materials Article Structural and Magnetic Properties of FePd Thin Film Synthesized by Electrodeposition Method Gabriele Barrera 1, *, Federico Scaglione 2 , Matteo Cialone 2,3 , Federica Celegato 1 , Marco Coïsson 1 , Paola Rizzi 2 and Paola Tiberto 1 1 Istituto Nazionale di Ricerca Metrologica (INRiM), Advanced Materials Metrology and Life Sciences, Strada delle Cacce 91, I-10135 Torino, Italy; f.celegato@inrim.it (F.C.); m.coisson@inrim.it (M.C.); p.tiberto@inrim.it (P.T.) 2 Dipartimento di Chimica e Centro Interdipartimentale NIS (Nanostructured Surfaces and Interfaces), University of Turin, Via Pietro Giuria 7, I-10125 Torino, Italy; federico.scaglione@unito.it (F.S.); matteo.cialone@uab.cat (M.C.); paola.rizzi@unito.it (P.R.) 3 Departament de Física, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain * Correspondence: g.barrera@inrim.it  Received: 7 February 2020; Accepted: 21 March 2020; Published: 23 March 2020  Abstract: Bimetallic nanomaterials in the form of thin film constituted by magnetic and noble elements show promising properties in different application fields such as catalysts and magnetic driven applications. In order to tailor the chemical and physical properties of these alloys to meet the applications requirements, it is of great importance scientific interest to study the interplay between properties and morphology, surface properties, microstructure, spatial confinement and magnetic features. In this manuscript, FePd thin films are prepared by electrodeposition which is a versatile and widely used technique. Compositional, morphological, surface and magnetic properties are described as a function of deposition time (i.e., film thickness). Chemical etching in hydrochloric acid was used to enhance the surface roughness and help decoupling crystalline grains with direct consequences on to the magnetic properties. X-ray diffraction, SEM/AFM images, contact angle and magnetic measurements have been carried out with the aim of providing a comprehensive characterisation of the fundamental properties of these bimetallic thin films. Keywords: FePd alloy; electrodeposition technique; magnetic properties; structural characterisation 1. Introduction Bimetallic nanomaterials, composed by a noble element and a magnetic metal, have attracted a growing interest in the scientific community because they show together the properties of both metals in addition to confinement effects [1–4]. Noble metals play a central role in the area of catalysis [5,6]; whereas the magnetic metal elements (Fe, Co, Ni) with specific magnetic properties offer the possibility to easily recover and reuse the bimetallic nanostructured catalysts after the completion of the reaction and/or the chemical processes [7,8]. In particular, the magnetic separation process provides a practical and useful technique for recycling the magnetic nanostructured catalysts [7,8]. Additionally, the magnetic–noble bimetallic nanomaterials exhibit not only a simple combination of the properties associated with their single elements but they perform enhanced catalyst activity, modified catalyst selectivity or improved catalyst stability with respect to the monometallic catalysts: Sun et al. [9] reveal that the addition of Fe significantly enhances the hydrodeoxygenation activity of Pd/C; Zhao and Gorte [10] found that the activity of Pd/ceria catalysts increases by as much as an order of magnitude upon the addition of a controlled amount of Fe for the water-gas shift; Mu at al. [11] demonstrated that the synergistic effect between Pt and Ni enhances the activity to CO oxidation; Materials 2020, 13, 1454; doi:10.3390/ma13061454 www.mdpi.com/journal/materials Materials 2020, 13, 1454 2 of 15 additional enhanced catalyst activity by means of magnetic–noble nanomaterials can be found in literature [12–16]. In view of their applications, magnetic–noble bimetallic nanomaterials should be produced in large quantities and at low costs. While nanoparticles may relatively comply well to these constraints, other forms of nanomaterials, such as thin films, could be more difficult to adapt, as their preparation techniques often require large and complex setups with well-controlled vacuum conditions. Electrodeposition technique helps overcoming some of these constraints, being a relatively low-cost and scalable approach for growing metallic thin films, even in the form of bimetallic alloys [17]. Within this context, it is of great importance and scientific interest to design and develop new multifunctional magnetic–noble bimetallic nanomaterials and to study the interplay between the properties and their microstructure, synthesis technique or spatial confinement, as the multiple available degrees of freedom could be exploited to tailor the properties to the desired application [2,15,18–20]. In the case of thin films, both bulk (e.g., grain size) and surface (e.g., roughness, surface-to-volume ratio) characteristics turn out to play fundamental roles in determining the resulting properties of the nanomaterial and to define the catalytic activity, therefore an adequate control during the growth process or the subsequent chemical etching is strictly required [21,22]. In this manuscript, we deal with this large subject by focussing on bimetallic thin films combining the noble metal, Pd, with the magnetic transition metal, Fe [23–27]. The former is attracting a wide interest for its catalytic properties, whereas the second, among the magnetic ones, is the least expensive. The FePd bimetallic nanomaterials are prepared in thin films form by electrodeposition, which allows obtaining a Fe-rich composition, and offers the possibility to easily control the film thickness by simply picking the desired deposition time. The surface (e.g., roughness) and magnetic properties depend on the film thickness, as specific microstructure and morphological features develop as a function of deposition time. The Fe-rich composition of the alloy combined with the higher removal rate of the Fe atoms than the one of the Pd atoms through chemical etching in hydrochloric acid allow to refine the roughness on the thickest films and to help the decoupling their grains. Such changes resulted in the ability to fine tune the magnetic properties and increase the surface-to-volume ratio. X-ray diffraction, as well as roughness, contact angle and magnetic measurements have been carried out with the aim of providing a comprehensive characterisation of the fundamental properties of these magnetic–noble bimetallic thin film. 2. Materials and Methods Binary FePd alloy was potentiostatically electrodeposited in a cylindrical electrochemical cell [28], in which a thin foil of platinum acts as counter electrode, a conductive substrate as working electrode and an Ag|AgCl electrode as a reference electrode (VAg|AgCl = + 0.239 V vs. standard hydrogen electrode at 25 ◦ C). The reference electrode and the working one were mounted into the electrochemical cell in a horizontal arrangement. The depositions were