Size Dependence Of Nanoparticles Research Articles

Magnetite crystallization in a sodium-calcium-silicate glass with high iron oxide concentration–Effect on the magnetic properties

Glasses with mol% composition 16 Na2O∙10 CaO∙54 SiO2∙20 Fe2O3 were prepared by the melt-quenching technique. Heat treatment at 580 °C for 1 and 3 h resulted in the crystallization solely of magnetite and additionally, of hematite for longer times. Transmission electron microscopy shows growth mainly of cubic magnetite with crystallite sizes ≤ 16 nm. X-ray photoelectron spectroscopy reveals the presence of Fe3+ and Fe2+ ions in both the glass and the glass-ceramics. Mössbauer spectroscopy of the glass detects octahedrally and tetrahedrally coordinated Fe3+, while Fe2+ solely occurs in octahedral coordination. In the glass-ceramics, magnetite was detected as well as Fe3+ in both tetrahedral and octahedral coordination. Magnetization curves of the heat-treated samples recorded at room temperature and below show ferrimagnetic behaviour. From the thermomagnetic curves recorded in the temperature range from 300 to 900 K, the Curie and superparamagnetic blocking temperatures are determined. Thermomagnetic measurements in the range from 5 to 300 K reveal the nanoparticle size dependence of the observed Verwey transition.

Au144 loaded hierarchical In2O3 nanospheres for effective detection of acetone in simulated exhalation

Loading noble metal nanoparticles (NPs) on metal oxides is an effective way to solve the challenge of low sensitivity and poor selectivity of semiconductor sensors in detecting acetone in breath. However, noble metal NPs synthesized by traditional methods suffer from problems including irregular grain shape and uneven grain size, leading to a decrease in gas sensing property. Herein, we loaded atomically precise Au144(SC2H4Ph)60 nanoclusters (NCs) on In2O3 nanospheres to construct Au-loaded In2O3 materials (xAu144/IO) for detecting acetone. The gas sensing properties of xAu144/IO were optimized, and the correlation between acetone responses and the size of Au NPs was studied. Among xAu144/IO, 3Au144/IO has higher responses to acetone with good selectivity, short response/recovery time, low detection limit, and excellent long-term stability. In addition, the dynamic responses to acetone exhibit the size dependence of gold nanoparticles. More attractively, the tests of simulated exhalation indicate that the sensor based on 3Au144/IO can distinguish diabetic patients from healthy people, which is further verified by detecting the breath of a diabetic. The strategy of loading atomically precise nanoclusters adopted in this work provides a new method for the design and preparation of highly selective acetone sensing materials for breath analysis.

Understanding the effect of ambient gas pressure on the nanoparticle formation in electrically exploding wires

In this paper, a computational model characterizing the preparation of metallic nanoparticles by electrically exploding wires from the onset of current flowing through the wire to the final moment of nanoparticle formation in a gaseous environment is constructed. The computational model consists of a 1D magnetohydrodynamic model, a simplified magnetohydrodynamic model with two-temperature approximation, and a set of general dynamic equations based on the nodal approach, corresponding to the phase transition stage, plasma evolution stage, and nanoparticle growth stage, respectively. The numerical investigation on the formation of nanoparticles is performed with “cold-start” conditions. The computational predictions for the dependence of nanoparticle size on proportion under argon gas pressure of 10 kPa demonstrate that the nanoparticles of 21 nm in diameter account for the maximum proportion of 4.3%. It coincides with the experimental measurements for nanoparticles of 19 nm in diameter with the maximum proportion of 3.5%. The computational model is employed to reveal the influence of ambient gas pressures on the process of nanoparticle formation. The variation trends for parameters of exploding products, cooling rate, and nanoparticle diameter with the largest proportion on ambient gas pressures are discussed. The size distribution of nanoparticles under different argon gas pressures matches relatively well with relevant experimental data. This computational model bridges the gap between the electrically exploding wires and the growth of nanoparticles, providing theoretical support for the regulation and control technology in nanoparticle synthesization by electrically exploding wires.

Selecting High-Performance Gold Nanorods for Photothermal Conversion.

