Brandon L. Weeks

Ionic Polymers as a New Structural Motif for High-Energy-Density Materials

Energetic materials have been used for nearly two centuries in military affairs and to cut labor costs and expedite laborious processes in mining, tunneling, construction, demolition, and agriculture, making a tremendous contribution to the world economy. Yet there has been little advancement in the development of altogether new energetic motifs despite long-standing research efforts to develop superior materials. We report the discovery of new energetic compounds of exceptionally high energy content and novel polymeric structure which avoid the use of lead and mercury salts common in conventional primary explosives. Laboratory tests indicate the remarkable performance of these Ni- and Co-based energetic materials, while DFT calculations indicate that these are possibly the most powerful metal-based energetic materials known to date, with heats of detonation comparable with those of the most powerful organic-based high explosives currently in use.

Cellulose derived magnetic mesoporous carbon nanocomposites with enhanced hexavalent chromium removal

Magnetic carbon–iron nanoadsorbents fabricated by carbonizing cellulose and reducing Fe3O4 nanoparticles or Fe(NO3)3 (the products are denoted as MC–O and MC–N, respectively) have demonstrated great Cr(VI) removal. MC–N with a higher proportion of zero-valence iron (ZVI) and bigger specific surface area exhibited better resistance to oxygen and acid than MC–O due to its smaller pore size. The Cr(VI) removal was highly pH-dependent. For example, 4.0 mg L−1 Cr(VI) neutral solution was completely purified by 2.5 g L−1 MC–O and MC–N within 10 min. 1000 mg L−1 Cr(VI) solution at pH 1.0 was completely removed by both nanoadsorbents in 10 min. The MC–O nanoadsorbents had a higher removal percentage (98.1%) than MC–N (93.5%) at pH 7.0, while MC–N had a removal capacity of 327.5 mg g−1, much higher than 293.8 mg g−1 of MC–O at pH 1.0. A chemical adsorption was revealed from the pseudo-second-order kinetic study. Monolayer adsorption of Cr(VI) was revealed by a better fitting of the Langmuir model isotherm, rather than multilayer adsorption for the Freundlich model. These nanoadsorbents could be easily separated from solution by using a permanent magnet after being treated with Cr(VI). Finally, the Cr(VI) removal mechanisms were proposed considering the Cr(VI) reduction and precipitation of Cr(III).

Metal-Organic Frameworks (MOFs) as Safer, Structurally Reinforced Energetics

Second-generation cobalt and zinc coordination architectures were obtained through efforts to stabilize extremely sensitive and energetic transition-metal hydrazine perchlorate ionic polymers. Partial ligand substitution by the tridentate hydrazinecarboxylate anion afforded polymeric 2D-sheet structures never before observed for energetic materials. Carefully balanced reaction conditions allowed the retention of the noncoordinating perchlorate anion in the presence of a strongly chelating hydrazinecarboxylate ligand. High-quality X-ray single-crystal structure determination revealed that the metal coordination preferences lead to different structural motifs and energetic properties, despite the nearly isoformulaic nature of the two compounds. Energetic tests indicate highly decreased sensitivity and DFT calculations suggest a high explosive performance for these remarkable structures.

Polyethylenimine Facilitated Ethyl Cellulose for Hexavalent Chromium Removal with a Wide pH Range

Ethyl cellulose (EC) composites modified with 20.0 wt % polyethylenimine (PEI) (PEI/ECs) demonstrated effective hexavalent chromium, [Cr(VI)], removal from solutions with a wide pH range. For example, 4.0 mg/L Cr(VI) solution with a pH below 3.0 was completely purified by 3.0 g/L PEI/ECs within 5 min, much faster than the as-received EC (2 h) and activated carbon (several hours). These PEI/ECs adsorbents has overcome the low pH limitation of Cr(VI) removal; for example, 4.0 mg/L Cr(VI) solution with a pH of 11.0 was completely purified within 15 min. These adsorbents followed chemical adsorption as revealed from the pseudo-second-order kinetic study. These PEI/ECs following the isotherm Langmuir model have a maximum adsorption capacity of 36.8 mg/g, much higher than pure EC (12 mg/g), tetrabutylammonium-modified celluloses (16.67 mg/g), and magnetic carbon (16 mg/g). The reduction of Cr(VI) to Cr(III) by the oxidation of amine groups and hydroxyl groups of PEI/ECs was verified as the main mechanism for the Cr(VI) removal.

Cr(VI) removal by magnetic carbon nanocomposites derived from cellulose at different carbonization temperatures

Magnetic carbon nanoadsorbents fabricated by using cellulose and Fe(NO3)3 as the carbon and iron precursors have demonstrated great Cr(VI) removal performance. The magnetic carbon synthesized at a carbonization temperature of 700 °C and a heating rate of 10 °C min−1 (MC7–10) has a greater Cr(VI) removal capacity (22.8 mg g−1) in neutral solutions due to its larger specific surface area (247.14 m2 g−1). Moreover, the magnetic carbon fabricated at a carbonization temperature of 800 °C and a heating rate of 10 °C min−1 (MC8–10) has the highest Cr(VI) removal capacity (278.8 mg g−1) in acidic solutions, much higher than those of cellulose (12.0 mg g−1), zero valent iron (ZVI)/chitosan (55.8 mg g−1) and ZVI doped ordered mesoporous carbon (256.86 mg g−1). The high removal capacity was attributed to its higher content of ZVI and specific surface area (136.27 m2 g−1) of MC8–10. The reduction of Cr(VI) to Cr(III) by the oxidation of carbon layer as well as the ZVI in the acidic solution was verified as the main mechanism for Cr(VI) removal. Moreover, the nanoadsorbents could be easily separated from solution by using a permanent magnet after being treated with Cr(VI).

