A one-pot synthesis of nitrogen doped porous MXene/TiO2 heterogeneous film for high-performance flexible energy storage
Graphical abstract
Introduction
In recent years, the two-dimensional (2D) material family has aroused extraordinary attention in the area of material science and technologies [1], [2], [3]. In the explorations of novel 2D materials, a new burgeoning group of transition metal carbides, collectively known as MXene, has been considered as a unique one among others due to its plenty of striking physical and chemical properties, such as excellent charge transfer kinetics, simple preparation and tunable redox chemistry at the metal oxide/hydroxide surfaces, which make MXene an ideal candidate for energy storage applications, especially in the area of flexible energy storage [4], [5], [6], [7], [8], [9], [10].
As a result, the delaminated MXene nanosheets can be easily assembled into freestanding “paper” electrodes with excellent energy storage performances, which are on the topmost level among other conventional graphene or carbon nanotube based freestanding electrodes [11], [12]. However, the emergence of a new material brings both excitement and challenges. One of the insurmountable barriers faced by delaminated MXene layers is that their conductive 2D structure is inclined to disassemble in the presence of water and oxygen or even oxygen-containing surface groups [7], especially at elevated-temperature treatment, which poses strict restrictions on the selection of modification method for MXene. The other is their tendency to re-stack during the film-forming process on account of hydrogen bonding or van der Waals forces, leading to substantial loss of electrochemical active area for energy storage [13], [14].
To address these problems, a common concept is to introduce “spacers”, which usually are low-dimensional inorganic nanostructures dispersing between the adjacent MXene layers, to isolate them from self-restacking, thereby protecting the electroactive sites from being sacrificed [15], [16], [17]. However, it is difficult to ensure that the implanted nanoparticles can be uniformly loaded between MXenes, since these inorganic nanoparticles trend to aggregate during the hybridizing process, which will impose stress on the whole hybridized assembly and increased the structural instability. Additionally, weak intrinsic interactions between MXenes and the particles can cause detachments and separations, leading to the isolation of electrical contacts.
Beyond that, the restacking risk of MXenes can also be reduced by introducing heteroatom dopants. It has been reported that N-doping of MXene layers can effectually release their restacking through the formation of wrinkles and the increased electrostatic repulsion, which can promote the infiltration and accessibility of electrolyte between layers [18], [19], [20]. However, the doping process of MXenes is usually triggered in harsh reaction conditions such as high temperature with protective atmosphere to avoid MXenes from oxidation [21], [22], while the sustainable hydrothermal doping with moderate conditions will result in severe destruction of MXene’s 2D structure.
Here, in this work, a protective hydrothermal method was applied to synthesis nitrogen doped porous MXene/TiO2 heterostructure in one pot, which enables a well preserved conductivity of MXene and controlled in-situ generation of uniformly dispersed TiO2 nanoparticles. In the meantime, the protective agent can also functions as N source during the hydrothermal process and introduce N heteroatom into MXene, offering a sustainable and easy way for N-doping and modulation of MXene. This method provides a well-established solution for the problems mentioned above. Since the oxidation of MXene layers is protected and controlled, the 2D structure and conductivity of MXene layers are maintained well after the treatment, while the oxidation product, the in-situ generated TiO2 nanoparticles, which functions as spacers, are uniformly and firmly attached on the MXene layers to avoid MXene re-stacking and maintain the assembled film structural stable. Besides, additional structural and functional benefits can be obtained by N doping. This unique hybridized structure provides a chance to integrate several physical and chemical advantages in a complementary easy way. As a result, the assembled freestanding film electrode based on the N-doped porous MXene/TiO2 heterogeneous layers demonstrates excellent energy storage performances.
Section snippets
Synthesis of Ti3C2Tx MXene
Ti3C2Tx MXene was prepared in a etchant solution to remove Al atom from Ti3AlC2. The etchant solution was prepared by mixing 0.8 g of LiF with 12 mL of 9.5 M HCl under continuous stirring until the LiF was completely dissolved. Then 0.5 g of Ti3AlC2 powder was slowly added into the etchant and reacted at 40 °C for 24 h. After the etching process, the resultant is repeatedly washed by centrifugation until pH ≥ 6, and the upper dark green dispersion is freeze-dried to obtain Ti3C2Tx MXene.
Preparation of N-doped porous MXene/TiO2 heterogeneous film
N-doped
Result and discussions
Fig. 1 illustrates the procedure for the synthesis of N-doped porous MXene/TiO2 heterogeneous film through a one pot process, that is, a protective hydrothermal treatment method, which can control the oxidation of MXene and simultaneously dope N atoms into MXene framework. Specifically, the etched-Ti3C2Tx colloidal suspension was first stirred with excess cysteine (CYS) solution to absorb CYS on the thin MXene layers, followed by hydrothermal treatment to obtain the N-doped porous MXene/TiO2.
Conclusions
In conclusion, a protective hydrothermal method was proposed in this work to construct porous MXene/TiO2 and, in the meantime, introduce N atom into MXene lattice through a more sustainable and easy way. The controlled partial oxidation of MXene to TiO2 formed a porous MXene/TiO2 heterostructure, which combines the advantages of large surface area, uniformly dispersed and strongly restrained TiO2 on MXene layer, high electrochemical activity and well-preserved conductivity. Besides, N doping in
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 work was supported by National Natural Science Foundation of China (51973118, 21805193), the Science and Technology Innovation Commission of Shenzhen (JCYJ20170818093832350, JCYJ20180507184711069, JCYJ20180305125319991), Key-Area Research and Development Program of Guangdong Province (2019B010929002, 2019B010941001), the Program for Guangdong Introducing Innovative and Enterpreneurial Teams (2019ZT08C642), China Postdoctoral Science Foundation (2019M650212).
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These authors contributed equally to this work.