Surface charge modification via protonation of graphitic carbon nitride (g-C₃N₄) for electrostatic self-assembly construction of 2D/2D reduced graphene oxide (rGO)/g-C₃N₄ nanostructures toward enhanced photocatalytic reduction of carbon dioxide to methane

Highlights

• Unprecedented rGO-hybridized protonated g-C₃N₄ (pCN) photocatalysts were prepared.

• rGO/pCN was developed by a sonication-aided and electrostatic self-assembly route.

• The rGO/pCN showed superior activity and recyclability toward CO₂ reduction to CH₄.

• Intimate 2D/2D interfacial contact of rGO and pCN renders solar energy conversion.

Abstract

In this work, we reported a 2D/2D hybrid heterojunction photocatalyst with effective interfacial contact by incorporating reduced graphene oxide (rGO) and protonated g-C₃N₄ (pCN) synthesized by a novel combined ultrasonic dispersion and electrostatic self-assembly strategy followed by a NaBH₄-reduction process. The resulting 2D rGO-hybridized pCN (rGO/pCN) nanostructures formed an intimate contact across the heterojunction interface as supported by the electron microscopy analysis. The rGO/pure g-C₃N₄ (rGO/CN) developed without the modification of surface charge on g-C₃N₄ has also been prepared for comparison. Compared with pure g-C₃N₄ and rGO/CN, the rGO/pCN photocatalysts demonstrated a remarkable enhancement on the CO₂ reduction in the presence of H₂O vapor to CH₄ under a low-power energy-saving daylight bulb at ambient temperature and atmospheric pressure. The optimized 15 wt% rGO/pCN (15rGO/pCN) exhibited the highest CH₄ evolution of 13.93 µmol g_catalyst⁻¹ with a photochemical quantum yield of 0.560%, which was 5.4- and 1.7-folds enhancement over pCN and 15rGO/CN samples, respectively. This was ascribed to the addition of rGO with pCN in a controlled ratio as well as sufficient interfacial contact between rGO and pCN across the rGO/pCN heterojunction for efficient charge transfer to suppress the recombination of electron–hole pairs as evidenced by the electron microscopy, zeta potential and photoluminescence studies. In addition, the 15rGO/pCN possessed a moderately high stability after three successive cycles with no obvious change in the production of CH₄ from CO₂ reduction. Lastly, a visible-light photocatalytic mechanism associated with rGO/pCN hybrid nanoarchitectures was presented.

Introduction

In the past decades, the substantial rise in CO₂ and the concern about the energy supply have gained immerse attention, which are considered the biggest challenges of the century. The conversion of CO₂ into renewable fuels by artificial photosynthesis has been regarded as one of the most compelling strategies to circumvent both energy and environmental problems simultaneously. Since the discovery of photoreduction of CO₂ in a semiconductor aqueous suspension by Inoue and co-workers [1], there has been a strenuous endeavor in constructing more effective and environmentally friendly photocatalysts to achieve CO₂ conversion more economically. Up to now, most of the photocatalytic CO₂ reduction reactions employing metal oxide semiconductor photocatalysts are performed under UV light or high-power light illumination and the product yields are considerably low for practical applications [2], [3].

Most recently, our research group has successfully reported a one-step solvothermal approach to fabricate nitrogen-doped TiO₂ with exposed {001} facets supported on the graphene, noble metal-modified reduced graphene oxide (rGO)/TiO₂ ternary nanostructures and also a dopant-free strategy to engineer oxygen-rich TiO₂ for remarkably enhanced visible-light photoreduction of CO₂ to CH₄ utilizing a low-power energy-saving daylight lamp at room temperature and atmospheric pressure [4], [5], [6]. However, the as-developed photocatalysts comprise metal-containing semiconductors as the main constituent, which would eventually incur economic cost concern for large scale applications. Thus, it is our long-term interest in the search for robust and metal-free visible-light-driven photocatalysts, which are of great economic value for practical benefits. Recently, two-dimensional (2D) graphitic carbon nitride (g-C₃N₄), which is an analog of graphite, has been reported with the merits of high chemical stability, “earth-abundant” nature as well as possessing an appropriate electronic structure with medium band gap energies [7], [8], [9]. Nevertheless, the efficiency of g-C₃N₄ is still limited due to its high recombination rate of photogenerated electron–hole pairs and low electrical conductivity. To address the problems, many attempts have been made to increase the photocatalytic performance of g-C₃N₄ such as designing an appropriate textural porosity [10], metal doping [11], [12], [13], non-metal doping [14], [15] and coupling with other semiconductors [16], [17]. The g-C₃N₄-based photocatalysts have been extensively investigated on the liquid-phase degradation of pollutants and H₂ production from water splitting. However, the utilization of g-C₃N₄-based nanocomposites on the photoreduction of CO₂ is relatively scarce and still in its infancy. Among the various reduction co-catalysts, carbon-based nanostructured materials such as carbon nanotubes [18], fullerene [19] and graphene [4] have received incessant research interest lately. In particular, graphene, a 2D sp²-conjugated carbon atoms packed in a honeycomb lattice, has become a hot spot on scientific research because of its extraordinary physical, chemical and catalytic properties [20], [21], [22], [23]. More importantly, the identical sp²-bonded π structure and carbon network are exhibited by graphene and g-C₃N₄, which endow them the compatible materials to develop nanostructures [24], [25].

