Carbon Sponge-based

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MOF-Derived Porous ZnO Nanocages/rGO/Carbon Sponge-based Photocatalytic Microreactor for Efficient Degradation of Water Pollutants and Hydrogen Evolution Yiping Su, Shun Li, Dongsheng He, Dongfang Yu, Fei Liu, Ningning Shao, and Zuotai Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02287 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Sustainable Chemistry & Engineering

MOF-Derived Porous ZnO Nanocages/rGO/Carbon Sponge-based Photocatalytic Microreactor for Efficient Degradation of Water Pollutants and Hydrogen Evolution

Yiping Su,† Shun Li,*,† Dongsheng He,‡ Dongfang Yu,† Fei Liu,† Ningning Shao,† and Zuotai Zhang*,†

†

School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of

Soil and Groundwater Pollution Control, Southern University of Science and Technology (SUSTech), 1088 Xueyuan Road, Shenzhen 518055, PR China ‡

Materials Characterization and Preparation Center (MCPC), Southern University of Science

and Technology (SUSTech), 1088 Xueyuan Road, Shenzhen 518055, PR China

*Corresponding author: Emails: [email protected]; [email protected]

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Abstract Semiconductor photocatalysts are of great importance for addressing current environmental and energy crises. However, traditional powdery photocatalytic materials suffer from drawbacks including difficult separation and weak durability, which do not conform to the practical demand. Herein, we developed a simple dipping-pyrolysis route to fabricate a novel 3D structure photocatalytic microreactor, in which ZIF-8 derived hollow ZnO nanocages are uniformly and firmly distributed throughout the surface of reduced graphene oxide (rGO) coated carbon sponge frameworks. The obtained carbon network based-porous ZnO bulky photocatalysts exhibit excellent absorption and photocatalytic degradation performance for organic pollutant, as well as high solar hydrogen production activity. With robust structure, good maneuverability and convenient recyclability, these monolithic microdevices show a promising prospect toward industrial photocatalytic applications in sustainable environmental remediation and renewable energy generation. Furthermore, our work also demonstrates a versatile synthetic strategy that can be extended to the design of a broad series of porous functional metal oxide/carbon foam bulk composites with high photocatalytic performance and general adaptability.

Key words: Carbon foam; Hollow ZnO nanocages; Photocatalysis; Pollutants degradation; Hydrogen production

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Introduction The direct conversion of abundant sunlight into chemical energy by semiconductor photocatalysis has emerged as one of the most attractive strategies for environmental purification and renewable energy generation.1 In the overwhelming majority of photocatalytic reaction processes, nanosized semiconductor photocatalysts are used in powdery form due to the high photocatalytic activity and relative simplicity of fabrication. However, these nanomaterials suffer from numerous disadvantages such as easy agglomeration, difficult and costly post-separation, as well as weak durability, which strongly hinder their practical photocatalytic applications.2-4 Therefore, to effectively prevent undesirable aggregation or release into environments of the nanoscale photocatalysts and at the same time maintain their activity, it is essential to anchor them on the surface of suitable substrates or frameworks with open cell structure that are solid and robust to avoid any structural deformation or collapse in the course of service.5-8 To this end, the design and engineering fabrication of free standing monolithic photocatalysts have attracted great attention owing to their mass productability, low manufacturing cost, simple processibility, as well as multiple functionalities.2, 9, 10 Combining the advantages of photocatalyts and absorbent materials, these 3D monolithic composite photocatalysts exhibit superior performance than a single material, making these integrate more applicable to cope with the facing problems of the current photocatalyst for practical solar-powered water purification and hydrogen production. A great number of works have attempted to combine or immobilize micro/nanostructured semiconductor photocatalysts on various substrates or support including hydrogels,8 cellulose nanofibers,11 ceramics,12 carbon materials,7, 13-16 polymers17, 18 and so on. In particular, bulky materials with 3D networks can expose more catalyst surface. Their unique open 3



