3. Thermodynamic Analysis
Electrochemical deposition (or electrodeposition) is a simple and versatile chemical solution deposition method based on redox reactions happening in an electrochemical cell
[31][32]. An electrochemical cell is made of a vessel containing an electrolytic bath with a working electrode, a counter electrode, and a reference electrode. The reference electrode is needed because it provides the stable ground voltage needed to accurately measure the electrochemical potentials by providing an isolated and stable chemical reaction that produces a known voltage. The reference electrode is placed very close to the working electrode in order to determine its potential against the stable reference electrode. The fundamental reference is the standard hydrogen electrode (SHE), for which the equilibrium between H
+ and H
2 gas is defined as zero. Standard electrode potentials (equilibrium potentials of an electrode reaction),
E0, are expressed relative to SHE and can be found tabulated. In practice, other, more convenient reference electrodes are in use, for example, silver chloride (Ag/AgCl) or saturated calomel (Hg
2Cl
2). The chemical reactions at the electrode surfaces involve electron and ion transfer and are ideally controlled by the electrode potential. The voltage and current flowing between the electrodes control the chemical processes, and the working electrode usually acts as a substrate.
In principle, to define the optimal conditions for electrodeposition, the determination and analysis of the potential–pH diagram, also known as the Pourbaix diagram, is useful. Below, the thermodynamic analysis, as presented in
[33], is reported. Symbols are explicated in
Table 1. now consider a hypothetical system where the redox reactions occurring in the solution are expressed generically as
When pressure and temperature are constant, the molar Gibbs energy change for the reaction can be written as
Figure 2. General
E–pH diagram for the Me–Se–H
2O system. Adapted from
[33].
The thermodynamic data useful for the derivation of the
E–pH diagram for FeSe can be found in
Table 2. Once the temperature and concentration of species in solution are defined, a diagram such as that in
Figure 2 can be derived. From this diagram, it can be infered that the stability of MeSe is defined by lines 4-3-2-1-5-6-7-8, and this area can extend to a wide range of potential and pH. Moreover, the redox potential for the formation of MeSe is higher than that of the reduction of Me
2+ to pure metal, which means that Me
2+ metal ions are more easily deposited as MeSe than pure metal. This behavior is due to the release of Gibbs free energy during the formation of metal selenides, which will result in a positive shift in deposition potential for MeSe. This process is known as the Kroger mechanism or induced underpotential deposition, and it is often exploited in the electrodeposition of metal selenides
[34].
Table 2. Thermodynamic data of substances of interest for the Fe–Se–H
2O system (from
[33]).
Substance |
(kJ/mol) |
Substance |
(kJ/mol) |
(kJ/mol) |
|
|
|
FeSe |
−87.533 |
Fe |
−8.31 |
Fe2+ |
−35.585 |
Fe3+ |
64.332 |
Se |
−12.592 |
HSe− |
−14.035 |
H2Se (g) |
−35.950 |
SeO32− |
−525.577 |
H2SeO3 |
−569.233 |
HSeO3− |
−560.894 |
Potential–pH formulas for FeSe0.96 |
Fe2+ + 2e− = Fe |
E = −0.5985 |
Fe2+ + 0.96Se + 2e− = FeSe0.96 |
E = −0.249 |
FeSe0.96 + 1.92H+ + 1.92e− = Fe + 0.96H2Se |
E = −0.278 − 0.059pH |
FeSe0.96 + 0.96H+ + 1.92e− = Fe + 0.96HSe− |
E = −0.424 − 0.030pH |
Fe2+ + 0.96SeO32− +5.76H+ + 5.84e− = FeSe0.96 + 2.88H2O |
E = 0.430 − 0.058pH |
Notwithstanding the availability of thermodynamic data in the literature, it becomes extremely complicated to adapt the theoretical model to real experiments due to a larger number of variables to consider, mainly the presence of other species in the solution, such as the electrolyte and the metal and selenium counter ions, that might be involved in a series of collateral reactions or the presence of the working electrode that might not behave just as an inert surface. This leads to the necessity to approach the system in a more practical way, which confines thermodynamic analysis to a precious tool to unravel the complexity of experimental data rather than use it as a predictive method for experimental design.
4. Electrodeposition of FeSe
Although electrodeposition is restricted to a number of elementary metals, it has attracted great interest in the metallurgy of alloys. The simultaneous deposition of more than one elemental precursor in the same electrolyte in order to create an alloy or a chemical compound is called co-deposition. It is a widely employed technique for the fabrication of thin films
[35] of compounds such as sulfides, tellurides, and selenides (PbSe
[36], NiSe
[37], ZnSe
[38], or CdSe
[39]).
