The size and dispersion effect of modified graphene oxide sheets on the photocatalytic H2 generation activity of TiO2 nanorods

Nano graphene oxide (NGO) was produced by further refluxing graphene oxide (GO) sheets in HNO₃, and carboxylic acid functionalized graphene oxide (GO–COOH) was obtained by a simple etherification reaction between GO and chloroacetic acid. The GO, GO–COOH and NGO sheets are combined with TiO₂ nanorods by a two-phase assembling method, and confirmed by transmission electronic microscopy. The GO–TiO₂, GO–COOH–TiO₂ and NGO–TiO₂ composites are used in a comparative study of photocatalytic H₂ generation activity under UV light irradiation. The H₂ generation rate of TiO₂ nanorods was slightly increased from 15 to 30 mL h⁻¹ g⁻¹ by replacing oleic acid ligands with hydrophilic dopamine, and significantly increased to 105 mL h⁻¹ g⁻¹ after combining with GO sheets. The further comparative study shows that GO–COOH–TiO₂ composite has higher H₂ generation rate of 180 mL h⁻¹ g⁻¹ than that of GO–TiO₂ and NGO–TiO₂ composites.     

Sub-20 nm anatase particles uniformly anchored on graphene oxide and reduced graphene oxide nanosheets and their photocatalytic oxidation and Li-ion storage capabilities

Nanosized anatase TiO₂ particles anchored on nanocarbon substrates have great potential for practical applications in high-performance lithium ion batteries and efficient photocatalysts. The synthesis of this material usually utilizes calcination to crystallize amorphous titania, which normally causes the formation of aggregates and some side effects. In this work, we demonstrated that sub-20 nm anatase particles uniformly anchored on graphene oxide and reduced graphene oxide nanosheets in aqueous solution at a temperature of 90 °C and atmospheric pressure, without further calcination. The photocatalytic oxidation activity and electrochemical properties of graphene oxide/anatase TiO₂ (GO/A) and reduced graphene oxide/anatase TiO₂ (RGO/A) were comparatively investigated. We found that GO/A showed higher photocatalytic oxidation activity than RGO/A under UV light irradiation. Graphene oxide accepted electrons and suffered reduction, which finally decreased GO/A’s photocatalytic oxidation activity to an extent similar to RGO/A. We also found that, as anode material for Li-ion battery, the specific capacity of RGO/A was nearly three times that of GO/A at the same current rate. This study will inspire better design of metal oxide/nanocarbon nanocomposites for high performance lithium ion battery and photocatalysis applications.     

Direct multipulse laser processing of titanium oxide–graphene oxide nanocomposite thin films

Nanocomposite thin films consisting of titanium oxide (TiO₂) nanoparticles (NPs) and graphene oxide (GO) platelets were deposited by a spin-coating technique. The obtained films were submitted to direct laser irradiation using a frequency quadrupled Nd:YAG (λ=266 nm, τ_FWHM≅3 ns, ν=10 Hz) laser source. The effect of the laser processing conditions, as laser fluence value and number of subsequent laser pulses incident onto the same target location, on the surface morphology, crystalline structure, and chemical composition of the TiO₂/GO nanocomposite thin films was systematically investigated. The laser fluence values were maintained below the vaporization threshold of the irradiated composite material. With the increase of the laser fluence and number of incident laser pulses melting and coalescence of the TiO₂ NPs into inter-connected aggregates as well as rippling of the GO platelets take place. The gradual reduction of GO platelets and the onset of anatase to rutile phase transition were observed at high laser fluence values.     

Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets

Monodispersed flower-like titanate superstructure was successfully prepared by simple hydrothermal process without any surfactant or template. N₂-sorption analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) observation of as-synthesized product revealed the formation of flower-like titanate with diameter of about 250–450 nm and BET surface area (S_BET) of 350.7 m² g^?1. Upon thermal treatment at 500 °C, the titanate nanosheets were converted into anatase TiO₂ with moderate deformation of their structures. The as-prepared flower-like titanate showed high photocatalytic activity for H₂ evolution from water splitting reaction. Moreover, the sample heat treated at 500 °C exhibited higher photocatalytic activity than that of commercial TiO₂ anatase powder (ST-01).     

