Figure 1. a) Schematic of the synthesis of
MoSe2 bulk powder and MoSe2 directly
grown on a carbon paper. b) Secondary electron microscope (SEM) image of
MoSe2-I-36h. c) High-resolution transmission electron
microscope (HR-TEM) image. d) Masked fast Fourier transform (FFT) of the
marked red box in c); green atom: Mo, yellow atom: Se, e) Cross-section
image of TEM of MoSe2-I-36h-Pt.
MoSe2-I-36h before and after Pt decoration.
Figure 1 (a) presents the schematic of MoSe2powder synthesis and in-situ grown MoSe2 on a carbon
paper. In brief, to produce the powder-type MoSe2, a
solution containing 1.645 g
Na2MoO4∙2H2O, 1.5492 g
Se powder, and 0.2595 g NaBH4 dissolved for 0.5 h in a
mixture of 25 mL deionized water and 25 mL ethanol was prepared.
Subsequently, the dark-colored solution was transferred to a 100 mL
stainless steel autoclave and heated to 200°C, maintained at varying
synthesis time. To obtain in-situ grown MoSe2 on a
carbon paper, the carbon paper was added to the mixed solution. Pt
nanoparticles were decorated on the MoSe2 surface using
chronopotentiometry at 10 mA cm−2 for 100 h. It was
confirmed using ICP-MS that the amount of Pt loading was about 0.15
wt%. (Table S1 ) Powder MoSe2 and in-situ grown
MoSe2 were denoted as MoSe2-P-X and
MoSe2-I-X, respectively, where X represents the
synthesis time.
Through SEM analysis, we observed variations in morphologies of
MoSe2 powder dependent on the synthesis time. For
MoSe2-P-1 h, irregular particles shapes/sizes with
undesired composition of MoSe2.86 were observed, as
shown in Figure S1 (a) and Figure S2 (a). The
compositions of MoSe2-P-6h, MoSe2-P-12h,
MoSe2-P-24h, MoSe2-P-36h, and
MoSe2-P-48h were analyzed by SEM-EDS shown inFig. S2 (b)–(f). The obtained compositions were
MoSe2.05, MoSe1.97,
MoSe2.02, MoSe1.96,
MoSe2.00, respectively. MoSe2 nanosheets
in Fig. S1 (b)–(f) were generated with increasing synthesis
time to 6 h. The flower-like MoSe2 nanosheet was
obtained from 12 h onwards. Figure S3 shows XRD spectra of
MoSe2 powder samples. For MoSe2-P-2H,
peaks at 13.1°, 31.4°, 37.8°, and 55.94° matched to (002), (100), (103),
and (110) planes of MoSe2-2H (JCPDS 00-029-0914). In the
case of MoSe-P-1 h, the peaks were corresponded to Se (JCPDS
01-086-2246) revealing the incomplete synthesis of
MoSe2. When the synthesis time was more than 6 h, we
observed the absence of the Se peak as well as the peak corresponding to
(103) plane of MoSe2 originating from the 2H phase.
Moreover, the shift and broadening of peaks corresponding to (002) and
(100) planes of MoSe2 were observed, indicating the
presence of mixed 1T and 2H phases of MoSe2.[6-a] Figure S4 exhibits Raman analysis
of the MoSe2-P samples. Intensity of a peak at 239
cm-1, corresponding to A1g of
2H-MoSe2, is diminished with increasing synthesis time,
whereas a peak at 281 cm-1, matching to
E12g of 1T-MoSe2, was
increased with the synthesis time, indicating the 1T phase of
MoSe2 increased as synthesis time increased.[14] Additionally, the peaks, which are related to
1T phase, is clearly observed for the synthesis time of 36 h
(J1=105 cm-1, J2=149
cm-1, J3=224 cm-1)
revealing the presence of mixed 1T and 2H phase of
MoSe2. [15] In the case of
MoSe2-P-6h, the peaks at 121, 198, 277, and 352
cm-1 is matched to MoOx revealing the
incomplete formation of MoSe2.[16] Figure S5 (a) and (b) , shows the TEM
images of the MoSe2-P-36h and its selected area electron
diffraction (SAED) pattern, exhibiting that synthesized
MoSe2 was polycrystalline in nature, reflected by the
ring patterns in SAED with respective lattice spacing values of 0.67 and
0.28 nm, which correspond to (0 0 2) and (1 0 0) planes of 1T+2H phases
of MoS2. Furthermore, TEM-EDS elemental mapping analysis
reveals the even distribution of elements Mo and Se in samples with
higher intensity of Se being observed, as shown in Figure
S5 (c)-(e). We determined that the optimized synthesis time was 36 h
based on the detailed material analysis revealing the presence of the
1T/2H mixed phases of MoSe2 and revealed the complete
transformation of precursors into MoSe2 structures
without the trace of unreacted Se. The composition of
MoSe2-P-36 h was MoSe2.24 which was
confirmed using inductively coupled plasma-mass spectrometry (ICP-MS).
(Table S1 ) Moreover, 36 h sample exhibited highest HER
performances under the 0.5 M H2SO4electrolyte, compared to samples synthesized at different time, where
lowest overpotential of 177mV at 10mA/cm2 and lowest
charge transfer resistance was observed (Rct= 22Ω).
Furthermore, highest electrical double layer capacitance (59.06
mF/cm2) was observed for the 36 h samples compared to
the other samples with different synthesis time (Figure S6 andS7 ).
After the optimization of the MoSe2 powder synthesis, we
conducted experiments to directly grow the 1T/2H mixed phase
MoSe2 on a conductive support such as carbon paper.
Morphology and the microstructure of MoSe2-I-36h was
investigated using SEM and HR-TEM. As shown in Figure 1 (b),
dense and flower-like MoSe2 nanosheets were grown on the
carbon paper. In addition, through the HR-TEM analysis, the resultant
in-situ grown samples exhibited heterointerface of 1T/2H phases of
MoSe2 as shown in Figure 1 (c) and (d).
Elemental composition mapping was conducted using TEM-EDS elemental
mapping technique where Mo and Se was well distributed along the samples
with higher intensity being observed for the Se, like the 36h powder
case. (Figure S8 ). After the Pt nanoparticle decoration, Pt
clusters were observed on the MoSe2 surface
(Figure 1 (e) and Figure S9 ). Furthermore, decorated Pt
nanoparticle exhibited lattice spacing value of 0.20 nm, which
corresponds to (200) plane, which was confirmed via HR-TEM analysis
(Figure S10) . [17]