Investigation of the Structural Changes and Catalytic Properties of FeNi Nanostructures as a Result of Exposure to Gamma Radiation

The morphological features, as well as the elemental composition of the synthesized nanostructures, were studied using the scanning electron microscopy method performed using “Hitachi TM3030” scanning electron microscope (Hitachi Ltd., Chiyoda, Tokyo, Japan). Shooting mode - LEI, Current – 20 μA, Accelerate voltage – 2 kV, WD= 8 mm. 

The study of the elemental composition, as well as the mapping of the structures under study in order to determine the equiprobable distribution of elements in the structure, was carried out using the energy dispersive analysis method performed using the EDA Bruker Flash MAN SVE installation (Bruker, Karlsruhe, Germany), при ускоряющем напряжении 15 kV.

The influence of the ratio of the components of the electrolyte solution and the difference in the applied potentials on the elemental composition and isotropy of the distribution of the components in the structure of the nanotubes was determined using the energy dispersive analysis method by taking 10 spectra from different sections of the synthesized nanostructures, as well as mapping the elements along the entire length. After taking 10 spectra from different sections of the nanostructures, we calculated the average values of both components, as well as determine the magnitude of the error, which was no more than 2 at. %.

A scale bar has been added.

The data are presented in a more complete form, which displays the distribution of each element separately.

As can be seen from the data presented, the elemental ratio of the components of iron and nickel in the structure is close to permalloy compounds, which are characterized by the ratio of the components Fe:Ni = 20:80. A slight deviation of this ratio for the synthesized nanostructures may be due to nucleation processes that occur during the preparation of nanostructures. According to the mapping data, the distribution of iron in nickel is equally probable over the entire length of the nanostructures, which indicates the isotropy of the growth rate of nanostructures and the same rate of reduction of metal ions from electrolyte solutions during synthesis.

The crystallographic characteristics of nanostructures before and after directed modification were studied using a D8 Advance Eco powder X-ray diffractometer (Bruker, Karlsruhe, Germany). Conditions for recording diffractograms: 2θ = 35–80°, step 0.01°, Bragg-Brentano geometry, spectrum acquisition time 3 s, sample table rotation speed 20 rpm, X-ray radiation Cu-Kα, λ = 1.54 Å. For X-ray diffraction, all samples before and after modification were in a polymer matrix, which was mounted on the sample holder perpendicular to the x-ray beam. The rotation of the sample table was carried out to minimize the effect of the difference in textures at different shooting angles. An analysis of the structural characteristics, as well as the phase composition, was carried out using TOPAS v.5.0 software based on the Rietveld method. The size of the crystallites, which was calculated according to the Scherrer equation:

where k = 0.9 is the dimensionless particle shape coefficient (Scherrer constant), λ=1,54Å is the X-ray wavelength, β is the half-width of the reflex at half maximum (FWHM), and θ is the diffraction angle (Bragg angle).

Microstresses were estimated based on an analysis of the displacement of diffraction peaks calculated according to equation (2):

where d_pristine and d_irr are the interplanar distances before and after irradiation. The lattice stresses and macrostresses in the structure were estimated for all diffraction peaks.

The degree of crystallinity or structural ordering was determined by approximating the diffraction lines by the required number of pseudo-Voigt symmetric functions, and also by measuring the width of the recorded lines at half their height (FWHM). The pseudo-Voigt functions used to approximate the X-ray peak profile in the diffractogram (3) [43,44]:

where x is the variable corresponding to the angle of reflection 2θ; x₀ - sets the position of the maximum of the function; η is the specific fraction of the Lorentz function; A is the normalizing factor; b_G and b_L are the parameters of the Gauss functions G(x, x₀, b_G) and Lorentz L(x, x₀, b_L).

An analysis of the angular dependence of physical broadening allows one to evaluate the influence of factors such as the size effect associated with crushing or recrystallization of crystallites as a result of external influences, and a distorting factor depending on the degree of deformation of the crystal structure and its change as a result of external influences. To assess the impact, the Williamson-Hall method was used, which is based on relation (4) [45]:

where β is physical broadening of the diffraction maximum, λ is the x-ray wavelength (1.54 Å), D is the crystallite size, θ is the Bragg diffraction angle, ε is the magnitude of microstresses in the grating.

Microstresses were estimated based on an analysis of the displacement of diffraction peaks calculated according to equation (5):

where d_pristine and d_irr are the interplanar distances before and after irradiation.

In the case of Figure 4, the dots indicate the experimental data, the red lines indicate the approximation lines of the results obtained, which are necessary to determine the rate of degradation reaction.

In the case of Figure 8, the dots indicate the experimental data, the red lines indicate the approximation lines of the results.

Kinetic curves are a collection of data on changes in the elemental composition and mass of samples during oxidation and degradation, obtained using methods of energy dispersive analysis, x-ray phase analysis and by weighing samples before and after corrosion tests.

