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Construction of Three-Dimensional Hemin-Functionalized Graphene Hydrogel with High Mechanical Stability and Adsorption Capacity for Enhancing Photodegradation of Methylene Blue Yuewu Zhao, Yuanjian Zhang, Anran Liu, Zhenzhen Wei, and Songqin Liu* Key Laboratory of Environmental Medicine Engineering, Ministry of Education, Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China S Supporting Information *
ABSTRACT: A three-dimensional hemin-functionalized graphene hydrogel (Hem/GH) was prepared by a facile self-assembly approach. The as-prepared Hem/GH showed good mechanical strength with a storage modulus of 609−642 kPa and a high adsorption capacity to organic dye contaminants (341 mg g−1 for rhodamine B). Moreover, Hem/GH could be used as a photosensitizer for the photocatalytic degradation of organic dyes and displayed superior photodegradation activity of methylene blue (MB). This result was better than that of counterparts such as graphene hydrogel (GH) and commercial catalyst P25. The excellent cycling performance of the Hem/GH was well maintained even after multiple cycles on adsorption process and photocatalytic reaction. Interestingly, after the photodegradation of MB, a light-induced pH change of the solution from alkaline pH 8.99 to acidic pH 3.82 was observed, and 10 wt % total organic carbon remained. The liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) analysis confirmed the generation of acidic degradation products. The photocatalytic mechanism was further investigated by trapping experiments, which revealed that the MB degradation was driven mainly by the participation of O2•− radicals in the photocatalytic reaction. As an extended application, visually intuitive observation showed the as-prepared Hem/GH also had strong antibacterial properties. These results suggest that Hem/GH could be potentially used for practical application due to its high adsorption ability, excellent photocatalytic activity, and strong antibacterial properties. KEYWORDS: self-assembly, hydrogel, adsorption ability, photocatalytic activity, antibacterial properties, cycling performance techniques.15 Inspired by these methods, a number of 3D nanostructured functionalized graphene derivatives including atom-doped materials,16 metal/metal oxides17,18 polymers,19−21 organic/inorganic materials,22,23 and biomolecules24,25 have been successfully prepared. For instance, Zhao et al. reported the N-dopted 3D graphene framework and found that it possessed ultralow density, good adsorption capacity, and high specific capacitance.16 Tang et al. demonstrated the combination of noble-metal nanocrystal and single-layered graphene oxide can effectively improve the mechanical properties, and the prepared 3D macrostructures have been utilized as fixedbed catalysts for a Heck reaction.18 Chen et al. also synthesized graphene oxide−chitosan composite hydrogels and explored them as broad-spectrum absorbents for water purification.26 Therefore, the 3D functionalized graphene hydrogels has great prospects for extensive applications.
1. INTRODUCTION Graphene, a single-layer two-dimensional carbon nanomaterial, has elicited much attention not only in electronics and mechanics but also in adsorption property.1−3 Gan et al. reported the photothermal effect of graphene-based nanocomposites, which ensured a high photocatalytic activity to organic pollutants.4 This encouraged the development of relatively simple, low-cost, and effective graphene-based materials for removing organic contaminants. The graphene hybrid with anatase TiO2 nanoparticles was also reported to possess superior photocatalytic degradation of methylene blue (MB),5−7 indicating that the combined graphene and other materials might facilitate surface adsorption and charge transfer of electrons.8−10 On the other hand, 3D nanostructures can effectively improve the performance of graphene, which endowed the graphene with a wide range of useful applications including dye adsorption, sensors, supercapacitors, and catalysts.11−14 According to the major strategies in 3D nanostructure preparation, the graphene hydrogels could be easily obtained by the reductioninduced self-assembly, cross-linked, and template-directed © 2017 American Chemical Society
Received: August 31, 2016 Accepted: January 12, 2017 Published: January 12, 2017 4006
DOI: 10.1021/acsami.6b10959 ACS Appl. Mater. Interfaces 2017, 9, 4006−4014
Research Article
ACS Applied Materials & Interfaces It is also noted that porphyrin has been extensively used as a photosensitizer that can react with molecular oxygen and translate into singlet oxygen (1O2) by energy transfer (ET) and superoxide anion (O2•−) by electron transfer under proper irradiation, which was found to exhibit apparent photocatalytic activity for degradation of organic pollutants.27−29 This encouraged the development of a graphene−porphyrin hybrid composite for an efficient adsorption and photodegradation of organic contaminants from aqueous solution. Herein, we report a facile self-assembly approach to prepare a highly active heminfunctionalized graphene hydrogel (Hem/GH). The π−π interaction between graphene and hemin gave the Hem/GH a relatively large absorption volume and high mechanical stability. As a result, Hem/GH exhibited higher photocatalytic performance for MB dye in comparison with graphene hydrogel or commercial catalyst P25 and excellent cycling performance. The mechanism investigation revealed that the MB degradation was driven mainly by the participation of O2•− radicals.
