Accumulation Of Atoms Research Articles

In Situ Kinetic Observations on Crystal Nucleation and Growth.

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Friction dependence on the textured structure of an amorphous carbon surface: A reactive molecular dynamics study

Amorphous carbon (a-C) film provides ultralow friction when combined with lubricating oil. However, it is unclear how textured surfaces affect the friction behavior of a-C at atomic scale, specifically the transformation of interfacial structure and the flow of lubricating oil, owing to the limitations of experimental characterization. Here, a-C films with different textured surfaces were fabricated, and the friction behaviors of mating a-C surfaces with textured parameters, including nanoscale depth and width, were examined by reactive molecular dynamics simulation. Results indicate that compared to that under dry condition, the introduction of a textured surface reduces the friction coefficient of a-C under oil lubrication condition. However, its efficiency highly depends on the depth and width of textured surface, exhibiting different effects on hydrodynamic lubrication. The increase in textured depth causes the friction coefficient decreased first and then increased, attributing to the lubrication from hydrodynamic to boundary state as primary mechanism. The regional aggregation of lubricant atoms with a textured width on the a-C surface occurs at the friction interface following the increased friction coefficient. These results provide insights into the underlying friction mechanism of a-C films induced by surface texturing and enable the development of highly efficient lubrication systems for applications.

In situ Observation of Structural Evolution and Phase Engineering of Amorphous Materials during Crystal Nucleation.

The nucleation pathway determines the structures and thus properties of formed nanomaterials, which is governed by the free energy of the intermediate phase during nucleation. The amorphous structure, as one of the intermediate phases during nucleation, plays an important role in modulating the nucleation pathway. However, the process and mechanism of crystal nucleation from amorphous structures still need to be fully investigated. Here, in situ aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is employed to conduct real-time imaging of the nucleation of ultrathin amorphous nanosheets (NSs). The results indicate that their nucleation contains three distinct stages, i.e., aggregation of atoms, crystallization to form lattice-expanded nanocrystals, and relaxation of the lattice-expanded nanocrystals to form final nanocrystals. In particular, the crystallization processes of various amorphous materials are investigated systematically to form corresponding nanocrystals with unconventional crystalline phases, including face-centered-cubic (fcc) Ru, hexagonal-close-packed (hcp) Rh, and a new intermetallic IrCo alloy. In situ electron energy-loss spectroscopy (EELS) analysis unveils that the doped carbon in the original amorphous NSs can migrate to the surface during the nucleation process, stabilizing the obtained unconventional crystal phases transformed from the amorphous structures, which is also proven by density functional theory (DFT) calculations.

Multicomponent Copper-Zinc Alloy Layer Enabling Ultra-Stable Zinc Metal Anode of Aqueous Zn-ion Battery.

Constructing stable surface modification layer is an effective strategy to suppress dendrite growth and side reactions of Zinc (Zn) metal anode in aqueous Zn-ion battery. Herein, a multicomponent Cu-Zn alloy interlayer with superior Zn affinity, high toughness and effective inhibition effect on lattice distortion is constructed on Zn foil (Cu-Zn@Zn) to fabricate ultra-stable Zn metal anode. Owning to the advantages of high binding energy of Cu-Zn alloy layer with Zn atoms and less contact area between metallic Zn and electrolyte, the as-prepared Cu-Zn@Zn electrode not only restricts the aggregation of Zn atoms, but also suppresses the pernicious hydrogen evolution and corrosion, leading to homogeneous Zn deposition and outstanding electrochemical performances. Accordingly, the symmetric battery with Cu-Zn@Zn electrode exhibits an ultra-long cycle life of 5496 h at 1 mA cm-2 for 1 mAh cm-2 , and the Cu-Zn@Zn//V2 O5 pouch cell demonstrates excellent cycling stability with a capacity retention of 88 % after 600 cycles.