macropore systems also benefit the fast diffusion of reactants to the catalyst surface. Case studies indicate carbon9 and polyurethane foam3 present good structure adaptability, in which both the interconnected fibrils and open cell pores are ideal as scaffolds for the immobilization of nanosized photocatalyst, leading to a composite absorber with photocatalytic functionality. For example, Ge et al. fabricated robust 3D Ag/AgCl nanowire networks on the backbone of melamine polymer foam as photoreactor for continuous high throughput degradation of methyl orange.5 In addition, carbon foam supported TiO2 and CuO microreactors were developed for simultaneous H2 production and pollutant spill remediation.19 Very recently, a new kind of bulky composites consisting of BiOX (X = Cl, Br, I) and polyurethane foam were reported by Wang et al, demonstrating efficient photocatalytic activity towards removal of organic azo dyes, antibiotic and phenol, with high stability and easy recyclability.3 Despite of these progresses, porous network or matrix absorber supported semiconductor nanomaterials applied in photocatalysis is still in their early stage of investigation. It is urgently required to explore new approaches, materials and systems with enhanced absorption performance, photocatalytic activity and stability to meet the practical demand. Recently, significant effort has been devoted to the catalytic applications of porous materials owing to their outstanding properties including abundant exposed active sites and efficient transportation of reactants through the interior space. Among them, metal-organic frameworks (MOFs) as templates derived porous materials such as metal oxides, carbons or their hybrids have received considerable attention in a wide range of applications.20-27 To some extent, the resulting derivatives can maintain the initial intriguing structural features of parent MOFs, such as large surface area, open-framework structure and surface permeability, which can endow them with 4


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enhanced harvesting of exciting light and charge transportation, thus enabling highly improved photocatalytic performance.28-31 However, in essence, MOFs derived nanomaterials usually exist in the form of micro-/nano-powder and the poor processability becomes a big challenge for them to be used in real industrial applications. Therefore, the elaborate design and facile fabrication of monolithic composite consisting of porous or hollow structures derived from MOFs and suitable 3D foam support is of great importance, but challenging in practice. Herein, we report a simple dipping-pyrolysis method to prepare carbon sponge supported porous ZnO nanocage photocatalytic microreactors by simply calcining melamine foam (MF) and ZIF-8 polyhedra architectures as solid precursor in air at suitable temperature. To the best of our knowledge, this is the first report of free-standing monolithic composite combining a 3D carbon foam support with functional MOFs derivatives that can effectively degrade water pollutants and producing hydrogen under solar irradiation. MF was chosen as the substrate for its attractive and practical attributes of light weight, effective absorption and low cost commercially availability.32, 33

The carbonization of the MF foam support and formation of ZIF-8 derived ZnO hollow cages

could be accomplished via a simple pyrolysis treatment in a single step. In our design, graphene oxide (GO) was coated on the surface of MF as a pre-modification step, which could provide abundant possible binding sites for implanting ZIF-8 on the surface, to form robust 3D MOFs-derived porous ZnO nanocages/reduced GO (rGO)/carbon sponge (ZRCs) networks. Moreover, both rGO and carbon can serve as a conducting medium for rapid electron-hole separation and charge transport, effective solar light absorption, and an interconnected framework for fast interfacial charge transfer.34 We anticipate that the obtained 3D structuring ZRCs monolithic photocatalyst and the synthetic strategy will promote the practical application of 5



solar-mediated high efficient, low cost and durable water-purification and hydrogen production. Experimental Methods Preparation of graphene oxide (GO), ZIF-8, and ZIF-8 derived ZnO nanocages. The GO was prepared via the oxidation of graphite powder (325 mesh, Qingdao Huatai Lubricant Sealing Science and Technology Co. Ltd., Qingdao, China) according to a modified Hummers method.35 The GO solution was firstly filtered and washed with HCl (250 ml, 1:10) aqueous solution to remove metal ions, followed by repeated washing with distilled water and centrifugation to remove the acid.36 Finally, the GO hydrogel was freeze-dried. ZIF-8 [Zn(2-methylimidazole)2·2H2O] was prepared by solution precipitation following the method as described by Torad et al.37 In a typical synthesis, Zn(NO3)2 (1.42 g) was dissolved in 10 ml methanol, and dimethylimidazole (2.46 g) was added in 90 ml methanol, respectively. After mixing these two solutions together, the solution immediately turned into milk-like suspension. Then the suspension was stirred and aged for 4 h at room temperature to complete the crystallization. Finally, the ZIF-8 powders were collected and washed several times by distilled water, and were dried at 90 °C overnight. ZIF-8 derived ZnO (ZnOZIF-8) nanocages were prepared by calcining the as-synthesized ZIF-8 powder in air at 350 °C for 3 h with a heating rate of 2 °C min-1. Preparation of ZnOZIF-8/rGO/carbon sponge (ZRCs) and ZnOZIF-8/carbon sponge (ZCs). A facile dipping-pyrolysis method was employed to prepare the ZRCs photocatalyst. First, the as-obtained GO aerogel was dispersed in water (2 g/L) by ultrasonication for 1 h. Meanwhile, the MF was cut into blocks and ultrasonically cleaned with acetone and water to remove contaminants, and was dried at 60 °C in an oven. The MF sponge was then immersed into the GO solution for 1 6


