Electrodeposition of Iron Selenide: A Review

Abstract

In recent years, metal selenide materials have attracted attention due to their wide application prospects. In this family of materials, FeSe is particularly studied since it is both a semiconductor used in solar cells and a superconductor with a critical transition temperature, T_c, of 8 K. For any envisaged application, the possibility of preparing large-area FeSe thin films at low cost is extremely appealing, and one possible technique suitable for this purpose is electrodeposition. Several groups have reported successful electrodeposition of FeSe, but the investigated systems are different in many aspects, and the results are difficult to compare. The aim of this review is to collect the available information on FeSe electrodeposition and the thermodynamic laws controlling this process; to catalog the literature pointing out the differences in the experimental procedure and how they influence the results; and to draw general conclusions, if any, on this topic.

1. Introduction

Transition metal chalcogenides have been widely studied because of the peculiar electronic properties that make them interesting for a large variety of applications, for example, as anode materials for sodium-ion batteries, cathode materials for lithium-air batteries [1,2], photo-absorbers for photovoltaics [3], and non-linear optical materials for bioimaging [4]. More recently, their relevant role as electrocatalysts in oxygen evolution reactions (OERs), hydrogen evolution reactions (HERs), and overall water splitting was demonstrated [5,6,7,8]. In the case of FeSe, much attention was raised because of its potential application as a solar cell absorber, and even more after the discovery of superconductivity in this family of compounds. The search for new superconducting materials fuelled research on this topic, and much effort is still being spent on the fabrication of FeSe thin films for a variety of applications, such as the design of a conductor [9,10,11,12,13] or its use as a low-resistivity coating in cavities for axions detection [14]. Several techniques are successfully employed for the growth of FeSe films, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and chemical vapor deposition (CVD). A detailed review of the fabrication methods and applications for FeSe can be found in [15]. These methods have been essential for the study of the properties of the material but are not suitable for the growth of larger-scale samples, for whatever application, or for the foreseen industrial scale-up. Electrodeposition has several advantages over the aforementioned thin-film deposition techniques. As an electric (and not a thermal) process, it can be carried out at room temperature (or close) and at ambient pressure. It can provide large, thick, and uniform films of complex, curved shapes, and it can easily be scaled up due to the inexpensive equipment and standard laboratory conditions required for the process. On the other hand, the crystalline quality and the microstructure of the deposit are not comparable to those obtained via other techniques such as MBE, PLD, or CVD. However, its potential led several authors to investigate the possibility of growing FeSe via electrodeposition, with the aim of applying it to large-scale applications such as dye-sensitized solar cells [16] or superconducting tapes [17,18,19]. This review wants to summarize and analyze the literature available on this topic so as to provide anyone approaching this technique with a solid background and the essential tools for developing their own experiments. The first part of this review will be dedicated to recollecting basic knowledge of the thermodynamic laws behind electrodeposition and the practical technique, then a general discussion of the electrodeposition of metal selenides will be given. The available literature on FeSe electrodeposition will be summarized, and eventually, conclusions on the presented results and issues will be drawn.

2. FeSe Structure

Fe–Se is a complex system that can exist in many different phases corresponding to different structures and stoichiometries, as reported in the phase diagram in [20]. However, FeSe mainly exhibits two crystalline forms: tetragonal PbO-like phase (P4/nmm space group, b in the phase diagram in Figure 1 (Upper panel)) and hexagonal NiAs-type phase (P6₃/mmc space group, g in the phase diagram in Figure 1 (Upper panel)) [21]. Both structures show alternate Fe/Se layers, but in the tetragonal phase, tetrahedral coordination is found between Fe and Se, whereas in the hexagonal phase, Fe coordinates Se octahedrally. Distinct bond regimes can be found in both structures: within the Fe–Se layer, the bonding regime is of the mixed metallic-covalent type, with Fe–Fe metallic bonds and Fe–Se covalent bonds. Instead, in the c-axis direction, the main interaction is through van der Waals forces [22,23]. The X-ray patterns and structures of these two phases are shown in Figure 1. The phase stability of these structures is limited to a very narrow region of the Fe–Se phase diagram [20], and the interconversion of one phase into the other occurs around 300 °C and/or high pressure. The properties of this material are strongly dependent on its crystalline structure and microstructure; for example, the hexagonal FeSe phase (a = 3.519 Å; c = 5.684 Å) is a ferrimagnet up to approximately 400 K, whereas tetragonal FeSe (a = 3.689 Å; c = 5.854 Å) undergoes a structural transition to an orthorhombic FeSe phase (space group: Cmma) below 90–100 K and shows a superconducting transition temperature at Tc ≈ 8 K, as reported by [24,25]. Superconductivity is also strongly dependent on the lattice parameter, on the Fe–Se distance, and, consequently, on every factor influencing those parameters, such as temperature, strain, intercalation, pressure, and atom substitutions [26]. On the other hand, crystallite size also plays a major role in tuning electrical properties such as band gap, absorption coefficient, absorbance/transmittance, et cetera [21,27,28,29,30]. These dependencies need to be taken into account when planning the use and preparation of FeSe for a specific application.

