Simultaneous Site Adsorption Shift and Efficient CO Oxidation Induced

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Simultaneous Site Adsorption Shift and Efficient CO Oxidation Induced by V and Co in Pt Catalyst Rafia Ahmad and Abhishek K. Singh* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: A complete solution for CO poisoning of Pt catalysts requires a design that entirely prevents CO adsorption on Pt atoms. Here, we explore the CO adsorption sites and oxidation capability of 3d transition metal doped small magic clusters of Pt4. Among Pt3M, only the Pt3V freestanding cluster entirely eliminates the prospect of Pt poisoning by inverting the adsorption site of CO to V atom. The V d-band center lies closer to the Fermi level than that of Pt atoms, resulting in a larger number of empty d-antibonding states, thereby making V comparatively more reactive toward CO. The inversion of the CO adsorption site is also observed for larger PtnVm clusters and becomes possible for PtnCom clusters for sizes larger than m + n = 12 atoms. Formation and removal of CO2 via the Langmuir−Hinshelwood mechanism occurs with low reaction barriers, exhibiting a high catalytic activity for the Ptn(V/Co)m clusters. A maximum catalytic efficiency is attained for Pt41V14, which at room temperature gives a CO2 turnover frequency comparable to the conventional catalysts. The oxidation of CO becomes more favorable by the Mars van Krevelan mechanism for the cluster supported on vacancy prone Li-doped MgO(100). Our results present a rationale design of Pt poisoning free fuel cells and automobile exhaust catalysts, which can be entirely protected from CO poisoning and maintain longterm high catalytic efficiency.

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replace monometallic with bimetallic catalysts.33−45 The developments of these bimetallic catalysts have been based on empirical grounds, and a detailed fundamental knowledge of the improved catalytic performance is still debated.34−38 A broadly discussed reason is the electronic modification, particularly of the d-states of Pt in the alloyed systems, which are mainly responsible for the enhanced catalytic activity.11,46−49 The enhancements, however, are overshadowed by the fact that Pt in the bimetallic system eventually gets poisoned by CO.50 The reason for this poisoning is essentially due to CO adsorbing selectively on the Pt atoms. This problem received attention in a recent work, where it was shown that the CO adsorption site inverts from Pt to Co for Pt3Co/Li− MgO(100). However, on the free-standing cluster of Pt3Co, CO still prefers Pt over Co.51 Therefore, a bimetallic catalyst that can inherently (free-standing case) avoid CO adsorption on Pt, inverting it to the second component, will provide a solution to the above challenge. Here, by performing first-principles density functional calculations on free-standing Pt3M (M = 3d transition metals), we show that only Pt3V exhibits the site inversion of CO adsorption from Pt to V. This is because the d-band center of V is closer to the Fermi level than that of Pt, while this property is opposite in the other clusters. Hence, V possesses a larger

INTRODUCTION Transition and noble metal nanoparticles possess a larger surface-to-volume ratio and higher concentration of partially coordinated surface sites than the corresponding bulk materials.1,2 These features often result in improved physical and chemical properties compared to the bulk counterparts. It is for these reasons that heterogeneous catalysis on these nanoparticles has received intense investigation in the scientific community.3−14 There are several important heterogeneous catalytic processes, where removal of carbon monoxide is either desired or absolutely necessary, such as in the postprocessing of syngas (H2 + CO) to produce hydrogen as an energy source for fuel cells.15−21 A byproduct of this reaction is CO and even trace amounts of CO (>10 ppm (ppm)) poisons a fuel cell electrode, drastically degrading its efficiency. Another process of technological importance in pollution control requires CO oxidation in automotive exhaust catalysts (the three-way catalysts), to prevent its emission in the environment. Platinum used as a fuel cell catalyst demonstrates remarkable efficiency in reducing traces of CO from the H2 feed gas to negligibly few ppm levels.22−28 Moreover, the best catalytic converters of automotives have been reported to be Pt nanoparticles.4,29−32 However, extensive commercialization of economical automobile exhausts and day-to-day use of fuel cells are severely hampered by high cost, low abundance, and CO poisoning of Pt catalysts. One of the strategies to reduce the usage of Pt and increase the efficiency and selectivity of electrocatalytic processes is to © XXXX American Chemical Society

