Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 13419−13427
Computational Screening of Electrocatalytic Activity of Transition Metal-Doped CdS Nanotubes for Water Splitting Priyanka Garg,† Akhil S. Nair,† Kuber Singh Rawat,† and Biswarup Pathak*,†,‡ †
Discipline of Chemistry and ‡Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore. Madhya Pradesh 453552, India
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S Supporting Information *
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 (TM) doping in the desirable tuning of the properties of nanostructures, we have studied the catalytic activity of late TM (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au)-doped CdS nanotubes (TM@CdS NTs) 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 the results are compared with pristine CdS NT and bulk CdS. The doping of TMs is found to be accompanied by an enhancement of impurity d states near the Fermi level, suggesting an efficient electrocatalytic activity. Majority of TM-doped structures are associated with significant stability and are observed to improve both OER and HER activities. Activity analysis places Pd@CdS and Ru@CdS as optimal catalysts for OER and HER, respectively, with the lowest overpotential, outperforming pristine CdS NTs as well as bulk CdS. The origin of the activity trend is attributed to the differences in the interaction with the reaction intermediates across the series of doped NT structures. A complete scrutiny of energetics of elementary reactions for all TM@CdS NT structures is provided, and an activity plot is constructed 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 a catalyst without any greenhouse gas emission.1−3 Especially, the semiconductorbased water-splitting technology has attracted a great deal of 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) have deprived the electrochemical performance.8,9 Therefore, it is highly important to explore catalysts with enhanced HER and OER activities. Cadmium sulphide (CdS) is a visible light-driven semiconductor material,10,11 which is widely studied because of its perfect band edge position for the overall water-splitting reaction.12−14 However, the rapid electron−hole recombination and the less photostability of CdS do not favor 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 © 2019 American Chemical Society
similar approach has been used for CdS to synthesize it in various nanostructure forms such as nanowires,20 nanotubes (NTs),21,22 nanorods (NRs),23,24 nanospheres,25 and nanobelts.26 Particularly, hollow structured materials such as NTs27 have emerged as promising catalysts for photocatalytic processes28 after the discovery of carbon NTs29 owing to the exposure of both the inner and outer surfaces for catalysis and their high light-harvesting power, resulting in a superior catalytic activity.30,31 In spite of the significant enhancement in photocatalytic activity exhibited by these CdS nanostructures including NTs, the electro-catalytic activity of CdS nanostructures has not been improved considerably for the OER/HER because of the high overpotential compared to the state-of-theart catalysts.27 Therefore, it is extremely important to look for alternative methods to improve the performance of CdS for the 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 (TM) doping provides an active site for the HER and OER, which significantly reduces the reaction overpotential.34,35 This is because the dopant atom enhances impurity states near the Fermi level or changes the electronic properties, which directly Received: February 19, 2019 Revised: April 18, 2019 Published: May 9, 2019 13419
DOI: 10.1021/acs.jpcc.9b01589 J. Phys. Chem. C 2019, 123, 13419−13427
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Figure 1. (a−c) Cross-sectional and side views of the CdS NT with different exposed facets. (d) Possible adsorption sites in the CdS NT for OER/ HER intermediates.
Figure 2. Optimized structures of TM-doped CdS NTs. Here, gray-shaded area metals are stable at the (11̅00) facet, whereas green-shaded area metals are stable at the (101̅0) facet.
Cd site in the CdS NT 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 TMs 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 of Cds nanotubes (CdS NT) toward 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 NT structures. Owing to the immense number of possible options for TM 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.
