Tuning of Structural, Morphological and Optical Properties of Pa 6 upon Stepwise Insertion of Amorphous Se, Cd2+ and Ag+ Ions

1. Introduction

Nanoscale sized inorganic materials within the polymers are interesting in optoelectronics as light emitting devices [1] or photovoltaics [2], separation-catalysis [3] and bioelectronics [4]. The semiconductor nanoparticles embedded in the polymer film are easy to process [5]. These composites are usually made by mixing inorganic and organic components, and the obtained structure has physical and chemical properties that are not present in the individual components [1].

Selenium and selenium-containing materials have become the subject of many scientific studies due to their exceptional physical-chemical properties and photoelectric characteristics [6,7,8]. As an important inorganic material, selenium has also received a lot of attention due to its good semiconductor properties. For an a-Se film with a thickness of 4200 Å, the value of the band gap is 1.76 eV [9,10]. Due to their high photoconductivity, various selenides are one of the key materials widely used in optoelectronic devices such as photocopiers, light emitting diodes, xerography, and solar cells [8,11,12,13].

Doping with impurity atoms is an effective strategy for optimising the properties required for the development of semiconductor technologies. Newly developed aliovalent doping provides extra electrons (n-type doping) or extra holes (p-type doping) to redefine their doping type for electronic requirements. For CdSe, the most studied nanocrystalline system, it has been proved that additional carriers from heterovalent dopants such as Ag, Cu and In [14,15] can enhance the electronic conductivity in films when they are excited by external energy. Incorporating aliovalent metal cations into binary, such as CdSe, semiconductors change the optoelectronic properties of the newly formed base material, providing greater performance in a number of applications, including solar cells, batteries, and LEDs [16,17,18].

Polyamides are widely used as technical polymers with applications ranging from synthetic fibres to industrial and structural applications. Typically, optoelectronic devices use a combination of highly conductive semiconductor components due to their electrical pumping capability and the strong interaction of light and matter of the organic component [19]. The efficient inorganic materials used include cadmium selenide (CdSe) [11] and silver selenide (Ag₂Se) [12,13]. CdSe and Ag₂Se are direct bandgap semiconducting materials with the corresponding bandgaps of 1.9 eV [20] and 1.8 eV [21], respectively.

Due to the high reduction potential of Se(IV)/Se(0), various reagents for reduction of selenium (IV) compounds in several types of aqueous solutions have been evaluated. N₂H₄ [6,22], (NH₂)₂CS [23], metallic Fe or Zn [24,25,26,27], CuCl [28], TiCl₃ [29], SnCl₂ [30], Na₂S₂O₄ [31], D-fructose [32], and ascorbic acid [33] have been proposed as quantitative reducing agents for Se(IV) in acidic solutions. In industrial applications, SO₂ is the preferred reagent for the reduction of Se(IV) to its elemental state. According to literature data [10,34], when working with hot (>80 °C) and relatively concentrated solutions of H₂SO₄ or HCl (pH <1), the complete reduction of selenium occurs very quickly. However, high toxicity (hydrazine and thiourea), low efficiency (zinc, D-fructose, ascorbic acid) and high cost (CuCl, TiCl₃, Na₂S₂O₄ and SnCl₂) limit the applicability of these reagents. The better candidate for this purpose as a strong reducing agent is sodium sulphite (Na₂SO₃), because of its fast reaction rate, low cost, and relatively low toxicity.

Solution-based chemical methods provide an excellent way to produce nano selenium and nano selenides. Each synthesis method has its own advantages in obtaining the appropriate structures and properties for each system, and hence the right combinations of processing along with suitable systems must be found to achieve the desired goal.

This work is aimed at studying the change in the properties of PA6 upon the stepwise introduction of amorphous Se, Cd²⁺ and Ag⁺ ions and understanding the factors that determine the properties of these light-absorbing materials. These composites in the PA6 membrane were prepared using various chemical synthesis methods: Se—by chemical bath deposition (CBD), Cd²⁺ and Ag⁺ selenide—by successive ionic layer adsorption and reaction (SILAR), and mixed Ag⁺-Cd²⁺ selenide composites were prepared using cation exchange (CE). For a deeper understanding of the relationship between the composition and consumer properties of polyamide-6-based selenium nanocomposites, the main attention was paid to the description of the microstructure, morphology, and physicochemical properties.

2. Materials and Methods

Semicrystalline flexible TECAMID^®6 (polyamide 6, PA 6) produced by Ensinger GmbH (Germany) was used as a thermoplastic matrix. Nominal characteristics provided by the manufacturer are: PA 6 film was opaque, 0.5 mm thick, density 1.13 g/cm³, moisture absorption 3 %, water absorption to equilibrium 9.5 %. Experiments were performed on 2′6 cm² cuts in a film. Prior to the experiments, PA 6 film cuts were washed with NaHCO₃ and heated in distilled water for 2 h at 100^oC to remove surface contamination. The criterion for the quality treatment of the PA 6 substrate surface was its uniform wetting with distilled water. After treatment, the substrates were stored in a desiccator.

