Philip A. Parilla

Cellulose crystallinity index: measurement techniques and their impact on interpreting cellulase performance

Although measurements of crystallinity index (CI) have a long history, it has been found that CI varies significantly depending on the choice of measurement method. In this study, four different techniques incorporating X-ray diffraction and solid-state 13C nuclear magnetic resonance (NMR) were compared using eight different cellulose preparations. We found that the simplest method, which is also the most widely used, and which involves measurement of just two heights in the X-ray diffractogram, produced significantly higher crystallinity values than did the other methods. Data in the literature for the cellulose preparation used (Avicel PH-101) support this observation. We believe that the alternative X-ray diffraction (XRD) and NMR methods presented here, which consider the contributions from amorphous and crystalline cellulose to the entire XRD and NMR spectra, provide a more accurate measure of the crystallinity of cellulose. Although celluloses having a high amorphous content are usually more easily digested by enzymes, it is unclear, based on studies published in the literature, whether CI actually provides a clear indication of the digestibility of a cellulose sample. Cellulose accessibility should be affected by crystallinity, but is also likely to be affected by several other parameters, such as lignin/hemicellulose contents and distribution, porosity, and particle size. Given the methodological dependency of cellulose CI values and the complex nature of cellulase interactions with amorphous and crystalline celluloses, we caution against trying to correlate relatively small changes in CI with changes in cellulose digestibility. In addition, the prediction of cellulase performance based on low levels of cellulose conversion may not include sufficient digestion of the crystalline component to be meaningful.

Large dielectric constant (ε/ε0>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (e/e0⩾6000) and tunability (Δe/e⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.

Designing Higher Surface Area Metal-Organic Frameworks: Are Triple Bonds Better than Phenyls?

We have synthesized, characterized, and computationally validated the high Brunauer-Emmett-Teller surface area and hydrogen uptake of a new, noncatenating metal-organic framework (MOF) material, NU-111. Our results imply that replacing the phenyl spacers of organic linkers with triple-bond spacers is an effective strategy for boosting molecule-accessible gravimetric surface areas of MOFs and related high-porosity materials.

Li ion diffusion measurements in V2O5 and Li(Co1−xAlx)O2 thin-film battery cathodes

Abstract Thin films of V 2 O 5 and LiCoO 2 were deposited by pulsed laser deposition (PLD) and the chemical diffusion coefficients, D were measured by potentiostatic intermittent titration technique (PITT). The PLD-grown V 2 O 5 and LiCoO 2 films are electrochemically similar to bulk powders and thin films produced by other techniques. In crystalline V 2 O 5 , the maximum and minimum D were found to be 1.7×10 −12 cm 2 s −1 and 5.8×10 −15 cm 2 s −1 respectively, with a general trend for D to rise in single-phase regions. In amorphous V 2 O 5 films, D was initially 5×10 −13 cm 2 s −1 and decreased steadily to 1.2×10 −13 cm 2 s −1 at Li 0.4 V 2 O 5 . The decrease in D then became more gradual with a final value of 5.52×10 −14 cm 2 s −1 at Li 1.5 V 2 O 5 . The chemical diffusion coefficient of Li in LiCoO 2 films ranged from 1×10 −12 –4×10 −11 cm 2 s −1 with a pronounced minimum at Li 0.7 CoO 2 . Thin films of LiCo 0.5 Al 0.5 O 2 also deposited by PLD exhibited limited cycling capabilities and an upper bound of D =9×10 −13 cm 2 s −1 .

Controlling single-wall nanotube diameters with variation in laser pulse power

Abstract We demonstrate that laser peak pulse power can be employed to tune carbon single wall nanotube (SWNT) diameters. The production of SWNTs was investigated at room temperature and at 1200°C. The diameters were shifted to smaller sizes in both cases as the pulse power was increased. SWNT size distributions and yields were studied with Raman spectroscopy and transmission electron microscopy. The evolution of the material quality with laser energy parameters offers insight in to SWNT formation mechanisms. These studies should aid in the development of methods for the rational control of SWNT growth.

