Designing dithiolene and bis(iminothiolato)-based 1D metal-organic-frameworks for electrocatalytic hydrogen evolution reaction

Hydrogen is considered as one of the most important clean and renewable energy resources to get rid of carbon-based fuels and to solve the problem of environmental hazzards caused for using fossil fuels. Hence, the large-scale production of hydrogen by water splitting through hydrogen evolution reaction (HER) demands inexpensive and efficient electrocatalysts to replace the scarce and expensive noble metal-based catalysts. In this work, using the density functional theory (DFT)-based computations, we have considered a family of one dimensional (1D) metal organic frameworks (MOFs), namely TM–dithiolene (TM–BTT), and TM–bis(iminothiolato) (TM–BIT), consisting of benzene-1,2,4,5-tetrathiolate (BTT) and benzene-bis(iminothiolato) (BIT) organic ligands, respectively, and a family of first row transition metals (TM = Mn, Fe, Co, and Ni), to find their catalytic activity toward HER. Using the Gibbs free energy for the adsorption of atomic hydrogen ( ΔGH∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {\Delta {\textit{G}}_{H^{*}}}$$\end{document}) as the key descriptor, we reached to the conclusion that Ni–bis(iminothiolato) MOF exhibits better catalytic activity toward HER over all other investigated 1D-MOFs.


Introduction
The growing human civilization and rapid industrialization to satisfy the increasing demand of the society results very fast depletion of fossil fuels and the world is at the stage of facing serious energy crisis. Another important concern is the environmental pollution because of burning of fossil fuels. So, the development of green, environmentally benign, and sustainable energy resources that can replace traditional fossil fuels is the need of the hour [1,2]. In this regard, generation of chemical fuels from either solar energy harvesting through artificial photosynthesis or electrochemical water splitting are the key challenges in this century. In the recent past, there are both experimental and computational exploration for designing active photovoltaic devices taking full advantages of tunable properties of nanoscale materials [3][4][5][6][7].
Apart from photovoltaic cell, the other important source of renewable energy is the molecular hydrogen because it is environmentally clean, safe and also has high charge density. As electrochemical water splitting is one of the primary pathways to generate hydrogen, the search for suitable electrocatalysts is an active area of recent research. To date, the most widely used catalyst for electrochemical hydrogen evolution reaction (HER) is the platinum metal. However, Pt being very precious and also of low abundance researchers are looking for non-precious and easily synthesizable electrocatalyst for large-scale generation of hydrogen fuels.
Two-dimensional (2D) materials such as 2D Mxenes, graphitic carbon nitrides, transition metal dichalcogenides, and transition metal-doped graphene have shown their potential to be an active catalyst for HER and thus, have promise to replace Pt-based catalysts. Very recently, the research on organic frameworks have gained huge interest because of their structural diversity and the opportunity to modulate their properties through variation of organic linkers, ligands and metal centers [8][9][10][11].
The organic frameworks, both covalent and metal organic frameworks (COFs and MOFs) offer a nice platform for designing suitable electrocatalysts for hydrogen evolution reaction. There are both experimental and computational reports on the design and catalytic performances of several COFs and MOFs for HER. Based on DFT-based computations Ball et al. have predicted that a silicon and phosphorus co-doped bipyridine-linked covalent triazine framework, followed by substitution of bipyridine hydrogens at the P-site with fluorine atoms (SiPF-Bpy-CTF) may be a potential electrocatalyst for HER [12]. Bhunia et al. [13] have reported that a pyrene-porphyrin-based COF can be used as a host or support to fabricate a water-splitting electrocatalyst. Sun et al. reported a single-atom strategy to construct excellent metal-organic frameworks (MOFs)-based electrocatalyst ( NiRu 0.13 − BDC ) for HER by introducing atomically dispersed Ru [14]. The density functional calculations of Wang et al. [15] have revealed that the monolayers of MOF consisting of transition metal (TM) atoms (TM = Fe,Cu and Zn) and 2,3,6,7,10,11-hexaiminotriphenylene ( C 18 H 12 N 6 ) functional group ( HITP ), namely Fe 3 (HITP) 2 , Cu 3 (HITP) 2 , and Zn 3 (HITP) 2 can serve as highly efficient HER electrocatalysts. Clough et al. [16] have reported the successful integration of cobalt dithiolene catalysts into a metal-organic surface and the MOF exhibits very good electrocatalytic performance for hydrogen generation from water. Song et al. [17] have predicted a two-dimensional (2D) metal-organic framework (MOF) bifunctional electrocatalyst, namely bis(iminothiolato)nickel ( NiIT ) monolayer, for overall water splitting. Liu et al. [18] through their first principles study have demonstrated that Ni-and Cr-based dithiolene MOFs possess better hydrogen evolution performances as compared to MOFs of other transition metals. Zaman et al. have nicely summarized the role of MOF materials in water splitting reaction in a recent review [19].
