Monolayered g-C₃N₄ nanosheet as an emerging cationic building block for bifunctional 2D superlattice hybrid catalysts with controlled defect structures

Highlights

• New type of 2D superlattice catalyst is synthesized with cationic g-C₃N₄ nanosheet.

• g-C₃N₄-MoS₂ superlattice exhibits strong interfacial coupling and N defect formation.

• g-C₃N₄-MoS₂ superlattice shows excellent bifunctionality as photo-/electro-catalysts.

• The formation of N defect improves a selectivity for photocatalytic N₂ fixation.

Abstract

New class of superlattice nanohybrids of interstratified graphitic-carbon nitride (g-C₃N₄)–transition metal dichalcogenide monolayers are synthesized by employing pH-controlled g-C₃N₄ nanosheet (NS) as an emerging cationic building block. The interstratification of g-C₃N₄ and MoS₂ NSs leads to strong interfacial electronic coupling, creation of nitrogen vacancy, and stabilization of 1T′-MoS₂ phase. The superlattice g-C₃N₄–MoS₂ nanohybrids display remarkably enhanced bifunctionality as photocatalysts for visible light-induced N₂ fixation and electrocatalysts for hydrogen evolution. Of prime importance is that the creation of nitrogen vacancy results in the significant improvement of selectivity for photocatalytic N₂ fixation (582 μmolL⁻¹h⁻¹) over competitive photocatalytic H₂ generation. This is attributable to promoted adsorption of nitrogen, provision of many active sites, and enhancement of charge transfer kinetics, charge separation, and visible light absorptivity. This study highlights that the application of g-C₃N₄ NS as a cationic building block provides valuable opportunity to widen the library of multifunctional NS-based superlattice nanohybrids.

Introduction

As an emerging alternative to graphene, highly anisotropic 2D nanosheets (NSs) of redoxable layered inorganic materials like transition metal dichalcogenide (TMD), layered double hydroxide (LDH), and transition metal oxide (TMO) have attracted a great deal of research activity because of their excellent electrochemistry-related functionalities [[1], [2], [3]]. The hybridization with foreign species can provide an effective way to further improve the functionalities of inorganic NSs via the synergistic tailoring of their electronic structures and chemical properties [4]. Such a hybridization effect can be maximized by the formation of heterolayered superlattice composed of oppositely-charged 2D NSs with extremely high surface-to-volume ratios. In this superlattice, a face-to-face contact between restacked monolayers leads to unusually strong interfacial electronic coupling between them [4,5]. As a new class of functional layered compounds, the 2D heterolayered superlattices show much superior catalyst/electrode performances over randomly-stacked nanocomposites, underscoring unique advantage of heterolayered hybridization [4,6,7]. In contrast with other synthetic methods like hydrothermal synthesis yielding randomly-combined nanocomposites [8,9], an electrostatically-driven self-assembly between oppositely-charged component NSs can provide an effective way of synthesizing layer-by-layer-ordered superlattice structure in terms of prominent electrostatic attraction. It is noteworthy that most of electrostatically-assembled superlattice nanohybrids contain exfoliated LDH NS as cationic component [10], because the exfoliated LDH NS is only an intrinsically positively-charged inorganic NS [3]. This fact seriously limits the widening of the library of inorganic NS-based superlattice nanohybrids. In contrast to other exfoliated inorganic NSs, a cationic form of graphitic-carbon nitride (g-C₃N₄) NS can be obtained by the tuning of suspension’s pH [11]. The positive surface charge and useful functionality of g-C₃N₄ NS render this material emerging cationic building block for synthesizing new class of multifunctional 2D superlattice nanohybrids [[12], [13], [14], [15]]. In one instance, the interstratification between photocatalytically-active g-C₃N₄ NS and electrocatalytically-active TMD NS is supposed to yield efficient bifunctional hybrid catalysts. The remarkable interfacial interaction between interstratified NSs enables not only to tailor the electronic and defect structures of component NSs but also to optimize the photocatalyst/electrocatalyst performances of the resulting nanohybrids. Of noteworthy is that the fine-control of crystal defect would be effective in improving the selectivity of nanohybrids for a specific catalytic reaction via the control of reactant adhesion. Although there are several reports about the synthesis of disorderly-coupled nanocomposites of g-C₃N₄ and MoS₂ [16,17], at the time of this submission, we are unaware of any other report about the synthesis and catalyst application of 2D heterolayered superlattice composed of alternating g-C₃N₄ and MoS₂ monolayers.

In this work, new class of artificial 2D superlattices of interstratified g-C₃N₄–TMD monolayers are synthesized by an electrostatically-driven self-assembly between cationic g-C₃N₄ and anionic MoS₂ NSs. The interfacial electronic coupling and the formation of nitrogen vacancy in the g-C₃N₄–MoS₂ nanohybrids are investigated with density functional theory (DFT) calculations and various spectroscopic analyses. The g-C₃N₄–MoS₂ nanohybrids show excellent bifunctionality as electrocatalysts for hydrogen evolution reaction (HER) and photocatalysts for visible-light-driven N₂ fixation.