Influence of heat treatment on thermally-reduced graphene oxide/TiO<Subscript>2</Subscript> composites for photocatalytic applications

Thermally-reduced graphene oxide/TiO₂ composites (TRGO/Ti) were prepared by the thermal reduction of graphene oxide/TiO₂ composite that was obtained from a simple, environmentally friendly, one-step colloidal blending method. The changes in structural and textural properties as well as their corresponding photocatalytic activities were investigated as a function of calcination temperature. The presence of stacked TRGO sheets significantly retarded both the aggregation and the crystalline phase transformation of TiO₂ as increasing the temperature from 200 to 600 °C. TRGO/Ti composites exhibited higher photocatalytic activity for the degradation of methylene blue in comparison with pure TiO₂ due to the increase in specific surface area and the formation of π-π conjugations between dye compounds and aromatic regions of TRGO. However, increasing the calcination temperature resulted in the lower photoactivity and slower kinetics, which can be ascribed to the decrease in surface area, the reduction of oxygen vacancies, and the loss of functional groups at the edges or on the basal planes of the TRGO sheets.     

In situ synthesis of ultrathin 2-D TiO2 with high energy facets on graphene oxide for enhancing photocatalytic activity

Two kinds of TiO₂ with novel structures, interpenetrating anatase TiO₂ tablets (IP-TiO₂), and overlapping anatase TiO₂ nanosheets (OL-TiO₂) with exposed {0 0 1} facets, are synthesized. The graphene oxide (GO) supported ultrathin TiO₂ nanosheets (OL-TiO₂/GO) is also prepared by one-pot hydrothermal method. The microscopic feature, morphology, phase, and nitrogen adsorption–desorption isotherms are characterized. The performance of photocatalytic degradation of methyl blue is also measured. Compared with IP-TiO₂, the OL-TiO₂ with GO possess higher photocatalytic efficiency. The GO can improve the photocatalytic property by increasing specific surface area, accelerating the separation of electron–hole pairs, as well as extending the electron life. The growth process of TiO₂ nanosheets on graphene oxide layers probably follows a step-growth mechanism with F⁻ as morphology controlling agent. The steps on the surface can improve the photocatalytic activity further due to the increase of dangling bonds of 5-coordinated Ti (Ti_5c) which are considered to be the active sites in the photocatalytic reaction.     

Microwave-assisted synthesis of Ag–TiO2/graphene composite for hydrogen production under visible light irradiation

Novel photocatalysts based on silver (Ag), TiO₂, and graphene were successfully synthesized by microwave-assisted hydrothermal method. The prepared photocatalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The influence of silver loading and graphene incorporation on photocatalytic hydrogen (H₂) production of as-prepared samples was investigated in methanolic aqueous solution under visible light irradiation (λ≥420 nm). The results showed that Ag–TiO₂/graphene composite had appreciably enhanced photocatalytic H₂ production performance under visible light illumination compared to pure TiO₂, Ag–TiO₂ and TiO₂/graphene samples. The enhanced photocatalytic hydrogen production activity of Ag–TiO₂/graphene composite under visible light irradiation could be attributed to increased visible light absorption, reduced recombination of photogenerated charge carriers and high specific surface area. This novel study provides more insight for the development of novel visible light responsive TiO²⁻ graphene based photocatalysts for energy applications.     

Fabrication of graphene films on TiO2 nanotube arrays for photocatalytic application

Graphene film was formed on the surface of titanium dioxide nanotube (TiO₂ NT) arrays through in situ electrochemical reduction of a graphene oxide dispersion by cyclic voltammetry. The residual oxygen-containing groups and other structural defects such as sp³-hybridized carbons in the electrodeposited graphene were further removed by photo-assisted reduction of the underlying TiO₂ NTs, thus achieving the maximum restoration of π-conjugation in the graphene planes. Spectroscopic, electrochemical, and photoelectrochemical techniques were used to characterize the graphene films, and the use of the resulting graphene–TiO₂ NT material in photocatalysis was investigated. The results showed that the graphene–TiO₂ NT material exhibited a greatly improved photocatalytic activity compared with unmodified TiO₂ NTs.     

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.     

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.     

