Analyzing the Li-Al-O Interphase of Atomic Layer-Deposited Al2O3 Films on Layered Oxide Cathodes Using Atomistic Simulations.

Alumina surface coatings are commonly applied to layered oxide cathode particles for lithium-ion battery applications. Atomic layer deposition (ALD) is one such surface coating technique, and ultrathin alumina ALD films (<2 nm) are shown to improve the electrochemical performance of LiNixMnyCo1-x-yO2 materials, with groups hypothesizing that a beneficial Li-Al-O product is being formed during the alumina ALD process. However, the atomic structure of these films is still not well understood, and quantifying the interface of ultrathin (∼1 nm) ALD films is an arduous experimental task. Here, we perform molecular dynamics simulations of amorphous alumina films of varying thickness in contact with the (0001) LiCoO2 (LCO) surface to quantify the film nanostructure. We calculate elemental mass density profiles through the films and observe that the Li-Al-O interphase extends ∼2 nm from the LCO surface. Additionally, we observe layering of Al and O atoms at the LCO-film interface that extends for ∼1.5 nm. To access the short-range order of the amorphous film, we calculated the Al coordination numbers through the film. We find that while [4]Al is the prevailing coordination environment, significant amounts of [6]Al exist at the interface between the LiCoO2 surface and the film. Taken together, these principal findings point to a pseudomorphic Li-Al-O overlayer that approximates the underlying layered LiCoO2 lattice but does not exactly replicate it. Additionally, with sufficient thickness, the Li-Al-O film transitions to an amorphous alumina structure. We anticipate that our findings on the ALD-like, Li-Al-O film nanostructure can be applied to other layered LiNixMnyCo1-x-yO2 materials because of their shared crystal structure with LiCoO2. This work provides insight into the nanostructure of amorphous ALD alumina films to help inform their use as protective coatings for Li-ion battery cathode active materials.

Highly dispersed Co deposited on Al2O3 particles via CoCp2 + H2 ALD.

Highly dispersed cobalt atoms were deposited on porous alumina particles using atomic layer deposition (ALD) with a CoCp2/H2 chemistry at approximately 7 wt%. H2 did not completely reduce the cyclopentadienyl organic ligands bound to deposited Co atoms at ALD reaction conditions. A sharp decline in Co deposited per cycle for two or more ALD cycles indicates that much of the Al2O3 surface is sterically blocked from further CoCp2 deposition after the first CoCp2 exposure. Temperature programmed reduction confirmed that the adsorbed precursor organic ligands persist after H2 exposures during ALD and temperatures as high as 500 °C are required to fully reduce the organic ligands to CH4. High resolution, element sensitive imaging showed that Co atoms were dispersed on the Al2O3 surface and could deposit in previously unobserved multiple growth morphologies, specifically layers that were continuous over several angstroms or discrete nanoparticles. Density functional theory calculations were used to examine Co atom adsorption, show the altered haptic binding of cracked Cp ligands, and to calculate the thermodynamics of Cp ligand decomposition. The lateral steric hindrance between organic ligands bound to deposited Co atoms, Cp ligand decomposition mechanism, and local Al2O3 surface termination all likely determine the observed Co growth morphology during initial ALD cycles.

High-Throughput Equilibrium Analysis of Active Materials for Solar Thermochemical Ammonia Synthesis.

Solar thermochemical ammonia (NH3) synthesis (STAS) is a potential route to produce NH3 from air, water, and concentrated sunlight. This process involves the chemical looping of an active redox pair that cycles between a metal nitride and its complementary metal oxide to yield NH3. To identify promising candidates for STAS cycles, we performed a high-throughput thermodynamic screening of 1,148 metal nitride/metal oxide pairs. This data-driven screening was based on Gibbs energies of crystalline metal oxides and nitrides at elevated temperatures, G(T), calculated using a recently introduced statistically learned descriptor and 0 K DFT formation energies tabulated in the Materials Project database. Using these predicted G(T) values, we assessed the viability of each of the STAS reactions-hydrolysis of the metal nitride, reduction of the metal oxide, and nitrogen fixation to reform the metal nitride-and analyzed a revised cycle that directly converts between metal oxides and nitrides, which alters the thermodynamics of the STAS cycle. For all 1148 redox pairs analyzed and each of the STAS-relevant reactions, we implemented a Gibbs energy minimization scheme to predict the equilibrium composition and yields of the STAS cycle, which reveals new active materials based on B, V, Fe, and Ce that warrant further investigation for their potential to mediate the STAS cycle. This work details a high-throughput approach to assessing the relevant temperature-dependent thermodynamics of thermochemical redox processes that leverages the wealth of publicly available temperature-independent thermodynamic data calculated using DFT. This approach is readily adaptable to discovering optimal materials for targeted thermochemical applications and enabling the predictive synthesis of new compounds using thermally controlled solid-state reactions.

