Linxi Hou

Template synthesis of CoSe2/Co3Se4 nanotubes: tuning of their crystal structures for photovoltaics and hydrogen evolution in alkaline medium

Tubular-structured nanomaterials with tailorable crystal structures and shell architectures whose properties can be tuned without changing their chemical compositions are attractive in electrochemical energy conversion and storage fields. Herein, we report the fabrication of tubular-structured orthorhombic CoSe2 (o-CoSe2) and cubic CoSe2 (c-CoSe2) by calcining monoclinic Co3Se4 nanotubes (Co3Se4 NTs) prepared by a facile precursor transformation method. Benefiting from advantageous structural features, including functional shells and well-defined interior voids, the tubular-structured o-CoSe2 showed a high power conversion efficiency of 9.34% as a counter electrode catalyst for dye-sensitized solar cells (DSSCs), superior to that of a Pt counter electrode (8.15%), under AM 1.5 G irradiation. In addition, the o-CoSe2 nanotubes also demonstrated excellent electrocatalytic activity in terms of low onset overpotential (∼54 mV) and small Tafel slope (∼65.9 mV per decade) as a hydrogen evolution reaction (HER) catalyst in alkaline medium. Hence, this work provides a promising strategy to selectively design and synthesize highly active electrocatalysts for energy conversion.

Hierarchical Porous Co9S8/Nitrogen-Doped Carbon@MoS2 Polyhedrons as pH Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction

The development of highly active and stable earth-abundant electrocatalysts to reduce or eliminate the reliance on noble-metal based ones for hydrogen evolution reaction (HER) over a broad pH range remains a great challenge. Herein, hierarchical porous Co9S8/N-doped carbon@MoS2 (Co9S8/NC@MoS2) polyhedrons have been synthesized by a facile hydrothermal approach using highly conductive Co/NC polyhedrons composed of cobalt nanoparticles embedded in N-doped carbon matrices as both the structural support and cobalt source. The Co/NC polyhedrons were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) in Ar atmosphere. Benefiting from the prominent synergistic effect of N-doped carbon enhancing the conductivity of the hybrid, MoS2 and Co9S8 providing abundant catalytically active sites as well as the well-defined polyhedral structure promoting mechanical stability, the as-synthesized Co9S8/NC@MoS2 shows excellent HER activity and good stability over a broad pH range, with onset overpotentials of 4, 38, and 45 mV, Tafel slopes of 60.3, 68.8, and 126.1 mV dec-1, and overpotentials of 67, 117, and 261 mV at 10 mA cm-2 in 1.0 M KOH, 0.5 M H2SO4, and 1.0 M phosphate buffer solution (PBS), respectively. This work provides a general and promising approach for the design and synthesis of inexpensive and efficient pH-universal HER electrocatalysts.

Heterogemini surfactant assisted synthesis of monodisperse icosahedral gold nanocrystals and their applications in electrochemical biosensing

Icosahedral nanocatalysts (NCs) have shown very interesting physical and chemical properties owing to their multiply twinned nanostructures. Herein, we introduce a novel heterogemini surfactant (C10OhpNC8) assisted seed mediated growth approach for the synthesis of monodisperse icosahedral gold (Au) NCs in aqueous solution at room temperature. Very small shape impurities were observed in the resultant icosahedral Au NCs. Significantly improved monodispersity (relative standard deviation (RSD) of <10%) has been achieved by using a binary mixture of C10OhpNC8 and PVP as structure directing agents. Interestingly, the size of icosahedral Au NCs can be tuned ranging from 40 nm to 190 nm, which guides the surface plasmon resonance (SPR) peak to be tuned throughout the whole visible region and even to the near infrared (NIR) region. Furthermore, the developed icosahedral Au NCs specific probe has been designed to be applied as an easy electrochemical biosensor and successfully used to detect the bacteria Escherichia coli O157:H7 (E. coli O157:H7) with a detection limit of ∼10 colony forming units (CFU) mL−1. Notably, a much higher sensitivity of these icosahedral Au NCs probes has been achieved compared to the traditional colloidal gold immunochromatography (detection limit ∼103 CFU mL−1).