Tunable photoluminescence and energy transfer of YBO3:Tb3+, Eu3+ for white light emitting diodes

Rare-earth (Eu 3+, Tb 3+) ion co-doped YBO 3 phosphors with a morphology of uniform flower-like assembly are fabricated by a facile hydrothermal method without the use of surfactant, organic ligands, or further sintering treatment. Photoluminescence (PL) studies have demonstrated that under the excitation of 365 nm ultraviolet (UV) light, YBO 3:Tb 3+, Eu 3+ emits a white light, including three emissions: a blue band attributed to self-trapped excitons (STEs), a green band due to the Tb 3+ transition of 5D 4– 7F j ( j = 6, 5, 4, and 3), and a red band due to the Eu 3+ transition of 5D 0– 7F j ( j = 0, 1, 2, 3, and 4). Energy transfers from host YBO 3 to Tb 3+, and Eu 3+ and Tb 3+ to Eu 3+, as well as tunable emission by varying the relative doping ratios are demonstrated. The combination of blue emission from STEs with green and red emissions from activators provides a novel and efficient technique to make white light emitting diodes (WLEDs).

Evidence of meniscus interface transport in dip-pen nanolithography: An annular diffusion model

Ring shaped dots were patterned with mercaptohexadecanoic acid ink by dip-pen nanolithography. These dots have an ink-free inner core surrounded by an inked annular region, making them different from the filled dots usually obtained. This suggests a different transport mechanism than the current hypothesis of bulk water meniscus transport. A meniscus interface ink transport model is proposed, and its general applicability is demonstrated by predicting the patterned dot radii of chemically diverse inks.

Calibration of AFM cantilever spring constants

In this paper we present two simple, reliable and readily applicable methods for calibrating cantilevers and measuring the thickness of thin gold films. The spring constant calibration requires knowledge of the Youngs modulus, density of the cantilever and resonant frequency. The thickness of thin gold layers was determined by measuring changes in the resonant frequency and Q-factor of beam shaped AFM cantilevers before and after coating. The techniques for measuring the spring constant and thin film thickness provide accuracy on the order of 10-15%.

Magnetic and optical properties of NaGdF4:Nd3+, Yb3+, Tm3+ nanocrystals with upconversion/downconversion luminescence from visible to the near-infrared second window

We have designed and synthesized NaGdF4:Nd3+, Yb3+, Tm3+ magnetic nanophosphors with combined dual-mode downconversion (DC) and upconversion (UC) photoluminescence upon 800 nm excitation. Hexagonal-phase NaGdF4:Nd3+, Yb3+, Tm3+ nanocrystals (NCs) with an average size of 21 nm were synthesized using a solvothermal approach. Nd3+, Yb3+, Tm3+ triple-doped NaGdF4 NCs exhibit a broad range of photoluminescence peaks covering a near infrared first/second window (860–900, 1,000, and 1,060 nm), and visible emission including blue (475 nm), green (520 and 542 nm) and yellow (587 nm) after excitation at 800 nm. A mechanism involving circulation of energy over Gd3+ sublattices as bridge ions and final trapping by the initial activator ions (Nd3+) has been proposed. Penetration depth studies indicate that NIR emission is easily detected even at a large tissue thickness of 10 mm. These paramagnetic nanophosphors demonstrate a large magnetization value of 1.88 emu/g at 20 kOe and longitudinal relaxivity value of 1.2537 mM−1·S−1 as a T1-weighted magnetic resonance imaging contrast agent. These NaGdF4:Nd3+, Yb3+, Tm3+ NCs are promising for applications in biological and magnetic resonance imaging.

Electropolymerized polyaniline/manganese iron oxide hybrids with an enhanced color switching response and electrochemical energy storage

Polyaniline (PANI) nanocomposites embedded with manganese iron oxide (MnFe2O4) nanoparticles were prepared as thin films by electropolymerizing aniline monomers onto indium tin oxide (ITO) glass slides pre-spin-coated with MnFe2O4 nanoparticles. The shift of the characteristic peaks of PANI/MnFe2O4 in UV-visible absorption spectra and Fourier transform infrared (FT-IR) spectra indicates the formation of composite films and the chemical interaction between the PANI matrix and MnFe2O4 particles. A coloration efficiency of 92.31 cm2 C−1 was obtained for the PANI/MnFe2O4 nanocomposite film, higher than that of the pristine PANI film, 80.13 cm2 C−1, suggesting a synergistic effect between the MnFe2O4 particles and the PANI matrix. An enhanced areal capacitance of 4.46 mF cm−2 was also achieved in the PANI/MnFe2O4 nanocomposite film compared with 3.95 mF cm−2 in the pristine PANI film by CV at a scan rate of 5 mV s−1. The enhanced performances of the composite films were attributed to the pseudocapacitive properties of MnFe2O4 and rougher morphology caused by the embedment of MnFe2O4 particles into the PANI matrix. Finally, the sulfuric acid (H2SO4) concentration and temperature effects on the supercapacitive behavior of the pristine PANI and PANI/MnFe2O4 nanocomposite films were studied, suggesting the positive effects of decreasing H2SO4 concentration and increasing temperature in a low temperature range; higher temperatures can destroy the PANI structure and cause the degradation of PANI.