TiO2-reduced Graphene Oxide Nanocomposites Research Articles

A Selective Electrochemical Sensor Based on Titanium Dioxide-Reduced Graphene Oxide Nanocomposite (TiO2- RGO/GCE) for the Efficient Determination of Nitrite

ABSTRACT A nitrite sensing platform has been constructed using a TiO2-reduced graphene oxide (RGO) nanocomposite incorporated into a glassy carbon electrode. Reduced graphene oxide is integrated in a simple technique for the manufacture of titanium dioxide (TiO2). Spectroscopic and electrochemical techniques have all been used to characterise the TiO2- RGO nanocomposite. In 3 mM and pH 6.0 phosphate buffer solutions, the TiO2-reduced graphene oxide nanocomposites tailored glassy carbon electrode demonstrated outstanding electro-catalytic nitrite oxidation activity. The square root of scan rate is also proportional to the oxidation peak current, showing that the nitrite redox reaction involves a diffusion-controlled mechanism. Using linear sweep voltammetry, the sensitivity and detection limit for nitrite at the modified electrode were determined to be 4.853 μA/μM cm2 and 0.006 µM respectively, over a concentration range of 10 - 2000 µM. Further, the nanocomposite is used to measure NO2- in water and buttermilk samples quantitatively.

Porous TiO2/rGO nanocomposites prepared by cold sintering as efficient electrocatalyst for nitrogen reduction reaction under ambient conditions

Haber–Bosch process as the current dominant artificial NH3 production process in industry, requires relatively high temperature (350–550 °C) and pressure (150–350 atm). Electrocatalytic nitrogen reduction reaction (NRR) as a green and sustainable strategy for ammonia production has raised intensive research interest in recent years but still remains a significant challenge because of the lack of high performance electrocatalysts. In this work, porous TiO2-reduced graphene oxide (TiO2/rGO) nanocomposite as self-supporting efficient electrocatalyst for NRR under ambient conditions were prepared by cold sintering associated with sacrificial template method. The porous TiO2/rGO nanocomposite with grain size of ∼40 nm were prepared by cold sintering process at 220 °C and 147 MPa. Given the 220 °C as cold sintering temperature, anatase TiO2 were preserved as the final phase which exhibit much better NRR electrocatalytic performance than the rutile phase. The oxygen vacancy densities in the nanocomposites were also tuned by heat treatment at 450 °C under different atmosphere, while samples heat treated under H2/Ar atmosphere gave the best electrocatalytic NRR performance with a FE of 8.88 % and an NH3 yield of 7.75 μg h−1 cm−2 at ambient conditions. Experiments also shows that the addition of rGO significantly improved the electrocatalytic NRR performance especially the conductivity. This work not only designed a framework of ceramic nanocomposites based self-supporting and durable electrocatalysts system but also paves a feasible way towards preparing electrocatalysts that are sensitive to high temperature fabrication process.

Impact of water matrix and oxidant agent on the solar assisted photodegradation of a complex mix of pesticides over titania-reduced graphene oxide nanocomposites

New titania-reduced graphene oxide (TiO2-rGO) nanocomposites have been studied to assess their application in the solar assisted photodegradation of a complex mixing of pesticides (pyrimethanil, isoproturon, alachlor and methomyl) at solar pilot plant, opening the opportunity to extend the use of these hybrid photocatalysts to complex wastewater effluents. TiO2-rGO nanocomposites were prepared by hydrothermal method from two commercial TiO2, Hombikat UV-100 (HBK) and Aeroxide® P25 (P25). The effect of two different water matrices (simulated and natural) on TiO2-rGO solar photoefficiency with two pesticides concentrations (5 mg·L−1 and 200 μg·L−1), and the additional use of an electron scavenging as oxidant agent, such as hydrogen peroxide in a comparison with oxygen from air, has been studied in compound parabolic collectors (CPC) photoreactors. Best photocatalytic performances were always found with oxygen over both TiO2-rGO nanocomposites, regardless of the water matrix employed. The use of H2O2 as oxidant agent has not improved the final photocatalytic efficiency in any case. The generation of secondary highly reactive species in the reaction media from reaction of SO4●- /HO radicals, and the more negatively surface charge with low SBET of P25-rGO nanocomposite, besides its lower hydrodynamic size with lower particle aggregation in the reaction conditions, have been responsible to promote the photo-oxidation of the complex pesticides mix studied. Finally, HBK TiO2-reduced graphene oxide (HBK-rGO) nanocomposite has presented excellent solar photoefficiency in the photodegradation of 200 μg·L−1 of each pesticide with the oxygen form air, especially under natural water matrix, because of its large negatively charge surface area where probably recombination of electrons and holes is prevented.

