Hydrolysis of metal salts forms a basis of many approaches to the synthesis of nanosized metal hydroxides, including the methods of precipitation [1, 2], sol-gel [3–5] and layer-by-layer synthesis [6], the interaction on the metal salt solution‒gaseous reagent boundary [7, 8], etc. The approaches affording metal hydroxides with predictable morphology are of special importance. A procedure for the synthesis of closed hollow microcapsules with size of several hundreds of nanometers, walls of which are built of Zr(OH)4, via the hydrolysis of an alcoholic solution of Zr(OC3H7)4 at the surface of an aqueous solution of NH4OH, followed by complete immersion of the microdrops in the solution, has been suggested [9]. The development of nano- and microsized nanomaterials with such morphology is an important issue of preparative chemistry, since such materials exhibit a novel set of functional properties and form a basement for encapsulation of various compounds [10] during the development of novel electrodes for supercapacitors [11], various electrolyzers [12], sensors [13], etc.

Herein we elaborated the conditions for the synthesis of the open microcapsules with walls of double nickel-iron hydroxides vis fast hydrolysis of the metal salts in the microdrops of their solution applied at a surface of an alkaline solution. Such microcapsules can be used as components of electrodes for various electrochemical devices. It could be expected that they were formed due to rapid hydrolysis at the microdrop–solution boundary instantly upon the immersion of a salt solution microdrop into the alkali solution. The microcapsules remained at the alkali–air interface without submersion into the alkali solution. The solution of the mixture of NiSO4 and FeSO4 (the concentrations ratio 4 : 1) was sputtered as an aerosol onto the surface of the NaOH using an ultrasound device. The precursor salts ratio of 4 : 1 was chosen in view of the expectation of that ratio in the obtained double nickel–iron hydroxide. The minimal values of the overpotential in the reaction of oxygen evolution during water electrolysis have been earlier observed for the double nickel–iron hydroxide of this composition [14, 15].

The hydrolysis of the mixture of NiSO4 and FeSO4 in the microdrops sputtered at the surface of the alkali solution during 8 min resulted in the formation of open vase-like microcapsules with size of 1‒10 μm (Figs. 1a, 1b). The outer surface of the microcapsules consisted predominantly of spherical nanoparticles with size of 10‒20 nm (Fig. 1c), whereas the inner surface contained the same nanoparticles along with the planer ones, with the lateral size of 50 nm (Fig. 1d).

Fig. 1.
figure 1

Scanning electron microscopy images of the microcapsules applied on a nickel foil surface: (a, b) general view of the microcapsules, (c) outer surface of the microcapsule, and (d) inner surface of the microcapsule.

Full size image

Investigation of the microcapsule walls by means of transmission electron microscopy (Fig. 2a) confirmed the states size of the microcapsules. Moreover, analysis of the high-resolution images revealed that the nanoparticles were crystalline (Fig. 2b). Direct information on the interplanar distances could not be obtained due to the so-called moiré pattern effect. However, the electron diffraction study revealed that the nanoparticles were characterized by the interplanar distances of 0.235, 0.200, 0.170, 0.140, 0.120, and 0.100 nm (Fig. 2c). Those values were close to the interplanar distances in β-Ni(OH)2 with trigonal crystalline structure and the –3m point groups [16].

Fig. 2.
figure 2

Transmission electron microscopy image of the microcapsule wall in the bright field (a) and high-resolution mode (b). The electron diffraction pattern (c).

Full size image

The ratio of the nickel and iron atoms concentration in the walls (3.7) was determined by X-ray spectral microanalysis. The obtained value was close to their ratio in the starting solution. From the obtained data of the X-ray photoelectron spectroscopy it followed that the layer consisted of the nickel, iron, and oxygen atoms, the position of the Fe2p3/2 electron peak being 712.4 eV. That fact evidenced the degree of oxidation of iron cations 3+ [17]. Likely, the Fe(II) cations were oxidized during the synthesis or upon the obtained samples keeping in air. It is important to note that the content of sodium, sulfur, and boron atoms, which could be incorporated in the double hydroxide, did not exceed several percent, which confirmed efficient separation of the side products by washing. According to the data obtained by means of diffuse reflection IR spectroscopy, the walls substance revealed a broad band of the M‒O bonds stretching with maximum at 630 cm–1, which, in view of the results in Ref. 18, allowed the conclusion about the formation of layered double nickel-iron hydroxide.

