1 Introduction

Nowadays, the promising and green photocatalysis technology-based semiconductors have capture tremendous concern because it can solve the problems of energy shortage and environmental pollution [1]. Various semiconductor materials have been widely applied on account of their high-light-harvesting efficiency, fast mobility of charge carriers, and various morphologies to photodegrade the pollutants [2,3,4,5,6,7,8,9,10,11,12]. Nevertheless, there remain some crucial deficiencies of many traditional catalysts, including high recombination rate of photogenerated carriers, limited visible-light response, and insufficient specific surface area [13,14,15], which limits their development and wide application. Therefore, development of novel, cost-effective, and highly efficient visible-light photocatalyst are important for practical applications.

Compared with reported ultraviolet-light photocatalysts, CdS is regarded as a remarkable photocatalyst and shows an outstanding potential in various photocatalytic process. CdS has unique energy levels and a narrow band gap about 2.4 eV, which can effectively make use of visible light to degrade organic pollutants [16,17,18]. However, the application of CdS is restricted because there are some drawbacks associated with CdS, namely, high recombination rate of photogenerated charge carriers and severe photocorrosion when irradiated by visible light [19]. As reported, to enhance the photocatalytic efficiency of CdS, several feasible methods have been introduced, such as, doping or semiconductor coupling to tune the band gap width and suppress photocorrosion of CdS [20,21,22,23]. Regrettably, to the best of our knowledge, satisfactory photocatalytic efficiency based on CdS has not been achieved until now. As a consequence, it is very much requisite to find a successful strategy to improve the photocatalytic activity of CdS.

As is well known, the photocatalytic activity of nanomaterials is strongly dependent on its size, shape, and morphology [24, 25]. Herein, it is of great importance for the formation of CdS with controllable morphology and structure to achieve excellent photocatalytic activity. CdS films have been deposited by a variety of physical and chemical techniques including close spaced sublimation method, electrodeposition, magnetron sputtering, chemical bath deposition method, successive ionic layer adsorption and reaction, and homogeneous precipitation method [26,27,28,29,30,31,32,33,34,35]. Compared with the above methods, homogeneous precipitation method is an efficient technique to control the dimensions and size of CdS nanoparticle. In addition, annealing atmosphere also plays a crucial role in constructing definite physicochemical properties of final products. CdS films with different shapes/sizes or colors can be produced by changing the parameters for heat treatment [35,36,37]. However, it is worth noting that the comparative study of thermal annealing effects under various atmospheres on CdS films prepared by homogeneous precipitation method is very limited. Thus, it is highly desirable to explore the relationship between annealing atmosphere and catalytic performance of CdS films.

Herein, in this study, CdS films annealed under various atmospheres are successfully fabricated and then used for the degradation of MB. The effects of annealing atmosphere on the structure, morphology, UV–Vis absorption, and photoelectrochemical properties of CdS films are thoroughly characterized by various analysis techniques. The results show that annealing atmosphere plays a role in the structure, morphology, UV–Vis absorption, and photoelectrochemical properties of CdS films, which greatly influences the photocatalytic activity of CdS films. Under the irradiation of visible light, the obtained N2 annealed CdS films exhibit excellent catalytic activity and stability in photocatalytic degradation. This work may provide a new strategy for the design of highly photoactive CdS nanomaterials under visible-light irradiation.

2 Experimental section

2.1 Materials preparation

CdS films are prepared on FTO using a homogeneous precipitation method [38]. Cadmium chloride, urea, thiourea, and deionized water are used for the preparation of CdS films. The deposition temperature is set at 90 °C for 40 min. After completion of the reaction, the as-deposited films are rinsed thoroughly with deionized water for several times to remove the loosely adhered CdS particles. During annealing process, vacuum, N2, and air atmosphere with different oxygen content and activity in chemistry are conducted. It is identified that N2 is a relatively inert gas in comparison with vacuum and air, which can be used to reduce the oxygen content during annealing process. In contrast, the air has a high oxygen content. The vacuum is an active gas with strong reducibility, which may react with the as-prepared CdS films. Therefore, the samples are annealed in vacuum, N2, and air atmosphere at 400 °C for 20 min, respectively.

