Investigation of Free-Standing Plasmonic Mesoporous Ag/CMK-8-Nafion Composite Membrane for the Removal of Organic Pollutants with 254-nm UV Irradiation
© The Author(s). 2017
Received: 7 February 2017
Accepted: 2 May 2017
Published: 19 May 2017
“Carbon-based material” has demonstrated a great potential on water purification due to its strong physical adsorption to organic pollutants in the water. Three-dimensional cubic ordered mesoporous carbon (CMK-8), one of the well-known ordered mesoporous carbons, was prepared by using nanocasting method with mesoporous silica (KIT-6) as the template. In this study, CMK-8 blended with Nafion polymer to form a free-standing mesoporous CMK-8-Nafion composite membrane. The synthesis of high crystallinity CMK-8 was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). More than 80% methyl orange (MO) removal efficiency was observed under 254-nm UV irradiation after 120 min. Ninety-two percent recycling performance was remained after four recycling tests, which indicated a reliable servicing lifetime for the water purification. Furthermore, an additional layer of plasmonic silver nanoparticles (Ag NPs) was integrated into this CMK-8-Nafion membrane for higher pollutant removal efficiency, attributing from the generation of plasmon-resonance hot electrons from Ag NPs. A 4-in. CMK-8-Nafion composite membrane was also fabricated for the demonstration of potential large-scale utilization.
In addition to activated carbon and carbon nanotubes, ordered mesoporous carbon is another effective material used for removing pollutants from wastewater [16–19]. Ordered mesoporous carbon has been receiving much attention because of its high surface area, high conductivity, and highly uniform and regular pore sizes, which facilitate mass transport [20–23]. Moreover, ordered mesoporous carbon has been successfully employed in energy storage devices such as fuel cells [24–26] and supercapacitors . Different structures, sizes, and shapes of ordered mesoporous carbon can be explicitly synthesized by varying fabrication parameters and surfactant concentrations [28, 29]. In this study, we propose the application of a free-standing CMK-8-Nafion composite membrane in the photo-induced decomposition of methyl orange (MO). Mesoporous carbon CMK-8 can not only adsorb MO  but can also effectively absorb photons due to their blackbody property [31, 32], contributing to the additional photo-induced decomposition of MO . This unique dual mechanism consisting of physical adsorption and photocatalytic decomposition is discussed under different experimental conditions. Finally, we also layered silver plasmonic NPs [33, 34] onto this free-standing mesoporous CMK-8-Nafion composite membrane to further enhance the removal of organic water pollutants.
High-quality samples of mesoporous silica KIT-6 and the corresponding mesoporous carbon CMK-8 were prepared using a process similar to that used in previous studies . CMK-8 has a reversed cubic structure, which was then replicated using KIT-6 as a hard template. A dilute H2SO4(aq) solution was added to sucrose solution with weight ratios of 1 g KIT-6/1.25 g sucrose/5 g H2O/0.14 g H2SO4. The colloid mixture was dried at 333 K for 6 h and dehydrated at 433 K for 6 h. The aforementioned steps were repeated again with a mixture of 0.8 g sucrose/3.2 g H2O/0.09 g H2SO4. The resultant dark brown powders were carbonized under argon atmosphere at 1173 K for 1 h. The silica template was removed with 1 M hydrofluoric acid in a solution of 50% ethanol and 50% H2O, and CMK-8 was finally collected. For the fabrication of CMK-8-Nafion composite membranes, designated amounts of CMK-8 were mixed with Nafion solution at a solid-content ratio of 30%, and this CMK-8-Nafion precursor was ultrasonically agitated for 10 min before the casting step. Each mesoporous CMK-8-Nafion membrane was formed by pouring CMK-8-Nafion precursor solution in a 4-in. petri dish and was then solidified at 323 K for 40 min. In addition, for the deposition of the silver NP layer, a precursor composed of silver acetylacetonate weighing 0.035 g [Ag(acac); 98%, Acros] was dissolved in 30 mL deionized water mixed with 5 mL 99.5% alcohol and the prepared mesoporous CMK-8-Nafion membrane was then immersed in the solution for 15 h . After examination of several samples, the average thickness of the Ag/CMK-8-Nafion membrane was 0.3–0.4 mm. The microstructures and morphologies of KIT-6 and CMK-8 were examined using a scanning electron microscope (SEM; FE-SEM; HITACHI S-4800) and a transmission electron microscope (JEOL JEM-2100F). The pore sizes and specific surface areas were analyzed using N2 adsorption/desorption analysis under 77 K (Micromeritics; ASAP2020). The mesostructures of KIT-6 and CMK-8 were confirmed by small-angle (2θ of 0.5°–8°) powdered X-ray diffraction (XRD) by using Cu Kα radiation (λ = 0.154 nm; scan rate of 1°/min). The chemical states of silver NPs were examined using an X-ray photoelectron spectroscope (XPS; ULVAC-PHI Versa-probe) with Al Kα X-rays and a 45° photoelectron takeoff angle. A 1-eV flooding electron source and 7-eV Ar+ was applied for charge compensation during spectrum acquisition. Finally, a UV-Vis integrating sphere was used to evaluate the performance of organic pollutant decomposition by the free-standing mesoporous CMK-8-Nafion membranes under UV irradiation at 254 nm.
Results and Discussion
According to the Al Kα X-ray photon energy (1486.6 eV), the AgM5N5N5 binding energy was calculated as 1133.8 eV. As presented in Fig. 2d, the KE of AgMNN was 358.8 eV (6.0 eV added to the KE data on M5N5N5 to obtain the KE of M4N5N5) [37, 38]. The auger interpretation of the binding energy (367.39 eV; Fig. 2c) and the KE of AgMNN (358.8 eV) indicate the existence of metallic silver NPs in our sample, suggesting that the sample has active plasmon resonance and substantial hot electron generation.
Free-standing CMK-8-Nafion membranes were fabricated for improving MO decomposition in wastewater. The basic membrane removed pollutants with an efficiency level of more than 80% after 120 min of UV irradiation at 254 nm. A reliability test indicated that the basic CMK-8-Nafion membrane still retained 92% of its original efficiency after four consecutive MO decomposition processes. Furthermore, with the integration of a silver NP layer, 98% MO decomposition efficiency was achieved, which was approximately 20% higher than that of the basic CMK-8-Nafion membrane. Finally, we demonstrated the feasibility of fabricating a 4-in. free-standing CMK-8-Nafion membrane for high-throughput wastewater treatment.
The authors acknowledge financial support from the Ministry of Science and Technology Foundation (MOST 105-2221-E-035-015-). The authors appreciate the support of the Precision Instrument Support Center of Feng Chia University for providing the facilities for production and measurement.
HLC carried out the experiments and drafted the manuscript. MSC participated in the design of the study and performed the analysis. CJP and CJL participated in the measurements. SNL conceived the study and participated in its design. CMT and WHH supervised the overall study and polished the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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