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Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom
Accepted Manuscript Research paper Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom W.X. Zhang, J.W. Zhao, W.H. He, L.J. Luan, C. He PII: DOI: Reference:
S0009-2614(17)30203-8 http://dx.doi.org/10.1016/j.cplett.2017.02.078 CPLETT 34591
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
21 December 2016 24 February 2017 26 February 2017
Please cite this article as: W.X. Zhang, J.W. Zhao, W.H. He, L.J. Luan, C. He, Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/ j.cplett.2017.02.078
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Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom W.X. Zhang 1∗, J.W. Zhao 1, W.H. He 1, L.J. Luan 1, C. He2* 1
School of Materials Science and Engineering, Chang’an University, Xi’an 710064,
China; 2
State Key Laboratory for Mechanical Behavior of Materials, School of Materials
Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
Abstract In this paper, the electronic, magnetic properties and the effect of Si-doped blue phosphorene on the adsorption of a H2O molecule have been investigated using density functional theory with van der Waals corrections. The results indicate that doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O molecules. The dissociative energy barrier is reduced from 2.45 eV on pristine blue phosphorene to 0.19 eV on Si-doped blue phosphorene, which indicates a smooth dissociative adsorption. In addition, the dissociative adsorption of H2O molecules can convert the Si-doped blue phosphorene from hydrophobic to hydrophilic with a semiconductor-to-metal transition.
Keywords: Blue phosphorene; Electronic properties; First principles calculations; Adsorption
*
Corresponding Authors: W. X. Zhang ([email protected]) or C. He ([email protected])
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1. Introduction Stimulated by the isolation of graphene from graphite in 2004, there has been tremendous research interest in atomic two-dimensional (2D) crystal due to its extraordinary structural and electronic properties as well as promising applications in nanoelectronics, [1-4] supercapacitors, [5,6] and biomaterial supports. [7,8] The impressive progress in graphene research has motivated scientists to explore other 2D atomic based materials. [9] Among them, black phosphorene-the few-layer black phosphorus, has been successfully isolated by mechanical exfoliation from black phosphorus. [10-13] Unlike graphene, black phosphorene is direct band semiconductor and exhibits high carrier mobility, [12-14] which would facilitate the application in optoelectronics. [15] Interestingly, many different layered phosphorene allotropes have been predicted by ab initio density functional calculations. [16-18] As a typical allotrope of black phosphorus, blue phosphorene has been theoretically predicted to share layered structure and high stability with the black phosphorene. [19-21] In addition, blue phosphorus should be exfoliated easily to form quasi-2D structures with a band gap of ~ 2eV, [16,17] which is also a promising material for applications in nanoelectronics and spintronics. Nevertheless, some important issues remain largely unexplored, for example, how to effectively tailor the electronics, magnetic properties, and the surface wettability of blue phosphorene to make it suitable for electrode material and biomaterial supports. It is desirable that blue phosphorene is hydrophilic and conductive because the hydrophilicity improves the wetting between blue phosphorene and polar electrolytes or biological molecules, while the conductivity enhances the transport of free carriers. The surface
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wettability is influenced by both the morphology [22] and chemical composition [23] of solid surfaces. For graphene-like materials, the wettability is mainly determined by the surface’s chemical composition, in particular, by its hydrophobic/hydrophilic character. [24] Generally, hydrophilic behavior depends on the interaction between the material and water molecules. Previous studies have shown that the dissociative adsorption of H2O molecules can produce hydrophilic OH groups and convert materials to hydrophilic. [23,25,26] Jiang et al. predicted that doping Al into graphene could facilitate the dissociation of H2O molecules to produce hydrophilic OH groups, while keeping graphene conductive. [25] Recently, the fascinating electronic and magnetic properties of the substitutional doping in some 2D materials, such as graphene, BN sheets and MoS2 monolayer, maybe experimentally achieved by filling the vacancies created by the electron beam with substitutional atoms. [27-29] Sun et al. have reported that the substitution of nonmetallic dopant F, Cl, B and N blue phosphorene are nonmagnetic due to the saturation or pairing of valence electron of dopant and its neighboring P atoms, while the substitutional doping of C, Si and O in blue phosphorene can induce the magnetism. [21,30] Therefore, the substitutional doping of nonmetal atoms has been considered as a promising approach to tune electronic structures and the magnetic states of 2D materials. [31-35] Because the interaction of a H2O molecule with blue phosphorene is of great technological interest, and the effect of doping Si on the wettability of blue phosphorene is an interesting question. Therefore, in this paper, the dissociative adsorption of a H2O molecule on Si-doped blue phosphorene is extensive carried out to understand the effect of the Si dopant on the wettability and conductivity of blue phosphorene based on
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first-principles calculations within density functional theory (DFT). These studies provide us a deep understanding of the novel properties of monolayer blue phosphorene, which is essential to enrich them for future in supercapacitors and biomaterial supports. 2. Computational methods The simulations were based on density functional theory (DFT), which was provided by DMOL3. [36-38] The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof scheme (PBE) [39] was adopted for the exchange-correlation potential to optimize geometrical structures and calculate properties for both spin-polarized and spin-unpolarized cases. [40] In order to take into account the contributions of the van der Waals (vdW) interactions, the DFT-D (D stands for dispersion) approach within the Grimme scheme was adopted for the vdW correction. [41] Double numerical atomic orbital plus polarization (DNP) is chosen as the basis set, with the global orbital cutoff 4.4 Å. Similar functional have been successfully used to study the structural and electronic properties of GNRs, Si and Cu nanowires. [40,42,43] The vacuum space between blue phosphorene was greater than 20 Å to ensure no interactions between different layers. Periodic boundary conditions was adopted for all utilized models in this work. The Brillouin zone was sampled by 6 × 6 × 1 (10 × 10 × 1) k-points for all structures in the geometry optimization (electronic) calculations, [44] which brings out the convergence tolerance of energy of 1.0 × 10-5 Ha (1 Ha = 27.2114 eV), maximum force of 0.002 Ha/Å, and maximum displacement of 0.005 Å.
