Conformal oxide nanocoatings on electrodeposited 3 D porous Ni films by atomic layer deposition

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Introduction
During the last decades, much progress has been made towards the development of novel synthetic approaches to prepare nanocomposite materials with tunable composition and microstructure.Multiphase composite materials are interesting since they combine the properties of the different constituents, often in a synergetic manner.The generation of large amounts of interfaces (as it is the case in nanocomposites) allows exploring novel interfacial coupling effects.In turn, porous structures, with much larger surface area-to-ratio compared to their fully dense counterparts, are very appealing to enhance certain physico-chemical properties.In terms of applications, nanostructured porous materials have received considerable attention since they hold great promise in areas like drug delivery systems, 1,2 batteries, 3 electrocatalysis, 4-6 supercapacitors 7 or magnetic micro/nanoelectromechanical systems (MEMS/NEMS). 8Recently, coating porous surfaces with different organic and inorganic materials has become an important route to obtain functionalized multiphase nanocomposites. 9,10Depositing an insulating layer can reduce electrical shorting in MEMS/NEMS, while bio-MEMS/NEMS can benefit from the coating with a hydrophilic layer, which allows greater wetting with aqueous biological fluids. 11In turn, coating porous materials with a biocompatible layer makes them more amenable to be used in vivo, such as for drug delivery or diagnosis. 12Moreover, besides its protective role, a coating can also serve as an integrated functional unit.For example, 3D porous Ni films can function as scaffolds to anchor Co(OH) 2 13 or Si 14 to produce nanoporous composites with superior supercapacitor performance.A magnetic porous film can also host second phases that could provide electrical insulation, biocompatible surfaces, hydrophilic/hydrophobic properties or even enhanced magnetic performance.
In general, the synthesis of porous nanocomposite films is accomplished by chemical or electro-chemical reactions that occur at surface level.Nanocasting procedures using suitable precursors allow the filling of parent templates with oxide second phase materials of interest. 15Nanoporous ceramic or polymeric templates (e.g., alumina or polycarbonate) can be filled with certain metals and alloys by electrodeposition.16   Contrary to these examples, the complete or partial filling or coating of metallic porous templates with oxide materials is more challenging.The preparation procedures usually involve some heating steps, which can easily deteriorate the properties of the porous metallic framework, from both morphological (collapse of the pore structure) and compositional (oxidation) points of view.This is certainly deleterious for the subsequent implementation of these materials into real devices.Thus, the choice of an appropriate Therefore, it is a powerful technique that allows precise control over the composition and physical properties of nanoscaled materials and novel nanostructures. 20To date, ALD of magnetic nanotubes and nanowires inside anodized alumina templates has been reported. 21,22,23Interestingly, Al 2 O 3 ALD films were successfully grown on a variety of magnetic substrates, including Co, Ni, NiFe and NiMn.

24
The aforementioned studies have proved the potential of ALD to deposit a wide variety of materials into narrow and highaspect ratio pores.
In this work, we propose the combination of electrodeposition and ALD to obtain Ni/Al 2 O 3 , and Ni/Co 2 FeO 4 magnetic nanocomposite porous films.3D porous Ni films are prepared by one-step hydrogen bubble assisted galvanostatic electrodeposition, [25][26][27]  ); meanwhile, Co 2 FeO 4 is ferrimagnetic.For the latter, conformal coating could result in strong magnetic exchange coupling between the ferromagnetic metallic porous matrix and the guest metal oxide component.Remarkably, the porous structure of the Ni matrices is maintained after the ALD process, which demonstrates that ALD permits the deposition of different types of oxide nanocoatings at relatively low, non-damaging temperatures.Structural and magnetic characterization reveals that Ni is not severely oxidized during the process.In addition, the resulting nanocomposite may become either more hydrophilic or hydrophobic than the parent Ni film, depending on the applied nanocoating.The proposed synthetic protocol could be readily extended to fabricate other 3D porous metal supported composite nanostructures for a variety of technological applications.

