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Understanding the Diverse Coordination Modes of Thiocyanate Anion on Solid Surfaces Zheng Wang, Tongfa Liu, Xia Long, Yuan Li, Fu-Quan Bai, and Shihe Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01457 • Publication Date (Web): 16 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Understanding the Diverse Coordination Modes of Thiocyanate Anion on Solid Surfaces Zheng Wang†, Tongfa Liu†, Xia Long†, Yuan Li‡, Fuquan Bai‡, Shihe Yang*†§ † Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China. E-mail: [email protected] ‡ Laboratory of Theoretical and Computational Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People's Republic of China §Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail: [email protected]

Thiocyanate anion (SCN) can coordinate to solid surfaces in various coordination modes, endowing the solid surfaces with different structures, electronic and catalytic properties, but the rules that dictate the coordination preference are poorly understood due to the multiple coordination sites of both SCN and the solid surfaces as well as the SCN-lattice mismatch. In this article, extensive DFT calculations are performed to study the coordination modes of SCN on some selected solid surfaces. In particular, the influence of surface stress, coverage and counter cations is discussed at length. It is found that the time honored principle of hard soft acid base can only partly account for the preference of sulfur or nitrogen coordination to the solid surfaces due to its failure to treat the Lewis acids and bases located at the borderline and to deal with the multiple coordination sites and the SCN-lattice mismatch. Specific to solid surfaces, the coordination number and coordination strength can be gradually changed by applying stress. Moreover, the interactions of SCN with the counter cations and solvent (water) can modify its coordination mode, which generally favors sulfur coordination owing to the stronger interaction between the more electronegative dangling nitrogen and the counter cations or water. The coordination of SCN could modify the electronic and geometric properties of the solid surface, which would benefit the design of catalysts. For example, the SCN-coordination improved the conductivity of -Ni(OH)2, which is potentially useful in electrocatalysis. This work sheds light on the coordination mode of SCN with significant implications on a variety of applications that can exploit the coordination chemistry. ABSTRACT:

1. Introduction Structure and bonding of molecules on solid surfaces are of fundamental and practical importance in catalysis, molecular devices, organic electronics and solar cells.1 In heterogeneous catalysis, the reactions take place on solid surfaces, and clearly, the nature of the surfaces is critical to their catalytic properties. Specifically, reactants need to be absorbed on solid surfaces to undergo reactions, and the generated products need to desorb from the solid

surfaces, leaving empty the space for further reactions. One can envision that molecules can be suitably chosen for coordination to solid surfaces to influence the catalytic reactions thereon. Similarly, the optical properties of materials, such as CuSCN, can also be modulated through ligand modifications.2-20 In functional devices such as perovskite solar cells, molecular coordination to surfaces and interfaces has been demonstrated to drastically modulate the device characteristics. Conceivably, the crystallinity of the perovskite layer and the energy landscape of its interfaces

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with the electron transport layer (ETL) and with the hole transport layer (HTL) can be modified, thus sharply influencing the cell performance. Although there have been extensive theoretical studies on coordination modes in complexes, the nature of coordination modes on solid surfaces is much less studied from a fundamental point of view. Thiocyanate anion (SCN) is a well-known inorganic species, and widely used in solid state chemistry. There have been some reports of SCN anion acting as a reactant on solid surfaces,21-25 such as the synthesis of -hydroxy thiocyanate on Al-MCM-41 surface (a mesoporous 26 aluminosilicate) using NH4SCN, thiocyanation of aromatic compounds in aqueous media by silica sulfuric acid and silica boron sulfonic acids using KSCN27 and oxidation of SCN anion by K2FeO4.28 More interesting is the fact that the catalytic properties of different catalysts can be modified by SCN anion in a radically diametrical way. For example, the reactivity of Ag/TiO2 for hydrogen production can be boosted up by SCN anion, whereas that of bare TiO2, Au/TiO2 and Pt/TiO2 is actually retarded by SCN anion.29 Recently, it has been noted that SCN anion can be utilized to poison the FeNx/C catalyst in experiments designed to probe the catalytically active sites of the catalyst. 30 Recently, a surface-chemistry strategy has been employed to achieve a sulfur incorporated metallic phase of Ni(OH)2 maintaining the long range order of -Ni(OH)2 with H2S as sulfur source, denoted as -Ni(OH)2-S.31 Considering that SCN anion can coordinate to -Ni(OH)2 either by S or N, and coordination by S or N definitely will induce different influence on -Ni(OH)2, leading to different electronic properties. More importantly, -Ni(OH)2 as the backbone of transition metal based layered-double hydroxides (LDH) has aroused extensive interest due to its excellent activities towards oxygen evolution reactions (OER). What can be learned about the impact of SCN anion on -Ni(OH)2 in terms of structure

