Comprehensive View of the Ligand–Gold Interface from First Principles

A large number of ligands have been used to stabilize and functionalize the gold surfaces and nanoclusters, but there has been no systematic comparison about their binding strengths with gold. In this work, we studied the interaction between 27 ligands of six different types (thiolates, phosphines, amines, aryl radicals, alkynyls, and N-heterocyclic carbenes) with the model Au surfaces by first-principles density functional theory (DFT). On the perfect Au(111), we found the order of binding strengths to be bulky N-heterocyclic carbenes (NHCs) ≈ alkynyls > thiolates ≈ phosphines > aryls ≈ less sterically bulky NHCs > alkylamines. The much stronger interaction of bulky carbenes to Au than the less sterically bulky NHCs arises from the van der Waals (vdW) attraction of bulky side groups with gold surface via the short Au···HCH₂R contact. Further, we showed that the presence of a gold adatom on Au(111) leads to enhanced binding and a similar order for most of the ligands examined. Overall, bulky NHCs and alkynyl groups form the strongest interaction to both Au(111) and Au_ad–Au­(111). This suggests that NHCs can be employed as alternatives to the currently widely used thiolates and the emerging alkynyl ligands for the preparation of more stable self-assembled monolayer structures on metal surfaces. Further, this insight allowed us to design viable magic-number gold clusters with NHCs as the protecting ligands

Revisiting Structural Models for Au₁₈(SR)₁₄

Au₁₈(SR)₁₄ (−SR being a thiolate group) is a small, stable thiolated gold nanocluster experimentally identified in 2005, but its structure remains elusive. A previously proposed model for Au₁₈(SR)₁₄ based on density functional theory (DFT) structural optimization consists of a Au₈ core protected by two −RS–Au–SR–Au–SR– (dimer) and two −RS–Au–SR–Au–SR–Au–SR– (trimer) motifs. Here we revisit structure prediction for Au₁₈(SR)₁₄ from extensive exploration of the possible isomers for Au₁₈(SCH₃)₁₄ by applying structural hypotheses based on both “staple motifs” and “ring and staple motifs”. Three isomers based on the staple motifs are found to be more stable than the best previous model. The two lowest-energy Au₁₈(SCH₃)₁₄ isomers (I and II) also have a Au₈ core protected by two dimer and two trimer motifs, but the core geometry and electronic properties are different. The third lowest-energy isomer (III) consists of a Au₈ core protected by two −RS–Au–SR– (monomer) and two −RS–Au–SR–Au–SR–Au–SR–Au–SR– (tetramer) motifs. By changing R from CH₃ to the experimentally used cyclohexanyl group (C₆H₁₁), we found that isomer III is the most stable for Au₁₈(SC₆H₁₁)₁₄. The computed X-ray diffraction (XRD) pattern and optical spectrum of isomer III are in good agreement with the experimental data. This work suggests that Au₁₈(SR)₁₄ may have monomer and tetramer motifs in the protective layer

Mechanism of Hydrogen Evolution Reaction on 1T-MoS₂ from First Principles

The 1T phase of transition-metal dichalcogenides (TMDs) has been demonstrated in recent experiments to display excellent catalytic activity for hydrogen evolution reaction (HER), but the catalytic mechanism has not been elucidated so far. Herein, using 1T MoS₂ as the prototypical TMD material, we studied the HER activity on its basal plane from periodic density functional theory (DFT) calculations. Compared to the nonreactive basal plane of 2H phase MoS₂, the catalytic activity of the basal plane of 1T phase MoS₂ mainly arises from its affinity for binding H at the surface S sites. Using the binding free energy (Δ<i>G</i>_H) of H as the descriptor, we found that the optimum evolution of H₂ will proceed at surface H coverage of 12.5% ∼ 25%. Within this coverage, we examined the reaction energy and barrier for the three elementary steps of the HER process. The Volmer step was found to be facile, whereas the subsequent Heyrovsky reaction is kinetically more favorable than the Tafel reaction. Our results suggest that at low overpotential, HER can take place readily on the basal plane of 1T MoS₂ via the Volmer–Heyrovsky mechanism. We further screened the dopants for the HER activity and found that substitutional doping of the Mo atom by metals such as Mn, Cr, Cu, Ni, and Fe can make 1T MoS₂ a better HER catalyst

Are MXenes Promising Anode Materials for Li Ion Batteries? Computational Studies on Electronic Properties and Li Storage Capability of Ti₃C₂ and Ti₃C₂X₂ (X = F, OH) Monolayer

Density functional theory (DFT) computations were performed to investigate the electronic properties and Li storage capability of Ti₃C₂, one representative MXene (M represents transition metals, and X is either C or/and N) material, and its fluorinated and hydroxylated derivatives. The Ti₃C₂ monolayer acts as a magnetic metal, while its derived Ti₃C₂F₂ and Ti₃C₂(OH)₂ in their stable conformations are semiconductors with small band gaps. Li adsorption forms a strong Coulomb interaction with Ti₃C₂-based hosts but well preserves its structural integrity. The bare Ti₃C₂ monolayer exhibits a low barrier for Li diffusion and high Li storage capacity (up to Ti₃C₂Li₂ stoichiometry). The surface functionalization of F and OH blocks Li transport and decreases Li storage capacity, which should be avoided in experiments. The exceptional properties, including good electronic conductivity, fast Li diffusion, low operating voltage, and high theoretical Li storage capacity, make Ti₃C₂ MXene a promising anode material for Li ion batteries

