Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

Properties of composite films of titania nanofibers and Safranin O dye

Titania (TiO₂) nanofibers and composite thin films of titania nanofibers and Safranin O dye (SAF) were studied. TiO₂ nanofibers were prepared by electrospinning technique from titanium tetra-isopropoxide precursor solution in ethanol. Surface topology of the nanofibers was observed using scanning electron microscopy (SEM), their crystal structure was studied by X-ray diffraction (XRD) and the chemical composition by X-ray photoelectron spectroscopy (XPS). Properties of the TiO₂ nanofibers were studied in dependence on the values of relative air humidity in the range from 15% to 55%. It was necessary to maintain the relative humidity lower than 30% during electrospinning in order to obtain high quality nanofiber films. The average minimum diameter of the as-prepared TiO₂ nanofibers was found to be around 100 nm. Nanofiber diameter diminishes to about 50 nm after annealing at 420 °C for 1 h. The as-prepared titania nanofiber films were completely amorphous while anatase crystal phase was detected in the films after annealing. In order to prepare the composite films, solution of SAF dye with poly(vinylpyrrolidone) in ethanol/water was dropped off on the prepared titania nanofibers surface. Opto-electrical properties of SAF dye and the resulting nanocomposite films were studied by UV–Vis spectroscopy and current–voltage characteristics. Safranin O is characterized by two strong absorption peaks; one at 274 nm and a wide band with splitting between 420 nm and 600 nm. The optical energy band gap of titania nanofibers was estimated from the UV–Vis measurements to be 3.4 eV. The charge transport in the composite films is influenced by the space charge limited currents due to the very high resistance of the materials.     

Temperature dependence of the resistance of self-assembled polyaniline nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid

Hollow polyaniline (PANi) nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) have been synthesized and their electrical resistance measured as a function of temperature. The average length of the nanotubes was in the range 2–5 μm and the average diameter was in the range 200–400 nm. Longer reaction times did not affect the morphology of the nanotubes. The resistance of AMPSA doped nanotubes were measured as a function of temperature and compared to emeraldine base PANi doped with AMPSA in the solid state. Dissolving the PANi nanotubes in dichloroacetic acid leads to better polymer chain ordering.     

Polyaniline nanofibers synthesized in compressed CO2

Polyaniline (PANI) nanofibers were synthesized in compressed liquid carbon dioxide without any template or surfactant. The polymerization of aniline took place at the interface between CO₂ and aqueous solution in a high-pressure stirred reactor. The prepared PANI nanofibers were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), electrical conductivity (EC), Fourier-transform infrared (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses. The yield of polymerization was high enough to reach 63.04% while maintaining small diameters of the PANI nanofibers. This result is very important for the preparation of the PANI nanofibers because no other previous investigations have achieved both high yield and small diameter of fibers at the same time. Through SEM and TEM analyses, we observed that the PANI nanofibers had diameter range of 30–70 nm and a length range of 0.3–1 μm, which caused them to disperse well in various solvents such as water, ethanol, 2-propanol, m-cresol and toluene. The electrical conductivity of the PANI nanofibers was 4.34 S/cm at 20 °C. The XRD diffraction pattern showed that the PANI nanofibers had crystalline one-dimensional structures, which gave high thermal stabilities as confirmed by TGA.     

Novel properties of polyaniline nanofibers coated with polycatechol

Polyaniline nanofibers coated with polycatechol, FcPAn/polycatechol, have been prepared with two steps by using repeated potential cycling. First, aniline in a solution containing ferrocenesulfonic acid (Fc) was polymerized on a platinum electrode to form polyaniline nanofibers (FcPAn); second, catechol was polymerized on the polyaniline nanofibers to form FcPAn/polycatechol. The scanning electron microscopy (SEM) images show that the diameters of FcPAn and FcPAn/polycatechol fibers are in the range of 40–60 and 70–90 nm, respectively. The IR and XPS spectra of FcPAn/polycatechol indicate that OH groups and ferrocenesulfonic acid are contained in FcPAn/polycatechol. FcPAn/polycatechol has a good electrochemical activity in the wide pH range. The cyclic voltammogram identifies that FcPAn/polycatechol in the Na₂SO₄ solution with pH 11.0 still has the electrochemical activity at the scan rate of 60 mV s⁻¹. The conductivity of FcPAn/polycatechol is 0.48 S cm⁻¹, which is slightly affected by water. FcPAn/polycatechol nanofibers with the diameter of 70–90 nm have a higher catalytic activity to the electrochemical oxidation of ascorbic acid, compared with FcPAn/polycatechol fibers with the diameter of 140–210 nm.     