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 H₂ 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 (Hg₂Cl₂). 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. List of symbols.

a, b, c, n, m	Stoichiometric Number
${\mathrm{G}}_{298}^{*}$	Standard Gibbs free energy at 298 K	kJ mol−1
${∆}_{\mathrm{r}}{\mathrm{G}}_{\mathrm{m}}^{*}$	Standard molar reactive Gibbs free energy	kJ mol−1
${∆}_{\mathrm{r}}{\mathrm{G}}_{\mathrm{m}}$	Molar reactive Gibbs free energy	kJ mol−1
$\mathrm{R}$	Molar gas constant	8.314 J K−1 mol−1 at 298 K
$\mathrm{T}$	Temperature	K
$a_X^x$	Activity of substance X
F	Faraday’s constant	96,485 C mol−1
${\mathrm{E}}^0$	Standard electrode potential	V
E	Electrode Potential	V

. Let us now consider a hypothetical system where the redox reactions occurring in the solution are expressed generically as

$$a\mathrm{A}+m{\mathrm{H}}^{+}+\mathrm{n}{\mathrm{e}}^{-}=b\mathrm{B}+c{\mathrm{H}}_2\mathrm{O}.$$

When pressure and temperature are constant, the molar Gibbs energy change for the reaction can be written as

$${∆}_{\mathrm{r}}{G}_{\mathrm{m}}={∆}_{\mathrm{r}}{G}_{\mathrm{m}}^{*}+RT\ln \left(\frac{a_{\mathrm{B}}^{\mathrm{b}}{a}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{c}}}{a_{\mathrm{A}}^{\mathrm{a}}{a}_{{\mathrm{H}}^{+}}^{\mathrm{m}}}\right)$$

If $a_{{\mathrm{H}}_2\mathrm{O}}$ = 1, −lga_H+ = pH, combining with ${∆}_{\mathrm{r}}{G}_{\mathrm{m}}=-\mathrm{n}FE$ and ${∆}_{\mathrm{r}}{G}_{\mathrm{m}}^{*}=-\mathrm{n}F{E}^{*}$, we obtain

$$\mathrm{n}FE=\mathrm{n}F{E}^{*}-2.303RT\mathrm{pH}m+2.303RT\lg \left(\frac{a_{\mathrm{A}}^{\mathrm{a}}}{a_{\mathrm{B}}^{\mathrm{b}}}\right)$$

That can be further simplified for reactions not involving electrons (n = 0) in

$$\mathrm{pH}=\frac{1}{m}\lg \left(\frac{a_{\mathrm{A}}^{\mathrm{a}}}{a_{\mathrm{B}}^{\mathrm{b}}}\right)$$

or, for reaction not involving H⁺/OH⁻ (m = 0), in

$$\mathrm{E}=\frac{-{∆}_{\mathrm{r}}{G}_{\mathrm{m}}^{*}}{\mathrm{n}F}-\frac{2.303RT}{\mathrm{n}F}\lg \left(\frac{a_{\mathrm{A}}^{\mathrm{a}}}{a_{\mathrm{B}}^{\mathrm{b}}}\right)$$

If both electrons and H⁺/OH⁻ are involved, we have

$$\mathrm{E}=\frac{-{∆}_{\mathrm{r}}{G}_{\mathrm{m}}^{*}}{\mathrm{n}F}-\frac{2.303RTm}{\mathrm{n}F}\mathrm{pH}+\frac{2.303RT}{\mathrm{n}F}\lg \left(\frac{a_{\mathrm{A}}^{\mathrm{a}}}{a_{\mathrm{B}}^{\mathrm{b}}}\right)$$

From these general equations and thermodynamic data available in the literature, it is possible to draw the potential–pH diagram for the general metal selenide system Me–Se–H₂O (see Figure 2), where, for every reaction, the $∆{\mathrm{G}}_{298}^{*}$ can be calculated by $∆{\mathrm{G}}_{298}^{*}=\sum ∆{\mathrm{G}}_{298}^{*}\left(\mathrm{products}\right)-\sum ∆{\mathrm{G}}_{298}^{*}\left(\mathrm{reactants}\right)$. The reactions of interest are:

Me²⁺ + Se + 2e⁻ = MeSe

(1)

MeSe + 2H⁺ = Me²⁺ + H₂Se

(2)

MeSe + 2H⁺ + 2e⁻ = Me + H₂Se

(3)

MeSe + H⁺ + 2e⁻ = Me + HSe

(4)

Me²⁺ + H₂SeO₃ + 4H⁺ + 6e⁻ = MeSe + 3H₂O

(5)

Me²⁺ + HSeO₃⁻ + 5H⁺ + 6e⁻ = MeSe + 3H₂O

(6)

Me²⁺ + SeO₃²⁻ + 6H⁺ + 6e⁻ = MeSe + 3H₂O

(7)

Me(OH)₂ + SeO₃²⁻ + 8H⁺ + 6e⁻ = MeSe + 5H₂O

(8)

Me²⁺ + 2e⁻ = Me

(9)

The thermodynamic data useful for the derivation of the E–pH diagram for FeSe can be found in

Table 2. Thermodynamic data of substances of interest for the Fe–Se–H₂O system (from [33]).

Substance	$G_{298}^{*}$ (kJ/mol)	Substance	$G_{298}^{*}$ (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

. 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, we infer 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 Me²⁺ to pure metal, which means that Me²⁺ 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].