Multicomponent Copper‐Zinc Alloy Layer Enabling Ultra‐Stable Zinc Metal Anode of Aqueous Zn‐ion Battery

AbstractConstructing stable surface modification layer is an effective strategy to suppress dendrite growth and side reactions of Zinc (Zn) metal anode in aqueous Zn‐ion battery. Herein, a multicomponent Cu−Zn alloy interlayer with superior Zn affinity, high toughness and effective inhibition effect on lattice distortion is constructed on Zn foil (Cu−Zn@Zn) to fabricate ultra‐stable Zn metal anode. Owning to the advantages of high binding energy of Cu−Zn alloy layer with Zn atoms and less contact area between metallic Zn and electrolyte, the as‐prepared Cu−Zn@Zn electrode not only restricts the aggregation of Zn atoms, but also suppresses the pernicious hydrogen evolution and corrosion, leading to homogeneous Zn deposition and outstanding electrochemical performances. Accordingly, the symmetric battery with Cu−Zn@Zn electrode exhibits an ultra‐long cycle life of 5496 h at 1 mA cm−2 for 1 mAh cm−2, and the Cu−Zn@Zn//V2O5 pouch cell demonstrates excellent cycling stability with a capacity retention of 88 % after 600 cycles.

Electrostatic Spinning Strategy to Prepare Cage-like PAN-Fiber Network-Wrapped Co–N–C Structures for the Oxygen Reduction Reaction

Zeolitic imidazolate framework (ZIF)-derived carbon materials are highly desirable cathode catalysts for the oxygen reduction reaction (ORR). Unfortunately, ZIF-derived materials undergo microscale migration, structural collapse, and aggregation of Co atoms during high-temperature pyrolysis. Here, we used an electrospinning technique to combine polyacrylonitrile (PAN) with carbonized ZIF-67 that can obtain cage-like nanocomposite catalysts (Co–N–C@PAN) by repyrolysis. It is found that the obtained composite Co–N–C@PAN has a hierarchical structure, and a high surface area and pore space with a volume is favorable for the adequate disclosure of ORR active sites. More importantly, this cage-like structure forms a stable Co–N–C structure and allows Co nanoparticles to be uniformly found in the Co–N–C substrate, which improves the active site of Co–NX. The as-prepared samples exhibit superior performance for the ORR with an onset potential (Eonset) of 1.05 V versus the reversible hydrogen electrode (RHE) and a half-wave potential (E1/2) of 0.93 V versus the RHE, both of which are higher than commercial Pt/C. In addition, Co–N–C@PAN was applied as a cathode for zinc–air batteries and display a superior power density (132 mW cm–2) at a discharge current density of 219 mA cm–2. Furthermore, at a constant discharge current density of 10 mA cm–2, a specific capacity of 761 mA h gZn–1 was obtained. This study may provide an idea for the design and synthesis of nanostructures prepared by electrostatic spinning/ZIF composites.

Philosophical Theories of Colour in Ancient Greek Thought – and Their Relevance Today

Our modern scientific explanation of colour as a subjective impression has replaced a ‘pre-theoretical’ notion of colour as an intrinsic property of objects, which was mainstream in ancient thought. Why have we lost such pre-theoretical notion, and what have we lost by losing it? I argue that most ancient Greek philosophers exploited this pre-theoretical assumption – one that was obvious to them – in terms and ways that are still worthy of attention in the context of contemporary philosophy of colour. I offer an in-depth analysis of the theories of a number of early Greek philosophers, as well as of Plato in the Timaeus, showing that they tend to share the idea that colour is an essential property of external bodies and therefore plays a causal role in perception. Then I focus on a comparison between Democritus’ stance and Aristotle’s in order to highlight a significant contrast between the two. Democritus' theory is an exception in ancient thought because it traces the colour phenomenon back to a refraction of atmospheric light through atomic aggregates. Aristotle, instead, posits the presence of colour and light in the bodies themselves. Finally, I interpret Aristotle's account as the most complex attempt in ancient thought to weld together what we call the ‘manifest image’ of colour and its ‘scientific image’.