Core–shell structured and electro-magnetic functionalized polyaniline composites

Core–shell structured and electro-magnetic functionalized polyaniline (PANI) composites were chemically prepared by micro-spherical hydroxyl iron (Fe(OH)) magnet as the hard-template associated with a self-assembly process in the presence of β-naphthalene sulfonic acid (β-NSA) as the dopant. Field emitting scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements indicate that the electro-magnetic functionalized PANI–β-NSA/Fe(OH) composites have a novel core–shell structure, in which micro-spherical Fe(OH) magnet (0.5–5 μm in diameter) is as the core, and self-assembled PANI–β-NSA nanofibers (30–50 nm in diameter) formed on the surface of the Fe(OH) microspheres is as the shell (60 nm in thickness). Moreover, the electro-magnetic properties of the core–shell micro/nanostructured composites are adjustable by changing the ratio of Fe(OH) to aniline monomer.     

Polyaniline,a dynamic block copolymer: key to attaining its intrinsic conductivity?

Photoluminescence studies of spun films of polyaniline base (emeraldine oxidation state) show that films prepared from NMP solution in air having a relative humidity between 43±2 and 57±2% (21°C) exhibit photoluminescence at 401 nm, which persists when the films are held in a dynamic vacuum. Films prepared from NMP solution containing water show similar behavior to those prepared in the above humidity range. When protonated (“doped”) with HCl the above photoluminescence disappears and is replaced by a photoluminescence peak at 467 nm. The reverse behavior occurs on deprotonation with NH₃ vapor, the 467 nm peak disappearing and the 401 nm peak reappearing. These results, together with photoluminescence studies on the phenyl/phenyl end-capped tetramer of aniline in both the emeraldine and leucoemeraldine oxidation states, lead to the conclusion that in solution the emeraldine base is a dynamic block copolymer in which reduced (benzenoid/amine) and oxidized (quinoid/imine) units are constantly interchanging positions via a tautomeric (hydrogen migration) process between nitrogen atoms. This process is promoted by water. The resulting production of long sequences of the reduced and oxidized units, which separate long sequences of the emeraldine base, persist when the NMP solvent is removed, resulting in microphase segregation of these sequences in the solid film. Since only the emeraldine base sequences can be doped by acids, the conductivity of the doped film is less than that of its predicted intrinsic conductivity.     

Synthesis of polyaniline nanotubes using Mn2O3 nanofibers as oxidant and their ammonia sensing properties

In this work, a new method for the synthesis of polyaniline (PANI) nanotubes was presented. Experimentally, Mn₂O₃ nanofibers prepared by electrospinning technique were used as the oxidant template to initiate the polymerization of aniline in acid solution. After reaction, polyaniline shells were formed on the Mn₂O₃ nanofiber surface, and the Mn₂O₃ nanofibers were spontaneously removed. As a result, PANI nanotubes were obtained. As-prepared PANI nanotubes show an average diameter of 80 nm and inner diameter of 38 nm. The final PANI nanotubes were characterized by SEM, EDX, TEM, FTIR and XRD. The gas sensing of as-obtained PANI nanotubes was also investigated. It was found that the PANI nanotube sensing device could detect as low as 25 ppb NH₃ in air at room temperature with good reversibility.     

Strengthening mechanisms in a high-strength bulk nanostructured Cu–Zn–Al alloy processed via cryomilling and spark plasma sintering

A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (～200–400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.     