affects the interaction of intermediates with the catalyst surface.36,37 For example, Nørskov and co-workers34 have studied a series of TMs doped in TiO2 by first-principles calculation for OER, where they have confirmed that TMsubstituted TiO2 shows better catalytic activity. Furthermore, Bothra and Pati38 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 TMs 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 lightdriven water-splitting activity of TM-substituted CdS nanostructure catalysts,43−46 the electrocatalytic activity of TMdoped CdS nanostructures for water splitting has not been addressed to the best of our knowledge. Hence, a computational screening study of the CdS NT doped with a series of TMs is worthwhile and is expected to provide sound insights toward 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
2. MODELLING AND COMPUTATIONAL METHODS In the past studies, CdS NT structures have been realized via various experimental techniques where the dominant phase of the synthesized NT is hexagonal over cubic.48,49 In this context, we have modeled a hexagonal-shaped facetted (Figure 1) CdS NT structure. The CdS NT is formed by a facetted CdS NR exposed with four (11̅00) and four (101̅0) planes because these are the planes involved during the growth morphology of the CdS NR.50 After that, a CdS NT of diameter 1.2 nm is made by removing the core atoms from the CdS NR. 13420
DOI: 10.1021/acs.jpcc.9b01589 J. Phys. Chem. C 2019, 123, 13419−13427
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The Journal of Physical Chemistry C The spin-polarized DFT calculations are carried out by the projector-augmented wave51 method as implemented in the Vienna Ab initio Simulation Package (VASP).52 The exchange−correlation potential is described by the generalized gradient approximation53 using the Perdew−Burke−Ernzerhof54 functional. The valence atomic orbitals are defined by plane-wave basis sets with cutoff energies of 470 eV. In the selfconsistent calculations, the energy convergence criterion is set to 10−4 eV between two consecutive relaxation steps, and the atomic forces of all systems are minimized to less than 0.01 eV Å−1 without any symmetry constraints. The unit cell and the 1 × 1 × 2 supercell of CdS NT structures contain 36 and 72 atoms, respectively, and are employed later 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
Figure 3. Formation energy (Ef) of different TM@CdS NT systems.
NTs), whereas a lesser size is similarly 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 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 candidates Fe@CdS, Pd@ CdS, and Cu@CdS NTs identified from the formation energy calculations are very much close to that of the 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 TM (Fe, CO, Ni, and 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 a result of a cumulative effect of size dependency of the TM, structural changes associated with TM doping, and the strength of the interaction of TM with nearby atoms. 3.2. Electronic Structure. The electronic properties of materials are very crucial for electrocatalytic performance because the electrical conductivity of catalysts may decide the charge transfer between OER/HER intermediates and the catalyst surface. In this context, we have done the density of states (DOS) analysis of different TM@CdS NTs for identifying the effect of TM doping on CdS NTs. The partial DOS (PDOS) plot shows (Figure S1) that both the valence band (VB) and conduction band are dominantly formed by hybridized S (3p), Cd (4d), and TM (3d/4d/5d) orbitals, where the maximum contribution comes from S (3p) orbitals. Furthermore, most of the TM@CdS NT structures show semiconducting properties like CdS NT except Cu, Ru, and Rh@CdS NT. Most of the metals show p−d mixing at the VB 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 TMs 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 that between Ag d and S p orbitals. The extent of p−d mixing can give information about the bonding between the TM and surrounding S atoms, which can be also correlated with 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
