Modulation of Phosphorene for Optimal Hydrogen Evolution Reaction

Research Article

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 www.acsami.org

Modulation of Phosphorene for Optimal Hydrogen Evolution Reaction † † † † † ‡ † § Jiang Lu, Xue Zhang, Danni Liu, Na Yang, Hao Huang, Shaowei Jin, Jiahong Wang,*, , § † Paul K. Chu, and Xue-Feng Yu † Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, P. R. China ‡ National Supercomputing Center, Shenzhen, Guangdong 518055, P. R. China § Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

*S Supporting Information

ABSTRACT: Economical and highly effective catalysts are crucial to the electrocatalytic hydrogen evolution reaction (HER), and few-layer black phosphorus (phosphorene) is a promising candidate because of the high carrier mobility, large specific surface area, and tunable physicochemical characteristics. However, the HER activity of phosphorene is limited by the weak hydrogen adsorption ability on the basal plane. In this work, optimal active sites are created to modulate the electronic structure of phosphorene to improve the HER activity and the effectiveness is investigated theoretically by density-functional theory calculation and verified experimentally. The edges and defects affect the electronic density of states, and a linear relationship between the HER activity and lowest unoccupied states ε ε ( LUS) is discovered. The medium LUS value corresponds to the suitable hydrogen adsorption strength. Experiments are designed and performed to verify the prediction, and our results show that a smaller phosphorene moiety with more edges and defects exhibits better HER activity and surface doping with metal adatoms improves the catalytic performance. The results suggest that modified phosphorene has large potential in efficient HER and provides a convenient standard to explore ideal electrocatalysts. KEYWORDS: two-dimensional materials, phosphorene, hydrogen evolution reaction, electronic structure, first-principle calculation

1. INTRODUCTION by sonication, ball-milling, or electrochemical intercalation.19,21 Another way is to introduce extrinsic active sites by metal Electrochemical hydrogen evolution reaction (HER) is a 22 7 1 doping, surface functionalization, and heterostructure desirable method to produce hydrogen for renewable energy. 6,17−20 At present, the most common commercial catalyst is platinum construction to enhance electron transfer for better but in spite of its high efficiency, large-scale commercial HER activity. In spite of recent experimental advances, a adoption is hampered by its high cost.2 Two-dimensional (2D) systematic theoretical study on pristine phosphorene has materials have attracted considerable attention as replacement seldom been performed but a better theoretical understanding of platinum because of its large specific surface area, unique is necessary for the rational and efficient design of optimal physicochemical properties, and abundant and tunable active − phosphorene-based catalysts. sites.3 5 Owing to the unique electronic structure, few-layer In the work described in this paper, a series of samples are black phosphorus (BP) sheets (also named phosphorene) are designed and prepared to improve the HER activity of 6,7 promising in electrochemical HER. The BP also has proved phosphorene and to reveal the relationship between the as an efficient electrocatalyst for the oxygen evolution 8,9 structure and activity. As shown in Figure 1, the typical edges reaction. In addition, the high carrier mobility, layer- (edge zigzag (E-Z), edge armchair (E-A)), point defects dependent electronic structure, and lone electron pairs of BP (Stone-Wales (D-SW1, D-SW2), single vacancy (D-SV1, D- are beneficial to its applications in biomedicine, sensors, and − SV2), double vacancy (D-DV1, D-DV2)), metal doping, and catalysis.10 15 Since Pumera revealed the HER behavior of BP under acidic conditions,16 synthesis of nanoscale BP has been strain, which can potentially boost the HER activity, are − fi explored from the standpoint of catalysis.7,17 21 Unfortunately, considered in the rst-principles calculation. It is found that ε the activity of either bulk or nanosized BP is not satisfactory the lowest unoccupied state ( LUS) can be used to design the for practical applications,7 and there has been much effort to optimal phosphorene-based HER catalysts and experiments are optimize hydrogen adsorption/desorption to improve the HER activity. A well-known strategy is to reduce the size of BP and Received: August 1, 2019 produce more surface area and edges by, for example, Accepted: September 25, 2019 exfoliation of bulk BP into smaller and thinner nanosheets Published: September 25, 2019

