Chapter 2 Experimental Part
2.4. Electrochemical Measurements
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.
23
Chapter 3
Results & Discussion
3.1. Characterization of LLC Mesophases
3.1.1. Time Dependent Solutions
The solution of ZnCo-1.5.60 composition, containing non-ionic surfactant (P123), CTAB as a charged surfactant and Zn(NO3)2.6H2O and Co(NO3)2.6H2O salts, was prepared to investigate the effect of aging the solution. The aging time with stirring for the solution was started after the addition of salts to clear aqueous surfactant solution. The solution was drop casted on microscope slides at the given aging time upon mixing all components.
Lyotropic liquid crystalline mesophase (LLCM) forms by aging 15-20 min upon drop casting.
Time dependent XRD patterns of LLCM from 1 hour, 2 days and, 20 days aged solutions were collected. In Figure 3.1 low angle XRD patterns and in Figure 3.2 high angle XRD
24
patterns of the LLCMs are given. One of the common characterization method for LLCMs is small angle XRD measurement. Line at around 1.75° is an indicator of the mesophase formation. On the other hand, in Figure 3.1 line at 3° and high angle lines in Figure 3.2 belong to an unknown crystal, which shows the instability of a mesophase. It was shown that the crystal formation gets faster with increasing aging of the precursor solution.
While 1-hour aged solution forms stable mesophases for 90 min, LLCM from 2 days and 20 days aged solutions are stable up to 45 min and 25 min after gelation, respectively.
Therefore, aging the precursor solution fastens the formation of the crystals. (Remember that one measurement at high angle between 10-80° takes 50 minutes.)
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.
25
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.
3.1.2. Optimization of CTAB Ratio
Solution containing P123, CTAB, Zn(NO3)2.6H2O and, Co(NO3)2.6H2O were prepared and used to evaluate the stability of the mesophases. In addition to non-ionic surfactant (P123), CTAB was also used as a charged surfactant in the solutions. 1.3.60, ZnCo-1.5.60 and, ZnCo-1.8.60 solutions (Zn = Zn(NO3)2.6H2O, Co = Co(NO3)2.6H2O, 3, 5 and, 8 are CTAB/P123 mole ratio and 60 is total salt/P123 mole ratio) were used to determine how CTAB ratio effects the stability. Drop casting method was applied on microscope slides. All solutions were used in the first hour after adding all components. The LLCM forms by aging for 15-20 min upon drop casting.
Time dependent small and high angle XRD patterns of the LLCMs were collected from the drop casted samples. In Figure 3.3, time dependent small angle XRD patterns of each compositions are given and high angle XRD patterns of the samples are shown in Figure 3.4. High angle lines’ showing up in the patterns is an indicator for instability of the LLCMs. The ZnCo-1.5.60 sample shows no high angle diffraction line(s) up to 80 min after gelation. However, the ZnCo-1.3.60 and ZnCo-1.8.60 samples diffracts at high angles in 10 to 20 min after gelation. (Remember that one measurement at high angle between 10-80° takes 50 minutes.)
26
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.
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.
Diffraction line in between 1-2° indicates the formation of lyotropic liquid crystalline mesophases, which can be seen in fresh samples’ patterns. In Figure 3.3, upon aging of ZnCo-1.5.60 and ZnCo-1.5.90 a line at around 2.9° is visible. It is also another indicator for instability (Figure 3.3) and originate from the unknown complex. As a result of these experiment, 5 (CTAB/P123) mole ratio gives the best stability for the mesophases. (Until this point, CTAB ratio was chosen to be 5, and it will not be indicated in the abbreviations.)
