Low Coordination Number Research Articles

Dual Pt-Ni atoms dispersed on N-doped carbon nanostructure with novel (NiPt)-N4C2 configurations for synergistic electrocatalytic hydrogen evolution reaction

Single-atom catalysts with high atomic utilization efficiency, well-defined active centers, and catalytic sites with low atomic coordination numbers are promising to boost electrocatalytic performance. Dual-atom catalysts (DACs) with neighboring heteroatoms are expected to exert the synergistic effect of the two atoms, further improving the activity. Herein, we report a PtNi-NC catalyst consisting of PtNi dual atoms anchored on a nitrogen-doped carbon substrate synthesized through atomic layer deposition. X-ray absorption spectroscopy confirms the existence of the PtNi dual atoms. The as-prepared PtNi-NC catalyst exhibits extraordinary catalytic activity, with an overpotential of 30 mV for hydrogen evolution reaction (HER) in acidic media at a current density of 10 mA cm−2, which is comparable to that of commercial 20 wt% Pt/C. Notably, PtNi-NC possesses a much higher mass activity, about 21 times, than 20 wt% Pt/C. Density functional theory calculations reveal that PtNi dual atoms generate a synergistic effect by modulating the local electronic structure and optimizing the charge distribution, contributing to optimized adsorption properties and enhanced electrocatalytic performance. This work provides a new avenue for the fabrication of DACs, revealing their potential applications in electrocatalytic HER and beyond.

Identification and localization of Pt species in Pt-NaA zeolite catalysts prepared by direct synthesis

Several characterization techniques were employed to elucidate the Pt species present in the structure of Pt-NaA catalysts. Three types of Pt particles were identified: nanoparticles located at the surface of the crystals (with a higher size and flattened form), 1–2 nm particles inside the structure, and sub-nanometric particles, lower than 1 nm, confined in the pores. The low PtPt coordination number (EXAFS) is compatible with both flattened dihedral-like morphology, as well as the presence of sub-nanometric particles. The Pt0.6_NaA catalyst exhibited higher activity (in the WGS) than the other catalysts, which could be related to the presence of small particles.

Few-atom-layer metallene quantum dots toward CO2 electroreduction at ampere-level current density and Zn-CO2 battery

Electrocatalytic CO 2 reduction reaction (CO 2 RR) toward commodity chemicals and fuels presents an enticing prospect wherein the emerging metallenes as catalysts have aroused great interest. Here, we report the synthesis of a kind of tiny metallene quantum dot (QD) called the stanene QD (Sn-ene QD), which features a few atom layers and rich edge centers and provides superior CO 2 RR activity by delivering ampere-level current densities with near-unity selectivity for formate production. Operando attenuated total reflection-infrared (ATR-IR) spectroscopy and theoretical calculations deciphered that the rich edge centers of Sn-ene QDs hold high reactivity, and the residual cyano ligands with electron-withdrawing nature favor the stabilization of the CO 2 RR intermediate. Furthermore, the Zn-CO 2 battery assembled with Sn-ene QDs catalyst affords a high power density of 1.25 mW cm −2 and an outstanding stability of ∼100 h. This work highlights the great potential of metallene QDs as a new platform for heterogeneous catalysis and energy conversion. • Few-atom-layer stanene quantum dots (Sn-ene QDs) with rich edge centers were prepared • Sn-ene QDs afford a superior activity toward CO 2 electroreduction to formate • The rich edge centers of Sn-ene QDs were deciphered to hold a high reactivity • Sn-ene QDs show excellent Zn-CO 2 battery performance Metallenes, the newly emerging two-dimensional metallic materials of atomic thickness, have been regarded as ascendant CO 2 RR electrocatalysts because of their plentiful accessible atoms and unique catalytic properties. When the lateral size of metallenes reduces to less than 10 nm, producing metallene quantum dots (QDs), the edge atoms with lower coordination numbers will occupy a large portion of the total atoms, which could exhibit a higher catalytic performance. Here, we delicately fabricated the few-atom-layer stanene QDs with rich edge centers via an in situ electrochemical reduction strategy. Such a unique metallene QD catalyst affords an excellent performance for CO 2 electroreduction to formate and Zn-CO 2 batteries. Few-atom-layer stanene quantum dots with rich edge centers are pioneeringly fabricated by a fast electrochemical self-reconstruction strategy and are shown to afford an excellent electrocatalytic performance for CO 2 electroreduction to formate and Zn-CO 2 batteries.

