Thermostable 1T‐MoS2 Nanosheets Achieved by Spontaneous Intercalation of Cu Single Atoms at Room Temperature and Their Enhanced HER Performance

A simple strategy to fabricate Cu single atoms (SAs) layer‐intercalated MoS2 only by stirring Cu metals with MoS2 nanosheets solution at room temperature is reported. An ultra‐high concentration (Cu: Mo = 98 at%) of Cu SAs is achieved and the intercalated Cu atoms strongly enhance the stability of the thermodynamically unstable 1T‐phase dominant MoS2. Notably, the as‐synthesized MoS2/Cu‐SAs exhibit a surprisingly high proportion of the metallic phase (64%) even after annealing at 800 °C in 5% H2/Ar foaming gas, indicating extraordinary thermostability of the Cu intercalated 1 T‐MoS2. In addition to, the as‐prepared MoS2/SAs exhibit outstanding catalytic performance owing to the improved electrical conductivity and the highly active unsaturated Cu SAs. This strategy is confirmed as a universal method for producing SAs of other metals and other 2D nanosheets can also be used as the host for SAs intercalation other than MoS2. This study may provide an effective strategy to fabricate facile and low‐cost SAs catalysts.


Introduction
The demand for environmental protection has promoted the research of nontoxic catalysts to be utilized in the field of sustainable energy and restricting the dimensions of catalysts down to nanometre-scale or even single atoms (SAs) has recently attracted A simple strategy to fabricate Cu single atoms (SAs) layer-intercalated MoS 2 only by stirring Cu metals with MoS 2 nanosheets solution at room temperature is reported. An ultra-high concentration (Cu: Mo = 98 at%) of Cu SAs is achieved and the intercalated Cu atoms strongly enhance the stability of the thermodynamically unstable 1T-phase dominant MoS 2 . Notably, the as-synthesized MoS 2 /Cu-SAs exhibit a surprisingly high proportion of the metallic phase (64%) even after annealing at 800°C in 5% H 2 /Ar foaming gas, indicating extraordinary thermostability of the Cu intercalated 1 T-MoS 2 . In addition to, the as-prepared MoS 2 /SAs exhibit outstanding catalytic performance owing to the improved electrical conductivity and the highly active unsaturated Cu SAs. This strategy is confirmed as a universal method for producing SAs of other metals and other 2D nanosheets can also be used as the host for SAs intercalation other than MoS 2 . This study may provide an effective strategy to fabricate facile and low-cost SAs catalysts.
Transition metal dichalcogenides (TMDs) are one of the 2D materials that demonstrated a variety of applications, [9,10] with the layered structure that can act as the supportive frame for the decoration of SAs, including Cu SAs intercalated NbS 2 , [11] Ni SAs incorporated WS 2 , [12] Pt SAs decorated VS 2 , [13] Mn/Ni/ Rh SAs doped MoS 2 , [14] etc. Because of the much higher dissociation energy of the metal-sulfur bonding than the metallic bonding, SAs could be readily anchored on the edge or basal S site of the supportive TMDs. [11] Among the variety of TMDs family members, MoS 2 is considered as one of the most promising candidates for incorporating SAs to achieve enhanced catalytic property. It is well known that MoS 2 can exhibit several polymorphs including mainly octahedral prismatic coordinated 1T phase (or the disordered 1T' and 1T") and trigonal prismatic coordinated 2H phase. Compared to the semiconductive 2H MoS 2 , the metallic 1T phase is a more suitable catalyst for HER and has been intensively studied owing to the abundant active sites, superior electrical conductivity, as well as moderate hydrogen binding energy [15] . However, the high energy configuration of 1T-MoS 2 leads to thermodynamical instability, and it can quickly transform to the 2H phase at room temperature. [16] It has been reported that the intercalated metal atoms in the interlayer, such as Li, Na, and K, could stabilize the 1T phase of MoS 2 . In addition to, the space confinement has been proved to effectively improve the catalyst activity due to the unique electrons transfer in a confined space. [17] Based on these theories, in this project, we rationally designed copper SA layer-intercalated 1T-MoS 2 nanosheets with a facile strategy to decay bulk Cu powder to SAs that are incorporated in the interlayers of 1T-phase dominant MoS 2 nanosheets with a high concentration of Cu SAs (up to Cu: Mo = 98at%) by spontaneous intercalation at room temperature. Both the theoretical calculations and the experimental validation confirm that the bonding between Cu and MoS 2 contributes to the high concentration of uniformly dispersed SAs. Meanwhile, the extra electrons provided by the