Enhanced photocatalytic activities of three-dimensional graphene-based aerogel embedding TiO₂ nanoparticles and loading MoS₂ nanosheets as Co-catalyst

Highlights

• Novel 3D MoS₂/P25/graphene ternary aerogel was obtained by hydrothermal process.

• MoS₂/P25/graphene-aerogel with porous structure has efficient charge separation.

• Photocurrent of ternary aerogel was 6 times higher than that of P25 at 0.6 V.

• Photodegradation MO of MoS₂/P25/graphene-aerogel was completed within 15 min.

Abstract

A novel graphene-based three-dimensional (3D) aerogel embedded with two types of functional nanomaterials had been prepared by a facile one-pot hydrothermal process. During the hydrothermal reaction, graphene, TiO₂ nanoparticles and MoS₂ nanosheets were self-assembled into the 3D interconnected networks aerogel, where the uniformly dispersed TiO₂ nanoparticles were densely anchored onto the graphene nanosheets and decorated with the ultrathin MoS₂ nanosheets. The UV–vis DRS and PL spectra measurement shows that the MoS₂/P25/graphene aerogel exhibits enhanced light absorption and efficient charge separation properties. As a new photocatalyst, the photocatalytic activity was evaluated by photoelectrochemical test and photodegradation methyl orange (MO) under UV irradiation, an improvement of photocurrent was observed, as 6 times higher for MoS₂/P25/graphene aerogel (37.45 mA/cm²) than pure P25 at +0.6 V, and the fastest photodegradation of MoS₂/P25/graphene aerogel was found within 15 min. The improved photocatalytic activity is attributed to the porous structure, good electrical conductivity and the maximization of accessible sites of the unique 3D graphene aerogel, the increasing active adsorption sites and photocatalytic reaction centers for the introduction of MoS₂ nanosheets, and the positive synergetic effect between the three components in this hybrid. This work demonstrates that the as-prepared MoS₂/P25/graphene aerogel may have a great potential application in photoelectrochemical hydrogen production and pollution removal.

Introduction

As one of the most investigated functional material in semiconductor photocatalysis, titanium dioxide (TiO₂) has been widely used in the fields of environmental pollutant degradation and energy conversion for its non-toxicity, effectiveness, low cost, and chemical stability [1], [2], [3], [4], [5], [6]. However, the photocatalytic activities of titanium dioxide are seriously limited by the photocatalytic sensitivity in the UV region and the fast recombination of photogenerated electron–hole pairs [5], [7], [8]. Therefore, many positive and effective approaches have been employed to improve the photocatalytic activity of TiO₂, such as modifying the surface [9], optimizing the structure [10], doping of metal or nonmetal elements [11], [12], [13], [14], [15], [16], and coupling of other photosensitive materials [17], [18] to modulate the optical absorption and facilitate the charge transfer.

Graphene, a flat monolayer of carbon atoms, was first reported by Andre Geim and Kostya Novoselov a decade ago [19], leading to a revolution in the graphene related research field. Owing to its fast two-dimensional (2D) electron-transfer kinetics, large surface areas, and superior mechanical, thermal and chemical stability [20], [21], [22], [23], graphene was found to be an attractive supporting material to load functional materials. Many attempts have been paid to composite the TiO₂ with graphene, these TiO₂/graphene composites have been reported to exhibit enhanced photocatalytic activity, but they still suffer from the low usage of natural sunlight, recombination of the electron–hole pairs and the limited amount of active reaction centers [24], [25], [26], [27], [28], [29], [30]. Recently, molybdenum disulfide (MoS₂), a quasi-two-dimensional transition metal dichalcogenide with layered structure, has been highlighted as a promising hydrogen evolution catalyst. For instance, Chhowalla's [31] group reported the chemically exfoliated MoS₂ nanosheets possessed excellent activity for hydrogen evolution reaction. Lu's [32] group reported the limited-layered MoS₂, as a high active co-catalyst, loaded on reduced graphene oxide (RGO) sheets as an alternative of Pt for hydrogen evolution. In particular, the synergetic effect of MoS₂ and graphene as co-catalysts for TiO₂ nanoparticles which exhibited superior photocatalytic H₂ production activity, was first reported by Yu's [33] group. Those findings explicitly indicate the significant potential of ultrathin MoS₂ nanosheets in photocatalysis. However, it has been reported that large accessible surface areas in the above 2D graphene-based macrostructures are inevitably sacrificed given that graphene nanosheets are particularly prone to form irreversible agglomerates, limiting the accessible surface areas for electrolyte ion infiltration and leading to a huge loss of electro-active sites [34], [35], [36]. To overcome these obstacles, 3D graphene macrostructures (hydrogels and aerogels) have been developed recently, which are composed of a unique 3D porous graphene skeleton [37], [38], [39], [40], [41]. It is noted that this graphene aerogels or hydrogels can not only preserve the large accessible surface areas of the graphene sheets but also possesses highly porous structures with pore sizes of several micrometers, making the most possible for embedding nanoparticles in the 3D graphene nanocomposites during a reduction and self-assembly process owning to its porous structure, good electrical conductivity and the maximization of accessible sites. Till now, most of these composites are consisted of graphene and a single type of nanostructure, and very few studies are concentrated in the assembly of 3D graphene-based aerogel with deposited multifarious types of nanomaterials. Dramatically, a series of unique ternary graphene-based nanomaterials has been reported with significantly enhanced performance, benefitting from the advantages of each component and synergistic interaction among the three components [33], [42], [43], [44], [45].

In this work, aim at solving the fast recombination of photogenerated electron–hole pairs and the limited photocatalytic reaction sites of TiO₂ under UV irradiation. To improve the photocatalytic activity of TiO₂ (P25, 20% rutile and 80% anatase), we embedded it in 3D graphene structure and decorated with MoS₂ nanosheets to enhance light absorption, accelerate charge separation and increase reaction sites, we firstly synthesize a ternary 3D graphene-based MoS₂/P25/graphene nanocomposite aerogel via a facile one-step hydrothermal reaction. Such 3D MoS₂/P25/graphene aerogel shows an interconnected macroporous framework of graphene sheets with uniform deposition of TiO₂ nanoparticles and MoS₂ nanosheets. Taking photoelectrochemical measurement and photodegradation methyl orange (MO) (0.02 g/L) test of the as-prepared samples under UV irradiation, the ternary MoS2/P25/graphene-aerogel shows the best photocatalytic properties, which may benefit from the porous structure, good electrical conductivity and the increasing accessible sites of 3D graphene aerogel, as well as the novel optoelectronic and catalytic properties of MoS₂ nanosheets, and the positive synergetic effect of this hybrid. The durability was also investigated via a more than 80 cycles photoresponse measurement and four cycling runs in the photodegradation of MO. These excellent properties demonstrate that this 3D ternary graphene-based structure is an efficient method to improve the photocatalytic activity of TiO₂.