**5. Electrochemical synthesis of praseodymium oxides**

Praseodymium oxide is a versatile and useful material, which is made up of a series of oxide compounds with the general formula PrnO2n-2 (n=4, 5, 6, 7, 8, 9, 10, 12) [84-86]. Pr2O3 has a hexagonal structure and belongs to the A-type rare earth structure, while, PrO2 and Pr6O11 have a fluorite structure (similar to CeO2) where the metals are eight-coordinate. The mixed valence states in praseodymium oxide compounds make them useful materials with several important applications. Two areas of interest for praseodymium oxide films include the use of this material as an ethanol sensor and as a catalyst. [87-90]. Tsang et al. found that Pr6O11 could detect ethanol in air and the optimal sensitivity (100%) was obtained at about 250 ˚C – 300 ˚C [87]. The Pr6O11 sensors gave a linear response to ethanol concentrations in the range of 200– 8000 ppm. Praseodymium oxide is a promising candidate to substitute silicon dioxide as a high-K dielectric, with a dielectric constant up to 10 times higher than SiO2 (dielectric constant is around 30) and very low leakage currents. Among the different compositions of praseody‐ mium oxide, Pr6O11 has the highest K value and Pr2O3 is a good dielectric [91-93]. Praseody‐ mium oxide films can be formed by several different methods including molecular beam epitaxy, pulsed-laser deposition (PLD), sputtering, electrocrystallization of molten salts, chemical vapor deposition (CVD), and spin coating [92, 94-99].

However, the literature for the electrochemical deposition of praseodymium oxide is quite sparse. There are only a couple of references using electrodeposition and even in these the initial deposition component is praseodymium hydroxide, which is converted to Pr6O11 or PrO2 by sintering [100, 101]. In both instances, Pr(NO3)3 and H2O2 were used as electrolyte and the deposition occurred by the base generation method. The reactions are [101]:

$$\rm{H\_2O\_2(aq) + 2e^- \to 2OH^-(aq)}\tag{11}$$

$$\mathrm{NO}\_3^-\mathrm{(aq)} + \mathrm{H}\_2\mathrm{O} \text{(l)} + 2\mathrm{e}^\cdot \to \mathrm{NO}\_2^-\mathrm{(aq)} + 2\mathrm{OH}^- \text{(aq)}\tag{12}$$

$$2\text{H}\_2\text{O}(\text{l}) + 2\text{e}^\cdot \rightarrow 2\text{OH}^\cdot(\text{aq}) + \text{H}\_2(\text{g})\tag{13}$$

$$2\text{Pr}(\text{OH})\_3\text{(s)} \rightarrow 2\text{PrO}\_2\text{(s)} \text{ or } \text{PrO}\_{11}\text{(s)} + 2\text{H}\_2\text{O(l)} + \text{H}\_2\text{(g)}\tag{14}$$

where the electrochemical generation of hydroxide ions increases the pH at the electrode surface and causes formation of Pr hydroxide on the surface. After deposition, the film is dried and sintered to produce Pr oxide.

Shrestha et al. electrodeposited an ultrathin layer of praseodymium hydroxide on ITO by applying a cathodic sweeping voltage followed by thermal annealing at 500°C for 1 h [100]. The predominant phase of the annealed film was Pr6O11. The XRD pattern of the deposit was

indexed as Pr6O11 (JCPDS 06-0329). The SEM showed the surface covered with small and uniform globular shaped Pr6O11 particles. The deposited particles did not seem to undergo aggregation into larger islands on the ITO surface, suggesting that the colloidal particles once formed near the cathode quickly accumulated on the ITO surface in the short electrodeposition time. The surface coverage increased with the number of electrodeposition cycles.

**5. Electrochemical synthesis of praseodymium oxides**

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chemical vapor deposition (CVD), and spin coating [92, 94-99].

and sintered to produce Pr oxide.