Experimental section 2.1 Preparation of the 3D porous Ni film
All solvents and chemicals were of analytical grade and used without further purification.Deposition of 3D porous Ni film was carried out in a double-jacketed single-compartment cell with an electrolyte containing 2 M NH 4 Cl and 0.1 M NiCl 2 at a pH value of 3.5.The working electrode (WE) was Si/Ti(25 nm)/Au(125 nm) with an active area of 0.25 cm 2 (Ti and Au were grown by evaporation).Prior to deposition, the WE was cleaned with acetone, followed by diluted sulphuric acid and finally rinsed in water.A platinum wire served as the counter electrode and a dual-junction Ag|AgCl 3M KCl (E = + 0.210 V versus standard hydrogen electrode) was used as the reference electrode.The electrolyte was bubbled with nitrogen to deaerate the solution before electrodeposition.Electrodeposition was performed galvanostatically at j = -1 A cm -2 for 150 s with a gentle stirring speed of 300 rpm at room temperature.A cathodic polarization curve was recorded in order to identify the limiting current density regime.The resulting porous Ni films were washed with Milli-Q water and dried in air.The cathodic current efficiency was estimated to be ca.76% on the basis of the pH drop after each deposition step.

Characterization of the structure and physical properties
Scanning electron microscopy (SEM) images and energydispersive X-ray spectroscopy (EDX) analyses were acquired using a Zeiss Merlin microscope operated at 3 kV and 15 kV, respectively.Transmission electron microscopy (TEM) and STEM-EDX analyses were performed on a Tecnai F20 HRTEM /STEM microscope operated at 200 kV.Cross sectional specimens were prepared by embedding the composites in EPON TM epoxy resin.Subsequently, a very thin slide was cut using a microtome apparatus and placed onto a carbon-coated Cu TEM grid.

