Enhanced Mechanical and Durability Properties of Cement Mortar by Using Alumina Nanocoating on Carbon Nanofibers.

This study evaluated the effect of carbon nanofibers (CNFs) coated by aluminum oxide Al2O3 as a reinforcement on compressive strength, frost resistance, and drying shrinkage of cement mortars. Three weight ratios of 0.125%, 0.25%, and 0.5% of Al2O3/CNFs and bare CNF cement mortars were compared with reference cement mortar samples. The reactive porous and high surface area layer of alumina induced the hydration reaction and promoted the production of well-distributed hydration gel. Derivative thermal analysis-differential thermogravimetric (TGA-DTG) and X-ray powder diffraction (XRD) characterization showed that Al2O3/CNFs reinforcement led to greater hydration gel production than bare CNFs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to study the coating and microstructure of the cement mortars evaluated in this paper. The results show that the optimum enhancement of the cement mortar properties was obtained at ratios of 0.125% for Al2O3/CNFs and 0.25% for CNFs. This enhancement was greater with Al2O3/CNFs-reinforced specimens in terms of high compressive strength, less compressive strength degradation after 150 cycles, and less drying shrinkage. The low use of the CNFs in Al2O3/CNFs samples indicates the coating is an economical and promising approach for improving the performance of cement mortars.

Removal of Congo red dyes from aqueous solutions by porous γ-alumina nanoshells.

Porous alumina has been shown to be an excellent adsorbent for Congo Red (CR) dye. In this work, highly porous g-Al2O3 nanoshells were synthesized from alumina coated carbon black (CB) obtained from a new deposition technique and used for removal of CR dye from aqueous solutions. Adsorption experiments were conducted in a batch mode and a series of parameters were investigated, including contact time, initial dye concentrations, ionic strength and pH of the solutions. It was found that equilibrium for CR adsorption can be reached within 30 min, much faster than reported by other studies in the literature on similar adsorbents. It was also found that the adsorption capacity of Al2O3 nanoshells is 44.8 % higher than that of alumina/CB. The adsorption capacity of Al2O3 nanoshells was more favorable at lower pH, and the optimal adsorption ability was achieved at pH 4.0 with a removal efficiency at 98.6 %. The Al2O3 nanoshells have a maximum adsorption capacity of 370.4 mg g-1 (25 °C; pH 7; no salt added), better than or comparable to those reported in the literature. A pseudo-second-order kinetics model can best fit the kinetics of CR adsorption, which follows the Langmuir isotherm. The high adsorption capacity is attributed to the strong hydrogen-bonding interactions between the anionic dye and Al2O3 nanoshells surface as well as to the electrostatic interactions between CR dye and the Al2O3 nanoshells.

Inorganic Ruddlesden-Popper Faults in Cesium Lead Bromide Perovskite Nanocrystals for Enhanced Optoelectronic Performance.

While the layered hybrid Ruddlesden-Popper (RP) halide perovskites have already established themselves as the frontrunners among the candidates in optoelectronics, their all-inorganic counterparts remain least explored in the RP-type perovskite family. Herein, we study and compare the optoelectronic properties of all-inorganic CsPbBr3 perovskite nanocrystals (PNCs) with and without RP planar faults. We find that the RP-CsPbBr3 PNCs possess both higher exciton binding energy and longer exciton lifetimes. The former is ascribed to a quantum confinement effect in the PNCs induced by the RP faults. The latter is attributed to a spatial electron-hole separation across the RP faults. A striking difference is found in the up-conversion photoluminescence response in the two types of CsPbBr3 PNCs. For the first time, all-inorganic RP-CsPbBr3 PNCs are tested in light-emitting devices and shown to significantly outperform the non-RP CsPbBr3 PNCs.

Cryo-ePDF: Overcoming Electron Beam Damage to Study the Local Atomic Structure of Amorphous ALD Aluminum Oxide Thin Films within a TEM.