Particle atomic layer deposition.

The functionalization of fine primary particles by atomic layer deposition (particle ALD) provides for nearly perfect nanothick films to be deposited conformally on both external and internal particle surfaces, including nanoparticle surfaces. Film thickness is easily controlled from several angstroms to nanometers by the number of self-limiting surface reactions that are carried out sequentially. Films can be continuous or semi-continuous. This review starts with a short early history of particle ALD. The discussion includes agitated reactor processing, both atomic and molecular layer deposition (MLD), coating of both inorganic and polymer particles, nanoparticles, and nanotubes. A number of applications are presented, and a path forward, including likely near-term commercial products, is given.

Pyrolysis of human feces: Gas yield analysis and kinetic modeling.

Pyrolysis of human feces renders the waste free of pathogens and is a potential method of treating fecal sludge waste collected from non-sewered systems. Slow pyrolysis experiments were conducted on human feces and the char yield and gas evolution quantified at 1-10 °C/min heating rates. Char yield ranged from 35.1 to 35.8% (dry mass basis), while the gas yield ranged from 17.2 to 29.6% (dry mass basis). The pyrolysis gases detected were CO, CO2, CH4, C2H6, and H2. These non-condensable gases contained a higher heating value (HHV) ranging from 7.2 to 22.8 MJ/Nm3. Kinetic analysis was done by a pyrolysis reaction model free method (Isoconversional) as well as a DAEM (Distributed Activated Energy Model) method that assumes many irreversible first order reactions. Both yielded very close values for activation energy ranging from 141 kJ/mol to 409 kJ/mol, with half of the biomass conversion happening at 241.5 ±â€¯2.9 kJ/mol. The findings of the research provide useful technical information that can guide the design of a pyrolysis system to treat fecal waste. Social acceptance and scale-up issues need to be addressed through further research.

Physical descriptor for the Gibbs energy of inorganic crystalline solids and temperature-dependent materials chemistry.

The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Despite this fundamental and ubiquitous role, G has been tabulated for only a small fraction of known inorganic compounds, impeding a comprehensive perspective on the effects of temperature and composition on materials stability and synthesizability. Here, we use the SISSO (sure independence screening and sparsifying operator) approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV atom-1 (~1 kcal mol-1) resolution, and with minimal computational cost, for temperatures ranging from 300-1800 K. We then apply this descriptor to ~30,000 known materials curated from the Inorganic Crystal Structure Database (ICSD). Using the resulting predicted thermochemical data, we generate thousands of temperature-dependent phase diagrams to provide insights into the effects of temperature and composition on materials synthesizability and stability and to establish the temperature-dependent scale of metastability for inorganic compounds.

Fecal sludge as a fuel: characterization, cofire limits, and evaluation of quality improvement measures.

In many low-income cities, a high proportion of fecal sludge, the excreta and blackwater collected from onsite sanitation systems such as pit latrines, is not safely managed. This constitutes a major danger to environmental and human health. The water, sanitation, and hygiene sector has recognized that valorization of treated fecal sludge could offset the upfront cost of treatment by using it as a fuel source. The few quantitative studies on fecal sludge fuel published to date have focused on heating value, moisture, ash fraction, and heavy metals. However, other factors impacting fuel utility, specifically ash speciation, have not been adequately quantified for fecal sludge. This study contributes to closing that gap and shows the value of more detailed quantification. It first characterizes fecal sludge samples from Colorado and Uganda, confirms that the fuel is better if cofired with other biomass, and outlines a framework for determining safe cofire ratios. Second, the study evaluates two methods for improving fecal sludge as a fuel: carbonization and ash leaching. Carbonization of fecal sludge did not improve fuel quality, but leaching showed promise in ash reduction.

