In Situ Growth of Interfacially Nanoengineered 2D–2D WS2/Ti3C2Tx MXene for the Enhanced Performance of Hydrogen Evolution Reactions

In line with current research goals involving water splitting for hydrogen production, this work aims to develop a noble-metal-free electrocatalyst for a superior hydrogen evolution reaction (HER). A single-step interfacial activation of Ti3C2Tx MXene layers was employed by uniformly growing embedded WS2 two-dimensional (2D) nanopetal-like sheets through a facile solvothermal method. We exploited the interactions between WS2 nanopetals and Ti3C2Tx nanolayers to enhance HER performance. A much safer method was adopted to synthesize the base material, Ti3C2Tx MXene, by etching its MAX phase through mild in situ HF formation. Consequently, WS2 nanopetals were grown between the MXene layers and on edges in a one-step solvothermal method, resulting in a 2D–2D nanocomposite with enhanced interactions between WS2 and Ti3C2Tx MXene. The resulting 2D–2D nanocomposite was thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses before being utilized as working electrodes for HER application. Among various loadings of WS2 into MXene, the 5% WS2–Ti3C2Tx MXene sample exhibited the best activity toward HER, with a low overpotential value of 66.0 mV at a current density of −10 mA cm–2 in a 1 M KOH electrolyte and a remarkable Tafel slope of 46.7 mV·dec–1. The intercalation of 2D WS2 nanopetals enhances active sites for hydrogen adsorption, promotes charge transfer, and helps attain an electrochemical stability of 50 h, boosting HER reduction potential. Furthermore, theoretical calculations confirmed that 2D–2D interactions between 1T/2H-WS2 and Ti3C2Tx MXene realign the active centers for HER, thereby reducing the overpotential barrier.


S3
Table S1.Summary of optimization parameters used for in-situ HF formation for chemical etching of MAX phase to produce the MXene.

Electrochemical Measurements
The potential was converted using Eq.(S2) to a Reversible Hydrogen Electrode (RHE) The value of double layer capacitance (C dl ) was determined by plotting the change in current density (∆j) against the scan rates.∆j is the average difference between the cathodic current (j c ) and the anodic current (j a ), as shown in Eq. (S3).The slope of this plot was used to calculate the C dl value. 2,33) ∆j = j c -j a S11 Furthermore, the CV curves were obtained by varying the scan rate from 10 to 100 mVs -1 for each sample.The active sites of the electrocatalysts were identified by determining the electrochemical surface area (ECSA) using Eq.S4. 4 (S4) Where, the specific capacitance (Cs) is given by Eq. 5. (S5) Where Q is the capacitive charge was computed from the area beneath the CV sweep at 100 mVs -1 for the 5% WS 2 -MXene, MXene, and WS 2 samples.Scan rate is equal to k (0.1 Vs -1 ), m is the mass of the catalyst deposited (3 mg), and ∆V (V 2 -V 1 ) is the potential window (0.2 V) for CV scan.The turnover frequency (TOF) has been calculated using the Eq.S6. 5 (S6)

S12
Where i is the current at -0.8 V vs RHE from LSV, F is the Faraday's constant (96500 C.mol 1 ) and n is the active site density that is calculated from Eq. (S7).

S21
The binding nature and thermodynamic stability are assessed using partial density of states (PDOS) and charge density difference (CDD).Figure S16(b) illustrates the total density of states of 2H-WS 2 , Ti 3 C 2 F 2 and hybrid structure 2H-WS 2 -Ti 3 C 2 F 2 .We found that the total density of states for the hybrid structure WS 2 -Ti 3 C 2 F 2 exhibits better metallic behavior in comparison to Ti 3 C 2 F 2 and WS 2 , which have a broader band gap.The presence of strong peaks near to the Fermi level for both the hybrid structure and Ti 3 C 2 F 2 specifies a high chemical reactivity, which improves HER performance.We have anticipated the spin-polarized density of states projected on the several atomic contributions (Ti-3d, C-2p, F-2p, S-3d, H-1s and W-5d) to better understand the binding nature between the hybrid structure as shown in Figure S16c.
According to the PDOS, the major peaks of Ti-3d orbital close to the Fermi level (E F ) is evidence of the high reactivity which might be responsible for the activation of adsorbates during catalysis reactions. 83,84ose to the E F , the extreme closeness of W-5d orbital with the S-3p is because of their relatively robust p-d orbital hybridization, that consequently influence the electron transfer from the WS 2 to Ti 3 C 2 F 2 . 85reover, the CDD of the adsorbed H* atom on the hybrid structure and between the two layers of WS 2 ,

S4Figure S1 .Figure S2 .
Figure S1.XRD pattern showing the MAX precursor, and MXene formed by in-situ HF formation indicating enhanced interlayer spacing of the 002 plane.

Figure S9 .
Figure S9.Cyclic Voltammetry plots in the non-faradaic region at the scan rate of 100 mVs -1 for pure Ti 3 C 2 T x MXene; WS 2 ; and 5% WS 2 -Ti 3 C 2 T x MXene; exhibiting the highest area of the loop in 5% WS 2 -Ti 3 C 2 T x MXene.

Ti 3 C 2 F
2 and WS 2 -Ti 3 C 2 F 2 (see Figure S17) were also measured to get insight into the nature of chemical bonding between the adsorbate and interatomic layers, the results are presented in Figure S16 (d) and S18.As seen from FigureS16(d), the adsorption of hydrogen on S and F sites affects the distribution of significant charge on the beneath atomic layers (W and Ti).We found that the S site's hydrogen adsorption influences the charge distribution of both the S and W atoms, but the F site only distributes the F layer with no impact on the Ti layer.As previously indicated by PDOS, this is mainly attributed to strong p-d orbital hybridization between W and S. The HER activities on both sites (S and F site) are further confirm the preceding arguments by calculating the PDOS (FigureS16(e)).Spin-polarized partial density of states results show that the adsorbed H-1s orbital above the E F is lower on the S-site compared to the F-site, indicating a better interaction with the H* and improves the HER performance.Likewise, the average electron density difference plot (FigureS16(f)) further confirmed that the hybrid structure WS 2 -Ti 3 C 2 F 2 have more charge distribution than Ti 3 C 2 F 2 and WS 2 , and has better catalytic activity for hybrid structures.Since the electron transfer has a significant impact on HER performance, the WS 2 and Ti 3 C 2 F 2 work functions were calculated using DFT.In order to calculate the Fermi level (E F ) and work function ( ), the  average potential profiles along the z-axis of Ti 3 C 2 F 2 and WS 2 were considered (FigureS19).The calculated Fermi-level and work function of Ti 3 C 2 F 2 ( eV) and 2H-WS 2 (   = -0.194eV, = 4.135   ), respectively.When these two materials are in contact, charge transfer from = -3.58eV, = 5.478  WS 2 to Ti 3 C 2 F 2 according to the E F and difference.The Schottky junction between Ti 3 C 2 F 2 and WS 2  creates an electric field when they are in contact (Figure S19a-b).Electric fields at interfaces increase charge separation, transport, and promote better HER activities. 86This is manifested by the WS 2 -Ti 3 C 2 F 2 S22 composite's reduced resistance.Additionally, the charge accumulation across the interface of WS 2 -Ti 3 C 2 F 2is greater than that across the stack of Ti 3 C 3 F 2 and WS 2 individually (FigureS18).

Table S4 .
Table showing the resistance to charge transfer for the as-prepared electrocatalyst.Comparison of HER performance of the 2D-2D heterostructure electrocatalyst in 1 M