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and electronic structure can be easily generalized to other TM-LDH, greatly simplifying the calculations, also the understandings. Besides, Ni(OH)2 is anti-ferromagnetic, Ni2+ is octahedrally coordinated with an electronic configuration of t2g6eg2, which can be easily modulated by ligands, facilitating the study of the influence of SCN on the solid surfaces. Therefore, we submit that Ni(OH)2 would provide an ideal platform to study the coordination of SCN on LDH. The SCN anion also has a place in the feverish field of perovskite solar cells for improving the quality of perovskite films and their interfaces with ETL or HTL. For example, the SCN incorporated MAPbI3-x(SCN)x perovskite exhibits a larger-sized crystals, fewer traps and better water stability than MAPbI3.32-42 Moreover, SCN anion can even be utilized to prepare low-dimensional perovskite, such as, MA2Pb(SCN)2I 43-46 and Cs2PbI2(SCN)247 imparting better thermal stability. Besides, SCN anion is a good candidate for passivation of the interface between perovskite and ETL or HTL, which can reduce the number of trap states for efficiency gain and improve the cell stability. For example, NH4SCN,48 Guanidinium thiocyanate (GUSCN),49 methylammonium thiocyanate (MASCN),50-51 KSCN,52 NaSCN52 and formamidinium thiocyanate (FASCN)53 were used to boost the performance and stability of perovskite solar cells. Despite that SCN anion has been widely used in catalysis and perovskite solar cells, a thorough understanding of the coordination modes of SCN anion on solid surfaces is lacking owing to multiple choices of the coordination donor atoms imparted by the resonance structures shown in Scheme 1.54-57 For example, it can coordinate to a solid surface via nitrogen through a monocoordinated μ1-N, bi-coordinated μ2-N and tricoordinated μ3-N depending on the specific solid surface under study. Similar coordination modes can be found for sulfur. In addition, SCN can coordinate a solid surface via both sulfur and nitrogen, κ1-S-κ1-N (Scheme 2).

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Since different coordination modes of SCN on solid surfaces can influence the surface structure and electronic properties, they will have significant influence on the optical, electronic and catalytic properties of the corresponding surfaces. Although the well-established principle of hard soft acids bases (HSAB)58-60 has been employed for ages to understand the preferred coordination mode of SCN anion, it usually fails for those acids and bases at the borderline (neither strong acid/base nor weak acid/base). More importantly, the coordination in a heterogeneous phase is more complex due to the many more coordination sites and thus the much higher coordination number. It is thus essential to study the coordination chemistry of SCN to have a thorough and systematic understanding of the role SCN can play in tailoring solid surfaces, and put the commonly practiced trial and error studies of this unique anion on a firm fundamental basis. Toward this end, we performed thorough DFT calculations of the coordination modes of SCN on several representative classes of solid surfaces, such as the surfaces of -Ni(OH)2 and perovskites to gain a better and deeper understanding of the corresponding surface coordination chemistry. We anticipate that this work will illuminate the previously underexplored aspects of surface coordination chemistry of SCN, which will aid in designing SCN coordination derived catalysts and other functional materials at large.

projector augmented wave (PAW) potentials65 for the plane-wave basis set. The convergence criteria was set at 0.01 eV/Å for maximum forces as well as 10-5 eV for the relative energies. To obtain solvation-corrected relative energies, we performed single-point calculations for all species studied here using the implicit solvation model with water as the solvent.66-67 The total bonding energy decomposition analysis (EDA) was based on the natural orbitals for chemical valence (NOCV).68-70 All the structures obtained are shown by VESTA.71 S

C

All the density functional theory (DFT) calculations were performed using Vienna ab initio simulation package (VASP).61-62 The generalized gradient approximation with the Perdew, Burke, and Ernzerhof (PBE) functional63 including Hubbard U corrections64 was used to describe the electronic exchange and correlation, in which U values for Ni, Pd and Pt are chosen as 3.2, 4.04 and 7.5, respectively. The cut off energy is 400 eV, and a vacuum layer was set with a thickness of 20 Å, which is sufficient to prevent interactions between periodic images of the slabs. The core-valence interactions were modeled by

C

N

Scheme 1: The resonance structures of SCN anion. S

S

S

C

C

C

N

N

N

TM

TM

TM

TM

2-N

1-N

TM

N

N

C

C

C

S

S

S

TM

TM

TM

2-S

TM

3-N

N

1-S

2. Computational Details

S

N

TM

TM

TM

3-S

N

TM

C S

TM 1-S-1-N

Scheme 2: Possible coordination modes of SCN anion on solid surfaces.



