Supporting Information for Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting Zhengfei Dai,,, Hongbo Geng,,, Jiong Wang, Yubo Luo, Bing Li, ǁ Yun Zong, ǁ Jun Yang, Yuanyuan Guo, Yun Zheng, Xin Wang, and Qingyu Yan *, State Key Laboratory for Mechanical Behavior of Materials, Xi an Jiaotong University, Xi an, Shaanxi 710049, People s Republic of China School of Materials Science and Engineering and School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ǁ Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 1
S1. Calculation of interfacial angle. As is known to us, the interfacial angle hexagonal crystal system (hcp) can be calculated as cos 2 1 3a h1h 2 k1k2 h1k 2 h2k1 l 2 1l2 2 4c 2 2 2 2 3a 2 2 2 3a 2 h1 k1 h1k 1 l 2 1 h2 k2 h2k2 l 2 2 4c 4c. Equation S1 where (h 1, k 1, l 1 ) = (1, 0, 2), (h 2, k 2, l 2 ) = (2, 2, 0), a = b = 1.001 nm, and c = 1.538 nm. It is calculated the cos φ = 0.57, and thus φ is determined as 55.24. 2
The synthetic process for the cobalt monophosphosulfide (Co-S-P) yolk shell spheres is illustrated in Figure S1. It starts from the uniform sized cobalt-precursor solid spheres prepared by a hydrothermal process. After that, a hydrothermal sulfuration reaction was carried out at 180 C with different duration to convert Co-precursor into cobalt sulfide (Figure S1i). Thioacetamide (CH 3 CSNH 2 ) was employed as a slow S release precursor for gradually generating the H 2 S reactant to convert the metal-organic complex precursor to metal sulfides.the formation of yolk-shell structure could be explained as an anion exchange sulfurization reaction of the Co-precursor. The reaction between the faster outward diffused Co 2+ cations and the slower inward diffused S 2 ions affords the formation the cobalt sulfide yolk shell structures. Subsequently, the phosphorization of cobalt sulfide was achieved in the presence of NaH 2 PO 2 at 320 C for 2 hours in Ar atmosphere. The PH 3 gas, generated from thermal decomposition of NaH 2 PO 2, reacted with the sulfurized Co-precursor to form the final yolk shell Co-S-P products. Figure S1. Zhengfei Dai et al. Figure S1. Schematic illustration of the synthetic process for Co-S-P yolk shell spheres. I: Co-precursor spheres are converted into Co-S yolk shell structure via a hydrothermal sulfuration reaction; II: Co-S-P yolk shell spheres are obtained by further phosphorization in the presence of NaH 2 PO 2. 3
The morphology and phase of the cobalt precursor were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), as illustrated in Figure S2. It is revealed that the as-synthesized cobalt-precursor is spherical and uniform in size with smooth surface (Figures S2a-b). The TEM image (Figure S2b) illustrates that these spheres are solid with an average diameter of ca. 450 nm. These Coprecursor spheres (Figure S2c) are amorphous as revealed by the XRD pattern, where no obvious diffraction peak can be detected. Figure S2. Zhengfei Dai et al. Figure S2. Phase and morphology of the Co-precursor spheres. (a) SEM image, inset: an expanded SEM view. (b) TEM image. (c) XRD pattern. 4
