Measurement of apparent diffusion coefficients within ultrathin nafion Langmuir-Schaefer films: comparison of a novel scanning electrochemical microscopy approach with cyclic voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.     

Preparation,characterization and electrochemical properties of Nafion® doped poly(ortho-anisidine) Langmuir–Schaefer films

Langmuir–Schaefer (LS) films of poly(ortho-anisidine) (POAS) were fabricated by utilizing water and water acidified HCl as subphases, respectively. The uniformity of the films formation and the doping with Nafion were verified by UV–Vis spectroscopy. The morphology and the thickness of the POAS, HCl post-doping POAS and Nafion post-doping POAS LS films were investigated using atomic force microscopy. The electrochemical properties of POAS LS films, HCl post-doping POAS and Nafion post-doping POAS were investigated and compared with our previously published work. The electrochemical switching time of HCl post-doping POAS and Nafion post-doping POAS LS films were also estimated.     

Fabrication of chiral Langmuir-Schaefer films of achiral amphiphilic Schiff base derivatives through an interfacial organization

Supramolecular chirality in the Langmuir-Schaefer (LS) films of two achiral amphiphilic Schiff bases, 2-(2'-benzimidazolyliminomethyl)-4-octadecyloxyphenol (BSC18) and 2-(2'-benzthiazolyliminomethyl)-4-octadecyloxyphenol (TSC18), was investigated. Both of these amphiphiles could form LS films from the water surface or coordinate with Ag(I) in the subphase to form Ag(I)-coordinated LS films. Although both of these amphiphiles were achiral, TSC18 formed a chiral LS film from the water surface, while BSC18 formed a chiral Ag(I)-coordinated LS film from the aqueous AgNO3 subphase. The supramolecular chirality in these LS films was suggested to be due to a cooperative stereoregular pi-pi stacking of the functional groups together with the long alkyl chains in a helical sense. The relationship between the chirality of the LS films and the molecular structures of TSC18 and BSC18 as well as their H-bond or coordination behaviors was discussed. The Schiff base films showed a reversible color change upon exposure to HCl and NH3 gas alternatively; however, the supramolecular chirality was irreversible during these processes.     

Effect of substituent position in coumarin derivatives on the interfacial assembly: reversible photodimerization and supramolecular chirality

The Langmuir and Langmuir-Schaefer (LS) films of two coumarin derivatives, 4-octadecyloxylcoumarin (4-CUMC18) and 7-octadecyloxylcoumarin (7-CUMC18), were newly synthesized, and their interfacial assemblies were investigated. Owing to the different substituent position of the long octadecyloxy chain in the coumarin parent, the two compounds showed completely different behaviors in the interfacial assemblies. When they were spread at the air/water interface, 7-CUMC18 formed a monolayer while 4-CUMC18 formed a multilayer film on the water surface. The spreading films on the water surface were transferred onto solid substrates by a Langmuir-Schaefer method, and the transferred films were characterized by UV-vis, Fourier transfer infrared, X-ray diffraction, circular dichroism, and atomic force microscopy spectra. Different packing of the molecules in the multilayer films was observed. While coumarin groups stacked in a face-to-face way in 7-CUMC18 film, they stacked in a head-to-tail manner in 4-CUMC18 film. Furthermore, distinct properties of the multilayer films were observed. It is revealed that a reversible [2+2] photodimerization and photocleavage could be induced in the LS film of 7-CUMC18 under photoirradiation with UV light of 365 and 254 nm, respectively. No photodimerization occurred in the 4-CUMC18 film. However, the film of 4-CUMC18 showed a supramolecular chirality although the compound itself is achiral.     

Supramolecular chirality from an achiral azobenzene derivative through the interfacial assembly: effect of the configuration of azobenzene unit

The Langmuir-Schaefer (LS) films of an achiral azobenzene derivative, 4-octyl-4'-(5-carboxypentamethyleneoxy) azobenzene (C8AzoC5), were fabricated and their optical activities were investigated. It was found that the LS film of the trans-C8AzoC5 showed strong Cotton effect, while that of cis-C8AzoC5 did not. The characterization of the LS films by UV-vis, Fourier transform infrared (FT-IR) spectra, and X-ray diffraction (XRD) revealed that this interesting phenomenon was due to the different packing of the azobenzene unit in the LS film. The planar conjugated trans-azobenzene favored ordered cooperative packing in a helical sense and produced the supramolecular chirality, while the cis-isomer did not due to the bulky twisted configuration.     

