Homochiral Nanopropeller via Chiral Active Surface Growth.

Under the control of chiral ligand glutathione and in the presence of hexadecyltrimethylammonium bromide, Au deposition on Au seeds is known to give chiral nanostructures. We have previously shown that the protruding chiral patterns, as opposed to flat facets, are likely caused by active surface growth, where nonuniform ligand coverage could be responsible for the focused growth at a few active sites. By pushing the limit of such a growth mode, here, we use decahedral seeds to prepare homochiral nanopropellers with intricate patterns of deep valleys and protruding ridges. Control experiments show that the focused growth depends on the rates of Au deposition by changing either the seed concentration or the reductant concentration, consistent with the proposed mechanism. The dynamic growth competition between the ligand-deficient active sites and the ligand-rich surfaces gradually focuses the growth onto a few active sites, causing the expansion of grooves, squeezing of steep ridges, and a surprising 36° rotation of the pentagonal outline. The imbalanced deposition on the prochiral slopes is responsible for the tilted grooves, the twisted walls, and thus the well-separated and distorted blades, which become the origin of the chiroptical responses.

Synthesis of N-substituted phthalimides via Pd-catalyzed [4+1] cycloaddition reaction.

A novel Pd-catalyzed assembly of N-substituted phthalimides by merging of [4+1] cycloaddition and difluorocarbene transfer carbonylation from 2-iodo-N-phenylbenzamides and difluorocarbene precursors is disclosed. Difluorocarbene acts as a carbonyl source and simultaneously forms one C-C bond, one C-N bond and one CO bond to produce N-substituted phthalimides in high yields.

Gold nanohexagrams via active surface growth under sole CTAB control.

The synthesis of homochiral nanostructures involves not only the chiral ligand, but also CTAB. The latter is often treated as a weak ligand unable to compete with the thiol-based chiral ligand. Here, we show that CTAB alone is able to induce Active Surface Growth on Au nanoplates, giving curved tips and steep ridges in the resulting nano-hexagrams. The growth materials (Au0) are diverted to a few active sites, whereas the rest of the Au surfaces are inhibited. Modulation of the growth rate by the ratio of ascorbic acid to Au precursor gives a continuous change of the growth modes, explaining the main trends of shape evolution and the inequivalent growth of the equivalent surfaces. With only CTAB as the ligand, the fact that ridges and spikes could be formed in defiance of facet control suggests that the role of CTAB cannot be ignored in the chiral synthesis and that the precise modulation of the Active Surface Growth could be the key to rational synthetic controls.

Correction: Single atomic Fe-N4 active sites and neighboring graphitic nitrogen for efficient and stable electrochemical CO2 reduction.

Correction for 'Single atomic Fe-N4 active sites and neighboring graphitic nitrogen for efficient and stable electrochemical CO2 reduction' by Leta Takele Menisa et al., Nanoscale Horiz., 2022, https://doi.org/10.1039/D2NH00143H.

Single atomic Fe-N4 active sites and neighboring graphitic nitrogen for efficient and stable electrochemical CO2 reduction.

Single atomic Fe-Nx moieties have shown great performance in CO2-to-CO conversion. However, understanding the structural descriptors that determine the activity of Fe-Nx remains vague, and promising strategies to enhance their catalytic activity are still not clear. Herein, we used a high-temperature pyrolysis strategy and post-synthesis acid treatment for the direct growth of a single Fe-Nx site adjacent to graphitic nitrogen for the electrochemical CO2 reduction reaction. This strategy could significantly reduce the amount of pyridinic and pyrrolic N atoms, while graphitic N surrounding the Fe-Nx site predominantly increases. An experimental study combined with density functional theory revealed that the increase in the neighboring graphitic N decreases the number of electrons transferred between CO and the catalyst for FeN4-2N-3 and FeN4-4N-3, which results in the decrease of the adsorption strength of CO and the energy barrier for desorbing CO*. The as-synthesized Fe-Nx neighbored by graphitic nitrogen exhibited maximum faradaic efficiency of 91% at a lower overpotential of 390 mV. Due to the increase in the graphitic N, the catalysts perform efficiently for 35 h without any drop in current density.

From flat to deep concave: an unusual mode of facet control.

Au particles with rhombic dodecahedron outlines and deep cavities are obtained by epitaxial growth from a triangular nanoplate. An unusual "wrapping" growth that combines ligand-promoted facet-selective growth and site-specific deposition is proposed. Such a templateless growth not only allows the extreme defect-tolerance, but also broadens the synthetic control at the nanoscale.