Received: March 30, 2017 Revised: May 26, 2017 Published: May 30, 2017 A

DOI: 10.1021/acs.jpcc.7b02998 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

In order to understand the bond length trend and effect of M doping, the charge redistribution from Pt to the transition metals is calculated as

affinity toward CO than Pt atoms in the bimetallic cluster. This study is further extended to larger sized clusters of Ptn(V/Co)m, for (n, m = 4, 2; 9, 3; 15, 5; 41, 14; and 59, 20), which are experimentally easier to realize. The site inversion for CO adsorption on V is preserved for all PtnVm. This inversion to Co from Pt is also observed beyond the 12-atom cluster for PtnCom. A subsequent peroxo-like O2 adsorption, on the adjacent Pt sites, leads to the efficient oxidation of CO to CO2 with low Langmuir−Hinshelwood barriers on these clusters. The maximum catalytic activity is observed for the 55-atom cluster of Pt41(V/Co)14 due to optimal CO adsorption and best reaction kinetics. The CO2 TOF at room temperature exhibits a good catalytic activity of Pt41V14 for CO oxidation. Finally, the Mars van Krevelan mechanism of CO oxidation on Pt41V14/Lidoped MgO(100) proceeds via a reaction barrier of 0.14 eV. The observed variations of catalytic properties of small metal clusters by altering the cluster size could prove to be universal for a variety of metals and will be essential to the design of nanostructured materials for various chemical applications.

Δρ = ρ(Pt3M) − ρ(Pt3) − ρ(M)

where ρ(Pt3M), ρ(Pt3), and ρ(M) are the charge densities of isolated Pt3M, Pt3, and M systems, respectively, and shown in Figure 1. A uniformly increasing charge redistribution trend is

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METHOD The first-principles calculations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).52 Electron−ion interactions were described using all-electron projector augmented wave (PAW) pseudopotentials.53,54 Electronic exchange and correlation were approximated by a Perdew−Burke−Ernzerhof generalized gradient approximation.55 The periodic images are separated by a 15 Å vacuum along three directions for clusters and along the z-direction for clusters supported on MgO. For all of the free-standing cluster calculations, the Brillouin zone is sampled by the gamma point. For the cluster supported on Li-doped MgO(100), a well converged k-mesh of 7 × 7 × 1 was employed. A conjugate gradient scheme is used to relax the structures until the component of the forces on each atom was ≤0.005 eV/Å. The cutoff energy was set to 400 eV to ensure the accuracy of the results. The minimization of the reaction pathways and the search of the transition states (TS) have been performed with the steepest-descent nudged elastic band method (NEB)56,57 and the dimer method.58 The CO oxidation reaction pathway and barrier is then calculated by the NEB method through the identified TS.

Figure 1. Charge redistribution in the bimetallic clusters of Pt3M, where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, respectively. The apex atom of each tetrahedron is the M atom, and the base of the tetrahedra is the three Pt atoms. The isosurface level is set to 0.1146 eV Å −3.

observed for Pt3M as M varies from Sc to Zn. This is expected from the trend in electronegativity (M (electronegativity) = Sc (1.3), Ti (1.5), Cr (1.6), Mn (1.5), Fe (1.8), Co (1.9), Ni (1.9), and Cu (1.9)) difference between M and Pt (2.2). A deviation from this trend is for V (electronegativity = 1.6) where the Pt3V cluster shows the maximum charge redistribution. This indicates the largest electron transfer from Pt to V, hence having the smallest bond length. This maximum charge redistribution can be attributed to the trigonal planar geometry, which arises due to sd2 hybridization. The valence electronic configuration of V is 3d34s2; therefore, it undergoes sd2 hybridization when one of the s-electrons is excited to the dsubshell. The three sd2 hybridized V orbitals can then form strong σ bonds with 6s orbitals of the three Pt (5d96s1) atoms. The Pt3Zn cluster on the other hand has most of the electrons accumulated on Pt, indicating least charge transfer to Zn (electronegativity = 1.6), therefore having the bond length 2.67 Å, closest to Pt−Pt. The least charge redistribution is a consequence of the completely filled d-orbital of Zn ([Ar] 3d104s2). Next, CO adsorption was performed on each cluster followed by a complete geometrical relaxation. The adsorption energy determines the CO coverage, its residence time on the surface, the possibility of poisoning, and catalytic efficiency.69,70 Moreover, the ratio of desorption to oxidation of CO controls the catalyst selectivity of the transformation of natural gas to syngas. Therefore, an efficient catalyst for CO oxidation should adsorb CO neither too strongly such as to poison it nor too weakly to make the reaction with activated oxygen difficult.51 CO adsorbs on the given bimetallic clusters with carbon end-on mode in a slightly tilted geometry with respect to the clusters. The adsorption energies of CO on all of the Pt3M are given in Table S1. The lowest CO adsorption energy among all of the Pt3M is for Pt3V, with a value of −1.46 eV. This is neither very strong to poison the V site nor very weak to promote desorption rather that oxidation.

