1. Introduction
In the light of a growing demand for clean energy conversion and storage systems, the role of hydrogen technology has become progressively more important. Consequently, substantial scientific and industrial research efforts have been dedicated to the development of hydrogen-based fuel cells and electrolysis equipment in the recent past. To ensure an adequate applicability, efficiency, and a broad availability of sustainable technological solutions, however, robust, active catalyst materials exhibiting structural stability over a large number of electrochemical cycles need to be established to reduce the overpotential, a figure of merit for efficient electrochemical water-splitting, as much as possible. These requirements are met, for instance, by some transition metal oxides including Co
3O
4 [
1]. Besides the chemical composition of the catalyst, nanoscale structuring of the material can significantly affect the catalytic performance, e.g., due to the stabilization of stationary active centers and the creation of a high number of reaction sites [
2,
3].
Electrodeposition has established itself as one of the most effective methods for nano-structuring of inorganic materials. This technique stands out from other deposition methods due to its high deposition rates, low cost and effort, precisely controllable experimental parameters, and remarkable product purity [
4]. As another advantage, precipitation occurs directly on the conductive substrate, such that the deposited structures themselves form the electrode surface. Hence, further use as an active electrode material, e.g., in water-splitting applications, does not require additional preparation steps. Moreover, structure-directing additives can be used conveniently and their influence on the formation and transformation of catalyst structures can be easily analyzed.
A well-described property of thin films and coating is their instability towards dewetting, as exemplarily reported by Reiter for polystyrene films on silicon when annealed above the glass transition temperature [
5]. Dewetting of as-deposited thin films occurs spontaneously as a result of surface energy minimization, thus generating a large driving force for agglomeration of thin layers into particles with dimensions on the micron and nanometer scale [
6]. In view of technological applications relying on robust homogenous coatings, dewetting is usually considered as unfavorable and needs to be inhibited [
6]. Recent examples for a targeted prevention of dewetting include the stabilization of silver thin films by implantation of silicon and indium atoms [
7] and the stabilization of polymer bilayers by adding gold nanoparticles to the top layer [
8]. In some cases, however, thin film dewetting processes are intentionally induced to achieve the formation of desired structures, as reported, for instance, by Bonvicini et al. for metallic and bimetallic nanostructures [
9]. In our here-presented work, thin film dewetting is actively used as a strategy for the formation of self-supported hierarchical organic–inorganic microscrolls with coiled morphologies.
When considering synthetic strategies for the preparation of inorganic materials with complex hierarchical structures, nature can serve as a rich source of inspiration, as biogenic minerals, such as bones, teeth, or mollusk shells characteristically demonstrate elaborate structural organization down to the nanometer scale. An important principle of bio-mineralization is the induction and regulation of mineral deposition by soluble and insoluble organic matrices [
10]. A well-known example is that of vertebrate bones, whose astounding mechanical properties originate from the combination of organic molecules (collagen) and inorganic minerals (hydroxyapatite) [
11]. Similar organic/inorganic nanocomposite structures are also observed in sea urchin spines, mussel shells, and corals [
12,
13,
14,
15,
16,
17]. Even though the content of occluded organics is usually very minor in these systems, they exert strong effects on the architecture and properties of the material [
18]. The elaborate hierarchical structures of biominerals, which exhibit remarkable adaptation to specific functions, while being formed under ambient conditions, have sparked imagination in view of artificial materials with complex architecture. A particularly facile concept inspired from biological mineralization is the use of water-soluble polyelectrolyte additives, mimicking highly acidic proteins associated with mollusk shells, to direct the crystallization of synthetic materials. An intriguing example is the formation of calcite mesocrystals mediated by the negatively charged polymer poly(4-styrene sulfonate) (PSS) [
19], where the additive drives the precipitation of the solid along a non-classical particle-based mechanism relying on the aggregation of initially stabilized primary units (amorphous or crystalline) [
20,
21]. As the influence of organic additives can lead to complex crystalline nanostructures with well-ordered superordinate arrangements [
22], a substantial number of studies have been devoted towards the translation of bioinspired crystallization concepts into functional inorganic materials [
23,
24], including Co
3O
4 [
25,
26].