Tunable electrorheological characteristics and mechanism of a series of graphene-like molybdenum disulfide coated core–shell structured polystyrene microspheres

Core–shell structured molybdenum disulfide (MoS2) coated polystyrene (PS) microspheres are synthesized with the help of hexadecyl trimethyl ammonium bromide (CTAB) through negative–positive electrostatic attraction. The morphology of the composite particles is studied by scanning electron microscopy (SEM), which apparently provides evidence of MoS2-coated PS. The microspheres’ structure and chemical components are investigated by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy, respectively. The MoS2/PS composite particles show better thermal stability than PS according to thermogravimetric analysis (TGA). Novel electrorheological (ER) fluids based on the MoS2/PS composite dispersed in silicone oil are prepared and further examined by a rotational rheometer in a controlled shear rate mode under various electric field strengths. The influence of factors such as the electric field strength, the particle sizes, the proportions of the MoS2/PS composite and the functional groups on the surface of PS on ER properties is investigated. The related mechanism of these effects on ER behaviors is also analyzed in detail, aiming to find whether the graphene analogue MoS2 is superior to graphene when making them into ER fluids. MoS2 has reversible and tunable electrorheological characteristics and can transform its phase from a liquid-like to a solid-like state when exposed to an external electric field.

Nickel Cobalt Sulfide Double-Shelled Hollow Nanospheres as Superior Bifunctional Electrocatalysts for Photovoltaics and Alkaline Hydrogen Evolution

Transition metal chalcogenides with hollow nanostructures have been considered as promising substitutes as precious metal electrocatalysts for energy conversion and storage. We synthesized NiCo2S4 double-shelled ball-in-ball hollow spheres (BHSs) via a simple solvothermal route and applied them in both dye-sensitized solar cells (DSSCs) and hydrogen evolution reactions (HERs) at the same time, which were clean and sustainable ways to convert energy. Benefiting from their remarkable structure features and advantageous chemical compositions, NiCo2S4 BHSs composed of tiny crystals possessed large surface area, well-defined interior voids, and high catalytic activity. The DSSC with NiCo2S4 BHSs under 100 mW cm-2 irradiation possessed a power conversion efficiency of 9.49% (Pt, 8.30%). Besides, NiCo2S4 BHSs as a HER catalyst also possessed a small onset overpotential (27.9 mV) and a low overpotential (89.7 mV at 10 mA cm-2) under alkaline conditions. Therefore, this work offers a sensible strategy to synthesize bifunctional electrocatalysts for DSSCs and HERs.

Preparation and characterization of poly(vinyl alcohol)/sodium alginate hydrogel with high toughness and electric conductivity

Development of bio-based hydrogels with good mechanical properties and high electrical conductivity is of great importance for their excellent biocompatibility and biodegradability. Novel electrically conducive and tough poly(vinyl alcohol)/sodium alginate (PVA/SA) composite hydrogel was obtained by a simple method in this paper. PVA and SA were firstly dissolved in distilled water to form the composite solution and the pure PVA/SA hydrogel was obtained through the freezing/thawing process. The pure PVA/SA hydrogels were subsequently immersed into the saturated NaCl aqueous solution to increase the gel strength and conductivity. The effect of the immersing time on the thermal and mechanical properties of PVA/SA hydrogel was studied. The swelling properties and the antiseptic properties of the obtained PVA/SA hydrogel were also studied. This paper provided a novel way for the preparation of tough hydrogel electrolyte.

Distinctive Performance of Gemini Surfactant in the Preparation of Hierarchically Porous Carbons via High-Internal-Phase Emulsion Template

The superior capability of gemini surfactant (GS) in the preparation of hierarchically porous carbons via high-internal-phase emulsion (HIPE) template followed by pyrolysis was confirmed in this work. Polymerized HIPEs (polyHIPEs) of phenol-formaldehyde resin were prepared by cross-linking the continuous phase of HIPEs stabilized by GS. Nonionic surfactant and cationic surfactant were also selected to stabilize HIPE for comparison. From scanning electron microscope observations, polyHIPEs with open-cell pore architectures were obtained with GS as emulsifier (polyHIPEs-GS) and the derived carbon foams (carboHIPEs-GS) well retained the original pore architectures, whereas polyHIPEs obtained using contrastive surfactants showed closed-cell porous structures and notable differences were observed for the derived carboHIPEs. Nitrogen adsorption/desorption measurements indicated that polyHIPEs-GS and carboHIPEs-GS both exhibited hierarchically porous architectures with much higher surface areas (SA) than those of the corresponding contrast samples. Mercury intrusion porosimetry results indicated that carboHIPEs-GS possessed higher SA and higher porosity than that of the contrast samples. The open-cell pore architecture and high SA are favorable to many applications, like energy storage. carboHIPE-GS expectably showed a higher capacitance than that of contrast samples when used as the electrode material of supercapacitor.

