Doping monolayer graphene with single atom substitutions

Synergistic Effects of Carbon Vacancies in Conjunction with Phosphorus Dopant across Bilayer Graphene for the Enhanced Hydrogen Evolution Reaction

Bilayer graphene (BLG) exhibits distinct physical properties under external influences, such as torsion and structural defects, setting it apart from monolayer graphene. In this study, we explore the synergistic effects of carbon vacancies, in conjunction with phosphorus dopants, across BLG, focusing on their impact on structural, magnetic, electrical, and hydrogen adsorption properties. Our findings reveal that the substitutional doping of a phosphorus atom into a single carbon vacancy in a graphene layer induces substantial structural distortion in BLG. In contrast, doping phosphorus into a double vacancy maintains the flat structure of graphene layers. These distinct layer structures affect the electron distribution and spin arrangement, leading to varied electronic configurations and intriguing magnetic behaviors. Furthermore, the presence of abundant unsaturated electrons around the vacancy promotes the capture and bonding of hydrogen atoms. Hydrogen adsorption on BLG results in substantial orbital hybridization, accompanied by significant charge transfer. The calculated Gibbs free energies for hydrogen adsorption on BLG range from -0.08 to 0.09 eV, indicating exceptional catalytic activity for the hydrogen evolution reaction. These findings carry implications for optimizing the properties of graphene layers, making them highly desirable for applications such as catalysis.

General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts

Single-atom catalysts (SACs) possess the potential to involve the merits of both homogeneous and heterogeneous catalysts altogether and thus have gained considerable attention. However, the large-scale synthesis of SACs with rich isolate-metal sites by simple and low-cost strategies has remained challenging. In this work, we report a facile one-step pyrolysis that automatically produces SACs with high metal loading (5.2-15.9 wt %) supported on two-dimensional nitro-oxygenated carbon (M1-2D-NOC) without using any solvents and sacrificial templates. The method is also generic to various transition metals and can be scaled up to several grams based on the capacity of the containers and furnaces. The high density of active sites with N/O coordination geometry endows them with impressive catalytic activities and stability, as demonstrated in the oxygen reduction reaction (ORR). For example, Fe1-2D-NOC exhibits an onset potential of 0.985 V vs RHE, a half-wave potential of 0.826 V, and a Tafel slope of -40.860 mV/dec. Combining the theoretical and experimental studies, the high ORR activity could be attributed its unique FeO-N3O structure, which facilitates effective charge transfer between the surface and the intermediates along the reaction, and uniform dispersion of this active site on thin 2D nanocarbon supports that maximize the exposure to the reactants.

Large-Scale Synthesis of High Energy Thermal Battery Cathode Ni0.5Co0.5S2 by a Simple Sintering Technique

With the synergies of multiple elements, bimetallic sulfides exhibit excellent performance as splendid electrode materials and effective catalysts. However, large-scale synthesis of high-performance single-phase multicomponent sulfides has always been a challenge. Based on thermodynamic calculations, the intermediate phases NiS2 and Co3S4 are devoted to the synthesis of single-phase Ni0.5Co0.5S2. Because the reaction from NiS2 and Co3S4 to Ni0.5Co0.5S2 goes through a lower energy, it thermodynamically contributes to achieving a single-phase structure. Thus, single-phase Ni0.5Co0.5S2 can be simply and quickly prepared by two-step sintering and successfully scalable for mass production. This technique can extend to the whole ingredients Ni1-xCoxS2. Ni0.5Co0.5S2 demonstrates excellent thermal stability and good conductivity. It delivers a specific capacity of 671 mAh·g-1 and a specific energy of 1173 Wh·kg-1 when applied to a thermal battery cathode, which are increased by 18.6% and 25.0%, respectively, compared to pristine NiS2 (566 mAh·g-1) and CoS2 (537 mAh·g-1). This work proposes an innovative sintering method, which is applicable for cost-efficient and large-scale synthesis of single-phase multicomponent sulfides.

Modifying the electronic and magnetic properties of the scandium nitride semiconductor monolayer via vacancies and doping

In this work, the effects of vacancies and doping on the electronic and magnetic properties of the stable scandium nitride (ScN) monolayer are investigated using first-principles calculations. The pristine monolayer is a two-dimensional (2D) indirect-gap semiconductor material with an energy gap of 1.59(2.84) eV as calculated using the GGA-PBE (HSE06) functional. The projected density of states, charge distribution, and electron localization function assert its ionic character generated by the charge transfer from the Sc atoms to the N atoms. The monolayer is magnetized by a single Sc vacancy with a total magnetic moment of 3.00μB, while a single N vacancy causes a weaker magnetization with a total magnetic moment of 0.52μB. In both cases, the magnetism originates mainly from the atoms closest to the defect site. Significant magnetization is also reached by doping with acceptor impurities. Specifically, a total magnetic moment of 2.00μB is obtained by doping with alkali metals (Li and Na) in the Sc sublattice and with B in the N sublattice. Doping with alkaline earth metals (Be and Mg) in the Sc sublattice and with C in the N sublattice induces a value of 1.00μB. In these cases, either magnetic semiconducting or half-metallicity characteristics arise in the ScN monolayer, making it a prospective 2D spintronic material. In contrast, no magnetism is induced by doping with donor impurities (O and F atoms) in the N sublattice. An O impurity metallizes the monolayer; meanwhile, F doping leads to a large band-gap reduction of the order of 82%, widening the working regime of the monolayer in optoelectronic devices. The results presented herein may introduce efficient methods to functionalize the ScN monolayer for optoelectronic and spintronic applications.

