The Role of Cu2O Mesoporous Spheres in the Removal of Organic Pollutants: Photocatalyst or Adsorbent?

Cuprous oxide (Cu2O) is a kind of functional and promising energy material, which has been used as solar cells, adsorbents, and lithium ion battery electrode materials. And its mainly application is toward photocatalytic reactivity of organics' decomposition under visible light irradiation. However, the real role of Cu2O remains debated, for these reports don't give a powerful evidence to prove its significant photocatalytic properties. Here, we describe a one-pot synthesis method to construct Cu2O mesoporous spheres with large specific surface area at room temperature. This mesoporous microstructure is controlled through an Ostwald ripening-based dissolved partly process. And Cu2O mesoporous spheres are used to study the property of removing organic pollutants. We distinguish the real role of Cu2O by carefully measure the reduced portion of photocatalyst or adsorbent, while it is reacting with organic pollutants. At last, we find out the reason that has caused the concentration of organic pollutants to decrease is mainly based on the adsorption property of Cu2O, rather than its photocatalytic effect. However, the photocatalytic effect of Cu2O is very low under visible light irradiation.

Anomalous enhancement of valley polarization in multilayer WS2 at room temperature.

We probe the polarization of the "A" exciton photoluminescence from monolayer and multilayer WS2 at 10 K and 295 K under near-resonant and off-resonant conditions. The monolayer WS2 exhibits relatively low valley polarization, around 24% at 10 K and 8% at 295 K, while all multilayer WS2 samples show very high valley polarization, which is a more or less constant value of around 80% at 10 K under near-resonant excitation. At room temperature, it is observed experimentally that valley polarization in multilayer WS2 monotonously increases with shrinking of the indirect bandgap energy. The phonon-assisted intervalley scattering via the K-Γ-K(K') valleys is identified as the primary valley polarization relaxation channel, which could be gradually suppressed as the thickness increases, leading to a valley polarization of up to 70% in multilayer WS2 (>3 unit layers).

Bound exciton and free exciton states in GaSe thin slab.

The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission.

Distinctive in-Plane Cleavage Behaviors of Two-Dimensional Layered Materials.

Mechanical exfoliation from bulk layered crystal is widely used for preparing two-dimensional (2D) layered materials, which involves not only out-of-plane interlayer cleavage but also in-plane fracture. Through a statistical analysis on the exfoliated 2D flakes, we reveal the in-plane cleavage behaviors of six representative layered materials, including graphene, h-BN, 2H phase MoS2, 1T phase PtS2, FePS3, and black phosphorus. In addition to the well-known interlayer cleavage, these 2D layered materials show a distinctive tendency to fracture along certain in-plane crystallography orientations. With theoretical modeling and analysis, these distinct in-plane cleavage behaviors can be understood as a result of the competition between the release of the elastic energy and the increase of the surface energy during the fracture process. More importantly, these in-plane cleavage behaviors provide a fast and noninvasive method using optical microscopy to identify the lattice direction of mechanical exfoliated 2D layered materials.

Universal synthesis of air stable, phase pure, controllable FeSe2 nanocrystals.

Iron diselenium (FeSe2) is a promising semiconductor for thin-film solar cells because it has a suitable band gap (E(g) = 1.0 eV) and high absorption coefficient. Despite these prospects, the controllable synthesis of FeSe2 nanostructures and the diversity of their geometries has hardly been studied previously. Here, we described a successful synthesis of phase-pure, high-quality, and stable orthorhombic FeSe2 nanocrystals (NCs) in aqueous solvents. A variety of morphologies of the FeSe2 NCs were achieved by adjusting synthetic methods. FeSe2 nanoparticles with diameters of 30-100 nm were synthesized in the presence of ethylenediamine (en). Moreover, the synthetic approach developed for nanoparticles proved to be quite universal and could be modified to produce nanowires and octahedrons, with which structure the material could display high crystallinity. The diameter of the FeSe2 nanowires was 300-500 nm with a length exceeding 2 µm. The octahedrons displayed lateral dimensions of 1 µm. Meanwhile, the probable growth mechanism and fabrication process of the NCs were proposed. Polycrystalline FeSe2 thin films were fabricated by modifying the sedimentation method. The obvious photoconductivity of FeSe2 has already been observed, and it was considered to be one candidate of solar cell for the very first time.