Microporous and Mesoporous Materials 267 (2018) 185–191 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso MoO3 nanoparticle formation on zeolitic imidazolate framework-8 by rotary chemical vapor deposition T Matteo Cipriana, Peng Xua, Somboon Chaemchuena, Rong Tua, Serge Zhuiykovb, Philippe M. Heynderickxb,d, Francis Verpoorta,b,c,∗ a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China Center for Environmental and Energy Research (CEER), Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 406-840, South Korea c National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation d Department of Green Chemistry and Technology (BW24), Faculty of Bioscience Engineering, Ghent University, 753 Coupure Links, Ghent B-9000, Belgium b A R T I C LE I N FO A B S T R A C T Keywords: MoO3 nanoparticles ZIF-8 Photocatalysis Rotary chemical vapor deposition For the first time, MoO3 nanoparticles (NPs) with a size ranging from 1.5 to 60 nm were deposited on spray dried zeolitic imidazolate framework-8 (ZIF-8) by rotary chemical vapor deposition (RCVD) in order to improve its photocatalytic performance. A direct effect of the deposition time on the metal oxide loading was observed. In a time frame between 0.9 and 2.7 ks the metal oxide loading can be increased from 1 to 3 wt%. All the MoO3-NPs/ ZIF-8 catalysts were tested towards the methylene blue photodegradation using sunlight. MoO3-NPs/ZIF-8 3 wt% RCVD reached a conversion of 82% and 95% after 180 and 300 min, respectively. 1. Introduction Zeolitic Imidazole Frameworks (ZIFs) [1,2] are an interesting subclass of metal-organic frameworks (MOFs) having an extended 3D crystalline structure consisting of metal ions (e.g. Zn, Co, In) bridged in a tetrahedral fashion via the imidazolate linker. They combine the advantages of MOFs with higher stability and framework diversity. Zeolitic Imidazolate Framework 8 (ZIF-8), which is constructed with Zn (II) ions and 2-methylimidazole ligands, has received significant attention due to its high thermal stability in aqueous solutions [3], application in gas uptake and separation, as well as drug delivery [4–6]. Additionally, this new class of materials created an increasing attention as photocatalyst with promising results [7,8]. MOFs structural key feature is the highly tunable specific surface area and porosity, which with the encapsulation of active species and nanoparticles, can enhance their catalytic performances [9–11]. Titanium oxide (TiO2) is an efficient photocatalyst under UV light and extensive research has been executed to modify its electronic structure by applying nanoparticles to improve the photo response under solar light radiation [12–15]. Among the n-type semiconductors, molybdenum oxide (MoO3) is an ideal candidate as doping agent and has generated much research interest due to its wide application in various fields such as photoluminescence [16], optical fibers [17], scintillation materials [18] and photocatalyst for dye degradation [19,20]. A monolayer-dispersed MoO3 on TiO2 has been synthesized by ∗ impregnation technique [21], reaching pour performances. To further advance the photochromic properties, numerous strategies such as metalorganic decomposition [22], hydrothermal method [23], surfactant micelle nucleation [24] and sol-gel process [25] have been researched achieving MoO3-TiO2 nanostructures with a significant improvement. However, with these techniques, the use of surfactants and high temperature second stage thermal treatments are required. A new versatile technique is the rotary chemical vapor deposition (RCVD) [26,27]. This one step solvent free process is proven to be effective towards a uniform nanoparticle deposition on powder supports [28–30]. The rotary reactor can ensure the contact between the precursor gasses and the powder, in combination with a stable floating state and a uniform temperature distribution. In the present study, MoO3 nanoparticles (NPs) were generated for the first time on spray dried ZIF-8 by RCVD at 250 °C using Mo(CO)6 as precursor. Subsequently, MoO3-NPs/ZIF-8 was characterized and its photocatalytic performance was evaluated against methylene blue degradation. 2. Experimental section 2.1. Materials Zinc acetate ((Zn(OAc)2·2H2O); ≥99%), 2-methylimidazole (2mIm; 98%), molybdenum oxide (MoO3; 99.5%) and methylene blue (MB) were purchased from Aladdin Ltd. All reagents and solvents were Corresponding author. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China. E-mail address: Francis.verpoort@ugent.be (F. Verpoort). https://doi.org/10.1016/j.micromeso.2018.03.028 Received 18 January 2018; Received in revised form 7 March 2018; Accepted 24 March 2018 Available online 29 March 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved. Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. range 0.01 < P/P0 < 0.05 and pore volume P/P0 0.15. Prior to the adsorption measurements, the samples were evacuated at 200 °C under vacuum for a period of 3 h. analytical grade and used directly without further purification. 2.2. ZIF-8 spray dry synthesis ZIF-8 was synthesized by spray dry apparatus (AF-8000, AFind Scientific Instruments Co.) with a modified procedure as reported elsewhere [31]. In a typical experiment, a solution of Zn(OAc)2·2H2O (16 mmol) and 2-methylimidazole (16 mmol) are dissolved in 50 mL of methanol and spray-dried at a feed rate of 11.5 mL min−1, a flow rate of 4.6 × 106 mL min−1 and an inlet temperature of 180 °C. Once the precursor solution is nebulized, a dry white powder appears in the spray dryer collector. The ZIF-8 (300 mg) was then suspended in methanol overnight and dried at 60 °C in vacuum oven. The characteristic cubic structure of the ZIF-8 was confirmed with emission scanning electron microscopy (SEM – Phenom, Ted Pella Inc.), showing an average diameter of 10 ± 0.5 μm. The ZIF-8 crystalline phase was identified by a X-ray diffractometer equipped with Cu-Kα (λ = 0.15406 nm) (XRD – Rigaku Ultima III, Japan) at 40 kV and 40 mA. The diffraction angle ranges from 5 to 45° at a scan speed of 2°/ min. The Brunauer-Emmett-Teller (BET – ASAP 2020, Micromeritics) gave a specific surface area of 1600 m2 g−1. It is worth mentioning that the feed rate was critical for the quality of the crystals, a lower flow rate led to the formation of needle shaped crystals. 2.5. Photocatalytic degradation The photocatalytic activity of MoO3-NPs/ZIF-8 was evaluated in terms of degradation of methylene blue (MB) under artificial sun light irradiation using a PL-XQ 350W Xenon light source. A general procedure was carried out as follows: 50 mg of the catalyst powder was added into 200 mL dye aqueous solution (10 mg/L) and the suspension was magnetically stirred for 30 min in a dark environment. Only then the suspension is placed into a water-cooled reactor at 25 °C and exposed to the light source. At specific time intervals, 3 mL aliquots of the dye aqueous solution were pipetted and filtered through 0.22 micron Millipore filters to remove the suspended catalyst particles. The dye degradation progress was assessed by UV–Vis spectroscopy (UV-1800 Shimadzu) maximum absorbance at 664 nm [34]. 3. Results and discussion 3.1. Preparation and characterization of MoO3-NPs/ZIF-8 catalyst The metal oxide loading was carefully controlled by the ZIF-8 residence time in the reactor chamber. The resulting MoO3-NPs/ZIF-8 was retrieved as a light yellow powder. As presented in Fig. 1, the PXRD pattern remained unchanged after the nanoparticles integration to the ZIF-8 structure. A decrease in intensity of the Bragg peaks can be noticed throughout the spectrum, which can be explained by the presence of MoO3 nanoparticles on the surface interacting with the framework atoms, causing a change in the charge distribution and electrostatic field [35]. The absence of MoO3 diffraction peaks is due to the low metal oxide loading. Furthermore, no other assignable Bragg peaks of impurities or any other phases can be identified, confirming that the ZIF-8 structure remains unaltered after the nanoparticle deposition procedure. The ZIF-8 and MoO3-NPs/ZIF-8 morphology were characterized by SEM. In Fig. 2(a) it can be clearly observed that ZIF-8 exhibits the characteristic truncated cube structure {100, 110}, and undoubtedly unaltered after the metal oxide deposition. Fig. 2(b and c) reveals how the nanoparticles (60 nm in average) are distributed on the ZIF-8 surface. The presence of MoO3 is further confirmed by SEM-EDX analysis (Fig. 2(d) taken from the region as indicated in Fig. 2(c)) [36–38]. Transmission electron microscopy (TEM) was performed to investigate the textural properties