Mingrun Li

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3

When Alpha met Beta: a tuneable α-β surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or β-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-β phase junction.

Selective conversion of syngas to light olefins.

Small olefins from syngas The conversion of coal or natural gas to liquid fuels or chemicals often proceeds through the production of CO and H2. This mixture, known as syngas, is then converted to hydrocarbons with Fischer-Tropsch catalysts. For the light olefins (ethylene to butylenes) needed for chemical and polymer synthesis, conventional catalysts are mechanistically limited to <60% conversion and deactivate through carbon buildup. Jiao et al. developed a bifunctional catalyst that achieves higher conversions and avoids deactivation (see the Perspective by de Jong). A zinc-chromium oxide creates ketene intermediates that are then coupled over a zeolite. Science, this issue p. 1065, see also p. 1030 A composite catalyst circumvents conventional limitations on the Fischer-Tropsch synthesis of light olefins from syngas. [Also see Perspective by de Jong] Although considerable progress has been made in direct synthesis gas (syngas) conversion to light olefins (C2=–C4=) via Fischer-Tropsch synthesis (FTS), the wide product distribution remains a challenge, with a theoretical limit of only 58% for C2–C4 hydrocarbons. We present a process that reaches C2=–C4= selectivity as high as 80% and C2–C4 94% at carbon monoxide (CO) conversion of 17%. This is enabled by a bifunctional catalyst affording two types of active sites with complementary properties. The partially reduced oxide surface (ZnCrOx) activates CO and H2, and C−C coupling is subsequently manipulated within the confined acidic pores of zeolites. No obvious deactivation is observed within 110 hours. Furthermore, this composite catalyst and the process may allow use of coal- and biomass-derived syngas with a low H2/CO ratio.

Fabrication of superior-performance SnO2@C composites for lithium-ion anodes using tubular mesoporous carbon with thin carbon walls and high pore volume

A tubular composite, including ultrafine SnO2 particles encapsulated in ordered tubular mesoporous carbon with thin walls and high pore volume, is fabricated through the in situ hydrolysis method. It is observed that up to 80 wt% of SnO2 particles with size between 4–5 nm are highly dispersed and homogeneously encapsulated in the mesopore channels and no bulky aggregates are visible. The tubular composite exhibits a considerably high reversible capacity of 978 mA h g−1 and a high initial efficiency of 71% at a current density of 200 mA g−1 between 0.005–3 V. Its reversible capacity even increases up to 1039 mA h g−1 after 100 cycles, which is much higher than the conventional theoretical capacity of SnO2 (782 mA h g−1), meanwhile, it also displays fast discharge/charge kinetics at a high current density of 1500 mA g−1. The excellent electrochemical performance is ascribed to its unique mesostructure by recruiting tubular mesoporous carbon with thin carbon walls (∼2 nm) and high pore volume (2.16 cm3 g−1). This tubular nanostructure provides confined nanospace for hosting immobilized ultrafine SnO2 with high loading, compensates volume expansion of SnO2, warrants efficient contact between nanoparticles and carbon matrix before and after Li+ insertion. We believe this special structure model might be extended for the fabrication of other cathode and anode electrode materials, to achieve high performance LIBs.

Enhancement of the Performance of a Platinum Nanocatalyst Confined within Carbon Nanotubes for Asymmetric Hydrogenation

Enhancement of the Performance of a Platinum Nanocatalyst Confined within Carbon Nanotubes for Asymmetric Hydrogenation

Spinel ZnMn2O4 nanoplate assemblies fabricated via "escape-by-crafty-scheme" strategy