performed at room temperature without stirring the solution. According to the previous work of Konczak et al. [29], the electrolyte composition is 0.01 M Pd(NH3 )4 Cl2 , 0.06 M sulfosalicylic acid (SSA), 0.05 M Fe2 (SO4 )3 7H2 O, 0.3 M (NH4 )2 SO4 (pH = 5). The magnetic FePd thin film was deposited on the working electrode constituted by a Si/SiO2 substrate made conductive by the deposition of an Au layer. The adhesion of Au layer on SiO2 surface was ensured by a thin Ti layer deposited between them. Prior to the FePd electrodeposition, the substrates were cleaned consecutively with acetone, isopropanol and de-ionized water in an ultrasonic bath. The deposition potential was fixed at E0 = −1.2 V vs. VAg|AgCl selecting three different deposition times: 500, 150 and 50 s; the three electrodeposited FePd thin film samples were labelled S500, S150 and S50, respectively. Scanning Electron Microscopy (SEM) was used to study the morphology of the electrodeposited samples. A software analysis of SEM images enables to calculate the size distribution of the grains. SEM was equipped by energy dispersive x-ray spectrometer (EDS) in order to evaluate the stoichiometry of the FePd alloy. Materials 2020, 13, 1454 3 of 15 Thickness and surface morphology of all samples were characterised by means of Atomic Force Microscopy (AFM) operating in intermittent contact mode with the oscillation frequency and amplitude set point achieved by the tuning of the cantilever. The film thickness was evaluated measuring the height of a well-defined step obtained by a lithographic process between the FePd film and the underlying substrate, see Figure S2 in the Supplementary Materials; the measured thickness was 51, 400 and 631 nm for the S50, S150 and S500 sample, respectively. From 2D AFM profiles, roughness parameters are calculated such as the root mean square roughness (Rq ), the mean slope of the roughness motifs (1/Kr ) and the steepness of the sample surface (Rq /ξ) [30–35]. The 1/Kr parameter is defined as: Kr = AR/R, where R is the mean depth of the roughness motifs and AR is the mean spacing of the roughness motifs [30]; whereas the ξ parameter is the correlation length that reflects the average distance between consecutive peaks on the surface [34]. In the manuscript, roughness (Rq ) is reported as the average value and standard deviation of several measurements in different points of the sample surface. The structure of the as deposited FePd thin films was investigated by grazing angle X-ray diffraction (XRD) with a Cu-Kα radiation. The incident X-ray beam was set at a constant grazing angle of 0.6◦ for all measurements. The moving detector, instead, was rotated around the sample with a step size of 0.02 degrees and an acquisition time of 40 s per step, which were found to provide a good signal-to-noise ratio. A substrate pattern got before electrodeposition was collected and used as a reference in the reflections assignment. Wettability properties of the samples surface were investigated by means of contact angle measurements performed at room temperature in which a droplet, with a volume of approximately 1 µL, is released from the tip of a calibrated micropipette on the sample surface. Each contact angle value is calculated as the average of at least six measurements in different points of the sample surface. Magnetic measurements were performed using an Alternating Gradient Field Magnetometer (AGFM) working in the field interval −18 kOe ≤ H ≤ 18 kOe in the parallel configuration (applied magnetic field in the sample plane). Hysteresis loops were obtained at room temperature for all samples and the diamagnetic signal of sample holder and film substrate was adequately subtracted. First Order Reversal Curves (FORC) [36–38] have been measured with the same AGFM instrument, with the aim of identifying reversible and irreversible magnetisation reversal processes in the samples, and their dependence on inter-grain coupling. Such measurement technique results in a FORC-distribution [39] that can be used to express the spreading of the irreversible magnetisation reversal mechanisms taking place in the sample, as a function of local coercive field HC values and mutual magnetic interactions among grains, expressed in terms of a so-called bias field HB . Therefore, the peak of the FORC distributions measured for all the studied samples will be analysed to identify the spreading of local coercive field values, due to the grain size distribution in the sample, and the presence of inter-grain interactions affecting the magnetisation reversal processes. All as-deposited samples were chemically treated for 5 h in a 2 molar aqueous solution of hydrochloric acid without applying any external potential and labelled S500_5h, S150_5h and S50_5h, respectively. The morphological and surface transformations and the magnetic variations induced by the chemical treatment were characterised by means of the aforementioned experimental techniques. 3. Results and Discussion Current density (j) vs. time curves recorded during the potentiostatic deposition of FePd thin film samples are shown in Figure 1a. At the beginning of the electrodeposition process, the j(t) curves show a reduction of their absolute value reaching a minimum (|j|min ) at around 20 s (indicated by the grey vertical dashed line in Figure 1a); this first portion of curve corresponds to the accumulation of a capacitive charge at the interface between the electrode and the solution [40]. Later, the FePd deposition occurs at an increasingly negative current density, which stays approximately constant Materials2020, Materials 2020,13, 13,xxFOR FORPEER PEERREVIEW REVIEW 44of of16 16 (indicatedby (indicated by Materials 2020, the 13,the greyvertical grey 1454 verticaldashed dashedlinelinein inFigure Figure1a); 1a);this thisfirst firstportion portionof ofcurve curvecorresponds correspondsto to the the 4 of 15 accumulation of a capacitive charge at the interface between the electrode and accumulation of a capacitive charge at the interface between the electrode and the solution [40]. the solution [40]. Later, the Later, the FePd FePd deposition deposition occurs occurs at at an an increasingly increasingly negative negative current current density, density, which which stays stays only in the approximately time interval constant 25–60 only ins (time the interval time between interval 25–60the s grey (time vertical interval dashed between approximately constant only in the time interval 25–60 s (time interval between the grey vertical and dotted the grey lines verticalin Figure dashed 1a). dashedand anddotted dottedlines linesin inFigure Figure1a). 