Functional Materials Letters

The biggest challenge for today’s supercapacitor systems readily possessing high power density is their low energy density. Their electrode materials with controllable structure, specific surface area, electronic conductivity, and oxidation state, have long been highlighted. Architecture engineering of functional electrode materials toward powerful supercapacitor systems is becoming a big fashion in the community. The construction of ion-accessible tunnel structures can microscopically increase the specific capacitance and materials utilization; stiff 3D structures with high specific surface area can macroscopically assure high specific capacitance. Many exciting findings in electrode materials mainly focus on the construction of ice-folded graphene paper, in situ functionalized graphene, in situ crystallizing colloidal ionic particles and polymorphic metal oxides. This feature paper highlights some recent architecture engineering strategies toward high-energy supercapacitor electrode systems, including electric double-layer capacitance (EDLC) and pseudocapacitance.

We fabricated a sensitive air flow detector that mimic the sensing mechanism found at the tail of some insects. [see Y. Yang, A. Klein, H. Bleckmann and C. Liu, Appl. Phys. Lett. 99(2) (2011); J. J. Heys, T. Gedeon, B. C. Knott and Y. Kim, J. Biomech. 41(5), 977 (2008); J. Tao and X. Yu, Smart Mat. Struct. 21(11) (2012)]. Our bionic airflow sensor uses a polyvinylidene difluoride (PVDF) microfiber with a molybdenum core which we produced with the hot extrusion tensile method. The surface of the fiber is partially coated with conductive silver adhesive that serve as surface electrodes. A third electrode, the metal core is used to polarize polyvinylidene difluoride (PVDF) under the surface electrodes. The cantilever beam structure of the prepared symmetric electrodes of metal core piezoelectric fiber (SMPF) is used as the artificial hair airflow sensor. The surface electrodes are used to measure output voltage. Our theoretical and experimental results show that the SMPF responds fast to air flow changes, the output charge has an exponential correlation with airflow velocity and a cosine relation with the direction of airflow. Our bionic airflow sensor with directional sensing ability can also measure air flow amplitude. [see H. Droogendijk, R. G. P. Sanders and G. J. M. Krijnen, New J. Phys. 15 (2013)]. By using two surface electrodes, our sensing circuit further improves sensitivity.

A novel 3D metal–organic framework (MOF) {[Zn₃(bpdc)₃ (p-4-bpmb)](DMF)₂}_n ( 1) (H${}_2\mathrm{\text{bpdc}}=\mathrm{\text{biphenyl}}$-4,4′-dicarboxylic acid; p-4-$\mathrm{\text{bpmb}}=1$,4-bis(pyridine-4-ylmethoxy)benzene) has been solvothermally synthesized and structurally characterized. Compound 1 exhibits a 3D pillar-layered framework based on the trinuclear {Zn₃} building blocks, consisting of Zn-dicarboxylate layers and bipyridyl-derivative pillars. Furthermore, it features three-fold interpenetrating 8-connected hex-type topology. In addition, its thermal stability and luminescent property have also been investigated.

We investigated how high magnetic field gradients affected the magnetostrictive performance of Tb${}_{0.27}$Dy${}_{0.73}$Fe${}_{1.95}$ during solidification. At high applied magnetic field gradients, the magnetostriction exhibited a gradient distribution throughout the alloy. Increasing the magnetic field gradient also increased the magnetostriction gradient. We attributed the graded magnetostrictive performance to the gradient distribution of (Tb, Dy)Fe₂ phase in the alloy and its orientation.

Nanostructured LiFePO₄ is appealing cathode material for rechargeable lithium batteries. Herein, however, we report the intriguing anode properties of carbon coated LiFePO₄ nanocrystals. In the potential range of 0–3.0 V, the LiFePO₄ nanocrystal electrodes afford high reversible capacity of 373 mAhg${}^{-1}$ at a current rate of 0.05 Ag${}^{-1}$ and retains 239 mAhg${}^{-1}$ at a much higher rate of 1.25 Ag${}^{-1}$. In addition, it is capable of sustaining 1000 cycles at 1.25 Ag${}^{-1}$ without any capacity fading. Such superior properties indicate that nanostructured LiFePO₄ could also be promising anode for rechargeable battery applications.

The nano-sized $\gamma$-MnO₂ precursor is synthesized using a room temperature, liquid-phase reaction route with the assistance of ultrasonic waves. The MnO₂ precursor as an electrode material in lithium manganese dioxide primary batteries displays a low capacity of 140mAhg${}^{-1}$ (45.5% for the theoretical capacity of MnO${}_2)$ at 20mAg${}^{-1}$. Therefore, the doped MnO₂ with cationic V or/and Ti are prepared at high temperature. After the heat treatment, the $\gamma$ phase precursor powder gradually converts into the $\beta$-MnO₂ and exhibits a higher specific surface area with a larger pore volume and pore size, providing significantly more electrochemically active sites for the redox reaction. The doped MnO₂ matrix has advantage of the ideal lattice parameters and the higher conductivity, resulting in an enhancement of the Li${}^{+}$ diffusion kinetics in the tunnel structure. Especially for co-doped MnO₂ with V and Ti, the modified material shows an outstanding electrochemical capacity of 190mAhg${}^{-1}$ (61.7% for the theoretical capacity) at 20mAg${}^{-1}$ and 169mAhg${}^{-1}$ for a higher power output of 100mAg${}^{-1}$.

