State-of-the-art review of morphological advancements in graphitic carbon nitride (g-CN) for sustainable hydrogen production

Highlights

• Hydrogen as alternate energy vector can combat increasing global energy demands.

• Literature review of g-C₃N₄ for solar-to-hydrogen production via water splitting.

• The role of morphology, pore texture/surface area and heterojunction formation in H₂ production is evaluated.

• Highlights important state-of-the-art developments in high surface area g-C₃N₄ for H₂ production.

• Focuses of water-splitting fundamentals, efficiencies, stabilities and advancements.

Abstract

Considering the technological benefits, the generation of hydrogen (H₂) via solar-powered enabled water splitting is not only an ideal route to harvest and stock the sustainable sun-energy for meeting the increasing energy demands but also to mitigate the global warming by reducing carbon footprints. Ideally, the photocatalyst involved in the process of solar-to-hydrogen (STH) production should remain unaffected by the undesirable catalytic processes and charge separation and transportation taking place at its surface. In the quest of lowering down the cost of producing H₂, the challenge of developing a cheaper photocatalyst material which can efficiently split water into hydrogen has become more prominent. Although, the metal-free semiconductor graphitic carbon nitride (g-CN or g-C₃N₄), owing to its 2D architecture and apposite band-energy gap and relatively lower production cost has shown immense potential in H₂ production via water splitting, yet the concerns for its low specific surface area (SSA) and rich defect density have limited its photocatalytic performance and water-splitting efficiency. This mini review features the recent research accomplishments made in the design strategies of g-CN nanostructures based on its pore texture/surface area tailoring, dimensionality tuning, band-gap modulation, defect control, metal-doping and semiconductor heterojunction formation and the corresponding application in H₂ generation. The reviewing of important state-of-the-art developments and prospect of high surface area g-C₃N₄ can provide new avenues in designing the g-CN with high SSA for utilization in H₂ evolution, fuel cell, solar cell, supercapacitor and lithium battery.

Introduction

As energy demands continue to increase worldwide, the focus has shifted from exploiting the finite non-renewable resources (petroleum products, natural gas etc) to the generation of cleaner, greener alternative fuels, such as H₂ [[1], [2], [3]]. Currently, 85% of global H₂ production is fulfilled by the combustion of fossil fuels, resulting in approximately 500 metric tonnes of carbon dioxide (CO₂) per year as a byproduct [4,5]. In the quest to lower CO₂ emissions and associated global warming, the H₂ production via photocatalytic water splitting has emerged as an ideal means to overcome energy and environmental threats imposed by the continuous usage of fossil fuels [[6], [7], [8]]. Despite continuous efforts, the reported STH conversion efficiency falls far below the threshold target which is required for a highly cost-effective and efficient solar hydrogen production system for commercial applications [[9], [10], [11], [12]]. Till now, a variety of process technologies have been devised to produce H₂ including thermal (coal gasification, pyrolysis of biomass, renewable liquid and biooil processing, steam reforming of methane etc), electrolytic (water splitting with the help of energy resources), and photolytic (solar-driven water splitting using electrochemical and biological materials) [13,14]. The development of semiconductors based materials for photocatalytic water splitting into pure H₂ and O₂ by utilizing the full solar spectrum through proficiently harvesting ultraviolet–visible (UV–Vis) and near-infrared (NIR) light was considered as a promising method for storing the solar energy through catalytic chemical reactions [15,16]. In this quest, Fujishima and Honda [17] in 1972 reported for the first time the H₂ production by means of water splitting using semiconductor titanium dioxide (TiO₂) as photocatalyst. However, its wide band-energy gap of 3.2 eV allowed for the faster electron (e⁻)/hole(h⁺) recombination rate leading to poor photocatalytic performance which was improved by coupling of TiO₂ with other semiconductor in the later years [[18], [19], [20]]. Thereafter, numerous catalytically active semiconducting materials were developed, yet, none demonstrated the desired performance for water splitting and the stated efficiencies were too low to be practical [[21], [22], [23]].

