SYNTHESIS & CHARACTERIZATION OF MESOPOROUS ZINC COBALTITE THIN FILMS AND ITS ELECTROCHEMICAL APPLICATION FOR OER

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SYNTHESIS & CHARACTERIZATION OF MESOPOROUS ZINC COBALTITE THIN FILMS AND ITS ELECTROCHEMICAL APPLICATION FOR

OER

A THESIS SUBMITTED TO

THE GRADUATE SCHOOL OF ENGINEERING AND SCIENCE OF BILKENT UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE IN

CHEMISTRY

By

Nesibe Akmanşen Kalaycı

July 2021

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SYNTHESIS & CHARACTERIZATION OF

MESOPOROUS ZINC COBAL TITE THIN FILMS

AND ITS ELECTROCHEMICAL APPLICATION FOR OER

By Nesibe Akınanşen Kalaycı July 2021

We certify that we have read this thesis and that in our opinion it is fully adequate, in scope and in quality, as a thesis for the degree of Master of Science.

Ömer Dağ (Advisor)

Ferdi Karadaş

Emre Büküşoğlu

Approved for the Graduate School ofEngineering and Science:

Ezhan Karaşan

V .

Director of the Graduate School

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ABSTRACT

SYNTHESIS & CHARACTERIZATION OF MESOPOROUS ZINC COBALTITE THIN FILMS AND ITS ELECTROCHEMICAL APPLICATION FOR OER

Nesibe Akmanşen Kalaycı M.Sc. in Chemistry Advisor : Ömer Dağ

July, 2021

Transition metal cobaltite materials were widely used as electrode material due to their excellent electrochemical performance, flexibility to be prepared with different morphologies and, high surface area. In this thesis, mesoporous zinc cobaltite thin films were synthesized in cubic spinel structure via molten-salt assisted surfactant assembly (MASA) method with a high surface area and its electro-catalytic performance in oxygen evolution reaction (OER) was analyzed. Clear and homogenous aqueous solution of surfactants (P123 and CTAB), zinc nitrate hexahydrate and cobalt(II) nitrate hexahydrate (as precursors) are coated on glass substrate to obtain mesophases, thereafter mesophases are calcined to synthesize mesoporous zinc cobaltite (denoted as m-ZnCo) as powder. m- ZnCo-60 (with a total salt/P123 ratio of 60) samples were synthesized with a smooth film morphology and maximum surface area of 102 m2/g.

The mesophases with different compositions were analyzed using X-Ray Diffraction (XRD) technique. The line(s) between 1.5 and 2°, 2θ, in the XRD pattern is an indication for the formation of ordered lyotropic liquid crystalline mesophase. Aging of the

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mesophase was monitored via XRD and POM techniques to establish its stability. The stable mesophases were used to synthesize m-ZnCo film and powder samples. The powder samples were collected after calcination process and characterized by XRD, N2

adsorption-desorption, SEM, HR-TEM techniques.

The precursor solutions were spin coated on half of 1cm x 2cm size FTO glasses, then calcined and used in three-electrode system as working electrodes. The electrocatalytic performance of the materials was analyzed by cyclic voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA) experiments for oxygen evolution reaction (OER). All electrodes were stable up to 100 mA/cm2 current density and displayed minimum Tafel slope value of 41 mV/dec.

Mesoporous zinc cobaltite materials were also synthesized through precursor solutions without CTAB. Removing CTAB from the synthesis results films with rougher surface and reduced crystallinity. Same techniques were also employed for characterization. The prepared electrodes of non-CTAB samples exhibited a lower Tafel slope of 40 mV/dec and overpotential of 256 mV at 1mA/cm2 current density.

In addition, silica templated mesoporous zinc cobaltite was synthesized by adding TMOS to the precursor solution of ZnCo-60 to increase the surface area, the calcined samples were denoted m-ZnCo-60-S20-300 (S20 is represents 20 TMOS/P123 mole ratio). The m- ZnCo-60-S20-300 sample has the highest specific surface area of 215 m2/g. However, despite having higher surface area due to high resistance of silica material, silicated samples exhibited higher overpotential values.

Keywords: Molten-salt assisted self-assembly, mesoporous thin film, zinc cobaltite, oxygen evolution reaction, spinel structure

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ÖZET

MEZOGÖZENEKLİ ÇİNKO KOBALTİT İNCE FİLMLERİN SENTEZİ VE KARAKTERİZASYONU VE OKSİJEN AÇIĞA ÇIKIŞ REAKSİYONU İÇİN

ELEKTROKİMYASAL UYGULAMALARI

Nesibe Akmanşen Kalaycı Kimya, Yüksek Lisans Tez Danışmanı : Ömer Dağ

Temmuz, 2021

Geçiş metal kobaltit malzemeler, mükemmel elektrokimyasal performasları ve farklı morfoloji ve yüzey alanlarıyla hazırlanabilme rahatlığından dolayı, elektrot malzemesi olarak sıkça kullanılmıştır. Bu tez çalışmasında, eriyik-tuz destekli kendiliğinden oluşma metoduyla, yüksek yüzey alanlı mezogözenekli çinko kobaltit ince filmler kübik spinel yapıda sentezlendi ve bunun oksijen açığa çıkış tepkimesinde elektro-katalitik performansı analiz edildi. Yüzey aktif maddeler (P123 ve CTAB), çinko nitrat hekzahidrat ve kobalt(II) nitrat hekzahidrattan oluşan berrak ve homojen sulu çözelti (öncü madde olarak), mezofaz elde etmek için cam alt katman üzerine kaplandı, sonrasında kalsinasyon ile mezogözenekli çinko kobaltit (m-ZnCo olarak ifade edildi) toz şeklinde sentezlendi.

m-ZnCo-60 (toplam tuz/P123 60 oranı ile) numuneleri pürüzsüz film morfolojisinde ve en yüksek102 m2/g yüzey alanı ile sentezlendi.

Farklı kompozisyondaki mezofazlar x-ray difraksiyon (XRD) tekniği kullanılarak analiz edildi. XRD deseninde, 1.5 ve 2°, 2θ, arasındaki çizgiler düzenli lyotropik sıvı kristal mesofaz oluşumunun bir göstergesidir. Zamana bağlı mezofazların yaşlandırılması, kararlılıklarını belirlemek için XRD ve POM teknikleriyle izlendi. Kararlı mezofazlar m-

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ZnCo film ve toz numunelerinin sentezinde kullanıldı. Toz numuneler, kalsinasyon prosesinden sonra toplandı ve XRD, N2 adsorpsiyon-desorpsiyon, SEM, HR-TEM teknikleriyle karakterize edildi.

Öncü çözeltiler döndürmeli kaplama ile 1 cm x 2 cm boyutlarında FTO camlarının yarısına kaplandı, daha sonra kalsine edildi ve üç-elektrot sisteminde çalışma elektrodu olarak kullanıldı. Malzemelerin oksijen açığa çıkış tepkimesi elektro-katalizinde elektrot malzemesi olarak performansı, dönüşümlü voltametri, kronopotansiyometri ve kronoamperometri deneyleri ile test edildi. Tüm elektrotlar 100 mA/cm2 akım yoğunluğuna kadar kararlı idi ve en düşük 41 mV/dec Tafel eğimi değeri verdiler.

Mesogözenekli çinko kobaltit malzemeler, CTAB içermeyen öncü çözelti üzerinden de sentezlendi. Sentezden CTAB’ın ayrılması düşük kristallik ve daha pürüzlü yüzey ile sonuçlandı. Karakterizasyon için aynı teknikler kullanıldı. CTAB’sız ürünlerin elektrotları en düşük 40 mV/dec Tafel eğimi ve 1 mA/cm2 akım yoğunluğunda 256 mV aşırı potansiyel değeri verdi.