Results and discussion
Fig. 1 schematically shows the preparation process followed to obtain the porous composite films.First, a hierarchically porous Ni film is electrodeposited onto the Au surface at a sufficiently negative current density.Ni possesses a high overpotential toward hydrogen evolution during electrodeposition in acidic media. 33The generated hydrogen bubbles are absorbed onto the WE and then are liberated from the freshly grown Ni deposit to the electrolyte-air interface, acting as a dynamic template during the deposition.Hence, metal electrodeposition occurs between the hydrogen bubbles, yielding a film with a 3D porous architecture.The porous Ni film acts as a backbone to deposit Al 2 O 3 and Co 2 FeO 4 by ALD.Shown in Fig. 2 are some representative SEM images of the as-deposited 3D porous Ni film.At low magnification (Fig. 2a), pores with an on-top circular shape are seen all over the surface, whose sizes range from 5 µm to 15 µm.Higher magnification observations (Fig. 2b) reveal that the pore walls consist of numerous tiny, interwoven, little protruding dendrites.The cross-sectional view of the deposits (Fig. 2c) confirms the ramified nature of the pore walls.The deposited Ni thickness is approximately 40 µm.Interestingly, the macropores extend from the outermost surface almost down to the substrate, having a depth of ∼30 µm.This demonstrates the rather high aspect-ratio of the pores.A representative EDX spectrum of the Ni film is shown in Fig. 2d.Ni element together with a very low oxygen signal are detected, which proves that the porous layer is almost entirely metallic.
To obtain a smooth coating that perfectly replicates the template surface it is important to identify the deposition conditions to be within the ALD window.This window is a Please do not adjust margins Please do not adjust margins temperature region where the growth rate is constant and assures a tight control of the process and high reproducibility.Working outside this window means that undesired processes can occur, including decomposition or desorption of the precursor (temperature too high), precursor condensation or insufficient reactivity (temperature too low).As a result, the growth rate is modified and the level of impurities in the film can increase.For Al 2 O 3 , which is the most widely studied material in ALD because it has a behavior close to ideal (i.e., the ALD window is well established), the following reaction mechanism has been proposed: 34 (1) diffusion of the aluminum precursor (Al(CH 3 ) 3 ) into the near surface region of the host material, (2) reaction and saturation of the substrate surface with Al-CH 3 species and purge to eliminate reaction products and excess of the precursor (3) diffusion of the oxygen precursor into the Al-CH 3 surface, (4) reaction and saturation of the surface with Al-OH species followed by purge to eliminate the reaction products.This is defined as the first ALD cycle and it is repeated as many times as required to obtain the desired thickness.This process allows the formation of a dense Al 2 O 3 film that coats the host material.According to the literature, the growth of a continuous thin layer of Al 2 O 3 by ALD is possible at a temperature as low as 33℃ 35 but the reaction kinetics is slow in this case (since the reaction is thermally activated) and hence higher temperatures are sometimes required. 36The completion of the reaction, i.e. full coalescence of the Al 2 O 3 clusters, needs longer time at lower temperatures.
It should be noted that such an ideal behavior is in many materials not easy to achieve.For high aspect ratio structures, exposure mode (i.e. the precursor is left longer time in the reaction time) are routinely used in order to ensure conformal coating.Here, as detailed in Table 1, several experiments have been performed varying the deposition temperature, oxygen source, precursor pulse and exposure time, to identify the optimal ALD conditions for both Al 2 O 3 and Co 2 FeO 4.
SEM images and the corresponding EDX mappings for A1, A2 and A3 composites are shown in Fig. 3. Fig. 3a reveals that the surface of A1 composite is rather rough, featuring small clusters instead of a continuous shell.Nevertheless, Al, O and Ni elements were homogeneously distributed in the corresponding EDX mapping image (Fig. 3b) (note that shadowing effects during EDX analysis preclude detection of elements inside the pores).Thus, from the morphology observed in Fig. 3a it is suggested that these deposition conditions are not optimal.It is likely that the reaction temperature is too low (precursor condensation-insufficient reactivity).When the temperature is increased to 200℃ and, simultaneously, water is replaced by ozone (more reactive oxygen source), the Ni surface becomes smoother.An Al 2 O 3 layer coating the Ni grains is apparently visible from Fig. 3c and  3e.Remarkably, the porosity of the Ni matrix is preserved after the Al 2 O 3 deposition, indicating that the oxide nanolayer is extremely conformal to the Ni skeleton.EDX elemental distribution images of A2 and A3 composites (Fig. 3d, f) show that Al and O are distributed in a parallel way, which indicate that the relative Al 2 O 3 coverage is completely uniform for reactant exposure times of 30 s.This finding is similar to that Please do not adjust margins Please do not adjust margins reported by George et al. 20 who also demonstrated that a reactant exposure of 30 s was sufficient to obtain a nearly conformal coating in high aspect-ratio structures.
