Preparation of TiO2/carbon nanotubes photocatalysts: The influence of the method of oxidation of the carbon nanotubes on the photocatalytic activity of the nanocomposites

A method for the preparation of efficient TiO₂/multi-wall carbon nanotubes nanocomposite photocatalysts by precipitation of anatase TiO₂ nanoparticles onto differently oxidized carbon nanotubes is presented. The precursor compound titanium(IV) bromide was hydrolyzed producing pure anatase phase TiO₂ nanoparticles decorated on the surface of the oxidized carbon nanotubes. The oxidative treatment of the carbon nanotubes influenced the type, quantity and distribution of oxygen-containing functional groups, which had a significant influence on the electron transfer properties, i.e., the photocatalytic activity of the synthesized nanocomposites. The results of C.I. Reactive Orange 16 photodegradation in the presence of all the synthesized nanocomposites showed their better photocatalytic activity in comparison to the commercial photocatalyst Degussa P-25.     

Photocatalytic activity of TiO2 supported on multi-walled carbon nanotubes under simulated solar irradiation

TiO₂ particles supported on multi-walled carbon nanotubes (MWCNTs) were prepared using a sol–gel method to investigate their photocatalytic activity under simulated solar irradiation for the degradation of methyl orange (MO) in aqueous solution. The prepared composites were analyzed using XRD, SEM, EDS and UV–vis absorption spectroscopy. The results of this study indicated that there was little difference in the shape and structure of MWCNTs/TiO₂ composite and pure TiO₂ particles. The composite exhibited enhanced absorption properties in the visible light range compared to pure TiO₂. The degradation of MO by MWCNTs/TiO₂ composite photocatalysts was investigated under irradiation with simulated solar light. The results of this study indicated that MWCNTs played a significant role in improving photocatalytic performance. Different amounts of MWCNTs had different effects on photodegradation efficiency, and the most efficient MO photodegradation was observed for a 2% MWCNT/TiO₂ mass ratio. Photocatalytic reaction kinetics were described using the Langmuir–Hinshelwood (L–H) model. The photocatalyst was reused for eight cycles, and it retained over 95.2% photocatalytic degradation efficiency. Possible decomposition mechanisms were also discussed. The results of this study indicated that photocatalytic reactions with TiO₂ particles supported on MWCNTs under simulated solar light irradiation are feasible and effective for degrading organic dye pollutants.     

Photocatalytic properties of mesoporous TiO2 nanocomposites modified with carbon nanotubes and copper

The photocatalytic activity of mesoporous TiO₂ modified by the addition of carbon nanotubes (CNTs) and Cu is reported. Nanocomposites of carbon nanotubes (CNTs) containing varying amounts of Cu were formed by treatment with Cu²⁺ then reduced to Cu⁰ using NaBH₄ as the reducing agent. The mesoporous TiO₂, synthesized by a sol-gel method from titanium isopropoxide, was combined with the CNT/Cu nanocomposites to form the photocatalysts which were characterized by XRD, SEM, TEM, FTIR, XPS and BET surface area analysis. The photocatalytic properties of the mesoporous TiO₂ composites were studied by measuring the degradation of methyl orange (MO) which was optimal in the sample containing 20 wt% of the Cu-CNT nanocomposite. The degradation efficiency for MO was a synergistic effect of photo-degradation of TiO₂ and may be due to improvement of the electrical conductivity of the system by the presence of the CNT/Cu networks, since the photodegradation of MO and the photocatalytic activity of the photoactive systems increased with increasing copper content.     

Modified microwave method for the synthesis of visible light-responsive TiO2/MWCNTs nanocatalysts

Recently, TiO₂/multi-walled carbon nanotube (MWCNT) hybrid nanocatalysts have been a subject of high interest due to their excellent structures, large surface areas and peculiar optical properties, which enhance their photocatalytic performance. In this work, a modified microwave technique was used to rapidly synthesise a TiO₂/MWCNT nanocatalyst with a large surface area. X-ray powder diffraction, field-emission scanning electron microscopy, transmission electron microscopy and Brunauer-Emmett-Teller measurements were used to characterise the structure, morphology and the surface area of the sample. The photocatalytic activity of the hybrid nanocatalysts was evaluated through a comparison of the degradation of methylene blue dye under irradiation with ultraviolet and visible light. The results showed that the TiO₂/MWCNT hybrid nanocatalysts degraded 34.9% of the methylene blue (MB) under irradiation with ultraviolet light, whereas 96.3% of the MB was degraded under irradiation with visible light.     

