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

Abstract

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.

Introduction

Titanium dioxide (TiO₂) has long been targeted for various applications, particularly in environmental pollution control, conversion and energy storage, sensors, photovoltaics and Li batteries because of its unique photo-electric properties [1], [2], [3], [4]. TiO₂ has the advantage of good chemical stability, nontoxicity and relatively low cost. However, many problems remain unresolved in the TiO₂ photocatalyst system for practical applications, such as narrow spectrum response range and low separation probability of the photoinduced electron–hole pairs. Therefore, many techniques have been examined to extend the spectral response of TiO₂ into the visible region and to enhance its photocatalytic activity. Doping with metallic cations or non-metallic anions has been widely used for the modification of TiO₂ to improve its photocatalytic activity or to extend its light absorption into visible region [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. It is known that metal ion doping can modify the surface properties of TiO₂, hinder the recombination of photogenerated electron–hole pairs and increase the amount of the active sites. Especially, photocatalytic activity of TiO₂ can be significantly enhanced by doping with the lanthanide ions having 4f configuration. Among them, neodymium doping has attracted more interest due to comparatively large (Nd⁺³) ion which produces a localized charge perturbation during substitutional doping into TiO₂ lattice and increases its photocatalytic activity [7], [10].

Another common method for enhancing the photocatalytic efficiency of TiO₂ is immobilization TiO₂ nanoparticles on a co-adsorbent surface such as mesoporous materials, zeolites, alumina, silica or carbon based materials [15], [16], [17], [18]. Among these, carbonaceous materials including activated carbon, carbon nanotubes, and graphene are of great interest due to their unique pore structure, electronic properties and adsorption capacity [19], [20], [21], [22], [23]. Recently, graphene with its unique structure of one-atom thick planar sheets of sp²-bonded carbon atoms closely packed in a honeycomb crystal lattice has attracted a great deal of scientific interest due to its excellent mechanical, electrical, thermal, optical and surface properties [23]. As compared with other carbonaceous materials, graphene has many advantages, including high surface area, high transparency and good interfacial contact with adsorbents. Therefore, it is advantageous to investigate simple and effective approaches for preparing graphene based composites and expand their practical applications. Due to conjugated structure of graphene, the combination of TiO₂ and graphene may be an ideal preference to achieve an enhanced charge separation in electron-transfer processes. In a recent investigation, P25–TiO₂ dispersed on graphene nanosheet was reported to show enhanced photocatalytic activity [24]. Williams et al. mixed ultrasonically TiO₂ particles and graphene oxide (GO) colloids, followed by UV-assisted photocatalytic reduction of GO to prepare TiO₂–graphene composites [25]. In another case, TiO₂–graphene composite materials were prepared by self-assembly of TiO₂ nanoparticles grown on graphene by a one-step approach with the assistance of an anionic surfactant [21]. Zhang et al. [22] prepared P25–graphene composite for methyl blue degradation using a hydrothermal method. Sonophototcatalytic activity of graphene oxide based Pt–TiO₂ composites for DBS degradation was investigated by Neppolian et al. [26]. Farhangi et al. [27] prepared Fe doped TiO₂ nanowires on graphene sheets for photodegradation of 17β-estradiol (E₂). Nevertheless, understanding of metal doped TiO₂/graphene photocatalysis system is unclear.

In this study, we have synthesized Nd doped TiO₂ (Nd/TiO₂) nanoparticles by the sol–gel method and then successfully decorated on graphene sheets by hydrothermal process. The effect of neodymium doping on TiO₂ and graphene–TiO₂ (GR–TiO₂) composite catalysts was investigated by degradation of methyl orange dye under visible-light illumination as a model reaction. The prepared GR–Nd/TiO₂ composites showed extended visible-light absorption and enhanced photocatalytic activity than those of pure TiO_2.