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Communication

Synthesizing Nanoporous Stainless Steel Films via Vacuum Thermal Dealloying

by
Xiaotao Liu
1,2,*,
Xiaomeng Zhang
2,3,
Maria Kosmidou
2,
Michael J. Detisch
2 and
Thomas John Balk
2
1
State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
2
Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA
3
Gaona Aero Material Co., Ltd., Beijing 100081, China
*
Author to whom correspondence should be addressed.
Metals 2023, 13(7), 1255; https://doi.org/10.3390/met13071255
Submission received: 12 June 2023 / Revised: 4 July 2023 / Accepted: 7 July 2023 / Published: 10 July 2023
(This article belongs to the Section Metallic Functional Materials)
Browse Figures
Versions Notes

Abstract

:
Vacuum thermal dealloying is a recently developed technique and was newly introduced to produce nanoporous metals, due to its intriguing advantages, i.e., preventing oxidation and producing no chemical waste, etc. Here, we report on the fabrication of nanoporous stainless steel films by vacuum thermal dealloying of sputtered stainless steel–magnesium precursor films. It was found that crack-free nanoporous stainless steel films can be successfully attained under a broad temperature range of 450–600 °C, with a dealloying time of 0.5–2 h. The resulting structure and ligaments were temperature- and time-dependent, and moreover, the condition of “600 °C + 2 h” generated the most homogeneous structure. Moreover, small amounts of residual Mg were found at pore sites in the resultant structures, suggesting that the dealloying was not fully complete.

1. Introduction

Nanoporous materials can be applied in numerous fields, e.g., sensors [1,2,3], actuators [4,5], catalysts [6,7,8,9,10], biomaterials [11,12,13], and energy conversion and storage [14,15,16,17], due to their unique bi-continuous open porous structure and high surface-area-to-volume ratio. As a result, this type of material has received significant attention, leading to a wide variety of approaches for the fabrication of nanoporous metals, which have emerged in the past decades. The porous nanostructure is basically obtained through dealloying, which is a robust method for fabricating 3D bicontinuous porous materials. It is generally believed that the formation of nanopores results from two kinetically competing processes at dealloying fronts, wherein a sacrificial (less stable) component is selectively removed from the precursor alloy while leaving the remaining (more stable) component to form a porous microstructure [4,18,19]. In light of previous investigations, the vast majority of nanoporous metals, for instance nanoporous Au, Cu, Ni, Pd, Ti, and Si [20,21,22,23], are synthesized through the techniques of chemical/electrochemical dealloying [4,19,24,25] and liquid metal dealloying [10,26,27,28,29], which are also the most extensively employed dealloying methods. However, each of them has its own limitation. More specifically, the technique of chemical/electrochemical dealloying can be efficiently applied only for fabrication of nanoporous noble metals, while the approach of liquid metal dealloying inherently requires a proper liquid metal to act as the etchant, as well as high temperature experimental conditions. Furthermore, as indicated by Kosmidou et al. [30] and Zhao et al. [31], neither technique is ideal for preparing nanoporous refractory metals, as any oxidation that occurs during the dealloying process is undesired.
In contrast, the technique of vacuum thermal dealloying adeptly overcomes these obstacles. As reported in literature [30], Kosmidou et al. succeeded in synthesizing unoxidized nanoporous refractory metals by employing this method. Unlike traditional dealloying approaches, vacuum thermal dealloying takes advantage of the vapor pressure difference between the two components in the precursor materials to selectively remove one and leave the other to form a nanoporous structure. This technique was initially developed over 10 years ago and has been recently introduced to materials synthesis, especially for fabricating nanoporous metals [30]. As demonstrated by Han et al. [22], the mechanism of vacuum thermal dealloying is a surface diffusion-governed process. This differs from the techniques of chemical/electrochemical dealloying, which are interface diffusion-controlled processes. Additionally, the pores that form in vacuum thermal-dealloyed structures are internal closed pores, which can be attributed to the Kirkendall effect. The pores generated in chemical/electrochemical dealloyed structures are, however, open pores. To date, vacuum thermal dealloying has not yet been widely applied in the fabrication of nanoporous materials, though this technique possesses very conspicuous virtues and advantages, compared to the conventional dealloying approaches.
Nanoporous stainless steel has been put into use in the field of energy harvesting, an example being the anode in microbial fuel cells, due to its enhanced bacterial catalytic activity and stability [15]. In recent years, this material has also started to be applied in the biomedical field, such as being used as body implants, because cell selectivity can be achieved by its specific surface nanofeatures [32,33,34]. According to a literature study regarding the fabrication of nanoporous stainless steel materials, Mokhtari et al. [11] succeeded in preparing a fully porous ferritic stainless steel via the approach of liquid metal dealloying while utilizing the Incoloy 800 precursor alloy. Abbas et al. [15] also fabricated the nanoporous stainless steel 316L. Nevertheless, the preparation process was rather complicated. The sample needed to be ultrasonically cleaned, anodized, then rinsed thoroughly with DI water and dried in air, followed by annealing in a tube furnace. Moreover, the anodization condition was required to be finely optimized for the generation of a nanoporous structure. It should be noted that both forms of nanoporous stainless steels were bulk materials rather than thin films. So far, the fabrication of a nanoporous stainless steel film has not been reported.
In this paper, we, for the first time, prepared nanoporous stainless steel films via the vacuum thermal dealloying approach. More specifically, stainless steel–magnesium precursor films were obtained by magnetron sputtering, where highly purified stainless steel and magnesium target materials were utilized. After that, nanoporous stainless steel thin films were generated by dealloying the precursor films at elevated temperatures and under vacuum conditions. In addition, microstructural characteristics of the resulting films and the chemical evolution taking place during the dealloying process were investigated and discussed. This work not only provides a novel way to synthesize nanoporous stainless steel films, it also offers an in-depth understanding of the relation between the microstructure of resultant films and the dealloying parameters.

2. Materials and Methods

The fabrication of nanoporous stainless steel involves two procedures, namely preparing the precursor stainless steel–Mg films through the technique of magnetron sputtering and then vacuum thermal dealloying the obtained precursor films. The precursor films were deposited using the facility of the magnetron sputtering system (ORION sputtering system, AJA International Inc., Scituate, MA, USA). The high-purity magnesium and 316 stainless steel (99.95%) target materials were purchased from the Kurt J. Lesker company. The nominal compositions of the 316 stainless steel target materials are listed in Table 1. Before deposition, the main chamber of the sputtering system was evacuated to ~10−7 torr and filled with Ar gas until it reached a pressure of 2.5 mtorr. During the preparation of precursor stainless steel–Mg films, the substrate was rotated to obtain a uniform single-composition film. As measured by EDS, the composition of the prepared precursor film was Fe 37.3%, Cr 9.5%, Ni 6.7%, Mo 1.4%, and Mn 1.1% (with the balance of Mg), expressed in wt.%, which was roughly equal to (stainless steel)25Mg75, apart from the trace elements of Si, P, C, and S, whose concentrations were not able to be quantified by EDS. It is noted that the substrate utilized was a commercially purchased (100) Si wafer, and a thin interlayer of Ta (~10 nm) was deposited to enhance adhesion between the film and the substrate.
The vacuum thermal dealloying process was performed in the main chamber of the sputtering system under high vacuum (less than 10−6 torr), and the heater inside the chamber was a quartz lamp (AJA model SHQ-X). A total of 9 sets of precursor films were dealloyed under the conditions of 450 °C, 525 °C, and 600 °C for 0.5 h, 1 h, and 2 h, respectively. During the vacuum thermal dealloying, the chamber was heated to a set temperature, with a constant ramp rate of 10 °C/min, and then held for about 20 min to stabilize the temperature. As explained in a separate study by the authors [30], the sample temperature was about 100 °C lower than the set temperature of the quartz heater. Upon the completion of dealloying, samples were cooled to room temperature inside the chamber.
Focused ion beam-scanning electron microscopy (FIB-SEM, Nanolab 660 and Helios G4) was applied to study the morphology of the sample before and after dealloying. Chemical composition was determined through the energy dispersive spectroscopy technique (EDS, X-MaxN 80 mm2 detector). The TEM lamella was prepared using the FIB (Helios G4) and was then investigated by transmission electron microscopy (FEI TITAN X). High-resolution EDS mapping was acquired using the Bruker EDS detector attached to the TEM system (FEI TITAN X).