intercalated Cu atoms favor the thermostability of 1T-MoS 2 . The as-synthesized MoS 2 /Cu-SAs exhibit a surprisingly high proportion of the metallic phase (64%) even after annealing at 800°C, indicating extraordinary thermostability of the Cu intercalated 1T-MoS 2 . In addition, the as-prepared sample Cu 0.39 MoS 2 shows outstanding HER performance with a low onset potential of 104 mV and a small Tafel slope of 52 mV/dec, benefiting from modified electronic structure, the improved electrical conductivity of MoS 2 , and the activated Cu SACs which synergistically promote the catalytic property. Moreover, the desirable HER performance of MoS 2 /Cu-SAs is still maintained after annealing at 800°C owing to the excellent thermostability, differing from the unintercalated 1T-dominant MoS 2 that is completely transformed to the less-active 2H phase after the annealing treatment. The sandwiched structure of metal atoms intercalated in TMDs shows enormous potential in fabricating SAs, and we applied the strategy to successfully synthesize Ti, Fe, Ni, and Zn SAs intercalated MoS 2 nanosheets, boosting in-depth investigation of developing applicable catalysts toward commercialization.

Characterization of MoS 2 /Cu-SAs
The fabrication process of MoS 2 /Cu-SAs samples was illustrated in Figure 1. The 1T-phase dominant MoS 2 was synthesized using a hydrothermal method, followed by spontaneous incorporation of Cu SAs in the interlayers of 1T-dominant MoS 2 nanosheets (see the Material Preparation section for a detailed description). The original products mixture including MoS 2 /Cu SAs and Cu powder after intercalation from 1 to 7 days was recorded and examined with X-Ray diffraction (XRD) measurement. It can be observed in Figure 2a that the Cu-related peaks at 43.4°, 50.6°, and 74.2°are gradually weakened with time, indicating the decomposition of the Cu powders. The pure sample of Cu 0.39 MoS 2 is obtained through centrifuging and cleaning by acetone, acidic solution, and DI water, and there are no bulk Cu-related patterns or other by-products observed in the final product except for the MoS 2 phase. Combined with the following X-Ray-absorption fine-structure (EXAFS) spectra, we can conclude that the bulk Cu powder has completely transformed into  Cu SAs. Further high-resolution transmission electron microscopy (HRTEM) characteristic analysis confirms the intercalated Cu exists as the form of SAs, which will be discussed later. The structures of the as-synthesized MoS 2 /Cu-SAs with different Cu concentrations were also investigated, as shown in Figure 2b. The broad peaks centered at around 9.7°, 33.0°, and 57.1°in  the XRD patterns of the Cu-SAs intercalated MoS 2 correspond to (002), (100), and (110) lattice planes of 1T-MoS 2 . Notably, the (002) lattice plane of 1T-MoS 2 has an expansion compared to 2H-MoS 2 (2θ = 14.4°, JCPDS Card No. 04-008-2232). Compared to the pristine MoS 2 , the (002) peak of MoS 2 /Cu-SAs slightly shifts to a lower angle from %10.6°to %9.7°, indicating the expanded interlayer due to the intercalation of foreign species. [18] The as-made MoS 2 /Cu-SAs samples were further annealed to examine the stability of the 1T phase. The intercalated MoS 2 nanosheets exhibit high stability with 1T-metallic phase even at 800°C, which has never been reported in any previous research as far as we know. After annealing at 400°C for 30 min, the unintercalated 1T-phase dominant MoS 2 completely transforms to the pristine 2H phase, which is confirmed by the peak shift of the XRD pattern, and the X-Ray photoelectron spectroscopy (XPS) binding energy, as discussed later. From the XRD pattern in Figure 2c, it could be observed that the peaks of the (002) plane of pristine MoS 2 shifted from %10.6°to %14.0°after annealing, indicating interlayer shrink from 0.82 to 0.62 nm. The change of lattice constant is consistent with the phase transformation from 1T to 2H under high-temperature environment that has been previously reported, [19] and the critical temperature is usually at around 100-400°C. [20] However, the quantification of 1T phase of the Cu-SAs intercalated MoS 2 is surprisingly maintained at a high level after being annealed even at 800°C. As evidenced from the XRD pattern of the annealed sample Cu 0.39 MoS 2 in Figure 2d, the peak at %9.7°still exhibits a high intensity, indicating the d-spacing of interlayers remains expanded (0.89 nm) compared to that of unintercalated MoS 2 (0.62 nm) after annealing at 400-800°C. It should be noted that after annealing at 800°C, the Cu pattern appears again, indicating the formation of Cu clusters, which may arise from sintering by high-temperature treatment.
