Seed-assembly-mediated fabrication and application of highly branched gold nanoshells having hollow and porous morphologies.

Highly branched gold nanoshells (BAuNSs) having hollow and porous morphologies have been fabricated via a seed-assembly-mediated strategy. Gold seed assemblies can be prepared by removal of SiO2nanotemplates with help of polyvinylpyrrolidone (PVP) molecules, which weakly link gold nanoparticles together even after SiO2etching. L-3,4-dihydroxy phenylalanine (L-DOPA) and AgNO3are employed as shape-directing agents to induce the anisotropic growth of gold. BAuNSs exhibit 7.4 and 4.4 times stronger activities than SiO2@Au nanoparticles in catalysis and surface-enhanced Raman scattering (SERS) applications, respectively, due to their large surface areas and numerous hot spots. It is necessary to find the optimal amount of gold deposition in fabrication to effectively utilize the hollow and porous morpologies of BAuNSs for catalysis and SERS applications. Overgrown nanobranches can fill the nanopores and nanogaps of BAuNSs, resulting in decrease of activities in applications. Overall, the seed-assembly-mediated fabrciation can be employed to produce plasmonic nanostructures having unique morphologies and high application activities.

Photothermal structural modification of porous gold nanoshells via pulsed-laser irradiation: effects of laser wavelengths and surface conditions.

Pulsed lasers are promising candidates for fabricating plasmonic nanoparticles having unique structures, and there is a strong need for studies on the detailed effects of various experimental conditions on laser-induced fabrications. In this work, we demonstrate the effects of laser wavelengths and nanoparticle surface conditions, as well as laser fluences, in the structural modification of porous gold nanoshells induced by picosecond pulse irradiation. Laser wavelengths play a critical role in the modification because irradiating laser pulses excite not only porous gold nanoshells but also gold nanospheres, which have been produced via the melting of irradiated porous gold nanoshells. Significantly different localized surface plasmon resonances of gold nanospheres and porous gold nanoshells make the effect of laser wavelengths noticeable. The polyvinylpyrrolidone (PVP) concentration of colloid containing porous gold nanoshells also modifies the deformation of the structures. Gold nanostructures become positively charged by the irradiation, strengthening gold-PVP attraction. The stronger binding affinity of PVP is considered to reduce the deformation of irradiated porous gold nanoshells.

Boosting Visible-Light Photocatalytic Redox Reaction by Charge Separation in SnO2 /ZnSe(N2 H4 )0.5 Heterojunction Nanocatalysts.

In this work, environmentally friendly photocatalysts with attractive catalytic properties are reported that have been prepared by introducing SnO2 quantum dots (QDs) directly onto ZnSe(N2 H4 )0.5 substrates to induce advantageous charge separation. The SnO2 /ZnSe(N2 H4 )0.5 nanocomposites could be easily synthesized through a one-pot hydrothermal process. Owing to the absence of capping ligands, the attached SnO2 QDs displayed superior photocatalytic properties, generating many exposed reactive surfaces. Moreover, the addition of a specified amount of SnO2 boosted the visible-light photocatalytic activity; however, the presence of excess SnO2 QDs in the substrate resulted in aggregation and deteriorated the performance. The spectroscopic data revealed that the SnO2 QDs act as a photocatalytic mediator and enhance the charge separation within the type II band alignment system of the SnO2 /ZnSe(N2 H4 )0.5 heterojunction photocatalysts. The separated charges in the heterojunction nanocomposites promote radical generation and react with pollutants, resulting in enhanced photocatalytic performance.

Laser-induced fabrication of porous gold nanoshells.

Pulse-laser irradiation is a promising approach to fabricate gold nanostructures with unique morphologies. Hollow and porous gold nanoshells with high surface-enhanced Raman scattering efficiency have been produced via irradiating SiO2@Au@SiO2 nanoparticles with nanosecond laser pulses; the synthetic strategy mainly relies on the laser-induced surface melting of gold nanoparticles.

Facile fabrication of porous ZnS nanostructures with a controlled amount of S vacancies for enhanced photocatalytic performances.

