Gold nanorod-seeded growth of silver nanostructures: from homogeneous coating to anisotropic coating

Single crystalline gold nanorods (Au NRs) dominated by {110} side facets were employed as seeds to tailor the deposition of Ag. Apart from homogeneous coating, anisotropic coating of Ag was observed and resulted in an orange slice-like shape for the Au@Ag nanocrystal. Different growth rates for the {110} side facets were responsible for this shape: among the four {110} facets, two of the neighboring {110} facets grew more quickly and another two grew more slowly, thus inducing the anisotropic deposition of Ag around the Au NR. This growth behavior is believed to be a consequence of competition between the strong stabilization of cetyltrimethylammomium bromide (CTAB) molecules to the {110} facets of Ag and minimization of the overall surface energy. Although the reason for the anisotropic coating remains to be clarified, our results lead to one important conclusion: The interaction of CTAB and metal can be utilized to tune the shapes of bimetallic structures.     

Structural evolution of gold nanorods during controlled secondary growth

Single-crystalline gold nanorods synthesized by the Ag(I)-mediated seeded-growth method (see: El-Sayed, M. A.; Nikoobakht, B. Chem. Mater. 2003, 15, 1957) were used as seeds for the preferential overgrowth of gold on particular crystallographic facets by systematic variation of the conditions during overgrowth. The results support previous reports about the relevance of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and Ag(I) in stabilizing anisotropic particle shapes and demonstrate that the regulation of the amount of ascorbic acid facilitates the preferential overgrowth of {111} crystal facets to form Xi-type particle shapes. Interestingly, secondary overgrowth is found to inevitably result in a loss of particle shape anisotropy. A mechanism based on surface reconstruction is proposed to rationalize the "shape-reversal" that is generally observed in the nanorod growth process, that is, the initial increase and subsequent decrease of particle anisotropy with increasing reaction time. High-resolution electron microscopy analysis of gold nanorods reveals clear evidence for (1 x 2) missing row surface reconstruction of high energetic {110} facets that form during the initial phase during particle growth.     

Mechanism of silver(I)-assisted growth of gold nanorods and bipyramids

The seed-mediated growth of gold nanostructures is shown to be strongly dependent on the gold seed nanocrystal structure. The gold seed solutions can be prepared such that the seeds are either single crystalline or multiply twinned. With added silver(I) in the cetyltrimethylammonium bromide (CTAB) aqueous growth solutions, the two types of seeds yield either nanorods or elongated bipyramidal nanoparticles, in good yields. The gold nanorods are single crystalline, with a structure similar to those synthesized electrochemically (Yu, Y. Y. et al. J. Phys. Chem. B 1997, 101, 6661). In contrast, the gold bipyramids are pentatwinned. These bipyramids are strikingly monodisperse in shape. This leads to the sharpest ensemble longitudinal plasmon resonance reported so far for metal colloid solutions, with an inhomogeneous width as narrow as 0.13 eV for a resonance at approximately 1.5 eV. Ag(I) plays an essential role in the growth mechanism. Ag(I) slows down the growth of the gold nanostructures. Ag(I) also leads to high-energy side facets that are {110} for the single crystalline gold nanorods and unusually highly stepped {11n} (n approximately 7) for the bipyramid. To rationalize these observations, it is proposed that it is the underpotential deposition of Ag(I) that leads to the dominance of the facets with the more open surface structures. This forms the basis for the one-dimensional growth mechanism of single crystal nanorods, while it affects the shape of the nanostructures growing along a single twinning axis.     