A three-phase composite film was produced by inserting multi-walled carbon nanotubes (MWCNTs) and BaTiO₃ nanoparticles into polyimide (PI). The combination of in-situ polymerization and water-based preparation involved in the experiment ensured fillers’ homogeneous dispersion in the matrix, which led to flexible shape of the composite films. The dielectric properties of composite films as a function of the frequency and the volume fraction of MWCNTs were studied. Such composite film displayed a high dielectric constant (314.07), low dielectric loss and excellent flexibility at 100Hz in the neighborhood of percolation threshold (9.02 vol%) owing to the special microcapacitor structure. The experimental results were highly consistent with the power law of percolation theory.

In this work, La_0.5Sr_0.5CoO₃ (LSCO) perovskite oxide with perfect crystallinity was successfully synthesized via a sol–gel method and then used as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The DSSCs with LSCO CEs exhibited excellent electrocatalytic activity for the triiodide reduction and yielded a power conversion efficiency of 7.17%, which is greater than that of the Pt electrode (7.06%). Compared with the hydrothermal method and solvothermal method, sol–gel method is more suitable for large scale preparation. This work should open up a new class of CE materials for low-cost and high-efficiency DSSCs.

We prepared In_xPb_xCo₄Sb${}_{12}$ by high-pressure and high-temperature (HPHT) method. Samples were characterized by X-ray diffraction (XRD), electron microprobe analysis and thermoelectric properties measurements. The Seebeck coefficient, electrical resistivity and thermal conductivity of In_xPb_xCo₄Sb${}_{12}$ samples were all performed in the temperature range of 323–723K. With the increasing synthetic pressure, the Seebeck coefficient of In${}_{0.5}$Pb${}_{0.1}$Co₄Sb${}_{12}$ samples, which synthesized between 1.5 GPa–2.3 GPa, showed an obvious increase while the thermal conductivity exhibited a substantial reduction.

A cobalt (Co)/graphene sheets (GRs) composite was synthesized via a one-pot chemical method. The composite shows high saturation magnetizations (M_s), which leads it to be conveniently separated from aqueous solution by an external magnetic field. Compared to the pure Co and some references, the catalytic activity of the as-obtained composite was significantly enhanced for the generation of H₂ gas by hydrolysis of NaBH₄ solution. Effects of NaBH₄ initial concentration, the composite and reaction temperature on the H₂ generation rate were investigated. The H₂ generation rate is independent with the initial NaBH₄ concentration, increased with the reaction temperature increasing. The composite can be continuously used several times with about the same catalytic activity.

To examine the effect of crystal size and orientation effect on ZT, polycrystalline Na_xCo₂O₄ materials were prepared by pressing layered crystals obtained in sol–gel (SG) synthesis, molten salt synthesis (MSS) with and without additional ball milling (BM) treatment and 1:1 molar ratio mixture (Mixture) of BM powder and MSS powders. We found that the orientation effect and crystal size for four samples follow Mixture < SG < BM < MSS and BM < Mixture < SG < MSS, respectively. Electrical conductivity was obviously enhanced in the highly orientated BM and MSS samples when compared with SG and Mixture. It appears that the crystal size plays a dominant role in thermal conductivity rather than Seebeck coefficient by controlling the phonon scattering at grain boundaries. Thermal conductivity for BM was significantly decreased in comparison to MSS, although both BM and MSS show comparable orientation effect. The maximum ZT value is developed to near 0.51 at 814K upon increasing the electrical resistivity and decreasing the thermal conductivity, which are mainly governed by the condition of crystal size and orientation effect.

Er${}^{3+}$/Yb${}^{3+}$ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering method. The Y₂O₃: Er${}^{3+}$:Yb${}^{3+}$ nonopowders were prepared by co-precipitation method in advance. The morphology of powders and ceramics as investigated. The powders exhibited well dispersive condition with 100nm in diameter. The compactness of ceramics increased obviously with the increasing temperature. No pore was found in the ceramic sintered at $1800^{\circ }$C. The ceramic samples showed intense near infrared luminescence peaking at 1535nm under the excitation of 980nm lasers, assigned to the ${}^4{I}_{13∕2}{\to }^4{I}_{15∕2}$ transition of Er${}^{3+}$. The intense up-conversion emissions were observed under the excitation of 980nm and 1550nm lasers.

