Chapter 4 Ultrathin few-layer MoS 2 plates embedded in nanoporous graphene film for flexible
4.3 Results and discussion
The fabrication process for the MoS2/NGF is illustrated in Figure 4.1a. Prior to fabrication procedure of MoS2/NGF, Ammonium tetrathiomolybdate ((NH4)2MoS4, ATTM)) was dissolved in DMF.
Preparation of MoS2plates offers the two-dimensional MoS2structure could provide a growth sites for nanoporous graphene film with maximize the surface area with improved ion diffusion.(see Figure 4.2) The ATTM solution was transferred to a Teflon lined autoclave and subjected to hydrothermal treatment at 200 ºC for 12 h. The ATTM/ PVA/NiCl2•6H2O complex was first formed by facile mixing process.
The mixture of MoS2 and PVA/NiCl2•6H2O solution was spin-coated on a SiO2/Si substrate. The prepared composite film was placed in a CVD furnace for the formation of ultrathin few-layer MoS2
plates embedded in nanoporous graphene film. To prevent the chlorination etching at an elevated growth temperature, the reaction temperature kept under 800 C.⁰ 36After the growth of MoS2/NGF on SiO2/Si substrate, the PMMA layer was coated by spin-coating to support the MoS2/NGF layers. The PMMA/MoS2/NGF/SiO2 was etched by using 10 wt% HF for 48 h to get a freestanding PMMA/MoS2/NGF. Then, as shown in Figure 4.1b, the freestanding PMMA/MoS2/NGF was reverse- transferred onto a PET film. An interdigitated microstructure, which consisted of 20 fingers as symmetric electrodes, was carved by plotter-assisted cutting. After that, a gel-type polymer electrolyte was directly coated onto the as-prepared micro-supercapacitor device for the further electrochemical measurement. The detailed dimensional parameters are shown in Figure 4.3.
Figure 4.1b-ii shows the cutting blade trajectory which generates the interspaces for electrode/electrolyte interface. The plotter cutting strategy is promising for mass production and integration of on-chip micro-supercapacitors onto arbitrary substrate without using of conventional electron beam or laser scribing. The simple cutting process can be easily achieved reasonable interspace for micro-supercapacitors.37 The electrolyte impregnates between adjacent fingers reduces the contribution of the equivalent series resistance(ESR) (Figure 4.1c-iii).38 For MoS2/NGF, the cross- sectional SEM shows 1.8 μm of active material layer was obtained without mechanical failure during micro fabrication process(Figure 4.4). The thicknesses of MoS2/NGF, which can be easily controlled by adjusting the spin-coating rate, were 800 nm and 1.7 μm for 2,000 rpm and 3,000 rpm,
respectively(Figure 4.4). The interdigitated-shaped morphology of MoS2/NGF mSC was found to be well preserved after cutting, where the MoS2/NGF films were uniformly covered on the PET film(Figure 4.1d-e). The magnified SEM and TEM images reveal that the ultrathin MoS2was randomly embedded by nanoporous graphene film, which effectively boost the diffusion kinetics of electrons and ions (Figure 4.1f, 4.2(low-mag TEM)).5To verify the applicability of the MoS2/NGF mSC electrodes in epidermal electronics, a transferred electrode was attached onto the skin for the epidermal mSC, which suggests various utilizations of our mSC.
Figure 4.1. Schematic diagram of synthesis and fabrication process of MoS2/NGF micro-supercapacitor.
a) Synthesis process of MoS2/NGF. b) Micro-supercapacitor electrode fabrication process on a PET substrate. c) Optical image of as-prepared MoS2/NGF mSC. d-f) SEM images of MoS2/NGF and TEM image of MoS2/NGF. g) Schematic diagram of an epidermal mSC directly attached to the hand.
Figure 4.2. Low-magnification TEM images of MoS2/NGF.
Figure 4.3. Dimensional information of single, series and parallel integrated MoS2/NGF micro- supercapacitor and cross-sectional schematic image of micro-supercapacitor
Figure 4.4. Cross-sectional SEM images of MoS2/NGF film.
