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Dec 13, 2016 - In this study, a 4 in. CMK-8-Nafion membrane was fabricated using three-dimensional cubic ordered mesoporous carbon CMK-8 blended with ...
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Plasmon Induced Hot Electrons on the Mesoporous Carbon for Decomposition of Organic Pollutant under Outdoor Sun Light Irradiation Hsin-Liang Chen, Chia-Jui Li, Chien-Jung Peng, Hoang-Jyh Leu, and Wei-Hsuan Hung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11360 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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Plasmon Induced Hot Electrons on the Mesoporous Carbon for Decomposition of Organic Pollutant under Outdoor Sun Light Irradiation

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Hsin Liang Chen , Chia-Jui Li , Chien-Jung Peng , Hoang-Jyh Leu , and Wei-Hsuan Hung *

Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan *

E-mail: [email protected];

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These authors contributed equally to this work.

Abstract In this study, a 4-inch CMK-8-Nafion membrane was fabricated using three-dimensional cubic ordered mesoporous carbon CMK-8 blended with a Nafion polymer. Plasmon-resonance hot electrons and holes from Au nanoparticles (NPs), combined with this CMK-8-Nafion membrane, resulted in the effective decomposition of methyl orange (MO) due to the synergetic work of hot carriers with mesoporous carbon; a sample of Au/CMK-8-Nafion exposed to outdoor sun light radiation for 150 minutes successfully removed 97% of MO. Fourier transform infrared spectroscopy (FTIR) was employed to examine the generation of hydroxyl groups (OH─) during the decomposition. Finally, the spatial distribution of hydroxyl groups was also investigated across the different coverage densities of plasmonic Au NPs. Keywords: Mesoporous carbon; Photocatalytic water purification; Plasmonic nanoparticles; Hot electron transfer; FTIR spectroscopy

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Figure 1. Illustration of the enhanced MO decomposition mechanism and reaction route of the Au/CMK-8-Nafion system. (a) CMK-8 in dark, (b) CMK-8 and (c) Au/CMK-8 under sun light irradiation.

Introduction Plasmonic nanoparticles (NPs) have been gaining significant attention due to their extraordinary energy coupling channels between photon and plasmon1-3, resulting in strong light scattering4-6 and near field effects4-7 for the improvement of solar spectrum utilization. In addition to enhanced incident photon absorption during the plasmon resonant process, the hot electrons and holes generated from the plasmon decay have been utilized and made impacts on many fields. The intimate interaction of the plasmonic NPs with supported materials and surrounding agents becomes a significant key to utilize these hot carriers efficiently. Several plasmonic metal interfaces have been investigated to improve the extraction rate of free hot carriers (i.e., hot electrons and holes). Z. Fang et al. developed a plasmonic graphene-antenna photodetector with 20% internal quantum efficiency 2

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for visible and near-infrared photons by integrating two single layer graphene sheets with plasmonic nanoparticles; the additional hot electrons generated in the designed plasmonic antenna structure contribute to the improvement of the detected photocurrent.8 Mukherjee et al. reported a novel reaction route for hydrogen dissociation with the existence of hot electrons, which can be directly transferred to the pre-absorbed H2 molecules, substantially reducing the barrier for H2 dissociation.9, 10 Furthermore, plasmonic metal-semiconductor structures are other intensively studied systems because of the possibility for photorcurrent generation at photon energies below the band gap of the semiconductors from the transfer of hot electrons at the metal and semiconductor interface.11-13 In addition, carbon-based materials have demonstrated a great potential for water purification due to the strong physical adsorption of organic pollutants in the water. With the rapid development of nanotechnology, carbonaceous-based materials with varying structural dimensions, such as fullerene C60 (0-D)14-18, carbon nanotubes (1-D)15, 19-24, graphene (2D)25-29, and mesoporous carbon (3-D)30-34, are effectively fabricated and widely used in a variety of applications. Even though several investigations of plasmonic hot electrons and holes on low dimension carbon materials have been reported, the demonstration of hot electrons and holes generated with mesoporous carbon still remains insufficient. Mesoporous carbon materials are as known for their high conductivity and uniform structural porosity, which can provide effective 3D channels for draining hot carriers from resonant plasmonic NPs. To demonstrate this concept, a gold NPs-CMK-8-Nafion composite membrane was fabricated to improve the photo-induced decomposition of methyl orange (MO) exposed to solar radiation. The merits of plasmonic NPs with mesoporous carbon for the removal of organic pollutants can be observed in the three scenarios depicted in Figure 1. (i) High surface physical adsorption sites: Most carbon materials possess an exceptional adsorption capability for various organic contaminants via many possible types of bonding, such as electrostatic interactions, π-π bonding, hydrogen bonding, and hydrophobic interactions, shown in Figure 1a. (ii) Blackbody photon absorption enhancement: In addition to superior adsorption of organics, mesoporous carbon is also considered as a blackbody material, which can contribute to enhanced photo-induced decomposition for the preadsorbed methyl orange shown in Figure 1b. (iii) Plasmon-induced hot carrier generation: These high energy electrons and holes from the plasmon decay process will react with aqueous solutions at the surfaces of Au NPs and mesoporous carbon to generate •OH and •O2, accelerating the decomposition process of organic pollutants in wastewater.35-37

