P Dopants Triggered New Basal Plane Active Sites and Enlarged

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P Dopants Triggered New Basal Plane Active Sites and Enlarged Interlayer Spacing in MoS2 Nanosheets towards Electrocatalytic Hydrogen Evolution Peitao Liu, Jingyi Zhu, Jingyan Zhang, Pinxian Xi, Kun Tao, Desheng Xue, and Daqiang Gao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00111 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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ACS Energy Letters

P Dopants Triggered New Basal Plane Active Sites and Enlarged Interlayer Spacing in MoS2 Nanosheets towards Electrocatalytic Hydrogen Evolution Peitao Liua‡ , Jingyi Zhu b‡, Jingyan Zhanga , Pinxian Xic, Kun Taoa, Daqiang Gaoa*, Desheng Xuea*. a

Key Laboratory for Magnetism and Magnetic Materials of MOE, Key Laboratory of Special

Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, P. R. China. b

Department of Physics and Astronomy, Clemson Nanomaterials Center and COMSET, Clemson University, Clemson, SC 29634, USA c

Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and The Research Center of Biomedical Nanotechnology, Lanzhou University, Lanzhou, 730000, P. R. China

‡ The two authors have equal contribution to this work * Corresponding author: [email protected],[email protected].

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MoS2-based transitional metal chalcogenides are considered as cost-effective, highly active and stable materials with great potential in application of electrocatalytic hydrogen production. However, their limited quantity of active sites poor conductivity has hampered the efficiency of hydrogen production. Combining calculations and experiments, we demonstrate that P dopants could be the new active sites in the basal plane of MoS2, as well as, help improve the intrinsic electronic conductivity, leading to a significantly improved activity for hydrogen evolution. Further, the P-doped MoS2 nanosheets show the enlarged interlayer spacing, facilitating the hydrogen adsorption and release progress. Experimental results indicate that the P-doped MoS2 nanosheets with enlarged interlayer spacing exhibits remarkable electrocatalytic activity and good long-term operational stability (with Tafel slope of 34 mV/dec, and an extremely low overpotential ~ 43 mV at 10 mA/cm2) . Our method demonstrated a facile technology to improve the electrocatalytic efficiency of MoS2 for hydrogen evolution reaction through non-metal doping, which could be explored to enhance and understand the catalytic properties of other transitional metal chalcogenides.

TOC GRAPHICS

P dopants could be the new active sites in the basal plane of MoS2 and can help to improve the intrinsic electronic conductivity, leading to a


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significantly improved activity for hydrogen evolution in P-doped MoS2 nanosheets.

As the growing demand and production of clean energy globally, hydrogen has gained increasing attention as the clean and renewable energy carrier for energy storage and conversion1,2. Recently, electrocatalytic water splitting has been considered as the most low-cost and effective way to store the electricity generated from other renewable energy souces in form of hydrogen3,4. To achieve this, it is necessary to develop durable and active electrocatalysts which can minimize the needed kinetic overpotential of the hydrogen evolution reaction (HER), and ultimately improve the efficiency of HER. During the past years, Pt-base metals are acknowledged as the most popular HER electrocatalyst, but their large-scale applications are seriously impeded by the high-cost and scarcity of Pt. Hence, developing high efficient novel electrocatalysts made of inexpensive and earth abundant elements for water splitting is of great significance5,6. During the past decade, as an earth abundance, highly active, and electrochemically stable material, MoS2 has been demonstrated to be a promising electrocatalyst for HER both computationally and experimentally

7–9

. Although MoS2 can possess the structure of two

dimensional (2D) layers similarly to that of graphene, the HER activity of MoS2 is considered to originate from the edges rather than its easily exposed basal planes7,10–12. Inspired by this understanding, various strategies have been developed to synthesize nanostructured MoS2 with more exposed edge sites to improve their HER activity, including nanostructured MoS2 of hollow spheres13, defective nanosheets14, edge-exposed porous films15,16, double-gyroid structures17, vertically aligned film18, amorphous films19 and nanoflower20. However, it seems that the HER activity of MoS2 by using expose edges has reached its limitation up to now,


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because the conductivity in such nano-structured materials is relatively low owing to their poor inter-particle (or inter-domain) electron transport, which always reduces the total HER activity of the catalysts21. Therefore, exploring effective methods to arouse the HER activity of inert basal plane sites in MoS2, as well as, to increase the intrinsic electronic conductivity is necessary and urgent. Chemical doping is known as an effective way to enhance the catalysts' performance via multiple mechanisms in electrochemical reactions

22–24

. Recently, Xie et. al has developed

oxygen-incorporated ultrathin MoS2 nanosheets with rich active sites as well as good intrinsic conductivity as efficient HER catalyst25. Our recent results also indicate that N dopants enhance the HER activity of WS2 by modulating the surface electronic state of WS2 effectively without changing the catalysts' original active sites26. These results provide us the new approaches to optimize the HER properties of MoS2 by regulating the active sites and the intrinsic conductivity of MoS2 simultaneous. Most recently, Deng et. al has aroused the HER activity of S atoms in the basal plane MoS2 via single Pt metal atom doping. Similar, the effects are also predicted for other metal atoms27. However, understanding on the HER of non-metal elements doped MoS2 catalysts are still inconclusive up to now28. It is necessary to unravel the HER mechanism for non-metal elements doped MoS2 catalysts, which is considered as the prerequisite for further optimizing their HER performance. Besides, Jiang et. al synthesized assembled 1T-MoSe2 nanosheets with an 81% expansion in (002) interlayer spacing of 1.17 nm, which are better than those of the 2H-MoSe2 counterpart with normal interlayer spacing value of 0.64 nm29. Further, Tang and co-workers utilized a mircowave-assisted method to synthesize MoS2 nanosheets which exhibited edge-terminated structure and enlarged interlayer spacing ~ 0.94 nm, resulted in a good HER activity30. These


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results indicate that increase the interlayer spacing may achieved by introducing proper dopants or varying the synthesis conditions. The enlarged interlayer spacing of MoS2 will increase the density of exposed active sites and modify the electronic structure to optimize the hydrogen adsorption free energy, which can truly enhance the activity of HER. Considering the recently reported catalysts of transition metal phoshpides in HER31,32. Especially, Kibsgaard et. al produced a molybdenum phosphosulfide (MoP/S) catalyst with outstanding HER activity by introducing S into surface region of MoP33. Ye et.al also reported the approach to improve the HER activity of MoS2 and MoP via formation of a bulk MoS2(1–x)Px (x = 0 to 1) solid solution34. A recent work by Song and co-workers also suggested the improved performance of CoS2 after P doping35. In addition, a dramatic enhancement of the catalytic activity for oxygen reduction reaction have been achieved with thermolytically phosphorusdoped ultrathin MoS2 nanosheets

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. Ye et. al also showed that intercalating phosphorus d from

p-type to n-type by increasing the content of intercalated P in MoS2 could be utilized to tune the electrical transport property37. Here, we select phosphorus (P) element as the dopants to explore the effect of P doping on the quantity of active sites and the electronic conductivity of MoS2 for their HER properties. Through experimental and first-principle calculations, our results demonstrate a dual functional strategy of both arousing inert sites and improving electrical conductive. The P dopant in MoS2 matrix has proven to speed up the slow HER kinetics. More importantly, the synthesized P-doped MoS2 nanosheets possess expanded (002) interlayer spacings as large as 0.91 nm as compared to that of the pristine MoS2 (0.65 nm). When used them as an integrated cathode in 0.5 M H2SO4, at a benchmark current density of 10 mA/cm2, the overpotential for HER with P-doped MoS2 nanosheets is extremely low ~ 43 mV. Besides, their Tafel slope is as small as 34 mV/dec, indicates a remarkable electrocatalytic activity and good