Chirality amplification of porphyrin assemblies exclusively constructed from achiral porphyrin derivatives.

Two achiral porphyrin derivatives, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (TPPOMe) and 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphine (TPPOH), were spread onto an air/water interface. The spreading films were transferred onto solid substrates by the Langmuir-Schaefer (LS) method. Although both of the porphyrin derivatives are achiral species, the transferred LS multilayer films shows macroscopic supramolecular chirality, which is suggested to be due to the spontaneous symmetry breaking that occurs at the air/water interface. A strong CD signal is observed from the as-deposited TPPOH LS film, while a relatively weak CD signal is detected from that of TPPOMe. Interestingly, when the TPPOMe LS film was annealed in high vacuum, a significant amplification of the supramolecular chirality is observed. Atomic force microscopy observations confirm that TPPOMe form more ordered aggregates upon annealing. It is suggested that the small amount of chiral assemblies formed in the as-deposited LS film grow into larger ones following the "sergeants and soldiers" principle during the annealing process.     

Drying and nondrying layer-by-layer assembly for the fabrication of sodium silicate/TiO2 nanoparticle composite films

Influences of drying and nondrying steps on structures of layer-by-layer (LbL) assembled sodium silicate/TiO(2) nanoparticles films (donated as silicate/TiO(2) films) have been systematically investigated. The nondrying LbL assembly produces highly porous silicate/TiO(2) films with large thickness. In contrast, the silicate/TiO(2) films fabricated with a drying step after each layer deposition are flat and thin without porous structures. In situ atomic force microscopy (AFM) measurements confirm that the sodium silicate and TiO(2) nanoparticles are deposited in their aggregated forms. A N(2) drying step can disintegrate the aggregated silicate and TiO(2) nanoparticles to produce thin silicate/TiO(2) films with compact structures. Without the drying steps, the aggregated silicate and TiO(2) nanoparticles are well retained, and their LbL assembly produces highly porous silicate/TiO(2) films of large thickness. The highly porous silicate/TiO(2) films are demonstrated to be useful as reusable film adsorbents for dye removal from wastewater because they can adsorb a large amount of cationic organic dyes and decompose them under UV irradiation. The present study is meaningful for exploring drying/nondrying steps for tailoring structure and functions of LbL assembled films.     

Photocatalytic reactivities of Nafion-coated TiO2 for the degradation of charged organic compounds under UV or visible light

Nafion (perfluorinated polymer with sulfonate groups)-coated TiO2 particles (Nf/TiO2) were prepared and their reactivities for the photocatalytic degradation (PCD) of charged organic substrates were investigated. The presence of Nafion adlayers drastically changed the positive TiO2 surface charge to a negative one over the entire pH range and significantly influenced the PCD kinetics and mechanisms. The UV-induced PCD of tetramethylammonium (TMA; cationic substrate) was greatly enhanced in the presence of Nafion adlayers on TiO2 because the ion-exchange sites within the Nafion can hold cationic substrates. On the other hand, despite the unfavorable electrostatic interaction between the Nf/TiO2 and anionic substrates, the PCD of dichloroacetate (DCA) and acid orange 7 (AO7) with Nf/TiO2 was not significantly inhibited. The visible-light-sensitized degradation of dyes was enhanced with Nf/TiO2 not only for cationic dyes (methylene blue (MB) and rhodamine B (RhB)) whose uptake on Nf/TiO2 is enhanced, but also for an anionic dye (AO7) that is less adsorbed on Nf/TiO2. The unexpected behavior in AO7 degradation seems to be related to the role of the Nafion layer in retarding the charge recombination. These observations indicate that Nf/TiO2 can enhance the PCD reactivity for cationic substrates without sacrificing the PCD reactivity for anionic substrates. In addition, it was found that the sensitized degradation of RhB followed a different path when the surface of TiO2 was coated with Nafion. The N-de-ethylation of RhB that leads to the generation of rhodamine-110 was a prevailing path with Nf/TiO2, whereas the cleavage of the chromophoric ring structure was dominant with pure TiO2. The effects of Nafion adlayers on the photoinduced electron transfer and PCD kinetics and mechanisms are discussed.     