Ek olarak, yüzey alanını artırmak için ZnCo-60 öncü çözeltisinde TMOS ekleyerek silika şablonlu mezogözenekli çinko kobaltit sentezlendi ve kalsine edilmiş numuneler m-ZnCo- 60-S20-300 olarak gösterildi (S20, 20 TMOS/P123 mol oranını ifade eder). m-ZnCo-60- S20-300 numunesi, 215 m2/g ile en yüksek yüzey alanına sahip olmuştur. Fakat, yüksek yüzey alanına sahip olmasına rağmen, silika malzemesinin yüksek direncinden dolayı, silikalı numuneler daha yüksek aşırı potansiyel değerleri göstermişlerdir.

Anahtar kelimeler: Eriyik-tuz destekli kendiliğinden oluşma, mezogözenekli ince film, çinko kobaltit, oksijen açığa çıkış reaksiyonu, spinel yapılar

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Acknowledgement

I want to special thanks to my supervisor Prof. Ömer Dağ. During my undergraduate and graduate study, he taught me how to question the problems in our field and create a systematic experimental route to answer the questions. He was always supportive and understanding towards me.

I would also thank TÜBİTAK for supporting financially during my graduate study under the project number 118Z820.

I am also very lucky to have such kind and helpful labmates: Irmak Karakaya, Işıl Ulu, Assel Amizhanova, Ezgi Yılmaz Topuzlu and, İpek Şahbenderoglu. I also thank my alumni labmate Muammer Yusuf Yaman. They played an important role for me to become a part of a great research group, we worked together as a team.

In addition, special thanks to my dearest friends Feyza Özel, Sare Kılıç, Pelin Abay, Beyza Nur Ercoşkun, Elif Beyza Yalvaç, Verda Nur Yavuz, Ayşe Beyza Ünüvar and, Hatice Kübra Poyraz. They made my Bilkent years unique and unforgettable.

I owe a big thank to my family. During my whole life journey, my parents, Ömer and Fatma Akmanşen, were always with me and, I have always felt their support. I am also very lucky to have such lovely sisters, Melike, Asude and, Sare, they brought me to life when I felt sad during my work.

My dearest husband, Yusuf Hakan Kalaycı, deserves a unique thank since he made big sacrifices while I was writing my thesis. He also supported me with remarkable academic advices and I know that his support will last forever.

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Contents

Chapter 1 Introduction ... 1

1.1. Mesoporous Materials & Metal Oxides ... 1

1.2. Lyotropic Liquid Crystalline Mesophases ... 5

1.3. Molten Salt-Assisted Self-Assembly (MASA) Method ... 8

1.4. Zinc Cobaltite Nanomaterials – Synthesis and Applications ... 10

1.5. Analyzing OER Performance ... 12

1.5.1. Cyclic Voltammetry ... 12

1.5.2. Tafel Equation and Tafel Slope Analysis ... 13

1.5.3. Choronopotentiometry Experiments and Overpotential Values ... 13

Chapter 2 Experimental Part ... 15

2.1. Chemicals ... 15

2.2. Synthesis of Materials ... 16

2.2.1. Preparation of Aqueous Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123-CTAB Solution 16 2.2.2. Preparation of Aqueous Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123 and TMOS Solution 17 2.2.3. Preparation of LLC Mesophases on Different Substrates ... 18

2.2.4. Preparation of Mesoporous ZnCo2O4 Thin Films ... 18

2.3. Instrumentation ... 18 2.3.1. X-Ray Diffractometer (XRD) & X-Ray Diffractometer for Powder (XRD- MPD) 18

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2.3.2. Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR)

Spectroscopy ... 19

2.3.3. X-Ray Photoelectron Spectroscopy (XPS) ... 19

2.3.4. N2 Adsorption-Desorption Measurements ... 19

2.3.5. Polarized Optical Microscope (POM)... 19

2.3.6. Scanning Electron Microscope (SEM) – Energy Dispersive X-Ray Spectroscopy (EDX) ... 20

2.3.7. Transmission Electron Microscope (TEM) ... 20

2.4. Electrochemical Measurements ... 20

2.4.1. Three-Electrode System ... 21

2.4.2. Cyclic Voltammetry ... 21

2.4.3. Chronoamperometry (CA) & Tafel Slope Analysis ... 21

2.4.4. Chronopotentiometry (CP) and Overpotentials for Water Oxidation ... 22

Chapter 3 Results & Discussion ... 23

3.1. Characterization of LLC Mesophases ... 23

3.1.1. Time Dependent Solutions ... 23

3.1.2. Optimization of CTAB Ratio ... 25

3.1.3. Optimization of Salt Ratio ... 26

3.1.4. The Unknown Crystal ... 28

3.1.5. LLC Mesophases with No CTAB ... 31

3.2. Characterization of Mesoporous ZnCo2O4 Thin Films ... 32

3.3. Mesoporous ZnCo2O4 Thin Films as Electrocatalyst for OER ... 42

3.4. Investigation of the CTAB Effect ... 50

3.4.1. Mesoporous ZnCo2O4 Thin Films from precursor without CTAB as Electrocatalyst for OER ... 53

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3.4.2. Investigation of Crystal Growth Effects ... 61

3.5. Mesoporous ZnCo2O4 Thin Films with Silica Templating ... 63

Chapter 4 Conclusion ... 86

Chapter 5 Future Work ... 89 5.1. Further Analysis of Mesoporous ZnCo2O4 Thin Films with Silica Templating89

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List of Figures

Figure 3. 1. Low angle XRD patterns of LLCM of ZnCo-1.5.60 from a) 1 hour, b) 2 days, and, c) 20 days aged solutions, drop casted on microscope slides. ... 24 Figure 3. 2. High angle XRD patterns of LLCM of ZnCo-1.5.60 from a) 1 hour, b) 2 days, and, c) 20 days aged solutions, drop casted on microscope slides. ... 25 Figure 3. 3. Small angle XRD patterns of LLC of a) ZnCo-1.3.60, b) ZnCo-1.5.60, and, c) ZnCo-1.8.60, drop casted on microscope slides. ... 26 Figure 3. 4. High angle XRD patterns of LLC of a) ZnCo-1.3.60, b) ZnCo-1.5.60, and, c) ZnCo-1.8.60, drop casted on microscope slides. ... 26 Figure 3. 5. Small angle XRD patterns of LLC of a) ZnCo-30, b) ZnCo-60, and, c) ZnCo- 90, spin-coated on microscope slides. ... 27 Figure 3. 6. High angle XRD patterns of LLC of a) ZnCo-30, b) ZnCo-60, and, c) ZnCo- 90, spin-coated on microscope slides. ... 27 Figure 3. 7. POM images of gelation process for ZnCo-60, drop casted. ... 28 Figure 3. 8. Small angle XRD patterns of ZnCo-60 LLC, centrifuged and spin-coated on microscope slides. ... 29 Figure 3. 9. a) Small angle and b) high angle XRD patterns of LLC of ZnCo-1.5.60 from 1 year aged centrifuged solution, drop casted on microscope slides. ... 30 Figure 3. 10. Photo of centrifuged ZnCo-60 precursor solution. ... 30 Figure 3. 11. Small angle XRD patterns of LLC of a) ZnCo-1.0.10, b) ZnCo-1.0.20, and, c) 1.0.30, spin coated. ... 32 Figure 3. 12. High angle XRD patterns of LLC of a) ZnCo-1.0.10, b) ZnCo-1.0.20, and, c) ZnCo-1.0.30, spin coated. ... 32 Figure 3. 13. (a) PXRD patterns of directly calcined m-ZnCo-30 and, (b) m-ZnCo-60 33 Figure 3. 14. SEM images of m-ZnCo-60-350 at a) 200000x magnification, b) 20000x magnification and of m-ZnCo-60-500 at c) 200000x magnification, d) 20000x magnification. ... 35