In this work, we establish a new paradigm on identifying optimal arbitrarily shaped metallic nanostructures for photothermal applications. Crucial thermo-optical parameters that rule plasmonic heating are appraised, exploring a nanoparticle size-dependence approach. Our results indicate two distinct figures of merit for the optimization of metallic nanoheaters, under both non-cumulative femtosecond and continuum laser excitation. As a case study, gold nanorods are evaluated for infrared photothermal conversion in water, and the influence of the particle length and diameter are depicted. For non-cumulative femtosecond pulses, efficient photothermal conversion is observed for gold nanorods of small volumes. For continuous wave (CW) excitation at 800 nm and 1064 nm, the optimal gold nanorod dimensions (in water) are, respectively, 90 × 25nm and 150 × 30 nm. Figure of Merit (FoM) variations up to 700% were found considering structures with the same peak wavelength. The effect of collective heating is also appraised. The designing of high-performance plasmonic nanoparticles, based on quantifying FoM, allows a rational use of nanoheaters for localized photothermal applications.

Deterministic approximation for the nucleation-growth type model of nanoparticle formation: A validation against stochastic simulations

A deterministic approximation method is introduced here for a complicated kinetic model that describes nanoparticle formation with nucleation and growth steps. The model starts from monomer units, some of which combine in a nucleation reaction, and the number of monomer units (n) in a kinetically effective nucleus is an unbounded integer parameter in the model. The growth steps are second-order processes between a particle and a monomer unit, the rate constant of which depends on the size of the growing nanoparticle in a way that is given in a kernel function. Four different kernels are considered: diffusion kernel (size independence), Brownian kernel (direct proportionality to the linear size), surface kernel (direct proportionality to the surface), mass kernel (direct proportionality to the volume or mass). Aggregation or coagulation are not part of the model. The approximation method works well for all four kernels and all integer values of n, and yields analytical formulas for the average and the most important moments (zeroth, first, and the one that is relevant for the kernel function) of the nanoparticle size distribution at every stage of the process. The approximation is validated against stochastic simulations using the Gillespie algorithm within the framework of the continuous time discrete state approach to stochastic kinetics. The comparison shows that the obtained analytical formulas are applicable for all practically important cases of the model and provide a way to interpret the dependence of average nanoparticle size on the ratio of the rate constants of nucleation and particle growth.

In Situ Studies of Single-Nanoparticle-Level Nickel Thermal Oxidation: From Early Oxide Nucleation to Diffusion-Balanced Oxide Thickening.

High-temperature oxidation mechanisms of metallic nanoparticles have been extensively investigated; however, it is challenging to determine whether the kinetic modeling is applicable at the nanoscale and how the differences in nanoparticle size influence the oxidation mechanisms. In this work, we study thermal oxidation of pristine Ni nanoparticles ranging from 4 to 50 nm in 1 bar 1%O2/N2 at 600 °C using in situ gas-cell environmental transmission electron microscopy. Real-space in situ oxidation videos revealed an unexpected nanoparticle surface refacetting before oxidation and a strong Ni nanoparticle size dependence, leading to distinct structural development during the oxidation and different final NiO morphology. By quantifying the NiO thickness/volume change in real space, individual nanoparticle-level oxidation kinetics was established and directly correlated with nanoparticle microstructural evolution with specified fast and slow oxidation directions. Thus, for the size-dependent Ni nanoparticle oxidation, we propose a unified oxidation theory with a two-stage oxidation process: stage 1: dominated by the early NiO nucleation (Avrami-Erofeev model) and stage 2: the Wagner diffusion-balanced NiO shell thickening (Wanger model). In particular, to what extent the oxidation would proceed into stage 2 dictates the final NiO morphology, which depends on the Ni starting radius with respect to the critical thickness under given oxidation conditions. The overall oxidation duration is controlled by both the diffusivity of Ni2+ in NiO and the Ni in Ni self-diffusion. We also compare the single-particle kinetic curve with the collective one and discuss the effects of nanoparticle size differences on kinetic model analysis.

The Effect of Gold Nano Particles with Different Sizes on Streptococcus Species.