1a). Figure Figure 1.1. (a) Figure 1. (a)Current (a) Currentdensity Current densityasas density a afunction as a functionof of function time of timerecorded time recorded recorded during the potentiostatic during during deposition the potentiostatic the potentiostatic of FePd deposition deposition of of thin FePd film thin samples; film (b) samples;root mean (b) root square mean roughness square (R roughness q ) as a (Rfunction ) as a of variation function FePd thin film samples; (b) root mean square roughness (Rq) as a function of variation of current q of of current variation of density current density∆ values density values values ∆𝑗 (see j (see ∆𝑗 text for text (see details) textfor over fordetails) details) theover deposition over time. time. thedeposition the deposition time. After that, After that, that, the the strong the strong increase strong increase increase inin |j|(t) over in |j|(t) overtime, time,observed observedfor forthe thedeposition deposition of of S150 and S150 and S500 and S500 S500 After |j|(t) over time, observed for the deposition of S150 samples, samples, indicates indicates that that the deposition the deposition conditions are conditions changing are and changing that and the surface that the area surfaceof the areaworking of the samples, indicates that the deposition conditions are changing and that the surface area of the electrode working iselectrode increasing is [40,41]. As a [40,41]. increasing consequence,As a the formation and consequence, the growth of anand formation increasingly growth rough of an working electrode is increasing [40,41]. As a consequence, the formation and growth of an film over deposition increasinglyrough roughfilm time filmover is expected. overdeposition depositiontime timeisisexpected. expected. increasingly This This hypothesis hypothesis is is corroborated corroborated by by the the results results of surface ofsurface analysis surfaceanalysis analysisof of the ofthe samples thesamples obtained samplesobtained obtainedby by the bythe the This hypothesis is corroborated by the results of AFM; AFM; representative representative images images for for all all samples samples are are reported reported in in Figure Figure 2.2. Indeed, Indeed, the the root-mean-square root-mean-square AFM; representative images for all samples are reported in Figure 2. Indeed, the root-mean-square roughness roughness(R qq)q))values (R valuesmeasured measured from the the from AFMAFM images turn out images turntoout monotonically out increaseincrease to monotonically monotonically as a function as aa roughness (R values measured from the AFM images turn to increase as of electrodeposition function of time. electrodeposition time. function of electrodeposition time. Figure 2. Figure Figure 2.2.Root Rootmean Root mean mean square square roughness roughness square roughness (Rq) (Rq)(Rq) for as-deposited as-deposited for as-deposited for and chemical chemical and chemical and etched samples etched etched samples samples as aa as a function as of electrodeposition function of time. At electrodeposition the right, time. Atthe corresponding the right, the AFM images, corresponding identified AFM by images, function of electrodeposition time. At the right, the corresponding AFM images, identified by thethe same identifiedsymbols by the and samecolours symbols used andin the graph. colours used in same symbols and colours used in the graph.the graph.   In addition, Inaddition, In the addition,the Rqqqincrease theRR increaseisisisalso increase alsolinearly also linearlycorrelated linearly correlatedwith correlated with with the the ∆ j = 𝑗𝑗 jstop −𝑗𝑗jmin ⁄⁄ ∆𝑗 the∆𝑗 /𝑗𝑗j parameter parameter parameter (see (see Figure Figure 1b), 1b), which which takes takes into into account account the the variation variation of of current current density density values values over over the the deposition deposition (see Figure 1b), which takes into account the variation of current density values over the deposition time; jstop is the current density value at which the electrodeposition process is stopped. This correlation indicates that the progressive increase of working electrode surface area is developed by means of an Materials 2020, 13, x FOR PEER REVIEW 5 of 16 time; j is the current density value at which the electrodeposition process is stopped. This Materials 2020, 13, 1454 5 of 15 correlation indicates that the progressive increase of working electrode surface area is developed by means of an increasing of surface roughness. This behaviour makes the growth in thickness increasingas nonlinear of asurface functionroughness. This behaviour of the deposition time. makes the growth in thickness nonlinear as a function of theThe deposition grazing time. angle patterns of electrodeposited FePd thin films are reported in Figure 3. The The grazing contribution of theangle patterns substrate of electrodeposited is evident in all samples, FePd thin films independent are reported of their thickness.inReflections Figure 3. The contribution peaks, labelled as of the substrate black is evident triangles in all3,samples, in Figure independent are associated to anofα-(Fe, their thickness. Pd) solidReflections peaks, solution phase: labelled as black triangles in Figure 3, are associated to an Pd) solid however their position is shifted to lower angles at decreasing deposition time, meaning that α-(Fe, solution phase: however theira position isof variation shifted to lower angles stoichiometry and aatdifferent decreasing deposition ratio time, meaning of deposition that aplace. are taking variation Thisofissue, stoichiometry already reported in literature and a different ratio of [42], is dueare deposition to ataking local place. increase ofissue, This pH ofalready the near-electrode layer, that reported in literature [42],can cause is due to the precipitation a local increase of of pHiron hydroxide of the at quite near-electrode negative layer, that canpotential, cause thewhen the electrodeposition precipitation of iron hydroxide bath does at quite not contain negative complexing potential, when agents: in this work, bath the electrodeposition the addition does notof sulfosalicilic contain complexingacid agents: and Fe3+inions as an this work, iron source instead the addition of Fe2+ avoids of sulfosalicilic acid and iron 3+ ions as an Fehydroxide precipitation [29] butof iron source instead not 2+ avoids Fethe local pHironincrease; hydroxide as aprecipitation consequence, [29] abut change not theinlocal thepH pHincrease; occurs.asFurthermore, a consequence, the hydrogen a change in theevolution pH occurs. at Furthermore, the sample surface favours the hydrogen the inclusion evolution at the of hydrogen sample surfaceinto the deposit favours [42]. of hydrogen into the deposit [42]. the inclusion Figure 3. Figure Grazing angle 3. Grazing angle XRD XRD pattern pattern of: of: (a) (a)substrate substrateand andelectrodeposited electrodeposited FePd FePd thin thin films films (b) (b) S50, S50, (c) (c) S150, (d) S150, (d) S500. S500. When deposition When deposition isisperformed performedfor forlonger longertimestimes (sample (sample S500, S500, cyan cyan pattern pattern in Figure in Figure 3), 3), the the fluctuation fluctuation of the of the electrolyte electrolyte composition composition in in proximity proximity ofof theworking the workingelectrode electrodebecomes