Guanjun Xiao

Facile fabrication of faceted copper nanocrystals with high catalytic activity for p-nitrophenol reduction

This article reports a reproducible and facile approach to synthesize faceted copper nanocrystals (Cu NCs) using an inexpensive copper oxide as a precursor. By simply prolonging the reaction time, Cu cubes and polyhedrons were successfully produced, and the mean size could be effectively controlled in the range of 9 to 21 nm. The catalytic activities of the Cu cubes and polyhedrons were investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4. The kinetics of the reduction reaction at different temperatures were investigated to determine the activation parameters. Our investigations indicate that Cu nanocubes exhibit higher catalytic activity than Cu polyhedrons, which can be ascribed to three features: the higher surface-to-volume ratio, the higher surface energy of the {100} facet, and the lower redox potential. In addition, these catalysts can be easily recycled with a slight decrease of the catalytic activities, and are stable in the air. Therefore, this facile route provides a useful platform for the fabrication of Cu catalysts which have the potential to replace noble metals for certain catalytic applications.

Recent advances in IV–VI semiconductor nanocrystals: synthesis, mechanism, and applications

This review is focused on the recent developments of the synthesis, mechanism and applications of IV–VI semiconductor nanocrystals (NCs), including germanium-, tin- and lead-based chalcogenides NCs. First of all, we systematically introduce a series of investigations on the preparation with controllable size and shape via a wide variety of methods. Corresponding growth mechanisms are also discussed. Moreover, the promising potential of IV–VI semiconductor NCs as building blocks with respect to energy, sensors and catalysis is highlighted. For the purpose of enhancing the performance to satisfy the practical applications, tailored nanocomposites by combining noble metals or graphene etc. are further developed. Finally, we present some concluding remarks and perspectives for future developments. We hope this article can provide researchers with the key snapshots of the recent advances and the future challenges, thus achieving a great progress in IV–VI semiconductor NCs.

Shape and size controlled synthesis and properties of colloidal IV–VI SnSe nanocrystals

Colloidal IV–VI SnSe nanocrystals with small and uniform size distribution were synthesized by a facile and phosphine-free method. Simple Sn6O4(OH)4 was introduced as a tin precursor to synthesize the SnSe nanocrystals. By changing the reaction temperature and Sn/Se molar ratio, SnSe nanocrystals with different shapes and sizes were achieved. The influence of reaction temperature and Sn/Se molar ratio to the shape and size of SnSe nanocrystals has been discussed detail. Similar to other IV–VI tin chalcogenides, SnSe shows potential as energy storage material. The performance of SnSe nanocrystals as an anode material for lithium ion batteries has been investigated. A mechanism for SnSe as anode material has been proposed based on its performance. The influence of the shape and size of the SnSe nanocrystals on the performance of lithium ion batteries has been discussed in detail.

One-step solution synthesis of bismuth sulfide (Bi2S3) with various hierarchical architectures and their photoresponse properties

In this paper, we introduce a facile and phosphine-free one-step solution method to synthesize size- and shape-controlled bismuth sulfide (Bi2S3) with hierarchical architectures. Changing variables, such as the reaction temperature, the ratio of precursors, and the concentration of oleic acid were observed to influence the resultant shape of Bi2S3 microstructures. For the formation of Bi2S3 hierarchical architectures, the crystal splitting growth mechanism played the dominant role. The absorption spectra were recorded at room temperature, which revealed that the obtained Bi2S3 product was a direct band gap semiconductor and the band gap Eg was estimated to be about 1.9 eV. Furthermore, the I–V characteristics of the Bi2S3-based device show a significant increase by ca. 1 order of magnitude compared with the dark state, indicating an enhanced conductivity and high sensitivity. The response and decay times are estimated to be about 0.5 and 0.8 s, respectively, which are short enough for it to be an excellent candidate for high-speed and high-sensitivity photodetectors or optical switches. Thus the Bi2S3 hierarchies as building blocks may offer the potential for monolithic, low-cost and large-scale integration with CMOS electronics.

Solution synthesis of copper selenide nanocrystals and their electrical transport properties

In this paper, we developed a one-pot solution strategy to synthesize copper selenide NCs with controllable shape and structure. By changing the precursors in the reaction, copper selenide NCs (Cu2−xSe nanoparticles, nanorods and CuSe nanoplates) with various morphologies could be achieved. We proposed a possible mechanism to explain the influence of precursors on the shape of copper selenide NCs and we found that the chemical activities of precursors played key roles in the morphologies and crystal structures of the final products. Moreover, the electrical transport properties of as-prepared products were investigated. The morphologies of copper selenide NCs have a great influence on the electrical transport properties. The copper selenide NCs with nanorods display the best electrochemical performance compared with the other two types. We believe that copper selenide NCs would be promising candidates for electrical transport materials.

