Yanqing Wang

Vortex-like excitations and the onset of superconducting phase fluctuationin underdoped La 2- x Sr x CuO 4

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap) opens up at a temperature significantly higher than the critical temperature (by 100–220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La2-xSrxCuO4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.

Corrigendum: Nitrogen-doped porous carbon monoliths from polyacrylonitrile (PAN) and carbon nanotubes as electrodes for supercapacitors

Nitrogen-doped porous activated carbon monoliths (NDP-ACMs) have long been the most desirable materials for supercapacitors. Unique to the conventional template based Lewis acid/base activation methods, herein, we report on a simple yet practicable novel approach to production of the three-dimensional NDP-ACMs (3D-NDP-ACMs). Polyacrylonitrile (PAN) contained carbon nanotubes (CNTs), being pre-dispersed into a tubular level of dispersions, were used as the starting material and the 3D-NDP-ACMs were obtained via a template-free process. First, a continuous mesoporous PAN/CNT based 3D monolith was established by using a template-free temperature-induced phase separation (TTPS). Second, a nitrogen-doped 3D-ACM with a surface area of 613.8 m2/g and a pore volume 0.366 cm3/g was obtained. A typical supercapacitor with our 3D-NDP-ACMs as the functioning electrodes gave a specific capacitance stabilized at 216 F/g even after 3000 cycles, demonstrating the advantageous performance of the PAN/CNT based 3D-NDP-ACMs.

Morphology-controlled synthesis of sunlight-driven plasmonic photocatalysts Ag@AgX (X = Cl, Br) with graphene oxide template

Novel cubic Ag@AgX@Graphene (X = Cl, Br) nanocomposites are facilely manipulated by means of a graphene oxide (GO) sheet-assisted assembly protocol, where GO sheets act as an amphiphilic template for hetero-growth of AgX nanoparticles. A morphology transformation of AgX nanoparticles from sphere to cube-like shape was accomplished by involving GO. With further UV irradiation, the reduction of GO to graphene and the generation of Ag nanocrystals on AgX occur simultaneously. We have demonstrated that the thus-produced Ag@AgX@Graphene nanocomposites could be employed as stable plasmonic photocatalysts to decompose acridine orange as a typical dye pollutant under sunlight irradiation. Compared with the bare quasi-spherical Ag@AgX, such graphene-interfaced cubic Ag@AgX nanocomposites display distinctly higher adsorptive capacity, smaller crystal size and reinforced electron–hole pair separation owing to the interfacial contact between Ag@AgX and graphene sheet components, resulting in an enhanced photocatalytic decomposition performance. This investigation provides new possibilities for the development of morphology-controlled plasmonic photocatalysts and facilitates their practical application in environmental issues.

Hydrogen evolution reaction activity of nickel phosphide is highly sensitive to electrolyte pH

The nickel phosphide (Ni2P) family of materials have become a hot subject in hydrogen evolution reaction (HER) electrocatalyst research. Various studies have reported their high activity, high stability, and high faradaic efficiency. To date, there have been no systematic studies regarding the influence of pH on the HER performance of Ni2P. Here we show that the pH of electrolytes can strongly influence the HER activity of Ni2P electrocatalysts. Tests in 19 electrolytes with pH ranging from 0.52 to 13.53 show that Ni2P is much more active in strongly acidic and basic electrolytes. With the increase of pH, the lower H+ concentration reduces the formation of adsorbed H atoms in the Volmer reaction, resulting in poorer activities. However, the high activity observed in the strongly basic electrolytes is not the intrinsic property of Ni2P. We found that Ni oxides/hydroxides are formed in strongly basic electrolytes under applied potentials, resulting in improved activities. Furthermore, the specific activity based on the electrochemically active surface area of recently reported Ni2P catalysts is not high and requires significant improvements for practical applications.