Nanoaggregates of copper porphyrazine in floating layers and Langmuir-Schaefer films

Aggregation behavior of unsubstituted copper porphyrazine (CuPaz) on the water surface was studied by analysis of compression curves, Brewster angle microscopy (BAM), and optical spectroscopy. The structure and stability of the CuPaz aqua aggregates in the floating layers are determined by hydration degree that depends on initial surface concentration and surface pressure. Langmuir-Schaefer (LS) films of CuPaz were prepared by deposition of the variously structured floating layers and studied by X-ray scattering technique and optical spectroscopy. Stable and labile structures were detected and compared with the floating CuPaz aqua aggregates. Conditions of formation of the stable four-stacked nanoaggregates in LS films were determined. A model comprising both nucleation of CuPaz on the water surface and structural transformations in the solid films is proposed.     

Optical and electrochemical properties of poly(o-toluidine) multiwalled carbon nanotubes composite Langmuir-Schaefer films

Conducting poly(o-toluidine) (POT) with multiwalled carbon nanotubes (MWNTs) nanocomposite (POT-MWNTs) was synthesized by oxidative polymerization. Chloroform solutions of the material were used for the optical characterizations by means of UV-visible spectroscopy and for the fabrication of Langmuir-Schaefer (LS) films. LS films were fabricated at the air-liquid interface by using 0.1 M HCl aqueous solution as the subphase to study the electrochemical properties of the nanocomposite by means of cyclic voltammetry and photoelectrochemical techniques. The optical characterizations gave proof that the presence of MWNTs inside the polymeric matrix produced no change in the (pi-pi*) transition of POT structure, indicating that the polymeric chains were simply wrapped around and not doped by MWNTs. The electrochemical investigations highlighted significant changes in the redox properties of POT-MWNTs LS films with respect to pure POT. The cyclic voltammetric study also revealed high electrochemical stability, confirmed by the estimation of the diffusion coefficient and the photoelectrochemical response of the nanocomposite LS films. This characteristic turned out to be more evident than that obtained in our earlier studied poly(o-anisidine)-MWNTs (POAS-MWNTs) system.     

TiO_2/CdS/Ru(bpy)_2(NCS)_2作为太阳电池光阳极的探讨

将 Cd S纳米粒子复合在 Ti O2 纳米多孔膜上 ,用染料 Ru( bpy) 2 ( NCS) 2 对此复合半导体纳米膜电极进行敏化 ,测量了不同 Cd S复合量的 ITO/Ti O2 /Cd S/Ru( bpy) 2 ( NCS) 2 光阳极组成光电池的能量转换效率 .实验证明 ,ITO/Ti O2 /Cd S/Ru( bpy) 2 ( NCS) 2 作为太阳电池光阳极的能量转换效率与 Ti O2 /Cd S复合半导体中 Cd S的含量有关 .当 Cd S复合时间为 5 min的电池的短路电流为 5 .2 3A/m2 ,开路电压为 0 .71 6 V,能量转换效率为 0 .77% .     

Molecular organization and effective energy transfer in iridium metallosurfactant-porphyrin assemblies embedded in Langmuir-Schaefer films

