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1.
J Am Chem Soc ; 2020 Mar 13.
Artigo em Inglês | MEDLINE | ID: mdl-32126754

RESUMO

Because of the typical instability of copper nanoclusters, atom-precise structural elucidation of these nanoclusters has remained elusive. Herein, we report an air- and moisture-stable 23-copper nanocluster (SD/Cu23a or SD/Cu23b) isolated from the reaction of Cu(CF3COO)2, tBuC≡CH, Cu powder, and Ph2SiH2 using a gradient reduction (CuII → CuI → Cu0) strategy (GRS), which is competent for controlling the kinetics of the reduction reaction, thus avoiding formation of pure CuI complexes or large Cu0 nanoparticles. The solid-state structure of the Cu23 nanocluster shows a rare [Cu4]0 tetrahedral kernel surrounded by an outer Cu19 shell, which is protected by tBuC≡C- and CF3COO- ligands. The Cu23nanocluster is a rare four-electron superatom with a 1S21P2 electronic shell closure and can be crystallized in two polymorphs (R3c and R3̅) depending on the solvent used. The crystallization of SD/Cu23a in the R3c space group is mainly governed by van der Waals forces and C-H···F interactions, whereas additional intermolecular C-H···Clchloroform interactions are responsible for the R3̅ space group of SD/Cu23b. This work not only shows the ingenuity of a gradient reduction strategy for the synthesis of copper nanoclusters but also provides a better fundamental understanding of how to produce the polymorphic copper nanoclusters in a precisely tunable fashion.

2.
Phys Chem Chem Phys ; 22(9): 5272-5285, 2020 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-32095793

RESUMO

We investigate the excited electron dynamics in [Au25(SR)18]-1 (R = CH3, C2H5, C3H7, MPA, PET) [MPA = mercaptopropanoic acid, PET = phenylethylthiol] nanoparticles to understand how different ligands affect the excited state dynamics in this system. The population dynamics of the core and higher excited states lying in the energy range 0.00-2.20 eV are studied using a surface hopping method with decoherence correction in a real-time DFT approach. All of the ligated clusters follow a similar trend in decay for the core states (S1-S6). The observed time constants are on the picosecond time scale (2-19 ps), which agrees with the experimental time scale, and this study confirms that the time constants observed experimentally could originate from core-to-core transitions and not from core-to-semiring transitions. In the presence of higher excited states, R = H, CH3, C2H5, C3H7, and PET demonstrate similar relaxations trends whereas R = MPA shows slightly different relaxation of the core states due to a smaller gap between the LUMO+1 and LUMO+2 gap in its electronic structure. The S1 (HOMO → LUMO) state gives the slowest decay in all ligated clusters, while S7 has a relatively long decay. Furthermore, separate electron and hole relaxations were performed on the [Au25(SCH3)18]-1 nanocluster to understand how independent electron and hole relaxations contribute to the overall relaxation dynamics.

3.
J Am Chem Soc ; 141(47): 18715-18726, 2019 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-31679332

RESUMO

The [Au25(SR)18]- and [Au13(dppe)5Cl2]3+ [dppe = 1,2-bis(diphenylphosphino)ethane] nanoclusters both possess a 13-atom icosahedral core with 8 delocalized superatomic electrons (8e), but their emission properties and time-resolved electron dynamics differ significantly. In this work, experimental photoluminescence and photoluminescence decay measurements are combined with time-dependent density functional theory calculations of radiative and nonradiative decay properties and lifetimes to elucidate the similarities and differences in the emission of these two nanoclusters with similar cores. In this work, the photodynamic properties of [Au13(dppe)5Cl2]3+ are elucidated theoretically for the first time. [Au13(dppe)5Cl2]3+ exhibits a single strong emission peak compared to the weaker bimodal luminescence of [Au25(SR)18]- (modeled here as [Au25(SH)18]-). The strongly emissive state is found to arise from deexcitation out of the S1 state, similar to what is seen for [Au25(SH)18]-. Both theory and experiment exhibit microsecond lifetimes for this state. Transient absorption measurements and theoretical calculations demonstrate that the excited-state lifetimes for higher excited states are typically less than 1 ps. The decay times for the higher excited states of [Au13(dppe)5Cl2]3+ and its model compound [Au13(pe)5Cl2]3+ [pe = 1,2-bis(phosphino)ethane] are observed to be shorter than the lifetimes of the corresponding states of [Au25(SR)18]-; this occurs because the energy gap separating degenerate sets of unoccupied orbitals is only ∼0.2 eV in [Au13(dppe)5Cl2]3+ compared to a ∼0.6 eV energy gap in [Au25(SH)18]-.