Interfacing CrOx and CuS for synergistically enhanced water oxidation catalysis

The sluggish kinetics associated with the oxygen evolution reaction (OER) limits the sustainability of fuel production and chemical synthesis. Developing catalysts based on Earth abundant elements with a reasonable strategy could solve the challenge. Here, we present a heterostructure built from CrOx and CuS whose interface gives rise to the advent of new functionalities in catalytic activity. Using X-ray photoelectron and absorption spectroscopies, we identified the multiple oxidation states and low coordination number of Cr metal in CrOx-CuS heterostructure. Benefitting from these features, CrOx-CuS generates oxygen gas through water splitting with a low over potential of 190 mV vs RHE at a current density of 10 mA cm−2. The catalyst shows no evident deactivation after a 36-hours operation in alkaline medium. The high catalytic activity, inspired by first principles calculations, and long-time durability make it one of the most effective OER electrocatalysts.

(Invited) Flexocatalysis of Single Pt Atoms Loaded Graphitic Carbon Nitride (g-C3N4)

Single-atom catalyst as an advanced co-catalyst shows promising potential in energy fields such as water splitting, degradation of pollutants, chemical conversion field, etc. It takes advantage of increased active sites and sub-nanometer size effects with electron confinement by reducing the size of supported noble metals; furthermore, a single-atom catalyst maximizes the atomic efficiency of metals and provides a low atomic coordination number for the active center. In this work, we discovered that the isolated single atoms (SAs) stabilized by the support of g-C3N4 as named by SA-Pt/g-C3N4. Single-atom catalyst is synthesized by anchoring Pt atoms in graphitic carbon nitride (g-C3N4) with different Pt-loading [0.01wt% to 1wt% H₂PtCl₆·(H₂O)₆]. Under flexocatalytic process, the k rate constant of the SA-Pt/g-C3N4 with 0.01wt% of Pt is 3×10-3 s-1 for decomposition of dye molecules, which is 2 times the pristine g-C3N4. In addition, the flexocatalytic hydrogen evolution of SA-Pt/g-C3N4 provides H2 production, reaching nearly 125 μmolgh-1, which is 312% of the pristine g-C3N4 (~40 μmolgh-1). The working mechanism of flexocatalytic activity suggests the mechanical strain-induced flexoelectric potential (flexopotential) and proceeding with redox's electrochemical reaction. In addition, the single-atom act as a critical role in attracting electrons to accelerate the reaction process. The single-atom catalyst of 2D materials is highly potential for applying in water splitting and degradation of pollutants through the flexocatalytic process.

Tuning optical dispersion and localized surface plasmon resonance of 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag glass system

The optical dispersion properties of the 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag glass system were studied using the Wemple–DiDomenico fitting model. The single oscillator energy Eo and dispersion energy Ed decrease with increasing Bi2O3 concentration, with a maximum at x = 11 mol%.The large increase at x = 11 mol% can be attributed to the role of Bi2O3 as a glass former, which induces the formation of bonds in the glass network, thereby contributing to average bond strength and increasing Eo. The role of Bi2O3 as glass former may also induce minimal transformation of the glass network TeO2 to lower coordination number Nc, contributing to a maximum in Ed. Analytical expressions were also used to compute the complex dielectric properties of the glass samples. The εr and εi initially decreased at x ≤ 7 mol% before a maximum at x = 11 mol% possibly due to a decrease in polarization caused by blocking of Bi3+ ions to Li+ ions. By contrast, the maximum of εr and εi at x = 11 mol% may be attributed to the high field strength F of Bi3+ and Bi5+ ions, which increases the electronic polarizability of the glass samples. The behavior of the localized surface plasmon resonance was studied using coupling polarization approach, which revealed that the localized surface plasmon resonance of the Ag NP is dependent on the dielectric properties of Er3+ host glass. The electrical χ and Clausius–Mossotti polarizability αL showed a maximum at x = 13 mol%, which could be attributed to high real εr and imaginary dielectric constant εi, thus contributing to the polarizability of the glass samples and leading to strong localized surface plasmon resonance.

