Elsevier

Catalysis Today

Volume 225, 15 April 2014, Pages 185-190
Catalysis Today

SiO2/carbon nitride composite materials: The role of surfaces for enhanced photocatalysis

https://doi.org/10.1016/j.cattod.2013.12.013Get rights and content

Highlights

  • Carbon nitride is a powerful visible light catalyst made from simple organic molecules, only.

  • Its activity can be remarkable improved when materials interfaces to silica nanoparticles, an inert insulator, are present.

  • This is coupled to the suppression of fluorescence and significant shortening of fluorescence lifetime.

  • This effect is quantified for a variety of systems.

  • The final figure of merit is the model reaction of rhodamin B decomposition where decomposition speed is remarkably accelerated.

Abstract

The effect of SiO2 nanoparticles on carbon nitride (C3N4) photoactivity performance is described. The composite SiO2–C3N4 materials exhibit a higher activity in the photo degradation of RhB dye. A detailed analysis of the chemical and optical properties of the composite C3N4 materials shows that the photo activity increases with higher SiO2 concentration. We found out that the presence of SiO2 nanoparticles strongly affects the fluorescence intensity of the matrix and life time by the creation of new energy states for charge transfer within the C3N4. Furthermore, the use of SiO2 in the synthesis of C3N4 leads to new morphology with higher surface area which results in another, secondary improvement of C3N4 photoactivity. The effect of different surfaces within C3N4 on its chemical and electronic properties is discussed and a tentative mechanism is proposed. The utilization of SiO2 nanoparticles improves both photophysical and chemical properties of C3N4 and opens new possibilities for further enhancement of C3N4 catalytic properties by the formation of composites with many other materials.

Introduction

Photocatalysis has attracted great interest over the last decades, especially for its potential to produce clean and cheap renewable energy without dependence on fossil fuels and without carbon dioxide emission [1], [2]. Photocatalysis applications span from many fields such as: solar fuel production [3], [4], water splitting [5], photo-degradation of pollutants [6], [7], and catalysis of other chemical reactions e.g., for the production of fine chemicals. The photocatalytic operation usually involves photoactive semiconductors, mostly the ones, which consist on metal-based semiconductors like TiO2[8], ZnO [9], Fe2O3[10], CdS [11], and many more. For efficient photocatalysis, the internal recombination rate of the charge carriers should be sufficiently long to allow electron/hole migration to the surface of the catalyst, in order to perform the desired reaction.

As an alternative to well-known metal based semiconductors, carbon nitride (C3N4) has recently attracted considerable attention due to its unique optical, chemical, and catalytic properties, along with its low price and remarkably high stability towards oxidation (at up to 550 °C), which makes it a very attractive material for photocatalytic applications [12], [13]. The synthesis of bulk-C3N4 usually involves simple condensation reactions of monomers such as cyanamide (CA) or dicyandiamide (DCDA), which react under solid state chemistry conditions to form the condensed structure [14]. Nevertheless, due to fast photoinduced charge carrier recombination and its low surface area, bulk-C3N4 exhibits still rather low photocatalytic activity. One approach to enhance C3N4 photoactivity is by chemical modification, such as the integration of hetero-atoms within its structure [12], the use of sol–gel methods in order to give rise to diverse configurations [15], the change of starting monomer [16], [17], etc. These modifications can lead to higher surface area [18], different crystal structure, and various morphologies [19], [20]. Furthermore, the electronic and optical properties of the C3N4 materials can be tuned by these modifications in order to increase photocatalytic performance [21], [22].

An alternative approach to increase semiconductor photo-activity is by combining it with other materials like TiO2[23], ZnO [24], but also insulators as SiO2[25]. The composite materials can form a hetero-junction with different electronic, optical, and chemical properties compared to the starting semiconductor [26], [27]. For example, SiO2 has been successfully used in order to increase the photocatalytic performance of other semiconductors [28], [29]; by increasing the surface area of the catalyst [30], [31] and by its influence on the charge carrier recombination potentially due to surface terminating structures [32], [33]. The higher concentration of charge carriers in the semiconductor surface increases their probability to react with the desired species in the solution, resulting in the enhancement of the photo-activity.

Here we present the preparation and an in-deep analysis of SiO2–C3N4 composite materials for photocatalytic applications. The SiO2–C3N4 was prepared by co-pyrolysis of different ratios of DCDA and 15 nm SiO2 nanoparticles. The chemical structure, morphology, and optical properties of the resulting carbon nitrides were characterized by XRD, FT-IR, SEM, UV–vis absorption, steady-state and time-resolved fluorescence spectroscopy. The photocatalytic activity of SiO2–C3N4 was tested by measuring the degradation of rhodamine B (RhB) dye under visible light illumination in the presence of the composite catalysis.

Section snippets

Synthesis of SiO2–C3N4

The SiO2–C3N4 samples were prepared by grinding different weight ratios of 15 nm commercial SiO2 (0.5 g) and DCDA (varied weight). Then the grinded powder was placed in a crucible and calcined at 550 °C under nitrogen flow with a heating rate of 2.3 K min−1.

Characterization

X-ray diffraction-patterns were measured on a Bruker D8 advance instrument using Cu Kα radiation. Nitrogen sorption measurements were accomplished with N2 at 77 K after degassing the samples at 150 °C under vacuum for 20 h using a Quantachrome

Results and discussion

The composite SiO2–C3N4 materials were formed by mixing 0.5 g of 15 nm SiO2 particles with different amount of dicyandiamide (DCDA), followed by thermally polymerization at 550 °C under nitrogen. For simplicity, the resulting C3N4 are denoted as SiO2-xDCDA, with x indicating the weight ratio. Fig. 1 shows the FT-IR spectra and the XRD pattern of different composite SiO2–C3N4 with respect to pure SiO2 and C3N4. For all the composite materials the FT-IR spectra (Fig. 1a) show a strong peak, of which

Conclusion

In conclusion, we showed that the modification of carbon nitride with SiO2 nanoparticles leads to significant improvement of the C3N4 photocatalytic activity in the photodegradation of RhB. The SiO2–C3N4 composite materials were characterized by X-ray diffraction, FT-IR, SEM, and nitrogen sorption measurements while the photophysical properties of the modified C3N4 were carefully examined by UV–vis absorption, steady-state emission and time-resolved fluorescence spectroscopy. The strong

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