Statement of the Problem: Streptococcus mutans, Streptococcus sanguinis, and Streptococcus salivarius are most common etiologic bacteria for dental caries. Different sizes of gold nanoparticles may have different antibacterial effects on these species.Purpose: This study aimed to compare the antibacterial effect of chlorhexidine and three sizes of gold nano particles (25, 60, 90nm) against clinical and standard strains of Streptococcus mutans, Streptococcus sanguinis, and Streptococcus salivarius.Materials and Method: In this cross-sectional study, the specimens were collected from 75 children aged 3-5 years old. Antibacterial effect of chlorhexidine and three sizes of gold nano particles (25, 60, 90nm) were investigated by evaluating the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against three bacterial strains.Results: The MIC and MBC of gold nanoparticles with different sizes against Streptococcus mutans, Streptococcus sanguinis, and Streptococcus salivarius were statistically different. The MIC and MBC of smaller gold nano particles (25nm) were significantly lower (p<0.001) than larger ones. Patient-derived bacteria had significantly higher values of MIC and MBC in comparison to standard species (p<0.001).Conclusion:The results of this study confirmed the significant size-dependency of gold nano particles for antibacterial activity. As the size of gold nano particles decrease, the antibacterial properties enhance.

Size dependence of CdS nanoparticles on the precursor concentration and visible light driven photocatalytic degradation of methylene blue

The effect of precursor concentration on the size of the nanoparticles and their visible light driven photocatalytic degradation of methylene blue are demonstrated.

Nanoparticle dose enhancement of synchrotron radiation in PRESAGE dosimeters.

The physical absorbed dose enhancement by the inclusion of gold and bismuth nanoparticles fabricated into water-equivalent PRESAGE dosimeters was investigated. Nanoparticle-loaded water-equivalent PRESAGE dosimeters were irradiated with superficial, synchrotron and megavoltage X-ray beams. The change in optical density of the dosimeters was measured using UV-Vis spectrophotometry pre- and post-irradiation using a wavelength of 630 nm. Dose enhancement was measured for 5 nm and 50 nm monodispersed gold nanoparticles, 5-50 nm polydispersed bismuth nanoparticles, and 80 nm monodispersed bismuth nanoparticles at concentrations from 0.25 mM to 2 mM. The dose enhancement was highest for the 95.3 keV mean energy synchrotron beam (16-32%) followed by the 150 kVp superficial beam (12-21%) then the 6 MV beam (2-5%). The bismuth nanoparticle-loaded dosimeters produced a larger dose enhancement than the gold nanoparticle-loaded dosimeters in the synchrotron beam for the same concentration. For the superficial and megavoltage beams the dose enhancement was similar for both species of nanoparticles. The dose enhancement increased with nanoparticle concentration in the dosimeters; however, there was no observed nanoparticle size dependence on the dose enhancement.

Mechanisms of Pulsed Laser-Induced Dewetting of Thin Platinum Films on Tantalum Substrates—A Quantitative Study

The mechanism of pulsed laser-induced dewetting (PLiD) of Pt thin films in the thickness range from 3.2 to 8.9 nm was investigated on smooth chemically polished Ta (PT) and patterned dimpled Ta (DT) substrates, both of which have a thin Ta2O5 layer on the surface. Complete dewetting was achieved for a film thickness of 3.2–4.6 nm. Beyond that, a distinct morphological evolution in the intermediate stages from bicontinuous to polygonal structures was observed with the change in thin-film thickness, enabling the determination of the transition thickness for Pt on Ta2O5/DT substrates to be between 5.9 and 7.7 nm. Furthermore, by examining the dependence of Pt nanoparticle (NP) size (D), nearest neighbor interparticle distance (L), and NP surface density (N) on the Pt film thickness (h), quantitative evidence was provided to demonstrate that PLiD of Pt thin films on Ta2O5/Ta substrates occurred via the spinodal dewetting mechanism. In particular, a power-law fit to the data obtained from Pt NPs on the Ta2O5/PT substrate showed that L has a quadratic dependence on h, that is, L = 5.13 h2.04. Also, the thickness dependence of D and N on Ta2O5/PT showed a power-law relationship of D = 3.74 h1.76 and N = 2.93 × 1012 h-4.03, respectively. These results are in excellent agreement with predictions from the spinodal dewetting theory. It has also been shown that the patterned DT surface underneath the Pt films exerted extra control on the NP morphology, causing a deviation from the power law as predicted by the spinodal dewetting mechanism. From the thickness dependence of L, D, and N, the Hamaker constant (A) of Pt, the proportional constant (a) correlating the characteristic spinodal length scale (Λ) and L, and the geometric factor (f (θ)) were determined.

