Enhanced visible/near-infrared light harvesting and superior charge separation via 0D/2D all-carbon hybrid architecture for photocatalytic oxygen evolution

Abstract

Since the water splitting rate-limiting oxygen evolution remains sluggish, engineering a rational architecture for photocatalysts to fulfill water oxidation needs becomes a vital issue. Here, we detail a 0D/2D all-carbon hybrid strategy for constructing a heterostructure of carbon dots (CDots) and reduced graphene oxide (rGO) to enhance the photocatalytic water oxidation of monoclinic-BiVO₄ nanosheets (CBrG). Given the visible-light-harvesting ability and up-conversion characteristics of 0D CDots, more photogenerated electron-hole pairs participated in water oxidation under visible and near-infrared light irradiation. Meanwhile, 0D CDots behaved as electron acceptors on 2D rGO to suppress the recombination of electron-hole pairs. This nature licenses for the feasible electron transfer from excited m-BiVO₄ to 0D CDots via electron transfer channels of 2D rGO, facilitating the separated holes to migrate onto the m-BiVO₄ surface for water oxidation. Compared with the rGO decorated m-BiVO₄ nanosheets (BrG), these merits endow the CBrG with an over 212% enhancement in O₂ yield under visible light irradiation as well as notable O₂ yield under near-infrared light irradiation, and a 1.57-fold increase in apparent quantum efficiency. The enhancement is also verified by the significant growth of ·OH radicals derived from OH⁻/H₂O oxidation and ·OOH/·O₂⁻ radicals originated from O₂ reduction. This work paves a new way for the 0D/2D all-carbon hybrid architecture applied in solar energy conversion.

Introduction

With the advancement of human society, there is growing global energy demand. To this end, chemical fuels with high specific energy are regarded as very promising energy storage media. Solar energy, the inexhaustible energy, has attracted tremendous attention. Especially the utilization of solar energy to drive the thermodynamically uphill reactions to store it in chemical bonds [1,2]. Of these reactions, photocatalytic water splitting into molecular hydrogen and oxygen provides a prominent and clean method to utilize solar energy to obtain fuels [3,4]. For the water splitting process, the half-reaction of water oxidation into diatomic O₂ gas has been well recognized as the rate-limiting step due to its 4H⁺/4e⁻ transfer process and high activation energy barrier (≈700 mV) for O-O bond formation. Namely, the water oxidation half-reaction remains sluggish to achieve highly efficient overall water splitting [5,6]. By far intense efforts have been dedicated to developing water oxidation photocatalysts (WOPs) comprising various inorganic and organic systems, whereas the most widely studied WOPs are only a few metal oxides (e.g., WO₃, TiO₂, and Fe₂O₃, etc.) in the past decades, whose valence band (VB) is mainly composed of an O 2p orbital located around +3 V_RHE and positive enough to oxidize water [3]. Nevertheless, several demerits always restrict their practical application in oxygen-generating photocatalytic systems, including inferior solar harvesting efficiency, low charge carrier mobility, slow reaction kinetics, unsatisfied apparent quantum efficiency, and so forth.

To explore highly active WOPs to accelerate the water splitting process, we should specifically address the three critical intermediate steps during photocatalysis – light absorption, charge separation and transfer, and surface catalytic reactions [7]. For the first two steps, bismuth vanadate, especially the monoclinic scheelite structure (m-BiVO₄), has stood out as frontier WOPs since Kudo et al. used m-BiVO₄ to achieve water oxidation under visible light irradiation [8]. Compared with other metal oxides whose VB consists only of O 2p orbitals and can only respond to ultraviolet light, the VB dominated by a hybrid orbital of Bi 6s and O 2p results in the decreased band gap of m-BiVO₄, thereby substantially expanding the light absorption to the visible light region [9]. Also, the local environment at the Bi and V sites in m-BiVO₄ exhibits remarkable deformation, as opposed to the tetragonal and zircon-type forms with symmetric structures. The BiO₈ dodecahedron distortion causes an improved lone-pair effect of the Bi 6s orbital, and the VO₄ tetrahedral distortion induces an internal electric field, boosting the separation of photogenerated carriers [10]. Hence these two merits, as well as earth-abundance and non-toxicity, make m-BiVO₄ a potential candidate for photocatalysis [7,11]. In terms of surface catalytic reactions, some 2D support materials (e.g., reduced graphene oxide (rGO), C₃N₄ nanosheets, and metal-organic frameworks, etc.) have abundant surface properties, which can provide more possibilities to enhance the water oxidation performance of m-BiVO₄ through accommodating m-BiVO₄ [12]. Ideal support materials should satisfy the following features: (i) specific active centers to allow the nucleation of crystals; (ii) large surface area to support the high loading of catalytic sites; (iii) high charge carrier mobility to facilitate the photoexcited electron transfer.