© 2019 American Chemical Society 37787 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article Δ sites is as large as 1.24 eV, whereas the typical GH* values of the E-A site (0.125 ML) and E-Z site (0.25 ML) are −0.45 and − Δ 0.27 eV, respectively. Although the GH* values of the edge sites of E-A and E-Z are more optimal than that of the basal Δ site, the absolute values of GH* are still much larger than that of Pt. It has been suggested that the hydrogen adsorption energy can be modulated by hydrogen coverage,24,27,28 and therefore, different hydrogen coverages (0.125−1 ML) on the edges are studied. The optimized models with different hydrogen coverages are summarized in Figure S1. Besides reconstruction on the edges, the basic structures of Δ phosphorene are complete and the corresponding GH* values are shown in Figure S2 and Table S1. When the Δ hydrogen coverage is 0.875 ML on the armchair edge, GH* has the smallest absolute value of 0.06 eV. Figure 1. Typical models considered in our theoretical assessment: Defects are regarded as active sites for various HER catalysts, two typical edges, three typical point defects (stone-wales, single and in the case of phosphorene, special localized electron states vacancy, and double vacancy), metal doping, and strain. are generated from the puckered lattice arrangement and − diverse P atom rings.29 31 Six kinds of typical point defects are performed to verify the enhanced HER effect rendered by the studied, including the Stone-Wales (D-SW1, D-SW2), single edges and metal doping. vacancy (D-SV1, D-SV2), and double vacancy (D-DV1, D- DV2),32 and all of the active sites for hydrogen adsorption Δ 2. RESULTS AND DISCUSSION around defects are marked in Figure S3. The EH* values of all In the classical electrocatalytic HER, hydrogen undergoes three of the sites are calculated and shown in Table S1, and only the changes of state in the process including the initial reactant most stable adsorption sites on the defects are discussed below. state (H+), intermediate adsorbed state (H*), and final The hydrogen adsorption ability of most defects is much Δ product state (H ). According to the Sabatier principle, the stronger than that of the basal site. In detail, EH* is 0.59 eV 2 − optimal HER activity results from the optimal adsorption for SW1-h7, 0.42 eV for SW2-h3, 0.10 eV for DV1-h1, 0.28 Δ 23 eV for SV1-h2, −0.31 eV for DV2-h2, and −0.43 eV for SV2- energy of the intermediate adsorbed state ( EH*). The * |Δ | h6. The variable hydrogen adsorption energy suggests that the absolute H adsorption Gibbs free energy ( GH* )is employed as a general parameter to evaluate the HER activity introduction of point defects is an effective way to modulate |Δ | of catalysts, and GH* = 0 usually indicates the ideal HER the hydrogen adsorption activity. As shown in Figure 3b, the activity.24 free energy diagram suggests that SV1-h2 and DV2-h2 show Δ − Before investigating phosphorene structures with different the optimal GH* values of 0.01 and 0.02 eV, respectively. constructed surface sites, the HER activity of intrinsic Figure 3a shows the optimized structures of the phosphorene phosphorene is first studied. As shown in Figure 2a, the defects after hydrogen was adsorbed at the optimal HER sites. hydrogen atom (H atom) prefers to adsorb onto the top site of Additionally, hydrogen-adsorption-induced reconstruction the phosphorus atom (P atom) on the top surface. Since it has plays an important role in HER activity. With regard to D- been suggested that the edge atoms of 2D materials contribute SV1 (Figure S4), some optimal sites are produced after − to better HER activity than the basal sites,25 27 the two typical hydrogen adsorption, for instance, h2, h3, h4, and h6. When edges of zigzag (E-Z) and armchair (E-A) are investigated. In the hydrogen atom adsorbs on h2, the pristine symmetric contrast to the original edge configurations in the upper graphs three-coordinated phosphorus atom turns into an asymmetric in Figure 2b,c, the edge atomic configurations are significantly five-coordinated one. As the hydrogen atom binds to the h3 reconstructed because of the strong bonding between the site, the strong P−H interaction breaks the P−P bond and the hydrogen atoms and edge atoms (lower graphs in Figure 2b,c). pristine ringlike structure transforms into a new (5|9) Δ fi The calculated Gibbs free energy ( GH*) of the basal and edge con guration. The situations of h4 and h6 sites are similar Δ fi sites on phosphorene are shown in Figure 2d. GH* of basal to that of the h3 site. Reconstruction of the adjacent ve-atom

Figure 2. (a) Structure of the basal site of phosphorene; (b) zigzag edge (E-Z); (c) armchair edge (E-A) structures before and after hydrogen adsorption. The green balls on the edges show clear reconstruction compared to the inner light purple P atoms of phosphorene, and the blue balls represent H atoms; (d) free-energy diagram for the HER of the diﬀerent phosphorene models and Pt reference.

37788 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article

Figure 3. (a) Optimized structures of the phosphorene defects (D-SW1, D-SW2, D-SV1, D-SV2, D-DV1, and D-DV2) with the hydrogen being adsorbed at the optimal HER sites; (b) free-energy diagram for the HER showing the optimal active sites in the phosphorene defects as well as Pt reference. Note that the adsorption sites are shown in Figure S3.

Figure 4. (a) DOS of nine typical phosphorene models with the black dotted line indicating the Fermi level; (b) relationship between the lowest ε Δ unoccupied state ( LUS) and hydrogen adsorption energy ( EH*) for the most stable adsorption sites in the phosphorene models. − Δ ε ring and nine-atom ring ocuurs due to the strong P H positive GH*) and a smaller LUS translates into a stronger Δ bonding, and the construction of these sites creates new one (more negative GH*). configurations that favor hydrogen adsorption. The partial charge density of the LUS state is investigated to To investigate the underlying mechanism of the enhanced reveal the physical pictures of active sites in the different HER activity of the phosphorene edges and point defects, the phosphorene models. As shown in Figure 5a, the B model is fi electronic structures of the configurations are determined by determined rst and a small amount of charges are distributed the density of states (DOS).33 It is found that the lowest uniformly. As a result, the basal plane is hard to attract H ε atoms. In contrast, DV2 has a new defect-doping-induced band unoccupied state (LUS) ( LUS) exhibits a high correlation with 34 below the intrinsic CBM (Figure 5b,c) so that DV2 has a lower the HER activity of phosphorene. The LUS corresponds to ε the conduction band minimum (CBM) in typical semi- LUS and more charges accumulate around the defects. Combining the top-view graph and side-view graph, the h2 conductors as well as the Fermi levels of metals and heavily site on the upper surface has the largest and more appropriate metal-doped semiconductors. As shown in Figure 4a, the LUS Δ density for hydrogen adsorption compared to the other sites, of the sites with highly positive GH* (B, D-SW1, D-SW2, D- thus representing the most stable hydrogen adsorption sites. DV1) is far away from the Fermi level and there are no With regard to SV1, LUS is a new band above the Fermi level occupied states at the Fermi level position. In contrast, the and the most stable hydrogen adsorption site of the SV1 model Δ sites with more negative GH* (D-SV1, D-SV2, E-Z) exhibit a is the h1 site with a lot of charge aggregation. Besides, the ε smaller LUS. Accordingly, a linear relationship (Figure 4b) Fermi level of SV2 is across the valence band, meaning that ε Δ fi between LUS and EH* is tted for the most stable adsorption phosphorene with the SV2 defect behaves as a metal with ε sites of edges and defect models. As a result, LUS can be used metallic behavior. The Fermi level is accordingly considered as to describe the activity of hydrogen adsorption. That is, a larger the LUS state in the SV2 model. The LUS partial charge ε − LUS corresponds to weaker hydrogen adsorption (more density calculated among ( 0.5, 0) eV below the Fermi level is

37789 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article

Figure 5. (a−c) Electronic bands of the B, D-DV2, and D-SV1 models, and the corresponding partial charge density of the lowest unoccupied state (LUS) at the lowest point (red circle), respectively. The isosurface value is 0.003 e A−3. The black and red lines in (c) correspond to spin-up and spin-down bands, respectively. (d) Electronic bands of the D-SV2 model and corresponding LUS partial charge density represented by the partial charge density among (−0.5, 0) eV below the Fermi level. The isosurface value is 0.02 e A−3. The blue dotted line represents the Fermi level.

ε ε fi Figure 6. (a) Models showing tensile and compressive strain applied along the zigzag and armchair directions ( A, Z), and samples doped with ve Δ Δ ff kinds of metal atoms (Au, Co, Al, Ni, and Pd); (b) GH* vs %x-strain; (c) GH* of phosphorene with di erent metal dopants (Au, Co, Al, Ni, and Pd); (d) LUS partial charge density at the γ point for the six metal-doped phosphorene samples, strained phosphorene along the armchair direction, and strained phosphorene along the zigzag direction with the isosurface value being 0.003 e A−3. shown in Figure 5d, in which the h3 site is the most stable all of the adatoms favor the hollow site of the hexagon, which is adsorption site. The results demonstrate that there is obvious similar to previous reports.39,40 Thepossiblehydrogen charge accumulation around the defects, thus affecting the adsorption models are shown in Figures S5 and S6.As adsorption ability of the site to capture hydrogen and the LUS shown in Figure 6a,c, the phosphorene samples under tensile of different models is critical to the HER activity as well. and compressive strain exhibit better HER activity than the As the above results demonstrate that the HER performance intrinsic one and the metal-doped ones show more apparent can be improved by modulating the electronic structure, the enhancement in the HER activity. In the metal-doped strain and adatom doping are considered as the other two phosphorene, Co-BP has the optimal HER activity with a ff 35−38 Δ e ective processes. Tensile and compressive strain along GH* of 0.09 eV. To clarify the charge distribution in the ε ε the zigzag and armchair directions ( Z, A) are applied to the strained phosphorene and apparent HER enhancement pristine phosphorene. There are five different kinds of metal observed from the metal-doped phosphorene, the LUS partial atoms (Au, Co, Al, Ni, and Pd) adsorbed on phosphorene, the charge density graphs are shown in Figure 6b−d. In general, models are established on the basal phosphorene surface, and because the strain is applied to the whole structure, the charge

37790 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article

Figure 7. (a−c) SEM images of L-BP and the corresponding EDS map of P. (d−f) SEM images of S-BP and the corresponding EDS map of P. (g− i) SEM image of L-BP(Co) and the corresponding EDS map of Co and P. (j) Polarization curves of L-BP, S-BP, and L-BP(Co) for the HER acquired at a scanning rate of 2 mV s−1. (k) Linear sweep voltammetry (LSV) curves before and after 500 continuous CV cycles. (l) Stability assessment of L-BP(Co). is well distributed without obvious localization in the plane. In different sizes are prepared to have different ratios of exposed ε − the armchair compressive model ( A ( 15%)), the distance edges. In general, the smaller ones have more edges and between adjacent P atoms in the upper layer is reduced along defects. To investigate the effects of metal doping, Co-doped the armchair direction and therefore the repulsive interaction phosphorene (L-BP(Co)) is synthesized similar to the large- between two adjacent P atoms is enhanced, causing out-of- phosphorene sheets. These sheets have similar crystal plane P−P bonding to have stronger charge accumulation on structures (Figure S7a), and their morphology and elemental the external surface. With regard to the armchair tensile model composition are determined by scanning electron microscopy ε ε − ( A (15%)) and zigzag tensile model ( Z ( 15%)), the partial (SEM) and energy-dispersive X-ray spectroscopy (EDS) charge density tends to increase around the plane and (Figure 7a−i). The lateral lengths of L-BP and S-BP are 10 interlayer, respectively, and consequently, the hydrogen μm and 1 μm, respectively. The elemental ratio of Co:P in ε − adsorption ability is weaker than that of the A ( 15%) EDS is about 1:25, wherein the small content of Co results in model. There are more localized charge density distributions the fact that there is no signal that belongs to the cobalt for the metal-doped phosphorene, especially around the metal compound (Figure S7b). X-ray photoelectron spectroscopy atoms which carry more electrons than P atoms. For instance, (XPS) reveals L-BP(Co) peaks at 779.0 and 793.9 eV Au-doped phosphorene has the most uneven LUS partial corresponding to cationic Co (Figure S8). The Raman peaks charge density and the strongest hydrogen adsorption. On the of L-BP and S-BP are located at the same positions, while the other hand, Pd-doped phosphorene shows the smallest Raman peaks of L-BP(Co) red-shift slightly (Figure S9). The ff distribution di erence and a small improvement in the HER HER polarization curves are collected in 0.5 M H2SO4, and as Δ slightly. More details of GH* are presented in Tables S1 and shown in Figure 7j, L-BP shows poor HER activity with an S3. onset potential of 350 mV and a potential of 615 mV to drive A series of experiments are designed and performed to verify 20 mA cm−2. S-BP exhibits better HER activity with onset the theoretical prediction. Large-phosphorene (L-BP) and potentials of 319 and 511 mV to attain 20 mA cm−2, and L- small-phosphorene (S-BP) sheets exfoliated from bulk BP are BP(Co) shows the best results with an onset potential of 211 prepared, and phosphorene sheets with the same weight but mV and the same current densities at a potential of 389 mV.

37791 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article

The current densities are normalized to the corresponding doping, and strain can be employed to modulate the electronic electrochemically active surface area (EASA) (Figure S10), and structure and construct more active sites. the EASAs of L-BP, S-BP, and L-BP(Co) are calculated to be 0.071, 0.10, and 0.36 cm2, respectively. When comparing with 3. CONCLUSIONS other previously reported BP-based electrocatalysts, the L- −1 Modulation of the electronic structure of phosphorene is BP(Co) has the smallest Tafel slope (47 mV dec ) and a demonstrated to improve HER activity based on the density- decent onset overpotential (Table S4). These results suggest functional theory (DFT) calculation and experiments. The that a smaller phosphorene moiety with more exposed edges edges, defects, strain, and metal doping are investigated, and a and defects delivers better HER performance than larger linear relationship is observed between the lowest occupied phosphorene. The metal doping is an effective means to ε state and hydrogen adsorption energy. A small LUS leads to improve the hydrogen evolution activity. weak H atom adsorption and hinders H activation (B, SW1, Two important issues of phosphorene are long-term stability SW2, and DV1), and strong H adsorption caused by a large and ease of oxidation. In previous studies, metal-ion ε LUS inhibits desorption of H2 (E-A, E-Z, and SV2). A medium modification and graphene oxide aerogels were employed to ε − LUS observed from the DV2 defect model has a suitable P H enhance the stability and physicochemical performance of 41,42 bonding strength because the sites in the defect with proper BP. In this work, two long-term stability tests were taken negative charges produce favorable HER effects. Furthermore, fi to con rm the good stability of the optimal catalyst of L- both straining and metal doping can improve the activity of BP(Co). First, the LSV curves recorded before and after 500 HER of phosphorene by modulating the charge distribution on cycles of cyclic voltammetry exhibit a little decrease of the the surface. The different HER activities of phosphorene sheets HER activity, demonstrating the good durability of L-BP(Co) with different sizes confirm that the exposed edges introduce − (Figure 7k). Second, the i t curve of L-BP(Co) demonstrates more active sites and Co doping corroborates the remarkable that the catalytic current density remains at around 10 mA improvement in the HER performance. This study investigates −2 cm after a 10 h chronoamperometry test (Figure 7l), which different phosphorene structures and mechanisms to enhance also suggests the good stability of the L-BP(Co) electrode for HER activity. All in all, edges, defects, and doping improve the HER. Although the BP nanosheets may decompose in an their performance, and our results provide insights into how to oxygen- and water-rich environment, the reducing operating design and optimize the structure of 2D materials to improve condition of the negative electric potential on the HER the catalytic activity. In particular, the results reveal more working electrode can suppress the degradation process. opportunities for defect-induced active sites on 2D materials. We have shown that modulation of the electronic structure Meanwhile, the controllable construction of specific defects in of phosphorene is a good way to enhance its HER 2D materials would have some applications on the detection ε performance, and LUS and the corresponding partial charge and separation of small molecules or ions. The modulation of density distribution are key parameters. Although we have the phosphorene electronic structure will promote the studied the majority of modulating methods, including basal, development of phosphorene-based nanodevices and the typical edges, defects, strain, and metal doping, there are still discovery of new photochemical reactions. other ways to change the electronic structure of phosphorene. 43,44 For instance, there are still other kinds of edges and 4. METHODS 32,45 defects that may have optimal HER activity. The model 4.1. First-Principles Calculation. First-principles calculation was can also be enlarged; when more atoms are used to construct performed using density-functional theory (DFT) implemented in the the strain model, ripple deformation in phosphorene should be Vienna Ab Initio Simulation Package code.49 The projector 46 considered according to a previous report, which may lead to augmented wave potentials and generalized gradient approximation new active sites. The doped atoms can also be extended to of the Perdew−Burke−Ernzerhof form were used for exchange other transition metal, noble metal, or nonmetal elements. The correlation.50 van der Waals (vdW) interactions were not considered here, but it has been tested, showing little discrepancy when the heteroatoms can change the surface molecular absorption 51 states and introduce new energy levels in the electronic optB88 exchange functional (optB88-vdW) was considered. structure. Both of them can bring positive influences on the The calculated lattice parameters of the BP unit cell were a = 4.62 Å and b = 3.30 Å in good agreement with the reported values.12,32 HER activity of doped phosphorene. What is more, to create × ff Supercells containing 4 5 primitive cells were used for the basal, more active sites combining the di erent optimal models point defect, strained, and metal-doped BP and 4 × 4 supercells for together may be a good choice, such as doping metal adatoms edge BP. A vacuum space of 15 Å was added to avoid interactions on the phosphorene edges or defects. Such a combination between adjacent layers and edges, and the energy cutoff for the would induce new metal-phosphorus bonds and lead to better plane-wave basis set was set to 500 eV in the calculation. k-point performance. sampling was implemented by the Monkhorst−Pack scheme with the Additionally, although we have used monolayer phosphor- grids of 3 × 3 × 1 and 3 × 1 × 1 for 4 × 5 (basal, defect, dope, strain) and 4 × 4 (edge) supercells in the structure optimization, respectively. ene as a simulation model to present the most distinct atomic × × × × structure and the most adjustable electronic structure in the In the DOS calculation, 6 6 1 and 6 1 1 k-grids were used for the 4 × 5 and 4 × 4 supercells and the convergence criteria for energy present work, we can make a prejudgment on the HER activity −5 −1 ff and force are 10 and 0.02 eV Å , respectively. of di erent layers of phosphorene. The band gap of To evaluate the electrocatalytic HER activity, the free energies fi Δ phosphorene is signi cantly dependent on its layer number ( GH*) of hydrogen adsorption were calculated as follows: such that the band gap of monolayer phosphorene is 1.51 eV 47,48 Δ= +Δ−Δ and that of three-layered phosphorene is 0.80 eV. GEHH** E ZPE TS However, the changes mainly result from the apparent increase Δ Δ Δ where EH*, EZPE, and S are the hydrogen adsorption energy, of valence band minimum and the slight decrease of LUS zero-point energy (ZPE), and entropy differences between the ff Δ (CBM), which suggests the HER activity of di erent-layered adsorbed state and gas phase, respectively. Here, T = 300 K, EZPE ‑ − ‑ − Δ * − phosphorene will be nearly unchanged. Accordingly, defects, = EZPE H 1/2 EZPE H2, and EZPE H2 = 0.27 eV; T S= T (SH 1/2

