Computational Screening of Electrocatalytic Activity of Transition Metal

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C: Energy Conversion and Storage; Energy and Charge Transport

Computational Screening of Electrocatalytic Activity of Transition Metal Anchored CdS Nanotubes for Water Splitting Priyanka Garg, Akhil S Nair, Kuber Singh Rawat, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01589 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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The Journal of Physical Chemistry

Computational Screening of Electrocatalytic Activity of Transition Metal Anchored CdS Nanotubes for Water Splitting Priyanka Garg, † Akhil S. Nair, † Kuber Singh Rawat, † Biswarup Pathak†,#,* †Discipline

of Chemistry and #Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore. M.P. 453552, India Email: [email protected] Abstract Exploring novel catalyst materials for water splitting reaction is far-reaching in the current research scenario. CdS derived nanostructures have been identified as potential catalysts for water splitting for decades. Realizing the competence of transition metal doping in the desirable tuning of the properties of nanostructures, we have studied the catalytic activity of late transition metal (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) doped CdS nanotubes (TM@CdS NT) for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Systematic screening of stability as well as activity among the doped NT structures is carried out and compared the results with pristine CdS NT and bulk CdS. The doping of transition metals is found to be accompanied with an enhancement of impurity d states near the Fermi level, suggesting an efficient electrocatalytic activity. Majority of transition metal doped structures are associated with significant stability and are observed to improve both the OER as well as HER activity. Activity analysis places Pd@CdS and Ru@CdS as optimal catalysts for OER and HER, respectively with the lowest overpotential, outperforming pristine CdS NT as well as bulk CdS. The origin of the activity trend is attributed to the differences in the interaction to the reaction intermediates across the series of doped nanotube structures. A complete scrutiny

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of energetics of elementary reactions for all the TM@CdS NT structures is provided and an activity plot is constructed in order to have a correlation between overpotential and adsorption energetics.

1. Introduction The increasing consumption of fossil reserves makes hydrogen carrier an ideal future energy source which can be produced from water using solar energy and catalyst without any greenhouse gas emission.1-3 Especially, semiconductor-based water splitting technology has attracted a great attention among the scientific community for its potential to utilize solar energy for H2 production.4-5 The complete process of water splitting consists of two half-cell reactions: oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode.6-7 The slow kinetics and high overpotential associated with OER (HER) which deprived electrochemical performance.8-9 Therefore, it is highly important to explore catalysts with enhanced HER and OER activity. Cadmium sulphide (CdS) is a visible light driven semiconductor material,10-11 which is widely studied due to its perfect band edge position for overall water splitting reaction.12-14 However, rapid electron-hole recombination and less photostability of CdS do not favour the wide applications in the field of solar energy conversion.14-15 Hence, a significant extent of attempts has been done to enhance the performance of CdS by structural engineering and surface modifications.16-18 In the recent years, a controllable synthesis of nanostructure materials from the bulk has been evidenced in improving the material properties extensively.19 Therefore, a similar approach has been used for CdS to synthesize it in various nanostructure forms such as nanowires,20 nanotubes,21-22 nanorods,23-24 nanospheres25 and nanobelts.26 Particularly, hollow 2 ACS Paragon Plus Environment