High-pressure deformation of metallic glass nanoparticles

The atomistic structure of metallic glasses is closely related to properties such as strength and ductility. Here, Ni1-xBx metallic glass nanoparticles of two different sizes are compressed under quasi-hydrostatic high-pressure conditions in order to understand structural changes under stress. The structural changes in the nanoparticles were tracked using in situ high-pressure X-ray diffraction (XRD). The ambient pressure pair-distribution functions generated from XRD showed that the smaller sized nanoparticles had a more compact amorphous structure with lower coordination number. XRD showed that the amorphous structure was stable up to the maximum pressures achieved. The bulk modulus of the smaller and larger sized nanoparticles was found to be 183 GPa and 178 GPa, respectively. The size-dependent high-pressure behavior was related to compositional differences between the nanoparticles. These results show that Ni1-xBx metallic glass nanoparticles are highly stable under pressure.

Theoretical understanding of water adsorption on stepped iron surfaces

Nanoscale zero-valent iron (nZVI) has attracted significant interest due to its high capability in groundwater remediation. Understanding the interaction between water and nZVI is essential to optimizing its performance. While stable stepped surfaces may play a unique role in nZVI during their practical applications due to their higher reactivity, the interactions between water and stepped surfaces are still rarely investigated. Here we applied density functional theory (DFT) to study the adsorption properties of water on stable stepped Fe(210) and Fe(211) surfaces with the consideration of the van der Waals correction. Our results revealed that the adsorption strength of molecular water is enhanced on surface Fe atoms with lower coordination numbers (CN). Accordingly, the water has a stronger interaction on the stepped surface after the dissociation into H and OH. The stepped surface can facilitate water dissociation by greatly reducing dissociation energy barriers. Since water adsorption and dissociation are the first stage of iron oxidation by water, our DFT results demonstrate that the water adsorption on stepped iron surface may determine the lifetime of nZVI. Our findings, therefore, suggests the passivation of stepped surface can be a practical approach to improve the performance of nZVI.

Tuning crystal-phase of bimetallic single-nanoparticle for catalytic hydrogenation

Bimetallic nanoparticles afford geometric variation and electron redistribution via strong metal-metal interactions that substantially promote the activity and selectivity in catalysis. Quantitatively describing the atomic configuration of the catalytically active sites, however, is experimentally challenged by the averaging ensemble effect that is caused by the interplay between particle size and crystal-phase at elevated temperatures and under reactive gases. Here, we report that the intrinsic activity of the body-centered cubic PdCu nanoparticle, for acetylene hydrogenation, is one order of magnitude greater than that of the face-centered cubic one. This finding is based on precisely identifying the atomic structures of the active sites over the same-sized but crystal-phase-varied single-particles. The densely-populated Pd-Cu bond on the chemically ordered nanoparticle possesses isolated Pd site with a lower coordination number and a high-lying valence d-band center, and thus greatly expedites the dissociation of H2 over Pd atom and efficiently accommodates the activated H atoms on the particle top/subsurfaces.

The effect of Ru-Ru coordination numbers on CO2 methanation over Ru supported catalyst

CO2 methanation might be an efficient way to reduce the greenhouse effect and solve the energy problems. The supported Ru catalysts are one of the most widely used methanation catalysts, but the effect of the Ru particles' micro-morphology on the CO2 methanation was rarely studied theoretically. The Run/Ru(0001) (n = 1, 2, 3, 4) was built for modeling the defected Ru sites with low Ru-Ru coordination numbers (CN = 3, 4, 5, and 5.5) in the present work. On the Run/Ru(0001) (n = 1, 2, 3, 4), the most favored pathway for CH4 formation is the COH route: CO2 → CO → COH → C → CH → CH2 → CH3 → CH4. Based on the microkinetic modeling results, we find the selectivity for CH4 increases when the CN of Ru increases from 3 to 5.5, and the selectivity is almost 100% over Ru4/Ru(0001). However, the selectivity for CO is the opposite, that is, the Ru ensemble with lower CN is favorable for CO formation. This study reveals the strong relationship between the Ru-Ru CN and the CO2 methanation reactivity/selectivity and finds the model catalysts with about fiver Ru-Ru CN exhibit the best methanation property. Consequently, metal nanoparticles with more fractions of five-coordinated Ru-Ru configurations would be beneficial for CO2 methanation.

