Mechanism of silver ion reduction in concentrated solutions of amphiphilic invertible polyesters in nonpolar solvent at room temperature

Fast formation and efficient stabilization of silver nanoparticles from [Ag(NH3)2]OH are achieved in concentrated nonpolar solutions of amphiphilic invertible polyesters based on poly(ethylene oxide) (PEO) and aliphatic dicarboxylic acids. Surface-modified silver nanoparticles able to be dispersed in both a polar and nonpolar medium are developed in the form of a ready-to-use colloidal solution with an enhanced silver concentration. The PEO fragments of polyesters form cavities (also called pseudo-crown ester structures) that can bind metal ions. The reduction of bound metal ions proceeds via oxidation of polyoxyethylene fragments. No chemical reducing agents are necessary in this approach. The polyesters act simultaneously as an efficient reducing agent and stabilizer. The main focus of the present research is to clarify the chemical mechanism of silver ion reduction in amphiphilic polyester solutions. A one-electron reduction mechanism is proposed to explain the formation of silver nanoparticles. The effect of the poly(ethylene oxide) fragment length and the polyester concentration are explored by examining several amphiphilic polyesters.     

Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods

We demonstrate the controlled growth of high aspect ratio anatase TiO2 nanorods by hydrolysis of titanium tetraisopropoxide (TTIP) in oleic acid (OLEA) as surfactant at a temperature as low as 80 degrees C. Chemical modification of TTIP by OLEA is proven to be a rational strategy to tune the reactivity of the precursor toward water. The most influential factors in shape control of the nanoparticles are investigated by simply manipulating their growth kinetics. The presence of tertiary amines or quaternary ammonium hydroxides as catalysts is essential to promote fast crystallization under mild conditions. The novelty of the present approach relies on the large-scale production of organic-capped TiO2 nanocrystals to which standard processing of colloidal nanocrystals, such as surface ligand exchange, can be applied for the first time. Concentrated colloidal titania dispersions can be prepared for a number of fundamental studies in homogeneous solutions and represent a new source of easily processable oxide material for many technological applications.     

Organic photovoltaic devices with colloidal TiO2 nanorods as key functional components

We report on a novel approach to integrate colloidal anatase TiO(2) nanorods as key functional components into polymer bulk heterojunction (BHJ) photovoltaic devices by means of mild, all-solution-based processing techniques. The successful integration of colloidal nanoparticles in organic solar cells relies on the ability to remove the long chain insulating ligands, which indeed severely reduces the charge transport. To this aim we have exploited the concomitant mechanisms of UV-light-driven photocatalytic removal of adsorbed capping ligands and hydrophilicization of TiO(2) surfaces in both solid-state and liquid-phase conditions. We have demonstrated the successful integration of the UV-irradiated films and colloidal solutions of TiO(2) nanorods in inverted and conventional solar cell geometries, respectively. The inverted devices show a power conversion efficiency of 2.3% that is a ca. three times improvement over their corresponding cell counterparts incorporating untreated TiO(2), demonstrating the excellent electron-collecting property of the UV-irradiated TiO(2) films. The integration of UV-treated TiO(2) solutions in conventional devices results in doubled power conversion efficiency for the thinner active layer and in maximum power conversion efficiency of 2.8% for 110 nm thick devices. In addition, we have demonstrated, with the support of device characterizations and optical simulations, that the TiO(2) nanocrystal buffer layer acts both as electron-transporting/hole-blocking material and optical spacer.     

Preparation of silver nanocrystals in the presence of aniline

The preparation and characterization of silver nanocrystals by chemical reduction of silver ions in the presence of aniline using hydrazine monohydrate (N(2)H(4).H(2)O) or sodium citrate as the reducing agent are described. A high yield of aniline-derivatized hexagonal silver nanoparticles is obtained by the reduction of Ag(2)SO(4) with N(2)H(4).H(2)O. An alternative strategy is the reduction of Ag(+) by citrate in the presence of aniline, by which the size and morphology of the Ag nanocrystals can be controlled to a certain degree by changing the concentration ratio of aniline to Ag(+). It is believed that the amount of aniline added in the starting solutions for the preparation influences the morphology of the Ag nanoparticles. In addition, the long Ag nanorods with a high mean aspect ratio are prepared in the presence of aniline at a low concentration or o-anisidine.     