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 Fe²⁺ 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, we 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 Fe²⁺ and Se²⁻ in the solution bulk, followed by FeSe precipitation onto the electrode surface [43,44]. The redox reaction for the formation of FeSe is

Fe²⁺ + Se + 2e⁻ → FeSe

where Se is first reduced to Se²⁻ and then reacts with iron(II) ions. Another interpretation sees the reduction process and solid-state reactions in both Fe and Se after plating.

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 Fe²⁺ that reaches the interface through the double layer and precipitates ad FeS_x on the electrode [45]. Therefore, we can hypothesize a similar mechanism for FeSe, such as:

Se + 2e⁻ → (Se²⁻)_ads

(Se²⁻)_ads → FeSe

While on the counter electrode, oxygen evolution is observed (supposing an aqueous medium). However, due to the unstable precursor used, the mechanism, in practice, can be far more complicated than the previous hypothesis. Iron(II) salts, which are preferred since the Fe oxidation state is the one required for FeSe formation, are highly unstable [46] and often oxidized during storage or inside the precursor solution if not properly degassed. Moreover, precipitation of species after oxidation occurs, which means not having complete control of the real amount of Fe(II) in the solution. Fe(III) salts are more reliable from this point of view and were also employed by some groups [47], but the electrochemistry of the system is complicated by the necessity of controlling the reduction of Fe(III). On the other hand, Se shows a very complicated electrochemistry with the existence of different solution species and their reduction products in acid media [48]. Depending on the solution pH and temperature, the nature of the dissolved Se species varies. When the Se precursor is dissolved in water, either SeO₂, H₂SeO_3, or the respective salt Na₂SeO₃, it can participate in equilibrium reactions such as

H₂SeO₃ ⇌ HSeO₃⁻ + H⁺ (pK = 2.72)

HSeO₃⁻ ⇌ SeO₃⁻ + H⁺ (pK = 8.32)

If the conditions are met, the species in solution can be part of several redox reactions, such as

SeO₄²⁻ + 4H⁺ + 2e⁻ ⇌ H₂SeO₃ + H₂O 1.150 V

SeO₄²⁻ + H₂O + 2e⁻ ⇌ SeO₃²⁻ + 2OH⁻ 0.050 V

H₂SeO₃ + 4H⁺ + 4e⁻ ⇌ Se + 3H₂O 0.740 V

SeO₃²⁻ + 3H₂O + 4e⁻ ⇌ Se + 6OH⁻ −0.366 V

HSeO₃⁻ + 4e⁻ + 5H⁺ ⇌ Se + 3H₂O 0.778 V

H₂SeO₃ + 6H⁺ + 6e⁻ ⇌ H₂Se + 3H₂O 0.360 V

SeO₃²⁻ + 6H⁺ + 6e⁻ ⇌ Se²⁻ + 3H₂O 0.276 V

HSeO₃⁻ + 6e⁻ + 7H⁺ ⇌ H₂Se + 3H₂O 0.386 V

Se + 2H⁺ + 2e⁻ ⇌ H₂Se⁻ −0.369 V

Se + 2e² ⇌ Se²⁻ −0.920 V

Moreover, because of the multiple oxidation states, Se can engage in numerous self-exchange reactions: Se(VI)/Se(IV), Se(IV)/Se(0), Se(IV)/Se(-II), Se(0)/Se(-II), et cetera. The presence of so many species at the same time makes the identification of the FeSe deposition potential and the selective promotion of this reaction a complicated matter.

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)

In the following paragraphs, the available literature is summarized. Taking into account the mentioned difficulties in comparing results, the authors chose, for clarity, to categorize the literature according to the precursor salts used, focusing on their characterization in solution and then summarizing the experimental results. Relevant parameters of the mentioned studies are reported in Table 2.

5. Summary of the Literature

5.1. FeCl₃ + SeO₂ + TEA

The first example of electrodeposition of FeSe was reported in 2006 [47]. In this work, the aqueous electrolytic bath was prepared from FeCl₃ and SeO₂ plus triethylamine (TEA) as a complexing agent. The deposition mechanism and film growth were investigated by cyclic voltammetry and chronoamperometry.

5.1.1. Characterization of Precursors

The electrochemical reactions taking place were studied via CV. In order to find the suitable reduction potentials of Fe, Se, and FeSe formation, the precursors were first analyzed separately and subsequently in combination. Figure 4a–c shows the cyclic voltammograms recorded on THE stainless steel substrate from electrolytic solutions containing 0.1 M FeCl₃ + TEA, 0.1 M SeO₂, and 0.1 M FeCl₃ + TEA + 0.1 M SeO₂. As regards the Fe precursor in Figure 1 (Upper panel), the cathodic current increases gradually up to −0.7 V vs. SCE and then rapidly up to −0.85 V vs. SCE. Therefore, this value is considered to be the potential of Fe³⁺ reduction to elemental Fe. By further lowering the potential, a reduction in the solvent is observed. In the reverse scan, an oxidation peak around −0.25 V vs. SCE is thought to be related to the oxidation of surface species. As regards the SeO₂ solution, the formation of elemental red Se is observed around the electrode at −0.53 V vs. SCE. When the complete solution is analyzed, a rapid increase in current is recorded after −0.35 V vs. SCE. In the reverse scan, a crossover between the current branches occurs at −1.05 V vs. SCE, which is a signal of a nucleation and growth process. This potential was selected for the electrodeposition of FeSe. The complete precursor solution was also analyzed via chronoamperometry, which is a technique used to investigate the nucleation process of the electrodeposited films. The authors show that the film deposition occurs through an intermediate growth mechanism where, during the first couple of minutes, there is a progressive shift from progressive nucleation to instantaneous nucleation.