3. Results 3.1 The limits of applying the principle of hard soft acids bases (HSAB) to solid surfaces As nitrogen atom in SCN anion is harder than sulfur atom, one may intuitively think that the preference of coordination modes of SCN anion can be explained and predicted by HSAB. Therefore, before getting into the details of DFT calculations of coordination modes of SCN anion on selected solid surfaces, let’s first revisit the time-honored rule: HSAB.58-60 HSAB states that hard acids prefer to bind to hard bases and soft acids prefer to bind soft bases. Hard acids are those cations that are difficult to be polarized, such as Al3+, Fe3+, Ti4+ Sn4+ as well as H+, Li+, Na+, K+, while soft acids are generally those low-valent or highly polarizable cations, such as Pd2+, Pt2+ Hg2+ and Au+. Except for those hard and soft acids, there are still some Lewis acids located at the borderline. Unfortunately, Ni2+, Pb2+ and Sn2+ in this study, all located at the borderline, hinting that HSAB can’t well explain the preference of these acids towards Lewis base. Hard Lewis bases are those that are difficult to be polarized, and usually have a smaller radius, such as OH, F and NO3 and NH3. And soft bases are those that are highly polarizable, such as CN, H and I. Indeed, similar to the division of Lewis acids, there are also some Lewis bases located at the borderline, such as Br, N3 and NO2. Klopman applied perturbation theory to have a deeper understanding of the nature of HSAB.72 As depicted in Equation 1, the total energy of interactions between Lewis acids and Lewis bases are divided into three terms, electrostatic interaction term, the covalent bonding interaction term and the solvation term. For the hard-hard interaction, the electrostatic interaction dominates the bonding, and the covalent bonding orbital contributes little. This is regarded as chargecontrolled reaction, as depicted in the orbital interaction diagram in Scheme 3a. As for the softsoft interaction term, the covalent bonding term contributes significantly to the interaction energy

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due to the smaller energy difference between the HOMO of Lewis base and LUMO of Lewis acid. This is treated as frontier molecular orbital controlled reaction, as shown in Scheme 3b. In most situations, both electrostatic interaction and covalent bonding interaction play important roles in chemical bonding, and the corresponding orbital interaction diagrams can be constructed as depicted in Scheme 3c. Here the chemical bonding can shift its preference towards either the chargecontrolled or frontier molecular orbital controlled type depending on the charge and energy of the frontier molecular orbitals of Lewis acid and base. Usually, the HSAB expressed below fails to explain this scenario. occ

Etotal  

q A qB  RAB

unocc

2 (C Am ) 2  (CBn ) 2  2 m

n

Em*  En*

 Esol (1)

Where △ Etotal is the total bonding energy, A represents Lewis acid, B stands for Lewis base. qA and qB are the charge of Lewis acid and base, respectively. RAB is the distance between Lewis acid A and B. ε is the dielectric constant of solvent. △Esol is the solvation energy. Em* and En* are the energies of the LUMO of acid and HOMO of Lewis base, respectively. As mentioned above, SCN anion is special in that it can coordinate to metal center through nitrogen or sulfur atom. Since the electrons of sulfur lies higher in energy than that of nitrogen due to the smaller electronegativity of sulfur, sulfur has a stronger preference to coordinate metal to maximize the covalent bonding interaction energy in terms of frontier molecular orbital interaction. However, sulfur bearing with smaller negative charge than nitrogen, thus has a smaller tendency to form electrostatic interaction with metal in terms of electrostatic interaction. Therefore, it is expected that sulfur prefers metal that has low-lying LUMO to maximize the covalent bonding interaction term, while nitrogen favors metal bearing with a high positive charge to maximize the electrostatic interaction, as depicted in Scheme 3d. 4


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Coordination of SCN anion to organometallic complexes has been well studied owing to its relatively simple bonding options, and the related crystals have been reported.73 Most of the experimental facts can be well explained by HSAB. The early and middle transition metals in the periodic table usually prefer coordination via nitrogen (denoted as SCN-N coordination mode) than sulfur (denoted as SCN-S mode), whereas the late transition metal congeners, such as Rh, Ru and Hg, favor SCN-S mode. One may naively think that this coordination behavior of SCN anion may be easily generalized to common inorganic solid materials. However, there are some big differences between organometallic complexes and inorganic solid materials, which prevent such a curt generalization of HSAB to inorganic solid materials. First of all, a higher coordination number is ubiquitous in inorganic solid materials, especially for those compounds with multiple coordination choices. To allow a higher coordination number of SCN anion via SCN-N mode, nitrogen must change its hybridization mode from sp, sp2 and sp3. As the p orbital lies higher in energy, the energy of the hybridized orbital will go up with increasing hybridization, leading to a gradual decrease of hardness of nitrogen, and thus gradually disfavoring the hardhard interaction. Second, the coordination number can be different for SCN-N or SCN-S coordination mode leading to different coordination ability due to the presence of coordination-lattice mismatch on inorganic solid surfaces. Moreover, the coordination-lattice mismatch can be facilely introduced by applying stress. Third, the electrons are more delocalized in inorganic solid materials, which can be easily influenced by the ligands causing a change of electric conductivity, magnetism, etc.

