Characterization of as-synthesized ZnCo 2 O 4

The crystal structure was identified by pXRD. In the diffraction pattern in figure 3.7, only typical reflections of the cubic spinel structure (ICSD PDF-number 01-080-1532) are visible.^[121]Using the Scherrer equation, a mean crystallite size of 18.2 nm was calculated.

10 20 30 40 50

Intensity (arb. units)

2 θ (degree)

Figure 3.7. pXRD pattern of ZnCo₂O₄. The bars show the reference pattern of a cubic spinel structure (ICSD PDF-number 01-080-1532).^[121]

44

3.2 ZnCo₂O₄containingCo³⁺in octahedral coordination

Figure 3.8. TEM micrographs and SAED patterns of as-synthesized ZnCo₂O₄and after reaction.

(a) Shows the as-synthesized nanostructures. (b) Shows the corresponding SAED pattern. (d) Shows the nanostructures after 1 h electrochemical reaction at 10mA cm⁻². The corresponding SAED pattern is shown in (c). Adapted from Wahl et al. with permission from WILEY-VCH.^[125]

The morphology of ZnCo₂O₄nanoparticles was assessed by HRTEM. In figure 3.8a, elongated structures with spherical particle domains are visible. The width of the structures varies between 9 and 20 nm and the spherical particles have diameters of 15 to 20 nm, showing a good agreement with the crystallite sizes estimated from the pXRD pattern.

The Debye-Scherrer rings in the SAED pattern in figure 3.8b can be assigned to the cubic spinel structure. This confirms that the structures are indeed ZnCo₂O₄in the spinel phase.

The EDX analysis in table 3.1 gave a Co/Zn ratio of 80 %/20 %, indicating that more than half of the tetrahedral Co was replaced by Zn. A uniform distribution of the elements over the sample could be verified by EDX-mapping in figure A.1.

The XPS spectrum in figure 3.9a shows the Co2p_3/2 peak at a binding energy of 780.6 eV. Shifted by 5.7 eV, a weak shoulder is visible. From the position, it can be assigned to a shake-up satellite, indicating Co²⁺. At a chemical shift of 9.6 eV, the shake-up satellite indicating Co³⁺is visible.

The Co2p_1/2 peak is shifted by 14.9 eV compared to the binding energy of Co2p_3/2, corresponding to the reported values for a Co³⁺species. From XPS, the main oxidation state in the sample is Co³⁺, but it indicates also the presence of some Co²⁺.^[122]

The spectrum at the Zn edge in figure 3.9b shows the Zn2p_3/2 peak at a binding energy of 1021.7 eV and the Zn2p_1/2 spin-orbit component shifted by 23.1 eV at 1044.8 eV. This is typical for Zn²⁺in an oxygen environment.^[126]

The oxidation state and the local atomic environment of ZnCo₂O₄ was further assessed by XAFS. In the XANES spectrum at the Co-edge in figure 3.10a, the edge energy of ZnCo₂O₄lies

3 Evaluating the Activity of Cobalt in Different Oxygen Environments

after reaction 14.9 eV 9.6 eV 5.7 eV

Figure 3.9. XPS spectra of ZnCo₂O₄. (a) Shows the spectra on the Co edge before and after 1 h electrochemical reaction. (b) Shows the spectra on the Zn edge before and after 1 h electrochemical reaction.

at 7720.6 eV, which points to a predominant oxidation state of +III. Small pre-edge features indicate an octahedral environment around the central Co atoms.

a)

Figure 3.10. XANES spectra and EXAFS of ZnCo₂O₄. (a) Shows the XANES spectra before and after 1 h electrochemical reaction at 10mA cm⁻² on the Co-edge. (b) Shows the EXAFS as the magnitude of the FT of spectra before and after reaction (before reaction: k-range: 4.0-13.6, R-range: 1.4-4.0 Å; after reaction: k-range: 4.0-12.62,R-range: 1.4-4.0 Å). The EXAFS before the reaction is shifted by six units on the y-axis.