Metal-free bifunctional carbon electrocatalysts derived from zeolitic imidazolate frameworks for efficient water splitting

Metal-free carbon catalysts have attracted great interest because of their high electrical conductivity, tailorable porosity and surface area, affordability, and sustainability. In particular, their bifunctional activity for hydrogen and oxygen evolution reactions (HER and OER) is attractive for electrochemical splitting of water. However, pristine carbon materials have low activities for HER/OER. Here, a high-performance carbon electrocatalyst is demonstrated by first pyrolyzing a metal–organic framework (MOF), i.e., zeolitic imidazolate framework-8 (ZIF-8), followed by optimized cathodic polarization treatment (CPT). Pyrolyzing ZIF-8 produces a highly N-doped (8.4 at%) carbon material having a large specific surface area of 1017 m2 g−1 with micro and mesopores. CPT in 0.5 M H2SO4 for up to 8 hours modulates the composition of N- and O-containing surface functional groups of the pyrolyzed ZIF-8 without sacrificing its large surface area and pore size distribution. After the 6-hour CPT, this material shows an excellent HER activity in 0.5 M H2SO4 electrolyte with an overpotential of 155 mV, a Tafel slope of 54.7 mV dec−1, and an exchange current density of 0.063 mA cm−2. And the 4-hour CPT results in excellent OER activity in 0.1 M KOH electrolyte with an overpotential of 476 mV and a Tafel slope of 78.5 mV dec−1. In a demonstration, these two carbon electrocatalysts steadily run a two-electrode water electrolyzer at a current density of 10 mA cm−2 over 8 hours under a potential of 1.82 V with a Faradaic efficiency of 98.0–99.1% in 0.1 M KOH electrolyte. The superior activity of the designed carbon electrocatalysts can be attributed to the functional group composition modulation achieved by CPT. High-performance metal-free carbon electrocatalysts derived from MOFs show excellent potentials for energy and environmental applications.

Polyelectrolyte-Induced Dispersion of Graphene Sheets in the Hybrid AgCl/PDDA/Graphene Nanocomposites

Cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) was used to stabilize graphene sheets in the self-assembly of AgCl/PDDA/Graphene heterostructure. The resultant AgCl/PDDA/Graphene nanocomposites were characterized by Scanning electron microscopy, Atomic force microscopy and X-ray photoelectron spectroscopy. The results showed that AgCl nanoparticles with sizes of 500 nm uniformly positioned on the PDDA stabilized graphene sheets surface. This work presents a facile and environmentally friendly approach to the synthesis of AgCl/PDDA/Graphene and opens up a new possibility for preparing graphene-based nanomaterials for large-scale applications.

Nano-RuO2-Decorated Holey Graphene Composite Fibers for Micro-Supercapacitors with Ultrahigh Energy Density

Compactness and versatility of fiber-based micro-supercapacitors (FMSCs) make them promising for emerging wearable electronic devices as energy storage solutions. But, increasing the energy storage capacity of microscale fiber electrodes, while retaining their high power density, remains a significant challenge. Here, this issue is addressed by incorporating ultrahigh mass loading of ruthenium oxide (RuO2 ) nanoparticles (up to 42.5 wt%) uniformly on nanocarbon-based microfibers composed largely of holey reduced graphene oxide (HrGO) with a lower amount of single-walled carbon nanotubes as nanospacers. This facile approach involes (1) space-confined hydrothermal assembly of highly porous but 3D interconnected carbon structure, (2) impregnating wet carbon structures with aqueous Ru3+ ions, and (3) anchoring RuO2 nanoparticles on HrGO surfaces. Solid-state FMSCs assembled using those fibers demonstrate a specific volumetric capacitance of 199 F cm-3 at 2 mV s-1 . Fabricated FMSCs also deliver an ultrahigh energy density of 27.3 mWh cm-3 , the highest among those reported for FMSCs to date. Furthermore, integrating 20 pieces of FMSCs with two commercial flexible solar cells as a self-powering energy system, a light-emitting diode panel can be lit up stably. The current work highlights the excellent potential of nano-RuO2 -decorated HrGO composite fibers for constructing micro-supercapacitors with high energy density for wearable electronic devices.