Preparation and characterization of cerium‐doped titanium dioxide/ultrahigh‐molecular‐weight polyethylene porous composites with excellent photocatalytic activity

Porous ultrahigh‐molecular‐weight polyethylene (UHMWPE)‐based composites filled with surface‐modified Ce‐doped TiO₂ nanoparticles (Ce–TiO₂/UHMWPE) were prepared by template dissolution. The composites were characterized by Fourier transform infrared spectroscopy, ultraviolet (UV)–visible spectroscopy, diffuse reflectance spectra, and scanning electron microscopy); the photocatalytic activity was also evaluated by the decomposition of methyl orange under UV exposure. The results demonstrate that the severe aggregation of Ce–TiO₂ nanoparticles could be reduced by surface modification via a silane coupling agent (KH570). The Ce–TiO₂/UHMWPE porous composites exhibited a uniform pore size. Doping with Ce⁴⁺ effectively extended the spectral response from the UV to the visible region and enhanced the surface hydroxyl groups of the TiO₂ attached to the matrix. With a degradation rate of 85.3%, the 1.5 vol % Ce–TiO₂/UHMWPE sample showed the best photocatalytic activity. The excellent permeability of the porous composites is encouraging for their possible use in wastewater treatment. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Fabrication of CdO–graphene embedded mesoporous TiO2 composite for the visible-light response and its organic dye remediation

ABSTRACT

In this study, mesoporous CdO–graphene and mesoporous TiO₂ were successfully synthesized using a facile method. The results indicated that photocatalytic activity can be enhanced with the combination of mesoporous CdO–graphene and mesoporous TiO₂. The photocatalytic performance of the mesoporous CdO–graphene–TiO₂ photocatalyst presents significantly improved photocatalytic activity of safranine O, reactive black B, and GA due to the improvement of the surface area and the small average pore size distribution of the as-prepared mesoporous CdO–graphene–TiO₂. The photodegradation rate was optimized by safranine O at a pH solution of 11 and by GA at the dosage amount of 0.07 g photocatalyst.     

Graphene–WO3 nanobelt composite: Elevated conduction band toward photocatalytic reduction of CO2 into hydrocarbon fuels

Graphene oxide (GO), tungsten trioxide (WO₃) and graphene–WO₃ nanobelt composites (GW) were synthesized and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV–vis diffuse reflection spectra (DRS) and X-ray photoelectron spectroscopy (XPS) valence band spectra. We demonstrated that the graphene can elevate the conduction band of WO₃ toward photocatalytic reduction of CO₂ into hydrocarbon fuels under visible-light irradiation. And the photocatalytic activity of GW is higher than that of GO, WO₃ and P25 TiO₂.     

Covalently functionalized graphene by thiourea for enhancing H2-evolution performance of TiO2 photocatalyst

Graphene as a well-known electron cocatalyst can enhance the hydrogen-production performance of photocatalysts due to its excellent conductivity. For highly efficient graphene-modified TiO₂ photocatalyst, besides the speedy electron transfer via graphene, it is significant to raise the interfacial hydrogen-generation reaction rate on the graphene surface. In this paper, thiourea (TU) can covalently functionalize graphene and act as effective H⁺-adsorbed active sites to improve the hydrogen-production efficiency of graphene-modified TiO₂ (TiO₂/rGO-TU). The TiO₂/rGO-TU sample was successfully prepared by a facile nucleophilic substitution reaction between the thiol (-SH) of thiourea and carboxyl (-COOH) of graphene. The TiO₂/rGO-TU could possess the maximum H₂-generation rate of 241.83 μmol h^?1 g^?1, which was 2.33 and 6.60 times greater than that of TiO₂/rGO and TiO₂, respectively. The enhanced photocatalytic activity of TiO₂/rGO-TU can be ascribed to the synergistic effect of graphene and thiourea, namely, the graphene functions as a cocatalyst to capture the photoexcited electrons of TiO₂ and the thiourea acts as effectual H⁺-adsorbed active sites to promote interfacial hydrogen generation. This study presents a feasible strategy for developing grephene-based photocatalysts for prospective applications in the hydrogen-production field.     

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.     

Novel synthesis of microcrystalline titanium(IV) oxide having high thermal stability and ultra-high photocatalytic activity: thermal decomposition of titanium(IV) alkoxide in organic solvents