5.1.2. Characterization of the Samples

Once the optimal conditions for deposition were defined, the obtained samples of stainless steel and FTO glass were subjected to structural, morphological, compositional, and optical measurements to investigate their properties. The surface morphology shows a uniform, pinhole-free surface with evenly distributed spherical grains rich in iron. X-ray analysis confirms the presence of monoclinic Fe₃Se₄, with lattice parameters in good agreement with the literature (JCPDS data file nr. 71-2250) (Figure 5). In addition, a study of thickness vs. deposition time is presented. It shows that thickness reaches a maximum value after 15 min of deposition and then starts decreasing. This behavior was explained by an increase in the rate of dissolution after attaining maximum thickness. An optical absorption study of samples deposited on FTO revealed a direct transition bandgap of 1.23 eV, slightly higher than what is reported elsewhere, probably due to deviation from stoichiometry, point defects, and dislocations in the material.

5.2. FeSO₄ + SeO₂

A similar system, where FeSO₄ was used instead of FeCl₃, was studied by Thanikaikarasan et al. [49,50,51], Chen et al. [43], and Demura et al. [17]. Slight differences in the experimental conditions strongly influence the reported results. Also, in these cases, the first step is cyclic voltammetry of the precursor in the electrolytic bath and the complete solution of Fe + Se to study the precursors’ reactivity and assess the optimal deposition potential.

5.2.1. Characterization of Precursors

In [49,50], the precursors were kept in aqueous solutions at pH = 3, and the two precursors were studied separately before the complete solution (Figure 6a–c). CV on ITO of the solution containing only FeSO₄ shows a reduction peak attributed to the reduction of Fe²⁺ at −0.9 V vs. SCE and the oxidation of surface species as in [47] around 0.3 V vs. SCE in the reverse scan. In the SeO₂ solutions, the reduction of Se⁴⁺ to Se occurs at −0.54 V vs. SCE. When the complete solution of Fe + Se (with Fe:Se = 10:1) is analyzed, a reduction wave to −0.725 V vs. SCE, followed by a second cathodic wave at −1.1 V vs. SCE, is recorded. This is attributed to the formation of the desired FeSe on the electrode. The whole CV profile is in accordance with what was previously observed in [47]. This value of −1.1 vs. SCE is considered the optimal value for FeSe deposition. The same potential value was obtained when the same study was performed with another working electrode, SnO₂ [50]. In [43] with CV, they identify the reduction peaks of Fe²⁺ to Fe and of Se to Se²⁻ at −0.64 and −0.87 V vs. Ag/AgCl, respectively. The potential for electrodeposition of FeSe was instead determined empirically by comparing the results of XRD and SEM analysis, as shown in Section 5.2.4.

5.2.2. Effect of Bath Temperature

Further knowledge is gained through the study of the temperature dependence of the structural properties of the samples deposited on ITO. In more detail, the deposition was performed at 3, 50, 70, and 90 °C. Higher temperatures mean higher solubility of the precursors, higher ion mobility, and a lower viscosity of the solution, which results in thicker films. However, the film’s crystalline quality increases only up to 70°. At temperatures above this limit, higher current densities and hydrogen evolution cause the formation of rougher deposits with poor adhesion. The same behavior vs. temperature was observed in [50] on samples deposited on SnO₂.

5.2.3. Characterization of the Samples

As regards FeSe deposited on ITO, from the analysis of the X-ray diffractogram, polycrystalline hexagonal FeSe is found. Morphological analysis of the deposited films shows that the surface changes from coarse-like in the films deposited at low temperatures to a uniform distribution of spherical grains at higher temperatures. This is due to the increase in grain nucleation with respect to grain growth at 70 °C. Also, EDX analysis confirms 70 °C as the optimal temperature for growth, revealing a Fe:Se ratio close to 50:50 with respect to 30 °C or 90 °C, in which the ratio is closer to 60:40. Structural and microstructural analysis of these samples is taken further in a second paper by Thanikaikarasan et al. on this topic [51]. From X-ray diffraction data, the dependence of crystallite size, dislocation density, stacking fault probability, and microstrain is evaluated as a function of bath temperature, deposition potential, and pH. Again, a T of 70 °C, V = −1.1 vs. SCE, and a pH value of 3 are confirmed to be the optimal conditions for FeSe growth. From the analysis of the absorption spectrum, a direct bandgap of 1.24 eV is extrapolated.

Hexagonal FeSe is also obtained on SnO₂, with a maximum degree of crystallinity for the sample deposited at −1.1 V vs. SCE, which also corresponds to a Fe:Se ratio of 51.55:48.45. Magnetic properties were investigated via vibrating sample magnetometry, and the values of coercitivity, saturation magnetization, and retentivity were extrapolated from the hysteresis loop. They were found to be 255 Oe, 39 emu/cc, and 330 emu/cc, respectively. Furthermore, reflectance measurement led to an estimate of the optical bandgap that was found to be in the range of 1.19–1.21.