Electronic and magnetic properties of transition-metal-doped monolayer B2S2 within GGA + U framework

Considering the significant role of magnetism induction in two-dimensional (2D) semiconductor materials, we systematically investigate the effects of various dopants from the 3d and 4d transition metal (TM) series, including Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag and Cd, on the electronic and magnetic properties of monolayer B2S2 through first-principles calculations. The calculated formation energies indicate that substitutional doping at the B site with various TM atoms could be achieved under S-rich growth conditions. What matters is that with the exception of systems doped with Cu, Tc, and Ag elements, which exhibit non-magnetic semiconductor properties, all other doped systems demonstrate magnetism. Specifically, the Cr-, Ni- and Pd-doped monolayers are magnetic half-metals, while the rest are magnetic semiconductors. We have also performed calculations of magnetic couplings between two TM atoms with an impurity concentration of 3.12%, revealing the prevalence of weak magnetic coupling in the majority of the magnetic systems examined. Moreover, the monolayers doped with Cr, Zr and Pd atoms exhibit ferromagnetic ground states. These findings strongly support the high potential for inducing magnetism in the B2S2 monolayer through B-site doping.

Binary Metal-Oxide Active Sites Derived from Cu-Doped MIL-88 with Enhanced Electroactivity for Nitrate Reduction

Nitrate-to-ammonia electrochemical conversion is important for decreasing water pollution and increasing the production of valuable ammonia. However, achieving high ammonium production without undesirable byproducts is difficult. Cu-doped MIL-88-derived bimetallic oxide catalysts with electrocatalytically active Fe-O-Cu bridges, which have high NO3- adsorption energy and facilitate N-intermediate hydrogenation, are developed for NH4+ production. Cu doping promotes hybridization between the O 2p of NO3- and Fe-Cu 3d, facilitating the adsorption and reduction of NO3- with a low Tafel slope (62.1 mV dec-1) and high ammonia yield (1698.8 μg·h-1·cm-2). The cathode efficiency is stable for seven cycles. Cu adjacent to Fe sites inhibits hydrogen evolution, promotes NO3- adsorption, and decreases the intermediate adsorption energy barrier. This study provides new opportunities for fabricating diverse binary metal oxides with new interfaces as efficient cathode materials for selective electroreduction.

Half-metallic and magnetic semiconductor behavior in CdO monolayer induced by acceptor impurities

In this work, a doping approach is explored as a possible method to induce novel features in the CdO monolayer for spintronic applications. Monolayer CdO is a two-dimensional (2D) non-magnetic semiconductor material with a band gap of 0.82 eV. In monolayer CdO, a single Cd vacancy leads to magnetization of the monolayer with a total magnetic moment of -2μ_B, whereas its non-magnetic nature is preserved upon creating a single O vacancy. Doping the Cd sublattice with Cu-Ag and Au induces half-metallic character with a total magnetic moment of -1 and 1μ_B, respectively. Dopants and their neighboring O atoms produce mainly magnetic properties. By contrast, doping with N, P, and As at the O sublattice leads to the emergence of magnetic semiconductor behavior with a total magnetic moment of 1μ_B. Herein, magnetism originates mainly from the spin-asymmetric charge distribution in the outermost orbitals of the dopants. Bader charge analysis and charge density difference calculations indicate charge transfer from Cu, Ag and Au dopants to the host monolayer, whereas the N, P and As dopants exhibit important charge gains. These results suggest that doping with acceptor impurities is an efficient approach to functionalize the CdO monolayer to generate spin currents in spintronic devices.

An Atomistic View of Platinum Cluster Growth on Pristine and Defective Graphene Supports

Density functional theory (DFT) is used to systematically investigate the electronic structure of platinum clusters grown on different graphene substrates. Platinum clusters with 1 to 10 atoms and graphene vacancy defect supports with 0 to 5 missing C atoms are investigated. Calculations show that Pt clusters bind more strongly as the vacancy size increases. For a given defect size, increasing the cluster size leads to more endothermic energy of formation, suggesting a templating effect that limits cluster growth. The opposite trend is observed for defect-free graphene where the formation energy becomes more exothermic with increasing cluster size. Calculations show that oxidation of the defect weakens binding of the Pt cluster, hence it is suggested that oxygen-free graphene supports are critical for successful attachment of Pt to carbon-based substrates. However, once the combined material is formed, oxygen adsorption is more favorable on the cluster than on the support, indicating resistance to oxidative support degradation. Finally, while highly-symmetric defects are found to encourage formation of symmetric Pt clusters, calculations also reveal that cluster stability in this size range mostly depends on the number of and ratio between PtC, PtPt, and PtO bonds; the actual cluster geometry seems secondary.

