Songliu Yuan

Hierarchical porous Ni/NiO core–shells with superior conductivity for electrochemical pseudo-capacitors and glucose sensors

Although the NiO nanostructures potentially hold outstanding electrochemical activity in theory, dual enhancements in both electrical conductivity and electrolyte transport are two challenging issues for designing high performance electrodes. In this work, hierarchical porous Ni/NiO core–shells are synthesized. The interconnected Ni skeletons with favorable electrical conductivity are uniformly covered by the continuous NiO scarfskins that hold both high energy storage capacity and efficient catalysis. The hierarchical porous Ni/NiO electrode exhibits superior pseudo-capacitive performance evidenced by an areal capacitance up to 255 mF cm−2. Meanwhile, this conductive electrode also exhibits electrocatalytic activity for glucose oxidation with a sensitivity of 4.49 mA mM−1 cm−2 and a reliable detection limit of 10 μM. On the other hand, hierarchical porosities enhance the effective transport of electrolytes and ions within the interconnected porous channels, making dramatic contributions to a superior storage stability of 4000 cycles and prompt an amperometric response time of 1.5 s. These concepts of the hierarchical metal–metal oxide core–shell open an avenue to design high-performance materials for energy storage and electrochemical catalysis.

Hierarchically porous Co3O4/C nanowire arrays derived from a metal–organic framework for high performance supercapacitors and the oxygen evolution reaction

Transition metal oxides with hierarchically porous structures supported by conductive substrates have been considered as promising electrodes for electrochemical energy storage and catalysis. Herein we configure porous Co3O4/C nanowire arrays (NAs) by thermally annealing a Co-based metal–organic framework (Co-MOF) in Ar and air, respectively. The hybrid Co3O4/C NAs demonstrate a high specific capacitance of 1.32 F cm−2 at a current density of 1 mA cm−2, which is much superior to that of bare Co3O4 NAs. A highly stable symmetric supercapacitor based on Co3O4/C exhibits an excellent durability with only 21.7% capacitance decay after 5000 cycles. Besides electrochemical energy storage, the Co3O4/C hybrids demonstrate an outstanding electrochemical catalysis ability for the oxygen evolution reaction, identified by the high current density of 30 mA cm−2 at low overpotential (η30 = 318 mV) and a small Tafel slope (81 mV dec−1). The electrical conductivity of the interconnected C infrastructures and ion diffusion within the hierarchical pores are intrinsic causes to promote the pseudo-capacitive performance and enhance catalytic activity. The synthesis strategy reported here opens an avenue to design high performance electrodes for energy storage and electrochemical catalysis.

Hierarchical Nanoporous Gold-Platinum with Heterogeneous Interfaces for Methanol Electrooxidation

The electrocatalysts utilized as the prospective electrodes in fuel cells and high efficient energy conversion devices require both the interconnected channels for efficient electrolyte transportation and the superior catalytic activity with long service life. In this work, nanoporous gold with the rigid skeletons in three dimensions is partially decorated by porous platinum shell containing nanoscale interstitials, aiming to create the heterogeneous gold-platinum interfaces and facilitate the electrolyte transportation as well. In comparison with no catalytic activity of bare nanoporous gold, the catalytic activity of hierarchical nanoporous gold-platinum towards electrochemical oxidation of methanol increases with the loading level of platinum shells, resulting in the highest electrochemical area of 70.4u2005m2·g−1 after the normalization by the mass of platinum. Heterogeneous gold-platinum interfaces affect the tolerance of the absorbed intermediate species because of the oxidization by the oxygenated species absorbed on the gold surface and the enhanced ion transportation within the porous platinum shell.

Nanoparticle monolayer-based flexible strain gauge with ultrafast dynamic response for acoustic vibration detection

The relatively poor dynamic response of current flexible strain gauges has prevented their wide adoption in portable electronics. In this work, we present a greatly improved flexible strain gauge, where one strip of Au nanoparticle (NP) monolayer assembled on a polyethylene terephthalate film is utilized as the active unit. The proposed flexible gauge is capable of responding to applied stimuli without detectable hysteresis via electron tunneling between adjacent nanoparticles within the Au NP monolayer. Based on experimental quantification of the time and frequency domain dependence of the electrical resistance of the proposed strain gauge, acoustic vibrations in the frequency range of 1 to 20,000 Hz could be reliably detected. In addition to being used to measure musical tone, audible speech, and creature vocalization, as demonstrated in this study, the ultrafast dynamic response of this flexible strain gauge can be used in a wide range of applications, including miniaturized vibratory sensors, safe entrance guard management systems, and ultrasensitive pressure sensors.

Planar integration of flexible micro-supercapacitors with ultrafast charge and discharge based on interdigital nanoporous gold electrodes on a chip

Flexible, wearable, implantable and easily reconfigurable micro-fabricated pseudocapacitors with impressive volumetric stack capacitance and energy densities are desired for electronic devices. In this work, scratching technology at the micron-scale enables construction of the planar electrode systems directly based on nanoporous gold films. We demonstrate that both nanoporous channels with high ion-accessible ability and interconnected skeletons with high conductivity enable the design of pseudocapacitive micro-supercapacitors with high performances. These planar devices show several attractive features including ultrafast charge/discharge (high rate), large capacitance (1.27 mF cm−2, 127 F cm−3), and ultrahigh energy density (0.045 W h cm−3) while maintaining a high power density (22.21 W cm−3). Especially, the superb cyclability and mechanical flexibility give them great potential for future microelectronics with a tiny volume. The design concept reported here provides an avenue to integrate planar micro-supercapacitors into large-scale devices with a small environmental footprint.