In situ hydrothermal synthesis of TiO2–RGO nanocomposites for 4-nitrophenol degradation under sunlight irradiation

Different amounts (0, 1.0, 3.0, 5.0 and 10.0 wt%) of reduced graphene oxide (RGO) were successfully immobilized to the surface of TiO2 nanoparticles through a hydrothermal process. With addition of RGO, the particle size decreased and the surface area and pore volume increased, resulting in improvement of the reactants’ diffusion and contact area. RGO could be hybridized with titanium atoms, leading to decreasing of the gap energy of TiO2 and more efficient utilization of the solar energy. Hence, the photocatalytic activity of TiO2–RGO composites for 4-nitrophenol degradation was improved accordingly. However, excess amount of RGO (≥ 10.0 wt%) brought about easier recombination of photoelectrons and holes, causing a lower quantum efficiency and photocatalytic activity. The ·OH radicals were the main active species during the degradation process, but the involvement of ·O2− radicals could not be neglected. The pathways for mineralizing of 4-nitrophenol over TiO2–RGO composites under sunlight irradiation were also proposed.

A highly sensitive and selective molecularly imprinted electrochemical sensor modified with TiO2-reduced graphene oxide nanocomposite for determination of podophyllotoxin in real samples

To obtain an electrochemical sensor with high sensitivity and good selectivity for determination of podophyllotoxin (PPT), a kind of graphene composite ([email protected]2) with multi-layer structure and excellent conductivity was prepared by hydrothermal treatment of reduced graphene oxide (rGO) and TiO2 nanoparticles. The composite was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and a sensor using [email protected]2 as a signal amplification element was fabricated by in-situ polymerization of PPT imprinted membrane (PPT-MIM) on the [email protected]2 modified glassy carbon electrode (PPT-MIM/[email protected]2/GCE). The structure and electrochemical performance of PPT-MIM/[email protected]2/GCE as well as the mechanism with superior conductivity of [email protected]2 exceeding the ones of the mixture of TiO2 and rGO (TiO2-rGO) or the pure TiO2 and rGO were explained with different technologies. Under the optimal conditions, the present sensor showed a linear response for the determination of PPT by differential pulse voltammetry (DPV) in the range of 1.0 × 10−9 − 1.0 × 10−7 mol L−1 (R2 = 0.9980) and 1.0 × 10−7 − 3.0 × 10−4 mol L−1 (R2 = 0.9991) with the limit of quantification is 1.0 × 10−9 mol L−1 and the limit of detection of 3.0 × 10−10 mol L−1. The sensor not only possesses high sensitivity and good selectivity, but also exhibits excellent repeatability and stability. It has been used to detect PPT in real samples (including Podophyllum hexandrum, human urine and human whole blood) with satisfied recoveries of 92.0%–102.3%.

