Full Length ArticleNovel ZnO nanoparticles modified WO3 nanosheet arrays for enhanced photocatalytic properties under solar light illumination
Graphical abstract
Schematic diagram of the band alignment-induced interfacial charge separation at the WO3/ZnO heterojunction interfaces favoring for improving the photocatalytic degradation of MB performance: CB, the bottom of conduction band (vs. NHE), VB, the top of valence band (vs. NHE).
Introduction
Nanometer-scaled semiconductor materials have been extensively studied in removal of hazardous organic pollutants due to the effective reduction-oxidation ability and large surface area [1], [2], [3]. However, the nanosized powder materials are difficult to reuse in waste water and they may suffer from secondary pollution for filtration procedure required to separate the catalyst from the reaction solution. Semiconductor nanostructures grown on stainless steel substrate can solve this challenge. In addition, the intimate contact between heterojunction and stainless steel substrate can accelerate the electron transition, which can enhance photocatalytic efficiency. Various metal oxide semiconductors, such as TiO2 [4], [5] and ZnO [6], have been widely investigated as photocatalysts in pollutant degradation [7]. Whereas these semiconductors are only excited under the ultraviolet light for their wide band gaps, leading to the restriction of the practical application [8]. Among these semiconductor photocatalysts, ZnO exhibits high photocatalytic capability, chemically stable and high electron mobility [9]. Neverthless, owing to its wide band gap of 3.3 eV, ZnO suffers as its light absorption is restricted. Hence, the modification of ZnO with narrow band-gap semiconductors can improve the photocatalytic performance due to harvesting more solar light, promoting interfacial electron transfer and reducing the charge recombination [10], [11].
Tungsten trioxide (WO3) possesses outstanding optoelectronic properties for narrow band gap of 2.4–2.8 eV and favorable hole diffusion lengths, which can theoretically utilize ∼12% solar light [12], [13], [14], [15]. However, WO3 displays poor photocatalytic efficiency for the decomposition of organic pollutants due to sluggish kinetics and slow charge transition at the semiconductor-electrolyte interface [16], [17]. In order to solve these problems, several techniques have been employed to decorate WO3, including metal implantation [18], [19] and coupling with other semiconductors [20], [21]. The most effective way is to construct hybrid heterojunctions by combining other semiconductors with appropriate band structures. Furthermore, hybridizing ZnO with WO3 can form a more negative conduction band level, which can promote the separation of photo-induced carriers. Zheng et al. [22] synthesized WO3 nanorod array (WNR) and ZnO nanosheet array (ZNS) composite structures on FTO substrates by a simple and effective hydrothermal technique. The WNR-ZNS composite structures had higher degradation efficiency than that of the WNRs or ZNRs individually under the visible light. Thein et al. [23] prepared highly UV driven WOx@ZnO nanocomposites via liquid impregnation method. The photocatalytic enhancement of WOx@ZnO nanocomposites in degrading RhB dye under UV light suggested that deposition of WOx could have prolonged the separation of electrons (in WOx) from the holes. Adhikari et al. [24] dispersed ZnO nanobelts into WO3 nanocuboids by physical mixing. The WO3-ZnO composites with the optimum amount of WO3 and ZnO exhibited the maximum photodegradation efficiency for the degradation of both the cationic and anionic dyes. Therefore, the main challenge is constructing semiconductor composites with well-matched band structures and strong contact [25], [26], [27].
In this paper, we fabricated WO3/ZnO NSAs through combining hydrothermal growth of monoclinic WO3 NSAs on the 316L stainless steel substrate and impregnation route for ZnO nanoparticles deposition on the WO3 NSAs. The effect of hydrothermal time and the concentration of the precursor solution on the structure and photoelectrochemical properties was studied. The as-synthesized WO3/ZnO NSAs exhibited excellent photocatalytic activities for degrading MB under solar light illumination, which was much higher than those of individual WO3. Additionally, the 30-WZ NSAs showed the maximal photocatalyst efficiency and photocurrent density, which could be ascribed to the synergistic effect of the WO3/ZnO heterostructures and effective charge separation at the heterostructure interface. The possible photodegradation mechanism of the WO3/ZnO composite was also discussed.
Section snippets
Chemicals and materials
All chemicals were analytical grade reagents and used without further purification. Electrolyte was freshly prepared from double distilled water (DDW). DDW was used throughout this study. AISI 316L stainless steel foil (composition of Cr: 17–19, Ni: 11–13, Mn: 2, C: 0.015, and Fe balanced, in wt.%) was cut into specimens of 35 × 10 × 0.5 mm3. All stainless steel specimens were mechanically polished with sandpapers (grade 1000, 1500 and 2000) and diamond pastes (from 3.5 to 0.25 μm). Then, the
Morphology and crystalline structure
The surface morphology and structure of the as-synthesized WO3 NSAs are characterized by FESEM as shown in Fig. 1. Fig. 1a exhibits the as-prepared WO3 nanosheets with thickness of about 60 nm grow on the 316L stainless steel substrate. When the hydrothermal time increases to 2 h, the thickness and density of the WO3 nanosheets raise (Fig. 1b). With the increase of reaction time to 3 h, WO3 nanosheets have a square cross section of about 100 nm width and 1.3 μm length (Fig. 1c and Fig. 1f).
Conclusions
In summary, WO3/ZnO NSAs were successfully fabricated on stainless steel substrate via a hydrothermal-impregnation method. The influence of hydrothermal reaction time and the concentration of Zn(NO3)2 precursor on the structure, morphology and photoelectrochemical properties of the as-prepared samples was investigated. The sample synthesized for 3 h and immersed into 30 mM of Zn(NO3)2 precursor displayed the highest photocatalytic activity with a degradation rate of 0.0258 min−1, which was 3.39
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51171133, No. 51471122, and No. 51774217) and the Key Program of Natural Science Foundation of Hubei Province of China (No. 2015CFA128).
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