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Materials Chemistry of Iron Phosphosulfide Nanoparticles: Synthesis, Solid State Chemistry, Surface Structure and Electrocatalysis for Hydrogen Evolution Reaction Zishan Wu, Xiaolin Li, Wen Liu, Yiren Zhong, quan gan, Xueming Li, and Hailiang Wang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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1 Materials Chemistry of Iron Phosphosulfide Nanoparticles: Synthesis, Solid State Chemistry, Surface Structure and Electrocatalysis for Hydrogen Evolution Reaction Zishan Wu,†,‡,|| Xiaolin Li,†,‡,⁑,|| Wen Liu,†,‡ Yiren Zhong,†,‡ Quan Gan,†,‡,§ Xueming Li,⁑ Hailiang Wang*,†,‡ †

Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States ⁑ College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, The People’s Republic of China § Department of Chemistry, Nankai University, Tianjin 300071, The People’s Republic of China ‡

Abstract Transition metal phosphosulfides represent an emerging category of earth-abundant electrocatalyst materials and some metal phosphosulfides have been shown to outperform the corresponding sulfides and phosphides. To fully realize the potential and benefit energy storage and conversion, it is necessary to study the chemistry of unknown phosphosulfide materials. In this article, we report on the materials chemistry of iron phosphosulfides. We systematically investigate the materials synthesis, solid state chemistry, surface structures and electrocatalytic properties of iron phosphosulfide nanoparticles supported on carbon nanotubes. Two types of iron phosphosulfide nanomaterials, adopting either the FeS or the FeP crystal structure, are successfully synthesized by two distinct synthetic routes designed in accordance with the different thermodynamic properties of the two structures. The compositions (i.e. P/S ratios) of the phosphosulfides can be adjusted within certain ranges without phase separation occurring. We discover that all the phosphosulfide nanoparticles exhibit higher P/S ratios on the surface than in the bulk, and that the presence of P atoms suppresses the oxidation of Fe and S atoms on the surface. We further find that there is a positive correlation between the P content of the iron phosphosulfide nanomaterials and their electrocatalytic activity for the hydrogen evolution reaction, which renders highperformance electrocatalysts for hydrogen production and the understanding that the Fe atoms coordinated by P atoms are the most active catalytic sites in the materials. Keywords: iron phosphosulfides; solid state chemistry; phosphorus-rich surface; electrocatalysis; hydrogen evolution reaction Introduction With increased concerns about unsustainable consumption of fossil fuels, utilizing renewable and clean energy sources becomes necessary. Hydrogen is considered as a clean fuel since water is the sole product of hydrogen combustion.1-2 Much effort has been devoted to developing high-performance and low-cost catalysts for hydrogen generation from electrolysis of water. Transition metal sulfides and phosphides based on earth-abundant elements have emerged as a category of electrocatalysts that may replace platinum which is excellent in catalyzing the hydrogen evolution reaction (HER) but imposes high cost.3-7 Recently, transition metal phosphosulfides have attracted research interest as they exhibit enhanced

