Review of MXene-based nanocomposites for photocatalysis
Introduction
The demand for water and energy has increased owing to rapid worldwide population growth and growing industrialization, necessitating new sources of sustainable water and renewable energy (Handoko et al., 2019; Jun et al., 2019b). As water usage has increased with an increasing population, water quality has also worsened over the past few decades, mainly because of various human activities and the inappropriate use of natural water resources (Bhatnagar et al., 2015). In particular, many previous studies have reported that both conventional and emerging contaminants, such as heavy metals, dyes, endocrine disruptors, and pharmaceutically active compounds, have been detected at a wide range of concentrations (<1 μg L−1 to mg L−1) in industrial wastewater, municipal wastewater effluents, and drinking water worldwide (Holkar et al., 2016; Joseph et al., 2019a; Snyder et al., 2003, 2007; Yoon et al., 2010). There are numerous approaches to treating various wastewaters containing different contaminants: biodegradation (Pereira et al., 2014), chlorination/ozonation (Qu et al., 2015; Westerhoff et al., 2005), sonodegradation (Han et al., 2019; Im et al., 2015), adsorption (Joseph et al., 2011; Jun et al., 2019a), and membrane separation (Xu et al., 2012; Yoon and Lueptow, 2005). While these processes have several disadvantages (e.g., high cost, harmful byproducts, and/or fairly low degrees of removal, photocatalytic degradation as an advanced catalysis process has shown important advantages (e.g., low cost, environmental friendliness, reusability, and complete degradation) (Trojanowicz et al., 2018).
Over the past decade, significant progress has been made in photocatalytic CO2 reduction, H2/O2 evolution, and nitrogen fixation (Li and Wu, 2019). While both electrocatalytic and photocatalytic CO2 reduction processes have been widely studied, photocatalytic CO2 reduction mainly stimulated by natural photosynthesis is somewhat more challenging than electrocatalytic CO2 reduction, owing to the additional processes of photocarrier production and charge transfer (Handoko et al., 2013). In general, photocatalytic water splitting includes the production of electrons (e−) and holes (h+) through the absorption of solar energy by photocatalysts, relocation of produced electrons and holes to the catalyst surface, and oxidation–reduction of H2O to produce hydrogen and oxygen on the catalyst surface (Tang et al., 2008). Research on the photocatalytic fixation of nitrogen has also increased, particularly at atmospheric pressure and room temperature, since photocatalytic reaction uses pure and intense solar energy as a moving force and unrestricted and abundant water and nitrogen as raw resources (Chen et al., 2018). Although remarkable accomplishments have been made in this field during the past decade (Jiang et al., 2020; Kumar et al., 2020; Zhang et al., 2020b), the majority of photocatalytic studies were conducted with conventional and noble-metal-containing photocatalysts, such as TiO2/ZnO/CuS/Fe2O3/CdS and Au/Pt/Pd, respectively.
In recent years, a significant amount of effort has also been made in developing highly efficient and strong photocatalysts with various carbon-based or two-dimensional (2D) nanomaterials, such as carbon nanotubes (Di et al., 2015), graphene oxides (Bafaqeer et al., 2018), metal–organic frameworks (Luo et al., 2020), and MXenes (Low et al., 2018), without the need for expensive metal content. Among these nanomaterials, MXene/MXene-based nanocomposites have attracted increasing attention in various areas, including the energy and environment, owing to their unique properties (nontoxicity, large surface area, great strength, large interlayer spacing, high melting point, outstanding oxidation resistance, environmental flexibility, extraordinary electrical/thermal conductivity, hydrophilicity, and outstanding biocompatibility) (Lukatskaya et al., 2013; Zhong et al., 2016). Briefly, in 2011, a new 2D member (Ti3C2Tx, multilayered MXenes) was first introduced by researchers at Drexel University (Naguib et al., 2011). In general, MXene (Mn+1XnTx) is fabricated by etching MAX phase (Mn+1AXn) where M represents an early transition metal (e.g., Ti, Cr, Nb, V), X represents carbon and/or nitrogen, T represents surface functional groups, A represents an element from groups (e.g., Cd, Al, P, As), and n varies from 1 to 3, determining the number of atomic layers in the unit cell (Khazaei et al., 2014; Zhao et al., 2012).