A two-step process that differs in important details from previous methods used to prepare ZnMn2O4 nanoplate assemblies has been reported. This material was prepared by thermal transformation of metal–organic nanoparticles into metal–oxide nanoparticles based on the “escape-by-crafty-scheme” strategy. Firstly, the nanoscale mixed-metal–organic frameworks (MMOFs) precursor, ZnMn2–ptcda (ptcda = perylene-3,4,9,10-tetracarboxylic dianhydride), containing Zn2+ and Mn2+, was prepared by the designed soft chemical assembly of mixed metal ions and organic ligands at a molecular scale. In a second step, the MMOFs are thermally transformed into spinel structured ZnMn2O4 with morphology inherited from the MMOFs precursors. The well-crystallized spinel structure can be formed by thermal treatment of ZnMn2–ptcda at 350 °C, and is formed at temperatures ≥450 °C using the co-precipitation method. This “escape-by-crafty-scheme” strategy can be extended to the preparation of other spinel metal–oxide nanoparticles, e.g. CoMn2O4, and NiMn2O4, with well-defined morphology inherited from the metal–organic precursors. The ZnMn2O4 nanoplate assemblies thermally treated at 450 °C have potential application in lithium ion batteries as anode materials, which show high specific capacity and good cyclability.

Visible emission characteristics from different defects of ZnS nanocrystals

Various sized ZnS nanocrystals were prepared by treatment under H(2)S atmosphere. Resonance Raman spectra indicate that the electron-phonon coupling increases with increasing the size of ZnS. Surface and interfacial defects are formed during the treatment processes. Blue, green and orange emissions are observed for these ZnS. The blue emission (430 nm) from ZnS without treatment is attributed to surface states. ZnS sintered at 873 K displays orange luminescence (620 nm) while ZnS treated at 1173 K shows green emission (515 nm). The green luminescence is assigned to the electron transfer from sulfur vacancies to interstitial sulfur states, and the orange emission is caused by the recombination between interstitial zinc states and zinc vacancies. The lifetimes of the orange emission are much slower than that of the green luminescence and sensitively dependent on the treatment temperature. Controlling defect formation makes ZnS a potential material for photoelectrical applications.

Solution‐Phase Synthesis and Characterization of Single‐Crystalline SnSe Nanowires

Tin(II) selenide is an important binary IV–VI semiconductor compound with a wide range of potential applications (e.g. memory switching devices, infrared optoelectronic devices, and anode materials for rechargeable lithium batteries). Bulk SnSe has both an indirect band gap at 0.90 eV and a direct band gap at 1.30 eV. Owing to the quantum confinement effect, tunable band gaps of SnSe nanostructured materials (e.g. thin films and nanocrystals) 3] have been demonstrated, which makes them capable of absorbing a major portion of solar energy. As an earth-abundant, environmentally benign, and chemically stable material, SnSe is placed among the most promising candidates for solar cells. Most recently, the interest in the controllable synthesis of colloidal tin chalcogenide (SnX; X = S, Se, Te) nanoparticles (NPs) has grown. However, nanowires (NWs) are expected to have properties superior to those of nanoparticles owing to their anisotropic geometry and the exciton confinement in two dimensions. It has been demonstrated that, for photovoltaic applications, high-aspect-ratio nanowires have the potential for enhancement of electron transport owing to their direct electron pathway that does not rely on electron hopping, in contrast to nanoparticles. Furthermore, as remarkably powerful building blocks in nanoscale electronic and optoelectronic devices, the synthesis of versatile semiconductor nanowires with superior controllability in structure and dimension is pivotal for exploiting their device applications. In 2003, Qian and co-workers reported the tentative synthesis of SnSe nanowires with a relatively large mean diameter of 50 nm, but the yield was low and the morphology (including irregular crystals) and purity (including tin oxides) were hard to control. In 2006, Zhao et al. developed a template route to synthesize SnSe nanowires, but only polycrystalline products were obtained after a long reaction time (e.g. 36 h), and a tedious process was required to remove the hard template. To date, high-yield synthesis of monocrystalline SnSe nanowires with relatively small diameters, decent aspect ratios, and good quality has remained a challenge; optical and electrical properties and optoelectronic applications of monocrystalline SnSe nanowires have not been described. We present herein a facile, solution-phase synthetic approach to colloidal SnSe nanowires, which have a mean diameter of approximately 20.8 nm, lengths tunable from hundreds of nanometers to tens of micrometers, and more importantly, a monocrystalline structure. Their optical and electric properties are examined by UV/Vis/NIR spectroscopy, cyclic voltammetry (CV), and transient photocurrent measurements, and a quantum confinement effect is clearly revealed. Furthermore, the fabrication of hybrid solar cells based on a blend of SnSe nanowires and poly(3hexylthiophene) (P3HT) is demonstrated. Colloidal SnSe nanowires were prepared from commercially available Sn[N(SiMe3)2]2, and trioctylphosphine selenide (TOP-Se) in oleylamine (OLA) or OLA/TOPO (trioctylphosphine oxide) solvent mixture by using monodisperse bismuth nanoparticles to catalyze the nanowire growth through the solution-liquid-solid (SLS) mechanism. In a typical synthesis, an injection solution consisting of Sn[N(SiMe3)2]2 (0.2 mmol) and Bi-nanoparticle catalysts (2.4 mmol) in octadecene (ODE, 600 mL) was introduced into a three-neck flask that contained a solution of TOP-Se (0.2 mmol) in OLA (4 g) or TOPO/OLA mixture (3.5 g/2.5 g) at 290 8C. The resulting reaction solution was kept at this temperature for 1–2 min before being cooled to room temperature. The product was cleaned and precipitated using hexane and isopropyl alcohol as the solvent and antisolvent, respectively. Finally the purified nanowires were dispersed in common organic solvents such as toluene. The scanning electron microscopy (SEM) image in Figure 1a shows the ensemble of randomly aligned SnSe nanowires with lengths exceeding 10 mm. The TEM image in Figure 1b displays smooth nanowires with mean diameters of (20.8 2.2) nm ( 10.6%), which correlates to the size of monodisperse Bi-nanoparticle catalysts (Figure S1 in the Supporting Information). The frozen Bi-nanoparticle seeds were usually observed at the nanowire tips (see arrows in the [*] Dr. S. Liu, Prof. W.-H. Zhang, Prof. C. Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian 116023 (China) and Dalian National Laboratory for Clean Energy Dalian 116023 (China) E-mail: whzhang@dicp.ac.cn canli@dicp.ac.cn

