Nanoporous metal foams (metal nanofoams) are a relatively new class of materials intensively investigated by the research community [1
]. These three-dimensional structures built by interconnected nanoparticles and/or nanosized filaments represent a unique combination of properties typical for nanostructured (e.g., ultralow density, high porosity, and surface area [3
]) and bulk metals (such as high thermal and electrical conductivity [1
Among techniques used for the preparation of metal nanofoams, electrodeposition seems to be exceptionally encouraging [4
]. The formation of metallic nanofoams by electrodeposition is based on the electrochemical reduction of metal ions at high current densities accompanied by the intensive formation of hydrogen bubbles playing a role of a dynamic template that is responsible for the metal foaming [9
]. The approach allows us to obtain deposits with much higher surface area compared to a standard plain foil [10
] and, in contrast to other reported methods, does not require either sophisticated equipment or complex procedures. Another attractive feature of this process is the formation of hierarchically organized micro/nanostructures [11
] with higher accessibility of the inner surface to external agents compared to bulk particles of nanoporous solids (as additional microstructuring should shorten the length of nanopores thus helping to mitigate possible diffusion limitations within them). Moreover, electrodeposition leads to the formation of nanofoams as integral coatings adhered to the surfaces of bulk supporting electrodes. The materials obtained in such a way are considered as promising for a number of applications, in particular for a broad scope of electrochemical ones [11
]. Nanofoams of a set of metals (Cu, Ag, Pt, Pd, etc.) [11
] and alloys [6
] were obtained by electrodeposition, in particular, successful fabrication of Sn nanofoams has been reported [4
The search for environmentally-friendly strategies of energy conversion and storage is nowadays one of the most urgent needs of mankind. Thus, the range of materials currently being studied for these applications is still extensively growing. Among them, tin oxides (mainly SnO2
, but also Sn3
and SnO), attract great scientific attention due to their unique semiconducting, optical, and electronic properties [16
], which make them promising candidates for photoelectrochemical (PEC) [19
] and photovoltaic applications [20
]. They also have been considered as favorable materials for various energy storage systems, especially Li-ion and Na-ion batteries [21
]. It is well known that precisely designed nanomorphologies of such oxides can offer some enhanced features (e.g., electron mobility, surface area) that make them even more attractive for the purposes mentioned above [23
Among diverse methods of synthesis of nanostructured tin oxides, one of the most attractive is electrochemical oxidation (anodization) of Sn because of its simplicity, low-cost, high effectiveness, and possibility of controlling and tuning the morphology of nanostructures [24
]. Despite extensive studies on the influence of various anodizing conditions (e.g., applied potential, electrolyte composition, process duration) on the growth and morphology of porous anodic SnOx
layers, which have already been performed [24
], nanostructured anodic tin oxide films have been obtained mostly on tin foils [24
] or smooth Sn layers electrochemically deposited on conductive supports [25
]. To the best of our knowledge, no researches concerning possibilities of fabrication of nanoporous SnOx
on micro/nano-structured metallic substrates have been reported, except the approach proposed by Wu et al. [33
] based on the oxidation of Sn nanowire arrays prepared using template-assisted electrodeposition. In particular, the formation of porous tin oxide layers by anodization of Sn nanofoams has not been studied so far, while it seems to be especially promising. Such a process is expected to yield hierarchical Sn/SnOx
systems consisting of thin films of nanoporous SnOx
generated on dendrites with preserved metal cores. On the one hand, the metal core can act as an effective current collector when the Sn/SnOx
system is used as a photoelectrode in photoelectrochemical systems [34
]. On the other hand, when it is used in Li- or Na-ion batteries, a combination of the micro/nanoporous structure of Sn dendrites and nanoporous morphology of the oxide film not only provides shorter diffusion paths for lithium or sodium ions but also suppresses volume changes occurring during intercalation/deintercalation that results in enhanced integrity of the electrode [35
Another important property of such hierarchical Sn/SnOx
systems is their wettability. Cao et al. [37
] reported that self-passivated electrochemically deposited nanoporous Sn foams exhibit hydrophobic or even superhydrophobic behavior without any further surface modification. Such kind of conductive superhydrophobic metal surfaces can exhibit enhanced corrosion resistance and offer potential use in, e.g., microfluidic devices, and water-oil separation, etc. [38
]. Nevertheless, for several applications, including photoelectrochemical water splitting, good wettability of the nanostructured electrode is strongly desirable. Since we recently confirmed the hydrophilic nature of nanoporous SnOx
electrochemically grown on the surface of Sn foil [40
], it can be expected that the presence of such kind of porous oxide film can strongly affect the wetting behavior of Sn foams. However, the factors which govern the wetting behavior of these materials remain obscure.