Figure 7 shows the results of changes in mass loss during degradation. According to the data obtained, the change in mass occurs in two stages. The first stage is described by a positive weight gain, which is due to the formation of an oxide film on the surface of the nanotubes. In this case, an increase in the radiation dose and, consequently, a change in the structural characteristics of nanotubes leads to an increase in the number of days at which a positive change in mass is observed, which indicates that the formed oxide film reduces the rate of degradation of nanotubes. The second stage of the change is characteristic of a negative weight gain, which is caused by the processes of degradation and partial destruction of the structure of nanotubes with the dissolution of oxide inclusions and the formation of ulcerative pores. It should be noted that an increase in the test temperature leads to a sharper drop in the mass loss coefficient, which indicates an increase in the corrosion rate of nanotubes.

Figure 7. Dynamics of mass loss due to corrosion: a) at a test temperature of 25°C; b) at a test temperature of 36°C; c) at a test temperature of 40°C.

According to the data of x-ray phase analysis, the degradation processes are associated with the formation of oxide and hydroxide phases in the structure of nanotubes, the presence of which leads not only to a deterioration in structural characteristics, but also with an increase in the test time to partial amorphization and degradation of nanotubes. Figure 8 presents data on changes in the concentration of impurity oxide inclusions in the structure of nanotubes.

Figure 8. Graph of the change in the concentration of oxide phases in the structure of nanotubes: a) at a test temperature of 25°C; b) at a test temperature of 36°C; c) at a test temperature of 40°C

As is known, the degradation of iron-containing nanostructures occurs by the following mechanisms:

Degradation by nickel oxidation occurs as a result of the loss of electrons in the interaction with the medium and the transition to nickel (II) oxide:

Ni⁰ – 2e = Ni²⁺.

The nickel (II) oxide formed refers to bertollides with oxygen stoichiometry. With a large amount of oxygen in the structure, it transforms into nickel (III) oxide Ni₂O₃·H₂O or NiOOH. In this case, nickel ions are completely oxidized:

Ni²⁺ – 1e = Ni³⁺.

But since the oxidation state of +3 is not characteristic of nickel, compounds with this valence are unstable; therefore, hydrated forms of nickel (II) oxide fall apart with the elimination of oxygen, which leads to the destruction of crystalline and chemical bonds.

The applicability of targeted modification of nanostructures by irradiation with gamma radiation in order to increase the rate of catalytic activity was evaluated using the reaction of the para-nitroaniline (PNA) reducing agent to para-phenyldiamine (PPD). The reaction was carried out by dispersing nanostructures (0.1 mg) in an aqueous solution of a mixture of PNA (10^-4 М) and NaBH₄ (6х10^-2 М). Evaluation of the catalytic reduction ability was carried out on a UV spectrometer at room temperature in the time range from 3 to 30 minutes in increments of 3 minutes. Based on the obtained UV spectra, the reaction rate constant was calculated. Obtaining UV spectra, as well as assessing changes in the catalytic activity of the studied structures, was carried out by the in situ method. Samples in a volume of 10 μl were taken after 3 minutes of measurements and placed in a spectrometer to record UV spectra, based on which the reaction rate constant was calculated. 

The radiation dose was controlled by two methods: dose calculation taking into account the structural features of the nanostructures, their geometry and length, and also using film detectors that were placed near the samples and behind the film samples to the nanotubes in order to determine the dose accumulated during the irradiation. Placing film detectors in two places made it possible to compare the accumulated doses of both the nanostructures themselves and the detector without them.

The crystallographic characteristics of nanostructures before and after directed modification were studied using a D8 Advance Eco powder X-ray diffractometer (Bruker, Karlsruhe, Germany). Conditions for recording diffractograms: 2θ = 35–80°, step 0.01°, Bragg-Brentano geometry, spectrum acquisition time 3 s, sample table rotation speed 20 rpm, X-ray radiation Cu-Kα, λ = 1.54 Å. For X-ray diffraction, all samples before and after modification were in a polymer matrix, which was mounted on the sample holder perpendicular to the x-ray beam. The rotation of the sample table was carried out to minimize the effect of the difference in textures at different shooting angles.

Figure 2. X-ray diffraction patterns of the studied samples before and after irradiation

The degree of crystallinity or structural ordering was determined by approximating the diffraction lines by the required number of pseudo-Voigt symmetric functions, and also by measuring the width of the recorded lines at half their height (FWHM). The pseudo-Voigt functions used to approximate the X-ray peak profile in the diffractogram (3) [43,44]:

where x is the variable corresponding to the angle of reflection 2θ; x₀ - sets the position of the maximum of the function; η is the specific fraction of the Lorentz function; A is the normalizing factor; b_G and b_L are the parameters of the Gauss functions G(x, x₀, b_G) and Lorentz L(x, x₀, b_L).

An analysis of the angular dependence of physical broadening allows one to evaluate the influence of factors such as the size effect associated with crushing or recrystallization of crystallites as a result of external influences, and a distorting factor depending on the degree of deformation of the crystal structure and its change as a result of external influences. To assess the impact, the Williamson-Hall method was used, which is based on relation (4) [45]:

where β is physical broadening of the diffraction maximum, λ is the x-ray wavelength (1.54 Å), D is the crystallite size, θ is the Bragg diffraction angle, ε is the magnitude of microstresses in the grating.