Superatom Generation and Deposition of Alkali-like Ta@Si16 and Halogen-like Al13 via Atomic Aggregation

Surface modification with uniform nanostructures is a promising way to fabricate functionalized surface, and the synthetic electrode functionalization can be performed by depositing atomically precise nanoclusters with a specific number of atoms and compositions. During atomic nucleation and growth in gas phase, atomic aggregates exhibit novel electronic properties, mimicking atoms at a specific number of atoms and composition, which is called superatom (SA). Tantalum-encapsulating Si16 cage nanoclusters (Ta@Si16) and aluminum 13-mer (Al13) are promising SAs for designing their assembled materials with tunable properties, where one-electron-excessive Ta@Si16 and one-electron-deficient Al13 exhibit alkali- and halogen-like characteristics, respectively, owing to the electron shell closing. In this study, with an intense nanocluster ion source, Ta@Si16 + and Al13 − were generated during the formation of TaSi n + cations and Al n − anions, where Ta@Si16 + was remarkably generated compared with the neighboring species. SA deposition provided uniformly decorated surfaces exhibiting nanofunctional properties superior to those obtained by conventional atom deposition. With mass spectrometry, Ta@Si16 + and Al13 − SAs were size-selectively soft-landed on the substrate surfaces predeposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66). The electronic states of Ta@Si16 and Al13 SAs immobilized on the organic substrates were characterized by X-ray photoelectron spectroscopy and oxidative reactivities under O2 exposure.

Pronounced inhibitory mechanism of Cr on vacancy-like defects in Fe9Cr alloy irradiation with helium ions

• First-principles calculations combined with positron annihilation spectroscopy. • The He atoms aggregate among (1 1 0) planes in the perfect Fe and Fe9Cr models. • Fe atoms around the He clusters are inclined to emit spontaneously. • The doping of Cr repels the He atoms and weakens the growth of He clusters. First-principles calculations and positron annihilation spectroscopy were used to investigate the effect of Cr atoms on He-induced vacancy-like defects in pure Fe and Fe9Cr alloys. The He atom is not energetically favorable at both the surface and the subsurface, but it becomes stable under the third atomic layer from the surface. The diffusion of He atoms to the vacuum and aggregation of He atoms into clusters coexist. We observed that the interstitial He n clusters prefer to stay along the (1 1 0) plane. Moreover, the small He n clusters induce strong lattice distortions, and the emission of a self-interstitial Fe atom located at the adjacent He n cluster is energetically favorable for n > 2 and vacancy formation energies of Fe9Cr are higher than that of Fe. This suggests that Cr atoms could effectively enhance the stability of Fe atoms around He n clusters but that the Fe atoms would still like to shift from the vicinity of He n to the surfaces of the models. In the experiment, Fe and Fe9Cr alloys were irradiated with He ions to a dose of 1 × 10 16 cm 2 and the positron annihilation results showed that the Fe9Cr alloy has a smaller relative S (ΔS) parameter value than pure Fe. The experimental results matched well with the theoretical analysis. These results would help provide additional insights into the behavior of He in irradiated Fe and Fe9Cr surfaces, necessary for the interpretation of the growth of the He bubble.

Isolation anchoring strategy for in situ synthesis of iron single-atom catalysts towards long-term rechargeable zinc-air battery

The reasonable design and construction of single-atom electrocatalysts with low-cost and excellent intrinsic activity for oxygen reduction reaction (ORR) is indispensable in metal-air batteries. Herein, a facile isolation anchoring strategy was reported for in situ formation of porous N-doped carbon catalysts (Fe1/NC) rich in Fe single atoms. The d-glucose, zinc gluconate and N-rich histidine were deployed as spatial isolation agents and anchoring agents to inhibit iron atom aggregation during the high-temperature pyrolysis. The as-synthesized Fe1/NC catalyst shows remarkable stability and superior ORR electrocatalytic activity under both alkaline and acidic conditions owing to highly dispersed Fe–N4 sites, especially with a half-wave potential up to 0.91 V in 0.1 M KOH, exceeding 60 mV compared with Pt/C. Notably, the Fe1/NC-based Zn-air battery exhibits maximal power density of 164 mW cm−2, large specific capacities of 769 mA h gZn−1, excellent cycle stability over 300 h, and great potential in renewable energy conversion electronics.