of the MoO3 nanoparticles deposited on the ZIF8 surface (Fig. 3). The diameter of the supported nanoparticles (Fig. 3(c)) confirms the size distribution depicted by SEM analysis, see Fig. 4(B), while Fig. 3(d) reveals nanoparticles with a smaller diameter, between 1.5 and 6 nm. Furthermore, considering that the pore size of the as-synthesized ZIF-8 is 2.4 nm (Table 1), we can deduce that the precursor gas can penetrate in to the ZIF-8 framework where the MoO3 nanoparticle can grow into the pore without exceeding its size [39]. The nanoparticle size distribution calculated from the TEM (inside the ZIF-8 pores) and SEM analysis (on the ZIF-8 surface) is reported in Fig. 4(A and B), respectively. The specific surface areas were investigated by N2 adsorption-desorption analysis. The pore volume and the specific surface area were extrapolated for the ZIF-8 and MoO3-NPs/ZIF-8 via the t-plot method. The isotherms display a Type I shaped curve which is characteristic for microporous materials, Fig. 5(a) [37]. As shown in Table 1, the BET and pore volume of ZIF-8 amounts to 1710 m2/g and 0.625 mL/g, respectively. After the metal oxide loading of 1.0, 2.0 and 3.0 wt% the BET and pore volume demonstrate a small decrease in a linear fashion. The change in specific surface area and pore volume can be attributed to some extent to the encapsulation of MoO3 nanoparticles into the ZIF-8 surface pores and cavities. Moreover, the incorporation of 2.3. Catalyst preparation via RCVD In order to functionalize the ZIF-8 for the photodegradation of methylene blue, molybdenum oxide was deposited on the ZIF-8 surface and in the pores via rotary chemical vapor deposition (RCVD) using Mo (CO)6 as precursor. Typically, ZIF-8 is thermally stable up to 400 °C [32], however, if the deposition is attempted at temperatures higher than 300 °C the ZIF-8 undergoes through a mild degradation [33], turning its characteristic white color to light brown. For this reason, the temperature of the RCVD reactor chamber was set at 250 °C to guarantee the ZIF-8 structural integrity and to ensure the precursor degradation in order to form the metal oxide nanoparticles. The RCVD is mainly constituted of a precursor chamber and a reactor where the support is placed (Fig. s1 and s2). Inside the rotary reactor, to ensure a proper dispersion of the ZIF-8, four blades are attached to the inner wall in order to increase the contact time of the reactant gas and powder. The temperature in the evaporator was set at 85 °C and the precursor was carried into the rotary reactor by oxygen with a flow rate of 8.3 × 10−7 m3 s−1, while the supply rate (Rs) was 2.2 × 10−7 kg s−1. The ZIF-8 powder was fed into the reactor and preheated at 250 °C. The total pressure of the RCVD apparatus was kept at 1.0 × 104 Pa. The deposition time was varied between 0.6 and 1.8 ks to control the amount of precipitated nanoparticles. 2.4. Characterization Powder X-ray diffraction (PXRD) patterns were performed on a Rigaku Ultima III diffractometer in a wide angle range (2θ = 5−30°) with Cu-Kα (λ = 0.15406 nm) 40 kV and 40 mA at a scan speed of 2°/ min. The ZIF-8 morphology was confirmed with emission Scanning Electron Microscopy (SEM – Phenom, Ted Pella Inc.), showing an average diameter of 10 ± 0.5 μm. The MoO3 nanoparticles morphology was observed by Transmission Electron Microscopy (FETEM – JEM-2100F, Jeol) and Scanning Electron Microscope (SEM – Phenom, Ted Pella Inc.). The ZIF-8 and MoO3-NPs/ZIF-8 powder were crushed and dispersed in water-free ethanol, followed by ultra-sonication before dropping onto a Cu-grid with lacey carbon film. The Mo loading on the ZIF catalyst was determined by Inductive Coupled Plasma Atomic Emission (ICP-AES, Optima 4300 DV, PerkinElmer Inc). The specific surface areas of the samples were determined by the Brunauer-EmmettTeller (BET) method using ASAP 2020 Micromeritics apparatus. The linearized BET and Langmuir equation were fit to the data within the 186 Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. Fig. 1. A) PXRD patterns of the synthesized ZIF-8 compared with the simulated pattern and B) display (a) ZIF-8 and MoO3-NPs/ZIF-8 from 1 to 3 wt% (b,c,d). Fig. 2. SEM of ZIF-8 (a), MoO3-NPs/ZIF-8 (b,c) and EDX spectrum (d). 