Evaluation of the Applicability of Modifying CdSe Thin Films by the Addition of Cobalt and Nickel to Enhance the Efficiency of Photocatalytic Decomposition of Organic Dyes

Abstract

This article presents the results of an assessment of the use of CdSe, NiCdSe, and CoCdSe thin films as a basis for photocatalysts used for the decomposition of the organic dyes rhodamine B, cargo red, and indigo carmine. Interest in this area was determined by the need to solve a number of issues related to increasing the efficiency purifying aqueous media from the negative effects of organic dyes, which cannot be disposed of using traditional methods associated with adsorption or filtration. The use of the electrochemical synthesis method to obtain thin films of a given thickness showed that the addition of nickel or cobalt sulfates to the standard electrolyte solution used to obtain CdSe films results in the formation of CdSe films with a higher degree of structural ordering (the crystallinity degree was more than 50%), as well as a decline in the band gap. When analyzing data on the photocatalytic decomposition of organic dyes, it was found that a change in the structure of the films due to the introduction of nickel and cobalt leads, in the case of the decomposition of the rhodamine B dye, to a more efficient decomposition, and in the case of the cargo red and indigo carmine dyes, not only to their complete decomposition and mineralization, but also to a reduction in the time of photocatalytic reactions (decomposition growth rate). Moreover, an analysis of cyclic tests demonstrated that NiCdSe and CoCdSe films maintain 90% of their photocatalytic decomposition efficiency compared to that achieved during the first decomposition cycle, while CdSe degrades after three consecutive cycles and its efficiency reduces by more than 2.5–3 times.

1. Introduction

Exploring novel catalysts, including photocatalysts, has emerged as a promising field of research in addressing environmental challenges related to water source contamination by organic dyes and heavy metals. The significance of this research area primarily stems from the imperative to tackle issues associated with the disposal of organic dyes, which cannot be treated effectively through conventional, traditional purification methods, necessitating innovative approaches for their decomposition into benign constituents [1,2,3,4,5].

As a rule, in most cases, the disposal of organic dyes such as rhodamine B, cargo red, and indigo carmine, which are the most common types of dyes used in the textile and printing industries, requires quite complex technological solutions, since these dyes have a very high threshold of resistance to decomposition by classical methods and are also difficult to filter and capture from aqueous media [6,7,8]. Moreover, their entry into water sources and subsequent accumulation in them can lead to mutations in living organisms living in these environments [9,10].

In turn, the inability to recycle and neutralize these dyes using classical methods opens up the possibility of searching for alternative methods for solving these issues, primarily related to the possibility of using cheap recycling methods using natural energy sources, for example solar energy [11,12,13].

One of the most promising approaches for addressing the challenges of recycling organic dyes through photocatalysis methods involves the use of thin films based on cadmium–selenium chalcogenide compounds. These films exhibit a range of unique characteristics that enhance photodecomposition, owing to the synergistic combination of their optical and electronic properties, characterized by small band gap values [14,15,16]. Furthermore, the transition from microsized grains to nanostructured ones allows for their growth in specific surface areas, facilitating the accelerated photocatalytic decomposition of dyes upon their interaction with the film’s surface. Additionally, it is worth noting that film-based photocatalysts, unlike powdered catalysts, can be easily recovered from aqueous solutions without the loss of useful mass and catalyst volume [17,18]. Despite the significant potential of CdSe films as photocatalysts, several unresolved challenges persist. These issues pertain to their durability under prolonged usage and the enhancement of decomposition efficiency. In this regard, the optical properties of the films are of utmost importance, particularly band gap width, as its reduction can initiate photon release with lower energy input. To address these concerns, a series of studies [19,20,21,22] have suggested incorporating elements from the iron subgroup into the composition of CdSe films. This approach aims to alter the films’ properties and boost their photocatalytic activity.

The aim of this work is to determine the efficiency of ACdSe (A–Co, Ni)-type thin films obtained by electrochemical deposition when used as a basis for photocatalysts utilized for the recycling (decomposition and mineralization) of organic dyes.