In this article we propose to study the electronic structure of a family one dimensional (1D) metal organic frameworks, to investigate their HER activity. These MOFs consist of benzene-1,2,4,5-tetrathiolate (BTT) and Benzene-bis(iminothiolato) (BIT) as organic ligands and a family of first row transition metals (TM = Mn, Fe, Co, and Ni). Sun et al. [20] have successfully synthesized 2D bis(iminothiolato)nickel nanosheet. Wang et al. have studied the electrolyte effects on electrocatalytic hydrogen evolution performance of 1D cobalt-dithiolene MOFs [21]. Downes et al. have synthesized cobalt dithiolene coordination polymer (CP) based on benzene-1,2,4,5-tetrathiolate (BTT) and studied the hydrogen evolution activity of this MOF [22]. In a subsequent paper the same group extended their work to synthesize 1D nickel, iron, and zinc dithiolene coordination polymers based on benzene-1,2,4,5-tetrathiolate (BTT) frameworks and have investigated their H 2 -evolving activities under fully aqueous conditions [23]. The authors have showed that among the different 1D TM-BTT MOFs they have studied, Ni-BTT MOF is an active electrocatalyst for the hydrogen evolution reaction (HER).

Computational details
All the spin-polarized calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab inito Simulation Package (VASP) [24,25] within the projector augmented wave (PAW) [26] formalism. PAW potentials were used to describe electron-ion interactions. We utilized the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) [27] for the exchange-correlation functional. The kinetic-energy cutoff for the plane wave basis set was 400 eV. 2 × 1 × 1 supercell is considered for the calculation of preferred magnetic ground state. The K-point grid for sampling the Brillouin zone was taken as 8 × 1 × 1 for geometry optimization while a denser [28] k-point mesh of 40 × 1 × 1 was used for electronic structure calculations. All the atoms were allowed to relax until the Hellmann-Feynman forces on each atom become smaller than 0.01 eV/Å. For the electronic self-consistency loop the convergence criterion was set to 1 × 10 −6 eV. We used DFT-D3 dispersion correction scheme that helps to account for nonbonded interactions between adsorbates [37,38]. The Brillouin zone was integrated by the Gaussian smearing method with a smearing width of 0.05 eV. A vacuum space up to 15 Å was applied along y and z-directions to avoid the interactions between neighboring images. VESTA software was used in drawing the structures and plotting the electron localization function(ELF)/spin densities [39].
In acidic medium (pH = 0), H + can act as a proton donor, and the overall HER can be represented as [29], where the catalyst is defined by the asterisk. The catalytic property of HER was described via using the Gibbs free energy difference of hydrogen adsorption ( ΔG H * ), which can be computed by the following equation: where ΔE H , ΔE ZPE , and TΔS H are the difference in adsorption energy, zero-point energy and entropy between the atomic hydrogen adsorption and hydrogen at the gas phase at 298.15 K, respectively. ΔE H is calculated using the following equation: where E H * , E are the total energies of the hydrogen adsorbed catalyst and bare catalyst, respectively. E H 2 is the energy of hydrogen molecule in gas phase. The difference in zeropoint energy between atomic hydrogen adsorption on the catalyst and hydrogen in the gas phase is calculated by the following equation: where E H * ZPE represents the zero point energy of the adsorbed atomic hydrogen without any contribution from the catalyst and E where S H 2 is the entropy of the hydrogen molecule in gas phase.