Coordination polymer-derived non-precious metal catalyst for propane dehydrogenation: Highly dispersed zinc anchored on N-doped carbon

Highly dispersed non-precious metal catalysts supported on carbon have been intensively explored in the field of heterogeneous catalysis and material science owing to its low price, special electronic and geometric structure. However, their application and improvement of catalytic efficiency in alkane dehydrogenation were chronically limited due to spontaneous aggregation of metal atoms under the harsh conditions, low loading content and the complex preparation process. A new type of lamellar coordination polymer and a series of well-dispersed zinc-based catalyst anchored on N-doped carbon nanosheets were prepared via a facile combination of self-assembly and controlled pyrolysis. These carbonized composites showed superior initial activity (the propane conversion at 34% and the olefin selectivity at 80%) and acceptable catalytic stability (the deactivation rate of 0.082 h−1) in propane DDH reactions (600 °C, 2.4 kPa C3H8, He balance). The structure analysis results indicated the zinc loading in the optimized sample can be up to 19 wt%, which was also effectively retained after reactions. The systematic investigation on structure–function relations revealed that the catalytic activity originated from highly dispersed Zn, and the improvement of its performance could be ascribed to the mutual promotion between zinc loading and specific surface area. This work provided new guidelines to enable further development of the carbon-supported zinc-based catalyst in alkane dehydrogenation reactions and promoted the rational design of highly dispersed metal materials.

Cu–Au nanoparticles produced by the aggregation of gas‐phase metal atoms for CO oxidation

AbstractAlloy nanoparticles (nanoalloys) are widely applied in heterogeneous catalysts, advanced electrodes, biomaterials, and other areas. The properties of nanoalloys can be tuned to a significant extent by their structures and compositions, which are governed by the employed synthetic procedure. Often such synthesis occurs in non‐equilibrium conditions and yields nanoalloys with structures and properties that are different from those obtained in thermodynamic equilibrium. In this work, we characterize how the non‐equilibrium conditions during the synthesis of Cu–Au alloys via physical vapor deposition (PVD) affect their morphology, composition, electronic structure, and reactivity in CO oxidation. We used molecular dynamics to simulate the PVD synthesis of Cu–Au nanoalloys through the non‐isothermal aggregation of Cu and Au atoms at a 3:1 ratio in the Ar atmosphere to obtain realistic structures of Cu–Au nanoparticles. Due to the different aggregation kinetics of Au and Cu atoms, the average Au concentration in the obtained Cu–Au particles varied between 14% and 50% depending on the nanoparticle size and the aggregation time. Density functional simulations revealed that the reactivity of the obtained Cu–Au clusters toward CO and oxygen as well as Brønsted–Evans–Polanyi relations for CO oxidation significantly depend on whether the clusters had fcc, icosahedral, or amorphous structures and do not strongly correlate with the d‐band centers of the adsorption sites. Our study highlights the importance of the non‐equilibrium character of nanoalloy structure and composition for their electronic structure and catalytic properties. The performed analysis of the reactivity of Cu–Au clusters with realistic structures in CO oxidation will help the optimization of Cu–Au catalysts for this societally important reaction.