Synthesis of dual-metal single atom in porous carbon with efficient oxygen reduction reaction in both acidic and alkaline electrolytes

The rational design and fabrication of platinum group metal-free (PGM-free) electrocatalysts for oxygen reduction reaction (ORR) via economically feasible approach is essential for reducing the cost of fuel cells and metal-air batteries. Catalysts must have very high activity, and excellent mass diffusion of reactants. Herein, we display a high-performing dual-metal single atom catalyst (DM-SAC) composed of Fe and Ni SA active sites immobilized in porous carbon nanospheres (Fe/Ni-N-PCS), prepared via defects/vacancies anchoring strategy. The abundant and accessible edge-hosted Fe and Ni SA active sites can promote the adsorption/desorption behavior for ORR intermediates attributing to possible synergistic effects between dual-metal SA active sites. Thus, the as-developed Fe/Ni-N-PCS DM-SAC exhibits impressive ORR electrocatalytic performance in both alkaline (E_onset = 1.04 V, E_1/2 = 0.9 V) and acid solutions (E_onset = 0.87 V, E_1/2 = 0.71 V), and high stability, outperforming SACs with solo Fe-N_x or Ni-N_x active sites, and benchmark PGM. Fe/Ni-N-PCS also exhibits superior oxygen evolution reaction (OER) performance with low overpotential and long-term stability. Zn-air battery with Fe/Ni-N-PCS cathode yields encouraging performance, including working potential, peak power density, and the stability of charge and discharge cycles. Our synthesis method may promote the fabrication of other DM-SAC and the great promise in practical applications.

Defective Graphene/Plasmonic Nanoparticle Hybrids for Surface-Enhanced Raman Scattering Sensors

Two-dimensional-zero-dimensional plasmonic hybrids involving defective graphene and transition metals (DGR-TM) have drawn significant interest due to their near-field plasmonic effects in the wide range of the UV-vis-NIR spectrum. In the present work, we carried out extensive investigations on resonance Raman spectroscopy (RRS) and localized surface plasmon resonance (LSPR) from the various DGR-TM hybrids (Au, Ag, and Cu) using micro-Raman, spatial Raman mapping analysis, high-resolution transmission electron microscopy (HRTEM), and LSPR absorption measurements on defective CVD graphene layers. Further, electric field (E) mappings of samples were calculated using the finite domain time difference (FDTD) method to support the experimental findings. The spatial distribution of various in-plane and edge defects and defect-mediated interaction of plasmonic nanoparticles (NPs) with graphene were investigated on the basis of the RRS and LSPR and correlated with the quantitative analysis from HRTEM, excitation wavelength-dependent micro-Raman, and E-field enhancement features of defective graphene and defective graphene-Au hybrids before and after rapid thermal annealing (RTA). Excitation wavelength-dependent surface-enhanced Raman scattering (SERS) and LSPR-induced broadband absorption from DGR-Au plasmonic hybrids reveal the electron and phonon interaction on the graphene surface, which leads to the charge transfer from TM NPs to graphene. This is believed to be responsible for the reduction in the SERS signal, which was observed from the wavelength-dependent Raman spectroscopy/mappings. We implemented defective graphene and DGR-Au plasmonic hybrids as efficient SERS sensors to detect the Fluorescein and Rhodamine 6G molecules with a detection limit down to 10^-9 M. Defective graphene and Au plasmonic hybrids showed an impressive Raman enhancement in the order of 10⁸, which is significant for its practical application.

The effect of vacancy defects on the electromechanical properties of monolayer NiTe2 from first principles calculations

The electromechanical properties of monolayer 1-T NiTe₂ under charge actuation were investigated using first-principles density functional theory (DFT) calculations. Monolayer 1-T NiTe₂ in its pristine form has a work area density per cycle of up to 5.38 MJ m^-3 nm upon charge injection and it can generate a strain and a stress of 1.51% and 0.96 N m^-1, respectively. We found that defects in the form of vacancies can be exploited to modulate the electromechanical properties of this material. The presence of Ni-vacancies can further enhance the generated stress by 22.5%. On the other hand, with Te-vacancies, it is possible to improve the work area density per cycle by at least 145% and also to enhance the induced strain from 1.51% to 2.92%. The effect of charge polarity on the contraction and expansion of monolayer 1T-NiTe₂ was investigated. Due to its excellent environmental stability and good electromechanical properties, monolayer NiTe₂ is considered to be a promising electrode material for electroactive polymer (EAP) based actuators.

New Folding 2D-Layered Nitro-Oxygenated Carbon Containing Ultra High-Loading Copper Single Atoms

The discoveries of 2D nanomaterials have made huge impacts on the scientific community. Their unique properties unlock new technologies and bring significant advances to diverse applications. Herein, an unprecedented 2D-stacked material consisting of copper (Cu) on nitro-oxygenated carbon is disclosed. Unlike any known 2D stacked structures that are usually constructed by stacking of separate 2D layers, this material forms a continuously folded 2D-stacked structure. Interestingly, advanced characterizations indicate that Cu atoms inside the structure are in an atomically-dispersed form with extraordinarily high Cu loading up to 15.9 ± 1.2 wt.%, which is among the highest reported metal loading for single-atom catalysts on 2D supports. Facile exfoliation results in thin 2D nanosheets that maximize the exposure of the unique active sites (two neighboring Cu single atoms), leading to impressive catalytic performance, as demonstrated in the electrochemical oxygen reduction reaction.

Identifying and manipulating single atoms with scanning transmission electron microscopy

The manipulation of individual atoms has developed from visionary speculation into an established experimental science. Using focused electron irradiation in a scanning transmission electron microscope instead of a physical tip in a scanning probe microscope confers several benefits, including thermal stability of the manipulated structures, the ability to reach into bulk crystals, and the chemical identification of single atoms. However, energetic electron irradiation also presents unique challenges, with an inevitable possibility of irradiation damage. Understanding the underlying mechanisms will undoubtedly continue to play an important role to guide experiments. Great progress has been made in several materials including graphene, carbon nanotubes, and crystalline silicon in the eight years since the discovery of electron-beam manipulation, but the important challenges that remain will determine how far we can expect to progress in the near future.