3. RESULTS AND DISCUSSION 3.1. Doping of TMs. 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 (11̅00) and (101̅0) surfaces with a doping concentration of 2.7%. Previous experimental studies have demonstrated that such a low concentration of TMs in CdS is favorable.58,59 We have investigated both the (11̅00) and (101̅0) facets of the CdS NT for TM doping. Our results indicate that Fe, Ru, Pd, Os, Ag, Ir, and Au metals prefer to be adsorbed on the (101̅0) facet while the rest of metals (Co, Ni, Cu, Rh, and Pt) are adsorbed on the (11̅00) facet of the CdS NT (Figure 2). Following these studies, we have considered the most preferred doped sites for the OER and HER studies. 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 the stability of TMdoped nanostructures.60−62 The Ef for TM@CdS NT (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) systems is calculated as follows Ef = [E TM@CdS NT − ECdS NT] − x[E TM − ECd ] (1) where ETM@CdS NT and ECdS NT are the energies of TM-doped CdS NTs and pristine CdS NTs, respectively. On the other hand, the ETM and ECd are the chemical potentials of dopant TM and Cd atoms, respectively. The chemical potentials of TM and Cd atoms have been calculated from per atom energy of their stable bulk structure. The stable bulk phases for all considered TMs and Cd are listed in Table S1. Hence, a lesser positive formation energy value means higher stability, implying the higher thermodynamic plausibility of formation of TM@CdS NT via doping TM into CdS NTs. 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 the 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 13421
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The Journal of Physical Chemistry C impurity d states of TMs increase the electrical conductivity compared to pure CdS NTs, 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 TMs, which leads to improved catalytic performance of CdS.44,58,63−65 3.3. OER. The OER is a four-electron water oxidation reaction. Here, we have considered the OER mechanism proposed by Nørskov and co-workers for which the elementary steps are as follows66−68
Table 1. Preferred Adsorption Sites and Adsorption Energies (Ead in eV) of OER and HER Intermediates Adsorption on the TM@CdS NT Surfacea system
HO*
Fe@CdS NT
−3.76 (bFe−Cd) −3.69 (bCo−Cd) −3.46 (tNi)
Co@CdS NT Ni@CdS NT Cu@CdS NT
* + H 2O → HO* + (H+ + e−)
(2)
HO* → O* + (H+ + e−)
(3)
Ru@CdS NT Rh@CdS NT
O* + H 2O → HOO* + (H+ + e−) +
−
HOO* → * + O2 + (H + e )
(4)
Pd@CdS NT
(5)
Ag@CdS NT
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 model, according to which the free-energy change of an electrochemical reaction is calculated as69 ΔG = ΔE + ΔZPE − T ΔS + ΔG U
Os@CdS NT Ir@CdS NT Pt@CdS NT Au@CdS NT 27
bulk CdS
(6)
where ΔE, ΔZPE, and ΔS are the difference in the total energy, zero-point energy, and entropy change between the final and initial states, respectively. ΔGU = −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 ΔZPE correction for free gaseous molecules have been taken from the NIST database,70 whereas the ΔZPE correction is calculated for adsorbed intermediates via vibrational frequency calculation using density functional perturbation theory71 implemented in VASP. As DFT is inadequate to accurately calculate the bond energy of molecular oxygen (O2), we have calculated the energy of O2 from the standard Gibbs free energy of water formation (2H2 + O2 = 2H2O), that is, −4.92 eV.68,72 3.3.1. Adsorption of Intermediates. Because 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 TM@CdS NT systems. These intermediates show different adsorption patterns and interactions for different doped metals. As a result, the potential determining step (PDS) and viable reaction paths can vary from one surface to another. Here, we have investigated all possible adsorption sites in TM@CdS NTs and the possible adsorption orientation of HO and HOO intermediates, and the energetically most stable structures (Figures S2 and S3) are considered for further investigations. The extent of adsorption of intermediates has been evaluated by adsorption energy calculations (Text S1). The adsorption energy data (Table 1) suggest that all TM@ CdS NTs show maximum binding strength toward O*. We have observed that all 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 the OER catalyst. 3.3.2. Free-Energy Analysis. The reaction free-energy profiles of TM@CdS NT systems are shown in Figures 4
CdS NT a
−2.83 (bCu−Cd) −3.90 (bRu−Cd) −3.24 (bRh−Cd) −3.28 (bPd−Cd) −2.77 (bAg−Cd) −4.05 (bOs−Cd) −3.56 (bIr−Cd) −3.17 (tPt) −3.24 (bAu−Cd) −1.58 (TCd) −2.36 (bCd−Cd)
O*
HOO*
−4.92 (bFe−Cd) −4.69 (bCo−Cd) −5.18 (bNi−S) −4.03 (h) −6.18 (tRu) −4.79 (bRh−Cd) −4.25 (bPd−Cd) −4.28 (bAg−S) −6.92 (tOs) −5.44 (bIr−Cd) −4.75 (bPt−Cd) −4.42 (bAu−S) −4.49 (BCd−S) −4.35 (bCd−S)
H*
−2.07 (tFe)
−1.66 (tFe)
−2.73 (bCo−Cd) −1.44 (tNi)
−2.11 (tCo)
−1.61 (bCu−Cd) −2.73 (bRu−Cd) −2.04 (bRh−Cd) −1.37 (bPd−Cd) −1.49 (bAg−Cd) −3.41 (bOs−Cd) −1.91 (bIr−Cd) −1.74 (bPt−Cd) −1.83 (bAu−Cd) −0.46 (BCd−Cd) −0.45 (bCd−Cd)
−2.54 (tS) −3.78 (bCd−S) −3.00 (tRu) −2.91 (tRh) −2.29 (tS) −3.76 (bCd−S) −2.39 (tS) −5.27 (tS) −3.71 (tPt) −2.29 (tS) −1.93 (TS) −1.52 (tS)
Here t, b, and h denote top, bridge, and hollow sites, respectively.
