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Cite this: Dalton Trans., 2014, 43, 16981 Received 22nd August 2014, Accepted 25th September 2014 DOI: 10.1039/c4dt02557a

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Fabrication of Au/ZnO nanoparticles derived from ZIF-8 with visible light photocatalytic hydrogen production and degradation dye activities† Liu He,a,b Lu Li,a,b Tingting Wang,*c Hong Gao,b Guangzhe Li,a,b Xiaotong Wu,a,b Zhongmin Sua,b and Chungang Wang*a,b

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A facile two-step strategy was developed to fabricate yellow fluorescent glutathione-Au nanoclusters/zeolitic imidazolate framework-8 nanoparticles (GSH-Au NCs/ZIF-8 NPs) and their derived Au/ZnO NPs, which exhibited visible light photocatalytic hydrogen evolution together with degradation of Rhodamine B (RhB).

Photocatalysis with semiconductor nanomaterials provides a “Green technology” route to solve the renewable energy and environmental pollution problems.1 Among the numerous semiconductor nanomaterials, ZnO nanoparticles (NPs) have gained tremendous research interest on account of their wide applications in dye-sensitized solar cells, photocatalysis and optoelectronic devices.2 In recent years, composite hybrid nanostructures formed from noble metals especially for Au and semiconductor NPs have attracted great attention for their enhanced or even new functionalities coming from each component.3 Zeolitic Imidazolate Frameworks (ZIFs),4 known as a subclass of metal–organic frameworks (MOFs),5 are crystalline tridimensional networks consisting of metal ions or metal clusters bridged tetrahedrally via the imidazolate type of linker. Recently, zeolitic imidazolate framework-8 (ZIF-8) has received great attention due to its potential applications in gas uptake, separation and drug delivery.6 So far, hybrid materials based on ZIF-8 and foreign NPs with versatile shapes and functions have been synthesized, which have outstanding benefits that derive from both the NPs (such as Au, Pt and Fe3O4 NPs) and ZIF-8 with gas adsorption, active catalytic and magnetic

a Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail: [email protected] b Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, Harbin, 150025, P.R. China c School of Chemistry & Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedures, PL, PXRD, TGA, EDX, XPS spectra and N2 sorption isotherm spectra. See DOI: 10.1039/c4dt02557a

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properties.7 Furthermore, ZIF-8 can still be utilized as a zinc precursor to construct ZnO NPs.8 Recently, Au nanoclusters (Au NCs) have gained lot of attention for their extensive applications in bio-nanotechnology and catalysis because of their distinctive fluorescence properties, small size and high catalytic activity.9 To the best of our knowledge, however, no attempts have been made to construct glutathione-Au NCs/ZIF-8 hybrid NPs (designated as GSH-Au NCs/ ZIF-8) and their derived Au/ZnO NPs. Herein, we report a simple and straightforward two-step method to synthesize yellow fluorescent GSH-Au NCs/ZIF-8 NPs at room temperature. Importantly, the resulting GSH-Au NCs/ZIF-8 NPs acted as templates to form Au/ZnO NPs, presenting excellent visible light photocatalytic hydrogen production and degradation of Rhodamine B (RhB). The procedure is illustrated in Scheme 1. In brief, the entire experimental process involves (1) the attachment of the as-prepared fluorescent GSH-Au NCs on ZIF-8 to

Scheme 1 Schematic representation of the synthetic route for the controllable fabrication of the rhombic dodecahedral GSH-Au NCs/ZIF-8 NPs and their derived Au/ZnO NPs for the visible light photocatalytic hydrogen production together with degradation of RhB.

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Fig. 1 HRTEM image of (a) GSH-Au NCs (inset: HRTEM image of a single GSH-Au NC). FE-SEM images of (b) ZIF-8 and (c) GSH-Au NCs/ ZIF-8 (insets: the corresponding TEM images). PL spectra of (d) bare GSH-Au NCs, (e) pure ZIF-8 and (f) GSH-Au NCs/ZIF-8 NPs with excitation wavelength at 365 nm (insets: photographs of the corresponding samples suspended in methanol solution illuminated under ambient light (left) and under UV light (365 nm) (right)).