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To examine the stability of doped blue phosphorene, ab initio molecular dynamics methods (ab initio-MD) simulation at 300 K, where the canonical ensemble was used and the time step was taken to be 1 fs, is also performed with the Castep code. [45] Ab initio-MD simulations are performed in the NPT statistical ensemble with the norm-conserving pseudopotentials and generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof scheme (PBE) exchange-correlation functional. Pressure P and temperature T are constants and N is the atom number. T is imposed by the Nose algorithm. [46]. To investigate the minimum energy pathway for H2O dissociative adsorption on blue phosphorene, linear synchronous transit/quadratic synchronous transit (LST/QST) [46] and nudged elastic band (NEB) [47] tools of the Dmol3 module was used, which have been well validated to determine the structure of the transition state and the minimum energy pathway. For a H2O molecule adsorbed on blue phosphorene for both physisorption and dissociative adsorption, the adsorption energy Ead is defined as: Ead = EH2O/blue phosphorene (Eblue phosphorene + EH2O), where EH2O/blue phosphorene, Eblue phosphorene and EH2O are total energies of the H2O/blue phosphorene system, the isolate blue phosphorene and a H2O molecule in the supercell with the same lattice of blue phosphorene, respectively. 3. Results and discussion The accuracy of our calculation procedure is tested by monolayer blue phosphorene. The full relaxed lattice constant of blue phosphorene is 3.30 Å, which agrees well with 5
other theoretical results (3.28-3.32 Å). [19-21,30,48-50] The P-P bond length is 2.28 Å, which is slightly larger than the P-P covalent bond length (~2.2 Å) of black phosphorus. [50] The two inequivalent P atoms are distributed in two planes with the buckling height of 1.25 Å (see the side view of left panel of Fig. 1(a)). All the above parameters agree well with the recent theoretical results. [19-21,30, 48-50] A 4 × 4 × 1 supercell of blue phosphorene containing 32 atoms is constructed (Fig. 1a). According to the band structure as shown in Fig. 1(b), the blue phosphorene is a semiconductor with an indirect band gap (Eg) of 1.97 eV, where the highest occupied states appear at the middle region along the M-F line, and the lowest unoccupied states are located between the G and M points. The calculated result is also in good agreement with previous theoretical results ~ 2.0 eV. [20,21,30] Similar to black phosphorene, each P atom in blue phosphorene has three equivalent covalent bonds with its neighbors, which means that three electrons of P atom are saturated, while the other two electrons are a single lone pair. As a result, the five valence electrons of each P atom are all paired, and thus the pristine blue phosphorene is intrinsically nonmagnetic. [20] As previously theoretical results mentioned, doping blue phosphorene can tune its chemical and physical properties. [20,49] Next we investigate the Si-doped blue phosphorene system P31Si, in which a substitutional Si atom occupies P site in the supercell, as shown in Fig. 1(c). For Si-doped blue phosphorene, the dopant atom moves a little inside of the plane, which is maintain the 3-fold symmetry. The distance between the Si atom and blue phosphorene is lP-Si = 2.27 Å, which is consistent with the reported 6
result of lP-Si = 2.26 Å. [21] Fig. 1 also shows the atomic charge near the dopant atom obtained by Mulliken analysis. The Mulliken population of the Si atom is 0.281 e, and the Si atom forms an electron-deficient position. Therefore, the electron distribution in blue phosphorene is changed by the dopant, and thus the interaction reactivity between small molecules and blue phosphorene would be changed. In order to investigate stability of electronic and magnetic state of Si-doped blue phosphorene, we have calculated the energy difference between the spin-polarized and spin-unpolarized states for P31Si system. The relative energy difference ΔESpin between Esp and Eusp is 217 meV. Therefore, noticeably, the calculated spin-polarized state of P31Si system is energetically lower than that of spin-unpolarized state, indicating that the ground state of P31Si system is magnetic. As shown in Fig. 1(c), in P31Si system, dopant Si is covalently bonded with its three nearest P atoms with an unpaired valence electron, thus inducing a net magnetic moment of approximately 0.78 µB. The magnetic moment at Si atom is about 0.366 μB and the rest magnetic moment mainly comes from the spin-polarized P atoms. The spin polarization is also evident in the band structure (Fig. 1d). The P31Si system behaves as an indirect semiconductor and the values of Eg of 1.34 and 0.53 eV in the spin-up and spin-down states, respectively. Consequently, Si-substituted system is a diluted magnetic semiconductor and may have potential application in spintronics. [52] To further study the electronic and magnetic structure properties of Si-doped blue phosphorene systems, we have performed the density of states (DOS) and the partials 7
density of states (PDOS) calculations in Fig. 2 and found that the majority and minority density of states are asymmetrical for Si-doped system, indicating a magnetic character. The PDOS of P31Si show that the 3s, 3p states of dopant Si, the 3p states of the nearest P atom around dopant overlap near the Fermi level, which indicate that the unpaired electron induced by dopant Si not only occupy the s, p orbitals, but also partially occupy the 3p orbitals of the nearest P atom. Stability and experimental feasibility are important properties of new 2D crystals. Next, we have investigated stability of the substitutional system by calculated binding energy. The binding energy (Eb) is calculated as a difference between the energy of a blue phosphorene sheet with a single vacancy plus energy of the isolated atom and the substitutional system. P31Si system is -5.16 eV, which is corresponding to Tang’s result. [31] Thus, the system is rather stable and could be artificially fabricated. To further validate the structural stability of Si-doped blue phosphorene, we performed ab-initio Molecular Dynamics simulations (MD) at 300 K with a time step of 1 fs. The simulation time was limited to 4 ps, as reported by Tománek et al. [16] The fluctuations of total energy with time during the simulation are shown in Fig. 3. The structure of Si-doped system maintains integrity at the end of the MD simulation, confirming that substitutional Si-doped blue phosphorene is stable at 300 K. Furthermore, we calculated the phonon dispersion curves of P31Si system as shown in Fig. 4, where all branches of the phonon dispersion curves have positive frequencies and no imaginary phonon modes are found, also confirming the stability of Si-doped blue phosphorene. 8
According to the DFT-D calculations and the equation of Ead, the favorable adsorption site of a H2O molecule on pristine blue phosphorene is at the hollow site with two OH bonds parallel to the blue phosphorene surface, which is shown as initial structure (IS) in the inset of Fig. 5(a). This configuration is also adopted as the initial structure in the subsequent transition state search calculations. In order to investigate the effect of the size of the simulation cell, we performed calculations on a 4 × 4 supercell with H2O adsorbed on the bridge site, and found the adsorption energy Ead = -335 meV, which is 4.7 % higher than the result obtained with the 3 × 3 supercell as shown in Fig. S1, (Electronic Supplementary Information, ESI) (Ead = -319 meV). In addition, the result obtained with the 5 × 5 supercell as shown in Fig. S2. The adsorption energy Ead =-349 meV, which is 3.8% higher than the result obtained with the 4 × 4 supercell. The differences are thus acceptable. For the H2O dissociative adsorption on blue phosphorene, the most favorable configuration is that H and OH are chemisorbed on the adjacent P atom in the six-membered ring, which is shown as the final structure (FS) in Fig. 5(a). Firstly, we investigate the pathway from IS to FS as shown in Fig. 5(a) for the reaction of the H2O molecule dissociative adsorption on pristine blue phosphorene. After LST/QST and NEB calculations, it is found that the reaction barrier Ebar = ETS − EIS is 2.45 eV as shown in Fig. 5(a), where ETS and EIS are respectively the energy of the transition state ETS and the initial state EIS. From Fig. 5(a), it is also known that the reaction consists of two steps: H2O is firstly dissociated into H and OH, meanwhile OH group binds with the nearest P atom (from IS to transition structure TS), which needs 9
