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ABSTRACT: Nearly monodisperse lead chalcogenide (PbE, E = S, Se, or Te) semiconductor quantum dots of controllable shape have been produced via a novel synthesis which includes the occurrence of in situ formed Pb(0) particles. Tunable size and shape are achieved through appropriate choice of the precursor type and the stabilizer. As precursor, we use, on the one hand, lead oxide or lead acetate, on the other hand, tellurium, selenium, or sulfur powder dissolved in trioctylphosphine (TOP), tributylphosphine (TBP), or 1-octadecene (ODE). Oleic acid (OA) and various amines, as well as TOP and TBP are used for stabilization. With respect to possible application in hybrid solar cells, the surface of as-synthesized spherical PbSe nanocrystals was investigated by nuclear magnetic resonance (NMR), mass spectrometry (MS) and thermogravimetric analysis (TGA). As an important result, it was found that the surface is not mostly covered by oleic acid after synthesis, but by a phosphorus compound. We also applied a ligand exchange procedure with hexylamine and found evidence for the successful attachment of hexylamine to the nanocrystal surface. Additionally, charge separation between these nanoparticles and the conjugated polymer poly(3-hexylthiophene) (P3HT) is studied by electron spin resonance and photoinduced absorption spectroscopy. The spectra obtained suggest that charges can be produced successfully by photoinduced charge transfer.
Physical Chemistry Chemical Physics 07/2012; 14(33):11706-14. · 3.57 Impact Factor
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ABSTRACT: Many physical and chemical properties of semiconducting nanocrystals strongly depend on their spatial dimensions and crystallographic structure. For these reasons, achieving a high degree of size and shape control plays an important role with respect to their application potential. In this report we present a facile route for the direct colloidal synthesis of copper(I) sulfide nanorods. A high reactivity of the starting materials is essential to obtain nanorods. We achieve this by using a thiol that thermally decomposes easily and serves as the sulfur source. The thiol is mixed in a noncoordinating solvent, which acts as the reaction medium. Adjustment of the nucleation temperature makes it possible to tailor uniform nanorods with lengths from 10 to 100 nm. The nanorods are single crystalline, and the growth direction is shown to occur along the a-axis of djurleite. The growth process and character of the nanorods were investigated through UV-vis and NIR absorption spectroscopy, transmission electron microscopy, and powder X-ray diffraction measurements.
ACS Nano 06/2012; 6(7):5889-96. · 10.77 Impact Factor
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ABSTRACT: CuInS2 (CIS) nanocrystals were successfully synthesized through a hot-injection technique employing a reaction of copper (I) acetate
and indium (III) acetate with tert-dodecanethiol as a source of sulfur, and trioctylphosphine oxide and 1-dodecanethiol were used as ligands. The reaction medium
was a mixture of two solvents: oleylamine and 1-octadecene. Varying the ratio between both solvents leads to the formation
of wurtzite CuInS2 particles with shapes ranging from triangular to rod-shaped with length up to 50nm. Oleylamine turned out to influence the
reaction condition in two opposite ways: by leading to monomer depletion before the injection of the sulfur precursor, and
at the same time increasing the activity of the monomers remaining in solution. By changing the sulfur source from tert-dodecanethiol to sulfur dissolved in oleylamine, triangular particles with zinc blend structure and a smaller size (~5nm)
were synthesized. The final materials were characterized by powder X-ray diffraction (XRD), transmission electron microscopy
(TEM), energy dispersive X-ray analysis (EDX), and absorption spectroscopy (UV–Vis).