3. Results and Discussion

For tuning the dealloying parameters, nine sets of precursor films were dealloyed at temperatures of 450 °C, 525 °C, and 600 °C for 0.5 h, 1 h, and 2 h, individually. Note that the highest selected dealloying temperature was 600 °C because at this temperature, the corresponding vapor pressure of Mg was on the order of 1 torr, which was significantly higher than the chamber pressure (less than 10−6 torr), to ensure that dealloying would occur. Plan view micrographs of the resulting structures after vacuum thermal dealloying are presented in Figure 1. For the sake of comparison, all of the SEM micrographs were acquired at the same magnification. It appeared that the nanoporous structures exhibited isolated islands and pores between ligaments under all of the given experimental conditions. However, the resulting structures were clearly different in terms of the sizes of the pores and islands, indicating that the dealloying process was temperature- and time-dependent.
At a temperature of 450 °C, pores in the resultant structure after dealloying for 0.5 h (Figure 1a) could barely be distinguished, implying a very low level of dealloying. Under the same temperature, the nanoscale pores became much more distinct as the dealloying time increased to 1 h and 2 h (Figure 1b,c). As the temperature rose to 525 °C, the sizes of the pores and islands became larger, as displayed by Figure 1d–f. One interesting phenomenon was that partial pores and island surfaces were covered by a darker material exhibiting contrast with the ligaments, especially for the sample that was dealloyed for 1h (Figure 1e), which was probably the MgO. When the dealloying temperature increased to 600 °C, the surface of the dealloyed films became cleaner, and moreover, the pores and islands tended to be more homogeneous, as exhibited in Figure 1g–i. Aside from the sample corresponding to the dealloying time of 0.5 h, on whose surface a certain quantity of residual Mg was found, the other two films, which were dealloyed for 1 h and 2 h individually, presented debris-free surfaces, indicating that the dealloying process was nearly complete. It is also worth noting that no cracks were observed on the planar surfaces of any of the dealloyed films, suggesting that the surface tensile stress during the dealloying process was not excessive. Otherwise, micro-cracks would likely appear.
Cross-sections of the obtained structures were investigated using FIB-SEM, as shown in the micrographs displayed in Figure 2. The morphologies and sizes of the ligaments in the resulting structures corresponding to various dealloying conditions were different, implying that dealloying temperature and time together governed the final microstructure. More specifically, for a dealloying time of 0.5 h at 450 °C, 525 °C, and 600 °C, ligaments of dealloyed films were smaller and tightly arranged, and moreover, partial ligaments had not yet grown across the sample thickness, as shown in Figure 2a,d,g. As the dealloying time increased to 1 h, dealloyed samples exhibited more sparsely distributed ligaments (Figure 2b,e,h), suggesting that more Mg was removed. It is worth noting that some ligaments in these three fabricated films were “abnormally” larger in size, which can probably be attributed to inhomogeneous dealloying. When the dealloying time increased to 2 h, the resulting specimens displayed more homogeneously nanoporous microstructures, especially for the sample dealloyed at 600 °C (Figure 2i). The microstructure presented in Figure 2c (450 °C, 2 h) exhibited a more compact morphology, implying that less Mg was evaporated than the microstructures corresponding to dealloying temperatures of 525 °C and 600 °C. Since the films were constrained by the Si substrate, their lateral dimensions were fixed, and they could shrink only in thickness (without cracking). As such, it provided a good way to evaluate the extent of dealloying by measuring the change in thickness before versus after vacuum thermal dealloying. We first measured the thickness of the precursor film (Figure 2j), which was 173 ± 2 nm, as indicated by the red line in Figure 2k. The thicknesses of dealloyed films were determined using Image J, and are shown in the histogram in Figure 2k. Each column represents the average thickness based on 10 measurements, and the Y error bar denotes standard deviation. This showed that the thickness change in the films dealloyed at a temperature of 450 °C or 525 °C was insensitive to the dealloying time, suggesting that these two temperatures were, perhaps, not the optimum temperatures for driving the thermal dealloying process. However, at a temperature of 600 °C, the thickness variation in the resulting films was clearly a function of the dealloying time. With a longer dealloying time, the resultant film became much thinner, indicating a higher level of completion for the dealloying process.
