Controlled Synthesis And Electrochemical Properties Of MoSe₂-based Nanostructures With Electronic Modulation

Nowadays, the use of traditional fossil energy has brought serious environmental concerns to human society, and the energy crisis has become a serious problem which restricts the development of society with the rapid increase of energy consumption. In this regard, all countries in the world are working to develop clean and sustainable alternative energy sources. Electrochemical energy conversion is an important approach to develop new energy sources, and the design of high efficiency electrocatalysts is critical for the practical application of this technology. In this field, the electrocatalytic water splitting into hydrogen (hydrogen evolution reaction, HER) is regarded as one of the most promising way to obtain clean hydrogen energy. Therefore, the design of low cost and efficient HER electrocatalyst is the key factors to promote the development of hydrogen fuels and energy conversion.In this dissertation, based on the analysis of the restrictive factors of electrode materials for the HER performance, we design novel efficient HER electrocatalysts and realize the performance optimization. We use the two-dimensional MoSe2 nanosheets as model material to regulate its electronic structure, realizing the HER performance optimization of MoSe2 nanosheets. The exploration of the correlation between the material structure and electrocatalytic properties provides the direction for the design of novel efficient electrocatalyst. The dissertation includes the following several aspects:1. We develop a bottom-up colloidal route to synthesize Mo-rich MoSe2-x nanosheets with Se vacancy to expose more active site for efficient hydrogen evolution. In detail, we report a facile fast strategy to synthesize scalable hierarchical ultrathin MoSe2-x (x～0.47) nanosheets with 2-5 Se-Mo-Se atomic layers within 20 min under mild condition from reaction of MoO2(acac)2 with dibenzyl diselenide. Meanwhile, the pathway can be extended as a general strategy to prepare other metal selenides, such as ultrathin WSe2 and SnSe nanosheets, and PbSe nanocrystals. Engineering the active edge sites is an effective strategy to optimize the HER activity. The chemical composition analyses indicate the as-obtained MoSe2-x nanosheets possess Mo-rich feature with Se vacancy to expose more active site for efficient hydrogen evolution. The large surface area allows for the easy access of electrolytes, thus facilitating fast interfacial charge transfer and electrochemical reactions. The Mo-rich MoSe2-x nanosheets show excellent HER performance with a small onset overpotential of-170 mV, large cathodic currents, and a Tafel slope of 98 mV/decade. It will pave a new way to synthesize scalable nanostructured materials for intriguing nanodevices and energy conversion applications.2. We develop a colloidal epitaxial growth strategy to synthesize MoSe2-NiSe nanohybrids with electronic structure modulation for enhanced hydrogen evolution. In detail, the MoSe2-NiSe nanohybrids with well-defined heterointerfaces have been successfully synthesized for the first time by in situ growth of metallic NiSe nanocrystallites on the MoSe2 nanosheets. Engineering conductivity is another effective strategy to optimize the HER activity. XPS、EELS and UPS analyses indicate band alignment at the heterointerface give rise to the electrons transferring from the metallic NiSe nanocrystallites to the MoSe2 matrix, achieving the electronic modulation of the MoSe2-NiSe nanohybrids for efficient electrocatalytic activity. The MoSe2-NiSe nanohybrids exhibit excellent HER catalytic properties with a low onset overpotential of 150 mV, a large cathodic current density (10 mA/cm2 at an overpotential of 210 mV) and a small Tafel slope of 56 mV/decade. We anticipate that constructing hybrid structures will be a powerful tool for achieving high performance electrocatalysts in solids.3. We synthesize MoSe2/TiO2 nanocomposites for photoelectrochemical water splitting. The MoSe2/TiO2 nanocomposites exhibit higher photocurrent density, lower interfacial charge-transfer resistance and better photostability than pure TiO2. The 2D morphology can not only allows for the easy access of electrolyte, but also ensure tighter contact with electrode to facilitate fast electron transfer. Moreover, a type II band alignment is formed at the interface and the valence band of MoSe2 is lower than that of TiO2. On irradiation, the photogenerated holes can be transferred easily from the valence band of TiO2 to that of MoSe2, which can effectively retard the photogenerated electron-hole recombination and prolong the lifetime of electron-hole pairs, leading to the enhanced photoelectrochemical and photocatalytic activity. This strategy will pave a new way to develop new efficient PEC catalysts.