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structured materials such as nanotubes27 have emerged as promising catalysts for photocatalytic processes28 after the discovery of carbon nanotubes29 owing to the exposure of both the inner and outer surfaces for catalysis and their high light harvesting power resulting in superior catalytic activity.30-31 In spite of the significant enhancement in photocatalytic activity exhibited by these CdS nanostructures including nanotube, the electro-catalytic activity of CdS nanostructures has not been improved considerably for OER/HER because of the high overpotential compared to the state-of-the-art catalysts.27 Therefore, it is extremely important to look for alternative methods to improve the performance of CdS for water-splitting reaction. Doping heteroatoms is an efficient approach to tune the electronic properties of materials for various applications.32-33 In particular, transition metal doping provides an active site for HER and OER reaction, which significantly reduces the reaction overpotential.34-35 This is because, dopant atom enhances impurity states near the Fermi level or changes the electronic properties, which directly affects the interaction of intermediates with catalyst surface.36-37 For example, Nørskov and coworkers34 have studied a series of TMs doped in TiO2 by first principles calculation for OER where they have confirmed that TM substituted TiO2 shows better catalytic activity. Furthermore, Pati and coworkers38 have studied cobalt oxide (Co3O4) doped with Fe, Ni, and Cu to enhance the OER activity by means of density functional theory (DFT) calculations. Furthermore, theoretical report on doping BaTiO3 with Fe and Ni reveals that such doping improves electrical conductivity which reduces the reaction overpotential for OER from 1.22 to 0.86 V.39 Another important study has been done on 1T-MoS2, where the HER activity has been triggered by doping with transition metals at the Mo site.35,

40

Thus, a broad spectrum of

experimental research has been associated with an in-depth examination of TM doping in some semiconductors which have led to higher catalytic activities.41-42 Irrespective of the considerable



amount of experimental reports on solar light driven water splitting activity of TM substituted CdS nanostructure catalysts,43-46 the electrocatalytic activity of TM doped CdS nanostructures for water splitting has not been addressed to the best of our knowledge. Hence, a computational screening study of the CdS nanotube doped with a series of TMs is worthwhile and is expected to provide sound insights towards the origin of electrocatalytic activity trends. Motivated by these studies, we have considered a sequence of late TMs from 3d (Fe, Co, Ni, and Cu), 4d (Ru, Rh, Pd, and Ag), and 5d (Os, Ir, Pt, and Au) series and doped them at the Cd site in CdS nanotube structure.The rationale behind the choice of these TMs lies in their ability to balance the adsorption and desorption processes for a number of catalytic reactions. On the other hand, early TM are more reactive to bind with reaction intermediates too strongly and results in catalyst surface poisoning.47 The primary objective of our study is to identify the effect of TM doping in the catalytic activity CdS NT towards OER and HER. A systematic study of the energetic stability, OER/HER reaction overpotential and activity plot is done for understanding the catalytic properties of TM doped CdS nanotube structures. Owing to the immense number of possible options for transition metal doping, a screening study is necessary to identify the potential candidate materials for catalysis to guide the experimental endeavors in an efficient and economical path. 2. Modelling and Computational Methods In the past studies, CdS NT structures have been realized via various experimental techniques where the dominant phase of synthesized nanotube is hexagonal over cubic.48-49 In this context, we have modelled a hexagonal-shaped facetted (Figure 1) CdS nanotubes (CdS NT) structure. The CdS NT is formed by facetted CdS nanorod (NR) exposed with four (1100) and four (1010)


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planes, because these are the planes involved during the growth morphology of CdS NR.50 After that CdS NT of diameter 1.2 nm is made by removing the core atoms from CdS NR.

Figure 1: (a-c) Cross sectional and side views of CdS NT with different exposed facets. (d) Possible adsorption sites in CdS NT for OER/HER intermediates. The spin polarized DFT calculations are carried out by the projector-augmented wave (PAW)51 method as implemented in the Vienna Ab initio Simulation Package (VASP).52 The exchangecorrelation potential is described by the generalized gradient approximation (GGA)53 using Perdew, Burke and Ernzerhof (PBE)54 functional. The valence atomic orbitals are defined by plane-wave basis sets with cut-off energies of 470 eV. In the self-consistent calculations, the energy convergence criteria is set to 10-4 eV between two consecutive relaxation steps, and the atomic forces of all the systems are minimized to less than 0.01 eV Å-1 without any symmetry constraints. The unit cell and the 1×1×2 supercell CdS NT structures contain 36 and 72 atoms respectively, and later is employed for the catalytic study. The Brillouin zone is sampled with 1×1×2 and 1×1×7 gamma centered k-point mesh for geometry optimization and electronic state calculations, respectively. We have added ~16 Å vacuum in the x-y-directions to decouple the spurious interactions between two periodic images. Grimme’s D3 type of semiempirical method