Pd single-atom-site stabilized by supported phosphomolybdic acid: design, characterizations and tandem Suzuki-Miyaura cross coupling/nitro hydrogenation reaction

Herein, a single-metal (Pd) site with high surface energy was stabilized and dispersed on a support (zirconia) via a stabilizing agent (phosphomolybdic acid) using a wet chemistry method. HRTEM and HAADF-STEM showed a highly uniform dispersion of Pd SASc on PMA/ZrO₂. The Pd SASc showed superior catalytic activity (>99% conversion) for the Suzuki-Miyaura cross-coupling reaction, which was further feasible for catalyzing mechanistically different nitro hydrogenation reactions in tandem fusion under mild reaction conditions. This catalyst showed outstanding activity (100% conversion and 99% selectivity) with a substrate/catalyst ratio of 927 and TON of 918 using a very low amount of Pd (0.94 × 10^-3 mmol) for the tandem Suzuki-Miyaura cross-coupling/nitro hydrogenation reaction. It also exhibited superior stability and reusability for up to three cycles without any change in its activity.

Emerging Heterogeneous Supports for Efficient Electrocatalysis

Electrocatalysis plays a fundamental role in many fields, such as metallurgy, medicine, chemical industry, and energy conversion. Anchoring active electrocatalysts with controllable loading and uniform dispersion onto suitable supports has become an attractive topic. This is because the supports can not only have the potential to improve catalytic activity and stability through the interaction between support and catalytic center, but also can reduce precious metal consumption by improving atomic utilization. Herein, recent theoretical and experimental progresses concerning the development of supports to anchor electrocatalytic materials are first reviewed. Next, their controllable syntheses, characterization techniques, metal-support electronic interactions, and structure-performance relationships are presented. Some representative carbon supports and non-carbonaceous supports, as well as recently reported star supports such as 2D supports, single atom catalysts, and self-supported catalysts are also summarized. In addition, the significant role of support in stabilizing and regulating catalytic active sites is particularly emphasized. Finally, challenges, opportunities, key problems, and further promising solutions for supported catalysts are proposed.

Electronic Properties and Electrocatalytic Water Splitting Activity for Precious-Metal-Adsorbed Silicene with Nonmetal Doping

Since nonmetal (NM)-doped two-dimensional (2D) materials can effectively modulate their physical properties and chemical activities, they have received a lot of attention from researchers. Therefore, the stability, electronic properties, and electrocatalytic water splitting activity of precious-metal (PM)-adsorbed silicene doped with two NM atoms are investigated based on density functional theory (DFT) in this paper. The results show that NM doping can effectively improve the stability of PM-adsorbed silicene and exhibit rich electronic properties. Meanwhile, by comparing the free energies of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) intermediates of 15 more stable NM-doped systems, it can be concluded that the electrocatalytic water splitting activity of the NM-doped systems is more influenced by the temperature. Moreover, the Si-S2-Ir-doped system exhibits good HER performance when the temperature is 300 K, while the Si-N2-Pt-doped system shows excellent OER activity. Our theoretical study shows that NM doping can effectively promote the stability and electrocatalytic water splitting of PM-adsorbed silicene, which can help in the application of silicene in electrocatalytic water splitting.

Fe@χ3-borophene as a promising catalyst for CO oxidation reaction: A first-principles study

A novel single-atom catalyst of Fe adsorbed on χ₃-borophene has been proposed as a potential catalyst for CO oxidation reaction (COOR). Quantitative pictures have been provided of both the stability of Fe@χ₃-borophene and various kinetic reaction pathways using first-principles calculations. Strong adsorption energy of -3.19 eV and large diffusion potential of 3.51 eV indicates that Fe@χ₃-borophene is highly stable. By exploring reaction mechanisms for COOR, both Eley-Ridel (E-R) and trimolecule E-R (TER) were identified as possible reaction paths. Low reaction barriers with 0.49 eV of E-R and 0.57 eV of TER suggest that Fe@χ₃-borophene is a very promising catalyst for COOR. Charge transfer between the χ₃-borophene and CO, O₂ and CO₂ gas molecules plays a key role in lowering the energy barrier during the reactions. Our results propose that Fe@χ₃-borophene can be a good candidate of single-atom catalyst for COOR with both high stability and catalytic activity.

Thermal transport in porous graphene with coupling effect of nanopore shape and defect concentration

Thermal conductivity of porous graphene can be affected by defect concentration, nanopore shape and distribution, and it is hard to clarify the effects due to the correlation of those factors. In this work, molecular dynamics simulation is used to compare the thermal conductivity of graphene with three shapes of regularly arranged nanopores. The results prove the dominant role of defect concentration under certain circumstances in reducing thermal conductivity, while the coupling effect of nanopore shape should be noticed. When the atoms at the local phonon scattering area around each nanopore are properly removed, the abnormal increment of thermal conductivity can be detected with the increase of defect concentration. Heat flux vector angles can effectively characterize the local phonon scattering area, which can be used to describe the effect of nanopore shape. The coupling effect of defect concentration and pore shape with similar heat flux path is clarified according to this process. By adjusting vertex angle of triangle defect, there is a balanced state of the effect factors between the variation of defect concentration and the same phonon scattering area. It provides a possible way to describe the weighing factors of the coupling effect. The results suggest a feasible approach to optimize and regulate thermal properties of porous graphene in nanodevice.