Laser-induced silver nanocrystal formation in polyvinylpyrrolidone solutions

We report unusual laser-induced shape conversions of silver nanoparticles dispersed in polyvinylpyrrolidone (PVP) aqueous solutions. Silver nanocrystals such as nanoplates and nanorods were formed using laser irradiation for colloidal silver nanoparticles prepared using laser ablation in aqueous solutions of PVP. Differing from the nanocrystal formation observed in neat water and halide solutions, which were induced by weak laser or fluorescent-light irradiation, the nanocrystal formation in PVP solutions was induced by strong laser irradiation. On the other hand, nanocrystal formation was not observed in polyvinylalcohol (PVA) solutions, in which fusion of nanoparticles were prominent. It is proposed that the nanocrystals were formed from fragmented nanoparticles protected by PVP via a ripening process.     

Electrochemistry of conductive polymers 39. Contacts between conducting polymers and noble metal nanoparticles studied by current-sensing atomic force microscopy

Electrical properties of contacts formed between conducting polymers and noble metal nanoparticles have been examined using current-sensing atomic force microscopy (CS-AFM). Contacts formed between electrochemically prepared pi-conjugated polymer films such as polypyrrole (PPy), poly(3-methylthiophene) (P3MeT), as well as poly(3,4-ethylenedioxythiophene) (PEDOT) and noble metal nanoparticles including platinum (Pt), gold (Au), and silver (Ag) have been examined. The Pt nanoparticles were electrochemically deposited on a pre-coated PPy film surface by reducing a platinum precursor (PtCl62-) at a constant potential. Both current and scanning electron microscopic images of the film showed the presence of Pt islands. The Au and Ag nanoparticles were dispersed on the P3MeT and PEDOT film surfaces simply by dipping the polymer films into colloid solutions containing Au or Ag particles for specified periods (5 to approximately 10 min). The deposition of Au or Ag particles resulted from either their physical adsorption or chemical bonding between particles and the polymer surface depending on the polymer. When compared with PPy, P3MeT and PEDOT showed a stronger binding to Au or Ag nanoparticles when dipped in their colloidal solutions for the same period. This indicates that Au and Ag particles are predominantly linked with the sulfur atoms via chemical bonding. Of the two, PEDOT was more conductive at the sites where the particles are connected to the polymer. It appears that PEDOT has better aligned sulfur atoms on the surface and is strongly bonded to Au and Ag nanoparticles due to their strong affinity to gold and silver. The current-voltage curves obtained at the metal islands demonstrate that the contacts between these metal islands and polymers are ohmic.     

Observed Dramatically Improved Catalysis of Ag Shell on Au/Ag Core‐shell Nanorods is Due to Silver Impurities Released During Etching Process

Core/shell bimetallic nanoparticles are highly popular in electrocatalysis; it is argued that the core metal enhances the catalytic properties of the shell. We have investigated the electrocatalytic properties of Au/Ag core‐shell nanorods (Au/Ag NRs) where Ag shell was thinned by aging in the presence of cetyltrimethylammonium bromide. We observed excellent electrocatalysis toward hydrogen peroxide electroreduction upon decreasing the Ag shell thickness, which would, at first, appear to imply a strong synergistic effect of the Au core with the Ag shell for electrocatalysis. We show, however, that this electrocatalysis is not caused by particular Au/Ag core/shell structures but rather by the presence of residual silver impurities in the form of Ag nanoparticles (Ag NPs) formed during the preparation of the thin‐layer silver shell/gold core nanorods.     

Converting Ag2SCdS and Ag2SZnS into AgCdS and AgZnS Nanoheterostructures by Selective Extraction of Sulfur

A mild three‐step solution strategy is developed to prepare Ag? MS (M=Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag₂S? MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag₂S nanoparticles as catalysts, then the Ag₂S sections of the heterostructures are converted into silver nanoparticles by selective extraction of sulfur. Notably, for the prepared Ag? CdS heterostructures, the localized surface plasmon resonance of silver remarkably intensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light‐emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal–semiconductor hybrid nanostructures with desirable performances.     

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.     