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping.

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in nitrogen (N2) to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH3, and NH3 desorption. We found activation barriers (Ea) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al-N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal-nitrogen bonds and, thus, more-facile NH3 liberation.

Worst-case losses from a cylindrical calorimeter for solar simulator calibration.

High-flux solar simulators consist of lamps that mimic concentrated sunlight from a field of heliostats or parabolic dish. These installations are used to test promising solar-thermal technologies for commercial potential. Solar simulators can be calibrated with cylindrical calorimeters, devices that approximate black body absorbers. Calorimeter accuracy is crucial to solar simulator characterization and maintenance. To discover the worst-case performance of a cylindrical calorimeter during flux measurement Monte Carlo ray tracing was coupled to finite volume simulations. Results indicated that the calorimeter can exhibit an observer effect that distorts the solar simulator flux profile. Furthermore, the proposed design was sensitive to changes in calorimeter optical properties, changes that can result from oxidation and/or photobleaching over time. Design fidelity and robustness were substantially improved through the use of a beveled (conical) calorimeter aperture.

Solvent Control of Surface Plasmon-Mediated Chemical Deposition of Au Nanoparticles from Alkylgold Phosphine Complexes.

Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuPPh3 onto the Ag surface because the PPh3 ligand is large compared to the size of the exposed surface between adsorbed benzenes. Instead, the Au-P bond of MeAuPPh3 dissociates in solution (Ea = 38.5 kcal/mol) in the plasmon heated near-surface region followed by the adsorption of the MeAu fragment on Ag in the interstitial space of the benzene monolayer. The adsorbed benzene forces the Au precursor to react through the higher energy path of dissociation in solution rather than dissociatively adsorbing onto the bare surface. This requires a higher temperature if the reaction is to proceed at a reasonable rate and enables the control of deposition by the light induced SPR heating of the surface and nearby solution.

Efficient generation of H2 by splitting water with an isothermal redox cycle.

Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Co-processing methane in high temperature steam gasification of biomass.

High temperature steam gasification/reforming of biomass-methane mixtures was carried out in an indirectly heated entrained flow reactor to analyze the feasibility of controlling the output composition of the major synthesis gas products: H(2), CO, CO(2), CH(4). A 2(3) factorial experimental design was carried out and compared to thermodynamic equilibrium predictions. Experiments demonstrated the product gas composition is mostly dependent on temperature and that excess steam contributes to CO(2) formation. Results showed that with two carbon-containing reactants it is possible to control the gas composition of the major products. At 1500 °C, the equilibrium results accurately predicted the syngas composition and can be used to guide optimization of the syngas for downstream liquid fuel synthesis technologies.

Rapid silica atomic layer deposition on large quantities of cohesive nanoparticles.

Conformal silica films were deposited on anatase titania nanoparticles using rapid silica atomic layer deposition (ALD) in a fluidized bed reactor. Alternating doses of tris(tert-pentoxy)silanol (TPS) and trimethylaluminum (TMA) precursor vapors were used at 175 degrees C. In situ mass spectroscopy verified the growth mechanism through a siloxane polymerization process. Transmission electron microscopy revealed highly conformal and uniform silica nanofilms on the surface of titania nanoparticles. A growth rate of approximately 1.8 nm/cycle was achieved for an underdosed and incomplete polymerization reaction. Primary nanoparticles were coated despite their strong tendency to form dynamic agglomerates during fluidization. Methylene blue oxidation tests indicated that the photoactivity of anatase titania particles was mitigated with the ALD films.

Thermochemical production of fuels with concentrated solar energy.

This review article develops some of the underlying science for converting concentrated solar energy into chemical fuels and presents examples of solar thermochemical processes and reactors.

Thermochemical production of fuels with concentrated solar energy.

This review article develops some of the underlying science for converting concentrated solar energy into chemical fuels and presents examples of solar thermochemical processes and reactors.

Ultra-thin microporous-mesoporous metal oxide films prepared by molecular layer deposition (MLD).

Porous aluminium oxide films with precisely controlled thickness down to several angstroms are deposited on particle surfaces from dense aluminium alkoxide hybrid polymer films by molecular layer deposition. Porous structures are obtained by either mild water etching at room temperature or calcination in air at elevated temperatures.