Closely packed nanoparticle monolayer as a strain gauge fabricated by convective assembly at a confined angle

The reliability and sensitivity of a strain gauge made from a nanoparticle monolayer intrinsically depend on electron tunneling between the adjacent nanoparticles, so that creating nanoscale interstitials with uniform distribution and tuning the interparticle separation reversibly during cyclic mechanical stress are two vital issues for performance enhancement. In this work, one assembly technique is initialized to fabricate parallel nanoparticle strips by precisely tailoring the contact angle of a gold colloid on a substrate. The assembly of a nanoparticle monolayer with a close-packed pattern can be simultaneously switched on and off by independently varying the contact angle across a threshold value of 4.2°. This nanoparticle strip shows a reversible and reliable electrical response even if a mechanical strain as small as 0.027% is periodically supplied, implying well-controlled electron tunneling between the adjacent nanoparticles.

Electrochemical Biosensor Based on Nanoporous Au/CoO Core–Shell Material with Synergistic Catalysis

An ultrathin CoO layer is deposited on the skeleton surfaces of a nanoporous gold (NPG) film by using atomic layer deposition, creating a flexible electrode. Detailed characterization demonstrates the superior performance of the flexible NPG/CoO hybrids for electrochemical catalysis. The NPG/CoO hybrid not only achieves high catalytic activity for glucose oxidation and H2O2 reduction, but also exhibits a linear dependence of the electrical signal on the concentration of glucose and H2O2 molecules in the electrolyte. Meanwhile, the sensitivity for H2O2 reduction can be as high as 62.5u2005μAu2009mm(-1) u2009cm(-2) with linear dependence on the concentration in the range of 0.1-92.9u2005mm. The high sensitivity is proposed to result from the synergistic effect of Au and CoO at the interfaces, and the high conductivity of the gold skeleton with a large surface area. The superior electrochemical performance of this hybrid electrode is promising for future potential applications in various transitional-metal-oxide-based electrochemical electrodes.

Ultrasensitive strain gauge with tunable temperature coefficient of resistivity

We demonstrate an ultrasensitive strain gauge based on a discontinuous metal film with a record detection limit as low as 8.3 × 10–6. Constructed by well-tunable crevices on the nanometer scale within the film, this gauge exhibits an ultrafast dynamic response to vibrations with a frequency range of 1 Hz to 10 kHz. More importantly, the temperature coefficient of resistivity (TCR) of the metal film is tunable owing to the cancellation effect caused by the possibility of tunneling across the nanoscale crevices (showing a negative temperature dependence) and the electron conduction within the metal islands (showing a positive temperature dependence). Consequently, a nullified TCR is achievable when the crevice size can be precisely controlled. Thus, a fabrication strategy to precisely control the nanoscale crevices was developed in this study through the real-time tracking of the electrical conductivity during thermal evaporation. The ultrasensitive strain gauge with a tunable thermal drift introduces numerous opportunities for precision devices and wearable electronics with superior reliability.

Topography-specific isotropic tunneling in nanoparticle monolayer with sub-nm scale crevices

Material used in flexible devices may experience anisotropic strain with identical magnitude, outputting coherent signals that tend to have a serious impact on device reliability. In this work, the surface topography of the nanoparticles (NPs) is proposed to be a parameter to control the performance of strain gauge based on tunneling behavior. In contrast to anisotropic tunneling in a monolayer of spherical NPs, electron tunneling in a monolayer of urchin-like NPs actually exhibits a nearly isotropic response to strain with different loading orientations. Isotropic tunneling of the urchin-like NPs is caused by the interlocked pikes of these urchin-like NPs in a random manner during external mechanical stimulus. Topography-dependent isotropic tunneling in two dimensions reported here opens a new opportunity to create highly reliable electronics with superior performance.

Negative magnetization and the sign reversal of exchange bias field in Co(Cr1-xMnx)2O4 (0≤x≤0.6)

A series of Co(Cr1-xMnx)2O4 (0u2009≤u2009xu2009≤u20090.6) ceramic samples have been synthesized by using the sol-gel method. The magnetic properties of the ceramics are experimentally studied through different protocols of dc magnetization measurements. It is found that Mn-doping continuously decreases the total magnetization for x in the range of 0u2009≤u2009xu2009≤u20090.2 and the net magnetization becomes negative in the range of 0.3u2009≤u2009xu2009≤u20090.5. The net magnetization reverses and becomes positive upon further increasing x to 0.6. This unusual magnetic phenomenon in the system for xu2009=u20090.3–0.5 can be called as negative magnetization. It is regarded as arising from the competition of the two magnetic sublattices at different crystallographic sites. For the sample xu2009=u20090.3, the magnetic switching effect near the compensation temperature Tcomp has been studied, and it shows potential applications in the spintronic devices. The magnetic configuration of the sample could be changed under a high magnetic field, and the spin is reoriented at TS...