Size dependency of gold nanoparticles interacting with model membranes

The rapid development of nanotechnology has led to an increase in the number and variety of engineered nanomaterials in the environment. Gold nanoparticles (AuNPs) are an example of a commonly studied nanomaterial whose highly tailorable properties have generated significant interest through a wide range of research fields. In the present work, we characterise the AuNP-lipid membrane interaction by coupling qualitative data with quantitative measurements of the enthalpy change of interaction. We investigate the interactions between citrate-stabilised AuNPs ranging from 5 to 60 nm in diameter and large unilamellar vesicles acting as a model membrane system. Our results reveal the existence of two critical AuNP diameters which determine their fate when in contact with a lipid membrane. The results provide new insights into the size dependent interaction between AuNPs and lipid bilayers which is of direct relevance to nanotoxicology and to the design of NP vectors.

Increased Intraband Transitions in Smaller Gold Nanorods Enhance Light Emission.

Photoinduced light emission from plasmonic nanoparticles has attracted considerable interest within the scientific community because of its potential applications in sensing, imaging, and nanothermometry. One of the suggested mechanisms for the light emission from plasmonic nanoparticles is the plasmon-enhanced radiative recombination of hot carriers through inter- and intraband transitions. Here, we investigate the nanoparticle size dependence on the photoluminescence through a systematic analysis of gold nanorods with similar aspect ratios. Using single-particle emission and scattering spectroscopy along with correlated scanning electron microscopy and electromagnetic simulations, we calculate the emission quantum yields and Purcell enhancement factors for individual gold nanorods. Our results show strong size-dependent quantum yields in gold nanorods, with higher quantum yields for smaller gold nanorods. Furthermore, by determining the relative contributions to the photoluminescence from inter- and intraband transitions, we deduce that the observed size dependence predominantly originates from the size dependence of intraband transitions. Specifically, within the framework of Fermi's golden rule for radiative recombination of excited charge carriers, we demonstrate that the Purcell factor enhancement alone cannot explain the emission size dependence and that changes in the transition matrix elements must also occur. Those changes are due to electric field confinement enhancing intraband transitions. These results provide vital insight into the intraband relaxation in metallic nanoconfined systems and therefore are of direct importance to the rapidly developing field of plasmonic photocatalysis.

Multi-scale simulations of polymeric nanoparticle aggregation during rapid solvent exchange.

Using a multi-scale approach which combines both molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations, we study a simple and scalable method for fabricating charge-stabilized nanoparticles through a rapid solvent exchange, i.e., Flash NanoPrecipitation (FNP). This multi-scale approach is based on microscopic information from MD simulations and uses a KMC algorithm to access macroscopic length- and time scales, which allows direct comparison with experiments and quantitative predictions. We find good agreement of our simulation results with the experiments. In addition, the model allows us to understand the aggregation mechanism on both microscopic and macroscopic levels and determine dependence of nanoparticle size on processing parameters such as the mixing rate and the polymer feed concentration. It also provides an estimate for the characteristic growth time of nanoparticles in the FNP process. Our results thus give useful insights into tailoring the FNP technique for fabricating nanoparticles with a specific set of desirable properties for various applications.

Molecular Orientations Change Reaction Kinetics and Mechanism: A Review on Catalytic Alcohol Oxidation in Gas Phase and Liquid Phase on Size-Controlled Pt Nanoparticles