Combustion synthesis of graphene oxide–TiO2 hybrid materials for photodegradation of methyl orange

Graphene oxide (GO)–TiO₂ hybrid materials with enhanced photocatalytic properties were synthesized by a one-step combustion method using urea and titanyl nitrate as the fuel and oxidizer, respectively. During the synthesis procedure, the precursors containing GO, fuel, and oxidizer were maintained at different combustion temperatures (300–450 °C) for 10 min to ignite the combustion reaction. The effects of combustion temperatures on the weight loss, chemical status and photocatalytic properties were studied by thermogravimetry and differential scanning calorimetry, X-ray photoelectron spectroscopy, Raman, and photoluminescence. GO in the GO–TiO₂ hybrids were not oxidized, but thermally reduced by decomposition of partial oxygen-containing groups. Meantime, the nitrogen doping of GO was achieved. Compared to the neat TiO₂ obtained at same condition, GO–TiO₂ hybrid obtained at 350 °C exhibited enhanced photodegradation performance, which is attributed to the effective photo-generated electron transferring from TiO₂ to partially reduced GO, which confirmed by the photoluminescence quenching of TiO₂.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Structural,magnetic, and textural properties of iron oxide-reduced graphene oxide hybrids and their use for the electrochemical detection of chromium

Superparamagnetic Fe₃O₄ nanoparticles were anchored on reduced graphene oxide (RGO) nanosheets by co-precipitation of iron salts in the presence of different amounts of graphene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were observed for the three hybrids of Fe₃O₄ and RGO. The structure, morphology and microstructure of the hybrids were examined by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, Raman and X-ray photoelectron spectroscopy. TEM images reveal lattice fringes (d₃₁₁ = 0.26 nm) of Fe₃O₄ nanoparticles with clear stacked layers of RGO nanosheets. The textural properties including the pore size distribution and loading of Fe₃O₄ nanoparticles to form Fe₃O₄–RGO hybrids have been controlled by changing the concentration of GO. An observed maximum (～10 nm) in pore size distribution for the sample with 0.25 mg ml^?1 of GO is different from that prepared using 1.0 mg ml^?1 GO. The superparamagnetic behavior is also lost in the latter and it exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chromium ion was assessed and a novel electrode system using cyclic voltammetry for the preparation of an electrochemical sensor platform is proposed. The textural properties seem to influence the electrochemical and magnetic behavior of the hybrids.     

Visible light driven photodynamic anticancer activity of graphene oxide/TiO2 hybrid

Graphene oxide/TiO₂ hybrid (GOT) was prepared by using Ti(OC₄H₉)₄ and graphene oxide (GO) as reactants. Superior adsorption and photocatalysis performance under visible radiation were achieved in the presence of the GOT rather than in unmodified TiO₂. GO that no toxicity in vitro acted as electron sink in GOT efficiently enhance the photodynamic activities. Consistent with photocatalytic performance of TiO₂, GOT generated reactive oxygen species after visible light irradiation both in cell free condition and in vitro. No dark cytotoxicity was observed using 0–100 μg/mL GOT during long incubation time. In parallel, following exposure of cells to GOT and irradiation, a marked decrease in mitochondrial membrane potential, cell viability, activities of superoxide dismutase, catalase and glutathione peroxidase, as well as increased malondialdehyde production were observed. Moreover, GOT caused significant elevation in caspase-3 activity, and induced apoptotic death. The results indicated that GOT had excellent photodynamic anticancer activity without dark cytotoxicity.     

Photocatalytic activity of rubber sheet impregnated with TiO2 particles and its recyclability

The preparation of rubber sheet impregnated with titanium dioxide particles is presented. This method is simple and low cost based on the use of commercial TiO₂ powder directly mixing with rubber latex (60% HA) and distilled water. The morphology and roughness of the sheet surface increased with increasing amount of distilled water. Sheet impregnated with anatase (Imp-An) showed uniform, small grains with dense structure and well surface coverage more than one with P25 (Imp-P25). Their photocatalytic activities were evaluated using methylene blue (MB) as a model organic dye compound. These impregnated sheets could degrade MB solution under UV-light irradiation. Comparing with the commercial TiO₂ samples in powder form (anatase from Carlo Erba and Degussa P25) the efficiencies of photocatalytic degradation of MB fall in the decreasing order as: P25 (powder) > anatase (Carlo Erba) (powder) > Imp-An sheet > Imp-P25 sheet. However, the impregnated sheet has an advantage over the loose powder that the catalyst sheet can be recovered after used and can be reused.     

Enhanced photocatalytic degradation of rutile/anatase TiO2 heterojunction nanoflowers

Rutile/anatase TiO₂ heterojunction nanoflowers were prepared via a facile one-step hydrothermal approach using titanium tetrachloride and urea as the raw materials, cetyl trimethyl ammonium bromide (CTAB) as the template. The prepared TiO₂ nanoflowers were characterized by XRD, SEM, TEM and BET analyses. The photocatalytic performance of the as-prepared TiO₂ samples for methyl blue degradation under simulated solar light was investigated. TiO₂ heterojunction nanoflowers with mixed rutile/anatase phase (prepared with 3 mmol CTAB) give the highest photocatalytic activity. In addition, TiO₂ nanoflowers show excellent stability after 9 cycles under the same conditions. These results suggested that the mixed phase anatase/rutile TiO₂ heterojunction nanoflowers have great potential for the future photodegradation of real dye waste water.     