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Figure 3. 15. SEM images of a) m-ZnCo-30-400 and b) m-ZnCo-60-400 at a 25000x magnification. ... 36 Figure 3. 16. SEM images of m-ZnCo-60-t, where t is a) 250, b) 300 and, c) 450 oC, recorded at a 100000x magnification (scale bara are 1 μm). ... 36 Figure 3. 17. HR-TEM images and analysis of m-ZnCo-60-350: a) TEM images, b) back FFT of the selected areas in green squares and rectangles, c) histograms along the lines in panels b) and d) FFTs of the same selected areas in panels. ... 37 Figure 3. 18. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-30-t (t are indicated in each panel). ... 38 Figure 3. 19. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-60-t (t are indicated in each panel). ... 38 Figure 3. 20. PXRD patterns of annealed m-ZnCo-60 samples. ... 40 Figure 3. 21. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-60-t-an. ... 41 Figure 3. 22. Cyclic voltammograms of fresh m-ZnCo-60-t electrodes. ... 42 Figure 3. 23. Multiple CVs of m-ZnCo-60 electrodes calcined at a)300°C, b)350°C, c)400°C, d)450°C and, e)500°C. ... 44 Figure 3. 24. Tafel slope analysis of m-ZnCo-60-t thin film electrodes. ... 45 Figure 3. 25. Chronopotentiometry results of the m-ZnCo-60-t electrodes at 1 mA/cm2 current density for 2 hours. ... 46 Figure 3. 26. Multistep chronopotentiometry results of m-ZnCo-60-t electrodes with 10 to 100 mAcm-2 current density. Each step takes 30 min. ... 47 Figure 3. 27. Cyclic voltammograms of m-ZnCo-60-t electrodes after multistep chronopotentiometry experiment. ... 48 Figure 3. 28. XPS results of m-ZnCo-60 electrodes before and after multi-step chronopotentiometry experiment. ... 49 Figure 3. 29. PXRD patterns of m-ZnCo-1.0.10, m-ZnCo-1.0.20 and, m-ZnCo-1.0.30, directly calcined at 350°C. ... 50 Figure 3. 30. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-1.0.n (1.0.n are indicated in each panel). ... 51

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Figure 3. 31. SEM images of a) m-ZnCo-1.0.10, b) m-ZnCo-1.0.20 and, c) m-ZnCo- 1.0.30 at 100 000x magnification (scale bars are 1 μm), d) m-ZnCo-1.0.10 at 50 000x magnification (scale bar is 2 μm), e) m-ZnCo-1.0.20 and, f) m-ZnCo-1.0.20 at 10 000x magnification (scale bars are 10 μm). ... 53 Figure 3. 32. Cyclic voltammograms of fresh m-ZnCo-1.0.n-350 electrodes (1.0.n are indicated in the panel). ... 54 Figure 3. 33. Multiple CVs of a) m-ZnCo-1.0.10, b) m-ZnCo-1.0.20 and, c) m-ZnCo- 1.0.30 electrodes calcined at 350°C. ... 55 Figure 3. 34. Tafel slope analysis of m-ZnCo-1.0.10, 20 and, 30 electrodes calcined at 350°C. ... 56 Figure 3. 35. Chronopotentiometry results of m-ZnCo-1.0.10, 20 and, 30 electrodes at 1 mA/cm2 current density for 2 hours. ... 57 Figure 3. 36. Multistep chronopotentiometry results of m-ZnCo-1.0.10, 20 and, 30 electrodes at from 10 to 100 mA/cm2 current density. Each step takes 30 min. ... 58 Figure 3. 37. Cyclic voltammograms of m-ZnCo-1.0.10, 20 and, 30 electrodes after multistep chronopotentiometry experiment. ... 59 Figure 3. 38. XPS results of m-ZnCo-1.0.10, 20 and, 30 electrodes before and after multi- step chronopotentiometry experiment. ... 60 Figure 3. 39. PXRD patterns of m-ZnCo-60-t samples, where t is 300 and 700. t is given in the plot. ... 61 Figure 3. 40. PXRD patterns of m-ZnCo-1.0.20-700 and m-ZnCo-60-700-(not centrifuged) samples. ... 62 Figure 3. 41. PXRD patterns of m-ZnCo-60-350, m-ZnCo-60-700 and, m-ZnCo-60-800 samples from calcination of 1 day aged ZnCo-60 mesophases. ... 63 Figure 3. 42. PXRD patterns of m-ZnCo-60-S5, m-ZnCo-60-S10 and, m-ZnCo-60-S20, directly calcined at 350°C. ... 65 Figure 3. 43. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-60-Sx (x are indicated in each panel). ... 66 Figure 3. 44. SEM images of m-ZnCo-60-S5-350 at a) 50 000x and b) 10 000x magnification and m-ZnCo-60-S10-350 at c) 100 000x and d) 10 000x magnification.. 68

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Figure 3. 45. SEM images of m-ZnCo-60-S20-350 at a) 50 000x and b) 15 000x and c) 2 000x. ... 69 Figure 3. 46. a) Cyclic voltammograms of fresh m-ZnCo-60-S5, m-ZnCo-60-S10 and m- ZnCo-60-S20 electrodes and b) with narrower scale. ... 70 Figure 3. 47. Multiple cyclic voltammograms of fresh a) m-ZnCo-60-S5 electrode and b) with narrower scale, c) m-ZnCo-60-S10 electrode and d) with narrower scale and e) m- ZnCo-60-S20 electrode and with f) with narrower scale. ... 72 Figure 3. 48. Chronopotentiometry results of the m-ZnCo-60-S5, m-ZnCo-60-S10 and, m-ZnCo-60-S20 electrodes at 1 mA/cm2 current density for 2 hours. ... 72 Figure 3. 49. Multistep chronopotentiometry results of the m-ZnCo-60-S5, m-ZnCo-60- S10 and, m-ZnCo-60-S20 electrodes with 10 to 100 mA cm-2 current density. Each step takes 30 min. ... 73 Figure 3. 50. Cyclic voltammograms of m-ZnCo-60-S5, m-ZnCo-60-S10 and, m-ZnCo- 60-S20 electrodes after mCP experiment... 74 Figure 3. 51. XPS results of m-ZnCo-60-S5, m-ZnCo-60-S10 and, m-ZnCo-60-S20 electrodes before and after multi-step chronopotentiometry experiment. Normalized except Si region. ... 76 Figure 3. 52. PXRD patterns of m-ZnCo-60-S20-t, t is indicated in the figure. Directly calcined. ... 77 Figure 3. 53. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-60-S20-t, t is indicated in each plot. Directly calcined. ... 78 Figure 3. 54. SEM images of a) m-ZnCo-60-S20-300 (scale bar is 1 μm), b) m-ZnCo-60- S20-500 (scale bar is 4 μm), c) and d) m-ZnCo-60-S20-600 (scale bars are 1 and 10 μm).

... 80 Figure 3. 55. SEM images of m-ZnCo-60-S20-400 with scale bar of a) 500 nm, b) 2 μm and, c) 10 μm. ... 80 Figure 3. 56. Multiple cyclic voltammograms (600 cycles) of m-ZnCo-60-S20-300 electrode a) between 1st and 25th cycles b) between 50th and 600th cycles. ... 81 Figure 3. 57. Multiple cyclic voltammograms (600 cycles) of m-ZnCo-60-S20-300 electrode between a) 1st and 25th cycles and b) 50th and 600th cycles. ... 81

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Figure 3. 58. XPS spectra of fresh m-ZnCo-60-300, m-ZnCo-60-S20-300 electrodes and, m-ZnCo-60-S20-300 electrode after 600 CVs. Normalized spectra, except for Si2p region.