4.
J Phys Chem A ; 123(45): 9712-9720, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31603684

RESUMO

Thiolate-stabilized gold nanoclusters have drawn significant attention for their extraordinary properties and their applications in many fields such as catalysis, sensing, biomedicine, etc. However, due to the size, complexity, and conformational flexibility of thiolate ligands, accurate structure prediction can be a challenge using computational approaches. Substitution of thiolate ligands with chloride ligands provides a possible alternative. In this work, the stabilities of a series of gold thiolate and chloride clusters with 1:1 stoichiometry (AunLn; L = SH, Cl; n = 2-9) and the analogs of some experimentally observed gold nanoclusters (AunLm; L = SH, Cl; n = 18-133) are examined, and binding energies, HOMO-LUMO gaps, and absorption spectra are determined using density functional theory (DFT). We observed that the optimized geometries of gold nanoclusters for both types of ligands converged to the same local minimum structure as the experimentally observed structures. The average binding energy per gold atom in gold clusters converges after Au4L4. The binding energies of chloride-stabilized gold clusters and nanoclusters average 87.5% and 95.7% of the binding energies of thiolate-stabilized systems for the clusters and nanoclusters, respectively. Typically, thiolates are found to be more stable than the chlorides. However, higher HOMO-LUMO gaps in Au2Cl2, Au38Cl24, and Au102Cl44 compared to their thiolate analogs suggest systems of particular interest for investigating the possible existence of chloride-based gold nanoclusters. Absorption spectra are very similar regardless of the ligand used. This study also demonstrates that in theoretical studies on large nanoclusters, complex thiolate ligands can be replaced by Cl ligands to predict structural and electronic properties with reasonable accuracy and reduced computational effort.

5.
Phys Chem Chem Phys ; 21(41): 23065-23075, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31602447

RESUMO

We perform a theoretical investigation of the electronic structure and optical properties of atomic nanowire and nanorod dimers using DFT and TDDFT. In both systems at separation distances larger than 0.75 nm, optical spectra show a single feature that resembles the bonding dipole plasmon (BDP) mode. A configuration interaction (CI) analysis shows that the BDP mode arises from constructive coupling of transitions, whereas the destructive coupling does not produce significant oscillator strength for such separation distances. At shorter separation distances, both constructive and destructive coupling produce oscillator strength due to wave-function overlap, which results in multiple features in the calculated spectra. Our analysis shows that a charge-transfer plasmon (CTP) mode arises from destructive coupling of transitions, whereas the BDP results from constructive coupling of the same transitions at shorter separation distances. Furthermore, the coupling elements between these transitions are shown to depend heavily on the amount of exact Hartree-Fock exchange (HFX) in the functional, which affects the splitting of CTP and BDP modes. With 50% HFX or more, the CTP and BDP modes mainly merge into a single feature in the spectra. These findings suggest that the effects of exact exchange must be assessed during the prediction of CTP modes in plasmonic systems.

