Electrocatalytic Hydrogen Evolution Reaction on Edges of a Few Layer Molybdenum Disulfide Nanodots (1507.00061v1)

Abstract: The design and development of inexpensive highly efficient electrocatalysts for hydrogen production, underpins several emerging clean-energy technologies. In this work, for the first time, molybdenum disulfide (MoS2) nanodots have been synthesized by ionic liquid assisted grinding exfoliation of bulk platelets and isolated by sequential centrifugation. The nanodots have a thickness of up to 7 layers (4 nm) and an average lateral size smaller than 20 nm. Detailed structural characterization established that the nanodots retained the crystalline quality and low oxidation states of the bulk material. The small lateral size and reduced number of layers provided these nanodots with an easier path for the electron transport and plentiful active sites for the catalysis of hydrogen evolution reaction (HER) in acidic electrolyte. The MoS2 nanodots exhibited good durability and a Tafel slope of 61 mVdec-1 with an estimated onset potential of -0.09 V vs RHE, which are considered among the best values achieved for 2H phase. It is envisaged that this work may provide a simplistic route to synthesize a wide range of 2D layered nanodots that have applications in water splitting and other energy related technologies. KEYWORDS: MoS2 nanosheets, hydrogen evolution reaction, electrocatalysis, edges, nanodots, ionic liquid exfoliation, water splitting

• The paper introduces an ionic liquid-assisted grinding method to synthesize few-layer MoS2 nanodots with lateral sizes below 20 nm and thicknesses around 7 layers.
• The paper demonstrates that meticulous structural and optical characterizations confirm enhanced quantum confinement and active edge exposure.
• The paper reports outstanding electrocatalytic performance with a Tafel slope of 61 mV/dec and an onset potential as low as -0.09 V vs. RHE, outperforming bulk MoS2.

Electrocatalytic Hydrogen Evolution Reaction on Edges of Few-Layer Molybdenum Disulfide Nanodots: A Contemporary Approach

The research paper discussed herein offers a comprehensive exploration into the synthesis and characterization of molybdenum disulfide (MoS$_2$) nanodots focusing on their electrocatalytic capabilities for the hydrogen evolution reaction (HER). The empirical paper is situated within the broader objective of developing cost-effective and efficient alternatives to noble metal catalysts like platinum for hydrogen production technologies. The work introduces an innovative approach leveraging ionic liquid assisted grinding exfoliation to synthesize few-layer MoS$_2$ nanodots, demonstrating significant enhancements in catalytic performance.

Synthesis and Methodology

One of the key contributions of this research lies in the method of obtaining nanodots, achieving high yields with consistent quality. Through ionic liquid-assisted grinding and subsequent sequential centrifugation, a new route for generating nanodots with a lateral size smaller than 20 nm and thickness of approximately 7 layers was chronologically defined. The mechanical processes involved, including compressive, torsional, and shear forces facilitated by a room-temperature ionic liquid (RTIL), were crucial for exfoliation and the prevention of restacking of the nanosheets. The syntheses were optimized to retain the crystalline integrity of MoS$_2$, important for maintaining active catalytic sites.

Structural and Optical Characterization

Characterization of these synthesized nanodots through techniques such as SEM, TEM, XRD, and Raman spectroscopy indicated successful exfoliation and preservation of high crystalline quality. A noticeable reduction in lateral size and thickness was confirmed, which enhanced the optical properties due to quantum confinement effects, evident from the UV-Vis absorption spectra. The optical analyses, notably the observed blue shift, corroborate the effective synthesis of thinner MoS$_2$ nanosheets.

Electrocatalytic Performance

The MoS$_2$ nanodots showcased impressive electrocatalytic activity with a Tafel slope of 61 mV/dec, which is significantly competitive among 2H-phase catalysts. The reduced number of layers and increased exposure of active edge sites are implicated in their enhanced catalytic capabilities. With an onset potential as low as -0.09 V vs. RHE and overpotentials reduced to -248 mV at 10 mA/cm$^2$, the nanodots outperform bulk MoS$_2$ significantly, attributed to the efficient electron transport and abundant active sites facilitated by the nanosized dimensions.

Implications and Future Perspectives

The implications of this work are multifaceted, addressing both practical and theoretical frontiers. Practically, the ionic liquid-assisted method provides a scalable alternative for the synthesis of MoS$_2$ nanodots, which could be generalized to other 2D materials for a variety of applications beyond HER, such as in energy storage and other catalytic processes. Theoretically, the insight it offers into the relationship between structural properties—including edge site exposure and oxidation states—and catalytic performance paves the way for future computational studies and experimental optimizations.

Moving forward, further exploration into the long-term stability of these nanodots under operational conditions is essential, although preliminary data indicates promising resistance to degradation. Moreover, integrating these nanodots into composite structures or arrays may enhance accessibility and loading on electrodes, potentially broadening their application in practical energy conversion devices.

In sum, this paper not only illuminates a novel synthesis pathway but also underscores the critical role of structural refinement in advancing the performance of robust, earth-abundant hydrogen evolution catalysts.