Selenious acid (H₂SeO₃, 99.0%), sodium sulphite heptahydrate (Na₂SO₃∙7H₂O, 99.0%), sulphur acid (H₂SO₄, 96%), cadmium nitrate tetrahydrate (Cd(NO₃)₂∙4H₂O, 99.997% trace metals basis), silver nitrate (AgNO₃, 99.0%) were purchased from Aldrich Chemical Co and used as received. All solutions were prepared using distilled water.

The first step involved the synthesis of Se via the CBD process. For Se deposition/insertion, the cleaned PA 6 films cuts were vertically immersed into a beaker with 0.1 M H₂SeO₃ and 0.15 M Na₂SO₃ solutions and exposed in a thermostatic vessel for 24 h at 20±1 ^oC. The concentrations of the H₂SeO₃ and Na₂SO₃ solutions (pH 2 with the addition of H₂SO₄), the deposition temperature and exposure time were chosen to give excellent composites in terms of continuity, smoothness and adhesion to the substrate. The solution was clear at first, became slightly turbid after two hours and turned into a light red liquid after a few hours. The Se precipitate was very fine, but it aggregated into large clusters within ∼24 h. This proposed mechanism for the formation of Se films is based on nucleation and the Ostwald ripening process [35]. This process is simple, controllable and reproducible. After deposition/insertion of the Se, the upper layer (Figure 1) was removed from the PA6 surface with filter paper, samples were thoroughly washed with distilled water, and stored in a desiccator between the individual processing steps. Then obtained composites (in the text referred to as a-Se/PA 6) were used as the initial composite for the stepwise insertion of Cd²⁺ and Ag⁺ ions. For this, half of the a-Se/PA6 composites were treated for 3 h with a 0.1 M solution of Cd(NO₃)₂ (pH 5.4) at a temperature of 80±1 ^oC. The second half of a-Se/PA6 composites were treated for 2 h with a 0.1 M solution of AgNO₃ (pH 6.35) at a temperature of 20±1 ^oC. The obtained composites were labelled as Cd-Se/PA 6 and Ag-Se/PA 6, respectively. Then one part of the Cd-Se/PA 6 composites were treated for 10 min with a 0.05 M AgNO₃ solution (pH 6.45, temperature of 20±1 ^oC) and washed well as described above. After immersion in the AgNO₃ solution, the resulting composites (throughout the text labelled as Ag-Cd-Se/PA 6) were black uniform and highly reflective with good adhesion. In all cases samples were treated in a thermostated vessel. During the deposition process, the beaker was kept undisturbed. All deposited films were evaluated for adhesion by subjecting them to a steady stream of distilled water. After each deposition step, the samples were washed first with hot and then cold distilled water, dried at ambient temperature, and stored in a desiccator until analysis. Figure 1 shows schematic outline of the approach and optical micrographs image of PA 6 membrane and obtained composites.

The porosity of the PA 6 film samples was measured by the BET (abbreviated from Brunauer-Emmett-Teller theory) method using nitrogen adsorption porosimeter Nova 2200 E-Series (Quantachrome Instruments, Boynton Beach, FL, USA). Before analysis, the film samples were degassed under vacuum at 100 °C.

Solution pH was measured using a pH-meter WTW330 (WTW GmbH, Weilheim, Germany). Optical microscopy images were obtained using an Olympus CX31 optical microscope and a C-5050 camera (Japan).

The structure of the composites was analysed using X-ray diffraction method (XRD) on a Bruker Advance D8 diffractometer. The XRD patterns were produced with CuKα (λ = 0.154178 nm) radiation in the 2θ scanning mode. The XRD data were analysed by observing characteristic peaks in the X-ray diffraction patterns using Search Match software and PDF-2 and PDF-4 database. Crystallite size (D_c) values were calculated from XRD patterns based on Bragg and Scherrer equation [36]:

$$D_C=\frac{K\lambda }{\beta (2\theta )\mathit{cos}\theta }1,$$

(1)

where K—constant with a normal value of 0.89, λ –wavelength of CuKα radiation, β(2θ) represents the full width at the half-maximum, and θ—Bragg angle. The dislocation density (δ) was evaluated by using the equation [37]:

$$\delta =\frac{1}{D^2}(\mathrm{line}·{\mathrm{m}}^{-2}),$$

(2)

The strain values (ε) were calculated from the following relation:

$$\epsilon =\frac{\beta \mathit{cos}\theta }{4}({\mathrm{line}}^{-2}·{\mathrm{m}}^{-4}),$$

(3)

Surface topography imaging was conducted using atomic force microscopy (AFM) using a NanoWizard^®3 NanoScience microscope, data were analysed using JPKSPM Data Processing software. The AFM images (scanning area 5×5 μm²) were collected using an AppNano production pyramidal-shaped i-type silicon cantilever (0.01–0.025 ohm/cm, spring constant of 2 N/m) working in contact mode.

Ultraviolet-visible (UV-Vis) absorption spectra (200 to 1100 nm) were recorded on a Spectronic Genesys 8 UV-Vis spectrophotometer with compensation for PA6 absorption. The energy optical band gaps (𝐸_𝑔) were determined by applying the absorption spectrum fitting method [38]. The fundamental absorption edge (λ_g) can be found from the plot:

$${\left(\frac{A}{\lambda }\right)}^{1/m}=f\frac{1}{\lambda },$$

(4)

where A—absorption, λ—wavelength, m—index which takes values 0.5, 1.5, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. Extrapolating of straight-line portion to cut the energy axis gives λ_g. Then employing the obtained value of λ_g the 𝐸_𝑔 can be directly calculated from the following relation [38]:

$$E_g=\frac{1239.83}{{\lambda }_g},$$

(5)

The Urbach energy (E_U) is related to the absorption coefficient by the following exponential equation [39]:

$${\alpha ={\alpha }_0·e}^{h\nu /E_U},$$

(6)

E_U was calculated using the slope determined from the linear part of the plot between ln(A) versus 1/λ according to the following relation:

$$E_U·tg\alpha =1239.83,$$

(7)

The steepness parameter $\sigma$ is related with E_U the by following equation:

$$\sigma =\frac{k·T}{E_U},$$

(8)

where k is the Boltzmann constant, 8.617343·10^‒5 eV/K and T the absolute temperature assumed to be 298 K. The following equation is applied for calculating the interaction strength of electron–phonon (E_e-p) [40]:

𝐸𝑒−𝑝=23𝜎,

(9)

The optical electronegativity χ is related with E_g by following equation:

$${\chi =2.688}^{}·E_g,$$

(10)

The changes in the chemical structure and the binding configuration of the samples were analysed using attenuated total reflectance (ATR) Fourier transform infra-red spectroscopy (FTIR). The ATR-FTIR spectra were recorded on a Perkin Elmer FT-IR Spectrum GX system (Perkin Elmer, Waltham, MA, USA) in the wavenumber range 4000–600 cm⁻¹. Spectra were recorded by averaging 64 scans with a wavenumber resolution of 1 cm⁻¹ at room temperature

3. Results and Discussions

Adsorption membranes are one of the best ways to obtain semiconductor composites from aqueous solutions. The established pore size less than 2 nm indicates that the PA 6 film is microporous. Therefore, it can adsorb Se micro particles and Cd²⁺ and Ag⁺ cations followed by their diffusion into the polymer membrane. In most cases, adsorption membranes are charged. There is an attraction of charges between the ions and the surface of the adsorption membrane.

3.1. XRD Analysis

To determine the structural properties of the PA 6 and the obtained composites, XRD patterns were analysed. The XRD pattern of PA 6 sample heated in distilled water is shown in Figure 2and 4, while XDR pattern of the obtained composites is shown in Figure 4.

PA 6 crystallizes into stable α- and γ-, as well as unstable β-forms [41]. XRD studies of the PA 6 sample revealed the presence of a main peaks at 2θ 9.52°, 20.26°, 23.56° and 28.64°. The small diffraction peak at 2θ = 9.52°, which was attributed to the (020) reflection of the γ crystalline form [40]. The other two peaks at 2θ = 20.26° and 23.56° were assigned to the (200) reflection and (002)/(202) reflections of the α crystalline form of PA 6 [41]. The peak at 28.64° appears after polymer treatment in hot water and cannot be assigned for neither forms of the crystalline PA 6. When the polymer is exposed directly to water, water initiates degradation/hydrolysis reactions in the polymer.

Mixing H₂SeO₃ and Na₂SO₃ solutions leads to the release of Se according to the following reaction [42]:

Na₂SeO_3(aq) + 2Na₂SO_3(aq) + H₂SO_4(aq) → Se_(s) + 3Na₂SO_4(aq) + H₂O_(l)

(11)

A red film was observed on PA 6 (Figure 3). The precipitate, characterised by XRD analysis (Figure 3 and Figure 4), was identified as red amorphous selenium (a-Se).

Because intensities of the diffraction peaks of PA 6 are very high and overlap with main peaks of the materials inserted/deposited on the polymer, the XRD patterns of the obtained composites were given in the 2θ assortment of 30–65°. As can be seen from Figure 4, in the a- Se/PA6 composite, Se formed at the end of reaction (9) is in the amorphous form, i.e., not detected by XRD analysis.

The obtained positions of the diffraction peaks at 2θ 42.74°, 43.46°, 48.68°, 51.09° in the Cd-Se/PA6 composite (Figure 4) were compared with standard values and mainly indexed as CdSe (PDF 77-2307). The peaks at 2θ 32.66°, 33.49°, 37.64°, 38.46°, 45.16°, 52.66°, 53.77°, 54.83° and 59.73° are indexed as Se₈ (PDF no. 71-528) with hexagonal and monoclinic unit cells, respectively (Figure 4, Cd-Se/PA6 composite). It was found that CdSe has a polycrystalline nature with predominant (110) orientation plane. These experimental results agree well with the literature data. It is well known [43] that in the case of hexagonal lattices the strongest chemical bonds are formed by atoms located in a semi crystalline configuration on crystallographic planes (110) or (101).

Based on the analysis of the XRD results, it can be concluded that the immersion of the a-Se/PA6 nanocomposite in a Cd(NO₃)₂ solution at a temperature of 80±1 ^oC contributes to the gradual crystallisation of red a-Se into grey cr-Se. It is very reactive and reacts with many metal cations [44,45]. The solubility of a-Se (2.2∙10^‒8 mol/dm³ [46]) was approximately an order of magnitude higher than the solubility of cr-Se (1.5∙10^‒9 mol/dm³ [46]). The formation of CdSe can be explained by the following reaction [47,48,49]:

2Se_(s) + 3Cd²⁺_(aq) + 3H₂O_(l) → 2CdSe_(s) + CdSeO_3(s) + 6H⁺_(aq),

(12)

The formation of the Cd-Se/PA 6 composite leads to a colour change of from red to brown (Figure 1).

The Ag-Se-6/PA 6 nanocomposite shows diffraction peaks corresponding to the orthorhombic phase of Ag₂Se naumannite (PDF No. 01-071-2410). As can be seen from Figure 4, the dominant peak (121) of the orthorhombic system represents the preferred orientation along this plane. The identified peak positions are in good agreement with those indicated in the literature for Ag₂Se nanowires [50] and nanoparticles [51]. The diffraction pattern of the Ag-Se/PA 6 nanocomposite shows two sharp lines of approximately the same intensity along the (112) and (121) planes respectively. The reaction involved in this process can be summarised as the following [47,48,49]:

3Se_(s) + 6Ag⁺_(aq) + 3H₂O_(l) → 2Ag₂Se_(s) + Ag₂SeO_3(s) + 6H⁺_(aq),

(13)

The formation of Ag₂Se leads to a colour change from red to black. Together with the Ag₂Se phase, a minor amount of monoclinic Se₈ (PDF No. 71-528) at 2θ 32.66° Se₈ may remain unreacted in the deposited film phase. Ag-Se/PA6 composite (Figure 4) XRD analysis showed that not only Ag₂Se but also the metallic Ag phase (PDF No. 04-003-1472) was identified in the diffractogram. Typically, the metallic structure of Ag is depicted by a sharp XRD peak at 2θ 38.12° corresponding to the preferred (111) texture. Metallic Ag is the most likely impurity in chemically deposited Ag₂Se layers [50] as shown in the reaction as follows:

Se_(s) + 4AgNO_3(aq) + 3H₂O_(l) → 4Ag_(s) + H₂SeO_3(aq) + 4HNO_3(aq).,

(14)

As discussed in [52], excess Ag can be introduced in several ways: in the form of adsorbed metal chains, in the form of point defects, or in the form of three-dimensional nano- or micro-inhomogeneities.

The XRD pattern of the Ag-Cd-Se/PA 6 composite shows the multiphase crystalline composition of the film. Compared to the XRD pattern of the Cd-Se/PA 6 composite, Se₈ and CdSe phases remain. The XRD peaks corresponding to CdSe are not shifted, which indicates the same structure, regardless of the cation exchange with Ag⁺ ions and the appearance of new phases associated with orthorhombic Ag₂Se and metallic Ag. On Figure 4 also shows that the intensity of the peaks of the Ag-Cd-Se/PA 6 composite increases with the appearance of new phase, which additionally shows an increase in crystallinity. Broadening of the diffraction peaks is associated primarily with the finite sizes of the crystallites. The formation of Ag₂Se in Cd-Se/PA6 composite leads to a colour change from brown to black.

Since the other phases were not found in Cd-Se/PA6, Ag-Se/PA6 and Ag-Cd-Se/PA6 nanocomposites, it could be assumed that the SeO₃^2–, SO₃^2– and SO₄^2– ions diffused from the a-Se/PA 6, reacted with the Cd²⁺ and Ag⁺ ions to form these compounds in solution near the sample area. In another case, by-products of Equations (10) and (11) due to the sufficiently high solubility (Table 2, CdSeO₃ (K_SP = 5.0·10^‒9 (mol/dm³)²) and Ag₂SeO₃ (K_SP = 1.35·10^‒16 (mol/dm³)³) were removed from the surface of the samples by rinsing the as-synthesized composites with excess hot water.

The structural parameters of PA 6 and the prepared composites calculated from XRD data are listed in Table 1.

A theoretical framework provides a general background to support our investigation. The sequence of phase formation in multicomponent systems depends on the structure of the starting material (SM). It is common knowledge that the exchange of cations can be achieved through simple mutual diffusion with other cations [53]. In addition, considering phase formation, the driving force of the solid-state interaction is also determined by the change in the Gibbs free energy, since the system always tends to the lowest possible free energy state [54]. The feasibilities of possible CE reactions were studied by thermodynamic deduction based on the thermodynamic law. CE reactions based on major difference in solubility product (K_SP) between the starting material (SM) and the final product (FP). Gibbs free energy (ΔG) for insoluble materials expressed according to Equation (15):

$$∆G=RT\times ln\frac{J_{CP}}{K_{SP}},$$

(15)

where R—universal gas constant (8.319 J/mol·K), T—temperature (K), K_SP—the solubility product of the SM (in our case CdSe), I_CP—the product of the concentration of FP ions (in our case Ag₂Se). I_CP can be calculated according to Equation 16:

$$I_{CP}={C_{{Ag}^{+}}^2\times C}_{{Se}^{2–}},$$

(16)

where $C_{{Ag}^{+}}$ and $C_{{Se}^{2–}}$—the concentration of ions (mol/dm³). In this way, the reaction of transition from a material with higher K_SP (CdSe) to a lower one (Ag₂Se) can proceed spontaneously. However, it is not easy to judge whether the CE reaction can proceed or not since this process is very complex and is associated with many factors: barrier to the activation energy of the phase transition, change in surface free energy, temperature, the active concentrations of ions in the solution, etc. [55]. Therefore, a more correct criterion should be the difference in solubility (S) of the SM and the FP (see Table 2).

The stoichiometric reactions at an interface between the Cd-Se/PA 6 and Ag⁺ salt aqueous solution can be considered as follows:

$${\mathrm{C}\mathrm{d}\mathrm{S}\mathrm{e}}_{\left(\mathrm{s}\right)}+{{2\mathrm{A}\mathrm{g}}_{\left(\mathrm{a}\mathrm{q}\right)}^{+}}_{}\to {\mathrm{A}\mathrm{g}}_2{\mathrm{S}\mathrm{e}}_{\left(\mathrm{s}\right)}+{\mathrm{C}\mathrm{d}}_{\left(\mathrm{a}\mathrm{q}\right)}^{2+},∆G_{reac,298}^{°}=-144.60$$

(17)

The negative values of ΔG°_reac,298 indicate that thermodynamic conditions are provided for the proceeding of this reaction. However, it should be emphasised that in the case of solid-phase interaction in multicomponent systems, phase formation at the precursor/solution interface is a dynamic non-equilibrium process that requires careful consideration of the mechanistic pathways along which they proceed [57]. Apparently, the determining criterion for the possibility of a thermodynamic reaction in hybrid materials is the solubility of CdSe in the starting Cd-Se/PA6 material. The higher solubility of CdSe (Table 2) favours the transformation into Ag₂Se. When Cd-Se/PA6 is immersed in AgNO₃ solution, Ag⁺ reacts with Se^2‒ ions formed from the ionisation of dissolved CdSe leading to the formation of Ag₂Se particles, which results in the reduction of Cd²⁺ ions in the solution. Before explaining the formation of the Ag₂Se phase, it is necessary to discuss the active species that might be involved in the reaction path. Based on the discussion presented above, we speculated that the formation of Ag₂Se phases in the Cd-Se/PA6 composite could be explained through complex mechanism reactions: Ag⁺ ions were attracted to the surfaces of metal chalcogenides particles due to the large specific area, then adsorbed Ag⁺ replaced Cd²⁺ ions from the CdSe lattice and formed Ag₂Se. In parallel, some Ag⁺ ions featuring relatively high ion diffusivity [58,59] propagate into the interior of the polymer and react with the cr-Se to form Ag₂Se according to reaction (13). The obtained composites (denoted in the text as a-Se/PA6, Cd-Se/PA6, Ag-Se/PA6 and Ag- Cd-Se/PA 6) were homogeneous with good adhesion.

3.2. AFM Analysis

The morphological changes of the PA 6 surface before and after subsequent chemical incorporation of a-Se, and Cd²⁺ and Ag⁺ cations were characterized by AFM.