Facile preparation of disposable immunosensor for Shigella flexneri based on multi-wall carbon nanotubes/chitosan composite

Based on multi-wall carbon nanotubes (MWCNT)/chitosan/horseradish peroxidase labeled antibodies to Shigella flexneri (HRP-anti-S. flexneri) biocomposite film on a screen-printed electrode (SPE) surface, a disposable immunosensor has been developed for the rapid detection of S. flexneri. The HRP-anti-S. flexneri can be entrapped into MWCNT/chitosan composite matrix without other cross-linking agent. Thionine and H₂O₂ were used as the mediator and substrate, respectively. The surface morphologies of modified films were characterized by atomic force microscope (AFM). Cyclic voltammery (CV) was carried out to characterize the electrochemical properties of the immobilization of materials on the electrode surface and quantified S. flexneri. Due to the strong electrocatalytic properties of MWCNT and HRP toward H₂O₂, the response signal was significantly amplified. S. flexneri could be detected by the decrease of the reduction peak current before and after immunoreaction. Under optimal conditions, S. flexneri could be detected in the range of 10⁴ to 10¹⁰ cfu mL⁻¹, with a detection limit of 2.3 × 10³ cfu mL⁻¹ (S/N = 3). Furthermore, the proposed immunosensor exhibited a satisfactory specificity, reproducibility, stability and accuracy, indicating that the proposed immunosensor has potential application for a facile, rapid and harmless immunoassay.     

Fabrication of superhydrophobic surfaces with non-aligned alkyl-modified multi-wall carbon nanotubes

Films of superhydrophobic multi-wall carbon nanotubes (MWCNTs) have been obtained by using alkyl-modified MWCNTs (MWCNT(COOC₁₈H₃₇)_n) and a simple and effective preparation method. The films show both a high contact angle and a small sliding angle for water droplets. A particular characteristic is that on the superhydrophobic surface the alkyl-modified MWCNTs are not intentionally aligned, thus avoiding the preparation techniques using aligned carbon nanotubes to produce the same effect.     

Melt crystallization of poly(ethylene terephthalate): Comparing addition of graphene vs. carbon nanotubes

Poly(ethylene terephthalate) (PET)-based nanocomposites with graphene or multi-wall carbon nanotubes (MWCNT) were prepared by melt mixing. Aspect ratio, A_f, and interparticle distance, λ, of graphene in the nanocomposites were obtained from melt rheology and transmission electron microscopy respectively. λ of PET/graphene nanocomposites was much smaller than λ in PET/MWCNT. For PET/graphene with highest A_f, λ became <1 μm at more than 0.5 wt% graphene. Non-isothermal crystallization behavior from the melt was investigated by differential scanning calorimetry. The crystallization temperatures suggest that the nucleation effect of graphene was stronger than that of MWCNT. The half crystallization time of PET/graphene became longer than PET/MWCNT with increasing graphene loading, suggesting that confinement by graphene suppressed the crystal growth rate. XRD analysis indicated that smaller crystals formed in PET/graphene than in PET/MWCNT. From Raman spectroscopy, the π–π interaction between PET and graphene was stronger than that between PET and MWCNT. This stronger interaction in PET/graphene appears to result in formation of crystals with higher perfection.     