TiO2-reduced graphene oxide nanocomposites for the trace removal of diclofenac

Solar photocatalysis using TiO2-reduced graphene oxide (T-RGO) nanocomposite as catalyst is explored for the removal of the last traces of diclofenac (DCF) pollutant from water. T-RGO nanocomposites of different compositions were synthesised by a modified process involving solvothermal treatment of titanium isopropoxide and graphene oxide (GO) in isopropanol medium. The prepared catalysts have been characterised by powder X-ray diffraction, infrared spectroscopy, Raman spectroscopy, transmission electron microscopy, photoluminescence emission spectroscopy, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption–desorption measurements. The degradation of DCF is highly facile under solar irradiation in the presence of T-RGO with more than 98% of the 25 mg/L DCF aqueous solution getting degraded in 60 min followed by complete mineralisation in 100 min. Relevant reaction parameters such as catalyst loading, RGO content in the composite, concentration of DCF and the influence of pH on the degradation of the pollutant were identified and optimised. Reaction intermediates were identified by using LC–MS technique. The degradation followed Langmuir–Hinshelwood mechanism and pseudo first order kinetics. The stability and reusability of the catalyst are established. The efficiency of the catalyst in various real water matrices has also been proved thereby affirming its potential for commercial applications.

Enhanced Transport of TiO2-Reduced Graphene Oxide Nanocomposites in Saturated Porous Media: the Impact of Loaded TiO2 Shape and Solution Conditions

Laboratory sand column experiments were conducted to model the transport behavior of TiO₂ nanoparticle-reduced graphene oxide nanocomposite (TiO₂ NP/rGO) and TiO₂ nanowire-reduced graphene oxide nanocomposite (TiO₂ NW/rGO) using different electrolyte solutions and pH values. The breakthrough curve of TiO₂/rGO nanocomposite shows that the mobility is highly sensitive to ionic strength and pH. Experimental results found that the zeta potential of TiO₂ NW/rGO is more negative due to more hydroxide ions in solution from the TiO₂ NWs. The mobility of TiO₂ NW/rGO is slightly greater than that of TiO₂ NP/rGO at lower ionic strength (1–50 mM NaCl and 1–5 mM CaCl₂), whereas at 10 mM CaCl₂, TiO₂ NW/rGO had weak transport because of physical straining. The ratio of the hydrodynamic diameter (4214 nm) to sand diameter was as high as 0.83. Mobility increased for both TiO₂ NP/rGO and TiO₂ NW/rGO with respect to ionic strength because of electrostatic repulsions. When the pH was 9 with a 10 mM NaCl background solution, the stronger energy barrier between the nanocomposite and sand contributed to the enhanced transport behavior. However, with a solution at pH 3–6, the ripening effect controlled the transport of TiO₂ NW/rGO. The normalized concentrations rapidly climbed to a maximum (0.05 and 0.14) and then decreased gradually after 2 pore volumes. In general, these behaviors may well predict the fate of carbon-based nanoparticles with tailwater or wastewater flowing into soil environments. Graphical Abstract

Synthesis of TiO₂-Reduced Graphene Oxide Nanocomposites Offering Highly Enhanced Photocatalytic Activity.

Photogenerated electron-hole recombination significantly restricts the catalytic efficiency of titanium dioxide (TiO₂). Various approaches have been developed to overcome this problem, yet it remains challenging. Recently, graphene modification of TiO₂ has been considered as an effective alternative to prevent electron-hole recombination and consequently enhance the photocatalytic performance of TiO₂. This study reports an efficient but simple hydrothermal method utilizing titanium (IV) butoxide (TBT) and graphene oxide (GO) to prepare TiO₂-reduced graphene oxide (RGO) nanocomposites under mild reaction conditions. This method possesses several advantageous features, including no requirement of high temperature for TiO₂ crystallization and a one-step hydrothermal reaction for mild reduction of GO without a reducing agent, which consequently makes the production of TiO₂-RGO nanocomposites possible in a green and an efficient synthetic route. Moreover, the as-synthesized nanocomposites were characterized by numerous advanced techniques (SEM, TEM, BET, XRD, XPS, and UV-vis spectroscopy). In particular, the photocatalytic activities of the synthesized TiO₂-RGO nanocomposites were evaluated by degrading the organic molecules (methylene blue, MB), and it was found that the photocatalytic activity of TiO₂-RGO nanocomposites is ~4.5 times higher compared to that of pure TiO₂. These findings would be useful for designing reduced graphene oxide-metal oxide hybrids with desirable functionalities in various applications for energy storage devices and environmental remediation.