2. Materials and Methods

Samples of thin films were obtained by electrochemical deposition by altering the electrolyte composition to obtain structures based on cadmium–selenium compounds with nickel or cobalt. To synthesize CdSe thin films, an electrolyte solution of 0.5 M CdSO₄·8H₂O and 5 mM SeO₂ was used. To obtain thin films of the ACdSe type (A–Co, Ni), 0.5 M CoSO₄·7H₂O or 0.5 M NiSO₄·7H₂O were added to the initial electrolyte solution. The synthesis procedure was conducted under a potential difference of 1.5 V within a three-electrode cell configuration. This cell included a copper cathode and anode composed of metal plates with a surface area of 4 cm². An AgCl electrode was applied as a reference electrode. The deposition process took place on polymer substrates constructed from PET with a thickness of 12 μm. These substrates were initially coated with a 30 nm thick gold layer using magnetron sputtering, serving as a conductive layer to activate grain nucleation processes for the subsequent formation of thin films on the dielectric substrate. The deposition duration was set at 10 min, resulting in films approximately 1 μm in thickness, as determined through the side-cleavage method.

An assessment of the morphological characteristics of the resultant films, considering their composition variations, was conducted through the atomic force microscopy technique in semi-contact mode. This approach enabled the examination of grain morphology, encompassing size, shape, and roughness evaluation. The atomic force microscope used for these investigations was the Smart SPM (AIST-NT, Zelenograd, Russia). A silicon needle with an approximate diameter of 20 nm served as the cantilever during the analysis.

Elemental distribution and quantification of their ratios in the resultant films were accomplished through energy-dispersive analysis. This technique was executed using a Hitachi TM3030 scanning electron microscope (Tokyo, Japan). The homogeneity of elemental distribution was evaluated using the mapping method, and the determination of elemental ratios was conducted by analyzing samples in 20 distinct regions, followed by the calculation of average values. This approach was used to exclude the occurrence of heterogeneities in the composition. Elemental analysis was assessed by changing the energy dispersive spectra in samples from different areas in order to determine the uniformity of the distribution of elements and the consistency of the uniformity of composition in different areas of the films. Measurements were performed at more than 15 points on the sample surface.

The assessment of the optical properties of the examined films, including UV-Vis transmission measurements and the investigation of band gap variations concerning film composition, was conducted utilizing a SPECORD 250 PLUS spectrometer (Analytik Jena, Jena, Germany). Measurements were executed within a wavelength range spanning from 250 to 800 nm, with a 1 nm increment.

The evaluation of the photocatalytic performance of the examined films involved conducting experiments focused on the photocatalytic decomposition of organic dyes, namely rhodamine B, cargo red, and indigo carmine. To induce photocatalytic decomposition processes and simulate sunlight, a xenon lamp rated at 500 W, emitting 2100 lm, was utilized. A light filter with a wavelength of 420 nm was employed. Dye decomposition efficiency assessments relied on optical techniques, encompassing measurements of the optical density of the solutions within a wavelength range spanning 400 to 700 nm. The temperature of the medium was 25 ± 2 °C; the temperature was maintained by placing the container with the medium in a water bath. When conducting experiments on photocatalytic decomposition, the pH level of the solution was controlled and maintained within a range of 5.2–5.5. Dye concentration in the model solutions was 5, 10, and 15 mg/L. The time for photocatalytic decomposition reactions was 300 min. Dye degradation efficiency was assessed by alterations in optical density C₀ (initial concentration) and C_i (after a certain time period). Formula (1) was used for the calculations:

$$Degradation\_efficiency=(1-\frac{C_i}{C_0})\times 100\%$$

(1)

Photodegradation efficiency was assessed based on the change in chemical oxygen demand (COD) values in a model environment before and after the photodegradation reaction. Formula (2) was used for the calculations.

$$\begin{array}{l}\\ \eta =\frac{(CO{D}_{initial}-CO{D}_{final})}{CO{D}_{initial}}\times 100\%,\end{array}$$

(2)

where COD_initial and COD_final are COD concentrations in the initial state and after the photocatalytic reaction.

Determination of the strength characteristics of the studied films before and after photocatalytic decomposition was carried out using the microindentation method, implemented using a Duroline M1 microhardness tester (Metkon, Bursa, Turkey). Indentation was performed using a Vickers diamond pyramid with a load of 10 N on the indenter, which made it possible to determine the hardness values of the films under study, as well as to establish the hardening effect for films obtained from electrolyte solutions with the addition of nickel and cobalt sulfates, associated with a change in the hardness of the films.

3. Results

Figure 1 shows the results of assessing changes in the morphological features of the studied thin films obtained by 3D construction of film surface reliefs when analyzed using atomic force microscopy. A general view of the presented data is as follows: changes in electrolyte composition lead to the formation of different morphologies of the resulting thin films, associated both with dimensional factors and the uniformity of layer-by-layer filling of layers during the electrochemical reduction of elements. In the case of CdSe, the morphology of the resulting films is characterized by a strong anisotropy of inhomogeneous layer-by-layer film formation, with the formation of larger agglomerates consisting of finely dispersed grains, the sizes of which vary from 70 to 80 nm. At the same time, analysis of the height profile of these inclusions indicates a heterogeneous formation of films from the electrolyte solution.

When nickel sulfate is added to the electrolyte composition, analysis of the morphological features of the resulting film surfaces showed the following: Unlike CdSe films, the addition of nickel leads to the formation of more uniform particles, on the order of 40–45 nm in size, while there is virtually no heterogeneity in layer-by-layer film formation, which leads to lower roughness values (about 10 nm). In the case of CdSe films, roughness is on the order of 35–40 nm, and in inhomogeneous areas, it could be on the order of 70–100 nm. Also, unlike CdSe films, the addition of nickel, according to morphological characteristics, results in the formation of more densely packed grains, which allows one to judge an increase in the strength characteristics of these films, as well as a growth in resistance to degradation and corrosion.

In the case of adding cobalt sulfate to the electrolyte composition, the morphological features of the resulting films are characteristic of the formation of structures of the “core–shell” type, where the core is a larger CdSe particle surrounded by a halo of compounds containing cobalt [23,24]. Figure 1d–f presents data on the distribution of grain sizes obtained by processing the morphological features of the resulting films depending on the composition of electrolytes used for production. As can be seen from the presented data, the addition of nickel and cobalt to the electrolyte composition leads to a decrease in grain size, which is associated with their denser packing, and also, as a consequence, an increase in specific surface area, which can also have a positive effect on the change in the photocatalytic activity of these films.

Figure 2 illustrates the results of the morphological features of the films obtained, as well as data on the distribution of elements in the composition of the films (mapping data). Figure 3 shows the results of changes in the elemental composition of the synthesized films, calculated on the basis of mapping data and analysis of energy-dispersive spectra. The overall impression of the presented mapping data indicates that in the case of CdSe films, the distribution of the elements cadmium and selenium is equally probable and isotropic throughout the entire volume under study. According to elemental analysis data (see data in Figure 3), the ratio of cadmium and selenium is 45% and 54%. This ratio of elements indicates that the resulting structures have an equally probable distribution of elements, characteristic of the formation of the CdSe phase.

In the case of adding nickel to the composition of ceramics (data in Figure 2b), as well as in the case of CdSe films, the distribution of elements is equally probable and isotropic, while the distribution of nickel is also equally likely isotropic throughout the entire volume, which indicates that nickel in the film samples is present in the entire volume, and not in the form of any individual phase inclusions containing nickel. At the same time, an analysis of the distribution of element ratios (see data in Figure 3b) showed the presence of nickel in the composition of more than 17%, which partially replaces selenium and, to a greater extent, cadmium. When cobalt sulfate is added to the electrolyte solution, a similar pattern of elemental distribution is observed. At the same time, cobalt, according to elemental analysis data, displaces cadmium and selenium to an equal extent.

Figure 4 illustrates the results of X-ray phase analysis of the examined thin films in relation to their electrolyte composition. A general observation of the presented X-ray diffraction patterns reveals the presence of low-intensity, broad reflections, which are indicative of significantly broadened diffraction lines. The shape of these reflections suggests a structure in the films that is nearly amorphous in nature. According to the information presented in the X-ray diffraction patterns, the inclusion of nickel or cobalt sulfates in the electrolyte mixture used to generate CdSe films results in the emergence of a reflection at 2θ = 38–39°, along with an enhancement in the intensity of the primary diffraction reflection occurring at 2θ = 52–56°. Both of these diffraction reflections correspond to the hexagonal CdSe phase with the spatial system P63mc(186). However, variations in the shape of these reflections suggest the development of grains with differing sizes and textural orientations. Furthermore, it is noteworthy that the inclusion of nickel and cobalt not only results in the formation of an additional textural direction, typical of larger grains measuring around 20–25 nm, but also contributes to an increased level of structural ordering when compared to CdSe films. CdSe films typically exhibit an almost amorphous structure, with a crystallinity degree of approximately 54–56%. Examination of the crystal lattice parameters of CdSe revealed the following values: a = 4.2116 Å and c = 6.8901 Å. It is worth noting that the amorphous character of the resulting CdSe films aligns well with existing literature data derived from experiments investigating the structural characteristics of films produced through electrochemical deposition [25,26].

Table 1. Structural parameter data calculated from the evaluation of X-ray diffraction patterns.

Parameter	Thin Film
	CdSe	NiCdSe	CoCdSe
Lattice parameter, Å	a = 4.2116 Å, c = 6.8901 Å.	a = 4.2012 Å, c = 6.8432 Å	a = 4.2042 Å, c = 6.8535 Å
Packing density of crystal lattice, c/a *	1.636	1.629	1.630
Volume parameter, Å3	105.84	104.60	104.91
Crystallinity degree, %	43.6	51.2	53.4

* This value characterizes the packing density of the crystal lattice and its perfection degree; the lower this value, the higher the degree of structural ordering.

provides the results of evaluating the structural attributes of the examined films in relation to variations in their production conditions. The general trend observed in the changes in structural parameters suggests a beneficial impact of substituting cadmium and selenium with nickel or cobalt, leading to improvements in structural characteristics. The introduction of nickel or cobalt into the ceramics results in a reduction in crystal lattice parameters and compaction, as indicated by alterations in crystal lattice volume. The decrease in crystal lattice parameters when nickel or cobalt sulfates are introduced into the electrolyte composition can be attributed to the partial substitution of cadmium and selenium ions in octahedral and tetrahedral positions by nickel or cobalt ions. In this case, the reduction in parameters can be attributed to the variation in ionic radii for the respective elements: nickel Ni²⁺ has a radius of 69 pm, cobalt Co²⁺ has a radius of 82 pm, cadmium Cd²⁺ has a radius of 92 pm, and selenium Se²⁻ has a radius of 184 pm. Considering this notable difference, the substitution effect can indeed result in a reduction in parameters and the creation of a denser crystal structure. The assessment of the c/a ratio in hexagonal structures, which signifies the compaction of the crystal lattice, reveals that a decrease in this value indicates a denser arrangement of atoms within the structure. Moreover, slight variations in this value when changing the substituting component from nickel to cobalt suggest that the addition of cobalt, with its larger ionic radius, leads to less pronounced lattice compaction and parameter alterations compared to the use of nickel. It should also be noted that changing the electrolyte composition by adding nickel or cobalt sulfates leads to an increase in the degree of crystallinity from 43.6% for CdSe films to 51–53% for films obtained from electrolytes to which nickel and cobalt were added. Thus, the addition of nickel and cobalt sulfates to the electrolyte composition, leading to a variation in the structural features as well as an increased crystallinity degree of the films, makes it possible to obtain more ordered structures based on CdSe compounds. This effect of structural ordering is akin to the changes observed in thermally annealed amorphous-like CdSe films [27,28].

Figure 5 displays information regarding variations in the optical properties of the examined films, contingent on their electrolyte composition. The recorded dependencies show alterations in the magnitude of transmission spectra, as depicted in Figure 5a, and exhibit the presence of a fundamental absorption edge spanning a 300–350 nm range. Furthermore, the films display commendable transparency in the visible light and near-IR regions. However, the inclusion of nickel or cobalt sulfate in the electrolyte mixture results in a shift in the fundamental absorption edge towards shorter wavelengths, accompanied by a reduction in film transmittance. This reduction can be attributed to the influence of substituting cadmium and selenium with nickel or cobalt, resulting in the formation of additional absorption centers that contribute to decreased transmittance. Additionally, changes in transmittance are influenced by the size factor of the films, as larger grain sizes, as indicated by morphological studies, lead to reduced transmittance in films containing nickel and cobalt. Analysis of the change in the transmittance of the resulting films when nickel and cobalt are added to them, determined at a wavelength of 550 nm, indicates a decrease in transmittance with a change in the composition of the films. At the same time, the change in the transmittance value for CoCdSe films is greatest in comparison with CdSe films for which the transmittance value is 104%, which is due to re-emission effects caused by the luminescence effect.

The shift in the fundamental absorption edge, reflecting a change in the band gap (E_g) in the Tauc plot, is presented in Figure 5b. In the case of CdSe films, the value of E_g is 1.71 eV, which is a slightly different value from the literature data on bulk CdSe samples [29]. Moreover, similar differences between E_g values for films and bulk samples are given in [30], according to which, in the case of thin films, the band gap is smaller due to size effects as well as structural features. The shift in the fundamental absorption edge and, as a consequence, the decrease in the E_g value of NiCdSe and CoCdSe films can be explained by several factors. Firstly, in the case of NiCdSe and CoCdSe films, according to their morphological features, increased grain size and film uniformity are observed, which in turn results in a reduction in E_g value, since, according to [31], E_g value is inversely proportional to grain size. Secondly, the addition of nickel or cobalt sulfates to the composition of electrolytes, according to mapping and X-ray diffraction data, leads to a partial replacement of cadmium and selenium atoms with nickel or cobalt atoms (depending on the electrolyte type). In this case, this substitution results in the emergence of an sp-d exchange interaction between localized electrons in d-orbitals and band electrons [32]. The degree of structural ordering, which is higher in the case of NiCdSe and CoCdSe films compared to CdSe films, also plays a significant role in altering their optical properties. Consequently, the adjustment of the band gap in NiCdSe and CoCdSe films will enhance light absorption across a broader range of the electromagnetic spectrum, thereby increasing the availability of photons for photocatalytic reactions.

The results illustrating the degradation degree of organic dyes, namely rhodamine B, cargo red, and indigo carmine, with varying concentrations in model solutions during their photocatalytic decomposition using the synthesized thin films, are presented in Figure 6. These data depict a connection between the percentage change in dye concentration within the solution over a specific time interval, enabling an assessment of degradation rate and decomposition efficiency trends over time. Altering dye concentrations within the range of 5 to 15 mg/L made it possible to ascertain decomposition efficiency under conditions where dye concentrations might surpass permissible limits, particularly in heavily polluted water sources.