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2 catalytic activity and stability compared with the corresponding sulfide and phosphide counterparts as HER electrocatalyst materials. For example, the Jaramillo group doped sulfur into molybdenum phosphide (MoP) and the obtained phosphosulfide which retains the original MoP crystal structure is more active than both MoS2 and MoP.8 The Tour group started from MoS2 and found MoS0.94P0.53 to be the most active composition, although the material is mixed phases of MoS2 and MoP.9 In another study, Jin et al. synthesized cobalt phosphosulfide with a ternary structure (CoPS) featuring distinct (PS)3anions which induce optimal binding energy for hydrogen atoms on the Co sites and thus generate high activity.10 Our group also developed a highly-active HER catalyst based on cobalt phosphosulfide nanoparticles (CoS|P) anchored on carbon nanotubes (CNTs), where P randomly substitutes for S in the mother CoS2 structure for realizing chemical stability and catalytic durability.11 While exemplary transition metal phosphosulfides have already shown high performance and promising potential as HER catalysts, knowledge of their materials chemistry and structure-reactivity correlation is largely missing. The solid state chemistry of phosphosulfides, including crystal structure, composition and surface structure as well as their dependence on the P/S ratio and the metal identity, is hardly understood. It is also unclear how the material structures determine their electrocatalytic activities. Missing of such knowledge is hampering rational design and controlled synthesis of high-performance electrocatalysts based on metal phosphosulfides. Thus, there is an urgent need to synthesize new metal phosphosulfide materials with controlled crystal structures and systematically varied compositions to study their solid state chemistry, surface chemistry and electrocatalysis. Here we report the first study on iron phosphosulfides (chemical formula: FePxS1-x). It is also the first complete study on a metal phosphosulfide nanomaterial system to our best knowledge. We synthesize iron phosphosulfide nanoparticles on CNTs and systematically study their solid state chemistry, surface structure and catalytic properties for HER. We find iron phosphosulfides can adopt two crystal structures, either the FeS or the FeP crystal structure, both with a tunable range of P:S ratio. We successfully unlock these two phases for iron phosphosulfides, by designing distinct synthetic routes based on the different thermodynamic properties of the two structures. For all the iron phosphosulfide nanoparticles exposed to ambient conditions, the surface is P-richer than the bulk, and there the P atoms protect the S and Fe atoms from being oxidized. P substitution for S atoms in the FeS structure drastically increases the HER electrocatalytic activity, while S substitution for P in the FeP only slightly decreases the activity, together leading to the conclusion that the Fe atoms coordinated by P atoms are the most active catalytic sites. Results and Discussion • Materials Synthesis and Solid State Chemistry We used mildly-oxidized multi-wall CNTs as supports for iron phosphosulfide nanoparticles, for the reason that the CNTs can assist nanoparticle size control, nanoparticle dispersion and electron conduction, all of which are desired for electrocatalytic performance optimization and comparison. The iron phosphosulfides adopting the FeS crystal structure (FeS|P) were synthesized by converting presynthesized FeS/CNT with PH3 gas (generated from thermal decomposition of NaH2PO2·H2O) at 300 °C (see Supporting Information for experimental details). As we varied the amount of NaH2PO2·H2O (5, 10, 25, 50 or 100 mg) with respect to that of the FeS/CNT (5 mg), we could obtain a series of FeS|P/CNT

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3 materials with different P doping levels (Table S1). Scanning electron microscopy (SEM) images show that the FeS|P nanoparticles are in the size range of 10-40 nm and are well anchored on CNTs, resembling the FeS/CNT material (Figure S1). As the amount of NaH2PO2·H2O was increased to 100 mg, the P:S ratio increased monotonically to 19% (Figure 1a), as determined by energy dispersive X-ray spectroscopy (EDX). When we further increased the amount of NaH2PO2·H2O to 300 mg, the increase of the P:S ratio in the FeS|P/CNT material was negligible (Table S1), indicating that the maximum allowable P:S ratio without altering the mother FeS crystal structure is ~20%, at least in our system. The FeS|P/CNT materials were analyzed with X-ray diffraction (XRD). The diffraction patterns shown in Figure 1b match well with that of the pure FeS (PDF# 01-075-0600, hexagonal lattice, P63/mmc (194) space group, Figure S2). It is observed that the diffraction peaks shift to lower 2θ angles as more P is introduced into the FeS structure (Figure 1b, S3a, b). Lattice refinement of the XRD data provided lattice parameters of the FeS|P/CNT materials (Figure S4a-c). A roughly linear relationship between the unit cell volume and the P:S ratio is found for the FeS|P/CNT materials (Figure 1c), which is a strong indication that P has been doped into the FeS structure to form a solid-solution-type phase.12-14 The expansion of unit cell with P doping might be due to replacement of S atoms (atomic radius 106 pm) with larger P atoms (atomic radius 108 pm).15 Lattice parameter changes caused by different radii of dopant and parent atoms have been observed in the literature.16-18 Raman spectra of the FeS|P/CNT materials match with that of the FeS/CNT, confirming successful P substitution in the FeS structure and excluding the possibility of FeP formation.

Figure 1. (a) P:S ratios derived from EDX analysis, (b) XRD patterns, (c) correlation of unit cell volume with P:S ratio, and (d) Raman spectra of the FeS|P/CNT materials synthesized with various amounts of NaH2PO2·H2O.

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The FeS|P nanoparticles were further characterized by high-resolution transmission electron microscopy (HR-TEM) and EDX mapping. Fig. 2a and b show HR-TEM images of the FeS|P-100mg nanoparticles (synthesized using 100 mg of NaH2PO2·H2O) anchored on CNTs. Lattice fringes are clearly observed, suggesting that these FeS|P nanoparticles are crystalline. The measured d spacing of 2.64 Å matches with the interplanar distance of the (112) planes of the crystal (Fig. 2b). EDX mapping under scanning transmission electron microscopy (STEM) mode shows that the Fe, S and P elements appear at nearly the same places (Fig. 2c, S5), verifying successful introduction of P into the FeS structure at individual nanoparticle level.

Figure 2. (a, b) HR-TEM images of FeS|P-100mg nanoparticles on CNTs. (c) STEM image recorded with a high-angle annular dark field (HAADF) detector and the corresponding EDX elemental maps of FeS|P-100mg nanoparticles on CNTs. With the success of doping P into FeS structure to achieve the corresponding iron phosphides, we then attempted to dope FeP with S by reacting FeP/CNT with H2S gas (generated from thermal decomposition of NaHS·xH2O), so as to explore iron phosphosulfides in the FeP crystal structure. However, S substitution for P did not take place even if we increased the amount of NaHS·xH2O or varied the reaction temperature between 300 °C and 600 °C (Table S2). As we changed the S precursor to S8 powder, FePS3 was formed instead of S-doped FeP (Figure S6). We also tried converting the FeP/CNT hydrothermally with thioacetamide, but no successful P substitution was observed (Table S2). The difficulty of replacing P with S in the FeP structure can be partially rationalized by the substantially higher thermodynamic stability of FeP (enthalpy of formation -276.2 kJ/mol) compared to FeS (enthalpy of formation -100.5 kJ/mol).19-20 Based on the understanding, we designed a new reaction route to synthesize S-substituted FeP. The route employs direct conversion of pre-synthesized FeOx/CNT with mixed PH3 and H2S gases (generated from thermal decomposition of a mixture of NaH2PO2·H2O and NaHS·xH2O). Since FeP has

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5 significantly better thermodynamic stability than FeS, we expect that in the presence of both PH3 and H2S, the FeOx/CNT will preferably react with PH3 to form the FeP basic structure while more slowly react with H2S to incorporate S dopants. Under such conditions, the resulting materials may adopt the FeP crystal structure with S partially replacing the P atoms. Using this new method (Table S1), we successfully synthesized iron phosphosulfides adopting the FeP crystal structure (FeP|S). By adjusting the amount of NaHS·xH2O (5, 10, 20, or 30 mg) with respect to that of NaH2PO2·H2O (200 mg) and FeOx/CNT (10 mg), we were able to prepare a series of FeP|S/CNT materials with different S:P ratios in the range of 8-21% (Fig. 3a). Phase segregation started to occur when 50 mg of NaHS·xH2O was used (Table S1 and Figure S7). SEM imaging reveals FeP|S nanoparticles in the size range of 10-40 nm anchored on CNTs, resembling the morphology and microstructure of the FeP/CNT (Figure S8). The XRD patterns of the FeP|S/CNT materials match well with that of the FeP/CNT (Fig. 3b), which is an orthorhombic lattice with the Pnma (62) space group (PDF#01-089-2746, Figure S2). As the S:P ratio increases, the diffraction peaks shift to lower 2θ angles (Figure 3b, S3c, d). The nearly linear dependence of unit cell volume on S:P ratio (Figure 3c, S4d-f) is strong evidence that S substitutes for P in the FeP structure forming a solid-solution-type phase. It is noted that the unit cell volume expands as S is incorporated into the FeP structure, which we believe is caused by the replacement of stronger and shorter Fe-P bonds with weaker and longer Fe-S bonds. The difference in bond strength is partially reflected in the difference in enthalpy of formation for FeP and FeS. In the literature, there have been examples that unit cell volume changes are not simply governed by the atomic radii of dopants.11, 21 The FeP|S/CNT samples exhibit almost identical Raman spectra as the FeP/CNT (Figure 3d), further confirming that they are single-phase materials.

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6 Figure 3. (a) S:P ratios derived from EDX analysis, (b) XRD patterns, (c) correlation of unit cell volume with S:P ratio, and (d) Raman spectra of the FeP|S/CNT materials synthesized with various amounts of NaHS·xH2O mixed with 200 mg of NaH2PO2·H2O. The FeP|S nanoparticles were further characterized by HR-TEM and STEM-EDX. Figure 4a and b show HR-TEM images of the FeP|S-30mg (referring to 30 mg of NaHS·xH2O used in the synthesis) nanoparticles with clear lattice fringes. The recorded d spacing of 2.60 Å belongs to the (200) planes of the crystal (Figure 4b). EDX mapping results reveal that the Fe, P and S elements coexist in the nanoparticles anchored on CNTs (Fig. 4c, S5), verifying uniform P doping.

Figure 4. (a, b) HR-TEM images, and (c) STEM image and the corresponding EDX elemental maps of FeP|S-30mg nanoparticles on CNTs. • Surface Structure Electrocatalysis takes place on catalyst surface. Therefore, it is crucial to understand surface structures and properties of electrocatalyst materials. We employed X-ray photoelectron spectroscopy (XPS) to probe the surface composition, coordination environment and oxidation state of the FeS|P and FeP|S nanoparticles supported on CNTs. The core level XPS spectra of Fe, S and P in the FeS|P/CNT materials and the FeS/CNT starting material are shown in Fig. 5a-c, with regions of different chemical states labelled for each element.22-23 The FeS nanoparticles are considerably oxidized on the surface, with evident components at binding energy of ~168 eV in the S 2p region and at ~711 eV in the Fe 2p3/2 region (Fig. 5a, b), corresponding to S and Fe atoms bonded to O atoms respectively.24-25 Interestingly, with the S atoms being partially substituted by P in the FeS|P nanoparticles, both the surface Fe and S atoms become less and less oxidized as the P doping level increases. With a bulk P:S ratio of 19%, the FeS|P-100mg nanoparticles feature completely reduced

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7 S atoms on the surface (Fig. 5b). The surface P atoms in the FeS|P nanoparticles are significantly oxidized, as indicated by the sizable components at ~134 eV in the P 2p spectra (Fig. 5c). The intensity ratio of the reduced to oxidized P 2p peaks, however, remains almost unchanged when the P doping level increases. Similar phenomena of S oxidation suppression by P substitution have also been observed for our previously reported cobalt phosphosulfide nanoparticles.11 The XPS spectra also allow for deriving the surface compositions of the nanoparticles. The surface-rich P:S ratios derived from the XPS results for the FeS|P nanoparticles are plotted in Figure 5d, in comparison with the bulk P:S ratios derived from the EDX results. It is notable that the surface P:S ratios are dramatically higher than the corresponding bulk P:S ratios. For example, for the FeS|P-100mg nanoparticles with a bulk P:S ratio of 19%, the surface P:S ratio is as high as 150%. It should be noted that the probing depth of XPS is in the nanometer or subnanometer range, and therefore the information derived from XPS is surface-rich but not necessarily based on surface atoms only. It is likely that the surface atomic layer of the FeS|P-100mg nanoparticles is completely P-dominated. Based on these observations, we conclude that the P atoms enrich on the nanoparticle surface and act as sacrificing agents to partially protect the Fe and S atoms from being oxidized.

Figure 5. (a) Fe 2p, (b) S 2p, and (c) P 2p core-level XPS spectra of the FeS|P/CNT materials synthesized with various amounts of NaH2PO2·H2O. Regions of different chemical states of an element are separated with dashed black lines and labeled. (d) Red line: plot of surface P:S ratios calculated from XPS spectra

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8 vs bulk P:S ratios obtained in EDX analysis for the FeS|P nanoparticles. The black dashed line is a counterfactual line if the surface and bulk P:S ratios would be equal. We also performed XPS analysis for the FeP|S/CNT and FeP/CNT materials. The Fe 2p, S 2p and P 2p core-level XPS spectra are displayed in Fig. 6a-c with different chemical states labeled.22-23 For the FeP nanoparticles, both the Fe and P atoms near the surface are partially oxidized. Upon partial S substitution for P, the Fe and P atoms become slightly more reduced, although there is no further noticeable dependence on the P doping level (Figure 6a, c). On the other hand, none of the FeP|S/CNT samples show any oxidized S peak at ~168 eV in the S 2p spectra (Figure 6b). This is reasonable since the FeP|S nanoparticles are already P-rich and the P amount is sufficient to suppress any oxidation of S. An obvious trend in the S 2p spectra is that the peak intensity rises with increasing S doping level (Figure 6b), while the intensity of the P 2p peaks remains almost the same (Figure 6c). The surface S:P ratios of the FeP|S nanoparticles with different S doping levels are plotted in Fig. 6d. It is clear that the surface S:P ratios are smaller than the corresponding bulk S:P ratios, suggesting the tendency of P atoms to accumulate near surface. For example, the FeP|S-30mg nanoparticles have a bulk S:P ratio of 21%, but the surface S:P ratio is only 14%. Considering the probing depth of XPS, it is plausible that the surface monolayer of the FeP|S nanoparticles is dominated by P.

Figure 6. (a) Fe 2p, (b) S 2p, and (c) P 2p core-level XPS spectra of the FeP|S/CNT materials synthesized with various amounts of NaHS·xH2O mixed with 200 mg of NaH2PO2·H2O. Regions of different chemical states of an element are separated with dashed black lines and labeled. (d) Red line: plot of surface S:P ratios calculated from XPS spectra vs bulk S:P ratios obtained in EDX analysis for the FeP|S

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9 nanoparticles. The black dashed line is a counterfactual line if the surface and bulk S:P ratios would be equal. • Electrocatalytic Reactivity To assess the electrocatalytic reactivity of the newly developed iron phosphosulfide materials with distinct crystal structures and varied compositions, we performed HER measurements catalyzed by the FeS|P and FeP|S nanoparticles anchored on CNTs. The FeS/CNT catalyst shows poor activity, with the onset potential being more negative than -400 mV vs the reversible hydrogen electrode (RHE) and the current density being only ~10 mA/cm2 at an overpotential as high as 500 mV (Fig. 7a, Table S3). P substitution for S in the FeS structure drastically enhances the HER catalytic activity. With a P:S ratio of 7%, the FeS|P-5mg nanoparticles on CNTs exhibits an overpotential ~260 mV smaller than that of the FeS/CNT. The catalytic activity increases with the P doping level and becomes stagnant after the P:S ratio reaches higher than 15% (Figure 7a, Table S3). Please note that even though the bulk P:S ratio is only 15%, the nanoparticle surface is already dominated by P (Figure 5d). On the other hand, the FeP/CNT material itself is a highly active HER electrocatalyst, achieving current densities of 10 and 100 mA/cm2 at overpotentials of 79 and 154 mV respectively (Figure 7b, Table S3), comparable to the best non-noble metal based HER catalysts reported in the literature.10-11, 26-29 Nevertheless, S substitution for P in the FeP structure slightly undermines the HER catalytic performance, and the activity decreases with the increase of the P doping level (Figure 7b, Table S3).