TiO2-reduced graphene oxide nanocomposites: Microsecond charge carrier kinetics

In this work, transient absorption spectroscopy studies in the microsecond time scale were carried out to investigate the dynamics of photogenerated electron-hole pairs in TiO2-rGO nanocomposites, prepared by a hydrothermal method, under different atmospheres: N2, O2, and N2 saturated in CH3OH. Under N2 atmosphere, the transient absorption signal detected in the region between 450 and 700 nm dropped as the rGO mass concentration in the composite was raised. The electron transfer from TiO2 to rGO was confirmed by using a model based on fractal surfaces which describes the decay kinetics. In the presence of methanol as hole acceptor, P25-rGO 0.5% and 1% were able to reach the maximum transient absorption faster than the other studied nanocomposites. However, after 10 μs, the P25-rGO 0.1% nanocomposite yielded the highest transient absorption signal and the best conversion and initial reaction rate in the photocatalytic degradation of dichloroacetic acid in aqueous suspensions. The effect of rGO on free electrons was investigated by detecting the transient signal at 980 nm under N2 saturated in CH3OH, for the different samples. It was found that the measured signals followed the same response than at 660 nm further evincing the electron transfer process. No sensitization effect of rGO was observed when the samples were excited at 450 nm.

Reduced graphene oxide modified titania photoanodes for fabrication of the efficient dye-sensitized solar cell

In the present investigation, a novel photoanode material TiO2-reduced graphene oxide (rGO) nanocomposite has been prepared and coated by doctor blade technique for the fabrication of dye-sensitized solar cells. The reduction of graphene oxide to reduced graphene oxide and the crystallization of the material has been achieved by one step thermal annealing of photoanodes at 400 °C. The UV–Visible absorption and photoluminescence studies were confirming the following attributes that, the reduction in band gap energy and the existence of longer lifetime of charge carriers in TiO2-rGO nanocomposite photoanodes respectively. Fourier transform infra-red characterization was confirming the bonding between TiO2 and rGO. The X-ray diffraction pattern fortified the formation of anatase titania with reduced crystallite size due to the presence of graphene. The scanning electron microscopy images of TiO2-rGO nanocomposite photoanodes revealed the presence of spherical nanoparticles and agglomeration of graphene sheets in TiO2 matrix. In addition, Raman and transmission electron microscopy analysis ensured the interaction between TiO2 and graphene. The solar cell with TiO2-rGO nanocomposite photoanode has 6.61% efficiency which was 30% higher than that of the pristine TiO2 nanoparticles based device. The electrochemical impedance spectroscopy analysis proposed the reduction in charge transfer resistance which was achieved in the newly developed photoanode employed devices.

The characterization of TiO2-reduced graphene oxide nanocomposites and their performance in electrochemical determination for removing heavy metals ions of cadmium(II), lead(II) and copper(II)

In this paper, we have studied various mass ratios of titanium dioxide/reduced graphene oxide (TiO2/rGO) nanocomposites modified glassy carbon electrode for the electrochemical detection of Cd(II), Pb(II) and Cu(II). The nanocomposite properties were discussed by FESEM, TEM, XRD, FT-IR, BET and UV–Vis results. UV–Vis spectra for absorption edge of TiO2/rGO showed a slight red-shift and a further narrowed band gap compared to pure TiO2. The simultaneous detection of heavy metal ions such as Pb2+ and Cu2+ carried out using cyclic voltammetry and square wave anodic stripping voltammetry methods. This work showed that the TiO2/rGO (1:1) has high potential as the effective adsorbent of Cd(II), Pb(II) and Cu(II) ions from aqueous solutions compared to a TiO2 electrode, because of the TiO2/rGO (1:1) inherited both features of the GO and TiO2. Improved electrochemical performance can be attributed to the presence of graphene oxide to enhance electrochemical properties.