Among various supports, the 2D carbon material rGO can meet the strict criteria above – offering oxygen-containing functional groups (OFGs) as nucleation centers of m-BiVO₄ crystals, providing abundant reactive sites with ultra-large surface area, and possessing high conductivity to act as electron transfer channels [13,14]. Moreover, the coupling of m-BiVO₄ and rGO to form a hybrid structure necessarily results in the generation of an interface that affects the electron transfer efficiency [10]. Noteworthy is that a small Mott-Schottky barrier caused by the higher Fermi level of rGO than that of m-BiVO₄ favors the photoexcited electron transfer from m-BiVO₄ to rGO across their contact interface [15,16]. Although the photocatalytic activity of m-BiVO₄-rGO composite is indeed much higher than that of bare m-BiVO₄, it still falls short of expectations including the following drawbacks: (i) limited visible light harvesting and scarce infrared light utilization; (ii) insufficient charge carrier separation efficiency. For the former, the m-BiVO₄ with a fixed absorption edge can only absorb part of visible light. Considering the composition of solar irradiation spectrum (i.e., ca. 7% ultraviolet light, ca. 50% visible light and ca. 43% infrared light), how to make full use of visible light and even infrared light is a challenging issue that must be considered in the field of photocatalysis [12]. As to the latter, the electron storage ability of conjugated sp²-bonded carbon network within rGO is extremely low, which provides new opportunities for the recombination of electron-hole pairs, unless there are additional electron acceptors loaded on rGO [17]. Therefore, unearthing optimal materials that can be combined with m-BiVO₄-rGO for overcoming these referred drawbacks is imperative.

In principle, light-harvesting ability and charge carrier separation efficiency can be both optimized by employing the emerging 0D quasi-spherical carbon material – carbon dots (CDots; monodisperse graphite nanoparticles less than 10 nm in diameter) – in the field of photocatalysis. CDots, first discovered in 2004 [18], consist of sp²/sp³ hybridized carbon atoms and exhibit excellent light absorption property, unique up-conversion characteristics, and electron-acceptor nature [[19], [20], [21]]. Consequently, a series of CDots-based photocatalysts, including CDots coupled with TiO₂ [22], C₃N₄ [23], or Bi₂WO₆ [24], were desirably fabricated in efforts to promote their performance for visible/near-infrared-light-driven photocatalytic reactions. These works together suggest that with CDots incorporated into m-BiVO₄-rGO should be expected for encouraging improvement in the photocatalytic activity of m-BiVO₄-rGO, whereas no works have been hitherto done to combine them and unveiled for the related mechanism. Hopefully, both 0D CDots and 2D rGO are rich in OFGs on each surface, providing the prerequisite of anchoring 0D CDots on 2D rGO, to form a 0D/2D all-carbon hybrid architecture with chemical bonding. Given the unique optical properties and electronic nature of CDots, we aim to tackle the intrinsic limitations of m-BiVO₄-rGO via the deliberate incorporation of CDots. Using the water splitting rate-limiting oxygen evolution as a model reaction, this well-designed 0D/2D all-carbon hybrid architecture would fully exert superiority of 0D CDots with 2D support material rGO, for significantly enhancing the photocatalytic water oxidation activity of m-BiVO₄-rGO.

Herein, deeming the m-BiVO₄-rGO as a prototype, we first developed a 0D/2D all-carbon hybrid strategy to obtain carbon dots (CDots) and reduced graphene oxide (rGO) co-decorated m-BiVO₄ nanosheets (CBrG) for triggering photocatalytic water oxidation reaction. To stress the applicability and superiority of 0D CDots on 2D rGO, a systematic investigation into the physical structures, chemical status, optical and electronic properties, and photocatalytic performances was carried out, supplemented by the mechanism exploration for the photocatalytic process. As anticipated, this 0D/2D all-carbon hybrid architecture endows the CBrG with an over 212% enhancement in O₂ yield under visible light irradiation and notable O₂ yield under near-infrared light irradiation, as well as a 1.57-fold increase in apparent quantum efficiency relative to the counterpart BrG. The improvement is also corroborated by the significant growth of ·OH radicals derived from OH⁻/H₂O oxidation and ·OOH/·O₂⁻ radicals originated from O₂ reduction.