3.1.3. Optimization of Salt Ratio
Clear and homogeneous solutions of ZnCo-30, ZnCo-60 and, ZnCo-90 (Zn = Zn(NO3)2.6H2O, Co = Co(NO3)2.6H2O, 30, 60 and, 90 are total salt/P123 mole ratio) were prepared and used to evaluate the stability of the mesophases. In addition to non-ionic surfactant (P123), 5 mole ratio of CTAB (CTAB/P123) was also used as a charged surfactant in the solutions. Drop casting method was applied on microscope slides. All solutions were coated in the first hour after adding the last component. After aging for 15-20 min upon drop casting, lyotropic liquid crystalline mesophase forms.
27
Figure 3. 5. Small angle XRD patterns of LLC of a) 30, b) 60 and, c) ZnCo-90, spin-coated on microscope slides
Figure 3. 6. High angle XRD patterns of LLC of a) 30, b) 60 and, c) ZnCo-90, spin-coated on microscope slides.
In Figure 3.5, time dependent small angle XRD patterns of each compositions with different salt ratio are given and high angle XRD patterns of the samples are shown in Figure 3.6. As described above, line at around 2.9° and high angle lines in the patterns are indicator for instability of the LLCMs. The ZnCo-30 and ZnCo-60 samples shows no high angle diffraction line(s) upon aging in the first hour. However, the ZnCo-90 sample diffracts at high angles in 10 min after gelation.
Therefore, the LLC mesophases should be calcined before the complex formation to avoid formation of bulk crystalline domains. Since the ZnCo-30 and ZnCo-60 samples are more stable, they were used for further investigations.
28
3.1.4. The Unknown Crystal
The gelation process of ZnCo-60 sample was monitored by polarized optical microscopy (POM). After drop casting on a microscope slide, in 5 min formation of LLCM is observed, then the crystals were observed.
Figure 3. 7. POM images of gelation process for ZnCo-60, drop casted.
In Figure 3.7.a, formation of LLCMs can be seen in the left side. The fan texture of LLCM of ZnCo-60 is a characteristic feature of hexagonal mesophases. In Figure 3.7.b, c and, d, the growth of the crystal on the fan texture is observable.
29
Besides speeding the crystal formation, the aged precursor solutions form white precipitate over the time. In order to minimize crystallization during gelation, solutions should be centrifuged at 6000 rpm for 10 minutes before using for coating. The LLCM, obtained from the centrifuged solutions were also analyzed by using x-ray diffractometer, the patterns are given below.
Figure 3. 8. Small angle XRD patterns of ZnCo-60 LLC, centrifuged and spin-coated on microscope slides.
The XRD patterns were collected over time. There is weak line between 1 and 1.5°, which is related to mesophase. In addition, because there is no line at around 3° for all patterns, there is no formation of the crystal after centrifugation. Therefore, centrifuged aqueous solutions of salts and surfactants are used in further steps.
Furthermore, time dependent small and high XRD patterns of LLCMs from 1 year aged, centrifuged solution are obtained (see Figure 3.9). When the crystal uptake time of LLCM from non-centrifuged 20 days aged solution (see Figure 3.1.c and 3.2.c) and from 1 year
30
aged, centrifuged solution, it can be said that centrifugation postponed the crystal formation.
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
Figure 3. 10. Photo of centrifuged ZnCo-60 precursor solution.
As it is seen in Figure 3.10, the amount of precipitate is exceedingly small with respect to the remaining solution. In previous studies, it was shown that adding charged surfactant is necessary to stabilize samples with higher salt/surfactant mole ratio (like those,
31
investigated in this work). However, higher CTAB to salt ratio leads to a formation of a surfactant complex, (CTA)2[MBr4] [74]. To investigate whether the crystal is surfactant complex or not, the atomic percentage by element of the precipitate was obtained via EDAX. The result from the EDAX gave us an idea of being another different surfactant crystal due to lack of high bromine signals (see Table 3.1).