2.2 Characterization

The morphologies and element distributions of synthesized samples are measured by the field emission scanning electron microscope (FESEM, JEOL JSM-6700F) and energy-dispersive X-ray spectroscopy (EDX). The crystal structure of samples is characterized by X-ray diffraction (XRD, D/max-2500/PC) with Cu Kα radiation (λ = 1.5418 Å). The photoluminescence (PL) spectra of samples are conducted on a Perkinelmer LS55 fluorescence spectrometer. The UV–Vis absorption spectra are recorded on a UV-3150 double-beam spectrophotometer in the region of 350–600 nm. The Brunauer–Emmett–Teller (BET) measurement is performed via nitrogen (N2) adsorption–desorption apparatus (BELSORP-max). Photoelectrochemical activity of samples is carried out on an electrochemical workstation (CHI660E) using a standard three-electrode system. CdS films, Pt wire, and Ag|AgCl|KCl(sat) are used as working, counter and reference electrode, respectively. The electrolyte contains a mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution. The photocurrent density–voltage (J–V) characteristics and transient photocurrent responses of samples are measured under AM 1.5 illumination provided by a CEL-S500 simulator at 100 mW cm−2. The electrochemical impedance spectroscopy (EIS) measurements are performed at the open circuit potential (− 0.8 V). The photocatalytic performance of CdS films is evaluated by the degradation of MB under visible-light irradiation. In a typical operation, the samples are firstly placed into 200 mL MB aqueous solution (5 mg/L), and remain still in darkness for 15 min, and are then exposed to light from a 500 W Xe lamp equipped with a 420 nm cut-off filter. During the irradiation, 5 mL of the reaction solution is collected at particular intervals. The degradation efficiency of MB is determined by the UV–Vis absorption spectroscopy. The total organic carbon (TOC) analysis of the reaction solution is performed with the Multi N/C 3100 analyzer.

3 Results and discussion

The morphology and elemental composition of CdS films are confirmed by FESEM and EDX, as presented in Fig. 1a–c. It can be observed that after 40 min deposition, a dense and pinhole-free CdS film with uniform nanoparticles is formed on the substrate (Fig. 1a). The thickness of CdS film is about 200 nm (Fig. 1b). The as-prepared films consist of cadmium and sulfur with a molar ratio of 1:1.07, indicating that the films prepared by a homogeneous precipitation method possess proper stoichiometry (Fig. 1c). In addition, during the experimental process, we find that the thickness of CdS film has a direct influence on its properties. CdS films with different thickness are prepared by controlling the corresponding reaction time. From Fig. 2, it can be observed that with the increase in CdS film thickness, the crystallinity of the product improves. The film deposited for 20 min has poor crystallinity, which can be attributed to its uneven morphology (shown in Fig. 3). Significantly, although the film prepared for 60 min shows better crystallinity, the film exhibits increased series resistance in comparison with the film prepared for 40 min (shown in Fig. 4), which can be attributed to its larger thickness (shown in Fig. 3). Hence, in this paper, CdS films deposited for 40 min are used for further investigation.

Fig. 1
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a Top-view and b cross-sectional FESEM images of CdS film; c EDX spectrum of CdS film

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Fig. 2
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XRD patterns of CdS thin films prepared at different time: a 20 min, b 40 min, and c 60 min, respectively

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Fig. 3
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FESEM images of CdS thin film prepared at a 20 min and b 60 min, respectively

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Fig. 4
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EIS of CdS thin films prepared at a 40 min and b 60 min, respectively

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Figure 5 depicts the XRD patterns of CdS films annealed in vacuum, N2, and air. As shown, besides the diffraction peaks of FTO substrate, all the diffraction peaks can be aligned to hexagonal CdS structure (JCPDF, no. 80-6). This XRD results confirm that the environment of annealing treatment shows no significant effect on the crystal structure of CdS. However, it is noteworthy that after air-annealing treatment, an additional peak corresponding to CdO can be observed (Fig. 5c). The formation of CdO can be attributed to the oxidation of CdS when the film is annealed in air atmosphere. Besides, the XRD peaks of N2 annealed CdS films become narrower, suggesting an improved crystalline quality. Based on Debye–Scherrer’s formula, the average crystallite size (D) of vacuum and N2 annealed CdS film is calculated: \(D = \frac{k\lambda }{{\beta (hkl){ \cos }\theta }}.\) In this formula, shape factor (\(k\)) is 0.9, \(\lambda\) is the wavelength of the X-ray source, \(\beta (hkl)\) is the “full width at half maximum”, and \(\theta\) is the diffracting angle. The average crystallite sizes for vacuum and N2 annealed CdS films are found to be approximately 80 nm and 180 nm, respectively. Thus, it can be concluded that the enhanced crystallinity of N2 annealed CdS films can be attributed to its improved crystallite size. Additionally, to further examine the effect of annealing atmospheres on the crystallinity of samples in details, PL spectra of CdS films are employed at room temperature. Generally speaking, the film with better crystallinity suggests less recombination of photogenerated carriers, thus leading to a lower PL intensity. As depicted in Fig. 6, PL intensity of the N2 annealed CdS films decreases significantly. Therefore, the PL result combining with XRD result indicates CdS film annealed in N2 has better crystallinity, which can be attributed to be one of the reasons for its superior photocatalytic activity.