Catalytic oxidation of alcohols is an essential process for energy conversion, production of fine chemicals and pharmaceutical intermediates. Although it has been broadly utilized in industry, the basic understanding for catalytic alcohol oxidations at a molecular level, especially under both gas and liquid phases, is still lacking. In this paper, we systematically summarized our work on catalytic alcohol oxidation over size-controlled Pt nanoparticles. The studied alcohols included methanol, ethanol, 1-propanol, 2-propanol, and 2-butanol. The turnover rates of different alcohols on Pt nanoparticles and also the apparent activation energy in gas and liquid phase reactions were compared. The Pt nanoparticle size dependence of reaction rates and product selectivity was also carefully examined. Water showed very distinct effects for gas and liquid phase alcohol oxidations, either as an inhibitor or as a promoter depending on alcohol type and reaction phase. A deep understanding of different alcohol molecular orientations on Pt surface in gas and liquid phase reactions was established using sum-frequency generation spectroscopy analysis for in situ alcohol oxidations, as well as density functional theory calculation. This approach can not only explain the entirely different behaviors of alcohol oxidations in gas and liquid phases, but can also provide guidance for future catalyst/process design.

(Invited) Studies of Li Insertion in Magnetite at Multiple Length Scales

Magnetite, Fe3O4 is a low-cost inverse-spinel material with high theoretical capacity, 926 mAh/g. Magnetite exhibits solid-state transport resistances, that have been alleviated through nanostructuring. However, the nanoparticles have been observed to form secondary structural motifs upon electrode fabrication. Physics-based modeling has revealed that resistances on both the nano (crystal) scale and on the scale of the secondary micron-sized structural motifs both contribute to the overpotential observed upon discharge.1,2 These resistances are found to be a function of cycle rate and also electrode fabrication methods, as synthetic methods have been utilized to eliminate the secondary structural motifs.3 Modeling the interaction of lithium and the Fe3O4 crystal lattice at the atomic scale has also been employed utilizing DFT, and it has been found that atomic scale effects have measurable performance implications at the device-scale. DFT calculations have revealed that the high potentials (~3.0 V) initially observed at low discharge concentrations of lithium into magnetite (x~1.0) cannot be due to Li insertion into the pristine Fe3O4 lattice, due to phase instability and poor voltage prediction agreement of the resultant intercalated structures, LixFe3O4. Instead, DFT calculations and pre-edge XANES experiments have indicated that there exist Fe cationic defects, the concentration of which occur as a function of nanoparticle size, where smaller particles are observed to have a higher volume fraction of defects.4 DFT has predicted that lithium insertion into these Fe cationic defects are responsible for the initial high intercalation voltages. The nanoparticle size dependence of cationic defects, leads to the prediction of a nanoparticle-size-dependent reversible potential, corroborated experimentally. Additionally, the experimental observation of increased capacity of smaller nanoparticles over larger nanoparticles can also be partly explained by the increased concentration of cationic defects in the smaller particles. Here, we show that modeling at the atomic scale gives insight into the high potentials occurring at low lithium concentrations and the nanoparticle size-dependence of the reversible potential. Coarse grained modeling at the nano and micron scales have identified the unexpected impact of the secondary micron sized motifs on the overpotentials. The use of modeling at atomic, nano, and micron scales has revealed that all three length scales have nontrivial and measurable implications at the device scale. Understanding of these complex nonequilibrium devices over multiple length and time scales and is a requirement if we are to realize the next generation of lithium-ion batteries. In addition to atomic scale modeling, continuum level modeling of batteries is an established method for interrogating performance. Previous efforts have focused on obtaining good model agreement with experimentally observed electrochemical measurements; where good agreement is usually used to validate the model and the concentration profiles predicted by the models are analyzed. Efforts have been made to directly interrogate the concentration profiles derived from battery operation, however, because these systems are highly dynamic even short time gaps between operation and characterization can result in significantly different profiles. Operando EDXRD characterization of battery electrodes has enabled us to directly compare the battery environment during operation to the results predicted from simulations. The final part of my talk will be about how to leverage techniques from the rapidly advancing field of computer science, specifically machine learning, to assist in development of physics-based modeling. The utility of physics-based models is well known, however, good models require significant time and expertise to develop, and while the results of the models are accessible to experimentalists, model development is practically restricted to those with the expertise. Using relatively simple and well-established computer science techniques such as sampling and cross-validating we believe we can significantly reduce the time necessary for model development and outsource parameter estimation and model selection to a computer program. (1) Knehr, K. W.; Brady, N. W.; Cama, C. A.; Bock, D. C.; Lin, Z.; Lininger, C. N.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; West, A. C., J. Electrochem. Soc. 2015, 162 (14), A2817–A2826. (2) Knehr, K. W.; Brady, N. W.; Lininger, C. N.; Cama, C. A.; Bock, D. C.; Lin, Z.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; West, A. C., ECS Trans. 2015, 69 (1), 7–19. (3) Bock, D. C.; Pelliccione, C. J.; Zhang, W.; Wang, J.; Knehr, K. W.; Wang, J.; Wang, F.; West, A. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. D ACS Appl. Mater. Interfaces 2016, 8 (18), 11418–11430. (4) Menard, M. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S., Electrochim. Acta 2013, 94 (42), 320–326.

