[Show abstract][Hide abstract] ABSTRACT: The conductivity enhancement of single-walled carbon nanotube (SWCNT) films was achieved by increasing the bundle length in an aerosol CVD synthesis method with the help of two carbon sources. Carbon monoxide provides carbon at temperatures below 900 °C, while ethylene takes over at higher temperatures. The significant decrease in the sheet resistance at the 90% transmittance was observed from 3500 to 7500 Ω/sq. for pure CO system via 1909 and 1709 Ω/sq. for CO–H2 system to 291 and 358 Ω/sq. in the presence of C2H4 at 900 and 1100 °C, respectively. Doping the film with a gold chloride solution in acetonitrile allowed us to create the transparent conductive films with the sheet resistance as low as 73 Ω/sq. at a transmittance of 90%.
[Show abstract][Hide abstract] ABSTRACT: A mixture of single-walled and double-walled carbon nanotubes (CNTs) was synthesised by an aerosol CVD (floating catalyst) method for fabrication of highly conductive and transparent films. The feedstock solution contained ferrocene as a catalyst particle precursor, toluene and ethylene as carbon sources and thiophene as a promoter was introduced in the reactor in a hydrogen atmosphere and heated up in the temperature range of 1000–1200 °C. The product was collected downstream of the reactor by filtering the flow in the form of thin films with adjustable thicknesses (transmittance) and subsequently transferred on a desirable substrate by a dry transfer technique. This method allowed us to fabricate excellent quality CNT films with a high optoelectronic performance: with a sheet resistance of 86 Ω/sq. at the transmittance of T = 90%. To our best knowledge, for the first time the CNT transparent and conductive thin films were fabricated from direct filtration of gas phase grown CNTs using hydrocarbon as a carbon source.
Chemical Engineering Journal 11/2014; 255:134–140. · 4.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The majority of the nanowire synthesis methods utilize catalyst particles to guide the nanowire geometry. In contrast, catalyst-free methods are attractive for facile fabrication of pure nanowires without the need for catalyst preparation. Nonetheless, how a nanowire growth is guided without a catalyst is still widely disputed and unclear. Here we show that the nanowire growth during metal oxidation is limited by a nucleation of a new layer. On the basis of in situ transmission electron microscope investigations we found that the growth occurs layer by layer at the lowest specific surface energy planes. Atomic layers nucleate at the edge of twin boundary ridges and form a long range ordering along the twin boundary. We anticipate our study to be a starting point to employ defects for nanowire growth control and consequently shaping the geometry of nanowires in a similar manner as in the catalyst assisted growth method.
[Show abstract][Hide abstract] ABSTRACT: Closed-edged bilayer graphene nanoribbons were formed by spontaneous collapse of large-diameter single-walled carbon nanotube (SWNTs) grown on gold nanoparticles by chemical vapor deposition. Such bilayer graphene nanoribbons could adopt different stacking configurations, such as AB-stacking or stacking order with any rotation angle, correlated with the chiral angles of their parent rounded SWNTs. Based on electron diffraction characterizations on a good number of collapsed and uncollapsed SWNTs, the threshold diameter for SWNTs to collapse was precisely determined to be 5.1 nm, independent of SWNT's chiral angle. The determination is consistent with that calculated by both classical Adaptive Intermolecular Reactive Empirical Bond Order force field and density functional theory after having taken stacking effect and thermal fluctuation into account.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate efficient surface passivation of GaAs nanowires using ultrathin in-situ grown epitaxial InP and GaP capping layers, with metallo-organic vapor phase epitaxy as the growth system. The passivation increased photoluminescence intensity by three orders of magnitude compared to unpassivated nanowires, and the effect remained strong after a month of storage in air. Effective passivation was acquired over a wide range of growth temperatures, although the highest studied temperatures caused additional detrimental effects such as etching and GaAsP formation. The capping layer thickness was in the order of few monolayers. Therefore, the impact on any other properties of the nanowires besides the surface states was minuscule. As a simple and effective method the studied capping layers offer an excellent way for nanowire passivation.
