nanomaterials Article Electrochemical Synthesis, Magnetic and Optical Characterisation of FePd Dense and Mesoporous Nanowires Deepti Raj 1 , Gabriele Barrera 2 , Federico Scaglione 1 , Federica Celegato 2 , Matteo Cialone 1,3 , Marco Coïsson 2 , Paola Tiberto 2 , Jordi Sort 4,5 , Paola Rizzi 1, * and Eva Pellicer 4 1 Dipartimento di Chimica e Centro Interdipartimentale NIS (Nanostructured Interfaces and Surfaces), Università di Torino, Via Pietro Giuria 7, 10125 Torino, Italy 2 Istituto Nazionale di Ricerca Metrologica (INRIM), Str. delle Cacce 91, 10135 Torino, Italy 3 Physics Department, University of Genova, Via Dodecaneso 33, 16146 Genova, Italy 4 Universitat Autònoma de Barcelona, Campus de la UAB, Bellaterra, 08193 Barcelona, Spain 5 Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain * Correspondence: paola.rizzi@unito.it Abstract: Dense and mesoporous FePd nanowires (NWs) with 45 to 60 at.% Pd content were suc- cessfully fabricated by template- and micelle-assisted pulsed potentiostatic electrodeposition using nanoporous anodic alumina and polycarbonate templates of varying pore sizes. An FePd electrolyte was utilized for obtaining dense NWs while a block copolymer, P-123, was added to this electrolyte as the micelle-forming surfactant to produce mesoporous NWs. The structural and magnetic properties of the NWs were investigated by electron microscopy, X-ray diffraction, and vibrating sample mag- netometry. The as-prepared NWs were single phase with a face-centered cubic structure exhibiting 3.1 µm to 7.1 µm of length. Mesoporous NWs revealed a core-shell structure where the porosity was only witnessed in the internal volume of the NW while the outer surface remained non-porous. Magnetic measurements revealed that the samples displayed a soft ferromagnetic behavior that depended on the shape anisotropy and the interwire dipolar interactions. The mesoporous core and dense shell structure of the NWs were seen to be slightly affecting the magnetic properties. Moreover, mesoporous NWs performed excellently as SERS substrates for the detection of 4,40 -bipyridine, show- Citation: Raj, D.; Barrera, G.; Scaglione, F.; Celegato, F.; Cialone, ing a low detection limit of 10−12 M. The signal enhancement can be attributed to the mesoporous M.; Coïsson, M.; Tiberto, P.; Sort, J.; morphology as well as the close proximity of the embedded NWs being conducive to localized Rizzi, P.; Pellicer, E. Electrochemical surface plasmon resonance. Synthesis, Magnetic and Optical Characterisation of FePd Dense and Keywords: nanowires; FePd; mesoporous; electrodeposition; SERS Mesoporous Nanowires. Nanomaterials 2023, 13, 403. https:// doi.org/10.3390/nano13030403 Academic Editor: Olivier Thomas 1. Introduction Metallic nanostructures hold promising potential for applications in different fields, Received: 20 December 2022 such as nanoactuators, sensing, catalysis, etc. [1]. Besides, mesoporous metallic nanostruc- Revised: 13 January 2023 tures have been increasingly showcasing their capability in an abundance of applications Accepted: 16 January 2023 including voltage-control of magnetism, electrocatalysis, sensing, etc., owing to their Published: 19 January 2023 porous architecture (pore sizes in the range of 2 nm to 50 nm), high thermal and electri- cal conductance, and other interesting properties [2–4]. In the same thread, dense and mesoporous nanowires (NWs) possess an array of applications credited to their elongated Copyright: © 2023 by the authors. shape and anisotropic physical and magnetic properties, such as in biomedicine, biotech- Licensee MDPI, Basel, Switzerland. nology, ultrahigh-density magnetic storage, storage media applications, electrocatalysis, This article is an open access article surface-enhanced Raman spectroscopy and microelectromechanical systems (MEMS) [5–8]. distributed under the terms and Specifically, bimetallic alloys with a magnetic transition element (Fe, Co, Ni) have garnered conditions of the Creative Commons immense attention for their multifaceted nature [9,10]. FePd, being one such bimetallic alloy, Attribution (CC BY) license (https:// holds excellent promise where properties of the transition metal (Fe) merge with that of the creativecommons.org/licenses/by/ noble metal (Pd) [11–13]. The intrinsic magnetic properties of Fe can be easily exploited in 4.0/). Nanomaterials 2023, 13, 403. https://doi.org/10.3390/nano13030403 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2023, 13, 403 2 of 18 an FePd alloy. This alloy also possesses enhanced catalytic efficiency among other applica- tory capabilities [14]. There have been numerous applications displayed by FePd alloys, e.g., hydrogen separation, environmental remediation, membrane hydrogenation reactions, sensors and actuators [15], magnetic drug delivery [16], and catalysis [17,18]. They have also garnered extensive attention attributed to their extraordinary magnetostrictive and shape memory effects [19]. Besides, FePd alloy can be dealloyed by readily dissolving Fe, leading to the formation of a nanostructured Pd-rich material with its own unique properties (such as a large surface area to volume ratio) [10,20]. For the fabrication of NWs, electrodeposition within nanoporous templates is an impressively proficient method [15]. Not only it is a cost-effective and high throughput synthesis technique, but it also allows for fine-tuning of the attributes of the target sample, such as chemical composition, and physical and mechanical properties, by simply and conveniently altering the deposition parameters [21]. For co-depositing two metals, their respective reduction potentials should be (nearly) the same [22]. But that is not the case for co-depositing Fe and Pd as both have a large difference in their reduction potentials, which makes it challenging. Thus, the FePd electrodeposition essentially requires a suitable complexing agent that would shift the reduction potential of Pd towards the less-noble Fe while keeping the electrolyte chemically stable [22–24]. In addition, hydrogen evolution takes place during the process as a side reaction [15]. The generated gas bubbles might block the pores and thus interrupt the metal deposition [15]. Hence, to avoid this from happening, the pulse technique is employed, which has been shown to minimize the blockage of pores [15]. Furthermore, the formation of a high number of OH- ions within the pores as a result of hydrogen evolution leads to facile precipitation of hydroxides, in particular Fe(OH)3 [15]. To prevent this, 5-sulfosalicylic acid is added, which forms a stable Fe3+ complex precluding hydroxide formation [15]. The pulse plating technique also decreases the occurrence of concentration gradients and keeps the pH constant. During the off-time, no metal deposition takes place. This allows for the balancing of concentration gradients of metal ions as well as the rise of hydrogen bubbles [25]. Notably, dense Fe-Pd NWs have been previously electrodeposited but no studies have focussed on fabricating their mesoporous counterparts, to the best of our knowledge [19,22,26–29]. When it comes to the synthesis of mesoporous metallic nanostructures, typical pro- cesses include soft-templating, hard-templating, and solution-phase approaches [30]. Specif- ically, soft templates such as micelle assemblies have come to the forefront in recent years. Micelle assemblies were first brought to light by Yamauchi and co-workers for the elec- trodeposition of Pt metal from diluted (1.0 wt.