Figure S3. Zhengfei Dai et al. Figure S3. Phase and structure of the Cobalt phosphide spheres. (a) SEM image. (b) TEM image. (c) XRD pattern. 5
Intensity (a.u.) Intensity (a.u.) Figure S4. Zhengfei Dai et al. S 0.83 P 0.17 0.6 S 0.58 P 0.42 S 0.71 P 0.29 20 30 40 50 60 70 2 theta (degree) S 0.71 P 0.29 S 0.83 P 0.17 S 20 30 40 50 60 70 50 52 54 56 58 60 2 theta (degree) 2 theta (degree) Figure S4. XRD patterns of (a) S 0.83 P 0.17 and (b) S 0.71 P 0.29, and the high magnification XRD patterns of different samples in the 2θ range of 50-60. 6
Intensity (a.u.) Intensity (a. u.) Intensity (a.u.) Figure S5. Zhengfei Dai et al. (a) F 2g (1) Si T g E g A 1g F 2g(2) S 0.58 P 0.42 (b) S 0.58 P 0.42 S 0.71 P 0.29 S 0.83 P 0.17 S S 0.71 P 0.29 S 0.83 P 0.17 S 100 200 300 400 500 600 700 800 900 Raman Shift (cm -1 ) (c) Co 3+ (sat.) Co 2+ (sat.) Co 2+ 3200 3250 3300 3350 3400 3450 3500 3550 Field (G) Co 3+ S 0.58 P 0.42 S 0.71 P 0.29 S 0.83 P 0.17 S 790 785 780 775 770 Binding Energy (ev) Figure S5. (a) Raman spectra, (b) room-temperature EPR spectra, and (c) Co 2p XPS spectra of cobalt sulfide and cobalt monophosphosulfides. 7
Figure S6. Zhengfei Dai et al. Figure S6. Morphology and structure of the Cobalt sulfide obtained from 2 h sulfurization. (a) SEM image. (b) TEM image. (c) HRTEM image. (d) SAED pattern. 8
Figure S7. Zhengfei Dai et al. Figure S7. Low-magnification SEM image of the S 0.58 P 0.42 yolk-shell spheres (a) and the size distribution (b). 9
Voulume Adsorbed (cm 3 g -1 ) STP Figure S8. Zhengfei Dai et al. 100 80 Adsorption Desorption 60 40 20 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P 0 ) Figure S8. Nitrogen adsorption-desorption isotherms of S 0.58 P 0.42 yolk-shell spheres. 10
Figure S9. Zhengfei Dai et al. Figure S9. As-synthesized pyrite-structured CoS P and the OER propeties in 1 M KOH. (a) XRD pattern, indicating the pyrite-structure cobalt phosphosulphide. (b) ir-corrected polarization curves in 1 M KOH, showing the overpotential is about 350 mv. Inset is the the corresponding Tafel plot (87 mv dec -1 ). The synthesis of pyrite-structured CoS P is based on the sulfurization and then phosphorization from the commercial Co 3 O 4 nanopowder, according to previous report. [S5] 11
Mass Activity (ma mg -1 ) Specific Activity (ma cm -2 ) Mass Activity (ma mg -1 ) Specific Activity (ma cm -2 ) Figure S10. Zhengfei Dai et al. 400 300 200 (a) 0.8 250 (b) Mass Activity Mass Activity Specific Activity 200 0.6 Specific Activity @ 0.5 M H 2 SO 4 @ 1 M KOH 0.2 V overpotential 150 0.2 V overpotential 0.4 Co-S 58 -P 42 Co-S 71 -P 29 Co-S 58 -P 42 0.5 0.4 0.3 100 0.2 100 Co-S 83 -P 17 Co-S 71 -P 29 0.2 50 Co-S 83 -P 17 0.1 0 Co-S Co-P 0.0 0 Co-S Co-P 0.0 Catalysts Catalysts Figure S10. Specific and mass activities of different HER catalysts at 0.2 V overpotential (a) in 0.5 M H 2 SO 4 and (b) 1 M KOH, respectively. 12