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RESULTS AND DISCUSSION In order to find the possibility of attaining a complete prevention of CO poisoning of the Pt site in bimetallic clusters, Pt3M (M = Sc, Ti, V, Mn, Cr, Fe, Co, Ni, Cu, and Zn) are studied as possible CO oxidation catalysts. The tetrahedral structure has been shown to be the global minima of the fouratom Pt cluster,59−62 and has also been reported as a CO oxidation catalyst.63 The low cost and abundant 3d transition metals have been extensively alloyed with Pt, Au, and Pd to improve the efficiency of fuel cell catalysts.32,34,36,37,64−67 The completely optimized structures of Pt3M clusters are shown in Figure S1. The tetrahedral geometry of Pt4 is retained by all of the bimetallic Pt3M clusters except for Pt3V. It attains a trigonal planar geometry with a Pt−V bond length of 2.15 Å; this is in agreement with previous studies.68 The other bimetallic clusters Pt3M (M = Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) have increasing Pt−M bond lengths, as reported in Table S1. These bond lengths are shorter compared to 2.7 Å for Pt−Pt in the pure platinum cluster. B


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therefore, a good first indicator of the metal−adsorbate bond strength. The d-band center is the weighted average of the dstates over the total number of the states given as75

The adsorption energy trends can be understood from the density of d-states of Pt and M atoms in Pt3M (M = Sc, Ti, V, Mn, Cr, Fe, Co, Ni, Cu, and Zn), as shown in Figure 2. The

E

∫E 2 (E·n(E) dE) 1

∫ n(E) dE

(1)

where E1 and E2 are upper and lower energy cutoffs of the dstates. The higher the center of the d-states are in energy, the higher in energy will be the antibonding (d−σ)* states and stronger will be the bonding. This is because higher energies of antibondong states lead to a decrease in their filling with electrons. This means that the metal−adsorbate system is less destabilized, translating to stronger binding between the metal and the adsorbate. Therefore, in the context of chemisorption of molecules to a metal surface, a higher d-band center results in stronger bonding. This model has successfully been employed in various studies to describe the adsorbing energy trends on transition metals.11,46−49,51 Figure 3 shows the distance of the d-band center from the Fermi energy for Pt3M. The numerical values of the d-band

Figure 2. Density of d-states for Pt and M atoms in Pt3M, where M is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, respectively. The cyan line indicates the Fermi level.

maximum overlap of Pt and V states below the Fermi level indicates a strong bond. The bonds of other Pt3M are not as strong, shown by the decreasing overlap of Pt and M d-states above the Fermi level. The interaction between Zn and Pt is the least, as seen from the negligible DOS overlap. Furthermore, the more reactive nature of V in Pt3V than Pt atoms is attributed to the absence of empty d-antibonding states of Pt. Whereas these states for V are more than those for Pt, they are less in number as compared to M of other bimetallic clusters. This is the reason for the relatively lower adsorption energy of CO on V of Pt3V, compared to other Pt3M. A more detailed electronic picture is shown by the l-decomposed density of states (lDOS) of the bimetallic catalysts shown in Figures S2, S3, and S4. lDOS is called local density of states, giving the electronic contributions decomposed into individual angular momenta contributions of a given orbital (e.g., for the d-orbital: dxy, dyz, dxz, dx2−y2, dz2). Further insights into this site inversion can be gained by comparing the d-band centers (dBC)71−73 of the constituent atoms in Pt3M. It is well-known that CO binds on transition metals by interactions between the 5σ (highest occupied molecular orbital) orbital of CO and unfilled d-orbitals of metals (eg) and a subsequent π-backdonation of filled d-orbitals (t2g) with the antibonding 2π* (lowest unoccupied molecular orbital) orbitals of CO. Since the electronic structures of transition metals are dominated by d-orbitals, the position of the d-band center with respect to the Fermi level is a critical factor, which determines the strength of the interaction of the metal with the adsorbate. The d-band model gives a qualitative assessment of metal− adsorbate bonding at a transition-metal surface.46 The hybridization of the metal d-states with the σ orbital of the adsorbate produces bonding (d−σ) and antibonding (d−σ)* states. The strength of the bond is inversely proportional to the filling of these antibonding states (essentially, leading to destabilization of the adsorbate molecule).74 By construction, the antibonding states are always above the d-level of the pure metal. The center of the pure metallic d-states relative to the Fermi level is,

Figure 3. d-Band center positions from the Fermi level of Pt3M, where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, respectively.

center are not of significance; the qualitative trend is of importance. The lower the d-band centers from the Fermi energy, the less the empty antibonding states are available for CO adsorption.71,72 The figure shows that V in Pt3V exclusively has the d-band center higher in energy than that of the Pt atom. The values listed in Table S1 also show that the relative position of dBC of Pt compared to that of M is negative only in the case of Pt3V; the absolute values are not of importance, rather the trend is accurate. This is the distinguished feature of only Pt3V among all Pt3M, where the CO adsorption site inverts to V from Pt and explains why it completely avoids poisoning of Pt. In order to study CO oxidation, we adsorb O2 next on CO adsorbed bimetallic clusters, followed by a complete geometrical relaxation. Oxygen adsorbs on Pt atoms in the peroxolike state with moderate adsorption energies, as given in Table S1, and the O−O bond is activated from 1.22 Å (free O2 molecule) to 1.29 Å on Pt3V. This process is accompanied by the transfer of −0.319 |e| Bader charge from the Pt atom to the 2π* orbital of O2. In order to provide insights into the design of new catalysts, it is important to predict the transition state and reaction C


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Figure 4. Potential energy surface plotted for the (a) x−y, (b) x−z, and (c) y−z planes to identify the TS for the L−H mechanism reaction pathway and barrier for CO oxidation on free-standing Pt3V. (d) L−H mechanism of CO oxidation on Pt3V. IS1 and IS2 are the starting points of the first and second halfs of the L−H mechanism of CO oxidation, respectively. TS11, TS12, and TS21 are the first and second transition states of the first and second half reactions, respectively. FS1 and FS2 are the final states of the first and second halves of the L−H mechanism. Gray, pink, blue, red, and brown atoms represent Pt, V, O, and C, respectively.

proceeds through a lower energy barrier of 0.37 eV (TS21) than the L−H first half reaction. The final release of CO2 (FS2) is a downhill reaction and leaves the Pt3V cluster undistorted, showing that it also fulfils the stability criteria for efficient catalysis. Although the L−H mechanism is widely known for CO oxidation, it is intensively dependent on both oxygen and CO coverages. The variations of pressure cause a difference in adsorbate coverages, which effectively limits the CO oxidation.81,82 Also, the free-standing clusters need a substrate for practical implementation as a catalyst. The free-standing cluster of Pt3V is then supported on an oxide, as this is known to enhance the catalytic activity of nanoclusters.83,84 The substrate considered here is an easily synthesized oxide surface MgO(100) and is prone to vacancy formation by Li doping.85,86 Complete relaxation of the Pt3V was performed on this surface, followed by CO adsorption on both Pt and V sites. The binding energy of clusters on MgO(100) is calculated as