Table 3. 1. Atomic percentage by elements of precipitate from centrifuged ZnCo-60 solution via EDAX
Atomic % by Element
Element C K O K Br L Co K Zn K
Percentage (%) 70.90 25.41 0.16 0.81 2.72
3.1.5. LLC Mesophases with No CTAB
The mesophases were also prepared from solutions without CTAB in order to understand the effect of CTAB to crystal formation. Clear and homogeneous solutions of ZnCo-1.0.10, ZnCo-1.0.20 and, ZnCo-1.0.30 (Zn = Zn(NO3)2.6H2O, Co = Co(NO3)2.6H2O, 10, 20 and, 30 are total salt/P123 mole ratio) were prepared. Then, the prepared solutions were spin-coated on microscope slides and the stability of the mesophases was checked over time. The mesophases are formed immediately after spin-coating.
32
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.
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.
While ZnCo-1.0.10 and ZnCo-1.0.20 samples formed no crystal in 3-5 days, ZnCo-1.0.30 sample showed high angle lines upon 1 day aging. It can be concluded that, removing CTAB slowed the crystal formation process, and the crystal could be originated from non-ionic surfactant, P123. Further analysis of no CTAB samples will be discussed in section 3.4.
3.2. Characterization of Mesoporous ZnCo
2O
4Thin Films
The ZnCo-30 and ZnCo-60 solutions were used for the preparation of LLC-1 and LLC-2 (see 2.1.2), respectively, and immediately calcined at a specific temperature for 1 hour. In this thesis, m-ZnCo-n-t abbreviation will be used, where m stands for mesoporous, n is salt/P123 ratio and t is calcination temperature. In this section, the calcination process of P123-CTAB-Zn(NO3)2.6H2O-Co(NO3)2.6H2O LLC mesophase that produces a mesoporous ZnCo2O4 thin films will be discussed.
After calcination of LLC-1-ZnCo-30 and LLC-2-ZnCo-60 samples, their powder forms were analyzed using powder-XRD (PXRD). The PXRD patterns of directly calcined m-ZnCo-30 and m-ZnCo-60 are shown in Figure 3.13. The PXRD patterns can be indexed to a cubic spinel structure of ZnCo2O4 (PDF card no of 00-023-1390).
33
Figure 3. 13. (a) PXRD patterns of directly calcined m-ZnCo-30 and (b) m-ZnCo-60 Increasing calcination temperature results sharper lines, which means larger ZnCo2O4
crystals. To support this, Scherrer’s equation and (311) lines were used to calculate crystallite size for the m-ZnCo-60-t.
𝐷(𝑛𝑚) = 𝐾𝜆
𝛽𝑐𝑜𝑠𝜃 = 0.94 × 0.154056 𝐹𝑊𝐻𝑀 𝑐𝑜𝑠𝜃
,where D is the mean size of the crystalline domains, K is a dimensionless shape factor, 𝜆 is the X-Ray wavelength, 𝛽 is the line broadening at half the maximum intensity (FWHM) and 𝜃 is the Bragg angle.
Table 3. 2. Calculated crystals size using Scherer’s equation for m-ZnCo-60-t Calcination Temperature (°C) Crystallinity Size (nm)
250 ⁰C 5.7
300 ⁰C 5.8
350 ⁰C 6.5
400 ⁰C 8.4
450 ⁰C 11.5
500 ⁰C 11.8
550 ⁰C 13.7
34
Particle size (pore-wall thickness) shows a direct proportionality with increasing calcination temperature and is obvious in the PXRD patterns; they display sharper lines with increasing annealing temperature. The particle size is in nanometer scale even after calcination at 500°C. Thus, our method for preparation of zinc cobaltite materials prevents the formation of bulk crystals.
SEM images are also recorded for all samples at all temperatures. Images are collected at a 20000x and 200000x magnification. The film morphology of ZnCo-60-350 and m-ZnCo-60-500 is shown in Figure 3.14. They show smooth film morphology and pores.
Calcination at 500°C leads to formation of bigger nanoparticles and pores, compare Figure 3.14a and c. This observation is consistent with the calculated crystal size. Images in Figure 3.14 b and d clearly show the pore structure and film thickness. The film thickness is around 400 nm but it can be changed with dilution of the initial solution or spin coating speed.