Originally, MXenes have been broadly examined as vehicles for energy storage/delivery in devices such as Li/Na-ion batteries and supercapacitors, owing to their lamellar structures and exceptional electrical conductivity (Jun et al., 2019b). In recent years, MXenes and MXene-based nanomaterials (e.g., Ti3C2–TiO2, N-doped TiO2@C, In2S3/TiO2@Ti3C2Tx, Ti3C2Tx/Bi2WO6, and BiOBr/Ti3C2) have been used as new photocatalysts in energy and environmental applications (Cao et al., 2018; Cheng et al., 2018; Huang et al., 2019; Li et al., 2020; Wang et al., 2018a). However, only very limited reviews have been published on the use of MXenes as innovative photocatalysts for photocatalytic degradation for contaminants or CO2 reduction/H2 evolution/N2 fixation (Jun et al., 2019b; Li and Wu, 2019); these are significantly different from our current review, as our review contains all recent studies and developments. To our knowledge, no comprehensive review on MXene-based catalysts for photocatalysis in photocatalytic degradation and CO2 reduction/H2 evolution/N2 fixation has been performed. Therefore, it is very valuable to discover how MXenes and MXene-based nanomaterials technically improve photocatalytic reaction. This review is organized as follows: (i) preparation techniques for MXene-based photocatalysts; (ii) photocatalytic degradation; (iii) CO2 reduction, H2 evolution, and N2 fixation, and (iv) regeneration of MXene-based photocatalysts. We also briefly describe the areas for future research on MXene-based catalysts for photocatalysis.
Considering the rapid growth in the application of MXene-based nanomaterials, several review studies on their synthesis for various applications have been reported (Alhabeb et al., 2017; Jun et al., 2019b; Xiao et al., 2018b). In particular, MXene-based photocatalysts are prepared by replacing noble metal co-catalysts to improve the charge-separation capacity of the photocatalyst, as shown in Fig. 1. The most widely used techniques for fabricating photocatalyst composites are mechanical mixing, self-assembly, and in-situ decoration/oxidation (Sun et al., 2019a). Mechanical mixing is the simplest technique for preparing photocatalyst composites, which involves mixing different components in solution or crushing powders. Interestingly, negatively charged MXenes are readily combined with positively charged photocatalysts, owing to electrostatic attraction, resulting in self-assembled photocatalyst composites (Ye et al., 2018). Unlike mechanical mixing and self-assembly, in-situ decoration techniques include the direct fabrication of different components onto the MXene surface. Consequently, in-situ prepared materials and MXenes with strong chemical bonding could provide a significant benefit in some designs (Sun et al., 2019a).
Since the late 1960s, TiO2 has been widely used as one of the most favorable and environmentally friendly photocatalysts, owing to its capable photoactivity, strong oxidizing power, low cost, safety, and high chemical/photo-stability (Kumar and Devi, 2011). However, there are two main limitations in the use of TiO2 as a photocatalyst: (i) Anatase TiO2 absorbs very little sunlight in the UV region (approximately 5%), owing to its large band gap energy (approximately 3.2 eV), and (ii) in addition, the photocatalytic activity of TiO2 is restricted by the fast recombination of photo-induced electrons and holes (Hisatomi et al., 2014). Therefore, numerous approaches have been employed in decreasing its band gap energy, inhibiting its electron–hole recombination activity, and improving its absorption of organic contaminants. A recent study has shown that N doping and TiO2-loading C materials derived from transition metal carbide Ti3C2 enhance these two issues (Huang et al., 2019). The new two-dimensional (2D)-layered Ti3C2 MXene was used as a C skeleton and Ti source. Since Ti3C2 MXene is negatively charged and readily oxidized, the N-containing cationic species could be accumulated on the negatively charged very thin Ti3C2 surface, owing to electrostatic attraction. Then, in-situ conversion of Ti3C2 into N-doped C-supported TiO2 is completed by adjusting the oxidation conditions, as shown in Fig. 2a. Numerous characterization methods have been employed to confirm the successful preparation of N-doped TiO2/MXene photocatalyst as follows: X-ray diffraction (XRD) patterns to verify the transformation of Ti3AlC2 to Ti3C2 and the oxidation of Ti3C2 to TiO2, Raman spectra to recognize the presence of C and C layer in the photocatalyst, and scanning/transmission electron microscopy (SEM/TEM) images to confirm a classic accordion-like multilayer Ti3C2, successfully demonstrating Al layer etching. In addition, the high-resolution TEM image of N-doped TiO2/MXene demonstrated that the (101)/(110) crystal plane of the anatase/rutile lattice with spacing of 0.352/0.325 nm, respectively, was consistent with XRD results (Huang et al., 2019).