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h. After washing with copious amounts of distilled water, the GO-coated MF sponge was dried in a vacuum oven for 10 h at 60 °C. Subsequently, the obtained samples were dipped in pre-prepared ZIF-8 solution (1 g/L) for 1 h at room temperature and dried at 60 °C. This procedure was repeated for three times. Finally, the ZRCs was obtained by direct calcination of the obtained sample at 350 °C for 3 h with a heating rate of 2 °C/min. For better comparation, ZCs without GO coating was also prepared under the same experimental conditions. Characterizations. X-ray powder diffraction (XRD) was performed on a Rigaku SmartLab X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.154 nm) operated at 45 kV. Characterization of sample morphology and chemical composition were carried out by a field emission scanning electron microscopy (FESEM, Zeiss Merlin) equipped with an energy-dispersive X-ray spectrometer (EDX). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by a FEI Talos F200X electron microscopy. X-ray photoelectron spectra (XPS) were recorded using an ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific) fitted with an Al Kα radiation exiting source. UV-vis diffuse reflectance spectra (DRS) were measured by a UV-vis spectrometer (Shimadzu, UV-2600) equipped with an integrating sphere attachment. Fourier transform infrared (FTIR) spectra were obtained on a Nicolet iS50 FT-IR spectrometer from 400 to 4000 cm-1 at room temperature. Thermogravimetric (TG) analysis was carried out using a Setsys EVO Easy 1750 (SETARAM) Jupiter thermoanalyser under an air atmosphere. The specific surface areas and pore size distribution were calculated from the N2 physisorption data obtained at 77 K (Micromeritics ASAP 2020 M system) using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method, respectively. 7



Photodegradation evaluation. The photocatalytic activities of the samples were tested by decomposition of rhodamine B (RhB) under simulated solar light irradiation with a 300 W Xe lamp (light intensity: ca. 200 mW cm−2). The irradiation distance between the mixture solution and the lamp was about 15 cm. In a typical experiment, 50 mg sponge photocatalyst was put into 100 ml of RhB (10 mg/L) aqueous solution. Prior to irradiation, the sample was soaked in the solution for 0.5 h under dark to obtain the adsorption-desorption equilibrium. The concentration of RhB solution was analyzed using a UV-vis spectrophotometer (Cary 5000) by monitoring the maximal absorbance peak value. Photocatalytic H2 evolution. The photocatalytic H2 production reaction was carried out in a Pyrex reaction cell connected to a closed gas circulation system (Labsolar-6A, Beijing Perfectlight Co. Ltd.). Typically, 50 mg of the as-synthesized sponge photocatalyst was placed into 120 ml of aqueous solution containing 25% volume of methanol as a sacrificial reagent. The suspension was then degassed and irradiated by a Xe lamp (300 W, PLS-SXE300, Beijing Perfectlight Co. Ltd.). The amount of generated H2 was determined with a gas chromatography (Techcomp GC-7900). Results and discussion The fabrication procedures of the ZRCs photocatalytic microreactor are illustrated in Fig. 1. Briefly, a MF sponge (Fig. 1a) was firstly coated with GO through dipping and drying process. It is clearly seen that the sponge turns grey after drying, as shown in Fig. 1b. Then the GO coated MF sponge was immersed into pre-prepared ZIF-8 suspension and was dried in air (Fig. 1c). Finally, black ZRCs photocatalyst was obtained by heating the obtained ZIF-8/GO/MF sponge sample at 350 °C for 3 h. As shown in Fig. 1d, the carbonization process caused a significant volume shrinkage, which is owing to the release of a great number of nitrogen from the MF during 8


