Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films

Flexible solid-state supercapacitors are of considerable interest as mobile power supply for future flexible electronics. Graphene or carbon nanotubes based thin films have been used to fabricate flexible solid-state supercapacitors with high gravimetric specific capacitances (80–200 F/g), but usually with a rather low overall or areal specific capacitance (3–50 mF/cm²) due to the ultrasmall electrode thickness (typically a few micrometers) and ultralow mass loading, which is not desirable for practical applications. Here we report the exploration of a three-dimensional (3D) graphene hydrogel for the fabrication of high-performance solid-state flexible supercapacitors. With a highly interconnected 3D network structure, graphene hydrogel exhibits exceptional electrical conductivity and mechanical robustness to make it an excellent material for flexible energy storage devices. Our studies demonstrate that flexible supercapacitors with a 120 μm thick graphene hydrogel thin film can exhibit excellent capacitive characteristics, including a high gravimetric specific capacitance of 186 F/g (up to 196 F/g for a 42 μm thick electrode), an unprecedented areal specific capacitance of 372 mF/cm² (up to 402 mF/cm² for a 185 μm thick electrode), low leakage current (10.6 μA), excellent cycling stability, and extraordinary mechanical flexibility. This study demonstrates the exciting potential of 3D graphene macrostructures for high-performance flexible energy storage devices

Tuning the Catalytic Activity of a Metal–Organic Framework Derived Copper and Nitrogen Co-Doped Carbon Composite for Oxygen Reduction Reaction

An efficient non-noble metal catalyst for the oxygen reduction reaction (ORR) is of great importance for the fabrication of cost-effective fuel cells. Nitrogen-doped carbons with various transition metal co-dopants have emerged as attractive candidates to replace the expensive platinum catalysts. Here we report the preparation of various copper- and nitrogen-doped carbon materials as highly efficient ORR catalysts by pyrolyzing porphyrin based metal organic frameworks and investigate the effects of air impurities during the thermal carbonization process. Our results indicate that the introduction of air impurities can significantly improve ORR activity in nitrogen-doped carbon and the addition of copper co-dopant further enhances the ORR activity to exceed that of platinum. Systematic structural characterization and electrochemical studies demonstrate that the air-impurity-treated samples show considerably higher surface area and electron transfer numbers, suggesting that the partial etching of the carbon by air leads to increased porosity and accessibility to highly active ORR sites. Our study represents the first example of using air or oxygen impurities to tailor the ORR activity of metal and nitrogen co-doped carbon materials and open up a new avenue to engineer the catalytic activity of these materials

Solution Processable Colloidal Nanoplates as Building Blocks for High-Performance Electronic Thin Films on Flexible Substrates

Low-temperature solution-processed electronic materials on plastic substrates are of considerable interest for flexible electronics. Solution dispersible inorganic nanostructures (e.g., zero-dimensional (0D) quantum dots or one-dimensional (1D) nanowires) have emerged as interesting ink materials for low-temperature solution processing of electronic thin films on flexible substrates, but usually with limited performance due to the large number of grain boundaries (0D) or incomplete surface coverage (1D). Here, we report two-dimensional (2D) colloidal nanoplates of layered materials as a new ink material for solution assembly of high-performance electronic thin films. The 2D colloidal nanoplates exhibit few dangling bonds and represent an ideal geometry for the assembly of highly uniform continuous thin films with greatly reduced grain boundaries dictated by large-area conformal plane–plane contact with atomically flat/clean interfaces. It can therefore promise efficient charge transport across neighboring nanoplates and throughout the entire thin film to enable unprecedented electronic performance. We show that Bi₂Se₃ and Bi₂Te₃ nanoplates can be synthesized with well-controlled thickness (6–15 nm) and lateral dimension (0.5–3 μm) and can be used for the assembly of highly uniform continuous thin films with a full surface coverage and an excellent room temperature carrier mobility >100 cm²·V^–1·s^–1, approaching that of chemical vapor deposition grown materials. Our study demonstrates a general strategy to using 2D nanoplates as a unique building block for the construction of high-performance electronic thin films on plastic substrates for future flexible electronics and optoelectronics

Solution Processable Holey Graphene Oxide and Its Derived Macrostructures for High-Performance Supercapacitors