energy of 2.45 eV; then H atom move to bind with the adjacent P atom from TS to FS while releasing energy of 1.20 eV. As a result, the reaction energy Er = EFS - EIS (1.25 eV) is required in total for the H2O dissociative adsorption on blue phosphorene, where the H2O dissociation with a large Ebar value is the rate-limiting step. Therefore, the dissociative adsorption of water on pristine blue phosphorene is difficult and the pristine blue phosphorene is strongly hydrophobic. Then we study the adsorption of H2O molecules on the Si-doped blue phosphorene system. The obtained adsorption energy Ead_IS based on Ead equation is listed in Table 1. In addition, Ead_IS of pristine and Si-doped blue phosphorene have been corrected by dipole correction in the calculations, as shown in Table 1. In Si-doped blue phosphorene systems, H2O chemically binds to the Si atom with a relatively strong adsorption energy of Ead_IS = -1.33 eV. The corresponding Si-O bond length is DSi-O = 1.928 Å. The structure of H2O adsorption on the Si-doped blue phosphorene system is the initial structure in the subsequent search calculations for transition states as shown in Fig. 5(b). To further study the adsorption properties of H2O molecule on blue phosphorene systems, we have performed the density of states (DOS) and the partials density of states (PDOS) calculations in Fig. 6. PDOS exhibits the integral intensity of H2O molecule adsorbed on pristine and Si-doped blue phosphorene for different bands. The valence state of H2O molecule adsorbed on blue phosphorene can be obtained via distribution of P-3p and O-2p electrons in the nonbonding band and bonding bands where the band from –2 to 0.9 eV denotes nonbonding band, and the band from –12 to –2 eV denotes bonding 10
bands, which is occupied by the px, py, and pz orbitals directly overlapping with p orbital of the O along the tetrahedron directions. Integration of each band of PDOS is listed in Table 2. The antibonding band consisting of the states from 1 to 4 eV are also included. The total density does not change within the precision of the calculation. In the Si-doped blue phosphorene, the bonding bands obviously extend to -1 eV and nonbonding bands significantly shorten. The integral intensity of bonding bands for H2O molecule adsorbed on Si-doped blue phosphorene is 33.58 eV wider than that H2O molecule adsorbed on blue phosphorene, or the participating of electrons in mixing of P-Si-O-H of H2O molecule adsorbed on Si-doped blue phosphorene is stronger than correlation of H2O molecule on blue phosphorene. It indicates that the structure of H2O molecule adsorbed on Si-doped blue phosphorene is more stable, and doping Si is beneficial for the H2O molecule adsorption, which is agreement with the results above. For the dissociative adsorption of H2O on the Si-doped blue phosphorene system, the favorite FS is shown in Fig. 5(b), which is that OH and H are chemisorbed on the Si atom and the second nearest P atom, respectively. The adsorption energies Ead_FS for the dissociative chemical adsorption based on Ead equation are listed in Table 1 as well. It shows that the dissociative adsorption energy of H2O on Si-doped blue phosphorene Ead_FS = -1.52 eV, which is much stronger than that on pristine blue phosphorene with Ead_FS = -0.915 eV. Therefore, the hydrophilic stability of blue phosphorene after Si doping is significantly enhanced compared with that of pristine blue phosphorene. The adsorption positions of H and OH for the dissociative 11
adsorption of the H2O molecule can be explained by the Mulliken analysis, and the atom charges near the dopant atom are shown in Fig. 1(c). It is found that the Si atom transfers electrons to the P atoms nearby and is positively charged, which is preferable for the adsorption of negatively charged OH. Meanwhile, P atoms near the dopant atom with negative charges are preferable for the adsorption of positively charged H atom, which makes the dissociative adsorption of the H2O molecule on blue phosphorene easier.After LST/QST and NEB calculations, the detailed reaction pathway and the energy barrier for the dissociative adsorption of a H2O molecule on the Si-doped blue phosphorene system are shown in Fig. 5(b). Besides the initial and final structures, the atomic structure of the transition state TS is also shown in Fig. 5(b). It is found that the energy barrier decreases significantly after Si atom doped. Ebar of the Si doped system decreases from 2.45 eV in the pristine blue phosphorene system to 0.19 eV. In addition, the reaction process is exothermic on Si-doped blue phosphorene with negative reaction energy Er of -0.58 eV. Thus, doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O on blue phosphorene and change the blue phosphorene from hydrophobic to hydrophilic spontaneously due to the presence of OH group. To understand the mechanism for the decrease of Ebar in the Si-doped blue phosphorene system, atomic charge transfers (QT) from H2O to pristine and Si-doped blue phosphorene are analyzed to understand the nature of the interaction [53] and the corresponding results are shown in Table 1. As H2O molecule interacts with the blue phosphorene surface and a small charge of 0.041 e is transferred from H2O to blue 12
phosphorene. Since the interaction between P and H2O is very weak and there is a strong coupling between O and H atoms, a large Ebar is observed for H2O dissociative adsorption on pristine blue phosphorene as shown in Fig. 5(a). While H2O molecule is adsorbed on Si-doped blue phosphorene, 0.235 e is transferred from H2O to Si-doped blue phosphorene. As a result, the Si-O interaction enhances while the O-H interaction decreases, which imply that the H2O dissociation becomes easier. Therefore, the Si dopant can lower Ebar for the dissociative adsorption of H2O on blue phosphorene, which induces the transition of wettability of blue phosphorene from hydrophobic to hydrophilic spontaneously in the presence of water vapor at room temperature. Generally, once oxygen-containing groups are introduced into graphene for obtaining hydrophilic property, [54] the electrical conductivity of graphene will greatly be reduced. Therefore, the band structures of the final structure FS with different concentration of OH group are also investigated, as shown in Fig. 7, which show that the dissociative adsorption of H2O molecules generally maintain the conductivity of blue phosphorene. After the adsorption of H and the OH group on pristine blue phosphorene with the 4 × 4 supercell, the band gap is about 1.90 eV and the Fermi level lies on the top of the valence band maximum (VBM), indicating an intrinsic semiconductor character in Fig. 7(a). For the final structure of the Si-doped blue phosphorene system with the 4 × 4 supercell, the Fermi level lies within the valence band and the conductivity improves significantly, as shown in Fig. 7(b). Moreover, in order to gain more insights into the electronic structure of undoped and Si-doped blue phosphorene, the corresponding charge 13
density isosurfaces of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) have been explored in Fig. 6. For undoped blue phosphorene, LUMO orbitals are primarily localized around the specific P atoms, which is bonded with H and OH groups, while for HOMO orbitals, the charge densities are mainly attributed by the nearby P atoms with OH group. In the Si-doped blue phosphorene system with the 4 × 4 supercell, it is found that LUMO orbitals is localized around the P atom, which is bonded with H atom. And the charge densities of HOMO orbitals are mainly attributed by the other P atoms. We further consider the effect of a larger and lower doping concentration on the electrical conductivity, the corresponding result with the 3 × 3 and 5 × 5 supercell systems shown in Figs. S3 and S4, respectively, indicating that the Si-doped FS is also conductive. Therefore, the dissociative adsorption of the H2O molecule on Si-doped blue phosphorene changes blue phosphorene to hydrophilic while achieves a semiconductor to metal transition. 4. Conclusions In summary, the effect of Si dopant on the dissociative adsorption of H2O molecule on blue phosphorene is considered for exploring the application of blue phosphorene as electrode material of supercapacitors and biomaterial supports, where hydrophilic and conductive blue phosphorene is required. The dissociative barrier of H2O molecule is efficiently decreased by Si-substituted doped on the blue phosphorene, with changing the wettability of blue phosphorene from hydrophobic to hydrophilic spontaneously. As a result, doping Si into blue phosphorene is an efficient method to tune the wettability of 14
blue phosphorene from hydrophobic to hydrophilic while achieving a transition from semiconductor to metal.
