Browsing by Subject "Oxides"

Now showing 1 - 13 of 13

Open Access
Chiral ceramic nanoparticles and peptide catalysis
(American Chemical Society, 2017) Jiang S.; Chekini, M.; Qu, Z.-B.; Wang Y.; Yeltik A.; Liu, Y.; Kotlyar, A.; Zhang, T.; Li, B.; Demir, Hilmi Volkan; Kotov, N. A.
The chirality of nanoparticles (NPs) and their assemblies has been investigated predominantly for noble metals and II-VI semiconductors. However, ceramic NPs represent the majority of nanoscale materials in nature. The robustness and other innate properties of ceramics offer technological opportunities in catalysis, biomedical sciences, and optics. Here we report the preparation of chiral ceramic NPs, as represented by tungsten oxide hydrate, WO3-x·H2O, dispersed in ethanol. The chirality of the metal oxide core, with an average size of ca. 1.6 nm, is imparted by proline (Pro) and aspartic acid (Asp) ligands via bio-to-nano chirality transfer. The amino acids are attached to the NP surface through C-O-W linkages formed from dissociated carboxyl groups and through amino groups weakly coordinated to the NP surface. Surprisingly, the dominant circular dichroism bands for NPs coated by Pro and Asp are different despite the similarity in the geometry of the NPs; they are positioned at 400-700 nm and 500-1100 nm for Pro- and Asp-modified NPs, respectively. The differences in the spectral positions of the main chiroptical band for the two types of NPs are associated with the molecular binding of the two amino acids to the NP surface; Asp has one additional C-O-W linkage compared to Pro, resulting in stronger distortion of the inorganic crystal lattice and greater intensity of CD bands associated with the chirality of the inorganic core. The chirality of WO3-x·H2O atomic structure is confirmed by atomistic molecular dynamics simulations. The proximity of the amino acids to the mineral surface is associated with the catalytic abilities of WO3-x·H2O NPs. We found that NPs facilitate formation of peptide bonds, leading to Asp-Asp and Asp-Pro dipeptides. The chiroptical activity, chemical reactivity, and biocompatibility of tungsten oxide create a unique combination of properties relevant to chiral optics, chemical technologies, and biomedicine.
Open Access
Electrospun Fe2O3 entrenched SiO2 supported N and S dual incorporated TiO2 nanofibers derived from mixed polymeric template/surfactant: enriched mesoporosity within nanofibers, effective charge separation, and visible light photocatalysis activity
(American Chemical Society, 2019) Pradhan, Amaresh C.; Uyar, Tamer
The α-Fe2O3 promoted and SiO2 supported N and S dual incorporated TiO2 nanofibers (FeSiNST NFs) along with neat oxide NFs have been synthesized by electrospinning via sol–gel. The keen approach is that mixed polyvinylpyrrolidone (PVP) as template and cetyltrimethylammonium bromide (CTAB) as surfactant are responsible for the creation of mesoporosity within NFs. The photoluminescence (PL) spectrum and UV–visible diffuse reflectance spectroscopic (DRS) result revealed the role of α-Fe2O3 as catalytic promoter in FeSiNST NFs by suppressing electron–hole (e––h+) recombination, red shifting, and oxygen vacancies (Ovs). The design of FeSiNST NFs by combining with SiO2 as catalytic support and N and S as visible light absorbers in TiO2, beautifies the present study. The high photocurrent (3.2 mA/cm2), high Efb value (−1.0 V), and low Rct value (∼74 Ω) support the enhanced photocatalysis (photoreduction and photodegradation) by FeSiNST in visible light. Charge transfer phenomena, Ovs, mesoporosity, and separation of e––h+ are the vital factors for an effective photocatalysis achievement.
Open Access
Elucidating the barriers on direct water splitting: key role of oxygen vacancy density and coordination over PbTiO3 and TiO2
(American Chemical Society, 2021-01-28) Ersen, M.; Ellialtıoğlu, Ş.; Gülseren, Oğuz; Uner, D.