3.2 Coordination engineering nanocatalysts based on -Ni(OH)2.

of

We first studied the coordination modes of SCN on the (001) surface of -Ni(OH)2, in which one hydroxide anion was replaced by one SCN anion to maintain the charge neutrality of the system. To compare the two coordination modes

conveniently, the relative energy difference between SCN-S and SCN-N modes without and with solvation corrections are listed together in Table 1, and expressed in eV. Table 1 shows that the μ3-N mode is more stable than μ2-S mode by 0.75 eV. To get a deeper insight into the more stable coordination mode of SCN-N over SCN-S, we performed the total bonding energy decomposition analysis (EDA) was based on the natural orbitals for chemical valence (NOCV).68-70 The resulted decomposition energies are given in Table 2, expressed in eV. In Table 2, we can find that the proportions of ΔEelstat (standing for electrostatic interaction term) and ΔEorb(corresponding to covalent bonding interaction term) are similar, but the values of SCN-N are larger than SCN-S. The larger value of orbital interaction energy (ΔEorb) in SCN-N can be understood by the fact there is one more coordination number of SCN-N mode than SCNS mode due to the coordination-lattice mismatch as evidence by Figure 1a. While the larger value of electrostatic energy in SCN-N is attributed that nitrogen is more negative than sulfur, thus bearing with more negative charge. Support for the argument of higher electrostatic interaction energy of SCN-N mode consists of the Bader charge of Ni, N and S is ~1.16, -1.18 and -0.10 in SCN-S mode, while ~1.22, -1.35 and +0.16 in SCN-N mode. From the Bader charge analysis, we can see that there is a strong charge separation in SCN-S mode, in other words, the dipole in SCN-S mode is much larger than SCN-N mode, hinting the energy SCN-S mode is overestimated in gas phase calculations. Moreover, the solvation term in equation 1 can influence the bonding interaction energy, thus affecting its favored coordination mode. Therefore, a solvation correction was made to include the solvation energy, and it was found that the energy difference between SCN-S and SCN-N mode decreased to 0.53 eV after solvation correction. It is expected that the explicit solvent, water, can also influence the relative stability of μ2-S mode and μ3-N mode, considering the fact that -Ni(OH)2 was prepared in aqueous conditions and the hydrogen bonds between N and water is stronger than S and water. Therefore, we also studied the effect of explicit solvent. When 5



two molecules of water were added into the system, the energy difference between SCN-S and SCN-N becomes -0.62, which can be further reduced to -0.40 eV after an implicit solvation correction.

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Figure 1: The structures of SCN anion on the (001) surface of -Ni(OH)2 with SCN-S (a) and SCN-N (b) coordination modes.

(b)

(a) A

A B

B

(c)

(d)

A

A SCN-S B

SCN-N

Scheme 3: Schematic illustration of principle of hard soft acids bases (HSAB).

The lower coordination number of sulfur in SCN-S mode can be attributed to that the vacant site is not large enough to accommodate sulfur atom, since sulfur atom is much larger than nitrogen and oxygen atom. To verify this hypothesis, surface stress was applied to the (001) surface of -Ni(OH)2 to enlarge the volume of the vacant site. Satisfactorily, S atom becomes tricoordinated as the surface stress increased to 5% (Figure S1). However, the coordination mode of SCN-N gradually changed from μ3-N to μ2-N when the surface stress increased (Figure S2). As a result, the energy difference between SCN-N and SCN-S mode diminished with the increase of surface stress (Figure 2).

Figure 2: The energy difference between SCN-N and SCN-S mode vs surface strain for -Ni(OH)2 – SCN-S (red line), and -Ni(OH)2 –SCN-N (black line) in gas phase.

