- Email: [email protected]
The mechanism of negative and positive hydrogen ions production on the Ni surface
Vacuum 171 (2020) 108982
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
The mechanism of negative and positive hydrogen ions production on the Ni surface ShuangWen Zhao a, KaiYuan Wang a, JianChun Wu b, ChangYong Zhan a, Yu Zou a, * a b
Key Laboratory of Radiation and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, 610064, China School of Material Science and Engineering, Jiangsu University, 301, Xuefu Road, Zhenjiang, 212013, Jiangsu Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ion surface interaction Porous Ni sheet Negative hydrogen ions Constrained density functional theory
The studies about surface production of hydrogen negative ions play an important role in many technologies. In the present paper, a low energy molecular hydrogen ion beam was irradiated to the porous Ni sheet. It was found that negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ion beam was lower than 180 eV. When the energy of the incident ions increased, the positive ions dominated at the back of the porous Ni sheet. The production of negative hydrogen on Ni surface can’t be explained by comparing the electron affinity of Ni and H. The charge transferring during the process of H atoms reflected from Ni surface was calculated by using constrained DFT (cDFT). The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, the influence of incident energy on the production of negative and positive hydrogen ions is unable to be explained by cDFT. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms.
1. Introduction The research and the design of hydrogen negative ion sources have been extensively performed in connection with neutral beam injection heating [1–3] and ion guns for proton accelerators [4–7]. Particularly, in magnetically confined fusion devices (tokamaks), negative ion can be used to generate a fast neutral beam through interaction with a stripping gas target. Surface production of the negative hydrogen in low-pressure caesium-free plasma is of interest from a fundamental point of view since the use of caesium complicates the ion-source operation and re quires the careful stabilization of caesium injection and discharge pa rameters [8]. Therefore alternative solutions would be highly valuable. One of the solutions to caesium was found in well-controlled beam experiments. Substantial fractions of fast atoms or ions were converted to negative ions during grazing scattering from a clean and flat mono crystalline surface of alkali metal halides. Due to the bandgap of the insulator, the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional forma tion [9]. There is an extensive literature on charge exchange in experi ments at grazing incidence. A variety of different phenomena are observed, when atoms and ions are impinging on surfaces over a
projectile energy scale ranging from the thermal (meV) to the high en ergy (MeV) domain [10]. Another solution to caesium is using the sur face plasma sources, where negative ions were created on a negatively biased cathode facing the extractor. The cathode was usually made of low work function materials and it has been shown that barium could be as efficient as caesium [11–13]. Besides barium, the carbon materials are also be used as a surface material. Negative ions formed on the graphite [14], diamond and hydrogenated-diamond [15] surfaces upon positive ion bombardment were detected according to their energy by the mass spectrometer. In recent years, the studies about interactions of hydrogen atoms with metal surfaces play an essential role in many technologies, including heterogeneous catalysis, hydrogen storage, etc. In our previ ous study, the mechanism on the production of atomic hydrogen ions in a channel of porous Ni sheet was studied [16]. Molecular hydrogen ions produced by using the Penning source can be split into atomic hydrogen ions with a thin porous Ni sheet. It was subsequently found that negative atomic hydrogen ions can be detected behind the porous Ni sheet. The production of negative and positive hydrogen ions by using a Penning ion source and a porous nickel sheet is studied in the present paper.
* Corresponding author. E-mail address: [email protected] (Y. Zou). https://doi.org/10.1016/j.vacuum.2019.108982 Received 8 July 2019; Received in revised form 29 September 2019; Accepted 30 September 2019 Available online 1 October 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 1. The porous Ni sheet.
Fig. 4. Hydrogen located on the (a) top site, (b) bridge site, (c) three fold site, (d) four fold site of the cubic octahedral Ni13.
Fig. 2. Schematic diagram of analyzing composition of the ions by using the omegatron. The omegatron consists of a pair of high-frequency plates (1), an ion collector (2), two trapping plates (3), and a plasma collection plate (4), a Keithley 2450 (5).
Fig. 5. The influence of Vp on the measured resonance current vs. RF fre quency. The dot-lines indicate the experimental resonant frequencies for Hþ 3, þ Hþ 2 , H and H , respectively.
from Xiamen Tob New Energy Technology Co., Ltd. The structural pa rameters are a pore size of 0.45 mm, a specific surface area of 1.86 m2/g, and a porosity of 96.6%. The porous Ni is 1.4 mm thick and appears completely opaque, as shown in Fig. 1. The hydrogen plasma is gener ated by the Penning source [17]. The area of the beam is 0.785 cm2. The operating pressure in the source is about 0.2 Pa. The composition of ions is analyzed by the omegatron mass spectrometer [18,19] through the use of ion cyclotron resonance. Fig. 2 shows a schematic of the key features of the omegatron probe. The schematic diagram is not drawn to scale. Plasma potential for the Penning source was measured using a Langmuir probe. The incident energy was controlled by setting Vp. In this paper, Vo, Ve and Vc were set equally to Vp, which allows positive ions or negative ions diffuse from the back of porous Ni to inward of the omegatron. Vo, Ve, Vc and Vp controls the potential of trapping plates, plasma collection plate, ion collector and porous Ni respectively. In an omegatron, a high-frequency electric field is set up normal to a uniform d.c. magnetic field and ions are excited at their various ion cyclotron resonances, causing their orbital radii to increase. The high-frequency voltage Vpp (Voltage Peak-Peak) was 5 V. The current (Ic) is measured when they strike a collector in the omegatron. A spectrum of Ic as a function of the frequency is obtained by slowly sweeping the frequency. The composition and charge of the ions were determined from Ic vs frequency curve. The current Ic was recorded by a Keithley 2450. Based on the calculating formulas of cyclotron frequency,
Fig. 3. Hydrogen atoms located on the (a) top site, (b) bridge site, (c) center site of the icosahedral Ni13.
2. Experiments and calculations 2.1. Experiments The experiment measures the composition and charge of the ions after passing thorough the porous Ni. The porous Ni sheet was purchased 2
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 6. The electron density contour line map of hydrogen on the surface of icosahedron Ni13.
f¼
qB 2π M
2.2. Computational details
(1)
The constrained density functional theory (cDFT) [20] is an external potential to KS (Kohn-Sham) equations which forces predefined atoms to carry a specified charge or magnetization. The cDFT calculation uses
þ þ the ion cyclotron frequencies of Hþ, Hþ 2 and H3 are calculated to be H , þ þ 912 kHz; H2 , 456 kHz; and H3 , 304 kHz assuming B ¼ 600 G, respectively.