Fig. 5
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XRD patterns of CdS thin films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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Fig. 6
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PL spectra of CdS thin films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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FESEM images are obtained to investigate the influence of annealing atmospheres on the morphologies of as-synthesized CdS films (Fig. 7a–c). Obviously, annealing atmospheres significantly affect the surface morphologies of films. As shown in Fig. 7a, a large number of small packed particles can be observed on the surface of the film annealed in vacuum, and these particles are connected tightly and have a clear boundary between each. When the film is annealed in N2, apparently, many small packed particles disappear, and the surface of CdS film becomes much smoother and more uniform (Fig. 7b). This different phenomenon may be explained as follows. In fact, the reorganization of the film occurs at all annealing process. The recrystallization process may densify the film and some of the small grains coalesce together to form a continuous film with few grain boundaries [39, 40]. Nevertheless, when the film is annealed in vacuum atmosphere, besides reorganization of the film, some of the sulfur will evaporate from the film [41, 42]. This effect leaves larger unfilled inter-granular volume on the surface of CdS film. Therefore, an entirely different microstructure and morphology of CdS film can be created. Similar change in surface features can also occur when the film is annealed in air (Fig. 7c). The grain size of CdS nanoparticles increases, but the interval between the particles becomes much wider, and some pinholes can be observed. More importantly, as shown from the low magnification image (the inset in Fig. 7c), the heat treatment under air atmosphere results in some cubes on the surface of CdS films. EDX data are obtained to further confirm the composition of these cubes, as illustrated in Fig. 7d. Notably, the EDX spectrum describes the presence of cadmium and oxygen, indicating that the composition of this structure is CdO. This result is also consistent with the aforementioned analysis based on the XRD measurement (Fig. 5). The formation of CdO can be attributed to its lower requirement for activation energy in comparison with that of sulfur oxides formation during annealing process. Hence, when CdS film is annealed in air, the formation of CdO on the surface of films can be observed. Similar phenomenon has also been observed by other researchers [43, 44]. In addition, to verify the likely uptake and influence of residual gas or ambient gas on the studied samples, the cross-sectional elemental mapping images of CdS film annealed in N2 is also exhibited in the inset of Fig. 7b. It is clear that the elements of cadmium and sulfur are well distributed, and no other elements such as C, N, H, and O appear, suggesting the high purity of the film. Indeed, during heat treatment process, a range of different reactions such as oxygen absorption or desorption, creation and annihilation of vacancy–interstitial pairs of cadmium ions, and sulfur evaporation will occur on the surface of CdS film at various atmospheres. As for CdS film, the structure and morphology changes may be ascribed to the different evaporation of sulfur atoms from the CdS films under different annealing atmospheres [45]. A uniform dense CdS film with better structural quality can be obtained when annealed in N2.

Fig. 7
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FESEM images of CdS thin films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively; d EDX spectrum of CdO

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The N2 adsorption–desorption, BET surface area and pore size distribution are conducted to verify the specific surface area and pore structures of as-synthesized samples. Figure 8 shows the absorption–desorption isotherms of CdS films annealed in vacuum, N2, and air, respectively. As shown in Fig. 8, typical type-IV characteristics of the isotherms of various samples can be observed, suggesting the mesoporous existence. This can facilitate the adsorption of organic pollutants. The BET specific surface areas are calculated to be 20.409 m2 g−1, 21.749 m2 g−1 and 25.899 m2 g−1 for vacuum, N2 and air annealed CdS film, respectively. Among the series of samples, the air annealed CdS film exhibits the largest specific surface area, which can be attributed to the large cubes on the surface of films. This result also reveals that the excellent photocatalytic performance of N2 annealed CdS film can be due to its efficient electron transfer process. The pore size distribution of samples is calculated by using the Barrett–Joyner–Halenda (BJH) method according to the desorption branch. The estimated pore size of vacuum, N2, and air annealed CdS films is approximately 18.12 nm, 14.877 nm, and 15.855 nm, respectively. According to the results, it can be seen that all the samples have mesoporous structure, facilitating the diffusion of the organic pollutants.