Humidity-Responsive Gold Aerogel for Real-Time Monitoring of Human Breath

Humidity sensors have received considerable attention in recent years because of their significance and wide applications in agriculture, industries, goods stores, and medical fields. However, the conventional humidity sensors usually possessed a complex sensing mechanism and low sensitivity and required a time-consuming, labor-intensive process. The exploration for an ideal sensing material to amplify the sensitivity of humidity sensors is still a big challenge. Herein, we developed a simple, low-cost, and scalable fabrication strategy to construct a highly sensitive humidity sensor based on polymer/gold nanoparticle (AuNP) hybrid materials. The hybrid polymer/AuNP aerogel was prepared by a simple freeze-drying method. By taking advantage of the conductivity of AuNPs and high surface area of the highly porous structure, the hybrid poly- N-isopropylacrylamide (PNIPAm)/AuNP aerogel showed high sensitivity to water molecules. Interestingly, the hybrid PNIPAm/AuNP aerogel-based humidity sensor can be used to detect human breath in different states, such as normal breath, fast breath, and deep breath, or in different individuals such as persons with illness, persons who are smoking, and persons who are normal, which is promising in practical flexible wearable devices for human health monitoring. In addition, the humidity sensor can be used in whistle tune recognition.

Stepwise synthesis of CoS2–C@CoS2 yolk–shell nanocages with much enhanced electrocatalytic performances both in solar cells and hydrogen evolution reactions

Yolk–shell nanocages possessing complex internal structure and excellent structural tenability are advantageous for the preparation of advanced catalysts. Due to outstanding features such as superior electrical, magnetic, optical and catalytic properties, non-noble metal materials with hollow nanostructures have been considered potential substitutes for highly active and stable abundant noble-metal electrocatalysts. In this study, the as-prepared CoS2–C@CoS2 yolk–shell nanocages were applied in both dye-sensitized solar cells (DSSCs) and hydrogen evolution reactions (HERs). Due to the remarkable structural features and advantageous chemical compositions, CoS2–C@CoS2 yolk–shell nanocages comprising tiny crystals possessed large surface area (161 m2 g−1), well-defined interior voids, excellent conductivity and high catalytic activity. The DSSC with CoS2–C@CoS2 achieved a high power conversion efficiency (PCE) of 9.32% under AM 1.5 G irradiation, and it outperformed the DSSC with Pt (8.24%). Besides, CoS2–C@CoS2 as a kind of HER catalyst also exhibited a low onset overpotential of 19.0 mV, small Tafel slope of 51.9 mV dec−1 and low overpotential of 79.1 mV at a current density of 10 mA cm−2 under acidic conditions (0.5 M H2SO4).

Influences of nonsolvent on the morphologies and electrochemical properties of carbon nanofibres from electrospun polyacrylonitrile nanofibres

The influences of nonsolvent on the morphologies and electrochemical properties of carbon nanofibres (CNFs) obtained via pre-oxidation and carbonization of electrospun polyacrylonitrile (PAN) nanofibres were mainly studied. Volatile methanol (MeOH) and acetonitrile (MeCN) were introduced into PAN solutions as the nonsolvent for PAN, which may produce porous structures via inducement of phase separation. The morphologies of the prepared nanofibres were observed via scanning electron microscopy. It was found that PAN nanofibres possessed corrugated and rough surfaces. PAN fibres obtained in the presence of nonsolvent showed larger diameters and wider distributions than those obtained without nonsolvent. After thermal treatments, inter-bonded CNFs were prepared. The structures of CNFs were confirmed by X-ray diffraction and Raman spectrometry. Then the electrochemical properties of CNFs were examined by an electrochemical method in a three-electrode system. Based on chronopotentiometry, CNFs exhibited the highest capacity up to