Interfacial Assembly of Ti3C2Tx/ZnIn2S4 Heterojunction for High‐Performance Photodetectors

Abstract Two‐dimensional (2D) materials have emerged as prospective candidates for electronics and optoelectronics applications as they can be easily fabricated through liquid exfoliation and used to fabricate various structures by further subsequent processing methods in addition to their extraordinary and unique optoelectronic properties. Herein, the Ti3C2Tx/ZIS heterostructure with nanometer‐thick Ti3C2Tx‐MXene and ZnIn2S4 (ZIS) films is fabricated by successive interfacial assembly of liquid exfoliated 2D MXene and ZnIn2S4 nanoflakes. Benefiting from the superior light‐harvesting capability and low dark current of ZnIn2S4, the limited absorbance, large scattering coefficient, and high dark current disadvantages of MXene are ameliorated. Meanwhile, the separation and transport of photogenerated carriers in ZnIn2S4 are improved due to the excellent electrical conductivity of Ti3C2Tx nanoflakes. As a result, the as‐prepared Ti3C2Tx/ZIS heterostructure photodetector has excellent optoelectronic characteristics in terms of a high responsivity of 1.04 mA W−1, a large specific detectivity up to 1 × 1011 Jones, a huge on/off ratio at around 105, and an ultralow dark current at ≈10−12 A. This work demonstrates a convenient method to construct heterostructured photodetectors by liquid exfoliated 2D nanoflakes, the as‐fabricated Ti3C2Tx/ZIS heterostructured photodetectors show promising application potential for low‐cost, reliable, and high‐performance photodetectors.


Synthesis of Exfoliated Ti 3 C 2 T x Nanoflakes:
LiF (1 g) was dissolved in 10 mL of HCl (9 mol·L −1 ) completely by continuous stirring in the Teflon tube (50 mL) at 35 °C in a water heating bath, and then 0.5 g of Ti 3 AlC 2 was added with gentle stirring. The reaction was allowed to continue for 24 h with 300 rpm stirring at 35 °C. The resulting product was centrifuged (at 3500 rpm for 5 min each cycle) several times with deionized (DI) water until the pH of ∼6 is achieved. Finally, the dark green Ti 3 C 2 T x MXene aqueous solution was obtained from the supernatant after centrifugation at 3500 rpm for an hour for further use.
Synthesis of Exfoliated ZnIn 2 S 4 Nanoflakes: ZnIn 2 S 4 nanoflowers were fabricated by a facile low-temperature refluxing method followed by a moderate exfoliation. In detail, 0.3 mmol of ZnCl 2 and 0.6 mmol of InCl 3 ·4H 2 O were added to 30 ml DI water and stirred for 30 min. Subsequently, an excess amount of thioacetamide (TAA, 1.8 mmol) was added to the above solution and stirred for another 30 min. The solution was then heated to 95 °C and maintained at that temperature for 5 h under vigorous stirring.
The resulting precipitation was collected by centrifugation 2 times and re-dispersed into 50 ml DI water. The dispersion was sonicated continuously for 2 hours and then centrifuged at 5000 rpm for 5min to remove aggregates. After that, ZnIn 2 S 4 nanoflakes were obtained from the supernatant after centrifugation.

Fabrication of Assembled Ti 3 C 2 T x (ZnIn 2 S 4 ) Films:
The assembled MXene thin Films were prepared by the method of liquid/liquid interface assembly. Glass substrate was ultrasonic cleaned in acetone, ethanol, and water sequentially for the next operation.
Firstly, 3 mL of the prepared Ti 3 C 2 T x nanoflakes solution (0.002 mg/mL), 0.5 mL of n-hexane, and 200 μL of hydroxyl acid (1M) were mixed to form the liquid-liquid interface and reduce electrostatic repulsion between individual 2D Ti 3 C 2 T x nanoflakes. And then, methanol was injected rapidly into the two-phase interface to induce the dispersed 2D Ti 3 C 2 T x nanoflakes being captured at the interface. After evaporation of n-hexane, the nanomembrane was exposed to the "soft" air−water interface and transferred onto the pre-cleaned glass substrate. ZnIn 2 S 4 Films were fabricated by the same interfacial assembly procedure.
Device Fabrication: By repeating the above process, the multi-layered Ti 3 C 2 T x /ZIS heterojunction photodetector was constructed. After drying in air thoroughly, we put Au pads (prepared by thermally evaporating Au on the glass substrate with a shadow mask technique in the vacuum) onto the film to form the Ti 3 C 2 T x /ZIS heterojunction photodetector, where ZnIn 2 S 4 and Ti 3 C 2 T x were deposited at two separate sides of the substrate with an overlap width of 1 mm in between.