Boosted ammonium production by single cobalt atom catalysts with high Faradic efficiencies

Efficient n = O bond activation is crucial for the catalytic reduction of nitrogen compounds, which is highly affected by the construction of active centers. In this study, n = O bond activation was achieved by a single-atom catalyst (SAC) with phosphorus anchored on a Co active center to form intermediate N-species for further hydrogenation and reduction. Unique phosphorus-doped discontinuous active sites exhibit better n = O activation performance than conventional N-cooperated single-atom sites, with a high Faradic efficiency of 92.0% and a maximum ammonia yield rate of 433.3 μg NH4·h^-1·cm^-2. This approach of constructing environmental sites through heteroatom modification significantly improves atom efficiency and will guide the design of future functional SACs with wide-ranging applications.

Beam-driven Dynamics of Aluminium Dopants in Graphene

Substituting heteroatoms into graphene can tune its properties for applications ranging from catalysis to spintronics. The further recent discovery that covalent impurities in graphene can be manipulated at atomic precision using a focused electron beam may open avenues towards sub-nanometer device architectures. However, the preparation of clean samples with a high density of dopants is still very challenging. Here, we report vacancy-mediated substitution of aluminium into laser-cleaned graphene, and without removal from our ultra-high vacuum apparatus, study their dynamics under 60 keV electron irradiation using aberration-corrected scanning transmission electron microscopy and spectroscopy. Three- and four-coordinated Al sites are identified, showing excellent agreement with ab initio predictions including binding energies and electron energy-loss spectrum simulations. We show that the direct exchange of carbon and aluminium atoms predicted earlier occurs under electron irradiation, although unexpectedly it is less probable than the same process for silicon. We also observe a previously unknown nitrogen-aluminium exchange that occurs at Al─N double-dopant sites at graphene divacancies created by our plasma treatment.

Orbital Dependence in Single-Atom Electrocatalytic Reactions

Transition metal single-atom catalysts supported on N-doped graphene (TM-N-C) could serve as an ideal model for studying orbital dependence in electrocatalytic reactions because the atom on the catalytic active site has discrete single-atom-like orbitals. In this work, the catalytic efficiency of Fe-N-C for the oxygen evolution reaction (OER) under a small structural perturbation has been comprehensively investigated with density functional theory calculations. The results suggest that the subtle local environment of a single atom can significantly modulate the catalytic reactivity. Further analysis demonstrates that the energy level of the TM d_z² orbital center, rather than the d-band center, is responsible for the OER catalytic efficiency as the d_z² orbital participates mainly in the reactions. Essentially, the d-band theory can be extended to the sub-d orbital level, and a small perturbation of the crystal field, induced by lattice strain or z-direction displacement of the TM atom, can prominently change the sub-d orbital associated with the reaction and in turn affect the catalytic activity.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

SiC monolayers as promising substrates for the development of highly stable single atom catalysts (Pd1/SiC): A density functional theory study

Single-atom catalysts have been touted as highly efficient catalysts, but the catalytic single-atom sites are unstable and tends to aggregate into nanoparticles during chemical reactions. In this study, we show that SiC monolayers are promising substrates for the development of highly stable single-atom catalysts (Pd 1 /SiC) within the density functional theory. In the presence of Si-vacancy, the diffusion barrier energy of Pd 1 atom embedded on SiC monolayer is substantially enhanced from 2.3 to 7.8 eV, which is much higher than the reported diffusion barrier energies of graphene, boron nitride and defective MgO of the same catalytic system. Ab initio molecular dynamic at 500K also confirms the enhanced stability of Pd1/SiC monolayer (Si-vacancy) such that the Pd 1 atom remains embedded in the vacancy. Additionally, the Pd 1 /SiC monolayer (Si-vacancy) catalysts show a ~34% reduction of activation barrier energy for CO oxidation as compared to the pristine catalysts. This work implies that nanostructured SiC materials are potential substrates for the synthesis of highly stable single-atom catalysts.

Single Carbon Vacancy Traps Atomic Platinum for Hydrogen Evolution Catalysis

The coordinated configuration of atomic platinum (Pt) has always been identified as an active site with high intrinsic activity for hydrogen evolution reaction (HER). Herein, we purposely synthesize single vacancies in a carbon matrix (defective graphene) that can trap atomic Pt to form the Pt-C₃ configuration, which gives exceptionally high reactivity for HER in both acidic and alkaline solutions. The intrinsic activity of Pt-C₃ site is valued with a turnover frequency (TOF) of 26.41 s^-1 and mass activity of 26.05 A g^-1 at 100 mV, respectively, which are both nearly 18 times higher than those of commercial 20 wt % Pt/C. It is revealed that the optimal coordination Pt-C₃ has a stronger electron-capture ability and lower Gibbs free energy difference (ΔG), resulting in promoting the reduction of adsorbed H⁺ and the acceleration of H₂ desorption, thus exhibiting the extraordinary HER activity. This work provides a new insight on the unique coordinated configuration of dispersive atomic Pt in defective C matrix for superior HER performance.

