Annealing Effects on Photocatalytic Activity of Zn 0 . 2 Cd 0 . 8 S Films Prepared by Chemical Bath Deposition

Zn 0.2 Cd 0.8 S alloyed films were prepared on glass substrates at room temperature using chemical bath deposition method. The obtained filmswere annealed at temperatures ranging from 200C to 500Cwith heating rate of 5C/min and annealed at 400Cwith heating rate of 2C/min and 10C/min. The films were characterized by X-ray diffraction, scanning electron microscopy, energydispersive spectroscopy, and UV-VIS spectrophotometer. The increasing of annealing temperature increases the crystallinity and the mean grain size of Zn 0.2 Cd 0.8 S alloyed films and significantly enhances the absorption in the visible region.The efficient visible light photocatalytic activity for annealed Zn 0.2 Cd 0.8 S alloyed films is associated with the larger size grain and the higher crystallinity.

Heat treatment is often used to tune the structure and properties of materials.By using appropriate parameters for heat treatment, different shapes [25] or colors [26] of nanomaterials with controllable properties can be produced.However, the systematic effects of annealing on the structural and photocatalytic properties of Cd 1− Zn  S alloyed films were rarely reported.In order to clarify this issue, Zn 0.2 Cd 0.8 S alloyed films are synthesized by a simple chemical bath deposition and the effects of annealing temperature and heating rate on the morphology, composition, and optical properties are comprehensively explored in this work.It is interesting that shape and particle size can be controlled by annealing temperature and heating rate which greatly influences the photocatalytic activity of Zn 0.2 Cd 0.8 S alloyed films.The photocatalytic activity of Zn 0.2 Cd 0.8 S alloyed films under visible light irradiation is greatly enhanced by increase of annealing temperature and heating rate.films.The suspension was magnetically stirred and placed at 3 cm from the lamp.During irradiation, the samples were withdrawn at regular time intervals and centrifuged to remove the catalysts.The absorption spectra of the solution were determined using UV-VIS spectrophotometer.

Results and Discussions
3.1.Crystal Structure and Morphology.SEM images of Zn 0.2 Cd 0.8 S alloyed films at different annealing conditions are delineated in Figure 1.As can be seen in Figures 1(a) and 1(b), as the annealing temperature is lower than 300 ∘ C, Zn 0.2 Cd 0.8 S alloyed films are dense, uniform, and homogeneous without detectable pores and covered well with glass substrate.However, the grains become bigger and cracks appear with increasing the annealing temperature or the heating rate as shown in Figures 1(c), 1(d), and 1(c2), which may be due to the increasing stress with the increasing temperature, which is similar to that reported for ZnS [11].Zn 0.2 Cd 0.8 S alloyed films were characterized by XRD to obtain information about the structure of the products.Figure 2 shows that the XRD patterns can be indexed as hexagonal structures (JCPDS No.49-1302) with the five characteristic diffraction peaks matching with the (100), (002), (101), (110), and (112) crystalline plane of CdS.The strong diffraction peaks indicate that all samples showed good crystalline structure and large mean grain size.With increasing the annealing temperature and heating rate, the CdS diffraction peaks become more distinct and the full width at half maximum narrows.This implies the increase of the crystallinity and the mean grain size of Zn 0.2 Cd 0.8 S.

Optical Properties.
The optical absorption spectra of Zn 0.2 Cd 0.8 S alloyed films annealed at different temperatures with different heating rates are illustrated in Figure 3.The absorption edge of Zn 0.2 Cd 0.8 S alloyed films is about 520 nm and obviously shifts to longer wavelength, with increasing the annealing temperature and heating rate, which greatly enhance the absorption of Zn 0.2 Cd 0.8 S alloyed films in the visible region.
The degradation efficiency is shown in Figure 4.The plot inserted in Figure 4(a) illustrates the typical irradiation time dependent UV-vis spectra of methyl orange solution during photocatalytic degradation.The spectra of methyl orange exhibit a main band with a maximum at 464 nm in the visible region.The absorption peaks of methyl orange gradually decrease with irradiation time during the photocatalytic reaction, indicating the degradation of methyl orange.Therefore, the decrease of absorbance corresponding to the decrease of methyl orange concentration is recorded.The degradation efficiency (%) is calculated according the following equation: Here  0 is the initial concentration of methyl orange (10 mg/L) and  is the concentration of methyl orange after an irradiation time.It can be clearly seen from Figure 4(a) that the degradation efficiency increases continuously with irradiation time.It is known that the photoactivity of semiconductors is particle size-dependent [27,28] and crystallinity-dependent [29,30].The grain size increases in Figure 1 and the peak intensity of XRD patterns in Figure 2 increases with the increasing temperature and heating rate, suggesting the larger grain size and the higher crystallinity.Thus, the higher photoactivity can be attributed to the larger size and the higher crystallinity of annealed Zn 0.2 Cd 0.8 S alloyed films, which decreases electron-hole recombination centers, facilitates the separation of electron-hole pairs, and therefore enhances the photoactivity of Zn 0.2 Cd 0.8 S alloyed films.

Conclusions
Zn 0.2 Cd 0.8 S alloyed films prepared by simple chemical bath deposition method were studied as photocatalysts for degradation of methyl orange under visible light irradiation.The grain size increases and crystallinity is enhanced for the Zn 0.2 Cd 0.8 S alloyed films with increasing annealing temperature and heating rate.Moreover the absorption edge obviously shifts to longer wavelength with increasing the annealing temperature and the heating rate.Increasing the annealing temperature and the heating rate can enhance photocatalytic activities of Zn 0.2 Cd 0.8 S alloyed films under visible light irradiation.This work provides a simple and efficient method to enhance the photocatalytic activity of semiconductors.
2.1.Preparations of Zn0.2 Cd 0.8 S Thin Films.All the chemicals are analytical reagents and used without further purification.The glass slides were cleaned with acetone, alcohol, and deionized water, respectively.0.005 M ZnSO 4 ⋅ 7H 2 O and The degradation efficiency of methyl orange for Zn 0.2 Cd 0.8 S films is 18.50%, 51.08%, 57.01%, and 63.75%, respectively, for the samples annealed at 200 ∘ C, 300 ∘ C, 400 ∘ C, and 500 ∘ C after 4 h visible light irradiation.The degradation efficiency of methyl orange for Zn 0.2 Cd 0.8 S alloyed films is 32.64%, 57.01%, 65.78%, respectively, for the samples annealed at 400 ∘ C with heating rate of 2 ∘ C/min, 5 ∘ C/min, and 10 ∘ C/min (Figure4(b)) after 4 h visible light irradiation.It is obvious that the degradation efficiency increases with an increase of heating rate.These results suggest that the photocatalytic activity of Zn 0.2 Cd 0.8 S alloyed films can be extremely improved by appropriate annealing conditions.