Boron Nitride- and Graphene-Supported Trimetallic Yolk–Shell and Hollow Nanoparticles

In this study, the stability of yolk–shell and hollow Ni@PdAu and PdAu@Ni nanoparticles in free state and supported on boron nitride and graphene substrates was investigated at 300 K using molecular dynamics simulations. To this end, analyses of several parameters such as relative stability, surface energy, coordination number, number of surface atoms, bond length, displacement vector, and atomic strain were employed. Results show that the yolk–shell and hollow Ni@PdAu nanoparticles are more stable than yolk–shell and hollow PdAu@Ni nanoparticles in free and supported states due to the less surface energy. Furthermore, results show that the hollow nanoparticles are more stable than yolk–shell nanoparticles in all states. In addition, results show that the void and hollow core in yolk–shell and hollow nanoparticles are unstable and collapse due to the contraction and expansion of the core and shell, which lead to the formation of the core–shell and quasi–Janus structures in hollow and yolk–shell nanoparticles, respectively. Moreover, nanoparticles create a quasi–ellipsoid structure with low density and high coordination number on the boron nitride. The yolk–shell and hollow nanoparticles are the most stable on the boron nitride due to the smaller displacement and variation of the atomic bond length and lower atomic strain, which are caused by the strong nonbonded interaction, good atomic match, and placement of the Ni, Au, and Pd atoms on the middle position of the boron nitride hexagonal structure. Generally, this study demonstrated that the stability of yolk–shell and hollow nanoparticles can be tuned by the substrate–nanoparticle interaction.

Thermal steam reduction etching to construct POM@CuICuII-BTC with hierarchical porosity for adsorption property enhancement

Host MOFs with atomically precise characteristics and functional-oriented guest molecules are the key to assemble multifunctional host–guest MOF composites. We selected O−rich POM tungstophosphoric acid (TPA) and CuII-BTC with high stability and adjustable structure to synthesize host–guest TPA@CuII-BTC. Then thermal steam reduction etching was used to construct TPA@CuICuII-BTC with hierarchical pores at molecular scale, enlarging the pore space and making partially inaccessible sites of TPA@CuII-BTC more expose. Under the thermal methanol atmosphere, partial six-coordinated metal node Cu2+ was reduced slowly into Cu+ with lower coordination number, accompanied by the fracture of partial Cu−O coordination bonds. The generated TPA@CuICuII-BTC not only obtained larger pore and exposed more space in contrast to TPA@CuII-BTC, but also possessed more free hydroxyl oxygen sites due to the breaking of partial coordination bonds, promoting the contact and interaction of the composite with external substances. The Pb2+ adsorption experiment showed that TPA@CuICuII-BTC exhibited best adsorption ability to external Pb2+, and the adsorption capacity was 614.95 mg·g−1, which was 180.25 mg·g−1 and 243.45 mg·g−1 higher than that of TPA@CuII-BTC and CuII-BTC. This work has great reference significance for the construction of MOF composites with hierarchical porosity and the study of MOF composites on adsorption performance improvement.

Activators lattice migration strategy customized for tunable luminescence of Ce3+ doped β-Ca3(PO4)2

The luminescent properties of phosphors are determined by the electronic configuration of their activators and the crystal environment in which they are embedded. By manipulating the site-selective occupancy of activators, we can coordinately control the local crystal field and electron cloud distribution to master the movement of emission wavelength and obtain a color-tunable phosphor. The system of β-Ca3(PO4)2 is an excellent carrier for crystal-site engineering to modify luminescent performance. It has five distinct crystal sites and a strong photoluminescence response. However, due to low doping concentration and structural complexity, the specific occupation of Ce remains unknown. We discussed the specific position of Ce in this study and proposed a novel activators lattice migration strategy for controlling the relative strength of crystal field splitting (CFS) and the nephelauxetic effect (NE) to control luminescence properties. By combining X-ray Absorption Fine Structure (XAFS) with DFT calculation, we predicted the local environment of each crystal lattice and migrated Ce3+ from M(3) to a strengthened crystal field site with a lower coordination number, shorter bonding bond length, and greater polyhedral distortion, M(5), resulting in phosphors with tunable luminescence emission. Relevant parameters ΔD(Ce3+) were identified for dealing with interference caused by the electronegativity difference during a structural modification. Some potential applications emerged as a supplement and an intriguing QR-code and response program for epidemic prevention were demonstrated. This structure–activity relationship-based strategy provides new inspiration for designing luminescent materials with tunable emission and a broader range of applications.