Recently, two dimensional (2D) materials have geared up their extensive utilities in solar energy conversion and storage devices due to their excellent optoelectronic and electrocatalytic properties [[24], [25], [26], [27]]. Among the various 2D materials reported so far, the polymeric graphitic carbon nitride (g-CN or g-C₃N₄), a metal-free semiconductor, has generated enormous interests in the energy [28,29] and environmental [[30], [31], [32], [33]] applications due to its appropriate and tunable visible light band-energy gap (2.7 eV) [34], easy processability [35], high chemical/thermal stability [36] and lower production cost due to the cheap and earth abundant precursors (urea or melamine) [37]. Therefore, g-CN based materials have been widely utilized for the photocatalytic degradation of numerous environmental pollutants, including dyes, antibiotics and harmful organics [10,[38], [39], [40]]. Besides, the g-CN photocatalyst has also been extensively explored for CO₂ reduction and as an electrocatalyst for H₂ evolution [32,41,42].

Despite the above mentioned advantages, the g-CN suffers from low crystallinity and high degree of disorders and defects [32,43,44]. Nevertheless, numerous efforts have been invested for enhancing the g-CN photocatalytic efficiency, since in its pristine form, these defects serves as the recombination centers which lowers its water splitting efficiency [30,45]. Thus, minimum defect structure with a higher degree of crystallinity is required for enhancing the g-CN performance in the process of water splitting [31,46]. The other ways including structural alteration of g-CN to produce 0D (quantum dots), 1D (nanowires and nanorods), 2D (nanosheets and nanomesh) and 3D structures (sphere, core-shell etc) have also been developed which not only assisted in improving the optical and physicochemical properties but also enhances the amount of redox sites thereby tuning the diffusion distance of charge carriers [38,39]. In addition, numerous efforts have been made in the elemental/molecular doping and functionalization of g-CN (Scheme 1), which reduces the band-energy gap thus allowing the complete utilization of the solar spectra. The heterojunction formation of g-CN with other semiconductors is also an attractive way to promote the electron-hole separation and transfer of charge carriers to those sites, where hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) take place while also improving the surface morphology which is responsible for increasing surface area of the nanocomposite [22,41]. Additionally, the H₂ production efficiency has been reported to improve by using sacrificial agents (methanol etc). These sacrificial agents, owing to their higher reduction potential than oxidation potential of H₂ to H⁺, acts as holes scavenger and electron donor and known to enhance the STH efficiency to greater extent [30,39,41].

After the first report on H₂ production using g-CN by Wang et al. [47], the world has witnessed significant advancement in this critically important field. Hitherto, numerous brilliant reviews concentric on the fundamental properties and designing strategies of g-CN for a variety of technologically throughput applications have been reported in the energy and environmental domain [27,30,32,39,[48], [49], [50], [51]]. In the context of g-CN modification via metal/non-metal doping, copolymerization, functionalization and heterojunction formation, prominent reviews featuring the important developments in the incipient field of H₂ generation, carbon reduction/conversion, nanosensors, solar cells, supercapacitors, and battery materials have been reported [[52], [53], [54], [55], [56], [57], [58]]. Albeit significant reviews focusing on the photocatalytic H₂ evolution via water splitting can be located in the rich collection of literature database, howbeit, none of them emphasized on the morphological tuning of g-CN for augmenting H₂ evolution rate [45,56,[59], [60], [61], [62], [63]]. Hence, it is imperative to acutely unfold the various design strategies of g-CN with a holistic overview of their preparation mechanism for H₂ production application. This mini-review is an effort to highlight the current advancements in designing g-CN nanostructures for H₂ production via photocatalytic water splitting. The first phase of this review deals with the fundamentals and principles of photocatalytic water splitting which is followed by a detailed discussion on the engineering strategies of realizing diverse morphologies and functionalizing g-CN with different materials to improve its photocatalytic performance.