KeywordsNanocrystals–Copper indium disulfide–Morphology–Shape-control–Semiconductors
Journal of Nanoparticle Research 04/2012; 13(11):5815-5824. · 3.29 Impact Factor
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Holger Borchert,
Florian Witt,
Alina Chanaewa,
Fabian Werner,
Jan Dorn,
Thomas Dufaux,
Marta Kruszynska,
Anne Jandke,
Michael Höltig,
Tamara Alfere,
Jens Böttcher,
Christoph Gimmler,
Christian Klinke,
Marko Burghard,
Alf Mews,
Horst Weller,
Jürgen Parisi
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ABSTRACT: One factor limiting the performance of polymer-based bulk heterojunction solar cells is inefficient charge transport in the disordered donor/acceptor blends. The incorporation of carbon nanotubes (CNTs) into the active layer is considered as a promising concept to improve charge transport toward the electrodes. Whereas disordered ternary blends of polymer, fullerenes and CNTs, were already examined in the past, there is much less work on ordered vertically oriented arrays of nanotubes for solar cell applications. We focused on the fabrication of corresponding arrays with spatial dimensions that are suitable for application in polymer-based solar cells. We demonstrate in this contribution that CNTs can be grown on ITO covered glass at temperatures below 600 °C by static pressure chemical vapor deposition (CVD). Using short growth times, we were able to obtain CNT arrays with a rather uniform length limited to about 200 nm. With a plasma-enhanced CVD process, we were also able to produce wall-like carbon nanostructures (multilayered graphene sheets) with controllable height on ITO. Both types of carbon nanostructures were investigated in test solar cells, in order to explore their suitability for application in organic photovoltaics. Efficiencies remained low so far (0.2%), but the successful incorporation of the new ordered structures into solar cells could be demonstrated, and issues for further optimization are discussed.
12/2011;
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ABSTRACT: PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.
Langmuir 08/2011; 27(17):11052-61. · 4.19 Impact Factor
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ABSTRACT: Alkylamines were recently found to be suitable ligands for CdSe quantum dots with respect to applications in polymer/nanoparticle solar cells. However, the physical origin of the superior performance with respect to more widely used pyridine-capped quantum dots still remains unclear. Here, we report about the details of the surface modification procedure of CdSe quantum dots with alkylamines and subsequent application of the nanoparticles for the fabrication of hybrid CdSe/poly-3-hexylthiophene (P3HT) solar cells. As-synthesized nanocrystals were subjected to a pyridine treatment as an intermediate step to remove the high molecular weight species from the sample, and then the exchanges with octylamine and butylamine were carried out. Investigation based on nuclear magnetic resonance (NMR), X-ray spectroscopy (EDX), and thermal gravimetric analysis (TGA) demonstrated that pyridine ligand exchange as intermediate step is an effective procedure to reduce undesirable impurities which otherwise impede further surface modification and, therefore, the final performance of the CdSe/P3HT hybrid cells. Laboratory samples with butylamine- and octylamine-capped CdSe nanoparticles and P3HT were prepared and characterized by current–voltage (I–V) and external quantum efficiency (EQE) measurements. In order to find out the optimum parameters of the butylamine-capped CdSe/P3HT samples, we looked at the influence of the active layer thickness and annealing temperature on the solar cell performance. Power conversion efficiency (PCE) of 2.0% was reached for butylamine-stabilized CdSe quantum dots and P3HT. The superior performance with respect to pyridine-capped quantum dots was found to be mainly due to a higher photocurrent. Deeper analysis of the photocurrent improvement was performed by detailed comparison of the EQE spectra and investigations on the charge carrier generation and recombination processes by light-induced electron spin resonance (l-ESR). Therefrom, we have done a model based on different charge carrier trap states at the nanocrystal surface.
06/2011;
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ABSTRACT: Cu(2)S-CuInS(2) hybrid nanostructures as well as pure CuInS(2) (CIS) nanocrystals were synthesized by methods of colloidal chemistry. The structure, the shape and the composition of these nanomaterials were investigated with transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). By changing the reaction conditions, CuInS(2) nanorods with different aspect ratio, dimeric nanorods as well as hexagonal discs and P-shaped particles could be synthesized. Under our reaction conditions, CIS nanoparticles crystallize in the hexagonal wurtzite structure, as confirmed by Rietveld analysis of the X-ray diffraction patterns. The formation of Cu(2)S-CuInS(2) hybrid nanostructures turned out to be an essential intermediate step in the growth of CIS nanoparticles, the copper sulphide part of the hybrid material playing an important role in the shape control of the CIS nanocrystals. By a treatment of Cu(2)S-CuInS(2) with 1,10-phenanthroline, Cu(2)S parts of the hybrid nanostructures could be removed, and pure CIS nanoparticles with shapes not accessible with other methods can be obtained. Our synthetic procedure turned out to be suitable to synthesize also other compounds, like CuInS(2)-ZnS alloys, and to modify, in this way, the optical properties of the nanocrystals.
