The dependence of the photocatalytic activity of TiO2/carbon nanotubes nanocomposites on the modification of the carbon nanotubes

Nanostructural TiO₂/modified multi-wall carbon nanotubes photocatalysts were prepared by hydrolysis of Ti(iso-OC₃H₇)₄ providing chemical bonding of anatase TiO₂ nanoparticles onto oxidized- or amino-functionalized multi-wall carbon nanotubes (MWCNT). The processes of functionalization of the MWCNT and the deposition of TiO₂ influence the photocatalytic activity of the synthesized nanocomposites. The phase composition, crystallite size, and the structural and surface properties of the obtained TiO₂/modified-MWCNT nanocomposite were analyzed from XRD, FEG-SEM, TEM/HRTEM and FTIR data, as well low temperature N₂ adsorption. In the photocatalytic study, the TiO₂/oxidized-MWCNT catalyst showed the highest and the TiO₂/amino functionalized-MWCNT catalysts somewhat lower degradation rates, indicating that the enhancement of photocatalysis was supported by the more effective electron transfer properties of the oxygen- than amino-containing functional groups, which support the efficient charge transportation and separation of the photogenerated electron-hole pairs.     

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.     

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.     

Hydrothermal synthesis of multiwalled TiO2 nanotubes and its photocatalytic activities for Orange II removal

The solvothermal synthesis of multiwalled TiO₂ nanotubes (MWTNTs) with the reaction of titanium dioxide (TiO₂) nanoparticles and sodium hydroxide aqueous solution at moderate temperature is presented. The kinetic and isotherm studies were investigated for Orange II removal mechanism. MWTNTs were prepared in length 70–200 nm, average diameter 4–10 nm, and inter-shell spacing 0.78 nm. Different characterizations were performed to confirm anatase and mesoporous structure of MWTNTs. An improvement in properties as compared to commercial TiO₂ was observed; specific surface area 244.81 m²/g and band gap 3.0 eV. An excellent photocatalytic activity for Orange II removal was exhibited using synthesized MWTNTs.     

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.     

Nanocomposites of styrene–butadiene rubber and synthetic anatase obtained by a colloidal route and their photooxidation

Photodegradable styrene–butadiene rubber (SBR)/TiO₂ nanocomposites were prepared by a colloidal route through the simple mixing of a commercial polymer latex and synthetic anatase nanoparticles. Stable colloids of pure anatase TiO₂ nanoparticles with an average diameter of 7 nm were prepared by a solvothermal route from the hydrolysis of titanium alkoxide by hydrogen peroxide in the presence of oleic acid. The photocatalytic degradation of the SBR–TiO₂ nanocomposites was carried out in ambient air at room temperature under a UV lamp and was monitored by Fourier transform infrared and UV–visible spectroscopies and differential scanning calorimetry. The results show that the SBR–TiO₂ nanocomposites were photocatalytically degraded under UV light, which indicate that the butadiene chains in the nanocomposite were oxidized during UV irradiation. Thermal analysis measurements indicated that crosslinking reactions occurred. The presence of anatase TiO₂ nanoparticles was found to accelerate the photocatalytic process, and the degradation mechanism was similar to that of the pure SBR polymer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Formation of the Ti4O7 phase through interaction between coated carbon and TiO2

Fine particles ofphotocatalytic anatase TiO₂ prepared through hydrolysis of titanium tetraisopropoxide were coated by carbon. A reduced phase, Ti₄O₇, was formed through interaction between TiO₂ and the coating carbon. EXAFS analysis on this Ti₄O₇ phase showed an intermediate Ti-Ti distance between those in anatase and rutile, which agreed with the structure composed of two-dimensional slabs of Ti-O octahedra separated by a shear plane. This carbon-coated Ti₄O₇ was confirmed to have photocatalytic activity, even though a little lower than anatase, examining the decomposition of methylene blue in water under LTV irradiation.     