Remarks on the reactions of a tetracarboxylic porphyrin with gold and silver ions: A spectrophotometric,TEM and SEM study

The coproporphyrin-I (CPI) behaves as a reducing agent for silver and gold metal ions and as stabilizing agent for the formed colloidal metallic nanoparticles. The peculiarity of silver and gold nanoparticles obtained in the reactions of monomeric form of CPI with their metal ions has been studied. The optical properties of the colloidal forms of these metals have been investigated by UV–Vis spectrophotometry, and their morphology by TEM and SEM measurements. The structures and the size distributions of Ag and Au particles have been characterized and determined by computerized TEM images.     

UV-switchable polyoxometalate sandwiched between TiO2 and metal nanoparticles for enhanced visible and solar light photococatalysis

To improve the photocatalytic efficiency of TiO(2)-based nanomaterials, we demonstrate a facile, generalized, highly localized reduction approach to the decoration of TiO(2)-polyoxometalate composites with a range of metal nanoparticles including Cu, Ag, Pt, and Au. The synthesis of nanocomposite photococatalysts reported in this study has been achieved by utilizing the unique ability of the TiO(2)-bound PTA (phosphotungstic acid) molecules (a polyoxometalate, POM) to act as a highly localized UV-switchable reducing agent that specifically reduces metal ions to their nanoparticulate forms directly and only onto the TiO(2) surface. This leads to the metal contaminant-free synthesis of TiO(2)-PTA-metal nanocomposites, which is a significant advantage of the proposed approach. The study further demonstrates that polyoxometalates are regenerable photoactive molecules with outstanding electron-transfer ability and the deposition of metal nanoparticles on the TiO(2)-PTA cocatalytic surface can have a dramatic effect on increasing the overall photocatalytic performance of the composite system. Moreover, it is observed that the photococatalytic performance of the TiO(2)-PTA-metal nanoparticles can be fine tuned by choosing the composition of metal nanoparticles in the nanocomposite. Interestingly, the photococatalysts reported here are found to be active under visible and simulated solar-light conditions. The underlying reaction mechanism for enhanced solar-light photococatalysis has been proposed.     

Preparation of highly uniform Ag/TiO2 and Au/TiO2 supported nanoparticle catalysts by photodeposition

Photodeposition of Ag nanoparticles on commercial TiO2 particles and nanoparticles was performed in order to provide direct visualization of the spatial distribution of photoactive sites on sub-micrometer-scale and nanoscale TiO2 particle surfaces and to create materials for potential catalytic applications. HRTEM (high-resolution transmission electron microscopy) and HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) were used to characterize these materials. The size and spatial distributions of the Ag nanoparticles on the commercial TiO2 were not uniform; the concentration of Ag was higher on grain boundaries and at the edges of these submicrometer particles. In the case of TiO2 nanoparticles, the size distribution of the Ag nanoparticles deposited was relatively uniform and independent of irradiation time and photon energy. The amount of Ag deposited on TiO2 nanoparticles was at least 6 times higher than that on the commercial samples for comparable irradiation conditions. Compared to the case of Ag photodeposition, the difference in the amount of Au photodeposited on TiO2 particles and nanoparticles was even greater, especially at low precursor concentrations. Photodeposition on TiO2 nanoparticles is suggested as a potential method for the preparation of Au/TiO2 catalysts, as loadings in excess of 10 wt % of uniform 1 nm metal particles were achieved in this work.     

Ag／TiO2复合纳米催化剂的制备和表征及其光催化活性

 采用光还原沉积贵金属法，制备了Ag／TiO2复合纳米催化剂．通过调节溶液的pH值控制TiO2表面负载银的形貌，利用AAS，XRD，TEM和XPS等手段对样品进行了表征．以苯胺氧化为模型反应，考察了Ag／TiO2复合纳米催化剂样品的光催化活性以及银沉积量和沉积形貌对催化剂活性的影响．结果表明，通过调控光还原沉积条件，可在平均粒径为24nm左右的TiO2颗粒上获得3nm左右均匀分散的银粒子；在TiO2上沉积适量的具有较高分散度的金属Ag，能有效提高TiO2对苯胺氧化反应的光催化活性．     

Defect-mediated formation of Ag cluster-doped TiO2 nanoparticles for efficient photodegradation of pentachlorophenol

A novel strategy was designed to prepare Ag cluster-doped TiO(2) nanoparticles (Ag/TiO(2) NPs) without addition of any chemical reducing agent and/or organic additive. A defect-rich TiO(x) species was generated by laser ablation in liquid (LAL) of a Ti target. The silver ions could be reduced and deposited on the surface of TiO(2) NPs through the removal of oxygen vacancies and defects; the TiO(x) species evolved into anatase NPs in a hydrothermal treatment process. The derived Ag/TiO(2) NPs are approximately 25 nm in size, with narrow size distribution. The Ag clusters are highly dispersed inside TiO(2) and less than 3 nm in size. The doped amount can be tuned by changing the concentration of Ag(+) ions. The as-synthesized Ag/TiO(2) NPs display improved photocatalytic efficiency toward pentachlorophenol (PCP) degradation.     