Fabrication and Characterization of MIM Diodes Based on Nb/Nb2O5 Via a Rapid Screening Technique

Metal–insulator–metal (MIM) structures are gaining signifi cant attention due to their applications in varied electronic devices such as rectennas for energy harvesting, [ 1–4 ] high-frequency detectors/infrared photo-detection, [ 5–7 ] high-frequency mixers, [ 8–10 ] as well as applications in static memory and switching devices. [ 11 , 12 ] Ideally, for most of these applications, the MIM structure should exhibit current–voltage ( I–V ) characteristics with high asymmetry ( f ASYM > 1), strong nonlinearity ( f NL > 3), fast responsivity ( f RES > 7 V − 1 ), low hysteresis and low turn-on voltage (close to zero bias). [ 3 ] Despite the widespread utility and simple architecture of MIM devices, there is a signifi cant lack of understanding as to which materials properties produce the desired device performance. Although it is commonly stated that a high work-function difference ( Δ φ ) between the metal electrodes is responsible for high f ASYM and f NL , [ 3 , 5 , 7 ]

Fast-Switching Electrochromic Li+-Doped NiO Films by Ultrasonic Spray Deposition

A low cost, high throughput deposition method for films of nickel oxide NiO and lithium-doped nickel oxide with improved electrochromic performance is demonstrated. This method is based on ultrasonic spray deposition of aqueous-based precursor solutions in air at atmospheric pressure, which represents a significant cost savings compared to vacuum deposition methods. The resultant materials are characterized by X-ray diffraction, Raman spectroscopy, electron microscopy, and electrochemical measurements. Electrochromic performance is demonstrated with in situ optical transmission measurements during electrochemical characterization. Nickel oxide materials color anodically and are thereby ideally suited to be used as counter electrode for the well-known tungsten oxide WO3 system in “smart” window applications. The coloration of nickel oxide materials is known to be slow when compared to WO3 and thereby limits the overall response time of a NiO/WO3 tandem device. The analysis of potential step response data shows that our lithium-doped nickel oxide material achieves 90% of its total coloration change in 29 s, which is comparable to reported measurements for WO3. These results significantly mitigate a potential bottleneck to the adoption of metal oxide electrochromic windows not only by demonstrating similar performance between NiO and WO3, but by achieving this result via low cost, highly scalable processing methods.

Electronic Structure and Optical Properties of α-CH3NH3PbBr3 Perovskite Single Crystal

The electronic structure and related optical properties of an emerging thin-film photovoltaic material CH3NH3PbBr3 are studied. A block-shaped α-phase CH3NH3PbBr3 single crystal with the natural ⟨100⟩ surface is synthesized solvothermally. The room-temperature dielectric function ε = ε1 + iε2 spectrum of CH3NH3PbBr3 is determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Data are modeled with a series of Tauc-Lorentz oscillators, which show the absorption edge with a strong excitonic transition at ∼2.3 eV and several above-bandgap optical structures associated with the electronic interband transitions. The energy band structure and ε data of CH3NH3PbBr3 for the CH3NH3(+) molecules oriented in the ⟨111⟩ and ⟨100⟩ directions are obtained from first-principles calculations. The overall shape of ε data shows a qualitatively good agreement with experimental results. Electronic origins of major optical structures are discussed.

LiCoO2 and LiCo1−xAlxO2 thin film cathodes grown by pulsed laser ablation

Abstract LiCoO 2 and LiCo 0.5 Al 0.5 O 2 thin films have been grown by pulsed laser ablation on SnO 2 -coated glass substrates. For both stoichiometries, the resultant films are dense and uniaxially textured films with the Li and Co layers parallel to the substrate. In general, to grow LiCo 0.5 Al 0.5 O 2 films, a laser flux roughly 80 mJ pulse −1 higher than that used for LiCoO 2 films is required to achieve a similar deposition rate. LiCoO 2 films grown at T s =600°C and p [O 2 ]=2000 mTorr have a typical grain size of ∼100 nm. For constant current cycling between 3.8 and 4.2 V at 5 μA, the LiCoO 2 films have an initial discharge capacity of ∼0.33 Li per LiCoO 2 (89 (mA h) g −1 ) decreasing to ∼0.18 Li/LiCoO 2 (49 (mA h) g −1 ) after 100 cycles and have a continued capacity loss of ∼0.25% per cycle. The LiCo 0.5 Al 0.5 O 2 films grown to date have roughly 3 times less capacity than the LiCoO 2 films and apparently a large asymmetry between Li extraction and reintercalation.

Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage

This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3-5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ∼7.3%, phosphorus ∼8.1%, and nitrogen ∼28.1%) had surface areas as high as 900 m(2) g(-1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol(-1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively.