Multiscale structural and electronic control of molybdenum disulfide foam for highly efficient hydrogen production

Hydrogen production through water splitting has been considered as a green, pure and high-efficient technique. As an important half-reaction involved, hydrogen evolution reaction is a complex electrochemical process involving liquid-solid-gas three-phase interface behaviour. Therefore, new concepts and strategies of material design are needed to smooth each pivotal step. Here we report a multiscale structural and electronic control of molybdenum disulfide foam to synergistically promote the hydrogen evolution process. The optimized three-dimensional molybdenum disulfide foam with uniform mesopores, vertically aligned two-dimensional layers and cobalt atoms doping demonstrated a high hydrogen evolution activity and stability. In addition, density functional theory calculations indicate that molybdenum disulfide with moderate cobalt doping content possesses the optimal activity. This study demonstrates the validity of multiscale control in molybdenum disulfide via overall consideration of the mass transport, and the accessibility, quantity and capability of active sites towards electrocatalytic hydrogen evolution, which may also be extended to other energy-related processes.

1. Is there change for the structure after doping with Co atoms? 2. Some relative literatures such as Adv. Funct. Mater. 2013, 23, 5326, etc. need be added.

Reviewer #3 (Remarks to the Author)
The manuscript contributed by Deng et al. reports an very interesting MoS2 containing multiscale control in terms of morphological structure and electronic structure. Such 3D mesoporous MoS2 foam was synthesized with the use assembled SiO2 photonic crystals as temples. The resulting MoS2 foam exhibit unique structures at three different length scales: 1) the macroscopic mesoscale pores with size of 30 nm provide channels for the transport of H3O+ and H2; 2) the nanoscale orientation of MoS2 nanosheets increases the number of edges to provide more catalytically active sites; 3) the atomic level control over doping changes electronic structures to benefit HER activity. Synergistic effects on these multiscale control make the reported MoS2 foam to exhibit HER catalytic activity superior to most MoS2. Figure 2g highlights the presence of many edge features. What parameter causes this well-aligned structure? What structure should be observed if the sample is rotated by 90 degrees? It is suggested to take a 3D tomography to highlight the anisotropic orientation of the MoS2 nanosheets.

Reviewer # 1
The paper presents results for a new nano-structuring strategy to increase the number of edge sites in MoS 2 . It also presents results suggesting additional effect of doping with Co. I find the paper interesting and it could become publishable in Nature Comm. I would suggest that the authors consider the following comments:

Comment: 1) Tone down the heavily inflated language.
Response: Thanks for the reviewer's comment. We have modified the language to avoid some inflated expression in the revised manuscript.
Comment: 2) Shorten the paper considerably; the basic idea and results are quite simple and will come though more clearly by a concise presentation.
Response: According to the reviewer's suggestion, we have shortened the revised manuscript reasonably and integrally to be more concise.

Comment: 3)
The overpotential is comparable to the best reported in the literature, not "outperforming" them, since that would indicate something much better. Please substantiate the suggestion that the Co is incorporated in the basal planes and not at the edges.
Our recent work have demonstrated that a series of transition metal atoms can be successfully doped into the basal planes of MoS 2 , including Pt, Co and Ni etc. (Energy Environ. Sci. 2015, 8, 1594. The prepared method of Co doped mPF-MoS 2 in this manuscript is similar to the previous one by one-pot homogeneous chemical reaction, except that the SiO 2 nanospheres were used as the mesoporous template. Therefore, owning to the similar synthesized process and 2D basal planes are the dominant domains, most Co atoms should be doped into the basal planes instead of the edges. Nevertheless, unlike the Pt dopants in previous study, Co atoms within the MoS 2 matrix can hardly be distinguished from Mo atoms by spherical aberration-corrected HAADF-STEM to observe their positions. Furthermore, additional DFT calculations also showed Co dopant within the basal plane led to lower surface energy (γ) of 0.35 eV compared to at the edge (S-edge is 0.56 eV and Mo-edge is 0.37 eV), suggesting that doping Co atom into the basal plane is more preferred. Considering few Co atoms at the edges may make contribution to the activity enhancement, we modified the expression about the effect of Co dopants to be more reasonable in the revised manuscript (line 3-5, paragraph 1, page 3; line 7-8, paragraph 1, page 10).

Reviewer # 2
The foam appeared when the Co doping contents exceeded 16.7% ( Figure S13). Therefore, there is an optimum doping content (16.7% from our experience) which will provide significant Co contents but still preserving the integrated mesoporous vertically aligned framework. The change for the structure of MoS 2 after doping with Co atoms has been highlighted in yellow in the revised manuscript (line 6-13, paragraph 1, page 9).  Figure 2g highlights the presence of many edge features.

Comment: 1) TEM image shown in
What parameter causes this well-aligned structure? What structure should be observed if the sample is rotated by 90 degrees? It is suggested to take a 3D tomography to highlight the anisotropic orientation of the MoS 2 nanosheets.

Response:
Thanks for the reviewer's comment and suggestion. The well-defined colloidal SiO 2 nanoshpheres template is the important parameter for the formation of well-aligned MoS 2 layers. Above the SiO 2 surface, the diffusion along the aligned layers is expected to be much faster than the diffusion across the layers, resulting in the oriented MoS 2 nanosheets with many edge features. Without the restriction of SiO 2 substrate, the diffusion process will be isotropic, leading to random-oriented MoS 2 nanosheets ( Figure S3).
According to the reviewer's comment, we have performed 3D tomography in HAADF-STEM mode. The bright-field TEM and HAADF-STEM images ( Figure S4 and S5) taken at different tilting angles show the projected structures of the mesoporous MoS 2 foam along different orientations, indicating that the mesopores are interconnected throughout the entire 3D MoS 2 framework. Furthermore, the video reconstructed from the tilting series images shows more clearly the 3D structure of the mesoporous MoS 2 foam. We have added the TEM and HAADF-STEM images into Supporting Information of the revised manuscript as Figure S4 and S5, and one sentence about the description of 3D MoS 2 foam in the revised manuscript (line 5-7, paragraph 1, page 5). In addition, the video has been uploaded as a single supporting material.  Figure S1a), the MoS 2 layers were grown around the SiO 2 nanosphere and almost all the MoS 2 layers presented as oriented alignment.
After etching the SiO 2 nanosphere, the mesopore has been well remained deriving from the residual spaces and vertically aligned MoS 2 layers with a large fraction of exposed edge sites were obtained (Figure 2g). Furthermore, no obvious difference among the diffraction peaks of MoS 2 in the XRD patterns were found before and after the removal of SiO 2 template ( Figure   S1b). In addition, Raman spectra ( Figure S6), XPS spectra ( Figure S7) and XANES spectra (inset of Figure 2i) showed no obvious difference of the MoS 2 prepared with or without SiO 2 and HF etching. So combined literatures and our experimental results, it is believed that the MoS 2 is stable within the HF solution. We have added a sentence in the revised manuscript to clarify the influence of HF etching on the structure of MoS 2 (line 6-7, paragraph 1, page 4).
The manuscript has been revised and could be published on Nature Communications.
Reviewer #3 (Remarks to the Author) The comments and questions are clearly addressed in the revised manuscript. It is ready to be publish.