6.
J Chem Phys ; 151(9): 094702, 2019 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-31492077

RESUMO

Experimental findings of Au18(GSH)14 as a photosensitizer with the highest potential compared to other glutathione-protected clusters demand understanding the photophysics and relaxation dynamics of the Au18(SR)14 cluster. To this end, we perform ab initio real-time nonadiabatic molecular dynamics simulations on Au18(SH)14 to investigate its relaxation dynamics compared to the well-studied [Au25(SR)18]-1 relaxation dynamics. In this work, the excitations covering up to ∼2.6 eV in the optical absorption spectrum are analyzed to understand the electronic relaxation process of the Au18(SH)14 cluster. The ground state growth times of Au18(SH)14 are several orders of magnitude shorter than the growth times observed for the [Au25(SH)18]-1 nanocluster. The S1 (HOMO-LUMO) state gives the slowest decay time (∼11 ps) among all the states (S1-S30) considered similar to [Au25(SH)18]-1. However, the S1 state in Au18(SH)14 is a semiring-to-core charge transfer state, whereas S1 in the [Au25(SH)18]-1 cluster is a core-to-core transition. The remaining higher excited states have very short decay time constants less than 1.4 ps except for S2 which has the second slowest decay of 6.4 ps. The hole relaxations are faster than the electron relaxations in Au18(SH)14 due to the closely packed HOMOs in the electronic structure. Radiative relaxations are also examined using the time-dependent density functional theory method, and the excited state emission energy and lifetime are found to be in good agreement with experiment.

7.
J Phys Chem A ; 123(29): 6152-6159, 2019 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-31241935

RESUMO

Understanding the role of Ca2+ ion in the oxygen-evolving complex of photosystem II is essential to design commercially viable and efficient water oxidation catalysts. To this end, small pure manganese oxide and calcium-doped manganese oxide model complexes saturated with water-derived ligands are investigated in this work. Density functional theory calculations are performed to investigate the water oxidation process on Mn2(µ-OH)(µ-O)(H2O)3(OH)5 (Mn2O4·6H2O) and CaMnO(µ-OH)2(H2O)5(OH)2 (CaMnO3·7H2O) complexes. Many reaction pathways are considered, and the three lowest energy water oxidation mechanisms on CaMnO3·7H2O have highest reaction energy steps of 1.37, 1.67, and 1.81 eV compared to the highest reaction energy step of 2.25 eV for the lowest energy mechanism of the pure Mn2O4·6H2O complex. Doping of the manganese dimer complex with calcium decreases the highest reaction energy of the water oxidation process. Consequently, the inclusion of calcium appears to improve the catalyst's efficiency for water splitting.

8.
Dalton Trans ; 48(11): 3635-3640, 2019 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-30747941

RESUMO

A diphosphine-protected 18-gold-atom nanocluster was isolated via a facile reduction of an AuI precursor by NaBH4. Its composition was identified as {[Au18(dppm)6Cl4]·C6H6·3Cl·PF6} (SD/Au18, SD = SunDi; dppm = bis-(diphenylphosphino)methane) by X-ray single crystal structural analysis. This nanocluster possesses a prolate shape and is built from an Au10 kernel (bi-octahedral Au6 units sharing one edge) fused with two Au7 caps via sharing six gold atoms. The identity of the Au18 cluster is further demonstrated by ESI-MS. The number of valence electrons of [Au18(dppm)6Cl4]4+ is 10 (n* = 18-4-4), which does not match with the known magic numbers according to the spherical jellium model, and elongated models must be considered. The special stability of the Au18 cluster likely arises from geometrical factors in the metallic core. Two charge states are reported for this system. This work not only presents the structure elucidation of a diphosphine-protected Au18 nanocluster, but also provides an important insight into the growth pattern of gold nanoclusters and the charge states they can achieve.