Coexisting Single-Atomic Fe and Ni Sites on Hierarchically Ordered Porous Carbon as a Highly Efficient ORR Electrocatalyst.

The development of oxygen reduction reaction (ORR) electrocatalysts based on earth-abundant nonprecious materials is critically important for sustainable large-scale applications of fuel cells and metal-air batteries. Herein, a hetero-single-atom (h-SA) ORR electrocatalyst is presented, which has atomically dispersed Fe and Ni coanchored to a microsized nitrogen-doped graphitic carbon support with unique trimodal-porous structure configured by highly ordered macropores interconnected through mesopores. Extended X-ray absorption fine structure spectra confirm that Fe- and Ni-SAs are affixed to the carbon support via FeN4 and NiN4 coordination bonds. The resultant Fe/Ni h-SA electrocatalyst exhibits an outstanding ORR activity, outperforming SA electrocatalysts with only Fe- or Ni-SAs, and the benchmark Pt/C. The obtained experimental results indicate that the achieved outstanding ORR performance results from the synergetic enhancement induced by the coexisting FeN4 and NiN4 sites, and the superior mass-transfer capability promoted by the trimodal-porous-structured carbon support.

Insight into atomically dispersed porous M-N-C single-site catalysts for electrochemical CO2 reduction.

Transition metal single-site catalysts have unique activities for electrochemical CO2 reduction. However, the exact active center and reaction mechanism remain unclear due to a number of challenges in the controllable synthesis of single-atom catalysts (SACs) and defects in metal supports. Here we combine both experimental and theoretical calculations to systematically explore the mechanistic reaction path of selected transition metal single sites on nitrogen-doped porous carbon. Facile pyrolysis was employed to prepare a fullerene type carbon with 0.35 nm interlayer distances to support the family of M-N-C (M = Ni, Fe, Co and Cu). Experimentally, Ni and Fe outperform the other metals with high faradaic efficiency up to >97% and 86.8%, respectively. The theoretical calculations reveal that Ni-N-C exhibits optimum activity for CO2 reduction to CO at a higher overpotential because of the moderate *CO binding energy at the Ni site, which accommodates *COOH formation and *CO desorption. Furthermore, the strong binding energy of *CO on the Fe site enables the catalyst to reduce CO2 beyond CO. A remarkable current density of 17.6 mA cm-2 has been achieved with the Ni-N-C catalyst and a record of 5.74 s-1 TOF has been realized at -0.8 V vs. RHE for the Ni-N-C catalyst.

MOF-derived nitrogen-doped nanoporous carbon for electroreduction of CO2 to CO: the calcining temperature effect and the mechanism.

Nitrogen-doped carbon materials are promising electrocatalysts for electroreduction of CO2. However, the low current density and moderate faradaic efficiency of these materials limit their practical application. Here, we report the MOF-derived nitrogen-doped nanoporous carbon (NC) as a highly efficient and stable electrocatalyst for the conversion of CO2 to CO. The NC catalysts were prepared by calcining ZIF-8 at different temperatures in argon (Ar). The catalytic performances show that the higher pyrolysis temperature result in a better CO2 electroreduction activity of the catalysts. The NC catalyst with the best performance achieves high selectivity with 95.4% CO faradaic efficiency (FE) at -0.5 V vs. reversible hydrogen electrode (RHE). The catalyst also maintains long-term stability of 20 h operation, after which the FE of CO is still greater than 90%. The experiments show that higher pyrolysis temperature reduces the total nitrogen (N) but changes the nature and density of N defects. Density functional theory calculations reveal that higher pyrolysis temperature leads to enhanced activity by promoting the formation of low multiple pyridinic N, which provides more efficient active sites.

Self-Assembly of Chiral Gold Clusters into Crystalline Nanocubes of Exceptional Optical Activity.

Self-assembly of inorganic nanoparticles into ordered structures is of interest in both science and technology because it is expected to generate new properties through collective behavior; however, such nanoparticle assemblies with characteristics distinct from those of individual building blocks are rare. Herein we use atomically precise Au clusters to make ordered assemblies with emerging optical activity. Chiral Au clusters with strong circular dichroism (CD) but free of circularly polarized luminescence (CPL) are synthesized and organized into uniform body-centered cubic (BCC) packing nanocubes. Once the ordered structure is formed, the CD intensity is significantly enhanced and a remarkable CPL response appears. Both experiment and theory calculation disclose that the CPL originates from restricted intramolecular rotation and the ordered stacking of the chiral stabilizers, which are fastened in the crystalline lattices.