Acknowledgements The authors acknowledge supports by National Key Basic Research and Development Program (Grant No. 2015CB655105), National Natural Science Foundation of China (NSFC, Nos. 51301020 and 51471124), China Postdoctoral Science Foundation (No. 2015M582585), the special fund for basic scientific research of central colleges of Chang’an University (No. 310831162002), and National Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 201310710064).
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[34] C. Xia, Y. Peng, H. Zhang, T. Wang, S. Wei, and Y. Jia, The characteristics of n- and p-type dopants in SnS2 monolayer nanosheets, Phys. Chem. Chem. Phys. 16 (2014) 19674. [35] D. Çakır, H. Sahin, F. M. Peeters, Doping of rhenium disulfide monolayers: a systematic first principles study, Phys. Chem. Chem. Phys. 16 (2014) 16771. [36] H. Y. He, J. Hu, B. C. Pan, Hydrogen in Ag-doped ZnO: theoretical calculations, J. Chem. Phys. 130 (2009) 204516. [37] G. A. Gelves, B. Lin, U. Sundararaj, J. A. Haber, Low Electrical Percolation Threshold of Silver and Copper Nanowires in Polystyrene Composites, Adv. Funct. Mater. 16 (2006) 2423. [38] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508. [39] J. P. Perdew, K. Burke, M. Ernzerhof, ERRATA:Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865. [40] Y. F. Zhu, Q. Q. Dai, M. Zhao, Q. Jiang, Physicochemical insight into gap openings in graphene, Sci. Rep. 3 (2013) 1524. [41] S. Grimme, Semiempirical GGA-type density functional constructed with a long-range dispersion correction, J. Comput. Chem. 27 (2006) 1787. [42] R. Q. Zhang, W. T. Zheng, and Q. Jiang, External Electric Field Modulated Electronic and Structural Properties of <111> Si Nanowires, J. Phys. Chem. C, 113 (2009) 10384. [43] C. He, X. L. Wu, G. Liu, W. X. Zhang, Elastic and transport properties of nanolayered crystalline Cu/amorphous Cu-Zr multilayers, Mater. Design 106 (2016) 133. [44] J. Y. Dai, J. M. Yuan, P. Giannozzi, Gas adsorption on graphene doped with B, N, Al, and S: A theoretical study, Appl. Phys. Lett. 95 (2009) 232105.
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[45] M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J.Hasnip, S. J. Clark, M. C. Payne, First-principles simulation: ideas, illustrations and the CASTEP code, J. Phys.: Condens. Matter 14 (2002) 2717. [46] T. A. Halgren, W. N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225. [47] G. Henkelman, H. Jonsson, Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points, J. Chem. Phys. 113 (2000) 9978. [48] T. Hu, J. S. Hong, Electronic structure and magnetic properties of zigzag blue phosphorene nanoribbons，J. Appl. Phys. 118 (2015) 054301. [49] Y. Ding, Y. L. Wang, Structural, electronic, and magnetic properties of adatom adsorptions on black and blue phosphorene: a first-principles study, J. Phys. Chem. C 119 (2015) 10610. [50] Z. Y. Zhang, M. S. Si, S. L. Peng, F. Zhang, Y. H. Wang, D. S. Xue, Bandgap engineering in van der Waals heterostructures of blue phosphorene and MoS2: A first principles calculation, J. Solid State Chem. 231 (2015) 64. [51] Y. Du, C. Ouyang, S. Shi, M. Lei, Ab initio studies on atomic and electronic structures of black phosphorus, J. Appl. Phys. 107 (2010) 093718. [52] I. Žutić, J.Fabian, S. DasSarma, Spintronics: Fundamentals and applications, Rev. Mod. Phys. 76 (2004) 323. [53] S. Li, Y. F. Wu, W. Liu, Y. H. Zhao, Control of band structure of van der Waals heterostructures: Silicene on ultrathin silicon nanosheets, Chem. Phys. Lett. 609 (2014) 161. [54] J. Rafiee, M. A. Rafiee, Z. Z. Yu, N. Koratkar, Superhydrophobic to superhydrophilic wetting control in graphene films, Adv. Mater. 22 (2010) 2151.
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Table 1 The adsorption energy in the initial structure (Ead_IS) and the final structure (Ead_FS), the distance of the O atom above the blue phosphorene D (for Si-doped blue phosphorene, D is the Si-O bond length), the O-H bond length lO-H, and the Mulliken atomic charge QH2O of H2O adsorption on substrates. The adsorption energies Ead_IS are obtained by using dipole correction in eV (in the parentheses).