Visible photocatalytic activity and photoelectrochemical behavior of TiO2 nanoparticles modified with metal porphyrins containing hydroxyl group

TiO₂ nanoparticles modified with 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin (HTPP), 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin zinc (ZnHTPP) and trans-dichloro-5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin tin (SnHTPP) were prepared in order to improve the visible photocatalytic activity of TiO₂ nanoparticles. The photocatalytic activity of the modified TiO₂ nanoparticles was investigated by carrying out the photodegradation of methyl orange in aqueous solution under visible light irradiation. The TiO₂ nanoparticles modified with SnHTPP show the highest visible photocatalytic activity with a degradation ratio of 86% of methyl orange after 180 min irradiation among three catalysts. This result indicates that the central metal ions in porphyrins can significantly influence the sensitization efficiency of porphyrins. In addition, the photoelectrochemical behavior of the modified TiO₂ nanoparticles was examined and related to their photocatalytic activity. Finally, the photocatalytic mechanism was discussed preliminarily.     

Synthesis, characterization and enhanced photocatalytic activity of TiO2/SiO2 nanocomposite in an aqueous solution and acrylic-based coatings

In this study, TiO₂/SiO₂ nanocomposites were synthesized via a sol-gel route by adding tetraethylorthosilicate (TEOS) to a solution containing different molar ratios of Degussa P25 TiO₂ nanoparticles. FTIR, TGA, EDAX and XRD techniques were used to characterize the modified nanoparticles. Photocatalytic activity of the nanoparticles in an aqueous solution and into the acrylic based coating was evaluated using colour coordinate data measurements, SEM analysis, gloss measurements and FTIR spectroscopy, in the presence of Rhodamine B (Rh.B) dyestuff, as a pollutant model, before and after exposure to the UVA (340 nm) irradiation and compared to their unmodified counterparts.The results showed that silica grafting effectively reduced the photocatalytic activity of the TiO₂ nanoparticles as evidenced by absorption spectra and colour changes of Rh.B aqueous solutions during the UVA irradiation. The results revealed the effectiveness of sol-gel route for preparation of TiO₂/SiO₂ nanocomposites. The optimum result was obtained with 1% molar ratio of TiO₂:TEOS. Addition of TiO₂/SiO₂ nanocomposites into the acrylic based coating revealed reduction of photo-degradation of Rh.B compared to untreated nanoparticles. Finally, inclusion of TEOS treated TiO₂ nanoparticles into the aqueous organic coatings, provides photocatalytic property and as a result, it can possibly be considered for self-cleaning coatings.     

Fabrication of a TiO2/carbon nanowall heterojunction and its photocatalytic ability

A two-dimensional TiO₂/carbon nanowall composite material was fabricated by growing carbon nanowalls on a Ti sheet with hot filament chemical vapor deposition, followed by metal-organic chemical vapor deposition using titanium isopropoxide as TiO₂ precursor and argon as carrier gas. Scanning electron microscopy showed that TiO₂ was uniformly coated on the entire carbon nanowall and a TiO₂/carbon nanowall composite was obtained. Raman spectroscopy and X-ray diffraction indicated that the crystal phase of the TiO₂ coating was anatase. The asymmetry of the current-voltage plot for the material revealed that a heterojunction was formed between the TiO₂ and the carbon nanowall. As a result of this heterojunction, enhanced separation of photogenerated electrons and holes was confirmed by surface photovoltage and photocurrent measurements. The investigation of photocatalytic ability showed that the TiO₂/carbon nanowall had a higher photocatalytic activity than TiO₂ nanotubes for the degradation of phenol under UV light irradiation.     

A highly active bi-crystalline photocatalyst consisting of TiO2 (B) nanotube and anatase particle for producing H2 gas from neat ethanol

Sodium titanate nanotubes (NaTNTs) are converted into monoclinic TiO₂ (B) nanotubes by rinsing with 0.10 M HCl followed by drying at 573 K. As calcination temperature is increased to 673 K, these TiO₂ (B) nanotubes start to transform into anatase nanoparticles producing a bi-crystalline mixture consisting of TiO₂ (B) nanotubes and anatase nanoparticles. The primary particle size of the anatase particles was estimated to be around 10 nm using Scherrer equation. After being promoted with 1% Pt, this bi-crystalline material becomes a very active photocatalyst producing 20% more H₂ gas than 1% Pt/Degussa P-25 TiO₂ in the photocatalytic dehydrogenation of neat ethanol after 2 h of UV light irradiation.     