To further assess the conformal coating of Ni with Al 2 O 3 , sample A3 was embedded in resin and sliced in order to gain insight into Ni/Al 2 O 3 interface.Sample preparation was very challenging since the material was prone to break into several pieces during the slicing owing to their 3D porous structure.Nevertheless, reasonably large isolated fragments of the material could be found by SEM, as shown in Fig. 4a.These fragments, featuring a dark contrast, are surrounded by a brighter-contrast shell, as indicated with the solid orange line in Fig. 4a.Note that during the preparation of composite slices for TEM observations, not only regions close to the film surface were analyzed but also slices close the Au surface.In all cases, the ALD layer was found conformally coat the Ni film.STEM-EDX line scan analysis was performed in order to determine the composition profile across the interface (Fig. 4b).An EDX line scan was done along the red arrow depicted in the inset STEM image of Fig. 4b  The typical peaks of crystalline Al 2 O 3 in the range 25°-55° are not observed in the XRD pattern.Instead, two peaks at 33° and 37° were detected, which can be attributed to a Ni x O y phase.This result further confirms the hypothesis that the Ni outermost surface was slightly oxidized during ALD (i.e., the interface can be described as Al 2 O 3 /(Al,Ni) x O y /Ni).Based on these findings, 3D porous Ni supported cobalt ferrite composite films (Co 2 FeO 4 ) were prepared using a similar protocol (Table 1).Metallocenes are ideal precursors for ALD owing to their thermo stability, high volatility and reactivity toward oxidation to a certain degree. 37Fig. 5 depicts typical SEM images of the as-prepared 3D porous Ni-supported cobalt ferrite.As for Al 2 O 3 coating, the porous morphology provided by the Ni matrix remains unchanged (Fig. 5a,b) and Co, Fe and O elements are evenly distributed (Fig. 5c).A Ni/Co 2 FeO 4 specimen for cross sectional view was also prepared to assess Please do not adjust margins Please do not adjust margins the quality of the oxide nanocoating.As displayed in the TEM image (Fig. 6a), the sliced porous film shows a nanosheet morphology, being the interface between the metal and the oxide layer less defined compared to Al 2 O 3 case.For this reason, a more detailed characterization was carried out near the edge.When the electron beam was spotted onto the red dot "b" in Fig. 6a, Ni, Fe, Co and O signals appeared in the EDX spectrum (Fig. 6b).The relative proportion between Co, Fe and O yielded a composition close to Co 2 FeO 4 , as expected.When the electron beam was swept from the body to the edge (red arrow in Fig. 6a), the Ni signal gradually decreases down to negligible levels (Fig. 6d).Meanwhile, Co, Fe and O signals simultaneously increase at the particle edge until a maximum value, leading to a Co/Fe/O atomic ratio of 2:1:4, in agreement with previous studies of ALD Co 2 FeO 4 thin films. 32The interface from pure Ni to Co 2 FeO 4 is not well defined (as it was also the case for Ni/Al 2 O 3 ), but rather a transition layer is formed which embraces the four elements.The thickness of this transition layer is around 5 nm.).This can be ascribed to the presence of a small amount of NiO (in agreement with the results from XRD and STEM-EDX), which is antiferromagnetic and hence exhibits virtually zero net magnetization.Note that the porosity degree is not taken into account in the magnetization (M) normalization; that is, the volume is calculated from the real "geometrical" thickness.When comparing the experimental and tabulated M s value for pure Ni (491 emu cm -3 ), it comes out that the porosity degree of the Ni layer is approximately 67 vol%.Although NiO is antiferromagnetic, no exchange bias effects (e.g., loop shift 38 or enhanced coercivity 39 ) are observed, mainly because the relative amount of NiO is very low compared to that of Ni and exchange bias effects are known to be inversely proportional to the thickness of the ferromagnetic counterpart. 38Similarly, because of the In fact, variations of the contact angle in solids not only depend on surface roughness, but are also related to the surface energy of the investigated materials. 41Metal oxides have lower surface energy than pure metals since the latter tend to react with the atoms (molecules) from the surrounding in an attempt to form a passive layer (e.g., metal oxide) with lower energy level.In general, most molecular liquids form lower contact angles on materials with a higher surface energy.However, this is opposite to what is observed here.Nonetheless, the wettability of a surface is also determined by the outermost chemical groups at the solid.Metal oxide surfaces are often fully or partly covered with OH groups which are usually formed by the interaction of water with the metal ions at the surface. 42The surface-anchored hydroxyl groups participate in hydrogen bonding with the static aqueous droplets, thus increasing the wettability of the material.Thus, our results reveal that 3D porous Ni structures Please do not adjust margins Please do not adjust margins coated with metal oxide nanolayers are slightly more hydrophilic than the parent Ni template.
In biological systems, relatively hydrophilic coatings allow the formation of tightly adherent layers of aqueous biological fluids with high lubricity to the material.Therefore, our oxide coatings would prevent from Ni ion leaching, which is a concern since Ni can pose cytotoxicity problems.This combination of metals and oxides in a single 3D porous structure could also be appealing as building blocks in MEMS/NEMS.The use of Al 2 O 3 would possibly reduce the tendency toward electrical shorting 29 since Al 2 O 3 has a high dielectric constant (in the range of 7-10) and an electrical resistance of about 1015 Ω•cm.Likewise, Ni/Co 2 FeO 4 is a material with potential applications in spintronics and as spring-magnet layered composites.Moreover, gas sensor based on cobalt ferrite showed high response, good selectivity to low concentration of ethanol. 43Yet the investigation on gassensing properties of cobalt ferrite is really limited, and further experimental proof is nevertheless still necessary.This indicates that the here-presented synthetic strategy of combining electrodeposition and ALD is very convenient to produce magnetic nanocomposite porous films with potential applications in a wide range of technological fields.