[Show abstract][Hide abstract] ABSTRACT: The interaction between high surface area nano-carbon catalyst supports for proton exchange membrane fuel cells (PEMFCs) and perfluorinated sulfonic acid (Nafion®) ionomer was studied 19 fluorine nuclear magnetic resonance spectroscopy (19F-NMR). The method was developed and improved for more efficient, user-friendly and systematic studies based on our earlier experience. In this work, multi-walled carbon nanotubes (MWCNTs) from Showa Denko® and the corresponding acid modified products were explored. The adsorption at low concentration was found to follow a Langmuir isotherm. Adsorption equilibrium constant (Keq.) and maximum surface coverage (Γmax) were determined based on the model. In general, the ionomer showed stronger adsorption for MWCNTs (Keq. = 21 − 377 depending on treatment) comparing to Vulcan (Keq. = 18), and slightly lower monolayer coverage. The interaction was found to be strongly affected by surface composition, morphology, porosity and oxygen containing functional groups, which are varied with purification and functionalization treatments. The modification of the surface properties was also studied with HR-TEM, BET, porosity measurement, EDXS, XPS, Raman and TG. The results will contribute to optimize electrode preparation with novel nano-carbon catalyst supports and durable catalyst for low temperature (LT) PEMFCs.
[Show abstract][Hide abstract] ABSTRACT: Encapsulation of coronene inside single-walled carbon nanotubes (SWNTs) was studied under various conditions. Under high vacuum, two main types of molecular encapsulation were observed by using transmission electron microscopy: coronene dimers and molecular stacking columns perpendicular or tilted (45-60°) with regard to the axis of the SWNTs. A relatively small number of short nanoribbons or polymerized coronene molecular chains were observed. However, experiments performed under an argon atmosphere (0.17 MPa) revealed reactions between the coronene molecules and the formation of hydrogen-terminated graphene nanoribbons. It was also observed that the morphology of the encapsulated products depend on the diameter of the SWNTs. The experimental results are explained by using density functional theory calculations through the energies of the coronene molecules inside the SWNTs, which depend on the orientation of the molecules and the diameter of the tubes.
[Show abstract][Hide abstract] ABSTRACT: Stable high-concentration aqueous dispersions (>1 mg ml−1) of single and few-layer graphene flakes were produced by direct exfoliation of graphite using cellulose nanocrystals (CNC). Biodegradable and widely available from renewable sources, CNC have proven to be very efficient graphene stabilizers even at low concentrations (0.2 mg ml−1), thus enabling remarkably high graphene/CNC ratios (up to 3.8).
[Show abstract][Hide abstract] ABSTRACT: Low-temperature chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWNTs) was achieved on two different types of CoxMg1−xO catalysts prepared by different techniques: atomic layer deposition (ALD) and impregnation. The chirality distribution of SWNTs grown on the ALD-prepared CoxMg1−xO catalyst is wider than that of SWNTs grown on the impregnation-prepared CoxMg1−xO catalyst. The different chirality distributions of SWNTs are related to their different growth modes. The ALD-prepared CoxMg1−xO catalyzes the growth of SWNTs by “tip growth” mode, as revealed by in situ environmental transmission electron microscopy studies. In contrast, SWNTs grow on the impregnation-prepared CoxMg1−xO by “base growth” mode. “Base growth” is attributed to strong metal–support interactions between the epitaxially formed Co nanoparticles and the underlying MgO support, accounting for the synthesis of SWNTs with high chiral-selectivity. In addition, impregnation-prepared CoxMg1−xO catalysts calcinated at different temperatures were systematically studied and their catalytic performances in synthesizing carbon nanotubes were elucidated. This work illustrates the influence of metal–support interactions and catalyst reducibility on the chirality-distribution of the synthesized SWNTs.
[Show abstract][Hide abstract] ABSTRACT: The effect of thermal post-treatments and the use of Ti adhesion layer on the performance of thin film diamond like carbon bioelectrodes (DLC) have been investigated in this work. The following results were obtained: (i) The microstructure of the DLC layer after the deposition was amorphous and thermal annealing had no marked effect on the structure, (ii) formation of oxygen containing SiOx and Ti[O,C] layers were detected at the Si/Ti and Ti/DLC interfaces with the help of transmission electron microscope (TEM), (iii) thermal post-treatments increased the polar fraction of the surface energy, (iv) cyclic voltammetry (CV) measurements showed that the DLC films had wide water windows and were stable in contact with dilute sulphuric acid and phosphate buffered saline (PBS) solutions, (v) use of Ti interlayer between Pt(Ir) microwire and DLC layer was crucial for the electrodes to survive the electrochemical measurements without the loss of adhesion of the DLC layer, (vi) DLC electrodes with small exposed Pt areas were an order of magnitude more sensitive towards dopamine than Pt electrodes and (vii) thermal post-treatments did not markedly change the electrochemical behavior of the electrodes despite the significant increase in the polar nature of the surfaces. It can be concluded that thin DLC bioelectrodes are stable under physiological conditions and can detect dopamine in micro molar range, but their sensitivity must be further improved.