The annealed samples were also examined by Raman spectra to determine the phases, as shown in Figure 2e-f. It shows that both MoS 2 and Cu 0.39 MoS 2 exhibit distinct peaks at the wavenumbers of %150, %230, and %325 cm À1 , which corresponds to the J 1 , J 2 , and J 3 MoÀS phonon modes of 1T phase MoS 2 , respectively. [21] After annealing at 400-800°C, the samples show intense peaks at the wavenumbers between %382 and %407 cm À1 , representing the E 1 2 Â g and A 1 Â g vibration modes of 2H phase MoS 2 , respectively. [22] For the unintercalated MoS 2 , the representative 1T phase peaks disappear after annealing at 400-800°C, transforming to pure 2H phase, while for Cu 0.39 MoS 2 -400/600/800, the integrated intensity of the J 1 , J 2 , and J 3 peaks are still strong, indicating superior thermostability of the metallic 1T phase with Cu SAs intercalation. The Raman analysis is in good agreement with the XRD measurement and the specific ratio of the 1T/2H phase will be discussed in the following part.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements were applied to examine the surface morphology. As shown in Figure S1, S2, Supporting Information), all the samples exhibit similar nanoflower morphology stacked by the curly layers of MoS 2 nanosheets, which is consistent with our previous research. [23] No Cu clusters or other impurities can be observed in the SEM or large-scale TEM images. To further verify the location of Cu atoms in MoS 2 , we carried out HRTEM analysis ( Figure 3). Compared to the pristine 1T-dominant MoS 2 , Cu atoms are monodispersed in the interlayer of the (002) MoS 2 when the concentration of Cu is at a relatively low level (Cu x MoS 2 , x < 0.2) (Figure 3b,c). In Figure 3d,e, the isolated atoms tend to form an atomic layer gradually with the increased concentration of Cu, which is also displayed in the corresponding atom intensity profile inserted in Figure 3e. In the Cu 0.98 MoS 2 sample, there is an expansion in terms of the (002) plane of 1T-dominant MoS 2 after intercalation (0.82-0.89 nm), consistent with the peak shift in the XRD patterns. From the image, Cu atoms are uniformly distributed in the interlayer of MoS 2 , which forms a perfect lattice structure of a new compound-CuMoS 2 , similar to that done by a chemical vapor deposition method. [24] However, from later EXAFS measurement, the sample still keeps SA nature, which will be discussed later ( Figure S8, Supporting Information). In addition to, the HRTEM image ( Figure S3, Supporting Information) of the annealed sample Cu 0.39 MoS 2 -800 reveals that the atomic Cu layer still sits at the interlayer of MoS 2 after high-temperature treatment at 800°C. In this case, the remaining intercalated Cu atoms stabilized the 1T phase of MoS 2 . In addition, the energy www.advancedsciencenews.com www.small-structures.com dispersive X-Ray (EDX) spectra and element mapping ( Figure S4, S5, Supporting Information) exhibited a uniform display of Cu atoms in MoS 2 with an increased intensity of Cu from Cu 0.10 MoS 2 , Cu 0.18 MoS 2 , Cu 0.39 MoS 2 , and Cu 0.98 MoS 2 , which is consistent with the analyses by XPS and will be discussed later.
