ReviewTwo-dimensional MXene-based and MXene-derived photocatalysts: Recent developments and perspectives
Graphical abstract
Introduction
Two-dimensional (2D) materials have drawn immense attention since the discovery of monolayer graphene in 2004 [1]. To date, many types of 2D materials also exhibit excellent catalytic, electronic, and optoelectronic properties [2], [3], [4], [5], [6], such as transition metal dichalcogenides (e.g., MoS2 [7], WS2 [8]), g-C3N4 [9], [10], layered double hydroxides (LDHs) [11], [12], and black phosphorous [13], [14]. Moreover, the 2D structure also minimizes the migration distances between charge carriers and the reaction interface, inhibiting the possibility of charge carrier recombination, which improves the photocatalytic performance [15]. In recent years, MXenes have become one of the most popular 2D materials owing to their diverse elemental compositions, unique 2D structure, large surface area, abundant surface terminations, and excellent photoelectronic properties [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Generally, Mn+1AXn (n = 1–3) precursors are usually etched to prepare Mn+1XnTx (MXenes, Fig. 1), where M represents a transition metal (e.g., Ti, Nb, Ta, and Mo), A represents group 13 and 14 elements from the periodic table (e.g., Si, Ga, and Al), X stands for C and/or N elements, and Tx is –O, –F, –OH, etc. [27], [28], [29], [30], [31], [32], [33] Experimentally, the composition and surface properties of MXenes vary with different etching methods [34], [35], [36]. Since the first report of the Ti3C2 MXene in 2011 [37], >20 kinds of MXenes have been successfully obtained, such as Ti3C2, Ti3CN, TiNbC, V2C, Mo2C, Nb2C and Y2CF2 [38], [39], [40], [41].
The most widely used methods for the synthesis of MXenes are direct HF etching and indirect in situ HF etching (e.g., using a mixture of HCl and LiF). To enhance the yield of MXenes, delamination with various organic intercalants (e.g., dimethyl sulfoxide, isopropylamine, tetramethylammonium hydroxide), high-temperature etching, and chemical vapor deposition have also been explored [42], [43], [44]. The timeline of MXenes prepared by various etching methods is presented in Fig. 2 [44]. Until now, MXenes have been used for various applications, such as absorbents, supercapacitors, gas sensors, and electrocatalysts. Apart from those applications, MXenes have become highly desirable materials in recent years as co-catalysts or precursors for photocatalytic environmental remediation and energy conversion.
A literature investigation revealed that a small number of reviews have summarized the progress in the preparation and applications of MXenes, and only a few of them refer to MXene-based and MXene-derived photocatalysts in photocatalytic environmental remediation and artificial photosynthesis. The significant difference of these two types of photocatalysts is that a certain component in the latter is from the oxidization of MXene precursor. Importantly, research on MXenes for photocatalysis is progressing rapidly. In this review, we summarize the latest progress on the preparation and applications of 2D MXene-based and MXene-derived photocatalysts in the photocatalytic field. First, we introduce the synthesis process of MXene-based and MXene-derived photocatalysts in detail. Then, we present a systematic review of recent studies addressing the environment and energy applications of MXene-based and MXene-derived photocatalysts, including organic pollutant degradation, water splitting for H2 evolution, CO2 reduction, nitrogen fixation, hydrogen peroxide (H2O2) production, antibacterial application, desulfurization and denitrogenation in fuels. Finally, a prospective outlook for future research is proposed from the current research challenges at the forefront of environmental and energy science.
Section snippets
Preparation of MXene-based photocatalysts
For the past few years, the construction of composites has been an effective way to synthesize highly efficient photocatalysts. Owing to their unique characteristics, MXenes have been regarded as promising research objects for the preparation of photocatalysts. The most widely used methods for the synthesis of MXene-based and MXene-derived photocatalysts include mechanical/ultrasonic mixing, electrostatic self-assembly, hydrothermal/solvothermal treatment, and calcination processes.
Preparation of MXene-derived photocatalysts
Calcination and hydrothermal oxidation strategies are most widely adopted for the preparation of MXene-derived photocatalysts by the in-situ oxidation of the MXene precursors. In recent decades, TiO2 has been the most-investigated semiconductor photocatalyst owing to its environmental friendliness, low cost, redox potential, and outstanding photostability. However, individual TiO2 still has two disadvantages that are difficult to overcome: a wide band gap and a high photogenerated carrier
Water and air purification
With the rapid improvements of modern industrialization, organic pollutants in water have dramatically increased, causing enormous harm to human health and the natural environment. Various methods, including adsorption, biotreatment, peroxymonosulfate activation, Fenton oxidation, and photocatalytic degradation have been widely used for water treatment [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94]. Among these methods, photocatalysis is a
Facilitating charge separation and transfer
How to improve the separation efficiency of photogenerated charge carriers have been a research hotspot in the photocatalytic field. Generally, MXene as a co-catalyst could restrict the recombination of photogenerated electrons and hole in the photocatalytic system [176], [177], [178], [179]. Experimentally, the Schottky junction could be formed between the semiconductor photocatalysts and Ti3C2. For instance, Ran et al. [180] found that electrons could transfer from CdS to Ti3C2 in the CdS/Ti3C
Conclusions and outlook
In summary, research on MXene has bloomed since the first work appeared in 2011. Among the MXene family, Ti3C2 is the most studied in the photocatalytic field. As stated in this review paper, a variety of synthesis methods including mechanical/ultrasonic mixing, electrostatic self-assembly, hydrothermal/solvothermal methods, and calcination approaches have been adopted for fabricating MXene-based and MXene-derived photocatalysts. Benefiting from the flexible adjustability of the elemental
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (NSFC No. 51472194) and the Natural Science Foundation of Hubei Province (2016CFA078).
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