Noteworthy Chemistry

July 29, 2013

Use chemical gradients to move a droplet on graphene. Modifying the surface of graphene has dramatically expanded the scope of its applications. S. C. Hernández, P. E. Sheehan, S. G. Walton, and colleagues at Nova Research (Alexandria, VA), the Naval Research Laboratory (Washington, DC.), and the Defense Threat Reduction Agency (Fort Belvoir, VA) developed a method for directing the motion of droplets on graphene surfaces modified with chemical gradients.

The authors used electron-beam–generated plasmas, which can precisely modify graphene surfaces with great uniformity. The electron beam generates sheetlike plasmas in O2–Ar or SF6–Ar background gases. The authors then applied the plasmas to graphene samples that were masked with a canopy-shaped structure. The masking regulates the plasma to gradually penetrate under the canopy and create chemical gradients with changing oxygen or fluorine content (see figure).

Representation of oxygen (A) and fluorine (B) gradients on graphene

The oxygen gradients make the surfaces gradually more hydrophilic, whereas fluorine gives the surfaces hydrophobicity gradients. The fluorinated graphene reduces adhesive force toward a diethylphosphonoacetic acid–coated atomic-force microscope tip compared with pristine graphene; the oxygenated graphene shows the reverse phenomenon.

The researchers placed droplets of water and dimethyl methylphosphonate (DMMP, a nerve agent simulant) on the modified graphenes. Both droplets were “pulled” from low to high oxygen content and “pushed” from high to low fluorine content. The authors attribute the directed motion of the droplets to increasing or decreasing hydrophobicity or hydrophilicity, depending on the surface gradient. (ACS Nano 2013, 7, 4746–4755; Xin Su)

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Optimize a Buchwald–Hartwig amination. A. J. Briggs and co-workers at Hoffmann-La Roche (Nutley, NJ) improved the synthesis of an aminopyridine component of a Bruton’s tyrosine kinase inhibitor. The Buchwald–Hartwig amination of 5-[1-(azetidin-1-yl)-2-methylpropan-3-yloxy]-2-chloropyridine with lithium hexamethyldisilazide (LiHMDS) in THF stalled when it was scaled up from 0.1 to 0.4 g starting material, although good conversion could be achieved by additional charges of reactants.

The researchers adjusted the ligand, catalyst, and base, but a second charge of reactants was still required to make the reaction to go to completion. They tried NaHMDS in place of LiHMDS, but it gave only a trace of product, in line with literature reports.

The big breakthrough came when the solvent was changed from THF to toluene. This averted the need for additional charges of reagent and allowed the reaction to be scaled up to produce 440 g of product in one run. (Org. Process Res. Dev. 2013, 17, 876–880; Will Watson)

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A microbelt array shows polarized fluorescence and optical nonlinearity. Although many inorganic micromaterials have been developed with controllable morphologies, tunable properties, and useful applications, preparing organic analogues with satisfactory performance remains a challenge. Organic π-conjugated systems, for example, usually photoluminesce weakly when made into 1-D microstructures. Significant optical nonlinearity has rarely been observed in 1-D organic microcrystal array systems.

D. Yan and co-workers at Beijing University of Chemical Technology and the University of Cambridge (UK) prepared an organic “microbelt” array that gives polarized fluorescence and nonlinear optical effects.

Cationic and anionic components of a 1-D microbelt array

The 1-D microbelt has a periodically layered structure, with the guanidinium cation (1) and a stilbene-based sulfonate anion (2) assembled uniformly through hydrogen-bonding interactions within the microcrystals. The microbelt exhibits enhanced luminescence compared with the pure stilbene-based chromophore. It has high emission efficiency (46%) and large polarization anisotropy (0.71).

The authors used an evaporation-induced deposition process to make the1-D array. The regularly oriented array emits highly polarized fluorescence and shows strong second harmonic responses to excitation by near-IR laser pulses. (J. Mater. Chem. C 2013, 1, 4138–4145; Ben Zhong Tang)

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Diblock copolymers’ dispersity affects their properties. M. K. Mahanthappa and collaborators at the University of Wisconsin—Madison explored the microstructure and thin-film behavior of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA; 29–43 kDa; volume fractions [fM] 0.35–0.63). The copolymers had low-dispersity (Đ ≤ 1.1) polystyrene blocks and high-dispersity (Đ ≈ 1.7) PMMA blocks.

