High surface area g-C₃N₄ and g-C₃N₄-TiO₂ photocatalytic activity under UV and Visible light: Impact of individual component

Highlights

• High surface area g-C₃N₄ is prepared using a single-step approach.

• Effect of surface area, irradiance, catalyst concentration on g-C₃N₄ photocatalysis.

• Same g-C₃N₄ photonic efficiency for the HCOOH degradation under Visible and UV.

• Optimized g-C₃N4 -TiO₂ concentrations to prepare a UV–visible active photocatalyst.

Abstract

Using sunlight to activate a catalyst and clean up aqueous effluents is an important issue. Graphitic carbon nitride (g-C₃ N₄) is a visible light photocatalyst, whose potential is restricted by its small surface area. This study counters this deficiency by developing a single-step calcination approach that results in g-C₃ N₄ with high surface area (200 m²/g). Then, the impact of surface area, irradiance and catalyst concentration of g-C3N4 on the HCOOH photocatalytic degradation is determined. Results showed that the performance increases linearly with surface area and irradiance, whereas it reaches a plateau as the concentration is increased. A relative photonic efficiency of around 5.5%, independent of the light source was found. Finally, g-C₃ N₄ -TiO₂ mixture was prepared using mechanical grinding and optimized to obtain an active photocatalyst in both visible and ultraviolet regions.

Introduction

Graphitic carbon nitride (g-C₃N₄) is a metal-free visible-light photocatalyst that is cost-effective [1], non-toxic [2] and has high thermal and chemical stability [3]. It has been used as a photocatalyst in various applications such as water-splitting, humidity sensing [4], lithium batteries [5], hydrogen storage [6] and degradation of pollutants [7], [8]. However, the potential of g-C₃N₄ as a photocatalyst is restricted by its small surface area and a large number of electron and hole pair recombination [9], [10]. Moreover, the photocatalytic activity of g-C₃N₄ under UV irradiation is low compared to that of TiO₂. Detailed literature survey on the synthesis, properties, improvement and applications of g- C₃N₄ have been provided by Li and co-workers [10] and Zhu and co-workers [11].

Several studies have focused on improving photocatalytic performance of g-C₃N₄ by increasing its surface area to enhance adsorption and create more active sites for photocatalytic reactions [12], [13], [14], [15]. Many reports in the literature describes a single-step calcination approach for the synthesis of g-C₃N₄. However, this method leads to a final product with low surface area within the range of 5 and 50 m²/g. In general, methods such as thermal or chemical exfoliation, sonication etc. are needed to improve the surface area of g-C₃N₄ obtained after calcination. For example, Xu and co-workers [16] chemically exfoliated bulk g-C₃N₄ to improve its surface area from 43 m²/g to 206 m²/g whereas Dong and co-workers [17] thermally exfoliated g-C₃N₄ at different temperatures (450–550 °C) for 2 h and improved its surface area from 27 m²/g to 151 m²/g. Ou and co-workers [1] on the other hand used sonication to exfoliate a sample of bulk g-C₃N₄ for 15 h and prepared crystalline carbon nitride nanosheets with a surface area of 203 m²/g.

Another strategy to improve the photocatalytic performance of g-C₃N₄ is to increase its charge separation [10], which can be accomplished by modifying it via elemental doping [18], [19], [20], [21], protonation by strong acids [22], or by coupling it with a semiconducting material such as CdS [23], MoS₂[24], Ag₃PO₄ [25], BiVO₄ [26], ZnO [27], CeO₂ [28], ZnWO₄ [29] and TiO₂ [30], [31], [32]. The two-dimensional sheets of g-C₃N₄ consisting tri-s-triazine rings connected by tertiary amine favor its coupling with other semiconducting materials and create a heterojunction to enhance its photocatalytic performance [33]. In particular, g-C₃N₄-TiO₂ has enjoyed great research interest for various photocatalytic applications [34], [35], [36].

Several approaches exist to synthesize g-C₃N₄-TiO₂, including sol-gel method [22], [37], [38], hydrothermal-sonification [39], calcination [40], solvo-thermal method [41], and mechanical mixing followed by calcination [42], [43].

A large number of publications deal with the photocatalytic performance of g-C₃N₄-TiO_2, generally to show an improvement of its performance under (UV)-visible light. However, most of these publications used dyes [18], [20], [22], [25], [30], [32], [34], [37], [41], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55] as pollutant and cannot be considered as true photocatalysis due to the possibility of dye-photo-sensitization reactions under visible light. Only a few reports have described the performance of g-C₃N₄-TiO₂ on the photocatalytic degradation of other organic pollutants such as phenol [55], DBNP (dinitro-butyl-phenol) [56], 2,4 dinitrophenol [45], acetaldehyde [57], NO₂ [42], N₂O [43], and herbicide imazapyr [38]. An overview of the types of dyes and light sources used during catalysis is provided in Table S1.

The above-mentioned publications show better photocatalytic activities of g-C₃N₄-TiO₂ compared to its components, which are generally attributed to the presence of a heterojunction between g-C₃N₄ and TiO₂ reducing electron and hole pair recombination due to close interfacial connection and favorable CB and VB levels [37]. In addition, the large surface area, narrow bandgap energy, small particle size and wider optical absorption of g-C₃N₄-TiO₂ lead to its high photocatalytic performance [38]. However, as mentioned before, very few publications used pollutants other than dyes and by considering only these works, it is not clear if the performance of g-C₃N₄-TiO₂ is actually better than the individual catalysts under visible light. Further, it is also not yet known if the improved performance of g-C₃N₄-TiO₂ obtained under visible light is also preserved under UV irradiation.

In this work, we elaborate g-C₃N₄ materials with surface area varying from 30 m²/g to 200 m²/g using a single-step calcination method. To our knowledge, there is no report so far that describes single step synthesis of g-C₃N₄ with high surface area. Secondly, we study in depth the performance of g-C₃N₄ by investigating the impact of the surface area, irradiance and concentration of g-C₃N₄. Third, using different light sources, the correlation between the impact of the number of photons that can be absorbed by g- C₃N₄ and the rate of disappearance of formic acid is established. Finally, g-C₃N₄-TiO₂ mixture is prepared by simple mechanical mixing and preliminary results on their photocatalytic performance is presented. The role of TiO₂ and g-C₃N₄ in the photoactivity of g-C₃N₄-TiO₂ is determined by studying the degradation of formic acid and measuring photonic efficiencies at different concentration ratios. We also investigated and compared the performance of the number of UV and Visible photons on the photocatalytic degradation of two pollutants. Moreover, we compared the impact of the number of UV and/or visible photons on the performance of g-C₃N₄ alone or present in a mixture.