becomesstronger, stronger, causing the deposition of a further phase identified as almost pure palladium causing the deposition of a further phase identified as almost pure palladium [43]. [43]. AA significant significantamount amountofofa tetragonal a tetragonalPd1.5 PdH1.52H phase appears in all samples due to the high affinity 2 phase appears in all samples due to the high affinity between hydrogen and palladium [44,45]. Theof formation between hydrogen and palladium [44,45]. The formation this metal hydride of this usually induces usually metal hydride a lattice distortion followed by surface cracking [46]; however, SEM images reported in induces a lattice distortion followed by surface cracking [46]; however, SEM images reported Figure 4 do not showin this evidence. Figure 4 do not show this evidence. A shift of reflections of the Pd1.5 H2 phase is observed on decreasing the time of deposition. This is possibly due to change in stoichiometry of the hydride phase as a consequence of the increase of pH and, therefore, the amount of Pd electrodeposited. The ratio of Fe:Pd was determined using EDS resulting in Fe68 Pd32 , Fe66 Pd34 and Fe69 Pd31 for S50, S150 and S500 sample, respectively. However, these stoichiometry values of as-deposited FePd alloy are affected by a high error because the EDS technique is not able to detect the H atoms present in the samples (Pd1.5 H2 phase observed by XRD) and, as a consequence, it is not possible to distinguish the correct fraction of the Pd atoms present in the FePd alloy from those in the Pd1.5 H2 metal hydride. Therefore, these EDS data are only used as a reference value to obtain the variation of Fe content in each sample after the chemical etching, as discussed in the following. If the Rq value (shown in Figure 2) mainly provides information on the change of the morphology along the vertical direction, the lateral growth of the grains as a function of electrodeposition time was studied by means of SEM measurements. Materials 2020, 13, 1454 6 of 15 Materials 2020, 13, x FOR PEER REVIEW 6 of 16 Figure 4. Figure 4. SEM SEMimages imagesofofas-deposited as-deposited(a)(a)S50, S50,(b)(b) S150, S150,(c)(c) S500 samples S500 samples andand etched (d) (d) etched S50_5h, (e) S50_5h, S150_5h, (e) (f) S500_5h S150_5h, samples (f) S500_5h samplesin ain2 aM2 aqueous solution M aqueous of HCl solution forfor of HCl 5 h.5 h. A shift The of reflections surface of the Pd1.5Hsamples of electrodeposited 2 phase is observed shows on decreasing a rough the time of deposition. granular morphology constitutedThis by is possibly due to change in stoichiometry of the hydride phase as a consequence spherically shaped grains with different size as a function of deposition time (see Figure 4a–c). of the increase of pHparticular, In and, therefore, the amount the grain of Pd thin size of FePd electrodeposited. films is observed to increase significantly as a function of deposition time. In the early stages of the using The ratio of Fe:Pd was determined EDS resulting deposition, in Fe68Pd32centres several nucleation , Fe66Pd on andAu 34 the Fe69 Pd31 for substrate S50, S150 and S500 sample, respectively. However, these stoichiometry values of are localized and, for short deposition time intervals, the grains do not have a chance to grow and as-deposited FePd the alloyappears film are affected dense by(seeaFigure high error 4a). Asbecause the EDS the a consequence, technique is not S50 sample able to displays detect small theuniformly grains H atoms present in the distributed over samples (Pd1.5H2surface the substrate phase characterised observed by XRD) and, as size by a narrow a consequence, distributionit(Figure is not possible 5a) withto a distinguish mean value the < D correct > ≈ 85 fraction nm, as Materials 2020, 13, x FOR PEER REVIEW of shownthe Pd in atoms Figure present 5c. in the FePd alloy from those in the Pd 1.5H2 7 of 16 metal hydride. Therefore, these EDS data are only used as a reference value to obtain the variation of Fe content in each sample after the chemical etching, as discussed in the following. If the Rq value (shown in Figure 2) mainly provides information on the change of the morphology along the vertical direction, the lateral growth of the grains as a function of electrodeposition time was studied by means of SEM measurements. The surface of electrodeposited samples shows a rough granular morphology constituted by spherically shaped grains with different size as a function of deposition time (see Figure 4a–c). In particular, the grain size of FePd thin films is observed to increase significantly as a function of deposition time. In the early stages of the deposition, several nucleation centres on the Au substrate are localized and, for short deposition time intervals, the grains do not have a chance to grow and the film appears dense (see Figure 4a). As a consequence, the S50 sample displays small grains uniformly distributed over the substrate surface characterised by a narrow size distribution (Figure 5a) with a mean value < D > ≈ 85 nm, as shown in Figure 5c. 5. Grain Figure 5. Figure Grain size sizedistributions distributionsof:of: (a)(a) as-deposited samples as-deposited and and samples (b) etched samples (b) etched in a 2 M samples inaqueous a 2 M solution of aqueous HCl forof solution 5 h;HCl (c) grain for 5mean h; (c) grain < diameter D > evolution mean diameter as< aDfunction of electrodeposition > evolution as a functiontime. of electrodeposition time. On increasing the deposition time, the grains are able to grow in spherical shape and their size distribution becomes wider (see Figure 5a) and the mean value increases to < D > ≈ 133 and 395 nm for the S150 and S500 sample, respectively, as shown in Figure 5c. The observed simultaneous increase of surface roughness and of the grain size indicates a 3D growth in the films as a function of Materials 2020, 13, 1454 7 of 15 On increasing the deposition time, the grains are able to grow in spherical shape and their size distribution becomes wider (see Figure 5a) and the mean value increases to < D > ≈ 133 and 395 nm for the S150 and S500 sample, respectively, as shown in Figure 5c. The observed simultaneous increase of surface roughness and of the grain size indicates a 3D growth in the films as a function of the electrodeposition time, and is probably related, as already mentioned above, to the current density variation j(t) during the potentiostatic electrodeposition. The |j(t)| increase over the deposition time (see Figure 1a) promotes an increasingly rapid growth of independent grains over the coalescence growth mechanism by inhibiting a compliant film deposition. The electrodeposited FePd alloy samples were chemically treated for 5 h in a 2 molar aqueous solution of HCl acid in order to induce a free corrosion of the sample surface promoting a variation of its morphological and physical properties. The measured FePd alloy stoichiometry after the chemical treatment is Fe66 Pd34 , Fe63 Pd37 and Fe60 Pd40 for S50_5h, S150_5h and S500_5h sample, respectively. The observed variation of the stoichiometry with respect to the initial values indicates that a selective dissolution of the FePd alloy occurs in which the removal rate of the Fe atoms is higher than the one of the Pd atoms in all samples. This effect is similar to a dealloying process [18,47–50] in which the less noble element of the alloy, Fe in FePd alloy, is selectively removed by the 2M HCl solution. The percentage reduction of Fe contained Materials in the FePd2020,alloy 13, x FOR PEER before REVIEW and after chemical etching for each sample is shown in Figure 6. 