5.2.4. Effect of Deposition Potential

In [50], a study of the effect of potential on the structural properties of FeSe deposited on SnO₂ is provided (Figure 7a). They prepared samples in a range of −0.1–1.3 V vs. SCE. Crystallinity, thickness, and grain size increase up to −1.1 V vs. SCE and decrease at −1.3 V, whereas stacking fault probability and dislocation density have the opposite trend. Chen et al. [43] instead found out that the potential also has an effect on the deposited FeSe crystalline phase. They worked again with a solution of FeSO₄ and SeO₂ in a 10:1 molar ratio on ITO and anodized aluminum oxide (AAO) as working electrodes, not controlling the pH or the bath temperature as was conducted in [49,50]. They analyzed the elemental composition of the deposits via EDX and X-ray patterns as a function of the deposition potential. The Fe:Se ratio shifts from 0.44 to 1.4, moving the deposition potential from −0.8 V to −1.2 V vs. Ag/AgCl; this is reflected in the X-ray diffractogram (Figure 7b) by the gradual shift from the orthorhombic FeSe₂ structure to the tetragonal FeSe at −1.0 V vs. Ag/AgCl. Demura et al. [17] used FeSO₄ + SeO₂ in a solution with Fe:Se 2:1 on the Fe electrode. The potential for electrodeposition was estimated to be around −1.75 V vs. Ag/AgCl via CV. The difference in this value with respect to those already mentioned for the same precursors can be attributed to other experimental differences, such as the nature of the working electrode, the different solution concentrations and Fe:Se ratios, the presence of stirring in the solution, et cetera. A series of samples were deposited around that value of potential and analyzed. The electric potential dependence of the composition ratio of Fe and Se was assessed by EDX. The measured 55% to 45% Fe:Se ratio is basically stationary between −2.5 and −1.50 V vs. Ag/AgCl, whereas at −1.0 V, Se was dominantly detected. From X-ray analysis (Figure 7c), clear signals of tetragonal FeSe, however broad, were found for the samples deposited between −2.0 and −1.5 V vs. Ag/AgCl. At −1.0 V instead, only Se and hexagonal FeSe peaks were identified.

5.2.5. Effect of Solution pH

The dependence of the Fe:Se ratio vs. solution pH was also studied in [17] by adjusting the acidity of the solution with HNO₃ and analyzing the deposits via XRD. Sharp peaks belonging to the tetragonal FeSe phase were visible between pH 2.1 and 2.9. Thus, the optimal conditions for FeSe deposition were identified at pH 2.3 and −1.75 V vs. Ag/AgCl. Since the aim of this work was to apply electrodeposition to the growth of superconducting samples, they were subject to magnetization measurements. Samples obtained in these conditions show a superconducting transition temperature T_c of 3.5 K.

5.3. Na₂SeO₃ + (NH₄)₂Fe(SO₄)₂ in Na₂SO₄

In [52], FeSe is electrodeposited on a glassy carbon electrode from a solution of Na₂SeO₃ and (NH₄)₂Fe(SO₄)₂ in 0.1 M Na₂SO₄, pH 2.1.

5.3.1. Characterization of the Precursors

The electrochemical behavior of the precursor was investigated via CV (Figure 8a–c). (NH₄)₂Fe(SO₄)₂ voltammetry shows two reduction peaks for Fe³⁺/Fe²⁺ and Fe²⁺/Fe at 0.45 and −0.5 V vs. Ag/AgCl, respectively. Since the precursor salt iron is present as Fe²⁺, we can suppose that the presence of Fe³⁺ is due either to partial oxidation during the CV scans or to air exposure. At lower potentials, a sharp increase in current is due to hydrogen evolution. In the reverse scan, the oxidation peak of Fe/Fe²⁺ is missing, probably due to hydrogen evolution occurring preferentially on iron rather than on GCE. This also causes the hysteresis on the return scan. As regards Na₂SeO₃, no redox process is visible at V > −0.5 V vs. Ag/AgCl. This can be attributed to the adsorption of poorly conducting Se_(red) on the electrode. For V < −0.5, the current increase corresponds to the reduction of Se or H₂SeO₃ to H₂Se. This process is accompanied by gas evolution (H₂) and the appearance of a red precipitate, Se_(red). When the solution containing both precursors is analyzed, significant changes occur in the voltammetric response. The oxidation peak (A’ in Figure 5) of Fe³⁺/Fe²⁺ decreases in intensity, and the process becomes more irreversible due to electrode passivation. A new multiple reduction peak appears around −0.7 V vs. Ag/AgCl (D in Figure 5), whose corresponding oxidation peaks are attributed to FeSe oxidation to Fe²⁺ and Se (D’ in Figure 5). A new pair of peaks attributed to the Fe/Fe²⁺ transition appears (E/E’). The overall charge imbalance in the cyclic voltammograms is caused by the formation of gaseous products and side reactions. A subsequent study of the effect of Se concentration on the electrochemical processes in the solution reveals that higher concentrations promote FeSe formation, visible as a progressive increase in the intensities of peaks D/D’. However, these concentrations should be avoided due to the accompanying H₂Se evolution.