Influence of TIO2-rGO optical properties on the photocatalytic activity and efficiency to photodegrade an emerging pollutant

TiO2-reduced graphene oxide (TiO2-rGO) photocatalysts with different mass ratios (GO:TiO2 0.1-0.5-1%) were synthesized via a hydrothermal method and compared through their physico-chemical properties and photocatalytic activity in the degradation of an emerging contaminant, clofibric acid. The optical properties of the TiO2-rGO nanocomposites were first estimated in order to calculate the local volumetric rate of photon absorption inside a photocatalytic reactor. Radiation models were solved using the Monte Carlo method. The effect of rGO as well as the photocatalyst loadings on the radiation absorption was evaluated. The lowest photodegradation rate found in P25-rGO 1% was ascribed to an excess of rGO that could well favor charge carriers recombination leading to detrimental photoactivity. A GO/TiO2 mass ratio of 0.5% provided the fastest initial photodegradation rate under the operating conditions studied here. Finally, the photo-efficiency of all these photocatalysts was also analyzed by calculating the quantum efficiency parameter. The highest value of quantum efficiency was achieved with P25-rGO 0.5% at 100 mg L−1, with an increase of 11% compared to the value obtained for P25-rGO 0%.

Preparation of TiO<sub>2</sub>-Reduced Graphene Oxide Nanocomposites for Sunlight Degradation of Methylene Blue

Photogenerated electron/hole recombination greatly limits the catalytic efficiency of TiO2, and recently modification with graphene substance has been regarded as an effective way to enhance the photocatalytic performance of TiO2. When referring to the fabrication of graphene based materials, the reduction process of graphene oxide has been demonstrated to be a key step. Therefore, it is highly required to develop an efficient and simple route for the GO reduction and the formation of TiO2-reduced graphene oxide (RGO) nanocomposites. In this study, TiO2-RGO nanocomposites were prepared by a facile and efficient one-step hydrothermal method using titanium (IV) butoxide (TBT) and graphene oxide (GO) without reducing agents. This method shows several unique features, including no requirement of harsh chemicals and high temperature involved, one-step hydrothermal reaction for mild reduction of GO and crystallization of TiO2 running in parallel, and the production of TiO2-RGO nanocomposites in a green and efficient synthetic route. In addition, the photocatalytic activities of the synthesized composites were systematically evaluated by degrading methylene blue (MB) under sun light irradiation. The TiO2-RGO nanocomposites show a superior photocatalytic activity to the synthesized pure TiO2. It is also found that the concentration of RGO in the nanocomposites plays a key role in the photocatalytic activity. Specifically, the composite with 1 wt % RGO shows the best photocatalytic activity, probably due to the reduction of the electron-hole recombination rate.

Microstructure and Photocatalytic Properties of TiO2–Reduced Graphene Oxide Nanocomposites Prepared by Solvothermal Method

TiO2–reduced graphene oxide (RGO) nanocomposites have been prepared by a solvothermal method using Ti(SO4)2 and graphene oxide (GO) in aqueous/ethanol suspension and their microstructure and optical and photocatalytic properties investigated. The results show that, for all cases, graphene oxide was partially reduced to RGO during the solvothermal process, and anatase TiO2 nanoparticles (~ 10 nm) accumulated on the surface of RGO sheets. The formal valence state Ti4+ was identified in all cases, whereas Ti–O–C bond can form in the TiO2–RGO nanocomposites due to the strong chemical bonding effect. On increasing the mass ratio (x) of GO/Ti(SO4)2, the bandgap of the nanocomposites became narrow, thus extending the absorption wavelength range. TiO2–RGO nanocomposites exhibited good photogenerated electron transfer properties as proven by photoluminescence spectra. However, excessive addition of GO (x ≥ 1/40) decreased the photoelectron transfer performance. The photocatalytic activity was studied under ultraviolet (UV) and visible light, being greatly improved with addition of GO at x = 3/200 then decreased at x ≥ 1/40. TiO2–RGO nanocomposite (x ≤ 3/200) also exhibited better photocatalytic performance than pure TiO2 under visible light, indicating increased absorption.