As mentioned above, -Ni(OH)2-S is metallic in sharp contrast to pristine -Ni(OH)2, which has a poor conductivity. To find out whether SCN can modify the electronic properties of -Ni(OH)2, we also analyzed the density of states (DOS) of Ni(OH)2, SCN coordinated -Ni(OH)2 via SCNS mode (Ni(OH)2-SCN-S) and SCN coordinated -Ni(OH)2 via SCN-N mode (Ni(OH)2-SCN-N), which is depicted in Figure 3a, 3b and 3c, respectively. Clearly, the -Ni(OH)2 has a band gap of 1.05 eV, and is anti-ferromagnetic. In contrast, Ni(OH)2-SCN-S and Ni(OH)2-SCN-N still remain anti-ferromagnetic with a decreased band gap, especially the former. Since conductivity and band gap can be related by equation (2), it is expected that the conductivity of Ni(OH)2-SCN-S is much higher than the pristine -Ni(OH)2.

 =C e



Eg 2 k BT

(2)

Where σ is the conductivity, Eg the band gap, kB the Boltzmann constant and T the temperature. 6 ACS Paragon Plus Environment

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Table 1: The calculated relative electronic energy differences between SCN-N and SCN-S for different systems. The energies are given in eV.

Systems

SCN-S

SCN-N

SCN-S-Sol

SCN-N-Sol

(001) surface of -Ni(OH)2

0.0

-0.75

0.0

-0.53

(001) surface of -Ni(OH)2-H2O

0.0

-0.62

0.0

-0.40

(001) surface of -Ni(SH)2

0.0

-0.15

0.0

0.01

(001) surface of -Ni(SeH)2

0.0

-0.07

0.0

0.09

Table 2: EDA-NOCV results for SCN-N and SCN-S coordination modes of -Ni(OH)2. The energies are given in eV, and the values in parentheses show the contribution of ΔEelstat or ΔEorb to the total attractive interactions.

Systems

SCN-S

SCN-N

ΔEtotal

-12.06

-12.39

ΔEpauli

3.91

8.45

ΔEelstat

-4.77 (29.9%)

-6.70 (32.2%)

ΔEorb

-11.20 (70.1%)

-14.13 (67.8%)

It has been suggested that FeOOH has a high theoretical OER activity, but the real activity is limited by its poor conductivity.74 Considering that -Ni(OH)2 can be easily oxidized to form NiOOH, and -NiOOH shares a similar structure with FeOOH, we propose that incorporation of SCN anion can improve the conductivity of FeOOH so that the OER activity of FeOOH can be greatly boosted. Furthermore, as those TM-LDHs with a high OER activity almost always has a relatively high conductivity,75 we can expect that the conductivity of TM-LDH can also be improved by incorporation of SCN anion, which could be a simple and useful strategy to increase the OER reactivity of TM-LDH. For the SCN-S mode on the (001) surface of Ni(OH)2, one of Ni2+ in -Ni(OH)2 adopts a square pyramidal structure, and this may prompt one to think that the electronic configuration of Ni2+ could change. However, this did not turn out to be

the case after we carefully checked the electronic configuration; Ni2+ cation still retained its high spin configuration, with the two single electrons occupying the two degenerate eg orbitals. One may wonder whether the electronic configuration of metal would change if the second or third row transition metals with higher ligand field were employed. To answer this question, we studied the coordination mode of SCN anion in -Ni(OH)2 and -Ni(OH)2, in which Ni2+ was substituted by Pd2+ and Pt2+, respectively. Interestingly, it was found that SCN can coordinate to palladium via μ3-S (Figure 4a) and μ2-S modes (Figure 4b), with the latter one is more stable by 0.39 eV (0.38 eV after solvation correction), even though it has one less coordination number. It should be noted that the geometry of one Pd2+ in -Pd(OH)2 with μ2-S mode becomes square planar, which is ubiquitous exist in organometallic compounds, and thus has a 7



d8 low spin configuration, in sharp contrast to that in -Pd(OH)2 with μ3-S mode in which Pd2+ bears with a d8 high spin configuration. The ligand field splitting of Pd2+ in square planar and octahedral geometry is illustrated in Figure S3. Here, μ2-S mode is even more stable than μ3-N mode, attributing to the significant different ligand field splitting as well as softness of Pd2+. -Pt(OH)2 shares an analogous situation with -Pd(OH)2 except that the energy difference between μ2-S and μ3-N in -Pt(OH)2 is larger due to the softer nature of Pt2+ and larger crystal splitting field strength.

Figure 3: The density of states and partial charge density of -Ni(OH)2 (a), SCN coordinated Ni(OH)2via SCN-S mode (b) and SCN coordinated -Ni(OH)2via SCN-N mode (c).

