Surface morphology

Figure 7.1. (a-1, b-1, c-1) are micrographs illustrating the surface morphologies of copper cobalt oxide thin films synthesised using different copper to cobalt concentrations ratios. Generally, all surfaces exhibit coarse surface morphologies except for the [Cu]/[Co]=2 sample which has a smoother surface (Figure 7.1.c-1). With closer examination (Figure 7.1. a-2, b-2, c-2), the surfaces consist of grain-like nanoparticles with sizes ca 10 - 60 nm. For

[Cu]/[Co] = 1, the particles appear to agglomerate to form a coral-like morphology embedded within pores/trenches (Figure 7.1.b-2). Interestingly, relatively homogeneous particle sizes and arrangement are observed for [Cu]/[Co] = 0.5 as compared to [Cu]/[Co] = 1, implying a more pronounced particle agglomeration for the latter. Such morphologies were previously reported by others researchers [171, 172, 200] for their porous copper–cobalt oxide layers synthesised via thermal decomposition of copper and cobalt nitrate precursors for electro-

catalytic application. Marsan and co-researchers [171] suggested that the porous/rough morphology of the copper-cobalt oxide surface was attributed to the higher evolution of gas volumes (NO2, O2) during the decomposition of the concentrated nitrate coating. In line with

their analysis, the observed morphologies of our copper-cobalt oxide coatings could be attributed to the evolution of O2 from the high temperature decomposition of copper and/or

cobalt oxides which ultimately form the copper cobalt oxide [150, 173].

These porous and rough surface structures may explain the different absorptance performances exhibited by the coatings as described in Chapter Six. Low porosity of coating exhibited by [Cu]/[Co]=2 sample has a direct correlation to the decrease of ~10% in absorptance value as seen in Table 7.1. The higher porosity and rougher surface are more conducive structures for higher absorption of incidental solar radiation due to the higher multiple reflections, resonant scattering and relaxation mechanisms that occurred on the

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surface and inside the pore/aggregate [8, 42]. However, the influence of surface morphology to the optical behaviour is not significant. The optical property of material mainly depends on its electronic structure or band structure as that will be discussed in Section 8.3.3.

Table 7.1. Correlation between the [Cu]/[Co] ratio and the porosity

[Cu]/[Co] ratio Porosity (qualitatively) Absorptance (see Section 6.3.2)

0.5 good-fair 86.77% 1.0 fair 83.4% 2.0 low 74.13%

Figure 7.1. Surface morphologies of copper cobalt oxide coatings synthesised using a) [Cu]/[Co]=0.5, b) [Cu]/[Co]=1 and c) [Cu]/[Co]=2.

116 7.3.2. Synchrotron radiation XPS study

High resolution SR-XPS was used to afford detailed information regarding the electronic structure of the thin film coatings. Figure 7.2 shows the Cu 2p SR-XPS spectra and

the decoupling of Cu 2p3/2 peaks of copper cobalt oxide film synthesised using different

concentrations ratios. In each spectrum, the two main peaks of Cu 2p3/2and Cu 2p1/2and the

satellites on the high energy side of each of the main peaks can be observed (Figure 7.2a). The presence of these satellites represents evidence of an open 3d9 shell of Cu2+ [228]. In

each spectrum, the binding energy difference between Cu 2p1/2and Cu 2p3/2 which is around

19.9 eV, and the shake satellite on the high energy side of Cu 2p3/2 peak, confirm the

presence of Cu2+ ions. The shorter separation between the Cu 2p3/2 line and its satellite peak,

and the higher value of satellite intensity to Cu 2p3/2 main peak intensity (Isat/Imain) ratio,

signify a decrease in the covalent character of the Cu-O bond in copper cobaltite as compared to CuO [172]. Relatively higher intensity of Cu 2p satellites in the sample with a [Cu]/[Co]

ratio of 2 also indicates the higher number of Cu2+ ions which are not incorporated into the

copper cobalt spinel structure compared with the other two concentration ratios.