187 Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. Fig. 3. TEM pattern for the ZIF-8 (a), MoO3-NPs/ZIF-8 on the scale of 0.5 μm (b), 200 nm (c) and 20 nm (d); insert: HRTEM lattice image of MoO3. Fig. 4. Nanoparticle size distribution derived from TEM (A) and SEM (B) analyses. 188 Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. Table 1 Texture properties of spray dried ZIF-8 and photocatalysts. Catalyst Metal loading (wt%) BET surface area (m2/g) Pore volume (mL/g) Average Pore width (nm) ZIF-8 MoO3-NPs/ZIF-8 1 wt% MoO3-NPs/ZIF-8 2 wt% MoO3-NPs/ZIF-8 3 wt% – 1.05 1.97 3.01 1710 1642 1580 1529 0.625 0.585 0.554 0.547 1.772 1.283 1.246 1.227 evaluated for the degradation of MB in aqueous solution under solar light. The photocatalytic degradation was carried out at 25 °C and to effectively dissipate the intense heat sourcing from the solar lamp, a water-cooled jacked glass reactor was used. Methylene blue exhibits a strong absorption at 664 nm, a change in maximum absorbance was used to monitor the dye's degradation (Fig. S4). As reference materials, MoO3 and ZIF-8 were also tested under the same experimental conditions. Fig. 8 displays the degradation curves for the MoO3-NPs/ZIF-8, ZIF-8 and MoO3. ZIF-8 synthesized by spray drying is an active catalyst towards MB photodegradation, while in absence of the photocatalyst, the degradation due to the metal oxide (MoO3) is rather insignificant (Fig. 8). However, under identical conditions MoO3-NPs/ZIF-8 3 wt% requires 180 min and 300 min of irradiation to achieve 82% and 95% dye degradation, respectively. In comparison, ZIF-8 achieved 60% dye degradation after 180 min and an additional 120 min to reach comparable results with MoO3NPs/ZIF-8 3 wt% photocatalysts. The photocatalytic performance of as-synthesized MoO3-NPs/ZIF-8 is determined from the MB photodegradation kinetics. The rate constant for the degradation reaction was calculated applying equation (1): nanoparticles does alter the pore size distribution, as seen in Fig. 5(b). The pore volume distribution of ZIF-8 shows two peaks at 1.7 and 1.85 nm which substantially diminished after the nanoparticle deposition, consistent with the fact that the introduced nanoparticle can occupy the ZIF-8 cavities [40,41]. To further confirm the metal deposition onto ZIF-8, a mapping of the prepared MoO3-NPs/ZIF-8 catalysts was executed using EDS. As shown in Fig. 6, molybdenum is uniformly dispersed on the ZIF-8 surface. The quantitative amount of molybdenum oxide was determined by Inductively Coupled Plasma (ICP) analysis, confirming a metal loading between 1 and 3 wt% (Table 1). The X-ray photoelectron spectroscopy (XPS) was used to further characterize the surface composition and the metal oxidation state of the resulting MoO3-NPs/ZIF-8. The XPS spectrum of the as-prepared MoO3NPs/ZIF-8 exhibited the presence of two well-resolved spectral lines at 232.8 and 235.9 eV [42–44], respectively, corresponding to Mo3d5/2 and Mo3d3/2 in the Mo3d region, see Fig. 7. This result confirms that molybdenum is present only as Mo6+ on the ZIF-8. 3.2. Photocatalytic activity Methylene blue is a thiazine type of dye and is largely used as industrial colorant (Fig. S3). It has some harmful effects on humans and animals. This dye causes eye burns, breathing disorders, heart rate increases, shock, cyanosis, jaundice, quadriplegia, tissue necrosis, nausea, vomiting, mental confusion, painful micturition and methemoglobinemia. In this study, MB was selected as a model dye [45]. Its photodegradation has gained an increasing interest. Yang et al. reported MoO3/TiO2 (mass ratio of 0.25) synthesized via impregnation followed by calcination at 500 °C [21]. Although MoO3 was able to increase the TiO2 activity, after 150 min the highest degradation reached only 38% under artificial solar light. A substantial improvement has been achieve by Khan et al. [46] with their MoO3-TiO2 3 wt% nanocomposite. The catalyst was synthesized in a surfactant free environment via femtosecond laser ablation from a mixed water solution of the two precursors. The obtained catalyst successfully degraded 95% of the MB after 120 min of light exposure. The photocatalytic activity of the as-prepared MoO3-NPs/ZIF-8 was -ln(C/C0) = kt (1) where C is the MB concentration at any given time (t), C0 is the initial concentration, k is the rate constant. The photocatalysts