Free MoS2 Nanoflowers Grown on Graphene by Microwave-Assisted Synthesis as Highly Efficient Non-Noble-Metal Electrocatalysts for the Hydrogen Evolution Reaction
August
Free MoS2 Nanoflowers Grown on Graphene by Microwave-Assisted Synthesis as Highly Efficient Non-Noble-Metal Electrocatalysts for the Hydrogen Evolution Reaction
Jiamu Cao 1 2
Xuelin Zhang 1 2
Yufeng Zhang 1 2
Jing Zhou 0 2
Yinuo Chen 1 2
Xiaowei Liu 1 2
0 School of Electronics and Information Engineering, Harbin Institute of Technology , Harbin, 150001 , P. R. China
1 MEMS Center, School of Astronautics, Harbin Institute of Technology , Harbin, 150001 , P. R. China
2 Editor: Yuriy Dedkov, SPECS Surface Nano Analysis GmbH , GERMANY
Advanced approaches to preparing non-noble-metal electrocatalysts for the hydrogen evolution reaction (HER) are considered to be a significant breakthrough in promoting the exploration of renewable resources. In this work, a hybrid material of MoS2 nanoflowers (NFs) on reduced graphene oxide (rGO) was synthesized as a HER catalyst via an environmentally friendly, efficient approach that is also suitable for mass production. Small-sized MoS2 NFs with a diameter of ca. 190 nm and an abundance of exposed edges were prepared by a hydrothermal method and were subsequently supported on rGO by microwaveassisted synthesis. The results show that MoS2 NFs were distributed uniformly on the remarkably reduced GO and preserved the outstanding original structural features perfectly. Electrochemical tests show that the as-prepared hybrid material exhibited excellent HER activity, with a small Tafel slope of 80 mV/decade and a low overpotential of 170 mV.
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OPEN ACCESS
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: This research was funded by National
Natural Science Foundation of China - No.61404037
and National Natural Science Foundation of China
No.61376113. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
Introduction
Recently, nanosized MoS2 has attracted extensive attention as an effective hydrogen evolution
reaction (HER) catalyst because of its excellent electrocatalytic properties [
1–5
].
Computational and experimental studies have confirmed that the HER activity of nanosized MoS2 stems
from its sulfur edges [
6
]. As a result, the electroactivity of nanosized MoS2 increases with
increasing number of exposed edges [
7,8
]. Different MoS2 nanostructures, including
nanoparticles (NP), nanosheets (NS) and nanoflowers (NF), have been extensively studied to improve
the activity [
9,10
]. However, the intrinsic poor conductivity of MoS2 greatly diminishes its
electroactivity by limiting the electron-transfer efficiency. Moreover, the strong van der Waals
interactions among lamellar MoS2 particles can result in their aggregation, decreasing the
number of the exposed edges as well as the electroactivity.
A commonly adopted solution to the aforementioned problems is to fabricate nanosized
MoS2 on a highly conductive substrate [
11
]. Graphene nanosheets can act as good substrates
because of their large surface area, excellent electrical conductivity and stable chemical
properties [
12,13
]. A hybrid catalyst of MoS2 NFs supported on reduced graphene oxide (rGO) shows
HER activity with an overpotential of -190 mV and a Tafel slope of 95 mV per decade [14]. To
further enhance the conductivity, Cu nanoparticles have been incorporated into MoS2/rGO
hybrids, which resulted in a decreased Tafel slope of 90 mV per decade [
15
]. Notably, however,
MoS2 NFs are still intended to assemble in large quantities on rGO during the fabrication
process. Another potential danger is that an inadequate rGO reduction will leave too many
oxygen-containing functional groups on the graphene plane, leading to decreased conductivity.
Therefore, the development of a hybrid catalyst characterized by a good dispersion of MoS2
NFs and sufficient reduction of GO is necessary.
In this study, MoS2 NFs prepared by a hydrothermal method were supported on rGO
using a microwave-assisted synthesis method with ethylene glycol (EG) as the reducing agent
[
16,17
]. The physical properties of the as-prepared hybrid were characterized by transmission
electron microscopy (TEM), energy-dispersive spectrometry (EDS) and X-ray photoelectron
spectroscopy (XPS). The HER activity of the hybrid was evaluated by linear sweep voltammetry
in 0.5 M H2SO4 solution at room temperature.
Experimental
Materials Preparation
Graphene oxide (GO) was prepared following the Hummer’s method. In the preparation of
MoS2 NFs, 0.25 mol Na2MoO4 2H2O and 0.98 mol CH4N2S were dissolved in 30 ml of
deionized (DI) water and sonicated to form a clear solution. The precursor solution was transferred
into a 50 ml Teflon-lined autoclave, which was subsequently maintained at 185°C for 20 h. The
product was collected by centrifugation at 6000 rpm, washed with DI water and ethanol, and
vacuum-dried at 60°C.
The MoS2 N (...truncated)