Minsong Wei

Titanium Disulfide Coated Carbon Nanotube Hybrid Electrodes Enable High Energy Density Symmetric Pseudocapacitors

While electrochemical supercapacitors often show high power density and long operation lifetimes, they are plagued by limited energy density. Pseudocapacitive materials, in contrast, operate by fast surface redox reactions and are shown to enhance energy storage of supercapacitors. Furthermore, several reported systems exhibit high capacitance but restricted electrochemical voltage windows, usually no more than 1 V in aqueous electrolytes. Here, it is demonstrated that vertically aligned carbon nanotubes (VACNTs) with uniformly coated, pseudocapacitive titanium disulfide (TiS2 ) composite electrodes can extend the stable working range to over 3 V to achieve a high capacitance of 195 F g-1 in an Li-rich electrolyte. A symmetric cell demonstrates an energy density of 60.9 Wh kg-1 -the highest among symmetric pseudocapacitors using metal oxides, conducting polymers, 2D transition metal carbides (MXene), and other transition metal dichalcogenides. Nanostructures prepared by an atomic layer deposition/sulfurization process facilitate ion transportation and surface reactions to result in a high power density of 1250 W kg-1 with stable operation over 10 000 cycles. A flexible solid-state supercapacitor prepared by transferring the TiS2 -VACNT composite film onto Kapton tape is demonstrated to power a 2.2 V light emitting diode (LED) for 1 min.

A Solar-Blind UV Detector Based on Graphene-Microcrystalline Diamond Heterojunctions

An ultraviolet detector is demonstrated through a whole-wafer, thin diamond film transfer process to realize the heterojunction between graphene and microcrystalline diamond (MCD). Conventional direct transfer processes fail to deposit graphene onto the top surface of the MCD film. However, it is found that the 2 µm thick MCD diamond film can be easily peeled off from the growth silicon substrate to expose its smooth backside for the graphene transfer process for high-quality graphene/MCD heterojunctions. A vertical graphene/MCD/metal structure is constructed as the photodiode device using graphene as the transparent top electrode for solar-blind ultraviolet sensing with high responsivity and gain factor. As such, this material system and device architecture could serve as the platform for next-generation optoelectronic systems.

Laser‐Induced Molybdenum Carbide–Graphene Composites for 3D Foldable Paper Electronics

Versatile and low-cost manufacturing processes/materials are essential for the development of paper electronics. Here, a direct-write laser patterning process is developed to make conductive molybdenum carbide-graphene (MCG) composites directly on paper substrates. The hierarchically porous MCG structures are converted from fibrous paper soaked with the gelatin-mediated inks containing molybdenum ions. The resulting Mo3 C2 and graphene composites are mechanically stable and electrochemically active for various potential applications, such as electrochemical ion detectors and gas sensors, energy harvesters, and supercapacitors. Experimentally, the electrical conductivity of the composite is resilient to mechanical deformation with less than 5% degradation after 750 cycles of 180° repeated folding tests. As such, the direct laser conversion of MCGs on papers can be applicable for paper-based electronics, including the 3D origami folding structures.

Foldable paper electronics by direct-write laser patterning

We report a laser-ablation aided, direct-write fabrication technique that could convert non-conductive paper rinsed with metal ions and polymer solution into conductive metal carbide and graphene with a typical sheet resistance of 45.3 Ω/□. As fabricated paper electronics inherit the microfiber network from paper and have nanoscale pores and 2D metal carbide flakes due to the laser ablation process. This conducive porous structure could be potentially utilized for sensor and capacitor applications, which usually need large specific area. As preliminary demonstrations, we show a wireless moisture sensor and a supercapacitor fabricated with this foldable paper based electronics. Experimentally, the moisture changes are successfully detected in ambient environment by a paper-based moisture sensor and the paper-based supercapacitor has a measured capacitance of 1.2 mF/cm2. As such, this laser converted paper electronics could be useful for multiple applications such as sensors and energy storage devices.

Self-Assembly of Large-Area 2D Polycrystalline Transition Metal Carbides for Hydrogen Electrocatalysis

Low-dimensional (0/1/2 dimension) transition metal carbides (TMCs) possess intriguing electrical, mechanical, and electrochemical properties, and they serve as convenient supports for transition metal catalysts. Large-area single-crystalline 2D TMC sheets are generally prepared by exfoliating MXene sheets from MAX phases. Here, a versatile bottom-up method is reported for preparing ultrathin TMC sheets (≈10 nm in thickness and >100 μm in lateral size) with metal nanoparticle decoration. A gelatin hydrogel is employed as a scaffold to coordinate metal ions (Mo5+ , W6+ , Co2+ ), resulting in ultrathin-film morphologies of diverse TMC sheets. Carbonization of the scaffold at 600 °C presents a facile route to the corresponding MoCx , WCx , CoCx , and to metal-rich hybrids (Mo2- x Wx C and W/Mo2 C-Co). Among these materials, the Mo2 C-Co hybrid provides excellent hydrogen evolution reaction (HER) efficiency (Tafel slope of 39 mV dec-1 and 48 mVj = 10 mA cm-2 in overpotential in 0.5 m H2 SO4 ). Such performance makes Mo2 C-Co a viable noble-metal-free catalyst for the HER, and is competitive with the standard platinum on carbon support. This template-assisted, self-assembling, scalable, and low-cost manufacturing process presents a new tactic to construct low-dimensional TMCs with applications in various clean-energy-related fields.

Synthesis of single layer MOS 2 array for surface Raman enhancement spectroscopy

We present direct synthesis of high quality single-layer MoS2 array on substrates with pre-deposited sulfur seeds for the first time with strong Raman signals of diluted Rhodamine 6G (R6G) due to Surface-Enhanced Raman Scattering (SERS). This work has following unprecedented accomplishments: (1) the usages of sulfur terminated substrate for the synthesis of CVD MoS2 array; (2) SERS using the monolayer MoS2 array with partially bowtie structures. (3) First time demonstration of SERS and photoluminescence (PL) on MoS2 covered with Al2O3, which address the possibility of the physical enhancement mechanism. As such, the newly developed processes can enable large scale growth of MoS2 for applications in bio-chemical sensing, and bring great inspiration in the 2D chalcogenide SERS mechanism.

Flexible Harsh environment micro supercapacitors using direct-write 2D transition metal carbides

2D transition metal carbides (MXenes) materials have drawn great interests for supercapacitor applications due to their unique properties, such as metallic conductivity and large specific surface areas. However, it is rather difficult to fabricate MXenes by using the process from the MAX phase, which requires a high temperature and HF etching processes. Here, we demonstrate a direct-write and fast conversion of molybdenum carbide from the Mo ions polymer composite on top of a flexible polymer substrate by an infrared laser beam. XRD results validate that the material is successfully converted to Mo3C2. The as-converted Mo3C2 has highly porous, 3D sponge-like structure generated by the localized heating effects. Preliminary testing results show that a micro supercapacitor using as-fabricated Mo3C2 as electrodes has a high measured specific capacitance of 50 F/g. Electrochemical tests of flexible micro supercapacitors at both low and high temperatures from −20 to 300°C have shown repeatable and stable performances. This laser conversion method has great potential for ultra-fast and low-cost synthesis of transition metal carbides material and the Mo3C2-based micro supercapacitor provides a promising alternative for harsh environment applications.