Fraction Of Surface Atoms Research Articles

Direct formation of copper nanoparticles from atoms at graphitic step edges lowers overpotential and improves selectivity of electrocatalytic CO2 reduction

A key strategy for minimizing our reliance on precious metals is to increase the fraction of surface atoms and improve the metal-support interface. In this work, we employ a solvent/ligand/counterion-free method to deposit copper in the atomic form directly onto a nanotextured surface of graphitized carbon nanofibers (GNFs). Our results demonstrate that under these conditions, copper atoms coalesce into nanoparticles securely anchored to the graphitic step edges, limiting their growth to 2–5 nm. The resultant hybrid Cu/GNF material displays high selectivity in the CO2 reduction reaction (CO2RR) for formate production with a faradaic efficiency of ~94% at -0.38 V vs RHE and a high turnover frequency of 2.78 × 106 h-1. The Cu nanoparticles adhered to the graphitic step edges significantly enhance electron transfer to CO2. Long-term CO2RR tests coupled with atomic-scale elucidation of changes in Cu/GNF reveal nanoparticles coarsening, and a simultaneous increase in the fraction of single Cu atoms. These changes in the catalyst structure make the onset of the CO2 reduction potential more negative, leading to less formate production at -0.38 V vs RHE, correlating with a less efficient competition of CO2 with H2O for adsorption on single Cu atoms on the graphitic surfaces, revealed by density functional theory calculations.

Block Copolymer-Templated, Single-Step Synthesis of Transition Metal Oxide Nanostructures for Sensing Applications.

The synthesis of transition metal oxide nanostructures, thanks to their high surface-to-volume ratio and the resulting large fraction of surface atoms with high catalytic activity, is of prime importance for the development of new sensors and catalytic materials. Here, we report an economical, time-efficient, and easily scalable method of fabricating nanowires composed of vanadium, chromium, manganese, iron, and cobalt oxides by employing simultaneous block copolymer (BCP) self-assembly and selective sequestration of metal-organic acetylacetonate complexes within one of the BCP blocks. We discuss the mechanism and the primary factors that are responsible for the sequestration and conformal replication of the BCP template by the inorganic material that is obtained after the polymer template is removed. X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (PXRD) studies indicate that the metal oxidation state in the nanowires produced by plasma ashing the BCP template closely matches that of the precursor complex and that their structure is amorphous, thus requiring high-temperature annealing in order to sinter them into a crystalline form. Finally, we demonstrate how the developed nanowire array fabrication scheme can be used to rapidly pattern a multilayered iron oxide nanomesh, which we then used to construct a prototype volatile organic compound sensor.

Nanothermodynamics on the Example of Metallic Nanoparticles

After analyzing the problem of extending the Gibbs surface excess method to nanoscale objects, two different approaches to the application of the Gibbs method for finding the specific surface energy of metal nanoparticles are being considered. The first approach involves the use of the local coordination approximation to estimate the specific surface energy of icosahedral FCC metal nanoparticles (magic nanoclusters). For the first time, we have drawn attention to the fact that for such a nanocluster, it is possible to accurately calculate both the fraction of surface atoms and the values of the first coordination number in the inner region of the nanoparticle and on its surface (faces, edges, and vertices). The second approach implemented by us earlier for spherical Au nanoparticles and here for FCC Pt nanoparticles, involves the complex application of the Gibbs method adapted for nanoparticles and the results of molecular dynamics simulation. Estimates using both approaches agree with the experimental values of the surface energy corresponding to the flat surface of the bulk phases of the corresponding metals. In the final section of the work, the limits of applicability of thermodynamics to nanosystems are discussed.

Manganese Oxide Nanoparticles: An Insight into Structure, Synthesis and Applications

AbstractManganese oxide nanoparticles with different crystal phases, morphologies, and structural diversity along with their exceptional properties like high specific surface area, a high fraction of surface atoms, non‐toxic nature, and excellent redox properties are drawing attention for various applications in storage science, especially in the batteries, super‐capacitors, energy conversion, and the environmental catalysis field. Precise control of particle size, morphology, surface area, Mnx+ oxidation state, etc. is the utmost important aspect to explore their application to the full potential. Here, the emphasis is on the recent trends in manganese oxide research – structure, synthesis, and applications. The structure of numerous crystalline phases of manganese oxide nanoparticles are summerized and several facile chemical synthesis processes to achieve the desired crystalline/amorphous structure are discoursed. Temperature and different synthesis conditions dependent phase transformations of β‐MnO2, α‐Mn2O3, and Mn3O4 from α‐MnO2 are discussed as well. The pragmatic approach directs that the application field is mostly controlled by the morphologies.