display the pseudo-first order reaction kinetics and their rate constant is reported in Table 2. The molybdenum oxide showed a negligible activity, while deposited as nanoparticles on ZIF-8, a substantial improvement of the ZIF-8 performance was observed. Jing et al. [47] recently reported the methylene blue photodegradation by ZIF-8. In their research ZIF-8 was synthesized by slow evaporation leading to a BET and pore volume of 1799 m2/g and 0.6866 cm3/g, respectively. The increase in porous properties had a direct effect on the photocatalytic performance, 80% of MB was degraded after 80 min under similar conditions. In comparison with the reaction settings used in this report, a higher catalyst concentration (0.5 g/L) was used in the study of Jing, while in this work the amount of catalyst was set a 0.25 g/L. Furthermore, Lee et al. [48] studied the Fig. 5. N2 adsorption-desorption isotherms (a) and pore size distribution (b) for ZIF-8 and MoO3NPs/ZIF-8 1-3 wt%. 189 Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. Fig. 6. SEM images of the MoO3-NPs/ZIF-8 (a,b) and corresponding elemental mapping of Zn (c) and Mo (d). Fig. 8. Degradation curves of MB. catalytic outcome towards the same reaction using ZIF-8 synthesized via different methods. Each method led to a specific array of physical and texture properties (BET, pore volume, particle size). As a direct consequence, the ZIF-8 catalytic performance can vary greatly, from 57% to 98%. The stability of a catalyst is essential, therefore, the structural stability of the used MoO3-NPs/ZIF-8 was examined by PXRD. As shown in Fig. S5, the distinctive diffraction peaks of the catalyst are well defined, Fig. 7. XPS spectrum of Mo3d core level doublet in MoO3-NPs/ZIF-8. 190 Microporous and Mesoporous Materials 267 (2018) 185–191 M. Ciprian et al. Table 2 Calculated rate constants and R2 for the photodegradation reaction of MB over different catalyst. Catalyst k (min−1) R2 ZIF8 MoO3-NPs/ZIF-8 1 wt% MoO3-NPs/ZIF-8 2 wt% MoO3-NPs/ZIF-8 3 wt% MoO3 0.0055 0.0068 0.0080 0.0097 0.0003 0.9695 0.9828 0.9852 0.9887 0.9912 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] suggesting that the high crystallinity of the sample is preserved and no significant crystallographic degradation occurred. The FTIR spectroscopy was used to further evaluate the molecular integrity of the asprepared catalyst after its use (Fig. S6). The ZIF-8 characteristic band at 420 cm−1 (Zn-N stretching), 1146 and 995 cm−1 (C-N stretching) and 1584 cm−1 (C=N stretching) are observed [49] and exhibiting an unchanged pattern, confirming that the chemical structure of MoO3-NPs/ ZIF-8 remained unchanged. The MoO3-NPs/ZIF-8 morphology remained virtually unaltered after the reaction course, as shown in Fig. S7, and the metal content is further confirmed by ICP-AES analysis (Table S1). The combination of these results proved that the photocatalyst was stable under the reaction conditions and could be used multiple times (Fig S8). [19] [20] [21] [22] [23] [24] [25] [26] [27] 4. Conclusion [28] [29] [30] [31] In summary, MoO3-NPs were deposited on spray dried ZIF-8 via rotary chemical vapor deposition for the first time. The prepared MoO3NPs/ZIF-8 was evaluated for the photodegradation under sunlight of MB dye pollutant. Most importantly, the degradation performance of the spray dried ZIF-8 catalyst has been significantly improved by the MoO3 nanoparticles deposition. The MoO3-NPs/ZIF-8 3 wt% achieved an efficiency of 82% and 95% after 180 and 300 min, respectively, following a pseudo first order reaction kinetic with a constant of 0.0097 min−1. In light of these results, MoO3-NPs are able to enhance ZIF-8 performance and find application as an effective dye photocatalyst. [32] [33] [34] [35] [36] [37] [38] [39] [40] Acknowledgement The authors are grateful to the State Key Lab of Advanced Technology for Materials Synthesis and Processing for financial support (Wuhan University of Technology). F.V. acknowledges the support from the Tomsk Polytechnic University Competitiveness Enhancement Program grant. [41] [42] [43] [44] References [45] [1] Z. Li, H.C. Zeng, Chem. Mater. 25 (9) (2013) 1761–1768. [2] D. Fairen-Jimenez, S.A. Moggach, M.T. Wharmby, P.A. Wright, S. Parsons, T. Düren, J. Am. Chem. 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