Triblock copolymer-mediated synthesis of catalytically active gold nanostructures

The design of nanostructures based on poly(ethylene oxide)-poly(propylene)-poly(ethylene oxide) (PEO-PPO-PEO) and metal nanoparticles is becoming an important research topic due to their multiple functionalities in different fields, including nanomedicine and catalysis. In this work, water-soluble gold nanoparticles have been prepared through a green aqueous synthesis method using Pluronic F127 as both reducing and stabilizing agents. The size dependence (varying from 2 to 70 nm) and stability of gold nanoparticles were systematically studied by varying some parameters of synthesis, which were the polymer concentration, temperature, and exposure to UV-A light, being monitored by UV-Vis spectroscopy and TEM. Also, an elaborated study regarding to the kinetic of formation (nucleation and growth) was presented. Finally, the as-prepared Pluronic-capped gold nanoparticles have shown excellent catalytic activity towards the reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride, in which a higher catalytic performance was exhibited when compared with gold nanoparticles prepared by classical reduction method using sodium citrate.

An Experimental and First Principles Study of the Energetics of Lithium Insertion into Fe3O4 and γ-Fe2O3

Inverse spinel magnetite, Fe3O4, has been studied as an electrode material for lithium-ion batteries due to its low cost, high abundance, and low environmental impact.1 Experimental discharge profiles of Fe3O4 initially show a concentration dependent voltage profile. The lithium concentration range of that profile varies as a function of nanoparticle size, with smaller particles exhibiting a larger intercalation regime.2,3 Thackeray et al. proposed initial lithium intercalation into magnetite on 16c sites, where the lattice accommodates a critical concentration of lithium without significant structural rearrangements. Above a critical concentration of lithium, Thackeray et al. proposed that with further discharge, lithium insertion to 16c sites leads to Fe8a displacement to vacant 16c sites in the material.4 It is also known that magnetite is iron deficient, Fe3-δO4 in the bulk and in the nanoparticulate forms with vacancies on the octahedral 16d sites.5–7 The most drastic manifestation of this oxidation process is maghemite, with δ=1/3, the topotactic oxidative product written as (Fe1 3+)8a[Fe1.66 3+♦0.33]16dO4 with ♦ as a 16d iron defect, or γ-Fe2O3. Menard et al. showed that the defects in nanoparticulate magnetite increase with decreasing nanoparticle size, but a thorough investigation of how defects affect the electrochemistry is yet to be presented.2 Using DFT+U calculations, we have investigated lithium intercalation in Fe3O4 magnetite. Our results show that stoichiometric Fe3O4 is not an intercalation material, with no stable phases for the LixFe3O4 0<x<1 convex hull. Resultantly, a voltage plateau at approximately 2.0 V is predicted, corresponding to a continuous phase transformation from inverse spinel Fe3O4 to rock-salt LiFe3O4with Li and Fe occupying 16c sites. This rules out the original hypothesis for a critical concentration of lithium hosted on the 16c sites resulting in a concentration dependent voltage at low concentrations of lithium in magnetite. We have performed DFT+U calculations on lithiated Fe3-δO4 with δ=0,1/8, and 1/3 to understand how 16d iron defects affect the open circuit potential of magnetite and therefore the discharge behaviour. In the presence of 16d iron defects, we have found that even at low concentrations of lithium, insertion on vacant 16c sites causes significant Fe8a rearrangements. When lithium is inserted on 16c positions, its proximity to Fe8a induces Coulombic repulsion between the Li16c and the Fe8a, and the resultant material is thermodynamically unstable against phase segregation. On the other hand, when lithium is inserted on vacant 16d sites available in the δ=1/8 and 1/3 cases, a structurally stable intercalated material results and DFT+U predicted voltages align with experimental observation. We further hypothesize that the concentration of defects is a function of nanoparticle size. Then we are able to explain the experimentally observed size-dependent open circuit potential and the variation of discharge capacity with particle size in a natural way. Finally, with DFT+U calculations we are able to provide a mechanism for intercalation and phase-segregation that is consistent with the nanoparticle size dependence observed in the electrochemical experiments. This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673. This research used resources of the Center for Functional Nanomaterials, a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory, Contract No. DE-SC0012704. The Extreme Science and Engineering Discovery Environment (XSEDE) was used, supported by National Science Foundation grant number ACI-1548562. The Yeti Shared HPC Cluster at Columbia University was used, which includes support from Empire State Development's Division of Science, Technology, and Innovation, contract number C090171. C. N. L. acknowledges the support of the National Science Foundation Graduate Research Fellowship under Grant No. DGE-11-44155. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. (1) Ito, S.; Nakaoka, K.; Kawamura, M.; Ui, K.; Fujimoto, K.; Koura, N. J. Power Sources 2005, 146, 319–322. (2) Menard, M. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Electrochim. Acta 2013, 94, 320–326. (3) Menard, M. C.; Takeuchi, K. J.; Marschilok, C.; Takeuchi, E. S. 2013, 15, 18539–18548. (4) Thackeray, M. M.; David, W. I. F.; Goodenough, J. B. Mater. Res. Bull. 1982, 17, 785–793. (5) Daniels, J. M.; Rosencwaig, a. J. Phys. Chem. Solids 1969, 30, 1561–1571. (6) Dieckmann, R. Berichte der Bunsen-Gesellschaft fur Phys. Chemie 1982, 86, 112–118. (7) Johnson, C. E.; Johnson, J. A.; Hah, H. Y.; Cole, M.; Gray, S.; Kolesnichenko, V.; Kucheryavy, P.; Goloverda, G. Hyperfine Interact. 2016, 237, 1–10.