Effect of Re aggregation doping configurations on the thermal and mechanical properties of W-Re alloys: A molecular dynamics study

Thermal and mechanical properties of plasma facing materials (PFMs) are critical for the magnetic confinement fusion devices. In this study, by using molecular dynamics (MD) simulations, we investigate the impact of different dopant configurations and doping levels on the thermal and mechanical properties of W-Re alloys. In the studied models, Re atoms are either randomly distributed in the system, or aggregated with certain shapes (sphere, cone, cylinder, prism, and plane). Accounting for the thermal conductivity and mechanical properties (including the Elastic modulus, Poisson's ratio, and Cauchy pressures, etc.) for the models with different Re aggregation configurations and concentrations, we find that W-Re alloys with spherical Re aggregation configuration exhibit larger thermal conductivity compared with other aggregation configurations regardless of the Re concentration, which also bear good mechanical properties. Also, the thermal conductivity of the spherically doped W-Re alloys as a function of temperature were investigated. The obtained results indicate that as the temperature increases, the phonon scattering increases and the thermal conductivity of the system in general decreases. Furthermore, the normalized phonon density of states (DOS) was calculated to better understand the effect of different doping configurations and concentrations on the thermal conductivity of W-Re alloys. The results show that the model with spherical Re configuration bears a wider phonon frequency domain, indicating inelastic scattering at the W-Re interface. However, when more Re atoms aggregate, the small structural size differences between W and Re atoms are magnified. In this case, the W-Re alloy lattice showed non-negligible distortions, and the whole system exhibited anomalous lattice constants and mechanical properties. Our investigation yields new insights into the physical mechanisms governing heat transport in alloys, and provides guidance to the design of plasma facing materials in terms of thermal transport.

Selective catalytic oxidation of formaldehyde on single V- and Cr-atom decorated magnetic C4N3 substrate: A first principles study

Formaldehyde (HCHO) is the most common indoor hazardous pollutant and has attracted great concern because its long-term exposure has adverse health effects on humans. Retention and catalytic oxidation of highly hazardous HCHO is an efficient and environmentally friendly method to use for air remediation, but a major obstacle to this procedure is the lack of an appropriate catalyst. Herein, two-dimensional magnetic C4N3 material with a 3d-transition metal as activate sites was systemically investigated in HCHO oxidation using density functional theory calculations. The results show that V-C4N3 and Cr-C4N3 have high structural stability and shallow activation barriers for O2 decomposition; these characteristics provide the necessary precursors for the subsequent oxidation reaction. Moreover, the V-C4N3 and Cr-C4N3 catalysts have unique selective adsorption and catalysis toward HCHO in a mixture of some typical in-door volatile organic compounds (VOCs) and air. The corresponding dynamic barrier for each reaction step was investigated and the mechanism involved in HCHO oxidation was revealed in detail. Aggregation of metal atoms in the V-C4N3 and Cr-C4N3 catalysts is prevented by enormous diffusion resistance, and this is further confirmed by AIMD simulations. These results provide insightful guidance for developing advanced magnetic catalysts for HCHO oxidation to improve the remediation of air contaminants.