Scalable preparation of solution processable graphene and its bulk materials with high specific surface areas and designed porosities is essential for many practical applications. Herein, we report a scalable approach to produce aqueous dispersions of holey graphene oxide with abundant in-plane nanopores via a convenient mild defect-etching reaction and demonstrate that the holey graphene oxide can function as a versatile building block for the assembly of macrostructures including holey graphene hydrogels with a three-dimensional hierarchical porosity and holey graphene papers with a compact but porous layered structure. These holey graphene macrostructures exhibit significantly improved specific surface area and ion diffusion rate compared to the nonholey counterparts and can be directly used as binder-free supercapacitor electrodes with ultrahigh specific capacitances of 283 F/g and 234 F/cm³, excellent rate capabilities, and superior cycling stabilities. Our study defines a scalable pathway to solution processable holey graphene materials and will greatly impact the applications of graphene in diverse technological areas

Large Area Growth and Electrical Properties of p‑Type WSe₂ Atomic Layers

Transition metal dichacogenides represent a unique class of two-dimensional layered materials that can be exfoliated into single or few atomic layers. Tungsten diselenide (WSe₂) is one typical example with p-type semiconductor characteristics. Bulk WSe₂ has an indirect band gap (∼1.2 eV), which transits into a direct band gap (∼1.65 eV) in monolayers. Monolayer WSe₂, therefore, is of considerable interest as a new electronic material for functional electronics and optoelectronics. However, the controllable synthesis of large-area WSe₂ atomic layers remains a challenge. The studies on WSe₂ are largely limited by relatively small lateral size of exfoliated flakes and poor yield, which has significantly restricted the large-scale applications of the WSe₂ atomic layers. Here, we report a systematic study of chemical vapor deposition approach for large area growth of atomically thin WSe₂ film with the lateral dimensions up to ∼1 cm². Microphotoluminescence mapping indicates distinct layer dependent efficiency. The monolayer area exhibits much stronger light emission than bilayer or multilayers, consistent with the expected transition to direct band gap in the monolayer limit. The transmission electron microscopy studies demonstrate excellent crystalline quality of the atomically thin WSe₂. Electrical transport studies further show that the p-type WSe₂ field-effect transistors exhibit excellent electronic characteristics with effective hole carrier mobility up to 100 cm² V^–1 s^–1 for monolayer and up to 350 cm² V^–1 s^–1 for few-layer materials at room temperature, comparable or well above that of previously reported mobility values for the synthetic WSe₂ and comparable to the best exfoliated materials

Significantly Enhanced Visible Light Photoelectrochemical Activity in TiO₂ Nanowire Arrays by Nitrogen Implantation

Titanium oxide (TiO₂) represents one of most widely studied materials for photoelectrochemical (PEC) water splitting but is severely limited by its poor efficiency in the visible light range. Here, we report a significant enhancement of visible light photoactivity in nitrogen-implanted TiO₂ (N-TiO₂) nanowire arrays. Our systematic studies show that a post-implantation thermal annealing treatment can selectively enrich the substitutional nitrogen dopants, which is essential for activating the nitrogen implanted TiO₂ to achieve greatly enhanced visible light photoactivity. An incident photon to electron conversion efficiency (IPCE) of ∼10% is achieved at 450 nm in N-TiO₂ without any other cocatalyst, far exceeding that in pristine TiO₂ nanowires (∼0.2%). The integration of oxygen evolution reaction (OER) cocatalyst with N-TiO₂ can further increase the IPCE at 450 nm to ∼17% and deliver an unprecedented overall photocurrent density of 1.9 mA/cm², by integrating the IPCE spectrum with standard AM 1.5G solar spectrum. Systematic photoelectrochemical and electrochemical studies demonstrated that the enhanced PEC performance can be attributed to the significantly improved visible light absorption and more efficient charge separation. Our studies demonstrate the implantation approach can be used to reliably dope TiO₂ to achieve the best performed N-TiO₂ photoelectrodes to date and may be extended to fundamentally modify other semiconductor materials for PEC water splitting

Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe₂ Single Crystals and Their Thickness-Dependent Electronic Properties

The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe₂) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe₂ nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 μm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-ray-diffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies show characteristics <i>E</i>_g and <i>A</i>_1g vibration modes at ∼109 and ∼155 cm^–1, with a systematic red shift with increasing nanosheet thickness. Electrical transport studies show the 2D PtTe₂ nanosheets display an excellent conductivity up to 2.5 × 10⁶ S m^–1 and show strong thickness-tunable electrical properties, with both the conductivity and its temperature dependence varying considerably with the thickness. Moreover, 2D PtTe₂ nanosheets show an extraordinary breakdown current density up to 5.7 × 10⁷ A/cm², the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides to date