37792 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 ACS Applied Materials & Interfaces Research Article

≈− − Author Contributions SH2) 1/2 TSH2 = 0.205 eV, considering the small entropy changes in solid compared to gas.24 The typical adsorption sites of the J.L., X.Z., and D.L. contributed equally. The manuscript was Δ Δ nine models were calculated, and GH* = EH* + 0.29 eV for BP. written through contributions of all authors. All authors have The detailed calculation results are listed in Table S2. For the metal- given approval to the final version of the manuscript. doped BP, the former correction is not suitable and the individual Notes correction for each site is listed in Table S3. fi Hydrogen coverage on the BP edge is defined as the fraction of H The authors declare no competing nancial interest. atoms to P atoms at the edge.27 In addition, the differential hydrogen adsorption energy is calculated as follows: ■ ACKNOWLEDGMENTS This work was jointly supported by the National Natural Δ=EEHBPHBP(2)HH*+()nn − E +−/2 −1/2E2 Science Foundation of China (51702352), Key Research where EBP+nH and EBP+(n−2)H are the total energy for BP with n and (n Program of Frontier Sciences, CAS, (QYZDB-SSW-SLH034), − 2) hydrogen atoms adsorbed onto the two edges, respectively, and Shenzhen Science and Technology Research Funding E is the energy of the hydrogen molecule in the gas phase. H2 (JCYJ20170413165807008 and JCYJ20170818162909200), 4.2. Experimental Section. Large-phosphorene (L-BP), small- China Postdoctoral Science Foundation (2018T110897), and phosphorene (S-BP), and Co-doped phosphorene (L-BP(Co)) were City University of Hong Kong Strategic Research Grant (SRG) synthesized from bulk BP via an electrochemical system, characterized ff no. 7005105. The calculation was carried out at the National via X-ray di raction (XRD), SEM, XPS, Raman, and measured Supercomputing Center in Shenzhen and the National electrochemical activity. More details are in the Supporting Information. Supercomputer Center in Guangzhou, China. ■ ASSOCIATED CONTENT ■ ABBREVIATIONS *S Supporting Information HER, hydrogen evolution reaction The Supporting Information is available free of charge on the 2D, two-dimensional ACS Publications website at DOI: 10.1021/acsami.9b13666. BP, black phosphorus DOS, density of states ff Reconstruction of BP edges (E-Z, E-A) with di erent LUS, lowest unoccupied state Δ hydrogen coverages; Gibbs free energies ( GH*) for E-Z CBM, conduction band minimum ff and E-A edges at di erent hydrogen coverages; atomic DFT, density-functional theory. structures of the phosphorene defects (D-SW1, D-SW2, D-SV1, D-SV2, D-DV1, and D-DV2) with the ■ REFERENCES adsorption sites labeled in the front side (upper) and back side (lower); four optimized structures of D-SV1 (1) Shih, C. F.; Zhang, T.; Li, J.; Bai, C. Powering the Future with Liquid Sunshine. Joule 2018, 2, 1925−1949. models after H adsorption; optimized structures of (2) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; strained BP after H adsorption; typical adsorption sites Δ * Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. 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37795 DOI: 10.1021/acsami.9b13666 ACS Appl. Mater. Interfaces 2019, 11, 37787−37795 Supporting Information