Formation of Free Hydrogen Gas By Annealing ALD-Al2O3/Si Stacked Structure

1. IntroductionAtomic layer depositions (ALD) of Al2O3 films are very essential technique to realize next generation electronic devices thanks to their high-k properties [1]. However, ALD-Al2O3 films usually suffer blistering during thermal treatments, which have caused serious problems for decades in the field of device fabrication [2]. Many groups have studied the cause of this phenomenon, and various models have been proposed. such as, stress at the interface between Al2O3 and Si [3], diffusion of Si atoms at the interface[4], residual H2O in the film after the ALD process [5], and aggregation of hydrogen atoms at the Al2O3/Si interface [6], and so on. However, the actual mechanism has hitherto remained unclear due to a lack of conclusive evidence. Here, in this study, we have tried to reveal the origin of this phenomenon using spectroscopic measurements [7]. As a result, we have succeeded in directly identifying the gas inside the blisters, which can be a concrete evidence to reveal the roots of a phenomenon that has long been obscured.2. Experimental ProcedureIn the experiment, (100) oriented Si substrates with resistivity of 0.1-0.5 W∙cm were carried out. After cleaning the substrates by acetone, sulfuric-acid hydrogen-peroxide mixture solution, and hydrofluoric acid solution, all the substrates were loaded into ALD chamber which was heated up to 300oC. Then, Al2O3 layer with ~108 nm thickness was deposited on the substrates with conventional thermal ALD process using trimethylaluminum (TMA) and H2O as precursor gasses. After deposition, samples were annealed in rapid thermal annealing (RTA) furnace for 5 min with 400 – 1000 oC at N2 ambient. Finally the samples were characterized by laser microscopy, field emission scanning electron microscopy (FE-SEM), spectroscopic ellipsometry, and Raman spectroscopy.3. Results and DiscussionsFigs. 1(a) and 1(b) show typical microscopic image of sample before, and after RTA (500oC, 5 min), respectively. We can clearly see that several blisters appear by annealing the samples, while there was no such sign before annealing. From the microscopic observations, we have summarized density of blisters as a function of RTA temperature in Fig. 2. Here, we can clearly see that by increasing RTA temperature, blister density increases, which is largely consistent with a previous result reported by O. Beldarrain et al, where 400-cycle Al2O3 layers were grown at 200 oC.To discuss the origin of the blister, we have conducted cross-sectional SEM observations of the formed blisters. Fig. 3 shows a typical SEM image of a blister after annealed at 500oC. The cross-sectional SEM observations clearly show that the blisters are completely hollow underneath. The surface of the flat Si substrate can also be seen under the blister, indicating that the upper Al2O3 film has completely delaminated from the interface between Si and Al2O3. From these results, we have speculated that blistering of Al2O3 after annealing is due to outgassing from the Al2O3/Si interface. Here, we came up with an idea that by identifying the gas inside the blisters, we can clarify the cause of this phenomenon.To identify the content inside blisters, Raman spectroscopy was carried out. The 532 nm wavelength laser used for Raman measurements can pass completely through Al2O3, making it possible to measure the gas inside without destroying the blisters. Fig. 4 shows a typical result of the Raman measurement of a blister of a sample after RTA (500oC). Here, as a result, characteristic spectra with peaks at positions ~4126, ~4144, ~4156, and ~4162 cm-1 were successfully observed. These four peaks can be assigned as peaks due to the difference in the rotational-quantum-number (J) of H2 as shown in Fig. 4, which is typical spectra of free H2 gas. These results provide evidence that blister formation is caused by the generation of hydrogen gas from the Al2O3/Si interface, a finding that is of great importance in clarifying the physics of blister formation mechanisms.In the presentation, we will also present details on the physics of hydrogen gas generation and methods to suppress blister formation.[References][1] R. Zhang, et al., IEEE Trans. Electron Devices 59, 335-341, (2012).[2] G Dingemans, et al., J. Vac. Sci. Technol. A, 30, 040802, (2012).[3] J. Thurn., et al., J. Mater. Sci. 39, 4799, (2004)[4] M. Broas, et al., Appl. Phys. Lett. 111, 141606, (2017)[5] O. Beldarrain, et al., J. Vac. Sci. Technol. A, 31, 01A128 (2013)[6] D. G. Xie, et al., Nat. Mater., 14, 899, (2015).[7] R. Matsumura, et al., accepted to ACS Applied Materials & Interfaces, https://doi.org/10.1021/acsami.1c20660 Figure 1

(Invited) Nucleation and Growth of Small Atomic Aggregates into Superatom Nanoclusters