In a recent work, Oh et al. [26] reported the fabrication of graphene oxide (GO)-assisted production of g-C₃N₄ using a one-pot aqueous solution process. In addition, Li et al. [24] and Xiang et al. [27] fabricated graphene/g-C₃N₄ by heating cyanamide or melamine in the dispersion of graphite oxide followed by thermal calcination at 550 °C for 4 h in an inert environment. This approach is typically known as in situ immobilization of g-C₃N₄ onto the graphene supports. However, the sublimation amount of the melamine or cyanamide is difficult to be controlled during the annealing process [28]. On top of that, thermal polymerization of the nitrogen-rich precursors (>500 °C) for the development of g-C₃N₄ inside the carbon support resulted in a significant decrease of the N content [29]. Therefore, an intimate interfacial contact between rGO and g-C₃N₄ synthesized at relatively low temperatures is of paramount significance to ensure effective charge separation in the heterojunction. As a matter of fact, the development of synergistic 2D/2D heterojunction interface of rGO and g-C₃N₄ afforded by the surface charge promoted self-assembly method has been lacking and it remains a challenge hitherto. As a result, the electrostatic self-assembly technique, by coupling g-C₃N₄ with rGO is anticipated to be effective for controlling the morphology of the g-C₃N₄ and rGO in a uniform manner and also increasing the contact area for efficient transportation of charge across the heterointerface in comparison with 0D nanomaterials, which are merely in point contact [25]. Moreover, the 2D/2D heterojunction reduces the charge transport distance and time, thus promoting the separation of electron–hole pairs across the contacting interface for enhanced activity [30]. This work addresses the principal issues raised in several reports: what causes the effective and intimate formation of heterojunction interface of 2D/2D hybrid nanocomposites toward improved photoredox processes? Is it the importance of surface charge modification between two components?

Herein, we demonstrated a robust approach to prepare proton-functionalized/protonated g-C₃N₄ (pCN) by the sonication-exfoliation of acidified bulk g-C₃N₄. Since both GO and unmodified g-C₃N₄ exhibit negative polarity [28], [31], they will not be successfully coupled together owing to the strong and mutual electrostatic repulsion in the dispersion. Hence, the acid pretreatment with HCl can easily alter the g-C₃N₄ to possess positive polarity. Upon reduction of GO using NaBH₄ as a reducing agent, the resulting rGO/protonated g-C₃N₄ (rGO/pCN) heterointerfaces could be successfully obtained by the π–π stacking and electrostatic attraction of oppositely charged materials to form the hybrid nanostructures. To the best of our knowledge, this is considered the first report on the development of metal-free rGO/pCN hybrid nanostructures via a combined sonication-assisted and surface charge modification strategy for the visible-light photocatalytic reduction of CO₂ to CH₄. The as-fabricated 2D/2D rGO/pCN nanocomposites demonstrated significantly improved photoactivity in comparison to pure g-C₃N₄. The remarkably enhanced photocatalytic performance was ascribed to the exceptional 2D/2D morphological structure associated with rGO and pCN and also intimate interfacial coupling, thereby successfully harnessing the electron conductivity of rGO to facilitate the charge transfer and separation over rGO/pCN. As a whole, this work not only highlights the utilization of rGO as an ideal substrate for various applications, but also emphasizes the rational significance of well-contacted 2D/2D heterojunction interface between rGO and semiconductor photocatalysts by a simple self-assembly approach in view of the electrostatic interaction, rather than combining the rGO “gold rush”.