Modulation of Phosphorene for Optimal

Hydrogen Evolution Reaction

Jiang Lu‡1, Xue Zhang‡1, Danni Liu‡1, Na Yang1, Hao Huang1, Shaowei Jin2, Jiahong

Wang*1,3, Paul K. Chu3, Xue-Feng Yu1

1Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, P. R. China

2National Supercomputing Center, Shenzhen, Guangdong 518055, P. R. China

3Department of Physics, Department of Materials Science and Engineering, and

Department of Biomedical Engineering, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong, China

S-1 * Corresponding Author: [email protected]

‡ These authors contributed equally to this work

S-2 Figure S1. Reconstruction of BP edges (E-Z, E-A) with different hydrogen coverages.

S-3 Figure S2. The Gibbs free energies (ΔGH*) for E-Z and E-A edges at different hydrogen coverage.

S-4 Figure S3. Atomic structures of the phosphorene defects (D-SW1, D-SW2, D-SV1, D-SV2, D-

DV1, D-DV2) with the adsorption sites labeled in the front side (upper) and back side (lower).

Note that the adsorption sites of front and back side is equal for two defects (D-SW1 and

D-DV2).

S-5 Figure S4. Four optimized structures of D-SV1 models after H adsorption.

S-6 Figure S5. Optimized structures of strained BP after H adsorption. (a) Straining along

Armchair (E-A) direction; (b) Straining along Zigzag (E-Z) direction;

Note: E-Z (-15%) model is unstable when H adsorbed and this result is ignored in this work.

S-7 Figure S6. Typical adsorption sites of H atom at Metal-doped BP.

S-8 Table S1. The ΔGH* values of the H* adsorbed on all the possible sites of different models.

Models Site ΔEH*/eV ΔGH*/eV Models Site ΔEH*/eV ΔGH*/eV

(Coverage) (Coverage)

B 1 0.95 1.24 14 -0.71 -0.42

E-A 0.125 ML -0.74 -0.45 15 -0.13 0.16

0.25 ML -0.49 -0.20 16 0.34 0.63

0.375 ML -0.59 -0.30 17 0.24 0.53

0.5 ML -0.64 -0.35 D-SV2 1 -0.64 -0.35

0.625 ML -0.64 -0.35 2 0.07 0.36

0.75 ML -0.52 -0.23 3 -1.02 -0.73

0.875 ML -0.23 0.06 4 0.02 0.31

1 ML -0.99 -0.70 5 -0.11 0.18

S-9 E-Z 0.25 ML -0.56 -0.27 6 -0.43 -0.14

0.5 ML -0.77 -0.48 7 -0.12 0.17

0.75 ML -0.70 -0.41 8 -0.58 -0.29

1 ML -0.84 -0.55 9 0.10 0.39

D-SW1 1 1.15 1.44 10 -0.63 -0.34

2 0.92 1.21 11 -0.03 0.26

3 1.00 1.29 12 -0.52 -0.23

4 1.01 1.30 D-DV1 1 0.10 0.39

5 1.00 1.29 2 0.27 0.56

6 0.64 0.93 3 0.86 1.15

7 0.59 0.88 4 0.77 1.06

D-SW2 1 0.53 0.82 5 1.68 1.97

2 0.72 1.01 6 1.02 1.31

S-10 3 0.42 0.71 7 0.90 1.19

4 1.02 1.31 8 0.70 0.99

5 0.66 0.95 9 0.24 0.53

6 0.81 1.10 10 0.65 0.94

7 0.82 1.11 11 0.56 0.85

8 0.63 0.92 D-DV2 1 0.26 0.55

9 0.60 0.89 2 -0.31 -0.02

10 0.77 1.06 3 0.21 0.50

11 0.79 1.08 4 0.47 0.76

D-SV1 1 -0.73 -0.44 5 0.84 1.13

2 -0.28 0.01 S-A -15% 0.31 0.60

3 -0.19 0.10 -10% 0.70 0.99

4 -0.12 0.17 -5% 0.90 1.19

S-11 5 0.11 0.40 5% 0.97 1.26

6 -0.12 0.17 10% 0.80 1.09

7 0.18 0.47 15% 0.62 0.91

8 0.20 0.49 S-Z -10% 0.72 1.01

9 0.12 0.41 -5% 0.94 1.23

10 0.11 0.40 5% 0.96 1.25

11 -0.71 -0.42 10% 0.86 1.15

12 0.14 0.43 15% 0.78 1.07

13 0.24 0.53

Note: S-Z and S-A respect strain in armchair and zigzag direction, respectively.