The authors synthesized the polydisperse PS-b-PMMA materials via a nitroxide-mediated polymerization with post-synthesis removal of the nitroxide ends to improve thermal stability. At fM <0.45, a hexagonally packed cylindrical morphology persists with limited long-range order. Unlike in narrow-dispersity copolymers, the cylindrical-to-lamellar transition was at fM ≈0.45–0.50, with lamellar microstructures dominant at fM >0.5.

The highly disperse MMA block increased configurational entropy and dilation behavior, which stabilizes the phase-separated state, increases the order–disorder transition temperature to >240 °C (independent of overall molecular weight), and produces larger domain spacings (27–38 nm) compared with low-dispersity PS-b-PMMA.

The authors also probed the thin-film microstructure of disperse PS-b-PMMA and showed that incommensurability, surface energetics, preferential block attraction, and composition work in concert with dispersity-related bulk morphologies to influence long-range ordering and domain alignment. (Macromolecules 2013, 46, 4472–4480; LaShanda Korley)

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How do inorganics in a biomass feedstock affect the environment? Any element in the periodic table might be present in biomass feedstocks, according to P. Thy and co-workers at the University of California, Davis. [Presumably only naturally occurring elements.—Ed.] Inorganic components can make up >20% of the dry weight of some feedstocks. Sorting out the inorganic components is important for determining the composition of the flue gas, identifying soil-modifying elements in ash-based fertilizers, and identifying toxic trace elements that are concentrated in the ash. In addition, controlling inorganic inputs and removing contaminants from the synthesis gas (syngas) are critical to operating reactors effectively and avoiding catalyst deactivation. Some inorganic residues offer the prospect of an additional income stream if they can be recovered and used effectively.

The researchers conducted a broad feasibility study of syngas-generation feedstocks from California agricultural, forestry, and energy crop sources. They performed chemical analyses of each feedstock and ash residue, and they examined the effects of pretreatments and pyrolysis temperature on the ash composition. The composition of the ash was influenced in part by the minerals naturally present in the soil where the plant matter was grown and by the salinity level of the irrigation water.

The organic components of the feedstocks decomposed between 250 and 450 °C. Hydroxides, carbonates, halides, and sulfates began to decompose at >1000 °C, a typical temperature for commercial boilers. As a result, the remaining ash was enriched in less-volatile trace elements such as copper, selenium, and cadmium, which reached significant levels in some samples. Pyrolysis produces a greater ash fraction than does combustion, indicating the effect of partial oxygen pressure on the mineralogy of the resulting ash.

Pretreating the feedstock by leaching it with water reduces the ash content and the ratio of fixed carbon to volatile matter. Leaching can reduce alkali metal, chlorine, and sulfur levels significantly. Organic matter extraction during leaching can affect concentrations of inorganic elements, but it can also reduce the fuel value of the feedstock. (Energy & Fuels, 2013, 27, 3969–3987; Nancy McGuire)

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Make lithium-ion battery anodes from rice husks. Rice husks, one of the most abundant agricultural byproducts, are rich in nanosized silica (≈20% dry weight SiO2), making them ideal inexpensive candidates for silicon-based nanomaterials. Y. Cui and coauthors at Stanford University (CA), the Huazhong University of Science and Technology (Wuhan, China), and SLAC National Accelerator Laboratory (Menlo Park, CA) report the efficient conversion of rice husks into nanostructured silicon with excellent performance as lithium-ion-battery anodes.

To retain the nanostructure of SiO2 particles, the authors first leached rice husks with HCl and then heated them in air at 700 °C. The now amorphous SiO2 nanoparticles were ≈80 nm in diam. They were reduced to nanosilicon by the magnesiothermic reaction (see figure) in the presence of 2.5 equiv magnesium powder at a ramp rate of 5 °C/min. Higher temperatures and faster ramp rates caused the particles to fuse.

Magnesiothermic reactions for reducing SiO2 to silicon

This process produced silicon nanoparticles in 5 wt% yield and >99.6%purity, sufficient for use in lithium-ion battery anodes. The nanosilicon anode reached a discharge capacity of 2790 mA·h/g for the first cycle at C/50, 6 times greater than that for graphite. (C/50 is the current draw at which the battery lasts for 50 h.) The anode exhibited capacity retention rates as high as 95% and 86% after 200 and 300 cycles, respectively.

The authors believe that the high porosity of nanosilicon is responsible for its exceptional capacity and stability. This method for producing nanosilicon is inexpensive, atom-economic, and easily scalable. (Sci. Rep. 2013, 3, No. 1919; Xin Su)

[See Noteworthy Chemistry for March 25, 2013 for another use of rice husks.—Ed.]

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