Polyaniline composite by in situ polymerization on a swollen PVA gel

Interpenetrating polyaniline (PANI) formation in a 3D network of poly(vinyl alcohol) (PVA) hydrogel was developed. Polymerization was effected by immersing swollen PVA hydrogel previously soaked with ammonium persulfate (APS) in 1 M HCl solution of aniline hydrochloride (AnHCl). Gradual transformation of the swollen gel from colorless transparent gel to an opaque green color indicated the formation of PANI emeraldine salt (ES) on the surface and bulk of the PVA gel matrix. Characterization by UV–vis spectra, ATR-FTIR spectra and X-ray diffraction analyses supported the formation of PANI–PVA composite film. The surface morphology of the film was studied by FESEM. Electrical conductivity of the film was measured by four-probe method.     

Influence of synthesis temperature on electrochemical polymerization of o-anisidine on low carbon steel

The influence of synthesis temperature on the electrochemical polymerization (ECP) of o-anisidine (OA) on low carbon steel (LCS) has been investigated. The ECP of OA was carried out on LCS substrate under galvanostatic conditions from aqueous solution of oxalic acid at various temperatures between 0 and 40 °C. The resulting poly(o-anisidine) (POA) coatings were characterized by potential–time (E–t) curves, UV–VIS absorption spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements. The E–t curves reveal that the synthesis of POA coating at 0, 15 and 27 °C on LCS occurs in three stages, an induction time for ECP, passivation of LCS electrode surface via the formation of iron oxalate (FeC₂O₄·2H₂O) interphase and the dissolution of interphase and followed by ECP of OA. However, at 40 °C with an applied current density ∼0.66 mA/cm², the E–t curve shows the undesirable behavior which is attributed to non-decomposition of interphase during the third growth stage. As a consequence, at 40 °C the ECP of OA has not occurred on the LCS electrode surface. The XRD, SEM and optical absorption spectroscopy support this observation. It has been found that, as the synthesis temperature is increased, at low current densities, the induction time is observed to decrease, whereas it does not show any significant change at higher current densities. The optical absorption spectroscopy reveals the formation of mixed phase of emeraldine salt (ES) and pernigraniline base (PB) at 27 °C, whereas at lower temperatures the major portion of the coating constitutes the ES phase. The surface morphology, as revealed by SEM is observed to depend on the synthesis temperature.     

Raman study of polyaniline nanofibers prepared by interfacial polymerization

Polyaniline nanofibers were synthesized by interfacial polymerization of aniline. Polyaniline so formed was studied using transmission electron microscopy, scanning electron microscopy, UV–vis and Raman spectroscopy. TEM of polymer dispersion shows the presence of fibers having diameter around 30–80 nm. Strikingly different Raman spectra were observed from the polyaniline film, which were related to two dissimilar areas on the film. SEM of polyaniline films shows fibrous as well as flake like structures scattered in fiber matrix. Raman spectra of the films heat treated from 25 °C to 300 °C were taken to study the structural variations induced by the change of temperature at these two regions. Many applications of polymer films are dependent on the homogeneity of the systems. In this work we intend to employ Raman Spectroscopy as an indispensable tool for looking into the non-uniformities of the polyaniline films.     

Spectroscopic investigation of the interactions between emeraldine base polyaniline and Eu(III) ions

The interactions of emeraldine base form of polyaniline (EB-PANI) and Eu(III) ions in 1-methyl-2-pyrrolidinone (NMP) solution and in films have been investigated by UV–vis–NIR, resonance Raman, luminescence and electron paramagnetic resonance (EPR) spectroscopies. These spectroscopic techniques allowed to characterize quinone and semiquinone segments in the polymeric chains, and the oxidation state of europium ions in Eu-PANI samples. For high values of Eu(III)/N molar ratio (24/1) the presence of a weak polaronic absorption band at 980 nm in UV–vis–NIR spectrum and the observation of bands at 1330 and 1378 (ν_C–N+) cm^?1 due to emeraldine salt in the Raman spectrum at 1064 nm indicate a low doping degree. Oxidation of EB-PANI to pernigraniline base (PB-PANI) occurs in diluted solutions. The experimental data showed that the solvent plays an important role on the nature of formed species. The narrow EPR signal at g = 2.006 (line width 8G) confirms the presence of PANI radical cations in Eu-PANI film. The absence of broad signal characteristic of Eu(II) in EPR spectrum suggested that europium ions are primarily at Eu(III) oxidation state. The luminescence spectra of Eu-PANI film presented emission bands at 405 and 418 nm assigned to PANI moieties and bands at 594, 615 and 701 nm assigned to ⁵D₀ → ⁷F_J (J = 1, 2 and 4, respectively) transitions of Eu(III). EPR and photoluminescence data confirm that europium ions are mainly in Eu(III) oxidation state in Eu(III)/PANI films.     

