Elsevier

Ceramics International

Volume 41, Issue 9, Part A, November 2015, Pages 10634-10643
Ceramics International

Preparation and photocatalytic performance of magnetic TiO2–Fe3O4/graphene (RGO) composites under VIS-light irradiation

https://doi.org/10.1016/j.ceramint.2015.04.163Get rights and content

Abstract

In this work, TiO2–Fe3O4/grapheme (RGO) composites with good magnetism and photocatalytic activity were prepared by a facile hydrothermal method with RGO and magnetic TiO2 as starting materials in ethanol–water solvent. The structural and magnetic features of the prepared composite photocatalysts were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), transmission electron microscopy (TEM), thermogravimetric and differential thermogravimetry analysis (TG-DTG), UV–vis diffuse reflectance spectra (UV–vis/DRS) and vibrating sample magnetometry (VSM). The results showed that the TiO2 coated Fe3O4 nanoparticles with a strong response to external magnetic fields were dispersed uniformly on the surface of RGO nanosheets. The composite catalysts can cause an obvious red shift of UV–vis spectra compared with pure TiO2. The adsorption and photocatalytic activity of the composite catalysts were evaluated by choosing methylene blue (MB) as organic pollutants. The photocatalytic degradation of MB by TiO2–Fe3O4/RGO composites under visible light irradiation was examined by varying the operational parameters such as catalyst amount, irradiation time, pH and initial MB concentration. The photocatalytic reactions obeyed pseudo-first-order kinetics according to Langmuir–Hinshelwood model. The repeatability of photocatalytic activity was also tested. A plausible mechanism was proposed and discussed on the basis of experimental results.

Introduction

Titanium dioxide (TiO2) has been considered as the most widely used semiconductor oxide photocatalyst due to its appealing attributes such as low cost, corrosion resistance, excellent chemical stability, non-toxicity and high photocatalytic activity [1], [2]. However, uses of TiO2 photocatalysts in applications such as wastewater treatment still have been limited because there are major bottleneck drawbacks associated with TiO2 photocatalysis. TiO2 has a high-energy gap (Eg=3.2 eV) and for this reason, it can be only triggered by near UV radiation (λ≤380 nm), which accounts for only about 3–4% of the solar spectrum [3], [4], [5]. Thus, the possibility of utilizing solar light as an energy source in TiO2 photocatalysis is limited. Besides, the photogenerated electron and hole pairs are liable to recombination, resulting in low quantum efficiency of TiO2. The problem of separation and recovery of nanometer sized TiO2 particles from aqueous solution would also hinder the application of this technology [6].

In recent years, many attempts have been made to not only extend the spectral response of TiO2 into the visible region but also reduce the recombination of photogenerated electron and hole pairs, thus enhancing its photocatalytic activity. Among those attempts, modification TiO2 with suitable non-metal atoms is one of the most efficient methods for improving the above two performances of TiO2. There have been many papers published about improved TiO2 photocatalysts doped with carbon [7], [8], nitrogen [9], [10], sulfur [11], halogen [12] or using codoped materials [13], [14]. Graphene, a new carbonaceous material on applications of optical, electronic, intercalation materials, biomaterials and catalytic fields has attracted considerable attention since its discovery. Graphene oxide (GO) can be obtained using traditional Hummers method [15], then the produced GO can also be reduced into graphene (RGO) under the action of reducing agent [16]. Because of its single-atom thick sheet which arranged by sp2-bonded carbon atoms in a hexagonal lattice [17], grapheme shows outstanding mechanical, thermal, optical, and electrical characteristics such as high thermal conductivity (~5000 W m−1 K−1) [18], excellent mobility of charge carriers (200,000 cm2 V−1 s−1) [19] and large specific surface area (~2600 m2 g−1) [20]. Graphene-based materials have been widely used as transparent conducting electrodes [21], supercapacitors [22], optoelectronic devices [23] and catalysts [24]. In the literature we can find that Graphene is a very attractive dopant for modification of TiO2 to give TiO2 additional functionalities, such as structure, surface area, adsorption capacity, and photocatalytic activity [25], [26], [27]. Graphene-doped other kind of photocatalysts such as MoS2, ZnS, Bi2O3, ZnO, Fe2O3, SnO2 and Ta2O5 also showed an enhanced photocatalytic performance [28], [29], [30], [31], [32], [33]. However, most methods for preparing these modification TiO2 materials are high temperature processes, resulting in expensive preparation cost, high energy consumption and most important the low surface area due to the undesirable sintering of nanocrystallites [2]. Strategies that have been investigated to address the obstacle of separation and recovery of suspended fine TiO2 catalyst particles from water include the synthesis of magnetic TiO2 photocatalysts [34], [35]. As a typical magnetic material, Fe3O4 can be used to solve the problem of TiO2 photocatalyst recovery when it is combined with TiO2. The magnetic TiO2–Fe3O4 composites showed rapid aggregation under the influence of a magnetic field, which can be useful for separation of these particles in applications [36], [37], [38]. Meanwhile, according to the energy level theory, the presence of Fe3O4 can also enhance the photocatalytic activity of TiO2 [39], [40]