ZnS nanostructures of barbell-shaped porous and hollow nanoplates with a controlled amount of S vacancies have been facilely fabricated via the hydrothermal treatment of ZnS(en)0.5 (en = ethylenediamine) nanoplates. The amount of S vacancies as well as the morphologies of ZnS nanostructures have been controlled by adjusting the hydrolysis time; the layered structure of ZnS(en)0.5 nanoplates decomposes to yield discrete ZnS nanoparticles at two end facets of template nanoplates, producing barbell-shaped porous and hollow ZnS nanoplates with abundant S vacancies finally. The photocatalytic activity of ZnS nanostructures prepared via hydrolysis for 4 h is 8.2 times higher than that of commercial ZnS. The photocatalytic activity of ZnS nanostructures increases with the increase of emission at 390 nm arising from sulfur vacancies, suggesting that the high photocatalytic efficiency of ZnS nanostructures results mainly from the high amount of sulfur vacancies. Surface defects such as sulfur vacancies can trap photogenerated electrons to block the recombination of charges, enhancing the photocatalytic efficiency of the as-prepared ZnS nanostructures. It has also been found that both ˙OH and ˙O2- act as the major reactive species in the photocatalytic decomposition of rhodamine B via our prepared ZnS nanostructures. Barbell-shaped porous and hollow ZnS nanoplates are suggested to have great applicability to photocatalysts in waste-water treatment.

Impurity Location-Dependent Relaxation Dynamics of Cu:CdS Quantum Dots.

Various types of 2% Cu-incorporated CdS (Cu:CdS) quantum dots (QDs) with very similar sizes have been prepared via a water soluble colloidal method. The locations of Cu impurities in CdS host nanocrystals have been controlled by adopting three different synthetic ways of doping, exchange, and adsorption to understand the impurity location-dependent relaxation dynamics of charge carriers. The oxidation state of incorporated Cu impurities has been found to be +1 and the band-gap energy of Cu:CdS QDs decreases as Cu2S forms at the surfaces of CdS QDs. Broad and red-shifted emission with a large Stokes shift has been observed for Cu:CdS QDs as newly produced Cu-related defects become luminescent centers. The energetically favored hole trapping of thiol molecules, as well as the local environment, inhibits the radiative recombination processes of Cu:CdS QDs, thus resulting in low photoluminescence. Upon excitation, an electron is promoted to the conduction band, leaving a hole on the valence band. The hole is transferred to the Cu+ d-state, changing Cu+ into Cu2+, which then participates in radiative recombination with an electron. Electrons in the conduction band are ensnared into shallow-trap sites within 52 ns. The electrons can be further captured on the time scale of 260 ns into deep-trap sites, where electrons recombine with holes in 820 ns. Our in-depth analysis of carrier relaxation has shown that the possibilities of both nonradiative recombination and energy transfer to Cu impurities become high when Cu ions are located at the surface of CdS QDs.

Formation and decay of charge carriers in aggregate nanofibers consisting of poly(3-hexylthiophene)-coated gold nanoparticles.

Thin nanofibers (NFs) of J-dominant aggregates with a thickness of 15 nm and thick NFs of H-dominant aggregates with a thickness of 25 nm were fabricated by the self-assembly of poly(3-hexylthiophene)-coated gold nanoparticles. The formation and decay dynamics of the charge carriers, which are dependent on the aggregate types of NFs, was investigated by time-resolved emission and transient-absorption spectroscopy. With increasing excitation energy, the fraction of the fast emission decay component decreased, suggesting that the fast formation of polaron pairs (PP), localized (LP), and delocalized polarons (DP) results from higher singlet exciton states produced by the singlet fusion. The faster decay dynamics of DP and LP in the thick NFs than in thin NFs is due to the increased delocalization of DP and LP. As the interchain aggregation is weaker than intrachain aggregation, PP decays faster in thin NFs than in thick NFs. In both thin and thick NFs, although triplet (T1) excitons were barely observed with excitation at 532 nm on a nanosecond time scale, they were observed with excitation at 355 nm, showing that T1 excitons within NFs are generated mainly through the singlet fission from a higher singlet exciton state rather than through intersystem crossing.

Dislocation-driven growth of porous CdSe nanorods from CdSe·(ethylenediamine)(0.5) nanorods.

Porous CdSe nanorods having a novel flute-like morphology have been prepared facilely via the hydrothermal treatment of CdSe·(en)0.5 (en = ethylenediamine) nanorods as sacrificial templates. During the hydrothermal process, various crystalline imperfections such as stacking faults and twinning planes appear due to lattice mismatches between orthorhombic CdSe·(en)0.5 and hexagonal wurtzite porous CdSe nanorods and subsequently disappear to release mismatched strains. In the self-healing process of defects, due to the imbalance of in-and-out atomic diffusion, point defects of atomic vacancies are heavily generated in CdSe nanorods to produce volume defects of voids eventually. The photoluminescence of CdSe nanorods shifts to the red region and decreases in intensity with the increase of the hydrolysis time as surface states and selenium vacancies increase. The mean lifetime of photoluminescence increases with the increase of the hydrothermal-treatment time as the fractional amplitude of the surface-state-related component increases.