Self-assembled polyaniline 12-tungstophosphate micro/nanostructures

Polyaniline (PANI) micro/nanostructures were synthesized by the external-template-free oxidative polymerization of aniline in aqueous solution of 12-tungstophosphoric acid (WPA), using ammonium peroxydisulfate (APS) as an oxidant and starting the oxidation of aniline from slightly acidic media (pH 5.4–5.9). The effect of the initial weight ratio of WPA to aniline on molecular structure, morphology, and physicochemical properties of polyaniline 12-tungstophosphate (PANI-WPA) was investigated by FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), scanning and transmission electron microscopies (SEM and TEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), and conductivity measurements. The morphological change of polymerization products during a single polymerization process, from non-conducting submicro-/microspherical oligoaniline intermediates to semiconducting PANI-WPA consisted of self-assembled nanotubes and/or nanorods co-existing with submicro-/microspheres, has been revealed by SEM and TEM. The average diameter of nanorods in PANI-WPA samples decreased with increasing the initial WPA/aniline weight ratio. The incorporation of 12-tungstophosphate counter-ions into PANI matrix has been proved by FTIR, Raman and ICP-OES spectroscopies, TGA and DTA analysis. Electrical conductivity of PANI-WPA increased in the range (2.5–5.3) × 10^?3 S cm^?1 with the increase of the initial WPA/aniline weight ratio. The presence of branched structures and phenazine units besides the ordinary paramagnetic and diamagnetic emeraldine salt structural features in PANI-WPA was proved by FTIR and Raman spectroscopies.     

Loss of pseudoelasticity in nickel–titanium sub-micron compression pillars

Nickel–titanium (NiTi) is capable of undergoing pseudoelastic deformation wherein relatively large amounts of inelastic deformation are recovered upon load removal due to a martensitic phase transformation. This study investigates pseudoelastic as well as plastic deformation in sub-micron diameter NiTi compression pillars. Pillars ranging in diameter from approximately 2 μm to 200 nm were prepared using focused ion beam micro-machining of aged [1 1 1] single crystal NiTi. Results reveal pseudoelasticity in all samples tested with diameters between 2 μm and 400 nm, although permanent strain was introduced at relatively low strains compared to bulk. Decreased sample size generally showed a smaller stress–strain hysteresis, with a full loss of recoverable pseudoelastic strain for samples with a diameter smaller than 200 nm. In addition, plastic flow stress of the martensite was shown to be independent of sample diameter for the aged NiTi material. Lastly, it is observed that crystallographic orientation has a stronger influence on martensite plastic flow strength than pillar size.     

Novel flexible,freestanding and transparent organic/inorganic hybrid materials formed between polyaniline and polyphosphate gel

In this paper we report the synthesis and characterization of new polyaniline/polyphosphate hybrid materials. The synthetic approach involves a single-step procedure in which the polyphosphate inorganic host structure (arising from a sol–gel transition) develops first, providing a restricted environment in which the conducting polymer is entrapped within the gel network. Polyaniline was formed as the green, conducting emeraldine salt form. The samples were characterized by thermogravimetric analysis and UV–vis, Raman, EPR and ³¹P-MAS-NMR spectroscopies. We have synthesized homogeneous, flexible, freestanding and transparent materials in which the conducting polymer presents a coiled conformation typical of primary doped polyaniline. Upon exposition to ammonia vapors the polymer can be converted to the emeraldine base form, which can be reverted by exposition to HCl vapors. Powdered samples present dc conductivities up to 0.2 S cm⁻¹.     