Structure
Ead_IS (eV)
D (Å)
lO-H (Å)
QH2O (e)
Ead_FS
pristine
-0.335 (-0.322)
2.674
0.978
0.041
-0.915
Si-doped
-1.33 (-1.178)
1.928
0.985
0.235
-1.52
21
Table 2 Integration intensities of 3p bands in H2O molecule adsorbed on pristine and Si-doped blue phosphorene. Structure
Antibonding
Nonbonding
Bonding
Total
pristine
23.05
35.43
65.28
123.76
Si-doped
19.25
5.83
98.86
123.94
22
Captions Fig. 1 Optimized geometry (a), (c) and the band structure (b), (d) for 4 × 4 × 1 monolayer pristine and Si-doped blue phosphorene, respectively. The pink and yellow balls denote P and Si atoms, respectively. The supercell is represented by the green lines. Fermi level is set to zero. Black and red lines denote up-spin and down-spin band respectively. Fig. 2 Total DOS of Si-doped blue phosphorene, and corresponding partial DOS of the s, p orbitals doping Si atom and one of the neighboring P atom around Si. The Fermi level is indicated by the vertical dashed line. Fig. 3 Changes of temperature and energy with time obtained from Molecular Dynamics simulations of Si-doped blue phosphorene system. Fig. 4 The calculated phonon dispersion curves of P31Si system. Fig. 5 The reaction pathway of a H2O molecule dissociative adsorption on pristine and Si-doped blue phosphorene. IS, TS and FS represent initial structure, transition structure and final structure, respectively. Their atomic structures are given in the insets. The energy of IS is taken to be zero. The units of Ebar and Er are eV, where Ebar is the energy barrier and Er is the reaction energy. The pink, white, red and yellow atoms are P, H, O, and Si in this figure and below, respectively. Fig. 6 The partial DOS of H2O molecule adsorbed on blue phosphorene and Si-doped blue phosphorene. The Fermi level is indicated by the vertical dashed line. Fig. 7 The band structures, charge densities of HOMO and LUMO of the FS in pristine (a) and Si-doped blue phosphorene (b) with the 4 × 4 supercells. Blue and yellow denote 23
positive and negative wave function contours in charge densities, respectively, and the isosurface values are ±0.03 e/Å3.
24
Fig. 1
25
Fig. 2
26
Fig. 3
27
Fig. 4
28
Fig.5
29
Fig.6
30
Fig.7
31
Graphical Abstract
Doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O molecules. The dissociative energy barrier is reduced from 2.45 eV on pristine blue phosphorene to 0.19 eV on Si-doped blue phosphorene.
32
Highlights. 1. Doping Si into blue phosphorene can facilitate the dissociative of H2O molecules. 2. The dissociative energy barrier is reduced to 0.19 eV on Si-doped blue phosphorene. 3. The dissociative of H2O can convert the Si-doped blue phosphorene to hydrophilic.
33
S0009-2614(17)30203-8 http://dx.doi.org/10.1016/j.cplett.2017.02.078 CPLETT 34591
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
21 December 2016 24 February 2017 26 February 2017
Please cite this article as: W.X. Zhang, J.W. Zhao, W.H. He, L.J. Luan, C. He, Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/ j.cplett.2017.02.078
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Enhanced hydrophilic and conductive properties of blue phosphorene doped with Si atom W.X. Zhang 1∗, J.W. Zhao 1, W.H. He 1, L.J. Luan 1, C. He2* 1
School of Materials Science and Engineering, Chang’an University, Xi’an 710064,
China; 2
State Key Laboratory for Mechanical Behavior of Materials, School of Materials
Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
Abstract In this paper, the electronic, magnetic properties and the effect of Si-doped blue phosphorene on the adsorption of a H2O molecule have been investigated using density functional theory with van der Waals corrections. The results indicate that doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O molecules. The dissociative energy barrier is reduced from 2.45 eV on pristine blue phosphorene to 0.19 eV on Si-doped blue phosphorene, which indicates a smooth dissociative adsorption. In addition, the dissociative adsorption of H2O molecules can convert the Si-doped blue phosphorene from hydrophobic to hydrophilic with a semiconductor-to-metal transition.
Keywords: Blue phosphorene; Electronic properties; First principles calculations; Adsorption
*
Corresponding Authors: W. X. Zhang ([email protected]) or C. He ([email protected])
1
1. Introduction Stimulated by the isolation of graphene from graphite in 2004, there has been tremendous research interest in atomic two-dimensional (2D) crystal due to its extraordinary structural and electronic properties as well as promising applications in nanoelectronics, [1-4] supercapacitors, [5,6] and biomaterial supports. [7,8] The impressive progress in graphene research has motivated scientists to explore other 2D atomic based materials. [9] Among them, black phosphorene-the few-layer black phosphorus, has been successfully isolated by mechanical exfoliation from black phosphorus. [10-13] Unlike graphene, black phosphorene is direct band semiconductor and exhibits high carrier mobility, [12-14] which would facilitate the application in optoelectronics. [15] Interestingly, many different layered phosphorene allotropes have been predicted by ab initio density functional calculations. [16-18] As a typical allotrope of black phosphorus, blue phosphorene has been theoretically predicted to share layered structure and high stability with the black phosphorene. [19-21] In addition, blue phosphorus should be exfoliated easily to form quasi-2D structures with a band gap of ~ 2eV, [16,17] which is also a promising material for applications in nanoelectronics and spintronics. Nevertheless, some important issues remain largely unexplored, for example, how to effectively tailor the electronics, magnetic properties, and the surface wettability of blue phosphorene to make it suitable for electrode material and biomaterial supports. It is desirable that blue phosphorene is hydrophilic and conductive because the hydrophilicity improves the wetting between blue phosphorene and polar electrolytes or biological molecules, while the conductivity enhances the transport of free carriers. The surface
2
wettability is influenced by both the morphology [22] and chemical composition [23] of solid surfaces. For graphene-like materials, the wettability is mainly determined by the surface’s chemical composition, in particular, by its hydrophobic/hydrophilic character. [24] Generally, hydrophilic behavior depends on the interaction between the material and water molecules. Previous studies have shown that the dissociative adsorption of H2O molecules can produce hydrophilic OH groups and convert materials to hydrophilic. [23,25,26] Jiang et al. predicted that doping Al into graphene could facilitate the dissociation of H2O molecules to produce hydrophilic OH groups, while keeping graphene conductive. [25] Recently, the fascinating electronic and magnetic properties of the substitutional doping in some 2D materials, such as graphene, BN sheets and MoS2 monolayer, maybe experimentally achieved by filling the vacancies created by the electron beam with substitutional atoms. [27-29] Sun et al. have reported that the substitution of nonmetallic dopant F, Cl, B and N blue phosphorene are nonmagnetic due to the saturation or pairing of valence electron of dopant and its neighboring P atoms, while the substitutional doping of C, Si and O in blue phosphorene can induce the magnetism. [21,30] Therefore, the substitutional doping of nonmetal atoms has been considered as a promising approach to tune electronic structures and the magnetic states of 2D materials. [31-35] Because the interaction of a H2O molecule with blue phosphorene is of great technological interest, and the effect of doping Si on the wettability of blue phosphorene is an interesting question. Therefore, in this paper, the dissociative adsorption of a H2O molecule on Si-doped blue phosphorene is extensive carried out to understand the effect of the Si dopant on the wettability and conductivity of blue phosphorene based on
3
first-principles calculations within density functional theory (DFT). These studies provide us a deep understanding of the novel properties of monolayer blue phosphorene, which is essential to enrich them for future in supercapacitors and biomaterial supports. 2. Computational methods The simulations were based on density functional theory (DFT), which was provided by DMOL3. [36-38] The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof scheme (PBE) [39] was adopted for the exchange-correlation potential to optimize geometrical structures and calculate properties for both spin-polarized and spin-unpolarized cases. [40] In order to take into account the contributions of the van der Waals (vdW) interactions, the DFT-D (D stands for dispersion) approach within the Grimme scheme was adopted for the vdW correction. [41] Double numerical atomic orbital plus polarization (DNP) is chosen as the basis set, with the global orbital cutoff 4.4 Å. Similar functional have been successfully used to study the structural and electronic properties of GNRs, Si and Cu nanowires. [40,42,43] The vacuum space between blue phosphorene was greater than 20 Å to ensure no interactions between different layers. Periodic boundary conditions was adopted for all utilized models in this work. The Brillouin zone was sampled by 6 × 6 × 1 (10 × 10 × 1) k-points for all structures in the geometry optimization (electronic) calculations, [44] which brings out the convergence tolerance of energy of 1.0 × 10-5 Ha (1 Ha = 27.2114 eV), maximum force of 0.002 Ha/Å, and maximum displacement of 0.005 Å.