In this work, using the state-of-the-art first-principles calculations based on density functional theory, we found that the concentration and coordination of surface oxygen vacancies with respect to each other were critical for the direct water-splitting reaction on the (001) surfaces of PbTiO3 and TiO2. For the water-splitting reaction to happen on TiO2-terminated surfaces, it is necessary to have two neighboring O vacancies acting as active sites that host two adsorbing water molecules. However, eventual dissociation of O–H bonds is possible only in the presence of an additional nearest-neighbor O vacancy. Unfortunately, this necessary third vacancy inhibits the formation of molecular hydrogen by trapping the dissociated H atoms on TiO2-terminated surfaces. Formation of up to three O vacancies is energetically less costly on both terminations of PbTiO3(001) surfaces compared with those on TiO2; the presence of Pb leads to weaker O bonds over these surfaces. Molecular hydrogen formation is more favorable on the PbO-terminated surface of PbTiO3, requiring only two neighboring oxygen vacancies. However, the hydrogen molecule is retained near the surface by weak van der Waals forces. Our study indicates two barriers leading to low productivity of direct water-splitting processes. First and foremost, there is an entropic barrier imposed by the requirement of at least two nearest-neighbor O vacancies, sterically hindering the process. Furthermore, there are also enthalpic barriers of formation on TiO2-terminated surfaces or removal of H2 molecules from the PbO-terminated surface. The requirement dictating three nearest-neighbor oxygen vacancies for hydrogen evolution is also consistent with the chemical intuition: The nearest neighbor of the formed hydrogen should be reduced enough to inhibit spontaneous oxidation under ambient conditions.
Open Access
Graphene nanoreactors : photoreduction of prussian blue in aqueous solution
(American Chemical Society, 2017) Nappini, Silvia; Matruglio, Alessia; Naumenko, Denys; Dal Zilio, Simone; Lazzarino, Marco; De Groot, Frank M.F.; Kocabaş, Coşkun; Balcı, Osman; Magnano, Elena
Prussian dyes are characterized by interesting photomagnetic properties due to the photoinduced electron transfer involved in the Fe oxidation and spin state changes. Ferromagnetic Prussian blue (PB) in contact with titanium dioxide (TiO2) can be reduced to paramagnetic Prussian white (PW) upon UV band gap excitation of TiO2. This process is promoted by the presence of a hole scavenger, such as water, fundamental to ensure the overall charge balance and the continuity of the process. In order to clarify the photoinduced reduction mechanism and the role of water, an innovative system of graphene nanobubbles (GNBs) filled with a PB aqueous solution was developed, enabling the application of electron spectroscopies to the liquid phase, up to now limited by the vacuum required to overcome the short electron inelastic mean free path in dense medium. In this work GNBs formed on the photocatalytic substrate are able to act as "nanoreactors", and they can control and take part in the reaction. The evolution of Fe L2,3 edge X-ray absorption spectra measured in total electron yield through the graphene membrane revealed the electron reduction from PB (FeIII-CN-FeII) to PW (FeII-CN-FeII) upon UV irradiation, shedding light on the photoinduced electron transfer mechanism in liquid phase. The results, confirmed also by Raman spectroscopy, unequivocally demonstrate that the reaction occurs preferentially in aqueous solution, where water acts as hole scavenger.