There are no Cu clusters observed in EDX mapping or HRTEM images of the as-made MoS 2 /Cu-SAs, agreeing well with the XRD analysis. XPS measurement was used to further verify the composition, valence state, and chemical bonding of the samples. We have investigated the electronic state of 1T and 2H phase MoS 2 in previous work and understand that there is a binding energy shift of the Mo (3d) and the S (2p) orbitals of 1T-phase MoS 2 owing to the electron donation from intercalated species, such as N 2 H 4 , NH 4 þ , K þ , Li þ , and some other functional groups. [10,20,25] Generally, the binding energy of the Mo (3d) peak of 1T-phase MoS 2 shifts to a higher value from 0.7 to 1.0 eV relative to that of 2H-phase MoS 2 . The evolution of core-level spectra of Mo 4þ with the increase of Cu atoms concentration is shown in Figure 4a, where Mo demonstrates a doublet peak corresponding to Mo (3d 5/2 ) and Mo (3d 3/2 ). Specifically, the peak splits are observed in the XPS spectra (Figure 4a,b), and the two peaks with the binding energy of %229.1 and %229.9 eV correspond to 1T and 2H of Mo 4þ (3d 5/2 ), indicating the coexistence of the two phases in MoS 2 /Cu-SAs. With the increase in the corporation of Cu SAs, 1T phase ratio increases, indicating SAs of Cu benefits the stabalization of 1T phase. The peaks from blue and yellow curves correspond to MoO 2 and MoO 3 , which has been observed previously, maybe due to the partially formation of oxide. [26] The increase in intercalated concentration leads to less portion of MoO 2 and MoO 3 phases, suggesting an electron transfer from Cu atoms to MoS 2 . [20] From XPS analysis, the percentage of the 1T phase for MoS 2 and Cu 0.98 MoS 2 is estimated to be 61% and 89%, respectively, by the fitting of Mo (3d) core level spectra (Table S1, Supporting Information), and we propose that the orbital hybridization among the intercalated Cu and the Mo atoms can assist the maintenance of the 1T structure as the extra Cu SAs may provide more electrons to occupy the Mo valence orbitals. [27] The superior thermostability of Cu intercalated 1T-dominant MoS 2 was further confirmed by the XPS measurement. After annealing treatment at 800°C, the unintercalated 1T-dominant MoS 2 is fully transformed into 2H-MoS 2 , as shown in Figure 4b   As illustrated in the Cu (2p) spectra in Figure 4c, the copper (2p 3/2 ) peak of samples MoS 2 /Cu-SAs is situated at 932.9 eV, suggesting the ionic Cu 1þ state of the intercalated Cu SAs. The binding energy is in good agreement with Cu 2 S, implying Cu atoms form covalent bonds with MoS 2 . [28] There is no peak corresponding to zero-valent Cu 0 with a binding energy of 932.3 eV, and the absence of Cu-Cu bonds evidence the complete decay of metal powders to SAs. [29] The atomic ratios of Mo, S, and Cu are obtained from the XPS survey scan, as shown in Figure S6, Supporting Information, and the accurate compositions, including phase quantification, are listed in Table S1, Supporting Information. The XPS results match well with the XRD analysis that the intercalation limit of Cu-SAs into 1T-dominant MoS 2 can be as high as Cu: Mo = 98 at% without forming clusters or nanoparticles. After high-temperature annealing, the copper (2p 3/2 ) peak of sample Cu 0.39 MoS 2 -800 exhibits a mixture of Cu 0 , Cu 1þ , and Cu 2þ , suggesting a combination of immobilized intercalated Cu SAs (Cu 1þ and Cu 2þ ) and some immigrated deintercalated nanoparticles (Cu 0 ). The peak split after annealing is also consistent with the zero-valent Cu peak observed in the XRD result (Figure 4c).
