Growth of Hydrothermally Derived CdS-Based Nanostructures with Various Crystal Features and Photoactivated Properties
© The Author(s). 2016
- Received: 7 April 2016
- Accepted: 17 May 2016
- Published: 23 May 2016
CdS crystallites with rod- and flower-like architectures were synthesized using a facile hydrothermal growth method. The hexagonal crystal structure of CdS dominated the growth mechanisms of the rod- and flower-like crystallites under specific growth conditions, as indicated by structural analyses. The flower-like CdS crystallites had a higher crystal defect density and lower optical band gap value compared with the rod-like CdS crystallites. The substantial differences in microstructures and optical properties between the rod- and flower-like CdS crystallites revealed that the flower-like CdS crystallites exhibited superior photoactivity, and this performance could be further enhanced through appropriate thermal annealing in ambient air. A postannealing procedure conducted in ambient air oxidized the surfaces of the flower-like CdS crystallites and formed a CdO phase. The formation of heterointerfaces between the CdS and CdO phases mainly contributed to the improved photoactivity of the synthesized flower-like CdS crystallites.
- Crystal growth
Semiconductor photocatalysts have attracted considerable attention because of their successful utilization of solar energy for solving environmental remediation problems. However, because of their wide band gap, most photocatalysts are active only under ultraviolet light, which occupies several percentages of the received solar energy [1, 2]. The development of efficient photocatalysts for visible light irradiation has therefore become a critical scientific challenge for the photocatalyst community [3–5]. Compared with the abundant wide band gap semiconductor photocatalysts, photocatalysts with a visible-light band gap are relatively limited in number. Among the reported candidates of semiconductors with a visible-light band gap, CdS is a vital member of the II–VI semiconductor group with a direct band gap of approximately 2.4 eV, and it has high refraction index, excellent transport properties, and suitable energetics for harvesting solar light for photoactivated device applications. Several studies have fabricated CdS photocatalysts with various morphologies and scales for photoactivated device applications [5–8]. Among various synthesis methods, the hydrothermal synthesis route has been considered one of the most promising synthetic routes for fabricating binary semiconductor crystals because of its low process cost, easy process control, and high product reproducibility .
The photocatalytic activity of semiconductors is highly associated with their microstructures, which are substantially dominated by the crystal growth mechanisms under specific growth conditions. The crystal morphology, size, and composition are major factors influencing the efficiency of light absorption and photocatalytic performance [10–12]. Therefore, understanding the correlation between the crystal growth mechanism and the crystal feature is crucial in designing semiconductor photocatalysts with high photocatalytic performance. Recently, semiconductor heterostructures comprising two compounds have attracted research attention and have been used to improve photoactivated properties compared with the corresponding individual constituents [10, 13–15]. A suitable band alignment between the constituents is an effective method for separating photoinduced electron–hole pairs and enhancing photocatalytic performance [14, 16, 17]. Preparing two semiconductor compounds for heterostructures usually requires a complex and incompatible two-step process, and thus hinders the actual applications of such semiconductor heterostructures in photodegradation. Therefore, the selection of a simple synthesis process and constituent compound for a CdS-based heterostructure is crucial to realize its photocatalytic applications. CdO is an n-type semiconductor with a band gap in the visible-light region [18, 19]. Incorporating CdO into CdS to form a heterostructure might be favorable for improving the photoactivated properties of CdS-based materials because of the special band alignment structure between CdO and CdS . The current study investigated the crystal growth, crystal features, and optical properties of rod- and flower-like CdS crystallites synthesized using a facile hydrothermal method. Moreover, a simple postannealing procedure was adopted to fabricate the CdS–CdO heterostructure. The photocatalytic performance levels of the CdS crystallites with various morphologies and CdO to form a heterostructure were compared and discussed on the basis of the differences in their microstructures.
In this study, CdS crystallites with various morphologies were synthesized using a hydrothermal method. Ethylenediamine solution was used as a surfactant to hydrothermally synthesize high-density rod-like CdS crystallites. Moreover, flower-like CdS crystallites were synthesized without ethylenediamine-assisted growth. The precursor solution for the hydrothermal synthesis comprised cadmium nitrate and thiourea with a molar ratio of 1:5 and was balanced with ethylenediamine solution or deionized water. The precursor solution was stirred for 10 min and then transferred into a Teflon autoclave to undergo a hydrothermal synthesis reaction. The temperature for the hydrothermal synthesis reactions was fixed at 170 °C and maintained for different durations. Finally, the reaction system was cooled to room temperature naturally, and the final precipitates were then washed in deionized water and dried in an oven. The CdO@CdS heterostructure was prepared by subjecting the flower-like CdS crystallites to thermal postannealing at 400 °C for 5 min in ambient air.
Sample crystal structures were investigated by X-ray diffraction (XRD; Bruker D2 PHASER) using Cu Kα radiation. The morphologies of the as-synthesized samples were characterized by scanning electron microscopy (SEM; Hitachi S-4800), and high-resolution transmission electron microscopy (HRTEM; Philips Tecnai F20 G2) was used to investigate the detailed microstructures of the samples. Room-temperature-dependent photoluminescence (PL; HORIBA HR800) spectra were obtained using the 325-nm line of a He–Cd laser. The diffuse reflectance spectra of the samples were recorded by using UV–Vis spectrophotometer (Jasco V750). Photocatalytic activity of as-prepared samples were performed by comparing the degradation of 10−6 M aqueous solution of methylene blue (MB) with various CdS samples as catalysts under visible light irradiation (λ > 420 nm). For each photodegradation test, 50 mg of CdS catalysts powdered sample was dispersed in 20 mL of MB solution. After the photodegradation reaction, the supernatant solution was measured by UV–Vis spectrophotometer in the wavelength range of 400–700 nm and analyzed the photodegradation ratio at the maximum absorption wavelength (~663 nm) of MB aqueous solution. The photodegradation size is defined as (C/C o), where C o is the concentration of aqueous MB without irradiation after dark adsorption equilibrium and C is the concentration of aqueous MB corresponding to a given visible light irradiation duration.
In this study, rod- and flower-like CdS crystallites were synthesized using a facile hydrothermal growth method. XRD analyses showed that the rod- and flower-like CdS crystallites have mainly hexagonal phases. The faster crystal growth rate on the c-axis than on the other crystallographic planes in the hexagonal CdS phase under the given growth condition might account for the anisotropic growth behavior observed in the rod- and flower-like architectures of the CdS crystallites. The flower-like CdS crystallites exhibited more defective crystal features than did the rod-like CdS crystallites. Moreover, the flower-like CdS crystallites exhibited a broader visible light absorption range than that of the rod-like CdS crystallites. Therefore, the photoactivity of the flower-like CdS crystallites is higher than that of the rod-like CdS crystallites. The postannealing of the flower-like CdS crystallites in ambient air also resulted in the formation of a CdO phase on the surfaces of these crystallites. In this study, the existence of heterointerfaces in the CdO@CdS heterostructure was beneficial for the separation of photogenerated charge carriers, and therefore, the photoactivity of the flower-like CdS crystallites was further improved.
This work is supported by the Ministry of Science and Technology of Taiwan (Grant Nos. MOST 104-2221-E-019-041 and MOST 102-2221-E-019-006-MY3).
YCL designed the experiments and drafted the manuscript. TWL carried out the sample preparations, material analyses, and characterization tests. Both authors read and approved the final manuscript.
YCL is a professor of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan). TWL is a graduate student of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan).
The authors declare that they have no competing interests.
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