Therefore, here we present, for the very first time, an effective strategy for the fabrication of hierarchical Sn/SnOx micro/nanostructures by electrodeposition of Sn foams followed by their one-step anodization in an alkaline electrolyte. We demonstrate that it is possible to obtain crack-free continuous porous oxide layers on a nanoporous metal foam substrate. The wettability of the prepared composites was evaluated and compared with the Sn/SnOx composites obtained via aerial oxidation of the tin nanofoams. Finally, the photoelectrochemical activity of the obtained material is also shown.
2. Materials and Methods
2.1. Substrate Preparation
Sn foil (99.99%, Goodfellow Cambridge Ltd., Huntingdon, UK, GB) was cut into coupons (c.a. 1 cm × 2 cm), washed in acetone and ethanol (both Chempur, Piekary Śląskie, Poland) to remove grease and other surface impurities. After that, the samples were treated with 60, 220, and 800 grit sandpaper in order to define a reproducible surface roughness (increased compared to the as-received foil) and cleaned ultrasonically in isopropanol (Chempur, Piekary Śląskie, Poland). Before electrodeposition, the working surface area was activated by immersing in 36–38% HCl (Sigma Aldrich, St. Louis, MO, USA) for 5 min in order to remove the superficial oxide layer that could emerge from the aerial tin oxidation. Finally, the samples were rinsed in isopropanol one more time.
2.2. Fabrication of Sn Foams
Tin foams were fabricated by cathodic electrodeposition in a typical two-electrode configuration with a Sn plate and platinum mesh used as cathode and anode, respectively. Electrodeposition was carried out at room temperature under the constant voltage of 6 V provided by a DC power supply (Array 3646A, Array Electronic Co., Ltd., Taiwan) for 60 s in the electrolyte containing 20 mM SnCl2·2H2O and 1.5 M H2SO4 (Sigma Aldrich). The distance between electrodes was fixed at 1 cm. After deposition, the samples were carefully rinsed with isopropanol to remove residues of the electrolyte. The working surface area of samples was defined by insulating the part of the surface with paraffin.
2.3. Synthesis of SnOx Layers on Sn Foams
The anodization was also carried out at room temperature in a two-electrode system with the prepared tin foams used as anodes and platinum mesh serving as a cathode. The process was performed in 1 M NaOH (Sigma Aldrich) at the constant potential of 4 V for 10, 20, and 30 min. Then, as anodized SnOx foams were rinsed in isopropanol and water, and dried in air. After that, some samples were annealed at 200 °C for 2 h in air with a heating rate of 2 °C min−1 using a muffle furnace (FCF 5SHM Z, Czylok, Poland).
2.4. Materials Characterization
The morphology of the materials was verified using an optical microscope (Delta Optical Evolution 100 Trino Plan with attached Delta Optical DLT-Cam PRO 3 MP digital USB camera as well as with an external LED lamp for upper illumination, Delta Optical, Poland) and Field-Emission Scanning Microscope (FE-SEM/EDS, Hitachi S-4700, Japan with Noran System 7). All geometrical parameters, including the thickness of films and pore sizes, were verified right from FE-SEM images using WSxM v.12.0 software [41
X-ray diffraction (XRD) measurements were performed using the X-ray diffractometer Rigaku Mini Flex II with monochromatic Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 20°–80° with a scan speed of 0.5° min−1 and a step size equaled to 0.02°.
Photoelectrochemical (PEC) measurements were carried out using a photoelectric spectrometer combined with a potentiostat (Instytut Fotonowy, Kraków, Poland) equipped with the 150 V Xe arc lamp. PEC tests were performed in a three-electrode cell with a quartz window. An Sn/SnOx sample serving as a working electrode was illuminated with a monochromatic light in the wavelength range of 250–500 nm. A Pt wire and saturated calomel electrode (SCE) were used as a counter and reference electrodes, respectively. Photocurrents were recorded in a borate buffer solution (pH ≈ 7.5) at the potential of 0.9 V vs. SCE.
The wettability of the tested samples was verified with the means of water contact angle (WCA) measurements using an OCA25 goniometer (Data Physics, San Jose, CA, USA) with an automatic dosing system. On each Sn/SnOx surface, three separate droplets were put, and 2 min-long movies were recorded in order to monitor the changes in the wettability. Where possible, CA was calculated based on the sessile drop method. The average CA is based on ten consecutive measurements on each droplet. All of the measurements were conducted at room temperature and humidity.