The decoupling of Cu 2p3/2 peak and its satellite in each coating is shown in Figure

7.2.b-d. The decoupling provide five curve-fitting components except for sample with [Cu]/[Co] ratio of 2 which does not contain any component below the binding energy of 932 eV (Figure 7.2. d). The quantitative analysis results are presented in Table 7.2. It is commonly recognised that the Cu 2p3/2photoelectron peaks at ca 933.3–934.0 eV are due to

the Cu2+. Many researchers [229] identify that the Cu 2p3/2 photoelectron peak at ca 932.5–

932.8 eV is attributed to the tetrahedral Cu+ with its counterpart peak from the octahedral Cu+

located below the tetrahedral one. From Table 7.2, it can be observed that the tetrahedral Cu+

is the more prominent oxidation state in each sample except for the [Cu]/[Co] = 2 sample whereby the numbers of tetrahedral Cu+ and octahedral Cu2+ are relatively balanced. The

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increase of copper content tends to promote the formation of octahedral Cu2+ and to reduce

the formation of octahedral Cu+.It is shown that for the [Cu]/[Co] = 2 sample, there is no

reduction of Cu2+ in the octahedral environment compared to the other two samples. In

Figure 7.2. a) Cu 2p SR-XPS spectra of copper cobalt thin film coatings synthesised using different [Cu]/[Co] concentration ratios, b-d) decoupling of their corresponding Cu 2p3/2 peak.

addition, the absence of octahedral Cu+ implies that there are less amounts of typical

monophasic Cu-Co mixed oxides in the coating for the [Cu]/[Co] = 2 sample compared to the other two samples [172]. The effect of the absence of the octahedral Cu+ on the electronic structure of CuxCoyOz films can be explained by studying the local environment of

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components via NEXAFS study as seen in Section 7.3.3. The presence of Cu2+ ions in an

octahedral environment is in contrast with the copper spinel structure, where copper occupies predominantly the tetrahedral sites [228]. This suggests that in the copper cobalt system, the Cu2+ ions themselves are ‘guests’ which partially substitute the tetrahedral Co2+ in the cobalt

structure host [229]. Based on these observations, it can be suggested that in the copper- cobalt system with a [Cu]/[Co] ratio of 2, the elevated concentrations of copper have increased the competitiveness of octahedral Cu2+ ions which facilitates occupation of the

Co2+ sites in the cobalt structure host while minimizing the driving-force of Cu2+ to undergo

reductions.

Table 7.2. Binding energies and the percentage compositions derived from the decoupling of Cu 2p3/2peak and its satellites in the copper cobalt film coatings.

Film coatings Binding energy and percentage of the components of Cu 2p3/2 photoelectron line

Binding energy and the percentage of satellites p q r Satellite I Satellite II [Cu]/[Co]=0.5 931.5 eV (3.7 at%) 932.8 eV (45.5 at%) 933.8 eV (35.7 at%) 940.9 eV (11.9 at%) 943.6 eV (3.2 at%) [Cu]/[Co]=1 931.5 eV (2.8 at%) 932.9 eV (51.4 at%) 933.9 eV (35.8 at%) 941.5 eV (7.6 at%) 944 eV (2.5 at%) [Cu]/[Co]=2 - 932.7eV (40.6 at%) 933.8 eV (42.0 at%) 941.0 eV (13 at%) 943.6 eV (4.4 at%) Attributions: Octahedral Cu+ Tetrahedral Cu+ Octahedral Cu2+ Paramagneti c Cu2+ Paramagneti c Cu2+

The profile of the Co 2p spectra is shown in Figure 7.3a. In each spectrum, the two

main peaks are attributed to Co 2p3/2 and Co 2p1/2, and the weak satellites located on the high

energy side of each these main peaks are also found. Qualitatively, the presence of satellite on the high energy side of the Co 2p3/2 peak indicates the presence of Co2+ ions. The Co 2p3/2

peak and Co 2p1/2 peaks separated by a spin-orbit splitting of ~15 eV correspond to the mixed

Co2+ and Co3+ ions, while the weak intensity satellite located in between the Co 2p3/2 and Co

2p1/2 indicates that the Co ions are present in a partial spinel-type lattice arrangement. The

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octahedral lattice sites with diamagnetic, filled t2g and empty eg levels, and 2+ cations are in

tetrahedral sites [218]. The observed asymmetry in the Co 2p1/2 peak confirms the existence

of both Co2+ and Co3+ ions.

