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Article

From Pigment to Photocatalyst: CdSe/CdS Solutions Mimicking Cadmium Red for Visible-Light Dye Degradation

by
Julia Łacic
1 and
Anna Magdalena Kusior
2,*
1
Faculty of Energy and Fuels, AGH University of Krakow, al. Mickiwiecza 30, 30-059 Krakow, Poland
2
Faculty of Materials Science and Ceramics, AGH University of Krakow, al. Mickiewicza 30, 30-059 Krakow, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 883; https://doi.org/10.3390/catal15090883
Submission received: 30 July 2025 / Revised: 1 September 2025 / Accepted: 10 September 2025 / Published: 15 September 2025
(This article belongs to the Special Issue Catalysis Accelerating Energy and Environmental Sustainability)
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Abstract

This study explores the dual functionality of cadmium-based pigments (CdS, CdSe, and CdS1−xSex solid solutions) as historical colorants and visible-light photocatalysts. Synthesized pigments here replicated hues of traditional cadmium reds. At the same time, their photocatalytic efficiency was evaluated using model dyes, such as indigo carmine (anionic) and fuchsine (cationic), as a representative of heritage materials. Structural and optical characterization confirmed tunable bandgaps (1.63–2.28 eV) and phase-dependent microstructures, with CdS1−xSex composites exhibiting compositional heterogeneity. Photocatalytic tests revealed specific degradation mechanisms. Indigo carmine degradation was dominated by superoxide radicals (O2•−), while fuchsine degradation relied on photogenerated electrons (e′). Scavenger experiments highlighted the synergistic role of reactive oxygen species (ROS) and charge carriers, with CdS and CdSe showing the highest activity. Intermediate composites displayed selective reactivity, suggesting trade-offs between phase homogeneity and surface interactions. Reduced photocatalytic efficiency in composites aligns with cultural heritage needs, where pigment stability under light exposure is critical. This work bridges material science and conservation, demonstrating how the compositional tuning of CdS1−xSex can balance color fidelity, photocatalytic activity, and longevity in art preservation.

Graphical Abstract

1. Introduction

Cadmium-based pigments exhibit a wide range of colors, from bright yellow to orange to deep red, owing to their compositional tunability through the formation of continuous solid solutions (CdS1−xSex). This compositional control enables precise band gap engineering, allowing the modulation of the observed color across the visible spectrum—from yellow (CdS) to black (CdSe) [1,2]. The striking hue of these pigments is directly related to their semiconductor nature, wherein the energy difference between the valence and conduction bands determines the observed color. For instance, CdS has a band gap of approximately 2.42 eV [3,4] toward longer wavelengths, resulting in an orange to red tone [5,6]. These optical properties make cadmium pigments not only artistically attractive but also scientifically relevant in the context of photocatalysis and material science.
On an industrial scale, their production began around 1900, primarily for use in artists’ paints, ceramics, and decorative applications. They replaced the oil colors based on toxic chrome yellow, PbCrO4 [7], which was widely used in 19th-century European painting, notably by Vincent Van Gogh, who used it to create his stunning yellows [8]. However, their application fields were limited due to the relatively high costs of raw materials and production, resulting in high prices. Beyond their aesthetic value, cadmium pigments are of great scientific interest due to their unique optoelectronic properties. Their distinct band structures and luminescent behavior, such as band-edge photoluminescence in the visible range and deep-trap luminescence in the red/infrared region, render them ideal candidates for non-destructive pigment identification and degradation analysis using advanced imaging spectroscopy techniques [2,9]. These methods, including photoluminescence (PL) mapping and synchrotron-based spectroscopy, have proven particularly valuable in cultural heritage and conservation science, offering insight into pigment composition and photoinduced degradation phenomena.
Considering the fundamental properties of the two base materials that form the basis of cadmium red, CdS and CdSe can crystallize in either cubic or hexagonal structures [10,11,12,13]. Cadmium sulfide has a relatively wide bandgap ranging from 2.4 to 3.5 eV, which makes it suitable for optoelectronic applications [12,14]. It also exhibits high photoactivity and photoconductivity [14,15,16] and has been widely used for photoelectrochemical water splitting to generate hydrogen [17,18,19]. Its practical use, however, is limited by the rapid recombination of photogenerated carriers [17,18]. CdS has also been employed in the photodegradation of organic compounds such as Victoria Blue B and Rhodamine B [20,21,22]. In contrast, CdSe possesses a narrower bandgap (1.76–1.88 eV) [11], enabling excellent light absorption. Owing to its good electrical conductivity, CdSe is applied in solar cells and LEDs [23,24,25]. It has also been reported as an active photocatalyst in CO2 reduction (with higher sensitivity to red light) and the degradation of organic dyes [26,27,28]. Nonetheless, both semiconductors suffer from intrinsic drawbacks such as limited stability and susceptibility to photocorrosion [29,30]. Considering that many sulfide-based compounds are employed in photocatalytic processes, each metal sulfide possesses distinct properties and suffers from challenges that govern its suitability for specific applications in photocatalysis and beyond [31,32,33].
As inorganic semiconductors, cadmium pigments are thermally stable, highly resistant to migration, and nearly insoluble in water, alkalis, and most organic solvents. However, under prolonged light exposure and in the presence of moisture and oxygen, cadmium sulfide is subject to slow photooxidation, leading to the formation of cadmium sulfate [34]. Environmental and health concerns have led to the development of alternative pigments, such as bismuth vanadate and high-value organic pigments, which are less problematic [1].
Importantly, cadmium pigments serve as a model system to explore the relationship between pigment composition and photocatalytic behavior. The concept of “color-driven” band engineering and electronic properties is governed by deliberate anion substitution (S → Se), mirroring strategies used in modern photocatalysis. Such engineered materials can facilitate the visible-light-driven degradation of industrial dyes or emerging organic pollutants. Moreover, the formation of heterojunctions within CdS−CdSe composites enhances charge carrier separation and reduces recombination losses, improving photocatalytic efficiency [35,36,37,38,39,40]. Furthermore, the substitution of sulfur by selenium results in predictable changes in the crystal lattice, narrowing the band gap and enhancing light absorption in the visible range [41,42]. These materials are capable of generating reactive oxygen species (ROS), including hydroxyl radicals (•OH), superoxide anions (O2−), and hydrogen peroxide, upon exposure to light irradiation. While these species are crucial for environmental photocatalysis, they may also contribute to the degradation of paintings by oxidizing binders and pigment matrices.
Considering heritage and conservation science, the photoactivity of cadmium pigments is a double-edged sword. On the one hand, it provides a mechanistic explanation for the gradual degradation observed in historical artworks, as cadmium pigments lead to the oxidation of the oil binder in paintings. On the other hand, it offers a model for studying degradation pathways and formulating preservation strategies. This process is exacerbated by the presence of moisture, which triggers the formation of degradation products such as cadmium sulfate [43]. Under light exposure, photogenerated holes (h+) in CdS oxidize sulfide ions to sulfate. In contrast, photogenerated electrons (e−) can reduce O2 to form reactive oxygen species, which further accelerate the degradation, resulting in whitish crusts on the surface of the paintings [44,45,46]. These reactions are accelerated under conditions of high humidity and light exposure, leading to the formation of white sulfate crusts and visual discoloration of the painted surface [47]. Additionally, interactions with neighboring pigments lead to discoloration; for example, copper-containing malachite may result in undesired color changes due to secondary reactions forming copper sulfides [48]. By considering the behavior of cadmium-based pigments in the presence of other pigments, along with their photoactivity and reactivity under light exposure, and by analyzing the underlying reaction mechanisms, it becomes possible to develop more effective strategies for preventing degradation and preserving the integrity of artworks.
The novelty of this work lies in demonstrating the dual role of cadmium pigments as both historical colorants and functional photocatalysts, by synthesizing CdS, CdSe, and their solid solution (CdS1−xSex). The synthesis targeted compositions that reproduce the hues characteristic of historical cadmium reds, thereby providing a direct link between material functionality and historical usage. These materials were subsequently evaluated for their photocatalytic performance under visible light irradiation. To simulate organic dye degradation processes, fuchsine (FC) and indigo carmine (IC, also known as Saxon Blue) were selected as model dyes [49,50]. These compounds represent synthetic organic dyes historically used alongside cadmium pigments in artworks and printing inks. Their molecular structures and color stability make them ideal probes for photocatalytic evaluation. Additionally, scavenger experiments were employed to identify the reactive species involved in the photodegradation process, and plausible degradation pathways were proposed. Furthermore, the novelty of this work lies in the systematic synthesis and optical characterization of CdS1−xSex solid solutions, combined with an experimental evaluation of their photocatalytic activity toward organic dyes. While CdS and CdSe have been widely studied individually, their solid solutions remain less explored, particularly concerning how compositional tuning affects the band structure and photocatalytic response. Here, the band gap evolution was analyzed using complementary optical methods (derivative reflectance and Tauc analysis), and the photocatalytic mechanism was rationalized by correlating the semiconductor band edges with the experimentally determined the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) levels of the dyes. This interdisciplinary approach aims to bridge the gap between conservation science and material engineering by linking pigment composition, structural properties, and photocatalytic behavior. The results are relevant not only to the design of efficient visible-light photocatalysts but also to the preservation and interpretation of pigment-based heritage materials.

2. Results and Discussion

2.1. Cadmium Pigments: Microstructure and Phase Composition

The microstructures of the synthesized cadmium-based pigments were examined using scanning electron microscopy (SEM), as shown in Figure 1. All samples were prepared via a two-step synthesis process—the initial dissolution of precursors at moderate temperature (60 °C), followed by hydrothermal treatment in a Teflon-lined autoclave at 160 °C for 24 h. The resulting powders were labeled according to their estimated chemical composition, defined by the molar percentage of CdS. Grain size distributions were determined using ImageJ v1.54p software based on SEM image analysis. The as-synthesized materials exhibited a range of colors, bright yellow for pure CdS, transitioning through orange-red and brown to nearly black for CdSe, corresponding to their compositional variation and related band gap shifts. Pure CdS displayed aggregated and irregularly shaped grains with particle sizes ranging from approximately 80 to 235 nm, characteristic of polycrystalline sulfide systems. In contrast, CdSe powders exhibited a more uniform and refined grain morphology, consisting of faceted particles with sizes ranging from 35 to 45 nm. The CdS1−xSex composites revealed heterogeneous microstructures, with visible changes in particle morphology as a function of selenium content. Notably, increasing Se substitution resulted in smoother surface textures and a progressive reduction in particle size. For example, in the 90 mol% CdS composite, particles ranged from 80 to 250 nm, whereas in the 50 mol% CdS sample, finer grains with dimensions between 25 and 40 nm were observed.
The structural properties of the synthesized CdS, CdSe, and their composite powders were examined by X-ray diffraction (XRD), as presented in Figure 2a. The diffraction pattern of CdS corresponds predominantly to the cubic phase (74.8%, #PDF 98−016−8373), with a minor contribution from the hexagonal form (25.2%, #PDF 98−062−0313). A similar biphasic character was observed for CdSe, which exhibits both cubic (44.4%, #PDF 98−062−0436) and hexagonal (50.3%, #PDF 98−062−0438) crystal structures. Notably, the CdSe sample also shows minor diffraction peaks associated with elemental selenium (5.2%, #PDF 98−016−4269), suggesting the incomplete reaction or partial crystallization of unreacted selenium during synthesis. In the case of composites, a shift of the diffraction peaks relative to the pure CdS and CdSe phases is observed, indicating the formation of a solid solution (CdS1−xSex) through the progressive substitution of sulfur atoms by selenium in the crystal lattice. This gradual shift of the reflections towards lower 2θ angles with increasing Se content is consistent with the larger ionic radius of Se2− (198 pm) relative to S2− (184 pm), resulting in an expansion of the crystal lattice, as further confirmed by the refined cell parameters (Table 1).
This structural evolution confirms the successful incorporation of selenium into the CdS host lattice. For the samples assigned as 75 mol% and 50 mol% CdS, additional phases were identified as CdSe (51.3%) and CdSSe (21.3%), respectively, suggesting the coexistence of multiphase domains or incomplete homogenization.
Figure 2b presents the Raman spectra of CdS, CdSe, and CdS1−xSex. The CdS sample exhibits a prominent longitudinal optical (LO) phonon mode at 305 cm−1 and its overtone at 610 cm−1, which are characteristic of the hexagonal wurtzite CdS structure [51]. The CdSe spectrum is dominated by a strong LO phonon mode near 210 cm−1, in agreement with the hexagonal structure of CdSe. In the CdS1−xSex composite materials, both characteristic phonon bands of CdS and CdSe are discernible, with their position and relative intensities varying depending on the S/Se molar ratio. This behavior reflects changes in local lattice dynamics and possible structural disorder induced by compositional inhomogeneity. Moreover, in samples with higher selenium content, additional acoustic phonon features are observed around 272 cm−1 and at 416 cm−1, which are indicative of cadmium selenide-rich domains or secondary phases [52,53].
The presence of multiple vibrational modes can influence electron–phonon coupling, a key parameter affecting the photogenerated charge carrier dynamics. Enhanced phonon scattering in these composite systems may facilitate exciton dissociation and contribute to the suppression of carrier recombination, thereby improving the photocatalytic activity.
Collectively, the XRD and Raman analyses confirm the successful formation of CdS1−xSex solid solutions with tunable structural properties. The observed shifts in diffraction peaks and Raman modes are consistent with modifications to the band structure resulting from anion substitution.
This structural adjustment leads to bandgap narrowing, resulting in a red shift in the absorption of light. The diffuse reflectance, Rdiff, as a function of the wavelength, is presented in Figure 3a.
As expected, the fundamental absorption edge systematically shifts with composition. The band gap energy (Eg) was estimated from the first derivative of the reflectance spectra (dRdiff/dλ), by identifying the maximum slope corresponding to the absorption onset. For pure CdSe, the estimated Eg is approximately 1.63 eV, while for CdS, it increases to 2.28 eV, consistent with values in the literature and the known compositional dependence of the band gap. In the case of CdS1−xSex solid solutions, two distinct optical transitions are observed, indicating the presence of compositional or structural heterogeneity. Specifically, the extracted optical transition corresponds to 1.66 and 1.90 eV for 50 mol% CdS, 1.67 and 2.09 eV for 75 mol% CdS, and 1.71 and 2.20 eV for 90 mol% CdS. These dual features suggest the coexistence of regions with different local compositions or degrees of crystallinity within the same material. To confirm the above results, the spectra were also analyzed by plotting the absorption coefficient versus phonon energy using Tauc plot. In the Tauc relation [54],
(αhν)1/n=A(hν − Eg)
n corresponds to the Tauc exponent, α is the absorption coefficient (a function of wavelength α(λ)), h is Planck’s constant, ν is the frequency, and A is a proportionality constant. CdS and CdSe materials are known as the direct bandgap semiconductors [55,56,57]. Therefore, the Tauc exponent value of ½ associated with a direct allowed transition was chosen for calculation. The results are shown in Figure 3b. The obtained results are similar to those from the first derivative method. The estimated Eg value for CdSe is 1.68 eV. Notably, as the sulfur content increases, the overall band gap widens, enabling absorption in the higher-energy (shorter-wavelength) region of the visible spectrum, with values of 1.78, 2.00, 2.20 to 2.31 eV for 50 mol%, 75 mol%, 90 mol% and CdS powders, respectively. This tunability of the optical properties through controlled S/Se substitution offers significant potential for the rational design of light-active materials. In the context of photocatalysis and pigment stability, a wider band gap may enhance photostability but reduce visible-light absorption. Although the typical bulk materials for CdS are characterized by the 2.42 eV bandgap, in the literature, several papers reporting smaller values can be found. The reduced Eg may be correlated with the particle size [58,59], and may also be related to the change in the synthesis technique [60,61,62,63], the presence of the defects [64,65], or different materials/additives [62,66].

2.2. Photocatalytic Activity

The photocatalytic activity of the synthesized cadmium-based pigments was evaluated using two model organic dyes: anionic indigo carmine and cationic fuchsine. The degradation efficiency, monitored under visible-light irradiation, is shown in Figure 4a,d. In all cases, the extent of photodegradation increases with illumination time, confirming the light-induced activity of the materials.
All tested photocatalysts exhibit measurable dye degradation, although the effectiveness is strongly dependent on both the chemical composition of the pigment and the nature of the dye. For indigo carmine, the cadmium-based materials exhibit a notable adsorption capacity in the dark, with dye uptake ranging from 3.66 to 5.80 µg, indicating favorable electrostatic and surface interactions with the anionic dye. This pre-adsorption step likely facilitates subsequent photocatalytic degradation. Among the tested samples, the highest photodegradation efficiency is observed for the pure CdSe and CdS powders. In contrast, the lowest photocatalytic activity is exhibited by the 50 mol% CdS sample. According to XRD data, this particular composition is biphasic, containing both CdSe and a mixed CdSSe phase (21.3%), which indicates the incomplete incorporation of sulfur and selenium into a uniform solid solution. Interestingly, despite the shift in diffraction peaks toward higher 2θ angles, which is commonly interpreted as indicative of solid solution formation, this structural adjustment does not translate into enhanced photoactivity. This unexpected behavior suggests that structural disorder or inefficient charge separation at the interface of multiple phases may hinder photocatalytic performance in the 50 mol% CdS sample. To better elucidate the underlying degradation mechanisms and differentiate between surface adsorption and actual photocatalytic activity, kinetic modeling was performed using the Lagergren kinetics model [67,68,69].
Firstly, considering that the dye’s diffusion through the boundary is the primary mechanism, the pseudo-first order was applied,
ln(qe − qt) = lnqe − k1t
where qe is the amount of adsorbed at time 0, qt is the amount of photodegraded dye at time t, and k1 corresponds to the rate constant. The linear relation between ln(qe − qt) vs t should yield a slope of k1 and an intercept of lnqe. However, none of the observed curves (Figure 4b) fulfill the model requirements.
The pseudo-second-order kinetic model highlights the dominant role of chemisorption as the rate-controlling step in the overall photodegradation process. The kinetic equation is expressed as
tqt−1 = (k2qe2)−1 + tqe−1
where k2 is defined as a pseudo-second-rate constant. According to this model, plotting tqt vs. t should yield a linear relationship, from which the slope and the intercept can be used to calculate k2 and qe, respectively.
As shown in Figure 4c, the experimental data for all samples exhibit linear correlation with the applied Lagergen model, suggesting that the photodegradation process proceeds predominantly via surface-controlled chemisorption. This indicates that surface interactions between the photocatalyst and the dye molecules play a critical role in determining the reaction rate and efficiency.
A similar trend is observed in the case of fuchsine photodegradation (Figure 4d). Notably, the CdSe powder exhibits strong adsorption behavior, removing nearly 80% of the dye from the solution before irradiation. This suggests a high affinity between the CdSe surface and the cationic dye, which may be attributed to electrostatic interactions and the specific surface properties of the CdSe. In contrast, for CdS and the 90 mol% CdS samples, an increase in absorbance is observed after the dark adsorption phase, indicating the possible leaching of surface species, such as sulfur ions or residual impurities, into the solution. However, this phenomenon does not significantly affect the photocatalytic degradation efficiency.
These observations underscore the importance of surface composition, phase homogeneity, and defect chemistry in determining the photocatalytic performance of cadmium-based pigment materials. A comparison of the pseudo-second-order rate constants for both dyes (Figure 5a) provides indirect insight into how the photocatalyst’s activity influences the surrounding dye molecules and the kinetics of their degradation. In the case of the anionic dye, IC, the highest k2 values are observed for the pure CdS and CdSe powders. In contrast, the composite materials exhibit significantly lower rate constants, despite having comparable values among themselves. This suggests that solid solutions or phase mixtures formed at intermediate compositions may exhibit less favorable surface properties or lower efficiency in charge carrier separation. From the perspective of cultural heritage protection, this reduced activity is advantageous, as it suggests lower reactivity under light exposure, potentially prolonging the stability and visual integrity of artworks such as paintings, dyed textiles, or glazed ceramics.
In the case of fuchsine photodegradation, the trend is reversed. The 50 mol% CdS sample demonstrates the highest k2 value, outperforming even the pure phases. Although CdS and CdSe show better adsorption affinity and moderately high photoactivity, the intermediate composition (50 mol% CdS) exhibits a unique balance. Its k2 value suggests the presence of highly selective and reactive surface sites. This points to a possible synergistic effect or optimized interface in the composite, which may enhance dye–surface interactions and facilitate more efficient degradation pathways for cationic species. Therefore, while high k2 values typically indicate faster photodegradation, in the context of photocatalyst design, the selectivity and interaction mechanism with specific dye molecules are equally critical for evaluating material performance.
The zeta potential values of the obtained powders were measured in aqueous suspensions containing indigo carmine and FC, as well as in distilled water, to assess the influence of the dyes on the surface charge of the materials (Figure 5b). The observed values vary depending on both the chemical composition of the powders and the nature of the surrounding medium. For CdS and CdSe samples, the presence of the anionic dye (indigo carmine) causes a shift of the zeta potential towards more positive values, while exposure to the cationic dye (fuchsine) results in a reversal to more negative values. This behavior suggests the strong electrostatic adsorption of dye molecules onto the powder surface, effectively reversing the native surface charge. Such interactions are consistent with the high photocatalytic activity of these materials, confirming that efficient dye adsorption is a prerequisite for effective photodegradation.
In the case of the 50 mol% and 90 mol% CdS samples, a pronounced shift in zeta potential is observed in both dye environments, towards more positive values in the anionic dye and more negative values in the cationic one. This behavior implies non-selective adsorption mechanisms or the dominance of factors beyond simple electrostatic interactions. Possible explanations include the presence of surface functional groups with variable charge states, the formation of a compact dye layer that screens the inherent charge of the powder, or specific molecular interactions involving dipole moments or hydrogen bonding within the electrical double layer. In contrast, the zeta potential of the 75 mol% CdS sample remains nearly unchanged regardless of the dye present. This indicates an electrically neutral surface under the experimental conditions, suggesting the minimal or weak adsorption of dye molecules. The lack of surface charge alteration aligns well with the low photocatalytic activity and the reduced k2 rate constants observed for this composition. Such behavior may arise from the limited availability of active sites or steric hindrance that prevents effective dye–surface interaction, further underscoring the role of surface properties in modulating photocatalytic performance.
The identification of the ROS or charge carriers (electrons (e’) and holes (h+), responsible for the photodegradation mechanism of dyes, was carried out by introducing specific scavengers, namely, p-benzoquinone (pBq) for O2•−, methanol (MetOH) for SO4•−/•OH, K2Cr2O7 for e’, and disodium ethylenediaminetetraacetate (EDTA–Na) for h+. The obtained results are shown in Figure 6.
Regardless of the dye type, the highest photocatalytic activity was exhibited by CdS and CdSe powders. However, in the case of indigo carmine, the key role in the photodegradation mechanism is attributed to superoxide radicals (O2•−), whereas for fuchsine, the dominant contribution comes from photogenerated electrons (e’). Notably, for CdS, each scavenger test resulted in a noticeable suppression of the reaction rate, indicating that the presence of all reactive species, like O2•−, •OH/SO4•−, e−, and h+, is crucial for efficient degradation. A similar pattern is observed for fuchsine, underscoring the importance of a synergistic interaction between charge carriers and ROS. Interestingly, for CdSe in the presence of indigo carmine, the addition of charge carrier scavengers accelerates the dye decomposition process. On the one hand, this suggests that trapping excess charge carriers reduces electron–hole recombination, thereby enhancing the availability of e’ or h⁺ for redox reactions. This behavior implies that, before scavenger addition, these carriers may have participated in competing side reactions or recombined prematurely, limiting overall efficiency. In contrast, the lack of change in fuchsine concentration upon the addition of p-benzoquinone indicates that superoxide radicals are not significantly involved in its degradation mechanism.
The CdS1−xSex composite samples, as predicted from the analysis of the pseudo-second-order rate constant (k2), exhibit similar behavior in the presence of indigo carmine. As expected for this anionic dye, the dominant role of superoxide radicals in the photodegradation mechanism is indisputable. However, what is particularly noteworthy is the necessity of photogenerated electrons (e’) in this process. Electrons participate in the multistep reduction of dissolved molecular oxygen, leading to the formation of various reactive oxygen species, including superoxide anions, hydrogen peroxide, hydroperoxyl radicals, and hydroxyl radicals, which in turn facilitate the degradation of dye molecules [70,71,72]. Nevertheless, the simultaneous involvement of multiple reaction pathways may introduce competitive or interfering processes, such as charge carrier recombination or non-productive ROS generation. As a result, despite the theoretical enhancement of photoactivity due to band structure tuning in CdS1−xSex, the observed dye photodegradation rate is the slowest among the tested systems. This highlights the complexity of optimizing photocatalytic materials, where improved charge dynamics must be balanced with selective reactivity and minimized side reactions.
A comparison of the results obtained for the selected powders with individual scavengers during the photodegradation of fuchsine reveals a more complex relationship between the charge carriers and the photocatalytic response. For CdS1−xSex samples, particularly those compositions close to pure CdS and CdSe, two primary reaction pathways dominate, those involving superoxide radicals (O2•−) and those involving electrons (e’). This dual activity may give rise to competing mechanisms or undesirable side reactions, ultimately slowing the overall photodegradation process. The observed acceleration in photocatalytic decomposition upon the addition of a hole (h+) scavenger further supports the key role of electrons in the fuchsine degradation mechanism. It is worth noting that, for CdSe powder, capturing the O2•− does not affect the overall reaction effectiveness.
Interestingly, for the sample marked as 75 mol% CdS, only superoxide radicals were identified as the dominant reactive species responsible for dye degradation. However, the addition of other scavengers in this case unexpectedly enhanced the degradation rate. This observation suggests that in the absence of scavenger molecules, competing side reactions or premature recombination events may occur. The scavengers thus act not only as tools for mechanistic identification, but also as modifiers that reveal latent photoactivity by suppressing unfavorable processes. These findings highlight the delicate balance between carrier generation, recombination, and dye interaction, particularly in mixed systems.
To better understand the role of particular ROS in the photocatalytic degradation of dyes, the energy levels of the valence (EVB) and conduction (ECB) band edges for CdS and CdSe were calculated, and the HOMO and LUMO levels of indigo carmine were experimentally determined. The obtained schematic energy diagram for IC and photocatalysts is presented in Figure 7a.
Linking the calculated energy levels with the measured zeta potential shifts highlights the interplay between the electronic structure and surface properties of the photocatalysts in IC solution. According to the zeta potential measurements, in an indigo carmine solution, both materials exhibit a shift of the zeta potential towards more positive values. pH analysis shows that the IC solution has an initial pH of about 6.1, which decreases to 4.8 upon the addition of CdS.
In contrast, the presence of CdSe shifts the pH towards more alkaline values (8.9). This takes into account the following equilibrium [70]:
O 2 • − + H + ⇄ H O 2 •
It can be assumed that in the CdS–dye system, the predominant form is HO2•, while in the CdSe–dye system, O2•− radicals dominate [73]. The positions of the conduction and valence band edges for CdS allow both direct reactions responsible for dye photodegradation and the generation of specific species such as H2O2 and HO2•, thereby facilitating the observed photocatalytic process (Figure 7b). In the case of CdSe, the evaluated band alignment indicates that photogenerated electrons and O2•− radicals play a crucial role in pigment degradation. Moreover, the presence of metallic selenium in the powder composition enhances electron transfer and promotes possible redox reactions [74] (Figure 7c). The observed effect of accelerating the photodegradation process, when the scavenger captures electrons, may be related to the higher oxidizing ability of the HO2• [75]. It is worth noting that for both materials, reactions devoted to the generation of the •OH radicals are not possible. Considering the tunable band gap values between the two base materials, and the fact that the pH of the photocatalyst–dye system for CdS1−xSex materials remains similar to that of CdS (4.2 for 90 mol% and 50 mol%, and 4.1 for 75 mol%), together with the results of the scavenger tests, it may be assumed that the CdS component determines the photodegradation pathways of indigo carmine. However, their photocatalytic activity can be influenced by competitive reactions, which limit the generation of reactive oxygen species.
For fuchsine, the HOMO and LUMO energy levels used in this study were sourced from the scientific literature. These values provide a reliable foundation for our discussion on the compound’s reactivity. Based on the data determined from CV analysis and the absorption edge of fuchsine, the positions were set at −5.37 and −3.08 eV for HOMO and LUMO, respectively [76,77]. The results are shown in Figure 8a.
A similar trend in pH variation was observed for the semiconductor–fuchsine system. The initial pH of the FC dye solution was determined as 6.74. Upon the addition of CdS, the pH shifted towards acidic values (4.53), whereas in the presence of CdSe it increased towards alkaline conditions (9.15). These results suggest that the predominant reactive oxygen species are HO2• in the CdS system and O2•− in the CdSe system, respectively. Although in both cases the direct degradation of the dye by photogenerated electrons is not feasible, the electrons play a crucial role in the generation of specific ROS, thereby accelerating the degradation pathways (Figure 8b,c). By correlating the constructed energy diagram with the scavenger test, the central role of electrons in the photocatalytic mechanism is confirmed. Moreover, as demonstrated in the case of solid solutions (Figure 6b), the effective exclusion of photogenerated holes reduces the probability of recombination processes. It enhances the fraction of electron-driven reactions, further promoting the efficiency of the photocatalytic degradation.
From the perspective of cultural heritage preservation, the observed slowdown in the photodegradation process of indigo carmine and fuchsine dyes by CdS1−xSex is worth noting. While the rapid degradation of pollutants is advantageous in environmental applications, in the case of pigments used in artworks, ceramics, or historical textiles, limited photocatalytic activity combined with structural stability of the pigment is essential. The reduced ability to generate aggressive reactive oxygen species lowers the risk of uncontrolled oxidation or degradation of adjacent components in the painted layer. Therefore, these materials exhibit potential as a new generation of pigments with optimized durability and minimal impact on the long-term integrity of artworks. The analyzed compositions not only provide color as mineral pigments, but also limit photocatalytic processes, especially if the artwork is stored in unprotected conditions.

3. Materials and Methods

3.1. Materials

Cadmium sulfate octahydrate (3CdSO4·8H2O, POCH, Gliwice, Poland, CAS: 7790-84-3), selenium powder (Se, Acros Organics, Geel, Belgium, CAS: 7782-49-2), thioacetamide (TAA, POCH, Gliwice, Poland, CAS: 62-55-5), sodium hydroxide (NaOH, POCH, Gliwice, Poland, CAS: 1310-73-2), sodium borohydride (NaBH4, POL-AURA, Zawroty, Poland, CAS: 16940-66-2), indigo carmine (IC, POCH, Gliwice, Poland, CAS: 860-22-0), fuchsine (FC, Michrome, London, UK).

3.2. Synthesis

The cadmium-based powders were synthesized via a two-step synthesis process: initial dissolution of precursors at a moderate temperature (60 °C), followed by hydrothermal treatment in a Teflon-lined autoclave at 160 °C for 24 h. Briefly, CdSe was prepared by mixing 0.5 mol of NaBH4 and 0.5 mol of selenium powder in 1.0 M NaOH at 60 °C under continuous stirring for 1 h, until the selenium was completely dissolved. Subsequently, 0.5 mol of cadmium sulfate was added dropwise. The resulting solution was then transferred to a Teflon-lined autoclave (LPP Equipment Sp. z o.o., Święcice, Poland) and maintained at 160 °C for 24 h.
CdS was synthesized by mixing 0.5 mol of 3CdSO4·8H2O and 0.5 mol of thioacetamide (TAA) in 100 mL of distilled water at 60 °C for 1 h, followed by hydrothermal treatment in an autoclave at 160 °C for 24 h.
The composites were obtained by combining both precursor solutions in specific proportions. The resulting mixtures were subjected to hydrothermal treatment under the same conditions. A summary of the chemical compositions and synthesis parameters is presented in Table 2.

3.3. Materials Characterization

The sample’s morphology was examined using scanning electron microscopy (SEM, ThermoFisher Apreo 2, Waltham, MA, USA).
The crystal structure of the obtained materials was analyzed by X’Pert MOD diffractometer (Malvern Panalytical Ltd., Malvern, UK). Phase identification was performed using X’Pert High Score Plus software (version 3.0.4) and the Powder Diffraction File (PDF—2).
The optical properties of the prepared materials were analyzed using a double-beam Jasco UV-ViS-NIR V-670 spectrophotometer (Jasco, Oklahoma City, OK, USA) with a 150 mm integrating sphere. The band gap values were determined from the derivative of diffuse reflectance spectra and calculated as in our previous work [78].
The electrophoretic light scattering (ELS) technique was used to characterize zeta potentials. The values were determined using Zetasizer Pro (Malvern Panalytical Ltd., Malvern, UK).
The Raman studies were performed using a WITec Alpha 300 M+ spectrometer (WITec GmbH, Ulm, Germany) equipped with a 100× objective, a 785 nm laser, and a 1800 grating. Data processing was carried out using WITec FIVE Plus software (v.5.1).

3.4. Photocatalytic Analysis

The photocatalytic experiments were conducted in a photoreactor (MSDM, Krakow, Poland), equipped with an LED lamp (light temperature: 6750 K). For each measurement, photocatalyst (0.125 g of cadmium pigments) was dispersed in the dye solution (5 × 10−5 mol/L, 100 mL). The suspensions were continuously stirred for 30 min in the dark at room temperature to establish adsorption–desorption equilibrium. After that time, the light was turned on. At given time intervals, the suspensions were collected (4 mL), filtered, and analyzed using a Biotech Vis-77200 spectrophotometer (A-BioTech, Wrocław, Poland).
The photocatalytic tests with the scavengers were performed alongside the typical photodegradation analysis; however, before illumination, 1.5 mL of 10 mM scavenger solution was added to the system, and then stirred for 1 min and placed in a photoreactor. The amount of the photodegraded dye from the solution (q) was calculated according to the following equation:
q = (C0 − Ct)VMd
Here, C0 is the initial dye concentration, Ct is the dye concentration at time t (min), V is the volume of the solution, and Md is the molar weight of the dye. p-benzoquinone, methanol, K2Cr2O7 and EDTA-Na were used as the scavenger of the O2•−, SO4•−/•OH, e’ and h+.
HOMO and LUMO energy levels of IC were analyzed by cyclic voltammetry using ferrocene as an internal standard. A detailed description of the method was provided in our previous works [71,75]. Briefly, tetrabutylammonium hexafluorophosphate (0.1 M TBAF6) dissolved in acetonitrile was employed as the supporting electrolyte. At the end of the dye measurements (1 mM, 15 mL solution), 3 mg of ferrocene was added. The electrochemical analysis was performed with a Gamry1010E potentiostat (Gamry Instruments, Warminster, PA, USA) in a three-electrode configuration, using Ag/AgCl as the reference electrode, Pt wire as the counter electrode, and a glassy carbon electrode (GCE) as the working electrode. Cyclic voltammograms were recorded in the potential range of −1400 to 1400 mV with scan rates between 10 and 200 mV s−1. Based on the peak positions and the bandgap derived from UV-Vis spectroscopy, the HOMO and LUMO levels were determined.
The energy levels of the valence band (EVB) and conduction band (ECB) edges of the semiconductors were calculated according to the following equations [79]:
ECB = −χ(CdX) + 0.5 Eg − 2.303RT/F(IEP − pH)
EVB = ECB − Eg
χ(CdX) = (χ(Cd) χ(X))
Here, CdX denotes CdS or CdSe, Eg corresponds to the bandgap energy, R is the universal gas constant, T is the absolute temperature, F is the Faraday constant, IEP is the isoelectric point, and χ(Cd), χ(X) represent the Mulliken electronegativity of Cd and X, calculated using Equation (8).

4. Conclusions

This study presents a comprehensive analysis of cadmium-based pigments CdS, CdSe, and their solid solutions CdS1−xSex as materials situated at the intersection of cultural heritage and photocatalytic science. The structural and spectroscopic investigations, including XRD and Raman analyses, confirmed the successful formation of solid solutions with progressive selenium substitution, leading to observable changes in lattice parameters, vibrational modes, and grain morphology. These modifications correlated directly with the optical properties of the materials, particularly in terms of the band gap narrowing and red-shifting of light absorption, enabling the controlled tuning of the photocatalytic response.
Photocatalytic tests using indigo carmine and fuchsine, organic dyes with historical relevance, revealed material-dependent degradation efficiencies. Pure CdS and CdSe exhibited the highest degradation rates, while the CdS1−xSex powders showed intermediate behaviors due to the coexistence of multiple active phases and charge transfer pathways. Kinetic modeling based on pseudo-second-order reactions confirms that surface-mediated interactions are critical for efficient dye degradation. Electrostatic attraction and surface affinity, reflected in zeta potential shifts, further emphasize the pivotal role of pigment-dye interactions. Scavenger experiments have revealed the key role of electrons and superoxide radicals in driving the photodegradation reactions, although their relative contributions varied depending on the dye and pigment composition. Significantly, materials with slower photocatalytic decomposition rates, particularly CdS1−xSex with intermediate compositions, exhibited the reduced generation of reactive species. From the perspective of cultural heritage conservation, this property may be advantageous, as it limits the risk of unintentional photochemical damage to artworks and decorative objects containing cadmium pigments. The results contribute to a deeper understanding of how pigment composition can be engineered not only for color performance, but also for environmental responsiveness and long-term material stability. Therefore, these cadmium-based composites can be regarded not only as colorants but as functionally stable pigment candidates suitable for long-term use in cultural heritage applications.
This interdisciplinary investigation bridges material science and conservation chemistry by establishing a link between pigment composition, structural attributes, photocatalytic performance, and implications for heritage preservation. The findings underscore the potential of tailoring semiconductor pigments with tunable band structures to balance color functionality and chemical stability in both applied and historical contexts.

Author Contributions

Conceptualization, A.M.K.; methodology, A.M.K.; validation, A.M.K.; formal analysis, J.Ł.; investigation, J.Ł.; resources, A.M.K.; data curation, J.Ł.; writing—original draft preparation, A.M.K.; writing—review and editing, A.M.K.; visualization, A.M.K.; supervision, A.M.K.; project administration, A.M.K.; funding acquisition, A.M.K. All authors have read and agreed to the published version of the manuscript.

Funding

A.M.K. acknowledges the financial support of the Polish Ministry of Education and Science within the framework of Subvention for Science 2025.