Open Access
MIMIM photodetectors using plasmonically enhanced MIM absorbers
(SPIE, 2017) Dereshgi, S. Abedini; Okyay, Ali Kemal
We demonstrate super absorbing metal-insulator-metal (MIM) stacks and MIMIM photosensitive devices operating at visible and near-infrared (VIS-NIR) spectrum, where absorbing (top) MIM and photocollecting (bottom) MIM can be optimized separately. We investigate different bottom metals in absorbing MIM with nanoparticles realized by dewetting of silver thin film on top. While gold and silver have conventionally been considered the most appropriate plasmonic absorbers, we demonstrate different absorbing metals like aluminum and specifically chromium, with its plasma frequency happening at 850 nm, as more efficient layers for absorption. Absorption in chromium hits 82 percent around 1000 nm. We provide convincing evidences by doing reflection experiment and computational simulations for absorbing MIM part. We also suggest for the first time investigating electric loss tangent of metal or coherently, surface plasmon quality factor of absorbing metals which are reliable tools for engineering different metal layers. They reveal that despite the fact that gold and silver are good plasmonic scatterers in VIS-NIR and reliable absorbers in VIS region, they are not proper choices as absorbers for NIR applications. Once the most optimum absorbing design is pointed out, we integrate it on top of another metal-insulator to form an MIMIM photodetector with tunneling photocurrent path. The final optimized sample consisting of silver - hafnium oxide - chromium - aluminum oxide - silver nanoparticles (from bottom to top) has a dark current of 7nA and a photoresponsivity peak of 0.962 mA/W at 1000 nm and a full width at half maximum of 300 nm, while applied bias is 50 mV and device areas are 300 μm x 600 μm. This photoresponse shows 70 times enhancement compared to former reported spin coated rare nanoparticle MIMIMs.
Open Access
Multispectral plasmonic structures using native aluminum oxide and aluminum
(OSA, 2017) Ayaş, Sencer; Bakan, Gökan; Dana, Aykutlu
We report the use of native aluminum oxide to fabricate periodic metal-insulator-metal resonators with simultaneous resonances in the visible and IR wavelengths. The cavity size is in the order of λ3/25000 in the NIR.
Open Access
Nonlinear laser lithography for indefinitely large-area nanostructuring with femtosecond pulses
(Nature publishing group, 2013) Öktem, B.; Pavlov, I.; Ilday, S.; Kalaycıoǧlu, H.; Rybak, A.; Yavaş, S.; Erdoǧan, M.; Ilday F. Ö.
Dynamical systems based on the interplay of nonlinear feedback mechanisms are ubiquitous in nature. Well-understood examples from photonics include mode locking and a broad class of fractal optics, including self-similarity. In addition to the fundamental interest in such systems, fascinating technical functionalities that are difficult or even impossible to achieve with linear systems can emerge naturally from them if the right control tools can be applied. Here, we demonstrate a method that exploits positive nonlocal feedback to initiate, and negative local feedback to regulate, the growth of ultrafast laser-induced metal-oxide nanostructures with unprecedented uniformity, at high speed, low cost and on non-planar or flexible surfaces. The nonlocal nature of the feedback allows us to stitch the nanostructures seamlessly, enabling coverage of indefinitely large areas with subnanometre uniformity in periodicity. We demonstrate our approach through the fabrication of titanium dioxide and tungsten oxide nanostructures, but it can also be extended to a large variety of other materials.
Open Access
Photocatalytic conversion of nitric oxide on titanium dioxide: cryotrapping of reaction products for online monitoring by mass spectrometry
(American Chemical Society, 2016) Lu, W.; Olaitan, A. D.; Brantley, M. R.; Zekavat, B.; Erdogan, D. A.; Ozensoy, E.; Solouki, T.
Details of coupling a catalytic reaction chamber to a liquid nitrogen-cooled cryofocuser/triple quadrupole mass spectrometer for online monitoring of nitric oxide (NO) photocatalytic reaction products are presented. Cryogenic trapping of catalytic reaction products, via cryofocusing prior to mass spectrometry analysis, allows unambiguous characterization of nitrous oxide (N2O) and nitrogen oxide species (i.e., NO and nitrogen dioxide (NO2)) at low concentrations. Results are presented, indicating that the major photocatalytic reaction product of NO in the presence of titanium dioxide (TiO2) P25 and pure anatase catalysts when exposed to ultraviolet (UV) light (at a wavelength of 365 nm) is N2O. However, in the presence of rutile-rich TiO2 catalyst and UV light, the conversion of NO to N2O was less than 5% of that observed with the P25 or pure anatase TiO2 catalysts.
Open Access
The substrate temperature dependent electrical properties of titanium dioxide thin films
(2010) Yildiz, A.; Lisesivdin, S.B.; Kasap, M.; Mardare, D.
Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13-320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conductioninthe films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behaviorisdiscussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared. © Springer Science+Business Media, LLC 2009.
Open Access
Tailoring insoluble nanobelts into soluble anti-UV nanopotpourris
(Royal Society of Chemistry, 2011) Wang, J.; Sun, X. W.; Jiao, Z.; Khoo, E.; Lee, P. S.; Ma, J.; Demir, Hilmi Volkan
Soluble, transparent and anti-UV nanopotpourris have been prepared by tailoring long nanobelts. The strains and layered structures facilitate the breaking of the as-synthesized nanobelts under an applied mechanical action. The developed tailoring process of nanobelts is a general top-down secondary processing of layered nanostructures at the nanoscale level, which can be expended to the modifications of layered nanowires, nanotubes and hierarchical nanostructures. By tailoring, the size, morphology and solubility are modified, which may open up an area of advanced processing of nanomaterials and hint at some potential applications. Because of the excellent solubility of the tailored nanopotpourris, they are easily dispersed in cosmetics or polymer films, which are quite useful for some anti-UV protection applications, such as anti-UV sunscreen creams and anti-UV window films for vehicles and buildings.
Open Access
The tight-binding approach to the corundum-structure d compounds
(Institute of Physics Publishing Ltd., 1994) Ivanov, V. A.
The analysis of electronic structures has been carried out for the transition-metal compounds showing the corundum-type crystal symmetry using the suggested tight-binding method for interacting bands. With the self-consistent field approximation, the branches of the electronic spectra and energy gaps have been analytically calculated. The role of the electron correlations was found to be decisive for the dielectrization of spectra for which no additional assumptions, e.g. the existence of spin- or charge-density waves, was necessary. The data obtained provide an explanation for the appearance of the insulator state in such compounds as Ti2O3, V2O 3, Cr2O3, alpha -Mn2O3 and alpha -Fe2O3. The calculated values of band gaps agree reasonably with the experimental data available. The Peierls problem is solved for the corundum-structure d compounds.
Open Access
Toxicity of lanthanum oxide (La2O3) nanoparticles in aquatic environments
(Royal Society of Chemistry, 2015) Balusamy, B.; Taştan, B. E.; Ergen, S. F.; Uyar, Tamer; Tekinay, T.
This study demonstrates the acute toxicity of lanthanum oxide nanoparticles (La2O3 NP) on two sentinel aquatic species, fresh-water microalgae Chlorella sp. and the crustacean Daphnia magna. The morphology, size and charge of the nanoparticles were systematically studied. The algal growth inhibition assay confirmed absence of toxic effects of La2O3 NP on Chlorella sp., even at higher concentration (1000 mg L-1) after 72 h exposure. Similarly, no significant toxic effects were observed on D. magna at concentrations of 250 mg L-1 or less, and considerable toxic effects were noted in higher concentrations (effective concentration [EC50] 500 mg L-1; lethal dose [LD50] 1000 mg L-1). In addition, attachment of La2O3 NP on aquatic species was demonstrated using microscopy analysis. This study proved to be beneficial in understanding acute toxicity in order to provide environmental protection as part of risk assessment strategies.
Open Access
Visible photoluminescence from SiOx films grown by low temperature plasma enhanced chemical vapor deposition
(Pergamon Press, 1995) Timofeev, F. N.; Aydınlı, Atilla; Ellialtioglu, R.; Turkoglu, K.; Gure, M.; Mikhailov, V. N.; Lavrova, O. A.
a-SiOx films of varying stoichiometry have been prepared by low temperature plasma enhanced chemical vapor deposition. The majority of films showed photoluminescence (PL) and films prepared in a narrow range of gas flows exhibited much stronger PL after annealing. Peak PL energies ranging from the ultraviolet to the near infrared have been observed. PL, infrared and X-ray diffraction on selected samples indicate formation of Si clusters in the films. The effects of annealing on the PL properties of the films have been found to depend on initial stoichiometry of the films. © 1995.