is used to account for dispersion interactions.55 The Bader charge calculations are performed using near-grid algorithm refine-edge method proposed by Henkelman et al.56-57 3. Results & Discussion 3.1 Doping of Transition Metals In the present study, we have replaced one Cd atom with 3d (Fe, Co, Ni, and Cu), 4d (Ru, Rh, Pd, and Ag), and 5d (Os, Ir, Pt, and Au) series TMs one after another from (1100) and (1010) surfaces with a doping concentration of 2.7%. Previous experimental studies have demonstrated that such low concentration of TMs in CdS is favorable.58-59 We have investigated both the (11 00) and (1010) facets of CdS NT for TM doping. Our results indicate that Fe, Ru, Pd, Os, Ag, Ir and Au metals prefer to be adsorbed on the (1010) facet while rest of metals (Co, Ni, Cu, Rh, and Pt) on the (1100) facet of CdS NT (Figure 2). Following these studies, we have considered most preferred doped sites for OER and HER study.


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Figure 2: Optimized structures of transition metal doped CdS NT. Here, grey shaded area metals are stable at the (1100) facet, whereas green shaded area metals are stable at the (1010) facet. The stability of TM@CdS NT is very important from catalytic perspectives. Therefore, we have evaluated the formation energy (Ef) of TM@CdS NTs which has been identified as an important descriptor for stability of TM doped nanostructures. 60-62 The Ef for TM@CdS NT (TM= Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt and Au) systems are calculated as follows: Ef = [ETM@CdS NT –ECdS NT]-x[ETM-ECd]

(1)

where, ETM@CdS NT, and ECdS NT are the energies of transition metal doped CdS NT and pristine CdS NT, respectively. On the other hand, the ETM, and ECd are the chemical potential of dopant TM and Cd atoms respectively. The chemical potential of TM and Cd atoms has been calculated 7 ACS Paragon Plus Environment


from per atom energy of their stable bulk structure. The stable bulk phases for all the considered TMs and Cd are listed in Table S1. Hence, a lesser positive the formation energy value means higher the stability implying the higher thermodynamic plausibility of formation of TM@CdS NT via doping TM into CdS NT. The bar diagram (Figure 3) shows an order of stability of Fe > Pd > Cu > Au ~ Ag ~ Ni > Co > Pt > Rh > Ir > Ru > Os. We have observed that the stability of TM@CdS NT is affected by the size difference between Cd and TM atoms. We have observed that the most stable candidates suggested by formation energy calculation possess a size similarity between the doped TM and Cd (Fe@CdS, Pd@CdS and Cu@CdS NTs) whereas a lesser size similarly is observed for less stable systems (Os@CdS NT, Ru@ CdS NT,and Ir@CdS NT). Nevertheless, the total stability of the NT structure does not depend exclusively on the size dependency of the TM, but is also influenced by the structural changes associated with doping as well as the interaction of doped TMs with nearby atoms. The degree of local structural distortion associated with TM doping is measured by changes in the Cd-S bond lengths after doping where a general correlation with changes in the Cd-S bond length and formation energy of TM@CdS NTs is observed. The average Cd-S bond lengths near to the TM atom observed for the most stable Fe@CdS, Pd@CdS and Cu@CdS NTs identified from formation energy calculations are very much close to that of pristine CdS NT whereas the least stable candidates such as Os@CdS, Ru@CdS and Ir@CdS NT show considerably higher Cd-S bond lengths leading to less structural integrity. Moreover, the relatively higher stability of first row TMs (Fe, CO, Ni, Cu) doped NTs is expected to arise from the strong TM-S interaction owing to their higher electronegativity difference with respect to S (Table S2) in spite of the size differences with Cd. Therefore, the stability trend observed among TM@CdS NT systems can be attributed to arise as the result of a cumulative effect of size dependency of the TM, structural changes


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associated with TM doping and the strength of interaction of TM with nearby atoms.