barrier. The reaction rate or efficiency of the catalyst decreases exponentially with the energy barrier.76,77 Therefore, after CO and O2 have been adsorbed on the Pt3V cluster, a reaction mechanism minimum energy pathway (MEP) calculation is done, as shown in Figure 4. The kinetics of CO oxidation by molecular O2 has been largely reported to proceed via the Langmuir−Hinshelwood (L−H)78,79 mechanism, where two adjacently adsorbed gas molecules react directly to give a product. The reaction barrier for the L−H mechanism is often quite lower than the desorption energies of CO and O2.80 The L−H mechanism of CO oxidation on the clusters is divided into two half reactions. The first half of the reaction CO* + O*2 → CO2* + O* involves the breaking of the O−O bond. The second half of the reaction CO* + O* →CO2* is dependent on the breaking of the Pt−O bond. The potential energy surface profile for the first half of the CO oxidation reaction on Pt3V is shown in Figure 4a−c. The intermediate states are identified from the saddle points of Figure 4a−c. These intermediate states are then optimized by the dimer method to identify the correct transition states. The barrier is then calculated from this transition state, and the energy profile is shown in Figure 4d. Initially, the V atom adsorbs the CO by an adsorption energy of −1.46 eV. The initial step (IS1) is where O2 adsorbs along with CO by an adsorption energy of −3.55 eV. The rate limiting step of the reaction is the formation of a bent CO2 (TS12) after the O−O bond breaking (TS11), where an energy barrier of 0.74 eV is required. The final release of the gas-phase CO2 (FS1) shows a slight rearrangement of the cluster, which ensures that the Pt atom is still connected with the O atom. For the second half reaction, again a potential energy surface helps to narrow down the search for intermediate states, which are further optimized by the dimer method to get the transition state. The oxidation of the next adsorbed CO by Pt3VO* (IS2)

E b = E Pt3V/Li − MgO(100) − E Li − MgO(100) − E Pt3V

(2)

where EPt3V/Li−MgO(100), ELi−MgO(100), and EPt3V are the energies of Pt3V supported on Li-doped MgO(100), bare support of Lidoped MgO(100), and Pt3V cluster, respectively. The binding energy was −6.33 eV, stronger than Pt−Pt (−5.8 eV) and Pt− V (−6.0 eV) cohesive energy, which shows that the cluster formation on the Li-doped MgO is favorable. Interestingly, CO adsorbs with an adsorption energy of −1.31 eV on the supported Pt3V, which is weaker as compared to the freestanding cluster, and V atom is still the preferable site. Oxygen is adsorbed on Pt atoms with a strong adsorption energy of −2.80 eV in the peroxo8-like state. This is known to promote oxidation reactions as oxygen becomes more reactive in the peroxo state.87 However, in practical cases, this peroxo-like D


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The Journal of Physical Chemistry C oxygen will assist CO oxidation efficiently at lower temperatures. To further analyze the CO adsorption site preference on supported Pt3V, the 2D topology of the electron localization function (ELF) was plotted for the systems with adsorption sites as Pt and V, respectively. Becke and Edgecombe introduced the ELF as a “simple measure of electron localization in atomic and molecular systems”.88 The 2D ELF basins represent important quantities for the discussion of 1 chemical bonding. The expression for ELF is η = 2, 1+

Furthermore, the adsorption of CO on Pt3V/Li-doped MgO(100) activates a spontaneous oxygen vacancy. The MEP is depicted in Figure S5, showing that the CO oxidation barrier is dependent on capturing the O atom from the vacancy. A small reaction barrier of 0.13 eV is observed, as seen in Figure S5, showing better reaction kinetics than the free-standing case. Therefore, the Pt3V cluster can be successfully employed as a CO oxidation catalyst. Next, we show the validity of the above study on larger size clusters, which are easily synthesized and experimentally more popular.61,90−96 Here, besides V, we also study Co doped Pt clusters, based on a previous study of Pt3Co,51 where CO site inversion possibility was observed. Initially, the pristine clusters, which have been reported before, of ascending size Pt6, Pt12, Pt20,Pt55, and Pt79 are relaxed in the octahedral,97 icosahedral,98 crowned shaped truncated icosahedral,99 close-packed fcc icosahedral,100 and truncated octahedral101 geometries, respectively. Ptn(V/Co)m (for n; m = 6; 2, 9; 3, 15; 5, 41; 14, and 59; 20) are then obtained by replacing Pt atoms in the abovementioned clusters with V and Co to obtain random bimetallic clusters, respectively. These structures retain the geometry of pristine Pt clusters after complete optimization. We study the CO adsorption on the pristine clusters first, and their completely relaxed stuctures are as shown in Figure 6.

