Facile construction of MoO3@ZIF-8 core-shell nanorods for efficient photoreduction of aqueous Cr (VI)

https://doi.org/10.1016/j.apcatb.2018.08.077Get rights and content

Highlights

  • MoO3@ZIF-8 core-shell nanorods were prepared by a facile method.

  • Properties of MoO3@ZIF-8 core-shell nanorods were confirmed by imperative measurements.

  • Enhanced photocatalytic performance were measured under the same condition.

  • The mechanism of the enhanced photocatalytic performance was discussed in detail.

Abstract

Recently, hexavalent chromium (Cr(VI)) in wastewater has become a threat to the ecosystem and human health. The synthesis of high-performance and recyclable photocatalysts still remains a challenge. The photoreduction efficiency of Cr(VI) is inhibited by the high rate of recombination of electron-hole pairs and the low adsorption capacity of Cr(VI). In this study, three-dimensional (3D) MoO3/ZIF-8 core-shell nanorod composite photocatalysts were prepared via a facile two-step method and applied to the reduction of Cr(VI). The chemical state of the elements, microstructure, surface elements, and optical properties of the MoO3/ZIF-8 core-shell nanorods were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy. The as-prepared MoO3@ZIF-8 catalysts displayed superior photocatalytic activity for Cr(VI) reduction under visible light, compared to the pure ZIF-8 and MoO3 nanowires. Further, MoO3@ZIF-8 (with 15% of ZIF-8) exhibited the best photocatalytic activity, and promoted 100% reduction of Cr(VI) (15 mg L−1) within 45 min. The 3-D core-shell structure not only provided a large surface area, but also separated the electron-hole pairs effectively. The photocatalytic activity of the composite remained almost unchanged after four cycles. The mechanism of Cr(VI) reduction is also discussed in detail.

Introduction

Recently, rapid industrialization has posed a serious threat to the environment, resulting in wastewater containing heavy ions becoming a primary concern for human beings. Chromium, as a typical heavy metal contaminant, is generated by commercial operations such as the leather tanning, textile manufacturing, and steel fabrication industries [[1], [2], [3], [4], [5]]. In contrast with other heavy metals, chromium exists in two main oxidation states: Cr(VI) and Cr(III). Cr(VI) has been regarded as a carcinogen due to its acute toxicity to organisms, whereas Cr(III) is nontoxic and acts as an essential trace metal for human beings [[6], [7], [8]]. Therefore, the reduction of Cr(VI) to Cr(III) is deemed to be an effective method for water treatment. Various pioneering works have been performed to resolve the issue of chromium reduction, and the approaches can be classified into three types: microbial reduction, chemical reduction, and photocatalytic reduction [[9], [10], [11], [12]]. The photocatalytic reduction method is cost effective without the discharge of any perilous chemicals [[13], [14], [15], [16], [17], [18]].

Metal-organic frameworks (MOFs), as a new class of organic-inorganic hybrid materials with high surface area and large pore volume, have attracted much attention in various applications such as gas capture and storage, catalysis, drug delivery, and trace metal ion sensing [[19], [20], [21], [22], [23], [24], [25]]. Ongoing efforts are still being devoted to the modification of MOFs as photocatalysts. Most MOFs, such as ZIF-8 (bandgap: 5.1 eV), can only adsorb UV light due to the large bandgap, which limits the scope of application. To solve this problem, many strategies such as metal loading, dye sensing, organic linker decoration, and semiconductor combination have been developed to reduce the bandgap to expand the visible light utilization [[26], [27], [28], [29], [30], [31]]. The combination of MOFs with semiconductors is regarded as an efficient way to improve the photocatalytic performance. With this in mind, various semiconductors have been used to construct coherent interfacial connections between MOFs and semiconductors in order to retard the recombination of photo-induced electron-hole pairs, thereby enhancing the photocatalytic activity. In 2014, Wu and co-workers developed a ternary UiO-66/CdS/RGO photocatalyst with a photocatalytic hydrogen evolution rate 13.8 times higher than that of commercial CdS [32]. The reduced electron-hole recombination rate and close contact of the catalyst components were the crucial factors giving rise to the enhanced photocatalytic activity. The modification of MOFs for the photoreduction of Cr(VI) is currently at the forefront of research. Various MOFs have been explored for Cr(VI) removal, including ZIF-8, UiO-66, and MIL-53. MOFs have also been loaded with noble metals such as Au, Pd, and Pt, leading to efficient photocatalytic activity [[33], [34], [35], [36]]. However, the formation of new core-shell photocatalysts remains underexplored.

