Abstract:

A series of nitrogen-doped MoS₂ (N-MoS₂) nanocatalysts were synthesized by hydrothermal method with sodium molybdate as molybdenum source, L-cysteine as sulfur source and reducing agent, and dicyandiamide as nitrogen source. The crystal structure, morphology, elemental mapping and electronic properties of N-MoS₂ with different N doping contents were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The characterization results revealed that all synthesized N-MoS₂ catalysts were flower-like spheres composed of nanosheets, and N atoms were successfully doped into MoS₂ lattice and uniformly distributed in the N-MoS₂ nanocatalysts. The doping of N atoms increases the electron densities of Mo and S atoms adjacent to N atoms, and forms more unsaturated coordination sites with high catalytic activity. The linear sweep voltammetry curves and the Tafel slopes of all catalysts were tested in acidic medium with an electrochemical workstation to evaluate their electrocatalytic hydrogen evolution (HER) performance. The results suggest that the hydrogen evolution reactions on MoS₂ catalyst and N-MoS₂ catalysts both proceed via the Volmer-Heyrovsky mechanism, but the rate-determining step of MoS₂ catalyst is Volmer reaction, and that of N-MoS₂ catalysts is Heyrovsky reaction. Compared with MoS₂ catalyst, N-MoS₂ catalysts exhibit better HER performance with lower Tafel slopes and faster hydrogen evolution reaction rates. Especially, N-MoS₂-0.1 (N/Mo=0.1) exhibits the lowest Tafel slope of 60mV/dec. The improved HER activity of N-MoS₂ nanocatalysts mainly results from the increased exposure of unsaturated coordination sites and the weakening of Mo-H* due to the electron-rich Mo atoms.

Key words: nitrogen doping, molybdenum disulfide, catalyst, electrochemistry, hydrogen

摘要：

以钼酸钠为钼源，L-半胱氨酸为硫源和还原剂，双氰胺为氮源，采用水热法合成了一系列氮掺杂二硫化钼纳米催化剂（N-MoS₂）。通过XRD、SEM、XPS、Raman等手段表征了不同N掺杂量的N-MoS₂催化剂的形貌、元素分布、晶体结构和电子性质。表征结果表明，合成的N-MoS₂催化剂均为纳米片层组成的花球，N原子均匀掺杂进了MoS₂晶格中。N掺杂使得N原子周围的Mo和S原子的电子密度增加，生成更多具有催化活性的不饱和配位点。采用电化学工作站在酸性介质中测试了催化剂的线性扫描伏安曲线和塔菲尔斜率，评价了N-MoS₂纳米催化剂的电催化析氢（HER）性能。结果表明，MoS₂催化剂和N-MoS₂催化剂上的析氢反应均通过Volmer-Heyrovsky路径进行。MoS₂催化剂上析氢反应速控步骤为Volmer反应，N-MoS₂催化剂上析氢反应的速控步骤为Heyrovsky反应。与MoS₂催化剂相比，N-MoS₂催化剂的塔菲尔斜率较低，析氢反应速率较快，显示出更好的电催化析氢性能。尤其当N与Mo的原子为0.1时（N/Mo=0.1），制备的N-MoS₂-0.1催化剂表现出最好的电催化析氢性能，其塔菲尔斜率为60mV/dec。N-MoS₂催化剂的电催化活性提高可归结为不饱和配位点暴露量的增加和富电子的Mo对Mo-H*的弱化。

关键词: 氮掺杂, 二硫化钼, 催化剂, 电化学, 氢