Photochemical fabrication of SnO2 dense layers on reduced graphene oxide sheets for application in photocatalytic degradation of p-Nitrophenol
Graphical abstract
Introduction
Tin dioxide (SnO2), an n-type semiconductor with a band gap of ∼3.57 eV, has attracted great interest in the fields of gas-sensor [1], [2], lithium rechargeable battery [3], sensitized solar cells [4], and photocatalysis [5], [6], [7], [8] because of its excellent electronic and optical properties and super stability. SnO2 can be modified with other semiconductors [9], [10], [11], noble metals [12], [13], or doped elements [14], [15] to improve the electron-hole separation efficiency for efficient photocatalysis. Combining SnO2 with graphene materials has become a new trend considering improved understanding of the properties of graphene materials [16], [17], [18].
Graphene, a single layer of sp2–bonded carbon atoms, has been making a profound impact in many areas of science and technology ever since its discovery [19]. Graphene can be used as a photocatalyst support material because of its unique properties, such as excellent electrical conductivity, high theoretical specific surface area, exceptional transparency, and adsorption activity [20]. Graphene oxide (GO), a derivative of graphene, is used as starting material for easy and efficient construction of graphene-based composites; GO is further changed into reduced graphene oxide (rGO) after combining with the target materials. In addition to many semiconductor–rGO composite photocatalysts [21], [22], [23], [24], various SnO2–rGO composites have been fabricated to enhance the photocatalytic activity of SnO2 through chemical bath method [25], hydrothermal method [26], and solid-state chemical method [27]. For most composites, the surface of the rGO sheet is generally covered with discrete semiconductor nanoparticles and a large proportion of the surface is exposed. The composite exhibits high flexibility and contains many wrinkles induced by the flexible rGO sheets. A composite with a regular morphology is difficult to obtain. Moreover, such composite sheets, like soft cloth, are easy to stack and aggregate, resulting in the decrease of active sites. Although changing the morphology of a semiconductor (e.g., using nanorods with larger sizes instead of fine particles) or fabricating a specific assembly of semiconductor and rGO sheets could reduce the aggregation and enhance the activity to a certain extent [28], [29], [30], [31], the exposure of rGO surfaces was still difficult to prevent. The exposed rGO surfaces might adsorb some unpredictable molecules when the composites are used or preserved, resulting in negative effects on their activity and stability. In addition, the number of semiconductor particles on a single rGO sheet is too small, and many rGO sheets had to be used to meet the requirements of the photocatalytic activity of a composite. These disadvantages not only waste rGO, but also add chances of aggregations among composite units and the total exposed surface areas of rGO sheets. The fabrication of semiconductor dense layers that could completely cover the two sides of an rGO sheet and eliminate the flexibility of the corresponding composite is thereby necessary but still a challenge.
Herein, a new room-temperature photochemical method, which irradiates ultraviolet (UV) light to the GO sheet-dispersed surfactant-free solution of SnSO4 and H2SO4, was introduced to prepare SnO2 dense layers that completely cover the rGO sheets. Different from most semiconductor-rGO composites, the obtained composite unit was composed of two SnO2 dense layers and an rGO sheet sandwiched in between. To distinguish the sheet-like structure of rGO or GO, the shape of the composite unit in this study was called “flake”. The flake exhibited a fine non-flexible flat structure. The large area of close contacts between the SnO2 layer and rGO sheet was beneficial to the separation of electron-hole when the composite was used in photocatalysis. The exposure of the rGO surfaces was avoided as much as possible, increasing the stability of the composite. To explain the photocatalytic activity of such composite, p-nitrophenol (PNP), one of the most hazardous refractory pollutants with high stability and solubility in water [32], [33], was chosen as the target pollutant to be photodegraded. The composite exhibited 12 times higher activity than the pure SnO2 particles because of its specific structure. Moreover, after exposure to ambient condition for more than 12 months, the composite still exhibited the nearly constant activity and had similar UV and solar photocatalytic activity. These observations demonstrate that the new SnO2-rGO composite obtained by the photochemical method is promising and practical for environmental remediation applications.
Section snippets
Preparation of GO
GO was synthesized from graphite powder by modified Hummers’ method [34]. Briefly, 3.0 g of powdered flake graphite and 1.5 g of NaNO3 were added into 69 mL of concentrated H2SO4 into a flake cooled in an ice bath under agitation. Then 3.0 g of KMnO4 was added to the suspension slowly maintaining the vigorous agitation at a temperature below 10 °C. The flask was taken out and placed into a water bath at a temperature of 35 °C. After 30 min, 90 mL of deionized water was slowly stirred into the paste to
Morphology and composition
Morphological and structural features of GO, SnO2-rGO (SR–6 h), and pure SnO2 were examined with SEM and TEM. Fig. 1A shows the SEM image of GO pristine sheets. The GO sheets are very thin, full of wrinkles, and edges are difficult to discern, which can also be reflected in the TEM image (Fig. 1B). The high resolution TEM image (HRTEM) of a GO sheet (inset of Fig. 1B) shows no lattice fringes, confirming the disordered nature of GO as described elsewhere [35]. Fig. 1C shows the SEM image of SR–6
Conclusion
SnO2 dense layers on rGO sheets were fabricated by UV irradiation of a mixed aqueous solution of GO, SnSO4, and H2SO4. The irradiation time was essential and 6 h of irradiation was optimal for obtaining the dense layers and the composite unit with a flat flake-like structure composed of two dense layers and an rGO sheet sandwiched in between. During the formation of the dense layers, the GO was gradually reduced by electrons generated from the UV excitation of SnO2. The dense layers had high
Acknowledgments
This work was co-supported by the National Natural Science Foundation of China (No. 21571068), the Research Project of Chinese Ministry of Education (No. 213029A), the Natural Science Foundation of Guangdong Province (No. 2015A030313387), the Science and Technology Program of Guangzhou (No. 201607010301) and the Special funds for Discipline Construction in Guangdong Province (No. 2013KJCX0057).
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