Electronic effects of metal hexacyanoferrates: An XPS and FTIR study

Materials Chemistry and Physics 203 (2018) 73e81

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Electronic effects of metal hexacyanoferrates: An XPS and FTIR study Stefaans J. Gerber, E. Erasmus* Department of Chemistry, PO Box 339, University of the Free State, Bloemfontein 9300, South Africa

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Preparation of metal hexacyanoferrates: KxMy[Fe(CN)6]z$qH2O, M ¼ Fe, Co, Ni and Cu.
Yields of the compounds are dependent on the Pauling scale electronegativity of the metal.
Systematic XPS study of the Fe 2p3/2 photoelectron lines and their satellite structures.
Amount of charge transfer is indicated by intensity of shake-up peaks.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2017 Received in revised form 11 September 2017 Accepted 16 September 2017 Available online 22 September 2017

A series four metal hexacyanoferrates, with a general formula of KxMy[Fe(CN)6]z$qH2O, with x, y, z and q representing stoichiometric numbers and M ¼ Fe (1), Co (2), Ni (3) and Cu (4), were prepared by a simple co-precipitation reaction. The yields of 1e4 were found to be a function of the Pauling scale electronegativity, sM, the order of increasing yield of the metal hexacyanoferrates were found to be 1 (M ¼ Fe, 26% yield, sM ¼ 1.83) < 2 (M ¼ Co, 43% yield, sM ¼ 1.88) < 4 (M ¼ Cu, 85% yield, sM ¼ 1.90) < 3 (M ¼ Ni, 94% yield, sM ¼ 1.91). Multiple peaks of the cyano-group stretching frequencies, v(C≡N), were observed in the 1900 e2200 cm1 area of the infrared spectroscopy of 1e4. Each stretching frequency at the different wavenumbers represents a different possible combination of the different oxidation states e.g. FeII-C≡NFeII, FeIII-C≡N-FeII, FeIII-C≡N-FeIII and FeII-C≡N-FeIII. Infrared spectroscopy also confirmed that the prepared metal hexacyanoferrates exhibited interstitial sites where water molecules or potassium ions could be trapped. X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of each metal as well as the different oxidation states in which they occur. XPS was very useful in calculating the ratio between the metals as well as the ratio of each oxidation state of the different metals in 1e4. These ratios were then used to derive the stoichiometry of each metal hexacyanoferrate. All compounds contained FeII and FeIII which delivered 2p3/2 photoelectron lines at ca. 708 eV for FeII and 710 eV for FeIII. Secondary (satellite) peaks were also found in all compounds at a few eV higher than the main 2p photoelectron lines that were ascribed to the charge transfer that exists between the iron and the CN ligand. The XPS spectra of all non-iron metals also showed charge transfer peaks at a few eV higher than the main 2p photoelectron lines. © 2017 Elsevier B.V. All rights reserved.

Keywords: XPS FTIR Hexacyanoferrates FeII FeIII Charge transfer

* Corresponding author. E-mail address: [email protected] (E. Erasmus). https://doi.org/10.1016/j.matchemphys.2017.09.029 0254-0584/© 2017 Elsevier B.V. All rights reserved.

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1. Introduction Transition metal hexacyanoferrates have undergone intensive study due to their exceptional characteristics such as mixed valency, water insolubility, high ionic conductivity as well as showing exceptional redox mediator properties [1]. Although these metal hexacyanoferrate compounds have been investigated for different types of applications (including grid scale energy storage, biosensing and electrocatalysis) [2e4], there does not exist characteristic study on the electron distribution and charge transfer within these coordination compounds. A coherent study that compare electronegativity with the different types of characteristic properties in finding the explanation of the individual characteristic properties are in need of attention. This would give insight into the electronic effects and metal/oxidation state stoichiometry of these compounds and it would also provide a better understanding of how the compounds form and what their catalytic capabilities would be limited to if applicable. The electron distribution and charge transfer within metal hexacyanoferrate can be studied using X-ray Photoelectron Spectroscopy (XPS). XPS data can be used to determine the oxidation states of the different metals, the ratios of the different oxidation states (e.g. II or III) of the different metals in the metal hexacyanoferrate. The substructures of the main metal photoelectron lines and satellite peaks can be explained by charge transfer from the ligand to the metal as well as inner-sphere reorganisation. In a multi-technique approach, other X-ray spectroscopy methods in a such as XAS, SEM-EDX, XANES and EXAFS was used to characterise the structure of copper hexacyanoferrates electrodeposited on a carbon surface [5]. This gave information on the structure of the electrodeposited copper hexacyanoferraed on a glassy carbon electrode, the ratio of potassium and copper ions to the iron ions as well as the bond lengths. Both metals in metal hexacyanoferrates exist in both the II and III oxidation state, resulting in a multitude of stoichiometric situations. In this study four different metal hexacyanoferrates were prepared (Fe, Co, Ni and Cu) and characterised by ATR FTIR and XPS. Their cyano-stretching frequencies will be used to determine metal oxidation state chains (e.g. FeII-C≡N-MII, FeII-C≡N-MIII or FeIII-C≡NMII) present and the XPS will be used to elucidate the metal ratios as well as the oxidation state ratios of each metal, to predict a more accurate stoichiometry of the prepared compounds. XPS binding energies will be compared to electronegativities of the metals to give insight into the electronic properties of the compounds. 2. Results and discussion 2.1. Preparation of metal hexacyanoferrates, 1e4 A series of metal hexacyanoferrate complexes with the general formula of KxMy[Fe(CN)6]z$qH2O, with M ¼ Fe (1), Co (2), Ni (3) and Cu (4), and x, y, z and q representing stoichiometric numbers, were

Fig. 1. Reaction scheme for preparation of iron hexacyanoferrate, 1, as an example.