One of the challenges in nucleation and growth of small atomic aggregates is to develop the novel research field of solid-state chemistry and material science using gas-phase-synthesized superatom nanoclusters. Among various superatoms, the greatest advantage of the central-atom-encapsulating binary superatoms, such as M@Si16, is that their electronic structures can be designed by optimizing the central metal atom M while retaining the geometrical symmetry of the Si16 cage [1], as shown in the figure below. We have systematically investigated the nucleation and growth of M@Si16 superatoms with gas-phase mass spectrometry, and then performed the surface immobilization of the M@Si16 cations and anions on a substrate monodispersively with the size-selective soft-landing. Initial products prepared via the surface immobilization of M@Si16 superatoms on solid surfaces decorated with monolayer films of C60 molecules were investigated using scanning tunneling microscopy (STM) [2] and X-ray photoelectron spectroscopy (XPS) [3]. Furthermore, we have developed a large-scale synthesis method for M@Si16 (M = Ti and Ta) by scaling up the clean dry-process with a high-power impulse magnetron sputtering (HiPIMS, nanojima®) [4] and by a direct liquid embedded trapping (DiLET) method [5]. The spectroscopic results reveal that the structures of soft-landed and isolated M@Si16 superatoms are the metal-encapsulating tetrahedral silicon-cage (METS) [5,6].Furthermore, superatoms of group-5 metals (M = V, Nb, and Ta) encapsulating Si16 cage nanoclusters (M@Si16) can be efficiently generated to form assembled films [7]. Temperature-dependent current–voltage (I–V) characteristics of the M@Si16 assembled films revealed that the electrical conduction mechanism is not band transport, but hopping transport with Efros–Shklovskii variable range hopping for all central M atoms [8]. The results show that electrons involved in conduction are strongly correlated to localized electronic states.[1] K. Koyasu, A. Nakajima, et al. J. Am. Chem. Soc. 127, 4998 (2005).[2] M. Nakaya, A. Nakajima, et al. Nanoscale, 6, 14702 (2014).[3] M. Shibuta, A. Nakajima, et al. J. Am. Chem. Soc. 137, 14015 (2015).[4] C. H. Zhang, A. Nakajima, et al. J. Phys. Chem. A 117, 10211 (2013).[5] H. Tsunoyama, A. Nakajima, et al. J. Phys. Chem. C 121, 20507 (2017).[6] H. Tsunoyama, A. Nakajima, et al. Acc. Chem. Res. 51, 1735 (2018).[7] M. Shibuta, A. Nakajima, et al. J. Phys. Chem. C, 124, 28108 (2020).[8] T. Yokoyama, A. Nakajima, et al. J. Phys. Chem. C, 125, 18420 (2021). Figure 1

Hydrogen storage in C-doped h-BN decorated with titanium atom: A computational study

Here, a novel candidate is explored with a high capacity of 6.33 wt% for hydrogen storage at room temperature and a mild pressure of 1 ∼ 5 atm. It is found that carbon atoms doped in h-BN with a concentration of 25 at% can eliminate the aggregation of titanium atoms on h-BN. The configuration of double carbon doped h-BN with two Ti atoms decorated double sides (2Ti/2C-BN) is able to stably adsorb up to 6 H2 molecules. The average adsorption energy of −0.58 eV between physical and chemical adsorption allows H2 molecules to be adsorbed under ambient condition.

Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia

Restructuring is ubiquitous in thermocatalysis and of pivotal importance to identify the real active site, yet it is less explored in electrocatalysis. Herein, by using operando X-ray absorption spectroscopy in conjunction with advanced electron microscopy, we reveal the restructuring of the as-synthesized Cu-N4 single-atom site to the nanoparticles of ∼5 nm during the electrochemical reduction of nitrate to ammonia, a green ammonia production route upon combined with the plasma-assisted oxidation of nitrogen. The reduction of Cu2+ to Cu+ and Cu0 and the subsequent aggregation of Cu0 single atoms is found to occur concurrently with the enhancement of the NH3 production rate, both of them are driven by the applied potential switching from 0.00 to -1.00 V versus RHE. The maximum production rate of ammonia reaches 4.5 mg cm-2 h-1 (12.5 molNH3 gCu-1 h-1) with a Faradaic efficiency of 84.7% at -1.00 V versus RHE, outperforming most of the other Cu catalysts reported previously. After electrolysis, the aggregated Cu nanoparticles are reversibly disintegrated into single atoms and then restored to the Cu-N4 structure upon being exposed to an ambient atmosphere, which masks the potential-induced restructuring during the reaction. The synchronous changes of the Cu0 percentage and the ammonia Faradaic efficiency with the applied potential suggests that the Cu nanoparticles are the genuine active sites for nitrate reduction to ammonia, which is corroborated with both the post-deposited Cu NP catalyst and density functional theory calculations.