The chemical composition of co-deposited films can be controlled via the chemical composition of the solution and the galvanic potential between the electrodes. The electrodeposition of alloys relies on the similar reduction potential of the components. The co-deposition of a metal and a non-metal, instead, is, in principle, a more challenging issue because of the largely different standard (equilibrium) potentials of the metal and non-metal atoms. For Fe and Se, the potential difference
E0 = E
metal − E
nonmetal is 1.225 V. Therefore, preferential plating of the more noble element (Se in this case) is expected to occur, and its more positive potential inhibits FeSe alloy formation. Fortunately, one can overcome the difference in potential
[40]. A shift in the deposition potentials of the constituents is achieved by changing the concentration or activity of the ions in the solution, for example, by using complexants or additives. In fact, co-deposition has been a successful methodology for forming II ± VI compounds. Stoichiometry is maintained by having the more noble element as the limiting reagent and choosing the potential where the less noble element will underpotentially deposit only on the more noble element
[34].
The electrodeposition of metal selenides is generally performed under acidic conditions at pH 1–3. This pH range is defined by the stability of the chemical species involved, and this information can be derived from Pourbaix diagrams (
Figure 3 [41]) that describe the stability zones for an element as a function of pH and potential. Comparing Pourbaix diagrams for a compound’s constituent elements gives an indication of the probability of forming a stable compound electrochemically and the potential and pH that might be used. The overlap of the conditions where both elements exist in their elemental state is a good indication of where to start. In more detail, in the FeSe case, the pH is limited by the hydrolysis of FeSe and/or hydrogen evolution at low pH values, not to mention the necessity to reduce Se(II) or Se(IV), a process that consumes large amounts of H
+ and that would be hindered at higher pH
[33]. The increase in pH of the deposition solution also results in the acceleration of Fe
2+ oxidation and further precipitation of Fe(OH)
3, as well as slowing down the electroreduction of selenium species. To complicate the matter further, no overlap of the stability zone is found for Fe and Se
[42]; therefore, the formation of FeSe via solid-state reaction seems unlikely; instead, it might expect a reaction of the more noble element (Se) that, after reduction, reacts with the less noble metal ions in the solution and precipitates on the electrode. However, the mechanism of cathodic electrodeposition of selenides from Fe and Se precursors in solution is not well understood. A hypothesis involves a direct reaction between Fe
2+ and Se
2− in the solution bulk, followed by FeSe precipitation onto the electrode surface
[43][44]. The redox reaction for the formation of FeSe is
Figure 3. Pourbaix diagram for Fe (
left) and Se (
right) at 25 °C and 1 atm. Adapted from
[41].
The insufficient understanding of the electrochemical reactions occurring in the deposition solution, along with conflicting data on the reaction mechanism, hinders the optimization of electrodeposition conditions. An educated guess on the reactions occurring at the working electrode requires parallelism with the closest iron chalcogenide, FeS
2, whose electrodeposition was suggested to happen in two steps: first the reduction and adsorption of sulfur on the electrode, and then the reaction with Fe
2+ that reaches the interface through the double layer and precipitates ad FeS
x on the electrode
[45]. Therefore, it can be hypothesized as a similar mechanism for FeSe, such as:
The ideal voltage necessary for the desired reaction of FeSe deposition to occur is often experimentally identified via cyclic voltammetry (CV). In CV, the current (or current density J) is measured as a function of the applied potential, which is swept in the range of interest in a direct and reverse scan at constant speed. The voltammogram will then show sudden increases in current in correspondence with redox reactions occurring in the electrolyte. The identification of these reactions is not always straightforward, but in general, it is possible to discern the required potential for deposition of the desired phase. To make the identification process easier, the study of the CV of the single reactants in the conditions selected for deposition can be helpful, as can a comparison with theoretical data available in the literature. It should be considered, though, that this value may change when experimental conditions are changed. Therefore, direct comparison between the available data is tricky and should be approached accordingly. For example, some experimental conditions influencing the electrodeposition process and potential are:
-
The electrolyte: i.e., the precursors and the solution. Different precursors give different results even if the element to be deposited is in the same oxidation state. Solubility, solvation effects, and other phenomena indirectly influence the reactions in the solution/on the electrode.
-
Electrode materials, working electrode potential (with respect to the reference electrode); even if the working electrode (substrate) is non-reacting, different materials will give different working potentials and, therefore, different results.
-
The pH of the solution influences the stability of the electrodes, the precursor salts, the conductivity of the solution, et cetera.
-
Additives/complexing agents: used to increase the solubility of precursors, they influence the adsorption of metal ions at the substrate surface, film nucleation, and growth.
-
The operation temperature (usually between room temperature and T < 100 °C)