%) non-ionic surfactant solutions [31]. This procedure has also been extended to bimetallic systems [30]. Among all soft-templating methods, electrochemical deposition is the most suitable and straight-forward technique to reduce the metal ions in a solution [30]. So far, commercial amphiphilic polymers like P-123, and F-127 of Brij-58, have been used as common non-ionic surfactant assemblies above their critical micelle concentration (cmc) [30]. P-123 forms micelles in acidic water above its critical micelle concentration, with the core made of poly(propylene oxide) (PPO) block and the shell of poly(ethylene oxide) (PEO) units. During the electrodeposition process, P-123 micelle assemblies gather spontaneously at the solid–liquid interface, and the cationic species (e.g., Pd(II)) can coordinate with the shell of these micelle self-assemblies [32]. In this way, porosity can be generated in the electrodeposited NWs, which is critically dependent on the extent and strength of this coordination [30]. Surface-enhanced Raman scattering (SERS) is a well-known and remarkable sensing technique that facilitates trace-level molecular detection and also finds a vast range of usage in analytical applications, such as in catalysis, biology, biomedicine, materials science, food science, art conservation, environmental analysis, etc. [33,34]. SERS utilizes a nanostruc- tured metallic substrate, which adsorbs the analyte molecules and amplifies the Raman signals by factors up to 108 or even larger. Both electromagnetic and chemical enhance- ments have been attributed as the reason behind SERS enhancement, but the former is more widely accepted [35]. This is based on the phenomenon of localized surface-plasmon reso- Nanomaterials 2023, 13, 403 3 of 18 nance, LSPR, which involves the excitement of the surface-electron oscillations caused by resonant coupling of the incident light and surface-plasmons of the substrate, resulting in overall signal enhancement [36–38]. It also encompasses the generation of ‘hotspots’ which are regions of highly intense electric field The origin of chemical enhancement lies in the resonant effect arising from the analyte-substrate charge-transfer complex [39]. Although Au and Ag-based substrates have been established as the best for SERS, it is still crucial to develop alternative SERS-active substrates to make this technique more cost-effective, precise, and sustainable [40]. In this paper, we present the successful fabrication of a series of dense and mesoporous FePd NWs with different compositions and diameters. The structural and magnetic proper- ties of the obtained NWs were investigated. Mesoporous NWs were tested as SERS-active substrates and an excellent performance was recorded. 2. Materials and Methods The FePd NWs with varying compositions and diameters, both dense and mesoporous, were fabricated by pulsed potentiostatic template-assisted electrodeposition. Commercial Anodisc 25 (Whatman, Cytiva, Marlborough, MA, USA) was used as the Anodised Alu- minium Oxide (AAO) template discs with a diameter of 25 mm, membrane thickness of 60 µm, and average pore sizes of 20 nm and 100 nm. Additionally, commercially avail- able Polycarbonate (PC) (Whatman, Cytiva, Marlborough, MA, USA) templates of pore size 30, 50, and 100 nm were used. These membranes act as hard/soft templates for the growth of the NWs. A 30 nm thick Au layer was sputter-deposited onto the backside of the membranes using an AJA international sputtering system (AJA International, Inc. Scituate, MA, USA) under an argon atmosphere to provide electrical contact and serve as working electrodes for electrodeposition. To synthesize the dense FePd NWs, an aqueous electrolyte containing 0.01 M Tetraam- minepalladium(II) chloride Pd(NH3 )4 Cl2 (Merck KGaA, Darmstadt, Germany), 0.05 M Iron(III) sulfate hydrate Fe2 (SO4 )3 xH2 O (Merck KGaA, Darmstadt, Germany), 0.06 M 5- Sulfosalicylic acid dihydrate (SSA) C7 H6 O6 S·2H2 O (Merck KGaA, Darmstadt, Germany), and 0.3 M Ammonium sulfate (NH4 )2 SO4 (Merck KGaA, Darmstadt, Germany) was used. This electrolyte was taken from earlier publications [23,41,42] and will be referred to as the FePd electrolyte in future mentions. For the preparation of mesoporous NWs, a micelle- forming surfactant was added to the aqueous FePd electrolyte, i.e., P123 triblock co-polymer (Merck KGaA, Darmstadt, Germany), which induces porosity by its micellar action. First, it was dissolved in the Pd(NH3 )4 Cl2 (1:1 ratio, e.g., 15 mg P-123 in 15 mL Pd(NH3 )4 Cl2 ) and stirred at room temperature for two days [30]. Afterwards, when P123 was completely dissolved, 0.05 M Fe2 (SO4 )3 xH2 O, 0.06 M SSA, and 0.3 M (NH4 )2 SO4 were added to this (Pd(NH3 )4 Cl2 + P123) aqueous solution. This electrolyte will be referred to as FePd+P123 in subsequent sections of this paper. The pH of both the aqueous electrolytes was adjusted to 5.0 for all experiments using ammonium hydroxide. All the solutions were prepared from reagents grade chemicals and de-ionized water. All the electrodeposition experiments were carried out at room temperature under stagnant conditions. A typical three-electrode setup was used—a Pt spiral as counter electrode and a saturated Ag/AgCl (3 M) reference electrode. The metalized membranes were placed vertically within the electrolyte using a custom-made holder. All electrode po- tentials were in reference to the potential of the Ag/AgCl electrode. Electrodeposition was carried out in pulsed potential mode using PGSTAT302N Autolab potentiostat/galvanostat (Metrohm-Autolab B. V. Utrecht, The Netherlands). Two alternating potential steps were applied—the deposition potential Eon and the resting potential Eoff . The duration for which Eon was applied was termed here as ton , which was set at 30 s, while that for Eoff was toff at 90 s. The whole electrodeposition was repeated 60 times for 2 h. Here, Eon was varied from −1.05 V to −1.3 V to obtain NWs with different compositions while Eoff was kept constant at −0.46 V vs. Ag/AgCl. A representative schematic diagram of pulsed poten- tiostatic electrodeposition is provided in Figure S1 of the Supplementary File. Following Nanomaterials 2023, 13, 403 4 of 18 electrodeposition, the samples were repeatedly rinsed with de-ionized water. To release the NWs, the AAO template was dissolved in 10% w/w NaOH solution at 80 ◦ C for 10 min while the PC template was dissolved in CHCl3 at room temperature for a few minutes. The supernatant was then removed and the freed NWs were washed several times in distilled water. Afterward, the NWs were kept dispersed in ethanol and sonicated for a few minutes. The evolution of the morphology and the stoichiometry was studied via a Zeiss Merlin field-emission scanning electron microscopy (FESEM) (Zeiss Group, Oberkochen, Ger- many) equipped with an energy-dispersive X-ray spectrometer (EDS) (Oxford Instruments, Abingdon, UK) and a JEOL JEM-2011 High-resolution transmission electron microscope (HRTEM) (JEOL Ltd., Tokyo, Japan) operated at 200 kV. To observe them under FESEM, a small blob of ethanol containing the dispersed NWs was dropped onto the carbon tape cov- ering a FESEM stub. The structure of the samples was examined with a Panalytical X-pert X-ray Diffractometer in Bragg–Brentano geometry (Panalytical, Almelo, The Netherlands) with monochromatic Cu Kα radiation. Room-temperature magnetic measurements were performed using a vibrating sample magnetometer (VSM) (Lake Shore Cryotronics Inc., Westerville, OH, USA) operating with an applied magnetic field (H) in the range ±15 kOe. The hysteresis curves were obtained by applying H along the direction parallel (PA) and perpendicular (PE) to the major axis of the NWs. The dia-/para-magnetic signal of the sample-holder and the porous membranes was duly taken into account and properly subtracted. SERS measurements were carried out using a Renishaw inVia Raman Microscope (Renishaw, Wotton-under-Edge, England) with 785 nm laser line (50% power, 20 × ULWD objective, 10 acquisitions with acquisition time of 40 s each), and 4,40 -bipyridine (bipy) as the probe molecule. The NWs were kept embedded in the template and subjected to a 6-min treatment in 10% w/w NaOH at 80 ◦ C to partially dissolve the template and expose the NWs for the SERS measurement. Then they were rinsed with distilled water and air-dried. Freshly prepared ethanol solutions of bipy of low concentrations were utilized namely, 10−10 M and 10−12 M. The NWs were incubated in each solution for 20 min followed by air-drying. Finally, the measurements were performed. 3. Results and Discussion 3.1. Dense Nanowires Dense and long NWs of different diameters and compositions were prepared using the FePd electrolyte with varying deposition potentials. Figure 1 exhibits the FESEM images of dense NWs obtained from electrodepositing at –1.2 V vs. Ag/AgCl over Au sputtered-AAO membranes of pore sizes of 20 and 100 nm. In Figure 1a, an array of NWs attached to the Au layer can be observed, showing the typical branched morphology at the top inherited from the AAO membrane. They are continuous with slightly different lengths, the longest being 4.7 µm. The mean diameter was calculated to be 157 nm with a standard deviation of 22 nm. As evidenced in Figure 1b, a magnified look at a single NW proves that these NWs are clearly dense. Remarkably, the NW diameters did not match the nominal pore size. This observation has been previously reported for AnodiscTM membranes from Whatman. The difference was particularly large for the 20 nm pore-size membranes [43–45]. Figure 1c shows a couple of dense NWs obtained at the same Eon value on an AAO membrane of 100 nm in diameter, featuring a composition of Fe56 Pd44 ; hence, similar to the previous case. The flat surface on which these NWs lay was the carbon tape used to observe them under the FESEM. Nanomaterials 2023, 13, Nanomaterials 2023, 13, 403 x FOR PEER REVIEW 55 of of 18 Figure 1. Figure 1. FESEM FESEMimages imagesofofFePd FePdNWs NWs obtained by by obtained electrodeposition at Eat electrodeposition = on on E –1.2 =V on AAO −1.2 V on mem- AAO branes of (a,b) 20 nm and (c) 100 nm in diameter. membranes of (a,b) 20 nm and (c) 100 nm in diameter. Nanomaterials 2023,13, Nanomaterials2023, 13,403 x FOR PEER REVIEW 6 6of of 18 For For the the sake sake of of comparison, comparison, another another set set of of dense dense NWs was obtained using the PC membrane membrane for for electrodeposition. electrodeposition. The The electrodepositing electrodepositing electrolyte electrolyte was was the the same, same, i.e., i.e., FePd. FePd. Figure 2 assembles assembles all the dense dense NWs NWs obtained obtained from electrodepositing electrodepositing over over PC membranes membranesat atdifferent differentdeposition depositionpotentials. potentials. Figure Figure 2a2a shows the FESEM image of dense NWs produced at E = − 1.05 V attached to NWs produced at Eonon = –1.05 V attached to the Au layer. The the Au layer. The composition, composition, as as determined determined by EDS, is Fe Pd by EDS, is Fe5555Pd45 . The NWs are cylindrical in shape, with a rough 45. The NWs are cylindrical in shape, with a rough texture on the texture on the outer outer surface, surface,as asshown shownin inthe theinset insetofof Figure Figure 2a.2a. The The NW in the same inset has a small diameter of of59 59nm nmatatthethetiptipthat thatbecomes becomes gradually gradually larger throughout larger throughout thethe length, length,going goingup toup161 nm. to 161 This nm. cigar-like shaped This cigar-like morphology shaped morphology on NWs on NWsgrown on PC grown ontemplates PC templates has been previously has been previ- described [46]. Figure ously described 2b shows [46]. Figure 2b showsthe FESEM the FESEM image of NWs image of NWs obtained obtained at at EonEon==−–1.15 1.15 V V where the NWs were attached to the Au layer, after dissolving where the NWs were attached to the Au layer, after dissolving the template. The chemical the template. The chemical composition composition of of the theNWs NWsremained remainedvirtually virtually thethe same same(Fe (Fe5151Pd Pd49 ), and 49), and waswas similar similar to to the the previous value found on NWs grown on AAO previous value found on NWs grown on AAO templates (Fe47Pd4753) (see templates (Fe Pd ) (see Table 1). 53 Table 1). The long- The longest est NWNW holdsholds a length a length of 4.6 of 4.6 µ m.µm. Similarly, Similarly, these these NWsNWs possess possess a cylindrical a cylindrical shape shape but but a asmoother smoothersurfacesurfacetexture, texture, visible visible in in thethe inset inset of Figure of Figure 2b.2b. TheThe twotwo NWs NWs displayed displayed in in this this insetinset havehave a small a small diameter diameter at the at the tip tip of ~51of ~51 nm,nm, while while a widera wider diameter diameter of 149 of 149 nm nm on on average is determined for this sample. The FESEM images average is determined for this sample. The FESEM images of NWs electrodeposited at Eon of NWs electrodeposited at on ofV− ofE–1.2 1.2 –1.3 and V and V on−1.3 V on a PC a PC membrane membrane of 100 pore of 100 nominal nominal size pore size are in are depicted depicted Figure in Figure 2c,d, respectively. The achieved compositions 2c,d, respectively. The achieved compositions were Fe55Pd45 and Fe were Fe Pd 5560Pd and Fe 60 Pd40 4540, suggesting a, suggesting a slight enrichment in Fe upon making the applied slight enrichment in Fe upon making the applied potential more negative (Table 1). This potential more negative (Table 1). This observation hasobservation also been made has also beenauthors by other made by other when authors when depositing FePd depositing NWs from aFePd am- NWs from a ammonium-sulfosalicyclic electrolyte [25]. The monium-sulfosalicyclic electrolyte [25]. The longest NWs were 5.2 and 7.1 µ m in length, longest NWs were 5.2 and 7.1 µm in length, respectively. respectively. Figure2.2. FESEM Figure FESEM images of of FePd FePdNWsNWsobtained obtainedby byelectrodeposition electrodepositiononon PCPC membrane at E membrane atonEof on(a) of –1.05 (a) V, (b) −1.05 –1.15 V, (b) V, (c) −1.15 V, –1.2 (c) −V, 1.2and (d) –1.3 V, and (d) −V.1.3 V. Nanomaterials 2023, 13, 403 7 of 18 Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 18 Table 1. Eon values used for the growth of dense and mesoporous FePd NWs, the resulting chemical composition, template used, mean diameter, and maximum length. Table 1. Eon values used for the growth of dense and mesoporous FePd NWs, the resulting chemical composition, template used, mean diameter, and maximum Template, length. Mean Pore Maximum NW Type –Eon (V) Composition Size (nm) Diameter (nm) Length (µm) NW Type –Eon (V) Composition Template, Pore Size (nm) Mean Diameter (nm) Maximum Length (µm) 1.05 Fe55 Pd45 PC, 50 135 4.6 1.05 Fe55Pd45 PC, 50 135 4.6 Fe51 Pd49 PC, 50 113 4.7 Fe51Pd49 1.15 PC, 50 113 4.7 1.15 Fe Pd AAO, 20 174 5.7 Fe47Pd53 AAO, 20 47 53 174 5.7 Fe52Pd48 Fe PC, 30 52 48 Pd PC, 30 65 65 3.1 3.1 Dense Fe55Pd45 PC, 100 Fe55 Pd45 PC,222 100 222 5.2 5.2 Dense Fe50Pd50 1.20AAO, 20Fe50 Pd50 1.20 157 AAO, 20 157 4.7 4.7 (Dense-A) (Dense-A) Fe56Pd44 Fe Pd AAO, 100 56 44 AAO,197100 197 3.5 3.5 (Dense-B) (Dense-B) 1.30 Fe60Pd40 1.30PC, 100 Fe60 Pd40 PC,156 100 156 7.1 7.1 Fe40Pd60 AAO, 20Fe40 Pd60 168 AAO, 20 168 3.2 3.2 (Porous-A) (Porous-A) Mesoporous 1.20 Mesoporous 1.20 Fe40Pd60 Fe Pd AAO, 100 40 60 AAO,197100 197 2.6 2.6 (Porous-B) (Porous-B) 3.2. Mesoporous Nanowires Along with the dense NWs, mesoporous counterparts have also been produced by electrodepositing from from the theFePd FePd++P123 P123 electrolyte at at electrolyte =− EonE=on–1.2 V1.2 vs.VAg/AgCl over 20over vs. Ag/AgCl and 100and 20 nm Au-sputtered AAO templates; 100 nm Au-sputtered from here onwards AAO templates; from herethe former will onwards the be addressed former as will be Porous-A while addressed the latter as Porous-A as Porous-B while the lattersamples. Figure as Porous-B 3 displays samples. the 3pulsed Figure potentiostatic displays the pulsed potentiostatic transientsand transients for Porous-A for Porous-B Porous-Awhereand Porous-B where current vs. timecurrent vs. time is recorded. The is recorded. electrodep- The electrodeposition osition in both cases in both cases appeared to beappeared homogeneousto be homogeneous and consistent.and consistent. 3. Pulsed potentiostatic transients recorded during the deposition of FePd NWs on AAO Figure 3. 100 nm templates of 20 and 100 nm in in diameter, diameter, both both for for EEon on = = –1.2 −1.2V.V. Figure Figures4a,b 4a,bdisplay displaythe FESEM the FESEM images images of of thethe first kind, first i.e.,i.e., kind, Porous-A. In Figure Porous-A. 4a the In Figure 4a collection of FePd NWs can be seen attached to the Au layer. In the inset, the collection of FePd NWs can be seen attached to the Au layer. In the inset, a free NW a free NW can be canviewed in a in be viewed well-defined way.way. a well-defined It hasIt the hascharacteristic branched the characteristic structure branched formed structure as a formed result of the AAO membrane. The surface is regular and smooth. No as a result of the AAO membrane. The surface is regular and smooth. No mesoporosity mesoporosity could could be observed at this level. But as we increased the magnification further, as show- cased in Figure 4b, a clear mesoporosity was evidenced at the top of the NW. Interestingly Nanomaterials 2023, 13, 403 8 of 18 Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 18 be observed at this level. But as we increased the magnification further, as showcased in enough, Figure 4b,this mesoporosity a clear mesoporosity was was onlyevidenced present throughout at the top the inside of the NW.the NW while the Interestingly outer enough, this mesoporosity was only present throughout the inside the NW while the outer gives part was dense. This essentially formed a core-shell structure. The inset of Figure 4b part an even was dense.better Thislook at the top essentially of the aNW, formed highlighting core-shell the mesoporosity. structure. The inset of Figure The composition 4b gives an was determined even better look at to the be Fe top40Pd , slightly of 60the enriched in the NW, highlighting Pd mesoporosity. as compared toThe thecomposition composition previously was determineddetermined to be Fefor40 dense Pd 60 , counterparts slightly enriched obtained in Pd at as the same compared Eto For on. the the Porous-B composition sample, similar observations have been made. In Figure 4c the previously determined for dense counterparts obtained at the same Eon. For the Porous-B top view of the NW array is apparent which in Figure 4d is visible at a higher magnification sample, similar observations have been made. In Figure 4c the top view of the NW array and in a better clarity. isThe inset ofwhich apparent Figurein4dFigure further 4dreveals is visibletheatpresence a higherofmagnification mesoporosity and throughout in a better theclarity. inside of the The NW. inset For this4dsample of Figure furthertoo,reveals the core-shell the presence structure is obvious.throughout of mesoporosity At this stage, theitinside is not clear of theyet NW. why Formesoporosity this sample too, did the not core-shell span the entire NW.isWe structure hypothesize obvious. At this that the micelles stage, it is not do not clear yetefficiently interact with why mesoporosity didthe notsurface span the of AAO entirechannels NW. We and a continuous, hypothesize non-porous that the micelles shell do notforms. Meanwhile, efficiently micelles interact with could be the surface efficiently of AAO adsorbed channels on the as-grown and a continuous, dense non-porous FePdforms. shell shell and mesoporosity Meanwhile, micelleswould couldthen develop. Figure be efficiently adsorbed5 shows on thethe EDS maps as-grown dense of FePd a Po- shell rous-B andNW,mesoporosity establishing would then develop. the presence of Pd Figure and Fe5 homogeneously shows the EDS maps spread of throughout a Porous-B NW, establishing the NW, the presence so suggesting of Pd and the presence of aFe homogeneously single phase formed spread duringthroughout the NW, the synthesis. No so suggesting oxidation the presence is observed and theof aoverall single phase formedofduring composition the NWs the comes synthesis. out toNobe oxidation Fe40Pd60. isTable observed and the the 1 summarizes overall composition similarities of the NWs and differences comesPorous-A between out to beand Fe40Porous-B Pd60 . Table 1 sam- summarizes ples. the similarities and differences between Porous-A and Porous-B samples. Figure4.4.SEM Figure SEMimages imagesofofFe Fe4040Pd Pd60 60 mesoporous NWs: (a) mesoporous NWs: Porous-A, inset (a) Porous-A, inset shows shows aa single single free free NW NWatat higher magnification; (b) magnified view of Porous-A; (c) Porous-B; (d) magnification of Porous-B; higher magnification; (b) magnified view of Porous-A; (c) Porous-B; (d) magnification of Porous-B; and insets of (b) and (d) give a closer look at the mesoporosity. and insets of (b,d) give a closer look at the mesoporosity. From the Fe-Pd phase diagram, it is apparent that the Fe40 Pd60 alloy in stable con- ditions must be constituted by two phases: FePd and FePd3 . Nevertheless, the EDS map (Figure 5) shows a homogeneous distribution of Fe and Pd atoms along the sample, in- dicating the formation of just one phase with composition Fe40 Pd60 . To determine the structure of the phase observed by the EDS map, XRD was performed on the as-deposited Nanomaterials 2023, 13, 403 9 of 18 Porous-A NW array and is reported in Figure 6. The NWs were kept embedded in their AAO template for obtaining the XRD pattern. Reflections related to an fcc FePd crystalline phase were detected and no secondary phases were present. This result confirmed the EDS Figure 5. EDS observation and maps of Porous-B showed that the NW (a)electrodeposition use of showing the presence and distribution can promote of Pd (b) and Fe (c the crystallization throughout of the length an FePd