The Role of Steps on Silver Nanoparticles in Electrocatalytic Oxygen Reduction

Hydrogen fuel cell technology is an essential component of a green economy. However, it is limited in practicality and affordability by the oxygen reduction reaction (ORR). Nanoscale silver particles have been proposed as a cost-effective solution to this problem. However, previous computational studies focused on clean and flat surfaces. High-index surfaces can be used to model active steps presented in nanoparticles. Here, we used the stable stepped Ag(322) surface as a model to understand the ORR performance of steps on Ag nanoparticles. Our density functional theory (DFT) results demonstrate a small dissociation energy barrier for O2 molecules on the Ag(322) surface, which can be ascribed to the existence of low-coordination number surface atoms. Consequently, the adsorption of OOH* led to the associative pathway becoming ineffective. Alternatively, the unusual dissociative mechanism is energetically favored on Ag(322) for ORR. Our findings reveal the importance of the coordination numbers of active sites for catalytic performance, which can further guide electrocatalysts’ design.

Nickel oxide nanoparticles with oxygen vacancies for boosting biomass-upgrading

The catalytic activity may be improved by introducing oxygen vacancies (Vo) into oxides, but the exact role of oxygen vacancy in 5-hydroxymethylfurfural oxidation reaction (HMFOR) is not clear. It is very important to study the reaction mechanism of oxygen enriched vacancy electrocatalyst and the dynamic behavior of oxygen vacancy. Here, a facile laser ablation strategy is used to fabricate nickel oxide (Vo-NiO) with different oxygen vacancy content for HMFOR. Series characterization show that abundant Vo, low coordination numbers and electron rich structures of Ni sites in Vo-NiO electrode. In-situ Raman electrochemical spectra show that Vo can promote low valent Ni preoxidation at low oxidation potential, which is conducive to the catalyst surface reconstruction and synergistic promote HMF oxidation. Theory calculation confirm the adsorption energy of 5-hydroxymethylfurfural (HMF) on Vo-NiO surface and the density of states increases, which increases the electron conduction rate in Vo-NiO, and promotes HMFOR.

Characterization of a Ternary Neodymium-HEDTA-Citrate Complex in the Actinide Lanthanide Separation Process.

The actinide lanthanide separation (ALSEP) process is a modern solvent extraction approach used for the separation of the minor actinides americium and curium from the lanthanide fission products for transmutation, a process that can significantly reduce the long-term radioactivity and heat loading of nuclear waste. This process, inspired by existing chemistry, uses the aminopolycarboxylate N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA) to selectively separate the actinides by stripping them from the organic phase while leaving the lanthanides behind. HEDTA is used in this separation as it has been shown to exhibit faster extraction kinetics than other aminopolycarboxylates, but its lower coordination number can allow for the formation of higher order complexes with the typically 8- to 9-coordinate f-elements. ALSEP uses a carboxylic acid buffer in the aqueous phase to control the pH of the system during metal stripping, and this buffer has the ability to complex actinide(III) and lanthanide(III) ions. The presence of a previously uncharacterized ternary lanthanide-HEDTA-citrate complex was detected during single-phase spectroscopy experiments. A combination of partitioning experiments and spectrophotometric titrations led to the identification of a 1:1:1 complex containing a partially protonated citrate ligand and determination of the stability constant of its neodymium complex.