4
To examine the stability of doped blue phosphorene, ab initio molecular dynamics methods (ab initio-MD) simulation at 300 K, where the canonical ensemble was used and the time step was taken to be 1 fs, is also performed with the Castep code. [45] Ab initio-MD simulations are performed in the NPT statistical ensemble with the norm-conserving pseudopotentials and generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof scheme (PBE) exchange-correlation functional. Pressure P and temperature T are constants and N is the atom number. T is imposed by the Nose algorithm. [46]. To investigate the minimum energy pathway for H2O dissociative adsorption on blue phosphorene, linear synchronous transit/quadratic synchronous transit (LST/QST) [46] and nudged elastic band (NEB) [47] tools of the Dmol3 module was used, which have been well validated to determine the structure of the transition state and the minimum energy pathway. For a H2O molecule adsorbed on blue phosphorene for both physisorption and dissociative adsorption, the adsorption energy Ead is defined as: Ead = EH2O/blue phosphorene (Eblue phosphorene + EH2O), where EH2O/blue phosphorene, Eblue phosphorene and EH2O are total energies of the H2O/blue phosphorene system, the isolate blue phosphorene and a H2O molecule in the supercell with the same lattice of blue phosphorene, respectively. 3. Results and discussion The accuracy of our calculation procedure is tested by monolayer blue phosphorene. The full relaxed lattice constant of blue phosphorene is 3.30 Å, which agrees well with 5
other theoretical results (3.28-3.32 Å). [19-21,30,48-50] The P-P bond length is 2.28 Å, which is slightly larger than the P-P covalent bond length (~2.2 Å) of black phosphorus. [50] The two inequivalent P atoms are distributed in two planes with the buckling height of 1.25 Å (see the side view of left panel of Fig. 1(a)). All the above parameters agree well with the recent theoretical results. [19-21,30, 48-50] A 4 × 4 × 1 supercell of blue phosphorene containing 32 atoms is constructed (Fig. 1a). According to the band structure as shown in Fig. 1(b), the blue phosphorene is a semiconductor with an indirect band gap (Eg) of 1.97 eV, where the highest occupied states appear at the middle region along the M-F line, and the lowest unoccupied states are located between the G and M points. The calculated result is also in good agreement with previous theoretical results ~ 2.0 eV. [20,21,30] Similar to black phosphorene, each P atom in blue phosphorene has three equivalent covalent bonds with its neighbors, which means that three electrons of P atom are saturated, while the other two electrons are a single lone pair. As a result, the five valence electrons of each P atom are all paired, and thus the pristine blue phosphorene is intrinsically nonmagnetic. [20] As previously theoretical results mentioned, doping blue phosphorene can tune its chemical and physical properties. [20,49] Next we investigate the Si-doped blue phosphorene system P31Si, in which a substitutional Si atom occupies P site in the supercell, as shown in Fig. 1(c). For Si-doped blue phosphorene, the dopant atom moves a little inside of the plane, which is maintain the 3-fold symmetry. The distance between the Si atom and blue phosphorene is lP-Si = 2.27 Å, which is consistent with the reported 6
result of lP-Si = 2.26 Å. [21] Fig. 1 also shows the atomic charge near the dopant atom obtained by Mulliken analysis. The Mulliken population of the Si atom is 0.281 e, and the Si atom forms an electron-deficient position. Therefore, the electron distribution in blue phosphorene is changed by the dopant, and thus the interaction reactivity between small molecules and blue phosphorene would be changed. In order to investigate stability of electronic and magnetic state of Si-doped blue phosphorene, we have calculated the energy difference between the spin-polarized and spin-unpolarized states for P31Si system. The relative energy difference ΔESpin between Esp and Eusp is 217 meV. Therefore, noticeably, the calculated spin-polarized state of P31Si system is energetically lower than that of spin-unpolarized state, indicating that the ground state of P31Si system is magnetic. As shown in Fig. 1(c), in P31Si system, dopant Si is covalently bonded with its three nearest P atoms with an unpaired valence electron, thus inducing a net magnetic moment of approximately 0.78 µB. The magnetic moment at Si atom is about 0.366 μB and the rest magnetic moment mainly comes from the spin-polarized P atoms. The spin polarization is also evident in the band structure (Fig. 1d). The P31Si system behaves as an indirect semiconductor and the values of Eg of 1.34 and 0.53 eV in the spin-up and spin-down states, respectively. Consequently, Si-substituted system is a diluted magnetic semiconductor and may have potential application in spintronics. [52] To further study the electronic and magnetic structure properties of Si-doped blue phosphorene systems, we have performed the density of states (DOS) and the partials 7
density of states (PDOS) calculations in Fig. 2 and found that the majority and minority density of states are asymmetrical for Si-doped system, indicating a magnetic character. The PDOS of P31Si show that the 3s, 3p states of dopant Si, the 3p states of the nearest P atom around dopant overlap near the Fermi level, which indicate that the unpaired electron induced by dopant Si not only occupy the s, p orbitals, but also partially occupy the 3p orbitals of the nearest P atom. Stability and experimental feasibility are important properties of new 2D crystals. Next, we have investigated stability of the substitutional system by calculated binding energy. The binding energy (Eb) is calculated as a difference between the energy of a blue phosphorene sheet with a single vacancy plus energy of the isolated atom and the substitutional system. P31Si system is -5.16 eV, which is corresponding to Tang’s result. [31] Thus, the system is rather stable and could be artificially fabricated. To further validate the structural stability of Si-doped blue phosphorene, we performed ab-initio Molecular Dynamics simulations (MD) at 300 K with a time step of 1 fs. The simulation time was limited to 4 ps, as reported by Tománek et al. [16] The fluctuations of total energy with time during the simulation are shown in Fig. 3. The structure of Si-doped system maintains integrity at the end of the MD simulation, confirming that substitutional Si-doped blue phosphorene is stable at 300 K. Furthermore, we calculated the phonon dispersion curves of P31Si system as shown in Fig. 4, where all branches of the phonon dispersion curves have positive frequencies and no imaginary phonon modes are found, also confirming the stability of Si-doped blue phosphorene. 8