( ) Dσ Dσo

where Dσ and Doσ represent the curvature of the electron pair density for electrons of identical σ spins (the Fermi hole), for the actual system and a homogeneous electron gas with the same density, respectively.89 The analytical form of ELF confines its values between 0 and 1 representing limits for delocalized and highly localized electrons, respectively. Figure 5a shows the ELF of the first layer of Li-doped MgO, when CO

Figure 5. Electron localization function plots in inverse gray scale for (a) CO adsorption on Pt atom of Pt3V/Li-doped MgO(100) and (b) CO adsorption on V atom of Pt3V/Li-doped MgO(100). The electron localization function (ELF) is shown for the topmost layer of the surface. The site of Pt3V binding on the surface is highlighted in blue. The gray, pink, brown, and red atoms signify Pt, V, C, and O, respectively.

Figure 6. Summary of CO adsorption energy for the PtnVm, for n; m = 6; 2, 9; 3, 15; 5, 41; 14, and 59; 20. The adsorption energies of CO on pristine Pt clusters are compared with PtnVm for both Pt and V sites. The gray, pink, red, and brown spheres represent Pt, V, O, and C atoms, respectively.

is close to the Pt site. This system exhibits no adsorption of the CO on Pt (adsorption energy 20 bar, increases the TOF beyond 7.5 molecules site−1 s−1. The catalytic efficiency follows an increasing trend with increasing PCO2, as the TOF increases more than 7 molecules site−1 s−1 for an ambient pressure range of PCO2, PCO, and PO2 ((=10 bar)) at 400, 300, and 200 K (Figure 8c−e). However, the TOF increases to >10 molecules site−1 s−1 with the decreasing temperature at PCO and PO2 > 300 bar. This trend continues for higher PCO2 = 500 bar as well (Figure 8g−i). At 200 K, the TOF is >20 molecules site−1 s−1 at ambient PCO and PO2. This system is, therefore, a highly active CO oxidation catalyst, preserving the site inversion without compromising on the CO2 TOF. Finally, we assess the catalytic activity on a support and perform CO oxidation by the MvK mechanism on Pt41V14 supported on Li-doped MgO(100). Complete relaxation of the Pt41V14 was done on this surface, followed by CO adsorption on both the Pt and V sites. The cluster has a strong binding energy of −7.33 eV, which shows that the cluster formation on the Li-doped MgO(100) is favorable. Moreover, CO adsorbs on the V site rather than Pt. The adsorption energy is weaker on the supported Pt41V14, as −1.11 eV, compared to the freestanding cluster. Furthermore, CO adsorption on Pt41V14/Lidoped MgO(100) again activates a spontaneous oxygen vacancy of the support. This promotes the MvK mechanism to proceed via a reaction barrier of 0.14 eV, and the MEP is G


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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.7b02998. The first figure shows the completely optimized geometries of Pt3M. The second, third, and fourth figures are of the local density of s-, p-, and d-states (lDOS) for M and Pt atoms in Pt3M, respectively. The fifth figure shows the MEP and reaction barrier for CO oxidation on Pt3V cluster supported on Li-doped MgO by the MvK mechanism. The sixth figure shows a graphical table of CO adsorption on Ptn for n = 6, 12, 20, 55, and 79 and PtnCom, for n; m = 6; 2, 9;3, 15;5, 41;14, and 59;20. The seventh figure shows a partial density of states (PDOS) plot of Pt and Co d-states in CO adsorption on Ptn and PtnCom. The eighth figure shows the L−H mechanism reaction pathway and barrier for CO oxidation on free-standing Ptm(V/Co)n. For the first half of the L−H mechanism of CO oxidation for CO* + O*2 → CO*2 + O*. The table lists values of Pt−M bond length, d-band center position of Pt with respect to M in Pt3M, adsorption energy of CO, and adsorption energy of O2. Thermodynamic calculation details are also included. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-80-22932449. ORCID

Abhishek K. Singh: 0000-0002-7631-6744 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by DST Nanomission. The authors acknowledge the computational facilities of Supercomputer Education and Research Center and Materials Research Center, both in Indian Institute of Science, Bangalore, provided for this work. R.A. acknowledges support from DST through INSPIRE Fellowship. The authors thank Mr. Soham Chattopadhyay for scientific discussions and useful insights.

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