Conclusions
The possibility to combine electrodeposition with ALD to prepare 3D porous magnetic metals conformally coated with metal oxide nanolayers, namely Ni/Al 2 O 3 and Ni/Co 2 FeO 4 , is demonstrated.Due to the nature of the presented approach the host (metal) and guest (metal oxide) materials can be chosen with certain degree of freedom.We demonstrate that both the hierarchical porosity and magnetic properties of the parent metallic Ni template are maintained after ALD step.Moreover, the presence of a nanometer-thick layer of Al 2 O 3 or Co 2 FeO 4 covering the Ni scaffold improves surface wettability.The procedure could be extended to prepare other magnetic compositions obtained using the same protocol.Owing to the synergies emerged between the host and the guest components, these nanocomposite magnetic porous films are promising candidates for widespread technology areas, such as biological applications, magnetic sensors or magnetic micro/nano-electromechanical systems (MEMS/NEMS), amongst others.

Fig. 2
Fig. 2 SEM images of the 3D porous Ni film: a) on-top general view of the material (inset shows a detail of a macropore); b) on-top zoomed detail of the pore wall; c) cross-sectional view of the Ni film; d) EDX spectrum of the Ni film.

Fig. 3
Fig. 3 On-top SEM images of a) A1, c) A2 and e) A3 nanocomposites.EDX mapping distribution of Al, O, and Ni elements in b) A1, d) A2 and f) A3 composites, obtained from the zoomed SEM images shown on the left.

Fig. 4
Fig. 4 a) SEM image of A3 nanocomposite slice; b) line-scan STEM-EDX analysis across the interface between Ni and Al2O3, as indicated by the red arrow in the insert STEM image; c) HRTEM image of the area enclosed with the red dotted square in a); d) EDX elemental distribution of O, Al and Ni in the interfacial area enclosed within the red rectangle.
, which embraces a translucent thin layer and a denser bright region.As the electron beam is scanned towards the Ni matrix, Al and O signals first appear at 35 nm from the initial scanning point and vanish at approximately 100 nm.Conversely, the Ni signal monotonically increases from around 60 nm, which indicates that the Al 2 O 3 coating has a thickness of about 25 nm.Remarkably, there is no abrupt switching from Al and O signals to Ni signal but, instead, they coexist within a few nanometers interval.This suggests the formation of a mixed Al/Ni oxide at the interface.This was further proved by STEM-EDX elemental distribution mapping (Fig. 4d), i.e., Ni-oxide or some mixed (Al,Ni) x O y phases may exist at the interface region.Hence, the structure of the interface can be defined as Al 2 O 3 /(Al,Ni) x O y /Ni.Fig. 4c actually corresponds to the HRTEM of the region enclosed in the small red box in Fig. 4a.It is likely that the asdeposited Al 2 O 3 layer is amorphous since lattice fringes were not detected.The amorphous nature of Al 2 O 3 was further confirmed by θ-2θ scan X-ray diffraction (XRD) (Fig. S1).For comparison, the XRD data of uncoated Ni is shown.Besides a peak coming from the Au surface, the diffraction peaks corresponding to Ni (111) and Ni (200) reflections of facecentered cubic (fcc) structure (PCPDF 04-0850) remain virtually unchanged after ALD coating of Ni scaffold with Al 2 O 3 .Hence, the 3D porous Ni withstands the ALD process both morphologically and crystallographically to a great extent.

Fig. 6
Fig. 6 a) TEM image of the cross sectional view of Ni/Co2FeO4 sample; b) EDX spectrum corresponding to the red dot "b" in a); c) HRTEM image of the area enclosed with the red square labeled as "c" in panel a); d) line-scan STEM-EDX analysis across the edge depicted with the red arrow "d" in panel a).
Fig. 6c shows a HRTEM image of the area enclosed in the red box in Fig. 6a.The porous Ni template is slightly brighter than the Co 2 FeO 4 nanocoating, which is about 5 nm thick.Both Ni and Co 2 FeO 4 are polycrystalline with clear lattic fringes.The interplanar distance of d = 0.202 nm can be assigned to the (111) fcc phase of Ni, whereas d = 0.246 nm matches the (311) fcc of Co 2 FeO 4 .The formation of crystalline cobalt ferrite is also confirmed by the XRD pattern shown in Fig. S2.Namely, the small peak at 36.02° after ALD can be ascribed to the (311) reflection of Co 2 FeO 4 .

