Electrochromic (EC) materials can undergo an electrochemical oxidation reaction with inserted ions under an applied electric potential, causing a reversible change in light absorption, showing great application prospects in the fields of displays, smart windows, and optical camouflage. In recent years, with the increasing interest in the introduction of electrochromic into the flexible and wearable electronics field, it has become more and more important to incorporate flexible functions into EC devices while maintaining high energy efficiency, rapid response and long-term durability. . However, this is a very challenging task because: (a) EC performance and mechanical properties are closely related to the selected materials, nanostructures and assembly methods; (b) the transparent conductive electrode and EC materials need to be optimized together , And how to integrate them together.
Recently, two-dimensional transition metal carbides, nitrides and carbonitrides (MXenes for short) have become promising nanoelectronic materials with good electrochemical stability. Due to its unique optical and electrical properties, MXenes and its derivatives have shown potential applications in the fields of energy storage, catalysis and electronics. More importantly, two-dimensional nanosheets of MXenes and its derivatives can be prepared in a scalable manner in the liquid phase, which makes it easy to construct self-assembled membranes and hybrid structures, and is therefore suitable for large-scale applications.
The self-assembled MXene/TMO heterostructure with MXenes as flexible transparent electrodes and TMOs (derived from MXene) as the flexible EC layer is a promising high-performance flexible EC device. In addition to the advantages of easy preparation and integration of the above-mentioned different functional components of MXene derivatives, more importantly, the nanosheet network of TMO can not only achieve excellent EC performance, but also achieve reliable mechanical and chemical stability in various EC materials. .
In view of this, as a proof of concept, Professor Wang Hongzhi of Donghua University, Professor Xi Ling of Boston University, and Professor Yuxuan Lin of the University of California, Berkeley, successfully assembled Ti3C2Tx (T = O, OH or F) and derived TiO2 nanosheets into a liquid -Uniform nano-thick film at the liquid interface, and fabricated EC devices based on TiO2/Ti3C2Tx heterostructure on a flexible substrate. Due to the greatly improved ion diffusion and electronic conductivity of the assembled single-layer TiO2 film, and the 2D nature of the MXene electrode and the TiO2 EC layer, the device is the best flexibility with the fastest coloring speed, the highest coloring efficiency and excellent electrochemical stability One of EC devices.

Article highlights:
1. The realization of single crystal 2D TiO2 thin sheets with large lateral size and nanometer thickness, and the liquid/liquid interface self-assembly technology make the mass production of MXene and 2D TMO thin films with excellent uniformity.
2. Due to the unique electrical, electrochemical and mechanical properties of networked 2D nanostructures, EC devices based on self-assembled 2D-TMO/MXene heterostructures simultaneously achieve excellent electrochromic efficiency, fast coloring speed and excellent mechanical flexibility .

Figure 1 Preparation process of TiO2/Ti3C2Tx heterostructure on flexible substrate

Figure 2 Electrochromic properties of TiO2/Ti3C2Tx heterostructure

Figure 3 Demonstration of mechanical properties of TiO2/Ti3C2Tx heterostructure and large-area flexible EC device
Original link:
https://doi.org/10.1038/s41467-021-21852-7