Chapter 3. In-situ phase transition of TMDs in perovskite oxide/TMDs heterostructure and excellent
3.4 Experimental details
500 mg of bulk MoSe2 powder (325 mesh powder, purity > 99.9%, Alfa Aesar) was dispersed in isopropyl alcohol (IPA) (350mL) and deionized water (150mL) and then exfoliated to prepare MoSe2
flakes via 5 hours through tip ultrasonic treatment (VC 505). The generated suspension was centrifuged, and the supernatant was collected and dried in a vacuum oven at 100 °C for 12 hours.
Synthesis of LSC
La0.5Sr0.5CoO3-δ (LSC) perovskite oxide was synthesized by a general sol-gel process. An aqueous solution was formed by dissolving a stoichiometric amount of metal nitrate precursor and citric acid in deionized water. After the nitrate precursor was completely dissolved, an appropriate amount of polyethylene glycol (Mw = 400) was added. All chemical reagents were purchased from Sigma-Aldrich and used as they were received without further refining. After the viscous resin was formed, the solution was heated to 300 °C. Then, the produced powder was pre-fired at 600 °C for 4 hours and fired at 950 °C for 4 hours.
５６ Synthesis of catalysts
LSC and MoSe2 were highly energy-milled with 10 wt.% of Ketjen black EC-600JD (KB) by planetary ball mill systems (PM-200, Retsch, Germany) to find the optimal ratio of LSC&MoSe2
catalysts. The following weight ratio was investigated using LSC:MoSe2:KB of 9:0:1, 8:1:1, 7:2:1, 6:3:1, 5:4:1, and 0:9:1 for the milling process. The total weight of the catalyst was maintained at 500 mg, and each catalyst was dispersed in ethanol and ball-milled at 400 rpm for 2 hours using Zr-ball. Therefore, LSC 300 mg, MoSe2 150 mg, and KB 50 mg were used for LSC:MoSe2:KB=6:3:1 synthesis. Then, for further analysis, the solvent was dried in an oven at 70 °C to collect powder.
Material Characteristics
Structural and morphological characteristics were performed through probe formation (STEM) Cs (spherical aberration) calibrator of 200 kV through HR-TEM (JEM-2100F, JEOL) and XRD (D8 Advance, Bruker) at a scan rate of 1° min-1, respectively. Fluorescent emission and UV-vis NIR spectra were obtained with Fluorometer (Cary Eclipse, Varian) and UV-vis NIR spectrophotometers (Cary 5000, Agilent). The BET surface area and pore size of LSC&MoSe2 and LSC were investigated through a physical adsorption analyzer (ASAP 2420, Micromeritics Instruments) with N2 desorption/adsorption.
Surface adsorption capacity was analyzed by TGA (Q500, TA). LSC&MoSe2 and LSC used in TGA analysis were exposed to humid air for 24 hours to absorb moisture. UPS and XPS measurements were performed using HeI (21.2 eV) discharge lamps and monochromatic Al-Kα radiation sources (ESCALAB 250Xi, Thermo Fisher Scientific) under ultra-high vacuum conditions (< 10-10 Torr), respectively. Through Nano 230 FE-SEM (Nova Nano SEM, FEI), SEM images of electrodes before and after total water electrolysis were obtained.
Electrocatalyst measurement Half-cell analysis
The half-cell measurement was carried out with a general three-electrode configuration using a Pt wire and an Ag/AgCl electrode as a counter electrode and a reference electrode, respectively. Rotating disk electrode (RDE) tests were performed using 20% by weight Pt/C (Alfa Aesar), IrO2 (Alfa Aesar),
５７
LSC, MoSe2 and LSC&MoSe2 catalysts in RRDE-3A (ALS). Each catalyst was prepared in an ink form by dispersing 10 mg of a 1mL binder solution (45:45:10=ethanol:isopropyl alcohol:5 wt% nafion solution (Sigma-Aldrich, volume ratio). The bath ultrasound process follows. Then, HER and OER profiles were investigated in an aqueous N2 saturated 1 M KOH solution at a scan rate of 10 mV s-1 by drop coating 5 μL of each catalyst ink on a glassy carbon disk electrode having an area of 0.13 cm2. The correction of the reversible hydrogen electrode (RHE) was experimentally determined at a scan rate of 1mV s-1 at H2 saturated 1M KOH, where platinum wires were used as working and counter electrodes, and Ag/AgCl was used as reference electrodes. All half-cell profiles were iR compensated by measuring the resistance of the solution (1 M KOH). All electrochemical tests were performed using Biological VMP3.