Seed‐Mediated Growth of Silver Nanocubes in Aqueous Solution with Tunable Size and Their Conversion to Au Nanocages with Efficient Photothermal Property

Two seed‐mediated approaches for the growth of silver nanocubes in aqueous solution have been developed. Addition of a silver‐seed solution to a mixture of cetyltrimethylammonium chloride (CTAC), silver trifluoroacetate, and ascorbic acid and heating the solution at 60 °C for 1.5 h produces uniform Ag nanocubes with tunable sizes from 23 to 60 nm by simply adjusting the volume of silver‐seed solution introduced. Alternatively, the silver‐seed solution can be injected into a mixture of cetyltrimethylammonium bromide (CTAB), silver nitrate, copper sulfate, and ascorbic acid and heated to 80 °C for 2 h to generate 46 nm silver nanocubes. Plate‐like Ag nanocrystals exposing {111} surfaces can be synthesized by reducing Ag(NH₃)₂⁺ with ascorbic acid in a CTAC solution. Relatively large Ag nanocubes were converted to cuboctahedral Au/Ag and Au nanocages and nanoframes with empty {111} faces through a galvanic replacement reaction. The nanocages showed a progressive plasmonic band red‐shift with increasing Au content. The nanocages exhibited high and stable photothermal efficiency with solution temperatures quickly reaching beyond 100 °C when irradiated with an 808 nm laser for large heat and water vapor generation.     

Facile synthesis of gold-silver nanocages with controllable pores on the surface

Gold-silver alloy nanocages with controllable pores on the surface have been synthesized via galvanic replacement reaction between truncated Ag nanocubes and aqueous HAuCl4. Unlike in the previous studies, the initiation of replacement reaction started in a controllable way, simultaneously from eight corners of the truncated Ag nanocubes where {111} facets were exposed. The formation of cubic nanocages with pores at all the corners was determined by the capping agent, poly(vinyl pyrrolidone) (PVP), which preferentially covered the {100} facets of a truncated Ag nanocube.     

Facile synthesis of gold nanoplates by thermally reducing AuCl_4~- with aniline

We herein report a one-step,wet-chemical approach to synthesizing gold nanoplates in large quantities via the AuCl_4~-thermal reduction process by aniline,without introducing additional capping agent or suffactant.It is found that the reduction kinetics of AuCl_4~-is greatly altered by varying the initial molar ratio of aniline to AuCl_4~-.Moreover,further investigation reveals that the in- situ formed polyaniline could serve effectively as a capping agent to preferably adsorb the{111}facets of gold crys...     

Facile synthesis of gold nanoplates by thermally reducing AuCl4ˉwith aniline

We herein report a one-step,wet-chemical approach to synthesizing gold nanoplates in large quantities via the AuCl4-thermal reduction process by aniline,without introducing additional capping agent or surfactant.It is found that the reduction kinetics of AuCl4-is greatly altered by varying the initial molar ratio of aniline to AuCl4-.Moreover,further investigation reveals that the in-situ formed polyaniline could serve effectively as a capping agent to preferably adsorb the { 111 } facets of gold crystals during a slow reduction process,directing the formation of gold nanoplates.     