incubated at 25 °C for 15 h, and the colony-forming units for B. subtilis on an LB agar plate could be observed. 2.4. Characterizations. The morphology and size of the samples were observed by scanning electron microscopy (SEM; JEOL JSM5610LV, Japan) operated at an acceleration voltage of 10 kV. Raman spectra were recorded on an Invia Raman spectrometer (Renishaw, UK) with a 514 nm laser. The hemin content was analyzed using an ICP optical emission spectrometer (Varian 710-ES, USA). X-ray powder diffraction (XRD; D8 Advance, Bruker, Germany) was characterized with high-intensity Cu Kα radiation (λ = 0.15406 nm), and the data were collected in the 2θ range of 20−80° at a step size of 0.02°. X-ray photoelectron spectroscopy (XPS) was acquired using a Kratos Analytical Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα source at 1486.6 eV. Fourier transform infrared spectroscopy (FT-IR) was carried out using a Nicolet 4700 FTIR spectrometer (Thermo, USA) equipped with an attenuated total reflection (ATR) setup. The total organic carbon (TOC) was measured with a Tekmar Dohrmann Apollo 9000 TOC analyzer. The photocatalytic products were analyzed by liquid chromatography/ time-of-flight mass spectrometry (LC/TOF-MS, Agilent Technologies Inc., USA). UV−vis absorption spectra were recorded on a 2450 UV− visible spectrophotometer (Shimadzu, Japan).
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of 3D Hemin/ Graphene Hydrogel. Scheme 1 illustrates the simple
2.1. Synthesis of GO Sheets and Hemin/Graphene Hydrogel. GO was prepared from natural graphite powder (325 mesh) by using a modification of Hümmer’s method.30,31 The hemin/graphene hydrogel was obtained by hydrothermal reaction using GO aqueous dispersion as the precursor and hemin as the surface functionalization agent. In a typical procedure, 0, 5, 6.7, 10, 20, and 40 mg of hemin were dissolved in 5 mL of methanol to form a homogeneous solution, respectively, followed by the addition of 5 mL of GO (4 mg mL−1) aqueous dispersions under continuous stirring. After that, the reaction mixture was placed in an oven at 130 °C for 3 h. The resulting product was taken out by using tweezers and washed with methanol and deionized water several times. Finally, the as-prepared hemin/ graphene hydrogels were freeze-dried overnight for the following experiment. According to the mass ratio between hemin and GO, the hemin/graphene hydrogel samples with ratios of 1:4, 1:3, 1:2, 1:1, and 2:1 have been named Hem/GH-1, Hem/GH-2, Hem/GH-3, Hem/ GH-4, and Hem/GH-5 in all of these samples. The hemin content of the as-prepared hydrogel was analyzed using an ICP optical emission spectrometer. 2.2. Dye Adsorption and Photocatalytic Measurements. Methylene blue (MB) as a heteropolyaromatic dye was used to evaluate the adsorption and photocatalytic performance of the asprepared Hem/GH samples. The absorption maximum was measured by dispersing 10 mg of Hem/GH samples into 80 mL of MB aqueous solution at concentrations of 1, 5, 10, 20, 30, 40, and 50 mg L−1 under magnetic stirring for 3 h in the dark at room temperature to reach the adsorption−desorption equilibration. From the difference in the absorbance before and after adsorption, the amount of dyes adsorbed by the Hem/GH catalyst could be estimated. For the photocatalytic measurement, 80 mL of MB aqueous solution (50 mg L−1) along with 10 mg of Hem/GH was exposed to the visible light irradiation produced by a 150 W Xe lamp. (The illuminating light intensity was 16 mW cm−2.) At given time intervals, specified analytical samples (700 μL) were pipetted from the reaction system and monitored using a UV−vis spectroscopy. The degree of photocatalytic degradation was calculated by measuring the absorbance of MB solution and the total organic carbon (TOC) analysis. 2.3. Antibacterial Activity of the Prepared Hemin/Graphene Hydrogel. Bacterial activation of Hem/GH was investigated according to our previous work.32 Briefly, 10 μL of Bacillus subtilis solution, obtained from Hubei Qiming Organisms Corp. (Hubei, China), was diluted with 2 mL of sterilized phosphate buffer solution (PBS, 0.1 M, pH 7.0). Then, 10 μL of this bacterial suspension was spread on the solid Luria−Bertani (LB) culture medium and irradiated for 0, 30, 60, 90, 120, and 150 min in the self-made reactor (Scheme S1), respectively. After that, the solid LB culture medium was
Scheme 1. Synthetic Route and 3D Interconnected Structure of the Hemin-Functionalized Graphene Hydrogela
a
In the process of the hydrothermal reaction, GO could be easily reduced to rGO and interact with hemin to form Hem/rGO composite, whereas the generated Hem/rGO sheets can simultaneously self-assemble due to the overlapping and coalescing interaction. Therefore, the 3D hemin-functionalized graphene hydrogel was prepared by a facile in situ self-assembly process.