3
S. Zhao et al.
Vacuum 171 (2020) 108982
energy calculations by spin-polarized cDFT method in the PBE0 [28] scheme that introduces a 25% HF exchange term based on Per dew Burke Ernzerhof (PBE) functional [29]. The Los Alamos National Laboratory double-ζ(LANL2DZ) [30] and effective core potentials (ECP) and basis set were used for Ni where the outer most 18 electrons of free Ni atom (3s23p63d94s1) are treated explicitly. The LANL2DZ basis sets have 22 functions of types 3s3p2d. The standard 6-31G** [31] all electron basis set is selected for the H atom. After constraining the charge, we calculated the energy of the nickel-hydrogen system for the four spin multiplicities. The chosen number of spin multiplicities is equal to the number of alpha electrons minus beta electrons plus 1. Then, the charges were analyzed with the Lowdin charge. Considering the calculation accuracy and speed, for all atomic calculations, we used medium mesh precision with a total energy target accuracy of 10 5 hartree (Ha). In all calculations, the maximum number of self-consistent field (SCF) iterations is set to 2000, and the maxiter of geometry opti mization steps is set to 500. Ni clusters are typical ferromagnetic transition-metal clusters in the 3d group. The resultant spin magnetic moment in Ni with 8 electrons in the d shell of the Ni atom will be 2 Bohr magneton (μB). However, in the cluster, the average magnetic moment per atom will be different from the value of a free atom due to the in teractions among the electrons. The magnetic moment obtained from our calculation of the icosahedral Ni13 and cubic octahedral Ni13 is 0.94 μB and 0.89 μB respectively, which is in good agreement with the experimental results [32,33].
Table 1 The bond forming distances of hydrogen atoms on Ni13 clusters. icosahedron octahedron
Top site
Bridge site
Center site
Four-fold site
2.6 Å 2.4 Å
1.4 Å 1.4 Å
1.4 Å 1.2 Å
1.6 Å
Fig. 7. LMOs map of the hydrogen on the top site at the critical distance of icosahedral nickel cluster.
3. Results and discussions The kinetic energy of the hydrogen beam was controlled by the electrical potential difference between the Penning source and the porous Ni. The potential of hydrogen plasma leaving Penning source is 29.6 V, which was measured by a Langmuir probe. In such a situation, the energy of the incident beam irradiated to Ni sheet is about 80 eV, 130 eV and 180 eV when Vp is 50 V, 100 V and 150 V respectively. The current density of hydrogen ions beam incident on a porous nickel plate is 38 μA/cm2. The current density of negative and positive hydrogen ions from the back of the porous nickel plate are 1.54 μA/cm2 and 5.73 μA/cm2, respectively. Because the Ni sheet and the collector had the same potential, the ions released at the back of Ni sheet defused into the omegatron. It was found in Fig. 5 that the charge of ions measured behind the porous Ni get the influence of incident energy. When electrical potential of the porous Ni was set to 50 and 100 V, a negative resonant current (Ic) was measured. The measurement using the omegatron mass spectrometer has ruled out the possibility of the existence of electron causing the negative current. The ion is identified as H according to the resonance frequency and the field. After increase the potential to 150 V, the measured Ic was caused by positive hydrogen ions. It should be pointed out that Ic could be a net current of negative ions and positive ions. In other words, low incident energy could cause the production of dominant negative ions; high incident energy could cause the production of dominant positive ions. During the process of hydrogen comes near or turn away from Ni surface, charge transfer could happen only when the electronic cloud of hydrogen atoms overlap with the electronic cloud of Ni surface. In other words, the problem concerns theoretical justification of the chemical bond forming or breaking. Due to difference of the surface symmetry, the bond forming distance should be different for the Ni clusters shown in Figs. 3 and 4. In this paper, two approaches were used. It is known that it’s electron density between the nuclei that holds two atoms together in a bond. The first one, the critical distance of bond forming or breaking was studied by plotting the electron distribution density map [34] of Ni13–H system. The electron densities being defined in equation (2) was calculated using Multiwfn [35].
Fig. 8. LMOs map of the hydrogen on top site at the critical distance of cubic octahedral nickel cluster.
Gaussian basis sets for finite systems, which is unable to handle the periodic boundary conditions of a crystal Ni. The next best substitution for a porous Ni surface is a Ni cluster. There are as yet a lot of experi ences [21–25] on how to model a surface by a finite cluster. The Ni13 cluster provides adsorption sites similar to the planar Ni (111) surface [26], including the 1-fold top, 2-fold bridge, and 3-fold adsorption sites. Based on the restriction of computation tool and above related reports, we selected Ni13 cluster which has a magic number of atoms and exhibits high binding energy and structural stability as compared with those of other members in the Ni cluster family. The Ni13 cluster has two con figurations of an icosahedron and a cubic octahedron. Geometrically, the Ni13 cluster consists of a core nickel atom surrounded by 12 neigh bours. For the octahedral Ni13, the surface Ni–Ni bond length is 2.49 Å, and the bond angles are 60� and 90� respectively; for the icosahedral Ni13, the surface Ni–Ni bond length is 2.63 Å, and the bond angles are 60� and 108� , respectively. The hydrogen atoms located on the top site, the bridge site and the center site respectively for the icosahedral configuration, as shown in Fig. 3. For the cubic octahedral configura tion, hydrogen atoms located on the high symmetry sites are the top site, the bridge site, the three fold site and the four fold site, as shown in Fig. 4. NWChem program package [27] has been used to perform total 4
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 9. The charge of the hydrogen vs. energy of the H–Ni13 system. The hydrogen atom is above the bridge site of the icosahedron (a) and cubic octahedron (b) Ni13.
Fig. 10. The charge of hydrogen when hydrogen atom is at different height from icosahedron Ni13 (a) and cubic octahedron Ni13 (b).
� �2 � X ��X � ρðrÞ ¼ ηi jφi ðrÞj ¼ ηi � Cl;i χ i ðrÞ� � � i l 2
Ni13 no longer existing is above 1.4 Å above the center site. A similar analysis is made on the critical distance that the hydrogen atom can transfer charge on the surface of the cubic octahedron Ni13. The results are summarized in Table 1. Not only the charge density but also the localized molecular orbitals (LMOs) [37] gives valuable information about the chemical bond. There are many ways to localize molecular orbitals. The Pipek-Mezey (PM) localization was used in this paper by using Multiwfn software. The orbitals corresponding to Ni–H bond for H at the top site of the icosa hedral nickel cluster and octahedral nickel cluster are shown in Fig. 7 and Fig. 8 respectively. The blue isosurface and green isosurfaces clearly portray the 1s orbital of H and 3d orbital of Ni. They form a sigma bond. After moving the hydrogen atoms in Figs. 7a and 8a 0.2 Å away from the top site nickel, the occupied LMO of Ni–H can’t be found as shown in Figs. 7b and 8b, which means that Ni–H bond breaks. The slightly dis torted molecular orbital of hydrogen atoms due to the charge polariza tion effect of nickel also represent the interaction between Ni–H is weak
(2)
where ηi is occupation number of orbital {i}, ϕ is orbital wave function, χ is the basis function, and C is a coefficient matrix. The results are shown in Fig. 6. According to the experimental data of L.W. Wang et al. [36], the average bond length of Ni–H is 1.54 Å, and H–H is 0.741 Å. Hydrogen atoms were put above the icosahedron surface on top site (Fig. 6a), bridge site (Fig. 6b), and center site (Fig. 6c) with distance around a bond length of Ni–H from 0.8 Å to 2.8 Å. It could be found from Fig. 5a that the critical distance is 2.6 Å for the top site. When the hydrogen atom is located at 1.4 Å above the bridge, the electron density contours overlap; while at 1.6 Å, the electron density is small, and the electron density lines do not overlap, indicating that there is basically no electron transfer at this height. Similarly, charge transfer between hydrogen and 5
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 11. The charge of hydrogen when hydrogen atom is at different height from compressed icosahedron Ni13 (a) and compressed cubic octahedron Ni13 (b).