Suspension high velocity oxy-fuel spraying of a rutile TiO2 feedstock: Microstructure,phase evolution and photocatalytic behaviour

Nano-structured TiO₂ coatings were produced by suspension high velocity oxy fuel (SHVOF) thermal spraying using water-based suspensions containing 30 wt% of submicron rutile powders (~180 nm). By changing the flame heat powers from 40 kW to 101 kW, TiO₂ coatings were obtained with distinctive microstructures, phases and photocatalytic behaviour. Spraying with low power (40 kW) resulted in a more porous microstructure with the presence of un-melted nano-particles and a lower content of the anatase phase; meanwhile, high powers (72/101 kW) resulted in denser coatings and rougher surfaces with distinctive humps but not necessarily with a higher content of anatase. Linear sweep voltammetry (LSV) was used to evaluate the photocatalytic performance. Surprisingly, coatings with the lowest anatase content (~20%) using 40 kW showed the best photocatalytic behaviour with the highest photo-conversion efficiency. It was suggested that this was partially owing to the increased specific surface area of the un-melted nano-particles. More importantly, the structural arrangement of the similarly sized TiO₂ nano-crystallites between rutile and antase phases also created catalytic “hot spots” at the rutile−anatase interface and greatly improved the photo-activity.     

Kinetics of anatase transition to rutile TiO2 from titanium dioxide precursor powders synthesized by a sol-gel process

Kinetics of anatase transition to rutile TiO₂ from titanium dioxide precursor powders synthesized by a sol-gel process have been studied using differential thermal analysis (DTA), X-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano beam electron diffraction (NBED) and high resolution TEM (HRTEM). The DTA result shows residual organic matter decomposed at 436 K. The transition temperature for amorphous precursor powders converted to anatase TiO₂ occurred at 739 K. Moreover, the full anatase transition to rutile TiO₂ occurred at 1001 K. The activation energy of anatase TiO₂ formation was 128.9 kJ/mol. On the other hand, the activation energy of anatase transition to rutile TiO₂ was 328.4 kJ/mol. Mesoporous structures can be observed in the TEM image.     

TiO2 nanotubes modified with electrochemically reduced graphene oxide for photoelectrochemical water splitting

The photocatalytic water splitting into hydrogen and oxygen using solar light is a promising method to provide clean energy carriers in the future. Herein we report on an experimental investigation of TiO₂ nanotubes (NTs) modified with electrochemically reduced graphene oxide (ERGO) for photoelectrochemical water splitting. A photocurrent density of 1.44 mA cm⁻² at 1.23 V vs. RHE has been achieved for ERGO–TiO₂ NTs photoanode under standard reporting conditions, i.e., simulated AM 1.5G sunlight (intensity 100 mW cm⁻²), which is notably increased by ∼140% compared to the bare TiO₂ NTs. This efficiency is nearly ten times higher than that of the P25 nanoparticles based device. The enhanced photocurrent densities can be attributed to the reduced graphene oxide and Ti^{3 +} self-doping produced by an electrochemical reduction treatment. The ERGO modified photoanodes show excellent stability during light soaking under full sunlight.     

Preparation of scale-like nickel cobaltite nanosheets assembled on nitrogen-doped reduced graphene oxide for high-performance supercapacitors

Ultrathin scale-like nickel cobaltite (NiCo₂O₄) nanosheets supported on nitrogen-doped reduced graphene oxide (N-rGO) are successfully synthesized through a facile co-precipitation of Ni²⁺ and Co²⁺ in the presence of sodium citrate and hexamethylenetetramine and subsequent calcination treatment. The composition and morphology of NiCo₂O₄ nanosheets@nitrogen-doped reduced graphene oxide (denoted as NiCo₂O₄ NSs@N-rGO) were characterized by Scanning electron microscope, Transmission electron microscope, X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller and thermogravimetric analysis. The thickness of NiCo₂O₄ nanosheets anchored on the reduced graphene oxide is around 4 nm. The capacitance of NiCo₂O₄ NSs@N-rGO is evaluated by cyclic voltammogram and galvanostatic charge/discharge with the result that the NiCo₂O₄ NSs@N-rGO could deliver a specific capacitance of 1540 F g⁻¹ after 1000 cycles at 10 A g⁻¹.     