LaCo1–xFexO3 Nanoparticles in Cyclohexene Oxidation

The identification of structure–activity relationships is challenging for complex oxides like lanthanum-based perovskite nanoparticles. The purpose of this study is to examine the structure of LaCo1–xFexO3 nanoparticles and develop a detailed structural model to identify subtle changes of the structure after catalysis. Therefore, small nanoparticles with a significant fraction of surface atoms and varying iron content (x = 0, 0.5, 1) are synthesized in the gas phase by chemical vapor synthesis and tested in cyclohexene oxidation in the liquid phase. The crystal structure is examined by X-ray and selected area electron diffraction. Additionally, the local structure of the cations is probed by X-ray absorption spectroscopy at the Co, Fe, La K, and La L3-edges. For a quantitative analysis of the local structure before and after catalysis, an atomistic model is refined by the available extended X-ray absorption fine structure spectra using Reverse Monte Carlo methods. The produced nanoparticles are small with a coherent diffraction domain size between 5 and 10 nm, highly crystalline, and consist mainly of the perovskite phase with a secondary spinel phase. In cyclohexene oxidation, the cobalt-containing samples exhibit significant catalytic activities. The active samples show structural changes after catalysis accompanied by reconstruction of the local structure surrounding the cations in the perovskite phase, which resembles edge-sharing cobalt octahedra.

Electrochemical Sensing of Favipiravir with an Innovative Water-Dispersible Molecularly Imprinted Polymer Based on the Bimetallic Metal-Organic Framework: Comparison of Morphological Effects.

Molecularly imprinted polymers (MIPs) are widely used as modifiers in electrochemical sensors due to their high sensitivity and promise of inexpensive mass manufacturing. Here, we propose and demonstrate a novel MIP-sensor that can measure the electrochemical activity of favipiravir (FAV) as an antiviral drug, thereby enabling quantification of the concentration of FAV in biological and river water samples and in real-time. MOF nanoparticles’ application with various shapes to determine FAV at nanomolar concentrations was described. Two different MOF nanoparticle shapes (dodecahedron and sheets) were systematically compared to evaluate the electrochemical performance of FAV. After carefully examining two different morphologies of MIP-Co-Ni@MOF, the nanosheet form showed a higher performance and efficiency than the nanododecahedron. When MIP-Co/Ni@MOF-based and NIP-Co/Ni@MOF electrodes (nanosheets) were used instead, the minimum target concentrations detected were 7.5 × 10−11 (MIP-Co-Ni@MOF) and 8.17 × 10−9 M (NIP-Co-Ni@MOF), respectively. This is a significant improvement (>102), which is assigned to the large active surface area and high fraction of surface atoms, increasing the amount of greater analyte adsorption during binding. Therefore, water-dispersible MIP-Co-Ni@MOF nanosheets were successfully applied for trace-level determination of FAV in biological and water samples. Our findings seem to provide useful guidance in the molecularly imprinted polymer design of MOF-based materials to help establish quantitative rules in designing MOF-based sensors for point of care (POC) systems.

Temperature-dependent magnetism in Fe foams via spin-lattice dynamics

Spin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature dependence of the magnetization in foams is determined as a function of the fraction of surface atoms in foams, nsurf. The Curie temperature of foams decreases approximately linearly with nsurf, while the critical exponent of the magnetization increases considerably more strongly. If the data are plotted as a function of the fraction of surface atoms, reasonable agreement with recent data on vacancy-filled Fe crystals and novel data on void-filled crystals is observed for the critical temperature. Critical temperature and critical exponent also depend on the coordination of surface atoms. Although the decrease we find is relatively small, it hints to the possibility of improved usage of topology to taylor magnetic properties.

Fraction of Surface Atoms in the Nanoparticles and Critical Nuclei of a New Phase

Fraction of Surface Atoms in the Nanoparticles and Critical Nuclei of a New Phase

Optical Imaging of Redox and Molecular Diffusion in 2D van der Waals Space.