prepared by a simple co-precipitation reaction according to the reaction scheme presented in Fig. 1. The water soluble starting materials, a metal halide (e.g. FeIIICl3 for the preparation of 1) and the potassium hexacyanoferrate (K3[FeIII(CN)6]) are mixed together at room temperature in ambient air. This resulted in the formation of water soluble KCl and the water-insoluble metal hexacyanoferrate (1e4), which precipitate. The yields obtained varied from 26% for 1e94% for 3, and seems to be dependent on the Pauling scale electronegativity, sM [6], of M. In general, as sM increase the yield obtained for 1e4 also increases. The order of yield (from low to high) of the metal hexacyanoferrates is: 1 (M ¼ Fe, 26% yield, sM ¼ 1.83) < 2 (M ¼ Co, 43% yield, sM ¼ 1.88) < 4 (M ¼ Cu, 85% yield, sM ¼ 1.90) < 3 (M ¼ Ni, 94% yield, sM ¼ 1.91). Thus metals that are more electron withdrawing (Ni, sM ¼ 1.91) are more likely to form a coordination bond with the lone pair electrons on the nitrogen atom of the cyano-group as compared to metals which are slightly less electron withdrawing (Fe, sM ¼ 1.83). The standard method of preparation of the metal hexacyanoferrates is the mixing the reagents a metal halide and the potassium hexacyanoferrate in a predetermined stoichiometry (e.g. a 1:1 ratio) for a long enough time, so that all the starting material is reacted and the supernatant is free of unreacted species. However, we selected a short reaction time of 15 min for the preparation of 1e4. Keeping the reaction time constant allowed the observation of metal dependent yields. This shows that the product composition is controlled by the reaction kinetic constants and sedimentation parameters. As a result, the reaction yields become dependent on the electronegativity of the metal M. 2.2. Infra-red spectroscopy Attenuated Total Reflection Fourier Transformed Infrared (ATR FTIR) is useful in detecting the presence of certain functional groups like carbonyls (v(CO) located between 1600 and 1800 cm1) [7,8], bound carbon monoxide (v(C≡O) located between 1900 and 2200 cm1) [9,10], cyanates (v(-NCO) located at ca. 2250 cm1) [11], hydroxyl groups (v(OH) located between 3650 and 3100 cm1 [12], and d(HOH) detected at ca. 1600 cm1) [13] or cyano-groups (v(C≡N) located between 2300 and 2000 cm1) [14]. The value of the wavenumber is also an indication of the chemical environment surrounding these functional groups as well as bond strength [7]. The cyano stretching frequency, v(C≡N), as measured by ATR FTIR was used to determine the different chemical and electronic environments of the C≡N bonds in the different hexacyanoferrate complexes, 1e4. Table 1 reports the v(C≡N) frequencies detected for 1e4, including the assignment based on the oxidation state of the metals responsible for the value of the frequency. The comparative ATR FTIR spectra of the v(C≡N) for 1e4 is shown in Fig. 2. Three different C≡N stretching frequencies was observed for each metal hexacyanoferrate, 1e4, at ca. 2089, 2116 and 2164 cm1, which was assigned to the FeII-C≡N-MII, FeII-C≡N-MIII and FeIII-C≡N-MII chain, respectively. Each of these C≡N groups experiences its own chemical and electronic environment, due to the two metals in different oxidation states. The assignments were made in correlation with reported results [11,15e17]. The n(C≡N) value assigned to the FeIII-C≡N-MII (2164 cm1) moiety is red-shifted by ca. 75 cm1 more than FeII-C≡N-MII (2089 cm1). It has been reported previously that this shift in frequency can be due to the change in the electronic state of the Ncoordinated cations from high-spin (MII) to low-spin (MIII) [18]. It was suggested that the decrease in eg electrons with antibonding character leaded to the increase in back-bonding accompanied by a partial depopulation of the p(N≡C) orbital in order to compensate for the charge deficit at the central ion caused by the back-bonding.

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Table 1 The ATR FTIR measured v(C≡N) stretching frequencies detected for KxMy[Fe(CN)6]z$qH2O, 1e4, as well as the electronegativity of the metal ions. No

M

cM2þa

(cM2þþcFe2þ)b

(cM2þþcFe3þ)c

v(C≡N)Fe2þ/M2þ/cm1

v(C≡N)Fe2þ/M3þ/cm1

v(C≡N)Fe3þ/M2þ/cm1

1 2 3 4

Fe Co Ni Cu

2.636 2.706 2.891 2.952

5.272 5.342 5.527 5.588

6.471 6.541 6.726 6.787

2079 2086 2095 2097

2114 2119 2118 2115

2155 2160 2165 2169

a b c

Data from Ref. [34]. The combination of the metal ions' electronegativity, cFe3þ ¼ 2.636. The combination of the metal ions' electronegativity cFe3þ ¼ 3.835.

Fig. 2. Comparative ATR FTIR spectra of the v(C≡N) area of 1e4.

The decrease of electrons in the antibonding p(N≡C) results in the shift of n(C≡N) toward higher frequency [19]. From literature it is known that Berlin green (FeIII-C≡N-FeIII) displays C≡N stretching frequencies at 2089 (strong) and 2152 (weak) cm1, while Prussian white (FeII-C≡N-FeII) displays C≡N stretching frequencies at 2095 (strong) and 21609 (weak) cm1 [20]. Compound 1 display C≡N stretching frequencies (2079 and 2155 cm1) that correlates good with both Berlin green and Prussian white. It is thus possible to conclude that the 1, exists as a combination of Berlin green and Prussian white, but mixed with the other possible species (FeII-C≡N-FeIII, Prussian blue, and FeIII-C≡NFeII, Turnbull blue) as well. The v(C≡N) measured for 2e4 is also in correlation with data reported for the mixed valent species of FeII/ III -C≡N-MII/III [11,20e23]. In Fe-C≡N-M, the carbon of the C≡N bond binds to the iron through a s-bond by donation of electrons from its s orbital to the metal's orbital and in return it accepts electrons from the metal in a back-donation to its antibonding p-orbital. The s-donation has a tendency to increase the v(C≡N) value, while the back bonding tends to decrease the v(C≡N) value. The v(C≡N) value of cyano complexes are governed by the electronegativity, the oxidation state and the coordination number of the metal bonded directly to the cyanide bond [24,25].