[Show abstract][Hide abstract] ABSTRACT: Atomic layer deposition (ALD) is a thin layer synthesis method applied in this study for preparing carbon-supported mono-metallic Pt- and bi-metallic PtCo catalysts. The catalyst characterization confirmed that small metal particles with a narrow particle size distribution and high metal dispersion were obtained. The location of the metals on the surface was controlled by alternating the ALD cycles, and the formation of bi-metallic PtCo particles on the support was observed. The prepared catalysts proved to be active for methanol oxidation and oxygen reduction in an acidic media. In addition, the durability of the catalysts in electrochemical oxidation was enhanced by varying the metal cycle order in the catalyst preparation. After the deposition of Co on the catalyst, one ALD cycle of Pt favored the catalyst durability in the methanol oxidation reaction.
Applied Catalysis B: Environmental. 01/2014; s 148–149:11–21.
[Show abstract][Hide abstract] ABSTRACT: Aimed at utilizing high-magnetization nanospheres for magnetic field-enhanced cellular labeling, core-shell structured sandwich-like magnetic mesoporous silica nanospheres were developed. While the magnetite cluster core can provide a high magnetic response for overcoming Brownian motion in cell culture media, the layered silica shell facilitates an efficient fluorescent dye labeling. However, the problem of particle aggregation in cell media, which is strongly enhanced under a magnetic field, significantly impeded the uptake by cells, resulting in difficulties in the precise analysis of the degree of particle internalization by fluorescence-based techniques (flow cytometry and confocal microscopy). To overcome this, reflection-based assessment was employed. Further, emphasis was put on utilizing the unique role of surface-hyperbranched polyethylenimine (PEI) in efficient prevention of particle aggregation prior to cell internalization in the presence of an external magnetic field. The interparticle attraction forces originating from magnetic dipole-dipole interactions are hereby balanced by the steric and electrostatic repulsion forces provided by the PEI functionalization, which leads to dispersed nanospheres in cell culture media during the magnetic-field induced cell labeling. As a consequence, PEI functionalization and the presence of the magnetic field synergistically enhanced the efficiency of MRI-fluorescence dual-mode labeling for cellular tracking.
[Show abstract][Hide abstract] ABSTRACT: Plasmonic core-shell Au@SiO2 nanoparticles have previously been shown to enhance the performance of dye-sensitized solar cells (DSSCs). A thin silica coating can provide a better stability during thermal processing and chemical stability to survive the corrosive electrolyte used in DSSCs. However, the thickness and completeness of the silica shell has proven crucial for the performance of the plasmonic particles and is largely controlled by the linking chemistry between the gold core and silica shell. We have evaluated four different silica coating procedures of ∼15nm gold nanoparticles for usage in DSSCs. The chemical stability of these core-shell nanoparticles was assessed by dispersing the particles in iodide/triiodide electrolyte solution and the thermal stability by heating the particles up to 500°C. In order to maintain stable gold cores a complete silica coating was required, which was best obtained by using a mercaptosilane as a linker. In situ TEM characterization indicated that the heating process only had minor effects on the core-shell particles. The final step was to evaluate how the stable Au@SiO2 nanoparticles were influencing a real DSSC device when mixed into the TiO2 photoanode. The plasmon-incorporated DSSCs showed a ∼10% increase in efficiency compared to devices without core-shell nanoparticles.
Journal of Colloid and Interface Science 12/2013; · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report a pathway to grow GaAs nanowires on a variety of substrates using a combination of atomic layer deposition and metallo-organic vapor phase epitaxy (MOVPE). GaAs nanowires were grown via MOVPE at 430–540C on an atomic-layer-deposited Al:ZnO buffer layer. The resulting nanowires were affected only by the properties of the buffer layer, allowing nanowire growth on anumber of substrates that withstand 400C. The growth occurred in two phases: initial in-plane growth and subsequent out-plane growth. The nanowires grown exhibited a strong photoluminescence signal both at room temperature and at 12 K. The 12 K photoluminescence peak was at 1.47 eV, which was attributed to Zn autodoping from the buffer layer. The crystal structure was zincblende plagued with either twin planes or diagonal defect planes, which were related to perturbations in the seed particle during the growth. The used method combines substrates with variable properties to nanowire growth on a transparent and conductive Al:ZnO buffer layer.
Journal of Applied Physics 08/2013; 114(084309). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report a new phenomenon related to Al-induced carrier confinement at the interface in core–shell GaAs/AlxGa1–xAs nanowires grown using metal–organic vapor phase epitaxy with Au as catalyst. All AlxGa1–xAs shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to 66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron–hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell–core–shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.