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calcination as N2 and NH3 gas.19 The morphology of these hybrid sponges was examined by scanning electron microscopy (SEM). As shown in Fig. 2a, the pristine MF sponge shows an interconnected highly porous structure with a clean and smooth skeleton. Figure 2b reveals that a thin layer of GO sheets have been evenly coated on the backbone of the MF sponge. The GO modified interconnected fibrils offer an ideal template for anchoring nanosized photocatalytic materials. Different from the MF sponge without GO coating (Fig. S1), polyhedron-shaped ZIF-8 structures with grain size around tens of nanometers can be easily immobilized throughout the surface of the GO coated MF fibril uniformly and firmly (Figs. 2c and 2d). Consequently, after thermal treatment, well scattered porous ZnO nanocages were formed on the rGO surface, as shown in Figs. 2e and 2f, demonstrating the successful loading of MOFs-derived ZnO hollow structures on the rGO coated carbon foam. The SEM image of Fig. 2e also reveal that the obtained carbon foam inherits a similar interconnected network architecture from MF polymer, with reduced size of the open cell and diameter of the fibrils. Consistent with previous report, MF derived porous carbon foam drastically increase the surface-area-to-volume ratio, thus bringing the contaminants or water in closer proximity to the photocatalytic materials.19 Meanwhile, these ZnO structures maintain similar porous structural features of the ZIF-8 precursor template, which is also beneficial for enhancing the photocatalytic performance.16 To get a deeper understanding of structural and morphological characteristics of the ZnO hollow nanocages grown on the carbon foam, we further investigated freestanding ZIF-8 and its derived ZnO nanostructures. Both TEM image (Fig. S2a) and SEM image (Fig. S2b) of ZIF-8 nanoparticles exhibit typical hexagonal facets with sizes range from 30 to 100 nm. After heat 9



treatment, ZnOZIF-8 almost retain the same morphology of ZIF-8 (Fig. 3a), indicating that framed porous ZnO can be obtained by controlling the thermal treatment of ZIF-8. The dodecahedral external surfaces of ZIF-8 transforms into external concave surfaces structure, which is caused by the decomposition of the organic ligands of ZIF-8 frameworks. The HRTEM image of ZnOZIF-8 (Fig. 3b) displays resolved interplanar distance of about 0.25 nm, corresponding to characteristic lattice fringe of (101) crystallographic plane of ZnO. The identified amorphous area in ZnOZIF-8 particles confirms the intimate interfacial contact between ZnO and carbon framework after the pyrolysis process. In addition, the high-angle dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. 3c) and corresponding elemental mapping images of ZIF-8 derived ZnO demonstrate a homogeneous distribution of Zn (Fig. 3d) and N (Fig. 3e) elements. The existence of N element is attributed to the incomplete decomposition of ZIF-8 as well as doping into ZnO lattice, which will be discussed in detail in the following parts. X-ray diffraction (XRD) was adopted to determine the crystal structures of the as-prepared samples. Figure 4a clearly presents the structure changes of ZIF-8 before and after annealing. For the ZIF-8 derived ZnO sample, a superimposition pattern of ZnO and ZIF-8 was observed, in which the distinct diffraction peaks (2θ) at 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, 66.3°, 67.9°, 69.0°, 73.5° and 76.9° could be attributed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) crystal planes of ZnO (JCPDS No.79-0206). Except for those diffraction peaks assigned to ZnO, the residual diffraction peaks appears at 2θ = 12.7° and 18.0° match well with the simulated XRD pattern of ZIF-8,38 as shown in the dashed box in Fig. 4a. Compared to MF polymer derived carbon sponge that only shows a broad amorphous peak, ZRCs sample clearly exhibits diffraction peaks from ZnO as well, shown in Fig. 4b. In addition, peaks 10