Heavily Cr${}^{3+}$-modified Li₄Ti₅O${}_{12}$ powders with a designed nominal composition of Li₃Cr₇Ti₂O${}_{16}$ have been prepared by one-step solid-state reaction. X-ray diffraction (XRD) combined with Rietveld refinement indicates that these powders contain 96.5wt.% spinel Li${}_{0.759}$Cr${}_{1.724}$Ti${}_{0.517}$O₄ and 3.5wt.% Cr₂O₃. Due to the combination of Ti${}^{3+}$/Ti${}^{4+}$ and Cr${}^{2+}$/Cr${}^{3+}$ redox couples in Li${}_{0.759}$Cr${}_{1.724}$Ti${}_{0.517}$O₄ and the existence of Cr₂O₃, the composite exhibits a large first-cycle discharge capacity of 315mAh$\cdot {\mathrm{\text{g}}}^{-1}$ at a small current density of 62.5mA$\cdot {\mathrm{\text{g}}}^{-1}$. Li${}_{0.759}$Cr${}_{1.724}$Ti${}_{0.517}$O₄ shows an improved Li${}^{+}$ ion diffusion coefficient and electronic conductivity, respectively arising from the small O${}^{2-}$ ion fractional coefficient and unpaired 3d electrons in Cr${}^{3+}$ ions. The majority of Cr₂O₃ is reduced to Cr after the first two lithiation processes, which benefits the electrical conduction between the Li${}_{0.759}$Cr${}_{1.724}$Ti${}_{0.517}$O₄ particles. Consequently, the composite exhibits a good rate performance and cyclability. Its capacity at 1000mA$\cdot {\mathrm{\text{g}}}^{-1}$ is as large as 141mAh$\cdot {\mathrm{\text{g}}}^{-1}$ with large retention of 90.1% after 100 cycles.

Nd-doped TiO₂ mesoporous microspheres with possessing regular micro/nanostructure were synthesized by a simple and facile method. The structure and optical properties of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption isotherms and UV-Visible absorbance spectroscopy. It was revealed that Nd-doped TiO₂ mesoporous microspheres are composed of primary nanoparticles with a particle size of $\sim$25nm. The photocatalytic activities of all the samples were evaluated by degradation methyl orange (MO) in aqueous solution as a model reaction under xenon lamp light irradiation. The results showed that the doped samples demonstrated a higher photocatalytic activity than TiO₂ mesoporous microspheres, and the MO of 10mg/mL almost could be completely degraded by the Nd-doped TiO₂ mesoporous sample (the dosage of Nd salt to TiO₂ is 6%) under xenon lamp light irradiation within 1h.

Silver nanoparticles superficially decorated with hierarchically smaller nanoparticles are obtained for the first time in the course of aging of silver globules in aerated aqueous ammonia. The process is caused at the beginning by gentle etching of metallic silver across extended defects and smoothing of their edges with oxygen dissolved in aqueous ammonia due to silver (I) stabilization within a soluble ammonia complex for a 1–2 day incubation period, then the complex accumulates as an intermediate product and gradually decays into nanosilver because of the ligand loss for the remaining 4–5 days. The as-formed metallic silver deposits onto larger silver particles heterogeneously decorating their surface; this broadens the initial plasmonic peak and causes a weak red shift because of nanostructuring. The latter makes a substantial contribution to the overall optical properties of the nanoparticles of a complex morphology and has to be considered as an important factor affecting optical characteristics of nanoparticle ensembles and their possible plasmonic applications.

Monolithic carbon has been synthesized via a sustainable biomimetic route utilizing intrafibrillar silicified collagen sponge as precursor and morphogenesis template. The mineralized silica in the biohybrid prevents collapse of the carbon during pyrolysis. Upon biosilica removal results show that the carbon monoliths inherit the porous fiber structure of the mother collagen. The carbon nanofiber framework facilitates the construction of a high electrical conductive pathway, while the internal spaces developed among the intertwined fibrillar network and pores within nanofiber walls offer room for sulfur storage. The as-obtained carbon-sulfur cathode exhibits an accessible discharge capacity approaching 800mAh g${}^{-1}$ in Li–S battery.

A new soft chemistry preparation method of submicron — thick porous coatings of metallic silver is suggested for possible surface enhanced Raman spectroscopy (SERS) applications. The method is based on facile deposition of diamminesilver (I) aerosols forming instantly a nanostructured layer by fast decomposition and self–reduction of [Ag(NH${}_3{)}_2{]}^{+}$ aqueous solutions onto surfaces of inorganic substrates under mild conditions of 280–300${}^{\circ }$C in air. A strong difference in overall microstructures and related SERS signals of model analytes is found for substrates with different deposition time and in comparison with a standard magnetron deposition technique. It is demonstrated that the suggested method is predominant for formation of robust SERS substrates with a stable and reproducible SERS enhancement.

BaSi₂ has been known as a wide gap semiconductor with an intrinsic low lattice thermal conductivity, therefore may be a promising environmental friendly thermoelectric (TE) material with abundant constituent elements. In this work, Ni_xBa${}_{1-x}$Si₂ compounds with 0$\le x\le 0.3$ are synthesized by arc melting. The resistivity, thermal conductivity and Seebeck coefficient are measured in the temperature range of 300–850K. The resistivity of the undoped BaSi₂ is as high as 480mOhm$\cdot$cm at room temperature. Through Ni-doping, both resistivity and the Seebeck coefficient decrease dramatically. Meanwhile, the total thermal conductivity has no apparent difference due to Ni-doping. The lattice thermal conductivity decreases with increasing temperature monotonously to be lower than 1W/m$\cdot$K at high temperatures.