S-12 Table S2. The ΔEH*, EZPE-H, ΔEZPE, ΔEZPE – T ΔS and ΔGH* values of the H* adsorbed on the typical P site of different models.

Models Site ΔEH*/eV EZPE-H/eV ΔEZPE/eV (ΔEZPE – T ΔGH*/eV

ΔS)/eV

B 1 0.95 0.21 0.08 0.28 1.24

E-A 0.125 ML -0.74 0.23 0.09 0.29 -0.44

E-Z 0.25 ML -0.56 0.22 0.09 0.29 -0.27

D-1 2 0.92 0.21 0.08 0.28 1.20

D-2 3 0.42 0.22 0.08 0.28 0.70

D-3 2 -0.28 0.23 0.09 0.30 0.02

D-4 5 -0.11 0.23 0.09 0.30 0.18

D-5 1 0.10 0.22 0.08 0.29 0.39

D-6 2 -0.31 0.22 0.09 0.29 -0.02

S-13 Note: (ΔEZPE – T ΔS) is calculated 0.29 ( ± 0.01) eV, that is ΔGH* = ΔEH* + 0.29 eV, which was then used in all the conversion relation of different models.

S-14 Table S3. The ΔEH*, EZPE-H, ΔEZPE, ΔEZPE – T ΔS and ΔGH* values of the H* adsorbed on the typical P site of doped-BP models.

Models Site ΔEH*/eV EZPE-H/eV ΔEZPE/eV (ΔEZPE – ΔGH*/eV

TΔS)/eV

Au-BP 1 -0.80 0.18 0.05 0.25 -0.54

2 0.80 0.18 0.05 0.25 1.06

Co-BP 1 -0.16 0.18 0.05 0.25 0.09

2 0.20 0.15 0.01 0.22 0.41

3 0.43 0.22 0.09 0.29 0.72

Al-BP 1 -0.04 0.17 0.03 0.24 0.20

2 0.43 0.23 0.09 0.30 0.73

3 0.65 0.15 0.02 0.22 0.87

Ni-BP 1 0.40 0.16 0.03 0.23 0.63

S-15 2 0.63 0.15 0.01 0.22 0.85

3 1.15 0.19 0.06 0.26 1.41

Pd-BP 1 0.65 0.17 0.03 0.24 0.88

2 1.04 0.14 0.01 0.21 1.25

3 1.14 0.20 0.06 0.27 1.40

Note: We’ve tested all the typical sites around the doped atom on the BP surface, and selected the above typical sites with some replicate results removed.

Experimental Section

1. Preparation of large black phosphorene (L-BP) sheets and small black phosphorene

(S-BP) sheets

Typically, large black phosphorene (L-BP) sheets with 10 μm lateral dimension were obtained by a simple electrochemical prepared process in a two-electrode setup by direct-current power supply, where Pt foil (1 cm × 1 cm × 0.1 cm) was chosen as

S-16 positive electrode, black phosphorus (BP) crystal (0.5 cm × 0.5 cm × 0.1 cm) was used as negative electrode and N,N-dimethylformamide (DMF) contained 5 mM Tetra-n- octylammonium bromidectrode was used as electrolyte. BP crystal was expanded by applying voltage at 10 V for 20 min and then dispersed in N,N-Dimethylformamide

(DMF) by sonication for 20 min. The result L-BP sheets dispersion was let to stand 30 min to remove thick-layered BP, and then washed by ethanol at least three times used centrifuged at 9000 rpm for 10 min to get thin layered BP sheets ethanol dispersion.

Small black phosphorene (S-BP) sheets with 1 μm lateral dimension were prepared by similar method only changed applying voltage as 20 V.

2. Preparation of Co doped black phosphorene (L-BP(Co)) sheets

Co doped phosphorene (L-BP(Co)) sheets were synthesized via a facile hydrothermal

reaction, CoCl2 (1 mmol) and urea (3 mmol) were dissolved in 40 mL BP sheets DMF dispersion under vigorous stirring for 30 min. Then the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, sealed and maintained at 180 °C for 1 h in an electric oven. After the autoclave cooled

S-17 down at room temperature, the solution was collected and washed by ethanol several

times centrifuged at 9000 rpm for 10 min to obtain L-BP(Co) sheets ethanol dispersion.

3. Characterizations

Scanning electron microscopy (SEM) measurements were performed on Zeiss Supra

55 scanning electron microscope at an accelerating voltage of 2 kV. X-ray diffraction

(XRD) data were acquired on a Bruker D2 X-ray diffractometer with Cu Kɑ radiation (k =

1.5418 Å). The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher ESCALAB 250Xi XPS. Raman scattering was performed on a

Horiba Jobin-Yvon Lab Ram HR VIS high-resolution confocal Raman microscope equipped with a 532 nm laser as the excitation source at room temperature.

4. Electrode Preparation

Prior to modification, the glassy carbon electrodes (GCE, 3 mm in diameter) were respectively polished with alumina slurry, and cleaned by ultrapure water, then dried by

nitrogen. The concentration of the ethanol dispersions of L-BP, S-BP, L-BP(Co), and

Pt/C was tuned as 100 μg mL-1, respectively. 990 μL dispersion with 10 µL Nafion

S-18 solution (0.05 wt %) were sonicated for 3 min to form an ink. Afterwards, 10 µL of the

ink were loaded onto the cleared GCE and dried at room temperature.