Since sulfur and selenium are softer atoms than oxygen, one may ask whether the relative stability between SCN-N and SCN-S can switch over. Bearing this question in mind, the coordination mode of SCN anion on -Ni(SH)2 and Ni(SeH)2 was comparatively studied, in which the oxygen atoms in -Ni(OH)2 are all replaced by sulfur and selenium atoms, respectively. From Table 1, we can observe that SCN-N mode is a little more stable than SCN-S mode in gas phase, but their stability is reversed after solvation correction for both -Ni(SH)2 and Ni(SeH)2 to the extent that the stability of SCN-N and SCN-S mode becomes similar. Said another way, the energy differences between SCN-N and

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SCN-S mode on -Ni(SH)2 and -Ni(SeH)2 are close. This is due to the similar electronegativity of S(2.58) and Se(2.55), suggesting that HASB still plays a dominant role here. On the other hand, it is notable that the energy difference between SCN-S and SCN-N mode in gas phase is smaller on -Ni(SeH)2 than on Ni(SH)2, indicating that SCN-N mode is comparatively less favorable than SCN-S mode on -Ni(SeH)2. The lower preference of SCN-N mode on -Ni(SeH)2 is due to the larger coordination-lattice mismatch for SCN-N mode than SCN-S mode (the ionic radii of N3-, S2- and Se2- are 1.46 Å, 1.84 Å and 1.98 Å, respectively).76-77

Figure 4: The structures of SCN anion on the (001) surface of -Pd(OH)2 with μ3-S (a) and μ2-S (b) coordination modes.

Given that one coordination mode in -Ni(OH)2 is also possible, we next studied the coordination of SCN anion on the (221) surface of -Ni(OH)2, in which the Ni2+ on the surface adopts square pyramidal structure with one vacant site for coordination for each metal. Interestingly, although SCN-S is a little more stable in gas phase, it becomes less stable after solvation correction, in contrast to the cases in the (001) surfaces of -Ni(XH)2, X=O, S, Se. On checking the structures, we observed that the SCN-S mode has two coordination sites, S and N, adopting κ1S-κ1-N mode, while SCN-N mode only has one coordinate site, namely μ1-N, as shown in Figure 5a and 5b. The higher coordination number of SCN-S mode account for its increased stability compared to SCN-N mode. Here, sulfur is sp3 hybridized, thus has a smaller bond angles, facilitating the dangling nitrogen atom in 8


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SCN approaching the Ni2+ on the (221) surface of -Ni(OH)2 to adopt κ1-S-κ1-N mode. Whereas, nitrogen is sp2 hybridized, has a larger bond angle, thus, the dangling sulfur in SCN is difficult to approach Ni2+ on the (221) surface of -Ni(OH)2, thus favoring single coordination. One may intuitively think that the surface coverage should have a large influence on

coordination of SCN on the (221) surface of Ni(OH)2, so we further studied two SCN anions coordinated to the (221) surface of -Ni(OH)2. Expectedly, κ1-S-κ1-N mode disappears with the a coverage of 100% of SCN, the SCN-S mode exits in μ1-S mode. And the SCN-N mode is much more stable than SCN-S by 0.89 eV (Table 3 and Figure 5d), and SCN-S mode becomes single coordination (Figure 5c).

Table 3: The calculated relative electronic energy differences between SCN-N and SCN-S for -Pd(OH)2 and -Pt(OH)2. The energies are given in eV.

Systems

μ3-S

μ2-S

μ3-N

μ3-S-Sol

μ2-S-Sol

μ3-N-Sol

(001) surface of -Pd(OH)2

0.0

-0.39

-0.48

0.0

-0.38

-0.25

(001) surface of -Pt(OH)2

0.0

-0.57

-0.47

0.0

-0.43

-0.24

Table 4: The calculated relative electronic energy differences between SCN-N and SCN-S for the (211) surface of -Ni(OH)2. The energies are given in eV.

Systems

SCN-S

SCN-N

SCN-S-Sol

SCN-N-Sol

(211) surface of -Ni(OH)2

0.0

0.07

0.0

-0.06

(211) surface of -Ni(OH)2 -100 % coverage

0.0

-1.21

0.0

-0.89

Figure 5: The structures of SCN anion on the (211) surface of -Ni(OH)2 with κ1-S-κ1-N (a) and μ1-N (b) μ1-S (100% coverage) (c) and μ1-N (100% coverage) (d) coordination modes. 9 ACS Paragon Plus Environment


