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]