In a recent study on polymer-mediated precipitation of Co(OH)
2, an important precursor to cobalt(II,III) oxide, Gruen et al. used polyethyleneimine (PEI) as an organic structure-directing polyelectrolyte to achieve the deposition of a mineral film at the air/solution interface. Intriguingly, upon drying, the resulting two-dimensional sheets of amorphous cobalt hydroxide occluding a very minor content of organics underwent strain-induced self-rolling into free-standing spirals with up to six twists, which were subsequently pseudomorphically converted into crystalline Co
3O
4 coils with mesoscale channels [
27]. As the self-rolling phenomenon of the largely inorganic mineral films critically relied on the presence of PEI, the polymer additive was discussed to facilitate slight differences in the deposition regimes between the air- and the solution-exposed side of the film, such that non-uniform structural motifs with a different capacity for water absorption emerged along the surface normal direction.
In general, self-folding of two-dimensional materials is based on compositional or structural heterogeneities (e.g., density, morphology) [
28] along the cross-section. While strain-induced self-rolling of heterogeneous two-dimensional bilayer, multilayer, or gradient systems is a common phenomenon in nature, especially in the plant kingdom [
29,
30], and has been translated into artificial soft matter systems [
31,
32], this concept is scarcely reported for geological and synthetic minerals due to the high bending strain which needs to be accommodated by these brittle materials to introduce curvature.
Remarkable examples for self-rolling in inorganic precipitation systems include manganese oxide microtubules, prepared by Tolstoy et al., where the density of sheet packing decreases within the film [
33]. Lepidocrocite (γ-FeOOH) microtubes which are formed by drying-induced self-rolling due to a moisture gradient along the cross-section were reported by Strykanova et al. In a next step, the authors were able to coat the inner surface of the lepidocrocite microtubes with silver nanoparticles to form an inorganic composite [
34]. Hybrid inorganic–organic microscrolls were also produced by Takahashi et al., who used differences in hydrophilicity as a driving force for the self-rolling process. Again, a modification of hybrid films with nanoparticles was successfully achieved, which also allows the controlled release of entrapped particles [
35].
On an even smaller length scale, bilayer structures originating from heterogeneities in lattice constants within a thin inorganic film can equally build up strain, which is eventually released by self-rolling into nanotubes, as reported by Schmidt et al. [
36]. From these examples it becomes evident that the experimental conditions allowing for the deposition of non-uniform inorganic films range from gas–solution interfacially directed techniques [
27,
28,
33,
34] over spin-coating [
35] to selective etching techniques [
36].
In this work, we explore the fabrication of three-dimensional self-supported cobalt hydroxide microscrolls based on the strain-induced self-rolling of electrochemically deposited thin films with heterogenous nanostructure.
2. Materials and Methods
2.1. Chemicals and Materials
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and potassium sulfate (K2SO4) was purchased in analytical grade from Merck KGaA (Darmstadt, Germany). Poly(sodium 4-styrene sulfonate) (PSS, average Mw 75,000 g mol−1), poly(allylamine hydrochloride) (PAH, average Mw 17,500 g mol−1), and potassium chloride (KCl, 3 mol L−1, BioUltra) were obtained from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Sodium hydroxide solutions (NaOH, 0.1 mol L−1 and 1 mol L−1 in water) were purchased from Grüssing GmbH (Filsum, Germany). Hellmanex III solution was purchased from Hellma GmbH & Co. KG (Müllheim, Germany). All chemicals were applied directly as received from the suppliers without further purification, and aqueous solutions were prepared with deionized water (resistivity 18.2 MΩ cm, Milli-Q Advantage A10, Merck KGaA, Darmstadt, Germany).
Glass slides coated with a conductive layer of fluorine-doped tin oxide (FTO, item number TEC 8A, glass thickness: 2.2 mm, resistance: 8 Ω/sq) were purchased from XOP Glass (XOP Física SL, Castellón, Spain) and cleaned according to the following procedure: The substrates were cleaned in a ultrasonic bath for 10 min, sequentially with the use of an aqueous Hellmanex III solution (5 wt%), acetone, ethanol, and aqueous NaOH solution (1 mol L−1). After each step, the substrates were rinsed with the corresponding solvent and air-dried at room temperature.
