enhancement is stronger in wavelengths below 550 nm. As expected, this is attributed to down-shifting via Si-NPs, which is verified by their PL characteristics. The results presented here can serve as a beacon for future performance enhanced devices in a wide range of applications based on Si-NPs including PVs and LED applications.