It is known that the electronegativity of one fragment of a molecule (in this case different metals with different oxidation states) can influence the physical properties of another part of a molecule, such as reactivity, infra-red spectroscopy, electrochemical properties and kinetics [26e33]. To determine if there is a link between the metals' oxidation state and the v(C≡N), relationships was established between the combined electronegativity of the metal ions ((cFe2þ þ cM2þ) or (cFe3þ þ cM2þ)) [34] and the C≡N stretching frequencies observed for each chain (v(C≡N)Fe2þ/M2þ or v(C≡N)Fe3þ/M2þ), see Fig. 3. The directly proportional relationships obtained indicates that the C≡N stretching frequencies increases as the electronegativity of the metal ions increases. Since the electronegativity of Cu2þ (cCu2þ ¼ 2.952), in 4, is higher than the electronegativity of Fe2þ (cFe2þ ¼ 2.636), in 1, the C≡N ligand in 4 is less electron rich than the C≡N ligand in 4, which means that the C≡N bond strength in 4 is higher, leading to a higher wavenumber for the v(C≡N), explaining the observed trend in Fig. 3. Working backwards, a higher v(C≡N) means a stronger C≡N bond and therefore weaker p back-bonding from the iron to carbon and a stronger s-bond. Thus the order of s-bond strength from high to low is 4 (M ¼ Cu) > 3 (M ¼ Ni) > 2 (M ¼ Co) > 1 (M ¼ Fe). The three peaks present in the ATR FTIR shows that the oxidation II and III of all the metals present in the sample exists, however due the limitation of the ATR FTIR technique, the relative percentage of each oxidation state cannot be determined. The broad OH stretching frequency with a maximum at ca. 3350 cm1 and the sharp OH stretching frequency with a maximum at ca. 3640 cm1 detected by ATR FTIR is assigned to the presence of water molecules adsorbed onto the outside of the metal hexacyanoferrate particle [35]. The H-O-H bending frequency detected at ca. 1605 cm1 confirms the intercalation of water molecules inside the interstitial spaces of the metal hexacyano ferrates (see Figs. S1 and S2 as well as Table S1 in the Supplementary Information), which is characteristic of coordinated water existing inside the structure [36,37]. 2.3. Thermogravimetric analyses The TGA spectra of the four metal hexacyanoferrates (1e4), showed the weight loss upon heating could be categorised into three stages, room temperature to ~200  C (stage 1), ~200e~300  C (stage 2) and from ~300  C onwards, see Fig. 4. These three stage is grouped as three global pyrolytic processes, each comprising of a few overlapping steps. The total weight loss ranged between 32 and 46%, while the weight loss for the three individual stages ranged from 2 to 34%. The weight loss during each stage is explained in accordance with literature [38]. The first stage up to ~200  C is due to the evaporation of water adsorbed onto the external surface of the particles. During the second stage ~200e~300  C, water which was trapped inside the interstitial spaces is removed. The third stage extending from ~300  C upto 500  C (it was only measured up to 500  C), decomposition of the organic binder takes place causing further weight loss.

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Fig. 3. Left: Relationship between the combined electronegativity of the Fe2þ and M2þ ions and the v(C≡N)Fe2þ/M2þ stretching frequency of the CN group in FeII-C≡N-MII. Right: Relationship between the combined electronegativity of the Fe3þ and M2þ ions and the v(C≡N)Fe3þ/M2þ stretching frequency of the CN group in FeIII-C≡N-MII.

Fig. 4. The comparative TGA thermograms of the metal hexacyanoferrates 1e4.

As mentioned in the FTIR section, external water can be detected at 3600 - 3100 cm1, which is the OH stretching frequencies of the water molecules adsorbed on the outside, while the internal water can be detected at ca. 1600 cm1, which is the d(HOH) bending

vibration of water molecules that are trapped within the interstitial spaces [37]. It should thus be possible to confirm the three different stages of mass loss by ATR FTIR. As an example, 4 will be discussed, the ATR FTIR measured after heating to different temperatures are shown in Fig. 5 [16]. After 4 was subjected to temperatures up to 200  C the broad peak at ca. 3500 cm1 disappeared. Since the OH stretching frequencies detected at 3600 - 3100 cm1 is associated with water molecules adsorbed on the outside, stage one of the mass loss is confirmed to be loss of the external water from the metal hexacyanoferrate. After 4 was subjected to temperatures up to 300  C the peak indicating internal water molecules at ca. 1500 cm1 disappeared [39], validating that stage two is the removal of the water trapped inside the metal hexacyanoferrate polymer structure. The third stage starting at ca. 300  C was only measured up to ca 500  C. Following the heating of 4 at 500  C for 0.2 h, the ATR FTIR revealed that the C≡N stretching frequencies between 2050 and 2200 cm1 disappeared, thereby confirming that the third stage of mass loss during the TGA belongs to the decomposition of the metal hexacyanoferrate by losing the CN organic linker. Comparing the measure % weight left (100% - total weight loss %) and the calculated amount of metal oxide remain after heating, it was found that for 1 and 4 not all the metal hexacyanoferrate sample decomposed to the metal oxide, and that some CN-groups still remain (see Table 2). For 2 and 3 decomposition to a combination of the different possible oxides, are within experimental error the same as the calculate %. 2.4. X-ray photoelectron spectroscopy of the main Fe 2p photoelectron lines

Fig. 5. Comparative ATR FTIR spectra of copper hexacyanoferrate (4) at (A) room temperature, (B) 200  C, (C) 300  C and (D) 500  C.

X-ray photoelectron spectroscopy (XPS) is a surface analysis technique used to analyse the top 5e10 nm of a sample, however the bulk of a sample can be investigated by removing the top layers of a sample by means of sputtering. It is a very valuable characterisation technique used to determine the composition of a sample as well as the oxidation state of metals within the sample. It is also useful to give insight into the chemical and electronic environment of the elements under investigation [40e42]. Additionally, the binding energy position as well as the substructure of the photoelectron lines of the elements under investigation are affected by final-state effects, such as shake-up -, shake-down peaks and multiplet splitting [43,44]. The source of these final state effects are crystal field splitting and charge transfer [45,46]. Thus the XPS technique can be used to complement ATR FTIR, since the metal's oxidation state and relative percentage of each oxidation state can be determined.

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Table 2 Total weight loss determined by thermogravimetric analysis. Complex

