Xinyi Yang

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.

A Protocol to Fabricate Nanostructured New Phase: B31-Type MnS Synthesized under High Pressure

Synthesis of nanomaterials with target crystal structures, especially those new structures that cannot be crystallized in their bulk counterparts, is of considerable interest owing to their strongly structure-dependent properties. Here, we have successfully synthesized and identified new-phase nanocrystals (NCs) associated with orthorhombic MnP-type (B31) MnS by utilizing an effective high-pressure technique. It is particularly worth noting that the generated new structured MnS NCs were captured as expected by quenching the high-pressure phase to the ambient conditions at room temperature. Likewise, the commercially available bulk rocksalt (RS) MnS material underwent unambiguously a reversible phase transition when the pressure was released completely. First-principles calculations further supported that the B31-MnS was more energetically preferable than the RS one under high pressure, which can be plausibly interpreted by the structural buckling with respect to zigzagged arrangements within B31 unit cell. Our findings represent a significant step forward in a deeper understanding of the high-pressure phase diagram of MnS and even provide a promising strategy to prepare desired nanomaterials with new structures that do not exist in their bulk counterparts, thus greatly increasing the choice of materials for a variety of applications.

Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals

Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr3 NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ∼1.2 GPa, where an isostructural phase transformation regarding the Pbnm space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr3 trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr3 nanowires are between those for CsPbBr3 nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb-Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br-Pb bond length and Pb-Br-Pb bond angle for the PbBr6 octahedral framework. Furthermore, the significant distortion of the lead-bromide octahedron to accommodate the Jahn-Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr3 NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems.

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.

A feasible approach to synthesize Cu2O microcrystals and their enhanced non-enzymatic sensor performance

We have introduced potassium bromide (KBr) as an additive to synthesize cuprous oxide (Cu2O) microcrystals with various well-defined shapes. Here, the bromide ions play a pivotal role in controlling the shape of the Cu2O microcrystals, from concave cubic into short hexapod shapes. As a typical representative, the obtained Cu2O microcrystals were further utilized in a non-enzymatic amperometric glucose sensor. And the sensor constructed by the extended hexapod Cu2O microcrystals show the best performance, exhibiting remarkable sensitivity (97 μA mM−1 cm−2), significant selectivity and a wide linear response (up to 14.3 mM) towards glucose detection. Compared with the previous sensors that were constructed by the Cu-based materials, this detection range is much closer to the glucose range in human serum. The wide range can be ascribed to the “clean surface” (with no organic capping agent adsorbed on the surface) and more rich {111} facets exposed for the extended hexapod structure, which maximize the accessible electroactive surface for the efficient transfer of electrons, as well as the product molecules. This work provides a green and feasible approach to enhance the Cu2O sensor performance, which can be extended to other applications such as solar-energy conversion and catalysis.

Synthesis of dendritic iridium nanostructures based on the oriented attachment mechanism and their enhanced CO and ammonia catalytic activities

Branched iridium nanodendrites (Ir NDs) have been synthesized by a simple method based on the oriented attachment mechanism. Transmission electron microscopy images reveal the temporal growth process from small particles to NDs. Precursor concentrations and reaction temperatures have a limited effect on the morphology of Ir NDs. Metal oxide and hydroxide-supported Ir NDs exhibit enhanced activity for catalytic CO oxidation. Particularly, the Fe(OH)x-supported Ir NDs catalyst with a 4 wt% Ir loading show superior CO oxidation catalytic activity with a full conversion of CO at 120 °C. Furthermore, compared with Ir NPs and commercial Ir black, Ir NDs exhibit higher activity and stability for ammonia oxidation. The specific activity and mass activity of Ir NDs for ammonia oxidation are 1.7 and 7 times higher than that of Ir NPs. The improved catalytic activities of Ir NDs are attributed not only to their large specific surface area, but also to their considerably high index facets and rich edge and corner atoms. Hence, the obtained Ir NDs provide a promising alternative for direct ammonia fuel cells and proton-exchange membrane fuel cells.