Current Density (ma cm -2 ) Current Density (ma cm -2 ) Figure S11. Zhengfei Dai et al. 0-5 (a) @ 0.135 V overpotential 0.5 M H 2 SO 4-10 -15-20 0 5 10 15 20 Time (h.) 0 (b) @0.135 V overpotential -5 1 M KOH -10-15 -20 0 5 10 15 20 Time (h.) Figure S11. Chronopotentiometry curves of the S 0.83 P 0.17 catalyst for HER in different electrolytes for 20 h. (a) 0.5 M H 2 SO 4 under 0.135 V overpotential. (b) 1 M KOH under 0.135 V overpotential. 13
The P/S ratio in the cobalt monophosphosulfide can be adjusted by varying the first-step sulfurization time (0~6 h). To investigate this, we fixed the phosphorization duration as 2 hours and varied the sulfurization time for all the samples. Table S1 shows the elemental Co, P and S atomic percentage derived from ICP analysis for different samples. Increasing the first-step sulfurization time will increase the crystallinity of the cobalt sulfides and the difficulty in phosphorus substitution, resulting in a decrease of phosphorus percentage in the cobalt monophosphosulfide. For instance, without the sulfurization (0 hour), the direct phosphorization at 400 C in Ar furnace produced the yolk-shell cobalt phosphide (CoP) spheres (JCPDS No. 29-0497) as shown in the XRD pattern in Figure S3. For 6 hourssulfurization sample, the P/S atomic ratio was reduced to be 17 : 83 ( S 0.87 P 0.13 ). Except for the 0 h-sulfurization sample, all other samples show a similar crystal structure of 0 S. Table S1. The elemental Co, P and S atomic percentage from ICP analysis for different samples obtained by different sulfurization/phosphorization processes. Sample 1 st sulfurization time 2 nd phosphorization time Formula 1 2 hours 0 hours S 2 2 hours 2 hours S 0.71 P 0.29 3 4 hours 2 hours S 0.58 P 0.42 4 6 hours 2 hours S 0.83 P 0.17 5 0 hours 2 hours CoP 14
Table S2. Comparison of OER activities of metal sulfides and phosphides catalysts in alkaline conditions. Materials η10 Electrolyte Tafel slope Ref. Ni 2 P NPs 290 mv 1M KOH 59 mv dec -1 [S2] Ni 2 P nanowires 330 mv 1M KOH 47 mv dec -1 [S2] Co-S/Ti mesh 361 mv 1M KOH 64 mv dec -1 [S3] Co 9 S 8 NPs/C 294 mv 1M KOH 50.7 mv dec 1 [S4] NiP nanosheets 300 mv 1M KOH 64 mv dec 1 [S5] Mn-Co 320 mv 1M KOH 52 mv dec 1 [S6] oxyphosphide NiCoP/C 330 mv 1M KOH 96 mv dec 1 [S7] nanoboxes Ni 2 P 347 mv 1M KOH 63 mv dec 1 [S8] nanosheets Ni 3 S 2 Ni foam 187 mv 1M KOH 159.3 mv dec 1 [S9] CoS 2 /N,S-GO 380 mv 1M KOH 75 mv dec 1 [S10] N-doped 280 mv 1M KOH 82.7 mv dec 1 [S11] Co 9 S 8 /Graphene S 0.58 P 0.42 266 mv 1M KOH 48 mv dec 1 This work REFERENCES [S1] Liu, W.; Hu, E.; Jiang, H.; Xiang, Y.; Weng, Z.; Li, M.; Fan, Q.; Yu, X.; Altman, E. I.; Wang, H. A Highly Active and Stable Hydrogen Evolution Catalyst Based on Pyrite- Structured Cobalt Phosphosulfide. Nat. Commun. 2016, 7, 10771. [S2] Stern, L. A., Feng, L., Song, F., Hu, X. Ni 2 P as a Janus Catalyst for Water Splitting: the Oxygen Evolution Activity of Ni 2 P Nanoparticles. Energ. Environ. Sci. 2015, 8, 2347 2351. [S3] Liu, T., Liang, Y., Liu, Q., Sun, X., He, Y., Asiri, A. M. Electrodeposition of Cobalt- Sulfide Nanosheets Film as an Efficient Electrocatalyst for Oxygen Evolution Reaction. Electrochem. Commun. 2015, 60, 92 96. 15
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