To investigate the electronic feature and verify Cu SAs in MoS 2 nanosheets, we applied EXAFS measurements in Singapore synchrotron light sources (SSLS). As shown in Figure 5a, the absorption edge of the as-made MoS 2 /Cu-SAs is higher than that of the standard Cu foil and lower than that of CuO, indicating that Cu atoms exhibit a cationic state of close to þ1 state similar to that of Cu 2 S. [30] This matches well with the XPS results. The K-edge EXAFS spectra after Fourier transformation (FT) were plotted with the relevant reference samples, as shown in Figure 5b. Compared to the standard Cu foil with a Cu-Cu bonding length of %2.24 Å, there is no detectable Cu-Cu firstor second-shell interactions in the EXAFS analysis, confirming that Cu atoms are isolated from each other, signaling the SA state. In addition, the R space of MoS 2 /Cu-SAs was quite different from that of CuO or Cu 2 O references but similar to that of the previously reported Cu 2 S with peak slightly shifting to a shorter bonding length of %1.72 Å, indicating that the oscillation of Cu in the as-made MoS 2 /Cu-SAs corresponds to the Cu-S coordination. [31] Figure S7, Supporting Information shows the FT of Cu 0.98 MoS 2 and the fitting results indicate that Cu has a coordination number of 3, which is smaller than CuS and much smaller than Cu bulk, which is 12, confirming the SA status after heaving doping of Cu atoms in MoS 2 .

Formation and Stabilization Mechanism Study on the Self-Intercalated Cu SAs into MoS 2
To understand the formation and stabilization mechanism of Cu intercalation, first-principles calculations were applied to investigate the change of thermodynamic formation energy (ΔG) before and after the intercalation of Cu SAs into 1T and 2H MoS 2 , and the optimized structure, density of states (DOS) plot, and band structure of the stable MoS 2 /Cu-SAs in 1T-MoS 2 are summarized in Figure 6. Under the two conditions of Cu x MoS 2 (x = 0.25 or 0.5), the calculated ΔG values are negative (À0.22 eV, x = 0.5) for intercalating Cu atoms into 1T-MoS 2 , indicating a spontaneous reaction with a more stable structure when Cu atoms are inserted into the interlayer of 1T-MoS 2 . In contrast, the ΔG values are positive (0.8 eV) for the intercalation of Cu atoms into 2H-MoS 2 , implying the process is thermodynamically nonspontaneous and the structure of Cu SAsintercalated 2H MoS 2 is unfavorable, which is consistent with the experimental and theoretical calculations of the reported work ( Figure S8, Supporting Information). [11] Moreover, DOS and band structure calculations indicate that the Cu intercalation significantly influences the energy Fermi level (E F ) of the MoS 2 system. From the calculations, both undoped and doped 1T phase are metallic. For undoped 1-T MoS 2 , the position of E F lies in the pseudogap, and the electron density is low. After being doped with Cu, the Fermi level is at the antibonding area and the electron density is higher (Figure 6b), which suggests the possible high activity. After heavy doping (Figure 6c), the Fermi level lies in pseudogap area again, suggesting the chemical activity may be reduced.