Permeability of boron- and nitrogen-doped graphene nanoflakes for protium/deuterium ions

Two-dimensional (2D) monolayer nanomaterials are the thinnest possible membranes with interesting selective permeation characteristics. Among two-dimensional materials, graphenes and hexagonal boron nitride (h-BN) are the most promising membrane materials, which can even allow the separation of proton isotopes. The current work aims at understanding the higher reported permeability of h-BN by sequential doping of B and N atoms in graphene nanoflakes. The kinetic barriers were calculated with two different models of graphenes; coronene and dodecabenzocoronene via zero-point energy calculations at the transition state for proton permeability. The lower barriers for h-BN are mainly due to boron atoms. The trends of kinetic barriers are B < BN < N-doped graphenes. The permeation selectivity of graphene models increases with doping. Our studies suggest that boron-doped graphene models show an energy barrier of 0.04 eV for the permeation of proton, much lower than that of the model graphene and h-BN sheet, while nitrogen-doped graphenes have a very high energy barrier up to 7.44 eV for permeation. Therefore, boron-doped graphene models are suitable candidates for proton permeation. Moreover, the presence of carbon atoms in the periphery of BN sheets has significant negative effects on the permeation of proton isotopes, an unexplored dimension of the remote neighboring effect in 2-D materials. This study illustrates the need for permeation study through other hetero-2D surfaces, where interesting hidden chemistry is still unexplored.

Two-dimensional (2D) monolayer nanomaterials are the thinnest possible membranes with interesting selective permeation characteristics.

The adsorption behaviors of N2O on penta-graphene and Ni-doped penta-graphene

In order to develop the adsorption application of penta-graphene (PG) to N₂O gas molecule, we calculated the sensing properties of PG and Ni-doped PG to N₂O molecule via first-principles calculations. Based on the calculated adsorption energy, charge transfer, band gap, density of states and partial density of states, we observed that this gas molecule was weakly physically adsorbed on the surface of intrinsic PG, while the adsorption behaviors on the surface of Ni-doped PG were greatly influenced by the doping sites and adsorption orientations. With the Ni atom doped at the sp² hybridized carbon site, strong chemical adsorption between the gas molecule and the substrate was induced. The adsorption structure of the N₂O molecule with its N atom close to the substrate exhibited better stability. Moreover, an external perpendicular electric field could enhance the adsorption performance of the N₂O molecule and adjust the charge transfer between the molecule and substrate. Our results broaden the adsorption applications of PG and indicate that Ni-doped PG is a potential candidate for N₂O gas sensors.

N₂O molecule is chemically adsorbed on the surface of Ni-doped penta-graphene only when the Ni atom is doped at the sp² hybridized carbon site. External perpendicular electric field can enhance the adsorption performance.

Highly dispersed rhodium atoms supported on defect-rich Co(OH)2 for the chemoselective hydrogenation of nitroarenes

0.54% Rh/Co(OH)2 exhibited 100% selectivity for –NO2 hydrogenation at >96% conversion for nitroarene hydrogenation. Its excellent catalytic performance is due to the interfacial effect of Rh–Co(OH)2 and Rh in the form of single atoms and nanoclusters.

The Adsorption Behavior of Gas Molecules on Co/N Co-Doped Graphene

Herein, we have used density functional theory (DFT) to investigate the adsorption behavior of gas molecules on Co/N3 co-doped graphene (Co/N3-gra). We have investigated the geometric stability, electric properties, and magnetic properties comprehensively upon the interaction between Co/N3-gra and gas molecules. The binding energy of Co is -5.13 eV, which is big enough for application in gas adsorption. For the adsorption of C2H4, CO, NO2, and SO2 on Co/N-gra, the molecules may act as donors or acceptors of electrons, which can lead to charge transfer (range from 0.38 to 0.7 e) and eventually change the conductivity of Co/N-gra. The CO adsorbed Co/N3-gra complex exhibits a semiconductor property and the NO2/SO2 adsorption can regulate the magnetic properties of Co/N3-gra. Moreover, the Co/N3-gra system can be applied as a gas sensor of CO and SO2 with high stability. Thus, we assume that our results can pave the way for the further study of gas sensor and spintronic devices.

New insights into NO adsorption on alkali metal and transition metal doped graphene nanoribbon surface: A DFT approach

The aim of this article is to investigate the sensing performance of NO gas molecule on the graphene nanoribbon domain for the determination of structural and electronic properties. Effect of an alkali metal (lithium) and a transition metal (iron) on the armchair oriented graphene nanoribbon (ArGNR) surface for the sensing purpose of NO gas has been performed through the quantum mechanics based Density Functional Theory (DFT) calculations. Various configurations of ArGNR doped with Li and Fe atoms such as one-edge doped, center doped, both-edge doped Li-ArGNR and Fe-ArGNR have been simulated, and a detailed comparative study of lithium and iron doping on different configurations of ArGNRs for the adsorption energy, stability analysis, band gap analysis and density of states analysis has been quantitatively evaluated. By comparing the adsorption energy of NO, it is found that Li doping enhances the strength of NO adsorption on the different variants of ArGNR. Computational results predict that the undoped ArGNR is insensitive to the NO gas adsorption with adsorption energy of about -0.41 eV. Our results determine that substitutional doping of Li doping at one edge doped and both-edge doped position increases the adsorption abilities of ArGNRs in these configurations with adsorption energies of approximately -6.92 eV and -9.64 eV that is 16 and 23 times greater than the pristine ArGNR (Pr-ArGNR). Band nature for both type of doping estimates the changing behavior of ArGNRs from semiconductor to metallic transition after the adsorption of NO molecule. It is concluded that the Li doping at one edge and both edge position of ArGNR makes it an excellent potential sensing material for the sensing purpose of NO gas as compared to the Fe doped configurations.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Emerging properties of carbon based 2D material beyond graphene