Fig. 8
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Nitrogen adsorption–desorption isotherm of CdS films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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The influence of annealing atmosphere on the optical absorption performance of CdS film is examined using the UV–Vis spectrometer. As is shown in Fig. 9, compared to the air annealed CdS film, the absorption edge of CdS films annealed in N2 and vacuum obviously shifts to longer wavelength, and an obvious increase in absorbance of films can also be observed. These changes can be attributed to their better growth of the nanocrystalline grain and increased particle size after heat treatment. Among all the films, the N2 annealed CdS films exhibit the highest absorptivity, suggesting that when the sample is irradiated, it can produce many effective photogenerated carriers and further enhance the photocatalytic activity. The air annealed CdS films show a poor absorptivity, which can be related to the formation of CdO [42]. As discussed above, it can be concluded that annealing atmosphere plays a key role in the enhancement of crystallinity and optical absorption ability of CdS films, which makes CdS films potential application future in the degradation of pollutants.

Fig. 9
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UV–Vis absorption spectra of CdS films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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It is widely recognized that charge separation is a very important process for photodegradation of pollutants. Many photogenerated carriers should be transfer to the surface of sample, and then create a large number of free radicals. These generated radicals are good oxidizers to decompose the organic dye [46,47,48]. Hence, photocatalytic activity is strongly dependent on the separation and transfer efficiency of photogenerated carriers in the catalysts [49]. In this work, the J–V characteristics, photo-current tests, and EIS measurements are measured to get more insight into the interface charge transfer [50, 51]. Shown in Fig. 10 are J–V curves of CdS films. It can be observed that the open circuit photopotential of all the samples is approximately the same, and the current density has remarkable differences. In particular, CdS films annealed in N2 exhibit the highest photocurrent with the value of 2.91 mA cm−2. The superior performance arises from its enhanced optical absorption, which can generate more photogenerated carriers between the interface of CdS films and the polysulfide electrolyte. In contrast, one significant decrease in photocurrent should be noted when the films are annealed in air, illustrating lower carrier-separation efficiency. This behavior can be attributed to the grain boundaries formed by CdO cubes. Figure 11a presents the photocurrent transient response for all the samples, which are obtained by several on–off treatment cycles of intermittent visible-light illumination. Obviously, under the visible-light irradiation, CdS films annealed in N2 reveal the best photocurrent response, suggesting that the films have superior separation efficiency of photogenerated carriers. In contrast, the transient photocurrent response of CdS films annealed in air is poor, indicating the appearance of some recombination centers of photogenerated carriers in films. These phenomenon can be explained by EIS measurement. As is shown in Fig. 11b, CdS films annealed in N2 show the smallest arc radius of the semicircle than the other two samples, suggesting the enhanced separation and transfer efficiency of photogenerated charge. For air annealed CdS films, the sample has a relatively large diameter over the semicircular Nyquist plot, that is, the resistance is higher during charge transfer process as compared to N2 annealed CdS films. Consequently, annealing atmosphere has a great influence on the separation and transfer of photogenerated carriers and finally may influence the photocatalytic performance of samples.

Fig. 10
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The J–V characteristics of CdS films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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Fig. 11
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a Transient photocurrent responses and b EIS of CdS films annealed in three different atmospheres: (a) vacuum, (b) N2, and (c) air, respectively

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To understand the effect of annealing atmosphere on the visible-light photocatalytic degradation activity of the as-prepared CdS films, the photocatalytic experiments are conducted. Figure 12a–c presents the changes in the absorption spectra of MB versus irradiation time with the presence of CdS films under visible-light irradiation. The typical progressive decrease in the peak intensity of MB at 664 nm (characteristic absorption peak for MB) is observed, which indicates the photodegradation of MB under visible-light illumination. Upon comparing the photocatalytic removal of MB, CdS films annealed in N2 have excellent photocatalytic degradation efficiency to MB than other samples, and the absorption at wavelength of 664 nm almost completely disappears after 60 min of irradiation. Moreover, no new absorption bands can be observed during the process of photodegradation, which confirm the complete photodegradation of the MB aqueous solution. In this work, C/C0 is calculated to evaluate the degradation rate. As presented in Fig. 12d, during the dark period, all the materials show strong adsorption because of their mesoporous structure, and the obtained N2 annealed CdS films show the highest adsorption ability, which is is beneficial for photodegradation of MB. When all the films are illuminated with visible light, for comparison, CdS films annealed in N2 show the highest photocatalytic degradation efficiency than others, and in the first 15 min, a sharp decrease in the concentration of MB can be indicated by the decrease in the intensity of absorption peaks (Fig. 12b). The removal rate of MB reaches maximum value of 99.98% after 60 min. The k values of MB degradation are also estimated. The values are 0.026/min, 0.044/min, and 0.012/min for the vacuum, N2 and air annealed CdS films, respectively. It can be seen that the N2 annealed CdS films exhibit the highest degradation rate constant. More importantly, the mineralization ability of the as-prepared CdS films is also studied. As is clearly depicted in Fig. 13, compared to other samples, the N2 annealed CdS films display excellent TOC mineralization capacity and the mineralization efficiency reaches about 69.81% within 60 min of reaction, indicating a promising application potential for removal of organics from contaminated water. This observation is also in accordance with the photocatalytic degradation results as shown in Fig. 12. The above results demonstrate that annealing conditions have a great influence on the photocatalytic activity of CdS films. The N2 annealed CdS films exhibit high photocatalytic activity, which could be attributed to the efficient visible-light utilization and high-efficiency charge separation and transfer in films. Therefore, appropriate annealing conditions can significantly improve the photoactivity of CdS, which can also be expected to be used for preparing other catalysts.