35
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
Both m-ZnCo-30 and m-ZnCo-60 compositions are successful in terms of forming film structure. They display remarkably similar film morphology and pore structure (see Figure 3.15). Figure 3.7 displays SEM images of m-ZnCo-60 films that are calcined at different temperature. All samples have film morphology with cracks that are a result of film-thickness (thicker films cracks due to stress exposed during calcination).
36
Figure 3. 15. SEM images of a) m-ZnCo-30-400 and b) m-ZnCo-60-400 at a 25000x magnification
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 bars are 1 μm)
Another calculation is done by using Bragg’s law and (111) line of m-ZnCo-60-350. The d-spacing, which is calculated from Bragg’s law is consistent with the observed spacing between the lattice fringes in transmission-electron microscopy (TEM) image. The d-spacing is 0.461 nm from TEM image analysis. Sponge-like pores and film morphology are displayed in Figure 3.17.
37
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.
N2 adsorption-desorption isotherms were also collected using the same powder samples.
Figure 3.18 and 3.19 display isotherms and pore size distribution plots of m-ZnCo-30 and m-ZnCo-60, respectively.
38
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).
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).
Table 3. 3. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-n-t (n:30 & 60).
Sample SBET (m2/g) VP (cm3/g) DP (nm)
m-ZnCo-30-250 80 0.18 8.3
m-ZnCo-30-300 93 0.26 7.7
m-ZnCo-30-350 82 0.70 24.3
m-ZnCo-30-400 72 0.39 14.7
m-ZnCo-30-450 53 0.60 31.6
m-ZnCo-30-500 50 0.55 30.5
m-ZnCo-60-250 79 0.24 9.9
m-ZnCo-60-300 92 0.22 6.8
m-ZnCo-60-350 102 0.33 9.6
39
m-ZnCo-60-400 70 0.28 11.3
m-ZnCo-60-450 53 0.30 16.6
m-ZnCo-60-500 44 0.30 19.2
Table 3.3 shows the Branauer-Emmett-Teller (BET) surface area, Barrett-Joyner-Halenda (BJH) pore volume, and BJH pore-size of m-ZnCo-30 and m-ZnCo-60-t, calcined at different temperature. The trend shows an inverse proportionality between BET surface area and calcination temperature. And pore width is getting wider as calcination temperature increases. In comparison between the ZnCo-30 and ZnCo-60, the m-60 has a more uniform pore-size distribution (Figure 3.18&3.19 (b)). Since, ZnCo-60 samples are used for annealing process to decrease surfactant amount and to obtain more uniform film structure. The isotherms are typical for the mesoporous materials, as type IV. As the calcination temperature is increased, both the crystallinity and pore size increase as a general trend. In terms of surface area, the values reach to a maximum at 300°C for m-ZnCo-30 and at 350°C for m-ZnCo-60 samples. It can be concluded that, until those indicated temperatures, not all surfactant is completely burned out from the mesostructure.
So far, only direct calcination method is discussed (in this study always direct calcination method is used, unless it is indicated). In order to compare direct calcination and annealing methods, the ZnCo-60 LLC mesophases (LLC-2) on microscope slides are first calcined at 250°C for 1 hour. Then the scraped powder is used for annealing with 50°C increment for 1 hour at each steps. The annotation of m-ZnCo-60-t-an is used for annealed samples, where t is the calcination temperature of the final step.
The annealed powder samples are analyzed by using multi-purpose X-Ray diffractometer.
XRD patterns of annealed samples are given in Figure 3.20, indexed to spinel structure of ZnCo2O4 (PDF card no of 00-023-1390).
40
Figure 3. 20. PXRD patterns of annealed m-ZnCo-60 samples.
