Chengzhi Wang

Enhancing visible-light photoelectrochemical water splitting through transition-metal doped TiO2 nanorod arrays

Extending the photoresponse from the ultraviolet (UV) to the visible light region, while maintaining a high photocatalytic activity has been an important challenge for TiO2. We demonstrate the use of transition-metal doping treatment as a facile and effective strategy to substantially improve the performance of TiO2 nanorods in the visible light region for photoelectrochemical (PEC) water splitting. The effect of Fe, Mn and Co as dopants on the PEC performance of the TiO2 nanorods was investigated, wherein the Fe doping is the most effective route to enhance the photoactivity of TiO2. The photocurrent density of Fe–TiO2 sample significantly increases with bias voltage and reaches 2.92 mA cm−2 at 0.25 V vs. Ag/AgCl, which is five times higher than that of the undoped TiO2. Even under visible light illumination (>420 nm), the photocurrent density of Fe–TiO2 is as high as 0.96 mA cm−2 at 0.25 V vs. Ag/AgCl. Incident-photon-to-current-conversion (IPCE) efficiency (up to ∼18%) measurements reveal that the Fe–TiO2 nanorod sample significantly improve the photoresponse not only in the UV region, but also in the visible light region. Fe doping not only enhances the visible light absorption of TiO2 nanorods by creating impurity states near the conduction band, but also obviously increases carrier density of TiO2, leading to effective carrier separation and transportation and relatively long electron lifetime. Because of their relatively high photocatalytic activity, the Fe–TiO2 nanorods can serve as a promising candidate in various areas, such as solar water splitting, dye-sensitized solar cells, and photocatalysis.

The effect of the phase structure on physicochemical properties of TMO materials: a case of spinel to bunsenite

It is worthwhile to comprehensively investigate the relationship between different phase structures and physicochemical properties of TMO materials. For investigating the phase structure effect, spinel Co3O4, NiCo2O4, and bunsenite NiO microflowers were rationally synthesized through a facile solvothermal method combined with a post-annealing process. The phase structures of final products were controlled by adjusting the Ni/Co addition ratio. The SEM and TEM results revealed that these Ni/Co oxides exhibited similar flower-like morphology; this provided convenience for investigation of their physicochemical properties in view of their phase structure. Compared with Co3O4 that exhibited superparamagnetic behavior, NiCo2O4 exhibited ferromagnetic characteristics because of the incorporation of nickel into the spinel structure. Co3O4 and NiCo2O4 demonstrated thermal catalytic ability higher than that of bunsenite NiO due to the more efficient electron transfer ability of the spinel structure. In view of the phase structures, this study provided a prospective case of research on the physicochemical properties of transition-metal oxide (TMO) materials.

The synthesis of FeCoS2 and an insight into its physicochemical performance

In this work, FeCoS2 was synthesised via a modulated hydrothermal method. With the hydrothermal temperature being increased, the microstructure of the as-obtained FeCoS2 evolved from the initial ultrathin nanosheets to the final microspheres, which were also assembled from these nanosheets. Meanwhile, X-ray diffraction results illustrated that the degree of the [101] preferred orientation was gradually reduced with increasing hydrothermal temperature. When being evaluated as a photocatalyst toward the photodegradation of methylene blue, FeCoS2 presented itself as a promising candidate for the photocatalytic degradation. When the as-obtained FeCoS2 was tested as an anode material for Na-ion batteries, it could deliver a first discharge capacity of 806 mA h g−1 at 50 mA g−1. In addition, the magnetic properties of the temperature-dependent FeCoS2 products were also investigated, demonstrating that FeCoS2 nanosheets obtained at 170 °C exhibited the most evident ferromagnetic characteristics. The current research illustrates FeCoS2s potential for diverse applications in modern materials science and devices.

Neat Design for the Structure of Electrode to Optimize the Lithium Ion Battery Performance

The appearance of mechanical cracks originated from anisotropic expansion and shrinkage of electrode particles during Li+ de/intercalation is a major cause of the capacity fading in Li-ion batteries. Well-designed and controlled nanostructures of electrodes have shown a prominent prospect for solving this obstacle. Here, we report a novel and convenient strategy for the preparation of graphene nanoscroll wrapping Nb2O5 nanoparticles (denoted as T-Nb2O5/G). First, high energy ball milling is conducted to acquire softly agglomerated T-Nb2O5 nanoparticles owing to its spontaneous reduction of surface energy among these single particles. Then freeze-drying leads to the formation of graphene nanoscroll, which easily realizes the in situ wrapping over softly agglomerated T-Nb2O5 nanoparticles. Extended cycling tests demonstrate that such T-Nb2O5/G yields a high reversible specific capacity of 222 mA h g-1 over 700 cycles at 1C. The dominated surface capacitive insertion processes possessing favorable kinetics enable T-Nb2O5/G to exhibit excellent rate performance, which achieve a capacity of 110 mA h g-1 at 10C. A combined ex situ X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigation reveal that the long-term cycling stability of T-Nb2O5/G is attributed to the excellent structural stability of the electrode, in which the synergistic effect between the softly agglomerated T-Nb2O5 nanoparticles and graphene nanoscroll prevents the formation of mechanical cracks.

Hydrothermal One-Step Synthesis of Highly Dispersed M-Phase VO2 Nanocrystals and Application to Flexible Thermochromic Film

Preparation of ultrafine highly dispersed VO2(M) nanoparticles that are essential materials to fabricate thermochromic flexible films remains a challenge, preventing effective use of their promising properties. Here, we report an original hydrothermal approach by controlling oxidizing atmosphere of reaction with hydrogen peroxide to prepare ultrafine VO2(M) nanoparticles free from annealing. Hydrogen peroxide is separated from precursor solution in a reactor, which creates a moderate oxygenation environment, enabling the formation of stoichiometric VO2(M) nanoparticles. The obtained VO2(M) nanoparticles are well-dispersed, highly uniform, and single-phase, with an average particle size ∼30 nm. The flexible thermochromic films fabricated with the VO2(M) nanoparticles exhibit excellent thermochromic performance with a solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. While the films prepared with annealed nanoparticles show reduced transmittance due to light scattering of the large size particles resulting from agglomeration and growth during annealing. This work demonstrates a promising technique to realize moderate oxidizing atmosphere by hydrothermal process for preparing well-dispersed stoichiometric nano-oxides.