Mixed Langmuir monolayers and Langmuir-Schaefer (LS) films containing the cationic metallosurfactant bis(2-phenylpyridine)(4,4'-diheptadecyl-2,2'-bipyridine)-iridium(III) chloride (Ir-complex) and the anionic tetrakis(4-sulfonatophenyl)porphyrin (TSPP) in 4:1 molar ratio have been successfully prepared by the co-spreading method at the air-water interface. The presence of both luminescent species at the interface, as well as the organization of the TSPP underneath the Ir-complex matrix in Langmuir and LS films, is inferred by surface techniques such as π-A isotherms, reflection spectroscopy, Brewster angle microscopy (BAM) and UV-visible absorption spectroscopy. A red-shift in the absorption band of the porphyrin under the compression of the mixed monolayer suggests the J-aggregation of the TSPP under the Ir-complex matrix. To date, this is the first report of Langmuir and/or LS films containing these two types of species together. Furthermore, the intermolecular energy transfer between Ir-complex and TSPP molecules in solution and in transferred mixed films is investigated through steady-state fluorescence and lifetime measurements. These results indicate that effective intermolecular energy transfer occurs from the Ir-complex to the TSPP molecules in LS films. The influence of the spatial proximity of donor and acceptor molecules has been studied by the insertion of lipid interlayers among them.     

New Cu-based catalysts supported on TiO2 films for Ullmann S(N)Ar-type C-O coupling reactions

New routes for the preparation of highly active TiO(2)-supported Cu and CuZn catalysts have been developed for C-O coupling reactions. Slurries of a titania precursor were dip-coated onto glass beads to obtain either structured mesoporous or non-porous titania thin films. The Cu and CuZn nanoparticles, synthesized using a reduction by solvent method, were deposited onto calcined films to obtain a Cu loading of 2 wt%. The catalysts were characterized by inductively coupled plasma (ICP) spectroscopy, temperature-programmed oxidation/reduction (TPO/TPR) techniques, (63)Cu nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (S/TEM-EDX) and X-ray photo-electron spectroscopy (XPS). The activity and stability of the catalysts obtained have been studied in the C-O Ullmann coupling of 4-chloropyridine and potassium phenolate. The titania-supported nanoparticles retained catalyst activity for up to 12 h. However, catalyst deactivation was observed for longer operation times due to oxidation of the Cu nanoparticles. The oxidation rate could be significantly reduced over the CuZn/TiO(2) catalytic films due to the presence of Zn. The 4-phenoxypyridine yield was 64% on the Cu/nonporous TiO(2) at 120 °C. The highest product yield of 84% was obtained on the Cu/mesoporous TiO(2) at 140 °C, corresponding to an initial reaction rate of 104 mmol g(cat) (-1) s(-1). The activation energy on the Cu/mesoporous TiO(2) catalyst was found to be (144±5) kJ mol(-1), which is close to the value obtained for the reaction over unsupported CuZn nanoparticles (123±3 kJ mol(-1)) and almost twice the value observed over the catalysts deposited onto the non-porous TiO(2) support (75±2 kJ mol(-1)).     

CdS敏化对TiO2纳米薄膜电极光生电荷转移特性的影响

1991年Gratzel等[1]以敏化的TiO2纳米薄膜电极组成的液体结光电化学太阳能电池(PEC),其光电转换效率(IPCE)达到10%. 最近,选用固态电解质使这种PEC的IPCE达到33%[2].于是用有机染料[3,4]及窄带隙半导体纳米微粒[5]敏化的电极受到了广泛关注.     

Atypical Film-Forming Behavior of Soluble Tetra-3-Nitro-Substituted Copper Phthalocyanine

Thin films of metal phthalocyanines (MPc) are known to exhibit excellent physical properties but poorly controlled morphologies. Therefore, the present work seeks to understand the film growth mechanism of a model compound for potentially usable MPc, specifically, copper tetra(3-nitro-5-tert-butyl)phthalocyanine (CuPc*). The Langmuir-Schaefer (LS) technique was applied to prepare a series of CuPc* films under different processing conditions. The film growth was examined by Brewster angle microscopy (BAM) on the water surface and small-angle X-ray scattering (SAXS) from the solid films. Neutron reflectometry (NR) measurements of the water uptake into the films and computer simulation of hydrated CuPc* were performed to substantiate an idea of colloidal MPc-water aggregates as nanoscale precursors of smooth solid films. This idea appears fruitful in terms of materials chemistry.     