Understanding charge transfer (CT) between two chemical entities and the subsequent change in their charge densities is essential not only for molecular species but also for various low-dimensional materials. Because of their extremely high fraction of surface atoms, two-dimensional (2-D) materials are most susceptible to charge exchange and exhibit drastically different physicochemical properties depending on their charge density. In this regard, spontaneous and uncontrollable ionization of graphene in the ambient air has caused much confusion and technical difficulty in achieving experimental reproducibility since its first report in 2004. Moreover, the same ambient hole doping was soon observed in 2-D semiconductors, which implied that a common mechanism should be operative and apply to other low-dimensional materials universally. Notably, a similar CT reaction has long been known for carbon nanotubes but is still controversial in its mechanism.In this Account, we review our breakthroughs in unraveling the chemical origin and mechanistic requirements of the hidden CT reactions using 2-D crystals. As a first step, we have developed in situ optical methods to quantify charge density using Raman and photoluminescence (PL) spectroscopy and imaging. To overcome the multimodal sensitivity of Raman frequencies, we established a novel analytical method based on theory and experiments with excellent resolution for the charge density (∼1 × 1012 cm-2) and lattice strain (∼0.02%) of graphene. For 2-D transition-metal dichalcogenides, PL spectroscopy and imaging provided a high precision and sensitivity that enabled rapid kinetic measurements in a spatially resolved manner.Using gas- and temperature-controlled in situ measurements, we revealed that the electrical holes are injected by the oxygen reduction reaction (ORR) O2 + 4H+ + 4e- ⇄ 2H2O, which was independently verified by the pH dependence in HCl solutions. In addition to oxygen and water vapor, the overall CT reaction requires hydrophilic dielectric substrates, which assist the hydration of the sample-substrate interface. We also found that the CT reaction is substantially enhanced when samples are thermally annealed. The amplification is due to the interfacial hydrophilicity increased by the thermal hydroxylation of substrates, which indicates that the CT reaction is localized at the interface and boosted by interfacial water.The interface-localized CT allowed us to study and control molecular diffusion through the 2-D van der Waals space between samples and substrates. Wide-field PL imaging showed how fast oxygen molecules diffuse through the interfacial space, subsequently inducing the CT reaction. By increasing the 2-D gap spacing, the diffusion kinetics could be accelerated. The rate of CT could also be enhanced by introducing defects on the basal plane of 2-D crystals, which demonstrates the decisive role of defects as CT centers.Because of their unique geometry, low-dimensional materials are highly susceptible to external perturbation including charge exchange. Because the vulnerability can be exploited to modify material properties, the complete mechanism of the fundamental charge exchange summarized in this Account will be essential to exploring material and device properties of other low-dimensional materials.

Uncompensated magnetic moment and surface and size effects in few-nanometer antiferromagnetic NiO particles

-The analysis of the M(H) magnetization curves of antiferromagnetic nanoparticles yields information about magnetic subsystems formed in these objects, which are characterized by a large fraction of surface atoms. However, in the conventionally investigated experimental magnetic field range of up to 60–90 kOe, this analysis often faces the ambiguity of distinguishing the Langevin function-simulated contribution of uncompensated magnetic moments μun of particles against the background of a linear-in-field dependence (the antiferromagnetic susceptibility and other contributions). Here, this problem has been solved using a pulsed technique, which makes it possible to significantly broaden the range of external fields in which the μun contribution approaches the saturation. Nanoparticles of a typical NiO antiferromagnet with an average size of <d> ~ 4.5 nm have been investigated. Based on the thorough examination of the M(H) magnetization curves measured in pulsed fields of up to 250 kOe, a model of the magnetic state of NiO nanoparticles of such a small size has been proposed. The average moment is ~130 μB (μB is the Bohr magneton) per particle, which corresponds to 60–70 decompensated spins of nickel atoms localized, according to the Néel hypothesis (μun~<d>3/2), both on the surface and in the bulk of a particle. A part of the surface spins unrelated to the antiferromagnetic core form another subsystem, which behaves as free paramagnetic atoms. Along with the antiferromagnetic core, an additional linear-in-field contribution has been detected, which is apparently related to superantiferromagnetism, i.e., the size effect inherent to small antiferromagnetic particles.

Identifying the Accuracy of Various Approaches for Determining the Fraction of Surface Atoms in a Nanoparticle to Deepen Students’ Understanding of Size-Dependent Properties

As the size of a nanoparticle is reduced, the fraction of surface atoms increases. It is interesting to note that various chemical and physical properties depend on the fraction of surface atoms. One of the most obvious questions that arise is how could one possibly estimate the fraction of surface atoms in a nanoparticle? Herein, we discuss various approaches for the same and attempt to figure out an accurate approach. We believe that the analysis presented here would benefit the undergraduate and graduate students who have taken nanomaterials courses, with a better understanding.