Synthesis of Cu–Ir nanocages with enhanced electrocatalytic activity for the oxygen evolution reaction

Iridium (Ir) is widely used as a catalyst in polymer electrolyte membrane water electrolyzers (PEMWEs). However, high cost and limited catalytic performance of Ir hamper its large-scale industrial application. Here, based on a modified galvanic replacement, we introduce Cu nanoparticles as a template to prepare single-crystalline Cu–Ir polyhedral nanocages (NCs). Alloying Ir with 3d transition metal Cu not only significantly reduces the loading of Ir but also remarkably enhances its catalytic activity by forming a unique NC structure and tuning the d-band structure of Ir. The as-prepared single-crystalline Cu1.11Ir NCs exhibit enhanced catalytic activity toward the oxygen evolution reaction (OER) in 0.05 M H2SO4, with a smaller overpotential (286 mV) required for a current density of 10 mA cm−2 and a Tafel slope of 43.8 mV per decade. The mass activity can reach 73 mA mgIr−1 at an overpotential of 0.28 V for Cu1.11Ir NCs. Hence, the obtained Cu1.11Ir NCs would be a promising electrocatalyst for practical electrocatalytic water splitting systems.

Synthesis of narrow band gap SnTe nanocrystals: nanoparticles and single crystal nanowires via oriented attachment

SnTe nanocrystals with different shapes and sizes are synthesized by a simple and facile method. The length of the fatty chain in amine has an important effect on the shape and size of SnTe nanocrystals. When oleylamine (OLA) is used as ligand, SnTe nanoparticles with size of 4 nm and high crystallinity are produced. However, when octylamine (OTA) is used as ligand, larger SnTe nanoparticles with low crystallinity are achieved, which would transform into single crystal SnTe nanowires with increasing reaction time. The driving force of shape evolution of SnTe nanocrystals is reducing the high surface free energy. An oriented attachment mechanism is proposed to explain the transition from nanoparticles to nanowires, and oriented attachment of nanoparticles to single crystal nanowires is proposed to reduce the interface energy by the greatest amount.

Unexpected Room‐Temperature Ferromagnetism in Nanostructured Bi2Te3

There is an urgent need for the development in the field of the magnetism of topological insulators, owing to the necessity for the realization of the quantum anomalous Hall effect. Herein, we discuss experimentally fabricated nanostructured hierarchical architectures of the topological insulator Bi2Te3 without the introduction of any exotic magnetic dopants, in which intriguing room-temperature ferromagnetism was identified. First-principles calculations demonstrated that the intrinsic point defect with respect to the antisite Te site is responsible for the creation of a magnetic moment. Such a mechanism, which is different from that of a vacancy defect, provides new insights into the origins of magnetism. Our findings may pave the way for developing future Bi2Te3-based dissipationless spintronics and fault-tolerant quantum computation.

Synthesis, optical properties and growth process of In2S3 nanoparticles

Cubic beta-In(2)S(3) nanoparticles (NPs) have been synthesized by a simple and facile way, which is 6 nm in size. Absorption and emission spectra of In(2)S(3) NPs show obvious blue peak shift compared to band gap of bulk In(2)S(3), indicating the strong quantum size confinement effect. The fluorescence quantum yield of In(2)S(3) NPs is found to be 10%. During the synthesis process, the absorption spectra have no peak shift, which is responding to transition from valence band to the conduction band levels. This absorption spectra show that the nucleation and growth process of In(2)S(3) NPs is very quick. The PL lifetime spectra and time resolved spectra give two emission processes in In(2)S(3) NPs, which would be excitonic recombination and electron-hole recombination via defects levels. The blue shift of emission peaks show the emission process in In(2)S(3) NPs is from mainly electron-holes recombination via defects levels to excitonic recombination. The Stokes shift becomes smaller which is mainly contributed by blue shift of emission and smaller contribution from the UV-Vis absorption. The absorption and emission spectra show the size and crystallinity of In(2)S(3) NPs have no changes (HRTEM images provide enough proofs); however the surface-related defects changed greatly in the reaction process.

Shape-controlled synthesis of PbS nanostructures from −20 to 240 °C: the competitive process between growth kinetics and thermodynamics

The traditional concept of the synthesis of semiconductor nanocrystals (NCs) by solvent routes usually performed under high temperatures, causes the semiconductor materials to nucleate and grow into various shaped NCs in solution. Therefore, these methods are named as “solvent-thermal approachs”. In this work, we describe a simple and reproducible strategy for the synthesis of PbS NCs at temperatures even as low as −20 °C by using frozen and solidified precursors. With the aid of alkylamines, nano-sized PbS could also nucleate and grow at such low temperatures within a short time (a few seconds). The experimental results not only break peoples traditional thinking but also provide a significant and novel direction in the engineering of the synthesis of NCs. In addition, we further systematically investigated the effect of two types of temperatures (the mixing temperature of the precursors and the ripening temperature of the PbS NCs). Combining this with different alkylamines, we found an obvious competition between a growth kinetic process caused by the alkylamines and a thermodynamic process induced by the temperature, which formed variously shaped monodispersed PbS NCs, including flower-, star-, sphere-, truncated octahedron-, cuboctahedron-, quasi cube-, cube-shaped and some hollow PbS NCs. Furthermore, this competition process could also provide a facile and cost-effective route to synthesize size-tunable but shape-permanent PbS NCs and their self-assembly superlattices in the same reaction systems, which is still a major challenge at present. Afterward, both the formation mechanisms of the PbS nanostructures synthesized below room temperature and the shape transformation depending on two types of temperature and alkylamines are systematically discussed.