Experimental Methods High-quality mesoporous silica KIT-6 and the corresponding mesoporous carbon CMK-8 were prepared by a procedure similar to previously reported works.30, 31 Typically, a dilute H2SO4(aq) solution was added to a sucrose solution with weight ratios of 1 g KIT-6 / 1.25 g sucrose / 5 g H2O / 0.14 g H2SO4. The mixture colloid was dried at 70°C for 6 hours and dehydrated at 160°C for 6 3

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hours. The aforementioned steps were repeated with 0.8 g sucrose / 3.2 g H2O / 0.09 g H2SO4. The resultant dark brown powders were carbonized under argon at 900°C for 1 hour. Finally, CMK-8 was collected after the silica template was removed with a 1 M HF solution containing 50% ethanol and 50% H2O. CMK-8 exhibited a reversed cubic structure, which was then replicated by using KIT-6 as the hard template. To prepare the CMK-8-Nafion composite membrane, the designated amount of CMK-8 was mixed with a Nafion solution at the solid-content ratio of 30%; this CMK-8-Nafion mixture was ultrasonically agitated for 10 min before the casting process. The mesoporous CMK-8-Nafion membrane was formed by pouring the CMK-8-Nafion precursor solution in a 4-inch petri dish which was solidified at 60°C for 40 min. After examination of several samples, the average thickness of the Au/CMK-8-Nafion membrane was 0.3-0.4 mm. The layer of Au NPs was deposited on top of the CMK-8-Nafion surface by the standard sputtering process with a 10 second operation time. The Au NP layer is not thick enough to form a continuous film but instead produces an island-like morphology that is known to exhibit strong plasmonic resonance.38-40 In addition, the microstructure and morphologies of KIT-6 and CMK-8 were examined by the scanning electron microscopy (FE-SEM; HITACHI S-4800) and transmission electron microscopy (JEOL JEM-2100F). The pore size and specific surface area were analyzed by N2 adsorption/desorption analysis at 77 K (Micromeritics; ASAP2020). The meso-structure of KIT-6 and CMK-8 were also confirmed by small angle (2θ of 0.5-8°) powdered X-ray diffraction (XRD) using Cu Kα radiation (λ=0.154 nm; scan rate of 1°/min). UV-Vis connected to an integrating sphere was employed to study the performance of organic pollutant decomposition by these free-standing mesoporous CMK-8-Nafion membranes, after exposure to solar radiation for 150 minutes. Finally, Fourier transform infrared spectroscopy (FTIR) was employed to analyze the concentration of hydroxyl groups on the varying sample surfaces during the decomposition.

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Results and Discussion

Figure 2. Characterization of KIT-6 and CMK-8. (a), (b) High magnification TEM images; (c), (d) Small angle of XRD patterns; (e) N2 adsorption/desorption isotherms (77 K) of CMK-8 and (f) SEM images and corresponding EDX elemental mappings of C (red), Au (green) of Au/CMK-8-Nafion membranes in the center, middle and edge regions. Figure 2a-b show the high resolution TEM (HRTEM) microstructure images of mesoporous silica KIT-6 and the corresponding 3-D structure of ordered mesoporous carbon CMK-8, which exhibits a well-ordered honeycomb structure with uniform pore size. Long-range ordering porosity of KIT-6 and CMK-8 was also observed in the XRD patterns shown in Figure 2c-d, consistent with the HRTEM results. Due to the high surface area of the cubic CMK-8, fast mass transfer kinetics become possible, which significantly benefits organic molecule adsorption. The N2 adsorption/desorption isotherms (77 K) were examined to determine the specific surface area of CMK-8. It presented a common type-IV isotherm with a broad hysteresis loop, which exhibited the 5