No

Total weight loss %

Measure % weight left

% Metal oxide calculated

Fe-C≡N-Fe

1

46.23

53.77

Fe-C≡N-Co

2

46.29

53.71

Fe-C≡N-Ni

3

40.58

59.42

Fe-C≡N-Cu

4

32.43

67.57

40.9 44.6 43.9 27.6 30.1 29.7 26.8 29.3 28.9 24.1 26.3 26.0

XPS has been performed on 1e4 to determine the oxidation states and the ratios of the oxidation states of the metals in the metal complexes. The substructures of the main metal photoelectron lines will be explained by charge transfer from the ligand to the metal as well as inner-sphere reorganisation. XPS data of the metal hexacyanoferrates, 1e4, with respect to Fe 2p, M 2p (M ¼ Co (2), Ni (3) and Cu (4)) photoelectron lines and Infrared data are presented in Tables 3 and 4, while the comparative Fe 2p XPS spectra of the different compounds are depicted in Fig. 6 the XPS of Co, Ni and Cu are shown in Figs. S3eS5 in the Supplementary Information. The Fe 2p photoelectron lines of 1e4 will be discussed first. The binding energy reported in literature for the Fe 2p3/2 photoelectron line of FeIII in K3[FeIII(CN)6] and sodium cobalt hexacyanoferrate is located at 709.60 eV and 709.0 eV respectively [15,47], while FeII in K4[FeII(CN)6] at 708.50 eV [48]. The peaks of the Fe 2p3/2 and Fe 2p1/2 photoelectron lines showed splitting of the main photoelectron lines (both Fe 2p3/2 and Fe 2p1/2) into three distinct substructures. The maximum binding energy of the Fe 2p3/2 and Fe 2p1/2 photoelectron lines are located between 708.18 - 710.48 eV and 721.14e723.36 eV respectively (with a spin orbit splitting of ca. 13.2 eV), charge corrected against C 1s at 284.8 eV (the lowest binding energy of the simulated adventitious C 1s photoelectron line) [49]. The three distinct substructures of the Fe 2p3/2 photoelectron lines are assigned to the presence of FeII at ca. 708.4 eV and FeIII at ca. 710.1 eV, in accordance with the reported literature values [31,50]. The third substructure (peak) located at ca. 712.3 eV is attributed to the charge transfer from the C≡N group to the iron. In an attempt to find a link between the binding energy of the Fe 2p3/2 photoelectron lines of 1e4 and the different metals in the

FeO Fe2O3 Fe3O4 FeO Fe2O3 Fe3O4 FeO Fe2O3 Fe3O4 FeO Fe2O3 Fe3O4

28.8 31.9 30.8 27.9 30.9

CoO Co2O3 Co3O4 NiO Ni2O3

24.0 Cu2O 26.7 CuO 29.4 Cu2O3

metal hexacyanoferrates, a correlation graph was constructed between the binding energy of the FeII 2p3/2 and FeIII 2p3/2 photoelectron lines and the Pauling electronegativity of the metal, sM [5], see Fig. 7 (a) and (b) respectively. The Pauling electronegativity scale was used in this case and not the electronegativity of the metal ions, since photoelectron lines e.g. the FeII 2p3/2 photoelectron line, is the amalgamated result of the influence of both MII and MIII in the FeII-C≡N-MII and FeII-C≡N-MIII chains. It is evident that as the Pauling electronegativity of the metal increases, the binding energy of the FeII 2p3/2 and FeIII 2p3/2 photoelectron lines also increases. The higher the sM, the more electron density is pulled away from the iron (both FeII and FeIII), causing the iron to bind tighter to its own electrons, which triggers the higher binding energy observed for the FeII 2p3/2 and FeIII 2p3/2 photoelectron lines. The difference in binding energy measured for the FeII 2p3/2 and III Fe 2p3/2 photoelectron lines (DBE2) was also found to be controlled by the Pauling electronegativity of the metal, sM, see Fig. 7(c). An increase in sM is accompanied by an increase in DBE2. Thus as more electron density is pulled away from the iron by the other metal (Fe in 1, Co in 2, Ni in 3 and Cu in 4) the difference in electronic environment between the FeII and FeIII becomes larger. The electronic environment of the FeIII (indicated by the binding energy of the FeIII 2p3/2 photoelectron line) is thus more affected by electron density (induced by sM) than the FeII. This tuning effect of the electronic environments of the FeII and FeIII in 1e4, could potentially be very useful in a variety of different applications. The ratio of the amount of FeII and FeIII present in 1e4, varies depending on the sM of the other metal in the metal hexacyanoferrate. As the electron density on the iron decreases (higher sM), the less FeII and the more FeIII is present in the metal hexacyanoferrate. This is consistent with the other metal (M ¼ Fe in 1,

Table 3 The maximum binding energy (BE) of the FeII and FeIII 2p3/2 photoelectron lines, the spin orbit splitting of maximum binding energies of the Fe 2p photoelectron lines, DBE1, binding energy of the satellite Fe 2p3/2 photoelectron line, as well as the peak separations between main Fe 2p3/2 and the satellite Fe 2p3/2, DBE2, the ratio area % between main Fe 2p3/2 and the satellite Fe 2p3/2, Iratio for KxMy[Fe(CN)6]z$qH2O, 1e4. No

M

sM

BE FeII 2p3/2main /eV

BE FeIII 2p3/2main /eV

DBE1a/eV

DBE2b/eV

Iratioc

BE Fe 2p3/2maind /eV

BE Fe 2p3/2satele /eV

DBE3f/eV

Iratio

1 2 3 4

Fe Co Ni Cu

1.83 1.88 1.91 1.90

708.44 708.52 708.59 708.58

709.44 710.03 710.25 710.09

12.95 13.12 13.42 13.24

1.22 1.47 1.66 1.51

1.72 1.44 0.52 0.88

708.82 709.27 709.42 709.28

712.65 712.00 711.88 711.76

3.83 2.73 2.46 2.48

0.56 0.42 0.17 0.19

a b c d e f g

DBE1 ¼ BE Fe2p1/2main e BE Fe2p3/2main. DBE2 ¼ BEFeIII 2p3/2 main e BE FeII 2p3/2 main.

Iratio ¼ ratio between the intensities of the FeII and FeIII Fe 2p3/2 photoelectron lines (Iratio ¼ (IFeII 2p3/2 main)/(IFeIII 2p3/2 main)). Maximum binding energy of the main Fe 2p3/2 envelope. Maximum binding energy of the satellite Fe 2p3/2 envelope. DBE3 ¼ BEFe2p3/2satele BE Fe2p3/2main. Iratio satel ¼ ratio between the intensities of the satellite and main Fe 2p3/2 photoelectron lines (Iratio ¼ (IFe2p3/2satel)/(IFe2p3/2main)).

g satel

v(C≡N)Fe2þ/M2þ/cm1 2079 2086 2095 2097

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Table 4 The Binding energies of the maximum binding energy (BE) of the nitrogen coordinated metal (M) 2p3/2 photoelectron lines, the spin orbit splitting of maximum binding energies of M 2p photoelectron lines, DBE1, binding energy of the satellite M 2p3/2 photoelectron line, as well as the peak separations between main M 2p3/2 and the M metal 2p3/2, DBE2, the ratio area % between main M 2p3/2 and the satellite M 2p3/2, Iratio. M ¼ Co, Ni or Cu. No

M

sM

BE MII 2p3/2main /eV

BE MIII 2p3/2main /eV

DBE1a/eV

DBE2b/eV

Iratioc

BE M 2p3/2maind /eV

BE M 2p3/2satele /eV

DBE3f/eV

Iratiog

2 3 4

Co Ni Cu

1.88 1.91 1.90

782.01 855.78 933.00

784.81 856.97 935.71

15.46 17.53 19.73

2.80 1.19 2.71

2.89 0.48 1.24

782.16 856.20 933.97

786.42 863.19 943.71

4.25 6.98 9.74

0.29 0.13 0.24

a b c d e f g

DBE1 ¼ BE M2p1/2main e BE M2p3/2main. DBE2 ¼ BE MIII 2p3/2 main e BE MII 2p3/2 main.