9.
J Am Chem Soc ; 141(10): 4460-4467, 2019 Mar 13.
Artigo em Inglês | MEDLINE | ID: mdl-30779559

RESUMO

The elaborate selection of capping ligands is of great importance in the synthesis of atomically precise metal nanoclusters. Organic thiolates, alkynyls, phosphines, and/or their combinations are the ligands most widely utilized to protect metal nanoclusters, while inorganic oxo anions have been almost neglected in this field. Herein, the first CrO42-/ tBuC≡C- co-capped Ag48 nanocluster (SD/Ag48, SD = SunDi) was synthesized and structurally characterized by single-crystal X-ray diffraction. The pseudo-5-fold symmetric metal skeleton of SD/Ag48 shows a core-shell structure composed of a Ag23 cylinder encircled by an outer Ag25 shell. Unprecedentedly, coexistence of inorganic (CrO42-) and organic ( tBuC≡C-) ligands was observed on the surface of SD/Ag48. The inorganic CrO42- anion plays three important roles in the construction of silver nanoclusters: (i) passivating the Ag23 kernel; (ii) connecting the core and shell; and (iii) protecting the Ag25 shell. This nanocluster belongs to a 14e superatom system and exhibits successive molecule-like absorption bands from the visible to the ultraviolet region. This work not only establishes a fresh inorganic ligand strategy in the synthesis of silver nanoclusters but also provides a new insight into the important surface coordination chemistry of CrO42- in the shape control of silver nanoclusters.

10.
Angew Chem Int Ed Engl ; 58(1): 195-199, 2019 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-30411441

RESUMO

Two pure silver nanoparticles (Ag210 (i PrPhS)71 (Ph3 P)5 Cl and Ag211 (i PrPhS)71 (Ph3 P)6 Cl labeled as SD/Ag210 and SD/Ag211 (SD=SunDi), were found to co-crystallize in forming compound 1. Single-crystal X-ray diffraction (SCXRD) revealed that they differ by only one Ag(PPh3 ). Their four-shell nanoparticles consist of three pure Ag metal shells (Ag19 @Ag52 @Ag45 ) shielded by a silver-organic Ag89 (i PrPhS)71 Cl[Ag(Ph3 P)]n outermost shell. The number (n) of Ag(Ph3 P) is five for SD/Ag210 and six for SD/Ag211. The pseudo-fivefold symmetric Ag nanoparticles exhibit surface plasmon absorption similar to a true metallic state but at the nanoscale. This work exemplifies the important effects of phosphine in stabilizing large silver nanoparticles; and offers a platform to investigate the origin of differences in nanoscale metal materials, even differing by only one metal atom; it also sheds light on the regioselective binding of auxiliary Ph3 P on the surface of silver nanoparticles.

11.
Acc Chem Res ; 51(12): 3065-3073, 2018 Dec 18.
Artigo em Inglês | MEDLINE | ID: mdl-30444598

RESUMO

Ligand-protected noble metal nanoclusters are of interest for their potential applications in areas such as bioimaging, catalysis, photocatalysis, and solar energy harvesting. These nanoclusters can be prepared with atomic precision, which means that their stoichiometries can be ascertained; the properties of these nanoclusters can vary significantly depending on the exact stoichiometry and geometric structure of the system. This leads to important questions such as: What are the general principles that underlie the physical properties of these nanoclusters? Do these principles hold for all systems? What properties can be "tuned" by varying the size and composition of the system? In this Account, we describe research that has been performed to analyze the electronic structure, linear optical absorption, and excited state dynamics of thiolate-stabilized noble metal nanoclusters. We focus primarily on two systems, Au25(SR)18- and Au38(SR)24, as models for understanding the principles underlying the electronic structure, optical properties, luminescence, and transient absorption in these systems. In these nanoclusters, the orbitals near the HOMO-LUMO gap primarily arise from atomic 6sp orbitals located on Au atoms in the gold core. The resulting nanocluster orbitals are delocalized throughout the core of these systems. Below the core-based orbitals lies a set of orbitals that are primarily composed of Au 5d and S 3p atomic orbitals from atoms located around the exterior gold-thiolate oligomer motifs. This set of orbitals has a higher density of states than the set arising from the core 6sp orbitals. Optical absorption peaks in the near-infrared and visible regions of the absorption spectrum arise from excitations between core orbitals (lowest energy peaks) and excitations from oligomer-based orbitals to core-based orbitals (higher energy peaks). Nanoclusters with different stoichiometries have varying gaps between the core orbitals themselves as well as between the band of oligomer-based orbitals and the band of core orbitals. These gaps can slow down nonradiative electron transfer between excited states that have different character; the excited state electron and hole dynamics depend on these gaps. Nanoclusters with different stoichiometries also exhibit different luminescence properties. Depending on factors that may include the symmetry of the system and the rigidity of the core, the nanocluster can undergo large or small nuclear changes upon photoexcitation, which affects the observed Stokes shift in these systems. This dependence on stoichiometry and composition suggests that the size and the corresponding geometry of the nanocluster is an important variable that can be used to tune the properties of interest. How does doping affect these principles? Replacement of gold atoms with silver atoms changes the energetics of the sp and d atomic orbitals that make up the nanocluster orbitals. Silver atoms have higher energy sp orbitals, and the resulting nanocluster orbitals are shifted in energy as well. This affects the HOMO-LUMO gap, the oscillator strength for transitions, the spacings between the different bands of orbitals, and, as a consequence, the Stokes shift and excited state dynamics of these systems. This suggests that nanocluster doping is one way to control and tune properties for use in potential applications.