phase rich inofPdthe byNW. hindering the formation of FePd3 . From a rough estimation of the lattice parameter for Porous-A, a0 comes out to be 0.388 nm, which is in accordance 023, 13, x FOR PEER REVIEW From with the a0 ofthe Fe-Pd 0.386 phase nm for diagram, the FePd phaseitreported is apparent in thethat the Fe literature 40Pd and 960ofalloy related 18to in stable condi a phase of composition tions must beFeconstituted 50 Pd50 ; the difference in lattice FePd by two phases: parameter and observed for Porous-A sample FePd3. Nevertheless, the EDS map was due to (Figure 5)ashows changeain composition from homogeneous Fe50 Pd50 to distribution ofFe Pd60 Pd Fe40and [47,48]. atoms along the sample, indi cating the formation of just one phase with composition Fe40Pd60. To determine the struc ture of the phase observed by the EDS map, XRD was performed on the as-deposited Po rous-A NW array and is reported in Figure 6. The NWs were kept embedded in their AAO template for obtaining the XRD pattern. Reflections related to an fcc FePd crystallin phase were detected and no secondary phases were present. This result confirmed th EDS observation and showed that the use of electrodeposition can promote the crystalli zation of an FePd phase rich in Pd by hindering the formation of FePd3. From a rough estimation of the lattice parameter for Porous-A, a0 comes out to be 0.388 nm, which is in accordance with the a0 of 0.386 nm for the FePd phase reported in the literature and related to a phase of composition Fe50Pd50; the difference in lattice parameter observed for Porous Figure 5. EDS mapsFigure of Porous-B NW (a) 5. EDS maps showingNW of Porous-B the (a) presence and showing thedistribution presence andof Pd (b) and distribution ofFe Pd(c) (b) and Fe (c) A sample was throughout the length of the NW. due to a change in composition from Fe 50Pd50 to Fe40Pd60 [47,48]. throughout the length of the NW. From the Fe-Pd phase diagram, it is apparent that the Fe40Pd60 alloy in stable condi- tions must be constituted by two phases: FePd and FePd3. Nevertheless, the EDS map (Figure 5) shows a homogeneous distribution of Fe and Pd atoms along the sample, indi- cating the formation of just one phase with composition Fe40Pd60. To determine the struc- ture of the phase observed by the EDS map, XRD was performed on the as-deposited Po- rous-A NW array and is reported in Figure 6. The NWs were kept embedded in their AAO template for obtaining the XRD pattern. Reflections related to an fcc FePd crystalline phase were detected and no secondary phases were present. This result confirmed the EDS observation and showed that the use of electrodeposition can promote the crystalli- zation of an FePd phase rich in Pd by hindering the formation of FePd3. From a rough estimation of the lattice parameter for Porous-A, a0 comes out to be 0.388 nm, which is in accordance with the a0 of 0.386 nm for the FePd phase reported in the literature and related to a phase of composition Fe50Pd50; the difference in lattice parameter observed for Porous- A sample was due to a change in composition from Fe50Pd50 to Fe40Pd60 [47,48]. Figure6.6.XRD Figure XRDpattern of the pattern as-deposited of the Porous-A as-deposited embedded Porous-A in the AAO embedded template. in the AAO template. The morphology of the NWs was also observed by TEM after completely dissolving The morphology of the NWs was also observed by TEM after completely dissolving the template, as described previously in the case of the FESEM sample preparation. A very the template, as described previously in the case of the FESEM sample preparation. A very small amount of ethanol containing the dispersed NWs was dropped onto a Cu grid to small be amount observed of ethanol under containing TEM. Figure 7 showsthethedispersed NWs bright-field TEMwas dropped image onto a Cu grid to b of the as-deposited observed under TEM. Figure 7 shows the bright-field TEM image mesoporous FePd NWs, Porous-A. From the high-resolution (HR) bright-field TEM of the as-deposited image mes oporous in FePd Figure 7c, NWs, Porous-A. the sharply From defined lattice the high-resolution planes (HR)nm with distances of 0.219 bright-field TEM image in can be observed. Figure This 7c, thelattice calculated sharply defined space was in lattice planes accordance withwith that distances of 0.219 obtained from XRD nm can be observed measurement corresponding This calculated to lattice the (111) plane space of in was theaccordance FePd phase.with Notethat thatobtained the innerfrom mesoporosity of XRD measuremen the NWs could not be disclosed by TEM because of both the relatively large diameter corresponding to the (111) plane of the FePd phase. Note that the inner mesoporosity of o the NWs and the occurrence of a non-porous shell. the NWs could not be disclosed by TEM because of both the relatively large diameter o the NWs and the occurrence of a non-porous shell. Figure 6. XRD pattern of the as-deposited Porous-A embedded in the AAO template. The morphology of the NWs was also observed by TEM after completely dissolving Nanomaterials2023, Nanomaterials 2023,13, 13,403 x FOR PEER REVIEW 10 of 18 10 of 18 Figure7.7.Bright-field Figure Bright-fieldTEM TEMimages imagesof ofPorous-A Porous-ANWs: NWs:(a) (a)AAbunch bunchof ofNWs, NWs,(b) (b)diameter diameterof ofaasingle single NW and (c) close-up view of the wall. NW and (c) close-up view of the wall. 3.3.Magnetic 3.3. MagneticStudies Studies Allthe All theinvestigated investigatedNWsNWswerewere ferromagnetic ferromagnetic despite despite their their relatively relatively large large Pd con- Pd content. tent. Ferromagnetic Ferromagnetic properties properties in1−Fe in Fex Pd Pd1−x alloys x xalloys are well are well documented, documented, even foreven for Pd per- Pd percentages exceeding 80 at.% [49]. centages exceeding 80The at.%room-temperature hysteresis loops [49]. The room-temperature of the Dense-A hysteresis loops of and the Dense-B Dense-A samples are shown and Dense-B samplesin are Figure shown8. The curves in Figure 8. obtained The curves byobtained applyingbyHapplying along the Hdirection along the parallel direction(PA) and perpendicular parallel (PE) to the(PE) (PA) and perpendicular major to theaxismajor of theaxis NWs were of the compared NWs were com-for both paredsamples; for bothallsamples; curves were normalized all curves to the magnetization were normalized value at H value to the magnetization = 15 kOe. at H The = 15 values of the kOe. The magnetic values properties of the magnetic such as saturating properties such field (Hs ), coercive as saturating fieldfield (Hs),(H c ), magnetic coercive field susceptibility evaluated at the coercive field (Hc), magnetic susceptibility evaluated at the coercive(χ Hc ), and the squareness ratio (M field (χHc), and the squareness r /Ms )ratio are reported in Table (Mr/Ms) are 2. in Table 2. reported Nanomaterials 2023,13, Nanomaterials2023, 13,403 x FOR PEER REVIEW 11 of 18 11 18 Figure8.8.Room-temperature Figure Room-temperaturehysteresis hysteresisloops loopsofof(a)(a) Dense-A Dense-A and and (b)(b) Dense-B Dense-B NW NW arrays arrays by by apply- applying ing H along the direction parallel (PA) and perpendicular (PE) to the major axis of the NWs. Inset: H along the direction parallel (PA) and perpendicular (PE) to the major axis of the NWs. Inset: enlargement of hysteresis loops at a low magnetic field. enlargement of hysteresis loops at a low magnetic field. Both samples were ferromagnetic with well-defined hysteretic behavior. In the Table 2. Magnetic properties evaluated from hysteresis loops reported in Figures 8 and 9. Saturation Dense-A NW array, the hysteresis loops measured along the PA and PE direction showed field (Hs ), coercive field (Hc ), magnetic susceptibility evaluated at the coercive field χHc , and the differences in the magnetic behavior (see Figure 8a), indicating an anisotropic behavior of normalized magnetization remanence (Mr /Ms ). The nomenclature for the NW arrays is the same as this sample. In particular, the PA magnetization approaches the saturation at lower Hs, in Table 1. with a reversal mechanism over a narrower field interval with respect to PE one. The co- ercive field slightly decreases from H thes PA to PE direction. Hc TheseχHc results suggest that the PA Sample direction, i.e.,Halong Direction the major (kOe) axis of the NWs,(Oe) is the easy axis (Oeof Mr /MsCon- −1 magnetization. ) versely, the hysteresis PAloops of the Dense-B ≈5 NW array 115 sample measured 8.4 × 10−4 along1.0the × PA10−1 and PE Dense-A directions appeared almost superimposed, suggesting an almost−4isotropic magnetic PE ≈8 107 5.1 × 10 × 10−2 behavior (see Figure 8b). A slight preference for the PA direction as the easy 5.6axis of mag- −3 10−2 2). netization can be inferred PA from the≈15 inset of Figure108b and the 4.1χ×Hc10values 3.6 ×Table (see Dense-B However, both curves ≈15 of magnetization PEdisplay a reversal 10 1.4 × 10−3 by a1.4steep characterized × 10− 2 jump with a χHc higher than PA that of the>15 Dense-A sample. 128 Moreover, 10−4the magnetization 9.7 ×after 1.1 × 10−1 Porous-A field jump, a slow approach to saturation ending 140at H ≈ 15 kOe is 10 observed indicating PE >15 4.4 × −4 8.5 × 10−2 a higher difficulty of the magnetic moments to reach the complete alignment with the ap- PA >15 62 3.9 × 10−4 2.0 × 10−2 plied magnetic Porous-B field. The coercivity values were coincident for both PA and PE directions in the Dense-B sample PEbut resulted >15 in them being one 71 order of4.6 × 10−4 lower magnitude 10−2that 3.9 ×than measured in the Dense-A sample. Both samples were ferromagnetic with well-defined hysteretic behavior. In the Dense- ATable NW2.array, Magnetic properties evaluated the hysteresis from hysteresis loops measured along loops the PA reported and PEindirection Figures 8 and 9. Saturation showed differ- field (H ), coercive field (H ), magnetic susceptibility evaluated at the ences in the magnetic behavior (see Figure 8a), indicating an anisotropic behavior of this s c coercive field χ Hc , and the sample. In particular, the PA magnetization approaches the saturation at lower Hs , with as normalized magnetization remanence (M r/M s). The nomenclature for the NW arrays is the same a in Table mechanism reversal 1. over a narrower field interval with respect to PE one. The coercive field slightly decreases from the PA to PE Hsdirection. These Hc results suggest χHc that the PA direction, i.e.,Sample along the majorH Direction axis of the NWs, is the easy axis of magnetization. Mr/Ms the Conversely, (kOe) (Oe) (Oe−1) hysteresis loops of the Dense-B NW array sample measured along the−4PA and PE directions PA ≈5 115 8.4 × 10 1.0 × 10−1 appeared Dense-A almost superimposed, suggesting an almost isotropic magnetic behavior (see PE Figure 8b). A slight preference ≈8 PA direction for the 107as the easy5.1 × 10 axis −4 5.6 × 10can of magnetization −2 be inferred from the PAof Figure 8b≈15 inset and the χ Hc 10 values (see 4.1 ×2). Table 10 However,3.6 −3 × 10curves both −2 Dense-B display a reversal of PE magnetization ≈15 characterized 10by a steep1.4 × 10with jump −3 a χHc1.4 × 10−2than higher that of the Dense-A sample. PA Moreover, >15 after the magnetization 128 9.7field × 10jump, a slow −4 1.1 ×approach 10−1 Porous-A to saturation ending at PEH ≈ 15 kOe>15 is observed indicating 140 a higher 4.4 × difficulty 10−4 of 8.5 the×magnetic 10−2 moments to reach the PAcomplete alignment >15 with the 62 applied magnetic 3.9 × 10 −4 field. The coercivity 2.0 × 10−2 Porous-B PE >15 71 4.6 × 10−4 3.9 × 10−2 Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 18 Nanomaterials 2023, 13, 403 The room-temperature hysteresis loops of the mesoporous NWs are shown in Figure 12 of 18 9. The main magnetic anisotropic features observed in the dense NWs are preserved: the Porous-A sample displays isotropic behavior with a magnetic easy-direction along the major-axis values wereof the NWs,for coincident whereas both PAanand almost anisotropic PE directions behavior in the is found Dense-B sampleinbut theresulted Porous-Bin sample. them being one order of magnitude lower than that measured in the Dense-A sample. Figure 9. Room-temperature hysteresis loops of (a) Porous-A sample and (b) Porous-B sample by Figure 9. Room-temperature hysteresis loops of (a) Porous-A sample and (b) Porous-B sample by applying H along the direction parallel (PA) and perpendicular (PE) to the major axis of the NWs. applying H along the direction parallel (PA) and perpendicular (PE) to the major axis of the NWs. Inset: enlargement of hysteresis loops at a low magnetic field. Inset: enlargement of hysteresis loops at a low magnetic field. However, The the core-shell room-temperature structureloops hysteresis of theofNWs, i.e., mesoporous the mesoporous NWs are core and dense shown shell, in Figure 9. slightly affected the magnetic properties; see Table 2. Specifically, The main magnetic anisotropic features observed in the dense NWs are preserved: the all curves of mesopo- rous NWssample Porous-A showeddisplays an increase of the behavior isotropic coercive field, withespecially a magnetic foreasy-direction the Porous-B sample, along the as well as more robustness of the magnetic disorder, which hindered major-axis of the NWs, whereas an almost anisotropic behavior is found in the Porous-B a deep magnetic satu- ration up to 15 kOe. The χHc values were comparable in the Dense-A and Porous-A NW sample. arrays, whereasthe However, ancore-shell evident decrease structurewas of thefound NWs, ini.e., Porous-B samplecore mesoporous with andrespect denseto the shell, Dense-Baffected slightly one. the magnetic properties; see Table 2. Specifically, all curves of mesoporous NWsAll these an showed magnetic increasefeatures suggested of the coercive thatespecially field, the formation of the for the mesoporous Porous-B sample, structure as well broadened the local magnetic anisotropy values, which influenced as more robustness of the magnetic disorder, which hindered a deep magnetic saturation the overall magnetiza- tiontoprocess up 15 kOe.distributing The χHc valuesthe irreversible processes were comparable within in the a larger Dense-A and portion of the Porous-A NW hysteresis arrays, whereas loop andan evident the hindering decrease domain was found wall in Porous-B motion. This effect sample with respect was mainly evidenttointhe theDense-B Porous- one. B sample. All theseFigures In both magnetic features 8 and suggested 9, the hysteresis that the formation loops of the Dense-A of the mesoporous (or porous A) structure sample broadened show a distinct the local magnetic anisotropic anisotropy behavior, withvalues, an easy which influenced oriented axis preferably the overall magnetiza- parallel to the tion