Figure 7.3. a) Co 2p SR-XPS spectra of copper cobalt thin film coatings synthesised using different [Cu]/[Co] concentration ratios, b-d) decoupling of their corresponding Co

2p3/2 peak.

In every spectrum, the decoupling of the Co 2p3/2 peak and the satellite on the high

energy side of this peak provides five curve-fitting components (Figure 7.3. b-d). The binding energy and the percentage of each component are tabulated in Table 7.3. The peaks in the

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region of 779.1–780.0 eV are mostly due to Co3+ in an octahedral environment and mixed

Co2+,3+ bonding states. Due to the covalence and final state effects, the binding energy of

Co2+ is higher than the Co3+ [230] and it is found mostly above 780.0 eV with a characteristic

shake-up satellite [229]. In Table 7.3, it can be seen that in all samples, the tetrahedral Co2+

ions dominate. However, in a copper-cobalt mixed oxide system, the Co2+ ions are partially

substituted by Cu2+ ions, forming a lower crystallization of copper-cobalt spinel particles

[162]. If Cu2+ ions and octahedral Co3+ ions are present in the copper-cobalt oxide system,

then the oxide could be represented by Cu2+Co23+O4, a form of copper–cobalt spinel structure.

Table 7.3. Binding energies and the percentage compositions derived from the decoupling of Co 2p3/2peak and its satellites in the copper cobalt film coatings.

Film coatings Binding energy and the percentage of the components of Co 2p3/2 photoelectron

line

Binding energy and the percentage of satellites p q r Satellite I Satellite II [Cu]/[Co]= 0.5 779.2 eV (10.4 at%) 780 eV (22.9 at%) 780.8 eV (51.75 at%) 785.9 eV (8.4 at%) 789.1eV (6.6 at%) [Cu]/[Co]= 1 779.4 eV (9.7 at%) 780.1 eV (26.6 at%) 781.1eV (45.8 at%) 786 eV (11.5 at%) 789.3 eV (6.5 at%) [Cu]/[Co]= 2 779.2 eV (8.4 at%) 779.9 eV (24.8 at%) 780.7 eV (52.7 at%) 786.1 eV (8.4 at%) 789.4 eV (5.7 at%) Attributions Octahedral Co3+ Mixed Co2+,3+ Tetrahedral Co2+ Paramagneti c Co2+ Paramagneti c Co2+

Figure 7.4 shows the O 1s SR-XPS spectra of copper cobalt oxide film coatings and

the corresponding curve-fitting resulting from the decoupling of the 1s peak. The O 1s

spectrum exhibits a strong peak with a shoulder at the high binding energy side of O 1s peak,

except for the [Cu]/[Co] = 2 sample, where a relatively lower intensity peak is identified (Figure 7.4.a). The apparent shoulder at the higher energy side of the O 1s main peak is a

typical feature of copper-cobalt mixed oxides [162]. The decoupling of O 1s photoelectron

spectrum in each sample results in four curve-fittings grouped into three components (Figure 7.4.b-d). The curve-fitting peaks at binding energy (BE) at around 529.4-529.5 eV (labelled

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“p”) are mostly attributed to the lattice O2-

in a Co3O4 spinel structure. The presence of this

spinel as the main structure reaffirms that in the mixed copper-cobalt system here the cobalt oxide itself becomes the spinel host with copper ions as “guests”, and then a copper cobalt oxide spinel structure is formed as elucidated earlier.

Figure 7.4. a) O 1s SR-XPS spectra of copper cobalt thin film coatings synthesised using different [Cu]/[Co] ratios, b-d) decoupling of their corresponding O 1s peaks and shoulders.

The curve-fitting peaks at BE in range of 530.4-531.4 eV (labelled “q” and “r”) may be attributed to surface oxygen from a wide variety of species such as chemisorbed oxygen

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O-, oxygen containing surface contamination, and/or OH-like species, as hydroxyl, carbonate

groups, etc [146, 162, 172, 198-201], while the curve-fitting peaks at BE above 531.4 eV (labelled “s”) could be assigned to subsurface O-

species [177, 178, 231]. Dupin et al. [177]

suggested that the subsurface (bulk structure near the surface) oxygen ions had lower electron density than the lattice O2- ions. They could be associated with sites where the coordination

number of oxygen ions was smaller than in a regular site, with higher a covalence of the M-O bonds [177].