Electrocatalytic Property of MoS 2 /Cu-SAs
The HER measurements of the as-prepared samples were carried out by the three-electrode cell on an electrochemical workstation. As illustrated in the linear sweep voltammetry (LSV) curves of Figure 7a, the HER performance of the Cu intercalated MoS 2 is strongly improved compared to the pristine MoS 2 nanosheets.   Figure 7b). The results are also comparable with Ref. [31]. Moreover, the OER performance was tested, and the Cu intercalated MoS 2 samples exhibit enhanced OER performance with higher current density and lower Tafel slopes compared to the pristine 1T-dominant MoS 2 nanosheets (Figure 7d,e). In addition to, the sample Cu 0.39 MoS 2 exhibits outstanding stability for HER and OER performance with neglectable degradation at the current density of 10 mA cm À2 after 12 h running (Figure 7c). After the stability test, the sample was examined with XRD, TEM, and XPS measurements, and there were no obvious changes in morphology, phase, or chemical bonding ( Figure S9-S11, Supporting Information). Apart from that, the electrocatalytic properties of the high-temperature annealed samples were also tested (Figure 7f ). After annealing at 800°C, sample 1T-dominant MoS 2 is completely transformed into the 2H phase with very poor catalytic activity, while the samples intercalated with Cu SAs still exhibit outstanding performance. The thermal-stable catalytic performance of the samples owes to the maintained 1T structure induced by the intercalation of Cu SAs, though there is inevitable negative effect on HER due to removing defects and vacancies by the incorporation of Cu atoms that originally serve as extra-active sites ( Figure S9-S11, Supporting Information). Figure S12, (Supporting Information) shows the electrochemical impedance spectroscopy (EIS) measurement of the samples. Cu intercalated samples exhibit lower resistance and Cu 0.39 MoS 2 displays the low resistance both before and after annealing, suggesting the electron charge transfer kinetics play important role in HER performance. In addition, the electrochemical active surface area (ECSA) was calculated by the C dl derived from the cyclic In addition, as discussed previously, doping of Cu SAs can reduce EIS value, which enhances charge transfer. Moreover, the defects (i.e., edges state) may be slightly reduced by higher doping concentration (i.e. 98% doping), which influence the HER performance. As discussed in Ref. [31], Cu incorporation can enhance the charges of MoS 2 and the electronic structure is changed after doping, which led to easy break of H-S bond, hence, enhancing HER performance. Therefore, the enhancement of HER performance may be associated with improved electrical www.advancedsciencenews.com www.small-structures.com conductivity, rich electron charges in MoS 2 , and richer active sites with Cu intercalation.
To understand the real mechanism of metal as the catalytic center, we applied in situ EXAFS measurement to observe the change of Cu bonding during the electrochemical reaction process. As illustrated in Figure 7g, the peak at 8,997 eV of as-prepared Cu 0.39 MoS 2 is higher than the sample in solution and with an applied current, suggesting that Cu valence state is from 1þ to more metallic. The small scale of the spectra of Figure 7g, as shown in Figure 7h, confirms the valence state change. It means though Cu is bonded with S to show 1þ state, after dispersing in solution or by applying an electric current, Cu becomes a more metallic state, which may benefit HER performance. Figure 7i is the FT of the samples of as-prepared, in solution, and applied À1, À3, and À6 mA current states. It should be noted that we cannot apply a current lower than À6 mA since it will induce too many hydrogen bubbles, strongly affecting the spectra measurement. After the catalyst is dispersed into solution, the Cu-Cu distance becomes slightly larger, which may be resulted from solution-induced volume expansion. It is seen that by applying a current of À1, À3, and -6 mA, the distance of Cu does not change and Cu keeps in SA states. No agglomeration is observed, confirming the high stability of the SA intercalation in MoS 2 during HER.