Fig. 7 3 )
Fig. 7 Room temperature hysteresis loops of uncoated Ni, Ni/Al2O3 and Ni/Co2FeO4 composite porous films.Room-temperature magnetic hysteresis loops of the uncoated Ni, Ni/Al 2 O 3 and Ni/Co 2 FeO 4 porous films are shown in Fig. 7.The saturation magnetization (M s ) for the Ni/Al 2 O 3 nanocomposite films (159.1 emu cm -3) is slightly lower than for pure Ni (162.0 emu cm -3 relatively low volume fraction of Co 2 FeO 4 , its contribution to the overall hysteresis loop of the Ni/Co 2 FeO 4 film is also very small.The hysteresis loops also reveal that the H c values of the composite films (around 73 Oe) are smaller compared to H c of uncoated Ni (118 Oe).This decrease of coercivity is probably related to thermally-induced microstructural changes that occur in metallic Ni during the ALD process.Actually, Fig. S1 and S2 reveal that the width of the XRD peaks of Ni becomes narrower after ALD (particularly for Ni/Co 2 FeO 4 ), which indicates that the crystallite size of Ni increases.The average crystallite size for Ni, estimated from XRD Rietveld refinements, increases from approximately 35 nm (for uncoated Ni) to 45 nm in the case of Ni/Al 2 O 3 and 60 nm for Ni/ Co 2 FeO 4 composites.In general, grain boundaries hinder and pin the propagation of magnetic domain walls.Hence, H c , in polycrystalline magnetic alloys, is inversely proportional to the grain size. 40Also the release of microstrains associated with the ALD thermal treatments could contribute to reduce the coercivity.The wettability of Ni, Ni/Al 2 O 3 and Ni/Co 2 FeO 4 films was characterized by the sessile drop technique, using 7 µL of 5 wt% NaCl droplets.Fig. 8 shows the shape of NaCl droplets deposited onto the three different surfaces.The contact angle attains the highest value at the surface of uncoated Ni (139°) (Fig. 8a).The contact angle values decrease to 119° and 102° for Ni/Al 2 O 3 and Ni/Co 2 FeO 4 , respectively.Variations in surface roughness could account for the observed differences among the three samples.However, surface roughness is similar since both Al 2 O 3 and Co 2 FeO 4 coatings are really thin and conformal.
Post-print of:Zhang, G. etal."Conformal oxide nanocoatings on electrodeposited 3D porous Ni films by atomic layer deposition" in Journal of Materials Chemistry C, vol 4, issue 37 (2016), p. 8655-8662.The final version is available at: DOI 10.1039/c6tc02656g Please do not adjust margins Please do not adjust margins technique to grow this type of composites is of paramount importance.Very recently, we have demonstrated that porous CuNi supported ZnO hybrid films can be successfully prepared at relatively low temperature by combining electrodeposition with sol-gel drop casting in which (i) the matrix (or host) and the coating can be chosen independently, and (ii) the ferromagnetic properties of CuNi are preserved.

Table 1 .
ALD parameters used in this work.A1, A2 and A3 refer to Al2O3 coatings grown in porous Ni at the indicated experimental conditions.CFO/Ni stands for Co2FeO4 coating onto Ni. "Expos" represents the exposure time. 2.

2 Fabrication of Ni supported Al 2 O 3 /Co 2 FeO 4
32), ferrocene (Fe(Cp) 2 ) and O 3 at 250℃.31Pulse and purge times were optimized for each material, as described in Table1.Note that due to the well-known lower reactivity of Please do not adjust margins Please do not adjust margins Fe(II) as a d6ion respect to Co(II) with d 7 electronic configuration,32the pulse length of Fe(Cp) 2 is longer than Co(Cp) 2 being both heated at 90°C.In this case, precursor pule ratio is 1Co(Cp) 2 :2Fe(Cp) 2 .