A novel composite of microporous Ti3C2/TiO2-x was fabricated simply by mixing multilayer Ti3C2 with TiO2-x in 30% H2O2, which is basically composed of greatly fragmented/porous mono-layered MXene layers as the skeleton and TiO2-x nanodots, surrounded equally on the layer surfaces (Cheng et al., 2018). Titania–carbon nanosheets were prepared by high-energy ball milling of 2D Ti3C2Tx serving both as a T/C source and a structure-directing agent (Li et al., 2018a). The multi-stacked structure of Ti3C2Tx MXene is destroyed by the high-energy ball-milling technique, which effectively develops free-standing nanosheets. Wang et al. fabricated quasi-core-shell In2S3/anatase TiO2@metallic Ti3C2Tx mesoporous nanohybrids containing a double heterostructure (well-structured type-II heterojunction and non-noble-metal-based Schottky barrier), which have promising charge transfer channels with effective photocatalysis activity (Wang et al., 2018a). TiO2/Ti3C2 nanocomposites were prepared by a simple calcination technique with TiO2 in situ formation on highly conductive MXene under different temperature conditions (350–650 °C), exhibiting an exceptional rice-crust-like structure (Low et al., 2018). Surface alkalization of Ti3C2 MXene prepared through a simple KOH treatment was physically mixed with TiO2 to fabricate Ti3C2–OH/TiO2 nanohybrids, which were used as a photocatalyst for CO2 reduction (Ye et al., 2018).
For the degradation of organic pollutants, Ag3PO4 has been considered as exhibiting the greatest quantum effectiveness, approximately 90%, at a wavelength of approximately 420 nm in water photooxidation among silver-based photocatalysts, which include Ag3PO4, AgI, and AgBr (Dong et al., 2013; Shao et al., 2017). However, owing to the relatively high degree of carrier recombination and the low potential of the Ag3PO4 conduction band (Guo et al., 2014), its removal behavior to organic contaminants could still be enhanced through incorporation with MXenes. Ag3PO4/Ti3C2 interface composite was fabricated as a Schottky catalyst by an electrostatic self-assembly technique, which was employed to improve photocatalytic performance and anti-photocorrosion activities (Cai et al., 2018). The fabrication flowchart of Ag3PO4/Ti3C2 is described in detail in Fig. 2b. As a family of transition-metal oxide bismuth ferrites, BiFeO3 has a crystal structure with multiferroic characteristics and a band cap of 2.01 eV (Li et al., 2014). To produce the solar energy for photocatalytic activity, BiFeO3/Ti3C2 nanohybrid was fabricated using a simple and low-cost double-solvent solvothermal method (Iqbal et al., 2019b). The nanoparticle sizes of BiFeO3 and BiFeO3/Ti3C2 determined with Scherrer’s formula are 45 and 43 nm, respectively (Ma et al., 2014). As the lattice constant or lattice distortion that occurs upon doping or hybrid formation decreases, the particle size decreases. This decrease in particle size results in an improvement in the surface-to-volume ratio of the fabricated nanomaterial (Irfan et al., 2017). Compared with pure BiFeO3, the BiFeO3/Ti3C2 photocatalyst showed a relatively high Brunauer–Emmett–Teller surface area of 147 m2 g−1, a small band gap of 1.96 eV calculated from the Tauc plot, and a small recombination time based on the photoluminescence spectra (Iqbal et al., 2019b).