8 of 16 Figure 6. Percentage Figure 6. Percentagereduction reductionof ofFeFecontained containedininthe theFePd FePdalloy alloybefore beforeand andafter chemical after etching chemical asas etching a function a functionof of thethe electrodeposition electrodepositiontime. time. The Fe reduction rate is not constant among samples, but it monotonically increases as a function The Fe reduction rate is not constant among samples, but it monotonically increases as a of the electrodeposition time. This effect can be ascribed mainly to the different fraction of sample function of the electrodeposition time. This effect can be ascribed mainly to the different fraction of surface area exposed to the aggressive 2 M HCl solution. The previous characterisations of Rq and sample surface area exposed to the aggressive 2 M HCl solution. The previous characterisations of Rq < D > values indicate that the as-deposited S500 sample shows the biggest exposed surface area, and < D > values indicate that the as-deposited S500 sample shows the biggest exposed surface area, in agreement with the highest Fe dissolution rate during the chemical process. On the other hand, in agreement with the highest Fe dissolution rate during the chemical process. On the other hand, the as-deposited S50 sample characterised by a low Rq and small grains size exposes a lower surface the as-deposited S50 sample characterised by a low Rq and small grains size exposes a lower surface area to the acid solution making the chemical etching very slow. area to the acid solution making the chemical etching very slow. The morphological transformations of the surface of FePd thin films induced by the chemical The morphological transformations of the surface of FePd thin films induced by the chemical etching were characterised by the aforementioned AFM and SEM techniques. A primary consequence etching were characterised by the aforementioned AFM and SEM techniques. A primary of the selective Fe dissolution is the increase of the average surface roughness (Rq ) and of the associated consequence of the selective Fe dissolution is the increase of the average surface roughness (Rq) and standard deviation. This effect is substantially clear for the S150_5h and S500_5h samples whose Rq of the associated standard deviation. This effect is substantially clear for the S150_5h and S500_5h values increase to 63 and 340 nm, respectively, as shown in the Figure 2. On the contrary, the S50_5h samples whose Rq values increase to 63 and 340 nm, respectively, as shown in the Figure 2. On the contrary, the S50_5h sample keeps almost the same roughness properties as the as-deposited S50 sample indicating a different response to the chemical etching. In addition to the effect on surface roughness, also the grain size is affected by the chemical etching. SEM images in Figure 4d–f show that the arrangement of the spherical grains in S150_5h and S500_5h sample appear less dense with a slightly higher separation among them. No significant change is visible in the S50 sample, where the structure does not seem to be altered by treatment in Materials 2020, 13, 1454 8 of 15 sample keeps almost the same roughness properties as the as-deposited S50 sample indicating a different response to the chemical etching. In addition to the effect on surface roughness, also the grain size is affected by the chemical etching. SEM images in Figure 4d–f show that the arrangement of the spherical grains in S150_5h and S500_5h sample appear less dense with a slightly higher separation among them. No significant change is visible in the S50 sample, where the structure does not seem to be altered by treatment in HCl acid (see Figure 4a,d). In addition, Figure 5b shows the variation of grain size distribution induced by the chemical treatments. The grain size distribution of the S500_5h sample is uniform over a much wider dimensional range than the one of as-deposited S500 sample and with an average grain size reduced to < D > ≈ 282 nm (see Figure 5c). In particular, this enlargement of the distribution induced by the chemical etching occurs completely towards smaller grain sizes while keeping the size of the larger grains unchanged. This change in the distribution shape indicates that the corrosion rate is not uniform on each grain but the as-deposited larger grains are more resistant. This evidence can be explained by assuming that the alloy stoichiometry of the as-deposited larger grains is richer in palladium or consists in pure palladium (as observed in XRD pattern), hindering the entire corrosion process. The Materials chemical 2020, 13, x FORtreatment PEER REVIEW induces similar effects, but with less intensity, also on the grain9 size of 16 distribution of the S150_5h sample (see Figure 5b,c). The average size < D > is slightly reduced and the contrary, thebecomes distribution effects slightly inducedmore by the same chemical asymmetric towardsetching on the lower size whilegrain size distribution maintaining of the a rather narrow S50_5h peak andsample a valuesare dispersion marginal since the average comparable (see to the Figure ones of as5b,c), the dispersion deposited of theOn S150 sample. values and the the contrary, distribution the are comparable effects induced by the sametochemical those of the as-deposited etching on the grain size sample. In agreement distribution to the of the S50_5h EDS sample characterisation are marginal since (see theFigure 6),(see average the Figure topographic anddispersion 5b,c), the morphologicalof the features values andof the the etched samples distribution are surface confirm comparable thatof the to those etching rate sample. the as-deposited of the surface is strongly In agreement correlated to the EDS with the (see characterisation surface area Figure 6), induced the of the as-deposited topographic samples. and morphological features of the etched samples surface confirm that the etching rate of theThe surface surface morphology is strongly of thewith correlated sample stronglyarea the surface influences inducedits of wettability properties the as-deposited [31–35,51] samples. which Thearesurface characterised by measuring morphology the contact of the sample strongly angle between influences its awettability drop of water and the properties surface [31–35,51] itself, see which Figure 7a. The are characterised bymeasured measuringcontact angleangle the contact values are reported between a drop offor all samples water in Figure and the surface 7b itself, where see no significant Figure differences 7a. The measured are observed contact among angle values areas-deposited reported for and etched samples. all samples in FigureIn7bparticular, where no the contactdifferences significant angle values increaseamong are observed as a as-deposited function of and electrodeposition etched samples.time indicating In particular, theacontact more hydrophobic angle surface. as a function of electrodeposition time indicating a more hydrophobic surface. values increase Figure 7.7.