5.3.2. Effect of Bath Temperature

The effect of bath temperature on the CV was also investigated; the CV of the precursor solution was recorded at 25, 40, 60, and 80 °C. Current densities increase for all processes, probably because at this temperature, the insulating Se_(red) layer is not deposited. At higher temperatures, the peak height for the processes involving FeSe formation/dissolution increases even further, suggesting that elevated temperatures promote the processes. On the other hand, the shift of the peak relative to FeSe formation to lower potentials is accompanied by a decrease in crystallinity evidenced by X-rays, with only small FeSe (Fe:Se = 1) crystallites embedded in an amorphous matrix (Fe:Se = 0.08–0.26).

5.3.3. Effect of Deposition Mode

Electrodeposition of films in potentiostatic mode in the range of −0.7–(−1.0) V vs. Ag/AgCl of a solution of 0.03 M (NH₄)₂Fe(SO₄)₂ + 0.015 M Na₂SeO₃ + 0.1 M Na₂SO₄ resulted in the formation of a dense layer near the electrode and a loose deposit on top of it (Figure 9, left panel), the latter consisting mainly of monoclinic Se, from X-ray and EDX measurements. The former instead shows a Fe:Se atomic ratio that decreases from 2.5 to 1.2 with a deposition potential shift from −1.0 V to −0.7, but the poor crystallinity of this layer makes XRD measurements impossible. Due to unsatisfactory results in the potentiostatic deposition mode, with very loose deposits and uncontrolled stoichiometry, Laurinavchyute et al. [42] used a different approach with pulsed deposition, already employed, for example, in the deposition of CdSe. By alternating the deposition potential between V (FeSe deposition) = −1.05 vs. Ag/AgCl and V (excess Fe oxidation), they managed to obtain much smoother and more compact films with a Fe:Se ratio close to unity (Figure 9, right panel).

5.4. FeSO₄ + H₂SeO₃ in Na₂SO₄

Pesko et al. [16] studied a system composed of FeSO₄ + H₂SeO₃ in Na₂SO₄ for the application of FeSe as a counter electrode in solar cells.

5.4.1. Characterization of the Precursors

They analyzed the electrochemical behavior of the precursor via multi-cycle CV (Figure 10, left). At first glance, a drop in the cathodic current is visible in the first three cycles, followed by an increase in the following cycles. This is explained by the initial deposition of an insulating material such as amorphous selenium, followed by the formation of the conducting material FeSe. Regarding the attribution of the observed peaks, Pesko et al. tentatively assign the reduction peak at −0.77 V vs. Ag/AgCl (peak 2 in Figure 10, left panel) to the direct reduction in selenous acid to hydrogen selenide, combined with the synproportionation of both compounds to give Se(s). The peak at −1.2 V vs. Ag/AgCl (peak 1 in Figure 10, left panel), instead, is attributed to the formation of FeSe according to the reactions Fe²⁺ + Se + 2e⁻ → FeSe(s) (−0.96 V vs. SHE) and Fe²⁺ + H₂SeO₃ + 4H⁺ + 6e⁻ → FeSe(s) + 3H₂O. Peak 3 in Figure 11 is instead attributed to the reaction FeSe → Fe²⁺ + Se + 2e⁻ at −0.77 V vs. Ag/AgCl (peak 2 in Figure 11, left panel) to the direct reduction in selenous acid to hydrogen selenide, combined with the synproportionation of both compounds to give Se(s). The peak at −1.2 V vs. Ag/AgCl (peak 1 in Figure 10, left panel), instead, is attributed to the formation of FeSe according to the reactions Fe²⁺ + Se + 2e⁻ → FeSe(s) (−0.96 V vs. SHE) and Fe²⁺ + H₂SeO₃ + 4H⁺ + 6e⁻ → FeSe(s) + 3H₂O. Peak 3 in Figure 11 is instead attributed to the reaction FeSe → Fe²⁺ + Se + 2e⁻.

5.4.2. Effect of Bath Temperature

The analysis of the temperature dependence (20, 40, 60, and 80 °C) of these electrochemical processes was also performed. Contrary to what was previously reported by [52], in this case, the higher temperature seems to favor the deposition of selenium instead of FeSe, as evidenced by the progressive disappearance of the reduction peak at −1.2 V vs. Ag/AgCl (Figure 7c).

5.4.3. Characterization of the Samples

As regards the characterization of the deposits, X-ray diffraction of the films deposited on Pt/Au at 20 °C shows signals of poorly crystalline FeSe. In the samples deposited at higher temperatures or subjected to thermal annealing, the formation of FeSe₂ is observed. The results of the X-ray analysis are confirmed by the EDX measurements, which show a general excess of selenium in the deposits. The SEM images of the morphology of the films (Figure 11) prepared from this precursor solution, either on Au or Pt, at 20 °C or 80 °C, with or without a recrystallization thermal treatment, show a granular microstructure with spherical grains decorated by smaller platelets. This feature, though not positive for current transport applications, could be successfully exploited in DSSCs, where a counter electrode with a highly developed surface would provide a huge number of active sites for the redox reactions of I⁻/I³⁻, which is a typical redox couple for electrolytes used in DSSCs. The CV scans of the annealed FeSe on gold in an electrolytic bath containing I⁻/I³⁻ show high stability, making this material a possible candidate for application in DSSCs, even though there is room for improvement mainly as regards the material’s purity.