5. Electrochemical measurements

All Electrochemical results were obtained in a standard three-electrode setup by CHI

760E electrochemical analyzer (CH Instruments, Inc., Shanghai) at ambient

temperature using GCE as working electrode, a graphite rod as counter electrode and a

saturated calomele electrode (SCE) as the reference electrode. For all measurements,

the obtained potential was calibrated with respect to reversible hydrogen electrode

(RHE) as formula, E(RHE) = E(SCE) + (0.059 pH + 0.242) V. Cyclic voltammogram

curves were obtained by cyclic voltammetry from -0.043 to 0.087 V vs RHE at scan rate

of 5, 50, 100, 150, 200 mV s-1 where there was no Faradic current. The double-layer

capacitance can be calculated as formula, Cdl = j/r, where j was the current density and r was the scan rate. Electrochemically active surface area (EASA) can be calculated

as: EASA = Cdl/Cs, where Cs was the specific capacitance value for a flat standard with

1 cm2 of real surface area. The general

S-19 -2 -2 -2 value of Cs was between 20 μF cm and 60 μF cm . Here, 60 μF cm was used as the average value. Linear sweep voltammetry was performed to get polarization curves

-1 from 0 to -0.8 V with a scan rate of 2 mV s in 0.5 M H2SO4. Before evaluate the HER activity of all result product, Pt/C (20%) with same loading mass was tested firstly to detect the measurement system. IR compensation was applied for all polarization curves by impedance measurements.

S-20 Figure S7. (a) XRD patterns of L-BP, S-BP and L-BP(Co). (b) The magnified XRD patterns of L-BP(Co).

As shown in Figure S7, L-BP and S-BP show diffraction peaks characteristic of BP

(JCPDS No.73-1358). After Co doping, L-BP(Co) shows typical diffraction peaks of BP without signals belong to cobalt or cobalt compound.

S-21 Figure S8. (a) P 2p XPS spectrum of L-BP. (b) P 2p XPS spectrum of S-BP. (c, d) Co

2p and P 2p XPS spectra of L-BP(Co), respectively.

Figure S8 display the X-ray photoelectron spectroscopy (XPS) spectra of L-BP, S-BP

and L-BP(Co). The binding energies (BEs) of P 2p1/2 and P 2p3/2 of L-BP, S-BP and L-

BP(Co) appear at 129.1 and 130.2 eV, respectively, consistent with the characteristic of

S-22 BP. After doped Co, the L-BP(Co) shows two peaks at 779.0 and 793.9 eV

corresponding to Co 2p3/2 and Co 2p1/2, respectively.

Figure S9. Raman scattering spectra of the L-BP, S-BP and L-BP(Co).

Figure S9 shows the Raman spectra of L-BP, S-BP and L-BP(Co). The Raman peaks of

L-BP and S-BP locate at same position and the Raman peaks of L-BP(Co) red-shift

slightly.

S-23 Figure S10 (a-c) CV conducted at potential from -0.043 to 0.087 V vs RHE at scan rates of 5, 50, 100, 150 and 200 mV s-1. (d) The capacitive currents at 0.022 V as a function

of scan rate for L-BP, S-BP and L-BP(Co) [∆ j0 = 1/2 (ja - jc)].

To assess the EASA of L-BP, S-BP and L-BP(Co), the CV cycles was measured at different scan rates during the potential from -0.043 to 0.087 V vs RHE, where there is

S-24 no Faradic current. The EASA of L-BP, S-BP and L-BP(Co) are 0.071, 0.10 and 0.36 cm2, respectively.

S-25 Table S4. Comparison of the HER performance of this work and previous jobs

Tafel Onset Slope Ref Catalysts Electrolyte Overpotential Overpotential (mV (mV) dec-1)

-2 S1 NH2-BP 1.0 M KOH 290 mV@10 mA cm N/A 63 -2 Ni2P@BP 0.5 M H2SO4 107 mV@10 mA cm N/A 38.6 S2 -2 BP 0.5 M H2SO4 600 mV@10 mA cm N/A N/A -2 MoS2−BP nanosheet 0.5 M H2SO4 85 mV@10 mA cm 85 68 S3 BP 0.5 M H2SO4 N/A N/A 161 -2 BP/Co2P 0.5 M H2SO4 340 mV@100 mA cm 105 62 -2 S4 BP 0.5 M H2SO4 600 [email protected] mA cm 389 125 -2 BP/Co2P 1.0 M KOH 336 mV@10 mA cm 173 72 EBP@1:4 1.0 M KOH 191 mV@100 mAcm-2 N/A 76 S5 EBP@1:8 1.0 M KOH 210 mV@100 mA cm-2 N/A N/A EBP 1.0 M KOH 370 mV@10 mA cm-2 N/A 135

-2 S6 BP(Co) 0.5 M H2SO4 294 mV@10 mA cm N/A 107 S7 BPQDs/Mxene N/A N/A 190 83

-2 L-BP(Co) 0.5 M H2SO4 389 mV@20 mA cm 194 47 This -2 L-BP 0.5 M H2SO4 615 mV@20 mA cm 355 91 work -2 S-BP 0.5 M H2SO4 511 mV@20 mA cm 299 85

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