Xining Zang

Uniformly Embedded Metal Oxide Nanoparticles in Vertically Aligned Carbon Nanotube Forests as Pseudocapacitor Electrodes for Enhanced Energy Storage

Carbon nanotube (CNT) forests were grown directly on a silicon substrate using a Fe/Al/Mo stacking layer which functioned as both the catalyst material and subsequently a conductive current collecting layer in pseudocapacitor applications. A vacuum-assisted, in situ electrodeposition process has been used to achieve the three-dimensional functionalization of CNT forests with inserted nickel nanoparticles as pseudocapacitor electrodes. Experimental results have shown the measured specific capacitance of 1.26 F/cm(3), which is 5.7 times higher than pure CNT forest samples, and the oxidized nickel nanoparticle/CNT supercapacitor retained 94.2% of its initial capacitance after 10,000 cyclic voltammetry tests.

Direct-Write, Self-Aligned Electrospinning on Paper for Controllable Fabrication of Three-Dimensional Structures.

Electrospinning, a process that converts a solution or melt droplet into an ejected jet under a high electric field, is a well-established technique to produce one-dimensional (1D) fibers or two-dimensional (2D) randomly arranged fibrous meshes. Nevertheless, the direct electrospinning of fibers into controllable three-dimensional (3D) architectures is still a nascent technology. Here, we apply near-field electrospinning (NFES) to directly write arbitrarily shaped 3D structures through consistent and spatially controlled fiber-by-fiber stacking of polyvinylidene fluoride (PVDF) fibers. An element central to the success of this 3D electrospinning is the use of a printing paper placed on the grounded conductive plate and acting as a fiber collector. Once deposited on the paper, residual solvents from near-field electrospun fibers can infiltrate the paper substrate, enhancing the charge transfer between the deposited fibers and the ground plate via the fibrous network within the paper. Such charge transfer grounds the deposited fibers and turns them into locally fabricated electrical poles, which attract subsequent in-flight fibers to deposit in a self-aligned manner on top of each other. This process enables the design and controlled fabrication of electrospun 3D structures such as grids, walls, hollow cylinders, and other 3D logos. As such, this technique has the potential to advance the existing electrospinning technologies in constructing 3D structures for biomedical, microelectronics, and MEMS/NMES applications.

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.

High aspect-ratio 3D microstructures via near-field electrospinning for energy storage applications

High-aspect-ratio, three dimensional (3D) microstructures have been constructed by a direct-write method via self-aligned near-field electrospinning process. Using papers as the collectors, the process enables both design and fabrication of a variety of 3D structures, such as walls, grids and other configurations based on layer-by-layer deposition of electrospun micro/nano fibers. Afterwards, these fabricated polymeric structures can be mechanically separated from the paper substrate for various applications. In this work, we have successfully fabricated 20-layer, ca. 100 μm in thickness, 2 × 2 cm2 3D grid-structures made of PVDF (Polyvinylidene fluoride) electrospun fibers. After a coating process using the dip-and-dry process in the CNT (carbon nanotube) ink, conductive and flexible electrodes made of CNT coated polymer grid-structures have been demonstrated and assembled as a micro-supercapacitor.

A hybrid supercapacitor using vertically aligned CNT-polypyrrole nanocomposite

We have successfully demonstrated, for the first time, the fabrication of vertically aligned carbon nanotube (VACNT)-polypyrrole (PPY) nanocomposites as a “hybrid supercapacitor” material directly integrated on silicon-based electrodes. In contrast to previous works, three distinctive achievements have been accomplished: (1) a “hybrid supercapacitor” using VACNT forest with electroplated PPY and dodecylbenzenesulfonate (DBS) as a dopant in acetonitrile, (2) realizing 500% higher capacitance as compared to the capacitance of electrodes made of VACNT or DBS-doped PPY alone, and (3) highly reversible cycling between -1 V and +1 V with improved knee frequency at 797 Hz. As such this hybrid nanocomposite could become a new class of material for future supercapacitors.

Graphene synthesis via droplet CVD AND its photonic applications

The process of “droplet CVD” for the synthesis of graphene sheets and its photonic applications have been demonstrated. Metal (Cu or Ni) droplets are naturally transformed from thin films under high temperature and used as the catalysts to grow graphene via the chemical vapor deposition (CVD) process. This work has achieved several advancements: (1) first demonstration of “droplet CVD” for graphene synthesis; (2) constructions of continuous graphene sheets with discontinuous metal droplets - readily available for graphene sensing applications; and (3) preliminary proof-of-concept demonstration of a photonic sensor based on the droplet CVD graphene. As such, this new class of fabrication process could open up various graphene-based device/system applications, including photonic sensors.

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.

Carbon SP 2 -SP 3 technology: Graphene-on-diamond thin film UV detector

This work presents a graphene-diamond-metal thin film system as ultraviolet (UV) light sensor on a flexible substrate. New scientific and engineering breakthroughs are: (1) first experimental investigation of electrical and optical properties of carbon-based sp2-sp3 (graphene-diamond) junction; (2) a peel-and-stick fabrication process to make flexible diamond thin films from CVD-grown micro crystalline diamond (MCD) wafers; and (3) a sandwich-like vertical sensor structure with graphene as a transparent top electrode, and metal (Ti-Au) as an ohmic bottom contact electrode. As such, the proposed detector/architecture can open up a new class of scheme to build diamond-based, attachable, portable and wearable optoelectronic systems.