Fig. 12
figure 12

Absorption changes to an aqueous solution of MB in the presence of CdS films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively; d comparison of the photocatalytic activity of CdS films for the photocatalytic decomposition of MB under visible-light irradiation

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Fig. 13
figure 13

TOC removal ratio of CdS films annealed in three different atmospheres: a vacuum, b N2, and c air, respectively

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Besides activity, stability is also a vital parameter for practical application of photocatalysts. The stability of as-prepared CdS films is investigated by five repeated MB degradation experiments, as depicted in Fig. 14a. As compared with the first time degradation ratio, the photocatalytic activity of the CdS films annealed in N2 still maintains a high level even after using for five cycles. This result describes favorable recycling activity of CdS film annealed in N2. For CdS films annealed in vacuum, the photocatalytic activity of the sample is reduced slightly by recycling photocatalytic reactions, and a significant change in color of the sample can be observed, suggesting that the sample is destroyed during the degradation of MB (Fig. 14b). Similarly, for the air annealed CdS films, the rate of MB degradation decreases significantly and CdS films clearly peel-off from the substrate after five degradation cycles, as shown in Fig. 14b. This phenomenon can be attributed to the photocorrosion of CdS [52] during the reaction. Meanwhile, to confirm the stability of the photocatalyst, XRD patterns of fresh and used N2 annealed CdS films (after five cycles) are shown in Fig. 15. One can notice that no change in crystal planes can be observed of N2 annealed CdS films before and after five times repeat tests. This result also indicates excellent stability of N2 annealed CdS films. Moreover, RhB as an another dye pollutant is used to evaluate the photocatalytic activity of CdS. It can be seen that the obtained N2 annealed CdS films can also exhibit excellent photocatalytic degradation rate for RhB of approximately 93% in 60 min under visible light irradiation (Fig. 16). This result indicates that the as-prepared CdS films can also have excellent photocatalytic performance for other colorless pollutants. Overall, due to the excellent photocatalytic activity and favorable recycling performance, the CdS films annealed in N2 is an excellent photocatalyst. More importantly, in comparison with the conventional powder photocatalysts, CdS films grown on FTO substrate are easier to use, and many problems in the photo-catalysis process such as the aggregation and the bleeding of the conventional powder photocatalyst can be avoided.

Fig. 14
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a Cycling runs for photocatalytic degradation of MB over CdS films annealed in: a: vacuum, b: N2, and c: air, respectively. b Figures of as-synthesized CdS films after the first time and fifth degradation cycles

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Fig. 15
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XRD patterns of a fresh and b used CdS thin films (after five cycles)

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Fig. 16
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The photocatalytic activity of CdS films for the photocatalytic decomposition of RhB under visible-light irradiation

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4 Conclusions

In summary, CdS films annealed under different atmosphere are successfully synthesized and used for efficient MB photodegradation under visible-light irradiation. The morphology, crystallization, UV–Vis absorption, photoelectrochemical and photocatalytic properties of the as-obtained CdS films are characterized. Among all the synthesized samples, the N2 annealed CdS films exhibit higher crystallinity, remarkable photoabsorption capacity and photoelectrochemical performance, which are all beneficial for the enhancement of photocatalytic capability. In addition, according to the results of recycling photocatalytic experiments, the N2 annealed CdS films have good repeatability. Therefore, CdS films with high photocatalytic efficiency and stability are potentially applicable in removal of organic pollutants.