Figure 7. Linear sweep voltammograms of the (a) FeS|P/CNT and (b) FeP|S/CNT materials at a scan rate of 5 mV/s in 0.5 M H2SO4. Catalyst mass loading is 1.6 mg/cm2. The data are all iR corrected. The results of the electrochemical measurements suggest that there is a positive correlation between the P content in iron phosphosulfide nanoparticles and their HER catalytic activity. It can be further inferred that for both crystal structures, Fe atoms coordinated by P atoms are more active for catalyzing HER than those coordinated by S atoms. This is different from some reported cases where the co-existence of P and S synergistically increases the catalytic activity of the phosphosulfide over the corresponding phosphide and sulfide.8, 10 From the linear sweep voltammograms, we calculated the nominal Tafel slopes (potential vs. log current density) for the catalysts. While the FeS/CNT exhibits a much larger slope, all the iron phosphosulfide catalysts show comparable slopes as the FeP/CNT (Table S3). This is consistent with the

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10 finding that all the iron phosphosulfide nanoparticles are relatively P-rich, which explains the observation that a low level of P substitution can drastically increase the catalytic activity of FeS while the activity of FeP is relatively insensitive to S substitution. It is reasonable to infer that the Fe-P sites are the major active sites for HER. It is also interesting to note that the FeS|P-100mg/CNT (the most active HER catalyst based on FeS|P nanoparticles) is still inferior in activity compared to the FeP|S-30mg/CNT (the least active HER catalyst based on FeP|S nanoparticles), even though both types of nanoparticles have Fe and P dominated surface (Figure S5). This implies that the electrocatalytic performance is not solely determined by the surface monolayer of the catalyst. The inner layers can also influence the catalysis, possibly by electronic interaction or conductivity modification: electron transfer between the inner core and the surface layer of the nanoparticles may modify the adsorption energy of certain reaction intermediates on the nanoparticle surface;30 the nanoparticles with different doping levels can have different conductivities and thus affect the electrocatalytic activity.31 Finally, the FeS|P-100mg/CNT and FeP|S-30mg/CNT materials after HER catalysis were characterized by XRD and EDX. While the crystal structure and composition of the FeP|S-30mg nanoparticles remain unchanged (Figure S9), the FeS|P-100mg nanoparticles undergo substantial material dissolution accompanied by considerable increase in the P:S ratio (Figure S10). These results suggest that the responses of the iron phosphosulfide nanoparticles to the HER working conditions are dependent on their structures and compositions. Future studies need to be performed to understand structural evolution of metal phosphosulfide materials under electrochemical conditions. Conclusion We study the materials synthesis, solid state chemistry, surface structures and HER electrocatalytic properties of iron phosphosulfide nanoparticles anchored on CNTs. Two types of iron phosphosulfides, with distinct crystal structures, tunable P/S ratios and adjustable electrocatalytic activity, are successfully synthesized and characterized. We find iron phosphosulfides can take the FeS or the FeP crystal structure, and the nanoparticles are P-rich on the surface. While P substitution for S atoms in the FeS structure drastically increases the HER electrocatalytic activity, S substitution for P in the FeP slightly decreases the activity. This is the first systematic study of transition metal phosphosulfide materials chemistry and will provide new opportunities for discovering high-performance and low-cost electrocatalyst materials based on metal phosphosulfides. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publicaitons website. Experimental details and supplementary characterization results are provided (PDF). Acknowledgements This work is partially supported by the Petroleum Research Funds from the American Chemical Society and the Global Innovation Initiative from the Institute of International Education. X.L. thanks the Chinese Scholarship Council for financial support. Author Information