... 82 Figure 3. 59. Chronopotentiometry results of the m-ZnCo-60-S20-t electrodes, where t is 300, 400, 500 and, 600, at 1 mA/cm2 current density for 2 hours. ... 83 Figure 3. 60. Tafel slope analysis of the m-ZnCo-60-S20-t electrodes, where t is 300, 400, 500 and, 600, at 1 mA/cm2 current density for 2 hours. ... 84 Figure 5. 1. ATR-IR spectra of fresh m-ZnCo-60-S20-350. ... 90

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List of Tables

Table 2. 1. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123-CTAB

aqueous solution. ... 16

Table 2. 2. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123 aqueous solution. ... 16

Table 2. 3. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123-CTAB- TMOS aqueous solution. ... 17

Table 3. 1. Atomic percentage by elements of precipitate from centrifuged ZnCo-60 solution via EDAX. ... 31

Table 3. 2. Calculated crystals size using Scherer’s equation for m-ZnCo-60-t. ... 33

Table 3. 3. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-n-t (n:30 & 60). ... 38

Table 3. 4. N2 (77 K) Adsorption−Desorption Data of Annealed m-ZnCo-60-t-an. ... 41

Table 3. 5. Calculated crystals size using Scherer’s equation for m-ZnCo-1.0.n. ... 51

Table 3. 6. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-1.0.n (n: 10, 20 &30) . 52 Table 3. 7. Calculated crystals size using Scherrer’s equation for m-ZnCo-60-Sx, where x is 5, 10 and, 20 directly calcined at 350°C. ... 65

Table 3. 8. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-60-Sx-350, where x is 5, 10 and, 20. ... 66

Table 3. 9. Calculated crystals size using Scherrer’s equation for m-ZnCo-60-S20-t. ... 77

Table 3. 10. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-60-S20-t, (t: 300, 350, 400, 500 & 600). ... 78

Table 3. 11. Overpotential values of m-ZnCo electrodes in this thesis... 84

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Abbreviations

OER : Oxygen Evolution Reaction LC : Liquid Crystal

LLC : Lyotropic Liquid Crystal

LLCM : Lyotropic Liquid Crystalline Mesophase EISA : Evaporation Induced Self Assembly MASA : Molten salt Assisted Self Assembly CMC : Critical Micellar Concentration CTAB : Cetyltrimethylammonium bromide SDS : Sodium dodecyl sulfate

PEO : Poly(ethylene oxide) PPO : Poly(propylene oxide) TLC : Thermotropic Liquid Crystal XRD : X-Ray Diffraction

PXRD : Powder – X-Ray Diffaction POM : Polarized Optical Microscopy SEM : Scanning Electron Microscopy TEM : Transmission Electron Microscopy EDX : Energy Dispersive X-Ray Spectroscopy

ATR-FTIR : Attenuated Total Reflection Fourier-Transform Infrared

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BET : Brunauer, Emmett and Teller BJH : Barrett-Joyner-Halenda SC : Supercapacitor

FTO : Fluorine Doped Tin Oxide NCNT : N-doped Carbon Nanotube GCE : Glassy Carbon Electrode CV : Cyclic Voltammetry CA : Chronoamperometry CP : Chronopotentiometry

mCP : Multi-step chronopotentiometry NHE : Normal Hydrogen Electrode RE : Reference Electrode

WE : Working Electrode CE : Counter Electrode

FWHM : Full Width Half Maxima

ICP-MS : Inductively Coupled Plasma Mass Spectrometry

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1

Chapter 1

Introduction

1.1. Mesoporous Materials & Metal Oxides

Porous materials have been attracted many researchers due to their high surface area and enhanced catalytic activity for industrial processes. Until 1992, while microporous materials with crystalline form and uniform pore size distribution and high catalytic activity are well known, only a few examples of amorphous mesoporous materials existed with a wide distribution of pore size [1]. The first mesoporous molecular sieves family with uniform pore distribution, which is called M41S (MCM-41 with an hexagonal pore structure, MCM-48, cubic and, MCM-50 lamella), was discovered in 1992 by researchers from Mobile R&D Corporation [2], [3]. The first mesoporous materials were synthesized using a liquid-crystal 'templating' mechanism, in which the inorganic walls between micelles are formed by silica materials. Beck et al. provided detailed information about MCM-41, a member of the group M41S, that it is formed by hexagonal arrays, also some

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2

of the members of the group have cubic phase [1], [3]. A cationic surfactant was used and it was shown that pore size is in relation with surfactant chain length [1]. Furthermore, they suggested that the cooperative organization between charged surfactants and organic- inorganic species was directed by electrostatic interactions [1], [3]. An extensive work has been done for the synthesis, characterization, formation of mechanism of such materials [2] over last almost 30 years.

Also, Kuroda and co-workers introduced the surfactant directed rearrangement of layered polysilicate, kanemite (NaHSi2O5•3H2O), to prepare microporous (micro pores, 2-4 nm in diameter)[4] and highly ordered mesoporous (FSM-16) [5] materials. They have used the advantage of layered materials due to the molecular design of nanoscale modifications of layered host materials.

In 1995, Pinnavaia and co-workers used neutral surfactants to prepare the first example of mesoporous silica materials [6]. In this case, mesoporous molecular sieves that is based on hydrogen bonding and self-assembly between neutral primary amine surfactants and neutral inorganic precursors were synthesized [6]. Besides, it (hexagonal mesoporous molecular sieves (denoted HMS)) could be considered as a member of M41S family, it differentiates in terms of physical properties. In the following year, the same group demonstrated that electrostatically templated MCM-41 silicas have smaller pore-wall thickness than neutral HMS mesostructures prepared from surfactants of the same chain length [7]. In comparison between two templating methods, neutral templating route resulted with a smaller particle size and defect structure in channel packing, which provides a better access to the mesopores as a complementary textural mesoporosity.

The mechanistic properties of the MCM-41 synthesis gave the idea that the similar procedure can be extended through non-silica-based mesostructured materials [8]. Stucky and co-workers extended the study to the synthesis of mesoporous metal oxides and the study became the pioneering work [9]–[11]. However, the first reported non-siliceous mesostructured materials by Huo et al.[9] were failed in terms of removal of the templates [12]. Following, in 1995, Ying’s group introduced the first mesoporous metal oxides with

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open pores by calcination [13]. Additionally, in the same year, Attard et al. employed lyotropic liquid crystals to synthesize mesoporous silica [14]. In this new method, liquid crystals of poly(oxoethylene) surfactants were used in higher concentration compared to the preparation of M41S. However, the developments on the synthesis of non-siliceous materials got faster with the introduction of evaporation-induced self-assembly (EISA)[15] and nano-casting[16] methods.

Nano-casting method, in other words hard-templating method (HTM), that has been used to prepare mesoporous metal oxides and metals uses mesoporous silica as a hard template.[17] HTM has some advantage over the synthesis through lyotropic liquid crystals and makes it possible to prepare materials which are hard to be synthesized by soft-templating methods [18]. This method can be described in four steps: (1) preparation of hard-template; (2) impregnation of a precursor into the pore system of the template; (3) transformation of the precursor into the target product by heat treatment (or other); and (4) template removal [12].

This method was originally used for templating by using porous Al2O3 membranes [19].

However, the hard-templating method is used widely with mesoporous silica or carbon as hard templates to obtain mesostructured metal oxides [20]–[26].

Silicas are ideal hard templates, since they have uniformly distributed pore size and highly ordered nanoscale structures [12]. However, mesoporous carbon material, CMK-1, that was synthesized for the first time by Ryoo et al.[16] was employed successfully for the first time for replication of ordered mesoporous materials [12]. Later, mesoporous carbon has found its place in the literature as a hard template for the synthesis of mesoporous metal oxides [22]–[24], [27]. Although it is advantageous for the formation of porous metal oxides due to ease of removing the template by combustion[20], inert atmosphere is required in the heat treatment stage[28] of the synthesis of mesoporous metal oxides.

Whereas, the silica framework can be removed by dissolving either in NaOH solution or HF solution.

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In the beginning of the 21st century, Zhu et al. synthesized the first nanostructured metal oxide, Cr2O3, by nano-casting using SBA-15 (silica, it is also introduced to the literature by Stucky et.al. using pluronics, tri-block copolymers, as templates as the hard template [29]. In the following years, Zhao and co-workers were able to synthesize a series of ordered mesoporous metal oxides with microwave-digested mesoporous silica, namely Co3O4, In2O3, Cr2O3, NiO, Fe2O3, MnxOy, CeO2 and, WO3 [30], [31].