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from ZIF-8 were also detected in the ZRCs sample (dashed box in Fig. 4b) besides that of ZnO and amorphous carbon, demonstrating the incomplete pyrolyzation of the MOFs precursor template due to the relatively low calcination temperature used in the present study. These results are in good agreement with the elemental mapping images as discussed earlier. The structural characteristics of the as-prepared ZIF-8, ZnOZIF-8 and commercial pure ZnO nanoparticles were further examined by Fourier transform infrared (FTIR) spectroscopy, as demonstrated in Fig. S3. For ZIF-8 sample, a sharp peak at 423 cm-1 assigned to Zn-N stretch mode was detected.39 Besides, the bands in the spectral region of 500-1350 cm−1 and 1350-1500 cm−1 are attributed to the plane bending and stretching of imidazole ring, respectively. The C=N stretch mode at 1584 cm−1 was also observed.40 Compared to that of ZnO, the spectrum of ZnOZIF-8 displays additional adsorption bands that are associated with the ZIF-8 structure. These results again suggest that ZIF-8 did not completely decompose after annealing, consistent with the XRD characterizations. Thermogravimetric analysis (TG) was used to evaluate the composition of the as-prepared ZRCs sample (Fig. S4). Pure ZnO exhibits a constant weight below 750 °C, while the TG curve of ZRCs reveals ca. 67% weight loss up to 700 °C, mainly owing to the oxidation of the carbon. Estimated from the TG measurement, ZnO photocatalyst shares only ca. 33% of the mass in the ZRCs. Our results are similar with previous reports on ZIF-8 derived ZnO by Liu et al.,41 in which the major mass losses of ca. 64.5% occurred between 300 and 500 °C, and the ZIF-8 framework was completely destroyed above 500 °C. Therefore, to keep both the structural feature of the MOFs framework precursor and the MF 3D foam network, the heating rate and temperature must be controlled carefully. After optimizing, a slower heating rate (2°C min-1) and a moderate 11



calcination temperature (350 °C) were chosen in the present study, by which the MF derived carbon sponge would not collapse and the beneficial porous morphology of the ZIF-8 could be remained in the final monolithic carbon sponge supported ZnO photocatalytic microreactors. The N2 adsorption measurements for the carbon sponge and ZRCs were carried out and the surface areas and pore size distribution were determined by the BET and BJH methods, respectively. Pristine sponge derived carbon sponge shows a specific surface area of 4.29 m2 g−1 (Fig. S5a) without the characteristic pore structure (inset of Fig. S5a). By contrast, ZRCs shows an enhanced specific surface area of 7.46 m2 g−1 (Fig. S5b) and a well-defined mesopore distribution (inset of Fig. S5b), which is originated from ZIF-8 derived porous ZnO. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical states of the ZRCs sample, as shown in Fig. 5. The survey scan spectrum in Fig. 5a confirms the existence of the characteristic C 1s peak in addition to O 1s and Zn 2p peaks. As shown in Fig. 5b, binding energies of 1020.8 and 1044 eV are indicative of Zn 2p3/2 and Zn 2p1/2 peaks, respectively. The high-resolution spectrum of C 1s peak is presented in Fig. 5c, which can be fitted into four peaks. The main peak at 284.8 eV can be ascribed to adventitious carbon and sp 2-hybridized carbon from the carbon layers (C–C).41 The other two peaks at 286.2 eV and 288.9 eV should be originated from the hydroxyl carbon (C–O) and carboxyl carbon (O=C–O), respectively.42 This may induce a long-tail absorption in the visible-light region for the present carbon-modified ZnO,43, 44 which will be discussed in the following parts. Moreover, the peak area at 288.9 eV of ZRCs is larger than that of ZCs, which indicates the existence of more carboxyl carbon on the surface of ZRCs after introducing rGO layer. The presence of Zn–C peak located at 283.6 eV indicate that part of carbon is successfully doped into the ZnO lattice.45, 46 According to previous 12