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11 Corresponding Author * [email protected] Author Contributions || These two authors contributed equally. References 1. Tachibana, Y.; Vayssieres, L.; Durrant, J. R., Nat Photonics 2012, 6, 511-518. 2. Dresselhaus, M. S.; Thomas, I. L., Nature 2001, 414, 332-337. 3. Faber, M. S.; Jin, S., Energ Environ Sci 2014, 7, 3519-3542. 4. Zeng, M.; Li, Y. G., J Mater Chem A 2015, 3, 14942-14962. 5. Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X., Angew Chem Int Ed Engl 2014, 53, 1285512859. 6. Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N. S.; Ding, Q.; Jin, S., J Am Chem Soc 2014, 136, 10053-10061. 7. Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H., J Am Chem Soc 2011, 133, 7296-7299. 8. Kibsgaard, J.; Jaramillo, T. F., Angew Chem Int Ed Engl 2014, 53, 14433-14437. 9. Ye, R.; del Angel-Vicente, P.; Liu, Y.; Arellano-Jimenez, M. J.; Peng, Z.; Wang, T.; Li, Y.; Yakobson, B. I.; Wei, S. H.; Yacaman, M. J.; Tour, J. M., Adv Mater 2016, 28, 1427-1432. 10. Caban-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S., Nat Mater 2015, 14, 1245-1251. 11. Liu, W.; Hu, E.; Jiang, H.; Xiang, Y.; Weng, Z.; Li, M.; Fan, Q.; Yu, X.; Altman, E. I.; Wang, H., Nat Commun 2016, 7, 10771. 12. Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H., Nature 2001, 414, 625-627. 13. Zhang, J.; Gao, L., Inorganic Chemistry Communications 2004, 7, 91-93. 14. Toraya, H., J Am Ceram Soc 1989, 72, 662-664. 15. Speight, J. G., Lange's Handbook of Chemistry. McGraw-Hill Scientific, Technical & Medical: 2017. 16. Zhuo, J. Q.; Caban-Acevedo, M.; Liang, H. F.; Samad, L.; Ding, Q.; Fu, Y. P.; Li, M. X.; Jin, S., Acs Catal 2015, 5, 6355-6361. 17. Wang, H.; Yang, Y.; Liang, Y.; Cui, L. F.; Casalongue, H. S.; Li, Y.; Hong, G.; Cui, Y.; Dai, H., Angew Chem Int Ed Engl 2011, 50, 7364-7368. 18. Liang, Y.; Wang, H.; Zhou, J.; Li, Y.; Wang, J.; Regier, T.; Dai, H., J Am Chem Soc 2012, 134, 35173523. 19. Predel, B., Fe-S (Iron-Sulfur). In Dy-Er – Fr-Mo, Madelung, O., Ed. Springer Berlin Heidelberg: 1995; Vol. 5E, p 2. 20. Predel, B., Fe-P (Iron-Phosphorus). In Dy-Er – Fr-Mo, Madelung, O., Ed. Springer Berlin Heidelberg: 1995; Vol. 5E, p 4. 21. King, H. W., J Mater Sci 1966, 1, 79-90. 22. Wagner, C. D., Handbook of x-ray photoelectron spectroscopy : a reference book of standard data for use in x-ray photoelectron spectroscopy. Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, Minn., 1979. 23. Morgan, W. E.; Van Wazer, J. R.; Stec, W. J., J Am Chem Soc 1973, 95, 751-755. 24. Audi, A. A.; Sherwood, P. M. A., Surface and Interface Analysis 2000, 29, 265-275. 25. Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S., Surface and Interface Analysis 2004, 36, 1564-1574. 26. Gao, M. R.; Liang, J. X.; Zheng, Y. R.; Xu, Y. F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S. H., Nat Commun 2015, 6, 5982. 27. Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X., J Am Chem Soc 2014, 136, 7587-7590.

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12 28. Callejas, J. F.; McEnaney, J. M.; Read, C. G.; Crompton, J. C.; Biacchi, A. J.; Popczun, E. J.; Gordon, T. R.; Lewis, N. S.; Schaak, R. E., Acs Nano 2014, 8, 11101-11107. 29. Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X., Angew Chem Int Ed Engl 2014, 53, 6710-6714. 30. Li, X.; Liu, W.; Zhang, M.; Zhong, Y.; Weng, Z.; Mi, Y.; Zhou, Y.; Li, M.; Cha, J. J.; Tang, Z.; Jiang, H.; Li, X.; Wang, H., Nano Lett 2017, 17, 2057-2063. 31. Liu, P.; Zhu, J.; Zhang, J.; Xi, P.; Tao, K.; Gao, D.; Xue, D., ACS Energy Letters 2017, 2, 745-752.

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