MoO3 has been widely investigated as an electrode material for lithium-ion batteries. It has also attracted considerable interest as a promising material because of its low cost, non-toxicity, high adsorbability, and environmental biocompatibility [[37], [38], [39]]. Nevertheless, the commercial production of MoO3 materials has been hindered due to the inherent electrical conductivity of this oxide, which results in low charge transfer ability. To improve its photocatalytic performance, MoO3 is often combined with carbon materials or loaded with noble metals or metal oxides to further inhibit the rapid electron−hole recombination. Unfortunately, the photocatalytic activity of existing MoO3 species is still far from adequate for practical application. The development of a suitable strategy for converting MoO3 into practical photocatalysts remains a challenge. Thus, combining MoO3 with MOFs is a novel approach. To date, only one kind of MoO3/MOF has been reported, i.e., MoO3/TMU-5, which displayed enhanced photo-oxidative desulfurization properties [40]. To the best of our knowledge, there is no report on MoO3/MOF-based photocatalysts for Cr(VI) photoreduction.

Based on the aforementioned considerations, we developed a series of MoO3@ZIF-8 core-shell nanorod photocatalysts by a facile two-step method. The photocatalytic activity of the series of MoO3@ZIF-8 photocatalysts is characterized by Cr(VI) reduction under visible light. It is proved that a new bond is formed between MoO3 and ZIF-8, which enhances the efficiency of separation of the photo-induced electron−hole pairs. The synergistic effect between MoO3 and ZIF-8 has proven to boost the photocatalytic activity. Moreover, a possible mechanism for the photocatalytic process is proposed on the basis of the experimental results.

Section snippets

Chemicals

Chemicals without special descriptions were obtained from commercial companies and used without further purification. Molybdenum powder, Zn(NO3)2·6H2O, 2-methylimidazole, methanol, hydrogen peroxide, Tert-Butanol (TBA), Benzoquinone (BQ), ethylene glycol, Polyvinylpyrrolidone (PVP) (M.W. 130,000) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Sigma-Aldrich. Ultra-pure water with a resistance of 18.2 MΩ was prepared using a water purification system.

Preparation of MoO3 nanowires

MoO3 nanowires were

Structure and composition

X-ray diffraction (XRD) served to detect the phases of the as prepared catalysts. As shown in Fig. 1, the MoO3 nanowires with various ZIF-8 weight percentages showed clear crystal peaks. The diffraction peaks at 12.6, 25.4, and 38.8° could be indexed to the (0 2 0), (0 4 0), and (0 6 0) planes of MoO3 (PDF No. 05-0508) [17], respectively. Meanwhile, the peaks of ZIF-8 are consistent with the previous reports. Interestingly, comparing with the other samples, the peaks of (0 2 0), (0 4 0), and (0

Conclusions

In summary, a heterogeneous MoO3@ZIF-8 core-shell nanorods heterojunction photocatalyst was successfully synthesized using a facile method. The formation of ZIF-8 NPs on the surface of MoO3 nanowires was confirmed by various characterization methods. Interestingly, MoO3@ZIF-8 core-shell nanorods, as a novel photocatalyst, demonstrated excellent stability and activity for Cr (VI) degradation. The enhanced photocatalytic activity is mainly derived from the synergistic effect and lower electron-

Acknowledgment

This work was supported by an Inha University Research Grant, South Korea.

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