As the optimal value for HER is ΔG H * = 0 , the smaller value of ΔG H * will lead to better catalytic activity. The theoretical overpotential η for HER is determined by the following equation:

Results and discussion
In the search for suitable electrocatalysts based on 1D-MOFs, we have first investigated the electronic properties of TM-dithiolene and TM-bis(iminothiolato) MOFs and thereafter, based on the calculated value of the HER catalytic activity descriptor, namely, the Gibbs free energy for the adsorption of atomic hydrogen, we have pointed out the most effective catalyst for HER among the studied 1D-MOFs.
T h e 1 D M O F s o f T M -d i t h i o l e n e a n d TM-bis(iminothiolato) consist of benzene-1,2,4,5-tetrathiolate (BTT) and Benzene-bis(iminothiolato) (BIT) as organic ligands and a family of first row transition metals (TM = Mn, Fe, Co, and Ni). The BTT and BIT ligands are linked together by the four-coordinated TM atoms, where the TM atoms form a square planar configuration (Fig. 1). All the atoms in TM-dithiolene and in TMbis(iminothiolato) MOFs are coplanar.
The  [21,30]. The TM-S, C-S, and C-C bond lengths in all the 1D MOFs remain close to those of monoanionic TM-BTT species [31] implying not much structural changes when the monomeric units are embedded into extended MOF structure and this fact is in nice agreement with previous research works [32].
We have performed spin polarized calculations to determine the magnetic ground state structure of these 1D  For TM-BIT MOFs (TM = Mn, Fe, Co, and Ni), the binding energy is calculated by the equation: where E system is the total energy of TM-BTT and TM-BIT MOFs. E C , E H , E S , E N , and E TM are the energies of a single carbon, hydrogen, sulfur, nitrogen, and transition metal atoms, respectively. n C , n H , n S , n N , n TM are the total numbers of carbon, hydrogen, sulfur, nitrogen, and transition metal atoms, respectively. Here, n is the total number of atoms in the system.  negative binding energies of these MOFs indicate that these MOFs are energetically stable.
To understand the nature of bonding in these 1D MOFs, we have shown the electron localization function (ELF) of 1D TM-BTT (TM = Mn, Fe, Co, and Ni) and TM-BIT (TM = Mn, Fe, Co, and Ni) MOFs in Fig. 4. The ELF ranges from 0 to 1, corresponding to electron delocalization and accumulation, respectively, while the intermediate value of 0.5 indicates an electron gas like states [15]. The ELF plots (Fig. 4a-f) of all the investigated 1D MOFs clearly indicate that electrons mainly accumulate on the H, S, and N atoms. The electrons around the dithiolene and TM-S bonds tend to be equably distributed, implying the conjugated -bond features of the TM-BTT (TM = Mn, Fe, Co, and Ni) framework. Also for TM-BIT MOFs (TM = Mn, Fe, Co, and Ni), electron densities are around the bis(iminothiolato) ligand and TM-S/N bonds, implying the conjugated -bond features of the framework.
To get a better understanding of electronic properties of these 1D MOFs, we have presented the band structures,  Fig. 5. A material must have very low band gap to be an active electrocatalyst for HER. The low band gap value results in an increase in electrical conductivity of the material [33][34][35][36]. The increased electrical conductivity facilitates charge transfer during HER. The band structure plot clearly reveals that these 1D MOFs are very low band gap semiconductors except Ni-BTT, which is metallic in nature. The low band gap and the metallic nature of all the studied 1D-MOFs may accelerate the charge transfer kinetics during HER.