One can see that the surface morphology of all nanocomposites varies depending on the incorporated element. AFM was used to elucidate the formation mechanism and homogeneity of obtained composites. On a selected small scan area of 5 µm × 5 µm, 2D height data (Figure 5A) and 3D topographic (Figure 5B) AFM images of the respective PA 6 membrane, a-Se/PA 6, Cd-Se/PA 6, Ag-Se/PA 6 and Ag-Cd-Se/PA 6 nanocomposites with the corresponding measured topographic parameters are shown in Figure 5. The morphological pattern of PA 6 membrane revealed a wavy surface. a- Se/PA 6, Cd-Se/PA 6, Ag-Se/PA 6 and Ag-Cd-Se/PA 6 composites had a rough surface. The 3D topographic images (Figure 5B) show an irregular pattern resembling a valley of nodules. Since the composites did not have a pronounced texture but were characterized by a complex relief morphology, the roughness parameters were calculated from profilograms obtained for different surface areas (Figure 5C). The length of the scan areas was approximately 4.5–6.5 μm. Surface roughness is an important indicator of the quantitative characteristics of the sample surface. The average height, Z_mean (156.9 nm), and the average surface roughness, R_a (27.61 nm), of the a-Se/PA 6 composite are clearly different from those of the unreacted PA 6 membrane (Z_mean 60.4 nm, R_a 10.78 nm) (Figure 5A,B). As shown in Figure 5 local areas of the surface of the PA 6 membrane and the a-Se/PA 6 composite have different morphological structures. Simultaneously, isolated solid particles of amorphous Se (Figure 5) functioned as seeds to start the formation of CdSe and Ag₂Se. When a-Se/PA 6 composite was exposed to an aqueous Cd(NO₃)₂ and AgNO₃ solutions, a-Se reacts with Cd²⁺ and Ag⁺ cations to form insoluble CdSe and Ag₂Se nanoparticles that deposit in situ on the a-Se/PA 6 and produce the Cd-Se/PA 6 and Ag-Se/PA 6 composites (according to Equations (12) and (13)). The average height Z_mean (53.57 nm) and the average surface roughness R_a (9.82 nm) of the Cd-Se/PA 6 composite significantly decreased compared to the a-Se/PA 6 composite. It implies that CdSe formation proceeded mainly through deposited a-Se layers within the PA 6 surface. The average height Z_mean (35.10 nm) and the average surface roughness R_a (8.25 nm) of the Ag-Se/PA 6 composite significantly decreased compared to the a-Se/PA 6 composite. After exposure of the Cd-Se/PA 6 composite in an aqueous solution of AgNO₃, submicron pyramidal particles, presumably belonging to Ag₂Se (according to Equation 13), are found on the surface of the resulting Ag-Cd-Se/PA 6 composite (Figure 5). Pyramid-like structures with a lateral size of about 150 nm are also visible. Some of these particles aggregate on the composite surface. Small dark spots at the boundaries of grains and agglomerates may be due to depressions in the layers. From the roughness values presented in Figure 5, we can conclude that the quantitative parameters of the surface of the Ag-Se/PA 6 and Ag-Cd-Se/PA 6 composited decreased compared to the Cd-Se/PA 6 composite. For Ag-Cd-Se/PA 6 composite R_a decreased from 9.82 nm to 8.36 nm, and R_q decreased from 12.17 nm to 11.55 nm. In addition, the peak-to-valley roughness R_t of the surface of the Cd-Se/PA 6 and Ag-Cd-Se/PA 6 composites remain virtually unchanged. That confirmed our assumption about the formation of compact films. RMS is defined as the standard deviation of the surface height profile from the average height. The profilograms indicate a slightly asymmetric surface with more peaks than valleys (Figure 5C). It is important to note that the RMS of the deposited films decreased during the formation of CdSe, Ag₂Se and mixed CdSe-Ag₂Se nanoparticles compared to the a-Se films deposited on the PA 6 surface.

3.3. ATR-FTIR Analysis

The FTIR study allows to discover the conformational structural changes of polymer chain as well as and the complexation and interaction between the polymeric matrices and inorganic nanoparticles. The ATR-FTIR spectra of the PA 6 and obtained nanocomposites within a wave number range from 650 to 1650 cm⁻¹ are displayed in Figure 6. PA 6 consists of (CH₂)₅ segments separated by a parallel or antiparallel arrangement of secondary amide groups [60]. The bands related to NH–CO fragments are mainly affected by the mutual arrangement of H–bonds, while the vibrations from CH₂ groups reflect the effect of the chain conformation [61]. In PA 6 spectrum the absorption bands at 834 cm^‒1, 929 cm^‒1, 960 cm^{‒ 1}, 1031 cm^{‒ 1} and 1200 cm^{‒ 1} (Table 3), confirm predominantly the α-crystalline phase. The spectral features at 1476 cm⁻¹ and 1417 cm^‒1 represent CH₂ scissoring vibrations next to −NH and >C=O groups, respectively. The band at 1463 cm⁻¹ corresponds to CH₂ scissoring vibrations. Two typical absorption bands at 1633 and 1535 cm^‒1 represents the amide I and II group, respectively.

The FTIR vibration bands of Se₈ rings are reported at 487.9 cm⁻¹ and 737.7 cm⁻¹ [62]. These peaks are absent in the FTIR spectrum of a-Se/PA 6, which confirms the XRD data that the inserted selenium is amorphous. Incorporation of solid inorganic nanoparticles into flexible polyamide network causes chain stretching. This should cause a conformational change in the chains of the polymer matrix [61,63]. It can be seen that most of the characteristic vibration bands of PA 6 in the wavenumber region from 650 to 1650 cm⁻¹ are retained in the FTIR spectrum of a-Se/PA 6; however the intensity of the peaks changes: some of them become more intense and some of them expand. It can be concluded that as a-Se nanoparticles are introduced between the polymer chains and stretched, the position and intensity of the peaks change. Thus, the peak of neat PA 6 at 1124 cm⁻¹ in a-Se/PA 6 spectrum shifts to 1112 cm⁻¹, and two new small peaks are recorded at 974 cm⁻¹ and 1390 cm⁻¹. The shoulder at 974 cm⁻¹ shows the appearance of a small amount of the γ crystalline form of PA 6, and C–C symmetry vibration causes IR absorption at the 1390 cm^–1 [64].

CdSe and Ag₂Se have no specific vibrations in the spectral region between 650 and 1650 cm^{– 1}, but exhibit absorption in the region of 200–400 cm^–1 [65,66,67]. Thus, it is possible to avoid the problem of overlapping of metal selenides and PA 6 characteristic peaks. The IR spectra of the Cd-Se/PA 6, Ag-Se/PA 6 and Ag-Cd-Se/PA 6 composites are very similar to the a-Se/PA 6 spectrum, but show an even greater shift of the peak at 1112 cm⁻¹ to lower frequencies (Table 3) and the appearance of another new peak at 877 cm⁻¹. By analogy with data reported for CdSe [61], those weak vibrations can be attributed to the Cd–Se or Ag–Se bond. Another dimension of the functionality and complexity of inorganic-organic materials is introduced if the selenides are combined with each other. For example, the negative energies of chemical band formation of CdSe and Ag₂Se promote the creation of Ag–Cd–Se– interfacial bonds, which leads to non-selective nucleation in Ag₂Se/CdSe hetero-nanostructures [68]. Thus, the peak at 1108 cm⁻¹ in the IR spectrum of the Ag-Se/PA 6 composite shifts to 1094 cm⁻¹ in the IR spectrum of the Ag-Cd-Se/PA 6 composite.

The shift and broadening of the C–C stretching vibration peaks, accompanied by the appearance of new small peaks of low intensity, allows us to conclude that we are observing the formation of intermediate structures inside the polyamide chains. Thus, we can be conclude, that the spectral changes after insertion of a-Se and ions of Cd²⁺ and Ag⁺ ions with respect to PA 6 sample belong to the bands related to the C–C conformation of the main chain. The peak at 1124 cm⁻¹, arising from the all-trans C–C conformation of the main chain, loses its spectral position. In addition, the peak is broadened, and its intensity noticeably increases in all spectra of the obtained composites (Figure 6).

3.4. UV-Vis Analysis

The experimental absorption spectra are presented in Figure 7.

The absorption spectrum of PA6, (Figure 7.) displays absorption peak at 225 nm. The color of the obtained a-Se/PA6 nanocomposite becomes red, suggesting the change of optical properties. In the UV-Vis absorption spectrum of the a-Se/PA6 nanocomposite, a number of peaks appear at 320 nm, 370 nm, and 475 nm, which characterize the formation of a band structure: red a-Se nanoparticles are introduced into the system of PA 6 micropores at the first stage, and then they combine into aggregates and agglomerates. According to the literature data [69,70] bulk a-Se absorbs 475 nm light.

In the UV-Vis spectrum of the Cd-Se/PA6 nanocomposite, peaks appear at the same 320 nm and 370 nm, which are related to Se nanoparticles and agglomerates, and a new peak at 535 nm. According to the literature data, CdSe quantum dots have a wide absorption band in the spectral range of 410–545 nm [71], and the absorption maximum of CdSe nanocrystals, depending on the particle size, is in the range from 492 to 578 nm [72]. Therefore, the peak at 535 nm can be attributed to CdSe.

In Ag-Se/PA6 UV-Vis spectrum the peaks appear at 320 nm, 365 nm and 475 nm. Ag₂Se nanoparticles absorb in a wide spectral region from 300 to 600 nm [73], while two sharp peaks are observed in the spectrum of quantum dots at 440 and 607 nm, respectively [74]. Monodisperse silver nanoparticles, depending on the size, shape, and distribution in nanostructures, have two absorption bands; one broad peak in the range 420-430 nm and a shoulder at 580-590 nm [75]. Therefore, a broad peak from 380 to 590 nm with a maximum at 475 nm can be attributed to the Ag₂Se phase with a small amount of Ag nanoparticles.

In Ag-Cd-Se/PA6 nanocomposite UV-Vis spectrum the peaks appear at 315 nm and 555 nm. According to the literature data [76,77,78], the introduction of Ag⁺ ions into CdSe nanocrystals via the CE reaction does not lead to noticeable changes in the absorption spectra of CdSe in the UV-visible region. In contrast, other authors suggested that the formation of the CdSe-Ag₂Se core-shell leads to the complete disappearance of the CdSe absorption peak in the UV-Vis spectra [79]. Thus, it can be stated that the absorption spectrum of the Ag-Cd-Se/PA6 composite is a superposition of the individual absorption spectra of the CdSe, Ag₂Se, and Ag phases, its absorption edge is shifted to the longer wavelength region, and their absorption increases in the range of 315–555 nm compared to with the region (320–535 nm) of Cd-Se/PA6 and Ag-Se/PA6 composites. This red shift of the λ_g may be related to the common complex alignment of the valence and conduction bands, as well as to the presence of multiple defects and vacancies. The determination of the phase composition of the obtained composites according to the data of optical studies is in good agreement with the XRD data.