3.3 Coordination engineering for halide perovskite solar cells. While the diverse coordination modes of SCN on -Ni(OH)2 are fascinating, more extensive research interest has been directed on the coordination of SCN anion to the halide perovskites with the general formula of ABC3 (A is a monovalent cation, B stands for divalent cation and C represents halide) in recent years amidst the rising of perovskite solar cells.78-81 However, a systematic investigation of the coordination modes is lacking. As different anions C and different cations A have different influences on the stability and polaron mobility of perovskite, SCN anion coordinate to Pb2+ in SCN-N or SCNS mode, of course, have different influences on perovskite. In addition, the perovskite is usually sandwiched between layers of HTL and ETL, understanding the coordination mode of SCN anion on the surface of perovskite can help in choosing and designing suitable ETL and HTL that can bind strongly with SCN to reduce defects and to improve stability of the interfaces . Therefore, we also performed calculations on SCN coordination mode on Pb-based perovskite. Since the perovskite exits in the solar cells in a solvent free condition, therefore, we have not applied solvation corrections for perovskite studies. Here, only the coordination modes of μ1N or μ1-S have been examined since the Pb2+ cations keep away from each other preventing the multiple coordination of SCN. The reported single crystal of MA2Pb(SCN)2I2 with coordination mode of μ1-S is further studied,82 and the relative energies and structures are depicted in Table 5 and Figure 6, respectively. It shows that the μ1-N mode is more stable than μ1-S mode in MA2Pb(SCN)2I2 crystal, in line with the experimental observation.82 On the other hand, the relative stability between μ1-N mode and μ1-S mode gets closer to each other on MA2Pb(SCN)2I2 (001) surface. Careful examination of μ1-N mode in MA2Pb(SCN)2I2 and MA2Pb(SCN)2I2 found that there are fewer hydrogen bonds formed between N and H atoms from SCN anion and MA cation, respectively, on

Page 10 of 17

(001) surface of MA2Pb(SCN)2I2. To demonstrate the role of hydrogen bonds stabilizing μ1-S mode, we added two water molecules to the (001) surface of MA2Pb(SCN)2I2, and found the relative stability between μ1-S and μ1-N becomes 1.11 eV, proving that hydrogen bonds formed between MA cation and SCN stabilized μ1-N mode. Subsequently, Cs2Pb(SCN)2I2 was obtained by substituting MA cation with Cs cation, in which there exists no hydrogen bonds in Cs2Pb(SCN)2I2. Then the observed stability difference between μ1S and μ1-N drops to only 0.25 eV, much smaller than that in MA2Pb(SCN)2I2 (0.85 eV), demonstrating that the formed several hydrogen bonds, between MA and SCN in MA2Pb(SCN)2I2, can strongly stabilize the μ1-N mode. Then, we studied the coordination of SCN on the surfaces of some commonly studied halide perovskite, CsPbI3, MAPbI3, FAPbI3. The neural species KSCN or MSCN (M=Cs, MA and FA) are employed to maintain the charge neutrality. From Table 5, we can observe that SCN-N mode is generally more stable than SCN-S mode, in contrast to that in MA2Pb(SCN)2I2 and Cs2Pb(SCN)2I2. That could be attributed to the increased ionicity of CsPbI3, MAPbI3, FAPbI3 since the minus charge in SCN is well delocalized and stabilized through conjugation as shown in Scheme 1. And the cation on MSCN has an influence on the relative stability of SCN-N and SCN-S modes. For example, SCN-N mode is more stable than SCN-S mode by 0.12 eV if MASCN is absorbed and 0.03 eV if KSCN is absorbed due to the better stabilization of nitrogen by potassium cation than MA cation. Similar situations can be observed for CsPbI3, FAPbI3, and FAPbI3. Experimentally, it was discovered that absorption of FASCN on the surface of FASnI3 leads to a red shift of C-N vibration. The author claimed that it is attributed to the coordination via SCN-S mode.53 In contrast, we suspected that it is due to the coordination via SCN-N mode based on the following arguments. First, a red shift of C-N vibration suggests a longer C-N bond. Second, coordination via SCN-S leads to a triple bond of 10


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C-N, while coordination via SCN-N results of a double bond of C-N. We thus studied the coordination of SCN on the (001) surface of FASnI3. Indeed, our calculations indicated that SCN-N mode is more stable than SCN-S mode for the (001) surface of FASnI3 with the coordination of FASCN, KSCN and SCN, and the last one has the largest energy difference. So we examined the

structure of SCN-N, SCN-S mode (Figure 7a and 7b), and found that for the SCN-S mode, C-N bond length becomes 1.177 Å, shorter than that in KSCN (1.181 Å), suggesting the C-N bond becomes stronger which will lead to blue shift of vibration. While C-N bond length in SCN-N mode is 1.212 Å, indicating a red shift of C-N vibration.