at this distance. The critical distances of breaking the Ni–H bonds were studied by plotting the LMO for H atoms at the bridge site, the center site and the four-fold site respectively. The results show that the critical distances obtained by electron density contour line map and LMOs graph are consistent. After comparing the electron density contour line map and LMOs maps between nickel and hydrogen, we conclude the maximum distance is 2.6 Å at top site. The bridge site is 1.4 Å, and the three fold site is 1.4 Å, and the four fold site is 1.6 Å. The above methods are the same as the farthest distance obtained from the graph of electron contour line density. After the range of bond-forming distance had been calculated, the charge distributions on several H–Ni13 configurations were studied with different distance between H and Ni13. In DFT, the energy E is obtained through the minimization of the DFT energy functional with respect to the electron density. In constrained DFT, this minimization is carried out under additional constraint on the charge density. These constraints are imposed on atom groups specified by the user. In this paper, charge was constrained to Ni13 and H, respectively. The total energy of a system was calculated when 1 to 1 unit charge constrains on a hydrogen atom and the nickel cluster respectively. When the charge constraint on H is 1, it means that the nickel cluster transfers an electron to H, and vice versa. It was found that when too much positive or negative charge was bound on the hydrogen atom, the energy of the system would increase sharply. By adjusting the confinement of the charged quantity, the charge on the H at the lowest energy of the system can always be found. Several typical curves of the system energy vs. the constrained charge on hydrogen are shown in Fig. 9. Fig. 9a and b are incident H atoms at the bridge site of the icosahedron and the cubic octahedron, respectively. From the view of thermodynamics, charge distributions of the H–Ni13 configurations tend to occupy low energy states. From the curves, it can be found that hydrogen tends to become positively charged. The charge of hydrogen atoms were calculated with the same method for the systems shown in Figs. 3 and 4. The results are summarized in Fig. 10. It was found that hydrogen atoms became charged positively or negatively ions when they left from the surface of nickel clusters at different sites. When hydrogen leaves from cubic octahedral and icosahedral nickel clusters at bridge site, the hydrogen carries positive charge; when hydrogen leaves from cubic octahedral and icosahedral nickel clusters at the top site, hydrogen carries negative charges; when hydrogen leaves from cubic octahedral and icosahedral nickel clusters at
the central vacancies, positive hydrogen ions are obtained. In general, when the hydrogen atoms are away from the nickel surface, whether it is an octahedron or an icosahedral Ni13 cluster, the hydrogen atom will become positive and negative hydrogen ions on the surface of the nickel cluster. Considering the electron density maps of the bridge site shown in Fig. 6, it can be found that the electron density between nickel and hydrogen decreases as hydrogen moves away from the nickel surface, which is consistent with the overall trend of hydrogen-constrained electricity we get. The electron density at the top site of the two con figurations is the highest, which causes the reflecting H leaving from here charged negatively. While the electron density of the bridge and vacancy is less than that of the top position, therefore the charge of hydrogen is positive. Since the incident hydrogen atoms were accelerated, the surface structure should be distorted due to collision. We assumed that the surface atoms of the Ni cluster were slightly pressed into the nickel cluster. The charge transfer was studied based on this situation. It is found from Fig. 11 that the distortion does not cause significant changes of charge on the leaving hydrogen atoms. Although the production of the positive and negative hydrogen atoms is explained by using the cDFT calculation, the influence of incident energy on the changes of the yield of Hþ and H is failed to explain. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms [38]. 4. Conclusion The incident energy of the hydrogen ions has an influence on the production of negative and positive hydrogen ions at the back of the porous Ni sheet. In our experiment, the current density of hydrogen ions beam produced by a penning source is 38 μA/cm2.The current density of negative and positive hydrogen ions from the back of the porous nickel plate are 1.54 μA/cm2 and 5.73 μA/cm2, respectively. The negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ions was ca. 80 eV and 130 eV. When the energy of incident ions increased to 180 eV, the positive ions dominated at the back of the porous Ni sheet. The mechanism for the production of negative and positive ions was studied by using cDFT. It is found that when the hydrogen atoms leave the Ni surface from top sites, the charge of hydrogen is negative, and the hydrogen atoms mainly get a positive 6
Vacuum 171 (2020) 108982
S. Zhao et al.
charge at the bridge site, center site etc. where the electron density is lower. The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, we failed to explain the influence of incident energy on the changes in the yield of Hþ and H .
[14] J.P.J. Dubois, K. Achkasov, D. Kogut, A. Ahmad, J.M. Layet, A. Simonin, G. Cartry, Negative-ion surface production in hydrogen plasmas: determination of the negative-ion energy and angle distribution function using mass spectrometry, J. Appl. Phys. 119 (2016) 193301. https://doi.org/10.1063/1.4948949. [15] Gilles Cartry, et al., Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas, New J. Phys. 19 (2017), 025010. https://doi.org/10.1088/1367-2630/aa5f1f. [16] Shuangwen Zhao, Changyong Zhan, Zhiyou Zhang, Yu Zou, Splitting of low energy molecular hydrogen ion beam in the channel of porous Ni and its mechanism study, Phys. Plasmas ,(unpublished results). [17] Da-Zhi Jin, Zhong-Hai Yang, Ping-Ying Tang, Kun-xiang Xiao, Jing-yi Dai, Hydrogen plasma diagnosis in penning ion source by optical emission spectroscopy, Vacuum 83 (2009) 451–453. https://doi.org/10.1016/j.vacuum.200 8.05.009. [18] A. Tonegawa, T. Nishijima, H. Ishioka, A. Nakanowatari, K. Kawamura, Omegatron mass spectrometer in a hydrogen/deuterium detached plasma, AIP Conf. proc. 988 (2008) 234–237. https://doi.org/10.1063/1.1699459. [19] Tatsuya Hayashi, Toshikio Takimoto, Akira Tonegawa, Yoshihito Matsumura, Kohnosuke Sato, Kazutaka Kawamura, Retention and permeation properties of deuterium in tungsten under irradiation of the D–He mixture plasma, Fusion Eng. Des. 136 (2018) 545–548. https://doi.org/10.1016/j.fusengdes.2018.03.017. [20] Qin Wu, Troy VanVoorhis, Direct optimization method to study constrained systems within density-functional