High-performance ultraviolet photodetectors based on solution-grown ZnS nanobelts sandwiched between graphene layers.

Ultraviolet (UV) light photodetectors constructed from solely inorganic semiconductors still remain unsatisfactory because of their low electrical performances. To overcome this limitation, the hybridization is one of the key approaches that have been recently adopted to enhance the photocurrent. High-performance UV photodetectors showing stable on-off switching and excellent spectral selectivity have been fabricated based on the hybrid structure of solution-grown ZnS nanobelts and CVD-grown graphene. Sandwiched structures and multilayer stacking strategies have been applied to expand effective junction between graphene and photoactive ZnS nanobelts. A multiply sandwich-structured photodetector of graphene/ZnS has shown a photocurrent of 0.115 mA under illumination of 1.2 mWcm(-2) in air at a bias of 1.0 V, which is higher 10(7) times than literature values. The multiple-sandwich structure of UV-light sensors with graphene having high conductivity, flexibility, and impermeability is suggested to be beneficial for the facile fabrication of UV photodetectors with extremely efficient performances.

Direct observation of valence band splitting using room temperature photoluminescence of CdS hollow submicrospheres.

The room-temperature photoluminescence spectra of CdS hollow submicrospheres prepared via employing silica hard templates and microwave irradiation present a profound insight into the quantum confinement-induced splitting of electron and hole states in the valence band.

Laser-induced silver nanojoining of gold nanoparticles.

Gold nanoparticles have been silver-joined to fabricate nanowires by irradiating gold nanospheres of 25 nm in diameter and silver nanospheres of 8 nm in diameter held together on a carbon-coated copper grid with a 30 ps laser pulse of 532 nm for 20 min at a fluence of 3.0 mJ/cm2. Laser-induced nanojoining of silver nanoparticles as well as that of gold nanoparticles has also been carried out by varying the wavelength and fluence of irradiation laser pulses. Irradiation at an optimum condition of laser fluence is essential for the proper silver nanojoining of gold nanospheres to produce gold@silver core-shell composite nanowires. The excitation of the surface plasmon resonances of the base-metallic gold nanospheres rather than the filler-metallic silver nanospheres paves the way for the silver nanojoining of gold nanoparticles.

Electrophoretic separation of gold nanoparticles according to bifunctional molecules-induced charge and size.

Gold nanospheres modified with bifunctional molecules have been separated and characterized by using agarose gel electrophoresis as well as optical spectroscopy and electron microscopy. The electrophoretic mobility of a gold nanosphere capped with 11-mercaptoundecanoic acid (MUA) has been found to depend on the number of MUA molecules per gold nanosphere, indicating that it increases with the surface charge of the nanoparticle. The extinction spectrum of gold nanospheres capped with MUA at an MUA molecules per gold nanosphere value of 1000 and connected via 1,6-hexanedithiol (HDT) decreases by 33% in magnitude and shifts to the red as largely as 22 nm with the increase of the molar ratio of HDT to MUA (R(HM)). Gold nanospheres capped with MUA and connected via HDT have been separated successfully using gel electrophoresis and characterized by measuring reflectance spectra of discrete electrophoretic bands directly in the gel and by monitoring transmission electron microscope images of gold nanoparticles collected from the discrete bands. Electrophoretic mobility has been found to decrease substantially with the increment of HDT to MUA, indicating that the size of aggregated gold nanoparticles increases with the concentration of HDT.

Direct observation of conformation-dependent pathways in the excited-state proton transfer of 7-hydroxyquinoline in bulk alcohols.

The excited-state proton transfer (ESPT) of 7-hydroxyquinoline (7HQ) in bulk alcoholic solvents has been explored with variation of protic hydrogen atoms as well as alcohols. By measuring time-resolved kinetic profiles at two different excitation wavelengths, we have observed conformation-specific pathways in the ESPT of 7HQ. There are two possible rotamers of 7HQ according to the configuration of its hydroxyl group, which are trans and cis. On one hand, trans-7HQ cannot undergo proton transfer within its excited-state lifetime because the internal rotation of the hydroxyl group to form cis-7HQ hardly occurs. On the other hand, some cis-7HQ molecules exist as cyclic complexes of 7HQ·(alcohol)(2) at the moment of excitation, and the cyclic complexes can undergo ESPT rapidly via tunneling. However, the other cis-7HQ molecules should undergo solvent reorganization to form cyclic 7HQ·(alcohol)(2) complexes prior to intrinsic ESPT, and the solvent reorganization becomes the rate-determining step. In contrast to proton transfer, where intrinsic ESPT and solvent reorganization have been observed separately in time-resolved kinetic profiles, intrinsic deuteron transfer is too slow to be distinguished kinetically from solvent reorganization.