Figure 3: The formation energy (Ef) of different TM@CdS NT systems. 3.2. Electronic Structure The electronic properties of materials are very crucial for electrocatalytic performance because electrical conductivity of catalysts may decide the charge transfer between OER/HER intermediates and catalysts surface. In this context, we have done density of states (DOS) analysis of different TM@CdS NT for identifying the effect of TM doping on CdS NT. The partial density of states (PDOS) plot shows (Figure S1) both the valence band (VB) and conduction band (CB) are dominantly formed by hybridized S (3p), Cd (4d), and TM (3d/4d/5d) orbitals, where maximum contribution comes from S (3p) orbitals. Furthermore, most of the TM@CdS NT structures show semiconducting properties like CdS NT except Co, Ru, and Rh@CdS NT. Most of the metals show p-d mixing at the valence band region which indicates the strong interaction of S with TM atoms. We have observed that there is a significant change in the extent of p-d mixing while transition metals are introduced into the pristine CdS NT. Moreover, there are considerable differences in the extent of p-d mixing within the different TM doped NTs as well. For example, in Pd@CdS NT, the p-d mixing between Pd-d and S-p orbitals is stronger compared to Ag-d and S-p orbitals. The extent of p-d mixing can give the information 9 ACS Paragon Plus Environment


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about the bonding between TM and surrounding S atoms which can be also correlated to the structural integrity of the TM@CdS system as evident from the higher stability of Pd@CdS than Ag@CdS NT. Furthermore, metal impurity d states lie near the Fermi level and these impurity d states of TMs increase the electrical conductivity compared to pure CdS NT which bring about significant electrocatalytic performance.36 Moreover, a couple of experimental and theoretical reports have also shown the presence of impurity d states after doping with transition metals which leads to improved catalytic performance of CdS.44, 58, 63-65 3.3. Oxygen Evolution Reaction (OER) The OER is a four-electron water oxidation reaction. Here, we have considered OER mechanism proposed by Nørskov and co-workers for which the elementary steps are as follows:66-68

* + H2O → HO* + (H+ + e-)

(2)

HO* → O* + (H+ + e-)

(3)

O* + H2O → HOO* + (H+ + e-)

(4)

HOO* → * + O2 + (H+ + e-)

(5)

Where, X* (X = O, HO, and HOO) and * represent the TM@CdS NT and adsorbed species on TM@CdS NT, respectively. To compute the free energy change of proton coupled electron transfer oxidation/reduction reactions, we have used the computational hydrogen electrode (CHE) model according to which the free energy change of an electrochemical reaction is calculated as:69 ∆G = ∆E + ∆ZPE - T∆S + ∆GU 10 ACS Paragon Plus Environment

(6)

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Where, ∆E, ∆ZPE, and ∆S are the difference in total energy, zero-point energy, and entropy change between final and initial states, respectively. The ∆GU = -eU, where U is the potential; e is the number of transferred electrons and T denotes the temperature which is 300 K. The values of (ΔS) and ∆ZPE correction for free gaseous molecules have been taken from the NIST database,70 whereas the ∆ZPE correction is calculated for adsorbed intermediates via vibrational frequency calculation using density functional perturbation theory (DFPT)71 implemented in VASP. As DFT is inadequate to accurately calculate the bond energy of molecular oxygen (O2), thus we have calculated the energy of O2 from standard Gibbs energy of water formation (2H2 + O2 = 2H2O) that is -4.92 eV.68, 72 3.3.1: Adsorption of Intermediates Since the free energies of elementary steps of OER depend on the adsorption energies of intermediates, we have investigated the adsorption characteristics of intermediates on all the TM@CdS NT systems. These intermediates show different adsorption patterns and interactions for different doped metals. As a result, the potential determining step and viable reaction paths can vary from one surface to another. Here, we have investigated all the possible adsorption sites in TM@CdS NT and possible adsorption orientation of HO and HOO intermediates and the energetically most stable structures (Figure S2-S3) are considered for further investigations. The extent of adsorption of intermediates has been evaluated by adsorption energy calculation (Text S1). The adsorption energy data (Table 1) suggests that all TM@CdS NTs show maximum binding strength towards O*. We have observed that all the OER intermediates strongly bind with Os@CdS NT as compared to any other TM@CdS NT. Such a strong adsorption limits the practical utility of Os@CdS NT for OER catalyst. 11 ACS Paragon Plus Environment