Atomic layer deposition (ALD) provides uniform and conformal thin films that are of interest for a range of applications. To better understand the properties of amorphous ALD films, we need an improved understanding of their local atomic structure. Previous work demonstrated measurement of how the local atomic structure of ALD-grown aluminum oxide (AlO x ) evolves in operando during growth by employing synchrotron high-energy X-ray diffraction (HE-XRD). In this work, we report on efforts to employ electron diffraction pair distribution function (ePDF) measurements using more broadly available transmission electron microscope (TEM) instrumentation to study the atomic structure of amorphous ALD-AlO x . We observe electron beam damage in the ALD-coated samples during ePDF at ambient temperature and successfully mitigate this beam damage using ePDF at cryogenic temperatures (cryo-ePDF). We employ cryo-ePDF and reverse Monte Carlo (RMC) modeling to obtain structural models of ALD-AlO x coatings formed at a range of deposition temperatures from 150 to 332 °C. From these model structures, we derive structural metrics including stoichiometry, pair distances, and coordination environments in the ALD-AlO x films as a function of deposition temperature. The structural variations we observe with growth temperature are consistent with temperature-dependent changes in the surface hydroxyl density on the growth surface. The sample preparation and cryo-ePDF procedures we report here can be used for the routine measurement of ALD-grown amorphous thin films to improve our understanding of the atomic structure of these materials, establish structure-property relationships, and help accelerate the timescale for the application of ALD to address technological needs.

Enhanced Third Harmonic Generation in Lead Bromide Perovskites with Ruddlesden-Popper Planar Faults.

Lead halide perovskites provide a test bed for exploring nonlinear optical properties. Although the underlying centrosymmetric crystal structure of 3D lead halide perovskites precludes the phenomenon of second harmonic generation, the third and higher-order harmonic generation are allowed. In this work, we probe the third harmonic generation (THG) from CsPbBr3 nanocrystals (NCs) and compare it to the THG from CsPbBr3 NCs with Ruddlesden-Popper planar faults (RP-CsPbBr3), formed via postsynthetic fusion-growth. The THG from CsPbBr3 NCs is negligible compared with that of RP-CsPbBr3 NCs within a wide range of femtosecond excitation wavelengths. We further compare the THG from a thin film of RP-CsPbBr3 with that of a single crystal of methylammonium lead bromide (MAPbBr3). The THG efficiency of RP-CsPbBr3 is found to be three times greater than that of MAPbBr3. An effective third-order susceptibility of the order of 10-18 m2 V-2 is obtained for a RP-CsPbBr3 film, opening up the prospect of inorganic halide perovskite NCs with planar defects for a range of nonlinear optical applications.

Atomic Structure and Electrical Activity of Grain Boundaries and Ruddlesden-Popper Faults in Cesium Lead Bromide Perovskite.

To evaluate the role of planar defects in lead-halide perovskites-cheap, versatile semiconducting materials-it is critical to examine their structure, including defects, at the atomic scale and develop a detailed understanding of their impact on electronic properties. In this study, postsynthesis nanocrystal fusion, aberration-corrected scanning transmission electron microscopy, and first-principles calculations are combined to study the nature of different planar defects formed in CsPbBr3 nanocrystals. Two types of prevalent planar defects from atomic resolution imaging are observed: previously unreported Br-rich [001](210)∑5 grain boundaries (GBs) and Ruddlesden-Popper (RP) planar faults. The first-principles calculations reveal that neither of these planar faults induce deep defect levels, but their Br-deficient counterparts do. It is found that the ∑5 GB repels electrons and attracts holes, similar to an n-p-n junction, and the RP planar defects repel both electrons and holes, similar to a semiconductor-insulator-semiconductor junction. Finally, the potential applications of these findings and their implications to understand the planar defects in organic-inorganic lead-halide perovskites that have led to solar cells with extremely high photoconversion efficiencies are discussed.

Substantially enhanced rate capability of lithium storage in Na2Ti6O13 with self-doping and carbon-coating.

Na2Ti6O13 (NTO) has recently been reported for lithium ion storage and showed very promising results. In this work, we report substantially enhanced rate capability in NTO nanowires by Ti(iii) self-doping and carbon-coating. Ti(iii) doping and carbon coating were found to work in synergy to increase the electrochemical performances of the material. For 300 cycles at 1C (1C = 200 mA g-1) the charge capacity of the electrode is 206 mA h g-1, much higher than that (89 mA h g-1) of the pristine NTO electrode. For 500 cycles at 5C the electrode can still deliver a charge capacity of 180.5 mA h g-1 with a high coulombic efficiency of 99%. At 20C the capacity of the electrode is 2.6 times that of the pristine NTO. These results clearly demonstrate that the Ti(iii) self-doping and uniform carbon coating significantly enhanced the kinetic processes in the NTO nanowire crystal, making it possible for fast charge and discharge in Li-ion batteries.

