Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is © The Royal Society of Chemistry 2019

Zinc- nanospheres as efficient oxidation electrocatalyst displaying low overpotentialfor evolution

Maira Sadaqata, Laraib Nisara, Noor-Ul-Ain Babarb, Fayyaz Hussainc, Muhammad Naeem Ashiq*a, Afzal Shahd, Muhammad Fahad Ehsane, Muhammad Najam-Ul-Haqa, Khurram Saleem Joya*b,f

a. Institute of Chemical Sciences, Bahauddin Zakariya University (BZU) Multan 60800, Pakistan b. Department of Chemistry, University of Engineering and Technology (UET), GT Road, Lahore 54890, Punjab, Pakistan c. School of Electronics Engineering Chungbuk National University, Cheongju, 28644 South Korea d. Department of Chemistry, Quaid-I-Azam University Islamabad 45320, Pakistan e. School of Natural Sciences, Department of Chemistry, National University of Science and Technology (NUST) H-12, Islamabad 44000, Pakistan f. Department of Chemistry, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia

Chemicals and Reagents All the chemicals used in our experiments including Chloride (99.8%), tellurium powder (5 N),KOH (analytical grade, >85%) and Hydrazine monohydrate were purchased from Sinopharm Chemicals and were used as received without any further modification.

Preparation of Glassy carbon electrode

To analyze the activity of hydrothermally synthesized ZnTe on Glassy carbon electrode, a homogeneous catalyst ink was prepared by adding 2mg of catalyst powder in 60.0 μL of water and 2 drops of Nafion solution were added followed by ultrasonication for 30 min. 4 μL of the dispersion was drop casted on a on glassy carbon (0.07cm2) surface. The drop-casted film was then dried at room temperature and finally heated at 60ᵒC for 30 minutes in an oven.

Calculation of mass loading on NF

The deposited mass of ZnTe-NS/NF was calculated as reported previously [1-2]. After the growth of electrocatalyst on NF, the increased in weight of NF (10mg) was directly judged by weighing the deposited NF and bare NF. The mass loading was calculated as follow.

ZnTeloading = 10* (MZnTe/MTe)

ZnTeloading = 10* (193.01/127.06)

ZnTeloading = 10*(1.51)

ZnTeloading = 15mg

Where M is molecular or atomic weight. Thus, the mass loading of ZnTe-NS on Nickel foam was almost equal to 15mg.

Mass loading on Glassy carbon electrode

The mass loading on Glassy carbon was evaluated by above mentioned formula.

ZnTeloading = 0.14* (MZnTe/MTe)

ZnTeloading = 0.14* (193.01/127.06)

ZnTeloading = 0.14*(1.51)

ZnTeloading = 0.211mg

Thus total mass loading on NF and Glassy carbon is 15mg and 0.906mg respectively. Functional Theory (DFT) Calculations The theoretical confirmation of hydroxyl ion (OH-) adsorption most favorable active site on ZnTe (001) surface were performed using generalized gradient approximation method along with Perdew, Burke and Ernzerhof (PBE) functionals [3-5]. All these calculations have been carried out with help of Vienna ab initio simulation package (VASP) [6, 7] based on density functional theory. Convergence tests for total energy of the system with respect to electron wave functions were conducted using plane-waves with cut- off energy of 500 eV. The ionic positions, cell volume and lattice parameters of the system were fully relaxed with conjugate gradient method until Hellmann Feynman forces became smaller than 0.02 eV/Å. The (001) surface of ZnTe were designated to estimation the favorable adsorption sites for OH- ion. The K-point mesh 9×9×1 was found to be sufficient to a self-consistent convergence criterion of 1×10-4 eV. A relatively large vacuum gap of ~20.0 Å was set between free surfaces to prevent the interactions of the system and its periodically repeated images. The Van-dar-waals corrections were applied perpendicular to surface plane to achieve better results for theoretical influence of Van-dar-waals interaction between layer and OH- ion [8, 9]. The bottom few layers of model was fixed, and few top layers was set to be free to move in all directions as shown in Figures S9-S12. The OH- adsorption energy (ΔEad) was calculated from well known equation 1:

ΔE(ad) = E(ZnX-OH-) – [E(pure) + E(OH-) ] 1 where, E(pure) is the total energy of slab models for ZnTe and E(OH-) is total energy of a isolated (OH-) ion, and E(ZnX-OH-) is the total energy of ZnX surface with bound OH– (X = TOP of Zn, Tetrahedron and Square pyramidal adsorption sites indicating by ZN, TH and SQ).

Owing to the difference in coordination three distinct Zn sites on (001) plane were selected to adsorb the OH- ions as presented in figure S9-S11. These active Zn sites have been denoted as ZN (top of the ZnTe), TH (tetrahedral sites on (001) plane), SQ (square pyramidal site on (001) plane).

Table1. Comparison of electrocatalytic parameters for OER

Catalys Total Loadin Onset η10Ac TOF Tafel Rct Rs(Ω t loading/ g Potential/ m/cm2 @ Slope/ (Ω ) mg area/ V / mV 10mA mVde ) cm-2 cm-2 c-1 (sec-1) ZnTe/N 15 0.5 1.41 210 0.250 62 55 9.8 F 0 ZnTe/G 0.211 0.07 1.42 287 0.035 67 67 11.1 C 0

NF=nickel foam, GC=Glassy carbon, Rct= charge transfer resistence, Rs= solution resistence.

Table S2. Comparison of electrocatalytic activity in alkaline media for ZnTe-NS with state-of-the-art

RuO2 and IrO2 based catalytic systems.