(a) (a) Water Water dropdrop on theon the samples samples surface; surface; (b) contact(b) contact angle angle evolution as aevolution function ofas a function of electrodeposition electrodeposition time for as-depositedtime foretched and as-deposited samples.and etched samples. Several models describe the connection between the wettability behaviour of a surface and its roughness by means of some parameters calculated from 2D AFM profiles such as the mean slope of the roughness motifs (1/Kr) and the measure of the steepness of the sample surface (Rq) [30–35]. Both 1/Kr and Rq/quantities, defined in the experimental section, include information both on the horizontal distance and on the vertical profile of the peaks in order to give an overall description of Materials 2020, 13, 1454 9 of 15 Several models describe the connection between the wettability behaviour of a surface and its roughness by means of some parameters calculated from 2D AFM profiles such as the mean slope of the roughness motifs (1/Kr ) and the measure of the steepness of the sample surface (Rq ) [30–35]. Both 1/Kr and Rq /quantities, defined in the experimental section, include information both on the horizontal distance and on the vertical profile of the peaks in order to give an overall description of the contact surface on which the drop is released. The 1/Kr and Rq /parameters were calculated for all studied samples both before and after the etching treatment and their dependence on the measured contact angle is shown in Figure 8. 10 of 16 Materials 2020, 13, x FOR PEER REVIEW Figure 8. Contact angle values as a function of: 1/Kr parameter (dotted line) and Rq /ξ parameter Figure 8. Contact angle values as a function of: 1/Kr parameter (dotted line) and Rq/ξ parameter (dashed line) for as-deposited (full symbols) and etched (empty symbols) samples. (dashed line) for as-deposited (full symbols) and etched (empty symbols) samples. The 1/Kr and R/parameters monotonically increase as a function of contact angle and, therefore, The 1/Kr and R/parameters monotonically increase as a function of contact angle and, therefore, as a function of the deposition time (see Figure 7b) for both as-deposited and etched samples. as a function of the deposition time (see Figure 7b) for both as-deposited and etched samples. This This development indicates a surface evolution over the deposition time, in which the vertical growth development indicates a surface evolution over the deposition time, in which the vertical growth of of the peaks (expressed by R and Rq for Kr and Rq /ξ, respectively) is faster than the increase in the peaks (expressed by R and Rq for Kr and Rq/ξ, respectively) is faster than the increase in their their horizontal distance (expressed by AR and ξ for Kr and Rq /ξ, respectively). This behaviour horizontal distance (expressed by AR and ξ for Kr and Rq/ξ, respectively). This behaviour disadvantages the spreading penetration of the droplet into the sharp local irregularities on the surface, disadvantages the spreading penetration of the droplet into the sharp local irregularities on the increasing the volume of air between the solid surface and the droplet [30]; as a consequence, the contact surface, increasing the volume of air between the solid surface and the droplet [30]; as a angle value increases. Therefore, the wettability of the FePd sample surface decreases as a function of consequence, the contact angle value increases. Therefore, the wettability of the FePd sample surface deposition time becoming more hydrophobic (> 90◦ ). Similar wettability properties of film surface are decreases as a function of deposition time becoming more hydrophobic (> 90°). Similar wettability observed after the chemical etching. properties of film surface are observed after the chemical etching. Room-temperature hysteresis loops of S50, S150 and S500 samples are shown in Figure 9. Room-temperature hysteresis loops of S50, S150 and S500 samples are shown in Figure 9. The The magnetic field is applied in the FePd thin film plane and the curves are normalized to the magnetic field is applied in the FePd thin film plane and the curves are normalized to the magnetisation value at H = 10 kOe. The magnetisation reversal process of the S50 sample displays the magnetisation value at H = 10 kOe. The magnetisation reversal process of the S50 sample displays typical features of a soft magnetic material [52]: a single irreversible magnetisation jump with a small the typical features of a soft magnetic material [52]: a single irreversible magnetisation jump with a coercive field (Hc ≈ 6.3 Oe). This is also evident by analysing the FORC distribution as a function small coercive field (Hc ≈ 6.3 Oe). This is also evident by analysing the FORC distribution as a of the coercive field, reported in Figure 10: the S50 sample is characterised by a sharp coercivity function of the coercive field, reported in Figure 10: the S50 sample is characterised by a sharp distribution at low fields. By increasing the deposition time, the magnetisation reversal as observed coercivity distribution at low fields. By increasing the deposition time, the magnetisation reversal as in the hysteresis loops of Figure 9a takes place on an increasingly wider magnetic field range; the observed in the hysteresis loops of Figure 9a takes place on an increasingly wider magnetic field single irreversible magnetisation jump reduces its intensity whereas a reversible mechanism becomes range; the single irreversible magnetisation jump reduces its intensity whereas a reversible clearly visible with a slow approach of magnetisation to saturation. In addition, the coercive field mechanism becomes clearly visible with a slow approach of magnetisation to saturation. In addition, slightly increases to 11.7 Oe for the S150 sample and to 23.7 Oe for the S500 sample. This is reflected the coercive field slightly increases to 11.7 Oe for the S150 sample and to 23.7 Oe for the S500 sample. in the FORC distributions of Figure 10, whose intensities decrease as the deposition time increases, This is reflected in the FORC distributions of Figure 10, whose intensities decrease as the deposition indicating a progressively reduced contribution of the irreversible processes to the magnetisation time increases, indicating a progressively reduced contribution of the irreversible processes to the magnetisation reversal, at the advantage