Multi-wall carbon nanotubes by catalytic decomposition of carbon monoxide on Ni/MgO

The redox behavior of the catalyst and the catalytic decomposition of carbon monoxide (CO) were investigated in the synthesis process of multi-wall carbon nanotubes (MWCNT) using Ni/MgO catalyst. The surface morphology of the heated Ni layer was observed by TEM to confirm the formation of NiO particles (50 nm or less) and NiO (222). The chemical reaction behavior of the catalyst in CO the atmosphere was displayed via TG-DSC analysis, and the reduction of NiO was revealed due to the mass decrease of 2.71 wt% and the exothermic peak at around 400°C. The deposition of carbon was identified with an increase in mass and the exothermic peak near 600°C. Ni (111) and carbon (002) facets was taken place in a diffraction pattern of carbon deposited catalyst, indicating the reduction in NiO and the graphitic carbon deposition. The crystallinity of the graphitic carbon was analyzed as the ratios of 0.998 for I_D/I_G and 0.26 for sp³/sp² in Raman and photoelectron spectra. The encapsulated Ni in MWCNT was observed through TEM-EDS, verifying the activation of the catalyst by CO.     

Multi-walled carbon nanotubes modified by sulfated TiO2 - A promising support for Pt catalyst in a direct ethanol fuel cell

We report the synthesis of multi-walled carbon nanotubes coated with sulfated TiO₂ (S-TiO₂/MWCNTs) as a promising support for Pt catalyst in a direct ethanol fuel cell. Highly dispersed Pt nanoparticles were supported on the S-TiO₂/MWCNT composites by NaBH₄ reduction procedure (Pt-S-TiO₂/MWCNTs). The presence and nature of the catalyst were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy. The size of the sulfated TiO₂ product was about 8 nm, and that of the Pt nanoparticle on the S-TiO₂/MWCNT composites was about 5 nm. The Pt-S-TiO₂/MWCNTs were used to study the electrochemical ethanol oxidation reaction using cyclic voltammetry, chronoamperometry and impedance spectroscopy. The results show that Pt-S-TiO₂/MWCNT catalysts show higher catalytic activity for ethanol oxidation compared with Pt supported on non-sulfated TiO₂/MWCNT composites and commercial Pt/C catalysts.     

Rutile TiO2-modified multi-wall carbon nanotubes in TiO2 film electrodes for dye-sensitized solar cells

The beneficial influence of incorporation of acid-treated and rutile TiO₂ (r-TiO₂)-modified multi-wall carbon nanotubes (MWNTs) in TiO₂ films on photocurrent–voltage characteristics of dye-sensitized solar cells (DSSCs) was studied. Two different routes were adopted for the modification of acid-treated MWNTs (a-MWNTs) with r-TiO₂. The films and MWNTs were characterized by electron microscopy, energy dispersive X-ray spectroscopy, XRD and Raman spectroscopy. In the case of incorporation of a-MWNTs with r-TiO₂ modification, short-circuit photocurrent (J _sc) of the pertinent DSSC increased by 35% compared with that of a cell with bare TiO₂ film. The open-circuit voltage remained almost the same for all cases. The enhanced J _sc is explained by the increased surface area of the film, enhanced cluster formation of TiO₂ particles around a-MWNTs, and improved interconnectivity of TiO₂ particles in the presence of a-MWNTs.     

Charge transfer properties and photoelectrocatalytic activity of TiO2/MWCNT hybrid

The vertically aligned multiwalled carbon nanotube (MWCNT) arrays on tantalum foils were successfully coated with TiO₂ nanoparticles by a hydrothermal process. The prepared TiO₂/MWCNT hybrid was characterized by scanning electron microscopy and transmission electron microscopy. The charge transfer properties and photocatalytic degradation of rhodamine B with and without bias potential under UV irradiation were investigated. The MWCNTs promoted the separation of photoinduced carriers in the TiO₂, thus enhanced photocatalytic activity. Applying bias potential on the photoanode further enhanced its catalytic activity. The efficient charge transportation and high photoelectrocatalytic activity towards degradation of rhodamine B made this hybrid material promising for photocatalyst and for the development of photoelectrical devices.     

Morphological and crystallite size impact on electrochemical performance of electrospun rutile and rutile/multiwall carbon nanotube nanofibers for lithium ion batteries

A sol–gel based route was used to produce TiO₂ based nanocomposites. Sols were electrospun into continuous nanofibers and calcined to get rutile phase. Fibers with diameter around 100 nm and crystallites size between 10 and 50 nm were obtained. The morphological impact and crystallites size dependence of the electrochemical performance for as-synthesized materials are reported. Enhancements using inert calcination atmosphere and incorporation of multi-wall carbon nanotubes (MWCNT) into the system are also presented.     