Journal of the American Chemical Society 10/2010; 132(45):15976-86. · 9.91 Impact Factor
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ABSTRACT: So-called charge transfer complexes (CTCs) in plastic solar cell materials have recently become one of the main focuses in research on organic photovoltaics. Because these states influence the relevant parameters of organic solar cells, it is of fundamental interest to understand fully the underlying processes. In the present work, we show that CTC states can be captured by light-induced electron spin resonance (l-ESR) with a much higher sensitivity than is possible with previously applied methods, such as photoinduced absorption spectroscopy. We take advantage of the l-ESR technique to demonstrate the existence of CTC states by their direct excitation with low-energy photons in an organic polymer/fullerene blend and, for the first time, also in hybrid blends of poly(3-hexylthiophene) and CdSe nanoparticles. Moreover, the recombination kinetics is analyzed and discussed.Keywords (keywords): polaron pairs; electron paramagnetic resonance; photoinduced absorption; CdSe quantum dots; organic solar cells; charge transfer
09/2010;
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ABSTRACT: We propose a simple chemiluminescence (CL) method for investigation of the surface of Co-based nanocrystals (NCs). Using a combination of CL and spin-trap electron paramagnetic resonance techniques, we systematically studied the generation of reactive oxygen species (ROS) at the surface of differently sized CoPt(3) spherical NCs and CoPt(3)/Au nanodumbbells. We have shown that differently sized CoPt(3) NCs can promote the formation of ROS and as a result can lead to the oxidation of luminol accompanied by the emission of the light. CL allows monitoring the stability of transition-metal-based NCs against oxidation and dissolution. We found by CL that cobalt ions slowly leach from the surface of CoPt(3) NCs even under very mild conditions; however, the amount of the leached cobalt ions does not exceed the maximal concentration of cobalt at the NC surface indicating that only surface atoms can go into solution.
Journal of the American Chemical Society 07/2010; 132(26):9102-10. · 9.91 Impact Factor
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ABSTRACT: Colloidal CdSe quantum dots (QDs) are suitable as electron acceptors in polymer/nanoparticle bulk heterojunction hybrid solar cells. For this application, a thick organic ligand shell which is typically surrounding the QDs after synthesis needs to be removed. Ligand exchange with pyridine is the most widely used method for this purpose. Although this approach is already 15 years old, detailed studies on the effectiveness of ligand exchange with pyridine for solar cell applications are still missing. In the present work hybrid solar cells were prepared from CdSe QDs initially capped with oleic acid (OA), and the impact of single and multiple pyridine treatment was thoroughly investigated. NMR was applied to determine the composition of the ligand shell as well as to distinguish the bound and free ligands before and after ligand exchange. It is shown that after a single pyridine treatment some amount of OA is still present in the samples. By using thermal gravimetric analysis (TGA) we could obtain also quantitative information about the effectiveness of subsequent pyridine treatments. In a series of one-, two-, and threefold ligand exchange, the estimated surface coverage by OA decreased from 26% to 12%, whereas that of pyridine increased from 54% to 80%. Laboratory solar cells with pyridine-capped CdSe QDs and poly(3-hexylthiophene) (P3HT) were characterized by current−voltage (I−V) measurements, and in order to get deeper insight into charge carrier generation and recombination processes, CdSe/P3HT blends were studied by light-induced electron spin resonance (l-ESR). Although repeated pyridine treatment was found to have a beneficial effect in the sense that more complete ligand exchange was achieved, which in turn enabled more efficient charge transfer, the performance of the solar cells was found to be reduced. This fact correlates with increased aggregation tendency of repeatedly pyridine-treated particles, negatively influencing the morphology of the blends, as well as with a larger amount of surface defects in particles stabilized by the weak pyridine ligand shell.
06/2010;
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ABSTRACT: The influence of the Cd-to-Se precursor ratio during the synthesis of CdSe nanoparticles (nc-CdSe) on the efficiency of solar cells made from semiconducting polymer/nanocrystal-blends (hybrid solar cells) was investigated. In these hybrid solar cells regioregular poly-(3-hexylthiophene 2,5 diyl) (P3HT) was used as the electron donor material while the acceptor was established by CdSe nanocrystals prepared via colloidal synthesis. Furthermore, the influence of the nc-CdSe-to-P3HT ratio in the semiconductor blend on the solar cell efficiency was investigated. CdSe:P3HT ratios of 8:1 to 10:1 were found to give the best results concerning the overall device performance as derived from current–voltage characterization. These findings were correlated with structural investigations of the active layer by means of atomic force microscopy (tapping mode AFM). Furthermore, the external quantum efficiency (EQE) of the hybrid solar cells was determined and also used to estimate the short circuit current density Jsc under standardized solar irradiation. The Jsc values from the EQE measurements were compared to the values obtained from the IV curves. Differences in these values could be explained by an intensity-dependent influence of trap states.