{111}晶面暴露对介孔金红石TiO2单晶光催化产氢的促进作用

光催化反应发生在半导体材料的表面,材料表面的原子/电子结构直接影响光催化剂的活性或选择性。因此,发展具有特定晶面的半导体光催化剂受到各国学者的普遍关注,被认为是调控光催化材料性能的有效途径之一。自2008年yang等首次合成高表面能{001}晶面占优的锐钛矿TiO2单晶以来,控制合成暴露不同晶面TiO2晶体的研究得到了迅猛的发展,已发展了多种方法合成了具有不同晶面的TiO2晶体。研究表明,选择性地暴露特定的活性晶面能够显著地提高光催化剂的活性或者改变光催化反应的选择性。但是,含有完整晶面构型的TiO2单晶样品的颗粒尺寸一般都较大,通常为几微米,因而显著增加了光生载流子传输与分离的难度,并且导致材料较小的比表面积,限制了对光催化活性的进一步提高。能否在合成含特定晶面单晶的同时增加多孔结构成为有效解决这一问题的关键。最近, Crossland等采用晶种模板法成功合成了介孔的锐钛矿TiO2单晶,并且通过光电器件研究证实了采用该思路可进一步提高材料的光电性能。金红石TiO2在光催化全分解水方面具有独特的优势,然而关于多孔单晶金红石TiO2的研究相对较少,尤其是合成热力学不稳定的高表面能{111}晶面完全暴露的多孔金红石单晶面临较大的技术挑战因而一直未见文献报道。本文利用晶种模板法,以TiCl4溶液为含Ti前驱体、NaF为形貌控制剂、采用水热处理制备出不同比例{111}晶面的介孔金红石单晶。我们前期工作表明, NaF可作为形貌控制剂合成低表面能{110)晶面占优的介孔金红石单晶。本文发现,通过改变NaF的添加量,可有效调变{111}/{110}晶面比例,最终合成完全暴露{111}高表面能的介孔金红石TiO2单晶。扫描电镜结果显示,当添加20 mg NaF时,合成{110}占优的具有高长径比的介孔晶体;当NaF用量增加到40 mg时{110}晶面进一步缩短;至80 mg时则制备出{111})高能面完全暴露的金红石TiO2晶体。值得注意的是,对比研究表明,不采用模板合成了与多孔晶体完全相对应的不同{111}/(110}晶面比例的实心金红石晶体。透射电镜及选区电子衍射以及结合X射线衍射进一步证实,多孔的金红石TiO2晶体与实心金红石单晶均都为单晶结构,孔结构贯穿于样品内部且具有较高的晶面结晶性。氮气吸附实验发现,虽然三个不同晶面比例介孔金红石单晶样品间的形貌具有显著的差异,但比表面积非常相近(分别为24,25,28 m2/g),孔径也都为50 nm左右,该值与所用SiO2模板球的直径以及TEM观察结果相一致。光催化产氢性能结果表明,选择性的暴露活性晶面显著提高了光催化活性,仅含高能面{111}的介孔金红石单晶样品具有最高的产氢速率(约800μmol h–1 g–1),比常规{110}晶面占优的介孔单晶样品速率提高了约一倍。尤其比实心单晶样品的产氢速率提高了至少一个数量级,这应归结于介孔结构特性所导致的表面反应活性位增加、电子传输距离缩短以及光吸收增强协同作用的结果。     

Synthesis and mechanistic study of palladium nanobars and nanorods

This paper describes a simple and versatile method for growing highly anisotropic nanostructures of Pd, single-crystal nanobars bounded by {100} facets and single-crystal nanorods with their side surfaces enclosed by {100} and {110} facets. According to thermodynamic arguments, Pd atoms should nucleate and grow in a solution phase to form cuboctahedrons of spherical shape with their surfaces bounded by a mix of {111} and {100} facets. Anisotropic nanostructures can only form under kinetically controlled conditions, while the cubic symmetry is broken. In the present system, we found that one-dimensional growth could be induced and maintained through an interplay of the following processes: (i) speedy reduction of the precursor to ensure prompt addition of atoms to the seed; (ii) chemisorption of bromide on the seed to promote the formation of {100} and {110} facets; and (iii) localized oxidative etching on one specific face of the seed to initiate preferential growth on this face. Experimentally, the anisotropic growth can be achieved by varying the type and concentration of reducing agent, as well as by adjusting the reaction temperature. This methodology developed for Pd has also been extended to both Au and Pt. As expected for a kinetically controlled product, the anisotropic nanostructure evolved into the thermodynamically favored shape during an aging process.     

Dipyrromethane as a new organic reagent for the synthesis of gold nanoparticles: preparation and application

Condensation reaction of several ketones with pyrrole in the presence of ferric hydrogen sulfate as a green homogenous acidic catalyst furnished the corresponding pure dipyrromethanes in good yields. Gold nanoparticles were produced through reduction of HAuCl₄ with substituted dipyrromethanes as new reducing agents at room temperature with the exclusion of any capping agent or surfactant. Gold nanoparticles were characterized by transmission electron microscopy, scanning electron microscopy, XRD and UV–visible absorption spectroscopic measurements. It is proposed that in situ formed oxidative products of dipyrromethane, such as polydipyrromethane could serve effectively as a capping agent to preferably adsorb the {111} facets of gold crystals during the reduction process, which leads to the formation of gold nanoparticles.     