2.2. Electrochemical Deposition and Characterization of Cobalt Hydroxide Thin Films and Their Conversion into Cobalt (II,III) Oxide
All electrochemical depositions and electrochemical measurements were performed within a three-electrode electrochemical cell with a PGSTAT204 potentiostat (Metrohm Autolab B.V., Utrecht, The Netherlands). The setup comprised a silver/silver chloride reference electrode (Ag|AgCl, item number 6.0726.100, Metrohm, inner filling: potassium chloride (KCl, 3 mol L−1), outer filling: potassium sulfate (K2SO4, 0.1 mol L−1)) and a platinum counter electrode (Pt, item number 3.109.0790, Metrohm).
Deposition of Co(OH)2. Thin films of cobalt hydroxide were produced by cathodic electrochemical deposition (CED) on FTO-coated glass slides connected as working electrode (
Figure 1). The exposed areas of both the working electrode and counter electrode were 1 cm
2 and the distance between both electrodes was adjusted to 2 cm. All three electrodes were centrally positioned in the reaction container and immersed in an aqueous cobalt nitrate solution ([Co(NO
3)
2·6H
2O] = 50 mmol L
−1), both under polymer-free conditions and in the presence of PSS or PAH as structure-directing additives ([polymer] = 4.2 mg mL
−1). To induce galvanostatic Co(OH)
2 deposition, a constant current of −0.5 mA was applied to the FTO substrate for 30 min.
As illustrated in
Figure 1a, the principle of the CED is based on the formation of hydroxide anions near the working electrode according to Equations (1a) and (1b) followed by basic precipitation of solid Co(OH)
2 in the presence of Co
2+ directly on the FTO surface (Equation (2)).
The reaction potentials provided in
Figure 1a are derived from the literature and were recalculated to account for the pH value of the Co(NO
3)
2 electrolyte solution (pH = 3.9 at start of the electrodeposition) [
37]. Detailed calculations are presented in the
Supporting Information.
Figure 1b shows the potential profiles for a series of CED experiments performed under the same conditions and their average curve. The electrochemical system stabilizes after a few minutes, such that a relatively constant potential is measured during electrodeposition using the setup shown as inset. The deposited films initially appear smooth and a homogeneous green coating of the entire immersed electrode surface is observed directly after the CED (
Figure 1c). During the first minutes of drying, however, the polymer-mediated solid Co(OH)
2 layers undergo complete dewetting from the substrate accompanied by conspicuous self-rolling into spiraling morphologies (
Figure 1d). For further investigation, the dewetted films were left to dry in air at room temperature.
Pseudomorphic Conversion into Co3O4. To achieve transformation of the hydroxide precursor into catalytically active Co3O4, selected samples were subjected to a thermal treatment performed under air atmosphere in a Carbolite Gero ELF 1100 oven (Carbolite Gero GmbH & Co. KG, Neuhausen, Germany). The temperature was gradually increased with a heating rate of 5 K min−1 and the target temperature of 300 °C was held for 2 h.
Oxygen evolution electrocatalysis. The catalytic properties of non-scrolled Co(OH)
2 films were tested by electrochemical water-splitting experiments using the anodic oxygen evolution as a model reaction. An aqueous solution of sodium hydroxide (0.1 mol L
−1) was used as electrolyte/reagent. Linear potential sweeps were performed in the window between 0.0 V and 2.0 V vs. Ag|AgCl with a scan rate of 10 mV s
−1. The measured potential
Emeas was corrected with respect to the cell resistance and the current
i (Equation (3a)). Additionally, the cell potential was converted to the potential vs. the reversible hydrogen electrode (RHE) using a standard potential
of 0.205 V and a pH value of 13. (Equation (3b)). The potential at a threshold current density of 10 mA cm
−2 was then used to determine the overpotential
η (Equation (3c)).
2.3. Structural and Compositional Characterization of the Products
The structure and composition of electrochemically deposited cobalt hydroxide precipitates prepared in the absence and presence of structure-directing polymer additives were analyzed using a wide range of imaging, scattering, and spectroscopy methods.
General morphologies and detailed structures of the samples were imaged by digital light microscopy (DLM) using a VHX-950F with a VH-Z250R lens system from Keyence Deutschland GmbH (Neu-Isenburg, Germany) and by scanning electron microscopy (SEM) using a Zeiss LEO 1530 VP Gemini from Carl Zeiss AG (Oberkochen, Germany) operated at an acceleration voltage of 3 kV. In preparation for SEM measurements, film fragments were mounted on a standard sample holder by conductive adhesion graphite-pads and sputtered with a conductive layer of platinum (layer thickness: 1.3 nm) using a Cressington Sputter Coater 208 HR from Cressington Scientific Instruments UK (Watford, United Kingdom). Cross-sectional views were achieved by manual positioning of individual spirals. To investigate the elemental composition of polymer/mineral spirals, energy-dispersive X-ray spectroscopy (EDX) maps were recorded by an UltraDry SDD detector from Thermo Fischer Scientific (Waltham, MA, United States of America) at an acceleration voltage of 20 kV.