Universal Study of Ti, Fe, Ni, and Zn SAs Intercalated 1T-Dominant MoS 2
The strategy of metal atoms intercalated in 1T-dominant MoS 2 was also applied in the synthesis of Ti, Fe, Ni, and Zn SAs intercalated MoS 2 nanosheets. XRD analyses showed that MoS 2 (002) peaks all shifted to a lower angle, confirming that these elements were successfully intercalated into MoS 2 interlayer like that of Cu. In addition, after annealing at 800°C, a large ratio of 1T phase could be still maintained similar to that of Cu intercalations ( Figure S14, Supporting Information). However, the peaks related to metallic Ti, Fe, Ni, and Zn appeared after 800°C annealing, indicating some atoms were agglomerated to form nanoclusters. It also, in turn, proved that these metal atoms were monodispersed in MoS 2 interlayers before annealing. Furthermore, HRTEM analyses showed that Ti, Fe, Ni, and Zn SAs in the SA state were successfully intercalated into MoS 2 interlayer as that of Cu. However, the concentrations of these elements are lower ( Figure S15,S16, Supporting Information). Parameters may need to be optimized for achieving high loading of SAs. As discussed, the decomposed Cu powders are bonded with S in MoS 2 , making Cu stay in a single-atom state. The bonding energy of Cu-S may play a critical role in the formation of SAs. [11] As shown in Table S2 (Supporting  Information), Cu-S bonding energy is much higher than Cu-Cu, which favors the decomposition of Cu powders. Similarly, Ti-S, Fe-S, Ni-S, and Zn-S bonding energies are all higher than their metallic bonding energies, supporting the assumption of mechanisms. Nevertheless, the high selectivity of Cu is still unclear and further investigation should be involved.

Conclusion
We present a facile method to synthesize Cu SAs intercalated in 1T-phase dominant MoS 2 nanosheets via mixing copper powders at room temperature. Different from other reported single-atom doping, Cu SAs can achieve a doping concentration of nearly 100%, which attributes to the higher chemical bonding energy of Cu-S compared to Cu-Cu. The high doping concentration of Cu SAs and the extra electrons provided by the intercalated Cu atoms contribute to the superior stability of the thermodynamically unstable 1T-phase MoS 2 , even stable after annealing treatment at 800°C. In addition, the as-prepared sample exhibits enhanced catalytic performance owing to the improved electrical conductivity, the highly active unsaturated Cu SAs as well as the modified electronic structure. The facile approach is successfully applied in fabricating SAs using other transition or post-transition metals, such as Ti, Fe, Ni, Zn, etc. In contrast, these metals may be possible to be intercalated into other TMDs, such as VS 2 , WS 2, NbS 2 , etc. This work has provided a simple route for synthesizing a high concentration of SAs, which may open a new approach toward the fabrication of low-cost catalysts for a variety of applications including energy devices (i.e., battery and supercapacitors, fuel cells), sensors, microwave absorption, SA-based ferromagnetism, etc. Moreover, the high stability of 1T-MoS 2 may make it suitable catalyst applied at high temperature, such as ammonia synthesis and CO 2 conversion.

Experimental Section
Material Preparation: All the chemicals were purchased from Sigma-Aldrich. Graphite paper was purchased from Beijing Jinglong Special Carbon (0.5 mm thick). 1T-phase dominant MoS 2 nanosheets were first synthesized by the same hydrothermal method demonstrated in our previous work. [10] In detail, the molar ratio of N 2 H 4 : Mo was set to be 4:1 in this project. The as-prepared 1T-MoS 2 was vacuum-dried in an oven at 50°C for 6 h and the black loose powder was applied to synthesize the Cu SAs intercalated samples.
The Cu intercalated MoS 2 samples were fabricated as follows: Cu powders were weighed and mixed with 0.5 mmol of as-prepared MoS 2 nanosheets with a molar ratio of Cu: Mo = 0, 0.1, 0.2, 1.0, and 2.0. The mixture was well-ground in a mortar before putting in a small jar, and 2 mL of hexane solution was added to the container. The jar was carefully sealed with a magnetic stirrer inside, and the whole set was magnetically stirred for 7 days at room temperature. At the end of the reaction, the solution was rinsed with acetone three times followed by HNO 3 , ethanal, and DI water to remove hexane and Cu powder residuals. Then, it was centrifuged at a low speed of 1,000 rpm for 5 min to precipitate the unintercalated Cu residuals, and the top suspension was collected and centrifuged at a high speed of 12,000 rpm for 30 min to obtain the SAs intercalated MoS 2 . At last, the paste was dried in a vacuum oven at 50°C for 6 h. The as-prepared samples were labeled as MoS 2 and Cu x MoS 2 , x = 0.10, 0.18, 0.39, and 0.98 as identified by XPS, respectively.