A similar trend is observed in terms of sharpness of the XRD lines, crystallinity, BET surface area and, pore-size distribution with increasing calcination temperature, compared to directly calcined m-ZnCo-60-t samples. If pore-size distribution plots of m-ZnCo-60-t and m-ZnCo-60-t-an samples are compared (see Figures 3.19.(b) and 3.21.(b)), a more uniform pore-size distribution is observed from the directly calcined samples.
41
Figure 3. 21. a) N2 (77 K) adsorption-desorption isotherms and b) pore size distribution plots of m-ZnCo-60-t-an.
Table 3. 4. N2 (77 K) Adsorption−Desorption Data of Annealed m-ZnCo-60-t-an.
Sample SBET (m2/g) VP (cm3/g) DP (nm)
m-ZnCo-60-250 79 0.24 9.9
m-ZnCo-60-300-an 95 0.38 13.6
m-ZnCo-60-350-an 75 0.25 11.5
m-ZnCo-60-400-an 58 0.28 14.1
m-ZnCo-60-450-an 56 0.43 23.7
Therefore, the optimized method for synthesizing mesoporous ZnCo2O4 thin films, the direct calcination should be selected as a calcination method for ZnCo-60 mesophases for further investigations.
42
3.3. Mesoporous ZnCo
2O
4Thin Films as Electrocatalyst for OER
To investigate the electrochemical performance of mesoporous ZnCo2O4 thin films as electrocatalyst for oxygen evolution reaction (OER), 1cm x 2cm FTO glasses are used as a substrate for the synthesis. Half the glass is coated (1cm x 1cm) and used as working electrode in three-electrode system.
Electrochemical experiment series is started by collecting cyclic voltammogram (CV).
Cycle starts from -0.4 V and goes to maximum of 1.0 V (vs NHE), then turn back to the initial point, while current density values are being collected.
Figure 3. 22. Cyclic voltammograms of fresh m-ZnCo-60-t electrodes
In Figure 3.22, cyclic voltammograms of fresh m-ZnCo-60 films with different calcination temperature are given. The reversible peaks at around 0.6 V belongs to the Co3+/Co4+
43
redox couple. In the region of higher voltages than 0.6 V, a sharp increase in current density is observed due to water oxidation reaction. Thus, the higher current density with the same voltage implies higher performance for OER. At 1.0 V, while the highest current density is obtained from the m-ZnCo-60-350 electrode, the next best performing electrode is m-ZnCo-60-500. Notice that the trend does not follow the calcination temperature.
The electrodes’ stability is monitored by performing multiple CVs (typically 100 cycles) between -0.4 V and 1.0 V vs NHE, see Figure 3.23.
44
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
There is no big change in current density at 1.0 V for all electrodes between the cycles throughout the experiment, indicating the physical stability of the electrodes. While m-ZnCo-60-450 electrode shows the best stability, there is also an increase in the
45
performance of m-ZnCo-60-350 and m-ZnCo-60-500 electrodes. On the other hand, electrodes of m-ZnCo-60-300 and m-ZnCo-60-400 displayed a decrease in the current density in OER region after several cycles.
Figure 3. 24. Tafel slope analysis of m-ZnCo-60-t thin film electrodes
Tafel slope values for each electrode is determined by analyzing multi-step chronoamperometry data. Simply, a lower Tafel slope indicates higher reaction rate. A lowest Tafel slope (41.5 mV/dec) is obtained from the m-ZnCo2O4-60-350 electrode of with. In overall, Tafel slope is calculated to be ~50 mV/dec.
As a next step, chronopotentiometry (CP) experiments are conducted to evaluate the overpotential values, which is the extra voltage that is needed to make OER occurs at specific current density. The experiments were performed at current density of 1 mAcm-2
46
for 2 hours and, from 10 mAcm-2 to 100 mAcm-2 with a 10 mAcm-2 increment, as multi-step CP with a 30 min multi-steps.
Figure 3. 25. Chronopotentiometry results of the m-ZnCo-60-t electrodes at 1 mA/cm2 current density for 2 hours.