theory, Phys. Rev. A 72 (2005), 024502, htt ps://doi.org/10.1103/PhysRevA.72.024502. [21] P.E.M. Siegbahn, Margareta R.A. Blomberg, The dissociation of H2 on the Ni(100) surface, J. Chem. Phys. 81 (1984) 2103. https://doi.org/10.1063/1.447834. [22] C.W. Bauschlicher, P.S. Bagus, H.F. Schaefer, Model study in chemisorption: molecular orbital cluster theory for atomic hydrogen on Be(0001), IBM J. Res. Dev. 22 (1978) 213. https://doi.org/10.1147/rd.223.0213. [23] T.H. Upton, W.A. Goddard, Chemisorption of atomic hydrogen on large-nickelcluster surfaces, Phys. Rev. Lett. 42 (1979) 472. T. H. Upton and W. A. Goddard, Critical Reviews in Solid State and Materials Sciences (CRC, Boca Raton, FL, 1981), and references therein, https://doi.org/10.1103/PhysRevLett.42.472. [24] K. Hermann, H.J. Hass, Chemisorption properties in the surface cluster approach: size dependence of adsorbate binding energies for CO/Al(100), Surf. Sci. 189–190 (1987) 426–430. https://doi.org/10.1016/S0039-6028(87)80463-6. [25] T. Jacob, D. Geschke, S. Fritzsche, W.-D. Sepp, B. Fricke, J. Anton, S. Varga, Adsorption on surfaces simulated by an embedded cluster approach within the relativistic density functional theory, Surf. Sci. 486 (2001) 194–202. https://doi. org/10.1016/S0039-6028(01)01046-9. [26] Bin Liu, Mark T. Lusk, James F. Ely, Influence of nickel catalyst geometry on the dissociation barriers of H2 and CH4: Ni13 versus Ni(111), J. Phys. Chem. C 113 (2009) 13715–13722. https://doi.org/10.1021/jp900319. [27] M. Valiev, E. Bylaska, N. Govind, K. Kowalski, T. Straatsma, H. van Dam, D. Wang, J. Nieplocha, E. Apra, T. Windus, et al., NWChem:a comprehensive and scalable open-source solution for larges cale molecular simulations, Comput. Phys. Commun. 181 (2010) 1477–1489. https://doi.org/10.1016/j.cpc.2010.04.018. [28] Carlo Adamo, Vincenzo Barone, Toward reliable density functional methods without adjustable parameters: the PBE0 model, J. Chem. Phys. 110 (1999) 6158–6170. https://doi.org/10.1063/1.478522. [29] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. https://doi.org/10.1103/Ph ysRevLett.77.3865. [30] P.J. Hay, W.R. Wadt, Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg, J. Chem. Phys. 82 (1985) 270–283. https://doi.org/10.1063/1.448799. [31] P.C. Hariharan, J.A. Pople, The influence of polarization functions on molecular orbital hydrogenation energies, Theor. Chim. Acta 28 (1973) 213–222. https://doi. org/10.1007/BF00533485. [32] S.E. Apsel, J.W. Emmert, J. Deng, L.A. Bloomfield, Surface-enhanced magnetism in nickel clusters, Phys. Rev. Lett. 76 (1996) 1441–1444. https://doi.org/10.1103/Ph ysRevLett.76.1441. [33] Wei Song, Wen-Cai Lu, C.Z. Wang, K.M. Ho, Magnetic and electronic properties of the nickel clusters Nin (n⩽30), Comput. Theor. Chem. 978 (2011) 41–46. https ://doi.org/10.1016/j.comptc.2011.09.028. [34] L.U. Tian, Qinxue CHEN, Revealing molecular electronic structure via analysis of valence electron density[J], Acta Phys. - Chim. Sin. 34 (2018) 503–513. htt ps://doi.org/10.3866/PKU.WHXB201709252. [35] Tian Lu, Feiwu Chen, Multiwfn: a multifunctional wavefunction analyzer, J. Comput. Chem. 33 (2012) 580–592. https://doi.org/10.1002/jcc.22885. [36] W.R. Myers, L.-W. Wang, T.J. Richardson, M.D. Rubin, Calculation of thermodynamic, electronic, and optical properties of monoclinic Mg2NiH4, J. Appl. Phys. 91 (2002) 4879. https://doi.org/10.1063/1.1454206. [37] J. Pipek, P.G. Mezey, A fast intrinsic localization procedure applicable for ab initio and semiempirical linear combination of atomic orbital wave functions, J. Chem. Phys. 90 (1989) 4916. https://doi.org/10.1063/1.456588. [38] Hatcher Ryan, Matthew Beck, Alan Tackett, Sokrates T. Pantelides, Dynamical effects in the interaction of ion beams with solids, Phys. Rev. Lett. 100 (2008) 103201. https://doi.org/10.1103/PhysRevLett.100.103201.
Funding This study was financially sponsored by National Natural Science Foundation of China (Nos. 11205107 and 11405111) and Open Research Fund of Key Laboratory of Radiation and Technology of Edu cation Ministry of China (2018SCURPT12). Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “The mechanism of negative and positive hydrogen ions pro duction on the Ni surface”. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.vacuum.2019.108982. References [1] U. Fantz, P. Franzen, D. Wünderlich, Development of negative hydrogen ion sources for fusion: experiments and modeling, Chem. Phys. 398 (2012) 7–16. https://doi.org/10.1016/j.chemphys.2011.05.006. [2] L. Schiesko, P. McNeely, U. Fantz, P. Franzen, NNBI Team, Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source, Plasma Phys. Control. Fusion 53 (2011), 085029. https://doi.org/10.1088/0741-3335/53/8/085029. [3] Ryuta Endo, Shogo Ishihara, Toshikio Takimoto, Akira Tonegawa, Kohnosuke Sato, Kazutaka Kawamura, Production of hydrogen negative ions in high density sheet plasma, AIP Conf. proc. 2018 (2011), 020009. https://doi.org/10.1063/ 1.5053251. [4] A. Ueno, et al., Interesting experimental results in Japan proton accelerator research complex H ion-source development (invited), Rev. Sci. Instrum. 81 (2010), 02A720. https://doi.org/10.1063/1.3271243. [5] D.P. Moehs, J. Peters, J. Sherman, Negative hydrogen ion sources for accelerators, IEEE Trans. Plasma Sci. 33 (2005) 1786–1798. https://doi.org/10.1109/TPS.200 5.860067. [6] J. Lettry, D. Aguglia, P. Andersson, S. Bertolo, A. Butterworth, Y. Coutron, et al., Status and operation of the Linac4 ion source prototypes, Rev. Sci. Instrum. 85 (2014), 02B122. https://doi.org/10.1063/1.4848975. [7] A. Ueno, K. Ikegami, Y. Kondo, Surface production dominating Cs-free H ion source for high intensity and high energy proton accelerators, Rev. Sci. Instrum. 75 (2004) 1714. https://doi.org/10.1063/1.1699459. [8] A. Latuszy� nski, K. Kornarzy� nski, A. Dro�zdziel, K. Pyszniak, D. Maçzka, Negative ion beams from a plasma type source with additional surface ionization, Vacuum 47 (1996) 1219. https://doi.org/10.1016/0042-207X(96)00130-3. [9] C. Auth, A. Mertens, H. Winter, A.G. Borisov, V. Sidis, Formation of negative ions in grazing scattering from insulator surfaces, Phys. Rev. A 57 (1998) 351–361. https://doi.org/10.1103/PhysRevA.57.351. [10] H. Winter, Collisions of atoms and ions with surfaces under grazing incidence, Phys. Rep. 367 (2002), 387-582, https://doi.org/10.1016/S0370-1573(02) 00010-8. [11] R.M.A. Heeren, et al., Angular and energy distributions of surface produced H and D ions in a barium surface conversion source, J. Appl. Phys. 75 (1994) 4340. htt ps://doi.org/10.1063/1.355977. [12] C.F.A. van Os, W.B. Kunkel, C. Leguijt, J. Los, Modeling of H surface conversion sources; binary (H-Ba) and ternary (H-Cs/W) converter arrangements, J. Appl. Phys. 70 (1991) 2575. https://doi.org/10.1063/1.349365. [13] C.F.A. van Os, P.W. van Amersfoort, J. Los, Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam, J. Appl. Phys. 64 (1988) 3863. https://doi.org/10.1063/1.341340.