12.
J Phys Chem C Nanomater Interfaces ; 122(41): 23639-23650, 2018 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-30364415

RESUMO

Here, we perform theoretical investigation using time-dependent density functional theory (TD-DFT) and time-dependent density functional tight binding (TD-DFTB) for the electronic structure and optical properties of silver nanorods. TD-DFTB generally performs well for the accurate description of optical properties with respect to the size and type of dimer assembly of silver nanorods compared to TD-DFT. However, the energies and intensities of the longitudinal and transverse peaks of the nanorods are somewhat underestimated with TD-DFTB compared to the values calculated at the TD-DFT level. By exploiting the computational efficiency of TD-DFTB, we also extend our investigation to longer nanorods and their dimers containing up to ∼2000 atoms. Our results show that the coupling between nanorods and the resulting optical properties of the dimer assemblies are quite dependent on the length of the monomers. In all cases, the energy shifts in dimers as a function of the gap distance deviate significantly from the dipole-dipole interaction model. Moreover, a comparison of the best-fit curves for the dependence of the fractional shifts (Δλ/λ0) on nanorod length indicates that the parameters of the plasmon ruler equation depend on the length of the nanorods and the type of the assembly rather than approaching a universal value. These insights are enabled by the computational efficiency of TD-DFTB and its ability to treat quantum mechanical effects in large nanorod dimer systems.

13.
Chem Sci ; 9(5): 1251-1258, 2018 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-29675171

RESUMO

Due to distinctive quantum confinement effects, ultrasmall gold nanoparticles usually exhibit interesting electronic structure and molecular-like properties. However, the lack of atomically-precise structural information makes the understanding of them almost impossible, such as understanding the relationships between their compositions and unique properties. Herein, by reducing a diphosphine AuI precursor (Au2(dppm)2Cl2; dppm = Ph2PCH2PPh2) with or without a S2- releasing reagent, we enriched our knowledge of the members in the families of Au13 and Au8 by the structural determinations of two new dppm-protected gold nanoclusters, [Au13(dppm)6]5+ (SD/Au1) and [Au8(dppm)4S2]2+ (SD/Au2), respectively. Within SD/Au1, the Au13 kernel significantly deviates from the ideal Ih icosahedron by the elongation of three surface Au-Au bonds to ∼3.5 Å, giving it C3 symmetry, whereas SD/Au2 has a novel heart-shaped C2 symmetric Au8S2 core (central Au4 tetrahedron + two Au2S units) protected by four µ2-dppm ligands in the outer shell. Of note, SD/Au1 represents a rare Au13 nanocluster with an opened icosahedral geometry, and SD/Au2 shows a new edge-shared "core + 4exo" structure type that has never been observed before. The electronic structures and optical absorption spectra of these systems are correlated with time-dependent density functional theory (TDDFT) calculations. Based on the spherical jellium model, the stability of the Au13 and Au8 nanoclusters can be ascribed to 8- and 2-electron superatoms with 1S21P6 and 1S2 configurations, respectively. Interestingly, the cluster SD/Au2 exhibits bright yellow luminescence with an emission maximum at 591 nm that slightly hypsochromically shifts to 581 nm upon cooling to 93 K. Our findings not only enrich the family of diphosphine-protected ultrasmall gold nanoclusters, but also demonstrate the rich variations of gold kernels during the transformation from a simple AuI precursor to Au nanoclusters.