While hard-templating method is based on use of mesoporous silica/carbon, soft- templating method is based on molecular/block-copolymer surfactants. The pathway of this general method can be summarized in three main steps: (1) preparation of solution of all ingredients; (2) co-assembling surfactants (or block-copolymers) and inorganic species; (3) removal of the surfactant (generally by calcining). When the surfactant in a proper solvent reaches a critical concentration (known as critical micellar concentration, CMC), they form supramolecular structures (known as micelles), therefore the micelle formation is concentration-dependent [32]. These structures can also be called as lyotropic liquid crystals, or mesophases at much higher concentrations (this topic will be discussed in the next section of this thesis).

The method to prepare first mesoporous material (M41S family) by Kresge et al. is also classified as a soft-templating method [3]. Then, in 1994, the first preparation of ordered mesostructured metal oxides (WO3,Sb2O5,Fe2O3, etc.) using soft-templating method was reported [9]. Both cationic and anionic surfactants are used as soft template. This method, i.e. “ligand-assisted” method, was applied to obtain ordered mesoporous TiO2[13]; Nb2O5

[33], [34]; ZrO2[35], [36]; and VOx [37].

Another widely used soft-templating method, evaporation-induced self-assembly (EISA), was introduced to synthesize mesostructured materials by Brinker’s group [15], [38], [39].

In the EISA route, non-aqueous solvents (like ethanol, propanol etc.) are used, therefore hydrolysis and condensation rates are slowed down. Thus, the control of macroscopic form (monoliths and thin films) could be synthesized, therefore, the EISA method is highly advantageous to form ordered mesostructures [15], [40]. Evaporation of solvent

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drives the co-assembly of surfactant and inorganic precursors, then ordered mesophases are formed. Addition of metal chlorides into alcohol in acidic media results with chloroalkoxy precursors [32] in the initial evaporation step. Later, with a thermal process, these precursors are converted into metal oxides.

As described above, the cationic surfactant (CTAB) in basic media is used to obtain M41S family [3], then SBA-type mesoporous silica with thicker pore walls was synthesized with the use of non-ionic PEO-PPO-PEO triblock copolymers[39]. After the pioneering work with Pluronic family, they have been extensively used to synthesize metal oxides and metal oxide thin films [41]–[45]. By applying EISA method and using amphiphilic block co-polymer non-ionic surfactants, Yang et al. produced mesoporous metal oxides (TiO2, ZrO2, Nb2O5, Al2O3, WO3, HfO2 and, SnO2) and mixed oxides (such as SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5 and, ZrW2O8) [46]–[48].

In comparison between hard- and soft-templating methods, hard-templating provides opportunity to design of highly mesoporous materials with different compositions in powder form. Moreover, the hard-templating is a highly-costly method and the removal of template may create other problems [49]. On the other hand, soft-templating method can lead to formation of undesired carbonaceous residues and collapsing of the pore structure [32]. Although, the EISA method resulted with thicker pore walls, it requires long time, which makes hard to prepare membranes or monoliths [50] and limited to a group of metal precursors. Therefore, new synthetic approaches are needed to design mesoporous metal oxides in thin film forms, which are desired in practical applications of these materials.

1.2. Lyotropic Liquid Crystalline Mesophases

In soft-templating method, surfactants can self-assemble in solution phase or lyotropic liquid crystalline mesophase. Lyotropic liquid crystals (LLCs), as a soft template, are more versatile and advantageous than hard templates. Liquid crystalline phase was detected in 1888 by Reinitzer to Lehmann. In comparison with liquids and solids, liquid crystals have higher state of order than liquids and have higher mobility than solids.

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Moreover, liquid crystals flow like a liquid and have a long range orientational order, so they can be defined as mesophases [51] and have very similar structure with those mesostructured solids. Note also that the term “mesostructured” is for the mesoporous materials before surfactant removal.

One of the two types of liquid crystals is thermotropic liquid crystal (TLC) and it displays change in phase with temperature. Our main concern is lyotropic liquid crystals, which has the name related to “solve” in Greek. The phase of LLCs is dependent to the amount of solvent. A well-known mixture, which shows LLC phases is soap and water. Surface active agents have long hydrophobic tail groups and hydrophilic polar head groups, namely amphiphilic molecules. Amphiphilic molecules aggregate and form structures with a hydrophilic shell and hydrophobic core are in aqueous solutions. Therefore, they form micelles, which are small aggregates with “finite” size and shape. When the concentration of surfactant goes beyond the CMC and with further concentration of micelles, the LLC mesophases appear through self-organization of micelles. Below CMC, molecules exist in the solution as solvated individual molecules. Molecular structure, concentration and temperature strongly affect the mesostructures of the LLCs [52].

Furthermore, it is possible to affect the morphology of the material with the change of composition of the LLC mesophases. For instance, mesoporous core-shell, nanofibers or rod-shaped silica, TiO2 hollow structures were prepared by using the micelles in solution phase and the mesophases [53]–[55].

LLC-mediated synthesis provides an easy control on the structure of materials by creating different mesophase structures, i.e. lamellar, 2D hexagonal (p6mm), 3D cubic (Ia3d) or disordered mesostructures. The formation of phases can be observed through polarized optical microscopy; while hexagonal phase (the long cylindrical micelles are assembled into a hexagonal structure) exhibits a fan texture, it is not possible to visualize a cubic phase because cubic phase is isotrophic. Another way for characterizing LLCs is small angle X-Ray diffraction (XRD). Due to much larger unit cells, much longer d-spacing between regular micella domains, the mesophases diffract at small angles, typically 1-5°, 2θ.

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The synthesis through LLCs can be conducted with or without a polymerizing agent. After the discovery of M41S[3] with lower surfactant concentration, Attard et al. introduced the usage of liquid crystals of poly(oxoethylene) surfactants to prepare mesoporous silica in 1995 [14]. Surfactant concentration is quite high in this new method compared to the preparation of M41S. Later, in 1997, the same group reported the synthesis of mesoporous Pt microparticles from LLC mesophases[56] and Pt films by electrodeposition of LLC phases[57]. For the synthesis of Pt microparticles, hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6.xH2O) and non-ionic surfactant (C16(EO)8) was used in the presence of water as the solvent. After the removal of the surfactant, following the reduction, mesoporous Pt microparticles were obtained [56].

LLC-mediated synthesis is also applied to obtain metal sulfides. Hexagonal LLC template and the colloidal template were used to prepare mesoporous ZnS hollow microspheres [58]. It is also possible to obtain 1-D[59], [60] and 2-D[61] nanostructures via LLC templating method. Kijima et al. synthesized the AgBr and SnO2 micro wires from LLCs that contain CTAB, oligoethylene alkyl ether and AgNO3 or SnF2 [59]. Later, the same group reported the synthesis of Pt-nano sheets by using LLCs of non-ionic surfactant. [61]

In addition, mesoporous metal oxides and mixed metal oxides [46]–[48] and mesoporous spinels [62], [63] were prepared using LLC mesophases via soft-templating methods.

Yang et al. and Tian et al. applied EISA method to obtain mesoporous metal oxides (TiO2, ZrO2, Nb2O5, Al2O3, WO3, HfO2 and, SnO2) and mixed metal oxides (such as SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5 and, ZrW2O8) [46]–[48] and mesoporous ZnAl2O4 thin films [63], respectively.

Until the discovery of a new type LLCs, lyotropic liquid crystals are formed with low concentration of metal salts and excess amount of water that limits the metal ion concentration in the LLC mesophase. Therefore, only mesostructured solids, produced by this approach was limited and yielded very small amount of powders in an extremely surfactant mediated gels.