and 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 freeenergy analysis by the following formulae66,67 ηOER = ΔGmax /e − 1.23 V
(7)
where ΔGmax is the maximum free energy change of elementary steps 2−5 of OER. The overpotential is identified as the additional bias required to enable all 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 Table 2 that all systems have HOO* formation as the PDS except for Pd@CdS NT. As the O* intermediate strongly adsorbs on the other catalyst surface, HOO* formation becomes the least energetically favorable step, whereas Pd shows quite a 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 the bulk CdS surface (1.65 V).27 Figure 4 shows the free-energy profile of OER on the most active Pd@CdS NT and the least active Os@CdS NT surface with respect to pristine CdS NTs. The reason behind the lesser activity of Os compared to pristine CdS NTs is the strong interaction with the O* intermediate, as evident from their adsorption energy values. Furthermore, we have observed 13422
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Figure 4. Reaction free-energy profile for OER at different potentials for Pd@CdS NT, pristine CdS NT, and Os@CdS NT structures, respectively.
evident from their high overpotential values. On the other hand, Pd@CdS NT is the most active catalyst with very low overpotential because it shows less interaction with the O* intermediate over the pristine CdS NT and other doped CdS NT structures too. This is further confirmed by observing the PDOS (Figure S5) of Os@CdS NT + O and Pd@CdS NT + O. The PDOS of Os@CdS NT + O suggests that the O (2p) orbital shows very strong overlapping with Os (5d) orbitals. On the other hand, in the 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 a bond length of 1.92 Å (Pd−O), 2.35 Å (Cd−O), and 2.79 Å (S−O). This gives rise to a strong interaction of the O atom on the Os@CdS NT surface and a weak interaction with the Pd@CdS NT surface, so that it shows a different PDS compared to others. Thus, Pd@CdS NT is identified as the optimal candidate for OER among the TMdoped CdS NT structures. Furthermore, 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 the acidic medium for OER of different benchmark catalysts such as RuO2, IrO2, Co3O4, and MnO2 belong in 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 the shallow content of TMs (2.7%) significantly improves the reaction overpotential compared to that of the pristine CdS NT without compromising the stability. In this respect, we believe that our study can inspire experimentalists for further exploration of TM@CdS NT systems. Also, our study provides insights for the engineering 1D TM-doped CdS-based composites with the optimal performance of water-splitting reaction at the macroscopic level. We have correlated the activity of TM@CdS NT catalysts with the well-known OER activity descriptor ΔGO* − ΔGHO* (Figure 5), the adsorption free energy difference between O* and HO*, for a comprehensive analysis of the 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 ΔGO* − ΔGHO* value of 1.82 eV, which is the highest among all other TM@CdS NT structures. This value is slightly higher than 1.5 eV at which the maximum activity has been observed for many previously reported bench mark catalysts such as RuO2,74 which can be attributed to the
Table 2. Calculated Free-Energy Change (ΔG in eV) of Elementary Steps of OER and Reaction Overpotential (ηOER in V)a
a
The red values indicate the PDS.
some negative ΔG values such as the O* formation step in Os@CdS NT and the O2 formation step in pristine CdS NTs. The spontaneity of this step is determined by the binding energy differences between HO* and O*, which is the maximum (2.87 eV) for Os@CdS NT among all 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 the pristine CdS NT for the O2 formation from HOO* can be attributed to the very weak binding of the HOO* (−0.45 eV) intermediate, which also results in 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 is 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 13423
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pristine CdS NT and bulk CdS surface (1.20 V), respectively.27 This implies that all TM@CdS NTs are efficient for reducing the reaction overpotential of HER. Remarkably, the Ni@CdS NT and Ru@CdS NT show minimum ΔGH or value close to zero, which indicates that these are the most active catalysts among all 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 a significant reduction of ΔGH. Furthermore, these metals (Ru and Ni) are also efficient for enhancing the HER activity of some previous reported catalysts.77 A 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 the cocatalyst.43−45 For example, Ni-doped CdS increases the H2 evolution rate about 10 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 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 ΔGH value of −0.50 eV. These results illustrate that doping CdS with these TMs is an efficient way to enhance the HER performance. Thus, both OER and HER activities 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.