process distributing the irreversible processes within a larger NWs’ long axis. Conversely, the hysteresis loops of sample B measured parallel and per- portion of the hysteresis loop and hindering pendicular the domain to the NWs’ wall motion. axis, almost coincide, This effect was indicating nomainly evident in the clear anisotropy. ThisPorous-B dissim- sample. ilar behavior between both samples is mainly attributed to the different average interwire In both distance Figures between the8two andsamples 9, the hysteresis (i.e., D = 390loops nm forof the Dense-A Dense-A and(or D =porous 270 nmA) forsample Dense- show a distinct anisotropic behavior, with an easy axis preferably B). More specifically, the overall magnetic behavior of the NWs depends on the energy oriented parallel to the NWs’ long axis. Conversely, the hysteresis loops of sample balance between three contributions: shape anisotropy, the dipolar field created between B measured parallel and perpendicular neighboring NWs, to theandNWs’ axis, almost coincide, the magnetocrystalline indicating anisotropy. Fornoan clear arrayanisotropy. of ordered NWs, This dissimilar behavior between both samples is mainly the resulting effective anisotropy field can be expressed as [50–55]: attributed to the different average interwire distance between the two samples (i.e., D = 390 nm for Dense-A and D = 270 nm for Dense-B). More specifically, the overall magnetic MS r 2Lof the NWs depends on the 6.3behavior energy balance betweenHthree K ,eff =contributions: ( N⊥ − N|| )MSshape − anisotropy, +the Hmcdipolar field created (1) D3 between neighboring NWs, and the magnetocrystalline anisotropy. For an array of ordered NWs,For thearesulting two-dimensional effective array of cylindrical anisotropy field canobjects with radius be expressed r, length L, aspect ratio as [50–55]: Ar = L/2r, and separation (i.e., interwire distance) D, the total dipolar field when all NWs are oriented along their long axis can be expressed 6.3M πr2 L asS[51–53]: HK,eff = (N⊥ − N|| )MS − 3 + Hmc (1) D Nanomaterials 2023, 13, 403 13 of 18 For a two-dimensional array of cylindrical objects with radius r, length L, aspect ratio Ar = L/2r, and separation (i.e., interwire distance) D, the total dipolar field when all NWs are oriented along their long axis can be expressed as [51–53]: 4.2MS πr2 L Hdip,0 = (2) D3 If all moments are aligned perpendicular to the NWs direction, then the dipolar field becomes: −2.1MS πr2 L Hdip,90 = (3) D3 Thus, the self-demagnetizing field of one NW can be expressed as: Hdemag = (N⊥ − N|| )MS (4) where N|| and N⊥ are the demagnetizing factors along the NW axis and its perpendicular direction and can be calculated as [54]:     4π   r Ar q ln Ar + A2r − 1 − 1  N|| = 2 (5) Ar − 1     2 A2r − 1 4π − N|| N⊥ = (6) 2 Finally, Hmc is the magnetocrystalline anisotropy contribution. In a first approximation, due to the shape anisotropy, an isolated NW with a large aspect ratio should display a square hysteresis loop when measured with a magnetic field applied parallel to the NW long axis, and a tilted (i.e., hard-axis loop) along the perpendicular direction, thereby behaving as a dipole with a clear anisotropic behavior. The other extreme would be when the NWs would be placed almost in direct contact with each other, thus resembling a continuous ferromagnetic film where, due to the shape anisotropy, the easy axis direction would be parallel to the film plane (i.e., perpendicular to the NW axis). In other words, interwire dipolar interactions can induce a crossover of the easy axis, from being parallel to the NWs’ long axis to being perpendicular to them. Such crossover in the magnetic easy axis has been observed in the literature in arrays of Co or Fe NWs [50,56]. In our case, since the interwire distance in the Dense/Porous-A sample was larger, the magnetic easy axis was still parallel to the NWs’ long axis. However, in the Dense/Porous- B sample, where the interwire distance was smaller, the easy axis direction became less defined and the NWs exhibited sheared hysteresis loops along both directions of measure- ment, thus suggesting that the magnetic easy axis was neither parallel nor perpendicular to the NW axis. Examination of Equation (1) predicts that if HK,eff is negative the magnetic easy axis will be perpendicular to the NWs axis; otherwise, for positive HK,eff values, the magnetic easy axis will be in the parallel-to-NWs direction [52]. In a first approximation, if one neglects Hmc (which would be a plausible assumption for the case of polycrystalline Fe [50]), and if one assumes representative values of r and L for the herein investigated NWs, the values of D that would lead to an overall zero HK,eff (i.e., Dcr ) would be in the range between 420 and 470 nm. Larger values of D would result in the NWs exhibiting clear anisotropy along the NW axis, whereas lower values of D would result in effective magnetic anisotropy along the perpendicular direction. This critical value of D that would lead to the crossover behavior was not far from the experimental D values in our work. Yet, our interwire separation was smaller than Dcr for both samples. This suggests that Hmc should possibly be taken into account for a more quantitative description of our Nanomaterials 2023,13, Nanomaterials2023, 13,403 x FOR PEER REVIEW 14 of 14 of 18 18 observations. the agreement Adding betweena positive Hmcmodel this simple term inand Equation (1) would the obtained result in a results. experimental lower DIncr , addi- thus improving the agreement between this simple model and the obtained experimental results. tion, strictly speaking, Equations (2)–(6) are valid only for ordered square arrays of cylin- In addition, drical strictly dots [51]. speaking, In our Equations case, the NWs are (2)–(6) arerandomly disposed valid onlyinside for ordered square the pores of thearrays AAO of cylindrical dots [51]. In our case, the NWs are disposed randomly inside the pores of templates, which has been reported to cause a tilt of the hysteresis loops and an increase the AAO templates, which has been reported to cause a tilt of the hysteresis loops and an of the saturation field [55]. Finally, magnetic curling effects cannot be completely disre- increase of the saturation field [55]. Finally, magnetic curling effects cannot be completely garded since the diameter of the NWs is relatively large, and they are relatively long. Such disregarded since the diameter of the NWs is relatively large, and they are relatively long. inhomogeneous magnetization reversal configurations tend to decrease the dipolar field Such inhomogeneous magnetization reversal configurations tend to decrease the dipolar and the coercivity, as compared to coherent rotation in single-domain states [56]. field and the coercivity, as compared to coherent rotation in single-domain states [56]. 