溶胶-凝胶法制备改性TiO2纳米薄膜及其防腐蚀性能

应用溶胶-凝胶法和浸渍提拉技术在316L不锈钢表面分别制备TiO2纳米膜和 B-Fe-Ce改性的TiO2纳米膜. 采用场发射扫描电子显微镜(FE-SEM)、原子力显微镜(AFM)、拉曼光谱法和能量分散谱(EDS)对薄膜进行表征,通过电化学阻抗谱(EIS)和动电位阳极极化曲线的测试考察薄膜的耐蚀性及对不锈钢的保护性能. 结果表明:两种纳米薄膜均含锐钛矿型的TiO2纳米颗粒,纯TiO2纳米膜与改性后的纳米膜中颗粒直径分别约为15和10 nm. TiO2/316L不锈钢和 B-Fe-Ce-TiO2/316L不锈钢膜电极浸泡在0.5 mo.lL-1 NaCl溶液后,后者的电化学反应电阻较大,动电位阳极极化曲线的稳定钝化区较宽,击穿电位更高,说明改性的纳米膜的耐蚀性及其保护性能更好.     

电沉积法制备介孔TiO_2/CdS薄膜光电极

采用阴极恒电位沉积法,在介孔TiO2薄膜上制备了介孔TiO2/CdS薄膜光电极,用XRD,SEM,Raman,SPS和UV-Vis等多种手段对薄膜电极进行了表征.结果表明,CdS成功沉积到介孔TiO2的表面和孔道内,形成了异质结结构.通过光电流作用谱考察了该复合体薄膜电极的光电性能,结果表明,与单纯的介孔TiO2薄膜相比,其光电转换效率显著提高,这是由于CdS具有吸收可见光的特性以及CdS与介孔TiO2形成异质结从而使得光生载流子更容易分离的结果.     

Photocatalytic Nanocomposite Films Fabricated by Layer‐by‐Layer Self‐assembly of TiO2 Nanoparticles and Lignosulfonates

Photocatalytic multilayer nanocomposite films composed of anatase TiO₂ nanoparticles and lignosulfonates (LS) were fabricated on quartz slides by the layer‐by‐layer (LBL) self‐assembly technique. X‐ray photoelectron spectroscopy (XPS), UV‐vis spectroscopy and atomic force microscopy (AFM) were used to characterize the TiO₂/LS multilayer nanocomposite films. Moreover, the photocatalytic properties (decomposition of methyl orange and bacteria) of multilayer nanocomposite films were investigated. XPS results indicated that the intensities of titanium and sulfur peaks increased with the LBL deposition process. A linear increase in absorbance at 280 nm was found by UV‐Vis spectroscopy, suggesting that stepwise multilayer growth occurs on the substrate and this deposition process is highly reproducible. AFM images showed that quartz slide was completely covered by TiO₂ nanoparticles when a 10‐bilayer multilayer film was formed. The decomposition efficiency of methyl orange by TiO₂/LS multilayer films under the same UV irradiation time increased linearly with the number of TiO₂ layers, and the results of decomposition of bacteria under UV irradiation showed that TiO₂/LS multilayer nanocomposite films exhibited excellent decomposition activity of bacteria (Escherichia coil).     

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.     

Electrophoretic deposition of TiO2 nanoparticles using organic dyes

Electrophoretic deposition method has been developed for the deposition of TiO(2) nanoparticles modified with organic dyes. Alizarin red, alizarin yellow and pyrocatechol violet dyes were used for the dispersion and charging of TiO(2) in ethanol and anodic electrophoretic deposition of TiO(2) films. The deposition yield was varied by the variation of dye concentration in suspensions and deposition time. Aurintricarboxylic acid dye was used for the deposition of TiO(2) from aqueous suspensions. It was found that thin films of pure aurintricarboxylic acid and composite aurintricarboxylic acid TiO(2) films can be obtained. The deposition yield was studied by quartz crystal microbalance. Dye film thickness was varied in the range of 0.1-2 μm by variation in the deposition time at a constant voltage. The composition of the films and the amount of the deposited material can be varied by the variation of TiO(2) and dye concentration in suspensions and deposition time. The films were studied by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential thermal analysis and electron microscopy. The deposition mechanisms were discussed. The electrophoretic deposition method offers advantages for the fabrication of dye-sensitized TiO(2) films.