Thermal decomposition of titanium(IV) tetra-tert-butoxide (TTB) in inert organic solvents at 573 K yielded microcrystalline anatase (titanium(IV) oxide, TiO₂) powders with a crystallite size of ca. 9 nm and a surface area of <100 m² g^-1. Primary and secondary alkoxides of titanium(IV), however, were not decomposed under similar conditions, indicating that the thermal stability of C-O bonds in the alkoxides was a decisive factor for their decomposition. The TiO₂ prepared from TTB by this manner was thermally stable upon calcination in air and retained high surface area of ca. 100 m² g^-1 even after calcination at 823 K. The as-prepared TiO₂ powders, without calcination, exhibited much higher rate of carbon dioxide formation than any other active photocatalysts such as Degussa P-25 and Ishihara ST-01 in the photocatalytic mineralization of acetic acid in aerated aqueous solutions. The higher activity of the present TiO₂ photocatalysts is attributed to both high crystallinity and large surface of the present product. The calcination of the as-prepared TiO₂ in air reduced the photocatalytic activity, but it was still higher than the other commercially available TiO₂'s. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of the status of Sm on the properties of Sm contained N doped TiO2 photocatalysts

Nitrogen-doped TiO₂ (TiO_2?xN_y) nanoparticles with and without adding Sm³⁺ were synthesized by thermohydrolysis of TiCl₃. The samples were characterized by X-ray diffraction, specific surface area determination, UV–Vis diffuse reflectance spectroscopy. The photocatalytic activity of the sample was investigated by employing the oxidative destruction of nitric oxide as a probe reaction using a flow reactor. Although the doping of Sm³⁺ in the lattice of titania was not useful to improve the photocatalytic activity, loading of samarium oxides on the surface of titania resulted in an improvement of the photocatalytic activity of the nitrogen-doped TiO₂. The beneficial effect was explained by an increased separation efficiency of the photogenerated electron–hole pairs.     

Photocatalytic reduction synthesis of SrTiO3-graphene nanocomposites and their enhanced photocatalytic activity

SrTiO₃-graphene nanocomposites were prepared via photocatalytic reduction of graphene oxide by UV light-irradiated SrTiO₃ nanoparticles. Fourier transformed infrared spectroscopy analysis indicates that graphene oxide is reduced into graphene. Transmission electron microscope observation shows that SrTiO₃ nanoparticles are well assembled onto graphene sheets. The photocatalytic activity of as-prepared SrTiO₃-graphene composites was evaluated by the degradation of acid orange 7 (AO7) under a 254-nm UV irradiation, revealing that the composites exhibit significantly enhanced photocatalytic activity compared to the bare SrTiO₃ nanoparticles. This can be explained by the fact that photogenerated electrons are captured by graphene, leading to an increased separation and availability of electrons and holes for the photocatalytic reaction. Hydroxyl (·OH) radicals were detected by the photoluminescence technique using terephthalic acid as a probe molecule and were found to be produced over the irradiated SrTiO₃ nanoparticles and SrTiO₃-graphene composites; especially, an enhanced yield is observed for the latter. The influence of ethanol, KI, and N₂ on the photocatalytic efficiency was also investigated. Based on the experimental results, ·OH, h⁺, and H₂O₂ are suggested to be the main active species in the photocatalytic degradation of AO7 by SrTiO₃-graphene composites.

PACS

61.46. + w; 78.67.Bf; 78.66.Sq     

Direct Solvothermal Formation of Nanocrystalline TiO2 on Porous SiO2 Adsorbent and Photocatalytic Removal of Nitrogen Oxides in Air over TiO2–SiO2 Composites

Solvothermal decomposition of titanium(IV) tert-butoxide (TTB) in toluene at 573 K in the presence of silica gel (SiO₂) with continuous stirring yielded a titanium(IV) oxide (TiO₂)–SiO₂ composite in which agglomerates of nanocrystalline TiO₂ were deposited on the surfaces of SiO₂ particles. Various TiO₂–SiO₂ composites having different TiO₂ contents can be synthesized by changing the ratio of TTB and SiO₂, and the composites had large surface areas corresponding to porous properties of SiO₂. These TiO₂–SiO₂ composites were used for photocatalytic removal of nitrogen oxides in air and their photocatalytic performances were compared with those of other TiO₂–SiO₂ samples prepared by different methods. Solvothermally synthesized 74 wt.%TiO₂–SiO₂ composite exhibited excellent photocatalytic performance (almost stoichiometric removal of NO _x (98%) and very low NO₂ release (0.3%)) attributable to high photocatalytic activity of TiO₂ and high adsorption property of SiO₂. Lesser performance of 74 wt.%TiO₂–SiO₂ composites prepared by other methods suggested that pore-mouth plugging of SiO₂ by TiO₂ and lower level of mixing of TiO₂ and SiO₂ decreased photocatalytic performance of the composites.     

