Enhanced photocatalytic activity of ZnO microflower arrays synthesized by one-step etching approach
Graphical abstract
Introduction
Water contamination and shortages are becoming more and more serious with the development of industry and economy. Photocatalysis, with semiconductor-assisted, has attracted more concern for complete elimination of pollutants in the waste water due to its efficiency and broad applicability [1], [2], [3]. Photocatalysis reaction is a complicated process and the photocatalyst activity of semiconductor materials is influenced by many factors [4], [5], [6], such as size [7], morphology [8], [9], orientation [10], [11], defects [12], [13], [14] and exposed crystal facets [15], [16]. Usually, the photocatalysis reaction occurs at the interface between catalyst and organic pollutants [17], so photocatalytic performance are strongly dependent on the surface structure of materials. And a large surface/volume (S/V) ratio of material is required to improve the activity [18], [19]. For example, decreasing particle size is an effective way to enhance surface area of materials. Alternatively, hollow structure material is considered the most promising morphology used in the fields of photocatalysis because of its high surface area. Therefore, exploring the fabrication of hollow structure catalysts whose active surfaces are extended to the inner surfaces is essential for the development of advanced photocatalysts.
ZnO with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV is an important semiconductor material and has been widely used as photocatalyst for photodegradation organic pollutants in waste water [20], [21], [22]. Various synthesis routes, such as thermal evaporation [23], [24], sonochemical [25], electrochemical [26], hydrothermal oxidation [27] and sol–gel method [28], have been developed to fabricate ZnO with various morphologies. Recently, a great quantity of ZnO tubes with different sizes has been prepared by various chemical methods [29], [30], [31]. Generally, ZnO tubes have been synthesized by a two-step etching process. However, most of the synthesis techniques are demand complicated controlling processes and those methods are unfavorable for low-cost and large-scale production. Therefore, how to fabricate a new type ZnO hollow architectures with desired functionality by a simply and low-cost synthetic strategy still remains a great challenge in the area of materials research. Moreover, understanding of the relationship between microstructure and the photocatalytic activity of ZnO is also significant.
In this paper, a new type of ZnO microflower arrays with partial hollow structure was synthesized via a novel one-step etching route in reaction solution without any assistant reagents. The formation mechanism of ZnO microflowers was analyzed, and the photocatalytic performance was studied for the degradation of dyes. ZnO microflowers exhibited the superior photocatalytic activity in the degradation of rhodamine B (RhB) compared to other samples in our experiments.
Section snippets
Materials
All reagents, including zinc nitrate hexahydrate (Zn(NO3)2·6H2O), methenamine (C6H12N4), rhodamine B (RhB) and absolute propanol, were analytical grade, purchased from Sinopharm Chemical Regent Co., Ltd., and used without further purification.
Preparation
Si (1 0 0) wafers were used as the substrate. Before deposited ZnO films, ZnO seed layers were spin coated onto Si wafers according to the literature [32]. In a typical ZnO films preparation, the seed-coated substrate was immersed into a solution of 0.1 M
Characterizations
Fig. 1 presents SEM images of the as-prepared ZnO arrays grown on Si substrate when the reaction time was fixed at 5 h. As shown in Fig. 1a–c, ZnO microflower arrays grow in large-scale and exhibit high orientation perpendicular to the substrate. The high-magnification image in Fig. 1d of a single microflower reveals that the inner and outer wall surfaces are quite smooth with the average thickness of 50 nm. However, a small quantity of microrods and partial tubular microstructure (like morning
Conclusions
In summary, major ZnO microflowers structure on Si substrate have been successfully synthesized via a simple one-step etching route based on chemical bath disposition (CBD) method by employing the propanol/deionized water as solvent. It is found that the reaction time play the most important role in the formation of microflowers, and the formation is mainly due to the preferential etching along c-axis. ZnO with different morphologies such as microrods and microcandles can be obtained by
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant No. 61178074, 61008051, 11204104 and 11254001), program for the development of Science and technology of Jilin province (Item No. 201115219, 201105084, 20100113, 201215225 and 201215222), the Twentieth Five-Year Program for Science and Technology of Education Department of Jilin Province (Item No. 20110170, 20110169, 20120174 and 20130447), program for the master students’ scientific and innovative research of
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