Figure 4.5a shows Raman spectra of NGF and MoS2/NGF. NGF displays a D peak at around 1347 cm-
1, a sharp G peak at 1588 cm-1, and a 2D peak at 2697 cm-1. Compared to pristine NGF, MoS2/NGF exhibited a much higher D peak at around 1351 cm-1, implying that the ultrathin MoS2promoted the formation of defect sites and the carbon-sulfide interfaces of MoS2/NGF. The MoS2/NGF also exhibited two peaks at 397.7 cm-1, and 407.6 cm-1 of in-plane E2g1 and out of plane A1g, respectively(Figure 4.6).
The X-ray diffraction (XRD) was performed to investigate the structural information of NGF and MoS2/NGF. As shown in Figure 4.5b, a strong X-ray diffraction peak at 33.0° corresponds to the (100) plane of single-phase MoS2, the peaks at 44.6° and 54.7° reflect the presence of Ni. Figure 4.5c shows the XPS C 1s spectrum of MoS2/NGF. C 1s peaks of the as-prepared MoS2/NGF were deconvoluted into four peaks at a binding energies of 284.0 (sp2 carbon peak), 284.8 (sp3), 285.3 (ether), 286.1 (carboxyl) eV, which suggests the presence of functional groups at edge sites and defects in the inner pores. The result of O 1s spectra indicates a presence of different oxygen functional groups such as C- O, C=O and Ni-O at a binding energy of 532.0 eV, 529.2 eV and 532.8 eV, respectively(Figure 4.5d).
The Mo 3d and S2p XPS spectra of MoS2/NGF are shown in Figures 4.5e-f. The binding energies of Mo 3d5/2and S 2p3/2of MoS2/NGF are consistent with the reported ultrathin MoS2.39Mo 3d and S 2p spectra display peaks at 232.9, and 229.5, which can be assigned to the Mo 3d3/2, Mo 3d5/2, 163.5 and 162.4 eV, which can be assigned to the S 2p1/2, and S 2p3/2 in MoS2, respectively. The shift of ultrathin MoS2is due to the annealing process.22
Figure 4.5a) Raman spectra of NGF and MoS2/NGF. b) XRD spectrum of the NGF and MoS2/NGF. c- f) X-ray photo-electron spectroscopy (XPS) of the MoS2/NGF: c) C1s, d) O1s and e) Mo 3d and f) S2p.
Figure 4.6 Raman spectra of MoS2/NGF.
The electrochemical performances of the mSC were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge curves (CDC), and electrochemical impedance spectroscopy (EIS).
Figure 4.7a shows the CV curves of a single unit mSC in the potential window of 0-1.0V. MoS2/NGF showed a larger area than NGF mSC, indicating the improved capacitive performances are originated from pseudocapacitive behavior. Owing to the faradaic charge transfer process of ultrathin MoS2layers with the electrolyte ions, during the redox process, resulting that the total capacitance enhancement of the MoS2/NGF. (MoS2+ H++ e ↔ MoS-SH+)40Figure 4.7b shows the galvanostatic charge/discharge curves (GCD) at a current density of 1 A cm-3. NGF and MoS2/NGF retained a quasi-triangular shape, indicating the capacitive behavior of the MoS2/NGF and discharge plateau was not observed. The specific capacitance value of the MoS2/NGF mSC was 55 F cm-3at a current density of 0.5 A cm-3, which is 275 times higher than NGF. Furthermore, the specific capacitance of the mSC was maintained at 2 F cm-3, when the high current density applied as 5 A cm-3. The electrochemical cycle stability of the mSC was measured at a current density of 5 A cm-3. The specific capacitance of the MoS2/NGF mSC maintained 82.2% of the initial value after 20,000 consecutive cycles. As shown in Figure 4.7d, the electrochemical impedance spectroscopy (EIS) was performed to comprehensively understand the capacitive behavior of MoS2/NGF mSC. The intercept of the Nyquist plot with the real axis at high frequencies shows the equivalent series resistance of the mSC. The MoS2/NGF mSC exhibited a slightly higher equivalent circuit resistance of 215 Ω compared to 113 Ω of NGF mSC, which confirmed that interconnection between highly conductive nanoporous graphene and ultrathin MoS2 effectively provided the abundant reactive sites. This consequently contribute to the high specific capacitance of the MoS2/NGF mSC. Figure 4.7e shows the Bode plots for the MoS2/NGF mSC. The response frequency of the mSC at -45° phase angle (f0) was 8.19 Hz. Thus, the relaxation time constant τ0was
of the mSC in this study was comparable to other reported values in devices fabricated by high cost mSCs (see Table 4.1).