hydrothermal process for fabrication of a 3D hemin/graphene hydrogel (Hem/GH) composite. Graphene oxide aqueous dispersion was used as the precursor, and hemin was used as the surface functionalization agent. Comparing with graphene hydrogels (GH) prepared at the same temperature, the asprepared Hem/GH has a significant large volume and high water content (99%, Scheme 1). This indicated that the doped planar hemin molecules were beneficial in forming a porous structure. The Hem/GH-3 showed an approximate type IV Langmuir isotherm with a Brunauer−Emmett−Teller surface area of 297.8 m2 g−1, which was much higher than that of GH (185.6 m2 g−1; Figure S1). Similarly, the Au nanoparticle-doped graphene hydrogels also exhibit high water contents.33,34 Meanwhile, the hemin contained in the 3D Hem/GH composite increases with the increasing amount of hemin 4007
DOI: 10.1021/acsami.6b10959 ACS Appl. Mater. Interfaces 2017, 9, 4006−4014
Research Article
ACS Applied Materials & Interfaces
Figure 1. (A) SEM images of the freeze-dried Hem/GH-3. (B) Element mapping images for C, O, Fe, and Cl. The four elements (C, O, Fe, and Cl) can be observed to exist homogeneously within the sample, and demonstrate the uniform distribution of these elements across the entire Hem/GH3. (C) Raman spectra of GO, GH, hemin, and Hem/GH.
sample, demonstrating the uniform distribution of these elements across the entire Hem/GH. The XRD patterns show that the Hem/GH has a weaker diffraction peak (002), indicating the π−π stacking between graphene sheets and hemin, which keeps the neighboring graphene sheets further separated (Figure S6). Raman spectra for GO and GH display two prominent peaks at 1350 and 1594 cm−1, which correspond to the disorder of the graphite (D band) and to the ordered sp2 bonding carbon atoms (G band, Figure 1C). The intensity ratio of D to G peak (ID/IG) for GH was increased significantly compared with GO, indicating the decrease in the average size of the sp2 domains and improvement of the structural defects.41,42 On the other hand, the Raman spectrum of hemin showed three characteristic peaks at 1365, 1565, and 1625 cm−1, assigned to symmetric pyrrole-half-ring stretching and asymmetric C−C stretching.43,44 In the presence of hemin, the Raman spectra of freeze-dried Hem/GH-3 showed that the three characteristic peaks of hemin remained with the D band and G band of graphene, confirming the successful formation of Hem/GH. Additionally, the UV−vis absorption spectrum of Hem/GH-3 showed a broad absorption at 265 nm corresponding to structural defects of the reduced graphene oxide, which is produced by the attachments of hemin molecules and hydrothermal reduction reaction.42 The well-defined absorption peak at 421 nm ascribed to the Soret band of hemin, which is 22 nm red-shifted from the hemin free in solution, indicates the existence of the cation−π interactions between iron centers and GO.45,46 The XPS measurements explored that the deconvolution of C1s spectrum of GO had three types of carbon bonds at 284.6 eV (CCC), 286.1 eV (CO), and 288.8 eV (CO). Compared with GO, the peaks related to the oxidized carbon species were greatly weakened, indicating that the GO has been well deoxygenated to form Hem/GH-3 (Figure S6). Moreover, the new peak at 285.2 eV (CN) comes from the π−π conjugated hemin, which also confirmed the Hem/GH-3 was successfully prepared. All of these results illustrated that the hemin and reduced GO coexisted in the prepared composite hydrogel. 3.2. Adsorption Capacity of Hemin/Graphene Hydrogel. The adsorption capacity of the Hem/GH toward the organic dye from aqueous solution was investigated by using MB as a model probe. The adsorption of MB at different initial concentrations was studied in the dark at room temperature for 3 h (Figure S7). The adsorption isotherm shows that the maximum adsorption capacity of MB to the Hem/GH-3 was 99.2 mg g−1 (Figure 2B), which outperforms many other
during the preparation of the hydrogel (Figure S2). The largest value of the doped hemin was 34% of the total mass value of Hem/GH. The as-prepared Hem/GH-3 hydrogel possesses good mechanical strength with a storage modulus of 609−642 kPa, which was 1.4 times larger than that of GH without hemin dopping. This confirmed that the π−π conjugation between graphene and hemin can effectively increase the mechanical stability of the hydrogel, making it more stable to maintaining its structure and for reuse in the practical application. We speculated that the hemin molecules were adsorbed on both sides of the graphene by π−π conjugation. Some hemin molecules placed between two layers of graphene improve the mechanical stability of the prepared hydrogels.35,36 On the other hand, some of the hemin molecules may adsorb on the surface of hydrogels, which makes them more conducive to photodegradation. It is speculated that the overlapping and coalescing of graphene sheets due to π−π conjugation and the regional diminishing of oxygenic functional groups lead to the generation of a cross-linked 3D Hem/GH network.37−39 With the hydrothermal reaction, the GO was reduced to rGO,34,37 whereas hemin attached onto graphene sheets through π−π stacking interaction to obtain Hem/rGO sheets simultaneously. The hemin coated on graphene can serve as intercalated molecules to keep the neighboring graphene sheets separated and finally form hydrogel with large size (Scheme 1). Meanwhile, the hydrothermal reaction temperature and starting GO concentration play important roles in the preparation of hydrogels. At a hydrothermal reaction temperature 130 °C, the gelation process can be completed within 3 h. At the same time, when the starting GO concentration was