Ground-state proton transport along a blended-alcohol chain: accelerated by accumulated proton-donating ability.

The ground-state reverse proton transfer (GSRPT) of 7-hydroxyquinoline (7HQ) along a hydrogen (H)-bonded mixed-alcohol chain made of different two alcohol molecules having dissimilar proton-donating abilities, designed as a biomimetic system of a proton wire composed of various amino acids, has been investigated in nonpolar aprotic media of n-alkanes using time-resolved transient-absorption spectroscopy with variation of alcohol combinations and medium viscosities. Solvent-inventory experiments have been carried out by varying the composition of alcohols systematically in the heterogeneous H-bonded alcohol chain to understand the molecular dynamics and the elementary mechanisms of GSRPT. Similarly to excited-state proton transfer, GSRPT takes place concertedly without accumulating any reaction intermediate but asymmetrically via a rate-determining tunneling process, and GSRPT is accelerated by the accumulated proton-donating abilities of two alcohol molecules participating in the H-bond chain by push-ahead effect. However, in the ground state, the reorganization of the H-bond bridge in a cyclic 7HQ·(alcohol)(2) complex to form an optimal precursor configuration for efficient proton tunneling takes place prior to intrinsic proton transfer, and the rate constant of GSRPT is governed mainly by configurational optimization. Consequently, the large contribution of the configurational optimization to GSRPT leads to the weaker push-ahead effect and the less-asymmetric character of GSRPT than the respective ones of excited-state proton transfer whose rate constant is determined mostly by tunneling.

Excited-state hydrogen relay along a blended-alcohol chain as a model system of a proton wire: deuterium effect on the reaction dynamics.

The excited-state deuteron transfer (ESDT) of deuterated 7-hydroxyquinoline (7DQ) along a heterogeneous hydrogen (H)-bonded chain composed of two deuterated alcohol (ROD) molecules having different acidities, as a model system of a proton wire consisting of diverse amino acids, has been investigated. To understand dynamic differences between deuteron transfer and proton transfer, solvent-inventory experiments have been performed with variation of the combination as well as the composition of alcohols in a H-bonded mixed-alcohol chain. Deuteron transfer from the adjacent ROD molecule to the basic imino group of 7DQ via tunneling, which is the rate-determining step, initiates ESDT, and subsequent barrierless deuteron relay from the acidic enolic group of 7DQ to the alkoxide moiety along the H-bonded chain completes ESDT. Whereas the acceleration of the reaction has been observed in excited-state proton transfer because of the accumulated proton-donating abilities of two alcohol molecules in a H-bonded chain by a push-ahead effect, such acceleration is not observed in ESDT. Because the energy barrier of deuteron relay is much higher than that of proton relay due to the low zero-point energy of 7DQ·(ROD)(2) and a deuteron is twice as heavy as a proton, it is hard for a deuteron to pass through the barrier via tunneling. Moreover, both the H-bonding ability and the acidity of ROD molecules are so weak that their deuteron-donating abilities cannot be accumulated at the rate-determining step of ESDT. Consequently, the rate constant of ESDT is determined mostly by the acidity of the ROD molecule H-bonded directly to the imino group of 7DQ.

Excited-state proton-relay dynamics of 7-hydroxyquinoline controlled by solvent reorganization in room temperature ionic liquids.

The excited-state triple proton relay of 7-hydroxyquinoline (7HQ) along a hydrogen-bonded methanol chain in room temperature ionic liquids (RTILs) has been investigated using picosecond time-resolved fluorescence spectroscopy. The rate constant of the proton relay in a methanol-added RTIL is found to be slower by an order of magnitude than that in bulk methanol and to have unity in its kinetic isotope effect. These suggest that the excited-state tautomerization dynamics of 7HQ in methanol-added RTILs is mainly controlled by the solvent reorganization dynamics to form a cyclically hydrogen-bonded complex of 7HQ·(CH(3)OH)(2) upon absorption of a photon due to high viscosity values of RTILs. Because the cyclic complex of 7HQ·(CH(3)OH)(2) at the ground state is unstable in RTILs, the collision-induced slow formation of the cyclic complex should take place upon excitation prior to undergoing subsequent intrinsic proton transfer rapidly.