5.5. Fe(NO₃)₃ + H₂SeO₃

Another system recently studied [44] uses electrolytic solutions containing Fe(NO₃)₃ and H₂SeO₃.

5.5.1. Characterization of the Precursors

The CV of the iron precursor vs. Pt (Figure 12a) identifies the reduction process Fe³⁺/Fe²⁺ in the range 0.6−0.1 V vs. Ag/AgCl and Fe²⁺/Fe around −0.2–(−0.9). The selenium ions in solution instead show a two-stage reduction process between 0.5 and 0.1 V vs. Ag/AgCl and below 0.1 (Figure 12b). When analyzing the solution for co-deposition with Fe:Se = 14:1, the potential for the formation of the FeSe film was identified at −0.38 V vs. Ag/AgCl on the Pt electrode and at −0.3 V vs. Ag/AgCl on the Ni electrode (Figure 12c,d). Electrodeposition at this potential for 30 min yields amorphous samples of FeSe, as seen from the X-ray diffraction. Thermal treatment at 450 °C for 1 h in Ar increases the crystallinity of the deposited film, whose composition was found to be 42.2% Fe and 57.8% Se.

5.5.2. Effect of Bath Temperature

Moreover, the effect on the co-deposition of several important parameters was studied so as to determine the optimal conditions for electrodeposition. First of all, the bath temperature was increased from room temperature to 75 °C, and the polarization curves showed a positive effect on the deposition process, with a shift of the deposition potential of +0.05 V. However, the films deposited at high temperatures are severely deteriorated, break off from the electrode surface, and the composition is no longer close to unity.

5.5.3. Effect of Precursor Concentration

Secondly, the effect of the concentration of the two precursors was examined. The polarization curves of solutions containing different amounts of Fe (or Se) salt and constant Se (or Fe) were recorded, but no significant change was observed in any case.

5.6. FeCl₂ + SeO₂ in Na₂SO₄

In a series of dedicated studies, Demura et al. [18,19] and Yamashita et al. [53,54] investigated the possibility of electrodeposition of FeSe starting from FeCl₂ + SeO₂ 2:1 in Na₂SO₄, with the pH of the solution adjusted via H₂SO₄ addition.

5.6.1. Effect of Deposition Potential/Temperature

The cyclic voltammogram on a Fe electrode (Figure 13, left panel) shows an anomaly around −0.9 V vs. Ag/AgCl, which is considered to correspond to the electrodeposition of FeSe [18]. A series of samples were deposited on Fe at potentials around this value, and the −0.9 V was confirmed to be optimal for tetragonal FeSe deposition by XRD. Another work by this group [53] was dedicated to the determination of the temperature/voltage FeSe phase diagram, depositing with the same solution on ITO. They found out that the applied voltage for the deposition of single-phase FeSe moves to higher bias voltages with increasing temperature. Also, the crystallinity of FeSe increases at higher temperatures, with the best samples obtained at 70 °C. By depositing in these conditions, a critical transition temperature of 8 K is obtained from magnetization measurements.

5.6.2. Effect of Solution pH

The dependence of the deposited film quality on the solution pH was investigated, and the optimal conditions (Fe:Se ratio via EDX and crystallinity via XRD) were found to be at pH = 2.1. The samples were subjected to magnetization measurements and showed a T_c = 8.1 K. This knowledge was then transferred to deposition on rolling-assisted biaxially textured Ni-W substrates (RABiTS) in view of the long-length deposition of FeSe superconducting films [18]. The same kind of optimization of the process presented before was performed, which led to the identification of the ideal potential (−1.1 vs. Ag/AgCl, CV, and XRD patterns at different voltages are shown in Figure 14) and pH (2.1) for the deposition of crystalline FeSe with a measured T_c of 8.0 K from magnetization measurements. Further optimization of the process was conducted in [54], where the deposition temperature was increased up to 70 °C and the deposition of insulating red Se on the substrate was prevented by applying the voltage before dipping the electrode in the electrochemical bath for deposition. Thus, samples were obtained with a critical transition temperature, measured via the four-probe method, of 2.5 K (with the onset at 8.4 K).

6. Discussion

From this review, it becomes clear how electrodeposition is an appealing technique that attracts attention for several reasons. First of all, it is cheap compared to physical techniques that require expensive vacuum equipment; it allows for the deposition of thick samples (up to a few microns); it is quick and suitable for the deposition of larger areas, an essential feature for an envisioned industrial scale-up. In the available literature, however, it is difficult to find common features and draw general conclusions on the optimal conditions for FeSe growth due to the experimental differences: precursors, pH, temperature, substrate, and concentrations. All these parameters have an effect on the reactions occurring in the cell. A summary of the conditions used for deposition in the literature is reported in

Table 3. Comparison of relevant parameters of the studies mentioned in the literature.