Regarding the photocatalytic decomposition of rhodamine B, the most rapid degradation is observed after 180 min for CdSe films and 120 min for NiCdSe and CoCdSe films. Beyond these time points, there is a decline in decomposition rate across all three cases, and decomposition efficiency, as indicated by the alteration in the optical density of the model solutions, remains within a range of 40–45% for CdSe films and 60–65% for NiCdSe and CoCdSe films. Importantly, an increase in dye concentration does not yield significant alterations in decomposition efficiency. This suggests that the process is not directly contingent on concentration and that these films can function effectively irrespective of whether the dye concentration is low or high. Simultaneously, the relatively modest values of photocatalytic decomposition efficiency (40–45% for CdSe and 60–65% for NiCdSe and CoCdSe) highlight the significant resistance of rhodamine B to decomposition when using these catalysts and the limited effectiveness of the chosen films for its decomposition. It is noteworthy that rhodamine B dye is known for its remarkable stability during disposal, whether through conventional methods or photocatalytic decomposition [33,34]. As the duration of the process reaches 180 min for CdSe films and 120 min for NiCdSe and CoCdSe films, decomposition efficiency experiences a sharp decline. This can be attributed to the accumulation of adsorbed dye molecules on the films’ surface during decomposition reactions, resulting in film shielding and a reduction in the photodecomposition rate. Also, one of the factors influencing changes in the efficiency of photocatalytic decomposition for NiCdSe and CoCdSe films may be the size factor associated with a decrease in grain size in these films, which in turn leads to an increase in the specific surface area that interacts with the dye during its absorption.

When it comes to the degradation of cargo red and indigo carmine dyes, the most efficient films are NiCdSe films (for the photocatalytic decomposition of cargo red) and CoCdSe films (for the photocatalytic decomposition of indigo carmine). In these instances, complete decomposition (surpassing 98%) is attained within 210–240 min, whereas for CdSe films, the level of decomposition for these dyes hovers around 60–70%. This variation in efficiency could be attributed to both the structural features of the films (NiCdSe and CoCdSe films exhibit a uniform coarse-grained structure) and their optical characteristics (differences in the band gap, as indicated in Figure 5b). It is important to highlight that with an increase in dye concentration, the efficiency of decomposition remains unaltered, and the decomposition rate (concentration change over time) remains consistent. Consequently, we can deduce that, when it comes to the photocatalytic decomposition of cargo red and indigo carmine dyes, NiCdSe and CoCdSe films prove to be the most effective. Regardless of variations in dye concentration, these films achieve a decomposition level exceeding 98%, making them suitable for applications involving high dye concentrations in aqueous solutions.

Figure 7 illustrates the obtained dependencies that assess the efficiency of the photocatalytic decomposition of organic dyes while altering the dye concentration in the solution. Based on the collected data, it is evident that modifying the dye concentration does not result in a substantial decline in decomposition efficiency (except for CdSe films when used in the decomposition of rhodamine B dye). This observation suggests the potential application of these films as photocatalysts in heavily polluted environments.

Simultaneously, an assessment of the mineralization degree reveals that during the decomposition of rhodamine B, the mineralization degree reaches approximately 35% when CdSe films are used as catalysts, whereas with NiCdSe and CoCdSe films, it reaches about 55–57%. These relatively low mineralization values during the decomposition of rhodamine B highlight its notable resistance to decomposition during photocatalytic reactions. This observation aligns with several experimental findings [35,36] that suggest a low mineralization degree (decomposition of organic dyes into harmless inorganic oxides) in the case of rhodamine B decomposition. For the decomposition of cargo red and indigo carmine dyes, the mineralization degree exceeds 95%, indicating the high effectiveness of the catalysts, not only as adsorbents but also in their ability to break down dyes into harmless components.

The outcomes of a comparative analysis assessing the efficiency of photocatalytic decomposition in cyclic tests are illustrated in Figure 8. These experiments were conducted to ascertain the feasibility of using the resultant films repeatedly. This approach aims to enhance purification efficiency while lowering the cost of photocatalysts’ manufacturing. To evaluate the sustainability of degradation efficiency, these films were subjected to five consecutive cycles, during which the dye concentration (absorbance data) in the solution was measured at uniform time intervals.

As is evident from the data presented, in the case of NiCdSe and CoCdSe films, with an increase in the number of successive cycles of photocatalytic decomposition, the efficiency (decomposition degree) remains within the error limits for all three dyes. Moreover, in the case of using CdSe films, the efficiency of photocatalytic decomposition is maintained only for three consecutive cycles, after which further use of films is not advisable, since a sharp deterioration in the efficiency of decomposition is observed.

The decline in the films’ photocatalytic activity observed during cyclic tests can be attributed to alterations in their optical characteristics, specifically a rise in the band gap. This increase, in turn, results in a reduced rate of photon generation (their emission from the film’s surface). Therefore,

Table 2. Data on changes in the optical and structural characteristics of films after 5 test cycles.

Parameter	Thin Film
	CdSe	NiCdSe	CoCdSe
during the decomposition of rhodamine B
Eg, eV	1.92 ± 0.12	1.53 ± 0.05	1.37 ± 0.03
Crystallinity degree, %	38.6	50.2	51.6
during the decomposition of cargo red
Eg, eV	1.91 ± 0.09	1.52 ± 0.06	1.36 ± 0.05
Crystallinity degree, %	38.7	50.4	51.8
during the decomposition of indigo carmine
Eg, eV	1.87 ± 0.08	1.52 ± 0.06	1.36 ± 0.07
Crystallinity degree, %	39.4	50.7	51.9

provides information regarding the values of E_g (band gap) and the crystallinity degree of the films after undergoing five cycles of consecutive testing.

The table data reveal that the most significant alterations in optical and structural parameters are evident in CdSe films following five consecutive cycles, indicating their limited stability as photocatalysts during prolonged testing. Conversely, NiCdSe and CoCdSe films display minimal reductions in crystallinity and only slight increases in the band gap.

Moreover, the decline in photocatalytic activity can be attributed to the degradation of film surfaces during cyclic tests. Figure 9 illustrates alterations in the morphological characteristics of thin films after five cycles of assessments for the photocatalytic decomposition of organic dyes, highlighting the film degradation linked to destructive processes and surface destruction.

For CdSe films, successive cyclic tests lead to a destruction of the surface and its embrittlement due to the formation of a dendritic porous structure. Surface destruction is associated with the accumulation of dye on the surface during its adsorption, which leads to a deterioration of the structure and a decrease in photocatalytic activity (the so-called material screening effect due to the polymolecular dye on the surface).

In contrast, NiCdSe and CoCdSe films do not exhibit such significant deterioration in their structure. However, they do develop longitudinal surface microcracks, indicating the accumulation of deformation and distortion related to film degradation processes. An increased concentration of these distortions leads to film cracking.

An examination of the mechanical properties, specifically hardness and hardening, of the films before and after undergoing photocatalytic dye decomposition tests is detailed in Figure 10. The analysis reveals that the CdSe films exhibit significantly low hardness values, measuring less than 100 HV. This low hardness is attributed to their amorphous-like structure and structural disorder in the crystal lattice, evident in the broadened parameters.

Additionally, when nickel or cobalt are introduced into the film composition, there is an increase in hardness, surpassing 100 HV. This signifies a positive strengthening effect resulting from alterations in structural parameters, such as increased ordering, and dimensional factors. Furthermore, the analysis of hardening data, derived from a comparative assessment of hardness values between NiCdSe and CoCdSe films and CdSe films, reveals a hardening effect of approximately 30–40%, which holds significant importance for such films. Analysis of data concerning hardness changes after cyclic testing reveals a decrease in sample hardness, consistent with the observed morphological studies of film surface degradation during photocatalytic decomposition. Notably, the most significant softening occurred in CdSe films, with a reduction of over 60% from the initial value. In contrast, NiCdSe and CoCdSe films experienced a reduction in hardness of no more than 20% from their initial values. This suggests a relatively high resistance of these films to degradation, along with the formation of feather-like oxide inclusions on the surface. It is important to highlight that the primary changes observed in the case of NiCdSe and CoCdSe films, which are related to softening, can be observed in the SEM images presented in the figure. These changes manifest as the formation of surface cracks, indicating their high stability due to their denser crystal packing and a higher structural ordering degree (degree of crystallinity).