of reversible rotation mechanisms, which do not contribute to the FORC curves intensity. At the same time, the FORC distribution becomes wider as a function of the coercive field as the deposition time increases, in agreement with hysteresis loops. Materials 2020, 13, 1454 10 of 15 reversal, at the advantage of reversible rotation mechanisms, which do not contribute to the FORC curves intensity. At the same time, the FORC distribution becomes wider as a function of the coercive field as2020, Materials the deposition time 13, x FOR PEER increases, in agreement with hysteresis loops. REVIEW 11 of 16 Figure 9. Figure (a) Room 9. (a) Room temperature temperature hysteresis hysteresis loops loops of of as-deposited as-deposited samples; samples; (b)(b) room room temperature temperature hysteresisloops hysteresis loopsof ofetched etchedsamples samplesin inaa22M M aqueous aqueoussolution solutionof ofHCl HClforfor55h; h; (c) (c) evolution evolutionofof coercive coercive field as a function of electrodeposition time for as-deposited field as a function of electrodeposition time for as-deposited and etched samples. This evolution of magnetic properties is fully compatible with the morphology changes observed by AFM and SEM characterisation and previously discussed. In the case of the S500 sample, the large grain size and the consequent high roughness imply a dominant role of the grain shape and crystal anisotropy in orienting the magnetisation. Therefore, at remanence the magnetisation is mostly randomly oriented in space, following the local easy axes of individual grains; upon increasing the applied field, reversible rotation processes bring the sample magnetisation to saturation. The same picture holds for the S150 sample, where, however, the small grain size and roughness are responsible for a smaller shape and crystal anisotropy energies; as a consequence, the reversible rotation processes affect a smaller fraction of the total magnetisation and the saturation is approached in a narrower field range. The S50 sample, instead, is characterised by smaller grain size and roughness, and a more compact morphology; in this case, inter-grain interactions overcome the single-grain shape and crystal anisotropy energies, inducing the formation of domain structures that cover several grains; their anisotropies are then largely averaged out, resulting in a magnetically soft behaviour with a fast approach to saturation determined by the absence of significant local anisotropies hindering the domain wall motion and forcing magnetisation rotation. The magnetic behaviour above described is isotropic in the film plane for all the as-deposited samples as confirmed by the prefect superposition of the hysteresis loops measured in two in-plane orthogonal direction, see Figure S1 in the Supplementary Materials. Figure 10. First Order Reversal Curves (FORC) distribution profiles as a function of the coercive field, taken at a bias field HB = 0 for all samples, except S150. Full symbols correspond to as-deposited samples; the equivalent open symbols correspond to the samples submitted to chemical etching. This evolution of magnetic properties is fully compatible with the morphology changes observed by AFM and SEM characterisation and previously discussed. In the case of the S500 Figure 9. (a) Room temperature hysteresis loops of as-deposited samples; (b) room temperature Materials 2020, 13,loops hysteresis 1454 of etched samples in a 2 M aqueous solution of HCl for 5 h; (c) evolution of coercive 11 of 15 field as a function of electrodeposition time for as-deposited and etched samples. Figure Figure 10. First First Order OrderReversal ReversalCurves Curves(FORC) (FORC) distribution distribution profiles profiles as aasfunction a function ofcoercive of the the coercive field, field, takentaken at a bias at a bias HB =H0B =for fieldfield 0 for all all samples, samples, except except S150.Full S150. Fullsymbols symbolscorrespond correspond to as-deposited as-deposited samples; samples; the the equivalent open symbols correspond to the samples submitted to chemical etching. Once evolution This the samplesof are submitted magnetic to chemical properties etching, is fully some important compatible changes with the in the hysteresis morphology changes loops take place, as shown in Figure 9b, as well as in FORC distributions vs. coercivity, observed by AFM and SEM characterisation and previously discussed. In the case of the S500 depicted in Figure 10. Overall, the sample deposited for 50 s remains the softer and with the steeper approach to saturation, whereas for the other two samples the contribution of magnetisation rotation toward saturation seems to have increased even further. Indeed, while for all three deposition times the FORC distribution has a lower intensity after HCl etching, the decrease is way more pronounced for the S150_5h and S500_5h samples with respect to the S50_h one, indicating, for them, a predominant role of reversible rotation processes in the magnetisation reversal. However, the most significant change with respect to the non-etched samples is the large variation of coercive field, summarised in Figure 9c. As discussed earlier, the effect of the chemical etching process is to reduce and spread the grains size, make the grains more far apart, and increase the roughness. This is reflected by the FORC distribution of the three etched samples, that with respect to their as-deposited counterparts display a much larger spreading of coercive field values, that are due to the increased size distribution of the grains induced by exposition to HCl. As the grains are also more far apart, they also become more magnetically decoupled; as a result, their reciprocal interactions become weaker with respect to the as-deposited films. For the S50_5h sample this means that the mechanism of local averaging out of the magnetic anisotropy is less efficient, and the coercive field increases. However, inter-grains coupling is still largely present, due to the more compact morphology of this sample, that is little affected by the chemical etching. As a consequence, the hysteresis loop still displays a fast approach to saturation after the coercive field has been overcome, with just a slightly more pronounced contribution of magnetisation rotation toward saturation. At the opposite end, the S500_5h sample incurs in a similar evolution with respect to its as-deposited counterpart: the coercive field increases, and the loop branches remain separated over a larger field interval, indicating that irreversible processes are spread over a larger portion of the hysteresis loop, in agreement with the much wider grains size distribution measured for this sample. Materials 2020, 13, 1454 12 of 15 The effects of the chemical etching on the magnetic properties are particularly evident in the S150_5h sample, displaying the largest coercivity value and the most significant change of loop and FORC distribution shape with respect to the as-deposited specimen. While