7
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
The mechanism of negative and positive hydrogen ions production on the Ni surface ShuangWen Zhao a, KaiYuan Wang a, JianChun Wu b, ChangYong Zhan a, Yu Zou a, * a b
Key Laboratory of Radiation and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, 610064, China School of Material Science and Engineering, Jiangsu University, 301, Xuefu Road, Zhenjiang, 212013, Jiangsu Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ion surface interaction Porous Ni sheet Negative hydrogen ions Constrained density functional theory
The studies about surface production of hydrogen negative ions play an important role in many technologies. In the present paper, a low energy molecular hydrogen ion beam was irradiated to the porous Ni sheet. It was found that negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ion beam was lower than 180 eV. When the energy of the incident ions increased, the positive ions dominated at the back of the porous Ni sheet. The production of negative hydrogen on Ni surface can’t be explained by comparing the electron affinity of Ni and H. The charge transferring during the process of H atoms reflected from Ni surface was calculated by using constrained DFT (cDFT). The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, the influence of incident energy on the production of negative and positive hydrogen ions is unable to be explained by cDFT. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms.
1. Introduction The research and the design of hydrogen negative ion sources have been extensively performed in connection with neutral beam injection heating [1–3] and ion guns for proton accelerators [4–7]. Particularly, in magnetically confined fusion devices (tokamaks), negative ion can be used to generate a fast neutral beam through interaction with a stripping gas target. Surface production of the negative hydrogen in low-pressure caesium-free plasma is of interest from a fundamental point of view since the use of caesium complicates the ion-source operation and re quires the careful stabilization of caesium injection and discharge pa rameters [8]. Therefore alternative solutions would be highly valuable. One of the solutions to caesium was found in well-controlled beam experiments. Substantial fractions of fast atoms or ions were converted to negative ions during grazing scattering from a clean and flat mono crystalline surface of alkali metal halides. Due to the bandgap of the insulator, the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional forma tion [9]. There is an extensive literature on charge exchange in experi ments at grazing incidence. A variety of different phenomena are observed, when atoms and ions are impinging on surfaces over a
projectile energy scale ranging from the thermal (meV) to the high en ergy (MeV) domain [10]. Another solution to caesium is using the sur face plasma sources, where negative ions were created on a negatively biased cathode facing the extractor. The cathode was usually made of low work function materials and it has been shown that barium could be as efficient as caesium [11–13]. Besides barium, the carbon materials are also be used as a surface material. Negative ions formed on the graphite [14], diamond and hydrogenated-diamond [15] surfaces upon positive ion bombardment were detected according to their energy by the mass spectrometer. In recent years, the studies about interactions of hydrogen atoms with metal surfaces play an essential role in many technologies, including heterogeneous catalysis, hydrogen storage, etc. In our previ ous study, the mechanism on the production of atomic hydrogen ions in a channel of porous Ni sheet was studied [16]. Molecular hydrogen ions produced by using the Penning source can be split into atomic hydrogen ions with a thin porous Ni sheet. It was subsequently found that negative atomic hydrogen ions can be detected behind the porous Ni sheet. The production of negative and positive hydrogen ions by using a Penning ion source and a porous nickel sheet is studied in the present paper.
* Corresponding author. E-mail address: [email protected] (Y. Zou). https://doi.org/10.1016/j.vacuum.2019.108982 Received 8 July 2019; Received in revised form 29 September 2019; Accepted 30 September 2019 Available online 1 October 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 1. The porous Ni sheet.
Fig. 4. Hydrogen located on the (a) top site, (b) bridge site, (c) three fold site, (d) four fold site of the cubic octahedral Ni13.
Fig. 2. Schematic diagram of analyzing composition of the ions by using the omegatron. The omegatron consists of a pair of high-frequency plates (1), an ion collector (2), two trapping plates (3), and a plasma collection plate (4), a Keithley 2450 (5).
Fig. 5. The influence of Vp on the measured resonance current vs. RF fre quency. The dot-lines indicate the experimental resonant frequencies for Hþ 3, þ Hþ 2 , H and H , respectively.
from Xiamen Tob New Energy Technology Co., Ltd. The structural pa rameters are a pore size of 0.45 mm, a specific surface area of 1.86 m2/g, and a porosity of 96.6%. The porous Ni is 1.4 mm thick and appears completely opaque, as shown in Fig. 1. The hydrogen plasma is gener ated by the Penning source [17]. The area of the beam is 0.785 cm2. The operating pressure in the source is about 0.2 Pa. The composition of ions is analyzed by the omegatron mass spectrometer [18,19] through the use of ion cyclotron resonance. Fig. 2 shows a schematic of the key features of the omegatron probe. The schematic diagram is not drawn to scale. Plasma potential for the Penning source was measured using a Langmuir probe. The incident energy was controlled by setting Vp. In this paper, Vo, Ve and Vc were set equally to Vp, which allows positive ions or negative ions diffuse from the back of porous Ni to inward of the omegatron. Vo, Ve, Vc and Vp controls the potential of trapping plates, plasma collection plate, ion collector and porous Ni respectively. In an omegatron, a high-frequency electric field is set up normal to a uniform d.c. magnetic field and ions are excited at their various ion cyclotron resonances, causing their orbital radii to increase. The high-frequency voltage Vpp (Voltage Peak-Peak) was 5 V. The current (Ic) is measured when they strike a collector in the omegatron. A spectrum of Ic as a function of the frequency is obtained by slowly sweeping the frequency. The composition and charge of the ions were determined from Ic vs frequency curve. The current Ic was recorded by a Keithley 2450. Based on the calculating formulas of cyclotron frequency,
Fig. 3. Hydrogen atoms located on the (a) top site, (b) bridge site, (c) center site of the icosahedral Ni13.
2. Experiments and calculations 2.1. Experiments The experiment measures the composition and charge of the ions after passing thorough the porous Ni. The porous Ni sheet was purchased 2
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 6. The electron density contour line map of hydrogen on the surface of icosahedron Ni13.
f¼
qB 2π M
2.2. Computational details
(1)
The constrained density functional theory (cDFT) [20] is an external potential to KS (Kohn-Sham) equations which forces predefined atoms to carry a specified charge or magnetization. The cDFT calculation uses
þ þ the ion cyclotron frequencies of Hþ, Hþ 2 and H3 are calculated to be H , þ þ 912 kHz; H2 , 456 kHz; and H3 , 304 kHz assuming B ¼ 600 G, respectively.
3
S. Zhao et al.
Vacuum 171 (2020) 108982
energy calculations by spin-polarized cDFT method in the PBE0 [28] scheme that introduces a 25% HF exchange term based on Per dew Burke Ernzerhof (PBE) functional [29]. The Los Alamos National Laboratory double-ζ(LANL2DZ) [30] and effective core potentials (ECP) and basis set were used for Ni where the outer most 18 electrons of free Ni atom (3s23p63d94s1) are treated explicitly. The LANL2DZ basis sets have 22 functions of types 3s3p2d. The standard 6-31G** [31] all electron basis set is selected for the H atom. After constraining the charge, we calculated the energy of the nickel-hydrogen system for the four spin multiplicities. The chosen number of spin multiplicities is equal to the number of alpha electrons minus beta electrons plus 1. Then, the charges were analyzed with the Lowdin charge. Considering the calculation accuracy and speed, for all atomic calculations, we used medium mesh precision with a total energy target accuracy of 10 5 hartree (Ha). In all calculations, the maximum number of self-consistent field (SCF) iterations is set to 2000, and the maxiter of geometry opti mization steps is set to 500. Ni clusters are typical ferromagnetic transition-metal clusters in the 3d group. The resultant spin magnetic moment in Ni with 8 electrons in the d shell of the Ni atom will be 2 Bohr magneton (μB). However, in the cluster, the average magnetic moment per atom will be different from the value of a free atom due to the in teractions among the electrons. The magnetic moment obtained from our calculation of the icosahedral Ni13 and cubic octahedral Ni13 is 0.94 μB and 0.89 μB respectively, which is in good agreement with the experimental results [32,33].
Table 1 The bond forming distances of hydrogen atoms on Ni13 clusters. icosahedron octahedron
Top site
Bridge site
Center site
Four-fold site
2.6 Å 2.4 Å
1.4 Å 1.4 Å
1.4 Å 1.2 Å
1.6 Å
Fig. 7. LMOs map of the hydrogen on the top site at the critical distance of icosahedral nickel cluster.
3. Results and discussions The kinetic energy of the hydrogen beam was controlled by the electrical potential difference between the Penning source and the porous Ni. The potential of hydrogen plasma leaving Penning source is 29.6 V, which was measured by a Langmuir probe. In such a situation, the energy of the incident beam irradiated to Ni sheet is about 80 eV, 130 eV and 180 eV when Vp is 50 V, 100 V and 150 V respectively. The current density of hydrogen ions beam incident on a porous nickel plate is 38 μA/cm2. The current density of negative and positive hydrogen ions from the back of the porous nickel plate are 1.54 μA/cm2 and 5.73 μA/cm2, respectively. Because the Ni sheet and the collector had the same potential, the ions released at the back of Ni sheet defused into the omegatron. It was found in Fig. 5 that the charge of ions measured behind the porous Ni get the influence of incident energy. When electrical potential of the porous Ni was set to 50 and 100 V, a negative resonant current (Ic) was measured. The measurement using the omegatron mass spectrometer has ruled out the possibility of the existence of electron causing the negative current. The ion is identified as H according to the resonance frequency and the field. After increase the potential to 150 V, the measured Ic was caused by positive hydrogen ions. It should be pointed out that Ic could be a net current of negative ions and positive ions. In other words, low incident energy could cause the production of dominant negative ions; high incident energy could cause the production of dominant positive ions. During the process of hydrogen comes near or turn away from Ni surface, charge transfer could happen only when the electronic cloud of hydrogen atoms overlap with the electronic cloud of Ni surface. In other words, the problem concerns theoretical justification of the chemical bond forming or breaking. Due to difference of the surface symmetry, the bond forming distance should be different for the Ni clusters shown in Figs. 3 and 4. In this paper, two approaches were used. It is known that it’s electron density between the nuclei that holds two atoms together in a bond. The first one, the critical distance of bond forming or breaking was studied by plotting the electron distribution density map [34] of Ni13–H system. The electron densities being defined in equation (2) was calculated using Multiwfn [35].
Fig. 8. LMOs map of the hydrogen on top site at the critical distance of cubic octahedral nickel cluster.
Gaussian basis sets for finite systems, which is unable to handle the periodic boundary conditions of a crystal Ni. The next best substitution for a porous Ni surface is a Ni cluster. There are as yet a lot of experi ences [21–25] on how to model a surface by a finite cluster. The Ni13 cluster provides adsorption sites similar to the planar Ni (111) surface [26], including the 1-fold top, 2-fold bridge, and 3-fold adsorption sites. Based on the restriction of computation tool and above related reports, we selected Ni13 cluster which has a magic number of atoms and exhibits high binding energy and structural stability as compared with those of other members in the Ni cluster family. The Ni13 cluster has two con figurations of an icosahedron and a cubic octahedron. Geometrically, the Ni13 cluster consists of a core nickel atom surrounded by 12 neigh bours. For the octahedral Ni13, the surface Ni–Ni bond length is 2.49 Å, and the bond angles are 60� and 90� respectively; for the icosahedral Ni13, the surface Ni–Ni bond length is 2.63 Å, and the bond angles are 60� and 108� , respectively. The hydrogen atoms located on the top site, the bridge site and the center site respectively for the icosahedral configuration, as shown in Fig. 3. For the cubic octahedral configura tion, hydrogen atoms located on the high symmetry sites are the top site, the bridge site, the three fold site and the four fold site, as shown in Fig. 4. NWChem program package [27] has been used to perform total 4
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 9. The charge of the hydrogen vs. energy of the H–Ni13 system. The hydrogen atom is above the bridge site of the icosahedron (a) and cubic octahedron (b) Ni13.
Fig. 10. The charge of hydrogen when hydrogen atom is at different height from icosahedron Ni13 (a) and cubic octahedron Ni13 (b).
� �2 � X ��X � ρðrÞ ¼ ηi jφi ðrÞj ¼ ηi � Cl;i χ i ðrÞ� � � i l 2
Ni13 no longer existing is above 1.4 Å above the center site. A similar analysis is made on the critical distance that the hydrogen atom can transfer charge on the surface of the cubic octahedron Ni13. The results are summarized in Table 1. Not only the charge density but also the localized molecular orbitals (LMOs) [37] gives valuable information about the chemical bond. There are many ways to localize molecular orbitals. The Pipek-Mezey (PM) localization was used in this paper by using Multiwfn software. The orbitals corresponding to Ni–H bond for H at the top site of the icosa hedral nickel cluster and octahedral nickel cluster are shown in Fig. 7 and Fig. 8 respectively. The blue isosurface and green isosurfaces clearly portray the 1s orbital of H and 3d orbital of Ni. They form a sigma bond. After moving the hydrogen atoms in Figs. 7a and 8a 0.2 Å away from the top site nickel, the occupied LMO of Ni–H can’t be found as shown in Figs. 7b and 8b, which means that Ni–H bond breaks. The slightly dis torted molecular orbital of hydrogen atoms due to the charge polariza tion effect of nickel also represent the interaction between Ni–H is weak
(2)
where ηi is occupation number of orbital {i}, ϕ is orbital wave function, χ is the basis function, and C is a coefficient matrix. The results are shown in Fig. 6. According to the experimental data of L.W. Wang et al. [36], the average bond length of Ni–H is 1.54 Å, and H–H is 0.741 Å. Hydrogen atoms were put above the icosahedron surface on top site (Fig. 6a), bridge site (Fig. 6b), and center site (Fig. 6c) with distance around a bond length of Ni–H from 0.8 Å to 2.8 Å. It could be found from Fig. 5a that the critical distance is 2.6 Å for the top site. When the hydrogen atom is located at 1.4 Å above the bridge, the electron density contours overlap; while at 1.6 Å, the electron density is small, and the electron density lines do not overlap, indicating that there is basically no electron transfer at this height. Similarly, charge transfer between hydrogen and 5
S. Zhao et al.
Vacuum 171 (2020) 108982
Fig. 11. The charge of hydrogen when hydrogen atom is at different height from compressed icosahedron Ni13 (a) and compressed cubic octahedron Ni13 (b).