	Substrate	Precursors	pH	Fe:Se	Tdep (°C)	Potential (V)	Dep Time (min)	Thermal Treatment	Phase
[43]	ITO/AAO	FeSo4 + SeO2	N.D.	10:1	25	−1 vs. Ag/AgCl	60	No	FeSetetra
[47]	Stainless steel/FTO	FeCl3 + SeO2+ additives	N.D.	1:1	25	−1.05 vs. SCE	15	No	Fe3Se4
[52]	Glassy carbon	Na2SeO3 + (NH4)2 Fe(SO4)2	2.1	2:1 to 6:1	25	−1.1 vs. Ag/AgCl	60	No	FexSey
[44]	Ni	Fe(NO3)3 + H2SeO3	N.D.	14:1	25–35	0.67 vs. Ag/Agcl	60	450 °C, 1 h	Fe42.2Se57.8
[16]	Pt/Au	FeSO4 + H2SeO3	2	5:1	25	−1.2 vs. Ag/AgCl	60	400 °C, 96 h	FeSe + FeSe2
[49,51]	ITO	FeSO4 + SeO2	3.0	10:1	30–90	−1.1 vs. SCE	10–60	No	FeSetetra
[17]	Fe	FeSO4 + SeO2	2.3	2:1	20	−1.75 vs. Ag/AgCl	60	No	FeSetetra
[19]	Fe	FeCl2 + SeO2	2.1	2:1	20	−0.9 vs. Ag/AgCl	60	No	FeSetetra
[18]	Ni Rabits	FeCl2 + SeO2	2.1	2:1	20	−0.8 to −1.0 vs. Ag/AgCl	60	No	FeSetetra
[53]	ITO	FeCl2 + SeO2	2.1	2:1	70	−1.7 vs. Ag/AgCl	5	No	FeSetetra
[54]	Ni Rabits	FeCl2 + SeO2	2.1	2:1	70	−0.9 to −1.1 vs. Ag/AgCl	5	No	FeSetetra

N.D. = no data, information not provided in the reference.

. A phenomenon observed by almost everyone is the preferential deposition of amorphous red selenium, which is consistent with what is expected from the redox potentials of Fe and Se. For this reason, the more noble element, Se, is used in lower concentrations as a limiting agent, but what is observed is that the original Fe:Se ratio in the precursor solution has no direct effect on the Fe:Se ratio of the deposits, which seems more sensitive to potential and temperature. Another common finding is that higher temperatures promote the deposition process (e.g., higher current densities are recorded in high-temperature CVs), but there is no unanimity on the structural effects of temperature, which only in some cases promotes the formation of the desired phase. pH control, though essential for the stability of the precursor solution, does not seem to have a crucial effect on the structure and composition of the deposits. The FeSe deposits, when SEM images are presented, appear extremely granular, formed by small platelet crystals and globular particles. Such coarse morphology is probably due to a fast, complex growth mechanism, as often observed in solution growth processes [55,56], as well as the inclusion of electrolyte salts in the deposit, the evolution of gaseous species at the electrodes during deposition, and/or the absorption and desorption of species on the electrodes. Furthermore, the morphology of the samples is often that of a granular deposit with no connection between adjacent grains and not that of a thin film, and this is a great drawback if the material’s capabilities of current transport want to be exploited. Working at low temperatures, though a great advantage in view of large-scale production, does not promote the formation of an interface between the substrate and FeSe, not even when a well-matched biaxially textured substrate is used, as in [54]. Indeed, the samples tend to be loosely adhered to the substrate or to peel off and crack. In order to obtain a more controlled growth of the desired material, it is probably necessary to sacrifice speed to promote a slower 2D growth mode, layer by layer, with respect to a fast 3D mode. A parameter whose influence has not yet been studied for the deposition of FeSe is the deposition rate. This could be conducted, for example, by lowering the ionic concentration of the solutions, thus limiting their current transport capabilities, or by working with low applied current densities. Another approach would require changing reaction kinetics in more indirect ways, for example, with the use of additives or different electrolytes in the buffer solution to suppress dendritic-like growth and favor the formation of a more compact film [57,58]. A possible, more sophisticated evolution of this idea could be to adopt a different approach and to employ electrochemical atomic layer epitaxy (EC-ALE) [59,60]. EC-ALE has been developed by analogy with atomic layer epitaxy (ALE). ALE is a methodology used initially to improve epitaxy in the growth of thin films. The principle of ALE is to use surface-limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2D, layer by layer, epitaxial. Surface-limited reactions are developed for the deposition of each component element, and a cycle is composed of them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. In addition, EC-ALE offers a way of better understanding compound electrodeposition by breaking it down into its component pieces. It allows compound electrodeposition to be divided into a series of individually controllable steps, resulting in an opportunity to learn more about the mechanisms and gain a series of new control points for electrodeposition. All these possibilities suggest that there is much room for improvement as regards the electrodeposition of FeSe films, even for applications in which structural requirements are more demanding.

7. Conclusions

From the analysis of the literature, it is possible to understand why electrodeposition is an appealing technique for the growth of FeSe since it is quick, cheap, does not require expensive equipment, and allows for the growth of thick layers on large/curved surfaces. However, the different experimental conditions used in the mentioned papers lead to different results; therefore, comparisons are not easily made. All in all, the results presented in the literature show how it is possible to use electrodeposition to obtain the desired FeSe phase, even though some critical issues arise. For example, the obtained samples consist mainly of crystals embedded in an amorphous phase or poorly crystalline films; therefore, they lack the crystallinity required for some of the foreseen applications. However, there are several possibilities worth trying to overcome this issue, such as the slowing down of the deposition rate or the use of electrochemical atomic layer epitaxy, which means that there is still room for improvement in the electrodeposition of FeSe films.