Table 5: The calculated relative electronic energy differences between SCN-N and SCN-S for selected perovskites and perovskite surfaces. The energies are given in eV.

Systems

SCN-S

SCN-N

Crystal of MA2Pb(SCN)2I2

0.0

0.85

(001) surface of MA2Pb(SCN)2I2-MASCN

0.0

0.56

(001) surface of MA2Pb(SCN)2I2-MASCH-H2O

0.0

1.11

Crystal of Cs2Pb(SCN)2I2

0.0

0.25

(001) surface of Cs2Pb(SCN)2I2-CsSCN

0.0

-0.03

(001) surface of MAPbI3-KSCN

0.0

-0.03

(001) surface of MAPbI3-MASCN

0.0

-0.12

(001) surface of CsPbI3-KSCN

0.0

0.03

(001) surface of CsPbI3-CsSCN

0.0

-0.06

(001) surface of FAPbI3-KSCN

0.0

0.01

(001) surface of FAPbI3-FASCN

0.0

-0.11

(001) surface of FASnI3-KSCN

0.0

-0.03

(001) surface of FASnI3-FASCN

0.0

-0.16

(001) surface of FASnI3- SCN

0.0

-0.42



Page 12 of 17

4. Conclusions

Figure 6: The structures of MA2Pb(SCN)2I2 perovskite with μ1-S (a) and μ1-N (b) coordination modes.

Figure 7: The structures of SCN anion on the (001) surface of FASnI3 perovskite with SCN-S (a) and SCN-N (b) coordination modes. Selected bond lengths are given in Angstroms.

DFT calculations have been performed to study the coordination modes of SCN anion on surfaces of the selected materials. It turns out that the coordination of SCN anion only partly follows the principle of hard and soft acids and bases (HSAB), which when referring to the SCN case stipulates that nitrogen favors binding to hard metals, while sulfur favors binding to softer metals. This principle fails when treating the Lewis acids and bases located at the borderline, and when facing with multiple coordination sites and coordination-lattice mismatch on solid surfaces. For the same valent metal, the anions can influence the hardness of the metal, for example, Ni2+ in -Ni(OH)2 is harder than -Ni(SH)2, significantly raising(lowering) the preference of the N(S)-coordination of SCN anion to the former than to the latter. And surface strain can also influence the coordination number and coordination strength of the SCN anion, a case which is absent in homogeneous systems and mono-coordinated species. In turn, the coordination of SCN anion can influence the electronic properties of the metal in question, for instance, by changing the ligand filed splitting of the transition metal. One salient example is the reduction of the band gap of -Ni(OH)2, leading to a conductivity enhancement. And for solid surfaces with multiple single coordination sites, the SCN-S mode could dominate because it allows multiple coordination of SCN due to the smaller bond angle of SCN-S. On the other hand, SCN prefers binding to perovskite in the μ1-N mode for perovskites with higher an ionic character, such as FAPbI3, MAPbI3, CsPbI3 and FASnI3. Meanwhile, the counter ion in MSCN (M=MA, FA, Cs, K) also has an influence on the preference of the coordination modes of SCN. For low dimensional perovskite, the SCN-S mode is more favorable due to its comparatively lower iconicity. In addition, hydrogen bonds and cations strongly favor the μ1S mode over the μ1-N mode due to the favorable stabilization they impart on nitrogen in SCN. Taken together, the rich coordination chemistry of SCN on solid surfaces we have uncovered could be used to guide the rational design of novel 12


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materials at different length scale for electronic and photonic devices. ASSOCIATED CONTENT The structures of SCN anion on the (001) surface of Ni(OH)2 with the strain increased from 0% to 9% for SCN-S and SCN-N modes are given in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (S. Y.).

Notes

Any additional relevant notes should be placed here.

ORCID

Zheng Wang: 0000-0002-7560-2618 Tongfa Liu: 0000-0001-6633-0502 Xia Long: 0000-0002-9705-1589 Fuquan Bai: 0000-0001-9398-1407

ACKNOWLEDGMENT This work was financially supported by China Postdoctoral Science Foundation Grant (No. 2018M631240), the Shenzhen Peacock Plan Program (KQTD2016053015544057), the National Science Foundation of China (contract No. 21703003), the Nanshan Pilot Plan (LHTD20170001) and the RGC of Hong Kong (GRF No. 16312216). The calculations were performed on the Tianhe-2 supercomputer system in Guangzhou.

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Insert Table of Contents artwork here TOC Graphic N

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Which atom coordinate to solid surface? nitrogen, sulfur or both? What's the coordination number? A DFT study.


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