The experimental UV-Vis spectra better correspond to the rectilinear section on the graphs 𝐴²·𝜆^–2 = 𝑓(𝜆^–1) (Figure 8), which confirms the direct transitions of the obtained nanocomposites.

All the calculated optical parameters are given in Table 4 for comparison.

The decrease in E_g of each subsequent nanocomposite compared to PA 6 is due to a change in the they optical properties. E_g of nanocomposites can be affected many factors, such as composition, particle size and distribution, shape, agglomeration/aggregation state, solubility, surface morphology/topography, structure crystallinity, and many different structural defects or deformations [80,81]. The wavelength at which the signal gets saturated is interpreted as being the value of the absorption edge or the E_g of the semiconductor. For a-Se/PA6 composite E_g is 2.23± 0.02 eV. E_g measured for bulk a-Se is 1.99 ± 0.02 eV [82]. Introduction of Cd²⁺ ions proseed at 80 ^oC, and at heat treatment amorphous Se crystallizes. E_g measured for cr-Se is 1.85 ± 0.02 eV [82]. An increase in the crystallinity of the obtained composites (Figure 4) and the carrier concentration is accompanied by a decrease in E_g from 2.05±0.02 eV for the Cd-Se/PA 6 composite to 1.64±0.02 eV for the Ag-Cd-Se/PA 6 composite (Table 4). The obtained values are in good agreement with the literature data: CdSe and Ag₂Se have a band gap of 1.9 eV [20] and 1.8 eV [21], respectively.

One of the sources of unfavorable recombination is the subband absorption tail, which is characterized by the Urbach energy. Absorption tails are defined as densities of states that extend from the bands into the band gap of the photoactive layer. Tail states inside the band gap act as traps and recombination centers and trap a charge carrier. The E_U is influenced by thermal and structural disturbances in semiconductor materials [83,84,85]. In this work, we analyze structure-dependent E_U. The Urbach energy plots were constructed and shown in Figure 9, and the calculated values are given in Table 4. E_U reflects the density of states in the band tails and, hence, the local microstructural disorder [86]. As can be seen from the values presented in Table 4, the E_U of PA 6 is 1.31 eV, which is higher as compared with obtained nanocomposites. The relatively high value of E_U indicates a greater propensity for the transformation of weak structural bonds into defects [87]. The a-Se/PA 6 nanocomposite (amorphous phase) is more structurally disordered. With the stepwise incorporation of Cd²⁺ and Ag⁺ ions, E_U decreases. This may be due to an increase in the structural ordering of nanocomposites, which is confirmed by an increase in crystallinity detected by XRD analysis. In addition, it was previously reported [88] that thicker selenide films have less structural randomness, since with an increase in the thickness and density of the layers, structural defects are minimized, which minimizes the E_U. The formation of compact these films was confirmed by AFM analysis.

The σ characterizes the broadening of the optical absorption edge arising from the electron-phonon interactions [40,89]. The steepness parameter of PA 6 is lower than that of the obtained nanocomposites at the same temperature (Table 4). The lower differences in electronegativity decrease the orbital overlap, and decrease the band gap.

4. Conclusions

To sum up, the amorphous Se films conjugated with PA 6 were successfully obtained by exposing the PA 6 membrane to an acidified aqueous solution of H₂SeO₃ and Na₂SO₃. The surface morphology of the a-Se/PA6 consists of granular amorphous Se nanoparticles. This texture ensures optimal deposition of the next phase. The use of prepared a-Se/PA 6 composites opens the way for the stepwise incorporation of Cd²⁺ and Ag⁺ ions by a chemical methods. The positions of the diffraction peaks in the Cd-Se/PA 6 composite were mainly indexed as hexagonal CdSe and monoclinic Se₈. XRD clearly shows that the Cd-Se/PA 6 and Ag-Se/PA 6 films are polycrystalline and exhibit a phase transformation from a-Se to a Se₈ monoclinic structure. The XRD pattern of the Ag-Cd-Se/PA 6 composite obtained by the CE method showed a complex crystalline composition of the film with peaks of monoclinic Se₈, hexagonal CdSe, orthorhombic Ag₂Se and metallic Ag. The obtained composites have an inhomogeneous surface morphology in the form of nanosized particles. ATR-FTIR spectroscopy revealed the interaction between PA 6 and inorganic components, which causes obvious conformational changes in PA 6 chains. The resulting composites differ in color, which affects and their optical properties. The optical spectra show that the nanostructured films have a high absorption coefficient in the visible region. The studied composites are direct band gap semiconductors, which exhibit a redshift of the band gap from 2.23 eV for the a-Se/PA 6 to 1.64 eV for the Ag-Cd-Se/PA 6 composite due to a change in chemical composition and can be promising in science for implantable microdevices, as well as for solar electronics. E_U and σ studies describe the broadening of solar absorption edges around optical band gaps, which correlates with the polycrystalline composition and heterogeneous surface morphology.