Iratio ¼ ratio between the intensities of the MII and MIII Fe 2p3/2 photoelectron lines (Iratio ¼ (IMII 2p3/2 main)/(IMIII 2p3/2 main)). Maximum binding energy of the main M 2p3/2 envelope. Maximum binding energy of the satellite M 2p3/2 envelope. DBE3 ¼ BEM2p3/2satele BE M2p3/2main. Iratio ¼ ratio between the intensities of the satellite and main M 2p3/2 photoelectron lines (Iratio ¼ (IM2p3/2satel)/(IM2p3/2main)).

2.5. X-ray photoelectron spectroscopy of the satellite Fe 2p photoelectron lines

Fig. 6. Comparative XPS spectra showing peak fittings for the main FeII 2p (dashed black fittings), FeIII 2p (solid black fittings) and satellite Fe 2p peaks (solid grey fittings) for complexes 1e4 as well as for K4[FeII(CN)6] and K3[FeIII(CN)6]. The green dashed line represents the ca. position of FeII, while the blue line represents the ca. position of FeIII. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The satellite peak, Fe 2p3/2satel, located at ca. 2.9 eV higher than the maximum of the main Fe 2p3/2 envelope (the combined FeII 2p3/ III 2 and Fe 2p3/2 photoelectron lines), see Fig. 6, has been reported to be the result of charge transfer from the ligand to the metal in octahedral first row transition metal complexes [51]. An inversely proportional relationship was been established between the maximum binding energy of the Fe 2p3/2 satellite peak and sM, see Fig. 7 (a and b). A larger satellite charge transfer peak indicates that more charge (electron density) is transferred from the ligand to the metal. The Iratio satel (Iratio ¼ (IFe2p3/2satel)/(IFe2p3/2main), which is the ratio of the intensity of the satellite photoelectron line and the main photoelectron line), is an indicator of the amount of charge transferred, the larger the satellite peak (as indicated by a large Iratio satel) the more charge is transferred from the ligand (C≡N-group) to the metal (in this case the Fe). The influence of the electron withdrawing property of the other metal (Fe in 1, Co in 2, Ni in 3 and Cu in 4) as measured by sM, shows an inversely proportional relationship, see Fig. 7(b). Thus, as more electron density is pulled towards the other metal (Fe in 1, Co in 2, Ni in 3 and Cu in 4) away from the ligand (C≡N-group), the less charge is transferred by the C≡N-group towards the iron (see Fig. 8). The binding energy of the Fe 2 p photoelectron lines is influenced by the Pauling electronegativity of the metal, sM, (as discussed earlier) transmitted through the C≡N bond, which unavoidably also influences the s-bond between the Fe and the C≡N. As the amount of electrons donated from the s-orbital of the carbon to the Fe increases (indicated by a lower v(C≡N)),1 the electron density on the Fe increases, which can be seen by the decrease in binding energy of the Fe 2p photoelectron lines (see Fig. S3(a) and (b)). The linearity of the relationship between v(C≡N) and Fe 2p binding energy (in a certain oxidation state) confirms the order of s-bond strength predicted by v(C≡N), to be from high to low is 4 (M ¼ Cu) > 3 (M ¼ Ni) > 2 (M ¼ Co) > 1 (M ¼ Fe). All of this indicated that the charge transfer detected by XPS can be related to v(C≡N), which is a measure of the s-bond strength. During an electron transfer process, such as photoemission (XPS), the sample undergoes inner-sphere reorganization to compensate for the positively charged species that formed, which is associated with a change in the geometry or distances of the coordination shell of the complex. As an electron is emitted from the

Co in 2, Ni in 3 and Cu in 4) drawing the electron density away from the iron creating an additional dþ on the FeII, up to a point where the FeII loses its electron to the “structure” and becomes FeIII. 1 A lower v(C≡N) indicates a stronger C≡N bond and accordingly a weaker sbond to the metal.

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79

Fig. 7. The relationships of 1e4 between the Pauling electronegativity of the metal, sM, and (a) the binding energy of the FeII Fe 2p3/2 photoelectron line, (b) the binding energy of the FeIII Fe 2p3/2 photoelectron line, (c) the difference in binding energy of the FeII 2p3/2 and FeIII 2p3/photoelectron lines (DBE2 ¼ BEFeIII 2p3/2 main e BE FeII 2p3/2 main) (d) Iratio ¼ ratio between the intensities of the FeII and FeIII Fe 2p3/2 photoelectron lines (Iratio ¼ (IFeII 2p3/2 main)/(IFeIII 2p3/2 main)).

Fig. 8. The relationships of 1e4 between the Pauling electronegativity of the metal, sM, and (a) the maximum binding energy of the satellite Fe 2p3/2 (BEFe 2p3/2 satel) photoelectron line, (b) the Iratio satel ¼ ratio between the intensities of the satellite and main Fe 2p3/2 photoelectron lines (Iratio ¼ (IFe 2p3/2 satel)/(IFeIII 2p3/2 main)).