14.
Annu Rev Phys Chem ; 69: 205-229, 2018 04 20.
Artigo em Inglês | MEDLINE | ID: mdl-29490202

RESUMO

Ligand-stabilized gold and silver nanoparticles are of tremendous current interest in sensing, catalysis, and energy applications. Experimental and theoretical studies have closely interacted to elucidate properties such as the geometric and electronic structures of these fascinating systems. In this review, the interplay between theory and experiment is described; areas such as optical absorption and doping, where the theory-experiment connections are well established, are discussed in detail; and the current status of these connections in newer fields of study, such as luminescence, transient absorption, and the effects of solvent and the surrounding environment, are highlighted. Close communication between theory and experiment has been extremely valuable for developing an understanding of these nanocluster systems in the past decade and will undoubtedly continue to play a major role in future years.

15.
Nanoscale ; 9(41): 15825-15834, 2017 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-29019494

RESUMO

We perform a theoretical investigation using density functional theory (DFT) and time-dependent DFT (TDDFT) on the doping of the Au25(SR)18-1 nanocluster with group IX transition metals (M = cobalt, rhodium and iridium). Different doping motifs, charge states and spin multiplicities were considered for the single-atom doped nanoclusters. Our results show that the interaction (or the lack of interaction) between the d-type energy levels that mainly originate from the dopant atom and the super-atomic levels plays an important role in the energetics, the electronic structure and the optical properties of the doped systems. The evaluated MAu24(SR)18q (q = -1, -3) systems favor an endohedral disposition of the doping atom typically in a singlet ground state, with either a 6- or 8-valence electron icosahedral core. For the sake of comparison, the role of the d energy levels in the electronic structure of a variety of doped Au25(SR)18-1 nanoclusters was investigated for dopant atoms from other families such as Cd, Ag and Pd. Finally, the effect of spin-orbit coupling (SOC) on the electronic structure and absorption spectra was determined. The information in this study regarding the relative energetics of the d-based and super-atom energy levels can be useful to extend our understanding of the preferred doping modes of different transition metals in protected gold nanoclusters.

16.
J Phys Chem A ; 121(14): 2715-2718, 2017 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-28403616
17.
J Phys Chem A ; 120(48): 9625-9635, 2016 Dec 08.
Artigo em Inglês | MEDLINE | ID: mdl-27933920

RESUMO

Magnetic circular dichroism (MCD) spectroscopy is a source of important data about the electronic structure and optical properties of different chemical systems. Theoretical simulation of the MCD spectra can be used to assist in the understanding of empirically measured MCD spectra. In the present paper, a theoretical investigation of electronic and optical properties of phosphine-protected gold clusters with a Au93+ core with D2h symmetry was performed with time-dependent density functional theory. The influence of ligands on the optical properties of the gold core was investigated. Simulations of the optical absorption and MCD spectra were performed for the bare gold Au93+ cluster as well as for ligand-protected Au9(PH3)83+ and Au9(PPh3)83+ species. MCD spectra were calculated at a temperature of 298 K and a magnetic field of 7 T. A comparative analysis of theoretical and experimental data was also performed. The obtained results show that the theoretically simulated MCD spectrum for the Au9(PPh3)83+ ion in gas phase exhibits a reasonable agreement with experimental results for the [Au9(PPh3)8](NO3)3 system, although with a red shift of up to 0.5 µm-1. Overall, MCD provides significant additional details about the electronic structure of the considered systems compared to the absorption spectra.