Praseodymium oxide is a versatile and useful material, which is made up of a series of oxide compounds with the general formula PrnO2n-2 (n=4, 5, 6, 7, 8, 9, 10, 12) [84-86]. Pr2O3 has a hexagonal structure and belongs to the A-type rare earth structure, while, PrO2 and Pr6O11 have a fluorite structure (similar to CeO2) where the metals are eight-coordinate. The mixed valence states in praseodymium oxide compounds make them useful materials with several important applications. Two areas of interest for praseodymium oxide films include the use of this material as an ethanol sensor and as a catalyst. [87-90]. Tsang et al. found that Pr6O11 could detect ethanol in air and the optimal sensitivity (100%) was obtained at about 250 ˚C – 300 ˚C [87]. The Pr6O11 sensors gave a linear response to ethanol concentrations in the range of 200– 8000 ppm. Praseodymium oxide is a promising candidate to substitute silicon dioxide as a high-K dielectric, with a dielectric constant up to 10 times higher than SiO2 (dielectric constant is around 30) and very low leakage currents. Among the different compositions of praseody‐ mium oxide, Pr6O11 has the highest K value and Pr2O3 is a good dielectric [91-93]. Praseody‐ mium oxide films can be formed by several different methods including molecular beam epitaxy, pulsed-laser deposition (PLD), sputtering, electrocrystallization of molten salts,

However, the literature for the electrochemical deposition of praseodymium oxide is quite sparse. There are only a couple of references using electrodeposition and even in these the initial deposition component is praseodymium hydroxide, which is converted to Pr6O11 or PrO2 by sintering [100, 101]. In both instances, Pr(NO3)3 and H2O2 were used as electrolyte and

( ) ( ) () - -

where the electrochemical generation of hydroxide ions increases the pH at the electrode surface and causes formation of Pr hydroxide on the surface. After deposition, the film is dried

Shrestha et al. electrodeposited an ultrathin layer of praseodymium hydroxide on ITO by applying a cathodic sweeping voltage followed by thermal annealing at 500°C for 1 h [100]. The predominant phase of the annealed film was Pr6O11. The XRD pattern of the deposit was

( ) ( ) - - H O aq + 2e 2OH aq 2 2 ® (11)

2 2 2H O l + 2e 2OH aq + H g ® (13)

( ) () () () - -- - NO aq + H O l + 2e NO aq + 2OH aq 3 2 ® <sup>2</sup> (12)

( )( ) ( ) ( ) () ( ) <sup>2</sup> 6 11 <sup>2</sup> <sup>2</sup> <sup>3</sup> 2Pr OH s 2PrO s or Pr O s + 2H O l + H g ® (14)

the deposition occurred by the base generation method. The reactions are [101]:

Golden et al. studied using both potentiostatic and galvanostatic control for the cathodic deposition method to produce Pr oxide films. For the potentiostatic method, a potential from -1.0 V to -1.3 V was maintained while for the galvanostatic method, the deposition current density was set at 0.8 mA/cm2 . The electrolyte solution contained praseodymium nitrate, ammonium nitrate, and potassium chloride. A simplified Pourbaix diagram is shown in Figure 5. Within the aqueous region, at acidic pHs the Pr3+ ion is stable and at basic pHs the Pr hydroxide is stable. PrO2 is stable at pH above 8, but is at an overpotential above O2 evolution in the aqueous solution. Therefore, increasing the local pH at the electrode surface by gener‐ ating base is likely to produce Pr hydroxide nuclei for deposition.

**Figure 5.** Simplified Pourbaix diagram for the Pr system in aqueous solution.

The XRD pattern for the as deposited film on a stainless steel substrate is shown in Figure 6. Both potentiostatic and galvanostatic methods gave similar results. The XRD pattern matches the reflections for Pr(OH)3 with some Pr(NO3)3 contamination. Pr compounds have a light green color when the valence of Pr is +3, or dark brown/black color when the valence is +4. Since the film on the stainless steel substrate is a light green color, the valence state of Pr in the film is probably closer to +3. The newly deposited films were sintered at 600 o C, and the film color turned black. Figure 7 is the XRD pattern of a sintered electrodeposited film. The XRD pattern matches a random orientation of Pr6O11 face-centered cubic (fcc) structure (PDF #42-1121). Table 1 shows a comparison of the experimental data to the ICDD database for Pr6O11.

**Figure 6.** XRD pattern of the electrodeposited film on a stainless steel substrate using galvanostatic method with ap‐ plied current density of 0.8 mA/cm2 , electrolyte solution composed of 0.1 M NH4NO3 and 0.1 M Pr(NO3)3, pH=3.48).