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long-term operational stability. The calculation results firstly indicate that P dopants could modulate the surface electronic state of MoS2 to improve its intrinsic electrical conductivity, benefiting the faster electron transfer between the basal plane active sites. Secondly, the P dopeants could be new active sites with the Gibbs free energy (∆GH*) of 0.04 eV, as well as cause the drop of ∆GH* for neighboring S atoms in basal plane to 0.43 eV compared to pristine MoS2 (2.2 eV), indicating that the P-doped MoS2 becomes basal plane active. What's more, the MoS2 nanosheet with interlayer spacing value enlarged to 0.91 nm presents a lower ∆GH* value of -0.09 eV compared with the ordinary MoS2 nanosheet, which facilitates a more preferable proton adsorption and faster hydrogen release processes. This finding of arousing the catalytic activity of inert basal plane atoms and improving the electrical conductivity of catalysts simultaneously shows the function of "kill two birds with one stone" for the catalysts' HER performance, which could be developed to other non-metal-doped MoS2 as catalysts for sustainable electrocatalytic hydrogen production. The P atoms doped few-layer MoS2 nanosheets are prepared via a one-pot chemical reaction by using sodium molybdate dihydrate [Na2MoO4·2H2O], thioacetamide [CH3CSNH2] and ammonium dihydrogen phosphate [NH4H2PO4] as precursors (see the Experimental section for details). To investigate the structural information of the as-obtained samples, characterization by X-ray diffraction (XRD) is first performed (See Figure 1a). As can be seen, the diffraction peaks of pristine MoS2 match with the standard pattern of 2H-MoS2 (JCPDS card no. 75–1539) very well. The broad peak centered at 14.1° is the (002) diffraction peak of MoS2 with interlayer spacing ~ 0.65 nm. For P-doped samples, there gradually appears a new diffraction peak at 9.8°, which agrees well with the calculated X-ray diffraction pattern using interlayer spacing value of ~0.91 nm along the c axis, indicating that using NH4H2PO4 as precursors can also extend the c


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axis of the pristine MoS2. Scanning electron microscopy (SEM) image of the pure MoS2 shown in Figure S1 (Supporting Information) reveals the flower-like shape of the sample, which is consistent with previous results. After P element doping Figure S1 (Supporting Information), the sample's morphology also shows the flower-like shape, consisting of many two dimensional nanosheets. The SEM and transmission electron microscopy (TEM) results of sample P3 are shown in Figure 1b and Figure 1c. To evaluate microscopic structure of P-doped MoS2 nanosheets, high-resolution transmission electron microscopy (HRTEM) at the internal area of the nanosheets is carried out. The fringe of 0.27 nm shown in inset of Figure 2c is corresponding to the (100) lattice plane of the 2H-MoS2 sample. To further give the information of the extended interlayer spacing for P-doped MoS2, HRTEM images of the edge area are performed. As shown in Figure 1e (P3), the spacing between two adjacent monolayers are ranging from 0.65 to 0.91 nm, indicating the highly expanded interlayer spacing of (002) planes, much larger than that of the pristine MoS2 nanosheets of ~0.65 nm as shown in Figure 1f (P0).


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Figure 1. (a) XRD results for pure and P-doped MoS2 nanosheets (samples P0 – P3). (b) SEM image, (c) TEM image, (d) HRTEM at inner plane and (e) stand-up edges for P-doped MoS2 nanosheets (P3). (f) The HRTEM result at stand-up edges for pure MoS2 nanosheets.

Energy-dispersive X-ray (EDX), and element mapping analyses are utilized to verify the existence of phosphorus heterogeneous atoms and the elemental composition of in the MoS2 plane of the samples. Element mapping analysis and EDX results of sample P3, shown in Figure 2a and Figure 2b, reveal that there are Mo, S and P elements exist in the sample and the P elements has uniform distribution in the plane of MoS2. The wide X-ray Photoelectron Spectroscopy (XPS) spectrum reveals the sample are consistent with Mo, S and P element. Further, the element analysis gives the concentration of P dopants in the three samples, that is 1.6, 3.5, and 5.1 at.% for sample P1, P2 and P3, respectively. To further confirm the microstructure of the samples, Raman and high resolution XPS measurements are employed. In the Raman spectra shown in Figure 2c, two characteristic peaks at 381 and 404 cm–1 in the range of 200 to 600 cm-1 arises from the E and A vibrational modes of hexagonal MoS2