Based on the magnetic responsive characters of Fe3O4 and the enhancements in photocatalytic activity of TiO2 doped with graphene, in this work, the magnetic nanometer TiO2–Fe3O4/RGO composite photocatalysts were prepared by a simple hydrothermal synthesis method. The hydrothermal reaction condition is much milder than that of conventional methods. The photocatalytic activity of the magnetic TiO2–Fe3O4/RGO composite photocatalysts prepared by hydrothermal synthesis method was investigated for the photodegradation of a model organic dye methylene blue (MB) in a suspension system. The main objective of our work is to evaluate the potential application of this kind of material in the disposal of organic pollutants in water. The results reported here are critical and necessary inputs in the development of photocatalytic degradation/separation process that can use the novel visible light and magnetosensitive composite photocatalysts from an academic as well as industrial point of view.

Section snippets

Materials and reagents

Natural graphite powder (CP, ≥95%), tetrabutyl titanate (TBOT, AR grade), polyethylene glycol (PEG), methylene blue (MB), chitosan (CP) were from Sinopharm Chemical Reagent Co. Ltd., China. All other reagents used in this study, including sodium borohydride, FeCl2·4H2O, concentrated sulfuric acid, potassium permanganate, concentrated hydrochloric acid, hydrogen peroxide, absolute ethyl ethanol and acetic acid were purchased from Tianjin Kermel Chemical reagent Co. Ltd., China. All these

Structural characterization

The XRD patterns of TiO2, Fe3O4, RGO and TiO2–Fe3O4/RGO composite are illustrated in Fig. 1. The intensified diffraction peaks of TiO2 at 2θ=25.3°, 37.8°, 48.2°, 54.0°, 55.1° and 62.7° revealed a pure anatase phase with tetragonal structure [41]. A series of characteristic peaks of pure Fe3O4 at around 2θ of 30.2°, 35.5°, 43.3°, 53.2°, 57.2° and 63.1° are related to the reflection of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of magnetite Fe3O4, well indexed to the typical cubic inverse

Conclusions

TiO2–Fe3O4/RGO composite photocatalysts were successfully prepared using a simple hydrothermal method and characterized by XRD, FT-IR, SEM, TG-DTG, UV–vis and VSM techniques. Titania coated Fe3O4 nanoparticles are found to be evenly dispersed on RGO nanosheets. The TiO2–Fe3O4/RGO composite can be activated by absorbing both the ultraviolet and visible light. The composite catalyst has a high adsorption capacity, good magnetism at room temperature and excellent photocatalytic activity for

Acknowledgement

We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant no. 21377018 and 20977013) and the Fundamental Research Funds for the Central Universities (DUT15ZD118).

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