Soft x-ray spectroscopy for probing electronic and chemical states of battery materials

The formidable challenge of developing high-performance battery system stems from the complication of battery operations, both mechanically and electronically. In the electrodes and at the electrode–electrolyte interfaces, chemical reactions take place with evolving electron states. In addition to the extensive studies of material synthesis, electrochemical, structural, and mechanical properties, soft x-ray spectroscopy provides unique opportunities for revealing the critical electron states in batteries. This review discusses some of the recent soft x-ray spectroscopic results on battery binder,transition-metal based positive electrodes, and the solid-electrolyte-interphase. By virtue of soft x-ray's sensitivity to electron states, the electronic property, the redox during electrochemical operations, and the chemical species of the interphases could be fingerprinted by soft x-ray spectroscopy. Understanding and innovating battery technologies need a multimodal approach, and soft x-ray spectroscopy is one of the incisive tools to probe the chemical and physical evolutions in batteries.     

Fabrication and electrochemical properties of free-standing single-walled carbon nanotube film electrodes

An easily manipulative approach was presented to fabricate electrodes using free-standing single-walled carbon nanotube (SWCNT) films grown directly by chemical vapor deposition.Electrochemical properties of the electrodes were investigated.In comparison with the post-deposited SWCNT papers,the directly grown SWCNT film electrodes manifested enhanced electrochemical properties and sensitivity of sensors as well as excellent electrocatalytic activities.A transition from macroelectrode to nanoelectrode behaviours was observed with the increase of scan rate.The heat treatment of the SWCNT film electrodes increased the current signals of electrochemical analyser and background current,because the heat-treatment of the SWCNTs in air could create more oxide defects on the walls of the SWCNTs and make the surfaces of SWCNTs more hydrophilic.The excellent electrochemical properties of the directly grown and heat-treated free-standing SWCNT film electrodes show the potentials in biological and electrocatalytic applications.     

Synthesis of Pr-doped ZnO nanoparticles: Their structural,optical, and photocatalytic properties

Undoped and praseodymium-doped zinc oxide(Pr-doped ZnO)(with 2.0-mol%–6.0-mol% Pr) nanoparticles as sunlight-driven photocatalysts are synthesized by means of co-precipitation with nitrates followed by thermal annealing.The structure, morphology, and chemical bonding of the photocatalysts are studied by x-ray diffraction(XRD), scanning electron microscopy(SEM) with energy dispersive x-ray emission spectroscopy(EDS), x-ray photoelectron spectroscopy(XPS), and Fourier transform infrared spectroscopy(FTIR), respectively. The optical properties are studied by photoluminescence(PL) and UV-vis diffuse reflectance spectroscopy(UV-vis DRS). We find that Pr doping does not change the crystallinity of ZnO; but it reduces the bandgap slightly, and restrains the recombination of the photogenerated electron–hole pairs. The photocatalytic performance of the photocatalysts is investigated by the photodegradation reaction of 10-mg/L rhodamine B(RhB) solution under simulated sunlight irradiation, showing a degradation rate of 93.75% in ZnO doped with6.0-mol% Pr.     

Photoelectrocatalytic oxidation of methane into methanol and formic acid over ZnO/graphene/polyaniline catalyst

ZnO/graphene/polyaniline(PANI) composite is synthesized and used for photoelectrocatalytic oxidation of methane under simulated sun light illumination with ambient conditions. The photoelectrochemical(PEC) performance of pure ZnO, ZnO/graphene, ZnO/PANI, and ZnO/graphene/PANI photoanodes is investigated by cyclic voltammetry(CV),chronoamerometry(J–t) and electrochemical impedance spectroscopy(EIS). The yields of methane oxidation products,mainly methanol(CH_3OH) and formic acid(HCOOH), catalysed by the synthesized ZnO/graphene/PANI composite are 2.76 and 3.20 times those of pure ZnO, respectively. The mechanism of the photoelectrocatalytic process converting methane into methanol and formic acid is proposed on the basis of the experimental results. The enhanced photoelectrocatalytic activity of the ZnO/graphene/PANI composite can be attributed to the fact that graphene can efficiently transfer photo-generated electrons from the inner region to the surface reaction to form free radicals due to its superior electrical conductivity as an inter-media layer. Meanwhile, the introduction of PANI promotes solar energy harvesting by extending the visible light absorption and enhances charge separation efficiency due to its conducting polymer characteristics.In addition, the PANI can create a favorable π-conjunction structure together with graphene layers, which can achieve a more effective charge separation. This research demonstrates that the fabricated ZnO/graphene/PANI composite promises to implement the visible-light photoelectrocatalytic methane oxidation.     