46

3.2 ZnCo₂O₄containingCo³⁺in octahedral coordination

The local atomic structure was examined by fitting the spinel crystal structure to the EXAFS. The magnitude of the FT can be found in figure 3.10b. In the first shell at a reduced distance of 1.92 Å, six O atoms are located. The second shell holds six Co atoms at a distance of 2.86 Å. In the third shell, six Zn atoms are located at a distance of 3.34 Å. A multileg scattering path within the first two shells also contributes to the third shell. AnR-factor of 0.013 for this fit indicates, that mostly Co_oh³⁺is present in the sample.

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 0.40

0.45 0.50 0.55 0.60 0.65

Kubelka-Munk (arb. units)

Wavelength (nm)

*

* **

*

* *

* * * *

*

*

*

Figure 3.11. DRUV spectrum of ZnCo₂O₄nanoparticles. Asterisks mark the band positions. Black squares mark band positions that might be due to artifacts.

The interpretation of the DRUV spectrum of ZnCo₂O₄in figure 3.11 is based on ad⁶configuration in a strong octahedral field. Therefore, the Tanabe-Sugano diagram ofd⁶is used. The ground term is¹A_1g(I).

The spectrum shows a lower intensity compared to other measured spectra in this chapter. It is composed of 3 main regions. The first lies below 1000 nm, and from the Tanabe-Sugano diagram, spin-allowed transitions are expected here. The transition ¹A_1g(I) ¹T_1g(I) lies at 731 nm (13680 cm⁻¹) and has a small shoulder at 658 nm. At 462 nm (21645 cm⁻¹), the transition¹A_1g(I) ¹T_2g(I) can be assigned.^[127]Several small bands lie on top of this transition, which may be due to splitting of it or to transitions to states of higher energy. Due to the general low intensity of the spectrum, it is not clear whether the two bands marked by black squares arise from artifacts or are real bands, as they are too sharp ford-dtransitions. Further assignment of the bands in this region is not possible, as the Tanabe-Sugano diagram predicts several possible transitions in this region.

3 Evaluating the Activity of Cobalt in Different Oxygen Environments

From the two assigned transitions, a Racah parameter B = 673 cm⁻¹ and a ∆/B = 22.9 can be approximated, which would result in a ligand field splitting parameter ∆o = 15400 cm⁻¹. From this∆/Bvalue, the transitions¹A_1g(I) ³T_1g(H) and¹A_1g(I) ⁵T_2g(D) are expected around 1384 nm (7725cm⁻¹) and 1783 nm (5610cm⁻¹), respectively. Indeed, in the second region of the spectrum between 1000 nm and 1950 nm, two wide bands and several shoulders can be found. Thus, the band with an intensity maximum at 1327 nm (7536cm⁻¹) can be assigned to the transition¹A_1g(I) ³T_1g(H), while the second intensity maximum at 1531 nm (6532cm⁻¹) and the shoulders at higher wavelengths can be assigned to the fine structure of the transition

1A_1g(I) ⁵T_2g(D). The transitions in the third region above 1950 nm to the infrared (IR) region are not described in the Tanabe-Sugano diagram and thus cannot be assigned.

1.0 1.1 1.2 1.3 1.4 1.5 0.00

0.05 0.10 0.15

Current density (mAcm-2 )

Potential (V vs. RHE)

Figure 3.12. Cyclic voltammogram of ZnCo₂O₄, recorded in 1 M KOH at 1600 rpm with a scan rate of 20mV s⁻¹.

The CV in figure 3.12 shows an oxidation peak and a related reduction peak at around 1.44 V.

This can be attributed to the redox couple Co^3+/4+. After that, at around 1.5 V, the OER starts.

Interestingly, no obvious signal for the redox pair Co^2+/3+is visible. In the region between 1.0 and 1.1 V there is a very weak redox feature, indicating a nearly suppressed reduction to Co²⁺in the cathodic scan.

The activity of ZnCo₂O₄was accessed by LSV and is shown in figure 3.37. An averageηof 369 mV towards the OER was recorded at a current density of 10mA cm⁻². The ECSA was derived from the C_dl and is 1.07cm²(see also figure B.3).

48

3.2 ZnCo₂O₄containingCo³⁺in octahedral coordination