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In 2001, a new type of LLC mesophase that contains transition metal salt at extremely high concentrations, CnEOm or pluronic-type nonionic surfactant was investigated by Dag and coworkers [64]. It has been discovered that C12EO10 and metal salts ([M(H2O)n](NO3)2 , where M is Co, Ni, Zn and, Cd) mixture at different mole ratios can form an LLC mesophase. The salts of transition metals exist in molten form in the non- ion surfactants and the metal concentration can go beyond the usual value in the salt/water/CnEOm system.

In the following publications, the effect of counter ions (Cl-, NO3- and, ClO4-) on the LLC mesophase of different transition metal salts and non-ionic surfactant, C12EO10, were investigated [65]. In addition, our group showed that in the cobalt chloride, cobalt nitrate, and cobalt perchlorate salts and pluronic P65 LLC phases, there is a cobalt catalyzed surfactant oxidation induced phase separation in the LLC mesophases [66]. In 2008, it has been reported that addition of a charged surfactants improved the salt uptake of the LLC mesophases [67]. C12EO10-CTAB or C12EO10-SDS, non-ionic and ionic surfactant couple and, [Zn(H2O)6](NO3)2 were used to show the role of a charged surfactant in the LLCs. In the two surfactants system, it is possible to maintain even higher amount of salt ions in the mesophases in the presence of a small amount of water that prevents the crystallization of salt ions. Following this invention, two surfactants system, which contains pluronic copolymer as a non-ionic surfactant and CTAB as the charged surfactant was employed to synthesize first example of a mesostructured metal sulfide film [68]. In addition, by taking the advantage of salt-surfactant LLC mesophase, in which the salt is in the molten phase, mesoporous SiO2-ZnO and SiO2–CdO thin films were synthesized, and the process was named as molten salt-assisted self-assemly (MASA) [69].

1.3. Molten Salt-Assisted Self-Assembly (MASA) Method

After the discovery of the new LLCs with exceptionally high salt concentration, our group introduced a new synthesis method for mesoporous materials. This is called molten salt- assisted self-assembly since the salt species exist in the molten phase in between micellar

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structures. In this assembly process, the salt species act as a solvent upon melting in a confined nanospace and induce a self-assembly of surfactants molecules and ingredients into mesostructured LLC mesophases [70]. Note that in a confined space of hydrophilic domains the melting point of the salt species drops down due to soft confinement effect (SCE) or nanospace effect (NE), therefore the molten salt does not crystallize at even under liquid nitrogen temperatures [70]. Also notice that in the nanospace (hydrophilic domains of the mesophase), there is no space for salt seeds to form for further growth. It means without seeding step, the growth of salt crystals is impossible. Therefore, the salt- surfactant mesophases are stable indefinitely.

In previous works, the effect of charged surfactant on increasing the metal salt in the mesophase was extensively investigated [67] and shown that CTAB is essential in the MASA process. Stable salt-C12EO10 LLC mesophases at higher salt concentrations, which is needed for the synthesis of stable mesostructured materials can only be obtained with the addition of CTAB [69]. To synthesize a desired mesoporous thin film, the LLC mesophases are first spin coated from their precursor solutions over a glass substrate, then calcined for the removal of surfactants and transformation of metal salt into its oxide. This method differs from EISA method due to the unique character of salt-surfactant LLCs with relatively high concentration. Furthermore, in the EISA the ingredients immediately turn into a mesostructured solid form during evaporation step. Therefore, it is limited to metal alkoxides that undergo hydrolysis and condensation reactions during evaporation step and surfactant and polymerized metal oxyhydroxy nanospecies assemble together into a mesostructured solid. The end product of EISA is a semisolid mesostructured solid, but the MASA yields a stable LLC mesophase that requires further steps to produce mesostructured and/or mesoporous metal oxides, but it is not limited to metal alkoxides.

The homogenous solution of two surfactants, solvent (water or ethanol) and, non-volatile molten salt is spin-coated into a LLC mesophase. Homogeneity of the solutions is very important for spin-coating process. In addition to spin coating, dip coating, spray coating or drop casting methods can be used to alternate the film thickness or to synthesize monoliths and powders.

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In 2011, MASA process was first applied to synthesize mesoporous ZnO and CdO coated silica thin films [69]. Later, this process was extended to synthesize many mesoporous metal titanates (CdTiO3, Zn2TiO4, CoTiO3, MnTiO3 and, Li4Ti5O12) [71]–[73], metal lithiates (LiCoO2, LiMnO2, LiMn2-xCoxO4) [74], [75], NiO [76] and metal cobaltites (MnCo2O4, NiCo2O4 and, ZnCo2O4) [77] as thin films. As a non-ionic surfactant, P123[73], [75], [77] and mostly C12EO10 [69], [71], [72], [74], [76] were used for the synthesis of these materials. In this work, we will discuss the synthesis of mesoporous ZnCo2O4 thin films from P123-CTAB-[Zn(H2O)6](NO3)2-water LLCs.

1.4. Zinc Cobaltite Nanomaterials – Synthesis and Applications

Transition metal cobaltites attracted many researchers due to their high charge storage and electron transport, abundance, safety and, low cost. They can be prepared in different morphologies, such as nanorods, nanosheets, nanospheres, with large surface areas, so materials can have an excellent electrochemical performance.

Their ability to transfer electrons rapidly increased their theoretical specific capacities, therefore they are widely used in supercapacitors. Because of the advantages of these electrode materials’ structure, transition metal cobaltites are used in other application areas such as lithium ion batteries, electrocatalysis and, electrosensing as an electrode material.

Venkatachalam et al. synthesized hexagonal nanostructured ZnCo2O4 with double hydroxide mediated synthesis by using hydrothermal method at 200°C [78]. The product had 31.6 m2 g-1 surface area and showed good electrochemical properties as a material for SCs. Xiao and coworkers have fabricated mesoporous ZnCo2O4 nanosheets with 191.64 m2/g surface area using hydrothermal method followed by a calcination process as a supercapacitor material [79]. Another work to synthesize porous ZnCo2O4 nanostructures hydrothermally was reported in 2017 [80]. Xu and coworkers have used the hydrothermally synthesized and heat-treated materials in the preparation of

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supercapacitors due to the porosity and the convenient ion transport. In addition, solvothermal and deposition methods were also used to obtain ZnCo2O4 for SCs. A self- assembled hierarchical peony-like ZnCo2O4 microstructures with highest 42.23 m2/g specific surface area was prepared through a facile solvothermal method and thermal annealing treatment [81]. Also, in 2018 in the work of Gao et al. hydrothermal and followed by deposition method were used to synthesize ZnCo2O4-rGO composite electrode [82].

ZnCo2O4 is also widely used as a material for lithium-ion batteries. Carbone et al. reported the preparation of ZnCo2O4 nanorods by step-wise synthetic procedure of CTAB assisted co-precipitation, autoclave treatment, calcination and be-spoke alkaline etching [83].

Solvothermal[84], electrostatic spray deposition[85], polyol process[86], molecular sieve templating[87] and widely hydrothermal synthesis methods[88]–[92] have been used for preparation of ZnCo2O4 with different morphologies as an anode material of lithium ion batteries. In 2015, Guo et al. applied typical hydrothermal synthesis method to obtain mesoporous zinc cobaltite microspheres with specific surface area of 39.52 m2/g [92].

Zhen and co-workers reported the preparation of ultrathin mesoporous ZnCo2O4

nanosheets with surface area of 52.7 m2/g via a facile hydrothermal method followed by annealing treatment [89]. Another important work was conducted by Yuan et al. to produce 3D porous ZnCo2O4 thin films by using electrostatic spray deposition and annealing for lithium-ion battery anodes [85].