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. No

other clear vibration modes are observed for sample P3, indicating that there are no other Pbased new phase formed. For the XPS results in Figure 2d, the two peaks at binding energy of 229.7 and 232.8 eV shown are arisen from Mo 3d5/2 and Mo 3d3/2, respectively, which is consistent with the reported 2H-MoS2

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. The XPS of S 2p spectrum in Figure 2e shows two

peaks at 162.5 and 163.7 eV corresponding to S 2p3/2 and S 2p1/2, respectively. As the increasing of the P dopants, a shoulder peak appears at ~161.3 eV, which is 1T metal phase related MoS2 peaks according to the previous reports

9,40

. Figure 2f shows the P 2p XPS spectra, where the

fitted peaks at 130.6 and 129.8 eV are corresponding to the binding energy of P 2p1/2 and P 2p3/2


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, which has further confirmed the existence of P element in P3. Thus, the above EDX, XRD

and Raman characterizations demonstrate that P atoms are successfully doped into the matrix of the MoS2.

Figure 2. (a) Element mapping and (b) EDX results for P-doped MoS2 nanosheets (sample P3). (c) Raman, (d) Mo 3d XPS spectra, (e) S 2p XPS spectra and (f) P 2p XPS spectra of pure and Pdoped MoS2 nanosheets (sample P0 and P3).

As expected, the electrocatalytic HER activity of P-doped MoS2 nanosheets is investigated, where glassy carbon electrodes (GC) modified with P-doped MoS2 nanosheets are prepared for cyclic voltammetry (CV) and linear-sweep voltammetry (LSV) tests with electrolyte of Arsaturated 0.5 M H2SO4. Similar tests are also carried out on commercial Pt/C catalyst and bare GC with 20 wt% Pt on Vulcan carbon black. Figure 3a shows the polarization curves obtained by LSV after iR-correction for samples P0 – P3, carbon black and the commercial Pt/C catalyst,


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with scan rate of 2 mV/s. Pt/C exhibits onset potential (the applied electric field with the sudden increase of the current intensity) of nearly 0 V, indicating its high HER activity. However, the carbon black shows insignificant activity for HER. It can be seen that pure MoS2 nanosheets show the HER performance comparable to our previous report42. After doping P atoms, the onset overpotential and overpotential decrease gradually. Notably, Sample P3 exhibites the least ηonset of 15 mV among all P-doped MoS2 nanosheets, while the pure MoS2 nanosheets display ηonset of 88 mV. Figure 3b shows that, compareing to other MoS2 nanosheets, the sample P3 also needs the lowest overpotential ~ 43 mV to afford a geometric current density at -10 mA/cm2. In contrast, the other pristine and P-doped MoS2 samples need overpotentials of 140 mV (P0), 105 mV (P1), 75 mV (P2) to achieve -10 mA/cm2, respectively, indicating that doping more P atoms into MoS2 matrix could enhance the HER activity of MoS2 nanosheets. The intrinsic HER activities of the catalysts are further assessed by studying Tafel slopes. Calculated from the polarization curves, Tafel plots of MoS2, P-doped MoS2, and Pt/C are shown in Figure 3c. The Tafel equation is employed to fit the linear portion of the plots: = log| | + , Here η, j, b and a denotes the overpotential, the Tafel slope, the current density and a constant, respectively43. The Tafel slope of Pt/C catalyst is ∼30 mV/dec, concurring with the value as reported44. The Tafel slopes are calculated to be 56 (P1), 53 (P2) and 34 (P3) mV/dec for catalysts of P-doped MoS2 nanosheets, respectively, which are obvious lower than that of pure MoS2 nanosheets (100 mV/dec), revealing significently enhanced catalytic activity of P-doped MoS2 nanosheets than that of pure MoS2. Further, molecular hydrogen is produced for different concentration of P dopants with Volmer-Heyrovsky or Volmer-Tafel mechanism, which transitions from the Volmer-Heyrovsky mechanism (H + H + e → H ) to the Volmer-Tafel


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mechanism (H + H → H ) gradually with the increasing concentration of P dopants, as shown by the decreasing Tafel values from 100 to 34 mV/dec 45,46. That is, for sample P0, P1 and P2, the Heyrovsky reaction dominates the rate-determining, while the Tafel reaction mainly determines the reaction rate for sample P3.