Graphene modified Nd/TiO2 photocatalyst for methyl orange degradation under visible light irradiation

A novel nanoscale GR–Nd/TiO₂ composite photocatalyst was synthesized by the hydrothermal method. Its crystal structure, surface morphology, chemical composition and optical properties were studied using XRD, TEM, and XPS, DRS and PL spectroscopy. It was found that graphene and neodymium modification shifts the absorption edge of TiO₂ to visible-light region. The results of photoluminescence (PL) emission spectra show that GR–Nd/TiO₂ composites possess better charge separation capability than do Nd/TiO₂ and pure TiO₂. The photocatalytic activity of prepared samples was investigated by degradation of methyl orange (MO) dye under visible light irradiation. The results show that the GR–Nd/TiO₂ composite can effectively photodegrade MO, showing an impressive photocatalytic activity enhancement over that of pure TiO₂. The enhanced photocatalytic activity of the composite catalyst might be attributed to the large adsorptivity of dyes, extended light absorption range and efficient charge separation due to Nd doping and graphene incorporation.     

Synthesis and enhanced photocatalytic activity of Mn/TiO2 mesoporous materials using the impregnation method through CVC process

TiO₂ particles were prepared by chemical vapor condensation and used to synthesize MnO_x/TiO₂ mesoporous materials by impregnation. The Mn-doped TiO₂ particles were smaller (8.5?nm vs. 10.5?nm) and had greater surface areas (203.7?m²?g^?1 vs. 134.4?m²?g^?1) than undoped particles. They were also smaller and had a greater surface area than similarly doped commercial P25, indicating highly dispersed Mn species on the surfaces of the crystalline TiO₂. The resulting materials?? photocatalytic activities towards methylene blue decomposition were compared: the synthesized TiO₂ particles with MnO₂ showed higher photocatalytic activity than the similarly doped commercial P25.     

Optimizing graphene-TiO2 interface properties via Fermi level modulation for photocatalytic degradation of volatile organic compounds

The interfacial interaction between graphene and semiconductors significantly affects volatile organic compound (VOC) adsorption and charge separation. Herein, we optimize the interaction between TiO₂ and reduced graphene oxide (rGO) by reducing the Fermi level of GO with Cu²⁺. The surface photocurrent (SPC) and surface photovoltage (SPV) of the prepared materials were tested under different atmospheres to study the effect of enhanced interfacial interactions on charge separation. The results revealed that Cu²⁺ treatment tended to induce the TiO₂ nanoparticles to form a thicker and more uniform layer on the rGO surface. The composition of TiO₂ nanoparticles and rGO generated a 2D arranged porous structure that had both hydrophobic rGO and hydrophilic TiO₂ as the pore walls. The amount of adsorbed toluene for the optimized TiO₂-graphene (t-TiO₂/rGO) was 1.21-fold higher than that for TiO₂/rGO. The SPC and SPV results showed that the optimized contact between TiO₂ and rGO significantly enhanced photogenerated electron mobility and toluene-induced hole utilization. Given the advantages in adsorption and charge separation, the photocatalytic reaction rates of t-TiO₂/rGO were 1.47-fold and 1.91-fold higher than those of TiO₂/rGO and porous TiO₂, respectively.     

Synthesis and characterization of novel PbS–graphene/TiO2 composite with enhanced photocatalytic activity

In this study novel material PbS–graphene/TiO₂ composites were prepared by sol–gel method. The “as-prepared” composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS) and Raman spectroscopic analysis. The photocatalytic activities were investigated by the degradation of methylene blue (MB) as a standard dye. We observed that coupling of PbS with TiO₂ extends the photoresponse to visible region. This revealed that the excellent photoinduced charge separation abilities and transport properties of graphene make these hybrids as potential candidates for developing high-performance next-generation devices.     

Modification of polypropylene filter with metal oxide and reduced graphene oxide for water treatment

A hydrothermal method for the synthesis of reduced graphene oxide/titanium dioxide filter (RGO/TiO₂) and reduced graphene oxide/zinc oxide filter (RGO/ZnO) by using polypropylene (PP) porous filter is reported. Field emission scanning electron microscopy illustrated that the nanoparticles were uniformly distributed on the reduced graphene oxide nanosheets. Flexural tests showed that the physical properties of the modified filters have greater strength than the original filter. Thermogravimetric analysis revealed that the thermal property of the modified filters is the same as that of the original filter. Under a halogen lamp, the modified filter exhibited excellent photocatalytic degradation of methylene blue. The RGO/TiO₂ filter maintained its ability to degrade MB efficiently, even after five cycles of photocatalysis.