form GSH-Au NCs/ZIF-8 NPs; (2) the fabrication of Au/ZnO NPs by the calcination of the GSH-Au NCs/ZIF-8 NPs; (3) the visible light photocatalytic hydrogen production as well as degradation of RhB using the resulting Au/ZnO NPs. The GSH-Au NCs were obtained by an established method.10 Fig. 1a displays the high-resolution electron transmission microscopy (HRTEM) image of the as-synthesized water soluble GSH-Au NCs with an average size of 3.5 nm. The close-up image of a single GSH-Au NC is provided in the inset of Fig. 1a. The GSH-Au NCs exhibit yellow light under irradiation with a 365 nm ultraviolet (UV) lamp (inset of Fig. 1d), which is consistent with the emission peak at 572 nm in the photoluminescence (PL) spectrum (Fig. 1d). Subsequently, the GSH-Au NCs/ZIF-8 was prepared by directly mixing GSH-Au NCs and the as-synthesized ZIF-8 at room temperature using methanol as a solvent with stirring for 12 h. Fig. 1b and c show the field-emission scanning electron microscopy (FE-SEM) images of the as-prepared pure ZIF-8 and GSH-Au NCs/ZIF-8, respectively, which reveal the uniform sodalite zeolite-type structure. Their corresponding TEM images are shown in the insets of Fig. 1b and c, displaying a hexagonal shape. The obtained GSH-Au NCs/ZIF-8 NPs with an average size of 110 nm have a rough surface due to the successful attachment of GSH-Au NCs (inset of Fig. 1c). We consider that the formation of GSH-Au NCs/ZIF-8 may result from the coordination interactions between Zn2+ ions and the GSH on the GSH-Au NCs surface. To prove the attachment of GSH-Au NCs does not affect the crystalline form of ZIF-8, powder X-ray diffraction (PXRD) measurements were performed. As expected, both the GSH-Au NC/ZIF-8 and pure ZIF-8 NPs show the characteristic reflection at 2θ = 7.3° attributed to the 110 peak of ZIF-8 (Fig. S1†). Moreover, it is found that the optical properties of the GSH-Au NCs are retained after the formation of GSH-Au NCs/ZIF-8 in comparison with the PL spectra of bare GSH-Au NCs, pure ZIF-8 and GSH-Au NCs/ZIF-8 (Fig. 1d–f ). It can be seen that the peak at 572 nm of the

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GSH-Au NCs/ZIF-8 is observed in the PL spectrum (Fig. 1f ), in agreement with that of the pure GSH-Au NCs (Fig. 1d). For pure ZIF-8, however, no obvious peak at 572 nm appears (Fig. 1e). The results demonstrate that the yellow fluorescence of GSH-Au NCs/ZIF-8 originates from the GSH-Au NCs rather than from ZIF-8, which is further confirmed by the photographs of the corresponding samples excited using a UV lamp at 365 nm. Under the same conditions, the GSH-Au NCs and GSH-Au NCs/ZIF-8 emit yellow light (insets of Fig. 1d and f ), but no luminescence can be observed in the case of pure ZIF-8 (inset of Fig. 1e). It is worth noting that the fluorescence intensity of the product does not decrease obviously after washing with methanol ten times, indicating that the GSH-Au NCs/ ZIF-8 NPs possess good stability (Fig. S2†). To investigate the thermal stability between ZIF-8 and GSH-Au NCs/ZIF-8 NPs for further obtaining Au/ZnO NPs through calcination method, thermogravimetric analysis (TGA) was carried out. As illustrated in Fig. S3,† the weight loss of GSH-Au NCs/ZIF-8 NPs decreases compared with ZIF-8 NPs resulting from the attachment of GSH-Au NCs. In addition, GSH-Au NCs/ZIF-8 NPs undergo a weight loss starting at ∼220 °C, followed by a gradual loss and finally reach a stable platform at about 520 °C. The weight loss should originate from the oxidation of the organic imidazole and the decomposition of GSH. In the calcining process, the GSH-Au NCs tend to fuse into Au NPs. Based on the above data, the GSH-Au NCs/ ZIF-8 NPs as templates were calcined to prepare Au/ZnO NPs at 550 °C in air. As expected, the PXRD pattern of the calcined product shows characteristic broad diffraction peaks at {100}, {002}, {101}, {102}, {110}, {103} and {112}, representing the successful formation of ZnO (Fig. S4a†). As for the weak reflection at 2θ = 37.8° and 45°, they are attributed to the {111} and {200} lattice planes of Au.11 The Au element was also identified in the energy dispersive X-ray (EDX) spectrum (Fig. S4b†), indicating the presence of Au in the resulting product. Besides, X-ray photoelectron spectroscopy (XPS) was used to determine the surface information on the calcined product (Fig. S5†). The peaks at 83 and 88.2 eV are assigned to Au 4f7/2 and Au 4f5/2 of Au in Au/ZnO NPs (Fig. S5a†).12 The Zn 2p1/2 and Zn 2p3/2 peaks at binding energies of 1044.3 and 1021.1 eV match well with pure ZnO NPs, which demonstrate that the Zn element mainly exists in the Zn2+ state (Fig. S5c†). And the counterpart of Zn is O2−, whose binding energy of O 1s appears at 530.05 eV (Fig. S5b†). In addition, the N2 adsorption isotherms (Fig. S6a†) and pore size distribution curves (Fig. S6b†) suggest that both Au/ZnO and ZnO NPs possess the porous structure. Fig. 2a and b show the TEM images of ZnO and Au/ZnO NPs that are obtained by the calcination of ZIF-8 and GSH-Au NCs/ZIF-8, respectively. It is clearly seen that their morphology changed from a hexagonal shape to an ellipsoidal shape with an average diameter of 90 nm owing to the transformation of ZIF-8 into ZnO. The Au/ZnO NPs have a relatively rough surface by comparing the corresponding FE-SEM images in the insets, derived from the existence of Au with ∼15 nm on the surface of ZnO NPs during the calcination process. HRTEM images reveal lattice distances of 0.26 and 0.23 nm