In fact, CdSe chemical compounds fall into the category of “potentially hazardous elements” (Category 1–4 according to Regulation (EC) No. 1272/2008). In this case, experiments to assess the stability of their long-term use were carried out not only to establish the dependence of changes in the absorption efficiency of dyes, but also to determine the preservation of the stability of the films themselves. With the long-term use of CdSe films, it was found that after five cycles, they become partially embrittled through the formation of microcracks, which can lead to the contamination of aquatic environments with film destruction products. In this regard, it was found that the long-term use of CdSe films is impractical (since a decrease in their effectiveness was observed) and carries a potential danger of contamination due to their embrittlement. In the case of NiCdSe and CoCdSe films, such obvious destruction was not observed, which indicates their increased resistance to degradation processes and opens up prospects for their further use.

As is known, the use of photocatalysts for the purpose of decomposing organic dyes and their subsequent utilization is based on the mechanisms of photodegradation, the effectiveness of which depends on the ability of the catalyst material to produce free charge carriers and the formation of OH⁻ ionic groups, which are strong oxidizing agents. In this regard, much attention is paid to materials with small band gaps (semiconductor or metal oxide materials), as well as a large specific surface area (mainly materials with small particle sizes). Among potential photocatalysts, in addition to CdSe films, much attention is paid to oxide materials based on ZnO, TiO₂, CuO, ZrO₂, etc. [37,38,39]. So, for example, in work [38], the use of ZnO materials of various morphologies for the decomposition of rhodamine B was considered. As was shown by the authors [38], a decrease in particle size leads to an increase in the efficiency of decomposition, which also coincides with the results of this work, according to which the formation of finer grains in NiCdSe and CoCdSe also affects decomposition efficiency. The highly efficient influence of composite structure formation achieved by the modification of CdSe materials by adding ZnO to them on the subsequent formation of nanocomposites was demonstrated in [40]. According to the results of [40], the formation of a nanocomposite of the ZnO/CdSe type leads to the formation of stable photocatalysts capable of operating for more than 15 consecutive cycles, which also confirms the assumption made in this study about the influence of nickel and cobalt on increasing the stability of long-term photocatalyst operations. The results obtained in this work on the efficiency of the photocatalytic decomposition of organic dyes are in good agreement with the results of work [41], which examined three types of photocatalysts based on CdSe modified with graphene and TiO₂. Thus, summing up a brief comparative analysis of the results of photocatalytic decomposition efficiency achieved by various types of materials, we can conclude that the greatest prospects in this area of research in the near future will be variations of composite photocatalysts that combine a set of dimensional, optical, and electronic characteristics of the components used to obtain composites. Much attention should also be paid to modifying materials by adding various elements to them in order to increase both the decomposition efficiency and the stability of photocatalysts.

4. Conclusions

This work studied the effect of adding nickel and cobalt sulfates to the electrolyte composition used to obtain CdSe films, in order to obtain films such as NiCdSe and CoCdSe, which have great prospects for use as a basis for photocatalysts. According to data on changes in structural, optical, and strength properties, it was established that the partial replacement of cadmium and selenium with nickel or cobalt leads to a decrease in the band gap, as well as the strengthening of films associated with changes in grain size and crystallinity degree.

When evaluating the potential of these films as the foundation for developing photocatalysts, it was determined that NiCdSe and CoCdSe films exhibit the highest effectiveness in decomposing cargo red and indigo carmine dyes, achieving decomposition and mineralization efficiencies of over 98% and 95%, respectively. Furthermore, these films can undergo five consecutive decomposition cycles without a decrease in their effectiveness. The enhancement in decomposition efficiency observed in NiCdSe and CoCdSe films compared to CdSe films can be attributed to alterations in the band gap, which is 1.5 eV for NiCdSe films and 1.34 eV for CoCdSe films, as well as improvements in their crystal structure and morphological characteristics, including grain coarsening and enhanced uniformity of film layers.