A novel 2D zinc(II)-organic framework for efficient catalytic cycloaddition of CO2 with epoxides

The catalytic cycloaddition of CO2 with epoxides is an essential approach for CO2 fixation, but noble metal free heterogeneous systems with milder conditions are still required. Hence, a novel zinc-organic framework based on imidazole carboxylic acid with a two-dimensional (2D) layer structure was designed and synthesized for this reaction. The MOF was determined to be {[Zn(dibpca)(OAc)]·2.5H2O}n (H-dibpca = 3′,5′-di(1H-imidazol-1-yl)-[1,1′-biphenyl]-4-carboxylic acid) by SC-XRD analysis, simplified as Zn(dibpca)(OAc), and further characterized by XPS, SEM, EDS, PXRD, FT-IR, TG and N2-sorption analysis. The developed Zn(dibpca)(OAc), along with n-Bu4NBr (TBAB) as a co-catalyst, exhibited efficient activity and high selectivity for the catalytic cycloaddition of CO2 with epoxides. The conditions are very mild: no catalyst activation, atmospheric CO2 pressure, low catalyst loading and no solvent. Multiple exposed Lewis acid sites on Zn(dibpca)(OAc), caused by the long rigid ligand and low coordination numbers, may contribute to the efficient catalytic performance. Various epoxides with different electronic and steric effects were evaluated and a slight influence was shown for both the yield and selectivity. In addition, the heterogeneous catalyst also showed good performance after being reused at least five times.

Theoretical insights into CO oxidation activities on CeO2(111) steps

Direct CO oxidation on four types of CeO2(111) surface steps were studied by using density functional theory (DFT) calculations. Our results showed that CO adsorption at step edges can be enhanced through the alignment of C-O and surface Ce-O bonds. Reactivities of various step edges on the CeO2(111) surface were found to be similar to those of the corresponding extended facets (i.e., {110} and {100}). In general, the oxygen sites at step edges with lower coordination numbers can give smaller vacancy formation energies and higher reactivities. In particular, nearly all the O species at the lower position of the step edges exhibit superior catalytic activities than those on the upper position. Moreover, it was found that the unusual decrease of the Ce 4f orbital energy level after CO adsorption on the Type II* step can contribute to its excellent catalytic activity towards CO oxidation, which is caused by the synergistic effect of structural and electronic distortions on the low-coordinated surface Ce5c at the step edge. Our results disclosed the unique role CeO2(111) steps play that influences the reactant adsorption, activation barriers, product desorption and thus the CO catalytic oxidation activities overall. These findings may inspire persistent research interests on the structure-reactivity relationships over surface defect sites on rare-earth oxide catalysts.

Uncovering a Vital Band Gap Mechanism of Pnictides

Pnictides are superior infrared (IR) nonlinear optical (NLO) material candidates, but the exploration of NLO pnictides is still tardy due to lack of rational material design strategies. An in‐depth understanding structure–performance relationship is urgent for designing novel and eminent pnictide NLO materials. Herein, this work unravels a vital band gap mechanism of pnictides, namely P atom with low coordination numbers (2 CN) will cause the decrease of band gap due to the delocalization of non‐bonding electron pairs. Accordingly, a general design paradigm for NLO pnictides, ionicity–covalency–metallicity regulation is proposed for designing wide‐band gap NLO pnictides with maintained SHG effect. Driven by this idea, millimeter‐level crystals of MgSiP2 are synthesized with a wide band gap (2.34 eV), a strong NLO performance (3.5 x AgGaS2), and a wide IR transparency range (0.53–10.3 µm). This work provides an essential guidance for the future design and synthesis of NLO pnictides, and also opens a new perspective at Zintl chemistry important for other material fields.

Simple hexagonal structured gold with eight-coordination formed with ordered structural vacancies

Phase engineering of nanomaterials attracts increasing research interest, as crystal structures of metals often determine their physical and chemical properties. Metals tend to form close-packed crystal structures to maximize their space filling and atomic coordination numbers due to the isotropic metallic bonding. Hitherto no metals yet crystallize with the simple hexagonal packing which has low coordination number and packing fraction. Here, we fabricate a novel simple hexagonal structured gold through in situ selective etching on ordered Cu-Au intermetallic compound, where the new crystal structure has only 8 nearest neighbors for each atom and a low packing fraction of 0.60. The simple hexagonal structured Au demonstrates an electrical conductivity of 389 S/m, approximately 105 times lower than that of face-centered cubic structured gold, according to first principles calculation, associated with localized valence electrons. The discovery of simple hexagonal structure suggests that metals can form more diverse crystal structures by changing effective atomic volume, which provides a pathway to engineer new structures of metals for novel physical properties and applications.