Figure 3.25 shows that all overpotential values are relatively close to each other, except m-ZnCo-60-450. In 2 hours, duration, performance of the electrodes increased, as a result overpotentials decreased over time. Lowest overpotential is obtained from the m-ZnCo2O4-500, likely due to better crystallinity.
47
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.
Multi-step chronopotentiometry (mCP) results are compatible with CVs and Tafel slope values, discussed above. The best performance belongs to ZnCo-60-350, where m-ZnCo-60-500 follows it. Note that calcining LLCs with a 60 salts/P123 ratio at 350°C gives the most uniform pore size distribution and the highest surface area (see Figure 3.19 b and Table 3.2). In the light of this information, being the best performing electrode is not a surprising result for m-ZnCo-60-350. In addition, CV of the same electrodes are recorded after mCP experiments and the highest current density at 1V is observed in the m-ZnCo-60-350 (see Figure 3.22 & 3.27). However, a biggest decrease is observed in the m-ZnCo-60-500, where the rest is relatively stable.
48
Figure 3. 27. Cyclic voltammograms of m-ZnCo-60-t electrodes after multistep chronopotentiometry experiment.
Before (fresh) and after mCP, x-ray photoemission spectra of m-ZnCo-60-t electrodes are collected and normalized (see Figure 3.28). Regions of O1s, Co2p and, Zn2p are analyzed in detail. There is no change in Co2p and Zn2p after the experiment. Co2p spectra gives information about the oxidation state of the cobalt ions in the structure via satellite peaks.
The peaks observed indicate Co3+ ions, where there is no sign for the existence of Co2+
ions. Spectrum of Zn2p shows that there are only Zn2+ ions in the electrodes. Thus, the material preserves the metal ion composition after OER experiments. On the other hand, O1s spectra show differences between before and after mCP. Peak at 528.8 eV is due to lattice oxygens, where peaks at 530.5, 531.2 and, 532.3 eV are due to hydroxy, peroxy and, coordinated water species, respectively.
49
Figure 3. 28. XPS results of m-ZnCo-60 electrodes before and after multi-step chronopotentiometry experiment.
50
3.4. Investigation of the CTAB Effect
Precursor solutions were also prepared without adding charged surfactant, CTAB. The ZnCo-1.0.10, ZnCo-1.0.20 and, ZnCo-1.0.30 solutions were used for the preparation of mesophases by spin coating or drop casting, on microscope slides. The mesophases were directly calcined immediately after coating at 350°C for 1 hour.
The obtained powder after calcination process were analyzed via PXRD. Calcination of all compositions was successful to form a cubic spinel structure of ZnCo2O4. The PXRD patterns of m-ZnCo-1.0.n, n is 10, 20 and, 30, are given in Figure 3.29 and indexed to ZnCo2O4 (PDF card no of 00-023-1390).
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.
51
Table 3. 5. Calculated crystals size using Scherer’s equation for m-ZnCo-1.0.n.
Sample Crystallinity Size (nm)
m-ZnCo-1.0.10 6.0
m-ZnCo-1.0.20 5.8
m-ZnCo-1.0.30 6.6
The calculated crystal size values, using Scherrer’s Equation, were lower than m-ZnCo-30-350 and m-ZnCo-60-350 samples. Addition of CTAB stabilizes the existence of higher salt amount and give wider distance between micelles. The XRD lines of fresh ZnCo-60 and ZnCo-1.0.20, see Figure 3.1.a and 3.11.b, are at 1.67 and 1.75° respectively. The calculated distance between micelles are 52.9 and 50.4 Å for ZnCo-60 and ZnCo-1.0.20.
Therefore, removing CTAB from the mesophases decreases the stabilized salt amount in between micelles.
The same powder samples were used to obtain N2 adsorption-desorption isotherms.
Isotherms and pore size distribution plots are given in Figure 3.30 for all three samples.
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).