ZnCo2O4 materials are also prepared for electrocatalytic purposes. Kim et al. synthesized ZnCo2O4 thin film electrodes via electro deposition and heat treatment at 550°C [93]. In this study, the activities of Co3O4 and ZnCo2O4 were compared and it was concluded that there is no electrocatalytic activity of Co2+ ion. In addition, the higher performance of ZnCo2O4 over Co3O4 is due to easy access of cobalt ions on the surface and loss of zinc ions from the near surface, where is active for OER [94]. However, in 2019 Sun et al.

showed that the intrinsic activity of Co3O4 is higher than ZnCo2O4, and added that cobalt in octahedral sites plays the key role in OER catalysis [95]. In 2016, Wang et al. reported

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the preparation of hollow porous ZnCo2O4 microspheres which have a better oxygen reduction reaction (ORR) performance than bulk ZnCo2O4 [96]. As a solution for the low conductivity of ZnCo2O4, Pu et al. synthesized ZnCo2O4/N-doped carbon nanotube (NCNT) composites [97].

OER electrocatalysis is also an important application area for ZnCo2O4 materials.

ZnCo2O4 micro-spindles and truncated drums with 65.85 m2/g specific surface area were prepared by a solvent thermal method, and then annealing treatment to be used as an electrode material [98]. The material was coated on the substrate of glassy carbon electrode (GCE) and the electrodes exhibited an overpotential of 389 mV at 10 mA/cm2 current density. In one of the recent studies, ZnCo2O4 nanowires were synthesized with NiFe layered double hydroxide nanosheets on Ni foam to prepare electrode[99]. The prepared electrode needs only 249 mV at 10 mA/cm2current density.

1.5. Analyzing OER Performance

Some techniques that are widely used to analyze electrochemical performance of the materials are cyclic voltammetry, Tafel slope analysis and, chronopotentiometry for overpotential values, and they were used in this thesis work. Brief background information on these techniques will be given in this section.

1.5.1. Cyclic Voltammetry

Cyclic voltammetry is a technique that where voltage is swept linearly in time scale, while current is recorded [100]. Generally, voltage is swept from negative voltage to positive voltage, therefore oxidation occurs first, then, and the oxidized species are reduced while potential is sweeping through negative voltage. A silver/silver chloride electrode (Ag/AgCl) can be used as a reference electrode, where the potential of working electrode is controlled versus a reference electrode [100]. This method was widely used to analyze, redox active species on the surface of a material.

The reached current by the peak observed while sweeping to positive voltage is called cathodic current and the voltage is called cathodic peak voltage. In contrast, the recorded

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current at the peak in reduction region is called anodic current at anodic peak voltage.

Mean value of the cathodic and anodic peak voltages is equal to oxidation potential of related redox couple.

1.5.2. Tafel Equation and Tafel Slope Analysis

Tafel slope is an important kinetic parameter of a related electrochemical reaction mechanism, therefore, for OER mechanism [101]. Tafel equation, which relates to the overpotential and current density, was first introduced by Julius Tafel in 1905 [102].

𝜂 = 𝑎 + 𝑏 𝑙𝑜𝑔𝐽

Tafel equation, where 𝜂 is the overpotential (V) and 𝐽 is current density (A/cm2); b is the Tafel slope (V/dec) and, a is an empirical value. Therefore 1 tafel equals to 1 V per decade of current [102].

It was reported that two of the best performing electrode materials for OER, bulk Ru electrode and RuO2 electrode, displayed Tafel slope of 39 and 41 mV/dec [103], [104].

As relation between Tafel-slope value and the mechanism, a Tafel-slope of 40 mV/dec is consistent with the second electron transfer step in the mechanism as a rate determining step [101], [105]. In addition, Reiser et al. added that Tafel-slope of 60 mV/dec demonstrate a similar rate determining step. However, when the value is changed to 120 mV/des, such as for MnCo2O4 [77], it was suggested that the OER mechanism is changed or slightly modified for the electrode.

1.5.3. Choronopotentiometry Experiments and Overpotential Values

Overpotential values are the extra required potential over theoretically calculated potential to make the reaction happen. The theoretically calculated values are obtained by using Nernst equation for a specific pH value.

Water oxidation, or oxygen evolution reaction in acidic media is given below:

2𝐻2𝑂(𝑙) ⇌ 𝑂2(𝑔)+ 4𝐻+ + 4𝑒̅ 𝐸0 = 1.229 𝑉 𝑣𝑠 𝑁𝐻𝐸

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However, the electrochemical experiments in this thesis work was conducted in 1 M KOH, which has a 14 pH value. The standard potential value was calculated by pH value and Nernst Equation, given below:

𝐸 = 𝐸0 −𝑅𝑇

𝑛𝐹𝑙𝑜𝑔𝑃𝑂2[𝐻+]4 𝐸 = 1.229 −0.0592

4 𝑙𝑜𝑔10𝑃𝑂2[𝐻+]4, 𝑤ℎ𝑒𝑟𝑒 [𝐻+] = 10−14𝑀 𝑎𝑛𝑑 𝑃𝑂2 = 1𝑎𝑡𝑚 𝐸 = 0.401 𝑉

Therefore, the minimum required potential is lower in the basic media.

Overpotential values are calculated by subtracting 0.401 V from the recorded potential and used in the analysis.

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Chapter 2

Experimental Part

2.1. Chemicals

In this study, all the chemicals listed below are used as it is purchased. No further purification is needed.

Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 98%, Sigma-Aldrich) and cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O, >98%, Fluka) are used as metal salts. Surfactants are Pluronic® P-123 (Poly(ethylene glycol)-block-poly(propylene glycol)-block- poly(ethylene glycol), (MW~5800g/mol) 99.9%, Sigma-Aldrich) as non-ionic and, hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br, denoted as CTAB, >98%, Sigma-Aldrich) as charged surfactant. Tetramethyl orthosilicate (Si(OCH3)4, denoted as TMOS, 99%, Sigma-Aldrich) is also used.

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2.2. Synthesis of Materials

2.2.1. Preparation of Aqueous Zn(NO

3

)

2

.6H

2

O-Co(NO

3

)

2

.6H

2

O-P123- CTAB Solution

P123 and water is mixed in 15 ml-glass vial to obtain a clear solution. Then, CTAB is added to the solution, and the resulted mixture is stirred for 1 day. As the final step, metal salts are added to the solution, and stirred for 2-3 hours. The red solution is centrifuged at 6000 rpm for 10 min, the mother liquor is used for preparing mesophases.

Table 2. 1. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123-CTAB aqueous solution.

Salts/P123 (mole ratio)

[Zn(H2O)6](NO3)2

(g)

[Co(H2O)6](NO3)2

(g)

CTAB (g)

P123 (g)

DI water (g)

30 0.513 1.003 0.314 1 5

60 1.026 2.006 0.314 1 5

90 1.539 3.009 0.314 1 5

Preparation of Aqueous Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123 Solution P123 and water is mixed in 15 ml-glass vial to obtain clear solution. Then, metal salts are added to the solution, and stirred for 2-3 hours until it becomes homogenous.

Table 2. 2. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123 aqueous solution.

Salts/P123 (mole ratio)

[Zn(H2O)6](NO3)2

(g)

[Co(H2O)6](NO3)2

(g)

P123 (g)

DI water (g)

10 0.171 0.335 1 5

20 0.342 0.669 1 5

30 0.513 1.003 1 5

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2.2.2. Preparation of Aqueous Zn(NO

3

)

2

.6H

2

O-Co(NO

3

)

2

.6H

2

O-P123 and TMOS Solution

P123 and water is mixed in 15 ml-glass vial to obtain clear solution. Then CTAB is added to the solution, and the resulted mixture is stirred for 1 day. As the final step, metal salts are added to the solution, and stirred for 2-3 hours.

The resulted solution is centrifuged at 6000 rpm for 10 min, the mother liquor is used for further steps.

Just before coating, a few drops of conc. nitric acid and TMOS is added, then stirrer for 5 minutes.

Composition of the samples are denoted as ZnCo-n-Sx, where n is salts/P123, x is TMOS/P123 and S stands for silica.

Table 2. 3. Amounts of chemicals for Zn(NO3)2.6H2O-Co(NO3)2.6H2O-P123-CTAB- TMOS aqueous solution.