at this distance. The critical distances of breaking the Ni–H bonds were studied by plotting the LMO for H atoms at the bridge site, the center site and the four-fold site respectively. The results show that the critical distances obtained by electron density contour line map and LMOs graph are consistent. After comparing the electron density contour line map and LMOs maps between nickel and hydrogen, we conclude the maximum distance is 2.6 Å at top site. The bridge site is 1.4 Å, and the three fold site is 1.4 Å, and the four fold site is 1.6 Å. The above methods are the same as the farthest distance obtained from the graph of electron contour line density. After the range of bond-forming distance had been calculated, the charge distributions on several H–Ni13 configurations were studied with different distance between H and Ni13. In DFT, the energy E is obtained through the minimization of the DFT energy functional with respect to the electron density. In constrained DFT, this minimization is carried out under additional constraint on the charge density. These constraints are imposed on atom groups specified by the user. In this paper, charge was constrained to Ni13 and H, respectively. The total energy of a system was calculated when 1 to 1 unit charge constrains on a hydrogen atom and the nickel cluster respectively. When the charge constraint on H is 1, it means that the nickel cluster transfers an electron to H, and vice versa. It was found that when too much positive or negative charge was bound on the hydrogen atom, the energy of the system would increase sharply. By adjusting the confinement of the charged quantity, the charge on the H at the lowest energy of the system can always be found. Several typical curves of the system energy vs. the constrained charge on hydrogen are shown in Fig. 9. Fig. 9a and b are incident H atoms at the bridge site of the icosahedron and the cubic octahedron, respectively. From the view of thermodynamics, charge distributions of the H–Ni13 configurations tend to occupy low energy states. From the curves, it can be found that hydrogen tends to become positively charged. The charge of hydrogen atoms were calculated with the same method for the systems shown in Figs. 3 and 4. The results are summarized in Fig. 10. It was found that hydrogen atoms became charged positively or negatively ions when they left from the surface of nickel clusters at different sites. When hydrogen leaves from cubic octahedral and icosahedral nickel clusters at bridge site, the hydrogen carries positive charge; when hydrogen leaves from cubic octahedral and icosahedral nickel clusters at the top site, hydrogen carries negative charges; when hydrogen leaves from cubic octahedral and icosahedral nickel clusters at
the central vacancies, positive hydrogen ions are obtained. In general, when the hydrogen atoms are away from the nickel surface, whether it is an octahedron or an icosahedral Ni13 cluster, the hydrogen atom will become positive and negative hydrogen ions on the surface of the nickel cluster. Considering the electron density maps of the bridge site shown in Fig. 6, it can be found that the electron density between nickel and hydrogen decreases as hydrogen moves away from the nickel surface, which is consistent with the overall trend of hydrogen-constrained electricity we get. The electron density at the top site of the two con figurations is the highest, which causes the reflecting H leaving from here charged negatively. While the electron density of the bridge and vacancy is less than that of the top position, therefore the charge of hydrogen is positive. Since the incident hydrogen atoms were accelerated, the surface structure should be distorted due to collision. We assumed that the surface atoms of the Ni cluster were slightly pressed into the nickel cluster. The charge transfer was studied based on this situation. It is found from Fig. 11 that the distortion does not cause significant changes of charge on the leaving hydrogen atoms. Although the production of the positive and negative hydrogen atoms is explained by using the cDFT calculation, the influence of incident energy on the changes of the yield of Hþ and H is failed to explain. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms [38]. 4. Conclusion The incident energy of the hydrogen ions has an influence on the production of negative and positive hydrogen ions at the back of the porous Ni sheet. In our experiment, the current density of hydrogen ions beam produced by a penning source is 38 μA/cm2.The current density of negative and positive hydrogen ions from the back of the porous nickel plate are 1.54 μA/cm2 and 5.73 μA/cm2, respectively. The negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ions was ca. 80 eV and 130 eV. When the energy of incident ions increased to 180 eV, the positive ions dominated at the back of the porous Ni sheet. The mechanism for the production of negative and positive ions was studied by using cDFT. It is found that when the hydrogen atoms leave the Ni surface from top sites, the charge of hydrogen is negative, and the hydrogen atoms mainly get a positive 6
Vacuum 171 (2020) 108982
S. Zhao et al.
charge at the bridge site, center site etc. where the electron density is lower. The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, we failed to explain the influence of incident energy on the changes in the yield of Hþ and H .
[14] J.P.J. Dubois, K. Achkasov, D. Kogut, A. Ahmad, J.M. Layet, A. Simonin, G. Cartry, Negative-ion surface production in hydrogen plasmas: determination of the negative-ion energy and angle distribution function using mass spectrometry, J. Appl. Phys. 119 (2016) 193301. https://doi.org/10.1063/1.4948949. [15] Gilles Cartry, et al., Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas, New J. Phys. 19 (2017), 025010. https://doi.org/10.1088/1367-2630/aa5f1f. [16] Shuangwen Zhao, Changyong Zhan, Zhiyou Zhang, Yu Zou, Splitting of low energy molecular hydrogen ion beam in the channel of porous Ni and its mechanism study, Phys. Plasmas ,(unpublished results). [17] Da-Zhi Jin, Zhong-Hai Yang, Ping-Ying Tang, Kun-xiang Xiao, Jing-yi Dai, Hydrogen plasma diagnosis in penning ion source by optical emission spectroscopy, Vacuum 83 (2009) 451–453. https://doi.org/10.1016/j.vacuum.200 8.05.009. [18] A. Tonegawa, T. Nishijima, H. Ishioka, A. Nakanowatari, K. Kawamura, Omegatron mass spectrometer in a hydrogen/deuterium detached plasma, AIP Conf. proc. 988 (2008) 234–237. https://doi.org/10.1063/1.1699459. [19] Tatsuya Hayashi, Toshikio Takimoto, Akira Tonegawa, Yoshihito Matsumura, Kohnosuke Sato, Kazutaka Kawamura, Retention and permeation properties of deuterium in tungsten under irradiation of the D–He mixture plasma, Fusion Eng. Des. 136 (2018) 545–548. https://doi.org/10.1016/j.fusengdes.2018.03.017. [20] Qin Wu, Troy VanVoorhis, Direct optimization method to study constrained systems within density-functional