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Table 1: Preferred adsorption sites and adsorption energies (Ead in eV) of OER and HER intermediates adsorption on TM@CdS NT surface. Here t, b, and h denote top, bridge, and hollow sites, respectively. System

HO*

O*

HOO*

H*

Fe@CdS NT

-3.76 (bFe-Cd)

-4.92 (bFe-Cd)

-2.07 (tFe)

-1.66 (tFe)

Co@CdS NT

-3.69 (bCo-Cd)

-4.69 (bCo-Cd)

-2.73 (bCo-Cd)

-2.11 (tCo)

Ni@CdS NT

-3.46 (tNi)

-5.18 (bNi-S)

-1.44 (tNi)

-2.54 (tS)

Cu@CdS NT

-2.83 (bCu-Cd)

-4.03 (h)

-1.61 (bCu-Cd)

-3.78 (bCd-S)

Ru@CdS NT

-3.90 (bRu-Cd)

-6.18 (tRu)

-2.73 (bRu-Cd)

-3.00 (tRu)

Rh@CdS NT

-3.24 (bRh-Cd)

-4.79 (bRh-Cd)

-2.04 (bRh-Cd)

-2.91 (tRh)

Pd@CdS NT

-3.28 (bPd-Cd)

-4.25 (bPd-Cd)

-1.37 (bPd-Cd)

-2.29 (tS)

Ag@CdS NT

-2.77 (bAg-Cd)

-4.28 (bAg-S)

-1.49 (bAg-Cd)

-3.76 (bCd-S)

Os@CdS NT

-4.05 (bOs-Cd)

-6.92 (tOs)

-3.41 (bOs-Cd)

-2.39 (tS)

Ir@CdS NT

-3.56 (bIr-Cd)

-5.44 (bIr-Cd)

-1.91 (bIr-Cd)

-5.27 (tS)

Pt@CdS NT

-3.17 (tPt)

-4.75 (bPt-Cd)

-1.74 (bPt-Cd)

-3.71 (tPt)

Au@CdS NT

-3.24 (bAu-Cd)

-4.42 (bAu-S)

-1.83 (bAu-Cd)

-2.29 (tS)

-1.58 (TCd)

-4.49 (BCd-S)

-0.46 (BCd-Cd)

-1.93 (TS)

-2.36 (bCd-Cd)

-4.35 (bCd-S)

-0.45 (bCd-Cd)

-1.52 (tS)

27Bulk

CdS

CdS NT

3.3.2 Free Energy Analysis


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The reaction free energy profiles of TM@CdS NT systems is shown in Figure 4 and Figure S4 at different potentials of OER. The free energy diagrams provide an understanding of energetics associated with each elementary step of OER across the series of TM@CdS NTs considered. Overpotential (ηOER), an important activity descriptor for OER can be deduced from the free energy analysis by the following formulae:66-67 ηOER = ∆Gmax/e - 1.23 V

(7)

Where, ∆Gmax is the maximum free energy change of elementary steps (1)-(4) of OER. The overpotential is identified as the additional bias required to enable all the OER elementary steps to be energetically downhill. The TM@CdS NT with the lowest overpotential can perform as potential catalysts for OER and hence is our choice of interest. It is noteworthy from the Table 2 that all the systems have HOO* formation as the potential determining step (PDS) except for Pd@CdS NT. As O* intermediate strongly adsorbs on the other catalysts surface, HOO* formation becomes the least energetically favourable step, whereas Pd shows quite strong interaction with HO* and hence O* formation turns out to be the PDS as compared to others. Furthermore, the reaction overpotential values as shown in Table 2 indicate that Co@CdS NT, Pd@CdS NT, Pt@CdS NT and Au@CdS NT require very low overpotential for OER. Interestingly, most of these metals are efficient to reduce the reaction overpotential significantly compared to pure CdS NT (1.55 V) and bulk CdS surface (1.65 V).26 Figure 4 shows the free energy profile of OER on the most active Pd@CdS NT and least active Os@CdS NT surface with respect to pristine CdS NT. The reason behind the lesser activity of Os compared to pristine CdS NT is the strong interaction with O* intermediate as evident from their adsorption energy values. Furthermore, we have observed some negative ΔG values such as