For nanoporous materials, their unique bi-continuous microstructure makes them suitable for a wide range of applications. Normally, the ligament size is the most relevant parameter, since it directly governs materials’ mechanical properties or other functionalities. As discussed above, the degree of completion of the vacuum thermal dealloying process is dependent upon both the dealloying temperature and time. This is, however, reasonable because the thermal dealloying process fundamentally relies on the surface diffusion of the metal adatoms. Variation in the ligament size of the developed, nanoporous microstructure was determined quantitatively. For each dealloyed structure, no less than 200 ligaments were measured, and then the average value coupled with standard deviation were calculated, as presented in Figure 3. As can be seen, the films dealloyed at 450 °C possessed smaller ligament sizes than those in films dealloyed at 525 °C. This was possibly due to their more tightly arranged structures (see Figure 2a,c), despite roughly equivalent degrees of dealloying completion at these two temperatures, as evidenced by Figure 2j. Note that the case of “450 °C, 1 h” was an exception, which could be attributed to the abnormally coarse ligaments (Figure 2b). In addition, the ligament size of the films dealloyed at 600 °C was smaller than that of the films dealloyed at 525 °C. This was probably because the structure resulting from dealloying at 600 °C was more uniform and without the abnormal coarse ligaments. Furthermore, for each of the dealloying temperatures, the ligament size became larger as the dealloying time increased.
Conventionally, the precursor materials utilized for fabricating nanoporous metals are binary solid solution alloys, e.g., AuxAgl−x. The precursor film in the current study was, however, sputtered stainless steel–magnesium, where the former component contained multiple elements, as listed in Table 1. It was, therefore, worth studying the element migration/removal before and after vacuum thermal dealloying, which is crucial for understanding the mechanism of this dealloying technique. As discussed above, the film dealloyed at a temperature of 600 °C for 2 h exhibited the most ideal version of the desired nanoporous structure. A TEM lamella was extracted from this sample, followed by milling to an electron-transparent thickness using the FIB-SEM. Afterwards (S)TEM and high-resolution EDS mapping were applied to study the morphology of the nanoporous structure and the chemical evolution caused by the dealloying process. The results are summarized and presented in Figure 4, in which the first two high-angle annular dark-field (HAADF) STEM micrographs (Figure 4a,b) revealed that most of the pores extended across the sample thickness, while some isolated voids were also visible in the inner part of the sample. In addition, the nanostructure of the obtained nanoporous stainless steel film was rather homogeneous, as evidenced by open porosity and ligaments with comparable sizes [30]. As suggested by Han et al. [21,22], the appearance of these internal isolated pores can be ascribed to the Kirkendall effect, which originates from differing diffusion rates between participating elements.
In addition to morphology analysis, high-resolution EDS mapping was performed with the aim of probing the chemical variation after dealloying, as exhibited in Figure 4c–j. Note that the elements C, P, and S were not mapped, since the initial concentrations of these trace elements were less than 0.1 wt%. As shown by the EDS mapping results in Figure 4c–j, it appeared that the ligaments primarily consisted of Fe, Cr, Ni, Mn, and Mo, which came from the stainless steel. Moreover, some residual Mg was also found in the pores (Figure 4d), indicating that the dealloying process, even for the case of 600 °C + 2 h, was not fully complete. By comparing the elemental maps of Fe, Mg, and O (Figure 4c–e), it was observed that oxygen coincided with Mg, suggesting that the chemical state of the residual Mg was oxide (MgO). This can be attributed to the higher chemical affinity of oxygen to magnesium versus iron. As a result, oxygen was preferentially alloyed with Mg rather than Fe (or the other metallic elements in the stainless steel). Furthermore, Figure 4j suggests that the Si in the dealloyed film had possibly disappeared, although this may be an artifact. One possible explanation for this observation is that the Si signal generated from the substrate was much stronger than that emanating from the nanoporous stainless steel film, resulting in almost zero Si contrast in the film area.