Temperature-induced restructuring of self-assembled PtPd nanoparticle superlattices.

PtPd nanoparticle superlattices were prepared at room temperature and investigated for their restructuring at elevated temperatures. The PtPd nanoparticles were spherical with a diameter of 3.5 +/- 0.3 nm and were capped with surfactants of oleic acids and oleylamine. Images of the nanoparticle superlattices were taken using a transmission electron microscope. Analysis of the images with fast Fourier transform (FFT) revealed whether the superlattices were crystalline or amorphous. It was found that the critical temperature that induced a phase transition of PtPd superlattices from a crystalline state to an amorphous state falls in the range of 100-110 degrees C. Meanwhile, heat-induced restructuring transformed less-ordered superlattices into better-ordered ones, similar to annealing. It was especially pronounced for 3D superlattices, which also showed well-defined grain boundaries after restructuring.

Deposition of metallic nanoparticles on carbon nanotubes via a fast evaporation process.

A new technique was developed for the deposition of colloidal metal nanoparticles on carbon nanotubes. It involves fast evaporation of a suspension containing sonochemically functionalized carbon nanotubes and colloidal nanoparticles. It was demonstrated that metallic nanoparticles with different sizes and concentrations can be deposited on the carbon nanotubes with only a few agglomerates. The technique does not seem to be limited by what the nanoparticles are, and therefore would be applicable to the deposition of other nanoparticles on carbon nanotubes. PtPd and CoPt(3) alloy nanoparticles were used to demonstrate the deposition process. It was found that the surfactants used to disperse the nanoparticles can hinder the nanoparticle deposition. When the nanoparticles were washed with ethanol, they could be well deposited on the carbon nanotubes. The obtained carbon nanotube supported metal nanoparticles were characterized by transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and cyclic voltammetry.

Polymer-mediated synthesis of highly dispersed Pt nanoparticles on carbon black.

A new method was developed to prepare highly dispersed Pt nanoparticles on carbon black to use as proton exchange membrane (PEM) fuel cell catalysts. This method involves using a polymer, poly(vinylpyrrolidone) (PVP), to prevent particle aggregation and thereby reduce nanoparticle sizes to achieve high dispersion. It was found that Pt nanoparticles mediated by PVP are smaller than those obtained without PVP and have a narrower size distribution. Well-dispersed Pt nanoparticles with metal loadings from 5 to 35 wt % were obtained on carbon black (Vulcan XC-72R). It was found that well-dispersed Pt nanoparticles on carbon black could be synthesized at a PVP monomers-to-Pt atoms ratio of 0.1 under our experimental conditions. Larger amounts of PVP did not produce smaller nanoparticles, but rather reduced the Pt mass loading on carbon black. The morphology of the Pt nanoparticles that were supported on carbon black was characterized with transmission electron microscopy and X-ray diffraction. Their active surface areas were determined using cyclic voltammetry in a sulfuric acid solution. High Pt dispersion was obtained for the catalysts synthesized with PVP mediation, even at Pt loadings up to 35 wt %. The catalysts prepared with PVP mediation generally showed larger active specific areas than did those prepared without PVP.

Sonochemical oxidation of multiwalled carbon nanotubes.

Functionalization of carbon nanotubes (CNTs) is important for enhancing deposition of metal nanoparticles in the fabrication of supported catalysts. A facile approach for oxidizing CNTs is presented using a sonochemical method to promote the density of surface functional groups. This was successfully employed in a previous study [J. Phys. Chem. B 2004, 108, 19255] to prepare highly dispersed, high-loading Pt nanoparticles on CNTs as fuel cell catalysts. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, cyclic voltammetry, and settling speeds were used to characterize the degree of surface functionalization and coverage. The sonochemical method effectively functionalized the CNTs. A mixture of -C-O-/-C=O and -COO- was observed along with evidence for weakly bound CO at longer treatment times. The integrated XPS C 1s core level peak area ratios of the oxidized-to-graphitic C oxidation states, as well as the atom % oxygen from the O 1s level, showed an increase in peak intensity (attributed to -CO(x)()) with increased sonication times from 1 to 8 h; the increase in C surface oxidation correlated well with the measured atom %. Most of the CNT surface oxidation occurred between 1 and 2 h. The sonochemically treated CNTs were also studied by cyclic voltammetry and settling experiments, and the results were consistent with the XPS observations.