Chemically activating MoS2 via spontaneous atomic palladium interfacial doping towards efficient hydrogen evolution

Lacking strategies to simultaneously address the intrinsic activity, site density, electrical transport, and stability problems of chalcogels is restricting their application in catalytic hydrogen production. Herein, we resolve these challenges concurrently through chemically activating the molybdenum disulfide (MoS2) surface basal plane by doping with a low content of atomic palladium using a spontaneous interfacial redox technique. Palladium substitution occurs at the molybdenum site, simultaneously introducing sulfur vacancy and converting the 2H into the stabilized 1T structure. Theoretical calculations demonstrate the sulfur atoms next to the palladium sites exhibit low hydrogen adsorption energy at –0.02 eV. The final MoS2 doped with only 1wt% of palladium demonstrates exchange current density of 805 μA cm−2 and 78 mV overpotential at 10 mA cm−2, accompanied by a good stability. The combined advantages of our surface activating technique open the possibility of manipulating the catalytic performance of MoS2 to rival platinum.

To further confirm our conclusion, we used the standard 2H MoS 2 sample (Mo: S=1:2, without Mo (III) species) instead of the chemically synthesized MoS 2 (homemade pristine MoS 2 with stoichiometry at 1:1.87) to react with Pd (II). As shown in Fig. 1a and Supplementary Fig. 2 (Supplementary Fig. 9 and Supplementary Fig. 11)

Supplementary Note 4: Pd doped configurations
Four possible doped sites of 2H-MoS 2 and 1T-MoS 2 were taken into account, as shown in Supplementary Fig. 16 Nevertheless, the increase content of of 1T metal phase causing by a small amount of Pd-doping well compensates the conductivity reduction of MoS 2 , and makes the active sites get electrons more easily. In addition, it is well known that the in-plane surface of 1T-MoS 2 is catalytically active for the HER 3 . In literature, the traditional way to induce phase transitions is lithium insertion 4 , and where the active sites of surface are covered by lithium and thereby results in activity loss. However, in our system, the surface of 1T-MoS 2 is fully exposed, with active sites of surface been fully utilized.

Supplementary Note 6: TOF calculation
The TOF was calcualted according the formula: The DFT calculations suggest that the S atoms near the Pd dopants in the MoS 2 are the prior adsorption sites, but the sulfur vacancies also may be as catalytically active sites. In the following we use the number of the sulfur vacancies and the S atoms near the Pd dopants in the MoS 2 as the number of active sites, which can be estimated from the number of Pd.
The number of the sulfur vacancies is estimated using defect reaction. is highly dispersed as few-layer thick sheets. We assume in the following calculation that all Pd atoms are surface exposed.
From ICP the atomic percentages of Pd were determined to be 1%. Using the deposited mass, molar masses and above atomic percentages, the number of Pd atoms, the total S vacancies, and the TOFs were calculated to be: Taking into the account of the multicrystalline nature of our testing electrode, we can confirm that the much higher average TOF was obtained. Thus, the high average TOF of the 1%Pd-MoS 2 reflect that the increased catalytic activity is originated from both increase in active site density and the intrinsic activity of each site.
In addition, we also adopted another strategy by comparing the activity of our This is impossible as the degree of the defects in the reported sample is already very high (83.7%) and the catalyst loading used for tests were similar between our tests (0.15 mg cm -2 vs 0.22 mg cm -2 ). (see Supplementary Table 12 for details) Therefore, it is actually the huge improvement in catalytic activity that leads us to look for the active sites with high intrinsic activity, where DFT calculations were adopted and the Pd adjacent S atoms were confirmed as supreme active sites towards HER.
Furthermore，the effect of Pd doping can also be clearly evidenced by comparing the catalytic activity before and after Pd doping. It is noted that electron paramagnetic resonance (EPR) can be used to measure unpaired electrons on coordinatively unsaturated defective sites and reflect basal sulfur vacancies present in these layers 7 .
We have also compared the EPR signals before and after Pd doping. As shown in Fig.   2f, the 1%Pd-MoS 2 exhibits approximately 3 times as many unsaturated sites than the pristine MoS 2 . However, the overpotential (20 mA cm -2 ) was reduced by 266 mV, corresponding to an increase in active site density by 2113 times (calculated from the Tafel slope), if the increase in site density is the only reason for the catalytic performance enhancement. This huge difference unambiguously points to the generation of more active HER sites after Pd doping.

Supplementary Methods
Electrical resistances measurements. The electrical resistances of samples were determined by using a homemade button cell. The powder samples were pressed in the mold with certain pressure and time (~10 MPa, 5 min). The sample is inserted between two smooth polished steel discs. Electrochemical impedance spectroscopy (EIS) performed at high frequency using Princeton Applied Research PARATAT MC.
The operating frequency range was between 10 mHz and 10 kHz, the DC potential was 0 V compared to an open circuit, and the AC amplitude was 10 mV. In this case, the phase angle between the voltage applied and the current induced is zero; the impedance of the sample as a function of frequency is present as a horizontal line. The value of resistance of sample is equal to the impedance; and the resistance can be directly read from the |Z| -axis in the Bode. The electronic conductivity was calculated according the formula: RS l   Here, the l is the thickness of specimen, R is electrical resistance of specimen, S is the area of specimen. l and S can get from the mould. R is obtained from Bode spectra (Bode spectra are read from the electrochemical impedance spectroscopy).
Computational details. The formation energy (E f ) for different S-vacancies were calculated to evaluate their structural stability, according to the formula E f =E SV + E S -E MoS2 , where E SV is the total energy for MoS 2 with S-vacancy, E s is the energy for single S atom and E MoS2 is the total energy for MoS 2 without vacancy, respectively.
The Gibbs free energy of H adsorption is calculated by ∆G H = ∆E H + T∆S + ∆E ZPE .
∆E H is the adsorption energy of the H atom. T∆S is the gas-phase entropy contribution of a hydrogen molecule at 298 K (It is a constant, 0.40 eV). ∆E ZPE is the zero-point energy difference between the adsorbed state of the system and the gas phase state.
"Additionally, the potential dependence of hydrogen adsorption is including using the computational hydrogen electrode model 8,9 , where ∆G H + (aq) + ∆G e-= 1/2∆G H(aq) at a potential of U=0 V versus RHE.