O* formation step in Os@CdS NT and O2 formation step in pristine CdS NT. The spontaneity of this step is determined by the binding energy differences between HO* and O*, which is the maximum (2.87 eV) among all the TM@CdS systems studied. O* is highly stabilized on the Os@CdS surface which makes the O* formation exergonic and hence associated with a negative ΔG value. Similarly, the negative ΔG associated with pristine CdS NT for the O2 formation from HOO* can be attributed to the very weak binding of HOO* (-0.45 eV) intermediate, which also results the removal of that intermediate to be exergonic. The OER is the endergonic reaction at zero potential and an efficient catalyst should be able to give a less overpotential for OER to be energetically downhill. However, if one of the steps of OER is exergonic (spontaneous) at zero potential, it leads to catalyst surface poisoning/empty site generation as one of the intermediate being strongly/weakly adsorbed to the catalyst surface. Therefore, both Os@CdS NT as well as pristine CdS NT are identified as less active for OER which is also evident from their high overpotential values. On the other hand, Pd@CdS NT is most active catalyst with very low overpotential because it shows less interaction with O* intermediate over pristine CdS NT and other doped CdS NT structures too. This is further confirmed by observing PDOS (Figure S5) of Os@CdS NT+O and Pd@CdS NT+O. The PDOS of Os@CdS NT+O suggests that O (2p) orbital shows very strong overlapping with Os (5d) orbitals. On the other hand, in case of Pd@CdS NT+O, the O (2p) orbitals show weak overlapping with Pd and Cd (4d) orbitals compared to Os@CdS NT because the O atom adsorbs at the center position between three metals (two Cd and one Pd) with bond length of 1.92 Å (Pd-O), 2.35 Å (Cd-O), and 2.79 Å (SO), respectively. This gives rise to strong interaction of O atom on Os@CdS NT surface and weak interaction with Pd@CdS NT surface, so that it shows different PDS compared to others.


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Thus, Pd@CdS NT is identified as the optimal candidate for OER among the transition metal doped CdS NT structures. Table 2: Calculated free energy change (∆G in eV) of elementary steps of OER and reaction overpotential (ηOER in V). The red values indicate the potential determining step. TM@CdS NT