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reports, the N elements can be incorporated into the ZnO lattice in at least two chemical states, either as a N2 molecule (ON2) or a N atom (ON) occupying an O site, depending on the synthetic methods.47, 48 Two deconvoluted peaks centered at 398.8 and 398.0 eV were identified for the N 1s spectrum (Fig. 5d). The peak located at 398.8 eV represents a typical N 1s binding energy in amines, and the peak of 398.0 eV can be attributed to the N 1s of oxynitride (O-Zn-N).49, 50 These results indicate that the N element was successfully incorporated into O sites of ZnO.45 UV-vis diffuse reflectance spectrum (DRS) was measured to investigate the optical absorption capability of the ZRCs, which are key factors for determining their photocatalytic activity. The commercial pure ZnO powder and as-prepared ZIF-8 derivatives were also investigated for comparison. Different from ZnO, ZnOZIF-8 shows dark yellow color (inset of Fig. 6a), possibly due to the presence of the residual ZIF-8,51 which has been confirmed by both XRD and FITR analysis. As shown in Fig. 6a, the absorption edge of the ZnOZIF-8 sample clearly shifted to the visible region, indicating the doping of C and/or N into the lattice of ZnO. For ZRCs sample, the long-tail absorption in the visible-light region should be induced by the carbon network foam and rGO layer. According to Tauc’s equation, for direct bandgap semiconductors, the band gap (Eg) can be estimated from a plot of (αhν)2 versus the photo energy (hv). As can be seen in Fig. 6b, commercial ZnO presents an absorption edge at 385 nm, which corresponds to an Eg of 3.22 eV. While the Eg of ZnOZIF-8 and ZRCs were estimated to be 3.0 eV and 2.84 eV, respectively. The carbon modications either by doping or surface coating will affect the light absorption properties.41 As discussed above, the relatively low sintering temperature resulted in incomplete decomposition of ZIF-8 frameworks and lower degree of crystallinity of ZnOZIF-8, which leads to the formation of localized defect states by favoring the doping process. Hence, visible-light 13



responsive porous ZnO loaded carbon foam photocatalysts with C and N doping and carbon modication were fabricated by calcining the pre-synthesized ZIF-8/GO/MF sponge in air at suitable calcination temperature. As a demonstration of potential applications, we assessed the photocatalytic performance of these monolithic materials for both pollutant degradation and hydrogen production from water. The performance of photocatalytic carbon sponge device was tested by means of degradation of RhB, a common dye pollutant. Figure 7a shows the degradation of RhB in the absence and presence of different photocatalysts. Negligible degradation was found in the blank test without catalyst or absorbent, demonstrating that RhB is stable under illumination. The carbon foam absorbed more than 20% of the RhB dye molecule, but no degradation occurred under illumination. The ZCs exhibited a degradation efficiency of 89% within 2 h under UV-vis irradiation. Figure 7b shows the UV-vis spectra of RhB photodegradation over the irradiation time for ZRCs photocatalyst, and the inset shows clearly the color changes of RhB dye before and after photocatalytic reaction. Enhancement in the degradation of RhB was obtained for the ZRCs sample with an efficiency of 99% under the same conditions, which can be attributed to the increased density of active sites as well as low recombination of electron-hole pairs by introducing of rGO layer on the surface of the carbon sponge. To quantitatively analyze the catalytic efficiency, the RhB degradation rate constants were extracted using the pseudo-first-order kinetics by plotting Ln(C/C0) vs. degradation time t (C indicates the dye concentration at the time of t, whereas C0 is the initial dye concentration), as shown in Fig. 7c. The results reveal that the photocatalytic behaviours for degradation of RhB over as-prepared photocatalysts are well fitted with pseudo-fist-order kinetics. The ZRCs sample 14


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exhibits the highest photodegradation rate constant (0.0229 min−1), which is slightly higher than that of ZCs (0.0168 min−1). Moreover, the photocatalytic stability of ZRCs sample was examined upon repeated photoreactions under the same conditions, as exhibited in Fig. 7d. It can be noticed that the ZRCs sample did not show apparent decrease of the photocatalytic activity for the degradation of RhB after three cycles, indicating that the 3D monolithic foam catalyst prepared in the current work possesses excellent photostability under light illumination. To better assess the activity of ZRCs in the present work, we have compared our results with relevant reports using ZnO based nanomaterials as photocatalysts. The degradation efficiency is higher than that of reported ZnO hollow spheres,52 nanoflowers53 and nanowires,54 which elucidates that the present ZRCs sample is highly promising for the photodegradation of RhB. In addition, the hydroxyl radicals (•OH), holes, and superoxide radicals (•O2−) during the photodegradation of RhB over ZRCs were investigated with the addition of isopropanol (•OH scavenger),55 EDTA-2Na (holes scavenger),56 p-benzoquinone (•O2− scavenger).57 The addition of trapping agents could decrease the photodegradation rate of RhB. Among them, isopropanol decreased the rate most significantly (Fig. 8), indicating that the •OH are the major oxidative species in the mineralization of RhB by ZRCs. The photocatalytic efficiency of these carbon sponge devices as well as related control samples towards water splitting was evaluated by monitoring the time dependent hydrogen generation under simulated sunlight. The hydrogen evolution amount is linearly dependent on time in both cases in 5 h, as shown in Fig. 9a. ZCs displays a hydrogen generation rate of about 67 µmol•g−1. For ZRCs sample, improvement of H2 generation rate up to 73 µmol•g−1 was achieved after introducing rGO, which is in agreement with the photodegradation results. Note that the 15