Inspired by previous research works [21], we have only considered metal sites and sulpher sites as possible adsorption sites for atomic hydrogen over TM-BTT MOFs (TM = Mn, Fe, Co, and Ni). As nicely explained by Wang et al. [21] and also evident from other studies the metal ion can exist in different oxidation states in these MOFs ( [TM III (bdt) 2 ] −1 ↔ [TM II (bdt)(bdt ⋅ )] −1 (bdt = 1,2-benzenedithiolate)) and because of the resonance forms of MOF both metal and S center can act as active catalytic sites for hydrogen adsorption. Wang et al. [21] in their study with Co-BTT 1D MOF have considered many other catalytic sites and concludes that the most preferred catalytic sites are either metal atoms or S atoms. The key descriptor for assessing the HER catalytic activity is the ΔG H * , which we have calculated using Eq. 2. To be an ideal catalyst for HER, the value of this parameter is desired to be close to zero, that means the atomic hydrogen should be adsorbed onto the catalyst neither too strongly nor too weakly. We calculated the ΔG H * values of a H atom adsorbing on the TM, and S atoms of TM-BTT MOFs and on the TM, S, and N atoms of TM-BIT MOFs to evaluate the HER catalytic activity. From Table 1, one can see that the optimal active sites of TM-BTT MOFs for HER are on the top of transition metal atom for Fe-BTT and Co-BTT MOFs and on the top of S atoms for Mn-BIT and Ni-BTT MOFs. Among these 1D TM-BTT MOFs, Ni-BTT is expected to show better HER catalytic activity as because it has low Gibbs free energy for atomic hydrogen adsorption on S atom. Following Liu et al. [18] we may conclude that the better HER activity is due to resonant electron transfer between metal atom and the ligand. Our calculated magnetic moment 0.1 B /unit for Ni-BTT MOFs confirms that [Ni II (bdt)(bdt ⋅ )] −1 form is the dominated form. Hence, due to the existence of great portion of bdt ⋅ radical in Ni-BTT MOF, enhances the reactivity of the relevant S atoms. In case of TM-BIT MOFs, the optimal active sites are on the top of N-atoms (Table 2).
In Fig. 6, we have shown the Gibbs free energies of hydrogen adsorption at metal center and the corresponding optimized geometry for all three 1D TM-BTT MOFs, and the same for adsorption at S center is shown in Fig. 7. In Figs. 8, 9, and 10, we have shown the Gibbs free energies  (Figs. 6, 7) clearly indicate that HER catalytic activity of the 1D TM-BTT MOFs are very much dependent on which site the hydrogen is adsorbed. For metal center, Co-BTT would exhibit better HER catalytic activity as it has the lowest ΔG H * followed by Fe, Mn, and Ni 1D MOF. However, Ni-BTT has the lowest ΔG H * at S center and thus expected to show better HER catalytic activity at S center. For 1D TM-BIT MOFs (Figs. 8, 9, and 10), N center is the most active catalytic sites for atomic hydrogen adsorption, it is also evident from ELF plots (Fig. 4). Among 1D TM-BIT MOFs, Ni-BIT has the lowest ΔG H * at N center and thus expected to show better HER catalytic activity at N center. Our computational results agree well with the recent experimental observation of Downes et al. [23] where the authors have studied the HER activity of different 1D TM-BTT MOFs and established that Ni-BTT shows better HER activity.

Conclusion
In summary, we have explored the electronic structure of TM-BTT and TM-BIT 1D MOFs by using spin-polarized density functional theory-based computations and also assessed their catalytic activities toward hydrogen evolution reaction. Our study reveals that these 1D MOFs are very low band gap semiconductor and Ni-BIT shows metallic behavior. By calculating the ΔG H * , the key descriptor for HER catalytic activity, we have predicted that catalytic performance