Catalyst Water Electrolyte Current Overpotential Tafel Slope Reference Electrolysis at respective density (j) test current density

(η)

-2 - [10] RuO2@NF OER 1.0 M KOH 20 mAcm 280mV 92.7mVdec 1

-2 -1 [11] IrO2 OER 1.0 M KOH 10 mAcm 340mV 80mVdec

ZnTe-NS/NF OER 1.0 M KOH 10 mAcm-2 210mV 62mVdec-1 This Work

Figure S1. Energy dispersive X-ray spectrum of ZnTe.

50

40

30

20

10

Quantity Adsorbed (cm³/g STP) (cm³/g Adsorbed Quantity

0

0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po)

Figure S2. BET Sorption isotherm of ZnTe.

0.0014

0.0012

0.0010

0.0008

dv/d 0.0006

0.0004

0.0002

0.0000

0 20 40 60 80 100 120 140 Pore diameter (nm)

Figure S3. The corresponding pore size distribution curve of ZnTe.

Figure S4. Overpotential needed to achieve current density of 10 mA cm2- on x-axis and Tafel slope value at y-axis for ZnTe-NS/NF and for various transition , -phosphide/ and metals- telluride based electrocatalysts reported to date.

Figure S5. SEM images of ZnTe-NS coated on NF substrate (a) bare NF (b,c) at low magnification and (d ) at high magnification suggesting the full coverage of Nickel foam substrate with Nano-spheres of ZnTe.

Figure S6. (a) LSV curve measured in 1M KOH solution, (b) Tafel slope of ZnTe/GC obtained from linear region of LSV curve.

Figure S7. Nyquist plot for ZnTe-NS/GC and simplest Randles circuit demonstrating the solution resistance and charge transfer resistance for electrode-electrolyte interphase.

Figure S8. Cyclic voltammogram of ZnTe-NS deposited on (a) Nickel foam (b) Glassy Carbon electrode in 1M KOH solution.

Figure S9. Optimized structure (side view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the top of Zn (ZN active site), where white, red, gray and brown balls represent , oxygen, zinc and tellurium respectively.

Figure S10. Optimized structure (side view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the square pyramidal (SQ active site), where white, red, gray and brown balls represent hydrogen, oxygen, zinc and tellurium atoms respectively.

Figure S11. Optimized structure (side view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the tetrahedral site (TH active site), where white, red, gray and brown balls represent hydrogen, oxygen, zinc and tellurium atoms respectively.

Figure S12a. Optimized structure (Front view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the top of Zn (ZN active site), where white, gray and brown balls represent hydrogen, zinc and tellurium atoms respectively as indicating by red color hexagonal shape.

Figure S12b. Optimized structure (Front view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the tetrahedral site (TH active site), where white, gray and brown balls represent hydrogen, zinc and tellurium atoms respectively as indicating by red color tetrahedral shape.

Figure S12c. Optimized structure (Front view) of ZnTe (001) surface with physio absorbed hydroxyl molecule at the square pyramidal (SQ active site), where white, gray and brown balls represent hydrogen, zinc and tellurium atoms respectively as indicating by red color rectangular shape.

Figure S13. Consecutive cyclic voltammetry profiles for ZnTe-NS/NF at the scan rate of 50 mV s1- in 1.0 M KOH electrolyte solution

Figure S14. LSV curves of synthesized ZnTe-NS deposited on NF before and after chronoamperometry for 24 hours.

Figure S15. Electrocatalytic stability, chronoamperometry of ZnTe-NS/GC at the applied potential of 1.53 V vs. RHE for more than 10 hours in 1.0 M KOH electrolyte solution;

Figure S16. Comparison of (a) XPS survey spectrum (b)Zn 2p, (c) Te 3d and (d) O1s XPS spectra of ZnTe-NS catalyst (before and after) for 24 h of stability in 1M KOH solution.

Figure S17. SEM images of ZnTe-NS after 24 hours of stability test (a) at low magnification (b) at high magnification.

Figure S18. Comparison of XRD pattern of ZnTe-NS (before and after) prolonged stability test. Powder material was scratched from the NF substrate to avoid the interference of NF.

Reference. 1. U. De Silva, J. Masud, N. Zhang, Y. Hong, W. P. Liyanage, M. A. Zaeem and M. Nath, Journal of Materials Chemistry A, 2018, 6, 7608-7622. 2. a) C. Tang, N. Cheng, Z. Pu, W. Xing and X. Sun, Angewandte Chemie International Edition, 2015, 54, 9351-9355; b) N-.U-.A. Baber, K. S. Joya, M. A. Ehsan, M. Khan and M. Sharif, ChemCatChem, 2019 (DOI: 10.1002/cctc.201900202). 3. J. P. Perdew, K. Burke and M. Ernzerhof, Physical review letters, 1996, 77, 3865. 4. A. H. Larsen, M. Vanin, J. J. Mortensen, K. S. Thygesen and K. W. Jacobsen, Physical Review B, 2009, 80, 195112. 5. G. Kresse and D. Joubert, Physical Review B, 1999, 59, 1758. 6. T. Björkman, Computer Physics Communications, 2011, 182, 1183-1186. 7. G. Kresse and J. Furthmüller, Physical review B, 1996, 54, 11169. 8. S. Grimme, Journal of computational chemistry, 2006, 27, 1787-1799. 9. T. Kerber, M. Sierka and J. Sauer, Journal of computational chemistry, 2008, 29, 2088-2097. 10. S. Shit, S. Chhetri, W. Jang, N. C. Murmu, H. Koo, P. Samanta and T. Kuila, ACS applied materials & interfaces, 2018, 10, 27712-27722. 11. X. Wu, S. Han, D. He, C. Yu, C. Lei, W. Liu, G. Zheng, X. Zhang and L. Lei, ACS Sustainable Chemistry & Engineering, 2018, 6, 8672-8678.