Figure 4.7 (a) Cyclic voltammogram profiles of the NGF and MoS2/NGF mSC (scan rate = 100 mV s- 1). b) Galvanostatic charge/discharge curves of the NGF and MoS2/NGF mSC (current density = 1 A cm-3). c) Cycle stability of the NGF and MoS2/NGF mSC. d) Nyquist plot of the NGF and MoS2/NGF mSC. The inset is a close-up image at the high frequency region. e) Bode plot of the NGF and MoS2/NGF mSC. f) Ragone plot for NGF and MoS2/NGF mSC with previous data.
Table 4.1. Comparison of micro-supercapacitors.
Active
Material Method Current
Collector
Potential
Window Electrolyte Specific Capacitance Co3O4
Lithography /Sputtering deposition
Cr 2V LiPON
14 F/cm3@ RT 37 F/cm3 @
90 C
MoS2-LIG CO2Laser beam MoS2-LIG 1V PVP/NaCl 16mF/cm2
MnO2 Pt magnetron
sputtering Pt 0.8V PVA/H3PO4 20 mF/cm2
MnOx Electron beam
evaporation Au/Cr 0.8 V PVA/H2SO4 32.8 F/cm3
MWCNT Plasma jet etching MWCNT 0.8 V PVA/H3PO4 2.02 F/cm3
rGO Pulsed UV laser rGO 1.2 V 0.1M
Na2SO4 288.7 mF/cm3
GO ink 3D Printing Au 1V PVA/H2SO4 828.06
mF/cm3 Cu(OH)2@
FeOOH nanotube
Screen printing Cu 1.5 V
Fumed silica [EMIM][BF
4] ionogel
32.2 F/cm3
MWCNT /Mn3O4
Photolithography LBL assembly
E-beam
Ti/Au 1.2 V PMMA-PC-
LiClO4 8.9 F/cm3 Photoresist
derived porous carbon
Photolithography Cu/Ni tape 0.8 V 0.5 M
H2SO4 11 F/cm3
MoS2/NGF Film transfer NGF 0 to 1 V PVA/H3PO4 55 F/cm3
Figure 4.8a displays the cyclic voltammogram of a single MoS2/NGF mSCs and integrated mSCs in series and parallel configuration. These integrations enhanced the operating voltage window in series configuration or a multiplied capacitive current in the parallel configuration. In the 3-mSC series connection, its potential window extended up to 3V, which was tripled than the single unit mSC. In the 3-mSC parallel connection, the output current of the integrated mSC provided nearly tripled than single unit mSC.
During the bending test, the mSC did not show degradation of electrochemical performances and maintained its original GCD curves shape (Figure 4.8b). The cycle stability also did not show much degradation with multiple bending deformation of the mSC after 2,000 cycles as shown in Figure 4.8c.
It is worthy to note that almost no change was observed along with imperceptible capacitance loss while undergoing bending deformations, which not only demonstrates high bendability and long-term bending durability but also exhibits the robustness to play a crucial role in wearable energy storage device. To further demonstrate the practical and scalable application, the integrated supercapacitor was transferred onto human skin using a wet transfer paper. Figure 4.8d and its inset shows the red LED operated by the integrated supercapacitor transferred onto the human skin. These results indicate that a MoS2/NGF mSC integrated onto a human skin can be successfully applied to a micro energy device. Furthermore, the electrode attached onto the back of the hand showed good contact with the skin surface and was not delaminated. Attributed to the structural robustness of the nanoporous graphene scaffold backbone, MoS2/NGF mSC exhibited an excellent mechanical stability and endured bonding force with the target surface including genuine skin for applications as electronic skins.
The enhanced electrochemical performances of MoS2/NGF mSCs can be ascribed to three points: 1) nanoporous graphene film can greatly enhance the conductivity of the ultrathin MoS2hybrid structure
Figure 4.8 a) Cyclic voltammogram of single, series, and parallel integrated MoS2/NGF mSCs. b) Capacitance retention of MoS2/NGF mSCs with 180⁰bending angle. c) Cycle stability of mSCs after 2,000 cycles of bending. d) Digital camera image of a supercapacitor prototype device of and LED lighting up by an integrated mSC on human skin.