grains borders etching, spreading of grains size, and their magnetic decoupling are still responsible for the observed variations of loop shape and coercive field values, in the S150_5h sample their effects are enhanced. Indeed, while for the 50 s deposition time the etching process is not sufficient to decouple the grains, and for the 500 s deposition time they were already largely independent in the as-prepared state, the specimen deposited for 150 s incurs in a more complex evolution: in its as-prepared state, and differently from all the other samples, the FORC distribution as a function of coercivity has its peak at a bias field of -8 Oe, as indicated in Figure 10. The presence of such a bias field indicates that the S150 sample is in a state where the dominant ferromagnetic behaviour is attributed to the magnetic properties of the individual grains plus the effects of their mutual interactions; conversely, the S50 sample magnetic properties are attributed to a collective ferromagnetic behaviour of the grains, that lose their independence in favour of an averaging out of the local anisotropies, whereas for the S500 one the magnetic properties are mostly due to the sum of those of the individual grains. After chemical etching, the significant magnetic decoupling of the grains occurring in the S150_5h sample induces the disappearance of the bias field in the peak of the FORC distribution, that goes back to HB = 0 Oe, and the particularly enhanced changes of loop and FORC distribution shapes. After the chemical etching, the isotropic magnetic behaviour in the film plane of all sample is preserved, see Figure S1 in the Supplementary Materials. 4. Conclusions Potentiostatically electrodeposited FePd alloy thin films are characterised by different morphological, structural and magnetic properties as a function of electrodeposition time. In addition to the desired α-(Fe, Pd) solid solution phase, XRD structural analysis reveals the formation of a pure palladium phase in the S500 sample and a tetragonal Pd1.5 H2 phase in all samples due to the high affinity between hydrogen and palladium. The increase of |j|(t) over deposition time indicates a progressive increase of surface area of the working electrode with direct and strong influence on the surface roughness and the grain size distribution of the FePd thin film. The FePd film surface appears dense and flat with small and uniformly distributed grains for deposition time of about 50 s; for longer deposition time, an increasingly rapid growth of independent spherical grains is observed favouring a reduction of film density with a strong increase of surface roughness and grain size. Magnetic properties are fully compatible with the structural and morphological properties of the as-deposited samples. Soft magnetic features of hysteresis loop with a single irreversible magnetisation jump and small coercive field are observed in S50 sample indicating that the magnetic anisotropy on the film plane can be easily overcome. For longer deposition times, the increase of the roughness and of the grain size strongly affects the magnetisation processes in which reversible rotations appear to reach the magnetic saturation. As-deposited samples show different response to chemical treatment (5 h in a 2 M aqueous solution of HCl). In general, the observed variation of the Fe:Pd ratio induced by the chemical etching indicates that a selective dissolution of Fe in the FePd alloy occurs in which the removal rate of the Fe atoms is higher than the one of the Pd atoms in all samples. However, differences arise among the different samples. The flat and homogeneous surface of the S50 sample allows slow chemical etching and, as consequence, the roughness and the average grain size keep almost the same values after the treatment. On the contrary, the chemical etching is more efficient on the S500 sample surface characterised by the biggest exposed surface area. In this case, a huge increase of the surface roughness and a remarkable reduction of the grain size is observed with a slightly higher reciprocal distance; however, the corrosion rate is not uniform on each grain but the as-deposited larger grains appear more corrosion Materials 2020, 13, 1454 13 of 15 resistant. The most significant change in the magnetic properties of the etched samples with respect to the non-etched ones is the large increase of coercive field. This effect is due to the morphological evolution of grains that induce weaker inter-grain interactions with respect to the as-deposited films. Single-grain anisotropy (both shape and crystal) becomes more important over collective behaviour in the etched samples, thanks to the magnetic decoupling of the grains, that is particularly evident in the S150_5h sample. Even though the in-plane isotropic behaviour of all samples is not modified by the chemical treatment, etched samples develop a magnetic behaviour more dominated by single-grain properties, which results in an increased coercivity and a reduction of the irreversible contributions to the magnetisation reversal, in favour of reversible rotations. A hydrophobic behaviour of the surface is observed in all FePd thin films. As a function of electrodeposition time, the 1/Kr and R/ parameters indicate that the vertical growth of the surface peaks develops faster than the increase in their horizontal distance disadvantaging the spreading penetration of the water drop into the sharp local irregularities on the surface and, as a consequence, the contact angle value is observed to increase. No significant differences on the wettability properties of film surface are observed after the chemical etching. In conclusion, this work provides a comprehensive characterisation of the fundamental properties of these FePd metallic thin films and demonstrates the ability to fine tune these properties opening the way to further advanced studies focused on their possible use as catalysts with improved features of activity, selectivity and stability combined to the possibility to recover and reuse them thanks to their specific magnetic properties. Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/6/1454/s1, Figure S1: (a) Room temperature hysteresis loops of as-deposited samples; (b) room temperature hysteresis loops of etched samples in a 2 M aqueous solution of HCl for 5 h. 0◦ and 90◦ labels indicate the two orthogonal in-plane directions along which the hysteresis loops are measured, Figure S2: FM profiles of the step height between the substrate and the electrodeposited S500, S150 and S50 samples. Author Contributions: Conceptualization, P.T. and P.R.; preparation of the samples, M.C. (Matteo Cialone), F.S. and F.C.; SEM characterisation and morphological data analysis, F.C. and G.B.; AFM characterisation and data analysis, F.C. and G.B.; XRD characterisation and structural data analysis, F.S., M.C. (Matteo Cialone) and P.R.; EDS characterisation and data analysis, F.C., G.B. and M.C. 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