Confocal Raman microscopy was performed with a WITec alpha 300 RA+ instrument from WITec GmbH (Oxford Instruments, Abingdon, United Kingdom). For the spatially resolved acquisition of Raman spectra, the excitation lasers with wavelengths of 532 nm and 785 nm, respectively, were operated at 1 mW.
Dynamic light scattering (DLS) on PSS solutions ([PSS] = 4.2 g L−1) subjected to different Co2+ ion concentrations ([Co2+] = 0 mmol L−1, 50 mmol L−1), was performed with a Zetasizer Nano-ZS ZEN3600 (wavelength λ = 638 nm) from Malvern Panalytical (Malvern, United Kingdom) in automatic mode at 25 °C. Samples were measured in disposable PMMA cuvettes purchased by Brand GmbH (Wertheim, Germany). Data acquisition and processing were conducted with the Zetasizer Software (version: 7.13) provided by the supplier.
For small-angle X-ray scattering (SAXS) measurements, which were performed using a Double Ganesha AIR system from SAXSLAB/Xenocs, deposited microscrolls were scratched off the FTO substrates, crushed with a mortar, and filled in glass capillaries (Ø = 1 mm, Hilgenberg, Malsfeld, Germany). Monochromatic radiation (λ = 1.54 Å) was provided by a rotating anode (generator: Micro 7 HFM with rotating copper anode) from Rigaku Corporation (Tokyo, Japan). Two-dimensional scattering patterns were collected with a position-sensitive PILATUS 300 K detector from Dectris (Philadelphia, PA, USA) which was placed at different distances from the sample. The radially averaged profiles of the scattering intensity I(q) versus the modulus of the scattering vector q were corrected for background, an assumed sample thickness of 1 mm, accumulation time as well as absorption, and subsequently merged.
The electrodeposited precipitates before and after calcination were investigated by powder X-ray diffraction (PXRD) performed with a STOE STADI P Mythen2 4K diffractometer (Stoe & Cie. GmbH, Darmstadt, Germany) equipped with a Ge (111) monochromator and four Dectris MYTHEN2 R 1K detectors in Debye–Scherrer geometry. Diffractograms were recorded in an angular range of 2θ = 2°–140° at room temperature using Ag–Kα radiation (λ = 0.56 Å). Prior to the measurements, all samples were sealed in glass capillaries (Ø = 1 mm, Hilgenberg, Malsfeld, Germany).
4. Discussion
Our results revealed intriguing self-supported microscroll morphologies formed upon drying of electrodeposited cobalt hydroxide films. The formation of these structures critically requires the presence of a structure-directing polyelectrolyte. In this, we would like to highlight that the self-rolling phenomenon mediated by the sulfonate-functionalized negatively charged polymer PSS could be reproduced with amine-functionalized poly(allylamine hydrochloride) (PAH), as summarized in
Figure S9 in the Supporting Information. This important observation suggests that the mechanism for self-rolling does not rely solely on the exact chemical nature of the polymer additive (in particular, its net charge), but rather on its ability to interact with Co
2+ ions or early-stage species of mineralization in solution and near the electrode surface to interfere with the precipitation and crystallization process. Therefore, we suspect that a general polymer—metal—interaction promotes the additive-mediated electrodeposition of mineral films with heterogeneous nanostructure. Such interplays are well-documented in solution-based Ca
2+/polyelectrolyte systems, where crystallization into calcium carbonate can proceed via colloidal clusters formed from an initially deposited amorphous precursor phase. The polyelectrolyte stabilizes this precursor phase and can eventually be found occluded in the final crystallized product [
16]. Such non-classical crystallization mechanisms have also been observed in a variety of precipitation systems and are not unique to calcium carbonate [
20].