Synthesis and properties of Ce-doped TiO2-reduced graphene oxide nanocomposite

A nanocomposite of Ce-doped TiO2-reduced graphene oxide (CedT/RGO) was synthesized to evaluate the synergistic effects of constituents on electrical and photoelectric properties. The nanoparticles of titania and CedT were synthesized via the sol-gel method. Then, RGO was prepared using a new version derived from the Hammer's method and hydrothermal methods. Finally, CedT/RGO nanocomposite was synthesized applying hydrothermal method. The nanoparticles and nanocomposite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Field emission scanning electron microscopy (FESEM), Energy dispersive X-ray spectroscopy and elemental mapping. Then, diffuse reflectance and photoluminescence spectroscopy were conducted to study photoelectric and photoluminescence behavior. Finally, electrical conductivity measurement showed that the electrical conductivity of CedT/RGO nanocomposite was increased from 4.6 (μS/cm) to 81.3 (μS/cm). In addition, the absorption spectrum of the nanocomposite was shifted toward visible wavelengths. The band gap of the titania was reduced from ∼3.2 eV to ∼2.65 eV in CedT nanoparticles and it was further reduced to ∼1.95 eV in the CedT/RGO nanocomposite. The FESEM results of the nanocomposite indicated the formation of CedT nanoparticles, smaller than 50 nm, on the RGO sheets. These data indicate that this nanocomposite can be suitable for electrical, photoelectric, photocatalytic and sensing applications.

Enhanced photocatalytic activities of TiO2-reduced graphene oxide nanocomposites controlled by Ti[sbnd]O[sbnd]C interfacial chemical bond

In this study, a hydrothermal-prepared TiO2-RG nanocomposites series exhibited excellent photocatalytic activities, of which the results can be largely influenced by a TiOC interfacial chemical bond between TiO2 nanoparticles and RG nanosheets. A stirring TiO2-RG sample was prepared as contrast analysis using the same wet process except for heating. The characteristics of the composites were tested by X-ray diffraction (XRD), Raman, FT-IR, Differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) analysis. The photocatalytic properties were verified by UV–vis diffuse reflectance spectra (DRS) testing as well as degrading Rhodamine B (RhB). Our research results show that the better TiOC interfacial chemical bond was formed, the narrower band gap of the nanocomposite was then obtained. The best hydrothermal synthesizing temperature was found out as 120°C. The TiO2-RG nanocomposite, as inexpensive, nontoxic and highly photo-catalytically active, may let it have a potential use in various photocatalytic applications.

Non-light-driven reduced graphene oxide anchored TiO2 nanocatalysts with enhanced catalytic oxidation performance

As a single-atom-thick carbon material with high surface and good conductivity, graphene provides an ideal platform for designing composite nanomaterials for high-performance catalytic system. Herein, we obtained a TiO2-reduced graphene oxide nanocomposite (TiO2-RGO) with graphene oxide and tetrabutyl titanate using a facile in situ hydrothermal method. The merit of this method is that the nanocomposites could be produced directly from graphene oxide in the hydrothermal reaction, where the reduction of graphene oxide and the decoration of TiO2 occurred simultaneously. TiO2 nanoparticles anchored on graphene sheets as spacers to keep the neighboring sheets separated. The in situ growth route provides a desirable platform for constructing graphene-supported nanocomposites with improved properties. As one of the major applications of the nanocomposites, we investigate the performance of as-prepared TiO2-RGO as effective non-light-driven catalysts for activating H2O2 in oxidative degradation of the dye. The system was employed in oxidation degradation not only to reach high degradation efficiency but also to avoid any energy consumption. Meanwhile, the proposed catalytic system processes broad-spectrum oxidative degradation activity for different model organic pollutants. Overall, this work could provide new insights into the fabrication of a TiO2-RGO as high performance catalysts and facilitate their application in the environmental protection issues.