The role of graphene and europium on TiO2 performance for photocatalytic hydrogen evolution

Highly efficient Eu-TiO₂/graphene composites were synthesized by a two-step method such as sol-gel and hydrothermal process. The synthesized photocatalysts were characterized by XRD, TEM, XPS, UV–vis diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. The results confirmed that anatase Eu-TiO₂ nanoparticles with average 10 nm sizes were successfully deposited on two-dimensional graphene sheets. The UV–visible spectroscopy showed a red shift in the absorption edge of TiO₂ due to Eu doping and graphene incorporation. Moreover, effective charge separation in Eu-TiO₂/graphene composites was confirmed by PL emission spectroscopy compared to TiO₂/graphene, Eu-TiO₂ and pure TiO₂. The photocatalytic activity for H₂ evolution over prepared composites was studied under visible light irradiation (λ ≥ 400 nm). The results demonstrate that photocatalytic performance of the photocatalysts for hydrogen production increases with increasing doping concentration of Eu upto 2 at%. However, further increase in doping content above this optimum level has decreased the performance of photocatalyst. The enhanced photocatalytic performance for H₂ evolution is attributed to extended visible light absorption, suppressed recombination of electron-hole pairs due to synergistic effects of Eu and graphene.     

Study on properties and synthesis of sulfonated coal supported nanostructure TiO2 photocatalysts

Nano-TiO₂ was synthesized with titanium sulfate by a simple economical microwave-assisted-hydrolysis method, and it was successfully coated on semi-organic-matter sulfonated coal (SC). SC as a semi-organic matter was first chosen as the substrate of nano-TiO₂. Their surface morphology and structure were investigated by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The hybrid nano-composite photocatalyst exhibited a good photocatalytic activity in degrading methylene blue (MB). Results revealed that under the assistance of microwave-process, the anatase TiO₂ can be synthesized at low temperature (around 100 °C), presents a strong crystalline and has a good photocatalytic performance.     

Graphene nanosheets based on controlled exfoliation process for enhanced lithium storage in lithium-ion battery

A facile and rapid approach was used for the fabrication of chemically derived graphene nanosheets based on the reduction of graphite oxide (GO) in tube furnace assembly at different temperatures. The morphologies, microstructures, specific surface areas and other features of GO and graphene nanosheets were characterized. Structure characterization indicates that the platelet thickness of graphene nanosheets obtained at 300 °C was 1.62 nm, which corresponds to an approximately 5 layers stacking of the monoatomic graphene nanosheets. Electrochemical performances of the as-prepared graphene nanosheets were performed, the result of which could prove the above observation that graphene nanosheets (5 layers) obtained at 300 °C actually displayed the most remarkable electrochemical performances: the first discharge and charge capacities of graphene nanosheets were as high as 2137 mAh/g and 994 mAh/g, respectively, and after 100 cycles graphene nanosheets still possessed a high capacity of 478 mAh/g.     

Iron-doped TiO2 nanotubes with high photocatalytic activity under visible light synthesized by an ultrasonic-assisted sol-hydrothermal method

A series of iron-doped anatase TiO₂ nanotubes (Fe/TiO₂ NTs) catalysts with iron concentrations ranging from 0.88 to 7.00 wt% were prepared by an ultrasonic-assisted sol-hydrothermal process. The structures and the properties of the fabricated Fe/TiO₂ NTs were characterized in detail and photocatalytic activity was examined using a reactive brilliant red X-3B aqueous solution as pollutant under visible light. The lengths of the NTs were determined to range from 20 nm to 100 nm. The incorporation of the iron ions (Fe³⁺) into the TiO₂ nanotubes shifted the photon absorbing zone from the ultraviolet (UV) to the visible wavelengths, reducing the band gap energy from 3.2 to 2.75 eV. The photocatalytic activity of the Fe/TiO₂ NTs was 2–4 times higher than the values measured for the pure TiO₂ nanotubes.     

Effect of carbon doping on WO3/TiO2 coupled oxide and its photocatalytic activity on diclofenac degradation

The improved photocatalyst carbon-doped WO₃/TiO₂ mixed oxide was synthesized in this study using the sol–gel method. The catalyst was thoroughly characterized by X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy, N₂ adsorption desorption analysis, scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic efficiency of the prepared materials was evaluated with respect to the degradation of sodium diclofenac (DCF) in a batch reactor irradiated under simulated solar light. The progress of the degradation process of the drug was evaluated by high-performance liquid chromatography (HPLC), whereas mineralization was monitored by total organic carbon analysis (TOC) and ion chromatography (IC). The results of the photocatalytic evaluation indicated that the modified catalyst with tungsten and carbon (TWC) exhibited higher photocatalytic activity than TiO₂ (T) and WO₃/TiO₂ (TW) in the degradation and mineralization of diclofenac (TWC>TW>T). Complete degradation of diclofenac occurred at 250 kJ m⁻² of accumulated energy, whereas 82.4% mineralization at 400 kJ m⁻² was achieved using the photocatalytic system WO₃/TiO₂-C. The improvement in the photocatalytic activity was attributed to the synergistic effect between carbon and WO₃ incorporated into the TiO₂ structure.