Graphene turns out to be the pioneering material for setting up boulevard to a new zoo of recently proposed carbon based novel two dimensional (2D) analogues. It is evident that their electronic, optical and other related properties are utterly different from that of graphene because of the distinct intriguing morphology. For instance, the revolutionary emergence of Dirac cones in graphene is particularly hard to find in most of the other 2D materials. As a consequence the crystal symmetries indeed act as a major role for predicting electronic band structure. Since tight binding calculations have become an indispensable tool in electronic band structure calculation, we indicate the implication of such method in graphene's allotropes beyond hexagonal symmetry. It is to be noted that some of these graphene allotropes successfully overcome the inherent drawback of the zero band gap nature of graphene. As a result, these 2D nanomaterials exhibit great potential in a broad spectrum of applications, viz nanoelectronics, nanooptics, gas sensors, gas storages, catalysis, and other specific applications. The miniaturization of high performance graphene allotrope based gas sensors to microscopic or even nanosized range has also been critically discussed. In addition, various optical properties like the dielectric functions, optical conductivity, electron energy loss spectra reveal that these systems can be used in opto-electronic devices. Nonetheless, the honeycomb lattice of graphene is not superconducting. However, it is proposed that the tetragonal form of graphene can be intruded to form new hybrid 2D materials to achieve novel superconducting device at attainable conditions. These dynamic experimental prospects demand further functionalization of these systems to enhance the efficiency and the field of multifunctionality. This topical review aims to highlight the latest advances in carbon based 2D materials beyond graphene from the basic theoretical as well as future application perspectives.

Tailoring the catalytic performance of single platinum anchored on graphene by vacancy engineering for propane dehydrogenation: a theoretical study

Propane dehydrogenation (PDH) is an effective approach to produce propylene. Downsizing the Pt species to single atom catalysts (SACs) has become a hotspot, owing to the maximum utilization and excellent catalytic behavior. However, the agglomeration of SACs is the decisive limitation for high temperature PDH. Herein, single Pt atoms were anchored on graphene with different types of vacancies, and their catalytic performances on PDH were explored based on density functional theory (DFT). As the vacancy size increased, the catalytic activity decreased. It was because the combined site of the detached H atom in propane would transfer from the Pt atom to the C atom around vacancies, thus increasing the migration distance and lowering the activity. However, with the increase of vacancy size, the selectivity to propylene was improved, owing to the enhanced repulsion between C atoms in graphene and propylene. Therefore, instead of stabilizing the single atom, vacancies in carbon materials can also tailor the catalytic performance by geometric disturbance. This fundamental work opens up the possibility for purposeful SAC design in PDH.

Atomically dispersed Fe atoms anchored on S and N-codoped carbon for efficient electrochemical denitrification

Nitrate, a widespread contaminant in natural water, is a threat to ecological safety and human health. Although direct nitrate removal by electrochemical methods is efficient, the development of low-cost electrocatalysts with high reactivity remains challenging. Herein, bifunctional single-atom catalysts (SACs) were prepared with Cu or Fe active centers on an N-doped or S, N-codoped carbon basal plane for N₂ or NH₄ ⁺ production. The maximum nitrate removal capacity was 7,822 mg N ⋅ g^-1 Fe, which was the highest among previous studies. A high ammonia Faradic efficiency (78.4%) was achieved at a low potential (-0.57 versus reversible hydrogen electrode), and the nitrogen selectivity was 100% on S-modified Fe SACs. Theoretical and experimental investigations of the S-doping charge-transfer effect revealed that strong metal-support interactions were beneficial for anchoring single atoms and enhancing cyclability. S-doping altered the coordination environment of single-atom centers and created numerous defects with higher conductivity, which played a key role in improving the catalyst activity. Moreover, interactions between defects and single-atom sites improved the catalytic performance. Thus, these findings offer an avenue for high active SAC design.

Designing Undercoordinated Ni-Nx and Fe-Nx on Holey Graphene for Electrochemical CO2 Conversion to Syngas

In this study, we propose a top-down approach for the controlled preparation of undercoordinated Ni-N_x (Ni-hG) and Fe-N_x (Fe-hG) catalysts within a holey graphene framework, for the electrochemical CO₂ reduction reaction (CO₂RR) to synthesis gas (syngas). Through the heat treatment of commercial-grade nitrogen-doped graphene, we prepared a defective holey graphene, which was then used as a platform to incorporate undercoordinated single atoms via carbon defect restoration, confirmed by a range of characterization techniques. We reveal that these Ni-hG and Fe-hG catalysts can be combined in any proportion to produce a desired syngas ratio (1-10) across a wide potential range (-0.6 to -1.1 V vs RHE), required commercially for the Fischer-Tropsch (F-T) synthesis of liquid fuels and chemicals. These findings are in agreement with our density functional theory calculations, which reveal that CO selectivity increases with a reduction in N coordination with Ni, while unsaturated Fe-N_x sites favor the hydrogen evolution reaction (HER). The potential of these catalysts for scale up is further demonstrated by the unchanged selectivity at elevated temperature and stability in a high-throughput gas diffusion electrolyzer, displaying a high-mass-normalized activity of 275 mA mg^-1 at a cell voltage of 2.5 V. Our results provide valuable insights into the implementation of a simple top-down approach for fabricating active undercoordinated single atom catalysts for decarbonized syngas generation.