Single-step solvothermal synthesis of mesoporous anatase TiO2-reduced graphene oxide nanocomposites for the abatement of organic pollutants

In the present work, we have fabricated a novel mesoporous TiO2–rGO nanocomposite by a facile one-step solvothermal method using titanic sulfate as the TiO2 source. The as-prepared composites were characterized by transmission electron microscopy, X-ray diffraction; UV–Vis diffuse reflectance spectra, X-ray photoelectron spectroscopy and photoluminence spectra. In situ nucleation and anchoring of TiO2 nanoparticles onto a graphene sheet is favorable fpr forming an intimate interfacial contact, and the chemically bonded TiO2–rGO nanocomposites commendably enhanced their photocatalytic activity in the photodegradation of rhodamine B and phenol. The high photocatalytic activity of the as-synthesized nanocomposites are primarily ascribed to the mesoporous structure, efficient charge transportation and separation with enhanced visible light absorption, which come from the appealing nanoarchitecture, for instance, ultra-dispersed and ultra-small TiO2 nanocrystals along with intimate and absolute interfacial contact between the TiO2 nanocrystals and the graphene sheet.

Enhanced photocatalytic activities of low-bandgap TiO2-reduced graphene oxide nanocomposites

In this study, a hydrothermal method was successfully used to prepare a reduced graphene oxide (RG)–titanium dioxide (TiO2) hybrid in 10–20 nm, starting from commercial TiO2 P25 nanoparticles and liquid-exfoliated graphene oxide (GO). Compared to TiO2, an obvious red shift of light absorption (from 3.1 to 2.6 eV) of the as-prepared RG–TiO2 was observed by UV–Vis analysis, and an enhanced photocatalytic degradation of the Rhodamine B (Rh. B) was also observed under Xe lamp exposure test by using the as-prepared RG–TiO2. Multiple characterizations of this RG–TiO2 nanocomposite confirmed that its photocatalytic enhancement could be ascribed to two approaches. Firstly, RG extended the mean free path and photogenerated electrons’ lifetime of TiO2, which minimized electron–hole pairs’ recombination. Secondly, RG expanded the light absorption spectrum of TiO2 from UV range to UV and visible light range. The explication of these improvements was concluded as the energy gap changing and a likelihood of up-conversion photoluminescence mechanism (UCPL). Due to the low-cost, nonpoisonous and excellent photocatalytic properties of RG–TiO2, this material can be applied well in sewage treatment and other related fields.

One-step solvothermal synthesis of TiO2-reduced graphene oxide nanocomposites with enhanced visible light photoreduction of Cr(VI)

Hexavalent chromium, Cr(VI), is a mutagenic and carcinogenic heavy metal environmental pollutant. Photoreduction is one of the remediation methods of the hexavalent chromium Cr(VI), which necessitates design of an efficient catalyst for visible light performance. Here, we report a one-step solvothermal synthesis of TiO2-reduced graphene oxide (TiO2-xRGO) composite catalysts using a mild reducing agent, dimethylformamide (DMF). Nanoscale TiO2 particles in the size range of 4–9 nm were formed on the reduced graphene sheets. The formation of the composite catalysts was accompanied by the appearance of a large fluorescence quenching, which indicates an efficient separation of photogenerated electrons and holes. The composites displayed excellent photoreduction of Cr(VI) in the visible light, which was found to be a function of the weight percentage of RGO in the composite. At the optimum composition of TiO2-xRGO, a maximum removal rate of 96% was recorded, which was higher than that of the pristine TiO2, which showed no appreciable catalytic activity under the same condition. The performance degraded with increasing RGO content in the composite, which can be attributed to the higher electron-hole recombination on the RGO surface. The Cr(VI) photoreduction also exhibited a pH dependence. The highest removal rate was observed in the acidic medium.