FeII/III centre of 1e4, the molecule simultaneously undergoes an electronic transition, which is accompanied by a vibrational excitation [52]. The wavenumber of vibration peaks as measured by FTIR such as the v(C≡N), can be used as a measure of this vibrational excitation. Seeing as the wavenumber is directly proportional to the photon energy, v(C≡N) can in turn be used an indication of the inner-sphere reorganisation energy. Here, a link was established between the energy of inner-sphere reorganisation (as expressed by v(C≡N)) and the binding energies of the FeII and FeIII 2p3/2 photoelectron lines (see Fig. S3(a) and (b)). Seeing as the wavenumber is directly proportional to the photon energy, and by implication the energy of inner-sphere reorganisation, the binding energy of the main Fe photoelectron lines is also directly proportional to the energy of the inner-sphere reorganisation. Thus, the easier it is too emit an electron (lower BEFe 2p3/2main), the less energy is required for inner-sphere reorganisation (low wavenumber). 2.6. X-ray photoelectron spectroscopy of other photoelectron lines The XPS spectra of the simulated main M 2p peaks as well as the satellite peaks of M 2p photoelectron lines of the different prepared complexes can be found in the Supplementary Information. XPS examination of the 2p metal peaks revealed the presence of distinct substructures, see Table 2 (for Fe) and Table 3 (for Co, Ni and Cu). Regarding the Co 2p3/2 photoelectron line, the two main peaks are assigned to CoII and CoIII based on the location of their binding energies at ca. 782.01 and 784.81 eV respectively, which is 4 and 6 eV higher than 778.30 eV the for Co0 [53], as well as literature [33,54]. In reference to the position at ca. 855.77 and 858.01 eV for the two distinguishable Ni 2p3/2 photoelectron line, they were

allocated to NiII and NiIII, respectively, since Ni0 is normally positioned at ca. 852.80 eV [55], and the allocations are also in accordance with reported values from literature [56]. As for the Cu 2p3/2 photoelectron line, the two main peaks found at ca. 933.00 and 935.71 eV were assigned to CuI and CuII, which correlates well reported data from literature [57]. The XPS spectra of all non-iron metals also showed charge transfer peaks at a few eV higher than the main 2p photoelectron lines. In comparison to the charge transfer of the iron bound to the carbon (Fe-C≡N-M) the charge transfer from the nitrogen to the metal (Fe-C≡N-M) is much less as indicated by the Iratio satel ca. 0.34 eV for Fe and ca. 0.22 eV for M. This shows influence of the Fe oxidation state on the C≡N bond is more than that of the oxidation state of M, which confirms the allocation of the v(C≡N) made earlier namely: ca. 2089 cm1 belongs to the FeII-C≡N-MII chain, 2116 cm1 belongs to the FeII-C≡N-MIII chain and 2164 cm1 belongs to the FeIII-C≡N-MII chain.

2.7. X-ray photoelectron spectroscopy related to stoichiometry Other elements that was detected by the XPS was carbon (C 1s located ca. 284.8 eV, see Table S2), oxygen (O s1 located at ca. 531.5 eV), potassium (K 2p3/2 located ca. 293.6 eV), nitrogen (N 1s located ca. 398.1 eV) and Co (for 2), Ni (for 3) and Cu (for 4). The atomic ratios was obtained between the iron, potassium and the other metals, from which it is possible to derive the a quantified stoichiometric formula for KMy[Fe(CN)6]z: 1: K1Fe1.8[Fe(CN)6]1 2: K0.29Co1.09[Fe(CN)6]1 FeIII0.41(CN)6]1 3: K0.46Ni0.87[Fe(CN)6]1 FeIII0.66(CN)6]1 4: K0.10Cu1.02[Fe(CN)6]1 FeIII0.53(CN)6]1

or

K0.29CoII0.81/CoIII0.28[FeII0.59/

or

K0.46NiII0.28/NiIII0.59[FeII0.34/

or

K0.10CuII0.56/CuIII0.46[FeII0.47/

The stoichiometric formula derived from the XPS correlated very well with the derived stoichiometry ratios between the carbon, nitrogen and the different determined from the ICP and elemental analysis (data given in the experimental section) which was found to be: 1: 2: 3: 4:

Fe:C:N ¼ 1:0.98:1.04 Fe:Co:C:N ¼ 1:1.10:0.95:0.97 Fe:Ni:C:N ¼ 1:0.88:0.96:0.98 Fe:Cu:C:N ¼ 1:1.10:0.97:0.99

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3. Experimental 3.1. General Solid reagents (Merck and Aldrich) for preparative purposes were used without further purification. Liquid reagents (Merck and Sigma Aldrich) were also used as received without further purification thereof. Filtration and vacuum evaporation were conducted using a water aspirator. 3.2. Elemental analysis The elemental analyses were performed at the University of the Free State on a Leco Truspec Micro analyser. ICP-OES analysis was performed with a Shimadzu ICPS-7510 Inductively Coupled Plasma e Optical emission spectroscopy with a radical-sequential plasma spectrometer for the wet chemical analysis. The vertically orientated ICP-OES with the radical viewing plasma was found to be suitable due to its better detection limits. The emission intensity measurements were made using the default conditions as indicated below.

Parameter

Condition

RF power Coolant gas flow rate Plasma gas flow rate Auxiliary gas flow rate Carrier gas flow rate Sample uptake method Type of spray chamber Type of nebulizer Injector tube diameter

1.2 kW 14.0 L/min 45 L/min 0.5 L/min 0.7 L/min Peristaltic pump Glass cyclonic Concentric 3.0 mm

The neat samples (~0.01 g) were accurately weighed and quantitatively transferred. Sodium phosphate salts (flux) (1.0 g) were added and mixed thoroughly. The mixture was placed in an oven at 900  C and fused for 5 min and left to cool to room temperature and dissolved in distilled water. These dilute solutions were analysed for C, N, H, Fe, Co, Ni and Cu concentrations or percentiles using ICP-OES. 3.3. Thermogravimetric analyses Thermogravimetric analyses (TGA) were performed under an Argon environment on a Mettler Toledo TGA/SDTA851 and analysed with STAR SW v8.10 software. 3.4. Attenuated Total Reflection Fourier Transformed Infrared Infrared spectra of neat samples were recorded with a Thermo Scientific IR spectrometer and NICOLET iS50 ATR attachment, running OMNIC software (Version 9.2.86). 3.5. X-ray photoelectron spectroscopy XPS data was recorded on a PHI 5000 Versaprobe system, with a monochromatic Al Ka X-ray source. Powered samples were mounted on the sample holder by means of carbon tape. Spectra were obtained using the aluminium anode (Al Ka ¼ 1486.6 eV), operating at 50 mm, 12.5 W and 15 kV energy (97 X-ray beam). A low energy neutraliser electron gun was used to minimise charging of the samples. The instrument work function was calibrated to give a