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Fig. 2 TEM images of ZnO (a) and Au/ZnO NPs (b) (insets: FE-SEM images of the corresponding samples). (c) and (d) are the corresponding HRTEM images. (e) UV-Vis spectrum of ZnO (black), GSH-Au NCs/ZIF-8 (blue) and Au/ZnO (red) (insets: photographs of the corresponding samples suspended in methanol solution). (f ) DRS spectrum of ZnO (black) and Au/ZnO (red).

Fig. 3 (a) Time courses of hydrogen evolution over Au/ZnO NPs (black) and ZnO NPs (blue) in aqueous solution under visible light irradiation (>400 nm cutoff filter) of a 500 W Xe lamp; (b) time-circle photocatalytic hydrogen evolution over Au/ZnO NPs; (c) the corresponding mechanism for the generation of hydrogen on the surface of Au/ZnO NPs under visible light exposure; (d) UV-Vis spectroscopic changes of the RhB aqueous solution in the presence of the Au/ZnO NPs photocatalyst upon exposure to visible light; (e) recycled performance of the Au/ZnO NPs for photo-degradation of RhB.

that match well with the {002} facet of ZnO and {111} facet of Au, respectively (Fig. 2c and d).13 Furthermore, the maximum absorption peak at around 536 nm of Au/ZnO NPs is observed in the UV-Vis absorption spectroscopy (Fig. 2e), indicating the formation of Au NPs. For pure GSH-Au NCs/ZIF-8 and ZnO, however, no obvious peak at 536 nm appears. Meanwhile, the diffuse reflectance spectrum (DRS) of Au/ZnO NPs exhibits two broad peaks at around 350 and 536 nm, resulting from ZnO and Au NPs, respectively (Fig. 2f ). Moreover, typical plots of (F(R)hν)2 versus photon energy for the synthesized ZnO and Au/ZnO NPs are shown in Fig. S7.† The energy of the band gaps can be estimated to be 3.21 eV for ZnO NPs and 3.17 eV for Au/ZnO NPs. The substantial role of the coexistence of Au in improving photocatalytic activity of ZnO was investigated by monitoring visible light photocatalytic hydrogen evolution with a 500 W xenon lamp irradiation. Na2S and Na2SO3 were added as sacrificial agents that consume the photo-generated holes from the photocatalytic surface.14 As shown in Fig. 3a, the hydrogen evolution rate for pure ZnO NPs obtained by direct calcination of ZIF-8 is 0.01 μmol h−1 g−1. Meanwhile, with the coexistence of Au, Au/ZnO NPs give a remarkably improved hydrogen evolution rate of 29.8 μmol h−1 g−1. Besides, the catalyst can be separated easily and remained unchanged by the PXRD measurement (Fig. S8a†). Subsequently, the time-circle photo-

catalytic hydrogen evolution was conducted to estimate the stability of Au/ZnO NPs as photocatalysts. No noticeable activity degradation in four cycles of photocatalytic reactions is detected (Fig. 3b), suggesting that the Au/ZnO NPs have an excellent stability and can be used for continuous photocatalytic hydrogen evolution. The great improvement of photocatalytic activity of the Au/ ZnO NPs can be ascribed to the following two reasons as the schematic diagram shown in Fig. 3c. First, the ZnO NPs act as electron and hole sources. When the ZnO catalysts absorb photons of energy greater than or equal to the band gap, photoelectrons may be promoted from the valence band (VB) to the conduction band (CB), leaving behind the same amount of electron vacancies or holes in the VB. Since the energy level of the bottom of the CB of ZnO is higher than the new Fermi energy level of the Au/ZnO, the photoelectrons could transfer from ZnO to Au driven by the energy difference. Au NPs act as a sink for the storage of photo-generated electrons and induce a shift of the Fermi level toward more negative potentials. The transfer of electrons to Au NPs continues until the Fermi level reaches close to the CB edge of ZnO. Thus, the recombination of photoelectrons and holes prior to the superoxide activation process is avoided and the photo-induced generation of electron–hole pairs will continue.15 Second, surface plasmon