Noble-Ag-based nanophotocatalysts, such as Ag2WO4, have been widely used in various energy and environmental fields (McEvoy and Zhang, 2014). However, their common application is totally influenced by their photostability associated with a very high photo-induced electron–hole recombination rate (Li et al., 2019a). To solve this issue, Fang et al. successfully fabricated Ag2WO4/Ti3C2 Schottky nanocomposites through electrostatic interactions (Fang et al., 2019b). The crystal structure of Ag2WO4 incorporated with Ti3C2 was very similar to that of pure Ag2WO4, which significantly improved the photocatalytic performance and corrosion resistance of Ag2WO4. Metal sulfides, including FeS2 ZnS, In2S3, and CdS, have shown great potential as photocatalysts (Wu et al., 2018). Pure CdS shows low photocatalytic performance, owing to the photocorrosion and high degree of recombination of photo-induced charge carriers (Zhang et al., 2018c). Ternary Ti3C2–OH/ln2S3/CdS nanocomposites were fabricated via a facile hydrothermal synthesis technique, which was employed to determine their photocatalytic performance (Fang et al., 2019a). An alkaline treatment was conducted to achieve Ti3C2–OH particles with sufficient –OH functional groups. SEM images showed that the Ti3C2–OH/ln2S3/CdS nanocomposites have a series of nano-sheets with a thickness of approximately 20–30 nm, which are somewhat similar to the spherical structure of clean CdS. In addition, the high-resolution SEM image showed that the thickness and pore size of the nanosheets in Ti3C2–OH/ln2S3/CdS nanocomposites were considerably smaller than those in CdS catalyst (Fang et al., 2019a). Ti3C2–OH/Bi2WO6:Yb3+/Tm3+ nanocomposites containing a 2D/2D heterojunction with a grid-like porous structure were fabricated by a facile hydrothermal method (Fang et al., 2020). In addition, various other materials, such as Bi0.90Gd0.10Fe0.80Sn0.20O3 (Tariq et al., 2018), Ag2O, Ag, PdO, Pd, and Au (Wojciechowski et al., 2019), and a-Fe2O3 (Zhang et al., 2018a) were incorporated with Ti3C2 MXene and evaluated as photocatalysts for the removal of various organic contaminants.
Section snippets
MXene/TiO2-based composite
Porous 2D-layered N-doped TiO2/Ti3C2 MXene, which showed extraordinary stability, very high degree of electron transfer, and outstanding visible-light photocatalytic performance, was employed to determine its performance for phenol degradation (Huang et al., 2019). The degradation of phenol with a contact time of 3 h under visible-light irradiation (λ > 420 nm) was <10%, 38.7%, and 96% in the presence of TiO2, TiO2/Ti3C2 MXene, and N-doped TiO2/Ti3C2 MXene, respectively (Fig. 3a). The main
Stability of MXene-based photocatalysts
Determining the stability of photocatalysts is very important, since the photocorrosion of catalysts strictly limits their application. The anti-photocorrosion test of Ag3PO4/Ti3C2 was conducted through eight cycles for tetracycline hydrochloride removal (Cai et al., 2018). Clearly, the photocatalytic degradation of pure Ag3PO4 decreased by approximately 90% after eight cycles, demonstrating significant Ag3PO4 decomposition due to photocorrosion. However, a relatively small reduction
Conclusions and areas of future study
The fascinating properties of MXenes, such as adjustable morphology and bandgap configuration, high electrical conductivity, hydrophilicity, thermal strength, and large specific surface area, stimulate the use of MXene-based photocatalysts. In this review, we have explored the broad applications of MXene-based nanocomposites (particularly Ti3C2Tx), incorporated with various co-catalysts such as TiO2, BiVO4, Ag3PO4, In2S3, CdS, BiFeO3, BiOBr, Fe2O3, RuO2, C3N4, AgInS, and MIL-100(Fe) as
Credit author statement
Jong Kwon Im: Overall literature review, writing Sections 1, 2, and 3.1. Erica Jungmin Sohn: Partial literature review, writing Section 3.2. Sewoon Kim: Partial literature review, writing Section 4. Min Jang and Ahjeong Son: Funding acquisition and conceptualization. Kyung-Duk Zoh: Review and editing, Yeomin Yoon: Overall supervision.
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.
Acknowledgements
This research was funded by the Korea Ministry of Environment (The SEM projects; 2018002470005, South Korea) and the U.S. National Science Foundation (OIA-1632824).
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