Anomalously slow proton transport of a water molecule.

Unusually low proton-transporting ability of a water molecule has been observed in the excited-state proton transfer (ESPT) of a 7-azaindole (7AI) molecule complexed cyclically with a water molecule in diethyl ether and dipropyl ether. In contrast with ultrafast (<1 ps) proton diffusion along a systematically structured hydrogen-bond network in an aqueous solution, the proton transport of a water monomer has been observed to be extremely slow (∼1 ns) because it is hard for a monomeric water molecule alone to accept or donate a proton. Thus, polar ether molecules surrounding a cyclic hydrogen-bonded 1:1 7AI-water complex (Nc) play a crucial role in the ESPT of Nc. The proton acceptance of a water molecule from the acidic amino group of 7AI via tunneling to form a hydronium ion, which is the rate-determining step, initiates ESPT, and the subsequent rapid proton donation of the hydronium ion to the basic imino group of 7AI takes place barrierlessly to complete ESPT without accumulating any intermediate. Due to the anomalously weak proton-transporting ability of a water monomer, the elaborate reorganization of the hydrogen-bond bridge in Nc to form an optimized precursor configuration is required for proton tunneling.

Solvent effect on the excited-state proton transfer of 7-hydroxyquinoline along a hydrogen-bonded ethanol dimer.

We have studied the solvent effect on structures and potential energy surfaces along proton transfer in the ground and the excited states of 7-hydroxyquinoline interacting with an ethanol dimer using ab initio calculations. The proton transfer is forbidden in the ground state not only in vacuum but also in solvents of n-heptane, ethanol, and dimethyl sulfoxide. In the excited state, although the proton transfer is forbidden in vacuum, it is possible in solvent due to its greatly reduced barrier (∼10 kcal mol(-1)) and highly stabilized product. It has also been found from the calculations that the proton-transfer barrier in the excited state decreases as the dielectric constant of a solvent increases. Our calculations are consistent with experimental results that the proton transfer does not take place in the ground state and that the excited-state proton-transfer rate increases as the solvent polarity increases. Our calculated absorption and emission properties are in excellent agreement with experimental results. Projection factors (reflecting geometrical change from the ground state to the excited state) and reorganization energies for several low frequency vibrations in connection with the excited-state proton transfer are discussed as well.

Excited-state double proton transfer of 7-azaindole dimers in a low-temperature organic glass.

The excited-state double proton transfer of model DNA base pairs, 7-azaindole (7AI) dimers, is explored in a low-temperature organic glass of n-dodecane using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the conformations of 7AI dimers at the moment of excitation; whereas planar conformers tautomerize rapidly (<10 ps), twisted conformers undergo double proton transfer to form tautomeric dimers on the time scale of 250 ps at 8 K. The proton transfer is found to consist of two orthogonal steps: precursor-configurational optimization and intrinsic proton transfer via tunneling. The rate is almost isotope independent at cryogenic temperatures because configurational optimization is the rate-determining step of the overall proton transfer. This optimization is assisted by lattice vibrations below 150 K or by librational motions above 150 K.

Ground-state proton-transfer dynamics governed by configurational optimization.

The ground-state proton transfer (GSPT) of 7-hydroxyquinoline along a hydrogen-bonded alcohol chain has been investigated in n-alkanes using time-resolved transient-absorption spectroscopy with variation of alcohols, media, isotopes, and temperatures. As a 7-hydroxyquinoline molecule associates with two alcohol molecules via hydrogen bonding to form a cyclic complex in a nonpolar aprotic medium, the intrinsic GSPT dynamics of the cyclic complex in a n-alkane has been observed directly without being interfered with by solvent association to form the cyclic complex. GSPT occurs concertedly without accumulating any reaction intermediate and yet asymmetrically with a rate-determining tunneling process. Both the rate constant and the kinetic isotope effect of GSPT increase rapidly with the proton-donating ability of the alcohol but decrease greatly with the molecular size of the alcohol. The reorganization of the hydrogen-bond bridge to form an optimal precursor configuration for efficient proton tunneling takes place prior to intrinsic GSPT, and configurational optimization becomes more important as the molecular size of the alcohol increases. Consequently, the larger contribution of configurational optimization to GSPT leads to the weaker asymmetric character of GSPT.