Facile design and nanostructural evaluation of silver-modified titania used as disinfectant

Fundamental research has been carried out to define optimal "green" synthesis conditions for the production of titania (TiO(2)) and silver (Ag) nanocomposites (TANCs) ranging from 12.7-22.8 nm in diameter. A bottom-up colloidal approach was employed to accurately control TANC monodispersity and composition. TANCs were found to be effective at inactivating Escherichia coli (E. coli) in water. The presence of Ag in the nanocomposites induced a decrease in TiO(2) band gap energy, which favoured valence to conduction band electron transfer and allowed for electron excitation using visible light. Aggregation of ultra-fine particles was prevented through the use of a long-chain polymer as evidenced by electrophoretic mobility studies. The TANCs catalyzed oxidation of bacterial membranes and cell death or disinfection. Theoretically, the TANC mode of E. coli disinfection is via water photolysis, which results in production of hydroxyl radicals and hydrogen peroxide. These interact with the outer membrane polysaccharides and lipids, leading to lipid peroxidation, membrane weakening and resulted in cell death. Our overarching goals were to optimize the variables involved in TANC "green" synthesis and to characterize its nanostructure. High resolution (HR) transmission and scanning electron microscopic (TEM and SEM) studies demonstrated that TANCs were highly crystalline and mono-dispersive. Elemental composition of Ag and Ti, as measured by X-ray energy dispersive (EDS) and X-ray photoelectron spectroscopy (XPS) confirmed sample purity. Ultraviolet-visible (UV-VIS) spectroscopy showed that the energy band-gap of Ag modified TiO(2) was in the visible range.     

Surface-enhanced Raman scattering investigation of the adsorption of 2-mercaptobenzoxazole on smooth copper surfaces doped with silver colloidal nanoparticles

The adsorption of 2-mercaptobenzoxazole on copper has been investigated by means of surface-enhanced Raman scattering (SERS) by doping smooth copper surfaces with silver colloidal nanoparticles. The metal surfaces have been characterized by means of atomic force microscopy measurements. The compound adsorbs on the Cu/Ag surfaces in its ionized thiolic form, adopting a tilted orientation with respect to the metal surface. The anion is chemisorbed through the sulfur and nitrogen atoms on the smooth copper surface, and the silver colloidal nanoparticles only enhance the Raman signal due to the electromagnetic mechanism. SERS data have been interpreted with the help of DFT calculations on models of the ligand bound to copper adclusters.     

A facile synthesis and characterization of Ag, Au and Pt nanoparticles using a natural hydrocolloid gum kondagogu (Cochlospermum gossypium)

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

Preparation of Ag nanoparticles termini‐protected side‐chain liquid crystalline azobenzene polymers by RAFT polymerization

A new synthetic approach to prepare Ag nanoparticles protected side‐chain liquid crystalline (LC) azobenzene polymers was reported. It is based on the reduction of silver ions in presence of a LC polymer polymerized by RAFT. The formation of Ag colloidal nanoparticles was confirmed by TEM and UV analysis. At the same time, according to the results of DSC, XPS, and FTIR spectra, Ag nanoparticles were protected by the side‐chain LC azobenzene polymers through surface attachment interactions between thiol groups and Ag. The out‐plane orientation of side‐chain LC is confirmed by surface‐enhanced Raman spectra analysis and scanning near‐field optical microscope, resulting from the large electromagnetic field arising from the excitation of surface plasmon polariton of Ag nanoparticles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5380–5386, 2007     