Total water electrolysis test
The entire water electrolysis test was conducted with a three-electrode configuration using the Ag/AgCl reference electrode. The prepared catalyst ink was electrically sprayed on a Ni mesh current collector having a catalyst loading density of 1 mg cm-2 to prepare a cathode and a anode. The measurement was carried out in a degassed 1M KOH aqueous solution. Current density has been normalized to the geometric area of the catalyst. All electrochemical tests were performed using Biological VMP3.
Calculation details
Spin-polarized DFT calculations were performed using the Vienna Ab initio simulation package (VASP) [81] within the Projector-Augmented Wave (PAW) method [82]. Electronized exchange- correlation energy was treated with generalized gradient application (GGA) using the Perdew-Burke- Ernzerhof (PBE) function [83]. The DFT+U method within Dudarev's approach [84] was also adopted as U = 4.3 eV and J = 1.0 eV for Co-3d and U = 4.0 eV for Mo-4d. The energy cutoff for the planar wave base set was set to 400 eV, and the PAW data set was used with the following valence electron state: 5s2, 5p6, 5d1, 6s2 for La; 4s2, 4p6, 5s2 for Sr; 3d8,4d1 for Co; 2s2, 2p4 for O; 4s2, 4p6,4d6 for Mo;
and 4s2,4p4 for Se, respectively.Shape optimization was performed using the conjugated gradient (CG) method until the net force of each atom reached less than 0.02 eVÅ-1and the total energy changed within 10-6 eV per atom. The dipole slab correction was also applied to all slab model calculations. The Monkhorst-Pack method of k-point grid [85] was set to points for geometric optimization and 3 2
５８
1 k-points in the Brillouin area for POS analysis, respectively. Vader analysis [76] was used to calculate the atomic charge.
Model system for calculation
In order to construct LSC & MoSe2 heterogeneous structures, each slab model for the LSC surface and MoSe2 layer was separately modeled in advance. First, the unit cell structure of LaCoO3 (LCO) was completely mitigated by optimizing both the atomic position and the lattice parameters that matched the experimentally reported values [86]. After that, the LCO bulk structure was cut into two plausible ends (i.e., CoO2 and LaO ends) along the plane (001). To remove the virtual dipole moment from the slab, we considered a symmetric slab model of LCO(001) consisting of five atomic layers. The bottom two layers were fixed to the bulk position. Next, a supercell structure of 2√2 3√2 1was created, and then half of La atoms were replaced by Sr atoms, resulting in stoichiometry of the La0.5Sr0.5CoO3
system (i.e., a = 10.91 Å, b = 16.37 Å, c = 25.00 Å, 156 atoms). For MoSe2 slab models, a 2 5 5 1 1 supercell structure of the orthorhombic unit cell for 2H-MoSe2 was created (i.e., a = 11.49 Å, b = 16.78 Å, c = 25.00 Å, 60 atoms). Finally, LSC and MoSe2 heterogeneous structures were constructed by combining LSC and MoSe2 slab with a minimized lattice mismatch of less than 3% (i.e., a = 11.20 Å, b = 16.57 Å, c = 35.00 Å, 216 atoms). Vacuum has been sufficiently applied to avoid self-interaction in the z-direction.
Calculation of surface energy
In order to evaluate the relative stability of complementary endings on the surface of LSC (001), the surface energy () was calculated by the sum of cutting energy (u) and relaxation energy (r) previously reported by Heifets et al. [87],
u r
= + (1) The cleavage energy can be obtained as follows,
u =(Eslabu (CoO -t.)2 +Eslabu ((La,Sr)O-t.)−NEbulk) / 4A (2)
５９
The CoO2- and (La, Sr)O-terminal slab energy that is not relaxed, the total energy of the bulk unit cell, N is the formula unit of the slab model, and A is the surface part. In the denominator, the acquisition of four factors occur on the four split surfaces of the two ends. Next, we can calculate the relaxation energy for each CoO2-t. And (La, Sr)O-t. as follows,
(X) (X) (X = CoO -t. or (La,Sr)O-t.)2
( r u ) / 2
r Eslab Eslab A
= − (3)
where Eslabr (X) is a slab energy after relaxation.