According to the DFT-D calculations and the equation of Ead, the favorable adsorption site of a H2O molecule on pristine blue phosphorene is at the hollow site with two OH bonds parallel to the blue phosphorene surface, which is shown as initial structure (IS) in the inset of Fig. 5(a). This configuration is also adopted as the initial structure in the subsequent transition state search calculations. In order to investigate the effect of the size of the simulation cell, we performed calculations on a 4 × 4 supercell with H2O adsorbed on the bridge site, and found the adsorption energy Ead = -335 meV, which is 4.7 % higher than the result obtained with the 3 × 3 supercell as shown in Fig. S1, (Electronic Supplementary Information, ESI) (Ead = -319 meV). In addition, the result obtained with the 5 × 5 supercell as shown in Fig. S2. The adsorption energy Ead =-349 meV, which is 3.8% higher than the result obtained with the 4 × 4 supercell. The differences are thus acceptable. For the H2O dissociative adsorption on blue phosphorene, the most favorable configuration is that H and OH are chemisorbed on the adjacent P atom in the six-membered ring, which is shown as the final structure (FS) in Fig. 5(a). Firstly, we investigate the pathway from IS to FS as shown in Fig. 5(a) for the reaction of the H2O molecule dissociative adsorption on pristine blue phosphorene. After LST/QST and NEB calculations, it is found that the reaction barrier Ebar = ETS − EIS is 2.45 eV as shown in Fig. 5(a), where ETS and EIS are respectively the energy of the transition state ETS and the initial state EIS. From Fig. 5(a), it is also known that the reaction consists of two steps: H2O is firstly dissociated into H and OH, meanwhile OH group binds with the nearest P atom (from IS to transition structure TS), which needs 9
energy of 2.45 eV; then H atom move to bind with the adjacent P atom from TS to FS while releasing energy of 1.20 eV. As a result, the reaction energy Er = EFS - EIS (1.25 eV) is required in total for the H2O dissociative adsorption on blue phosphorene, where the H2O dissociation with a large Ebar value is the rate-limiting step. Therefore, the dissociative adsorption of water on pristine blue phosphorene is difficult and the pristine blue phosphorene is strongly hydrophobic. Then we study the adsorption of H2O molecules on the Si-doped blue phosphorene system. The obtained adsorption energy Ead_IS based on Ead equation is listed in Table 1. In addition, Ead_IS of pristine and Si-doped blue phosphorene have been corrected by dipole correction in the calculations, as shown in Table 1. In Si-doped blue phosphorene systems, H2O chemically binds to the Si atom with a relatively strong adsorption energy of Ead_IS = -1.33 eV. The corresponding Si-O bond length is DSi-O = 1.928 Å. The structure of H2O adsorption on the Si-doped blue phosphorene system is the initial structure in the subsequent search calculations for transition states as shown in Fig. 5(b). To further study the adsorption properties of H2O molecule on blue phosphorene systems, we have performed the density of states (DOS) and the partials density of states (PDOS) calculations in Fig. 6. PDOS exhibits the integral intensity of H2O molecule adsorbed on pristine and Si-doped blue phosphorene for different bands. The valence state of H2O molecule adsorbed on blue phosphorene can be obtained via distribution of P-3p and O-2p electrons in the nonbonding band and bonding bands where the band from –2 to 0.9 eV denotes nonbonding band, and the band from –12 to –2 eV denotes bonding 10
bands, which is occupied by the px, py, and pz orbitals directly overlapping with p orbital of the O along the tetrahedron directions. Integration of each band of PDOS is listed in Table 2. The antibonding band consisting of the states from 1 to 4 eV are also included. The total density does not change within the precision of the calculation. In the Si-doped blue phosphorene, the bonding bands obviously extend to -1 eV and nonbonding bands significantly shorten. The integral intensity of bonding bands for H2O molecule adsorbed on Si-doped blue phosphorene is 33.58 eV wider than that H2O molecule adsorbed on blue phosphorene, or the participating of electrons in mixing of P-Si-O-H of H2O molecule adsorbed on Si-doped blue phosphorene is stronger than correlation of H2O molecule on blue phosphorene. It indicates that the structure of H2O molecule adsorbed on Si-doped blue phosphorene is more stable, and doping Si is beneficial for the H2O molecule adsorption, which is agreement with the results above. For the dissociative adsorption of H2O on the Si-doped blue phosphorene system, the favorite FS is shown in Fig. 5(b), which is that OH and H are chemisorbed on the Si atom and the second nearest P atom, respectively. The adsorption energies Ead_FS for the dissociative chemical adsorption based on Ead equation are listed in Table 1 as well. It shows that the dissociative adsorption energy of H2O on Si-doped blue phosphorene Ead_FS = -1.52 eV, which is much stronger than that on pristine blue phosphorene with Ead_FS = -0.915 eV. Therefore, the hydrophilic stability of blue phosphorene after Si doping is significantly enhanced compared with that of pristine blue phosphorene. The adsorption positions of H and OH for the dissociative 11
adsorption of the H2O molecule can be explained by the Mulliken analysis, and the atom charges near the dopant atom are shown in Fig. 1(c). It is found that the Si atom transfers electrons to the P atoms nearby and is positively charged, which is preferable for the adsorption of negatively charged OH. Meanwhile, P atoms near the dopant atom with negative charges are preferable for the adsorption of positively charged H atom, which makes the dissociative adsorption of the H2O molecule on blue phosphorene easier.After LST/QST and NEB calculations, the detailed reaction pathway and the energy barrier for the dissociative adsorption of a H2O molecule on the Si-doped blue phosphorene system are shown in Fig. 5(b). Besides the initial and final structures, the atomic structure of the transition state TS is also shown in Fig. 5(b). It is found that the energy barrier decreases significantly after Si atom doped. Ebar of the Si doped system decreases from 2.45 eV in the pristine blue phosphorene system to 0.19 eV. In addition, the reaction process is exothermic on Si-doped blue phosphorene with negative reaction energy Er of -0.58 eV. Thus, doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O on blue phosphorene and change the blue phosphorene from hydrophobic to hydrophilic spontaneously due to the presence of OH group. To understand the mechanism for the decrease of Ebar in the Si-doped blue phosphorene system, atomic charge transfers (QT) from H2O to pristine and Si-doped blue phosphorene are analyzed to understand the nature of the interaction [53] and the corresponding results are shown in Table 1. As H2O molecule interacts with the blue phosphorene surface and a small charge of 0.041 e is transferred from H2O to blue 12