Physica Status Solidi (A) Applications and Materials 09/2009; 206(12):2700 - 2708. · 1.46 Impact Factor
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Birte Jürgens, Holger Borchert,
Kirsten Ahrenstorf,
Patrick Sonström,
Angelika Pretorius,
Marco Schowalter,
Katharina Gries,
Volkmar Zielasek,
Andreas Rosenauer,
Horst Weller,
Marcus Bäumer
Angewandte Chemie International Edition 11/2008; 47(46):8946-9. · 13.45 Impact Factor
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ABSTRACT: Bimetallic nanoparticles often turn out to be superior to the corresponding monometallic systems with respect to their catalytic properties. To study such effects for the methanol decomposition reaction, model catalysts were prepared by physical vapor deposition of Pd and Co under ultrahigh-vacuum (UHV) conditions. Monometallic Pd and Co particles as well as CoPd core-shell particles were generated on an epitaxial alumina film grown on NiAl(110). The interaction with methanol is examined by temperature-programmed desorption of methanol and carbon monoxide and by X-ray photoelectron spectroscopy. The decomposition of methanol proceeds in two reaction pathways independent of the particle composition: complete dehydrogenation towards carbon monoxide and hydrogen, and C--O bond scission yielding carbon deposits. Pd is the most active material studied here. The relative importance of the two channels varies for the different particle systems: on Pd dehydrogenation is preferred, whereas the C--O bond cleavage is more pronounced on Co. The bimetallic clusters show a moderate performance for both pathways. Carbon deposition poisons the model catalysts by blocking the adsorption sites for methoxide, which is the first intermediate product during methanol decomposition. In particular on Co, large amounts of carbon deposits can also be caused by dissociation of the final product of the dehydrogenation pathway, carbon monoxide. A comparison with the results of methanol decomposition on Co, Pd, and CoPd catalysts in continuous-flow reactors demonstrates that the findings of the present UHV study are relevant for catalytic performance under high-pressure conditions.
ChemPhysChem 05/2008; 9(5):729-39. · 3.41 Impact Factor
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ABSTRACT: Nanostructured praseodymium oxides were successfully prepared via four different methods: two traditional methods (calcination of praseodymium nitrate and sol−gel method with propylene oxide) and two more sophisticated, modern techniques (citrate method and modified Pechini method). Powder X-ray diffraction revealed that all synthesis methods led to praseodymium oxide Pr6O11 with cubic fluorite-like structure. The temperature necessary for the formation of the crystalline oxide phase, however, was dependent on the method and synthesis parameters. The size of the nanocrystalline domains was in the range of some 10 nm in all cases. The catalytic properties of the nanostructured oxides were studied choosing CO oxidation as a first test reaction. According to infrared spectroscopy, the surface of all samples was covered with monodentate carbonate species after the synthesis. After exposure to CO, two types of bidentate carbonates were observed on the oxide surface, and under the feed of both CO and O2, carbon dioxide was observed by IR spectroscopy as product in the gas phase at temperatures from 300 °C on. The activity with respect to CO oxidation was further investigated in a catalytic test reactor. The maximum conversion of CO was reached at 550 °C, and it was 95−96% independent of the synthesis method. At moderate temperatures (350−500 °C), the activities of the catalysts prepared in the present work were dependent on the synthesis method and synthesis parameters, only to a small extent, but all of them were more active than commercial Pr6O11. The differences between the various samples prepared in this study can be explained by an influence of the synthesis on the oxygen ion mobility. Mechanistically, the results of our work suggest that CO oxidation occurs through the adsorption of CO as a bidentate carbonate, which is then transformed into a monodentate carbonate finally desorbing as CO2.