∆G at 0 V H2O →HO*

HO* →O*

O* →HOO*

HOO* →O2

ηOER

Fe@CdS NT

0.25

1.34

2.26

1.08

1.03

Co@CdS NT

0.46

1.31

1.87

1.28

0.64

Ni@CdS NT

1.20

0.83

2.29

0.59

1.06

Cu@CdS NT

1.09

0.67

2.62

0.54

1.39

Ru@CdS NT

0.01

0.08

3.35

1.49

2.12

Rh@CdS NT

0.50

1.01

2.26

1.15

1.03

Pd@CdS NT

1.25

1.76

1.47

0.43

0.53

Ag@CdS NT

1.48

0.75

2.32

0.36

1.09

Os@CdS NT

0.15

-0.24

3.47

1.54

2.24

Ir@CdS NT

0.52

0.71

2.89

0.79

1.66

Pt@CdS NT

1.32

1.06

1.89

0.65

0.66

Au@CdS NT

1.09

1.17

1.99

0.67

0.76

2.41

-0.10

2.88

-0.28

1.65

1.85

0.54

2.78

-0.26

1.55

27Bulk

CdS

CdS NT



What more, we have compared the obtained overpotential value (0.53 V) for Pd@CdS NT with some of the previous reported materials. The experimental overpotential in acidic medium for OER of different benchmark catalysts such as RuO2, IrO2, Co3O4, MnO2 belong to the range of 0.4-0.6 V to produce the current density of 10 mA cm-2.73-74 We have observed that our calculated overpotential (0.53 V) of OER for the best candidate Pd@CdS NT is comparable with the experimental overpotential obtained for these catalysts and hence is expected to perform as a potential catalyst for OER. Here, we observed that shallow content of TMs (2.7%) significantly improves the reaction overpotential compared to that of pristine CdS nanotube without compromising the stability. In this respect, we believe that our study can inspire experimentalists for the further exploration of TM@CdS NT systems. Also, our study provides insights for the engineering 1D TM doped CdS based composites with optimal performnace of water splitting reaction at the macroscopic level.

Figure 4: Reaction free energy profile for OER at different potentials for Pd@CdS NT, pristine CdS NT and Os@CdS NT structures, respectively. We have correlated the activity of TM@CdS NT catalysts with the well known OER activity descriptor ∆GO* - ∆GHO* (Figure 5), the adsorption free energy difference between O* and HO*, 16 ACS Paragon Plus Environment

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for a comprehensive analysis of activity trend. The adsorption free energies are obtained by incorporating zero-point energy correction to the calculated adsorption energy. It is found that Pd@CdS NT occupies the peak position of the plot with a ∆GO* - ∆GHO* value of 1.82 eV which is the highest among all the other TM@CdS NT structures. This value is slightly higher than 1.5 eV at which maximum activity has been observed for many previously reported bench mark catalysts such as RuO2,74 which can be attributed to the relatively low O* adsorption shown by Pd@CdS NT in comparison to HO* among the series of TM@CdS NTs considered which causes it to shift towards the peak position of activity plot. This difference in trend between the O* and HO* adsorption brings out the highest ∆GO* - ∆GHO* value for Pd@CdS NT.

Figure 5: Calculated negative overpotential plotted against ∆GO* - ∆GHO* for TM@CdS NT structures. 3.4. Hydrogen Evolution Reaction (HER) In the past decade, a significant amount of theoretical studies has been reported where TM doping is found to accelerate the efficiency of catalysts towards electrochemical HER, examples 17 ACS Paragon Plus Environment


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including 1T-MoS2, C2N, g-CN and so on.35-36, 75 In this context, the electrochemical HER on the TM doped CdS NT is scrutinized to check the effect of dopant on the HER activity. Here, we have used Volmer-Heyrovsky mechanism for HER which involves proton adsorption along with electron deduction and formation of H* followed by generation of molecular hydrogen in the next step. Hence, the possible HER mechanism is as follows:76 * + (H+ + e) → H*

(8)

H* + (H+ + e) → H2 + *

(9)

Here, H* indicates the adsorbed hydrogen atom on the doped CdS NT surface. Therefore, the Gibbs free energy change (∆GH) for HER can be evaluated by equation (6). The catalytic efficiency of TM@CdS NT for HER is examined by the ∆GH values of H* formation on the different TM@CdS NT. It is the ∆GH value which is the descriptor to evaluate the HER activity.35-36,

75

According to this approach, the ideal HER catalyst should exhibit a ∆GH value

close to zero. For comparison, we have compared our data (Table S3) with the pristine CdS NT, where we have seen that the ∆GH value (1.60 eV) is very high. This indicates that pristine CdS NT is not efficient for HER. Figure 6 shows that the ∆GH has been reduced significantly for all the TM@CdS NT compared to pristine CdS NT and bulk CdS surface (1.20 V), respectively.27 This implies that all the TM@CdS NTs are efficient for reducing the reaction overpotential of HER. Remarkably, the Ni@CdS NT and Ru@CdS NT show minimum ∆GH or value close to zero, which indicates that these are most active catalysts among all the TM@CdS NTs for HER. Figure S6 reveals that the active site for HER in Ni@CdS NT is S, whereas Ru acts as an active site in Ru@CdS NT for significant reduction of ∆GH. Furthermore, these metals (Ru, and Ni) are also efficient for enhancing the HER activity of some previous reported catalysts.77 A