Single-Layer [Cu₂Br(IN)₂]_<i>n</i> Coordination Polymer (CP): Electronic and Magnetic Properties, and Implication for Molecular Sensors

Inspired by the recent breakthrough in synthesizing the two-dimensional (2D) [Cu₂Br­(IN)₂]_<i>n</i> (IN = isonicotinato) single-layer coordination polymer (CP) (<i>Chem. Commun.</i> <b>2010</b>, <i>46</i>, 3262), we systematically investigated the structural, electronic, and magnetic properties of this periodic monolayer [Cu₂Br­(IN)₂]_<i>n</i> CP, as well as its possible application as molecular sensors by means of density functional theory computations. The pristine monolayer [Cu₂Br­(IN)₂]_<i>n</i> CP is ground-state antiferromagnetic with a band gap of 0.47 eV. Among various gas molecules (H₂, O₂, CO, CO₂, NO, NO₂, N₂, and NH₃), NO and NO₂ have strong interactions with the metal centers and can effectively modify the electronic structure of this monolayer [Cu₂Br­(IN)₂]_<i>n</i> CP, suggesting the feasibility of designing 2D CP-based molecular sensors to detect NO and NO₂ molecules

Frequency-electrode functions.

<p>The panel shows pure tone frequency matched to an electrode position for each of the 3 cochlear implant subjects with significant contralateral acoustic hearing. The x-axis is the electrode insertion depth from the round window. The y-axis is the pure-tone frequency matched to an electric stimulus delivered to a single electrode at a fixed stimulation rate (different symbols representing different rates). Error bars represent the frequency difference of two pure tones that were judged higher in pitch than the electric stimulus 50% and 70.7% of times (see text in the methods section). The upper dashed line represents the Greenwood function and the lower dashed line represents two octaves below the Greenwood function. The solid line represents the clinical frequency-electrode map used in the speech processor (Advanced Bionics for S1; Med El for S2 and S3).</p

Frequency discrimination for the non-implant ears.

<p>The Weber fraction is plotted as a function of standard frequency for S1 (open circles), S2 (triangles), and S3 (inverted triangles). The normal range (mean±2 SDs) is shown as the dashed outlined box, which was obtained using a similar paradigm in a previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Zeng5" target="_blank">[34]</a>.</p

Structural Evolution of Tc_<i>n</i> (<i>n</i> = 4–20) Clusters from First-Principles Global Minimization

We explore the structural evolution of Tc_<i>n</i> (<i>n</i> = 4–20) clusters using a first-principles global minimization technique, namely, basin-hopping from density functional theory geometry optimization (BH-DFT). Significantly more stable structures have been found in comparison with previous models, indicating the power of DFT-based basin hopping in finding new structures for clusters. The growth sequence and pattern for <i>n</i> from 4 to 20 are analyzed from the perspective of geometric shell formation. The binding energy per atom, relative stability, and magnetic moments are examined as a function of the cluster size. Several magic sizes of higher stability and symmetry are discovered. In particular, we find that Tc₁₉ prefers an O_h symmetry structure, resembling a piece of a face-centered-cubic metal, and its electrostatic potential map shows interesting features that indicate special reactivity of the corner atoms

Weber fractions for acoustic and electric hearing.

<p>Normal-hearing subjects show relatively constant values (the dashed outlined box in each panel). The individual panels represent Weber fractions for 3 unilaterally implanted subjects: The filled diamonds represent the data for the non-implant ear while triangles and circles represent that for the implant ear. In panel S3 (bottom), comparable data from previously published other studies are also plotted: The two open circles represent Weber fractions for modulation frequency discrimination of sinusoidally-amplitude-modulated noise for a group of normal-hearing subjects <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Zeng5" target="_blank">[34]</a>; the star represents the datum for a single implant subject from Fig. 4 of Boex et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Boex1" target="_blank">[14]</a>; the six crosses represent the data for two subjects from Fig. 2 of Carlyon et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Carlyon1" target="_blank">[19]</a>; the two open squares represent the data for a single subject from Fig. 22 of Eddington et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Eddington1" target="_blank">[35]</a>; the two open diamonds represent the data for two subjects from Fig. 1 of Reiss et al., in which the Weber fraction was estimated from the monotonic part of the psychometric function in S10 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Reiss1" target="_blank">[21]</a>; the three plus symbols represent the data from S1’s second estimates in Fig. 7 of Baumann and Nobbe <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088662#pone.0088662-Baumann1" target="_blank">[17]</a>.</p

Subject information.

<p>Age is reported at the time of test. HiRes90k is a product of Advanced Bionics Corporation (Valencia, CA) and Combi 40+ and PULSAR ci100 are products of Med El (Innsbruck, Austria). Implant speech is specified as the type of speech processing and keyword percent correct for HINT sentences in quiet (all via direct connection to prevent acoustic leakage). PTA (pure tone average) is the average of thresholds in dB HL at 500, 1000 and 2000 Hz for the non-implant ear. Acoustic melody indicates percent correct recognition of 12 familiar melodies delivered to the non-implant ear.</p