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resonance (SPR) is a collective oscillation of the free CB electrons at the interface between Au NPs and dielectrics driven by the electromagnetic field of incident light in the visible and infrared (IR) regions. When plasmonic-Au NPs absorb visible light, the electrons near the Fermi level are excited to the SPR state (generally above the CB minimum of the ZnO NPs), injected into the CB of the ZnO NPs and then participate in the photocatalytic reactions. Therefore, under appropriate visible light irradiation, the photocatalytic activities of Au/ZnO NPs could be greatly boosted. Simultaneously, since S2− and SO32− could be oxidized by photo-generated holes to Sn2− and SO42−, respectively, they can thus act independently as sacrificial reagents for photocatalytic hydrogen generation. However, the oxidation of S2− ions to yellow polysulfides Sn2− leads to a decrease in hydrogen formation over time, which comes from the high light absorption of the yellow polysulfide Sn2− in the visible region and then to the competitive reduction of Sn2− with H2O. Fortunately, SO32− could act as an Sn2− regenerating agent and maintain the solution colourless. Therefore, the S2−/SO32− mixture is most widely used as electron donors and added to the water/semiconductor suspension to improve the photocatalytic activity and stability for hydrogen evolution from water. The probable photocatalytic reaction mechanism in the presence of S2−/SO32− as the sacrificial reagent can be expressed as follows:16 Au=ZnO þ hν ! hþ þ e 2e þ 2H2 O ! H2 þ 2OH

In summary, we fabricated yellow fluorescent GSH-Au NCs/ ZIF-8 NPs and their derived Au/ZnO NPs. More importantly, the resulting Au/ZnO NPs can be used for visible light photocatalytic hydrogen production and degradation of RhB. Furthermore, the GSH-Au NCs/ZIF-8 NPs have potential applications in simultaneous fluorescence imaging and pH-responsive drug delivery for cancer cells. Both the controllable synthetic method and the unique applications make Au/ZnO NPs as potential semiconductor nanomaterials for solving renewable energy and environmental pollution problems. We believe that this synthetic strategy can be extended to construct various noble-metal (such as Ag, Pt NPs)/ZnO hybrid materials utilizing ZIF-8 as templates.

Acknowledgements This work was supported by the National Natural Science Foundation of China (grant no. 21173038, 21401013 and 21301027), the Natural Science Foundation and Science and Technology Development Planning of Jilin Province (201215003, 201201115, 20130522136JH and 20130521010JH), the Science Foundation for Young Teachers of Changchun University of Science and Technology (XQNJJ-2011-11), the Program for New Century Excellent Talents in University (NCET-13-0720) and the Fundamental Research Funds for the Central Universities. Supported by the Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China.

SO3 2 þ 2OH þ 2hþ ! SO4 2 þ H2 O 2S2 þ 2hþ ! S2 2 S2 2 þ SO3 2 ! S2 O3 2 þ S2 SO3 2 þ S2 þ 2hþ ! S2 O3 2 In addition, the as-prepared Au/ZnO NPs were also utilized for the degradation of RhB under direct visible light irradiation. As shown in Fig. 3d it can be found that the intensity of the characteristic adsorption peak decreases dramatically with the increase of irradiation time in the degradation process. This decrease in absorption is accompanied with a slight shift of the main absorption peak to lower wavelength due to the formation of the demethylated dyes.17 For comparison, the same photodegradation experiment of ZnO NPs and commercially available ZnO nanopowder were carried out, which shows no appreciable degradation of RhB after irradiating for 3 h (Fig. S9†). Moreover, the visible light photocatalytic cycle experiments were carried out for the sake of examining the stability of Au/ZnO NPs as photocatalysts. After four cycles, the photocatalytic activity of the Au/ZnO NPs has a slight decrease derived from the loss of Au/ZnO NPs during the recycle process (Fig. 3e). However, the catalyst still remains unaffected throughout PXRD measurements (Fig. S8b†).

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Dalton Trans., 2014, 43, 16981–16985 | 16985

ZnO nanoparticles derived from ZIF-8 with visible light photocatalytic hydrogen production and degradation dye activities.

A facile two-step strategy was developed to fabricate yellow fluorescent glutathione-Au nanoclusters/zeolitic imidazolate framework-8 nanoparticles (G...
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