Removal of stacking-fault tetrahedra by twin boundaries in nanotwinned metals

Stacking-fault tetrahedra are detrimental defects in neutron- or proton-irradiated structural metals with face-centered cubic structures. Their removal is very challenging and typically requires annealing at very high temperatures, incorporation of interstitials or interaction with mobile dislocations. Here we present an alternative solution to remove stacking-fault tetrahedra discovered during room temperature, in situ Kr ion irradiation of epitaxial nanotwinned Ag with an average twin spacing of ~8 nm. A large number of stacking-fault tetrahedra were removed during their interactions with abundant coherent twin boundaries. Consequently the density of stacking-fault tetrahedra in irradiated nanotwinned Ag was much lower than that in its bulk counterpart. Two fundamental interaction mechanisms were identified, and compared with predictions by molecular dynamics simulations. In situ studies also revealed a new phenomenon: radiation-induced frequent migration of coherent and incoherent twin boundaries. Potential migration mechanisms are discussed.

Effects of Zn2+ and Pb2+ dopants on the activity of Ga2O3-based photocatalysts for water splitting

Zn-doped and Pb-doped β-Ga2O3-based photocatalysts were prepared by an impregnation method. The photocatalyst based on the Zn-doped β-Ga2O3 shows a greatly enhanced activity in water splitting while the Pb-doped β-Ga2O3 one shows a dramatic decrease in activity. The effects of Zn(2+) and Pb(2+) dopants on the activity of Ga2O3-based photocatalysts for water splitting were investigated by HRTEM, XPS and time-resolved IR spectroscopy. A ZnGa2O4-β-Ga2O3 heterojunction is formed in the surface region of the Zn-doped β-Ga2O3 and a slower decay of photogenerated electrons is observed. The ZnGa2O4-β-Ga2O3 heterojunction exhibits type-II band alignment and facilitates charge separation, thus leading to an enhanced photocatalytic activity for water splitting. Unlike Zn(2+) ions, Pb(2+) ions are coordinated by oxygen atoms to form polyhedra as dopants, resulting in distorted surface structure and fast decay of photogenerated electrons of β-Ga2O3. These results suggest that the Pb dopants act as charge recombination centers expediting the recombination of photogenerated electrons and holes, thus decreasing the photocatalytic activity.