Theoretical study on CO2 reduction to methanol catalyzed by highly stable single-atom transition metal riveted bilayer 2D material

The main factor limiting the high activity of single-atom catalysts (SACs) is the structural instability, which is caused by atomic aggregation or attack of reaction intermediates. In this paper, the single-atom transition metal copper and palladium was riveted between two layers of the monolayer materials (carbon nitride and graphene) respectively to enhance the structural stability, and used C3N-TM-C3N and GR-TM-C3N (TM= Cu, Pd) in the theoretical study of CO2 reduction to methanol (CO2RR). It is found that a single TM atom can be firmly anchored between two C3N or GR-C3N layers, which can prevent the aggregation of TM atoms (TM= Cu, Pd) in SACs and inhibit the attack of reaction intermediates. The TM atom enhances the catalytic activity of the material system by providing electrons to the protective layer GR/C3N. And C3N-Cu-C3N material has the best performance and the lowest reaction energy barrier in CO2RR, which is only 2.094 eV. It provides a feasible method to improve the catalytic activity and maintain the stability for the practical application of SACs systems.

Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin Film Using Atomic Layer Deposition for HER Electrocatalysis Applications

The efficiency of hydrogen gas generation via electrochemical water splitting has been mostly limited by the availability of electrocatalyst materials that require lower overpotentials during the redox reaction. Noble metals have been used extensively as electrocatalysts due to their high activity and low overpotentials. However, the use of single noble metal electrocatalyst is limited due to atomic aggregation caused by its inherent high surface energy, which results in poor structural stability, and, hence, poor electrocatalytic performance and long-term stability. In addition, using noble metals as electrocatalysts also causes the cost to be unnecessarily high. These limitations in noble metal electrocatalysts could be enhanced by combining two noble metals in a core-shell structure (e.g., Rh@Ir) as a thin film over a base substrate. This could significantly enhance electrocatalytic activity due to the following: (1) the modification of the electronic structure, which increases electrical conductivity; (2) the optimization of the adsorption energy; and (3) the introduction of new active sites in the core-shell noble metal structure. The current state-of-the-art employs physical vapor deposition (PVD) or other deposition techniques to fabricate core-shell noble metals on flat 2D substrates. This method does not allow 3D substrates with high surface areas to be used. In the present work, atomic layer deposition (ALD) was used to fabricate nanoparticle thin films of Rh@Ir and Rh@Pt in a core-shell structure on glassy carbon electrodes. ALD enables the fabrication of nanoparticle thin film on three-dimensional substrates (a 2D functional film on a 3D substrate), resulting in a significantly increased surface area for a catalytic reaction to take place; hence, improving the performance of electrocatalysis. The Rh@Pt (with an overpotential of 139 mV and a Tafel slope of 84.8 mV/dec) and Rh@Ir (with an overpotential of 169 mV and a Tafel slope of 112 mV/dec) core-shell electrocatalyst exhibited a better electrocatalytic performances compared to the single metal Rh electrocatalyst (with an overpotential of 300 mV and a Tafel slope of 190 mV/dec). These represented a 54% and a 44% improvement in performance, respectively, illustrating the advantages of core-shell thin film nanostructures in enhancing the catalytic performance of an electrocatalyst. Both electrocatalysts also exhibited good long-term stability in the harsh acidic electrolyte conditions when subjected to chronopotentiometry studies.