The crystallite size and strain of the praseodymium oxide films were also calculated from the XRD data by examining the peak position and peak broadening of the reflections. The broadening of the peaks arises from three areas: instrumental broadening, crystallite size, and lattice strain. Contributions from these three factors can be determined by the Williamson– Hall method when at least 3 or 4 peaks exist in a XRD pattern [102]. Separation of the peak broadening due to crystallite size and lattice strain can be obtained by plotting Brcosθ versus sinθ, where the crystallite size is calculated from the y-intercept and strain from the slope. The calculated crystallite size ranged from 20 to 40 nm for the electrodeposited Pr oxide films.

The oxidation state of Pr in sintered praseodymium oxide films can be studied using XPS. The core-level binding energies for praseodymium oxide and the exact position of each peak from XPS are listed into Table 2. Figure 8 is the high resolution XPS spectra of the Pr 3d core level showing the 3d5/2 and 3d3/2 signals separated by 20.2 eV, with the peak positions at 933.3 eV and 953.5 eV, respectively. A strong shoulder can be seen on the lower BE sides of the Pr 3d5/2 and Pr 3d3/2 peaks, with comparable intensity and a distance between around 4.5 and 4.3 eV,

the reflections for Pr(OH)3 with some Pr(NO3)3 contamination. Pr compounds have a light green color when the valence of Pr is +3, or dark brown/black color when the valence is +4. Since the film on the stainless steel substrate is a light green color, the valence state of Pr in the

color turned black. Figure 7 is the XRD pattern of a sintered electrodeposited film. The XRD pattern matches a random orientation of Pr6O11 face-centered cubic (fcc) structure (PDF #42-1121). Table 1 shows a comparison of the experimental data to the ICDD database for

**Figure 6.** XRD pattern of the electrodeposited film on a stainless steel substrate using galvanostatic method with ap‐

The crystallite size and strain of the praseodymium oxide films were also calculated from the XRD data by examining the peak position and peak broadening of the reflections. The broadening of the peaks arises from three areas: instrumental broadening, crystallite size, and lattice strain. Contributions from these three factors can be determined by the Williamson– Hall method when at least 3 or 4 peaks exist in a XRD pattern [102]. Separation of the peak broadening due to crystallite size and lattice strain can be obtained by plotting Brcosθ versus sinθ, where the crystallite size is calculated from the y-intercept and strain from the slope. The calculated crystallite size ranged from 20 to 40 nm for the electrodeposited Pr oxide films. The oxidation state of Pr in sintered praseodymium oxide films can be studied using XPS. The core-level binding energies for praseodymium oxide and the exact position of each peak from XPS are listed into Table 2. Figure 8 is the high resolution XPS spectra of the Pr 3d core level showing the 3d5/2 and 3d3/2 signals separated by 20.2 eV, with the peak positions at 933.3 eV and 953.5 eV, respectively. A strong shoulder can be seen on the lower BE sides of the Pr 3d5/2 and Pr 3d3/2 peaks, with comparable intensity and a distance between around 4.5 and 4.3 eV,

, electrolyte solution composed of 0.1 M NH4NO3 and 0.1 M Pr(NO3)3, pH=3.48).

C, and the film

film is probably closer to +3. The newly deposited films were sintered at 600 o

Pr6O11.

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plied current density of 0.8 mA/cm2

**Figure 7.** XRD pattern of praseodymium oxide film obtained by sintering the deposited film at 600 °C for 1 h.


**Table 1.** Comparison of XRD data of the sintered electrodeposited film and PDF card. (Golden et al.).

this is consistent with results in literature for praseodymium [103, 104]. The XPS spectra of polycrystalline powders of Pr2O3, PrO2, and Pr6O11 all have an energy separation of the Pr 3d3/2 and Pr 3d5/2 core levels in the range of 17.5–23.0 eV. In addition, the Pr 3d5/2 core level spectra of these praseodymium oxides exhibit a shoulder at 4–4.5 eV on the lower binding energy side of the metal main peak. The relative intensities of the Pr 3d main peak and satellite peak vary with the Pr(III)/Pr(IV) ratio [105]. Specifically, the main peak intensity increases as the relative content of Pr(III) increases whereas the satellite peak intensity decreases. Therefore, the variation of relative intensities of the main and satellite peaks can be used to examine the change in Pr valence. The Pr 3d XPS spectra of Pr2O3 and PrO2 are almost identical in most respects, such as the shape of the peak and the overall fine structure. The main and satellite peaks for Pr6O11 are situated between PrO2 and Pr2O3. The Pr 3d5/2 binding energy is 935.0 eV for Pr(IV) and 932.9 eV for Pr(III). Our experimental results are 933.3 eV for Pr 3d5/2, which is an indication of a Pr +3/+4 mixture such as in Pr6O11. By curve fitting the Pr 3d XPS spectrum and determining the areas of the fitted peaks, a non-stoichiometric ratio is determined as PrO1.80, an indication of the mixed valence state of Pr(III) and Pr(IV) (Table 3).