Figure 5. Calculated negative overpotential plotted against ΔGO* − ΔGHO* for TM@CdS NT structures.
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 toward the peak position of the activity plot. This difference in trend between the O* and HO* adsorptions brings out the highest ΔGO* − ΔGHO* value for Pd@CdS NT. 3.4. 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 toward electrochemical HER, examples including 1T-MoS2, C2N, gCN, 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 the Volmer− Heyrovsky mechanism for the HER, which involves proton adsorption along with electron deduction and formation of H*, followed by the generation of molecular hydrogen in the next step. Hence, the possible HER mechanism is as follows76 * + (H+ + e) → H*
(8)
H* + (H+ + e) → H 2 + *
(9)
4. CONCLUSIONS The first-principles calculations provide significant insights into the stability and catalytic activity of TM@CdS NT. All TM@ CdS NTs are found to be associated with significant stability. The catalytic activity of TM@CdS NT toward OER and HER is confirmed by evaluating the reaction overpotential. It is observed that 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 the pristine CdS NT and bulk CdS surface too. Among the different NT structures considered, Pd@CdS NT shows the highest activity with the lowest overpotential of 0.53 V for OER because of the less interaction of OER intermediates with the Pd@CdS NT surface. Furthermore, the HER performance of TM@CdS NT is analyzed via Gibbs free energy change (ΔGH) values. All of these TMs are efficient to reduce the ΔGH over pure CdS NT, among which Ni@CdS NT and Ru@CdS NT are the best HER catalysts because of least ΔGH values. The observed activity trends are comparable with previous experimental reports. Hence, TM doping emerges as an efficient way to improve the performance of the CdS NT toward electrocatalytic water splitting. Our results could provide sound insights into the experimental design of TM-doped CdS NT catalysts for the water-splitting reaction.
Here, H* indicates the adsorbed hydrogen atom on the doped CdS NT surface. Therefore, the Gibbs free-energy change (ΔGH) for HER can be evaluated using eq 6. The catalytic efficiency of TM@CdS NT for HER is examined by the ΔGH values of H* formation on the different TM@CdS NTs. It is the ΔGH value which is the descriptor to evaluate the HER activity.35,36,75 According to this approach, the ideal HER catalyst should exhibit a ΔGH value close to zero. For comparison, we have compared our data (Table S3) with the pristine CdS NT, where we have seen that the ΔGH value (1.60 eV) is very high. This indicates that the pristine CdS NT is not efficient for the HER. Figure 6 shows that ΔGH has been reduced significantly for all TM@CdS NTs compared to the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01589.
Figure 6. Reaction free-energy profile for HER of TM@CdS NT. 13424
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Adsorption energy calculations, PDOS of pristine CdS NT and TM@CdS NT, adsorption of OER intermediates on TM@CdS NT, reaction free-energy profile for OER at different potentials for TM@CdS NT structures, PDOS of Pd@CdS NT+O and Os@CdS NT + O, adsorption of the HER intermediate on TM@CdS NT, atomic radius, list of crystal structures of stable bulk phases of considered transition metals and reaction freeenergy values (ΔGH) of HER (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Priyanka Garg: 0000-0002-9852-929X Akhil S. Nair: 0000-0001-5723-3970 Kuber Singh Rawat: 0000-0002-7308-4204 Biswarup Pathak: 0000-0002-9972-9947 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the IIT, Indore for the lab and computing facilities. This work is supported by the DST-SERB [grant number: EMR/2015/002057] and the CSIR [grant number: 01(2886)/ 17/EMR(II)]. P.G., and A.S.N. thank the MHRD, and K.S.R. thanks the UGC for the research fellowship.
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