Modeling of size and shape dependent band gap, dielectric constant and phonon frequency of semiconductor nanosolids

A bond theory model is extended to study the size and shape dependent optoelectronics properties of semiconductors solids at nanoscale. On structural miniaturization down to nano scale, the optical parameters no longer remain stable but become tunable. The fraction of surface atoms and the dangling bonds on the surface affects the properties of semiconductors at nanoscale. The theory is applied to study the size and shape dependent energy band gap, dielectric constant and phonon frequency of TiO2, CdS, CdSe, Si and GaN semiconductor nanosolids. We incorporated the relaxation factor, defined as the ratio of dangling bonds and the total bonds of atoms at nano scale. It is predicted that as the energy band increases with decrease in size, the effect becomes more when shape changes from spherical to tetrahedral. The model projects a decrease in phonon frequency and dielectric constants of semiconductor nanostructured materials with decrease in particle size. A good agreement between predicted results and the available experimental data is projected.

Investigation on mechanical behaviors of Cu-Ni binary alloy nanopillars: a molecular dynamics study.

Cu-Ni binary alloy has become the attention of scientific world for its potentials in nanodevices. It is indispensable to investigate on the mechanical properties of this material due to lack of previous work done regarding this binary alloy. Molecular dynamics (MD) studies were performed on nanopillar (NP) structures comprised of Cu-Ni binary alloy having an FCC unit cell with Cu atoms selectively replaced by Ni atoms. This selective replacement resulted in a better stress behavior than the randomly replaced alloy structure when both tension and compression load were applied. The effect of crystal orientation, NP dimensions, temperature, and strain rate on the stress-strain curve of Cu-Ni binary alloy NPs was thoroughly investigated under tensile loading. This investigation reveals significant influence of crystal orientation on ultimate strength and flow stress region. Among four different crystal orientations, <111> orientation shows maximum strength behavior under tensile loading, while <110> shows highest strength under compressive load. However, in both cases, i.e. tension and compression, the poorest stress behavior was observed for <001> orientation. Under tensile load, <111>-oriented binary alloy fails due to the formation of Shockley partials followed by formation of complex dislocation network. On the other hand, <110>-oriented binary alloy fails due to the formation of Lomer-Cottrell (LC) lock from the Shockley partials. Total dislocation length is calculated, and its effect on the stress-strain behavior of the Cu-Ni binary alloy is discussed. Highest Young's modulus and yield stress are observed on <111>-oriented binary alloy among other orientations, and these values for <111>-oriented NP was found to decrease with the increment of temperature. If the temperature is increased, yield stress and Young's modulus decrease. The effect of cross section width was also investigated in this study, and it was found that yield stress decreases with the increment of cross section width due to the effect of surface atom fraction. Increasing the strain rate causes the initiation of amorphous structure, resulting in superplastic behavior of the <111>-oriented Cu-Ni binary alloy NP.

Magnetic Nanoparticles for Biomedical Purposes: Modern Trends and Prospects

The presented paper is a review article discussing existing synthesis methods and different applications of nanosized magnetic nanoparticles. It was shown that, in addition to the spectrum of properties typical for nanomaterials (primarily a large specific surface area and a high fraction of surface atoms), magnetic nanoparticles also possess superparamagnetic properties that contribute to their formation of an important class of biomedical functional nanomaterials. This primarily concerns iron oxides magnetite and maghemite, for which in vitro and in vivo studies have shown low toxicity and high biocompatibility in comparison with other magnetic nanomaterials. Due to their exceptional chemical, biological, and physical properties, they are widely used in various areas, such as magnetic hyperthermia, targeted drug delivery, tissue engineering, magnetic separation of biological objects (cells, bacteria, viruses, DNA, and proteins), and magnetic diagnostics (they are used as agents for MRS and immunoassay). In addition to discussing the main problems and prospects of using nanoparticles of magnetic iron oxides for advanced biomedical applications, information is also reflected on their structure, production methods, and properties.

Size effects in the formation of an uncompensated ferromagnetic moment in NiO nanoparticles

The magnetic properties of samples of NiO nanoparticles with average sizes of 23, 8.5, and 4.5 nm were investigated. Using the magnetization curves measured in strong (up to 250 kOe) pulsed magnetic fields, the contributions of the free spin and ferromagnetic subsystems were extracted. It has been found that the ferromagnetic contribution increases with a decrease in the nanoparticle size and is proportional to the fraction of uncompensated exchange-coupled spins. It is demonstrated that the uncompensated spins form in the antiferromagnetic NiO oxide due to an increase in the fraction of surface atoms in the nanoparticles with decreasing particle size and defects in the bulk of particles.