Table 1. HER catalytic performance of pure and P-doped MoS2 nanosheets.

Concentration of P

Onset overpotential

η (mV) at

(at.%)

(mV vs. RHE)

mA/cm2

P0

0

88

P1

1.7±0.1

P2

Tafel slope

Exchange current density

(mV/dec)

(mA/cm2)

140

100

0.21

65

105

56

0.31

3.4±0.1

31

75

53

0.39

P3

5.0±0.1

15

43

34

0.49

Pt/C (20%)

-

0

10

30

0.78

Catalyst

J = 10


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Figure 3. (a) Polarization curves, (b) magnified low current density area, and (c) corresponding Tafel plots of pure and P-doped MoS2 nanosheets in 0.5 M H2SO4. Scan rate: 2 mV/s. In (a), the polarization curve of bare carbon black is also added to compare. (d) Durability measurement of P-doped MoS2 nanosheets (sample P3). The solid lines with hollow/solid square dots indicate polarization curves recorded initially/after 5000 CV sweeps (Scan rate: 100 mV/s), respectively. Inset: time-dependent current density curve for P-doped MoS2 nanosheets (sample P3) under a 100 mV static overpotential without iR corrected. (e) Nyquist plots of pure and P-doped MoS2 nanosheets. (f) The plots of capacitive currents in CV as function of scan rates for samples P0 – P3, the lines show the linear fitting of the plots.


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The exchange current density (j0) can be obtained from the intercept of the linear Tafel plots to the x-axis (where the thermaldynamic redox potential reaches 0). Sample P3 exhibits a j0 value of 0.49 mA/cm2, comparable to the j0 value of Pt/C (0.78 mA/cm2). Overall, Table S1 in the supporting information summarized the catalytic performances of sample P3 and other MoS2based catalyst as reported. It can be seen that P3 exhibits a small ηonset (~15 mV), a low Tafel slope (~34 mV/dec) and a small overpotential (~43 mV) needed at J = 10 mA/cm2, indicating that Sample P3 is superior to most of the others as HER catalyst. Besides, we further evaluate the stability and durability of the pure and P-doped MoS2 catalysts by a constant current measurement Figure S2 (Supporting Information). As shown in Figure 3d, the polarization curves of sample P3 before and after 5000 sweeps are almost overlapped, especially at the low overpotential region. Further, electrolysis measurements are performed with overpotential remains contant for up to 20 h and also show minimal change. The results above demonstrate the robust stability of the P-doped MoS2 nanosheets electrode for HER in strong acidic electrolyte. Above results indicate the HER activity of MoS2 nanosheets is largely enhanced by introducing P dopants. To reveal the HER mechanism, we analyze the HER properties of the catalysts from the aspects of electronic conductivity and active sites, which are considered to be the most critical factor for the HER catalysts. Firstly, electrochemical impedance spectroscopy (EIS) measurements are conducted in frequency range of 10 kHz to 0.01 Hz deeply understand the HER kinetics happening at the interface of electrode and electrolyte. Figure 3e shows the fitted results reveal that the P-doped MoS2 have smaller charge transfer resistance (Rct) than bare MoS2, indicating a faster Faradaic process in HER kinetics. The smaller Rct for P-doped MoS2 would be probably attributed to the modulation of electronic structure through doping with P dopants. To further confirm this, the samples’ carrier densities are determined by the