**Table 2.** Core-level binding energies for sintered electrodeposited praseodymium oxide measured by XPS.

**Figure 8.** High-resolution XPS spectra of Pr 3d core level showing the spin-orbit splitting of the 3d level.

The pH effect was also studied as one of the parameters for the electrodeposition of the Pr oxide films. When the pH value of the electrolyte solution was below 7, films could be electrodeposited onto the substrate; however, powders only formed in the solution when the pH was above 7, due to bulk formation of Pr(OH)3. XRD of the electrodeposited films showed that intensities of the reflections belonging to Pr(OH)3, such as (110), (101), (103), (321), and (220), increased with increasing pH (up to pH 7) of the electrolyte, and the intensities of the peaks due to Pr(NO3)3 decreased with increasing pH.

The morphology of the praseodymium oxide film is interesting and is shown in Figure 9. Before sintering, the films appeared smooth and continuous, but after sintering, cracks appeared in


the films (similar to CeO2 films) due to shrinkage and mismatch between the film and substrate. However, at higher magnification in the SEM images, the Pr oxide is distributed as platelets across the surface and the film has a high surface area. This may be beneficial in catalysis.

**Table 3.** Peak list of Pr 3d and calculation of Pr valence.

respects, such as the shape of the peak and the overall fine structure. The main and satellite peaks for Pr6O11 are situated between PrO2 and Pr2O3. The Pr 3d5/2 binding energy is 935.0 eV for Pr(IV) and 932.9 eV for Pr(III). Our experimental results are 933.3 eV for Pr 3d5/2, which is an indication of a Pr +3/+4 mixture such as in Pr6O11. By curve fitting the Pr 3d XPS spectrum and determining the areas of the fitted peaks, a non-stoichiometric ratio is determined as

PrO1.80, an indication of the mixed valence state of Pr(III) and Pr(IV) (Table 3).

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**Oxide** Pr 5p Pr 5s Pr 4d Pr 4p Pr 4s Pr 3d5/2 Pr 3d3/2 O1s **Peak position (eV)** 20.08 36.8 116.2 217.2 304.7 933.3 954.7 530.3

**Table 2.** Core-level binding energies for sintered electrodeposited praseodymium oxide measured by XPS.

**Figure 8.** High-resolution XPS spectra of Pr 3d core level showing the spin-orbit splitting of the 3d level.

peaks due to Pr(NO3)3 decreased with increasing pH.

The pH effect was also studied as one of the parameters for the electrodeposition of the Pr oxide films. When the pH value of the electrolyte solution was below 7, films could be electrodeposited onto the substrate; however, powders only formed in the solution when the pH was above 7, due to bulk formation of Pr(OH)3. XRD of the electrodeposited films showed that intensities of the reflections belonging to Pr(OH)3, such as (110), (101), (103), (321), and (220), increased with increasing pH (up to pH 7) of the electrolyte, and the intensities of the

The morphology of the praseodymium oxide film is interesting and is shown in Figure 9. Before sintering, the films appeared smooth and continuous, but after sintering, cracks appeared in

**Figure 9.** SEM images of Pr6O11 film obtained by cathodic electrodeposition on stainless steel in an electrolyte system composed of 0.1M Pr(NO3)3 and 0.1M NH4NO3 solution, j = 0.8 mA/cm2 , T= 25 o C, and after sintering at 600 o C for 1 h; (left) magnification 1000. (T. Golden); (right) magnification 10000.

In conclusion, Pr6O11 films were successfully obtained on stainless steel substrates (Gold‐ en et al.) using the base generation method and then sintered at 600 o C. X-ray diffraction showed the films matched the Pr6O11 fluorite structure and the crystallite size was calculat‐ ed as 20 to 40 nm. Scanning electron microscopy was utilized to study the surface texture and microstructure of deposits. As-deposited films had uniform morphology but sinter‐ ing caused cracking of the films. SEM showed interesting surface texture and platelet structure for the deposits. The oxidation state of Pr oxide was determined by XPS and revealed that the praseodymium oxide was non-stoichiometric with the oxidation state of Pr between +3 and +4.