4. Conclusions

Nanoporous stainless steel films were successfully fabricated by vacuum thermal dealloying of magnetron-sputtered stainless steel–magnesium precursor films. Various structures, in terms of ligament size and morphology, were achieved under different dealloying temperatures and times, which also indicates that the nanostructure of the dealloyed nanoporous stainless steel films can be tailored. Based on the analysis and discussion above, several conclusions can be drawn.
(1)
Crack-free nanoporous stainless steel films were successfully prepared, with dealloying temperatures ranging from 450 °C to 600 °C and dealloying times of 0.5 h to 2 h.
(2)
The resulting structures were influenced both by the dealloying temperature and the dealloying time. The ligament size increased with dealloying temperature for a given dealloying time. Higher dealloying temperature generally resulted in the coarsest ligaments, and deviations from this trend may reflect the existence of abnormally thick ligaments within the overall distribution of ligament sizes for a given film.
(3)
Magnesium was not completely removed, even in the dealloyed film corresponding to “600 °C + 2 h”, i.e., the film that experienced the highest degree of dealloying. Additionally, the elemental maps of residual Mg and oxygen matched very well, implying that residual magnesium exists as an oxide.

Author Contributions

X.L.: data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, supervision, validation, writing—original draft, and writing—review and editing; X.Z.: formal analysis, investigation, and methodology; M.K.: formal analysis and investigation; M.J.D.: formal analysis, investigation, and writing—review and editing; T.J.B.: project administration, resources, supervision, and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Young Elite Scientist Sponsorship Program of the China Association for Science and Technology (No. YESS20210054) and the Independent Innovation Research Fund of Huazhong University of Science and Technology (2172021XXJS010).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Acknowledgments

The authors acknowledge the support from the Electron Microscopy Center at the University of Kentucky.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 13 01255 g001 550
Figure 1. Plan view SEM micrographs of nanoporous stainless steel films after thermal dealloying under various conditions. The conditions were (a) 450 °C, 0.5 h; (b) 450 °C, 1 h; (c) 450 °C, 2 h; (d) 525 °C, 0.5 h; (e) 525 °C, 1 h; (f) 525 °C, 2 h; (g) 600 °C, 0.5 h; (h) 600 °C, 1 h; and (i) 600 °C, 2 h.
Figure 1. Plan view SEM micrographs of nanoporous stainless steel films after thermal dealloying under various conditions. The conditions were (a) 450 °C, 0.5 h; (b) 450 °C, 1 h; (c) 450 °C, 2 h; (d) 525 °C, 0.5 h; (e) 525 °C, 1 h; (f) 525 °C, 2 h; (g) 600 °C, 0.5 h; (h) 600 °C, 1 h; and (i) 600 °C, 2 h.
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Metals 13 01255 g002 550
Figure 2. Cross-sectional micrographs of the precursor films after vacuum thermal dealloying under various conditions. The conditions were (a) 450 °C, 0.5 h; (b) 450 °C, 1 h; (c) 450 °C, 2 h; (d) 525 °C, 0.5 h; (e) 525 °C, 1 h; (f) 525 °C, 2 h; (g) 600 °C, 0.5 h; (h) 600 °C, 1 h; and (i) 600 °C, 2 h. (j) Cross-section image of the precursor film and (k) the thickness change in films due to thermal dealloying.
Figure 2. Cross-sectional micrographs of the precursor films after vacuum thermal dealloying under various conditions. The conditions were (a) 450 °C, 0.5 h; (b) 450 °C, 1 h; (c) 450 °C, 2 h; (d) 525 °C, 0.5 h; (e) 525 °C, 1 h; (f) 525 °C, 2 h; (g) 600 °C, 0.5 h; (h) 600 °C, 1 h; and (i) 600 °C, 2 h. (j) Cross-section image of the precursor film and (k) the thickness change in films due to thermal dealloying.
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Figure 3. Variation in ligament size of the dealloyed films.
Figure 3. Variation in ligament size of the dealloyed films.
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Figure 4. STEM micrographs and high-resolution EDS mapping results of nanoporous films dealloyed at 600 °C for 2 h. (a) Low-magnification HAADF micrograph; (b) zoomed image corresponding to the red box in (a) and indicating the area for EDS mapping; (c–j) elemental maps of Fe, Mg, O, Cr, Ni, Mo, Mn, and Si, respectively.
Figure 4. STEM micrographs and high-resolution EDS mapping results of nanoporous films dealloyed at 600 °C for 2 h. (a) Low-magnification HAADF micrograph; (b) zoomed image corresponding to the red box in (a) and indicating the area for EDS mapping; (c–j) elemental maps of Fe, Mg, O, Cr, Ni, Mo, Mn, and Si, respectively.
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Table
Table 1. Nominal compositions of 316 stainless steel target materials (wt. %).
Table 1. Nominal compositions of 316 stainless steel target materials (wt. %).
NiCrMoSiMnCPSFe
12172.5120.080.0450.03bal.
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Liu, X.; Zhang, X.; Kosmidou, M.; Detisch, M.J.; Balk, T.J. Synthesizing Nanoporous Stainless Steel Films via Vacuum Thermal Dealloying. Metals 2023, 13, 1255. https://doi.org/10.3390/met13071255

AMA Style

Liu X, Zhang X, Kosmidou M, Detisch MJ, Balk TJ. Synthesizing Nanoporous Stainless Steel Films via Vacuum Thermal Dealloying. Metals. 2023; 13(7):1255. https://doi.org/10.3390/met13071255

Chicago/Turabian Style

Liu, Xiaotao, Xiaomeng Zhang, Maria Kosmidou, Michael J. Detisch, and Thomas John Balk. 2023. "Synthesizing Nanoporous Stainless Steel Films via Vacuum Thermal Dealloying" Metals 13, no. 7: 1255. https://doi.org/10.3390/met13071255

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