Review ArticleGraphitic C3N4 based noble-metal-free photocatalyst systems: A review
Graphical abstract
Introduction
Graphitic carbon nitride (g-C3N4) is a polymeric layered material, structurally analogous to graphene [1]. In contrast to the pure C constituent of graphene, g-C3N4 is composed of C and N with some impurity of H, which all are abundant raw materials. Semiconducting properties of g-C3N4 also drastically distinguish it from graphene. The band gap of bulk g-C3N4 is ∼2.7 eV, and it is a medium band gap semiconductor. Pertaining to its yellow color the optical absorption of g-C3N4 lies around 460 nm making it an interesting material for harvesting solar energy. Furthermore, the thermal and chemical stability of g-C3N4 in an aqueous suspension phase and under photocatalytic reaction condition makes it an interesting material [2].
This g-C3N4 is regarded as the oldest synthetic polymer first reported by Berzelius and Liebig in the year 1834 and named as ‘melon’ [1]. A flow sheet diagram is provided in Fig. 1 showing a summary of historic developments in understanding g-C3N4 and its application in photocatalysis. In 1922, Franklin found the empirical composition of ‘melon’ to be C3N4. Next, Pauling and Sturdivant derived tri-s-triazine type structure of C3N4 in the year 1937. By 1940 it was known that this material ‘melon’ has a graphite structure as reported by Redemann and Lucas [2]. Photocatalysis received enormous attention after Fujishima and Honda reported photolysis of water on TiO2 in 1972 [3]. A wide variety of materials, mainly inorganic semiconductors were evaluated for photocatalytic application. However, nobody paid attention to making use of g-C3N4 in photocatalysis until Wang et al. first reported in 2009 [4].
Since the pioneering photocatalytic studies by Wang et al. in 2009 [4], [5], g-C3N4 has become the focus of research on photocatalytic materials. Lately, a comprehensive review article is presented by Ong et al. [2]. Besides some other relevant reviews, feature articles and perspectives on g-C3N4 are important to read [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. A record of yearly publications on photoactive applications of g-C3N4 elucidates the magnitude of research interest in this material (Fig. 2). Wang et al. put forward this polymeric material essentially as a metal-free visible-light active photocatalyst [4]. An efficient overall water splitting ability using solar energy is considered a Holy Grail of photocatalysis. Once this is realized, a commercial scale production of green and renewable chemical fuel will become a viable process. There are several requirements for developing an efficient photocatalyst for water splitting under solar irradiation: (1) good properties for harvesting solar light, (2) a band gap of suitable energy with valence and conduction band appropriately positioned for the desired reactions, and (3) good stability of the photocatalyst under experimental conditions. In principal, this thermally and chemically stable polymeric semiconductor g-C3N4 fulfills the band gap requirement for harvesting solar energy with a band structure suitable for both water oxidation and reduction reactions. At present, there are a lot many ongoing efforts on the development of efficient and sustainable noble-metal-free photocatalyst systems.
Environmental pollution and sustainable supply of greener energy are two of the main global challenges of the current era. Considering the Sun as an almost inexhaustible and primary source of energy, lately, there are many interests in developing semiconductor materials for harvesting solar energy to produce cleaner fuels and resolve the environmental issues. Lately, visible light active photocatalysts are getting enormous attention for applications to environment and energy sectors. Production of green and renewable energy carrier, H2 from water, reduction of CO2, synthesis of fine chemicals and remediation of environmental pollutants are the main explored reactions.
Polymeric g-C3N4 is a metal-free medium band gap p-type semiconductor with a reliable chemical and thermal stability. Furthermore, its versatile optical and electronic properties with a band gap ∼2.7 eV make it an attractive candidate for harvesting solar energy. TiO2 is the most popular photocatalyst material due to its robust reactivity, availability, and chemical stability, however, it absorbs only UV light that limits its application [16]. Among various photoactive applications, g-C3N4 have been widely employed as a visible light active photocatalyst for degradation of organic pollutants, H2/O2 evolution half reactions, complete water splitting, reduction of CO2 and organic synthesis (Fig. 3). The semiconductor, g-C3N4 is composed of earth-abundant elements. However, the noble/precious metals which are mainly loaded as a co-catalyst e. g. Pt, to avoid the recombination of photogenerated electron-hole pairs are the costly constituents. A photocatalyst system is comprised of a semiconductor or a junction of semiconductors along with a sensitizer and/or a co-catalyst. Here in this review we will discuss g-C3N4 based photocatalyst systems which are altogether free from noble-metals and important for sustainable development.
Visible-light-driven photocatalyst systems are interesting for efficient harvesting of the solar spectrum. The band gap of pure g-C3N4 (∼2.7 eV) corresponds to absorption of blue-light up to 450 nm, hence inactive towards major part of the solar radiation comping to the earth. Furthermore, this relatively large band gap of the pure g-C3N4 over-energizes the ideally required potential (1.23 eV) for the most desired water splitting reaction. Wang et al. applied various methods of doping [17], [18], copolymerization [19], [20], and dye sensitization [21] to modify g-C3N4 for an optimum utilization of solar spectrum for a specific photoactive reaction. Next, the development of a noble-metal-free or precious-metal-free photocatalyst system is important for making it sustainable. Intrinsically, g-C3N4 is a layered material in which C3N4 sheets are connected by Van der Waal forces. Hence single and a few layer sheets of g-C3N4 are obtained upon the breakdown of these weak forces. Nanoscale materials offer unique regime of catalysis in between homogeneous catalysis and heterogeneous catalysis. However, the stability of nanoscale materials is challenging and needs attention for the important recycling of the material. Furthermore, nanomaterials based photocatalysts are more effective with the greatly enhanced availability of active surface sites [22]. Wang et al. have done extensive research on the development of g-C3N4 based nanoscale materials for photocatalytic applications e. g. H2 evolution [22], [23], [24], [25], [26], [27], [28], O2 evolution [29], [30], and CO2 reduction [31]. In this review, we are summarizing g-C3N4 based nanomaterials for all the various noble-metal-free applications in photocatalysis.
Section snippets
Overall water splitting
To mimic the natural photosynthesis, a semiconductor-based photocatalytic water splitting into H2 and O2 has been the focus of many researchers. However, the bottleneck of overall water splitting is the formation of oxygen–oxygen bond that requires the transfer of four electrons in a single step. The separation of photocatalytically cogenerated H2 and O2 is yet another challenging task. There are only a few studies on overall water splitting over g-C3N4 based noble-metal-free photocatalyst
Conclusions and future perspectives
In all the various photoactive applications, noble-metal-free nanoscale photocatalyst systems based on g-C3N4 demonstrated favorable results for its versatile applications. Especially, the good achievements in H2 evolution reaction surpassing that of with the Pt co-catalyst are highly promising towards a sustainable production of green and renewable energy. Pertaining to the high stability of g-C3N4 and its nanocomposites under different conditions, it has gained attention for a number of
Acknowledgements
The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for the financial support to conduct this research work.
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