Atomic layer deposition on gram quantities of multi-walled carbon nanotubes.

Atomic layer deposition (ALD) was employed to grow coaxial thin films of Al(2)O(3) and Al(2)O(3) /W bilayers on multi-walled carbon nanotubes (MWCNTs). Although the MWCNTs have an extremely high surface area, a rotary ALD reactor was successfully employed to perform ALD on gram quantities of MWCNTs. The uncoated and ALD-coated MWCNTs were characterized with transmission electron microscopy and x-ray photoelectron spectroscopy. Al(2)O(3) ALD on untreated MWCNTs was characterized by nucleation difficulties that resulted in the growth of isolated Al(2)O(3) nanospheres on the MWCNT surface. The formation of a physisorbed NO(2) monolayer provided an adhesion layer for the nucleation and growth of Al(2)O(3) ALD films. The NO(2) monolayer facilitated the growth of extremely conformal coaxial Al(2)O(3) ALD coatings on the MWCNTs. Cracks were also observed in the coaxial Al(2)O(3) ALD films on the MWCNTs. After cracking, the coaxial Al(2)O(3) ALD films were observed to slide on the surface of the MWCNTs and expose regions of bare MWCNTs. The Al(2)O(3) ALD film also served as a seed layer for the growth of W ALD on the MWCNTs. The W ALD films can significantly reduce the resistance of the W/Al(2)O(3)/MWCNT wire. The results demonstrate the potential for ALD films to tune the properties of gram quantities of very high surface area MWCNTs.

Atomic layer deposition of quantum-confined ZnO nanostructures.

The modulation of optoelectronic properties, such as the bandgap of a pure-component semiconductor material, is a useful ability that can be achieved by few techniques. Atomic layer deposition (ALD) was used here to experimentally demonstrate the ability to deposit films that exhibit quantum confinement on three-dimensional surfaces. Polycrystalline ZnO films ranging from approximately 1.5 to 15 nm in thickness were deposited via ALD using diethylzinc and hydrogen peroxide at 100 degrees C. Conformal, pinhole-free films were deposited on Si wafers and on nanosized spherical SiO(2) particles using an augmented central composite design strategy. Powder x-ray diffraction was used to measure the crystallite size of the films and monitor size evolution on the basis of the number of ALD cycles and thermal annealing post-treatments. The absorbance of the ZnO films on Si wafers and SiO(2) particles was measured using spectroscopic ellipsometry and diffuse transmittance techniques, respectively. Post-deposition annealing steps increased the crystallite size of the films, independently of the coating thickness. The ZnO bandgap was increasingly blue-shifted for films of decreasing crystallite size, approaching +0.3 eV at dimensions of 2-3 nm. The nonlinear bandgap response correlated well with the Brus model. This work represents an experimental demonstration of quantum confinement using ALD on two- and three-dimensional substrates.

Biocompatible interface films deposited within porous polymers by Atomic Layer Deposition (ALD).

Ultrathin ceramic films were deposited throughout highly porous poly(styrene-divinylbenzene) (PS-DVB) particles using a low-temperature atomic layer deposition (ALD) process. Alumina and titania films were deposited by alternating reactions of trimethylaluminum and H2O at 33 degrees C and of titanium tetrachloride and H2O2 (50 wt % in H2O) at 100 degrees C, respectively. Analytical characterization revealed that conformal alumina and titania films were grown on internal and external polymer surfaces. The improved bioactivity of the polymer substrates was revealed on the basis of the formation of hydroxyapatite (HA) in simulated body fluid. The accelerated formation of HA on the ALD-modified polymer surface was caused by the negatively charged surface provided by the ultrathin ceramic interface. The potential for ALD films to support cell attachment was demonstrated.

The role of surface basal planes of layered mixed metal oxides in selective transformation of lower alkanes: propane ammoxidation over surface ab planes of Mo-V-Te-Nb-O M1 phase.

The surface ab planes of the M1 phase exposed selectively after atomic layer deposition (ALD) of alumina followed by crushing showed significantly improved selectivity to acrylonitrile during propane ammoxidation. The results demonstrated the importance of surface ab planes for the activity and selectivity of the M1 phase in propane ammoxidation and general utility of surface modification by ALD in studies of catalytic behavior of surface planes in layered mixed metal oxides.