钯纳米颗粒形貌控制合成及其SPR／SERS性质

在水溶液中分别以十六烷基三甲基溴化铵（CTAB）和CTAB／柠檬酸钠混合剂为包覆剂合成钯纳米颗粒,并研究其形貌演变．钯纳米颗粒在成核阶段会形成具有不同孪晶结构的晶核,在生长阶段又会选择性的放大某一组晶面,这两个因素导致了钯纳米颗粒形貌的多样性．在合成中CTAB既会影响钯纳米颗粒的成核,也会影响颗粒晶面的选择性生长．通过改变CTAB和还原剂的量可以调控钯纳米颗粒的形貌．溶液中CTAB和还原剂浓度的改变,非常明显地影响合成产物中不同形貌钯纳米颗粒的产率．通过向溶液中引入柠檬酸离子调控纳米颗粒的成核与生长过程,首次合成出了星状钯二十面体和截面为五角星形的钯纳米棒．这些不同形貌的钯纳米颗粒有着不同的表面等离子体共振和表面增强拉曼散射性质．     

Experimental and numerical study of cetyltrimethylammonium bromide (CTAB)-directed synthesis of goethite nanorods

This study demonstrates a facile surfactant-directed approach to prepare goethite (α-FeOOH) nanorods in aqueous solution at room temperature. The obtained α-FeOOH nanorods have a diameter of ～20 nm and length up to 300 nm. Various experimental parameters have been investigated, such as surfactants, solution pH, and reaction temperature. It is observed that the surfactant, cetyltrimethylammonium bromide (CTAB), plays a key role in the growth of goethite nanorods at ambient conditions. The final product can be purified using diluted hydrochloric acid (HCl) to remove particles of other shapes. Molecular dynamics (MD) method is used to understand the underlying principles governing particle growth through the analysis of the interaction energies between crystal surfaces and surfactant molecules. The simulation results indicate that CTAB can strongly interact with {100}, {010}, and {110} planes, which benefits the growth of nanorods along [001] direction. Such simulations can provide useful information for the synthesis and shape control of other metal oxide materials.     

Nanocrystals composed of alternating shells of Pd and Pt can be obtained by sequentially adding different precursors

This paper describes a layer-by-layer epitaxial approach to the synthesis of multishelled nanocrystals composed of alternating shells of Pd and Pt by starting with seeds made of Pd or Pt nanocrystals. The synthesis was conducted by sequentially adding PtCl(4)(2-) and PdCl(4)(2-) salt precursors into a system containing either Pd or Pt seeds (in the shape of cuboctahedrons, octahedrons, plates, or cubes) together with a weak reducing agent such as citric acid (CA). The slow reduction kinetics associated with CA played an important role in the epitaxial growth of one metal on the other, resulting in the formation of Pd-Pt multishelled nanocrystals. Owing to the capping effect of CA for {111} facets of Pd and Pt, the multishelled nanocrystals tended to be enclosed by {111} facets in the form of octahedrons or thin plates, depending on the shapes of the Pd or Pt seeds: octahedrons for cuboctahedral, cubic, or octahedral seeds, and plates for platelike seeds.     

Seedless synthesis of octahedral gold nanoparticles in condensed surfactant phase

We report a seedless synthetic method of gold octahedral nanoparticles in an aqueous phase. Eight facets with {111} crystalline structures of octahedral nanoparticles could be formed in an aqueous medium when the gold salt was reduced by ascorbic acid at room temperature in the presence of cetyltrimethylammonium bromide as a shape-inducing agent, and hydrogen peroxide as a reaction promoter. The growth kinetics and surface crystalline structures were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy.     

Quantitative analysis of the role played by poly(vinylpyrrolidone) in seed-mediated growth of Ag nanocrystals

This article presents a quantitative analysis of the role played by poly(vinylpyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: (i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface, and (ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm(2) for PVP of 55,000 and 10,000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm(2), respectively, when 100 nm Ag cubes were used as the seeds.     