content of ZnO in the ZRCs (50 mg) is merely 16.5 mg estimated by TG analysis, demonstrating the advantage of carbon sponge based photocatalytic device. No obvious loss of photocatalytic performance was observed after being used repetitively for three times (Fig. 9b), indicating that the ZRCs has good stability and recyclability under solar irradiation, thus making such materials promising for industrial application. Additionally, the morphology of ZRCs after 20 h photocatalytic reaction was examined by SEM (Fig. S6), confirming that abundant porous ZnOZIF-8 were still maintained on the skeleton of carbon sponge. Moreover, the XRD characterizations of ZRCs at different intervals during reaction (Fig. S7) also shows that the diffraction peaks of ZnO remain unchanged. It is worth noting that the characteristic peaks from ZIF-8 gradually disappeared with photocatalytic reaction process, which indicates that the ZIF-8 are not stable under illumination. The concept of 3D foam photocatalytic microreactor was recently proposed,5, 19 in which the open pore structure readily soaks up contaminated water and performs photocatalysis within itself. Simplified reaction setup and working principal of our ZRCs photocatalytic microreactor are schematically illustrated in Fig. 10a. Herein, RhB aqueous/methanol (10:10, v/v) solution was used and the procedure is shown in Figs. 10(b-e). A piece of sponge reactor with size of 3 cm × 3 cm × 2 cm was saturated within the RhB aqueous/methanol mixture solution and was then irradiated with simulated sunlight. After 4 h, the degradation rate of RhB reached approximately 96.3%. As a result, a colorless transparent solution was expelled from the system by simply compressing the foam microreactor (Fig. 10e). Simultaneously, the evolved H2 from the microreactor was up to 3.19 µmol. Moreover, the reactor is able to return to its original form after the entire process, implying a favorable reusability of the foam microreactor. 16


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The as-prepared ZRCs 3D foam photocatalysts with ZIF-8 derived hollow ZnO structures and rGO modification possess several advantages in photocatalytic degradation of water pollutant and hydrogen evolution. First, the porous structure and high specific surface of the resulting MOFs-derived ZnO photocatalysts favor their accessibility to the target molecule. Second, the MF derived carbon foam with 3D porous structures consists of pores with different scales from mesopores to macropores, which provide a combination of high specific surface area and open channels for efficient diffusion of reactants. Third, surface carbon modification and N/C co-doping of ZnO porous structures enable the visible light absorption of the microreactor. Fourth, when the samples are irradiated by UV-visible light, the ZnOZIF-8 nanocages attached to the rGO sheets inject their excited electrons to the rGO due to band alignment and prevent the radiative electron-hole recombination. The graphene sheets act as a web for the electron percolation mechanism and facilitate the charge separation.58, 59 The recombination of photo-induced electrons and holes can be significantly reduced in this way, thus improving the photocatalytic activity of the ZRCs microreactors. The proposed reaction mechanism of the photocatalytic microreactor is illustrated in Fig. 10f. All the combined benefits together endow the as-prepared ZRCs with superior photocatalytic performance. Conclusions In summary, we have designed and fabricated a bulky photocatalytic microdevice consisting of carbon sponge, rGO layer and ZIF-8 derived porous N/C co-doped ZnO nanocages for simultaneous utilization in solar H2 evolution and pollutant degradation. The photocatalytic microreactor was obtained by a simple dipping-hydrolysis process, followed by the direct carbonization of ZIF-8/GO/MF sponge. The procedure is rapid, cost-effective and can be easily 17