Geometry-Modulated Magnetoplasmonic Optical Activity of Au Nanorod-Based Nanostructures.

Comprehension and modulation of optical activity at nanoscale have attracted tremendous interest in the past decades due to its potential application in many fields including chemical/biological sensing, artificial metamaterials, asymmetric catalysis, and so forth. As for the conventional molecular materials, magnetic field is among the most effective routes in inducing and manipulating their optical activity; whereas the magnetic optical activity at nanoscale calls for deeper understanding, especially for anisotropic noble metal nanoparticles. In this work, distinctly different magnetic circular dichroism (MCD) responses are demonstrated in gold nanorods (GNRs) with a derivative-shaped MCD signal corresponding to the transverse surface plasmon resonance (TSPR) band and a Gaussian-shaped signal at the position of the longitudinal surface plasmon resonance (LSPR) band. Furthermore, changing the aspect ratio of GNRs easily regulates such magnetoplasmonic CD response. More interestingly, GNR assemblies with different geometric configuration (end-to-end and side-by-side) show structure-sensitive magnetoplasmonic CD response. Armed with theoretical calculation, we clearly elucidate the intrinsic relationship of the resultant magnetoplasmonic CD response with the optical symmetry and geometry factor inside one-dimensional GNRs. This work not only greatly benefits our understanding toward the nature of SPR mode in anisotropic plasmonic nanostructures but also opens the way to achieve tunable magnetoplasmonic response, which will significantly advance the design and application of optical nanodevices.

Tunable chiral metal organic frameworks toward visible light-driven asymmetric catalysis.

A simple and effective strategy is developed to realize visible light-driven heterogeneous asymmetric catalysis. A chiral organic molecule, which only has very weak catalytic activity in asymmetric α-alkylation of aldehydes under visible light, is utilized as the ligand to coordinate with different types of metal ions, including Zn2+, Zr4+, and Ti4+, for construction of crystalline metal organic frameworks (MOFs). Impressively, when used as heterogeneous catalysts, all of the synthesized MOFs exhibit markedly enhanced activity. Furthermore, the asymmetric catalytic performance of these MOFs could be easily altered by selecting different metal ions, owing to the tunable electron transfer property between metal ions and chiral ligands. This work will provide a new approach for fabrication of heterogeneous catalysts and trigger more enthusiasm to conduct the asymmetric catalysis driven by visible light.

Gold Nanowire Chiral Ultrathin Films with Ultrastrong and Broadband Optical Activity.

An ultrastrong and broadband chiroptical response is key but remains challenging for many device applications. A simple and cost-effective bottom-up method is introduced to fabricate large-area long-range ordered chiral ultrathin films with the Langmuir-Schaeffer technique using gold nanowires as building blocks. Significantly, as-prepared ultrathin films display giant optical activity across a broad wavelength range covering visible and near infrared regions with an anisotropic factor of up to 0.285, which is the record value for bottom-up techniques. Detailed experimental result and theoretical analysis disclose that such remarkable optical activity originates from birefringence and dichroism of the well-aligned Au nanowire layers in the ultrathin films. The universality of this facile strategy for constructing chiral ultrathin films is further demonstrated with many other one-dimensional nanomaterials.

High-energy passively Q-switched 2 µm Tm(3+)-doped double-clad fiber laser using graphene-oxide-deposited fiber taper.

We have demonstrated a high-energy Q-switched double-clad thulium-doped fiber laser (TDFL) using a graphene-oxide-deposited tapered fiber (GODTF) device as a saturable absorber operating at a wavelength of 2 µm for the first time. Because of the side-interaction of the graphene-oxide with the evanescent field on the taper waist, the GODTF devices have potential for offering high laser damage threshold. Using a 788 nm laser diode as the pump source, the TDFL generated stable single transverse mode Q-switched pulses with a single pulse energy of 6.71 µJ (corresponding to an average power of 302 mW) at a wavelength of 2032 nm. This is significantly higher than the highest pulse energy/average power from any rare-earth-doped fiber lasers employing a graphene or graphene-oxide based Q-switch so far. The demonstrated TDFL in this paper represents an encouraging step towards the practical applications of graphene or graphene-oxide based Q-switched 2 µm TDFLs.