binding energy of 284.8 eV for the lowest binding energy peak of the carbon 1s envelope, corresponding to adventitious carbon. Survey scans were recorded at constant pass energy of 187.85 eV, while detailed region scans were recorded at constant pass energy of 29.35 eV for C and O, and 93.90 eV for the metals, with an energy step of 0.1 eV; the analyzer resolution is  0.5 eV. Charge neutralisation was enhanced by shooting the mounted sample with an Ar gun during data recording. The resolution of the PHI 5000 Versaprobe system is FWHM ¼ 0.53 eV at a pass energy of 23.5 eV and FWHM ¼ 1.44 eV at a pass energy of 93.90 eV. The background pressure was 2  108 mbar. Spectra have been charge corrected to the main line of the carbon 1s spectrum, which was set to 284.8 eV. XPS data was analysed utilising Multipak version 8.2c computer software [58], and applying Gaussian/Lorentz fits (the Gaussian/ Lorentz ratios were always > 95%). The photoelectron lines were charge corrected against the lowest binding energy of the fitted adventitious C 1s peak at 284.8 eV (the normal position of C-C according to the XPS Handbook [32]). The carbon peak of all the complexes were fitted to 3 peaks, namely C-C at 284.8 eV, C¼C at 285.5 eV and C¼O at 286.7 eV. 3.6. General preparation of 1e4 1e4 were all prepared according to the same synthetic procedure, 1 will be shown as an example. 3.7. Preparation of 1 Potassium hexacyanoferrate/ferricyanide (491 mg, 1.5 mmol, 0.1 M, 1 e.q.) was dissolved in water (15.2 ml), creating an aqueous solution with a concentration of 0.1 M. To this solution was added a 15 ml aqueous solution of iron(III)chloride (0.258 g, 1.59 mmol, 0.1 M, 1.06 e.q.). The combined solution was stirred at room temperature for 15 min. The resulting mixture was centrifuged at 8500 rpm for 30 min at 15  C. The excess water was discarded and the precipitate dried overnight in vacuo at 60  C. The dried powder was crushed to yield 0.344 g (26.69%) of pure 1. ATR FTIR v(C≡N) ¼ 2079, 2114, 2155 cm1. XPS: Binding energy ¼ 708.44 eV (FeII 2p3/2); 709.44 eV (FeIII 2p3/ 2). ICP: 30% Fe (calculated), 30.2% Fe (found). Elemental analysis: 20% C (calculated), 19.61% C (found), 20% N (calculated), and 20.75% N (found).

3.8. Charaterisation data for 2 Yield 42.84% ATR FTIR v(C≡N) ¼ 2086, 2119, 2160 cm1. XPS: Binding energy ¼ 708.52 eV (FeII 2p3/2); 710.03 eV (FeIII 2p3/ II III 2); 782.01 eV (Co 2p3/2); 784.81 eV (Co 2p3/2). ICP: 15% Fe (calculated), 14.97% Fe (found), 15% Co (calculated), and 16.49% Co (found). Elemental analysis: 20% C (calculated), 19.07% C (found), 20% N (calculated), and 19.44% N (found).

3.9. Charaterisation data for 3 Yield 94.01% ATR FTIR v(C≡N) ¼ 2097, 2115, 2169 cm1. XPS: Binding energy ¼ 708.59 eV (FeII 2p3/2); 710.25 eV (FeIII 2p3/ II III 2); 855.78 eV (Ni 2p3/2); 856.97 eV (Ni 2p3/2).

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ICP: 15% Fe (calculated), 14.88% Fe (found), 15% Ni (calculated), and 13.12% Ni (found). Elemental analysis, 20% C (calculated), 19.29% C (found), 20% N (calculated), and 19.54% N (found).

3.10. Charaterisation data for 4 Yield 84.45%. ATR FTIR v(C≡N) ¼ 2095, 2118, 2165 cm1. XPS: Binding energy ¼ 708.58 eV (FeII 2p3/2); 710.09 eV (FeIII 2p3/ II III 2); 933.00 eV (Cu 2p3/2); 935.71 eV (Cu 2p3/2). ICP: 15% Fe (calculated), 14.87% Fe (found), 16% Cu (calculated), and 16.42% Cu (found). Elemental analysis, 20% C (calculated), 19.37% C (found), 20% N (calculated), and 19.73% N (found). 4. Conclusion A series metal hexacyanoferrates, were prepared and the yields were found to be dependent on electronegativity of the metal. The cyano-group stretching frequencies, v(C≡N), as measured by infrared spectroscopy affirmed the presence of different possible combination of the different oxidation states of the Fe and the other metal (Co, Ni or Cu). ATR FTIR showed that water molecules were trapped in interstitial sites. X-ray photoelectron spectroscopy (XPS) was used to calculate the ratio between the different metals as well as the ratio of each oxidation state of the different metals, which broadens the knowledge of the stoichiometry of metal hexacyanoferrates. The photoelectron lines of all the metals showed substructures (satellite) ascribed to the charge transfer that exists between the CN ligand and the metal. The amount of charge transfer was also related to the electronegativity of the metals. Acknowledgements The authors acknowledge financial support from Sasol during the course of this study. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.matchemphys.2017.09.029. References [1] S.R. Ali, V.K. Bansal, A.A. Khan, S.K. Jain, M.A. Ansari, J. Mol. Cat. A Chem. 303 (2009) 60e61. [2] F. Ricci, G. Palleschi, Biosen. Bioelec. 21 (2005) 389e407. [3] N.A. Sitnikova, M.A. Komkova, I.V. Khomyakova, E.E. Karyakina, A.A. Karyakin, Anal. Chem. 86 (2014) 4131e4134. [4] C.D. Wessells, M.T. McDowell, S.V. Peddada, M. Pasta, R.A. Huggins, Y. Cui, Am. Chem. Soc. 2 (2012) 1688e1689. [5] M. Giogetti, L. Guadagnini, D. Tonelli, M. Minicucci, G. Aquilanti, Phys. Chem. Chem. Phys. 14 (2012) 5527e5537. [6] L. Pauling, J. Amer. Chem. Soc. 54 (1932) 3570e3582. [7] E. Erasmus, J. Conradie, A. Muller, J.C. Swarts, Inorg. Chim. Acta 360 (2007) 2277e2283. [8] E. Erasmus, J.O. Claassen, W.A. van der Westhuizen, Water SA 42 (2016) 442e448. [9] E. Erasmus, Inorg. Chim. Acta 451 (2016) 197e201. [10] E. Erasmus, Polyhedron 106 (2016) 18e26. [11] M.M. Conradie, J. Conradie, E. Erasmus, Polyhedron 79 (2014) 52e59.