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Mott−Schottky apporach47. The curve slopes in Figure S5 (Supporting Information) imply that the concentration of electrons in sample P3 is largest among all samples of MoS2 nanosheets, indicating the enhancement of electro conductivity as a results of P dopants. In addition to the charge transfer resistance, we also focused on the active sites of the P-doped MoS2 nanosheets. The electrochemical double-layer capacitances (Cdl) for all samples are measured from CV tests to estimate their effective surface areas (see Figure S6, Supporting Information)48. Here, the capacitance values Cdl are compared, because the electrochemically effective surface area is directly propotional to Cdl. Figure 3f plots the capacitive current densities (∆ = − , where and denotes current densities in CV at a certain potential: 0.15 V versus RHE) as a function of scan rates, and their linear fitted lines. The values of Cdl are equal to half of the slopes of these plots. The maximum Cdl value is measured to be 56 mF/cm2 for sample P3, compared to that of 11 mF/cm2 for pure MoS2, suggesting a high electrochemical active surface area of the P-doped MoS2, which certainly leads to their high catalytic activity. To further evaluate the intrinsic activity of active sites, the turnover frequency (the amount of H2 molecules evolved from one active site per second) is calculated by electrochemically quantifying the active sites using the phosphate solution with the pH=7 and the scan rate of 50 mV/s 49. The polarization curves shown in Figure S7 (Supporting Information) are normalized for the active sites expressed in terms of turnover frequency. The catalysts exhibit TOF of 0.11 (P0), 0.47 (P1), 0.71(P2), 1.4 (P3) H2/s at the overpotential of 100 mV. The trend is in good agreement with the double-layer measurement that P dopants increase the amount of active HER sites. For better understanding the enhanced performance on the catalytic HER of the P-doped MoS2 nanosheets, the density functional theory (DFT) calculations are carried out. Here, we also


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focus on the aspects of the electronic conductivity and active sites, which are known as the key factors to affect the HER catalytic performance of MoS2 system. A 4×4×1 supercell structural model of monolayer MoS2 with a single S atom replaced by one P atom is investigated owing to its lowest formation energy (Figure S10 supporting information). As can be seen from Figure 4a, P-doped MoS2 monolayer has a narrower bandgap (1.5 eV) comparing to that of the pristine MoS2 monolayer (1.8 eV)50. Besides, there are electronic states occuring at the Fermi level in Pdoped MoS2, indicating that more charge carriers can be introduced via P atoms doping, and therefore increase the conductivity of MoS2. Particularly, conducting charges are observed on all elements including Mo, S and P, which can be understood by the partial density of states (PDOS) plots in Figure 4b. As can be seen, at the Fermi level, the p orbital of P atom has strong hybridization with the d orbit of its neighbouring Mo atoms and the p orbit of S atoms, which can promote fast charge transfer in basal plane. Further, the PDOS plots of S atom neighboring to the P atom in P-doped MoS2 and pristine MoS2 shown in Figure 4c can further illustrate this charge transfer. It can be seen that, there are no electronic states around the Fermi level for S atom for pure MoS2, however, S atom neighboring to the doped P atom shows obvious electronic states around the Fermi level for the P-doped MoS2. In addition, we increase the P concentration by adding additional dopants until 3 into the MoS2 basal plane as shown in Figure 4d. Results indicate that increasing amount of S atoms become conductive, which can perfectly explained the experimental observation that conductivity of the P-doped MoS2 nanosheets increases with increasing of the P dopants.


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Figure 4. (a) Density of states (DOS) plots for pure and one P atom doped MoS2 monolayer. (b) The partial density of states (PDOS) result for one P atom doped MoS2 monolayer. (c) PDOS plots for in-plane S atoms for pure MoS2 and P-doped MoS2. The vertical dashed lines indicate the Fermi level. (d) Partial charge density of the P-doped MoS2 monolayer with 0 to 3 dopants for the bands within 0.3 eV below Fermi level. The isosurface value is set at 0.004 e/bohr3 for all the plots.

Moreover, we focused on the active sites of the P-doped MoS2. Generally, it is considered that the processes occur during the HER contain an initial H+ state, an intermediate H* state and the final 1/2 H2 state. To compromise the reaction barriers between the adsorption/desorption steps, an excellent catalyst for HER is expected to have a H adsorption free energy ∆GH* close to zero51. Clearly, in Figure 5a the pristine MoS2 surface is not active with the ∆GH* as high as 2.2