In view of the obtained results, it could be stated that when a microdrop of the solution of the NiSO4 and FeSO4 mixture achieves the surface of the 1 M alkali solution, fast hydrolysis of the salts occurs with the formation of the double nickel-iron hydroxide nanoparticles at the interface. Those nanoparticles formed sufficiently strong walls of a microcapsule. At that stage, a peculiar semispherical microboat appeared, its walls consisting of the nanoparticles and immersed in the alkali solution. That microboat was open in the upper part, since initially there was no contact between the nickel and iron cations with alkali in the upper part of the microdrop, their interaction in that part did not occur, and that led to the formation of the open vase-shaped microcapsules.

The relatively fast formation of the porous microcapsule wall was followed by the ions diffusion and levelling of their concentrations inside and outside the capsule, accompanied by relatively slow growth of the nanocrystals of the double hydroxide at the inner side of the capsule wall. We suggest that the planar nanocrystals of the double hydroxide and the rim in the upper part of the microcapsule (Fig. 1a) were formed at that stage. The microbubbles of hydrogen could be formed at the outer side of the microcapsule via the interaction of water with the BH4‒ anions, and those microbubbles likely prevented the microcapsules from the submersion in the alkali solution. The size of the obtained microcapsules corresponded to that of the microdrops in the aerosol generated by the applied device, which confirmed the sequence of the chemical processes during the microcapsules formation.

Transfer of the microcapsules to the nickel foil afforded an array of the microcapsules which specified unique 3D texture of its surface. The so prepared nickel electrodes exhibited active electrocatalytic properties towards the reaction of oxygen evolution during water electrolysis in an alkaline medium (Fig. 3). For example, for one of the specimens the overpotential value equaled 280 mV, the Tafel slope being of 69.1 mV/dec. That overpotential value was only several millivolt higher than the values obtained for the similar electrocatalysts applied on more expensive foamed nickel sample with developed interior surface and specially directed 3D morphology of the support [19].

Fig. 3.
figure 3

Polarization curve (a) and the Tafel slope (b) of the electrode based on nickel foil with the microcapsules of nickel-iron double hydroxide at the surface.

Full size image

EXPERIMENTAL

Nickel electrodes covered by microcapsules of double nickel(II)-iron(III) hydroxide. The synthesis was performed starting from NiSO4·7H2O (pure), FeSO4·7H2O (pure), NaBH4 (99.9%, Aldrich), and NaOH (chemical pure). The solutions were prepared via dissolution of weighed portions of the chemicals with at least 30 min stirring. The synthesis was performed from a mixtures of 0.4 M solution of NiSO4 with 0.1 M solution of FeSO4 and of 1 M solution of NaOH with 0.5 M solution of NaBH4. To obtain the microcapsules, 2 mL of the solution of the iron and nickel salts mixture was put in a chamber of an A&DUN-231 ultrasound nebulizer equipped with an ultrasound generator (power 10 W, frequency 2.5 MHz), whereas 3 mL of the alkaline solution of NaBH4 was put in a planar Teflon vessel (diameter 3 cm). The sputtering chamber and the Teflon vessel were connected via a Γ-shaped glass tube with inner diameter 15 mm and length 15 cm, having a conical flare to diameter of 6 cm over the Teflon vessel. The microcapsules were obtained during ultrasound-assisted sputtering of the aerosol of a mixture of NiSO4 and FeSO4 over the surface of the solution of a mixture of NaOH and NaBH4. Duration of the treatment with the aerosol was varied between 2 to 20 min. When the treatment was complete, the formed microcapsules were transferred from the solution surface to a nickel foil via the vertical elevator technique, washed three times with deionized water, and dried in air at 60°C.

Electron microscopy images were obtained using a ZeissMerlin scanning electron microscope and a ZeissLibra 200 transmission electron microscope. Composition of the microcapsules walls was determined by means of X-ray spectral microanalysis using an Oxford Instruments X-Max 80 probed (a component of the transmission electron microscope). IR spectra of the microcapsule applied onto the nickel foil were recorded using an FSM-2201 spectrophotometer equipped with a diffuse reflection attachment.

Electrocatalytic properties of the obtained electrodes composed of the nickel plate and the applied microcapsules surface were studied by means of cyclic voltammetry with linear scanning of the potential and the Tafel analysis using the oxygen evolution reaction as the example. Characteristics of the electrodes were measured using an Elins P-20X8 potentiometer in a three-electrode cell. The nickel foil/microcapsules sample was used as the working electrode, whereas platinum foil and silver chloride electrode acted as the auxiliary and the reference electrodes, respectively. The measurements were performed in 1 M aqueous solution of KОН at room temperature and atmospheric pressure, the scanning rate being 10 mV/s. Electrochemical measurements were performed with the IR compensation. The overpotential value was determined from the Nernst plot.