The samples 1T-dominant MoS 2 and Cu 0.39 MoS 2 were also annealed to examine the thermostability, and the quick annealing treatment was carried out in a tube furnace with 5% H 2 /Ar foaming gas at 400-800°C for 15 min. The annealed samples were taken out when the tube furnace is cooled down to room temperature and they were labeled as MoS 2 -400/ 600/800 and Cu 0.39 MoS 2 -400/600/800, respectively.
The sample Fe/Ni/Ti/Zn SAs intercalated MoS 2 nanosheets were synthesized with a similar strategy, while the magnetic stirring process for Fe/Ni intercalated MoS 2 was changed to mechanical stirring due to the magnetic properties of Fe and Ni. The annealing treatment is the same as Cu 0.39 MoS 2 -800.
Material Characterization: Microstructure and morphology were characterized by SEM (FEI Nova NanoSEM 450) and TEM (JEOL JEM-F200 Multi-Purpose FEG-S/TEM) equipped with EDX (Bruker SDD-EDX detector) for compositional analysis. XRD measurement was carried out with multi purpose diffractometer (PANalytical) Xpert Multipurpose XRD system with Cu-Kα radiation. XPS was applied to examine the composition and valence state using ThermoScientific ESCALAB 250i X-Ray photoelectron Spectrometer. We also applied in situ EXAFS measurement in SSLS to investigate the electronic feature and to verify the atomically dispersed Cu.
Electrochemical Measurements: The electrochemical measurements were carried out by using a three-electrode cell on an electrochemical workstation (Autolab PGSTAT302N). The working electrode was the as-made samples grown on carbon paper (sample loading amount $0.4 mg cm À2 ). The commercial 40% Pt/C and RuO 2 catalysts of the same loading mass were prepared for comparison. Ag/AgCl (3.0 M KCl) and graphite rod were applied as the reference electrode and counter electrode, respectively, in the three-electrode system. Thirty milliliters of 0.5 M H 2 SO 4 was used as an acidic electrolyte.
LSV and chronoamperometry were applied with a scan rate of 5.0 mV s À1 . The measurements were performed from À0.1 to À0.8 V (vs. Ag/AgCl) for HER test and from 1.1 to 1.8 V (vs. Ag/AgCl) for OER test in 0.5 M H 2 SO 4 . All the recorded potentials were then calibrated vs. RHE based on the Nernst equation The durability test was applied at the overpotential point of 10 mA cm À2 for 43,200 s (12 h). EIS measurements were carried out at an overpotential of À0.25 V vs. RHE over a frequency range from 100 kHz to 0.01 Hz with an amplitude of 0.005 V. The electrochemical double-layer capacitance C dl was derived from CV scans in the non-Faradaic region with increasing scan rates (5,10,15,20,25, and 30 mV s À1 ), and the ECSA was calculated with the following equation (2) Computational Methods: All the first-principles calculations are done by Vienna ab initio simulation package. [33] The core electrons of all the atoms are represented by projector augmented wave basis set. [34] Perdew, Burke, and Ernzerhof exchange-correlation functional within generalized gradient approximation with a kinetic energy cutoff of 620 eV was used in all the calculations. [35] 2 Â 2 supercell was used for the calculations. The geometrical structure of MoS 2 was optimized by relaxing atomic positions and lattice vectors without any symmetry restrictions. Conjugate gradient scheme is used for the optimizations. 5 Â 5 Â 1 and 9 Â 9 Â 1 Monkhorst-Pack k-points mesh was used for total energy and DOS calculation. Hexagonal high symmetry first Brillouin zone was considered for band structure calculation. In the Z-direction, vacuum space of 20 Å was used to avoid interactions between neighboring slabs. The van der Waals interaction is corrected for bilayer and Cu intercalation calculations. [36] Formation energy of the system is calculated using the following formula: where ΔE F is formation energy of the system, E MþCu is total energy of Cu intercalated MoS 2 , E M and E Cu are total energy of MoS 2 system and Cu atom, respectively. The negative formation energy indicates the structure is thermodynamically feasible.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.