Sample [Zn(H2O)6] (NO3)2 (g)

[Co(H2O)6] (NO3)2 (g)

TMOS (μl)

conc.

HNO3

(drop)

CTAB (g)

P123 (g)

DI water

(g) ZnCo-

60-S5

0.821 1.605 102 3 0.250 0.8 16

ZnCo- 60-S10

0.513 1.003 127 2 0.156 0.5 10

ZnCo- 60-S20

0.257 0.501 127 2 0.078 0.25 5

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2.2.3. Preparation of LLC Mesophases on Different Substrates

The solution is spin coated on microscope slides (7cm x 4cm) at 2000 rpm for 10 seconds.

(LLC-1). It is used for further XRD analysis.

The solution is spin coated on the half of FTO (fluorine-doped tin oxide) glasses with 1cm x 1cm size at 2000 rpm for 10 seconds. (LLC-2)

Another preparation method of LLC mesophases is drop coating on microscope slides.

(LLC-3)

2.2.4. Preparation of Mesoporous ZnCo

2

O

4

Thin Films

Each of the LLC mesophases are calcined at temperature between 250 and 700°C for 1 h in a furnace.

LCC-1 is used as a precursor of the material for SEM analysis.

The material prepared from LLC-2 is used for electrochemical experiments and XPS analysis.

For N2 adsorption/desorption and XRD analysis, drop coated solution on microscope slides (LLC-3) is calcined. The material is collected as powder by scratching from the slides.

2.3. Instrumentation

2.3.1. X-Ray Diffractometer (XRD) & X-Ray Diffractometer for Powder (XRD-MPD)

LLC1 and LLC3 were analyzed by XRD measurements by using Rigaku Miniflex x-ray diffractometer, equipped with an x-ray source, Cu Kα (1.54056 Å) operated at 30 kV and 15 mA. The data is collected over time after it is coated. For small angle measurements, the range of 1 and 5° is scanned with scan rate of 1 degree/min with a 0.01 step size. Also, from 10° to 80°-degree high angle measurements are performed with a scan rate of 1 degree/min or 1.4 degree/min.

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After calcination, collected powder by scratching is analyzed by using Panalytical Multipurpose X-Ray Diffractometer, equipped with a Cu Kα source (1.54056 Å), operated at 45 kV and 40 mA at high angle range between 10 and 80°.

2.3.2. Attenuated Total Reflection Fourier-Transform Infrared (ATR- FTIR) Spectroscopy

Mesoporous metal oxide powders and LLC mesophases are analyzed by using Bruker Alpha Platinum spectrometer with 4 cm-1 resolution and 64 scans in the range of 400-4000 cm-1.

2.3.3. X-Ray Photoelectron Spectroscopy (XPS)

Freshly calcined materials coated on FTO glasses are analyzed with Thermoscientific K- alpha instrument with Al Kα monochromatic source (1486.68 eV) for XPS analysis. The coated FTO glasses were analyzed after electrochemical experiments, too. The experiments were performed under ultra-high vacuum conditions with 400 µm spot size.

The data is calibrated according to C1s and O1s signals and normalized.

2.3.4. N

2

Adsorption-Desorption Measurements

After calcination of LLC3 mesophases, powder is obtained by scratching from microscope slides. The powder sample was evacuated under vacuum in tubes at 200°C for 2 hours to reach pressure less than 50 mTorr before low-temperature measurements.

The isotherms are collected using TriStar 3000 automated gas adsorption analyzer in the relative pressure range of 0.01 to 0.99 atm P/Po.

Surface area values are obtained from five points in the range of 0.05 and 0.3 atm, thanks to Brunauer-Emmett-Teller (BET) theory. The average pore size values are reported by Barrett-Joyner-Halenda (BJH) desorption isotherms.

2.3.5. Polarized Optical Microscope (POM)

LLC mesophases coated on microscope slides are imaged with ZEISS Axio Scope A1 Polarizing Optical Microscope at 5x, 10x, 20x and 50x magnification.

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2.3.6. Scanning Electron Microscope (SEM) – Energy Dispersive X-Ray Spectroscopy (EDX)

The LLC1, which is described above, is calcined at certain temperature for 1 h. Then, the material is scraped, and tiny particles of the material is dispersed on a weighing paper.

The powder is stuck onto carbon tape on the aluminum stab. In case of low conductivity, for samples with silica template, the material on carbon tape is coated with Au/Pd with 120 μm thickness. An Environmental-Scanning Electron Microscope (ESEM), FEI Quanta 200 FEG, is used for imaging under a high vacuum condition, at an operating voltage of 15-25 kV. Additionally, the same microscope is used for collecting EDAX data by using EDAX Genesis program.

2.3.7. Transmission Electron Microscope (TEM)

The solution that is used in preparation of LLC1 is diluted 10 times and spin coated on microscope slides at 2000 rpm for 10 seconds, then the obtained mesophase is calcined at 350°C for 1 h. After calcination, the material is scraped, and grinded in pure ethanol in a mortar. The grinded sample is transferred into a 15 ml glass vial, filled with pure ethanol.

The solution is sonicated for 15 minutes, then a drop of liquor is placed under an UV lamp on a carbon coated Cu grid with 300 mesh. The resulted grid is used for imaging by utilizing the high-resolution transmission electron microscope (HRTEM) of JEOL JEM 2100 F operating at a voltage of 200 kV.

2.4. Electrochemical Measurements

LLC3 mesophase on fluorine-doped tin oxide (FTO) glasses, whose preparation is described in 2.1.2, is calcined at a certain temperature for 1 h, and used as electrode for electrochemical measurements. The electrodes are used for investigation of electrocatalytic activity for oxygen evolution reaction (OER) and characterization of material by redox active species. A Gamry Instrument and Potentiostat – PCl4G750 are utilized for the measurements with three electrode system.

A systematic experiment on a single electrode is listed as;

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 3 cycles of cyclic voltammetry

 Multi-step chronoamperometry

 3 cycles of cyclic voltammetry

 Chronopotentiometry at 1mA for 2 hours

 3 cycles of cyclic voltammetry

 Chronopotentiometry at 10mA for 2 hours

 3 cycles of cyclic voltammetry

A second electrode was used for multi-step chronopotentiometry, and a third electrode was used for cyclic voltammetry for 100 cycles.

2.4.1. Three-Electrode System

Ag/AgCl in 3.5 M KCl electrode is used as reference electrode (RE), where platinum wire is counter electrode (CE). The electrodes, prepared by coating material on FTO glasses, are used as working electrode (WE). All electrochemical experiments are conducted in 1M KOH solution, which is purged with nitrogen gas before. pH value is excepted as to be 14.

2.4.2. Cyclic Voltammetry

Cyclic voltammograms are collected by applying voltage starting from -0.4 V to 1.0 V, then finishing at -0.4 V / Ag/AgCl (3.5 M KCl), with 50 mV/sec scan rate, and current density is recorded.

2.4.3. Chronoamperometry (CA) & Tafel Slope Analysis

Range of applied voltage values is determined according to CV of the same electrode. The potential values are applied for 300 seconds at each step with 0.01 V increment, and current density is recorded.

The resulted data is used for Tafel slope calculations. The slope is evaluated from the graph of overpotential (V) vs logj (log(mA/cm2)).

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Overpotential is calculated by subtracting 0.401 V from the recorded potential. 0.401 V is theoretically required potential for OER at pH 14.

2.4.4. Chronopotentiometry (CP) and Overpotentials for Water Oxidation

Chronopotentiometric data is collected with three different types of experiment, and potential values are recorded.

One is conducted by applying 1mA constant current on the working electrode for 2 hours.

(CP1mA)

The second one is conducted at 10 mA current for 2 hours. (CP10mA)

The third one is multi-step chronopotentiometry. Steps are in range of 10 to 100 mA with 10 mA increment, and the duration of each step is 30 minutes.

The recorded potential values are converted to and reported as overpotential values for further analysis.

Figure

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