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comparison of our results with the previous experimental reports suggest that some of the TM (Ni, Cu, and Ag) doped CdS nanostructures have been reported as good catalysts for photocatalytic and photoelectrochemical HER without any requirement of co-catalyst.43-45 For example, Ni doped CdS increases the H2 evolution rate about ten times compared to pure CdS.44, 78

Our results are in qualitative agreement with experimental reports where Ni doped CdS has

been found to bring out excellent HER activity which is identified as a candidate of potential interest in our results with a reduction of overpotential from 1.60 V to 0.28 V. It is also noteworthy that, Pd@CdS NT, which is identified as a potential catalyst for OER, is associated with a reasonable HER activity as well with a ∆GH value of -0.50 eV. These results illustrate that doping CdS with these TMs is an efficient way to enhance the HER performance.

Figure 6: Reaction free energy profile for HER of TM@CdS NT.



Thus, both OER and HER activity of CdS NT are observed to be enhanced via doping with TMs under consideration. Therefore, TM doping is an effective approach to improve the activity of pristine CdS catalysts for overall water-splitting. 4. Conclusion The first principles calculations provide significant insights regarding the stability and catalytic activity of TM@CdS NT. All the TM@CdS NTs are found to be associated with significant stability. The catalytic activity of TM@CdS NT to oxygen evolution and hydrogen evolution reaction (OER/HER) is confirmed by evaluating the reaction overpotential. It is observed most of the TM (Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, and Au) doped NTs are promising for reducing the reaction overpotential for OER compared to that of pristine CdS NT and bulk CdS surface too. Among the different NT structures considered, Pd@CdS NT shows highest activity with the lowest overpotential of 0.53 V for OER because of the less interaction of OER intermediates with Pd@CdS NT surface. Furthermore, the HER performance of TM@CdS NT is analyzed via Gibbs free energy change (∆GH) values. All of these TMs are efficient to reduce the ∆GH over pure CdS NT amongst which Ni@CdS NT and Ru@CdS NT are the best HER catalysts due to least ∆GH values. The observed activity trends are comparable with previous experimental reports. Hence, TM doping emerges as an efficient way to improve the performance of CdS NT towards electrocatalytic water splitting. Our results could provide sound insights to the experimental design of transition metal doped CdS nanotube catalysts for water splitting reaction. 5. Associated Contents: * Supporting Information


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Formation energy and adsorption energy calculations (Text S1-S2). PDOS of pristine CdS NT and TM@CdS NT (Figure S1), adsorption of OER intermediates on TM@CdS NT (Figure S2S3), reaction free energy profile for OER at different potentials for TM@CdS NT structures (Figure S4), PDOS of Pd@CdS NT+O and Os@CdS NT+O (Figure S5), adsorption of HER intermediate on TM@CdS NT (Figure S6) have given in Supporting Information. Atomic radius (Table S1) and reaction free energy values (∆GH) of HER (Table S2) have been given in Supporting Information. 6. Acknowledgments: We thank IIT Indore for the lab and computing facilities. This work is supported by DST-SERB [Grant number: EMR/2015/002057] and CSIR [Grant number:01(2886)/17/EMR(II)]. P.G., and A. S. N. thank MHRD, and K. S. R. thanks UGC for the research fellowship. References 1. Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729-15735. 2. Zhao, C.; Yu, C.; Liu, S.; Yang, J.; Fan, X.; Huang, H.; Qiu, J. 3D Porous N-Doped Graphene Frameworks Made of Interconnected Nanocages for Ultrahigh-Rate and LongLife Li-O2 Batteries. Adv. Funct. Mater. 2015, 25, 6913-6920. 3. Schlapbach, L.; Züttel, A. Hydrogen-storage Materials for Mobile Applications. Nature 2001, 414, 353-358. 4. Fujishima, A. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38.



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Computational Screening of Electrocatalytic Activity of Transition Metal

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