02/2008;
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ChemPhysChem 05/2007; 8(5):654-6. · 3.41 Impact Factor
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Angewandte Chemie International Edition 02/2007; 46(16):2923-6. · 13.45 Impact Factor
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Holger Borchert,
Yulia Borchert,
Vasiliy V Kaichev,
Igor P Prosvirin,
Galina M Alikina,
Anton I Lukashevich,
Vladimir I Zaikovskii,
Ella M Moroz,
Eugenii A Paukshtis,
Valerii I Bukhtiyarov,
Vladislav A Sadykov
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ABSTRACT: Nanostructured ceria doped with other rare earth elements is a good oxygen ion conductor, which gives rise to various catalytic applications such as the construction of membranes for syngas production by partial oxidation of methane. This article focuses on the Gd-doped cerium dioxides, which can be modified with Pt or Pd to enhance the reactivity of the lattice oxygen in interaction with methane. The aim of the work is the elucidation of correlations between the structural, electronic, and chemical properties of these nanomaterials. Detailed studies were performed for a series of samples with and without surface modification by noble metals using a complex combination of physicochemical methods: XRD, TEM, CH(4) TPR, XPS, SIMS, and FTIR spectroscopy of adsorbed CO. XPS and TPR data revealed that surface modification with noble metals enhances the reducibility of the doped ceria support, where the effect is more pronounced for Pd than for Pt. The formation of highly cationic Pd species due to strong metal support interactions provides a possible explanation for this behavior. Furthermore, the results obtained in the present work for the Gd-doped ceria system are compared to those obtained previously for the Pr-doped ceria system.
The Journal of Physical Chemistry B 12/2005; 109(43):20077-86. · 3.70 Impact Factor
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Vladislav A. Sadykov,
Yulia V. Frolova,
Galina M. Alikina,
Anton I. Lukashevich,
Vitalii S. Muzykantov,
Vladimir A. Rogov,
Ella M. Moroz,
Dmitrii A. Zyuzin,
Vyacheslav P. Ivanov, Holger Borchert,
Eugenii A. Paukshtis,
Valerii I. Bukhtiyarov,
Vasiliy V. Kaichev,
Stylianos Neophytides,
Erhard Kemnitz,
Kerstin Scheurell
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ABSTRACT: For samples of the Ce1-xGdxO2-y system prepared by the Pechini route and promoted with Pt, the mobility and reactivity of lattice oxygen were found to correlate with the density of surface and bulk anion vacancies.
Reaction Kinetics and Catalysis Letters 06/2005; 85(2):367-374. · 0.93 Impact Factor
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ABSTRACT: One of the most fundamental tasks in nanoscience is the accurate determination of particle sizes. Various methods have been developed to elucidate the mean particle diameter and the standard deviation for an ensemble of nanocrystals. However, good agreement between the results from different methods is not always encountered in the literature. In this study, we investigate colloidally prepared, highly monodisperse CoPt3 nanoparticles by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and powder X-ray diffraction (XRD). The results are compared in order to examine to which extent agreement is obtained by the different techniques when applied to small nanocrystals in the size range below 10 nm. In particular, the applicability of the simple Scherrer formula for size determination from the broadening of XRD reflections is checked. When the different techniques are correctly applied, the results from all methods are in good agreement.
Langmuir 04/2005; 21(5):1931-6. · 4.19 Impact Factor
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Holger Borchert,
Yulia V Frolova,
Vasiliy V Kaichev,
Igor P Prosvirin,
Galina M Alikina,
Anton I Lukashevich,
Vladimir I Zaikovskii,
Ella M Moroz,
Sergei N Trukhan,
Vyacheslav P Ivanov,
Eugenii A Paukshtis,
Valerii I Bukhtiyarov,
Vladislav A Sadykov
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ABSTRACT: Nanostructured doped ceria is a prospective material for catalytic applications such as the construction of membranes with mixed electronic and ionic conductivity for effective syngas production. In this article, the surface properties of nanostructured ceria doped with praseodymium have been studied by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy of adsorbed carbon monoxide. The effects of supporting 1.4 wt % Pt as well as structural changes upon the reduction of the samples with methane have been investigated. While in samples without supported platinum, mainly praseodymium cations are reduced in a methane atmosphere; stronger reduction of cerium cations was found in the case of surface modification with Pt. The structural differences correlate with results from temperature-programmed reaction experiments with methane. Explanations are discussed in terms of different reaction mechanisms.
The Journal of Physical Chemistry B 04/2005; 109(12):5728-38. · 3.70 Impact Factor