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typical characteristics of capillary condensation in mesoporous channels shown in Figure 2e. According to the Brunauer–Emmett–Teller (BET) method, the CMK-8 possessed a specific surface area of 840.67 m2 g-1, while an analysis of the isotherm desorption branch using the Barrett–Joyner– Halenda (BJH) method indicated an average pore size diameter of approximately 4 nm. Due to its high specific surface area and porous nature, CMK-8 is expected to provide a substantial number of active sites for physical adsorption of organic pollutants. Figure 2f shows SEM images of the center, middle and edge regions of the 4-inch Au/CMK-8-Nafion membrane and their corresponding EDX mapping of C and Au. According to these SEM images, different coverage densities of Au NPs were observed in the center, middle and edge regions, which were attributed to different sputtering angles. Consequently, the morphology of Au NPs in these three areas vary, and a closer proximity to the center results in coarser Au NPs, which could result in a significantly different population of hot carriers due to the different plasmon resonance conditions.

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Figure 3. (a) Photograph of MO decoloration under solar radiation. UV-Vis spectra of the evolution of MO decomposition: (b) under sunlight irradiation only, (c) with CMK-8-Nafion membrane, (d) with Au/CMK-8-Nafion, (e) the comparison of MO decomposition efficiency with different experimental conditions. Figure 3a shows the experimental setup for our MO decomposition test using outdoor solar radiation; all three samples were processed at the same time to provide the same exposure conditions. To investigate the MO decomposition efficiency and mechanism, three different experimental setups for testing the removal rate of methyl orange were used: (i) only sunlight, (ii) a CMK-8-Nafion membrane, or (iii) an Au/CMK-8-Nafion membrane. Figure 3(b-d) shows the changes in the UV-Vis spectra under these three conditions. The absorption peak of methyl orange at 463 nm is due to the conjugated structure of the azo bond. The decrease of peak intensity and the decoloration of the solution indicate the decomposition of methyl orange.39, 41, 42 MO showed negligible self-degradation after exposure to solar radiation for 150 minutes (Figure 3b). A noticeable decrease in the 463 nm peak was observed for the CMK-8-Nafion membrane trial (Figure 3c); this decrease is due not only to the strong physical adsorption ability of mesoporous carbon to the methyl orange but also the enhanced photo-induced decomposition for the preadsorbed methyl orange. Figure 3d shows the plasmon-enhanced decomposition of methyl orange with an Au/CMK-8-Nafion membrane. This membrane achieved a remarkable 97% decomposition of methyl orange within 150 minutes, a rate 7 times greater than that of CMK-8 only, indicating the importance of Au NPs. This decomposition enhancement is attributed to the hot carriers generated on the surface of the Au NPs from the plasmon decay process, which significantly elevate the population of active radicals in the solution and provide an additional reaction route for the MO decomposition process (see Figure 1c). The photons from solar radiation create hot electrons and holes which are excited from the surface of the gold NPs to produce superoxide and hydroxyl radicals, which are the primary oxidizing species in photocatalytic oxidation processes. Notably, the excellent molecule adsorption ability of the CMK-8 also provides a perfect reaction ground for these active oxidizing species with pre-absorbed MO molecules to accelerate the decomposition process. Figure 3e compares the decomposition efficiency of these three scenarios. For the further understanding of the adsorption properties of the samples, the comparison of the absorption spectra of methyl orange for Au/CMK-8-Nafion and CMK-8-Nafion without light irradiation are provided in Figure S1 in the supporting information. In addition, the recycling stability of the Au/CMK-8-Nafion is provided in Figure S2 in the supporting information.