Thionine-mediated chemistry of carbon nanotubes

Thionine can be employed as a kind of useful functional molecule for the non-covalent functionalization of carbon nanotubes, as it shows a strong interaction with either SWNTs or MWNTs. Attachment of thionine molecules onto the sidewalls of carbon nanotubes would improve the solubility and lower the thermal stability of original carbon nanotubes. More importantly, it may functionalize the surface of carbon nanotubes with rich NH₂ groups and therefore open up more opportunities for the surface chemistry of carbon nanotubes. It has been proved that through the modification of small thionine molecules, other kinds of species such as cytochrome C and TiO₂ nanoparticles could be easily and selectively introduced onto the surface of carbon nanotubes. With this approach, SWNTs or MWNTs can be tailored with desired functional structures and properties.     

Synthesis and Photocatalytic Activity of Anatase TiO2 Nanoparticles-coated Carbon Nanotubes

A simple and straightforward approach to prepare TiO₂-coated carbon nanotubes (CNTs) is presented. Anatase TiO₂ nanoparticles (NPs) with the average size ~8 nm were coated on CNTs from peroxo titanic acid (PTA) precursor even at low temperature of 100 °C. We demonstrate the effects of CNTs/TiO₂ molar ratio on the adsorption capability and photocatalytic efficiency under UV–visible irradiation. The samples showed not only good optical absorption in visible range, but also great adsorption capacity for methyl orange (MO) dye molecules. These properties facilitated the great enhancement of photocatalytic activity of TiO₂ NPs-coated CNTs photocatalysts. The TiO₂ NPs-coated CNTs exhibited 2.45 times higher photocatalytic activity for MO degradation than that of pure TiO₂.     

Equilibrium and dynamics of acetylene sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNT) were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and by adsorption of N₂ and acetylene. The dynamics of acetylene sorption was studied by frequency response spectroscopy. The average tube dimension and the shell number were determined from a statistical evaluation of the TEM images. This value agreed with that obtained from XRD via the Scherrer equation only if a shape factor of 0.49 was used. The diffusion of acetylene in the nanotubes of the MWCNT sample was found to be the rate-controlling step of the sorption process. Relationships between the tube dimensions and the equilibrium and dynamic sorption properties were demonstrated.     

The effect of incorporating carbon nanotubes in titania films used for the photocathode protection of 304 stainless steel

Composite films of TiO₂ and multiwall carbon nanotubes (MWCNTs) were prepared on 304 stainless steel (304 SS) by the sol–gel method and heat treatment. Their crystal phase and surface structure were characterized by X-ray diffraction and electron microscopy. The photoelectrochemical performance of the composite films in 3.0% NaCl solution was evaluated by electrochemical measurements under UV irradiation and dark conditions. Since highly conductive MWCNTs in the TiO₂ films can transport the photo-generated electrons easily and quickly to the metal substrate, the MWCNT/TiO₂ composite films exhibited three times the photocurrent and the half the charge transfer resistance of pure TiO₂ films, and provide a much better photocathode protection for 304 SS.     

The features of the magnetoresistance of carbon nanotubes modified with Fe

Two specimens of modified multi-wall carbon nanotubes (MWCNT) with a mass fraction of iron of 0.20 and 0.29 have been obtained by the method of metal reduction from aqueous salt solution. According to structural and phase investigations, modified MWCNT contain Fe₃C, α-Fe (ferromagnetic phase), and iron oxides FeO and Fe₂O₃ (antiferromagnetic phases with a Néel temperature of 188 K and 260 K, respectively) particles up to 8 nm in size. It is experimentally found that for MWCNT modified simultaneously by several ferromagnetic phases with different coercive forces, a giant magnetoresistive effect is observed at room temperature. For MWCNT modified with only one ferromagnetic phase, the giant magnetoresistance has not been experimentally detected. With temperature decrease, the magnetoresistance dependence on magnetic field for such modified nanotubes acquires a specific butterfly-like form, which is characteristic of the magnetic ordering of ferromagnetic and antiferromagnetic phases due to exchange anisotropy. For MWCNT modified with only one ferromagnetic phase there has been an effect of asymmetric magnetoresistance, which is related to the presence in the specimen of inhomogeneous transverse Hall voltages due to the pronounced spin-orbit interaction of conduction electrons and magnetic moments of the magnetic phase, and an anisotropic magnetoresistive effect.     

TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration

In this work, we investigated titanium dioxide (TiO₂) nanotubes and CNT–TiO₂ hybrid materials for the photocatalytic oxidation (PCO) of propene at low concentration (100 ppmv) in gaseous phase. The materials were prepared via sol–gel method using sacrificial multi-walled carbon nanotubes (CNT) as templates and subsequent heat treatments to obtain the desired crystalline phase (anatase, rutile or a mixture of both) and eventually to remove the carbon template. We also studied rutile nanotubes for the first time and demonstrate that the activity strongly depends on the crystalline composition, following rutile < anatase < anatase/rutile mixture. The enhanced activity of the anatase–rutile mixture is attributed to the decrease in the electron–hole pair recombination due to the multiphasic nature of the particles. The key result of this work is the exceptional performance of the CNT–TiO₂ hybrid, which yielded the highest observed photocatalytic activity. The improved performance is attributed to synergistic effects due to the hybrid nature of the material, resulting in small anatase crystalline sizes (CNT act as heat sinks) and a reduced electron–hole pair recombination rate (CNTs act as electron traps). These results demonstrate the great potential of hybrid materials and stimulate further research on CNT-inorganic hybrid materials in photocatalysis and related areas.     

Visible light photocatalytic degradation of thiophene using Ag–TiO2/multi-walled carbon nanotubes nanocomposite

Ag–TiO₂ nanocatalyst, supported on multi-walled carbon nanotubes, was synthesized successfully via a modified sol–gel method, and the prepared photocatalyst was used to remediate aqueous thiophene environmentally by photocatalytic oxidation under visible light. The prepared Ag–TiO₂/multi-walled carbon nanotubes nanocomposite photocatalyst was characterized through X-ray diffraction, Brunauer–Emmett–Teller (BET), transmission electron microscopy, and UV–vis spectra (UV–vis). The results showed that both Ag and TiO₂ nanoparticles were well-dispersed over the MWCNTs and formed a uniform nanocomposite. Ag doping can eliminate the recombination of electron–hole pairs in the catalyst, and the presence of MWCNTs in the TiO₂ composite can change surface properties to achieve sensitivity to visible light. The optimum mass ratio of MWCNT:TiO₂:Ag was 0.02:1.0:0.05, which resulted in the photocatalyst's experimental performance in oxidizing about 100% of the thiophene in a 600 mg/L solution within 30 min and with 1.4 g L⁻¹ amount of catalyst used.     

Preparation of ordered TiO2 nanofibers/nanotubes by magnetic field assisted electrospinning and the study of their photocatalytic properties

Ordered-and-oriented TiO₂ nanofibers and nanotubes were prepared by magnetic field-assisted electrospinning, and photocatalytic properties of all samples were analyzed under UV–Vis shine. TiO₂ nanofibers/nanotubes prepared by magnetic field-assisted electrospinning showed better degradation effect on rhodamine B, reduced the band gap, increased the contact area of organic pollutants with the sample and higher photocatalytic activity than TiO₂ nanofibers/nanotubes prepared by classical electrospinning. The product obtained after high temperature annealing was a mixed phase of rutile phase and anatase phase and could be advantageous to the segregate of photogenic electron hole pairs and enhance the high dye absorption capacity; Surface roughness could increase more active sites and accelerate the reaction rate of photocatalytic activity; the addition of magnetic field regulated the morphology of TiO₂, and narrowed the band gap to favor photocatalytic performance. The magnetic field-assisted electrospinning study prepared in this paper was an easy-to-use and versatile method for the preparation of ordered TiO₂ nanomaterials, which could be easily extended to practical applications or other materials for photocatalysis and water cleavage.