Microstresses were estimated based on an analysis of the displacement of diffraction peaks calculated according to equation (5):

where d_pristine and d_irr are the interplanar distances before and after irradiation.

This column has been deleted. Diffraction line width data added.

The density of the studied nanostructures was calculated using the formula (6):

where V₀ is the volume of the crystal cell, Z is the number of atoms in the crystal cell, A is the atomic weight of atoms.

The degree of disorder of the crystal lattice or the so-called integral porosity of the samples under study was found according to formula (7):

where р₀ is the density of the reference sample.

Corrected, the link is replaced with the corresponding one.

Figures 10a-b show the dynamics of changes in the intensity of spectral maxima characteristic of PNA and PPD compounds. As can be seen from the data presented, complete recovery is observed only for modified samples with doses of 300 and 500 kGy, while for unirradiated samples and irradiated with a dose of 100 kGy, the recovery reaction does not end in the allotted time for testing.

Figure 10. Graphs of the dependence of the changes in C_I/C₀ (a) and LnC_I/C₀ (b) reflecting the catalytic activity of the studied nanostructures; c) PNA-PPD reaction scheme; d) The graph of the change in the rate of reaction constant (dots indicate experimental data; red lines indicate approximation of the results)

As can be seen from the data presented, for the initial nanostructures, a partial restoration of the PNA–PPD reaction is observed over the entire cycle time (see the diagram in Figure 10c), which indicates a low rate of catalytic activity. In contrast to the initial nanostructures, the modified nanostructures show a complete recovery of the PNA–PPD reaction, as evidenced by an increase in the reduction rate (see the data in Figure 10d). The value of the reaction rate constant was calculated on the basis of data on the changes in the concentration of PNA and PPD compounds, which were estimated from the intensity values of the corresponding peaks in the spectra. As a result of the studies, it was found that an increase in the degree of ordering of the crystal structure, as well as a decrease in defective inclusions, leads to a sharp increase in the productivity of nanostructures for the PNA–PPD reduction reaction, which indicates the promise of using gamma radiation not only to increase corrosion resistance, but also catalytic activity nanostructures.

The authors are grateful for the comments made and will try to clarify the description of the methods.

1. The Rietveld method was applied to determine the phase composition of the studied nanostructures, as well as to determine the lattice parameters.

2. The Williamson-Hall method allowed us to estimate the contributions of dimensional factors and distortion of diffraction peaks. In this case, both crystallite size and microstress were calculated using this method.

3. The use of the Scherrer formula was used to estimate crystallite sizes without taking into account distortions of diffraction lines. Moreover, the use of this formula showed that the crystallite size is comparable to the sizes determined by the Williamson-Hall method. Also in the latest edition, the Scherrer equation is removed from the text of the article.

Regarding the representation of diffraction patterns, according to the remark of the previous reviewer, the representation of diffraction patterns was changed to represent them without subtracting the background, as well as smoothing, which led to a more pronounced manifestation of the entire diffraction pattern, as well as the reflection of weak reflections (200) and (220) for irradiated samples with doses above 300 kGy. The latest edition of the article presents diffraction patterns without the use of filters for smoothing and subtracting background lines, which made it possible to display all changes in the diffraction pattern. The authors also presented calculations of texture coefficients of the studied structures before and after irradiation.

The refinement of sizes and microstresses was carried out using the Williamson-Hall method.

Macrostrain were estimated based on an analysis of the displacement of diffraction peaks calculated according to equation (3):

Corrected.

The authors are grateful for the comment provided.

Regarding these comments.

The change in the orientation of crystallites was evaluated by assessing the change in the intensity of diffraction reflections, and determining the texture coefficients, the values ​​of which show a distinguished direction of orientation. In this case, the use of the rotation of the sample in measuring the diffraction pattern allows us to remove questions about the azimuthal variation of the intensity with respect to the position of the angle φ. In this shooting mode, the intensities of the diffraction peaks are isotropic relative to the position of the angle φ, but their ratio allows us to estimate the contribution to the preferred orientation of the crystallites.

Moreover, for irradiated samples, a decrease in the intensity of (200) and (220) diffraction reflections is observed while maintaining isotropy with respect to φ, which leads to a change in the values of texture coefficients, the values of which are presented in Table 2, reflecting the preferred orientation of crystallites in the structure of nanotubes. Texture coefficients were calculated using the Harris formula (6):

where I(hkl) – the experimentally obtained relative intensity; I₀(hkl) – relative intensity corresponding to a given plane according to the PDF-2 database; n – the number of planes.

Table 2. Texture coefficient data

The authors are grateful for the comments made and agree with the reviewer that the radiation dose calculations require separate work. In this regard, the description of dose measurements is corrected and presented in the following form.

The radiation dose was controlled by using film detectors that were placed near the samples and behind the film samples to the nanotubes in order to determine the dose accumulated during the irradiation. Placing film detectors in two places made it possible to compare the accumulated doses of both the nanostructures themselves and the detector without them.  The measurement error of the dose was not more than 2-3%.