theory, Phys. Rev. A 72 (2005), 024502, htt ps://doi.org/10.1103/PhysRevA.72.024502. [21] P.E.M. Siegbahn, Margareta R.A. Blomberg, The dissociation of H2 on the Ni(100) surface, J. Chem. Phys. 81 (1984) 2103. https://doi.org/10.1063/1.447834. [22] C.W. Bauschlicher, P.S. Bagus, H.F. Schaefer, Model study in chemisorption: molecular orbital cluster theory for atomic hydrogen on Be(0001), IBM J. Res. Dev. 22 (1978) 213. https://doi.org/10.1147/rd.223.0213. [23] T.H. Upton, W.A. Goddard, Chemisorption of atomic hydrogen on large-nickelcluster surfaces, Phys. Rev. Lett. 42 (1979) 472. T. H. Upton and W. A. Goddard, Critical Reviews in Solid State and Materials Sciences (CRC, Boca Raton, FL, 1981), and references therein, https://doi.org/10.1103/PhysRevLett.42.472. [24] K. Hermann, H.J. Hass, Chemisorption properties in the surface cluster approach: size dependence of adsorbate binding energies for CO/Al(100), Surf. Sci. 189–190 (1987) 426–430. https://doi.org/10.1016/S0039-6028(87)80463-6. [25] T. Jacob, D. Geschke, S. Fritzsche, W.-D. Sepp, B. Fricke, J. Anton, S. Varga, Adsorption on surfaces simulated by an embedded cluster approach within the relativistic density functional theory, Surf. Sci. 486 (2001) 194–202. https://doi. org/10.1016/S0039-6028(01)01046-9. [26] Bin Liu, Mark T. Lusk, James F. Ely, Influence of nickel catalyst geometry on the dissociation barriers of H2 and CH4: Ni13 versus Ni(111), J. Phys. Chem. C 113 (2009) 13715–13722. https://doi.org/10.1021/jp900319. [27] M. Valiev, E. Bylaska, N. Govind, K. Kowalski, T. Straatsma, H. van Dam, D. Wang, J. Nieplocha, E. Apra, T. Windus, et al., NWChem:a comprehensive and scalable open-source solution for larges cale molecular simulations, Comput. Phys. Commun. 181 (2010) 1477–1489. https://doi.org/10.1016/j.cpc.2010.04.018. [28] Carlo Adamo, Vincenzo Barone, Toward reliable density functional methods without adjustable parameters: the PBE0 model, J. Chem. Phys. 110 (1999) 6158–6170. https://doi.org/10.1063/1.478522. [29] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. https://doi.org/10.1103/Ph ysRevLett.77.3865. [30] P.J. Hay, W.R. Wadt, Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg, J. Chem. Phys. 82 (1985) 270–283. https://doi.org/10.1063/1.448799. [31] P.C. Hariharan, J.A. Pople, The influence of polarization functions on molecular orbital hydrogenation energies, Theor. Chim. Acta 28 (1973) 213–222. https://doi. org/10.1007/BF00533485. [32] S.E. Apsel, J.W. Emmert, J. Deng, L.A. Bloomfield, Surface-enhanced magnetism in nickel clusters, Phys. Rev. Lett. 76 (1996) 1441–1444. https://doi.org/10.1103/Ph ysRevLett.76.1441. [33] Wei Song, Wen-Cai Lu, C.Z. Wang, K.M. Ho, Magnetic and electronic properties of the nickel clusters Nin (n⩽30), Comput. Theor. Chem. 978 (2011) 41–46. https ://doi.org/10.1016/j.comptc.2011.09.028. [34] L.U. Tian, Qinxue CHEN, Revealing molecular electronic structure via analysis of valence electron density[J], Acta Phys. - Chim. Sin. 34 (2018) 503–513. htt ps://doi.org/10.3866/PKU.WHXB201709252. [35] Tian Lu, Feiwu Chen, Multiwfn: a multifunctional wavefunction analyzer, J. Comput. Chem. 33 (2012) 580–592. https://doi.org/10.1002/jcc.22885. [36] W.R. Myers, L.-W. Wang, T.J. Richardson, M.D. Rubin, Calculation of thermodynamic, electronic, and optical properties of monoclinic Mg2NiH4, J. Appl. Phys. 91 (2002) 4879. https://doi.org/10.1063/1.1454206. [37] J. Pipek, P.G. Mezey, A fast intrinsic localization procedure applicable for ab initio and semiempirical linear combination of atomic orbital wave functions, J. Chem. Phys. 90 (1989) 4916. https://doi.org/10.1063/1.456588. [38] Hatcher Ryan, Matthew Beck, Alan Tackett, Sokrates T. Pantelides, Dynamical effects in the interaction of ion beams with solids, Phys. Rev. Lett. 100 (2008) 103201. https://doi.org/10.1103/PhysRevLett.100.103201.
Funding This study was financially sponsored by National Natural Science Foundation of China (Nos. 11205107 and 11405111) and Open Research Fund of Key Laboratory of Radiation and Technology of Edu cation Ministry of China (2018SCURPT12). Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “The mechanism of negative and positive hydrogen ions pro duction on the Ni surface”. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.vacuum.2019.108982. References [1] U. Fantz, P. Franzen, D. Wünderlich, Development of negative hydrogen ion sources for fusion: experiments and modeling, Chem. Phys. 398 (2012) 7–16. https://doi.org/10.1016/j.chemphys.2011.05.006. [2] L. Schiesko, P. McNeely, U. Fantz, P. Franzen, NNBI Team, Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source, Plasma Phys. Control. Fusion 53 (2011), 085029. https://doi.org/10.1088/0741-3335/53/8/085029. [3] Ryuta Endo, Shogo Ishihara, Toshikio Takimoto, Akira Tonegawa, Kohnosuke Sato, Kazutaka Kawamura, Production of hydrogen negative ions in high density sheet plasma, AIP Conf. proc. 2018 (2011), 020009. https://doi.org/10.1063/ 1.5053251. [4] A. Ueno, et al., Interesting experimental results in Japan proton accelerator research complex H ion-source development (invited), Rev. Sci. Instrum. 81 (2010), 02A720. https://doi.org/10.1063/1.3271243. [5] D.P. Moehs, J. Peters, J. Sherman, Negative hydrogen ion sources for accelerators, IEEE Trans. Plasma Sci. 33 (2005) 1786–1798. https://doi.org/10.1109/TPS.200 5.860067. [6] J. Lettry, D. Aguglia, P. Andersson, S. Bertolo, A. Butterworth, Y. Coutron, et al., Status and operation of the Linac4 ion source prototypes, Rev. Sci. Instrum. 85 (2014), 02B122. https://doi.org/10.1063/1.4848975. [7] A. Ueno, K. Ikegami, Y. Kondo, Surface production dominating Cs-free H ion source for high intensity and high energy proton accelerators, Rev. Sci. Instrum. 75 (2004) 1714. https://doi.org/10.1063/1.1699459. [8] A. Latuszy� nski, K. Kornarzy� nski, A. Dro�zdziel, K. Pyszniak, D. Maçzka, Negative ion beams from a plasma type source with additional surface ionization, Vacuum 47 (1996) 1219. https://doi.org/10.1016/0042-207X(96)00130-3. [9] C. Auth, A. Mertens, H. Winter, A.G. Borisov, V. Sidis, Formation of negative ions in grazing scattering from insulator surfaces, Phys. Rev. A 57 (1998) 351–361. https://doi.org/10.1103/PhysRevA.57.351. [10] H. Winter, Collisions of atoms and ions with surfaces under grazing incidence, Phys. Rep. 367 (2002), 387-582, https://doi.org/10.1016/S0370-1573(02) 00010-8. [11] R.M.A. Heeren, et al., Angular and energy distributions of surface produced H and D ions in a barium surface conversion source, J. Appl. Phys. 75 (1994) 4340. htt ps://doi.org/10.1063/1.355977. [12] C.F.A. van Os, W.B. Kunkel, C. Leguijt, J. Los, Modeling of H surface conversion sources; binary (H-Ba) and ternary (H-Cs/W) converter arrangements, J. Appl. Phys. 70 (1991) 2575. https://doi.org/10.1063/1.349365. [13] C.F.A. van Os, P.W. van Amersfoort, J. Los, Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam, J. Appl. Phys. 64 (1988) 3863. https://doi.org/10.1063/1.341340.
7