Multi-functional polypyrrole nanofibers via a functional dopant-introduced process

This paper reports a functional dopant-introduced route to synthesize polypyrrole (PPy) nanofibers (60–100 nm in average diameter) in the presence of p-hydroxyl-azobenzene sulfonic acid (p-OH-ABSA) as a functional dopant. The nanofibers show a high conductivity (120–130 S/cm) and photoisomerization, which resulted from proton doping and photoisomerization of azobenzene moiety, respectively. Static and dynamic light scattering as well as freeze-fracture replication transmission electron microscope measurements (FFRTEM) showed that the self-assembled cylindrical micelles act as “soft-templates” during the formation of the nanofibers. Influence of polymerization conditions, such as the type of oxidant, the rate of oxidant addition, the concentration of reactants and polymerization time, on the fibrous morphology of PPy-(p-OH-ABSA) has been investigated. The characterizations of molecular structure, photoisomerization and electrical properties have been carried out. The method described in this study provides a simple and inexpensive route to prepare multi-functional nano-structured conducting polymers.     

Synthesis and characterization of electrospun polymer nanofibers incorporated with CdTe nanoparticles

CdTe nanoparticles were incorporated in polyvinyl alcohol (PVA) nanofibers by electrospinning. The hybrid nanofibers were characterized morphologically and optically by scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) measurements. The average diameter of the hybrid nanofibers was about 150 nm and the CdTe nanoparticles were present both inside them and on their surfaces. Compared with the PL of the CdTe nanoparticles dispersed in solution, the PL peak of the CdTe nanoparticles in the CdTe/PVA hybrid nanofibers was shifted from 525 to 426 nm. This blue shift of the PL peak results from the different quantum confinement effects, due to the different environment surrounding the CdTe nanoparticles, in the case of those dispersed in solution and those embedded in the PVA nanofibers.     

Preparation of spherical silica powder by oxygen–acetylene flame spheroidization process

A novel economic oxygen–acetylene flame spheroidization process and special equipments to prepare spherical silica powders are presented. Before spheroidization treatment, the raw silica is purified by mixed acids. In this spheroidization process, oxygen is used both as the carrier gas to transport the purified silica powders to the flame region, and as the combustion-supporting gas. The crystallized silica powders are melted and then transformed to amorphous silica after oxygen–acetylene flame spheroidization treatment. The powder size analysis indicates that the powder size increases and the size distribution become narrower after spheroidization treatment. By optimizing the process parameters, spherical silica powders were obtained. A scanning electron microscope (SEM) investigation reveals that the spheroidization efficiency of the 5 μm diameter powder is more than 95% at feeding rates below 60 g/min. The closer the length–diameter ratio of raw silica powder is to 1, the higher the spheroidization efficiency of the sample is. The fluidity value and apparent density of the powder with the spheroidization efficiency 95% is 70 s/50 g and 0.88 g/cm³, respectively. Oxygen–acetylene flame spheroidization process is a promising low-cost alternative for large-scale production of spherical silica powder.     

Synthesis and characterization of CaF2 nanocrystals

Calcium fluoride nanocrystals (CaF₂) were synthesized by two different techniques namely co-precipitation and hydrothermal. The synthesized nanocrystals were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared red spectroscopy (FTIR), scanning electron microscopy (SEM), optical absorption and photoluminescence (PL). The crystallite size estimated using Scherer's formula was found to be in the range 30–35 nm for nanocrystals synthesized by co-precipitation method where as in case of hydrothermally synthesized nanocrystals it is in the range 20–28 nm which is less compared to those obtained by co-precipitation method. The morphological features as studied using SEM revealed that the nanocrystals are agglomerated, crispy with porous. The SEM images of hydrothermally synthesized nanocrystals showed less agglomeration than those obtained by co-precipitation method and the images confirm the formation of nanoparticles. The optical absorption spectrum showed a strong absorption band peaked at 244 nm for nanocrystals synthesized by co-precipitation method and it is 218 nm peak in case of hydrothermally synthesized ones. The PL emission spectrum showed two prominent emission bands peaked at 330 and 600 nm when excited at 218 nm.