81

[12] E. Erasmus, Hem. Ind. 70 (2016) 595e601. [13] R.R. Sheha, J. Colloid. Intersurf. Sci. 338 (2012) 21e30. [14] M. Berrettoni, M. Ciabocco, M. Fantauzzi, M. Giorgetti, A. Rossi, E. Caponetti, RSC Adv. 5 (2015) 35435e35445. [15] R.O. Lezna, R. Romagnoli, N.R. de Tacconi, K.J. Rajeshwar, J. Phys. Chem. B 106 (2002) 3612e3621. [16] N. Shimamoto, S. Ohkoshi, O. Sato, K. Hashimoto, Inorg. Chem. 41 (2002) 678e684. [17] A. Zanotto, R. Matassa, M.L. Saladino, M. Berrettoni, M. Giogetti, S. Zamponi, E. Caponetti, Mat. Chem. Phys. 120 (2010) 118e122. [18] O. Sato, Y. Einaga, A. Fujishima, K. Hasimoto, Inorg. Chem. 38 (1999) 4405e4412. [19] Z.D. Reed, M.A. Duncan, J. Amer. Soc. Mass Spec. 21 (2010) 739e749. [20] T. Suemoto, R. Fukaya, A. Asahara, H. Watanabe, H. Torkor, S. Ohkoshi, Curr. Inorg. Chem. 6 (2016) 10e25.
ndez-Bertr
[21] E. Reguera, J. Ferna an, J. Balmaseda, Trans. Metal. Chem. 24 (1999) 648e654. [22] S.R. Ali, P. Chandra, M. Latwal, S.K. Jain, V.K. Bansal, S.P. Singh, Chin. J. Catal. 32 (2011) 1844e1849. [23] R.O. Lezna, R. Romagnoli, J. Phys. Chem. B 106 (2002) 3612e3621. € m, [24] D.O. Ojwang, J. Grins, D. Wardecki, M. Valvo, V. Renman, L. H€ aggstro T. Ericsson, T. Gustafsson, A. Mahmoud, R.P. Hermann, G. Svensson, Inorg. Chem. 55 (2016) 5924e5934. [25] C.W. Ng, J. Ding, Y. Shi, L.M. Gan, J. Phys. Chem. Sol. 62 (2001) 769e770. [26] E. Erasmus, Inorg. Chim. Acta 378 (2011) 95e101. [27] J. Conradie, J.C. Swarts, Organomet 28 (2009) 1018e1026. [28] E. Erasmus, J.C. Swarts, New J. Chem. 37 (2013) 2862e2873. [29] E. Erasmus, J. Electroanal Chem. 727 (2014) 1e7. [30] J. Conradie, J.C. Swarts, Euro. J. Inorg. Chem. 15 (2011) 2439e2449. [31] M. Trzebiatowska-Gusowska, A. Gagor, E. Coetsee, E. Erasmus, H.C. Swart, J.C. Swarts, J. Organomet. Chem. 745e746 (2013) 393e403. [32] T.J. Muller, J. Conradie, E. Erasmus, Polyhedron 33 (2012) 257e266. [33] E. Eramus, A.J. Muller, U. Siegert, J.C. Swarts, J. Organomet. Chem. 821 (2016) 62e70. [34] K. Li, M. Li, D. Xue, J. Phys. Chem. A 116 (2012) 4192e4198. [35] D.M. Gil, M.C. Navarro, M.C. Lagarrgue, J. Guimpel, R.E. Carbonio, M.I. Gomez, J. Mol. Struct. 1003 (2011) 129e133. [36] R.R. Sheha, J. Coll. Interf. Sci. 388 (2012) 21e30. [37] W. Xiaoyu, Y. Yukawa, Y. Masuda, J. All. Comp. 290 (1999) 85e90. [38] R.R. Sheha, J. Colloid Intersurf. Sci. 338 (2012) 21e30. [39] M.S. Rather, K. Majid, R.K. Wanchoo, M.L. Singla, J. Therm. Calorim. 112 (2013) 897. [40] B.E. Buitendach, E. Erasmus, M. Landman, J.W. Niemantsverdriet, J.C. Swarts, Inorg. Chem. 55 (2016) 1992e2000. [41] A. Jansen van Rensburg, M. Landman, E. Erasmus, D. van der Westhuizen, H. Ferreira, M.M. Conradie, J. Conradie, Electrochim. Acta 219 (2016) 204e213. [42] A. van As, C.C. Joubert, B.E. Buitendach, E. Erasmus, J. Conradie, A.N. Cammidge, I. Chambrier, M.J. Cook, J.C. Swarts, Inorg. Chem. 54 (2015) 5329e5341. [43] J. Conradie, E. Erasmus, Polyhedron 119 (2016) 142e150. [44] R. Liu, J. Conradie, E. Erasmus, J. Electron Spectrosc. Relat. Phenom. 206 (2016) 46e51. [45] J.W. Niemantsverdriet, Spectroscopy in Catalysis, third ed., WILEY-VCH, Weinheim, 2007, pp. 50e51. [46] B.E. Buitendach, E. Erasmus, J.W. Niemantsverdriet, J.C. Swarts, Molecules 21 (2016) 1427e1442. [47] N.G. Vannerberg, Chem. Scr. 9 (1976) 122e126. [48] K.B. Yatsimirskii, V.V. Nemoshalenko, Y.P. Nazarenko, V.G. Aleshin, V.V. Zhilinskaya, N.A. Tomashevsky, J. Electron Spectrosc. Relat. Phenom. 10 (1977) 239e245. [49] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy: a Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, ULVAC-PHI, Inc., Enzo, Chigasaki, Japan, 1995. [50] S. Sauter, G. Wittstock, R. Szargan, Phys. Chem. Chem. Phys. 3 (2001) 562e569. [51] J.S.H.Q. Perera, D.C. Frost, C.A. McDowell, J. Phys. Chem. 72 (1980) 5151e5158. [52] N.P. de Leon, W. Liang, Q. Gu, H. Park, Nano Lett. 8 (2008) 2963e2967. [53] C.J. Powell, J. Electron Spectrosc. Relat. Phenom. 185 (2012) 1e3. [54] M.M. Kaplun, Y.E. Smirnov, V. Mikli, V.V. Malev, Russ. J. Electrochem 37 (2001) 918e923. [55] A.B. Mandale, S. Badrinarayanan, S.K. Date, A.P.B. Sinha, J. Electron Spectrosc. Relat. Phenom. 33 (1984) 61e72. [56] T.R.I. Cataldi, R. Guascito, A.M. Salvi, J. Electroanal. Chem. 417 (1996) 83e88. [57] M.R. Riyanto, Othman, Int. J. Electrochem. Sci. 10 (2015) 4911e4921. [58] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., Enzo, Chigasaki, 1995, pp. 253e8522. Japan.