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.