Size-dependent cohesive energy, melting temperature, and Debye temperature of spherical metallic nanoparticles

It is necessary to theoretically evaluate the thermodynamic properties of metallic nanoparticles due to the lack of experimental data. Considering the surface effects and crystal structures, a simple theoretical model is developed to study the size dependence of thermodynamic properties of spherical metallic nanoparticles. Based on the model, we have considered Co and Cu nanoparticles for the study of size dependence of cohesive energy, Au and Cu nanoparticles for size dependence of melting temperature, and Cu, Co and Au nanoparticles for size dependence of Debye temperature, respectively. The results show that the size effects on melting temperature, cohesive energy and Debye temperature of the spherical metallic nanoparticles are predominant in the sizes ranging from about 3 nm to 20 nm. The present theoretical predictions are in agreement with available corresponding experimental and computer simulation results for the spherical metallic nanoparticles. The model could be used to determine the thermodynamic properties of other metallic nanoparticles to some extent.

Atomic simulations for configurations and solid-liquid interface of Li-Fe and Li-Cu icosahedra

The melting point of Li is lower than that of Fe (or Cu); thus, solid-liquid interfaces can be easily formed on Li-Fe and Li-Cu nanoalloys. In this work, the configurations and solid-liquid interfaces of Li-Fe and Li-Cu icosahedra are studied using Monte Carlo and molecular dynamics methods. The atomic interactions are described by the analytic embedded-atom method. The dependence of composition, temperature, and nanoparticle size on the configurations and thermal stabilities of nanoalloys is discussed. The behavior of the Li-Fe and Li-Cu nanoalloys in segregation, configuration, and thermal stability is investigated. A different behavior of surface segregation of Li atoms is observed for the two types of nanoalloys. The interface between the Li and Fe atoms is clear. Mixing of Li with Cu at larger nanoparticle sizes is found because of low heat of formation in the system. The configurations of the Li-Fe and Li-Cu nanoalloys are related to the competition between surface segregation and alloying. The thermal stability of Li in the two types of nanoalloys is enhanced by the support of the Fe (or Cu) solid substrate.

Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction

New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying.