Optical, Electrical and Photocatalytic Properties of the Ternary Semiconductors Zn_xCd_1−xS, Cu_xCd_1−xS and Cu_xZn_1−xS

1. Introduction

Current materials science requires the creation of new, simple, and low-cost ternary and quaternary semiconductor materials with controllable chemical and physical properties. The chalcogenides have received attention for their ability to relatively easily form binary, ternary, and quaternary compounds. The physical and chemical properties of these compounds principally depend on their compositions.

The chalcogenides have many applications, including photodetectors, photovoltaic devices, optical coatings, electro-optic modulators, field-effect transistors, sensors, transducers, light sources and lasers, and photophysical and photocatalytic applications [1–16]. Ternary and quaternary compounds can easily be obtained from solid-state reactions of bilayers of binary chalcogenides because of the unique physical and chemical properties of these species, including the ionic radii of Cd²⁺, Zn²⁺, Cu²⁺, and Cu¹⁺, which are 103, 83, 72, and 96 pm, respectively [17]. Additionally, wide solubility ranges can be expected for zinc and copper ions in CdS and CuS in films of ternary Zn_xCd_1−xS, Cu_xCd_1−xS, and Cu_xZn_1−xS compounds. Zn and Cu ions can fill substitutional or interstitial sites in the lattices of CdS and CuS [18].

2. Experimental Methods

3. Results and Discussion

Figure 1 shows SEM images of the ternary semiconductor compounds Zn_xCd_1−xS, Cu_xCd_1−xS, and Cu_xZn_1−xS annealed under vacuum at 250°C and 450°C. All films covered the substrate surface and were free of pinholes. For films of the semiconductor compound Zn_xCd_1−xS annealed at 250°C, the superficial morphology was composed of agglomerates approximately 160 nm in diameter with a well-defined spherical shape (Figure 1(a)). The spheres contained small particles measuring 8.5 nm. The spherical shapes of the agglomerates were lost in films annealed at 450°C and nanocracks are evident on the surface of these films. The Cu_xCd_1−xS thin films annealed at 250°C were composed of irregular agglomerates with approximately circular, asymmetric shapes, and a distribution of sizes and both small particles (Figure 1(c)) and nanocracks were evident. At an annealing temperature of 450°C, the agglomerates exhibited better-defined shapes and stacking faults (Figure 1(d)). The thin films of the Zn_xCu_1−xS semiconductor demonstrated a superficially amorphous nature (Figures 1(e) and 1(f)).

Table 1 shows the EDS analysis results of the three ternary compounds at the vacuum annealing temperatures of 250 and 450°C. The compounds containing cadmium are generally rich in cadmium at low temperature (250°C); however, at high temperature, where diffusion is favored, the cadmium concentration diminishes and, consequently, the respective concentrations of zinc and copper increase (Zn_xCd_1−xS and Cu_xCd_1−xS). The compound Cu_xZn_1−xS is rich in zinc at low temperature. The copper concentration increases at high temperature.

Figures 2(a) and 2(b) display the XRD results obtained over a 2θ range of 15 to 60 degrees for the ternary semiconductor alloys Cu_xCd_1−xS, Cu_xZn_1−xS, and Zn_xCd_1−xS annealed under vacuum. For Figure 2(a) in all cases, the thin films were amorphous independent of the annealing temperature. At 250°C, the samples were most amorphous, and at 450°C, small peaks appeared. Figure 2(a) (A) shows the XRD results obtained for thin films of Cu_xCd_1−xS, the patterns show two peaks at 44.28° and 52.44° and at 43.04° and 47.8° for films annealed under vacuum at 250°C and 450°C, respectively. Figure 2(a) (B) shows the XRD results obtained for nanocrystalline thin films of Cu_xZn_1−xS, which indicate a single one peak at 43.26° and 45.16° for films annealed under vacuum at 250°C and 450°C, respectively. The small peaks made it difficult to identify the phase.

As shown in Figure 2(b) (A), at 250–450°C Zn_xCd_1−xS shows peaks at 25.13°, 26.8°, 44.2°, and 52.4°, corresponding to the hexagonal wurtzite planes of (100), (002), (110), and (201) in thin films of the standard PDF Card 00-049-1302. Figure 2(b) (B) shows that the orientation of the films is in the plane (002), with the increase of annealing temperature, these peaks move to higher diffraction angle, indicative of an increase of Zn content in the films. The grains size was calculated from Scherrer’s equation for the peaks (002) as a function of the annealing temperature and varied for 16.316 nm, 19.95 nm, 19.43 nm, and 19.50 nm, respectively, for 250, 300, 350, and 400°C.

Figures 3(a), 3(b), and 3(c), respectively, show the UV-Vis transmission spectra as a function of annealing temperature for Zn_xCd_1−xS, Cu_xCd_1−xS, and Cu_xZn_1−xS films annealed under vacuum. The Zn_xCd_1−xS films exhibited high transmittances > 80% over an annealing temperature range of 250°C to 450°C. For the Cu_xCd_1−xS thin films, the transmittance was approximately 65%; higher transmittance was observed for the films annealed under vacuum at 300°C than for those annealed at 250°C. The Cu_xZn_1−xS thin films showed the lowest transmittance values of 35 to 50%; the highest transmittance was observed for the films annealed at 350°C and the lowest for those annealed at 300°C. The optical band gap, E_g, of each film was calculated using the Tauc equation by plotting (αhν)² as a function of hν, where α is the optical absorption coefficient and hν is the photon energy. In the graphs, the linear portion of the curve was extrapolated to (αhν)² = 0. The band gap is shown as a function of the annealing temperature in Figure 4. For Zn_xCd_1−xS, the E_g values decreased from 2.83 to 2.5 eV as a function of the vacuum annealing temperature. For the Cu_xCd_1−xS thin films, the E_g values decreased from 2.43 to 2.28 eV by increasing the annealing temperature. The Zn_xCu_1−xS thin films showed E_g values of 2.80 to 2.65 eV, with the increase of the annealing temperature. The E_g values are in agreement with the values reported by other research groups [21–23]. In general, the band gap decreases as the annealing temperature increases; this effect may be caused by both the greater diffusion between the bilayers and the formation of the ternary alloy.