Preparation and electrochemical capacitance of RuO2/TiO2 nanotubes composites

In this paper, RuO₂/TiO₂ nanotubes composites were synthesized by loading various amounts of RuO₂ on TiO₂ nanotubes. The symmetric supercapacitors based on these nanocomposites were fabricated by using gel polymer PVA-H₃PO₄-H₂O as electrolyte. The electrochemical capacitance performance of the nanocomposites in these supercapacitors was investigated by current-potential responses, galvanostatic charge-discharge tests and electrochemical impedance spectroscopy. The results show that the three dimensional nanotube network of TiO₂ offers a solid support structure for active materials RuO₂, allows the active material to be readily accessible (available) for electrochemical reactions, and improves the efficiency of the active materials. A maximum specific capacitance of 1263 F/g was obtained for the RuO₂ which was loading on TiO₂ nanotubes.     

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.     

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.     

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.     

Effect of TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO₂ nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO₂ nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl₄) treatment, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles. XRD patterns showed that the oxygen-annealed TiO₂ nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO₂ nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl₄ treatment, the efficiency of TiO₂ nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of R_k (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO₂ nanotubes were treated with TiCl₄. These results indicate that TiO₂ nanotubes treated with TiCl₄ can increase the surface area of TiO₂ nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

Study of the effect of rutile/anatase TiO2 nanoparticles synthesized by hydrothermal route in electrospun PVA/TiO2 nanocomposites

Mixed rutile–anatase TiO₂ nanoparticles were synthesized by hydrothermal treatment under acidic conditions and incorporated into poly(vinyl alcohol) (PVA). These nanocomposites were electrospun to produce nanofibers of PVA/TiO₂, which were characterized by scanning electron microscopy, transmission electron microscopy, X‐ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. The photocatalytic degradation of Rhodamine B and degradation of the polymer by UV‐C lamps were also investigated. The results showed that TiO₂ nanoparticles did not change the morphology and thermal behavior of the nanofiber polymer, but were effective in modifying the UV absorption of PVA without reducing its stability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A highly reactive catalyst for CO oxidation: CeO2 nanotubes synthesized using carbon nanotubes as removable templates

In this work, the nanocomposites, carbon nanotubes (CNTs) coated with nanosized ceria, were prepared by a facile solvothermal method. The obtained nanocomposites have a dense overlayer which is made of nanoparticles with the diameter of ～10 nm. Ceria nanotubes with a porous and hollow structure were fabricated by the removal of CNTs, which possess high surface area and remarkable thermal stability. The products were characterized by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The parameters affecting the formation of CeO₂ nanotubes were discussed in details. The key steps involved in the formation of the CeO₂ nanotubes are solvothermal modifications of CNTs and controlled calcinations. CeO₂ nanotubes have an excellent catalytic performance for the CO oxidation. The remains of the templates in the CeO₂ nanotubes are considered to play an important part in the enhanced catalytic activity for the formation of CeO_2?xC_x.     

Electrochemical lithium storage of titania nanotubes modified with NiO nanoparticles

The nanotubes of mixed TiO₂(B) and anatase phases, obtained by hydrothermal synthesis and subsequent calcination, are modified with NiO nanoparticles. In the modified products, NiO nanoparticles with poor crystallinity exist inside titania tubes and are attached to the outside surface of the nanotubes according to X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectra (EDS) analysis. The titania nanotubes, modified with 5 wt.% NiO in which NiO nanoparticles were distributed homogenously, exhibit the optimal cycle performance and a good capability for high rate discharge. The lithium ion diffusion is mainly related to the anatase phase, while the electrochemical reaction activity is attributed to the TiO₂(B) phase. Relative to titania nanotubes, NiO-modified nanotubes have a better electrochemical reaction activity, which is beneficial for the improvement of the high rate charge-discharge capability.