Facet effect of single-crystalline Ag3PO4 sub-microcrystals on photocatalytic properties

We recently reported that Ag(3)PO(4) exhibits excellent photooxidative capabilities for O(2) evolution from water and organic dye decomposition under visible-light irradiation. However, very little is known about the shape and facet effects of Ag(3)PO(4) crystals on their photocatalytic properties. Herein we have developed a facile and general route for high-yield fabrication of single-crystalline Ag(3)PO(4) rhombic dodecahedrons with only {110} facets exposed and cubes bounded entirely by {100} facets. Moreover, studies of their photocatalytic performance have indicated that rhombic dodecahedrons exhibit much higher activities than cubes for the degradation of organic contaminants, which may be primarily ascribed to the higher surface energy of {110} facets (1.31 J/m(2)) than of {100} facets (1.12 J/m(2)).     

Silicon Wafers with Facet‐Dependent Electrical Conductivity Properties

By breaking intrinsic Si (100) and (111) wafers to expose sharp {111} and {112} facets, electrical conductivity measurements on single and different silicon crystal faces were performed through contacts with two tungsten probes. While Si {100} and {110} faces are barely conductive at low applied voltages, as expected, the Si {112} surface is highly conductive and Si {111} surface also shows good conductivity. Asymmetrical I –V curves have been recorded for the {111}/{112}, {111}/{110}, and {112}/{110} facet combinations because of different degrees of conduction band bending at these crystal surfaces presenting different barrier heights to current flow. In particular, the {111}/{110} and {112}/{110} facet combinations give I –V curves resembling those of p–n junctions, suggesting a novel field effect transistor design is possible capitalizing on the pronounced facet‐dependent electrical conductivity properties of silicon.     

The Effects of Exposed Specific Facets and Sulfation on the Surface Acidity of Cu2O Solids

Cuprous oxide microcrystals with {111}, {111}/{100}, and {100} exposed facets were synthesized. ³¹P MAS NMR using trimethylphosphine as the probe molecule was employed to study the acidic properties of samples. It was found that the total acidic density of samples increases evidently after sulfation compared with the pristine cuprous oxide microcrystals. During sulfation, new {100} facets are formed at the expense of {111} facets and lead to the generation of two Lewis acid sites due to the different binding states of SO₄²⁻ on {111} and {100} facets. Moreover, DFT calculation was used to illustrate the binding models of SO₄²⁻ on {111} and {100} facets. Also, a Pechmann condensation reaction was applied to study the acidic catalytic activity of these samples. It was found that the sulfated {111} facet has better activity due to its higher Lewis acid density compared with the sulfated {100} facet.     

Facile synthesis and electrochemiluminescence application of concave trisoctahedral Pd@Au core-shell nanocrystals bound by {331} high-index facets

Concave trisoctahedral (TOH) Pd@Au core-shell nanocrystals bound by {331} facets have been synthesized for the first time. Pd nanocubes and cetyltrimethylammonium chloride were used as the structure-directing cores and capping agents, respectively. Their optical and electrocatalytic properties were investigated.     

Self-assembled silver nanochains for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) integrates high levels of sensitivity with spectroscopic precision and has tremendous potential for chemical and biomolecular sensing. The key to the wider application of Raman spectroscopy using roughened metallic surfaces is the development of highly enhancing substrates for analytical purposes. Here, we demonstrate a simple strategy for self-assembling silver nanochains on glass substrates for sensitive SERS substrates. The chain length of short Ag nanochains can be controlled by adjusting the concentration of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA). CTAB with appropriate concentration serves as the "glue" that can link the {100} facets of two neighboring Ag nanoparticles. MUA is found to be effective in "freezing up" the aggregation of Ag short chains and preventing them from further aggregating into a long chainlike network structure. The surface plasmon bands can be tuned over an extended wavelength range by controlling the length of the nanochains. The Ag monolayer, mainly composed of four-particle nanochains, exhibited the maximum SERS enhancement factor of around 2.6 x 108, indicating that a stronger SERS enhancement can be obtained in these interstitial sites of chainlike aggregated Ag nanoparticles.