The presence of Co(II)/polymer complexes in solution prior to electrodeposition can be regarded as an important indicator for a polymer-mediated non-classical crystallization process. These aggregates represent the primary species interacting with the electrode surface, where precipitation is induced by reaction with hydroxide ions. In support of such a hypothesis, dynamic light scattering (DLS) analysis, indeed, shows such complexes (
Figure S10). While an aqueous solution of PSS already contains aggregates with a size of 27 nm (presumably stabilized by Na
+ ions) even in the absence of Co(II), the signal becomes narrower and more intense when Co
2+ ions are added, thus indicating well-defined metal-bridged polymer aggregates.
When considering potential mechanisms for self-rolling of the deposited hybrid films, passive self-folding phenomena in two-dimensional biological or polymeric soft matter systems are usually achieved by a bilayer, multilayer, or gradient structure, in which different layers show a distinct response to an external stimulus. In inorganic films, non-uniform properties along the surface normal direction can be implemented via heterogeneities in growth regimes/nanostructure [
27], composition/hydrophilicity [
35], or lattice constants [
36]. Internal strain then leads to different displacements between the layers, resulting in deformation and bending of the structure.
In the here-presented precipitation system, the separation of the deposited polymer–inorganic hybrid films into two domains, comprising a disordered and less-dense nanostructure at the convex surface and a more ordered one at the concave surface, integrates well with the discussed concepts of self-rolling. Specifically, we propose that the differently structured layers exhibit different capacities for the absorption of intrastructural water, thus undergoing a non-uniform degree of volume-shrinkage during drying, resulting in the build-up of internal strain as a driving force for self-rolling. The deposited films show a strong anisotropy along the surface normal direction with regard to their drying behavior, an observation which is comparable to other self-rolling materials. In general, heterogeneities in the structure give rise to a non-uniform mechanical response of the material to external stimuli, such as electrical signals [
43], temperature changes [
44], the exposure to water in combination with swelling phenomena [
35], or, as presented in this work, the release of intrastructural water during drying. The internal stress that builds up within the material due to the anisotropic response to external conditions is ultimately released by a bending motion, which can create complex three-dimensional structures.
An important question in this context is the origin of the bilayer structure. Considering the formation of the film via CED, it is important to note that immediately after applying a potential to the working electrode, a first layer of Co(OH)2 nucleates directly on the FTO surface. Afterwards, deposition takes place at the newly formed Co(OH)2 surface.
As observed through SEM imaging (
Figure 3 and
Figure 6a), the first formed Co(OH)
2 layer appears less ordered than the subsequently formed material. This structural distinction could be explained by a stronger structure-directing effect of the polyelectrolyte in the later stages of the electrodeposition. However, this potential effect is not reflected in the composition of the film, as both EDX and Raman microscopy point to a homogenous distribution of polymer throughout the hybrid material. Hence, PSS is uniformly incorporated during the entire deposition procedure.
The deposition potential
EWE, in contrast, varies during the process and only stabilizes a few minutes into the electrodeposition (
Figure 6a). Here lies a possible cause for the bilayer formation. In particular, the less-ordered layer at the convex surface of the hybrid films is structurally similar to the cross-sections of Co(OH)
2 films deposited in the absence of polyelectrolytes (
Figure S3). Hence, the most dominant factor leading to the rapid formation of disordered material may be the strong tendency for precipitation due to the initially more negative electrode potential and the lower resistance at the FTO surface compared to the subsequently formed Co(OH)
2 surface. In other words, a first layer of highly disordered Co(OH)
2 is formed, because the driving force for uncontrolled rapid precipitation exceeds the stabilization effect of the organic additive. This layer is nucleated to the FTO substrate and upon dewetting becomes the convex surface of the microscroll (
Figure 6a, inset). The higher precipitation rate, however, does not affect the concentration of polymer occluded within the material. The subsequently deposited more-well-defined layer of Co(OH)
2 is in contact with the reactant solution during the CED process and eventually becomes the concave surface of the coiled film fragments. Here, the impact of the polyelectrolyte is enhanced due to the more positive potential and the lower driving force of un-regulated precipitation, resulting in generally slower reaction kinetics and a higher level of structural organization.
As schematically illustrated in
Figure 6b,c, our proposed mechanism proceeds via the deposition of a polymer/Co(OH)
2 hybrid film on FTO (
Figure 6b). Upon drying, internal strain builds up due to differences in drying speed and volume shrinkage between both layers. As a result, a microscroll is formed (
Figure 6c). Finely dispersed organic additives are occluded in both parts of the bilayer, which is in conformity with the results of EDX analysis.