Non-noble metal thickness-tunable Bi2MoO6 nanosheets for highly efficient visible-light-driven nitrobenzene reduction into aniline

High efficiency and selectivity is significant for photocatalytic organic transformation. In this work, the efficient photocatalytic reduction of nitrobenzene into aniline is achieved on non-noble metal thickness-tunable bismuth molybdate nanosheets prepared by the simple hydrothermal reaction. The photocatalytic activity of Bi2MoO6 strongly depends on the nanosheet thickness. The Bi2MoO6 monolayer displays remarkably enhanced photocatalytic activity for selective reduction of nitrobenzene than bilayer, multilayers and bulk Bi2MoO6 due to the unique properties of 2D materials with the large fraction of surface atoms and the ultrafast charge separation. The obtained conversion rate of 487.5 μmol g−1. h-1 (>95% conversion and >99% selectivity within 60 min reaction) on Bi2MoO6 monolayers is 5 times higher than that of Bi2MoO6 bulk, far exceeding most of common photocatalysts up to date. This work provides a thickness dependent nanosheet concept in the design of newly highly efficient catalysts for the photocatalytic reduction conversion of nitroarenes to anilines.

High temperature ferromagnetism in NbCo2 nanoparticles – Bulk magnetization and hyperfine field measurements

Abstract We report the observation of high temperature ferromagnetism in nanoparticles of NbCo2, prepared by high speed ball milling. The magnetic properties of the bulk and noncrystalline samples have been studied through measurements of bulk magnetization and hyperfine field of 100Rh, implanted into the sample. Compared to the low, temperature independent magnetic susceptibility ( χ ∼ 10 - 5 emu/gm) observed for bulk NbCo2, the nanoparticle samples exhibit very high value of magnetic susceptibility ( χ ∼ 10 - 3 emu/gm), whose temperature dependence indicates possible ferromagnetic ordering near or above 300 K. The saturation magnetic moment obtained from isothermal magnetization (M vs H) data, however, turns out to be quite small (0.10 μ B ). On the other hand, hyperfine field measurements performed for 100Rh probe using time differential perturbed angular correlation technique indicate strong paramagnetic behavior of nano-NbCo2. We believe that ferromagnetism observed in nano-NbCo2 most probably arise from a small fraction of surface atoms, while the bulk of the material remains paramagnetic.

Formation of the magnetic subsystems in antiferromagnetic NiO nanoparticles using the data of magnetic measurements in fields up to 250 kOe

Abstract It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μun, paramagnetic subsystem, and antiferromagnetic particle core. The μun value obtained for the investigated NiO particles is consistent with the Neel model, in which μun ∼ N1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.

Chemically reversible isomerization of inorganic clusters.

Structural transformations in molecules and solids have generally been studied in isolation, whereas intermediate systems have eluded characterization. We show that a pair of cadmium sulfide (CdS) cluster isomers provides an advantageous experimental platform to study isomerization in well-defined, atomically precise systems. The clusters coherently interconvert over an ~1-electron volt energy barrier with a 140-milli-electron volt shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand-binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines "phase" stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations and bridges these disparate length scales.

Rapid and reversible lithiation of doped biogenous iron oxide nanoparticles

Certain bacteria produce iron oxide material assembled with nanoparticles (NPs) that are doped with silicon (Fe:Si ~ 3:1) in ambient environment. Such biogenous iron oxides (BIOX) proved to be an excellent electrode material for lithium-ion batteries, but underlying atomistic mechanisms remain elusive. Here, quantum molecular dynamics simulations, combined with biomimetic synthesis and characterization, show rapid charging and discharging of NP within 100 fs, with associated surface lithiation and delithiation, respectively. The rapid electric response of NP is due to the large fraction of surface atoms. Furthermore, this study reveals an essential role of Si-doping, which reduces the strength of Li-O bonds, thereby achieving more gentle and reversible lithiation culminating in enhanced cyclability of batteries. Combined with recent developments in bio-doping technologies, such fundamental understanding may lead to energy-efficient and environment-friendly synthesis of a wide variety of doped BIOX materials with customized properties.