52
Table 3. 6. N2 (77 K) Adsorption−Desorption Data of m-ZnCo-1.0.n (n: 10, 20 &30).
Sample SBET (m2/g) VP (cm3/g) DP (nm)
m-ZnCo-1.0.10 89 0.74 6.5
m-ZnCo-1.0.20 94 0.31 6.4
m-ZnCo-1.0.30 87 0.42 6.4
The Branauer-Emmett-Teller (BET) surface area, Barrett-Joyner-Halenda (BJH) pore volume, and BJH pore-size values were obtained from N2 (77K) adsorption-desorption measurements and displayed in Table 3.6. Despite having high pore volume, the surface area of m-ZnCo-1.0.10 is remarkably close to the other samples without CTAB. This can be the result of the improvement in mesophase structure, because pores with larger size contributes to total volume. Therefore, m-ZnCo-1.0.20 and m-ZnCo-1.0.30 were more uniform and homogenous. In comparison with the samples with CTAB, which were calcined at the same temperature, removing CTAB resulted with lower surface area and wider pore size distribution (see Figure 3.18 and 3.19).
The film morphology of the samples without CTAB is shown in SEM images in Figure 3.31.
53
Figure 3. 31. SEM images of a) 1.0.10, b) 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).
The synthesis method is successful to form film structure from all compositions. However, it can be concluded that addition of CTAB results with smoother film and a uniform pore distribution, therefore higher quality film (see also Figure 3.16 and 3.17). Larger pores are visible in the structure of m-ZnCo-1.0.20, these might be formed during the removal of combustion products via calcination or defects in the mesophases.
3.4.1. Mesoporous ZnCo
2O
4Thin Films from precursor without CTAB as Electrocatalyst for OER
The electrocatalytic performance of samples without CTAB was also investigated via spin coating precursor solution on the half of 1cm x 2cm FTO glass substrate and calcining.
Electrodes were used as working electrode in three-electrode system.
54
Figure 3. 32. Cyclic voltammograms of fresh m-ZnCo-1.0.n-350 electrodes (1.0.n are indicated in the panel).
Cyclic voltammograms were obtained between -0.4V and 1.0V (vs NHE) potential. Figure 3.32 shows the CVs of these 3 electrodes. The reversible peaks at around 0.6 V are due to Co3+/Co4+ redox couple. In the water oxidation region, 1.0.10-350 and m-ZnCo-1.0.20-350 electrodes gave relatively close and the highest current density. The maximum current density at 1.0 V was around 20 mA/cm2, which was lower than the value of m-ZnCo-60-350, but higher than the value of m-ZnCo-60 electrodes with calcination temperature of 300, 400, 450 and, 500°C (see Figure 3.22).
55
Figure 3. 33. Multiple CVs of a) 1.0.10, b) 1.0.20 and, c) m-ZnCo-1.0.30 electrodes calcined at 350°C.
Multiple CVs with 100 cycles between -0.4 V and 1.0 V vs NHE were also collected to monitor the electrodes’ stability. The m-ZnCo-1.0.30 electrode displays the best stability in three compositions. As going through the higher number of cycles, the current density at 1.0V, which is related to the OER performance, was decreasing. However, there is not much change between the cycles for all electrodes.
The best performance in terms of higher current density at OER region was displayed by m-ZnCo-1.0.10 and m-ZnCo-1.0.30 electrodes. The larger pores of m-ZnCo-1.0.10 and higher salt ratio of m-ZnCo-1.0.30 would ease the process.
Tafel slope analysis was done using multi-step chronoamperometry data. All three electrodes gave relatively low Tafel slope (~ 40 mV/dec), which indicates the reaction rate
56
and the mechanism. The values are relatively lower than m-ZnCo-60 electrodes, but the difference is not higher than 15-20 mV/dec, therefore, there should not be dramatic mechanism change but improvement.
Figure 3. 34. Tafel slope analysis of m-ZnCo-1.0.10, 20 and, 30 electrodes calcined at 350°C.