TiO2浓度对核-壳结构Ag/TiO2纳米复合粒子结构以及三阶非线性光学性质的影响

采用光还原方法制备了核-壳结构的Ag/TiO2纳米复合粒子, 通过TEM、UV-Vis光谱和XRD表征了不同TiO2浓度下Ag/TiO2纳米复合粒子的结构和光学性质. UV-Vis光谱证明了银颗粒的存在, 且复合粒子中的银粒径随着TiO2含量的增加而增加, 同时随着TiO2浓度的增加, 银的吸收峰出现明显的增强和展宽；从TEM照片 发现, Ag/TiO2纳米复合粒子是一种以Ag为核, 外面包覆一层TiO2的核-壳结构, TiO2浓度和Ag+浓度的增加, 使得复合粒子的银颗粒粒径增大. 用Z-扫描技术, 以锁模Ti:sapphire飞秒激光器发出的脉宽为130 fs激光做光源, 在790 nm波长的光作用下, 研究了0.5%(w)Ag+含量, 不同TiO2浓度的Ag/TiO2纳米复合粒子的非线性光学特性. 结果发现, 在790 nm激光作用下, 0.25%(w)TiO2样品膜有双光子吸收和自聚焦非线性折射现象; 而当TiO2浓度为0.70%(w)时, 样品膜的非线性吸收由反饱和吸收转变为饱和吸收.     

高活性Ag修饰TiO2空心八面体光催化剂的简易模板诱导合成

贵金属修饰的TiO2空心纳米光催化材料由于具有大的比表面积、低的质量密度、良好的表面渗透性、强的光吸收能力以及界面电荷的快速转移,因而表现出优异的光催化性能.为了制备结构可控且精良的贵金属修饰TiO2空心纳米光催化材料,人们先后采用湿化学法、高温固相法、声化学法及光化学沉积法等.由于在制备过程中涉及多种异相材料的合成和结构控制,因此上述合成方法通常需要复杂且多步过程(有些过程甚至很容易导致结构破坏),从而限制了其在光催化领域的实际应用.本文开发了一种简单的模板诱导法,成功制备了Ag纳米粒子修饰的TiO2空心八面体光催化剂.合成过程包括模板指导沉积和原位模板溶解.以Ag2O八面体为模板, TiF4水溶液为前驱体,首先通过TiF4水解形成TiO2纳米粒子沉积在Ag2O模板表面,而TiF4水解产生的酸性HF水溶液可原位溶解碱性的Ag2O模板,从而一步制得TiO2空心八面体.同时,利用Ag2O以及Ag+离子的光敏特性,在光照条件下使Ag2O和Ag+离子生成Ag纳米颗粒,直接沉积在TiO2空心八面体的内外表面,得到Ag/TiO2光催化剂.由此可见,该合成方法具有明显的步骤少的优点.扫描电镜、透射电镜、能量散射和紫外-可见光谱等表征结果表明成功制得内外Ag纳米粒子修饰的TiO2空心八面体光催化剂,其中TiO2空心八面体的壁厚约为25 nm,边长约为1μm,而修饰在其表面的Ag纳米粒子尺寸仅为5–10 nm.此外,通过控制TiF4前驱体浓度,还可制备核壳结构的Ag2O@TiO2八面体以及不同壁厚Ag修饰的TiO2空心八面体,表明该方法在Ag/TiO2形貌合成上的多样性和可控性.为了获得良好的光催化活性,对制备的Ag纳米粒子修饰的TiO2空心八面体光催化剂进行了不同温度的热处理. X射线衍射结果表明,即使在600 oC高温下,制备的Ag/TiO2空心八面体光催化剂依然能够保持原有的特殊形貌和锐钛矿TiO2晶型.该温度明显高于TiO2从锐钛矿向金红石转变的温度.这说明合成的Ag/TiO2能在保持高活性锐钛矿相的同时获得更好的结晶性.这可能是由于分散在TiO2原始晶粒周围的Ag纳米粒子能阻止其进一步长大,从而抑制了其晶型转变.不同样品光催化降解甲基橙的结果表明,经600oC高温热处理的样品表现出最高的光催化活性,相应的反应速率常数为0.11 min？1.这应得益于该光催化剂具有以下三个特点：(1) TiO2空心八面体的薄壁结构能够缩短光照条件下光生电子和空穴从内部扩散到表面的距离;(2)由于贵金属Ag纳米粒子沉积在TiO2薄壁的内外两个表面以及TiO2和Ag形成肖脱基势垒,因此在它们接触的界面上能够发生更快的界面电荷转移;(3)高温处理导致TiO2良好的晶化以及高活性锐钛矿TiO2晶型的保持.