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Figure 4. FTIR spectra of (a) the hydroxyl group population on the three different surfaces after 120 mins, (b) evolution of the hydroxyl group from the Au/CMK-8-Nafion sample during decomposition, and (c) Spatially resolved mapping of hydroxyl group across different plasmon-enhanced regions. To investigate the MO decomposition efficiency and mechanism, Fourier transform infrared spectroscopy (FTIR) was employed to analyze the concentration of hydroxyl groups (OH─) on the different sample surfaces after 120 minute irradiation. All samples were dried on a hot plate at 80°C for 30 minutes prior to measurement. In Figure 4a, we examined the CMK-8-Nafion and Au/CMK-8-Nafion membranes under different conditions. First, a CMK-8-Nafion membrane was soaked in the MO solution and kept in the dark for 120 minutes; the FTIR spectrum curve marked in black on Figure 4a showed typical peaks at ~2854 cm-1 and ~2930 cm-1 due to the C-H stretching of 9

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methyl groups from adsorbed methyl orange on the surface of the CMK-8-Nafion membrane. Furthermore, there was a small broad peak from 3000-3700 cm-1 which belonged to the stretching vibration absorption of the hydroxyl groups. We also examined the CMK-8-Nafion membrane under after exposure to solar radiation for 120 minutes and the peak intensity of hydroxyl group absorption was 3 times higher than that in the dark, which implies that the enhanced photo-induced decomposition increased the hydroxyl radical generation. Remarkably, integrating plasmonic Au NPs observed a pronounced increase in the hydroxyl absorption peak (Figure 4a), which suggests that a large amount of hydroxyl group were formed at the surface of the Au/CMK-8-Nafion membrane. This significant increase of hydroxyl group we believed is due to the hot carrier generation from plasmon resonance decay, contributing the generation of OH radical and then these active radical could react with mesporous carbon and increase of the OH─ population at carbon surface. This result can also explain why Au/CMK-8-Nafion possessed the highest decomposition efficiency observed in Figure 3. For the further proof of the hot carrier enhanced mechanism, we also monitored the evolution of CO2 production with the sample of Au/CMK-8-Nafion membrane, which are provided in Figure S3 in the supporting information. We also examined the evolution of hydroxyl group from the Au/CMK-8-Nafion sample during the 120 minute decomposition reaction. As expected, the absorption peak of the hydroxyl groups gradually increased with time as shown in Figure 4b, which corresponds to the growth of hydroxyl radicals during sunlight exposure. Finally, the spatial mapping of hydroxyl group was performed by examining the different plasmonic resonant conditions across the Au/CMK-8-Nafion thin film. The highest intensity of hydroxyl population was observed in the center area of the thin film, implying that the region could have the highest generation of OH radicals from the plasmon decay. According to the SEM image observed in Figure 2, the average Au NPs size is approximately 30-50 nm, which is a large enough optical cross-section for interaction with incident photons, with respect to those in the middle area. In addition, the lowest concentration of hydroxyl group occurred at the edge due to the negligible coverage of Au NPs.

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Figure 5. Illustration of hot carriers excitation and transition to the mesoporous carbon. Figure 5 illustrates a possible explanation for the enhanced mechanism in this plasmonic NPs-mesoporous carbon system. Solar radiation provides the incident photons with enough energy to excite electrons above the Fermi level of metal; these high energy electrons separate from the plasmonic metal before recombination because of the internal electric field built from the work-function mismatch between mesoporous carbon and the plasmonic Au NPs. In addition, the gapless carbon material also provides a better transport channel for the hot electron than semiconductors due to its high conductivity. As a result, photogenerated holes remain trapped in the plasmonic Au NPs. Therefore, these separated electrons and holes would have a sufficiently long relaxation time to produce superoxide radical anions and hydroxyl radicals, both of which act as effective oxidizing species which contribute to photocatalytic oxidation processes.

Conclusions In this work, a 4-inch Au/CMK-8-Nafion composite membrane successfully decomposed 97% of methyl orange upon exposure to solar radiation for 150 minutes. This decomposition rate was seven times greater than that of CMK-8 alone, which indicates the importance of hot carriers from gold nanoparticles in this process. Hot carriers generated from plasmon decay were investigated in this plasmonic NPs-mesoporous carbon system by observing the concentration change of hydroxyl groups (OH─) using FTIR spectroscopy. Additionally, this Au/CMK-8-Nafion system will not leave any unnecessary legacy products in the cleaned water even after several recycling tests, which eliminate the additional effort on the recollection of photocatalyst. 11

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Associated Content Supporting Information. Absorption spectra under the dark condition; Recycling examination of Au/CMK-8 membrane; The changes of concentration of CO2 during the decomposition reaction.

Acknowledgments The authors acknowledge the financial support by the National Science Council Foundation (MOST 105-2221-E-035-015- and MOST 104-2221-E-035-043- to WHH). The authors appreciate the Precision Instrument Support Center of Feng Chia University in providing the fabrication and measurement facilities.

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