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eV, in agreement with the other previous reports. After doping P, the calculated ∆GH* shows a significant drop for neighboring S atoms, which is about 0.43, indicating that the basal plane S atoms of MoS2 are actived by the P dopants. Most importantly, the doped P atoms can also as the new active sites with the value of ∆GH* 0.04 eV, which is comparable to Pt catalyst. These calculated results indicate that the P dopants and the basal plane S atoms can possess a higher HER activity and thereby can significantly enhance the HER activity, concurring with our experimental observations that the samples' HER performance increase as the increasing of the P dopants. Further, the enhancement of the HER activity may be related to another factor, that is the enlarged interlayer spacing Figure S12 (Supporting Information). The free energy diagram for two different P-doped MoS2 nanosheets shown in Figure 5b indicate that the P-doped MoS2 nanosheets with expanded interlayer spacing ~ 0.91 nm performs a more preferable ∆GH* value of -0.11 eV, smaller than that of the MoS2 nanosheets with an ordinary interlayer spacing value of 0.65 nm (-0.36 eV). As mentioned above, a smaller ∆GH* value reveals faster proton/electron adsorption and hydrogen release processes, accordingly, the MoS2 nanosheets with enlarged interlayer spacing may also benefit the HER activity of MoS2 nanosheets. Further, we also calculate the ∆GH* values of MoS2 with different interface spacing Figure S13 and S14 (Supporting Information), the results show the similar variation that that the P MoS2 nanosheet with an enlarged interlayer spacing of 0.91 nm performs a more preferable ∆GH* value, consisting with our experiment results.


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Figure 5. (a) HER free energy diagram for P site and S sites in basal plane of pristine and Pdoped MoS2. Insets show the P-doped MoS2 basal plane with H adatom on the most active site of P site. (b) Calculated free energy diagram for HER on P-doped MoS2 with different interlayer spacing values as measured in experiments (i.e. 0.65 nm and 0.91 nm). Insets show the calculated models respectively.

Summarized above calculations results, we can conclude that 1) P dopants could modulate the surface electronic state of MoS2 to improve its intrinsic electrical conductivity, which will benefit the faster electron transfer between the basal plane active sites; 2) P dopants could be new active sites with the Gibbs free energy (∆GH*) of 0.04 eV, which is close to Pt. At the same time, the P dopants could cause a significant drop of ∆GH* for neighboring S atoms to 0.43 eV compared to pristine MoS2 of ∆GH*=2.20 eV, indicating that MoS2 becomes in-plain active; 3) comparing to the ordinary MoS2 nanosheet, the P-doped MoS2 nanosheet exhibits an expanded interlayer spacing value ~0.91 nm, which presents a more preferable ∆GH* ~ -0.09 eV, and provides faster proton/electron adsorption and hydrogen release processes. These multifunction of P dopants in MoS2 nanosheets lead them to be the excellent candidate in HER.


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In conclusion, the non-metal element of P-doped MoS2 nanosheets with enlarged interlayer spacing has been successfully prepared. The P-doped MoS2 nanosheets exhibited a small Tafel slope of 34 mV/dec, an extremely low overpotential of 43 mV at benchmark current density of 10 mA/cm2, indicating excellent HER activity and good stability superior to that of pure MoS2. Both the experiment and calculations study indicate that the P dopants in MoS2 matrix have enhanced the HER activity greatly by not only increasing the quantity of intrinsic basal plane active sites (S atoms in basal plane and dopant P atoms), but also improving the conductivity of the materials, besides, the enlarged interlayer spacing has enabled fast proton/electron adsorption and hydrogen release processes. Our finding provides a new strategy to enhance the MoS2-based catalysts via P doping. It is also expected that further improvements can be achieved with other non-metal elements dopants.

ASSOCIATED CONTENT Supporting Information. The Supporting Information associated with this article can be found in the online version. The Supporting information includes the following: Experimental Section, Calculation details, extra SEM pictures, XRD and XPS results, Raman, Stability test, Mott-Schottly plots, CV curves, Polarization curves, Calculated models and results.

AUTHOR INFORMATION


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Corresponding Authors

* Email: [email protected] (D. Q. Gao). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 11474137 and 21571089), the Fundamental Research Funds for the Central Universities (GrantNo.lzujbky-2014-27, No.lzujbky-2016-130 and lzujbky-2016-k02).

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