Structural design of carbon dots/porous materials composites and their applications

Highlights

• This review focuses on recent development of carbon dots/porous materials composites.

• Preparation, structures, and properties of carbon dots/porous materials composites are discussed.

• Challenges and future development of carbon dots/porous materials composites are described.

Abstract

Carbon dots (CDs), an emerging fluorescent carbon nanomaterial, have attracted great attention in recent year due to their features such as unique optical property, good biocompatibility and low cost and easy synthesis. When CDs are integrated with porous materials, it not only compensates for the shortcomings of monocomponent CDs in applications, but also produces new functionalities through synergistic interactions to extend new applications. At present, CDs have been assembled with various porous materials such as zeolites, carbonaceous porous materials, porous graphitic carbon nitride (g-C₃N₄), mesoporous SiO₂, porous metallic compounds and metal-organic frameworks (MOFs), which exhibit excellent performance and a wide range of applications. Therefore, recent research advances in CDs/porous materials composites should be presented to facilitate further development of the field. In this article, the preparation methods, porous structures, and properties of CDs/porous materials composites are classified and presented. Then, the applications of CDs/porous materials composites in optics, biomedicine, sensing, electrochemistry, and photocatalysis are also discussed. Finally, current challenges and future directions are briefly discussed to facilitate the next development of CDs/porous materials composites.

Introduction

Carbon dots (CDs), an emerging fluorescent carbon nanomaterial with a size smaller than 10 nm, typically consist of amorphous to nanocrystalline cores and abundant surface functional groups [1], [2]. CDs were first discovered occasionally by Xu et al. [3] in the purification of single-walled carbon nanotubes in 2004. Then Sun et al. [4] found that the fluorescence emissions of fluorescent carbon nanoparticles could be enhanced via surface passivation, and named these fluorescent carbon nanoparticles as “carbon dots”. Since then, more research work has been devoted to the synthesis, mechanism, and application of CDs. In particular, the meaning of CDs is no longer specific due to a variety of synthetic methods and the wide availability of raw materials, and can be categorized as graphene quantum dots (GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs), and carbonized polymer dots (CPDs) [5]. The widespread interest in CDs is inextricably associated with their unique advantages such as low cost, easy synthesis, biocompatibility, high water solubility, photostability and easy functionalization [6], [7], [1]. More importantly, CDs exhibit some unique properties such as effective optical absorption, tunable photoluminescence (PL), up-converted photoluminescence (UPCL), and photoinduced electron transfer [8]. Based on superior optical, biological, and electrochemical properties, CDs are favored in many fields, such as bioimaging [9], [10], [11], [12], [13], sensing [14], [15], [16], [17], [18], [19], [20], catalysis [21], [22], energy devices [23], [24], [25], optoelectronics [26], [27], [28], [29] and so on. However, in practical applications, monocomponent CDs cannot fully realize their potentials. A common phenomenon is that CDs often undergo the aggregation‐induced luminescence quenching effect in the solid state, which results from π–π interactions of CDs’ graphitized cores, excessive resonance energy transfer (RET), and interparticle coupled surface states [30], [31], [32]. As a result, luminescence quenching of solid-state CDs seriously affects the applications related to the optical properties, such as opto-electronic devices, printing [33], [34], [35]. For example, When CDs are used as the color converter of white light-emitting diodes (WLEDs), the aggregation‐induced luminescence quenching of CDs can cause WLEDs to exhibit lower luminous efficiency [36]. In the photocatalysis applications, monocomponent CDs exhibit poor photocatalytic activity due to their low photosensitization efficiency [37]. In addition, CDs are not easily recyclable because of their extremely small size.

Recently, it is an interesting research topic to combine CDs with porous materials. By integrating CDs with porous materials, not only can the above drawbacks of CDs be overcome, but also through their synergistic effect enhance existing performance or introduce other novel functionalities to extend the applications. Porous materials typically have high porosity (micropores (<2 nm), mesopores (2–50 nm), macropores (>50 nm) or their combination), which gives them high specific surface area and pore volume. On the one hand, CDs can be well dispersed and stabilized in the materials with nanoscale porosity, which can exhibit the full properties of CDs (PL, UPCL, photoinduced electron transfer, etc.). On the other hand, synergistic effects can be generated between CDs and porous materials, which are usually in two categories. One is the synergistic interaction between the CDs and the porous structure of the porous material. For example, the fluorescence sensor constructed by CDs and metal–organic frameworks (MOFs) mainly utilizes the fluorescence quenching properties of the target analyte to CDs and the ability of MOFs to accumulate the analyte relying on the porous structure [38]. Another is to focus on the synergistic interaction between CDs and the physical or chemical function of the porous materials, while the porosity of the porous material is only used as an aid to enhance performance. For example, when CDs-modified porous graphitic carbon nitride (g-C₃N₄) composites are applied for photocatalytic degradation, CDs can act as photosensitizers and promptly separate the photo-generated electron-hole pairs to enhance the photocatalytic activity provided by g-C₃N₄, while the porous structure of the composite is used to expose more active sites to further improve photocatalytic degradation efficiency [39].

In this review, the current progress of CDs-based porous materials composites is presented. Before describing the strategy of combining CDs with porous materials, the common synthesis methods of CDs are briefly introduced, and then further detailed strategies for incorporating CDs into various porous materials, such as zeolites, carbonaceous porous materials, porous g-C₃N₄, mesoporous SiO₂, porous metallic compounds and MOFs are described. Next, this review focuses on describing the porous structure of CDs/porous materials composites and the properties afforded by CDs (PL, photoinduced electron transfer and UPCL). In addition, this review also discusses the applications of CDs/porous materials composites in the fields of optics, biomedicine, sensing, electrochemistry, and photocatalysis. Finally, based on the current research progress, this review gives the current challenges and future directions in the hope of providing a little help to researchers in the field.