An environmentally friendly route to synthesize Cu micro/nanomaterials with “sustainable oxidation resistance” and promising catalytic performance

Practical application of nanostructured Cu has long been limited by the surface oxidation. Although conventional surface modification can slow down the oxidation rate, the formation of a surface oxide shell cannot completely be prevented. Here, we report an effective approach to achieve “sustainable oxidation resistance” for Cu micro/nanomaterials. Once the Cu is oxidized by the external environment, an ageing treatment would not only convert the oxidized sample back to an unoxidized state but also enhance the oxidation resistance. This approach takes advantage of the dual functions of the citrate group: one is its complexation with the Cu2+, which can facilitate the oxidative etching of Cu2O; the other is its interaction with the Cu surface, which can effectively enhance the Cu oxidation resistance. In the ageing process, the oxide layer was etched by the oxygen, whereas the formed Cu0 was protected by the citrate group. Since there is no long-chain or hydrophobic molecule capped on the surface, the adsorption and desorption of the reactant on the Cu surface could proceed smoothly, enabling Cu to be a preferable catalyst. In the reduction of 4-nitro-phenol (4-NP), the rate constant of the reaction catalyzed by the Cu particles is estimated to be 3.85 × 10−2 s−1. By comparison, rate constants for Ag and Au particles are much lower, which are 1.03 × 10−2 s−1 and 2.73 × 10−3 s−1, respectively. Since the Cu is significantly cheaper, this work provides a promising platform for the development of non-noble metal catalysts.

Unravelling a solution-based formation of single-crystalline kinked wurtzite nanowires: The case of MnSe

The search for a novel strategy to sculpt semiconductor nanowires (NWs) at the atomistic scale is crucial for the development of new paradigms in optics, electronics, and spintronics. Thus far, the fabrication of single-crystalline kinked semiconductor NWs has been achieved mainly through the vapor−liquid−solid growth technique. In this study, we developed a new strategy for sculpting single-crystalline kinked wurtzite (WZ) MnSe NWs by triggering the nonpolar axial-oriented growth, thereby switching—at the atomistic scale—the NW growth orientation along the nonpolar axes in a facile solution-based procedure. This presents substantial challenges owing to the dominant polar c axis growth in the solution-based synthesis of one-dimensional WZ nanocrystals. More significantly, the ability to continuously switch the nonpolar axial-growth orientation allowed us to craft the kinking landscape of types 150°, 120°, 90°, and 60°. A probabilistic analysis of kinked MnSe NWs reveals the correlations of the synergy and interplay between these two sets of nonpolar axial growth-orientation switching, which determine the actual kinked motifs. Furthermore, discriminating the side-facet structures of the kinked NWs significantly strengthened the spatially selected interaction of Au nanoparticles. We envisage that such a facile solution-based strategy can be useful for synthesizing other single-crystalline kinked WZ-type transition-metal dichalcogenide NWs to develop novel functional materials with finely tuned properties.

Phosphine-free engineering toward metal telluride nanocrystals: the role of Te precursor coordinated at room temperature

A colloidal strategy offers opportunities for the rational design and synthesis of metal telluride nanocrystals (NCs) with the desired crystal structure, uniform geometry, and composition. However, it remains a challenge to use the paradigm to construct metal telluride NCs by a phosphine-free synthesis procedure for promising applications such as luminescence, photovoltaics and thermoelectricity. Here, we developed a new strategy for fabricating metal telluride nanocrystals, e.g. CdTe and PbTe NCs, by using a highly reactive phosphine-free Te precursor. The ability to reduce a TeO2 powder with dodecanethiol (DDT) has been achieved in the presence of oleylamine (OLA) to generate a soluble alkylammonium telluride at room temperature. We provide direct experimental evidence that the OLA-Te complexes were formed in an order of second magnitude kinetic process based on an in situ UV-vis absorption test. In the case of the CdTe NC system, the straightforward measurement of luminescence and the fabrication of LED devices are presented that can semiquantitatively assess the quality of preparation and the reactivity of this air-stable precursor. The proposed strategy highlights several unique features of this solution-based green chemistry that can be useful for synthesizing other metal telluride NCs to develop novel functional materials.

Solution synthesis of conveyor-like MnSe nanostructured architectures with an unusual core/shell magnetic structure

We report for the first time one-dimensional (1D) wurtzite (WZ) MnSe nanoconveyors with a single-crystalline configuration fabricated by a solution-processed colloidal method. High-resolution transmission electron microscopy (HRTEM) measurements show that the stem of MnSe nanoconveyors grows along the [100] direction, while the teeth grow along the [0001] direction. We find that the initial WZ nanobelts with the [100] growth direction are crucial to the formation of nanoconveyors, whereas the teeth are a result of a self-catalyzed growth process induced by the Mn-terminated (0001) surface. The magnetic measurements suggest that 1D WZ MnSe nanoconveyors consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperature. Furthermore, the hysteresis measurements indicate that these nanoconveyors have 300 Oe coercive fields, which is attributed to the high surface-to-volume ratio of the nanoconveyors. This facile solution-based strategy can be anticipated to synthesize WZ metal chalcogenide nanomaterials with 1D hierarchical structures, for potential applications from spintronics to photocatalysis.