Direct growth of graphene films without catalyst on flexible glass substrates by PECVD

A hydrogen-plasma-etching-based plasma-enhanced chemical vapor deposition(PECVD) synthesis route without metal catalyst for preparing the graphene films on flexible glass is developed. The quality of the prepared graphene films is evaluated by scanning electron microscopy, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, ultraviolet-visible spectroscopy, and electrochemical measurements. In a radio frequency(RF) power range of50 W–300 W, the graphene growth rate increases with RF power increasing, while the intensity ratio of D-to G-Raman peak(ID/IG) decreases. When the RF power is higher than 300 W, the ID/IGrises again. By optimizing experimental parameters of hydrogen plasma etching and RF power, the properties of as-prepared flexible graphene on glass are modulated to be able to achieve the graphene's transparency, good electrical conductivity, and better macroscopic uniformity.Direct growth of graphene film without any metal catalyst on flexible glass can be a promising candidate for applications in flexible transparent optoelectronics.     

Effect of surface morphology on the electron mobility of epitaxial graphene grown on 0° and 8° Si-terminated 4H-SiC substrates

Graphene with different surface morphologies were fabricated on 8° -off-axis and on-axis 4H-SiC(0001) substrates by high-temperature thermal decompositions. Graphene grown on Si-terminated 8° -off-axis 4H-SiC(0001) shows lower Hall mobility than the counterpart of on-axis SiC substrates. The terrace width is not responsible for the different electron mobility of graphene grown on different substrates, as the terrace width is much larger than the mean free path of the electrons. The electron mobility of graphene remains unchanged with an increasing terrace width on Siterminated on-axis SiC. Interface scattering and short-range scattering are the main factors affecting the mobility of epitaxial graphene. After the optimization of the growth process, the Hall mobility of the graphene reaches 1770 cm 2 /V·s at a carrier density of 9.8.×10 12 cm 2 . Wafer-size graphene was successfully achieved with an excellent double-layer thickness uniformity of 89.7% on a 3-inch SiC substrate.     

Tight-binding electron–phonon coupling and band renormalization in graphene

The kink structure in the quasiparticle spectrum of electrons in graphene observed at 200 me V below the Fermi level by angle-resolved photoemission spectroscopy(ARPES)was claimed to be caused by a tight-binding electron–phonon(e–ph)coupling in the previous theoretical studies.However,we numerically find that the e–ph coupling effect in this approach is too weak to account for the ARPES data.The former agreement between this approach and the ARPES data is due to an enlargement of the coupling constant by almost four times.     

In-situ fabrication of ZnO nanoparticles sensors based on gas-sensing electrode for ppb-level H_2S detection at room temperature

The zinc oxide(ZnO)nanoparticles(NPs)sensors were prepared in-situ on the gas-sensing electrodes by a one-step simple sol-gel method for the detection of hydrogen sulfide(H₂S)gas.The sphere-like ZnO NPs were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),x-ray diffraction(XRD),energy dispersive x-ray analysis(EDX),and their H₂S sensing performance were measured at room temperature.Testing results indicate that the ZnO NPs exhibit excellent response to H₂S gas at room temperature.The response value of the optimal sample to750 ppb H₂S is 73.3%,the detection limit reaches to 30 ppb,and the response value is 7.5%.Furthermore,the effects of the calcining time and thickness of the film on the gas-sensing performance were investigated.Both calcining time and film thickness show a negative correlation with the H₂S sensing performance.The corresponding reaction mechanism of H₂S detection was also discussed.     

Thickness shear mode quartz crystal resonators with optimized elliptical electrodes

Quartz crystal resonators (QCRs) with circular electrodes have been widely used for various liquid and gas sensing applications. In this work,quartz crystal resonators with elliptical electrodes were studied and tested for liquid property measurement. Mindlin’s theory was used to optimize the dimension and geometry of the electrodes and a 5-MHz QCR with minimum series resistance and without any spurious modes was obtained. A series of AT-cut QCRs with elliptical electrodes of different sizes were fabricated and their sensing performances were compared to devices with circular electrodes. The experimental result shows that the device with elliptical electrodes can obtain lower resonance impedance and a higher Q factor,which results in a better loading capability. Even though the sensitivities of devices with elliptical and circular electrodes are found to be similar,the sensor with elliptical electrodes has much higher resolution due to a better frequency stability. The study indicates that the performance of QCRs with elliptical electrodes is superior to that of traditional QCRs with circular electrodes.     

Fabrication of VO_2 thin film by rapid thermal annealing in oxygen atmosphere and its metal–insulator phase transition properties

Vanadium dioxide thin films have been fabricated through sputtering vanadium thin films and rapid thermal annealing in oxygen. The microstructure and the metal–insulator transition properties of the vanadium dioxide thin films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and a spectrometer. It is found that the preferred orientation of the vanadium dioxide changes from(1ˉ11) to(011) with increasing thickness of the vanadium thin film after rapid thermal annealing. The vanadium dioxide thin films exhibit an obvious metal–insulator transition with increasing temperature, and the phase transition temperature decreases as the film thickness increases. The transition shows hysteretic behaviors, and the hysteresis width decreases as the film thickness increases due to the higher concentration carriers resulted from the uncompleted lattice. The fabrication of vanadium dioxide thin films with higher concentration carriers will facilitate the nature study of the metal–insulator transition.