Figure 5 shows the resistivity values of the ternary semiconductor compounds. The thin films of Zn_xCd_1−xS were highly resistive independent of thermal annealing; the resistivity values were greater than 10⁶ Ω·cm (results not shown). Figure 5 displays the resistivity values of the Cu_xCd_1−xS and Cu_xZn_1−xS films annealed under vacuum as a function of the annealing temperature. The values for Cu_xCd_1−xS decreased as the annealing temperature increased. The minimum resistivity occurred at 300°C (2.74 × 10⁻³ Ω·cm), above this temperature, the resistivity value increased. The resistivity values for the Cu_xZn_1−xS samples decreased as the annealing temperature increased. The lowest value was obtained at 350°C (4.11 × 10⁻⁴ Ω·cm) and, for temperatures above 350°C, the resistivity values increased.

The change of resistivity values with the change of the annealing temperatures could be attributed to the diffusion of copper ions on the matrix of CdS and ZnS, with the temperature increase and the presence of nanocracks on the surface of the films at elevated temperatures. The ionic radii of Cd²⁺, Zn²⁺, and Cu²⁺, are 103, 83, and 72 pm, respectively [17], on the other hand, the copper ions could be introduced more effectively with the increase of the temperature until the saturation, and possibly the segregation phenomena takes place at different temperatures (300 and 350°C for Cu_xCd_1−xS and Cu_xZn_1−xS, resp.). Copper ions are closer to the ionic radii of Zn²⁺ than Cd²⁺. Additionally, copper ions could be introduced more easily in the ZnS lattice occupied substitutional or interstitial sites in the lattices of ZnS than in CdS [18].

To demonstrate the prospective photocatalytic applications of the fabricated materials, the thin films were used as catalysts in the degradation of methylene blue (MB) under germicidal irradiation (λ = 252 nm) at room temperature. Figure 6 displays the normalized concentration of MB in aqueous solution as a function of the vacuum annealing temperature for the three different semiconductor alloys. The concentration was quantified based on the absorption at 653 nm.

The absorption is time-dependent, initially, the irradiation time was held constant at 3 h for all samples. The ternary semiconductor samples generally showed photocatalytic behavior. The sample with the best photobleaching performance was Zn_xCd_1−xS annealed at 300°C (see Figure 6).

As indicated in Figure 6, the best samples were Zn_xCd_1−xS and Cu_xCd_1−xS annealed at 300°C and 250°C, respectively. The temporal dependence of the photobleaching of MB by these samples was analyzed. Figure 7(a) shows the normalized MB concentration as a function of time in hours. The Zn_xCd_1−xS film demonstrated that it is an excellent candidate for photocatalytic applications. In 5 h, the mineralization of MB was nearly complete (82%) for the thin films of Zn_xCd_1−xS annealed under vacuum at 300°C. This can be attributed to the synergistic effect of thermal annealing, better crystallinity, and concentration of the Zn mole fraction to 0.3 and 0.33 for 250 and 300°C, respectively; similar results were found by an optimal Zn incorporation corresponding to x approximately 0.2–0.33 [24–27]. Additionally these results are comparable to the photocatalytic activities reported in the literature for thin films of other materials, such as ZnO and TiO₂, which have been shown to be excellent catalyzers [28–31].

The thin films of Zn_xCd_1−xS were obtained by chemical bath deposition and were annealed under vacuum at 300°C. Obtaining this film is very economical compared to the preparation of films of ZnO (sol-gel, sputtering at T ≥ 500°C) and TiO₂ (sol-gel, sputtering at T ≥ 450°C). Additionally, our films have high transmittances (≥85%) and band gaps (2.5 to 2.83 eV) closer to the absorption spectrum of visible light than gaps larger than 3.2 eV (ZnO and TiO₂).

For Cu_xCd_1−xS annealed at 250°C, the extent of mineralization after 5 h was approximately 50%. Figure 7(b) displays the absorption spectra of an aqueous solution of MB at different intervals in the presence of a sample of Zn_xCd_1−xS annealed at 300°C. The concentration was quantified based on the absorption at 653 nm. The absorption was time-dependent and the complete mineralization of MB was nearly reached after 5 h under the experimental conditions described above.

4. Conclusions

Simple and economical syntheses of ternary semiconductor alloys of Zn_xCd_1−xS, Cu_xCd_1−xS, and Cu_xZn_1−xS were obtained by the chemical bath deposition and subsequent annealing of stacked bilayers of binary semiconductor films. The three compounds showed good optical properties, superficial morphology, and photocatalytic activity. The band gap of the Cu_xCd_1−xS and Cu_xZn_1−xS samples was tuned from 2.3 to 2.85 eV. These samples demonstrated the lowest resistivity values of 2.74 × 10⁻³ and 4.11 × 10⁻⁴ Ω·cm, respectively, among all of the films that were fabricated. Extensive photodegradation of methylene blue was observed for the Zn_xCd_1−xS film annealed at 300°C in vacuum; this activity is comparable to that reported in the literature for thin films of ZnO and TiO₂.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.