scaled up. The obtained ZRCs based photocatalytic microreactor showed superior photocatalytic activity for pollutant degradation and hydrogen production under simulated sunlight, and no apparent decrease of the photocatalytic activity was noticed after three repeated cycles. With high stability, good recyclability and excellent adaptability, as well as multiple functionalities as both absorber and photocatalyst, these 3D structure microreactors are highly promising for environmental remediation and renewable energy generation. This work provides a general strategy for the processing of other carbon sponge based composite photocatalysts with enhanced activity for practical applications. Supporting Information SEM image of ZIF-8/MF sponge without GO coating, SEM and TEM images of ZIF-8, FTIR spectra of ZIF-8, ZnOZIF-8 and ZnO, TG curves of ZRCs, N2 adsorption-desorption isotherms and pore size distribution curves of ZRCs and carbon sponge, XRD patterns and SEM images of ZRCs before and after photocatalytic reaction. Acknowledgements This work was supported financially by Shenzhen Science and Technology Innovation Committee (ZDSYS201602261932201, JCYJ20170412154335393, KQTD2016022619584022). Additional support was provided by the Southern university of Science and Technology (Grant No. G01296001), Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (Grant No. 2017B030301012) and NSFC (No. 51602143). References [1] Fox, M.A.; Dulay, M.T.; Heterogeneous Photocatalysis. Chem. Rev. 1993, 93 (1), 341-357, DOI 10.1021/cr00017a016. [2] Chen, D.; Zhu, H.; Yang, S.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. Micro–nanocomposites in Environmental Management. Adv. Mater. 2016, 28 (47), 10443-10458, DOI 10.1002/adma.201601486. 18


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Figure 1. (a) Schematic illustration of the fabrication process. Digital images of (b) pristine MF sponge foam, (c) GO-coated MF sponge, (d) ZIF-8/GO/MF sponge, and (e) ZnOZIF-8/rGO/carbon sponge (ZRCs) (bar: 5 cm).

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Figure 2. SEM images of (a) MF sponge, (b) GO-coated MF sponge, (c and d) ZIF-8/GO/MF sponge, and (e and f) ZRCs.

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Figure 3. (a) SEM and (b) HRTEM images of ZnOZIF-8 (inset shows the TEM image). (c) HAADF-STEM and the element mapping images of (d) Zn and (e) N of ZnOZIF-8.

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Figure 4. (a) XRD patterns of ZIF-8 and ZnOZIF-8, and (b) ZRCs and carbon sponge.

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Figure 5. (a) XPS survey scan spectra of ZRCs. High-resolution XPS spectra for (b) Zn 2p, (c) C 1s, and (d) N 1s.

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Figure 6. (a) UV-vis DRS spectra of the ZRCs, ZnOZIF-8 and ZnO, and the inset shows the digital images of ZnOZIF-8 and ZnO. (b) Calculation of band gap energy by Tauc equation using a plot of (αhν)2 vs. photon energy (hν).

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Figure 7. (a) Photocatalytic activities of different photocatalysts for RhB degradation under UV-visible light irradiation. (b) Changes of the characteristic absorption of RhB under different irradiation time using ZRCs (inset shows the color changes of RhB dye before and after photocatalytic reaction). (c) The first-order-kinetic plots of RhB degradation over different samples. (d) Recycling experiments for photocatalytic degradation of RhB using ZRCs.

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Figure 8. Active species trapping degradation experiments for the ZRCs sample.

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Figure 9. (a) Irradiation-time dependence of H2 production for different samples under the illumination of simulated sunlight. (b) Photocatalytic hydrogen generation as the function of irradiation time in three consecutive cycles for the ZRCs sample.

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Figure 10. (a) Schematic illustration of photocatalytic reaction in ZRCs-based microreactor. Process of RhB degradation and H2 production: (b and c) ZRCs absorbs RhB/methanol aqueous solution; (d and e) RhB solution and clear water that was extruded out from the foam reactor before and after photocatalytic reaction process, respectively. (f) Proposed mechanism of simultaneous sunlight-driven photocatalytic degradation of RhB dye molecules and hydrogen production of the ZRCs sample.

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Table of Contents Graphic: For Table of Contents Use Only

Synopsis: A novel ZIF-8 derived ZnO nanocages/rGO/carbon sponge monolithic photocatalytic microreactor was fabricated, which exhibited excellent absorption and photocatalytic activity for degradation of pollutants and H2 production.

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Carbon Sponge-based

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