phosphorene. Since the interaction between P and H2O is very weak and there is a strong coupling between O and H atoms, a large Ebar is observed for H2O dissociative adsorption on pristine blue phosphorene as shown in Fig. 5(a). While H2O molecule is adsorbed on Si-doped blue phosphorene, 0.235 e is transferred from H2O to Si-doped blue phosphorene. As a result, the Si-O interaction enhances while the O-H interaction decreases, which imply that the H2O dissociation becomes easier. Therefore, the Si dopant can lower Ebar for the dissociative adsorption of H2O on blue phosphorene, which induces the transition of wettability of blue phosphorene from hydrophobic to hydrophilic spontaneously in the presence of water vapor at room temperature. Generally, once oxygen-containing groups are introduced into graphene for obtaining hydrophilic property, [54] the electrical conductivity of graphene will greatly be reduced. Therefore, the band structures of the final structure FS with different concentration of OH group are also investigated, as shown in Fig. 7, which show that the dissociative adsorption of H2O molecules generally maintain the conductivity of blue phosphorene. After the adsorption of H and the OH group on pristine blue phosphorene with the 4 × 4 supercell, the band gap is about 1.90 eV and the Fermi level lies on the top of the valence band maximum (VBM), indicating an intrinsic semiconductor character in Fig. 7(a). For the final structure of the Si-doped blue phosphorene system with the 4 × 4 supercell, the Fermi level lies within the valence band and the conductivity improves significantly, as shown in Fig. 7(b). Moreover, in order to gain more insights into the electronic structure of undoped and Si-doped blue phosphorene, the corresponding charge 13
density isosurfaces of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) have been explored in Fig. 6. For undoped blue phosphorene, LUMO orbitals are primarily localized around the specific P atoms, which is bonded with H and OH groups, while for HOMO orbitals, the charge densities are mainly attributed by the nearby P atoms with OH group. In the Si-doped blue phosphorene system with the 4 × 4 supercell, it is found that LUMO orbitals is localized around the P atom, which is bonded with H atom. And the charge densities of HOMO orbitals are mainly attributed by the other P atoms. We further consider the effect of a larger and lower doping concentration on the electrical conductivity, the corresponding result with the 3 × 3 and 5 × 5 supercell systems shown in Figs. S3 and S4, respectively, indicating that the Si-doped FS is also conductive. Therefore, the dissociative adsorption of the H2O molecule on Si-doped blue phosphorene changes blue phosphorene to hydrophilic while achieves a semiconductor to metal transition. 4. Conclusions In summary, the effect of Si dopant on the dissociative adsorption of H2O molecule on blue phosphorene is considered for exploring the application of blue phosphorene as electrode material of supercapacitors and biomaterial supports, where hydrophilic and conductive blue phosphorene is required. The dissociative barrier of H2O molecule is efficiently decreased by Si-substituted doped on the blue phosphorene, with changing the wettability of blue phosphorene from hydrophobic to hydrophilic spontaneously. As a result, doping Si into blue phosphorene is an efficient method to tune the wettability of 14
blue phosphorene from hydrophobic to hydrophilic while achieving a transition from semiconductor to metal.
Acknowledgements The authors acknowledge supports by National Key Basic Research and Development Program (Grant No. 2015CB655105), National Natural Science Foundation of China (NSFC, Nos. 51301020 and 51471124), China Postdoctoral Science Foundation (No. 2015M582585), the special fund for basic scientific research of central colleges of Chang’an University (No. 310831162002), and National Training Programs of Innovation and Entrepreneurship for Undergraduates (No. 201310710064).
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Table 1 The adsorption energy in the initial structure (Ead_IS) and the final structure (Ead_FS), the distance of the O atom above the blue phosphorene D (for Si-doped blue phosphorene, D is the Si-O bond length), the O-H bond length lO-H, and the Mulliken atomic charge QH2O of H2O adsorption on substrates. The adsorption energies Ead_IS are obtained by using dipole correction in eV (in the parentheses).
Structure
Ead_IS (eV)
D (Å)
lO-H (Å)
QH2O (e)
Ead_FS
pristine
-0.335 (-0.322)
2.674
0.978
0.041
-0.915
Si-doped
-1.33 (-1.178)
1.928
0.985
0.235
-1.52
21
Table 2 Integration intensities of 3p bands in H2O molecule adsorbed on pristine and Si-doped blue phosphorene. Structure
Antibonding
Nonbonding
Bonding
Total
pristine
23.05
35.43
65.28
123.76
Si-doped
19.25
5.83
98.86
123.94
22
Captions Fig. 1 Optimized geometry (a), (c) and the band structure (b), (d) for 4 × 4 × 1 monolayer pristine and Si-doped blue phosphorene, respectively. The pink and yellow balls denote P and Si atoms, respectively. The supercell is represented by the green lines. Fermi level is set to zero. Black and red lines denote up-spin and down-spin band respectively. Fig. 2 Total DOS of Si-doped blue phosphorene, and corresponding partial DOS of the s, p orbitals doping Si atom and one of the neighboring P atom around Si. The Fermi level is indicated by the vertical dashed line. Fig. 3 Changes of temperature and energy with time obtained from Molecular Dynamics simulations of Si-doped blue phosphorene system. Fig. 4 The calculated phonon dispersion curves of P31Si system. Fig. 5 The reaction pathway of a H2O molecule dissociative adsorption on pristine and Si-doped blue phosphorene. IS, TS and FS represent initial structure, transition structure and final structure, respectively. Their atomic structures are given in the insets. The energy of IS is taken to be zero. The units of Ebar and Er are eV, where Ebar is the energy barrier and Er is the reaction energy. The pink, white, red and yellow atoms are P, H, O, and Si in this figure and below, respectively. Fig. 6 The partial DOS of H2O molecule adsorbed on blue phosphorene and Si-doped blue phosphorene. The Fermi level is indicated by the vertical dashed line. Fig. 7 The band structures, charge densities of HOMO and LUMO of the FS in pristine (a) and Si-doped blue phosphorene (b) with the 4 × 4 supercells. Blue and yellow denote 23
positive and negative wave function contours in charge densities, respectively, and the isosurface values are ±0.03 e/Å3.
24
Fig. 1
25
Fig. 2
26
Fig. 3
27
Fig. 4
28
Fig.5
29
Fig.6
30
Fig.7
31
Graphical Abstract
Doping Si into blue phosphorene can facilitate the dissociative adsorption of H2O molecules. The dissociative energy barrier is reduced from 2.45 eV on pristine blue phosphorene to 0.19 eV on Si-doped blue phosphorene.
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Highlights. 1. Doping Si into blue phosphorene can facilitate the dissociative of H2O molecules. 2. The dissociative energy barrier is reduced to 0.19 eV on Si-doped blue phosphorene. 3. The dissociative of H2O can convert the Si-doped blue phosphorene to hydrophilic.
33