3.4.SERS 3.4. SERSStudies Studies ToTocompare compare the the influence influence of of chemical chemicalcomposition compositionand andmorphology morphology ononthetheSERS SERS en- hancement, two enhancement, different two different samples samples were taken were into account. taken The first into account. The being firstPd-rich Porous- being Pd-rich A, fabricated in this study and held against a nanoporous FePd Porous-A, fabricated in this study and held against a nanoporous FePd thin film that thin film that displayed notable SERS displayed activity, notable SERSasactivity, previously reported by as previously Cialoneby reported et Cialone al. [57]. The et al.latter [57].sample, The latter ob- tained by chemically dealloying a sputtered thin film, had a composition sample, obtained by chemically dealloying a sputtered thin film, had a composition of of Fe 60Pd 40 , po- rous Fe morphology, with a roughness of (3.4 ± 0.3) nm measured by atomic force micros- 60 Pd40 , porous morphology, with a roughness of (3.4 ± 0.3) nm measured by atomic copy microscopy force and a bcc α(Fe,Pd)and a bcc phase (XRD pattern α(Fe,Pd) provided phase (XRD in Figure pattern S3 ofin provided theFigure Supplementary S3 of the File). Supplementary File). TheSERS The SERSmeasurements measurements were were performed performed using using Porous-A Porous-A as as aa substrate substrate for for the the de-de- tection of 0 4,4′-bipyridine (bipy) probe molecule using the same tection of 4,4 -bipyridine (bipy) probe molecule using the same conditions adopted forconditions adopted for the nanoporous FePd thin film. Figure 10a,b represent the spectra obtained the nanoporous FePd thin film. Figure 10a,b represent the spectra obtained for Porous-A for Porous-A with bipybipy with concentration concentration of 10−10of M10− 10 M and 10and 10−12 M respectively. −12 M respectively. The sample Theshows sample well-enhanced shows well- signals with representative bipyridine peaks atpeaks 1605 cm −1, 1550 − 1 , −1, −1,−11474 −1−1,, enhanced signals with representative bipyridine at 1605 cm cm 15501474 cmcm , 1419cm cm and cm 1419 − 1 1325 cm, and. Accordingly, −1 − 1 we witnessed 1325 cm . Accordingly, weawitnessed minor difference a minorindifference signal intensities in signalbetween intensi- the between ties two spectra, the two approximately around 75around spectra, approximately a.u., given thegiven 75 a.u., low the concentrations low concentrationsof bipy [58,59]. of A remarkably bipy [58,59]. low detection A remarkably limit of 10 low detection −12 Mof limit 10−recorded. was 12 M wasThe reason behind recorded. The reason such behind suchresults impressive impressive lies inresults lies in the morphology the mesoporous mesoporous of morphology Porous-A asofwell Porous-A as well as the adjacent as the adjacent arrays of the NWs arrays of the NWs embedded embedded in close in close proximity, which proximity, which conducive are exceedingly are exceedingly to the conducive to the enhancement electromagnetic electromagnetic enhancement effect and can be effect regardedandas can be regarded a facile area foras a facile the area generation for the generation of hotspots (Figure ofS2hotspots (Figure S2 of theFile). of the Supplementary Supplementary File). Figure10. Figure 10. (a,b) (a,b) SERS SERS spectra spectra shown shownby byPorous-A Porous-ANWs NWsforforbipy concentration bipy concentration of of 10−10 10−M10and 10−12 M and M−12 10 respectively. (c,d) (c,d) M respectively. SERSSERS spectra shown spectra by nanoporous shown FePd by nanoporous thinthin FePd film forfor film bipy concentration bipy of concentration 10−6 − M6 and 10−12 M − respectively. 12 of 10 M and 10 M respectively. Nanomaterials 2023, 13, 403 15 of 18 Regarding the SERS results acquired by Cialone et. al., Figure 10c,d display the spectra obtained by the nanoporous thin film for the same analyte, i.e., 4,40 -bipyridine at concentra- tions of 10−6 M and 10−12 M respectively. The signals are quite evidently enhanced and the detection limit, in this case, was also found to be 10−12 M. Similar to Porous-A, the SERS performance, in this case, can be accredited to the nanoporous morphology of the as-dealloyed thin film which facilitated the LSPR phenomenon, producing amplified SERS signals combined with palladium clusters present on the surface. Hence, Porous-A showed excellent SERS performance akin to Cialone et. al.’s nanoporous FePd thin film, making these mesoporous NWs a promising candidate as a SERS active substrate. 4. Conclusions In conclusion, successful fabrication of dense and mesoporous FePd NWs with Pd content ranging from 45 to 60 at.% has been achieved. A template- and micelle-assisted pulsed potentiostatic electrodeposition method was adopted utilizing AAO and PC tem- plates of variable pore sizes. For dense NWs, an FePd electrolyte was used, while for the mesoporous counterparts, this electrolyte additionally consisted of a block copolymer, P-123, acting as a micelle-forming surfactant. Single-phase fcc NWs were obtained dis- playing a range of lengths, from 3.1 µm to 7.1 µm. Upon applying progressively negative potentials, the NWs were seen to become richer in Fe content. The NWs obtained from the AAO membranes had the typical branched structure at the bottom while those obtained from PC membranes showed cigar-like shape ends, in addition to the main cylindrical morphology. A core-shell structure was noticed in the case of the mesoporous NWs—a porous internal volume and denser non-porous outer surface. This structure was deemed to have a slight influence on the magnetic properties of the mesoporous NWs. Overall, the samples exhibited a soft ferromagnetic behavior. The shapes of the hysteresis loops (when measured parallel and perpendicular to the NWs long axis) were shown to be influenced by the shape anisotropy and interwire dipolar interactions, which in turn depended on the NWs dimensions. Besides, mesoporous NWs acting as SERS substrates exhibited excellent SERS performance with a low detection limit of 10−12 M for 4,40 -bipyridine probe molecule. The mesoporosity and the adjacent embedding of the NWs provided an easy ground for the LSPR effect as well as hotspot generation. The SERS performance of these NWs was found to be comparable with that of a nanoporous FePd thin film in the literature with similar composition and morphology. Thus, these NWs can be potential candidates as active substrates for a wide range of SERS-based applications. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13030403/s1, Figure S1: Pulsed potentiostatic electrodeposi- tion scheme: during ton , the potential Eon is applied at the selected deposition sites; during toff , the electrodes are brought back to the initial open-circuit potential Eoff ; Figure S2: SEM image of the top surface of Porous-A nanowires partially embedded in the AAO template; Figure S3: (a) XRD pattern of the as-deposited Porous-A embedded in the AAO template. (b) XRD pattern of Cialone et. al’s nanoporous (NP) FePd thin film. Author Contributions: Conceptualization, D.R., G.B., P.T., J.S., P.R. and E.P.; methodology, D.R., J.S. and E.P.; formal analysis, D.R., G.B., J.S. and E.P.; investigation, D.R., G.B., F.S., F.C., M.C. (Matteo Cialone) and E.P.; writing—original draft preparation, D.R., G.B. and J.S.; writing—review and editing, D.R., G.B., F.S., M.C. (Matteo Cialone) M.C. (Marco Coïsson), P.T., J.S., P.R. and E.P.; supervision, P.T., J.S., P.R. and E.P.; funding acquisition, P.T., J.S., P.R. and E.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Spanish Government (Research State Agency, PID2020- 116844RB-C21) and the Generalitat de Catalunya (2017-SGR-292). Data Availability Statement: Data will be made available on request. Conflicts of Interest: The authors declare no conflict of interest. Nanomaterials 2023, 13, 403 16 of 18 References 1. 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