18.
Nanoscale ; 8(45): 18905-18911, 2016 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-27747330

RESUMO

Here we report the single-crystal structure, experimental and theoretical characterization of a 41-metal atom Au-Ag alloy nanocluster [Au3Ag38(SCH2Ph)24X5]2- (1, X = Cl or Br). The nanocluster 1 is co-protected by thiolate and halogen atoms and features an all-metallic face-fused biicosahedral Au2@AuAg20 rod-like kernel enwrapped by the outermost Ag18(SCH2Ph)24X3 shell. Two sites on the surface of the biicosahedral kernel are partially occupied by Au and Ag atoms. The outer Ag18(SCH2Ph)24X3 shell is composed of two Ag6S6 cycles at the two poles and one Ag6S2X3 arc at the equator with both 2- and 3-coordinated Ag atoms, which has not been observed in gold or silver nanoclusters ever before. Theoretical calculations elucidate its electronic structure as well as optical properties, thus producing informative correlations between its structure and properties. This nanocluster exhibits near-infrared (NIR) emission around 825 nm. This work (i) snapshots a rare crystal structure of an Au-doped silver alloyed nanocluster; (ii) gives a deep insight to understand how the capping ligand or anions affect the structure of the alloy nanocluster; and (iii) provides precise information about gold atom doping site that is very significant in the recognition of potential active catalytic sites of the alloy nanoparticles.

19.
J Am Chem Soc ; 138(35): 11202-10, 2016 09 07.
Artigo em Inglês | MEDLINE | ID: mdl-27524386

RESUMO

Understanding fundamental behavior of luminescent nanomaterials upon photoexcitation is necessary to expand photocatalytic and biological imaging applications. Despite the significant amount of experimental work into the luminescence of Au25(SR)18(-) clusters, the origin of photoluminescence in these clusters still remains unclear. In this study, the geometric and electronic structural changes of the Au25(SR)18(-) (R = H, CH3, CH2CH3, CH2CH2CH3) nanoclusters upon photoexcitation are discussed using time-dependent density functional theory (TD-DFT) methods. Geometric relaxations in the optimized excited states of up to 0.33 Å impart remarkable effects on the energy levels of the frontier orbitals of Au25(SR)18(-) nanoclusters. This gives rise to a Stokes shift of 0.49 eV for Au25(SH)18(-) in agreement with experiments. Even larger Stokes shifts are predicted for longer ligands. Vibrational frequencies in the 75-80 cm(-1) range are calculated for the nuclear motion involved in the excited-state nuclear relaxation; this value is in excellent agreement with vibrational beating observed in time-resolved spectroscopy experiments. Several excited states around 0.8, 1.15, and 1.25 eV are calculated for the Au25(SH)18(-) nanocluster. Considering the typical underestimation of DFT excitation energies, these states are likely responsible for the emission observed experimentally in the 1.15-1.55 eV range. All excited states arise from core-based orbitals; charge-transfer states or other "semi-ring" or ligand-based states are not implicated.

20.
J Phys Chem A ; 120(15): 2480-92, 2016 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-27015543

RESUMO

Understanding the factors that affect efficiency of manganese oxides as water oxidation catalysts is an essential step toward developing commercially viable electrocatalysts. It is important to understand the performance of the smallest versions of these catalysts, which will in return be advantageous with bottom up catalytic design. Density functional theory calculations have been employed to investigate water oxidation processes on Mn2(µ-OH)(µ-O)(H2O)3(OH)5 (Mn2O4·6H2O), Mn2(µ-OH)2(H2O)3(OH)4 (Mn2O3·6H2O), and Mn2(µ-OH)2(H2O)4(OH)4 (Mn2O3·7H2O) complexes. The effect of different oxidation states of manganese is considered in this study. Thermodynamically, the lowest energy pathway for the fully saturated Mn2O4·6H2O complex occurs through a nucleophilic attack of a solvent water molecule to a Mn(IV1/2)O oxo moiety. The lowest energy pathway on the Mn2O3·6H2O complex proceeds with an attack of Mn(VI)O group to the surface hydroxo group on the same manganese atom; this pathway is related to the third lowest energy pathway on the Mn2O4·6H2O complex. The water oxidation process on the fully saturated Mn2O3·7H2O complex also involves a nucleophilic attack from a solvent water molecule to a Mn(V)O moiety. The formation of these manganese oxo groups can be used as a descriptor for selecting a manganese-based water splitting catalyst due to the high electrochemical potentials required for the generation of these groups.

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