Prediction and Understanding: Quantum Chemical Framework of Transition Metals Enclosed in a B12N12 Inorganic Nanocluster for Adsorption and Removal of DDT from the Environment

Emission of harmful pollutants from different sources into the environment is a major problem nowadays. Organochlorine pesticides such as DDT (C₁₄H₉Cl₅) are toxic, bio-accumulative, and regularly seen in water bodies, air, biota, and sediments. Various systems can be considered for minimizing the DDT (dichloro-diphenyl-trichloroethane) pollution. However, due to simplicity and acceptability, the adsorption method is the most popular method. Adsorption is gradually employed for the removal of both organic and inorganic pollutants found in soil and water. Thus, in this regard, efforts are being made to design inorganic nanoclusters (B₁₂N₁₂) encapsulated with late transition metals (Zn, Cu, Ni, Co, and Fe) for effective adsorption of DDT. In this context, detailed thermodynamics and quantum chemical study of all the designed systems have been carried out with the aid of density functional theory. The adsorption energy of DDT on metals cocooned in a nanocluster is found to be higher, and better adsorption energy values as compared to that of the pristine B₁₂N₁₂-DDT nanocluster have been reported. Further, analysis of the dipole moment, frontier molecular orbitals, molecular electrostatic potential plots, energy band gap, Q_NBO, and Fermi level suggested that the late-transition-metal-encapsulated inorganic B₁₂N₁₂ nanoclusters are efficient candidates for effective DDT adsorption. Lastly, the study of global descriptors of reactivity confirmed that the designed quantum mechanical systems are quite stable in nature with a good electrophilic index. Therefore, the recommendation has been made for these novel kinds of systems to deal with the development of DDT sensors.

Fabricating polyoxometalates-stabilized single-atom site catalysts in confined space with enhanced activity for alkynes diboration

Effecting the synergistic function of single metal atom sites and their supports is of great importance to achieve high-performance catalysts. Herein, we successfully fabricate polyoxometalates (POMs)-stabilized atomically dispersed platinum sites by employing three-dimensional metal-organic frameworks (MOFs) as the finite spatial skeleton to govern the accessible quantity, spatial dispersion, and mobility of metal precursors around each POM unit. The isolated single platinum atoms (Pt₁) are steadily anchored in the square-planar sites on the surface of monodispersed Keggin-type phosphomolybdic acid (PMo) in the cavities of various MOFs, including MIL-101, HKUST-1, and ZIF-67. In contrast, either the absence of POMs or MOFs yielded only platinum nanoparticles. Pt₁-PMo@MIL-101 are seven times more active than the corresponding nanoparticles in the diboration of phenylacetylene, which can be attributed to the synergistic effect of the preconcentration of organic reaction substrates by porous MOFs skeleton and the decreased desorption energy of products on isolated Pt atom sites.

It is of great significance to exert the synergistic effect between single atom and support. Here, the authors prepare polyoxometalates-stabilized single-atom site catalysts in confined space with enhanced activity for alkynes diboration.

Antiferromagnetic spin ordering in two-dimensional honeycomb lattice of SiP3

Magnetism in low-dimensional materials has been of sustained interest due to its intriguing quantum mechanical origin and promising device applications. Here, we propose a buckled honeycomb lattice of stoichiometry SiP₃, a two-dimensional binary group-IV and V material that exhibits an antiferromagnetic ground state with itinerant electrons. Here we perform elemental Si substitution in pristine blue phosphorene to downshift the Fermi energy and induce the Fermi instability that results in a spin polarized ground state. Within first-principles calculations, we observe antiferromagnetic spin alignment between adjacent ferromagnetic triangular domains where each Si atom is coupled with three neighboring P atoms with a ferromagnetic interaction. Such unique spin structure and resulting magnetic ground state are unprecedented in defect-free two-dimensional materials made of only p-block elements. This metal-free magnetism can be exploited for advanced spintronic and memory storage applications.

CO oxidation over defective and nonmetal doped MoS2monolayers

Defective (missing S atoms) and nonmetal (C- and N-) doped MoS₂monolayers in the 2H and 1T' phases have been evaluated for catalyzing CO oxidation based on first-principles calculations. For the reaction 2CO + O₂→ 2CO₂, the oxidization of the first CO molecule is fairly easy and sometimes is even spontaneous, as the O₂ molecule is highly activated or dissociates upon adsorption. However, for the defective (2H-), C-doped (1T'-), and N-doped (2H- and 1T'-) MoS₂monolayers, the remaining O^*adatom often refuses to react with other CO molecules and is hard to be removed (barrier > 1.20 eV). Only when over the C-doped 2H- and defective 1T'-MoS₂monolayers, the removal of the second O^*adatom requires to overcome moderate barriers (0.74 and 0.88 eV, respectively) by reacting with another CO molecule via the Eley-Rideal mechanism and the catalysts are recovered. The barriers can be further reduced by applying either tensile or compressive strain to the MoS₂nanosheet. In contrast, the Langmuir-Hinshelwood mechanism is followed over the metal-containing MoS₂nanosheets, as the bigger size of metal dopants allow the co-adsorption of CO and O₂. Therefore, the C-doped 2H- and defective 1T'-MoS₂monolayers are promising nonmetal-doped catalysts for CO oxidation.