Data Availability Statement

The dataset is available on request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The SEM images and photographs of the obtained cadmium pigment powder with different molar ratios of CdSe to CdS.
Figure 1. The SEM images and photographs of the obtained cadmium pigment powder with different molar ratios of CdSe to CdS.
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Figure 2. (a) XRD pattern and (b) Raman spectra of the obtained cadmium pigments of CdS, CdSe, and CdS1−xS powders. The green symbols correspond to the selenium diffraction patterns.
Figure 2. (a) XRD pattern and (b) Raman spectra of the obtained cadmium pigments of CdS, CdSe, and CdS1−xS powders. The green symbols correspond to the selenium diffraction patterns.
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Figure 3. (a) Optical spectra of the obtained cadmium pigments with the dRdiff/dλ analysis (insert), and (b) analysis of the bandgap energy using the Kubelka–Munk function.
Figure 3. (a) Optical spectra of the obtained cadmium pigments with the dRdiff/dλ analysis (insert), and (b) analysis of the bandgap energy using the Kubelka–Munk function.
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Figure 4. Photocatalytic activity of the materials obtained, determined by (a,d) the amount of photodecomposed dye (µg) and the (b,c,e,f) analysis of the mechanism determining the catalytic process using the Lagergren kinetics model for (a–c) indigo carmine and (d–f) fuchsine (powder concentration: 1.25 g L−1, dyes concentration 5 × 10−5 mol L−1). The blue and pink squares indicate the stabilization of the adsorption–desorption equilibrium between the dye and the powder.
Figure 4. Photocatalytic activity of the materials obtained, determined by (a,d) the amount of photodecomposed dye (µg) and the (b,c,e,f) analysis of the mechanism determining the catalytic process using the Lagergren kinetics model for (a–c) indigo carmine and (d–f) fuchsine (powder concentration: 1.25 g L−1, dyes concentration 5 × 10−5 mol L−1). The blue and pink squares indicate the stabilization of the adsorption–desorption equilibrium between the dye and the powder.
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Figure 5. (a) The pseudo-second-rate constant for various dyes following chemical composition and (b) the changes of the value of the zeta potential depending on the solution and the used powder. The arrows indicate the directions of change.
Figure 5. (a) The pseudo-second-rate constant for various dyes following chemical composition and (b) the changes of the value of the zeta potential depending on the solution and the used powder. The arrows indicate the directions of change.
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Figure 6. Summary of the scavenger test, comparison of the results obtained for the selected powders with the selected scavenger in (a) indigo carmine and (b) fuchsine after 30 min reaction. The dye concentration reflects the amount of dye remaining in the solution to be decomposed. The dashed red line represents the recorded changes in dye concentration in the absence of the scavenger.
Figure 6. Summary of the scavenger test, comparison of the results obtained for the selected powders with the selected scavenger in (a) indigo carmine and (b) fuchsine after 30 min reaction. The dye concentration reflects the amount of dye remaining in the solution to be decomposed. The dashed red line represents the recorded changes in dye concentration in the absence of the scavenger.
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Figure 7. (a) Calculated valence and conduction band edge levels of CdS and CdSe materials (pH = 7), with experimentally determined energies of the dye HOMO and LUMO levels for indigo carmine, with probable pathways of photocatalytic degradation of dye for (b) CdS and (c) CdSe.
Figure 7. (a) Calculated valence and conduction band edge levels of CdS and CdSe materials (pH = 7), with experimentally determined energies of the dye HOMO and LUMO levels for indigo carmine, with probable pathways of photocatalytic degradation of dye for (b) CdS and (c) CdSe.
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Figure 8. (a) Calculated valence and conduction band edge levels of CdS and CdSe materials (pH = 7), with energies of the dye’s HOMO and LUMO levels for fuchsine, with probable pathways of photocatalytic degradation of dye for (b) CdS and (c) CdSe.
Figure 8. (a) Calculated valence and conduction band edge levels of CdS and CdSe materials (pH = 7), with energies of the dye’s HOMO and LUMO levels for fuchsine, with probable pathways of photocatalytic degradation of dye for (b) CdS and (c) CdSe.
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Table 1. Phase composition and refined cell parameters.
Table 1. Phase composition and refined cell parameters.
SampleCell Parameters (Å)
CdS (C)CdS (H)CdSSeCdSe (C)CdSe (H)Se
CdSe---a = b = c = 6.08806a = b = 4.14255
c = 6.37272		a = b = 3.89157
c = 6.5.18936
50 mol% CdS--a = b = 4.20701
c = 6.92681		a = b = c = 5.96577--
75 mol% CdSa = b = c = 5.86438a = b = 4.16639
c = 6.79831		-a = b = c = 5.91300--
90 mol% CdSa = b = c = 5.85225a = b = 4.15547
c = 6.76906		----
CdSa = b = c = 5.84177a = b = 4.14255
c = 6.73272		----
C, cubic structure; H, hexagonal structure.
Table 2. Summary of the chemical compositions and synthesis parameters.
Table 2. Summary of the chemical compositions and synthesis parameters.
SampleSolution A—CompositionSolution B—CompositionTimeTemperature
1M NaOHNaBH4SeCd2+ *H2OTAACd2+ *(h)(°C)
CdSe100 mL0.5 mol0.5 mol0.5 mol---24160
50 mol% CdS50 mL500.5 mol0.5 mol
75 mol% CdS25 mL75
90 mol% CdS10 mL90
CdS----100
* 3CdSO4·8H2O.
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MDPI and ACS Style

Łacic, J.; Kusior, A.M. From Pigment to Photocatalyst: CdSe/CdS Solutions Mimicking Cadmium Red for Visible-Light Dye Degradation. Catalysts 2025, 15, 883. https://doi.org/10.3390/catal15090883

AMA Style

Łacic J, Kusior AM. From Pigment to Photocatalyst: CdSe/CdS Solutions Mimicking Cadmium Red for Visible-Light Dye Degradation. Catalysts. 2025; 15(9):883. https://doi.org/10.3390/catal15090883

Chicago/Turabian Style

Łacic, Julia, and Anna Magdalena Kusior. 2025. "From Pigment to Photocatalyst: CdSe/CdS Solutions Mimicking Cadmium Red for Visible-Light Dye Degradation" Catalysts 15, no. 9: 883. https://doi.org/10.3390/catal15090883

APA Style

Łacic, J., & Kusior, A. M. (2025). From Pigment to Photocatalyst: CdSe/CdS Solutions Mimicking Cadmium Red for Visible-Light Dye Degradation. Catalysts, 15(9), 883. https://doi.org/10.3390/catal15090883

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